Wet clutch tribology : friction characteristics in all-wheel drive differentials

Abstract: In recent past, several electronically controlled automotive transmission systems, where wet clutches are used as intelligent differentials, have emerged on the market. These applications generally impose high stresses on the transmission fluids. The fluids must retain the desired frictional characteristics at different torque levels during its lifetime in an environment where the ambient and system temperatures vary in a wide temperature range. This thesis is focused on transmission fluids for wet clutches in all-wheel drive systems. The all-wheel drive system, featuring a wet multi-plate clutch with a sintered bronze based friction material, is described and compared to other systems on the market. A comprehensive literature review section where the state-of-art in this field is presented gives an insight to many of the problems commonly experienced in this type of application. Different methods to investigate the function of wet clutch transmission fluids are also presented. Test equipments designed during this thesis work in order to determine the frictional characteristics of transmission fluids are described. These equipments can also be used to measure the actual temperature experienced by the fluid in the contact zone using an infrared temperature measurement method. 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 characteristics of transmission fluids. The friction decreases with increasing temperature, and it is therefore necessary to measure the correct temperature in the clutch to obtain realistic measurements of friction. The friction-velocity relationship is a good indicator of the fluids ability to suppress friction induced vibrations. It is, however, important to measure the friction-velocity relationship at constant temperature, or compensate the relationship accordingly. The influence of clutch disc pressure on friction is quite moderate, as compared to the influence of temperature and sliding velocity. The influence of different oil additives on friction has also been investigated. These additives have considerable influence on friction, and this must be considered when formulating new transmission fluids.