| Gas distribution cam mechanism is an important part of CNG engine. Tappet is prone to fly off from cam and the cam/tappet system prone to wear failure because of unreasonable design, manufacture, and usage; as a result, the engine cannot work well. The aim of this paper is to study the relationship between contact stress and wear failure for the cam and tappet of 6110 medium-sized CNG engine. A kinematic elastodynamics calculation of the cam and tappet was first carried out. The results show that the tappet can work well under the conditons of the idle speed(500r/min) and nominal speed(1000r/min), while under the condition of overspeed(1500r/min), the tappet will fly off from the cam at the rotation angle of 55°～125°. In addition, the contact stress of the cam and tappet was calculated by means of the Hertz formula: the maximum contact stress (440MPa) is less than the maximum allowable stress (823MPa) and the former is in the position of peak acceleration (36°of rotation angle) the second-largest contact stress is 350MPa in the positon of the peach sharp of the cam.To verify the theoretical results, the dynamics analysis of cam-drive mechanism and calculation of the contact stress between the cam and tappet were carried out by using the FE software ANSYS/LS-DYNA. Compared with the result of calculation, the lift and velocity curve have better match, but the acceleration curve is distorted. The reason is that the calculation results are obtained under the condition of ideal smooth contact surface, however, the simulation results are obtained from the meshed roughly surface and the rough surface will lead to great instantaneous acceleration. The simulation results of contact stress under the conditions of idle speed and nominal speed are agreement with those of kinematics and kinetics. The maximum contact stress is at the site of peach sharp, the stress value(640MPa) is greater than that (440MPa) of theoretical calculation, but less than maximum allowable stress; the maximum contact stress will be 380MPa at the maximum acceleration moment, and this value is less than that of theoretical calculation. Due to the different contact mode, the position of maximum stress is not in line with the calculation. The friction between cam and tappet include both sliding and rolling friction, and the fatigue wear failure of contact surface will occur because of cyclic contact and separation between cam and tappet.Experimental analyses of fatigued cam-drive mechanism were conducted by means of chemical composition and hardness test, microstructure SEM observation.The main wear failure forms, mechanisms and the effecting factors have been determined. The main wear failure include fatigue spalling, ploughing, abrasive wear, and wear mechanism is contact fatigue failure. Corresponding to the value of contact stress, the minimum film thickness was calculated under different conditions. The results show that the location of the minimum film thickness (0.1μm) is at the peach sharp, it is the most dangerous position. In addition, themaximum contact stress is obtained when the tappet did not rotate and the value (1050MPa) will be greater than maximum allowable stress.In order to improve the wear resistance of the cam and tappet, nanoindentation test simulation was carried out on a magnesium matrix sample with a super-hard ceramic coating material. And the results show that the microhardness (93.7GPa) of the ceramic will be 37.2 times of the microhardness (2.52GPa) of the magnesium matrix. This shows that cam and tappet with coating will be more wear resistant.
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