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Screws, Fasteners, and the Design of Nonpermanent Joints 411
Figure 8–1 Major diameter
Pitch diameter
Terminology of screw threads. Minor diameter
Sharp vee threads shown for Pitch p
clarity; the crests and roots are
actually flattened or rounded
during the forming operation. 45° chamfer

Root
Crest Thread angle 2α

Figure 8–2 H
8 p
Basic profile for metric M 8 Internal threads
and MJ threads.
d major diameter
d r minor diameter p p 3H
8
d p pitch diameter H 5H 2 2
8
p pitch p H 60°
√ 3 4
H 2 p 4
H 60° d
4 30°
d p
p
External threads d r

threads. The MJ profile has a rounded fillet at the root of the external thread and a
larger minor diameter of both the internal and external threads. This profile is espe-
cially useful where high fatigue strength is required.
Tables 8–1 and 8–2 will be useful in specifying and designing threaded parts.
Note that the thread size is specified by giving the pitch p for metric sizes and by
giving the number of threads per inch N for the Unified sizes. The screw sizes in
1
Table 8–2 with diameter under in are numbered or gauge sizes. The second column
4
in Table 8–2 shows that a No. 8 screw has a nominal major diameter of 0.1640 in.
A great many tensile tests of threaded rods have shown that an unthreaded rod
having a diameter equal to the mean of the pitch diameter and minor diameter will have
the same tensile strength as the threaded rod. The area of this unthreaded rod is called
the tensile-stress area A t of the threaded rod; values of A t are listed in both tables.
Two major Unified thread series are in common use: UN and UNR. The differ-
ence between these is simply that a root radius must be used in the UNR series.
Because of reduced thread stress-concentration factors, UNR series threads have
improved fatigue strengths. Unified threads are specified by stating the nominal major
diameter, the number of threads per inch, and the thread series, for example, 5 8 in-18
UNRF or 0.625 in-18 UNRF.
Metric threads are specified by writing the diameter and pitch in millimeters, in
that order. Thus, M12 × 1.75 is a thread having a nominal major diameter of 12 mm
and a pitch of 1.75 mm. Note that the letter M, which precedes the diameter, is the
clue to the metric designation.