Fits & Clearances
Drill sizes and hole preparation; clearance, transition and interference fits; schedules of fits and clearances; limits of bow, twist and wear; go/no-go gauging.
Fits, clearances & tolerances
FreeFits & clearances — summary notes
Free- A fit is the dimensional relationship between a shaft and a hole, and there are three classes: clearance, transition and interference.
- A CLEARANCE fit leaves the shaft smaller than the hole, so the parts assemble freely and can rotate or slide — a plain bearing.
- An INTERFERENCE (press) fit makes the shaft larger than the hole, so the parts are forced together and held by friction — bushes and bearing races.
- A TRANSITION fit falls between the two, giving either slight clearance or slight interference within tolerance, where accurate location matters more than free movement.
- Clearance = hole size − shaft size. A positive result is a clearance fit, a negative result an interference fit.
- Fits are specified on a hole basis (hole size fixed, shaft varied) or a shaft basis. The hole basis is the more common, because holes are harder to vary than shafts.
- Measuring: micrometers and bore gauges for absolute sizes, feeler gauges for running clearances, dial gauges for end-float, backlash and run-out, and go/no-go gauges for a quick pass/fail. Compare the result against the schedule of fits and clearances in the maintenance manual.
- Components must be clean and at a stable temperature when measured, because thermal expansion changes the reading — and the instrument itself must be in calibration.
- ⚠ Exam trap: an interference fit has the SHAFT LARGER than the hole. Reversing that is the standard distractor.
- Clearance
- clearance = hole size − shaft size
- Sign rule
- positive → clearance fit · negative → interference fit
- Interference fitting
- heat the hole or chill the shaft to assemble
A hole measures 25.02 mm and its shaft 25.05 mm. Identify the fit.
Clearance = 25.02 − 25.05 = −0.03 mm. The negative result means the shaft is larger than the hole, so this is an interference (press) fit.
Why is a bearing housing sometimes heated before a race is fitted?
Heating expands the housing bore, temporarily turning an interference fit into a clearance so the race can be positioned without force. On cooling the bore contracts onto the race and the interference is restored, giving the grip the design relies on.
Fits & clearances concept map
FreeFits & Clearances
Fits & clearances quiz
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