Bearing Fit Tolerances: Shaft and Housing Seats (ISO 286)

Published 28 September 2026. Last checked 28 September 2026 by The CheckedCalc editor.

A rolling bearing is a precision part bought with its bore and outside diameter already made to a tolerance. The designer only controls the other half of each fit: the shaft seat and the housing bore. Those seats take ISO 286 tolerance classes. The right class depends on which ring sees a rotating load, and the fit that results must include the bearing's own tolerance. The seat limits are computed from ISO 286-1:2010 by the same tested code behind the press fit calculator and the ISO 286 fit tolerance chart.

The rule that decides the fit: which ring sees a rotating load

A ring that is loaded all the way round its circumference as it turns relative to the load will creep on its seat if the fit is loose. Creep means slow rotation of the ring on the shaft or in the housing, and it wears the seat, causes fretting corrosion and fills the bearing with debris. So the ring with the rotating load gets an interference fit. The ring whose load stays at one point (a stationary or point load) can have a loose or transition fit. That makes assembly easier and lets a floating bearing slide axially as the shaft grows with temperature.

Load size moves the choice further. A heavier load, shock, a hollow shaft, or a thin-wall or light-alloy housing each calls for a tighter fit on the ring with the rotating load. The ring and seat deflect more, and the fit loosens under load. The fit also eats into the bearing's radial internal clearance: an interference fit expands the inner ring or shrinks the outer ring. ISO 5753-1 defines the internal clearance groups. A larger group such as C3 is normally chosen where heavy interference fits or a hot inner ring would otherwise leave too little clearance in operation.

Recommended tolerance classes by load condition

The classes below are the general guidance published by bearing manufacturers for solid steel shafts and cast iron or steel housings. They follow the NTN recommendations for radial bearings with a cylindrical bore (NTN catalogue Table 7.2). SKF and Schaeffler publish comparable tables that differ in detail by bearing type and size, so treat this as the starting point and check the catalogue for your bearing. NTN classes the load by comparing P, the equivalent dynamic load, with C, the basic dynamic load rating. Light is up to P = 0.06 C, normal from 0.06 C to 0.12 C and heavy above 0.12 C.

j and J versus js and JS: ISO 286-1 prefers the symmetric js and JS classes and allows j and J where an application needs them. Bearing catalogues use both; the numbers differ by a few micrometres, so read the class on your drawing literally. For light-alloy housings, a tighter class than listed is normally required.

Why a bearing fit is tighter than the class letter suggests

ISO 492 puts the tolerance of the bearing's mean bore diameter entirely below nominal: for the Normal tolerance class, a 40 mm bore is 0 / −12 µm. Ordinary holes in the hole-basis system go the other way, with H placing the tolerance above nominal. So a k5 shaft, a transition fit in an H hole, becomes an interference fit in a bearing bore. Worked through at 40 mm, the k5 seat is +13 / +2 µm. The loosest pair is the largest bore on the smallest shaft, 0 − (2) = 2 I. The tightest is the smallest bore on the largest shaft, -12 − 13 = 25 I (µm).

The outer ring is also toleranced below nominal, which works the other way in the housing. An 80 mm outside diameter is 0 / −13 µm, so an H7 housing (+30 / 0 µm) gives 43 C at the loosest and zero clearance at the tightest. In the fit tables below, C marks clearance and I interference, in µm, loosest first. The bearing ring tolerances are the ISO 492 Normal class values for radial bearings other than tapered roller bearings, as tabulated by NTN (catalogue Table 6.4, class 0, which NTN states matches ISO 492). Precision classes 6, 5 and 4 have narrower ring tolerances and give a narrower fit range.

Checks the tables do not make

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