A girth gear (ring gear) is a large-diameter gear ring mounted around the shell of a rotating drum — a ball mill, rotary kiln or dryer — that transmits drive torque from one or two pinions to turn the entire machine. Diameters commonly run 3 to 10+ meters, with weights from a few tons to over 100.
Cast in halves or quarters
Most girth gears are cast steel rings produced in two or four segments, flanged and bolted together around the mill shell — a single 8-meter ring cannot travel by road. Segment joint machining is critical: the assembled ring must run true within tight radial and axial tolerance or tooth loading becomes uneven.
Material and tooth quality
Typical grades are ZG310-570, ZG35CrMo or GS-20Mn5 class steels, normalized and tempered for a tooth-surface hardness matched to the pinion (the pinion is sacrificially harder). Tooth geometry is hobbed — in our case on 5m, 8m and 10m CNC gear hobbing machines — and verified for profile, pitch and runout.
Why sourcing matters
A girth gear failure stops the entire production line, and replacements take months. UT-verified internal soundness in the rim and tooth zone, documented heat treatment, and accurate joint machining are what you are actually buying — the gear teeth are only as good as the casting beneath them.