Thread galling is a serious problem that affects the performance of fasteners, screws and bolts. It occurs when the threads of mating fasteners rub against each other and generate friction that causes heat. The result is a bump, scar or gall on the surface of the threads. Eventually, the galling can lead to structural distortion of the fasteners and may cause them to fail. Understanding the causes of galling and incorporating appropriate countermeasures is essential in reducing or eliminating this costly issue.
The most effective method for avoiding galling is to design your assemblies with components of different materials and hardnesses. Choosing different types of threads, such as coarse or fine, can help reduce the likelihood of galling. Also, using cold rolled or lathe-cut threads can produce smoother surfaces that have less tendency to create excessive friction and heat. Using different metal grades for mating fasteners can also reduce the risk of galling. For example, using a type 304 stainless steel nut with a type 316 stainless steel bolt can eliminate the potential for galling because they have different corrosion resistance and hardness properties.
Another method of preventing galling is to use the right lubricant for your assembly. A dry film lubricant, such as those found in antiseize compounds, can help reduce the friction that can cause galling by coating the mating threads. These lubricants can be pre-applied or added at the time of assembly.
Using the right tools to prevent galling also helps. Using power tools that can be turned off at the point of maximum torque or slowing down the speed of tool use can reduce friction and heat, which can cause galling. Keeping the surfaces of the fasteners clean from dirt, nicks and debris also helps reduce the risk of galling because rougher surfaces tend to generate more heat and friction.
The best method for reducing the chances of galling is to make sure the fasteners are in good condition before use. A quality inspection should include assessing whether the threads of the fasteners are damaged or contaminated, and if necessary, replacement parts should be used.
In addition, a thorough cleaning of the threads of the fasteners should be carried out before installation. Using cleaners, such as WD-40 or liquid degreasers, can also help remove any contamination from the threads of the fasteners that could increase their susceptibility to galling.