Question 1

How is bolt tightening torque calculated?

Accepted Answer

The common short-form equation is T = K · F · d, where T is torque, K is the nut factor (torque coefficient), F is the bolt preload (clamp force) and d is the nominal bolt diameter. For example an M10 bolt with K = 0.2, d = 0.010 m and a preload of 20 kN needs T = 0.2 × 20000 × 0.010 = 40 N·m.

Question 2

What is the nut factor K?

Accepted Answer

K (also called the torque coefficient) lumps together thread and under-head friction. Typical values are about 0.20 for plain/as-received steel, 0.18 for zinc plating, 0.12–0.16 for lightly lubricated threads and 0.10 or less for wax, MoS2 or PTFE coatings. Because friction dominates, K is the largest source of uncertainty in torque control.

Question 3

What preload should I aim for?

Accepted Answer

A common target is 65–75% of the bolt's proof load for reusable joints, where proof load = tensile stress area × proof strength. Some permanent joints go up to about 90% of proof. Higher preload resists fatigue and loosening but leaves less margin against overload and yield.

Question 4

What is the tensile stress area?

Accepted Answer

The tensile stress area As is the effective area that carries the axial load in a threaded fastener, based on the average of the pitch and minor diameters: As = (π/4) × (d − 0.9382·P)² for ISO metric threads, where P is the pitch. Bolt stress is preload divided by As.

Question 5

Why does the actual clamp force vary so much?

Accepted Answer

Torque mostly fights friction — typically only about 10–15% of the input torque becomes preload. Small changes in surface finish, lubrication, plating and tightening speed swing K and therefore the achieved preload by ±25% or more. For critical joints use angle control, tension indicators or direct preload measurement.

Bolt Torque& Preload Calculator

Bolt Torque
& Preload Calculator