Elastohydrodynamic Lubrication Film Research Articles

Effect of Sliding Direction on EHL Film Shape Under High Sliding Conditions

ABSTRACTIt is well known that a sliding speed influences a lubricant film thickness of elastohydrodynamic rolling–sliding contacts significantly. The effect of sliding is described quite well for unidirectional rolling and sliding; however, there are a limited number of papers dealing with sliding in different directions. This study describes how the sliding direction influences elastohydrodynamic film shape under high sliding conditions. An optical ball-on-disc tribometer together with thin-film colorimetric interferometry method is used for a film thickness measurement. The results show that the sliding direction influences lubricant film shape and the effect is connected with dimple phenomena. The temperature–viscosity wedge effect is discussed as a possible mechanism. The results are important for a film thickness prediction under high sliding conditions and provide experimental evidence for an extension of elastohydrodynamic lubrication (EHL) theory.

Factors Affecting Surface Roughness Measurement Results, and the Impact on Predicted Life of Elastohydrodynamic Lubricated Rolling-Element Bearings

This work investigates the sensitivity of the surface roughness lubricant life adjustment factor for rolling-element bearings to variations in measured roughness. Roughness measurement results using different values of stylus tip dimension and short wavelength cutoff filter were obtained for several bearing raceway surfaces and used as the inputs for a lubricant life adjustment factor based on the lambda ratio—the ratio of elastohydrodynamic lubrication (EHL) film thickness to the composite surface roughness. The effect on predicted bearing life is shown to be significant. As one means of verification of surface roughness measurements, an ISO type D random/repeating precision roughness specimen was also investigated and shown to have excellent sensitivity to stylus tip dimension, demonstrating its effectiveness as a tool for assessment of stylus radius and condition.

Experimental and Numerical Investigation of Inlet Dimple in Point and Elliptical Contact Elastohydrodynamic Lubrication Films

The experimental observation of the inlet dimple of the elastohydrodynamic lubrication (EHL) film was carried out in point and elliptical contacts, respectively, and then, the numerical analysis under point contacts was conducted. The inlet dimple appears and then moves upstream toward the inlet and finally disappears with the increase of the entrainment speed. In the meantime, the dimple depth increases at the beginning and then decreases. The high load leads to a wide entrainment speed range where dimple exists. The varying range of the entrainment speed corresponding to the dimple appearance is smaller at a smaller included angle between the minor axis of elliptical contact and the entrainment direction.

Pressure–viscosity response in the inlet zone for quantitative elastohydrodynamics

The descriptions employed in classical elastohydrodynamic lubrication (EHL) of the piezoviscous effect at low pressures characteristic of the inlet zone are simply inaccurate for low viscosity liquids. This article offers the 1952 McEwen equation for accurately describing the pressure–viscosity effect at low inlet pressures for those liquids with sufficiently high inflection pressure that the faster-than-exponential response does not significantly affect the viscosity in the inlet. The parameters of this model have significance for the classical EHL film thickness formulas and these parameters are listed for several low viscosity liquids including a refrigerant.

H-20 弾性流体潤滑膜における蛍光スペクトルの測定

H-20 弾性流体潤滑膜における蛍光スペクトルの測定

Effects of lubricant viscoelasticity on film thickness in elastohydrodynamic line contacts during start-up

By means of numerical simulation the elastohydrodynamic lubricating film formation during start-up of motion of a line contact from rest is studied. The transition from solid contact to lubricated contact is of importance in investigating the start-up process and its effects on bearing performance. The numerical results of Holmes et al. (2002) and others, concerning point contact and Newtonian lubricant, are not in good agreement with the known experimental data. The numerical film thickness is significantly smaller than the experimental values and the experimental velocity at which lubricant travels through the conjunction does not equal the entrainment velocity of numerical results. In the current paper, a lubricant with viscoelastic properties is considered. The Maxwell model of a lubricant at pure rolling of elastic cylinders is used. Also, as well as in work of Usov (2008) the model assumes the division of the whole area of contact into three zones: the dry contact of surfaces, the transient zone and the zone in which the surfaces are separated by a liquid lubricant layer. The rigid body model is assumed for the lubricant in the transient zone. The effects of viscoelasticity and size of transient zone on film thickness and the velocity at which the lubricant moves through the contact are studied. It is shown that viscoelasticity raises film thickness in a start of the motion and lowers the velocity at which the lubricant moves through the contact. This velocity is lower than the entrainment velocity in the first half of the contact, as in the experimental results with point contact. In the second half of the contact, this velocity may be lower or higher than the entrainment velocity depending on the size of the transient zone. The film thickness may increase considerably at the start of motion when the size of the transient zone increases.

Film Forming Behavior of Greases Under Starved and Fully Flooded EHL Conditions

ABSTRACTImproving knowledge on the film forming behavior of greases is essential to be able to develop efficient greases. This article examines how operating conditions (e.g., temperature, lubrication condition [fully flooded/starved]) and base oil viscosity influence the film forming properties of greases by comparing the behavior of two lithium-based greases and their respective base oils in rolling point contact. It is found that the onset and degree of starvation is controlled by speed (u) × viscosity (ν)/load (W) factor (uν/W) and temperature and that low uν/W values promote entrainment of thickener into contact. Thus, grease with low base oil viscosity shows significant thickener entrainment in the low speed region compared to the one with high base oil viscosity, which leads to the formation of thickener-rich viscous material during extended running with the low base oil viscosity grease. The results suggest that the shape of the film thickness versus speed curve is viscosity and uν/W range dependent. Furthermore, for the test conditions used in this study, grease-lubricated contacts appear to shift from the initial fully flooded condition to starved condition over a prolonged running of 2 h. The results from this study concur with those reported in the literature that fully flooded oil elastohydrodynamic lubrication (EHL) theory or film thickness cannot be directly applied or taken as a guideline in grease-lubricated contacts.

Film Thickness and Rolling Resistance in Starved Elastohydrodynamic Lubrication of Point Contacts With Reflow

Elastohydrodynamic lubrication (EHL) film thickness and rolling resistance play a critical role in determining friction, wear, life, and other tribological characteristics of rolling bearings. Although film thickness formulas are widely used and experimentally verified, accurate prediction of the film thickness is still difficult under starved conditions. This paper presents a numerical study of starved EHL point contacts using a nonuniform inlet film thickness obtained from a modified Coyne–Elrod boundary condition. An experimental verification of the numerical results is also presented. Based on the results of a parametric study, inlet distance formulas are obtained as a function of the initial film thickness, the fully flooded central film thickness, and the capillary number. By using the inlet distance formulas and the Hamrock–Dowson formulas, the central film thickness, the minimum film thickness, and the viscous rolling resistance can be calculated.

Understanding micropitting in gears

Micropitting is a serious form of erosive wear, which can occur on the teeth of transmission gears. It is associated with roughness effects and surface fatigue and has become a particular problem in the speed-increasing gearboxes of wind turbines. This paper reviews the contributions which the authors have made towards an understanding of the basic mechanism of micropitting in gears based on analysis of the contact mechanics and elastohydrodynamic lubrication (EHL) of gear tooth surfaces under realistic operating conditions. Results are presented which demonstrate the crucial influence of EHL film thickness in relation to roughness (the ‘Λ ratio’) on predicted contact and near-surface fatigue. The important effect of plastic deformation, which takes place during the initial stage of running-in of gears, has also been investigated, and it has been shown that significant residual effects occur, which can contribute to the early formation of surface-initiated cracks.

Elastohydrodynamic lubrication properties and friction behaviors of several ester base stocks

Abstract This paper reports a series of studies on the lubricant properties, elastohydrodynamic film thickness, and coefficients of friction of several commercially available ester base stocks, i.e., diisooctyl phthalate (DIOP), diisodecyl phthalate (DIDP), diisotridecyl phthalate (DITDP), diisooctyl sebacate (DOS), diisotridecyl sebacate (DTDS), trihydroxymethylpropyl trioleate (TMPTO), and pentaerythritol tetraoleate (PETO). The results include densities and viscosities from 303 to 398 K, and elastohydrodynamic lubricant film thicknesses and friction in the boundary, mixed and full-film lubrication regimes measured at several temperatures, loads, and speeds. These ester base stocks have different lubrication abilities owing to their chain lengths, geometric configurations, and molecular rigidity. This study provides quantitative insight into the use of ester-based lubricants for low friction through the entire lubrication regime (boundary to full film) by utilization of suitable type and size of the ester base stocks.

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.

Tilting 상태인 테이퍼 로울러의 탄성유체윤활 해석

Tapered roller bearings are widely used in equipment where high combined thrust and radial loads are experienced. A certain amount of tilting between the tapered rollers and the races always occurs because of bending moment load conditions and shaft deflection. It is now well understood that a coherent elastohydrodynamic lubrication (EHL) film separates the rollers and races. In spite of extensive study on EHL problems for over half a century, relatively few studies have focused on the finite line contacts problem. This study presents a complete numerical analysis of the effects of roller tilting on the EHL characteristics in a tapered roller bearing. We systematically analyze this highly nonlinear problem using finite differences with fully non-uniform grids and the Newton-Raphson method. Detailed EHL pressure distributions and film shapes are presented under moderate loads and material parameters. A very small roller tilting significantly affects the pressure distributions and film shapes near both ends of the roller. Moreover, the effect of tilting on the EHL characteristics at the small end is much greater than that at the large end. Therefore, in designing optimum profiles for tapered roller bearings, the profile radius should be larger at the small end.

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.

Transient Elastohydrodynamic Lubrication Film Thickness During Normal Approach Considering Shear-Thinning and Linear Piezoviscous Oils

Transient film thickness behavior is investigated using full elastohydrodynamic lubrication (EHL) line contact simulations during film collapse due to sudden halt and impact loading. Due attention is given to realistic shear-thinning behavior and comparisons are made with a largely ignored class of EHL lubricants that exhibit linear pressure–viscosity dependence at low pressures. The EHL film collapse is found to be governed by the piezoviscous response and the linear P–V oils exhibit rapidly collapsing EHL entrapment. Under impact loading, the transient film thickness deviates markedly from the corresponding steady-state behavior and this departure is a function of lubricant rheology.

Calculation of Spur Gear Dynamic Transmission Error in Consideration of the Progressive Engagement of Compliant Profile-modified Teeth

A simple yet accurate model is developed for the dynamical simulation of profile-modified gears, considering the effects of progressive tooth engagement, stiffness, elastohydrodynamic lubricant film formation and hysteresis. The real path of contact, stiffness and elastohydrodynamic lubricant film thickness are calculated for various operating conditions and the results are input to the dynamical simulation, resulting in a prediction of the dynamic transmission error.

A critical evaluation of film thickness‐derived pressure–viscosity coefficients

AbstractA single parameter, the pressure–viscosity coefficient, α, quantifies the pressure dependence of the viscosity of the liquid in elastohydrodynamic lubrication (EHL). Most published values of α have not been obtained from measurements of viscosity as a function of pressure. Rather, these effective pressure–viscosity coefficients have been derived from the measurement of the EHL film thickness, a more difficult procedure. In this article, five well‐characterized liquids that should be Newtonian in the EHL inlet are identified for which film‐derived coefficients have been reported. These coefficients are compared with coefficients derived from published viscosity correlations and new viscosity measurements. The film‐derived coefficients are found to not be an accurate representation of the piezoviscous response. The procedure of deriving a pressure–viscosity coefficient from a film thickness measurement does not offer an alternative to the simpler and easier viscometer measurement. Copyright © 2014 John Wiley & Sons, Ltd.

Model for Elastohydrodynamic Lubrication of Multilayered Materials

A novel model is constructed for solving elastohydrodynamic lubrication (EHL) of multilayered materials. Because the film thickness equation needs the term of the deformation caused by pressure, the key problem for the EHL of elastic multilayered materials is to develop a method for calculating their surface deformations, or displacements, caused by pressure. The elastic displacements and stresses can be calculated by employing the discrete-convolution and fast Fourier transform (DC-FFT) method with influence coefficients. For the contact of layered materials, the frequency response functions (FRFs), relating pressure to surface displacements and stress components, derived from the Papkovich–Neuber potentials are applied. The influence coefficients can be obtained by employing FRFs. The EHL of functionally graded material (FGM) can also be well solved using a multilayer material system. The effects of material layers and property gradient on EHL film thickness and pressure are further investigated.

Molecular interaction originating from polar functional group in lubricants and its relationship with their traction property under elastohydrodynamic lubrication

AbstractThe correlation between molecular interaction and traction properties was investigated using a traction tester and in situ observation of elastohydrodynamic lubrication film with a micro‐Fourier transform infrared spectrometer. The sample oils used were polypropylene glycols (PPGs) with the end‐group of alcohol or ether and a synthetic hydrocarbon oil, poly‐α‐olefin. From the traction tests, it was found that the traction coefficient of PPG was sensitive to the end‐group. PPG with alcohol as the end‐group showed a higher traction coefficient than that with the ether group. In situ observation with a micro‐Fourier transform infrared was performed in order to investigate the molecular interaction of the lubricant oil. It was found that the hydrogen bonding of hydroxyl groups in PPG was strengthened by high pressure in the Hertzian contact region. These results suggest that the rheological properties in the elastohydrodynamic lubrication contact region were affected by the strengthened hydrogen bonding. Copyright © 2014 John Wiley & Sons, Ltd.

An Experimental Investigation into the Onset of Smearing Damage in Nonconformal Contacts with Application to Roller Bearings

The onset of smearing damage was studied under controlled conditions in a custom test rig that simulates the passage of a rolling element through loaded and unloaded zones of a rolling bearing. The setup includes a spherical roller that is intermittently loaded between two bearing raceways driven at a prescribed speed. The roller is free to accelerate during the loading phase. Contact load, roller speed and acceleration, and electrical contact resistance are recorded during the test. Contact shear stress, friction coefficient, frictional power intensity, and elastohydrodynamic film thickness are calculated from the recorded kinematics data. Results suggest that the first onset of smearing occurs early in the loading phase where the roller is near stationary and the frictional power intensity is high. The raceway speed at the onset of damage decreases with increasing load and increasing lubricant supply temperature. The maximum frictional power intensity is found to be relatively constant at all contact conditions that led to smearing. An existing thermomechanical contact model is used to estimate the contact temperature distribution under smearing conditions and the potential for elastohydrodynamic lubrication (EHL) film thickness reduction due to forward heat conduction.

Steady State EHL Line Contact Analysis with Surface Roughness and Linear Piezo-viscosity

A special form of hydrodynamic lubrication, elastohydrodynamic lubrication (EHL), is generally encountered in mechanical components involving highly stressed non-conformal contact such as gears, cams, roller bearing etc. EHL is characterized with substantial elastic deformation of the interacting surface and piezo-viscous rise in lubricant viscosity which assist largely in evolution of load carrying fluid film.In the current paper, the effect of surface roughness of different magnitude in EHL line contact on pressure distribution is considered. At the same time graph between film thickness with rolling direction is plotted which gives reader the knowledge of EHL film thickness. Steady state effect of EHL with linear piezo-viscosity response at low pressure is taken into consideration. Full isothermal, Newtonian simulation of the EHL problem is discussed. It investigate the pressure distribution of sinusoidal waviness in the line contact, more precisely it study the effect of linear piezo-viscous fluid on surface roughness of different amplitude in steady state EHL line contact. Graphs plotted gives the clear idea to the designers, the behaviour of surface roughness with several amplitude and wavelength by linear piezo-viscous liquid like 2, 3-Dimethylpentane, L7808 (high temperature lubricant) and water/glycol solution of two water percentage and exponential fluid like santotrac 40 (S 40).The Reynolds equation, which governs the generation of pressure in the lubricated contact, is discretized using finite differences and solved along with the load balance equation using Newton-Raphson technique. A computer code is developed for implementation of solution algorithm. The results are obtained to study the linear and exponential piezo-viscous effect on surface roughness on the EHL characteristics.