Contact Elastohydrodynamic Lubrication Research Articles

Finite line contact stiffness under elastohydrodynamic lubrication considering linear and nonlinear force models

The aim of this study is to analyze the stiffness of finite line contact under elastohydrodynamic lubrication (EHL) with linear and nonlinear force approximations. The numerical solution of the finite line contact is compared with experimental results of optical interferometry. Four types of profiles are evaluated: non-profiled roller, crowned roller, dub-off profile and logarithmic profile. For each type of roller, the geometry values vary in order to identify the behavior of the reduced contact force parameters. The results of the force model showed good correspondence with the numerical solution of the EHL contact.

Heavily loaded line EHL contacts with thin adsorbed soft layers

Studies of thin soft polymeric or metal layers adsorbed on the surfaces of lubricated solids showed lower friction coefficients and energy losses. The degree to which this reduction happens depends on the coating material nature, its rheology, and coating thicknesses, as well as other factors. To an extent, the application of such soft coatings is in its infancy and it is still not clear which parameters affect the performance of lubricated joints, and how. This paper attempts to model such coated lubricated contacts and to understand how soft coatings affect the joint performance. The paper presents a model of heavily loaded elastohydrodynamically lubricated (EHL) contacts of solids made of elastic homogeneous materials, the surfaces of which are covered by thin coating layers made of soft elastic material. The model incorporates not only normal but also tangential contact stresses and displacements. First, the effect of tangential contact stresses and displacements on the problem parameters is determined, and then the EHL problem is formulated and analyzed. It is necessary to account for variations in the solid surface linear velocities caused by tangential contact stresses. The deformation and displacement processes in thin coating layers are asymptotically modeled by certain boundary conditions. This is done using the Fourier transform with respect to the coordinate along the contact and making the necessary simplifications of the equations. The formulated EHL problem incorporates the normal and tangential contact surface displacements caused by normal and tangential contact stresses. The difference between the classic EHL problems and the problem at hand is due to variations in the linear surface velocities compared with the nominal ones. This causes significant changes in the contact friction parameters. The solution of the EHL problem is obtained numerically. A parametric analysis of the EHL problem solution is performed.

Numerical Study on Effect of Dimples on Tribo-Characteristics in Non-Newtonian Thermal Elastohydrodynamic Lubrication Point Contacts With Different Mechanical and Thermal Properties

Abstract The transient behavior of tribo-characteristics caused by micro-dimples on point contact surfaces with different mechanical and thermal properties was investigated based on non-Newtonian thermal elastohydrodynamic lubrication (EHL) analysis. The dimples were assumed to exist on both contact surfaces and the surface shapes of the contact bodies were evaluated separately. It is pointed out that surface texturing due to the micro-dimples is not necessarily beneficial in EHL contacts under fully flooded conditions since the micro-dimples provide a high pressure and a thin minimum film thickness as compared with the case of contacts with smooth surfaces, although the friction coefficient of surfaces with micro-dimples is always lower than that of the smooth surfaces. In order to obtain relatively good tribo-characteristics, the velocity of the surface with low thermal conductivity should be faster than that with high thermal conductivity, and the wavelengths of micro-dimples in the direction of motion on both surfaces should be different.

An improved stiffness model for line contact elastohydrodynamic lubrication and its application in gear pairs

PurposeThe purpose of this paper is to propose an simple and efficient stiffness model for line contact under elastohydrodynamic lubrication (EHL) and to investigate the gear meshing stiffness by the proposed model.Design/methodology/approachThe method combines the surface contact stiffness and film stiffness as EHL contact stiffness. The EHL contact stiffness can be calculated by the external load and displacement of the load action point. The displacement is the sum of deformation of the film and contact surface and is equal to the distance of the mutual approach of two contact bodies.FindingsThe conclusion is drawn that the contact stiffness calculated by the proposed model is smaller than that by the minimum film model and larger than that by the mean film model. It is also concluded that the gear meshing stiffness under EHL is slightly smaller than that under dry contact.Originality/valueThe EHL contact stiffness can be obtained by the increment of external load and mutual approach directly. The calculation of oil film stiffness and surface contact stiffness separately is avoided.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-11-2019-0465

Linear complementarity solution of 2D boundary slip EHL contact

Recent researches proved that bearing friction can be largely reduced with an oleophilic (no-slip) surface sliding against an oleophobic (slip) surface. The present work investigates the applicability of the slip/no-slip concept to 2D elastohydrodynamic lubrication (EHL) problems. Numerical analysis of isothermal EHL point contact is conducted under pure rolling conditions with the critical shear stress slip model. The challenge of determining the slip direction upon the attainment of the critical shear stress value in 2D problems is resolved by using a linear complementarity principle. The effect of boundary slip on bearing performance in terms of film thickness and friction is evaluated.

Comparison of rolling contact fatigue life between elastohydrodynamic lubricated point contacts pre and post running-in treatment

Abstract Rolling contact micropitting fatigue life is affected by stress fields in shallow subsurface. It is well known that there is a significant dependence, of stress distribution in subsurface, on the surface microstructures. Therefore rolling contact micropitting fatigue lives can be enhanced through modifying surface topography. In this study, running-in method was employed as the method to alter surface microstructures to explore the effect of various morphological characteristics on micropitting fatigue lives. Firstly, various rough surfaces were formed by applying experimental running in processes on a series of ball-on-disc pairs. Meanwhile, the evolutions of topography during the running-in processes were recorded. Secondly, rolling contact fatigue lives of these rough surfaces under elastohydrodynamic lubricated condition were calculated and compared based on Zaretsky's fatigue model combined with elastic contact mechanics. The results have indicated that a running-in process has significantly positive effect on prolonging micropitting fatigue lives. Theoretical analysis of the relationship between topographical parameters and micropitting fatigue lives to ascertain optimum morphological characteristics which are beneficial in reducing micropitting fatigue failures is the key originality of this work. This work provides guidelines on surface morphology for achieving maximum pitting fatigue life for interacting systems. In turn enhanced durability of the systems will lead to significant cost savings.

Influence of Realistic Lubricant Density-Pressure Dependence on the Stiffness of Elastohydrodynamic Lubricated Contacts

Abstract The analysis of the dynamic response of elastohydrodynamic lubricated contacts has often invoked a universal law for lubricant density–pressure dependence, even though the densities of many lubricants exhibit a substantial deviation from this widely adopted law. The current work investigates the influence of real lubricant density–pressure behavior on the stiffness of elastohydrodynamic lubricated contacts. It is shown that accounting for the real lubricant density–pressure dependence is crucial for an accurate estimation of the oil film stiffness, under steady-state considerations. The influence on the overall stiffness of the contact is found to be negligible though. Finally, an analytical correction procedure is provided, allowing a correction of oil film stiffness predictions that are based on the universal law for lubricant density–pressure dependence (or any other unrealistic law), to account for the real lubricant density–pressure response.

Friction reduction in EHL contacts by surface microtexturing – tribological performance, manufacturing and tailored design

Purpose This paper aims to derive tailor-made microtextures for elastohydrodynamically lubricated (EHL) contacts under consideration of manufacturing possibilities. Design/methodology/approach Component tests were used for the evaluation of the influence of surface texturing on the friction behavior in the cam/tappet contact. Furthermore, the manufacturing possibilities and limitations of a combined μEDM and micro-coining process and the feasibility of integration into a forming process were studied. Finally, a methodology based on transient EHL simulations and a meta-model of optimal prognosis was exemplarily used for microtexture optimization. Findings It was found that surface texturing in EHL contacts with high amount of sliding is promising. Moreover, the combination of μEDM and micro coining and the integration into established production processes allow the manufacturing of microtextures with desirable structural parameters and sufficient accuracy. Originality/value This paper gives a holistic view on surface microtexturing over several phases of the product life cycle, from the design, over efficient manufacturing to application-related testing.

The role of the thermal conductivity of steel in quantitative elastohydrodynamic friction

Abstract Numerical models for thermal elastohydrodynamic lubricated contacts of steel surfaces have always employed a value for the thermal conductivity of steel corresponding to its soft annealed alloy state. However, steel in elastohydrodynamic lubricated contacts is usually hardened. It has been known for more than a century now (not within the Tribology community though) that the thermal conductivity of steel could be reduced by a factor of more than two when it is hardened. Only recently did the Tribology community realize this “mistake”, of which the impact on friction predictions is investigated in this work. The mistake is found to lead to significant overestimations of friction in the thermo-viscous regime.

Elastohydrodynamic lubrication line contact of piezoelectric materials

This paper investigates the elastohydrodynamic lubrication (EHL) line contact between a rigid cylindrical punch and a transversely isotropic piezoelectric half-plane. The punch is supposed to be a perfect conductor, and its electric potential is constant within the contact region. The lubricant is assumed as a kind of Newtonian fluid. The lubricant density and viscosity are considered as the function of the fluid pressure. An iterative method is developed to solve the film thickness equation, Reynolds equation, flow rheology equation and load balance equation simultaneously. The effects of the total electric charge, maximum normal pressure, entraining velocity and material properties on the EHL film thickness and fluid pressure are discussed. Furthermore, the EHL contact of the insulating punch is also discussed to analyze the influences of conductivity of punch and piezoelectric effect of materials. The present results could be applied to improve the wear and contact damage of piezoelectric devices.

Transient analysis of surface roughness features in thermal elastohydrodynamic contacts

Abstract Understanding the influence of surface roughness in elastohydrodynamically lubricated (EHL) contacts is essential to improve durability and friction performance of machine elements employing non-conformal contacting surfaces. In this work, the transient event of a surface feature passing through a thermal EHL line contact operating under different sliding conditions is investigated with the purpose of providing a deeper understanding of surface roughness influence. This is achieved by solving the EHL problem in space and time. It was seen that sliding influences the temperature rise in the contact significantly, especially in the vicinity of the asperity. However, due to the characteristic behaviour of EHL contacts, the local temperature rise mainly influence the film thickness during exiting of inlet perturbations and the asperity.

Numerical simulation of mixed lubrication considering surface forces

Abstract Mixed lubrication describes a lubrication regime where elastohydrodynamic lubrication and asperity contact coexist. Surface spacing is expected to be small during the interaction between nano-scale smooth surfaces. When alkali metal salt solution is applied as lubricant, short-range surface forces are incorporated into an existing model to simulate mixed lubricated non-conformal sliding contact. The new model is examined in a wide range of sliding velocities to demonstrate the effect of surface forces at different lubrication regime, from boundary lubrication to full film hydrodynamic lubrication. It is revealed that surface forces reduce friction coefficient particularly at mixed lubrication regime. Reduction of surface roughness improves contribution of surface forces and makes it easier to achieve superlubricity at low speed.

Effects of inhomogeneity on rolling contact fatigue life in elastohydrodynamically lubricated point contacts

PurposeThis paper aims to investigate the effects of inhomogeneities on the rolling contact fatigue (RCF) life in elastohydrodynamically lubricated (EHL) point contacts.Design/methodology/approachA numerical model for predicting the RCF life of inhomogeneous materials in EHL contacts was established by combining the EHL model and the inclusion model through the eigen-displacement and then connecting to the RCF life model through the subsurface stresses. Effects of the type, size, location and orientation of a single inhomogeneity and the distribution of multiple inhomogeneities on the RCF life were investigated.FindingsThe RCF life of a half-space containing manganese sulfide (MnS) inhomogeneity or the mixed inhomogeneity of aluminium oxide (Al2O3) and calcium oxide (CaO) was longer than that for the case of Al2O3inhomogeneity. For a single ellipsoidal MnS inhomogeneity, increases of its semi-axis length and decreases of its horizontal distance between the inhomogeneity and the contact center shortened the RCF life. Furthermore, the relationship between the depth of a single MnS inhomogeneity and the RCF life was found. For the half-space containing multiple inhomogeneitites, the RCF life decreased remarkably compared with the homogeneous half-space and showed discreteness.Originality/valueThis paper implements the prediction of the RCF life of inhomogeneous materials under EHL condition.

Superlubricity in EHL Contacts with Water-Containing Gear Fluids

Fluid friction in elastohydrodynamically lubricated (EHL) contacts depends strongly on the lubricant considered. Synthetic oils can have significantly lower fluid friction than mineral oils. Water-containing fluids have the potential to significantly reduce fluid friction further. The aim of this study is to investigate the film formation and frictional behavior of highly-loaded EHL contacts with water-containing fluids. Comparisons are made with mineral and polyalphaolefin oils. Measurements at an optical EHL tribometer show good lubricant film formation of the considered water-containing gear fluids. Measurements at a twin-disk test rig show coefficients of friction smaller than 0.01, which is referred to as superlubricity, for all considered operating conditions.

Film Thickness and Shape Evaluation in a Cam-Follower Line Contact with Digital Image Processing

Film thickness is the most important parameter of a lubricated contact. Its evaluation in a cam-follower contact is not easy due to the continuous variations of speed, load and geometry during the camshaft rotation. In this work, experimental apparatus with a system for film thickness and shape estimation using optical interferometry, is described. The basic principles of the interferometric techniques and the color spaces used to describe the color components of the fringes of the interference images are reported. Programs for calibration and image analysis, previously developed for point contacts, have been improved and specifically modified for line contacts. The essential steps of the calibration procedure are illustrated. Some experimental interference images obtained with both Hertzian and elastohydrodynamic lubricated cam-follower line contacts are analyzed. The results show program is capable of being used in very different conditions. The methodology developed seems to be promising for a quasi-automatic analysis of large numbers of interference images recorded during camshaft rotation.

Investigation on metallic contact conditions using ECR technique by performing rolling/sliding experiments

Understanding asperity contact conditions in rough surface elastohydrodynamic lubrication (EHL) contacts is of particular interest in tribology due to the drastic change in asperity characteristics during the wear process leading to significant losses. This work attempts to understand in situ change in surface topography during the wear process without disturbing the contact configuration. Measurements of electrical contact resistance (ECR) are one of the promising techniques for achieving this. To understand the behavior of asperity-to-asperity contacts, twin disc experiments are performed under rolling/sliding conditions. The effect of load and speed on friction coefficient and contact potential is presented. It is shown that transient representation of contact potential and friction coefficient is more appropriate than the average value of the friction coefficient and contact potential in the mixed-elastohydrodynamic lubrication regime. The variation of average friction coefficient and contact potential for different film thickness with respect to surface roughness (lambda ratio) is presented. Semi-quantitative techniques are used for theoretical prediction of the electrical contact resistance, number of contacting asperity and the real contact area. The trend for variation of numerically determined contact resistance with lambda ratio is found to be same as for experimentally determined average contact potential. It is also observed that at low lambda ratio, significant real contact area is formed which causes severe asperity-to-asperity contacts. Whereas, small real contact area is obtained at high lambda ratio which shows less asperity interactions.

Theoretical analysis of a rolling-sliding elastohydrodynamic lubrication line contact with a subsurface inclusion

This work investigates the transient effects of a single subsurface inclusion over the pressure, film thickness, and von Mises stress in a line elastohydrodynamic lubrication contact. Results are obtained with a fully-coupled finite element model for either a stiff or a soft inclusion moving at the speed of the surface. Two cases analyzed consider the inclusion moving either at the same speed as the mean velocity of the lubricant or moving slower. Two additional cases investigate reducing either the size of the inclusion or its stiffness differential with respect to the matrix. It is shown that the well-known two-wave elastohydrodynamic lubrication mechanism induced by surface features is also applicable to the inclusions. Also, that the effects of the inclusion become weaker both when its size is reduced and when its stiffness approaches that of the matrix. A direct comparison with predictions by the semi-analytical model of Morales-Espejel et al. ( Proc IMechE, Part J: J Engineering Tribology 2017; 231) shows reasonable qualitative agreement. Quantitatively some differences are observed which, after accounting for the semi-analytical model's simplicity, physical agreement, and computational efficiency, may then be considered as reasonable for engineering applications.

A homogenized micro-elastohydrodynamic lubrication model: Accounting for non-negligible microscopic quantities

Abstract In rough elastohydrodynamic lubricated contacts the geometry often exhibits two clearly separated scales: a macroscopic scale –the one of the bearing– and a microscopic scale, that of the surface roughness. In numerical simulation of lubricated contacts, this difference in scales leads to large systems of equations to solve. Assuming periodicity or pseudo-periodicity of the small scale, several methods to decouple the macro scale from the micro scale have been proposed, the formal approach being the homogenization theory. However, the approximation errors due to the classical asymptotic assumptions can be considerable. In this work we introduce a homogenized model which takes into account the non-negligible pressures and deformations of the micro scale, thus extending the applicability of the classical asymptotic homogenized approaches.

Effects of Nanoscale Ripple Texture on Friction and Film Thickness in EHL Contacts

The effects of nanoscale ripple texture on the film thickness and friction in elastohydrodynamically lubricated (EHL) contacts were investigated through ball-on-disc experiments and numerical simulations of line contacts. The texturing was produced by femtosecond LASER irradiations and the ripple texture was in the form of sinusoidal waviness with nanoscale amplitudes and wavelengths. The experimental and numerical results indicate that the orientation of the ripples with respect to the entrainment direction has little to no effect on their capability to form a lubricating film. In the EHL regime, the ripples were found to reduce the central and minimum film thickness by half of their peak-to-peak amplitude as compared to a smooth contact. The transition from EHL to mixed lubrication regime was attributed to micro-EHL effects although the subsequent friction increase was found to be largely due to the onset of asperity contacts. In the mixed lubrication regime, the coefficient of friction was mainly determined by surface roughness and its value increased with an increase in the ripple amplitude.

Local Effects in EHL Contacts with Oil-Impregnated Sintered Materials

Oil-impregnated open-pored sintered materials can be used to provide the intrinsic self-lubrication of tribological contacts. Although its general functionality was recently confirmed for highly-loaded contacts, detailed analyses on the local effects of surface porosity in tribological contacts are required in order to understand and improve its operating behavior. In this context, this study investigates the influence of different surface finishes at a twin-disk test rig, and the local effects of surface porosity in elastohydrodynamically lubricated (EHL) contacts at an optical tribometer, based on thin film colorimetric interferometry. The results show the detrimental influence of high surface porosity on the operating behavior. Local observations of the lubricant film in EHL contacts indicate the presence of “open” pores, resulting in local film breakdown, and “closed” pores, transporting the additional lubricant into the pressurized zone. An appropriate surface finish technique to manufacture a low permeable layer with an adequate mechanical strength is demanded.