Quantitative Viscosity Mapping Using Fluorescence Lifetime Measurements

Lubricant viscosity is a key driver in both the tribological performance and energy efficiency of a lubricated contact. Elastohydrodynamic (EHD) lubrication produces very high pressures and shear rates, conditions hard to replicate using conventional rheometry. In situ rheological measurements within a typical contact are therefore important to investigate how a fluid behaves under such conditions. Molecular rotors provide such an opportunity to extract the local viscosity of a fluid under EHD lubrication. The validity of such an application is shown by comparing local viscosity measurements obtained using molecular rotors and fluorescence lifetime measurements, in a model EHD lubricant, with reference measurements using conventional rheometry techniques. The appropriateness of standard methods used in tribology for high-pressure rheometry (combining friction and film thickness measurements) has been verified when the flow of EHD lubricant is homogeneous and linear. A simple procedure for calibrating the fluorescence lifetime of molecular rotors at elevated pressure for viscosity measurements is proposed.     

Influence of pressure and temperature dependence of thermal properties of a lubricant on the behaviour of circular TEHD contacts

The aim of this paper is to study the effects of pressure and temperature dependence of a conventional lubricant's thermal properties on the behaviour of heavily loaded thermal elastohydrodynamic lubrication (TEHL) contacts. For this purpose, a typical mineral oil (Shell T9) is selected and the dependence of its transport properties on pressure and temperature is investigated. Appropriate models are then developed for these dependencies. The latter are included in a TEHL solver in order to investigate their effect on the behaviour of circular EHD contacts. The results reveal the necessity of a thermal analysis including the pressure and temperature dependence of thermal properties for a good estimation of film thicknesses and mostly traction coefficients in circular EHD contacts operating under severe conditions. Numerical results are compared with experiments, showing a very good agreement over the considered ranges. This thorough validation of a thermal EHL framework for the calculation of film thickness and friction offers a previously unavailable opportunity to investigate the effects of variations in material properties.     

Effect of Base Oil Structure on Elastohydrodynamic Friction

The EHD friction properties of a wide range of base fluids have been measured and compared in mixed sliding–rolling conditions at three temperatures and two pressures. The use of tungsten carbide ball and disc specimens enabled high mean contact pressures of 1.5 and 2.0 GPa to be obtained, comparable to those present in many rolling bearings. The measurements confirm the importance of molecular structure of the base fluid in determining EHD friction. Liquids having linear-shaped molecules with flexible bonds give considerably lower friction than liquids based on molecules with bulky side groups or rings. EHD friction also increases with viscosity for liquids having similar molecular structures. Using pure ester fluids, it is shown that quite small differences in molecular structure can have considerable effects on EHD friction. The importance of temperature rise in reducing EHD friction at slide–roll ratios above about 5% has been shown. By measuring EHD friction at several temperatures and pressures as well as EHD film thickness, approximate corrections to measured EHD friction data have been made to obtain isothermal shear stress and thus EHD friction curves. These show that under the conditions tested most low molecular weight base fluids do not reach a limiting friction coefficient and thus shear stress. However, two high traction base fluids appear to reach limiting values, while three linear polymeric base fluids may also do so. Constants of best fit to a linear/logarithmic isothermal shear stress/strain rate relationship have been provided to enable reconstruction of isothermal EHD friction behaviour for most of the fluids tested.     

Non-newtonian thermal analysis of an EHD contact lubricated with MIL-L-23699 oil

A thermal and non-Newtonian fluid model under elastohydrodynamic lubrication conditions is proposed, integrating some particularities, such as the separation between hydrodynamic and dissipative phenomena inside the contact. The concept of apparent viscosity is used to introduce the non-Newtonian behaviour of the lubricant and the thermal behaviour of the contact into the Reynolds equation, acting as a link element between the hydrodynamic and dissipative components of the EHD film, independently of the rheological and thermal models considered. The apparent viscosity enables the application of the rheological model better adapted to each lubricant, without appealing to special formulations of the EHD problem.The Newton–Raphson technique is used to obtain the lubricant film geometry and the pressure distribution inside the EHD contact. The shear stresses developed in the fluid film are evaluated assuming the non-linear Maxwell rheological model. The surfaces and lubricant temperature distributions are determined using the simplified Houpert's method, applied to the inlet contact zone, and the thermal method proposed by Tevaarwerk is applied in the high pressure contact zone.The non-Newtonian thermal EHD model is applied to the analysis of a contact lubricated with MIL-L-23699 oil. Significant results are obtained for the centre and minimum film thickness, for the inlet shear heating and film thickness reduction factor (φ_T), for the temperature rise of the lubricant and of the surfaces and for the friction coefficient inside the contact, considering wide ranges of the operating conditions (maximum Hertzian pressure, inlet oil temperature, rolling speed and slide-to-roll ratio).Finally, the numerical traction curves determined are compared with the corresponding experimental results, showing very good correlation.     

The behaviour of friction modifiers under boundary and mixed EHD conditions

The behaviour of a range of model and commercial friction modifiers (FMs) has been evaluated under elastohydrodynamic (EHD) and boundary lubrication conditions. Using a series of long‐chain carboxylic acids, it has been shown that measured boundary friction coefficients (BFCs) decrease with increasing chain length, unsaturation level, temperature, and concentration. Base oil polarity was found to have no effect under these conditions. Commercial oleate esters in synthetic base fluids gave lower BFCs than nitrogen‐containing compounds under the same conditions. This difference was observed over a range of concentrations and temperatures. The friction performance of formulated oils under mixed and full‐film EHD conditions was found to be dependent on FM, base oil, and detergent type. Under boundary conditions, friction was found to vary with FM type, but the effect of changing the base oil and the detergent system was negligible.     

Thin film lubrication of sliding point contacts of AISI 52100 steel

The mechanism of thin film lubrication of sliding point contacts of AISI 52100 steel has been studied as a function of load, sliding speed, composition and temperature of the lubricant.Below certain critical combinations of Hertzian pressure, speed and temperature the surfaces are kept apart by an elastohydrodynamic lubricant film. The load carrying capacity of this film depends primarily on the effective viscosity of the lubricant in the contact region which decreases with bulk oil temperature and with increasing sliding speed, because of friction induced thermal effects. After breakdown of the EHD film, boundary lubrication may still prevent severe adhesive wear. The transition from the boundary lubricated regime towards the regime of severe adhesive wear is a function of load (normal force), speed and bulk oil temperature and possibly depends on the conjunction temperature. Irrespective of the initial lubrication condition, oxidation of the steel surfaces leads to the (re)establishment of low friction, mild wear conditions.     

Application of Spectroscopic Reflectometry to Elastohydrodynamic Lubrication Films Study

Optical measurement techniques have been successfully used for elastohydrodynamic (EHD) lubricant films studies for several decades and have significantly helped to understand the lubrication mechanisms within highly loaded machine contacts. Nevertheless, there are still many phenomena waiting for the explanation and new experimental approaches and measurements techniques are developed. Recent studies have provided promising results as to the application of spectroscopic reflectometry to the study of EHD films. Nevertheless, some simplifications were introduced. The main aim of this study was to develop a physically correct approach that could provide the additional information about the properties of lubricant film within highly loaded contacts. The principal part of this article was devoted to the effort to develop and verify the optical design suitable for such applications. This verification was carried out within lubricated contact formed between a steel barrel and sapphire disc without any semi-reflective layer. This simplified optical arrangement has enabled to obtain the correct lubricant film data and verify the applicability of the spectroscopic reflectometry for EHD lubrication films study. It represents the first step in this application of spectroscopic reflectometry and further research in the field of the behavior of thin solid films under high contact pressures is necessary to enable thin film measurements.     

In Situ Quantification of Oil Film Formation and Breakdown in EHD Contacts

Abstract

A measurement method using electrical impedance has been developed for simultaneous quantifications of the thickness (h) and breakdown ratio (α) of oil films in elastohydrodynamic (EHD) contacts. First, based on simplified geometrical and electrical models of EHD contacts, theoretical expressions of h and α were derived as explicit functions of the measured electrical impedance by using the Lambert function. Then, to verify the proposed measurement principle, oil film thickness measurements were conducted by using the electrical method together with the optical interferometry method in a ball-on-disc-type apparatus, which utilized the lubricated contact between a steel ball and a glass disc with a transparent conductive layer (i.e., an indium tin oxide layer). As a result, it was confirmed that the measured h-values obtained by the electrical method agreed well with those obtained by the optical method, under various test conditions with changing the entrainment speed, slide-to-roll ratio, normal load, and viscosity. Besides, it was also confirmed that the measured α-values obtained by the electrical method showed consistent correlations with the film parameter and the friction coefficient. It is hoped that the developed electrical method will be applied to practical metal-to-metal contacts (e.g., the contacts in practical ball bearings) to understand invisible behaviors of oil films in EHD contacts.     

Computational fluid dynamics and full elasticity model for sliding line thermal elastohydrodynamic contacts

Classically, the EHD problem is solved using the Reynolds assumptions to model the fluid behaviour, and the Boussinesq elastic deformation equation to model the solid response, both being coupled with the load balance equation. The development of an alternative approach is presented here in order to solve at once the Navier-Stokes equations (mass conservation and momentum equilibrium), the full elasticity and energy equations for the line EHD problem in a fluid-structure interaction approach.The Finite Element Method is used to solve the mathematical formulation in a fully coupled way, inspired from Habchi et al. (2008) [1]. After linearisation with the Newton procedure, all the physical quantities (pressure, velocity field, deformations and temperature) are solved together in a unique system. An important benefit of this approach is the possibility to implement in a simple manner the non-Newtonian and thermal effects; in fact all the quantities can vary through the film thickness. The extension to non-Newtonian rheology and the pressure and temperature dependencies for the viscosity and density are taken into account in a direct way to allow an acceptable prediction of the friction coefficient. Gradients across the film thickness and temperature fields in both the fluid and the two solids are naturally computed and analysed. As a case study, we focus first on the pure sliding cylinder-on-plane contact. It is shown that thermal effects due to friction in the central zone of the contact play a role in heating the lubricant at the inlet zone, via heat conduction in the solids. By increasing the Slide-to-Roll Ratio (SRR), the occurrence of dimples and the subsequent effects in different parts of the contact under zero entrainment velocity conditions are then studied.     

A Generalized Differential Colorimetric Interferometry Method: Extension to the Film Thickness Measurement of Any Point Contact Geometry

Whereas industrial elastohydrodynamic (EHD) contacts are generally noncircular, most experimental observations are made on sphere-on-plane conjunctions. The circular case is indeed a specific elliptical case, and it was widely investigated. The differential colorimetric interferometry (DCI) technique was often used to perform precise film thickness measurements in circular EHD contacts. From a single picture of the dynamic contact, it enables mapping the film thickness of the full conjunction, and postprocessing can be applied afterwards. Moreover, it is possible to record sequences at relatively high-frequency sampling. However, until now, the method could not be directly applied to noncircular conjunctions. In the present article, a generalized DCI method is proposed and assessed by several static and EHD validation cases for elliptical and torus-on-plane contact geometries. This new method no longer necessitates particular requirements on the contact shape while retaining the advantages of the original DCI method. It allows precise film thickness measurements in realistic industrial EHD contacts and opens the way for new experimental observations.     

The effect of surface texturing on thin EHD lubrication films

Surface texturing has been successfully used for conformal contacts in many tribological applications in an effort to diminish friction and wear. However, the use of such a surface modifications are still in nascent as far as highly loaded contacts between non-conformal surfaces are concerned. It is mainly caused by the fact that the presence of such micro-features within these contacts can significantly influence the pressure distribution within the contact. Nevertheless, it has been shown in recent studies that the surface texturing can also have beneficial tribological effects if the depth of micro-features is properly designed. This paper is devoted to the experimental study of the effect of the micro-dents of various depths on thin lubrication films to find an experimental evidence of the micro-feature depth threshold for surface texturing applications in highly loaded non-conformal surfaces. The behaviour of an array of micro-dents within thin EHD contacts has been studied by thin film colorimetric interferometry. The influence of surface texturing on lubricant film formation has been observed under sliding/rolling conditions. The significant effect of micro-dents depth on lubricant film thickness is observed for positive slide-to-roll ratio when the disc is moving faster than the micro-textured ball. The presence of deep micro-dents within lubricated contact results in film thickness reduction downstream. As the depth of micro-dents is reduced, this effect diminishes and beneficial effect of micro-dents on film thickness formation has been observed. No significant influence of micro-dents depth on lubricant film shape has been observed in case of negative slide-to-roll conditions when micro-dents do not cause film thickness reduction regardless of their depths.     

Oscillations induced in EHD film thickness by a step in entrainment speed

Transient elastohydrodynamic (EHD) lubrication conditions occur in the contacts of many machine elements, such as gears, cams, and reciprocating devices, as a result of their working cycles. These conditions also occur in rolling‐element bearings at the onset or cessation of motion. The aspect of film thickness in elastohydrodynamically lubricated contacts subjected to a very rapid change in entrainment speed has not received much attention from researchers, probably because it is seen as less problematic than a sudden fall of the entrainment speed, which theoretically can lead to film failure. For a sudden stop, however, it has been shown previously that the lubricant forms an entrapment, which is able to protect the contact in many cases when the motion resumes. In this paper, EHD film behaviour under sudden acceleration is investigated; the study covers three cases ‐ starting from zero film, starting from an entrapped film, and starting from a continuous, steady film.     

History,Origins and Prediction of Elastohydrodynamic Friction

There is currently considerable debate concerning the most appropriate rheological model to describe the behaviour of lubricant films in rolling–sliding, elastohydrodynamic contacts. This is an important issue since an accurate model is required to predict friction in such contacts. This paper reviews the origins of this debate, which primarily concerns a divergence of views between researchers using high pressure, high shear rate viscometry and those concerned with the measurement and analysis of elastohydrodynamic friction; the former advocate a Carreau-based shear stress/strain rate model while the latter generally favour an Eyring-based one. The crucial importance of accounting for shear heating effects in analysing both viscometric and friction data is discussed. The main criticisms levied by advocates of a Carreau-based model against Eyring’s model are discussed in some detail. Finally, the ability of both types of rheological model to fit elastohydrodynamic friction measurements for a quite simple, well-defined base fluid is tested, using previously measured pressure–viscosity behaviour for the fluid. Both models appear to fit the experimental data over a wide temperature range quite well, though fit of the Eyring model appears slightly closer than that of the Carreau–Yasuda model. Friction data from a wider range of well-defined fluid types are needed to identify categorically the most appropriate model to describe elastohydrodynamic friction.     

Elastohydrodynamic Film Thickness Mapping by Computer Differential Colorimetry

This paper describes a new experimental technique for the study of elastohydrodynamic (EHD) lubricant films. This technique, which is based on the computer processing of EHD chromatic interferograms, uses a combination of image analysis and differential colorimetry for film thickness evaluation. This approach overcomes some major limitations of conventional optical interferometry and allows the precise mapping of lubricant film thickness distribution in EHD contacts, including transient and quasistatic phenomena. The technique has been used for the evaluation of chromatic interference patterns obtained from a conventional optical test rig for rolling point contacts. Three-dimensional representations of lubricant film thickness and shape with high accuracy and spatial resolution have been obtained. The technique's accuracy has been checked and a comparison with conventional monochromatic interferometry has been done for validation. The technique's resolution has been confirmed through the observation of local film thickening just before the EHD exit constriction for both pure rolling and sliding conditions.     

Numerical modeling an elastohydrodynamic contact of shaped rollers with allowance for misalignment of their axes

A method of numerically solving an elastohydrodynamic (EHD) contact of shaped rollers with allowance for misalignment of their axes in a plane perpendicular to the rolling direction is advanced. The mode of EHD lubrication is typical of such friction assemblies as roller bearings and gearings, in which the contacting elastic bodies are separated by a lubricant film and deformed under the action of an external load. Results of numerical modeling demonstrate the significant effect of the misalignment angle on the distribution of pressure and thickness of the lubricant film in the EHD contact and can be used further to analyze friction in a contact area and the stress tensor in a subsurface layer. The mathematical model of the EHD contact is described through nonlinear integro-differential equations and inequalities. The computational algorithm is based on Newton’s method.     

The Lubricity of Gasoline

A study has been made of the lubricating properties of gasoline fuel. A conventional HFRR diesel fuel lubricity tester has been modified to measure gasoline wear. Using this test equipment, a number of features of gasoline lubricity have been investigated, including the comparative lubricating behavior of gasoline, the influence of detergent additives and oxygenates on wear and the wear behavior of a series of refinery streams employed in gasoline blending.

The lubricity of a range of pure organic chemicals known to be present in gasoline has also been studied. From these measurements it has been shown that, except for components such as dienes and diaromatics, the HFRR lubricating properties of most gasoline hydrocarbon constituents are broadly independent of chemical structure bur depend significantly on viscosity. Using these measurements, predictive wear equations based on gasoline group analysis have been developed.

Because it has been found that viscosity plays a role in determining the wear properties of gasoline, the elastohydrodynamic (EHD) film-forming and friction properties of gasoline have been measured and compared to those of diesel fuels. This shows that the combination of gasoline's very low viscosity and low pressure-viscosity coefficient results in very thin EHD film thickness generation and also very low friction in full-film EHD conditions.     

Mapping Shear Stress in Elastohydrodynamic Contacts

A new method has been, devised for investigating the theological properties of lubricant films in two-dimensional EHD contacts. A lubricated, sliding contact is produced between a sapphire flat and a steel ball. Thermal infrared emission microscopy is then employed to obtain 2-D maps of the variation of temperature rise due to friction across the contact. These maps are then used in conjunction with moving heal source theory to produce maps of energy dissipation and thus shear strength, of the lubricant film across the contact.

A series of mixtures of two lubricants, one giving high traction and one with low traction, have been studied using this technique to investigate the influence of lubricant, blending on shear stress and traction.     

Influence of the size,geometry, and material of the rollers on EHD traction

The influence of the parameters relating to the rolling elements on traction in EHD contacts is experimentally studied on a two-roller machine, in which experiments with point contacts are conducted for two paraffinic mineral oils, a synthetic naphthene, and a synthetic ester. Firstly, it is shown that the same traction curves are obtained in both internal and external contacts when the effective radius in rolling direction of the rollers is equal. The effect of an increase in the size of the rollers is to increase the film thickness and this results in a gentle decline in the maximum traction coefficient. As the effective radius in transverse direction is increased, so the traction decreases for a paraffinic mineral oil, while that for a synthetic naphthene remains constant independent of the geometry of the rollers. Finally, the effect of the material of the rollers is studied, employing rollers made of steel, ceramics, phosphorus bronze, brass, and aluminium alloy. The traction obtained under an identical normal load can be arranged according to the effective elastic modulus of the rollers for each oil. However, under identical contact pressure the same maximum traction curve is obtained independent of the material, but the decline in traction in the thermal region is slightly steeper with the ceramic than with the steel rollers because of the difference in the temperature rise of the fluid film.     

Current tools and techniques for EHL modelling

We describe the construction of genuine black box software to model in detail elastohydrodynamically lubricated contacts. Such models are necessarily built on a basis of sophisticated numerical software, since the operating parameters encountered in engine and machine components (e.g. an automotive valve train) span a very wide range of conditions from the nearly steady and hydrodynamic case to extreme EHD conditions with rapid transients. As transient phenomena are responsible for the non-failure of many machine components under EHD conditions, we regard the ability to model them as critical. The traditional correlations for lubricant oil film thickness (survey functions) do not account for transients. We are therefore using EHD modelling software as the core of several engine component simulation programs.     

Effect of wear on EHD film thickness in sliding contacts

A theoretical solution to the elastohydrodynamic (EHD) lubrication problem in sliding contacts, which takes into consideration the effect of the change in shape of the gap due to wear on the load‐carrying capacity, is presented. The model of such a contact is based on assumptions of Grubin and Ertel (von Mohrenstein). The resultant dimensionless Reynolds and film profile equations have been solved numerically for a number of cases with several values of thickness of the worn layer. Iteration of the EHD film thickness is performed by means of the secant method. Values of the calculated dimensionless film thickness are presented as a function of dimensionless wear. The conclusions concern the influence of the linear wear on the film thickness in heavily loaded sliding contacts. Copyright © 2006 John Wiley & Sons, Ltd.