Experiments and analysis of vibration signals from a failed component in a helicopter gearbox

This paper investigates and analyses a failed component in a helicopter gearbox after metal flaking triggered a chip detector warning. Examination of the gearbox revealed spalling in one of the rollers of a cylindrical roller bearing. In-flight data was acquired and a pool of techniques has been suggested in this paper for detecting bearing faults. They are envelope demodulation, cepstrum analysis and statistical methods such as zero order figure of merit (FM0), spectral entropy (SE) and energy in the enveloped spectrum. Systematic failure analysis was carried out, using in-flight data and also data collected from a test rig under controlled laboratory conditions, i.e., a helicopter gearbox fitted with the same defective bearing. The gearbox was run at different speed and load conditions. Multiple vibration accelerometers were mounted on the bearing housing in the axial and radial directions to capture vibration signals for maximum signal coverage. Analysis of in-flight signals is a complex one as there are a number of shaft rotational frequencies as well as gear mesh frequencies (GMF) and their harmonics and these signals can easily mask bearing fault frequencies. The methods suggested above attempt to address this problem and the results are promising