EHL film thickness in rolling element bearings evaluated by electrical capacitance method: a review

This paper reports direct measurements of lubricant film thickness in a radially-loaded grease-lubricated model rolling bearing. These measurements are performed on a recently developed set-up, termed the Sapphire Bearing Rig, which allows for direct observation and measurement of film thickness in a single ball-outer ring contact in a rotating bearing using optical interferometry. Measurements are made with a selection of greases covering different base oils and thickener types. The results show that starvation occurs as the bearing speed increases. This is confirmed by observations of the inlet meniscus. The bearing speed at which starvation occurs is determined by grease properties, particularly the base oil viscosity. When the bearing operates for a prolonged time at a constant speed, as is usually the case in practice, the initial film thickness is seen to decay in the first few hours of operation, to eventually reach a steady state value that is lower than that predicted by elastohydrodynamic lubrication (EHL) theory. For conditions tested here, this steady-state film thickness was largely independent of bearing speed, which suggests that it is mainly governed by the level of lubricant supply in the inlet. Such film behavior clearly cannot be predicted using the standard EHL theory. Stopping the bearing for a sufficiently long time leads to a temporary recovery of the film thickness upon restart. These measurements provide new insights into grease lubrication mechanisms of rolling bearings and can aid the development of new tools for improved prediction of grease film thickness in rolling bearings.

Many engineering estimates of the film thickness in the concentrated contacts of real machines have come from extrapolations of measurements of elastohydrodynamic lubrication film thickness performed in glass on steel elastohydrodynamic rigs. Such estimates are likely to have large errors due to shear dependence of viscosity. The classical film thickness formulas employed have not been validated except for some Newtonian reference liquids at room temperature because the real pressure-viscosity response measured in viscometers has been ignored. A blend of polyalpha olefin base oils with mild shear-thinning has been employed in a line contact calculation to assess the effects of shear-dependent viscosity on the power-law exponents of the classical film thickness formula. The exponents on the pressure-viscosity coefficient and on the elastic modulus of the solids are not sensitive to the non-Newtonian effect. The exponents on ambient pressure viscosity and velocity are slightly reduced by shear-thinning. The exponents on pressure and scale are substantially increased by the shear dependence. The usual practice of measuring film thickness in an elastohydrodynamic rig to obtain an effective “ α -value” to use in a classical Newtonian film thickness formula will overstated the film thickness of a liquid which is shear dependent in the elastohydrodynamic lubrication inlet.

<div class="htmlview paragraph">The objective of the research project was the investigation of lubricant film formation in elastohydrodynamic contacts for oils of different origin (paraffinic, naphthenic, polyalphaolefin) and various types of VI-improvers (polymethacrylate, olefin copolymer, styrene butadiene copolymer). An essential part of the research was to find out whether VI-improvers maintain their thickening effect under contact conditions of high pressure (p=10,000 bar) and temperature (<i>ϑ</i>=50-90 °C), high shear rate (γ̇=10<sup>6</sup> s<sup>-1</sup>) and short contact time, all these conditions occurring simultaneously. A twin disk machine was used to determine values of film thickness in a line contact. For these measurements, an electrical capacitance method was used. In addition to a thermal viscosity loss, high shear rate γ̇ leads for VI improved oils to temporary viscosity loss and thus reduced film thickness. Additional permanent viscosity loss due to shear thinning reduces measured film thickness further. Lubrication with non-Newtonian fluids results in higher film thickness compared to their base oil but, even for pure rolling, in thinner films as a Newtonian fluids of same nominal viscosity.</div>

Determining the film thickness during the bleeding phase of grease lubrication in a bearing is difficult due to the various replenishment and loss mechanisms involved. An additional factor that could influence the film thickness is the amount of grease in the bearing. In this study, film thickness is measured in deep-groove ball bearings using two different greases, each with initial grease filling percentages of 7.5%, 15%, and 30% of the bearing’s free volume. Film thickness is measured during the early stage of the bleeding phase at various loads and speeds using the electrical capacitance method. Results show that, after churning, values of temperature and film thickness are similar for the three different filling quantities. This indicates that bulk grease flow or oil bleed from the reservoirs formed in the unswept region does not impact the lubricant film thickness in the early stage of bearing operation.

Crucial role of solid body temperature on elastohydrodynamic film thickness and traction

Elastohydrodynamic lubrication (EHL) film thickness suggests the capability of a lubricant to support load under a set of hydrodynamic conditions. It is related to the rheological properties of a lubricant, contact geometry, materials properties, and operating conditions such as load, speed, and temperature. Film thickness variation with speed or load may follow the prediction made with the theory of EHL. Experimental techniques based on capacity [1-3]; electrical conductivity and contact resistance [4-6]; discharge voltage [7]; X-ray transmission [8]; indirect laser transmission [9,10]; R-C oscillation technique [11]; and optical interferometry [12–35] have been used in EHL experiments for film- thickness measurement. On the other hand, as a result of the rapid development of high-speed computers, in-depth and detailed numerical analyses of EHL problems intend to become a routine practice. More influential factors can be considered now in EHL calculations: several non-Newtonian properties of a lubricant [30,31,36–38], surface roughness [32,33,37,38], the thermal effect [34,39,40], and non-steady-state transitions [35,41,42] have been included in a number of simulation models. Moreover, mixed lubrication simulations based on digitized real rough surfaces have been conducted in wider ranges of speed and load [36,37]. This section reviews and discusses the existing techniques for EHL film-thickness measurements, views of measurement data, and comparisons of measurement results with EHL simulation solutions.

Point contact film thickness in elastohydrodynamic lubrication (EHL) is analyzed by image processing method for the images from an optical interferometer with monochromatic incident light. Interference between the reflected lights both on half mirrorCr coating of glass disk and on super finished ball makes circular fringes depending on the contact conditions such as sliding velocity, applied load, viscosity-pressure characteristics and viscosity of lubricant under ambient pressure. In this situation the film thickness is regarded as the difference of optical paths between those reflected lights, which make dark and bright fringes with monochromatic incident light. The film thickness is computed by numbering the dark and bright fringe orders and the intensity (gray scale image) in each fringe regime is mapped to the corresponding film thickness. In this work, we developed a measuring technique for EHL film thickness by dividing the image patterns into two typical types under the condition of monochromatic incident light. During the image processing, the captured image is converted into digitally formatted data over the contact area without any loss of the image information of interferogram and it is also interpreted with consistency regardless of the observer’ s experimental experience. It is expected that the developed image processing method will provide a valuable basis to develop the image processing technique for color fringes, which is generally used for the measurement of relatively thin films in higher resolution.

Application of the ultra-thin elastohydrodynamic oil film thickness technique to the study of automotive engine oils

Microoscillation is a typical case of transient motion, which occurs in many machine elements, including rolling or sliding element bearings, cams, and gears. Wear is easy to occur on the surface of such elements, particularly at the end point of the stroke, where the surfaces are momentarily static. In the present work, an experimental investigation is conducted to explore the grease film behavior of point contact lubrication during microoscillation in the case of pure rolling or pure sliding. The technique of relative optical interference intensity was used to monitor the variation of the grease film thickness and the motion of the grease in the contact area through analyzing the captured interferograms. Experimental results indicate that a crescent-shaped grease film can form along the motion direction in the contact area under microoscillation. The grease film is formed in the inlet region, and the film thickness remains while moving in the Hertzian contact area. In the case of pure rolling, the crescent-shaped grease film and the initial entrapped grease film are carried by a tow effect of moving interfaces in the contact area. However, in the case of pure sliding, there are relative motions in the sliding direction at the two interfaces of the grease/ball and the grease/disk in the Hertzian contact area. The shape of the entrapped grease remains almost unchanged while moving in the Hertzian contact area. During the repetition of microoscillations, the crescent-shaped grease film thickness drops gradually.

The theory of partial elastohydrodynamic contacts

A new high‐pressure viscometer that can measure viscosity at pressures up to 0.8 GPa has been developed in the authors' laboratory. The ‘modulus equation’ has been used to compare the behaviour of mineral and synthetic lubricants. Among the oils investigated there was one ester that biodegraded rapidly both before and after ageing in a long‐term test‐rig operation. To facilitate a comparison or application of the results to other oils, an analysis of the correlation between the viscosity—pressure coefficient and the kinematic viscosity measured at atmospheric pressure has been provided. A prediction of lubricant film thickness based on high‐pressure viscosity data is compared with film thickness measurements in a roller bearing.

ABSTRACTThis article describes the measurement of lubricant film thickness distribution in a roller bearing by using two juxtaposed transducers, which are excited by a new ultrasonic pulse receiver with a maximum pulse repetition rate (PRR) of 100 kHz. The experimental results demonstrate the feasibility and advantage of this method with high PRRs. The influence of rotating speed and radial load on the measured film thickness are shown to be consistent with the theoretical predictions. Skewing of the roller in the bearing is observed. A combined trigger signal is proposed to reduce the influence of the cyclical vibration of the rotor.

The surface topography plays significant role in lifetime of highly loaded machine parts with lubricated contacts. Many elements like gears, rolling bearings, cams and traction drives operate in mixed lubrication conditions, where the lubricant film behavior closely implies the main practical performance parameters such as friction wear, contact fatigue and scuffing. For prediction of wear and especially contact fatigue, the values and distribution of the pressure in rolling contact are often required. The usual theoretical approach based on numerical solution of physical-mathematical models built around the Reynolds equation can be extremely time consuming, especially when lubricant films are very thin, and contact load and required resolution very high. This study presents a further refined approach to our previously published experimental method, based on on inverse elasticity theory and fast convolution transformation between the lubricant film thickness map and the pressure distribution within the point contact. The experimental film thickness maps of EHD lubricated contacts with smooth and dented surfaces were processed using colorimetric interferometry and validated using numerical solution, in order to calibrate numerical parameters and to find limits of the new approach.

Simultaneous application of two laser-induced fluorescence approaches for film thickness measurements in annular gas-liquid flows

General Elastohydrodynamic Lubrication Code (GEHLC) is a FORTRAN code for calculating the lubrication performance of line and point contacts used in machine elements such as gears, rolling bearings, cam and follower systems, and traction drives, etc. It adopts the most recent results published by Chittenden and Dowson et al. (1), to evaluate isothermal film thickness of point contacts, and uses the thermal reduction factor (2) to account for the inlet heating effects on the film thickness. By using Bair and Winer's analysis (3), a viscoelastic fluid model has been employed in this program to calculate shear stress in the film with bisection technique and In predict the coefficient of friction. Using this program one can also obtain flash temperature distribution on the two solid surfaces from a simplified heat conduction analysis based on work by Bolt (4), Jaeger (5), and Archard (6), et al. Typical results are presented for elliptical contacts used in rolling bearings. Presented at the 42nd Annual Meetin...