Frictional Torque Transfer Behaviors of Friction Pads in Wet Clutch Engagement

Abstract

For stable frictional torque transfer, wet clutch pads have the functional requirements of a high traction and a low wear rate with positive μ-V frictional characteristics. However, because of the intrinsic negative μ-V frictional characteristics of a friction material that has a static friction coefficient greater than its kinetic friction coefficient, the frictional torque transfer has unstable vibrations during the clutch-pad engagement. To reduce the vibration mode during the engagement of wet clutch pads, the kinetic friction coefficient should be made greater than the static friction coefficient through modifications in the design parameters as well as in the characteristics of the friction materials. To obtain positive μ-V frictional characteristics, it is important to manipulate the static-friction coefficient, which largely develops in the boundary lubrication stage during the clutch-pad engagement. The formation of the boundary film is described by slip boundary conditions and hydrodynamic lubrication films. It also includes the elastic deformation of the wet pad material due to the contact pressure. The wet clutch pad material is made of a porous structure through which the lubricant can easily spread when the applied load and sliding speed are imposed. The lubrication and the direct contact of the surfaces are simultaneously considered for a frictional torque transfer that lasts for less than one second, depending on the working conditions. In this study, some of the computational results with a measured μ-V friction coefficient covering both the static and kinetic friction during the wet-clutch engagement are obtained for the lubrication and the direct-contact pressures.