Elastohydrodynamic Lubrication

Quantitative Compressibility Effects in Thermal Elastohydrodynamic Circular Contacts

[+] Author and Article Information
W. Habchi

Lebanese American University
Dep. of Ind. & Mech. Eng.
Byblos, Lebanon
; G.W. Woodruff School of Mech. Eng.
Center for High Pressure Rheology
Georgia Institute of Technology
Atlanta, GA 30332-0405

S. Bair

G.W. Woodruff School of Mech. Eng.
Center for High Pressure Rheology
Georgia Institute of Technology
Atlanta, GA 30332-0405

1At the time this work was done, the first author was holding a visiting scholar position at Georgia Institute of Technology.

Contributed by Tribology Division of ASME for publication in the Journal of Tribology. Manuscript received August 27, 2012; final manuscript received October 25, 2012; published online December 20, 2012. Assoc. Editor: Dong Zhu.

J. Tribol 135(1), 011502 (Dec 20, 2012) (10 pages) Paper No: TRIB-12-1136; doi: 10.1115/1.4023082 History: Received August 27, 2012; Revised October 25, 2012

This paper investigates the effects of lubricant compressibility on the film-forming performance of thermal elastohydrodynamic lubricated (EHL) circular contacts. Numerical film thickness predictions using the classical Dowson and Higginson relationship are compared to those that would be obtained using a more realistic compressibility model, all other parameters kept unchanged. This allows an isolation of the realistic compressibility effects on the film-forming performance. For realistic predictions, the authors consider two model liquids from the 1953 report of the ASME Research Committee on Lubrication, the most and the least compressible. The compressibility of these liquids is modeled using the Tait equation of state (EoS) while all other transport properties are kept unchanged for the sake of isolating compressibility effects. In addition, the same typical generalized-Newtonian behavior is assumed for both model liquids. The results reconfirm the well-known observations that minimum film thickness is very little affected by lubricant compressibility while central film thickness decreases linearly with the increase in volume compression of the lubricant. It is also observed that the relative errors on central film thicknesses induced by the use of the Dowson and Higginson relationship for compressibility increase with load and temperature and are very little affected by mean entrainment speed. Compressibility is shown to be a significant source of error in film-derived measurements of pressure-viscosity coefficients especially at high temperature. The thermodynamic scaling that provides an accurate and consistent framework for the correlation of the thermophysical properties of liquids with temperature and pressure requires an accurate equation of state. In brief, this paper highlights the importance of using realistic transport properties modeling based on thermodynamic scaling for an accurate numerical prediction of the performance of EHL contacts.

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Grahic Jump Location
Fig. 1

The ASME relative volumes of the naphthenic mineral oil (ASME 38) and the silicone oil (ASME 55) and their fit to the Tait EoS

Grahic Jump Location
Fig. 2

Comparison of the Tait EoS and Dowson and Higginson relationship for dimensionless density of ASME 38(left) and ASME 55 (right) lubricants as a function of pressure

Grahic Jump Location
Fig. 3

Comparison of dimensionless film thickness profiles along the central line of the contact in the x-direction obtained under pure-rolling conditions using the Tait EoS for both ASME 38 (left) and ASME 55(right) with respect to those obtained using the Dowson and Higginson relationship (T0 = 30 °C)

Grahic Jump Location
Fig. 4

Comparison of dimensionless film thickness profiles along the central line of the contact in the x-direction obtained using the Tait EoS for ASME 55 with respect to those obtained using the Dowson and Higginson relationship for SRR = 0 (left) and SRR = 0.5 (right) with T0 = 30 °C (top figures) and T0 = 100 °C (bottom figures)

Grahic Jump Location
Fig. 5

Comparison of effective pressure-viscosity coefficients using assumed compressibilities with the directly measured coefficient

Grahic Jump Location
Fig. 6

Comparison of real (certified by NIST) density variations of 2-methylpentane against pressure and temperature with those predicted by the Tait EoS



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