Thin Film Elastohydrodynamic Lubrication Research Articles

Effects of adsorption layers and elastic deformation on thin-film lubrication of circular contacts with non-Newtonian lubricants

PurposeThe modified Reynolds equation for non-Newtonian lubricant is derived using the viscous adsorption theory for thin-film elastohydrodynamic lubrication (TFEHL) of circular contacts. The proposed model can reasonably calculate the phenomenon in the thin-film lubrication (TFL) unexplained by the conventional EHL model. The differences between classical EHL and TFEHL with the non-Newtonian lubricants are discussed.Design/methodology/approachThe power-law lubricating film between the elastic surfaces is modeled in the form of three layers: two adsorption layers on each surface and one middle layer. The modified Reynolds equation with power-law fluid is derived for TFEHL of circular contacts using the viscous adsorption theory. The finite difference method and the Gauss–Seidel iteration method are used to solve the modified Reynolds equation, elasticity deformation, lubricant rheology equations and load balance equations simultaneously.FindingsThe simulation results reveal that the present model can reasonably calculate the pressure distribution, the film thickness, the velocity distribution and the average viscosity in TFL with non-Newtonian lubricants. The thickness and viscosity of the adsorption layer and the flow index significantly influence the lubrication characteristics of the contact conjunction.Originality/valueThe present model can reasonably predict the average viscosity, the turning point and the derivation (log film thickness vs log speed) phenomena in the TFEHL under constant load conditions.

POWER LAW FLUID MODEL INCORPORATED INTO THIN FILM ELASTOHYDRODYNAMIC LUBRICATION OF CIRCULAR CONTACTS

The modified Reynolds equation for power-law fluid is derived from the viscous adsorption theory for thin film elastohydrodynamic lubrication (TFEHL) of circular contacts. The lubricating film between solid surfaces is modeled as three fixed layers, which are two adsorption layers on each surface and a middle layer. The differences between classical EHL and TFEHL with power-law lubricants are discussed. Results show that the TFEHL power law model can reasonably calculate the pressure distribution, the film thickness, and the velocity distribution. The thickness and viscosity of the adsorption layer and the flow index significantly influence the lubrication characteristics of the contact conjunction.

A Layered-Rheology Model for Thin Film Elastohydrodynamic Lubrication of Circular Contacts

The modified Reynolds equation for power-law fluid is derived from the viscous adsorption theory for thin film elastohydrodynamic lubrication (TFEHL) of circular contacts. The lubricating film between solid surfaces is modeled as three fixed layers, which are two adsorption layers on each surface and a middle layer. The differences between classical EHL and TFEHL with non-Newtonian lubricants are discussed. Results show that the TFEHL power law model can reasonably calculate the pressure distribution, the film thickness, and the velocity distribution. The thickness and viscosity of the adsorption layer and the flow index influence significantly the lubrication characteristics of the contact conjunction.

The effect of nanoparticles on thin film elasto-hydrodynamic lubrication

Carefully conducted friction tests of a nano-lubricant in the thin film elasto-hydrodynamic lubrication regime showed that the presence of nanoparticles reduces friction. By using surface analyses techniques and molecular dynamics simulations, we explored the effectiveness of different interactions in the system, namely the interactions between nanoparticles with the lubricant or surfaces. Based on the results, the friction reduction mechanism was found to be that the nanoparticles induce an obstructed flow (plug flow) in the thin film between lubricated surfaces. This reduces friction by forcing only a few layers of lubricant molecules to slide on each other.

Effects of Surface Roughness and Surface Force on the Thin Film Elastohydrodynamic Lubrication of Circular Contacts

The effects of surface roughness and surface force on thin film elastohydrodynamic lubrication (TFEHL) circular contact problems are analyzed and discussed under constant load condition. The multi-level multi-integration (MLMI) algorithm and the Gauss-Seidel iterative method are used to simultaneously solve the average Reynolds type equation, surface force equations, the load balance equation, the rheology equations, and the elastic deformation equation. The simulation results reveal that the difference between the TFEHL model and the traditional EHL model increase with decreasing film thickness. The effects of surface forces become significant as the film thickness becomes thinner. The surface forces have obvious effects in the Hertzian contact region. The oscillation phenomena in pressure and film thickness come mainly from the action of solvation forces

Effects of surface forces and surface roughness on squeeze thin film of elastohydrodynamic lubricated spherical conjunction

ABSTRACTThe pure squeeze thin film elastohydrodynamic lubrication motion of circular contacts with effects of surface forces and surface roughness taken into account is explored under constant load conditions. The coupled transient stochastic Reynolds, elasticity deformation, the load balance, surface forces (hydrodynamic, solvation and van der Waals pressure) and lubricant rheology equations were solved simultaneously by using the finite difference method and the Gauss–Seidel iteration method. The simulation results reveal that the differences between radial type roughness and circular type roughness problems are apparent as the film thickness is thinner than 5 nm. The oscillation phenomena in pressure and film thickness come mainly from the action of solvation forces. The effects of surface forces become significant as the film thickness becomes thinner. The film thickness with circular type roughness is thicker than that with radial type roughness. Copyright © 2012 John Wiley & Sons, Ltd.

Effects of Electric Double Layer on Pure Squeeze Motion of Circular Contacts - An Elastohydrodynamic Lubrication Model

In this paper, the pure squeeze thin film elastohydrodynamic lubrication (TFEHL) motion of circular contacts with electric double layer (EDL) is explored under constant load condition. The coupled transient modified Reynolds, elasticity deformation, the load balance, and lubricant rheology equations were solved simultaneously by using the finite difference method and the Gauss-Seidel iteration method. The simulation results reveal that the effect of electric double layer produces an obvious increase in the film thickness and apparent viscosity. The greater the Zeta-potential, the greater the apparent viscosity, and the greater the film thickness.

The Establishment and Numerical Calculation of Thin Film Lubrication Model

Thin film lubrication (TFL) and elasto-hydrodynamic lubrication (EHL) are different lubrication states, their lubrication mechanisms are different, but the transition from EHL to TFL is a continuous process at the aspect of film thickness change, it is important to establish a physical-numerical model that can be suited for both EHL and TFL. Based on rheological, the change of micro thickness is expressed by the change of viscosity, then the micro problem of TFL can be turn to macro problem. The adsorption layer is divided into two parts formatted by static contact and shear force in the calculation. In a given static adsorption film, other oil film parts will be comprehensive consideration, modified formula of the overall average equivalent viscosity is gained by the method of combining numerical calculation and experiment, then numerical model is established for both EHL and TFL. TFL and EHL typical characteristics and the effectiveness of the model can be obviously shown by the results.

New film thickness formula for shear thinning fluids in thin film elastohydrodynamic lubrication line contacts

A new central film thickness formula pertaining to thin film elastohydrodynamic lubrication (EHL) line contacts has been developed for Carreau-type shear-thinning lubricants using an extensive set of full EHL simulations. The shear-thinning correction factors available in the literature are based on EHL simulations carried out for a particular value of load and piezo-viscous coefficient using the simplest exponential pressure–viscosity relationship. In the present work, therefore, the load and piezo-viscous coefficient are varied over a wide range so as to arrive at a more generic and accurate film thickness equation. Also, the pressure–viscosity relation employed herein is the Doolittle's free volume-based viscosity model, which is capable of replicating the experimental pressure–viscosity data with a high degree of accuracy. It has been demonstrated that the present film thickness equation accurately captures the recently discovered scale and load sensitivity effects.

Thin film elastohydrodynamic lubrication of circular contacts at pure squeeze motion

PurposeThe purpose of this paper is to explore the pure squeeze thin film elastohydrodynamic lubrication (TFEHL) motion of circular contacts with adsorption layers attached to each surface under constant load condition. The proposed model can reasonably calculate the pressure distributions, film thicknesses, normal squeeze velocities, and effective viscosities during the pure squeeze process under thin film lubrication.Design/methodology/approachThe transient modified Reynolds equation is derived in polar coordinates using viscous adsorption theory. The finite difference method and the Gauss‐Seidel iteration method are used to solve the transient modified Reynolds equation, the elasticity deformation equation, load balance equation, and lubricant rheology equations simultaneously.FindingsThe simulation results reveal that the thickness of the adsorption layer and the viscosity ratio significantly influence the lubrication characteristics of the contact conjunction in the thin film regime. In additional, the turning points in the film thickness which distinguish thin film lubrication from elastohydrodynamic lubrication curve is found. In thin film region, the effective viscosity predicted by present model is better than that predicted by traditional elastohydrodynamic theory.Originality/valueThe paper develops a numerical method for general applications with adsorption layers attached to each surface to investigate the pure squeeze action in a TFEHL spherical conjunction under constant load condition.

Effects of surface forces on pure squeeze thin film EHL motion of circular contacts

The pure squeeze thin film elastohydrodynamic lubrication (thin film EHL) motion of circular contacts with effects of surface forces taken into account is explored under constant load conditions. The difference between thin film EHL model and EHL model is apparent as the film thickness is thinner than 5 nm. The oscillation phenomena in pressure and film thickness come mainly from the action of solvation forces. The effects of surface forces become significant as the film thickness becomes thinner. Moreover, the viscosity is oscillatory due to its dependency on the hydrodynamic pressure which is affected by surface forces.

Rheological Characteristics for Thin Film Elastohydrodynamic Lubrication with Non-Newtonian Lubricants

AbstractThe modified Reynolds equation for power law fluid is derived from the viscous adsorption theory for thin film elastohydrodynamic lubrication (TFEHL). The differences between classical non-Newtonian EHL and non-Newtonian TFEHL are discussed. Results show that the proposed model can reasonably calculate the pressure distribution, the film thickness, the velocity distribution and the average viscosity under thin film lubrication. The thickness (δ), the viscosity (m1), and the flow index (n1) of the adsorption layer influence significantly the lubrication characteristics of the contact conjunction. Furthermore, the film thickness increases with the increase of n1 and the film thickness affected by m1 is greater than that affected by n1, but the effect of n1 produces a very small difference in the pressure distributions. In addition, the greater n1, the smaller the change of velocity distribution in the adsorption layer, and the greater the change of velocity distribution in the middle layer. The larger δ and n1, the larger the deviation on log (film thickness) vs. log (speed) produced in the very thin film regime. In the region of the flow index ratio between 1.0 and 1.3, the difference in film thickness is significant. When the flow index of the adsorption layer is 1.6 times greater than the flow index of the middle layer, the adsorption layer is generally looked upon as a “solid-like”.

Thin film elastohydrodynamic lubrication—a power-law fluid model

A 1-D modified Reynolds equation for power-law fluid is derived from the viscous adsorption theory for thin film elastohydrodynamic lubrication (TFEHL). The lubricating film between solid surfaces is modeled as three fixed layers, which are two adsorption layers on each surface and a middle layer between them. The comparisons between classical non-Newtonian EHL and non-Newtonian TFEHL are discussed. Results show that the TFEHL model can reasonably calculate the pressure distribution, the film thickness, the velocity distribution and the average viscosity. The thickness and viscosity of the adsorption layer and the flow index influence the lubrication characteristics of the contact conjunction significantly. The film thickness increases with the increase of flow index. As the flow index becomes greater, the dimple in the film shape moves towards the center of the contacts. The effect of flow index produces an obvious difference in the pressure distribution. The greater the flow index, the greater the pressure spike, and the pressure spike tends to move toward the center. The larger the flow index, the more the velocity varies in both the middle layer and adsorption layers along the Z-axis. The greater the thickness and viscosity of the adsorption layer and the flow index, the greater the deviation in central film thickness versus speed between EHL model and TFEHL model produced in the very thin film regime.

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.

Molecular dynamics characterization of thin film viscosity for EHL simulation

Molecular simulations were used to characterize changes in lubricant viscosity that may occur during thin film elastohydrodynamic lubrication (EHL). Molecular dynamics simulations were performed at variable wall speed and film thickness such that the effects of both parameters could be evaluated. Using this approach it was found that the viscosity of thin films under large shear is subject to both shear thinning and oscillation with film thickness. A composite model was developed that incorporated both effects. The expected impact that this model might have on an EHL interface was evaluated using a continuum simulation. An overall decrease in viscosity with some oscillation near the interface edges was predicted due to the molecularly modeled thin film effects.

Rheological Characteristics for Thin Film Elastohydrodynamic Lubrication

ABSTRACTThis paper uses three lubrication models to explore the differential phenomenon in the status of thin film lubrication (TFL). According to the viscous adsorption theory, the modified Reynolds equation for thin film elastohydrodynamic lubrication (TFEHL) is derived. In this theory, the film thickness between lubricated surfaces is simplified as three fixed layers across the film, and the viscosity and density of the lubricant vary with pressure in each layer. Under certain conditions, such as a rough or concentrated contact of a nominally flat surface, films may be of nanometer scale. The thin film elastohydrodynamic lubrication (EHL) analysis is performed on a surface forces (SF) model which includes van der waals and solvation forces. The results show that the proposed TFEHL model can reasonably calculate the film thickness and the average relative viscosity under thin film EHL. The adsorption layer thickness and the viscosity influence significantly the lubrication characteristics of the contact conjunction. The differences in pressure distribution and film shape between surface forces model and classical EHL model were obvious, especially in the Hertzian contact area. The solvation force has the greatest influence on pressure distribution.

Inverse approach for estimating rheological characteristics of adsorption layer in thin film EHL contacts

A modified Reynolds equation is derived for thin film elastohydrodynamic lubrication (TFEHL) by means of the viscous adsorption theory. This TFEHL theory can be used to explain the deviation between the measured film thickness and that predicted from the convenient elastohydrodynamic lubrication (EHL) theory under very thin film conditions. Results show that the thinner the film, the greater the ratio of the adsorption layer to the total film thickness becomes, and the greater the value of the pressure–viscosity index ( z′). An inverse approach is proposed to estimate the pressure distribution based upon the film thickness measurement and to determine the pressure–viscosity index of oil film, and the thickness ( δ) and the viscosity ratio ( η *) of the adsorption layer in TFEHL circular contacts. Based on TFEHL theory, the inverse approach can reduce z′ error, and provides a reasonably smooth curve of pressure profile by implementing the measurement error in the film thickness. This algorithm not only estimates the pressure, but also calibrates the film shape. Consequently, it predicts z′, η *, and δ with very good accuracy. It can also be used to evaluate the lubrication performance from a film thickness map obtained from an optical EHL tester. Results show that the estimated value of z′ is in very good agreement with the experimental data.

Surface Topography and Fatigue Life of Rolling Contact Bearing

The contact stresses of bearings were analyzed by a micro-macrocontact model in which the macrocontact was elastic, and the microcontact was elastic-plastic. Subsurface stress maps were calculated for real contact surfaces, by including roughness, waviness, and profile. The predicted subsurface stress maps display the local stress levels for various elastohydrodynamic lubrication (EHL) film thicknesses and show why enhanced-finish bearings have longer lives than the standard-finish bearings. It is believed that the present approach provides an alternative to the empirical methods that are used for predicting life under thin-film EHL conditions. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Diego, California, October 19–21, 1992

The Measurement and Study of Very Thin Lubricant Films in Concentrated Contacts

Optical interferometry is now a widely used technique for measuring the separating film thickness in model rolling and sliding elastohydrodynamic contacts. There are two limitations of the method as conventionally employed: first, it cannot easily be used to accurately measure films less than one quarter the wavelength of visible light, i.e. less than about 100 nm. Secondly, only certain, discrete thicknesses, spaced at least 50 nm apart can be determined. This paper describes work aimed at overcoming these limitations so as to make optical interferometry applicable to the study of boundary or very thin film elastohydrodynamic lubrication in rolling contacts. A combination of a solid spacer layer with spectrometric analysis of reflected light from the contact enables very thin lubricant films to be accurately measured. The approach is applied to the study of thin films formed in rolling contacts by low viscosity lubricants. Some anomalies in the relationship between film thickness and speed are found with...

A unified treatment of thick and thin film elastohydrodynamic problems by using higher order element methods

A unified mathematical treatment of thick and thin film elastohydrodynamic lubrication problems is presented. The construction of approximate solutions is discussed by using Ritz-Galerkin methods with piecewise polynomial basis functions. In particular for the line contact problem, smooth Hermite and cubic spline spaces are used to obtain solutions over a wide load-speed range. Pressure ‘spikes’ are obtained.