Swing Gear (Also Known As Ring Gear or Bull Gear) : External or internal gear with which swing pinion on revolving upperstructure meshes to provide swing motion. (Spec 2C)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}