Fretting Fatigue in a Pinion Shaft of the Planetary Gearbox

This paper reports the failure analysis of the pinion shaft of a bevel planetary gearbox in the agitator drive system of a petrochemical plant. The shaft failed after 4 years of service. The metallurgical failure analysis and fractographic analysis were conducted by means of standard investigation procedures to seek the failure cause. The analysis included visual examination, fractographic analysis by scanning electron microscope (SEM), chemical compositional analysis by optical emission spectroscopy (OES), and hardness testing. The results showed that the failure of the shaft was initiated by the micro-pits formed on the surface of the keyway by fretting. The crack propagated in fatigue and a relatively small final fracture zone was observed which is indicative of low stress concentration on the shaft at the point of failure. The lower hardness was observed near the crack initiation region which was the probable cause of the initiation of micro-pits. It was concluded that the shaft failed due to anomaly in the shaft design/manufacturing or incompatibility in the hardness of the keyway that resulted in fretting at the contact surface. It was recommended to revise the manufacturing procedures after consultation with the manufacturer.