gear

The toothed and interlocking wheels which make up a typical gear movement.

Gear ratio is calculated by dividing the number of teeth on the driver gear by the number of teeth on the driven gear (gear ratio = driver/driven); the idler gears are ignored. Idler gears change the direction of rotation but do not affect speed. A high driven to driver ratio (middle) is a speed-reducing ratio.

Different gears are used to perform different engineering functions depending on the change in direction of motion that is needed. Rack and pinion gears are the commonest gears and are used in car steering mechanisms.

Toothed wheel that transmits the turning movement of one shaft to another shaft. Gear wheels may be used in pairs, or in threes if both shafts are to turn in the same direction. The gear ratio – the ratio of the number of teeth on the two wheels – determines the torque ratio, the turning force on the output shaft compared with the turning force on the input shaft. The ratio of the angular velocities of the shafts is the inverse of the gear ratio.

The common type of gear for parallel shafts is the spur gear, with straight teeth parallel to the shaft axis. The helical gear has teeth cut along sections of a helix or corkscrew shape; the double form of the helix gear is the most efficient for energy transfer. Bevel gears, with tapering teeth set on the base of a cone, are used to connect intersecting shafts.