LUBRICATION FUNDAMENTALS Dr. Robert M. Gresham / Contributing Editor

A little about fretting

KEYKEY COCONCEPTNCEPTSS

• FrettiFretting corroscorrosionion is the damage to contacting susurfacesrfaces Jet engine manufacturers have devised exexperiencingperiencing slightt t rerrelativeel ateive reciprocating sliding mmotionotion tribological methods to reduce failure in ooff loloww amamplitude.plitude. their products due to fretting . • FalseFalse bbrinelling,rinelling, by cocontrast,ntrast, iiss ththee formatioformationn ooff a wwowornorn TWO WEAR MECHANISMS, FRETTING military equipment overseas, on ships ddepressionepression oonn the racewaracewayy CORROSION AND , are in (see Figure 1) and across the country on ooff a llubricatedububrica catedted rorollinlling elelementement some ways very similar and can cause intercontinental trucks, where for days, bbearingearing due to the sslightlight confusion—or cause one to fret about weeks or even months the vehicle or rocking of tthehe rolling ,elementselements,, fretting. Some people over the years have machine subtly rocks back and forth on mamaybeybe ononlyly a fracfractiontion of a considered these two mechanisms to be its bearings and gears. degree. the same, but in reality there are subtle Taking a closer look, fretting differences, and the two processes can corrosion is the damage to contacting • JetJet enggine mamanufacmanufacturer u actuturere s aarareee even happen simultaneously. They are surfaces experiencing slight relative aableble to yddelayelay the rate oof aufafailureilure e both caused by small relative motions, reciprocating sliding motion of low usually due to low amplitude vibration amplitude. Fretting corrosion is ooff jet engi,nesnes,, pparticularly while the components of a machine the fretting damage to unlubricated smasmalleller engenginesines eddueue to ththisis are relatively stationary or not rotating. contacting surfaces accompanied by frettingg kind ooff wearwear.. Indeed, one immediately envisions the corrosion, mostly oxidation that occurs difficulties in shipping cars, trucks and if the fretting occurs in air. Fretting

26 Looking for work? Check out STLE’s Career Center to find a job that’s right for you. Details at www.stle.org. Figure 1 | One can see the difficulties in shipping cars, trucks and military equipment overseas on ships, where the vehicle or machine subtly rocks back and forth on its bearings and gears.

corrosion of steel in air produces the red In true brinelling, the raceway is False Brinelling of Thrust iron oxide hematite (Fe2O3), as shown basically dented as true brinelling in Figure 2, where the wear scar is is the result of shock loading with PSA-35 Snubber surrounded by red iron oxide. little relative motion. Thus, one False brinelling, by contrast, is the can sometimes even see machining formation of a worn depression on the marks in the dent, and the dent will raceway of a lubricated rolling element occasionally have a slight shoulder due bearing due to the slight rocking of the to plastic deformation of the raceway. rolling elements, maybe only a fraction In my experience, we worked with jet of a degree. Lack of movement prevents engine manufacturers to delay the rate of Ball Grove Before Cleaning the formation of an elastohydrodynamic failure of jet engines, particularly smaller oil film and necessarily forces boundary engines, due to this fretting kind of wear lubrication on the contact area to prevent (see Figure 4). Jet engines are basically it. The worn depressions replicate the a big rotating turbine fan. Some of the shape and the position of the rolling fan blades—which are made from some elements. No sliding or skidding marks pretty exotic metals—are static, and some are found in false brinelling depressions. are mounted on the rotator and rotate Even in Figure 3 you also might see a within the engine. Some of the blades are slight reddening, indicating some iron located toward the front of the engine, Figure 3 | In false brinelling, you might oxide formation on the edges and the compressor, which compresses air see a slight reddening indicating some hinting that these processes could coming into the engine. Some are located iron oxide formation on the edges, happen simultaneously in some cases. toward the rear, the turbine, where the hinting that these processes could energy of combustion spins the rotator happen simultaneously. providing thrust and energy to compress more air into the engine. While there are a number of design reciprocating sliding motion of low variations, basically the blades are amplitude” against the surfaces of the mounted into the stator and rotor slot into which it is mounted. Because via a tongue-and-groove arrangement these blades are not made from iron, analogous to the way furniture drawers iron oxide is not seen. are constructed. This design is very Can you imagine the consequences strong but is subject to fretting wear of wear on the root section of the due to the vibration of the engine. And blade, the part that fits into the slot, while engine manufacturers strive to and the slot surfaces? As fretting wear Figure 2 | Fretting corrosion of steel minimize engine vibration, the problem continues, the root section gets smaller in air produces the red iron oxide is with the so-called root section of the and the slot gets larger. Clearly one can hematite (Fe2O3), where the wear scar is surrounded by red iron oxide. blade (the tongue of the tongue and see that if left long enough, the blade groove) “experiencing slight relative will eventually come loose and could

WWW.STLE.ORG & LUBRICATION TECHNOLOGY JULY 2015 • 27 boundary lubrication problem. These SFLs will vary themselves depending on what part of the engine they are used for. In the compressor section, where temperatures are comparatively low, conventional SFLs are used; back in the turbine section, where temperatures can be quite high, exotic ceramic-bonded SFLs are used. In each Figure 4 | While there are many Figure 5 | Wear of the root case, the job of the SFL is to lubricate different design variations of jet sections and slot surfaces have the “slight relative reciprocating sliding engine blades that are strong, many been reduced through the use of motion of low amplitude” that results are subject to fretting wear due solid film . (Photo courtesy in fretting wear due to inadequate to the vibration of the engine. of E/M Products Corp.) (Photo courtesy of E/M Products Corp.) boundary lubrication. Because of the work of tribologists, in conjunction with their vendors, the jet severely damage the engine. either hits its target or misses; either engine manufacturers are able to control Engine manufacturers have way the engine is destroyed. In some the rate of failure due to this problem. developed a number of techniques cases, wear of the root sections and slot And when we fly from place to place, we for delaying failure due to this wear surfaces have been reduced through the need not fret about fretting. mechanism. In some cases, a pin is use of solid film lubricants (think high- inserted through the root section of the tech slippery paint-like coatings, see Revised from another issue blade to hold it in place, which adds Figure 5). Solid film lubricants (SFLs) of TLT. Bob Gresham is STLE’s to expense but certainly reduces risk. are excellent boundary lubricants, director of professional In other cases, like a missile engine, probably the best there are. And, as development. You can reach it only runs for a few seconds and we said above, this is fundamentally a him at [email protected].

[email protected] NAPOLEON ENGINEERING SERVICES 877-870-3200 Your one-stop shop for engineered bearing products. www.nesbearings.com

28 • JULY 2015 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG