What is a traction fluid?

Traction fluid is a transmission fluid with properties specifically designed for transmitting torque between two smooth rolling elements. The fluids are generally based on special synthetic oils. The ability of a traction fluid to transfer torque is measured by its Coefficient of Traction, or Ct. While most fluids have some measurable Ct, the Ct of a fluid engineered as a traction fluid is 3x to 5x greater than the Ct of a typical standard lubricant. The synthetic base oil used in a traction fluid is tailored to give them high traction coefficients over wide operating ranges.

How does a fluid transmit torque?

Torque is transferred between the rolling elements through shear forces in a thin fluid film. Under normal conditions of hydrodynamic lubrication, the shear forces transmitted are very small. However, under specific operating conditions (i.e., certain relative surface speeds and contact pressures), the fluid transitions into the elastohydrodynamic lubrication (EHL) regime and exhibits shear force transmitting properties similar to those of elastic solids. In engineering terms, materials like steel and aluminum are elastic solids. So, in effect, the traction fluid serves the same purpose as the steel gear teeth of a conventional transmission.

Won't the rolling elements slip at higher torques?

Most traction drives, including the NuVinci^® CVP, incorporate mechanisms to dynamically control the contact pressure as torque is increased. The properties of the traction fluid and the geometric and material properties of the rolling elements and the pressure-control mechanisms are designed to manage internal forces to prohibit slipping. Therefore, design of a traction drive includes careful selection and testing of the traction fluid. All traction transmissions have a very small amount of what is called creep that occurs within the fluid under normal conditions. The amount of creep is generally small enough that precision instrumentation is required to measure it.

Does the traction fluid also provide lubrication?

Yes. In addition to a high traction coefficient, a traction fluid is designed to provide lubrication to protect elements from wear, dissipate heat, and dampen vibrations in the transmission, just as conventional transmission fluids do in gear type transmissions.

Why haven't I heard of traction fluids before?

Though traction CVTs have been used for vehicular and industrial applications for nearly a century, advances in material and fluid properties over the last two decades and a better understanding of the elastohydrodynamic phenomenon were required to enable wide-spread use of traction drives.  The NuVinci CVP and Valvoline Invaritorc™ traction fluid leverage these technology advances to provide a solution that is more broadly applicable than previous designs.