Clearance Fits
A clearance fit always provides a positive clearance between the hole and shaft. The maximum size of the shaft is always smaller than the minimum size of the hole. Used for bearings, sliding components, and free-running assemblies.
Common Specifications
| Fit | Description | Typical Applications |
|---|---|---|
| H7/h6 | Locational clearance (very small gap) | Accurate locating, spigots |
| H7/g6 | Small clearance fit | Precision rotating shafts, linkages |
| H7/f7 | Moderate clearance | Running and sliding fits, journal bearings |
| H8/f7 | Larger running clearance | General rotating assemblies |
Transition Fits
A transition fit may result in either a small clearance or a small interference depending on the actual manufacturing limits of the mating parts. Used for accurate location where slight tapping may be required for assembly.
Common Specifications
| Fit | Description | Typical Applications |
|---|---|---|
| H7/js6 | Near-zero transition fit | Accurate positioning, hand assembly |
| H7/k6 | Light transition/interference | Hubs, accurate machine tool assemblies |
| H7/m6 | Tighter transition fit | Locating components, driven fits |
Interference Fits
Produces a negative clearance (interference), meaning the shaft is intentionally manufactured larger than the hole. Assembly usually requires thermal expansion (heating the hole) or significant hydraulic pressing force.
Common Specifications
| Fit | Description | Typical Applications |
|---|---|---|
| H7/p6 | Light interference fit | Permanent or semi-permanent, pressed bushes |
| H7/s6 | Medium interference | Gears, hubs, rigid couplings |
| H7/u6 | Heavy interference | High-load permanent assemblies, shrink fits |
ISO Tolerance Grades (IT)
The IT (International Tolerance) grade determines the width of the tolerance zone. Lower IT numbers represent tighter, more expensive manufacturing tolerances, while higher numbers represent wider, economical tolerances.
Common Specifications
| Grade | Relative Precision | Typical Manufacturing Process |
|---|---|---|
| IT1 - IT4 | Ultra / Very High Precision | Lapping, honing, ultra-precision grinding |
| IT5 - IT7 | Precision | Grinding, fine broaching, precision turning |
| IT8 - IT9 | General Engineering | Turning, milling, reaming, boring |
| IT10 - IT11 | Medium Tolerance | Drilling, rough turning, milling |
| IT12 - IT16 | Coarse / Very Coarse | Casting, forging, flame cutting |
Hole vs Shaft Basis
In a Hole-Basis system, the hole's lower deviation is zero (H), and fits are achieved by altering the shaft. In a Shaft-Basis system, the shaft's upper deviation is zero (h). Hole-basis is generally preferred because holes are harder to manufacture to custom sizes than shafts.
Common Specifications
| System | Zero Reference | Example Fits |
|---|---|---|
| Hole-Basis (H) | Hole minimum limit is exact nominal | H7/g6, H7/p6 |
| Shaft-Basis (h) | Shaft maximum limit is exact nominal | G7/h6, P7/h6 |