Tutorial on the Basics of Bolted Joints
Why Bolt Preload is Important
Over the last fifty years great improvements have been made
by the fastener industry in improving the design and reliability
of their products. However, no matter how well designed and
made the fastener itself is, it cannot alone make the joint
more reliable. Fastener selection based upon an understanding
of the mechanics of how a threaded fastener sustains loading
and the influence that tightening procedures can play is also
needed. This article provides an introduction to the basics
of bolted joints and the major factors involved in the design
of such joints.
It is not widely understood how
a bolted joint carries a direct load. A fully tightened bolt
can survive in an application that an untightened, or loose
bolt, would fail in a matter of seconds. When a load is applied
to a joint containing a tightened bolt it does not sustain
the full effect of the load but usually only a small part
of it. This seems, at first sight, to be somewhat contrary
to common sense. Figure 1A shows a bolt and nut securing a
bracket to a support plate. 
With the nut loose on the bolt, if
a weight of 1 unit is added to the bracket, as shown in figure
1B, then the force in the bolt shank will increase by 1. However,
if the nut is now tightened and the weight applied, the force
in the bolt shank will not increase by 1 but usually by only
a small fraction of this amount. An understanding of why the
bolt does not sustain the full effect of the applied load
is fundamental to the subject.
A model can often be of help in understanding
why the bolt does not sustain the full effect of the applied
load. Figure 2 is an attempt to illustrate the load transfer
mechanism involved in a bolted joint by the use of a special
fastener. In the case of this fastener no significant load
increase would be sustained by the fastener until the applied
load exceeded the fastener's preload. (Preload is the term
used for a bolt's clamp force.)

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