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.)
With the special fastener shown, the bolt is free to move within its casing, a compression spring
is included within the casing so that if the bolt is pulled down the spring will compress. A scale on
the side of the casing indicates the force present in the spring and hence the force present in the
shank of the bolt. Figure 2A illustrates this special fastener in its untightened condition.
The bolt is now inserted through a hole in a support plate
and a bracket attached to the special fastener by securing
a nut to the threaded shank. If the nut is now rotated so
that the head of the bolt is pulled down, the spring will
be compressed. If the nut is rotated so that 2 force units
are indicated on the casing, the compressive force acting
on the spring will be 2 and the tensile force in the bolt
shank will also be 2. This is illustrated in figure 2b; this
is like a tightened bolt without any working load applied.
If a weight is now added to the bracket (figure 2c) of value
1, then the initial reaction is to think that the load in
the bolt must increase, otherwise what happens to the additional
force? Surprisingly it will keep at its existing value of
2 - it will not 'feel' any of the additional force. To visualise
why this is so - imagine what would happen if the load in
the bolt did increase. To do this it would compress the spring
more and a gap would be made between the bracket and the plate.
If such a gap was to form then it would mean that there would
be 2 units of force acting upwards - due to the spring, and
1 unit of force acting downwards from the applied weight.
Clearly this force imbalance would not occur. What does happen
is that the effect of the applied load is to decrease the
clamp force that exists between the plate and the bracket.
With no load applied the clamp force is 2 units, with the
load applied this decreases to 1 unit of force. The bolt would
not actually 'feel' any of the applied force until it exceeded
the bolts clamp force.
Older design procedures proposed calculation methods based upon the idea that the bolt will not
'feel' any of the applied load until it exceeds the bolts clamp force. That is, the bolt should
be sized so that its clamp force is equal to the external load after a factor of safety has been
included. With the special fastener used in this example the stiffness of the fastener is far smaller
than the stiffness of the plate and bracket it clamps. Practical fasteners differ from that shown in
figure 2 in that elongation of the fastener and compression of the clamped parts occurs upon tightening.
This compression results in the bolt sustaining a proportion of the applied load. As the applied force reduces
the clamp force existing within the joint an additional strain is felt by the bolt which increases the force
it sustains. The amount of the additional force the bolt sustains is smaller than the applied force to the joint.
The actual amount of force the bolt sustains depends upon the ratio of stiffnesses of the bolt to the joint
The best way to understand and visualise how the force sustained by the bolt depends upon the joint stiffness is
by the use of joint diagrams. These are the subject of the next page in this basics of bolted joints tutorial.
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