VIRGIL ATANASIU, IOAN DOROFTEI, MIHAI RADU IACOB, DUMITRU LEOHCHI NONLINEAR DYNAMICS OF STEEL/PLASTIC GEARS OF SERVOMECHANISM STRUCTURES The paper presents the main aspects of dynamic modelling of steel/plastic geared systems with high gear ratios in relation to the nonlinear parameters, such as time-varying mesh stiffness and load sharing alternation. Firstly, the specific aspects of geometrical design criteria for helical gear with small number of pinion teeth are presented in order to prevent the negative phenomena of the involute tooth generation and gear engagement. Then, the dynamic analysis is conducted to investigate the shared dynamic loads and dynamic transmission error of steel/plastic gear pairs along the line of action. The effects of the time-varying mesh stiffness are incorporated in the nonlinear dynamic model for numerical analysis. An improved model of the mesh stiffness is developed by including specific meshing characteristics of helical gear pairs. The variation of the dynamic transmission error is simulated and examined in relation to the teeth number, addendum modification coefficients and input torques. A computational algorithm is developed for instantaneous dynamic contact analysis.