Effect of EHL Contact Conditions on the Behavior of Traction Fluids

New infinitely variable transmission (IVT) systems are under development for the automotive industry as a means to achieving significant fuel economy benefits. These systems rely on the lubricating fluid to transmit the drive train loads across the interface of the transmission components. This requires the development of new fluids that exhibit high traction properties under elastohydrodynamic lubrication (EHL) conditions. However, it has been reported recently that the traction performance of some fluids can reduce dramatically as temperature is reduced. This may place severe operational limits on IVT systems and suggests that the low-temperature traction properties of fluids for these systems should be studied in order to understand the mechanism for the observed reduction in traction.

The work reported here is an experimental study aimed at identifying whether low temperature traction reduction is related to a fundamental change in rheological behavior specific to the fluids tested or to more generic changes in the EHL contact conditions. A series of model experiments were performed using a mini traction machine (MTM) on three high-viscosity polybutene samples. The results have been mapped against previously reported non-dimensional parameters used to identify different EHL regimes. The results show that dramatic reductions in traction occur when the contact transitions from the rigid piezo-viscous (RP) toward the rigid iso-viscous (RI) region. Similar results were also found for two other high-viscosity fluids of different molecular structure and lower traction properties. The results support the hypothesis that the reduction in traction observed at low temperature is due to a change in EHL contact conditions rather than being solely due to a change in the rheological performance of the test fluids.     

Tribological characterization of environmentally adapted ester based fluids

Fundamental properties of six synthetic ester base fluids, suitable for the formulation of environmentally adapted lubricants, have been investigated. High pressure viscosity data for the test fluids were obtained through experimental measurements with a high pressure Couette rheometer. The temperature, pressure and viscosity data η(p, T) were parameterized against the Roelands pressure–viscosity equation. Thermal conductivity and specific heat capacity data were obtained using a transient hotwire method, and the EHD friction coefficient, γ, was obtained experimentally as well. The results from these measurements are reported, and the correlation between thermal properties, molecular structure, and the fluid rheology parameters, of the test fluids are discussed.     

A non-averaging method of determining the rheological properties of traction fluids

Traction machines have been frequently used to study the rheological responses of lubricants in elastohydrodynamic lubrication (EHL) contacts. Fundamental properties are inferred from EHL traction measurements based on the average pressures and temperatures in the contact. This average approach leads to uncertainty in the accuracy of the results due to the highly nonlinear resonse of the fluid such as viscosity to both pressure and temperature. A non-averaging method is developed in this paper to study the elastic and plastic properties of traction fluids operating in EHL contacts at small slide-to-roll ratios. A precision line-contact traction rig is used to measure the EHL traction at a given oil temperature and Hertz pressure. By choosing a sensible pressure-property expression, the parameters of the expression can be determined through the initial slope and peak traction coefficient of the traction measurements. The elastic shear modulus and the limiting shear stress of the lubricant corresponding to a single pressure can then be calculated for a range of pressures and temperatures of practical interest. Two high-traction fluids are studied, and their elastic moduli and limiting shear stresses presented.     

Temporary and Permanent Viscosity Loss Correlated to Hydraulic System Performance

ABSTRACT

Straight- and multigrade fluids were evaluated in a hydraulic dynamometer that incorporated a pressure-compensated axial piston pump and a fixed displacement axial piston motor. Pump, motor, pressure compensator, and directional control valve internal flow losses were determined under various conditions of pressure, speed, and temperature. Fluid samples were collected before and at various times during the dynamometer experiments, and viscosity measurements were performed to probe for correlations between viscosity, operating time, and system leakage flow losses. The low shear rate viscosities of the multigrade fluids decreased linearly throughout the duration of testing due to polymer degradation. However, system flow losses did not exhibit a statistically significant increase as the multigrade fluids sheared down. The fluids were also characterized by their permanent viscosity loss produced in sonic shear and tapered bearing tests and by their temporary shear thinning measured in an ultra-high-shear viscometer at several temperatures. The effects of these viscous properties were analyzed using an empirical model to identify which measures of viscosity were most correlated with flow loss. The results suggested that the relative contributions of temporary and permanent viscosity loss change as the fluid is used. Further, analysis of torque loss and input power revealed that input power and losses are more useful indicators of the effect of fluids on hydraulic system performance than pump efficiency.     

Evaluation of some traction fluids with a four roller machine

Traction characteristics of eight fluids of various compositions are evaluated with a four-roller machine, in which rotational speeds of mating rollers are positively controlled. The fluids tested are paraffinic and naphthenic mineral oils, liquid paraffin, polyisobutylene, and synthetic branched hydrocarbons. When the coefficient of traction is determined as a function of slide/roll ratio all fluids show essentially similar behaviour. However, different compositions have a marked effect on the values of the coefficient. These values are found to show a reasonable correlation with the estimated viscosity of each fluid at the average Hertzian pressures in the contacts     

Thermal analysis of an Eyring fluid in elastohydrodynamic traction

Recent research into elastohydrodynamic (EHD) traction has shown that under high pressure and isothermal conditions the flow properties of typical lubricants follow the Eyring relation between shear stress τ and shear rate /.γ: $\text{η/.γ = τ}{}_0\text{sinh(}\text{τ}\text{τ}{}_0\text{)}$ where η is the Newtonian viscosity and τ₀ a representative stress. The maximum in the traction curve arises from shear heating, and Crook's thermal analysis for a Newtonian lubricant has been modified to apply to an Eyring lubricant.In an EHD contact the pressure and hence the viscosity η vary from inlet to outlet, but it is shown that under the conditions of maximum traction it is sufficiently accurate to use the average values of η and τ₀ associated with the average values of pressure and temperature through the length of the contact. A simple formula can then be derived for the maximum traction coefficient in terms of the properties of the fluid (viscosity, pressure and temperature indices, and the representative stress τ₀ and the operating conditions (contact pressure, speed and film thickness).     

Study on the fundamental molecular structures of synthetic traction fluids: Part 2

It is essential to obtain guidelines for molecular design through analysing quantitatively the correlation between molecular structures and traction coefficients of traction fluids for developing an excellent traction fluid with a high traction coefficient. Although it is well known that the hydrogenation of an aromatic compound greatly increases its traction coefficient there are few accounts explaining the reason. We have analysed the influence of the intermolecular force on traction, examined why aromatic and chlorinated compounds have low traction coefficients and obtained the following results. (1) Alicyclic compounds have high traction coefficients because they interlock with each other well under high pressure. (2) Aromatic compounds have low traction coefficients because it is difficult for them to get close to each other due to repulsion of π-electrons and they have little unevenness for interlocking. (3) Chlorocyclohexane has a low traction coefficient due to the repulsion of large chlorine atoms which have a negative charge. (4) The traction property is greatly influenced by intermolecular force and molecular size, and the influence of the molecular stiffness is even smaller.     

Tailor-making polyalphaolefins

The production of polyalphaolefins (PAOs), an important class of synthetic functional fluids, can be better controlled to form materials with given sets of desired properties than can the production of analogous petroleum based fluids. Until now, commercial PAOs have been based almost exclusively on 1-decene as the starting material. Furthermore, they have been sold on the basis of the viscosity at 100 °C. Work at the authors' company has shown the need to focus attention on other physical properties for various alternative applications. Important properties may or may not include low temperature viscosity, pour point, volatility, high temperature stability, flash and fire point, and oxidative stability. The current work shows how the physical properties of the PAO product can be controlled by the judicious choice of starting olefin, reaction catalyst and co-catalyst, reaction temperature, reaction time, and other pertinent reaction parameters. Comparisons are made between PAOs and hydrocracked mineral oils.     

Traction Characteristics of High-Temperature Powder-Lubricated Ceramics (Si3N4/αSiC)

As part of a high-temperature, dry-lubricated bearing technology and lubricant system development program, a high-speed, high-temperature disk-on-disk tribometer was utilized and a matrix of traction data covering a range of load, speed and temperature was obtained. The influence of dry powder lubricants, TiO₂ and MoS₂, on the traction coefficients between two ceramic materials, Si₃N₄ and SiC, was investigated. The most important results of this investigation are characteristic curves for the traction coefficient vs. the slide/roll ratio with dry powders which are reminiscent of fluids, and the observation of dry powder lubricants' lower traction coefficients and wear. Measured tractions are found to be a strong function of powder lubricant type and values decrease moderately with slide-to-roll ratio and load. The data, show a weak sensitivity to temperature.     

An In-Line Method for Measuring Oxidation in Gear Oils and Hydraulic Fluids

A study was conducted to examine degradation of a gear oil and a hydraulic fluid using an in-line polymeric bead matrix (PBM) system that correlates oil oxidation with a relative change in the solvent properties of a fluid. The solvent property is an inherent aspect of an oil and allows oxidation to be measured independent of the base type, additive package, and viscosity. Samples of gear oil were heated to simulate a typical oxidized condition. Hydraulic fluids from a turbine were run for a limited time to provide discrete oxidized samples and test the sensitivity of the method. Results for the gear oils with the PBM are shown to compare favorably with infrared (IR) spectroscopy at room temperature. Elevated temperatures available with the PBM were needed to resolve oxidation levels for the hydraulic fluids.     

Temperature‐dependent effects in base oils: Carbon‐13 NMR spin‐lattice relaxation time and viscometry studies

Carbon‐13 NMR spin‐lattice relaxation time (T₁) data have been used to study the molecular dynamic aspects of base oils with different physical properties. Relaxation time measurements have been carried out on a few model compounds and a number of mineral base oils at various temperatures (273–373 K). Effective correlation time (°_C) and rotational mobility data obtained for the model compounds and base oils have provided evidence of relationships between molecular flexibility and the temperature dependence of viscosity. It is possible to determine the average carbon alkyl chain lengths and the molecular weights of the base oils from the ratio of the T₁ values of C_β and C_int carbons and the optimised value of the microviscosity factor (f_r), respectively. A qualitative correlation between Arrhenius energies (E_a) for microscopic motion and macroscopic bulk properties such as viscosity and viscosity‐temperature characteristics has been observed. Base fluids having better viscosity‐temperature characteristics were also associated with lower values of E_a for micro‐ as well as macroscopic processes.     

Biobased Polyalphaolefin Base Oil: Chemical,Physical, and Tribological Properties

The properties of a biobased polyalphaolefin with a viscosity of 40 cSt at 100 °C (BPAO-40) were investigated relative to a commercial petroleum-based PAO of similar viscosity at 100 °C (PAO-40). BPAO-40 was synthesized by oligomerization of a mixture of alpha olefins, with and without terminal methyl esters. These olefins were obtained from vegetable oils via a biorefinery process. In contrast to BPAO-40, commercial PAO-40 is synthesized only from non-functionalized alpha olefins. Thus, BPAO-40 is not only biobased, but also has a unique chemical structure, which makes it a functionalized PAO. The effect of chemical structure (presence or lack of methyl ester functionalization) on chemical, physical, and tribological properties of these two base oils was investigated. The investigation showed that, relative to the commercial non-functionalized PAO-40, the functionalized BPAO-40 displayed the following properties: higher density at 40–100 °C, lower number average molecular weight, higher polydispersity index, higher viscosity index, lower oxidation stability (pressurized differential scanning calorimetry), higher total acid number, higher free fatty acid, lower four-ball anti-wear coefficient of friction (COF) and lower wear scar diameter (WSD), higher elastohydrodynamic (EHD) lubricant film thickness under boundary conditions (low speeds and high temperature), lower EHD traction coefficient at 40 and 100 °C, similar pressure–viscosity coefficient, lower COF, lower WSD, and higher relative film thickness on a high-frequency reciprocating rig tribometer under boundary conditions (low speeds).     

Development of methods for evaluating traction fluids

Two methods have been developed for evaluating the performance of potential traction fluids. One method employed a 4-ball machine modified to measure torque and slip under rolling conditions. Aadvantages of the technique were the cheapness and replaceability of the rolling components, and the small amount of test fluid required. The other method was developed using a commercial traction drive, a Kopp variator; it was instrumented to allow measurement of torque and slip. There was a satisfactory correlation between results obtained in both pieces of apparatus for several fluids.Fluids of widely varying compositions were examined in the 4-ball machine and clear indications of the necessary features for a good traction fluid are discussed. In general, fluids with higher pressure-viscosity coefficients were better tractants. There was a clear trend towards better traction with compositions containing high proportions of naphthenic material, an observation which confirmed the patent literature claims for such classes of compounds.     

Synthetic Fluids for High Capacity Traction Drives

A series of fluids having coefficients of traction in rolling contact devices higher by 30% or more than those of the best previously available oils have been developed. These fluids have good oxidation stability in the 300–350 F range, are compatible with standard engineering materials and have viscosity characteristics suitable for industrial or vehicular applications.

The screening test methods used for determination of coefficient of traction in the development of these fluids are described.     

Viscosity–temperature correlation for liquids

A new viscosity–temperature equation and corresponding chart have been developed to extend the range of the current ASTM viscosity–temperature charts. This new chart and equation extends the temperature and viscosity range for hydrocarbons and, for the first time, has the ability to extend to the low viscosity regime of halocarbons and low temperature fluids. The new equation and chart can linearize liquid viscosity data from 0.04 cSt and covers the temperature range from −210 to 500 °C for halocarbons and hydrocarbons. With a modification to the temperature scaling, the new equation also has the ability to fit liquid metal viscosity data. The new chart and equation cannot accurately linearize the viscosity with respect to temperature of fluids exhibiting strong molecular bonding (water, ammonia), fluids whose molecular structure consists of long coils (some long chained silicones), or fluid mixtures in which one fluid precipitates out of solution (wax precipitation).

Influence of Lubricant Properties on ARKL Temperature Rise and Transmission Efficiency

It has been suggested in the literature that the ability of a lubricant to produce low “end of test temperature” (EOTT) in the achsialrillenkugellager (ARKL)—that is, axial groove ball bearing—thrust bearing test is an indicator of its likely efficiency in vehicle transmissions. Based on this, the current study has used regression analysis to correlate the ARKL EOTT with a range of physical and friction properties of a set of 26 lubricants. The latter have been selected to span a wide variety of base oil and additives types. It has been found that the most important lubricant property in determining ARKL EOTT is the elastohydrodynamic (EHD) friction or traction coefficient at low slide–roll ratio. Lubricants with low EHD friction were found to give low EOTT. Two lubricant properties of secondary importance have been identified. One is the lubricant viscosity index (VI), where a high VI contributes to high EOTT. The second is the heat transfer capability of the fluid, where a high capability was found to correlate with lower EOTT.

The findings help clarify how low EOTT in the ARKL test can be achieved via lubricant composition. If the correlation between ARKL and transmissions that has been suggested in the literature is valid, then these findings are transferable to the design of high efficiency transmission lubricants.     

A Simple,New Type of Rotational Viscometer

Abstract

The objective of this study was to construct a new type of rotational viscometer and to obtain the temperature dependence of the viscosities of fluids. A beaker containing 80 mL of fluid was rotated and the variation of the angular velocity of the beaker was measured with a photogate using a frequency?voltage converter (FVC). Viscosity measurements were performed while heating the fluid. The variation of the viscosity with temperature was investigated for lubricants. The output of the FVC and the temperature probe (thermistor) were connected to a PC via an analog-to-digital converter (ADC).     

Thermofluids Considerations and the Dynamic Behavior of a Finger Seal Assembly

Finger seals represent a compliant seal configuration. What differentiates and makes them preferable to the brush seals is their potential hydrodynamic lifting capabilities, and thus their noncontacting nature. The fingers' compliance allows both axial and radial adjustment to rotor excursions without damage to the integrity of the seal. The work to be presented here concerns the mapping of the thermofluid and dynamic behavior of a repetitive section of the newly proposed design of a two-layer finger seal. The assembly contains four high-pressure and four low-pressure fingers arranged axially in a staggered configuration and subject to an axial pressure drop. The numerical three-dimensional temperature and pressure results were obtained using a customized Navier-Stokes–based commercial package, CFD-ACE+. The results were obtained in a parametric fashion where the high-pressure side, the speed of rotation, and the heat transfer coefficient are the controlling parameters; the gas compressibility and the viscosity are also considered in the model of the thermofluids seal behavior. The stiffness and damping characteristics of the padded/unpadded fingers and the fluid were obtained through numerical simulation and were then used to model the interaction with the motion of the shaft. It is shown that the proposed geometry provides satisfactory lifting capability for the fingers. The fingers follow the motion of the shaft and their stiffness is small when compared to that of the fluid; thus, the displacement transmissibility is in most cases close to 1. Lifting forces and seal leakages, as well as the interaction between the profiled backplate and the low-pressure fingers through Coulomb damping/friction forces, are also parametrically studied.     

High‐pressure short time behavior of traction fluids

The squeeze film formation ability of traction fluids is studied under impact load by dropping a steel ball‐bearing against a flat anvil made of mild steel. The effect of the pressure–viscosity coefficient and of the viscosity is investigated for plastic impact. The depth difference between the lubricated surface dent and the dry dent increases linearly with the product αη of the pressure–viscosity coefficient α and the viscosity η. The importance of the lubricant parameter αη is observed under the squeeze film formation ability from contact voltage or elastohydrodynamic lubrication central film thickness measurement at rolling condition. The intensity of each impact collision is measured by means of an acoustic emission (AE) sensor. The high‐pressure short‐time solidification of traction fluids was confirmed by dent analysis after the impact tests and AE analysis under impact loads. Copyright © 2006 John Wiley & Sons, Ltd.     

Pressure–Viscosity Coefficients for Polyalkylene Glycol Oils and Other Ester or Ionic Lubricants

We present in this article new viscosity and density data for polypropylene glycol monomethyl ether, which completes a series of articles where we have published dynamic viscosity data of poly(propylene glycol) dimethyl ethers, dipentaerythritol esters, and pentaerythritol esters. New dynamic viscosity measurements up to 60 MPa at five temperatures in the range of 303.15–373.15 K, and density values at temperatures ranging from 298.15 to 398.15 K up to 60 MPa are reported in addition to other physical properties that affect the behavior of the fluids in elastohydrodynamic lubrication regime, such as the viscosity index value, VI, the universal pressure–viscosity coefficient, α _film, and the temperature–viscosity coefficient, β. The experimental measurements were performed using a rotational automated viscometer Anton Paar Stabinger SVM3000, a rolling-ball viscometer Ruska 1602-830 for high pressures, and an automated Anton Paar DMA HPM vibrating-tube densimeter. Together with these data, we also present a comparison of the film-generating capability for the fluids above mentioned as well as for other five ionic liquids. We analyze the dependence of the molecular structure on the lubrication properties of these oils, which can help the lubricant engineers to develop products with enhanced performance.