SPRING 2022

98
THE DISTRIBUTOR’S LINK
ROB LaPOINTE FASTENER SCIENCE: HOW TIGHT IS RIGHT TIGHT? from page 10
FIGURE 2 SKIDMORE WILHELM BOLT TENSION CALIBRATOR.
Typically, the correct tension for fasteners is
somewhere between 50 – 80 % of the fastener’s ultimate
tensile strength. This amount of tension stretches the
fastener enough so that it holds the joint securely
and can accommodate additional loads added by the
application without overloading the fastener. For example,
applications may add cyclical loading or vibration to the
fastener, both adding to and subtracting from the load
applied by tightening the fastener. The fastener must
have enough springiness to accommodate a lower load
without loosening and a higher load without breaking. A
fastener’s springiness is provided to it by tensioning it.
Fortunately, there are many ways of quantitively measuring
the tension in a fastener. These include direct tension
measurement devices, tension indicating devices, fastener
length measurement while under tension, and using an
established torque-tension relationship. I’ll briefly expand
on each of these below.
Instruments that directly measure tension in a fastener,
such as in a Skidmore Bolt Tension Calibrator (Figure 2) or
similar device are the most direct way of knowing tension
in a fastener. However, most of the time, these types
of tension measuring instruments will not measure the
tension of the fastener when it’s tightened in an application,
but only when tightened in the instrument. This doesn’t do
us much good if we need to measure a fastener’s tension
while installed. There
are small in-line load
measuring instruments
that could be placed
in an installation, but
this is not typical due
to the cost of such
instruments and the
need for them to be
designed into the
application.
There are other devices available such as direct
tension indicating (DTI) washers (Figure 3 and 4) that are
calibrated to show a bolt has a specific tension when the
washer is compressed a specific distance. This is not a
direct measurement as alluded to by the name, but an
indirect indication of tension. There are other analogous
devices such as squirting washers where cells filled with
colored paint pop at a specified tension and indicate the
tension in the joint. One downside of this method is that
although they provide evidence that the joint was, at one
time, tensioned correctly, they do not indicate if the joint
had loosened since being correctly tensioned.
The tension in a fastener can also be known indirectly
by measuring the length of the fastener while under
tension and comparing that to its length while not under
tension. Using this method, we’re measuring the amount
that the fastener stretches when placed in tension.
There is an established connection between the tension
a fastener exerts and the amount of stretch that causes
that tension.
FIGURE 3 DIRECT TENSION
INDICATING (DTI) WASHER
FIGURE 4 MEASURING THE DISTANCE BETWEEN A DTI WASHER AND
A HARDENED WASHER TO DETERMINE BOLT TENSION.
CONTINUED ON PAGE 158