Detecting planetary gear bore crack

Abstract

Since 2009 there have been two fatal crashes of the Super Puma helicopter caused by fatigue cracks propagated from the bore of the planetary gear in the main transmission gearbox. The bore crack propagated through the gear rim causing the planet gear to breakup, which consequently destroyed the integrity of the transmission gearbox. For helicopter safety, it is imperative to develop methodologies for detecting such faults and to implement this capability into helicopter Health and Usage Monitoring Systems (HUMS). In this paper, a method is proposed to detect and track the propagation of a planetary gear bore-crack based on planet gear Synchronous Signal Averaging (SSA) and residual signal enveloping. The method has been initially validated using the vibration data generated from a small industrial planetary gearbox test rig with a notch inserted in the bore of one of its planetary gears. Results from this test show that the planetary gear bore notch is detectable with two different notch sizes using the residual signal of the composite planet SSA signal under three different load conditions. Furthermore, the diagnostic capability may be achievable using the squared envelope of the SSA residual signal, where the respective meshing of the defective section in the planet gear with the ring and sun gears are individually identifiable. Further bench testing will be conducted in the small test gearbox and in a full-scale Bell-206B helicopter main rotor gearbox with a very fine spark-eroded initial notch defect inserted in the bore of the planetary gear. The objective is to initiate a real fatigue crack from the bore notch and propagate the crack. The vibration data generated in this test will be used to further validate the proposed method.