Elastohydrodynamic Conditions Research Articles

Measurements of viscosity profile in elastohydrodynamic lubrication film using fluorescence method under various contact pressure conditions

This paper describes rheological properties of hydrodynamic and elastohydrodynamic lubrication (EHL) film under various contact pressure conditions using the fluorescence measurement method. In this measurement method, the viscosity and the information about the state of the lubricant (liquid or solid) can be obtained via the analysis of fluorescence spectrum of a pyrene excimer emission. Previous research revealed that the intensity of excimer emission decreased with increasing of viscosity. In addition, the intensity ratio between the excimer peak and lubricant 5P4E fluorescence peak was introduced for the analysis, and the ratio decreased with increasing viscosity of lubricant. In this study, the viscosity profile obtained from the relationship between the intensity ratio and viscosity showed that the state of lubricant in the lubrication film corresponded almost exactly to the lubrication regime proposed in previous researches. It is concluded that the viscosity measurement technique proposed in this study is useful method for practical uses in hydrodynamic and elastohydrodynamic conditions, while in the high viscosity range, the measured viscosity varies widely due to the variation of intensity ratio.

Transient non-Newtonian elastohydrodynamics of rough meshing hypoid gear teeth subjected to complex contact kinematics

Predictions are provided for mixed elastohydrodynamic conditions of meshing hyoid gear teeth pairs of light truck differentials. Under transient conditions, pertaining to vehicle cruising or in urban driving, the lubricant is subjected to non-Newtonian traction. The meshing teeth are subjected to complex kinematics, comprising rolling, sliding and squeeze film motions. Furthermore, instantaneously varying angled lubricant flow into the contact constitutes a precession of contact footprint, further complicating the prevailing conditions. The inclusion of all these interacting kinematics shows that the lubricant film thickness, shear characteristics and transmission efficiency are significantly affected by the ensuing transience. This cannot be adequately represented by the usual quasi-steady analyses. The in-depth detailed analysis of the hypoid gear pairs of vehicular differentials constitutes the main contribution of the paper.

Discussion on “Scale and contact geometry effects on friction in thermal EHL: Twin-disc versus ball-on-disc” by Liu, Zhang, Bader, Venner, Poll, Tribology International 154, 106694, 2021

Liu et al. recently published a study on friction in heavily loaded lubricated contacts. They compare measurements obtained from tests carried out on two tribometers, at constant entrainment speed, temperature and contact pressure. They observe deviations they attribute to the influence of scale and contact geometry effects.We show, by means of experiments conducted under the same conditions and with the same lubricant, that three alternative effects, somehow dependent on each other, can explain their results: very different elastohydrodynamic conditions, the pervasive presence of significant thermal effects and the occurrence of a mixed lubrication regime in some tests.This discussion provides an opportunity to reconsider how to conduct friction tests with in mind to focus on the lubricant response only. Proposals are presented in terms of operating conditions and lubricant selection, with the subsequent objective to better understand the mechanisms behind friction in highly loaded EHD contacts.

Investigation of the tribological performance of ionic liquids in non-conformal EHL contacts under electric field activation

Real-time external alteration of the internal properties of lubricants is highly desirable in all mechanical systems. However, fabricating a suitable and effective smart lubricant is a long-lasting experimental process. In this study, the film thickness and frictional response of ionic liquid-lubricated non-conformal contacts to an electric field excitation under elastohydrodynamic conditions were examined. Film thickness was evaluated using a “ball-on-disc” optical tribometer with an electric circuit. Friction tests were carried on a mini traction machine (MTM) tribometer with a “ball-on-disc” rotation module and an electric circuit for contact area excitation. The results demonstrate that there is a difference in the behaviour of the ionic liquid during electric field excitation at the evaluated film thicknesses. The results of evaluated film thicknesses demonstrate that there is a difference in the behaviour of the ionic liquid during electric field excitation. Therefore, the ionic liquids could be a new basis for the smart lubrication of mechanical components. Moreover, the proposed experimental approach can be used to identify electrosensitive fluids.

Digital twin by DEM for ball bearing operating under EHD conditions

In this paper we investigate the elastohydrodynamic (EHD) lubrication in ball bearings by means of a digital twin. Based on an original description involving the discrete element method (DEM), the digital twin integrates all the components of ball bearings and enables realistic behavior under mechanical loading and kinematic conditions. In order to check the standard indicators recommended by most ball bearing manufactures, a stiffness model for elliptical Hertzian contact and an improved EHD formulation for lubricated contact are implemented in the numerical tool. In addition, we have introduced into the discrete modeling an electrical capacitance model correlated to the fluid film thickness and the contact pressure. The numerical predictions of lubricant film capacitance provided by digital twin are in good accordance, both qualitatively and quantitatively, with the experimental data available in the literature. The coupling of the discrete method with the electrical approach enables efficient solutions to be provided in terms of lubrication regime in relation to the lubricant properties to optimize the bearing lifetime.

Effect of laser surface texturing on friction behaviour in elastohydrodynamically lubricated point contacts under different sliding-rolling conditions

The Laser Surface Texturing (LST) technique has been largely investigated to improve the tribological performance of lubricated contacts. The present contribution is aimed at scrutinizing the influence of three texture configurations fabricated by LST on the tribological performance of elastohydrodynamic (EHD) point contacts under different slide-to-roll ratios (SRR), entrainment velocities and inlet temperatures. Friction experiments were conducted through a series of ball-on-disk tests in the MTM-2 (Mini-Traction Machine) tribometer. Main results showed that the texture configurations promoted significant effects under boundary and mixed lubrication conditions, and also affected the full-film EHD regime at higher temperatures. Furthermore, the tribological performance of textured samples was strongly related to the texture depth. Shallower texture designs (∼0.5 μm) reduced friction compared to untextured material, whereas deeper features (>1 μm) generally led to detrimental results. In general, dimples configuration decreased the lift-off speed and promoted full-film EHD conditions for a larger range of speeds, whereas radial curved grooves yielded to friction reduction under mixed lubrication conditions, moving the transition from boundary to mixed regimes to lower speeds, especially for intermediate lubricant viscosity.

Quantitative measurements of film thickness in a radially loaded deep-groove ball bearing

Abstract The evaluation of the lubricant film thickness in machine elements working in elastohydrodynamic conditions is essential for the design aimed at improving their performance and durability. Among electrical methods for measuring the lubricant film thickness in these contacts the electrical capacitance is convenient to use because it relates directly to the film thickness by an inverse proportional relationship. Simultaneous measurements of optical film thickness and electrical capacitance have allowed the authors to perform quantitative evaluation of the film thickness in a model glass/steel contact, and develop a testing procedure which can be applied to steel/steel contacts. In the current paper a novel approach to film thickness measurements in rolling element bearings is presented. By replacing all but one steel ball with electrically insulating, ceramic balls evaluation of the lubricant film thickness in a radially loaded ball bearing has been achieved. The current procedure provides a valuable research tool for in-situ monitoring of lubrication condition, allowing studying the influence of operating parameters as well as the lubricant's chemistry.

The Effect of Nanoparticle Functionalization on Lubrication Performance of Nanofluids Dispersing Silica Nanoparticles in an Ionic Liquid

Recently, ionic liquids (ILs) have received an increasing attention as lubricants owing to their intriguing properties such as tunable viscosity, high thermal stability, low emissions, nonflammability, and corrosion resistance. In this work, we investigate how the incorporation of octadecyltrichlorosilane (OTS) functionalized silica nanoparticles (NPs) in 1-butyl-3-methylimidazolium (trifluoromethysulfony)imide influences the tribological properties and rheological properties of IL under boundary lubrication and elastohydrodynamic conditions, respectively. It was found that the coefficient of friction was depended on the concentration of NPs in IL with a concave upward functional trend with a minimum at 0.05 wt.% for bare silica NPs and at 0.10 wt.% for OTS-functionalized silica NPs. For steel–steel sliding contact, the presence of functionalized NPs in IL at the optimum concentration decreased the coefficient of friction by 37% compared to IL and 17% compared to IL with bare silica NPs. While IL with bare NPs demonstrated a shear thinning behavior for all concentrations, IL with functionalized NPs showed a Newtonian behavior at low concentrations and shear thinning behavior at high concentrations. Overall, this study provides new insights into the antifriction and antiwear additives for lubrication systems involving ILs.

A technique for forecasting the durability of rolling bearings and the optimum choice of lubricants under flood-lubrication and oil-starvation conditions

Effective conditions of friction in units that ensure performance and maximum longevity have been created by flood lubrication under the conditions of elastohydrodynamic (EHD) lubrication. In connection with this, prolonging the service life of friction units by ensuring local requirements to the amount of oil in the contact area and establishing the optimum choice of lubricating material in terms of its viscosity ratings is a topical problem for rolling bearings. Thus, it is suggested to determine the parameter of the lubricating layer (λ) values for the conditions of flood lubrication (λo) and progressing (λo.s ) and catastrophic (λ c.s ) oil starvation for a wide spectrum of lubricating materials of different viscosity, since, in oil starvation mode, the efficiency of EHD conditions is violated, the lubrication mode is disturbed, and the bearing capacity of the EHD lubricating layer is lost.

The behaviour of lubricated EHD contacts subjected to vibrations

Machine components containing contacts working in elastohydrodynamic (EHD) conditions are often subjected to vibrations. These may be originated from the mechanism or machine the contact is part of, the surrounding environment and within the contact itself. The influence of vibrations upon the behaviour of elastohydrodynamic films has been studied experimentally in a number of papers, but a comprehensive study of the effect of the parameters of the oscillatory motion upon the film thickness has not been carried out yet. In this study the authors evaluate the effect of the frequency of the oscillatory motion upon the EHD film thickness. Optical interferometry is used to measure lubricant film thickness in a ball-on-flat disc arrangement. A high – speed camera records the interferometric images for later analysis and conversion into film thickness maps. The disc runs at a constant angular velocity while the ball is driven by the traction forces developed in the EHD film. In steady state conditions, this would ensure pure rolling conditions, however in the present investigation the ball is subjected to harmonic vibrations in a direction perpendicular to the plane of the film. The contact under study is lubricated by basic oils and the temperature is kept at a constant value of 60°C. The aim of this paper is to understand how vibrations influence the lubricant film formation.

The effect of ageing on elastohydrodynamic friction in heavy-duty diesel engine oils

To further improve the efficiency of machine components found in automotive engine systems it is important to understand the friction generation in these components. Modelling and simulation of these components are crucial parts of the development process. Accurate simulation of the friction generated in these machine components is, amongst other things, dependent on realistic lubricant rheology and lubricant properties, where especially the latter may change during ageing of the lubricant. Many modern heavy-duty diesel engines are in operation for several hundred hours before the engine oil is changed. In this work, two engine oils, one 10 W-30 and one 5 W-20, have been aged in full heavy-duty diesel engine bench tests for 400 and 470 hours respectively. This roughly corresponds to the amount of ageing these oils are subjected to between oil drains in field conditions. The aged oils were subjected to a number of oil analyses showing, among other things, a maximum increase in oil viscosity of 12.9% for the 5 W-20 oil and 5.5% for the 10 W-30 oil, which is most likely primarily an effect of evaporation and oxidation. The aged oils were tested in a ball-on-disc test rig under elastohydrodynamic conditions where friction was measured and the performance was compared to fresh samples of the same oils. The results show that there is almost no difference in elastohydrodynamic friction when comparing the aged oils with the fresh oils. These results indicate that it is not necessary to include oil ageing in numerical elastohydrodynamic friction models as long as the oil is changed before the ageing has reached a critical level.

Film thickness and traction measurements of refrigerant R1233zd used as lubricant in elastohydrodynamic conditions

This study presents film thickness and friction measurements of the refrigerant R1233zd (E-1-chloro-3,3,3,trifluoropropene-1) in a tribological contact. For the film thickness, optical interferometry technique with glass–steel contact is used in a ball-disc configuration, whilst for the traction measurements (Stribeck and traction curves), a contact ceramic ball–steel disc is used. In both cases, the Wedeven Associates Machines-5 tribometer is employed. An attempt to model the film thickness results is also carried out. The model gives a reasonably good prediction with respect to the experiments.

A Simplified Thermal Analysis of Elastohydrodynamic Contacts

Under elastohydrodynamic (EHL) conditions, the temperature in the contact zone determines the load resistance of the lubricant film. Therefore, an efficient assessment of the scuffing risks requires accurate contact temperature predictions. The work presented in this paper develops a simple and prompt temperature model exploitable within any thermal EHL modeling approach. The model incorporates a multilayer lubricant film representation for the heat equation solution. The developments also include an original heat repartition factor expression and a simple formula for handling intermediate values of the Peclet number. The rolling/sliding conditions in the inlet cause temperature rises that also affect the film resistance in the contact zone; this study proposes an inlet temperature rise equation. This formula offers temperature predictions in agreement with reference values. The contact zone temperature predictions for the line of contact problem under sliding/rolling conditions agree remarkably well with published numerical results; for the evaluations presenting the higher absolute difference, the correspondence remained over 94% and 95% for the maximum and mean temperatures, respectively. Both line and elliptical contact conditions were tested and compared to experimental data available in the literature. The analysis evidenced the precision of the estimates; thus attesting to the accuracy of the model under any contact conditions. Finally, the results indicate that, depending on the pressure and speed combination, the shearing zone may occupy around 30% of the film thickness.

The effect of the polarity of the lubricant upon capacitance measurements of EHD contacts

Electrical methods, such as capacitance or resistance, allow the study of film formation in elastohydrodynamic (EHD) metal/metal contacts. This makes them more applicable to real machine elements, such as rolling element bearings, gears or cams. Measurements of electrical resistance give an indication of the amount of direct metal/metal contact, while electrical capacitance provides the separation between the surfaces. From the theory of capacitance it is known that electrical capacitance increases with decreasing film thickness, making it very suitable for studies in the thin film region. Measurements of film thickness by electrical capacitance were successfully performed by the authors on both glass/steel and steel/steel contacts with a nonpolar lubricant for films thinner than 20nm. All commercial lubricants contain polar additives, or are polar themselves; hence it is of practical interest to examine the influence of a lubricant's polarity on capacitance measurements. For this reason, three different types of fluids were studied: a nonpolar polyalphaolefin (PAO), glycerol—a strongly polar fluid, and polyethylene glycol (PEG) with polarity between that of PAO and glycerol. The results indicate that PAO shows a very good agreement between the film thickness measured with optical interferometry and evaluated from capacitance measurements. In case of glycerol and polyethylene glycol, however, it was found that the film thickness extracted from capacitance deviates from the optically measured values in the thinner film region. This suggests that EHD conditions may have an effect on the dielectric behaviour of polar lubricants, and that this effect becomes stronger as the polarity increases.

Elastohydrodynamic Study of Blends of Bio‐Based Esters with Polyalphaolefin in the Low Film Thickness Regime

AbstractThe film thickness in elastohydrodynamic (EHD) conditions for soybean oil (SBO), oleic estolide ester (EST) and their binary blends with polyalphaolefins (PAO2 or PAO40) were studied at 30 and 100 °C. Changes with time, for up to 200 min, were monitored. SBO and its blends with the lower viscosity PAO2 showed initially good agreement with the Hamrock–Dowson (H–D) equation down to 1–3 nm film thickness. 60 min or more after the start of the measurements, boundary layers with thickness up to 4.7 nm were observed. The blend of SBO with the more viscous PAO40 showed initially a good agreement with H–D at 100 °C. Negative deviations in film thickness were observed 15 min after the start of the measurements. At extended periods of time, up to 200 min, they were less pronounced but still detectable. EST–PAO2 blend showed initially formation of boundary layers with thickness around 2 nm. The boundary layer at 30 °C did not change for 200 min, while at 100 °C showed a decrease in thickness and/or viscosity with time. The EST and the EST–PAO40 blends showed good agreement with the H–D equation and did not display a boundary or fractionation layer within 200 min.

Film-forming properties of castor oil-polyol ester blends in elastohydrodynamic conditions

The viscosity and elastohydrodynamic (EHD) film thickness properties of binary blends of castor oil with polyol esters were determined experimentally. Predicted blend viscosities were calculated from the viscosities of the pure blend components. Measured viscosity values were closer to the values predicted using the Lederer model than the Arrhenius model. EHD film thickness data were mostly in agreement with the predictions of the Hamrock–Dowson model. Observed deviations of EHD film thickness were attributed to boundary film formation and thermal effects. Calculated effective pressure–viscosity coefficients, α, displayed a complex relationship with blend viscosity. At 40°C, the addition of 10% polyol esters resulted in a 12–17% drop in α of castor oil. Higher concentrations of polyol esters resulted in an increase of α. At 70 and 100°C, α displayed an almost linear dependence on blend composition. Copyright © 2011 John Wiley & Sons, Ltd.

Elastohydrodynamic Study of Deformable Blade-Organic Photoconductor Conjunction

In Liquid electro photography (LEP), deformable blades are used to remove ink residuals and liquid from the Organic Photoconductor (OPC) drum. The thin layer remaining after the blade can produce undesirable chemical residuals on its surface. When bombarded by ions at the charging station, this layer can oxidize and affect the OPC's electrical properties. In this work we investigate the blade-OPC drum conjunction under fully flooded inlet and outlet conditions; a theoretical model incorporating elastohydrodynamic (EHD) considerations was developed where a deformable blade is deflected by a moving surface (OPC). Unlike typical EHD problems which are widely discussed in the literature, our case consists of a contact line which causes singularity problem in the analysis. This singularity is due to the blade's sharp corner edge prior to its deformation. In order to overcome this singularity, we first solved the elastic-static deformation of the tip using finite element simulation (COMSOL) and combined hydrodynamics considerations afterwards. Experimental results were found with good agreement to our calculations. We found that for a 2 [mm] width blade (1.2 [mm] deflection), the film thickness remaining after the OPC surface wiping is about 25 [nm]. The contact nip length is 12 [μm], where the maximum conjunction pressure was found to be 4 [MPa] and 1.8 [MPa] under static and elastohydrodynamic conditions, respectively.

Elastohydrodynamic study of vegetable oil–polyalphaolefin blends

AbstractTwo polyalphaolefins, of higher and lower viscosity than vegetable oils, were used to make binary blends of varying compositions with soy bean and canola oils. The pure oils and the blends were used in viscosity and film thickness investigations. The effects of composition and temperature on viscosity were found to agree well with the theoretical predictions of a simple mixing law. The film thicknesses of the various blends under elastohydrodynamic conditions were measured at 20 N load, and varying entrainment speeds and temperatures. From the data, pressure–viscosity coefficients, α, as a function composition and temperature were obtained. The resulting α values were compared with theoretical predictions. Experimental values of α as a function of composition showed a slight negative or no deviation from the values predicted by an ideal mixing model. On the other hand, experimental values of α displayed a mild decrease with increasing temperature, while the model predicted a sharp decrease with increasing temperature. Published in 2008 by John Wiley & Sons, Ltd.

Effect of surface texturing on mixed lubricated non-conformal contacts

The behaviour of surface texturing based on shallow micro-dents was observed within mixed lubricated non-conformal contacts and compared with results obtained under thin film elastohydrodynamic conditions. Thin film colorimetric interferometry was used to observe the changes in lubrication film thickness. It was found that lubricant emitted by micro-dents could effectively lift off the real roughness features that provided an increase in average but also the local minimum film thicknesses. On the contrary to smooth contact conditions no film thickness reduction is obvious either downstream or upstream the micro-dent. The possible beneficial effect of surface texturing on mixed lubricated contact was checked through the qualitative wear test. It confirmed that an array of shallow micro-dents reduced asperity interactions of rubbing surfaces. Moreover, the effect of micro-dents on rolling contact fatigue was also considered in this study. It has been shown that individual dents would have to be much deeper compared to those used in surface texturing experiments to cause reduction in contact fatigue life. It can be suggested from the obtained results that properly designed surface texturing could help to increase the separation of rubbing surfaces under mixed lubrication conditions.

Analysis and optimization of dynamically loaded porous metal sliding bearings under conditions of elastohydrodynamic lubrication

PurposeThis paper aims to develop a simulating model of a journal porous metal bearing under elastohydrodynamic conditions and combined (radial, friction and thermal) load distribution and to carry out structural optimization.Design/methodology/approachThe structure analysis is carried out for each kind of load separately and for the combined load distribution of the bearing, where a dynamically loaded porous metal bearing is simulated. This simulating model is developed by finite elements method using the structure analysis module of the CATIA V5 software. Further, a parameter optimization of a porous metal bearing is presented considering the elastic deformations of the bearing shell.FindingsIt is revealed that the bearing, even at points with maximum displacements, could not reach the mounting clearance value during its operational life. Relatively small bearing dimensions produce very high values of eigenfrequency response (over 150 kHz) and common dynamic loads met in all sorts of sliding bearing are not dangerous for bearing damage compared with static loads. In the stage of structural optimization based on the correlation between stress and geometric bearing parameters like wall thickness and outer diameter, the influence of finite element dimension on calculated results can be also analyzed and a proper choice of the latter is achieved.Research limitations/implicationsThe present porous bearing optimization model with the aid of CATIA V5 module for optimum design uses only single objective optimization. For a complete optimum design a multi‐objective optimization has to be carried out.Practical implicationsThe analysis under dynamic load conditions proved that relatively small dimensions of bearing commonly used in micro technique and precision mechanics result in extended safe and reliable operation.Originality/valueThis paper provides a methodology for bearing stress and deformation analysis in the elastic range and on the basis of this analysis it is possible to develop an optimization model for porous bearings offering help to designers for the selection of optimal bearing dimensions considering the bearing load caused by dynamic radial force, friction and temperature variation.