In order to provide a profitable and cost-effective way in maintaining a machine, it is necessary to prepare an economical way to provide service manipulation is to prepare standard and stable spare parts, replacements, and consumables in stock. The problem of accurate prediction of load, deformation and stresses in 3D contacts is then revisited. Most research has used one, two or three pairs of meshing teeth to simulate the mating process. It is insufficient to describe the entire process of three contact zones during each mating pair. By implementing a combined 3D face-contact and FEM, contact stress analysis between two spur gear teeth was considered in eleven different contact positions during a full mating process. The proposed approach provides a complete and effective solution of the contact problem in quasi-dynamic way. Instead of applying more fixed boundary conditions to constrain and simplify material behavior in 2D models, point and line contact of tooth surfaces are substituted by a face contact model of teeth in this article.