The Experts: How do I take torsion into consideration when tightening?

Q:  How do I take torsion into consideration when tightening a bolted joint?

A:  Torsion is the twisting of the bolt when applying the tightening torque. When a bolt is tightened it is subjected to tensile stress as preload is introduced but also to torsion stress as a result of thread friction. Torsion generates a shearing stress that is perpendicular to the radius in the circular cross section of the bolt.

Bolts are designed to support a certain stress [N/mm2] before yielding, known as the yield strength of the material. Torsion increases the total stress in the bolt, which may break at a lower preload than expected; the greater torsion stress, the lower the tensile stress that can be introduced. Therefore, when designing a bolted joint, it is important to consider both tension and torsion in order to utilise maximal axial load in the bolt without yielding it.

It is possible to anticipate the amount of torsion stress using the following two methods:

  • By calculation, according to international standards and re­commendations.
  • By considering that torsion stress will represent 15% of the total stress. Nevertheless, in respect of thread friction, this usual value can be adjusted: from 10% if the fasteners are lubricated, to 20% in dry conditions and even more when using friction-based locking fasteners.

Since torsion originates from thread friction, it can be simply reduced by applying a lubricant to the threads of the fastener. There are also tightening methods utilising tensioning of the bolt either by hydraulic equipment or special nuts. Since no turning movement occurs between the threads, torsion can be completely eliminated and a higher axial tension obtained without any risk of the bolt yielding.

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