When a fastener is tightened or installed it experiences friction that causes the surface treatments on both the male and female threads to temporarily break down, creating microscopic high points on each. When these high points rub together they cause the materials to stick or seize – this problem is called “thread galling” and can make it difficult or impossible to loosen or remove the fastener.
There are many ways to prevent or reduce the occurrence of thread galling, including using lubricants like grease and oil, coatings that provide low-friction or a thin film of protection such as molybdenum disulfide, and techniques for increasing surface hardness on stainless steel fasteners that include induction and case hardening. Good housekeeping is also important to ensure that dirt or abrasive particles don’t get between mating surfaces.
A common trigger for galling is the use of improper or damaged hardware, often because of shipping damage where pieces bang into each other in transit. Another cause is reusing overtightened hardware which can create damage to the threads that makes them more susceptible to galling during future tightening. Also, if two different materials with differing hardness ratings are used together (such as a grade of stainless steel and copper), galling can occur because the soft material can deform or fuse with the harder material.
The best way to prevent galling is by using the right lubricant before installation. The most effective lubricant for this purpose is anti-seize, which can be applied directly to the threaded area of the bolt or screw before installing it. Some screws and nuts have an anti-seize coating already on them but it is always a good idea to apply additional lubricant during installation and especially before starting to tighten.
It is also a good practice to keep the temperature low during installation. The heat generated by power tools can increase the friction between mating surfaces and lead to increased chances of galling. Also, reducing the speed of installation can help to reduce the chance of galling as well.
Other considerations in galling prevention include the choice of mating materials and thread design. For example, coarse threads are less prone to galling than fine threads and cold rolled threads are better than machined threads as they have a smoother finish. Threads should also be properly matched in diameter and profile and adequate clearance must be provided between them to avoid interference during assembly.
Another method to reduce the occurrence of galling is by ensuring that both threaded surfaces have been cleaned prior to assembly and inspection with a low-power microscope or a hand lens. Rough surfaces tend to gall more than smooth ones so the threaded areas of the fastener should be sanded or chamfered where necessary to eliminate rough edges. Finally, electropolishing the threaded surface can be an effective technique to further improve galling resistance.