Drive system (girth gear drive train: motor, gear unit and pinion turning the ring gear of a grinding mill, rotary kiln or dryer)
Girth gear drive systems for mills, kilns and dryers · main motor, gear unit with auxiliary drive, spacer coupling, pinion and girth gear, single or dual pinion, power per pinion, gear materials and segments
A drive system here is the mechanical drive train of a large rotating drum: an electric motor and gear unit on a base frame turn a pinion that meshes with a girth gear bolted round a grinding mill, rotary kiln or dryer, with an auxiliary drive for slow turning and maintenance. Ask three things first: how much power the drum needs and whether one pinion or two must carry it, how the pinion is kept aligned with a girth gear that moves with the drum, and which girth gear material, section and segments suit its size.
Girth gear and pinion drive trains for horizontal grinding mills, rotary kilns, dryers, coolers and scrubbers: main motors, gear units, auxiliary or barring drives, couplings, pinions and girth gears, single- or dual-pinion. Rail-mounted ladle transfer cars and their travel drives are covered on the Molten metal (ladle) transfer car page; slewing drives on the Slewing drives page; conveyor drive units on their own page; gearless ring-motor drives are outside this page except where compared.
CNFX editorial research · Sources checked October 12, 2026 · 3 sources
Power per pinion: when one pinion is no longer enough
An FLSmidth patent on mill drives describes the conventional arrangement: "an electric motor drives, directly or via a gearbox, a pinion meshing with the girth gear", either as a single-pinion drive on one side of the mill or a dual-pinion drive with one pinion on each side. It states that with this concept mills of 9 MW (single pinion) or up to 18 MW (dual pinion) can be driven, that mills with four girth gear pinions may be driven with up to 40 MW, and that for very large mills only ring motors have been used so far. [3]
David Brown Santasalo rates its cast and fabricated girth gears "up to 10MW per pinion", with diameters up to 14 m, weights up to 105 tonnes and pitch up to module 50, for mills, kilns, dryers, coolers and scrubbers. [2]
Editorial calculation: the patent's figures work out to 9 MW per pinion for single and dual drives (18 ÷ 2) and 10 MW per pinion for four pinions (40 ÷ 4), matching the 10 MW per pinion David Brown Santasalo states. For torque, a drum taking an assumed 5 MW at an assumed 15 rpm needs 5,000,000 ÷ (15 × 2π ÷ 60) ≈ 3.2 MN·m at the girth gear. [3][2]
Give the drive supplier the drum type, diameter and speed, absorbed and installed power, number of pinions, girth gear diameter and module if existing, starting and inching requirements, motor supply, foundation layout, lubrication method, ambient and dust conditions, and whether the girth gear, pinion, gear unit and motors are all in scope.
02
Gear unit with auxiliary drive, spacer coupling to the pinion, and girth gear build
Flender says that ball mills and rotary kilns grinding iron ore, limestone and clinker produce large axial and radial forces that single-pinion drives must withstand permanently. For the girth gear and pinion drive, a standard gear unit with input shaft on both sides "is generally used as the main gear unit": the main motor connects to one side and the auxiliary drive to the other, and "the pinion at the mill's girth gear can be connected via a double-jointed coupling with spacer to ensure the adjustability of the pinion". Its MDSS single-pinion drives come in fixed and self-aligning designs. [1]
David Brown Santasalo supplies girth gears alone or with a mill drive gearbox, pinions, barring drive and couplings, brakes, pedestal bearings and clutches. It offers SG iron and cast steel gears with hardness up to 320 BHN and fabricated steel rims from 180 to 340 BHN, in 2, 4, 6 or 8 segments, with T-section rims described as "resilient to pinion misalignment" and Y-section rims for rim support across the face width, designed to AGMA standards. [2]
03
Main motor, gear unit, auxiliary drive, coupling, pinion and girth gear
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
How is this different from a ladle transfer car drive?
This page covers girth gear drives for rotating drums; travel drives of rail-mounted ladle cars belong to the Molten metal (ladle) transfer car page. [3][1]
How much power can one pinion carry?
David Brown Santasalo rates its girth gears up to 10 MW per pinion; an FLSmidth patent gives 9 MW for conventional single-pinion mill drives. [2][3]
Why does the gear unit have two input shafts?
Flender connects the main motor to one side and the auxiliary drive to the other. [1]
What girth gear materials are used?
David Brown Santasalo offers SG iron, cast steel and fabricated steel girth gears in 2 to 8 segments. [2]
Sources and what still needs a direct check
Sources checked 2026-10-12. Flender describes its single-pinion mill and kiln drives; David Brown Santasalo publishes its girth gear data sheet; an FLSmidth patent describes conventional mill drive limits.
