Experimental characterization of the sliding friction of a degrading wet friction clutch

Wet friction clutches have become widely used in today's transmissions, e. g. automatic transmissions, dual clutch transmissions, etc. It is known that the central role of this particular component is strongly determined by its friction characteristics. While a clutch is in use, degradation inevitably takes place that consequently deteriorates its friction characteristics. However, to the authors knowledge, how the friction characteristics evolve with the clutch degradation progression is not fully understood yet. A profound understanding of the clutch friction characteristics during its lifetime can improve our knowledge, which enable us to model and simulate the dynamic engagement behavior of a clutch with the progression of the degradation. Eventually, the gained knowledge can aid in deriving physical features, which are useful for clutch monitoring, prognostics and adaptive control. This study aims at experimentally investigating the sliding friction characteristics of a wet friction clutch during its lifetime. More precisely, the objective of the study is to understand how the Stribeck and the frictional lag (i.e. sliding hysteresis) parameters evolve as the degradation progresses. For this purpose, a novel test procedure is proposed and the test is carried out on a fully assembled (commercial) clutch using a modified SAE#2 test setup. The clutch consists of a predefined number of commercial friction and separator discs and is lubricated with a commercial automatic transmission fluid (ATF). Furthermore, a systematic methodology for the identification of the Stribeck and the frictional lag parameters is proposed. The experimental results reveal that the identified parameters evolve deterministically during the clutch lifetime.