The mesh equation is derived based on the principle of the mesh characteristics between plastic helical gear and steel worm. An analysis on the law of the deformation process of tooth profile and potential contact point in different kinds of load by means of Hertz contact theory is carried out. Using FEM to simulate the stress-strain status and the deformation process

mesh movement law of the plastic helical gear is obtained. The correctness of mesh analysis using Hertz contact theory is verified.