The lubrication of rolling bearings similar to that of sliding bearings mainly serves one purpose: to avoid or at least reduce metal-to-metal contact between the rolling and sliding contact surfaces, i.e. to reduce friction and wear in the bearing. Oil, adhering to the surfaces of the parts in rolling contact, is fed between the contact areas. The oil film separates the contact surfaces preventing metal-to-metal contact ( physical lubrication ). In addition to rolling, sliding occurs in the contact areas of the rolling bearings. The amount of sliding is, however, much less than in sliding bearings. This sliding is caused by elastic deformation of the bearing components and by the curved form of the functional surfaces. Under pure sliding contact conditions, existing for instance between rolling elements and cage or between roller faces and lip surfaces, the contact pressure, as a rule, is far lower than under rolling contact conditions. Sliding motions in rolling bearings play only a minor role. Even under unfavourable lubrication conditions energy losses due to friction, and wear are very low. Therefore, it is possible to lubricate rolling bearings with greases of different consistency and oils of different viscosity. This means that wide speed and load ranges do not create any problems. Sometimes, the contact surfaces are not completely separated by the lubricant film. Even in these cases, low-wear operation is possible, if the locally high temperature triggers chemical reactions betweenthe additives in the lubricant and the surfaces of the rolling elements or rings. The resulting tribochemical reaction layers have a lubricating effect ( chemical lubrication ). The lubricating effect is enhanced not only by such reactions of the additives but also by dry lubricants added to the oil or grease, and even by the grease thickener. In special cases, it is possible to lubricate rolling bearings with dry or solid lubricants only. Additional functions of rolling bearing lubricants are: protection against corrosion, heat dissipation from the bearing (oil lubrication), discharge of wear particles and contaminants from the bearing (oil circulation lubrication; the oil is filtered), enhancing the sealing effect of the bearing seals (grease collar, oil-air lubrication).