Viscosity Change in Automatic Transmission Fluid: Comparison between Field Data and Shear Stability Test Results

摘要

Automatic transmission fluid (ATF), used to lubricate automatic transmissions (ATs), performs diverse functions. ATF operates as a hydraulic fluid transmitting the power of a torque converter unit, as a lubricant protecting movable parts, such as gears and bearings, and as a hydraulic fluid in hydraulic circuits performing step shift control. The provision of all these functions in a wide temperature range, from a low to high, requires an insignificant viscosity change in ATF with a change of temperature, that is, a high viscosity index is required. Therefore, a viscosity index improver (VII) is usually added. However, VIIs contain polymer compounds, of which polymethacrylate is a typical representative. Under real operation conditions, part of the molecules is destroyed by mechanical shear, reducing viscosity in many cases. Consequently, the reasonable provision of viscosity characteristics of ATF for a long time requires selecting a VII with high shear stability. The evaluation of the shear stability characteristics of ATF is performed by using different shear stability test methods. In Japan, standard data are established by two test methods: the ultrasonic method (JASO M 247-95) [2] standardized in JASO M 315-04 (automatic transmission fluid standard) [1] enacted by Japanese Society of Automotive Engineers and the CEC-L-45-A99 method (common name: KRL (Kegelrollenlagerausseenring) [3] by using a taper roller bearing, which has started to be used on an international scale in recent years. However, in these shear stability test methods, measuring the dynamic viscosity is performed only during a standard test time (in JASO M 347-95, it amounts to 1 h, in CEC-L-45-A99, it amounts to 20 h) without considering a change of time during which the viscosity reduces.