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