Backlash and tooth stiffness variation are the important nonlinear factors to gears. This work studied initial behaviour by nonlinear model of helical gears with backlash and tooth stiffness variation, and excitation of helical gears focused on torque fluctuation. For this purpose, 12 DOF equations of motion of helical gears with the periodical change of mesh stiffness and backlash were derived. The Newmark beta method and the Newton-Raphson method were used to obtain nonlinear behaviour of helical gears. Many excitation frequencies initially caused the tooth separation and single-sided impacts of the gear pair and eventually led to the normal tooth contact. However, some special excitation frequencies caused the single-sided impacts in the entire time. Fluctuation torque increase made the single-sided impacts in the entire time, and damping increase reduced the duration of single-sided impacts. Backlash increase extended the duration of single-sided impacts and also increased mesh force.
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