Failure analysis of a pinion

Abstract: A pinion, which was a part of an air-motor driving a diesel generator (DG) of a power plant’s emergency power-supply system, was found to have failed into three parts during a routine check-up of the DG. This pinion, attached to the air-motor shaft is active only for a short duration during the start up, when the motion of the air-motor shaft is transferred to the ring gear attached to the DG which in turn starts the DG. Once the DG has started, the pinion disengages from the ring gear and the air-motor stops. The pinion that failed had been in operation for a long time and experienced several start-up operations before failure. A systematic failure analysis was carried out to find out the reasons for this unexpected fracture of the pinion during operation. The results indicate that the fracture was caused by fatigue with a fatigue crack initiating from the fillet of one of the pinion teeth. Due to misalignment that was present, the pinion did not mesh properly with the ring gear during the start-up operation and this led to a high stress concentration at the root of the pinion. The misalignment also led to severe wear and excessive heat generation at the mating surface. Misalignment, improper heat treatment, non-uniform distribution of sulphide inclusions and sharp corners would have all contributed to the failure of the pinion. [Copyright &y& Elsevier]