Detroit Metro Soap Box Derby

DMSBD Tech Tips
Tightening Threaded Fasteners - by Ian Carsten

For 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.

For 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 torque wrenches.

We 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.

An 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.

The 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.

For 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).

If we 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.

The 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 tightening.

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