GEAR HONINGI

Properties of Tooth Surfaces due toGear Honinq with Electroplated Tools

hardness in near-surface layers. Tensilere idual stresses are especially regardedas causing crack and crack growth.Consequently, a significant loss of

fatigue life occurs. On the other hand,compressive re idual stresses in near-surface layer have enhancing effects onfatigue life under dynamic load.

Different hard macilining processesfor gears were investigated at the

Institute for Production Engineering andMachine Tools, at the University ofHannover, in Germany, 10 evaluate their

effects on the properties of tooth sur-face. The emphasis of the result pre-sented in tills paper L based 011 theproces of gear honing. That process's

ICarslen MalrzeneU and Hans Kurt TOnshofl

mn z ~ an~mmI I"} (OJ

A 4.32 32 25 17.5

B 4.32 31 24.8 17.5...II'll C 2.48 65 26 20IIII01

10 3 45 15 20

IIE 2 9' 15 .20

origin goes back to jhe 1970s, whenFassler AG of Dabendorf, Switzerland,

fJrst applied the kinematics of an internalgeared 1001 meshing with an externalgeared workpiece. In the meantime, theprocess was adapted by several machine

tool manufacturers and has gainedhcreasing importance in tile 1990 .Inrecent years, machine tool manufacturer

Kapp GmbH of Coburg. Germany. devel-

oped the process ofCoronillg™, a powergear honing process with electroplatedtools. The research work presented herei,based on the Coroning process.

Gear honing includes several estab-

lished terms, such as shave grinding.Coronmg, power gear honing and spher-

Or.-Ing'. Carsten M,arzeneUis a projeet manager with automotivesupplier Robrrt Bosch. GmbH of Stuttgart,Germany. He works in the assembly-sys-tems & special-machinery department fIIUJ

specializes in the assembly of electronicthrottle systems. From July 1995 10 May2000. he II'(U (I scientific collaborator {It thl!Institute of Production Engineering andMac/line Tool. at the University ofHannover; ill GemlllllY. He worked ill rileinstitute's production-processes department.where he sperializrd ill gear machining,grinding. hard marhining, andhigh-pressure water peening.

Prof. O'rAng ..10r,~I:ng.E. h. HansKurt Tiinshoflis Ireatj of the Institute for ProductionEngineering Wid Mllclrille Tools .

..... "'.powertrlltlsmfss'on.com •.. w....g.eart·9chnology com' GEAR TECHNOLOGY' NOVEM'BERIDECEMBER 2001 43,

IntroductionIn recent years, the demands for load

capacity and fatigue life of gears con-stantly increased while weight and vol-

ume had 10 be reduced. To achieve rho e

aims. rno t of today's gear wheels are

heal treated oiooth surfaces will havehigh wear resi lance. A a con equenceor heal treatment, distortion unavoidably

occurs. With the high geometrical accu-

racy and quality required for gears, a Ilhard machining proce s is needed thatgenerate, favorable properties on thetooth surface and the near-surface mate- Figure I-the pTincip'le ot gear honingl

•

.rial with high reliability. r-:;;;;;;;;;;;;:;:::;:;::;::;::;:::;::;:::;;:;;::;;;-----------IHard machining proce se can modi-

fy properties such as surface roughness

and topography. residual stress slatematerial structure and hardness in a widerange. From grinding. for instance. it isknown that adverse process conditionsmay cause thermal overload that re ults

in annealing zones. rehardening zones. oreven grinding bum. Tho e effects usual-

ly are characterized by the occurrence ofhi~gh tensile residual stresses, as well asmodifications of material structure and Figure,2"""""Geomehieso. investigated gars ..

GEAR HON!IINIG

gear 'llOnmg (roughing I finishing)slack alk1Nance . 80 1 2Q ~mgear rotary frequency: 6B011mlnsMfI allQle : - 8 'abrasi\ie"s . 05" I 025

C8N profile grinding (roughmg I nnishlng)stock allowance: lao 120 urncutting speed . 40 mls

gear 8'mn = 4 32 mm: t= 31material:25 MoCr4 E

F,igure3--M,a,chining groove lines due to gear honing andlprofile grinding.

4r, ----------------, gearC mn =2 48 mm; Z =65material: 28 Cr ~

I-------F"I----l gear hoo!ngifOlJ9hlllQ I finishing)$\o¢k allowance 80 120 urnigear rotary frequency BOOllm,nabrastves . 054 I 025

hard shaving(~Ingle_stage)stock allowance : 1<1011m

CBN profile gnnding(rough,ng I fimshing)slock allowance . 80 I 20 11m

--~------::--.. -..----- .---' .." -~

Figure 4!-Surface roughness due to gear honing and ,alternative gear ma,chining processes.The, cutoH length wa,s 5.6 mm.

gear 'honing(roughing / finishing)atock allowance : 80 /20 11mgear rotary frequency; 800 11minabrasives • D54 1026

e -400'<Il

~ -6001---'!Iiro -800 I__------"=--------l~~-10001__-------------l<Il

I!! -1200 I__---------U:=.

; "OO~m

MPall-------------

-1600~==;;;;;---=-=-;;;;;;;;;;=======,X"lIY measurement

- "--- radiaOOri C,11(g XEC 5.76 TPII"'Bragg angle 2!1 166 438' penelrabon depth 5 11m

~ lattice pIaoe (211)

Iiigure 5-Resid'uaJ stresses due to' gear ihonin.91,and alternative gear machining processes,

ic honing. Still, the expression "gearhoning" win be used in this paper, as it ismost widely used.

Machine Tool Concept and ProcessCharaeterisities

All experiments all gear honing wereperformed on a Coroning machineVAC 65. manufactured by Kapp GmbH.The machine tool offers two spindles forthe workpiece (A-axis) as well as tor thetools (Csaxis). Each spindle has a nomi-nal power of 14 kW andenables rotaryfrequencies up to a maximum of800 \lmin. Gears up to tip diameters of220 mm. maximum modules of 6 mm,maximum widths of 80 mm and maxi-mum workpiece lengths of 500 rnm canbe machined in the working area.

Gear honing on the Coroning ma-chine VAC 65 removes maximum stockallowances of 130 um on the gear flanks.No prernachining process is necessaryafter heat treatment The heat-treatedworkpiece, whose permissible hardnessis limited to 62 HRC, undergoes a rough-ing operation and a following finishingoperation. The principle of gear honingand the working area of the machine toolare shown in figure L The kinematics isbased on the continuous meshing of aninternal geared tool with the workpieceto be machined. During gear honing, theaxes of the tool and the workpiece have adefined shaft angie that results in rnateri-aJ: removal. because of the relative motionbetween the flanks of the tool and theworkpiece.

A roughing tool and a finishing too!'aremounted in the Coroning head. The toolsrepresent internal geared metallic bodieswhose tooth flanks are electroplated with.asingle layer of abrasives. In our experi-ments, diamond grit of the specification054 (roughing tool) and D25 (finishingtool) was used. During tool life, no dress-ing operations are necessary. When thetools are worn, the abrasives are removedfrom the metrul.ic body and a new layer ofdiamondgrit can be applied.

To enable reduction of the pitch errorduring gear honing, the axes of the work-piece and the tools areel.ectronicaUycoupled via control of the machine 100L

The gears used for the presented44, NOVEMBERIDECEMBER2001 • GEAR TECHNOLOGY' www.geeflechn%gy.com • www.powetlranstnlss"on.cotn

experimeotalresearch work are shown inFigure 2. Gears A and B, with a largemodule of 4.32 mm,are made of casehardened 25MoCr4 teel and buiU fortruck gearboxe . Gear C, with a signifi-cantly smaller module of 2.48 mm, is apart of differential gearboxe for passen-ger cars and con i t of tempered 28Cr4steel, Gears D and E run in stationarygearboxes and are made of case hardened1.6MIICr5 steel.

To evaluate theproperties of the gearsmachined experimentally residual stressstate, surfaeeseughnes , surface topogra-phy. material lructure and microhard-De s were inve tigated. The residualtres analysis was performed on an X-

ray diffractometer using CrKa radiation.In order to obtain depth profiles of resid-ual stresses, 5U rface layers of tooth flankswere removed in several slep . by elec-trolytic polishing. This proces guaran-tees the absence of thermal and mechan-ical loads that would modifylhe residualslressstate. After poli hing, residualstress measurement was carried 'out in thedetermined depth before the cycle of pol-ishing and measuring began afresh.

For roughness measurement , .31 '0011-

tact tylus in irumem PerthorneterConcept was used. Photographs of thearface lopography were taken by light-

optical and canning electron micros-copy. Effects on hardness and materialstructure were detected by photographsof metallographic preparations andmicrohardne s mea urements. The mostintere ting results of the investigationson honed gear tooth flank. are presentedin the following paragraphs.Surface Roughness IUld Topography

The kinematics in gear noning is char-acterized by the me rung of the gear to bemachined with the internal geared toolunder a shaft angle, As a consequence,the relative motion between 'the tool andthe workpiece i composed of a roll and ascrew movement (Ref. 4), With a S:haft

angle of 0°, a mere ron movement occurs.OUl., a haft angle different from OPresultsin a screw component that causes an addi-tional lide movement in tip-root direction.With regard to the single grain contact ingear honing, curved groove line occur that

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presents a comparison of typical surface. tructures generated by gear honing andCBN profile grinding, The microscopic

jphotograph how tooth flank of gear B,.I which were machined alteraati vely by

I" the two processe .Due to the axial feed direction of the

i"".;grinding wheelvthegtcund tooth flankshows straight-lined grooves t:hat areI parnllelto the tooth trace. In gear hon;ng,

I~

Ij

Il

ii

1

I!

1

I

IIii

are not paralleJ 'to the tooth trace.Figure 3

High Speed Automatic Functional Tesler

machining grooves are olllyparallel. tothe tooth trace neal" the pitch circle. Intheareas near the tip and the root, the direc-lion of the machining grooves and tiletooth trace include a cutting angle thatincreases as the distance from lhepitchcircle increase,

Be ide the structure of the urtacegenerated. by different gear machiningproces es, surface roughness was aboinvestigated. Samples of gear C were

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GEAR HON~NGmachined by gear honing. bard shavingor CBN profile grinding. Though shavingis a soft machJn:ing process in most cases,hard shaving is an application suitable forhardened gears.

The roughness values after machiningare compared ill Figure 4. Lowest len pointheight values of only 1.6 urn were meas-ured at the ground variant. Gear honingand bard shaving led to comparativelyhigher ten point height values between 2.6

and 3..1 urn.In Figure 4, the surface roughness for

honing may seem high. That is due in partto a mi nomer, Gear honing with electro-plated tools refers to a "honing" process,but it i actually a grinding process using aprofiled grinding loot The rugh roughnessis also due in pan to the tool pccification,

Residual StressesThe residual stress [ate play an

important role concerning fatigue life of

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46 NOVEMBER/DECEMBER 2001' • GEAR TECHNOLOGY· www.glil8'rlechnology.com • www.po", .. ,Itl!nsrrllSSion.com

components under dynamic load. [II. geargrinding. modificauon of tile grinding

conditions. such as increasing wear ofthe grinding wheel or varying cuttingspeeds, can C3_Useresidual tresses l1talwidely vary from len, de to cornpre siverange (Ref. ~). Tensile re idual stressesare regarded as causing crack and forc-ing crack growth. 011 the other hand.compressive residual stresses in near-surface layers have enhancing effects onfatigue life under dynamic load.Therefore. the interaction. between dif-ferent hard machining proce ses and theresidual stress states generated are ofhigh importance.

To inve ligate those interactions,machining of gear C was done by thethree competing processes of hard shav-ing, CBNprofile grinding and gear bon-ing. The residual stresses mea ured at thesurface of tooth flanks in axial and tan-gential. directions are displayed inFigure 5. nne can see that all of the men-tioned processes induce compressiveresidual. stresses. The lowest compres-sive residual strcs es were found afterprofile grinding. Hard shaving induceslightly higher compressive stresses

between -474 MPa and -729 MPa. Butthe highest compressive residual stresseoccur due to gear honing, Depending onthe direction of measurement. they rangefrom=L ..217 MPa to -1,4.19 MPa .

Similar investigations were done forgear B. which was machined by gearhoning and CBN profile grinding. In thiscase, compre ive residual stre sesbetween -L03] MPa and -],,304 MPawere detected after gear honing. A forgrinding. lower compre sive Ires e ofaround -900. MPa could be measured.

When discussing tho e high compres-sive residual stress slates after machin-ing, tile question arises whether thestresses were generated by rnachiniog orwere already in the material due to thepreceding heal treatment. To clarify that,re idual stresses states after case harden-ing and after the roughing and finishingoperations of gear honing were investi-gated. The investigations were done ongear A. Figure 6 shows a comparison ofthe depth profiles of residual stres ob-

rained after case hardening as well asafter roughing and finishing. Directly atthe surface of the unmachined part, highcompressive residual stresses of about-850 MPa occur: The maximum com-pressive residual stress was found at adepth of 10 11m. Because the roughingoperation in gear honing removes a stockallowance of 80 pill (light grey area inthe diagram), compressive residualstresses in those layers are of no impor-tance. The surface generated by theroughing operation is equivalent to adepth, of 80 11m, where (he unmachinedpart shows only low compressive resid-ual stresses of about -455 Mfa.

In the diagram, the surface generatedby roughing lies at a depth of 80 urn, Thedepth profiles of residual stress inducedby roughing are marked by the dottedlines. High compressive residual stressesoccur directly atthe roughened surface.Dependent on the direction of measure-ment, they range from -,j ,026 MPa to-I, ,315 MPa:. Compared to the initialslate of the material (continuous lines),one can see an increase in compressiveresidual stresses of more than 600 MPainduced by tJIE~ roughing operation ingear honing.

After roughing. the maximum com-pressive residual stre s wa found at thesurface of the tooth flank, But a signifi-cant increase of compressive residualstress was also detected below the sur-face. The influencing of the residualstress state due to roughing reachesmaterial regions up to depths of almost40' um.

As the finishing operation removes anadditional stock allowance of 20 !Jill(dark grey area). the surface of the fin-ished workpiece can be found at a depthof 100 )lID, and the re idual tress pro-files after finishing are marked by thedashed lines. Again, the maximum cem-pres ive residual lIess occurs at the sur-face. although the values between -968MFa and -1,245 M~a are slightly lowerthan after roughing. The 'compressiveresidual stresses could be increased up todepths of about. 30 um.

High compressive residual stressescan be attributed to the high mechanical,

:GiEAR HONING

distance from the surface00 20 40 60 80 100 1'20 140 160 ~m 200

I X-tar measuremenl finlshlllllOCI<

I radla~o" ' Cf 'K" 110"'"111>:e ,.. ..... 1M... •... . .• ",-20() , Bragg angle.2O 166 ~3a" ?O IJfII .'

i laltlce plane . (211) .' ,,( ,,~./~I~":::XEC 576TPa"1 ~~ .....

-40(); penetration depth· 5 ~m ...!!I ~ .... I!r' gear'" m" =" 32 mm, Z = 32

~ ~Iore ~ "'ng~ P' ;/ 0 f malflrlal 25 MoCr-IE!II - l:or ~,lip gear hailing

~.. -eoo I V}r if """",' (roughing I ~nH'hlng)::: v-.. sllX;k allawance 80 I 20 urn- '/ radial reed , 0 I 0.4 mm/mln(11 !" _.. "- 8- 11mi.§ -800 ~'''/ .. ". . .•' gear ,,,,,,ry .. .,.,uency w n!II f' muglling only •.J:. '",...1'0I.III' ,mgallr.ur.e8. OM I 025CD ~ r / . 'i] 'I and <> aillngenual.~_.... 10,00 \\ / l" ~I,j fi",shlng - '1_~~mad1ll1lng

\0- U/ -;i ,.,i C "IDual Ib- <f ... "llIngenuaJ . '

- ,,'rough,ng onlyMPa r I X "a>JIII I

roughing sLock allawance 80 11m -1 D"llI:ngen1lal'~~ing and.140() i I I .."ulal I fLnlshing

iFigure~De,pth Iprofiles of resitJuaillstress after different production steps ..

gear hQlllng(rougtllng I ftnl$h.ng)stOCk allowance: 60120 !-1mradlall'eed 0.810 '" rTmimlnshan angle ·8'

, DMID25

CrKa156 ~3a"(211) .

·576 TPa,'depth 5 IJfII

mean cutting speed [mls]

Figur,e 7-Residuall stresses due to ,I'lifferent cutting s,peeds.

cuttingl speed

Dfl.4fD25

IFig,ure8-tJIaterial str-uctur,e due to gear Ihoning under various conditions.·www.powertral.!sm/sSJOI.!.com"WW ....g·6Iut·9ChI.!O/ogy.com·GElleR TECHNOLOGY' NOVEM,BERIDECEM!BER 2001 47

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GEAR HONING, 11 _

48 NOV'EMBERIDECEMBER 2001 • GEAR TECHNOLOGY' www.geart9chnotogy.com • www.powllr/rsn$ml$s/on.Com

forces in gear honing caused by machin-

ing with superhard diamond abrasives. In

roughing, the machining forces are high-er due to using the coarser diamond gritD54, which is why the increase in com-

pressive residual stress and the effect ondepth are higher than in finishing withD25 grit.

It has become apparent that gear hon-

ing with electroplated diamond toolsleaves high compressive residual stresses

in the near-surface material. Admittedly,the interactions between process layout

of gear honing and the induced residual

stresses had to be discovered. For thatreason, gear honing experiments withvarying cutting speeds were carried outusing gear A as the workpiece. The vari-ation of the cutting speed was realized by

changing the workpiece rotary frequen-cy. Figure 7 displays surface residual

stresses dependent on different mean cut-ting speeds and gear rotary frequencies.

Cutting speeds of o.n, 1.08 and

1.27 mls were used, which correspondwith rotary frequencies of 450, 680 and800 l/rnin. For all mentioned conditions.

high compressive residual stresses occurin axia] and tangential direction. While

no variations of residual stresses can bestated for cutting speeds of 0.71 and1.08 mfs, a slight decrease was found fora cutting speed of 1.27 mls ..Measurements of the spindle's consump-

tion showed II decrease with increasingrotary frequency. That indicates that

WHEREASPRODUCTIVE EFFICIENCY

OF GEAR HONINGIS AlLEASl

IN THE RANGE OF'GEAR GRINDING,

THE GEAR HONINGPROCESS

ALSO EFFECTSFAVORABLE PROPERTIES

IN THETOOTH FLANKS.

lower .machining forces can explain theslight decrease in compressive residual

stresses when using high cutting speeds.

Structural ModificationsThe generation of material properties

when machining with geometrical, unde-fined cutting edges is always call sed bythe interaction of mechanical and ther-

mal loads in the contact zone. While

mechanical forces Carl inducecompres-sive residual stresses that strengthen the

material, the occurrence of high thermal

loads shifts residual stresses to the ten-sile range and causes structural modifi-

cations, such as annealing zones or evenrehardening zones, so-called wbite lay-ers. Even though the phenomenon of

white layers. is not completely investigat-ed yet, their formation is regarded as

harmful to the workpiece (Ref. 1).Ingear honing. the already described

generation of high compressive residualstresses can be attributed to strong

mechanical forces. Thermal loads obvi-ously playa minor role. That assumptionis stressed, when one takes into consider-

ation the influence of cutting speeds in

gear honing, In general, the amount of

thermal load increases when using high-er cutting speeds. Whereas in grindinggear B, a cutting speed of 4() m/s was

used, the mean cutting speed in honing

of the same gear geometry only amountsto about I mis, which indicates low ther-

mal influencing of the near-surfacematerial.

To finish evaluating the thermaleffects in gear honing, modifications ofthe material structure near the surfacewere investigated. Figure 8 showsmicroscopic photographs of near-surface

structures due to gear honing under com-

pletely different conditions.Variations of the cutting speed, the

radial feed and the state of tool wearwere used in the gear honing experi-ments to create favorable and adverseprocess conditions. The three photo-

graphs on the left side display the struc-

tures generated as a consequence offavorable process conditions with lowestrhermal loads possible. Near the surface,no structural modifications can be seen.

The process conditions used for !he

specimens on the right side were chosento create high temperatures in the contact

zone. Gear honing with worn toolseffects high friction, which results fromthe low cuuing ability of the diamond

grits' blunt and rounded cutting edges. Ahigh cutting speed of I..27m1s all 0 caus-

es increasing friction, In spite of high

thermal loads due to increased friction,

the photographs on the ri.ght side show1:10 modifications of the structure at all.Effects like annealing zones or reharden-

ing zones can be avoided with high reli-ability, Those discoveries are stre sed by

additional microhardne s measurements,wh.ich were performed all the discussed.specimens. In all cases, no alteratiens of

the microhardness due to gear honing

could be measured. Those facts indicatethat, in contrast to grinding, the tempera-tures in gear honing are too low to causethermal damage even under the most

unfavorable process conditions.Conclusions

For ecological and economic reasons,

the demands fOJ fatigue life and load

capacity of gears constantly increase.

Under aspects of low development costsand risk, the design of gears oftenremains unchanged and the improved

performance of the product has to be

achieved by better quality due toimproved and efficient manufacturingprocesses, In recent years, the process of

gear honing with electroplated tools hasbeen established as a competitive finish-ing proces for hardened gears. tn con-trast to conventional gear honing withcorundum tools, high stock allowancesup to 130 urn can be removed, and

premachining processes after heat treat-ment. can be abolished.

Whereas productive efficiency of gearhoning is at least in the range of geargrinding, the gear honing process also

effects favorable properties in the tooth

flanks. Surface structures with curvedgroove lines are generated. The surfaceroughness due to gear honing is sli.ghtlyhigher compared with competing gearmachining processes. One further advan-tage of gear honing with electroplatedtools .is the generation of high compres-sive residual stresses directly at the sur-

_------------1 GiEARIHONINGface, as wen as jill tile near-surface mate-rial. Compressive residual stresses areregarded as a mean of preventing cracksand stopping crack growth under cyclicload and therefore increase fatiguestrength, In gear honing. 'the compre sivere idual tres lates can be reproducedwith ve.ry high reliability. Even whenvaI:'ying me cutting speed or Ille work-piece rotary frequency respectively.almost. the arne stress slates occur.

The cause for high compressive resid-ual stresses was found in the combination Iof mechanical and thermal loads in gearhoning. On the one hand. high mecbani-cal force that trengthen the materialemerge from machining with superhardabr sives, On 'the other hand. very lowcutting peed in the range of I mls re ultill low process temperatures. Therefore.unfavor:abl.e shiRing ofihe residual stres •es to tensile range as well as modifica-tions of fhe near- urtace material struc-

ture are avoided with high reliability.

Ack--'1owledgmenlsThe scientific work pre ented in this

paper reo LIltsfrom a cooperative researchproject with tile Institute for ProductionEngineering and Ma hine Tool.Hannover. and the companies KappGmbH. Coburg; Daimler-Benz AG,Gaggenau: Ford-Wer~ke _G. Ktiln; andLenze GmbH & Co KG. Extertal. Theproject was financed by the GermanFederal Ministry for Education andScience and upported by Re earch

entre J'Ulicl1.

Rcrcl'l!ncesI. Ton shoff, H. K.. II. Karpu chewski and C.Marzenell, uSurlace generation by hard nun hini ng."PnJ('l'~dilllfS of Ihe Fourth EU.TO/H!OJ1Ctmftf"t!nu Dli

.RuidJILJI Stresses. Vol. I, Cluny en Beurgogne, JW1e4-ti. 1996. p. '13--31.2. nnsholT. H.K., [. Inasaki, B. Karpuscbew~ti.and Th. Mandrysch, "Grinding process achieve-ments and their consequence 00 macbi~ I:OOI~~challenges and opponunities," C/RP AruUlI:S 199 •Volume 471211998, Verlag Teehnische Rundschau,Berne. Switzer-land. 1998.

. Tonshoff, H.K., B. Karpuschewski, :LIId F.Werner. "ProzeBnahe QualiHllssicherung in derFeinbearbeitung." P1"OCudlng.t of the 7.In/l'mQt;(Hla{rr BraunschweigrrFl!i"~ar~iru,,gsxDl'oql1jlU1l (FBK), .Braun!>c"weig.March 2 . 1993, p. 3.0/-3.19'.4. Brinksmeler, E.. . Berger and Ch. Schneider.''Technolo ische ' aehbildung des Schabsch leifens(VerLahnunll~honel1):' Jahrbuch Sell/eifel!. Honen,Ulp~n und Poliere», Volume 58. edited by H.K .

T osholl and E. Westkllmper, Vulkan· Verlag Essen,19%.5. Gosdin. M. "Nelles Verfahren fUr dieHwein'beMbeinmg von Zahnrlldem in groBeIlSerien," "',dus/rial Diamond Rel'il'l1' 1 (1994), p.15-17.6. Bausch, Th.. "Verfahrensmerkmafe beimSchleifen und HODen von Zahnrddern, PW'I II,"lruluslfial Diamond Review 2 (1994). p. 94--'98.

This pa,per was originally presented atthe ,.. WDrid COngress Ion Gearing BAdlPower Transmission in Paris, Man:h, 1999.

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