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AM A A AQO-36488

AIAA 2000-3245

TRIBOLOGICAL AND THERMODYNAMICAL ISSUES OF AZERO CLEARANCE AUXILIARY BEARING (ZCAB) FORSPACE APPLICATION

Mohsen Salehi, Rita Kaur, and Hooshang Heshmat

Mohawk Innovative Technology, Inc.®Albany, New YorkTel (518)862-4290 Fax. (518) 862-4293 e-mail: miti@albanv.net

36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit17-19 My, 2000

Huntsville, Alabama

For permission to copy or to republish, contact the American Institute of Aeronautics and Astronautics,1801 Alexander Bell Drive, Suite 500, Reston, VA, 20191-4344.

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2000-3245

TRIBOLOGICAL AND THERMODYNAMICAL ISSUES OF A ZERO CLEARANCE AUXILIARYBEARING (ZCAB) FOR SPACE APPLICATION

Mohsen Salehi, Ph.D.Member AIAA, STLE, Associate Member ASME

Rita Kaur, Ph.D.Member ASM

Hooshang Heshmat, Ph.D.Fellow ASME, STLE

Mohawk Innovative Technology, Inc.Albany, New York

Tel. (518)862-4290 Fax. (518) 862-4291e-mail: miti@albany.net

ABSTRACTA unique Zero Clearance Auxiliary Bearing (ZCAB) hasrecently been developed, incorporating a series ofinterconnected rollers that move radially inward whenactivated until the shaft is centered by all rollers and theinitial clearance is eliminated. The ZCAB is aimed tofunction as a back up bearing system for use in highspeed rotating machines both on land and in space. Twomain components of the ZCAB are the ball bearings andbearings sleeve which together serve as ZCAB rollers.During ZCAB operation the rollers will be in contactwith the rotor, from the onset of initial shock loadtransmission to the final steady operation,. Since theZCAB is targeted to operate in a high temperature andextreme environment, evaluation of the thermal andmechanical compatibility of the materials and lubricantsfor final selection for the ZCAB components is of vitalimportance in the development of this technology. Inorder to evaluate the operation of the ZCAB componentsand to determine the type of lubrication system, athermal analysis was performed for the ZCAB assembly.A thermal resistance network was used along with thecalculation of the power loss to predict the bulktemperature of the components. This paper also reviewsthe state of the art of liquid/solid lubrication for use inthe space ZCAB. The application of these lubricants inthe ZCAB present a unique challenge and specificexamples of the space-environment related problems andour approach to these issues are presented.

INTRODUCTIONActive magnetic bearings (AMB) are typically used forlow noise and high speed turbo machineries. Some ofthe advantages of using AMB are that they have nolimitation with regards to the speed, no oil or lubricationsystem is required, they can be precisely controlled andheat generation is low. However, due to materiallimitation for the laminations and coatings, their use islimited to low temperature applications (T<400 °F) andalso their performance under shock loads and their lowstiffness should be compensated by other componentssuch as rolling element bearings. Use of rolling elementbearings are susceptible to friction induced backwardwhirl (1). There are also problems associated withmaintaining high speed applications in gas turbinesystems due to high DN values versus acceptable lifetime, where D is the journal diameter in mm and N is therotor speed in rpm.

A Zero Clearance Auxiliary Bearing (ZCAB) wasintroduced in a patent by Heshmat and Chen (2) to beimplemented as a back up system for the magneticallysupported systems. A summary of the ZCAB ispresented, however, a more detailed description can befound in a paper by Chen et al.(3). The ZCAB consistsof a series of interconnected compliantly mounted rollerssurrounding the rotor with an initial clearance. Asshown in Fig.la, the ZCAB consists of the followingmajors components:« five to eight rollers with bearings inside each

roller, and the bearing is mounted on a pinCopyright © Mohawk Innovative Technology, Inc. 1

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which passes through a circumferential guidepath (slot) in the support plate (side plate);two drive rings which are connected to the pins.The drive ring is used to move all rollersinward in unison along the slots on the sideplates, thereby decreasing the clearancebetween shaft and rollers;two side plates on which circumferential guidepaths (slots) are made and the pins move insidethe slots;a compliant damper mount.

la) Ib)

Fig. 1 The ZCAB hardware and a schematic of theoperating mechanism

At the ZCAB open position, there exist an initialclearance between the shaft surface and surface of therollers. At this condition the ZCAB is open. When theZCAB is activated, the drive rings are rotated in adirection to force the roller to spiral inward in unison inthe slots made on the side plates. Since the rollerassemblies are connected through a continuous drivering, once one roller assembly begins to close or open, allthe roller assemblies close or open in unison. At thistime the ZCAB is in a closed position. Further rotationof the drive ring will results in applying additional loadon the rollers and increasing the stress at the contactpoints.

There are two mechanisms by which ZCAB can beactivated from the open position to the closed position,that is, the passive mechanism and the activemechanism. In a passive mechanism, when the rotorcontacts any one of the roller assemblies, the resultingtraction force causes the roller assembly to movecircumferentially in the direction of shaft rotation andradially inward, thereby closing around the shaft.

2000-3245

However, in an active mechanism, after the rotor-rollercontact, the drive ring is rotated by an actuator, causingthe rollers to move inward in unison and eliminate theinitial clearance.

The tested ZCAB in Fig. Ib shows the roller assemblies,the front drive ring and the support assembly. Thecomponents not shown are the torsional springs and theresilient damped mount. The traction force is rotorspeed dependent and has to be measured in tests. Theresilient damped mount can be made of elastomermaterial as in the test rig. For a high temperatureenvironment, corrugated steel ribbon was suggested bySchmied and Pradett (4), or specially designed foildamper may be applicable.

In a system where the ZCAB is employed as a backup forthe AMB the following design issues need to beconsidered: (a) the overall stiffness and damping; this isto prevent the shaft from contacting the AMB stator intransient conditions or when the AMB fails, so that themaximum static deflection will not be exceeded; (b)variation of stiffness and forces on the rollers while theZCAB is closing (the transient time from initialclearance to zero clearance), causing the forces on thebottom roller(s), the side rollers and some overall shaftdeflection; this also determines the force on the bearingsembedded in the rollers; :(c) the external force needed toclose the ZCAB and the impact resulting from this force;(d) the temperature rise in the components or themaximum rise temperature in the ZCAB; this can effectthe thermal growth and will determine the type oflubrication system that should be employed for the roller-shaft contact or for the ball bearings used for the rollersand (e) the lubrication process. The overall ZCABdynamic model is shown in Fig. 2.

Detailed dynamic issues have been addressed by Chenet al (3). Since in the ZCAB, mechanical componentsare in contact and heat is generated in the rollingelement bearings, a thermal model is essential forpredicting the temperature of the components under aspecific operating condition. The thermal analysis resultsalso can be used as a tool for determining the lubricationprocess and also the life time of the mechanicalcomponents.

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ic Model

shaft

clearanceCo 1

(Gap

MagneticBearing

Afier rotor drop:C0 + Max(fixJ < Gap

Fig.2 Dynamic Model of ZCAB

THERMAL ANALYSISAs mentioned earlier, when the ZCAB is in a closedposition, the shaft and the rollers will come into contactand due to the shaft load there will be a contact stress atthe shaft-to-roller contacts. As the ZCAB closes, theshaft load will be transferred to the ball bearing insidethe rollers and due to the shaft-roller diameter ratio theball bearing speeds are normally very high. Thisultimately causes a high rate of heat generation insidethe bearings, although convection from the side and topsurfaces of the rollers and also from the shaft will assistin dissipating this heat.

This analysis is based on the simulation of a ZCAB withan electrical circuit where the heat sources, heat flowpaths and different modes of heat transfer between thecomponents and surroundings has been mapped. For thisthermal resistance network, the governing heat transferequations are written and simultaneously solved usingthe Engineering Equation Solver (EES) program. Thesimulated electrical circuit is shown in Fig. 3.

Three different modes of heat transfer resistance aresimulated by the following equations:

K,AR -con, ^ (I)

JK Qo.

<U*

*T_T.;

Fig. 3 Thermal resistance network of ZCABassembly and physical model of heat flowpath in the ZCAB roller-frame assembly

The last equality in the above equation shows that theradiation resistance is a function of emissivity and shapefactor. The emissivity is a function of material and shapefactor (F,_2 ) which represents the amount of radiationheat emitted from surface 1 and received by surface 2.The convection heat transfer from the top surface of theroller or shaft was simulated by external convection overa cylinder and the following correlation was employed(5) for the side surfaces:

Nur,top = 0.076

= 0.664

(2)

(3)

where the Re and Pr are the Reynolds and Prandtlnumber, respectively.

The results of the analysis are compared with atemperature measurement from a point on the pinconnected to the bore of the ball bearing and exposed tothe natural convection to surroundings. It has to be takeninto consideration that in the outer space the effect ofconvection heat transfer is minimal and the effect ofradiation plays a more important role in the heat transferprocess.

THERMAL ANALYSIS TEST RESULTSThe results of thermal analysis, as discussed in theprevious section were employed to predict the

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2000-3245

temperature rise in the ZCAB components. In order tocompare the result of experiment with that for theanalysis, the temperature of the drive pin (Fig. 3) wasmeasured by installing thermocouples placed in differentaxial and radial positions. Table 1 shows the otherparameters for which the ZCAB thermal analysis wasperformed.

Table 1. The ZCAB parameters employed for thermal analysis

Load(N)

334

Dsh (cm)

10.16

Df (cm)

3.175

Tamb

(°C)

24

Bearing

SKFZS2300

The speed of the shaft was increased from 5000-30,000rpm while the applied load was kept constant. Asmall amount of oil was passed over the contact areabetween the shaft and the rollers. The results of thethermal analysis and experimental results for this testassembly are presented in Fig. 4.

T--8 535+73.58(1 -.-"•6"!XP<-4>*RPM)Load - 75!b

0 5000 10000 15000 20000 25000 30000 35000RPM

Fig. 4 Comparison of experimental and analysisfor temperature rise in ZCAB

The temperatures shown in Fig.4 are the steady statetemperatures, and show good agreement between theresults of the prediction by analysis and experimentalresults. These transient temperature measurementshelped in characterization of heat generation in the ballbearing and the contact areas. A preliminary correlationbetween the temperature and speed at a constant loadcould be fitted in the range of data presented.

SPACE TRIBOLOGICAL ISSUES FOR ZCABDeveloping a ZCAB operating at high speeds,approaching I00,000rpm, requires careful selection ofmaterials and lubricants since surface velocities at theshaft-to-ZCAB contact will approach from 70 to160m/sec.

MATERIAL SELECTION CRITERIAThe high shaft surface velocities initially result in highskidding and subsequently very high DN values for theZCAB roller support bearings. This situation can resultin extremely high heat generation and material wear.Compounding the challenges associated with the highspeed conditions are the fact that the system must operatein a vacuum, which affects thermal management of thesystem. Therefore, in identifying and selecting candidatematerials and lubricants for a ZCAB the entiretribomaterial system must be addressed in order to assurethe desired ZCAB life and durability for spaceapplications.

LUBRICANT SELECTION CRITERIAThe key issues affecting the choice of lubricant for usewith the ZCAB include the very low ambient pressures,radiation environment and the absence of a gravitationalfield. Radiation effects are not considered a majorproblem in space lubrication, since, radiation-resistantlubricants have been developed and effective shielding isemployed. However, the low pressure environment (10~16

torr) contributes to rapid evaporation of the lubricantand to counteract this, low vapor pressure fluids are used.Since lubrication normally takes place by means of a filmentrained between sliding or rolling surfaces, the loss ofthis film due to evaporation and mechanical working canresult in failure of the mechanism. The main problemassociated with zero gravity is that fluid reservoirs areineffective and there is also a complete absence ofconvection currents for cooling. The result of this is thatthe frictional surfaces will operate at higher temperaturesthan those in a terrestrial environment. Other detrimentaleffects are volatization, condensation of the volatiles,absence of reacting gases and poor thermal conductivity.

The ZCAB must operate under elastohydrodynamicregimes, whereby the contacting surfaces deformelastically and the hydrodynamic action caused by thehigh surface speeds creates the lubricant film. This typeof lubrication guarantees low and constant operating

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torques.

The two types of lubricants which were considered forthe ZCAB are liquid and solid lubricants and these areoutlined in Fig.5.

PerfluoropolyethersPFPE-Dematin S-200 (Datkin)PFPE-Fomttin Z2S(Montecatini Edison)PFFE-Brayco SliZ (Castrol)PFPE-Krytax 143AC (Dupont)

Synthetic HydrocarbonsMAC-2001 (Formulated)MAC-200laNYE- 186

PolyalpnaolefinsPAO-imPAO-182

Fig.5 Candidate Lubricants for ZCAB in Space

TRIBOLOGICAL TEST RESULTSThe objective of this part of the study was to investigatethe tribological characteristics of materials andlubricants for use in the Zero Clearance AuxiliaryBearing.

TEST SET-UPThe selection of the material combinations and lubricantto be used for the shaft and roller were effectivelyaccomplished by screening through the materials pin-on-disc tests, which was carried out using a tribometer asshown in Fig.6.

The tribometer consists of a test disk mounted on aspindle drive by a 1 HP, variable -speed, DC-controlledmotor, linked to the spindle through a belt with a stepdown ratio of 2.5. Maximum speed capability of themotor during low speed wear testing was set to 500 rpmand for high speed testing up to SOOOrpm. The test diskwas supported perpendicular to the drive spindle. Aholder was designed to attach the roller. A load cell wasattached to the base of the specimen holder to measurethe amount of frictional force of the roller, and athermocouple was attached near the contact interface ofthe roller and disc to measure the temperature duringtesting. An efficient interactive graphical user interfacedata acquisition software was designed and implementedto record all data from tests.

Initial tests were conducted under rolling conditions forsome selected materials and lubricants. The testconditions simulated the high sliding velocitiesencountered in the actual ZCAB assembly duringoperation. The following material and lubricants wereconsidered for initial screening:Rollers - C865G Bronze, Stelite 6B, AISI 52100Disc - M50, M2, S7Lubricant - Santotrac 50, MiL-L-7808K, NYE 186

The initial rolling tests results revealed that Stellite 6Band AISI 52100 Rollers and M50 Disc material withMiL-L-7808K lubricant gave overall good wear andfriction characteristics

SLIDING TESTSFurther sliding tests were conducted by locking therollers in the holder using the following materialcombinations:Stellite 6B rollers and M50 Discalso A1S! 52100 rollers and M50 Disc using MiL-L-7808K as the lubricant for both tests.

Figures 7 and 8 show the result from the surfaceobservation of the rollers and the discs after the slidingtests.

Fig. 6 Tribometer

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AlHf-S21O8 Roller (X3Si MSODise~(X35) - fAf</-i-/*ftSO

Fig .7 Surface Observations of A1SI 52 !00 RollersTested with M50 Disc using M1L-L-7808K

StatSse US JfeSer (X3S) MS® like (X3S) ' f,«iH-

Fig .8 Surface Observations of Stellite 6B RollersTested with M50 Disc using MiL-L-7808K

The results show that under sliding conditions usingAISI 52100 roller and M50 disc there is very little wearon both the roller and disc surface as shown in Fig. 6.However when Stellite 6B is used as the roller materialthere is significant material displacement occurring atthe contact interface of the roller and also sever wear onthe disc surface.

STARVATION TESTSFurther starvation tests were conducted by changing theoil feeding speed under rolling conditions on the samematerial combinations as used for the sliding tests:Stellite 6B rollers and M50 Discalso AISI 52100 rollers and M50 Disc using MiL-L-7808K as the lubricant for both tests. Figures 9 and 10show the result from the surface observation of the rollersand the discs after the starvation tests.

AlSl-S2l<m Ratio- - tt'J5,i ,««f> OIK- - (X3S)

Fig. 9 Surface Observations of AISI 52100 RollersTested with M50 Disc using MJL-L-7808K

Slflliu 6B Kaller - (X35) MSO Disc- (035; {Mit-l-?$0SK)

Fig .10 Surface Observations of Stellite 6B RollersTested with M50 Disc using MiL-L-7808K

The results show that under starvation conditions usingAISI 52100 roller and M50 disc there is severe wear onboth the roller and disc surface as shown in Fig. 9.Pits are clearly visible on the AISI 52100roller surfaceand a severely worn wear track at the interface of theM50 disc. However when Stellite 6B is used as the rollermaterial (Fig. 10) there is mild wear at the contactinterface of both the roller and disc surface.

These results clearly show that by simply changing thetest conditions and material combinations very differentwear and friction characteristics can be obtained.

SOLID LUBRICANTSSolid lubricants have also been considered for use as alubricant in a back-up bearing. These lubricants areknown to have low shear strength required for sliding,very low coefficient of friction in vacuum conditionsthan liquids, they have negligible vapor pressure and aregenerally insensitive to temperature.

The most common solid-film lubricants used in spaceapplications fall into one of three generally recognizedcategories. These are: lamellar solids, soft metals andpolymers. It has been reported that among the solidlubricants for roiling bearings used in vacuums, lamellarsolids such as Molybdenum disulfide and TungstenDisulfide, soft metals such as gold, silver and lead andpolymers like PTFE are effective, Solid lubricant filmshave generally been used when it is not convenient to useliquid lubricants or when contamination might be aproblem.

Molybdenum disulfide in its powdered mineral form isunsurpassed in its load-carrying capacity (as high as500,000psi). When placed under load, the crystallinestructure forms laminar platelets, which readily burnishonto metal surfaces. The smooth surface that results hasvery low friction and a very wide operating temperature

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range (approximately -375"F to 750°F). Molybdenumdisulfide is a highly effective vacuum lubricant functionalup to 2,000°F in the absence of oxygen. It can be used inextreme environments, such as in the presence of liquidoxygen, under radiation exposure, or in dirtycontaminated atmospheres. It works much better in theabsence of absorbed vapors. The problem associated withMolybdenum disulfide is that its life can be shortened ifit slides too much in air before it is used in vacuum, andlong term storage can sometimes induce decompositionto molybdenum oxide.

Heshmat et a!., (6, 7) have been investigating the use offine powders to lubricate rolling-element and slidingbearings. The current ongoing research by Higgs et al.,(8,9) have demonstrated the potential for using powdersto lubricate at high temperatures where liquids will notfunction. This testing was conducted on a self-containedpowder lubricated auxiliary bearing (PLAB) with a100mm bore diameter as shown in Fig. 11.

Fig. 11 PLAB

This test set-up has recently been used to evaluate thethermal performance of the powder lubricated auxiliarybearing at speeds up to 30,OQOrpm and loads up to 236N.Self-contained auxiliary bearings are needed formagnetic bearing supported rotors in space mechanismsand flywheel energy storage systems. This verysophisticated technology of a high speed, self-acting,solid/powder- lubricated journal bearing has recentlybeen tested up to a record breaking DM value of 3 million(DN = shaft diameter in mm x speed in rpm). The twomajor technology components for this system are the

s

130

i1204

110

100

Pad1-LPadl-TPad2-LPad2-TPadS-LPadS-TSpeed

E£ 90

t^

100 150

Time, sees

—--— 36000

30000

25000S

20000 &13

- 15000 809

10000

- 5000

! 0250

Fig.12 Temperature Change of Bearing Padsat High Speed Test of 30,000rpm

Molybdenum Disulfide powder pellet lubricant deliverysystem and the prototype journal bearing. The powdermaterials suitable for the expected operatingenvironments (speed, temperature and load), werereviewed and characterized for use in the lubricantdelivery system. The prototype bearing system was testedin a magnetically suspended rotor bearing test rig(magnetic bearing/rotor-bearing system). Three series oftests were conducted on this prototype bearing: shortduration low speed intermittent testing, high speedintermittent testing and impact or rotor drop testing(transient shock simulating magnetic bearing failure).The acquired test data spanned the range of the expectedoperating conditions, including lubricant feed rate,bearing temperatures and operational dynamicperformance. Fig. 12 shows the temperature change atthe contact interface of the pads and the shaft duringtesting at 30,000rpm. These results clearly show that thelubricant was operating within the temperaturelimitations of Molybdenum Disulfide, which has amaximum temperature capability up to about §50°F.These recent preliminary investigations have beenextremely successfully and will be discussed in moredetail in proceeding papers.

It has also been reported that soft metals, such as, gold,silver and lead are effective under vacuum conditions.Studies conducted by Takasi et al., (10), on solidlubricated ball bearings tested under high vacuum ofKT*Pa, temperature of 572°F and high speed conditionsof 9000rpm showed that tool steel balls and races coatedwith silver or lead gave good torque properties. However,the durability of such bearings was reported to be less

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2000-3245

than SOOhours.

PTFE is a fluorinated plastic resin and has long beenrecognized as an efficient solid lubricant (11). PTFE isa fluorinated plastic resin. It is an effective lubricant forapplications involving loads less than SOOQpsi. Itdisperses well in many carrier fluids and can be moldedinto a low surface energy solid. PTFE seems to lubricatebest on soft materials, like zinc and die-cast metals. Ingeneral, PTFE works well both in the presence of vaporsand in a vacuum environment. However, PTFE also hasa problem in that it tends to cold flow under load andmust be incorporated into some sort of a binder system tokeep it in place. Another disadvantage is that PTFE'sload carrying capacity is low, such that, it should not beemployed in applications where contact stresses exceed1200MPa(12).

CONCLUSIONS• The ZCAB provides a robust backup bearing

system for the magnetically supported rotorsystem. Due to structural integrity and thedesign concept implemented in the ZCAB, thisbackup system can be used in manyenvironments, such as, high temperature or forspace applications. In order to evaluate theperformance of the ZCAB a thermal analysis ofwas conducted using an electrical circuitnetwork. The thermal analysis included theheat sources/heat dissipation and heat transfermechanisms in the ZCAB. A prototype ZCABwas fabricated and tested. The predictedthermal analysis was compared with theexperimental observations resulting in goodagreement.

• The factors which influence the choice oflubricant for a vacuum environment have beendiscussed. This is an initial feasibility study,certainly more work needs to be done on theselubricants in a vacuum environment and inthe end-use mechanism to ascertain theirsuitability for space use.

• Liquid lubricants are frequently used becausethey are associated with low mechanical noise,no wear in the elastohydrodynamic regime, easeof replenishment, ability to remove wear debris,and an insensitivity to environmental factors.

The advantage of using solid lubricants are thatthey have negligible vapor pressures and someare quite insensitive to temperature, whichmakes them suitable for high temperatureapplications. Generally they have also beenknown to have lower coefficients of frictionthan liquid lubricants in vacuum conditions.The major disadvantage is their shorterlifetimes relative to liquids.

Initial screening tests have clearly validatedthat tribomaterial combination and operatingenvironment are important in ZCAB materialselection

The solid lubricant molybdenum disulfide hasbeen successfully evaluated in the Self-contained Powder Lubricated Auxiliary Bearingand the results are currently being used forfurther testing.

NOMENCLATUREA Cross section areaAg SilverAu GoldAMB Auxiliary magnetic bearingD DiameterFi_2 Shape factorFESS Flywheel Energy Storage Systemh Heat transfer coefficientk StiffnessK, Thermal conductivityMAC Multiply Atkytated CyctopentaneMoS2 Molybdenum DisulfideNu IMusselt numberPAO PolyalphaolefinPb LeadPFPE PerfluoropolyethersPLAB Powder lubricated auxiliary bearingPr Prandtl numberPTFE PolytetrafluoroethyleneQ heatR Thermal resistanceRe Reynolds numbert TimeT Temperaturex LengthXz Vertical Disp.of the ZCAB assembly (fig. 2)

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WS2 Tungsten DisulfideZCAB Zero Clearance Auxiliary Bearinge Emissivity

Subscriptamb Ambientcond Conductioncont Contactconv Convectionr Rollerrad Radiations, side Side surfacesh Shafttop Top surfacex Length, Axialz ZCAB

REFERENCES1. Harris, C. M, and Crede, C.E., "Shock and

Vibration Handbook", 2nd. ed., ch.5, MC-Graw Hill Book Co., (1976).

2. Heshmat, H., and Chen, H.M., "ZeroClearance Auxiliary Bearing for ProvidingRotor Shock Tolerance", US patent No. 5,752, 774, (1998).

3. Chen., H.M., Walton, J. and Heshmat, H.,"Zero Clearance Auxiliary Bearings forMagnetic Bearing Systems", Int. Gas Turbine& Aeroengine Congress & Exhibition, June 2-5,Orlando, Florida, (1997).

4. Schmied, J. and Pradetto, J., "Experience withMagnetic Bearings Supporting a 6 MWPipeline Compressor," ROMAG 91 MagneticBearings and Dry Gas Seals Conference,Washington D.C., March 13-15, (1991).

5. Incropera, P.P., and DeWitt, D.P.,"Introduction to Heat Transfer", New York,John Wiley & Sons, (1985).

6. Heshmat, H., Pinkus, O., & Godet, M., "On aCommon Tribological Mechanism betweenInteracting Surfaces," STLE TribologyTransaction, Vol 32, 1989, pp.32-41.

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7. Heshmat, H., "The Rheology andHydrodynamics of Dry Powder Lubrication,"Tribology Transactions, Vol.34, (1991),pp. 433.

8. Higgs, F.C.HI, Heshmat, C.A., & Heshmat,H., "Comparative Evaluation of MoS2 andWS2 as Powder Lubricants in High Speed,Multi-Pad Journal Bearings," Journal ofTribology, Vol. 121, (1999), pp.625-630.

9. Higgs, F.C.IH & Heshmat, H.,"Characterization of Pelletized MoS2Powder," Presented at ASME / STLEConference, Florida, (1999).

10. Takasi, O., Kazuki K., Satouri A., andTakeshi S., "Development of Ball Bearingswith Solid Film for High Vacuum, HighTemperature, High Speed Application,"STLE, Vol.49, No.4, (1989), pp.291-299.

11. Bowden, F. P & Tabor, D., "The Friction andLubrication of Solids OUP," (1950)

12. Roberts, E. W., "Thin Solid Lubricant Filmsin Space," Tribology International, Vol. 23,No.2, (1990), pp.95-104.

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