Contact Elastohydrodynamic Lubrication Research Articles

Thermal transient rough EHL line contact simulations by aid of computational fluid dynamics

Reynolds equation is the pre-dominantly used PDE for modelling the fluid flow or more accurately the fluid pressure in an elastohydrodynamic lubrication (EHL) contact. The equation is derived by combining the two conservation equations of momentum and continuity into a single equation for the fluid pressure. The numerical approach for theoretical investigations performed on EHL contacts in this work is somewhat different. The modelling of the fluid flow is based on a computational fluid dynamic (CFD) technique. The fluid flow is simulated by aid of the equations of momentum and continuity in a more complete form and when the thermodynamics is incorporated, the equation of energy. The aim of the investigation was to examine whether the CFD technique could be used to handle thermal transient rough EHL line contacts. It is shown that commercial CFD software can be modified to meet such requirements. The influence of thermal effects on the flow under sliding motion was investigated. The non-Newtonian model used in this work is the Ree–Eyring model. It is shown that the choice of the Eyring stress in the model influences flow in the contacts. If the thermal properties of the surrounding solids differ, it has been shown experimentally and theoretically that a dimple or increased central film thickness may appear in the EHL contacts. This work shows that the governing mechanisms that result in the dimple are also present in thermal transient rough EHL line contacts.

Simulation of dry and lubricated contacts in multi-body systems

The present paper investigates the modeling of counterformal elastohydrodynamic lubricated (EHL) contacts in multi-body system (MBS) environments. The aim is to obtain a representation of the dynamic behaviour of machine elements and characteristic values as the lubrication film thickness for lifetime calculations. Two contact models are presented for the study of cam-roller tappet and raceway-roller systems. The first model is a one-dimensional representation of the EHL contact behaviour. It is based on the calculation of the central lubrication film thickness by means of dimensionless values (functions of the operating conditions) and provides a simplified model of the mixed lubrication. The second one, still under development, investigates the modeling of 3D dry and lubricated contacts. The fluid-structure interaction is here fully considered and a discretised form of the contact problem is solved using an iterative algorithm. This paper proposes, in the latter case, some perspectives and a discussion of its implementation.

Contact pressure in indented elastohydrodynamic lubrication contacts

Indents perturb the pressure and stress distribution and increase the failure risk. Existing theory for roughness deformation is extended to pure-rolling elastohydrodynamic lubrication indented contact pressure perturbations. First, a study of shallow indents is proposed, then it is modified to account for non-linear behavior of deeper indents.

Optical characterization of elastohydrodynamic lubricated (EHL) contacts using surface plasmon resonance (SPR) effect

In this paper we report a novel optical technique based on the surface plasmon resonance (SPR) phenomenon for studying elastohydrodynamic lubricated (EHL) dimple. While it is well known that the refractive index (RI) variation in the lubricant may provide direct information on the pressure distribution inside a highly pressurized EHL dimple, one can obtain the pressure in the spectral SPR absorption curve, which corresponds to a unique color complexion in the SPR image. We have performed experiments to compare our SPR imaging technique with the one based on conventional optical interferometry. The new SPR technique provides an improvement of over 180 times in measurement accuracy. More importantly, SPR measures the RI directly and does not require any information on the lubricant film thickness. In light of these advantages, the SPR detection approach reported herein should be an attractive technique for studying EHL contacts.

On the elastohydrodynamic analysis of shear-thinning fluids

Realistic prediction of the characteristics of the elastohydrodynamic lubrication (EHL) contact requires consideration of the appropriate constitutive equation for the lubricant. In many applications, the lubricant exhibits a shear-thinning behaviour which significantly affects the film thickness. In this paper, we present a generalized formulation that can efficiently treat shear-thinning fluids with provision for compressibility in the EHL line contact. Specifically, the Carreau model and the sinh-law model are investigated. An extensive set of numerical solutions and comparison with experiments reveal that the Carreau equation properly captures the film thickness behaviour under both rolling and sliding conditions.

A theoretical study on the response of a point elasto-hydrodynamic lubrication contact to a normal harmonic vibration under thermal and non-Newtonian conditions

Time-dependent thermal and non-Newtonian elastohydrodynamic lubrication of an elliptical point contact subjected to a normal harmonic vibration was studied numerically in this work. The contact was idealized as between an infinite plane and a spherical roller. The normal vibration of the roller was described by specifying the centre of the spherical roller to the infinite plane (without deformation) as a cyclic function of time. The shear-thinning rheological property of the lubricant was described by the Eyring model. The time-dependent numerical solution was achieved instant after instant in each period of a vibration. The periodic errors were checked at the end of each vibration cycle until the responses of variables such as pressure, film thickness, and temperature were all periodic functions with the same frequency of the roller's vibration. At each instant, the pressure field was solved with a multi-grid method, the surface deflection produced by pressure was determined with a multi-level multi-integration technique, the non-Newtonian flow of the lubricant was considered by using the equivalent viscosity calculated according to the shear-strain rate along the entrainment direction only, and the temperature field was evaluated with a finite-difference scheme through a column-by-column relaxation process. The computing time for a cyclic solution was 12–15 h on a personal computer with a 3.0 GHz central processing unit. The effects of both the amplitude and the frequency of the vibration were investigated. It was shown that the time-dependent solution is significantly different from the steady-state solution, especially when the amplitude of vibration is large and the frequency of vibration is high. Corresponding to a typical thermal and non-Newtonian case, numerical solutions were also obtained under isothermal and Newtonian, isothermal and non-Newtonian, and thermal and Newtonian conditions. Comparisons between these solutions indicate that, under time-dependent conditions, the effects of thermal and non-Newtonian flow are similar to those under steady-state conditions.

Numerical analysis of the design parameters on the performance of thin film temperature sensors

An embedded thin film temperature sensor onto the surface of lubricated mechanical components is promising for real-time machine condition monitoring. The present study has been concerned with the performance of thin film temperature sensors due to the design parameters such as sensor film thickness, width, length, sensing materials, insulating and protecting layer. An improved three-dimensional heat conduction model has been developed to study the influence of those design parameters on the performance of thin film sensors designed for monitoring temperature distribution in elastohydrodynamic lubrication (EHL) contacts. The finite-volume method was utilized to solve the transient, three-dimensional heat conduction equation with time-dependent heat generation due to friction over the sensor. The time response to temperature change in a lubricated contact was predicted. This research work provided more deep insights for selection of the design parameters to ensure sufficient sensitivity, fast response time and the appropriate temperature range of the EHL temperature sensor to be used.

Rapid calculation of the pressures and clearances in rough, rolling-sliding elastohydrodynamically lubricated contacts. Part 2: General, non-sinusoidal roughness

Part 1 of this paper [1] described how the behaviour of low amplitude, sinusoidal roughness in elastohydrodynamically lubricated (EHL) contacts could be characterized by three complex quantities: attenuation of original profile, amplitude of complementary wave and its wave number and decay rate. This second part outlines how these results can be used to estimate, rapidly, the clearances and pressures in any rough EHL contact. The method is applied to a number of contacts for which accurate experimental results are available and it is shown that the process gives close estimates of the clearance and pressure distributions where the amplitude of the attenuated roughness is less than 50 per cent of the clearance and good indication of the behaviour of rougher surfaces.

Elastohydrodynamic modelling of heat partition in rolling-sliding point contacts

This article presents the results of a series of thermal elastohydrodynamic lubrication (EHL) analyses of a set of disc experiments carried out by Patching et al. The authors have previously calculated the heat partition at each load stage fromthe experimental data, and the present work seeks to compare the heat partition developed in a thermal EHL analysis of the lubricant film with that calculated from the experiments. These EHL analyses show that the heat partition depends on the non-Newtonian formulation used. Only when the dissipation takes place by wall slip occurring at the faster moving surface do the heat partition results approach those determined from the experimental results. It is concluded that this combination of experimental heat partition results with associated EHL analysis can prove to be a much more discerning test of rheological behaviour in EHL contacts than is provided by measurement of traction.

A study on starved micro-thermal elastohydrodynamic lubrication in simple sliding circular contacts

Numerical analyses based on Newtonian and Eyring fluid models are carried out to simulate the effects of inlet oil starvation in thermal elastohydrodynamic lubrication (TEHL) point contacts. It is assumed that the surface with transverse roughness is stationary and smooth surface is moving. Results of smooth TEHL contacts based on Newtonian fluid model are also presented as a reference. It is found that the temperature in the oil film increases suddenly at the inlet meniscus like the pressure build-up because of the discontinuous oil film at that boundary. The increasing degree of the inlet starvation raises the average temperature in the oil film for both smooth and rough contacts. The Newtonian maximum pressure and temperature for the rough contact occur at the macroscopic horseshoe-shaped constriction. Those of the Eyring fluid model still locate at the contact centre. As the degree of oil starvation increases, the differences in the pressure and film thickness distributions in rough elastohydrodynamic lubrication contacts between the Newtonian and Eyring fluid flow models decrease.

EHL Modeling for Nonhomogeneous Materials: The Effect of Material Inclusions

Inclusions are common in bearing materials and are a primary site for subsurface fatigue crack initiation in rolling element bearings. This paper presents a new approach for computing the pressure, film thickness, and subsurface stresses in an elastohydrodynamic lubrication (EHL) contact when inclusions are present in the elastic half-space. The approach is based on using the discrete element method to determine the surface elastic deformation in the EHL film thickness equation. The model is validated through comparison with the smooth EHL line contact results generated using linear elasticity. Studies are then carried out to investigate the effects of size, location, orientation, and elastic properties of inclusions on the EHL pressure and film thickness profiles. Both inclusions that are stiffer than and/or softer than the base material are seen to have effects on the pressure distribution within the lubricant film and to give rise to stress concentrations. For inclusions that are stiffer than the base material (hard inclusions), the pressure distribution within the lubricant film behaves as though there is a bump on the surface, whereas for inclusions that are less stiff than the base material (soft inclusions), the pressure distribution behaves in a manner similar to that of a dented surface. Inclusions close to the surface cause significant changes in the contact stresses that are very significant considering the stress life relationship. For inclusions that are located deep within the surface, there is little change in the EHL pressure and film thickness.

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.

Sixty years of EHL

AbstractIt is now 60 years since Ertel produced the first solution to the elastohydrodynamic lubrication (EHL) problem. There has been enormous progress since then, both in numerical modelling and in experimental research on EHL. The moving, rough surface EHL problem can now be solved on laptop‐level computers, while maps of film thickness, pressure and temperature can be obtained experimentally from within rolling/sliding contacts. However, there remain some important questions that have not been fully resolved. One of the most contentious is how to describe the rheological properties of lubricants under the very severe conditions present in thin film EHL contacts. A second is how to model mixed lubricated contact, where the fluid film can break down at asperity conjunctions. But perhaps the greatest challenge to researchers in EHL is to produce useful design equations for predicting the performance of machine components operating in EHL and thereby ensure that EHL theory becomes an integral part of the design process. Copyright © 2006 John Wiley & Sons, Ltd.

On the Mechanisms of the Reduction in EHL Traction at Low Temperature

Various experiments have revealed a dramatic traction reduction in elastohydrodynamic lubrication (EHL) contacts under subambient temperatures or with fluids of high viscosity and high pressure-viscosity coefficients. This paper conducts a systematic theoretical analysis to identify and analyze the mechanisms of this reduction. A thermal EHL model is used for the analysis that is capable of capturing many possible effects, including a thermally induced cross-film shear localization within the EHL film. The analysis is carried out for a wide range of ambient viscosity and pressure-viscosity coefficients. The results and analysis suggest that the thermal effects are responsible for the dramatic reduction of the EHL traction. Two thermal effects become particularly pronounced with high viscosity and high pressure-viscosity coefficients. One is the lubricant re-circulation induced inlet shear that generates and accumulates heat in a prolonged inlet region and significantly increases the film temperature in the region of high pressure. The other is the thermally induced cross-film shear localization that generates high temperature in a small central layer of the EHL film. Presented at the STLE Annual Meeting in Houston, Texas May 19-23, 2002 Review led by Greg Kostrzewsky

Roughness Amplitude Reduction Under Non-Newtonian EHD Lubrication Conditions

Abstract Over the last decade, the operating conditions of the Elastohydrodynamic lubricated (EHL) contact have become increasingly severe. Consequently, the average film thickness decreased and became comparable to the surface roughness. Under those conditions, the surface features can reduce the minimum film thickness and can thus increase wear. They can also increase the temperature and the pressure fluctuations, which directly affect the component life. In order to describe the roughness geometry inside an EHL contact, the amplitude reduction of harmonic waviness has been studied over the last decade. This theory currently allows a quantitative prediction of the waviness amplitude and includes the influence of wavelength and contact operating conditions. However, the model assumes a Newtonian behavior of the lubricant. The current paper contributes to the extension of the roughness amplitude reduction for EHL point contacts including non-Newtonian effects. A generalized model is derived that includes both types of behavior.

Rapid Calculation of the Pressures and Clearances in Rough, Elastohydrodynamically Lubricated Contacts Under Pure Rolling. Part 1: Low Amplitude, Sinusoidal Roughness

Modern elastohydrodynamically lubricated (EHL) solvers allow the calculation of the pressures and clearances in rough EHL contacts. However, the process is time consuming and the results give little insight into the physical behaviour of the system. The length of calculation also makes these methods unsuitable for use as a design tool. The investigation of the behaviour of low amplitude, sinusoidal roughness in EHL contacts provides greater understanding of the processes involved. The results also allow the effects of surface roughness to be examined rapidly. This suggests that it may be possible to develop the approach and create a ‘real-time’ design process for the analysis of different surface roughnesses under a range of operating conditions.

Dynamic performance of a thin film temperature sensor in a lubricated contact

A thin film temperature sensor integrated onto mechanical component surface is promising for real-time machine condition monitoring. In this article, a one-dimensional heat conduction model has been developed to study the dynamic performance of the thin film sensor designed for monitoring the temperature distribution in an elastohydrodynamic lubrication contact. A control volume approach was used to numerically analyse the effects of sensor film thickness (from 0.1 to 100 μm), sensing materials, and substrate materials on the transient time response of the thin film sensor. The numerical model was evaluated by an analytical solution in a semi-infinite domain. The sensor time constant is obtained on the basis of a constant heat load and sensor sensitivity is studied when a typical elastohydrodynamic pressure distribution in a lubricated contact is applied. Faster response time and short time delay of a thin film sensor are expected in a higher conductivity of substrate. It is also clear that the response time decreases with increasing sensor film thickness and with increasing conductivity of the substrate. The results show that when the thickness of the sensor is < 1 μm, the sensor is feasible to capture transient temperature profile in real time for machine health monitoring under various operating conditions. One of the experimental validations given in this article has proven the robustness of the developed model.

Occurrence of a Noncentral Dimple in Squeezing EHL Contacts

Previous studies about pure squeeze elastohydrodynamic lubrication (EHL) have disclosed a film profile with a central dimple. Two problems about pure squeeze EHL are numerically solved in this paper. One is for a very small initial impact gap, and the other is the response of a squeezed EHL conjunction under stepwise loads. None of them result in the familiar film with a central dimple, which can be attributed to the local squeeze effect generated in the periphery region. In the first problem, it has been found that when there is adequate oil present on the plate, with a decrease in the initial impact gap, a shallow circumferential dimple occurs at the periphery of the conjunction instead of the primary central dimple presented in previous studies. Correspondingly the minimum film thickness occurs at the central region. The effect of the initial impact velocity on the periphery dimple is also investigated. In the second problem, the response of a conjunction subjected to a prescribed stepwise load is studied. When the first step load is applied, a central dimple film is produced. When the applied load is increased with a second step load, a periphery dimple appears, similar to that in the first problem. The local squeeze effect for the present numerical periphery dimple has been observed in previous experiments under similar conditions.

Contact–fluid interfacial slippage in hydrodynamic lubricated contacts

Contact–fluid interfacial slippage in hydrodynamic lubricated contacts

Roughness in rolling-sliding elastohydrodynamic lubricated contacts

A simplified treatment of roughness effects in rolling-sliding elastohydrodynamic lubricated (EHL) contacts was developed by the author [4]. This analysis predicted that the original amplitude would be attenuated under the conjunction and that a decaying, complementary wave would be generated at the inlet. This complementary wave would be carried through the contact at the entrainment velocity and, for sinusoidal roughness, its wavelength would be given by λ u/v, where λ is the wavelength of the original roughness, u the entrainment velocity, and v the velocity of the rough surface. This simplified analysis predicted the degree of attenuation of the original roughness and the decay rate of the complementary wave but did not determine the amplitude of the complementary wave. This paper presents a more detailed account of this analytical approach including compressibility effects together with a more accurate allowance for the coupling between the decay of the complementary wave and its wavelength. It also extends the work, using a perturbation analysis, determining accurate values for the pressures and clearance variations produced, under rolling-sliding conjunctions, by low-amplitude roughness. The perturbation results are compared with the predictions of the simplified analysis. It will be shown that the analysis is remarkably accurate. In addition, it appears to be relatively straightforward to determine the magnitude of the complementary wave for any given operating condition. The effect of low-amplitude, sinusoidal roughness on EHL contacts can then be expressed in terms of the operating conditions together with a single value for the amplitude of the complementary wave. This simplification suggests that it may be possible to produce a method for the rapid analysis of roughness effects in rolling-sliding conjunctions without the need for any detailed EHL calculations.