Prediction of Anti-Shudder Properties of Automatic Transmission Fluids using a Modified SAE No. 2 Machine

One of the barriers to successful implementation of the continuous slip torque converter clutch (CSTCC) design for automatic transmissions (AT) is transmission shudder. An important factor contributing to shudder is the frictional characteristics of the automatic transmission fluid (ATF). In this paper, we report the development of a test that allows the precise measurement of the frictional characteristics of the ATF at different sliding speeds (μ-v data) using an SAE No. 2 friction machine. A conventional SAE No. 2 machine was modified, by installing a DC static drive motor, to simulate the sliding speeds, surface pressures, and fluid temperatures found in actual vehicle operating conditions. The μ-v data obtained with the machine showed good correlation with the vehicle shudder, i.e., the ATFs showing more negative gradient in the μ-v data caused more severe shudder in the vehicle. Initial shudder and anti-shudder durability evaluations were conducted using a variety of commercial and experimental ATFs. It was found that the highly friction-modified ATFs exhibited good initial anti-shudder performance. ATFs meeting major North American OEM requirements were not always acceptable for CSTCC applications.