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

Abstract

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.

Keywords:

• Order tracking
• autoregressive models
• spectral kurtosis
• complex morlet wavelets
• constant percentage bandwidth

Additional information

Notes on contributors

N Sawalhi

Nader Sawalhi was born in Canberra, Australia in 1974. He received the BSc degree in mechanical engineering from the University of Jordan, Amman, Jordan in 1997. He received the MEngSc degree and the PhD in 2001 and 2007 respectively, both in mechanical engineering, from the University of New South Wales (UNSW), Sydney, Australia.

Between 1997 and 2003, he was with Jordan Cement Factories, Amman, Jordan, as a reliability and predictive maintenance engineer. From 2003 to 2007 he worked on his PhD project at UNSW developing signal processing algorithms and simulation models for rolling element bearings in complex environments. The project, under the supervision of Prof Bob Randall, was fully supported by the Australian Defence Science and Technology Organisation (DSTO) as a part of their Centre of Expertise scheme. He spent three months on a sponsored internship with FAG Bearings, Schweinfurt, Germany, in 2004. Since 2007 he has been working at the School of Mechanical and Manufacturing Engineering at UNSW as a Research Associate. His research interests include signal processing, machine condition monitoring, diagnostics and prognostics.

Dr Sawalhi is a member of the Jordanian Engineering Association and is eligible for membership of Engineers Australia.

R B Randall

Bob Randall is a Professor in the School of Mechanical and Manufacturing Engineering at the University of New South Wales, Sydney, Australia. He has a degree in mechanical engineering from the University of Adelaide, and an Arts degree in mathematics and Swedish from the University of Melbourne. Prior to joining the University in 1988 as a Senior Lecturer he worked for the Danish company Bruel & Kjaer for 17 years where he was responsible for developing systems for machine condition monitoring and diagnostics. He previously had ten years' experience in the chemical and rubber industries in Australia, Canada and Sweden. He was promoted to Associate Professor in 1996 and to Professor in 2001. He is the author and/or co-author of the Bruel & Kjaer books Frequency Analysis and Mechanical Vibration and Shock Measurements, and the invited author of chapters on vibration measurement and analysis in the McGraw-Hill handbooks Shock and Vibration Handbook and Handbook of Acoustical Measurements and Noise Control as well as the Encyclopedia of Vibration (Academic Press), and the forthcoming Handbook of Noise and Vibration Control (Wiley). He was invited to contribute a chapter on “Signal Processing in Vibration Analysis”, in Structural Dynamics @ 2000: current status and future directions, (Eds. D.J. Ewins and D.J. Inman) Research Studies Press. He is a member of the editorial board of the journals Mechanical Systems and Signal Processing and The International Journal of Comadem. He is the author of more than 140 papers in the fields of vibration analysis and machine diagnostics, and has successfully supervised eleven PhD and three Masters projects in these areas. He is the Director of the DSTO Centre of Expertise in Helicopter Structures and Diagnostics located at the University of NSW, one of four such Centres of Expertise set up and funded in Australian universities by DSTO (Defence Science and Technology Organisation). The main function of the Centre has been to carry out research into diagnostics of helicopter gearboxes and gas turbines, as well as the structural dynamics of helicopter airframes.