A fit is not a property of the bearing. It is the relationship between the bearing tolerance and the tolerance you machine into the shaft and the housing. This page gives the tolerance unit, the IT grade values, and the combinations we recommend by application.
ISO 286-1 does not tabulate a tolerance for every diameter. It defines a tolerance unit that scales with size, then multiplies it by a factor for each grade:
i = 0.45 · ∛D + 0.001 · D
where i is in micrometres and D is the geometric mean of the size step in millimetres. IT5 is 7i, IT6 is 10i, IT7 is 16i, IT8 is 25i, IT9 is 40i, IT10 is 64i and IT11 is 100i. Because each grade is a fixed multiple of the same unit, the step from IT6 to IT7 is always a factor of 1.6 - which is why loosening one grade is a predictable, not an arbitrary, change.
The tabulated values in the standard are rounded to preferred numbers rather than to the exact product, so a hand calculation will differ by a micrometre or so. Use the table below for design and the standard itself for inspection.
| Size step (mm) | Mean D (mm) | Unit i (µm) | IT5 | IT6 | IT7 | IT8 | IT9 | IT10 | IT11 |
|---|---|---|---|---|---|---|---|---|---|
| 3 – 6 | 4.24 | 0.73 | 5 | 8 | 12 | 18 | 30 | 48 | 75 |
| 6 – 10 | 7.75 | 0.90 | 6 | 9 | 15 | 22 | 36 | 58 | 90 |
| 10 – 18 | 13.42 | 1.08 | 8 | 11 | 18 | 27 | 43 | 70 | 110 |
| 18 – 30 | 23.24 | 1.31 | 9 | 13 | 21 | 33 | 52 | 84 | 130 |
| 30 – 50 | 38.73 | 1.56 | 11 | 16 | 25 | 39 | 62 | 100 | 160 |
| 50 – 80 | 63.25 | 1.86 | 13 | 19 | 30 | 46 | 74 | 120 | 190 |
| 80 – 120 | 97.98 | 2.17 | 15 | 22 | 35 | 54 | 87 | 140 | 220 |
| 120 – 180 | 146.97 | 2.52 | 18 | 25 | 40 | 63 | 100 | 160 | 250 |
| 180 – 250 | 212.13 | 2.90 | 20 | 29 | 46 | 72 | 115 | 185 | 290 |
The one question that decides the whole fit: which ring rotates relative to the load? A ring carrying a load that rotates relative to it must be a firm interference fit, or the ring will creep, fret and wear the seat. A ring whose load direction is stationary relative to it can be a loose fit, which makes assembly easier and lets the ring unload itself if it gets hot.
In a normal electric motor the shaft turns and the load hangs below, so the inner ring sees a rotating load and takes the interference; the outer ring, in a stationary housing, takes the point load and stays on H7. In a wheel hub the housing turns instead, and it is the outer ring that needs the interference - M7 or N7 - while the shaft seat relaxes to g6 or h6.
| Application | Shaft | Housing | Why |
|---|---|---|---|
| Rotating inner ring load, light duty | h6 or j5 | H7 | Fans, light conveyors, small motors. Point load on the outer ring, so the housing can stay loose. |
| Rotating inner ring load, normal duty | j5 or k5 | H7 | Pumps, gearboxes, general machine tools. The usual industrial default. |
| Rotating inner ring load, heavy or shock duty | m5, m6 or n6 | H7 | Crushers, mills, vibrating screens. Firm interference stops fretting on the shaft. |
| Rotating outer ring load | g6 or h6 | M7 or N7 | Wheel hubs, belt tensioners, rope sheaves. The load now creeps around the housing bore instead. |
| Floating position, axial displacement required | h6 | H7 | One bearing of a pair must slide to absorb thermal expansion. Never put interference here. |
| Precision spindle, low runout | j5 or k5 | J6 or K6 | Grinding and milling spindles. Tight geometric tolerances matter more than the fit letter. |
| Split housing | keep the interference on the shaft | H7 (H8 on rough bores) | A split housing distorts the ring; take the interference on the shaft and use the loose end of H7. |
| Thin-walled or light alloy housing | j6 or k6 | K7 or M7 | A thin wall follows the ring, so the same fit gives less grip; compensate on the housing side. |
| Thrust bearing washers | j6 or k6 | H7 or J7 | Shaft washer needs a light interference, housing washer must stay free to self-align. |
Lower case is a shaft, upper case is a hole. The letter fixes where the tolerance zone sits relative to nominal; the number fixes how wide the zone is. A loose letter with a small number, such as h6, is a precise clearance fit; a tight letter with a large number, such as p6, is a precise interference fit.
| Zone | Applies to | Meaning |
|---|---|---|
| h | Shaft | Upper deviation zero. Always clearance - the standard choice for a floating or easily fitted seat. |
| g, f | Shaft | Larger clearance, used where the ring must slide or where the seat runs hot. |
| j, js | Shaft | Small clearance to light transition. Typical for normal rotating inner ring load. |
| k, m | Shaft | Light to medium interference. Normal to heavy inner ring load. |
| n, p | Shaft | Medium to heavy interference for shock loads and thin sections. |
| r, s, u | Shaft | Heavy interference, normally assembled by heating the bearing. |
| H | Housing | Lower deviation zero. Always clearance; the default housing seat. |
| J, JS | Housing | Transition, used for precision and light outer ring load. |
| K, M | Housing | Light interference, for normal rotating outer ring load. |
| N, P | Housing | Medium interference for heavy outer ring load and shock duty. |
| R, S | Housing | Heavy interference; the housing is usually heated before assembly. |
Tell us the application, the load direction and the housing material and we will recommend a shaft and housing tolerance.