Bearing Fits and Shaft Tolerance Calculator
Bearing Fits and Shaft Tolerance Calculator
The ring with the rotating load gets the interference fit — then the actual interference range from the ISO 286 bands AND the bearing’s own minus-toleranced bore, and how much internal clearance that interference eats.
Bearing fits and shaft tolerance
A 40 mm bore bearing (40 × 80 mm) on a rotating shaft with the load fixed in space, a normal load, a ground seat and CN internal clearance
The fit, the interference, and what it costs the clearance
- es, ei
- the shaft class’s upper and lower deviations, from ISO 286. Computed here, not looked up
- Δd_mp
- the bearing bore’s own deviation — ISO 492 Normal class, 0 to MINUS 8 to 30 µm by size. This is what makes a bearing fit tighter than the same shaft class in a plain hole
- Δd_eff
- effective interference: the measured interference less the roughness that flattens on assembly. About 2 µm on a ground seat and 3 µm on a turned one, whatever the diameter
- 0.80
- the fraction of the interference that appears as lost internal clearance, because the inner ring expands with the shaft. NSK gives 70 to 90 per cent; 80 is the value it calls general
- G_initial
- the bearing’s radial internal clearance before mounting, from its clearance class (ISO 5753 C2 to C5)
Worked example
A 40 mm bore bearing (40 × 80 mm) on a rotating shaft with the load fixed in space, a normal load, a ground seat and CN internal clearance
Settle the decision first, because everything else is table work. The shaft rotates and the load stands still, so the load sweeps round the INNER ring's circumference: the inner ring carries a circumferential load and must have an interference fit. The outer ring's load always presses on the same spot, so it carries a point load and can be loose. Get this backwards and the inner ring creeps on the shaft and fretts the seat away
For a 40 mm bore under a normal load the recommendation is a k6 shaft seat, whose ISO 286 band at 40 mm is 18 to 2 µm — so the seat is machined 40.0020 to 40.0180 mm
NOW THE PART A GENERAL FIT TABLE DOES NOT KNOW. A bearing bore is not a hole with a symmetric tolerance: ISO 492's Normal class makes it 0 to MINUS 12 µm at this size, never plus. So the bore is 39.9880 to 40.0000 mm, and the maximum interference is the largest shaft against the smallest bore: 18 + 12 = 30 µm. The minimum is the smallest shaft against the largest bore, which is just ei = 2 µm. The spread, 28 µm, is the IT6 band plus the bore tolerance
Roughness flattens on assembly, so not all of that interference is real. NSK's correction for a ground seat is d/(d+2): 30 × 40/42 = 28.57 µm effective, two micrometres less. A turned seat uses d/(d+3) and loses three
What it costs the bearing: the inner ring expands with the shaft and the internal clearance falls by about 0.80 of the effective interference — 22.86 µm. Compare that with what a CN bearing has to give: 6 to 20 µm at this bore. At the tight end of the class the fit has eaten ALL of it and 16.9 µm more, so the bearing is preloaded before it turns. With C3 (15 to 33 µm) there is -7.9 µm left at the tight end, which is why C3 is the normal choice with an interference fit and not an upgrade
Be honest about the stack, though. The maximum interference and the minimum clearance are independent tolerances and both landing at once is unlikely; at the MEAN of each the residual clearance is 0.8 µm, which is positive. Both figures are printed above, and the honest engineering answer is to specify the clearance class so that the WORST case is not a preload, then check the operating temperature difference as well — that is the preload and axial play page
The housing side: for a point load on the outer ring, H7. At 80 mm that is 0 to +30 µm, and the outer ring's own OD is 0 to minus 13 µm, so the fit runs from 0 µm of clearance to 43 µm. Loose enough to push in, loose enough to let the shaft grow, tight enough to locate
The decision: which ring gets the interference fit
| Inner ring (on the shaft) | Outer ring (in the housing) | |
|---|---|---|
| Rotating shaft, load fixed in space (a belt pull, a gear mesh, gravity) — the ordinary case | CIRCUMFERENTIAL load. The load sweeps round the inner ring’s circumference as it turns. Interference fit: k5, k6, m5, m6, n6 by size and load | POINT load. The load always presses on the same spot of the outer ring. Transition or loose fit: H7, or G7 if it must slide axially |
| Stationary shaft, rotating housing (a wheel hub on a fixed axle, an idler pulley, a tension roller) | POINT load. Loose or transition fit: g6, h6, js6 | CIRCUMFERENTIAL load. Interference fit: K7, M7, N7, P7 by load |
| Load rotating WITH the shaft (an unbalanced rotor, a centrifugal force, a crank pin) | POINT load — counter-intuitively. The load and the ring turn together, so it presses on the same spot of the inner ring | CIRCUMFERENTIAL load. Interference fit |
| Direction of load indeterminate, or shock loading | Treat as circumferential. Interference fit, and tighter than the load magnitude alone suggests | Treat as circumferential. K7 or tighter |
A k6 shaft, across the size range: what the fit does to the clearance
| Bore d (mm) | k6 band es/ei (µm) | Bore tol. (µm) | Max interference (µm) | Min interference (µm) | Clearance eaten (µm) | CN clearance (µm) | Worst residual, CN (µm) | Worst residual, C3 (µm) |
|---|---|---|---|---|---|---|---|---|
| 10 | 10 / 1 | 8 | 18 | 1 | 12.0 | 2–13 | -10.0 | -4.0 |
| 20 | 15 / 2 | 10 | 25 | 2 | 18.2 | 5–20 | -13.2 | -5.2 |
| 30 | 15 / 2 | 10 | 25 | 2 | 18.8 | 5–20 | -13.8 | -5.8 |
| 40 | 18 / 2 | 12 | 30 | 2 | 22.9 | 6–20 | -16.9 | -7.9 |
| 60 | 21 / 2 | 15 | 36 | 2 | 27.9 | 8–28 | -19.9 | -4.9 |
| 80 | 21 / 2 | 15 | 36 | 2 | 28.1 | 10–30 | -18.1 | -3.1 |
| 120 | 25 / 3 | 20 | 45 | 3 | 35.4 | 15–41 | -20.4 | 0.6 |
| 180 | 28 / 3 | 25 | 53 | 3 | 41.9 | 15–41 | -26.9 | -5.9 |
Housing classes, and why the outer ring is usually the loose one
| Class | When |
|---|---|
| H7 | the default: accurate location, outer ring still free to be pushed in and out |
| H8 | large or split housings, coarser machining |
| G7 | the outer ring must slide axially to take thermal growth |
| J7 | quieter running wanted, displacement still possible |
| K7 | light load, rotating outer ring |
| M7 | normal load, rotating outer ring |
| N7 | heavy load, rotating outer ring |
| P7 | heavy shock load, thin-walled housing |
The decision, the bearing’s own tolerance, and the clearance the fit eats
One rule decides a bearing fit, and the rest is table work. The ring whose load sweeps round its circumference gets an interference fit; the ring whose load always presses on the same spot can be a transition or a loose fit. Schaeffler’s own wording: for a circumferential load, “as damage to the bearing seating surface can occur, a tight fit should be used”, and for a point load, “there is no risk that the seating surface will be damaged and a loose fit is possible”. For the ordinary arrangement — a rotating shaft with the load standing still — that means an interference fit on the shaft and a loose one in the housing. Reverse it (a wheel hub on a fixed axle) and the interference belongs in the housing instead. Get it the wrong way round and the ring creeps on its seat, a few degrees per revolution, and fretts the seat into an abrasive grey powder that laps the shaft undersize. Nothing in a life calculation sees that coming.
A bearing bore is not a hole, and that is why this is not just an ISO 286 lookup. The general fit case belongs to the ISO 286 fit calculator and this page reuses exactly the same tolerance generator. What it adds is the bearing’s own rings. ISO 492’s Normal tolerance class makes the bore 0 to MINUS 8 to 30 micrometres by size — unilateral, never plus — and the outside diameter likewise. So the maximum interference on a k6 seat is not the k6 band’s upper deviation, it is that plus the whole bore tolerance: 18 + 12 = 30 micrometres at a 40 mm bore, where a plain H7 hole on the same shaft would have given 23 micrometres of CLEARANCE at worst. The bearing’s tolerance is always in the tightening direction, and a fit chosen from a general table without it will be tighter than intended every time.
Interference eats the internal clearance, and that is the constraint people miss. The inner ring expands with the shaft it is pressed onto, and the radial internal clearance falls by roughly eighty per cent of the effective interference — NSK gives the range as seventy to ninety per cent and calls eighty general. At a 40 mm bore, the worst case of a k6 fit eats about 23 micrometres, and a CN bearing at that size has only 6 to 20 micrometres to give. So the worst corner of the stack has a preloaded bearing before it has turned. That is the entire reason C3 clearance exists and the reason it is described as NORMAL for an interference fit rather than as a heavy-duty option. The preload and axial play page takes it further and adds the thermal term, which comes off the same budget.
Effective interference is less than measured interference. Surface roughness flattens on assembly, so a measured thirty micrometres does not give thirty micrometres of ring expansion. NSK’s correction is a ratio of a useful shape: d/(d+2) for a ground seat and d/(d+3) for a machined one, which removes exactly two or three micrometres whatever the diameter. On a 20 mm bearing that is a tenth of a k6 fit; on a 200 mm one it is nothing. The seat’s roughness matters for a second reason the correction does not capture: a rough seat carries the ring on a few high peaks that yield under load, so the fit relaxes in service, which is why bearing makers specify the seat finish as well as its tolerance.
What this page does not do. It does not compute the force or the temperature needed to get the ring on — that is the interference and shrink fit calculator, which does the two-material Lamé solution properly. It does not compute the fit at operating temperature when the two parts are different materials, which is the thermal effect on fit calculator. It does not give the general hole-and-shaft fit, which is the ISO 286 fit calculator. And it does not address the shaft’s own SLOPE at the bearing seat, which misaligns the rings and which no fit can correct — that is the shaft deflection and slope calculator, and the slope limit it reports is a bearing limit rather than a stiffness one.
Frequently asked questions
Which ring should have the interference fit?
The one carrying the circumferential load — the load that sweeps round that ring’s circumference as it turns. For a rotating shaft with a stationary load, which is the ordinary case, that is the inner ring: interference on the shaft, loose in the housing. For a rotating housing on a fixed shaft (a wheel hub, an idler pulley) it is the outer ring: interference in the housing, loose on the shaft. And for a load that rotates WITH the shaft — an unbalanced rotor, a crank pin — it is the outer ring again, which surprises people.
Why is the interference bigger than the shaft tolerance suggests?
Because the bearing’s own bore is toleranced 0 to MINUS. ISO 492’s Normal class gives a bore deviation of 0 to −8 µm up to 18 mm, 0 to −10 up to 30, 0 to −12 up to 50 and so on — unilateral, always in the tightening direction. So the maximum interference is the shaft class’s upper deviation PLUS the whole bore tolerance. On a 40 mm k6 seat that is 18 + 12 = 30 µm, where the same shaft in a plain H7 hole would have up to 23 µm of clearance. A fit picked from a general tolerance table without the bearing’s own band will be tighter than intended every time.
How much internal clearance does an interference fit use up?
About eighty per cent of the effective interference, because the inner ring expands with the shaft. NSK gives the range as seventy to ninety per cent depending on the bearing and the shaft design and calls eighty the general figure; this page prints the 80 per cent value and both bounds. The effective interference is itself less than the measured one, by two micrometres for a ground seat and three for a turned one. At a 40 mm bore a k6 fit can therefore eat 23 µm of clearance, against the 6 to 20 µm that a CN bearing has.
Do I need C3 clearance with an interference fit?
Usually yes, and it is not an upgrade — it is the normal choice for that condition. The arithmetic is on this page: with the worst-case interference of an ordinary k6 seat and the minimum clearance of a CN bearing, the residual clearance is negative at every bore size from 10 to 180 mm, meaning the bearing is preloaded before it turns. C3 roughly doubles the clearance available and brings the worst case back to positive or nearly so. The same reasoning applies to a hot inner ring, which is why C3 is also normal for an electric motor and for anything driven through a hot coupling.
Should the outer ring be tight or loose?
Loose, in the ordinary arrangement, and for two reasons. It carries a point load so it does not need interference; and something in a two-bearing arrangement must be free to move axially as the shaft grows with temperature, and a sliding outer ring is the cheapest way to allow it. H7 is the default and G7 where the sliding matters. If both outer rings are tight in a rigid housing, a 500 mm steel shaft 30 °C above the housing grows about 0.18 mm and that growth becomes an axial load on the bearings that nothing was sized for.
What actually goes wrong if the fit is too loose?
Creep, then fretting corrosion, then a scrap shaft. A ring that is loose on a seat carrying a circumferential load rotates slowly relative to that seat, driven by the travelling bulge the load makes in it — a few degrees per revolution, which adds up. The relative movement is too small for the oxide to escape, so it work-hardens into an abrasive and laps the seat undersize; the fit then gets looser and begins to hammer. ISO 15243 names this separately from fatigue. The bearing usually survives longer than the shaft does.
Can I use this page for a general hole-and-shaft fit?
No, and there is a page that does: the ISO 286 fit calculator in this plugin owns the general case and this page reuses its tolerance generator rather than duplicating it. The difference is that this page adds the bearing’s own ring tolerances, the roughness correction, the clearance the fit eats and the clearance class it eats it out of — none of which exists for a plain hole. Use the general page for a hub, a bush or a dowel, and this one only for a rolling bearing.
Related calculators
References
- Schaeffler, Technical principles — Bearing types, bearing arrangements, fits (MH1 technical principles section). The source for the decision the fits page leads with, in the publisher’s own words: a ring carrying a circumferential load is one where “as damage to the bearing seating surface can occur, a tight fit should be used”, and a ring carrying a point load is one where “there is no risk that the seating surface will be damaged and a loose fit is possible”. It is also the source for the seating-surface IT grades by bearing tolerance class.
- NSK, Rolling Bearings catalogue E1102, section 9, “Fits and internal clearances”. The source for the effective-interference correction Δd = Δda d/(d+2) for a ground shaft and d/(d+3) for a machined one, for “the amount of this decrease is approximately 70 to 90 % of the interference”, for De = (4D+d)/5 for ball bearings and (3D+d)/4 for roller bearings, and for the recommended shaft and housing classes. NOTE: two readings of table 9.2 returned its condition headings SWAPPED — one put g6 under “rotating outer ring load” and the interference classes under the same heading in the other. Only the classes both readings agreed on are used here, the diameter steps are kept coarse, and the decision rule — which is the part that matters — is taken from Schaeffler’s own wording instead.
- NSK, Internal clearance — types and norms, and the NSK bearing-doctor chapter “Fits and internal clearance”. The source for “generally, the decrease in radial clearance is calculated to be approximately 80 % of the interference”, which is the figure this site uses, and for the residual-clearance bookkeeping.
- ISO 492, Rolling bearings — Radial bearings — Geometrical product specifications (GPS) and tolerance values. Cited by number. The Normal (P0) class bore and outside diameter deviations used here were read from TWO independent printings of the table — SMB Bearings’ tolerance tables and RKB’s tolerances and radial internal clearances catalogue — which agreed row for row from 2.5 mm to 250 mm. They matter because a bearing bore is a unilateral MINUS tolerance, so the interference on the shaft is always larger than the shaft class alone suggests.
- ISO 286-1 and -2, Geometrical product specifications (GPS) — ISO code system for tolerances on linear sizes. Cited by number, never reproduced. The tolerance bands behind the shaft and housing classes on the fits page are COMPUTED by this site’s own ISO 286 generator, which is the same machinery the general fit page uses and which reproduces published fundamental-deviation columns from three independent sources.
- ISO 5753-1, Rolling bearings — Internal clearance — Part 1: Radial internal clearance for radial bearings. Cited by number. The C2/CN/C3/C4/C5 clearance groups used here were read from SKF’s published table 3 for deep groove ball bearings and from NTN’s table 8.8 in its technical section on bearing internal clearance and preload; the two agreed exactly at every bore step from 6 to 120 mm, which is why the numbers are used.
- SKF, Radial internal clearance of deep groove ball bearings (table 3 of the published deep groove ball bearing technical section). Used as the second, independent printing of the ISO 5753 clearance groups; it agreed with NTN’s table exactly over the range this page covers. Its rows above 250 mm contain one gap that NTN’s does not (C4 ending at 237 µm where C5 starts at 255), which is why this page stops at 120 mm rather than transcribing the whole column.
- NTN, Ball and Roller Bearings, catalogue 2203-E, technical section 8, “Bearing internal clearance and preload”. The source for the radial internal clearance table (cross-checked against SKF’s), for the effective clearance relation Δe = Δ0 − (δf + δt), and for the thermal term δt = α ΔT De with α = 12.5 × 10−6/°C.
- ISO 15243:2017, Rolling bearings — Damage and failures — Terms, characteristics and causes. Cited by number. It is the document that gives the failure modes this batch keeps naming — surface-initiated and subsurface fatigue, smearing, fretting corrosion, false brinelling — their agreed names, which matters because a defect frequency identifies a LOCATION and not a cause.