Contact Stiffness Characteristics of a Paper-Based Wet Clutch at Different Degradation Levels

After clutch engagement in the post-lockup phase, the contact stiffness between friction materials and separators plays an important role in the dynamic behaviour of an Automatic Transmission (AT). The friction material deteriorates progressively during the service-life of a clutch, thus affecting the contact stiffness. The deterioration therefore changes the dynamic behaviour of the AT. In order to be able to predict the dynamic behaviour of the latter in the post-lockup phase, the contact stiffness characteristics at different degradation levels must be investigated. Consequently, this change in the dynamic behaviour can be used as a means to monitor clutch degradation. In this paper, both simple elastic contact model of rough surfaces, and experimental-setup tests are presented. Three identical paper-based friction materials with different degradation levels were used. Disc-on-disc experiments were performed on a newly developed rotational tribometer to simulate the representative post-lockup phase. In the experiments, those identical specimens were immersed in a fresh Automatic Transmission Fluid (ATF). The experimental results qualitatively agree with the presented model. In general, it can be concluded that, due to the friction material degradation, normal contact stiffness exhibits an increasing trend; in contrast, tangential contact stiffness exhibits a decreasing trend.