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.     

On the shear stress of elastohydrodynamic lubrication in elliptical contacts

Results of mathematical modelling of elastohydrodynamic lubrication of rolling contacts are presented. Effects of dimensionless parameters such as speed, normal load, elliptical parameters and coefficient of limiting shear stress on shear stress distributions have been studied. Moreover, profiles on hydrodynamic pressure and film thickness in EHD contacts have been studied. It has been found that shear stress profiles on two contact surfaces in entraining direction are similar with each other in some way. Shear stresses of fluid film on contact surfaces vary with many factors, which reveals the mechanism of traction in elastohydrodynamically lubricated contacts.     

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.     

Multilevel solution for thermal elastohydrodynamic lubrication of rolling/sliding circular contacts

A fast multigrid approach is presented for the analysis of thermal elastohydrodynamic lubrication (EHL) under rolling/sliding circular contacts at high loads and high slip ratios with low computing time on a personal computer. This fast solver combines directiteration, multigrid, Newton-Raphson, Gauss-Seidel iteration, and multilevel multi-integration methods into one working environment that can reduce the computational complexity from O(n³ to O(nlnn) for the thermal EHL problem under rolling/sliding circular contacts. Since the couped Reynolds and energy equations are simultaneously solved by the Newton-Raphson scheme, the iteration for the convergence solution is less than those of the classical approach. Results show that thermal effects on the pressure profile and film thickness are significant for a wide range of loads, speeds and slip ratios. The maximum midfilm and surface temperature rise in the Hertzian contact region increases with increasing slip ratio, dimensionless speed, and load. The minimum film thickness decreases with increasing load and slip ratio, and decreasing dimensionless speed.     

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.     

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.     

Computer simulation of starvation in thrust-loaded ball-bearings

Starvation of lubricant supply is an important effect in high speed ball-bearing operation but it is poorly understood as it is difficult to examine experimentally. The work here uses a modified version of an existing advanced computational method of predicting ball motion in thrust-loaded bearings to study some effects of starvation. The influence of reduced lubricant supply on cage drag, pocket friction, elastohydrodynamic (EHD) film thickness and EHD traction is illustrated. In particular it is shown that partial starvation may be beneficial since the reduced drag forces allow the bearing to run at lower loads without experiencing the gross skidding which can cause sliding wear.     

Failure criteria in thin film lubrication: Investigation of the different stages of film failure

The mechanisms of failure of sliding lubricated concentrated steel contacts have been studied using a newly developed tribometer which allows fast separation of the sliding surfaces at the different stages of film failure. The appearance of the corresponding contact topography was investigated by means of optical interference microscopy. The results indicate a gradual transition from full to partial elastohydrodynamic (EHD) lubrication as a function of increasing load and increasing bulk oil temperature until complete failure of the EHD film occurs at critical triplets of normal load, sliding velocity and bulk oil temperature.     

Experimental results and analytical film thickness predictions in EHD rolling point contacts

In this work, we consider several types of lubricants—including non-Newtonian fluids—that were studied in EHD pure rolling point contacts under various operating conditions, leading us to explore a wide range of dimensionless parameters. The experimental results are compared with predictions given by the usual analytical EHL relationships and by more recently developed models. This broad comparison conducted with particular emphasis on minimum film thickness (h_m) showed a fair agreement between experimental data and a few predictions including some obtained from extended models. Commonly used elastohydrodynamic lubrication (EHL) models did not systematically give accurate h_m estimation, whereas minimum film thickness not only is a yield value but also serves as a key parameter in estimating lubrication regimes.     

Pressure-viscosity effect on the solutions of a lubricated thick elastic bonded strip

Utilizing the numerical method developed by the author for isoviscous, fully flooded, elastohydrodynamic lubrication of a rigid cylinder rolling or sliding on an elastic strip (layer) which is attached to a rigid substrate (bonded strip), the influence on the pressure-viscosity coefficient, α, upon solutions is investigated. The present solutions are obtained for contacts operating in the transition region betweeen isoviscous-elastic and piezoviscous-elastic regimes where a pressure spike can be expected in some sense.New sets of results are presented for central and minimum film thicknesses in dimensionless form when the bonded strip is thick (0γ1, where γ = a/t is the ratio of the half contact width to strip thickness). It is shown that the film thickness depends not only on the values of α but also is influenced by Poisson's ratio, v.     

The Influence of Lubricant Upon EHD Film Behavior During Sudden Halting of Motion

Although steady state elastohydrodynamic (EHD) lubrication is quite well understood both from the theoretical and from the experimental point of view, studies of transient effects in EHD are currently far less developed. This paper describes an experimental investigation into EHD film behavior during sudden halting of motion. A technique has been devised which enables both central lubricant film thickness and film thickness profiles to be measured every millisecond during halting of a ball on flat, sliding contact. This has enabled detailed information of influence of lubricant on film collapse during halting to be obtained. It is shown that film collapse occurs in two stages. The first is a very rapid reduction in film thickness with only very small changes in film geometry and thus pressure distribution. This is followed, as soon as entrainment ceases, by the formation of a lubricant entrapment, and subsequent slow leakage of fluid from the central film region. This paper focussed on the formation of this entrapment and the influence of the rheological properties of the lubricant, i.e. viscosity and pressure-viscosity coefficient, on its development and behavior.     

In Situ Pressure and Film Thickness Measurements in Rolling/Sliding Lubricated Point Contacts

The physical conditions—pressure, temperature, shear stress—generated in an elastohydrodynamic contact govern the rheological behavior of the lubricant within the contact, and thus its tribological performance.This paper presents in situ measurements of pressure and film thickness in EHD point contacts using respectively Raman microspectrometry and differential colorimetric interferometry. A model lubricant—a polyphenylether oil (5P4E)—is used. The influence of load, temperature and sliding speed at different mean entrainment speeds on pressure and film thickness distributions is investigated. The analysis is based upon the relative values taken by the Moes dimensionless parameters M and L. In all cases, the pressure peak tends to increase and to move towards the center of the contact when the slide to roll ratio increases. When the sliding speed reaches large values (100 to 180%), several cases are encountered: When M > L, the pressure peak softens and moves towards the outlet; The Petrusevich peak disappears and the pressure profile remains almost constant as M and L are close to each other; When M < L, the peak disappears along with a strong modification of the pressure distribution. The maximum value is significantly reduced and the area of contact increases.Measured film thickness profiles confirm these tendencies. Increasing the slide to roll ratio leads to an important film thickness reduction and modifies the position of the constriction. Furthermore, film increase localized between the contact center and the constriction region is observed. This indicates the presence of important thermal effects within the contact and is discussed in the light of recently published papers.     

Formulas Used in Thermal Elastohydrodynamic Lubrication

A numerical solution to the problem of thermal compressible elastohydrodynamic lubrication of rolling/sliding contacts has been tested over a wide range of operating parameters consisting of dimensionless load, speed, slip, and material parameters. A least squares exponential curve fit was used to develop simple formulas for the amplitude and location of the pressure spike, the minimum film thickness, and the maximum lubricant temperature as a function of the operating parameters. These equations can be used to evaluate parameters affecting the performance of rolling/sliding heavily loaded lubricated contacts.     

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.     

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.     

滚锥包络环面蜗杆传动润滑研究

本给出了滚锥包络环面蜗杆传动啮合齿面间诱导法曲率和相对速度计算公式,进而给出蜗轮和蜗杆啮合齿面间弹流油膜厚度计算公式;以此为基础,研究了该种传动蜗轮和蜗杆齿面间弹流油膜厚度分布规律,分析了油膜厚度的影响因素和影响规律。研究工作对于该种传动的设计与制造具有指导意义。     

The film-forming properties of polyalkylene glycols

It is now recognised that, for many practical applications, an important property of a lubricant is its ability to generate thick, elastohydrodynamic (EHD) films in concentrated contacts. This paper describes a study of the EHD film- forming properties of polyalkylenie glycol lubricants. A wide range of polyglycol structures have been examined, with different monomer types, initiators, and molecular weights. Film thickness has been measured at several different temperatures using both conventional and ultra-thin film interferometry. From the measured film thicknesses, the effective pressure–viscosity coefficients of the lubricants have been evaluated. This has enabled a systematic investigation of the effect of polyalkylene glycol structure on both pressure–viscosity coefficient and EHD film formation.     

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.     

Influence of surface roughness features on mixed‐film lubrication

An optical technique (three‐dimensional spacer layer imaging) has been developed to map accurately lubricant film thickness in thin‐film elastohydrodynamic (EHD) contacts. This experimental technique has been used to study the influence of surface roughness features, asperity height, and slope on EHD film thickness and pressure. Single ridges transverse to the entrainment direction were used to represent asperities. It was found that the ridges with lower slopes generate films of greater minimum thickness. Below a certain entrainment speed, the minimum film thickness declined at a rate dependent on the ridge slope. At low speeds, the ridges with higher slopes entrapped a larger volume of lubricant ahead of the ridge and along the entrainment direction. For all speeds, the highest ridges entrapped the most lubricant. Both ridge slope and ridge height had a negligible effect on mean film thickness in the contact. Asperity pressure increased with higher ridge slope, but was not influenced by entrainment speed. An increase in pressure was found where lubricant is entrapped upstream of a ridge.     

Elastohydrodynamics and failure prediction

Many rolling and sliding contacts in industrial machineries are lubricated elastohydrodynamically to mitigate friction and wear. Predictions of tribological failures and performance have been greatly enhanced in the past two decades due to advancements in elastohydrodynamics. In this paper, the major physical features and principles of elastohydrodynamic lubrication (EHL) along with the major modes of tribological failures are summarized, first, for these contacts. Brief discussions are given to current predictability of lubricant film thickness, contact pressure, temperature, and friction in elastohydrodynamic contacts, and how these variables are used in predicting contact fatigue life and the threshold conditions for sliding failure.