How to Prevent Thread Galling

Thread galling is an unpleasant and often dangerous condition that results from direct metal-to-metal contact between fasteners. In extreme cases, it can cause a structural distortion of mating components. Even in less severe cases, galling causes a significant increase in friction and temperature that can damage or shear the threads of bolts, screws, and nuts.

When exposed to the high pressure and heat of tightening, the relatively soft materials of stainless steels, aluminum, and titanium can form thin protective oxide layers that reduce friction between adjacent surfaces. These oxide layers break down under the stress and movement of the fastener during tightening, allowing the thread surfaces to come into direct contact with each other. When this occurs, microscopic high points and asperities on the thread surface rub together, causing shearing and locking that can destroy the threads and cause the fastener to seize up.

The risk of thread galling can be reduced by designing mating components with different thread profiles and material hardnesses. In addition, using a threaded insert that provides additional lubrication can help prevent galling. The use of a special anti-galling lubricant can also significantly reduce the chance of galling. For example, Never-Seez lubricant can create an effective barrier between the surfaces of threaded parts that reduces the “shoveling” effect of high points and asperities. Additionally, reducing the tightening speed can greatly reduce the amount of heat and friction generated during installation.

Another method for reducing the chance of galling is to ensure that all fasteners are free from damage and debris. A dented bolt, for instance, can dramatically increase the friction and temperature that is produced during installation and increase the chances of galling. Inspect all fasteners, especially those with finer threads, before use. Make sure standard nuts will work with a particular bolt by attempting to put them on by hand. If they do not fit, this is a good indication that the threads are damaged, dented or dirty and will likely seize up during installation.

To further lower the risk of thread galling, the fasteners can be cleaned and electropolished prior to assembly. This will smooth any rough areas on the surface and remove any oxidation. A phosphate coating is a great choice for galvanized fasteners, as it will provide the best resistance to corrosion and galling. In addition, using a PTFE-based thread lubricant can further reduce the friction and the likelihood of galling.

Another method for preventing galling is to use slow-tightening tools, such as power tools, when installing fasteners. This will greatly reduce the amount of heat and friction generated. Lastly, lowering the torque specification can also help to decrease the risk of galling.

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