Bolt Preload from Tightening Torque
Bolt Preload from Tightening Torque
Most bolts are tightened to a torque, which produces tension through the nut factor — the basis of every torque-spec table.
The preload is F = T/(K·d). where T is the tightening torque, K the nut factor and d the bolt diameter.
Inputs
T := 90 N·m=90 Nm
K := 0.2=0.2
d := 12 mm=12 mm
Solve the torque–tension relation for preload.
F := T/(K·d)=37500 N
Results
The nut factor K (about 0.2 for dry steel) lumps thread and head friction; lubrication lowers it and raises the tension for the same torque, which is why specs state the condition.
Assumptions & limitations
- Nut factor from test or tables.
- Elastic tightening.
- Consistent lubrication condition.