Failure Analysis of Bearings of Accessory Gearbox of an aero-engine

During 100 h servicing of an aeroengine, chips were noticed in the oil system. Strip examination of the engine revealed severe damages to the cover side bearing on O3 axis of the engine accessory gearbox with cage broken and lying inside the casing along with the balls. In addition, on the cover side, the bush on O3 axis was found without nuts and tab washers. The damaged bearing along with the dislodged nuts and tab washers were sent to this laboratory for analyzing the cause(s) of failure. Examination revealed that there was spalling, smearing and flow of material in all elements of the cover side bearing. There was sliding in the bearing due to breakage of the cage resulting in loss of shape of the balls and flow of material out of inner raceway. Fractographic study confirmed that the cage of the bearing had failed by fatigue. This, along with the signatures of failure in the bearing as a whole indicated that the failure was progressive in nature, and it was caused due to rotational creep from either inner ring being loose or outer ring being loose or excessive axial load. Since there is remote possibility of excessive axial load on the bearing on O3 axis of the EAGB, looseness in the inner ring and/or outer ring or any problem(s) elsewhere which could give rise to similar situation appears to be the reason for the bearing failure. Laboratory examination did not reveal any damages to the dislodged nuts except for rubbing marks along all six edges of the nut faces. The Cu-plating on all the faces was found undisturbed. The threads were found to be in tact without any distress. This leads to the conclusion that the bush nuts have come out of the bolts by progressive unwinding. This is further substantiated by the presence of rubbing marks on all the six edges of the nuts, which is possible only when the nuts rotate around the axis, that is, the bolts, and they make contact periodically with the tab washer surface. The two tab washers provided for examination showed fretting damage on the contacting surfaces with the nuts as well as the bush. This is indicative of relative movement of the tab washers in assembly in service. Examination also revealed that the bending of the tab washers for effecting the locking of the nuts was oblique and also, the contact between the tab washer and the nut faces was inadequate. This was evident from the contact marks available on the tab washer surfaces. Evidences suggest that the nut surface was in partial contact with the tab washer and hence, the locking was not effective enough to offer adequate resistance for preventing unwinding of the nuts even under normal operating conditions. Once there was unwinding of the nuts, the bush on the cover side was as good as unsupported. This is similar to a situation wherein the outer ring of the bearing is loose. Hence, the looseness in the bush would result in progressive failure in the bearing as seen in the present case. The disturbances caused because of progressive failure in the cover side bearing had resulted in initiation of spalling on all rolling surfaces of the bearing on the casing side as well. Hence, the damages to this bearing are secondary in nature. Laboratory studies did not reveal any metallurgical deficiencies in the bearing elements or in the tab washers which could give rise to the present failure. Also, negligible discolouration even after complete destruction of the bearing indicates that there were no lubrication related issues which could be the cause of failure. Based on the above, it is concluded that the failure in the cover side bearing on O3 axis of EAGB was progressive in nature. The failure was resulted due to unwinding and dislodgement of the nuts meant for securing the bush in position. Investigation showed that the unwinding of the nuts was possible because of deficiency in locking mechanism. Therefore, the primary cause for the bearing failure is attributed to either improper assembly of tab washers or inappropriate choice of the tab washer type for the present application