Overrunning Clutch

overrunning clutch [′ō·və‚rən·iŋ ′kləch]

(mechanical engineering)

A clutch that allows the driven shaft to turn freely only under certain conditions; for example, a clutch in an engine starter that allows the crank to turn freely when the engine attempts to run.

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Overrunning Clutch 

(also freewheeling clutch), a device used to connect two coaxial shafts or a shaft to a freely moving part that is seated on the shaft. An overrunning clutch transmits rotary motion and torque from a driving member to a driven member in one direction only. There are overrunning versions of positive clutches, including adaptations of the ratchet and jaw types; there are also overrunning friction clutches, with circular cylindrical and eccentric rollers and with self-tensioning helical springs (Figure 1).

Figure 1. Overrunning clutches: (a) ratchet clutch; (b) jaw clutch; (c) friction clutch with cylindrical rollers; (d) friction clutch with self tensioning helical spring; (1) element transmitting rotary motion and torque in the direction indicated by the arrow; (2) element to which rotary motion and torque are transmitted

Overrunning clutches are used to prevent a reverse transmission of motion in a kinematic loop, such as motion from the driving wheel of a bicycle to the pedals. They are used to convert a rocking motion to a rotary motion, for instance, in the pulse type of continuously variable transmissions, or to impart to a slowly rotating shaft a faster rotation in the same direction, as, for example, in mechanisms for high-speed shifting in metal-cutting machines. Other applications include winding mechanisms and arresting devices, where reverse shaft rotation must be prevented.