A Clutch is a mechanical
device that provides for the transmission of power (and therefore usually motion) from
one component (the driving member) to another (the driven member) when engaged,
but can be disengaged.
are used whenever the transmission of power or motion must be controlled either
in amount or over time (e.g., electric screwdrivers limit how much torque is
transmitted through use of a clutch; clutches control whether automobiles
transmit engine power to the wheels).