A lumped mass gear-rattling model with backlash is established by considering the time-varying mesh stiffness, composite transmission error and the torque fluctuation. The concepts of “high speed impact” and “low speed contact” in gear-rattling are proposed firstly. And then a new method for researching gear-rattling is developed. The dynamical behavior of the gear system is studied. The calculating results show that there are three kinds of gear rattling in gear system. Gear pair is alternating in engaging and disengaging repeatedly in the first one or the second one. When the gear teeth cause to contact from disengaging state, there is no rebound in first kind but rebound in second one. And in the last one, the collision between gear teeth will occur again and again without engaging. Three kinds of rattling are different greatly in rattling level. According to the principle that no separation will occur if the response amplitude is not larger than the static response, the threshold is obtained by theoretical derivation. And the validity of the theoretical derivation is verified by the numerical method. The researching results of the rattling threshold show that the higher the rotation speed of gear system, the smaller the mean load and the greater the exciting amplitude, more easily the gear rattling will occur.
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