Bolt Tightening Torque and Preload Calculator

How is bolt tightening torque calculated?

The purpose of tightening is not torque but the tension it creates in the bolt. This tension is the preload, or clamp force, and it is what holds the parts together. The practical link between torque and preload is T = K × F × d, where d is the nominal diameter and K is the torque coefficient that lumps thread and under head friction into one number.

Preload is chosen as a share of the bolt's yield strength: F = p × R_e × A_s. The tensile stress area is the load carrying area of the threaded section, and for metric threads A_s = π / 4 × (d − 0.9382 × P)².

What does the property class mean?

For class 8.8 the first number gives the tensile strength: 8 × 100 = 800 MPa. The second is the ratio of yield to tensile strength: 800 × 0.8 = 640 MPa. The same rule gives a yield strength of 900 MPa for 10.9 and 1080 MPa for 12.9.

Example

An M10 × 1.5 bolt of class 8.8 has a tensile stress area of 58.0 mm². For a preload at 75 percent of yield, F = 0.75 × 640 × 58.0 = 27.8 kN. With K = 0.20 the tightening torque is 0.20 × 27 835 × 0.010 = 55.7 N·m.

Limits

Most of the torque goes into friction, so the result is very sensitive to K. Tightening a lubricated bolt to the torque meant for a dry one raises the preload dangerously. Critical joints use the manufacturer's torque, angle controlled tightening or direct preload measurement.