The link between tightening torque and resulting bolt tension, usually approximated by T = K x D x F. Roughly 85 to 90 percent of the torque goes into friction, about half under the nut or head face and most of the rest in the threads, leaving only 10 to 15 percent to stretch the bolt. Small changes in lubrication, plating or surface finish therefore cause large swings in preload at the same torque, which is why critical joints get a torque-tension study.
Same torque, new plating supplier, preload dropped by a third, so we're rerunning the torque-tension study on the new lot.
A torque wrench measures friction as much as tension; when preload really matters, measure stretch or use tension-indicating washers.
Also heard as
- torque-tension relationship
- T = KDF
Related terms
Nut factor
The empirical constant K in T = KDF that lumps all the friction and geometry between torque and tension.
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.
Click wrench
Shop name for an adjustable torque wrench that clicks and briefly gives way when the set torque is reached.