| Based on timing silent chain of the engine,thermal mixed lubrication of the pin shaft-chain plate pairs has been investigated from both theory and experiment to reveal the reason of wear for the pin shaft and the chain plate hole of the silent chain.Four parts are included in the thesis: Analysis of infinitely long line contact thermal mixed lubrication for pin shaft-chain plate pair of the silent chain;Modelling and characteristic analysis of simple sliding finite line contact thermal mixed lubrication for pin-plate pairs;Analysis of time-varying thermal mixed lubrication performance based on kinematic characteristics of pin-chain plate pairs;Experimental study on the frictional and lubricating performance with different commercial oils.Firstly,the geometric relationship and kinematic characteristics of the pin-plate contact pair in a silent chain were analyzed.Based on the average flow rate model,a thermal mixed lubrication model for the pin-plate contact pair was established,and the effects of the equivalent radius of curvature,the surface roughness,and the load on the non-Newtonian thermal mixed lubrication performance were discussed.The results show that with the increase of the equivalent radius of curvature,the film thickness ratio increases,the lubrication state changes from mixed lubrication to full film lubrication,the oil film pressure increases,the asperity contact pressure decreases,and the friction coefficient and friction heat both decrease,and the lubrication performance of the pin-chain plate pair will be improved effectively.As the surface roughness or the load increase,the film thickness ratio decreases,the friction heat increases,and full film lubrication translates into mixed lubrication,the wear is easily generated for the pin-chain plate pair.Secondly,a simple sliding finite line contact thermal mixed lubrication model based on a pin-plate pair was established,and a typical solution for the thermal mixed lubrication of a silent chain pin-plate pair was presented.The effects of the entrainment speed,the width of the chain plate,and the modified profile radius at the end part of the chain plate hole on lubricating performance were discussed.In addition,comparison of the lubricating performance between the middle and end part of the chain plate was carried out.The results show that with the increase in the entrainment speed,the film thickness ratio increases,the load ratio,the frictional coefficient and frictional heat decrease,and the lubrication performance of the contact area will be enhanced.However,different from the middle part,the film thickness ratio is smaller,whereas the mid-layer film temperature is higher at the end part of the chain plate in.When the middle part is in a mixed lubrication or full film lubrication state,the end part is in a boundary lubrication or mixed lubrication state.As the width of the chain plate increases,the film thickness ratio increases,the load ratio,the average frictional coefficient and the maximum mid-layer film temperature decrease,the lubrication performance of the pin-chain plate pair will be improve effectively.Moreover,as the radius of modified profile increase,the film thickness ratio increases,the load ratio and the maximum mid-layer film temperature rise decrease,and the stress concentration at the end part weakens.Therefore,the mixed lubrication performance of the pin shaft-chain plate pair can be improved by increasing the radius of the modified profile at the end part of the chain plate hole and reduce pin shaft wear.Thirdly,based on the kinematic characteristics of the pin-chain plate pair,a timevarying thermal mixed lubrication model for the pin-chain plate pair was established,and the thermal mixed lubrication performance under transient conditions was analyzed.Additionally,the influencing mechanism of the friction coefficient for asperity contact on the thermal mixed lubrication performance was revealed.The results show that during the meshing stage of the silent chain,the lubrication state of the pin-chain plate contact pair changes from boundary lubrication to full film lubrication,and then from full film lubrication to boundary lubrication;A full film lubrication is easily achieved for the pin-chain plate pair with higher acceleration.Under boundary and mixed lubrication,the friction coefficient of asperity has a significant impact on the temperature field;however,under the condition of full film lubrication,the frictional heat of asperities can be neglected.Finally,three commercial engine lubricating oils were selected to measure the frictional coefficients at various speeds,loads,and temperatures.With a line contact interferometry test rig,the effects of speeds and loads on the lubrication performance were studied.The results show that the friction coefficient of 0W-20 is low and stable under different temperatures,and the friction coefficient of 5W-40-old is higher than that of 5W-40-new at the same temperature.0W-20 has good and stable frictionreduction performance under medium and high load conditions;The friction coefficient of 5W-40-new does not vary significantly with load;However,the friction coefficient of 5W-40-old is higher under different loads.With different speeds,the friction coefficient of 0W-20 is larger,but its variation range is small and stable;the friction coefficient of 5W-40-new is low,but it changes in a wide range,and its friction-reduction performance is good at low and medium speeds.The antifriction performance of 5W-40-old is the worst.The variation trend of the minimum film thickness of 0W-20 under different accelerations is in good agreement with the theory,and verifies of the theoretical results.Under the same operating conditions,the film thickness of 5W-40-new is thicker than that of the other two oils,and when the load is slightly higher,the film thickness of 5W-40-old fluctuates greatly with the increase of acceleration,but the lubrication performance of 0W-20 is the most stable.Therefore,the reasonable selection and regular replacement of lubricating oil are of great significance to the reliability and service life of components. |
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