When applied torque and the resulting tension (preload) in
the bolt are measured during tightening and plotted on a graph,
there is a linear relationship between the torque and the
tension. The bolt tension is directly dependent, and proportional
to, the applied torque. This is illustrated by the graph,
which is based upon experimental results, that is shown in
the diagram above. From such test results it is possible to
establish the appropriate torque for a required bolt preload
that may be required.
One of the disadvantages of using torque control is that
there can be a significant variation in the bolt preload achieved
for a given torque value. There are several reasons for this
e.g. inaccuracy in applying the torque, dimensional variations
of the thread and hole size variation amoungst others. However,
the dominant factor is usually due to the frictional variation
that is present between the contact surfaces that are being
From tests, it is known that approximately 50% of the tightening
torque is dissipated in overcoming friction under the bolt
head or the nut face (whichever is the face that is rotated).
Typically only 10% to 15% of the overall torque is actually
used to tighten the bolt, the rest is used to overcome friction
in the threads and on the contact face that is being rotated
(nut face or bolt head). This is illustrated in the piechart
shown above. Relatively small changes in the nut face friction
can have a significant effect on the bolt preload. As more
torque is perhaps needed to overcome friction, less remains
for the bolt extension and hence as the effect of adversely
reducing the preload. If the friction under the nut face is
reduced, then, for a given torque, the bolt preload will be
diagram shown at the side is perhaps the most common situation
where the top and bottom plates of the joint are made from
the same material, have the same finish and the hole size
is the same through both of the plates. For such a joint,
when the nut face and bolt head sizes have the same diameter
and finish, it will not matter whether the bolt head or the
nut is tightened. Some people believe that by tightening the
bolt head rather than the nut it will affect the torsion in
bolt shank. The torsion in the shank of the bolt depends upon
the thread friction torque. For a given finish condition,
the thread friction has some scatter associated with it, but
will not depend on whether the nut or the bolt head is tightened.
If the thread friction torque remains the same, the torsion
in the shank will be the same irrespective of whether the
bolt head or the nut is tightened.
The diagram at the side shows the situation when the plates
comprising the joint are different materials (such as one
being steel and the other aluminium) or have different finishes
(such as one plate being galvanised and the other painted).
In such situations, it will, in general, be important as to
whether the bolt head or nut is tightened. The reason is that
each face will have a different friction coefficient. If the
tightened torque was determined either by testing or by looking
up the friction characteristics of the surface, say based
upon the nut face, then it is probable that the head face
would have a different friction coefficient. If it had a lower
friction value then the preload would be increased if the
bolt head was tightened. In the extreme case, if the frictional
differences were large, bolt breakage could occur.
The diagram at the side illustrates the case when the clearance
hole in the top plate differs from that used in the bottom
plate. Such situations are relatively common. There is an
effective friction radius on the part that is rotated (nut
or bolt head) that is usually taken as the mean of the clearance
hole and outer bearing face radii. Because this radii would
be greater for the bolt head than the nut in the situation
shown, less bolt preload would result by tightening the bolt
head rather than the nut, other factors such as friction being
the same. Hence another example of a situation as to whether
the nut or bolt head is tightened.
The drawing at the side the case when there are style and
dimensional differences between the bolt head and the nut.
The effect is similar to that which happens in the previous
case. Differences in the friction radii between the bolt head
and the nut-washer interface result in the preload being affected
by which item is tightened. In the case shown here, there
would probably also be differences between the friction coefficient
that is present when the nut is tightened on the washer and
the bolt head onto the joint. This would increase the variability
Washers are occasionally used as a means of minimising frictional
scatter besides the common reason of reducing the bearing
stress on the joint face. The friction condition between the
washer and nut face can be reasonably well defined and controlled,
more so than the joint surfaces usually can. By controlling
the friction, the preload can be more reliably achieved. To
do this consistently, a close fit is needed on the inside
diameter of the washer. One way in which this can be achieved
is by the use of a SEMS unit (in which a washer is held captive
on the bolt shank). The same can be achieved by using a KEPS
unit (a washer being held captive on a nut).
So in general, when using torque control, tightening the
bolt by rotating the bolt head or the nut can matter. It is
good practice to specify which part should be tightened so
that the bolt preload variation is minimised.
To assist the Engineer in overcoming the problems associated
with the use of threaded fasteners and bolted joints, Bolt
Science has developed a number of computer programs.
These programs are designed to be easy to use so that an engineer
without detailed knowledge in this field can solve problems
related to this subject.