A bearing is a machine element that supports a rotating or sliding part, carries its load and keeps friction low. There are two main types of bearings: rolling-element bearings (ball and roller bearings) and plain bearings (journal bearings, bushings and thrust pads), where the surfaces slide on a film of lubricant. Ball bearings suit light to moderate loads at high speed; roller bearings carry heavier loads; thrust bearings take load along the shaft axis.
Below, each type is described with the load direction it takes and where it is used, followed by a comparison table, how to read a bearing number such as 6205, and a worked bearing life calculation using real catalogue data.
Why do machines need bearings?
- Reduce friction. Rolling friction or a lubricant film replaces metal rubbing on metal, so less power is lost as heat and parts last longer.
- Support load. A bearing carries radial load (at right angles to the shaft, such as belt pull or gear force) and/or axial load, also called thrust (along the shaft, such as the push from a helical gear or a vertical shaft’s weight).
- Locate the shaft. It holds the shaft in position radially and axially so gears stay in mesh and seals stay concentric.
The two families: rolling-element vs plain bearings
A rolling-element bearing has an inner ring, an outer ring, a set of balls or rollers between them and a cage that keeps the rolling elements spaced. The load passes through the rolling elements, so starting friction is very low. A plain bearing has no rolling parts; the shaft (journal) turns inside a sleeve and the two surfaces are separated by oil, grease or a low-friction lining.
In short: rolling bearings are standard, easy to replace and good from rest to high speed. Plain bearings are compact, quiet, cope with shock load and, when properly lubricated, run almost indefinitely. That is why a car engine’s crankshaft runs in plain shell bearings while its gearbox and wheel hubs use rolling bearings.
Types of rolling-element bearings
1. Deep groove ball bearing
The most common bearing of all. Deep raceway grooves let it carry radial load plus a moderate axial load in both directions. Low friction and quiet running make it the default choice for electric motors, fans, pumps, household appliances and two-wheeler wheels. Sealed (2RS) and shielded (2Z) versions come greased for life.
2. Angular contact ball bearing
The raceways are offset so the line of contact through each ball is at an angle to the shaft axis. It takes combined radial and heavy axial load, but axial load in one direction only, so these bearings are usually mounted in pairs, back to back or face to face. Used in machine-tool spindles, pumps and compressors.
3. Self-aligning ball bearing
Two rows of balls run in a single spherical raceway in the outer ring, so the inner ring, balls and shaft can tilt relative to the housing. It tolerates shaft bending and housings that are not perfectly in line. Load capacity is modest. Used on long line shafts, agricultural machinery and textile machines.
4. Thrust ball bearing
Balls run between two flat washers, one fixed to the shaft and one seated in the housing. It carries axial load only and no radial load, and is limited to moderate speeds. Found in rotary tables, vertical low-speed shafts and car steering pivots.
5. Cylindrical roller bearing
Rollers make line contact with the rings instead of the point contact of a ball, so radial load capacity is much higher for the same size. Most designs (NU, N types) carry little or no axial load and allow the shaft to float axially, which suits the non-locating end of a shaft. Used in gearboxes, large electric motors, rolling mills and railway axleboxes.
6. Needle roller bearing
A cylindrical roller bearing with long, thin rollers (length several times the diameter). Its small cross-section gives high radial capacity where radial space is tight. Common in gearbox layshaft gears, universal joints, two-stroke connecting rod small ends and rocker arms.
7. Tapered roller bearing
Rollers and raceways are cone-shaped, with all the cone lines meeting at one point on the shaft axis. It carries heavy radial and heavy axial load together, but thrust in one direction only, so it is fitted in opposed pairs. The classic uses are wheel hubs of cars and trucks, differentials and gearbox shafts, where the preload can be set by adjusting the pair.
8. Spherical roller bearing
Two rows of barrel-shaped rollers run in a spherical outer raceway. It combines very high radial capacity with self-alignment and also takes some axial load in both directions. Used where shafts deflect under heavy load: conveyors, crushers, vibrating screens, paper machines and wind turbine main shafts.
9. Thrust roller bearing
Cylindrical, tapered, needle or spherical rollers placed between washers to carry large axial loads. The spherical roller thrust type also accepts some radial load and misalignment. Used in crane hooks, extruders, vertical pump motors and ship propeller shafts.
Types of plain (sliding) bearings
Journal or sleeve bearing
The shaft section inside the bearing is the journal; it turns in a sleeve or a pair of half-shells lined with babbitt, bronze or aluminium-tin. At rest, shaft and sleeve touch. As speed rises, the turning journal drags oil into the narrowing wedge-shaped gap on the loaded side, and pressure builds in that wedge until it lifts the shaft clear of the sleeve. This is hydrodynamic lubrication: the load rides on a pressurised oil film and the metals do not touch. Wear happens mainly during starting and stopping. Engine crankshafts, steam and gas turbines and large generators run on this principle.
Bushings
A bushing (bush) is a simple one-piece sleeve pressed into a housing. Sintered bronze bushes are oil-impregnated; wrapped steel-backed bushes carry a PTFE or polymer layer and need little or no lubrication. Used for slow or oscillating motion: hinges, pivots, suspension links, pump shafts and hydraulic cylinders.
A related type is the spherical plain bearing shown above: an inner ring with a spherical outside surface sliding in a matching outer ring. It allows tilt as well as rotation and is used in rod ends, hydraulic cylinder eyes and vehicle suspension joints. Do not confuse it with the spherical roller bearing described earlier.
Thrust pads and other fluid film bearings
For large axial loads, a thrust collar on the shaft runs against a ring of pads. In a tilting-pad design each pad pivots slightly so an oil wedge forms under it, the same hydrodynamic action as in a journal bearing. These carry the thrust of hydro turbines, large pumps and ship propeller shafts. In a hydrostatic bearing an external pump supplies the pressurised oil, so the film exists even at zero speed; it is used in large telescopes and precision machine tools.
Magnetic and air bearings
- Magnetic bearings hold the shaft in a magnetic field controlled by position sensors and electronics, with no contact at all. There is no lubricant and very little friction, so they suit high-speed turbo compressors and flywheel energy storage. They need backup rolling bearings to catch the shaft if power fails.
- Air (gas) bearings float the shaft on a thin film of air. Friction is tiny and speeds can be very high, but load capacity is low. Used in dental drills, precision measuring machines and some small turbomachines.
Comparison of bearing types
| Bearing type | Radial load | Axial load | Speed | Misalignment tolerance | Typical application |
|---|---|---|---|---|---|
| Deep groove ball | Moderate | Light to moderate, both directions | Very high | Low | Electric motors, fans, pumps |
| Angular contact ball | Moderate | High, one direction per bearing | Very high | Very low | Machine-tool spindles, compressors |
| Self-aligning ball | Light to moderate | Light | High | High | Line shafts, farm machinery |
| Thrust ball | None | Moderate | Low to moderate | None | Rotary tables, steering pivots |
| Cylindrical roller | High | None to light (type dependent) | High | Very low | Gearboxes, large motors, axleboxes |
| Needle roller | High for its size | None | Moderate to high | Very low | Gearbox gears, universal joints |
| Tapered roller | High | High, one direction per bearing | Moderate | Very low | Wheel hubs, differentials |
| Spherical roller | Very high | Moderate, both directions | Moderate | High | Conveyors, crushers, screens |
| Thrust roller | None to light | Very high | Low | Low (spherical type: high) | Crane hooks, extruders |
| Journal (hydrodynamic) | Very high | None (needs thrust pads) | High, once the film forms | Low | Crankshafts, turbines |
| Bushing | Moderate | None to light | Low | Low | Pivots, hinges, linkages |
Ball bearing vs roller bearing
A ball touches its raceway at a point; a roller touches along a line. So a roller bearing carries a much higher load than a ball bearing of the same size, while a ball bearing runs at higher speed with less friction and costs less. Choose ball bearings for light to moderate loads and high speed; choose roller bearings for heavy or shock loads. The worked example below puts numbers on this for two bearings with identical dimensions.
How to read a bearing number: 6205
ISO designations follow the same pattern across makers. For a deep groove ball bearing 6205:
- 6 = bearing type: single row deep groove ball bearing.
- 2 = dimension series: diameter series 2 (the width series digit 0 is left out). Series 2 is commonly called the light series; 3 is the medium series with a larger outside diameter for the same bore.
- 05 = bore code. From 04 upward, bore = code x 5, so 05 x 5 = 25 mm.
Codes 00, 01, 02 and 03 are exceptions: they mean 10, 12, 15 and 17 mm bores. So a 6203 has a 17 mm bore, not 15 mm. Suffixes add detail: 2RS or 2RSH = rubber seals on both sides, 2Z = metal shields both sides, C3 = greater than normal internal clearance. A 6205 measures 25 x 52 x 15 mm (bore x outside diameter x width).
Worked example: basic rating life L10
The basic rating life is the life that 90% of a large group of identical bearings will reach or exceed before fatigue appears:
L10 = (C/P)p million revolutions, and in hours L10h = L10 x 106 / (60 n)
where C = basic dynamic load rating from the catalogue (kN), P = equivalent dynamic load on the bearing (kN), n = speed in rpm, and the exponent p = 3 for ball bearings and 10/3 for roller bearings.
Data: a shaft carries a steady radial load P = 2 kN at n = 1500 rpm. From the SKF Rolling bearings catalogue (PUB BU/P1 17000/1 EN, 2018), the SKF 6205 has C = 14.8 kN (C0 = 7.8 kN), and the cylindrical roller bearing NU 205 ECP, with the same 25 x 52 x 15 mm envelope, has C = 32.5 kN.
- Ball bearing 6205: C/P = 14.8 / 2 = 7.4. L10 = 7.43 = 405.2 million revolutions.
- In hours: 405.2 x 106 / (60 x 1500) = 405.2 x 106 / 90 000 = about 4500 h.
- Roller bearing NU 205 ECP: C/P = 32.5 / 2 = 16.25. L10 = 16.2510/3 = about 10 870 million revolutions, or 10 870 x 106 / 90 000 = about 120 800 h.
Same size, but the roller bearing lasts about 27 times longer under this load, which is the point-contact vs line-contact difference in numbers.
Working backwards. Suppose the machine must run 20 000 h. Required L10 = 20 000 x 60 x 1500 / 106 = 1800 million revolutions. For a ball bearing, required C = P x L101/3 = 2 x 18001/3 = 2 x 12.16 = 24.3 kN. The 6205 (14.8 kN) is too small, and even the medium-series 6305 (C = 23.4 kN, 25 x 62 x 17 mm) gives only (23.4/2)3 x 106 / 90 000 = about 17 800 h. For a roller bearing the required C = 2 x 18000.3 = 19.0 kN, which the NU 205 ECP meets easily, but only if there is no axial load, since an NU bearing cannot take thrust.
L10 is a basic fatigue figure. Makers also publish a modified life that corrects it for lubrication, contamination and reliability, so real life can be longer or much shorter.
How to select a bearing
- Load direction and size: radial only, axial only or combined, and whether there are shocks.
- Speed: check the bearing’s limiting speed in the catalogue (for the SKF 6205 it is 18 000 rpm for the open bearing).
- Required life: use the L10 equation to find the minimum C, as above.
- Space: needle bearings where radial space is tight; light or medium series to suit the housing.
- Misalignment and shaft deflection: self-aligning ball or spherical roller bearings if the shaft bends or housings may be out of line.
- Locating and non-locating ends: one bearing fixes the shaft axially; the other (often a cylindrical roller) lets it expand with temperature.
- Fits, lubrication and sealing: the rotating ring usually needs an interference fit on its seat. See our guide to types of fits in engineering for the shaft and housing tolerances.
Why do bearings fail?
- Poor lubrication: too little, wrong grade or old grease. Leads to overheating, discoloured rings and smearing.
- Contamination: dirt or water entering past damaged seals dents the raceways and starts fatigue long before the rated life.
- Misalignment: a skewed shaft or housing loads the balls or rollers unevenly and shows up as a wear path running at an angle.
- Overload and wrong fitting: hammering a bearing on through the rolling elements leaves dents (brinelling); a fit that is too tight removes internal clearance and makes the bearing run hot.
- Fatigue: after its rated life, flaking (spalling) of the raceway is normal wear-out, which is what L10 predicts.
For lecture material on machine element design, including bearings, see NPTEL.
FAQs
What are the main types of bearings?
Rolling-element bearings (deep groove ball, angular contact, self-aligning ball, thrust ball, cylindrical, needle, tapered, spherical and thrust roller) and plain bearings (journal or sleeve bearings, bushings and thrust pads). Magnetic and air bearings are special types.
What is the difference between a ball bearing and a roller bearing?
Balls make point contact with the raceway, rollers make line contact. Roller bearings carry heavier loads; ball bearings run faster with lower friction and cost less.
Which bearing takes both radial and axial loads?
Tapered roller and angular contact ball bearings take heavy combined loads in one direction each and are fitted in pairs. Deep groove ball and spherical roller bearings also take some axial load in both directions.
What does the bearing number 6205 mean?
6 means a deep groove ball bearing, 2 is the light diameter series, and 05 x 5 gives a 25 mm bore. It measures 25 x 52 x 15 mm.
What is L10 life of a bearing?
The number of revolutions (in millions) that 90% of identical bearings will complete before fatigue appears: L10 = (C/P)^3 for ball and (C/P)^(10/3) for roller bearings.