Contact Elastohydrodynamic Lubrication Research Articles

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.

Measurement of EHL temperature by thin film sensors – Thermal insulation effects

The temperature of elastohydrodynamically lubricated (EHL) contacts strongly influences fluid friction. The higher the EHL temperature, the lower is the effective lubricant viscosity and hence shear stress. EHL contacts with Diamond-like Carbon (DLC) coatings have shown lower fluid friction than EHL contacts without DLC coatings. A strong thermal insulation effect resulting in higher EHL temperatures is indicated. This has been confirmed by theoretical investigations, but rarely validated experimentally. Hence, EHL temperatures were measured using thin film sensors for different bulk and coating materials on a twin-disk test rig. The measurements were analyzed by a thermal EHL (TEHL) simulation model. Simulated and measured EHL temperatures are in very good accordance and confirm the significance of thermal insulation effects.

Influence of Lubricant Pressure Response on Subsurface Stress in Elastohydrodynamically Lubricated Finite Line Contacts

In order to adapt to increasingly stringent CO2 regulations, the automotive industry must develop and evaluate low cost, low emission solutions in the powertrain technology. This often implies increased power density and the use of low viscosity oils, leading to additional challenges related to the durability of various machine elements. Therefore, an increased understanding of lubricated contacts becomes important where oil viscosity–pressure and compressibility–pressure behavior have been shown to influence the film thickness and pressure distribution in elastohydrodynamic lubrication (EHL) contacts, further influencing the durability. In this work, a finite line EHL contact is analyzed with focus on the oil compressibility–pressure and viscosity–pressure response, comparing two oils with relatively different behavior and its influence on subsurface stress concentrations in the contacting bodies. Results indicate that increased pressure gradients and pressure spikes, and therefore increased localized stress concentrations, can be expected for stiffer, less compressible oils, which under transient loading conditions not only affect the outlet but also the edges of the roller.

Mapping of the lubrication regimes in rough surface EHL contacts

Abstract Understanding film formation in rough surface elastohydrodynamically lubricated (EHL) contacts have been an ongoing pursuit in lubrication science for more than half a century. This study furthers that quest by establishing a single combined friction and electrical contact resistance map that forms a clear and comprehensive overview of the lubrication performance. A ball-on-disc machine was operated under a wide variety of heavily loaded rolling/sliding contact conditions. Results show that while sweeping the contact over the SRR- and entrainment speed-domain, the primary sweep direction significantly affects running-in and consequently the transition from EHL to the mixed lubrication regime. Such knowledge sheds new light into the mechanisms that governs EHL film formation and the concurrent interplay with the mixed lubricated friction coefficient.

An Isothermal Elastohydrodynamic lubrication of elliptical contact with Multigrid method

ABSTRACT The paper presents steady isothermal elastohydrodynamic lubrication (EHL) of elliptical contact with Roelands pressure–viscosity model over varying rotational speed. The governing Reynolds and film-thickness equation for elliptical contact in roller bearings are analysed by using multigrid method with full approximation scheme. The non-dimensional pressure distribution and film-thickness for elliptical contacts are obtained. The distinct friction models appropriate to lubricants in contact regions are predicted. The result shows that rolling contact weariness does not occur on the surfaces with longitudinal roughness and wear or scraping exists in circular EHL contacts with entrainment along major axis to that of minor axis. The computation predicts that minimum film thickness, central film thickness and elastic deformation increase as the rotational speed increases. The computed results are compared with earlier finding which are in good agreement.

Temperature Estimations in Elastohydrodynamic Contacts of Tribochemical Processes: A Comparative Study Using Modeling by Mechanical/Wear Process Mechanisms and Chemical Kinetic Modeling

Abstract This article deals with the subject of thermal Elastohydrodynamic Lubrication (EHL) and how temperatures can be estimated in elastohydrodynamic contacts. An attempt has been made to explain why it is essential to study temperature effects in EHL, and the process of thermal EHL modeling has been elucidated by solving a simple thermal EHL model. The temperatures obtained from this simple model seem to agree well with published literature. A comprehensive literature review of the numerous studies in thermal EHL modeling with particular emphasis on recent developments in this field has been presented. The second part of this comprehensive study includes a comparison between temperatures in the EHL contact as obtained by mechanical models and a chemical model, which will be discussed. The contact temperature required to consume the lubricant by oxidation during a microsample wear test is determined by a chemical kinetic model. Finally, the discrepancy between the temperatures obtained by both the chemical kinetic model and the mechanical model is discussed, presenting various plausible explanations to account for the temperature difference.

Modelling three-dimensional soft elastohydrodynamic lubrication contact of heterogeneous materials

Abstract This paper presents an approach for modelling three-dimensional soft elastohydrodynamic lubrication (EHL) contact of heterogeneous materials. In this approach, a hydrodynamic interface element is developed to achieve the coupling of elastic deformation of heterogeneous contacting bodies and the steady-state hydrodynamic lubrication at the contact interface. The deformation behaviors are simulated by the finite element method, while the hydrodynamic lubrication is dealt with by solving the Reynolds equation. This approach can enable the simulation of soft EHL contact between bodies with heterogeneous materials and complicated geometric configurations. Predicted lubrication performances of soft EHL contacts show good agreement with the experimental measurements reported in literature. Analysis and discussion are also conducted on various cases of soft EHL contact between an elastic ball and a coating-substrate system with or without inhomogeneities.

Elastohydrodynamic lubrication for the finite line contact under transient loading conditions

Abstract Research related to elastohydrodynamic lubrication (EHL) has led to improved performance and durability of machine elements where non-conformal contact geometries interact. Only a relatively small portion of the EHL literature has, however, dealt with the lubricating performance of finite line contacts under non-steady conditions, commonly found in many practical applications. The purpose of this work has thus been to further understand the behaviour of finite line EHL contacts under transient conditions by studying a finite length roller subjected to a time varying load using a full-system finite element approach. The transient load was shown to initiate oscillations in the system, governed by waves of lubricant moving through the contact, affecting both pressure and film thickness throughout the contact.

Thermal Elastohydrodynamic Lubrication of Ceramic Materials

ABSTRACTThe effect of thermal conductivity on point elastohydrodynamic lubrication (EHL) contacts was discussed with representative engineering ceramics and steel by a non-Newtonian thermal EHL analysis. Through this investigation, fundamentals of EHL characteristics of contact surfaces composed of different thermal conductivities were generally revealed and a combination of optimum ceramic materials has been proposed. Furthermore, when the contacting materials are the same, it has been pointed out that the equivalent elastic modulus should be rather small and the thermal conductivity of the contact material should be high to obtain a thick overall film thickness and low film pressure.

Influence of oil film stiffness on natural characteristics of single-rotor three-input helicopter main gearbox

The elastohydrodynamic lubrication (EHL) contact model is established, the oil film stiffness is calculated based on Dowson-Higginson empirical minimum thickness equation. The vibration model of single-rotor three-input helicopter main reducer is proposed by lumped mass method, and nonlinear factors like oil film stiffness, dynamic meshing force are taken into account. The influence of oil film stiffness on system natural frequency is analyzed as well. The results show that oil film stiffness of the internal and external meshing pairs in the planetary gear system have greater impact on the natural frequency, which tends to destabilize the system. Therefore, the planetary gear train is the most crucial branch regarding the system splash lubrication. When all the oil film stiffness in the system are greater than 4×109 N/m, the natural frequencies tend to be stable. This study can provide the theoretical reference for the lubrication characteristics in the single-rotor multi-input helicopter.

Tribological behavior of coated spur gear pairs with tooth surface roughness

Coating is an effective way to reduce friction and wear and to improve the contact-fatigue lives of gear components, which further guarantees a longer service life and better reliability of industrial machinery. The fact that the influence coefficient linking the tractions and stress components could not be expressed explicitly increases the difficulty of coated solids contact analysis. The complicated tribological behavior between tooth surfaces influenced by lubrication and surface roughness further adds difficulty to the coated gear pair contact problems. A numerical elastohydrodynamic lubricated (EHL) contact model of a coated gear pair is proposed by considering the coupled effects of gear kinematics, coating properties, lubrication, and surface roughness. The frequency response function and the discrete convolute, fast Fourier transformation (DC-FFT) method are combined to calculate the surface deformation and the subsurface stress fields at each meshing position along the line of action (LOA). The Ree-Eyring fluid is assumed to incorporate the non-Newtonian effect, which is represented in the generalized Reynolds equation. Influences of the ratio between the Young’s modulus of the coating and the substrate on the contact performance, such as pressure, film thickness, tooth friction coefficient, and subsurface stress field, are studied. The effect of the root mean square (RMS) value of the tooth surface roughness is studied by introducing the roughness data, deterministically measured by an optical profiler.

Regulation of lubricant supply by wettability gradient in rolling EHL contacts

Abstract Starvation often occurs in elastohydrodynamic lubricated (EHL) contacts due to limited lubricant replenishment at high speeds or restricted fresh oil charging. This work presents several conceptual studies for regulating lubricant replenishment/supply by means of wettability gradient, which is generated by an oleophilic track bounded by two oleophobic zones on solid surface. It was demonstrated that when the EHL contact was within the central oleophilic track, the wettability gradient on the two sides could drive the lubricant to form a stable oil reservoir leading to thick EHL films. Nevertheless, if there was a single step wettability gradient, the lubricant could be driven away easily from the lubrication track, leading to starvation despite a large amount of lubricant provided.

Analysis of Lubrication Characteristic of Single Droplet in the Elastohydrodynamic Lubrication Contact

Analysis of Lubrication Characteristic of Single Droplet in the Elastohydrodynamic Lubrication Contact

Observation on the deformation of dimpled surface in soft-EHL contacts

Abstract In order to study the deformation of a textured surface in soft elastohydrodynamic lubrication (EHL) contacts, observation of the deformed surface around the dimple was realized and conducted under a sliding condition with various loads by using optical interferometry. The results show that uneven deformation emerges around the dimple. Both the applied load and sliding velocity have significant influence on the deformation of the soft surface, especially on the deformation pattern. Contrary to the usual assumption, the effect of shear strains in the soft slider on the total surface deformation is increasingly noticeable, and it results in unexpectedly deformation patterns around the dimple.

Numerical study of surface dimple phenomenon by starvation in simple sliding contact

PurposeThis study aims to use a thermal elastohydrodynamic lubrication (EHL) algorithm incorporating an Eyring flow model to solve a steady-state contact in simple sliding motion.Design/methodology/approachA theoretical model was used to investigate the effect of starvation on the surface dimple phenomenon by gradually reducing the thickness of the inlet oil layer.FindingsThe increase in the starvation degree reduces the dimple depth, film thickness, the pressure peak and the temperature rise. Under the severe starvation condition, the dimple is eliminated so that the EHL contact becomes partly parched. In elliptical results, for the same starvation parameters, the oil replenishment is stronger than that in circular contact.Originality/valueThis paper fulfils an exploration to study how the oil starvation influences the surface dimple phenomenon.

Experimental study of elastohydrodynamic lubrication behaviour under single oil droplet supply

In this study, an oil droplet passing through an elastohydrodynamic lubrication contact is observed via interferometry, and the effects of droplet size, entrainment speed, load, and slide/roll ratio (SRR) are studied. The results show that the surface area of the droplet is enlarged because of the squeeze spread and capillary force at the inlet region, which influence the film growth. With a large droplet size, the spreading diameter increases and a thick film is generated. When the entrainment speed increases, the surface area enlargement of the oil droplet at the inlet is found to be insufficient, and only a local dimple-shaped film is formed. The SRR has a direct effect on the lubricant distribution. In particular, the load and viscosity effects exhibit different characteristics compared with the case of typical fully flooded lubrication.

Traction and local temperatures measured in an elastohydrodynamic lubrication contact

Traction in highly loaded elastohydrodynamic contacts is of great importance to reduce losses. The calculation of traction in these EHL contacts relies on the rheological models used. In this paper, results from traction experiments, which form an integration over Hertzian contacts with strongly inhomogeneous conditions, are presented. They are compared to data from laboratory measurements with homogeneous conditions. Due to the fact that the integral data do not directly represent local rheological fluid properties, further investigations are presented. Here, thermographic measurements are used to discern the contact temperature locally. Furthermore, a model for the maximum shear stress depending on pressure is proposed and compared to existing models.

Effect of oil starvation on the surface dimple in elastohydrodynamic lubrication contact under opposite sliding conditions

Optical interferometry experiments were performed to investigate the dimple phenomena in starved elastohydrodynamic lubrication of point contacts under opposite sliding condition by using a high viscosity polybutene (PB) lubricant. The effects of the bilateral starvation, left starvation, and right starvation on the variation of the oil film under a high slide-to-roll ratio were investigated. It was found that oil starvation could generate a starved constriction, which would make the shape of the oil film resemble the one in pure-rolling or rolling-sliding contact. However, the film thickness at the starved constriction is much lower. In addition, the influence of the applied load on the variation of the surface dimple in fully flooded and starved conditions was explored. It was found that the effect of oil starvation depended on the way the net mass flow through the contact zone. When the net mass flow was from left to right, a major constriction was formed on the right side, and vice versa. When symmetrically increasing the degree of oil starvation, the classical big dimple shrunk and became centralized before eventually disappearing and the contact approximated the nominally flat state of a heavily starved or almost dry contact.