SPRING 2022

THE DISTRIBUTOR’S LINK 159
ROB LaPOINTE FASTENER SCIENCE: HOW TIGHT IS RIGHT TIGHT? from page 158
Once the K-factor is known, the torque needed to
acquire the desired tension in the bolted joint can be
calculated. For example, if we have a ½-inch diameter,
grade 8 bolt and nut combination with a determined
K-factor of 0.12, we can use the torque-tension equation
above to calculate the torque needed to achieve this
tension. The ultimate tensile value for a 1/2-13, grade
8 fastener is 21,300 lbf minimum. Using 70% of that
value for our desired tension we get 14,910 lbf.
Using the equation solved for torque,
= KFd,
and substituting our values for the 1/2 inch grade 8,
we get...
= (0.12)(14,910 lbf)(0.5 in) = 984.6 lbf in
The result is given in pound inch units but can be
converted into pound foot units by dividing by 12. This
gives us 74.6 lbf ft of torque needed to tension this
fastener to 70 % of ultimate tensile. I have checked this
relationship many times by placing the bolt in a direct
tension device (Figure 2) and it is very accurate.
No matter how you determine right-tight for your
fastener application, you must be confident in the tools
and methods used to get the correct tension. The
quality of your bolted joint depends on correct tensioning
whether by feel, instrumentation or calculation.
For questions or additional information, contact me
at robl@aimtestlab.com, visit www.aimtestlab.com or call
(619) 396-2046.
ROB LaPOINTE / AIM TESTING LABORATORY