Question 56 QMED01 - Junior Engineer

The Correct Answer is B **Explanation for Option B (The Overrunning Clutch):** The overrunning clutch (also known as a sprag clutch or Bendix drive in some electric starters, though the principle is the same in hydraulic systems) is the mechanical device specifically designed to protect the starter from the engine's rotational speed once the engine fires and begins to accelerate. When the starter engages the engine's flywheel, it drives the engine. However, once the engine is running on its own power, it immediately speeds up, attempting to spin the starter gear much faster than the starter motor is designed to handle. The overrunning clutch is located between the starter drive gear and the starter motor shaft. When the speed of the output (the drive gear) exceeds the speed of the input (the starter shaft), the clutch disengages or freewheels, preventing the high rotational speed of the engine from being transferred back into and damaging the internal components of the starter motor. **Why the Other Options are Incorrect:** * **A) the immediate increase in hydraulic pressure:** While hydraulic pressure is necessary to operate the starter, an *increase* in pressure once the engine starts would not mechanically disconnect the starter from the engine. In fact, the hydraulic pressure used to spin the starter motor must drop (by releasing the start button) to stop the starter from continuing to run, but the physical protection from back-drive speed is mechanical, not hydraulic. * **C) closing the starting check valves:** Check valves in a hydraulic system prevent fluid from flowing backward. In a hydraulic starting system, they ensure high pressure is delivered to the motor or prevent backflow into the accumulator. They do not provide the mechanical protection needed to decouple the rotating gears when the engine overspeeds the starter. * **D) the pivoting of the shaft from being engaged with the flywheel:** The shaft often shifts or pivots to initially engage the flywheel ring gear (often done hydraulically or pneumatically in large systems). However, the act of pivoting *out* of engagement is typically done by releasing the starter button, which removes the pressure holding it in place. This release is often accompanied by the action of the overrunning clutch, but the clutch is the actual component providing speed protection *before* the starter fully retracts. Pivoting is the mechanism for engagement/disengagement, not speed protection.