Engineering fits

4 min readMay 11, 2022

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What is a Fit?

When designing a part or assembly, engineering fits are typically employed as part of the geometric dimensioning and tolerancing process. In engineering jargon, the “fit” is the clearance (or overlap) between two mating components. The magnitude of this clearance determines whether the parts may move or rotate independently of each other or are temporarily/ permanently linked at the other. As a result, a fit is utilised to explain the components’ dimensional connections. It determines if the components are slack or tight, which assists in the sliding or pressing characteristic. Engineering fits are sometimes referred to as “shaft and hole” pairings, however they are not confined to only round components.

Type of Fit

  1. Clearance: The hole is larger than the shaft, enabling the two parts to slide and / or rotate when assembled.
  2. Transition: The hole is fractionally smaller than the shaft and mild force is required to assemble/disassemble.
  3. Interference: The hole is smaller than the shaft and high force (eg. hammering/hydraulic press) or heat is required to assemble/disassemble the parts.
Types of Engineering Fits

Clearance Fit

A clearance fit is utilised when loose mating and unrestricted movement of components are required. As a result, they are suitable for creating items whose components must easily move in and out.

Clearance fittings feature a shaft that is smaller than the hole. This has two consequences. The first is a maximum clearance, in which the shaft has the smallest diameter and the hole has the largest diameter. The other is the minimal clearance, in which the shaft is at its maximum and the hole is at its smallest.

Clearance fits are further divided into five categories classified based on how loose they are. Below are the different types of fits under this category:

Loose running: Larger clearance where accuracy is not essential

Free running: Large clearance where accuracy is not essential and involves high running speeds, large…


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