A brief summary of different types of mechanical fits generally used in assemblies and their tolerance ranges.
Shafts and Holes are most common type of features used in mechanical design for assembly and interfaces
The interface between a shaft and hole is defined by a fit
Fit: Degree of looseness or tightness between two mating parts
Fit is defined as the state of connection and also the tolerancing scheme between a pair of male, female counterparts like hole and shaft.
Types of Fits
Over sized hole and under sized shaft
Hole is always larger than the shaft hence the shaft has a clearance with the hole
Example : Clearance holes for floating bolted fasteners
Over sized shaft and undersized hole
Shaft is always larger than the hole which leads to interference at interface
Example: Bearing assembly to shafts
Hole and shaft same size.
Also called as snug fit. Example: Plastic assembly
The fit system is characterized by the tolerance range of the hole and shaft i.e. the range of maximum to minimum size of hole and shaft
Here is a depiction of the tolerance zones for each fit.
The tolerance zone is defined in its location with respect to the Basic size or nominal size based on the fundamental deviation as shown.
Steps in defining fit (clearance, interference or transition) :