New Central Film Thickness Equation for Shear Thinning Lubricants in Elastohydrodynamic Lubricated Rolling/Sliding Point Contact Conditions

2014 ◽  
Vol 136 (4) ◽  
Author(s):  
Puneet Katyal ◽  
Punit Kumar
Keyword(s):  
High Pressure ◽  
Equation Of State ◽  
Film Thickness ◽  
Point Contact ◽  
Viscosity Coefficient ◽  
Shear Thinning ◽  
Numerical Approach ◽  
Present Formulation ◽  
Contact Conditions ◽  
Doolittle Equation

This paper offers central film thickness formula pertaining to shear-thinning lubricants under rolling/sliding point contact conditions. The shear-thinning behavior of the lubricants is modeled using Carreau viscosity equation and the piezo-viscous response employed herein is the free-volume based Doolittle equation in conjunction with Tait's equation of state for lubricant compressibility. The present formulation is based on reciprocal asymptotic isoviscous piezo-viscous coefficient as it is a more accurate measure of the high pressure piezo-viscous response of elastohydrodynamic lubricated (EHL) lubricants compared to the conventional pressure–viscosity coefficient. Comparisons between simulated, curve-fitted values, and experimental results validate both the employed numerical approach and rheological model.

Scale Effects in Generalized Newtonian Elastohydrodynamic Films

2008 ◽  
Author(s):  
I. I. Kudish ◽  
P. Kumar ◽  
M. M. Khonsary ◽  
S. Bair
Keyword(s):  
Film Thickness ◽  
Scale Effects ◽  
Point Contact ◽  
Elastohydrodynamic Lubrication ◽  
Viscosity Coefficient ◽  
Shear Thinning ◽  
Effective Pressure ◽  
Practical Advantage ◽  
Inlet Zone ◽  
Real Machine

The prediction of elastohydrodynamic lubrication (EHL) film thickness requires knowledge of the lubricant properties. Today, in many instances, the properties have been obtained from a measurement of the central film thickness in an optical EHL point contact simulator and the assumption of a classical Newtonian film thickness formula. This technique has the practical advantage of using an effective pressure-viscosity coefficient which compensates for shear-thinning. We have shown by a perturbation analysis and by a full EHL numerical solution that the practice of extrapolating from a laboratory scale measurement of film thickness to the film thickness of an operating contact within a real machine may substantially overestimate the film thickness in the real machine if the machine scale is smaller and the lubricant is shear-thinning in the inlet zone.

The High Pressure Rheology of Mixtures

2004 ◽  
Vol 126 (4) ◽  
pp. 697-702 ◽  
Author(s):  
Scott Bair
Keyword(s):  
Molecular Weight ◽  
High Pressure ◽  
Binary Systems ◽  
Viscosity Coefficient ◽  
Shear Thinning ◽  
Newtonian Viscosity ◽  
Double Shear ◽  
Ideal Mixing ◽  
Viscosity Behavior ◽  
Single Flow

The Newtonian mixing rules for several binary systems have been experimentally investigated. Some systems show non-ideal mixing response and for some systems the non-ideal response is pressure-dependent, yielding an opportunity for manipulation of the pressure-viscosity behavior to advantage. The mixing of differing molecular weight “straight cuts” can produce very different pressure-viscosity response. This behavior underscores the difficulty in predicting the pressure-viscosity coefficient based upon chemical structure and ambient viscosity since the molecular weight distribution is also important, but it also provides another opportunity to control the high-pressure response by blending. The first experimental observation of double shear-thinning within a single flow curve is reported. Blending then provides the capability of adjusting not only the Newtonian viscosity but also the non-Newtonian shear-thinning response as well.

scholarly journals The Unresolved Definition of The Pressure-Viscosity Coefficient

2021 ◽  
Author(s):  
Scott Bair
Keyword(s):  
High Temperature ◽  
Equation Of State ◽  
Low Temperature ◽  
Film Thickness ◽  
Pressure Dependence ◽  
Analytical Calculation ◽  
Elastohydrodynamic Lubrication ◽  
Viscosity Coefficient ◽  
Classical Approach ◽  
Definition Of

Abstract In the classical approach to elastohydrodynamic lubrication (EHL) a single parameter, the pressure-viscosity coefficient, quantifies the isothermal pressure dependence of the viscosity for use in prediction of film thickness. Many definitions are in current use. Progress toward a successful definition of this property has been hampered by the refusal of those working in classical EHL to acknowledge the existence of accurate measurements of the piezoviscous effect that have existed for nearly a century. The Hamrock and Dowson pressure-viscosity coefficient at high temperature requires knowledge of the piezoviscous response at pressures which exceed the inlet pressure and may exceed the Hertz pressure. The definition of pressure-viscosity coefficient and the assumed equation of state must limit the use of the classical formulas, including Hamrock and Dowson, to liquids with high Newtonian limit and to low temperature. Given that this problem has existed for at least fifty years without resolution, it is reasonable to conclude that there is no definition of pressure-viscosity coefficient that will quantify the piezoviscous response for an analytical calculation of EHL film thickness at temperatures above ambient.

On Three-Dimensional Flat-Top Defects Passing Through an EHL Point Contact: A Comparison of Modeling with Experiments

2005 ◽  
Vol 127 (1) ◽  
pp. 51-60 ◽  
Author(s):  
A. Fe´lix-Quin˜onez ◽  
P. Ehret ◽  
J. L. Summers
Keyword(s):  
High Pressure ◽  
Film Thickness ◽  
Three Dimensional ◽  
Point Contact ◽  
Numerical Results ◽  
Shaped Surface ◽  
Pure Rolling ◽  
Pressure Region ◽  
Inlet Zone ◽  
High Pressure Region

A direct comparison between experimental and numerical results for the passage of an array of 3D flat-top, square shaped surface features through an EHL point contact is presented. Results for pure rolling conditions show that the features’ deformation in the high-pressure region is governed by their ability to entrap lubricant both underneath and in the grooves during their passage through the inlet zone. Film perturbations associated with each defect occur as locally enhanced regions of lubricant and film thickness micro-constrictions. Under sliding conditions the features sustain further deformations as they traverse the high-pressure conjunction and meet the highly viscous lubricant entrapped in the grooves, which moves at a different velocity. Lubricant is also seen to accumulate just in front or behind the features depending on the slide-to-roll ratio. Overall, the results highlight the importance of understanding the effects of the defects structure and the lubricant rheology on the film thickness to unravel the effects of real roughness patterns.

A Quantitative Solution for the Full Shear-Thinning EHL Point Contact Problem Including Traction

2007 ◽  
Author(s):  
Yuchuan Liu ◽  
Q. Jane Wang ◽  
Scott Bair ◽  
Philippe Vergne
Keyword(s):  
Film Thickness ◽  
Point Contact ◽  
Elastohydrodynamic Lubrication ◽  
Shear Thinning ◽  
High Viscosity ◽  
Viscosity Models ◽  
Pure Rolling ◽  
Volume Viscosity ◽  
Traction Coefficient ◽  
Simulation And Experiment

We present a realistic elastohydrodynamic lubrication (EHL) simulation in point contact using a Carreau-like model for the shear-thinning response and the Doolittle-Tait free-volume viscosity model for the piezoviscous response. The liquid is a high viscosity polyalphaolefin which possesses a relatively low threshold for shear-thinning. As a result, the measured EHL film thickness is about one-half of the Newtonian prediction. We derived and numerically solved the two-dimensional generalized Reynolds equation for the modified Carreau model based on Greenwood [1]. Departing from many previous solutions, the viscosity models used for the pressure and shear dependence were obtained entirely from viscometer measurements. Truly remarkable agreement is found in the comparisons of simulation and experiment for traction coefficient and for film thickness in both pure rolling and sliding cases. This agreement validates the use of a generalized Newtonian model in EHL.

The Shear-Thinning Elastohydrodynamic Film Thickness of a Two-Component Mixture

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
Yuchuan Liu ◽  
Q. Jane Wang ◽  
Ivan Krupka ◽  
Martin Hartl ◽  
Scott Bair
Keyword(s):  
Film Thickness ◽  
Ambient Pressure ◽  
Point Contact ◽  
Elastohydrodynamic Lubrication ◽  
Shear Thinning ◽  
Component Mixture ◽  
Base Oil ◽  
Viscosity Models ◽  
Viscosity Behavior ◽  
Pure Rolling

Lubricant base oils are often blends of different molecular weight cuts to arrive at a specified ambient pressure viscosity and, to improve the temperature-viscosity behavior or to simply increase the viscosity, viscosity-modifying polymer additives are often added to the base oil. This paper investigates the effect of mixture rheology on elastohydrodynamic lubrication (EHL) film thickness using EHL contact measurements and a full numerical analysis for three synthetic lubricants including two single-component lubricants PAO650 and PAO100 and a mixture of these. The pressure and shear dependences of the viscosity of these lubricants were measured with high-pressure viscometers; viscosities were not adjusted to fit experiment. The point contact film thicknesses for these lubricants in pure rolling were measured using a thin-film colorimetric interferometry apparatus. Numerical simulations based on the measured rheology show very good agreement with the measurements of film thickness while the Newtonian prediction is up to twice the measurement. These results validate the use of realistic shear-thinning and pressure-viscosity models, which originate from viscosity measurements. It is conceivable that simulation may provide a means to “engineer” lubricants with the optimum balance of film thickness and friction through intelligent mixing of components.

Optical Interferometric Observations of the Effects of a Bump on Point Contact EHL

1992 ◽  
Vol 114 (4) ◽  
pp. 779-784 ◽  
Author(s):  
M. Kaneta ◽  
T. Sakai ◽  
H. Nishikawa
Keyword(s):  
Thin Film ◽  
Film Thickness ◽  
Numerical Simulations ◽  
Contact Region ◽  
Thickness Distribution ◽  
Point Contact ◽  
Elastohydrodynamic Lubrication ◽  
Interesting Phenomenon ◽  
Contact Conditions ◽  
Average Speed

The effects of surface kinematic conditions on micro-elastohydrodynamic lubrication (micro-EHL) are investigated under rolling and/or sliding point contact conditions using the optical interferometry technique. A long bump of chromium sputtered on the surface of a highly polished ball is used as a model asperity. It is shown that the film thickness distribution or the elastic deformation of the bump is influenced significantly by the surface kinematic conditions and the orientation of the bump. An interesting phenomenon is also found when contacting surfaces move with different speeds; the thin film formed on a transversely oriented bump existing at the entrance of the contact travels through the contact region at the average speed of the surfaces. The experimental results obtained qualitatively confirm published numerical simulations.

Scale Effects in Generalized Newtonian Elastohydrodynamic Films

2008 ◽  
Vol 130 (4) ◽  
Author(s):  
Ilya I. Kudish ◽  
P. Kumar ◽  
M. M. Khonsari ◽  
Scott Bair
Keyword(s):  
Film Thickness ◽  
Scale Effects ◽  
Elastohydrodynamic Lubrication ◽  
Viscosity Coefficient ◽  
Shear Thinning ◽  
Effective Pressure ◽  
Carreau Fluid ◽  
Practical Advantage ◽  
Inlet Zone ◽  
Real Machine

The estimation or prediction of elastohydrodynamic lubrication (EHL) film thickness requires knowledge of the lubricant properties. Today, in many instances, the lubricant properties have been obtained from a measurement of the central film thickness and the assumption of a classical Newtonian film-thickness formula. This technique has the practical advantage of using an effective pressure-viscosity coefficient, which compensates for shear-thinning. We have shown by a perturbation analysis of limiting cases for fluid with Carreau rheology (represented by Newtonian and power fluid) and by a full EHL numerical solution for Carreau fluid that the practice of extrapolating from a laboratory scale measurement of film thickness to the film thickness of an operating contact may substantially overestimate the film thickness in the real machine if the machine scale is smaller and the lubricant is shear-thinning within the inlet zone. The intention here is to show that errors result from extrapolation of Newtonian formulas to different scale and not to provide advice regarding quantitative engineering calculations.

Actual Eyring Models for Thixotropy and Shear-Thinning: Experimental Validation and Application to EHD

2004 ◽  
Vol 126 (4) ◽  
pp. 728-732 ◽  
Author(s):  
Scott Bair
Keyword(s):  
High Pressure ◽  
Film Thickness ◽  
Experimental Validation ◽  
Shear Thinning ◽  
Anomalous Behavior ◽  
Sliding Contact ◽  
Modern Methods ◽  
Eyring Model ◽  
Using Data ◽  
Limited Usefulness

The Eyring sinh law is presently the most well-accepted model for shear-thinning of EHD lubricants at high pressure. It was, however, not accepted for this purpose by Eyring, who found it to be of only limited usefulness for thixotropy. Then, it is extremely important that these models receive a critical review using data obtained by modern methods to evaluate the actual Eyring models and show how they differ from the model in use today. Data from high-pressure viscometers were used to validate the actual Eyring models. The Ree-Eyring model for shear-thinning obeys time-temperature-pressure superposition. A film thickness calculation shows it to suffer from the same anomalous behavior in sliding contact as does the sinh law.

EHL Circular Contact Film Thickness Correction Factor for Shear-Thinning Fluids

2008 ◽  
Vol 130 (4) ◽  
Author(s):  
Punit Kumar ◽  
M. M. Khonsari
Keyword(s):  
Film Thickness ◽  
Correction Factor ◽  
Point Contact ◽  
Elastohydrodynamic Lubrication ◽  
Shear Thinning ◽  
Correction Factors ◽  
Shear Thinning Fluids ◽  
Pure Rolling ◽  
Wide Range ◽  
Circular Contact

An extensive set of full elastohydrodynamic lubrication point contact simulations has been used to develop correction factors to account for the effect of shear-thinning lubricant behavior on the central and minimum film thickness in circular contacts under pure rolling condition. The film thickness for a shear-thinning lubricant can be easily obtained by dividing the corresponding Newtonian film thickness by the appropriate correction factor. Comparisons of the film thickness values obtained using the correction factors have been matched with the published experimental results pertaining to shear-thinning lubricants with a variety of realistic flow and piezoviscous properties under a wide range of operating speed. The good agreement between them establishes the validity and versatility of the correction factors developed in this paper.

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