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Kim Berglund (LTU)

Published: 14 October 2013

Predicting wet clutch service life performance

Engineers in industry today face many challenges; products should be efficient, cheap, reliable and  environmentally friendly. Wet clutches as parts of auto-matic transmissions and limited slip differentials in cars are no different. One important concern is how the performance changes during operation, which can lead to failure of the wet clutch as well as failure of other components. Changes in performance may also affect how the car behaves on the road. Consequently, the overall aim of this work is to establish methods of predicting the performance of wet clutches during ageing.
A wet clutch consists of friction and separator discs alternately positioned in a clutch pack and submerged in a lubricant. The friction discs are connected to either the input or output shaft, while the separator discs are connected to the remaining shaft. When the clutch pack is loaded, by e.g. hydraulic pressure, friction in the interfaces between separator and friction discs provides torque transfer. In automatic transmissions, the wet clutch is used to couple and decouple the input and output shaft. Engagement times are short and frictional power dissipation high during the engagement. For wet clutches used in limited slip differentials, the operating conditions are quite different. Wet clutches used in limited slip differentials are used to control the amount of torque transfer to parts of the vehicle’s drivetrain, and normally operate under limited slip conditions. Generally, this implies low frictional power dissipation over long periods of time. Although a lot of research have been performed when it comes to wet clutches, most research has been performed for wet clutches incorporated in automatic transmissions. Therefore this work focuses on wet clutches used in limited slip differentials. In this work, a test method and test bench was developed to evaluate the changes in performance over time for wet clutches used in limited slip differentials. The test method and test bench addresses the specific operating conditions of the limited slip differential.
Results in this study show that friction of the investigated clutch system generally increases with ageing. A model of how friction increases with time and lubricant temperature was developed. Results indicate that the model is capable of predicting  friction increase in both severe operating conditions and operating conditions closer to the real application. This model can be implemented in vehicle control software and be used to adjust for the changes in performance with time. The model to predict friction increase was also implemented in a dynamics model of a vehicle driveline. The dynamics model can be used to predict when failure occurs for a specific driveline and set of operating conditions.