Metallurgical failure analysis of inter-rotor and inter-shaft bearings of aeroengines

Engine No. 30109630400080 (hours done: 992:38:22): The inter-rotor bearing of this engine showed signs of degradation in terms of pitting and scoring marks. The damages observed were in the initiation stages. Considering a service life of 992:38:22 hr against a TTL of 1000 hr, it can be stated that the bearing performance is more or less satisfactory. However, in view of the possible variation in the operating conditions from one engine to another, the margin for safe life of the bearing for the stated TTL of 1000 hr needs to be evaluated. Initiation of damages was also observed on all elements of inter-shaft bearing. However, compared to the inter-rotor bearing, the damages in this case were negligible. Judging from the overall conditions of the rolling surfaces, it is stated that the bearing is in usable condition. However, whether or not the bearing can be used for another 1000 hrs cannot be commented from this study. Considering the failure history, it is suggested that the health of the bearing be monitored during further use. Engine Nos. 16-96-006 (hours done: 365:43:28) and 30109610600121 (hours done: 365:20:23): All the elements of the inter-shaft bearings of these two engines were found damaged in the form of spalling, gouging, adhesive wear and smearing of metallic material. The damages were very severe, and the conditions of the bearings were found to be unfit for any further use. Detailed examination revealed that the failure on the rolling surfaces was by surface contact fatigue which manifested in the form of spalling. Once the debris generated due to spalling, further damage to the bearings was autocatalytic. Analysis shows a few possibilities which could lead to failure of the inter-shaft bearings of the above engines by surface contact fatigue. These are discussed in this report. However, identification of the primary cause(s) for failures requires detailed system level analysis by the designer, manufacturer and the user. It is believed that the metallurgical evidences and the analysis presented in this report would form valuable inputs for such analysis