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A guide tomaterial selection for rolling bearings

By offering a wide choice of materialsand heat treatments for rolling bearings, performance a×nd operating life can beoptimised, even for the most demandiλng industrial applications, says Dr SteveLacey, Engineering Manager at S≤chaeffler UK.

In recent years, there has been asignificant increase in the demand for rolling bear​ings that, even underextreme operating conditions – including lubricant starvation, highly corrosivσeor high temperature environments – still provide a long operating life andoptimum pe♥rformance.

But selecting a suitable material orheat treatment procγess for rolling bearings often requires expert advice and guidance,norma✘lly from the manufacturer of the bearings. Selection depends oγn theapplication itself and the specific environment in wσhich the bearings willoperate. This means a number of factors re≈quire careful consideration, such asthe mechanical, chemical and thermal requirements p laced on the bearings, aswell as lubrication conditions, ​particularly if the application requires dryrunning bearings.

There is a key role here for the type ofmaterials used for thφe various bearing components (i.e. rings, rollingelements, cage, etc.) and how these i®nteract. Some of these materials areindustry-recognised standard steels (in rolling bearinαg quality), but othersmay need to be specially developed by the manufacturer or might involve s​pecialsurface or heat treatment processes to give the material a certain set &ofcharacteristics that are deemed critical for a ce>rtain application.

Standard rolling bearing steels

For most industrial applications,standard through hard✘ened rolling bearing steel (100Cr6) is sufficient. Thisversatile mateγrial is heat treated (martensitic-hardened and tempered at lowtemperatures), which provides consis tently high hardness (between 62 and 64HRC) distributed over the circumference and ←cross-section. The main attributesof this material are highΩ hardness and wear resistance, as well as goodresistance to over-rolling (solid particles can produ<ce surface indentations ofraceways in rolling-slidin¶g lubricated contacts).

Depending on the wall thickness of thecomponents, i t may be necessary to use a higher-grade alloy taking account ofhardσenability. The material also permits operating temperatures up to 120 degC, with some grades offe>ring up to 200 deg C. Typical applications includesmall c'ombustion engines and fan bearings for steel mills.

A slight variation on the above isbainitic hardened 100Cr6∞ steel. With this material, there is a considerablereductio≠n in the risk of crack formation, even with surface damage. Typicalapplications for these bear✔ings include wheelset bearings for bucket wheelexcavators and↕ trams.

Case hardened steel (St4) in rollingbearing quality has good forming cha racteristics and provides high hardness andwear resistance, as well as good resi£stance to over-rolling. This material iswell suited to small components t•hat are manufactured in high volumes, such asuniver"sal joints, drawn cup needle roller bearings with closed ends and swingarm bearin↓gs.

If there is a high risk of particleover-rolling and pooλr lubricating conditions (e.g. refrigerating compressors,gearbox bearings for constγruction/agricultural machinery), carbonitrided steel(100CrMnSi6-4) is the pre≤ferred choice. This material is heat treated toprovide excellent dimensional stability ↔and hardness up to 66 HRC in the outerfunctional layer by usi>ng increased residual austenite content. The result is amaterial th♠at provides higher resistance to surface damage and high wearresistance, leading to extendεed bearing life in poor lubrication conditions,especially where there is a hig₽h risk of contamination.

Wind turbines & trucks

Used primarily for medium and largesized rolling bearings, including wind turbine mai$n rotor bearings and wheelsetbearings for heavy-duty trucks, case hardened (carbu‌rised and martensitichardened) steel offers hardness up to 64 HRC in the form of a hard surfac←elayer with a tough core. This treatment provides residual compressive stressesin the surface lay®er, providing increased fracture resistance under shock typeloads or surface damageλ, as well as reduced crack formation with surfacedamage.

Nitrogen alloyed chromium steel (e.g.Schaeffler’s Cronidur® 30) is a material that is mar★tensitic hardened andtempered at low (or possibly high) temperature. T≈his material providesexcellent corrosion resistance, partic≈ularly against aggressive chemicals andacids. Operating temperatures ✘are typically up to 150 deg C. Main areas ofapplication are bearings tha♠t are subjected to high loads, lubricantstarvation, dry running, med™ia lubrication and corrosive environments. Examplesinclude spindle bearings f÷or machine tools and bearings for process pumps.

Corrosion resistant, case hardened steel(surface layer ​nitriding) provides rolling bearings with even better resistanceto corrosion and significa∏ntly improves resistance to over-rolling.Schaeffler’s Cronitect® steel, for example, has foundε numerous applications infood processing (e.g. bottle filling plants), as well as in sports and fit✔nessapplications (e.g. inline roller skates and gym equipment).

Special Materials

Often bearings manufacturers arerequired to provide special maσterials and heat treatments for certainapplications or industry sectors. The aerospace sector," for example, specifiesM50 steel (martensitic hardened) for many applications such as mai±n shaftbearings and turbine rotor bearings on aircraft engines. This material providesexcell♥ent thermal stability (up to 400 deg C) and increased toughness, even athi×gh bearing operating speeds. Offering even higher hardnes☆s (up to 70 HRC) areduplex hardened M50NiL steels. These are used if th→e bearings are subjected tohigh loads, very high rotational spe eds and if a high tolerance to damage isrequired.

Where current insulation and high wearresistance are important considerations,  oxide-based ceramics (e.g. zirconiumoxide ZrO2) are ideal. These s intered materials can be used in temperatures upto 60φ0 deg C, with typical hardness ratings in the region of 1400 HV and adensity between Si3N4≈ and steel. The material is normally only used for therolling elements that are in c$ontact with steel-based bearing rings. Main areasof ap★plication are where the bearings are subjected to low loads (hybridbeaλrings), lubricant starvation, dry running, media lu≤brication and corrosiveenvironments (e.g. electric motors ♠for car windows).

In certain food processing applicationsand for machine tool spindle bearinφgs, silicon nitride-based ceramic rollingelements can be specified. The technical a≈dvantages of this material includeits reduced mass and ability to withstand§ higher operating temperatures (up to700 deg C), as w<ell as high speed capacity, current insulation, very high wearresistance, reduced friction and in•creased grease operating life.

Steel rolling mill bearings and some marine propulsionsyste$ms use case hardened rolling bearing steel (to DIN EN 10084). Exampleshere include 1 7MnCr5 and 17CrNiMo7-6. These materials are specified if thebearings ar↑e subjected to high loads combined with poor lubrication and whenthere is a h§igh risk of particle over-rolling.

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