Micro-elastohydrodynamic Lubrication Research Articles

Understanding the Mechanism of Load-Carrying Capacity between Parallel Rough Surfaces through a Deterministic Mixed Lubrication Model

Experimental results have confirmed that parallel rough surfaces can be separated by a full fluid film. However, such a lift-off effect is not expected by the traditional Reynolds theory. This paper proposes a deterministic mixed lubrication model to understand the mechanism of the lift-off effect. The proposed model considered the interaction between asperities and the micro-elastohydrodynamic lubrication (micro-EHL) at asperities within parallel rough surfaces for the first time. The proposed model is verified by predicting the measured Stribeck curve taken from literature and experiments conducted in this work. The simulation results highlight that the micro-EHL effect at the asperity scale is critical in building load-carrying capacity between parallel rough surfaces. Finally, the drawbacks of the proposed model are addressed and the directions of future research are pointed out.

Duncan Dowson: Pioneer of elastohydrodynamic lubrication of gears

The paper briefly reviews Duncan Dowson's ground-breaking contribution to the theory of elastohydrodynamic lubrication in relation to the understanding of lubrication of gear tooth contacts. His early work with Higginson on numerical modelling of elastohydrodynamic lubrication finally explained how gears can operate successfully, and avoid wear, due to the generation of a stiff, protective oil film. The resulting minimum film thickness equation stands as a reliable reference formula for calculations in gear design standards. The paper includes examples of how elastohydrodynamic lubrication theory has been developed by the present authors and their co-workers, and applied to aid the design of engineering components such as worm gears, thrust rims and profile-modified helical gears. Also included is its extension to include the important effects of surface roughness at the asperity level (micro-elastohydrodynamic lubrication) and its relevance to the current, troublesome problem of micropitting.

Advanced morphological analysis of siloxane-hydrogel contact lenses.

The purpose of this work is to provide a better understanding of three-dimensional (3-D) surface texture of siloxane-hydrogel contact lenses (CLs) using atomic force microscopy (AFM) and stereometric analysis. The 3-D surface texture characterization of unworn/worn siloxane-hydrogel CLs made of Filcon V (I FDA group) was performed with stereometric analysis. The atomic force microscopy (AFM) measurements of surface roughness and micromorphology of CLs were made using a Nanoscope V MultiMode (Bruker) in intermittent-contact mode, in air, on square areas of 5 × 5μm. Stereometric study of 3-D surface texture was made according with ISO 25178-2:2012 for CLrins (taken from the blister and rinsed with deionized water); CLss (preserved for 12 hr in saline solution and rinsed with deionized water); CLworn-smooth (worn for 8hr and presenting the smooth type morphology), and CLworn-sharp (worn for 8hr and presenting the sharp-type morphology). The 3-D surface texture of siloxane-hydrogel CLs was found to have specific morphological characteristics. Statistical parameters revealed local geometrical and morphological spatial structures at nanometer scale attributed to the specific interactions at the CLs surface. Before wear, the surface micromorphology of Filcon V CLs is regular with uniformly distributed microasperities and relatively small heights (Sq=0.6nm). After 12 hr in saline, it is found that the micromorphology changes relatively easily, but retaining the main morphological characteristics (Sq=1.2nm). After 8hr of wear, there are two typical micromorphologies: smooth type, characterized by gutter structures and isolated microasperities (Sq=2.5nm), while the sharp type has an appearance with compactly arranged microasperities of hill type flanked by compactly arranged microregions of valley type (Sq=2.2nm). Surface statistical parameters allow manufacturers in developing the next generation of CLs with improved surface texture while improving biocompatibility and minimizing the impact of the material on corneal physiology. Furthermore, the micro-elastohydrodynamic lubrication due to surface texture at a nanometer scale between the back surface of the CL with the corneal surface and the front surface of the CL with the under-surface of the eyelid can be deeper and more nuanced to understand in light of modern tribological theories.

Lubrication analysis and wear mechanism of heavily loaded herringbone gears with profile modifications in full film and mixed lubrication point contacts

The transient non-Newtonian thermal micro-elastohydrodynamic lubrication point contact model and the adhesive wear model in mixed lubrication point contact are applied to a heavy-loaded modified herringbone gear drive respectively in steady running phase and start-stop phase. The fundamental parameters of both elastohydrodynamic point contact and wear model are derived from the meshing characteristic and kinematics analysis of herringbone gears. The effects of pinion torque, rotational speed and surface roughness on the maximum pressure and temperature rise and the minimum film thickness are discussed in gear steady running condition for full film lubrication contact. Meantime, the asperity load ratio, contact flash temperature and tooth wear depth are investigated in gear start-stop phase for mixed lubrication point contact. The results show that the lubrication performance of herringbone gear is deteriorated by substantially increased pressure and temperature rise resulting from a larger tooth surface roughness and rotational speed. The tooth wear depth of herringbone gear in start-stop phase is likely greatly reduced with a larger start-stop acceleration. Moreover, the lubrication and wear performances for the complete running cycle of herringbone gear drive from start-up to stop indicate that large tooth surface roughness rather than high load are more prone to herringbone gear thermo-mechanical and wear failures.

Misalignment-Induced Micro-Elastohydrodynamic Lubrication in Rotary Lip Seals

In literature the lubrication of rotary lip seals is explained by hydrodynamic action on a microscopic scale. This theory assumes perfect concentricity between the seal and the shaft which in reality seldomly occurs. Focusing on the stern tube seals application, an analysis is performed on the phenomena distorting the axisymmetric operation of rotary lip seals. Radial and angular shaft misalignments together with pressure and temperature gradients have been modelled. The model predictions are validated using a dedicated setup. Additionally, applying the soft-EHL film thickness expressions at the asperity level, an equivalent film thickness along the circumferential direction is estimated. The Reynolds PDE is solved to predict the misalignment-induced hydrodynamic pressure build-up. The film thickness variation derived and accompanying non-uniform contact pressure distribution was shown to be sufficient for hydrodynamic action and, depending on the minimum film thickness, the hydrodynamic pressure build-up can exceed the static contact pressure. Additionally, significant differences were observed between the radial and angular misalignment configurations.

Tribological and thermal properties of a crowned gear pair with high-speed and heavy-load in thermal micro-elastohydrodynamic lubrication

The oil film stiffness, temperature rise of oil film, flash temperature and frictional loss ratio of a crowned gear pair with non-Newtonian transient thermal elastohydrodynamic lubrication of rough surfaces in high-speed and heavy-load operating conditions are investigated. The pressure, film thickness, coefficient of friction and temperature rise are calculated along the action line of spur gears to verify the validity of full numerical solution. Subsequently, the effects of high-speed, heavy-load and roughness on those tribological and thermal properties of a crowned gear drive are discussed. The results show that in high-speed, heavy-load and rough surface contact, the crown modification is in favor of stability improvement of gear drive due to smoother curve of oil film stiffness. The temperature rise of the oil film and flash temperature on tooth surface are quite high which are prone to result in gear scuffing. Moreover, small flash temperature and frictional loss ratio occur in the domains near the start of active profile and the tip due to the effect of crown modification.

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.

On the complementary function amplitude for film thickness in Micro-EHL

Abstract Given recent published measurements of the amplitude of the complementary clearance ripple in micro-elastohydrodynamic lubrication conditions, the present article compares theoretically obtained amplitudes with the experimental ones from literature. Based on the results obtained, the article explores a possible explanation to why previous attempts of comparisons show deviations. The explanation seems to be that in previous literature attempts the amplitude of the theoretical overall deformed clearance was used rather than only the amplitude of the complementary function. In addition, theoretical and experimental results are included about changes in amplitude and shape of the complementary function due to non-Newtonian lubricant effects.

Experimental and Numerical Study of Micropitting Initiation in Real Rough Surfaces in a Micro-elastohydrodynamic Lubrication Regime

Micropitting is a form of surface fatigue damage that happens at the surface roughness scale in lubricated contacts in commonly used machine elements, such as gears and bearings. It occurs where the specific film thickness (ratio of smooth surface film thickness to composite surface roughness) is sufficiently low for the contacts to operate in the mixed lubrication regime, where the load is in part carried by direct asperity contacts. Micropitting is currently seen as a greater issue for gear designers than is regular pitting fatigue failure as the latter can be avoided by control of steel cleanliness. This paper describes the results of both theoretical and experimental studies of the onset of micropitting in test disks operated in the mixed lubrication regime. A series of twin disk mixed-lubrication experiments were performed in order to examine the evolution of micropitting damage during repeated cyclic loading of surface roughness asperities as they pass through the contact. Representative measurements of the surfaces used in the experimental work were then evaluated using a numerical model which combines a transient line contact micro-elastohydrodynamic lubrication (micro-EHL) simulation with a calculation of elastic sub-surface stresses. This model generated time-history of stresses within a block of material as it passes through the contact, based on the instantaneous surface contact pressure and traction at each point in the computing mesh at each timestep. This stress time-history was then used within a shear-strain-based fatigue model to calculate the cumulative damage experienced by the surface due to the loading sequence experienced during the experiments. The proposed micro-EHL model results and the experimental study were shown to agree well in terms of predicting the number of loading cycles that are required for the initial micropitting to occur.

Study on Dynamic Micro-elastohydrodynamic Lubrication with Surface Defects of Rollers in Rolling Bearings

Study on Dynamic Micro-elastohydrodynamic Lubrication with Surface Defects of Rollers in Rolling Bearings

Fractal Features and Surface Micromorphology of Unworn Surfaces of Rigid Gas Permeable Contact Lenses

ABSTRACTPurpose: The aim of this exploratory study was to investigate the micromorphology of surfaces of rigid gas permeable (RGP) contact lenses (CLs) using atomic force microscopy (AFM) followed by fractal analysis.Materials and methods: In order to characterize in a quantitative manner the micromorphology of surfaces of new and unworn RGP CLs made of twelve different materials, AFM was taken and then analyzed using fractal methods. Surface topography was sampled in an intermittent-contact mode in air, on square areas of 5 × 5 µm2 (MultiMode with Nanoscope V (Bruker). Spatial characteristics of 3-D surface texture were obtained using parameters defined in ISO 25178-2: 2012 norm.Results: The surface texture turned out to have complex 3-D nanoscale geometry. For quantitative characterization of the properties of surface geometry at nanometer level of CL on the global scale, a series of fractal parameters was used.Conclusions: Statistical and fractal parameters of 3-D surfaces can be used by manufacturers to assess the micromorphology of CLs in order to improve their 3-D surface texture characteristics. These parameters can also be used in an elastic–plastic finite element model with contact elements to simulate the friction, wear and micro-elastohydrodynamic lubrication at a nanometer scale between the CL with the corneal surface.

Morphological Properties of Siloxane-Hydrogel Contact Lens Surfaces

ABSTRACTPurpose: The aim of this study was to quantitatively characterize the micromorphology of contact lens (CL) surfaces using atomic force microscopy (AFM) and multifractal analysis.Materials and Methods: AFM and multifractal analysis were used to characterize the topography of new and worn siloxane-hydrogel CLs made of Filcon V (I FDA group). CL surface roughness was studied by AFM in intermittent-contact mode, in air, on square areas of 25 and 100 μm2, by using a Nanoscope V MultiMode (Bruker). Detailed surface characterization of the surface topography was obtained using statistical parameters of 3-D (three-dimensional) surface roughness, in accordance with ISO 25178-2: 2012.Results: Before wear, the surface was found to be characterized by out-of-plane and sharp structures, whilst after a wear of 8 h, two typical morphologies were observed. One morphology (sharp type) has a similar aspect as the unworn CLs and the other morphology (smooth type) is characterized by troughs and bumpy structures. The analysis of the AFM images revealed a multifractal geometry. The generalized dimension Dq and the singularity spectrum f(α) provided quantitative values that characterize the local scale properties of CL surface geometry at nanometer scale.Conclusions: Surface statistical parameters deduced by multifractal analysis can be used to assess the CL micromorphology and can be used by manufacturers in developing CLs with improved surface characteristics. These parameters can also be used in understanding the tribological interactions of the back surface of the CL with the corneal surface and the front surface of the CL with the under-surface of the eyelid (friction, wear, and micro-elastohydrodynamic lubrication at a nanometer scale).

Soft micro-elastohydrodynamic lubrication and friction at rough conformal contacts

Conformal surfaces in parallel sliding lack a macroscopic hydrodynamic pressure and fluid film formation mechanism. However, such a mechanism still exists on a microscopic level due to roughness. It is common to translate roughness into a variation of fluid film thickness which in turn yields a hydrodynamic pressure distribution resulting in a net hydrodynamic lift. Reynolds equation and a suitable cavitation algorithm suffice to describe this effect mathematically. In case one surface consists of a compliant material with low modulus of elasticity, the deformation of asperities due to pressures and shear stresses in the fluid cannot be neglected—in fact, besides cavitation, it significantly contributes to the net hydrodynamic lift. Therefore, a coupling between fluid dynamics and elastic solid body deformations needs to be introduced. An additional complication arises when the hydrodynamic lift and the subsequent separation of the mean lines of the contacting rough surfaces is not enough to prevent asperity contacts completely. This situation is known as mixed lubrication where part of the normal load is transmitted at asperity contacts. These contacts are commonly treated as solid body contacts with a Coulomb-like friction law or more sophisticated solid friction models. However, when considering asperities as contraformal Hertzian contacts, elastic deformation may allow for the existence of thin micro-elastohydrodynamic lubricant films preventing direct solid body contact even at speeds which otherwise would be regarded as deep within the mixed lubrication regime close to boundary lubrication. These films may not be able to prevent wear completely, but may reduce friction significantly in comparison to dry friction. In this paper, the existence of such effects is demonstrated both by simulation and by experiments with elastomeric radial lip seals.

Roughness evolution in mixed lubrication condition due to mild wear

Modelling of surface roughness evolution in time, as a function of the lubrication condition and other operating parameters (i.e. load, sliding/rolling ratio, and initial roughness of both contacting surfaces), is important for the understanding and prediction of various surface damage modes, such as surface distress, wear, and scuffing. Moreover, it is one of the key aspects in understanding friction and running-in phenomena. In the present study, a previously developed model for partial micro-elastohydrodynamic lubrication conditions is combined with a local Archard-type wear model to describe the surface topography evolution during the operation of a rolling/sliding lubricated contact. The model has been compared with a series of experimental results showing similar trends.

A parametric investigation on the microelastohydrodynamic lubrication of power law fluid lubricated line contact

A parametric study of microelastohydrodynamic lubrication contact between rough cylinder and plane lubricated with non-Newtonian power law fluid is carried out. A Gaussian rough surface is considered and modeled using stochastic approach. A theoretical solution of pressure distribution, fluid film thickness, and friction for given speeds, material properties, slide-to-roll ratio, and power law index, is obtained by solution of Reynolds equation using finite element method techniques. An isotropic roughness pattern is considered and results are obtained for wide range of operating parameters. The results indicate that the shear-thickening fluids provide thicker lubricant films as compared to shear-thinning fluids. The minimum fluid film thickness is observed to decrease with an increase of load and it increases with an increase of speed and material parameter. Slide-to-roll ratio has a marginal influence of the minimum film thickness. Under certain operating conditions, softer materials with shear-thickening lubricants yield thicker lubricant films than the harder materials lubricated with shear-thinning fluids. The coefficient of friction too is influenced by the rheology of lubricants with shear-thinning fluids providing larger values of coefficient of friction than the shear-thickening fluids.

Analysis of Micro-elastohydrodynamic lubrication of Angular Contact Ball Bearing Considering Dynamic Characteristics

Analysis of Micro-elastohydrodynamic lubrication of Angular Contact Ball Bearing Considering Dynamic Characteristics

Surface roughness effects in elastohydrodynamic lubrication: A review with contributions

Modern machine elements are required to work under increasingly severe conditions (thinner lubricating films, higher temperatures, less lubricant, etc.). Therefore, in heavily loaded lubricated contacts the effect of surface topography is now more relevant than say, 40 years ago. Yet, in the year 1977 the 4th Leeds-Lyon Symposium on Tribology was held with the theme ‘Surface Roughness Effects in Lubrication’! How much have we learnt since then? The objective of this paper is to discuss in detail the progress in micro-elastohydrodynamic lubrication (micro-EHL) and to give an overview of mainly modelling progress and understanding in the last 40 years. The paper starts with a quick review of the main methods, models and challenges in micro-elastohydrodynamic lubrication, from the days of the 4th Leeds-Lyon Symposium to current times. Then the focus shifts towards discussing in more detail the existing semi-analytical methodologies developed in recent times to model the micro-elastohydrodynamic lubrication problem. The global and local effects of roughness, the effects of sliding and the transition to dry contact will be included. Finally, examples of industrial applications will be presented where roughness plays a major role in function; at the end, new challenges in the modelling of surface roughness effects in elastohydrodynamic lubrication will be outlined, e.g. interactions with the subsurface material.

Analysis of Micro-Elastohydrodynamic Lubrication and Prediction of Surface Fatigue Damage in Micropitting Tests on Helical Gears

The paper describes results obtained from the micro-elastohydrodynamic lubrication (micro-EHL) modeling of the gear tooth contacts used in micropitting tests together with a contact fatigue and damage accumulation analysis of the surfaces involved. Tooth surface profiles were acquired from pairs of helical test gears and micro-EHL simulations were performed corresponding to surfaces that actually came into contact during the meshing cycle. Plane strain fatigue and damage accumulation analysis shows that the predicted damage is concentrated close to the tooth surfaces and this supports the view that micropitting arises from fatigue at the asperity contact level. A comparison of the micropitting performance of gears finish-ground by two alternative processes (generation-grinding and form-grinding) suggests that 3D “waviness” may be an important factor in explaining their different micropitting behavior.

A Multiscale Framework for EHL and Micro-EHL

In this article, a heterogeneous multiscale method is introduced to analyze the microelastohydrodynamic lubrication (micro-EHL) of bearings with topological features. Two scales are adopted in the analysis: the large-scale simulations describe the entire bearing domain, and the small-scale simulations describe the fluid–structure interaction (FSI) at the small-scale features. Conservation of mass and momentum of the lubricant and the bearing's elastic deformation are solved for. The relationship between the pressure gradient and mass flow is obtained from homogenized small-scale FSI simulations and applied on a global scale via a scattered data interpolation method. When the micro structure is periodic the exact model at micro scale is replaced by an effective derived equation, i.e., homogenized model. The elastic deformation of the textured bearing surface is addressed at both the large and small scales, by decomposing the displacement influence matrix into the diagonal terms and nondiagonal terms (sorted at the small scale and large scale, respectively). The multiscale method was demonstrated as being capable of modeling the global pressure and film thickness for a bearing with surface texture while maintaining the accuracy of the small-scale modeling features. The illustrative geometry was that of a linear converging pad bearing in two dimensions. The solutions were compared with those obtained using lubrication theory for the smooth surface case, and good agreement was obtained. The method was then demonstrated for geometries incorporating topographical features.

Modelling of elastohydrodynamic lubrication and fatigue of rough surfaces: The effect of lambda ratio

The article describes a numerical model for the analysis of elastohydrodynamic lubrication under conditions in which the nominal lubricant film thickness is small compared to the roughness present on the surfaces (lambda ratio less than unity). These conditions occur in most types of gear tooth contacts and in many other heavily loaded machine elements. In these situations, lubrication occurs at the roughness asperity level (micro-elastohydrodynamic lubrication), and in extreme cases ‘mixed’ lubrication behaviour occurs in which momentary solid contacts between the surfaces take place. In both micro-elastohydrodynamic lubrication and mixed lubrication regimes high, localised pressures are applied cyclically to asperity features as they move through the nominal contact leading to fatigue at the asperity level, which culminates in micropitting wear. Results of the modelling show the effect of lambda ratio in lubricated gear tooth contacts, and demonstrate the transition from full-film to micro-elastohydrodynamic lubrication and mixed lubrication, and the consequences in terms of predicted fatigue damage.