Boundary Layer Behaviour in Circular EHL Contacts in the Elastic-Piezoviscous Regime

Abstract

The solution of elastohydrodynamically lubricated contacts at high loads and/or low speeds can be described as a Hertzian pressure with inlet and outlet boundary layers: zones where significant pressure flow occurs. For the soft lubrication regime (elastic-isoviscous), a self-similar solution exists in the boundary layers satisfying localized equations. In this paper, the boundary layer behaviour in the elastic-piezoviscous regime is investigated. The lengthscale of the boundary layers and the scaling of pressure and film thickness are expressed in nondimensional parameters. The boundary layer width scales as 1= ffiffiffiffi ffi M p (equivalent to " k 3=8 ), the maximum pressure difference relative to the Hertzian solution as 1= ffiffiffiffi ffi M 3 p (equivalent to " k 1=4 ) and the film thickness as 1= ffiffiffiffi ffi M 16 p (equivalent to " k 3=64 ) with M the Moes non-dimensional load and " k a dimensionless speed parameter. The Moes dimensionless lubricant parameter L was fixed. These scalings differ from the isoviscous-elastic (soft lubrication) regime. With increasing load (decreasing speed), the solution exhibits an increasing degree of rotational symmetry. The pressure varies less than 10 % over an angle less than 45 degrees from the lubricant entrainment direction. The results provide additional fundamental understanding of the nature of elastohydrodynamic lubrication and give physical rationale to the finding of roughness deformation depending on the ''inlet length''. The findings may contribute to more efficient numerical solutions and to improved semi-analytical prediction methods for engineering based on physically correct asymptotic behaviour.

Professor Duncan Dowson, a source of inspiration

“…The basic idea is to identify rheological parameters based on very accurate film thickness measurements. The pressure spike zone is regarded as a boundary layer variation around the Hertzian pressure, see Biboulet et al 14 and Venner et al 15 A very similar phenomenon occurs during an EHL impact, see Lubrecht et al 16 Figure 2 shows the calculated difference between EHL pressure distributions and the Herztian pressure. In a sense, it represents the additional contribution of the lubricant once the "dry" contact contribution is removed.…”

Boundary layers: Unifying the impact and rolling EHL point contacts

“…This boundary layer, its scaling, and the effect on the EHL film are described in Refs. [44][45][46]. The current work extends this boundary layer analysis to the film thickness behaviour in the impact problem.…”

Solid viscoelasticity in contact mechanics and elastohydrodynamic lubrication

“…. For point contacts used in this study, the inlet length of elastic contacts may be proposed as following based on the similarity solutions by Snoeijer et al [139],…”