White ltching AreaCase-Hardened Cea

AUTHORS:

PROFESSOR DR. -ING. HANS WLNTERis the head of t~leLaboratory of Gear Researchand Gear Design at the Technical U,liversity ofMunich. He has held positioP1S in tile Gemra'lgear industry since 1956 working for Zahnrad-fabrik Fn'edn'chshafen and DEMAG Duisberg inInarlufacturing. development. desigrt andresearch. He received his doctorate from theTechm'cal Universi.ty of Municl1 and studied asan associate of Prof. Dt.-lng h.e. GustavNiemann'.

DR. -lNG. GERHARD KNAUER isa grad-uate engineer of FZG and is an engineer atZa}ml'ad!abrik Friedrichshajen.

MR. JOHN J. GAMEL is a senior designengineer with SO/Dr Turbines. Inc.. San Diego,CA. Prior to coming there. he worked at theNaval Ship Systems Engi11eerillg Station inPhiladelphia. PA. He smdied at FlG during1986.

Abstract:The phenomenon of white layers, which

arises from high stress, can be observed undera microscope after the white layers have beentreated with a weak nitric acid solution. Theiroccurrences in zones of high shear stress canprovide qualitatively valuable indications ofthe size and direction of the stress, and they canpoint out possible starting points for flankdamage. An investigation of this phenomenonis described.

IntroductionIt is known that on the surface and just

under the surface of rolling elements ofhardened steels, so called ~white layers" canbe observed, This phenomerton.also called

Professor Dr.-Ing. Hans Winter,Dr .-Ing ..Gerhard Knauer

Technical University of MunichJohn J. Gamel

Solar Turbines, Inc., San Diego, CA

"white etching areas (\NEA)," arises fromhigh stress. These white etching areas arestructured zones which are affected whenetched with diluted nitric acid and whichthen forma light contrast with the matrixwhen observed with a microscope.

In References 1, 2, 6, 7, 10, 12 and 13,the different forms of appearances havebeen described and their appearances inconnection with the actual stress have beenanalyzed. In the same way, the structure ofthese structured zones can be regarded asknown to a large degree. The conditionsfor the appearance of WEA on the surfaceare high friction and/or impact stress;whereas, WEA were observed beneath thefunctioning surface only inelements whi.chwere at the same time exposed to stronghydrostatic pressu re and a shear stress. r La)

In 'the cited reference, the origin of WEAbeneath the surface resulted because thepresence of a mechanical thermodynamicstate induced such high tensions, that in theareas, the shear resistance 'Of the material,had been exceeded. The flanks of the crackwhich run in the direction of the greatestshear stress are heated to the meltingtemperature in an extremely short timebecause of the plastic deformation of thestructure. In this process the carbidesdissolve and the carbon enters the gamma-matrix. After self-cooling, because of thesurrounding structures with considerablylower termperamres, we get a highly over-saturated martensite.

Untll new the investigations of WEAphenomena have been carried out largelywith rolling element bearings ..It is knownthat rolli element bearings are exposed tosubstantially higher Hertzian pressure thangears under similar conditions. This isprobably why the 'Occurrence of whiteetching areas on gears has 5.0 far beennoticed in 'On)y a few cases, and it is still notclear whether WEA can be regarded acause of flank damage. This article willdescribe, further some fonns of occurrencesof \IVEA. on the highly stressed flanks ofcase-hard ned gears. In addition, we willdiscuss w ether there is a connection withfatigue fa ilu

This research is based on a number ofoperating tests with case-hardened gearsforthe investigations of pit t ing resisrance.which h ve also been subsequently ana-lyzed through metallographic investiga-tions". It is, therefore, not the purpose ofthis article to analyze the microstructureand triggering mechanism of WEA (Werefer here rather to the abundance of ex-isting literature), but we will investigate theoccurrence of \IVEA. in connection with thestress .of the tooth flanks and the conse-quences of it.

•All operating tests and metaliOlfilphic investiga-tions were performed at the Gear ResearchLaboratory (FZd of the Technicat University ofMunich. West' Germany

White Etching Areas in theArea of the Tooth Flank.

Material Fatigue on Non-metallic.Inclusions Beneath the Surface

The sliding-rolling motion under highHertzian stress in the contact of two matingtooth flanks leads to a three-axis-pressure-stress state, which is superimposed by ashear stress resulting from the friction load.The main shear stress, which can beregarded as the cause of the .crack(6.111has,with ideal geometrical bodies, a path to thedepth, as shown in Fig. 1.

According to investigations of Refer-ences 4 and 5, micro-Hertaian fields of ten-sion are formed on machined surfaces dueto previous damage as well as scratchesand roughness. These fields of tension leadto paths of tension that are different fromthose that should be taken in an idealgeometrical body. In Fig. 1, the path of themain shear tension fora. rectangular notchwith a depth of 0.1 ~ and a length of 0.2~ has been added. According to this, themain shear stress directly beneath the sur-face is considerably greater than the max-imum, which follows the currenthypothesis about the endurance. Whenreaching a depth of 0.5 bH the influence ofthe surface roughness has decreased. Afterthat depth, the path of the tension followsthe pattern of the Hertzian pressuredistribution on the ideally smooth surface.In this area Ithemain shear stress is directedagainst the surface just under 450

• Inaddi-tion to this, in the area of non-metallic in-clusions peaks of tension occur, as pointedout in Fig. 2, which can lead to localized 'ex-ceeding of the allowable shear. (SeeReferences 3 and 4\. )

The calculation derived from Fig.. 2 isconfirmed by Fig. 3. An oxidation inclu-sion, whose E-modulus is about two timesas large as for steel, lies in the area of the in-ner point of single contact, about 0.14 mmbelow the surface of the flank in the area ofhigh main shear stress. (See Fig. 1.) In anundisturbed structure, the main shear ten-sion did not reach a value which would becritical for a case-hardened steel. However,the approximate 2.5-fold increase in ten-sion due to the inclusion (compare Fig. 2)led to the occurrence of cracks and WEA,which, in accordance with the path of mainshear stress, are approximately 450 withrespect to the surface, The gear data andthe operating conditions for this experi-ment are listed in.Table 1.

In different publications. for example,References 6 and 10, an increase of VVEAinconnection with a greater number ofrevolutions has been noticed. This leads tothe conclusion that also with non-metallicinclusions. WEA do not appearimmediately after the first stress cycle.They only occur after a certain period ofstress due to plastic deformation whichcauses change in the state of the residualstress. Possibly the crack expands veryquickly once a critical state of deformationis reached, and the crack travels in its en-vironment to temperature regions whichare above melting temperature. (SeeReference 10,) The high impact stress thatoccurs with roller element bearings canap-parently also be reached locally with gears.In Reference 6, it is noted that a minimumimpact stress of T45 = 725 n/mm2(105,2001b/in2), at the inner ring of aroller element bearing, leads to the occur-rence of WEA. Also, with case-hardenedgears, the main impact stress of this dimen-sion can occur in the area of inclusions.

The cracks which occur on non-metallicinclusions below the surface of case-hardened gear flanks generally extend verylittle. So far, it has not been provenwhether they extend to the surface andcause damage at the flank.

The strictly local limitation of WEA tonon-metallic inclusions and the connectionwith the relation (E inclusion I I:: steel)become apparent in the example of sulfurcontent in case-hardened gears made of(continued on page 22)

Table 1Gear Data and Operating Conditions

Module (Diametral Pitch)m = Smm (S.08Iin.)Number of teeth (1 = Pinion/

2 = gear)zl/zz = 17118Face widthb = 16mm (0.63 in.)Center distancea = 91.Smm (3.60 in.)TorqueM = 360Nm (265 ft-lb)K - factorK = U.l Nzmrrr' (16S0psi)Hertzian Pressure at rolling point CPc = 1440 N/mmRevolutions of the driving pinionn= 3000 min-1

NEe:-,

:z..5 600..'"~;; 400...c.."'"...." 200CI.~

1·1000

800

o0.1 0.2 0,3 o.~ A,S

Distance from surfgCI in IMI •

Fig. 1- The path of the main shear stress into thdepth with over laying of the Hertzian pressure andfriction stress on a tooth flank. (See Table 1.)

o

4

1~ 2.,:I:

\:;."Ii~

o 0,5 1,0 1,5 2,0

EllncLu5ionl/EtsteellF;ig, 2- The increase in tension in the surroundingso( non-metallic inclusions in the process of over-running bearing steel 100 Cr 6. Reference 3: FLndingsfrom optical tension. model tests in simulating thestress dlstribution on a con lac! line of a ball bearinginner ring 6309 for Fr - 19200 N radial load.

Fiig. 3 - WEA starting from an oxide inclusionbeneath the flank surface of a case-hardened gear of16 Mn CrS.

September/October 1989 19

INow....instead 'Of deciding which gear geometry to use,the Gear Manufactur,er may select the Klingelnberg'W800CNC {Wiener System) Spiral Bevel Gear grinder whichprovides precise tolerances and surface finishes for anygeometry.

With ,leading-edge software technO'logy, the W800CNCbecomes the industry's universal 'gear grinder producinglSpiral Bevel Gears up to 31.5" OD (800mm). Perfect forAircraft, Military and other applications requir,ing the ulitimateprecision, the W800CINC (Wiener Syst,em) is designed and'built for cormncous operation - with shari: or 'long runs,This high degme of versatility, short set-up time and theability to pmduce"quiet" gears with gr,eater loadbearingcharacteristlcs makes the WBOOCNC a must for aU manufac-turers of precision Spiral Bev,e'lGears.

This exciting!, new innovation will be shown at maj,orexpositions,. wor'ldwide. For further details or a copy of ourbrochure, contact: Klingelnberg Corporation

115200Foltz Industrial Par,kwayStr,ongsville, OH 44136Or phone (.216) 57.2-210Q..IFAX (.216) 572-09'85.oKLI~GELNBERG

- ....Puts" all toge"her.

,II

Th,e CNiC con'tr,o,lled'g,rilnd,e'r for aliiSpiirall Bevel Ge,ar'geometri:es.,

See us at: EMO (Hannover), Halil #3Booth #005AGMA Gear Expo '.89 (Pittsburgh)Booth #101CH~CLE A~15 ON IREADERIREfLY CARD

WHITE ETCIDNG AREAS ... (continued from pag,e 19)

16MnCrS (See Table Z), with globularlyformed sulfide inclusions in which a greatnumber of very small WEA can be noticedin the area of high impact stress. TheseWEA result exclusively from inclusions;whereas, such occurrences cannot befound with long strips of manganesesulfide. Globular sulfides are harder thanmanganese sulfides due to combinationwith the sulfide influencing elements. Thecracks that have been examined are neverlonger than some I'm and never have con-nection to the surface. (See Fig, 4.) The in-dusions themselves are spread out veryfinely,

The Stress of the Gear Flankat tile Begjnn.ing of Contact

It is a fact that WEA frequently occur insurface areas which are exposed to strongshock stress, In highly stressed gear pairs,

Table 2Material and Heat Treatment

~ig, 4 - White etching areas at globular form sulfideinclusions beneath the flank surface.

22 Gear Technology

the gear teeth that are in contact aredeformed, so that the followLng gear toothtip strikes against the flank of the drivingpinion in the area of the gear tooth root tooearly. (See Reference 8.) This leads to anLmpact shock. Shown in Fig. 5 is the toothstretching signal of the driving pinionunder the conditions described inTable 1,which shows a very steep increase at thebeginning of contact which points out 3.

very high shock-type stressat the begin-ning of contact,

Metallograph:icalexamina.tions showthat the greatest number of and the mostprominent WEA could be found in thisarea. (See Fig. 6.)

Fig, 7 shows WEA at the surface whichhave occurred due to high stress caused byth impact shock, Their extension in depthis between 30 and 5QJlm, Beneath the SUT-

face, the WEA are limited by a crack whichruns almost parallel to the surface, Due tothe contact shock, there occurs localJy highpressureas well as strong friction forces,because of which the flank surface is ex-posed to an extreme shear stress in thisarea, Shown in Fig, 8 is the path and thedirection of Ithemain shear stress relative tothe local pressure on the flank for two fric-tion values as a function of depth from thesurface.

Only with increasing depth does thedirection of the main shear stress comecloser to an angle of 45° relative to the sur-

---.20pm

1

S'S c·c· DO' r E

Path of contact _

A,'B,C.D.E Attuall)Qlh of contactA·,B',CP,E' Theordical path 01 ,(onlaU

rag. 5 - Tooth stretching signal in root of the pinienover the length of gearcontact, (See Table 1.)

fig. 6 - Position of the white etching areas in the areaor the ~nning .or contact on the f1alll of the driv-ing pinion.

fig_ 7 - Whi te etching areas in the area of the begi nning of contact on the surface of a case-hardened pin-ion made of 16 Mn Cr 5.

1---4'

20llm

Fig. 8 - The path and the direct jon ·of the main impactstress for two different friction values.

Fig. 9 - White etching area in the area of the surfacein the direction of the main shearstress,

Fig.10-location of the maximum impact stress withPH=1440 N/mm2 bH~0.20mm in relation to thefriction value.

face. Correspondingly, the angles of theWEA change as is pointed out in Fig. 9.

The position of the main shear stressminimum depends on the magnitude of thecoefficient of friction. As shown in Fig. 10from /L = 0.40 onwards, the maximummain shear stress lies at the surface. As frio-tion stress increases, the point at which thedirection of the main shear stress reachesthe 45° limit can be found as it travelsdeeper.

This connection, again, is valid only forideal geometrical bodies. Since tooth flankscan be considered as t,eclmicalsurfaces, wehave to take also Intoaccount the tension-increasing effect of the notches directly inthe area of the impact shock beneath thesurfaceaccording to RefereneeS. (See Fig.1.) Also favoring the formation of VVEAisthe local flash temperature, which again islargely dependent on friction value.

Caused by the effects of running-in, the[cantinued OM page 36)

u.s- gear makers need a com-petlUV8' edge. In these compet~li'18 'limes illa_k:es a~' !M!f!!geperiormance'. lind IlbovB II'I8ragesuppliers-. to survlve.

IWNOIS TOOlS ttelps pro-vide 'that compeU1i'18 ,edge:

We're organized to get. you t.he'gear generatlng loois you need -whmI )IOu need them

Our PIle. are surprl$lngl~competKivV and we are offering:an expanded line 01off·the-shellstanoard g9 T tools.

IFa;s! response to yourenglneer,ed special: and proto-type needs.

Our products" WhIChhawcon'linUed to sel the' quality stan-

dard fbr the' industry for morethan,1\) )'981'11, are your assur-ancs or reUabLe, durable, hlg"perlormllnce 'hobs. shaper cut-ters. master geal'll and WQf1I1geBr habs,

Our widelyaaclalml'dl G•• rSchool, Iso conllnu" 10keepAmerica', gear technologlllll upto date on Iha Iltale-or·lh"Bl't ingear maklng: and '11_Inspee1ion,

In \lllar making. oompathlwper[OITIlanoa StlLr1SwHit 1M Ilrllcut ,ILLINOIS TOOlS .. il helpeut It with lasler dellwrles al1ldlower costsl

III )'Ou',., lOoking 10< a com-paUU\l8!1dge. cailys.

ITrJ:1!l_:llinois Too~lsGear Tool SpersahstsAn 'illinois Tool Wo!b Company

Visit Us Booth 1120' 3601 W. TautlyAve./Llncolnwood. 11.1106451312-761·2100

GEAR TOOL HOTUNE: Poorte 1OI..L FREE 1..eQ0..628-2220. In illinOiS can: 1-800-626-2221.

CIRCUE .A~17 ON !READER :REPlY CARD

WHITE ETCHING AREAS ... (continued from page 23)stress in the area of the beginning of contact ...-----------------:11decreases with an increasing number ofstress cycles. It can therefore be expectedthat the WEA would occur at this spotfollowing a comparatively short amount oftime.

Specific Characteristics of theExamined White Etching Areas

Structure and the composition of WEAhave been met.alIogr-aphically examined ona broad basis in various research tests. Thefindings with case-hardened gears arelargely in agreement with the aforemen-tioned research. According to ourmeasurements on our sample, the micro-hardness of the WEA at the surface isbasedon an average of 1200HV11 dearly higherthan that of the surrounding matrix withabout 850HV1.

The WEA at the surface examined on thelight electron microscope show a partlyporous structure which points to a temper-ing process. (See Fig. 11.) Also, with thelight microscope, regions of a darker colorcan be seen within theWEA .. It is stillunclear whether within the surface near theWEA" as shown in Fig. 12" such hightemperatures can occur that can temper the

I

wiflank 5urfn:- II

- Ii

I

I1

P~rp'ndrcul(lrcut I----:1.1

I

Fig. 11- Porous structure in the surface area of a gearflank.

Z2000 Gear Tesler

'" Universal, multiparameter rolling flank 'geartester with computer and recorder.

IJi. Produces fulll documentation; RS-232interface gives it SPC capabilities.

IJi. Motor drive assures high, repeatability andeaseot operation.

III>Precision motion transfer for greater accuracy.III>Adjustable' measuring force makes lit suit-

able for plasticg,ears ..'" Features program stor.age for different gears.

!lft MET100 Production CourtNew Britain, Gl 06051(2031223-6784Telefax: (20312232979

CI:RCLE A-26 ON READER REPLY CARD

36 Gear Technology

Fig.]2- Partly tempered WEA at the flank surface.

:Fig. 13a - Crack near the surface (unetched slide).

Fig. 13b- View magnification. of the crack shown in.Fig. 13a that is bordered with white etching area.(Etched with HN03 .•)

(continued on page 39)

WH.ITE ETCHING .AREA5 ... (continued from page 36)

WEA structureYOIAsa condition for the formation of

WEA under high stress, we must also con-sider the initial structure and the heat treat-ment of the steel which influences thedistribution of the carbides.i!" The moreeven the carbides are spread out, the bet-ter conditionsare for the formation ofWEA. Table 21ists the heat-treatment datafDr the examined gears.

Possible Connection Between WhiteEltching Areas and Rank Damage

Fig. 13a.showsa crack beneath the sur-face of a tooth flank on an unetched slide.Fig. 13b shows the crack at the HNOJ -

etched slide in an area magnification.The fact tna.! the crack is bounded with

WEA, in accordance with the theorydescribed in Reference 10, points to a veryhigh crack propagation. The questionwhether the crack has a connection to thesurface cannot be answered by just a singleplane of a cut; however.we have to assumethat such cracks can lead to pitting. Figs. 141and 15show slides of tooth flanks of a driv-ing pinion with white etching areas near thebeginning of contact and a frontal view ofthe damaged flank.

On both pictures, the lower part of theflank is characterized by strong fatiguescratches and pores due to the high stress ofthe impact shock. Starting in this zone, pit-ting stretches out in the negative slippagearea which caused the failure of the pinion.

TheVIIEA that are shown as a.light elec-tron microscopical picture I:iedirectly onthe surface. (See fig. 16.) In the right halfof the picture the strongly fatigued toothflank around the WEA can be discerned ..

Based on the 'theory in Reference 10about the fonnation of WEA. there is thefollowing connection between WEA. andflank damage in case-hardened gears.

At hard, non-metallic inclusions, highshear stresses that lead to the formation ofIimlted cracks can occur locally ..If the in-clusions lie deeply, the crack does not ex-tend to the surface, but it can favor an out-break of pit.ting by growing together witha crack 'originating at the surface. It cannot,however, be regarded as the cause .of thepitting ..Inclusions below the surface cer-tainly do not primarily have an influenceon flank damage.

Inzones of high stress, fer example at 'thebeginning of contact, WEA occur at thesurface due to strong friction and high flashtemperature. Simultaneously, high shear

stresses lead to the formation of cracksfrom which pores and, later on, pittingmay arise. In this case WEA cannot defi-nitely be seen as the cause of the large pit-tings. Rather, WEA are an indicator for theoccurrence of high shear stress. Thus, as anexample, they allow for the conclusion ofhigh friction values at the beginning ofcontact.

Obviously, the material compositionand the state of the structure are the criteriafor the formation of WEA. A case-hard-ened outer layer with fine carbide distribu-tion, as could be found in the examinedgears, seems to favor the formation ofwhite etching areas.

CondusionIn this essay the phenomenon of white

etching areas on the flanks of highlystressed case-hardened gears has beendescribed. It can. be assumed that WEAarenot the cause of fatigue damage on 'toothflanks. Their occurrences in zones of highshear stresses, however,can providequalitatively valuable indications ,of the(continued on page 45)

fig.14-flank damage on a pinion. WEA at the sur-face In the area of initialpitting (Pc = 1600 N/mm2).

Rg.15 - Flank damage on 'the pinion 'WEA at the SW'-

faa? in the area of initial pitting. (Pc-I560N/mm2l.

Perpendicular cut Flanksul'face

SeJ:),tember/Octoberl989 39

WHETE. ETCHING AREAS ...{continued from page 39)

size and direction of the stress, and theycan point out possible starting points forflank damage.

In order to come to a quantitative con-clusion about the definite connection be-tween calculated paths of tension and theformation and direction of WEA on thetooth flanks, ,3 further .ana]ysis must ex-amine the influence of residual stress,which, especiallyat the surface, must notbe negleded.

It isa1so unclear whether the tempera-tures that locally occur at the flanks cancause a tempering process, and, thereby,the decay of the WEA structure over alonger time.

Ref~l'ences

1. BORG£SE, S.A., A 5t11dy o/the GrowthMechanism 0/ LmticuIar Carbides inCyclically Stressed 52100 Steel. ResearchLabor.atory, SKF Industries, Inc., 1968.

2. EYRE, T.S. and BAXTER, A. 'The For-mations of While Layers at Rubbing SIH'-faces." Metals and Materials, 1972,,,p.142-146.

3. KENGERER, F. WCAS, G. and uu-JEKVIST, B,. "Development of Roller Ele-ment Steel and Its Matnufacture." HeatTreating Technical R~port 3D, 1975, 2,pages 91·98. Un German)

4. JOACHIM, F.J. .Investigation of Pitting inHeat Treated and Normalized Gears.Dissertation, TU Munch n, 1984. ([nGerman)

5 ..KASER, W. Can tn'buting Factors to Pit-ting on Case-Hardened' Gears. Influenceof Case Depth and .Lubrication on theFlank l.oad Carryil1,g Capacity. Disserta-Ilion, TU Milnchen, 1989. (ln German)

6. MARTIN, J.A., 130RGESE:,B.A. andEBERHARD, A.D. "MicrostructuralAlterations ,of Rollil\g~Bea:ring SteelUnde:rgoing Cyclic Stressing:' Trans.Amer. Soc. Mech,. Eng .. 59, 1966, pp,555-567. ..

'I. MARTIN, I,A., BORCESE, S.A.. andEBERHARD', A.D. Third SummaryRepo.rt on Structural Studies of .BearingSteel Undergoing Cyclic Stressing.Research laboratory, S](f Industries,lnc., 1967.

S. NIEMANN. G. and WINTER, H.Machine Elemenfs, Volume 2, Springer-Verlag, 1983. (In German) .-

9. OSTER, P. The Stress 011 Gear Flanks inElastic-Hydrodynamic Terms. Disserta-tion, TU Miinch n, 1982. Un Gelman)

10. SCHLICHT. H. "Regarding th Forma-lion of White Etching Areas (WEAl inRolling Elements." Heat TreatingTeclmicalRep0r128 (1973) 2. pp, 112-123.(In German)

11. STAHu, G. "Possibilities and Limitationsof Rapid Surface Hardening on Steel,"Heat Treating Technical Ri?por134 (1979)2. PI'. SS-6J. (In German)

12. SWAHN, H., BE.CKER, P.c. andVINGSBO, O. "Martensite Decay Durin~Rolling Contact Fatigue in BaLIBearings."

MIior gut' mll\utlCtuNl'l In thelUto-motive IIKI Mlion Industry rely on the'quality 01 HoHmann IPreliur. F)lerSysttml lor IheIr eonvenlionallnd CBN'grinding appIicIflonJ.The comblllltion of h.igl'l ~ fIHratIon:,,coolant lemperalUlI' stablllD&n MdmJlt cOUtclIon ' urea manulacturtng atprecISIon gem.

MetalIrlrgical Tra,ISQctio'I5 A, Vol. 7A,1976, S. 1099-1110.

13. VOSKAMP, AP., et al., "GradualChanges in Residual Siress and Micro-structure During Contact Faligu in BallBearings." Metals Teclmology, 1980, S.14-21.

Acknowledgement: Reprinted willi permission of fhl.'

A merican Gear Manufacturers Assoaation. The opi·nions. statements and (onc/usiafl I1reserlti1d ,"lhispaper are those of the Authors ,m" ill· no way repre-sellt the positiol1 or opinion of tile AMERICA.NGEAR MANUFACTURERS ASSOCIATION.

THE L'EA'DE'RINI GEAR

'GRINIDINGIF:ILTRATION

Hottmann PrftuRe Fit.. otItr ft\II\yldY.n ..... uchlS:.' Incrutedlcooflnt IOd ~ IHI• Improved producI quality.lIgnlflcanl reckJctIon '" opemtng COlli

SEE US AT &,BOOTH #432-484 W

I IOFFMA'N\J FILTER CORPOR_ATIION22847 IHESLIP DRIV,E •P,HOINE ,(313) 348-7855

NOVII, M.IICHliQAN 48050• FAX (3,13) 348-7901

CIRCLE A~31 ,ON REAIDER R1PLV CARD'

September/October 1989 45