Bolt torque & preload
Bolt torque to clamp load with the T = K F d relation, tensile stress area, stress as a percentage of proof load, and a suggested torque for a target preload.
Bolt torque & preload
What clamp load a torque gives you, how hard the bolt is working, and the torque to hit a target preload. Covers the usual inch and metric grades.
Short-form torque relation: T = K · F · d, so F = T / (K · d). K is the nut factor, which bundles thread and bearing friction. 0.20 dry is the usual assumption.
Tensile stress area: A_t = 0.7854 · (d − 0.9743/n)². Bolt stress: σ = F / A_t, shown as a fraction of proof strength.
Proof load: F_p = A_t · S_proof. Torque for a target preload: T = K · (target% · F_p) · d. 75% of proof is the common target for reusable joints, 90% for permanent ones.
K varies by ±25% or more between lots, platings and lubricants, so torque is a loose way to control preload. If the joint matters, measure stretch or use a torque-angle method. Stainless values are typical for 18-8; the real yield depends on how cold worked the bolt is.
Related tools
More in Fasteners & fits: Tap drill & clearance hole, Thread engagement check, Thread insert drill chart, Press fit / interference
Engineering & Manufacturing
Need the part, fixture or drawing done for you? That is the day job.