Today, bearing lubricants serve several functions:creating a barrier between rolling contact surfaces,creating a barrier between sliding contact surfaces,protecting surfaces from corrosion,sealing against contaminants or providing heat transference (in the case of oil lubricant)
Lubricants take the form of either oil or grease. Oil lubricants are most common in high speed, high-temperature applications that need heat transfer away from working bearing surfaces. Bearing oils are either a natural mineral oil with additives to prevent rust and oxidation or a synthetic oil. In synthetic oils the base is usually polyalphaolefins (PAO), polyalkylene glycols (PAG) and esters. Although similar, synthetic and mineral oils offer different properties and are not interchangeable. Mineral oils are the more common of the two.
Grease consists of about 85% mineral or synthetic oil with thickeners rounding out the rest of the grease volume.
The thickeners are usually lithium, calcium or sodium-based metallic soaps. Formulations for higher-temperature applications often include polyurea. The higher viscosity of grease helps contain it within the bearing envelope. The most important considerations when choosing a grease are the base oil viscosity, rust-inhibiting capabilities, operating temperature range and load-carrying capabilities.