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The current research on gear system dynamics mainly utilizes linear spring damping model to calculate the contact force between gears.However,this linear model cannot correctly describe the energy transfer process of collision that often occurs in gear system.Focus on the contact-impact events,this paper proposes an improved gear contact force model for dynamic analysis in helical gear transmission system.In this model,a new factor associated with hysteresis damping is developed for contact-impact state,whereas the traditional linear damping factor is utilized for normal meshing state.For determining the selection strategy of these two damping factors,the fundamental contact mechanics of contact-impact event affected by supporting forces are analyzed.During this analysis,an effect factor is proposed for evaluating the influence of supporting forces on collision.Meanwhile,a new restitution of coefficient is deduced for calculating hysteresis damping factor,which suitable for both separation and non-separation states at the end of collision.In addition,the time-varying meshing stiffness (TVMS) is obtained based on the potential energy approach and the slice theory.Finally,a dynamic analysis of a helical gear system is carried out to better understand the contact force model proposed in this paper.The analysis results show that the contribution of supporting forces to the dynamic response of contact-impact event within gear pair is important.The supporting forces and dissipative energy are the main reasons for gear system to enter a steady contact state from repeated contact-impact state.This research proposes an improved contact force model which distinguishes meshing and collision states in gear system.