Page 277 - Aeronautical Engineer Data Book

Basic mechanical design 223
• Fundamental deviation: (FD) is the distance
(or ‘deviation’) of the nearest ‘end’ of the
tolerance band from the nominal or ‘basic’
size of a dimension.
• Tolerance band: (or ‘range’) is the size of
the tolerance band, i.e. the difference
between the maximum and minimum
acceptable size of a toleranced dimension.
The size of the tolerance band, and the
location of the FD, governs the system of
limits and fits applied to mating parts.
Tolerance values have a key influence on the
costs of a manufactured item so their choice
must be seen in terms of economics as well as
engineering practicality. Mass-produced items
are competitive and price sensitive, and over­
tolerancing can affect the economics of a
product range.

12.5 Limits and fits

12.5.1 Principles
In machine element design there is a variety of
different ways in which a shaft and hole are
required to fit together. Elements such as
bearings, location pins, pegs, spindles and axles
are typical examples. The shaft may be required
to be a tight fit in the hole, or to be looser, giving
a clearance to allow easy removal or rotation.
The system designed to establish a series of
useful fits between shafts and holes is termed
limits and fits. This involves a series of tolerance
grades so that machine elements can be made
with the correct degree of accuracy and be inter­
changeable with others of the same tolerance
grade. The standards ANSI B4.1/B4.3 contain
the recommended tolerances for a wide range
of engineering requirements. Each fit is desig­
nated by a combination of letters and numbers
(see Tables 12.3, 12.4 and 12.5).
Figure 12.8 shows the principles of a
shaft/hole fit. The ‘zero line’ indicates the basic
or ‘nominal’ size of the hole and shaft (it is the