Ball mill bearings: arrangement, lubrication and failures
A cement ball mill is a rotating drum of steel balls and cement clinker. It turns on trunnions at each end and is driven through a girth gear. Trunnion supports, trunnion seals, the gear drive and the jacking system are the key lubrication points.
Typical duty
| Speed | Load | Temperature | Typical life |
|---|---|---|---|
| Very low (mill shell about 15 rpm) | Very heavy, with continuous impact | Warm, 50 to 80 °C at the bearing | Planned around a shutdown |
How the bearings are arranged
Trunnion support · oil film bearings at each end
Used when: The two ends of the mill drum
The drum is carried on two hollow trunnions, each resting in a large trunnion bearing. A jacking system lifts the trunnion with a high-pressure oil pump before start so the oil film forms before rotation.
Girth gear drive · pinion bearings and open gear
Used when: Gear around the drum
A girth gear on the drum meshes with a pinion driven through a gearbox. The pinion shaft turns in heavy spherical roller bearings. The open gear is lubricated by spray.
Common failures and how to prevent them
| Failure | What you see | Why it happens | Prevention |
|---|---|---|---|
| Contact at start from a missing oil lift Adhesive wear (smearing & skidding) | Smeared trunnion surface | Jacking pump not run before start | Interlock the jacking pump with the start |
| Dust entering through trunnion seals Abrasive wear | Scored surfaces, dirty oil | Worn trunnion seals | Replace seals on condition and filter the oil |
| Oil too hot or flow too low Overheating & lubricant breakdown | High bearing temperature | Blocked cooler or low flow | Monitor oil temperature and flow |
Lubrication
Trunnion bearings
Circulating oil with filtration and temperature control; jacking oil at start.
Girth gear
Spray open-gear grease.
Gearbox and motor
Per their own schedules.
Sealing
- Trunnion seals: Keep fine cement dust out of the oil and the bearing.
What to watch
- Bearing oil temperature and flow.
- Trunnion seal condition.
- Gearbox vibration and oil analysis.
Questions
Which bearings are used in ball mills?
Trunnion support · oil film bearings at each end: The two ends of the mill drum. Girth gear drive · pinion bearings and open gear: Gear around the drum.
What are the common bearing failures in ball mills?
Contact at start from a missing oil lift: Jacking pump not run before start. Dust entering through trunnion seals: Worn trunnion seals. Oil too hot or flow too low: Blocked cooler or low flow.
How should ball mills bearings be lubricated?
Trunnion bearings: Circulating oil with filtration and temperature control; jacking oil at start. Girth gear: Spray open-gear grease.