Frequency analysis on vibration signatures for gearbox spalling defect detection

Gearboxes are widely used in rotary machinery for mechanical power transmission and are prone to faults. Vibration signature analysis usually provides a good potential for gearbox incipient fault detection. This article presents an application of frequency analysis on vibration signatures obtained from a faulty gearbox. Experiments were carried out on an accessory gearbox of a J85 engine, which serves on several aircrafts such as Canadair CT-114 Tutor. Real operating system of the gearbox is reconstructed on a test rig located at the National Research Council of Canada. A spalling defect was artificially introduced to a flank of one of the pinions of the gearbox at the meshing section. Vibrations of the gearbox under various operating conditions are measured by an accelerometer mounted on the gearbox housing. Raw signals are first time synchronous-averaged (TSA) to reduce noise and then the averaged signals are transformed into frequency domain for analysis. Order analysis, which represents all frequencies in terms of shafts' rotating frequency, is employed instead of traditional frequency analysis in this investigation. Testing results demonstrate that the fundamental and the second gear mesh frequency (GMF) contain the major features of the gearbox health condition. Analysis results also show that at a constant operating speed, the spalling defect attenuates the power of the fundamental GMF while strengthens the power of the second GMF. Accordingly, the magnitude ratio between the fundamental and the second GMF is introduced as an indicator for spalling defect detection. The fault indicator is capable of clearly distinguishing the normal pinion from the defected one. It is also observed that the proposed fault indicator is robust and is consistent as the load on the gear is varied.