Contact Fatigue Failure of an Axle Shaft Spur Pinion

One end of an axle shaft containing the integral spur pinion was submitted for examination, along with the report of a tooth pitting failure. The spur pinion, integral to the axle shaft, operated in a medium-size, off-highway truck at an open-pit mine, for “a relatively short time.” Only the pinion head had been returned. The shaft portion had been torch-cut away. Chemical analysis along with the microstructure confirmed the specified material was SAE 43BV12 Ni-Cr-Mo alloy steel. The mode of failure was surface contact fatigue through the shear plane subsurface at the lowest point of single-tooth contact. The cause of failure was tooth-tip interference from the mating gear teeth. Because the mating parts within the assembly had not been returned or examined, unanswered questions remained.