Girth gear is a term used for equipment such as rotary kilns and ball mills. It is typically a large diameter gear mounted onto the shell or end of the kiln or mill, and is driven by a pinion, or in larger applications two pinions. The motor drives the pinion and pinion drives the girth gear mounted on the cylinder shell, because of this, the ball mill start driving. The girth gear is connected to the cylinder shell by bolt.