Bearing fits: shaft and housing tolerance selection (k6, m6, H7, and when to use them)
The one rule that drives everything
The ring that rotates relative to the load direction gets the interference fit. Everything else — tolerance tables, clearance adjustments, mounting method — follows from this.
| Application | Load direction vs shaft | Interference fit on | Typical classes |
|---|---|---|---|
| Motor, pump, gearbox shaft | Fixed direction, shaft rotates | Inner ring | k6 / m6 (shaft), H7 (housing) |
| Conveyor pulley, wheel hub | Fixed direction, housing rotates | Outer ring | g6/h6 (shaft), K7/M7 (housing) |
| Vibrating screen | Rotating load | Both rings tight | m6/n6 + P6-class housing |
Get this wrong and the loose ring creeps: the seat wears, fretting corrosion appears (see the failure modes guide), and clearances effectively disappear as debris enters.
Common shaft classes and what they mean
For a standard rotating inner ring under radial load:
- j6 — near-zero to slight transition. Light loads, small bearings, where easy disassembly matters.
- k6 — the everyday default. Light to normal loads, electric motors up to medium size.
- m6 — firmer interference. Normal to heavy loads, shock, or when shaft stiffness is low.
- n6 — heavy loads and shock; mounting usually needs heating.
- Above that (p6, r6) is specialist territory — hollow shafts, very heavy shock — consult the maker’s fit tables with actual load numbers.
On the housing side with a stationary outer ring, H7 (clearance) is the default because the outer ring must be free to float axially in a floating-end arrangement. J7/K7 add slight grip where the housing is thin, split, or carries a rotating load.
Fits eat internal clearance
An interference fit expands the inner ring raceway and consumes radial internal clearance. The practical chain:
- Choose the fit from load and rotation (k6/m6 typical).
- Estimate clearance loss — a firm m6 fit on a 25 mm bore can consume roughly half to most of a CN clearance band.
- Compensate with clearance class: interference fits plus a hot-running shaft are the standard case for C3. Read bearing clearance C2/CN/C3/C4 before ordering the tighter fit with a CN bearing.
Geometry matters as much as tolerance
A tolerance class controls size, not form. Bearing seats also need:
- Cylindricity/roundness — an oval shaft seat distorts the ring, producing localized load zones and early spalling at two or four points around the race.
- Shoulder perpendicularity — a skewed shoulder cocks the ring, showing up as edge loading.
- Fillet radius smaller than the bearing chamfer — the bearing must seat fully against the shoulder; check the maximum fillet in the catalogue data.
Mounting practice follows from fit size
- Transition and light interference (j6/k6, small bearings): mechanical press on the correct ring — force through the press-fitted ring only, never through the rolling elements.
- m6 and above, or bore sizes above roughly 50–70 mm: induction heating to 80–110 °C (max 120 °C). A 6308 on m6 is already firmly in heating territory.
- Tapered-bore bearings on adapter sleeves use the sleeve drive-up to set the clearance reduction — a different procedure entirely, measured with feeler gauges on spherical roller bearings like 22210.
- Tapered roller bearings like 32208 add a second step: after seating, set end play or preload. See the TRB mounting section on its model page.
Never correct a wrong fit by hammering, shim stock under the ring, or knurling a worn shaft — the first destroys the bearing, the others fail within weeks.
Frequently asked questions
Which ring gets the interference fit?
The ring that rotates relative to the load direction. In the common case — shaft rotating, radial load fixed in direction — the inner ring gets interference (e.g. k6/m6) and the outer ring gets a loose or slight-transition fit (e.g. H7). If the housing rotates relative to the load (like a conveyor pulley), the rule reverses: interference on the outer ring.
What does k6 actually mean for a 25 mm shaft?
k6 is an ISO shaft tolerance class giving a small interference against a standard bearing bore. For 25 mm the k6 tolerance is roughly +2 to +15 µm against the bearing bore tolerance of 0 to −8 µm — so the assembly ranges from a slight interference to a small transition. Heavier loads step up to m6 or n6 for firmer interference.
Why does a tight fit reduce bearing clearance?
Pressing a ring onto an interference seat expands the inner ring (or compresses the outer ring), consuming part of the bearing's radial internal clearance. A k6/m6 fit on a normal-clearance bearing can eat most of a CN clearance — which is why interference fits plus hot shafts often call for C3 clearance. See the clearance guide for selection logic.
Can I use H7 on both shaft and housing?
H7 is a housing (hole) tolerance, not a shaft one. H7 on the housing gives the outer ring a clearance-to-transition fit — correct for a stationary outer ring that should be able to slide axially in floating arrangements. The shaft side needs a shaft class (j6, k6, m6...) chosen by the rotating-ring rule and load.
What happens if the fit is too loose?
The ring creeps on its seat, wearing the shaft or housing and producing reddish-brown fretting debris — a classic premature failure pattern. Once the seat is worn undersize, even a correct new bearing will not hold; the shaft must be repaired or built up.