Transmission

A transmission is an assembly located between the drive and the work unit of a mechanical device. It can have following purposes:
- transmitting kinetic energy
- altering speed
- changing the rolling direction

Transmission are everywhere in everyday life. They can be found in mechanical devices and machines, like cars, bicycles, clocks and watches, washing machines, mixers and blenders, or vacuum cleaners.

Different ways of transmission

According to the mechanical principle of energy conversion there are several distinguishable types of transmissions:
- belt drives (e.g. vee belt in a car)
- chain drives (e.g. bicycle chain)
- gear drives (e.g. lathe transmission)

Belt and chain drives transmit the rotary motion of a drive wheel to the driven wheel by means of belts or chains. This type of transmission is used to bridge large center distances.

In a belt drive an elastic belt is used for the transmission of the rotary motion. The force transport occurs by traction. The amount of transmittable force depends on the friction being effective between belt and pulley.
The belt is stretched more on the tractive side than on the opposite side. The tractive force increases gradually at the circumference of the driving pulley and decreases again at the circumference of the driven pulley. The extension in the belt behaves the same. The result is, that the speed of the driven pulley is slightly slower than at the drive pulley. This phenomenon is called slippage. It amounts up to about 2% of the drive speed, depending on the load.

In a chain drive, a steel chain serves as a medium to transmit the motion. It is working by form fit, therefore no slippage occurs.

Gear drives transmit rotary motions from one shaft to another by form fit and therefore without slippage. Gears are toothed interlocking wheels. Gear drives are suitable for transmission of very low powers, like in clocks and watches, up to very large powers, such as in rolling mills, depending on design. In addition to the transmission of rotary motion, mostly the speed, torque or rolling direction are changed.

Gear drives can be designed with multiple gear ratios from which the operator can choose. In a manual automotive transmission, typically four, five or six different gear ratios can be selected by using a gear shift.

Automatic transmissions automatically choose the optimal gear ratio for the current speed of the vehicle. They work using hydraulics to select gears.