Fluid coupling in the context of Hydraulic pump

A centrifugal clutch is an automatic clutch that uses centrifugal force to operate. The output shaft is disengaged at lower rotational speed and engages as the output increases to a certain speed. It is often used in mopeds, underbones, lawn mowers, go-karts, chainsaws, mini bikes, and some paramotors and boats to keep the engine from stalling when the output shaft is slowed or stopped abruptly, and to remove load when starting and idling. It has been superseded for automobile applications by the fluid coupling, torque converter and automated manual transmissions.

A torque converter is a device, usually implemented as a type of fluid coupling, that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the prime mover to the automatic gear train, which then drives the load. It is usually located between the engine's flexplate and the transmission. The equivalent device in a manual transmission is the mechanical clutch.

A torque converter serves to increase transmitted torque when the output rotational speed is low. In the fluid coupling embodiment, it uses a fluid, driven by the vanes of an input impeller, and directed through the vanes of a fixed stator, to drive an output turbine so that torque on the output is increased when the output shaft is rotating slower than the input shaft, thus providing the equivalent of an adaptive reduction gear. This is a feature beyond what a simple fluid coupling provides, which can match rotational speed but does not multiply torque.