Mechanical Technology Gr11 Learner's Guide

Mechanical Technology Grade 11 Learner’s Book

SAMPLE COPY

© Future Managers 2012 All rights reserved. No part of this book may be reproduced in any form, electronic, mechanical, photocopying, or otherwise, without prior permission of the copyright owner. ISBN 978-1-920133917 To copy any part of this publication, you may contact DALRO for information and copyright clearance. Any unauthorised copying could lead to civil liability and/or criminal sanctions.

Telephone: 086 12 DALRO (from within South Africa); +27 (0)11 712-8000 Telefax: +27 (0)11 403-9094 Postal Address: P O Box 31627, Braamfontein, 2017, South Africa www.dalro.co.za First published 2007 Revised 2012

Please note that this is a sample draft copy and may still undergo minor changes.

FutureManagers Published by Future Managers (Pty) Ltd PO Box 13194, Mowbray, 7705 Tel (021) 462 3572 Fax (021) 462 3681 E-mail: [email protected] Website: www.futuremanagers.net

Contents How to use this book Chapter 1: Chapter 2: Chapter 3: Chapter 4: Chapter 5: Chapter 6: Chapter 7: Chapter 8: Chapter 9:

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Safety........................................................................................................................................................................................ 1 Tools...................................................................................................................................................................................... 19 Materials............................................................................................................................................................................ 57 Terminology................................................................................................................................................................... 79 Joining methods....................................................................................................................................................... 119 Forces................................................................................................................................................................................. 157 Maintenance................................................................................................................................................................ 185 Systems and control.............................................................................................................................................. 209 Pumps............................................................................................................................................................................... 277

How to use this book Outline of the curriculum Mechanical Technology Grade 11 Learner’s Book has been based on the new FET Curiculum for Mechanical Technology which entails 9 topics.

Spider diagrams Each chapter is introduced by a spider diagram which is a diagrammatical summary of the content covered in a particular chapter. The following spider diagram is an example from Chapter 6:

Forces found in engineering components

Performing basic testing on basic mechanical principles

Forces

Basic calculations on stress

iv

Moments found in engineering components

Explanation of icons The following icons are used throughout the book to help you to recognise important concepts or activities. Icon

Description Topics

Assessment

Did you know?

Key word

Pause for thought

Caution!

✎

Note!

Besides the various icons, explanatory notes, Pause for thought and Did you know? boxes have been placed in the margin to give further insights.

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The importance of Mechanical Technology in the world South Africa and many other countries in the world currently suffer from huge skills shortages and desperately need skilled engineers, technologists, technicians and artisans. An introduction to Mechanical Technology aims to produce learners who have been exposed to knowledge, skills, attitudes and values (SKAVs) which will equip them for further study in Mechanical Engineering and related sectors. The subject Mechanical Technology focuses on technological processes from conceptual design through to the process of practical problem solving to produce or improve on products which can enhance our quality of life.

Explanation of the key words You may encounter many unfamiliar words in this course. For this reason, key words have been included in the margins, to explain the meanings of words that appear in bold print in the text. The key words also cover acronyms (words made up of the first letters of the name of something) and abbreviations that are used in the book.

Assessment activities The assessment activities comprise individual, pair and group tasks. Some are pen-and-paper activities and some are practical tasks. The solutions to some tasks can be found in the text but others will require you to do further research. It is very important that you read the instructions carefully before attempting any of the tasks.

Message from the authors You have the good fortune to be one of the first learners to choose Mechanical Technology as one of your FET subjects. It will definitely stand you in good stead for your future studies. To help you succeed in this subject, it is essential to apply the following principles: • Go through your notes and make sure that you understand the work. • Learn the important concepts and definitions. • Do as many problems as you can. • You will find that regular revision will help you to understand and remember the work better. Do not hesitate to refer to other relevant reading material to broaden your understanding of the subject. Above all, think and work hard. We wish you well for your studies this year. THE AUTHORS vi

Safety

1

Chapter 1

Safety Topic 1

Machine tools Handling and storage of gas cylinders

Grinding machines

Safety Joining equipment

Cutting machines

Press machines

Shearing machines

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1

Mechanical Technology

Safety Introduction

DID YOU KNOW?

Safety engineers work to prevent accidents. These experts design structures and equipment to make homes, schools, jobs, roads and communities safer.

People mistakenly believe that accidents will never happen to them. As a result, work accidents often occur because people are careless and take chances. In fact, every year in South Africa, nearly 220 000 serious industrial accidents occur. Existing legislation aims to make workplaces as safe as possible. All South African safety regulations are based on the Occupational Health and Safety (OHS) Act No. 85 of 1993. The handbook which deals with this Act is available from any local office of the Department of Labour.

Definition of an accident The Act defines an accident as an unplanned, uncontrolled event caused by unsafe activities and conditions.

Figure 1.1: Know safety! No accidents!

Accidents do not simply happen by chance; usually they result from carelessness. 2

Safety

1

Causes of accidents The causes of accidents include: • poor housekeeping • loose clothing • improper use of tools • inaccurate setting-up of machines.

Assessment

1. Name four causes of accidents. 2. What is the purpose of the Occupational Health and Safety Act?

General safety rules Responsibility for safety Although everyone in a workshop must practice safety, the Act makes the person in charge of machinery, ultimately responsible for safety. This person must: • install and properly maintain machinery • repair machinery • ensure that safety appliances, devices and guards are in good condition and properly used • stop anyone from using a dangerous machine.

Obedience to instructions Machines are to be used according to the manufacturers’ instructions. Do not use a machine without your teacher’s supervision.

Illumination and lighting Machinery and workspaces should be adequately illuminated. Artificial light, if it is used, must not shine in a machine operator’s eyes.

Assessment

1 . Name four responsibilities that a person has when in charge of machines.

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Power-driven machines Bench grinder Perspex shield Tool rest On/off switch

Head Wheel guard Maximum gap 3 mm Grinding wheel Stand

Figure 1.2: A bench grinder

stationary at rest and not rotating

4

When using a bench grinder, observe the following safety rules: • Only use a machine once the guards have been correctly fitted. • Ensure that there is no oil or grease on the floor around the machine. • Check that the tool rest is not more than 3 mm from the grinding wheel surface. • When starting the machine, do not stand in front of the wheel. Before you start the grinding, let the machine idle for a few seconds. • If the wheel is running unevenly, dress it with an emery-wheel dresser. • Grind only on the face of a straight grinding wheel and never on the side of the wheel. • Use the various wheels only for their intended purpose. • Approach the wheel carefully and gradually, and do not ‘jab’ materials onto it. • Never ‘force grind’ so that you cause the motor to slow or stop. • Adjust the tool rest only when the wheel is stationary. • Clamp workpieces and holding devices safely and firmly. • Never allow the wheel to stand in cutting fluid as this may cause it to run ‘off balance’ when you switch it on again.

Safety

1

Mounting of a grinding wheel The following steps are advised: • Select the correct type of wheel for the job. • Inspect the wheel for cracks and tap it to apply the ‘ringing test’. Never use a grinding wheel that is damaged or not properly dressed. • Make sure that the wheel’s speed does not exceed the manufacturer’s recommendation. Below is an example of a manufacturer’s recommendation.

• Never force the wheel onto the spindle. • Use only one smooth paper spacer on each side of the wheel. • Use true and correctly recessed flanges of the same size and at least one third the diameter of the wheel. • Gently tighten the grinding wheel with a spanner only enough to hold it firmly. • Replace the guards correctly. • Stand aside and set the machine in motion. Let the machine idle before you dress the wheel, using an emery-wheel dresser. • Finally stop the machine and reset the tool rest to within 2 mm of the wheel surface. Ensure that the tool rest is parallel to the wheel surface.

Grinding wheels All power-operated grinding machines should be clearly marked with the recommended speed of the spindle in revolutions per minute. This speed should not allow the peripheral speed of the wheel to exceed the manufacturer’s recommendation. Other safety measures are: • Every grinding wheel should have a guard that can withstand the force of a rupturing wheel. • Bench grinders should have a transparent shield to protect an operator’s eyes. • Each machine must carry a notice prohibiting persons from performing, inspecting or observing grinding work without suitable eye protection.

spindle the spindle of the bench grinder is the rotating shaft onto which the grinding wheel is attached

recessed flanges the recessed flanges fit onto the spindle of the bench grinder on either side of the grinding wheel

peripheral speed the peripheral speed of the grinding wheel is the speed along the circumference of the grinding wheel

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Angle grinder

Body

Handle Disk Safety guard Figure 1.3: An angle grinder

essential necessary

Observe the following safety precautions: • The safety precautions that are applicable to other types of grinders are also applicable to an angle grinder. • The safety guard must be in place before you can start the grinding process. • Protective shields must be placed around the object being grinded to protect people passing by. • Use the correct grinding wheel for the job. • Do not force the grinding wheel on the work. • Make certain that there are no cracks on the stone before you start a job. • Protective clothing and eye protection are essential when working with an angle grinder.

Surface grinder Vertical adjuster

Grinding stone

Horizontal table Horizontal adjuster On/off switch

Figure 1.4: A surface grinder

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Safety

1

Observe the following safety precautions when using a surface grinder: • The same safety precautions that are applicable to other types of grinders are applicable when using a surface grinder. • Protective clothes and eye protection are essential when working ith a surface grinder. w • Before operating the surface grinder, be sure you have been properly taught how to control it and that you understand the potential dangers associated with it. • Do not operate the surface grinder unless all guards and safety devices are in place and working correctly. • Understand the operating instructions applicable to your machine. • Never clean or adjust the machine whilst it is in motion. • Immediately report any dangerous aspect of the machine and stop using it until it has been repaired by a qualified person. • You may have to stop your machine in an emergency. Learn how to do this without having to stop and think about it.

Assessment

1. 2. 3.

Name five safety precautions to observe when working with a bench grinder. Name five steps to follow when installing a grinding wheel. Name five safety precautions to observe when working with a surface grinder.

Cutting machines A drill press Motor and gearbox Depth gauge Feed lever

Table

Column Base

Figure 1.5: A drill press

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Mechanical Technology Observe the following safety precautions when using a drill press: • Choose the correctly sharpened drill for the type of work you need to do and the material you are going to drill. • Do not leave the chuck key in the chuck when you are not at the machine. • Never leave the machine running if unattended. • Clamp the workpiece securely to the table and do not hold it by hand. • Never try to stop the workpiece by hand if it slips from the clamp. • A drill should run at the correct speed for the job. • Do not force a drill into the workpiece – this may cause broken or splintered drills and possible injury. • Use a brush or wooden rod to remove chips from the drill – and not your fingers, waste or rags. • When reaching around a revolving drill, be careful that your clothes do not get caught in the drill or drill chuck.

Power saws

Blade

Material clamp

Power saw switch

Figure 1.6: A power saw

Observe the following precautions when using a power saw: • See that all guards are in place. • Ensure that there is no oil, grease or obstacles around the machine. • Select the correct blade for the material to be cut. • When changing blades, ensure that the machine is switched off at the main switch. • When removing or replacing the blade, do it gently. Quick movements, such as pulling off the blade, may result in a severe cut to your hand. 8

Safety • • • •

1

Do not adjust guides whilst the machine is running. All material must be clamped properly before cutting commences. Long pieces of material must be supported at the ends. Always stop the machine if you leave it unattended.

Lathes and milling machines Toolpost Top slide or compound slide Cross slide Cross-feed handle Saddle or carriage Thread dial Power-feed control Leadscrew engager

Rack Leadscrew

Feed rod Carriage handwheel

Figure 1.7: A centre lathe

Adjustable overarm Arbor support

Arbor

Machine table Knee and saddle

Handwheel

Power-feed unit

Base

Figure 1.8: A milling machine

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Mechanical Technology Observe the following safety precautions when working with a metal lathe or milling machine: • Make sure that all guards are in place. • Do not use a machine or come close to its moving parts whilst wearing loose clothing. Keep any cleaning materials, such as waste and rags, away from rotating parts. • Check that there is no oil or grease on the floor around the machine. • Do not leave spanners or keys on rotary parts. Always disconnect, remove or stand clear of hand wheels, levers or chuck keys before setting your machine or feeds in motion. • Never apply a wrench to revolving work. • Always clamp workpieces and holding devices safely and firmly. A loose fit, especially of spanners and keys, may cause slipping and result in injury. • Do not use your hands to remove cuttings while a machine is in motion. Use a wire hook or a brush once the machine has stopped. • Never adjust the cutting tool while a machine is running. • Resist the habit of leaning on machinery. This dangerous, ‘automatic’ practice often results in serious injury. • Do not attempt to stop a machine by placing your hand on the chuck while the machine is slowing down. • Pay attention to cutting-fluid control before switching on a machine.

Assessment

1. 2. 3.

Name five safety precautions to follow when working with a drill press. A power saw is a power tool that is very dangerous. Name five safety precautions that must be observed when working with the saw. Name four safety precautions to follow when working with a lathe or a milling machine.

Guillotines Switch

Blade safety mechanism

Machine platform

Activating mechanism Figure 1.9: An electrical guillotine

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Safety

1

Safety mechanism Blade

Operating pedal

Figure 1.10: A manual guillotine

Manual and electrical guillotines Where the opening of a pair of shears or a guillotine at the point of operation is greater than 10 mm, the machine should have either a fixed guard that prevents hands or fingers from reaching through, over, under or around the guard or a self-adjusting guard which automatically adjusts to the thickness of the material being worked. Some machines have manual or automatic moving guards that completely enclose the point of operation so that the working stroke cannot be opened unless the ram or blade is stationary. Another safety device is the automatic sweep-away or push-away that pushes any part of the operator’s body out of the danger zone when the working stroke starts. Today there are electronic, presence-sensing devices which stop the working stroke if the device senses any foreign object in the danger zone.

DID YOU KNOW?

A guillotine is an instrument for inflicting capital punishment by decapitation, introduced into France in 1792 during the French Revolution.

A hydraulic press Pressure meter

Return springs Plunger Platform

Hydraulic press cylinder

Adjustment holes

Figure 1.11: A hydraulic press

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Mechanical Technology Observe the following safety precautions when using a hydraulic press: • The predetermined, maximum pressure must never be exceeded. This operating pressure is always less than the maximum safe pressure and is indicated by a pressure gauge on the apparatus. • Pressure gauges must be tested regularly and adjusted or replaced if any malfunction occurs. • The platform on which the workpiece rests must be rigid and square with the cylinder of the press. • The platform must rest on the supports provided and should not be supported by the cable by which it is raised or lowered. • Place objects to be pressed in or out in suitable jigs. Ensure that the direction of pressure is always at 90° to the platform. • Special tools and holding devices must be used to prevent damage to soft material. • Relieve the cylinder of all pressure after use by opening the return valve. Also remember: • The level of the hydraulic fluid in the reservoir should be checked regularly. If fluid has to be added frequently, it is an indication that an internal leakage is present. • Regularly inspect the apparatus for rigidity and tighten all bolts and nuts. • Pins and/or other equipment that keep the platform at a desired height on the frame must be inspected for damage. • When the apparatus is equipped with cables to alter the working height of the platform, the cable and pulleys must be inspected for damage and lubricated with grease.

Assessment

1. Name five precautions that one must observe when operating a hydraulic press? 2. Which safety devices are used in conjunction with guillotines.

General machinery protection clutch a clutch is a device that enables two shafts (or rotating members) to be connected or disconnected, either while at rest or when in motion 12

Where possible, the moving parts of machinery (including parts not mentioned specifically in this chapter) within reach of any person should be fenced or guarded. Fences and guard railings need to be at least 1 m high and of double rail construction.

Revolving machinery Make sure that shafts, pulleys, wheels, gears, couplings, collars, clutches and friction drums are fenced. Similarly, set screws, keys and bolts on revolving

Safety

1

shafts, couplings, collars, friction drums, clutches, wheels, pulleys and gears should be countersunk, enclosed or otherwise guarded. Never use a damaged pulley.

Projecting shaft ends All shaft or spindle ends must be enclosed by a cap or shroud if they protrude more than a quarter of their diameter.

Transmission belts All driving belts, ropes, chains or sprockets within normal reach must be guarded. This includes the underside of overhead driving belts, ropes or chains above passages and workplaces. Driving belts must never be adjusted while the machine is in motion.

shroud a shroud is a protective cap that fits over a projecting shaft

Conditions of safety appliances and machinery All safety appliances, devices or guards must be maintained in good working condition. Turn off machinery if safety is compromised.

Starting and stopping machinery • All machinery must be fitted with an efficient stopping and starting device. This device must be accessible for easy engaging. • Never start a machine while another person is repairing, cleaning, oiling or adjusting, or even dangerously close to it. • Machines with foot-operated pedals should have either an automatic locking device to stop the pedal being accidentally pressed or a stirrup guard over the pedal with only enough space for the operator’s foot.

Repairing and oiling machinery Never clean, repair, adjust or lubricate a machine while it is in motion. All repairs should be done by a competent person.

Machine tools Rotating stock bars which extend beyond the end of a machine should be guarded with either a fence or a supported tubular guard. Machines that use cutting lubricants should have splash guards and pans. 13

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Lighting • There must be adequate illumination (lighting) in the workplace. • The glare in any workplace must be reduced to a level that does not impair vision. • The lighting on rotating machinery must not cause a stroboscopic (flashing) effect. • Lights and lamps must be kept clean and maintained.

Ventilation Every workplace must be ventilated either by natural or mechanical means in such a way that: • the air breathed by employees is safe. • the air concentration of any explosive or flammable gas, vapour or dust does not exceed safety levels.

Assessment

1. 2.

State two safety precautions for each of the following: • transmission belts • projecting shaft ends • revolving shafts • general machine protection. Name two safety precautions that must be observed when considering each of the following: • ventilation • lighting • machine tools.

Joining equipment Arc, spot and gas welding The following general safety precautions are applicable to the welding processes listed above. Specific safety instructions for each apparatus are detailed in Chapter 5. When in doubt, the manufacturer’s instructions are always the final authority on safety precautions and procedures. African Oxygen (Afrox) freely supplies safety booklets on all aspects of welding safety at their outlets and depots.

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Safety

1

Welding or flame-cutting operations may not be undertaken, unless: • an operator has been instructed on how to use the apparatus safely; • a workplace is effectively partitioned off; • an operator uses protective equipment; • effective ventilation is provided or masks or hoods maintaining a supply of safe air for breathing are available and used by the persons performing such operations; • the insulation of electrical leads is in sound condition; • the electrode holder is completely insulated to prevent accidental contact with current-carrying parts; • the welder is completely insulated by boots, gloves or rubber mats; and • any vessel that contains any substance which, under the action of heat, may ignite or explode (or react to form dangerous or poisonous substances) must not be welded or heated until it has been properly cleaned. Where hot work involving welding, cutting, brazing or soldering operations is carried out at places other than workplaces, which have been specifically designated and equipped for such work, steps to ensure that proper and adequate fire precautions must be taken.

First aid, emergency equipment and procedures Injuries and emergencies should be dealt with swiftly and carefully. Where more than five employees are employed at a workplace, the employer must provide an accessible first-aid box. The Act stipulates what the box should contain and states that the position of the first-aid box be clearly signposted.

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Handling of gas cylinders Oxygen regulator Oxygen flashback arrestor Acetylene regulator Acetylene flashback arrestor Oxygen cylinder

Welding nozzle

Welding torch

Acetylene hose Oxygen hose

Acetylene cylinder

Figure 1.12: Oxy-acetylene welding apparatus

Gas cylinders must contain the following permanently marked, minimum particulars: • name of the manufacturer • country of origin • year of manufacturing • manufacture serial number • name, number and date of the design • design gauge pressure in Pascal • maximum permissible operating pressure in Pascal • operating temperature • mark of an approved inspection authority. Gas cylinders must be tested before being placed in service. People must use a portable gas container and fill, place in service, handle, modify, repair, or inspect and test any portable gas container in compliance with South African Bureau of Standards (SABS) standards. 16

Safety

1

The following safety precautions must be observed when handling gas cylinders: • Store full cylinders apart from empty ones. • Keep cylinders in a cool place and protect them from sunlight and other sources of heat. • Always store and use cylinders in an upright position. • Store oxygen cylinders away from fuel cylinders. • Never stack cylinders on top of each other. • Do not bang or work on cylinders. • Never allow cylinders to fall from any height. Cylinders must be chained to prevent this. • Do not allow oil or grease to come into contact with oxygen fittings as oxygen and oil form an inflammable mixture. • Keep the caps on the cylinders for protection. • The thread on an oxygen cylinder is a right-hand thread. • The thread on an acetylene cylinder is a left-hand thread.

Assessment

1. 2.

To join material by means of arc or gas welding can be a dangerous activity. Name six safety precautions that a worker must observe to secure a safe environment and do a safe job. The gas in the cylinders of the welding plant is highly inflammable. Name five safety precautions that must be observed when working with gas cylinders to prevent them from exploding.

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Tools

2

Chapter 2

Tools Topic 2

Joining equipment

Gas cylinders Dial indicators

Press machines

Inside micrometers Tools

Shearing machines

Telescopic gauges

Cutting machines

Grinding machines

Stocks and dies

2


Tools This chapter mainly deals with precision-measuring tools. These tools are usually used to test for trueness or when setting up machines for precision work.

A dial gauge Lock screw

linear motion motion along a straight line

graduated divided into degrees

DID YOU KNOW?

The production of artificial abrasives in the late 19th century led to the introduction of grinding machines. Norton of Massachusetts, USA, illustrated the potential of grinding machines. He made a machine that could grind an automobile crankshaft in 15 minutes, a process that previously had required five hours.

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Indicator pointer Body

Rotation indicator

Plunger Figure 2.1: A dial indicator

Dial indicators are used as precision-measuring tools in the setting up of work on machinery such as centre lathes or milling machines. (The use of dial indicators is referred to later in the text.) A dial gauge magnifies the actual linear motion of the plunger. This highly accurate and versatile measuring instrument has a plunger attached to a rack and pinion for magnification. A pointer on the graduated dial shows the amount of movement. The outer rim is rotated so as to set the pointer to zero and then clamped into position. The dial indicator is marked off in hundredths of a millimetre.

Uses of a dial indicator A dial indicator is used to determine: • the ‘runout’ of a flywheel • if a crankshaft is bent • if a workpiece in a lathe is running true • if two pieces of equipment are the same size • the bearing nip on a bearing shell used on a crankshaft • the end float on a crankshaft.

Tools

2

Care of a dial indicator • Over travel of the pointer on the scale must not result in damage to the dial indicator. • Care must be taken that the dial indicator is not knocked over since the sensitive dial will be damaged. • Store the dial indicator in a proper storage place after use. • Clean the dial indicator, including its magnetic base, after use.

Assessment

1. Make a drawing of a dial indicator and label all the parts. 2. Name four instances in which you would use a dial indicator.

An inside micrometer Handle Thimble Interchangeable rod

Lock

Figure 2.2: An inside micrometer

The function of an inside micrometer You can use an inside micrometer to measure an inside diameter or the inside of two parallel surfaces accurately.

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How to use an inside micrometer You can use an outside micrometer to test an inside micrometer for accuracy. The size of the hole that can be measured depends on the size of the body of the micrometer. Most common inside micrometers are able to measure holes of up to 50 mm in diameter. To increase the range of the micrometer, the provided set of lengthening rods is used. The rods cover these sizes: 50 – 75, 75 – 100, 100 – 125, 125 – 150, 150 – 175 and 175 – 200 millimetres. To allow the inside micrometer to measure a 50 mm bore diameter, a special arrangement must be made. This is because the rods in the micrometer only cover 25 mm ranges. To overcome this, a 12 mm spacer is fitted to the micrometer as an extension piece. To measure a bore of 50 mm, the main scale is made to move only 13 mm, as 12 mm is already accounted for by the extension piece.

Adjustment of a inside micrometer To test if a micrometer is accurate, do the following: • Place the 50 – 75 mm extension shaft in the micrometer. • Adjust the micrometer on 0 to get dimensions of 50 mm. • Measure the inside micrometer with an outside micrometer of which the accuracy is known. • If a small adjustment is necessary you can adjust it in the same way that you would adjust the outside micrometer. For bigger adjustments the necessary adjustment is made on the front of the extension shaft. • Loosen the locknut on the extension shaft with a key which will be supplied in the set. • Adjust the anvil in or out to get to the zero reading. • Check for correctness. If it is right, lock the lock nut on the anvil. • Test again for correctness. • Put the micrometer inside the hole. • Turn the micrometer till both ends touch the sides of the hole. • Move the micrometer handle to one side, and remove the micrometer. How to read the reading on the micrometer. • Determine the length of the extension shaft. • Ex 100–125 • The length of the micrometer with the extension is now 100 mm. 22

Tools

2

• Check if the 12 mm spacer has been used. If so, increase the reading by 12 mm • 100 + 12 = 112 mm • Determine the reading on the inside micrometer in the same way as on the outside micrometer. • Ex reading on micrometer = 10,68 mm • Add up. • 100 + 12 + 10,68 = 122,68 mm

Assessment

1. What is the function of an inside micrometer?

Care of the micrometer • Micrometers must be handled with care if the accuracy is to be maintained. Keep the face of the anvil and spindle clean to ensure accuracy. If the zero line on the thimble does not coincide with the zero line on the index line, loosen the cap on the end of the thimble. Reset the thimble to zero, adjust to the correct position and tighten the cap. • When taking a reading, use very light pressure to obtain a correct reading. Never touch the anvil or spindle with bare fingers since it will cause the anvil and spindle to tarnish. • Store micrometers in wooden boxes after use. Clean them after use.

A telescopic gauge Spring-loaded plungers

Lock

Figure 2.3: A telescopic gauge

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2


Function of a telescopic gauge A telescopic gauge provides a quick and accurate means of checking inside measurements. Small gauges have two plungers which are internally springloaded. A telescopic gauge is inserted into the item to be measured, locked and then removed to check the size, using an outside micrometer.

Care of the telescopic gauge • Do not over-tighten the locking screw. • Do not force the telescopic plungers into the bore. • Take care when removing the telescopic gauge after measurement was taken. • Store gauges safely away after use.

Assessment

1. State the function of a telescopic gauge.

A torque wrench Rotation adjuster

Nm. indicator

Nm. adjuster

Handle

Figure 2.4: A torque wrench

The most convenient torque wrench is the adjustable, double audible type. Just before the final torque is reached, a distinct but soft audible click is heard, and the wrench makes a louder click when the final torque is reached.

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Tools

2

Torque Torque is indicated or calculated in Newton metres (Nm).

Reasons for using a torque wrench on an engine • It prevents bolts or studs from breaking. • It prevents bolts and nuts from loosening. • It prevents castings from warping.

Some important applications A torque wrench is used to tighten: • cylinder head bolts or nuts • main or big-end bearings, bolts or nuts • front wheel-bearing nuts • rear axle assemblies • bolts and nuts on automatic gearboxes. Note: When a bolt or nut has been tightened beyond the specified torque, it should first be loosened (but not with a torque wrench) and thereafter be torqued again. This practice ensures that readings on the torque wrench are accurate. Screw threads on bolts, nuts and tap holes must be clean otherwise the torque at which they are tightened is less than that required. Uneven torques may also result when a number of bolts or nuts are tightened.

Care of the torque wrench • Clean torque wrench after use. • Make sure that sockets fit properly. • Store properly after use.

Assessment

1. Give three reasons for using a torque wrench on a motor engine. 2. Name five places where you will use a torque wrench on a motor engine.

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Taps and dies

Tap

Die

Figure 2.5: Taps and dies

The stock is the tool that holds the tap or die. Taps and dies cut internal and external threads. It is advisable to cut internal threads first, as the taps are non-adjustable for size, whereas dies are adjustable. Taps used for cutting internal threads are extremely brittle and easily broken. They are made from hardened and tempered cast steel and are generally available in sets of three, namely taper taps, second or intermediate taps and bottoming or plug taps.

Taper taps

Full thread Taper Pilot

Figure 2.6: A taper tap

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Tools

2

Taper taps are used to start cutting the thread. The taper enables the tap to take the cutting force gradually. The remaining portion of the tap cuts the threads to its full depth.

Second taps or intermediate taps

Figure 2.7: An intermediate tap

Intermediate or second taps are tapered for one-third of their length. After the first tap has cut the initial thread, the second is used, and finally the third tap. Each tap cuts a little more thread than the previous one.

Bottoming taps or plug taps

Figure 2.8: A bottoming or plug tap

Bottoming taps are of uniform thickness throughout their length. This kind of tap is the last of the set to be used and is necessary when a blind hole has to be tapped.

Tapping size Before tapping, a suitable hole must be drilled a little larger than the core diameter of the tap. The tapping size can be obtained by referring to tables. Tables are published in engineering handbooks and brochures printed by tap and die manufacturers. The tables indicate which tapping size to use with which tap.

Clearance size Clearance size is the size of the hole that must be drilled so that it (the hole) will clear the outside diameter of a screw or bolt.

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Tap wrenches T-handle type tap wrench

Tap wrench

Using a T-handle type tap wrench

Using a tap wrench

Figure 2.9: Taps and tap wrenches

Function/Uses Tap wrenches are used to hold the taps while cutting. They are usually adjustable. Two types of tap wrenches are commonly used. Tap wrenches are used for general work and T-handle type tap wrenches are used for light work. As taps are brittle, delicate and easily broken, it is essential that you take the following precautions: • Taps must be used in the correct order (taper tap first). • A tap must be entered squarely in the tap wrench. • The correct size tapping drill must be used. • A tap is at a right angle to the stock once cutting has started. Take care not to bear too heavily on only one handle of the tap wrench or to force the tap, as it is likely to break off in the hole. • The tap is turned forwards a part-turn and then turned backward about half a turn to break off the chippings. • A suitable lubricant must be used. For steel, bronze, copper and wrought iron, use cutting fluid or cutting paste. For aluminium, use paraffin or soluble oil. No lubricant is required for brass and cast iron. • When deep, blind holes are being tapped, withdraw the tap occasionally to clear the hole of chippings.

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External threads External threads are cut by dies that fit into die stocks or die holders. The face of the die has a chamfer lead to enable the cutting to start more easily. Ensure that the die is square with the work. Note the following: • After squaring the die, use a generous amount of cutting oil, otherwise the threads are likely to tear. • Turn the die backwards frequently (quarter or half a turn) in order to break the chippings. • After cutting a few threads, back the die off the work and test the thread for size. • Turning the spreading screw in or out, opens or closes the die. Always start with the die fairly open and then gradually release the spreading screw and tighten the set screw.

chamfer lead tapered opening

Tapping and clearance sizes ISO metric Diameter

Pitch

Tapping drill

Clearance drill

2

0,4

1,6

2,1

2,5

0,45

2,05

2,6

3

0,5

2,5

3,1

4

0,7

3,3

4,1

5

0,8

4,2

5,1

6

1,0

5,0

6,1

8

1,25

6,8

8,2

10

1,5

8,5

10,2

11

1,5

9,5

11,2

12

1,75

10,2

12,2

The tapping sizes given are for 75% depth of thread. For threads 3 mm to 12 mm diameter clearance = nominal dia. +0,1 mm For threads above 12 mm diameter clearance = nominal dia. +0,4 or 0,5 mm The tapping size can be found by subtracting the pitch from the nominal diameter.

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Care of taps • Taps should be lubricated when used on all materials except brass and cast iron. • Regulary remove cutting otherwise taps will clog up and can break. • Turn the tap about half a turn forward and reverse slightly to clear the thread. • When a blind hole is being tapped, the tap should be removed occassionally to clear the metal cuttings from the bottom of the hole.

Assessment

1. 2. 3. 4. 5. 6.

What do you use taps and dies for? What material are taps made from? Name the three types of taps you will use to cut an internal screw thread and explain what each tap is used for. What do you understand by ‘clearance size’? Name the safety precaution that you must observe when working with taps. Explain how you will cut an external screw thread.

Thread pitch gauge Selection of the thread pitch gauge • Ensure that you choose the correct thread pitch gauge, e.g. is it a metric or imperial thread? Is a V-screw thread or Acme thread to be measured?

Figure 2.10: A pitch gauge

Function of a pitch gauge A thread pitch gauge is used to compare the threads on a bolt to the teeth cut on the gauge in order to assess the pitch of the bolt. The pitch is normally displayed on each blade of the thread gauge.

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Care of the thread pitch gauge • Clean after use. • Avoid finger contact with the blade as it may tarnish and destroy the numbering on the blade. • Oil after use. • Store in wax paper after use.

Assessment

1. State the function of a pitch gauge.

Grinding machines Perspex shield Tool rest On/off switch

Head Wheel guard Maximum gap 3 mm Grinding wheel Stand

Figure 2.11: A bench grinder

Function of a grinder A grinder is used to take wrought edges off objects, to make objects smaller and to shape objects into certain forms.

Care of grinding machines • • • •

Never force-grind the workpiece in the grinding wheel. Ensure the grinding wheel is properly dressed. Ensure that the transparent shields are clean and in place. Always fit the correct wheel to the grinding machine according to the rated speed of wheel and machine.

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Angle grinder

Body

Disk

Handle

Safety guard Figure 2.12: An angle grinder

Function • To grind off sharp edges • To cut material.

Care of angle grinder • Never force-grind. • Ensure that the correct grinding disc is fitted, e.g. cutting vs. grinding; metal disc vs. masonry disc. • Do a physical inspection of the grinder before use. • Only use in dry conditions. • Replace broken or frayed wires immediately. • Clean and store after use.

Surface grinder Function Surface grinding is the precision grinding of a planed surface. While all grinding, strictly, is surface grinding, the term is commonly used to describe the grinding of flat surfaces. Many parts formerly milled, planed or hand scraped, are now precision ground. With the introduction of large, high-powered surface grinders, it is obvious that steel or other metals are actually machined away.

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Types of surface grinding machines Among the numerous types of surface grinders that are manufactured, the following are the principal ones: • Horizontal spindle reciprocating table • Horizontal spindle rotary table • Vertical spindle reciprocating table • Vertical spindle rotary table.

Care of surface grinder • • • •

Never force-grind a workpiece. Dress the wheel with a diamond wheel dresser regulary. Do not take big cuts as the wheel will bite into the workpiece. Inspect wheels for cracks regulary by doing the ring test.

Vertical adjuster

Grinding stone

Horizontal table Horizontal adjuster On/off switch

Figure 2.13: A surface grinder

Assessment

1. Name five safety precautions to observe when working with a bench grinder. 2. Name five steps to follow when installing a grinding wheel. 3. Name five safety precautions to observe when working with a surface grinder.

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Drilling machines The selection of the drilling machine depends on the operation, the size of the workpiece and the size of hole to be drilled. Function • Drilling machines are used mainly for drilling holes. • Their work includes reaming, countersinking, boring, spot facing, honing, lapping, and tapping. • The ability to drill accurately is essential. It requires a high degree or skill.

Portable drills DID YOU KNOW?

The American inventors Elias Howe, Eli Whitney, and J.P. Brown contributed to the development of the drilling machine. In 1798, Whitney took a contract with the United States government to manufacture several thousand muskets. He decided to make them on the principle of the interchangeable parts. He had to design and build his own machinery. One of these machines was a drilling machine. He thus invented a highly important manufacturing method.

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Figure 2.14: A portable drill

Function Manually held, electric, portable drills are recommended only for operations that cannot be performed on drilling machines, especially in assembly work. Portable drills are powered by small electric motors. They contain chucks that will handle drill bits from 1 mm to 13 mm in diameter.

Drilling machines It is not known who invented the drilling machine. However, Elias Howe, Eli Whitney and J.P. Brown are credited with contributing to its development. Drilling machines are used mainly for drilling holes. In addition, their functions include reaming, countersinking, boring, spot facing, honing, lapping and tapping. Machinists need to be able to use several types of complex and powerful drilling machines. Safety is the starting point for any machinist. If you are unsure about safety in the workshop, reread the safety precautions discussed in Chapter 1. Because of the great power exerted by these machines, holding devices for workpieces must be used to secure the work and to keep the operator safe.

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Drilling machines cut by rotating a multi-edged cutting tool. The combination of the straight-line and rotating motion of the tool produces a circular hole in the workpiece. Figure 2.19 shows some basic drilling operations.

Drill multiple diameters

Multiple drill, countersink and counterbore

Drill and countersink

Drill and counterbore

Drill and chamfer

Drill, countersink and counterbore

Figure 2.15: Some basic drilling operations

Three types of drilling machines are used in machine shops. They are the sensitive drilling machine, used for light drilling on small parts; the upright drill press, used for heavy-duty drilling; and the radial drill press, used for drilling large, heavy workpieces.

Sensitive drill presses Fanbelt and pulleys

Chuck

Table

Motor Feed lever

Up and down control arm

Column

Base

Figure 2.16: A sensitive drill press

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Mechanical Technology As the name implies, this machine allows you to ‘feel’ the cutting action as you hand-feed it into the work. This drill press is usually belt driven. A sensitive drilling machine is either bench- or floor-mounted. Since these drill presses are designed for light-duty applications only, they are used to drill holes up to 12,5 mm in diameter. The machine consists of a base, column (which holds the motor), vertical spindle and horizontal table. The spindle is provided with a morse-tapered hole to accommodate a taper-like drill sleeve. The size of a sensitive drilling machine is determined by the diameter of the largest workpiece that can be drilled from its centre.

Upright drill presses Motor and gearbox

Depth gauge

Feed lever

Table

Column

Base

Figure 2.17: An upright drill press (gear head)

An upright drill press is very similar to a sensitive drill press, but is used for heavier work. The machine has a column rising from the base. It carries a table for the workpiece and spindle head. The drive is very powerful. Some types are driven by belts and pulleys while others are gear driven. They can drill holes of 50 mm or more, in diameter and have the capacity to drill holes of 38 mm diameter in thick steel.

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Radial drilling machines These are the most versatile drilling machines. Machine size is determined by the diameter of the column and the length of the arm, measured from the centre of the spindle to the outer edge of the column. The machine is used for operations on large castings too heavy to be repositioned for drilling each hole. The work is clamped to the table or base, and the drill can then be positioned by swinging the arm and moving the head along the arm. This can also hold revolving fixtures and jigs for drilling. The arm and head can be raised or lowered on the column and then locked into place. A radial drilling machine is used for drilling large holes and for boring, reaming, counter-boring and countersinking. The radial drilling machine has a power-feed mechanism, a wide range of spindle speeds and a handfeed lever.

Care of drilling machines • • • • •

Clean after use, especially the T-slots of the table. Never force-drill. Oil machine regulary. Check rack on side of pillar column. Release the table lock before adjusting the table.

Drilling head Arm

Spindle Column Table

Base Figure 2.18: A radial arm drilling machine

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Assessment

1. 2. 3. 4.

Name the function of a drilling machine. How do you determine the size of a sensitive drilling machine? What is a radial drilling machine used for? Name five safety precaution on a drill press

Power saw Function Reciprocating power saws are used mainly for cutting to length metal of various sizes, kinds, and shapes. These sawing machines vary in design. Some are light-duty, crank-driven machines. Others are large, heavy-duty machines that are hydraulically driven. Actual cutting takes place in only one direction. The saw blade is lifted slightly on the return stroke. This saves wear on the saw teeth. On the lighter reciprocating machines, the saw is fed into the workpiece by gravity, through the weight of the saw frame and blade. The saw blade is usually lowered a fixed amount on each stroke. Some machines are designed with faster return strokes for the saw blade.

Horizontal band saws This saw has a continuous blade that travels in a horizontal plane or a plane slightly inclined from the horizontal. The blades are made of high-carbon steel with a flexible back and hardened teeth. They may have from six to 24 teeth per 25 mm that are raker set. Some of these machines have a hydraulically operated feed, an adjustable vice, an adjustable stock stop and a means of varying the cutting speed and downward pressure. They can cut stock square or at an angle. Blade tensioner

Coolant control

Blade guard Machine vice On/off switch Coolant reservoir Feed control Figure 2.19: A horizontal band saw machine

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Vertical band saw machines Vertical band sawing machines are available with either fixed or variable speeds. The proper cutting speed is vital in band machining. If the machine is operated too fast for the type of material being cut, the teeth are not allowed sufficient time to dig into the material. As a result, they merely rub over the work. This creates friction and dulls the cutting edge of the teeth. Blades for profile sawing are always raker set. Blade adjuster

Blade spot welder

Inspection light Blade

On/off switch

Table

Figure 2.20: A vertical band saw

This type of tooth provides the necessary side clearance. Profile or contour sawing is a fast, accurate and efficient method of producing intricately curved or irregular cuts in almost any machinable metal. Where internal contours are to be cut, it is necessary to first drill a hole within the contour to admit the saw blade. The blade is cut at a convenient point, threaded through the pilot hole, and rewelded on the butt welder. Machines have a built-in butt welder for this purpose. Observe the following precautions when using a power saw: • See that all guards are in place. • See that there is no oil, grease or obstacles around the machine. • Select the correct blade for the material to be cut. • When changing blades, ensure that the machine is switched off at the main switch.

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Mechanical Technology • When removing or replacing the blade, do it gently. Quick movements, such as pulling off the blade, may result in a severe cut to your hand. • Do not adjust guides whilst the machine is running. • All material must be clamped properly before cutting commences. • Long pieces of material must be supported at the ends. • Always stop the machine if you leave it unattended.

Care of power saws • Always select the proper blade. • Always clean cuttings out of slots and guides to prevent blades from becoming clogged. • Always adjust down pressure in order not to overload. • Check that the filler tank is in a serviceable state.

Assessment

1. 2. 3.

Describe the function of a power saw. What are the blades of a horizontal band saw made of? Name five safety precautions you must observe when working with a power saw.

Lathes and milling machines Centre lathes Function To cut a workpiece that is turning while the cutter is stationary. With the earliest lathes, the workpiece was held at either end by a forked stick, an unsatisfactory method that seldom produced accurate work. To overcome this, pins were later used to rotate the workpiece between centres. Wooden pins were first used but were later replaced by metal pins. With early lathes, the workpiece was rotated by pulling a rope around it. However, the cutting action was sporadic, and this method was succeeded by continuous rotation; a rope or belt was run around the workpiece and then around a wheel or pulley (which was driven by a pedal device).

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Rapid developments in the engine lathe followed James Watt’s improvement of the steam engine. However, the operator’s holding and guiding the tool by hand worked for turning wood, but was unsatisfactory for metal. The inability of early lathes to produce screw threads created serious problems and bottlenecks in the production of machine tools. The problem was overcome by an English mechanic, Henry Maudsley, who added the slide rest, lead screw and gears, to perfect the first practical, screw-cutting lathe. Between 1800 and 1830 the first lathes were built in the United States of America, with a wooden bed covered with iron. In 1850, solid iron beds were made, in New Haven, Connecticut. The centre lathe produces a cutting action by rotating the workpiece against the cutting edge of the tool. As the cutting tool is moved lengthwise and cross-wise to the axis of the workpiece, the shape of the workpiece is generated. The shape produced is basically cylindrical. All parts are designed to hold and rotate the workpiece, and to hold and control the movement of the tool. If you understand these two fundamentals, you will find it easy to learn the parts and functions of the centre lathe.

Care of centre lathes • • • • • •

Clean lathe after use. Cover lathe with canvas or plastic covers during holiday breaks. Check and fill up gear boxes. Oil lathe beds and parts to prevent corrosion. Always keep service records up to date. Do not turn wood on a metal lathe.

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Toolpost Top-slide or compound slide

Rack Leadscrew

Cross slide Cross-feed handle Saddle or carriage Thread dial Power-feed control Leadscrew engager Feed rod Carriage handwheel

Figure 2.21: The centre lathe

Milling machines Vertical milling machines are relatively new. The first vertical milling machine appeared in the 1860s and was closely related to the drill press, the basic difference being that the spindle assembly and pulleys moved vertically. The next significant step came in the mid-1880s with the adaptation of the ‘knee and column’ part of the horizontal milling machine. This step allowed the milling machine table to be raised or lowered in relation to the spindle. In the early 1920s, vertical milling machines appeared with power feeds on the spindle and were housed in a heavy-duty quill. Machines with automatic table cycles followed and, by 1920, electrical servo-mechanisms were used on vertical milling machines for operations such as die sinking. In 1927 hydraulic tracing controls were developed and applied to vertical milling machines. Control systems, not limited to vertical milling machines, that activate machine control movements from information stored on punched or magnetic tape called numerical control (NC), or from a computer numerical controlled machine (CNC), have been developed.

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A milling machine provides cutting action to a rotating cutting tool. In milling, a multi-toothed cutter rotating at a fixed position on the machine shapes the work as it is traversed across the cutter. The workpiece is firmly and safely secured in a machine vice or on the machine table, which can be adjusted to set the depth of the cut and can be traversed in at least two directions on the horizontal plane. Adjustable overarm Arbor support

Arbor

Machine table Knee and saddle

Handwheel

Power feed unit

Base

Figure 2.22: A universal milling machine

Care of milling machines • Clean after use. • Cover milling machine with canvas or plastic covers during holiday breaks. • Check and fill up gear boxes. • Oil moving parts. • Keep service record up to date.

Assessment

1. Name the function of a lath and a milling machine. 2. Name four safety precautions to follow when working with a lathe or a milling machine.

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Guillotines Switch

Blade safety mechanism

DID YOU KNOW?

‘A guillotine is an instrument for inflicting capital punishment by decapitation, introduced into France in 1792 during the French Revolution.

Machine platform

Activating mechanism

Figure 2.23: An electrical guillotine

Function It is a machine that cuts material by means of a mechanical or electrical method. Safety mechanism

Blade

Operating pedal

Figure 2.24: A manual guillotine

Manual and electrical guillotines Do not use the guillotine unless a teacher has instructed you in its safe use and operation and has given permission. Safety glasses must be worn at all times in work areas.

Long and loose hair must be contained.

Sturdy footwear must be worn at all times in work areas.

Close-fitting/protective clothing must be worn.

Rings and jewellery must not be worn.

Gloves must not be worn when using this machine.

Only one person may operate this machine at any one time. 44

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Pre-operational safety checks • Ensure fixed guards are in place to prevent hands or other parts of the body from entering the trapping space. • Guards or safety devices must never be removed or adjusted, except by an authorized person for maintenance purposes. • Working parts should be well lubricated and free of rust and dirt. • The area around the machine must be adequately lit and kept free of materials, which might cause slips or trips. • Be aware of other personnel in the immediate vicinity and ensure the area is clear before using equipment. • Familiarise yourself with and check all machine operations and controls. • Ensure that the cutting table is clear of scrap and tools. • Faulty equipment must not be used. Immediately report suspect machinery.

Operational safety checks • • • • • • • •

Do not attempt to cut material beyond the capacity of the machine. Never attempt to cut rod, strap or wire with this machine. Use correct lifting procedures when handling large sheets of material. Take extreme care during the initial feeding of the workpiece into the machine. The workpiece should always be held sufficiently far back from the edge being fed into the guillotine. Ensure fingers and limbs are clear before actuating the guillotine. Hold material firmly to prevent inaccurate cutting due to creep. Ensure that your feet are positioned to avoid contact with the footoperated lever when cutting.

Where the opening of a pair of shears or a guillotine at the point of operation is greater than 10 mm, the machine should have either a fixed guard that prevents hands or fingers from reaching through, over, under or around the guard or a self-adjusting guard which automatically adjusts to the thickness of the material being worked. Some machines have manual or automatic moving guards that completely enclose the point of operation so that the working stroke cannot be opened unless the ram or blade is stationary. Another safety device is the automatic sweep-away or push-away that pushes any part of the operator’s body out of the danger zone when the working stroke starts. Today there are electronic presence-sensing devices which stop the working stroke if the device senses any foreign object in the danger zone.

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Care of manual and electric guillotines • Always keep blades sharp and in good condition. • Ensure guards are in place and operational.

A hydraulic press Function • The hydraulic press is used when you want to press bearings on shafts or remove them from shafts. • You use it to push a shaft with a bearing into a housing like the water pump of a motor car. • You can also use it to press gears on shafts in a gearbox. • A hydraulic system uses force that is applied at one point and is transmitted to another point using an incompressible fluid. A simple hydraulic system consists of two pistons and an oil-filled pipe connecting them. The hydraulic press employs the principle of the multiplication of force within a closed system, which means that the force exerted on the smaller cylinder is transferred to the larger cylinder, in proportion to the diameter of the larger cylinder.

Piston/plunger

Small cylinder

Big cylinder

One-way valve Reservoir

Figure 2.25: Hydraulic cylinders

Hydraulic motor car crusher A large motor powers a pump that pushes hydraulic fluid to drive large cylinders. Using principles of force-multiplication, a hydraulic system can generate over 2,000 psi and impart more than 150 tons of crushing force onto a pile of scrap cars. 46

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Pressure gauge

Return springs

Plunger

Platform

Adjustment holes

Hydraulic press cylinder

Figure 2.26: Hydraulic press

Care of a hydraulic press Observe the following safety precautions when using a hydraulic press: • The predetermined, maximum pressure must never be exceeded. This operating pressure is always less than the maximum safe pressure and is indicated by a pressure gauge on the apparatus. • Pressure gauges must be tested regularly and adjusted or be replaced if any malfunction occurs. • The platform on which the workpiece rests must be rigid and square with the cylinder of the press. • The platform must rest on the supports provided and should not be supported by the cable by which it is raised or lowered. • Place objects to be pressed in or out in suitable jigs. Ensure that the direction of pressure is always at 90° to the platform. • Special tools and holding devices must be used to prevent damage to soft material. • Relieve the cylinder of all pressure after use by opening the return valve. Also remember: • The level of the hydraulic fluid in the reservoir should be checked regularly. If fluid has to be added frequently, it is an indication that an internal leakage is present. • Regularly inspect the apparatus for rigidity and tighten all bolts and nuts. • Pins and/or other equipment that keep the platform at a desired height on the frame must be inspected for damage.

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Mechanical Technology • When the apparatus is equipped with cables to alter the working height of the platform, the cables and pulleys must be inspected for damage and lubricated with grease.

Assessment

1. Name five precautions that one must observe when operating a hydraulic press. 2. Which safety devices are used in conjunction with guillotines?

Joining equipment Arc welding machines Arc welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or non-consumable electrodes. The welding region is usually protected by shielding gas and/or slag. There are 4 main types of arc welding machines namely, AC machines, inverters, MIG and TIG welding machines. Current adjuster On/off switch

Current scale

Electrode holder Electrode terminal Electrode

Arc gap Earth terminal

Earth clamp Figure 2.27: Arc welding equipment

Selection The appropriate welding machine must be selected according to what type of welding is to be performed. The use of each machine is, therefore, determined by what it is designed to weld, as described in the following section: 48

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AC welding machines AC (alternating current) welding machines are usually very basic and relatively inexpensive. They make use of a transformer to transform the voltage and current of an electrical outlet to that which can be used to strike an arc and weld with. They are either air or oil cooled.

Use They are primarily used for manual metal arc welding (stick welding) mild steel, stainless, cast iron and some alloys.

DC welding machines (inverters) Inverters use electronics to change the welding current from AC to DC. This allows them to weld a greater variety of material than an AC welding machine. The steady arc provided by the inverter results in neater weld beads and less spatter.

Use They are primarily used for manual metal arc welding (stick welding) mild steel, stainless, cast iron and a wide range of alloys.

MIG (Metal Inert Gas) welding machines Gas metal arc welding (GMAW), commonly called MIG (metal inert gas), is a semi-automatic or automatic welding process with a continuously fed, consumable wire acting as both electrode and filler metal, along with an inert or semi-inert, shielding gas flow around the wire to prevent the weld site from contamination.

Use MIG welding machines are commonly used in industries such as the automobile industry for its quality, versatility and speed in conjunction with production line robotics.

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TIG (Tungsten Inert Gas) welding machines Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding, is a manual welding process that uses a non-consumable electrode made of tungsten, an inert or semi-inert gas mixture, and a separate filler material.

Use TIG welding is especially useful for welding thin materials. This method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds. It can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle frames and aerospace applications.

General care of arc welding equipment Ensure that: • all connections are secure. • insulation and electrical leads are in sound condition. • electrode holders are properly insulated to prevent accidental contact with current carrying components. • machines are regularly serviced and well maintained.

Resistance (spot) welding machines Spot welding is a process in which contacting metal surfaces are joined by the heat obtained from resistance to electric current flow. Workpieces are held together under pressure exerted by electrodes. Typically the sheets are 0,5 to 3 mm thick. The process uses two shaped, copper alloy electrodes to concentrate welding current into a small “spot” and to simultaneously clamp the sheets together. Forcing a large current through the spot will melt the metal and form the weld. The advantage of spot welding is that a lot of energy can be delivered to the spot in a very short time (approximately ten milliseconds). That permits the welding to occur without excessive heating to the rest of the sheet and, thereby, minimises distortion.

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Figure 2.28: Two plates to be spot welded

Use Spot welding machines are mainly used in fusing sheet metal components together. A good example of this is where the body components of vehicles are rapidly spot welded together by a robotic process on a production line.

Care of spot welding machines Always ensure that: • the time and current settings are appropriate to the type and thickness of material being welded. • the copper alloy tips are kept in good condition and not damaged during operation. • the copper tips are constantly cooled (through the circular liquid cooling system) to prevent overheating.

Assessment

1. Explain how to establish an arc using a arc welder. 2. Name four safety precautions when working with an arc welder. 3. Explain what is meant by resistance welding.

Gas welding equipment As you will learn in more detail in chapter 5, gas welding equipment consists of oxygen and acetylene cylinders, regulators, reinforced hoses, flashback arrestors, a blow torch and nozzle.

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Uses Gas welding (oxy-acetylene) equipment is used for heating/bending, gas welding, brazing and silver soldering.

Care of gas welding equipment The following guidelines are limited to the care of equipment and are not the safety precautions for operation. These safe working procedures are detailed in chapter 5 and in the manufacturer’s guidelines. • Ensure that gas cylinders are secured in the upright position (chained) in a well-ventilated housing (away from sparks and open flames). • Store oxygen and fuel gas separately. • Never allow oil, grease or dirt to come into contact with threads of regulators or cylinders. • Ensure flashback arrestors are in place and in good order. • Never leave regulators lying around. • Mark empty cylinders with chalk and store separately. • Regularly check the condition of gas hoses and connections. • Routinely check for leaks with a soapy solution (as provided by the manufacturer of the welding equipment) and never a naked flame. • Only use oxygen-safe thread tape if thread tape is required. • Never leave a burning torch unattended. • Never wrap hoses around the welding cylinder. • Never allow the welding flame to come into contact with the gas cylinders. • Do not use welding equipment without supervision.

Acetylene gas and cylinders An acetylene cylinder is shorter than an oxygen cylinder and is colour-coded maroon. The cylinder is filled with a porous mixture of aluminium silicate and charcoal particles. This sponge-like medium is used to absorb the solvent acetone, which in turn absorbs more than its own volume of acetylene gas. The gas is pumped into the cylinder under great pressure and is absorbed by the liquid acetone. Usually 8,5 kg of acetylene gas is stored in a cylinder. Acetylene gas is commercially produced by reacting calcium carbide with water. A garlic odour is artificially added to acetylene gas as a safety feature, so that leaks can be detected before accidents occur. It is extremely important to ensure that acetylene gas cylinders do not have leaky valves or connections, as a concentration of only 2% acetylene gas in a confined space is enough to 52

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cause an explosion. Acetylene cylinders should always be stored and used in their vertical position. This ensures that no acetone liquid is withdrawn when a cylinder is used. Cylinders should be stored in ventilated and secure containers, away from hot work and not be exposed to naked lights or other fire hazards. They should also be correctly signposted with the required safety signage and regularly inspected for safety. In case of a fire, cylinders should be removed from the hazardous area and hosed down with water to keep them cool. Copper fittings should never be used in conjunction with any acetylene equipment as copper acetylides may form, which can explode on impact or pressure. For this reason, brass fittings are usually used. Fittings screwed to acetylene cylinders always have left-handed thread to prevent confusion with oxygen cylinders.

Oxygen (O2) gas and cylinders Oxygen cylinders are colour-coded black and contain industrial-grade oxygen. The oxygen gas itself is not flammable, but supports combustion. Oxygen makes up 21% of the air we breathe. Other gases in the air are nitrogen (78%), argon (0,9%) and CO2, hydrogen and water vapour (0,1%). To extract oxygen from air, we compress the air to liquify it. Nitrogen has a lower boiling point than oxygen, so when the pressure is released to a certain point, the oxygen can be separated from nitrogen, as they boil off at different temperatures. They are then stored in separate cylinders. The process of separating oxygen from nitrogen is called fractional distillation. Oxygen is stored in cylinders with a cylinder content of 11,6 kg, under huge pressure. For this reason, valve guards should always be installed around the cylinder valves and the cylinders should be chained in the upright position. If a cylinder were to be knocked over, breaking off the cylinder valve, the effect would be similar to that of a 75 kg steel rocket causing havock. Oil or grease should never be used in conjunction with oxygen fittings, as even a small droplet of oil can cause an explosion because of the pressure of the oxygen cylinder content. (This same principle is described under diesel or CI engines in the Grade 10 book.) Oxygen should never be inhaled or used to blow dust off your clothing. A small spark or smouldering ember from the welding process can cause your clothing to ignite and burn in the oxygen rich environment.

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Argon (Ar) cylinders Argon gas is used in MIG and TIG welding MIG stands for metal inert gas. MIG welding machines are direct current (DC) welding machines. Instead of using flux-coated electrodes, like metal arc welding machines, MIG machines use a continuous wire electrode feed. The wire electrode is shielded by an inert gas. The inert gas takes the place of the flux coating in metal arc welding electrodes. Inert gases are used to shield the molten pool because they do not react with the weld metal and they shield the molten pool from atmospheric gases. The cylinder normally contains 17 kg of inert shielding gas. The inert gas that is usually used is a mixture of argon and carbon dioxide (CO2). UHP (ultra high purity) argon is too expensive to be used on its own commercially, so it is mixed with CO2. The cylinder must be fitted with a regulator to reduce the cylinder pressure, as well as a flow meter to control the flow rate of shielding gas (in litres/minute).

Helium (He) cylinders Helium is the chemical element with atomic number 2 and an atomic mass of 4.002602 u, which is represented by the symbol He. It is a colourless, odourless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group on the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Next to hydrogen, it is the second most abundant element in the universe, and accounts for 24% of the elemental mass of our galaxy. An unknown, yellow, spectral line signature in sunlight was first observed during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with the discovery of the element with Norman Lockyer, who observed the same eclipse and was the first to propose that the line was due to a new element which he named helium. In 1903, large reserves of helium were found in the natural gas fields in parts of the United States, which is by far the largest supplier of the gas. Helium is used in cryogenics (its largest single use, accounting for about a quarter of production), particularly the cooling of superconducting magnets, with the main commercial application in MRI scanners. Helium's other industrial uses as a pressurising and purge gas, and a protective atmosphere 54

Tools

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for arc welding and processes such as growing crystals to make silicon wafers, account for half of its use. Economically minor uses, such as lifting gas in balloons and airships, are popularly known.

Nitrogen (N) cylinders Nitrogen is a chemical element that has the symbol N, atomic number 7 and atomic mass of 14.00674 u. Elemental nitrogen is a colourless, odourless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere. The element nitrogen was discovered as a separable component of air by Scottish physician Daniel Rutherford in 1772. Many industrially important compounds, such as ammonia, nitric acid, organic nitrates (propellants and explosives), and cyanides, contain nitrogen. The extremely strong bond in elemental nitrogen dominates nitrogen chemistry, causing difficulty for both organisms and industry in breaking the bond to convert the N2 into useful compounds, but at the same time causing release of large amounts of often useful energy when the compounds burn, explode, or decay back into nitrogen gas. Nitrogen occurs in all living organisms, and the nitrogen cycle describes movement of the element from air into the biosphere and organic compounds, then back into the atmosphere. Synthetically-produced nitrates are key ingredients of industrial fertilisers, and also key pollutants in causing the eutrophication of water systems. Nitrogen is a constituent element of amino acids and thus of proteins and nucleic acids (DNA and RNA). It resides in the chemical structure of almost all neurotransmitters, and is a defining component of alkaloids, biological molecules produced by many organisms. The human body contains about 3% by weight of nitrogen, a larger fraction than all elements save oxygen, carbon, and hydrogen. Uses of nitrogen • Nitrogen is used to preserve packaged foods by stopping the oxidation of food which causes it to go ‘off ’. • Light bulbs may contain nitrogen as a cheaper alternative to argon. • Nitrogen gas is often used on top of liquid explosives to keep them from exploding! • Nitrogen is used to produce many electrical parts, such as transistors, diodes and integrated circuits. • When dried and pressurised, nitrogen gas is used as a dielectric gas for high voltage equipment. • It is used to manufacture stainless steel. 55

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Mechanical Technology • It is used to reduce the fire hazard in military aircraft fuel systems. • Nitrogen gas is used to fill the tyres of aircraft and cars. • Nitrogen tanks are gradually replacing carbon dioxide tanks as the power source of paintball guns. • Liquid nitrogen (called cryopreservation because of the low temperature) is used to preserve blood, sperm and eggs, and other biological samples. It is also used to cool X-ray detectors and central processing units in computers when they are hot. • Nitrogen is a component of nearly every pharmacological drug. Laughing gas (nitrous oxide) can be used as an anaesthetic.

Assessment

1. Name six points that you must bear in mind in the care of gas welding equipment. 2. Describe the properties of helium gas. 3. Name six uses of nitrogen.

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Chapter 3

Materials Topic 3

Manufacture of steel

Heat-treatment processes

Materials

Properties of engineering materials 57

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Manufacture of steel Extracting metals from ores Iron ore: Iron is the most important element in steel. Most steel comprises at least 98% iron. The remaining two percent is carbon, silicon, sulfur, manganese, nickel, tungsten or other elements. Iron comes from iron ore, which exists in nature as a solid or as a powder. Iron ore varies in colour from red to yellow to black and is mostly mined in open-pit mines. Pig iron: Once mined, iron ore is put into a blast furnace. Heat in the blast furnace separates the iron from oxygen and other materials in the ore. The molten iron that comes out of the blast furnace is either used immediately or cast into solid slabs or blocks, called pigs, and stored for future use. Iron mine

Coke ovens

DID YOU

Coal mines

KNOW?

The name ‘pigs’ has an interesting history. When iron production started, a trough, called a ‘runner’, carried molten metal from the tapping hole on the side of the blast furnace. The runner sloped downward and had runners branching off from it. The side runners were called ‘sows’, after the name for a female pig. Moulds along the side runners received the molten iron like ‘rows of suckling piglets’.

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Iron ore enrichment

Limestone quarry Dust collector

Blast furnace Stove

Molten iron

Molten slag Figure 3.1: Producing iron

There are several methods for extracting iron from the ore. Smelting is the most important method. In smelting, a chemical process, reduction, separates the iron from the oxygen. The ore is dumped into a blast furnace and heated with coke and limestone. Oxygen escapes from the iron and combines with the carbon from the coke. Other impurities from the iron ore and coke become trapped in the molten limestone.

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A blast furnace is a tall, round structure about 30 m high and 9 m in diameter. The blast furnace is charged before smelting begins. In charging, the blast furnace is filled with coke, limestone, and iron ore and then ignited. Air is heated to 675 °C by smaller furnaces called stoves and is forced in through the bottom of the blast furnace. The blast of hot air intensifies the burning of the charge material. The temperature at the bottom of the furnace rises to well above the melting point of iron, which is 1 535 °C. This high temperature causes chemical reactions to occur, during which pure iron is released from the iron ore. The molten iron drops to the bottom of the blast furnace. The molten limestone traps the impurities from the iron ore and coke. The mixture, called slag, floats on the top of the molten iron. The slag is then drawn off through a hole in the furnace called a slag tap hole. The molten iron is drawn off near the bottom of the furnace and is either used immediately for making steel or stored as pig iron. Foundries make iron castings from re-melted pig iron.

Hopper

Small bell

Larger bell 250 °C

Steel casing

550 °C

Stack Refractory brick lining

850 °C Bosch Hot air supply from stoves

1150 °C

Melting zone Hot air supply from stoves

Slag tap hole 1500 °C

Hearth Iron tap hole

Figure 3.2: Smelting iron in a blast furnace

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Steel Different processes produce different kinds of steel, each process requiring a special furnace. Steel-making furnaces include open-hearth furnaces, basic oxygen furnaces and electric furnaces.

Open-hearth process Open-hearth furnaces are large, rectangular basins. To make steel, the openhearth furnace is charged with limestone and steel scrap. Iron ore may also be added. Gas, oil or coal is burned as fuel, and hot air is directed over the charge in the furnace. The temperature above the charge reaches about 1 650 °C and the charge melts. When the charge is nearly melted, molten pig iron from the blast furnace is added to the furnace. Heating continues, and the impurities combine with the oxygen. Some of the oxidised impurities bubble up through the molten metal as a gas. Others float to the top and combine with the molten limestone to form slag. After the impurities have been burnt away, alloying elements are added to bring the steel to the required composition. The steel is then drawn from the furnace into a ladle and poured into tall moulds to form ingots. Charging ladle Funnel Scrap metal Steel

Tap hole Charging machine Charging boxes Ladle Figure 3.3: An open-hearth furnace

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Slag thimble

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Bessemer process Sir Henry Bessemer’s breakthrough in the 1850s, that contaminants and pollutants could be removed from molten iron by their oxidation with an air blast through the hot molten metal, was a key step forward for industry in his time. The oxidation raises the temperature of the molten iron and keeps it in a molten state. The Bessemer process was named after its inventor and patentee, Sir Henry Bessemer. This process was the first, cheap, engineering manufacturing process for mass production of steel from pig iron. The Bessemer converter is a large, pear-shaped container in which molten pig iron is converted to steel by the Bessemer process. The Bessemer converters’ refractory lining were silica or ganister (hard rock containing silica that can resist high temperatures) bricks and this was the beginning of acid steel making. The operation of the Bessemer converter is illustrated in figure 3.4.

ganister hard rock containing silica that can resist high temperatures

Refractory lining

Charging of the Bessemer converter

Blowing of the Bessemer converter

Pouring of the Bessemer converter

Figure 3.4: The sequence of operations in the Bessemer converter

Mining and the environment Mining is not an environment-friendly industry. It causes severe scarring of the sensitive environment, e.g. big hole of Kimberley, as well as air and noise pollution, dust, and contaminated soil and water, e.g. ground water. Once ground water is contaminated, it is almost impossible to clean it. On the other hand, mining for iron ore, metals (semi- and precious metals) provides work for an estimated 600 000 South Africans and people from neighbouring states. Because of the nature of their work and living conditions, HIV/AIDS and lung diseases are the most important illnesses affecting mine workers. Accidents in foundries, factories, mines and work places are a great concern for trade unions and the Department of Labour.

PAUSE FOR THOUGHT

Scrap vendors, e.g. people pushing shopping trolleys filled with scrap metal, contribute to recycling, reducing our carbonfootprint, reducing our mining activity and add to our GDP (Gross domestic product).

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Mechanical Technology The Department of Environmental Affairs endeavours to control air and water pollution, global warming, and the release of greenhouse gases such as CFCs, carbon dioxide, water vapour, and nitrous oxides and methane. Many mining and industrial companies have become socially aware of the need to rehabilitate land, such as mine dumps, to minimise dust. They have planted trees and greened areas to decrease carbon dioxide levels in the air. The production of smokeless coke also helps. We can play a part by participating in Arbour Day, through tree planting and by remembering the three Rs (reduce, recycle and reuse).

Assessment

This is a group activity. Discuss the following topics in groups of five and nominate a spokesperson to report back to the class. • How can you decrease noise pollution in urban areas? (Hint: Think, for instance, of revving car engines and loud radios.) • How can you improve sanitation, and thus health, in rural areas? (Hint: Think of VIP latrines [toilets], the function of bacteria in these toilets, and preventing ground water from being contaminated.) How can we preserve our natural resources for future generations? Asian countries import scrap metal from South Africa and export highly sophisticated products back to South Africa. How can we, as South Africans, reuse our scrap metals and turn them profitably into usable products that will reduce our carbon foot print? Read up on the COP 17 conference that was held in Durban, South Africa, in December 2011. Formulate a report on how the outcome of the conference will benefit future generations of the world (global warming).

Properties of metals In engineering, metals must meet certain requirements and must, therefore, have certain characteristics. It is vital for the most suitable material to be used for the job. Often this will be specified in the drawing from which we are working, but sometimes we have to decide what to use. The following properties have to be considered: • Hardness refers to the material’s ability to resist penetration, scratching, abrasion, indentation and wear. Unfortunately the harder carbon steel tools are made, the more brittle they become, so some hardness must be sacrificed for toughness in the tempering process. 62

Materials • Plasticity refers to the material’s ability to change shape permanently – it is the reverse of elasticity. • Elasticity refers to the material’s ability to absorb forces and flex in different directions and return to its original shape when the load is removed. • Ductility refers to the material’s ability to change shape by stretching it along its length, or to be drawn into wire form. • Malleability refers to the material’s ability to be reshaped in all directions without cracking. Lead is a malleable material but lacks ductility because of low tensile strength. • Brittleness refers to the material’s behaviour when fractures occur with little or no deformation. Glass is a classic example of a material with this property. • Toughness refers to the material’s ability to withstand shock loads and remain intact after continual bending in opposite directions. • Strength refers to the material’s ability to withstand forces that are applied to it, without breaking, bending, shattering or deforming in any way. • Softness is the opposite property to hardness. Soft materials may be easily shaped by filing, drilling or machining in a lathe, milling machine or shaping machine. • Stiffness is the ability to withstand bending. • Flexibility refers to metals which remain bent after a bending force has been removed.

Methods of enhancing the properties of steel In Grade 10 you learnt about different materials and their uses and composition, as well as how to identify these different materials. In this chapter we will discuss heat treatments of metals, quenching media, and look at the different kinds of heat treatment furnaces.

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DID YOU KNOW?

Heat treatment is a word used to describe a process during which the mechanical and physical properties of a metal are changed by heating and then cooling it down again.

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crystalline having a chemical formation of a crystal or to resemble a crystal

Heat treatment is the heating and cooling of metals (under controlled conditions) in their solid state so as to change their properties. All metals have a crystalline grain structure while in their solid state and the kind of grain structure determines their properties. The size of grains in steel depends upon a number of factors, of which the main one is the furnace treatment the steel has received. To bring about the required grain structure and so produce the most wanted properties, the metals are heated and then cooled down in a number of ways. To refresh your memory, these properties (discussed in Grade 10) are strength, elasticity, plasticity, ductility, brittleness, toughness, hardness, softness, stiffness and flexibility, to name but a few. Grain structure

Before heating

Small grains

After heatiing

Large grains

Figure 3.5: Grain structure and changes to the grain structure

Assessment

This is an engineering activity. Use a drill press to drill holes into sample pieces of wood and metal. Which of the two is easier to drill through? Can the same drill bit be used on both materials?

History of heat treatment Metals become hardened when suddenly cooled from a heated condition. People have known about the process of hardening for many centuries, although the exact origin of metal hardening cannot be determined. During the fifteenth and sixteenth centuries, the practice of hardening metal became an art. A great deal of superstition and secrecy developed amongst the skilled workers who practised this art. Process secrets were handed down from generation to generation.

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Materials Even though skilled workers were involved in the process, they did not understand why the metal hardens. During the last century, scientists attempted to discover the reasons for metallic behaviour. They have developed the science of metallurgy. Heat treatment is part of metallurgy. It changes the structure and grain of metals by applying heat.

Heat treatment and the environment

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metallurgy the science of the production, purification, properties of metals and their application

Care for the environment includes concerns about how industry manufactures products and the effects that such products and manufacturing processes could have on the environment. Codes of practice provide companies and manufacturers with guidelines and requirements for adopting an environmental policy and implementing an environmentally responsible system. These codes help companies and manufacturers set their own environmental objectives. Steelmakers have to ensure that they minimise pollution from fumes and gases produced by the steelmaking process. Modern steel mills are very efficient at controlling pollution. The fumes that come from heating the metal are collected in hoods above the furnaces. Other gases are reused as fuel for heating. In the heat-treatment department of a metal treatment plant, a safe working environment is crucial. Hot metal is very dangerous and great care must be taken when using or handling it. The working area should be well ventilated and provided with exhaust hoods, because the fumes given off by the heated steel and the cyanide used for case-hardening are very toxic and hazardous.

toxic poisonous

Goggles must be worn when you are working on lead, cyanide, or nitrate pots. Always ensure that nothing damp or wet is introduced into a heating pot since an explosion could occur. A leather apron and gloves must be worn when you are working with tongs and hot metal. Never pick up a workpiece with bare hands, unless you are sure that it is not hot. Always use correctly shaped tongs that are in good working order to pick up hot workpieces and do not leave hot tongs where other people may accidentally be burnt by them.

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The heat-treatment process

soaking holding the metal at this predetermined, elevated temperature for a certain period to ensure uniform penetration of heat

DID YOU KNOW?

Room temperature is 20 °C to 25 °C

quenching to cool rapidly in a quenching medium

brine salted water

DID YOU KNOW?

Brine is the result of dissolving common rock salt in water. It is a very effective quenching medium as its ionic structure conducts heat very easily.

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The rate of heating is very important in any heat-treatment operation. Heat flows from the outside to the inside of the metal at a definite maximum rate. If metal is heated too fast, the outside of the part becomes hotter than the inside, and a uniform structure is very difficult to achieve. All heat treatment processes involve heating and cooling metal according to a time-temperature cycle that includes the following three steps: 1. heating the metal slowly to a certain temperature to ensure a uniform temperature 2. soaking the metal 3. cooling the metal at a certain rate to room temperature. The hardness that can be achieved from a specific treatment depends on the following three factors: • workpiece size • quenching rate • carbon content. Water is normally used as a quenching medium for low carbon and medium carbon steels. Rapid quenching is required to harden these steels. An ideal quenching medium for high carbon and alloy steels is oil. The quenching rate of oil is not as harsh as that of water. Where extreme cooling is needed, brine is used. The temperature at which steel is normally quenched for hardening is known as the hardening temperature. The hardening temperature depends mostly on the carbon content of the steel. The highest degree of hardness achievable in steel by means of direct hardening is determined mainly by the carbon content. Steel with low carbon content will not respond very much to the hardening process. Carbon steels in general are considered shallow hardening steels. The hardening temperature is, as a rule, 10 °C to 38 °C above the critical temperature.

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Assessment

1. What is meant by the term “heat treatment”? 2. What is the main factor determining the grain size of steel? 3. The hardness that can be obtained through a given heat treatment depends on which three factors? 4. Which safety precautions must be taken when conducting heat treatments? 5. List the three groups of plain carbon steels. 6. What is meant by soaking during heat treatment? 7. List four kinds of quenching mediums. Which cools most rapidly? Which one cools the least rapidly? DID YOU

Types of heat treatment

KNOW?

As we now know, heat treatment is any one of a number of controlled heating and cooling operations used to cause the desired change in the physical properties of metals. There are five basic heat-treatment processes to obtain characteristics like toughness, hardness and wear resistance. To obtain these characteristics, operations such as hardening, tempering, annealing, normalising and case-hardening are necessary.

Critical temperature is the temperature range in which steel undergoes structural change during heating and cooling.

Tempering This is a follow-on process from hardening. After a material has been quench-hardened, it is not always ready for immediate use. Tempering is a process generally applied to steel to relieve the strains induced during the hardening process and to reduce brittleness. It consists of heating the hardened steel to a temperature below its critical temperature (tempering temperatures are normally much lower than the hardening temperature), soaking it at this temperature for a period, and then quenching in water, brine, air or oil. During this process, the degrees of strength, hardness and ductility obtained depend directly upon the temperature to which the steel is heated. High tempering temperatures improve ductility at the expense of tensile, yield strength, and hardness.

Chisels and punches can be heated on an electric hot plate until the required colour shows, after which it can be cooled in water.

Figure 3.6: Tempering a punch on a hot plate

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Case-hardening Case-hardening is a surface-hardening process. The objective is to produce a hard case over a tough core. Case-hardening is an ideal heat treatment for parts which require a wear-resistant surface and, at same time, must be tough enough internally at the core to withstand the applied loads, such as gears, cams, cylinder sleeves, etc. The steels best suited to case-hardening are the low-carbon and low-alloy steels. If high-carbon steel is case-hardened, the hardness penetrates the core and causes brittleness. In case-hardening, the surface of the metal is changed chemically by inducing a high carbide or nitride content. The core is unaffected chemically. When heat-treated, the surface responds to hardening while the core toughens. The common methods of case-hardening are carburising, nitriding and cyaniding.

Course grain

Fine grain

Figure 3.7: Cross-sections of case-hardened steel shafts. Note the fine grain of the hardened case and the coarse grain of the unhardened core.

Carburising Carburising consists of holding the metal at a higher temperature (carburising temperature) while it is in contact with a solid or gaseous material rich in carbon (carburising mixture). Many types of carboncontaining substances may be used to introduce carbon into steel at the carburising temperature. These substances include solid materials (pack carburising), liquids (special kinds of salts are heated to form a molten salt bath) and gases (liquids or natural gas rich in carbon). The process requires several hours, to allow for the surface metal to absorb enough carbon to become high-carbon steel. The material is then quenched and tempered to the desired hardness.

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The length of time required for penetration of carbon during the carburising process varies with (1) the carburising temperature, (2) the carburising substance used, and (3) the depth of penetration required.

The part to be pack-carburised should be entirely covered with carburising compound. The metal box should have a snugly fitting lid.

DID YOU KNOW?

The carburising mixture generally consists of crushed bone, bone dust, charcoal, hoof clippings and strips of old leather.

Figure 3.8: Packing parts for case-hardening in a carburising compound

Nitriding Nitriding is a method of putting an extremely hard surface on a piece of steel. Nitriding consists of holding special alloy steel, at temperatures below the critical point, in anhydrous hot ammonia gas for some hours. Absorption of nitrogen as iron nitride into the surface of the steel produces a greater hardness than carburising, but the hardened area extends to a lesser depth.

Cyaniding This method is referred to as liquid-case hardening. Cyaniding is a rapid method of producing surface hardness on an iron-based alloy of low carbon content. It can be achieved by immersion of the steel in a molten bath of cyanide salt, or by applying powdered cyanide to the surface of the heated steel. The temperature of the steel during this process should range from 760 °C to 950 °C, depending on the type of steel, depth of case desired, type of cyanide compound, and the time exposed to the cyanide. The material is dumped directly from the cyanide pot into the quenching bath metal.

Caution Cyanide is a deadly gaseous poison. It is dangerous to use in a school Mechanical Technology room. Great care must be exercised when handling this material. Safety glasses or a face shield must be worn. An exhaust fan is necessary to extract fumes and good ventilation is a primary requirement.

Assessment

1. Explain the process of annealing. 2. To anneal a piece of steel, you should heat it to a temperature above its critical temperature. 3. List three case-hardening methods. 4. What is meant by carburising? How is it done? 69

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Mechanical Technology 5. Complete these sentences: a. In all heat treatments, … time and … temperature are both important in producing the desired changes in the metal. b. In annealing, the hot metal is cooled … c. In hardening, the hot metal is cooled ... d. In tempering, the hot metal is cooled … e. In order to fully anneal a casting, it must be heated to … above its critical temperature. f. Forgings that become distorted during heat treatment can usually be straightened during … cooling.

Hardening Hardening is the first step in the production of high-strength steel. Hardening causes a condition of extreme hardness in the steel to enable it to resist wear or cut other metals.

A pyrometer is a high temperature thermometer.

Hardening is achieved by heating the workpiece slightly higher than the critical temperature, and then rapidly cooling it by quenching the workpiece in a medium like water, brine or oil. This treatment produces a fine grain structure, which is very hard and of maximum tensile strength and minimum ductility. Usually, material in this condition is too brittle for most practical uses. It is desirable that all workpieces be heated in a furnace or oven equiped with a pyrometer.

Annealing Metals are annealed to relieve internal stresses which may have been set up during previous workings of the metal, to soften them in order to facilitate the machining processes, make them ductile, refine their grain structures and reduce brittleness. Metal is annealed by heating it to the prescribed temperature, soaking it at that temperature for the required time and then cooling it back to room temperature. The cooling rate of the metal from the annealing temperature varies greatly. Steel must be cooled very slowly to produce maximum softness.

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This can be done by burying the hot part in an insulating material such as sand, ashes, charcoal, lime or some other substance that does not conduct heat readily, or by shutting off the furnace (furnace door remains shut) and allowing the furnace and the part to cool off together (furnace cooling). The cooling period is sometimes as long as 24 hours.

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Annealing temperature

Grain growth

Recrystallization

Recovery

Figure 3.9: Changes in metal structures that take place during the annealing process

Normalising Normalising is a process whereby iron base alloys are heated to approximately 56 °C above the upper critical temperature (which is higher than either hardening and tempering temperatures), soaking the metal until it is uniformly heated, followed by cooling it down to room temperature in still air, away from draughts. This prevents the sudden cooling of a localised spot which might cause distortion. Ferrous metals are normalised to relieve the internal stresses produced by machining, forging or welding.

Forging is a method of metalworking in which the metal is hammered into the desired shape or is forced into a mould by pressure, usually after being treated so that it is in a more plastic state. Hot forging requires less force than cold forging, which is usually done at room temperature.

Scale a dark, thin layer of metal oxide on the surface of the metal

Steel is much tougher in the normalised state than in any other state. Lowcarbon steels do not usually require normalising, but no harmful effects result if these steels are normalised. Normalising is used to establish materials of the same nature with respect to grain size, composition and structure. This is a process which could cause severe scaling of the workpiece.

Assessment

This is a research assignment. There are many small tools, such as cold chisels, punches, screw drivers and scratch awls, that can be hardened in the Mechanical Technology room. Describe step-by-step the general procedure to heat-treat a cold chisel.

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Kinds of furnace Electric heat-treatment furnace This furnace is capable of providing heat to temperatures within a range of 149 °C to 1 260 °C. The figure below shows an electric heat-treatment furnace with two separate chambers: a high-temperature furnace for hardening and a low-temperature furnace for tempering.

Hardening furnace

Tempering furnace

Figure 3.10: An electric heat-treatment furnace with separate chambers

Gas-fired heat treatment furnace This furnace heats to temperatures up to 1 260 °C and an example of it is shown in Figure 3.11.

Figure 3.11: A gas-fired heat-treatment furnace with an automatic temperature controlling device

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This type of furnace heats lead, salts and cyanide baths for liquid casehardening processes and for other special heat-treatment processes.

Temperature indicating and control equipment Modern heat-treatment methods for industry require accurate measurement and temperature control. The estimation of temperature by colour has long since been discarded. The thermocouple operates on the principle that whenever two wires of dissimilar metals are joined together at one end and heated, an electromotive force (EMF) is generated at the welded end. The other end is connected to a pyrometer. Indicator

thermocouple the connection of dissimilar metal wires which develop a potential difference when heated

Hot area

Furnace

Leads

Thermocouple (dissimilar metals)

Hot junction

Figure 3.12: A pyrometer is used to tell the temperature inside the furnace accurately. The thermocouple is located in the back, with leads returning to the furnace.

Assessment

1. List the principal kinds of heat treatments. 2. What kind of grain structure must steel have before it is quenched for hardening? 3. Explain the difference between hardening and tempering. 4. What is a pyrometer? 5. Why is it important that hardened steel be tempered as soon as possible? 6. What is the purpose of tempering and how is it done? 7. Name one advantage of a high tempering temperature. 8. Name three disadvantages of a high tempering temperature.

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DID YOU KNOW?

EMF is electrical energy called electromotive force.

Many types of excellent pyrometers are available. Some indicate the temperature of the furnace by the position of the needle on the dial. These are known as indicating pyrometers. Other types permanently record the temperature on a graph. These are called recording pyrometers. When the hot junction of the thermocouple is heated, a small amount of electrical energy is developed. Voltage is measured in millivolts. The scale reading on the millivoltmeter usually indicates the temperature in degrees. The thermocouple is enclosed in a tube made of heat-resistant material. This prevents oxidation or damage.

Figure 3.13: A temperature-indicating furnace controller (Honeywell)

Assessment

Answer the following multiple-choice questions in your workbook. 1. As an industrial maintenance specialist, you will normally heat-treat only: a. aluminium and brass. b. copper and iron. c. steel and aluminium. d. iron and steel. 2. Distortion and cracking occur in welding because of: a. slow cooling of the hot metal. b. uneven heating and cooling. c. uniform heating and cooling. d. incomplete fusion of the parts. 3. When annealing, steel should be heated above its: a. melting temperature. b. critical temperature. c. transformation temperature. d. freezing temperature.

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4. Preliminary annealing of cast steel: a. relieves most stresses. b. hardens the metal. c. makes drilling and grinding impossible. d. reduces the critical temperature. 5. Medium-carbon steels are almost always: a. cold formed. b. hardened and tempered. c. melted and cast. d. distorted and cracked. 6. High carbon steels are: a. difficult to form and weld. b. difficult to heat and quench. c. easy to machine and grind. d. easy to temper and breaks. 7. Explain how normalising is done. 8. When might normalising be necessary? 9. How should a part be cooled for normalising heat treatment? 10.What safety precautions must be taken when using cyanide? 11. What method of case-hardening uses ammonia gas?

Temperature colours When clean, bright steel is heated, various colours appear at different temperatures, as indicated in the following temper colour chart. Temper colours are formed by different metal oxides forming at different temperatures. These colours are extremely helpful if the furnace used is not fitted with a temperature-indicating and control device. The temperature can be successfully estimated by observing the colour of the steel as it is heated. This is the way old-time blacksmiths determined the temperature of steel for heat-treatment purposes. This method, however, is not very accurate. Without skill and experience, the temperature reader can easily be wrong by as much as 10 °C to 15 °C in a range of 191 °C to 312 °C. The estimation of temperature by colour has long since been discarded.

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Mechanical Technology The table below gives the hardness of various tools as related to their oxide colours and the temperature at which they form. Temper colour chart Approximate temperature

Articles for which suitable

Very light yellow

193 °C

Tools that require maximum hardness, lathe centres, cutting tools for lathe and shapers

Light straw

220 °C

Steel cutting tools, files and paper cutters

Straw

240 °C

Punches, taps and dies, and hacksaw blades

Gold

258 °C

Shear blades, hammer faces, rivet sets, wood and cold chisels

Purple

260 °C

Axes, wood-cutting tools and striking faces of tools

Violet

282 °C

Spring and screwdrivers, centre punches

Pale blue

304 °C

Springs

Steel grey

327 °C

Cannot be used for cutting tools

Harder – Softer

Colour of oxide

Temperature-indicating material An inexpensive way to determine temperatures of heated steel is through the use of temperature-indicating pellets, crayons, or paints. These materials are made to melt at various temperatures from 38 °C to 1 371 °C. Simply select the crayon or other material that is designed to melt at the desired temperature. Rub the crayon or other identifying material on the workpiece. When it is heated to the desired temperature, the identifying material will melt, thus indicating the temperature of the workpiece.

Figure 3.14: Temperature-indicating materials: pellets, crayons and liquid. (Tempil®)

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Materials

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Quenching media The cooling rate of a workpiece depends on many factors. The size, composition and the initial temperature of the part, and final properties desired are the deciding factors in the selection of a quenching medium. The quenching of steel from temperatures above 700 °C is a very drastic treatment and this rapid cooling is often responsible for cracking and distortion of the work. Since cooling starts from the outside on immersion, a hard and contracting shell is immediately formed around the core, which cannot begin to cool and contract at the same moment. As heat is conducted away, the core begins to cool and, as it passes through the upper critical point, there is an expansion. The hard shell has already been affected by this change and is then contracting on the core as the slight expansion takes place. These are the causes of cracking. Brine cools twice as rapidly as water and tends to remove scale from the steel. This causes the steel to cool more uniformly. In general, seven quenching media are used to give different rates of cooling. These are listed in order of severity or speed of quenching: 1. water and salt (that is sodium chloride and also called brine) 2. tap water 3. fused or liquid salts 4. molten lead 5. soluble oil and water 6. oil (the oil used should have a high flashpoint otherwise there will be the danger of fire) 7. air.

Flashpoint the lowest temperature at which a substance gives off enough vapour to flash (ignite) when a small flame or spark is applied

Water

Oil

Figure 3.15: Portable quench tank

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Mechanical Technology In liquid quenching baths it is important that either the quenching medium or the steel being quenched should be agitated (moved around in the quenching medium). The vapour that forms around the part being quenched acts as an insulator and slows down the cooling rate. This can result in incomplete or spotty hardening of the parts. Moving the part around breaks down the vapour barrier. An up-and-down motion works best for long, slender parts held vertically in the quench. A ‘figure 8’ motion is sometimes used for heavier parts.

Assessment

1. Why should steel parts be agitated during quenching? 2. Name three factors that influence the quenching rate. 3. What two defects can occur in a part if it is quenched too rapidly? 4. Name three advantages of quenching a part in brine. 5. Name three ways the hardening temperature of steel can be determined when a temperature-controlled furnace is not available. 6. Why is it desirable to have a temperature-indicating and controlling device on a heat-treatment furnace? 7. What is the purpose of a pyrometer?

Time to reflect Maintenance specialists need a working knowledge of heat treatment for welding, repairing broken parts, cutting processes and sharpening of cutting tools. Heat treatment on the production line is usually done in a furnace, but the heat treatment by the maintenance specialist is done with a welding torch. Nearly all the heat treatment by the maintenance specialist involves iron and steel. Other metals are rarely heat-treated by anyone but a trained metallurgist. All heat treatment involves both time and temperature. In annealing and hardening, the metal is heated above the critical temperature and held there for some time, until its internal structure is uniform. Then the metal is cooled. Slow cooling anneals the metal, and fast cooling hardens it. In tempering, the metal is reheated to a lower temperature for a time, and then slowly cooled. Heat treatment can either soften a metal or harden it. Low-carbon steel is often worked cold, and then annealed to relieve internal stresses. Mediumcarbon steel is always hardened and tempered. Many high-carbon steels are brittle. They must be tempered before use. 78

Terminology

4

Chapter 4

Terminology Topic 4

Cutting procedures

Terminology

Using instructions and drawings and applying different cutting methods 79

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Terminology Introduction Communicating ideas and thoughts through drawings is not new and its importance has increased in the modern world. Pictorially representing ideas is an important method of communication between product designers and manufacturers. Technological design would be impossible were it not for the several different ways in which a drawing can represent an idea. The drawing also provides a testing phase for the idea. Many drawings are rejected at the early drawing board stage, thus saving the investment of money in a manufacturing plant or risking the production of an inferior item. Almost anything can be represented by drawing, even those designs that are quite impossible to fabricate. It is important that the designer is conscious of the possible problems that the machinist will encounter. A machinist must fully understand the symbols and terminology on the designer’s drawing. A good interpretation of these symbols and terms will enable the machinist to convert the designer’s ideas into useful products.

DID YOU KNOW?

SI is the abbreviation for the Systême Internationale of Units.

History of the Systême Internationale of Units The creation of the decimal metric system during the French Revolution and the subsequent using of two platinum rods representing the metre (and now stored in vaults in Paris and New York), is really the first step in the development of the present SI system. In 1832, Gauss promoted this metric system as a system of units for physical science. He was the first person to make absolute measurements of Earth’s magnetic force in terms of a decimal system based on the millimetre (length), gram (mass) and second (time).

Systems of measurement Throughout history there have been many systems of measurement. Prior to industrial operations, individuals were often responsible for completely manufacturing products. Since the same person made the necessary parts and did the assembly alone, that person needed only to his own particular system of measurement.

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Terminology However, as machines replaced people and mass production was established, the need for standardisation of measurement grew. Measurement throughout the world is still not fully standardised. Most measurement in the modern world does, however, conform to either the English (British Imperial) system or the metric (International System of Units). The metric system is now used chiefly by most industrialised nations of the world.

The British system of measurement The British Imperial system of measurement uses the units of inches, pounds and seconds to measure length, mass and time.

The metric system of measurement

DID YOU KNOW?

The basic unit for length in the metric system is the metre. Originally the length of a metre was defined by a natural standard, specifically, a portion of Earth’s circumference. Later, more convenient metal standards were constructed. Although the metric system had been used for many years in different countries, it still lacked complete standardisation; therefore, an attempt was made to modernise and standardise the metric system. This led to the International System of Units (SI). The SI metre is defined by a physical standard that can be reproduced anywhere with accuracy. Probably the biggest advantage of the metric system is its convenience in calculations. All subdivisions and multiples use 10 as a divisor or multiplier. Notice this in the following table. Decimal fraction of 1 m

Description of the decimal fraction

Name of unit

,000001

one millionth of a metre

micrometre

,001

one thousanth of a metre

millimetre

,01

one hundredth of a metre

centimetre

,1

one tenth of a metre

decimetre

1,00

unit metre

metre

10

10 metres

one decametre

100

100 metres

one hectometre

1000

1 000 metres

one kilometre

1 000 000

one million metres

one megametre

The British Imperial system is also referred to as the inch system.

NOTE!

1 metre = 1 650 763,73 wavelengths in a vacuum of orangered light spectrum of the Krypton086 atom.

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SI base units Base quantity

Name

Symbol

length

metre

m

mass

kilogram

kg

time

second

s

electric current

ampere

A

thermodynamic temperature

Kelvin

K

amount of substance

mole

mol

luminous intensity

candela

cd

SI derived units Other quantities, called derived quantities, are defined in relation to the seven base units. Derived quantity

Name

Symbol

area

metre square

m2

volume

cubic metre

m3

speed, velocity

metre per second

m/s

acceleration

metre per second squared

m/s2

wave number

reciprocal metre

m-1

density

kilogram per cubic metre

kg/m3

specific volume

cubic metre per kilogram

m3/kg

current density

ampere per square metre

A/m2

magnetic field strength

ampere per metre

A/m

concentration of a substance

mole per cubic metre

mol/m3

luminance

candela per square metre

cd/m2

mass fraction

kilogram per kilogram, which may be represented by the number 1

kg/kg = 1

To make the system easy to understand, some units have been given special names and symbols. This was done especially where the quantities end with several units. As you can see from the following list, these quantities are named after influential scientists.

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Terminology Derived quantity

Name

Symbol

Frequency

Hertz

Hz

Force

Newton

N

Pressure, stress

Pascal

Pa

Energy, work, quantity of heat

Joule

J

Power, radiant flux

Watt

W

4

Assessment

Write an essay of 500 words or more on the contribution of any one of the above-mentioned scientists to physical sciences.

SI prefixes

DID YOU

Prefixes, combined with the unit name, form smaller or larger units by factors of powers of 10. For example, exponent (base 10) of decimal number: En = 10n (where n refers to the number of powers the base number 10 should be raised by.) Prefix

Symbol

Factor

Yotta

Y

1024

E24

1 000 000 000 000 000 000 000 000

Zeta

Z

1021

E21

1 000 000 000 000 000 000 000

Exa

E

1018

E18

1 000 000 000 000 000 000

Peta

P

1015

E15

1 000 000 000 000 000

Tera

T

1012

E12

1 000 000 000 000

Giga

G

109

E9

1 000 000 000

Mega

M

106

E6

1 000 000

Kilo

k

103

E3

1 000

Hector

h

102

E2

1 00

Deca

da

101

E0

10

Deci

d

101

E-1

0,1

Centi

c

102

E-2

0,01

Milli

m

103

E-3

0,001

6

E-6

0,000 001

9

E-9

0,000 000 001

12

E-12

0,000 000 000 001

15

E-15

0,000 000 000 000 001

18

E-18

0,000 000 000 000 000 001

21

E-21

0,000 000 000 000 000 000 001

Micro Nano Pico Femto Atto Zepto

μ n p f a z

10 10

10 10

10 10

KNOW?

A prefix is a group of letters placed in front of a word to change its meaning.

Multiple SI unit with

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Mechanical Technology The bold prefixes in the table above are the most common prefixes for metric units.

Assessment

Complete the following questions: 1. (a) one metre (m) = … millimetres (mm). (b) 50 mm = … centimetres (cm) (c) five kilometres = … metres (d) 682 mm = … centimetres (cm) 2. What is meant by the abbreviation SI? 3. What is mass production? 4. What are the three elevations most commonly used in a drawing?

Symbols and abbreviations Besides the elevations and dimensions, more information is needed to manufacture a product. This includes things like the kind of material to be used, its treatment, the number of parts required, size and shape of each part, type of finish, and the tools and dies needed. Using symbols saves a lot of time and gives the worker much needed information. The two most common symbols used on machine drawings are those that show screw threads and finish marks. Screw thread symbols may be either regular or simplified. Finished or machined surfaces are indicated by a ‘V’-like mark. The point of the ‘V’ should rest on the line of the metal in a manner similar to that of a tool bit. When it is necessary to control surface roughness of finished surfaces, the ‘vee’ is used as a base for more elaborate surface quality symbols. Many abbreviations are placed on drawings. A few of the most common are listed in the following table.

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Abbreviation

Meaning

Abbreviation

Meaning

ASSY

assembly

M/C

machine

CBORE

counter bore

I/D

inside diameter

CHAM

chamfered

CYL

cylinder or cylindrical

CI

cast iron

O/D

outside diameter

CL

centre line

A

area

CP

circular pitch

M/CD

machined

CRS

centres

INC

included

Terminology CSK

countersink

Galv

galvanised

DIA / Ø

diameter

PCD

pitch circle diameter

DR

drill

SPEC

specification

FAO

finish all over

HEX

hexagon

GA

gauge

U’CUT

undercut

HDN

harden

MATL

material

LH

left hand

A/F

across flats

MOD

module

VOL

volume

NC

national coarse

M7 × 1

metric tap size and pitch

NF

national fine

RPM

revolutions per minute

P

pitch

M

metric thread nominal diameter

R

radius

B.D.C.

diameter bolt circle

RH

right hand

SAE

Society of Automotive Engineers

SQ

square

SPH

spherical

THD

thread

ISO

International Organisation of Standardisation

UNC

United NC

BSP

British standard pipe thread

UNF

United NF

BSW

British standard Whitworth thread

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Colour-coding of metals There are numerous different metals. It is often necessary to identify and distinguished between these metals. For this reason, steel manufacturers supply the various metals with colour codes. These colour codes are standardised by the SABS (South African Bureau of Standards) and are regularly used in industry. The following table provides the most frequently used metals and their colour codes.

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Mechanical Technology metal

colour code

carbon and alloy spring steel

black

case hardened steel

orange

cast steel

blue

high-carbon steel

brown

low-alloy steel

light purple

low-carbon steel

orange

pipeline steel

grey

silicon chrome steel

black

stainless steel

black

steel for lifting machines

green

steel for pressure containers

white

structural steel

red

Suppliers of steel use different conventions on different steel sections or sizes. The colours are usually painted on the ends of the stock.

Machining symbols Finishing marks indicate the particular surface of a rough casting or forging that is to be machined or ‘finished’. They are placed in all views, touching the visible or invisible lines that are the edge views of surfaces to be machined. When a surface is to be finished by the removal of material and it is not necessary to indicate surface quality, a bar is added to the check-mark portion of the texture symbol at the top of the short leg. When material removal is prohibited, a small circle is added to the ‘V’ in place of the horizontal bar. It is not necessary to show finish marks on holes. They are omitted, and a title note, ‘finish all over’ (FAO), is substituted if the piece is to be completely machined. Finish marks are not required when limited dimension are used.

Figure 4.1: Examples of rough and machined forging

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Terminology

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Surface texture symbols Surface texture, finish, roughness, and other surface characteristics are generally used interchangeably in the workshop. However, application of these terms to drawings must follow precise Code of Practice standards. Surface texture symbols and values provide specific standards according to which finished parts may be accurately produced, uniformly inspected and measured. Common surface characteristics are surface texture, surface finish, micrometer value, lay, waviness, and flaws. The following table is an example of these surface texture symbols. Symbol

Details The surface may be produced by any manufacturing process. The basic machining symbol, a 60º with uneven legs. Add a bar to the basic machining symbol. Material removal from the surface by machining is required. Add a circle to the basic machining symbol. The surface is to be produced by no machining process such as casting, forging, welding, rolling, powder metallurgy, etc. There is no subsequent machining of the surface. The addition of a decimal or metric value to the basic machining symbol indicates the amount of stock to be removed by any machining process. This symbol indicates (the permissible range of roughness) that the high and low limits are set for the roughness value.

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Mechanical Technology This symbol indicates that the required finish texture is to be obtained without the removal of material. This symbol indicates the suitable fabrication process, treatment or coating or preservation method to obtain a specific surface texture. Place it above the horizontal bar of the machining symbol. This symbol indicates the sampling length in millimetres (length cut off). Place it below the horizontal bar of the machining symbol This symbol indicates the required direction of lay (right of lay symbol). Place it to the right of and next to the machining symbol. This symbol indicates the required machining allowance to be made in a process, in millimetres. Place it to the left of the machining symbol. To avoid repetition of a symbol only one appropriate symbol may be used when the same surface texture is required on all the surfaces of an object. Place the suitable symbol, followed by the phrase “all over” near the drawing of the object.

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Terminology

4

Centre lathe Introduction • The centre lathe is one of the most multipurpose machines in the Mechanical Technology centre. Even though the centre lathe is mainly a machine for the producing cylindrical work, it is by nature much more than that. Being so versatile, it is a readily adaptable piece of machinery which can be used to carry out various other machining operations in addition to its fundamental operations. • The operational principles of the centre lathe were discussed at length in Gr. 10. These uses included facing, parallel turning, drilling, boring, taperturning, screw-thread cutting, parting and knurling. • You should remember that a centre lathe is a machine tool that cuts by rotating the workpiece against the cutting edge of the cutting tool. The cutting tool can move across the bed (facing) and also along the lathe bed (sliding). • A centre lathe enables the operator to execute a variety of processes on a workpiece. These processes may involve different actions carried out either by a cutting tool or by another kind of tool (e.g. a knurling tool, reamer, drill bit) acting on the turning workpiece. • In grade 11 you are required to do taper turning as centre lathe work. There are a number of ways taper turning can be done on a centre lathe: – The tail stock can be offset for longer external tapers. – The taper turning attachment can be used for external tapers and for short internal boring – The compound slide rest can be rotated round for turning short internal and external tapers. • Tapers can be expressed in three ways: – It can be expressed as an included angle. – As a given amount on the diameter per unit length, e.g. 12 mm per 300 mm. – As an incline, e.g. 1 in 25 on the diameter. • For grade 11 purposes you are only required to concentrate extensively on taper turning using the compound slide, set over method.

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Taper turning Did you know?

A taper is a shaft or hole that grows gradually smaller towards the one end.

Did you know?

A slight taper, like a Morse taper, is called a self-holding taper since it is held in and driven by friction.

Tapers are very useful machine elements that are used on machines because of their capacity to align and hold machine parts and to realign them when they are frequently disassembled and assembled. A well-known example of a taper is the Morse tapered shank of a drill bit. Taper turning is the process of producing a conical (pointed) profile which equally increases or decreases in diameter (externally or internally) as the cutting tool is fed longitudinally along the rotating workpiece on the lathe. There are, of course, many types of taper work that have to be carried out repeatedly on the centre lathe. These could be roughly arranged in categories as follows: • long, slow tapers under approximately 14° included angle. • long, steep tapers over approximately 14° included angle. • short, slow tapers • short, steep tapers • combinations of two or a number of the previous categories. There are a number of methods in which tapers can be turned: 1. The tailstock can be set over (off set) for longer external tapers. 2. The taper turning attachment can be used for external tapers and for boring short internal tapers. 3. A straight-edged turning tool (formed tool) can be used for very short tapers (this method is not well-known or generally in used). 4. The compound slide rest can be swivelled around for turning external tapers or short internal tapers. 5. Use of a tracer or computer numeral controlled lathe. These methods are basically mentioned to make you aware of the broad range of methods that can be employed for taper turning. It is expected that you familiarise yourself with the following method of taper turning:

The compound slide method of taper turning The compound slide method can be used for both internal and external short, steep tapers on the lathe, by hand feeding the compound slide. The same method can also be used for boring work, lathe centres and bevel gears, etc. This method is best described by H. C. Bogaard.

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Terminology

4

The compound slide is always set at an angle, equal to half the included angle of the desired angle, to the axis of the workpiece. The base of the compound slide is graduated in degrees to enable it to be easily set to cut the required taper. The following formula is used to calculate the angle to which the compound slide must be set. large diameter of taper – small diameter of taper 2 × length of taper D–d = 2×L

Tan θ =

Example 1 A taper 150 mm long, has to be turned on the end of a 75 mm diameter shaft. If the diameter of the small end of the taper is 60 mm, calculate the angle to which the compound slide must be set in order to cut this taper. Solution:

=

D–d 2×L

=

75 – 60 2 × 150

=

15 300

=

1 20

=

0,05

θ

= 2° 52´

The compound (top) slide must be set at the required angle for turning. The workpiece can either be held in the chuck or between centres. The circular graduated, compound slide base is slackened at the two holding down bolts. Sway the compound slide to the correct angle and clamp down at the two bolts when in position. Successive cutting operations are required to turn this taper by hand. 91

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Example 2 An internal taper 150 mm long, has to be bored in a bush. The large diameter of the bush hole is 60 mm. Calculate the small diameter of the taper hole if the included angle is 8°. Solution:

NOTE: θ/2 = 8°/2 = 4° Tan θ =

D–d 2×L

Tan 4° =

60 – d 2 × 150

0.0699 = 60 – d 300 300 × 0.0699 = 60 – d 20,97 = 60 – d 20,97 + d = 60 – 20,97 d = 39.03 mm The diameter at the small end of the tapered hole is 39,03 mm.

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Terminology

Example 3 A taper of 1 in 15 must be turned by means of the compound slide method at the end of a shaft. Calculate the angle at which the compound slide must be set to cut the taper, as well as the included angle of the taper. Express the angle in degrees (°) and minutes (‘).

4

DID YOU KNOW?

Solution:

1 degree (°) = 60 minutes (‘) 1 minute (‘) = 60 seconds (“)

NOTE: A taper of 1 in 15 means that the diameter increases 1 mm for every 15 mm of taper length, or 10 mm for every 150 mm of taper length, etc. Tan θ =

D–d 2×L

Tan θ =

1 2 × 15

0.Tan θ =

1 30

REMEMBER: That compound slide will only be set to half the included angle of the taper. 5 Tan θ = 150 Tan θ = 0,03333 Tan θ = 1,909°

Tan θ = 0,03333 Tan θ = 1,909° Tan θ = 1° 55‘

Tan θ = 1° 55‘

The angle is 1 degree and 55 minutes

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Mechanical Technology Advantages of the compound slide method • The main advantage of this method is the fact that compound slide can be set at any required angle. • Tapers with large angles can be turned. • This method can also be used for external and internal taper cutting. Disadvantages of the compound slide method • The compound slide can only move a short distance. • Only short tapers can be turned, because of the limited length of the compound slide. • The taper can be done in stages, but the accuracy of the taper is impaired. • This method can only be done by hand, which will make the surface texture irregular. • Taper turning using this method tends to be monotonous and can cause fatigue in the operator.

Cutting procedure for cutting a taper by using the compound slide method • Release the lock nuts of the compound slide. • Swing the compound slide to half the included angle. • Tighten the lock nuts (take care not to over tighten). • Mount the cutting tool in the tool holder in the toolpost; the cutting tool • • • •

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must be set centre height. Use the compound slide feed handle and feed the cutting tool slowly into the workpiece. Proceed to the end of cutting length, return to starting position, and feed the cutting tool in for the next cut. Repeat the procedure until the taper is completed. On completion, test the taper with a taper ring gauge for size and correct angle.

Terminology

4

Milling machine Vertical milling machines The vertical milling machine differs from the horizontal milling machine as follows. In the vertical milling machine, the spindle occupies a vertical position relative to the table. The head can be swivelled in the vertical plane up to an angle of 45º, making it possible to take angular cuts without disturbing the work from its setting. Cutters adapt to the spindle with taper shanks and drawbars hold them tightly, or they can be held in chucks. In vertical milling, end mills are used to machine flat, vertical and horizontal surfaces. Vertical milling is commonly used in lighter work of an assorted nature, which includes the milling of bevels, keyways, slides, chamfers, other angles, grooves, dies, jigs recesses, slots, tees and dovetail slots or keyways. End mills vary from 3,17 mm to 50,8 mm in diameter. Face milling is done with shell-end mills whose diameters run from 139,7 mm to 381 mm. The knee, saddle and table are similar to those on the horizontal machine. Motor

Overarm

Head clamp Vernier scale

Spindle

Saddle

Figure 4.2: A vertical milling machine

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Horizontal milling machines The most common type of milling machine is the horizontal milling machine.

Did you know?

The horizontal milling machine is also called the horizontal knee machine.

It is sometimes called the horizontal knee milling machine because of the suspended ‘knee”, which slides up-and-down the front of the milling machine and carries the cross-slide and table. Horizontal milling machines may be of the plain or universal type. The main difference between the two types is that the table of the universal type of milling machine can be swivelled about a point directly below the centre line of the arbor, whilst the table of the plain milling machine cannot be swivelled. Another key difference is that the arbor of the universal milling machine can rotate in the forward and reverse direction. In addition, the standard accessory of the universal milling machine includes a dividing head. Adjustable overarm

Arbor support

Horizontal spindle

Machine table

Knee and saddle

Power-feed unit

Handwheel Base

Figure 4.3: A universal horizontal milling machine

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Dividing head The dividing head is a supplementary component to the milling machine. The dividing head breaks up the circumference of a circular workpiece into a number of equal divisions. It is used for helical milling and is necessary for gear, flute and spline cutting as well as for indexing and cutting operations where accurate angular divisions are required. The dividing head consists of a worm shaft with a single-start worm that engages with a worm gear with 40 teeth. It is from this engaging that a 40 to 1 ratio of the dividing head is obtained.

Did you know?

The dividing head is also known as the indexing head. Figure 4.4: A dividing head

With milling work of this nature, a method to secure the workpiece and to rotate it so that each groove and gear tooth can be machined accurately is important. The accuracy of the spacing is of extreme importance. In milling operations like this, the highest level of accuracy and precision is vital. The dividing head is mounted on the milling machine table and used to work between centres in conjunction with the tailstock; or it can be fitted with a chuck for the direct mounting of work.

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The function of the dividing head The dividing head is a device for dividing the circumference of the workpiece into any number of equal parts and to hold the workpiece in the required position while cuts are being made. The purpose of the dividing head includes helical milling and is necessary for gear, flutes, keyways, squares, hexagons and spline cutting as well as for indexing and cutting operations where accurate, angular divisions are needed.

Figure 4.5: Section through dividing head showing worm and worm gear.

The construction of the dividing head The dividing head consists mainly of a spindle on which a (spur) gear with 40 teeth is keyed. A worm shaft with a single-start worm engages with a worm gear (worm wheel) with 40 teeth. The 40 to 1 ratio of the dividing head is obtained from the engaging of the worm wheel and worm. The worm and worm gear obtain a rotary movement of the spindle. A hole is bored through the entire length of the spindle. A tapered hole is provided on the front end of the spindle. This tapered hole holds the live centre, which is used to support the workpiece when working between centres. The front of the spindle is threaded to receive a chuck (similar to that of the main spindle of the centre lathe). The other principal parts of the dividing head are the index plates, sector arms, spindle, crank, and case. 98

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Figure 4:6: The main parts of a driving head

The operation of the dividing head The gear ratio in the dividing head is 40 to 1, i.e. it takes 40 revolutions (turns) of the worm to turn the worm wheel one revolution. That implies that one turn of the worm will rotate the worm wheel ¼0 th of a turn or 9 degrees of a revolution.

Figure 4:7: Operating principle of the dividing head

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Index plate The aim of the index plate with its confusion of holes is to enable one revolution of the crank to be further subdivided into fractions of a revolution, especially where the fraction is not a factor of 40. The index plate is provided with a number of circles each with a different number of holes. The hole circles in the index plate provide the indexing function. The pattern on the index plate consists of a number of equally spaced hole circles, and the crank pin is adjusted in and out so that the desired circle may be used. The index crank contains a pin on a springloaded plunger that engages the holes in the index plate. The Brown and Sharpe dividing head is provided with three index plates with the following circles of holes: Plate one : 15, 16, 17, 18, 19, and 20 holes Plate two : 21, 23, 27, 29, 31 and 33 holes Plate three : 37, 39, 41, 43, 47 and 49 holes The Cincinnati and Parkinson dividing heads are of larger diameter than the Brown and Sharpe plates and are reversible. : 24, 25, 28, 30, 34, 37, 38, 39, 41, 42, and 43 holes Side one Side two : 46, 47, 49, 51, 53, 54, 57, 58, 59, 62, and 66 holes

Sector arms Two sector arms rotate about the crank hub and can be adjusted to indicate the correct number of holes for a partial turn of the crank. For example, if you are indexing one turn and ten holes for each division, the sector arms can be set so that you will not have to count the ten extra holes each time.

The advantage of the sector arms The use of sector arms saves much time in counting holes each time when indexing is done. It removes the possibility of error in counting the number of holes for each move of the index pin. It eliminates fatigue caused by the monotony in counting holes.

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Terminology

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Indexing on the dividing head Rapid indexing Rapid indexing is the simplest form of indexing. In this kind of indexing the worm shaft is first disengaged from the worm wheel by an eccentric device in the dividing head. This method can be used for quick indexing when it is necessary to cut squares, pentagon, hexagons, flutes and dog clutches, as well as for mass production of parts where only a small number of divisions are required around a circumference or across a face, e.g. two flats on a bolt.

Did you know?

Rapid indexing is also known as direct indexing.

The workpiece is mounted in the chuck of the dividing head and is rotated the required amount. It is held in place by a pin that engages the slot or hole in the rapid index plate. This rapid index plate is mounted on the dividing head spindle. These plates normally have twenty-four, thirty, and thirty-six equally spaced holes. The number of divisions is limited to numbers that are fractions of these numbers. Care must be taken to engage the worm and the worm wheel in proper mesh when rapid indexing has been completed. The index head is, therefore, once again set up for plain or simple indexing.

Simple indexing When indexing must be done, the number of turns of the crank that is needed to move the workpiece the required distance in order to cut the number of teeth or grooves on the circumference of the workpiece, must be calculated. Because the dividing head has a 40 to 1 ratio, the indexing movement, i.e. the crank T = 40 N Number of turns = 40 where N = number of divisions (e.g. number of teeth N or grooves) Example 1 Calculate the indexing for the following: (i) 5 teeth (ii) 20 teeth (iii) 8 teeth

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To calculate the number of turns (i) Number of turns = 40 N Number of turns = 40 5 Number of turns = 8 full turns (ii) Number of turns = 40 N Number of turns = 40 20 Number of turns = 2 full turns (iii) Number of turns = 40 N Number of turns = 40 8 Number of turns = 5 full turns

Example 2 Calculate the indexing for the following: (i) 23 divisions (ii) 17 divisions (iii) 28 divisions (iv) 39 divisions (v) 98 divisions To calculate the number of turns (i) Number of turns = 40 N Number of turns = 40 23 Number of turns = 1 17 23 The number of turns is equal to 1 full turn of the crank, the seventeen and twenty-thirds of a full turn is obtained from the index plate. (Brown and Sharpe plate with a twenty-three hole circle). (See Brown and Sharpe table above.)

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(ii) Number of turns = 40 N Number of turns = 40 17 Number of turns = 2 6 17 The number of turns is equal to 2 full turns, 6 holes on a 17 hole-circle (iii) Number of turns = 40 N Number of turns = 40 28 Number of turns = 10  4 7 Number of turns = 1 3 7 Number of turns = 1 3 × 3 7 3 Number of turns = 1 9 21 The number of turns is equal to 1 full turn, 9 holes on a 21 hole-circle (iv) Number of turns = 40 N Number of turns = 40 39 Number of turns = 1 1 39 The number of turns is equal to 1 full turn, 1 hole on a 39 hole-circle (v) Number of turns = 40 N Number of turns = 40 98 Number of turns = 1 20 49 The number of turns is equal to 20 holes on a 49 hole-circle

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Practical examples Calculation on indexing for a square, pentagon and hexagon Machining a square Example A shaft with a diameter of 80 mm must be machined on a milling machine, a square on the one end and a hexagon on the other end. The shaft must be supported on the dividing head and the tailstock. A helical cutter must be used to machine the shaft. Calculate how much the cutter must be fed into the workpiece to cut the biggest possible square and hexagon on the ends of the workpiece. Also describe the procedure of the milling operation. The milling operation to cut a square: • Mount the shaft (workpiece) between the centres of the dividing head and the tailstock. • Mount the appropriate size helical milling cutter in the appropriate position on the arbor. • Support the shaft with a screw jack close to the tailstock end. • Move the milling machine table to bring the cutter in position above the shaft at the tailstock end. • Disengage the worm and the worm gear, and set up the rapid indexing for four divisions. (HINT: If a rapid index plate with 24 groves or holes is used, the 6th groove or hole will be used for indexing. Thus mark the 6th groove or hole with white chalk to avoid future confusion.) • Calculate the amount that the milling cutter must be fed into the shaft material to obtain the biggest possible square. • Calculate the distance x across flat sides.

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Sin θ = = = = x =

x 80 Sin θ × 80 Sin 45 × 80 0,7071 × 80 56,59 mm

Terminology

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To determine the depth of the cut, deduct (minus) x from the shaft diameter and divide by two. Depth of cut =

80 – x 2

= 80 – 56,59 2

Depth of cut = 11,705 mm • Lift the table slowly, holding a paper strip between the shaft and the rotating cutter until the rotating cutter grabs the paper strip. (HINT: Ordinary chalk can also be used instead of a paper strip. Lift the table slowly until the chalk is scrapped away.) • Move the table away from the milling cutter and then lift the table 11,705 mm. (HINT: It is advisable that 2 smaller cuts be taken and a finishing cut of 0,02 mm be taken last; but when the smaller cuts and finishing cut are taken, rotate the workpiece through the whole cycle (all four sides) before any adjustments are done to the table setting.) • Lock the knee in this position. • Set the table stoppers at the required length of cut. • Cut the first side of the square and repeat until the square is completely machined.

Machining a hexagon To machine a hexagon, the same procedure must be followed as for a square, except: • Set the indexing for six divisions (six flat sides). • Calculate the amount that the milling cutter must be fed into the shaft material to obtain the biggest possible hexagon. • Calculate the distance x across flat sides. x 80 = Sin θ × 80 = Sin 60 × 80 = 0,886 × 80 x = 69,28 mm

Sin θ =

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To determine the depth of the cut, deduct (minus) x from the shaft diameter and divide by two. Depth of cut = Diameter of shaft – distance across flat sides 2 Depth of cut =

80 – x 2

= 80 – 69,28 2

Depth of cut = 10,72 mm

Machining a pentagon Example Calculate the index to cut a pentagon using simple indexing. SOLUTION: Indexing Number of turns = 40 N

Number of turns = 40 5 Number of turns = 8 full turns of the crank

The cutting procedure will be the same as for the above examples.

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Centring of a milling cutter In some types of work the position of the milling cutter in relation to the workpiece is of no consequence. However, in many operations it is of the utmost importance that the centre of the milling cutter corresponds with the centre of the workpiece. To centre the milling cutter on the workpiece, measure the thickness of the cutter and mount the cutter on the arbor. Measure the diameter of the workpiece, position a square on the machine table against the workpiece, and measure from the square to the inner side of the milling cutter a distance equal to half the diameter of the workpiece minus half the width of the milling cutter. Example A 15 mm wide slot must be cut on a workpiece 45 mm in diameter. Draw a neat sketch and describe how a 15 mm wide side and face milling cutter can be centred on the workpiece to cut the slot. Measure from the side of the square to the side of the milling cutter a distance equal to the half workpiece diameter minus half the width of cutter: = 22,5 mm – 7,5 mm = 15 mm When the distance of 15 mm is measured between the square and the milling cutter, the centre of the milling cutter will coincide with the centre of the workpiece.

Cutter

Steel rule

Work Square

Figure 4.8: Centring a milling cutter

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Squaring the milling machine vice Before understanding how to centre a milling cutter with a dial test indicator (DTI), let us look at a very important aspect of successful milling operations, namely how to square a milling machine vice on the machine table accurately using a dial test indicator. Did you know?

Dial test indicators are also called dial indicators or dial gauges.

To do accurate milling on a workpiece clamped in a milling machine vice, it is of the utmost importance to align the vice squarely with the milling machine or parallel to the machine. • Firstly, clean the column or spacing collars on the arbor depending on where you will mount the DTI. (If the DTI is mounted on spacing collars on the arbor, use the split holder for DTI.) • The plunger of the indicator is adjusted to touch the solid jaw of the machine vice. • The milling machine table is adjusted in and out across the knee. (That is on the xx plane.) • If the needle, pointer or hand moves, that is an indication that the jaw is out of alignment and the machine vice must be adjusted on the milling machine table. • Repeat the procedure until there is no movement or deflection of the needle of the DTI anymore, and lock the vice securely on the milling machine table. • Upon completion, milling operations can commence.

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Figure 4.9: Squaring the milling machine vice using the fixed jaw of a vice or a parallel

NOTE: • Always use the dial test indicator readings near the extremes of the jaw/ parallel to achieve best accuracy. • Always tap the side of the machine vice at the handle end of the vice to correct any error. • Never tap the machine vice towards the dial test indicator unless the plunger is removed from the vice jaw or parallel.

Centring a dial test indicator to locate the centre of a worn hole If the need arises that the centre of a worn hole, e.g. worn bearing hole or worn cylinder hole of a motor vehicle engine, is to be determined, the spindle of the milling machine must be centred over the existing hole. Place the dial test indicator in the spindle of the milling machine. Bring the finger or plunger of dial test indicator to touch the inside of the hole, and read the inside surface. Move the handwheels for the “X” and “Y” axis until there is no movement of the hand on the dial (clock) of the dial test indicator when the spindle turns around. Now the spindle is in perfect alignment with the centre of the worn hole. Please take note that the hole is worn out and it may not be possible to reach a zero reading on the dial (clock) of the dial test indicator. This practice is common in the automotive engineering workshop. An example of this is the re-boring and the re-sleeving of cylinders of a motor vehicle engine. Note must also be taken that the boring out of worn bearing holes to larger diameters and sleeving them with simple bushes made on the centre lathe is a fairly common machining practice.

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Workpiece

Longitudinal feed handwheel

Cross-feed handwheel

Figure 4.10: Using a dial test indicator to find the centre of a hole

Centring a milling cutter using a dial test indicator Many vertical milling machine operations require that the milling cutter be positioned over the centre of the workpiece. It is important for the centre line of the milling cutter and the centre line of the workpiece to coincide. • Mount the dial test indicator in the milling machine spindle (after the milling machine spindle has been cleaned thoroughly). • For the centre line of the milling cutter and the centre line of the workpiece to overlap, use the following formula: 110

Terminology

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The diameter of workpiece – width of cutter 2 • Move the dial test indicator plunger contact point so that it touches the side of the workpiece. Set the indicator bezel (gauge) to zero. • Now rotate the milling machine spindle 180°. Compare the two dial indicator readings and split the difference between them by moving the milling machine table. Locate the cutter centre in one axis, then in the second axis. • In this case, the dial indicator readings must be identically 180° apart over the workpiece centre line. • Always double check the readings on the dial test indicator gauge and the machine micrometer dial settings before any machining can commence. Arbor with collars/spacers

Workpiece

Centre line of workpiece

Arbor support

Handwheel for cross feed table movement

Figure 4.11: Centring a milling cutter using a dial test indicator

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Milling machine cutters The metal-cutting versatility of the horizontal and vertical mill can be realised and utilised by understanding, identifying and selecting from the many types of milling cutters available for use on the milling machine. The general types of milling cutters can be classified as: • arbor cutters – plain, side, staggered-tooth, metal-slitting saw and form (profile) cutters • shank cutters – end mills, T-slot, Woodruff key seat and fly cutter.

Arbor cutters Plain milling cutters Plain cutters are cylindrical with teeth on the periphery only. These teeth can be straight or helical, depending on the width of the cutter. These cutters can be used with other cutters to produce surfaces in various forms.

Plain milling cutters

Plain milling cutter with helical teeth

Figure 4.12: Plain milling cutters and plain milling with helical teeth

Side milling cutters These cutters are similar to plain milling cutters. They do their main work with teeth formed on the end. The side and face cutters cut differently; they cut on both periphery and face. The teeth can be straight, helical or staggered.

Figure 4.13: A side milling cutter

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Staggered-tooth milling cutters Staggered-tooth milling cutters are narrow, cylindrical cutters that have staggered teeth with alternate teeth having opposite helix angles. These cutters are ground to cut only on the periphery. However, each tooth has a chip-clearance ground on the protruding side. The cutters have a free cutting action that makes them useful in milling deep slots.

Figure 4.14: A staggered-tooth milling cutter

Slitting saws Slitting saws are manufactured with diameters from 63 mm to 315 mm. Slitting saws are designed for many jobs, from parting off and slitting thin sections to cutting deep and narrow slots. The sides are slightly tapered toward the bore (hole) to prevent binding. Like other milling cutters, they are available with plain, staggered and side teeth. The thinner slitting saws must be used with extreme care and should not be used when a thicker saw would be more suitable.

Figure 4.15: Slitting saws

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Mechanical Technology Angular milling cutters These cutters offer a quick and convenient way of milling small, angular surfaces, notches, dovetails, flutes on milling cutters and reamer teeth and V-grooves without tilting the workpiece. Single-angle cutters have one angular surface. Double-angle cutters have V-shaped teeth. These cutters, with equal conical angles on both faces, are made with an included angle of 45°, 60° and 90°. With double unequal angle cutters, the angles are measured separately in a radial plane between the sides of the cones and their plane of intersection.

Double unequal angle cutter

Double equal angle cutter

Single angle cutter

Figure 4.16: Angular milling cutters

Form (profile) cutters A variety of form cutters are made for producing hollows, corner-rounded edges, gear cutters, formed tooth cutters and thread milling cutters. They are available in concave and convex styles. Clearance behind the cutting-edge is provided by relieving and these cutters are sharpened only on the front face of each tooth. In this way they will reproduce identical profiles after repeated sharpening. This is important with gear cutters, for example, which are given top relief only. Form cutters are often given side relief as well.

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Terminology

Single angle cutter

Side and face cutter

4

Single cornerrounding cutter Equal angle cutter

Cylindrical cutter

Convex cutter

Concave cutter

Figure 4.17: Form cutters

Shank cutters End mills End milling cutters have teeth on the end face as well as on the periphery. They are made in two distinct styles; the solid and the shell-end type. Except for the shell type, all end mills have either a straight shank or a tapered shank. The shank fits into the spindle of the milling machine for driving the cutter. Many end mills have carbide teeth; this makes an increase of production possible. Multi-flute end mills can be run at the same speed and feed as comparable two-lipped end mills. However, they have a longer cutting life and will produce a better finish. Shell-end mills Shell-end mills are made to be mounted on a short arbor. They have end and peripheral teeth. Shell-end mills are usually more economical than large, solid end mills. They are cheaper to replace when broken or worn out. Cutters of this type are intended for stabbing or surfacing cuts, either face or end milling operations.

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Mechanical Technology T-slot cutters Note

In cutting a T-slot, a groove for the narrow portion of the slot is first machined with an end mill or side mill.

T-slot cutters are a special type of end mill, having either straight or tapered shanks. They are designed for cutting T-slots in machine tables and similar applications. It is finished with the T-slot cutter.

Figure 4.18: Milling slots

Woodruff keyseat cutters (way cutters) Woodruff keyseat cutters are specially designed to cut keyways for Woodruff keys. Woodruff keys have the shape of a semi-circle. These cutters are available in all sizes. They are of two types: end-mill and arbor cutters.

Figure 4.19: Narrow-width Woodruff cutter

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Assessment

1. List the two categories of milling cutters, and give five examples of each category. 2. Why are slitting saws tapered towards the bore? 3. List seven profiles that can be produced by profile cutters.

This chapter covers only the basics in becoming a successful machinist. Operating a centre lathe can become an enormously rewarding career. In this chapter we have explained the work pertaining to working between centres. When you work with the centre lathe fitted with a chuck, the work will be very straightforward.

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Joining methods

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Chapter 5

Joining methods Topic 5

Working instructions and applying joining methods

Welding joint symbols

Joining methods Factors influencing welding joints

Applications

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Joining methods What are permanent joining applications?

dismantled taken apart

In Grade 10 you learnt that semi-permanent joining applications refer to methods used to join different objects together. These joins are secure but not necessarily permanent. Permanent joins are used when joints do not have to be dismantled or serviced. There are numerous ways of permanently joining engineering materials, but we will deal only with the main groups, namely soldering and fusion welding. After learning about these joining methods, you will use some of them by constructing welded joints.

Welding joint symbols Many parts that were formerly cast in foundries are now being constructed by welding. Complete welding and machining information is conveyed from the part designer to the welder, machinist, and machine technician by graphic symbols. The symbols indicate the required type of weld, specific welding and machine dimension, and other specifications.

Elements of a welding symbol Welding symbols comprise seven elements. These elements are: • reference line • arrow • weld symbol/s • supplementary symbol/s • dimension/s • tail • specification, process, or other reference/s.

Reference line The reference line is the line on which all the welding symbol elements are placed. The reference line should be drawn parallel to either the horizontal or the vertical axis of the drawing. If drawn parallel to the vertical axis, the information on it should be arranged to read from left to right when viewed from the right-hand side of the drawing.

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Arrow The arrow may be drawn from either end of the reference line. The head of the arrow should connect with the side of the joint which is to be regarded as the arrow side.

Figure 5.1: Symbols placed beneath the reference line refer to the arrow side

Welding symbol/s Welding symbols should be placed below, above, or on the reference line and not on the lines of the drawing.

Supplementary symbol/s Supplementary symbols indicate additional information about a specific weld symbol. A site-weld symbol indicates that the weld is to be done on site. The weld-all-around symbol means that the joint must be welded all the way around. Contour symbols denote the shape of the weld face. Finish symbols denote how the contour is to be achieved.

Dimension/s All dimensions are given in millimetres. Use the comma as the decimal mark and precede all fractions of the unit by a ‘0’, e.g. 0,6. Degree marks should be included in all cases. It is advisable to read up on additional dimension symbols like size of weld, length of weld, pitch of intermittent welds and symmetrical double weld symbol.

Tail of the welding symbol When the weld symbol, dimensions and supplementary symbols cannot give sufficient information to describe a weld, additional information in the form of notes, symbols, or references is given in the tail of the welding symbol. The tail comprises a V-shaped line which is placed at the end of the weld symbol. In cases where no such information is necessary, the tail may be omitted. 121

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Specification, process, or other reference/s When dimensions, profiles or the finish of all welded joints in a drawing are covered by a general note on the drawing, such information should not be repeated on the symbol.

Combining all the information

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Reference line

Basis of welding symbol and all elements. It is a straight line. Accommodates weld data.

Arrow line

Connects the reference line to the arrow-side member of the joint. Points to the joint where the weld is to be placed. It can point in any direction.

Basic weld symbol added

A variety of symbols can be used. Attached to reference line to show the kind of weld and the sides on which it is to be welded.

Additional welding symbol added

A combination of symbols can be added to the reference line. When welding is to be done on both sides of the joint, the weld symbol applying to the arrow side should be inverted and placed below the reference line, and the symbol applying to the other side of the joint above the reference line.

Supplementary symbols added

= Flush or flat contour symbol (indicates the shape of the required weld face). M = Machine finish (how the contour is formed). • = Site weld. = Weld all round.

Joining methods Dimensions added

10 = Size of the weld (always to the left of the weld symbol. No size dimension is required if the weld is a full penetration weld). 25 = Length of the weld (always to the right of the weld). 100 = Pitch of the weld.

Tail added

Tail provides additional welding process information or specification which is not otherwise shown by symbols.

Complete symbol

V-butt weld on the other side MIG welded 3 mm root gap 10 = Size of the weld 25 mm long weld 100 mm pitch Convex weld face Grinded finish Welded on site Welded all round

5

Weld symbols in more detail To ensure that a standard group of symbols can be understood in all manufacturing plants, conventional symbols are used. Each symbol resembles, in a general way, the type of weld it represents.

Stud

Plug or slot

Surfacing

Bead

Fillet

Corner

Edge

Flare V

Flare bevel

J-

Flange U-

Bevel Half V

v-

Square I-

Butt or grove

Fusion spot

Type of weld

Figure 5.2: Fusion welding symbols

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Mechanical Technology Examples of fusion welding symbols The diagrams below indicate how the following fusion welding symbols are depicted on a reference line: • fillet weld • square-butt weld • V-butt weld • bevel weld • U-butt weld • J-butt weld

Figure 5.3: A fillet weld (both sides)

Figure 5.4: A square-butt weld (arrow side)

Figure 5.5: A V-butt weld (both sides)

Figure 5.6: A single bevel weld (arrow side)

Figure 5.7: A U-butt weld (arrow side)

Figure 5.8: A J-butt weld (arrow side)

Resistance weld symbols

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Resistance welding is another method of fusing parts. The fusing temperature is produced in the particular area to be welded by applying force and passing electric current between two electrodes and the parts.

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Resistance welding does not require filler metal or fluxes. The symbols for general types of resistance welds follow. Types of Resistance Welds Spot

Projection

Seam

Foil Seam

Flush

Figure 5.9: Resistance welding symbols

Supplementary weld symbols in more detail General supplementary weld symbols are shown in the following table. These symbols convey additional information about the extent of welding, and the location and contour of the weld bead. The contour symbols are placed above or below the weld symbol. Weld all round

Site weld

Weld face Flush

Finish

Convex Concave Machine M

Grind

Chisel

Flame

G

C

F

Figure 5.10: Resistance welding symbols

Assessment

Copy the following diagrams into your workbook and complete them by indicating the correct welding symbol for each joint.

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Applications Soldering Solders are commonly classified as soft and hard solders, depending upon their melting points and strengths. These groups will be discussed in greater detail later in the chapter. adhesion bonding

Soldering is an adhesion process in which the metals to be joined are heated, but not melted. This allows metals to be joined together at temperatures far below their melting temperatures. The solder metal melts and flows between the metals being joined, but does not melt them. You can demonstrate this principle by dropping some water onto a piece of plate glass and floating another piece of glass on top of it. The water bonds the pieces of glass together very strongly by means of capillary action. The pieces are held together by the water without being fused together.

bismuth an element, found next to lead on the Periodic Table of Elements

Soft solder

flux a substance used in conjunction with solder to assist with the soldering process

Soft solder is sometimes called lead solder and is commonly used in joining electronic components. It is made of an alloy of lead, tin and sometimes bismuth. Soft solder melts at a very low temperature and is a good electrical conductor, hence its use in electronic components. One kind of soft solder, lead solder, is also used in joining copper plumbing pipes. The solder is either a solid bar or in hollow wire form, with an acid or resin core as a flux. Flux is used together with soldering to dissolve metal oxides and impurities on the metal surface. This allows the solder to flow into the joint.

Figure 5.11: Lead solder in wire form

The soldered joints are usually heated by an electric soldering iron or a LP gas blowtorch. When the surfaces are heated to the melting point of the solder, the solder is applied and runs freely, solidifying as the surfaces cool.

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Figure 5.12: Electric soldering iron (used for soldering electronic components)

Figure 5.13: Liquid petroleum (LP) gas blowtorch (used for soldering copper plumbing pipes)

Because very little heat is needed in soft soldered joints, the surrounding metal joint is not exposed to high temperatures and hence, stress and distortion are minimised.

Hard solder Hard solders are usually alloys of silver, copper, and zinc (silver solder) or of copper and zinc (brazing). They are a lot stronger than soft solders and have higher melting points. Hard solder is applied in a very similar way to soft solder, but since more heat is required, oxy-acetylene welding apparatus is usually used. Flux is usually in powder form and can be applied in many ways.

Hard solder flux The main component of commercial flux is borax. Borax, (or sodium tetraborate), is a chemical compound (Na2B4O7·10H2O), which is manufactured in the form of a white, crystal-like powder. The powder readily forms a solution when mixed with water. Borax is a very good flux because it dissolves metallic oxide and other undesirable substances to leave a clean metal surface. When heated, it melts, forming a glass-like, sticky liquid. Some commercial fluxes also contain small amounts of phosphorous and halogen salts.

DID YOU KNOW?

Halogens are the elements found in Group 7 of the Periodic Table of Elements. They are very reactive and are usually found as binary ionic compounds.

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Mechanical Technology When soldering, good ventilation is essential as most fumes from soldering fluxes are harmful or even poisonous. Be careful not to let flux come into contact with your skin.

The application of flux Caution Flux fumes can be very dangerous. Do not inhale them or allow flux to come into contact with your skin.

Flux is usually prepared by mixing it into a paste with a little water. It is then applied directly onto the soldered joint. It may also be applied in the following ways: • sprinkled directly onto a heated filler rod and soldered joint • pre-coated on filler rods • automatically fed through the blowtorch in an alcohol solution.

Silver solder Silver (Ag) is a precious metal and, therefore, very expensive. Some silver solders contain up to 50% Ag. The more silver there is in a solder alloy, the lower its melting temperature. Because silver solder has a relatively low melting point, it is particularly useful in soldering other metals with low melting points, such as copper and brass. Silver solder is used in many commercial processes where dissimilar metals need to be joined.

Brazing Brazing is an adhesion process in which the metals to be joined are heated but not melted; the brazing filler metal melts and flows at temperatures above 427 °C. A brazed joint is stronger than a silver-soldered joint and can be as strong as a welded joint. Brazing is used where mechanical strength and pressure-proof joints are desired and can be superior to welding as it does not affect the heat treatment of the original metals as much as fusion welding. It also warps the material less and makes it possible to join dissimilar metals, for example: • steel tube to cast iron • copper to steel • tool steel to low-carbon steel • tungsten carbide to low-carbon steel. Good ventilation is important when brazing, because fumes from the heated fluxes and vaporising filler metals (such as zinc and cadmium) may affect your respiratory system, eyes or skin.

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When brazing two pieces of metal, the joining surfaces are first cleaned mechanically and then coated with a borax flux that cleans them chemically and assists the solder in making a bond. The surfaces are then heated with an LP gas or oxy-acetylene blowtorch. When the surfaces are heated to the melting point of the brazing filler rod, the filler rod is applied and runs freely, solidifying as the surfaces cool. The many filler metal alloys include: 1. copper and zinc alloys (brass) 2. copper and tin alloys (bronze) 3. nickel and chromium alloys 4. copper and phosphorous alloys 5. copper and gold alloys 6. aluminium and silicon alloys 7. magnesium alloys. As more of Earth’s metals are refined and used commercially, brazing filler metal alloys are being developed. Such metals are palladium, titanium, beryllium and zirconium (the exotic group), and are now being used in the aerospace industry. Brazing is one of the methods of joining these exotic metals.

Soldering/welding apparatus Oxy-acetylene welding equipment can be used in both hard soldering and fusion welding. It is essential to use the equipment correctly and maintain it properly to ensure the safety of the operator. Before we look at the safety aspects, you first need to understand what equipment is used and how the apparatus is correctly assembled. Under no circumstances should oxy-acetylene equipment be used or handled without qualified supervision and the necessary safety equipment. General operating principles for most equipment are the same, though there can be slight variances among suppliers’ instructions. It is essential that you conform to a manufacturer’s operating and safety instructions. (Note that these instructions may exceed the scope of the instructions in this chapter.) We will begin by looking at the gas cylinders which store the fuel and oxygen gases.

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Figure 5.14: Oxy-acetylene cylinders

Gas cylinders Acetylene gas and cylinders

porous sponge-like

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An acetylene cylinder is shorter than an oxygen cylinder and is colourcoded maroon. The cylinder is filled with a porous mixture of aluminium silicate and charcoal particles. This sponge-like medium is used to absorb the solvent acetone, which in turn absorbs more than its own volume of acetylene gas. The gas is pumped under great pressure into the cylinder and is absorbed by the liquid acetone. Usually 8,5 kg of acetylene gas is stored in a cylinder. Acetylene gas is commercially produced by reacting calcium carbide with water.

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PAUSE FOR THOUGHT

Figure 5.15: A Soda Stream® machine

A garlic odour is artificially added to acetylene gas as a safety feature, so that leaks can be detected before accidents occur. It is extremely important to ensure that acetylene gas cylinders do not have leaky valves or connections as a concentration of only 2% acetylene gas in a confined space is enough to cause an explosion. Acetylene cylinders should always be stored and used in their vertical position. This ensures that no acetone liquid is withdrawn when a cylinder is used. Cylinders should be stored in ventilated and secure containers, away from hot work and not exposed to naked lights or other fire hazards. They should also be correctly signposted with the required safety signage and regularly inspected for safety. In case of a fire, cylinders should be removed from the hazardous area and hosed down with water to keep them cool. Copper fittings should never be used in conjunction with any acetylene equipment as copper acetylides may form, which can explode on impact or under pressure. For this reason, brass fittings are usually used. Fittings screwed to acetylene cylinders always have left-handed thread to prevent confusion with oxygen cylinders.

Acetylene gas is absorbed into acetone liquid in the same way that CO2 gas is absorbed into the water of a Soda Stream® bottle under pressure. When the pressure is released (the lid is removed from the Soda Stream® bottle), the gas is released.

acetylides organic compounds formed when acetylene comes into contact with copper

Oxygen gas and cylinders Oxygen cylinders are colour-coded black and contain industrial-grade oxygen. The oxygen gas itself is not flammable, but supports combustion. Oxygen makes up 21% of the air we breathe. Other gases in the air are nitrogen (78%), argon (0,9%) and CO2, hydrogen and water vapour (0,1%). To extract oxygen from air, we compress the air to liquify it. Nitrogen has a lower boiling point than oxygen, so when the pressure is released to a certain point, the oxygen can be separated from nitrogen, as they boil off at different temperatures. They are then stored in separate cylinders. The process of separating oxygen from nitrogen is called fractional distillation. Oxygen is stored in cylinders with a cylinder content of 11,6 kg, under huge 131

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Mechanical Technology pressure. For this reason, valve guards should always be installed around the cylinder valves and the cylinders should be chained in the upright position. If a cylinder were to be knocked over, breaking off the cylinder valve, the effect would be similar to that of a 75 kg steel rocket causing havock. Oil or grease should never be used in conjunction with oxygen fittings, as even a small droplet of oil can cause an explosion because of the pressure of the oxygen cylinder contents. (This same principle is described under diesel or CI engines in the Grade 10 book.) Oxygen should never be inhaled or used to blow dust off your clothing. A small spark or smouldering ember from the welding process can cause your clothing to ignite and burn in the oxygenrich environment.

Regulators sintered tiny metal fragments are compressed together

diaphragm a thin membrane inside the regulators, used to regulate gas pressure

Regulators are attached to the cylinders by means of screw threads and are primarily used to reduce the cylinder pressure to operating or working pressure. They contain two gauges, one to indicate the content of the cylinder, and the other to indicate reduced pressure. Regulators also have built-in safety features like sintered metal filters and safety gauges, with bulkheads which deflect to the rear in the case of diaphragm rupture. Oxygen regulators are usually colour-coded blue and acetylene regulators are colour-coded red.

Figure 5.16: Oxygen and acetylene regulators

Flashback arrestors Back-feeding is one of the most hazardous aspects of oxy-acetylene welding. It usually occurs when either oxygen or acetylene gas flows towards the cylinder of the other gas. The danger is that the explosive mixture produced could ignite or explode. Back-feeding usually occurs if there is a pressure difference between fuel and oxygen cylinders, as a result of one of the cylinders emptying before the other, and the consequent equalisation that takes place. 132

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Gases back-feeding

Welding torch

Oxygen cylinder

Acetylene cylinder

Figure 5.17: Back-feeding

To prevent back-feeding, it is essential to purge the oxygen and acetylene lines every time before igniting the welding torch. The practice of purging will be explained a little later. Purging prevents back-feeding and flashbacks. Remember, there is no substitute for caution and correct procedures when welding.

purge to flush out gas from gas welding equipment

The automatic type of flashback arrestor is a device which is placed between the regulator and gas hose leading to the blowtorch. It is used to prevent back-feeding. It is effectively a valve which cuts off the gas supply when: • a pressure difference develops between cylinder and blowtorch • the flow of gas changes direction • a flame burns backward towards the cylinder (flashback). Cartridge types of flashback arrestors are not automatic and have a more limited lifespan. It is wise to install cartridge types between the blowtorch and hoses as well as along the supply line for added protection. Purging can never be substituted by using flashback arrestors, as both should be used to ensure safety.

Figure 5.18: An automatic type flashback arrestor

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Figure 5.19: A cartridge type flashback arrester

Reinforced hoses Oxy-acetylene hoses are made of synthetic rubber and reinforced to be able to withstand high pressures. In South Africa the oxygen hoses are colourcoded blue and acetylene hoses are colour-coded red. The hoses are available in internal diameter sizes of 5mm and 8mm.

Blowtorch

orifice opening of a welding torch nozzle

There is a variety of welding blowtorches. Popular kinds are the DH torch and Servex Type-2 welding torch, distributed by African Oxygen (Afrox). Most blowtorches consist of three main parts: the stock, mixer and nozzle. The stock is where the stop-valves are located. The mixer is located between the stock and nozzle, and is responsible for mixing oxygen and fuel gas. The nozzle screws into the mixer or rotating adapter. It has a very small diameter orifice through which the mixed gases flow and burn at its tip. Nozzles are supplied in a wide variety of orifice sizes, depending on the size of welding flame required for the job. They are usually classified from size 1 upwards.

Assembly of oxy-acetylene equipment Assembling oxy-acetylene welding is very straightforward and safe, provided that it is done methodically and the safety precautions are taken. Make sure that you do any task of this kind with the permission and supervision of your teacher.

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Joining methods Step 1: Cylinders Make sure that the cylinders are upright and chained securely. Remove the protective plastic film around the cylinder valves. Make sure that there are no open flames and snift each cylinder to remove any dust or debris. This is also a good opportunity to check if the cylinder valve opens and closes properly. Step 2: Attach regulators Attach the regulators to the cylinders (oxygen – right-hand thread; acetylene – left-hand thread). Make sure that there is no oil or grease on the fittings being screwed onto the cylinders. Once you have attached the regulators, allow some gas to flow through them (purging) to blow out any dust or debris.

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DID YOU KNOW?

Snifting a cylinder involves opening the cylinder valve briefly to blow out any dust and debris.

Step 3: Attach flashback regulators Fasten the flashback regulators to the regulators and purge. Always use the correct spanner when fastening all fittings to prevent damage to the soft, brass fittings. Step 4: Attach the hoses Attach the hoses and purge. Step 5: Attach the blowtorch Attach the hoses to the blowtorch and purge. Step 6: Attach the nozzle Attach the nozzle to the blowtorch and purge both the oxygen and acetylene stop-valves. Use a nozzle reamer to clean the orifice of the nozzle. Step 7: Check for leaks Once the oxy-acetylene apparatus has been fully assembled, you must check thoroughly for gas leaks. First pressurise the system by adjusting the regulators to the required pressure and then use a liquid soap test solution, available from gas suppliers. Spray the solution over every connection in the apparatus. Any sign of soap bubbling is a clear indication of a gas leak. If tightening the connection does not solve the problem, apply some approved (oxygen-safe) thread tape to the connection. If the leak persists, shut down the apparatus and replace the faulty part with a safe and approved component.

persists continues

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Figure 5.20: Safetest leak detection solution

Start-up procedures

flame-retardant resists igniting

To begin with, the correct personal safety equipment must always be worn. This includes: • an overall (two-piece or a boiler suit) which is made from flame-retardant material • a chrome-leather apron • leather gloves • leather spats • flip-front, shaded goggles. The safety equipment is designed to reduce the risk of injuries when welding and it is important to follow the correct start-up procedures. When in doubt, ask your teacher for assistance. Step 1: Open the gas main Open the cylinder valve. One full anticlockwise turn is sufficient. Step 2: Set the regulators Adjust the gas regulators to the required pressure by turning the adjusting screw clockwise. Acetylene pressure should not exceed 50 kPa for welding and brazing. Oxygen pressure should not exceed 100 kPa for welding and brazing. If the pressure is accidentally adjusted too high, the pressure of the regulator can be reduced only once the gas main has been turned off and the system purged to depressurise it.

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Joining methods Step 3: Purge the system If you are right-handed, hold the welding torch in your left hand. This enables you to use your dominant hand to control the stop-valves. Anticlockwise turns open the valve; clockwise turns close it. A simple practice like this can prevent confusion if the gas needs to be shut off quickly. Purge the system by opening the oxygen stop-valve for 4 to 5 seconds and closing it again. Do the same with the acetylene stop-valve. Purging ensures that there are no gas mixtures in the separate gas lines and greatly improves the safety of the welding operation. Step 4: Ignite the acetylene gas Open the acetylene stop-valve on the blowtorch a quarter of a turn and ignite the gas with a flint-spark lighter. The flint lighter should always be held at a right angle to the nozzle to avoid accidental burns.

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dominant leading

Caution Remember that gas lines should be purged every time before igniting a welding torch.

Figure 5.21: A flint lighter

Never ignite a welding torch using matches or a gas cigarette lighter, because matches continue to burn after ignition and there is a risk of an explosion when using a cigarette lighter, as it stores propane as its fuel. Step 5: Adjust the welding flame Once the acetylene gas has been ignited, it will produce a large amount of black smoke and soot. Open the acetylene stop-valve until most of the smoke is gone and the flame burns fairly cleanly. At this point, you can open the oxygen stop-valve. Three conical flames will appear, as indicated in Figure 5.22. Together, this is known as a carburising flame.

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Mechanical Technology 1st flame cone

3rd flame cone

2nd flame cone

Figure 5.22: Adjusting a welding flame

recede retreat

If you slowly open the oxygen stop-valve, the second flame will recede towards the nozzle. Adjust the oxygen until the tip of the second flame cone touches the tip of the first flame cone. This flame is known as a neutral flame and is suitable for welding and brazing. An excess of oxygen produces a small, third flame cone and a loud hissing noise. This type of flame is sometimes used for brazing.

Adjust the oxygen until the tip of the second flame cone touches the tip of the first flame cone. This results in a neutral flame.

Figure 5.23: Adjusting the flame to form a neutral flame

Shutdown procedures Again, hold the welding torch in your non-dominant hand to avoid confusion when closing the stop-valves in the correct direction.

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Step 1: Shut off the acetylene and oxygen stop-valves Close the acetylene stop-valve first. This will extinguish the flame immediately, because the fuel supply is shut off. Once this has been done, shut off the oxygen stop-valve. Never shut off the oxygen valve first, because this will cause unnecessary smoke and soot. It will also increase the risk of flashbacks, as the acetylene flame will recede into the nozzle when it is shut down after the oxygen has been shut down. Step 2: Shut off the cylinder valves The cylinder valves should be shut off directly after the flame has been extinguished by turning the valves clockwise. Step 3: Purge the system Open the welding torch’s acetylene stop-valve to release the pressure in the regulators and hoses. Follow exactly the same procedure as for purging the oxygen line. Step 4: Reset regulators to zero outlet pressure This is done by turning the adjusting knobs on the regulator anticlockwise to release the pressure on the spring and diaphragm, and hence the regulated pressure. Step 5: Close the torch valves Close the torch valves. (Always ensure that the torch valves are closed when equipment is not being used.) Once this is done and all the safety equipment has been neatly stowed away, it is safe to leave the oxy-acetylene apparatus.

Brazing and gas welding techniques Brazing Once the joint has been set up, tack it together temporarily to reduce movement caused by expansion and contraction. Note that brazing is not a fusion welding technique. This means that the material to be joined is not actually melted. The brazing filler material flows into the joint and bonds the two materials together. For this reason the welding flame should not be used to melt the filler rod, as this will cause the molten metal from the filler rod to drop onto the joint surface rather than flow inbetween the joint surfaces. The correct method is to hold the blowtorch nozzle at 60° to the joint surface and heat the joint in small, circular movements. The heat in the joint then melts the filler material, allowing it to flow freely. The filler rod, in turn, should be held in the other hand at 30° to the joint surface and fed into the molten pool of brazing as it is consumed. It is usually best to follow the filler rod with the 139

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Mechanical Technology flame (from right to left if you are right-handed). Figure 5.24 indicates the correct angle to hold both rod and blowtorch. The filler rod should also be periodically dipped into brazing flux to help the brazing to flow. Remember to work in a well-ventilated area as brazing fumes can be harmful to your health. Take note that the tip of the brazing filler rod has been bent over. This is done for two reasons: firstly, to help you identify which is the heated end, and secondly, to reduce the hazard if you accidentally poke it into your eye.

Figure 5.24: Correct angles for brazing

Gas welding Unlike brazing, gas welding is a fusion process, where the welding flame actually melts the joint to be welded and mixes the two surfaces together. On cooling, the joint solidifies, producing a welded joint. Exactly the same technique as with brazing is applied, except that no flux is required and that the parent metal is in fact melted.

Leftward gas welding Leftward gas welding involves holding the blowtorch nozzle at 60° – 70° to the joint surface and heating the joint in small, circular movements until the parent metal starts to form a molten pool or keyhole. The filler rod, in turn, should be held in the other hand at 30° – 40° to the joint surface and fed into the molten pool as it is consumed. The movement and heat of the welding flame cause the molten pool to progress along the welded joint, forming a welding bead as it cools and solidifies. In leftward gas welding the rod precedes the flame (from right to left if you are right-handed). This method is most suitable for thin plate and general gas welding applications.

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Figure 5.25: Leftward gas welding

Rightward gas welding Rightward gas welding or backhand gas welding is usually used for welding material 4 mm thick and thicker. Here the rod follows the flame, which tends to anneal the welded joint as the welding progresses. The rod is held at 30° – 40°, with the base metal and the flame at 40° – 50°.

anneal soften through heat treatment

Figure 5.26: Rightward gas welding

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Welding joints There are five main types of welding joints. These joints can be brazed, gas welded or arc welded. The following diagrams give a pictorial representation of the joint layouts:

Figure 5.27: A butt joint

Figure 5.28: A lap joint

Figure 5.29: A T-fillet joint

Figure 5.30: An edge joint

Figure 5.31: A corner joint

Welding positions There are six main positions used when welding. You will find that the flat position is by far the easiest to weld in because gravity keeps the molten pool where you want it to be. The other five positions are much more difficult to master and require different welding methods and lots of practice. It is, however, very satisfying to master the art of welding. Skilled welders or coded welders are in great demand worldwide and they are generally well rewarded for their labour.

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The following diagrams give a pictorial representation of the different welding positions:

Figure 5.32: The flat position

Figure 5.34: The vertical position

Figure 5.36: The overhead position

Figure 5.33: The horizontal position

Figure 5.35: The oblique position

Figure 5.37: The all-round position

Arc welding Arc welding uses electric current instead of burning gases to achieve the high temperatures required to melt and fuse metals. An initial high voltage (open circuit voltage) is used to force an electric current across the gap between the electrode and parent metal, as shown in Figure 5.38.

Figure 5.38: An arc welding machine

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Safety precautions for arc welding and spot welding When arc welding and spot welding, similar personal protective equipment must be worn as when gas welding. The only difference involves facial equipment. The flip-front goggles used in gas welding must be replaced by a welding helmet with the correctly shaded lens. Arc welding helmets protect the entire face area and have a darker lens to protect the user’s eyes against the intense light produced when arc welding. Rubber-soled shoes should be worn and rubber mats provided to insulate a welding operator from electrocution. There should also be adequate ventilation to remove harmful fumes during the welding process. There are two main types of arc welding machine: AC welding machines, which are very common and are used for general purpose welding, and DC welding machines (or inverters), which are used for general purpose work and specialised welding such as TIG welding. DID YOU KNOW?

TIG is an abbreviation for Tungsten Inert Gas. This welding process uses a tungsten electrode which is not easily consumed, a shielding gas which is a blend of argon and helium, and a filler rod. TIG welding will be dealt with in more detail in Grade 12.

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Figure 5.39: An AC welding machine and an inverter

Establishing an arc Contact between the electrode and parent metal is not sufficient to start the welding process. An arc must be established so that the heat required for fusion between the electrode and parent metal can be obtained. This is done by first making contact with the parent metal by the electrode. Once contact occurs, the welding circuit is closed and current begins to flow. The electrode is then lifted off the parent metal in order to form a gap of between 3 – 4 mm. This is known as the arc length. The gap of air has a very high resistance and hence a large voltage drop occurs, resulting in high intensity light and heat developing. This heat is used to melt the parent metal and electrode tip, which also acts as filler material, and in so doing it forms fusion welds. Since the electrode is being consumed as the welding process proceeds, it must continuously be fed into the weld to maintain the arc length. If the arc length is not maintained, the circuit will be broken and welding will not be possible. The process of establishing an arc length when beginning welding is known as striking an arc. It is a bit like striking a match.

Joining methods The moment that contact is made with the weld surface, the electrode must be withdrawn to a gap of 3 – 4 mm. It is a lot more difficult in practice though and requires a lot of time mastering the skill. The size of electrode and current setting are the most important in establishing an arc. The rule of thumb is that the thinner the electrode, the less current is required; and the thicker the electrode, the more current is required. Your current settings will also depend on the type of welding machine you are using. Once you have practised and mastered the ability to successfully strike and maintain an arc, you will be ready to attempt arc welding.

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rule of thumb general rule

Arc welding runs Only attempt arc welding if you have mastered the ability to strike and maintain an arc. Probably the best way to practise welding is to find a piece of fairly thick, off-cut steel plate (thicker than 3 mm) and draw a few parallel lines on the plate (about 1 cm apart) with boilermaker’s chalk. Use the lines as guidelines for welding. After each weld run, chip off the slag deposit using a chipping hammer. Always remember to wear clear goggles when chipping the slag, as hot pieces of slag can injure your eyes. As you proceed, evaluate each weld with the help of your teacher and make the necessary adjustments to the welding current, speed of travel and arc length, until you are able to weld on your own. After you have gained confidence in basic welding, practise until you are able to produce a uniform weld bead with no defects. Only after this should you attempt welding joints.

Arc welding electrodes An electrode is a rod or wire covered with a coating of flux which is used for arc welding. Arc welding electrodes are produced in various sizes and materials for various purposes. We will only be looking at the electrodes used for welding mild steel. When bare wire is used as an electrode, it is found that the arc is difficult to control. The arc stream wanders over the molten pool. The molten metal takes in oxygen and nitrogen from the surrounding atmosphere and this causes a porous and brittle weld. Much of the metal is lost by volatilisation. When a wire is covered with a flux coating, it produces an inert gas, which will not combine with the molten metal during the welding process. These inert gases form a shield around the arc and the molten pool, completely excluding the atmosphere. The coating melts at a higher temperature than the metal core and thus the coating always extends a little beyond the core, concentrating and directing the arc stream, making it stable and easy to control. The loss of metal by volatilisation is also greatly reduced from 25% to 5%. The flux also produces a layer of slag over the completed weld, which must be removed with a chipping hammer after the weld has been completed.

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Figure 5.40: An arc welding electrode

Arc welding electrode flux The flux used in arc welding electrodes is chemically very different from the flux used in brazing. It consists of a variety of different compounds depending on the particular application. Flux is used to reduce oxidation, remove oxides being formed and remove any other impurities which may affect the quality of the weld. The use of flux results in a stronger and more ductile weld as well as easing the welding operation.

Care of arc welding electrodes All electrodes should be kept dry, since the flux coating tends to absorb moisture and the efficiency of the rod is greatly impaired if the coating is damp. To keep the electrodes dry, they should be stored in a special cupboard fitted with an electrical light globe to keep the interior warm and, therefore, dry. This practice is normally only observed in large scale industry. Care should also be taken not to damage the coatings by rough handling or over bending.

Resistance welding (spot welding) Resistance welding, or spot welding as it is more commonly known, does not use a consumable electrode to deposit a weld bead, as with other forms of welding.

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Figure 5.41: Two plates being spot welded

In this method of welding, the heating effect which occurs when a current flows through a resistance is used to fuse two plates together. Figure 5.35 shows two plates, A and B, to be welded together at point X. Two copper electrodes are pressed against the plates at this point. The electrical resistance is fairly high and when a heavy current is passed between the electrodes, this high resistance (it is high in comparison to the resistance of the rest of the circuit) causes intense heat to be produced. The heat is greatest at the point of highest resistance, which is the plane between the two plates. It is at this point that the metal of the two plates melts and fuses together, forming a weld nugget which is known as a spot weld. Spot welding is very quick and efficient, which is why it is used in mass production plants such as auto-body assembly. Another variation of resistance welding is seam welding, where the electrodes used are wheels instead of points. This allows a continuous resistance weld to be formed.

Heat evolved when resistance welding The amount of heat evolved when resistance welding takes place depends on the following factors: • the metal being welded (metals with a high electrical resistance are easier to weld than those with a low resistance) • the pressure between the plates, which depends upon the applied pressure and the area of the tip of the electrode • the current and the time for which it is flowing; by choosing the correct combination of current, time for which it flows, and electrode pressure, a small area is made molten and fusion takes place; the heat-affected zone is very small; the extent of the fusion zone or nugget will depend upon the heating effect. 147

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Mechanical Technology The timing of the current flow must be closely controlled otherwise the electrode will be forced through the sheets, due to the applied pressure. For this reason timing is performed electronically.

Electrode tips The thicker the sheet, the greater the tip diameter, giving a larger spot and developing a greater weld strength between the plates. The following formula is used to determine the correct electrode tip diameter for spot welding: Tip diameter = 2 mm + 2 t where t = plate thickness (single plate). The electrodes which carry the current and apply the pressure must have both high strength (to avoid the tips being deformed under the applied pressure) and good conductivity. Stronger electrode tips tend to have lower conductivity. The three main types of electrode tips are: 1. cold-worked copper 2. copper alloyed with cadmium, beryllium, etc. 3. sintered, for example, copper and tungsten: the latter giving the greatest mechanical strength but the lowest conductivity. If cold-worked copper is used, it must not heat up above its recrystallisation temperature (lowered by cold working) or the tips will be softened and are therefore usually water-cooled. For welding sheets made of high conductivity materials, the currents used are so large that water-cooled electrodes are essential. Steels are easily spot welded, but as the carbon content increases, the weld becomes harder and less ductile. Heat treatment will reduce the cracking tendency in any particular case. The high current with a low voltage is supplied by an AC transformer.

Assessment

1. Briefly describe the difference between brazing and gas welding. 2. What is the purpose of flux when soldering? 3. How is flux usually applied to a brazed joint? 4. Briefly describe the correct sequence for assembling oxy-acetylene welding apparatus. 5. Write a point-form summary to describe the correct start-up and shutdown procedures for oxy-acetylene welding. 6. Describe rightward and leftward gas welding techniques and state when each method should be used. 7. Make a neat, labelled sketch of an arc welding machine and all its components. 8. Sketch a typical cross-section of a spot weld and explain the basic principles of spot welding. 148

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A cross-sectional view of a welding joint The following 2 diagrams indicate a cross-sectional view of a fusion welded joint. The intense, localised heat generated by the weld causes recrystalisation in the area directly adjacent to the weld bead, thus softening the parent metal and resulting in a permanent bond of weakness in the welded joint. The weld metal will also be softer than the parent metal, unless the filler material is selected to have a specific strength matching the parent metal. In this case, only the heat-affected zone will be softened by recrystalisation.

Figure 5.42: A cross-section of a fusion welded joint

Figure 5.43: A longitudinal-section of a fusion welded joint (showing the electrode and slag)

Factors influencing the welding joint There are numerous factors that affect a welded joint. The following are, however, the primary factors: • type of material • number of welds • type of welding rod • size of weld • presence of oxygen/hydrogen • preparation 149

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Type of material Most welding is done on mild steel (low-carbon steel), which can be welded using all the welding methods listed in this chapter without too much complication. However, some alloy steels and non-ferrous metals require special processes and electrodes to be welded (e.g. TIG welding). Medium and high-carbon steels are also more difficult to weld because of greater brittleness and the tendency to build up residual stress which must be relieved through heat treatment.

Number of welds Generally speaking, the more welding runs that are required in a welding joint, the greater the heat in the parent metal. This can lead to stress and distortion if not managed correctly. It is, therefore, important to plan the sequence of welds to offset the effects of distortion and/or preheat the welding joint to minimise the effects of localised heating and cooling in the joint.

Type of welding rod There are a vast variety of electrodes and welding rods available to cater for a range of alloys and carbon steels. The electrode is coated in a mixture called flux, which gives off gases as it decomposes to prevent weld contamination, introduces deoxidisers to purify the weld, causes weld-protecting slag to form, improves the arc stability, and provides alloying elements to improve the weld quality. The appropriate type of welding rod and flux must, therefore, be used to suit the job at hand.

Size of weld The size of a weld is determined differently, depending on the type of weld. Butt welds are measured by their effective throat thickness and fillet welds (with even legs) are measured by the length of the leg. The size of the weld will affect how many weld runs will be needed to complete the joint.

Presence of oxygen/hydrogen Shielding gases are necessary for gas metal arc welding to protect the welding area from atmospheric gases, such as nitrogen and oxygen, which can cause fusion defects and porosity which weakens welding joints. Excessive hydrogen in welds is responsible for causing cracks. One of the 150

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principal sources of hydrogen is the moisture contained in the flux coating of electrodes. The electrode type also determines the amount of hydrogen generated. There can also be other significant sources of hydrogen, e.g. moisture from the atmosphere, impurities on the surface of the material or consumables such as oil, paint and rust, etc.

Preparation Preparation refers to how the welding joint is set up before welding. It is critical to first select the appropriate type of welding joint, as referred to in figures 5.27 and 5.31, for the job. Once a joint type has been selected, the parent material must be prepared accordingly. This will ensure good root penetration and a strong weld.

Using working instructions and applying complex but relevant joining methods Practical assessment

Conduct the following procedures in the presence of your teacher: 1. the correct start-up of the oxy-acetylene gas welding apparatus 2. setting the blowtorch to a neutral flame 3. shutting down the neutral flame and making the apparatus safe The following aspects will be assessed: • Put on the necessary safety equipment. • Turn on gas main. • Set regulators to the required pressure. • Purge oxygen. • Purge acetylene. • Open acetylene and ignite with flint lighter. • Adjust acetylene until smoke disappears. • Open oxygen until neutral flame is achieved. • Shut down acetylene to extinguish flame. • Shut down oxygen. • Turn off gas main. • Release spring pressure on regulators. • Purge oxygen and acetylene. • Stow away the safety equipment. Assessment task mark sheet. School:................................................................................................................... Name:.................................................................................................................... Grade:................................................................................................................... 151

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Mechanical Technology CATEGORY: The task must be followed in the given order

MARK: (one mark if correctly performed)

Put on the necessary safety equipment Open gas main Set acetylene regulator Set oxygen regulator Purge oxygen Purge acetylene Ignite acetylene with flint lighter Adjust until smoke disappears Open oxygen to neutral flame Shut off acetylene Shut off oxygen Turn off acetylene gas main Turn off oxygen gas main Release pressure in acetylene regulator Release pressure in oxygen regulator Purge acetylene valve Purge oxygen valve Stow away safety equipment TOTAL: /18 CONVERT TO: 50 marks

Practical assessment

Conduct the following procedure in the presence of your teacher. Use an LP gas blowtorch and lead solder (with flux) to solder a copper T-capillary joint as shown in Figure 5.44 below:

Figure 5.44: A copper T-capillary joint

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The following score sheet can be used for self-assessment, peer assessment and assessment by your teacher. NAME: YOUR NAME: PEER 1:

START RUN: /5 UNIFORMITY: /5 END RUN: /5 DEFECT-FREE: /5 OVERALL: /5 TOTAL: /25

PEER 2: TEACHER: /100 Convert to: /50


Conduct the following procedure in the presence of your teacher. Tack and braze together two pieces of off-cut, square tubing in a fillet joint as shown in Figure 5.45.

Figure 5.45: A fillet joint




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Conduct the following procedure in the presence of your teacher. Bend and spot weld together two pieces of off-cut, 1 mm, sheet metal in an edge joint as shown in Figure 5.46. Then gas weld the edge (without using a filler rod). Fuse the edges together to form a neat and even weld bead. Practise a few times before you attempt the assessment. Spot weld plates in position before fusion weld

Figure 5.46: An edge joint




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Conduct the following procedure in the presence of your teacher. Tack weld together two pieces of 50 × 6 mm, flat bar according to the welding symbol as shown in Figure 5.47. Practise a few times before you attempt the assessment.

Figure 5.47: Arc welding practical exercise

TOTAL: /20

OVERALL: /2

DEFECT-FREE: /3

END RUN: /3

UNIFORMITY: /2

START RUN: /3

Interpretation of tail info /1

Interpretation of dimension (60) /1



Arrow side orientation/1

Interpretation of weld symbol /1

NAME:


The following score sheet can be used for self-assessment, peer assessment and assessment by your teacher.

YOUR NAME: PEER 1: PEER 2: TEACHER: /80 Convert to: /50

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Forces

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Chapter 6

Forces Topic 6

Forces found in engineering components

Performing basic testing on basic mechanical principles

Forces

Basic calculations on stress

Moments found in engineering components

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Forces found in engineering components Introduction We will start this chapter by revising the basic concepts of systems of forces and determining resultants and equilibrants graphically and by means of calculation.

Resultant If a system of forces acts on a body and a single force can be found that has the same effect as the system, that single force is known as the resultant of the system.

Equilibrium When two or more forces act on a body and the body remains at rest, the forces are said to be in equilibrium.

Equilibrant If a system of forces acts on a body but a single force keeps the body at rest, the single force is known as the equilibrant of the system of forces. The equilibrant of a system of forces has the same magnitude and line of action as the resultant, but is opposite in direction.

Components of a force

P

If two or more forces have the same effect as a single force, these forces are called the components of the single force. If the diagonal of the parallelogram in the following figure represents R newtons and the sides P newtons and Q newtons, then P newtons and Q newtons are the components of R in their respective directions. It is often necessary, when solving problems, to replace a force by its components at right angles to each other. In other words, the x and y coordinates of the force are determined. This can be very useful R when forces (or vectors) are added together by taking Q the sum of their x and y components. Figure 6.1: Components of a force

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Forces The following diagram shows how to resolve any force into its vertical and horizontal components. The force in this case is a 100 N force in a direction of 30° north of east. The components are found by constructing a parallelogram, the sides of which are parallel to the x and y axes of the drawing. The x and y axes form what is known as a Cartesian plane. The x and y components are then simply measured off the x and y axes. In this case the x and y components of the 100 N force are x = 87 N and y = 50 N.

6 Did you know?

The Cartesian plane was named in honour of the mathematician Descartes who invented it.

y Cartesian plane

x

y

0N

10

0

x

Figure 6.2: How to resolve any force into its vertical and horizontal components

Introduction to triangle of forces In the previous section you learnt how to graphically determine the resultant and equilibrant of two forces (whose magnitude and direction were known), by means of the parallelogram of forces. Although useful, this method is limited to solving problems involving only two forces. When a system of forces contains three forces, a triangle of forces is used. Definition of triangle of forces

If three forces, whose lines of action meet at a point, can be represented in magnitude and direction by the sides of a triangle, taken in order, they are in equilibrium. The converse of this is also true, that is: ‘If three forces are in equilibrium, their vectors can be put together to form a triangle.’ Bow’s notation

Bow’s notation is a method which can be used to simplify problem solving where three or more forces are applied to a body in a system of forces. The following system of forces contains three forces. We will use Bow’s notation to construct a triangle of forces and show that they are in equilibrium. 159

6

Mechanical Technology We do this in two parts: Part 1: Construct a space diagram Step 1 Construct a space diagram, depicting the lines of action and direction of all the forces in the system. Step 2 Once this is done, label the spaces between all force lines, using capital letters. (Any letters will do, for example A, B and C.)

Did you know?

A force diagram or vector diagram is a scale drawing of a system of forces, where the length of the lines in the drawing represents the magnitude of the actual forces (to scale), and the direction of the lines represent the direction of the respective forces. Space diagrams differ from force diagrams in that they only represent the correct directions of the forces and not their magnitudes.

10

N

C 120° 10

120°

N

120° A

10 N

B

Figure 6.3: Space diagram

Part 2: Construct a force diagram Step 1 Moving clockwise around the system of forces in the space diagram, you will see that all the forces represented between two capital letters can be expressed as vectors in the force diagram, with lower case letters at either end. The first 10 N force can be referred to as the force between the spaces A and B or the force ab. The second and third 10 N forces are bc and ca respectively. Construct the forces in the force diagram at the same angles as those in the space diagram. a

a

c

b

Figure 6.4: Force diagram

a

c

b

Scale 4 cm = 10 N

Step 2 The forces are arranged from head to tail in the order in which they are taken.They are considered to be in equilibrium because the force diagram’s end point is the same as its starting point.

160

b

6

Forces Example 1 Two ropes, inclined at 30° and 45° to the vertical, support a load of 200 N. Determine the tensions, these are pulls, in both ropes. Draw the space diagram based on the information given. Part 1: Space diagram

C 45° 30°

A 200 N

B

Step 1 Set out the lines of action of the three forces. Step 2 Label the spaces between the forces A, B and C. Part 2: Force diagram Step 1 Choose a suitable scale and begin with force ab. (Begin with force ab in this case, because both its magnitude and direction are known.) Point a is projected vertically downwards to point b with a magnitude of 200 N. The arrow indicates the direction of the force. a

Scale 1cm = 50 N

b

Moving clockwise about the centre point, force bc is projected next. (Force bc has known direction but unknown magnitude.) Step 2 Begin at the known point b, and project the construction line bc in a direction of 45° to the vertical (as in the space diagram). Point c cannot be found at this stage because the magnitude of bc is not yet known. Moving clockwise about the centre point, the next force to be projected is force ca. (Force ca has known direction but unknown magnitude.)

a

b

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Mechanical Technology Step 3 Construct line ca, which passes through point a in a direction of 30° to the vertical. Point c can be found where line bc intersects with line ca. a

c

b

Step 4 To complete the vector diagram, the arrows must be introduced from head to tail, forming a triangle of forces. The lines bc and ca now represent the magnitude and direction of the two ropes. a

c

b

162

80 0

Example 2 The following space diagram shows two rods which are joined to a pin. The angle between the rods is 60°. The horizontal rod is pushing with a force of 1 000 N in a westerly direction. The inclined rod is pulling with a force of 800 N in a direction of 60° north of east. Determine the force exerted by the pin, that is the equilibrant.

N

Measuring lines bc and ca and using the scale, the tension in the ropes are found to be as follows: • Rope bc represents a pulling force of 104 N in a direction of 45° north of west. • Rope ca represents a pulling force of 147 N in a direction of 60° north of east.

60˚ 1 000 N

Forces

6

A method to find the unknown force, or equilibrant will now be introduced. Step 1 Draw the space diagram. Nothing as yet is known about the third force (the equilibrant), so indicate it in the space diagram as a curly line. Do this so that you can label the space diagram.

Step 2 Label the spaces L, M and N as follows:

L

M

N

Step 3 Construct the force diagram in the same way as in the previous example to determine the unknown force. First construct the two known forces, lm and mn. m

m

n

l

l

Scale 1cm = 3 N

Once these two force lines have been constructed, complete the triangle of forces to find the equilibrant. n

m

l

Measuring nl, it is found that the force exerted on the pin (or equilibrant) is 920 N in the direction of 49° south of east. 163

6


50

N

Example 3 Two forces of 50 N and 25 N act on the pin O, as shown in the diagram below. The line OX is horizontal. Determine the resultant force acting on the pin, as well as the horizontal and vertical components of this resultant.

B

A 45° O

X 30° C

25

N

Step 1 Construct a force diagram. Proceed as in the previous example; ca is the vector of the equilibrant of the two given forces. The force diagram has been drawn to a scale of 1 cm = 2 N. c b

a

The resultant of the two forces is equal in magnitude to the equilibrant, but opposite in direction. The resultant is, therefore, represented by the force ac. c b

a

47,8 N

c b

a 9,7 N

It is measured to be 49 N in a direction 78° north of east. Step 2 Determine the horizontal and vertical components of the resultant. Construct a Cartesian plane through the origin that is point a. Construct two parallel lines from point c to the x and y axis respectively, and measure the horizontal and vertical components. You will find: • The horizontal component is 9,7 N. • The vertical component is 47,8 N. You could probably have worked out some of the above examples using the parallelogram of forces. However, it is essential to be familiar with the use of the triangle of forces method and Bow’s notation. 164

Forces

6

Assessment

1. Use a suitable scale to solve the following exercises. You will need: • a sharp pencil or clutch pencil (HB) • set squares and ruler • a protractor • a note book (a) The diagram represents the lines of action of three forces which are in equilibrium. One of the forces is 80 N. N 80 Determine the other two forces. (b) Two ropes with lengths of 2 m and 4 m are attached to a body with a mass of 75° 800 kg. The free ends of the ropes are 45° attached to two points which are 5 m apart in the same horizontal line. Determine the tensions in the ropes. (c) A rod pushes vertically downwards on a pin with a force of 80 N, and a rope pulls upwards at a bearing of 30° with a force of 120 N. Determine the resultant force on the pin. (d) The diagram shows a mass suspended from the end of a jib, AB, which is held in position by a tie, BC. The lengths of AB and BC are 1,0 m and 1,2 m respectively. AB is horizontal and AC is vertical. Determine the magnitude of the mass and tension in the tie when the force in the jib is 480 N. C (e) A mass of 1 200 kg is supported by two ropes which are inclined at 30° and 45° respectively to the horizontal. Determine the tensions in both ropes. A B M

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(f)

A body with a mass of 1 000 kg is suspended by a rope. It is pulled in an easterly direction by the horizontal force of P, until the rope is inclined at 30° to the vertical, as shown in the diagram. Determine the magnitude of force P and the tension in the rope.

P

Did you know?

A derrick is a type of crane with a movable boom and a pulley at the top.

(g) The following diagram shows a mass suspended from a derrick. If the maximum load this derrick can carry is 4 000 N, what will the tension in cable YZ be? Z 60° Y

45°

M

X

Assessment 1.

166

Use a suitable scale to solve the following exercises. You will need: • a sharp pencil or clutch pencil (HB) • set squares and ruler • a protractor • a note book (a) The following four forces acting at a point are in equilibrium: 100 N acting east, 160 N acting 60˚ north of east, Q acting north-west and P acting south. Determine the values of Q and P. (b) Four forces, whose lines of action meet at a point, are in equilibrium. Three of these forces are known to be: 90 N acting south, 60 N acting east and 70 N acting north-west. Determine the fourth force.

Forces

(c)

The diagram shows a system of three forces. The line OX is horizontal. (i) Find the magnitude and the direction of the resultant. (ii) Obtain the horizontal and vertical components of the resultant in question (i).

70

N

6

X

O

45°

40 N

30° 50

N

Calculating forces Horizontal and vertical components of a force acting at an angle To be able to calculate the resultant of a system of forces it is often very useful to first resolve the force into its horizontal and vertical components. The horizontal component (X) of any force (F) can be obtained by using the simple formula: X = F cos θ The vertical component (Y) of any force (F) can be obtained by using the simple formula: Y = F sin θ

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6

Mechanical Technology Example 1 The 100 N force below is inclined at 30˚ to the horizontal. Determine (by calculation) the horizontal and vertical components of the force. Solution: X = F cos θ ∴ X = 100 cos 30˚ ∴ X = 86,6 N Y = F sin θ ∴ Y = 100 sin 30˚ ∴ Y = 50 N

Calculating the resultant of a parallelogram of forces To calculate the resultant of a parallelogram of forces, the forces in the system must first be resolved into their horizontal and vertical components (as with the previous example). The arithmetic sum of X and Y components are then used to find the magnitude of the resultant by Pythagoras, i.e. R2 = X2 + Y2. The direction of the resultant force is calculated by using the formula: tan0 = sum Y sum X Example 1 Calculate the magnitude and direction of the resultant force (R) in the parallelogram of forces below.

Solution: Force

Vertical component

Horizontal component

6N

Y = 6 sin 60˚

5,2 N

X = 6 cos 60˚

3N

4N

Y = 4 sin 0˚

0N

X = 4 cos 0˚

4N

Arithmetic sum of components 168

5,2 N

7N

Forces

6

Use Pythagoras to calculate R as follows: R2 = X2 + Y2 ∴ R2 = 72 + 5,22 ∴ R2 = 49 + 27,04 ∴ R2 = 76,04 ∴ R = 8,72 N Calculate the direction of the resultant as follows: tanθ = sum Y sum X ∴ tanθ = 5,2 7 ∴ tanθ = 0,74 ∴ θ = 36,6˚ The resultant, therefore, has a magnitude of 8,72 N in a direction of 36,6˚ North of East.

Calculating the resultant of a triangle of forces To calculate the resultant of a triangle of forces (or a polygon of forces for that matter), the forces in the system must first be resolved into their horizontal and vertical components in exactly the same way as before. Remember, however, to use the correct sign to indicate a + or – position on the Cartesian plain. The arithmetic sum of the X and Y components is then used to find the magnitude of the resultant by Pythagoras, i.e. R2 = X2 + Y2. The direction of the resultant force is calculated by using the formula: tan0 = sum Y sum X Example 1 Calculate the magnitude and direction of the resultant force (R) in the triangle of forces below.

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Solution: Force

Vertical component

Horizontal component

50 N

Y = 50 sin 90˚

50 N

X = 50 cos 90˚

0N

30 N

Y = 30 sin 45˚

-21,2 N

X = 30 cos 45˚

21,2 N

45 N

Y = 45 sin 0˚

0N

X = 45 cos 0˚

45 N

Arithmetic sum of components

28,8 N

66,2 N

Use Pythagoras to calculate R as follows: R2 = X2 + Y2 ∴ R2 = 66,22 + 28,82 ∴ R2 =4382,44 + 829,44 ∴ R2 = 5211,88 ∴ R = 72,2 N Calculate the direction of the resultant as follows: tanθ = sum Y sum X ∴ tanθ = 28,8 66,2 ∴ tanθ = 0,44 ∴ θ = 23,5˚ The resultant, therefore, has a magnitude of 72,2 N in a direction of 23˚ South of West.

170

Forces

6

Moments found in engineering components Moment of force The moment of a force is defined as: The moment of force at a given point is the product of the force and the perpendicular distance from the point to the line of action of the force. Figure 6.5 best illustrates this definition.

Figure 6.5: The moment of a force

You will notice from Figure 6.5 that the force is acting at right angles (perpendicular) to the point about which it is acting. Moments are categorised as clockwise or anticlockwise moments, depending on which rotational direction they cause at a given point. Figure 6.6 depicts a seesaw which is evenly balanced. The force on the left-hand side (A) will cause the seesaw to rotate anticlockwise, while the force on the right-hand side(B) will cause the seesaw to rotate clockwise.

Figure 6.6: Clockwise and anti-clockwise moments

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Calculating moments The moment of force is calculated as follows: moment = force (N) × distance (m). The units of bending moments are, therefore, Newton metres (Nm). Figure 6.7 demonstrates the calculation of a moment.

Figure 6.7: Calculating moments Moment = 2 N × 4 m = 8 Nm

The following example demonstrates how two forces with different magnitudes can have the same moments.

Figure 6.8: Calculating moments

The seesaw in Figure 6.8 is in equilibrium (it neither rotates clockwise or anticlockwise) even though different forces are applied on either side. The bending moments are calculated as follows: Left-hand side: moment of force (A) = 4 N × 2 m = 8 Nm Right-hand side: moment of force (B) = 2 N × 4 m = 8 Nm

172

As has been demonstrated, the left-hand moment is equal in magnitude to the right-hand moment, even though the magnitudes of their forces are different. Remember that moments are calculated by multiplying force by distance.

Forces

6

Torque Torque, in engineering and mechanics, is a twisting effort applied to an object that tends to make the object turn about its axis of rotation. The magnitude of a torque is equal to the magnitude of the applied force multiplied by the distance between the object’s axis of rotation and the point where the force is applied. In many ways, torque is the same as the moment of force. The units of torque are also Nm.

Distance from force to centre Figure 6.9: Torque on a round shaft

Torque wrenches are used on certain mechanical components, such as cylinder head bolts, to ensure a uniform degree of tension at a predetermined torque or moment. Modern torque wrenches measure the moment of force or torque on the object which is being tightened and indicate when the preset torque has been achieved.

DID YOU KNOW?

Diesel engines, such as tractor engines, produce their maximum torque at relatively low engine revolutions.

Figure 6.10: A tractor Figure 6.11: Setting a torque wrench

Figure 6.12: The moment of force on a torque wrench

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6


Beams

sheer to break because of sheer forces

A rigid horizontal member which is supported at both ends is called a beam. Beams have many uses in structural engineering. They are used in anything – from bridges to roof structures – and are often made of steel girders, timber or concrete. Loads tend to cause beams to bend or deflect and sheer. Beams normally carry enormous loads and, for this reason, must be designed to support the loads. When beams are designed, it is important to know what sheer forces and bending moments will be operating on the beam. This is determined using calculations and diagrams, which will be dealt with a little later. Figure 6.14 and 6.15 illustrate how forces can affect a beam which has been weakened by a cut line (X – X) – the beam will either bend or sheer.

Figure 6.14: A bending tendency (bending moment)

DID YOU KNOW?

A steel girder is a rolled steel section such as an I-beam.

Figure 6.13: An I-beam

Figure 6.15: A sliding tendency (sheer force)

174

Forces

6

Calculating reactions Reactions are counter-forces that are exerted when a beam is subjected to one or more vertical downward force which causes the beam to be in equilibrium. To calculate the sheer forces and bending moments in a beam, first calculate the reactions. Consider the beam in Figure 6.16. 6N

equilibrium a state which is achieved when the sum of all the up forces on a beam are equal to the sum of all the down forces

Figure 6.16: Calculating reactions

To calculate the reactions, apply the law of moments. This law is defined as follows: If a simple lever, capable of turning about a given hinge point or fulcrum, is loaded in such a manner as to keep it in equilibrium, the sum of all the clockwise moments taken about the fulcrum will equal the sum of all the anticlockwise moments.

fulcrum a hinge or pivot point

If the beam is in equilibrium, the 9 N and 6 N force (see Figure 6.16) must be resisted by the left and right end reactions, and the sum of the downward forces must equal the sum of the reactions. Because the point load is not in the centre of the beam, each of the reactions will not resist an equal force. Take moments about reaction left (RL) RR × 3 m = (9 N × 1 m) + (6 N × 2 m) ∴ RR = (9 N × 1 m) + (6 N × 2 m) 3m ∴ RR = 7 N Take moments about reaction right (RR): RL × 3 m = (9 N × 2 m) + (6 N × 1 m) ∴ RL = (9 N × 2 m) + (6 N × 1 m) 3m ∴ RL = 8 N

175

6

Mechanical Technology Test: The beam is in equilibrium; therefore, the sum of up forces must equal the sum of down forces. RL + RR = down forces ∴8N+7N=9N+6N 15 N = 15 N  Both these reactions must be calculated accurately. If there is an error in the first calculation, the rest of the calculations will also be incorrect if the result which has been obtained is subtracted from the sum of the down forces to determine the other reaction.

Calculating bending moments A bending moment is defined as follows: The bending moment at any point along a loaded beam is the algebraic sum of the moments of all the vertical forces acting to one side of the point on the beam. For the purpose of this text book, the algebraic sum of all moments will be taken (clockwise moments minus anti-clockwise moments) to the left of any given point and move from RL to RR. In other words, all the down moments will be subtracted from all the up moments at any given point and move from left to right when moving on to the next moment. Use the space on the right-hand side of the double page beneath the reaction calculations to calculate the bending moments. To begin with, the value for RL and RR will always be 0 Nm if the beam is in equilibrium and they will not have to be calculated. The bending moment at the 9 N force is calculated as follows: Hint: Place a sheet of paper on the point which you want to calculate, covering everything to the right of the point, as shown in Figure 6.17.

Hint:

BmA

Figure 6.17: Calculating bending moments

176

Forces

6

Calculate clockwise (upward) moments for BmA: = RL × 1 m (RL multiplied by the distance to the page) = 6 N × 1 m = 6 Nm Calculate anticlockwise (downward) moments for BmA: = 9 N × 0 m (the 9N force is at the page and therefore there is no distance to be multiplied) = 0 Nm Subtract anticlockwise (downward) moments from clockwise (upward) moments: = (6 × 1) − (9 × 0) = 6 Nm 9N

6N

BmA 1m

Hint:

BmB 1m

Figure 6.18: Calculating bending moments

Calculate clockwise (upward) moments for BmB: = RL × 2 m (RL multiplied by the distance to BmB) =8N×2m = 16 Nm Calculate anticlockwise (downward) moments for BmB: = (9 N × 1 m) + 6 N × 0 m) = 9 N + 0 Nm = 9 Nm Subtract anticlockwise (downward) moments from clockwise (upward) moments: = 16 Nm – 9 Nm = 7 Nm By shifting the page to the RR position and calculating the up and down moments, you will see their sum is 0 Nm.

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6


Example 1 A beam is subjected to two point loads and is supported at either end by RL and RR. 1. Calculate: a. the magnitudes of RL and RR. b. the bending moments at points A and B. 4N

3N

BmA

BmB 4m

2m

Figure 6.19: A beam with two point loads

1. Calculating reactions: Take moments about reaction left (RL) RR × 8 m = (4 N × 2 m) + (3 N × 6 m) ∴ RR = (8 + 18) 8 ∴ RR = 3,25 N Take moments about reaction right (RR) RL × 8 m = (3 N × 2 m) + (4 N × 6 m) ∴ RL = (6 + 24) 8 ∴ RL = 3,75 N Test: The sum of up forces must equal the sum of down forces. RL + RR = down forces ∴ 3,75 N + 3,25 N = 4 N + 3 N ∴7N=7N✓ 2. Calculating bending moments: = 7 Nm Bm A: (3,75 × 2) – (4 × 0) Bm B: (3,75 × 6) – (4 × 4) – (3 × 0) = 6,5 Nm

178

Forces

6

Assessment

Figure 6.20 represents a beam which is supported at either end by RL and RR.

4m

Figure 6.20: A beam with two point loads

1. Calculate the following for Figure 6.20: a. the magnitudes of RL and RR. b. the bending moments at points A and B.

Basic calculations on stress The stress calculations in the grade 11 curriculum are performed using the exact same principles covered in grade 10, but instead of using solid bars, the stress will be calculated in hollow tubes (square and round). The only difference in calculating stress in this case, is the difference in crosssectional area (hollow objects obviously have a smaller cross-sectional area). Calculating stress in square tubing and round tubing Example 1 Calculating the stress in square tubing Calculate the compressive stress in a 20 × 20 × 2 mm square tube if it is subjected to a compressive load of 10 kN.

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6


Step 1: Write down the formula. Stress =

Load Cross-sectional area

Step 2: Write down all the information you are given about the problem. Stress = ? Load = 10 kN Cross-sectional area = (20 × 20) – (16 × 16) outside area – inside area = 400 – 256 = 144 mm2 Step 3: Convert all the variables to their correct units. Stress = ? Load = 10 × 103 N (Correct units are Newtons.) Cross-sectional area =

144 106 (convert to m2)

To convert to the correct units, mm2 must be converted to m2. This is done by dividing mm2 by 106 because there are 1 000 × 1 000 mm in 1 m2 or 1 × 106 mm2 in 1 m2. Step 4: Substitute the variables into the formula and solve the equation. Stress = Load Cross-sectional area ∴ Stress =

10 × 103 144 106 (Substitute)

∴ Stress =

10 × 103 × 106 ____________ 144

∴ Stress = ________ 10 × 109 144

(Simplify)

(Simplify)

∴ Stress = 69 4444 4444 N/m2 or 69 4444 4444 Pa 180

Forces

6

Step 5: Convert the solution to the appropriate units according to engineering notation. Stress = 69 4444 4444 N/m2 or 69 4444 4444 Pa (Correct units) ∴ Stress = 69 ×106 Pa or 69 MPa

Example 2 Calculating the stress in round tubing Calculate the compressive stress in a Ø 25 × 2 mm round bar if it is subjected to a compressive load of 12 kN. Step 1: Write down the formula. Stress =

Load Cross-sectional area

Step 2: Write down all the information you are given about the problem. Stress = ? Load = 10 kN Cross-sectional area = (π × 252) – (π × 212) outside area – inside area 4 4 = 490,87 – 346,36 = 144,51 mm2 Step 3: Convert all the variables to their correct units. Stress = ? Load = 10 × 103 N (Correct units are Newtons.) Cross-sectional area =

144,51 106

(convert to m2)

To convert to the correct units, mm2 must be converted to m2. This is done by dividing mm2 by 106 because there are 1 000 × 1 000 mm in 1 m2 or 1 × 106 mm2 in 1 m2.

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6

Mechanical Technology Step 4: Substitute the variables into the formula and solve the equation. Stress = Load Cross-sectional area ∴ Stress =

10 × 103 144,51 106 (Substitute)

∴ Stress =

10 × 103 × 106 ____________ 144,51

(Simplify)

∴ Stress = ________ 10 × 109 144,51

(Simplify)

∴ Stress = 69 199 363 N/m2 or 69 199 363 Pa Step 5: Convert the solution to the appropriate units according to engineering notation. Stress = 69 199 363 N/m2 or 69 199 363 Pa ∴ Stress = 69 ×106 Pa or 69 MPa

(Correct units)

Performing basic testing on various mechanical principles Assessment

1. Calculate the compressive stress in a 50 × 50 × 3 mm square tube if it is

subjected to a load of 25 kN. 2. Calculate the tensile stress in a 60 × 3 mm round tube if it is subjected to a load of 30 kN.

182

Forces

6

Assessment

The equilibrium of three forces

Aim The aim of this activity is to illustrate the concept of the triangle of forces. Equipment The equipment consists of two load cells, which are interconnected by a cord via pulleys. The load cells register the tension on each side of the cord for a given load hung on them. A weight hanger can be hung on the cord in any position, using the cord retainer as a stop as required and giving various geometries. Load cells

Frame

Mass pieces on a string

Pulleys Equilibrium of three forces

Figure 6.21: Equilibrium of three forces

Method • Assemble the equipment as indicated in Figure 6.21. • Zero the load cell and compression cell. • Put the weight hanger in any position along the string and add a known mass (300 g). • Take the reading on the load cell. • Obtain the geometry of the system of forces by measurement and then by calculation. Ensure that the pulley is free-running. Try other geometries by rearranging the pulleys and load cells (as long as the pulleys allow the string to be attached to the load cell vertically). 183

6


Assessment

The reaction of a simply supported beam

Aim The aim is to determine the reactions on either side of a simply loaded beam. Supplement the moment of a force activity by taking the theory of moments one step further to include the measurement of the supporting forces on the beam. The sum of the reaction at the beam ends can be seen to equal the sum of the masses suspended from it, regardless of their position (that is, the vertical forces are in equilibrium), but only the distribution of the load at each end is affected. Equipment For this experiment the beam is suspended at each end by a load cell. Masses can be attached via the nylon screws and plastic hangers which can be considered to be light compared to the masses attached and thus ignored in calculations. Load cells

Frame Supported beam

Mass pieces

Reaction of a supported beam Figure 6.22: A supported-beam kit

Method • Assemble the equipment as indicated in Figure 6.22. • Zero the load cells. • Apply loads via the plastic hangers to the beam at any position. • Record the load cell reading. • Repeat for other loads and load positions. Use the calculation methods you learnt to confirm your experimental findings. 184

Maintenance

7

Chapter 7 Maintenance Topic 7

The effect of lack of maintenance on operating systems

Identification of signs of wear on components of mechanical systems Maintenance

Alignment

Analysis of the malfunction of operating systems

185

7


The effect of lack of maintenance on operating systems Analysis of the malfunction of operating systems

lubricant substance applied to the bearing or contact surfaces of machinery to reduce friction between moving parts

In grade 10 you learned about various types of maintenance, namely: preventative, predictive and reliability centred maintenance. Maintenance must be scheduled at the appropriate times to ensure that operating systems do not malfunction. The effects of lack of maintenance are more machine downtime, which may be very costly to repair, as well a loss of productivity. It can also result in very unsafe working conditions. Maintenance activities include, but are not limited to: inspection, measuring, cleaning, lubricating, adjusting and replacing parts.

Lack of lubrication or incorrect lubrication The purpose of any lubricant is to reduce friction. Without lubricants, friction between working surfaces becomes too great, resulting in loss of efficiency and eventual mechanical failure. Before discussing lubricants, it is important to first understand what friction is.

Friction Friction is a force that resists the movement of one object against another. It constantly affects people and objects. Friction force in itself is neither good nor bad; in fact, it is essential in our day-to-day lives. Without friction between your shoes and the ground, you would not be able to walk without slipping, and friction between tyres and the road enables bicycles and cars to roll along the ground.

Figure 7.1: Shoe soles rely on friction to provide grip

The exaggerated treads of the tyres of off-road vehicles use friction force by providing good grip on poor surfaces such as sand and gravel. Figure 7.2: An off-road vehicle

186

Maintenance There are, however, instances where friction is not as useful and needs to be overcome to achieve greater efficiency. It is important to note that friction can never be completely eliminated in mechanical components, but can be drastically reduced by using suitable lubricants and lubricating systems.

Causes of friction One of the reasons why friction occurs is that rough surfaces tend to catch on each other as they slide past each other. Even surfaces that seem to be very smooth can be rough when examined under a microscope. The many ridges and grooves of each surface can get stuck, effectively creating a type of mechanical bond between the two surfaces. Two surfaces in contact with each other also attract each other at a molecular level, forming chemical bonds. These bonds prevent objects from moving, even when an external force is applied. When an object is in motion, chemical bonds form and release as the molecules pass one another. Creating new bonds and breaking existing bonds remove energy from the motion of the object, so it requirs more energy to propel the object. Friction, therefore, causes a lack of efficiency because energy is ‘lost’ by the production of heat, light, noise and other forms of energy loss.

7

eliminated got rid of; done away with

chemical bonds forces of attraction between atoms or molecules

propel to push an object into motion

Scientists do not yet fully understand exactly how friction works because there are so many forces involved, acting at an atomic level. There is a field of science devoted to the understanding of energy transfer called thermodynamics. It is profoundly interesting and is one of the foundations of the study of physics. Despite not fully understanding friction, scientists and engineers have been able to find a way to describe frictional forces in a wide variety of applications. The force of friction between an object and any given surface is equal to a constant number, multiplied by the force the object exerts directly on the surface. The constant number is called the coefficient of friction for the two materials and is abbreviated µ. The force that the object exerts directly on the surface is called the normal force and is represented by the letter N. The amount of friction depends on this force; increasing the force increases the contact that the object has with the surface at a microscopic level. The greater the normal force, therefore, the greater the friction force. The following formula is used to calculate the force of friction between an object and a surface over which it passes: F=µ×N where F is the force of friction, µ is the coefficient of friction between the object and the surface, and N is the normal force. 187

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Mechanical Technology Scientists have measured the coefficient of friction for many combinations of engineering materials. The coefficient of friction depends on whether the object is initially moving or stationary, and on the type of material involved. The following coefficients of friction are examples of high and low coefficients: • rubber sliding on concrete – 0,8 (relatively high) • teflon® sliding on steel – 0,04 (relatively low). Example Rubber sliding on concrete on a horizontal plane has a coefficient of friction (µ) of 0,8. Calculate the friction force (F) if the rubber has a mass of 5 kg. Step1: Convert the mass of the rubber to force in Newtons. 5 kg × 9,81 = 49,05 N Step 2: Substitute all the values into the formula F = µ × N F = 0,8 × 49,05 N ∴ F = 39,24 N

Assessment

1. Research the coefficients of the following materials and calculate the static friction force in each case, assuming the object is resting on a flat surface: a. A 50 kg aluminium mass piece resting on a mild steel surface b. A 50 kg mild steel mass piece resting on a mild steel surface. 2. What can you deduce from the results of your research and calculations?

DID YOU KNOW?

Teflon® is a type of plastic with a relatively high melting temperature and low coefficient of friction. Because of this it is very suitable as a nonstick coating on frying pans and other utensils.

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Figure 7.3: A teflon-coated pan

The normal force is the force that the object exerts perpendicularly on the surface. When the object is on a level surface, the normal force is equal to the weight of the object, i.e. mass × 9,81. If the surface is inclined, only a fraction of the object’s weight will be applied directly onto the surface, therefore the normal force will be less than the object’s weight. In physical science you will learn about inclined planes in your work on vectors.

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Types of friction There are many types of friction. The coefficient of friction for engineering materials may differ depending on the type of friction involved.

Sliding friction Sliding friction is the force that resists the motion of an object as it moves along a surface. A book sliding off your table or brake callipers pushing against your bicycle wheel and slowing it down are both examples of sliding friction or kinetic friction. Sliding friction acts in the opposite direction to the direction of motion. It is a hindrance to an object already in motion and tends to slow it down. If another force is not continually applied, the sliding friction will eventually cause the object to become stationary. Gravity is responsible for this type of friction, because it is gravity which pulls the object onto the surface on which it slides. The coefficient of kinetic friction between the object and the surface on which it is moving (µk) is usually less than the coefficient of static friction. This means that more force is required to start an object sliding than it does to keep it sliding. The amount of friction depends on the nature of the surfaces in contact and on the weight of the object. In the following example (see Figure 7.4), 100 Newtons of force are needed to slide a block which exerts 50 Newtons on the surface and 200 Newtons of force are needed to slide a block which exerts 100 Newtons on the surface.

Figure 7.4: An example of sliding friction

It is important to note that friction does not depend on the amount of surface area in contact between an object and the surface, as demonstrated in the following example:

Figure 7.5: How surface area relates to sliding friction

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Static friction Static friction occurs between stationary objects. Static friction is the force that prevents an object from moving. It will prevent a book on your table from sliding off, even when the table is slightly inclined. It is also the force that allows you to pick up an object without it slipping through your fingers. Before an object can be moved, the force of static friction between the object and the surface on which it is resting must be overcome. This force depends on the coefficient of static friction (µs) between the object and the surface and the normal force (N) of the object.

Rolling friction

retards works against; hinders

Rolling friction retards the motion of a rolling object. For example, rolling friction slows down a cricket ball on the grass as it is driven away from the batsman. The coefficient of rolling friction between the two materials (µr) and the normal force (N) of the object is normally one hundredth that of sliding friction. Because of this, a rolling object experiences much less drag than the same object sliding along the ground.

Assessment

Research how the invention of the wheel has revolutionised the way people work. Write an essay of approximately 500 words and use illustrations to convey your findings.

Fluid friction

DID YOU KNOW?

The invention of the wheel has made the most significant contribution to the reduction of friction in human history.

Objects moving through a fluid (liquid or gas) experience fluid friction or drag. Drag greatly hinders the motion of the object (especially in very viscous fluids). The amount of drag depends on the object’s shape, material, and speed, as well as the fluid’s viscosity. As you learnt in Grade 10, viscosity is the measure of a fluid’s resistance to flow. It results from the friction that occurs between the fluid’s molecules. Studying viscosity is a study of its own. Drag also slows down an object, such as an aeroplane flying through the air. An aeroplane’s engines produce thrust which helps it to overcome drag and travel forward.

Figure 7.6: An airfoil

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Maintenance Figure 7.6 shows how air flowing over an airfoil results in lift. Studying fluid friction is a much more complex study, because there are more variables involved. Aerodynamics is the branch of physics which deals with the motion of air and fluids.

The effects of friction Friction helps us to convert one form of motion into another. For example, when a vehicle’s tyres push backward along the ground, friction with the ground makes the tyres roll forward. Friction allows gases to be compressed in the combustion chamber of an internal combustion engine. Besides friction enabling us to convert motion from one form to another, friction also converts some energy into heat, noise, and wear and tear on material. As wear increases, so too does vibration, heat and noise, which further damage the mechanical components. The loss in energy of these parts reduces the efficiency and power of a machine and will eventually result in mechanical failure.

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DID YOU KNOW?

Reducing an aeroplane’s airspeed may result in the aeroplane stalling when the drag becomes too great and the thrust too little.

Overloading friction As you learnt in Grade 10, the purpose of a lubricant is to provide a thin film or barrier between metal components to prevent the ridges and grooves of the respective surfaces from rubbing against each other. Correctly machined components should be as smooth as possible. Engine components are machined to exact tolerances in this respect. Metal surfaces without lubrication

Metal surfaces with a thin film of lubrication

Figure 7.7: The effect of lubrication between metal surfaces

Overloading friction occurs when the thin film of lubrication is compromised. This can occur as a result of a variety of factors, but the two main ones will be discussed.

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Overloading Overloading is the act of running a machine or device (such as an internal combustion engine) at a rate, burden or power level higher than that at which it was designed to operate, risking permanent damage to the machine. When any machine is overloaded, the lubricating barrier of oil is effectively squeezed out of the machine’s bearing surfaces. This results in metal on metal contact. The friction caused by the contact causes heat and expansion. The expansion further squeezes oil from the bearing surfaces, which worsens the condition, resulting in more heat being produced. If the process continues, the metal surfaces will become scratched and scored, and finally seize, causing massive engine failure and expensive repairs. Engine overloading is primarily caused by driver behaviour but can also be caused by any number of mechanical failure(s) which result(s) in a breakdown of the lubrication system or cause greater than normal friction. Some causative factors relating to driver behaviour are: over-revving, speeding and aggressive driving style, e.g. rapid acceleration/deceleration, incorrect gear selection, carrying loads exceeding the manufacturer’s specifications or a combination of the above. Most modern vehicles have sophisticated electronic engine control units (ECU) on board, which monitor the engine performance continually through sensors and actuators. They can also detect mechanical/electronic faults before catastrophic engine failure occurs. Some vehicles are also governed to prevent excessive speeding.

Overheating Overheating ultimately has the same effect as overloading. It can occur in a variety of ways. duty cycle the percentage of time prescribed by a manufacturer for the use of a particular machine

Extended use If a machine runs over extended periods, it will not be able to adequately maintain a safe working temperature. Machines should never operate over periods greater than their duty cycle for this reason. Inadequate maintenance Services should be set for routine intervals and the temperature should also be regularly monitored by inspecting the temperature gauge and paying attention to warning lights. Oil not only has the vital function of lubricating the engine, but also plays a part in dispersing heat. Over time, oil becomes contaminated with carbon particles and other impurities. When oil loses

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its ability to lubricate and cool adequately, it becomes darker in colour. Contaminated oil is usually drained during services and the machine is re-lubricated once all the necessary filters have been replaced. The exercise of replacing a motor vehicle’s oil is described on page 150 of the Grade 10 Learner’s Book. Insufficient lubrication If there is an oil leak, the level of oil in the machine’s sump will gradually drop. The dip stick in a motor vehicle’s engine indicates this level. The oil level should be between the upper- and lower-limit marks on the dip stick. If the oil level is too low, there may not be sufficient oil pressure to transport the oil to all the places which it needs to lubricate. Over-filling the oil sump is never wise as it results in too much oil pressure, which may damage the oil seals. Inadequate cooling Failure of an engine’s cooling system is an obvious cause of overheating. This can occur as a result of a variety of reasons: A loss of coolant fluid can occur due to a leak in the cooling system, such as the radiator and its hoses, or a damaged cylinder head gasket. Engine coolant is usually a substance called antifreeze, which is glycerol-based. Antifreeze freezes at a temperature lower than water’s freezing temperature of 0˚C. Similarly, it boils at a temperature higher than water’s boiling point of 100˚C. The use of antifreeze allows the engine’s cooling system to be effective over a greater range than water. It is particularly useful in countries, like South Africa, which are known for extreme seasonal weather conditions. If there is a loss of pressure in the cooling system due to a leak, the coolant will boil off at a lower than normal temperature, which will result in overheating. Mechanical failure can also occur in a cooling system. A faulty water pump or broken fan belt also results in overheating, because the coolant is not able to move around the heated engine components and disperse the heat effectively.

DID YOU KNOW?

A liquid under pressure boils at a higher temperature than its normal boiling temperature.

Modern vehicles are almost exclusively centrally controlled by computers, which constantly monitor and adjust the interaction of the various components. Electronic failure of things such as temperature sender units can result in overheating as a result of incorrect data being processed by the engine’s management computer. 193

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Balancing This topic covers a wide spectrum of engineering fields. This section of the chapter will focus on automotive components.

Balancing of automotive components Any object in a motor engine or on a motor vehicle which rotates needs to be engineered in such a manner that causes least vibration when it is revolving at speed. Objects that are out of balance cause unnecessary vibration, which in turn leads to excessive wear on bearings and bushes due to increased friction and inefficiency. In reality, however, it is very difficult to manufacture engine components which are perfectly balanced. For this reason, certain components need to be balanced on specialised machines after manufacture. These machines are able to detect any un-uniform weight distribution precisely, and rectify it by adding small mass pieces or removing stock, by means of drilling or grinding at strategic places. Balanced machine components have a far greater lifespan and produce much less noise and vibration.

Balancing of wheels It is essential that a wheel assembly be properly balanced. Unbalanced wheels will cause unnecessary tyre wear, poor driving and wear on the steering and suspension parts due to the continual shaking. The side-effect of unbalanced wheels is “wheel shimmy” (shaking of the wheel assembly from side to side). A wheel must be in static and dynamic balance. Unbalanced wheels are the result of centrifugal forces. These forces increase with the square of the speed of rotation. A mass of 30 g on the tread of a tyre 800 mm in diameter will exert a force of 5 Newtons at a speed of 50 km per hour. At 100 km per hour the force becomes 20 Newtons; at 150 km per hour the force becomes 45 Newtons. This fact illustrated that a wheel may not be seriously out of balance at low speeds, but becomes dangerous at high speeds.

Static balance A wheel is said to be in “static” balance when it has no heavy spot, i.e. it will remain at rest in any position when free to turn on a spindle through its centre. If a wheel in static balance is divided into exact halves, then the mass of each half of the wheel must be exactly the same.

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Mount the wheel so that it is free to turn on a spindle through its centre, the spindle being approximately horizontal and the wheel set turning slowly: • If the wheel is in balance, it will come to rest in any position. • If the wheel is out of balance, it will always come to rest with one point – “the heavy spot” – at the bottom. To correct static out of balance, a small mass piece is applied to the wheel rim diametrically opposite the “heavy spot”. The size and position of the mass piece to be fitted is found by trial and error.

Dynamic balance It is possible for a wheel to be in static balance, but still cause a vibration when rotated at high speed. It is said to be incorrect in respect of dynamic balance, i.e. there may be “heavy spots” on opposite sides of the central plane. Centrifugal forces will not balance each other, but will force a “couple” tending to vibrate the spindle and this causes “wheel wobble” or “shimmy”. Excessive unbalance will cause wear on wheel bearings. It is necessary to use a dynamic wheel balancing machine, which has provision for spinning the wheel at fairly high speed while mounted on an axle free to vibrate. Various means are adopted in these machines for locating: • the plane of unbalance • the strength of the unbalancing forces • the sense of direction of these forces (clockwise or counter-clockwise). Balancing such a condition is done by applying two equal mass pieces at the point which will produce a couple exactly equal to that at the heavy spots. Before attempting any balancing, the above checks should be followed carefully in addition the following: • The tyre must be carefully examined for bruises, cracks and damaged side walls. • The wheel rim must be examinded for damaged beads. • Remove foreign matter from the rim and tyre.

Figure 7.8: A dynamic wheel balancing machine

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Balancing of crankshafts When the crankshaft of an engine is rotating at high speed, it is liable to vibrate with varying intensity. This vibration has two main causes. The first is the action upon the shaft of unbalanced forces; and the second is the torsional or twisting effect of the power strokes upon the shaft by any one, or a combination of any, of the following: • The crankshaft and flywheel assembly is not statically balanced. • The crankshaft as well as the flywheel is not dynamically balanced • The mass of the pistons and connecting rods is not balanced while in motion, i.e. the reciprocating mass is not balanced For balancing purposes, the reciprocating mass is taken to be that of the pistons, gudgeon pins and the upper third of the connecting rod. The rotating mass is taken to be that of the crankpin, big-end and the lower twothirds of the connecting rod.

Static balance A crankshaft is in static balance when the mass in all directions from the centre of rotation is equal while the crankshaft is at rest. In practice the vibration of the crankshaft is reduced by careful balancing of the crankshaft and flywheel. This may be done by fitting balance mass pieces to the crank-webs, by removing metal from the crank-webs, or by arranging the crankpins on opposite sides of the crankshaft. When the main journals are supported by parallel and accurately levelled knife edges, the crankshaft should remain in any given position. The crankshaft is said to be statically balanced, i.e. balanced when stationary.

Dynamic balance A crankshaft is in dynamic balance when the centrifugal forces of rotation in all directions at any point are equal while the crankshaft is rotating. When the crankshaft of a multi-cylinder engine is rotating, it tends to rock from side to side, the motion being violent at certain speeds. This is because the crankpins cannot be arranged exactly opposite each other and side thrusts are produced which act alternately in opposite directions. This rocking effect can be reduced by removing metal from certain parts of the crank-webs, the positions and mass of metal to be removed being detected by the use of a special machine in which the crankshaft is rotated at various speeds. When this operation has been completed, the crankshaft is said to be dynamically balanced, i.e. balanced while rotating. 196

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Balance of reciprocating mass Vibrations as a result of reciprocating mass are caused when the piston has to change direction at a very high speed, at a rate of twice per cycle. A factor which contributes to these vibrations is the fact that the speed of the piston in the cylinder is not constant for a constant speed of the crankshaft. When the piston moves from TDC (top dead centre), it moves further and, therefore, faster during the first and last 90˚ of crankshaft rotation than during the second and third 90˚. Balancing of reciprocating mass is also performed with the crankshaft rotating, together with the assemblies of pistons and connecting rods fitted.

Torsional vibrations The torque or turning moment of the crankshaft is derived from the power strokes, each of which forces a crankpin to rotate about the centre of the main journals and so tends to twist the shaft, i.e. to subject it to torsional stress. Although the power strokes may occur at equal angles of crankshaft rotation, the torque produced by them alternates between high and low value. The alternation of the torque and of the corresponding torsional loads may cause the shaft to vibrate about its own centre line. This torsional vibration is sometimes referred to as shaft “wind-up”; the shaft alternately winds up and releases as it rotates. In addition to the torsional vibration, the shaft also has its own natural frequency of vibration. At certain speeds of rotation, known as critical speeds, the frequencies of these two vibrations may coincide and produce an excessive vibration called “resonance”. The shaft vibration may then be so violent that the shaft will be fractured. This must be avoided by careful attention to the design and balance of the crankshaft assembly, and by the fitting of a suitable torsional vibration damper.

Functions of balance mass pieces • •

To balance the mass of the piston, connecting rod, web and crank journal so that the assembly of moving parts will be in static equilibrium for all positions of the stroke. To provide an opposing centrifugal force to counteract centrifugal forces due to the piston, connecting rod, web and crank journal. • To counteract the inertia loads due to the moving parts during the accelerating and retarding portions of their travel, and in this way avoiding a considerable amount of vibration which would otherwise occur. 197

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Built-in features to improve engine balance The crankshaft Is carefully balanced to run smoothly at high speeds. Webs are extended and drilled to form balance mass pieces at points opposite to the connecting rods. Connecting rods and pistons Are kept as light as possible to reduce reciprocating mass and force. Flywheels Are carefully balanced and are usually fitted to the crankshaft flange in one position only. Vibration dampers Are usually fitted to the front end of the crankshaft to smooth out the engine vibrations. Function The vibration damper adds mass to the crankshaft on the opposite side of the normal flywheel in order to counteract the torsion of the crankshaft, i.e. to resist the sudden movement of the crankshaft during the twisting and untwisting oscillations. The friction-face type Secondary flywheel

Crankshaft flange

Crankshaft

Friction spring Friction disc

Figure 7.9: A combined friction and rubber vibration damper

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Construction This type consists of a flange fitted tightly on the end of the crankshaft. A secondary flywheel is placed against this flange and a friction disc is placed between them. A spring plate with retaining bolts holds the secondary flywheel in position. Two friction springs exert a frictional force against the secondary flywheel and flange. Assuming the crankshaft to be stationary, the secondary flywheel (dampening device) can be turned. Operation In actual operation it is the variable speed of the crankshaft (“twisting” and “untwisting”) which is transmitted to the secondary flywheel. When a front cylinder fires and the shaft tries to speed up, it tries to spin the secondary flywheel with it. In doing this, the friction between the secondary flywheel and the flange holds the flywheel back and the crankshaft does not speed up as much. When the firing pressure is removed from the crankshaft, its unwinding force is retarded because the secondary flywheel has now built up energy which keeps it turning. The unwinding force of the crankshaft cancels out the twist in the opposite direction, thus smoothing out the torsional vibration.

Figure 7.10: Balancing a crankshaft

Balancing is not confined to automotive components. It is essential to balance any engineering component which has a high mass or operates at high speeds, such as turbines and rotors. Failure to balance these components could result in engineering calamities.

calamities disasters

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Alignment

aligned brought into correct line

Alignment can refer to many aspects of engineering. As you learnt in Grade 10, it is essential that engineering components, like pulleys and gears, are correctly aligned. When components like these are not correctly aligned, the result is uneven wear, more operating noise, friction and a reduction in lifespan of the components. A practical illustration of alignment in engineering is the wheel alignment of motor vehicles.

Wheel alignment Wheel alignment basically consists of adjusting the angles of the wheels so that they are perpendicular to the road and parallel to one another. The purpose of these adjustments is to ensure maximum tyre life and optimal road holding. It is also done to ensure that the vehicle tracks straight and true when driven along a straight and level road. Wheel alignment should not be confused with wheel balancing. The two have nothing to do with each other except for the fact that they both affect driving-comfort and road-holding. If a wheel is out of balance, it will cause a vibration at high speed, that can be felt in the steering wheel. If the alignment is out, it can cause excessive tyre wear and steering or tracking problems. The important things you need to know about wheel alignment, are the concepts of camber, caster and toe-in.

Figure 7.11: A technician checking the wheel alignment of a vehicle

Computerised equipment used in wheel alignment employs lasers and prisms attached to all four wheels to measure the camber, caster and toe-in.

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Camber Positive camber Positive camber angle is the outward tilt of the wheel at the top away from the vehicle when viewed from the front. If a centre line is drawn through a cambered wheel, it would form an angle with the true vertical line. If excessive positive camber exists, the wheel will tend to roll outwards, causing hard steering and tyre wear. Camber brings the contact point of the tyre on the road more nearly under the king-pin thus providing for: • easier steering • the vehicle mass to be carried by larger inner wheel bearing • the centre line of the tilted wheel and the true vertical line contacting the road on the same plane as the tyre • a centre point on which the wheel can turn • this centre point bringing about easier steering and placing the mass of the vehicle on the inner wheel bearing. Reason for cambered wheels is to allow for: • deflection of the suspension under load • cambered road surfaces. Camber is adjusted by means of a cam or wedge plates on the suspension, depending on the design, and is measured in degrees. Usually not more than 1 degree positive is allowed. On some suspension systems the camber changes from positive to negative as the wheel rises or falls when hitting a bump or pothole. Negative camber Wheels with a permanent negative camber are used on some continental vehicles, particularly on the rear, to improve road holding. When employing negative camber on the front wheels, provision must be made for toe-out to counteract the camber effect. Negative camber

Positive camber

Coil spring

Shock absorber Drive shaft Stabiliser shaft

Figure 7.12: Camber angles

Figure 7.13: The camber wear-pattern on a tyre

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Caster Positive caster Positive caster is the backward tilt of the kingpin at the top, when viewed from the side, so that the centre line through the kingpin strikes the road at a point ahead of the vertical line through the wheel. Caster gives a self-centring action to the steering and keeps the wheels in the straight-ahead position. • By tilting the kingpin back at the top, its centre line strikes the road in front of the point at which the tyre contacts the road. • This places the point of resistance to forward motion behind the steering centre line. • The wheel is forced to trail behind the kingpin centre line. • Thus the straight-ahead position is maintained. Caster is measured in degrees and never exceeds 2 degrees. It may be adjusted on independent suspensions by tilting the upper members slightly to the rear of the lower members. On solid beam axles, wedges are placed below the spring saddles in order to tilt the axle. On late-model vehicles, there is often little or no positive caster. Positive caster makes turning the wheels from the straight-ahead position more difficult than if no caster angle is present. Positive caster also causes a mild tipping action of the wheels towards the outside cornering. This will affect the cornering ability of the vehicle. Negative caster Negative caster is the reverse of positive caster. It ensures easier turning of steering and provides better cornering qualities by providing a tipping action towards the inside of the turn. Proper tracking is still provided by kingpin inclination.

Figure 7.14: Caster angle viewed from the side of the vehicle

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Toe-in and toe-out Toe-in Toe-in of the front wheels is the difference in the distances between the wheel rims or tyre thread centres, measured at stub axle height, behind and in front of the axle or suspension. Since toe-in wheels tend to roll inwards, they will overcome the tendency of wheels with positive camber to roll outwards. • A positive cambered wheel engages the road at an angle. • This will cause the wheel to roll as though it were consented. • It will tend to roll outward as it is rolled forward. • To compensate for this outward roll, the wheels are adjusted to toe-in. Toe-in is measured in millimetres and is usually between 0,8 and 4,8 mm, depending on the camber angle. The larger the camber angle, the larger the toe-in. The adjustment is made by altering the length of the track rod.

Tyre

Upper control arms Toe-in

Engine

Figure 7.15: Toe-in

Toe-out Toe-in is accomplished by placing the front of the wheels further apart than the rear when viewed from the top. Toe-out is usually given to wheels where the suspension is designed with negative camber and on vehicles with front wheel drive.

Tyre

Upper control arms Toe-out

Engine

Figure 7.16: Toe-out

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Kingpin inclination On most modern designs, the kingpin is set at an angle relative to the true vertical line, as viewed from the front or back of the vehicle. This is the kingpin inclination or KPI (also called steering axis inclination or SAI). This has an important effect on the steering, making it tend to return to the straight-ahead or centre position. This is because the straight-ahead position is where the wheel is at its highest point relative to the suspended body of the vehicle – the weight of the vehicle tends to turn the kingpin to this position. A second effect of the kingpin inclination is to set the scrub radius of the steered wheel. This is the offset between the tyre’s contact point with the road surface and the projected axis of the steering down through the kingpin. Camber Kingpin or steering axis inclination

Ball joint

Drag rod Tyre Scrub or pivot angle radius Figure 7.17: Kingpin inclination

Ackermann principle (toe-out on turns) The intention of Ackermann geometry is to avoid the need for tyres to slip sideways when following the path around a curve. The geometrical solution to this is for all wheels to have their axles arranged as radii of a circle with a common centre point. As the rear wheels are fixed, this centre point must be on a line extended from the rear axle. Intersecting the axes of the front wheels on this line as well requires that the inside front wheel is turned, when steering, through a greater angle than the outside wheel. A linkage between these hubs moves the two wheels together and, by careful arrangement of the linkage dimensions, the Ackermann geometry can be approximated. This is achieved by not making the linkage a simple parallelogram, but by making the length of the track rod (the moving link between the hubs) shorter than that of the axle, so that the steering arms 204

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of the hubs appear to be “toe-out”. As the steering moves, the wheels turn, according to Ackermann, with the inner wheel turning further. If the track rod is placed ahead of the axle, it should instead be longer in comparison, thus preserving this same “toe-out”. A simple approximation to perfect Ackermann’s steering geometry may be generated by moving the steering pivot points inward so as to lie on a line drawn between the steering kingpins and the centre of the rear axle. With perfect Ackermann, at any angle of steering, the centre point of all of the circles traced by all wheels will lie at a common point. In other words, when the wheels are turned, the steering arm on the inside of the turn swivels more sharply, due to the angle of steering arm at this point. This causes the inner wheel to move through an angle of 23 degrees, while the outer wheel moves through 20 degrees. When the wheels are straight ahead, they are parallel again. Toe-out on turns gives true rolling motion to the front wheels without scuffing when negotiating a corner. 20˚ 23˚

Stearing control arms

Centre of turning wheel Rear axel Figure 7.18: The Ackermann principle (toe-out on turns)

Assessment

1. 2.

Name 2 different types of balancing and give a description of each one. Make use of a diagram to demonstrate the Ackermann principle with regard to toe-out turns.

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Four-wheel alignment

To true up a mechanism means to adjust it accurately.

There are two main types of four-wheel alignment. In each case, the technician will place an instrument on all four wheels. In the first type the rear toe and tracking are checked, but all adjustments are made at the front wheels. This is done on vehicles that do not have adjustments on the rear. The second type is a full four-wheel alignment where the adjustments are first made to true up the rear alignment, then the front is adjusted. A full four-wheel alignment will cost more than the other type because there is more work involved.

Important things that vehicles owners need to know about wheel alignment Before attempting any wheel alignment adjustment, the following should be examined and, if necessary, corrected or replaced: • The kerb mass (tank full of petrol, spare wheel and tools) must be checked against the manufacturer’s specifications • Tyre for uneven wear • Tyre pressure • Wheels for run-out; check wheel nuts with torque wrench • Hub bearings for correct prelate (with torque wrench) • Kingpins and bushes • Suspension ball joints for wear, locking and lifting • Suspension bushes for excessive free movement • Steering box play and whether secure on chassis • Tie-rod ends • Sagged springs which includes riding height • Ineffective shock absorbers • Spring U-bolts • Chassis for possible cracks and loose cross-members

Assessment

Visit your local tyre fitment and wheel alignment centre. Ask the manager if you may observe the wheel balancing and wheel alignment technicians performing their respective functions. Make rough notes of the process, ask questions and take photographs if you have a camera. When you arrive home, write a step-by-step report on how wheels are balanced and aligned.

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Identification of signs of wear on mechanical systems All mechanical systems are subject to wear over time. To keep mechanical systems performing optimally, it is necessary to evaluate and report on the condition of these components as they deteriorate. The following guidelines will assist in fault-finding on an engine with specific reference to fuel systems and ignition circuits. Most modern vehicles, however, make use of sophisticated electronic diagnostic tools to fault find.

The engine control unit (ECU) An engine control unit (ECU), most commonly called the power train control module (PCM), is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure the optimum running. It does this by reading values from a multitude of sensors within the engine bay, interpreting the data using multidimensional performance maps (called Look-up tables), and adjusting the engine actuators accordingly.

Fuel systems For an engine with fuel injection, an engine control unit (ECU) will determine the quantity of fuel to inject based on a number of parameters. If the throttle position sensor is showing the throttle pedal is pressed further down, the mass flow sensor will measure the amount of additional air being sucked into the engine and the ECU will inject more fuel into the engine. If the engine coolant temperature sensor is showing the engine has not warmed up yet, more fuel will be injected (causing the engine to run slightly ‘rich’ until the engine warms up).

Ignition circuits An ECU can adjust the exact timing of the spark (called ignition timing) to provide better power and economy. If the ECU detects knock, a condition which is potentially destructive to engines, and “judges” it to be the result of the ignition timing being too early in the compression stroke, it will delay (retard) the timing of the spark to prevent this. Since knock tends to occur more easily at lower rpm, the ECU controlling an automatic transmission will often downshift into a lower gear as a first attempt to alleviate knock. The ECU is mapped to be able to deliver spark at the optimal time for a multitude of operating conditions.

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Fault finding procedures on fuel systems and ignition circuits When a motor vehicle is brought in for service and repairs, it is connected to a PC or laptop via data cable to the ECU. Some ECU’s are programmable (usually when vehicles have been customised aftermarket) but most are set according to the manufacturers’ specifications and cannot be reprogrammed by the user. The software installed on the technician’s computer will indicate all the faults in the fuel and ignition system, as well as many other mappable parameters. The technician will then proceed to replace all the necessary parts and make the required adjustments, as indicated on the diagnostic programme to “tune up” the vehicle. The service technician will also monitor and report on the condition of all mechanical components, according the service schedule, to ensure that all the mechanical systems are functioning correctly and parts are repaired or replaced in good time, to prevent mechanical failure.

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Chapter 8

Systems and control Topic 8

Electric/ Electronic control

Hydraulics/ Pneumatics

Drives Systems and control

Operating principles of clutches, levers, linkages, cams and eccentrication

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Systems and control A mechanism is a system that changes one kind of movement into another kind of movement, e.g. rotary motion is converted into longitudinal motion. A mechanism can gather energy or information and allow smooth motion to take place.

Introduction Every imaginable task nowadays can be done by a machine or mechanism. Life was not always like that. The change started about 250 years ago, when the machinery and manufacturing industry began to take over from the primitive, traditional manufacturing industry. (This ‘old’ system is sometimes referred to as the domestic system.) These changes were part of the industrial revolution which began in England and eventually spread to all parts of the world. We hardly notice the mechanisms around us until one is lost or broken. For example, we often realise the importance of machines such as television sets or kettles only when we have to do without them, e.g. during a power failure. In this chapter we will have a closer look at mechanical, hydraulic, pneumatic, and electrical systems.

hydraulic the application of liquid laws to achieve mechanical advantage

pneumatic the science of the mechanical properties of gases, and their application to perform work

transmits conveys

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Mechanical drives Gears A gear is a toothed wheel which engages (or meshes) with other gears and transmits motion from one part of a mechanism to another. The evolution of gears from pin cogs to the modern involute type is one of the most interesting developments in mechanical engineering. You were introduced to gears in Grade 10. You should have a basic knowledge of gears and be able to identify types of gears. Gear drives are a positive mode of work transfer and are generally used to transmit motion in parallel planes and also provide positive (non-slip) power transmission. They are also used to increase torque in mechanical devices. Two or more gears running in mesh form a gear train. The smallest gear in the gear train is called the pinion. These gears are used in the following circumstances: • where space is limited • where a difference in speed is necessary (increase or decrease in speed) • where power is transmitted • where a change in the direction of transmission is required. Gears differ in shape according to their function.

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Spur gears The teeth of spur gears are straight and are cut parallel to the axis of the shaft. Spur gears are easy to manufacture and are, therefore, cheaper than other gears. They are commonly used to transmit power between parallel shafts. Because their teeth are straight, there is no axial load or pressure on them. Normally they are used under conditions of reasonable speed and moderate tooth load.

axial load load applied in the direction of the axle on which the gear is mounted Figure 8.1: A spur gear

Helical gears Single helical gears Helical gears resemble spur gears, but the teeth are cut at an angle, rather than parallel to the shaft axis. The angle is known as the helix angle and can vary from a few degrees to 45°. Two or more teeth are in mesh at the same time, depending on the helix angle.

The helix angle is the angle at which the teeth of a helical gear are slanted across the face of the gear.

Figure 8.2: A single helical gear

The ‘hand’ of the gear, either left or right, is the direction in which the teeth lean or slope, with the gear horizontal and the bore vertical. Helical gears of opposite hands will operate on parallel shafts. Helical gears of the same hands operate on shafts crossing at an angle, depending on the angle of the gears. Single helical gears are used where silent drives are required. Advantages of single helical gears include: • The design of the gear prevents concentrated loads on the tips of the teeth during initial meshing. • The contact between the meshing teeth takes place very gradually. • Helical gears are much quieter than spur gears because a number of teeth engage at the same time. • A helical gear is much stronger than a spur gear of the same size. • A helical gear can carry heavier loads. • A helical gear can transmit more power. • Helical gear drives are smooth and silent.

DID YOU KNOW?

For a pair of helix of helical gears to mesh, they must have the same helix angle, the same pitch and the same pressure angle.

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Mechanical Technology The disadvantages of single helical gears are: • Friction due to the sliding motion between the meshing teeth is high and, therefore, the wear and tear is also high. • Because of the high wear and tear, helical gears need to be well oiled, or even better, run in an oil-bath such as in gearboxes or sumps. • These gears are difficult and expensive to manufacture. • Thrust bearings must be included to neutralise the tendency towards axial thrust that the gears have.

Double helical gears

Herringbone gear

Herringbone gear

Double-opposed helical gear

Figure 8.3: Herringbone gears

axial-end thrust the thrust developed as a result of axial force

Herringbone gears are designed and used to neutralise and overcome axialend thrust. Although they are more expensive and difficult to cut, they are preferred to single helical gears because thrust bearings are not needed. They are used in gearboxes of overhead cranes. Herringbone gears have a larger contact area than other gears of the same size and are, therefore, much stronger than other gears as the tooth pressure is spread over a larger surface. This gear meshes perfectly, where large amounts of torque have to be transferred. Due to considerable friction between the teeth, this type of gear must also operate in an oil-bath. Herringbone gears consist of two opposite-hand helical gears. The axial force present with helical gears is eliminated and they can, therefore, be mounted without thrust bearing.

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Bevel gears

Figure 8.4: Two-speed final drive in high ratio

Please note that the pinion and crown wheel are bevel gears in mesh. Ask your teacher to show you the single epicyclic gear train. The teeth of bevel gears are cut on the outside of a cone. The teeth can be cut straight across the face of the gear, or they may have a spiral. Bevels are used to transmit power between two shafts that are situated at right angles to each other. These gears are used to change direction of transmission. Mitre gears are two bevel gears of equal size and dimension, with an equal number of teeth and their axes intersecting at right angles. Bevel gears are used in the lowering and raising mechanisms of drilling, milling and shaping machines tables.

Worm gears Worm gear drives consist of a worm meshing with a worm wheel (or gear). The centre lines are at 90º to each other and do not lie on the same level. The worm is the small driving gear and the driven gear is the worm wheel. The helical teeth on the worm are in the form of screw thread, and are often called a thread. The teeth on the worm wheel are helical. Normally a worm is made of high-carbon steel and the worm gear of brass or bronze. Worms may have single, double or triple threads.

Figure 8.5: A worm and worm gear

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Mechanical Technology Worm gear drives are used where: • an increase of power is needed. • the direction of the drive must change through 90˚ and the centre lines of the drive shafts are situated on the same plane. • high ratios of reduction in speed are required, e.g. 40 : 1, as in the dividing head of the milling machine. Examples of their uses are the dividing heads of milling machines and reduction gearboxes. The advantages of worm drives are: • The direction of rotation of the gears can be changed. • A power increase can easily be achieved with this system. • The drive is very quiet. • High speed ratios can be achieved. • Their speed can be reduced greatly, e.g. reduction gearboxes. The disadvantages of worm drives are: • A great deal of wear and tear is caused by friction. • A large amount of power is used to overcome the friction between the worm and the worm wheel. • Power can be transmitted only from the worm to the worm wheel and not from the worm wheel to the worm.

Rack and pinion Pinion

Rack

Figure 8.6: Rack and pinion gears

Figure 8.7: Circular motion of the gear is transmitted into linear motion of the rack

The gear rack is a piece of parallel-sided material with teeth cut on the flat surface, and the pinion is a small spur gear. The rack and pinion mesh together. This system is used to change rotary movement into reciprocating motion, and vice versa, depending on which is the driven component. A rack and pinion may be either spur or helical in tooth form. Rack and pinion gears are used in the feed mechanisms of milling machines and planning machines.

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There are numerous other gears and gear systems, e.g. hypoid gear, internal gears, epicyclic gears, planetary gears, spiroid® and helicon® gears. A fuller study of gears is intricate and outside the scope of the Grade 11 syllabus.

Pulleys In Grade 10 pulleys were discussed in general. You know that pulleys are wheels that turn readily on an axial. The axial is mounted on a frame. One or more pulleys enclosed in a frame are called a block. A series of pulleys with attached ropes or chains is known as a block and tackle.

Electrical motor

V-belt V-pulley

Figure 8.8: Pulley drive installation

Pulley surfaces should be smooth and clean without any rough spots or build-up of material. The size of the pulley depends on the grade of belting, the number of plies and the speed. In crowned pulleys the belt moves to the largest diameter of the pulley. A crowned pulley uses this tendency to centre the belt. The pulley has a greater diameter at the centre than at the rim.

DID YOU KNOW?

A pulley is also called a sheave.

Flanged pulleys are used to keep the belt on the dive where there is a combination of small pulleys at high speed, or when there are sudden starts under heavy loads. Idler pulleys have two main purposes: to increase the arc of contact on the driver pulley and to act as a belt take-up adjustment. Idler pulleys run on the slack side of the belt and should be as large as practical. Metallic fasteners or high spots in the belt will cause the idler to jump or bounce.

Figure 8.9: Driver and driven sheave alignment

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Belts Pulleys are normally belt-driven, and under high loads, slip generally occurs. Flat and V-belts transmit power by their grip on the pulley or sheave. Three factors that determine the potential of the grip are: 1. the area of contact 2. the tension 3. the coefficient of friction.

Figure 8.10: Exact representation of the arc of contact

The area of contact is determined by: • the belt width • the arc of contact is the amount of belt wrap on the smaller pulley.

Figure 8.11: The arc of contact of two equal-diameter sheaves or an open drive

Pulleys of equal size are rarely used, thus, with pulleys of unequal diameter, the effective arc of contact is less than 180°.

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Figure 8.12: The arc of contact of two unequal-diameter sheaves

The arc of contact can be increased slightly by increasing the centreto-centre distance between the sheaves.

Figure 8.13: Unequal-diameter sheaves with arc of contact slightly increased

An arc of contact larger than 180° can be achieved by using: • a crossed drive • a drive with an idler pulley or sheave. A crossed drive is not generally suggested for use with V-belts. The centreto-centre distance of the crossed-belt drive assembly must be long enough to limit the internal stress in the belt.

Crossed-belt drive Figure 8.14: Crossed-belt drive and drive with an idler sheave

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Chains

Figure 8.15: The basic construction of a roller chain

A roller-chain drive combines the positive action of gear drives with the wide choice of shaft centres of a belt drive. Finished steel roller-chains are efficient because there is no slippage, no initial tension and chains may travel in either direction. The shorter pitch allows for higher operating speeds of a roller chain. Increased power capacity can be achieved by using multiple chains, which are essentially parallel single chains assembled on pins common to all strands. Sprocket wheels with fewer than 16 teeth may be used for relatively slow speeds, but 18 to 24 teeth on a socket are needed for high-service speeds. Sprockets with fewer than 25 teeth, running at speeds of above 500 – 600 revolutions per minute, should be heat-treated to give a tough wear-resistant surface which, when tested, should have readings between 35 and 45 on the Rockwell C hardness scale. It is usually better to make the desired reduction in two or more steps. Idler sprockets may be used on either side of the standard roller-chain to take up slack, guide the chain around any obstructions, change the direction of rotation of a drive shaft and provide more wrap around one another. 218

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Advantage of roller chains The efficiency of roller chains is influenced by the following factors: • the speed of the chain • the ratio between the sizes of the sprockets • the centre distance between the shafts • the efficiency of the lubrication • the type of bearing used on the shafts • the nature of the loading • the ratio between the magnitude of the load and the size of the chain.

Figure 8.16: Connecting links: A – cotter type; B – spring type

Assessment

Answer the following questions in your workbook. 1. Give another name for the bevel gear. 2. Explain the term ‘meshing of gears’. 3. Compare the advantages of spur gears with those of helical gears. 4. Describe the design of the herringbone gear, and explain the advantage of this type of gear design. 5. Where would you use the following gears in the workshop: (Hint: think specifically of the machines in your workshop.) a. worm and worm gear? b. rack and pinion? 6. What are the main functions of idler sheaves? 7. How do sheave sizes influence the arc of contact? 8. Sketch and illustrate the components of a roller-chain.

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Cables

Figure 8.17: Cables

Wire rope is used for so many purposes today that it is not practical to specify every type of rope. Because of this widespread use, it is unlikely that one type of rope (cable), varying only in size and strength, will meet all requirements. To determine the breaking strength and safe working load for a wire rope, it is essential to know the construction of the rope. The most commonly used wire rope is 6 – 19 and 6 – 24 plow steel in ordinary lay. Galvanised wire rope is used under harsh environmental conditions, e.g. near saltwater or in chemical plants and pulp mills. Lang lay wire rope is mostly used on shovels and drag lines. Under ordinary conditions fibre core is used, but where additional strength or resistance to compression is required, independent wire rope core is chosen. On most motor vehicles, the handbrake activates only on the rear brakes. It is a lawful requirement that the hand control should work the brake mechanically (by links, cables or rods) if the pedal operates them hydraulically. The handbrake lever may pull on a single cable, which is coupled to a pivoted T-piece to transmit the pull identically or evenly to both rear brakes, or there may be two cables from the handbrake lever, one to each of the rear brakes. When disc brakes are used on the rear wheels, there are sometimes two pairs of brake pads straddling the disc, with one pair operated hydraulically from the pedal and the other pair cam-operated by the handbrake cables.

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Figure 8.18: Cables and linkages used to activate the handbrake of a vehicle

Pivot T-piece transmits pull to both rear brakes

Two cables: The handbrake lever operates two cables, one to each rear brake

Figure 8.19: Some vehicles have two independent handbrake cables and others have one in conjunction with a linkage

Assessment

This is an engineering activity. Research and design a block and tackle system (ordinary rope, not steel wire) to lift twice your body weight. Sketch your design and label your sketch.

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Threads The development of the screw thread goes back many centuries. The concept of the screw thread is linked to Archimedes (287 – 212 BC), the greatest mathematician and inventor of the ancient world. In the sixteenth century, screws were used in German watches and armour. In 1841, Sir Joseph Whitworth started appealing in England for a standard screw thread. He developed the Whitworth thread. In 1864, William Sellars, an American, developed a thread that was adopted in the United States of America. However, the Sellars bolt would not fit the Whitworth nut as the thread angles were different. interchangeability ability to replace a similar part which is manufactured to the same specifications

During World War 1, this proved to be a serious inconvenience. During World War 2, an agreement was reached between the Americans, British and Canadians regarding the unification of the American and British screw threads. The result was a standardised screw thread called the Unified screw thread. This allowed complete interchangeability of screw threads in the three above-mentioned countries. Screw threads are fundamental to industrial progress. They are used for hundreds of different functions. The following are the basic applications: • to hold parts together • to transmit motion • to transmit power • to adjust parts with reference to one another. Threads that are cut on the outside of a cylinder are called external screw threads, e.g. on bolts, and threads that are cut on the inside of cylinders are called internal threads, e.g. nuts. External screw threads may be manufactured by cutting. This cutting can be done by hand with a stock and die. The thread can also be cut on a centre lathe or a screw-cutting machine. It may also be formed on a milling machine, rolling between dies, die casting and grinding.

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Screw thread terminology

Figure 8.20: Screw thread terminology

• The major diameter is the largest or outside diameter. • The minor diameter is the smaller diameter measured at the root of the screw thread. • The pitch diameter (simple effective diameter) is the diameter of an imaginary cylinder that would pass through the threads at such points as to make equal the width of the threads and the width of the spaces between the threads. The pitch diameter is equal to the major diameter minus the single diameter. • The pitch (P) is the distance from a point on a screw thread to a corresponding point on the next screw thread, measured parallel to the axis of the screw thread. A thread pitch gauge is used to measure the pitch. • The lead (L) is the distance a screw thread advances axially in one turn. On a single-start screw thread the lead and the pitch are the same. On a double-start screw thread, the lead is twice the distance of the pitch. • The crest is the top surface where the adjacent sides or flanks of the thread join. • The root is the bottom surface where the adjacent sides or flanks of the thread join. • The sides or flanks are the surfaces of the screw thread connecting the root and the crest. • The axis of the screw thread is the imaginary centre line through the screw thread lengthwise. • The depth of the screw thread is the distance between the crest and the root of the screw thread, measured perpendicularly to the axis.

DID YOU KNOW?

Lead = pitch × number of starts of the screw thread. A single-start screw thread is composed of one ridge; multiplescrew threads are composed of two or more ridges running side by side.

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Mechanical Technology • The thread angle is the included angle between the sides of the thread measured in a plane through the axis

Screw thread forms or profiles The most commonly used screw thread forms are the V-shaped screw threads used in construction and engineering work where parts are assembled with screws or with nuts screwed onto studs or bolts. Multiple screw threads are used where quick motion rather than great strength is important. They are used on valve stems, fountain pens, and tooth paste caps. A right-hand screw thread requires a bolt or nut to be turned clockwise or to the right to tighten it. A left-hand screw thread requires a bolt or nut to be turned anticlockwise or to the left to tighten it.

ISO metric screw thread The basic form of the ISO metric screw thread has a nominal diameter ranging from 1,0 mm upwards in two series, ‘fine’ and ‘coarse’. The included angle of this screw thread is 60˚. The designation of these threads must be understood. The letter ‘M’ is followed by the nominal diameter and the pitch, both in millimetres. For example, M6 × 0, 75 indicates a nominal diameter of 6 mm and a pitch of 0,75 mm (fine series), while M6 × 1 indicates a nominal diameter of 6 mm and a pitch of 1 mm (coarse series).

debur remove the ragged edges

To find the proper tap drill size when using metric threads, subtract the pitch from the nominal diameter. Metric tap drills are needed for drilling tapping holes to receive metric screws. This thread form is mostly used for fasteners. It is necessary to make a slight flat on top of the thread in order to debur it. The greatest disadvantage of this screw thread is that it is easily damaged during handling.

Acme screw thread

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The Acme screw thread is classified as a motion and power-transmitting thread in machinery. The acme thread is often used instead of the square thread. The 29˚ included angle of an acme screw thread reduces friction. Although this screw thread is not a fastener, it is strong and capable of carrying heavy loads and is thus suitable for connecting mechanisms such as the half nuts of the centre lathe, machine jacks, brake screws and cross-slide screws.

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Square screw thread In this screw thread the opposite sides (flanks) of the ridge are square with each other and perpendicular to the centre axis of the threaded part. The depth of the screw thread is equal to the width of the spaces between the threads. It is expensive to manufacture. This thread is ideal for power transmission as well as where rapid sliding motion is required. The square screw thread is used for all types of machines, e.g. valve spindles, on the tool heads and tables of shaping machines, as well as the feed spindles of the milling machine, planning machines and slotting machines.

Buttress screw thread The buttress screw thread is designed for use in vices, jacks and presses where pressure is applied in one direction only. The front faces which resist the heavy axial loads are perpendicular to the axis of the screw thread. The included angle of the screw thread is 45º

Figure 8.21: Standard screw thread profiles

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Screw thread micrometer The screw thread micrometer is specifically designed to measure the pitch diameter of a screw thread. The anvil and spindle tips are shaped to match the form of the thread to be measured. The screw thread pitch gauge is used to measure the pitch of a screw thread.

Figure 8.22: A screw thread micrometer

Figure 8.23: A thread pitch gauge

Linkages DID YOU KNOW?

A linkage is a mechanism made by connecting levers together.

A link is the most common machine element. A link is a stiff bar that transmits force and velocity. An assemblage of links that produces a prescribed motion is called a linkage. To connect the levers together you can use any type of fastening which allows free movement, e.g. pins, screws and pop rivets. The linkage can be made to change the direction of the force, or make two or more things move at the same time, e.g. lazy tongs rivet gun, braai tongs, an umbrella, a kitchen dirt bin (foot-pedal type), a washing line and safety doors. A linkage that makes things move in opposite directions is called a reverse motion linkage. A linkage that makes the output move in the same direction as the input is called a push-pull linkage. Linkages are designed to: • change the directions of a force or motion. • cause two parts to move at once. • make objects move identically to each other.

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Figure 8.24: Different types of linkages

Assessment

1. Indicate the included angles of the following screw threads by copying and completing the table: Screw thread

Included angle

Use

Acme Square ISO metric V Buttress 2. Define the term ‘interchangeability of parts’. 3. Give three uses of screw threads. 4. Mention seven (7) ways to manufacture screw threads. 5. Sketch a V-metric screw thread and insert all screw thread terminologies. 6. Discuss the purpose of link design.

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Wheels and axles

Figure 8.25: A wheel and axle

Life without wheels and axles is unimaginable. The Sumerians, who lived in Mesopotamia (now known as Iraq), may have made their pottery using wheels as early as 3250 BC. Before that, people used wooden sleds to drag heavy loads along the ground. Still today, in remote rural areas in South Africa, some people move drums of water and firewood in this way. The wheel-and-axle is a wheel or crank, rigidly attached to an axle. The wheel-and-axle is a kind of lever. Round, disc-like objects such as wheels work by decreasing frictional force that keeps the objects back when they move over a plane. The axle is the pivot or fulcrum. A small effort pushing round a big wheel will raise a large weight. The control and adjustment devices on drill presses, centre lathes, milling machines and hand drills are examples of the wheel-and-axle operations.

Effort applied by the handwheel

Figure 8.26: A simple machine (wheel-and-axle)

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Basic velocity calculations Belt drives Power is generally transmitted from one pulley to the other by means of belts. These belts can be flat, V-shaped or wedge-shaped. When a belt is in use, one side is slack and the other tight. The slack side is not totally loose. There has to be a certain degree of tension in it or the belt will slip around, which will prevent it from driving the pulley. Power is transmitted by belts due to frictional resistance between the belts and pulleys. Belt slip is one of the main reasons for loss of power and occurs when this resistance is low. The advantage of belt drives is the absence of noise (as with chain drives and gear drives). Belts do not transmit shock. They tend to slip under sudden loads, thus preventing major damage to the system and saving the expense of fixing machines. All belt drives should be kept clean and dry, and free from dust, dirt, grease and oil. A belt pulley thus has two forces acting on it: • the tension T(A) on the tight side • the tension T(B) on the slack side. The effective tension turning the pulley is, for that reason (T(A) − T(B)). You are probably familiar with the formula: work done = force × distance moved Therefore, work done by belt = effective tension × distance moved per second Thus: work done/s = (T(A) − T(B)) × (π × D × N) m/s power = work done per second Therefore, power = Newton metres/second or watt or kilowatt Where: (T(A) − T(B)) = effective tension in the belt in Newton D = diameter of the pulley in metres (m) N = number of revolutions per second turned by the pulley π = pi = 22 7

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Mechanical Technology Ratio of tensions between T(A) and- T(B) The ratio of the two belt tensions is usually expressed as follows: T(A) : T(B) = 2 : 1 or T(A) 2 = or T(A) = 2 T(B) T(B) 1 Example 1 The effective diameter of a driving pulley is 200 mm. It rotates at 800 rpm. The tension in the tight side of the belt is 400 N and the ratio of tensions is 2 : 1. Calculate the power transmission. Solution Find the effective tension in the belt. You are given that: T(A) = 2 and T(A) = 400 N T(B) Thus T(A) = 2 T(B), or T(B) = T(A) 2 T(B) = 400 = 200 N 2 Effective tension in belt = T(A) − T(B) Therefore, effective tension in belt = 400 – 200 = 200 N Now find distance moved per second. Distance moved =π×D×N s Distance moved 200 800 =π× × s 1 000 60 Distance moved 8, 38 m/s = s Power transmitted by belt Power transmitted = work done per second Power = force in Newton × distance moved in m/s Power = (T(A) − T(B)) × (π × D × N) Power transmitted = 200 × 8, 38 kW 1 000 = 1,676 kW

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Rotational speeds of belt and gear drives Belt drives Circumferential speed of driving shaft (mm/min) = circumferential speed of driven shaft (mm/min) π × DA × NA = π × DB × NB where DA = diameter of pulley A in mm NA = revolutions per minute of pulley A DB = diameter of pulley B in mm NB = revolutions per minute of pulley B

A B

Figure 8.27: A belt drive

Example 2 A shaft revolving at 700 rpm has a 250 mm diameter pulley which drives a 120 mm diameter pulley on a second shaft by means of a driving belt. Calculate the speed of the driven shaft in rpm. (Simple drive) Solution Circumferential speed of driving shaft (mm/min) = Circumferential speed of driven shaft (mm/min) π × DA × NA = π × DB × NB π × 250 × 700 = π × 120 × NB NB = π × 250 × 700 π × 120 = 1 458,3 rpm

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Assessment

1. 2.

An electric motor rotates at a speed of 1 200 rpm, and drives a 1 500 mm diameter pulley at the speed of 250 rpm on a machine spindle by means of a driving belt. What is the size of the pulley mounted on the motor spindle? (Hint: This is a simple drive.) A flat-belt drive consists of a 200 mm diameter driving pulley and a 100 mm diameter driven pulley. Determine the speed of the driving pulley if the driven pulley rotates at 750 rpm.

Gears Gears are used to transmit power and motion from one shaft to the other. As can be seen at the beginning of the chapter, gears come in all shapes and sizes. They need very little maintenance and power is transferred directly. They are also used where slip is not allowed.

Gear A is called the driver and the gear B, driven by gear A, is called the driven gear. The gear teeth of gear A and gear B are engaged or meshed and thus ensures that no slip will take place. If 12 teeth of gear A (driver) pass a certain point on the horizontal centre line, then 12 teeth of gear B (driven) must also pass that point on the horizontal centre line, as the gear teeth of the driver drive the gear teeth of the driven gear. This is done because both gears have the same module. Both driver and driven move at the same circumferential speed; THEREFORE, if a marked spot on the driver tooth moves 7 m/s, a spot on the driven gear also moves 7 m/s.

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Rotational speeds of belt and gear drives Gear drives (Circumferential speed of driving shaft (mm/min) = circumferential speed of driven shaft (mm/min) TA × NA = TB × NB where TA = number of teeth on gear A NA = rotational frequency (revolutions) of gear A TB = number of teeth on gear B NB = rotational frequency (revolutions) of gear B If gear A and gear B are only two disc, the following formula applies: DA × NA = DB × NB where DA = diameter of gear A NA = rotational frequency (revolutions) of gear A DB = diameter of gear B NB = rotational frequency (revolutions) of gear B If gear A rotates clockwise, then gear B will rotates in the opposite direction (anticlockwise) at the same speed. If the need arises for the driver and driven gears to rotate in the same direction, then an idler gear (intermediate gear) can be added as can be seen below.

If more than two gears are used, the formula can be expanded as follows: TA × NA = TB × NB = Tc × Nc The formula above can be extended to: Revolutions of final driven gear = Number of teeth on all the drivers Revolutions of first driver gear Number of teeth on all the driven

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Example Below a gear train is shown. Calculate the rotational frequency (speed) of the driven shaft.

Solution: Revolutions of Product of number of teeth on all final driven gear = the drivers Revolutions of first driver gear Product of number of teeth on all the drivers T ND TA × C = TD NA TB T T ND = A × C × NA TB TD

25 40 ND = × 15 × 75 67

Rotational frequency of driven shaft = 2,985 rev/sec Say = 3 rev/sec

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Clutches A clutch is a device in which two shafts or rotating members may be connected or disconnected, either while at rest or when in relative motion. When clutches are used to connect two shafts, care must be taken to minimize misalignment. Clutch life can be extended and maintenance time reduced by having parallel and concentric contact surfaces. In an expanding or contracting ring type, the degree of concentricity, or internal clearance between the ring and the drum, will affect the balance of the operating parts. As with couplings, the closer to perfect alignment, the longer the service life. Clutches can be divided into three types, according to the means of power transmission: • mechanical – positive contact; friction type; over-running • hydraulic • electrical.

concentric circles having the same centre points

Clutches can be further grouped according to their shaft position: • all parts on the same shaft • connecting different shafts, or used as a coupling.

Figure 8.28: A mechanical clutch

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Multiple-plate clutches The greatest advantage of this type of clutch is that the overall diameter is materially reduced owing to the fact that the power transmitted is spread over a number of friction faces. The clutch consists of a number discs or rings which are keyed alternately to the driven and driving shafts. The contact surfaces are brought into or out of engagement by means of a system of toggle levers operated by a sliding shifter.

General Some disc-type clutches work in an oil bath. Disc clutches can also be automatically operated, by means of air or hydraulic cylinders, and have the following advantages: • less friction and wear • smooth engagement • lower working temperature • reduced operating cost • less shock when engaged • can transmit power at high speed • can be engaged when the machine is in motion • act as a safety device in case of overloading.

Levers Levers are the simplest (and one of the original) mechanisms. A lever is a rigid bar that is free to turn about a fixed point called a fulcrum. It can be described as a mechanism designed to lift loads or create movement. When we row a boat, we use oars as levers. The fulcrum is the pivot point. The effort force is exerted upon one lever arm. It tends to rotate the lever in one direction. The resistance force is exerted upon the other lever arm and tends to rotate the lever in the opposite direction. Levers are categorised by the placement of the fulcrum (pivot) relative to the effort and the load. In grade 10 the three levers were discussed very basically. In grade 11 the levers are discussed again, but now more industry-related examples will be used. 236

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All levers consist of a firm and unyielding arm or beam, which need not to be straight and may also have a range of shapes (examples are the accelerator, brake pedal or handbrake on a motor vehicle). The arm or beam is free to turn about a fixed pivot or point called a fulcrum or pivot point. The amount of movement about the fulcrum may also differ, in some cases the lever can revolve through 360° whereas other levers only cover small distances. Two forces (sometimes opposing forces) act in on the firm beam, and are called the effort and the load.

Types of levers Levers (first-class)

Figure 8.29: First-class lever

As can be seen, the pivot is in the middle, between the effort and the load. The bigger the distance between the pivot/fulcrum and the load, the greater the force that is exerted on the load, e.g. scissors, claw hammers, see-saws, crow bars, pliers, tin snips and pry bars. Most tools in the metal workshop use first-class levers. The tin snips, for instance, will cut material more easily, because you will have to use less force on the handles if the material is placed as deeply as possible inside the jaws and you apply effort as near the ends of the handles as possible.

Figure 8.30: Levers at work in using straight tin snips

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Figure 8.31: Levers at work in a clutch assembly

Levers (second-class)

Figure 8.32: Second-class lever

The load is in the middle, with the pivot/fulcrum and effort on either side. The nearer the load is to the pivot, the less effort is required to lift the load. The bench vice, nutcrackers wheelbarrows, bottle openers, and screw driver are examples of this type of lever.

Figure 8.33: Levers at work in a bench vice

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Levers (third-class)

Figure 8.34: Third-class lever

The effort is situated between the pivot/fulcrum and the load. This class of lever differs from the previous types because the effort is always greater than the load. The one and only advantage of this lever is that the load is moved a greater distance than is the case regarding the prior types of levers. If the effort is moved closer to the pivot/fulcrum it will move the load further than it can move itself. However, the force of the effort will have to be increased the closer it is moved to the pivot/fulcrum. Shovels, hand vice, tweezers, sugar tongs, fishing rod and staple remover are examples of third-class levers.

Figure 8.35: Levers at work in clamping a workpiece on a machine table

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Cams

Figure 8.36: A disc cam and follower

oscillating moving back and forth

DID YOU KNOW?

A cam is an irregular projection on a wheel or rotating shaft, shaped to transmit regular movement to another part in contact with it.

A cam is a plate, cylinder, or any solid material which has a curved outline or curved groove that, by its oscillating motion, gives a predetermined motion to another piece, called the follower, which is in contact with it. Cam mechanisms are commonly used to operate valves in motor vehicles, and stationary and marine internal combustion engines. They are also used in clocks, locks, printing machinery and nearly any kind of machinery that we generally regard as automatic machines. All cam mechanisms consist of at least three parts: • the cam, which has a contact surface (either curved or straight) • the follower, the motion of which is produced by contact with the cam surface • the frame, which supports the cam and guides the follower.

Eccentric action of the cam Eccentric action is when a rotating member whose axis of rotation is different or offset from the primary axis of the part or mechanism. Thus, when one turned section of a shaft centres on a different axis than the shaft, it is said to be eccentric or to have “run out”. For example, the throws or cranks on an engine crankshaft are eccentric to the main bearing axis. An example of this action in the automotive engineering sector is the camshaft driving method where the cam is used. The mechanical fuel pump on a motor vehicle obtains its oscillating action by running against an eccentric cam.

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Assessment

1. Identify and describe the three (3) types of levers by copying and completing the following table. Lever

Description

Application

2. Name the three parts of a cam.

Hydraulics/Pneumatics Earliest recorded history shows that devices such as pumps and waterwheels were known in ancient times. However, it was not until the 17th century that hydraulics and pneumatics were first used. Hydraulics and pneumatics are based on the principle discovered by the French scientist and philosopher Blaise Pascal (1623 – 1662). They relate to using confined fluids and air to transmit power, multiply force and modify motions.

confined restricted to a container

Hydraulic systems are systems that can cause a force or movement by compressing fluid. Pneumatic systems are systems that can cause a force or movement by compressing air. They can be used as links between mechanical parts, thus eliminating the need for shafts, gears and connecting rods. Machine tools, motor vehicles, aircraft, jacks, missiles and ships are just a few applications of hydraulic and pneumatic systems.

Valves Valves are used in hydraulic systems to control the operations of the actuators. As soon as the pump places the fluid under pressure, the valves are required to control pressure, as well as to control and check the direction of flow. Until recently, valves were the only method to control flow direction and pressure. Lately, pumps have been developed that can control flow and direction.

An actuator is part of a hydraulic or pneumatic system that causes movements

Hydraulic valves can be classified in the following three general categories: • flow-control valves • pressure-relief valves • directional-control valves.

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Mechanical Technology Pneumatic valves can also be classified in three general categories: • check valves • drain cock valves • pressure-reducing valves. The functions of valves include: • regulating pressure in the circuit • directing the hydraulic fluid and compressed air into lines or into specific directions • determining the amount of fluid and compressed air that will flow in different parts of the circuit.

Hydraulic valves Non-return valves (flow direction) These valves are mostly used in small, piston-type pumps and in small oil pipelines to provide one-directional oil flow. Most non-return valves are spring loaded and their operation is based on a balance between pressure and spring force. They open as soon as the pressure in the system is larger than the spring tension. One of their main functions is to act as pressure relief valves, and they are installed between the hydraulic pump and the fluid reservoir to avoid the extreme build-up of fluid pressure in the system.

The spring is compressed and free flow is allowed as the ball unseats

Flow is blocked as the valve seats

Figure 8.37: A non-return valve

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Rotary valves Rotary valves are used to control the direction of flow of fluid. These valves are normally used as pilot valves to control movement of spool-type valves. The supplementary ports and passages in the turning port of the valve let the liquid flow into various lines by means of controlling one valve, e.g. in the hydraulically controlled surface grinder, the forward and backward movement as well as the sideward movement of the table is controlled by one rotary valve.

Figure 8.38: A rotary valve

Spool valves This type of valve is mainly used, as a result of its fast and positive reaction, to control the direction of flow of fluid. Spool valves are versatile and can control fluid flow in a variety of parts of the hydraulic system. The contact area of spool valves must be extremely accurately machined, lapped, and fitted to prevent any leakages and to provide an efficient operation. Oil grooves are often machined around the pistons so that the spool is centralised and lubricated. This type of valve can be controlled mechanically, electronically and hydraulically.

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Figure 8.39: A spool valve

Pneumatic valves Check valves

DID YOU KNOW?

A one-way valve allows air to flow only in one direction.

The check valve is a one-way (non-return) valve. This valve is situated at the inlet of the air receiver. The check valve allows compressed air to flow from the compressor into the air receiver and prevents it from leaking back when the compressor is stopped

Drain valve (drain cock) When this valve is opened, all the water condensing in the air receiver is drained from it. It is recommended that this valve be opened at the start of each workday to get rid of the water in the air receiver. If this is not done, the water in the air receiver will be transported through the whole system by the compressed air. This can influence the system negatively, causing a water hammer and corrosion in the pipes or components.

Pressure-reducing valves DID YOU KNOW?

Pressure-reducing valves are also called safety valves

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A safety valve is mounted on the air receiver and is set at a predetermined pressure so that ‘blow down’ will occur long before the air receiver explodes. Blow down is when the safety valve permits compressed air to be released into the atmosphere if the pressure in the air receiver should rise above a safe level. The purpose of a safety valve is to: • protect the system against failure. • prevent exceeding of the maximum system pressure limit.

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Pipes Piping is a common term which includes the various kinds of conducting lines that carry hydraulic fluid between components. It also includes the fittings or connectors used between the conductors. Hydraulic systems nowadays use three types of conducting lines: steel pipes, steel tubing and flexible hose. Presently, pipes are relative economical, but tubing and hoses are more convenient for making connections and in servicing plumbing. It is likely that plastic plumbing will become more popular. Iron and steel pipes were first used in industrial hydraulic systems and are still widely used because of their low cost. Seamless steel pipes are recommended for hydraulic systems with the inside of the pipes free of rust, scale and dirt. Seamless tubing offers significant advantages over pipes for hydraulic plumbing. Tubing can be bent into any shape, is easier to work with and can be used over and over without any sealing problems. In low-volume systems, tubing will handle higher pressure and flow with less bulk and weight. It is, however, more expensive, and so are the fittings required to make tube connections. Flexible hose should be installed so that there is no kinking during operation. Some slack should always be present to relieve strain and permit absorption of pressure surges. Twists in the hose and unusually long loops are undesirable. Clamps may be required to avoid chafing or tangling between moving parts. Hose subjected to rubbing should be encased in a protective sleeve or guard. Pipes for pneumatic systems are generally made from aluminium, copper, stainless steel or steel. The piping acts as air conductors which carry compressed air. It may be necessary to direct the compressed air from the compressor to the actuator and also to specific points of application around the plant.

The operation of the hydraulic clutch The hydraulic clutch can be used to reduce the amount of pedal pressure needed. When the driver pushes down on the clutch pedal, a push rod is forced into a master cylinder. As the push rod moves down into the master cylinder, the rod forces a piston down into the cylinder. This action puts pressure on the hydraulic fluid in the cylinder, and some of the fluid is forced out. The fluid flows through a tube or pipe into a servo cylinder at the clutch. 245

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The servo also has a piston. The fluid flowing into the servo cylinder from the master cylinder, forces the piston in the servo cylinder to move. This movement is carried through a push rod to the release lever, thus releasing the clutch.

Figure 8.40: Hydraulically operated coil-spring clutch

DID YOU KNOW?

A burr is a thin edge of metal usually very sharp, left from a cutting or machining operation. The process to remove the sharp edge or corner caused by the cutting or machining operation is called DEBURR.

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The preparation of pipes, tubes and fittings before installation in a hydraulic system It does not matter whether the hydraulic installation is used in an industrial plant (factory), domestic, commercial or motor vehicle, there are general requirements the system must fulfill. When installing the various iron and steel pipes, tubes and fittings of the hydraulic system, it is necessary that they be absolutely clean, free from scale and all kinds of foreign matter. The tubing, pipes and fittings should be brushed with a boiler tube wire brush or cleaned with commercial pipe cleaning apparatus. The inside edge of the pipe, tube or fittings should be reamed after cutting to remove the burrs.

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Pressure gauges Pressure gauges indicate the amount of pressure in the hydraulic or pneumatic system, and are required by law as they are an integral part of a hydraulic or pneumatic system. Pressure gauges are needed for adjusting pressure control valves to required values and for determining the force being exerted by the cylinder or torque of a hydraulic motor. Two main types of pressure gauges are the Bourdon tube and the Schrader types. In a Bourdon tube gauge (named after the French inventor Eugène Bourdon) a sealed tube is formed in an arc or bent into the shape of a hook. When pressure is applied at the port opening, the tube tends to straighten. This actuates the linkage to the pointer gear and moves the pointer to indicate the pressure on the dial. In a Schrader gauge, the pressure is applied to a spring-loaded sleeve and piston. When the pressure moves the sleeve, it actuates the gauge needle through linkage. Gauges used for recording fast, unpredictable pressures generally employ electrostatic-sensing elements that can give an immediate reading. Pressure gauges indicate to a worker the amount of pressure in a system as well as sudden drops or rises in pressure in the system. It gives the worker ample time to take corrective or preventative action, e.g. start with fault-finding or shutting down the system. Most pressure gauges read zero at atmospheric pressure and are calibrated in kilograms per square centimetre, ignoring atmospheric pressure throughout their range. Pressure gauges in pneumatic systems indicate the pressure in the air receiver. The pressure gauge shows a blockage, leakage or damage in the system. This gauge verifies a safe and satisfactory air pressure.

verifies used to check or confirm

Figure 8.41: Pressure gauges

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Pistons Pistons are an integral part of hydraulic pumps. All piston pumps operate on the principle that a reciprocating piston in a bore will draw in fluid as it is pulled back (or retracted) and expel it on the forward stroke. Pistons can be single-acting or double-acting. The two basic designs of pistons are radial and axial. They are available as fixed or variable displacement models. A radial pump has the pistons arranged radially in a cylinder block, while in the axial units the pistons are parallel to each other and to the cylinder block. The latter may be further divided into in-line (swash plate or wobble plate) and bent-axis types.

Radial piston pumps In a radial pump the cylinder block rotates on a stationary swivel spindle and inside a circular reaction ring or rotor. As the block rotates, centrifugal force, charging pressure or some form of mechanical action causes the pistons to follow the inner surface of the ring which is off-set from the centre line of the cylinder block. As the pistons reciprocate in their bores, porting in the swivel spindle permits them to take in fluid as they move outward and discharge the fluid as they move in. The size, number of pistons and the length of the stroke determine pump displacement (delivery rate).

Figure 8.42: A radial piston pump

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Axial piston pumps In axial piston pumps, the cylinder block and drive shaft are on the same centre line and the pistons reciprocate parallel to the drive shaft. The simplest type of axial piston pump is the swash in-line design shown in the Figure 8.43. (An example of the bent-axis piston pump is shown in the Grade 10 Learner’s Book.) The cylinder block in this pump is turned by the drive shaft. The pistons fitted to the bores in the cylinders are connected through piston shoes and a retracting ring, so that the shoes bear against an angled swash plate.

swash plate a stationary canted (sloped) plate in an axial-type piston pump which causes the pistons to reciprocate as the cylinder barrel rotates.

As the block turns, the piston shoes follow the swash plate, causing the pistons to reciprocate. The ports are arranged in the valve plate so that the pistons pass the inlet as they are being pulled out, and pass the outlet as they are being forced back in. Piston pumps are extremely efficient units available in a wide range of capacities, from very small to high. Being variable and reversible, they are well-suited to large press applications and hydrostatics. Because of their closely fitted parts and finely machined and lapped surfaces, cleanliness and good quality, hydraulic fluids are essential for long service life.

Figure 8.43: A swash plate piston pump (axial)

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Reservoirs The storage space for the fluid is the oil reservoir. Hydraulic fluid is kept clean by strainers, filters and magnetic plugs to the degree required by the conditions. The reservoir, also called the sump or tank, should be deep and narrow instead of shallow and wide. It is used to store hydraulic fluid until it is required by the system. The reservoir also should provide a place for the air to separate out of the fluid and should permit contaminants to settle out as well. An added advantage of a well-designed reservoir is that it will help dispel any heat that is generated in the system. In the case of a solid tank, it helps to decrease the noise level. It is vital that every hydraulic system should have its own oil reservoir. The main functions of the reservoir are the following: • provides a fluid storage tank • promotes air separation from the fluid • supports the pump and electric motor • promotes heat dispersion • acts as a base plate for the mounting of control equipment. A reservoir must have the following components: • sight glass – fluid levels must be continuously checked, using the sight glass; maximum and minimum fluid levels must be clearly indicated • return line – the return line returns the oil to the return chamber of the reservoir • suction line – the suction strainer is connected to the suction line; the suction line connects the suction chamber to the inlet of the pump • strainer – its purpose is to prevent contamination from entering the hydraulic system • removable cover – generally two of them, one for the suction chamber, the other for the return chamber; makes cleaning the reservoir easy • baffle plate – divides the reservoir into two compartments (suction and return chambers); in the return chamber the fluid is allowed to settle, deposit any contamination, disperse air trapped in the fluid and disperse heat, before moving into the suction chamber • drain plug – enables the maintainer to get the hydraulic fluid out of the reservoir during fluid changes as cleanly as possibly; the drain plug is always fitted at the lowest point of the reservoir • magnetic plugs – optional in reservoirs; their purpose is to trap iron, steel or any magnetic matter in the fluid 250

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• filter/breather – plays an important role in keeping dirt out of the fluid; it consists of a mesh strainer to retain any foreign matter during filling of the reservoir, and a cap with a micron-sized element which filters the air drawn into the reservoir.

Figure 8.44: Diagram of hydraulic reservoir in a master cylinder

Figure 8.45: A hydraulic reservoir

In pneumatic systems the storage tank for compressed air is called an air receiver. It is important to remember that this air receiver is a vessel under pressure and should be treated with care. Air receivers should always be kept outside the workshop on a level, concrete surface. An air receiver stores compressed air for further use and should be fitted with the following: • a drain valve (drain cock) fitted at the lowest point at the bottom of the air receiver, to drain condensate at daily intervals (see ‘drain cock’ under ‘valves’) • a safety valve to release pressure when a build-up of pressure or dangerously high levels of pressure occur • a pressure valve to indicate operating pressure in the system.

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Hydraulic calculations The volume of liquid displaced by the ram (large piston) is equal to the liquid displaced at the plunger (small piston). Volume = Area of ram × height of displacement (h) = Area of plunger × height of displacement (H) Volume = π D2 h π d2 H = 4 4 Pressure on ram = Pressure on plunger Pressure (P) = Large force (F) Small force (f) = Large area (A) Small area (a)

P =

F A

=

f a

Example 1 Suppose a piston acts on the liquid with a force of 9 000 N and the area of the piston is 5 m2. Find the pressure in the liquid. Force Pressure = Area Pressure = 1 800 Nm-2 Pressure = 1 800 Pa

252

Pressure =

9 000 5

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Example 2 Calculate the diameter of plunger B in the sketch below.

Solution: Force = force area Area = 5000 N = 500 N 200 mm x – (unknown diameter) x = 500 N × 200 mm 5000 N

REMEMBER: The SI unit for force is Newton (N). To convert the downward force of 50 kg (mass) to Newton, you have to multiply it with the gravitational acceleration (g) = 9, 81 m/s2. For this course you may use 10. Mass × g = 500 kg × 10 = 5 000 N

x = 100 000 N/mm 5000 N x = 20 mm

Example 3 The pressure in the hydraulic fluid in a cylinder is 2, 4 MPa and the cylinder diameter is 320 mm. Calculate the force exerted by the piston. Pressure = Force Area Force = Pressure × Area Force

= 2,4 x106 × 0,080424771

Force

= 193019,4504 N

Force

= 193,0194504 kN

Area = πD2 4 Area =

π × 0,322 4

= 0,080424771 m2

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Example 4 The ram of a hydraulic press can produce a force of 15 kN when a force of 250 N is applied to the plunger. The diameter of the plunger is 80 mm. Calculate the pressure in the hydraulic fluid. Solution: Force = force area Area Pressure = Force Area Pressure

Pressure

=

250 N 0,00502655

Area = πD2 4 Area = π × 0,802 4 = 0,00502655 m2 = 5026,55 mm2

= 49735,9 Pa

Example 5 A force of 300 N is exerted on the 50 mm diameter plunger of a hydraulic motor vehicle jack. The diameter of the ram is 500 mm. Calculate the load that can be lifted by the hydraulic jack. Solution: Liquid area of plunger Area

= πD2 4

Area = π × 502 π × 50 × 50 4 × 106 OR Area = 4 × 106 Area

= 0,0019635 m2

Pressure in Jack Pressure =

Force Liquid Area

Pressure =

300 0,0019635

Pressure = 152788,3881 Pa

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Liquid area of ram Area = π × 5002 π × 500 × 500 4 × 106 OR Area = 4 × 106 Area

= 0,19635 m2

Load lifted by Jack Pressure = Force/Load Area Load

= Pressure × Liquid Area

Load

= 152788,3881 × 0,19635

Load = 30 000 N OR 30 kN

Mechanical brake system Brakes are friction clutches used to slow down turning or rotating speed by means of friction to slow down a revolving disc. Well-known to all of us are the brakes that are compulsory to slow or stop the motor vehicle. Basically all motor vehicles use brakes that operate by applying pressure on hydraulic fluid. This type of braking system is called a hydraulic braking system. Most motor vehicle brakes used today are hydraulically operated. Refer to the general hydraulic braking system discussion below. An example of mechanical brakes is the strap or band brake. This type of brake takes advantage of the high-tension force that is generated by leverage as the strap or band is pulled into the drum, e.g. the strap brake or tensioner on an exercise bicycle. These straps or bands can be made from rope, weaved nylon material or metal, depending of the application (where the strap brake is used). Friction increases as the pressure on the strap or band increases, resulting in a strong, frictional drag and a strong braking effect. By properly selecting the pivot point, strap or band brakes may by selflocking for one direction of rotation and free-running in the reverse (opposite) direction. These mechanical brakes are known as differential brakes and are used on hand-operated hoists to prevent accidental lowering 255

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Mechanical Technology of the load (built-in safety mechanism). General applications of strap or band brakes include winch brakes (as used on cable and hose drum winches on ships and by Eskom), transmission parking brakes (carriages, coaches and oxen wagons), and emergency stopping of press brakes.

Hydraulic brake systems A hydraulic system makes use of the fact that the fluid is not compressible. Pressure exerted on any place and on a fluid is spread evenly throughout the fluid. A piston and cylinder unit, controlled by a pedal, can be used to exert pressure at one end of the hydraulic line. The pressure in the fluid moves another piston at the other end of the line, which in turn applies brake power. If the second piston has a larger cross-sectional area than the first one, then the exerted force will be proportionally greater than the force exerted on the first cylinder. The distance travelled by the second piston will be smaller and in the same proportion as the force.

Figure 8.46: Layout of a hydraulic system

If, for example, the second piston has a cross-sectional area three times that of the first piston, the exerted force will be three times as great and the distance travelled 1 . 3

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In the majority of vehicles, most of the braking force is undertaken by the front wheels, because the weight is thrown forward when the brake is applied.

Pneumatics systems The term pneumatics is derived from the Greek word “pneumatiko” (“pneuma” means air or wind and pneo means breath). Pneumatics is the branch of Physics that studies the properties of gases. The pneumatic focus area involves compressors; cylinders (actuators), valves, conduit (piping), air receivers (storage tanks), filters for water tap, etc. Pneumatic systems (i.e. pressurised air or gas) are used to operate equipment such as automated assembly machines, packing machines and devices for clamping and lifting. Numerous tools used in industry today have been developed because of what man knows of the laws of pressure and the elasticity of air. Power created by pressurised air or gas (pneumatic power) is also used to drive tools requiring rotary motion and those requiring reciprocating motion, such as drills, nibblers, air grinders, rivet guns, impact wrenches, chipping hammers and chisels, to name but a few. Pneumatic tools are driven by air motors. These tools and machines can vary from the very small drilling machine used by a dentist to a huge jack hammer used to demolish concrete slabs or rocks on construction sites or road construction. A pneumatic conveyor is simply a high velocity airstream which is used to move material from one location to another. The material moved is light and bulky. The pneumatic conveyor is used as a delivery system only. The material is introduced into a moving airstream by means of a feeder. The feeder introduces a fixed amount of solids into the airstream and also acts as an air seal to maintain air pressure in the line. The air motors have many advantages: they are physically small and light for the high torque and power that they deliver and do not generate heat. They cannot be damaged by overloading. Advantages of pneumatic systems • The apparatus is relatively light and affordable. • Compressed air is stored relatively easily in high pressure cylinders. • Pneumatic apparatus is not sensitive to temperature between 0 °C and 80 °C. • Pneumatic motors or turbines can achieve very high speeds. 257

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Mechanical Technology Disadvantages of pneumatic systems • Pneumatic systems cannot function at the same high pressure at which hydraulics systems operate. • Pneumatic systems do not have the even action of hydraulics systems because gases are more likely to be compressed. • Compressed air as a source of energy is more expensive than hydraulics. • Air at high pressure can be very dangerous. • Air or gas, by their very nature, are elastic (i.e. it can be compressed) and this can cause erratic motion.

Electrical/electronic control Ignition system Construction of the ignition system The sketch below portrays the construction of the ignition system of a fourcylinder engine. The distributor contains breaker points, the rotor, breaker cam, and mechanical and vacuum advance mechanism.

Figure 8.47: Ignition system of a four-cylinder engine

DID YOU KNOW?

In a typical engine, the points may separate when the piston is 8 mm before top dead centre (TDC). 258

Ignition timing The sparks must reach the spark plugs in the cylinders at exactly the right time. The sparks must arrive a specific number of degrees before top dead centre (TDC) on the compression stroke. Adjusting the distributor to make the spark arrive at the right time is called ignition timing.

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You adjust the distributor by turning the distributor in its mounting. If you rotate the distributor in the direction opposite to the normal cam rotation, you move the contact points ahead. That is, the points will close and open earlier. This advances the spark, so the sparks appear at the spark plugs earlier. Turning the distributor in the direction of normal cam rotation retards (slows down) the sparks. The sparks appear at the spark plugs later. To time the ignition, check the markings on the crankshaft pulley with the engine running. Since the pulley turns rapidly, you cannot see the markings in normal light, but by using a special timing light, you can make the pulley appear to stand still. A stroboscopic timing lamp is used for checking the ignition timing of an engine. If the mark on the rapidly rotating wheel is seen by the light of the lamp that flashes once for every rotation of the wheel, as said above, the mark will appear stationary. Battery

Battery

High voltage spark plug lead

Ignition switch

Spark plug Ignition coil

Simplified diagram of the primary circuit of an ignition system

Ignition coil Distributor

Simplified diagram of the secondary circuit of an ignition system

Figure 8.48: A simple ignition system

Ignition timing operational practice As mentioned previously, it is the function of the distributor to deliver a high voltage spark to each spark plug at the right instance. The distributor can only perform this function correctly if the initial ignition timing was set correctly. The ignition timing is normally set on the number one cylinder according to the manufacturer’s specification. With the spark plug leads connected to the distributor cap according to the firing order of the engine, the ignition timing of all the other cylinders will be correct, owing to the equal angles of the breaker-cam lobes. The ignition timing may be adjusted statically by means of a test lamp or neon timing light while the engine is running.

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Mechanical Technology Static ignition timing The sketch below portrays the test lamp connected with the primary circuit in parallel. The following procedure should be followed for the correct adjustment of static ignition timing. With the ignition switch off, turn the crankshaft until the number one piston is at the end of the compression stroke. Bring the timing marks on the crank pulley of the fly wheel exactly in line, according to the manufacturer’s prescription.

Figure 8.49: Test lamp for static ignition timing

Loosen the clamp bolt of the distributor and switch on the ignition. Turn the distributor casing in the opposite direction to the rotor direction until the test lamp comes on. The test lamp shows that the contact points are beginning to open, indicating the delivery of the high voltage spark. Tighten the clamp bolt while the distributor casing is held firmly in place. The number one spark plug lead is connected to the side terminal in the distributor cap, in line with the rotor. The other spark-plug leads are now connected to the other side terminals according to the firing order and direction of rotation of the rotor. Ignition timing by means of a neon timing light In this case the ignition timing is adjusted while the engine is idling. Connect the timing light to the high-voltage lead off the number one spark plug. If prescribed by the manufacturer, remove the vacuum line of the vacuum advance mechanism. Adjust the idling speed according to the specification and direct the beam of the timing light onto the timing marks of the engine. With the distributor clamp bolts loosened, turn the distributor casing until the timing marks correspond with the manufacturer’s specifications. Tighten the clamp bolt without moving the distributor housing. 260

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The transistor-ignition system with contact points In order to understand the construction and operation of the transistorignition system with contact points, brief reference is made to the operation of the conventional ignition system. The construction and operation of the conventional ignition system has been discussed.

Figure 8.50: Conventional ignition system

The sketch above portrays a schematic layout of a conventional ignition system as used in a single-cylinder engine. Briefly, the operation is as follows. With the ignition switch on and the contact points closed, the primary circuit is completed and current flows through the ignition switch, primary coil and contact points. An electromagnetic field builds up around both the primary and secondary coils of the ignition coil. When the contact points are opened by the breaker cam, the flow of current in the primary circuit is interrupted and the magnetic field around both the primary and secondary coils collapses. Both the primary and secondary coils cut through the collapsing magnetic power lines. The self-induction charge of the primary coil is absorbed by the capacitor and prevents arcing between the contact points. The discharge of the capacitor accelerates the demagnetisation of the ignition-coil core, and secondary induction is increased. As a result of the high voltage of the secondary induction charge, a high-voltage or high tension spark is delivered across the spark plug electrodes. It is clear from the above that the primary circuit is controlled by the contact points. Worn contact points cause resistance in the primary circuit, resulting in a lower secondary induction and a weak high-tension spark. It is clear that worn contact points will result in power loss and increased fuel consumption. 261

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Mechanical Technology A transistor is used to control the primary circuit electronically in order to eliminate the above-mentioned problems. The illustration below shows a schematic layout of a transistor-ignition system with contact points as used in a single-cylinder engine.

Figure 8.51: Transistor-ignition system with contact points

Construction The battery is connected to the emitter of the transistor through the ignition switch. The collector of the transistor is earthed through the primary coil in order to complete the primary circuit. The contact points are situated in the base circuit. The base circuit is earthed when the contact points are closed, while open contact points interrupt the base circuit. The secondary circuit or high-tension circuit is similar to that of the conventional system. Operation With the ignition switch on and the contact points closed, battery current flows through the ignition switch, emitter, base and the contact points to the earth. The small base circuit is now completed and the transistor is activated. The primary circuit from the battery, through the ignition switch, emitter, collector and primary coil, is switched on electronically. As in the case of the conventional system, an electromagnetic field builds up around both the primary and secondary coils of the ignition coil. The rotating breaker cam opens the contact points and the base circuit is interrupted. The transistor is no longer activated and the flow of current in the primary circuit through the emitter and collector is interrupted.

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The magnetic field around both the primary and secondary coils collapses and both the primary and secondary coils cut through the collapsing magnetic power lines. The self-induction charge of the primary coil flows to the earth, while the secondary induction charge delivers a high-tension spark across the spark plug electrodes. It is thus clear that the contact points are only used to interrupt the small base current. Because the contact points are not exposed to the selfinduction charge of the primary coil, no capacitor is necessary. Wear of the contact points owing to arcing between the points is eliminated. An ignition coil with a greater number of primary windings may be used for the transistor-ignition system. The larger number of primary coil windings, together with the rapid electronic interruption of the primary circuit, promotes a high-tension spark of as much as 40 000 volts. When a conventional ignition system is converted to a transistor-ignition system, the ignition coil, contact points and capacitor may be retained. The ignition coil may be replaced by a coil with a greater number of primary windings and the capacitor may be removed, since it is no longer required. The transistor-ignition system without contact points In this case, the base current of the transistor is interrupted by means of a reverse current. Although several methods may be used to interrupt the base current, only one type is discussed here. The sketch below depicts a schematic layout of a transistor-ignition system without contact points, as used in a single-cylinder engine. This system is fully electronic, with the rotor being the only moving mechanical component. Construction The primary and secondary systems are identical to that of the transistorignition system with contact points. One end of the impulse-coil is connected to the transistor side of the ignition switch, while the other end of the impulse coil is connected to the control unit. The base of the transistor is also connected to the control unit. The base as well as the impulse coil are earthed by the control unit and are also interconnected. The engine-driven rotor contains a number of permanent magnets, depending on the number of engine cylinders.

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Mechanical Technology Operation With the ignition switch on, battery current flows through the emitter, base and control unit to the earth. The small base circuit is now completed, the transistor is activated and the primary circuit is switched on electronically. A magnetic field builds up around both the primary and secondary coils of the ignition coil. At the same time, battery current flows through the ignition switch, impulse coil and control unit to the earth. The rotor rotates and, as soon as one of the magnets passes the impulse coil, magnetic induction takes place in the impulse coil. The induction in the impulse coil causes a higher voltage on the impulse coil side than on the emitter side of the base. The reverse current in the base “blocks” the emitter/base circuit, and the primary circuit through the emitter and collector is switched off electronically.

Figure 8.52: Transistor-ignition system without contact points

The magnetic field around both the primary and secondary coils collapses, secondary induction takes place and a high-tension spark is delivered across the spark-plug electrodes. As soon as the magnet has passed the impulse coil, no further induction takes place. The voltage in the impulse coil lowers, the emitter/base circuit is restored and the primary circuit is switched on electronically. It is clear that the number impulses in the impulse coil is determined by the number of rotor magnets, which in turn determines the number of high-tension sparks delivered per rotor revolution.

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Advantages of the transistor-ignition system with contact points in comparison with the conventional system • The contact points do not erode as a result of arcing, since they are not subjected to the self-induction charge of the primary coil. • There is no change in the ignition timing owing to burning or corrosion of contact points. • An ignition coil with more primary winding improves coil saturation. • An ignition coil with more primary winding improves secondary induction. • Rapid interruption of the primary circuit improves secondary induction, especially at high revolutions. • The problem of contact points turning blue as a result of high current flow at low temperatures is eliminated. Additional advantages of the transistor-ignition system without contact points • Disturbance of ignition timing as a result of wear of the rubbing block on the movable contact point is eliminated. • At high revolutions, the movable contact point tends to bounce. This problem is eliminated in the absence of contact points. Disadvantages of the transistor-ignition system in comparison with the conventional ignition system • Transistors are sensitive and may be damaged by excessive engine vibration. • Transistors should be kept cool since they may be damaged by heat. • The cost of the transistor-ignition system is considerably higher than that of the conventional system.

Fuel injection Most modern motor vehicles have a petrol-injection system instead of the carburettors. In a carburettor system, the petrol is sucked into the combustion chamber and the subsequent mixture is distributed to the cylinders. In an injection system, the petrol is squirted under pressure through little injector nozzles, one for each cylinder. The petrol is pressurised by a mechanical or electrical pump. The injectors are located in the intake passages near the inlet valves. The quantity of fuel injected and timing of injection vary with the type of system used, but metering of the fuel must be accurate.

DID YOU KNOW?

The injection system replaced the conventional carburettor, which delivers fuel under pressure into the combustion chamber or into the air flow just as it enters each individual cylinder.

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throttle accelerator

Compared to carburation, fuel injection gives improved maximum engine power and acceleration, and fuel consumption can be reduced since mixture distribution is more efficient. The engine usually responds much more rapidly to throttle changes because of the very small time lapse between throttle movements and injection of the fuel. The disadvantages of fuel injection systems are their high cost; they are more expensive than multi-carburettor installations. Their servicing, although rarely needed, must be done by a specialist.

Fuel injector nozzle

Intake valve

Fuel injector nozzle Piston Injecting fuel into the intake manifold just behind the intake valve Figure 8.53: Fuel injection for petrol engines

Figure 8.54: Electronic fuel injection

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Injecting fuel directly into the combustion chamber of the engine

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Direct injection The sketch below portrays the position of the injector in a direct injection system used in a petrol engine. As with a compression-ignition engine, injection takes place directly in the combustion chamber. The injector is often situated so as to spray the injected fuel directly onto the hot exhaust valve. This helps to cool the exhaust valve and vapourise to fuel. The injection pressure is 4000 to 10 000 kPa. Injection takes place during the last section of the intake stroke when the inlet air speed is at its highest. With injection towards the end of the inlet stroke, scavenging with a larger valve overlap is possible without any loss of fuel through the open exhaust valve. Direct injection is commonly used in racing engines, where maximum power and high engine speeds are always required. In practice, it has been found that lubricating oil is washed off the cylinder walls at idling and low engine speeds in direct injection. The high degree of cylinder wear and oil pollution render the system unpopular in passenger motor vehicles. Figure 8.55: Injector for direct injection

Indirect injection Sketch A below illustrates the position of the injector for an indirect injection system in petrol engines. The injector is placed in the intake manifold. Often the injector is placed in the inlet port, as shown below in Sketch B. Fuel is injected during the intake stroke through the open inlet valve, while the air enters the cylinder. In both cases a low injection pressure of 160 to 360 kPa is used. Indirect injection can be classified into two categories, namely timed injection and continuous injection.

Figure 8.56: Injector for indirect fuel injection

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Timed injection As the name indicates, injection takes place over a specific period of time. As in the case of compression-ignition and direct injection, the injection period may increase or decrease depending on the speed and power required. Contrary to the situation in compression-ignition engines, injection takes place during the inlet stroke. The duration of injection depends on the amount of air that enters the cylinder. Air intake in the cylinders is controlled by a butterfly valve in the inlet manifold which is connected to the accelerator. The ratio of the volume of the air allowed into the cylinder of the timed injection (volume of fuel) supplies the correct air/fuel ratio for all engine speeds.

Continuous injection As the name indicates, injection takes place continuously while the engine is running. The fuel is injected continuously into the inlet manifold or behind the intake valve, and is taken into the cylinder with air as soon as the intake valve opens. The amount of fuel injected is increased or decreased depending on the amount of air allowed through the butterfly valve. In this way the correct air/fuel ratio for all engine speeds is ensured. A number of advantages of timed injection are lost with continuous injection. Cylinder scavenging and considerable valve overlap are not possible since fuel will escape through the open valve. The intake manifold should be designed to prevent one cylinder from using fuel from another cylinder. Continuous injection is popular because it is a simple, inexpensive system. A simple injection pump may be used since it is possible to feed two or more cylinders from the same pump element.

Starting circuit The system mostly used in passenger cars and other vehicles has starting contacts in the ignition switch. When the ignition key is turned against spring pressure past the ON position to START, the starting contacts close. This connects the starting-motor solenoid or magnetic switch to the battery. After the engine starts and the ignition key is released, spring pressure returns it to the ON position. 268

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Some modern vehicles have a foot-operated pedal connected to automatic devices that close the starting motor circuit when the accelerator pedal is depressed. The starter motor used in motor vehicles is a direct-current electric motor that is particularly designed to turn the crankshaft of the motor vehicle’s engine. The speed and torque of the starter motor should be adequate to start the engine with no difficulty. The electrical energy stored in the battery in a chemical form is transformed into mechanical energy, namely, rotation, by the starter motor. Operating principle The operation of the starter motor is based on the fact that like magnetic poles repel one another and unlike magnetic poles attract each other. When a current-carrying conductor with its own magnetic field is placed in another magnetic field, the magnetic lines of force of the magnetic field are distorted. The tendency of the magnetic lines to follow the shortest path or to straighten, results in a push-action against the conductor. This pushaction tends to move the conductor out of the magnetic field. The sketch below portrays a single current-carrying conductor placed in a magnetic field. The direction of the magnetic field around the conductor depends on the direction of current flow in the conductor. With the current flowing away, the magnetic lines of force are bent to the left around the conductor, as shown. The concentration of lines of force on the left, all trying to straighten themselves, performs a push-action to the right on the conductor.

Figure 8.57: Single current-carrying conductor

This push-action increases as the magnetic field strengthens and also as more current flows through the conductor. In both cases, more magnetic lines are distorted and more force is exerted on the conductor as they tend to pull straight.

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Mechanical Technology A U-shaped conductor is used to convert this push-action, which is exerted on a current-carrying conductor in a magnetic field, to a rotating motion.

Figure 8.58: Basic operating principles of an electric motor

The sketch above depicts a simple layout of an electric motor. A magnetic field develops between the north pole and the south pole of the electromagnet, as shown. The two ends of the U-shaped, current-carrying conductor are connected to two segments on which two brushes rest. Note that the electrical current flows through the two field coils and from there, through the U-shaped conductor, back to the battery. The field coils and conductor are connected in series, hence the name series motor. The full battery current through the field coils induces a strong magnetic field between the north and south poles of the field coils. At the same time, electrical current flows through the V-shaped conductor, causing a circular magnetic field around the conductor, as shown. The current-carrying conductor will distort the magnetic field of the field coils as described. With the current-flow direction as shown, the right side of the V-shaped conductor is lifted and the left side is depressed. This push-action causes the V-shaped conductor to rotate as indicated. The current-flow direction through the V-shaped conductor is altered every 180 degrees by means of brushes and segments. This change of direction ensures that the current-flow direction in the conductor remains constant in relation to the north and south poles. The push-action against the two legs of the V-shaped conductor is always in the same direction and, consequently, the V-shaped conductor keeps rotating.

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In the case of the starter motor, two field magnets, each with a north and south pole, are normally used. A considerable number of V-shaped conductors are connected to a considerable number of segments on the commutator, as in the generator. The large number of current-carrying conductors and the two field magnets perform a push-action on a number of conductors simultaneously, which increases the torque of the starter motor significantly. Construction The illustration below depicts a sectional view of a starter motor used in vehicles. The starter motor contains the same basic components as a generator, namely field coils, armature, commutator and brushes.

Figure 8.59: Starter motor

The most important differences in construction between the starter motor and the generator are the thicker field coil and armature windings, and the brushes of copper instead of carbon, as in the generator. The starter motor normally uses four field coils and four brushes. In the starter motor, the armature windings are connected in series to the field coils, while in the generator, the armature windings are connected to the field coils in parallel. The rear end of the armature shaft is equipped with a Bendix drive, as shown. The spur gear of the Bendix drive meshes with the flywheel ring gear in order to drive the crankshaft of the engine.

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Figure 8.60: Wiring diagram of a starter motor

Charging circuit The main function of the charging system is to keep the battery in a charged condition. The starter motor depletes the battery when it turns the engine. The charging system puts the current back. An added advantage is that it deals with electric loads, such as the ignition system and the lights, and safety devices when the car is running. An alternator generates the electricity required for the charging system and is driven by the belt drive. Connecting the alternator-charging system to the battery Figure 8.50 portrays a simplified alternator-charging system connected to a battery. The stator, as shown, consists of a single, U-shaped conductor connected through the diodes to the battery.

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Figure 8.61: The alternator

The rotor is an electromagnet with one north and one south-pole. The coil of the electromagnet is connected to the slip rings on which two brushes rest. One brush is earthed, while the other is connected to the battery through the voltage regulator. In the alternator, the rotating magnet consists of an electromagnet with a number of north and south poles. The increased electromagnetic impulses in the charging system supplement one another in such a way that the alternator output charge may, for all practical purposes, be regarded as a steady, direct current. Note that the diode offers some resistance, causing a voltage drop of approximately 1 volt in the charging circuit. Precautionary measures Certain precautionary measures should be taken when working on a vehicle equipped with an alternator. • Battery installation – When the battery is disconnected, remove the earth cable before removing the live cable. When the battery is connected, the reverse procedure is followed. First connect the live cable and then the earth cable. 273

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Mechanical Technology – Make absolutely sure that the polarity of the battery corresponds with the polarity of the alternator. Check the manufacturer’s instructions if there is any doubt. – Short-circuiting should be avoided at all times since the diodes may be damaged. – Some alternators will not charge if a totally discharged battery is installed. This type of alternator has no remnant magnetism. – When a second battery is used to jump-start a vehicle, corresponding battery poles should be connected to each other. – Check the electrolyte level in all the battery cells. • Arc welding – When arc welding on a vehicle equipped with an alternator, it is best to disconnect the battery completely. Abnormally high voltage and incorrect polarity in the circuit may damage the electronic components of the alternator. • Fan-belt adjustments – Adjust the fan belt according to specifications. A fan belt with inadequate tension will slip, while a fan belt with too much tension will place an abnormal load on the rotor shaft bearings. – When the fan belt is adjusted, precautions should be taken not to place any pressure on the stator frame. Pressure exerted with a lever on the stator frame may dent the mild steel frame. • Repairs – The battery should be disconnected before embarking on repairs to the charging system. – If the battery is required for tests, short circuiting should be avoided at all costs. – Use only the correct testing equipment to carry out tests. – Make sure that all electrical connections are soldered or properly secured.

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Assessment

See if you can find the following words in the word search. ANTILOCK BRAKE

TECHNOLOGY

TRACTION CONTROL

HYDRAULIC

REDUCTION

WINDSCREEN

IGNITION

SEATBELT

SAFETY FEATURE

TIMING LIGHT

INLET VALVE

DISTRIBUTOR

AIRBAGS

PULLEY

TDC

SPARK

BRAKE

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Chapter 9

Pumps Topic 9

Mono pumps Rotor pumps

Centrifugal pumps

Pumps

Vane pumps

Reciprocating pumps Gear pumps

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Pumps Function of a pump device an object or thing

A pump is a displacement device. It is called a displacement device because it displaces fluid from one location to another. The various types of pumps use the following:

reciprocating moving continuously forwards and backwards

• Reciprocating movement of a piston or a plunger or a bucket inside a cylinder or housing. This means that the piston, plunger or bucket moves to and fro inside the cylinder, like the piston in a car engine. • Rotating wheels with vanes such as in a centrifugal pump. • An air current, as in air-lift pumps. These pumps use the buoyancy of air in water to draw water to the surface through a pipe. • Rotating or slinging blades in a casing as meshing gears or rotary pumps with sliding blades.

buoyancy ability to keep afloat

For the purpose of the chapter the following will be discussed: • Mono pump • Centrifugal pump • Reciprocating pump • Gear pumps • Vane pumps • Rotor pumps

Mono pump Construction A mono pump is a type of positive displacement pump. It is a rotary pump with a helical, metal worm which rotates in a stator made from rubber. Specific designs involve the rotor of the pump being made of a steel, coated in a smooth, hard surface, normally chromium, with the body (the stator) made of a moulded elastomer inside a metal tube body. The elastomer core of the stator forms the required complex cavities. The rotor is held against the inside surface of the stator by angled link arms, bearings (which have to be in the fluid) allowing it to roll around the inner surface.

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Casing gasket Packing

Plug-in shaft Coupling rod pin

Guided bush

Coupling rod bush Retaining sleeve

Large holding band Small holding band

Rotor

Guide bush

Stator

The helical rotor and the two universal joints of the drive mechanism.

A cutaway drawing of the rubber stator

Shape of the cavities left between the rotor and the stator Figure 9.1: A mono pump and components

Operation A mono pump is a type of positive displacement pump. The progressive mono pump consists of a helical rotor and a twin helix, twice the wavelength and double the diameter helical hole in a rubber stator. The rotor seals tightly against the rubber stator as it rotates, forming a set of fixed-size cavities in-between. The cavities move when the rotor is rotated but their shape or volume does not change. The pumped material is moved inside the cavities. 279

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Mechanical Technology The liquid is forced through the space between the stator and the rotor. It can handle corrosive liquids and slurries. The pump also pushes against high pressures. It is self-priming and it cannot handle a wide range of flow rates. It can produce whatever head is impressed upon it by the system restrictions (pipes, bends, valves etc.), while a centrifugal pump develops a head which is uniquely determined for any given flow by the speed of rotation. Typical application areas • Food and drink pumping • Oil pumping • Slurry pumping • Sewage sludge pumping • Viscous chemical pumping • Storm flow screening Specific uses • Grout/cement pump • Lubrication oil pump • Marine diesel fuel pump • Mining slurry pump • Oilfield mud motors • Winery use

Assessment

1. Describe the typical design of a mono pump. 2. Name four typical areas where you will use a mono pump. 3. Name four specific uses of a mono pump.

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Pumps

Centrifugal pumps

Outlet Guide vane

Outlet

Direction of rotation

Impeller blades

Casing

Inlet or eye Impeller blades

Inlet or eye

Concentric casing

Figure 9.2: Different views of a centrifugal pump

Construction A centrifugal pump is a rotodynamic pump that uses a rotating impeller to increase the pressure of a fluid. Centrifugal pumps are commonly used to move liquids through a piping system. The fluid enters the pump impeller along or near the rotating axis and is accelerated by the impeller, flowing outward in a radial pattern into a diffuser or volute chamber (casing), from where it exits into the downstream piping system. Centrifugal pumps are used for large discharge through smaller heads. Operation The centrifugal pump consists of a casing which contains a rotating wheel with blades or vanes, as shown in figure 9.2 This rotating wheel is known as the impeller of the pump. If the pump casing is filled with fluid and the impeller is in operation, the impeller will sling the fluid outwards by centrifugal force, and force it out at the outlet. (Centrifugal force is the force which acts on a body that is moving in a circular path. It tends to force the body further from the centre of the circle it is describing.) This creates a vacuum at the centre, or eye, of the impeller. As a result of atmospheric pressure, fluid is again drawn through this eye into the pump casing. Centrifugal pumps are not suitable for pumping gases. They have no valves, pistons or plungers.

PAUSE FOR THOUGHT

The space shuttle uses many pumps to take off, control its orbit, and land. On 18 August 1994, Shuttle’s computers sensed an overheated fuel pump in Orbiter Engine 3 and automatically shut everything down. The countdown clock read 00:00:00. No shuttle launch had ever been aborted that late. The clock had never gone to zero before. The main engines had been firing for nearly 7 seconds. To cool the system and prevent any fires, 1,5 million litres of water were quickly poured onto the Shuttle. If the solid fuel boosters had been fired, the mission would have had to proceed – with possible disastrous consequences. 281

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Impellers The efficiency of the centrifugal pump depends on the form and the shape of the impeller. Impellers must withstand erosion, pitting and corrosion by acids, so they are commonly made of brass, bronze, plastic Teflon® and cast iron. Impellers with relatively small diameters are used to deliver large volumes of fluid at low pressure heads. Impellers with large diameters are used to pump smaller volumes of fluid at higher pressure heads. Different types of impellers are selected according to the use of the centrifugal pump. The three main types of impellers are open-vane, semiopen and closed impellers. Plate on one side

Both sides shrouded Figure 9.3: An open-vane impeller

Figure 9.4: A semi-open or ribbed impeller

Figure 9.5: An enclosed or shrouded impeller

The different types of impellers are used in the following instances: • Open-vane impellers are used to pump sandy, muddy or sewerage water. They guard against pumping foreign materials that can get stuck between the impeller and the casing. • Semi-open or ribbed impellers are used for medium pressure heads. They are suitable for pumping fluids that contain gravel, sand, or other dirt or sediments. There are several variations of this type of impeller. • Enclosed or shrouded impellers are suitable for most centrifugal pumps. Because the assembly is enclosed, they are very strong. The machining and balancing of these impellers do not present many problems and they are often used at high speeds. Because the impellers are enclosed, no slippage can take place over the sides of the impellers. The pump is, therefore, much more efficient. The initial cost of this pump is much higher.

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Advantages of centrifugal pumps over reciprocating pumps • Centrifugal pumps are more compact, which means they need less floor space. • The initial cost is relatively low. • Maintenance costs are low due to the rotating motion of the main parts. • Centrifugal pumps are quite adaptable. They can pump sandy, muddy and dirty fluids with ease. • The construction of the pump is simple and reliable. • The pump works at high speeds and it can, therefore, be connected directly to the motor. • Water hammer and shocks do not occur because the pump delivers a regular and continuous stream of fluid. There is, therefore, no need for air vessels. • There are almost no vibrations, thus there is no need for sturdy and heavy foundations. • The delivery of fluid can be regulated from no flow to full flow without switching off or damaging the pump. • Centrifugal pumps have no moving valves or sensitive parts.

Assessment

1. Define the term “centrifugal force” as it applies to centrifugal pumps. 2. Briefly describe the different types of impellers. 3. List the advantages of centrifugal pumps over reciprocating pumps.

Additional assessment

1. In groups: Discuss the differences between a centrifugal water pump and a reciprocating water pump with regard to the working principle of each. 2. Individually: Research the catalogues of different pump manufactures. Identify the main parts and functions of centrifugal and reciprocating pumps. Summarise your findings on a single A4 page for your portfolio. 3. In groups of two. Role play a conversation between a potential client and a salesman. The salesman tries to persuade the client to buy a centrifugal water pump rather than a reciprocating water pump for a particular application. Reverse roles, and repeat the role play.

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Reciprocating pumps DID YOU KNOW?

The Archimedes screw is at least 2 000 years old. The Archimedes screw (also called an Archimedes snail) was used for irrigation and powered by people, horses and mules. This pump is still used today! The helix revolves inside a tube (only the bottom of the tube is shown) and the water rises accordingly. It is believed to have been invented by Archimedes himself. Surf the internet if you want to read more about pumps.

Construction A reciprocating pump has three main moving elements. They are: • an inlet valve, also called admission valve • an outlet valve, also called a discharge valve • a plunger or a piston. Although a plunger and a piston perform the same work, they differ in two ways. The length of a plunger exceeds its stroke length. (The stroke is the distance the plunger moves in one direction before returning.) The length of the piston is shorter than its stroke length. The packing of the plunger is housed in a stuffing box with soft packing at the end of the cylinder. The piston has packing rings that are inserted on the rim to prevent leakage. These differences are shown in Figures 9.6 and 9.7. Shorter than stroke

Longer than stroke

Packing on piston rim

Stroke Stroke Figure 9.6: A piston pump

Seal

Figure 9.7: A plunger pump

Operation Reciprocating action is a backward and forward, or up and down movement, developed from a circular movement. Figure 9.8 shows the action of the pump. A connecting rod joined to a crank, which moves or rotates through a circular path, drives a cross head forward and backwards in a straight line. Outlet valve

Reciprocating movement

Pump casing

Inlet valve

Shaft bushes

Figure 9.8: A reciprocating pump

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Types of reciprocating pump These pumps are divided into two groups, depending on the way they pump fluids: pumps and force pumps. Force pumps will be discussed.

Force pumps Construction Force pumps use force to lift and push fluids from one level to the next. A force pump uses a plunger which pushes the fluid by force to its destination. This kind of pump has inlet and discharge valves and a closed cylinder. The pump may have a piston or a plunger. The pump’s function is to deliver water at a pressure above atmospheric pressure. This is because it pushes against the mass of the water in the pipe above the discharge valve. DID YOU

Types of force pump

KNOW?

Atmospheric pressure

Force pumps can be divided into single-acting and double-acting pumps. A single-acting pump delivers fluid from one side of the piston only. Figures 9.9 and 9.10 show the operation of a single-action plunger pump. This pump has one inlet and one discharge valve. It delivers only once for every complete revolution of the crank. The operating cycle of the single-acting plunger or piston pump is as follows: Plunger

Water is forced into the vacuum cyliner inlet valve open

Stroke 1

Discharge valve closed

Figure 9.9: Stroke 1 of the operating cycle of the single-action plunger or piston pump

The most amazing pump of all was not invented by people. It is your heart. Beating thousands of times an hour from long before birth until death, your heart is a continuous worker. This means that the average human heart beats about 2,5 billion times. It pumps about five litres of blood per minute. Despite all the work it does, this amazing ‘machine’ weighs less than a kilogram.

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Mechanical Technology Plunger

Cylinder Inlet valve closed

Stroke 2

Discharge valve open

Figure 9.10: Stroke 2 of the operating cycle of the single-action plunger or piston pump

Operation • Stroke 1 The plunger moves up. This is known as the up stroke or intake stroke. A vacuum is created in the cylinder. This causes the available atmospheric pressure to force the water into the cylinder. The inlet valve is open and the discharge valve is closed. • Stroke 2 The plunger moves downwards. The discharge valve is forced open, while the inlet valve is closed. The downward movement of the plunger forces the water out of the cylinder. The single-action pump needs to complete two strokes to pump one cylinder of water. However, the double-action pump delivers fluids on both sides of the plunger, either by passing the fluid through the piston or by using two sets of intake and discharge valves. The double-action pump will be discussed by focusing the intake and discharge valves. Construction The double-action pump has two inlet and two discharge valves, one at each end of the cylinder. Figure 9.11 shows the inlet valves and the discharge valves in a double-action pump. The pump draws in fluid at one end while it pumps out fluid at the other. This is done in one stroke. Fluid is discharged at every stroke. This pump has double the capacity of the single-action pump and delivers twice during every complete revolution of the crank. The operation cycle of the double-action plunger or piston is shown by Figures 9.11 and 9.12.

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Discharge valve no. 4 closed Discharge valve no. 2 open Plunger moving down

Cylinder

Inlet valve no. 1 closed

Stroke 1

Discharge valve no. 3 open

Figure 9.11: Stroke 1 of the operating cycle of the double-action plunger pump

Operation • Stroke 1 The plunger or piston moves downwards. It draws in fluid through inlet valve number 2. At the same time, it delivers fluid through discharge valve number 3. Discharge valve no. 4 opwn Discharge valve no. 2 closed Plunger moving up

Cylinder

Inlet valve no. 1 open

Stroke 2

Discharge valve no. 3 closed

Figure 9.12: Stroke 2 of the operating cycle of the double-action plunger pump

• Stroke 2 The plunger moves upwards. It draws in fluid through inlet number 1. At the same time it delivers fluid through discharge valve number 4.

Water hammer Water hammer in plunger or piston pumps is caused by a sudden change in the speed at which fluid is moving, together with a proportional change in pressure. This causes a loud hammering in the pipeline, which is called the “knock” sound. 287

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Mechanical Technology Water hammer is a force which can cause damage to the pipelines and pump fittings. There are usually two possibilities when water hammer occurs: • The valve or stopcock in the pipeline can suddenly close. • During the delivery stroke, the water moves up the delivery pipe. During the suction stroke, some of the water can fall back down the delivery pipe. With the next delivery stroke the water that is being pumped collides with the falling water of the previous delivery stroke. This makes a loud, knocking sound. Delivery pipe

Falling water Stuffing box Water that is being pumped Figure 9.13: Water hammer due to falling water Compressed air

Delivery pipe Air vessel

Non-return valve

Figure 9.14: Neutralising water hammer

Using an air vessel to neutralise water hammer

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During the delivery stroke, the piston pushes water up into the air vessel, as shown in Figure 9.14. This water compresses the air in the air vessel. During the suction stroke, the non-return valve prevents the water from falling back down the delivery pipe. The air, which is under pressure, therefore, continues to push the water up the delivery pipe. This is done with very little disruption to the water flow. With the next delivery stroke, the piston pushes more water up the delivery pipe and it gradually joins the water of the previous stroke.

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Pump slip Pump slip can be defined as the difference between the theoretical flow rate and the real flow rate. Thus, slip is a measure of the amount of fluid that is not delivered but is lost. Slip is caused when the fluid slips out of the pump system for some reason. Air then slips into the pump system and destroys the vacuum. The reasons for pump slip are related to: • Worn external packing, which allows the pump to draw in air during the suction stroke. It also allows water to slip out during the delivery stroke. • Worn internal packing, which allows the fluid to slip from the one fluid chamber to the other fluid chamber. This makes the operation inefficient. • A strainer exposed above the fluid level. The result is that air slips into the pump and the delivery of the fluid is irregular. • A faulty foot valve, which allows fluid to slip back into the suction chamber during the delivery stroke. • Faulty or loose flanges or joints, which allow fluid to slip in during the suction stroke and to slip out during the delivery stroke. • A weak or faulty seat or spring of a valve. Pump slip is common in reciprocating pumps. However, it can also occur in centrifugal pumps.

Assessment

1. 2. 3. 4. 5. 6. 7.

Define a reciprocating pump. List the three main moving elements in a reciprocating pump. Reciprocating pumps can be divided into single-acting or doubleacting pumps. Discuss these two types in detail. Explain the term “water hammer” and what causes it. Explain how water hammer can be neutralised. What is the meaning of the term “pump slip” with regard to a pump? List the reasons for pump slip.

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Gears and rotary pumps analogous can be compared with

The prime mover of a hydraulic system, analogous to the electric motor or internal combustion engine, is the hydraulic pump. There are many types of pumps with a wide range of flow capacities. The four basic designs are the gear, vane, rotor and piston pumps. All four types are positive-displacement pumps, which means that the pump discharges flow as long as the pump is being driven. If the discharge opening were to be fully restricted, then the pump would continue to force fluid until a failure occurred. To protect them against this problem, pump systems contain a pressure relief valve to allow flow to escape once an excessive discharge pressure is reached. Volumetric efficiency is the ratio of actual delivered flow to the theoretical value and is a measure of internal pump leakage and slippage. Vane pumps have low efficiency at low revolutions per minute. Both gear and vane pumps have comparable efficiencies at normal operational speeds. Piston pumps have nearly 100% volumetric efficiency.

Gear pump Inlet port

Driven gear

Driver Casing

Outlet port Figure 9.15: A gear pump

Construction Gear pumps are compact and inexpensive and have few moving parts. The most common type of gear pump is the gear-on-gear type. The pump consists of a housing with an inlet and an outlet port. The inside of the housing is machined to form a figure-of-eight. There are two gears. The drive gear fits on a shaft which is driven by the camshaft. The other gear is an idler gear, also called the driven gear. The gear teeth run very close to the inner 290

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walls of the housing. Clearance between the gear faces, the housing and the cover is minimal. The volumetric flow rate induced by gear rotation is called pump displacement and is expressed in litres per minute. For gear pumps, displacement is calculated as follows: Flow = displacement = A × W × T × 2 × rpm, where A is the cross-sectional area between two teeth and the housing, W is the width of the gear and T is the number of teeth (× 2, since there are two gears).

Operation • When the shaft drives the drive gear, it drives the driven gear. Small pockets of oil are trapped between the gear teeth and the pump housing. • The rotating spaces between the teeth carry the oil towards the outlet port. • At the same time a vacuum is created over the inlet port and oil is drawn from the sump. Oil cannot return between the gear teeth and pressure is built up, forcing the oil out through the outlet port, from where it is fed to the oil channels.

Advantages • • • •

The pump is very efficient and can develop a high pressure. There are no reciprocating parts which can cause vibrations. The drive is always positive. It has no valves or springs.

Disadvantages • Wear between the gears and the housing reduces the pump pressure. • When the gears wear, the pump tends to be noisy.

Assessment

1. Complete: The volumetric flow rate induced by gear rotation is called … and is expressed in … units. 2. The formula for flow is: A × W × T × 2 × rpm. Explain what each letter stands for. 3. Explain step-by-step the operation of the gear-type pump.

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Vane pump

Figure 9.16: A vane pump

Construction The pump consists of a cylindrical housing in which a rotor is fitted eccentrically in such a way that it almost touches the inside of the housing at a point between the ports. In the rotor, there are slots into which two or more vanes are fitted. They are held apart by a spring so that the tips of the vanes are always in contact with the housing. Inlet and outlet ports are cut into the housing. Vane pumps are efficient at speeds over 600 rpm, which correspond to a maximum sealing of the vanes against the cavity due to centrifugal load. Below this speed, leakage is high. Displacement for a vane pump is equal to: A × W × 2 (for a balanced pump) × rpm, where A is the cross-sectional area of the pumping cavity formed by two vanes and the cavity wall, and W is the width of the circular disk. The rotor is driven by a shaft which, in turn, is driven by the camshaft.

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Operation • When one of the vanes moves past the inlet port, the space between this vane, the rotor and the housing increases gradually. • This causes a vacuum in the space, which causes oil to be drawn from the sump. • When the next vane moves past the inlet port, the oil is trapped and is carried along by the rotating rotor. • Due to the eccentric rotor, the space now decreases and the oil is pressurised. • The first vane now moves past the outlet port while the space is still decreasing. • The decreasing space and the next vane force the oil through the outlet port to the oil channels.

Advantages • The movement of the components are mainly rotary, which ensures smooth operation. • The only reciprocating components are the vane, but its movement is short and relatively slow. • They are very efficient for slow-speed engines.

Disadvantages • Wear on the vanes is high. • The pump cannot develop a high pressure. • They can cause pulsation – the fewer the vanes, the higher the pulsating effect.

Assessment

1. Explain the construction of the vane pump. 2. Compare the advantages of the piston pump and the vane pump.

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Rotor pump

Figure 9.17: A rotor pump

Construction

eccentrically off centre

The cylindrical housing contains a close-fitting, outer rotor which can rotate freely. In the centre of this outer rotor there is a five-pointed opening. The four-pointed inner rotor is mounted eccentrically in the outer rotor on a shaft which is driven by the camshaft.

Operation • When the inner rotor is driven, it drives the outer rotor and, thereby, causes the spaces between the teeth of the two rotors to increase and decrease alternatively. • The intake port in the housing is placed at such a point that, when the space between the teeth is increasing, it coincides with the inlet port and oil is drawn in. • As the rotors rotate further, the oil is trapped between the teeth and carried by them. When the space between the teeth decreases, pressure is built up. At a later stage, the outlet port coincides with the decreasing space and the oil is delivered under pressure to the oil channel.

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Advantages • • • • • •

There are no valves or springs. All movements are rotary movements. Wear is minimal. Operation of the pump is silent. Large inlet and outlet parts ensure a steady flow of oil without pulsation. Efficiency is very high, especially at low revolutions.

Disadvantages • Manufacturing costs are high.

Assessment

1. Explain the construction of the rotor pump. 2. Compare the advantages and disadvantages of the gear pump with the rotor pump.

Plunger/Piston pump

Figure 9.18: A plunger oil pump

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Construction The pump consists of a casing with an inlet and an outlet port. The ports are controlled by means of an inlet valve and an outlet valve. A closefitting plunger fits in a cylinder in the casing. A spring keeps the plunger in constant contact with an eccentric cam on the camshaft. Displacement of a piston pump is calculated as follows: 2 B × S × × P × rpm 4

Operation • When the camshaft rotates from the position indicated, the cam forces the plunger against the spring tension into the cylinder. • The oil in the pump chamber is pressurised and the inlet valve is closed. • The pressure causes the outlet valve to open against spring tension and oil is forced under pressure to the oil channels. • Delivery is continued for 180° of camshaft rotation. • Further rotation of the camshaft allows the spring to force the plunger out of the cylinder. • The pressure in the pump chamber is relieved, after which a partial vacuum is created. • The outlet valve is closed and oil from the sump fills the pump chamber past the inlet valve.

Advantages • Wear is distributed over a large area and unless the wear is considerable, it will not affect the effective operation of the pump. • They are cheap to manufacture.

Disadvantages • • • • •

Valves are required, which may stick or not seal properly against the seats. The reciprocating parts cause vibrations. The plunger or valve springs can break or lose tension. Oil is supplied in pulsations. They are not suitable for high-speed engines.

Assessment

Explain the operation of the piston/plunger pump.

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Necessity of oil pressure in an engine One of the functions of engine oils is to prevent metal on metal contact between engine parts. In a modern engine, it is not only necessary to get the oil between the friction surfaces, but also to keep it there under pressure to act as an oil cushion between the surfaces. Oil is also necessary to cool the friction surfaces by a constant supply of cooler oil. The oil is thus drawn from the sump by the oil pump and forced under pressure to the parts.

How oil pressure is built up in the engine You know that the crankshaft and connecting rod have bearings in which they turn. You also know that there is a clearance in the moving parts to allow oil to enter. The bearings are specially machined to give the oil a wedging action so that it can move in between the moving parts. When the engine is started, the camshaft turns the oil pump and the oil is drawn up and forced into the oil channel until all the channels are filled, and the oil is forced in between the moving parts. If the clearance between the parts is within specification, the oil builds up a resistance to flow between the parts and it is this resistance which is actually the oil pressure. If the bearings are worn considerably and the oil flow is easy and does not offer high resistance, the oil pressure will drop. An oil pump is designed to supply more oil than is actually required, so that there is no danger of too little oil.

The necessity to regulate oil pressure The pressure which is supplied by the oil pump must be controlled because it rises in relation to engine revolutions. At high engine revolutions, the pressure can become so great that damage to pressure gauges, oil feed pipes and even bearings can occur. To ensure a constant oil pressure at any engine revolution, an oil pressure relief valve is designed as an integral part of the oil pump. It can also be fitted anywhere in the main oil channel.

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Functions of pressure-relief valves Pressure-relief valves regulate the engine oil pressure at all engine revolutions and temperatures. They also relieve excess oil into the sump.

Types of pressure relief valves Plunger

Drainage channel

Rebound spring

Figure 9.20: A plunger pressure-relief valve

Adjusting nut Lock nut

Pressure spring

Ball

Main oil passage

Figure 9.21: A ball pressure-relief valve

Both pressure-relief valves are so designed that they fit snugly onto a seat and are held in this position by a calibrated spring. The adjustment can be done by means of shims or an adjusting screw. Normally the pressure-relief valve is set by the manufacturer and needs no further attention or servicing.

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When the engine is started, it takes a few seconds for the oil to build up pressure. The pressure-relief valve is still in the closed position. When engine revolutions increase, more oil is pumped and the pressure increases. As the pressure rises to a point where it overcomes the tension of the spring, the ball or plunger is lifted off its seat to relieve the pressure. The oil pressure is thus kept at a constant level. The surplus oil flows back into the sump.

Assessment

1. Why do you think it is necessary to have oil pressure in an engine? 2. How is oil pressure built up in the engine? 3. Name two types of pressure-relief valves found on oil channels. 4. Explain why you think it is necessary to have a pressure-relief valve on an oil pump.

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Ver 0002

Mechanical Technology Gr11 Learner's Guide

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