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Knowledge Center

      Quality & Technology
      Cadenas 3D Portal
      Technical information

         Steel
         Stainless Steel
         Surface Treatments
         Self Locking

            Importance of quality fastening
            The interplay of forces in a bolted joint
            Why do threaded joints loosen?
            Loosening due to changes in length
            Self-loosening
            Locking threaded joints
            Conclusion

         Imperial Fasteners
         Anchoring
         Conversion Tables

      Assortment
      Safety

SELF LOOSENING 

Two phenomena are responsible for the self-loosening of a threaded joint:
  • Internal loosening torque.
  • The occurrence of displacements under the effect of a transverse load.
Internal loosening torque arises when the clamping force in a bolt connection tries to make the nut slide along the pitch of the thread. Without friction in the thread and the supporting surface of the nut, it would not be possible to maintain the clamping force in the bolt connection.
 
The greater the preload and pitch angle of the thread, the greater the internal loosening torque. The following formula represents this relationship:
 
 
In the formula, Mli is the internal loosening torque, Fv is the preload and P is the pitch of the thread.
 
In general, however, the friction between the contact surfaces is high enough that the internal loosening torque is only important if changing transverse forces act on the connection.
 
The friction generated by the clamping force disappears only if there is relative movement between the supporting surfaces of the bolt head or nut and the structural members. This friction occurs if the bolt is insufficiently elastic to absorb the relative movement between the clamped parts.
 
If displacement occurs with any change in the direction of the load, the friction is also negligible. At that moment, the internal loosening torque causes the nut or bolt to rotate in relation to the other.

IN SHORT

In order to prevent a thread connection from self-loosening, the following measures can be implemented:
  • Apply a high preload, which results in a lower likelihood of transverse displacement.
  • Ensure that there is a flexible connection by providing for a high clamping length-to-diameter ratio: lk/D ≥ 5. The permissible relative movement increases as a result.
  • Prevent structural members from displacing by using shoulder bolts, pins and clamping bushes.
  • Increase the friction between the contact surfaces by applying friction coatings or roughening the mating surfaces by blasting.
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