Frictional Response of Automatic Transmission Fluids in Band and Disc Clutches

The frictional response of automatic transmission fluids (ATF) in band clutches is significantly different from that in disc clutches. A research program was undertaken to investigate these differences and to explore effects of ATF chemistry on frictional response. Two disc clutch tests (the HEFCAD and the MERCON ^® friction durability procedures), as well as two band tests (the THM 700-R4 and the 125C), were run on different fluids. Vehicle shift-feel tests were also run to better gauge the results.

It was found that the band tests yielded high static friction relative to dynamic friction whereas the disc clutch test results frequently gave the reverse. The static-minus-dynamic value (S-D), as measured in some of these bench tests, appears to predict the shift quality of a given fluid. Fluids based on sulfur-phosphorus (S-P) chemistry seem to bridge the gap between the disc clutch and band clutch friction better than fluids based on ZDDP chemistry.

The findings are believed to be helpful to the formulators of DEXRON^® -III ATF 's. (DEXRON^® -III is a future specification intended for a new generation of ATF for GM transmissions.)