TL;DR
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.
What you might see
- knocking sound at the mill shell during rotation
- liner bolt torque below specification on periodic check
- material bypassing the liner to the shell
- liner movement visible on bore scope inspection
Likely causes
Liner bolts losing preload from mill vibration and thermal cycling
Liner bolt nut-side washer deformed allowing the bolt to back off
Liner thickness reduced from wear reducing the clamping stack height
Initial torquing to below specification during liner installation
Required tools
- Calibrated torque wrench rated for liner bolt torque values
- Replacement liner bolts and washers
- Confined-space entry equipment (gas monitor, harness, rescue line)
- LOTO kit
Safety first
- Confined-space entry into a grinding mill requires a formal permit, atmospheric testing, and a standby rescue team.
- Lock out all drives and physically block the pinion before any personnel enter the mill shell.
- Loose liner plates can shift unpredictably. Avoid standing directly below unsecured liner plates.
Procedure
- 1
Stop the mill and lock out all drives.
Warning: Confined-space entry into a SAG mill shell requires a permit, a rescue plan, and atmospheric testing. Do not enter without the full permit. - 2
Follow the confined-space entry procedure to enter the mill shell.[1]
- 3
Torque-check each liner bolt with a calibrated torque wrench. Start from the center of each liner plate and work toward the edges.[1]
- 4
Tighten all bolts that are below specification to the required torque.
- 5
Replace any bolt that is cracked, elongated beyond the yield mark, or has a damaged thread.
- 6
Inspect the liner plate for cracking at the bolt holes. A cracked liner must be replaced at the next liner change.
- 7
After re-torquing, run the mill for 24 hours, stop, and re-check torque on all bolts. New liners typically require a hot re-torque after the first run.
Sources
Metso Outotec Metso SAG / Premier mill Grinding Mill (SAG / Ball) general technical documentation, Metso Outotec
Metso Outotec grinding mill operation and maintenance manual, liner installation and bolt torque procedures (general)
More guides for Metso Outotec Metso SAG / Premier mill
How to diagnose girth gear and drive vibration on a Metso SAG / Premier mill
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.
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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