Friction and Lubrication Performance of Marine Cam-Tappet Pair

With the continuous improvement of power density, speed and other performance parameters of marine diesel engine, the cam-tappet pair is facing more severe working environment, especially the tribological characteristics of micro-contact area at tribo-interface, under the conditions of transient load impact, velocity impact, curvature change and local rough peak contact, interface friction and flash temperature change rapidly, which produce surface failure problems, e.g. wear and gluing. Based on the elastohydrodynamic lubrication analysis model and the fast-moving heat model with consideration on the effect of the transient condition and geometric change, transient surface roughness of cam-tappet pair and the non Newtonian fluid of lubricating oil, the quasi system numerical analysis method with better stability and fast convergence speed was used to analyze the friction and lubrication performance of cam-tappet pair. The classical fitting formula was used to verify the correctness of our method. It was assumed that the surfaces of cam and tappet had sinusoidal ripple, and the effects of sinusoidal wave amplitude, base circle radius and cam speed on friction lubrication state were discussed.　The results showed that during the operation cycle, the cam-tappet pair was affected by the surface roughness, resulting in drastic changes in the film state. In specific, the minimum film thickness decreased, the maximum film pressure increased, the friction coefficient and surface flash temperature rise increased significantly, resulting in the decrease of the film bearing capacity and lubrication failure easily, which was also the cause of wear and gluing failure of the tappet. When the base circle radius decreased, the slide-roll ratio of the surfaces increased, resulting in the drastic change of the slide-roll state and the increase of the surface flash temperature. Increasing the base circle radius reduced the maximum temperature rise of cam and tappet surface by about 10 ℃ and 20 ℃, respectively. The friction coefficient decreased, so increasing the base circle radius was beneficial to improve the lubrication condition of cam-tappet pair. Increasing the cam speed increased the film bearing capacity and reduced the surface temperature rise, which increased the film thickness by about 0.2 μm, reduced the film pressure and improved the lubrication state. The contact probability of rough peak decreased, resulting in the decrease of surface temperature rise. The maximum temperature rise decreased by about 10 ℃, so it should ensure that the cam operated within the range of high speed and avoid idle state.　The above work revealed that the influence of the roughness parameters, working condition change and geometric structure on its lubrication state and friction flash temperature characteristics were revealed, which provided theoretical guidance for the tribological optimization design of cam-tappet pair and the prediction of wear and gluing failure of marine diesel engine