Choosing Tolerance Zones and Fits

This article covers tolerance zones, how to select them, and fits — the types of fits, what the letters used for fits mean, that is, the meaning of the letters — and shares tolerance tables for the hole-basis and shaft-basis systems.

Choosing Tolerance Zones

As much as possible, tolerance zones should be chosen among the shaft and hole symbols given in the figures below that fit the requirement, especially among the boxed symbols. In fitting, the hole-basis system should be chosen first. 

The shaft-basis system should only be preferred when using it provides a clear economic advantage. After that, the tolerance symbols and values for the smallest and largest clearance or interference that will best meet or suit the operating conditions should be chosen.

Fits

The relationship that results from the difference between the dimensions of two parts to be assembled together, before assembly, is called a FIT. 

Generally, the relative positions of the tolerance zones of the hole and shaft to each other produce a clearance fit, a transition fit (clearance or interference), or an interference fit.

As you can see, in the hole-basis system, the hole’s dimension is taken as the basic size. In the shaft-basis system, the shaft’s dimension is taken as the basic size.

Fit Tolerance Tables for the Hole-Basis System

Fit Tolerance Tables for the Shaft-Basis System

Types of Fits 

The table below gives the fit types for precision, fine, medium, and coarse fit grades. For both the hole-basis and shaft-basis systems, it gives the letter equivalents in the tolerance tables for fit types like locational fits, interference (press) fits, drive fits, transition fits, running fits, sliding fits, rotating fits, and tight rotating fits, along with how each fit should be assembled (with a hammer, with a press, by hand, etc.).

In this article, we’ve covered the types of fits in tolerances and their letter equivalents. We’ve shared tolerance value tables for H5, H6, H8, H9, H10, H11, H12, and H13 for the hole-basis system, and tolerance tables for h5, h6, h8, h9, h10, h11, h12, and h13 for the shaft-basis system. We look forward to seeing you in our next articles.