Thread galling is an issue that affects the longevity and efficiency of a fastener, screw or bolt. It occurs when the smooth, rolling action of a threaded surface rubs against another rough surface. This rubbing causes the materials to build up into small bumps or “galls.” Galling can cause the two surfaces of the fastener to seize together. This is a serious problem and can result in lost time, money and equipment. Taking the necessary precautions and actions to avoid galling can prevent these problems from occurring.
The main factors in thread galling are friction and temperature. Stainless steel and aluminum fasteners are especially prone to galling due to their relatively soft metals and ability to wear off the protective oxide coatings on their exposed metal surfaces during tightening. These oxide coatings help reduce friction between the fasteners and prevent direct metal to metal contact which can lead to galling.
During tightening, these protective oxide layers can be sheared off by the pressure and friction applied to the fastener by the installer. The resulting direct contact of the fasteners can create heat, which can then melt and weld the two surfaces together in a process called cold-welding. This can cause the fasteners to bind and become permanently stuck in place, rendering them useless.
The best way to prevent thread galling is through lubrication. Adding an anti-seize compound to the surface of your fasteners can help to reduce friction between the surfaces and eliminate the need for direct contact between the threads. These compounds can be added to the nut or bolt before installation and will provide lasting protection against galling.
In addition, keeping the fasteners clean can also help to prevent thread galling. Any grease, dirt or debris that is present on the surface of a nut or bolt can increase friction between the mating parts and create an environment for galling to occur. Ensure that all fasteners are free of these contaminants before use to ensure that they can perform as expected.
Lastly, reducing the speed of installation can help to prevent thread galling. This is particularly true for fasteners with locking mechanisms such as nylon insert lock nuts or prevailing torque nuts that generate a high amount of friction and heat during installation. Slowing down the installation speed can significantly reduce the chance for galling to occur and will help to dissipate the built up heat.
While the unavoidable roughness and high-pressure conditions of a threaded fastener can make it seem like galling is inevitable, there are a number of things that can be done to minimize this problem. Using a lubricant like Never-Seez on the threaded surface of your fasteners can create an effective barrier between the surfaces and greatly reduce the “shoveling” effect that can occur. This lubricant can also help reduce friction between the components and dissipate the excess heat that can cause galling to occur. Similarly, slowing down the installation speed can dramatically reduce and in some cases completely solve the problem of thread galling.