Limits and Fits, Tolerance Dimensioning

Definitions:nominal size: The size designation used for generalidentification. The nominal size of a shaft and a hole are thesame. This value is often expressed as a fraction.basic size: The exact theoretical size of a part. This isthe value from which limit dimensions are computed. Basic size isa four decimal place equivalent to the nominal size. The number ofsignificant digits imply the accuracy of the dimension.

example: nominal size = 1 1/4basic size = 1.2500

design size: The ideal size for each component (shaft andhole) based upon a selected fit. The difference between the designsize of the shaft and the design size of the hole is equal to theallowance of the fit. The design size of a part corresponds to the Maximum Material Condition (MMC). That is, the largest shaft permitted by the limits and the smallest hole. Emphasis is placed upon the design size in the writing of the actual limit dimension, so the design size is placed in the top position of the pair.

tolerance: The total amount by which a dimension is allowed to vary. For fractional linear dimensions we have assumed a bilateral tolerance of 1/64 inch. For the fit of a shaft/holecombination, the tolerance is considered to be unilateral, that is, it is only applied in one direction from design size of the part. Standards for limits and fits state that tolerances are appliedsuch that the hole size can only vary larger from design size and the shaft size smaller.
basic hole system: Most common system for limit dimensions. In this system the design size of the hole is taken to be equivalent to the basic size for the pair (see above). This means that the lower (in size) limit of the hole dimension is equal to design size. The basic hole system is more frequently used since most hole generating devices are of fixed size (for example, drills, reams, etc.) When designing using purchased components with fixed outer diameters (bearings, bushings, etc.) a basic shaft system may be used.

allowance: The allowance is the intended difference in the sizes of mating parts. This allowance may be: positive (indicated with a "+" symbol), which means there is intended clearance between parts; negative("-"), for intentional interference: or "zero allowance" if the two parts are intended to be the "same size".This last case is common to selective assembly.

The extreme permissible values of a dimension are known as limits. The degree of tightness or looseness between two mating parts that are intended to act together is known as the fit of the parts. The character of the fit depends upon the use of the parts. Thus, the fit between members that move or rotate relative to each other, such as a shaft rotating in a bearing, is considerably different from the fit that is designed to prevent any relative motion between two parts, such as a wheel attached to an axle.

In selecting and specifying limits and fits for various applications, the interests of interchangeable manufacturing require that (1) standard definitions of terms relating to limits and fits be used; (2) preferred basic sizes be selected wherever possible to be reduce material and tool costs; (3) limits be based upon a series of preferred tolerances and allowances; and (4) a uniform system of applying tolerances (bilateral or unilateral) be used.