Causes of non-linearity in the tightening torque/tension relationship

Torque tightening: an imprecise method that is still very widespread

When tightening a screw, torque is applied, stretching the bolt and creating preload that generates a tightening force essential for the integrity of the assembly.

Manufacturers often use applied torque to evaluate and measure tightening quality, assuming a linear relationship between torque and tension (or preload).

So doubling the applied torque should theoretically double the preload, assuming the material and threads remain intact. This principle underlies tightening torque calculations. Experience and reality shows us that this is completely false!

Decorrelation between torque and preload

At TRAXX, thousands of tests show that the torque/preload relationship is often not linear but curvilinear. No linear relationship can be established between these two quantities. We can observe variations of plus or minus 30% between the torque applied and the tension obtained!

Torque tightening is therefore fundamentally inaccurate because much of the torque is dissipated by friction and other factors, making the method obsolete. Only a fraction of the applied torque translates into effective preload.

Torque tightening limits

1. Sensitivity to variations in friction : Difficulty predicting final tightening due to variations in friction (under the head of the screw, in the thread, etc.)
   
2. Difficulty achieving very tight tolerances : Torque tightening hardly achieves high precision.
3. Influence of lubrication: Applying lubricants to threads can alter friction levels, thereby affecting tightening accuracy.
4. Overvoltage detection : Torque is not very effective in detecting overvoltages.
5. Dependence on manufacturer specifications: Tightening to torque requires tightening specifications provided by the manufacturer, which may vary from manufacturer to manufacturer.

Alternatives to torque tightening

For applications requiring precision and reliability, methods such as hydraulic, electromechanical tightening, or ultrasonic bolt tension measurement may be preferable.

To find out more, visit the TRAXX company website.