A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft (driving shaft) to a driven shaft. The clutch acts as a mechanical linkage between the engine and transmission; and briefly disconnects, or separates the engine from the Transmission system, and therefore the drive wheels, whenever the pedal is depressed, allowing the driver to smoothly change gears.
In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). In these devices, one shaft is typically attached to an engine and other to power unit (the driving member).
While the other shaft (the driven member) provides output power for work and typically the motions involved are rotary, linear clutches are also possible.
In a torque-controlled drill, for instance, one shaft is driven by a motor, and the other drives a drill chuck. The clutch connects the two shafts so they may be locked together and spin at the same speed (engaged), locked together but spinning at different speeds (slipping), or unlocked and spinning at different speeds (disengaged).