SPRING 2022

158
THE DISTRIBUTOR’S LINK
ROB LaPOINTE FASTENER SCIENCE: HOW TIGHT IS RIGHT TIGHT? from page 98
FIGURE 5 TENSION INDICATING DEVICES INSTALLED IN BOLTS
KNOWN AS THE MAXBOLT BY VALLEY FORGE & BOLT.
It’s essentially a variation on Hooke’s law F = kx,
where F is the force of tension, k is the spring constant
and x is the amount of stretch. This relationship must
be developed and modeled mathematically to derive the
tension from the change in length for a particular fastener.
The changed length can be measured in a variety of
different ways including an application of a strain gauge
such as those developed by Valley Forge & Bolt (Figure
5), an application of sound ranging (Figure 7), or directly
measuring the length change of the bolt using calipers,
micrometers, indicators, or other suitable instrumentation
(Figure 6).
Determining the tension in a fastener by the amount
of torque applied to tighten it uses an established torquetension
relationship. Mathematically, the torque-tension
relationship is a direct relationship between torque
and tension and is similar to Hooke’s law above. The
simplified torque-tension relationship is F = / Kd, where
F is the force of tension, (the Greek letter tau) is torque
applied, K is a value representing the total resistance to
FIGURE 7 ULTRASONIC BOLT TENSION INSTRUMENT.
torque, and d is the nominal diameter of the fastener. This
method is the most used method for correctly tensioning a
bolt so I will develop this method most completely.
A correct application of the torque-tension relationship
is largely governed by correctly measuring the value K,
known as K-factor. The K-factor is a value that represents
the total resistance to torque which includes stretching the
fastener and the coefficient of total friction in a bolted joint.
One frictional component is found between the fastener’s
head or nut, if that’s what’s being turned to tighten, and
the material the head or nut is bearing against. Another
frictional component is between the external and internal
threads. Figure 8 diagrams the frictional components that
make up the K-factor. The K-factor is highly connected
to the bolting and joint materials as well as any surface
treatments done on the fasteners such as lubrication,
plating, coating, carburization or kolsterizing.
FIGURE 6 DETERMINING BOLT TENSION BY MEASURING LENGTH
CHANGE WITH AN INDICATOR.
FIGURE 8 CROSS-SECTION OF INTERNAL AND EXTERNAL BOLTED
JOINT SHOWING FRICTIONAL COMPONENTS IN RED.
CONTINUED ON PAGE 159