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sprag clutch[′sprag ‚kləch]
A clutch designed to transmit power in one direction only.
A mechanical clutch used in reciprocating engines and direct-shaft turbine engines. It helps remove the load of the rotor when the engine is being started. In helicopters, it allows the engine to drive the rotor, but when the engine power is reduced, or the engine fails, the clutch allows the rotor to freewheel and prevents it from driving the engine. When the clutch is partially engaged, the RPM needles are split, and when it is fully engaged, the needles are married.