Bearings and Bearing Selection

Bearing types

A bearing supports a rotating shaft, carries the radial and axial loads, and keeps the friction low. Rolling-element bearings use balls or rollers between an inner and an outer ring, and give very low friction at start-up; plain bearings let the shaft slide on a film of oil or a low-friction material. The right type depends on the size and direction of the load, the speed and the misalignment.

Type	Radial load	Axial load	Speed	Misalignment	Notes
Deep-groove ball	Good	Moderate, both directions	Very high	Small (about 0.1°)	The most common; cheap, quiet, often sealed
Angular-contact ball	Good	High, one direction (pairs for both)	Very high	Very small	Machine tool spindles, pump shafts; preload
Self-aligning ball	Moderate	Low	High	Large (2° to 3°)	Where the housing can not be aligned
Cylindrical roller	Very high	None or very little	High	Very small	Heavy radial loads; gearboxes, motors
Tapered roller	High	High, one direction (pair)	Moderate	Small	Wheel hubs, helical gearboxes; adjusted in pairs
Spherical roller	Very high	Moderate, both directions	Moderate	Large (up to 2°)	Heavy loads with shaft bending
Needle roller	Very high for the size	None	Moderate	None	Compact; gears and rocker arms
Thrust ball, thrust roller	None	Pure axial	Low to moderate	None	Vertical shafts, screw jacks
Linear ball and roller guides	Along a rail	—	Linear speed	Small	Machine slides (see Mechanical Transmission)
Plain / sleeve	High	Low (flanged types)	Depends on the film	Moderate	Cheap, quiet, damps shock, oscillating or slow motion

Rating life: the L10 equation

A rolling bearing is a fatigue machine: every ball or roller passes over the same small area of the ring, and after many millions of stress cycles a tiny crack grows into a flake (spalling). Bearing manufacturers define the basic rating life L₁₀ as the life that 90 percent of a large group of identical bearings reach or exceed, in millions of revolutions: L₁₀ = (C ÷ P)^p, with C the basic dynamic load rating from the catalog (a load that gives a life of one million revolutions), P the equivalent dynamic load on the bearing, and the exponent p = 3 for ball bearings and 10/3 for roller bearings. In hours, L_10h = L₁₀ × 10⁶ ÷ (60 n), with n the speed in rpm.

Reliability and modified life. L₁₀ is for 90 percent reliability. For a higher reliability multiply by a factor a₁ (ISO 281): 95 percent 0.64, 96 percent 0.55, 97 percent 0.47, 98 percent 0.37, 99 percent 0.25. The 6205 above would have 9 005 hours at 99 percent. The full ISO 281 method adds a lubrication and contamination factor a_ISO (a good clean film gives more than 1). Typical design lives: household appliances 2 000 to 4 000 hours, general machinery 8-hour days 20 000 hours, electric motors 20 000 to 40 000 hours, continuous heavy plant 50 000 to 100 000 hours.

Equivalent load and combined loads

The load in the life equation must be the equivalent load P = X F_r + Y F_a that combines the radial force F_r and the axial force F_a, with factors X and Y from the bearing catalog, which depend on the ratio F_a ÷ F_r and on F_a ÷ C₀ (C₀ is the static load rating). If the axial load is small (below a limit e), P is simply F_r; above it, X and Y rise. A bearing with an axial load of 400 N on a radial load of 1 000 N has F_a ÷ C₀ = 400 ÷ 7 800 = 0.051: read X and Y from the catalog for that ratio.

• Radial reactions come from the shaft analysis: split the gear force between the two bearings (see Shafts, Keys and Couplings) and add the effect of belts, chains and weights.
• Variable loads and speeds: use an equivalent (weighted) load: P = (Σ P_i^p × fraction of time)^1/p, with the speed weighting.
• Static load check: a bearing at rest or turning very slowly can be damaged by a load that dents the raceways (brinelling). Check the safety factor s₀ = C₀ ÷ P₀ ≥ 1 to 2 (higher for quiet running, lower for rough use).
• Axial loads from helical gears and worm drives (see Gear Types and Selection) need angular-contact, tapered or thrust bearings, mounted so that the load goes through the right ring.

Choosing a bearing size

• Bore: from the shaft diameter (see Shafts, Keys and Couplings).
• Type: from the load direction, the speed and the misalignment (the table above).
• Dynamic load rating C: from the required life and the equivalent load as in the example.
• Static rating C₀: check against shock and standstill loads.
• Speed limit: the catalog gives a reference speed and a limiting speed that depend on the lubricant and the cage; a heat balance may lower it.
• Cage and clearance: brass, steel or polyamide; normal (CN) or larger (C3) internal clearance for hot shafts or tight fits.
• Seals and shields: sealed bearings (2RS, ZZ) are greased for life: easy and cheap, but they run hotter at high speed.
• Quality class: ABEC or ISO tolerance classes (P0, P6, P5, P4) for accuracy and speed.

Arrangements: fixed and free bearings, preload

A shaft needs to be located axially without being over-constrained: because the shaft expands with temperature more than the housing, one bearing is fixed (it carries the axial load and locates the shaft) and the other is free (it can slide axially in its housing, or is a cylindrical roller bearing whose rings can move). Pairs of angular-contact or tapered roller bearings in an O (back to back) or X (face to face) arrangement take axial loads in both directions; they are set with a small preload (negative clearance) that removes play and raises the stiffness for spindles and gearboxes, at the cost of friction and heat.

• O arrangement (load lines diverge) gives a stiff support against tilting (long distance between the pressure centers); X arrangement tolerates a little shaft bending.
• Adjust tapered roller bearings to a small end play or preload, following the manufacturer’s torque and turn procedure.
• Do not fix both bearings of a long shaft rigidly: thermal growth would load them heavily and ruin them.

Fits, mounting and handling

A bearing ring that rotates relative to the load must be an interference fit on its seat, or it creeps and wears the shaft or housing. A ring that is stationary relative to the load can be a looser fit. The usual choices (see Fits and Tolerances for the meaning of the codes):

Load condition	Shaft seat (inner ring)	Housing seat (outer ring)
Rotating shaft, load fixed in direction (normal)	k5 or k6 (interference, light to normal load); m5 or m6 for heavy loads	H7 or J7 (clearance or slight transition)
Rotating outer ring (a wheel hub on a fixed axle)	g6 or h6 (loose)	N7 or P7 (interference)
Free (floating) bearing	As above	H7 with free axial movement
Light loads, easy mounting	h5 or j6	H7

• Mount by the ring that has the interference fit: press the inner ring by its own face with a sleeve, never through the balls or by hammering the outer ring. Heat the inner ring in oil or with an induction heater to 80 to 100 °C above the shaft for large interference.
• Keep them clean: leave the bearing in its wrapper until mounting; dirt is the commonest cause of early failure.
• Check the seat geometry: roundness, taper and the shoulder squareness (a run-out of a few microns) with the correct fillet radius to clear the bearing corner.
• Axial retention with a nut and a lock washer, a circlip or an end cover.
• Alignment: shaft and housings in line; misalignment beyond the bearing’s tolerance overloads the ends of the rollers.

Plain (sleeve) bearings

A plain bearing (bush, journal bearing) has no rolling elements: it is cheap, compact, silent and good with shock, dirt and slow or oscillating motion. Materials include bronze, sintered bronze (oil-impregnated), PTFE-lined steel, filled polymers (nylon, acetal, PEEK) and white metal. They are rated by the pressure p = F ÷ (d L), the sliding speed v and their product PV, which sets the heat generation.

Lubrication, seals and temperature

• Grease (lithium or polyurea, NLGI 2) is the choice for most bearings: simple and sealed. Fill 30 to 50 percent of the free space: too much grease overheats at speed. Relubricate at the interval from the speed factor (n × d_m) and the temperature (halve the interval for every 15 °C above 70 °C).
• Oil (bath, splash, circulation, mist) for high speed, high temperature or when the bearing is in a gearbox with oil (see Lubrication, Wear and Gear Failures).
• Seals and shields: contact seals keep dirt out and grease in, with extra friction; labyrinth seals suit speed.
• Temperature: a normal bearing runs 20 to 40 °C above ambient. Above about 80 to 90 °C check the grease, the clearance (C3) and the seal; above 120 °C use heat-stabilized bearings and high-temperature grease.
• Electrical currents through a bearing (from a variable-frequency drive) cause fluting damage: use insulated bearings or shaft grounding rings (see AC Motors and Variable-Frequency Drives).

Failure modes

Symptom	Likely cause	Prevention
Spalling (flaking) of the raceway	Normal end of fatigue life, or an overload	Size for the life; check loads and alignment
Dents in the raceway at ball spacing (brinelling)	Shock load or vibration at standstill, wrong mounting force	Mount through the correct ring; protect from shock
Fretting, brown rust on the fit surfaces	A loose fit with small movement	Correct the fit; use an anaerobic retaining compound
Scratches, dull, pitted surface	Contamination: dirt, water, metal particles	Seals, clean assembly, filtered oil
Blue or discolored, brittle rings	Overheating from lack of lubricant, a tight fit or too much grease	Lubricant type and quantity, clearance, load
Fluting (washboard) pattern	Electric current through the bearing	Insulated bearings, grounding rings
Smeared surface, skidding	Too light a load, too fast acceleration, too little grease	Preload, correct lubricant
Cracked ring	Too great an interference, a bad mounting method, a hard impact	Fit tolerances, correct tools
Noise, heat or vibration in service	Any of the above; first sign of failure	Monitor with vibration and temperature measurement (see Force, Load Cells and Vibration)

Related

• Gears and Power Transmission: the overview of this guide.
• Shafts, Keys and Couplings: the loads on the bearings from shaft analysis.
• Lubrication, Wear and Gear Failures: lubricants, film thickness and failure analysis.
• Gear Strength and Design Calculations, Belts and Pulleys and Chains and Sprockets: the sources of bearing load.
• Fits and Tolerances.

FAQ

What does L10 mean?

It is the life, in millions of revolutions (or hours at a given speed), that 90 percent of a group of identical bearings reach or exceed in the same conditions: a statistical figure for fatigue life, not a guarantee for one bearing.

Why does a small increase in load shorten the life so much?

Because the life varies as the load to the power of 3 (balls) or 10/3 (rollers): a 20 percent higher load cuts a ball bearing’s life by 42 percent.

Do I need two fixed bearings?

No. One bearing locates the shaft axially (fixed), the other is free to move, so that thermal expansion does not load them. Pairs of angular-contact or tapered bearings are fixed together as a unit.

How much grease should I put in?

About 30 to 50 percent of the free space in the bearing and a similar amount in the housing, for most speeds. More grease causes churning, heat and early failure.

References

• ISO 281, Rolling bearings: dynamic load ratings and rating life; ISO 76 (static load ratings); ISO 15 and ISO 492 (dimensions and tolerances).
• SKF, Rolling bearings catalog and Bearing maintenance handbook; Schaeffler (FAG, INA), NSK and Timken engineering handbooks.
• Harris, T. A. and Kotzalas, M. N., Essential Concepts of Bearing Technology, CRC Press.

Last reviewed: October 2026. Load ratings and limits are typical; use the bearing manufacturer’s catalog and calculation tools for a final selection.