Helicopter gearbox bearing blind fault identification using a range of analysis techniques

Vibration acceleration signals were obtained from an overload test of a Bell 206 helicopter main rotor gearbox in order to complete a blind bearing fault analysis where no knowledge of the fault was made available prior to the analysis. A range of diagnostic techniques was applied. These included power spectral density comparisons, constant percentage bandwidth (CPB) spectrum analysis, SK analysis to determine the frequency bands with maximum impulsiveness and to filter the signal to maximise that impulsiveness, and envelope analysis to determine the fault frequencies. Order tracking was used to compensate for speed fluctuations, while linear prediction using autoregressive models (AR) was used to remove the regular gear meshing contribution in the signals. As a result of applying these techniques, a fault in one of the planetary bearings was identified. A match with the cage frequency and the inner race ball pass frequency indicated deterioration associated with these components. Roller fault frequencies were not directly detected, but the fact that roller faults give a modulation at cage frequency shows that their effect was still detected. SK gave a good measure of the severity of the fault when compared to the amount of metal wear debris in the oil. Details of the test, as well as application of a statistical fault detection technique can be found in a companion paper submitted by the Defence Science and Technology Organisation (DSTO) Australia.