46 Overrun clutches

Various names have been used for these mechanisms such as freewheel, one way clutch and overrun clutch, each one signifying the nature of the device and is therefore equally appropriate.

A freewheel device is a means whereby torque is transmitted from one stationary or rotating member to another member, provided that input torque (drive speed) is greater than that of the output member. If the conditions are reversed and the output member's applied torque (or speed) becomes greater than that of the input, the output member will overrun the input member (rotate faster). Thus the lock between the two members will be automatically released. Immediately the drive will be discontinued which permits the input and output members to revolve independently to one another.

Overrun clutches can be used for a number of applications, such as starter motor pre-engagement drives, overdrives, torque converter stator release, automatic transmission drives and final differential drives.

Most overrun clutch devices take the form of either the roller and wedge or sprag lock to engage and disengage drive.

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