Knowledge · Glossary
Bearing glossary
56 technical terms — load ratings, clearance, fits, lubrication, failure modes and designs, explained for practitioners and cross-linked.
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Back-to-back arrangementThe back-to-back (O) arrangement pairs two angular contact ball bearings or tapered roller bearings so that their pressure lines diverge outward, forming an "O" shape. The pressure centers lie far apart, creating a wide effective support base.Basic rating life L10The basic rating life L10 is the number of revolutions — or operating hours L10h at constant speed — that 90 percent of a sufficiently large group of identical bearings will reach or exceed before the first signs of material fatigue. Per ISO 281 it is calculated as L10 = (C/P)^p, with life exponent p = 3 for ball bearings and p = 10/3 for roller bearings.Bearing designationA bearing designation is the standardised code that fully describes a rolling bearing. It consists of an optional prefix, the basic designation and optional suffixes. The basic designation encodes bearing type, dimension series and bore: in 6205, the 6 denotes the type, 2 the dimension series and 05 a 25 mm bore. Suffixes add execution details such as seals and shields (2RS, ZZ), internal clearance (C3), cage material or tolerance class. Basic designations are largely comparable across manufacturers, but suffixes can be brand-specific — when identifying an old bearing, cross-check dimensions and suffixes to find the technically equivalent replacement.Bearing steel 100Cr6100Cr6 is the classic through-hardening bearing steel and the standard material for rings and rolling elements of most rolling bearings. The hypereutectoid chromium steel — the designation indicates roughly one percent carbon with chromium addition — is hardened and tempered, typically reaching about 58–65 HRC. This hardness underpins load capacity in rolling contact, dimensional stability and resistance to indentation. Steel cleanliness is decisive for bearing life, since non-metallic inclusions act as crack initiators for subsurface fatigue; modern vacuum-treated melts have raised achievable life considerably. Its limits are corrosion and elevated temperature — stainless or hybrid bearings serve those applications.Bore codeThe bore code is formed by the last two digits of a rolling bearing's basic designation. From code 04 upwards, multiplying by 5 gives the bore in millimetres: a 6204 has a 20 mm bore, a 6310 a 50 mm bore. Four exceptions apply below that: 00 = 10 mm, 01 = 12 mm, 02 = 15 mm, 03 = 17 mm. For bores under 10 mm the diameter appears directly in the designation (e.g. 608 = 8 mm); intermediate bores are appended after a slash (e.g. 62/28 = 28 mm). The code is uniform across manufacturers and the quickest way to identify an unknown bearing.
C
CageThe cage keeps the rolling elements evenly spaced, prevents mutual contact and guides them through the unloaded zone. It carries no external load but strongly influences friction, limiting speed and noise. Common designs are pressed steel cages (robust standard), machined brass cages (high cage strength, for high temperatures and rib guidance) and glass-fibre-reinforced polyamide cages (quiet, good emergency-running properties, favourable under shock thanks to elasticity and low mass, but limited continuous temperature and sensitive to some oil additives). Cage material is coded in the manufacturer-specific designation suffixes. High cage strength, heat and rib guidance favour machined metal cages, while shock and high accelerations favour polyamide; cage wear is a typical secondary damage from lubricant starvation, and cage frequencies are a key condition-monitoring signal.Circumferential & point loadCircumferential and point load describe how the load direction moves relative to a bearing ring. If the ring rotates relative to the load, the load zone travels around its entire raceway (circumferential load); if it is stationary relative to the load, the same raceway spot is always loaded (point load). With a rotating shaft and stationary load, the inner ring sees circumferential load and the outer ring point load; rotating unbalance loads reverse this.Condition monitoringCondition monitoring is the condition-based surveillance of machinery aimed at detecting bearing damage long before failure. Vibration analysis is the key method for rolling bearings: raceway and rolling-element defects generate characteristic overrolling frequencies derived from bearing geometry and speed, visible in spectrum and envelope analysis. Temperature, lubricant condition and ultrasonic techniques complement it. The benefit is plannability — bearings are replaced when their condition requires it rather than on fixed intervals or after unplanned downtime. Early indicators such as incipient pitting, lubricant starvation or electrical erosion can be distinguished and addressed. Solutions range from handheld route devices to permanently installed online systems; trending against a clean baseline is essential.Contact angleThe contact angle is the angle between the line of contact — along which force passes from rolling element to raceways — and the bearing's radial plane. It sets the ratio of radial to axial load capacity: 0° bearings (cylindrical roller) carry essentially radial load only, 90° pure thrust bearings axial only. Angular contact ball and tapered roller bearings lie between; axial capacity rises with the angle while speed capability tends to fall (typical nominal angles for angular contact ball bearings: 15° to 40°). The angle also defines the pressure-cone apexes and thus the effective spread of adjusted O- or X-arrangements, governing tilting rigidity.Contact sealA contact seal presses an elastic sealing lip – typically NBR, or FKM for higher temperatures – against a counterface on the inner ring under defined preload. On sealed bearings, suffixes such as RS or 2RS (both sides) denote this design; the seal washer is usually steel-reinforced and anchored in the outer ring. Contact seals reliably exclude dust, dirt and splash water and retain the grease, enabling sealed-for-life, maintenance-free bearings. The trade-off is lip friction, which raises frictional torque and operating temperature and lowers the permissible speed compared with shields or open bearings. Low-friction contact variants offer a compromise.
D
Dimension seriesDimension series classify bearings of identical bore by cross-section. The diameter series defines the outside diameter relative to the bore, the width series the width; together they form the dimension series to ISO 15. For a 25 mm bore, deep groove ball bearings 61805, 61905, 6005, 6205 and 6305 share the same bore while outside diameter, width and load capacity increase from series to series. In practice: for more capacity on the same shaft, move to a heavier series if housing space allows; for compact, lightly loaded designs, choose a light series. The system applies across bearing types and structures every dimension table.Dynamic load rating CThe dynamic load rating C, defined in ISO 281, is the constant load of fixed direction that a rolling bearing can theoretically sustain for a basic rating life of one million revolutions — purely radial for radial bearings, purely axial and centric for thrust bearings. It is calculated by the manufacturer from bearing geometry, material and manufacturing quality and listed in every dimension table.
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Elastohydrodynamic lubricationElastohydrodynamic lubrication (EHD/EHL) is the lubrication regime in the rolling contact: under extreme contact pressure, an ultra-thin load-carrying lubricant film forms between rolling element and raceway. The steel surfaces deform elastically and the lubricant's viscosity rises sharply within the contact – this interplay is what enables full surface separation. Film thickness depends mainly on base oil operating viscosity, contact speed and temperature, while load has comparatively little influence. Film adequacy is assessed via the viscosity ratio, which feeds directly into modified bearing life calculation. Insufficient viscosity or speed causes mixed friction, surface distress and premature fatigue.Electrical flutingElectrical fluting occurs when current passes through a rolling bearing. Discharges through the lubricant film create microscopic melt craters which, under continued overrolling, organize into a grey washboard-like pattern across the raceway. ISO 15243 lists electrical erosion as a separate damage class. Affected bearings produce tonal noise and increased vibration, and the discharges also degrade the grease. The most common cause is high-frequency bearing currents in inverter-driven motors; faulty earthing or welding current routed through the bearing has the same effect. Remedies include insulated coated bearings, hybrid bearings with ceramic rolling elements, shaft grounding rings and proper earthing practice.Equivalent bearing load PThe equivalent bearing load P converts a combination of radial and axial forces into a single substitute load with the same effect on bearing life as the actual load combination. It follows the basic formula P = X·Fr + Y·Fa, with factors X and Y depending on bearing type, contact angle and the axial-to-radial load ratio, as tabulated in manufacturer catalogues.
F
Face-to-face arrangementIn the face-to-face (X) arrangement, the pressure lines of two angular contact or tapered roller bearings converge toward the bearing axis, crossing between the bearings to form an "X". The pressure centers lie closer together than in the back-to-back arrangement, giving a smaller effective support base.False brinellingFalse brinelling describes shallow wear depressions in raceways that form while the bearing is stationary. Unlike true brinelling caused by overload, the marks result from micro-movements between rolling elements and raceway under external vibration. Because there is no rolling motion, the lubricant film is displaced locally and fretting wear develops at rolling-element pitch. ISO 15243 assigns this damage to the wear and fretting category. Typical causes are transport without shaft locking, standstill vibration from nearby machines and small oscillating movements. Remedies include securing shafts during transport, preloading, periodic rotation during standstill, vibration isolation and greases with suitable anti-wear additives.Fatigue & pittingFatigue is the classical rolling-contact failure mode described in ISO 15243 and the basis of calculated bearing life. Repeated overrolling initiates micro-cracks below or at the raceway surface; when particles break out, small craters known as pitting appear, progressing to larger spalling. Symptoms include rising noise, vibration and temperature. ISO 15243 distinguishes subsurface-initiated fatigue under high load from surface-initiated fatigue caused by poor lubrication, contamination or overrolling of indentations. Countermeasures include adequate bearing sizing, clean mounting, effective sealing, a sufficient lubricant film, and early detection through condition monitoring to prevent consequential damage.Fatigue load limit PuThe fatigue load limit Pu is the load below which, under ideal conditions — full separation of the contact surfaces by the lubricant film and utmost cleanliness — no fatigue theoretically occurs in the raceways. It transfers the endurance-limit concept of classical fatigue theory to rolling contacts.FitA fit describes the dimensional relationship between a bearing ring and its seat – inner ring to shaft, outer ring to housing bore. Since bearing boundary dimensions are standardized to ISO tolerances, the fit is determined solely by the shaft and housing tolerance zones, producing clearance, transition or interference conditions.Floating & locating bearingThe locating/floating arrangement is the standard concept for supporting a shaft at two bearing positions. The locating bearing guides the shaft axially in both directions and carries thrust; the floating bearing supports radially only and permits axial displacement, absorbing thermal expansion and tolerances without cross-locating preload — a frequent cause of overheated bearings. Suitable floating bearings are NU/N cylindrical roller bearings, which allow displacement inside the bearing on the ribless raceway, or deep groove ball bearings with a sliding outer-ring fit (point load required). Alternatives are adjusted arrangements of paired angular contact bearings (O or X) for short, stiff shafts, and floating arrangements with limited end play.Fretting corrosionFretting corrosion appears as a reddish-brown to black deposit on the fit surfaces between bearing ring and shaft or housing. Micro-movements in the fit continuously remove protective oxide layers; the fine debris oxidizes and acts abrasively. ISO 15243 classifies it under frictional corrosion. Consequences include loss of fit, creeping rings, local stress concentrations up to ring fracture, and difficult dismounting. The main cause is a fit too loose for the load case — especially circumferential load on a ring with a clearance fit — plus seat form errors. Remedies: correct fit selection, accurate seat geometry, mounting paste, and seat inspection before fitting a new bearing.
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H
H1 food-grade lubricantH1 lubricants are greases and oils registered by NSF for use in the food and beverage industry where incidental, technically unavoidable contact with food cannot be ruled out. They are formulated from approved base oils such as medical white oils or selected synthetics, with physiologically harmless thickeners and additives. Category H2 covers points with no possible food contact; 3H covers direct contact. HACCP concepts often mandate H1 throughout the process area. Many manufacturers supply sealed deep groove ball bearings and housed units factory-filled with H1 grease, frequently in stainless steel for washdown areas. Do not mix H1 greases from different suppliers unverified.Housed bearing unitA housed bearing unit is a ready-to-mount combination of an insert bearing and a matching housing in cast iron, pressed steel, stainless steel or thermoplastic. Main types are pillow blocks (e.g. UCP series), square, oval and round flanged units (UCF, UCFL, UCFC) and take-up units. The insert bearing's spherical outer contour sits in a matching housing seat, allowing it to align itself.Hybrid bearingHybrid bearings combine steel rings with ceramic rolling elements, typically silicon nitride (Si3N4). The ceramic elements are electrically insulating, harder than steel and of lower density. Two use cases dominate: in electric motors and generators, hybrid bearings interrupt current passage and prevent electrical fluting of the raceways; in high-speed applications such as machine tool spindles, the lighter rolling elements reduce centrifugal loads and improve running behaviour. Hybrid deep groove ball bearings and spindle bearings are dimensionally interchangeable with their all-steel counterparts, so upgrading usually requires no design change. The higher price is offset by longer service life in demanding applications.
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Inner ringThe inner ring is the bearing ring mounted on the shaft; its outer surface carries the hardened, precision-finished raceway, while the bore forms the shaft seat. It is usually made of through-hardened bearing steel such as 100Cr6 and toleranced to ISO 492. In most applications it rotates with the shaft and therefore sees circumferential load, so it is mounted with an interference fit to prevent creeping and fretting corrosion. Mounting forces must be applied to the inner ring only, never through the rolling elements, and the fit must be chosen together with the internal clearance, since interference reduces radial play.Insert bearing (Y-bearing)An insert bearing (Y-bearing) is a sealed single-row deep groove ball bearing with an extended inner ring and a spherical (convex) outer ring surface, designed for mounting in housed bearing units. The spherical contour self-aligns in the housing seat, compensating static misalignment.Interference fitAn interference fit means the bearing ring seat has dimensional interference with its counterpart, creating surface pressure after mounting that fixes the ring by friction and prevents creep under load. It is generally required for the ring carrying circumferential load – typically the inner ring with a rotating shaft and stationary load direction.Internal clearanceInternal clearance is the total distance one bearing ring can be displaced relative to the other without external load — radially (radial clearance) or axially. It is measured in the unmounted state and graded for radial bearings into standardized classes: C2 (less than normal), CN (normal), C3, C4 and C5 (progressively larger), per ISO 5753.
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Labyrinth sealA labyrinth seal is a non-contact seal in which rotating and stationary parts form interlocking chambers and narrow gaps. The repeatedly deflected gap path presents high flow resistance to entering contaminants and escaping lubricant; the passages are often grease-packed so the grease acts as a barrier and flushes dirt outward during relubrication. With no rubbing contact, labyrinth seals run virtually friction- and wear-free with no lip-imposed speed or temperature limit, suiting high peripheral speeds and long service intervals, typically on plummer block housings, fans, pumps and gearboxes. Against standing water or pressure they are not tight on their own – contact seals or combined systems are then required.Limiting speedThe limiting speed is the speed a rolling bearing should not permanently exceed for mechanical-kinematic reasons: centrifugal forces on rolling elements and cage, cage stability and guidance, the sliding speed of contact seals, and the lubricant's ability to maintain a load-carrying film. Unlike the reference speed, it is a design limit rather than a thermal one.
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O
Oil lubricationOil lubrication supplies rolling bearings with liquid lubricant and is used where grease reaches its limits – high speeds, high operating temperatures, or where adjacent components such as gears are oil-lubricated anyway. Common methods include oil bath, circulating oil, oil jet and oil-air (minimal quantity) lubrication for high-speed spindles. Its key advantage is heat dissipation: circulating oil carries away friction heat and wear particles and can be filtered and cooled, enabling higher speeds than grease. The correct operating viscosity in the rolling contact is decisive for bearing life; oil analysis and scheduled changes support condition monitoring.One-way clutchA one-way clutch (freewheel) transmits torque in one direction of rotation and overruns freely in the other. Locking is achieved by sprags or rollers that wedge between inner and outer ring in the driving direction. Three basic functions cover most applications: backstops prevent reverse rotation, for example on inclined conveyors; overrunning clutches disengage a drive automatically when the driven side rotates faster; indexing freewheels convert oscillating motion into stepwise rotation. Designs range from clutch-only units without bearing support to complete ball-bearing-supported freewheels and sprag cages for customer-supplied rings. Torque, overrunning speed and suitable lubrication govern selection.Operating clearanceOperating clearance is the internal clearance that actually remains in a mounted bearing under operating conditions. It equals the initial clearance minus the reduction caused by fits — inner-ring expansion on an interference-fitted shaft, outer-ring contraction in the housing — and minus thermal effects when the inner ring runs hotter than the outer ring.Outer ringThe outer ring is the outer bearing ring seated in the housing bore; its inner surface carries the raceway, its outside diameter forms the housing seat. It is made of hardened bearing steel and toleranced to ISO 492. With a rotating shaft the outer ring is stationary and sees point load, so a loose-to-transition housing fit suffices — easing mounting and allowing axial displacement at floating bearing positions. If the housing rotates (wheel hubs, rope sheaves), the ring sees circumferential load and needs a tight fit. Housing roundness and stiffness matter: a distorted bore deforms the ring and falsifies operating clearance.
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RacewayThe raceway is the hardened, precision-finished surface on the inner and outer ring (or shaft and housing washer of thrust bearings) on which the rolling elements run. In ball bearings it is a groove whose radius is slightly larger than the ball radius (osculation); in roller bearings it is cylindrical, tapered or spherical. Contact areas are small, so Hertzian pressures are high. Surface finish and form accuracy govern running noise, friction and the elastohydrodynamic lubricant film. The raceway is also the primary damage zone: rolling contact fatigue, false brinelling and electrical fluting all appear here, and indented contamination particles act as fatigue initiators.Radial bearingRadial bearings primarily carry loads perpendicular to the shaft axis; their nominal contact angle lies between 0° and 45°. They form the largest bearing group: deep groove and angular contact ball bearings, cylindrical, tapered, spherical roller and needle roller bearings. Many also take limited axial load — deep groove ball bearings via raceway osculation, angular contact and tapered roller bearings in one direction by design. Type selection follows the load case: high speed favours ball bearings, heavy radial load cylindrical rollers, combined load tapered rollers, misalignment self-aligning types, tight radial space needle bearings. Sizing per ISO 281 uses the equivalent dynamic load against the dynamic load rating.Reference speedThe reference speed is a thermal rating: the speed at which, under standardized reference conditions for load, lubrication and heat dissipation (ISO 15312), the frictional heat generated in the bearing balances the heat removed via shaft and housing. It is not a mechanical limit but describes thermal equilibrium under standard conditions.Relubrication intervalThe relubrication interval is the period after which a grease-lubricated rolling bearing must receive fresh grease, because the grease service life is shorter than the intended bearing life. It depends on bearing type and size, speed, operating temperature, load, mounting orientation and contamination. Manufacturers provide diagrams and calculation methods with correction factors for real operating conditions; rising temperature, vertical shafts, vibration and dirt all shorten the interval. For very short intervals, automatic lubricators or centralized systems are more economical. Relubricate with the machine running or warm, allow spent grease to escape, and verify grease compatibility before any changeover.Rib, flangeA rib (flange) is a raised shoulder beside a raceway that guides the rolling elements axially and, depending on the design, carries axial load. In cylindrical roller bearings the rib layout defines the type: NU (two outer-ring ribs, ribless inner ring — axially free, ideal as floating bearing), N (reversed), NJ (one inner-ring rib, axial location in one direction), NUP (loose rib washer, both directions). Tapered roller bearings support their rollers on the large inner-ring rib. Roller-end/rib contact is sliding, not rolling: it limits permissible axial load, demands adequate lubricant viscosity, and fails first by smearing under starvation or axial overload.Rod endA rod end is a ready-to-mount machine element combining a spherical plain bearing with a housing eye and a threaded shank. Shanks come with male or female thread, in right-hand or left-hand versions; a right/left pair allows fine length adjustment of a linkage without dismantling the ends. Rod ends transmit tension and compression along the rod axis while compensating angular movement and misalignment. Relubricatable steel-on-steel versions with grease nipples and maintenance-free versions with self-lubricating liners are available. Typical applications include pneumatic and hydraulic cylinders, push rods, and linkages in agricultural and construction machinery.Rolling bearingRolling bearings support parts that move relative to each other — typically a rotating shaft in a stationary housing — with minimal friction. Rolling elements (balls or rollers) run between hardened raceways on the inner and outer ring, separated by a cage; rolling contact replaces the sliding friction of plain bearings. Key standards are DIN 625 (deep groove ball bearings), ISO 492 (tolerances) and ISO 281 (rating life). Ball bearings suit high speeds, roller bearings carry heavier loads via line contact. Selection depends on load, speed, misalignment, lubrication and environment; correct fits and clean mounting often govern service life more than calculated fatigue life.Rolling elementsRolling elements transfer load between inner and outer ring by rolling on the raceways. Common forms are balls, cylindrical rollers, needle rollers, tapered rollers and spherical rollers. Balls make point contact, favouring high speeds at moderate loads; rollers make line contact, carrying much higher loads for the same size but tolerating less misalignment. They are made from through-hardened bearing steel (typically 100Cr6) to extremely tight tolerances; hybrid bearings use silicon nitride ceramic balls — lighter, harder and electrically insulating. Standstill vibration can cause false brinelling, and current passage causes fluting, both appearing as patterns at rolling-element spacing.
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ShieldA shield is a non-contact steel closure anchored in the bearing's outer ring, forming a narrow gap to the inner ring without touching it. Common suffixes are Z (one side) and 2Z/ZZ (both sides); such bearings are factory grease-filled and normally lubricated for life. With no rubbing lip, shields add virtually no friction: shielded bearings reach the same speeds as open bearings and run cooler than contact-sealed ones. They retain grease and exclude coarse contamination but offer no protection against fine dust, moisture or splash water. Typical applications are electric motors and fans; in dirty or wet environments, contact seals (2RS) are the right choice.Solid lubricationSolid lubrication replaces oil or grease with dry lubricants such as graphite, molybdenum disulphide (MoS2) or PTFE, applied as bonded coatings on raceways, as powder or paste, or embedded in cage materials that transfer a lubricating film during operation. It is used where conventional lubricants fail or are prohibited: very high temperatures beyond grease limits, vacuum, radiation, or where oil outgassing and contamination are unacceptable, as in furnace and cleanroom applications. Limitations: solid lubricants dissipate no heat and the film is consumed rather than self-replenishing, so speeds and service life are well below oil- or grease-lubricated bearings.Spherical plain bearingSpherical plain bearings are sliding bearings with a spherical contact surface: an inner ring with a convex outer surface slides in an outer ring with a concave spherical bore. They carry very high static and shock loads while accommodating angular movement and misalignment between shaft and housing. Maintenance-type versions with steel-on-steel sliding contact require regular relubrication; maintenance-free versions use liners such as PTFE fabric. Radial types are most common, complemented by thrust and angular contact designs. Typical applications include hydraulic cylinder mountings, linkages, and construction and agricultural machinery — generally locations with oscillating rather than rotating motion.Stainless steel bearingStainless steel bearings use corrosion-resistant steel, usually martensitic chromium grades, for rings and rolling elements. They are the standard choice wherever moisture, cleaning agents or aggressive media would attack conventional 100Cr6 bearing steel — typically in food and beverage processing, packaging, medical equipment and outdoor applications. In hygiene-critical areas they are often sealed on both sides and greased with food-grade H1 lubricants. Corrosion-resistant steels generally reach slightly lower hardness than 100Cr6, so load ratings sit below those of comparable standard bearings; always check the manufacturer's catalogue values. Prefixes such as S, SS or W identify stainless versions.Static load rating C0The static load rating C0 is the load a stationary or very slowly rotating bearing can carry without inadmissible plastic deformation of rolling elements and raceways. The standard criterion is a permanent deformation of roughly 0.0001 of the rolling element diameter at the most heavily loaded contact; larger indentations cause noise, vibration and premature failure in later operation.
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Tandem arrangementIn a tandem arrangement, two or more angular contact or tapered roller bearings are mounted in the same orientation so their pressure lines run parallel and all bearings act in the same axial direction. The set behaves like a single angular contact bearing with increased capacity.Thrust bearingThrust bearings primarily carry loads along the shaft axis; their nominal contact angle exceeds 45°, reaching 90° for pure thrust types. Designs include thrust ball bearings (single or double acting), cylindrical, needle and tapered roller thrust bearings, and spherical roller thrust bearings — the only type that also accommodates significant radial load and misalignment. The rings are called shaft and housing washers. Thrust bearings are speed-sensitive because centrifugal force pushes the rolling elements outward; thrust ball bearings therefore require a minimum axial load per manufacturer data to prevent skidding. Mounting demands flat, square abutment faces, and pure thrust bearings are almost always combined with radial bearings.Tolerance classes P0–P4Tolerance classes define how precisely a bearing's bore, outside diameter, width and running accuracy are manufactured. ISO 492 grades them P0 (normal), P6, P5 and P4, with tolerances becoming finer in that order; P0 is not marked in the designation, higher grades appear as suffixes. The American ABEC scale corresponds approximately: ABEC 1 to P0, ABEC 3 to P6, ABEC 5 to P5, ABEC 7 to P4.