An explanation of frequency features enabling detection of faults in equally spaced planetary gearbox

Equally spaced planetary gearboxes are important power-train components for varied engineering systems. Their failures can result in significant capital losses and pose safety concerns. The vibration measurements perceived by a sensor mounted on the gearbox housing can provide valuable diagnostic information without normal gearbox operation interference. However, such vibration based monitoring techniques are difficult to implement in planetary gearboxes because of the complex nature of measured vibration spectra that is a result of planets revolving with respect to the stationary sensors mounted on the gearbox housing. Previous research with simulations and experiments using such measurements has reported distinct sideband patterns in the resulting vibration spectra, which differ significantly from the spectra of a normal fixed-axis/parallel gear pair system. In this paper, Fourier series analysis is used to explain these distinct sideband patterns that contain rich diagnostic information. The results obtained are useful to understand the cause of the observed vibration behavior in both healthy and faulty planetary gearboxes and identify the locations of additional frequency components introduced by the damaged gear in a complex measured vibration spectrum. Thus, the formulation presented in this paper can assist in developing robust feature extraction algorithms for early detection of planetary gearbox failures. The theoretical derivations presented in this paper are validated by both dynamic simulations and experiments on a dynamometer test bed using a 750 kW gearbox damaged during its operation while installed in a wind turbine. The predicted frequencies for observed faults in the annulus and sun gears of the gearbox are vividly presented in the experimentally measured frequency spectrum.