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Bearing Shaft & Housing Fits Explained

The "fit" is how tightly a bearing sits on its shaft and in its housing. Get it right and the bearing runs true for its full life; get it wrong and it either creeps and frets, or pinches itself and overheats. The whole subject comes down to one rule, plus a system of tolerance codes for fine-tuning.

The One Rule That Decides Everything

The ring that carries a rotating load needs a tight (interference) fit. The ring that carries a stationary load can be looser.

If a load always pushes on the same spot of a ring (a "point" or stationary load), that ring can be a looser transition or clearance fit — which has the bonus of letting it slide for location and thermal expansion. But if the load circulates around a ring (a rotating load), a loose fit lets the ring creep and spin on its seat, generating heat and fretting damage. That ring must be pinned down with an interference fit.

The most common case: the inner ring rotates while the load stays fixed in direction (gravity, a belt pull). The inner ring then sees a rotating load → interference fit on the shaft. The outer ring sees a stationary load → a looser fit in the housing.

The ISO 286 Tolerance Codes

Fits are specified with ISO 286 tolerance classes — a letter and a number, like k5 or H7:

• Lower-case letters are shafts (g6, h6, js6, k5, m6, n6, p6). Letters from k upwards give interference; g/h give clearance/near-clearance.
• Upper-case letters are housings (G7, H7, J7, K7, M7, N7, P7). H/G give clearance; M/N/P give interference.
• The number is the tolerance grade — smaller means tighter manufacturing tolerance (grade 5 is tighter than grade 6).

The bearing's own bore and outside diameter are made to ISO 492 with a tolerance that runs from zero downwards, so the fit you actually get is the combination of the seat tolerance and the bearing's ring tolerance.

Choosing the Grade: Load and Size

Within the interference family, a heavier load and a larger bore call for a tighter grade. Loads are judged against the dynamic rating C:

• Light: P ≤ 0.05·C  ·  Normal: 0.05–0.10·C  ·  Heavy / shock: P > 0.10·C

Typical recommendations for a solid steel shaft with the inner ring carrying the rotating load:

For the housing: a stationary outer-ring load that must move axially uses H7; a rotating outer-ring load uses K7 (light), M7 (normal) or N7/P7 (heavy).

Fit Changes the Internal Clearance

A tighter fit doesn't just hold the ring — it expands it, which reduces the bearing's internal clearance. Choose too tight a fit and you can squeeze the clearance to zero, leaving the bearing preloaded and prone to running hot. This is why bearings with tight fits (or that run warm) are often specified with C3 clearance to compensate.

When to Adjust

• Tighter for hollow shafts, thin-wall or light-alloy housings, and heavy vibration or shock.
• Tighter grade number (grade 5) when high running accuracy is needed.
• Always check the shaft and housing surfaces are clean, round and deburred — a perfect fit on paper is undone by a burr.

Frequently Asked Questions

Does the bearing inner ring need an interference fit?

Yes, when it carries a rotating load — which is the usual case. A loose inner ring under rotating load creeps on the shaft and frets. Common shaft fits are k5 or m6.

What's the difference between shaft fit and housing fit?

Shaft fits use lower-case ISO codes (k5, m6…) and housing fits use upper-case (H7, K7…). Which one is interference depends on which ring carries the rotating load.

How does fit affect bearing clearance?

An interference fit expands the ring and reduces internal clearance. Too tight a fit can remove the clearance entirely, causing preload and overheating — which is why C3 clearance is often used with tight fits.