TAHA Engineering Toolsby TAHA Knowledge Lab

Metrology & Quality

Fits Calculator – Clearance, Transition & Interference

Fit type and maximum/minimum clearance or interference of a hole and shaft.

Inputs

Fit diagram: the hole tolerance zone sits just above the zero line and the shaft tolerance zone lies below the zero line, showing minimum and maximum clearance.0holeESEI = 0shafteiesmin clearancemax
Hole and shaft tolerance zones drawn against the zero (nominal) line. Here the shaft zone is wholly below the hole zone, so there is always clearance: minimum clearance = EI − es, maximum clearance = ES − ei. Diagram not to scale.

Signed values, from the drawing or ISO 286-2. Example 25 H7: ES = +21 µm, EI = 0.

Example 25 g6: es = −7 µm, ei = −20 µm.

Result

Type of fit
Clearance fit
What this means
There is always a gap: the shaft can slide or turn in the hole.
Maximum clearance (negative = interference)
41
Minimum clearance (negative = interference)
7
Fit tolerance (hole + shaft tolerance)
34
Hole limits
25 to 25.021 mm
Shaft limits
24.98 to 24.993 mm
Fit system
Hole-basis (EI = 0, hole H)
Working
Max clearance = ES − ei = 21 − (-20) = 41 µm Min clearance = EI − es = 0 − (-7) = 7 µm

Clearance fit: clearance ranges from 7 µm to 41 µm (negative values are interference).

Educational estimate — check the assumptions below.

About this calculator

A fit is the relationship between a hole and the shaft that goes into it. Depending on their limits, the pair always has a gap (clearance fit, e.g. a sliding or running shaft), is always tight (interference fit, e.g. a pressed-in bearing or bush), or may be either (transition fit, for accurate location). Enter the limit deviations of the hole and the shaft to see which type you have and the extremes of clearance or interference.

Most fits use the hole-basis system: the hole is H (lower deviation 0) and the shaft letter is varied, because holes are made with fixed-size reamers and checked with fixed gauges. In the shaft-basis system the shaft is h (upper deviation 0) and the hole varies.

Formula

Maximum clearance = ES − ei    Minimum clearance = EI − es

Fit tolerance = (ES − EI) + (es − ei)

ES, EI = upper and lower deviations of the hole; es, ei = upper and lower deviations of the shaft. Clearance fit: minimum clearance ≥ 0. Interference fit: maximum clearance ≤ 0. Otherwise transition fit. A negative clearance is an interference.

Worked example

Nominal size 25 mm, deviations from ISO 286-2:

  • H7/g6 (hole +21/0 µm, shaft −7/−20 µm): max clearance 21 − (−20) = 41 µm, min clearance 0 − (−7) = 7 µm → clearance fit.
  • H7/k6 (shaft +15/+2 µm): max clearance 21 − 2 = 19 µm, min clearance 0 − 15 = −15 µm → transition fit (up to 15 µm interference).
  • H7/p6 (shaft +35/+22 µm): max clearance 21 − 22 = −1 µm → interference fit, interference 1 to 35 µm.

Assumptions and limitations

  • Deviations must be entered from the drawing or from ISO 286-2 tables for the actual size range; no fit tables are built in.
  • The extremes are worst cases. Real assemblies usually fall between them.
  • Form errors, surface roughness, temperature differences and the effect of assembly on an interference fit are not included. Press-fit force and hub stress need a separate calculation.

References

  • ISO 286-1: definitions of fits, hole-basis and shaft-basis systems.
  • ISO 286-2: tables of standard tolerance classes and limit deviations (source of the example values).