What is Galling?
Galling is damage that occurs on sliding metal surfaces when a combination of excessive force and high friction cause the transfer of material.
Often macroscopic, between the two surfaces the metal from one surface tears away and becomes embedded in the other surface. The most common galling example is thread stripping in bolts, nuts, and other parts and is most susceptible to aluminum, stainless steel, or other very hardened steel and alloys.
Why is Galling a Cause for Concern?
Galling is a sudden failure event and can cause all kinds of complicated repairs to machinery. The galling result can be as severe as friction welding of the two parts together so they can’t slide further. In threaded connections, this means stuck bolts.
Galling is common in many industrial applications such as sheet metal fabrication, engine bearings, pistons, air motors, and etc. Preventing galling in industrial settings can significantly reduce major unexpected outages and save vast amounts of money in lost production time.
How to Prevent Galling?
Galling is a function of three things, one must address each specific application where prevention is needed.
- Lubrication is a crucial ingredient to galling prevention. Any sliding surfaces in an environment with high forces possible and using susceptible materials mandate proper lubrication. Dry graphite-based lubricants, such as Neolube, are custom made for those applications where a simple wipe-on, no-mess, no-contamination lubricant is needed. In some other less demanding cases, a little oil or grease will work. In food processing applications, special lubricants must be used that are food grade.
- Reducing force is the second key to preventing galling in existing systems. For threaded connections, for example, the second any tension is felt in the movement of the two parts, the force must be removed. Reversing the direction of a threaded bolt and moving it back and forth can free up areas that are starting to gall. This doesn’t work inside pistons or sliding machinery where only lubrication is the solution once the design is set.
- Material selection in the design phase is also critical. Softer steels and other metals are much less susceptible to galling and should be used in sliding applications where possible. Corrosion, hardness, strength, and many other design considerations dictate when soft metals won’t work. Metal surface treating and coatings can be effective at mitigating the risk of galling as well.
Neolube #1 and Neolube #2 are great lubricants for reducing the probability of galling, particularly in stainless steel and other hardened alloys. The graphite layer that is dry and left on the sliding parts can retain its protective lubrication thickness in the contact zone where galling can occur. The ability to tightly control surface contamination with outside agents, like halogens, is the other significant benefit to a product like Neolube, which was developed for the Nuclear Power Industry but is also effective for thousands of different industrial applications.