Full Film Lubrication Regime Research Articles

Homogenization coefficients for modeling the partial and full-film lubrication regimes

Most mechanical systems have mating interfaces that require effective lubrication for smooth functioning. Thus, understanding and accurately modeling lubrication regimes is an essential research task. The present work proposes a set of normalized coefficients based on homogenization and stochastic theories to model the partial and full-film lubrication regimes between a smooth and a rough surface. The modified Greenwood and Williamson theory, considering inter-asperity-interactions, was used to model the contact-zones. Subsequently, the film-thickness was evaluated and the homogenized Reynolds equations were solved to compute the normalized coefficients. The estimated coefficients were compared with Patir and Cheng’s flow-factors, and the similarities and disagreements between both approaches were discussed. New sets of coefficients for running-in surfaces were proposed for modeling their lubrication regimes.

Effect of geometrical, operational and material parameters in the lubrication regime of hard-on-hard hip implants

The lubrication regime of a hip implant is influenced by various parameters, including material, geometry, roughness, relative angular movement, and loading circumstances. These factors impact friction and wear performance, ultimately limiting the implant's longevity. In this study, a ball-on-plane model is considered to predict the lubrication regime across the entire gait cycle for the metal-on-metal and ceramic-on-ceramic hip implants. According to ISO 14242-1, a normal walking gait cycle is considered for the loads and rotations in the hip implant for this study. A comprehensive analysis is done to estimate the lubrication regime by considering different material combinations, body weights, femoral head sizes, clearances, and roughness across the entire gait cycle. The correlation coefficients show that the geometrical parameters are dominant in affecting the lubrication film thickness compared to the operational and material parameters. The femoral head size and body weight are found to be the most dominant and least dominant parameter respectively. Among the hard-on-hard tribo-pairs, ceramics operate predominantly in the full-film lubrication regime compared to metals due to its fine surface finish. The present research suggests that in order to maximise the longevity of a hip implant, an orthopaedic surgeon should choose one with a larger femoral head diameter, less clearance, and an ultra-fine surface finish, regardless of any material tribo-pair.

Experimental study on the feasibility of alternative materials for tilting pad thrust bearings operating in transition to mixed friction

In hydrodynamic bearings traditional bearing alloys: Babbitts and bronzes are most frequently utilized. Polymer sliding layers are sometimes applied as a valuable alternative. Hard diamond-like carbon (DLC) coatings, which are also considered for certain applications may show some advantages, as well. Although material selection is of secondary importance in a full film lubrication regime it becomes important in mixed friction conditions, which is crucial for bearings with frequent starts and stops. Experimental research aimed at studying the performance of fluid film bearings in the specific operating regime, including the transition to mixed friction, is described in the paper. The tests were carried out on four tilting pad bearings of different material compositions: Steel/bronze, DLC/steel, steel/polyether ether ketone (PEEK), and steel/Babbitt. The tests comprised stopping under load and reproduction of the Stribeck curve by decreasing rotational speed to very low values, and observing the changes of friction force during the transition to mixed friction regime. Analysis of the transition conditions and other results showed clear differences between the tested bearings, illustrating the feasibility of less popular material compositions for bearings operating in specific conditions. More specifically, the DLC/steel bearing was demonstrating superior performance, i.e. lower friction, transition to mixed friction occurring at higher load, and more stable performance at start-stop regime over the other tested bearings.

Tribological variable-friction coefficient models for the simulation of dense suspensions of rough polydisperse particles

The rheology of concentrated suspensions of particles is complex and typically exhibits a shear-thickening behavior in the case of repulsive interactions. Despite the recent interest arisen, the causes of the shear-thickening remain unclear. Frictional contacts have been able to explain the discontinuous shear thickening in simulations. However, the interparticle friction coefficient is considered to be constant in most simulations and theoretical works reported to date despite the fact that tribological experiments demonstrate that the friction coefficient can not only be constant (boundary regime) but also decrease (mixed regime) or even increase (full-film lubrication regime), depending on the normal force and the relative velocity between the particles and the interstitial liquid between them. Interestingly, the transition between the boundary regime and the full-lubrication regime is governed by the particle average roughness. Particle-level simulations of suspensions of hard spheres were carried out using short-range lubrication and roughness-dependent frictional forces describing the full Stribeck curve. Suspensions with different particle’s roughness were simulated to show that the particle roughness is a key factor in the shear-thickening behavior; for sufficiently rough particles, the suspension exhibits a remarkable shear-thickening, while for sufficiently smooth particles, the discontinuous shear-thickening disappears.

Effect of Shear Strength of Oil on Lubrication of Cold Strip Rolling in Full-Fil Regime

Cold strip rolling is a major deformation process in industry. The need for high quality products and increased production speed, makes the application of the lubricant important. In the present study, oil property effect is investigated in cold rolling using oil lubricant in full-film lubrication regime. The model predicts decisive role of oil property in rolling parameters.

Lubrication and Wear Characteristics of Mechanical Face Seals under Random Vibration Loading.

The service life of mechanical face seals is related to the lubrication and wear characteristics. The stable analytical methods are commonly used, but they cannot address effects of random vibration loading, which, according to experimental studies, are important factors for lubrication and wear of mechanical face seals used in air and space vehicles. Hence, a dynamic model for mechanical face seals is proposed, with a focus on the effects of random vibration loading. The mechanical face seal in the axial direction is described as a mass-spring-damping system. Spectrum analysis specified for random vibration is then performed numerically to obtain the response power spectral density (PSD) of the mechanical face seal and calculate the root mean square (RMS) values under random vibration conditions. A lumped parameter model is then developed to examine how dynamic parameters such as stiffness and damping affect the lubrication regimes of mechanical face seals. Based on the dynamic model and Archard wear equation, a numerical wear simulation method is proposed. The results elucidated that the increase of input acceleration PSDs, the decrease of axial damping, and the increase of axial stiffness lead to the probability of the mechanical face seal operating under full film lubrication regime increase and finally the decrease of wear. This research provides a guideline for improving the adaptability of mechanical face seals under random vibration environments.

Mapping the friction coefficient of AISI 316L on UHMWPE lubricated with bovine serum to study the effect of loading and entrainment at high values of sliding-to-rolling ratio

From an experimental point of view, determining the lubrication regime established between contact surfaces of joint prosthesis using the theory of elastohydrodynamic lubrication is rather impractical due to the intrinsic hindrances to measure the thickness of the lubricant film at the contact area. This claim is supported by the results obtained by experts using optical interferometry to perform such measurements using synovial like fluids. Similarly, research has shown that the complex rheology of the synovial fluid complicates the direct application of EHL theory and plays an important role in the lubrication mechanisms at work in prosthetic devices. Since the natural response of all tribological systems is the frictional force, rather than trying to measure the thickness of the lubricant film to determine the lubrication regime, we measured the effect of loading and entrainment on the friction coefficient at the contact point of an AISI 316L stainless steel sphere loaded against an ultra-high molecular weight polyethylene disc, lubricated with fetal bovine serum solution, at high values of sliding-to-rolling ratio. Applying statistical analysis, we obtained a best-fit model that shows a smooth transition from mixed to full-film lubrication regime

Film thickness build-up in zero entrainment velocity wide point contacts

Abstract In this work, elastohydrodynamic wide point contacts are studied under Zero Entrainment Velocity (ZEV) conditions. Contrary to classical rolling-sliding contacts, no hydrodynamic lift (oil wedge effect) is expected in stationary isothermal ZEV contacts. However, both experiments and numerical simulations taking into account temperature gradients in the thickness of the lubricant show the occurrence of a full-film lubrication regime over a large range of surface velocities, contact loads and external temperatures. Numerical simulations, including thermal effects but neither transient nor wall slip effects, are in good agreement with experimental film thickness measurements. They allow for both a qualitative description of the mechanisms at stake and to a first tentative minimum film thickness prediction under those specific conditions.

Experimental and Numerical Investigations of the Stribeck Curves for Lubricated Counterformal Contacts

The Stribeck curve is an important means to demonstrate the frictional behavior of a lubricated interface during the entire transition from boundary and mixed to full-film lubrication. In the present study, a new test apparatus has been built that can operate under rolling–sliding conditions at a continuously variable speed in an extremely wide range, approximately from 0.00006 to 60 m/s, covering six orders of magnitude. Hence, a complete Stribeck curve can be measured to reveal its basic characteristics for lubricated counterformal contacts. The measured curves are compared with numerical simulation results obtained from an available unified mixed elastohydrodynamic lubrication (EHL) model that is also capable of handling cases during the entire transition. A modified empirical model for the limiting shear stress of lubricant is obtained, and a good agreement between the measured and calculated Stribeck curves is achieved for the tested base oils in all the three lubrication regimes, which thus well validates the simulation methods employed. Both the experimental and numerical results indicate that the Stribeck curves for counterformal contact interfaces behave differently from those for conformal contacts. When the rolling speed increases at a fixed slide-to-roll ratio, the friction continuously decreases even in the full-film lubrication regime due to the reduction of the lubricant limiting shear stress caused mainly by the rise of the surface flash temperature. In addition, the test results indicate that the boundary additives in a commodity lubricant may have considerable influence on the boundary lubrication friction but that on the friction in the mixed and full-film lubrication appears to be limited.

Investigation of Shear-Thinning Behavior on Film Thickness and Friction Coefficient of Polyalphaolefin Base Fluids With Varying Olefin Copolymer Content

This study investigates the rheological properties, elastohydrodynamic (EHD) film-forming capability, and friction coefficients of low molecular mass poly-α-olefin (PAO) base stocks with varying contents of high molecular mass olefin copolymers (OCPs) to assess their shear stability and their potential for energy-efficient lubrication. Several PAO–OCP mixtures were blended in order to examine the relationship between their additive content and tribological performance. Gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) spectroscopy were used to characterize the molecular masses and structures, respectively. Density, viscosity, EHD film thickness, and friction were measured at 303 K, 348 K, and 398 K. Film thickness and friction were studied at entrainment speeds relevant to the boundary, mixed, and full-film lubrication regimes. The PAO–OCP mixtures underwent temporary shear-thinning resulting in decreases in film thickness and hydrodynamic friction. These results demonstrate that the shear characteristics of PAO–OCP mixtures can be tuned with the OCP content and provide insight into the effects of additives on EHD characteristics.

Determination of rolling element bearing condition via acoustic emission

Acoustic emission is an emerging technique for condition monitoring of rolling element bearings and potentially offers advantages for detection of incipient damage at an early stage of failure. Before such a technique can be applied with confidence for health monitoring, it is vital to understand the variation of acoustic emission generation with operating conditions in a healthy bearing. This paper investigates the effects of increased speed and load on the generation of acoustic emission within cylindrical roller bearings, and it was found that the root mean square signal level increased significantly with increasing speed whereas increasing load had a far weaker effect. The AERMS value for each experiment was compared with the trend of the Lambda value. The bearing was operating under full film lubrication regime, so it was determined that increases in AERMS were not caused by asperity contact. By consideration of trends in frequency energy amplitude, it was determined that excitation of the bearings resonant frequencies were responsible for an increase of energy in the frequency range of 20–60 kHz. The excitation energy at 330 kHz (the acoustic emission sensor’s resonant frequency) increased with load, indicating a link between high-frequency emission and stress at the contact zone. Following characterisation of the bearing under normal operating conditions, an accelerated life test was conducted in order to induce fatigue failure. The frequency response demonstrated that throughout a period of constant wear, the energy amplitude at the bearings resonant frequency increased with time. As the bearing failure became more significant, the energy of the high-frequency components above 100 kHz was spread over a broader frequency range as multiple transient bursts of energy were released simultaneously by fatigue failure of the raceways. This paper demonstrates the potential of acoustic emission to provide an insight into the bearing’s behaviour under normal operation and provide early indication of bearing failure.

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.

Spontaneous Blinking from a Tribological Viewpoint

The mechanical forces between the lid wiper and the ocular surface, and between a contact lens and the lid wiper, are reported to be related to dry eye symptoms. Furthermore, the mechanical forces between these sliding partners are assumed to be related to the ocular signs of lid-wiper epitheliopathy (LWE) and lid-parallel conjunctival folds (LIPCOF). Recent literature provides some evidence that a contact lens with a low coefficient of friction (CoF) improves wearing comfort by reducing the mechanical forces between the contact lens surface and the lid wiper. This review discusses the mechanical forces during spontaneous blinks from a tribological perspective, at both low and high sliding velocities, in a healthy subject. It concludes that the coefficient of friction of the ocular surfaces appears to be strongly comparable to that of hydrophilic polymer brushes at low sliding velocity, and that, with increased sliding velocity, there is no wear at the sliding partners' surfaces thanks to the presence of a fluid film between the two sliding partners. In contrast, in the case of dry eye, the failure to maintain a full fluid film lubrication regime at high blinking speeds may lead to increased shear rates, resulting in deformation and wear of the sliding pairs. These shear rates are most likely related to tear film viscosity.

LUBRICATION PREDICTIONS FOR TOTAL KNEE PROSTHESIS MADE OF HARD MATERIALS

The lubricated contact zone in a total knee prosthesis is modeled, where femoral and tibial components are supposed to be made of hard materials (high stiffness and hardness) and lubricated by a non Newtonian fluid (synovial liquid). To determine optimal relations among geometric parameters, load and film thickness, a generalized Reynolds equation was solved by means of finite element method. Although mix lubrication regime is predominant, it is shown that increasing conformity, it could be assure a full-film lubrication regime in most of the gait cycle. These results might be a reference to make more durable total knee prostheses.

Studying The Effect of The Surface Roughness on the Maximum Eccentricity Ratio and The Load Carrying Capacity in The Journal Bearing

The focus of this investigation is to study the effect of the two dimensional isotropic surface roughness of both , journal and bearing on the maximum eccentricity ratio and the load carrying capacity of the finite length plain journal bearing , on the basis of full film lubrication regime to reduce the wear and heat produced by friction to extend the machine element life , in the steady state operation conditions. The appropriate equation to predict the suitable minimum film thickness which include the surface roughness is used . The Conventional Reynolds equation in two dimensional forms are solved numerically . The change of viscosity due to temperature and pressure , using adiabatic solution ,were taken into account. To investigate the effect of surface roughness on the maximum eccentricity ratio value and load capacity , it was assumed that there are two surfaces smooth and rough and are compared with each other . The results of this work show that the increase of the surface roughness case the maximum eccentricity ratio decreases at L / D ratios of 0.5 , 1 at range of roughness 0.3 – 1.2 Mm , which means that the limit of bearing to work with a steady state , full film lubrication . The load ratio at L / D = 0.5 is greater than at L / D = 1 for clearance ratio 0.001 , 0.002 . The surface roughness have no influence on the bearings that have 90 mm diameter or larger at clearance ratio 0.002 .

A Parametric Study on Fatigue Life for Mixed Elastohydrodynamic Lubrication Point Contacts

The lubrication characteristics and fatigue life are numerically analyzed under full film and mixed lubrication regimes, in which the three-dimensional sinusoidal surfaces with changeable wavelengths in x and y directions are used, the geometry changes of the contact areas are described by the various ellipticity, and the non-Newtonian flow of lubricant is described by the sinh-law rheology model. The results show that the influences of characteristic shear stress, wavelength ratio, and ellipticity on lubrication characteristics and fatigue life are remarkable. The effect of surface topography on lubrication characteristics has a close relationship with speed. Increasing the ellipticity and decreasing wavelength ratio and characteristic shear stress can prolong the fatigue life.

Energy Efficient Siloxane Lubricants Utilizing Temporary Shear-Thinning

This study investigates the rheologic properties, elastohydrodynamic film, and friction coefficients of several siloxane-based lubricants to assess their shear stability and their potential for energy efficient lubrication. Several siloxane-based polymers with alkyl, aryl, and alkyl-aryl branches were synthesized in order to examine the relationship between their molecular structures and tribological performance. Nuclear magnetic resonance spectroscopy and gel permeation chromatography were used to characterize the molecular structures and masses, respectively. Density, viscosity, elastohydrodynamic film thickness, and friction measurements were measured from 303 to 398 K. Film thickness and friction measurements were made at loads and speeds that cover the boundary, mixed, and full film lubrication regimes. These results illustrate that the shear characteristics of siloxane lubricants vary significantly with polymer length as well as branch structure and content. The findings provide quantitative insight into the features of siloxane molecular structure conducive to optimum film formation with minimum wear and elastohydrodynamic friction to enhance energy efficiency.

Spectrum loading effects on the running-in of lubricated bronze and surface-treated titanium against alloy steel

Lubricated bearing surfaces like those in engine connecting rods commonly experience transient loading in service. These transients can affect friction, wear rate, and regime of lubrication. It was therefore of interest to investigate the effects of periodically-varying loads on candidate bearing materials under non-steady-state conditions and running-in. Light weight titanium alloys could be used for bearing components in fuel-efficient vehicles if their sliding friction and wear characteristics were improved. Candidate surface treatments and coatings for Ti–6Al–4V alloy were tribo-tested under diesel oil lubrication with time-varying loads. A variable load bearing test (VLBT) apparatus was built for that purpose. Baseline tests of a bearing bronze (CDA 932) against alloy steel (AISI 8620) showed that wear during a variable load test was greater than that for the same duration test under a constant load equal to the average of the spectrum loads. Spectrum loading effects on the running-in and steady-state friction and wear behavior of bronze and several Ti alloy surface treatments and coatings are discussed. Results indicate that the progression of observable wear and surface damage may be difficult to quantify under the nearly full-film lubrication regimes as are found in engine bearings. Implications for screening candidate materials and coatings for lightweight engine bearings are discussed.

Traction Characteristics of Siloxanes with Aryl and Cyclohexyl Branches

The molecular structures, rheological properties, and friction coefficients of several new siloxane-based polymers were studied to explore their traction characteristics. The molecular structures including branch content were established by nuclear magnetic resonance spectroscopy, while the molecular mass distributions were determined by gel permeation chromatography. Density, viscosity, elastohydrodynamic film formation, and friction were investigated over a temperature range of 303–398 K. Film thickness and friction measurements were studied under the conditions that are representative of boundary, mixed, and full-film lubrication regimes, aiming at maximizing traction performance and temperature stability by simultaneous optimization of the size and content of ring-shaped branch structures. This study provides quantitative insight into the effect of siloxane molecular structure on the tribological performance for traction drive applications such as continuously variable transmissions.

Lubrication Properties of Polyalphaolefin and Polysiloxane Lubricants: Molecular Structure–Tribology Relationships

This study investigates the rheological properties, elastohydrodynamic film thickness, and friction coefficients of several commercially available polyalphaolefin (PAO) and polydimethylsiloxane (PDMS)-based lubricants to assess relationships between molecular structure and lubricant performance. Molecular structures and masses were determined by nuclear magnetic resonance spectroscopy and gel permeation chromatography, respectively. Density and viscosity are measured from 303 to 398 K, while elastohydrodynamic lubricant film thickness and friction measurements were made at temperatures, loads, and speeds that are representative of boundary, mixed, and full-film lubrication regimes. The results show that PDMS-based lubricants are thermally and oxidatively more stable than PAOs, while the viscosity of PDMS-based lubricants is generally less temperature sensitive than PAOs, except for highly branched polysiloxanes. In particular, this study provides quantitative insight into the use of PDMS-based lubricants to obtain low friction through the entire lubrication regime (boundary to full film) by optimal tuning of the molecular mass and chain branching.