Traction Behavior Under Extreme Conditions

Traction drive continuously variable transmissions (TCVTs) are under active investigation by a number of OEMs and suppliers. Along with advances in control systems and metallurgy, improved traction fluids will be key to successful implementation of this technology. Traction fluids will need to function over a wide range of temperatures and contact pressures. Contact pressures may reach as high as 4 GPa, while temperatures may range from about - 40 C to about 140 C. It is widely recognized that low temperature fluidity at start-up is an issue, since fluids which give high traction coefficients and adequate viscosity under normal operating temperatures generally exhibit high viscosity at low temperatures. However, fe published data are available on traction behavior at temperatures below about 20 C and at contact pressures above about 2 GPa. Since published data are an inadequate guide to traction behavior under extreme conditions, we have extended our traction measurements to contact pressures as high as 3.7 GPa and to temperatures as low as - 20 C, using point contact traction machines. The results show that traction can drop dramatically at temperatures below about 20 C, indicating potential operating problems at low temperatures. This effect needs to be taken into account when designing hardware and must be a focus of future fluid development.