The slewing bearings are the large-size bearings which are able to carry higher axial, radial loads and over turning moments. The slewing bearings are usually provided with mounting holes, internal gears or external gears, lubricant holes and seal equipments. There are various structures for slewing bearings, such as four-point contact ball bearings, double-row angular contact ball bearings, crossed cylindrical roller bearings, crossed tapered roller bearings and triple-row cylindrical roller bearings. The four-point contact ball slewing bearings possess higher static loads carrying capacity; the crossed cylindrical roller slewing bearings possess higher dynamic load carrying capacity. The crossed tapered roller slewing bearings have higher stiffness and operating accuracy by preloading.