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.
Thread Standards and Deﬁnitions
The terminology of screw threads, illustrated in Fig. 8–1, is explained as follows:
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