Effectiveness of Roelands Formula with Constant VPC in EHL of Higher Pair Contact

An essential issue concerning the elastohydrodynamic lubrication (EHL) problem in the higher pair contacts of gear and bearing is the viscosity calculation of lubricating oil, which has a non-negligible effect on the EHL characteristic parameters such as film thickness, friction coefficient, temperature rise of oil film and substrate and so on. Roelands empirical formula is commonly used in the calculation of lubricant viscosity, and considered to have higher accuracy than exponential function. However, the operation temperature and pressure of the lubricating oil have exceeded its application range in the contact of the high pair. In this paper, the effectiveness of Roelands formula with constant VPC (viscosity-pressure coefficient) in EHL analysis of higher pairs is investigated. The film thickness and temperature rise of EHL point contact are computed according to the fitted VPC expression varying with the temperature, and the results are compared with those with the constant VPC. The results show that the lubricant viscosity, film thickness and film temperature rise of EHL are highly sensitive to the VPC at low temperature and high pressure. Even a modest change in the VPC produces an important rise/reduction in the above characteristic parameters. The Roelands formula with constant VPC causes a certain deviation to the central film thickness (CFT) and maximum film temperature (MFT) in the EHL point contact when the reference temperature is small, and the deviation decreases gradually with the increasing of the reference temperature.