Page 280 - Aeronautical Engineer Data Book

226 Aeronautical Engineer’s Data Book
• If the deviation is small, the tolerance range
will be near the basic size, giving a tight fit.
• A large deviation gives a loose fit.
Various grades of deviation are designated by
letters, similar to the system of numbers used
for the tolerance ranges. Shaft deviations are
denoted by small letters and hole deviations by
capital letters. Most general engineering uses a
‘hole-based’ fit in which the larger part of the
available tolerance is allocated to the hole
(because it is more difficult to make an accurate
hole) and then the shaft is made to suit, to
achieve the desired fit.
Tables 12.4 and 12.5 show suggested clear­
ance and fit dimensions for various diameters
(ref.: ANSI B4.1 and 4.3).

Table 12.6 Metric fit classes
1. Easy running fit: H11-c11, H9-d10, H9-e9. These are
used for bearings where a significant clearance is
necessary.
2. Close running fit: H8-f7, H8-g6. This only allows a
small clearance, suitable for sliding spigot fits and
infrequently used journal bearings. This fit is not
suitable for continuously rotating bearings.
3. Sliding fit: H7-h6. Normally used as a locational fit in
which close-fitting items slide together. It incorporates
a very small clearance and can still be freely
assembled and disassembled.
4. Push fit: H7-k6. This is a transition fit, mid-way
between fits that have a guaranteed clearance and
those where there is metal interference. It is used
where accurate location is required, e.g. dowel and
bearing inner-race fixings.
5. Drive fit: H7-n6. This is a tighter grade of transition fit
than the H7–k6. It gives a tight assembly fit where the
hole and shaft may need to be pressed together.
6. Light press fit: H7-p6. This is used where a hole and
shaft need permanent, accurate assembly. The parts
need pressing together but the fit is not so tight that it
will overstress the hole bore.
7. Press fit: H7-s6. This is the tightest practical fit for
machine elements such as bearing bushes. Larger
interference fits are possible but are only suitable for
large heavy engineering components.