The determination of the pressure-viscosity coefficient for high lubricant parameter oils using accurate film thickness measurements

The pressure-viscosity coefficient alpha is a very important parameter in elastohydrodynamic (EHD) lubrication but almost never known. Unfortunately, its determination is problematic. At the 5th ATK in 2009 a method to assess alpha has been presented, by analyzing accurate film thickness measurements, through film thickness formulas from the literature. This was successful for a simple mineral Newtonian oil (HVI60). The Chittenden et al. (1985) formula proved to be the best out of 10 approximation formulas. The lubricant parameter L is a nondimensional group, linear in a. In this first study, L values were smaller than 7. This method is now used for two traction fluids, Santotrac S50 and S70. Traction fluids show extremely high alpha values, thereby extending the range to high values of L. They are known for their non-Newtonian behaviour. It was concluded that The Hamrock and Dowson (1977) and the Chittenden et al. (1985) approximation formulas for central film thickness performed best out of 12 models tested in a domain 1.7 <L <17. Also, the validity of these formulas transcends the regime where they were based upon, and can describe fluid film formation of both traction fluids under investigation. This implies that the method is even more general than assumed, in that it can also be used with high alpha value lubricants.