Pellet Mill Roller Bearing Failure: Causes and Checks
News 2026-10-29
A pellet mill roller bearing that has failed is usually the last item in a chain that started earlier. A bearing manufacturing defect, a sudden impact, and a seal failure can also produce a sudden failure, and the failure mode and the bearing record separate the chain failure from the sudden one. The bearing carries the roller load, and it fails when the lubrication is lost, the load is beyond the rating, the fit is wrong, or the contamination reaches the raceway. The failed bearing shows which of these happened, and the failure mode and the cause relation is read from the failure mode table, the bearing supplier documentation, and the maintenance record. A replacement bearing installed without correcting the chain fails the same way.
This article is written for plant operators and maintenance teams who have to decide why the bearing failed, whether the roller or the housing is also damaged, and what to correct before the replacement is fitted. It covers the failure modes, the checks on the failed bearing and the machine, the fit and the lubrication, when to stop the machine, and what to confirm with the supplier.
Failure Mode and What It Points To
The bearing surface and the damaged part point to the cause.
| What the bearing shows | Likely cause | What to check |
|---|---|---|
| Raceway discoloured, blue or brown, with the lubricant carbonised | Lubrication loss or overheating | The lubricant, the grease interval, and the bearing temperature |
| Raceway with a fatigue spall, or a flaking surface | Load beyond the rating or a fatigue life reached | The roller load, the clearance, and the bearing hours |
| Cage broken or deformed | Impact, misalignment, or a seizure | The roller rotation, the alignment, and the fit |
| One side of the raceway worn, the other side clean | Misalignment or an incorrect fit | The shaft and the housing fit, and the alignment |
| Rust or water marks on the raceway | Moisture ingress or a failed seal | The seal, the washdown, and the storage |
| Contamination embedded in the raceway | Dust, sand, or metal particles | The seal, the cleaning, and the material handling |
A discoloured raceway points to a lubrication or an overheating cause. A fatigue spall points to a load or a life cause. A broken cage points to an impact, a misalignment, or a seizure. A one-sided wear points to a misalignment or a fit. Rust points to moisture. Embedded contamination points to a seal or a cleaning cause.
Fatigue Spall: Load Beyond the Rating or Fatigue Life
The two causes look the same on the raceway, and the machine record separates them. A load beyond the rating usually comes with an overload record, a die blockage record, or an excessive feed record. A fatigue life reached usually comes with the bearing hours close to the rating life, with the normal lubricant and the normal load. The bearing supplier documentation and the maintenance record are the reference for the rating life and the load history.
Bearing Type and What It Changes
The bearing type changes the failure mode and the inspection. The machine manual and the bearing supplier documentation take precedence over this section.
| Bearing type | Where it is used | What to check when it fails |
|---|---|---|
| Deep groove ball bearing | A lighter roller load, or a smaller roller | The raceway and the ball condition, and the internal clearance |
| Cylindrical roller bearing | A roller with a high radial load | The roller and the raceway, and the axial guidance |
| Spherical roller bearing | A roller with a high load and a possible misalignment | The misalignment tolerance, the raceway, and the seal |
A deep groove ball bearing tolerates less load than a cylindrical or a spherical roller bearing, and a spherical roller bearing tolerates more misalignment than the other two. The bearing type should not be changed without the machine supplier and the bearing supplier confirming the load and the fit.
Causes of a Roller Bearing Failure
Lubrication Loss or Overheating
Why it fails: The bearing needs a lubricant film between the rolling elements and the raceway. When the lubricant is insufficient, degraded, or the wrong grade, the film breaks down, the metal contacts the metal, and the surface overheats. The raceway discolours, and the bearing fails.
What to check: The lubricant type and grade, the grease interval and quantity, the bearing temperature, and the seal condition.
What to do: Restore the lubricant to the machine manual specification, correct the interval and the quantity, and replace the seal that has failed. Do not mix different grease grades, because the thickener and the base oil can be incompatible, and the compatibility should be confirmed with the grease supplier documentation and the machine manual before the mix.
Load Beyond the Rating or a Fatigue Life
Why it fails: The bearing is rated for a load, and a load beyond the rating accelerates the fatigue. The roller load rises when the die is blocked, when the feed is excessive, or when the roller clearance is wrong. A bearing that has reached its fatigue life fails with a spall on the raceway, without a lubrication fault.
What to check: The roller load, the die condition, the feed, the roller clearance, and the bearing hours.
What to do: Correct the load cause before the replacement is fitted, and set the roller clearance to the machine manual. A replacement bearing under the same overload fails the same way.
Incorrect Fit or Misalignment
Why it fails: The bearing inner ring must fit the shaft, and the outer ring must fit the housing, within the tolerance stated in the machine manual. A fit that is too loose allows the ring to creep, and a fit that is too tight reduces the internal clearance. A misalignment loads one side of the raceway, and the bearing fails at that side.
What to check: The shaft and the housing dimensions, the fit tolerance, and the alignment of the roller assembly. The shaft is measured with a micrometer at several points, and the housing is measured with a bore gauge, and the two are compared with the manual tolerance.
What to do: Correct the shaft or the housing to the manual tolerance, and align the roller assembly. A bearing installed on a worn shaft or in a worn housing fails early, even with the correct lubricant.
Contamination and Moisture
Why it fails: The dust, the sand, and the metal particles that reach the raceway act as an abrasive, and the water that reaches the bearing causes rust. Both reduce the bearing life, and both come through a failed seal, a washdown, or a storage condition.
What to check: The seal condition, the cleaning method, the washdown practice, and the storage. The seal type changes the protection: a lip seal is common on the roller, a labyrinth seal is used where the dust is heavy, and a V-ring seal is used where the water is present.
What to do: Replace the seal, correct the cleaning and the washdown, and store the spare bearing in a dry place. The seal is part of the bearing life, not an accessory.
Checks on the Failed Bearing and the Machine
The checks support the cause and the replacement decision.
| Check | How to make it | What it tells you |
|---|---|---|
| Bearing rotation | Rotate by hand with the machine locked out | Seizure, roughness, or free rotation |
| Raceway and cage | Visual inspection after cleaning | The failure mode and the cause |
| Lubricant condition | Colour, texture, and contamination | The lubrication cause |
| Seal condition | Visual inspection | The contamination or moisture cause |
| Shaft and housing fit | Measure the shaft with a micrometer and the housing with a bore gauge, against the manual | The fit cause |
| Roller alignment | Check the roller assembly against the manual | The misalignment cause |
| Bearing temperature record | The record before the failure | The overheating cause |
The bearing is cleaned and inspected after the removal, and the raceway, the cage, the lubricant, and the seal are recorded. The shaft and the housing are measured before the replacement is fitted, because a new bearing in a worn housing fails early. The shaft and the roller are inspected after the bearing removal, and the surface condition, the dimension, and the alignment are recorded against the machine manual.
When to Stop the Machine
Stop the machine when any of the following applies.
| Condition | Action |
|---|---|
| Bearing noise with a temperature rise | Stop. Inspect the bearing before it seizes. |
| Roller does not rotate freely | Stop. Inspect the bearing and the shaft. |
| Grease leaking from the seal, or the seal damaged | Stop. Replace the seal before the contamination reaches the bearing. |
| Bearing housing hot to the touch, or the temperature above the manual limit | Stop. Inspect the bearing and the lubrication. |
| Vibration or a new noise from the roller assembly | Stop. Inspect the bearing, the roller, and the alignment. |
A bearing that is allowed to seize damages the shaft and the roller, and the repair becomes a shaft and roller replacement rather than a bearing replacement.
Preventing a Repeat Failure
The replacement bearing is the opportunity to correct the chain.
- Use the lubricant grade and the interval stated in the machine manual, and record the greasing.
- Check the seal at each maintenance, and replace it before it fails.
- Measure the shaft and the housing before the replacement, and correct the fit.
- Keep the roller clearance and the alignment to the manual specification.
- Keep the material cleaning stage working, so that the sand and the metal do not reach the die and the roller.
- Record the bearing temperature and the roller rotation at each shift.
What to Confirm with the Supplier
The replacement bearing is specified against the machine and the application.
| Item to confirm | What to provide |
|---|---|
| Machine model | The model and the roller configuration |
| Bearing designation | The bearing number and the supplier |
| Bearing type and clearance | The type and the internal clearance required |
| Lubricant | The grade and the interval stated in the manual |
| Seal | The seal type and the replacement interval |
| Failure record | The failure mode, the bearing hours, and the temperature record |
A bearing life figure should state the load, the speed, the lubricant, and the operating conditions it assumes. Where the figure does not state those conditions, it is a reference rather than a commitment, and the actual bearing record is the basis for the next replacement.
Frequently Asked Questions
How often should the roller bearing be greased?
The interval and the quantity should come from the machine manual, and they depend on the bearing type, the load, the speed, and the lubricant. The greasing should be recorded, and the interval should be shortened where the temperature or the load is high.
When should I stop the machine instead of monitoring the bearing?
Stop when the bearing noise is accompanied by a temperature rise, when the roller does not rotate freely, when the seal is leaking, or when the housing is hot. A bearing that is allowed to seize damages the shaft and the roller.
How do I know whether the bearing failed from lubrication or from overload?
A discoloured raceway with carbonised lubricant points to a lubrication or an overheating cause, and a fatigue spall with the normal lubricant colour points to a load or a life cause. The raceway, the lubricant, and the temperature record separate the two.
Can I replace only the bearing and keep the roller?
The roller can be kept when the shaft and the housing are within tolerance, the roller surface is not damaged, and the seal seat is intact. A roller with a damaged shaft or a worn housing needs the shaft or the housing corrected before the new bearing is fitted.
Why did the new bearing fail quickly?
A new bearing that fails quickly is running with the wrong lubricant, a wrong fit, a misalignment, or a damaged seal. The shaft and the housing should be measured, the alignment checked, and the seal replaced before the bearing is installed.
Request Roller and Bearing Support
Send the machine model, the bearing designation and type, the failure mode, the bearing hours, the lubricant grade and interval, the seal condition, the shaft and housing measurements, and the temperature record. A roller and bearing review can then be prepared against the machine and the application rather than against a general catalogue.
Bearing life, clearance, lubricant grade, and acceptance criteria in this article must be confirmed against the machine manual, the bearing supplier documentation, and the maintenance record for the same application. This article does not provide universal values for these items.
