CAT 01 Hydraulic Fundamental Student Guide

s l a t n e m a d n u F c i l e u d i u a r G t d n e y d u t H S

Titles you can't find anywhere else

Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.

























Caterpillar Service Technician Module APLTCL025 HYDRAULIC FUNDAMENTALS

Published by Asia Pacific Learning 1 Caterpillar Drive Tullamarine Victoria Australia 3043 Version 3.2, 2003

Copyright © 2003 Caterpillar of Australia Pty Ltd Melbourne, Australia. All rights reserved. Reproduction of any part of this work without the permission of the copyright owner is unlawful. Requests for permission or further information must be addressed to the Manager, Asia Pacific Learning, Australia.

This subject materials is issued by Caterpillar of Australia Pty Ltd on the understanding that:

1.

Caterpillar Caterpillar of Austral Australia ia Pty Ltd, Ltd, its offici officials, als, author author(s), (s), or any any other persons persons involve involved d in the preparation of this publication expressly disclaim all or any contractual, tortious, or other form of liability to any person (purchaser of this publication or not) in respect of the publication and any consequence arising from its use, including any omission made by any person in reliance upon the whole or any part of the contents of this publication.

2.

Caterpillar Caterpillar of Austra Australia lia Pty Pty Ltd express expressly ly disclaims disclaims all and any liabil liability ity to any any person person in respect of anything and of the consequences of anything done or omitted to be done by any such person in reliance, whether whole or partial, upon the whole or any part of the contents of this subject material.

Acknowledgements

A special thanks to the Caterpillar Family for their contribution in reviewing the curricula for this t his program, in particular:



























MODULE I NTRODUCTION Module Title Hydraulic Fundamentals.

Module Description This module covers the knowledge and skills of Hydraulic Fundamentals. Upon satisfactory completion of this module students will be able to competently service and repair basic hydraulic components.

Pre-Requisites The following modules must be completed prior to delivery of this module: 

Occupational Health and Safety



Mechanical Principles.

Learning & Development Deliver y of this facilitat ed module re quires acc ess to the Hydraulic Hydraulic Fundamentals Fundamentals Activity Work book. The successful completion of the curriculum provides the knowledge for competency assessment, on further lear ning outcomes, by an Accredited Workplace Assessor. Assessor.

Suggested References 

No references recommended.

Assessment Methods Classroom and Workshop To satisfactorily complete this module, students must demonstrate competence in all learning outcomes. Consequently, Consequently, activities and assessments will measure all the necessary module requirements. For this module, students are required to participate in classroom and practical workshop activities and satisfactorily complete the following: 

Activity Workbook



Knowledge Assessments



Practical Activities.

Workplace To demonstrate competence in this module students are required to satisfactorily complete the Workplace Assessment(s).



























H YDRAULIC FUNDAMENTALS

MODULE INTRODUCTION



























H YDRA ULIC F UNDAMENTALS

KNOWLEDGE AND SKILLS ASSESSMENT Explain hydraulic principles.

Assessment Criteria 1.1. Describe hydrodynamics hydrodynamics and hydrostatics in relation to hydraulic systems 1.1.1 Energy Energy principles principles applicable applicable 1.2. State the the physical physical aspects aspects of a gas 1.2.1 1.2 .1 Gas compre compresse sses s 1.3. State the the properties properties of a fluid fluid 1.3.1 Liquids Liquids virtually virtually incompress incompressible ible 1.3.2 Liquids Liquids conform conform to the shape shape of the contain container er 1.3.3 Liquids Liquids apply pressur pressure e in all direction directions s 1.3.4 Purposes of fluids fluids in a hydrauli hydraulic c system system 1.4. State Pascal’s Pascal’s Law and how hydraulic hydraulic force can be used to create a mechanical advantage 1.4.1 1.4 .1 Pasca Pascal’ l’s s Law 1.4.2 1.4 .2 Fluid Fluid power power 1.4.2.1 1.4.2.1 Force Force transmission transmission 1.4.2.2 1.4.2.2 Force Force transmitted transmitted through a liquid 1.5. Explain the basic basic principles of work, flow and pressure, energy transfer and power 1.5.1 1.5 .1 Fluid Fluid weight weight 1.5.2 1.5 .2 Atmosp Atmospheri heric c pressu pressure re 1.5.3 1.5 .3 Barome Barometric tric pressu pressure re 1.5. 1.5.4 4 Work ork 1.5. 1.5.5 5 Flo Flow 1.5.5.1 1.5.5.1 What is flow flow 1.5.5.2 1.5.5.2 Laminar Laminar flow 1.5.5.3 1.5.5.3 Turbulent urbulent flow 1.5.5.4 1.5.5 .4 Flow across across an orifice orifice 1.5.6 1.5 .6 Energ Energy y transf transfer er 1.5.6.1 1.5.6.1 Bernoulli’ Bernoulli’s s Law 1.5. 1.5.7 7 Pres Pressu sure re 1.5. 1.5.8 8 Pow ower er 1.5.9 Fluid powe powerr advan advantages tages 1.5.10Key hydraulic principles 1.6. Describe Describe series and parallel hydrauli hydraulic c circuits 1.6.1 1.6 .1 Series Series circui circuitt



























K NOWLEDGE AND S KILLS A SSESSMENT


1.7. Calculate force, force, pressure and area in a hydraulic system 1.7. 1.7.1 1 Formu Formula la 1.7. 1.7.2 2 Force orce 1.7. 1.7.3 3 Pres Pressu sure re 1.7. 1.7.4 4 Area rea 1.7.5 Imperial/Metri Imperial/Metric c conversion conversion factors factors 1.7.5. 1.7 .5.1 1 Len Length gth 1.7.5. 1.7 .5.2 2 Area Area 1.7.5.3 1.7.5.3 Volume 1.7.5. 1.7 .5.4 4 Mass Mass 1.7.5.5 1.7.5.5 Velocity elocity 1.7.5. 1.7 .5.6 6 Force Force 1.7.5. 1.7 .5.7 7 Pressu Pressure re 1.7.5.8 1.7.5.8 Torque 1.7.5.9 1.7.5.9 Temperature emperature 1.7.5.10Power 1.7.6 Prefixes Prefixes commonly commonly linked linked to base base units. units. Identify and explain the purpose and operation of basic hydraulic circuits and components.

Assessment Criteria 2.1. Demonstrate knowledge of the basic layout layout of a hydraulic hydraulic circuit 2.1. 2.1.1 1 Tank ank 2.1. 2.1.2 2 Pum ump p 2.1.3 Main Pressure Pressure Relief Relief Valve Valve (PRV (PRV)) 2.1. 2.1.4 4 Line Lines s 2.1.5 Flow control control valv valves es 2.1. 2.1.6 6 Actu Actuat ator ors s 2.1. 2.1.7 7 Filt Filter ers s 2.1. 2.1.8 8 Mo Moto torr 2.1.9 2.1 .9 Graphi Graphic c symbo symbols ls 2.2. Identify and describe the function function of hydraulic tanks 2.2. 2.2.1 1 Vent ented ed 2.2. 2.2.2 2 Clos Closed ed 2.3. Identify, Identify, describe the function and explain explain the operation of hydraulic pumps 2.3.1 Positiv Positive e and non positive positive displacement displacement 2.3. 2.3.2 2 Gear Gear 2.3.2.1 2.3.2.1 Construction Construction 2.3.2.2 2.3.2.2 Operation Operation 2.3. 2.3.3 3 Vane ane 2.3.3.1 2.3.3.1 Construction Construction































2.4. Identify, Identify, describe the function and explain explain the operation of hydraulic control valves 2.4.1 2.4 .1 Direct Direction ional al 2.4. 2.4.2 2 Pres Pressu sure re 2.4.3 2.4 .3 Flow-v Flow-volu olume me 2.4. 2.4.4 4 Pilo Pilott 2.4.5 2.4 .5 Open Open centre centred d 2.4.6 2.4 .6 Closed Closed centre centred d 2.4.7 2.4 .7 Graphi Graphic c symbo symbols ls 2.5. Identify, Identify, describe the function and explain explain the operation of hydraulic actuators 2.5. 2.5.1 1 Line Linear ar 2.5.1.1 2.5.1.1 Single Single acting acting – Con Constr struct uction ion – Cap end head – Body Bod y – Rod end head – Piston Pist on – Piston Pist on rods rod s – Seals Seal s – ‘O’ ring rin g with w ith backup back up ring rin g – Lip seal sea l – Lip seal sea l with w ith gar ter spring spr ing – Lip seal sea l with wi th rod wiper wipe r – ‘U’ packing pack ing – ‘V’ packing pack ing 2.5.1.2 2.5.1.2 Operation Operation – Integra Int egrated ted counte cou nterba rbalan lance ce car c artr tridge idge – Cushion Cush ion plunge plu ngerr – Stro ke limit li miting ing stop tub tube e – Adju stable sta ble stop s top valve – Therm The rmal al relief reli ef valve – Operati Ope rating ng pressur pre ssur es 2.5.1.3 2.5.1.3 Doub Double le acting acting – Con Constr struct uction ion































2.5.2. 2.5 .2.2 2 Vane – Con Constr struct uction ion – Operati Ope ration on 2.5.2. 2.5 .2.3 3 Piston Piston – Con Constr struct uction ion – Operati Ope ration on 2.5.2.4 2.5.2.4 Graphic Graphic symbols symbols 2.6. Explain the purpose and function of hydraulic oil, filters and coolers 2.6. 2.6.1 1 Oil Oil 2.6.1.1 2.6.1.1 Lubrication Lubrication 2.6.1.2 2.6.1.2 Friction Friction 2.6.1.3 2.6.1.3 Viscosity Viscosity effects effects – To a system sys tem – By temp t empera eratur ture e – By pres p ressur sure e – On lubric lub ricati ation on – Hydrody Hydr odynam namic ic lubric lub ricati ation on – Clearan Cle arance ce flow 2.6.1.4 2.6.1.4 Water Water in hydraulic hydraulic oil 2.6.1. 2.6 .1.5 5 Foami Foaming ng 2.6. 2.6.1. 1.6 6 Dirt Dirt 2.6.1.7 2.6.1.7 Cavitation Cavitation 2.6. 2.6.2 2 Filt Filter ers s 2.6.2.1 2.6.2.1 Reservoir Reservoir strainer strainer 2.6.2.2 2.6.2.2 Suction Suction filters filters – Locatio Loc atio n – Type 2.6.2.3 2.6.2.3 Pressure Pressure filters filters – Locatio Loc atio n – Type 2.6.2.4 2.6.2.4 Return line line filter filter – Locatio Loc atio n – Type 2.6.2.5 Full flow flow filters (including by-pass)






























TOPIC 1: Principles of Hydraulics Hydraulics Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Why Are Hydraulic Hydraulic Systems Systems Used? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Hydraulic Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Fluid Power Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Fluid Power Power Advantage Advantages s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Flow (Q) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Pressure Drop Drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Hydraulics Hydraulics Doing Work Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

TOPIC 2: Hydraulic Circuit Circuit & Components Graphic Symbols Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Hydraulic lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Hydraulic Tank Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Graphic Symbol Symbol - Hydraulic Hydraulic Tank Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Hydraulic Pumps Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Gear Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Vane Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Piston Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Graphic Symbol Symbol - Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Linear Actuators Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Telescopic Cylinders Cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Modified Cylinders Cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Cylinder Ratings Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Graphic Symbol Symbol - Hydraulic Hydraulic Cylinder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Rotary Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Graphics Symbol Symbol - Rotary Rotary Actuator 67






























TABLE OF CONTENTS






























TOPIC 1 Principles of Hydraulics Hydraulics INTRODUCTION Hydraulic systems are extremely important to the operation of heavy equipment. Hydraulic principles are used when designing hydraulic implement systems, steering systems, brake systems, power assisted steering, power train systems and automatic transmissions. An understanding of the basic hydraulic principles must be accomplished before continuing into machine systems. Hydraulics play a major role in mining, construction, agricultural and materials handling equipment. Hydraulics are used to operate implements to lift, push and move materials. It wasn’t until the 1950s that hydraulics were widely used on earthmoving equipment. Since then, this form of power has become standard to the o peration of machinery. In hydraulic systems, forces that are applied by the liquid are transmitted to a mechanical mechanism. To To understand how hydraulic systems operate, it is necessary to understand the principles of hydraulics. Hydraulics is the study of liquids in motion and pressure in pipes and cylinders.

WHY ARE HYDRAULIC SYSTEMS USED? There are many reasons. Some of these are that hydraulic systems are versatile, efficient and simple for the transmission of power. This is the hydraulic system’s job, as it changes power from one form to another. The science of hydraulics can be divided into two sciences: 

Hydrodynamics



Hydrostatics.
































Hydrodynamics This describes the science of moving liquids.

Figure Figure 1 - a & b

Applications of hydrodynamics: 



water wheel or turbine; the energy that is used is that created by the water’s motion mot ion (Figure (Figure 1a) torque conv converter erter (Figure (Figure 1b).

Hydrostatics This describes the science of liquids under pr essure. Applications of hydrostatics: 

hydraulic jack or hydraulic press



hydraulic cylinder actuation.
































Liquids Conform to Shape

Figure Figure 2

Liquids will conform to the shape of any container. Liquids will also flow in any direction through lines and hoses of various sizes and shapes. We have have three oddly shaped containers shown in Figure 2, all connected together and filled to the same level with liquid. The liquid has conformed to the shape of the containers.

A Liquid is Practically Incompressible
































Liquids Apply Pressure in All Directions

Figu Figure re 4

There is equal distribution of pressure in a liquid. The pressure measured at any point in a hydraulic cylinder or line will will be the same wherever wherever it is measured (Figure 4).
































Water would be unsuitable because: 1.

it free freeze zes s at cold cold temp tempera erature tures s and and boils boils at 100 100oC

2.

it causes causes corr corrosi osion on and rusti rusting ng and furni furnishe shes s little little lubric lubricati ation. on.

Purpose of the Fluid Many types of fluids are used in hydraulic systems for many reasons, depending on the task and the working environment, but all perform basic functions: First, the fluid is used to transmit forces and power through conduits (or lines) to an actuator where work can be done. Second, the fluid is a lubricating medium for the hydraulic components used in the circuit. Third , the fluid is a cooling medium, carrying heat away from the “hot spots” in the hydraulic circuit or components and discharging it elsewhere.

And fourth, the fluid seals clearances between the moving parts of components to increase efficiencies and reduce the heat created by excess leakage.

FLUID POWER In the seventeenth century, a French Philosopher and Mathematician named Blaise Pascal, formulated the fundamental law which forms the basis for hydraulics. Pascal’s Law states: “Pressure applied to a confined liquid is transmitted undiminished in all directions, and acts with equal force on a ll equal areas, and at right angles to those areas.”
































Figure 7 - Container Container bursting due to pressure pressure

Because liquid is essentially incompressible, and forces are transmitted undiminished throughout the liquid and act equally on equal areas of the bottle, and the area of the body of the bottle is much greater than the neck, the body will break with a relatively light force on the stopper. Figure Figure 7 illustrates this phenomenon. phenomenon.


































This relationship is represented by the following formula: Force = Pressure x Area. This formula allows the Force to be determined and the Pressure and the Area when two of the three are known.
































Figure 10 - Pressure created by weight weight

The same relationship is used to determine the pressure in a fluid resulting from a force applied to it. Figure 10 shows a weight being being supported by fluid over a 2 .01m area. By rearranging the above formula, the fluid pressure of 100,000Pa
































FLUID POWER ADVANTAGES Multiplying forces is only one advantage of using fluid to transmit power. As the diagram in Figure 11 shows, the forces do not have have to be transmitted in a straight line (linearly). Force can be transmitted around corners or in any other non-linear fashion while being amplified. Fluid power is truly a flexible power transmission concept. Actually, Actually, fluid power is the transmission of power from an essentially stationar y, rotary source (an electric motor or an internal combustion engine) to a remotely positioned rotary (circular) or linear (straight line) force amplifying device called an actuator. Fluid power can also be looked upon as part of the transformation process of converting a benign form of potential energy (electricity or fuel) to an active mechanical form (linear or rotary force and power). Once the basic energy is converted to fluid power, other advantages exist: 1.

Forces Forces can be easily easily altered altered by changin changing g their their direction direction or revers reversing ing them. them.

2.

Protective Protective devices devices can can be added added th that at will will allow allow the the load operating operating equipme equipment nt to stall, but prevent the prime mover (motor or engine) from being overloaded and the equipment components from being excessively stressed.
































PRESSURE

Figure 13 - Pressure at reservoir outlet outlet

The system fluid is forced out of the reservoir into the inlet side of a pump by the sum of several pressures pressures that act on the fluid (Figure 13). The first pressure is the one
































This is the same pressure felt on eardrums when swimming under water, and experience says that the pressure increases with depth. The pressure can be expressed as follows: Pressure (Pa) = water depth (m) x 9810 Pa per metre of depth. Other fluids behave the same as water, the difference being relative to the difference in weight of the fluids. The difference is usually defined by the Specific Gravity of the fluid (SG), which is the ratio of the fluid’s weight to the weight of water. SG = Weight of fluid

÷ Weight of water

A typical specific gravity for oil used in hydraulic systems is approximately 0.92, meaning the weight of the oil is 92% of the weight of water. The relationship of the first formula then becomes: Pressure (Pa) = Fluid Depth (m) x 9810 Pa/m water x SG.


































Atmospheric Pressure






























































































Laminar Flow
































PRESSURE DROP


































This direct relationship between flow and pressure drop is an important consideration































































Series & Parallel Circuits
































Restriction in Parallel
































HYDRAULICS DOING WORK






















































































































































































































































































































































































































































































































































































































































































































MODIFIED CYLINDERS There are many special configurations of linear actuators that customise them for particular applications. Some of these are proprietary to specific machine manufacturers, and are not typical beyond those applications. Others, though, are more generally available and may be considered standard options.

Counterbalance valve

Figure Figure 76

It frequently can be advantageous to incorporate some specific valve requirements within the cylinder, usually into the cap end head. The popularity of screw-in cartridge valves has made some of these features relatively easy and inexpensive to accomplish. Although the counterbalance valve valve (Figure 76) is probably the most popular type of integral valve, other types such as directional control, flow control and sequence valves can be found.

Cushions

Figure Figure 77

Frequent and abrupt end-of-stroke stops can damage a cylinder; cylinders that extend and/or retract at high speed can fail catastrophically in just a few strokes. Cylinder cushions are a fairly common feature on mobile equipment that helps to slow the piston down near the end of its stroke and reduce the impact. Cushions may be found




























Stroke limiting

Figure Figure 78

Although a simple stop tube (Figure 78) can be used to limit the travel travel of a cylinder, cylinder, it is frequently desirable to externally adjust the stroke travel.

Figure Figure 79

This is done with a stroke stroke control valve valve similar to that shown in Figure 79. The stroke is adjusted by locating the stop flange on the cylinder rod to activate the stop valve at different retraction positions. Reversing the flow direction will compress the valve spring and allow the cylinder to extend.

Thermal relief valves

Figure Figure 80

Cylinders that have cooled down, and are then subjected to heat from the sun (or other source) may be damaged by the resulting fluid expansion. Extremely high pressures can be developed as the fluid expands, unless there is a way to relieve the pressure and drain off a small amount of the fluid. A small integral relief valve or cartridge (Figure 80), set much higher than system pressure, will accomplish this this task




























CYLINDER RATINGS The ratings of a cylinder include its size and pressure capability. Principle size features are bore (piston diameter or inside body diameter), piston rod diameter and stroke length. The pressure rating is based on the size, design and materials used, and is established by the manufacturer. Refer to the cylinder nameplate or the manufacturer’s manufacturer’s catalogue for this information.

GRAPHIC SYMBOL - HYDRAULIC CYLINDER

Figure Figure 81

The symbols for a few basic types of hydraulic and pneumatic cylinders are shown in Figure 81. The rod size and length length of stroke is not reflected reflected by the symbol's symbol's dimensions. At the top two examples of single acting cylinders, or cylinders which can only be pressure activated in one direction. In the centre is a common example of a double acting cylinder while at the bottom an example of a double rod end cylinder similar to the steering cylinder used on the backhoe loaders and wheeled tractor scrapers is shown.

Figure Figure 82

The basic symbols for cylinders can also be modified to show other features and




























Figure Figure 83

The two symbols in Figure 83 represent fixed and adjustable adjustable cushions or snubbers. These are used to slow piston and rod travel near the end of the cylinder stroke. The top symbol has a fixed cushion in both directions of travel. The bottom symbol is an adjustable cushion on the extension side only, denoted by the variability arrow drawn through the cushion block. Note the cushion block symbol is only on the rod side of the cylinder. To To the left we see a cutaway drawing of a cylinder with fixed cushions on both the extension and retraction sides. Another example of how a cylinder may be cushioned is seen on the next slide.

ROTARY ACTUATORS Linear actuators convert fluid power to linear motion, rotary actuators convert fluid power to rotary motion. Fluid is pushed into the inlet of the rotary actuator and causes the output shaft to rotate. Resistance to rotation by an external load creates pressure in the hydraulic circuit and in the inlet of the motor.

Gear Type

Figure Figure 84

A cross sectional view of an external gear hydraulic motor is shown in Figure 84. This design is referred to as “external gear” design because the gear teeth are machined on the outside of the gears. One of the gears will be connected to an output shaft, and the other will be an




























Vane Motors

Figure Figure 85

A cross sectional view view of a balanced vane rotating group group is shown in Figure 85. The elements shown in the view are the cam (cam ring or displacement ring), rotor and vanes. The output shaft of the motor is connected to the centre of the rotor. The vanes slide in and out of the slots in the rotor so as to make contact with the cam surface.

Figure Figure 86

A form of spring, either a spring clip or a small coil spr ing, is placed under the vane to cause it to stay against against the cam surface (Figure (Figure 86). In addition, inlet fluid is also ported under the vanes so as to balance the pressure between the top and bottom and prevent pressure from pushing the vane back into the slot. Fluid entering the motor will pressure two opposite sides of the rotor assembly, and return fluid will exit two opposite sides. This way, way, equal pressures are always opposite each other, balancing forces across the rotor. This relieves any loading on the drive shaft and bearings caused by internal pressures and forces.






























Figure Figure 87

Figure 87 shows how the differential differential pressure across a vane will create create a force on the vane. The amount of vane that is exposed to pressure will determine the magnitude of the force (force equals pressure times area), and the distance from the centre of the exposed vane area to the centre of the drive shaft will determine the torque that is generated. Therefore, the torque output of a vane motor is dependent upon pressure, size of the vane (height extending above the rotor and width) and radius of the rotor (distance from the centreline of the dr ive shaft).

In-line Piston Motors

Fig

88






























Bent Axis Piston Motors

Figure Figure 89

A cross sectional view of a bent axis piston motor is shown shown in Figure 89. The main elements are a cylinder block, pistons and shoes, drive shaft and flange, a universal link and a valve plate. The piston shoes are lodged in the drive shaft flange, and the universal link maintains alignment between the cylinder block and the drive shaft so that they turn together. As fluid is forced through the valve plate into the cylinder block, pistons are forced out of the cylinder block, forcing the drive shaft flange to rotate. This causes the drive shaft to rotate along with the cylinder block and pistons. Pistons are forced back into the cylinder block by the drive shaft flange, and fluid is forced out through the valve plate and back to the reservoir. The entire operation is much the same as the in-line piston motor, except that the cylinder block and piston assembly is angled instead of a swash plate. The amount of torque that a motor will deliver is based on the force of the piston (pressure times the piston cross sectional area), the radius of the drive shaft flange (force times distance) and the angle of the cylinder block. The higher the cylinder block angle, the greater the torque output for any given pressure and piston size.






























GRAPHICS SYMBOL - ROTARY ACTUATOR

Figure Figure 90 - Fixed Displacem Displacement ent

The symbols used for motors are very similar to those used for pumps. Rather than creating flow or converting mechanical energy into fluid power though, motors do just the opposite and convert fluid power into mechanical energy. This is shown by using the same basic component symbol as a pump, but with the energy triangle reversed, pointing inward. This indicates that fluid power is taken into the component as the triangle's orientation shows. All other supplemental component elements used for motors are the same as those used for pumps. It is important to remember that graphic symbols such as these for pumps and motors do not indicate the kind of pump or motor? (gear, vane or piston type) being represented. The symbols only, only, designate the type of pump displacement and method of operation.

DIRECTIONAL CONTROL VALVES

Figure Figure 91

Directional Control Valves (DCV) are used to direct oil into separate circuits of a






























Simple Spool DCV

Figure Figure 92

The spool shown in the Figure 92, operates a double acting cylinder, cylinder, by directing flow to either end of the cylinder. Ports A and B are the ports to the cylinder Port P is pressure oil from the pump. Port T is drain oil to the tank.

Valve Centred Oil to the cylinder is blocked by the position of the spool.

Valve shifted left Oil can now flow from the P port to the cylinder A port and oil can also flow from the non-active side of a double acting cylinder through B to tank (T).

Valve shifted right Oil can now flow from the P port to the cylinder B port. Oil can also flow from the non-active






























Open Centre Directional Control Valve

Figure 93 DCV in Hold Position

Open centre valves have a passage designed in the valve body casting that allows all inlet flow, flow, when the spool is in neutral or centre position, to pass through a bypass area. This flow either exits the valve back to the tank or is available for another valve connected in series to the first valve. The advantage of an open centre valve is that there is little work done by the pump. When the valve is in neutral, minimal pressure is built up. The main disadvantage is that there is a small time lag when the valve opens as pressure needs to build up in all of the circuit. Figure 93 shows a cutaway cutaway diagram of a typical open centre directional control valve valve in the HOLD position. In the HOLD position, the pump oil flows into the valve body, around the valve spool and returns to the tank. The valve spool also blocks the oil in the line to the rod end and the head end of the cylinder. The above example of a directional control valve also contains a load check valve. In the hold position, the load check valve has spring tension behind it, to keep the valve closed and prevent drift from the implements attached to the cylinder.
































Figure Figure 94 - Raise Position Position

Figure 94 shows the valve valve spool at the instant the spool is moved to the RAISE position. When the valve spool is moved to the RAISE position, the valve spool blocks the pump oil flow to the tank. However, However, pump oil flow is open to the load check valve. The valve spool also connects the cylinder head end to the oil behind the load check valve and the cylinder rod end to the tank passage. The load check valve prevents prevents the oil in the head end of the cylinder from flowing into the pump oil passage. The blocked pump oil flow causes an increase in the oil pressure. This action prevents drift of the implement until pump pressure builds up.
































Check Valve

Figure Figure 96

The purpose of a check valve is to readily permit oil flow in one direction, but prevent (check) oil flow in the opposite direction. The check valve is sometimes called a ’one way’ check valve. Most check valves consist of a spring and a tapered seat valve as in the figure above. However, a round ball is sometimes used instead of the tapered seat valve. In some circuits, the check valve may be free floating (has n o spring). In the valve valve on the left (Figure 96), when the pump oil pressure overcomes overcomes the oil pressure in back of the check valve plus the check valve slight spring force, the check valve opens and allows the oil to flow to the implement. In the valve on the right, when the pressure of the pump oil is less than the oil pressure in the implement, the check valve closes and prevents implement oil flow back through the valve.

Closed centre, directional control valve
































Figure 98 - multiple spool valve with with centre bypass bypass construction

Figure 98 shows a multiple multiple located located bypass bypass passage. passage.

spool,

open

centre

valve, valve,

with with

a

centrally

In Figure 98 valve section 1 is in the neutral position and the bypass bypass feature is porting flow at the bypass pressure setting to the following valve sections. Valve Section 2 illustrates the bypass operation with the valve shifted out for flow through Port B; and, in Section 3, flow to Port A Before the flow in either section 2 or section 3 can be useable, the bypass must first be restricted or dosed down, or else the fluid will continue through the bypass and do no work (flow takes the path of least resistance). As illustrated in section 2, when the spool is shifted far enough, the bypass will be completely shut off and a flow will then pass to the work port (port “B”). Return flow, coming back through the return line, is free to flow to the outlet, or tank, passage to the reservoir. The reverse of this is illustrated in the third section. Multiple spool valves are typically either series design or series parallel design. Series design valves are typically the lowest cost and are used in many cost sensitive
































PRESSURE CONTROL VALVES Also known as relief valves. The function of a relief valve is to provide protection to a hydraulic system so that the system components do not malfunction, seize or burst and the fluid lines do not burst or leak at their connections. The relief valve performs this function by providing a means for system fluid to be diverted to the reservoir when the valve pressure setting is reached. The opening of the relief valve is accomplished when pressure of the fluid in the system exceeds a pressure set by a spring force in the relief valve. The spring holds the relief valve in its closed position. As the fluid pressure rises to a level that exceeds the force of this spring, the relief valve opens and creates a flow path to the reservoir. This action results in the “relieving”, or limiting, of fluid pressure in the system to the value of the spring force in the relief valve.
































Simple Pressure Relief Valve, Valve, Cracking Pressure

Figure Figure 100

Figure 100 above shows a simple relief relief valve in the ‘cracking ‘cracking pressure’ position. The simple relief valve (also called direct acting relief valve) is kept closed by spring force. The spring tension is set to the ’relief pressure’ setting. However, the relief pressure setting is not the pressure a t which the valve first begins to open. When a condition develops that causes a resistance to the normal oil flow in the circuit, excessive oil flow causes the oil pressure to increase. The increasing oil pressure is sensed a t the relief valve. When the force of the increasing oil pressure overcomes the force of the relief valve spring, the valve moves against the spring and begins to open. The pressure required to begin valve opening is called the ’cracking pressure’. The valve opens just enough to allow excess oil to flow through the valve.

Simple Pressure Relief Valve, Relief Pressure Setting


































Pilot Operated Relief Valve Closed Position

Figure Figure 102

The pilot operated relief valve (Figure 102) is often used in systems that require a
































Open Position

Figure Figure 103

When the system oil pressure exceeds the pilot valve spring setting (Figure 103) the pilot valve opens. The open pilot valve allows the oil in the unloading valve spring chamber to flow to the tank. The pilot valve opening (orifice) is larger than the unloading valve orifice.
































Non-compensated Flow Controls The simplest type of flow control is the non-compensated flow control. Noncompensated flow control valves are used as restrictors to control the rate of flow into a branch circuit at a given input pressure to the valve. As system pressure increases, flow through the non-compensated flow control will increase with a corresponding increase in pressure drop. If outlet or load pressure increases, flow may decrease proportionally. The simplest form of a non-compensated flow control is an orifice. An orifice can conveniently be inserted into a hydraulic line as a stand alone valve, or more typically, typically, into a hydraulic fitting as a means of restricting the flow into a branch circuit. Needle or glob type valves are also commonly used as simple flow restrictions that control the flow into a branch circuit. An orifice is a small opening in the oil flow path. Flow through an orifice is affected by several factors. Three of the most common are: 1.

The The tem tempe pera ratu ture re of the the oil oil
































Check Valve with a Fixed Orifice

Figure Figure 104

Figure 104 shows an example of a check valve valve with a fixed orifice that is commonly used in construction equipment.
































The oil flow through the needle valve must make a 90° turn and pass between the valve tip and the valve seat. The needle valve is one of the most frequently used variable orifices. When the valve stem is turned counter-clockwise, the orifice becomes larger and the flow increases through the valve. valve. When the valve stem is turned clockwise, the orifice becomes smaller and the flow decreases through the valve.


































Flow Dividers





























































































































Combined Orifice and Dump Valve Valve


































GRAPHIC SYMBOLS - VALVES
































unactivated position. Valves Valves which change position due to flow or pressure are in their neutral condition when there is no flow or pressure in the circuit. That is, the external































































The lower cutaway and symbol are representative of a tandem-type open centre valve. Pump supply is open to tank by means of a passageway inside of the spool,
































Graphic Sysmbols - Flow Control Valves Valves















































































































































































































































































































































































CAT 01 Hydraulic Fundamental Student Guide

Recommend Documents