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A guide tomaterial selection for rolling bearings



By offering a wide choice of materialsand heat treatments for rolling bearings, performance and operating life can beoptimised, even for the most demanding industrial applications, says Dr SteveLacey, Engineering Manager at Schaeffler UK.

In recent years, there has been asignificant increase in the demand for rolling bearings that, even underextreme operating conditions – including lubricant starvation, highly corrosiveor high temperature environments – still provide a long operating life andoptimum performance.

But selecting a suitable material orheat treatment process for rolling bearings often requires expert advice and guidance,normally from the manufacturer of the bearings. Selection depends on theapplication itself and the specific environment in which the bearings willoperate. This means a number of factors require careful consideration, such asthe mechanical, chemical and thermal requirements placed 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 the various bearing components (i.e. rings, rollingelements, cage, etc.) and how these interact. Some of these materials areindustry-recognised standard steels (in rolling bearing quality), but othersmay need to be specially developed by the manufacturer or might involve specialsurface or heat treatment processes to give the material a certain set ofcharacteristics that are deemed critical for a certain application.

Standard rolling bearing steels

For most industrial applications,standard through hardened rolling bearing steel (100Cr6) is sufficient. Thisversatile material is heat treated (martensitic-hardened and tempered at lowtemperatures), which provides consistently high hardness (between 62 and 64HRC) distributed over the circumference and cross-section. The main attributesof this material are high hardness and wear resistance, as well as goodresistance to over-rolling (solid particles can produce surface indentations ofraceways in rolling-sliding lubricated contacts).

Depending on the wall thickness of thecomponents, it may be necessary to use a higher-grade alloy taking account ofhardenability. The material also permits operating temperatures up to 120 degC, with some grades offering up to 200 deg C. Typical applications includesmall combustion engines and fan bearings for steel mills.

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

Case hardened steel (St4) in rollingbearing quality has good forming characteristics and provides high hardness andwear resistance, as well as good resistance to over-rolling. This material iswell suited to small components that are manufactured in high volumes, such asuniversal joints, drawn cup needle roller bearings with closed ends and swingarm bearings.

If there is a high risk of particleover-rolling and poor lubricating conditions (e.g. refrigerating compressors,gearbox bearings for construction/agricultural machinery), carbonitrided steel(100CrMnSi6-4) is the preferred choice. This material is heat treated toprovide excellent dimensional stability and hardness up to 66 HRC in the outerfunctional layer by using increased residual austenite content. The result is amaterial that provides higher resistance to surface damage and high wearresistance, leading to extended bearing life in poor lubrication conditions,especially where there is a high risk of contamination.

Wind turbines & trucks

Used primarily for medium and largesized rolling bearings, including wind turbine main 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 surfacelayer with a tough core. This treatment provides residual compressive stressesin the surface layer, providing increased fracture resistance under shock typeloads or surface damage, as well as reduced crack formation with surfacedamage.

Nitrogen alloyed chromium steel (e.g.Schaeffler’s Cronidur® 30) is a material that is martensitic hardened andtempered at low (or possibly high) temperature. This material providesexcellent corrosion resistance, particularly against aggressive chemicals andacids. Operating temperatures are typically up to 150 deg C. Main areas ofapplication are bearings that are subjected to high loads, lubricantstarvation, dry running, media lubrication and corrosive environments. Examplesinclude spindle bearings for machine tools and bearings for process pumps.

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

Special Materials

Often bearings manufacturers arerequired to provide special materials and heat treatments for certainapplications or industry sectors. The aerospace sector, for example, specifiesM50 steel (martensitic hardened) for many applications such as main shaftbearings and turbine rotor bearings on aircraft engines. This material providesexcellent thermal stability (up to 400 deg C) and increased toughness, even athigh bearing operating speeds. Offering even higher hardness (up to 70 HRC) areduplex hardened M50NiL steels. These are used if the bearings are subjected tohigh loads, very high rotational speeds 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 sintered materials can be used in temperatures upto 600 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 contact with steel-based bearing rings. Main areasof application are where the bearings are subjected to low loads (hybridbearings), lubricant starvation, dry running, media lubrication and corrosiveenvironments (e.g. electric motors for car windows).

In certain food processing applicationsand for machine tool spindle bearings, silicon nitride-based ceramic rollingelements can be specified. The technical advantages of this material includeits reduced mass and ability to withstand higher operating temperatures (up to700 deg C), as well as high speed capacity, current insulation, very high wearresistance, reduced friction and increased grease operating life.

Steel rolling mill bearings and some marine propulsionsystems use case hardened rolling bearing steel (to DIN EN 10084). Exampleshere include 17MnCr5 and 17CrNiMo7-6. These materials are specified if thebearings are subjected to high loads combined with poor lubrication and whenthere is a high risk of particle over-rolling.