Wet Clutch Performance in a Mineral-Based and in a Partial-Synthetic-Based Automatic Transmission Fluid

The effects of a mineral-based, commercial automatic transmission fluid (Fluid-M) and a partial-synthetic-based commercial fluid (Fluid-PS) on wet clutch performance were investigated from the viewpoints of compressibility, durability, and friction-pressure-speed-temperature (μ-P-v-T) characteristics. Furthermore, the physical and chemical properties of mineral and partial-synthetic fluids were compared by the analyses of viscometry, thermo-oxidative stability, and metal-to-metal wear preventive characteristics. Friction material specifications for partial-synthetic fluid applications will be different from those for mineral-based fluid applications. Size exclusion chromatography showed that Fluid-PS has a higher concentration and a lower molecular weight of viscosity index (VI) improver than Fluid-M. Chemical analyses indicated major differences in the antiwear and antioxidant additive packages of Fluid-M and Fluid-PS. Thermal oxidation tests indicated different degradation paths for the fluids. Both fluids formed acid, ketone, and ester. However, Fluid-M showed more tendency to polymerize to diester or polyester than Fluid-PS. Fluid-PS reduced friction and wear in metal-to-metal contacts in four-ball tests as compared to Fluid-M. The less metallic wear in Fluid-PS was explained with the superior viscometrics and antiwear additive characteristics of the fluid. Fluid-M and Fluid-PS had similar torque response curve performance in SAE tests at low temperatures. However, Fluid-PS tended to cause more "shudder" at high temperatures, especially with rigid friction materials. Fluid-PS reduced the compressibility of wet clutch systems as compared to Fluid-M. The differences in torque curve shape and compression modulus in the fluids were explained with the differences in base oil type and VI improver package and the other additives of the fluids. Fluid-PS' tendency to increase the rigidity of a wet clutch system can be compensated by tailoring the compressibility of friction materials. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference In San Francisco, California, October 13-17, 1996