Wet clutch tribology : friction characteristics in limited slip differentials

Abstract: In recent years, electronically controlled automotive transmission systems, where wet clutches are used as intelligent differentials, have appeared in the market. These applications impose great demands on the transmission fluids and friction materials used as well as on controllability and vibration preventive (anti-shudder) properties of the clutch systems. This thesis focuses on transmission fluids used in wet clutches in all- wheel drive systems. The investigated all-wheel drive system, featuring a wet multi-plate clutch with a sintered brass based friction material, is described. A comprehensive literature review section outlines the state-of-art in this field and gives an insight into many of the problems commonly experienced in this type of application. Different methods used to investigate the function of wet clutch transmission fluids are also presented. Test equipment designed during this thesis work in order to determine the frictional characteristics of transmission fluids is described. This equipment measures friction torque, normal load, oil temperatures and the actual temperature experienced by the fluid in the contact zone. Base oil type and viscosity have been found not to significantly influence friction characteristics of a wet-plate clutch , indicating that the torque is primarily transmitted by asperity contacts rather than fluid films. Oil additives, on the other hand, have a considerable influence on friction, again leading to the conclusion that tribolayers on contacting asperities rather than fluid films govern friction. From these observations it can be concluded that the lubrication regime under the conditions studied are boundary lubrication, moving into mixed lubrication at high velocities and low temperatures. Results show the influence of several operating parameters on the frictional behavior of the clutch. Temperature is shown to have significant influence on the friction of transmission fluids which decrease with increasing temperature. It is therefore necessary to measure the true temperatures in the clutch contact in order to obtain realistic measurements of friction. A method which excludes the influence of temperature on measured friction data have been developed and verified. The influence of clutch disc pressure on friction is quite moderate compared to the influence of temperature and sliding velocity. The influence of velocity on friction is governed by the transmission fluid and friction material used in the clutch. The friction-velocity relationship is a good indicator of the fluid’s ability to suppress friction induced vibrations. It is, however, important to measure the friction-velocity relationship at constant temperature, or to compensate the relationship for temperature effects. A successful method to develop transmission fluids has been developed. Formulated fluids allow good anti-shudder properties to be combined with good lubrication performance for other machine elements present in the transmission. Interactions between different additives must be considered which can, in many cases, completely alter the friction characteristics since additives compete for the same adsorption surface. Extreme pressure additives have been found to be particularly troublesome when used in combination with other additives as far as their ability to maintain good anti-shudder properties is concerned. Based on the knowledge of clutch performance obtained from the research presented in this thesis, a model to predict transmitted clutch torque has been developed. This accurately determines the transferred torque from the current operating conditions and the thermal history of the clutch. It can be concluded that thermal effects have a significant influence on the torque transferred by the clutch, and it is therefore necessary to have a thermal model of the clutch combined with a temperature dependant boundary friction model based on empirical friction data for the friction material/transmission fluid combination of interest.