Understanding Vibration Spectra of Planetary Gear Systems for Fault Detection

This paper explores the vibration spectra for planetary gear systems by studying a kinematic model of vibration and comparing the model with measurements of two helicopter transmissions made in flight. The model and flight data include systems with both uniformly and nonuniformly spaced planet gears. This model predicts vibration to occur only at frequencies that are integer multiples of the planet spacing repetition frequency and clustered around gear mesh harmonics. Vibration measurements show the model correctly predicts the frequencies with large components around the first several harmonics of the gear mesh frequency. Measurements do not confirm some of the more detailed features predicted by the model. Some features in the spectra from the numerically derived model can be used to separate the model data with and without planted faults. These features were not found useful for detecting faults in the vibration measurements of real gearboxes in flight due to added complexity in the spectra from real gearboxes.