Tolerancing: Types of Fits in Mechanical Assembly

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

Clearance Fit:

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

Interference Fit:

Over sized shaft and undersized hole

Shaft is always larger than the hole which leads to interference at interface

Example: Bearing assembly to shafts

Transition Fit:

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.

Fits nomenclature:

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) :

Define Tolerance grade – establishes the size of tolerance zone
Define the fundamental deviation and locate tolerance zone relative to Basic size

For a full course on Tolerance stackup analysis with and without GD&T