Clearance, transition and interference fits

A fit is the relationship between a hole and a shaft that are assembled, resulting from the difference of their sizes before assembly.

Types

  Clearance    Transition    Interference
   |hole|       |hole|         |shaft|
   |    |       |    |         |     |
                |shaft|        |hole|
   |shaft|      |     |
   (hole zone   (zones         (shaft zone
    above shaft  overlap)       above hole)

1. Clearance fit: the shaft is always smaller than the hole, so there is always a gap (minimum clearance $\ge 0$). The parts can slide or rotate. Example: shaft in a plain bearing (H7/g6, H8/f7), sliding fit of a piston in a cylinder (running fits).
2. Interference (force/press) fit: the shaft is always larger than the hole, so the assembly needs force (pressing, or heating or cooling the parts) and they are fixed together by friction. Example: bush in a housing, gear on a shaft, wheel on an axle (H7/p6, H7/s6).
3. Transition fit: the sizes overlap, so, depending on the actual dimensions, there may be a small clearance or a small interference. It is used for accurate location without much load: pulley on shaft with a key, spigot location, coupling hub (H7/k6, H7/n6).

Hole basis and shaft basis

• Hole basis system: the hole has zero lower deviation (H); the fits are obtained by changing the shaft's fundamental deviation. Examples: H7/g6 (clearance), H7/k6 (transition), H7/p6 (interference).
• Shaft basis system: the shaft has zero upper deviation (h); the fits are obtained by changing the hole's deviation. Examples: G7/h6 (clearance), K7/h6 (transition), P7/h6 (interference).

The minimum clearance is the difference between the lower limit of hole and the upper limit of shaft; the maximum clearance is the difference between the upper limit of hole and the lower limit of shaft.