Limits and Fits

The subject of limits and fits really deserves a chapter of its own. The subject is included here because the variability inherent in many of the fit classes is so useful in demonstrating the practical application of the statistical ideas presented.

The designer is free to adopt any geometry of lit for shafts and holes that will ensure the intended function. There is sufficient accumulated experience with commonly recurring situations to make standards useful. There are two standards for limits and fits in the United States, one based on inch units and the other based on metric units.

These differ in nomenclature, definitions, and organization. No point would be served by separately studying each of the two systems. The metric version is the newer of the two and is well organized, and so here we present only the metric version but include a

set of inch conversions to enable the same system to be used with either system of units. In using the standard, capital letters always refer to the hole; lowercase letters are used for the shaft.

The definitions illustrated in Fig. 2-6 are explained as follows: (fig attached)

- Basic size is the size to which limits or deviations are assigned and is the same for both members of the fit.
- Deviation is the algebraic difference between a size and the corresponding basic size.
- Upper deviation is the algebraic difference between the maximum limit and the corresponding basic size.
- lower deviation is the algebraic difference between the minimum limit and the cor responding basic size.
- Fundamental deviation is either the upper or the lower deviation, depending on which is closer to the basic size.
- Tolerance is the difference between the maximum and minimum size limits of a part.
- International tolerance grade numbers IT designate groups of tolerances such that the tolerances for a particular IT number have the same relative level of accuracy but vary depending on the basic size.
- Hole basis represents a system of tits corresponding to a basic hole size. The funda' mental deviation is H.
- Shaft basis represents a system of tits corresponding to a basic shaft size. The fundav mental deviation is h. The shaft-basis system is not included here.