ISO 286-2:2010
ISO 286-2 hole and shaft fit classes, IT tolerance grades and fundamental deviations in micrometres.
ISO 286-2 · 2010 · unit: mm
IT grade widths in micrometres, from the ISO 286-2 tolerance grade table. The second table gives the fundamental deviations of shafts for the h, g, f, e, d and c fits.
| Nominal size range (mm) | IT 4 (µm) | IT 5 (µm) | IT 6 (µm) | IT 7 (µm) | IT 8 (µm) | IT 9 (µm) |
|---|---|---|---|---|---|---|
| up to 3 | 3 | 4 | 6 | 10 | 14 | 25 |
| over 3 to 6 | 4 | 5 | 8 | 12 | 18 | 30 |
| over 6 to 10 | 4 | 6 | 9 | 15 | 22 | 36 |
| over 10 to 18 | 5 | 8 | 11 | 18 | 27 | 43 |
| over 18 to 30 | 6 | 9 | 13 | 21 | 33 | 52 |
| over 30 to 50 | 7 | 11 | 16 | 25 | 39 | 62 |
| over 50 to 80 | 8 | 13 | 19 | 30 | 46 | 74 |
| over 80 to 120 | 10 | 15 | 22 | 35 | 54 | 87 |
| over 120 to 180 | 12 | 18 | 25 | 40 | 63 | 100 |
| over 180 to 250 | 14 | 20 | 29 | 46 | 72 | 115 |
| over 250 to 315 | 16 | 23 | 32 | 52 | 81 | 130 |
| over 315 to 400 | 18 | 25 | 36 | 57 | 89 | 140 |
| over 400 to 500 | 20 | 27 | 40 | 63 | 97 | 155 |
ISO 286-2 · 2010 · unit: µm
Fundamental deviation of shafts h, g, f, e, d and c. Clearance fits use these values together with the IT tolerance grade.
| Nominal size range (mm) | shaft h (µm) | shaft g (µm) | shaft f (µm) | shaft e (µm) | shaft d (µm) | shaft c (µm) |
|---|---|---|---|---|---|---|
| up to 3 | 0 | -6 | -10 | -14 | -20 | -30 |
| over 3 to 6 | 0 | -6 | -10 | -20 | -30 | -40 |
| over 6 to 10 | 0 | -4 | -13 | -25 | -40 | -50 |
| over 10 to 18 | 0 | -5 | -16 | -32 | -50 | -60 |
| over 18 to 30 | 0 | -6 | -19 | -40 | -60 | -75 |
| over 30 to 50 | 0 | -7 | -22 | -50 | -70 | -90 |
| over 50 to 80 | 0 | -8 | -24 | -58 | -80 | -105 |
| over 80 to 120 | 0 | -9 | -28 | -68 | -90 | -120 |
| over 120 to 180 | 0 | -10 | -32 | -76 | -105 | -135 |
Fit classes are a pair: a letter that sets the position of the tolerance band and an IT grade that sets its width. Holes are normally the hole basis and shafts are the shaft basis. The tables below give the IT grade widths and the fundamental deviations of the shafts, both in micrometres. Confirm the edition named on the drawing and state the fit explicitly rather than relying on a general note.
Reference source
ISO 286-2:2010
The hole tolerance controls how far the hole may deviate from its nominal size, and the shaft tolerance does the same for the shaft. The two are independent, and the actual clearance or interference of the joint comes from the combination of both plus the fit class.
H is the hole basis: the deviation of the hole is zero at the lower limit, so the hole is always at or above the nominal size. The number is the IT grade, which sets how wide the tolerance band is. H7 therefore means a hole from the nominal size up to the nominal size plus the IT7 tolerance. A shaft marked g6 sits below the nominal size, which gives a clearance fit with an H7 hole.
Use a clearance fit with a clearly positive minimum clearance, for example H8/f7 or H9/e9 for a rotating shaft in a bearing housing. The joint must still be assembled by hand or with light press force, and you should keep enough clearance to absorb thermal growth and any coating thickness.
A clearance fit always has a gap, a transition fit can have a small gap or a small press fit, and an interference fit is always a press or thermal fit. Interference fits need the parts heated or cooled and are not practical for every size or material, so most machine designs stay in the clearance range.
Maximum clearance equals the largest hole minus the smallest shaft. Minimum clearance equals the smallest hole minus the largest shaft. For an H7/g6 fit, the shaft can never be larger than the nominal size and the hole can never be smaller, so the minimum clearance is always positive.
IT stands for International Tolerance grade. The grade number sets the tolerance band width for a given nominal size, and the band widens as the size grows. IT01 and IT0 are the super-precision grades, IT1 to IT5 are precision, IT6 to IT11 are the general engineering grades, and IT12 and above are for rough, large or cheap parts.
It is a practical convention, not a rule. A slightly coarser hole is easier to produce and to inspect, it allows a slightly larger bearing or fastener, and it keeps the assembly from becoming too tight. Always confirm the fit against the real component before freezing the drawing.
The fundamental deviation, written as a lower case letter for shafts and an upper case letter for holes, sets the position of the tolerance band relative to the nominal size. It defines whether the joint will be a clearance, transition or interference fit, while the IT grade sets how wide the band is.
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