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Bearing performance limits with grease lubrication: the interaction of bearing design, operating

conditions and grease properties

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2007 J. Phys. D: Appl. Phys. 40 5446

(http://iopscience.iop.org/0022-3727/40/18/S05)

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 40 (2007) 54465451 doi:10.1088/0022-3727/40/18/S05

Bearing performance limits with greaselubrication: the interaction of bearingdesign, operating conditions and greasepropertiesP M E Cann1 and A A Lubrecht2

1 Tribology Group, Department of Mechanical Engineering, Imperial College London, UK2 Laboratoire de Mecanique des Contacts et des Solides, UMR-CNRS 5514, INSA de Lyon,France

E-mail: p.cann@imperial.ac.uk

Received 23 February 2007, in final form 25 February 2007Published 30 August 2007Online at stacks.iop.org/JPhysD/40/5446

AbstractThe majority of rolling element bearings in use today are lubricated bygrease. Grease is a two-phase lubricant with complex rheological propertiesand poses severe challenges for the prediction for lubricating performance.Grease lubricated contacts are liable to starvation and as a result the filmthickness is reduced, which can result in surface damage or prematurebearing failure. It is important to know when starvation occurs and the effectof grease type, bearing design and operation on lubrication replenishment.The influence of bearing design and operation in controlling lubricantsupply to the contact zone is examined in this paper. The aim is to develop astarvation parameter capable of predicting the operating limits for aparticular bearing/grease system.

A number of bearing design parameters are examined in the paper; theseinclude cage design, ball spin and bearing size. Ball spin and cage effectscan be efficient mechanisms for maintaining the lubricant supply to thetrack. Increased bearing size, line contact geometries and high load result inreduced lubricant replenishment of the contact. Using this analysis it will bepossible to establish operating limits for families of bearings.(Some figures in this article are in colour only in the electronic version)

1. Introduction

The operation and life of rolling element bearings is highlydependent on the lubricant performance as both friction andsurface damage will be influenced by the properties andthickness of the lubricant layer. The critical contacts insuch bearings operate under elastohydrodynamic lubrication(EHL) conditions. In this case film thickness is determinedby lubricant properties, bearing geometry and operatingconditions and can be predicted assuming fully floodedconditions [1, 2]. However the actual film thickness canbe much less than predicted due to reduced availability oflubricant, this is the starved regime and is often encounteredwith grease lubrication [3]. Moreover bearings, particularly

those operating at high speeds or low temperatures, aresusceptible to starvation due to repeated overrolling of thecontact zone expelling lubricant from the track. Without anefficient replenishment mechanism the contact is prone tostarvation, enjoining a loss of film thickness. In contrastto the established film thickness rules applying to the fullyflooded condition, the starved regime dependence is less wellunderstood. Furthermore, some film thickness rules in the fullyflooded regime are inverted in the starved regime, as shownin table 1. This inversion can be disorientating and leads toconfusion in recommending remedies for the problem.

Bearing operation is intrinsically linked to lubricatingperformance; however, lubrication cannot remedy baddesign or manufacturing, poor installation or inappropriate

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Bearing performance limits with grease lubrication

Table 1. Comparison of film thickness dependence on contactparameters for fully flooded and equilibrium starved regime.

Increasing Fully flooded Starved filmparameters film thickness thickness

Speed u Increase decreaseViscosity 0 Increase decreaseTemperature T Decrease increaseLoad w Negligible decrease

Figure 1. Starvation curve for two viscosities 0.59 Pas (at 30 C)and 0.16 Pas (at 58 C).

application. Good lubrication is necessary but insufficientcondition for successful bearing operation. Bearing life isgenerally used to assess performance; however, other criteriacan be equally valid depending on the application, for example,friction [4], noise [5], or relubrication interval. Bearing testshave shown that friction is reduced for semi-starved contactsbut increases dramatically for starved conditions [6]. Noisein bearings is usually related to thickener distribution [5] andsolid content.

It is necessary to know under what conditions the transitionfrom the fully flooded regime to the starved regime occurs fora particular lubricant and application. The speed at which thistransition occurs is identified as the starvation speed. Figure 1shows typical film thickness versus speed results for twodifferent base oil viscosities. The starvation speed for the 0.59Pas oil is approximately 0.065 m s1, compared with 0.2 m s1for the 0.16 Pas oil. The starvation speed depends on thelubricant properties, operating conditions and lubricant layerthickness in the inlet [7]. A relationship has been developedfor fluid lubrication of a model EHL contact [7]; however, itis very difficult to extend this to bearings as it must includegrease and bearing parameters.

The most important factor determining lubrication levelin bearings is the lubricant supply to the contact zone. Thetraditional view has been that oil bleeds from the greasereservoir to replenish the rolling track [8]. It is oftenassumed that reflow occurs during the unloaded rotationperiod [9]. However optical studies of starved lubricationhave suggested that reflow occurs local to the contact andis driven by capillary forces associated with the converginggap [10]. The identification of the bled oil reflow as thedominant replenishment mechanism is too simplistic andignores the role of lubrication mechanisms specific to the

bearing. Furthermore, the role of the grease thickener in thelubrication process is disregarded.

Examination of used bearings has provided useful insightsinto lubrication mechanisms. In this study [11, 12] bearingsfrom R2F (modified) and R0F tests were dismantled and thelubricant distribution and condition assessed. The bearingswere run for different times and operating conditions. Thechemical composition of the lubricant film in different partsof the bearing was determined by IR reflection spectroscopy.The pattern of grease distribution was similar in both typesof bearing; however, it was apparent that the provision ofmobile lubricant was very dependent on bearing design andoperation. In the R2F(M) bearings an appreciable volumeof mobile lubricant was distributed throughout the bearing.This was generated through shear degradation of the greasepresent on one of the shields by the action of the cage. Inthe R0F bearings most of the grease was present on the sealswith a small amount in the cage pockets and a thin lubricantlayer on the raceways. IR analysis showed that the greasewas heavily degraded due to shearing, oxidation and thermaleffects. The thin layers present on the rolling track implieda limited amount of lubricant available for film formationand that efficient replenishment mechanisms are required tomaintain an adequate film thickness level. The observedpresence of thickener within the rolled track would also implythat there is a mechanism for redistribution of bulk grease. Theability of the bearing to promote bulk grease flow will dependon design and operation, for example action of the cage, spinand vibration level [13].

One conclusion from the foregoing review is that a numberof different lubrication mechanisms are possible in a bearing[13]. In this study the mechanisms are classified by the typeof lubricant (base oil or base oil and thickener) replenishingthe track and the physical origins of replenishment (capillaryflow or mechanical redistribution). The relative importance ofthe different mechanisms in maintaining a sufficient lubricantlevel will depend on the bearing, operational and lubricantproperties.

2. Development of lubrication performance modelfor bearings

The development of a model to predict lubrication levelrequires an understanding of the relubrication mechanismsin a bearing and the influence of the lubricant and bearingparameters. The factors controlling relubrication in a bearingcan be grouped into three broad categories. These are:

(a) Lubrication parameters: the physical and chemicalproperties of the grease: base oil and thickener properties,grease rheology, additive effects and oxidation/thermaldegradation.

(b) Operational parameters: load, speed, temperature andvibration level.

(c) Bearing parameters: design and materials: cage properties,size, type, surface finish, materials.

Figure 2 represents schematically the interaction of thevarious grease and bearing parameters and the resultinglubrication level.

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P M E Cann and A A Lubrecht

Figure 2. Interaction of lubricant, bearing and operating parameters.

Figure 3. Film thickness as a function of speed for different contactdimensions, oil viscosities and oil volumes.

2.1. Lubricant parameters

The lubricant parameters will include base oil viscosity, greaserheology and shear stability. The efficiency of the capillaryreflow mechanism is determined by the base oil properties(viscosity, volume, polarity). This is examined in the starvedlubrication model presented in an earlier paper [7] where adimensionless parameter (SD) was developed to define thefully flooded/ starved transition. The relationship betweenfilm thickness and oil properties was studied in an opticalEHL device. Four parameters were varied: oil volume, speed,contact dimensions (load) and viscosity. The film thicknessmeasured as a function of these four parameters is plotted infigure 3.

This figure clearly shows a wide variety of starvationspeeds and maximum film thickness at this velocity. Thusviscosity, contact dimension and oil volume all influencestarvation speed and maximum film thickness but in a differentfashion. The effect of base oil surface tension was notexamined but it would also be expected to affect the onset

Figure 4. Relative film thickness (starved/fully flooded) as afunction of the SD parameter.

of starvation. In the capillary reflow model increasing surfacetension increases contact replenishment [7].

These results were replotted as a relative film thickness(starved/fully flooded) against the dimensionless starvationparameter (SD):

SD = 0uahoils

,

where: 0 = atmospheric viscosity, u = entrainment speed,a = contact width, hoil = oil layer height and s = surfacetension.

The set of curves that appeared in figure 3 is reducedto a single (master) curve shown in figure 4. Furthermore,the transition from the fully flooded to the starved regimeoccurs around SD = 2. The spread in the results is relativelylarge especially at high speeds and is due to the accumulationof experimental uncertainties. The SD parameter predictslubricant replenishment for the capillary reflow mechanismalone and was limited to a model test device and base oil.

The shear stability of grease affects the availability of freeoil for replenishment. In the SD parameter this volume isrepresented by the oil layer height hoil. Grease with poorshear stability would be expected to release base oil quicklyand thereby counteract the starvation. Grease with high shearstability releases oil slowly and thus there is reduced volumeof lubricant available for reflow. The effect of grease shearstability was studied in an optical test device [14] and someresults are shown in figure 5. Film thickness was measuredas a function of time at constant speed for three greaseswith the same base oil but different shear stabilities. Shearstability was measured in a rotational viscometer. The filmthickness behaviour is very different with the least-shear stablegrease giving the highest film thickness. The most stablegrease shows a diminishing film over the test period. Themedium-stability grease initially decays and then recoversas the shearing process generates mobile lubricant, whichreplenishes the contact. The question of grease shear stabilityis an important one as this property is usually dictated bystorage requirements rather than lubrication considerations.The test results would suggest that increased shear stabilityis bad from a starvation point of view. However, a balance hasto be found, otherwise for low shear stability grease loss fromthe bearing or slumping can occur.

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Bearing performance limits with grease lubrication

Figure 5. Starved film thickness as a function of shear stability.Order of increasing shear stability: Grease A < Grease B 2) isconsidered. The effect of speed has been studied in bearing andmodel test devices [7, 6]. Increasing speed leads to a reduction

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P M E Cann and A A Lubrecht

Figure 7. Effect of a plastic cage (shown inset) on starvation speed.

in starved film thickness. However, additional phenomena canoccur at very high speeds in full bearings [6]. For instancecage instability or bearing vibration can provide an additionalredistribution mechanism. Thus the bearing film thicknesspartially recovers at higher speeds [6].

Load affects the starvation level through the changingtrack width. Increasing the load increases starvation [SD]due to the greater distance required for replenishment of thecentre of the track. A second effect is observed in the transientapplication of load [17]. Unloading the bearing causes areduction of the contact size and capillary forces pull mobilelubricant into the centre of the track. The effect of unloadingon film thickness in the starved regime is shown in figure 8.In this figure three different tests are shown where the loadis removed periodically after 2, 5 and 20 disc revolutions.In each case, upon reloading, the film thickness decreasesmonotonically with overolling. Different unloading periodsresult in different average film thicknesses. Although dynamicloading might be considered to be an infrequent event, it mustbe remembered that such replenishment will occur once everyrevolution for radially loaded bearings. Clearly the efficiencyof the unloading mechanisms as a method for replenishing thetrack will depend on the amount, proximity to the contact andviscosity of lubricant available.

The operating temperature of the bearing will stronglyinfluence starvation speed through the base oil viscosity.Decreasing temperature results in increasing oil viscosity andhence increasing likelihood of starvation. These effects areincluded in the starvation parameter through the viscosity.

2.4. The next step?

This paper has analysed the lubrication, operation and bearingsfactors that determine operating limits for grease lubricationof bearings. Taken singly many of these parameters aresimple to quantify and it is possible to develop basic rulesto predict lubrication performance. However in combinationthey represent a significant challenge. Thus at present it is notpossible to develop a predictive model from first principles,for example, the SD parameter requires information on thedistribution, volume and viscosity of the lubricant. Althoughthis can be measured in a model test device it is not practicable

Figure 8. The effect of periodic unloading on film thickness in thestarved regime.

with an operating bearing. However there are a number ofpossibilities. Firstly by developing some practical rules ofthumb, although these undoubtedly already exist throughexperience in the field they could be codified in a more formalfashion.

A second approach would be through a bearing testprogramme designed to establish operating limits for differentbearing types and lubrication factors identified in this paper.

3. Conclusion

The paper has summarized the effect of operating, bearing andgrease parameters on lubrication performance. At present itis not possible to predict lubrication level from first principlesfor every individual bearing, lubricant and operating parametercombination. However, it should be possible to predictperformance within a particular bearing or grease family from asingle reference bearing test and the approach developed in thisstudy. This predicted performance is based on the hypothesisthat the main failure mechanism remains unaltered.

The following conclusions can be drawn:

1. Efficient relubrication is the key to low temperature and/orhigh speed operation and extended bearing life.

2. The design of the bearing plays a critical role indetermining operating limits of lubricating grease.

Ball spin and cage effects can be efficient mechanismsfor maintaining the lubricant supply to the track.

Increased bearing size, line contact geometries andhigh load result in reduced lubricant replenishmentof the contact.

3. Using this analysis it will be possible to establish operatinglimits for families of bearings.

The next step in the development of this approach requiresinput from a bearing manufacturer supplying bearing test datato quantify the lubrication and bearing factors identified in thispaper.

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References

[1] Dowson D and Higginson G R 1966 The fundamentals ofroller and gear lubrication Elastohydrodynamic Lubrication(Oxford, UK: Pergamon)

[2] Hamrock B J and Dowson D 1976 Isothermal EHL of PointContacts: Fully Flooded Results ASME Trans. J. Lubr.Technol. 99 26477

[3] Wilson A R 1979 The relative thickness of grease andoil films in rolling bearings Proc. Inst. Mech. Eng.193 18591

[4] Wunsch F 1991 Grease starvation lubrication inhigh-speed spindle bearings NLGI Spokesman55 21721

[5] Astrom H and Venner C H 1994 Soap thickener inducedlocal pressure fluctuations in a grease lubricated EHDpoint contact Proc. Inst. Mech. Eng. J. Eng. Tribol. J208 1918

[6] Baly H, Poll G Cann P M and Lubrecht A A 2006 Correlationbetween model test devices and full bearing tests undergrease lubricated conditions IUTAM Symp. onElastohydrodynamics and Micro-elastohydrodynamics(Cardiff, UK) ed R W Snidle and P Evans Solid Mech. Appl.134 22940

[7] Cann P M E, Damiens B and Lubrecht A A 2004 TheTransition between Fully Flooded and Starved Regimes inEHL Tribol. Int. 37 85964

[8] Baker A E 1958 Grease bleedinga factor in ball bearingperformance NLGI Spokesman 22 2719

[9] Chiu Y P 1974 An analysis and prediction of lubricantstarvation in following contact systems ASLE Trans.17 2235

[10] Jacod B, Pubilier F, Cann P M E and Lubrecht A A 1998 Ananalysis of track replenishment mechanisms in the starvedregime Proc. 25th LeedsLyon Symp. on Tribology (Lyon,France) pp 48392

[11] Cann P M, Doner J P, Webster M N and Wikstrom V 2001Grease degradation in rolling element bearings STLETribol. Trans. 44 399404

[12] Cann P M, Webster M N, Doner J P, Lugt P and Wikstrom V2007 Grease distribution and degradation in R0F bearingtests Tribol. Trans. 50 111

[13] Cann P M and Lubrecht A A 1999 Analysis of greaselubrication in rolling element bearings Lubri. Sci.11 22745

[14] Merieux J-S, Hurley S, Lubrecht A A and Cann P M 1999Shear-degradation of grease and base oil availability instarved EHL lubrication Proc 26th LeedsLyon Symp. onTribology (Leeds, UK) pp 5818

[15] Damiens B, Cann P M and Lubrecht A 2004 Influence of cageclearance on bearing lubrication Tribol. Trans. 47 26

[16] Hurley S, Cann P M and Spikes H A 2000 Lubrication andreflow properties of thermally aged greases Tribol. Trans.43 2218

[17] Cann P M and Lubrecht A A 2004 The effect of transientloading on contact replenishment with lubricating greasesProc. 31st LeedsLyon Symp. on Tribology (Lyon, France)pp 74550

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1. Introduction2. Development of lubrication performance model for bearings2.1. Lubricant parameters2.2. Bearing parameters2.3. Operating parameters2.4. The next step?

3. Conclusion References