February 2011
e-Soft board
Theme: Gear Manufacturing Process The theme encompasses the following: • • • • • • • •
Introduction History Gear Manufacturing process Methods of forming gears Gear Generating Process Gear Shaping Classification of Gear References
Introduction:
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, magnitude, and direction of a power source. The most common situation is for a gear to mesh with another gear; however a gear can also mesh a non-rotating toothed part, called a rack, thereby producing translation instead of rotation. The gears in a transmission are analogous to the wheels in a pulley. An advantage of gears is that the teeth of a gear prevent slipping. Two meshing gears transmitting When two gears of unequal number of teeth are combined a mechanical rotational motion. Note that the advantage is produced, with both the rotational speeds and the torques of the smaller gear is rotating faster. two gears differing in a simple relationship. Although the larger gear is Gears are used extensively for transmission of power. They find application rotating less quickly, its torque is in: Automobiles, gear boxes, oil engines, machine tools, industrial proportionally greater. machinery, agricultural machinery, geared motors etc. To meet the strenuous service conditions the gears should have: robust construction, reliable performance, high efficiency, economy and long life. Also, the gears should be fatigue free and free from high stresses to avoid their frequent failures. The gear drives should be free form noise, chatter and should ensure high load carrying capacity at constant velocity ratio. To meet all the above conditions, the gear manufacture has become a highly specialized field.
History: th
According to historical records, gears had started as far as 400 to 200 BC in ancient China. Until the 17 century, th people began to study that they can transfer the movement of the tooth shape. In the 18 century, after the industrial revolution in Europe, gear drive has been used widely; first development of cycloid gear and then involutes gear is, until the early 20th century, involutes gear has play a dominant position in the application. Gears mesh with each other toothed machine parts, its mechanical transmission and the mechanical application of the field is extremely broad. Modern gear technology has been achieved: the gear module O.004 ~ 100 mm; gear diameter from 1 mm to 150 m; transmission power up to the 100 thousand kilowatts; speed up to hundreds of thousands of r / min; maximum peripheral speed of 300 m / sec.
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From the Knowledge Centre’S Desk
February 2011
The composition generally gear teeth, alveolar, French side addendum circle, tooth root circle, base circle, pitch circle. Gear can be classified by gear shape, tooth shapes, tooth surface. Such as gear tooth profile, including tooth profile curve, pressure angle, tooth height, and deflection. On the above mentioned involute gear, it is relatively easy to manufacture, so the modern use of gears, the involute gear absolute majority, while the cycloid gear and the circular gear is seldom used.
Gear Manufacturing process:
Gear manufacturing can be divided into two categories namely forming and machining as shown in flow chart. Forming consists of direct casting, molding, drawing, or extrusion of tooth forms in molten, powdered, or heat softened materials and machining involves roughing and finishing operations.
Materials used in gear manufacturing process The various materials used for gears include a wide variety of cast irons, non ferrous material &non - material materials the selection of the gear material depends upon: i. Type of service ii.
Peripheral speed
iii. Degree of accuracy required iv. Method of manufacture v. Required dimensions & weight of the drive vi. Allowable stress vii. Shock resistance viii. Wear resistance. 1. Cast iron is popular due to its good wearing properties, excellent machinability machinability & Ease of producing complicated shapes by the casting method. It is suitable where large gears of complicated shapes are needed. 2. Steel is sufficiently strong & highly resistant to wear by abrasion. 3. Cast steel is used where stress on gear is nigh & it is difficult to fabricate the gears. 4. Plain carbon steels find application for industrial gears where high toughness combined with high strength.
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From the Knowledge Centre’S Desk
February 2011
5. Alloy steels are used where high tooth strength & low tooth wear are required. 6. Aluminum is used where low inertia of rotating mass is desired. 7. Gears made of non -Metallic - Metallic materials give noiseless operation at high peripheral speeds.
Gear manufacture by casting method: Gear can be produced by the various casting processes. Send casting is economical and can take up large size and module, but the gears have rough surfaces and are inaccurate dimensionally. These gears are used in machinery where operating speed is low and where noise and accuracy of motion can be tolerated, for example, farm machinery and some hand operated devices. Send casting is suitable for one off or small batches. Large quantities of small gears are made by “Die - Casting”. These gears are fairly accurate and need little finishing. However the materials used are low melting ones, such as alloys of zinc, aluminum and copper so, there gears are suitable for light duty applications only (light loads at moderate speeds). Gears made by “Investment Casting” may be accurate with good surface finish. These can be made of strong materials to withstand heavy loads. Moderate size gears are currently being steel cast in metal moulds to produce performs which are later forged to size. Light gears of thermoplastics are made by “Injection Moulding”. This method is satiable for large volume production. However, gear tooth accuracy is no high and initial tool cost is high. These gears find use in instruments, household appliances etc for phosphor bronze worm wheel rims; “centrifugal casting” is used far more extensively than any other method. Centrifugal casting is also applied to the manufacture of steel gears. Both vertical and horizontal axis spinners are used. After casting, the gears are annealed or normalized to remove cooling stresses. They may then be heat treated, if required, to provide the needed properties. Centrifugally cast gears perform as well as rolled (discussed ahead) gears and are usually less expensive. “Shell moulding” is also sometimes used to produce small gears and the product is a good cast gear of somewhat lower accuracy than one made by investment casting but much superior to t o sand casting.
Methods of forming gears:
Roll forming:
In roll forming, the gears blank is mounted on a shaft & is pressed against hardened steel of rolling dies. The rolls are fed inward gradually during several revolutions which produce the gear teeth. The forming rolls are very accurately made & roll formed gear teeth usually home both by not and cold. In not roll forming, the not rolled gear is usually cold -rolled which compiles the gear with a smooth mirror finish. In cold roll forming, higher pressures are needed as compared to not rolling many of the gears produced by this process need no further finishing. It becomes stronger against tension & fatigue. Spur & helical gears are made by this process. Stamping:
Large quantities of gears are made by the method known as stamping ‘blanking’ or ‘fine blanking’. The gears are made in a punch press from sheet; up to 12.7mm think such gears find application in: toys, clocks 4 timers, watches, water & Electric maters & some business Equipment. After stamping, the gears are shaved; they give best finish & accuracy. The materials which can be stamped are: low, medium & high carbon steels stainless steel. This method is suitable for large volume production. Powder metallurgy:-
High quality gears can be made by powder metallurgy method. The metal powder is pressed in dies to convert into tooth shape, after which the product is sintered. After sintering, the gear may be coined to i n crease density & surface finish. This method is usually used for small gears. Gears made by powder metallurgy method find application in toys, instruments, small motor drivers etc. Extrusion:
Small sized gear can also be made by extrusion process. There is saving in material & machining time. This method can produce any shape of tooth & is i s suitable for high volume production gears produced by extrusion fi nd application in watches, clocks, type writers etc.
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From the Knowledge Centre’S Desk
February 2011
Gear Generating Process:
Gear Hobbing
Hobbing is the process of generating gear teeth by means of a rotating cutter called a hob. It is a continues indexing process in which both the cutting fool & work piece rotate in a constant relationship while the hob is being fed into work. The route gears the hob has essentially straight sides at a given pressure angle. The hob and the gear blank are connected by means of proper change gears. The ratio of hob & blank speed is such that during one revolution of the hob, the blank turns through as many teeth. The teeth of hob cut into the work piece in Successive order & each in a slightly different position. Each hob tooth cuts its own profile depending on the shape of cutter, but the accumulation on the shape of cutter, but the accumulation of these straight cuts produces a curved form of the gear teeth, thus the name generating process. One rotation of the work completes the cutting up to certain depth. Type of Hobbing 1) Axial Hobbing
This type of feeding method is mainly used for cutting spur or helical gears. In this type, firstly the gear blank is brought towards the hob to get the desired tooth depth. The table side is them clamped after that, the hob moves along the face of the blank to complete the job. Axial hobbing which is used to cut spur & helical gears can be obtained by ‘climb noting’ or ‘convential hobbing. 2) Radial Hobbing
This method of hobbing is mainly used for cutting worm wheels. In this method the hob & gear blank are set with their ones normal to Each other. The gear blank continues to rotate at a set speed about its vertical axes and the rotating hob is given a feed in a radial direction. As soon as the required depth of tooth is cut, feed motion is stopped. 3) Tangential hobbing
This is another common method used for cuffing worm wheel. In this method, the worm wheel blank is rotated in a vertical plane about horizontal axes. The hob is also held its axis or the blank. Before starting the cut the hob is set at full depth of die tooth and then it is rotated. The rotating hob is then fed forward axially. The front portion of the hob is tapered up to a certain length & gives the fed in tangential to the blank face & hence the name ‘Tangential feeding’.
Gear shaping
In gear shapers, the cutters reciprocate rapidly. The teeth are cut by the reciprocating motion of the cutter. The cutter can either be ‘rack - type t ype cutter’ or a rotary pinion type cutter’. Rack - type cutter generating process:
The rack cutter generating process is also called gear shaping process. In this method, the generating cutter has the form of a basic rack for a gear to be generated. The cutting action is similar to a shaping machine. The cutter reciprocates rapidly & removes metal only during the cutting stroke. The blank is rotated slowly but uniformly about its axis and between each cutting stroke of the cutter, the cutter advances along its length at a speed Equal to the rolling speed of the matching pitch lines. When the cutter & the blank have rolled a distance Equal to one pitch of the blank, the motion of the blank is arrested, the cutter is with drawn from the blank to give relief to the cutting Edges & the cutter is returned to its starting position. The blank is next indexed & the next cut is started following the same procedure. Pinion type cutter generating process
The pinion cutter generating process is fundamentally the same as the rack cutter generating process, and instead of using a rack cutter, it uses a pinion to generate the tooth profile. The cutting cycle is commenced after the cutter is fed radically into the gear blank Equal to the depth of tooth required. The cutter is then given reciprocating cutting motion parallel to its axis similar to the rack cutter and the cutter & the blank are made to rotate slowly about their axis at speeds which are Equal at the matching pitch surfaces. This rolling movement
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From the Knowledge Centre’S Desk
February 2011
blow the teeth on the blank are cut. The pinion cutter in a gear shaping rn/c may be reciprocated either in the vertical or in the horizontal axis.
Classification Classification of Gear
Spur Gears General: Spur gears are the most commonly used gear type. They are characterized by teeth which are perpendicular to the face of the gear. Spur gears are by far the most commonly available, and are generally the least expensive. The basic descriptive descriptive geometry for a spur gear is shown in the figure below. Limitations: Spur gears generally cannot be used when a direction change between the two shafts is required. Advantages: Spur gears are easy to find, inexpensive, and efficient.
Helical Gears General: Helical gears are similar to the spur gear except that the teeth are at an angle to the shaft, rather than parallel to it as in a spur gear. (See the references for more specific information). The resulting teeth are longer than the teeth on a spur gear of equivalent pitch diameter. The longer teeth cause helical gears to have the following differences from spur gears of the same size:
Tooth strength is greater because the teeth are longer,
Greater surface contact on the teeth allows a helical gear to carry more load than a spur gear
The longer surface of contact reduces the efficiency of a helical gear relative to a spur gear
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From the Knowledge Centre’S Desk
February 2011
Limitations: Helical gears have the major disadvantage that they are expensive and much more difficult to find (at least insofar as an ME3110 student is concerned). Helical gears are also slightly less efficient than a spur gear of the same size (see above). Advantages: Helical gears can be used on non parallel and even perpendicular shafts, and can carry higher loads than can spur gears.
Bevel Gears General: Bevel gears are primarily used to transfer power between intersecting shafts. The teeth of these gears are formed on a conical surface. Standard bevel gears have teeth which are cut straight and are all parallel to the line pointing the apex of the cone on which the teeth are based. Spiral bevel gears are also available which have teeth that form arcs. Hypocycloid bevel gears are a special type of spiral gear that will allow nonintersecting, non-parallel shafts to mesh. Straight tool bevel gears are generally considered the best choice for systems with speeds lower than 1000 feet per minute: they commonly become noisy above this point. Limitations: Limited availability. availability. Cannot be used for parallel shafts. Can become noisy at high speeds. Advantages: Excellent choice for intersecting shaft systems.
Worm Gears General: Worm gears are special gears that resemble screws, and can be used to drive spur gears or helical gears. Worm gears, like helical gears, allow two non-intersecting 'skew' shafts to mesh. Normally, the two shafts are at right angles to each other. A worm gear is equivalent to a V-type screw thread. Another way of looking at a worm gear is that it is a helical gear with a very high helix angle. Limitations: Low efficiency. The worm drives the drive gear primarily with slipping motion, thus there are high friction losses. Advantages: Will tolerate large loads and high speed ratios. Meshes are self locking (which can be either an advantage or a disadvantage).
Racks (straight gears) General: Racks are straight gears that are used to convert rotational motion to translational motion by means of a gear mesh. (They are in theory a gear with an infinite pitch diameter). In theory, the torque and angular velocity of the pinion gear are related to the Force and the velocity of the rack by the radius of the pinion gear, as is shown below: Perhaps the most well-known application of a rack is the rack and pinion steering system used on many cars in the past.
Limitations: Limited usefulness. Difficult to find. Advantages: The only gearing component that converts rotational motion to translational motion. Efficiently transmits power. Generally offers better precision than other conversion methods.
References
http://www.123eng.com/seminar/GEAR%20MFG..pdf http://www.public.asu.edu/~smurshed/acad http://www.public.asu.edu/~ smurshed/academic/assignments emic/assignments/gear_classification.pdf /gear_classification.pdf http://nptel.iitm.ac.in/courses/IIT-MADRAS/Machine_Des http://nptel.iitm.ac.in/courses/IIT-MAD RAS/Machine_Design_II/pdf/2_5.pdf ign_II/pdf/2_5.pdf http://www.ehow.com/list_7648221_problem http://www.ehow.com/list_76 48221_problems-brake-override.html s-brake-override.html
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