Tolerancing: Types of Fits in Mechanical Assembly
Mufaddal Rasheed 
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
have a look here : Basics of Tolerance analyses : Stackups and Fits
Explore more topics in GD&T :
What is virtual condition in GD&T
Factors in Specifying tolerance
GD&T Regardless of feature size
GD&T maximum material condition
What is datum feature modifier
What is actual mating envelope
What is 3-2-1 principle in tolerancing
Applying GD&T scheme to a bracket
GD&T applied to patterns of features
Tolerance stack up analysis of a simple part
What are material conditions in GD&T
Composite position tolerance in GD&T
What are datum targets in GD&T
Verification of manufactured GD&T drawing
What are simultaneous requirements in GD&T
Calculating the geometric tolerance of a part
Developing GD&T scheme for a part
Traditional tolerancing vs GD&T
Categories: : GD & Tolerancing