Metallurgical analysis of components of an aeroengine

A few components belonging to an aeroengine were sent to the laboratory for investigation. Evidences suggested that the primary failure in the engine was in the inter-rotor bearing. The damages seen on the other components of the engine were secondary in nature and they appeared to have occurred subsequent to the primary failure in the inter-rotor bearing. It is believed that the failure in the bearing was responsible for vibration and wobbling of the LP/HP spools. This had led to secondary damages in various other components such as breaking of the graphite sealing rings, dislodgement of the graphite sealing rings holding nut, etc. This in turn, led to loss of lubrication oil in the bearing. The bearing had then run under lubrication starvation condition until stop. During this time, the damages in the bearing were severe and this had led to temperature generation in excess of 8000C. Finally, when the rotors stopped, the LP/HP spools were under complete seizure. Due to the damages that took place after initiation of vibration and wobbling of the spools, the primary signatures of failure in the bearing were largely obliterated. However, from the gross evidences that were present on the elements of the failed bearing, it is considered probable that the failure was progressive in nature and spalling on the rolling surfaces of the bearing elements appears to have been the mechanism of failure. Since this mechanism of failure could not be unambiguously established from the evidences available on the failed bearing, an inter-rotor bearing of another engine (No. 30109610400059) which was rejected due to excessive vibration was subjected to detailed examination. Study revealed that there was initiation of spalling on all rolling surfaces of this bearing. In view of this finding and the repetitive nature of failure in the inter-rotor bearing of an engines, spalling is considered being the most probable failure mechanism in the bearing of engine No. 301020300052. Spalling in a bearing can occur due to various reasons, among which the deficiency in the material of construction in this case was ruled out from the laboratory test results. Evidences did not show any assembly related problems as well. However, analysis indicated that spalling was most probably associated with lubrication related issues. Although the information available with this laboratory was insufficient for establishing the exact problem, the possible causes that could give rise to spalling in the present bearing have been discussed in detail. A few of these possible causes could be eliminated based on the laboratory findings and the rest needs further evaluation. Also, a few recommendations have been suggested for narrowing down the probable causes. After the evaluation, if it is found that there are no lubrication related issues, it is suggested that the total technical life (TTL) of the bearing with the present design and prevalent operating conditions be reevaluated