Thread Galling

 

Thread galling mainly occurs with fasteners made of stainless steel and other alloys which self-generate an oxide surface film for corrosion protection.

 

Galling can occur when excessive speed and/or pressure is used to tighten a nut onto a thread. This is observed mostly on assemblies utilising Nylon Insert Nuts or when drawing a joint together using the mechanical effect of the thread. As the installation RPM speed increases, the heat generated with friction during tightening escalates the chances of thread galling.

 

The application of excessive pressure increases the friction between the mating threads and during fastener tightening the pressure builds between the contacting and sliding thread surfaces. Their high points are removed and the tiny particles of metal jam the thread.

 

This cumulative clogging-shearing-locking action causes increasing adhesion and friction. In the extreme, galling leads to complete seizing of the threads and if tightening is continued the fastener can be twisted off and sheared.

 

 

Some suggestions to help prevent thread galling..

 

  1. Tightening the nut more slowly without excessive pressure will frequently reduce the problem.

 

  1. Reduce the friction by applying a suitable lubricant to the thread.

    Notes: (i)Applying a lubricant will reduce friction on the mating threads which will have an effect on the tightening torque of the fastener, and experiments to check that the joint will perform as it should may need to be conducted.

    (ii) Awareness of the end use of the fasteners before settling on a lubricant. Stainless steel is frequently used in food related applications which may make some lubricants unacceptable. Lubricants can be applied at the point of assembly or pre-applied as a batch process similar to plating. Several chemical companies offer anti-galling lubricants.

 

  1. Use different stainless alloy grades for the bolt and the nut reduces the chance of galling because the mating of materials having different work hardening rates. If one of the components is A4 (316) and the other is A2 (304) they are less likely to gall than if they are both the same alloy grade.