Thread galling is a common cause of mechanical failure that can damage fasteners, screws, bolts and other metal parts. It happens when the threads of two or more different materials erode and stick together creating a lumpy surface called a gall. The result is reduced performance and structural distortion of the part.
A few simple measures can help prevent thread galling. Most of these involve using the right lubricants to reduce friction. A high-quality dry lubricant can help keep the threads smooth and clean. It can also prevent the buildup of heat that leads to galling. It’s important to follow the recommended torque specs when tightening any fastener. If you need to use a higher torque rating than what is specified, make sure you are applying the pressure evenly so that one area does not generate more heat than another.
In addition to using the right lubricants, preventing galling is important by keeping all components cool during installation. Heat and friction go hand-in-hand, and higher temperatures increase the likelihood of galling. By slowing down the installation process and allowing the heat to dissipate, you can greatly reduce the chances of galling.
Stainless steel and aluminum fasteners are particularly prone to thread galling. These materials self-generate a protective oxide film to prevent corrosion, but this layer is rubbed off during installation. The resulting metal-to-metal contact creates excessive heat and friction that can fuse the materials together in a process known as solid-phase welding (galling). Once this happens, it is very difficult to remove or separate the two parts.
To avoid galling, you should always perform a full quality check before installing new fasteners. This includes assessing the condition of the packaging, making sure that the bolts are clean and untampered with. It is also important to make sure that the proper nut and bolt are being used in conjunction with each other. If the wrong nut is being used, it can lead to increased friction and heat, leading to thread galling.
In general, harder materials tend to gall less than softer ones. However, even hard materials can gall if they are installed with incorrect tools or in the wrong application.
Using the correct lubricant and following the tips above can dramatically reduce your risk of thread galling. The best lubricants for reducing galling are molybdenum disulfide, graphite, mica and talc. There are also anti-seize compounds that can be pre-applied or applied at the point of assembly to reduce friction and protect against galling.