Double row ball slewing bearing has three seat rings . The steel ball and the spacer are directly arranged into the upper and lower races . Double rows of steel balls in different diameters are installed according to the force born .This kind of open mode fitting features are very convenient . The load angles of both upper and lower races are 90 °, which enable the bearing to bear large axial force and the tipping moment . But when the radial force is larger than 1/10 the axial force , the race should be designed . So it is suitable for the machines which require large working radius , such as : tower cranes , mobile cranes and mine machinery .