Page 448 - Shigley's Mechanical Engineering Design

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Screws, Fasteners, and the Design of Nonpermanent Joints 423
Figure 8–9 H W
Approx. 1 in
64
Hexagon-head bolt; note the
washer face, the fillet under the
head, the start of threads, and
the chamfer on both ends. Bolt
lengths are always measured R
from below the head.
30°

Figure 8–10
Typical cap-screw heads:
(a) fillister head; (b) flat head;
(c) hexagonal socket head. Cap
screws are also manufactured A A A
with hexagonal heads similar to
the one shown in Fig. 8–9, as 80 to 82°
well as a variety of other head H
styles. This illustration uses
one of the conventional H H
methods of representing D
D D
threads.

L L
L
l
l l

(a) (b) (c)

has elongated almost to the elastic limit. If the nut does not loosen, this bolt tension
remains as the preload or clamping force. When tightening, the mechanic should, if
possible, hold the bolt head stationary and twist the nut; in this way the bolt shank
will not feel the thread-friction torque.
The head of a hexagon-head cap screw is slightly thinner than that of a hexagon-
head bolt. Dimensions of hexagon-head cap screws are listed in Table A–30.
Hexagon-head cap screws are used in the same applications as bolts and also in appli-
cations in which one of the clamped members is threaded. Three other common cap-
screw head styles are shown in Fig. 8–10.
A variety of machine-screw head styles are shown in Fig. 8–11. Inch-series
machine screws are generally available in sizes from No. 0 to about 3 8 in.
Several styles of hexagonal nuts are illustrated in Fig. 8–12; their dimensions are
given in Table A–31. The material of the nut must be selected carefully to match that
of the bolt. During tightening, the first thread of the nut tends to take the entire load;
but yielding occurs, with some strengthening due to the cold work that takes place,
and the load is eventually divided over about three nut threads. For this reason you
should never reuse nuts; in fact, it can be dangerous to do so.