The primary purpose of the lubricant is to build a load-carrying film separating the bearing components in rolling and sliding contact in order to minimize friction and wear. The lubricant should also protect the bearing against corrosion. If required, it should also act as a sealant, and in case of circulating oil lubrication, as a coolant.
Due to deterioration and mechanical stressing, the lubricants become unuseable. Change of oil or grease or replenishment, i. e. bearing maintenance, has a favorable influence on the bearing service life. Under certain sealing and environmental conditions, appropriate lubricant selection allows for a lubrication for life. For detailed information see also FAG Publ. No. WL 81115 “The Lubrication of Rolling Bearings”.
Only high-grade greases – generally on a metal soap basis – should be used. Rolling bearing greases for extreme temperatures contain different thickeners and a synthetic oil instead of the mineral oil. Greases containing extreme pressure (EP) additives are used in highload and low-speed applications. High-speed bearings and bearings which ask for low friction, are lubricated with greases containing a thin synthetic base oil. The operating temperatures specified by the grease supplier should be kept in mind. Rolling bearing greases should be stable against deterioration and must not change their structure, even after long periods of operation.
For rolling bearing lubrication, mineral oils are generally used. They should have the following properties:
Utmost cleanliness, stability against deterioration, good viscosity temperature behavior and good water repellency. In addition, the lubricating oil must ensure satisfactory protection of the bearing against corrosion. Very high and very low operating temperatures require the use of synthetic oils. Oils for highly loaded and low-speed bearings should contain EP-additives.
3- Selection of Lubricant
Greases are generally preferred to oils because they simplify maintenance and can be used as sealants. The asset of oil is that it readily feeds into all areas of contact and carries off heat. Its disadvantage is that it involves a more complex design of the bearing location and especially of the sealing system.
4- Loads and speed
Under the given operating conditions, the lubricant must form a load carrying lubricating film. With oil, the load carrying capacity of the film is primarily a function of viscosity. The lower the bearing speed, the higher the oil viscosity in operating condition. Information on viscosity n1 can be seen in the FAG catalogue WL 41520. Consideration must be given to the fact that bearing temperatures depend on load and speed. The operating temperature required for determination of the nominal viscosity must be estimated.
5- Bearing Size
Small bearings are generally lubricated with a low-viscosity oil or with a very soft grease to minimize lubricant friction in the bearing. In large bearings, the lubricant friction plays a minor role, and the choice between oil or grease is, in this respect, of secondary importance.
The reaction of rolling bearing greases against moisture is different from one grease to the other. Only the water-repellent calcium base greases (Ca-greases) possess a safe sealing action against water. They are used, therefore, in labyrinths for operating temperatures not exceeding 50°C, acting as sealing agents.