Tightening Threaded Fasteners
- by Ian Carsten
derby cars, we want to know the maximum safe torque we can use on our screws
so we don’t tighten them beyond a safe limit. The most critical item is
kingpin tension. If a kingpin breaks while the car is racing, at the very
least, you’d lose the race. It could also cause a crash and possible injury.
reasons unknown to us, the makers of several popular torque wrenches include
a table of maximum torque values that significantly exceeds the maximum
values given by engineering manuals and screw manufacturers. It seems
sensible that the companies that engineer and manufacture threaded fasteners
are more likely to publish correct information than the companies building
know that applying a turning force, or torque, to a screw or nut, generates
a lengthwise pulling force called tension. The tension clamps two or more
components together and is also called clamping force. We can use a handbook
to find the screw manufacturer’s maximum safe torque and the resulting
tension for a given fastener. Also, the tension resulting from a given
torque or the required torque for a given tension can be easily calculated
from simple formulas.
often-overlooked factor is whether the fastener is dry or lubricated. It
makes a big difference. If you lubricate a fastener and apply the maximum
safe dry torque, the resulting tension can be dangerously high. An excellent
torque-tension calculator is available at http://www.fastenal.com/resources/online_calculators/torque/torque.asp.
It calculates the maximum safe torque for either dry or lubricated
fasteners. You can also examine some good related articles by going to
www.google.com and typing “fastener torque” in the subject line.
Consider the 1/4-inch diameter, 28 thread-per-inch, grade 8 screws used as
derby kingpins. Engineering handbooks and manufacturer’s data list a maximum
safe tension of 3,274 pounds. That is generated in a dry fastener at 164
inch-pounds of torque or only 123 inch-pounds in a lubricated one. Compare
that to the 185 inch-pounds of torque listed in the tables packaged with
many inch-pound torque wrenches. Many derby racers recommend this much
torque, but we wonder about the safety of such setups.
formula for the required torque is: t = kdf, where t is the torque in
inch-pounds, k is the coefficient of friction (k = .20 dry and k = .15
lubricated), d is the nominal diameter of the fastener in inches, and f is
the clamping force in pounds and is determined by the manufacturer.
example, for our 1/4-inch diameter, 28tpi, grade 8 kingpin used dry,
k = .20, d = .25 inch, f = 3,274 pounds. Then t = (.20)(.25 inch)(3,274
pounds) = 163.7 inch-pounds, which rounds off to 164 inch-pounds. Also, we
can solve our formula for tension if the input torque is known. Then f = t/(kd).
incorrectly used the dry torque value on a lubricated kingpin, then t = 164
inch-pounds, k = .15, d = .25 inch. Consequently, f = (164
inch-pound)/[(.15)(.25 inch)] = 4,373.3 pounds. That is about 34% greater
than the manufacturer’s maximum safe tension and is equivalent to using
218.7 inch-pounds of torque on a dry fastener. If it didn’t break while
tightening, it could break unexpectedly while racing. The key idea is:
always apply dry values to dry fasteners and lubricated values to lubricated
ones. Naturally these ideas apply to all threaded fasteners.
difference in tension in lubricated fasteners compared to dry ones can
create trouble when using a liquid thread-locking compound, such as Loctite©.
Until it hardens, thread-locking compound is slippery and acts as a
lubricant. Therefore, a fastener with freshly applied thread locking
compound should be tightened as though it were lubricated. Otherwise it may
be inadvertently over tensioned. If you choose to use lubricant on your
fasteners, you should be sure to apply the lubricant to the underside of the
screw head and the underside of the nut as well as the threads. The idea is
to lubricate all metal-to-metal surfaces that slide over each other during
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