The present paper concentrates on the investigations regarding the situations of frictional shear stress of gear teeth and the relevant frictional effects on bending stresses and transmission error in gear meshing. Sliding friction is one of the major reasons causing gear failure and vibration; the adequate consideration of frictional effects is essential for understanding gear contact behavior accurately. An analysis of tooth frictional effect on gear performance in spur gear is presented using finite element method. Nonlinear finite element model for gear tooth contact with rolling/sliding is then developed. The contact zones for multiple tooth pairs are identified and the associated integration situation is derived. The illustrated bending stress and transmission error results with static and dynamic boundary conditions indicate the significant effects due to the sliding friction between the surfaces of contacted gear teeth, and the friction effect can not be ignored. To understand the particular static and dynamic frictional effects on gear tooth contact analysis, some significant phenomena of gained results will also be discussed. The potentially significant contribution of tooth frictional shear stress is presented, particularly in the case of gear tooth contact analysis with both static and dynamic boundary conditions. 1. Introduction In modern world, many reliable gear manufacture techniques have existed and operated for modern industry; the advanced high performance gears products are indispensable components for lots of power-train transmission systems. As a basic transmission part, gears have been used for three thousand years in history, and the investigation of gear design techniques and product performance kept developing all the time. In 1892, Wilfred Lewis first presented a formula for evaluating the bending stress of gear teeth in which the tooth form entered into the equation. The formula still remains the basis for most gear design today, and the parameter proposed as “Lewis stress” can indicate the bending stress of loaded contacted gear teeth accurately. But in further investigation on the basis of the application of Lewis stress, the nonlinear responses due to frictional effects in meshing process have become obvious gradually, and many scientists have been tried to investigate any reliable techniques to predict frictional effect on gear performance. In 1982, Barakat  presented a technique for the determination of the instantaneous coefficient of friction at gear tooth contact by considering tooth fillet strains using finite
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