Research On Contact Wear And Friction Characteristics Of Cam/tappet Pair In Valve Trains Of Diesel Engine

With the development of speed and power of diesel engines,the contact load and relative sliding velocity between the cam and the tappet are increasing.Therefore,the frictional loss and wear of cam tappet pairs are more and more serious.The research on controlling and reducing the friction and wear is very important to solve these problems.Compared with the experimental study of friction and wear,numerical simulation is smalller in costs and easier to change parameters.In this paper,the numerical simulation of the friction and wear of the cam tappet pair in the valve train of one diesel engine was carried out.The main points of this paper are as follows:(1)The contact surfaces of the test specimens were measured.The double logarithmic graph is got from the test data.Judging from the slope of the line,the data has fractal characteristics.Then the rough surface profiles of friction pairs were characterized with W-M function method and constructed into the 3D geometric model of this pair.(2)The finite element model of the contact pair was developed including the oil lubricant using CEL technology.VOF method was used to determine the changing boundary of the oil material.The relationship between the friction coefficient and the oil film thickness was obtained using the improved Archard wear model.By changing the viscosity of lubricant,the effect of viscosity on the friction and wear characteristics was analyzed.It is found that the oil film thickness increases and the coefficient of friction and wear decreases with the increase of oil viscosity.(3)The rough surfaces of grinding processes and turning processes were also described by fractal method and the finite element models were established accordingly.Taking the use of CEL technology,the effects of grinding processes and turning processes on friction and wear characteristics were anlayzed.The relationships between the ratios of the mixed lubrication wear coefficient to the boundary wear coefficient and the thickness of the oil film was obtained for grinding contact pairs and turning contact pairs.(4)The oil film thickness of the contact point in the cam tappet pair in the valve train of one diesel engine under certain condition was calculated.According to the relationships between the coefficient of friction and wear and the thickness of the oil film in the mixed lubrication,the wear and friction coefficients of the cam and tappet contact pair were determined and the wear depth of the cam and friction power for one engine cycle were obtained as well.With the increase of engine speed,the friction power decreases.With the increase of spring preload,the friction power increases.