 Spur gears found on a piece of farm equipment. A gear is a toothed wheel designed to transmit torque to another gear or toothed component. The teeth of a gear are shaped to minimize wear, vibration and noise, and to maximize the efficiency of power transmission.
Different-sized gears are often used in pairs, allowing the torque of the driving gear to produce a larger torque in the driven gear at lower speed, or a smaller torque at higher speed. The larger gear is known as a wheel and the smaller as a pinion. This is the principle of the automobile gearbox. As a gearbox is not an amplifier or a servo system, the amount of power delivered by the output gear or shaft will never exceed the power applied to the input gear, regardless of the gear ratio. There is actually a loss of power due to friction.
The most common type of gear wheel, spur gears, are flat and have teeth projecting radially and in the plane of the wheel. These gears can be fitted only to parallel axles. Helical gears offer a refinement over spur gears. The teeth are cut at an angle, allowing for more gradual, hence smoother meshing between gear wheels. A disadvantage of helical gears is a resultant thrust along the axis of the gear, which needs to be accomodated by appropriate thrust bearings. Double helical gears overcome this problem by having teeth that are 'V' shaped. They can be directly interchanged with spur gears without any need for different bearings. Beveled gears have angled teeth, allowing torque to be transmitted between non-parallel but intersecting axles. If the axles are skewed, i.e. non-intersecting, then a worm gear can be used. This is a gear that resembles a screw, with parallel helical teeth, and mates with a normal spur gear. The worm gear can achieve a higher gear ratio than spur gears of a comparable size.
Torque can be converted to linear force by a rack and pinion. The pinion is a spur gear, and mates with a serrated bar that can be thought of as a spur gear with an infinitely large radius of curvature. Such a mechanism is used in automobiles to convert the rotation of the steering wheel into the left-to-right motion of the tie rod(s).
 A crown gearA crown gear is a special form of bevel gear which has teeth at right angles to the plane of the wheel, allowing it to drive axles at right angles to its own. A variation of this mechanism is used in the differential gear, a complex arrangement of gears that transmits power to two axles moving at variable speeds, such as those on a cornering automobile.
A contrate wheel has teeth at a right angle to the axis and meshes with a straight cut spur gear or pinion (used in clocks, some instrumentation, wind-up toys and Meccano).
Simple gears suffer from backlash, which is the error in motion that occurs when gears change direction. When moving forwards, the front face of the drive gear tooth pushes on the rear face of the driven gear. When the drive gear changes direction, its rear face is now pushing on the front face of the driven gear. Unless deliberately designed to eliminate it, there is slight 'slop' in any gearing where briefly neither face of the driving gear is pushing the driven gear. This means that input motion briefly causes no output motion. Assorted schemes exist to minimize or avoid problems this creates.
In some machines it is necessary to change the gear ratio to suit the task. There are several ways of doing this. For example:
The tooth form used for most applications is involute but there are other tooth forms such as cycloidal (used in mechanical clocks) or rack (used in automobile steering).
See also
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