Bearings for Vacuum Environments
Rolling-element bearings are often put to the test in industries known for harsh and extreme operating conditions, such as steel fabrication and food processing. However, machine builders are raising performance requirements of these bearings in additional applications like semiconductor fabrication, aerospace, and processing equipment for medical devices — typically ultraclean and vacuum environments. In these cases, bearings are expected to operate at high or low temperatures, and perform in highly corrosive environments with exposure to moisture and chemicals. The problem is that these environmental conditions have a major impact on bearing life and can offset calculations on expected life by a staggering 90%.
Contamination of mission-critical devices is of major concern in spacecraft and satellites. In the vacuum of space, outgassing materials can condense on optical elements such as lenses, solar cells, and navigation sensors. In the semiconductor industry, contamination from outgassing of materials can damage millions of dollars worth of wafers.
Sapce bearing components that are of concern in vacuum environments are the retainer and lubricant. Bearings used in these vacuum environments need to be ultraclean. Surfaces must be cleaned with solvents and, in some cases, baked to drive off volatile residues that could potentially outgas.
Proper lubrication is vital for good bearing life. For rolling-element bearings, oil and grease lubricants are the most effective, particularly at and above the critical rotational speed where the load-bearing fluid film forms. In addition to damaging nearby components, outgassing reduces the effectiveness of the oil or grease. Fortunately, lubricant technology has advanced, and low outgassing and low-vapor pressure oils and grease have been developed. These lubricants are typically produced with base oils, such as synthesized hydrocarbon or PFPE. Vapor pressure for these fluids is about 4.7 × 10 to 12 torr at 25°C.
The rate of outgassing increases at higher temperatures because the vapor pressure and rate of chemical reaction increases. Therefore, solid lubricants may be required at higher temperatures within a vacuum environment. Typical solid lubricants for vacuum applications include gold, silver, molybdenum disulfide (MoS2), and polytetraflouroethylene (PTFE).
Austenitic stainless steel AISI 304 cages can be used in vacuum environments. The cages can also be coated with PTFE to allow for transfer of PTFE to the raceways, and to reduce torque between the rolling element and the cage. When lighter, more rigid higher speed cages are required, snap in or full machined types are made of PTFE, Vespel, or PEEK.
When faced with extreme or harsh environments, designers and bearing users are advised to consult a bearing applications specialist for assistance with specifying the critical bearing attributes, calculating load capacity and bearing life, and providing a cost-benefit analysis.