Bearings are simple parts that are used to reduce friction when placed between two moving parts and are often applied with traditional lubricants to decrease friction. However, there are certain situations when lubricants cannot be used and in these instances, bearings made of graphite are used because they are self-lubricating.
Graphite is naturally a greasy mineral, and does not require any additional fluids to reduce friction when used as a bearing. It also has an extreme temperature resistance, more than any metal, and is able to maintain its structure and shape even in temperatures up to 5000°F. Therefore, machined graphite is commonly used in high heat environments, where oil, grease and metals tend to oxidize, often in load capacity systems.
Using graphite bearings dramatically opens up the temperature and chemical range of mechanical operations; some even fare well in cryogenic conditions as graphite is not susceptible to damage as a result of thermal shock. Mostly, they are used in the food, beverage, medical and pharmaceutical industries in machinery where contamination is a concern. They also exhibit high temperature stability and chemical resistance, and can therefore operate in underwater, outer space, underground, in liquid, gas and vacuum environments.
Graphite bearings are able to operate in extreme heat at temperatures significantly higher than any lubricants, which begin to carbonize at temperatures above 200º C. The bearings are fabricated via the extrusion process, which involves heating a mixture of graphite powder and additives until soft and forcing it through a hollow profile.
They are generally shaped like standard bushings—round profiles, thin and hollow. Graphite bearings are black or dark grey in color, and are usually smooth and uniform in thickness. Since graphite tends to be a porous material, many bearings are put through post processing secondary operations, where they are impregnated with wax, high purity linseed oil or a resin. This smoothes out the surface of the bearing and prevents any problems that may occur if water or gas seeps into the graphite’s exterior surface.
The wax, resin or oil reduces the porosity of graphite, making it less susceptible to interior wear and tear. Graphite bearings are long lasting, although as with all bearings or load carrying components, will need to be replaced from time to time. Due to these many advantages, including being dimensionally stable, graphite products are used in a large number of corrosive and abrasive applications.