| Along with the rapid development of vehicle transmission technology,the power density and speed of wet clutches increase.Therefore,the operating conditions of friction pairs become severe,and the early clutch damages due to the friction and wear problems have become the bottleneck for improving the service life of the vehicle transmissions.This paper focuses on the study of the local lubrication,friction and wear behaviors of the friction pairs under different influence factors,the high temperature friction and wear behaviors and wear mechanisms,and the effects of intermittent contact caused by thermal buckling on the friction and wear behaviors by numerical and experimental methods to improve the design and control policies of the wet clutches,and enhance the reliability of the vehicle transmissions.The main research works of this dissertation include:(1)A quantitative calculation method for analyzing the local lubrication,micro contact state,load bearing mechanism,friction,and wear in wet clutch friction interfaces is provided,in which the mixed lubrication model in point contacts proposed by Hu and Zhu,and the Archard wear model are used.The local pressure distribution,load sharing ratio of the lubricant film and asperity contacts,elastic deformation,local temperature rise,real contact area,friction coefficient,and wear loss are obtained.(2)The effects of sliding velocity,temperature,pressure,and material porosity on the local lubrication,friction and wear characteristics of friction pairs are investigated based on the simulations and wet pin-on-disc tests.A curve fitting formula of local friction coefficient for Cu-based friction pairs is proposed as a function of the sliding velocity,temperature,and pressure.At the same time,the three-dimensional curves of the friction coefficient are provided.The results show that as the regime changes from boundary to hydrocynamic lubrication,the friction coefficient decreases quickly from about 0.15 to about 0.04 at first,and then increases slowly,which is well consistent with the Stribeck curve.However,the wear loss increases quickly at first,and then decreases gradually.(3)The high temperature friction and wear behaviors of Cu-based wet clutches are investigated based on the high temperature dry pin-on-disc tests.The transitions of high temperature wear mechanism are revealed.The variation regularity of high temperature friction coefficient,wear factor,and friction vibration are obtained.The results show that when temperature is lower than 345?,there are main abrasive wear and ploughing wear,so that the friction coefficient is relatively stable and the wear is slight;when temperature is higher than 345?,then the main wear mechanism changes to adhesive wear,so that the friction coefficient begins to fluctuate strongly and the wear begins to grows sharply.(4)On the basis of the previous study on local friction and wear behaviors,a lubricated friction model on intermittent contact is proposed.First,the hydrodynamic pressure and contact pressure are obtained by solving the Reynolds equation and load balance equation.Then,the local surface temperature rise and friction coefficient are obtained by calculating the local heat flux.At last,the local friction torque and total output torque are obtained when the local operating condition and friction coefficient are known.(5)The effects of intermittent contact on the friction and wear characteristics of friction pairs are investigated based on the simulations and 1:1 bench tests.The local temperature rise,local and total friction coefficient,local friction torque,total output torque,wear volume,and friction vibration under different contact ratio are analyzed.The results show that as the contact ratio reduces,the surface temperature,local friction coefficient,and wear volume increase significantly.The outer radius temperature rise is higher than the inner radius,and the outer radius wear volume is larger than the inner radius.In addition,the total output torque grows significantly,and the vibration becomes stronger. |
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