Cut-off point (COP) for early gear fault diagnosis via Meshing impact modulation (MIM) analysis

Comparing the number and magnitude differences and variations of the fault-induced features in frequency domain, such as the sidebands of meshing frequency in the raw spectrum, or the rotation frequency of faulty gear in the envelope spectrum between healthy and faulty conditions are the underlying principle of most of classical fault diagnostic methods for gearbox. However, several inevitable factors, for instance, circumferential stiffness anisotropy of gear shaft (including coupling mechanisms like keyways) and gear meshing backlash, can also lead to similar features, which may challenge these classical methods, especially when the fault is in its early stage. In specific, the initial early gear fault may neither lead to the significant increment of sideband or rotation frequency nor stimulate the high amplitude resonance, more than the inevitable factors mentioned above. As such, a meshing impact modulation frequency band is constructed to create a unique feature which does not exist in healthy condition. Considering both the impact nature and the modulation mechanism of gear meshing process, an MIM (Meshing Impact Modulation) index is designed, with which can effectively identify the optimal meshing impact modulation response frequency band with high stability and robustness across entire lifespan of gears. Within the demodulation spectrum of this band, the presence or absence of distinct gear rotation frequency spectral lines and their mirage as sidebands around the meshing frequency can serve as the effective and robust indicators to discern whether the gear is in a healthy "before cut-off point (pre-COP) state " or in an early faulty "after cut-off point (post-COP) state ". The effectiveness and the superiority of the proposed algorithm is testified by the life-test data from a test rig using industrial gearbox.