Abstract: A nonlinear twodegreeoffreedom dynamics model for a spur gear pair system is developed by considering the timevarying stiffness，mesh damping，backlash and transmission error. The differential equations were solved by the fourthorder variable step size RungeKutta algorithm. The statistics of the proportion of tooth contact states in the iterative process were acquired，and the impact of the rotational speed and torque on dynamic transmission error was studied. It shown that the tooth mesh condition changes with the increase of rotational speed，which explains the cycle and chaos response of the dynamic transmission error and the error amplitude jump phenomena. With the increase of torque，jumping phenomenon of the rotational speed increased，the changes of amplitude also increased，but the tooth impact reduced.

Key words: gear dynamics, tooth contact state, dynamic transmission error, rotational speed, torque