Research On Friction-induced Oscillation Of Clutches In The Process Of Engagement

The engagement stability of clutches is a crucial index to evaluate the shift quality of vehicles as well as a significant factor to influence the performance and the service duration of the driveline system.In this research,based on friction characteristics and interface properties,the friction-induced oscillation of clutches(judder)is investigated from the perspective of variational friction coefficient and mode coupling between normal and tangential forces.According to theoretical modeling,stability analysis,numerical calculation and simulation,and experimental test,the mechanism and critical conditions of the clutch judder are obtained.Furthermore,the relationship between frictionally-induced thermal load and the oscillatory behavior is studied.The influence of different factors including clutch parameters and system parameters on the clutch judder is further analyzed.The frequency features of the clutch judder are evaluated via Fast Fourier Transfer(FFT).Main scientific work in this research is shown as follows:(1)The models of friction coefficient and external control pressure are developed based on experimental tests,which are then employed in a powertrain torsional vibration model.The critical conditions for the clutch judder are obtained through an integrated damping.Numerical simulations show that the clutch judder takes place only when the negatively sloped friction coefficient reaches a specific value;meanwhile,the fluctuation of the external control pressure also can induce the clutch judder and the variation history of control pressure substantially influence the judder behavior.The results further indicate that the clutch judder attenuates with the increase of the rotational inertia of the clutch driven side and the torsional stiffness of the transmission output shaft.(2)The analytical solution of a multi-degree-of-freedom(MDOF)torsional vibration model is obtained,and the influence of engine torque excitation and external drag load excitation on the clutch judder is investigated.In addition,the effect of the clutch oscillatory begavior on the dynamic response of the transmission is evaluated.The results indicate that the external excitation has notable effect on the clutch judder when the excitation frequency approaches or equals the system natural frequency,however,the effect is weak in other frequency ranges.Furthermore,the effect of external excitation,negatively sloped friction coefficient,and fluctuant pressure on the driveline dynamic response is remarkable.(3)The heat conduction equation and the thermoelastic equation are solved by an alternate finite volume method and finite element method(FVM-FEM)that the author proposed.The surface temperature distribution and the interface contact pressure distribution of the clutch friction pairs suffering frictionally-induced thermal load are obtained,and the contact pressure distribution is then formulated to study the thermal judder phenomenon.The results present that the friction interface contact pressure distributions are non-uniform due to frctional heat,even though the initial pressure is even,and moreover,the contact pressure distributions on different interfaces are dissimilar and the non-uniformity increases with the sliding process.Furthermore,the non-uniform contact pressure along the radial direction has little influence on the clutch judder,however,the pressure perturbation along the circumference contributes to the judder behavior substantially and poorer engagement stability takes place.(4)The frequency properties of the clutch judder under different circumstances are studied through FFT method.The amplitude characteristics are reflected by envelope lines.An evaluation model for the clutch judder is developed to predict the oscillatory behavior.The results indicate that the evaluation model possesses good prediction capabilities.(5)A test bench for multidisc clutches is designed and set up.The rotational velocities and torques of the clutch driving side and the driven side and the temperature of the mated steel disk under different control pressures and various speed differences are measured.Comparing the test data and the theoretical solutions validates that the judder behavior takes place during the clutch engagement.Furthermore,it is verified that the theoretical analysis is correct.