A gear or cogwheel is a rotating machine part with cutting teeth, and in the case of a gear, an insert tooth (called cogs) that meshes with another toothed part to transmit torque. Geared devices can change the speed, torque, and direction of the power source.

Gears almost always produce a change in torque, and the gear ratios create mechanical benefits. Therefore, it can be considered a simple machine. The teeth of the two meshing gears are all the same shape. Two or more meshing gears that working in a sequence, are called a gear train or transmission. The gears mesh with linear toothed part, called racks, to produce translation rather than rotation.

The gears of the transmission are similar to the wheels of a cross belt pulley system. The advantage of gears is that the gear teeth prevent slippage.

When two gears mesh with each other, if one gear is larger than the other, there is a mechanical advantage in that the rotation speed and torque of the two gears differ in proportion to the diameter.

In transmissions with multiple gear ratios, such as bicycles, motorcycles, and automobiles, the term “gear” refers to the gear ratio rather than “real gear.” The term describes similar devices where the gear ratio is continuous rather than discontinuous, or where the device does not include gears, such as a continuously variable transmission.

External gears are those with teeth formed on the outer surface of a cylinder or cone. Conversely, internal gears have teeth formed on the inner surface of a cylinder or cone. In the case of bevel gears, internal gears is the one with a pitch angle exceeding 90 degrees. The internal gear does not reverse the direction of the output shaft.

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