Bearing Fits Explained: Choosing Shaft and Housing Tolerances (ISO 286)

A rolling bearing is only as good as the two fits that hold it: the ring pressed onto the shaft and the ring seated in the housing. Get those tolerances wrong and even a perfectly selected bearing will creep, fret, run hot, or seize. This guide walks through how bearing fits work, how to read the ISO 286 shorthand (h6, k5, H7…), and how to pick a fit from the load and rotation conditions of your application.

Why fit matters more than most people think

The inner and outer rings of a bearing are thin, hardened, and dimensionally precise. When you press a ring onto a shaft with interference, that interference partially closes the internal radial clearance of the bearing. Too little interference and the ring can rotate relative to its seat — called creep — wearing the shaft and generating heat. Too much interference and you preload the raceways, kill the internal clearance, and drive the bearing toward premature spalling.

Reading the ISO 286 tolerance code

ISO 286 describes a fit with a letter and a number. The letter sets the position of the tolerance band relative to the nominal size; the number (the IT grade) sets how wide the band is. Capital letters denote holes (housing bores); lowercase letters denote shafts.

Lower IT numbers (5, 6) are tighter grades and cost more to machine. Bearing seats on the shaft are usually held to IT5–IT6; housing bores to IT6–IT7.

Choosing a fit from load and rotation

Start with two questions: which ring rotates, and how heavy is the load relative to the bearing’s dynamic rating? The four combinations map cleanly onto fit classes.

Rotating inner ring, stationary load (the common case)

A shaft-mounted gear, pulley, or fan — the inner ring turns under a load whose direction is fixed. The inner ring sees a rotating load and needs interference: k5 for light-to-normal loads, m5 or n6 as load climbs. The outer ring is under a stationary load and takes a clearance H7 bore so it can float axially for thermal expansion.

Rotating outer ring, stationary load

Wheel hubs and idler pulleys — here the outer ring rotates under the load and needs the interference (M7, N7), while the shaft can run a looser g6/h6.

Worked example: a 6205 on a 25 mm shaft

Take a deep-groove 6205 (25 mm bore, 52 mm OD) carrying a normal radial load with a rotating inner ring — a small pump. From the guidance above we select a k5 shaft and an H7 housing.

The shaft is always larger than the 25 mm bore by 2–11 µm, giving a light press that resists creep without crushing the internal clearance. The housing is 0–30 µm larger than the 52 mm OD, so the outer ring slides in and is free to grow with temperature. With that much interference on the inner ring, check that you still have adequate residual clearance — on a hot or heavily loaded job you would specify C3 internal clearance to compensate.

Practical checklist before you release the drawing

• Confirm which ring rotates relative to the load — that ring gets the interference fit.
• Hold shaft seats to IT5–IT6 and housing bores to IT6–IT7; call out cylindricity, not just diameter.
• Account for thermal growth — the non-locating (floating) ring must be able to shift axially.
• Step one grade tighter for hollow shafts, light-alloy housings, or hot service, and pair heavy interference with C3 internal clearance.
• Verify surface finish: a rough seat behaves like a looser fit once the peaks flatten.

Fits are the least glamorous part of a bearing arrangement and the fastest way to wreck one. When in doubt, follow the manufacturer’s mounting recommendation — the fit tables in an SKF or Timken general catalog are free, authoritative, and worth keeping on the shelf next to your copy of Machinery’s Handbook.

Explore the full set of rotation tools on the Bearings & Rotation page, including speed limits (DN value) and equivalent dynamic load.