TL;DR
Vibration at the girth gear is usually a backlash or alignment issue. Check the pinion-to-girth gear backlash and confirm the pinion shaft is parallel to the mill axis before pulling the gear.
What you might see
- vibration alarm on the pinion bearing
- tooth wear pattern uneven across the gear face
- periodic impact-type vibration at gear mesh
- girth gear grease thrown excessively at one sector
Likely causes
Pinion misaligned relative to the girth gear face causing uneven tooth loading
Girth gear section bolts loose allowing the ring to distort
Excessive or insufficient backlash between pinion and girth gear
Gear tooth wear or a cracked tooth creating a periodic impact
Required tools
- Dial indicator with magnetic base
- Precision level
- Borescope (if needed)
- Torque wrench
- LOTO kit
Safety first
- Lock out and physically block the pinion drive before any person works near the girth gear.
- A cracked girth gear tooth can eject fragments under load. Do not restart without engineering review.
Procedure
- 1
Stop the mill and lock out all drives.
- 2
Measure girth gear to pinion backlash with a dial indicator on the pinion tooth flank. Compare against the documented design specification.[1]
- 3
Inspect the girth gear teeth visually for cracks, spalling, or impact damage. Use a borescope if direct access is not possible.[1]
- 4
Check the pinion shaft alignment to the mill axis using a precision level. The pinion must be parallel to the girth gear face within the design tolerance.
- 5
Inspect the girth gear segment joint bolts for correct torque. Loose segment joints allow the gear to wobble during rotation.
- 6
Inspect the spray lubricant nozzles for the girth gear. Confirm the spray pattern covers the full tooth face width. Unlubricated areas show dry discoloration.
- 7
If a cracked tooth is found, mark the location and consult the mill manufacturer before restarting. A cracked girth gear tooth is a critical defect.
Sources
Metso Outotec Metso SAG / Premier mill Grinding Mill (SAG / Ball) general technical documentation, Metso Outotec
Metso Outotec grinding mill maintenance manual, girth gear alignment and inspection procedures (general)
More guides for Metso Outotec Metso SAG / Premier mill
How to detect and address liner bolt loosening on a Metso SAG / Premier mill
A knocking noise from the mill shell almost always means a loose or broken liner bolt. Stop the mill and torque-check all bolts before a liner shifts and damages the shell.
How to diagnose trunnion bearing failure on a Metso SAG / Premier mill
Trunnion bearing failure starts as elevated temperature and low lube pressure. Stop the mill, check lube flow to each bearing, and pull an oil sample for metallic contamination before the bearing seizes.
Stop fixing the same fault twice.
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