Gear Lubrication 1

7/30/2019 Gear Lubrication 1

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'1 8 GUill T!!,CHNOLOQY

factors mu st b e c on sid er ed : G ea r to oth geo metry G ear tooth motion (k in ema ti cs ) Gear tooth f orc es ( sta tic a nd dynamic) Gear t o o m material and su rface characte ri st ic s

( Ph ys ic al a nd c hemi ca l) Lubricant characteristics '(physical and

chemical) Envi ro nmen t c ha ra ct er is ti cs ( ph ys ic al an d

ehemicaf)

The Lubricationof G ears Part 1

Int roduct ionT his is a three-p artarticle exp la in ing the p rin- T he ch oice o f lu brica nt an d its a pp lica tio n m eiho d

c ip les o f g ea r Iub rica tio n. It review s current is as im po rtant ss the choice o f steel a lloy and heatknowledge of th e f ie ld o f g ea r tr ib ol og y a nd is in- treatment. The interrelation hip of m e followingt en de d f or b oth g ea r d es ig ne rs a nd g ea r o p er at or s ..P art 1 c l as si fi es gea r tooth failures i nt o f ive modesa nd exp la ms the fac to rs tha t a g ea r design er an do pe ra to r m us t c on sid er to a vo id g ea r fa ilu res . [td efin es th e n om en cla tu re a nd g iv es a . list o f ref-e re nc es fo r th os e in te re ste d in fu rth er r es ea rc h.It also contains an in-depth discussion o f thegear tooth f ail ur e m o de s that ar e influenced byIu br ic atio na nd g iv es m et ho ds f or p re ve nt in g g ea rtooth failures.

The s econd pa n g ives an equa tion fo r calculatingth e lubricant film th ic kn es s w h ic h determinesw heth er the g ea rs o pera te in thebo und ary, elas-t oh yd ro dy nam ic , o r f ul l- film l ub ric at io n r eg im e.A ls o g iv en is a n 'e qu at io n f or B lo k 's f la sh tem pe r-ature, which isused fo r predicting th e risk o f s c uf -fing. Finally , a reco mm endatio n fo r selectinglubricant type. , v is co sity , a nd a pp lic at io n m et ho d

Gear 'Ieeth Failure ModesTo o bta in o p tim um ..m in im um -w eig ht g ea rs et s

th e gea r d esign er m ust be aw are o f th e intric atedetails o f many competing modes o f f a ilu re . In itsn om e nc la tu re p ub lic atio n.t !' t he American GearManufactllrers Assoc ia ti on fAGMM ha s classified20 modes o f g ea r f ai lu re W Ide r t he b ro a d categories

a nd a c as e h isto ry , which demonstrates many of o f wear, s ur fa ce f at ig ue , p la st ic f low . b re ak ag e,the princip les of gear lubrica tion, m e given. and asso cia ted gear fa ilures. R eferences 2 thro ugh

6 a ls o g iv e g ea r fa il ur e m od es . F or o ur p ur po se s,'Gear Tn1JoIogy the basic categories are over lo a d, be nd ing :f at ig u e,

B eca usegea rs are suc h co mm on m ac hine co m- H ertzia n fati,g ue, wea r, a nd s cu ff in g.p on en ts . th ey m ay be taken for granted .. N ot 'T able 1 s ubdivides the five basic fa ilure m odes.gener:ally appreciated is that th.ey a re c om pl ex M a n r y gea r fa ilures are k no wn b y s ev era l n am essystems requiring know ledge from all th e a nd /o r q ua lif yin g te rm s, s uc h a s in itia l, m od er ate,enginee r ing cfuci :P l ines f o r t hei r successful d esign . destruc tive, etc . These names and terms are in-G ear design is a process of synthesis w here gear eluded in t he l is t s inp a re nt he se s. Th e term. "seer-g eome tr y, ma te ria ls , h ea t t re a tment , m anuf ac tu r- ing" has been u se d inth e past inth e U .S .A ., w hil eingm eth od s, a nd lu bric atio n a re s ele cte d to m ee t theterm " sc uf fm g " is u se d in. Eur op e to d es cr ib ethe req uirem ents o f agiven app lica tio n. The th e s ev ere fo rm o f a dh es iv e wear wh ich invol ve sdesigner m ust design the gearset w ith adequate tbe welding and tearing o f the surfaces o f gearstrength, wear re sis ta nc e. a nd s cu ffin g re sis ta nc e. te eth . 'T o a gr ee with current usage, th e term scuf-To do this, he o r she m ust eo nsidergear ttibo lo gy. fing w ill be used in this article w hen referring to


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Bask F aU ure M od es

OverloadBrittle fractureDuctile fractureP l a s ti c de fo rma t ion

c old f lo wbot flowinden ta tio n (ro lling, bruis ing, p een ing,brineUing)rippling (fish scal ing)ridgingb end in g. y ie ld in gtip-to-root interferenceBending Fatigue

Low-c y c le f a ti gu e 1000 c yc le s t o f ail ur e)H igh-c yc le fa tig ue 10 00 c yc les to fa ilure)

this fai lure mode. The te rm s co ri ng im p lie s s er ar-c hin g, a nd it will b e u se d to describe a br as iv e wea rrather than scuffing.

Lubrication - Related Failure ModesT hese a rticles a re concerned with gear tooth.

fai lures that. a re lnfluenced by l ubr ic a t ion . P i tt ingo r s cu ffin g m ay ca use the gea r t ee th t o deterioratea nd g en er at e d yn am i c fo rces. which intumcauseth e gear teeth to fa il by bending fa ti gue . Intheseea ses the b end ing failurre is secondary and no tdi:reo!:lyrelated to l ub ri ca t ion , w1 i ll ep i tt ing o r scuf-fing a re th e primary fa il ure m o de s, a nd both aredefinitely iniJiuenced by lubrication. The fai lurea na ly st m us t d is ce rn tile d if fe ren ce b etween pri-m ary a nd sec ond ary fai lure modes b ec au se th ew ro ng c orr ec tiv e a ctio n is l ikely to be recom -mended if a s ec ond a ry fa:i luremode is mistaken fo rth e primary fai lure mode. F or e xamp le , in cr ea s-i ng 't he size o fd le g ea r tee th to 'prevent reoccurenceof the above-men ti oned bending fa il u re would onlym ak e tb e s itu atio n w ors e by lowering thepittingand scuffing resistance. GodfreyO) gives a go odd es cr ip tio n o f l ub ric atio n- re la te d fa il ure m o de s.

W ith the above consideration~, o verlo ad a ndbending fa tigue a re judged to be u nre la te d to

TABLE 1

He rt zia n Fa ti gu eP itting (initial. superficial, destructive, spa1lioa)Micropitting (frosting. grey s ta in iDg . pe el ing )S ub -c a se f at ig ue ( ca se c ru sh in g)

Wea rAdhe si on ( no nn al , r un ni ng -i n, mild. modera te .severe . exces s ive )Ab ra si on ( sc o ri ng . s cr at ch in g. p low in g, cul l ing,gouging)CorrosionF retting - corrosionCavitationE le ct ric al d is ch ar ge d am a gePo li sh ing (bu rn ishing )

ScuffingScut fmg (scoring. gal l ing, seizing, welding.smearing . initial, mode ra t e, de st ruc ti ve )

l ub ri ca tio n and a r e e lim i na te d f rom further discus-sion to ge th er w il ih s ub -c as e, H ertzia n fatigue.AI lihough cor rosion , fr :e t ti llg-cor ros ion, cavit a tion ,an d 'e lectrical discharge d am ag e a re in fl ue nc ed b yl ub ric atio n, ru ey are n ot d is cu ss ed b ec au se th es efai lure modes occurrelativelyrarel.y ingearteeth,H en ce , th e fo llo win g fa il ure modes ar e includedin th e sc op e o f th is a rtic le: H ertz ia n f atig ue , in -c lu din g p itt in g a nd m ic ro p itt il lg ; w ea r, In cl ud in ga dh es io n, a br as io n, a nd polishing; and scuffing.

H ertzia nF atig ue - P i tt ing . P i tt ing is a. com-m on fa ilure m ode fo r gear teeth. b ec au se t he ya resub jec ted to h ig h Hertzian ccntaet stresses andmany stress cycles. For example, through-has-denedgears are t y p ic a l ly des igned to withstandcontact stresses of approxim ately 100,000 p i,w l li le lb e c ont ac t s tr es s es on c a :r bu ri ze d g ea rs mayr ea ch 3 00 .0 00 p si, In addition, a given to olh o na p in ion that is revol.ving at 3600 rpmaccumulatesover 5 million stress cy cles every 2 4 ho urs.

Pitting is a f at igue phenomenon(8 ) wh ich occu rswhen a fatigue c rack in it i ates e i ther a t th e surfaceo f th e g ear to oth o r a ta sm all dep th below th e S U J -face. The c ra c k u su al ly p ro pa ga te s fora shortdistance in a d ir ec ti on r oughl y p a ra Il e] to t il e tooth

I I E P P I d II I I1$ 1M p n n d J R l l in Gt:ARTECH. a g m r c 0 1 l S 'U l t in g j " 1 n T IInAl iMn" CA , His ar/iel,uprinltd h"~ has U/QfI IhtTU's J 990 Willm'DndchM f 1 M r ia l A . lL I t 'l rdf a, . t Il L batar tic le on 1 M Fa t/ iea t a s pt cuo f I ll b ri ca li on . M r , E r ri ch tl lois a mtlllb,r of ,15ME,AGMA , and is a Rtgisl"tdPrOft3Sioool Erlgifl!' in theStau of California..""eHI"'~'"IL,.tl li 9


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Nomencl at ur e T ab leSymbolsSymbol Description UnitsBM - therm al contac t Ibf/lin i J s oF ] Ve entra ining velocity in ls

coefficient V rl ' V r2 - ro lling v elo city inls~ - sem i-w idth of H ert- in ( p in io n . g e ar )zian contac t band WNr - no rm al o perating Ib fC - co ns tant hp/gpm load

(See Table 3.) WNr - no rm al unit load Ibf/incM s pe cif ic h ea t p er Ib f i n/ [lb oF] X w - w elding fac to r

unit mass Xr - load sharing fac to rd - o pera ting p itch in a - p ressure-viscosity in2 1 1 t > 2

d ia me te r o f pinion coefficientEI'~ - m odulus o f elastic ity Ibf/in2 A - s pec ific film

( pin io n . g ea r) th icknessEr - reduced modulus of Ibf/in2 AM - h ea t c on du ctiv ity Ibf/[s oF ]

elasticity /A m - mean c o ef fic ie nt o fh . m n - m inimum film in friction

thickness p.., - a bs ol ute v is co sity ReynsL m m minimum contac t in Obslin2)

length " 1 " 2 - P oisso n's ra tion - p inion speed rpm (pinion. gear)p - transm itted power hp "41) - k inem atic viscosity cStq - o il flow rate gpm of 40CS - average surface "i n (I I (12 - transverse radius o f in

ro ug hn es s, rm s cu rv a tu re ( p in io n ,Th - bu lk tem pera tu re OF gear)Tb, test - bulk tem pera ture o f OF (1M - density Ib/in3

t es t g ea rs ( I n - no rm al rela tive inTc - c on ta ct te mp er atu re OF r ad iu s o f c ur va tu reTf - flas h tem pera tu re OF 0 - co mp osite surface lLinTf, test - maximum flash OF r ou gh nes s, n ns

te mp er atu re o f te st 0.. 02 - surface roughness. lL ingears r ms (p in io n. g ea r)

Ts - scuffing temperature OF ! / I t , - base helix angle degV - opera ting p itch line ft lmin "'I, W :z - angular veloci ty radls

velocity (p inion. gear)

surface before turning or branC hing to the surface.W hen the crac ks have gro wn. to the extenlthat theysep ara te a p iece of the surface materia l . a p it isformed. W f s ever al p it s, g r ow togelher, the resu l tinglarger pit is often referred to as a "spall." Th.ereis no e nd ura nc e l im it fo r H en zia n fu tig ue , a nd p it-ting OCcur s even at low stresses if the gears areoperated long enough. Because there is no en-durance limit,gear teeth must be designed fo r asui ta b le , f in it e l i fetime.

To extend the pining lifeof a gear et, thed es ig ner mus t k ee p Ilheoontact stress low and th ematerial! strength and lubricant specific filmthickness high, There are seve.raIgeometricvariables, such as diameter, face width, numbero f teeth , p ressu re a ngle, etc . th at m ay be opt im-ized to lower the contact stress. Materia l . alloysand heat treatment are selected to obtain hardto oth s ur fa ce s w ith h ig h s tre ng th . M a xim um p .itling resistance isobtained with. carburized gear2 0 O EA II T EC HI NO LO OY


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M e tho d s fo r P r e ve n tin g P i ttin g1 . R e d u c e c o n t a c t s t r e s s e s b y re d u c in g lo a ds o r

o p tim i z in g g e ar g e o m e t r y .2 . U s e c le an steel, p r o p e r t y heat t r e a t e d t o h igh

h a rd n e s s , p r e fe r ra b ly b y ca rbu r i z i n g .3 . U s e sm o o th t o o t h su r f a ces p r o d u c e d b y c a r e f u l

g rin d in g o r h on in g .4 . U s e an ad eq ua te am o un t o f c o o l , c l e a n , a nd d ryl u b r i can t o f a d e q u a t e v i s c o s i t y .

teeth b ec aus e th ey h av e hard surfaces, and car-bur iz ing induces !beneficial c omp re ss iv e r es id ua lstres e wh ich e ff ec tive ly lower the load stresses.T he d ra wba ck s to using them a re that l hey a re re l-atively expensive to p ro du ce a nd that they must bef in ished by grinding. The details for obta ining highlubricant pecific filin t hi ckn es s w il l b e e xp l ai ne dla te r when eJas tohyd rodynamic (EHD) lubricationis d is cu ss ed . b ut g en er al . r ec om m en da tio ns a re touse a n a deq ua te sup ply o f c oo l. c lea n. a nd dryh ib ric a nt I lh a t has ad eq ua te v isc usity a ed a highp r es _u r e- vi se o i ty c o ef fi ci en t.

P itting m ay initia te a t the surface or a t a sub-s ur fa ce d efe ct, s uc h a s a . n on me ta ll ic in cl us io n.W i th g ea r teeth, p its a re m o st 'o fte n o f the surface-initiated type b ec au se th e h ib rie an tf ilm th ie kn es sis u sua l ly Low , r es u lt in g inrelatively high metal-t o-me ta l. c o nt ac t. T he in te ra ctio n b etw e en a sp er -i ti es o f con tac t at defects, such as nicks or furro w .,c re at es s ur fa ce -in it ia te d, r at he r t ha n s ub su rf ac e-initiated cracks . For ~gh .-speed gears with _mootbsu rfac e finish es, th e film thickness is greater,a nd s ub su rf ac e-in itia te d p itt in g, r ath er t ha n. s ur -face-initiated, may p re do m in ate . In th es e c as es ,p ittin g u sua lly starts a t a sub surfa ce inc lusio n,which acts as a po int of stress concentratioe,Cleaner te el s.: u eh a s th os e p ro du ce d b y v ae uu mm eltin g, p ro lo ng tb e p ittin g l ife by r ed uc in g th enumber .o f inclu ions,C onta min atio n fro m w ater .in the lu bric ant i.believed to p rom ote pitting through hydrogenembr itl lem en t o f t he m e ta l, a nd a br as iv e p a rt ic lein th e lub rica nt c ause p itting by in de nt in g th etooth surfaces, causing stress concentrationsa nd /o r d is ru ptin g t he l ub ric an t film..A t p r es en t,the influence o f lubricant additives on pittingis unresolved,

Hettzian Fatigue - M icropitting. On relat-ively soft gear tooth surfaces. such as thoseef

t hr ou gh -h ar de ne d g ea rs , H e rt zia :n fatigue formslarge p its with dimensiens on the order ofmi ll ime te rs . W i lb sur fa c e-ha rdened gears, suc h a sea rb urized , n itrid ed , In du ctio n-ha rd ened, a ndf lame -h a rd en ed, p it tin g m ay occur on a. muchsm al le r s ca le , ty pic .a ll y o n ly lO ~m d eep . To then ak ed e ye , th e a re as w he re m ic ro pittin g has oc-curred a pp ea r f ro s te d, and "frost ing" is a popu la rtenn for m icro pitting .. J apanese researchers(9)h ave referred to the failure m ode as " 'g r ey s t ai n-i ng " b ec au se : th e L ig ht-s ca tte rin g p ro per tie s o fm ic ro pirting g iv es th e g ear teeth a grey appear-a nc e . . U n der th e sc an nillg elec tro n m ic ro sco pe(SSM) immediately e vid en t is t ha t m ic ro p it tin gp ro cee ds b y th e same fatigue p ro ce ss a s c la ss ic alp inia g, exc ep t th e p its a re ex trem ely sm all.

Many times micropitt ing is not dest ruc tive to th egear to oth surface. It so metim es oceurs o nly inp a tc he s a nd m a y s to p a ft er th e t nOO lo gic al c o nd i-tio ns h av e im p ro v ed b y r un nin g- in ..T h m ic ro p it sm ay ac tually be rem oved b y l ig ht p ol is hin g w ea rduring running-in, inwh icb c a se t he m ic r op it tin gis said to "Ilea l." H ow ev er.jh ere h av e b een ex-amples(9-11) where m ic re pin in g h as e sc al at ed in-to fu ll -s ca le p i tt ing , leading to th e destru ctio n o fth e g ea r teeth,

T he sp ec ific film th ic kness is th e m os t im po r-tant p ara meter th at influ en ces m i.c ro pitting .D a ma ge s ee ms to o cc ur m os t re ad il y . 0 1 ' 1 gear teethw ith . r ou gh s urfa ce s, e sp ec ia ll y when they arel ub ric ate d w ith l ow v is co sity l.u.briCaJ1ts. Gearsf in is be d w ith s pec ia l g rin din g w he els to a mir-ro r-l ik e fin is h(t2 ) h av e e ff ec tiv el y e limi na te dmicropining. Slow -speed gears are prone tomicropi tt ing because t lt eir tiJ rn t hic kn es s is l ow ,

M e th od s fo r P re v e ntin g M ic ro p ittin g1 , U s e s m oo th t o o t h s u rfa c e s p ro d u c e d b y ca r e f u l

g r i n d i n g o r h o n i n g .2 . U s e a n a de qu ate am o u nt o f c o o l , c l e a n , and d ry

l u b r i can t o f th e h i ghes t v i s c o s i t y p e rm i s s i b l e .3. U s e h ig h s pe ed s ~ p o s s i b l e .4 . U s e c arb u riz e d s te e l w i t h p r o p e r c a m o n c o n t e n t

in th e s u rfa c e la ye r s.

To preven t m i c rop i tt ing , the specific film thicImesssh ou ld be m ax im ized by u sin g sm oo th gea r tooths ur fa ce s. h ig h-v is co sit y [ ub ric an ts , and highspeeds. Experiments(lO) h av e s ho wn th attl am e-


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, 2 2 QEA~ TECHN,OLOQY

h ar den ed a nd l nd uetio n- ha rd en ed g ea rs h av e l es s Wear i s considered excessive when t he t oo th pro-resistance to micropitting 'than c arb ur ize dg ea rs o f rues wear to the ,extent that higb dyna r o i c loadsth e same hardness, T his is probably due to the occur or the tooth thickness isreduced to th e e x-lower carbon content of the surface Layers of the tent that bending fatigue bec omes po ss ib le .flame-ha rdened and in duc tio n -h a rd en ed g ea rs .

W ear - Adhesion. Adhesive wear i s c la s si fi edas "mild" ifit L confined to th e o xide la yers o f th eg ea r to o th surfaces, If,h ow ev er, th e oxide layersar e disrupted, a nd b ar e m eta l! is e xp os ed . t he t ra ns -ition to severe ad hes iv e w ea r \1 WIlly occurs.S ev er e a dh es iv e w ea r is te rm ed s cu ff in g a nd w illbe disc ussed later, H ere w e assum e that scuffinghas been avoided t hr ou gh p ro p er design of th eg ea rs , s el ec tio n o f th e l ub ric an t, a nd control o f th er unn ing -in p r oc e ss .

When ne w gear units ar e first o pera te d, th e COD-tact between the gear t ee th is not optimum becauseo f u na vo id ab le manu fa cl iU li Il g i na cc ur ac ie s. If th etribological conditions a re f av or ab le , mild ad -hesive wear o cc urs d urin g ru nn in g- in a nd u su al lysubsides with tim e. resulting in a . satisfactoryl if et ime f o r the g ea rs. T he w ear th at o cc urs, d ur-rng nmning-in i s benef ic ia l if it sm oo ths the to othsu rf aces , i nc r ea s ing th e speei fi c fi lm th ickne ss , an ditf it increases the area of contac t by rem ovingmin or imp er fs ct io ns th roUgh l oc al wear, TD ensure' l it a t the wear ra te re ma in s u nd er c on tr ol , run innew gea rs et s by operating fo r at least t he f ir st tenho urs a t one-half l o ad . .T he am ount of w ear co nsidered to lerable de-pends on th e expe ct ed l if et ime fo r the gear andrequirements f or c on tr ol of noise an d vibration.

M e tho ds fo r P r ev en tin g A d he s iv e W e ar1. U s e s m o o th to o th s u rfa ce s .2 . I f p o s s i b l e , ru n in n ew g e a r s e t s b y o p e ra tin g th e

f i rs t te n ho urs a t o n e - h a I f l o a d .3. U se h ig h s p e e d s if p o s s ib le . O the rw is e ,re c o g n iz e th at h ig h ly lo a d e d , s I o w - s p e e d g ea r s

a re b o u n d a r y lu bric ate d a nd a re e sp ec ia llyp ro n e to e xc e ss iv e w e ar. F o r th es e c o nd n io n s,s p e c i f y n i t r i d e d g e a r s and th e h i ghes t permiss~b le l u br ic a n t v i s c o s i t y .

4 . F o r v e ry s lo w - sp e e d g e ars 1 0 f p m ) , a vo id u s-in g lu bric an ts w ith s u lp hu r- p ho sp ho ro usa d d n i v e s .

5 . U s e a n a de qu ate a m o un t o f c o o l , c l e a n , and d rylu br ic a nt o f the h ig h e st v is c o s ity p e r m is s ib le .

Many g ea rs , because o f p r ac ti c al limits on lubri-c an t v is co sity " speed, a nd tem pe ra tu re , musto pera te u nd er b ou nd ary -lub ric ated c on ditio nswhere som e w ear is inevitable .. H ighly loaded.slow' peed 10 0 fpm) , b ou nd ary -l ub ric ated g ea rsa re e sp e ci al ly p r on e to e xc e ss iv e we ar . Tests withslo w-speed!gears(!O j have haw D that. nitridedg ea rs h av e g oo d wear r es is ta n ce , wh il e c a rbu ri zedan d through-hardened gears have similar, butlow er w ear resistance, R eference 10 concludedt ha t l ub ric an t v is co sit y h as t he g re at es t in fl ue nc eon slow-speed, adhesive wear. and that higb -v is co sity Iu br ic an ts r ed uc e th e wear r a te s ign if i-candy. Th e same authors fo und tha t su lp hur-p ho p ho ruu additives ca n be det rimen ta l ! w i thslow-speed t o fpm ) gears, giving very highwea r ra te s.

A fe w g ea r u nits o pe ra te u nd er ideal conditionswith smo oth to oth s ur fa ce s, h ig h p itc h l in e s pee d,a nd high lubrica nt film thick ness. For example,t ur bin e g ea rs th at o p er ate d a lm o st c o ntin uo us ly a t30,000 fpm pitch line speed stillhad t he o r ig i na lm achining m arks o n their teeth, even after o p-e ra :~ g fo r .2 0 ye ars .. Mos t g ea rs , h ow ev er. o pe ra tebetween t he b ou nd ar y and f ul l- fi lm lub ri ca ti onregim es, und er elasto hyd ro dyna mic (E HD } COD-ditions, In th e EHD regime, with t he p r op e r typean d v isc osity o f lub ric ant,th e w ear ra te usuallyr educes dur ing running-in a nd a dhesive w ear v ir-t ua ll y c ea s es once runn lng -i a is completed. Ifth elubricant is properly maintained (cool , c le an , a nddry), t he g ea rs et should no t suffer an adhesivewea r f ail ure .

W ear - Abrasion. Abrasive wear on gearteeth is u su al ly c au se d by contam ination of thelu bric an t by ha rd , sha rp -ed ged p articles .. C on-tamin atio ne nte rs g ea rb ox es b y being built-in, in -Ilernally gener at ed , inges te d through breathers ands ea ls , o r ia ad ve nen tl y a dd ed d ur in g maintenance.

Many gear manufaclUrers. do not fully appreciateth e s ig n if ic a nc e o f c le a n a s semb ly ; it. isllOt unoom-mo ll . t o f in d s an d, ma ch in in g ,chips, g rind ing dus t,w eld sp la tter o r other debris in new gear bo xes.To r em o ve bwilHn con tamina t ion . d ra in a ndf lu s hth e gearbox l u br ic a nt . b ef o re s ta rt -u p a nd a gainafter th e firs t 5 0 h ou rs o f o pe ra tio n, refill with th er ecommended lub ri cant , and install a ne w oi l filter,


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M e th od s fo r P r ev en tin gA b ra s iv e W e a r

1 . R e m o v e b u ilt- in c on tam in atio n fro m n ew g e ar-bo xe s by d ra in in g an d flu sh in g the lu b r ic a n tbe f o r e s t a r t - u p a nd a ga in a fte r t h e f irst 5 0 ho u r so f o pe ra tio n . R e fi ll w ith th e re co m m en de d lu bri-c a n t a n d in sta ll a n ew fi lte r.

2. M i n i m i z e i n t e m a . l l y g en era te d w e ar debris by u s-in g s u rfa c e -ha rd en ed g ea r te e th , s m oo th to oths u rfa c es , a nd h ig h- vis co s ity lu br ic a nts .

3 . M i n im iz e in ge ste d c on tam in atio n by m a in ta in -in g o il- tig h t s e als an d u sin g fitte re d b re ath e rv en ts lo ca te d in c le an , n on -p re ss ur iz ed a re as .

4. M in im iz e c o n tam in a tio n tha t is a d d e d d u r i n gm ain te n a n c e b y u s in g g o o d h o u s e k e e p i n gp r o c e d u r e s .

5 . F o r c irc u la tin g o il s ys te m s, u se fin e filtra tio n .6 . F o r o i l ba th s ys te m s, c han g e the lu b r ic a n t a tle a s t e ve ry 2 5 0 0 ho u rs o r e v e ry s ix m o n ths .7 . M o n ito r the lu bric an t w ith s p e ctro grap hic a n d

fe rro grap hic a na lys is to ge th e r w ith a na lys is o fa c id n u m be r, v is c os ity , a n d w ate r c on te nt.

Internally g en era te d p artic le s a re u su al ly w ea rdebris from gears o r bearings du e to Hertzianfa t ig l . le pi t ting or adhesive an d abrasive wear.T he w ea r p an ic le s a re es pe cia lly a bras ive be-cause they becom e w ork -hardened when theya re tra pp ed b etw een me gearteeth, Internalilyg en er ate d w ea r d eb ris c al libe minimized by usinga cc ur at e, s ur fa ce -h ar de ne d g ea :r t ee th . ( with h ig hpitting resis tance), sm ooth surfaces, and highv is cosi ty lubr ic an ts .

Breather vents ar e used on gearboxes to v en t in -t er na l p re ss ur e, w h ic h may OCCUIi ' when air entersthrough seals, ot when air w il hJn d Ie g ea rb ox e x-pands ( o r c ont ra c ts ) d u rin g th e nonn a ll he a ti ng and,c oo lin g o f th e g ea r unit. T h - b rea ther vent sho uldbe located in a clean, non-pressurized area an ds ho ul d h av e a filte r to p re ve nt in gre ss o f a irb orn ec o nt am in an ts . I n e sp ec ia ll y h ar sh e nv is onm ea ts ,the gea rb ox. c an be complet.ely s ea le d, a nd th epressure v ar ia ti on c an be accommodated by a n ex -pansio n cham ber w ith a flexible diaphragm.

All. m aintenance p rocedures w hich involveopening any part o f the gearbox or lubricationsystem must be ca refu lly p erfo rm ed to preventc on tam in at io n o f th e g ea rb ox s ys tem.

A bra sive w ea r du e to fo reigll co nta min ants,suc h a s sa nd or in te rn al ly g en er at ed w ea r d eb ris ,

called three-body abrasio n, is a co mm on o ccur-rence. Two-body a bra sio n a ls o o cc ur s w he n h ardp artic le s o r a sp eritie s o n onegear tooth a br ad e th eopposing to oth s urf ac e ..Unless the tooth surfaceso f a s ur fa ce -h ar de ne d g ea r a re sm oo th ly f in is he d,m ey will act like files if the mating gear is ap-preciably softer. This isthe reason that a wormpinion is p ol is he d a fte r g rin din g b efo re it is nmwith a b ron ze wo rm whee]. Manu fa ctm e rs o f c om-p ute r d is k: d riv es h av e fo un d that stainless steelpLllion~mated wi th anod ized a lumin um ra ck s h Jl veex cessiv ely hig h w ea r ra tes. T he a no d.ized layero f th e a lum in um . ra ck is ex lrem e]y lhin a nd brit-t le . a nd it breaks up and impregnates t be r e la t ive lysoft stainless steel p inio n, T he a lum inu m oxidep a rt ic l es t he n act. like e me ry p ap er and wear th eteeth o f the rack very quickly .

The lubr ica t ion sys tem should be ca re fu l ly main-tained. and m onitored. to ensure that th e gearsreceive a n a deq 1l8 te amo unt o f c oo l , c lea n .an d dryl ub ric a nt . F o r c ir cu la tin g -o il s ys tem s, fine filtra-t ion r emoves con tamina t ion . F il t er s as fine as 3 , I U Dh ave s ig n if ic a at ly i nc r ea s ed g e ar l if e. Fo r oil-bathgearboxes, th e lubricant sh ou ld b e c han ged fre-q uently to rem ove c onta mina tio n. U nder no rm aloperating conditions th e l ub ric an t should bec ha ng ed a t l ea st every 2500 operating ho urso r sixmonths, whichever occurs f ir st. F o r c ritic al g ea r-b ox es a re gu la r p ro gr am o f lu br ic an t m on ito rin gcan . he lp p re ven t g ea r f ail ur es by sho win g w henmaintenance is r eq uir ed . T h e l ub ric an t m o nit or -in g s ho ul d i nc lu de s pe ct ro gr ap hi c a nd fenographica na ly sis o f c ont am i na ti on , a lo ng w ith a na lys is o fa cid n um be r, v is co sity , a nd w ate r c on te nt.

P o lis hin g w ea r. If th e extreme pressure (EP)a dd itiv es in th e l ub ric an t a re to o' c he mic a1 ty re -aedve, th ey m ay c au se p ol is hin g o f th e g ea r to oths ur fa ces u ntil th ey attain a bri,ght.mirror finish.A llh .o ugh the p olished g ea r teeth m ay lo ok good,p o lis hin g w ea r is u nd es ir ab le b ec au se it generallyre du ces g ea r a cc ur ac y b y w ea rin g the to oth p ro -files away from their ideal fo rm . E P a dd itivesused in . lubricants to p revent scuffing, such assulfe r and phosphorous , will be covered whens cu ffin g is d is cu ss ed . T he y fu nc tio n by formingiron s ul fid e a nd ir on phosphate films on areas ofme gear te eth w he re h ig h te mp era tu re s o cc ur,I d e a i l 1 y . , th e a dd itiv es should react only atte mp era tu res w he re th ere is a d an ge r o f w el din g,! fl he r at e o f r ea ct io n is too high, an d th ere is a OOR-t in uo us r emova l o f th e surface f i lms c aused by


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:2 4 I lE A R T EC H N OL OG Y

s cu ff in g d ur in g th e c rit ic al r un nin g-in p er io d.The basic mechanism of scuffing is no t

u nde rs to od c le a rl y, but by genera l ag reemen t, itis believed to be caused by i nt en s e f ri ct io n alheating generated by th e c om bin atio n o f highs liding velocity and intense surface p ressure,Blok'S


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high m eltin g p oin ts, allo win g th em to rem ain asso lid s o n th e g ea r tooth surfaces even at h ig h c on -tact temperatures. The rate 'o f r ea ctio n o f th e anti-scuff a dd il iv es i s greatest wher e th e g ea r to oth c on -tact te mp era tu re s a re h ig he st. B ec au se o f th e r ub -b in g a c ti on of th e g ea r te eth . th e s urf ac e f il ms a rerep ea tedly sc ra ped o ff an d refo rm ed .. In effect.s cu ff in g is p re ve nte d by substituting mild corro-sion ini ts p lace . Oocas ionaUy, ant i- scuffaddl tives,s uc h a s s ul fe r, a re to o c he mic al ly a ctiv e, c au sin gp olishing w ear and nec essita ting a cha nge to lessagg re s si ve add it ive s. Lub ri can ts with anti-scuff ad-d itiv es o f p ota ss ium b ora te do not 'C8:1ilSe polishingw ea r b ec au se they d ep o sit g la ss -l ik e b ou nd ar yf ilm s w ith ou t r ea ct in g with. t he me ta l: .For mineral oils w it hou t a n ti -s c uf f additives,

th e c ritic al s ou ffin gte mp era tu re in cr ea se s withincreasing visco sity , and ranges from 150C to300 C . T he inc rea sed sc u~ mg resista nc e o f high.viscosi ty lubricants i s be li eved to be due to dif-ferences inchemical composit ion r at he r th an in-c re as es i n v is c os it y. However , a v is c os it y i nc re as ea lso help s to reduce dle risk o f s cu ffin g b y in-

Fig. 1

PITCHPO INT

creasing t he lub ri can t film t hi ckn es s a nd r edu ci ngth e c ont ac t t empe ra tu re generated by metal-to-metal . contact.

Scuffing is control led by th e total contac ttemperature Tc, wh ich c ons is ts of th e sum of thegear bulk temperature Tb, and the flashtemperature Tf; i.e.,

Tc = Tb + TfThe bulk tem perature is the equilibrium

temp er atu re o f t he su rf ace of m e gea r t ee th beforethey e nt er lh e me sh ing z on e. T he f lash temperatureis th e lo ca l a nd instantaneous temperature r is e t ha to cc ur s o n dlegear teeth due to t he f ric tional heatingas th ey p ass through. th e meshing zone .

Anything th at re du ces e ith er th e b ulk tem p era -ture ortheflash temperature willreducethe totalc on ta ct temp era tu re a nd le ss en th e r is k o f s cu ff in g.Higher viscos ity lubr icants or sm oo th er to o th s ur -f ac es h el p by i ne re as in gi he s p ec if ic f ilm t hi ckn es s.which in rum reduces the frictional heat and,therrefore. th e fla sh te mp er atu re . A ls o, th e lubri-

L IN E O FACT ION

IIAIICII/A~IIL 1n1 2 5


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cant performs th e important f un c ti on o f r emov ingh ea t fro m th e g ea r teeth. It must be supplied to t hegear teeth in . such a. way that it removes heatrapidly an d k eep s a lo w b ulk . temperature. A . heatexchanger ca n be u se d w ith a circulating oi l systemto c oo l the lub rica nt before it is sp rayed at theg ears. T he gea r design er can. m axim ize scu ffin gresistance by optimizing the gear geometry so matt he gea r teelh are as sm aD as po ssible, consistent.with bending s t reng th requ ir emen t s, to reduce th etemperature rise caused by sliding, Fig, 1 showsthai t he r oUi ng vel o ci tie s, Vrl and Vr2, l inearlyincrease from zero at th e ia te rf ere ac e p oin ts . to am axim um at each en d of the path o f contact. Thesliding veloci ty is represented by t he d is tance be-tw een the Vrl and Vr2 lines. The am ount ofsliding is p ropo rt iona l t o the distance f rom t he p it chp oin t, P , and is zero w hen the g ea r teeth co nta cta tth e p itc h p oin t, a nd largest at th e e :n ds o f t hepath. A ddendem m odiflcation can be used tobalance and m in im iz el he t emp er at ur e rise that oc -c ur s in th e a dd en d'um an d dedendum of the gearteeth. The tem perature rise m ay a lso b e r ed uc edby m odify ing th e to oth p ro files, w ith slight t1 pand/o r ro ot relief to ease th e load at th e start an dend of the engagem ent path where the slid ingvelocit ies are the g re a te st . A I so ', lh e gear teeth mustbe accura te and held rigidly ingood alignm ent tominimize to oth ~ oa din g and, therefo re, th etemperature rise .

G ear m aterials sho uld be ch osen w ith their seuf-fing resistance inmind. Nitrided s te el s, s uc h .8 SNi traUoy 135M, .a re genera ll y found to have thehighest r es is ta n ce t o scuffing, whil e s ta in le ss s te el sare liable to scuffevenunder near-zero loads, Thethin o xid e l ay er on stainless steel. is hard an d brit-tle, and it breaks up ,easily under sliding lo ads, ex-po in gd Ie b are m eta l, m u promot ing scuff ing .Anodized aluminum has a lo w s cu ff in g r es is ta n ces im il ar to s ta in le ss s te el , H ardnes does not seemto be a r el ia b le i nd ic a ti on o f s c uf fin g resistance.Acknowledgemen t : RepriJJled by perm ission of S ociety ofT ri b ol og is ts a n d L u br ic at io n ' E n g in t! t! rs .Refereooes:L ANSI/AGMA ] 10.'04. "Nomenclature of GearT oo th F ailu re M od es ,ft ]9 80 .2, SHIPLEY, E,E. "Gea r Fa i lu re s, " Mach i n e Design ,D ec. 7 , 1967, P I', 1 52 -1 62 ,3. DUDLEY , D .W . "Gear W ear," We ar C on tr olHandbook. ASME,4. KU, P .M . "G ear F ailure M odes - Im po rtanc e ofLubrication and Mechanics," ASLE Trans., Vol.19 . N o.3, 19 75 ,l 'p . 239 -24 9.5, W U LP I, 0'.1. "HDW CDmponent s Fail."

6. - - - -"F.ailure A nalysis and Prevention,"( F ai lu r es o f Gears), Me t al s H a n d b oo k , Vol . W . 8 tbed. p p . 5 07 -5 24 .7. GOD FREY , D , "Reoognitionand So l ut io n o f Some

C om mo n W ear P ro blem s R elated to L ubricatio nand Hydraul ic Fluids," Lubr ic at io n E n g in e er in g ,F eb ., 1 98 7, pp , ]H 14.8. UTTMAN, W. E , " Th e M e ch an is m ofContaet

Fatigue," inJerrJisciplinary A p pr oa ch to lh e [d)rica-(ion of Conc en tr at ed COTrl ac ts , NASA SP-237.1 97 0, p p , 3 09 -3 77 .

9, UENO, T, et al, "Surface Durability of ease-Carburized Gears - O n a P henom eno n D f'iG reyStaining of Too th Surface," A SM E Pap , No ,80-C2fDET-27, 1980, pp , 1-1,

10. WINTER, H. and WEISS, T. " Some F ac to rs In-t lu enc ing th c P i tt in g . Mic ro p it ti ng (F ro s te d A rea s )and Slow Speed WearofSurface~Hardened Gears,"A SM EP ap . N O'. 80!.QfDET,89, 1980.,1'1',1-7.

11, SHIPLEY, B,B. uFai l~ r e Analys is o f Coa rs e -P i tch ,Hardened, and G round Gears," A GM A Pap . N o,P229,26,. 1982, P I'. 1 -2 -4 ,]2. TANAKA, S, e t a I. "Apprec iable I nc re as es i n Sur-face Durability of Gear Pairs with. Mirror-LikeFinish," A SM E P a,p er N o. 8 4-D ET -22 3, 1984,pp. 1-8.B. ADMAS , J.H. &GODFREY , D."Bora te GearL ub ric an t-E P F il m A na ly sis and Performance,"

LubricaJion Eng inee r , Vol. 37 , ND . I, Ja n, 19 81 ,pp, 16-2].14 , B LOK , H . "Les Tem peratures de Surface dansle s C o nd itio ns d e G ra is sa ge SOlIS Pression Ex -treme," Second Wo r ld P et ro le um Cong re ss , P .a ri s,J un e, 1 93 7.15 . B LO K, H . " Th e P osrulate A bo ut ihe Constancy ofScoring Temperature ." . In terdiscipl inary Approoch

to the UdJ ri ca t ion o fCof lCtmJ roJ ed Contacts. NASASP -2 37 , 1 91 70 ,p p . 1 53 -2 48 .1 6. O OW S ON , D, ME la st oo y dr o dy nam ic s," P ap e rNo. 10, P ro c. I nst. M e ch , E n grs., VoL 182, PT3A, 1 96 7, p p . 151-167,17. AGMA 200] -B88 . "Fund amen ta l Ra ti ng F a ct or sand C alculatio n M etho ds for Invo lute Sp ur andHelical Gear Teeth." .1988.18 . AKAZAWA, M ., TEJIM A, T. &NARITA, T."Ful l Salle T es t o f H jg h S pe ed , H ig h P ow er ed GearUnit - Helical Gears of 25,000 PSat 200 mIPLV," A SM E Paper No, 8O '-C2 /DET-4, 1 9 80 ,1.9. DRAGO, R .I. ' ICDmparative L oa d C ap ac ity

Evaluali .onofCBN-Finished G ea rs" A GM A P ap erNo. 88 FfM 8, Oct, 1988.2 0, A G MA 2 50 ,0 4, " AO M A S Ia J:l da rd Sp ec if ic atio n -

Lubrication .o f mndus tr ia l .Enc lo s ed Gear Drives."Sept., 1.98l.21 . A GM A 42 ] .0 6. "P ra ctic e fo r High-Speed Hclica l

and H er rin gb on e G ea r U r n e s , " Jan" 1.969,2 2. W E LL AU ER . ar.&H OLLO W AY , G .A . "A p-p lic atio n o f E H D O il Film Theory to lndllsttia.l GearDrives,~ Trw1S .ASME , J. Eng . Ind, . WI !. 9 8, S er ie sa, N o .2 , M a y.,. 1 97 6, P I', 6 26 -6 34 .23. D RAGO, R .I, Fundamentals .o f G ea r D e sl'g n,

Butterworth, 1988.24, AK1N , L, &T OW N SE ND , D . "S tudy of Lubri.-cant let F low P ben m ena in ,Spur Gears," NASATMX-7J572. Oet., 1974,25 , SCORlNG+ . .c om pu te r P ro gra m, G EA R TE CH

Sof tw a re , In c. 1985-1989~

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Gear Lubrication 1