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410 Mechanical Engineering Design
The helical-thread screw was undoubtably an extremely important mechanical inven-
tion. It is the basis of power screws, which change angular motion to linear motion
to transmit power or to develop large forces (presses, jacks, etc.), and threaded fas-
teners, an important element in nonpermanent joints.
This book presupposes a knowledge of the elementary methods of fastening. Typ-
ical methods of fastening or joining parts use such devices as bolts, nuts, cap screws,
setscrews, rivets, spring retainers, locking devices, pins, keys, welds, and adhesives.
Studies in engineering graphics and in metal processes often include instruction on var-
ious joining methods, and the curiosity of any person interested in mechanical engi-
neering naturally results in the acquisition of a good background knowledge of fasten-
ing methods. Contrary to first impressions, the subject is one of the most interesting in
the entire field of mechanical design.
One of the key targets of current design for manufacture is to reduce the number
of fasteners. However, there will always be a need for fasteners to facilitate disas-
sembly for whatever purposes. For example, jumbo jets such as Boeing’s 747 require
as many as 2.5 million fasteners, some of which cost several dollars apiece. To keep
costs down, aircraft manufacturers, and their subcontractors, constantly review new
fastener designs, installation techniques, and tooling.
The number of innovations in the fastener field over any period you might care
to mention has been tremendous. An overwhelming variety of fasteners are available
for the designer’s selection. Serious designers generally keep specific notebooks on
fasteners alone. Methods of joining parts are extremely important in the engineering
of a quality design, and it is necessary to have a thorough understanding of the per-
formance of fasteners and joints under all conditions of use and design.

8–1 Thread Standards and Definitions

The terminology of screw threads, illustrated in Fig. 8–1, is explained as follows:
The pitch is the distance between adjacent thread forms measured parallel to
the thread axis. The pitch in U.S. units is the reciprocal of the number of thread forms
per inch N.
The major diameter d is the largest diameter of a screw thread.
The minor (or root) diameter d r is the smallest diameter of a screw thread.
The pitch diameter d p is a theoretical diameter between the major and minor
diameters.
The lead l, not shown, is the distance the nut moves parallel to the screw axis when
the nut is given one turn. For a single thread, as in Fig. 8–1, the lead is the same as
the pitch.
A multiple-threaded product is one having two or more threads cut beside each
other (imagine two or more strings wound side by side around a pencil). Standard-
ized products such as screws, bolts, and nuts all have single threads; a double-threaded
screw has a lead equal to twice the pitch, a triple-threaded screw has a lead equal to
3 times the pitch, and so on.
All threads are made according to the right-hand rule unless otherwise noted.
That is, if the bolt is turned clockwise, the bolt advances toward the nut.
The American National (Unified) thread standard has been approved in this coun-
try and in Great Britain for use on all standard threaded products. The thread angle
is 60 and the crests of the thread may be either flat or rounded.
◦
Figure 8–2 shows the thread geometry of the metric M and MJ profiles. The M
profile replaces the inch class and is the basic ISO 68 profile with 60 symmetric
◦