Bearing fits: shaft and housing tolerances
The bearing has its own tolerances, but how it sits on the shaft and in the housing is set by their tolerances — and that fit decides whether the bearing runs true or spins on its seat, seizes or creeps. The bore of a 6204 is 20 mm, but a 20.00 mm shaft and a 19.99 mm shaft give completely different bearings in service. Here is the logic behind choosing a fit.
The one rule that drives everything
The ring that rotates relative to the load must have an interference (tight) fit. The ring that stays still relative to the load can have a looser fit.
In the usual case — a rotating shaft carrying a steady load — the inner ring turns under the load, so it needs a tight fit on the shaft or it will creep and spin on the seat, fretting both surfaces to powder. The outer ring is stationary relative to the load, so it can sit in the housing with a light transition or clearance fit that still lets it be assembled and lets the bearing take up thermal expansion.
What the codes mean
Fits are written as an ISO tolerance symbol — a letter and a number, like k5 or H7. The letter sets where the tolerance band sits relative to nominal (lower-case for shafts, upper-case for housings); the number sets how tight the band is. Roughly, for shafts:
| Shaft | Fit | Use |
|---|---|---|
| g6, h6 | Clearance / slide | Non-rotating inner ring, or easy dismount |
| j5, js5 | Transition | Light loads, needs support |
| k5, m5, m6 | Interference | Normal rotating-inner-ring duty |
| n6, p6 | Heavy interference | Heavy or shock loads |
And for housings, from loose to tight: H7 (clearance, outer ring can shift axially — good for a floating bearing), J7 and K7 (transition), M7 and N7 (interference, for a rotating outer ring or heavy load).
Why a tight fit eats internal clearance
An interference fit does not just hold the ring — it stretches it. Pressing an inner ring onto an oversized shaft expands it slightly, which shrinks the space between the raceways and rollers. So the fit and the bearing's internal clearance are two halves of one calculation: a heavy interference fit on a normal-clearance bearing can close the clearance to zero and preload it into early failure. That is exactly why heavily-fitted bearings are often specified C3 — the extra clearance is there to be eaten by the fit.
Too tight, too loose
- Too loose on a rotating ring: the ring creeps around the seat, generating heat, fretting corrosion and eventually a scored, undersized shaft. A common cause of a bearing that "keeps failing" in the same spot.
- Too tight: the clearance closes up, the bearing preloads, runs hot and fails — and it is hard to dismount without damage.
- Floating bearing seized in the housing: if the design uses one located and one floating bearing to absorb thermal growth, and the floating one is fitted too tight in the housing, the shaft cannot expand and loads build up axially.
Practical takeaways
- Identify which ring rotates under load — that one gets the interference fit.
- Measure the shaft and housing before fitting a new bearing; a worn seat changes the fit even with a perfect bearing. See how to replace a ball bearing.
- If a bearing runs hot straight after fitting, suspect too-tight fit or lost clearance before blaming the bearing.
- Match fit, clearance and precision together — they are specified as a set, not chosen independently.
General engineering reference only. Recommended fits depend on bearing type, load, speed and shaft material — use the manufacturer's fit tables for the specific case before machining a shaft or housing.