Failure of a Diesel Engine Timing Gear
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A truck diesel engine crankshaft timing gear cracked after a 380-h run test of the gear train. Fractographic analysis indicates that high stress-low cycle fatigue was the dominant failure mechanism of the gear. The external end of the gear is subjected to instant impact load when assembling the gear, causing the surface of the internal hole to bear the localized circumferential tensile stress. The surface of the internal hole of the gear also bore the homogeneous circumferential tensile stress resulting from hot-assembling gear. The crack initiated at the interface of the external end of the gear and the surface of the internal hole of the gear owing to the action of both stresses. Under the action of the alternating operating load, the fatigue crack propagated from the internal hole toward the external circle along the radial direction, and then extended from the external end toward the internal end of the gear along the axial direction until the radial cracking of the gear took place.
KeywordsGear High stress-low cyclic fatigue fracture Instant impact load Failure analysis
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