Thread galling mainly occurs with fasteners made of aluminium, titanium, stainless steel, and other alloys which self-generate an oxide surface film for corrosion protection.
Galling can occur when excessive pressure is used to tighten a nut onto a thread. In our experience this is observed most on assemblies utilising Nyloc nuts.
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. This causes the surface protective oxides to temporarily break down and the interfaced metal high points are removed, the tiny particles of metal jam the thread.
This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme, galling leads to seizing of the threads, if tightening is continued, the fastener can be twisted off and sheared.
In our experience the use of nyloc nuts exacerbates the problem; the integral nylon insert on the nut effectively sweeps up the broken metal particles into the nut as it is tightened, not allowing the nut to simply pass over them. This action adds to the overall clogging effect.
Some suggestions to help prevent thread galling:
Tightening the nut more slowly without excessive pressure, will frequently reduce or sometimes solve completely, the problem. As the installation RPM speed increases, the heat generated with friction during tightening escalates the chances of thread galling.
Reduce the friction by applying a suitable lubricant to the thread frequently eliminates thread galling. Typical lubricants are chloride-paraffine, molykoat lacquer, molybdenum disulphide, high pressure oil, graphite, mica, or talc, some assemblies where there is extreme pressure, corrosion resistant grease or waxes may also be effective.
Care must be taken, firstly:- 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.
Secondly:- 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.
Using different stainless alloy grades for the bolt and the nut reduces galling. The key here is the mating of materials having different hardnesses 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. This is because the different alloys work harden at different rates.