Failure Analysis of a Diesel Engine Gear System Consisting of Camshaft and Crankshaft Gears

A failure investigation was conducted on a diesel engine gear system consisting of a driven camshaft and drive crankshaft gears that were used in a truck. The gears are made from a nitrided 42CrMo steel. Adjacent teeth fracture and plastic deformation regions appeared on the gears after a 400 h run test of the gear system. Fractography indicates that fatigue fracture is the dominant failure mechanism for the gear teeth. Although the appearance of needle-like nitrides in the nitrided layer and the narrow depth of the compound layer may decrease the fatigue strength of the camshaft gear, these do not suffice to lead to the premature fracture of the gear teeth. Geometrical analysis of the gears was performed and compared with an analysis of unfailed gears that had experienced a run test for 1800 h. The comparison reveals that the small fillet radius at the root area of the camshaft gear concentrated the stresses and is mainly responsible for fatigue fracture of the teeth. The camshaft gear is the component that initiated trouble in the gear system. The appearance of severe plastic deformation on the gear faces is caused by the fractured teeth crushing the teeth faces and being embedded in the grooves between teeth.