Bearing Lubrication Basics
Lubrication reduces friction. It also prevents wear and corrosion, and guards against solid and liquid contamination. Theoretically, a properly lubricated bearing operating under ideal conditions will last forever. This is not possible in reality, of course. But a properly lubricated bearing has the best chance of achieving its maximum service life.
The lubricant forms a film between the bearing’s rolling and sliding surfaces, so that metal contact is minimized even under heavy load. Rolling bearings are normally lubricated with grease or oil. In special cases a solid lubricant is used.
Oil Versus Grease
Why one type of lubricant is used instead of another depends on bearing size, speed, type, load and operating environment.
Grease which is actually oil with a soap thickener added, is used for bearings operating under normal speed and temperature conditions. It is simpler and less expensive to apply than oil; offers better adhesion and better protection against moisture and contaminants. Approximately 90% of bearing applications employ grease.
Oil is used when speed and/or operating conditions make it impossible to use grease, or where heat needs to be transferred.
Lubricant Supply Systems
Oil and grease require different types of supply systems. Several oil and grease supply systems exist that meet the needs of various bearing applications. Oil supply systems include; oil baths, circulating oil systems, spray or mist systems, and the wick feed method. Grease supply systems include: housings (with or without grease fittings), grease chamber lubrication, and grease quantity regulator.
Lubricant cleanliness is essential, since ever minute contaminant particles can affect a bearing’s operating performance and shorten service life.
Filtering fresh oil before feeding it into the delivery system is an excellent lubrication practice. In oil circulating systems, filters can further guard against contamination when they are suitably located in the application.
Grease should be kept in their original containers until used. Never leave a container uncovered, since dust and other particles from the environment will quickly collect. Wash grease guns with clean solvent and dry thoroughly before use. Keep new bearings scrupulously clean during mounting. Inspect and thoroughly clean bearings that have been disassembled for relubrication (see article on bearing mounting).
Viscosity is the most important lubricant property that should be considered during lubricant selection. Viscosity is the ease with which a liquid flows, and is the guiding principle in lubricant selection. Film-forming ability (film thickness) is related to lube type, rotational speed, temperature and viscosity. Consistency is the degree of stiffness in a grease.
Lubricant can also have anti-oxidizing, rust inhibiting and anti-wear and anti-foaming properties.
Lubricant selection criteria differs for various bearing types and sizes. Contact your bearing manufacturer, equipment manufacturer or lubricant supplier for detailed selection information.
Oil change intervals depend on operating conditions and oil quantities. For most oil bath systems, yearly changes are acceptable. “Closed” mist systems exist that reuse the oil; here, oil sampling can help determine acceptable frequency of change intervals. Also, large circulating systems that filer and de-water oil are available, “make-up” oil is needed to maintain volume.
Greased bearings must be relubricated if the grease’s service life is shorter than the bearing’s expected service life. The relubrication interval is that time period at the end of which 99% of the bearings are still reliably lubricated.
Contact your bearing manufacturer for relubrication charts covering all types of rolling bearings.
Although care has been taken to ensure the accuracy of this data, SKF assumes no liability for errors or omissions.
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1998 SKF USA Inc.
The above information was supplied by the SKF USA, Inc. The SKF Bearing Maintenance Institute recommends the above bearing maintenance tips to maximize bearing performance. SKF USA, Inc. may be contacted at www.skfusa.com. Bearing Lubrication Basics