Thrust 3: Effectiveness
Title
3D.2: New Directions in Elastohydrodynamic Lubrication to Solve Fluid Power Problems
Project Leader
Scott Bair (Georgia Tech)
Statement of Project Goals
The goal of the project is to develop the tools that may be used by engineers to design more compact, reliable and energy efficient fluid power components by improving the film thickness and reducing mechanical loss in the full-films occurring between non-conforming rolling/sliding machine elements. A fundamental rheological foundation for the field of elastohydrodynamic lubrication (EHL) has been lacking since the inception. For example:
- The proper definition has not been found for a parameter (a pressure-viscosity coefficient) to quantify the piezoviscous strength of any Newtonian liquid, regardless of the nature of the piezoviscous function, so that Newtonian film thickness may be predicted.
- The properties of a liquid that must necessarily be included in a film thickness calculation when the Newtonian prediction is inaccurate have not been specified.
- The properties of a liquid that must necessarily be included in a full-film friction calculation have not been specified.
This project is providing the rheological foundation to solve these important problems.
Project’s Role in Support of the Strategic Plan
Compactness
More compact components must necessarily have smaller radius of curvature of the contacting elements. A clear strategy for making more compact components is also to increase the operating pressure. The resulting increase in contact pressure and decrease in radius of curvature of the sliding/rolling elements will result in diminished film thickness. The reduced film must impact the reliability.
An example can be made of the conversion from organic based fluids to water/glycol solutions. This usually results in having to reduce the operating pressure to retain the fatigue life of the concentrated contacts. Water/glycol produces a substantially thinner film than do organic based fluids (by an order-of-magnitude) [1]; however, present EHL theory is completely incapable of predicting the film reduction as there is currently no means to simulate the rheology of linear piezoviscous liquids. We have made the solution of this problem a priority.
The ability to predict film thickness of any liquid from properties that can be measured and associated with the chemistry of the liquid will enable the formulation of fluids for improved durability at smaller scales.
Efficiency
Surprisingly, there has been little progress within EHL over the last forty years in explaining the mechanism of mechanical dissipation in full EHL films. In very recent related work [2] using the temperature/pressure correlation devised by this project, the first experimentally validated EHL friction calculation was performed which included thermal-softening and shear-thinning. Fragility has been shown to be the principal property controlling friction. In particular, the results of this project may be used to rank the mechanical energy loss of contacts lubricated by fragile hydraulic oils.
Description and Explanation of Research Approach
A significant opportunity to investigate the elastohydrodynamic lubrication (EHL) problem using experimental film measurements, high pressure rheological measurements and numerical analysis (quantitative elastohydrodynamics) has recently appeared as a result of this project. In an exciting departure from previous methods, new film behavior regarding the effect of scale and load has been predicted from EHL simulation using measured rheological properties and the predictions have subsequently been experimentally validated [3]. Both film thickness and friction may now be predicted [4], at least for light loads, from primary properties rather than from fictitious properties adjusted to fit analysis to measurements of film thickness or friction. Film thickness may now be calculated from the properties of mixtures [5]. Thermal EHL calculations using measured rheology have revealed the importance of the high-pressure thermal properties of lubricants in calculations which have been experimentally validated [2].
An unfortunate aspect of EHL research over the last several decades has been the use of adjusted viscosity to validate hypotheses. Rather than test the predictions of theory by comparison of predictions with experiment using calculations based upon the measurable viscosity, in most cases, viscosity has been adjusted to ensure a successful outcome. As a result, many of the outstanding questions remain unanswered.
The present time is propitious for the EHL field to embrace a quantitative description of the temperature and pressure dependence of viscosity since there has been, over the last decade, an interest by the physics community in the pressure evolution of the dynamic properties of the supercooled liquids such as lubricants [6]. Fragility, a property strongly affecting EHL friction [7] and transient EHL film response [8] is now being intensely studied [6]. Fragile liquids experience greater changes in their properties (are more non-Arrhenius) as the glass transition is approached by cooling or compression than do strong liquids [9].
A description of the temperature and pressure dependence of viscosity is also necessary for the calculation of the relaxation times which determine the onset of shear-thinning response and the onset of time-dependent behavior in both shear and compression. For example, the shear-dependent viscosity of liquids is often described by the single-Newtonian Carreau law [10],
where n is the power-law exponent which in the limit of high shear rate is 
.
The generalized viscosity, η, departs from the low-shear Newtonian viscosity, μ, when the product of shear rate, 
, and relaxation time, 
, approaches unity. The commonly quoted form [11] of the Einstein-Debye relation for the rotational relaxation time of a molecule in terms of the universal gas constant, Rg, is 
Now, the molecular weight, M, is constant and the product of mass density and temperature, ρT, varies only slowly with temperature and pressure as compared with the viscosity. Therefore, for practical measurements and EHL calculations, it is sufficient to set λ proportional to μ. This simple rule also provides an alternative method of measurement of low-shear viscosity. Any measurement of relaxation time under conditions which overlap with a viscosity measurement will provide the constant of proportionality which will allow extrapolation of the viscosity data to the conditions of the relaxation time measurement [12].
An essential part of this program involves collaboration with partners around the world. A list of collaborators which have been instrumental to the progress made to date follows:
1. Ashlie Martini, Purdue University, simulation
2. Ivan Krupka, Brno University, Czech Republic, film thickness measurement
3. Riccardo Casilini, George Mason University, measurements of relaxation time
4. Mike Roland, Naval Research Laboratory, rheology
5. Michael Khonsari, Louisiana State University, simulation
6. Punit Kumar, National Institute of Technology, India, simulation
7. Philippe Vernge, INSA Lyon, France, film thickness and traction measurement
8. Kees Venner, Univ. of Twente, Netherlands, film thickness measurement and simulation
9. Paul Michael, MSOE, lubricant formulation
10. Arno Laesecke, NIST Boulder, viscosity correlations
References
[1] Ratoi-Salagean, M. and Spikes, H.A., "The Lubricant Film-Forming Properties of Modern Fire-Resistant Hydraulic Fluids," Tribology of Hydraulic Pump Testing, ASTM STP 1310, George S. Totten, Gary H. King and Donald J. Smolenski, Eds., American Society for Testing and Materials, 1996, pp. 21-37.
[2] Habchi, W., Vergne, P., Bair, S., Andersson, O., Eyheramendy, D., Morales-Espejel, G.: Influence of Pressure and Temperature Dependence of Lubricant Thermal Properties on the Behaviour of Circular TEHD Contacts. Tribology International 33(2), 127-135 (2009).
[3] Krupka, I, Bair, S., Kumar, P., Khonsari, M. M., Hartl, M. "An Experimental Validation of the Recently Discovered Scale Effect in Generalized Newtonian EHL" Tribology Letters 33, 2009, 127-135.
[4] Liu, Y., Wang, Q. J., Bair, S., Vergne, P. "A Quantitative Solution for the Full Shear-Thinning EHL Point Contact Problem including Traction. Tribology Letters , 2007, 28, 171-181.
[5] Liu, Y., Wang, Q., Krupka, I., Hartl, M., Bair, S.: The Shear-Thinning Elastohydrodynamic Film Thickness of a Two-Component Mixture" ASME, J. Tribology ,2008, 130, 021502.
[6] Drozd-Rzoska, A., Rzoska, S.J., Roland, C.M. and Imre, A.R., "On the Pressure Evolution of Dynamic Properties of Supercooled Liquids," J. Phys. Condens. Matter, 20, 2008, 244103, 11 pages.
[7] Bair, S., Roland, C.M. and Casalini, R., "Fragility and the Dynamic Crossover in Lubricants", Proc. Instn. Mech. Engrs. Part J, J. Engr. Trib., 2007, 221(7), 801-811.
[8] Martini, A. and Bair, S., "The role of Fragility in EHL Entrapment," Tribology International, 43, 2010, 277-282.
[9] Angell, C.A., "Relaxation in Liquids, Polymers and Plastic Crystals-Strong/Fragile Patterns and Problems", J. Non-Cryst. Solids, 131-133, 1991, 13-31.
[10] Carreau, P.J., "Rheological Equations from Molecular Network Theories", Trans. Soc. Rheology, Vol. 16, No. 1, 1972, pp. 99-127.
[11] Bair, S. and Winer, W.O., "A Quantitative Test of the Einstein-Debye Relation for Low Molecular Weight Liquids using the Shear Dependence of Viscosity", Tribology Letters, 26(3), 2007, pp.223-228.
[12] Bair, S. and Winer, W.O., "Some Observations on the Relationship Between Lubricant Mechanical and Dielectric Transitions under Pressure", ASME J. Lubrication Techn., 102(2), 1980, 229-235.