The tower crane is on boom rotary to ensure that its work coverage, rotational motion is generated through the upper, lower rotary bearings are respectively arranged on the rotary bearing inner and outer rings, and is composed of a rotary mechanism drives the small gear, the small gear and the big gear ring meshing rotary bearing, drives the rotary upper support relative to the lower support motion. The big gear ring two to four small gear drives the small gear, even on the large gear ring around.

　　Tower crane slewing inertia big, turning, braking often have the inertia impact. In order to ensure the smooth revolution, rotary mechanism characteristics should be soft, rotary deceleration must be small, small drive gear motor for maintaining synchronization between.