Failure analysis of driving gear components of a helicopter

In continuation of the earlier investigation on the IGB failure of a helicopter (NAL Report No: MT-FA-937-06-2005), a few more components were examined to establish the cause of excessive load on the gears. Observations revealed that the driving gear failure was initiated due to minute oscillatory motion (slip) between the shaft and the bore of the roller bearing (45 mm inner dia). This was evident from the fretting on the driving gear shaft as well as the inner race of the roller bearing. Analysis suggests that excessive surface undulations on the shaft and/or bore of the bearing would have promoted fretting. The excessive load on the tooth flank appears to be due to slip between the shaft and the roller bearing. This slip would have also resulted in improper meshing of the gears