When an external tensile force is applied to the joint it
has the effect of reducing some of the clamp force caused
by the bolt's preload and applying an additional force to
the bolt itself. This is illustrated in the joint diagram
shown above. The external force acts through the joint material
and then subsequently into the bolt. At first sight it may
seem a bit strange to place the applied force in the position
shown in the diagram. However, it should be realised that
the load on the bolt cannot be added without decreasing the
clamp force acting on the joint. As can be observed from a
study of the diagram, the actual amount of increase in the
bolt force is dependent upon the relative stiffness of the
bolt to the joint.
As an illustration of the importance of the relative stiffness of the bolt to the joint, presented
above is a joint diagram for a 'hard' joint (a low stiffness bolt with a high stiffness joint).
In this case, because of the steep stiffness slope of the joint, the bolt will only sustain a small
proportion of the applied force.
With a 'soft' joint (a high stiffness bolt with a low stiffness joint), because
the stiffness slope of the bolt is greater than that of the
joint, the bolt would sustain the majority of the applied
force. Study of these diagrams provides understanding of why
high performance bolts have shanks that have been reduced
to a diameter below that of the outside diameter of a thread.
By reducing the shank diameter in this manner the stiffness
of the fastener is reduced so that it will not sustain as
much of any applied force that it would otherwise do. If the
shank diameter is not reduced to a diameter below that of
the stress diameter (see stress area
in the glossary) then the strength of the fastener will
not normally be impaired.
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