Ball Bearing

What is a Ball Bearing

A ball bearing constrains relative motion to the desired rotation and reduces friction between moving parts by interposing hardened balls between two grooved raceways. The classic single-row deep groove ball bearing has five elements: an inner ring pressed onto the shaft, an outer ring seated in the housing, a row of precision balls, a cage (also called a retainer or separator) that keeps the balls evenly spaced, and optionally seals or shields that retain lubricant and exclude contamination. Load passes from shaft to inner ring, through the rolling balls, into the outer ring, and finally into the housing. Because the balls roll rather than slide, the coefficient of friction is roughly 0.001 to 0.005, an order of magnitude below a lubricated plain bearing.

The deep, closely conforming groove in each ring is what distinguishes a deep groove ball bearing from a simpler design. The groove radius is only slightly larger than the ball radius, giving a small contact ellipse that spreads load while keeping rolling resistance low. This geometry lets a single deep groove bearing carry radial load, axial load in both directions, and the combined loads that result, which is why it is the most widely used bearing type in the world. Angular contact, self-aligning, and thrust ball bearings are variations on the same rolling principle, each optimised for a different load direction or misalignment tolerance.

The history of the rolling bearing is long. Sketches of ball-supported turntables appear in Leonardo da Vinci's notebooks around 1500, and a recovered Roman ship from Lake Nemi (first century AD) carried a ball-bearing rotary platform. The modern radial ball bearing was patented by Jules Suriray, a Parisian bicycle mechanic, in 1869, and was fitted to the winning bicycle of the Paris-Rouen race that year. Industrial mass production followed at the turn of the twentieth century, and SKF was founded in Sweden in 1907 around Sven Wingquist's self-aligning ball bearing. Today the major makers, including SKF, Schaeffler (FAG and INA), Timken, NSK, NTN, and JTEKT (Koyo), build to a common set of ISO and national dimension and rating standards so that a bearing from one supplier is dimensionally interchangeable with another.

Four engineering quantities govern whether a chosen ball bearing will succeed: the basic dynamic load rating C (which sets fatigue life under rotation), the basic static load rating C0 (which guards against brinelling when stationary or shock-loaded), the limiting or reference speed, and the internal geometry expressed through clearance and precision class. The rest of this guide unpacks each of these in turn. A correctly sized bearing in a clean, well-lubricated, properly fitted housing can run for tens of thousands of hours; the same bearing starved of lubricant, contaminated, or misaligned can fail in days, so application engineering matters as much as the catalogue number.

Ball Bearing Types

Ball bearings divide into a handful of families distinguished by raceway geometry and the direction of load they best support. The wrong family is the most common selection error: fitting a deep groove bearing where an angular contact pair is needed leaves the shaft unable to take its thrust load, while fitting a self-aligning bearing where rigidity is required allows unwanted shaft tilt. The table below compares the five mainstream ball bearing types on load capability, misalignment tolerance, and typical use.

Deep groove ball bearings (ISO type 6) are the universal workhorse. The deep, conforming grooves let a single row carry substantial radial load together with bidirectional axial load, and the design tolerates high speed with low noise and vibration. They mount easily, need little maintenance, and are available open, shielded, or sealed across bore sizes from a few millimetres to over a metre. When a designer simply needs "a bearing" for a radial-load shaft position, this is almost always the starting point.

Angular contact ball bearings place the line of contact between ball and raceways at an angle to the bearing axis, commonly 15, 25, or 40 degrees. This contact angle lets them carry large axial load, but only in one direction, so single bearings are arranged in pairs: back-to-back (DB) for a rigid, moment-resisting span, face-to-face (DF) for easier mounting, or tandem (DT) to share heavy thrust in one direction. Preloading a pair removes internal play and raises stiffness, which is why angular contact bearings dominate machine-tool spindles and ball-screw supports.

Self-aligning ball bearings use two rows of balls running on a common spherical outer raceway, so the inner ring and ball set can swivel inside the outer ring. This swivel absorbs shaft misalignment or housing deflection of one to three degrees without inducing harmful edge loading, making them ideal for long line shafts and applications where bores cannot be machined perfectly coaxial. They carry radial load well but limited axial load. Thrust ball bearings are built solely for axial load: balls run between two washers (a shaft washer and a housing washer), carrying thrust in one direction (single-direction) or both (double-direction), but no radial load. They appear in jacks, crane hooks, swivels, and slow rotary tables.

Two further variants are worth knowing during selection. Double-row deep groove ball bearings place two ball rows in a single wider envelope to raise radial capacity where a larger-bore single-row bearing would not fit the available space, at the cost of tighter alignment requirements. Insert (Y) bearings are deep groove bearings with a spherical outside surface and an extended inner ring, locked to the shaft by grub screws or an eccentric collar; mounted in a pillow block or flange housing, they form the bolt-on plummer-block units common on conveyors and agricultural machinery. Choosing among these variants is a packaging and mounting decision layered on top of the load-direction decision: first answer which load the position must carry, then ask which envelope and mounting method fits the machine.

Materials, Cages and Clearance

A ball bearing's durability rests on its material set: the steel of the rings and balls, the material of the cage, and the seals or shields and lubricant that protect the contact surfaces. The default ring and ball material is through-hardened high-carbon chromium bearing steel, designated AISI 52100 in the United States, 100Cr6 (DIN 1.3505) in Europe, SUJ2 in Japan, and EN31 in the UK. It is heat-treated to a minimum hardness of about 58 HRC, the condition assumed when ISO 281 and ISO 76 load ratings are published, and it offers the best combination of rolling-contact fatigue strength, wear resistance, and cost.

Where corrosion is a concern, martensitic stainless steel AISI 440C replaces 52100, accepting a reduction in load rating and maximum hardness in exchange for resistance to water, mild acids, and washdown chemistry. For high speed and demanding spindles, hybrid bearings retain 52100 or stainless rings but substitute silicon nitride (Si3N4) ceramic balls. Ceramic balls are only about 40 percent the density of steel (roughly 3.2 versus 7.8 g/cm3), so centrifugal loading at high speed falls sharply, friction and heat drop, and the non-conductive balls block the electrical erosion that pits the raceways of inverter-driven motors. Full-ceramic bearings exist for corrosive, non-magnetic, or vacuum service but carry lower load and command a price premium. The table below compares the common rolling-element material options.

The cage spaces the balls and prevents them touching, and its material is chosen for speed and temperature. A pressed steel sheet cage is the inexpensive default for medium speeds. A machined or moulded brass cage suits heavy loads, higher temperatures, and oil-lubricated high-speed duty. A glass-fibre-reinforced polyamide 6.6 (PA66) cage runs quietly with low friction and good acceleration capability but is generally limited to about 120 degrees Celsius and can degrade in aggressive lubricants or aged oil; for higher temperature, PEEK or phenolic cages are used in precision and high-speed bearings.

Radial internal clearance is the total distance the inner ring can move relative to the outer ring in the radial direction before any load is applied, and it is standardised by ISO 5753. The classes, from tight to loose, are C2, CN (Normal, unmarked), C3, C4, and C5. Clearance is not a quality grade: it is an application choice. A press fit on the shaft expands the inner ring and consumes clearance, and so does thermal growth of the inner ring when it runs hotter than the outer ring. Engineers therefore select greater-than-normal C3 for electric motors, generators, and many gearbox positions so that the running, warmed-up bearing settles at a small positive operating clearance rather than going into damaging preload. C4 and C5 appear where fits are very tight or temperature differentials are extreme, such as some wind-turbine and hot-running generator positions. Seals (suffix such as 2RS1, contact rubber, dust and splash tight) and shields (suffix 2Z, non-contact sheet metal, lower friction) complete the package by retaining grease and excluding contamination.

Designation, Dimensions and Standards

Ball bearings are interchangeable across suppliers because their boundary dimensions, bore (d), outside diameter (D), and width (B), follow ISO 15, the general dimension plan for radial bearings. ISO 15:2017 lays out preferred diameter series (7, 8, 9, 0, 1, 2, 3, 4) and width series so that any catalogue 6205, whether from SKF, NSK, NTN, or Timken, shares the same 25 mm bore, 52 mm outside diameter, and 15 mm width. This is the single most useful fact in bearing procurement: a worn bearing can be replaced by reading its number, without measuring the housing.

The number itself encodes the geometry. In a basic designation such as 6205, the first digit is the type code (6 = single-row deep groove ball bearing; 7 = angular contact; 1 = self-aligning; 5 = thrust). The second digit is the ISO dimension series, which pairs a width series with a diameter series to fix the cross-section, so the 02-series is lighter and the 03-series heavier for the same bore. The last two digits are the bore code: codes 00, 01, 02, 03 correspond to 10, 12, 15, and 17 mm respectively, while codes 04 and above are multiplied by 5, so 05 equals a 25 mm bore. Bores of 22, 28, and 32 mm are exceptions written with a slash, for example 62/22. The table below decodes the most common deep groove series for a single bore.

Suffixes follow the basic number to describe non-dimensional features. Common SKF-style suffixes include 2RS1 (rubber contact seal both sides), 2Z (metal shield both sides), C3 (greater than normal radial clearance), P6 or P5 (tighter ISO 492 precision class), ETN9 (glass-fibre polyamide cage and reinforced design), and M (machined brass cage). Other makers use their own letters for the same features, which is why a cross-reference chart is part of every spare-parts workflow. A complete ordering description names the bore-OD-width through the basic number, then the seal or shield, clearance, precision, and cage.

Several standards underpin these numbers, and naming the right one resolves most procurement disputes. ISO 15 fixes boundary dimensions. ISO 492 defines dimensional and running-accuracy tolerance classes, which the ABMA labels ABEC (ABMA Std 20). ISO 5753 defines radial internal clearance classes. ISO 281 defines dynamic load ratings and the rating-life calculation, and ISO 76 defines static load ratings. National standards such as DIN 620, JIS B 1518, and ANSI/ABMA 9 mirror the ISO content, so a bearing marked to any of them can be cross-referenced with confidence.

Load Ratings, Life and Spec Parameters

The numbers that decide whether a bearing survives are its load ratings and the fatigue life they imply. A catalogue page lists many figures, but eight drive the selection: bore and outside diameter, basic dynamic load rating C, basic static load rating C0, fatigue load limit Pu, reference speed, limiting speed, mass, and the tolerance and clearance class. Each is defined below, with a worked anchor on the 6205 example.

Basic dynamic load rating C is the constant radial load under which a bearing attains an ISO 281 basic rating life of one million revolutions, for bearings of 52100-class steel hardened to at least 58 HRC under normal conditions. It is the headline strength figure: for a 6205 the published dynamic rating is on the order of 14 kN (about 1,430 kgf), and a heavier 6305 in the same bore reaches roughly 23 kN because its larger balls and raceways spread load over more contact area. Always compare C between competing bearings at the same bore before judging which is stronger.

Basic static load rating C0, defined by ISO 76, is the load that produces a permanent (plastic) deformation of 0.0001 times the ball diameter at the most heavily loaded ball-to-raceway contact. C0 governs a bearing that is stationary, oscillating slowly, or absorbing shock, where the risk is brinelling (permanent dents) rather than fatigue. A static safety factor s0 = C0/P0 of roughly 1 to 2 is applied, higher for smooth-running or vibration-sensitive duty. For a 6205, C0 is on the order of 7 to 8 kN.

Rating life is the heart of bearing engineering. ISO 281 defines the basic rating life L10, the number of millions of revolutions that 90 percent of a population of identical bearings will reach before the first rolling-contact fatigue spall, by the formula:

So L10 (in millions of revolutions) equals (C/P) raised to the power 3 for ball bearings, where P is the equivalent dynamic bearing load combining radial and axial components. Doubling the load cuts life to one eighth, the steepest sensitivity in machine design, which is why slight oversizing pays back. ISO 281 also defines the modified rating life Lnm = a1 x aISO x L10, where a1 corrects for reliabilities above 90 percent (a1 = 1 at 90 percent, falling for 95, 99, and 99.95 percent) and aISO is a single factor that folds in lubrication film thickness, viscosity ratio, and contamination per the systems approach of ISO/TR 1281. Life in hours follows by dividing L10 revolutions by 60 times the shaft speed in rpm.

Speed ratings come in two forms. The reference speed is a thermal reference under defined conditions, while the limiting speed is the mechanical ceiling set by cage strength, seal rubbing, and lubricant. Sealed bearings have a lower limiting speed than open or shielded ones because the contact seal generates heat. Fatigue load limit Pu is the load below which, under good lubrication and cleanliness, classical fatigue effectively does not occur, giving an in-principle infinite life; it anchors the aISO factor. Reading these eight figures across two catalogue pages, with C, C0, speed, and clearance aligned, is enough to choose between competing bearings of the same bore.

A short worked example ties the parameters together. Suppose a 25 mm shaft in a fan carries a steady radial load P of 2 kN at 1,500 rpm, and a 6205 with a dynamic rating C of about 14 kN is proposed. The basic rating life is L10 = (C/P) cubed = (14/2) cubed = 343 million revolutions. Converting to hours, divide by 60 times the speed: 343,000,000 / (60 x 1,500) is roughly 3,800 hours at 90 percent reliability before the first fatigue spall is statistically expected. If the duty demands higher reliability or runs in a contaminated, marginally lubricated environment, the modified life Lnm = a1 x aISO x L10 will be shorter, and stepping up to a 6305 (C of about 23 kN) lifts L10 by a factor of (23/14) cubed, roughly fourfold, for the same load. This is the everyday arithmetic that separates an adequate selection from a marginal one, and it is why engineers reach for the heavier dimension series whenever the housing envelope allows it.

Selection Decision Factors

To turn the preceding chapters into a single part number, follow the decision sequence below. Most selection mistakes come not from one wrong number but from settling a later step before an earlier one is fixed. These eight steps double as an RFQ template for quoting against multiple suppliers.

1. Bore and bearing type: Fix the bore from the shaft diameter, then pick the type from the load direction. Deep groove (6-series) for radial and light bidirectional thrust; angular contact (7-series) in pairs for combined or high thrust; self-aligning (1-series) where shafts misalign; thrust (5-series) for pure axial load.
2. Equivalent load and dynamic rating: Resolve radial and axial forces into the equivalent dynamic load P, then verify the required L10 in revolutions equals (C/P) cubed using the catalogue dynamic rating C. Add margin; do not size exactly at the minimum L10.
3. Static safety: Check the basic static rating C0 against the worst stationary, shock, or starting load with a safety factor s0 of about 1 to 2 per ISO 76, raising it for vibration-sensitive or smooth-running duty.
4. Speed: Confirm the operating rpm sits below the reference speed for thermal reasons and below the limiting speed for mechanical reasons. Sealed bearings derate; consider shields or hybrid ceramic balls at high speed.
5. Clearance and precision: Choose CN for normal fits and temperatures, C3 for press fits, hot running, or high speed, and a tighter ISO 492 class (P6, P5, P4) only when running accuracy and low vibration justify the cost.
6. Sealing and lubrication: Open for relubricable oil baths, shields (2Z) for clean low-friction grease retention, contact seals (2RS) for dusty or splash environments. Match grease base oil viscosity to speed and temperature, and define a relubrication interval.
7. Materials and environment: Standard 52100 for general duty, 440C stainless for corrosion and washdown, hybrid Si3N4 balls for high speed or inverter-fed motors prone to electrical erosion, full ceramic for chemical or non-magnetic niches.
8. Fit, mounting and total cost: Specify shaft and housing tolerances (the rotating ring takes the tighter interference fit), the mounting method, and then weigh purchase price against lubrication intervals, replacement labour, and downtime over the design life.

One dimension that buyers routinely overlook is serviceability and supply chain: local stock of the exact designation, availability of cross-references when a series is obsoleted, and the maker's mounting and lubrication documentation. SKF, Schaeffler (FAG/INA), NSK, NTN, JTEKT (Koyo), and Timken all publish full engineering catalogues, maintain regional distribution, and offer interchange tables, which keeps a five-to-ten-year production line supplied long after the original purchase order. A bearing that is theoretically optimal but unavailable in 48 hours when a line is down is the wrong bearing.