P. M. Cann

The Development of a Spacer Layer Imaging Method (SLIM) for Mapping Elastohydrodynamic Contacts

Optical interferometry has proved to be a valuable experimental tool in the study of elastohydrodynamic lubrication (EHD). It is a technique that gives detailed information on the lubricant film distribution within the contact; however, the sensitivity is limited and it is only recently, with the development of the spacer layer optical technique, that the study of the thin film lubrication regime has been possible. The limitation of the spacer layer technique is that generally only one measurement is taken from the center of the contact. The next logical step in the development of this technique is, therefore, a system that combines the mapping capabilities of the original optical method with the thin film capabilities of the spacer layer approach. This paper describes the development of a contact mapping technique that uses the spacer layer approach to visualize, and measure, thin lubricant films in concentrated contacts. The development of the technique is described and its application to both static an...

Film Thickness in Starved EHL Point Contacts

This paper presents a numerical study of the effects of inlet supply starvation on film thickness in EHL point contacts. Generally this problem is treated using the position of the inlet meniscus as the governing parameter; however, it is difficult to measure this in real applications. Thus, in this paper an alternative approach is adopted whereby the amount of oil present on the surfaces is used to define the degree of starvation. It is this property which determines both meniscus position and film thickness reduction. The effect of subsequent overrollings on film thickness decay can also be evaluated. In the simplest case a constant lubricant inlet film thickness in the Y direction is assumed and the film thickness distribution is computed as a function of the oil available. This yields an equation predicting the film thickness reduction, with respect to the fully flooded value, from the amount of lubricant initially available on the surface, as a function of the number of overrollings n. However, the constant inlet film thickness does not give a realistic description of starvation for all conditions. Some experimental studies show that the combination of side flow and replenishment action can generate large differences in local oil supply and that the side reservoirs play an important role in this replenishment mechanism. Thus the contact centre can be fully starved whilst the contact sides remain well lubricated. In these cases, a complete analysis with a realistic inlet distribution has been carried out and the numerical results agree well with experimental findings.

Starved Lubrication of Elliptical EHD Contacts

This paper focuses on the lubrication behavior of starved elliptical Elasto-HydroDynamic (EHD) contacts. Starvation is governed by the amount of lubricant available in the inlet region and can result in much thinner films than occurring under fully flooded conditions. Therefore, it would be desirable to be able to predict the onset and severity of starvation and to be able to relate film reduction directly to the operating conditions and lubricant properties. The aim of this work is to explore the influence of these parameters on starvation. A combined modeling and experimental approach has been employed. The numerical model has been developed from an earlier circular contact study [1]. In this model, the amount and distribution of the lubricant in the inlet region determines the onset of starvation and predicts the film decay in the contact. Numerical simulations for a uniform layer on the surface show that a single parameter, characteristic of the inlet length of the contact in the fully flooded regime, determines the starved behavior. Film thickness measurements under starved conditions were performed to validate this theory. For a circular contact excellent agreement was found. In theory the same mechanism applies to elliptic contacts, however, the behavior is more complicated.

Lubricant Film Thickness in Rough Surface, Mixed Elastohydrodynamic Contact

A spacer layer imaging method has been employed to map lubricant film thickness in very thin film, rough surface, rolling elastohydrodynamic (EHD) contacts. A series of model roughnesses have been produced by depositing tiny ridges and bumps on a steel ball surface and the influence of these features on film thickness has been investigated at a range of rolling speeds. It has been shown that all the model surface features studied form speed-dependent, micro-EHD lubricating films, but the detailed shape and thickness of these films depends upon the geometry of the feature and the rolling speed. All model surface features also produce a net increase in mean film thickness, compared to the smooth surface, under operating conditions where the film thickness is less than the out-of-contact height of the surface feature studied. For a real, random, rough surface, however, mean film thickness is less than the smooth surface case. The film thickness mapping technique has also been used to measure the effective roughness of surfaces in lubricated contact. This shows that surfaces based on 2-D array of tiny circular bumps become rougher as the rolling speed and thus film thickness increases. However, real, rough surfaces appear to show a decrease of in-contact roughness with increasing rolling speed.

In Lubro Studies of Lubricants in EHD Contacts Using FTIR Absorption Spectroscopy

Research into the physical and chemical processes occurring within lubricated concentrated contacts has traditionally been limited by the necessity to observe events out-of-contact, or under simulated conditions. This paper describes the application of a new technique involving infrared microreflection absorption spectroscopy. The method is applied to the direct determination of the conditions of pressure experienced by lubricants in contacts, to the formation of antiwear films by phosphorus additives and to the study of the alignment of molecules, including viscosity index improvers in contacts. Presented at the 45th Annual Meeting In Denver, Colorado May 7–10, 1990

Starved Grease Lubrication of Rolling Contacts

Many grease lubricated roller bearings operate in the starved elastohydrodynamic (EHL) regime where there is a limited supply of lubricant to the contact (1). Under these conditions the film thickness drops to a fraction of the fully flooded value (2) and, thus, it is difficult to predict lubrication performance, or bearing life, from conventional EHL models. In this regime film thickness depends on the ability of the grease to replenish the track rather than the usual EHL considerations. The conventional view of grease lubrication is that base oil bleeds from the bulk reservoir close to the track, replenishing the inlet and forming a fluid EHL film (3). Resupply, under starved conditions, will thus depend on both operating conditions and grease parameters. The aim of this paper is to evaluate the influence of these parameters on starved lubrication in a rolling contact. Starved film thickness has been measured for a series of greases and the results have been compared to the fully flooded values. These s...

Starvation and Reflow in a Grease-Lubricated Elastohydrodynamic Contact

The lubrication mechanisms of a grease in a rolling-element bearing has been studied through the measurement of film thickness in a rolling point contact. To simulate bearing conditions the contact runs under fully starved conditions; there is no attempt to maintain bulk flow of the grease into the inlet using an external supply. In consequence the film thickness drops off rapidly as the contact progressively starves. After a few minutes rolling (at constant speed) an equilibrium film thickness is attained which has two components: a residual film (hR) comprised of degraded grease thickener and a hydrodynamic component (hEHD) due to the liquid phase from the grease. The hydrodynamic contribution represents a balance between lubricant lost from the contact and replenishment from the grease close to the track. The ability of the grease to replenish the rolling track has been inferred from measurements of lubricant reflow around the static contact. These results are discussed in light of current starvation a...

The development and application of the spacer layer imaging method for measuring lubricant film thickness

Abstract This paper reviews the historical development of optical interferometry as applied to the study of lubricant films. The technique was first applied to lubricated contacts in the 1960s, when it played an important role in the validation of the elastohydrodynamic theory of lubrication. Initially the method was not suited to the study of mixed and boundary lubrication because it could not measure film thicknesses of less than about 50 nm. In the 1970s, however, this limitation was partially overcome by the use of a spacer layer and this, coupled in the early 1990s with spectrometric analysis of the interfered light, enabled films down to just 1 nm thick to be measured in lubricated contacts, well within the boundary lubrication regime. Recently a number of workers have applied colorimetric image analysis to optical interference images to enable accurate three-dimensional maps of film distribution in lubricated contacts to be determined. This approach, coupled with the use of a spacer layer, has led to the spacer layer imaging method, which can map film thickness in boundary and mixed lubricated contact. Some recent applications of this technique are described.

Grease Degradation in R0F Bearing Tests

This paper is the second in a series that examines grease lubrication mechanisms and failure in rolling element bearings. The aim of the work was to understand the grease condition changes during use and the relationship to lubrication performance and failure. R0F bearing tests were carried out with two lithium hydroxystearate greases and the effects of the temperature, the speed, and the additive package on lubrication life was studied. Post-test, one pair of bearings (fail and non-fail) was dismantled and grease distribution and condition assessed. IR spectroscopy was then used to determine the lubricant composition and the oxidation level of the grease remaining on the shields, the inner raceway, and the cage pockets. The additive package increased the grease life by 100-700% depending on the test condition. Most of the grease remaining in the bearing was found on the shields, with only trace amounts in the cage pockets or close to the rolling track. The IR analysis showed that the composition of the shield sample was similar to the fresh grease although the base oil oxidation was evident and this increased with the running time. The cage pocket and inner raceway films contained a number of chemical species; these included the base oil and the thickener and their oxidation products. The study concludes that after an initial running-in period the “active” lubricant is heavily degraded grease, which contains oxidized species from the base oil and the thickener. Different failure mechanisms are identified depending on the test condition. High-speed tests that fail relatively quickly are due to poor boundary lubrication performance or cage failure rather than the lubricant reaching its “oxidation” limit. Long-term tests at slower speeds suffer considerable base oil oxidation. Under these conditions, failure is due to a reduction in the amount and/or mobility of the raceway lubricant.

Grease Degradation in Rolling Element Bearings

Grease is degraded during use in rolling element bearings and as a result the lubrication performance can deteriorate. Under severe conditions this can result in lubrication failure and, thus, the grease life will effectively limit the bearing life. At present there is a lack of detailed information regarding the changes that occur in the grease and the way in which this degradation affects lubrication performance and failure. This paper reports an initial study into grease degradation in bearings. The aim of the work was to characterize the changes that occur to the chemical and physical properties during use. A series of bearing tests using the modified DIN 51 806 test designated R2F(M) have been carried out using two greases: additized and non-additized. The tests have been run for different temperature and speed conditions for up to 300 hours. The aim was to examine the grease during normal running rather than after failure. At the end of the tests the bearings were dismantled and grease taken from different parts of the bearing for infrared spectroscopic analysis. This technique can characterize the degree of oxidation or degradation of the grease both in the bulk sample and from thin grease layers remaining on the bearing surfaces. The analysis has shown that the condition of the grease varies depending on the distribution within the bearing. The lubricant remaining in the cage pocket region was heavily degraded and contained very little thickener. The grease on the seals contained different amounts of thickener depending on the seal position. The lubricant remaining on the inner raceway surface was predominately base oil although there was some thickener present. These results are discussed in the light of proposed bearing lubrication mechanisms. Presented at the 56th Annual Meeting Orlando, Florida May 20–24, 2001