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Bearing Glossary (A–Z)

176 terms used in bearing catalogues, drawings, quotations and condition monitoring reports - written for buyers and maintenance engineers rather than for the standards committee.

176 termsTypes, ratings, clearance, fits, lubrication, failure

#A B C D E F G H I J K L M N O P R S T U V W Z

Cross-check any term against the article that uses it: the technical library covers selection, mounting, lubrication and failure analysis in depth.

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2RS

Suffix for a bearing sealed on both sides with contact rubber seals for grease retention and dust exclusion.

2RSH

Suffix for a bearing with both sides sealed by a contact rubber seal of the RSH design, common in SKF deep groove bearings.

Suffix for a bearing shielded on both sides by non-contacting steel shields that retain grease but do not seal against liquid.

A

ABEC

Annular Bearing Engineers Committee tolerance classification for ball bearings used in the USA, ranging from ABEC 1 to ABEC 9 for increasing precision.

ABMA

American Bearing Manufacturers Association, which publishes tolerance and boundary dimension standards used for inch and metric bearings in North America.

Adapter Sleeve

Tapered sleeve fitted onto a cylindrical shaft so a bearing with a tapered bore can be mounted and locked in position.

Air-Oil Lubrication

Metered lubrication system where small quantities of oil are carried by compressed air directly to each bearing, common in high-speed spindles.

Angular Contact Ball Bearing

Ball bearing with raceways offset so the contact angle carries combined radial and axial loads, often mounted in pairs for higher stiffness.

Axial Internal Clearance

Total possible axial movement of one ring relative to the other when the bearing is unmounted and measured under a light measuring load.

Axial Load

Force acting along the bearing axis, parallel to the shaft centerline, also called thrust load.

B

Bainite Hardening

Heat treatment producing a bainitic microstructure in bearing steel, giving good dimensional stability and toughness for large or shock-loaded bearings.

Ball

Spherical rolling element that carries load between the inner and outer ring raceways in ball bearings.

Base Oil Viscosity

Viscosity of the oil contained in a grease, usually quoted in cSt at 40 C, which governs film thickness in the bearing.

Basic Rating Life

Life that 90 percent of a group of identical bearings reach or exceed before fatigue, expressed in millions of revolutions and denoted L10.

Bearing Steel

High-carbon chromium steel such as 100Cr6 or SAE 52100 used for rings and rolling elements because of its hardness and fatigue resistance.

Bore

Inner diameter of a bearing, denoted d for the inner ring bore, which normally forms the fit on the shaft.

Boundary Dimension

Envelope dimensions of a bearing, mainly bore, outside diameter and width, which must not be exceeded to guarantee interchangeability.

BPFI

Ball Pass Frequency Inner race, the vibration frequency generated when a rolling element passes a defect on the inner ring raceway.

BPFO

Ball Pass Frequency Outer race, the vibration frequency produced when rolling elements pass a defect on the stationary outer ring raceway.

Brass Cage

Machined or pressed cage made of brass, designated by suffix M, used where high strength and temperature resistance are required.

Brinelling

Permanent indentation of raceways caused by a load exceeding the material yield point, often from shock or improper mounting with hammer blows.

BSF

Ball Spin Frequency, the frequency at which a rolling element spins about its own axis, visible in vibration spectra when a ball defect exists.

C

C0a

Basic static axial load rating of a thrust bearing, the pure axial load it can carry without harmful permanent deformation of the raceways.

C0r

Basic static radial load rating, the pure radial load a bearing can carry without exceeding a defined permanent deformation of rolling elements.

Bearing internal clearance smaller than Normal, used to obtain a suitable operating clearance when interference fits reduce the mounted clearance.

Internal clearance larger than Normal, the most common choice for bearings running hot or mounted with heavy interference fits.

Internal clearance larger than C3, selected where thermal expansion or very tight fits on both rings would otherwise close the operating clearance.

Largest standard internal clearance group, used for heavily loaded bearings operating at high temperature where the rings expand substantially.

Basic dynamic axial load rating of a thrust bearing, used with the equivalent load to compute its rating life.

Cage

Component that separates the rolling elements and keeps them evenly spaced, also called a retainer or separator.

Cage Pocket

Machined recess in a cage that guides and holds an individual rolling element, its clearance affecting friction and noise.

Cage Suffix J

Cage code denoting a pressed steel cage, standard on many small and medium deep groove ball bearings.

Cage Suffix M

Cage code denoting a machined brass cage that is guided on the rolling elements or on the outer ring.

Cage Suffix TVH

Cage code for a glass fibre reinforced polyamide cage, common in bearings rated for moderate temperatures up to about 120 C.

Case-Hardened Steel

Steel whose surface is carburized to a hard wear-resistant layer while the core stays tough, used for tapered and spherical roller bearings.

Ceramic Ball

Ball made of silicon nitride, lighter and harder than steel, used in hybrid bearings for high speed and electrical insulation.

Ceramic Bearing

Bearing with ceramic rolling elements, all-ceramic or hybrid, offering low density, high stiffness and resistance to electrical currents.

Chamfer

Small relief at the corner of a bearing ring, denoted r, that prevents edge contact and eases mounting over shaft shoulders.

Clearance Fit

Fit where the shaft is always smaller than the bore, leaving a gap so the bearing can slide or rotate freely.

Special internal clearance group defined for electric motor bearings, giving reduced noise and vibration at normal operating temperature.

Normal internal clearance, the standard clearance group for bearings mounted with light fits and running near ambient temperature.

Combined Load

Simultaneous radial and axial load acting on a bearing, handled by contact angle and combined into an equivalent load P.

Contact Angle

Angle between the line joining ball and raceway contact points and the radial plane, typically 15 to 40 degrees in angular contact bearings.

Contact Seal

Seal lip that touches the rotating ring, giving good exclusion of dust and water but adding friction and limiting speed.

Contamination

Entry of dirt, water or wear particles into a bearing, one of the most common causes of early fatigue and surface damage.

Corner Radius

Radius of the rounded transition between a ring face and the bore or outside diameter, important for shoulder clearance.

Corrosion

Chemical attack of bearing steel by moisture or acids, producing rust that destroys surface finish and leads to premature failure.

Basic dynamic radial load rating, the constant radial load giving a rating life of one million revolutions in the standard formula.

Creep

Slow circumferential movement of a ring on its seat due to insufficient interference fit, causing wear and fretting of the shaft.

Cylindrical Roller Bearing

Bearing with cylindrical rollers giving high radial capacity and stiffness, available in NU, NJ, N and NUP designs for axial location.

D

Deep Groove Ball Bearing

Most widely used bearing type, with close-fitting ball grooves carrying radial and moderate axial loads at high speed.

Delta dmp (mean bore deviation)

Deviation of the mean bore diameter from nominal, a key ISO 492 tolerance measured in the single radial plane.

Delta Dmp (mean outside diameter deviation)

Deviation of the mean outside diameter from nominal, a boundary tolerance controlling the fit of the outer ring in the housing.

Dimensional Stability

Ability of a bearing to keep its dimensions during service and after heat treatment, often specified by a temperature suffix like S1.

DIN 620

German standard specifying tolerances for rolling bearings, with classes P0 to P2 corresponding roughly to normal up to high precision.

dn Value

Product of bearing bore diameter in mm and rotational speed in rpm, a simple indicator of the speed severity and lubrication demand.

Drawn Cup Needle Bearing

Thin outer shell formed by deep drawing, containing needle rollers, used where radial space is very limited.

Dynamic Load Rating

Radial or axial load a bearing can carry for a rating life of one million revolutions, denoted Cr or Ca.

E

Electrical Fluting

Washboard-like ridges on raceways caused by electric current passing through the bearing, common in inverter-driven motors.

End Play

Total axial movement of a shaft between its two axial limits, related to the internal clearance of the locating bearings.

Envelope Analysis

Vibration technique that demodulates high-frequency bursts to reveal bearing defect frequencies, also called envelope or envelope acceleration.

EP Additive

Extreme pressure additive in grease or oil that reacts with metal surfaces to prevent scuffing under heavy or shock loads.

Equivalent Dynamic Load

Constant load, radial for radial bearings, that would give the same rating life as the actual combined load; denoted P.

Equivalent Static Load

Constant static load that produces the same maximum rolling element contact stress as the actual static load combination.

F

False Brinelling

Shallow wear depressions caused by micro-vibration when an unmoving bearing is subjected to vibration, often during transport or standby.

Fatigue Spalling

Flaking of raceway or rolling element material after repeated stress cycles, the classic rolling contact fatigue failure of bearings.

Felt Seal

Seal made of felt rings, used in older or low-speed bearings to retain grease and exclude coarse dirt at low cost.

FKM

Fluorocarbon rubber, also known as Viton, used for seals where high temperature and chemical resistance are needed.

Flange

Projecting collar on a bearing ring or housing that provides axial location or a mounting face for bolts.

Four-Point Contact Bearing

Ball bearing with a single raceway groove that contacts each ball at four points, carrying axial load in both directions.

Fretting

Wear and material transfer between two surfaces under small oscillatory slip, common between ring and shaft when the fit is loose.

Fretting Corrosion

Oxidation-assisted fretting that produces reddish-brown debris at ring seats and can eventually loosen the fit and damage the shaft.

FTF

Fundamental Train Frequency, the rotation frequency of the cage, equal to the shaft speed minus the ball pass effect.

G

Grease

Semi-solid lubricant made of base oil, thickener and additives, retained in a bearing to provide long-term lubrication without a circulation system.

Grease Life

Operating time over which a grease retains its lubricating ability, often quoted as L10 for a defined speed and temperature.

Groove

Circular channel in a bearing ring in which the balls or rollers run, its radius and conformity controlling stress and friction.

H

Shaft tolerance with an upper deviation of zero, giving a clearance fit when used with a normal bearing bore in light-duty applications.

Common housing bore tolerance with a lower deviation of zero, producing a transition or interference fit with a standard bearing outer ring.

HNBR

Hydrogenated nitrile rubber with improved heat and oxidation resistance compared with NBR, used for demanding seal applications.

Housing Bore

Bore in the housing or machine frame that receives the bearing outer ring, its tolerance controlling the outer ring fit.

Hybrid Bearing

Bearing with steel rings and ceramic rolling elements, giving higher speed, lower friction and electrical insulation in motors.

Hydraulic Nut

Oil-pressure tool that expands a sleeve or forces a tapered seat apart to mount or dismount bearings with controlled force.

I

Induction Heater

Device that heats a bearing inner ring by induced currents so it expands and slides onto a shaft without force.

Inner Ring

Ring with the bore that fits on the shaft and normally carries the inner raceway, often the rotating ring.

Insert Bearing

Self-contained ball bearing with a wide inner ring and locking device, mounted in a cast housing to form a mounted unit.

Instrument Bearing

Small, high-precision bearing used in instruments and miniature mechanisms, often with low torque and low noise requirements.

Interference Fit

Fit in which the shaft is larger than the bore, producing a tight assembly that prevents ring creep under load.

ISO 15243

International standard that classifies rolling bearing damage and failure modes into fatigue, wear, corrosion, electrical, plastic and overheating groups.

ISO 492

International standard defining boundary dimensions and tolerances for radial bearings, with classes Normal, P6, P5, P4 and P2.

ISO 5753

International standard listing radial internal clearance values for bearings, grouped as C2, CN, C3, C4 and C5.

J

Shaft tolerance giving a light transition fit with a bearing bore, sometimes used for locational applications with minimum interference.

JIS B 1514

Japanese Industrial Standard that specifies tolerances and running accuracy for rolling bearings, similar in structure to ISO 492.

K

Shaft tolerance giving a light interference fit with a bearing bore, widely used for the rotating inner ring of normal-speed machines.

Kea

Permissible variation of the outside diameter of an assembled bearing, an ISO 492 running accuracy parameter linked to radial runout.

Kia

Permissible variation of the bore diameter of an assembled bearing, an ISO 492 running accuracy parameter linked to inner ring runout.

L

L10 Life

Basic rating life in millions of revolutions at which 90 percent of a group of identical bearings survive, the standard fatigue life measure.

Labyrinth Seal

Non-contact seal formed by a series of narrow gaps and grooves that make a tortuous path for contaminants, giving low friction.

Land

Shoulder surface or guiding surface of a bearing ring, such as the land that centers a cage or supports a seal lip.

Life Adjustment Factor

Factor a1 applied to the basic rating life for reliability other than 90 percent, for example 0.62 for 95 percent reliability.

Linear Bearing

Bearing that allows unlimited straight-line motion, usually with recirculating balls or rollers running on a hardened shaft or rail.

Lithium Complex Grease

Multi-purpose grease thickened with lithium complex soap, offering a high dropping point and good water resistance for bearing applications.

Lnm

Modified rating life calculated from the basic life, the life adjustment factor a1 and a life modification factor aISO.

Load Ratio

Ratio of axial to radial load or of applied load to rating, used to select bearing type and to check equivalent load.

Load Spectrum

Description of the varying loads and speeds a bearing experiences over a duty cycle, used to compute a weighted rating life.

Lock Nut

Nut threaded on a shaft or adapter sleeve to press the bearing inner ring against a shoulder and set axial location.

Lock Washer

Tabbed washer that engages a shaft groove and a lock nut slot to prevent the nut from loosening in service.

M

Shaft tolerance giving a medium interference fit with a bearing bore, used for heavy loads or high-speed rotating inner rings.

Machined Cage

Cage produced by machining from a solid ring of brass, steel or light alloy, giving high strength and precise pockets.

Mean Diameter

Arithmetic mean of the bore and outside diameter, denoted dm, used when computing the dn value of a bearing.

Miniature Bearing

Very small bearing, typically with an outside diameter below about 9 mm, used in instruments, medical devices and small motors.

Mounted Unit

Pre-assembled bearing in a cast iron or pressed steel housing, including pillow blocks and flange units, ready for bolting down.

Mounting Force

Axial force applied to mount a bearing, which should act on the interference-fitted ring only, never through the rolling elements.

N

NBR

Nitrile butadiene rubber, a general-purpose seal material with good oil resistance and moderate temperature capability.

Needle Roller Bearing

Bearing using long, slender cylindrical rollers, giving very high radial capacity in a small radial section height.

NLGI Grade

Consistency classification of grease from 000 fluid to 6 very hard, with NLGI 2 the usual choice for rolling bearings.

Non-Contact Seal

Seal that does not rub the rotating ring, such as a gap or labyrinth type, giving low friction and high speed capability.

O

Oil Bath

Lubrication method where the bearing or its housing partly dips into an oil sump, suitable for low and moderate speeds.

Oil Lubrication

Lubrication with liquid oil supplied by bath, circulation, jet or mist, preferred when high speed or heat removal is required.

Oil Mist

Lubrication system that atomizes oil into a low-pressure air stream and delivers a fine mist to the bearing.

Operating Clearance

Internal clearance present in a mounted bearing at its working temperature, after fits and thermal expansion have acted.

Outer Ring

Ring with the outside diameter that fits in the housing and normally carries the outer raceway, often the stationary ring.

Outside Diameter

Outer diameter of a bearing, denoted D, which forms the fit in the housing bore.

P

Pillow Block

Mounted bearing unit with a housing that bolts to a flat surface, supporting a shaft that runs parallel to the mounting face.

Pitch Diameter

Diameter of the circle passing through the centers of the rolling elements, also called the pitch circle diameter.

Polyamide Cage

Cage moulded from polyamide PA66, light and quiet but limited to about 120 C, common in mass-produced deep groove bearings.

Precision Class

Tolerance grade of a bearing such as Normal, P6, P5, P4 or P2, controlling bore, diameter and running accuracy.

Preload

Small axial or radial force applied to a bearing pair to remove clearance, increasing stiffness and reducing noise and runout.

Press Fit

Assembly by pushing a shaft into a slightly smaller bore, producing an interference fit without heating.

Pressed Cage

Cage stamped and formed from steel or brass sheet, low cost and widely used in small and medium bearings.

R

Raceway

Hardened and ground surface of a ring on which the rolling elements run, its geometry governing load capacity and life.

Radial Internal Clearance

Total radial movement of one ring relative to the other under a light measuring load, grouped by ISO 5753.

Radial Load

Force acting perpendicular to the bearing axis, taken mainly by the rolling elements in the loaded zone.

Radial Runout

Difference between the largest and smallest radial distance of a ring surface during one rotation, a key running accuracy measure.

Relubrication Interval

Time or operating hours between successive grease replenishments, estimated from speed, temperature and bearing size.

Rib

Raised axial flange on a roller bearing ring that guides the rollers or takes axial load, as in NJ and tapered designs.

Roller

Cylindrical, tapered or spherical rolling element that contacts the raceway along a line, giving higher load capacity than a ball.

Suffix for a single contact rubber seal fitted on one side of a bearing, giving improved dirt and moisture exclusion.

Run-To-Failure

Maintenance strategy that runs a bearing until damage forces replacement, used only for non-critical or cheap-to-replace machines.

Running Accuracy

Ability of a bearing to rotate with minimal runout, specified by ISO 492 parameters such as Kia, Kea and Sd.

S

Seal Friction

Torque loss caused by a seal lip rubbing the ring, which raises temperature and limits the permissible speed of the bearing.

Self-Aligning Ball Bearing

Double-row ball bearing with a spherical outer raceway that tolerates shaft misalignment of a few degrees without extra load.

Shaft Seat

Ground portion of a shaft on which the bearing inner ring is fitted, its diameter and roundness affecting fit and life.

Shaft Shoulder

Machined step on a shaft that axially locates the bearing inner ring and transmits mounting or operating forces.

Shoulder

Axial face of a ring adjacent to the raceway, used to guide rollers, support a cage or seat a seal.

Silicon Nitride

Ceramic material Si3N4 used for bearing balls and rollers, giving low density, high hardness and good electrical insulation.

Slewing Bearing

Large diameter bearing, often with integral gear teeth, that supports and rotates the upper structure of cranes and excavators.

Smearing

Localised surface damage from sliding under heavy load and poor lubrication, producing torn metal and a rapid temperature rise.

Solid Lubricant

Dry lubricant such as graphite or molybdenum disulphide used where oil or grease cannot survive, for example very high temperature.

Speed Rating

Maximum permissible rotational speed of a bearing for a given lubrication and load, limited by temperature and cage strength.

Spherical Plain Bearing

Plain bearing with a spherical sliding contact, often with a PTFE liner, that accommodates misalignment and transmits radial or axial load.

Spherical Roller Bearing

Self-aligning bearing with barrel-shaped rollers in two rows, designed for heavy radial and axial loads and shaft deflection.

Stainless Steel 316

Austenitic stainless steel with good corrosion resistance but lower hardness, used for bearing rings in food and chemical equipment.

Stainless Steel 440C

Martensitic stainless steel with high hardness and moderate corrosion resistance, a common material for stainless bearing rings and balls.

Static Load Rating

Load a bearing can carry while stationary or turning slowly without harmful permanent deformation, denoted C0r or C0a.

Static Safety Factor

Ratio of the static load rating to the equivalent static load, denoted s0; values above about 1.5 are common for normal service.

Steel Cage

Cage made of pressed or machined steel, offering high strength and temperature resistance at low cost for most bearings.

T

Tapered Roller Bearing

Bearing with conical rollers and raceways whose apexes meet on the axis, carrying high combined radial and axial loads.

Thermography

Infrared imaging technique that detects heat generated by a failing bearing, useful for condition monitoring of machines in service.

Thickener

Soap or non-soap agent that holds base oil in a grease structure, its type and content controlling consistency and temperature range.

Through-Hardened Steel

Bearing steel hardened throughout its section by quenching and tempering, used for rings and rollers of standard bearings.

Thrust Ball Bearing

Bearing with flat or sphered washers and balls, designed to carry axial load only, usually in one direction.

Thrust Roller Bearing

Bearing with cylindrical or spherical rollers for high axial load, including cylindrical, spherical and tapered thrust designs.

Total Runout

Combined radial and axial runout of a ring surface during rotation, controlled by ISO 492 for precision bearings.

Track Roller

Bearing with a thick outer ring, often crowned, used as a wheel running directly on a track, cam or guide rail.

Transition Fit

Fit that may result in either a small clearance or a small interference, depending on the actual sizes within tolerance.

U

Under-Lubrication

Insufficient lubricant film in a bearing, causing metal-to-metal contact, overheating, wear and early fatigue failure.

V

V-Ring

Elastic all-rubber seal ring stretched onto a shaft, its lip contacting a counterface, used as an external dirt and splash seal.

Vdmp

Permissible variation of the mean bore diameter within a single radial plane, an ISO 492 tolerance limiting bore ovality.

Vdp

Permissible variation of bore diameter within a single radial plane, an ISO 492 roundness tolerance for the inner ring.

Vibration Analysis

Condition monitoring method that measures bearing vibration to detect defects early and estimate remaining service life without dismantling.

Viscosity Ratio

Ratio of the actual operating viscosity to the required reference viscosity, denoted kappa, used in the modified rating life.

W

Water Resistance

Ability of a grease or seal to prevent water from reaching the bearing and to resist being washed out by water.

Wear

Progressive loss of surface material from rolling or sliding contact, caused by contamination, poor lubrication or inadequate hardness.

Width

Axial dimension of a bearing, denoted B for the inner ring or C for the outer ring, determining the mounting space.

Withdrawal Sleeve

Tapered sleeve used to mount a bearing with a tapered bore on a cylindrical shaft and to remove it during dismounting.

Z

Suffix for a single non-contacting steel shield on one side, used to keep coarse dirt out of the bearing.

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