Effect of Friction Material on the Relative Contribution of Thin-Film Friction to Overall Friction in Clutches

In order to prevent shudder in automatic transmissions, friction must decrease as the sliding speed between the friction plates in clutches decreases. Theoretical studies have shown that friction in wet clutches is a combination of boundary friction and the friction due to flow of fluid through the friction materials (thin-film friction). Therefore, these physical properties of oils should control the anti-shudder performance of automatic transmission fluids. Recently, we demonstrated that boundary and thin-film friction contribute to friction measured at low speeds in JASO SAE No.2 and LVFA tests. Two different friction materials are used in these tests and the relative effect of thin-film friction on low speed friction is greater in the JASO SAE No. 2 test than in the JASO LVFA test. This difference in the relative effect of thin-film friction on overall friction could be due to the different speed conditions used to measure low speed friction in the SAE and LVFA tests or could be due to the difference in the friction materials used in these tests.

Therefore, we have expanded our research to determine the effect of sliding speed and friction material on the relative effect of thin-film and boundary friction on overall friction measured in friction material-steel contacts. As sliding speed increases, the relative contribution of thin-film friction to overall friction increases. The relative contribution of thin-film friction to overall friction at each speed depends upon the friction material used. The relative effect of thin-film friction to overall friction is greater for friction materials in which the rate of fluid flow into the material is slow. The relative effect of thin-film friction on overall friction is also greater for friction materials with lower surface roughnesses.