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A gear is a toothed wheel designed to transmit torque to another gear or toothed component. The teeth of a gear are often specially shaped in order to minimise wear and maximise 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. Regardless of the gear ratio, the amount of power delivered by the output gear cannot exceed the power applied to the input gear, and there is usually 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. Bevelled 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 steering rods.

A crown gear

A 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. 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 minimise 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:

• manual gearbox ('stick shift' in the US)
• automatic gearbox
• derailleur gears
• hub gears (also called epicyclic or sun-and-planet gears)
• continuously variable transmission
• infinitely variable transmission

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).