Introduction to Gears and Gear Sets
The following information is about gears and gear sets. Gears and gear sets are used in machines for the following purposes:
- Transmit motion and energy
- Change speed and torque relationships
- Change direction of travel
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| Illustration 1 | g01084289 |
Many types of gears are used in Caterpillar machines to perform various functions.
Gear teeth in the mesh act as multiple levers that transfer torque from the engine flywheel to the other gears in the power train. When only two gears are used, the shafts rotate in opposite directions (Illustration 1).
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| Illustration 2 | g01084290 |
Two gears in the mesh are called a gear set. A third gear or idler gear (Illustration 2) is sometimes used between the drive gear and the driven gear. The idler gear changes the direction of the driven gear so the driven gear turns in the same direction as the drive gear.
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| Illustration 3 | g01084291 |
A gear train is three or more gears in mesh together (Illustration 3).
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| Illustration 4 | g01084293 |
When one gear is considerably smaller than another gear the smaller gear is called a pinion (Illustration 4).
Gear Splines
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| Illustration 5 | g01084294 |
Gears are usually mounted on shafts. Power is transferred to gears and from gears by shafts. Gears must be firmly fastened to shafts. Various methods are used to fasten gears to the shafts. Grooves that are known as splines, may be machined on the surface of the shaft and in the gear hub. When the gear is pushed into the shaft, the splines hold the gear so that the gear turns the shaft without slipping. Sometimes, splines are engineered so that the gear can slide sideways on the shaft. This sliding gear feature is often used in transmissions.
Gear Keys
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| Illustration 6 | g01084295 |
Keys are another method that is used to prevent gears from slipping on the gear shafts. In a simple key arrangement, a keyway is machined in the shaft and another keyway is machined in the hub of the gear. When the key is inserted the key locks the gear and the shaft together. A more elaborate variation of the key is a semi-circular type known as a Woodruff key.
Gear Mechanical Advantage
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| Illustration 7 | g01084296 |
Gears in machinery are frequently used to provide a speed advantage or a torque advantage. Gears cannot provide a power advantage. The actual power of a machine is determined by the capacity of the engine. However, the use of different sizes of gears permits engine power and speed to be used most efficiently in order to operate a machine under various load conditions. When gears are used to increase torque, output speed is reduced. When output speed is increased through gearing, torque is reduced.
Gear Ratio
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| Illustration 8 | g01084297 |
The rotational speed of gear-driven shafts depends on the number of teeth in each gear.
A pinion gear with 24 teeth driving a gear with 48 teeth will revolve twice as fast as the gear it is driving. Illustration 8 shows the gear ratio is 2:1.
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| Illustration 9 | g01084299 |
If the power flow is reversed, so that the larger gear is driving the smaller gear, the gear ratio is also reversed to 1:2 as shown in Illustration 9. By using a train of several gears, the ratio of driving to driven speed may be varied within wide limits.
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| Illustration 10 | g01084300 |
A single idler gear that is used to change the direction of rotation does not change the gear ratio (Illustration 10). The idler gear can have any number of teeth. So if a small idler wheel with 12 teeth is used between two gears with 48 teeth the ratio remains 1:1. The same is true if the idler gear has 48 teeth.
Gear Shape and Size
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| Illustration 11 | g01084301 |
The width of a gear across the teeth is called the face width. Wider gears have more contact area and can transmit more power.
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| Illustration 12 | g01084302 |
For a power train to operate properly, all gears in a gear train must have teeth that are compatible with one another in size and shape. The sides of gear teeth are not straight. Instead, gear teeth are machined with a profile that is designed to obtain maximum power transfer efficiency from the gear as it meshes with other gears. The sides of each tooth follow the shape of what is known as an involute curve (Illustration 12). The curved shape provides a rolling contact as opposed to sliding against other teeth in mesh.
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| Illustration 13 | g01084303 |
Gears teeth are cut with a profile so that when teeth mesh they produce a pressure angle that is calculated to allow smooth, full-depth engagement (Illustration 13).
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| Illustration 14 | g01084305 |
Smooth gear mesh is critical to proper gear operation. If gears mesh too tightly, binding occurs. This produces excessive friction and power loss. If the mesh is too loose, gears will be noisy and inefficient. A small amount of clearance (Illustration 14) is required between teeth in order to allow for lubrication and smooth, efficient operation. The clearance allows a slight backward movement of the gears that is called backlash.
Excessive backlash is usually an indication of wear in the gear teeth or the bearings that support the gears. Excessive backlash can result in broken gear teeth or gears that bounce under load. During equipment service operations, it is often necessary to measure and adjust backlash to the proper specification by using shims.
Straight Cut or Spur Gears
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| Illustration 15 | g01084306 |
Since the work of a gear is done by the teeth, gears are usually named according to the way the teeth are cut. As machinery has developed over the years many different gear patterns have been devised to perform specific tasks. For proper operation, meshing gears must have teeth of the same size and design. Also, at least one pair of teeth must be engaged at all times although gear tooth patterns allow for more than one pair of teeth to be engaged. The following gears discussed in this section are the most common gears found in modern industrial machines.
The teeth of straight cut or spur gears (Illustration 15) are cut straight and parallel with the axis of the gear rotation. Straight cut gears are prone to produce vibration. These gears also tend to be noisy in operation and are generally used in slower speed applications.
Straight spur gears are often used in manual transmissions. The straight teeth allow gears to be more easily slid in and out of mesh. This allows for easier shifting.
Helical Gears
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| Illustration 16 | g01084307 |
Helical gears have teeth that are not parallel to the axis of the shaft but are spiraled around the shaft in the form of a helix. Helical gears are suitable for heavy loads because the gear teeth come together at an acute angle, rather than at 90° as in a spur gear. Engagement of the gears begins and rolls down to the trailing edge. This allows for a smoother transfer of power than on a straight cut. This also permits quieter operation and the ability to handle greater thrust. So helical gears are more durable than straight gears.
A disadvantage of simple helical gears is that helical gears produce a sideways thrust that tends to push the gears along shafts. This produces additional load on the shaft bearings.
Herringbone Gears
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| Illustration 17 | g01084315 |
The thrust that is produced by helical gears can be balanced by using double helical, or herringbone gears. Herringbone gears have V-shaped teeth composed of half a right-handed helical tooth and half a left-handed helical tooth. The thrust that is produced by one side is counterbalanced by the thrust on the other side. Usually a small channel is machined between the two rows of teeth. This is to allow for easier alignment and to prevent oil from being trapped in the apex of the V.
Herringbone gears have the same advantages as helical gears, but are expensive. Herringbone gears are used in large turbines and generators.
Plain Bevel Gears
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| Illustration 18 | g01084319 |
Bevel gears permit the power flow in a gear train to turn a corner. The gear teeth are cut straight on a line with the shaft but, the gear theeth are beveled at an angle to the horizontal axis of the shaft. Bevel gear teeth are tapered in thickness and in height. The smaller driving gear is called the pinion while the larger driven gear is known as the ring gear.
Plain bevel gears are used in applications where speed is slower and there is no high impact present. For example, hand wheel type controls often use plain bevel gears.
Spiral Bevel Gears
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| Illustration 19 | g01084322 |
Spiral bevel gears are designed for applications where more strength is needed than a plain bevel gear can provide. Spiral gear teeth are cut obliquely on the angular faces of the gears. The teeth overlap considerably, so the teeth can carry greater loads. Spiral bevel gears reduce speed and increase force.
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| Illustration 20 | g01084327 |
The bevel gear and pinions (arrows) are a matched set. The bevel gear set in Illustration 20 is used in Track-type Tractors to transfer power from the transmission to the axles and final drive.
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| Illustration 21 | g01084334 |
The bevel gear set (1) in Illustration 21 is used in wheel machines to transfer power from the transmission to the differential. Note that the bevel gear in wheel machines is part of the differential assembly.
Hypoid Gears
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| Illustration 22 | g01084337 |
Hypoid gears are variations of helical bevel gears that are used when the axes of the two shafts are perpendicular, but do not intersect. The smaller pinion is located below the center of the larger ring gear that it drives. One of the most common uses of hypoid gearing is to connect the drive shaft and the rear axle in automobiles. Helical gearing that is used to transmit rotation between shafts that are not parallel is often incorrectly called spiral gearing.
Worm Gears
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| Illustration 23 | g01084341 |
Another variation of helical gearing is provided by the worm gear. This gear is also called the screw gear. A worm gear is a long, thin cylinder that has one or more continuous helical teeth that mesh with a helical gear. Worm gears differ from helical gears in that the teeth of the worm slide across the teeth of the driven gear instead of exerting a direct rolling pressure. Worm gears are used mainly to transmit rotation, with a large reduction in speed, from one shaft to another at a 90° angle.
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| Illustration 24 | g01084345 |
Illustration 24 is an example of a worm gear application.
Rack and Pinion Gear Set
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| Illustration 25 | g01084349 |
Rack and pinion gears can be used to convert straight line motion into rotary motion or rotary motion into straight line motion, depending on whether the rack or the pinion is driven. The teeth on the rack are straight cut while the teeth on the pinion are curved. Common uses of a rack and pinion gear set is in automotive steering systems or in an arbor press.
Illustration 26 and Illustration 27 are examples of different rack and pinion gear set applications.
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| Illustration 26 | g01084354 |
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| Illustration 27 | g01084359 |
Ring and Planet Gears
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| Illustration 28 | g01084364 |
Ring gears are used in planetary gear sets. The planetary gear set includes a ring gear with internal teeth which mates with teeth on smaller planetary gears. The planetary gears mate with a sun gear.
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| Illustration 29 | g01084367 |
Planetary gear sets are used in transmissions, torque dividers, and final drives. Planetary gear sets are so named because their operation resembles a small solar system. Illustration 29 shows the components of the planetary gear set.
Planetary gears (1) are also called planet gears, pinions, and idler gears. A sun gear (4) is also called a center gear. Around the sun gear (4), two or more planetary gears (1) rotate in constant mesh with the sun gear. The planetary gears are mounted in a carrier (2) and rotate on the gears own axes while rotating around the sun gear. The planetary gears are also in constant mesh with the inside teeth of a larger ring gear (3) that surrounds the planetary assembly.
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| Illustration 30 | g01084371 |
The planetary transmission shown in Illustration 30 and the planetary final drive shown in Illustration 31 are two examples of the planetary gear set that are used in the power train.
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| Illustration 31 | g01084374 |
Countershaft Gear Set
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| Illustration 32 | g01084377 |
Countershaft gears are used mainly in the manual and the power shift transmissions. Countershaft gear sets (Illustration 32) allow one set of gears to be shifted without disturbing the other gear ratios. The gears are mounted on parallel shafts. The direction of power cannot be changed unless an idler gear is added to the countershaft gear set. One gear on a shaft drives another gear on a second shaft. A countershaft gear set can be equipped with several gears and shafts in order to achieve different speeds.
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| Illustration 33 | g01084418 |
Advantages of the countershaft gear set include fewer parts and less weight. A countershaft gear set is generally less expensive than a planetary gear set. The countershaft transmission that is shown in Illustration 33 is an example of the countershaft gear set that is used in the power train.
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| Illustration 34 | g01084419 |
The bull gear final drive that is shown in Illustration 34 is another example of the countershaft gear set that is used in the power train.