Elastohydrodynamic Lubrication Regime Research Articles

Influence of foaming on tribological and rheological characteristics of foamed asphalt

The frictional characteristics of asphalt binder as a lubricant film in an asphalt-aggregate interaction play a major role in workability of warm-mix asphalt (WMA). However, no WMA research considered the frictional characteristics of asphalt to demonstrate the workability of WMA, rather most of the previous studies are limited to foaming and some of the rheological properties. Frictional evaluation can be conducted on asphalt binder through the tribological experimentation with a ball-on-three-plates apparatus assembled inside a dynamic shear rheometer (DSR). This study aims to evaluate the effect of foaming on tribological and rheological characteristics of foamed asphalt binder. The tribological and rheological tests were conducted on six foamed asphalts with varying foaming water contents (FWCs) and on a controlled un-foamed asphalt binder at different temperatures. These binders were also characterized by foaming and chemical properties. Test results show that the foaming process alters the frictional resistance of the foamed asphalt binders, specifically the foaming process reduces the coefficient of friction in the elasto-hydrodynamic and hydrodynamic lubrication regimes. The tribological characteristics are more relevant to the improved workability of foamed WMA over rheological characteristics.

Prediction of the Stribeck curve under full-film Elastohydrodynamic Lubrication

The Stribeck curve shows the friction coefficient as a function of speed, viscosity and load. The viscosity times speed over load parameter can be interpreted as a film thickness. The film thickness over roughness parameter unifies friction curves in the isoviscous rigid regime. In this paper, the Stribeck curve is predicted numerically in the full-film Elastohydrodynamic Lubrication regime. It is shown that the lambda ratio is not the most appropriate parameter. A more elaborate parameter including the operating conditions and based on the Amplitude Reduction Theory [1] gives much better results. For a complex surface topography, the full numerical simulation is time-consuming. A rapid prediction method is proposed. Good agreement is found between the full numerical simulation and the prediction.

Dynamics and lubrication analyses of scotch yoke mechanism

In this paper the contact problem of a scotch yoke mechanism has been investigated in details under the elastohydrodynamic lubrication regime and line contact model is used in the analyses. The non-Newtonian oil behavior of the lubricant and the variation of the load and the surface velocities throughout the operating cycles are considered in the analyses. The results reveal that the variation of these values makes the contact problem (depending on the position in the operating cycle) to operate under lightly and heavily loaded conditions. Under these conditions, extremely thin levels of film thickness have been predicted.

Shear heating, flow, and friction of confined molecular fluids at high pressure.

Understanding the molecular-scale behavior of fluids confined and sheared between solid surfaces is important for many applications, particularly tribology where this often governs the macroscopic frictional response. In this study, nonequilibrium molecular dynamics simulations are performed to investigate the effects of fluid and surface properties on the spatially resolved temperature and flow profiles, as well as friction. The severe pressure and shear rate conditions studied are representative of the elastohydrodynamic lubrication regime. In agreement with tribology experiments, flexible lubricant molecules give low friction, which increases linearly with logarithmic shear rate, while bulky traction fluids show higher friction, but a weaker shear rate dependence. Compared to lubricants, traction fluids show more significant shear heating and stronger shear localization. Models developed for macroscopic systems can be used to describe both the spatially resolved temperature profile shape and the mean film temperature rise. The thermal conductivity of the fluids increases with pressure and is significantly higher for lubricants compared to traction fluids, in agreement with experimental results. In a subset of simulations, the efficiency of the thermostat in one of the surfaces is reduced to represent surfaces with lower thermal conductivity. For these unsymmetrical systems, the flow and the temperature profiles become strongly asymmetric and some thermal slip can occur at the solid-fluid interface, despite the absence of velocity slip. The larger temperature rises and steeper velocity gradients in these cases lead to large reductions in friction, particularly at high pressure and shear rate.

Elastohydrodynamic Films Under Periodic Load Variation: An Experimental and Theoretical Approach

The elastohydrodynamic lubrication regime occurs in systems where large elastic deformations, the hydrodynamic action of a converging wedge and eventually large variation of viscosity of the fluid combine to determine the formation of a continuous fluid film that separates the solid surfaces. Experimental and theoretical works, over the past few decades, have elucidated the role of various working and material parameters on the lubricant film thickness which plays a crucial role in protecting the solid surfaces from direct contact and ultimately from failure. These mechanisms are well understood for steady-state conditions; however, elastohydrodynamic contacts most often experience transient conditions, including variation of geometry, velocity of surfaces or load. In this case, the mechanisms of film formation are more complex involving film squeeze in addition to the mechanisms mentioned above. Experimental and theoretical modelling of transient phenomena in elastohydrodynamic lubrication include sudden variation of entrainment speed or load and changing geometry. No systematic experimental study on the effect of harmonic load vibration upon the elastohydrodynamic films has been published before. In order to cover this gap, this paper presents the results of an experimental study and of a simple theoretical approach on the behaviour of the elastohydrodynamic film thickness under harmonic variation of load.

Film formation in EHL contacts with oil-impregnated sintered materials

PurposeThe purpose of this study is to investigate lubricant film-forming capability of oil-impregnated sintered material in highly loaded non-conformal contacts. This self-lubrication mechanism is well described in lightly loaded conformal contacts such as journal bearings; however, only a little has been published about the application to highly loaded contacts under elastohydrodynamic lubrication regime (EHL).Design/methodology/approachThin film colorimetric interferometry is used to describe the effect of different operating conditions on lubricant film formation in line contacts.FindingsUnder fully flooded conditions, the effect of porous structure can be mainly traced back to the different elastic properties. When the contact is lubricated only by oil bleeding from the oil-impregnated sintered material, starvation is likely to occur. It is indicated that lubricant film thickness is mainly governed by oil bleeding capacity. The relationship between oil starvation parameters corresponds well with classic starved EHL theory.Practical implicationsTo show practical, relevant limitations of the considered self-lubrication system, time tests were conducted. The findings indicate that EHL contact with oil-impregnated sintered material may provide about 40 per cent of fully flooded film thickness.Originality/valueFor the first time, the paper presents results on the EHL film-forming capability of oil-impregnated sintered material by measuring the lubricant film thickness directly. The present paper identifies the phenomena involved, which is necessary for the understanding of the behavior of this complex tribological system.

Ionic Liquid Additives for Mixed and Elastohydrodynamic Lubrication

ABSTRACTIonic liquids (ILs), both as pure lubricants and as lubricant additives, have been demonstrated extensively to exhibit excellent tribological performance in terms of friction and wear reduction in the boundary lubrication (BL) regime. Because engineering contacts experience boundary and mixed as well as full film lubrication depending on operating conditions, it is crucial to examine whether lubrication regimes other the BL regime can also benefit from the use of ILs. The objective of this work is to investigate the tribological performance of IL additives in the mixed lubrication (ML) and the elastohydrodynamic lubrication (EHL) regimes. Polyethylene glycol (PEG) was used as the base fluid. ILs were synthesized in situ by dissolving lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in PEG. Friction and film thickness measurements were employed to investigate the effectiveness of IL additives at room temperature, 60°C, and 80°C at various loads and slide–roll ratios (SRRs). The effect of IL additives on the rheological behavior of PEG was also investigated. The EHL film thickness increases with increasing IL concentration. EHL friction is, however, only mildly affected by IL additives. In the ML regime, IL additives can reduce friction and metal wear compared to pure PEG in mild conditions. It is conjectured that IL forms sacrificial layers and protects the rubbing surfaces.

Can a “pre-worn” bearing surface geometry reduce the wear of metal-on-metal hip replacements? – A numerical wear simulation study

Total Hip Replacement (THR) is generally a highly successful treatment for late stage hip joint diseases and wear, however, wear continues to be one of the major causes of metal-on-metal THR's failure. Hip replacements typically experience a two-stage wear; a higher initial wear rate in the beginning followed by a lower steady-state one with the surface profile changed. This alludes to the potential use of a cup with a non-spherical interior cavity with an initial geometry similar to a worn surface which may benefit from lower wear rate. In this paper wear is numerically simulated with a cup having a non-spherical geometry inspired by the initial stage of wear.A wear model was recently developed by the authors for the THR, which considered the lubricated contact in both elastohydrodynamic lubrication (EHL) and mixed lubrication regime, rather than a dry contact used in most of other studies of wear modelling in the academic literature. In this study the wear model has been updated by introducing the ‘λ ratio’ (the ratio of film thickness to surface roughness) and addressing the non-Newtonian shear-thinning properties of the synovial fluid. This wear model was able to describe the non-linear wear evolution process due to the change of worn profiles. Firstly the wear of a spherical hip joint was simulated until a steady-state wear rate is achieved. Then a non-spherical joint was proposed in which the cup bearing geometry was generated by the previously predicted worn profile from the spherical joint. At last the wear of this “pre-worn” hip bearing was simulated and compared to the spherical one. Approximately 40% reduction in the steady-state wear rate and 50% in the total accumulated wear has been observed in the non-spherical hip joint. This study presented a full numerical analysis of the relationship between lubrication, wear reduction and the geometry change, and quantitatively suggested the optimal geometry to reduce running-in wear.

Soft Matter Lubrication: Does Solid Viscoelasticity Matter?

Classical lubrication theory is unable to explain a variety of phenomena and experimental observations involving soft viscoelastic materials, which are ubiquitous and increasingly used in e.g. engineering and biomedical applications. These include unexpected ruptures of the lubricating film and a friction-speed dependence, which cannot be elucidated by means of conventional models, based on time-independent stress-strain constitutive laws for the lubricated solids. A new modeling framework, corroborated through experimental measurements enabled via an interferometric technique, is proposed to address these issues: Solid/fluid interactions are captured thanks to a coupling strategy that makes it possible to study the effect that solid viscoelasticity has on fluid film lubrication. It is shown that a newly defined visco-elasto-hydrodynamic lubrication (VEHL) regime can be experienced depending on the degree of coupling between the fluid flow and the solid hysteretic response. Pressure distributions show a marked asymmetry with a peak at the flow inlet, and correspondingly, the film thickness reveals a pronounced shrinkage at the flow outlet; friction is heavily influenced by the viscoelastic hysteresis which is experienced in addition to the viscous losses. These features show significant differences with respect to the classical elasto-hydrodynamic lubrication (EHL) regime response that would be predicted when solid viscoelasticity is neglected. A simple yet powerful criterion to assess the importance of viscoelastic solid contributions to soft matter lubrication is finally proposed.

Design and Experiment Study on a New Type of Lubrication Simulation System for Thrust-Ball Bearing in Confined Space

Many machine components work within an elastohydrodynamic lubrication (EHL) regime. Unconfined space is widely used in EHL formulas for to evaluate film thickness, which is related to operating conditions and material properties. In classical theoretical EHL studies, film pressure matches the loading balance and the location of the lubricated components can be adjusted. In the present study, the lubrication performance is analyzed based on a confined space. A thrust ball bearing lubrication simulation system is designed and used to examine the relationship between velocity and film thickness. It was found that the central film thickness and minimum film thickness increased as entrainment velocity increased. Fluctuations in the film thickness curve were observed, which may have arisen from slight gap variations in the ball-plate contact area.

Experimental investigation on the rheological properties of castor oil at different temperatures

Castor oil is one of the most enormous potential environment-friendly lubricants, and used as the base oil and additives in industrial applications. In this paper, the rheological property of castor oil was investigated using the optical interferometric test rig to analyze its flow properties under the micro gap. These results indicated that the central film thickness of castor oil in the point contact zone underwent the initial increased and then stable variation with the entrainment speed at different temperatures. The temperature and entrainment speed played an important role on the lubrication state and oil shape variation along the transverse and entrainment direction. At low temperature, the contact zone was almost in the thin lubrication regime at low entrainment speed, but was in the elastohydrodynamic lubrication regime at high entrainment speed. The side leakage went through the initial increased then decreased trend with the temperature and entrainment speed. These research results would further develop the engineering application of castor oil.

Numerical Simulations of Groove Topography Effects on Film Thickness and Friction in EHL Regime

The effects of depth and top width of transverse rectangular grooves on film thickness and friction in elastohydrodynamic lubrication (EHL) regime were investigated through numerical simulations. Results were obtained in the form of pressure profiles and Stribeck curves for central and minimum film thickness and for friction coefficient. The results indicate that grooves with narrow top widths reduce the minimum film thickness and that this reduction is greater for deeper grooves. Lubricant shearing inside these grooves was further identified as a dominant factor contributing to friction. Near the groove edges, however, no evidence of micro-EHL effect was observed. Based on the results, a groove volume parameter was proposed to characterise the groove lubrication efficiency. We found that the parameter was linearly related to the average central film thickness and by increasing the groove wavelength the film could be made thicker than that of a smooth contact.

The Effect of Ni Nanoparticles on the Lubrication of a DLC-Based Solid–Liquid Synergetic System in All Lubrication Regimes

Ni nanoparticles (with average diameter 7 nm) capped with oleylamine and oleic acid were added to a DLC/DIOS solid–liquid synergetic lubricating system. The tribological behavior rubbed with steel balls in three lubricating regimes (boundary, mixed and elasto-hydrodynamic lubrication) has been systematically studied by surface analyses techniques such as SEM, EDX and XPS. It was found that, in all the lubricating regimes, the system’s lubricating performance was substantially improved because of the addition of the Ni nanoparticles. For all the lubrication regimes, the COF decreased by 10.3–19.1%. In the boundary lubrication regime, the wear rate of DLC could be decreased by 50%. Ni nanocores facilitate the adequate oxidation of the steel surface and the transformation of C=C (sp2) to C–C (sp3) of the DLC sliding surface. The tribo-film containing ferrous oxides, Ni and its oxides, presents low shearing strength and good cohesion to the DLC coating, which greatly improves the lubricating behaviors of the DLC/DIOS solid–liquid synergetic system. In the elasto-hydrodynamic lubrication regime, the Ni nanoparticles cause the occurrence of obstructed or plug flow in the oil films, which in turn reduces friction by reducing the layers of lubricant molecules to slide on each other.

Effects of elastic ball and coating on pure squeeze EHL motion for constant load with couple stresses using FDM method

In this paper, an elastic sphere approaching a lubricated elastic coating surface with couple stress lubricant is explored under constant load condition. The finite difference method (FDM), Gaussian elimination, and Gauss-Seidel iteration method are used to solve the transient modified Reynolds equation with its boundary condition, the elasticity deformation equations of ball and coating layer, load balance equation, and lubricant rheology equations simultaneously. The transient pressure profiles, film shapes, and elastic deformation during the pure squeeze process under various operating conditions in the elastohydrodynamic lubrication (EHL) regime are discussed.

Comparison of tribological performance of roller follower and flat follower under mixed elastohydrodynamic lubrication regime

In this study, a numerical model is developed to show the performance improvement of a cam–follower mechanism when using a roller type follower compared to the flat-faced follower. Nonconformal geometry besides the thermal effects due to the shearing of the lubricant film results in formation of a thin film in which the asperities contribute in carrying the load. The numerical model is developed in which the geometry, load, speed, lubricant properties, and the surface roughness profile is taken as input and the film thickness and friction coefficient as a function of cam angle are predicted. The asperities are assumed to have elastic, elasto-plastic, and plastic deformation. Simulation results indicated that the thermal effects cannot be neglected. Surface roughness is also a key parameter that affects the pressure distribution, film thickness, and friction coefficient. Finally, asperity and hydrodynamic pressure is reported and the performance of the two mechanisms is compared. Roller follower has a considerable preference in terms of friction coefficient compared to flat-faced follower. The minimum film thickness, however, is slightly larger in the flat follower.

On the relevance of analytical film thickness EHD equations for isothermal point contacts: Qualitative or quantitative predictions?

Thin film and elastohydrodynamic lubrication regimes are rather young domains of tribology and they are still facing unresolved issues. As they rely upon a full separation of the moving surfaces by a thin (or very thin) fluid film, the knowledge of its thickness is of paramount importance, as for instance to developing lubricated mechanisms with long lasting and efficient designs. As a consequence, a large collection of formulae for point contacts have been proposed in the last 40 years. However, their accuracy and validity have rarely been investigated. The purpose of this paper is to offer an evaluation of the most widespread analytical formulae and to define whether they can be used as qualitative or quantitative predictions. The methodology is based on comparisons with a numerical model for two configurations, circular and elliptical, considering both central and minimum film thicknesses.

Thin film lubrication in the past 20 years

Thin film lubrication (TFL), a lubrication regime that fills the gap between boundary lubrication (BL) and elastohydrodynamic lubrication (EHL) regimes, was proposed 20 years ago. Since it was first recorded in the literature, TFL has gained substantial interest and has been advanced in the fields of theoretical and experimental research. Following the revelation of the TFL phenomenon and its central ideas, many studies have been conducted. This paper attempts to systematically review the major developments in terms of both the history and the advances in TFL. It begins with the description and definition of TFL, followed by the state-of-art studies on experimental technologies and their applications. Future prospects of relevant studies and applications are also discussed.

Graphene based composite grease for elastohydrodynamic lubricated point contact

This paper present tribological and dynamic evaluation of reduced graphene oxide (rGO) sheets as an additive in lithium grease. Highly dispersion mixing method is used to mix rGO in commercial lithium grease to prepare composite grease. Tribological contact under investigation is established by ball-on-disc configuration. Friction, noise and vibration responses are recorded for the point contact lubricated with composite grease and base lithium grease in rolling and sliding- induced-rolling conditions. Relative speed of disc with the speed of ball is varied in order to get sliding-induced-rolling contact. Observations are performed at different nornal loads and fixed speed in elastohydrodynamic lubrication (EHL) regime. Results show existence of an optimum concentration (0.4% w/w) of rGO in commercial lithium grease. Friction, noise and vibration are recorded minimum for concentration of 0.4% w/w of rGO in commercial lithium grease. Reduction in friction coefficient is recorded up to 30% and 20% for rolling contact and sliding-induced-rolling contact respectively at optimum concentration of rGO in lithium grease. The lamellar structure of rGO in base grease controls the lubricity of concentrated point contact.

Dynamic and tribological analysis of a toroidal continuously variable transmissions

The continuously variable transmission investigated in this paper works with contacts in the elastohydrodynamic regime of lubrication, thus the tangential forces between elements are transmitted through the shearing the lubricant film. The behaviour of the lubricant film when subjected to shear depends of the nature of the lubricant and the relative motion between the contacting surfaces. In this paper, a non-Newtonian behaviour is assumed for the lubricant while the relative motion is determined for every point on the contact area by kinematic methods. The net tractive force in the sliding direction and the spin torque are evaluated and from these the power losses in the contacts are calculated. The dynamic behaviour of the device is evaluated taking into account the behaviour of the lubricant as extracted from traction measurements. A simplified method for the evaluation of the dynamic response of the device to a rapid variation of the resistive torque is presented. The results show the importance of the rheological behaviour of the lubricant, measured by the slope of the linear region of lubricant traction curves.

Mixed lubrication of soft contacts: An engineering look

Mixed elastohydrodynamic lubrication of materials with low elastic modulus (soft materials) is investigated. Expressions for prediction of film thickness and the asperity load ratio in soft line-contact elastohydrodynamic lubrication are presented. The traction behavior of soft contact in mixed elastohydrodynamic lubrication regime is also studied in terms of the Stribeck curves.