An experimentally measured coefficient in the short-form torque equation, where torque equals K times nominal diameter times desired preload. It is not a friction coefficient but bundles thread friction, bearing-face friction and thread geometry into one number. Typical values run near 0.20 for dry plain steel and roughly 0.10 to 0.15 with moly paste or anti-seize, so a half-inch bolt targeted at 12,000 lb needs about 100 lbf-ft dry and much less lubricated.
Calc used a nut factor of point two, but these are zinc flake with a lubricated topcoat, so measure it on the tension tester first.
Never use a dry torque spec on a lubricated bolt; the same number on the wrench can take it past yield.
Also heard as
- K factor
- K
- torque coefficient
Related terms
Torque-tension
The relationship between the torque applied to a fastener and the preload it produces, dominated by friction.
Preload
The tension created in a bolt by tightening it, which clamps the joint and keeps it from slipping or opening.
Anti-seize
A grease loaded with metal or ceramic particles that stops threads from galling, rusting or bonding under heat.
Zinc flake
A baked-on coating of zinc and aluminum flakes in a binder, giving high corrosion resistance without plating hydrogen.