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Reference liquids can be useful in the new quantitative approach to elastohydrodynamic lubrication calculations only if the properties of these liquids are accurately modeled. The pressures in the contact sometimes exceed the pressures of the available viscometer measurements, requiring extrapolation. The previtreous piezoviscous response is always faster-than-exponential with an apparent divergence pressure. Therefore, any model used for extrapolation must possess these characteristics and up to now published work in EHL, where extrapolation of the viscosity of squalane is performed, has not. To date, the only model that has been shown to accurately extrapolate to high pressure is the Hybrid model. This expression was applied to six reference liquids for use in quantitative investigations in EHL. Viscosity measurements for trioctyl trimellitate were extended to 1.1 GPa. The results obtained with high pressure viscometers are preferable over molecular dynamics results if they are different. Molecular dynamics calculations are not currently capable of reliable results for viscosity at very high pressures. [ABSTRACT FROM AUTHOR]

Many engineering estimates of the film thickness in the concentrated contacts of real machines have come from extrapolations of measurements of elastohydrodynamic lubrication film thickness performed in glass on steel elastohydrodynamic rigs. Such estimates are likely to have large errors due to shear dependence of viscosity. The classical film thickness formulas employed have not been validated except for some Newtonian reference liquids at room temperature because the real pressure-viscosity response measured in viscometers has been ignored. A blend of polyalpha olefin base oils with mild shear-thinning has been employed in a line contact calculation to assess the effects of shear-dependent viscosity on the power-law exponents of the classical film thickness formula. The exponents on the pressure-viscosity coefficient and on the elastic modulus of the solids are not sensitive to the non-Newtonian effect. The exponents on ambient pressure viscosity and velocity are slightly reduced by shear-thinning. The exponents on pressure and scale are substantially increased by the shear dependence. The usual practice of measuring film thickness in an elastohydrodynamic rig to obtain an effective " α -value" to use in a classical Newtonian film thickness formula will overstated the film thickness of a liquid which is shear dependent in the elastohydrodynamic lubrication inlet. [ABSTRACT FROM AUTHOR]

The shear thinning observed in viscometers, which has been so successful in establishing the effects of shear-dependent viscosity on film thickness, should be used for evaluation of shear-thinning models because we have confidence in their accuracy. The field of EHL (elastohydrodynamic lubrication) may be the only one in science in which a model for shear-dependent viscosity would be evaluated by means other than viscometer measurements. One of the oils used by Johnson and Tevaarwerk [[4]] to generate traction curves was Shell Turbo 33, a mineral oil with a low inflection pressure at the experimental temperatures, so that the slower-than-exponential piezoviscous response did not appear [[5]]. For nearly a century, viscometer measurements at EHL pressures [[2]] have indicated that the pressure response, which is slower than exponential, is restricted to the lowest pressures and becomes faster than exponential or at least exponential above an inflection pressure. [Extracted from the article]

With the uncertainty regarding the global energy future, the ability to lubricate concentrated contacts with sufficiently thick liquid films while minimizing friction is of extreme importance. The assumptions of classical elastohydrodynamic lubrication have remained unchanged since the early days. It has not been possible to test many of these assumptions without the measurement of the viscosity at elastohydrodynamic lubrication (EHL) pressures, and viscometer measurements have been ignored. One of these assumptions has been the equivalence of a traction curve to a rheological flow curve for the lubricant. This notion should have been discarded forty years ago, simply because it required the pressure–viscosity behavior to be unlike the behavior observed in viscometers. At the heart of the problem is the fact that the pressure within the EHL contact is not homogeneous and the liquid properties are highly dependent on pressure, making the contact a very poor rheology laboratory. These past failures must be avoided in the future. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The behavior of lubricants at operational conditions, such as at high pressures, is a topic of great industrial interest. In particular, viscosity and the viscosity-pressure relation are especially important for applications and their determination by computational simulations is very desirable. In this study, we evaluate methods to compute these quantities based on fully atomistic molecular dynamics simulations, which are computationally demanding but also have the potential to be most accurate. We used the 9,10-dimethyloctadecane molecule, main component of PAO-2 base oil as the lubricant for our tests. The methods used for the viscosity simulations are the Green-Kubo equilibrium molecular dynamics (EMD-GK) and nonequilibrium molecular dynamics (NEMD), at pressures of up to 1.0 GPa and various temperatures (40-150 degrees Celsius). We present the theory behind these methods and investigate how the simulation parameters affect the results obtained, to ensure viscosity convergence with respect to the simulation intervals and all other parameters. We show that, by using each method in its regime of applicability, we can achieve good agreement between simulated and measured values. NEMD simulations at high pressures captured zero shear viscosity successfully; while at 40 degrees Celsius, EMD-GK is only applicable to pressures up to 0.3 GPa, where the viscosity is lower. In NEMD, longer and multiply repeated simulations improve the confidence interval of viscosity, which is essential at lower pressures. Another aspect of these methods is the choice of the utilized force field for the atomic interactions. This was investigated by selecting two different commonly used force fields. [ABSTRACT FROM AUTHOR]

The behaviour of elastohydrodynamic contacts at high slide-roll ratios (SRR) differs markedly from that found in rolling conjunctions and at low SRRs. Behaviour is dominated by viscosity reduction due to shear-thinning and by temperature rises in the fluid and mating surfaces. These have a major influence on both the performance of smooth contacts and the behaviour of rough surfaces. This paper presents a set of experimental results for these conditions where artificial roughness in the form of transverse ridges is present. These are then compared to the results of a perturbation analysis to see whether this rapid analysis technique can adequately predict the behaviour. It is shown that very powerful feedback mechanisms exist in both the thermal and shear thinning and that these make the behaviour of surface roughness relatively insensitive to changes in fluid properties. In contrast to most previous work, real temperature, pressure and shear dependences of viscosity as measured in viscometers have been employed. [ABSTRACT FROM AUTHOR]

In this paper, applying the density scaling idea to an associated liquid 4-methyl-2-pentanol used as an example, we identify different pressure-volume-temperature ranges within which molecular dynamics is dominated by either complex H-bonded networks most probably leading to supramolecular structures or non-specific intermolecular interactions like van der Waals forces. In this way, we show that the density scaling law for molecular dynamics near the glass transition provides a sensitive tool to detect thermodynamic regions characterized by intermolecular interactions of different type and complexity for a given material in the wide pressure-volume-temperature domain even if its typical form with constant scaling exponent is not obeyed. Moreover, we quantify the observed decoupling between dielectric and mechanical relaxations of the material in the density scaling regime. The suggested methods of analyses and their interpretations open new prospects for formulating models based on proper effective intermolecular potentials describing physicochemical phenomena near the glass transition. [ABSTRACT FROM AUTHOR]

The greatest challenge in high-shear viscometry is the recognition of the difference between constitutive behavior and thermal feedback. The failure to realize this distinction was the cause of a serious mistake which has had a profound effect upon the EHL field for four decades. A traction curve is not a rheological flow curve. The classical approach to EHL has ignored the measurement of viscosity in viscometers at elevated pressures. As a consequence, the field has not benefitted from the understanding gained from directly observing the thermal softening effects of viscous heating. The Eyring sinh-law appears in high-shear viscometry when viscous heating causes thermal softening. The thermal limit of pressurized thin film Couette viscometers with high thermal conductivity cylinders is well-defined by the Nahme–Griffith number. The Spikes and Jie equation overcorrects at low values of the Nahme–Griffith number and undercorrects at high values. The viscous heating in a shear-thinning liquid may cause an apparent transition from power-law to sinh-law above some viscous power. Surprisingly, the sinh-law in this instance extrapolates back to the correct low-shear viscosity. The same response has been observed in one set of NEMD simulations. [ABSTRACT FROM AUTHOR]

In the classical study of elastohydrodynamic lubrication (EHL) which does not employ real, measurable viscosity in analysis, the possibility of a glass transition has not been considered in many years. Indeed, the two rheological assumptions of classical EHL, the Newtonian inlet and the equivalence of a traction curve to a flow curve, would not have persisted so long had the pressure dependence of the viscosity been accurately stated. With the recent appearance of viscosity obtained from viscometers in EHL analysis, the possibility of a glass transition in the contact should be reexamined, especially for the fragile traction fluids. This article employs published data for a synthetic cycloaliphatic hydrocarbon to estimate the glass transition viscosity so that, when using real viscosities in EHL simulations, the state of the liquid may be assessed. Far into the glassy state the liquid should be treated as an elastic solid with a yield stress. [ABSTRACT FROM AUTHOR]

There is presently a heated debate within the field of elastohydrodynamic lubrication (EHL). Some have attributed this to a controversy regarding the shear dependence of viscosity. However, the real nature of the debate is, as it has been for more than 40 years, whether or not to correctly describe the piezoviscous effect. If real pressure dependence of viscosity were to be employed in all EHL analyses, then the true nature of shear dependence would become apparent and the debate would end. [ABSTRACT FROM AUTHOR]

An important part of the new quantitative approach to elastohydrodynamic lubrication (EHL) is the use of reference liquids with well-characterized thermophysical properties. New measurements are reported for the thermal and rheological properties of squalane to high pressure and of high shear rate and high-frequency viscosity of squalane thickened with polyisoprene (SQL + PIP) at ambient pressure. The glass transition viscosity of squalane at ambient pressure was found from published viscosity measurements and new glass transition measurements by transient hot wire. The glass transition viscosity so determined was incorporated into the improved Yasutomi model and the calculated glass transition temperatures as a function of pressure are comparable to those directly measured, although the hybrid model yields better agreement. The glass transition viscosity of squalane by any definition must be substantially lower than the “universal value” of 1012 Pa·s. The second Newtonian inflection cannot be characterized in steady shear at ambient pressure for SQL + PIP due to cavitation; however, acoustic viscometry with matching layer does characterize the second Newtonian inflection. To form the analogy between steady and oscillatory shear requires that the steady shear rate be compared with the ordinary frequency rather than the angular frequency for SQL + PIP. [ABSTRACT FROM AUTHOR]

To improve performance of machine components, lubrication is one of the most important factors. Especially for use in extreme environments, researchers look for other solutions rather than common lubricant base stocks like mineral oils or vegetable oils. One such example is ionic liquids. Ionic liquids have been defined as molten salts with melting points below 100 °C that are entirely ionic in nature, comprising both cationic and anionic species. The industrial use of ionic liquids is mostly as solvents, electrolytes, extractants and catalysts. In tribological applications, ionic liquids are mainly studied in boundary lubrication and in pure sliding contacts. In this work, the elastohydrodynamic performance of a bio-based, non-corrosive, [choline][L-proline] ionic liquid is evaluated in terms of pressure-viscosity response, film forming capability and friction. The results show a pressure-viscosity coefficient of below 8 GPa-1 at 25 °C, among the lowest reported for any ionic liquid. The ionic liquid generated up to 70% lower friction than a reference paraffin oil with a calculated difference in film thickness of 11%. It was also shown that this ionic liquid is very hygroscopic, which is believed to explain part of the low friction results, but also has to be considered in practical applications since the water content will influence the properties and thus the performance of the lubricant. [ABSTRACT FROM AUTHOR]

Application of the new quantitative elastohydrodynamics to the concentrated contacts operating within refrigerant compressors will require the accurate description of the viscosity of the lubricant that is diluted by the refrigerant. This article presents extensive viscosity measurements on polyolester and polyvinylether oils and their mixtures with low mass fractions (2-23%) of refrigerants R134a and R410a. Pressures up to 400 MPa and temperatures to 100°C were reached. Correlations are presented to predict the viscosity and pressure-viscosity coefficients at arbitrary temperatures and pressures and concentrations. Pressure-viscosity coefficients are compared with those derived from film thickness measurements using one of the popular definitions for the coefficient. Where comparisons can be made, the film-derived pressure-viscosity coefficients were mostly inaccurate. Part of the problem has been the lack of a single well-accepted and rigorous definition of the pressure-viscosity coefficient and the lack of an accurate film thickness formula that employs that definition. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Guegan and coworkers have addressed the important problem of friction of elastohydrodynamic contacts in the mixed regime. Any attempt at understanding friction in the mixed regime must address the friction in the full-film regime. To analyze fiction for the full film, they used a common, but unsupported, assumption that the friction curve is a rheological flow curve. The inconsistency can be demonstrated by integration over the contact with the assumed Eyring equation employing the known pressure dependence of the viscosity and also by a full numerical simulation which yields the same sinh friction curve without using the Eyring assumption. [ABSTRACT FROM AUTHOR]

The reliability and efficiency of refrigeration compressors is dependent on the lubricating effectiveness of the refrigeration oil, which is necessarily a solution of oil and refrigerant. In this article, the development of a high-pressure viscometer that can operate at elastohydrodynamic lubrication (EHL) inlet pressures for oil/refrigerant solutions is explained in detail. New viscosity measurements are presented for two compressor oils to 1.2 GPa. Preliminary results are given for a polyol ester with two concentrations of R134a refrigerant to 0.35 GPa. These are the first measurements of oil/refrigerant solutions to such high pressure. Temperature–pressure–viscosity correlations are applied to all materials for use in modeling. Correlations are provided for the temperature and pressure dependencies of the viscosity of the oils, refrigerant, and mixtures. The definition of pressure–viscosity coefficient is a pressing problem for elastohydrodynamics. [ABSTRACT FROM AUTHOR]

Perhaps the most thorough characterization of the elevated pressure properties of any commercial EHL lubricant is presented here for a gear oil. Compressibility, thermal conductivity, and low-shear viscosity were measured. Of particular interest is the shear dependence of viscosity, measured across four decades of stress, which shows two transitions each with a specific value of power-law exponent. An attempt to capture a suspected third transition at very high stress resulted in mechanical degradation of the liquid in the viscometer. Numerical simulations of a point contact between a steel ball and a glass disc showed good agreement over a range of slide-to-roll ratio for the measured central thickness. The agreement for the minimum thickness was excellent. A new result is that shear-thinning of a higher molecular weight component that occurs from 3 to 200 kPa had little effect on the film thickness and could therefore be neglected in a film thickness calculation. [ABSTRACT FROM AUTHOR]

The new quantitative elastohydrodynamics has resulted in rapid advances in understanding concentrated contact lubrication behaviour by employing real transport properties. A key component of this revolution has been the use of reference liquids. In this paper, two liquids, a triester and a perfluoropolyether, which are under consideration as possible viscosity standards are characterised to 1 GPa for conditions relevant to tribology. Two correlations are provided for interpolation and data smoothing, and an explanation for difficulties encountered with free volume theory is offered. The perfluoropolyether is only appropriate as a reference liquid when an identical lot is available. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

One obvious minimum requirement for any temperature-pressure-viscosity correlation is that the pressure-viscosity relation be capable of describing all possible trends in the pressure dependence of viscosity. Empirical models are employed because theoretical models do not provide experimental accuracy. If an empirical model is selected to determine outliers from any data set, failure to accommodate any aspect of the pressure response may lead to incorporating flawed data and to the exclusion of accurate data in the regression of correlation parameters. Two simple models have been thoroughly investigated, one for faster-than exponential pressure response and another for slower-than exponential pressure response. A test for the applicability of the McEwen model has been derived, analogous to the derivative Stickel analysis. A new model, a hybrid of the McEwen model and the Paluch (Johari and Whalley) model, has been derived and tested on two liquids. The purpose of this article is not to construct a reference correlation, but rather, to show the pit-falls of assuming a pressure-viscosity relation of limited applicability. [ABSTRACT FROM AUTHOR]

A single parameter, the pressure-viscosity coefficient, α, quantifies the pressure dependence of the viscosity of the liquid in elastohydrodynamic lubrication (EHL). Most published values of α have not been obtained from measurements of viscosity as a function of pressure. Rather, these effective pressure-viscosity coefficients have been derived from the measurement of the EHL film thickness, a more difficult procedure. In this article, five well-characterized liquids that should be Newtonian in the EHL inlet are identified for which film-derived coefficients have been reported. These coefficients are compared with coefficients derived from published viscosity correlations and new viscosity measurements. The film-derived coefficients are found to not be an accurate representation of the piezoviscous response. The procedure of deriving a pressure-viscosity coefficient from a film thickness measurement does not offer an alternative to the simpler and easier viscometer measurement. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Currently, the only aspect of non-Newtonian behavior being modeled in lubrication is the shear dependence of viscosity. However, shear thinning is accompanied by a large difference between the normal stress in the flow direction and the cross-film direction. This stress difference can increase the load capability of a lubricant film without increased frictional penalty. A commercial 10W-40 motor oil was characterized at elevated pressures. Three different high-pressure instruments were employed: a falling-body viscometer, a thin-film Couette viscometer, and a parallel-plate rheogoniometer. Ordinary shear thinning with a second Newtonian inflection was observed. A first normal stress difference of 0.6 MPa was measured under what may be mild conditions for a crankshaft journal bearing. Elevated pressures are essential to the measurement of rheological properties that govern hydrodynamic film thickness and friction in automotive components. Time–temperature–pressure superposition was validated for the first normal stress difference. The first normal stress difference in the terminal regime may be estimated from the upper-convected Maxwell model, where the shear modulus is assumed to be equal to the Newtonian limit shear stress obtained from a measurement of shear thinning. The first normal stress difference in the shear-thinning regime may be estimated from an extant empirical rule. These results will be of substantial importance when analytical techniques are developed for hydrodynamic lubrication with real non-Newtonian shear response. The results are immediately useful for calculating the shear stress for cavitation in ambient pressure high-shear viscometers. [ABSTRACT FROM PUBLISHER]

The potential for using an empirical expression to predict the piezoviscous response of a fluid from its pressure-volume behavior is explored. This approach is particularly promising since the variation of volume with pressure can be obtained relatively easily using atomistic simulations that are based on the molecular structure of the fluid. The accuracy of predictions made using the proposed method is evaluated, and its limitations are discussed in terms of sources of error and potential means of minimizing that error going forward. [ABSTRACT FROM AUTHOR]

The application of the new quantitative elastohydrodynamics, which employs real properties of the liquid lubricant, to practical tribology problems obviously requires property measurements at high pressure to parameterize the correlations. The thermal conductivity under elevated pressure has been shown to be essential for nonisothermal situations. A new transient hot-wire instrument for pressure to 600 MPa is described and validated. New measurements are presented of the temperature- and pressure-dependent thermal conductivity of a synthetic oil for jet aircraft engines. The density scaling law,, with exponent approximately equal to the standard value of 3 gives an excellent accounting of the results. The temperature dependence of the conductivity may be ignored in elastohydrodynamic lubrication (EHL) simulations without great loss of accuracy and it may be sufficient to describe the pressure dependence with a general expression,. [ABSTRACT FROM AUTHOR]

The relative volume and the limiting low shear viscosity have been accurately determined as functions of temperature and pressure for a series of eight branched perfluoropolyether liquids. The Murnaghan equation of state was fitted to the volumes and three correlations for viscosity were examined: the Doolittle free volume, the Bair and Casalini thermodynamic scaling, and the improved Yasutomi. The ambient-pressure bulk modulus increases gradually with molecular weight and is low compared with conventional lubricants. The Doolittle parameter increases with molecular mass as does the occupied volume fraction. The Ashurst–Hoover interaction parameter decreases with increasing molecular mass. The improved Yasutomi model provides accuracy comparable to models requiring an equation of state. When the kinematic viscosity at 40℃ is greater than about 200 mm2/s, fragility is independent of pressure. When the kinematic viscosity is less than about 200 mm2/s fragility increases with pressure. The pressure counterpart to the Angell plot and to the Vogel, Tammann, and Fulcher equation suggested by Drozd-Rzoska et al. is shown to be workable using stability limit pressures in the tensile domain. Clearly, progress in quantitative elastohydrodynamic lubrication must rely upon accurate measurements of the dynamic properties as a function of temperature and pressure and upon accurate temperature and pressure correlations for these properties. Only then can precise predictions of film thickness and friction be made. [ABSTRACT FROM PUBLISHER]

That classical elastohydrodynamic lubrication (EHL) is not a quantitative field can be illustrated by its failure to provide a consistent and rigorous definition of the viscosity-pressure coefficient. Indeed, if the pressure dependence of viscosity cannot be accurately described, then the viscosity-pressure coefficient cannot be defined. Classical EHL has employed fictional narratives to justify the pressure dependences that have been utilized. In this context, the purpose of this perspective article is to review specific and real needs from EHL and to show that data and models describing the viscosity-pressure dependence are already available and how they can properly be used. The final aim is to encourage researchers to change their philosophy of classical EHL to a quantitative approach, in which every hypothesis and every result, whether experimental or numerical, would be justified on the basis of acceptable physics. [ABSTRACT FROM AUTHOR]

The Yasutomi temperature–pressure–viscosity correlation is attractive for elastohydrodynamic lubrication analysis because it does not require an equation of state for the liquid. However, the original Yasutomi correlation suffers from the appearance of a zero in the function describing the pressure dependence of the relative free volume thermal expansivity. When the original correlation is employed, a liquid with a high inflection pressure lacks the inflection in the predicted pressure–viscosity behavior and a liquid with a low inflection pressure is predicted to have two inflections. A new function removes these concerns and provides improved precision without the need for an equation of state. The improved function gives a more faithful representation of viscosity at even low pressures. [ABSTRACT FROM PUBLISHER]

Until very recently, viscometers have not had a significant role in the development of the field of elastohydrodynamic lubrication (EHL). Viscosity has generally been treated as an adjustable parameter that assumed the values at elevated pressure necessary to validate whatever model was proposed. The description of the pressure dependence of viscosity in EHL requires the choice of a model that reproduces the way that viscosity changes with pressure and requires values for the parameters of the chosen model. Viscometers are necessary for both requirements if a quantitative understanding of the mechanisms of film generation and friction is sought. Four types of pressurized viscometers, two based on Poiseuille flow and two on Couette flow, are investigated here. They yield remarkably similar pressure responses. There is therefore no ambiguity in the measurement of the pressure dependence of viscosity in pressurized viscometers. A significant challenge for engineers wishing to apply the results archived in the EHL literature is the assessment of the extent to which the results would be altered if an accurate viscosity had been employed. An important step toward establishing EHL as a quantitative science would be the designation of reference liquids for the pressure–viscosity effect and the use of these liquids in experimental and analytical investigations of EHL. [ABSTRACT FROM AUTHOR]

Viscosity measurements have been carried out with a falling-cylinder instrument in the compressed liquid and supercritical regions of the hydrofluorocarbon 1,1,1,2-tetrafluoroethane (R134a). A highly pure sample was used. The measurement region at temperatures from 293.15 K to 438.15 K with pressures from 10 MPa to 400 MPa extends the existing data range substantially. With estimated uncertainties of 3.5 % for viscosity, the greater of 1 MPa or 0.5 % for pressure, and 0.4 K for temperature, the new results can be used to reconcile inconsistencies between literature data sets. They also provide a test of the extrapolation capability of existing viscosity correlations for this fluid and enable the development of a more accurate and wider ranging formulation. [ABSTRACT FROM AUTHOR]

Elastohydrodynamic lubrication (EHL) calculations using the real pressure and real shear dependence of viscosity have revealed that the shear and load dependences of film thickness are often greater than the classical prediction. Experimental measurements have confirmed the increased sensitivity to scale and load; however, the sensitivities are even greater than would be expected from shear-thinning. Time-dependent film thickness measurements and stress-history-dependent flow curves are employed to show that molecular degradation may occur at nominal rolling EHL stress levels and that this degradation affects the film thickness. [ABSTRACT FROM AUTHOR]

It has been recognized for about 50 years that the addition of polymer to motor oil will result in shear-dependent viscosity at relatively low shear stress. This shear-thinning can be shown to reduce the elastohydrodynamic film thickness and there has been an expectation of a reduced film in hydrodynamic lubrication as well. The expected associated increase in the journal bearing wear, however, has not been observed. The load-supporting benefit of the elastic properties contributed by the polymer has been used as an explanation for the success of the polymer blended oils in this application. The available measurements of elasticity in the form of normal stress differences have been inconclusive regarding their contribution to load support. These measurements have all been at atmospheric pressure, however, and at a very high shear rate. The oil in the load-carrying region of a bearing is at elevated pressure and the elevated pressure may enhance the elasticity. A new experimental technique for the measurement of elasticity in oils at elevated pressure was developed. AWeissenberg rheogoniometer of the parallel plate, torsional flow type has been miniaturized and placed in a 350-MPa pressure vessel. This is essentially a parallel face thrust washer that can support a load unrelated to the usual viscous action. The load capacity due to the liquid elasticity vanishes for low-molecular-weight liquids of the same viscosity. The device has been validated with an NIST non-Newtonian standard reference material and the preliminary measurements on commercial multigrade oils clearly show the effect of elevated pressure. The possibility exists to provide additional load support without a viscous frictional penalty. [ABSTRACT FROM AUTHOR]

EHL traction behavior in the high-pressure, low slide-to-roll ratio regime, where the response may be described as linear, is of importance for the operation of toroidal continuously variable transmissions that must reduce the mechanical loss by operating in or close to this regime and for defining the torque losses of a rolling element bearing. In this article, we take a novel approach to the problem by comparing traction in this low slide-to-roll regime for dry and lubricated contacts. New experimental measurements show that for very high contact pressures along with low slide-to-roll ratios, the lubricated traction curve varies little from that obtained in dry sliding contact. Longitudinal sliding, transverse sliding, and spin are investigated and the results are compared to the theory of Kalker. The implications for the calculation of traction from material properties and for the advantageous manipulation of the linear traction gradient are explored. [ABSTRACT FROM AUTHOR]

High-pressure viscometers, which have in the past been used mainly to obtain rheological properties for elastohydrodynamics, can be used to obtain flow curves for motor oils that extend much deeper into the shear-thinning regime than conventional high-temperature, high-shear (HTHS) viscometers and the constitutive information obtained should be immensely helpful in journal bearing calculations. A simple infinite journal bearing calculation shows that the effect of pressure on viscosity is at least as important as the effect of shear rate and that shear rate alone is a poor measure of the extent of shear thinning in a bearing under HTHS conditions. Then variability in piezoviscous response should be at least as important as variability in shear thinning. Sufficiently accurate theory exists to possibly design lubricants for both high load capacity and low friction by balancing pressure and shear rate effects. [ABSTRACT FROM AUTHOR]

Liquid failure has been observed in low Reynolds number (Stokes) shear flows where reduction of the hydrodynamic pressure should not occur. Cavitation in Stokes flows has implications in lubrication, polymer processing, and rheological measurements. Such cavitation can be predicted by a principal normal stress cavitation criterion (PNSCC). We present results of a direct experimental test of the PNSCC. Imaging of the cavitation events suggests that the cavitation is gaseous and originates from preexisting nuclei. Crevice-stabilized gas nuclei are assumed, and numerical simulations are used to investigate the cavitation event for a Newtonian liquid. The inception of cavitation from a preexisting nucleus, the persistence of suitable nuclei, and the growth and deformation of shed bubbles are considered. [ABSTRACT FROM AUTHOR]

Of the fields for which the piezoviscous property of liquids is of interest, elastohydrodynamics is unique in deriving its existence from this property. Even recent publications in the field, however, have based the description of the pressure variation of viscosity on myths rather than empirical data. The author offers a description of the construction and operation of an easy-to-use viscometer capable of accurate measurements to a pressure of 1 GPa. The classic measurements of Nobel laureate P. W. Bridgman are repeated and examples of measurements on lubricants are given. [ABSTRACT FROM AUTHOR]

A film thickness correction for shear-thinning in elastohydrodynamic contacts has been available that requires parameters obtained from flow curves generated for the specific liquid. For instances for which reduced accuracy is acceptable, a new rough film thickness correction has been formulated for pure rolling and a moderate amount of sliding based on Van Krevelen's graphical technique of estimating shear-thinning from the molecular weight. It has been shown to be useful for high-molecular-weight base oils, when lubricant specific shear-thinning data is absent, although it should most often overestimate the film thinning. [ABSTRACT FROM AUTHOR]

Presents a study that formulated a numerical simulation of elastohydrodynamic (EHL) lubrication. Application of EHL in solving practical quantitative problems by pressure-viscocity relationships; Assessment of pressure-viscosity response and comparison of free volume formulations; Introduction of empirical expressions that can describe some features over pressure range.

Recently, remarkable agreement was reported between nonequilibrium molecular dynamics simulation and high-pressure Couette rheometry on squalane. We have utilized the viscosity-strain rate relationship obtained from this unique combination of experimental and simulation data along with high-pressure viscometer measurements to calculate the viscous traction curve in the elastohydrodynamic lubrication (EHL) regime. A comparison with measured traction at 0.57 and 1.29 GPa shows excellent agreement, confirming the validity of the measurements and simulations. Thus, we present for the first time, a successful calculation of EHL traction from the liquid shear response obtained from both molecular dynamics and rheometry. [ABSTRACT FROM AUTHOR]