The mechanisms of wet clutch friction behaviour

Abstract

Wet clutches are used in automatic transmissions to bring about gear changes and also to reduce energy loss in the torque converter. These friction devices are susceptible to stick-slip effects, which result in the vehicle giving an unsteady ride. Stick-slip effects can be avoided by ensuring the wet clutch and lubricant combination produces a friction coefficient that increases with sliding speed. This friction characteristic is achieved by using a specific material on one of the two clutch surfaces and by using certain surface active chemicals, which are added to the lubricant. Although wet clutches have been studied throughout the industry for many decades, the mechanism of the generated friction is still not fully understood. In this thesis the friction mechanisms are clarified by the experimental study of the wet clutch in terms of its real area of contact, its flash temperatures and the friction characteristics, which are measured over a broad range of conditions. These results are used along with theoretical calculations to first clarify the lubrication regime, which is found to be predominantly boundary due to the roughness of the friction material and the small size of the contact units formed. The generated friction is then attributed to surface active additives, which form solid-like films on the clutch surfaces. These friction characteristics can be modified by varying the nature of the solid-like film, and when a close-packed film is formed, this displays the friction increasing with speed characteristic due to an activated shearing mechanism, which is linked to the speed of molecular rearrangement at the surface.