High-strength bolt tightening torque calculator: Fp,C, Mr and turning angle

Tightening torque calculator

Choose the diameter and property class, the tightening method and the set class by k. For K2 enter the k-factor from the set’s certificate; for the combined method also the grip thickness.

How to use it

• diameter and property class: M12–M36, classes 8.8 and 10.9, for which the standard gives the preload;
• torque method: K2 sets, two steps — 0.75 Mr, then 1.10 Mr;
• combined method: classes K2 or K1; the first step is 0.75 Mr (for K1 the value of the standard’s table), the second is a further turn of the nut by an angle that depends on the grip thickness;
• the k-factor: for K2 it is declared by the manufacturer of the set’s batch, it is not a fixed number in the standard; for K1 it is 0.125;
• class K0 (DTI, HRC): the torque is not calculated from k, the method and control are set by the standard for the set.

Examples

The examples are conditional: the k-factors are taken for illustration, in a project they come from the batch certificate.

How it is calculated

The preload Fp,C is taken from Table 18 of DSTU EN 1090-2:2019. The torque is Mr = k·d·Fp,C, where d is the thread diameter in millimetres, Fp,C is in kilonewtons and the result is in newton-metres. For the torque method the standard divides tightening into two steps, 0.75 Mr and 1.10 Mr, and the check is made with a torque of 1.05 Mr after 12–72 hours. The formulas, the k-classes and the mistakes are covered in the article Bolt tightening torque and preload.

In the combined method the first step is 0.75 Mr (for K1 the value of Table 20), the second is a further turn of 60°, 90° or 120° depending on the grip thickness: below 2 diameters, from 2 to 6 and from 6 to 10 diameters. The preload tables and the tightening methods are also in the article Bolted connections in steel structures.

What the calculator does not take into account

• the condition of the set: the calculation holds for a bolt as delivered, lubrication or galvanizing on site changes k;
• reuse of bolts and nuts, friction surfaces, coating of the contact faces;
• the set class without a certificate: without the declared k the torque cannot be found;
• wrench calibration and accuracy: the ±4 % tolerance is shown only as a range, the check is done by whoever tightens;
• DTI washers and HRC bolts, whose preload is controlled by the set itself.

What to send for agreement

We supply the fasteners for the joints together with the structures. To state the diameter, property class and tightening method in the KMD, send a drawing or a sketch of the joint, the set class and the project requirements. Erection and tightening on site are done by our partner construction company; the drawings are prepared by our department (KM and KMD design, fabrication).