Choosing a fit is not a matter of taste. The ISO 286 system defines, for every nominal size and tolerance grade, exactly how large a shaft and a housing bore may be, and therefore exactly how much interference or clearance results. A buyer or designer who understands the arithmetic can predict whether a bearing will grip its shaft without cracking the ring or spinning loose, and can catch a bad fit before it reaches the shop floor. This article works one real bearing through the tables to the numbers, then repeats the method in a short sequence that can be applied to any size.
The reason a fit must be calculated rather than guessed is that both parts of the joint carry a tolerance. The shaft is a range, the bearing ring is a range, and the fit is the difference between the two ranges at their extremes. Reporting one figure for the interference hides the worst case, and the worst case is exactly what decides whether the ring grips or the clearance closes.
Fundamental deviation plus IT grade
ISO 286 describes a fit with two things. The fundamental deviation is the position of the tolerance zone relative to the nominal size, written as a lowercase letter for a shaft (h, j, k, m, n) and an uppercase letter for a hole (H, J, K, M, N). The tolerance grade is the width of the zone, taken from the IT grades and written as a number, so IT5 is narrow and IT7 is wider. A shaft described as k5 is a shaft with the k deviation and the IT5 width. A housing described as H7 is a hole with the H deviation and the IT7 width. Read the letter and the number, and the whole zone is pinned down.
Two quantities are then needed to compute the fit: the limits of the shaft, and the limits of the mating bore. For a shaft, the lower deviation is the fundamental deviation and the upper limit sits one IT grade above it. For a hole such as H, the lower limit is the nominal size and the upper sits one IT grade above it. The numbers depend on the diameter band, so the same code on a different diameter gives a different width.
The ring's own ISO 492 tolerance
A bearing ring is not a nominally perfect hole or shaft, and its tolerance is defined separately by ISO 492. For a normal-class bearing the mean bore deviation runs from zero to a small negative figure, and the mean outside diameter deviation likewise runs from zero to a negative figure. That means a 25 mm nominal bore is a little under 25 mm in practice, and a 52 mm nominal outside diameter is a little under 52 mm. This is the step most often skipped. If the ring is treated as exactly 25 mm, the calculated interference is wrong for the whole bore, and on a tight fit the error can be the difference between a grip and a loose ring.
| Symbol | What it gives | Value used in this example |
|---|---|---|
| IT5 | Tolerance width, 18 to 30 mm band | 9 µm |
| IT6 | Tolerance width, 18 to 30 mm band | 10 µm |
| IT7 | Tolerance width, 50 to 65 mm band | 30 µm |
| k | Fundamental deviation, shaft | +2 µm at 25 mm |
| Δdmp | Mean bore deviation, normal class | 0 to −10 µm at 25 mm |
| ΔDmp | Mean outside diameter deviation, normal class | 0 to −13 µm at 52 mm |
A worked inner-ring interference
Take a 6205 with a 25 mm bore on a 25 mm shaft fitted k5. The catalogue defines r as fixed and the ring as a range, so the calculation proceeds in three moves.
Shaft k5: the limits
For the 18 to 30 mm diameter band, IT5 is 9 micrometres and the k deviation starts at +2 micrometres. The shaft limits are therefore +2 to +11 micrometres, that is 25.002 mm to 25.011 mm.
Bore deviation: the normal class
The 6205 bore in normal class is 0 to minus 10 micrometres, that is 24.990 mm to 25.000 mm. The ring is slightly under nominal at every point in its band.
Combining both ranges
The interference is the shaft minus the bore. The smallest interference is the smallest shaft against the largest bore: 25.002 minus 25.000, or 0.002 mm. The largest interference is the largest shaft against the smallest bore: 25.011 minus 24.990, or 0.021 mm. The guaranteed interference is therefore 2 micrometres minimum and 21 micrometres maximum. Both ends matter: the minimum decides whether the ring can slip, the maximum decides whether the ring is over-stressed and its clearance closed.
A worked housing clearance
Now the outer ring into the housing. The 6205 outside diameter is 52 mm. A housing H7 has a fundamental deviation of zero and, for the 50 to 65 mm band, an IT7 width of 30 micrometres, so the housing bore runs 52.000 mm to 52.030 mm. The bearing outside diameter in normal class is 0 to minus 13 micrometres, that is 51.987 mm to 52.000 mm. The result is a clearance fit: the outer ring is smaller than the housing bore in every combination, giving 0 to 0.043 mm of clearance. That is intentional. A stationary outer ring carrying a steady load is given a loose seat so it can be assembled by hand and can creep slightly rather than fret.
Shaft candidates at a 25 mm bore
| Shaft code | Shaft limits (mm) | Interference vs a normal bore (µm) | Character |
|---|---|---|---|
| h6 | 24.990 to 25.000 | 10 clearance to 10 interference | Free, transition; easy to mount |
| k5 | 25.002 to 25.011 | 2 to 21 interference | Light, reliable grip |
| k6 | 25.002 to 25.012 | 2 to 22 interference | Light, slightly wider band |
| m5 | 25.008 to 25.017 | 8 to 27 interference | Heavier grip, hot duty |
| m6 | 25.008 to 25.018 | 8 to 28 interference | Heavier grip, wider band |
Normal-class bore taken as 24.990 to 25.000 mm. Limits are for the 18 to 30 mm band and vary with diameter.
The table shows why the code alone is not enough. The letter and number set where the zone sits, but the actual micrometres depend on the band, so the same k5 on a 40 mm shaft gives a different interference. The right way to read the table is to see the trend, then look up the exact band for the size in hand.
Interference is not free
The clearance the interference consumes
As the inner ring is pressed onto the shaft it expands, and that expansion eats internal clearance. A few micrometres of interference on a small ring can consume a meaningful share of the bearing clearance, which is one reason a k5 fit on a motor shaft is often paired with C3 clearance rather than normal. The designer's job is to size the fit so the ring grips reliably without squeezing the running clearance toward zero. On a hot shaft the interference also grows as the metal expands, so a fit that is comfortable cold can become tight at operating temperature; the check has to be repeated warm.
Four steps to compute any fit
The same sequence works for any designation and size.
- Read the shaft or housing code. Identify the deviation letter and the IT grade, for example k5, m6 or h6, and note the diameter band.
- Look up the zone. Take the fundamental deviation and the IT width from the ISO 286 table for that band and write both limits.
- Add the ring's own tolerance. Apply the ISO 492 normal-class deviation to the bore or the outside diameter so the ring is treated as a range.
- Subtract at both extremes. Compute the interference or clearance at the smallest and largest combination, then check both ends against the load and the clearance requirement.
Where fit calculations go wrong
- Stopping at the nominal size. Comparing a shaft limit with 25.000 mm and ignoring the ring's negative tolerance overstates the interference and hides the loose end of the range.
- Reporting one number. A single figure looks safe at the average but can be loose at one extreme and dangerously tight at the other, so both ends must be computed.
- Forgetting the clearance the fit consumes. Pressing a ring on expands it and closes internal clearance, so the fit has to be coordinated with the C-class, not chosen in isolation.
- Ignoring temperature. A fit calculated at room temperature migrates across the band as the shaft or housing heats, especially when steel rings sit in aluminium housings.
- Trusting the arithmetic alone. A calculation assumes round, smooth, clean, concentric seats; an oval or tapered shaft does not deliver the interference the numbers predict.
- Applying a catalogue recommendation blindly. The standard combinations assume typical load direction and stiffness, so a machine that differs needs the fit recalculated.
- A fit is a range. It is defined by a fundamental deviation and an IT grade, and both limits matter.
- Add the ring's own tolerance. ISO 492 gives a normal-class bore and outside diameter a small negative deviation that changes the result.
- The 6205 worked case. On a 25 mm k5 shaft the interference is 2 to 21 micrometres against a normal bore.
- H7 gives clearance. On a 52 mm outside diameter the housing H7 produces 0 to 43 micrometres of clearance, as intended.
- Interference consumes clearance. Coordinate the fit with C3 or C4 so the running clearance stays positive.
The full fit guidance and the recommended combinations for rotating and stationary loads sit in the shaft and housing fit guide, and the way a fit eats clearance is worked in the clearance class guide. The tolerance standard behind the ring's own band is explained in the ISO 492 guide, and the effect of heat on a calculated fit is covered in the temperature and clearance article. How the ISO and ABMA languages differ is set out in the ABEC and ISO comparison. Dimension and fit references for each size are collected in the technical library, and the bearing selector lists the boundary data for the two families used here, the deep groove ball range and the cylindrical roller range. Chamfer and dimensional data as published by a maker is shown on the SKF listings.
Frequently asked questions
How is interference calculated from the ISO 286 tables?
Read the shaft code to get its fundamental deviation and IT width, which give the shaft limits. Read the bearing ring's own ISO 492 deviation to get the bore limits. Interference is the shaft minus the bore, computed at both extremes: smallest shaft against largest bore gives the minimum, largest shaft against smallest bore gives the maximum.
Why does the bearing bore have a tolerance of its own?
Because a ring is a manufactured part, not a nominally perfect hole. ISO 492 gives a normal-class bore a mean deviation from zero to a small negative figure, so a 25 mm nominal bore is actually a little under 25 mm. Ignoring that deviation makes the calculated interference wrong across the whole range.
What interference does a k5 fit give on a 25 mm shaft?
For the 18 to 30 mm band the k5 shaft runs from +2 to +11 micrometres, that is 25.002 to 25.011 mm. Against a normal-class bore of 24.990 to 25.000 mm the interference is 2 micrometres minimum and 21 micrometres maximum. The letter sets the position and the IT grade sets the width.
Why does H7 on a bearing outside diameter give clearance?
H7 places the housing bore at nominal and above, and the normal-class outside diameter sits at nominal and below, so the ring is always slightly smaller. On a 52 mm outside diameter that yields 0 to 43 micrometres of clearance. A stationary outer ring under a steady load is meant to have a loose seat.
Why can a tight fit damage a bearing?
Pressing the inner ring onto the shaft expands it and consumes internal clearance. Too much interference can close the running clearance to zero or preload the bearing, which raises friction and heat and shortens life. That is why a heavy fit is normally paired with a larger C3 or C4 clearance class.
Does a fit calculated at room temperature still hold when hot?
Not always. As a shaft or housing heats, its diameter grows, so the interference migrates across the band. The effect is largest when a steel ring sits in an aluminium housing, because aluminium expands faster. The fit should be checked at the operating temperature, not only cold.