Page 83 - Analysis and Design of Machine Elements

Detachable Joints and Fastening Methods
transportation, maintenance and so on. Normally, permanent joints are less expensive 61
than detachable joints.
Fastening methods use such devices as bolts, screws, nuts, keys, pins, rivets, welds
and adhesives for various applications. Typical application scenarios of the individual
devices are another consideration. Usually, threaded fasteners, welds, rivets, adhesive
bonds are used to connect plates; threaded fasteners and welds can join rods; keys
and splines can connect shafts with hubs; pins, setscrews are chosen for retaining
shaft-mounted components; interference fit for mounting rolling contact bearings;
couplings and clutches link shafts together [1–4]. In general, the use of threaded
fasteners remains the basic and the most widely used fastening method in the design
and construction of a machine.
Other considerations include size, thickness, geometries and weight of the compo-
nents to be joined. And the most important factor is the loading conditions, which
greatly influence the strength of joints.

3.2 Screw Threads

3.2.1 Types of Screw Threads

Screw threads are helical ridges formed by cutting or cold forming a groove onto the
surface of a cylindrical bar, producing a screw, bolt and stud; or internally in a cylindrical
hole, fabricating a nut [5]. Different types of screw threads are classified according to
various criteria, such as function, profile, pitch, thread position, directions and starts.
Figure 3.1a is basically the same for both Unified (inch series) and ISO (metric)
threads [3], used on screws for fastening. Square, Acme and buttress threads shown
from Figure 3.1b–d are power screws. Among them, the square thread has the greatest
∘
strength and efficiency, yet is more difficult to fabricate because of the 0 thread
angle. Acme and buttress profiles have small thread angles making them easier to
manufacture. Acme profile is selected to carry bidirectional loads, while buttress is for
unidirectional loading.
For most standard screw threads, at least two pitches are available; that is, coarse series
and fine series. Coarse threads are recommended for ordinary applications, especially
where rapid assembly or disassembly is required. Fine threads have better capability
of resisting loosening from vibrations because of their smaller lead angle. They have a
smaller thread depth and larger root diameter that provide higher static tensile strength.
Fine threads are used in automobiles, aircraft and other applications that are subject
to vibration. Extra-fine threads may be used for more precise adjustments or thin-wall
tubing applications.
According to the thread position, external threads are the threads on a screw, while
internal threads are the threads on a nut or threaded hole. Threaded fasteners work by
assembling the matching external and internal threads together.
There are also left-handed and right-handed threads, which are based on the direction
of helix line. Threads are usually made right-handed unless otherwise indicated. That is,
if the bolt is turned clockwise, the bolt advances towards the nut.
Screws can be multiple-threaded by having two or more threads cut beside each other.
Multiple threads have the advantages of smaller thread height and increased lead for