Effects of lubricant compressibility on the film thickness in EHL line and circular contacts

As a first step towards generalized (non-closed-form coefficients in Reynolds' equation) lubricant behaviour, this paper describes the incorporation of the Jacobson and Vinet density pressure equation in a multilevel solver for the pressure and the film thickness in an EHL contact. However, the use of this model is also of interest on its own account, as compression experiments (Jacobson and Vinet, Ramesh) have indicated limitations to the applicability of the widely used Dowson and Higginson equation. In this paper, results of the isothermal steady-state line and circular contact are presented and compared with results obtained assuming an incompressible lubricant and with results employing lubricant compressibility according to the Dowson and Higginson equation. The observed phenomena are traced back to Reynolds' equation, and it is shown that the reduction of the central film thickness due to compressibility can be predicted easily, regardless of the type of density pressure equation used. In addition it is shown that, using the Jacobson and Vinet equation, the minimum film thickness in a line contact, beyond a certain load, may occur in the centre of the contact, instead of at the exit. This phenomenon is analysed theoretically, and it is shown to be very unlikely to occur in the circular-contact problem.