Vibration-based monitoring of helicopter epicyclical gears

Epicyclical gears in helicopters are the most sophisticated part of the mechanical transmission monitored. This is a result of its numerous sub-components and its low accessibility for sensor mounting and finally, its complex, dynamical behavior. Some analytical and finite element models have suggested to anticipate the vibratory response. However, the external forces that apply to the system during the flight, and the complexness of the mechanical design and the multiple power transfer paths, make it quite difficult to make an accurate prediction of the vibratory response received by the accelerometer. Utilizing this prediction for a condition monitoring purpose would lead to poor performances. For this reason, a signal oriented model of vibrations for epicyclical gears is preferred. This model allows for the anticipation of the influence of some damages. Two indicators are proposed to detect the presence of a crack similar to that of a transverse crack on the carrier. The first one uses the cyclic coherence and the second one takes advantage of cyclostationary component separation. These indicators are then tested on data recorded from an Eurocopter's testing rig and have allowed the tracking of the growth of the crack.