Wet clutch tribological performance optimization methods

Abstract: Wet clutches are used in a variety of machinery such as in vehicles where they are used to distribute torque in the drivetrains. Clutches can be located in automatic transmissions or in limited slip differentials. The frictional behavior of a clutch is of great importance for overall vehicle behavior and has to be thoroughly investigated when designing new wet clutch applications. Frictional behavior is normally studied in test rigs where whole friction discs or complete clutches are tested under similar working conditions to those pertaining to the clutches in the drivetrain of the vehicle. However, today clutch behavior may be simulated with regard to some clutch applications and design of the clutch system is not limited to testing. This is an advantage as it is possible to simulate behavior that may not be possible or suitable to study in a test rig. Another advantage is that the design process is faster and more cost efficient than that which is possible when all tests are carried out in a laboratory. The torque transferred by the clutch during engagement can roughly be divided into full film torque and boundary lubrication torque. Full film torque originates from the part of the engagement where the clutch discs are completely separated by a lubricant film and the friction surfaces are not in contact, whereas boundary lubrication torque occurs when the lubricant film is so thin that the surfaces of the clutch discs are in direct contact, only separated by a thin additive film. The distribution between full film torque transfer and boundary lubrication torque transfer differs for different types of wet clutch and for differing operating conditions. When the clutch works in full film regime it is possible to simulate the friction quite well. However, the friction in the boundary lubrication regime is much more difficult to model and simulate since it is very dependent on the additives. Wet clutches are most commonly used in automatic transmissions for vehicles. As a result, most research into wet clutch testing and most simulations concerns wet clutches suitable for such applications. In an automatic transmission the wet clutch is often used to brake a rotating shaft to stand still relative to another shaft and the total engagement has a duration of fractions of a second. During most of the engagement the clutch is working in full film lubrication. In this thesis the focus is on wet clutches working under limited slip conditions: in other words this thesis studies clutches that are working with a small amount of slip over a long period without reaching a state of lock-up. These clutch types can be found, for example, in limited slip differentials. During this type of engagement the clutch mainly works under boundary lubrication conditions and much heat can be generated. The optimum method of designing a new wet clutch would be to simulate the clutch performance without having to do any measurements in the laboratory. This, however, is not yet possible, but an efficient way to design clutches can be achieved by combining simple measurements with efficient computer simulations. In this thesis, simple measurement techniques for wet clutch materials are developed and combined with a temperature simulation of a wet clutch, where the lubricant cooling flow, which is dependent on the surface roughness and groove pattern, is simulated. This method makes it possible to optimize a wet clutch for given working conditions with regard to lubricant, friction material, surface roughness and groove pattern. The simulations are validated to measured data from a test rig in which torque behavior from whole friction discs are investigated. Good agreement between simulations and measurements is achieved.