Pressure-viscosity Coefficient Research Articles

Examining the rheology of 9-octylheptadecane to giga-pascal pressures

The properties of alkanes in the C20–C40 mass range are of fundamental importance in industrial applications as they are important constituents of synthetic lubricant base stocks. In an extension to earlier work on alkanes in the C20–C40 carbon number range we present the results of molecular simulations for 9-octylheptadecane, a starlike isomer of C25. Both equilibrium (EMD) and nonequilibrium molecular dynamics (NEMD) simulations have been performed under ambient state conditions and to pressures in the gigapascal range. The EMD simulations focus on calculations of the rotational relaxation times, while the NEMD simulations reveal the dependence of the viscosity on strain rate. Additionally, we calculate the viscosity number and pressure–viscosity coefficient for 9-octylheptadecane and compare the results with those obtained experimentally.

The high-pressure, high-shear stress rheology of a polybutene

The shear rheology of a low molecular weight liquid polybutene has been studied under elevated pressure using the Couette geometry. Results can be described by a temperature and pressure shifted Carreau-Yasuda model. An interesting relationship was found between the model parameters and the temperature and pressure coefficients of viscosity.

Thin Film Colorimetric Interferometry

Measurement technique for the study of very thin lubrication films down to one nanometer in a point contact between a steel ball and a transparent disc is used to explore the relationship between central and minimum film thickness and rolling speed at the interface between elastohydrodynamic and boundary lubrication for a series of lubricating fluids. This technique based on the colorimetric interferometry combines powerful film thickness mapping capabilities with high accuracy. It was confirmed that both hexadecane and mineral base oil obey the linear relationship between log central and minimum film thickness and log rolling speed predicted by elastohydrodynamic theory down to approximately one nanometer. Conversely, squalane and additive-treated mineral base oil showed film thickness enhancement at slow speeds caused by boundary layers formation within the lubricant film. Obtained experimental data was used for the determination of pressure-viscosity coefficients of test fluids. The measurement technique also enabled us to produce information about the influence of boundary layers on film thickness shape. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Seattle, Washington, October 1–4, 2000

The Elastohydrodynamic Traction of Synthetic Base Oil Blends

The aim of this study was to investigate the elastohydrodynamic (EHD) properties of lubricant blends. Three base fluids of very similar viscosities, a polyalphaolefin, a diester and an alky lated aromatic, have been obtained and their EHD film thickness and traction behavior measured at a range of pressures. Blends of these fluids have been prepared and the influence of blending on film thickness and traction has been investigated. Traction measurements were conducted at film thicknesses between 100–200 nm and thermal analysis was incorporated to correct for in-contact shear heating. The blends showed a broadly linear relationship between the inlet pressure-viscosity coefficient and blend composition. Isothermal traction comparisons revealed that traction is not an additive property of lubricant blends. Presented at the 53rd Annual Meeting Detroit, Michigan May 17–21, 1998

Modeling of Micro-Pit Evolution in Rolling or Strip-Drawing

The micro-plasto-hydrodynamic lubrication (MPHL) model of pit evolution is extended to account for the variation of sliding speed and strain rate in rolling and drawing processes. Results show that all of the following factors are important: pit angle, lubricant viscosity and pressure viscosity coefficient, material yield stress and sliding speed. Theoretical predictions for the change in pit area during the deformation process are well correlated by a non-dimensional group of these parameters. The model agrees reasonably with the measured change in pit volume and area from drawing experiments on cold rolled stainless steel strip containing both artificial and stochastic roughness.

Kinetic theory of dense fluids of rigid biaxial ellipsoids

The transport equation for a one-particle distribution function f of a pure and dense fluid composed of hard biaxial ellipsoids has been derived by the Enskog method through a modification of the Taxman equation which describes the corresponding low-density fluid. The equation for f has been utilized in obtaining approximate equations of continuity, linear momentum, and energy of the dense fluid, and has then been solved through the Enskog infinite series expansion technique, and a second-order approximate formula for f has been achieved. Using this, results are derived for the hydrodynamic pressure, shear and bulk viscosity coefficients, and heat conductivity of the fluid. Fast exchange of energy between the translational and rotational motions is assumed throughout the calculation. The quantities ultimately appearing in the results, which cannot further be reduced analytically and require numerical evaluation, are the four-dimensional quadratures over the orientational coordinates of two interacting rigid ellipsoidal molecules. In the appropriate limit, all results reduce to those obtained by Enskog for a dense fluid of hard spheres, and a first-order modified Eucken-type formula for the dense fluid emerges.

Rheological Properties of Polyolester Under an EHD Contact in Some Refrigerant Environments

EHD traction of polyol ester in some refrigerants is analyzed using the non-Newtonian theory, from which available rheological parameters are determined. Then the rheological parameters are studied in elation to refrigerant concentration. As a result, the viscosity pressure coefficient of the mixtures is inversely correlated with refrigerant concentration in terms of mole per volume, while the representative stress characterizing the non-Newtonian is positively correlated. When the viscous volume relating to the unit of viscous flow is plotted as a function of molecular volume of refrigerant/POE mixture, the relation between them is well described by a single line independent of kinds of refrigerants.

On the concentrated contact as a viscometer

Lubricant viscosities from published concentrated contact-based measurements are compared with measurements by conventional viscometry for overlapping pressures and viscosities. Contrary to published claims there are important differences. The greater than exponential pressure-viscosity behaviour, which is known to occur at high pressures, is absent from contact measurements. Deficiencies in contact-based techniques are probably due to assumed constitutive behaviour and are not related to time scale. In general, contact-based methods are poorly conditioned for the measurement of pressure-viscosity coefficients.

Influence of degradation on the film-forming properties of polyolester oils

The film thicknesses of two polyolester degraded oils were measured over a wide range of temperatures to investigate the influence of degradation on film-forming properties. The results have been interpreted in the light of the idea that degradation of an ester lubricant can produce two different types of product which have opposing influences on film-forming properties. One is the formation of smaller, polar molecules, such as acids, which may result in a decrease in effective pressure—viscosity coefficient. The other is the formation of larger, bulky molecules such as sludge, which result in increased pressure—viscosity coefficient. The effective pressure—viscosity coefficient decrease with degradation of the oil thus is made up of an earlier stage involving a series of reactions, followed in a later stage by an increase.

The Pressure-Viscosity Coefficient of a Perfluorinated Polyether Over a Wide Temperature Range

An unbranched perfluoropolyether is the current ball bearing lubricant for deep space flight. We report measurements of viscosity to moderate pressures for the calculation of pressure-viscosity coefficients to temperatures as low as −40°C. A free volume model predicts that a minimum pressure-viscosity coefficient exists at higher temperature. The existence of a minimum αoT is confirmed by viscometry at temperatures to 180°C.

The Influence of Lubricant Upon EHD Film Behavior During Sudden Halting of Motion

Although steady state elastohydrodynamic (EHD) lubrication is quite well understood both from the theoretical and from the experimental point of view, studies of transient effects in EHD are currently far less developed. This paper describes an experimental investigation into EHD film behavior during sudden halting of motion. A technique has been devised which enables both central lubricant film thickness and film thickness profiles to be measured every millisecond during halting of a ball on flat, sliding contact. This has enabled detailed information of influence of lubricant on film collapse during halting to be obtained. It is shown that film collapse occurs in two stages. The first is a very rapid reduction in film thickness with only very small changes in film geometry and thus pressure distribution. This is followed, as soon as entrainment ceases, by the formation of a lubricant entrapment, and subsequent slow leakage of fluid from the central film region. This paper focussed on the formation of this entrapment and the influence of the rheological properties of the lubricant, i.e. viscosity and pressure-viscosity coefficient, on its development and behavior. Presented at the 54th Annual Meeting Las Vegas, Nevada May 23–27, 1999

A Study of Indentation Profiles of Sphere and Oiled Plate due to Impact Load.

This study was carried out to investigate the profile characteristics of the indentation caused by an impact load on an oiled plate by a sphere. The materials of the anvil and the sphere were either SUS304 or SUJ2 to investigate the effects of the handness on the identation profile. Each indentation profile was measured by the three-dimensional stylus profilometer. The deformed volume of a plate was confirmed to be smaller than that of a sphere. The profile of the indentation showed a clear entrapment of the oil. The center of the indentation becomes deeper and sharper with increasing viscosity and pressure viscosity coefficient of oil. Clear edge was recognized around an indentation on a sphere in the case of an oiled impact. On the contrary, an ambiguous edge was recognized in the case of dry impact.

Theoretical investigation on the dimple occurrence in the thermal EHL of simple sliding steel–glass circular contacts

The conical depression (surface dimple) phenomena observed by Kaneta et al. (Kaneta M, Nishikawa H, Kameishi K, Sakai T. Effects of elastic moduli of contact surfaces in elastohydrodynamic lubrication. ASME J. Tribol. 1992;114:75–80; Kaneta M, Nishikawa H, Kanada T, Matsuda K. Abnormal phenomena appearing in EHL contacts. ASME J. Tribol. 1996;118:886–892.) in optical interferometry experiments are simulated numerically by a complete solution to the simple sliding circular contact thermal elastohydrodynamic lubrication (TEHL) problem. Good agreement is displayed between the theoretical and experimental results. This agreement is explained by the “temperature–viscosity wedge” mechanism, which was first proposed by Cameron (Cameron A. Hydrodynamic lubrication of rotating discs in pure sliding, a new type of oil film formation. J. Inst. Petrol 1951;37:471.). Effects of the viscosity–pressure coefficient, the ambient viscosity, and the entrainment velocity on the behavior of the surface dimples are discussed.

Film-Forming Properties of Polyol Esters, Polyphenyl Ethers and Their Mixtures Over a Wide Range of Temperature

There is a growing interest in lubrication at very high temperature and in the development of synthetic lubricants to satisfy the requirements of high temperature systems. Polyolesters, polyphenylethers and their mixtures which have excellent stability at high temperature are possible candidates. In this study, the film-forming properties of these fluids were investigated over a wide range of temperature, from 40° to 240°C. As a result the following findings were made. The effective pressure-viscosity coefficients and thus elastohydrodynamic film-forming properties of PPEs are, on the whole, greater than those of POEs. Considering mixtures of POE and PPE, both thermo-oxidative stability and film forming properties can be improved by the addition of PPE to POE. Mixture of R4P2E and POE result in the effective improvement of thermo-oxidative stability as well as effective pressure-viscosity coefficient and are thus good candidates for high temperature usage. A good correlation between the effective pressure-viscosity coefficient and viscosity was also found Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in Orlando, Florida, October 11–13, 1999

An Inverse Analysis for Steady-State Elastohydrodynamic Lubrication of One-Layered Journal Bearings

This paper presents a solution to an inverse problem for the elastohydrodynamic lubrication of one-layered journal bearings. Eccentricity ratio, viscosity of the lubricant at atmospheric pressure, pressure-viscosity coefficient, and bulk modulus of the lubricant are the parameters to be estimated from a given pressure distribution. The Least-squares optimization technique is used to solve the proposed inverse problem. Results are presented only for infinite-width journal bearings. The results showed that the proposed inverse algorithm is efficient and accurate. [S0742-4787(00)00703-7]

Influence of lubricant properties on elastohydrodynamic lubrication

In hard EHL, encountered in, e.g., gears, rolling element bearings etc. the properties of the lubricant play a significant role in the forming of a lubricating film and reducing friction between the contacting surfaces. The influence of pressure and temperature on viscosity, limiting shear stress and density, has to be taken into account when creating lubricant models to be used in numerical calculations of film thickness and friction. This paper describes some experimental methods to determine these properties of a number of different lubricating oils, both from mineral, vegetable and synthetic origin. The results show that, apart from 5P4E, naphthenic mineral oil has the highest values of pressure–viscosity as well as limiting shear stress–pressure coefficients. It also exhibits the highest density increase with pressure. Rapeseed oil has a rather high pressure–viscosity coefficient but a low limiting shear stress–pressure coefficient as well as a low density increase with pressure. Thus, rapeseed oil is a strong alternative to naphthenic (and also paraffinic) oil since it has the ability to form a relatively thick oil film and at the same time give a low coefficient of friction. Ester, polyglycol and polyalphaolefin oils also exhibit low values of friction but are not equally good as rapeseed oil to form a lubricant film as a result of their lower pressure–viscosity coefficients.

Lubrication of rolling element bearings with HFC–polyolester mixtures

Manufacturers of refrigeration and air-conditioning compressors have had to re-evaluate their knowledge of compressor bearing lubrication following the introduction of hydrofluorocarbon (HFC) refrigerants and polyolester lubricants. The lack of anti wear protection in comparison to traditionally used refrigerants/lubricants makes the lubrication of bearings using these modern systems a much more difficult task than before. This paper presents results from ongoing research activities to develop methods and generate data that can be used to support engineers and chemists designing compressors and lubricants. Viscosity and pressure–viscosity data for three ISO VG 32 polyolester lubricants and also a film forming comparison between two ISO VG 68 polyolester lubricants are presented. This data shows that a high degree of branching has a negative effect on the performance of the lubricant in certain applications. Whilst a branched lubricant offers the benefit of a higher-pressure–viscosity coefficient under normal conditions, the reduction in viscosity and pressure–viscosity coefficient when diluted by refrigerant is far greater than for normal/linear lubricants. This characteristic results in a poorer film forming ability in applications where the lubricant is subjected to refrigerant gas with a low superheat.

Film‐forming tendencies of some base fluids and viscosity modifiers

AbstractThe film‐forming tendencies of selected mineral base oils and synthetic base fluids were investigated with and without conventional GL‐5 additive packages. A pressurised falling‐body viscometer and a concentrated contact simulator were used to measure low‐shear viscosities, central film thicknesses, and traction coefficients. Analysis of the mineral—based oils showed that a paraffinic base oil and a naphthenic base oil had similar film thicknesses, even though the naphthenic base oil has higher pressure‐viscosity coefficients. A very high viscosity index oil gave thinner film thicknesses and lower pressure—viscosity coefficients than the paraffinic or naphthenic base oils. Analysis of the synthetic base fluids showed that a PAO‐4 base fluid gave thicker film thicknesses than an ester base fluid. The analysis of fully‐formulated oils showed that the PAO‐4 oil containing a proprietary polyolefin provided a similar filnz thickness to the PAO‐4 oil containing a more expensive PAO‐100.

A high pressure sliding plate rheometer for polymer melts

A high-pressure sliding plate rheometer has been developed to investigate the effect of pressure on the rheological behavior of molten polymers and elastomers. The new rheometer operates at pressures up to 70 MPa and temperatures up to 225 °C. The sample is subjected to simple shear, and the resulting shear stress is measured locally using a shear stress transducer. This design eliminates the inhomogeneities in pressure and shear rate that occur in high pressure capillary and slit rheometers. Preliminary evaluation of the new instrument was carried out using a linear low density polyethylene. Viscosity curves were generated at pressures ranging from atmospheric pressure to 70 MPa, and the pressure coefficient of viscosity was determined. Experiments were also carried out in step strain and large amplitude oscillatory shear, demonstrating the new rheometer’s use to study the nonlinear viscoelastic behavior of molten polymers. Finally, this instrument was used to study strain-induced crystallization.

Elastohydrodynamic lubrication: A lubricant industry perspective

The twin challenges facing the automotive and lubricants industries as we approach the millennium are: how to reduce the levels of gaseous emissions and how to achieve increased durability. Improved durability is necessary in order to maintain low levels of emissions. The lubricant has an important part to play in both the improvement in durability by way of improved wear and in the reduction of emissions and conservation of non-renewable energy resources by way of the reduction of friction levels in engines. Examples are shown of how the application of electrohydrodynamic lubrication (EHL) equations permit the calculation of the oil film distribution between cam and follower contacts as a function of cam angle, cam speed and oil viscosity. The results are compared to post-engine test cam wear patterns. The position with of maximum wear can thus be predicted and critical operating conditions identified. The development of optical methods for studying EHL behaviour and simple, reliable, methods for measuring traction under EHL conditions represents a major advance and allows the development of lubricants that offer high levels of wear protection together with high levels of fuel efficiency. This paper shows the importance of selection of both base oils and additive packages to achieve these ends. The relationships between ambient pressure viscosity, pressure coefficient of viscosity, EHL film thicknesses and EHL traction coefficients are discussed. The use of such data in the prediction of fuel economy performance in industry engine tests is also discussed.