Slewing bearings are large size bearings which can carry higher axial, radial loads and overturning moments at the same time. Slewing bearings generally have mounting holes, internal or external gears, lubricant holes and sealing equipment. There are various structures of slewing ring bearings, such as four point contact ball slewing bearing, double row angular contact ball slewing bearings, crossed cylindrical roller slewing bearings, crossed tapered roller bearings and three row cylindrical roller slewing bearings, with gears or no gears. Four point contact ball slewing bearings possess high static loading capacity; crossed cylindrical roller slewing bearings can carry high dynamic loads; crossed tapered roller slewing bearings have high stiffness and rotation accuracy by preloading. Slewing ring bearings are widely used in cranes, excavation machines, construction machinery, port cargo handling machinery, large turnplates of high precision radars and missile launchers, etc.