Tribology of traction drives
To understand the reasons for the long and trouble-free operation of Torotrak’s full-toroidal traction drives, it is helpful to appreciate a little of the physics involved. Key to our technology is the tribology of thin film lubrication. (Tribology is the science of interacting surfaces in relative motion and is concerned with friction, lubrication and wear.) Tribology divides lubrication into four broad regimes, based upon the relative thickness of the oil film in each case, compared to the roughness of the surfaces.
When the film thickness is the same as the height of the surface roughness, contact between the two surfaces is inevitable; this is called boundary lubrication. By the time the film is approximately ten times the height of the surface roughness, full fluid film lubrication is developed and no contact occurs between the surfaces.
In between the two extremes exist two further regimes: mixed lubrication and elastohydrodynamic lubrication (called EHD or EHL). The type of lubrication used by the full-toroidal variator is EHD. Rolling the crown of each roller against the surface of the disc traps a microscopic oil film between them. The effect is rather like a car tyre aquaplaning on a wet road; the surfaces do not touch but are separated by the fluid. The actual thickness of the film is between one hundredth and one thousandth of the thickness of a human hair.