Question

What is the purpose of the overrunning clutch in a starter motor?

Accepted Answer

The overrunning clutch in a starter motor prevents the engine from driving the starter motor after the engine has started. A starter motor is used to crank the engine until it reaches a self-sustaining speed. Once the engine starts, it begins to rotate faster than the starter motor. Without an overrunning clutch, the engine would drive the starter motor at an excessively high speed, potentially damaging the starter motor. The overrunning clutch is a one-way clutch that allows the starter motor to transmit torque to the engine to start it, but prevents the engine from transmitting torque back to the starter motor once the engine is running. Typically, it uses a system of rollers or sprags that lock in one direction, allowing the starter motor to turn the engine, but slip or freewheel in the opposite direction, preventing the engine from overspeeding the starter. This protects the starter motor from damage and prolongs its lifespan. For example, if a driver keeps the ignition key engaged after the engine has started, the overrunning clutch will prevent the engine from spinning the starter motor at dangerously high speeds, saving the starter from destruction.

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What is the purpose of the overrunning clutch in a starter motor?

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