Failure investigation of a truck diesel engine gear train consisting of crankshaft and camshaft gears

A failure investigation has been conducted on a diesel engine gear train consisting of a drive crankshaft and a driven camshaft gears that were used in a truck. The gears are made from a nitrided 42CrMo steel. Adjacent teeth fracture regions appeared on the gears after a service of 4.2 10 4 km. Fractographic features indicate that multiple origins fatigue fracture was the dominant failure mechanism for the gear teeth. The crankshaft gear fracture first, followed by the camshaft gear. Low hardness in subsurface and core region of the nitrided crankshaft gear makes it difficult for the matrix to support the load by the engaged camshaft gear to lead to initiating the fatigue crack at the root fillet bearing the maximum tensile stress. The crankshaft gear is the component causing trouble for the failed gears train.