Limits and fits

Limits and fits

Dimensional tolerances are decided according to type of required fits. The type of required fits decides the limits.

Following are types of fits.

Clearance Fit-Clearance always exists in between maximum allowed shaft diameter and minimum allowed hole diameter.

Take example of shaft and bushing. In worst case shaft dimension will be 59.99 and bushing ID will be 60.01.Clearance of 0.02 mm. 

Interference Fit-Interference always exists in between minimum allowed shaft diameter and maximum allowed hole diameter.

Take example of shaft and bushing. In worst case shaft dimension will be 60.01 and bushing ID will be 59.99.Interference of 0.02 mm. 

Transition Fit- Close Interference or close clearance always exists in between shaft and hole diameter.

This will be clear from this example .Following extreme conditions are possible here.

1-Shaft diameter 60.01mm and hole diameter 60.02mm-close clearance of 0.01 mm.

2-Shaft diameter 60.01 mm and hole diameter 60.04mm-close clearance of 0.03 mm.

3-Shaft diameter 60.05 and hole diameter 60.02 mm-close interference of 0.03 mm.

4-Shaft diameter 60.05 and hole diameter 60.04 mm-close interference of 0.01 mm.

CLEARANCE FIT – TRANSITION FIT – INTERFERENCE FIT-This is sequence for assembly efforts. Clearance fit will need minimum efforts, transition will need medium and interference type fit will need maximum efforts. But on the other hand  alignment accuracy level increases from clearance type to interference type of fit.

We designers should be smart enough  to decide type of fit according to functionality of part. There are many but following are some points ,we shall keep in mind while deciding type of fit.

1-For how many times dismantling of parts is needed in total life of assembly.

2-What is role of part in assembly, is it going to move, revolve etc.etc.

3-What is expected accuracy level of assembly.

4-Ether assembly allows me lubrication or not.

5-Kind of disassembly needed-destructive or non destructive.

6-Beyond that what is raw material of mating parts.