device for quickly and easily connecting or disconnecting a pair of rotatable coaxial shafts. Clutches are usually placed between the driving motor and the input shaft to a machine and provide a convenient means for starting and stopping the machine and permitting the driving motor or engine to be started in an unloaded state (as in an automobile).

Mechanical clutches provide either a positive (no-slip) or a friction-dependent drive. Positive clutches are collars with jaws that interlock, one member being rigidly attached to its shaft while the other slides on its shaft.

Friction clutches have pairs of conical (see illustration), disk, or ring-shaped mating surfaces and means for pressing the surfaces together. The pressure may be created by a spring or a series of levers locked in position by the wedging action of a conical spool.

Automatic engagement is obtained with a centrifugal clutch in which the friction shoes are segments of rings that are pivoted to or carried around by the driving member and make firmer and firmer contact with the internal cylindrical surface of the driven member as the driver's speed increases.

An overrunning clutch transmits torque in one direction only and permits the driven shaft of a machine to freewheel, or keep on rotating when the driver is stopped. On bicycles, such clutches permit the rider to coast without moving the pedals.