bearing basics

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Chapter 1 - Bearing Basics

Course ObjectivesAttendees will learn the skills necessary tosuccessfully install and maintain rolling elementbearings in rotating machinery. Specifically, thefollowing topics are presented:• Understanding basic rolling element bearing

concepts• Identifying bearings for replacement• Choosing appropriate bearing removal

methods• Determining if associated components are

usable• Choosing appropriate installation methods• Properly installing and lubricating bearings• Maintaining installed bearings• Troubleshooting common bearing problems• Examining used bearings for root failure

cause• Introducing condition-based maintenance

concepts

SKF Circle of Life

Bearing QualityThe bearing manufacturer controls bearingquality. SKF achieves extended design life inapplications through investment in world classmanufacturing techniques and quality practices,including statistical process control, ISO 9000and ISO 14000 qualifications. Our EuropeanResearch Center continuously researchesadvanced bearing life extension andmanufacturing process improvements. Onaverage, SKF applies for 2 new patents everyday.Original equipment manufacturers choosebearing size based on design life considerations.Once the bearing or equipment is delivered, thecustomer controls the ultimate service life. SKFknows that service life can be extendedsignificantly by paying close attention toenvironment, installation, and maintenance.

EnvironmentControlling contamination is the key. Inaddition to training end users and manufacturersin the benefits of clean installation practices andoperating environments, SKF provides specialsolutions, such as optimized seals, lubricantsand coatings for extending bearing life incontaminated applications.

InstallationSKF is the only bearing company fielding acomprehensive maintenance tools catalog.However, getting the right tool for the job is notenough; training in proper techniques is, also,essential to successful installation.

MaintenanceProper application of precision maintenanceprocedures can safely extend bearing life.Reliability centered maintenance practicesimprove preventive and predictive maintenancesuccesses. Manufacturing plants still employing

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breakdown maintenance tactics for plant-criticalmachinery, are probably losing significantamounts of money that could be reinvested tocompete better in the marketplace.

SKF Literature Review

SKF has the most comprehensive variety ofliterature available for the proper installationand maintenance of bearings. Contact one ofyour local SKF authorized distributors, or visitour web site, www.skf.com, for moreinformation.

Ancient Bearing

One of the first recorded examples of a rollingelement bearing was found on the remains of aship dated circa 20 AD, in the reign of theRoman Emperor Caligula.

Reconstructed Ancient Bearing

This is a reconstruction of the bearing that wasused to support the rotating structure on thevessel’s prow. The desire to reduce friction andimprove application performance demonstratedby the ancients remains with us today.

Bearing Innovations

SKF has been responsible for all new bearingtypes developed this century, starting with thedevelopment in 1907 of the self-aligning ballbearing by Sven Wingquist. The invention ofthe spherical roller bearing in 1919 by SKFengineer Arvid Palmgren was followed by thespherical roller thrust bearing in 1939. The hub-bearing unit became available in 1972; althoughit was not strictly a new bearing type, beingbasically a double row angular ball bearing, theconcept was original. When the CARB wasannounced in 1995 it was the first completelynew bearing since 1939.Today, SKF continues to lead the world ininnovative bearing design.

Primary Function of Bearing

FrictionIn a rolling element bearing, friction is thedetermining factor for heat generation and thebearing’s operating temperature. Frictiondepends on load and several other factors, themost important of which are the bearing typeand size, operating speed, lubricant properties,and lubricant quantity.The total resistance to rolling in a bearing ismade up from:

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• The rolling and sliding friction in the rollingcontacts

• The rolling and sliding friction in the contactareas between rolling elements and cage

• The guiding surfaces for the rolling elementsor the cage

• The friction in the lubricant• The sliding friction of rubbing seals, in the

case of sealed bearings

Bearing LoadsLoads are transmitted between the stationarypart of a machine (commonly a housing) and arotating part of a machine (commonly a shaft)with minimum resistance.

Load Examples:Radial - belt tension, car wheel (straight line)Axial - drill press, pumpsCombined - right angle gearbox, car wheel(turning)

How Bearings Work

In the above graphic, the book on the tablecould be thought of as a plain bearing. Whenpencils are introduced, the contact area isreduced significantly, lowering friction. Thecontact area has changed from a flat, planesurface to a line – line contact.

Introducing marbles or balls reduces the contactarea further, to no more than a point. Pointcontact has less friction than line contact,resulting in improved rolling motion.Curl the book into a circle and it becomes theinner ring of the bearing. Curl the tabletopsurface into a circle and it becomes the outerring of the bearing. Add a cage to preventrolling element contact and we have the modernrolling element bearing.Rolling element bearings have lower frictionthan plain bearings, especially at higher speeds.There is a clear energy savings using rollingelement bearings over plain bearings as speedincreases.

Roller and Ball Raceway Contact

The difference between the two basic familiesof rolling bearings is in the contact area of therolling element. Balls have point contact, whilerollers have line contact. Stress levels in thebearing are directly proportional to the appliedload over the contact area of the rollingelements. The higher the stress level, theshorter the bearing life. Because friction also

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increases with greater contact, ball bearings aremost suited for light loads and higher speeds,while roller bearings are used for heavier loadsand lower speed applications.

Roller Types

The shape of the rolling element often identifiesthe applications for which it is most suitable.The various roller shapes give each bearingunique characteristics.

Bearing Components

The purpose of rings and rollers is to carry loadsand transmit power. Different shapesaccommodate various design requirements.The purpose of cages is to provide rollingelement guidance.Closures are used to control contamination andretain lubricants, reducing maintenance costsand extending bearing life.

Materials Used for SKF RollingBearingsA rolling bearing can only be as good as thematerial from which it is made. SKF has

developed steel making techniques that ensurethe highest quality and cleanliness of its steel,for maximum performance in operation. “Cleansteel” means that the presence of inclusions(which can initiate cracking and fatigue spalls)is at an extremely low level. Standard bearingsteels are of the “52100” grade, generallyconsidered high carbon steel. Compared tostainless steels, which contain 10 to 18 percentof chromium and other elements, bearing steelsare susceptible to corrosion. Stainless steels areavailable in some types and sizes, but bearinglife is reduced. New developments includeceramics (such as silicon nitride), which are anoption in harsh or extremely demandingenvironments.

Bearing rings and rolling elements

Through-hardening steelsCarbon chromium steelcontaining approximately1 % carbon and1,5 % chromium

Case-hardening steelsChromium-nickel alloyed steeland manganese-chromiumalloyed steel containingapproximately 0,15 % carbon

Heat Treatments

SKF bearings are predominately through-hardened (either Martensitic or Bainitic) steels.The most common through-hardening steel usedfor rolling bearings is carbon chromium steelcontaining approximately one percent carbonand 1.5 percent chromium. For bearingcomponents with large cross-sections, steelsalloyed with manganese and molybdenum areused for their superior through-hardeningproperties.SKF has played an important role in thedevelopment of through-hardening bearingsteels. Particular attention is paid to cleanliness.Modern bearing steels have such small contentsof macro and micro slags that it is unlikely thatbearings operating under ideal conditions willfail from fatigue.Case-hardening is used for large heavily loadedbearings subjected to shock loads.

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AdvantagesBainitic hardening

Tough structure

Automatic dimensionalstability

Compressive residualsurface stresses

Case hardening

Hard surface and soft coreresists through cracking

Highest compressiveresidual surface stresses

Martensite hardening

Less costly

Fixture can be used toreduce distortion

DisadvantagesBainitic hardening

Somewhat more costlythan martensite

Lower hardness

Case hardening

Time consuming andcostly process

Martensite hardening

Highest risk of propagationof surface cracks

Chromium-nickel and manganese-chromiumalloyed steels, with a carbon content ofapproximately 0.15 percent are the case-hardening steels most commonly used forrolling bearings.In most applications, there is virtually nodifference in behavior between bearings madeof through-hardened or case-hardened steels.ISO has acknowledged this fact by not making adistinction between steel types in the lifecalculation. Steel cleanliness and propermanufacturing methods, along with bearingdesign, are the decisive factors. However, insome applications a particular type of steel hascertain advantages.

Dimensional Stability

The graph below shows the differing effects oftemperature (over time) on the dimensionalstability of bearing rings that are Martensiticallyand Bainitically hardened.

Classification of Dimensional StabilityThis chart shows the classification of SKFbearings and their suitability for differentoperational temperatures. Note the effect on thehardness. The bearing life is reduced atelevated temperatures.

SKF rolling bearings can generally be used atoperating temperatures up to +125° C. Ifoperating temperatures are higher, the bearingsmust be subjected to a special heat treatment(stabilization) so inadmissible changes indimensions do not occur because of structuralchanges. However, bearings should not bestabilized for a higher temperature than theexpected operating temperature.For bearings required to operate at temperaturesin excess of 300° C, special steels with high hot-hardness are required. In such cases, it isadvisable to contact SKF.

Hardening Processes: Effects

Each of the three hardening treatments affectsthe structure of the steel, giving it different

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characteristics. Compressive stress reduces thetendency for cracking.

Effects of Different Heat TreatmentsShown here are typical applications for thedifferent treatments. The majority of bearingsare Martensitically hardened. This treatmentmakes the bearing highly resistant to wear, andis suitable for most applications. It is also theleast expensive process. In general, rollingelements (Rc 60 - 66) are slightly harder thanrings (Rc 58 - 62.)

Bearing Contact Angle

Single Cylindrical Angular Taper SphericalRow Roller Contact Roller Roller

Radial Loads Combined Loads

Bearing contact angles vary from zero, in thecase of purely radial bearings like the SingleRow Deep Groove Ball Bearing, to 90 degreesfor the Thrust Ball Bearing, which is intended toaccept loads only along the shaft axis (thrustloads). During actual operation, contact anglescan vary somewhat from the original design.

Bearing Terminology

In general, manufacturer’s catalogs refer tooutside diameter (O.D.) as “D,” inside diameter(I.D. or Bore) as “d” and width as “B.”Manufacturers’ part numbers are generallylocated on the bearing’s side faces.

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The lands are often used by manufacturers forcage guidance. Raceways are the normallyloaded “working area” of the bearing, where therolling elements, separated by a lubricating film,contact the inner and outer rings during normaloperation.Cages are generally not designed to transmitpower. They reduce friction by separating andguiding the rolling elements.

Ball and Roller Bearing Ranges

The basic range of bearing types is illustratedabove. Note, in the ball bearing range theraceway configuration gives the bearing its maincharacteristics. The roller shape defines themain features of the roller bearing group.

Load CapabilitiesThe greater the area of rolling element contact,the higher the load capability. This is theconverse of speed ratings.

Load MagnitudeThe magnitude of the load is the primary factorin determining the bearing size. Generally,roller bearings are able to support heavier loadsthan ball bearings with the same envelopedimensions and bearings, with a fullcomplement of rolling elements, can takeheavier loads than corresponding cagedbearings. Ball bearings are typically used whereloads are light or moderate; for heavy loads,where large-diameter shafts are used rollerbearings are more appropriate.

Load DirectionRadial Load – Most radial bearings can carrysome axial load in addition to radial loads, withthe exception of:• Cylindrical roller bearings, which have one

ring without flanges (NU and N types)• CARB™• Radial needle roller bearings, which are only

suitable for purely radial loadsAxial Load - Thrust ball bearings and four-point contact ball bearings are most suitable forlight and moderate pure axial loads. Singledirection thrust ball bearings can onlyaccommodate loads acting in one direction. Forloads acting in both directions, double directionbearings are necessary. Angular contact thrustball bearings can support moderate axial loadsat high speeds, and single direction bearings canalso accommodate simultaneously acting radialloads, while double direction bearings arenormally used only for purely axial loads. Formoderate and heavy axial loads acting in one

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direction, suitable bearings are needle rollerthrust bearings, single direction cylindrical andtaper roller thrust bearings, and spherical rollerthrust bearings, which can also accommodateradial loads. For heavy alternating axial loads,two cylindrical roller thrust bearings or twospherical roller thrust bearings can be mountedadjacent to each other.Combined Load - A combined load comprisesa radial and an axial load acting simultaneously.A bearing’s ability to carry axial load isdetermined by its angle of contact (alpha). Thegreater the angle, the more suitable the bearingfor axial loads. The axial load carrying capacityof deep groove ball bearings depends on thebearing internal clearance.For combined loads, single and double rowangular contact ball bearings and single rowtaper roller bearings are most commonly used,although deep groove ball bearings andspherical roller bearings are also suitable. Inaddition, self-aligning ball bearings andcylindrical roller bearings of the NJ and NUPtypes along with the NJ and NU types with HJangle rings can be used to a certain extent forcombined loads. Where axial loads pre -dominate, four-point contact ball bearings,spherical roller thrust bearings and crossed taperor crossed cylindrical roller bearings areappropriate.Single row angular contact ball bearings andtaper roller bearings, NJ type cylindrical rollerbearings and spherical roller thrust bearings canonly accommodate axial loads acting in onedirection. For alternating loads, these bearingsmust be combined with a second bearing.Consequently, single row angular contact ballbearings are available for universal pairing andas matched sets comprising two single rowbearings.When the axial component of combined loads islarge, it may be supported independently of theradial load by a second bearing. In addition tothe specialized thrust bearings, some radialbearings (e.g. deep groove or four-point contactball bearings) are suitable for this task. Tomake sure the bearing is only subjected to the

axial load in such cases, the outer ring must bemounted with a clearance fit in the housing.Moment Load - When the load actseccentrically on the bearing, tilting momentswill arise. Double row bearings (e.g. deepgroove or angular contact ball bearings) cantake up tilting moments, but paired single rowangular contact ball bearings or taper rollerbearings arranged face-to-face, or better stillback-to-back, are more suitable, as are crossedcylindrical and crossed taper roller bearings.

MisalignmentAngular misalignment between shaft andhousings occur, for example, when the shaftbends under the operating load, when thebearing seatings in the housing are notmachined at a single setting, or when shafts aresupported by bearings in separate housings thatare far apart.The so-called rigid bearings cannotaccommodate any misalignment, or can onlyaccommodate very slight misalignments withoutthe application of force. On the other hand,self-aligning bearings (e.g. self-aligning ballbearings, spherical roller bearings, sphericalroller thrust bearings, and CARB™) canaccommodate misalignments produced underoperating loads and can compensate foralignment errors resulting from machining ormounting. Values for the permissiblemisalignments are given in the introductorytexts of the table sections of the GeneralCatalog. If the expected misalignment exceedsthe permissible values, SKF should becontacted.Thrust ball bearings with sphered housingwashers and seating washers, as well as Y-bearings fitted in their appropriate housings, cancompensate for initial misalignment errorsarising from machining or mounting. They arenot intended to accept dynamic misalignment.

Relative Speed LimitationsThe relative speed capabilities of each basicbearing type can be seen. The chart aboveshows the limits for grease, oil, and the

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maximum speed limitations. As ageneralization, the larger the rollingelement/raceway contact the lower the speedrating. Not only the bearing type, but also manyother considerations, such as the type andquantity of lubrication, affect the limitingfactors. The bearing component materials andlubricant generally specify the upper operatingtemperature range of the bearing.

SpeedThere is a limit to the speed at which rollingbearings can operate. Generally, it is theoperating temperature, which can be permitted,with respect to the lubricant being used or to thematerial of the bearing components, that sets thelimit. The speed at which this bearingtemperature is reached depends on the frictionalheat generated in the bearing (including anyexternally applied heat) and the amount of heatthat can be transported away from the bearing.Bearing type and size, internal design, load,lubrication and cooling conditions, along withcage design, accuracy and internal clearance, allplay a part in determining the permissible speed.

Speed RatingsSpeed ratings for grease and oil lubrication arequoted in General Catalog bearing tables. Thespeed rating for a given bearing represents thespeed at which, under a load corresponding toan L10h life of 150,000 hours, there is a balancebetween the heat which can be removed fromthe bearing via the shaft and housing (andsometimes via the lubricant) and the heatgenerated in the bearing by friction at a giventemperature interval above ambient.

Speed ratings apply to bearings where the innerring rotates. Some reduction may be necessarywhere bearings operate with a rotating outerring. This is taken into account where trackrunner bearings are concerned, as these aregenerally required to operate under suchconditions (stationary inner ring).The speed rating is the first threshold to bereached with almost all bearing types. Beyondthis rating, other criteria have a strongerinfluence, depending on bearing type. Thesecriteria include the form stability or strength ofthe cage, lubrication of cage guiding surfaces,centrifugal and gyratory forces acting on therolling elements, and other speed-limitingfactors.Another threshold is set by the choice of grease.The base oil viscosity and thickener determinethe lubricant’s shear strength, which in turndetermines the permissible operating speed forthe particular bearing.For high speed bearing arrangements, allcomponents, particularly those which rotate,must have a higher than normal accuracy toallow for vibrating behavior. Special cagedesigns may also be required.Experience gained from practical applicationsindicates there are maximum speeds that shouldnot be exceeded, for technical reasons or

Bearing type Factorfn

Deep groove ball bearings, withoutseals

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Self-aligning ball bearings, withoutseals

1.5

Angular contact ball bearings, singlerow

1.5

Cylindrical roller bearings (with cage) 2.2Needle roller bearings, without seals 1.5CARB™ (with cage) 1.5Spherical roller bearings, without seals 1.5Taper roller bearings 2Thrust ball bearings 1.4Cylindrical roller thrust bearings 2Needle roller thrust bearings 2Spherical roller thrust bearings 3All other bearing types 1

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because of the very high costs involved.Guideline values for these maximum speeds areobtained for various bearing types bymultiplying the speed ratings given in thebearing tables by a factor fn, which can befound in the adjacent table.Remember that if bearings are to functionsatisfactorily, particularly at high speeds, theymust be subjected to a given minimum load.

Special CasesIn certain applications, the speed rating issuperseded in importance by otherconsiderations.Low Speeds - At very low speeds, it isimpossible for an elastohydrodynamic lubricantfilm to build up in the contacts between rollingelements and raceways. In such applications,lubricants with EP additives should generally beused.Oscillating Rotary Movements - With thistype of movement, the direction of rotationchanges before a complete revolution of thebearing has been accomplished. Sincerotational speed is zero at the point where thedirection of rotation is reversed, a fullhydrodynamic lubricant film cannot bemaintained. In such cases, it is important to usea lubricant containing effective EP additives toobtain a boundary lubrication able to supportloads.It is impossible to give a limit or rating for thespeed of such oscillating movements, as theupper limit is dictated by inertia forcers ratherthan a heat. With each reversal of direction,there is a danger of the rolling elementcomplement, because of its inertia, sliding for ashort period and damaging the raceways bysmearing. The permissible accelerations anddecelerations depend on the mass of the rollingelements and cage, the type and quantity oflubricant, the operational clearance, and thebearing load. For example, for connecting rodbearing arrangements, preloaded bearingsincorporating relatively small rolling elementswith a small mass are used. General guidelinescannot be given; therefore, it is necessary to

analyze the movements more precisely inindividual cases (it is advisable to contact theSKF application engineering service).Stationary Conditions - When rolling bearingsare at a standstill for longer periods, and arecaused to vibrate by externally applied forces,minute movements in the rolling element /raceway contacts produce surface damage. Thisdamage may be noticed as a marked increase inbearing noise during subsequent operation. Thiscan lead to premature fatigue failure of thebearing. Such damage must be avoided, forexample, by insulating the bearing arrangementfrom external vibrations, by unloading it bysuitable means during transportation, or byproviding some means of keeping the unloadedbearings in rotation at a slow speed. Experiencehas shown that oil lubrication is more favorablethan grease lubrication in such cases.

Bearing Cages

The development of cages was a key advance inbearing technology. It allowed rolling elementseparation and guidance, substantially reducingfriction and heat.

Cage Functions

Modern rolling element bearings did not existuntil the cage was invented in the late 1800’s. Itallowed the rolling elements to remainseparated, reducing friction and heat.

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Function of the Cage

Nearly all rolling element bearings have cageswhose main functions are to keep the bearingtogether and prevent contact between the contra-rotating rolling elements. The cage also carriesthe rolling element through the unloaded zoneof the bearing and helps to guide the rollingelements.

Materials for CagesWhere bearings are of a separable design, thecage also serves to retain the rolling elementswhen one bearing ring is removed duringmounting or dismounting. In some bearingtypes, e.g. needle roller bearings or cylindricalroller thrust bearings, the cage also serves toguide the rolling elements. It aligns them insuch a manner that they roll with a minimum offriction. In grease-lubricated bearings, some ofthe grease inside the bearing will adhere to thecage, forming a lubricant reservoir and ensuringgood lubrication of the bearing’s operatingsurfaces.The cage is centered either on the rollingelements or on one of the bearing rings, and isthus radially guided. Pressed steel or brasscages are generally centered on the rollingelements. Inner or outer ring centered machinedcages generally permit operation at higherspeeds and are necessary when movementsadditional to pure rotational are superimposed,particularly when high acceleration conditionsprevail. Suitable steps must be taken (e.g. oillubrication) to ensure there is sufficientlubricant supply to the cage’s guiding surfaces,and to the inside of the bearing.

Rolling bearing cages are mechanically stressedby friction, strain, and inertia forces. They mayalso be subject to the chemical action of certainlubricants, lubricant additives or products oftheir aging, organic solvents, coolants(halogenated hydrocarbons, ammonia), etc.Thus, the design and choice of material are ofparamount importance for the performance ofthe cage, and for the operational reliability ofthe bearing as a whole.

Cage Variants

The above graphic shows three variants of cagesfor a cylindrical roller bearing. Each has certainattributes that make it suitable for specificapplications. Cages can be located on the innerring, rolling elements or outer ring. Speed,lubrication requirements and exposure to highvibration are some of the influences in theselection of a particular cage type.

Standard CagesAs rolling bearings have developed, variouscage types and designs for the different bearingtypes and sizes have emerged. Cages differ inform, material, manufacturing methods, cost ofproduction, and operational limits.One particular cage design is established as thestandard for each of the bearings shown in theGeneral Catalog bearing tables. The standardcage is always proven in service, and is thedesign considered most suitable for the majorityof applications. With reference to the viabilityof production, the costs and different applicationareas of the bearings, the standard cage for thelarger bearings may be different from that forthe smaller bearings in the same series.

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In the text preceding each table section,information is provided regarding the standardcages with which the bearings are fitted, and thepossible alternatives. If a bearing with a non-standard cage is required, it is always advisableto check availability before ordering.

Polyamide Cages

Polyamide cages that are reinforced with glassfiber have some unique qualities. They arelightweight and flexible, and are very tolerant ofpoor lubrication conditions. They should not beused at high temperatures, over 100o C (210° F),and are vulnerable to heavy impact loads. Theyare only found in high volume types, sinceproduction tooling is very expensive.Some small- and medium-sized bearings arefitted with molded cages of heat-stabilized,glass fiber reinforced polyamide 6.6 as standard.This material is characterized by a favorablecombination of strength and elasticity. Thegood sliding properties of the plastic onlubricated steel surfaces, and the smoothness ofthe cage surfaces in contact with the rollingelements keep cage friction low, which in turn,keeps heat generation and wear in the bearing toa minimum. The low density of the materialkeeps the inertia of the cage small. Theinjection molding process used to produce thecages allows functionally suitable designs to bemade. The excellent running properties ofpolyamide cages under lubricant starvationconditions permit continued operation of thebearing for a time, with minimum risk of seizureand secondary damage.When using bearings with polyamide cages, thepermissible operating temperatures for thematerial and its resistance to the lubricant used

must be observed. At operating temperatures upto the values given in the General Catalog forthe various oils and greases used as bearinglubricants, cage properties are unaffected. If thepermissible temperature is exceeded, the cagematerial will age. The longer the cage isexposed to the excessive temperature, the moreaccelerated is this process. Brief periods at upto 20° C above the recommended maximumtemperatures can be tolerated, provided they areinterspersed with longer periods at operatingtemperatures below the recommended values,and provided the maximum operatingtemperature for the lubricant is not exceeded.When operating temperatures are constantlyabove 120° C, bearings fitted with metalliccages must be used. Polyamide cages are alsounsuitable for operating temperatures below -40° C, as they lose their elasticity.Organic solvents normally used to clean rollingbearings, such as white spirit or trichloroethane,do not affect cage properties, neither do dilutealkaline cleaners (e.g. soda) which are used atroom temperature for a short period of time.The fluorocarbons or ammonia used inrefrigeration do not attack polyamide.Additionally, in vacuum polyamide cagesbecome brittle as they dehydrate.

Window Type Sheet Steel Cages

These cages are strong and lightweight with notemperature limitations. In poor lubricationconditions, wear will be more rapid than withother cage materials.Pressed cages of steel sheet are standard formany ball bearings, spherical roller bearings,and most taper roller bearings. These cageshave relatively high strength and weigh little.

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They may be hardened and surface treated toreduce friction and wear.Machined steel cages are used for large-sizebearings or where the application conditions aresuch that there would be a danger of seasoncracking if pressed brass cages were used (e.g.in ammonia). To improve sliding and wearresistance properties, some machined steel cagesare surface hardened by carbonitriding.Steel cages can be used at operatingtemperatures up to 300° C. They are notaffected by the mineral or synthetic oil-basedlubricants normally used for rolling bearings orby organic solvents used to clean bearings, andthere is a risk of corrosion where water ispresent. Pressed steel cages are notrecommended for use with ammonia.

Window Type Solid Brass Cages

Solid brass cages are costly to machine and areusually restricted to small batch production.They are highly resistant to wear and shockloads, and are often found in hazardousenvironments.Pressed brass cages are used for some small-andmedium-sized bearings and as a standardalternative for many angular contact ballbearings. Machined brass cages from cast orwrought material are often used for larger-sizedbearings of all types.Brass cages should not be used at temperaturesin excess of 300° C. They are unaffected bymost commonly used bearing lubricants,including synthetic oils and greases, and can becleaned using normal organic solvents. The useof alkaline cleaning agents is not recommended.Ammonia (e.g. in refrigeration) causes seasoncracking in pressed brass; therefore pressed

brass cages are unsuitable for these applications.One-piece machined brass cages must be usedinstead.

Other Cage MaterialsIn addition to the materials described above,SKF bearing cages for special applications maybe made from other engineering plastics, lightalloys, or special cast iron.Closures extend the life of the bearing bykeeping lubricants in and contamination out.There are three basic types: shields, non-contactseals and land riding lip seals.

Lubricants

Most bearings are coated with slushingcompound for protection during packaging,transport and storage. Sealed bearings aregenerally filled 1/3 full with polyurea basedgrease. SKF slushes are compatible withstandard mineral and most synthetic grease.Special flouroelastomer greases may requireslush removal prior to installation. Contact SKFfor details.

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Bearing Basics

Rolling Bearings are PrecisionProducts

Although all standard bearings are highprecision products, even higher grades ofaccuracy are obtainable depending onapplication requirements. This graphic showsthe tolerance classes in which bearings areavailable.

Precision ClassesTwo general classes for precision tolerancesexist: ABEC (Annular Bearing engineersCommittee) and ISO (International StandardsOrganization). The basic equivalents arepresented in the following table:ABEC ISO1 Normal

3 P65 P57 P49 P3

Internal Clearances

All rolling bearings require internal radialclearances to help ensure satisfactory operation.There are some exceptions, such as machinetools. The illustration shows the clearance in adeep groove ball bearing. Clearance values aregenerally to ISO standard, and values for eachbearing type can be found in the SKF GeneralCatalogue.Bearing internal clearance is defined as the totaldistance through which one bearing ring can bemoved relative to the other in the radial

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direction (radial internal clearance) or in theaxial direction (axial internal clearance).It is necessary to distinguish between theinternal clearance of a bearing before mountingand the internal clearance in a mounted bearingwhich has reached its operating temperature(operational clearance). The initial internalclearance (before mounting) is normally greaterthan the operational clearance. The differentdegrees of interference in the fits and thedifferences in thermal expansion of the bearingrings and their associated components cause therings to expand or compress.A bearing’s radial internal clearance is ofconsiderable importance for satisfactoryoperation. As a rule, ball bearings should havean operational clearance that is virtually zero, orthere may be a slight preload. On the otherhand, cylindrical and spherical roller bearingsshould always have some residual clearance,however small, in operation. The same is trueof taper roller bearings, except in bearingarrangements where stiffness is desired (e.g.pinion bearing arrangements) and the bearingsare mounted with a certain degree of preload.The bearing internal clearance referred to asNormal has been selected to obtain a suitableoperational clearance when bearings aremounted with the recommended fits, and whenoperating conditions are normal. Whereoperating and mounting conditions differ fromthe normal (e.g. where interference fits are usedfor both bearing rings, unusual temperaturesprevail etc.), bearings with greater or smallerinternal clearance than Normal are required. Insuch cases, it is recommended that the residualclearance in the bearing should be checked afterit has been mounted.Bearings with an internal clearance other thanNormal are identified by the suffixes C1 to C5.**Reminder: Clearances are not the same asprecision classes.**

C1 - Less than C2 clearanceC2 - less than normal clearance (CN, C0) -normal clearance (not generallymarked)C3 - greater than normal clearance

(“standard”)C4 - greater than C3 clearanceC5 - greater than C4 clearanceExample:

A 6210 /C3 Ball bearing has from 18 - 36µm (microns) or 0.0007” - 0.0014” RadialInternal Clearance

For certain bearing types, radial clearance issometimes presented in catalogs as axialclearance, also called end play.

Example:

A 5210 / C3 Ball Bearing has33 - 54 µm (microns) or 0.0013”– 0.0021”Axial Internal Clearance

Special clearances are sometimes available (e.g.,C2H). Sometimes nomenclature is combined:ABEC: C783 = C78 (ABEC 5) + C3P62 = ISO P6 + C2

Effects of TemperatureExpansion rates vary with the material; forexample, aluminum expands faster than steel.

Internal Clearance: Range andReasonsInitial radial clearance is present to compensatefor the effects on the bearing of the interferencefits to shaft and/or housing, and the temperaturedifferentials that can occur between the innerand outer rings.

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Clearance rangeC1 Clearance less than C2C2 Clearance less than NormalC3 Clearance grater than NormalC4 Clearance grater than C3C5 Clearance grater than C4

COLD

WARM

Reducedradial

clearance

Expansion

Compression

Across the wide operational range of speed, loadand temperature in which bearings operate theradial internal clearance requirement variesgreatly. A range of clearances – from C1through C5 – is available to provide for theseconditions.Bearing components under load act like springs.When a bearing is mounted with a press fit onthe shaft, the shaft will compress slightly, andthe inner ring of the bearing will expandslightly, reducing clearance in the bearing.Once the application begins running, frictionheat may further reduce clearance. Theremainder is considered the operationalclearance in the bearing. In some cases, all theclearance is removed and the rings and rollingelements are in full contact with one another,with the various components undercompression. This condition is known asPreload.

Preload vs. ClearancePreload is sometimes desirable, as in heavilyloaded pumps. The designer will generally erron the side of safety, choosing an initial bearingclearance that leaves a slight operationalclearance. Relative bearing life decreases veryrapidly if the bearing is preloaded. Excessclearance reduces overall bearing life at a muchlower rate.

Effects of Internal Preload andClearance on Bearing Life

Clearances that are too large result in reducedbearing life. Bearings that have too little initialclearance or are preloaded can suffercatastrophic failure on start from excessivetemperature differentials occurring between therings.

Load Distribution due to ClearanceChanges

The optimum bearing life is achieved aroundzero clearance at operational speeds and loads.If clearances are too great, the stressconcentration within the bearing increases dueto reduced number of rolling elements inraceway contact in the load zone. An increasein the arc that the unloaded rollers move throughcreates additional loads on the cage and cagepockets.

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Bearing Selection Criteria

Why was a particular bearing chosen to do thejob? Sometimes the wrong bearing is selectedfor the application. Causes for improper bearingselection include lack of application knowledgeand unanticipated parasitic loads.The General Catalog provides guidance fordesigners in choosing the correct bearing type(s)to meet application requirements.Power density is roughly the amount of load thatcan be carried by a bearing in a given space.Generally, designers prefer greater powerdensity, as it is more economical.

Bearing Selection FactorsThe application engineer must consider manyfactors when determining the best bearingselection for a given application. The mostobvious and straightforward includeload/direction of load, speed, space,misalignment, required life, and operatingconditions.

Depending on its design, each type of bearingdisplays characteristic properties that make itmore, or less, appropriate for a givenapplication. For example, deep groove ballbearings can accommodate moderate radialloads along with axial loads. They have lowfriction and can be produced with highprecision. Therefore, they are preferred forsmall- and medium-sized electric motors.Spherical roller bearings can carry very heavyloads and are self-aligning. These propertiesmake them popular for application in heavyengineering, where heavy loads producedeformations and .However, in many cases several factors must beconsidered and weighed against each otherwhen selecting bearing type. No general rulescan be given. Special catalogs are available forbearing types that are not included in the SKFGeneral Catalog (e.g. fixed section bearings).

18

Example: Catalogue Selection

This example is for deep groove ball bearingswith shields, in the d 25 – 70 mm range.The bore size (d) denotes the bearing size. Theother boundary dimensions are the outsidediameter(D) and width (B).The basic load ratings are shown next –dynamic (C) and static (C0). The fatigue loadlimit shows the dynamic load, below whichinfinite life applies (under ideal operatingconditions).Next are the recommended speed ratings forgrease and oil, then the bearing mass, andfinally the designation.

Available SpaceIn many cases, one of the principal dimensionsof the bearing, generally the bore diameter, ispredetermined by machine design.For small-diameter shafts, all types of ballbearings can be used, the most popular beingdeep groove ball bearings. Needle rollerbearings are also suitable. For large-diametershafts, cylindrical, spherical, and taper rollerbearings are available, along with deep grooveball bearings.When radial space is limited, bearings with asmall cross section, particularly those with a lowcross-sectional height must be chosen (e.g.bearings of Diameter Series 8 or 9). Specialmention can be made of needle roller and cageassemblies, drawn cup needle roller bearings,and needle roller bearings without (or with)inner ring. Certain series of deep groove andangular contact ball bearings, cylindrical andspherical roller bearings; CARB™, and thebearings listed in the SKF catalog “Thin sectionbearings” are all suitable.When space is limited in the axial direction,certain series of single row cylindrical rollerbearings and deep groove ball bearings can beused for radial and combined loads. For purelyaxial loads, needle roller and cage thrustassemblies, needle roller thrust bearings, andcertain series of thrust ball bearings andcylindrical roller thrust bearings can be used.

Bearing Downsizing

Bearing size has decreased as bearings haveimproved. The newer bearings can handle thesame load, at higher speeds, than older designs.Maintenance-wise, however, more caution isnecessary with these bearings. The sameamount of contamination or handling damage tonewer, smaller bearings will cause a greaterreduction in life than on a larger, older design.

Bearing Life Determination

What is a cyclic shear stress? Cyclic shearstress is a similar to the concept of breaking awire. In order to break a wire, it needs only tobe bent repeatedly. The metal fatigues, thenbreaks. Bearings work the same way. Forexample, there are two pieces of wire of thesame diameter. By bending one quickly and theother slowly, using the same bending angle, thewire being bent quickly undergoes more stresses(fatigue cycles) over the same period of time,and will break first. Bearings fatigue the sameway with respect to speed - bearing life isinversely proportional to speed. If we doublethe speed, life is halved.With two new wires, one is bent 90 degrees, theother ten degrees, at the same speed. Since thewire being bent 90 degrees had a greater load, itwill fail much earlier. In the case of bearings,the life changes at a much greater rate whenload is altered than from a similar change inspeed. As will be seen during the exploration ofthe life equations, small changes in the appliedload result in large variations in predicted life.

19

However, the SKF New Life Method alsopredicts that if we stress the bearing below aspecific minimum load, it may never fail.

Bearing Fatigue Failure

Rolling loads generate cyclic elasticdeformation in the raceways and rollingelements. The resultant shear stresses, whichoccur just below the surface, cause metallurgicalchanges. These promote fractures to progress tothe surface, causing small pieces of the bearingmaterial to become detached. As damageincreases, the failed bearing will eventuallyneed to be replaced.

Fatigue LifeBearing fatigue life is defined as the momentwhen the onset of fatigue spalling on either theraceways or rolling elements occurs. It isimpossible to determine exactly when anyindividual bearing will fail, but it has beenestablished by testing that in any representativebatch of identical bearings, a pattern of failurewill emerge that is statistically significant. Thefirst 10 percent of bearing failures establish theL10 life. The spread of subsequent failure issuch that the median of failure will beapproximately five times greater than the first10 percent. Some bearings will continue to runfor much longer. These tests are conductedunder controlled conditions and do not representactual service life, which can be influenced bymany other factors.

Bearing Life Calculation

ISO Life Rating

ISO “Catalog Rated Bearing Life” (C) isdetermined by the overall size of the bearing,size, shape and number of rows of rollingelements, the contact angle of rolling elementwith the raceway, and the materials used in thebearing.

Basic Load RatingsThe basic dynamic load rating C is used forcalculations involving dynamically stressedbearings (e.g. when selecting a bearing which isto rotate under load). It expresses the bearingload that will give an ISO basic rating life(defined below) of one million revolutions.

20

The basic dynamic load ratings of SKF bearingshave been determined in accordance with themethods prescribed by ISO 281:1990 /Amd.1:2000 and ABMA Stds. 9 and 11. Valuesare based on the material and manufacturingtechniques used for SKF standard production.They apply to loads that are constant inmagnitude and direction, for radial bearingsradial loads, and for thrust bearings axial loadsthat act centrically.The basic static load rating C0 is used incalculations when bearings are to rotate at veryslow speeds, to be subjected to very slowoscillating movements, or to be stationary underload during certain periods. It must also betaken into account when heavy shock loads ofshort duration act on a rotating (dynamicallystressed) bearing.The basic static load rating is defined inaccordance with ISO 76-1990 as the static loadwhich corresponds to a calculated contact stress

at the center of the most heavily loaded rollingelement / raceway contact of:- 4,600 MPa for self-aligning ball bearings- 4,200 MPa for all other ball bearings- 4,000 MPa for all roller bearingsThis stress produces a total permanentdeformation of rolling element and raceway thatis approximately 0.0001 of the rolling elementdiameter. The loads are purely radial for radialbearings and centrically acting axial loads forthrust bearings.

Load Carrying Capacity and LifeThe size of a bearing used for an application isinitially selected based upon its load carryingcapacity in relation to the loads to be carried andthe requirements regarding life and reliability.Numerical values for “basic load ratings” areused in the calculations to express load carryingcapacity. Values for the basic dynamic loadrating C and the basic static load rating C0 arequoted in the General Catalog bearing tables.

The L10 life of a rolling bearing in a givenapplication can be established using the ISObasic life equation:

1/p10

p

10 LPCor

PCL =

=

where

L10 = basic rating life, millions ofrevolutionsC = basic dynamic load ratingP = equivalent dynamic bearing loadp = exponent of the life equation

p = 3 for ball bearingsp = 10/3 for roller bearings

21

For bearings operating at constant speed it maybe more convenient to deal with a basic ratinglife expressed in operating hours using theequation:

p

10 PC

n600000001L

=

or

1010h Ln600000001L =

whereL10h = basic rating, operating hoursn = rotational speed, r/min

When the speed is doubled, life is reduced by ½.When the load is doubled, life is reduced almost90 percent. A 20 percent increase in loadreduces roller bearing life by 54 percent. Ageneral conclusion can be made: altering theapplied load dramatically affects the life of thebearing, changes in speed affect life linearly.

The “P” value for “average” applications is fiveto ten percent of the C value, with the result thatdesigners generally choose load ratios (C/P) inthe “normal” range, between eight and 20. Thismeets the application’s required life withoutapplying more bearing than necessary to do thejob.

Bearing Capacity SelectionDynamic LoadsThere are several methods to estimate the properbearing size for a given rotating application. Theusual method involves selecting the proper loadratio, given a known load, desired life, andspeed. Several methods will be used, as

described in the General Catalog, for thefollowing example:What size bearing(s) would meet the followingrequirements:• Type: Deep Groove Ball Bearing• Life Required: 50,000 Hours• Speed: 3,600 RPM• Radial Load (P) : 1000 lbs.

Bearing Selection Using theNomogram

The Nomogram method gives 2 significantdigits of accuracy. (Note: there are two sets ofnomograms: the left set for Ball Bearings, the

right for Roller Bearings.)• Using the given speed (3600 RPM) and L10

operating hours (50,000), find C/P in themiddle column with a straightedge.

The value is 22.• To determine the required dynamic load

capacity, multiply the load ratio by the givenapplied load, in this case, 4450N (1,000 lbs).C/P x P = 22 x1000 = 97,900N (22,000 lbs)

22

• Now go to the Deep Groove Ball Bearingsection of the General Catalog and choosean appropriate bearing with at least 97,900N(22,000 lbs.) dynamic capacity (C). Onepossible solution would be an SKF 6219Deep Groove Ball Bearing.

Bearing Selection Using Load RatioCharts

In the SKF General Catalog, note that the chartis broken in half, the upper section for lowerspeed, the other for higher speeds. Using theinformation from the example, find the columnby locating the speed (3600 rpm) at the top ofthe chart. Find the desired operating hours(50,000) on the left side of the chart. Theintersection of the column and row gives theC/P value: 22.1. Multiply the C/P value by theload (1000 lb.) to get the required minimumbasic dynamic load rating.

=×

P

PC

22.1 x 1000 = 22,100 lbs.

An appropriate bearing may be selected fromthe General Catalog. Although a 6219 bearingwas previously selected, other bearings couldhave been chosen based on application

requirements (boundary dimensions, etc.). Notethat this is the simplest of calculations, using asingle, known load. Contact SKF ApplicationsEngineering Service for assistance with morecomplex bearing selection and designrequirements.

Adjusted Life EquationWith the adjusted rating life equation, theinfluence of bearing load on the life of a givenbearing is considered. Where the rollingbearings listed in this catalog are used inconventional applications, a calculation of thebasic rating life L10 is adequate, since therecommendations regarding requisite life arebased on experience and, in fact, considerfactors such as lubrication.It may, however, be desirable to consider otherfactors influencing bearing life in more detail.ISO and ABMA introduced a revised lifeequation to this end. This adjusted rating lifeequation is

p

na PCaaaL

= 321

or simply

whereLna = adjusted rating life, millions ofrevolutions (the index n represents thedifference between the requisite reliability1) and100 percent)a1 = life adjustment factor for reliabilitya2 = life adjustment factor for materiala3 = life adjustment factor for operatingconditionsThis life method was more frequently used priorto 1989. For historical purposes, the adjustedrating life is explained below.A calculation of the adjusted rating lifepresupposed that the operating conditions werewell defined and that the bearing loads could beaccurately calculated, i.e. the calculations

10321 LaaaLna =

23

should have considered the load spectrum, shaftdeflection, etc.For the generally accepted reliability of 90percent and for bearing materials to which the Cvalues correspond, and for normal operatingconditions, a1 = a2 = a3 = 1 and the equations forthe basic and adjusted rating lives becameidentical.1) Reliability is the probability that a bearingwill attain or exceed a specified life.

Life Adjustment Factor a1

The a1 factor for reliability is used to determinelives other than the L10 life, i.e. lives which areattained or exceeded with a greater probabilitythan 90 percent. Values of a1 are given in theaccompanying table. This life factor is also usedwith the SKF life method.


When determining SKF basic dynamic loadratings, the fact had been taken into account thatthe standard steels used by SKF had better lifeproperties than the material on which theequations given in ISO 281/I-1977 and ABMAStandards 9 and 11 were based. When usingthese load ratings (C values), therefore, a2 = 1.Higher values of a2 could, however, be appliedto SKF bearings made of special steels.

Reliability Lna a1

90 L10a 195 L5a 0.6296 L4a 0.5397 L3a 0.4498 L2a 0.3399 L1a 0.21


The operating conditions factor a3 wasessentially determined by bearing lubricationprovided bearing operating temperatures werenot excessive. Changes in material properties atelevated temperatures were accounted for byreducing the basic dynamic load ratings. The

efficacy of lubrication was primarily determinedby the degree of surface separation in the rollingcontacts of the bearing. If an adequate load-carrying lubricant film was to be formed, thelubricant must have had a given minimumviscosity at the operating temperature, i.e. thetemperature of the bearing in operation.Under the cleanliness conditions normallyprevailing in bearing arrangements which wereadequately sealed, the a3 factor was based on theviscosity ratio [kappa, κ]. This is defined as theratio of the actual viscosity ν to the viscosity ν1required for adequate lubrication, both valuesbeing kinematic viscosities at the operatingtemperature. κ is also used with the SKF lifemethod.The viscosity ν1 required at the operatingtemperature to ensure adequate lubrication canbe determined from the General Catalogprovided mineral oil is used. The diagram isalso valid for greases based on mineral oils and,in this case, the value of ν1 obtained gives therequisite base oil viscosity at the operatingtemperature.For these calculations, the viscosity shouldalways be expressed in mm2/s (cSt -centistokes), rather than in Saybolt UniversalSeconds (SUS), as the conversion between theseunits is nonlinear.

SKF New Life MethodPractical experience and modern research haveshown that, under special conditions, SKFbearings attain a much longer life than predictedby standardized life calculation methods,particularly when loads are light. These specialconditions apply when the rolling surfaces(raceways and rolling elements) are effectivelyseparated by a lubricant film and when surfacedamage caused by contaminants is limited.Even infinite life is possible under idealconditions.The SKF life method introduces the concept of afatigue load limit Pu, analogous to that usedwhen selecting other machine components.This fatigue load limit represents that load

24

below which fatigue will not occur in thebearing under ideal conditions.

The method represents an extension of the workof Lundberg and Palmgren, taking into accountthe fatigue load limit and several other factorsrelated to lubrication and contamination. Usingthe SKF method, it is possible to accuratelypredict the long bearing life referred to above.However, an accurate prediction can only bemade if the operating conditions are accuratelyknown and if the full theory is applied, whichrequires the use of a computer.

Due to complexity, a detailed description of thetheory is beyond the scope of this book.However, to pass on the benefits inherent in thedeeper understanding of bearing behavior onwhich the SKF life method is based, asimplified “catalogue” approach has beendevised. This approach enables users to exploitthe enhanced life potential of bearings, toundertake controlled downsizing, and torecognize the significance of contaminants. Seethe SKF General Catalogue for details.

These calculations account for life undercontrolled conditions only. There is noadjustment in the basic life calculation foroutside influences (contamination, poorlubrication, vibration, etc.) These externalfactors reduce actual bearing life below thatpredicted by the basic life equation.

Static LoadsBearing size should be selected on the basis ofthe basic static load rating (C0) when one of thefollowing conditions applies:1. The bearing is stationary and is subjected to

continuous or intermittent (shock) loads.

2. The bearing makes slow oscillating oralignment movements under load.

3. The bearing rotates under load at very slowspeed and is only required to have a short

life (the life equation in this case, for a givenequivalent load P would give such a lowrequisite basic dynamic load rating C, thatthe bearing selected on this basis would besubjected to considerable overloading inservice).

4. The bearing rotates and, in addition to thenormal operating loads, has to sustain heavyshock loads which act during a fraction of arevolution.

Additional guidance can be found in the SKFGeneral Catalogue or contact SKF applicationsEngineering Service for assistance.

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Correct Fits

Bearing manufacturers have, over many years ofdevelopment and experience, established arange of shaft and housing fits that apply to thevarious bearing applications. To achievesatisfactory bearing performance, it is importantthat the specified shaft and housing fits areapplied and bearings with the correct clearancevalues are used.

Types of Rotation

It is important to establish the type of rotation,i.e. which ring rotates and which is stationary,as this defines what type of fit is required foreach ring. To prevent creep, which can damagethe bearing and the shaft or housing, the rotatingring requires an interference fit, and thestationary ring a transition or “loose” fit. Forvibration applications and out-of-balance loads,the stationary ring can also require aninterference fit. This type of application isreferred to as “direction of rotationindeterminate.”The radial location of the bearing rings isextremely important to the performance of thebearing. The rings are relatively thin andwithout proper support from the shaft andhousing will distort and result in early failure.

Effect of Loose Fit

The effect of only a 0.01 mm clearance betweenshaft and inner ring bore can be very significantin a rotating inner ring application. If there isno wear, which would increase the effect, a

26

relative movement of approximately twokilometers will occur over 30 days in theexample given. In practice, wear would occur,and the movement would increaseproportionally.

Axial Location of Rolling Bearings

Axial location of the shaft must be achieved inall bearing arrangements. This is usually doneby axially fixing one bearing and allowing axialfreedom to the other bearing(s). This allows forchanges in length of the shaft caused by thermalexpansion or contraction. In some applications,this expansion is accommodated by the type ofbearings used; for example, cylindrical rollerbearings. Often the bearing outer ring is a looseor transition fit to allow the complete bearing tomove axially as required. This is anotherimportant reason why the correct fit must beused.

Axial Freedom in a Fan

This fan is a good practical example of how therequirement for axial freedom in the non-locatedbearing housing is achieved. The graphic shows

the method of axial locating the fixed bearing bythe use of locating rings. It must be stressedthat only one bearing per shaft section is locatedthis way.

Minimum Load

Although rolling bearings are primarilydesigned to minimize friction, they still requiresome friction to function correctly. They can beconsidered a friction gear machine.To ensure there is sufficient traction within thebearing to overcome the resistance to rolling, aminimum load condition must apply. This isusually achieved by the weight of the shaft.However, resistance to rolling can be affectedby all of the following: the drag of the lubricant(particularly in very cold conditions); thefriction within the cage pockets; the inertia ofthe rolling elements; and the amount of internalclearance in the bearing.

SKF Bearing DesignationsThe basic designation identifies:• Product type• Standard design• Size

The supplementary designation identifies:• Alternative design (variants)• Bearing components• Types of special bearings

27

Supplementary designations identify variationsand changes from standards, and can be specificto the manufacturer.

Width and Diameter Variations

Bearings can be designed with proportions thatvary significantly. For example, changes madein the width and/or height for a given borediameter will result in a needle roller bearing,rather than a cylindrical roller bearing. Toenable economic production of high qualitybearings, it is necessary to limit the number ofvariants to a standard range.

ISO StandardsManufacturers and users of rolling bearings are,for price, quality, and ease of replacementreasons, only interested in a limited number ofbearing sizes. The International Organizationfor Standardization (ISO) has thereforedetermined Dimension Plans for the boundarydimensions of metric rolling bearingsThe ISO Dimension Plan for radial bearings(except taper roller bearings) contains aprogressive series of standardized outsidediameters for every standard bore size arrangedin Diameter Series 7, 8, 9, 0, 1, 2, 3 and 4 (inorder of ascending size). Within each DiameterSeries, different Width Series have also beenestablished (Width Series 8, 0, 1, 2, 3, 4, 5, 6and 7 in order of increasing width). The Width

Series for radial bearings correspond to theHeight Series for thrust bearings (Height Series7, 9, 1 and 2 in ascending order of bearingheight).By combining a Diameter Series with a Widthor Height Series, we arrive at Dimension Series,designated by two figures. The first figureindicates the Width or Height Series, the secondthe Diameter Series.In the ISO Dimension Plan for single row metrictaper roller bearings, the boundary dimensionsare grouped for certain ranges of the contactangle (alpha), known as the Angle Series (AngleSeries 2, 3, 4, 5, 6 and 7 in order of increasingangle). Based on the relationship betweenoutside and bore diameters, and between thetotal bearing width and the cross-sectionalheight, Diameter and Width Series have alsobeen established. Here, Dimension Series areobtained by combining the Angle Series with aDiameter and a Width Series. These DimensionSeries are designated by a combination of onefigure (for the Angle Series) and two letters (thefirst for the Diameter Series, the second for theWidth Series).With very few exceptions dictated by rollingbearing development, the bearings listed in theGeneral Catalogue comply with ISO DimensionPlans. Experience has shown that therequirements of the vast majority of bearingarrangements are met using bearings with thesestandardized dimensions.Some sizes of English (non-metric) dimensionbearings are still being manufactured.

Chamfer DimensionsThe chamfer dimensions in the radial direction(r1, r3) and axial direction (r2, r4) quoted in thebearing tables are the prescribed minimumvalues according to ISO 582-1979 and ABMA20 (i.e. nominal values are no longer given).The appropriate maximum chamfer limitsimportant when dimensioning fillet radii arefound in the tables under “Limits for chamferdimensions” in the General Catalogue.Interchangeability of bearings with earlier andnew chamfer dimensions is assured provided the

28

fillets have been designed in accordance withthe recommendations contained in SKFpublications.

Diameter Series

The ISO Dimension Plans state that for eachstandard bore size, a series of outside diametersapply. These are arranged in ascendingorder:Radial bearings Diameter Series = 8, 9, 0, 1, 2, 3 and 4Taper roller bearings Diameter Series = 9, 0, 2 and 3Single row thrust bearings Diameter Series = 0, 1, 2, 3, 4 and 5

Relationship: Diameter and WidthSeriesFor each Diameter Series, there is a series ofbearing widths arranged in ascending order:Radial bearings Width Series = 0,1, 2, 3, 4, 5 and 6Taper roller bearings Width Series = 0, 2 and 3Thrust bearings Height Series = 7, 9 and 1, which correspond to the radial bearing Width Series

Dimension Series

Width Series

Dia

met

er S

erie

sThe combination of a Diameter Series with aWidth or Height Series results in a DimensionSeries, e.g.

Dimension Series 02 is the Width Series 0and the Diameter Series 2.Dimension Series 13 is the Width Series 1combined with the Diameter Series 3.

How the Designation System Works

Most bearing designations consist of fivefigures. As a rule, the first figure indicates the

type of bearing, the second figure the Width

Series and third figure the Diameter Series. Thetwo combined represent the Diameter Series.The first three figures together form the bearingseries designation.This combination therefore defines each bearingto a particular bearing series. Note in somecases the figure indicating the bearing typeand/or the first figure in the Dimension Series is

29

omitted. Where these omissions apply is shownin brackets in the illustration.The last two figures represent the bore diameterin mm divided by five. This rule applies tobearings with bore diameters of 20 to 490 mm.For bearings with a bore size less than 10 mm orequal to/greater than 500 mm, the actual boresize is separated from the basic designation by astroke, e.g. 618/8 (d = 8 mm), or 230/630 (d =630 mm).For bearings with a bore of 10, 12, 15, or 17mm: 00 = 10 mm; 01 = 12 mm, 02 = 15 mm; 03= 17 mm.

Bearing Type Designations:Cylindrical Roller Bearings

This is the basic bearing type designation for thecylindrical roller bearing. The letters indicatethe various designs, and are followed by three orfour figures.

Inch Size Taper Roller Bearings: Cupsand Cones

This is a section through a taper rolling bearingassembly showing that same-series cups andcones are interchangeable. Note that thecomplete bearing consists of two separate parts:

The outer ring (cup) and the inner ring androllers/cage assembly (cone). Each has aseparate designation. The designations of boththe cone and cup – separated by an obliquestroke – are required for a complete bearing.SKF inch size taper roller bearing designationsare preceded by a K, then letters to define thecapacity of the bearing, and then the seriesdesignation.

Suffixes: Seals and ShieldsShields Low-friction seals Seals

-Z and -2Z -RZ and -2RZ -RS1 and -2RS1

Contaminantexclusion

Silentrunning

Greaseretention

Speedcapability

The importance of suffixes to the overallbearing designation is illustrated in these deepgroove ball bearings examples. The outer ringis modified at the manufacturing stage to acceptthe seal or shield.Z or 2Z denotes one or two shields made ofsheet steel. They provide non-rubbingprotection, and the only temperature restrictionis the limitation imposed by the grease used.RS or 2RS indicate one or two seals made ofsheet steel and a rubberized cloth, whichcontacts the inner ring. This arrangement willwithstand temperatures up to 80 °C (176 °F).RS1 or 2RS1 consists of nitrile rubber moldedto a reinforcing plate, which can toleratetemperatures between –20 to +100 °C (–4 to+210 –F).

30

Characteristics of Different Seals

The N suffix bearing has an outer ring machinedto accept a snap ring. The suffix NR means thatthe bearing comes complete with a snap ring.The ring is designed to simplify the axiallocation of the bearing.

Cage Types

Cages are manufactured using differentmaterials and methods. Which of these areused depend on economic and bearingapplication considerations. For example, thecharacteristics of plastic cages make them idealfor many bearings due to their flexibility andlow weight and friction. However, high toolingcosts restrict their use to bearing types producedin high volume.How the cage is located is very important. Forinstance, in vibrating equipment applications,using an outer ring land-riding cage cansignificantly reduce stresses in the cage.Another example is the taper roller bearing,

where a rolling element centered cage is neededto prevent the rollers from falling out.

Tapered Bore Suffixes

When the letter K is used as part of the bearingsuffix, the bearing has a 1:12 tapered bore. Thebore is sized at the small end of the taperaccording to normal convention.When the suffix K30 is used, the bearing has astandard bore taper of 1:30; this is applied to thevery wide series spherical roller bearings, asshown.

Suffix K

Suffix K30

Taper 1:12

Taper 1:30

31

Date Codes

Letter Year Year Year Year Letter Year Year Year Year

A 1910 1934 1958 1981 N 1922 1946 1970 1993B 1911 1935 1959 1982 O 1923 1947 1971 1994C 1912 1936 1960 1983 P 1924 1948 1972 1995

D 1913 1937 1961 1984 R 1925 1949 1973 1996E 1914 1938 1962 1985 S 1926 1950 1974 1997F 1915 1939 1963 1986 T 1927 1951 1975 1998

G 1916 1940 1964 1987 U 1928 1952 1976 1999E 1917 1941 1965 1988 V 1929 1953 – –J 1918 1942 1966 1989 W 1930 1954 1977 2000

K 1919 1943 1967 1990 X 1931 1955 1978 2001L 1920 1944 1968 1991 Y 1932 1956 1979 2002M 1921 1945 1969 1992 Z 1933 1957 1980 2003

The Year Letter to be marked on a bearing or a bearing accessory shallcomprise a single letter in accordance with the table below

Due to SKF’s continual product development indesign, steel quality and manufacturingmethods, rolling bearings are regularly up-ratedfor higher capacity and improved speedcapability. Therefore, it is important to use thecorrect bearing for the application.Beware of old stock being used on newequipment. Date codes are indicated on allbearing boxes.

Tolerances (precision classes) for SKFBearings

All standard SKF bearings are manufactured toa very high standard of accuracy. It is certainthat they are among the most accurate andprecisely made bearings available in theindustry. Even so, for certain applications, suchas high speed machine tools, even higher levelsof precision are produced. These high precisionproducts are identified by a suffix following aseparating slash in the designation.

Comparative Precision Classes

Rolling bearings are made to a range ofprecision classes which take into account thedemands of the many and varied applications.For example, a standard fan application has amuch lower precision requirement than agrinding spindle.The precision classes range from (N) thestandard quality, to the very high ABEC 9 (SKFequivalent is PA 9).Significantly tighter tolerances exist betweeneach precision class. These affect both internaland external bearing dimensions, and greateraccuracy of shaft and housing is required.Bearing manufacturers’ recommendations onthe tolerance grades required for these higherquality bearings must be applied.

Tolerance DefinitionDefinition of a Tolerance: PermissibleLimit for the Variation of Size orDimensionAll manufactured parts are made to a nominal orbasic size. Limits must be applied to any sizevariation to ensure acceptable performance.

32

Tolerance Grades TableThe above graph represents the ISO range oftolerances in general engineering use. Part ofthis system is used for shaft and housingtolerances.Along the top of the matrix are the tolerancegrades identified by number (IT-01 through IT-16). With each grade increase, the tolerancevalue shown in microns increases by about 50percent. Down the left hand side the range ofnominal sizes are shown graded into 20 stepsfrom 0.0 mm to 3,150 mm.Examples:

The tolerance value on a nominal size of 100mm IT6 is 0,022 mmThe tolerance value on a nominal size of 200mm IT9 is 0,115 mm

How to Specify TolerancesThis illustrates some examples of specifictolerances:States the nominal dimension

The letter defines the tolerance position tozero (it defines the fit).The figure (IT tolerance band) represents thetolerance in microns.

Tables will be needed to translate thisinformation into actual dimensional values.

Roller Bearing

SKF Center for Learning – Bearing Technology 33

Chapter 2 - Ball Bearings

Ball Bearing Types

Deep groove ball bearings are used in aparticularly wide variety of applications. Theyare simple in design, non-separable, capable ofoperating at high and even very high speeds,and require little attention or maintenance inservice. These characteristics coupled with aprice advantage make deep groove ball bearingsthe most popular of all rolling bearings.A variation of the Single Row Deep Groove(SRDG) is the max type bearing, which isdiscussed later in this chapter. There are alsoseveral other ball bearing variants, which are notcovered in this course. Please refer to the SKFcatalog for details or contact your local supplier.

Ball Bearing Capabilities

The angular contact ball bearing has uni-directional load capability. For this reason, it isusually mounted in pairs in a back-to-back or

face-to-face arrangement. The self-aligning ballbearing’s dynamic alignment capability makes itan indispensable type for applications whereseparate housings are used, and significantmisalignment is present.

Single Row Deep Groove Ball Bearings

SKF single row deep groove ball bearings havedeep, uninterrupted raceways and a high degreeof conformity between the balls and raceways.This enables them to carry axial loads in bothdirections in addition to radial loads, even athigh speeds. Single row deep groove ballbearings are suitable for a variety ofapplications and are produced in a large numberof sizes and designs.SRDGBBs accept radial, axial, and combinedloads. Their thrust load limit is about 25 percentof the rated radial load. SRDGBBs areavailable in shielded and sealed designs forcontaminant exclusion. They accept only veryslight misalignment: ten minutes of arc (1/6th ofa degree) or less. Some SRDGBB sizes areavailable with filling slots for maximum

Roller Bearing


capacity. Typical applications include pumps,motors, agricultural equipment, and sports andrecreational equipment (in-line skates,skateboards). SRDGBBs are the most widelyused bearing type.Single row deep groove ball bearings of thebasic design, i.e. open at both sides, areproduced by SKF in several series and in a verywide range of diameters. Those bearings of thebasic design which are also produced asshielded or sealed bearings may, formanufacturing reasons, have recessed outer ringshoulders to take the shields or seals.

SRDGBB Sample Nomenclature• Example: 6217-2Z/C3GJN

– 6 - Single row deep groove ball bearing,Conrad style

– (0)2 - “Light” dimension series

– 17 - Multiply by 5 to get the bore in millimeters(works for bore designations 4-96)

– 2Z - Two shields

– C3 - Greater than standard radial internalclearance

– GJN - Special SKF approved grease

Sizes less than xx04:00 = 10 mm01= 12 mm02 = 15 mm03 = 17 mmOver 480 mm, a “/” is used: for example:619/560.The Conrad method is used to assembleSRDGBBs. The outer ring is distorted brieflyduring assembly to allow the introduction ofadditional balls.

Single Row Deep Groove ClosuresShields Low-friction seals Seals

-Z and -2Z -RZ and -2RZ -RS1 and -2RS1

Contaminantexclusion

Silentrunning

Greaseretention

Speedcapability

Bearings with shields or seals (also calledclosures or caps) at both sides are supplied asstandard filled with a grease which has goodrust inhibiting properties and which is suitablefor operating temperatures between -30 and+110° C (-20 and +230°F). The bearings arelubricated for life and maintenance-free.Cleaning is not required before installation. Formaintenance-free operation over long periods,the sealed bearings of the wide series 622, 623and 630 are particularly suitable.The quantity of grease supplied is appropriate tothe size of bearing and normally fills between25 and 35 percent of the free space in thebearing.

Metallic Shields (2Z):Bearings fitted with metallic shields offer fairgrease retention and the lowest frictional torqueof any bearing with closures. They provide fairsealing performance (particles > 0.0006”) andgenerally have the same speed rating as theequivalent open bearing.

Low Friction Seals (2RZ)Bearings of series 618, 619, and some smallsizes of series 60 and 62 are equipped with RZseals. They were developed to meet therequirements for positive sealing with aminimum of friction. The lips of the seals forma very narrow gap with the land of the inner ringshoulder, so the seals are essentially non-rubbing. Because of the low friction, the seals

Roller Bearing


permit operation at the same speeds as bearingsfitted with Z shields. Their sealing performanceis superior to that of Z shields. They offer goodgrease retention and low friction torque. RZsupplied bearings offer good sealingperformance (particles > 0.004”) and generallythe same speed rating as the equivalent openbearing.

Contact Seals (2RS1)Bearings containing rubbing seals withdesignation suffix RS1 or 2RS1 that seal againsta recess in the inner ring side face or against thecylindrical surface of the inner ring shoulder arethe most common. The standard seals are madeof nitrile rubber (NBR) with sheet steelreinforcement. For applications wheretemperatures or environmental conditions aresuch that the standard seals are unsuitable, sealsmade of other materials are also available. Theseals are inserted in recesses in the outer ringand provide good sealing at this positionwithout deforming the outer ring.With sealed bearings operating under extremeconditions, e.g. high speeds or hightemperatures, grease may leak out between thelip and the inner ring. For applications wherethis can be a disadvantage, it is necessary tomake other design arrangements. RS sealsprovide excellent grease retention and sealingperformance at the cost of high frictional torqueand reduced speed rating.

Bearings with Snap Ring GrooveDeep groove ball bearings with a snap ringgroove in the outer ring simplify arrangementdesign in many cases, as they can be retained inthe housing by a snap ring. This method ofaxial location is simple and saves space.Appropriate snap rings are listed in the bearingtables and can be supplied separately or alreadymounted on the bearings.

Deep Groove Ball Bearing SpeedRatings

Note that the ultimate speed limit is based onthe maximum centrifugal forces the cage canwithstand.

MisalignmentSingle row deep groove ball bearings have alimited ability to accommodate alignmenterrors. The permissible angular misalignmentbetween inner and outer rings, which will notproduce inadmissibly high additional stresses inthe bearing, depends on:• The radial internal clearance of the bearing

during operation• The bearing size• The bearing’s internal design• The forces and moments acting on itDue to the complex relationship between thesefactors, exact values for permissiblemisalignment cannot be provided. However,under normal service conditions, it is usuallybetween two and ten minutes of arc. Note thatmisalignment of the bearing rings causes anappreciable increase in noise level when thebearing is running.Double row deep groove ball bearings can onlyaccommodate angular misalignment up to abouttwo minutes of arc. Larger misalignmentsubjects the balls and raceways to increased andunacceptable loading, shortening bearing life.

TolerancesSKF single row deep groove ball bearings withan outer diameter less than or equal to 52 mm

Roller Bearing


are produced to P6 (ABEC2) tolerances asstandard. Bearings with an outer diametergreater than 52 mm are produced to Normaltolerances as standard. The availability ofbearings with higher accuracy should always bechecked before ordering.Large bearings with a bore diameter ofapproximately 400 mm and above havedimensional accuracy to tolerance class Normal,and running accuracy to tolerance class P6 asstandard.

Internal ClearanceSKF single row deep groove ball bearings areproduced with Normal radial internal clearanceas standard. Many of the bearings, particularlythe smaller sizes, are also available with radialinternal clearance greater or less than Normal.Check availability of bearings with clearanceother than Normal before ordering.

Maximum Capacity Ball Bearings

Maximum Capacity Ball Bearings, also called“max type” or “filling slot” ball bearings, referto extra balls added through notches in inner andouter rings. More rolling elements in the sameenvelope allow for a greater Catalog LoadRating. Typical applications include balancedmotors and pumps. Don’t thrust load filling slotbearings; doing so may cause the balls to rollover filling slots, increasing noise and reducinglife. Filling slot designs have even lower

misalignment capabilities (2-5 minutes of arc),and speed is limited compared to standarddesigns.

Basic DesignBearings of the basic design are open at bothsides. For manufacturing reasons, openbearings have recesses in the outer ringshoulders that are intended to take the shields.

Bearings with ShieldsDeep groove ball bearings with filling slots arealso available with a shield (non-rubbing seal) atone or both sides. Pressed steel shields form agap with the land of the inner ring shoulder.Bearings with shields at both sides are suppliedfilled with a grease that is suitable for theoperating temperature range -30 to +110° C (-20and +230°F). The bearings are lubricated forlife and maintenance-free. They should not beheated prior to mounting and must never bewashed.The grease quantity is appropriate to the bearingsize, and normally fills 25 to 35 percent of thefree space in the bearing.

Bearings with snap ring grooveDeep groove ball bearings used primarily inautomotive gearboxes and located in a limitedspace are supplied with snap ring groove in theouter ring together with snap ring. This methodof location saves space and offers designadvantages. In addition to the open bearings,bearings with one shield are available in thisdesign.

MisalignmentConditions pertaining to the possiblemisalignment of the inner ring relative to theouter ring correspond to those of the standardsingle row deep groove ball bearings. However,due to the filling slots, the permissiblemisalignment is limited to between two and fiveminutes of arc. Greater misalignment mightallow the balls to roll over the edge of the filling

Roller Bearing


slots. This would shorten bearing lifeconsiderably and cause noisy operation.

Internal ClearanceDeep groove ball bearings with filling slots aremanufactured with Normal radial internalclearance as standard. Alternatively, bearingswith a radial internal clearance greater orsmaller than Normal can be supplied.

Self Aligning Ball Bearings

The Self-aligning Ball Bearing (SABB) has thelowest friction of any bearing type. SABBs alsohave a high misalignment capacity, up to threedegrees depending on the series. SABBs arecapable of light to moderate radial load capacityand accept only very light thrust loads. SABBsare used in pairs or with Spherical Roller orCARB Bearings in fan and blower applications.SKF was started when Sven Wingquist inventedthe SABB in 1907. Other SABB variantsinclude designs with seals, tapered bore, or extracapacity.Self-aligning ball bearings have two rows ofballs with a common sphered raceway in theouter ring. This gives the bearings their self-aligning property, permitting angularmisalignment of the shaft relative to thehousing. They are, therefore, particularlysuitable for applications where misalignmentcan occur from errors in mounting or from shaftdeflection.For applications where the load carryingcapacity of self-aligning ball bearings isinsufficient, spherical roller bearings whichhave the same self-aligning property should beused instead.

SABB Nomenclature

Bearing Series

E-2RS1 E-2RS1

Light series Heavy series

SABBs have the lowest friction of any bearingdesign due to unique ball-to-raceway contact.

Basic DesignSelf-aligning ball bearings of the basic designare available with cylindrical and tapered bore(taper 1:12).For self-aligning ball bearings with a taperedbore, SKF supplies appropriate adapter sleevesfor securing the bearings simply and rapidly to

Roller Bearing


smooth or stepped shafts. The adapter sleevesare supplied complete with nut and lockingdevice.

Sealed BearingsSKF self-aligning ball bearings are alsoavailable in a sealed (2RS1) version withrubbing seals on both sides. Seals are made ofoil and wear-resistant synthetic rubber and havesheet steel reinforcement. The operatingtemperature range for the seals, with a suitablegrease, is -40 to +120° C. The outside diameterof the seal is retained in a recess in the outerring, giving a tight seal without deformation.The lip of the seal exerts a slight pressureagainst a recess in the inner ring shoulder.Sealed bearings are supplied as standard filledwith a lithium base grease which is suitable forthe operating temperature range -30 to +110° C.The bearings are lubricated for life and aremaintenance-free.

Radial Internal ClearanceSKF self-aligning ball bearings are availablewith Normal radial internal clearance asstandard. Bearings with a tapered bore can alsobe supplied as standard with a C3 radial internalclearance. Bearings having clearances that aregreater or less than standard are available onrequest.

Cage Designs

CagesSKF self-aligning ball bearings are suppliedwith a variety of cages. See the General Catalogfor more information. Bearings with glass fiberreinforced polyamide 6.6 cages (designationsuffix TN9) can be used at operatingtemperatures up to +120° C, with a suitable

lubricant. For bearing arrangements thatoperate continuously at elevated temperatures,or where conditions are difficult, bearings thatnormally have a polyamide cage can be suppliedwith a machined brass cage instead. Check foravailability before designing these into yourapplication.

Why Polyamide Cages?Although some might consider “plastic” cagestoo flimsy for use in bearings, today’spolyamide cage formulations have excellentproperties for use in bearings. Their primarylimitation over metallic cages is temperature.See the General Catalog for specific temperatureratings.

Lightand

flexible

Smoothrunning

Favourablein vibratingapplications

Longbearing

service life

Very highspeed rating

Gooddamping

properties

Allowsoptimiseddesign (E)

Lowoperating

temperatureIncreasedcarryingcapacity

Types of Misalignment

MisalignmentSelf-aligning ball bearings are designed toaccommodate angular misalignment of onebearing ring relative to the other withoutdetrimental effect. Guideline values for thepermissible misalignment under normaloperating conditions are given in the GeneralCatalog. Whether or not the maximumpermissible misalignment can be utilizeddepends on the design of the bearingarrangement, the type of seal, etc.

Roller Bearing


Special FeatureSelf-aligning ball bearings of series 14 as wellas some sizes of series 12 (K) and 13 (K) haveballs that protrude somewhat from the bearing.This should be considered when designingadjacent components.

Difference between Self Aligning BallBearings and Y Bearing UnitsY-Bearing Units are designed to accommodatestatic misalignment, while SABBsaccommodate static and dynamic misalignment.

Different Mounting Methods

Mounting Bearings with Tapered BoreBearings with a tapered bore are alwaysmounted with an interference fit on the shaft,adapter sleeve, or withdrawal sleeve. As ameasure of the degree of interference of the fit,either the reduction in radial internal clearanceof the bearing or the axial displacement of theinner ring on the tapered bearing seating can beused. The mounting of self-aligning ballbearings with tapered bore calls for experienceand skill, as they have a relatively small internalclearance, and a reliable measurement of theclearance reduction is not always possible.When mounting bearings with Normal radialinternal clearance, it is generally sufficient tocheck clearance reduction during the drive-upby turning and swiveling out the outer ring.When the bearing is properly mounted, the outer

ring can be easily turned but there should be aslight resistance when the ring is swiveled out.The bearing will then have the requisiteinterference fit. However, in some cases, theresidual internal clearance may be too small forthe application, and a bearing with C3 radialinternal clearance should be used instead.When mounting bearings with C3 clearance, thetightening angle (alpha) or the axialdisplacement can be used to measure the degreeof interference. These procedures also be usedto mount bearings with Normal clearance.An easy method of mounting bearings onadapter sleeves is based on the tightening angle(alpha) through which the nut is turned.Detailed procedures are available in the BearingMaintenance Handbook and later in thispublication.

Applications: Separator

The separator is a tough application well suitedto the SABB. A standard single row, deepgroove ball bearing could certainly handle thehigh-speed requirement, however, service lifewould be unacceptably low from the dynamicrotor flexing in the application.

Roller Bearing


Application: Air Fans

Air handling equipment often requires specialengineering work due to the nature of theapplication. Fan shafts are often oversized tomeet stiffness demands while speedrequirements are high for efficient operation.The Self Aligning Ball Bearing is often thebearing of choice due to its low friction andcompact size.

Angular Contact Ball Bearings

Angular Contact Ball Bearings (ACBB) are thefirst bearing type we have considered that isdesigned to accept both radial and axial loadssimultaneously. To accept these combinedloads, the bearings are built with specialraceway geometry. To understand the conceptmore easily, it is useful to understand theinfluence of contact angles within differentbearing types.Angular Contact Ball Bearings are available intwo types, single and double row designs.Single Row Angular Contact Ball Bearings(SRACBB) are typically used in pairs.

Double Row Angular Contact Ball Bearings(DRACBB) are also available in maxconfiguration (filling slot), and with closures(seals or shields) for contaminant exclusion.Typical Angular Contact Ball Bearingapplications include pumps and vertical electricmotors.

Single Row Deep Groove (SRDG) ball bearingsare designed with a zero contact angle. Theycan accept heavy radial loads perpendicular tothe shaft. Under this pure radial load, thecontact angle within the bearing is zero. Oncewe begin to add an axial (thrust) load to thebearing, the axis of rotation of the ball departsfrom the vertical. A contact angle is developedwithin the bearing. As axial load increases, thecontact angle increases. Eventually, as thethrust load increases, the bearing generates toomuch friction and heat from the high axialloading. Service life is reduced to anunacceptable value, and new designs must beadopted to accommodate the thrust loads.Angular Contact Ball Bearings are one answer.Angular Contact Ball Bearings (ACBB) areavailable with contact angles from 12 - 60degrees. Standard industrial applications, suchas pumps and gearboxes, generally use bearingswith contact angles in the 29 to 40 degree range.Machine tools are often found with 15 and 25degree ACBB designs.Pure thrust ball bearing designs (90 degreecontact angle) are used in applications whereradial load is minimal.

Roller Bearing


Single Row Angular Contact BallBearings

Single Row Angular Contact Ball Bearings(ACBB) can accommodate both radial andthrust loads in a single direction. Each bearingis universally ground for paired mounting and isavailable with various preload and clearanceranges. SKF SRACBB are produced toABEC-3 tolerances. The bearings are availablewith a variety of cage styles and materials, andcan operate at high speeds.

Bearings for Universal MatchingSome bearings are specially manufactured sothat, when mounted in random order butimmediately adjacent to each other, thepredetermined value of axial internal clearanceor preload, or an even distribution of load willbe attained, without shims or similar deviceshaving to be used.

Single Row Angular Contact BallBearing Arrangements

Paired MountingPaired mounting is used when the load carryingcapacity of a single bearing is inadequate(tandem arrangement) or when the bearingarrangement must carry axial loads in bothdirections (back-to-back and face-to-facearrangements). When arranged in tandem, the

load lines are in parallel and the radial and axialloads are evenly divided between the bearings.The bearing pair is only able to accommodateaxial loads in one direction; therefore, a thirdbearing is generally adjusted against the pair tocarry the axial loads acting in the oppositedirection.The load lines of bearings arrangedback-to-back diverge towards the bearing axis.Axial loads acting in both directions can beaccommodated, but only by one bearing in eachdirection. Bearings mounted back-to-backprovide a relatively stiff arrangement which canalso accommodate tilting moments.The load lines of bearings mounted face-to-faceconverge towards the bearing axis. Again, axialloads acting in both directions can be carried,but only by one bearing in each direction. Thisarrangement is not as stiff as the back-to-backone and is less suitable for the accommodationof tilting moments.Back-to-back is the most common arrangementproviding greater stiffness than the face-to-facearrangement.Bearings should be replaced in the sameconfiguration used by the manufacturer. Threeor more bearings may be used in variousarrangements to achieve the desired mix ofthrust and radial load carrying capacity.Do not mix sets of Angular Contact BallBearings from different manufacturers.Methods for achieving preload or clearance varywidely. Service life will be shortened.Radial load capacity of bearing sets is less thanwith the same number of single bearings.Minimum axial loads should be applied toprevent ball skidding. This may result in noise,vibration, heat, and reduced service life.

Roller Bearing


SRACBB Series

SKF and MRC SRACBB follow ISOdimensional guidelines. Some Manufacturersstill produce a 74xx SRACBB series. Highprecision SRACBB for machine tools areavailable in 70xx, 718xx and 719xx series.Contact SKF for details and catalogs.

Bearing Preload and Static LoadDeflection

When designing the SRACBB to be used inpairs or other set arrangements, preload andclearance curves from each bearing must be

used to achieve maximum design life. Actualservice life can be greatly reduced throughimproper pump operation (insufficient pressurehead, cavitations, etc.) or poor installationpractices (misalignment, pipe strain). ConsultSKF for assistance in achieving maximumservice life in your ACBB applications.

Bearing Preload, Spring Loaded

In some cases, the preload or clearancesavailable are insufficient to ensure properbearing set function throughout the application’s

Roller Bearing


operational regime. Spring preloading can beuseful in these applications. If the spring isdesigned to act on the outer ring of one of thetwo bearings, this outer ring must be capable ofaxial displacement. The preload force remainspractically constant even when there is axialdisplacement of the bearing as a result ofthermal expansion.It is also important when adjusting bearings topreload in an arrangement, that the establishedvalue of the preload force, determined either bycalculation or by experience, is achieved withthe least possible scatter. For example, forbearing arrangements with taper roller bearings,the bearings should be turned several times inboth directions during adjustment so that therollers do not skew and the roller ends are incorrect contact with the guide flange of the innerring. If this is not the case, the inspection ormeasurement results will be false and the finalpreload can be much smaller than the requisitevalue.

Clearance and Preload OptionsClearance designations§ CA - Axial clearance less than normal§ CB - Normal axial clearance§ CC - Axial clearance greater than normal

Preload designations§ GA - Light preload§ GB - Medium load§ GC - Heavy load

SKF produces six varieties of clearance (CA,CB, CC) or preload (GA, GB, GC) in ourSRACBB. Not all sizes are produced in allvariations. Check with your distributor beforespecification in applications.

Design of Bearing ArrangementsWhen designing bearing arrangements usingsingle row angular contact ball bearings, it isnecessary to pay attention to the specialcharacteristics of the bearings. Due to theirinternal design, they cannot be used singly.

They must be used either with a second bearingor as bearing sets.

MisalignmentSingle row angular contact ball bearings havelimited ability to accommodate errors inalignment. Their relationship betweeninfluencing factors is as complex as for singlerow deep groove ball bearings. When thebearings are paired, particularly in a back-to-back arrangement, angular misalignment canonly be accommodated between the balls andraceways by force, leading to increased ballloads and cage stresses as well as a reduction inbearing life. Any misalignment will also lead toan appreciable increase in running noise fromthe bearing.

Preloaded SRACBBQuestion: Why use preload?Answer: Light load (balanced pump).Problem Area: Bearings may unload due toincreased internal clearance and skid.Possible solutions:7309 BECBM – Bronze Cage may reduce ballmovement and enhance lubrication.7309 BEGBM – Preloading the bearing mayreduce skidding, temperature rise may occurMRC PUMPAC – Reduce the contact angle inbearing that sees less thrust

ACBB Cage Types

Roller Bearing


Cage Type CharacteristicsP: Polyamide Superior lubricity, low

friction, temperature limited.M:Machined Bronze

Excellent lubricity and ballretention, very robust, slightlyhigher cost

J: Pressed Steel Low friction, inexpensiveY: Pressed Brass Low friction, inexpensive.F: Machined Steel Excellent ball retention,

robust, high costBearings with polyamide cages are suitable forthe majority of applications, and can be usedwith a suitable lubricant at operatingtemperatures of up to +120° C.For applications in refrigeration plants usingammonia, brass is an unsuitable material.Bearing manufacturers produce bearings withmachined steel cages for this purpose.Check availability of bearings with cages otherthan standard manufacture before ordering.

MRC PumPac

Special SRACBB: problem solver forapplications with large thrust loads in onedirection.

Thrust loaddirection

Double Row Angular Contact BallBearings

Double Row Angular Contact Ball Bearings(DRACBB) are capable of radial loads, doubledirectional thrust loads, and combined loads.DRACBB are available with shields, seals, andsnap rings. They have an economical, compactdesign that can operate at high speeds. Thisbearing is often used in place of a pair ofSRACBB, due to cost and width considerations.In design, the double row angular contact ballbearings correspond in function to two singlerow angular contact ball bearings arrangedback-to-back, yet are narrower than two singlerow bearings. They can accommodate heavyradial loads and moderate axial loads acting inboth directions. The bearings shown in thissection have a contact angle of 24, 25, or 30degrees, which provides a relatively stiffbearing arrangement that can also take up tiltingmoments. The bearings are non-separable.The Conrad-type assembly method forDRACBB employs a slight elastic deflection of

Roller Bearing


the outer ring to insert the “last ball.” Thisresults in uninterrupted raceways with smoothrunning and significant thrust carrying capacityin both directions.Reminder: This bearing is designed primarilyfor axial loads.

DRACBB Available Styles:

The boundary dimensions of the 52 and 53series bearings conform to those of dimensionseries 32 and 33, respectively, of AFBMAStandard 20 and ISO Standard 15. (While mostdimensions are metric, the bearing width isbased on inches.) Series 54, although not

standardized by AFBMA or ISO, is recognizedas an industry series.

Cages for Double Row AngularContact Ball Bearings

Shielded and Sealed BearingsIn addition to open designs, most double rowball bearings are offered with shields or seals oneither one or both sides. In most sizes, openbearings also contain seal grooves on the innerand/or outer rings.Shields are non-contacting. They form a smallgap with a chamfer on the inner ring. They aremade from pressed steel and are fixed into anouter ring groove. This non-contacting closure

is designed to retain lubricant and exclude largerparticles of foreign matter. These shieldedbearings are primarily intended for applicationswith inner ring rotation. Single shieldedbearings can be used with oil or greaselubrication, while double shielded bearings arepacked with a predetermined quantity of grease,providing maintenance-free operation for thelife of the bearing.Most bearings are also available with either oneor two contacting seals. The seals consist of an

Roller Bearing


oil and wear resistant, non-metallic disk or anelastomer bonded to a metal plate. The O.D. ofeither of these two types of seal is firmly fixedinto an outer ring groove. The lip at the I.D. ofthe seal contacts an inner ring seal chamfer foreffective closure. This seal effectively excludessolid contaminants and moisture from the insidesurfaces of the bearing. The permissibleoperating temperature range for the seals with aproper lubricant is -40 to +120° C. Singlesealed bearings can be regreased, while doublesealed bearings are greased for life.

Snap RingsSnap rings in the outer ring can provide an easymethod of locating the bearing in theapplication. The snap rings and the snap ringgrooves in the outer rings generally conform toABMA Standard 20 and ISO Standard 464.

This graphic illustrates the wide number ofDRACBB variants. Check with your manufacturerfor availability of particular styles.

Filling Slot Type Double Row AngularContact Ball BearingsFilling slot bearing features are similar to thoseof the Conrad type; however, because of thegreater number of balls their radial load carryingcapacity is higher than that of the Conrad type.On the other hand, heavy thrust loads can beaccommodated in only one direction. Thesebearings should be mounted so the predominantaxial load acting on the shaft is directed awayfrom the filling slot.

PrimaryThrust

Filling Slot

Part number markings are normally locatedon either the side face or the O.D. The sideface marking is always on the side oppositethe filling slot, and the O.D. marking is offsetfrom the center away from the side with thefilling slot. Therefore, even double sealed orshielded bearings with the filling slotcovered from view can be oriented correctly.

MisalignmentAngular misalignment can only beaccommodated in double row ball bearingsbetween the ball and raceways by force. Thisproduces increased ball loads and can lead to areduction of bearing life.

Roller Bearing


Double Row Double Row Angular ContactBall Bearing Nomenclature

M suffix denotes Maximum capacity typeDRACBB.

Application: Process Pump

ANSI Standard Pumps are often supplied withDRACBB. The DRACBB accommodates thrustloads in both directions and locates the shaft.

SKF Pump Bearing

The SFK Pump Bearing is a problem solver fortough DRACBB applications where service lifeis compromised by poor operating conditions orproblematical loads. The SFK Pump Bearing isa drop-in replacement for the five most popularsizes of standard 53 and 33 series Double RowACBB. Avoiding the costly retrofit of aSRACBB pair has enhanced thrust capacity (40°angle of contact). ABEC-3 tolerances and “CB”axial internal clearance are standard. The splitinner ring allows additional rolling elements for

Roller Bearing


increased load capacity. The robust machinedbronze cage enhances reliability, especially inpoor lubrication conditions. All sizes aresupplied with a removable snap ring.

Four Point Contact Ball Bearings (QJBearings)Four-point contact ball bearings are single rowangular contact ball bearings with racewaysdesigned to enable axial loads in both directions.They need less axial space than double rowbearings. Four-point contact ball bearings havea contact angle of 35° and a two-part inner ring,allowing a large number of balls to beincorporated, thus providing a high loadcarrying capacity. The bearings are separable,allowing the inner ring halves and the outer ringwith ball and cage assembly to be mountedindividually.

FPACBB are single row ACBB designed toenable axial loads to be carried on bothdirections. They require less space than doublerow ball bearings. They eliminate the need foran additional thrust bearing. FPACBB must bepaired with radial bearings for handling largecombined loads.

Locating SlotsFour-point contact ball bearings are designed toaccommodate predominantly axial loads, andare arranged as thrust bearings with radialclearance in the housing in many applications.To permit simple location and prevent rotationof the outer ring, most bearings with an outsidediameter of 160 mm and above are providedwith two locating slots in the outer ring

CagesFour-point contact ball bearings have an outerring centered cage machined from brass asstandard. Some bearings can also be suppliedwith a ball-centered cage of glass fiberreinforced polyamide 6.6. Availability shouldbe checked before ordering.

Four Point ACBB NomenclatureExample: QJ 217 N2MA/C3QJ - Four point angular contact ball bearing(0)2 - Light duty bearing; (width) diameter series17 - Multiply by 5 gives the bore in “mm” except

for bore sizes less than 20 mmN2 - Anti-rotation notches (2) in outer ringMA - Machine brass cageC3 - Greater than normal axial clearance

Other Pertinent Suffixes:TN9 - Polyamide 6.6 cage (glass fiber reinforced)

Reminder: This bearing is designed primarilyfor axial loads.

MisalignmentFour-point contact ball bearings have only alimited ability to accommodate errors inalignment between outer and inner rings. Therelationships between the factors governing thepermissible values are as complex as for singlerow deep groove ball bearings. It should beremembered that any misalignment would leadto a considerable increase in bearing noise.If four-point contact ball bearings are combinedwith other radial bearings to act as thrustbearings, being mounted with radial clearance inthe housing, no misalignment of the outer ringin relation to the inner ring is possible.

Roller Bearing


FPACBB Application:

Screw Compressor

The screw compressor in this example uses fourcylindrical roller bearings to accommodate thelarge radial loads present in the application. Thelobes of the two shafts mesh closely, yet cannotbe allowed to make contact. The CylindricalRoller Bearings accommodate the high radialloads generated by the compressor, while thefour point contact bearings are used to locate theshafts axially. Note that the FPACBB areradially free in the housing. To permit simplelocation and prevent rotation of the outer ring,all bearings with an outside diameter of 160 mmand above are provided with two locating slotsin the outer ring (N2 design).

Roller Bearing


Roller Bearing Types

Introduction

The basic difference between ball and rollerbearings is the type of contact the rollingelement makes with the raceway. Ballbearings have “point” contact; rollerbearings have “line” contact.

Generally, ball bearings are selected forapplications having light to moderate loads andhigh speeds; roller bearings are used in heavierload conditions with slower speeds.Ball and roller bearings generally follow theISO dimension plan. One significant exceptionis U.S. Taper Roller bearings, which have theirown non-ISO numbering system.

Cylindrical Roller Bearings

The rollers of single row cylindrical rollerbearings with cage are guided between integralflanges on one of the bearing rings. The bearingring with integral flanges and the roller and cageassembly can be withdrawn from the other ring.This facilitates mounting and dismounting,

particularly where both rings requireinterference fits due to load conditions.Single row cylindrical roller bearings have highradial load carrying capacity and high-speedcapability. They are produced in differentdesigns, which differ in flange configuration.Bearings of the NU type have two integralflanges on the outer ring, and an inner ringwithout flanges. The N type has two integralflanges on the inner ring, and an outer ringwithout flanges. Axial displacement of the shaft,with respect to the housing, is permitted in bothdirections within certain limits. For example,changes in length due to thermal expansion canbe accommodated. The bearings are thereforesuitable as non-locating bearings.Cylindrical roller bearings of the NJ type havetwo integral flanges on the outer ring and oneintegral flange on the inner ring, allowing axiallocation to be provided for the shaft in onedirection.Type NUP cylindrical roller bearings have twointegral flanges on the outer ring, and oneintegral and one loose flange on the inner ring.This enables the bearings to locate a shaftaxially in both directions.NJ type bearings, when used with an HJ anglering, can also be used to locate the shaft axiallyin both directions. The combination of an NUbearing with an HJ angle ring will locate theshaft axially in one direction. It is not advisableto fit angle rings at both sides of an NU typebearing, as this can lead to axial compression ofthe rollers.Needle roller bearings are a specialized form ofCRB. Generally, if the roller length is twice thediameter, it is classified as a needle. Englishand metric sizes are available.

Roller Bearing


Cylindrical Roller Bearing Designs

Non-locating

Semi-locating

Fullylocating

Non-locating

MisalignmentThe ability of single row cylindrical rollerbearings to accommodate angularmisalignments of the inner ring with respect tothe outer ring, and tin order to compensate forerrors of alignment, is limited to a few minutesof arc. For SKF bearings with the logarithmiccontact profile, the actual values are:• Four minutes of arc for bearings of the

narrow series 10, 2, 3 and 4• Three minutes of arc for bearings of the

wide series 22 and 23.For these guideline values to apply, thepositions of the shaft and housing axes mustremain constant.

Cylindrical Roller BearingNomenclature

– More and largerrollers

– Logarithmic contactprofile plus optimisedsurface finish

– Improved roller end /flange contact geometry

– 3 cage variants: steel,polyamide, brass

NU NJ NUP

N NJ+HJNU+HJ

Radial Internal ClearanceSKF single row cylindrical roller bearings areproduced with Normal internal clearance asstandard. The majority of the bearings are alsoavailable with C3 radial internal clearance, andsome with the appreciably greater C4 clearance.

Reminder: The complete part number is markedon the removable ring.Example: The outer ring of an NU 217ECM/C3 will be marked 2NU17, and the innerring will be marked NU 217 ECM/C3.

Cage VariantsSingle row cylindrical roller bearings areproduced with a wide selection of cagematerials. Standard cages for various sizes arelisted in the General Catalog. As a rule, thecages of the NJ, NUP, and N type bearings arethe same as those of the NU type bearings.Bearings fitted with glass fiber reinforcedpolyamide 6.6 cages (designation suffix P) canbe used with a proper lubricant at operatingtemperatures up to +120° C. For bearingarrangements that operate at high temperaturescontinuously, or under arduous conditions, apressed steel cage (suffix J) or a machined brasscage (suffix M or MA) is available instead ofthe polyamide cage. Bearings that incorporate apressed steel cage as standard can also beordered with a machined brass cage.Check availability of bearings fitted with cagesother than the standard before ordering.

Roller Bearing


Minimum LoadAll ball and roller bearings must always besubjected to a given minimum load to assuresatisfactory operation. This is also true ofcylindrical roller bearings. Particularly if theyrun at high speeds where the inertia forces of therollers and cage and the friction of the lubricantcan have a detrimental influence on the rollingconditions in the bearing, which may causedamaging sliding movements to occur betweenthe rollers and the raceways.

Dynamic Axial Load CarryingCapacityBearings with flanges on both inner and outerrings can support axial loads in addition toradial loads. Their axial load carrying capacityis not primarily determined by the fatiguestrength of the material, but instead by the loadcarrying capacity of the sliding surfaces at theroller end and flange contact. Thus, it is mainlygoverned by the lubrication, operatingtemperature and heat dissipation from thebearing.

Full Complement Bearings

Full complement cylindrical roller bearingsincorporate the maximum number of rollers and,as a rule, have a low sectional height in

relathion to their width. This produces anextremely high load-carrying capacity, whichallows for space-saving designs. Fullcomplement cylindrical roller bearings aresuitable for very heavy radial loads; however,the different kinematic conditions in the bearingmean that they cannot operate at the same highspeeds as cylindrical roller bearings of theconventional caged type.

Precision Cylindrical Roller Bearings

Available with 1-12 tapered bore, super andultra precision (P4, RBEC-7 and P2, RBEC-9),NN-series double row cylindrical roller bearingsoffer high load capacity, excellent rigidity, andheat expansion capability for high speedmachine tools; these are often paired withspecial precision angular contact bearings forshaft location and thrust load capabilities.Ceramic and coated roller options are availablefor special applications.

Application: Screw Compressor

Roller Bearing


Cylindrical Roller Bearings are a logical choicefor the screw compressor due to their high loadcarrying capacity and speed capability.

Taper Roller Bearings

Taper roller bearings have tapered inner andouter ring raceways between which taperedrollers are arranged.

Geometry

If extended, the tapered surfaces convergetowards a single point on the bearing axis,ensuring optimum rolling conditions. Theirdesign makes them particularly suitable for theaccommodation of combined (radial and axial)loads.

Taper Roller Bearing ComponentsTaper roller bearings are generally of separabledesign, i.e. the inner ring with roller and cageassembly forms a unit that can be mountedseparately from the outer ring.

Taper roller bearing component nomenclaturevaries from traditional rolling bearings: theinner ring and roller assembly is called the cone,while the outer ring is referred to as the cup.Mixing cups and cones from differentmanufacturers results in differences in internalgeometry that can lead to catastrophic failures.

Misalignment

light load

heavy load

heavy load+misalignment

straight circular crowning logarithmic crowning

The ability of single row taper roller bearings toaccommodate angular misalignments of theinner ring, with respect to the outer ring, islimited to a few minutes of arc. Maximummisalignment for SKF TRB is three minutes ofarc. Therefore, it is only possible to compensatefor alignment errors of this magnitude.

Dimension Series

Roller Bearing


Inch SizesInch-size taper roller bearings are designatedwith a system that is completely different fromthe one used for metric taper roller bearings.The basic principles of this designation systemare described in the AFBMA Standard 19.The inner ring with roller and cage assembly,known as the cone, and the outer ring (cup) haveindividual designations. The complete bearingdesignation consists of the cone designationfollowed by the cup designation, the twodesignations being separated by an obliquestroke. The cones and cups belong to various“bearing series.” Any cup belonging to aparticular series can be assembled with any coneof the same series to form a complete bearing,provided they are from the same manufacturer.

Metric BearingsThe boundary dimensions of the metric taperroller bearings listed in the tables confirm toISO 355-1977, except for those bearingscarrying designations prefixed by J. Theseconform to AFBMA Standard 19.1.Single row metric taper roller bearings aremanufactured to normal tolerances as standard.

Load Capability

The axial load carrying capacity of TRB islargely determined by the contact angle (α,alpha), which corresponds to the angle of theouter ring raceway. The larger the angle, thelarger the axial load carrying capacity.

Design Range for Taper RollerBearings

In addition to the single row designs, double andmultirow Taper Roller Bearings are available.

Effects of Axial and Radial Loads

Single row taper roller bearings are able toaccommodate axial loads acting in one directiononly. Under radial loads, an axial force isproduced in the bearing that must becounteracted. Therefore, it is customary for thesingle row taper roller bearings to be adjustedagainst a second taper roller bearing.

Internal ClearanceThe internal clearance of single row taper rollerbearings can only be obtained after mounting. Itis determined by adjustment of the bearingagainst a second bearing providing location inthe opposite direction.

Life Performance and BearingAdjustmentPreloaded bearing arrangements in certainapplications mean enhanced operational

Roller Bearing


reliability and long service lives. A properlydimensioned preload has a favorable influenceon the load distribution in the bearings and,therefore, also on life.When selecting the preload force for a bearingarrangement it should be remembered that thestiffness is only marginally increased when thepreload exceeds a given optimum value,whereas friction and, consequently, heatgeneration increase, and there is a sharp drop inbearing life as a result of the additional,constantly acting load.

Taper Roller bearings achieve optimum lifewhen adjusted properly. The illustration showsthe effects of excess preload or clearance onrelative bearing life. Roller loading increases aspreload increases. Because of the risk which anexcessive preload implies for the operationalreliability of a bearing arrangement, andbecause of the complexity of the calculationsnormally required to establish the appropriatepreload force, it is important to followmanufacturers instructions for Taper Rollerbearing adjustment. Additional guidance foradjustment can be found in the SKF GeneralCatalog and Bearing Maintenance Handbook, orcontact SKF Applications Engineering Servicefor assistance.

CagesSKF taper roller bearings have a pressed steelcage as standard except for the large inch-sizebearings of series 157000, 168000, LL 687900,763000 and H 969200, which incorporate amachined brass cage

Misalignment and Stress Distribution

light load

heavy

heavy load+misalignment

straight circular crowning logarithmic crowning

The ability of single row taper roller bearings toaccommodate angular misalignments of the innerring with respect to the outer ring is limited to a fewminutes of arc. Edge loading is the result of excessmisalignment of taper roller bearings. The graphicshows the effects of various taper roller bearingdesigns and their reaction to load and misalignment.High peak stresses and uneven load distributionoccur in straight and circularly crowned rollers,which can lead to edge loading and reduced servicelife. SKF’s logarithmically crowned roller profilereduces peak stresses and improves load distributionunder all load and misalignment conditions. Longerservice life is the result.

Minimum LoadAll ball and roller bearings must always besubjected to a given minimum load to assuresatisfactory operation. This is also true of taperroller bearings, particularly if they run at highspeeds where inertia forces of the rollers andcage, and the friction in the lubricant can have adetrimental influence on the rolling conditionsin the bearing. Damaging sliding movementsmay occur between the rollers and raceways ifinsufficient minimum load is applied.

Application: Worm Gearbox

Roller Bearing


The taper roller bearings in this worm gearboxapplication were chosen for their ability toaccept combined loads and provide positiveshaft location. Preload adjustment of eachbearing set assures good engagement of the gearteeth. The natural stiffness of the bearing setsresists deflection under the combined loads.Since a high reduction is generally achieved inthe application, input and output speeds varygreatly, demanding careful lubrication analysisof bearing sets and gears. Lubricant additivepackages are often necessary to span the speedand load ranges in such applications.

Spherical Roller BearingsSpherical roller bearings have two rows ofrollers with a common sphered raceway in theouter ring. The two inner ring raceways areinclined at an angle to the bearing axis. Thebearings are self-aligning and consequentlyinsensitive to errors of alignment of the shaftrelative to the housing, and to shaft deflection.In addition to radial loads, the bearings can alsoaccommodate axial loads acting in bothdirections.

Spherical roller bearings are capable of carryinghigh radial loads and moderate axial loads inboth directions.

The sphered outer ring and two rows of rollersgive this bearing heavy radial load-carryingcapacity with good misalignment capability.Spherical roller bearings can accommodatemisalignment of one to 2.5 degrees dependingon the series. Maximum applicationmisalignment is generally limited by the sealingor housing arrangements rather than the bearingalignment capacity. SRBs have up to 50 timesgreater misalignment than other bearing types.Tapered bores allow easy mounting on taperedjournals or sleeves. Spherical Roller Bearingapplications include paper machines, continuouscasters in steel mills, shaker screens in sand &gravel quarries, coal mining equipment, andheavy-duty fans and blowers.SKF Explorer class bearings can give life up tothree times more than other SRBs. See literaturefor details.

Internal ClearanceSKF spherical roller bearings are manufacturedas standard with Normal radial internalclearance. Nearly all the bearings are alsoavailable with the larger C3 clearance and somecan be supplied with the even larger C4 or C5clearances. Some sizes can be delivered withC2 clearance, which is smaller than Normal.Check availability of bearings with radialinternal clearances other than Normal beforeordering.

MisalignmentSpherical roller bearings have a design that isinherently self-aligning. Misalignment betweenthe outer ring and inner ring can beaccommodated without any effect on thebearing. Under normal loads and operatingconditions, and when the inner ring rotates, theguideline values of misalignment given in theSKF General Catalog are permitted.

Appropriate HousingsAppropriate housings for spherical rollerbearings with cylindrical bore and for bearingswith adapter sleeves are available.

Roller Bearing


Minimum LoadAll ball and roller bearings must always besubjected to a given minimum load to assuresatisfactory operation. This is also true ofspherical roller bearings, particularly if they runat high speeds where the inertia forces of therollers and cage, and the friction in the lubricantcan have a detrimental influence on the rollingconditions in the bearing. Damaging slidingmovements may occur between the rollers andraceways.

Axial Load Carrying Capacity ofBearings Mounted on Adapter SleevesIf spherical roller bearings with adapter sleevesare mounted on smooth shafts with no fixedabutment, the magnitude of the axial load thatcan be supported is determined by the frictionbetween the shaft and sleeve.

Spherical Roller BearingNomenclature

Example: 22217 CCK/C3W332 - Double row spherical roller bearing22 - Width and diameter series17 - Multiply by 5, gives the bore in “mm” for bore

sizes over 20 mm and under 480 mmCC - Flangeless inner ring, pressed steel cage designK - 1:12 Tapered boreC3 - Greater than normal radial clearanceW33 - Lubrication groove and three holes in outer ringOther Pertinent Suffixes:E - New extra-capacity version (W33 omitted)M - Machined Bronze Cage

K30 is 1:30 taper on the bore, bearing series240xx, 241xx only.

Popular VariationsPrefixes and suffixes added to the basicdesignations are used to describe bearings thatreflect limited variations in the standard bearing.(Contact SKF for assistance with nomenclaturenot found on this table.) Commonly specifiedvariations used in applications requiring specialbearing design features include:Prefix

ECB Spherical roller bearing, airmelt carburizedsteel, I.R. only

EVB Spherical roller bearing, vacuum meltcarburized steel, I.R. only

EPVB Same as EVB, except with precision tolerancesSuffix

CA Separate guide ring; inner ring retaining flangesCAC Separate guide ring, inner ring retaining

flanges, improved internal designCC Improved internal designCO2 Extra close inner ring running accuracy; normal

internal clearanceCO4 Extra close outer ring running accuracy; normal

internal clearanceCO8 Extra close running accuracy of both inner and

outer ringsC1 Internal clearance less than C2C2 Internal clearance less than Normal

C3 Internal clearance greater than NormalC4 Internal clearance greater than C3C5 Internal clearance greater than C4C10 Reduced I.D. and O.D. tolerance centered on

meanC40 Reduced O.D. tolerance approaching nominal

dimensionE High capacity, improved internal designJ Pressed cage of sheet steelK Tapered bore, taper 1:12 on diameterK30 Tapered bore, taper 1:30 on diameterMC Machined brass cage, inner ring or guide ring

centeredM Machined brass roller centered cage and no

guide ringW4 Hi-point of eccentricity marked on inner ringW20 Three lubrication holes in outer ringW22 Special reduced O.D. tolerance, for outer ring;

normal tolerance for inner ringW26 Six lubrication holes in inner ringW31 Bearing inspected to special quality

requirementW33 Lubrication groove and 3 holes in outer ringW33X Lubrication groove and 6 holes in outer ringW55 Bearing with locating holes in the outer ring

Roller Bearing


W77 Bearing with W33 lubrication holes pluggedW502 Combination of W22, W33W504 Combination of W33, W55W506 Combination of W31, W33W507 Combination of W4, W31, W33W509 Combination of W26, W31, W33W513 Combination of W26, W33W525 Combination of W31, W77Y Pressed cage of sheet brass

Shaker Screen Duty Spherical RollerBearingsShaker Screens and other vibratory applicationsdemand special features not found in standardspherical roller bearings, including:• One-piece, heavy duty, centrifugally cast

machined brass roller centered cagebalanced for dynamic stability

• Increased cage section height providesgreater strength

• Open-end cage construction allows freeaccess of lubricant

• No center flange; improves lubrication, andreduces wear

• Optimum groove geometry, surface finish,and radial internal clearance provide betterlubrication, cooler running, and longer life

• Improved cage pocket design and sphere-ended rollers provide optimum rollerguidance, develop “oil wedge” for superiorlubrication and less wear

• Available with or without W33 lubricationfeature. This feature makes lubrication moreeffective with a circumferential groove and aseries of radial holes in the outer ring

• Reduced O.D. tolerance; W22 now standardfor more precise housing fit-up

• W502 feature available as a combination ofW22 O.D. tolerance plus the W33lubrication feature

• Vacuum-degassed or melt-refined type steelprovided in standard bearing for longer life

• Vacuum-melted type steels available forlonger life applications

• Available in the standard series 4523CACM2 and in the extra wide Series 4533CACM2

Standard SRBs cannot be substituted for ShakerScreen Bearings. The full part number with allprefixes and suffixes is required for preciseinterchanges between various manufacturers.

Shaft and Housing TolerancesA loose shaft fit and an interference housing fitare recommended for vibrating applicationswhen the bearing load rotates with respect to theouter ring, and is stationary with respect to theinner ring.

Spherical Roller Thrust BearingsIn spherical roller thrust bearings, the load istransmitted from one raceway to the other at anangle to the bearing axis. They are suitable (incontrast to other thrust bearings) for theaccommodation of radial loads in addition tosimultaneously acting axial loads. Anotherimportant characteristic of spherical roller thrustbearings is their self-aligning capability. Thismakes the bearings tolerant to shaft deflectionand to errors of alignment of the shaft relative tothe housing.Spherical roller thrust bearings are of separabledesign; the shaft washer with roller and cageassembly, and the housing washer can bemounted individually.

Roller Bearing


Spherical roller thrust bearings can handleheavy thrust loads with some radial load, andmisalignment of two to three degrees dependingon the series. SRTBs are highly susceptible todamage when inadequately loaded. A hardenedsteel spacer may be needed when replacingolder machined bronze caged bearings with thenewer extra-capacity “E” design (pressed steelcage). Contact SKF Customer Service for thesespacers.

MisalignmentDue to their design, spherical roller thrustbearings are self-aligning, i.e. they permit errorsof alignment of the shaft relative to the housing,and a certain degree of shaft bending has noeffect on the bearing. The permissiblemisalignment is reduced as the load increases

Minimum LoadMany applications using SRTBs use springsunder the separable ring to apply minimumloads throughout the machine load cycle.Unloaded rollers can lead to skidding,overheating, and poor performance.To assure the satisfactory operation of all balland roller bearings, they must always besubjected to a given minimum load. This isalso true of spherical roller thrust bearings,particularly if they run at high speeds wherethe inertia forces of the rollers and cage, andthe friction in the lubricant can have adetrimental influence on the rolling conditionsin the bearing, and may cause damaging

sliding movements to occur between therollers and the raceways.

LubricationIt is generally recommended that sphericalroller thrust bearings be oil lubricated. Greaselubrication can be used in special cases, forexample, under light loads and at low speeds,particularly where incorporating a pressedsteel cage.When using grease as the lubricant, it isnecessary to ensure the roller end / flangecontacts are adequately supplied with grease.Depending on the actual application, this canbe best done by completely filling the bearingand its housing with grease, or by regularrelubrication.

Mounted Bearing & Bearing Housing


Due to their internal design, spherical rollerthrust bearings have a pumping action, whichmay be exploited under certain conditions andshould be taken into consideration whenselecting lubrication method and seals.

CARB® Toroidal Roller BearingCapabilities

CARB.AVI

The “Compact Aligning Roller Bearing” is arelatively new bearing type developed by SKF.It combines the self-aligning properties of thespherical roller bearing, the compact section ofthe needle roller bearing, and the ability todisplace axially like the cylindrical rollerbearing. The ability to carry heavy radialloads and accommodate dynamicmisalignment – coupled with a high internalaxial displacement feature – make this aunique bearing.

CARB Accommodation

CARB is available in a range of ISODimension Series, equivalent to self-aligning ball and spherical roller bearingsused in standard bearing housings andother common types of assemblies. The

range also covers wide, low section seriesequivalent to needle roller bearings.

CARB Nomenclature

CARBs are available in a tapered bore, makingthem a drop-in for most spherical rollerbearing sizes. Applications include papermachines, gearboxes, fans, and pinions.Self Aligning Bearing Systems

SKF’s new self-aligning bearing systemconsists of a CARB® toroidal roller bearing asthe non-locating bearing in combination, witha double row spherical roller or self-aligningball bearing as the locating bearing. Thebearing arrangement accommodates bothmisalignment and axial adjustment internallyand without frictional resistance, with nopossibility of generating internal axial forcesin the bearing system. Due to the idealinteraction between the two bearings, theapplied load is always distributed consistentlyand in the assumed (theoretical) mannerbetween all load-carrying elements.The design characteristics of both bearings inthe new system are fully exploited; theyfunction as the machine designer intends andassumes they should. This is often not the case



with other bearing systems, which have somecompromise in the arrangement whichproduces non-ideal operating conditions.

The new compromise-free SKF systemdelivers increased reliability and performance,enabling designers to confidently optimize theselection of bearings and the machineconstruction as a whole.Both manufacturers and end users of machinesachieve significant cost reductions through aleaner design and improved productivity.Depending on the machine and application, thebenefits seen with SKF’s new self-aligningbearing system are:• safer, more optimized designs• increased bearing service life• extended maintenance intervals• lower running temperature• lower vibration and noise levels• greater throughput of the machine• same throughput with a lighter, orsimpler machine• improved product quality / less scrap

Thrust Bearings

Thrust Bearing CapabilitiesThe thrust ball bearing consists of shaft andhousing washers with cage and ball set, andhave two configurations: single and doubledirection. As it is very sensitive tomisalignment, the single direction type can beassembled with sphered housing seatingwashers (rings) to compensate for staticmisalignment.The cylindrical roller thrust bearing can carryvery heavy loads at slow speeds in onedirection only. Misalignment cannot betolerated.

Minimum LoadAll ball and roller bearings must be subjectedto a given minimum load to assure satisfactoryoperation. This is also true of thrust ballbearings. Particularly if they run at highspeeds, where the inertia forces of the ballsand cage, and the friction in the lubricant canhave a detrimental influence on the rollingconditions in the bearing, and may causedamaging sliding movements (skidding) tooccur between the balls and the raceways.

Review QuestionsBall Bearings

Double RowAngularContactBall Bearing(DRACBB)

Four PointAngularContactBall Bearing(FPACBB)

SingleRowDeepGrooveBallBearing(DGBB)

SingleRowAngularContactBallBearing(SRACB)

SelfAligningBallBearing(SABB)

Double RowAngularContactBall Bearing(DRACBB)

Four PointAngularContactBall Bearing(FPACBB)

SingleRowDeepGrooveBallBearing(DGBB)

SingleRowAngularContactBallBearing(SRACB)

SelfAligningBallBearing(SABB)

Which bearings can tolerate dynamicmisalignment? (SABB Only)1. Which bearings are not designed to accept

thrust loads? (SRDGBB, SABB)



2. Which of these bearings can be used as thenon-locating bearing? (None)

Roller Bearings

1. Which bearings can tolerate dynamicmisalignment? (SRB, SRTB, CARB)

Which bearings are not designed to acceptthrust loads? (CRB, CARB)

2. Which of these bearings can be used as thenon-locating bearing? (CRB, CARB,SRB?)



Chapter 4 - MountedBearingsOverview

Three basic types of mounted units areavailable: ball bearing units, roller bearingunits and split pillow blocks. Generally, ballbearing units are suitable for simple, lightlyloaded application conditions. Choose rollerbearing units in heavier loaded applications.Roller bearing mounted units mayaccommodate shaft expansion, while standardunit ball bearings do not.For more demanding applications, use splitpillow block housed units. They offer thebroadest range of options for bearing types andlubrication methods. Split Housingsaccommodate shaft expansion and have theheaviest load carrying capacities.Be sure to check size ranges before applicationspecification as manufacturers vary widely intheir production availability.

Load Carrying Capacity and LifeThe size of a bearing to be used for anapplication is initially selected on the basis ofits load carrying capacity in relation to theloads to be carried and the requirementsregarding life and reliability. Numerical valuestermed basic load ratings are used in thecalculations to express load carrying capacity.Values for the basic dynamic load rating C andthe basic static load rating C0 are quoted in the

bearing tables. The ability of wide inner ringbearings to carry axial loads does not primarilydepend on the internal geometry, but on themeans by which they are located to the shaft.Bearings with eccentric collars or set screwlocking can carry axial loads of up to 20percent of the basic dynamic load rating if softshafts are used and the set screw is properlytightened during mounting.

Mounted Ball Bearing Units

Ball Bearing Units consist of a precision wideinner ring bearing assembled into a housingmade of cast iron, pressed steel, or rubber.Units are prelubricated and are designed to slipfit on to the shaft. Locking method is eithereccentric self-locking or setscrew locking.On cast iron and pressed steel units, theoutside diameter of the bearing and the insidediameter of the housing are sphered, allowingthe bearing to swivel within the housing toaccommodate initial alignment.There are numerous bearing and housingcombinations to accommodate a diversity ofmounting surfaces, load requirements, shaftsizes, and dimensional requirements. Housingtypes include: pillow blocks, flange units,take-up units, flangette units, and rubbercartridges.



Inner Ring LengthThe inserts for Ball Bearing Units areavailable in wide and narrow width inner ringdesigns. When space permits, the wide innerring is the preferred choice. The wide innerring minimizes shaft misalignment relative tothe bearing bore, and the load is distributedover a greater shaft area. The narrow innerring is used when space is limited.Zone Hardening and ThroughHardeningThe raceways of wide inner ring bearings arehardened to between 58-62 Rockwell C toprovide strength and long life. The outer ringsof all insert bearings are through hardened,that is the entire ring is heat treated to increasethe hardness of the steel throughout. The innerrings of wide inner ring bearings witheccentric locking collars are also throughhardened. The inner rings of bearings with setscrew locking are induction zone hardened atthe raceway location.

CagesAll wide inner ring bearings are fitted with aninjection molded, heat stabilized, glass fiberreinforced polyamide 6.6 cage. This materialhas a favorable combination of strength andelasticity. The friction of these cages is lowand they have excellent running propertiesunder adverse conditions. Additionally, the

cages promote quiet operation. The operatingtemperature range for the cage exceeds that ofthe grease with which the bearings are filled.

Grub Screw Type

Set Screw Locking features two set screws 120degrees apart. This placement of the set screwsmaximizes holding power while minimizinginner ring distortion. Other factors such as:zone hardening of the bearing inner ring,drilling and tapping after heat treat, properscrew length, a knurled cup point set screw,and a forged set screw socket have beenincorporated to further enhance holding powerand vibration resistance.



Eccentric Collar Type

Eccentric Self-Locking features a machinedcollar and bearing inner ring which act asmating cams when assembled on the shaft.When the collar is engaged on the bearinginner ring, and turned in the direction of shaftrotation, it grips the shaft firmly with a bindingaction that further increases with rotation. Thesingle setscrew in the collar firmly secures theposition of the eccentric collar. This lockingmethod is recommended for all applicationsexcept where reversing shaft rotation isinvolved.

Inserts with Interference FitThese are sometimes recommended foreccentric and setscrew locking, but it is the

required mounting method for some series thathave no auxiliary locking device.

Grease FillGenerally speaking, wide inner ring bearingsare designed to operate without relubricationunder normal speed and operating conditions.All wide inner ring bearings are sealed at bothsides with rubbing contact seals, and are filledwith a special long life grease of NLGIconsistency 2. The grease has good corrosioninhibiting properties and is suitable foroperating temperatures between –20°C and120°C (–5°F and 250°F). However, underextreme conditions or in heavily contaminatedenvironments, it may be necessary torelubricate the bearings.

Mounted Products Seals

The M-Seal is standard on FM series housed units and YET series bearings. This low



friction contact seal is made up of a wearresistant elastomer lip bonded to a steel trashguard. The lip rides on the precision groundsurface of the inner ring providing positiveexclusion of dust, moisture, and othercontaminants.The Multi-function Seal (RF) is optional onTF and WF series housed units and YAR andYEL series bearings. It is the most effectiveseal ever produced for ball bearing units andwide inner ring bearings. It combines theproven sealing of the M-Seal with anadditional flinger-contact seal assembly. Thissealing arrangement provides four separatebarriers against the ingress of contamination: aflinger, a contact seal, a grease chamber, and asecond contact seal. Stainless RF seals arestandard on MRC Marathon series blocks. Alow-friction version is available on SKF air-handling unit ball products.

Housings

Cast Iron Housings

Pillow block housings, the most popularhousing design for mounted bearings, areavailable in several series. All are designed tosupport loads primarily directed through thebase.Four-bolt flange units are designed to supportshafts through walls, bulkheads, and other flatsurfaces. The housing mounts flush to thesurface and the load is carried by the fourmounting bolts.

Two-bolt cast iron flange units support shaftsthrough walls, bulkheads, and other flatsurfaces but are of a more compact two-boltdesign.Center Pull Cast Iron take-up units aretypically used in conjunction with a customersupplied frame for belt or shaft tensioning inconveyor, elevator, and power transmissionapplications. A wide slot for the frame guide isstandard.

Pressed Steel Housings

Pressed Steel Pillow Block housings arelightweight, low cost solutions for light-dutyapplications. Housings are of a heavy gaugetwo-piece stamped steel construction, whichallows for initial bearing alignment. Thesehousings are available in Two-bolt and Three-bolt Pressed Steel Flange Units.Rubber cartridges are generally used inheating, ventilating, and air conditioningapplications or other applications where lownoise is important. The conductive rubbercartridge, or tire, helps dampen noise andvibration as well as dissipate the build up ofstatic electricity.

Corrosion Resistant Solutions



MRC Marathon Series are available forapplications requiring additional corrosionresistance. The standard offering is comprisedof a glass fiber reinforced nylon housingpaired with a corrosion resistant, setscrewmounted insert bearing.Two additional housing series are available:cast iron (with a corrosion resistant coating)and stainless steel. Housings are available inpillow block, 2-, 3-, and 4-bolt flange, tappedbase, and take-up designs.The standard insert is made of 52100 steelwith a corrosion resistant coating. Stainlesssteel seals and setscrews are provided foradditional corrosion resistance.Optional inserts include stainless steel ballbearings and non-rolling element inserts in awide variety of polymer and stainless steelconfigurations.Marathon units are supplied with 100 percentfull grease pack and are not intended to beregreased.

MisalignmentBall bearing units can compensate for up to±5° of static misalignment. However, in thecast iron housings, when it is desirable torelubricate the bearings, initial errors inalignment should not exceed ±2° for basicbearings size 211 and smaller and ±1.5° forlarger sizes. Misalignment greater than thiswill prevent the lubrication holes in the outerring of the bearing from lining up with thegroove in the housing bore and the bearingswill not be relubricated.

Replacement ConsiderationsIdentifying the bearing by looking at thehousing alone is not always possible. For thisreason, housings, inserts, and complete unitshave discrete part numbers. When possible,record the housing and bearing part numbersand the manufacturer of the unit for completeidentification. If part numbers are notreadable, collect the following data:1. Shaft diameter (with a micrometer or

accurate dial caliper)

2. Length through the bore3. Mounting style (Eccentric collar, setscrew,

etc.)4. Bearing O.D.5. Housing dimensions: Length and width (or

diameter). For take-up housings, also,measure the slot width.

6. Overall height7. Bearing centerline height8. Bolt hole spacing and arrangement (2, 3 or

4 bolt)With the proper information, the correctreplacement bearing can be acquired.Loosely fitted insert bearings can be prone tofretting corrosion. Antifretting compoundbetween shaft and bearing inner ring is notrecommended, and may promote shaft creep.Use precision ground shafting in heavilyloaded applications, or specify a press fit asrecommended. See manufacturers’ catalogs orsupplied instructions for details.When replacing Unit Ball Bearings, it isgenerally recommended to replace the entireunit. Housing wear may reduce the service lifeof the replacement insert.Housings and inserts from differentmanufacturers are generally incompatible. Ifonly the insert is replaced, do not mix insertsand housings of different manufacturers. Lubegrooves, holes, or pins usually will not line up,preventing relubrication.



Collar Mounted Roller BearingUnit

Roller bearing units are available in a varietyof housing styles and two mounting methods.These units are simple and economic bearingarrangements, combining the high loadcapacity and self-aligning features with theconvenience of set screw mounting and arugged cast iron housing. They come pre-greased and fully assembled. Installation timeis reduced and there is no risk of bearing

contamination during assembly. SKF Rollerbearing units accommodate dynamic or staticmisalignment and can be supplied as free orheld assemblies. (Contact your manufacturerto confirm the ability of these features.)Roller bearing units offer many of theadvantages of the split housing series, but withthe convenience of just slipping them on theshaft. Running or static misalignment (up toone to one-half degrees), free or heldassemblies, two or four bolt bases, cast iron,and a variety of seal options make the rollerbearing unit a preferred alternative to the splithousing.Typical applications include:• Apron conveyors• Belt conveyors• Fans & blowers• Wastewater treatment equipment• Commercial laundry equipment• Bucket elevators• Chain conveyors



Collar Mounted units use a locking collardesign to firmly secure the bearing inner ringto the shaft. The two set screws extendthrough the inner ring of the bearing and lockinto the shaft. The elasticity of the lockingcollars maintains pressure on the set screwshelping to maintain a secure fit to the shafteven in severe power transmissionapplications. To mount, slide the unit on theshaft, bolt the housing securely to themounting surface, and tighten the two setscrews.

Cast iron flange unit roller bearings supportrotating shafts through walls, bulkheads, orother flat surfaces. The two smallest sizesmount to the surface with three bolts; all othersizes use four mounting bolts.

Piloted flange cast iron flange units areavailable in a variety of shaft sizes and have amachined extension on one side to fit into aprecisely machined bulkhead opening. Thisprovides additional location and support to theunit. All piloted flange units are supplied infour-bolt mounting arrangements.Cast iron, center pull take-up units aredesigned for use in premanufactured frames.

Take-up assemblies consist of a speciallydesigned, cast iron, take-up unit mounted in anall-steel top angle take-up frame. Primarily forbelt tensioning in conveyor applications, theunit is positioned in the frame by turning theprotected adjustment screw from either end ofthe frame.

Top mount take-up frames are designed foruse with unit roller pillow blocks. As withother take-up frames, top mounted assembliesare used primarily for belt tensioning inconveyor applications. The unit roller pillowblocks mount directly to the top of the frameand are positioned by turning the protectedadjustment screw at either end of the frame.Top mount take-up frames are available in avariety of lengths.Mounting Instructions are provided in everybox. Shaft diameter requirements andsetscrew torque recommendations are criticalin more heavily loaded applications.



Unit Roller Bearing Seals

The TriGard Seal is supplied as standard on allSKF unit roller bearings. A double lip moldedcontact seal rides on the ground surface of theinner ring. A steel outboard trash guardprotects the seal and excludes largercontaminants. The Trigard Seal providespositive contact and accommodates up to±11 2° misalignment with low frictionaltorque. Maximum continuous operatingtemperature is limited to 95°C (200°F).The two-piece all metal Labyrinth Seal (suffix– 18) accommodates higher operating speeds.An inboard flinger pressed on the inner ring ofthe bearing helps retain grease and excludecontaminants. An outboard stationary trashguard is pressed into the housing and has closerunning tolerance with the inner ring of thebearing to prevent the entrance ofcontaminants. The design permits ±11 2°misalignment with no frictional torque or sealtemperature limitation.The Garter Spring Seal (suffix – 3) consists ofa one piece molded nitrile rubber seal pressedinto the housing bore. It provides greatergrease retention and sealing protection. Thestainless steel garter spring holds the seal lipfirmly to the ground inner ring. The Garterspring seal provides positive contact evenunder misaligned conditions, while permitting±11 2° misalignment. The garter spring sealprovides good protection in moistenvironments and is limited to 95°C (200°F)maximum operating temperature.Standard steel End Covers (suffix Y) aresimply pushed into position in the housing

bore. They provide complete contaminantexclusion and protection for exposed shaftends.Details, such as speed ratings, dimensions, andrecommended applications, may be found inappropriate SKF Catalogs.

Pop-Release Roller BearingUnits

Pop Release spherical roller bearing units aresupplied completely assembled with bearings,seals, and grease. They are prelubricated andrequire no internal adjustment once mounted.

Pop Release units can be used in the sameapplications as collar mounted roller bearingunits.The bearings used in Pop Release Sphericalroller bearing units are based on the highcapacity SKF 22200 series spherical rollerbearing. They accommodate heavy radial andaxial loads, running and static misalignment,and high operating temperatures. Clearances inthe bearing are preset; no adjustment isrequired.The Pop Release spherical roller bearing unit



uses a patented multi–tapered sleevearrangement for easy shaft mounting. Thislocking system provides nearly 360 degrees ofinterference fit. The locking system alsocenters the shaft, reducing imbalance,vibration and fretting corrosion.To dismount, loosen the mounting set screwsand tighten the dismounting set screws untilthe multi tapered sleeve releases its grip,usually with a “POP”.

Pop-Release uses a stepped sleeve matched toa stepped inner ring. When the inner ring isdriven up the inclined planes of the sleeve, thesleeve firmly clamps to the shaft to provide anearly 360-degree interference fit. Thebearing stays entered on the shaft as well,reducing dynamic imbalance. Severalproblems can be addressed with Pop-Release:excess fretting corrosion, which often preventseasy unit removal, imbalance from offsetmounting with setscrews or collars, and shaftcreep from lack of proper press fit.

SKF Split Bearing HousingsOverview

Worldwide, SKF manufactures several two-types of Split Housings. The most commonstyles are the SNH/L Plummer Block Stylesand the SAF Split Pillow Block Style. Thestandard SNL/H blocks and accessories aredimensioned to fit SI (metric system) sizedshafts while the SAF standard product range isdimensioned to fit English (inch) shaft sizes.Either block can be special ordered for non-standard shaft sizes, if desired. There are also

differences in the types of seals offered in thetwo different styles. The two block styles arenot dimensionally interchangeable in terms ofcenterline height, bolthole spacing, width, etc.Both styles can be used with grease or oillubrication.

Split Pillow Blocks

SAF, SAW, SDAF DesignsSplit Pillow Blocks are the most versatilemounted assemblies and are designed forextremely demanding applications. They canaccommodate a large range of shaft sizes andhandle higher loads. They come standard withheavy gray cast iron housings, and providemore flexibility in sealing options andlubrication (grease or oil can be used).These assemblies are predominately designedfor radial loads that are carried directlythrough the base. Applications include:• Strip processing equipment steel mills• Conveyors that must accommodate larger

shaft sizes or special seals

• Fans and blowers• Felt rolls and drying cylinders within the

paper industry• Crushers, hammer mills, and kilns in the

minerals processing industries.



Standard split pillow blocks are made fromcast iron, cast steel, or ductile. Housings arehorizontally split, and the cap and base are amated set, which cannot be interchanged withother caps or bases. For accurate realignment,each cap is mated to the base with dowel pins.Split pillow blocks are available in two- andfour-bolt base mounting configurations, whichallows for moderate adjustments duringinstallation. Sealing options for split pillowblocks include the LER/LOR Triple ring seal,the contact seal, taconite seal, and the endplug.SKF Split Pillow Blocks come in three styles,SAF, SAW, and SDAF. A fourth type, knownas the Extended Range, is custom designed fordemanding applications, where requirementsinclude shaft sizes of 25-50 cm (10-20 inches).Cylindrical Bore Mounting

Cylindrical mounting involves mounting astraight bore bearing to a cylindrical shaftusing an interference fit. This mounting

method is preferred for precise location of theshaft. It is also better suited for high speedapplications. In cylindrical mounting, the shaftmust have precision tolerances at the bearingseat and machined threads for the locknut.Heating of the inner ring is usually necessaryfor mounting.

Taper Bore Mounting

In adapter mounting, a tapered bore bearing ismounted to a cylindrical shaft through the useof a tapered adapter sleeve. Adapter mountingis considered the general purpose mountingmethod and most applications of split pillowblocks use this method of attaching the bearingto the shaft. The shaft seatings do not need tobe machined to the precision tolerancesnecessary for cylindrical mounting, and themounting method does not require heating thebearing. Additionally, dismounting isconsiderably easier with a tapered adaptersleeve. For these reasons, the total costsassociated with adapter mounting are less thanfor cylindrical mounting.Adding Stabilizing Ring(s) fixes the bearing inthe housing, also called the “Held” Bearing.Other terminologies are “Fixed or Float” or thepreferred “Locating or Non-Locating.”



SAF Design Split Pillow Block

SAF style blocks are manufactured in castiron, cast steel, or ductile iron materials. Itfeatures a combination of five bearing types,four sealing options and two mountingmethods, enabling the SAF to be optimized fora wide variety of applications.The SAW accommodates the higher capacityseries 23200 bearings, or provides greaterthermal expansion of the shaft when the 22200bearing is used. It is dimensionallyinterchangeable with SAF. Although thebolthole spacing and center heights are thesame, the SAW block is about 25 mm (oneinch) wider.The SDAF style is used for applications thatinvolve heavy or shock loads. SDAF housingsare used extensively in steel mills, mineralprocessing, and other environments usingheavy-duty equipment.Split Pillow Blocks offer optional customizedfeatures, such as special bearings to meetunusual clearance and/or tolerancerequirements, machined base ends that provide

flat surface for jacking screws, holes forattachment of various condition monitoringdevices (thermocouples and vibrationdetection equipment), and special seals for usewith circulating oil lubrication and high-speedoperation.



Split housings are designed to accommodate avariety of bearings. Care must be taken to assurethe correct accessories are used duringassembly.

Standard Split Pillow BlockAssortmentSAF

Cast iron SAF split pillow blocks accommodateself-aligning ball bearings, spherical rollerbearings and toroidal (CARB™) roller bearings.All SAF housings are suitable for adapter orcylindrical mounting to a shaft. SAF housingsare available for a variety of shaft sizes andoffer a choice of two or four bolt base mounting.Their design accommodates grease or oillubrication. LER/LOR triple ring seals aresupplied as standard, but SAF housings alsoaccommodate a variety of optional seals such ascontact seals, taconite seals, and end plugs.

SAFS

The SAFS split pillow blocks are cast steelversions of the SAF block described above. Thecast steel material offers greater strength andshock resistance for extra heavy-dutyapplications. SAFS blocks are used withcylindrical or adapter mounted spherical rollerbearings. They are available in a variety of shaftsizes. They can be supplied in two or four boltbase mounting arrangements. The SAFS pillowblocks can be grease or oil lubricated and comestandard with LER/LOR triple ring seals.Contact seals, taconite seals, and end plugs arealso available. SAFS housings have the samebasic mounting dimensions as SAF housings butdiffer in some dimensions.

SDAF

The SDAF block is designed for applicationswhere heavy thrust loads and shock require ahousing of exceptionally sturdy construction.



The four-bolt cap and four-bolt base of theheavy-duty SDAF are of unusually ruggedproportions adequate to resist shock and heavyloading in any direction. The caps of these largehousings are supplied with tapped holes forlifting eyes. SDAF housings accommodateseveral series of spherical roller bearings,cylindrical, or adapter mounted. They areavailable in a variety of sizes. Triple ring sealsare standard, but optional taconite seals and endplugs are available. SDAF housings can begrease or oil lubricated.

Mounting BoltsMost split housing pillow blocks are available intwo and four mounting bolt designs. In manycases, this option is limited to certain shaft sizes.Check with the bearing manufacturer forspecific sizes available.

Two Bolt Four bolt

Nomenclature - Prefixes

S = Prefix letter for Split Pillow BlockA = American DesignF = Ring Seals

ExamplesSAFS = the last ‘S’ denotes cast steelSAFD = the ‘D” denotes ductile iron, availablefrom SKF Canada.SDAF = the ‘D’ denotes heavy duty series splitpillow blockSSAF = the first ‘S’ denotes a blank base whichis no longer availableSSDAF is available as a made to order special.

FSAF = Four bolt base SAF housing (see suffix–11 for steel)These can be combined. Example: SDAFD =Ductile Iron Heavy Duty Split Pillow BlockAmerican Design with Ring Seals.A three-digit size designation denotes HousingOnly, for example, SAF 517A five or six digit size designation denotes thecomplete assembly, which includes the SplitPillow Block, Bearing, Seals and StabilizingRings. For example, SAF 22517 is the completeassembly with Normal Internal ClearanceSpherical Roller Bearing. SAF C2517 indicatesthe complete assembly with CARB toroidalBearing.

Common size designations:SAF 222xx = cylindrical mount with a 222series bearingSAF 223xx = cylindrical mount with a 223series bearingSAF 225xx = adapter mount with a 222 seriesbearingSAF 226xx = adapter mount with a 223 seriesbearingSAF 230xx = 230 series bearing used withsuffix KAThe only letter allowed in the size field is theletter ‘C’ to signify CARB.



Special Application Solutions

TY-RPA

TY-RPA take-up assemblies consist of aspecially designed, cast iron, TY-R take-up unitmounted in an all steel PA take-up housing.These assemblies are used primarily for belttensioning in conveyor applications. The TY-Rhousing uses an adapter mounted sphericalroller bearing. Travel is adjusted by turning theprotected adjustment screw from either end ofthe frame. TY-RPA assemblies are available intravel lengths ranging from 12 to 36 inches, andaccommodate a variety of shaft sizes. Theseunits are intended for grease lubrication only,and are supplied as standard with one endclosed. Piston ring type seals are used at theopen ends. Versions with both ends open arealso available.

TFT

TFT top mount take-up frames are designed foruse with SAF split pillow blocks. As with theTY-RPA assemblies, TFT assemblies are usedprimarily for belt tensioning in conveyorapplications. An SAF block mounts directly tothe top of the TFT frame to complete theassembly. TFT frames are available in travellengths from 12 to 48 inches (30 to 120 cm) andaccommodate SAF blocks in a variety of shaftsizes.

SAW

SAW split pillow blocks are designed toaccommodate the higher capacity 23200 seriesspherical roller bearings. They can also be usedwith 22200 series bearings to allow for greateraxial travel or thermal shaft expansion. SAWblocks have the same basic mountingdimensions as their SAF counterparts, but areabout one inch wider to accommodate the widerbearing. A variety of shaft sizes are available inadapter mounting. SAW blocks are available infour-bolt base only and are suitable for grease oroil lubrication. LER/LOR triple ring seals arestandard, but, as with the SAF, other sealingoptions are available.

Extended Range

The Extended Range split pillow blocks providean extremely rugged housing style ofexceptionally sturdy construction for shaft sizesranging from nine to 20 inches (23 to 50 cm)and higher. These blocks are made-to-order andcan be supplied with a variety of customizedfeatures to suit the largest and most demandingapplications. Customized features can includespecial seals and grease shrouds, specialbearings and tolerances, specific boltholepatterns and machining for attachment ofauxiliary equipment such as temperature andvibration detectors. The blocks canaccommodate several series of high capacityspherical roller bearings in either cylindrical or



adapter mounted arrangements. As with otherSKF split pillow block housings, the ExtendedRange is suitable for grease or oil lubricationand has a variety of sealing options.

SealsSeveral seal designs are available for SKF splitpillow blocks. Guidance on choosing the correct sealmay be found in the SKF Bearing MaintenanceHandbook.

Triple Ring LER Seal

The LER and LOR seals are supplied for SAF,SDAF and SAW split pillow blocks. Theyconsist of a one-piece aluminum rotatinglabyrinth seal. The close running tolerancebetween shaft and housing surfaces providesprotection from contaminants while retaininggrease or oil.

PosiTrac (LOR) Seal

This lightweight seal is supplied with most SKFSAF-design split pillow blocks. PosiTracconsists of a lightweight aluminum ring with anO-ring integrated into the seal bore. This designblocks contaminants from migrating along the

shaft into the housing, while preventinglubricant leakage in the opposite direction. Thelabyrinth design offers good, large particleprotection, and acts as a rotating flinger duringoperation.

PosiTrac Plus Seal

PosiTrac Plus adds a nitrile rubber contactelement to the standard Positrac Seal. Fittingsnugly into the housing seal bore, it providespositive sealing contact with the outer diameterof the seal’s aluminum ring. The ring sealprovides a wearing surface, and virtuallyeliminates any possibility of shaft wear. Thiscontact seal is completely encased within thehousing and ring seal labyrinth, protecting itfrom large particle contamination. PosiTrac Plusseals are ordered separately.

Positrac is adaptable for purge and non-purgeoperations. Grease purging is essential in someapplications (high-contamination miningoperations, for example), but unacceptable inothers (papermaking, meat and fruit processing).PosiTrac adapts easily to either requirement.Reverse the seal orientation in the housing toconvert PosiTrac Plus from purgeable to non-purgeable operation.



Contact Seal

This one-piece nitrile rubber seal should beselected for dirty, dusty and wet environments.Contact seals fit into the center of the housinglabyrinth and provide positive contact evenunder misaligned conditions.

End Plug

End plugs are extruded nitrile rubber stripfabricated over a metal plate. They provide apositive fit in the housing labyrinth center forcomplete contaminant exclusion.

Taconite TER Seal

Taconite Seals should be selected for theharshest duty applications with extensiveparticle contamination, such as mines, quarries,and foundries. The design replaces the standardLOR/LER ring seal with an extended auxiliaryseal. Taconite seals should be checked forproper clearances before specifying extension.Three sealing areas are provided: an outboardshaft riding split contact seal, a grease cavitywith fitting to flush contaminants, and a feltinboard seal for grease retention and additionalsealing.

Taconite-TER-V ring seal

The taconite seal with V-ring option should beused when rougher shaft surfaces must beaccommodated.PosiTrac replaces existing SKF LER or A9508seals as they are identical in width. To easeassembly, the O-ring in the seal bore may also



lubricated with lightweight mineral oil.PosiTrac misalignment capability is equivalentto that of existing SKF LER seals, and has aoperating temperature range from 35°C to110°C (-30°F to 230°F).SDAF Extended Range Split Pillow BlockThe Extended Range Series of pillow blocks area rugged housing style of exceptionally sturdy4-bolt construction. The standard material isClass 40 gray iron, but when extra high strengthor impact resistance is necessary, housings canbe supplied in high strength ductile irons or caststeel, as specified below. Extended Rangehousings are machined according to therequirements of the individual application andare, therefore, highly customized bearingsolutions. These customized housings are eachgiven their own specific part number anddrawing so that there is a permanent record foreach housing.When no special features are required, housingsare designated according to the part numbers inthe following tables. However, since ExtendedRange housings are not stocked in a finalmachined state, it is recommended that eachapplication for Extended Range housings bereviewed with SKF Applications Engineering todetermine what features may be needed sincemany special features can often beaccommodated with little or no additional cost.These features can include, but are not limitedto: drilled base bolt holes, milled foot pads,special machining for lubrication and conditionmonitoring systems, special bore tolerances,machined foot ends, special seals, machining foruser supplied auxiliary seals, hydraulic assistedadapter sleeves, etc.

MountingBecause of their size, Extended Range housingsoften require special considerations whenmounting and dismounting. For bearingsmounted directly on the shaft, it isrecommended that hydraulic assist features beincluded in the shaft design. These will aid inmounting and dismounting the bearing. Forbearings mounted on adapter sleeves, the

sleeves can be supplied modified with hydraulicassist features. Additional guidance may befound in the SKF Bearing MaintenanceHandbook, or contact SKF service for assistancein mounting these specialized solutions.

SKF Housing MaterialsStandard material is ASTM A48B Class 40gray iron. Alternate high strength or impactresistant materials available include:1. Ductile Iron: ASTM A339, Grade 60-45-102. Ductile Iron: ASTM A536, Grade 80-60-033. Cast Steel: ASTM A27, Grade U-60-30

LubricationSplit housing designs provide effective bearinglubrication. Grease, static oil, circulating oil, air-oil, and oil mist systems can be accommodated.Guidance on initial grease lubrication amounts,relubrication intervals, and static oil fill levelsmay be found in the SKF Bearing MaintenanceHandbook or appropriate catalog.

Cap and Base AlignmentAll SKF Split Pillow Block Housings aremachined as matched sets: caps and basescannot be mixed. Dowel pins or groove pinscontrol the alignment of the cap to base.

To prevent mixing caps and bases duringassembly all SKF Split Pillow Block housingshave cap and base match marking. All SAF andSAFS 200 & 500 series housings have cap andbase labels. All SDAF, Extended Range, andSAFS 300 & 600 series housings are stamped.



Cap to Base OverhangAll SKF Split Pillow Block Housings aredesigned so that the cap will overhang the baseslightly to create a drip edge to prevent waterfrom wicking at the split line. The amount ofoverhang will vary based on casting tolerances.

Shaft Fits for Split Pillow BlocksShaft fit selection procedures for cylindricalbore bearings are the same as those previouslypresented. Consult the appropriate tables in theappropriate SKF catalog for guidance.Tapered Bore bearings may be mounted ontapered shafts, or adapter or withdrawal sleeves.

Housing Bore FitsAll SKF split pillow blocks are supplied withG7 housing bores as standard, except the SDAFextended range which have F7 housing bores.Special fits are available on a made-to-orderbasis.

Split Pillow Block Housing Loads

The selection of a rolling bearing pillow blockdepends not only on determining adequateservice life of the bearing for the operatingconditions of the application, but also on theadequacy of the housing to safely accommodatethe magnitude and characteristics of all appliedloading.

Split pillow blocks are designed for

predominantly vertical loads directed throughthe base. If the load direction is otherwise, caremust be taken to see that housing selected hassufficient strength. It must have an adequatefactor of safety against fracture and be properlyassembled to carry the loading involved.

Mounting AccessoriesAdapters

The adapter is used to hold and locate a taperedbore bearing on a cylindrical shaft seat. Itpermits relatively wide tolerances for the shaftdiameter, and makes it unnecessary to heat thebearing or use a press when mounting, in orderto obtain an interference fit of the inner ring onthe shaft. Most adapters are generally supplied completewith locknut and locking device.



Locknuts, Lockwashers andLockplatesLocknuts and lockwashers are commonly usedas effective means for holding bearing innerrings axially on the shaft. They are alsofrequently used to secure gears, belt pulleys, andother machine components. The locknuts areaccurately made to insure that the abutment facewill be square with the shaft axis, thus avoidingdistortion of the shaft during tightening.Lockwashers, which are made from selectedhigh quality steel, and are heat treated, mustpass a final inspection that requires theirsurfaces to be smooth and free from burrs.Lockplates are steel stampings that engage theslot of the sleeve and are secured to the end faceof larger nuts by two screws. This unit is used inlieu of lockwashers on larger size nuts.

Metric Plummer Blocks

SNL (SNH) blocks are constructed to metricdimensions, i.e., bolthole centers and base tocenterline of the shaft are measured inmillimeters, not inches. Using H, HA and HEseries adapter sleeves, tapered bore self aligningball, spherical roller, and CARB bearings maybe fitted to either metric or inch dimensionshafts.The SNL size range is SNL 505 to SNL 532 orshaft diameters, ¾” to 5-1/2” and 20 mm to 140mm.SNL Plummer Blocks are also available in anextended size range covering shaft diametersfrom 150 mm to 450 mm. The new rangeaccommodates series 231, 230 and 240spherical roller bearings along with C31 andC30 CARB bearings.Large SNL are available with several choices ofseals, including labyrinth, taconite, felt, etc.The new extended range SNL replaces the SDseries. Consult SKF for additional information.Split pillow block basic design, materials, andapplications have been covered in the previoussections. Detailed installation information canbe found in various SKF publications and is alsoprovided with each block.SNL pillow blocks are intended for use withself-aligning ball bearings, spherical rollerbearings, and CARB bearings of ISODimension series 02, 03, 22, 23 and 32. SNLsplit blocks can be fitted with a variety of seals.

Tapered Bore MountingSNL housings incorporate two dimension seriesin one block. For example, the SNL511-609can accommodate either a 22211EK or22309CCK/W33 spherical roller bearings. Bothbearings have the same outside diameter of 100mm. Using different adapter sleeves, differentsection height seals in the hub and fixing ringswith different widths, two series of bearing maybe accommodated in the same housing. Forexample: SNL 22511/1-11/16”TG with a22211EK bearing uses:

HA 311 adapter sleeveTSN 511G sealsFRB9.5/100 fixing rings.



SNL 22609/1-7/16”TGA with a 2309CCK/W33bearing uses:

HA2309 adapter sleeveTSN609GA sealsFRB4/100 fixing rings.

Tapered bore bearings are usually mounted onsmooth (plain) shafts with the aid of adapter orwithdrawal sleeves, but can also be mounted onstep shafts when an accurately machined spacersleeve is used to secure the bearing against theshoulder.Sleeve mounting on cylindrical shafts allows theuse of economically priced round bar (coldrolled) shafting (tolerance h9) withoutmachining.SNL pillow blocks can be used for mountingtapered bore self- aligning ball, spherical roller,and CARB bearings of the following dimensionseries:SNL5 series for 12EK, 22EK, 222K, 232CCK,C22K, C32K.SNL6 series for 13EK, 23EK, 213CCK, 223EKand C23K.

Cylindrical Bore MountingThere are two series of SNL pillow blocks formounting cylindrical bore bearings on steppedshafts, SNL2 and SNL3. In many cases, theSNL5/6 housings can be used instead of SNL2/3series since the housing dimensions are thesame. Detailed listings of SNL pillow blocksfor both adapter sleeve mounted and cylindricalbore mounted bearings can be found in the SKFcatalogs.Since the bearing fit and residual internalclearance after mounting is determined by theshaft size, form, and finish, it is necessary to useexpensive precision ground shafting. Step shaftmounting may be preferred when large shockloads can occur and to maximize the shaft sizeat the bearing position.SNL pillow blocks can be used for cylindricalbore mounting of self-aligning ball, sphericalroller, and CARB bearings of the followingdimension series:SNL2 series for 12E, 22E, 222E, 232CC, C22and C32.

SNL3 series for 13E, 23E, 213CC, 223CC andC23

Metric Block Seals

SNL pillow blocks can be fitted with a varietyof seals. The standard seals are double liprubber “G” seals (most popular), V-ring sealsand felt seals. Labyrinth seals and heavy-duty“taconite” seals are available as well as endcovers. SKF catalogs offer a comprehensiveoverview of the seal types available for SNLpillow blocks. Seal design features andsuitability for various operating conditionsprovided.

Double-lip Seal, Type GThe G seal is made of polyurethane, a wearresistant material with good elastic properties.The seals are intended for grease lubrication forspeeds up to 8 m/s (1600 ft/min).Permissible angular misalignment is up to 1.0ºfor shaft diameters 100 mm (3-15/16”) and upto 0.5º for larger sizes. The surface roughnessof the seal counterface on the shaft should notexceed 125 microns (0.005”) RMS.“G” type seals can accommodate axialmovement of the shaft. The seals are supplied inpacks of two. Double lip seals are identified bya prefix, TSN, followed by the size and suffix,i.e., TSN 511G. (Generally, G for metric shafts,GA and GE for inch dimension shafts).

V-ring Seals, Type AThe V-ring seals consist of two parts:



• The v-ring itself• A sheet steel sealing washer with rubber lip

which fits into the housing hub groove.The v-ring is made of nitrile rubber and has aseal lip which runs against the sealing washer.The v-ring seals can be operated at peripheralspeeds of 7 m/s (1400 ft/min). If the ring isprevented from moving by a support ring on theshaft, permissible angular misalignment is 1.5ºfor shaft diameter of 50 mm (~2”), decreasing to1.0º for shaft diameter of 150 mm (~6”) andabove.V-ring seals are identified by a prefix TSNfollowed by the size and suffix, i.e., TSN 511A.

Felt Seals, Type CFelt ring seals are simple and can be used atperipheral speeds up to 4 m/s (800 ft/min.). Theseal consists of two light alloy half-rings, inwhich felt strips are fitted, and o-section cords.The o-rings are placed in the grooves to preventthe seal rotation. The felt must be soaked in oilprior to use. Felt seals are identified by a prefixTSN, followed by the size and a suffix, i.e.,TSN 511C.

Labyrinth and Taconite SealsThese seals are functionally similar to those forSAF style spilt pillow blocks.

End CoversFor applications where the pillow block housingis at the end of the shaft an end cover is used.The standard end cover is made of plastic anddesignated with the prefix ASNH, i.e., ASNH511-609 indicates this end cover is used with anSNL 511-609 housing. The standard end coversare suitable for temperatures of –40ºC to+110ºC (-40ºF to +230ºF). For highertemperatures, a metal end cover should be used.

InterchangeabilityThe newest design SNL replaces SNH, whichreplaced SNA. The original SN series can still

be found in use in the market place. Althoughthe principal dimensions have not changed, theSNA, SNH and SNL hub opening diameter hasbeen enlarged. Seals and end covers for SNstyle will not fit SNA, SNH or SNL. The largerhub openings mean fewer pillow block sizes areneeded for a wider range of shaft sizes. This isaccomplished by using new seal designs withdifferent section heights to fit the hub openings.Dimensions of SNL conform to ISO 113:1994.

SNL Design CharacteristicsSNL pillow block housings are a furtherdevelopment of the well-proven SNH. SNLhave all the advantages of SNH, such as highload carrying capacity and machining quality,along with the following enhancements:SNL housing black provides additional heatdissipation. The housing is reinforced with ribsand extra material surrounding the attachmentboltholes. Attachment bolts can be preloadedfor better location without deforming thehousing base or bore. Heat conductivity isimproved through increased contact areabetween the base and base plate. Bearings willrun 5-10% cooler than in other housings. Capsand bases are individually marked to preventmixing. Housing dimples indicate the locationpoints for attachments, i.e., site glasses andcondition monitoring probes. Indentations arecast on the end and side faces of the housingbase (vertical to the center of the bore) forhousing alignment. Mounting instructions areincluded with the seal pack, inside the block.

Fixing RingsAlso called locating or stabilizing rings, fixingrings are needed to axially locate a bearing onthe shaft. Only one bearing on a shaft section isto be fixed. The same recommendations for useas those for SAF style pillow blocks should befollowed. Fixing rings for SNL pillow blocksare designated by the prefix FRB, i.e., FRB11.5/100. This designation shows the width ofthe ring is 11.5 mm and the outside diameter is



100 mm, thus the ring matches a housing boreof 100 mm, i.e., SNL511-609.Note: there are special considerations involvingaxial displacement versus internal clearance inCARB bearings when used in pillow blocks,including SNL. Consult SKF.

LubricationGrease LubricationSNL housings have dimples cast in the caps toindicate where holes can be drilled and tappedfor grease fittings. The two outer dimples arefor seal re-lubrication, while the others are forbearing re-lubrication. When spherical rollerbearings are re-lubricated via the W33 groove,the grease fitting should be placed in the centerof the cap. The SNL housing can be orderedwith a grease fitting or the cap can be drilledand tapped at the center dimple site. Whenbearings without the W33 feature are to be re-lubricated, such as self-aligning ball bearings orCARB bearings, fittings must be located to theright and, or left of the center. These twodimple sites should not be confused with outerdimples for seal lubrication. For information onhow to order SNL pillow blocks withlubrication features, such as grease fittings andgrease escape holes, as well as initial greasecharge (mass) consult the appropriate SKFcatalog.The general recommendation for moderatespeeds and light loads (C/P > 8.3) is one-third toone-half the volume of free space in the pillowblock base. The bearing itself should be filledcompletely with grease. For high speeds andheavier loads, consult SKF.

Oil LubricationSNL pillow blocks are suitable for oillubrication, which can be applied in severalways, such as oil bath, circulating oil (drysump), and circulating oil (wet sump).Operating conditions dictate which method oflubrication is best. Oil bath is easy to apply andcovers most applications. For oil circulation

applications, contact SKF ApplicationsEngineering Service

Housing Loads, Cap BoltLoadsRecommended safe housing loads, cap boltloads and cap bolt torque information iscontained in appropriate SKF catalogs and inassembly instructions. SNL cap bolts are madeof ISO Grade 8.8 steel.

bearing basics

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