Elastohydrodynamic Lubrication Film Research Articles

Effect of Impact Load on Transient Elastohydrodynamic Lubrication of Spur Gears

A full transient elastohydrodynamic lubrication(EHL) solution of involute spur gears is obtained under impact load based on the multi-grid (MG) method for solving the pressures, the multi-level multi-integration (MLMI) approach for evaluating the elastic deformations, which takes into account the variation of equivalent curvature , entertainment velocity and load on time along the line of action, the gear tooth surface is assumed to be smooth. The influences of impact load spectrum and the approach impact load when the teeth come into action on the EHL pressure and film thickness are analyzed in the paper. The results show that the approach impact load can strongly influence the approach point maximum pressure and minimum film thickness. The impact load can lead to instantaneous lubrication film deterioration between contact teeth of involute spur gears. The maximum pressure and the minimum film thickness all occur in the vicinity of approach point immediately after the impact load was feed. The approach impact load is seriously harmful to the gear lubrication.

Three-Dimensional Plasto-Elastohydrodynamic Lubrication (PEHL) for Surfaces with Irregularities

Elastohydrodynamic lubrication (EHL) is one of the most common types of lubrication, which widely exists in many machine elements such as gears, rolling bearings, cams and followers, metal rolling tools, and continuous variable transmissions. These components often transmit substantial power under heavy loading conditions that may possibly induce plastic deformation of contacting surfaces. Moreover, the roughness of machined surfaces is usually of the same order of magnitude as, or greater than, the average EHL film thickness. Consequently, most components operate in mixed lubrication with considerable asperity contacts, which may result in localized pressure peaks much higher than the Hertzian pressure, causing subsurface stress concentrations possibly exceeding the material yield limit. Plastic deformation, therefore, often takes place, which not only permanently changes the surface profiles and contact geometry, but alters material properties through work-hardening as well. Available mixed EHL models, however, do not consider plastic deformation, often yielding unrealistically high pressure spikes and subsurface stresses around asperity contact locations. Recently, a three-dimensional (3D) plasto-elastohydrodynamic lubrication (PEHL) model has been developed for investigating the effects of plastic deformation and material work-hardening on the EHL characteristics and subsurface stress/strain fields. The present paper is a continuation of the previous work done by Ren et al. (2010, “PEHL in point contacts,” ASME J. Tribol., 132(3), pp. 031501) that focused on model development and validation, as well as investigation of fundamental PEHL mechanisms in smooth surface contacts. This part of the study is mainly on the PEHL behavior involving simple surface irregularities, such as a single asperity or dent, which can be considered as basic elements of more complicated surface roughness. It is found that considerable plastic deformation may occur due to the pressure peaks caused by the surface irregularity, even though sometimes external loading is not heavy and the irregularity is concave. The plastic deformation may significantly affect contact and lubrication characteristics, resulting in considerable reductions in peak pressure and maximum subsurface stresses.

Study on EHL Film Thickness of Non-Rectangular Section Vane Seal

The sealing contact pressure is critical in determining the film thickness of elastohydrodynamic lubrication (EHL) on the seal. Contact pressure in sealing surface of non-rectangular section vane seal was obtained via FEA, and the polynomial equation of the contact pressure distribution was fitted with regression analysis. Therefore, the numerical model for the EHL film thickness can be developed and solved. Typical results of film thickness distribution and frictional force for the vane seal at room temperature were obtained. The EHL film thickness distributions follow the variation of the related contact pressure, and the sliding speed is also significant on the average film thickness and the hydrodynamic friction.

Low viscosity shear heating in piston skirts EHL in the low initial engine start up speeds

Absence of elastohydrodynamic lubricating (EHL) film causes piston wear in low speed cold initial engine start up, while shearing of low viscosity lubricant in few cycles affects its load carrying capacity. Shear heating effects are incorporated in 2-D hydrodynamic and EHL model by solving 2-D heat equation. EHL pressures are calculated using inverse solution technique. Comparative analysis is based on viscous dissipation coupled with piston motion, changes in pressure, film thickness and viscosity. This study suggests that the increase in temperature varies with speed to affect piston eccentricities, viscosity and film thickness. This optimizes low start up speed for a few engine cycles.

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.

A Study on Propagation of Crack in Twin Disk Test

Pitting is occurred from initiation of a surface crack under rolling-sliding contact. Lubricant has been said to have a crucial effect on pitting, however, few are known about contact condition, such as lubricant flow and pressure. In our twin disk tests, the contact electric resistance became conductive from insulated when the crack passed through contact region immediately before pitting. The contact electric resistance between two surfaces is influenced by insulating elastohydrodynamic lubrication (EHL) films. Thus, the reduction of the contact resistance is resulted from the breakdown of the EHL films due to local increase of the contact pressure. As a result of finite element method simulation of the contact pressure around the crack, the contact pressure at downstream edge of the crack was locally large. In the cross-sectional image of a crack immediately before pitting, we confirmed that the EHL films at the downstream edge of the crack collapsed with surface plastic deformation. When the crack passes through the contact region, the lubricant from upstream may be compressed into the crack due to local increase of the contact pressure.

Plasto-Elastohydrodynamic Lubrication (PEHL) in Point Contacts

Elastohydrodynamic lubrication (EHL) is an important branch of the lubrication theory, describing lubrication mechanisms in nonconformal contacts widely found in many mechanical components such as various gears, rolling bearings, cams and followers, metal-rolling tools, traction drives, and continuous variable transmissions. These components often transmit substantial power under heavy loading conditions. Also, the roughness of machined surfaces is usually of the same order of magnitude as, or greater than, the estimated average EHL film thickness. Consequently, most components operate in mixed lubrication regime with significant asperity contacts. Due to very high pressure concentrated in small areas, resulted from either heavy external loading or severe asperity contacts, or often a combination of both, subsurface stresses may exceed the material yield limit, causing considerable plastic deformation, which may not only permanently change the surface profiles and contact geometry but also alter material properties through work hardening as well. In the present study, a three-dimensional plasto-elastohydrodynamic lubrication (PEHL) model has been developed by taking into account plastic deformation and material work-hardening. The effects of surface/subsurface plastic deformation on lubricant film thickness, surface pressure distribution, and subsurface stress field have been investigated. This paper briefly describes the newly developed PEHL model and presents preliminary results and observed basic behavior of the PEHL in smooth-surface point contacts, in comparison with those from corresponding EHL solutions under the same conditions. The results indicate that plastic deformation may greatly affect contact and lubrication characteristics, resulting in significant reductions in lubricant film thickness, peak surface pressure and maximum subsurface stresses.

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.

Some important aspects of thermal elastohydrodynamic lubrication

Thermal effect in elastohydrodynamic lubrication (EHL) has been the subject of study for the last four decades; however, some important aspects related to the physical behaviour of the lubricant in response to pressure, temperature, and shear rate remain largely neglected. This paper presents a brief review of the thermal EHL literature and sheds light on the importance of accurate characterization of the lubricant properties such as viscosity, density, rheology, and thermal conductivity. Full thermal EHL line contact simulations under steady-state and transient conditions show that using the ambient value of thermal expansivity, which has been the usual trend, may overestimate the central film thickness and introduce unrealistic features in transient EHL characteristics. Also, it is demonstrated that the most extensively used rheological equation – the sinh law – for characterizing the behaviour of shear-thinning lubricants underestimates the effect of viscous heating on EHL traction and film thickness.

Influence of spinning on the rolling EHL films

Spinning cannot be ignored in some elastohydrodynamically lubricated contacts. In this paper, spinning is incorporated into an elastohydrodynamic lubrication (EHL) contact of pure rolling and its influences on EHL films are studied. Results show that with increase in spinning, the symmetry of the film shape gets lost, and the minimum film thicknesses, located respectively at the two side-lobes, decrease and show more dependence on loads. The speed indices of the film thickness at the side lobes are higher than those of the classical EHL theory predicted. Numerical work has also been carried out to clarify the experiment measurements.

The Effect of Load (Pressure) for Quantitative EHL Film Thickness

New quantitative numerical simulations of the elastohydrodynamic lubrication (EHL) film thickness using realistic pressure and shear-dependent rheology and realistic compressibility have indicated that the dependence of central film thickness upon Hertz pressure (or load) for the classic Newtonian, slightly compressible solution is merely a lower limit with magnitudes three times as great being possible. Experimental measurements of central film thickness employing Hertz pressures from 1.0 to 2.6 GPa confirm that for a neat mineral oil, the classical pressure dependence is accurate, while for two gear oils the experimental pressure dependence is much larger. Shear-dependent viscosity is a major factor and compressibility plays a lesser role, while there is evidence that mechanical degradation is also important. New experimental evidence of the enhanced scale sensitivity resulting from shear-thinning has also been obtained. These results for the pressure and scale dependence have dire implications for the usual practice of extrapolation of film thickness from experimental measurements at large scale and low pressure using effective pressure–viscosity coefficients. For machines of small scale and high pressure, the extrapolation will sometimes result in substantially overestimated film thicknesses.

On the Behavior of Friction in Lubricated Point Contact With Provision for Surface Roughness

This paper presents a simple approach to predict the behavior of friction coefficient in the sliding lubricated point contact. Based on the load-sharing concept, the total applied load is supported by the combination of hydrodynamic film and asperity contact. The asperity contact load is determined in terms of maximum Hertzian pressure in the point contact while the fluid hydrodynamic pressure is calculated through adapting the available numerical solutions of elastohydrodynamic lubrication (EHL) film thickness formula for smooth surfaces. The simulations presented cover the entire lubrication regime including full-film EHL, mixed-lubrication, and boundary-lubrication. The results of friction, when plotted as a function of the sum velocity, result in the familiar Stribeck-type curve. The simulations are verified by comparing the results with published experimental data. A parametric study is conducted to investigate the influence of operating condition on the behavior of friction coefficient. A series of simulations is performed under various operating conditions to explore the behavior of lift-off speed. An equation is proposed to predict the lift-off speed in sliding lubricated point contact, which takes into account the surface roughness.

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.

Influences of oil additives on the wear behavior of ball screws

The lubrication in a tribosystem has a significant influence on the resulting friction and wear behavior. The spectrum of the used base oils for lubrication preparation increases continuously. That affects especially the use of environmentally friendly substances in order to save the environment. Regardless of the chemical origin, the lubrication systems should primarily take over the function of positive influencing of the friction and wear behavior. The ball screw as a central component of the machine tool feed axis transforms the rotational motions of the servo motor to a linear motion of the slide and takes over the fine positioning function in machine tools. According to the roller bearing lubrication specifications a grease or oil lubrication for ball screws is indispensable. The lifetime calculation is based on an elastohydrodynamic lubrication film. Within the research activities of the collaborative research center (CRC 442) ball screws with additived oils and nonadditived synthetic ester, which are developed within the collaborative research center, are tested on the test stand. The aim is to clarify the question, which influence the oil additives have on the wear behavior and preload loss in ball screws for machine tool applications. Investigations have clarified that oil additives have a significant influence on the preloading of the ball screws which are operated in the mixed friction area. The investigations of the axial rigidity as well as the investigations of the no-load torques clearly showed that oil additives cause a lower preload loss in ball screws compared to nonadditived oils.

Effect of Perfluoropolyether Fluids on Life of Thrust Ball Bearings

Perfluoropolyether (PFPE) fluids are used satisfactorily on magnetic recording media, in the aerospace industry, and in satellite instruments. These oils have been used as hydraulic fluids, high-temperature liquid lubricants in turbine engines, and base oils for high-temperature greases. Despite considerable research on PFPE, the rheological characteristics at high pressure and their effects on the life of ball bearings have not been investigated completely. First, high-pressure density measurements of three kinds of commercial PFPE fluids (fluid K, fluid D, and fluid F) were made from temperatures 293 K to 333 K and pressures up to 1.2 GPa for estimation of the free volume. The density-pressure-temperature relation and viscosity-pressure-temperature relation of PFPE fluids were proposed based on the free volume and the phase diagram. Next, the effect of PFPE fluids on ball bearing fatigue life was determined by systematic tests using thrust ball bearings. The general tendency of PFPE fluids shows that bearing life increases with the film parameter. The elastohydrodynamic lubrication (EHL) film thicknesses were studied and compared with experimental measurements. The results show that for fluid F, which contains an acetal group (-OCF 2 O-), the viscosity was decreased by mechanical shear at the high shear rate in the EHL inlet and the EHL oil film was not easily formed. The beneficial effect of high film parameter is therefore reduced for fluid F by permanent viscosity loss; thus, contacts occur in ball bearings and shorten the bearing life.

Deterministic mixed lubrication modelling using roughness measurements in gear applications

The presence of surface roughness on the teeth of hardened and ground power transmission gears is an unavoidable consequence of their manufacture. The paper discusses the effect of surface roughness when the elastohydrodynamic lubricant film thickness developed between the gear tooth surfaces is small compared to the heights of the roughness features. The ratio of these quantities, called the Λ value, may be well below unity in typical applications. For such thin film conditions the moving roughness features cause the elastohydrodynamic contact between the gears to be highly transient in nature. Surface roughness features on the working surfaces of the gears move past each other during meshing and these asperity encounters are associated with extreme pressure perturbations, or with film breakdown and isolated asperity boundary lubrication events. The paper reviews approaches used to study this problem and describes a coupled approach to solving the elastic and hydrodynamic equations. This allows numerical solutions to be obtained for these extreme conditions so that transient contact events associated with mixed lubrication can be predicted in a unified numerical solution scheme. Typical results obtained from such an analysis are presented including surface fatigue modelling and contact strain energy calculations.

Occurrence of Wall Slip in Elastohydrodynamic Lubrication Contacts

Preliminary experimental work has been carried out to identify some of the boundary slip phenomena of highly pressurised polybutenes in an elastohydrodynamic lubrication (EHL) conjunction. The movement of the oil is signified using an entrapment that can be readily formed by the impact of a steel ball against a layer of oil on a glass block in an optical EHL test apparatus. The post-impact lateral movement of the entrapment was investigated under the conditions: (i) pure rolling, (ii) pure glass block sliding (steel ball stationary) and (iii) pure ball sliding (glass block stationary). It was observed that under pure rolling the entrapped oil travels within the contact region at the entrainment speed, which is correlated with EHL theory. Under pure glass block sliding conditions, the speed of the entrapped oil core is less than the entrainment speed, and in the extreme cases, this core can be nearly stationary. Under pure ball sliding conditions, the oil core moves at a speed greater than the entrainment speed. The observation indicates that the oil/steel ball interface can sustain higher shear stress than the oil/glass (chromium coated) interface and there is a boundary slip in terms of relative sliding at the latter interface under the experimental conditions. Furthermore, the amount of slip increases with an increase in the pressure. These experiments provide evidence of the existence of wall slippage, which leads to the abnormal EHL film profile characterised with an inlet dimple as reported earlier.

U.S. Air Force Perfluoropolyalkylether Experiences

The U.S. Air Force Research Laboratory (AFRL) received the first sample of perfluoropolyalkylether (PFPAE) liquid lubricant outside of private industry in the 1960s. The unique behavior of PFPAE fluids includes their excellent thermal stability (up to 345°C with appropriate additives) and excellent viscosity-temperature properties for the linear structures. Military applications are often cutting-edge technology requiring these unique properties, but other undesirable behavior such as tribo-corrosion has required investigations into basic understanding of PFPAE fluids. From the 1960s on, Air Force scientists have investigated many aspects of their behavior as summarized here. This presentation discusses various types of PFPAEs commercially available and highlights studies on elastohydrodynamic lubrication film formation, model compounds, additives, relative humidity relationship to wear, and potential U.S. military program applications.

On the role of lubricant rheology and piezo-viscous properties in line and point contact EHL

Accurate prediction of elastohydrodynamic lubrication (EHL) characteristics, i.e., film thickness and traction coefficient, requires the knowledge of lubricant rheology. The EHL analyses based on Newtonian fluid model may be useful primarily for film thickness prediction of mineral oils. However, for applications involving mineral oil/polymer blends and synthetic oils that exhibit shear-thinning behavior, the use of an appropriate non-Newtonian fluid model is required to predict the EHL behavior correctly. It is, thus, no surprise that characterization of non-Newtonian fluid behavior in EHL studies has been the focus of attention over last four decades. In this regard, several types of non-Newtonian EHL models have been presented and solved using different methods. Unfortunately, many of these studies suffer from various drawbacks such as use of inappropriate non-Newtonian model, approximations in the numerical solution and lack of sufficient experimental data pertaining to lubricant rheology as well as piezo-viscous properties. Over the years, some of these shortcomings have been transferred erroneously from one generation of researchers to the next. For example, notwithstanding the fact that the sinh-law lubricant model (commonly referred to as the “Ree–Eyring” model) was actually rejected for use as a shear-thinning model by Henry Eyring himself, it is still being widely used to describe the shear-thinning behavior of EHL lubricants. The present paper discusses the recent developments clearly indicating the incapability of sinh-law for the EHL applications. This paper also reviews the perturbation method often used to reduce the actual constitutive equation to a simplified form in order to derive a Reynolds-type equation to which normal EHL solvers can be applied. This approach is still in use even though appropriate procedures for implementing the generalized Newtonian approach are available that can conveniently and accurately allow the use of the exact constitutive equation in its original form. In the light of above facts, this paper presents a collective perspective in an effort to emphasize the importance of implementing realistic non-Newtonian and piezo-viscous models with accurate treatment methods in EHL applications.

An Experimental Validation of the Recently Discovered Scale Effect in Generalized Newtonian EHL

New quantitative numerical simulations of the elastohydrodynamic lubrication (EHL) film forming ability of generalized Newtonian liquids have elucidated a previously unrecognized property of EHL films. The dependency of the film thickness on the scale of the contact is greater when the viscosity is shear dependent within the inlet. Measurements of film thickness were performed in a ball on disc experiment using balls ranging from 5.5 to 35 mm in diameter. Three liquids were investigated with varying shear dependence in the range of stress important to film forming. The experimental results confirm the previous analytical findings. Numerical simulations using the measured viscosities show that the increased scale sensitivity is substantially the result of shear-thinning. However, the smallest scales produced films thinner than even the shear-dependent prediction, possibly indicating molecular degradation. It is quite likely that some machine components, which were designed using the effective viscous properties derived from a larger scale film thickness measurement, are operating with substantially lower film thickness than the designer had intended.