Mill pinion and counter shaft bearings: arrangement, lubrication and failures
A mill pinion drives the girth gear that turns the mill drum. It sits on a counter shaft that is fed by a coupling from the gearbox or motor. Two bearings carry the shaft, one located and one free. The gear force depends on torque and pinion diameter, and it often exceeds the weights by a factor of ten or more.
Typical duty
| Speed | Load | Temperature | Typical life |
|---|---|---|---|
| About 120 to 600 rpm | Very heavy radial load from the gear mesh, small axial load, shock at start | Bearings about 60 to 90 °C | 100,000 hours is a common design target |
How the bearings are arranged
Two bearings · located and free
Used when: Modern roller bearing arrangement
A spherical roller bearing holds the shaft axially at one end. A toroidal roller bearing at the other end lets the shaft grow with heat. The pinion is placed close to one bearing, so reactions are very uneven.
Three bearings · sleeve bearings
Used when: Older arrangements
Three white-metal sleeve bearings carry the counter shaft. The statics are indeterminate, so alignment of all three decides the load share.
Common failures and how to prevent them
| Failure | What you see | Why it happens | Prevention |
|---|---|---|---|
| Fatigue from an under-estimated gear load Subsurface-initiated fatigue | Spalling on the located bearing in months instead of years. | The gear force was computed with the wrong power, pitch diameter or pressure angle. | Use one set of inputs on every sheet, and recompute from torque. |
| Overheating of the located bearing Overheating & lubricant breakdown | Housing temperature above the other bearing. | Axial load not allowed for, or grease quantity wrong. | Check the equivalent load with the thrust and set the grease interval from the real temperature. |
| Dust damage at the seals Abrasive wear | Scoring and wear marks on the rollers. | Cement dust enters through worn seals near the gear. | Use labyrinth seals with a grease purge and check them at every stop. |
Lubrication
Grease
A high-viscosity EP grease is typical. Check the interval against the real temperature, not the assumed one.
Oil
Oil circulation is an option on the heaviest drives.
Sealing
- Labyrinth with grease purge: Best for the dusty mill house.
- Cover plates: Keep gear lubricant spray away from the bearing.
What to watch
- Bearing temperature on both ends, and the difference.
- Vibration at the gear mesh frequency and its sidebands.
- Shaft growth at the free bearing against its travel.
Questions
Which bearings are used in mill pinion and counter shaft?
Two bearings · located and free: Modern roller bearing arrangement. Three bearings · sleeve bearings: Older arrangements.
What are the common bearing failures in mill pinion and counter shaft?
Fatigue from an under-estimated gear load: The gear force was computed with the wrong power, pitch diameter or pressure angle. Overheating of the located bearing: Axial load not allowed for, or grease quantity wrong. Dust damage at the seals: Cement dust enters through worn seals near the gear.
How should mill pinion and counter shaft bearings be lubricated?
Grease: A high-viscosity EP grease is typical. Check the interval against the real temperature, not the assumed one. Oil: Oil circulation is an option on the heaviest drives.