Clutch Shudder Correlated to ATF Degradation through Local Friction vs. Velocity Measurements by a Scanning Force Microscope

Abstract

Shudder in continuously slipping wet clutch (CSWC) systems is an instability that manifests itself as a vibration of the automobile's drive train. Dynamometer experiments imply that the shudder is connected to degradation of the frictional properties of the clutch friction material-automatic transmission fluid (ATF) system. The poor friction characteristics are revealed in negatively sloped torque vs. slip-speed curves which can result in dynamic friction oscillation instabilities. The authors report local measurements of friction vs. velocity with the use of a scanning force microscope (SFM) on new and glazed wet clutch friction material with either new or degraded ATF acting as a lubricant. The friction material was a commonly used cellulose/phenolic resin material. Results in the very low-speed dynamic regime indicate that the fluid condition plays a critical role in determining the slope characteristics of the friction vs. velocity curve. ATF degraded in service imparts a flat or negative slope to the friction vs. velocity curve, whether on new or glazed (degraded) friction material. A positive slope is measured for fresh ATF. The SFM measurements are therefore consistent with dynamometer and vehicle experiments which measure gross properties of the friction clutch system. The ability of the SFM to reproduce the dynamometer results on a local scale implies that shudder is not due to the gross fluid flow or mechanical effects that are present in a complete torque converter. The frictional characteristics of the clutch plate materials and the ATF are paramount. This technique allows continuous monitoring of ATF fluid degradation in test fleet vehicles without ATF replenishment since only a few ccs of fluid are necessary for these tests.