How it’s calculated
The short-form torque equation relates the applied torque to the axial preload it produces. The nut factor K lumps together thread friction, under-head friction and thread geometry, so it is an empirical value, not a material constant. Assumes standard threads, a rigid joint and torque applied to the nut or head at a steady rate.
Example (inch units): a ½ in bolt, target preload 10,000 lbf, K = 0.20: T = 0.20 × 0.5 in × 10,000 lbf = 1,000 lbf·in = 83.3 lbf·ft. With K = 0.15 the same preload needs only 62.5 lbf·ft, which is why lubricating a fastener tightened to a “dry” torque can overload it.
NASA notes that K = 0.2 is commonly assumed but “should not be used blindly” and that 0.15 is a more realistic typical value for steel on steel. Torque-controlled tightening typically scatters preload by ±25% or more; critical joints should use the manufacturer’s torque specification, a measured K, or direct tension measurement.
Frequently asked questions
Should I reduce torque for lubricated bolts?
Yes. Lubrication lowers K, so the same torque produces more preload. NASA RP-1228 Table VI shows K falling from 0.250 at a friction coefficient of 0.20 to 0.133 at 0.10.
What preload should I target?
It is set by the joint design, commonly a fraction of the bolt’s proof load (proof load itself is usually about 75% of theoretical yield, per NASA RP-1228). Never exceed the bolt’s yield.
Does this work for metric bolts?
Yes. Enter the diameter in mm and preload in kN or N; the result is in N·m.
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Sources
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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