Page 421 - Shigley's Mechanical Engineering Design

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396 Mechanical Engineering Design
Figure 7–20
Upper deviation, u Max. size, d max
Definitions applied to a Lower deviation, Min. size, d
cylindrical fit. l min
International tolerance
grade, d (IT number)
Fundamental deviation,
(letter)
F
Basic size, D(d)
Lower deviation, l
Upper deviation, u

Fundamental deviation,
International tolerance F (letter)
grade, D (IT number)
Min. size, D
min
Max. size, D max

• Shaft basis represents a system of fits corresponding to a basic shaft size. The
fundamental deviation is h. The shaft-basis system is not included here.

The magnitude of the tolerance zone is the variation in part size and is the same
for both the internal and the external dimensions. The tolerance zones are specified in
international tolerance grade numbers, called IT numbers. The smaller grade numbers
specify a smaller tolerance zone. These range from IT0 to IT16, but only grades IT6 to
IT11 are needed for the preferred fits. These are listed in Tables A–11 to A–14 for basic
sizes up to 16 in or 400 mm.
The standard uses tolerance position letters, with capital letters for internal dimensions
(holes) and lowercase letters for external dimensions (shafts). As shown in Fig. 7–20, the
fundamental deviation locates the tolerance zone relative to the basic size.
Table 7–9 shows how the letters are combined with the tolerance grades to estab-
lish a preferred fit. The ISO symbol for the hole for a sliding fit with a basic size of
32 mm is 32H7. Inch units are not a part of the standard. However, the designation
3
(1 in) H7 includes the same information and is recommended for use here. In both
8
cases, the capital letter H establishes the fundamental deviation and the number 7 defines
a tolerance grade of IT7.
For the sliding fit, the corresponding shaft dimensions are defined by the symbol
3
32g6 [(1 in)g6].
8
The fundamental deviations for shafts are given in Tables A–11 and A–13. For
letter codes c, d, f, g, and h,
Upper deviation = fundamental deviation
Lower deviation = upper deviation − tolerance grade
For letter codes k, n, p, s, and u, the deviations for shafts are
Lower deviation = fundamental deviation
Upper deviation = lower deviation + tolerance grade