Bearing Fit Tolerances

Bearing bores and outside diameters are held to their own special tolerance classes (to ISO 492), which are tighter than general ISO 286 tolerances and mostly biased to one side of the nominal size. Because of this, a "k6" shaft or "H7" housing paired with a bearing doesn't behave quite like the same letters would on two ordinary machined parts — the fit that results depends on the bearing's own tolerance as much as the shaft or housing. This page covers the basic selection principle, shaft fit tables for ball bearings, cylindrical, tapered and spherical roller bearings, and needle roller bearings, plus housing fit guidance. Recommendations follow the general standards published by major bearing manufacturers (NTN, SKF) for cast iron or steel housings and solid steel shafts.

THE BASIC SELECTION PRINCIPLE

The single most important question when choosing a bearing fit isn't the load's magnitude — it's which ring rotates relative to the direction of the load. Whichever ring rotates relative to the load needs an interference (tight) fit, so it can't creep round on its seat; the ring that stays stationary relative to the load can use a looser, transition, or clearance fit, which also makes it easier to fit and remove.

An inner ring given a clearance fit while it's the ring that rotates relative to the load is one of the most common causes of premature bearing failure — it creeps against the shaft, and that tiny repeated slip quickly wears both surfaces and lets in abrasive debris.

SHAFT FIT — BALL BEARINGS

The table below covers cylindrical bore ball bearings on a solid steel shaft, with the inner ring rotating (or the load direction indeterminate). "Light load" broadly means an equivalent bearing load Pr ≤ 0.06 Cr, and "normal to heavy load" means Pr > 0.06 Cr, where Cr is the bearing's basic dynamic load rating — check your bearing's rating against the actual applied load to see which column applies.

SHAFT FIT — CYLINDRICAL, TAPERED AND SPHERICAL ROLLER BEARINGS

Roller bearings generally need a tighter shaft fit than a ball bearing of the same bore size under equivalent load, since roller contact concentrates load over a smaller area and the bearings themselves are typically used in heavier-duty applications. The two tables below cover cylindrical/tapered roller bearings and spherical roller bearings separately, again for a solid steel shaft with the inner ring rotating or the load direction indeterminate.

For any of the bearing types above, where the inner ring is stationary relative to the load (e.g. an idler pulley on a fixed shaft), use g6 if the inner ring needs to be moved axially for adjustment, or h6 if it doesn't. For a tapered bore bearing fitted with an adapter or withdrawal sleeve, the shaft itself only needs an h9 tolerance (with an IT5 roundness/form requirement) since the sleeve, not the shaft fit, takes up the running fit — this is one of the main practical advantages of using a sleeve over a directly tapered shaft seat.

SHAFT FIT — NEEDLE ROLLER BEARINGS

Needle roller bearings are typically used where radial space is tight, so their recommended fits are organised a little differently — primarily by load type and speed rather than a single diameter/load grid.

HOUSING FIT GUIDANCE

Housing fits follow the same "rotating ring gets the tight fit" logic, but are usually chosen a class or two looser than the equivalent shaft fit, since the outer ring is far more often the stationary member and split or removable housings need to come apart for maintenance.

For a typical electric motor or gearbox (rotating shaft, stationary housing, normal load), the H7/k6 or H7/m6 combination on housing/shaft is by far the most common pairing seen on general industrial equipment.

A few other factors can override the "standard" fit chosen from load and rotation alone: a difference in operating temperature between the inner and outer ring (heat conducted along a rotating shaft tends to expand the inner ring, loosening its fit, so a tighter class may be needed to compensate); a requirement for very high running accuracy, which is better solved with a higher-precision bearing and tighter-tolerance seats (IT5 instead of IT6) than by simply tightening the interference fit; and housings made from light alloy or plastic, which generally need a tighter fit than the equivalent cast iron or steel housing to achieve the same grip.

The maximum interference fitted to any bearing seat should generally stay below 1/1000 of the shaft or housing bore diameter — too tight a fit can induce enough stress into the rings to reduce bearing fatigue life, even though the fit is nominally "correct" for the load case.