Lessons · Engineering · Tolerance and fit: clearance, transition, interference
Clearance, transition, interference
A fit is the relationship between a hole and the shaft that goes in it. Clearance means the shaft is always smaller and it turns or slides. Interference means the shaft is always larger and it must be pressed or shrunk in. Transition may be either, depending where in tolerance the two parts land.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is where a drawing stops being a picture and becomes manufacturable. A dimension without a tolerance says nothing about whether the parts will go together at all.
How to think about it
Ask what the joint must do -- rotate, slide, or never move -- and choose the class of fit that guarantees it at both ends of tolerance. Then check the worst case: largest shaft in smallest hole, and smallest shaft in largest hole.
Worked example
Clearance: shaft always smaller. It moves.Bearings, sliding parts.
Interference: shaft always larger. It is pressed in.Locating parts that must not move.
Transition: could be either. Only for located parts.Assembly may need light force.
Always check both extremes of tolerance.The middle never fails; the ends do.
Your turn
Which fit guarantees the shaft can always turn in the hole?
The fit that always leaves a gap is a fit
Solve one, graded on the server
The trap
Checking the nominal sizes only. Both parts are made to a tolerance, and the fit is decided at the extremes, not at the number on the drawing.