Industrial MRO Reference Library · Technical Reference

Bearing Fits & Mounting

A rolling bearing needs the right fit on the shaft and in the housing. The ring that turns relative to the load gets an interference fit; the other ring usually gets a looser fit, and the tolerance classes come from ISO 286-2.

Ref MRO-0014ISO 286-2

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

The fit is how tight the bearing ring is on the shaft or in the housing. An interference fit (press fit) means the shaft is slightly larger than the bore, so the ring is stretched on. A clearance fit (loose fit) lets the ring slide on. A transition fit may be either, depending on where the parts fall in their tolerances. Bearing bore and OD tolerances are set by the bearing standard. The shaft and housing tolerances are chosen from ISO 286-2 classes such as k5 or H7.

How to read it

ISO 286 fit codes have a letter and a number. Lowercase letters are shaft tolerances; uppercase letters are hole (housing) tolerances. Letters further along the alphabet are tighter for shafts (j, k, m, n, p) and looser for housings (H is a common base). The number is the tolerance grade; a smaller number means a narrower band. The actual limits in microns depend on the diameter and come from ISO 286-2 tables.

Choosing which ring is tight

Load condition	Inner ring	Outer ring	Example
Shaft turns, load fixed in direction	Interference fit	Clearance or transition fit	Motors, pumps, gearboxes
Housing turns, load fixed in direction	Clearance or transition fit	Interference fit	Wheel hubs, idler pulleys on fixed axles
Load rotates with the shaft	Clearance or transition fit	Interference fit	Unbalanced vibrating shafts
Direction of load uncertain	Interference fit	Interference fit	Heavy shock or vibrating equipment

The bearing maker's fit tables give the recommended shaft and housing classes for each load size and bearing type.

Mounting methods

Cold press: for small bearings with light interference. Press only on the ring being fitted, using a fitting sleeve. Never push through the rolling elements.
Heating: an induction heater or oil bath expands the inner ring so it slides on. Stay under the maker's maximum temperature. Do not use an open flame.
Hydraulic or tapered sleeve: for tapered bores on adapter or withdrawal sleeves. Drive-up is controlled by clearance reduction or axial movement from the maker's tables.

Key points

Interference fits reduce internal clearance. That is why C3 clearance is common with tight fits or hot shafts. See the bearing internal clearance sheet.
Measure shaft and housing with a micrometer and bore gauge before mounting.
Shoulders and fillets must clear the ring chamfers so the ring seats square.
Locknuts and washers on tapered seats must be tightened to the drive-up the maker gives, not by feel.

Common mistakes

Hammering on the outer ring to mount the inner ring.
Using a loose fit on the rotating ring, which lets it creep and wear the shaft.
Heating with a torch.
Over-tight fits that remove all running clearance.

Governing standards: ISO 286-2 (shaft and hole tolerance tables); ISO 492 (bearing tolerances); bearing maker's fit and mounting tables.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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