Showing papers by "Scott Bair published in 2016"

Classical EHL Versus Quantitative EHL: A Perspective Part II—Super-Arrhenius Piezoviscosity, an Essential Component of Elastohydrodynamic Friction Missing from Classical EHL

Abstract: Ninety years of high-pressure measurements with many different types of viscometers have shown that faster-than-exponential (super-Arrhenius) pressure dependence of viscosity is universal for glass-forming liquids and, therefore, all typical liquid lubricants. Dielectric spectroscopy at elevated pressure also yields super-Arrhenius response in the dependence on pressure of the primary relaxation time. In contrast, classical elastohydrodynamic lubrication (EHL) has gone to great lengths to ignore this phenomenon, including fictional accounts of the results of viscometry. As a result of this, classical EHL is unable to quantitatively account for one of the most important properties affecting friction at low sliding velocity, the low-shear viscosity. Differences in friction between similar liquids at low sliding velocity can be explained by their different inflection pressures. Some observed liquid response to shear stress at high pressure can be explained with the measured super-Arrhenius pressure dependence. It should be clear that, had classical EHL employed realistic pressure dependence of viscosity from its beginning, the field would have been in a better position today to solve engineering problems which involve the differences among molecular structures.

The temperature and pressure dependence of viscosity and volume for two reference liquids

Abstract: 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.

Density scaling and decoupling in o-terphenyl, salol, and dibutyphthalate

Abstract: We present new viscosity and equation of state (EoS) results extending to high pressures for o-terphenyl, salol, and dibutylphthalate. Using these and data from the literature, we show that the three liquids all conform to density scaling; that is, their reduced viscosities and reorientational relaxation times are a function of the ratio of temperature and density with the latter raised to a constant. Moreover, the functional form of the dependence on this ratio is independent of the experimental probe of the dynamics. This means that there is no decoupling of the viscosities and relaxation times over the measured range of conditions. Previous literature at odds with these results was based on erroneous extrapolations of the EoS or problematic diamond anvil viscosity data. Thus, there are no exceptions to the experimental fact that every non-associated liquid complies with density scaling with an invariant scaling exponent.

Density Scaling and Decoupling in o-Terphenyl, Salol, and Dibutyphthalate

Abstract: We present new viscosity and equation of state (EoS) results extending to high pressures for o-terphenyl, salol, and dibutylphthalate. Using these and data from the literature, we show that the three liquids all conform to density scaling; that is, their reduce viscosities and reorientational relaxation times are a function of the ratio of temperature and density with the latter raised to a constant. Moreover, the functional form of the dependence on this ratio is independent of the experimental probe of the dynamics. This means that there is no decoupling of the viscosities and relaxation times over the measured range of conditions. Previous literature at odds with these results were based on erroneous extrapolations of the EoS or problematic diamond anvil viscosity data. Thus, there are no exceptions to the experimental fact that every non-associated liquid complies with density scaling with an invariant scaling exponent.

Quantitative elastohydrodynamic film forming for a gear oil with complex shear-thinning:

Abstract: 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.