Rolling bearings are the second most used machine components. They work in what it is called elastohydrodynamic lubrication regime. The geometry of rolling element bearings makes the direct measurement of the lubricant film thickness a challenging task. Optical interferometry is widely used in laboratory conditions for studying elastohydrodynamic lubrication however it cannot be used directly in rolling element bearings thus the only suitable methods are electrical techniques. Of these, film thickness measurement based on electrical capacitance of the contacts has been used in the past by a number of authors. One of the limitations of the capacitance method, when used in rolling bearings, is that it cannot distinguish between the contacts of every rolling element and raceway on one hand and on the other between the inner and outer ring contacts. In the present study the authors used an original test rig which can measure the film thickness for only one ball and separately for the inner and outer rings of a radial ball bearing. This paper thus shows for the first-time results of the lubricant film thickness, at the inner and outer raceways, in grease lubricated rolling bearings.

There are a number of widely used machine components, such as rolling element bearings, gears and cams, which operate in the lubrication regime known as Elastohydrodynamics (EHD), where lubricant film thickness is governed by hydrodynamic action of convergent geometry, elastic deformation between non-conformal contacting surfaces, and the increase of lubricant viscosity with pressure. Variable loading conditions occur not only in all the machine components mentioned above, but also in natural joints such as hip or knee joints of humans or many vertebrates. Experimental studies of the behaviour of EHD films under variable loading are scarce and to authors’ knowledge systematic studies of the evolution of lubricant film thickness in EHD contacts subjected to forced harmonic variation of load are even less common. The aim of the present study is to explore the effect of load amplitude on the EHD film behaviour. This is done in alternating cycles with the load varying about a fixed, preset value at various amplitudes. Experimental results are compared with a simple theoretical analysis based on the speed of change of contact’s dimensions, a semi-analytical solution which includes both speed variation and squeeze effect, and finally with a full numerical solution.

Oil lubrication of the contacts between high-elastic modulus, non-conformal surfaces is well understood in fully-flooded, steady state conditions, when there is generous supply of lubricant in the inlet and the parameters that govern the film thickness or the friction force do not vary in time. Semi-analytical or full numerical formulas have been derived which can predict the lubricant film thickness in terms of the working parameters and the properties of the materials involved. In case of lubricating greases, their semi-solid nature given by the presence of a solid phase makes the prediction of the film thickness and thus the design of various systems difficult. The equations developed for oil lubricants can be used, using the properties of the base oil however it was found experimentally that the actual film deviates strongly from theoretical values for values of the entrainment speed above a certain critical value. Grease-lubricated contacts are very important because over 90% of the second most numerous machine components that is, the rolling element bearings are lubricated with greases. An added actor of uncertainty is the variable load on the contacts between the rolling elements and the raceways, due to on one hand the passage of the former through the loaded region of the bearing and on the other due to vibrations likely to be experienced by the bearing. In this paper film thickness measurements were carried, in a test rig based on optical interferometry under both steady-state and variable loading conditions. It was found that severely starved contacts, which run under very thin films, show a degree of recovery of the film thickness towards fully flooded values, depending of the frequency of the variable load and the properties of the grease.

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 concur to determine the formation of a continuous fluid film that separates the solid surfaces. Experimental and theoretical work, 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.

This article describes experimental and theoretical studies on the cavitation phenomena in the grease lubrication film under pure sliding elastohydrodynamic contact. In-situ observation tests using the optical interferometry technique were conducted, and the growth of cavitation was captured using a high speed camera. The results showed that the cavity grew in two stages, which was similar to the behaviour in the base oil, and that the cavity growth rate in the initial stage was higher than that in the second stage. In the initial stage, the cavity growth time in the grease was longer than that in the base oil, and the cavity length after the growth depended on the base oil viscosity. It was also found in the test using diurea grease that small cavities were formed by the lumps of thickener. The cavity growth in the initial stage was discussed by numerical simulation of pressure distribution based on a simple rheological model.

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

Machine elements such as rolling element bearings or gears often experience vibrations due to for example geometrical inaccuracies, shock loading, rotating unbalanced masses, and others. These machine elements rely on a very thin lubricant film to protect the metallic surfaces from direct contact and eventual damage. Rapid variation of load in elastohydrodynamic contacts it is influenced by the so-called squeeze film effect, however, when both entrainment and squeeze are present, the conditions of film formation are more complex. It is expected that the lubricant film thickness is influenced by the amplitude and frequency of the vibrations. At the same time, as it is known that the film thickness is established in the inlet of the contact, it is equally important to evaluate what is the role played by the supply of lubricant to the contact under oscillatory conditions. To date there are not many studies on the effect of the oscillatory motion parameters upon the behaviour of the lubricant film. In this study the focus is on the effect of the frequency of vibrations and the supply of lubricant upon the film thickness.

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

It has been shown before that liquids can slip at a solid boundary, which prompted the idea that parallel-surfaces bearings can be achieved just by alternating slip and non-slip regions in the direction of fluid flow. The amount of slip at the wall depends on the surface tension at the liquid–solid interface, which in turn depends on the chemical state of the surface and its roughness. In the present study a heterogeneous surface was obtained by coating half of a circular glass disc with a coating repellant to glycerol. A rotating glass disc was placed at a known/calibrated distance and the gap was filled with glycerol. With the mobile surface moving from the direction of slip to non-slip region it can be theoretically shown that a pressure build up can be achieved. The pressure gradient in the two regions is constant, similar to that in a Rayleigh step bearing, with the maximum pressure at the separation line. The heterogeneous disc was placed on a holder supported by a load cell thus the force generated by this pressure increase can be measured accurately. Tests were carried out at different sliding speeds and gaps and the load carried was measured and subsequently compared with theoretical calculations. This allowed the slip coefficient to be evaluated.

The continuously variable transmission investigated in this paper works with contacts in the elastohydrodynamic regime of lubrication, thus the tangential forces are transmitted between elements through the shearing the lubricant film. The behavior 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 behavior 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 behavior of the device is evaluated taking into account the rheological behavior of the lubricant.

Electrical capacitance has been applied in the past for measuring the lubricant film thickness in rolling element bearings. The main difficulty arises from the fact that the measured capacitance is a combination of the capacitances of many rolling elements, which come in contact with both the inner and outer rings. Besides, the capacitance of the Hertzian contact itself and the surrounding area must also be separated. It results in a complex system which, in order to be solved for the film thickness at a particular location on the bearing many approximations have to be made. In the present study the authors use an experimental rig in which the capacitance of a single ball can be isolated. Moreover the capacitance of the ball – inner ring and ball – outer ring contacts can be measured separately.

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

The elastohydrodynamic (EHD) lubrication regime occurs in many machine elements where a combination of hydrodynamic effect, elastic deformation of the loaded surfaces and increase of the viscosity of the lubricant with pressure ensure the formation of a very thin, but continuous film of lubricant separating the contacting surfaces. Electrical methods to determine this film’s thickness have preceded optical methods, which are widely used today. Although they generally give more qualitative thickness information, electrical methods have the main advantage that they can be applied to metallic contacts in machines, which makes them useful tools in the study of elastohydrodynamically lubricated contacts. The present paper is part of a larger study on the application of electrical capacitance for the evaluation of film formation in EHD contacts. The main focus is on the quantitative measurements of film thickness using electrical capacitance. A new approach allowing the lubricant film thickness to be extracted from the measured capacitance is developed using a chromium-coated glass disc and subsequently applied to a steel-on-steel contact. The results show good agreement with optical measurements and theoretical models over a range of film thickness.

This paper reviews the electrical methods to evaluate lubrication, with the focus on elastohydrodynamic applications. The methods are based on the measurement of the electrical resistance and/or capacitance, and are used to determine the thickness of lubricating films as well as to detect asperity contact in the mixed lubrication regime. The experimental works are introduced in which these methods are used in conventional sliding and rolling/sliding tests and machine component tests including piston ring and cylinder liner, cam and follower, and gears. Also some works on the effect of electric field upon lubrication phenomena are introduced. Advantages and disadvantages of the electrical methods are discussed.

Greases are attractive solutions to lubrication problems due to certain advantages over oils, such as easier manipulation, no need of resupply and filtering systems and lower lubricant losses. They are widely used in various machine components which work in the elastohydrodynamic lubrication regime. Many experimental studies have revealed that, in the absence of mechanisms to replenish the track grease lubricated contacts suffer of starvation which translates in poor lubrication and thin film thickness. In the present paper the lubrication performance of greases in contacts subjected to lateral oscillations is analyzed. The contact under study is formed between a flat disc and a ball. The ball is driven in an oscillatory motion in a direction transverse to the rolling direction. The thickness of the EHD film was measured with the optical interferometry technique. It has been found that, in the conditions of the tests, lateral oscillations have a beneficial effect, improving grease's lubrication performance even at large rolling speeds.

Increasing environmental demands push the limits of lubrication of machine elements, towards thinner films, higher operating temperatures, lower friction and higher durability. In this context the need for a better understanding of the physical and chemical processes which take places at intimate, molecular levels of rubbing surfaces, becomes crucial. Grease have important advantages over oil lubricants such as lower lubricant losses, better corrosion protection, improved sealing, easy to handle and the elimination of complex and costly oil supply system. At the same time the complex nature of lubricating grease is little understood and only little knowledge about its chemistry and physics is accessible. In the present paper the relation between lubricating grease composition and characteristics and the cavitation length of point EHD contacts was evaluated.

In many machine elements working in the elastohydrodynamic lubrication regime the components are subjected to low amplitude oscillatory motions, which disturb the conditions for optimum formation of the lubricant film. Most of these applications involve combined vibrations in both vertical and longitudinal directions. For example, in gears and rolling element bearings vertical vibrations may be caused by changes in load, speed or geometry at the contact point, while the longitudinal vibration is often a direct result of the torsional vibrations of the shaft. Vibrations also transmit to these machine elements from other parts of the assembly or from the surrounding environment. Vibrations are usually harmful because they reduce the accuracy of motion and may impede the formation of a lubricant film, thus affecting the durability of machine components.The aim of this is study is to investigate the behaviour of EHD films during lateral oscillatory motion using the optical interferometry technique.

It is well-documented that parameters, such as film thickness and temperature in EHL contacts, can be measured experimentally using a range of techniques include optical interferometry, ultrasonics, capacitance and infrared emission. Considerably less is known, however, about the flow of lubricant through such contacts. Information about lubricant flow would greatly benefit the prediction of friction in machine components. This article describes initial steps to develop fluorescence as a means of observing lubricant flow. An EHL contact was produced between a steel ball and a glass disc and viewed using a fluorescence microscope. The entrained lubricant was dyed using a fluorescent species, so that when illuminated with laser light, a fluorescence intensity map could be viewed. When the contact was fully flooded with dyed lubricant, the fluorescence intensity within the contact correlated well with optical interferometric film thickness measurements under the same conditions. This suggests useful possibilities for mapping film thickness in contacts where conventional optical methods are impractical, such as between rough surfaces and within soft contacts. In order to observe how lubricant flows in an EHL contact, fluorescer-containing lubricant was placed on the out-of-contact track. The boundary between fluorescent and non-fluorescent lubricant was then entrained into the contact and the passage of the boundary through the contact was monitored.

Grease lubricants are often preferred in machine elements working under elastohydrodynamic conditions such as rolling element bearings and constant velocity joints due mainly to the fact that they do not need resupply and filtering systems and provide lower lubricant losses. At the same time grease are complex materials, with liquid and solid phases, which make difficult the prediction of their behaviour in those contacts. In the present paper dielectric properties of lubricating greases are studied, as a step towards the understanding of their behaviour and predicting their in-contact rheological properties.

The intense competition on the global markets faced by manufacturing companies is reflected by critical issues like decreasing the product-development-cycle time, lowering costs and increasing quality. The quest to meet the market requirements can be viewed as a multi-criteria decision-making problem set within the space of engineering characteristics for the designed system. In this paper one aspect of a particular design method: Quality Function Deployment (QFD) is used to asses different design concepts for transmission systems according to specific criteria linked with requirements. The engineering design process is based on a structured design methodology and multiple aspects of the design process are presented from the perspective of a set of criteria and objectives like: power density, ratio range, efficiency, drivability, response time, smoothness, noise and costs. This paper attempts to explore the intimate relation between the most important features for the system definition in specific applications and their relation to different concepts. Various concepts are assessed based on this relationship, whilst dimensions of individual components of a novel CVT are optimised to meet the derived requirements.

Many machine components, which work in the Elastohydrodynamic (EHD) lubrication regime, including transportation bearings, reciprocating engines, geared transmissions experience vibrations, which result in a rapid variation of contact load or entrainment speed. The bearings supporting the main spindle of machine tools are subjected to vibrations generated by the cutting process, the gearbox, geometrical inaccuracies and also vibrations from the surrounding environment. It is very important to understand how these vibrations affect the lubrication and functioning of the bearings supporting the main spindle, in order to maintain the desired precision and accuracy of the cutting process.

Tribological contacts of machine components working in the elastohydrodynamic regime always experience transient conditions due to variation of speed, load, geometry, or vibrations generated within the contacts or transmitted from the environment. Given the fact that the contacting bodies and lubricant, together with the other components of the mechanisms or machine, form a dynamic system comprising of springs and dampers, this will respond to dissipate the instability caused by the variation of the above parameters. At the lubricating film level, this is done by constant-amplitude, or dampened oscillations of its thickness.
This chapter is a review of experimental research carried out in the past decade into the non-steady state elastohydrodynamic lubrication, including transient loading, sudden variation of entrainment speed and variation of micro-geometry.

Continuously variable transmissions (CVT) are mechanical devices that allow a continuous variation of the output velocity by adjusting its internal geometrical configuration. Through the use of an epicyclic gear set, a CVT can deliver an infinite number of transmission ratios, allowing the vehicle's engine to operate within a higher efficiency envelope for longer periods of time. This offers several advantages over traditional transmissions such as better fuel efficiency, quieter operation, and lower mass. Current efforts to reduce vehicles' fuel consumption in order to protect the environment and save fuel have seen a recent revival in CVT research, especially in the automotive industry. This paper documents a successfully implemented search algorithm for the dimensional optimisation of a novel CVT design in order to determine the maximum theoretical efficiency of the device. In addition to showing an economical and simple method of calculating contact losses, this method of optimisation also proved to be a powerful tool which can be used to select dimensions of the transmission to fulfil any desired characteristics.