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Most of the machines and products have various joints such as welded joints, riveted
joints, bolted joints and adhesive joints for effective working, performance and
balancing. And bolted joint is one of the joint structures widely used because it is
easy to install and remove, produces high fastening power with small force applied.
Furthermore, the simplicity of its mechanism of developing and maintaining the
desired clamp force has made it very popular, and it has become one of the most
accepted engineering products. External loads acting on the bolted joint are of two
types. First, if the line of action of the forces on the joint is more or less parallel to
the axis of the bolt, the joint is known to be loaded in tension and is called tensile
joint. Second, if the line of action of the load is more or less perpendicular to the
axes of the bolt, the joint is called shear joint. Bolted joint is a combination of threaded
fasteners bolt and nut. The two partners are kept together by a combination of the friction of their threads, a slight stretch of the bolt, and compression of the parts.
High tensile nuts and bolts find wide application in joining part where continuous
rotation and wear and tear of the nuts and bolts occur, as the nuts and bolts are
subjected to variable stress in static and dynamic condition.
The cause of self-loosening of bolted joint is explained by the well-known law of
physics related to the effect of friction on two intersecting solid bodies (Junker,
1969). The maximum stress in the bolt always occurred within one pitch of the load
bearing face of the nut. Truncating threads increased the maximum bolt stresses.
Tapering the whole thread form reduced the maximum stress and produced a more
uniform load distribution in the bolt (Kenny and Patterson, 1985). A cantilever
configuration was used to analyze and test the loosening of threaded fasteners under
dynamic loading. This configuration introduced shear loadings on the bolt due to the
bending induced by the beam inertial forces (Kerley and James, 1987). The reliability
of bolted joint testing rig depends on several factors. These factors include the design,
materials and manufacturing methods as well as accurate analysis of the strength of
the structure. For efficient design of structure, knowledge of dynamic and static
response of forced vibration characteristics is essential (He and Oyadiji, 2001). The
tensile load is applied only to prevent the nut from loosening. When the shear load
on the joint changes, the corresponding stress field in the bolt also changes. Under
dynamic loading, this can lead to possible fatigue failure of the bolts (Fernando,
2001). The failure of threaded fasteners by vibration induced loosening caused due
to dynamic shear loads. Such failures can be avoided by proper joint design using
guidelines based on the understanding of loosening caused by dynamic loads (Pai
and Hess, 2002). The various 3/8 BSW locking screw fasteners with nylock nut, flat
washer, nylon washer, serrated washer and spring washer are tested under accelerated
vibrating conditions obtained in an indigenously made test rig (Mahato and Das,
2009). The superposition of forces implies that the original, i.e., assembly prestressing,
may not be preserved, but by improper installation and adverse operating conditions,
it gradually decreases, in extreme case right to zero. It is proven that the most frequent
spontaneous release of bolted joints occurs after the first loaded cycles (Dominick
and Zmindak, 2012).
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