A bearing that spins beautifully on the bench can fail in weeks once it is pressed into the wrong housing. Rating life gets all the attention, but the fit — how tightly the rings clamp onto the shaft and into the bore — decides whether that calculated life is ever realized. Get it too loose and the ring creeps and frets; too tight and you crush the internal clearance to zero. This article walks through the ISO 286 fit system, the rotating-load rule that governs which ring gets an interference fit, and a full worked example for a common deep-groove bearing.
Why the fit matters more than the number
A rolling bearing carries load through a moving contact. If the ring that sees a rotating load is only lightly held, the load vector sweeps around the raceway and slowly drags the ring around with it — a phenomenon called creep. Creep polishes the seat, generates heat and fine wear debris (fretting corrosion), and eventually enlarges the bore until the ring spins freely and the bearing destroys itself. The fix is an interference fit on the rotating ring: the seat is machined slightly larger than the bore so the ring is elastically stretched over it and cannot move.
The trade-off is internal clearance. Pressing a ring onto an oversized shaft expands its raceway and shrinks the radial gap between balls and races. Push the interference too far and the bearing preloads itself, runs hot, and loses life — the exact opposite of what you intended. Fit selection is the balance between “tight enough to stop creep” and “loose enough to keep clearance.”
The ISO 286 tolerance system in one minute
Fits are specified with ISO 286 tolerance codes: a letter for position and a number for the tolerance grade (IT grade). Lowercase letters describe the shaft; uppercase letters describe the housing bore. The letter sets where the tolerance band sits relative to the nominal (“zero line”); the grade sets how wide it is.
Reading a fit code
A shaft called Ø25 k5 has a band that sits mostly above the nominal (transition-to-interference), held to IT grade 5 (very tight). A housing called Ø52 H7 has its band starting at the nominal and opening upward (clearance), at the looser IT7. Lower-case = shaft, upper-case = bore, small number = tighter.
Bearing bores and outside diameters are themselves toleranced to a tight, one-sided band by the bearing standard (they run slightly under nominal). That is why the same letter code produces a tighter fit with a bearing than it would between two plain machined parts — the bearing manufacturer has already biased the ring dimension. Always select fits from a bearing fit table, not a generic engineering-fit chart.
The rotating-load rule
The single most important decision is which ring rotates relative to the load direction:
- Ring rotating under the load → interference fit. The load point moves around this ring, so it must be clamped to prevent creep. Most machines have a rotating shaft and a fixed load (belt tension, gear separation), so the inner ring gets the tight fit.
- Ring stationary relative to the load → loose (clearance) fit. The load always presses on the same spot, so creep is not a concern. A clearance fit here also lets the ring slide axially to absorb thermal growth — essential for the “floating” bearing in a two-bearing shaft.
Heavier loads, shock, and larger bores all push you toward tighter interference. Our bearing fit calculator applies the ISO 286 bands directly so you can see the resulting maximum and minimum interference in microns without hand-interpolating a table.
Worked example: a 6205 on a rotating shaft
Consider a 6205 deep-groove ball bearing (bore d = 25 mm, OD D = 52 mm) on the shaft of a small gearmotor. The shaft rotates, the load from the gear mesh is fixed in space, and the magnitude is moderate — well under 10% of the bearing’s dynamic rating C. This is the textbook “rotating inner ring, light-to-normal load” case.
Inner ring (rotating under load): select shaft k5. For a 25 mm nominal, k5 gives a shaft band of roughly +0.002 to +0.011 mm. Because the bearing bore runs 0 to −0.010 mm, the resulting fit is a light interference of about 2–21 µm — enough to stop creep without collapsing clearance.
Outer ring (stationary relative to load): select housing H7. For 52 mm, H7 is 0 to +0.030 mm; with the bearing OD at 0 to −0.013 mm the outer ring sits in a small clearance, so it can be pushed in by hand and can float axially. That floating outer ring becomes the thermal-expansion accommodation for the shaft.
| Condition | Shaft fit | Housing fit |
|---|---|---|
| Light load, rotating inner ring | j5 / k5 | H7 |
| Normal load, rotating inner ring | k5 / m5 | H7 / J7 |
| Heavy / shock load | m6 / n6 | K7 / M7 |
| Rotating outer ring, stationary shaft | g6 / h6 (loose) | N7 / P7 (tight) |
Check the clearance you have left
Interference reduces radial internal clearance by roughly 80% of the effective interference at the bore. If your 6205 starts with a Normal-clearance (CN) group and you apply 15 µm of interference, you consume about 12 µm of the internal clearance — acceptable for CN, but the same interference on a preloaded matched pair could drive the bearing into negative clearance. When shafts run hot or interference is heavy, specify a wider clearance group (C3) to keep a positive operating clearance. The mounted-clearance check is exactly the kind of thing worth confirming before you commit a print.
Fit is only one leg of bearing selection. Once the seat tolerances are set, confirm the load and speed still deliver the life you need with our L10 bearing life calculator, or start from the duty cycle and let the bearing selection wizard narrow the candidates.
Get the fit right on the first print
Enter your bore, housing, load direction, and clearance group into the free FicientDesign Bearing Fit Calculator — it returns the ISO 286 shaft and housing tolerances, the resulting interference in microns, and the clearance you will have after mounting.
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