Optimized wet clutches : simulation and tribotesting

Sammanfattning: Wet clutches are used in a variety of different machinery. Wet clutches and brakes are frequently used to distribute torque in vehicle drive-trains. The clutches can be located in e.g. automatic transmissions or limited slip differentials. Their frictional behavior is of great importance for the overall vehicle behavior and has to be thoroughly investigated when designing new wet clutch applications. Frictional behavior is normally investigated in test rigs where complete friction discs are tested under similar working conditions as for the clutches in the drive-train. However, it is today possible to simulate the clutch behavior and not limited ourselves to only use testings as a help in the design of the wet clutches. This is an advantage because it is possible to simulate behavior that is not possible to measure in test rigs. Another advantage is the faster and more cost efficient design process than when all tests are carried out in a laboratory.The torque transferred by the clutch during engagement can be roughly divided into full film torque and boundary lubrication torque. Full film torque originates from the part of the engagement where the friction 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 friction discs are in direct contact. The distribution between these two types of torque is different for different types of wet clutches and engagements. When the clutch works in full film regime, it is possible to simulate the friction quite well, whereas the friction in the boundary regime is much more difficult to model and simulate, since it is very additive dependent.The most common use of wet clutches is in automatic transmissions for vehicles. Hence, most research in wet clutch testing and simulations are performed on wet clutches suitable for these applications. Here, the wet clutch is often used to brake a rotating shaft to standstill and the total engagement will have a duration of fractions of a second. During most of the engagement the clutch will be working in full film lubrication.In this investigation, the focus is on wet clutches working under limited slip conditions, i.e. the clutches will have a limited slip during a long duration without any lock-up. During this kind of engagement the clutch will work mainly in boundary lubrication and generate much heat. Such clutches can be found in, e.g. limited slip differentials.The optimum when designing a new wet clutch would be to simulate the clutch during the whole engagement without having to do any measurements in the laboratory. This, however, is not yet possible but an efficient way to design clutches can be obtained by combining fast friction measurements with efficient computer simulations. In this work, a simple friction measurement technique for wet clutches working in boundary lubrication is developed, based on pin on disc measurements. These friction measurements are combined with a temperature simulation of a wet clutch, where the lubricant cooling flow, dependent of the surface groove pattern, is simulated. This method makes it possible to allow a wet clutch working in boundary lubrication to be optimized for given working conditions regarding lubricant, friction material and surface groove pattern. The simulations are validated by measured data from a test rig in which whole friction discs are investigated with the same working conditions.