28 D E C E M B E R 2 0 0 3 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

wo wear mechanisms, fretting cor-rosion and false brinelling, areboth caused by small relative

motions, usually due to vibration while thecomponents of a machine are stationary ornot rotating. An example of vibrating com-ponents is the gears and rolling elementbearings in the turbine of an electrical gen-erating windmill that is rotating but whichis being vibrated by breezes. Anotherexample is the rolling element bearings ofa machine being shipped by truck or rail-way. Some publications (1, 2, 3) statethat the two mechanisms are the same and

that the terms are synonymous. Theauthor proposes that they are two differentwear mechanisms.

Fretting corrosion. Fretting is the dam-age to contacting surfaces experiencingslight relative reciprocating sliding motionof low amplitude.

Fretting corrosion is the fretting dam-age to unlubricated contacting surfaces ac-companied by corrosion, mostly oxidationthat occurs if the fretting occurs in air.Fretting corrosion of steel in air producesthe iron oxide hematite. Hematite, the ore

Practical Applications

Contrary to popular opinion, these two wear mechanisms are not the same.

By Douglas Godfrey (Fellow, STLE)

Fretting Corrosion or False Brinelling?

T

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of iron, also jeweler’s rouge, is alpha Fe2O3,ferric oxide, a dark red or cocoa-coloredmaterial (4). Red films or reddish debris area symptom of the absence of lubricant.

Figure 1 shows a schematic and micro-graphs of fretting corrosion between steelspecimens. After removal of the oxide crust,sliding marks can be seen in the damagedspot by scanning electron microscopy (bot-tom micrograph). Their length correspondsto the amplitude of vibration.

Brinell metal hardness test. The brinellhardness test uses a 10-mm diameter hardball to make an impression or dent in ametal specimen. The load and dent sizedetermine the brinell hardness. Originalgrinding marks are visible microscopically inthe dent. The brinell hardness number is theload in kilograms divided by the area of theindentation in square millimeters (1).

True brinelling of a rolling elementbearing. True brinelling of a rolling ele-ment bearing occurs when it is not rotatingand is subjected to a force, impact or shockload great enough to cause the rolling ele-ments to plastically deform and indent theraceway (2, 4, 5). The dents in the racewaycorrespond to the position of the rolling ele-ments. The original grinding marks are stillvisible microscopically in the indentation,as seen in Figure 2.

False brinelling of a rolling elementbearing. False brinelling is the formationof a worn depression on the raceway of alubricated rolling element bearing due tothe slight rocking of the rolling elements,maybe only a fraction of a degree (4, 5, 6, 7).Conditions prevent the formation of an elas-tohydrodynamic oil film and impose bound-ary lubrication.

The worn depressions replicate the shapeand the position of the rolling elements.There is no shoulder around the depressionsuch as occurs with true brinelling. The orig-inal grinding marks in the depression areworn off. No sliding or skidding marks havebeen found in false brinelling depressions.

The oil meniscus or adjacent grease

around the contacts contains wear frag-ments, which, for steel on steel, is usuallythe black oxide Fe3O4, magnetite. Magnetiteis a common wear fragment of steel con-tacts under moderate boundary lubricationconditions. The dents occur due to thewearing off of the pre-existing and continu-ally reforming oxide film on steel.

Figure 3 shows a schematic and a photo-graph of typical false brinelling of a taperedrolling element bearing raceway. Falsebrinelling causes noisy and, in some cases,inhibiting performance. It can also reducerolling element bearing life by creatingstress raisers leading to fatigue failure.

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False brinellingcauses noisyand, in somecases, inhibitingperformance.

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30 D E C E M B E R 2 0 0 3 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

PREVENTIONFor both fretting corrosion and falsebrinelling the basic prevention method is tostop the vibration to cease the sliding orrocking motion. This may be done by usingvibration isolators and, in some cases, bytightening contacts to increase friction withgreater loads.

Fretting corrosion is prevented by keep-ing the metal surfaces apart by antiwearfilms, surface treatments, solid lubricants ora rubber or plastic gasket. Lubricating thecontacts with oil containing an antiwearadditive, or grease made with a base oil con-taining a soluble antiwear additive is bene-ficial. Another approach is to alter the sur-face finish providing microchannels so thatthe oil can reach the areas of real contact.

False brinelling is prevented mechanical-ly by not allowing a rolling element to rockin the same place a long time. This isaccomplished by continuously or at leastoccasionally rotating the bearing. Rotationdistributes the wear uniformly around theraceway. Since boundary lubrication is ineffect, antiwear additives in the oil are ben-eficial in reducing the rate of wear and, thus,the depth of the depressions. <<

Douglas Godfrey heads his own consulting company,Wear Analysis in San Rafael, Calif. You can con-tact him at douggodfr@aol.com.

False brinelling isprevented mechani-cally by not allowinga rolling element torock in the sameplace a long time.

REFERENCES

1. Metals Handbook, Desk Addition, American Society for Metals,(1985). Glossary of Definitions, pp 1-6, and 1-16.

2. Eschmann, Hasbargen and Weigand (1985), Ball and Roller Bear-ings, John Wiley and Sons, New York, pp 970.

3. Faultersack, M. (1982), “Analysis of Bearing Damage,” SAE Paper820626.

4. Harris, T. E. (1991), “Rolling Bearing Analysis, 3rd Edition,” JohnWiley and Sons, pp. 965-986 (Failure Investigation and Analysis).

5. The Barden Corp. “Bearing Failure: Causes and Cures”. Bulleting WAV10M 4/99.

6. Wilcock, D.F., Booser, E. R. (1957), “Bearing Design and Applica-tion”, McGraw Hill, New York, pp. 160-162.

7. The Timken Co., “Tapered Roller Bearing Damage Analysis,” Bul-letin 35M -11-95 Order No. 6347.

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