Bearings & Fits
A one-page reference for choosing a rolling bearing and the shaft and housing fits it sits in. It compares the common bearing types, gives the ISO 286 [4] tolerance classes for the shaft and housing, decodes metric bore numbers and states the L10 rating life formulas. Designers use it to detail bearing seats.
What the page covers
- Bearing Types: radial and axial load capacity and typical uses for seven types, from the SKF, Timken and NTN catalogues [1]–[3].
- ISO Fit Selection for Bearings: shaft and housing tolerance classes for a rotating shaft or a rotating housing [1], [4].
- Metric Bearing Bore Code: the bore diameter from the last two digits of a designation such as 6205.
- L10 Bearing Life: rating life in revolutions and hours [5].
Reading the fit codes
An ISO tolerance class is a letter and a number. The letter places the zone: lower case for a shaft, capitals for a housing bore, with h and H starting at the nominal size. Letters later in the alphabet (j, k, m, p for a shaft; K, M, N, P for a housing) give more interference. The number is the IT grade: a larger number is a wider tolerance. The bearing maker fixes the ring tolerances, so the shaft and housing set the fit. The ring that turns relative to the load needs interference, or it creeps and frets. The other ring gets clearance so it can slide for thermal growth.
Limits and common mistakes
- The fit table is a starting point. Shock loads, hollow shafts, thin housings and heat change the choice; check the maker’s tables [1]–[3].
- Interference reduces internal clearance; a tight fit may need a C3 bearing.
- L10 is the life 90 % of bearings reach, not the average. P is the equivalent load, not the radial load alone.
- Bore codes 00 to 03 are not × 5: 6203 is 17 mm, not 15.
Worked example
A pump shaft turns at 1800 rpm with a steady 1.5 kN radial load on a 6205 deep groove ball bearing in a fixed housing. Spec the seats and check the life.
- In Bearing Types, the Deep groove ball row reads High radial load and Low–Moderate (both directions) axial load.
- In Metric Bearing Bore Code, read the row 04 through 96: 6205 → 05 × 5 = 25 mm.
- In ISO Fit Selection for Bearings, read the row Rotating shaft, stationary housing (motors, pumps, gearboxes). A steady, normal load takes:
Shaft: ⌀25 k5
Housing: ⌀52 H7 - In L10 Bearing Life, use k = 3 for a ball bearing. With C = 14.8 kN for a 6205 from the SKF catalogue [1]:
L10 = (14.8 / 1.5)3 = 960.5 million rev
L10h = 106 × 960.5 / (60 × 1800) = 8,894 h
Sanity check: C/P is about 10, and 103 = 1,000 million revolutions. About 8,900 hours is one year of continuous duty. For longer, choose the heavier 6305. The 52 mm housing bore is the 6205’s outside diameter [1].
Bearing Types
| Type | Radial Load | Axial Load | Typical Use |
|---|---|---|---|
| Deep groove ball | High | Low–Moderate (both directions) | General-purpose shafts, motors, gearboxes, pumps — the default choice for light-to-moderate loads |
| Angular contact ball | Moderate–High | High (one direction; paired for both) | High-speed spindles, gearbox pinion shafts, precise axial location under combined load |
| Cylindrical roller | Very high | None (standard type) | Heavy radial loads at high speed — motors, gearboxes, machine tool spindles |
| Tapered roller | High | High (one direction; opposing pairs for both) | Wheel hubs, differentials, gearboxes with combined radial/thrust loads |
| Spherical roller | Very high | Moderate | Heavy machinery with misalignment or shaft deflection — conveyors, vibrating screens |
| Needle roller | High (compact envelope) | None (standard type) | Severe radial-space restrictions — transmissions, rocker arms, universal joints |
| Thrust (ball or roller) | None–Low | High | Axial-dominant loads — vertical shafts, gear thrust faces, crane hooks |
ISO Fit Selection for Bearings
| Application | Inner Ring (Shaft) Fit | Outer Ring (Housing) Fit | Why |
|---|---|---|---|
| Rotating shaft, stationary housing (motors, pumps, gearboxes) | j5/k5 (light-normal), m5/m6 (heavy), up to p7 (very heavy/large) | H7/H8 (clearance) or J7 (slight clearance) | Inner ring rotates with the load — needs interference; outer ring is stationary — clearance allows axial float |
| Stationary shaft, rotating housing (wheel hubs, conveyor pulleys) | h6 (clearance) | K7 (transition, normal-heavy) or M7/N7/P7 (heavier loads) | Housing rotates with the load — needs interference; shaft is stationary — clearance is fine |
Roller bearings typically run one tolerance grade tighter than ball bearings at each shaft-size break, since they need less internal clearance margin. A loose fit on the ring carrying the rotating load causes it to creep on its seat, generating wear debris and fretting. See the Press-Fit / Interference Fit calculator to check the resulting contact pressure and stress for a given interference.
Metric Bearing Bore Code
| Bore Code | Bore Diameter |
|---|---|
| 00 | 10 mm |
| 01 | 12 mm |
| 02 | 15 mm |
| 03 | 17 mm |
| 04 through 96 | Bore code × 5 mm (e.g. 6205 → 05 × 5 = 25 mm) |
Non-standard/odd bores (below 10 mm, or sizes like 22/28/32 mm that don't fit the coded scheme) are given uncoded, separated by a slash — e.g. 62/22 for a 22 mm bore.
L10 Bearing Life
| Quantity | Formula | Symbols |
|---|---|---|
| Rating life (90 % reliability), millions of revolutions | L10 = (C / P)k | C = dynamic load rating, P = equivalent load; k = 3 for ball and 10/3 for roller bearings |
| Rating life in hours | L10h = 106 L10 / (60 n) | n = speed (rpm) |
See the Bearing Life (L10) calculator for L10 from a bearing’s load rating and loads, with the reliability factor a1.