Vacuum and cleanroom environments present difficulties when specifying bearings due to the risk of contamination. Materials can give off a vapour, a process known as outgassing. The bearing materials and the lubricants must be carefully chosen to avoid such complications.
SMB Bearings specify stainless steel for vacuum or cleanroom applications as stainless steels used for the rings, balls and retainer exhibit low outgassing. They usually supply open or shielded stainless steel bearings as vacuum bearings as these will outgas less than a nitrile rubber sealed bearing. Nitrile rubber is often used in vacuum applications but there could be a problem if surfaces sensitive to contamination, such as optical components, are present.
Some of the bearings are available with Viton seals. Although Viton sealed bearings are often chosen for high temperature or chemical resistance, Viton is also a fluorocarbon so it performs well in a vacuum.
Plastic bearings made from PEEK also perform well in low load, semi-precision applications. These bearings are usually unlubricated. PEEK has low outgassing characteristics so is suitable for use in these environments.
Full ceramic silicon nitride bearings perform well in a vacuum. They can also be run without lubrication which removes the need for a vacuum compatible lubricant.
Great care must be taken when choosing the bearing lubricant for a vacuum or cleanroom as most greases will vaporize to an unacceptable level, contaminating the environment and preventing the grease from lubricating correctly. Special low volatility, perfluorinated greases with very low outgassing rates are designed to perform well under these conditions. These greases are regularly used in applications such as semiconductor manufacturing or in equipment used in deep space.
Another option, for low speeds, is no lubricant at all or a “dry” lubricant such as molybdenum disulphide or tungsten disulphide. SMB Bearings can treat the inner bearing surfaces to provide a very low outgassing lubricating film.