Thermal Characteristics Analysis And Comprehensive Optimization Of Friction Pairs In Wet Clutch

Under the off-road conditions of high load and poor road conditions,the reliability of automobile parts is particularly important.Wet clutch is an important component of automatic transmission?AT?,and its reliability will directly determine whether the transmission can work properly.In the process of wet clutch engagement,the thermal characteristics between friction pairs are the important factors affecting the reliability of wet clutch.Based on a pre-research project "Integrated Application Technology of XXXX Vehicle Electronically Controlled Hydraulic Automatic Transmission?AT?",this paper analyses the influence factors of thermal characteristics by means of theoretical analysis,experimental measurement,finite element simulation and comprehensive optimization.Combining with the actual use of the whole vehicle,this paper comprehensively considers reducing the temperature and stress between friction pairs and improving the shift quality,which provides a new idea for the design of wet clutch assembly.Firstly,the thermal characteristics of friction pairs are analyzed theoretically.Based on the simplified structure of grooveless friction disc,the theoretical model of heat generation process of friction pair is established.The effects of oil temperature,relative speed,oil pressure and friction coefficient on heat generation process are explored,and the effects of oil pressure,dynamic viscosity,relative speed and friction pair material on friction coefficient are also explored.By introducing the heat flow distribution coefficient,the heat generated is distributed to the friction sheet and the steel sheet.Based on the theory of heat transfer,theoretical models of heat transfer in friction disc and convection heat transfer between friction pairs are established,and corresponding convection heat transfer coefficients are calculated according to the physical properties of oil.Secondly,dynamic and static friction tests are carried out on SAE₂ test bench.The friction coefficients between friction pairs were measured under different oil pressures,initial rotational speeds and oil temperatures.The variation of dynamic friction coefficients,static friction coefficients and ratio of static to dynamic friction coefficients with oil pressures,initial rotational speeds and oil temperatures was explored.The significance of each factor was tested by variance analysis,and the influence of each factor on friction coefficients was determined.Thirdly,the friction pair of wet clutch is simulated by finite element method.The three-dimensional model of friction pair is established by CATIA software,which simplifies the parts that have little influence on thermal characteristics.In Hypermesh software,the friction disc is meshed.In ABAQUS software,the finite element model of friction pair is established,and all test conditions are simulated.The direct solid-thermal coupling method and indirect fluid-solid coupling method are used to solve the problem.The temperature field,stress field and heat flux field of friction pair are obtained respectively.The distribution of the three fields was analyzed and the influencing factors of the three fields were tested by variance analysis.Finally,the gear shifting process of friction pairs is optimized comprehensively.In LMS.AMESim software,the dynamic model of the whole vehicle shift process is established.The oil pressure of the torque phase and inertia phase is simplified to the oil pressure which changes linearly with time.Based on the idea of orthogonal experiment,the shift time and impact are obtained by simulation.Based on the above dynamic model and simulation results,six evaluation indexes describing the comprehensive characteristics of friction pairs in gear shifting process are determined,and the objective function is obtained by using analytic hierarchy process?AHP?and combining with the actual needs of the vehicle.The agent model is constructed based on Kriging model,and the objective function method is optimized by particle swarm optimization.The results show that the comprehensive characteristics of the friction pair are improved under the optimal solution condition,and the reliability of the vehicle in the actual off-road environment is improved.