Swing Gear (also known as Ring Gear or Bull Gear)
External or internal gear with which the swing pinion on the revolving upper-structure meshes to provide swing motion.
Source: API RP 2D, Operation and Maintenance of Offshore Cranes, Seventh Edition, December 2014. Global Standards
Source: API RP 2D, Operation and Maintenance of Offshore Cranes, Sixth Edition, May 2007. Global Standards