Page 467 - Shigley's Mechanical Engineering Design

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442 Mechanical Engineering Design
8
7
(85 kpsi up to a diameter of 1 in). Bowman recommends a preload of 75 percent of
proof load, which is about the same as the RB&W recommendations for reused bolts.
In view of these guidelines, it is recommended for both static and fatigue loading that
the following be used for preload:
for nonpermanent connections, reused fasteners
0.75F p
F i = (8–31)
0.90F p for permanent connections
where F p is the proof load, obtained from the equation

F p = A t S p (8–32)
Here S p is the proof strength obtained from Tables 8–9 to 8–11. For other materials,
an approximate value is S p = 0.85S y . Be very careful not to use a soft material in a
threaded fastener. For high-strength steel bolts used as structural steel connectors, if
advanced tightening methods are used, tighten to yield.
You can see that the RB&W recommendations on preload are in line with what
we have encountered in this chapter. The purposes of development were to give the
reader the perspective to appreciate Eqs. (8–31) and a methodology with which to
handle cases more specifically than the recommendations.

EXAMPLE 8–4 Figure 8–19 is a cross section of a grade 25 cast-iron pressure vessel. A total of N bolts
are to be used to resist a separating force of 36 kip.
(a) Determine k b , k m , and C.
(b) Find the number of bolts required for a load factor of 2 where the bolts may be
reused when the joint is taken apart.
(c) With the number of bolts obtained in part (b), determine the realized load factor
for overload, the yielding factor of safety, and the load factor for joint separation.

Solution (a) The grip is l = 1.50 in. From Table A–31, the nut thickness is 35 in. Adding two
64
2
threads beyond the nut of 11 in gives a bolt length of
35 2
L = + 1.50 + = 2.229 in
64 11
1
From Table A–17 the next fraction size bolt is L = 2 in. From Eq. (8–13), the thread
4
length is L T = 2(0.625) + 0.25 = 1.50 in. Thus, the length of the unthreaded portion
Figure 8–19 5 8 in-11 UNC × 2 in grade 5
1
4
finished hex head bolt
No. 25 CI
3 in
4
3 in
4

7 Russell, Burdsall &Ward Inc., Helpful Hints for Fastener Design and Application, Mentor, Ohio, 1965, p. 42.
8 Bowman Distribution–Barnes Group, Fastener Facts, Cleveland, 1985, p. 90.