Bolt Preload from Torque (T = KDF)

A bolt is not a hook, it is a clamp: tightening stretches the shank into a very stiff spring whose tension squeezes the joint together, and that preload is what actually carries the load and stops the joint from working loose. The nut factor K rolls thread friction, under-head friction and thread lead into one empirical number — about 0.20 for plain steel as-received, 0.15 lubricated, 0.10 or lower with anti-seize or wax. An M12 bolt tightened to 72 N·m with K = 0.20 gets F = 72 ÷ (0.20 × 0.012) = 30 000 N of preload. In field units, 75 ft·lbf on a 1/2-inch bolt at K = 0.20 gives 900 in·lbf ÷ (0.2 × 0.5) = 9000 lbf.

Here is the sobering part: only about 10 to 15% of the torque you apply ends up as bolt stretch. The rest is spent overcoming friction under the nut face and in the threads, which is why K is not a constant of nature — it drifts with plating, lubricant, surface finish, reuse and even how fast you pull the wrench. A torque wrench delivers preload accurate to roughly ±25% at best, and anti-seize on a spec written for dry threads can overload a bolt to failure. When it truly matters, control the stretch instead: turn-of-nut, load-indicating washers, ultrasonic bolt-length measurement, or hydraulic tensioners on flange bolting.