Basic size, limits, tolerance, allowance, types of fit and the hole basis system.
Interchangeability means any part from a lot can replace a similar part and fit correctly without extra fitting. It is possible only when parts are made within limits.
Terms
- Basic (nominal) size – the size given on the drawing, e.g. Ø25.
- Limits – the two extreme sizes allowed: upper (maximum) limit and lower (minimum) limit.
- Tolerance – difference between upper and lower limits.
- Deviation – difference between a limit and the basic size (upper / lower deviation).
- Allowance – intentional difference between the hole and shaft at maximum metal condition.
Example: Ø25 +0.03 / −0.02
Upper limit = 25.03 mm, Lower limit = 24.98 mm
Tolerance = 25.03 − 24.98 = 0.05 mm
Upper limit = 25.03 mm, Lower limit = 24.98 mm
Tolerance = 25.03 − 24.98 = 0.05 mm
Fits
Fit is the relation between a hole and a shaft that are to be assembled.
| Fit | Condition | Example |
|---|---|---|
| Clearance fit | Shaft always smaller than hole | Shaft in a bearing, sliding parts |
| Interference fit | Shaft always bigger than hole – pressed in | Gear on shaft, bush in housing |
| Transition fit | May give clearance or interference | Spigot, location of parts |
Hole basis system
The hole is kept constant (its lower deviation is zero – symbol H) and the shaft size is changed to get different fits. It is preferred because holes are made with standard drills and reamers. In the shaft basis system the shaft is constant (symbol h).
BIS system: IS 919, with 18 grades of tolerance (IT01 to IT16 / IT18). Example: H7/g6 – hole H7, shaft g6 (clearance fit).
Quick Revision
- Tolerance = upper limit − lower limit.
- Fits: clearance, interference, transition.
- Hole basis: hole constant, lower deviation zero (H).
- Capital letters for holes, small letters for shafts.
Your progress is saved on this device.