Tightening torque is the rotational force applied to a nut (or bolt if there is no access to the nut) during assembly to generate an axial prestressing force on the bolt. This force causes the joined parts to be compressed together, and in prestressed joints it is essential for the connection to work by friction and not by direct shear.

How it is calculated?

The torque required to achieve a given prestressing force can be estimated by the following formula:

 M=K∙Fₚ∙d 

Where:

• M = tightening torque [Nm]

• d = nominal bolt diameter [mm]

• Fₚ = required prestressing force [kN]

This preload force is tabulated according to the diameter and class of the material.

• K = coefficient of friction or factor relating the tightening torque to the prestressing force (unitless)

There are two types of k-value:

·         K₁: Normative and fixed value, defined in EN 1090-2. K₁ = 0.125.

·         K₂: Value determined by testing the performance of the complete system (bolt + nut + washer) according to the procedure   established in standard EN 14399-2.

With these data we can calculate the torque reference values where if K₁ is used it will be and if K₂ is used it will be.

·       Mr,1=K₁∙Fₚ∙d

·       Mr,2=K₂∙Fₚ∙d

There are different methods to ensure that the minimum prestressing force is achieved, depending on the method chosen the value per standard K₁ can be used or it is required to calculate by testing the K₂ value.

Combined method

This method is suitable for K₁ and K₂ sets.

1st Step

All joint assemblies shall be tightened with a torque wrench suitable for the application to a torque corresponding to 75% of the torque required to achieve the tabulated preload force (0.75Mr,1). The tightening torque should be applied continuously and smoothly.

K₁: In case of having a set K₁, the friction coefficient is constant. This value 0.75Mr,1 is tabulated as a function of the diameter and class of the material.

K₂: If we have an assembly for which the K₂ coefficient has been obtained by testing, the tightening torque will be 0.75Mr,2. It should be calculated for each assembly according to its K₂ value.

2nd Step

All joint assemblies shall be retightened by applying a partial turn according to the ratio between the bolt diameter and the thickness of the elements to be joined (washers included). This ratio is indicated in the following table. The position of the nut with respect to the thread threads after the 1st stage must be marked to be able to control the turn in the second stage.

Torque method

This method is only suitable for K₂ sets.

1st Step

Tighten all joint assemblies with a torque wrench suitable for the application to a torque corresponding to 75% of the torque required to achieve the tabulated preload force (0.75Mr,i where Mr,i = Mr,2).

2nd Step

Re-tighten all joint assemblies with a torque wrench set to a torque corresponding to 110% of the torque required to achieve the tabulated preload force (1.10Mr,i where Mr,i = Mr,2).