Research On Frictional Characteristics Of Friction Pairs Of Wet Multi-disk Clutches In DSG Tracked Vehicles

With the development of tracked vehicles towards higher power density and higher reliability,the wet clutches,which are the critical components in transmission system,always encounter the problems of ablation,wear,and breakage during the vehicle start-up and shift processes and have been one of the main factors that influence the performance and the service life of the integrated transmission assembly.In this work,the three-degree-of-freedom fixed-shaft wet direct shift gearbox(DSG)is taken as the research object to explore its applicability to the tracked vehicles.Taking the investigation of frictional characteristics of the wet friction pair as the entry point,the influences of main parameters on the working process of the wet clutches are analyzed and thermal mutation characteristics of frictional properties are identified through theoretical model,simulation analysis and experimental tests.After that,the thermal status and the wear status of the wet clutches are evaluated under the dynamic load of tracked vehicles.Main scientific work in this research is shown as follows:(1)A dynamic clearance varying model and a torque balance model are developed for the wet friction pair to explain the bearing mechanism in the normal and circumferential direction;based on the wet pin-on-disc test,the influences of interface pressure,slipping speed and temperature on the friction coefficient and the wear coefficient are fully discussed;after that,by developing the temperature prediction model,a theoretical coupling model is finally established for the engagement process of the wet friction pair.From the comparison between the simulation results and experimental data,the developed coupling model is sufficient to describe the changing trend of several critical parameters during the engagement process,including the viscous torque,the asperity torque,the oil film thickness,the contact status on the friction interface,the temperature rise,the friction coefficient and the wear amount,etc.(2)A comprehensive friction torque model in the full-gap variation range is proposed for the wet multi-disc clutches,capable of describing the clearance varying process,frictional behaviors and the dynamic response of the hydraulic execution structure.Then,the influences of main factors on the formation process of friction torque are studied.The obtained results indicate that,under the separation status,the bias effect of gyro will lead to the increase of the drag torque in the low rotation speed region.During the engagement process,the spline friction force will result in the attenuation of the transmitted friction torque,and the higher the temperature is,the more obvious the attenuation phenomenon is.In addition,both the relative speed and temperature have a great influence on the viscous torque and the asperity torque,however,the applied pressure has little influence.(3)Through the experimental research,the thermal instability conditions and thermal decay characteristics of the wet clutches are studied and the thermal safety boundary for control is obtained.A multi-degree-of-freedom model is developed for DSG to obtain the real-time dynamic load during the start-up process.Then,the starting characteristics with single/dual clutch and the thermal safety are investigated under different conditions.The results show that the proportion of viscous torque is large at the linkage point under the quick start-up and low temperature condition.However,the heavier the load is,the larger the proportion of asperity torque is.With the increase of temperature,its great influence on the friction coefficient and the output torque cannot be ignored,and the control pressure should be adjusted.(4)The dynamic model is established to study the shift process with two/four clutches in DSG.The power flow connection process during the shift process is analyzed to achieve the best possible torque relationship between two/four clutches and to achieve the optimized power flow transmission process.According to the shift frequency theoretical model of tracked vehicles,a comprehensive 10 km shift cycle spectrum is obtained to evaluate the thermal and wear status of wet clutches.Studies show that,during the shift process,only the engagement clutches are in the slipping status,the minimum thermal load can be obtained.According to this control strategy to shift,the temperature of clutches is within the controllable range and the wear rate is small,which satisfy the requirements of tracked vehicles.(5)The performance characteristics of wet clutches are studied on the SAE#2 test bench.The UMT Tribolab tester and the MOA II spectrometer are introduced to acquire the changing trend of the friction coefficient and wear coefficient respectively.In addition,to study the dynamic responses of DSG during the start-up and shift processes,a specific test bench is carried out.And,through comparison of experimental data and simulation results,the accuracy and the effectiveness of theoretical research are verified.