Galling is the abrasive contact between mating metal threaded surfaces, resulting in adhesion and seizing that can result in twisted fasteners and stripped or damaged threads. This is a problem that can occur in any application where ductile metals are put under high amounts of friction with one another. This may include applications with stainless steel and aluminum fasteners.
When the frictional heat of a fastener’s operation causes a protective oxide layer to wear away, it exposes the reactive surface underneath to a corrosive environment that can lead to the galling that occurs during installation or use. This can also cause the fastener to become stuck in place, requiring a difficult and time consuming effort to dislodge it.
There are several different ways to reduce the likelihood of galling in threaded metal applications. The first is to apply a lubricant to the mating metal surfaces during assembly. This will significantly reduce the amount of friction between the fastener and the mating threads. This can be done with either a standard oil-based grease or a specially designed anti-seize product.
The second method to prevent galling is to slow down the speed of the fastener during installation or use. This will allow more time for the frictional heat to dissipate, reducing the risk of galling. In addition, the use of a coarser thread can reduce the risk of galling because it offers more clearance between thread flanks.
A third way to reduce the risk of galling is to change the materials used in an assembly or a fastener. Stainless steel and aluminum are particularly susceptible to galling because of their low ability to dissipate frictional heat. Other metals such as martensitic stainless steel and hardened tool steel are less prone to galling because they have a much higher ductility that allows them to dissipate more frictional heat.
Other ways to prevent galling are to increase the axial alignment between the clamped components, or to eliminate extraneous loads that may be introduced by mistake or design. Using a helical or wedge type washer can prevent the galling that is caused by the axial load between the fasteners.
The most effective way to prevent galling is to design the system with as little rubbing between components as possible. This can be accomplished by making sure that the threaded holes are positioned correctly and by limiting the amount of side-to-side movement between the components. Lastly, specifying fastereners that have been electroplated with an anti-galling material can help to avoid this issue. This will usually require that the fasteners be specially made because the plating will add thickness to the threaded area, decreasing their tolerance. This can be cost prohibitive for high-volume applications. However, it is still a good choice for long term maintenance or repair of equipment that will be used frequently. This is especially true when the equipment will be subject to high levels of stress and vibration. If left unchecked, galling can make it difficult to remove or replace a fastener and will result in a failed connection or a seizing condition.