Bolt Tension Control with a Direct Tension Indicator

ACHIEVING THE MINIMUM required bolt tension in a high
strength bolted friction-type joint is a primary factor, since
the slip resistance of the joint is dependent on the bolt
tension. Current specifications
require that high strength
structural bolts be tightened to at least 70 percent of the
minimum required tensile strength.
For the past two decades installation has been primarily
controlled by either turn-of-nut or calibrated wrench
tightening. The turn-of-nut method depends upon strain
control as contrasted to the torque control of the calibrated
wrench method.
Since the turn-of-nut method is primarily strain control,
the effectiveness of the method depends on the starting
point and accuracy of the rotational measurements.
The variability of torque control is well known and was one of
the reasons for the development of. the turn-of-nut method.
Bolts tightened by the turn-of-nut method may have the
outer face of the nut match-marked with the protruding bolt
point before final tightening, so that an inspector can
visually note the nut rotation. If an impact wrench is used
for tightening, a slight peening of the bolt heads or nuts
gives an additional indication that the bolt has been
tightened. However, if the need for more inspection of bolt
tension is necessary, one must use a calibrated inspection
torque wrench with all its uncertainties.
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