Structure of a fit specification
ISO 286 defines the international system of tolerances and fits. A fit designation such as H7/k6 consists of two abbreviations: uppercase letter = bore (internal dimension, A–ZC), lowercase letter = shaft (external dimension, a–zc). Each abbreviation combines a letter (base dimension—determines the position of the tolerance zone relative to the zero line) with a number (IT grade—determines the width of the tolerance zone, ranging from IT1 (very tight) to IT18 (very wide)).
For shafts, the letters a–g lie below the zero line and create clearance; h lies exactly on the zero line with its upper dimension; k–zc lie above it and create interference. For bores, the situation is reversed: A–G are above the zero line (clearance), H lies on the zero line with its lower dimension, and K–ZC are predominantly below it (interference).
Standard bore system
In practice, the standard bore system predominates: The bore tolerance always remains H; only the shaft tolerance varies (g6, h6, k6, p6 …). The reason is related to manufacturing: reamers provide an H tolerance—a single reamer for H7⌀30 thus covers all fit types, because the adjustment is made exclusively on the shaft (which can be machined more flexibly).
Three types of fits: H7/g6, H7/k6, H7/p6
H7/g6 – Clearance fit: The bore’s tolerance zone completely overlaps that of the shaft, resulting in a positive clearance. Typical for plain bearings and guide pins.
H7/k6 – Transition fit: The tolerance zones overlap; depending on the individual dimensions, this results in slight clearance or slight interference. Standard for gears and hubs under normal loads—often combined with a key or pin.
H7/p6 – Press fit: The tolerance zone of the shaft completely overlaps that of the bore, always resulting in an interference fit and a force-fit connection—for hubs under high loads, without additional connecting elements.
Mnemonic
Worked example: Backlash for H7/g6
Given
For ⌀50, the following applies: H7 bore = 0 to +25 µm, g6 shaft = −9 to −25 µm.
Calculation
- Maximum clearance = ES(bore) − ei(shaft) = +25 − (−25) = 50 µm
- Minimum clearance = EI(bore) − es(shaft) = 0 − (−9) = 9 µm
The clearance is ⌀50 H7/g6, so it is always between 9 and 50 µm.
Description and values of the figure
The zero line corresponds to the nominal dimension ⌀50 mm. The bore tolerance H7 ranges from 0 to +25 µm, entirely above the zero line. The shaft tolerance g6 ranges from −25 to −9 µm, entirely below it. Because the two fields do not overlap—with the bore at the top and the shaft at the bottom—there is always clearance: a minimum of 9 µm and a maximum of 50 µm. This is a clearance fit (standard bore system).
| Characteristic | Bore H7 | Shaft g6 |
|---|---|---|
| Lower dimension (EI/ei) | 0 µm | −25 µm |
| Top dimension (ES/es) | +25 µm | −9 µm |
| Clearance | at least 9 µm | max. 50 µm |
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