If you’ve ever tried to tighten a bolt on a stainless steel threaded component and found that the nut doesn’t move, then you’ve experienced galling. While it may seem like a minor inconvenience, galling can lead to catastrophic failures for components that are designed with tight tolerances. The good news is that there are ways to reduce the chances of this frustrating event. By understanding what causes galling, you can make better choices in the design process to avoid costly failures down the road.
What is Galling?
Galling is a form of adhesive wear that results from the friction between two metal surfaces. It can occur at both large and microscopic levels and is most commonly seen on fasteners or machine parts that have to be removed or tightened frequently. Galling is a distinct phenomenon from both abrasive and cold welding wear, which are typically more related to material transfer and shear.
The conditions that most often produce galling are high loads and large apparent areas of contact, as well as low ductility and rougher surface areas. It is important to note that the occurrence of galling can also be accelerated by the presence of abrasives and other contaminants that increase the amount of friction between surfaces.
While the primary cause of galling is friction, other factors that contribute to its development include temperature and deformation. High temperatures can increase the viscosity of lubricants, leading to an insufficient layer between the materials that is unable to prevent adhesion. Deformation can also be a significant contributor to the formation of galling, especially if the material being deformed is very soft.
Fortunately, the occurrence of galling can be prevented by reducing the forces involved and using a suitable lubricant. In addition, slowing down the process of turning nuts or bolts can decrease the amount of heat generated and frictional heating between the threaded components.
Although a wide variety of alloys are capable of resisting galling, some are more susceptible than others. For example, aluminum and austenitic stainless steels tend to be more prone to galling than hardened metals like tool steels and martensitic stainless steels.
The most effective way to reduce the risk of galling is to use a lubricant that can adequately protect against frictional heat and deformation between the threaded components. This is especially important when working with high-strength alloys that are prone to galling, such as those used in aerospace applications. It’s also a good idea to inspect the hardware before installing it into a system, as damaged or dirty threads can lead to increased friction and galling. what is galling