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

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 non-dimensional parameters. The boundary layer width scales as 1/M−−√ (equivalent to λ¯3/8 ), the maximum pressure difference relative to the Hertzian solution as 1/M−−√3 (equivalent to λ¯1/4 ) and the film thickness as 1/M−−√16 (equivalent to λ¯3/64 ) with M the Moes non-dimensional load and λ¯ 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

Nurses' perceptions of aids and obstacles to the provision of optimal end of life care in ICU

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Prediction of the Stribeck curve under full-film Elastohydrodynamic Lubrication

The Stribeck curve shows the friction coefficient as a function of speed, viscosity and load. The viscosity times speed over load parameter can be interpreted as a film thickness. The film thickness over roughness parameter unifies friction curves in the isoviscous rigid regime. In this paper, the Stribeck curve is predicted numerically in the full-film Elastohydrodynamic Lubrication regime. It is shown that the lambda ratio is not the most appropriate parameter. A more elaborate parameter including the operating conditions and based on the Amplitude Reduction Theory [1] gives much better results. For a complex surface topography, the full numerical simulation is time-consuming. A rapid prediction method is proposed. Good agreement is found between the full numerical simulation and the prediction

Boundary layers: Unifying the impact and rolling EHL point contacts

Transient effects in elastohydrodynamic lubrication occur due to varying operating conditions and surface features moving through the contact. For rolling/sliding contacts the lubricated contact behaviour is determined by a unifying mechanism characterized by the inlet length (boundary layer). Roughness deformation depends on a single dimensionless parameter representing the ratio inlet length to wavelength. This behaviour is shown to generalize to the pure impact problem. The bell shaped film thickness near the periphery of the contact is directly related to the boundary layer velocity profile. Also, the waviness deformation under impact conditions is shown to depend on the same parameter as in rolling contacts when the rolling velocity is replaced by the local boundary layer velocity in the impact problem