Vibration fault features of planetary gear train with cracks under time-varying flexible transfer functions

Abstract Due to the multiple gear-pair meshes in planetary gear train, fault features of tooth root crack are weak and complicated in vibration signals. Influenced by the time-varying transfer paths of vibration signals, the fault features of crack are further weakened, which becomes hard to diagnose. To reveal the origination of crack fault features, a lumped-parameter-model of planetary gear train is firstly built to solve the excitation forces that generate vibration signals. Gear mesh stiffness that drives the excitation forces is used to equivalently reflect the crack characteristics. Secondly, a finite-element-model of flexible housing is used to calculate the time-varying transfer functions that determine the amplitude characteristics of vibration signals. Based on the signal convolution theorem, the full-band vibration signal is deduced by the convolution of excitation forces and flexible transfer functions. Simulations and experimental tests on two typical types of planetary gear trains show good consistency at crack fault frequency features, which effectively validates the proposed models. Significant fault features locating at resonance modulation sidebands are summarized as effective indicators for crack diagnosis.