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 show that the degree of starvation increases with increasing rolling speed, base oil viscosity and thickener content but decreases at higher temperatures. In many cases an increase in absolute film thickness is obtained when moving from high viscosity base oil to a low one, this result is the reverse of normally accepted EHL rules. Taking the fully flooded film thickness as a guide to lubrication performance is therefore not valid as grease film thickness in the starved regime is determined by local replenishment rather than bulk rheological properties.     

The transition between fully flooded and starved regimes in EHL

Lubricant starvation of the contact can occur in high-speed oil or grease lubricated bearings resulting in reduced elastohydrodynamic (EHL) film thickness (Lubricat. Sci. 11 (1999) 227). To achieve optimum bearing performance and component life, it is obviously desirable to be able to predict when starvation will occur and the resulting level of surface protection. For both oils and greases, the transition between fully flooded and starved behaviour is determined by lubricant loss and replenishment of the track (ASME Trans. J. Tribol. 120 (1998) 126; Cann PME, Chevalier F, Lubrecht AA. Track depletion and replenishment in a grease lubricated point contact: a quantitative analysis. Proceedings of the 23rd Leeds–Lyon Symposium on Tribology. 1996. p. 405–14). The current paper develops a criterion for the fully flooded and starved transition of an oil-lubricated contact based on four primary factors controlling lubrication level; these are volume of oil, contact dimensions, oil viscosity and speed. Experiments have been carried out using an optical EHL device to investigate the effect of each of these parameters on starvation. The results show how the boundary between the fully flooded and the starved regime and the film thickness in the starved regime depend on these parameters. A single dimensionless parameter (SD), based on replenishment local to the contact, has been established between the operating parameters and the transition from the fully flooded to starved regime.In this paper, the starvation criterion has been developed for an oil-lubricated contact with a fixed volume of oil present. The next stage is to extend the analysis to grease and the prediction of lubrication failure in bearings.     

On the film thickness behaviour of polymer greases at low and high speeds

Experimental batches of polymer thickened greases, as well as their base and bleed-oils were tribologically characterized through film thickness measurements over a wide range of entrainment speeds on a ball-on-disc test rig using optical interferometry. The results are in agreement with previous observations of several authors. Under fully flooded conditions and low speed it was observed that thickener lumps enter the contact producing a high film thickness plateau. The transition speed at which the film thickness increases with decreasing speed is dependent on the thickener content and operating temperature. At moderate to high speeds, all the tested greases show a film thickness much higher than the base and bleed-oils, even though the bleed-oil׳s film thickness is closer to the grease׳s.     

Film Thickness of Mechanically Worked Lubricating Grease at Very Low Speeds

Several different types of commercially available greases, aged in a roll stability tester, have been tested in a ball-on-disc machine where the film thickness was measured using the interferometry method. The grease film thickness at very low speed is thicker than the base oil film thickness due to the contribution of the thickener in this regime, which practically vanishes at higher speeds. The (mechanical) aging process reduces the thickener material, decreasing the grease film thickness at very low speeds (entrainment speed less than 0.01 m/s) with aging time, whereas no significant difference was found at higher speeds where the grease film thickness still approached the calculated base oil film thickness.     

Film thickness in a ball-on-disc contact lubricated with greases,bleed oils and base oils

Three different lubricating greases and their bleed and base oils were compared in terms of film thickness in a ball-on-disc test rig through optical interferometry. The theoretical values calculated according to Hamrock's equation are in close agreement with the base oil film thickness measurements, which validates the selected experimental methodology.The grease and bleed oil film thickness under fully flooded lubrication conditions presented quite similar behaviour and levels. Therefore, the grease film thickness under full film conditions might be predicted using their bleed oil properties, namely the viscosity and pressure-viscosity coefficient. The base and bleed oil lubricant parameter LP are proportional to the measured film thickness.A relationship between grease and the corresponding bleed oil film thickness was evidenced.     

The Slow-Speed Wear Behavior of Case-Carburized Gears Lubricated with NLGI 00 Grease under Boundary Lubrication Conditions

NLGI 00 greases are often used to lubricate gears running at low pitch line velocities, such as, for example, in large open gear drives. At low pitch line velocities, sliding wear, which under these operating conditions is referred to as slow speed wear, is often the limiting factor to gear lifetime. A thorough knowledge of the effect of different grease components on the wear behavior is therefore important when selecting a grease to effectively reduce gear wear in a given gear drive. In order to systematically investigate and analyze the influence of different grease components on the slow-speed wear behavior of case-carburized gears, systematic gear tests using the Gear Research Center's (FZG) back-to-back gear test rig were conducted. Primarily, the focus of the experimental investigations is on the influence of the base oil viscosity and type, the additive type, and also the type of soap thickener on the gear wear behavior at low pitch line velocities. To experimentally determine the influence of these different grease components on the wear behavior of case-carburized gears, a modified, more stringent wear test, based on the standard DGMK slow-speed wear test for gear oils, was developed. Different NLGI 00 greases with base oil viscosities between ν₄₀ = 70 and 1,200 mm²/s were investigated.

Base oil type and base oil viscosity were shown to have only a minor effect on the wear behavior under boundary lubrication conditions. On the other hand, the thickener type and especially the additive type play an important role in determining the wear behavior.     

Grease lubrication of rolling element bearings — role of the grease thickener

The contribution of grease thickener to lubricant film formation was examined in this paper. Lubricant film thickness and friction were measured for different grease thickener types in a bearing simulation device. The results showed that the greases formed thick (20–80nm), low friction surface layers at low speeds, which were much greater than the corresponding base oil film. These films appeared to be formed by the physical deposition of thickener in the track during overrolling of the grease. This was confirmed by infrared reflection analysis, which showed the deposited films to have increased thickener content. The ability of grease to form renewable physically deposited solid films has significant implications for optimising lubricant formulation for certain applications, e.g. bearings operating at high temperatures and low speeds where a conventional elastohydrodynamic lubricating film would be inadequate. Copyright © 2007 John Wiley & Sons, Ltd.     

偏心凸轮副时变乏油润滑数值分析

为研究乏油条件下偏心凸轮副的润滑状态,基于凸轮-挺杆机构建立时变乏油润滑模型,探究一个周期内6个典型瞬时(60°、120°、180°、240°、300°、360°)的压力和油膜厚度变化规律,并分析不同凸轮旋转角度下转速、初始载荷和润滑油黏度等参数对接触区润滑状态的影响。结果表明：当凸轮转至180°时,膜厚最小,压力最大,乏油状况最严重;限量供油下最小膜厚出现在凸轮转角为180°时,但是凸轮转角为0°时乏油速度最快,乏油程度更深;增大凸轮旋转速度时乏油速度更快,乏油程度更深;相同供油条件下,润滑油黏度越高使得接触区乏油情况越严重,乏油速度更快,乏油程度更深;载荷对接触区的润滑状态的影响较小,只在凸轮转角为0°接触区卷吸速度最大时,能够体现出载荷对接触区润滑状态的影响。     

Starvation and Reflow of Point Contact Lubricated with Greases of Different Chemical Formulation

Starved film thickness measurements are presented in grease-lubricated point contact under pure rolling condition. The starvation process of different types of greases is recorded and analysed in detail. Results show that the film decay rates for lithium greases and SiO₂ greases are quite similar regardless of the rolling speed. Urea grease shows strong adsorbability to the metal surface, which can be clearly observed during the test. Oil separation tests and scanning electron microscopy observations proved the differences in structure and stability of the tested grease. A close relationship between starved film thickness and oil bleeding rate is found in the present research.     

Tribological Performance of Some Pennzane®-Based Greases for Vacuum Applications

Commercial greases for space applications usually fulfill the requirements imposed by the severe conditions of use (load, ultrahigh vacuum, speed, etc.). The main requirement is their ability to create an elastohydrodynamic lubrication film, boundary film, or both, for the speed, load, and temperature conditions under which the mechanisms will operate. Three greases, all based on a multiply alkylated cyclopentane (Pennzane®) base oil, were studied. The thickeners were an n-octadecylterephthalamate soap, a lithium soap, and a urea derivative. A four-ball tribometer and a spiral-orbit tribometer were employed to evaluate the greases under ultrahigh vacuum. The results indicated that all three greases yielded very low wear rates and extended lifetimes. In addition, routine physical property data are reported for each grease.     

乏油工况下斜齿轮热弹流润滑特性研究*

建立斜齿轮的乏油热弹流润滑模型,并讨论供油量、转速和齿面粗糙度对润滑性能的影响。结果表明:乏油工况下增大入口区供油量,润滑区的膜厚增大而摩擦因数、温升和次表面应力幅值降低;随着供油量增大,乏油润滑特性逐步趋于全膜润滑状态下特性;随着转速升高,润滑膜厚增大但幅度有限,相应温度场增大和次表面应力场增大;齿面粗糙度会使油膜压力出现剧烈的波动,在油膜压力峰位置的次表面会出现应力集中。     

Numerical analysis of grease thermal elastohydrodynamic lubrication problems using the Herschel-Bulkley model

Grease thermal elastohydrodynamic lubrication (TEHL) problems of line contacts are analysed numerically. The effects of temperature and rheological parameters on grease TEHL are investigated using the Herschel-Bulkley model as a rheological model of greases. The pressure distribution, the shape of grease film, mean film temperature and surface temperature of solid wall in line contacts are obtained. It is found that thermal effects on the minimum film thickness become remarkable at high rolling speeds. The effect of yield stress of the Herschel-Bulkley model on minimum film thickness is negligible, while the flow index and viscosity parameter have significant effects on minimum film thickness.     

多点接触乏油弹流润滑模型及试验研究

为探讨多点接触乏油弹流润滑机制,基于球与滚道接触区域的排油和补油平衡,建立适用于不同润滑状态的油膜厚度计算模型,可以计算从充分供油、乏油到干涸乏油的中心膜厚以及油膜不平衡时中心膜厚随滚动次数的衰减。利用自制的球-盘接触光干涉弹流试验装置,通过安装双镜筒同时获取相邻球的油膜图像,研究多点接触中相邻球的轨道重合和不重合时前球尾迹对后球油膜图像和中心膜厚的影响。结果表明:乏油润滑条件下,前后球的轨道不重合时轨道之间可相互补油;前后球的轨道重合时,在给定供油条件下,随着滚动线速度增加,入口弯液面逐渐靠近接触区域,中心油膜厚度增加,与相同工况下乏油润滑模型计算的膜厚对比吻合较好,验证了所建乏油润滑模型的正确性。     

Influence of Cage Clearance on Bearing Lubrication©

The effect of the cage clearance on the lubricant supply and elastohydrodynamic (EHL) film thickness has been studied in a ball-on-disc device. A single pocket from a standard nylon cage was mounted around the ball. The cage was instrumented so that the clearance between the cage and ball could be altered. Film thickness measurements were made with and without the cage present and for different clearances. Two lubricants were tested: a lithium hydroxystearate grease and its base oil. Film thickness was measured with increasing speed to determine the onset of lubricant starvation. Without a cage present the grease lubricated contact starved at a very low speed, typically 0.02 m/s and the film thickness dropped to a fraction of the fully flooded value. Starvation did not occur within the speed range for the base oil.

The presence of the cage significantly changed the starvation response. For the base oil reducing the clearance induced starvation by locally removing the lubricant from the track. The grease gave a very different result as reducing cage clearance increased the starvation speed thus ensuring fully flooded behavior over a much greater speed range. The improvement in grease performance with the cage present is attributed to two effects. First, the cage with reduced clearance helps to redistribute the grease into the track. Second, the close conformity between cage and ball promotes shear degradation of the grease structure generating low-viscosity material, which improves replenishment.     

Asymptotic analysis of a lubricated heavily loaded spinning and rolling ball in a parabolic raceway

In the paper, the previously developed asymptotic approach to solution of the steady isothermal problem of elastohydrodynamic lubrication (EHL) for heavily loaded point contacts is applied to a lubricated point contact with rolling and spinning. It is shown that the whole contact region can be subdivided into three subregions. The central region can be subdivided into the Hertzian region and two adjacent boundary layers — the inlet and exit zones. The main results of the paper are threefold: (i) it is shown that in the central parts of the inlet and exit zones, the mechanisms and the equations controlling the behaviour of the lubrication contact parameters in heavily loaded point and line EHL contacts are identical, (ii) asymptotically precise formulas for the central and exit lubrication film thickness for pre‐critical and over‐critical starved and fully flooded lubrication regimes are analytically derived, and (iii) the inlet and exit zone asymptotically valid equations are uniform across all steady heavily loaded line and point EHL contacts for lubricants with the same rheology. These asymptotically valid equations were analysed and numerically solved in previously published work based on the stable methods utilising the specific regularisation approach developed for lubricated line contacts. Cases of pre‐critical and over‐critical lubrication regimes are considered. The formulas for the lubrication film thickness for pre‐critical and over‐critical starved and fully flooded lubrication regimes allow for simple analysis of the film thickness as a function of spinning angular speed, angle of the entrained lubricant and other pertinent contact characteristics. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Rheological properties of greases in ehd contacts

A rheological model of four characteristic parameters is proposed to describe the effective viscosity of greases in ehd contacts. Experimental determinations of film thickness have been achieved in line and point contacts. The proposed model fits the viscometric results better than a Herschel-Bulkley model and allows calculation of the grease viscosity at the high shear rates found in a typical ehd contact. Rules are given to find the effective viscosity of greases in a practical situation     

Thermal elastohydrodynamic lubrication of starved elliptical contacts

For the starved thermal elastohydrodynamic lubrication in elliptical contacts, a numerical solver based on the multi-grid technique was developed and numerical solutions were achieved. The influences of the thickness of oil-supply layer, elliptical ratio, entrainment velocity, slide-roll ratio, and the maximum Hertzian pressure on the lubrication behaviour were investigated. The thermal solutions were compared with isothermal solutions. The numerical results show that, for an oil-starved contact, the central and minimum film thicknesses predicted by both thermal and isothermal solutions change at the same trend as the thickness of the oil-supply layer increases. That is, as the thickness of the oil-supply layer increases, in the beginning both the central and minimum film thicknesses increase rapidly, however, their increasing rates become smaller gradually, at last, when the thickness of the oil-supply layer reaches a certain value, both the central and minimum film thicknesses almost stop increasing. Such situation was called a quasi-fully flooded state in this study. It has been found that the amount of the supplied oil for the quasi-fully flooded state differs with the elliptical ratio, and the maximum amount of the supplied oil is required by a circular contact.     

Rolling Bearing Performance and Film Formation Behavior of Four Multiply Alkylated Cyclopentane (MAC) Base Greases for Space Applications

Four multiply alkylated cyclopentane (MAC)-based greases, used for space applications, were used for rolling contact fatigue tests employing thrust ball bearings. These greases were R2000, ML, MU, and 5200 and were made from the same base oil 2001A. Each grease had a different thickener, which were sodium soap, Li soap, urea, and polytetrafluoroethylene (PTFE), respectively. The effect of the grease thickener on the ball bearings' fatigue life in vacuum and atmospheric environments was investigated via testing. For the greases R2000, MU, and ML it was found that the oil film formation in the experiments performed under vacuum conditions declined remarkably in comparison to under atmospheric conditions. For the 5200 grease, the oil film formation was almost the same as under atmospheric conditions. These results were supported by the occurrence of cage wear in the ball bearing fatigue tests. In the absence of sufficient films, high wear rates were measured.     

Laser-induced fluorescence for film thickness mapping in pure sliding lubricated, compliant, contacts

A laser-induced fluorescence (LIF) technique has been used to measure fluid film thickness in a compliant, sliding contact under low-load/low-pressure conditions. The soft contact between an elastomer hemisphere and a glass disc is lubricated by a liquid containing fluorescent dye. The contact is then illuminated with 532 nm laser light through the glass disc, and viewed with a fluorescence microscope. From the intensity of emitted radiation, film thickness maps of the contact are determined. Previous calibration procedures have used a separate calibration piece and test specimen with possible errors due to differences in reflectivity between the calibration and test specimens. In the work reported in this paper a new calibration process is employed using the actual test sample, thereby avoiding such errors.Results are reported for a sliding contact between PDMS and glass, lubricated with glycerol and water solutions under fully flooded and starved conditions. It was found that, for glycerol, the measured film thickness is somewhat lower than numerical predictions for both lubrication conditions. It is suggested that a combination of thermal effects and the hygroscopic nature of glycerol may cause the lubricant viscosity to drop resulting in thinner films than those predicted for fully flooded contacts. Starvation occurs above a critical entrainment speed and results in considerably thinner films than predicted by fully flooded I-EHL theory. A numerical study has been carried out to determine the effect of the observed starvation on film thickness. Predicted, starved film thickness values agree well with those obtained experimentally.