Slewing bearing is also called slewing ring, slewing ring bearing, turntable bearing, and rotary bearing. The double row ball slewing bearing has three seat-rings. The steel balls and the retainers may be directly arranged into the upper and lower racers. Two rows of steel balls with different diameters are fitted according to the force. Such open type fitting is extraordinary convenient, the loading angels at upper and lower races are 90°which can carry both of the axial force and capsizing moment. When the radial force is larger than 1/10 of the axial force, the races should be newly designed.