Quick answer
When a rotary‑kiln support roller bearing shows an abnormal temperature rise after a shutdown‑restart cycle, the first step is to verify that the bearing was fully cooled, properly lubricated, and that the kiln drive alignment has not shifted. Conduct a systematic visual‑inspection, oil‑analysis, and vibration check before re‑energising the drive. If the bearing temperature exceeds the OEM‑specified limit after a short‑run test, isolate the bearing, re‑apply the correct lubricant, and repeat the test. Persistent rise indicates mis‑alignment, inadequate lubrication, or bearing damage and requires corrective repair before returning the kiln to production.
Problem / job
A cement plant reports that the temperature of a support roller bearing on a rotary kiln spikes to a high value within minutes of restarting the kiln after a routine shutdown. The bearing is equipped with a temperature sensor that triggers an alarm when the temperature exceeds the preset alarm point. The plant needs a reliable diagnostic pathway to determine the root cause, decide on corrective action, and verify that the bearing can safely resume service.
Evidence
- Temperature sensor reading: alarm at X °C (value set by plant, verify against OEM limit).
- Shutdown duration: 4 h, kiln cooled to ambient.
- Restart sequence: drive motor engaged, kiln rotation achieved, temperature rise observed within 5–10 min.
- Lubrication record: last oil change 6 months ago, oil type matches OEM recommendation.
- Vibration data: baseline normal; post‑restart vibration amplitude increased.
- Visual inspection: no obvious external damage, but oil film appears thin on bearing race.
Decision
Proceed with a structured 7‑step workflow that isolates the bearing, validates lubrication, checks alignment, and uses targeted measurements (temperature, vibration, oil analysis) to confirm the bearing’s health. If any step reveals a deviation from OEM or site‑specific limits, schedule corrective maintenance before the next production run.
Practical 7‑step workflow
- Secure the kiln and isolate power. Apply lock‑out/tag‑out (LOTO) to the drive motor, gearbox, and any auxiliary power sources. Verify zero voltage with a calibrated tester before proceeding.
- Document the temperature event. Record the exact alarm temperature, time to alarm, ambient conditions, and the operating parameters at the moment of restart (kiln speed, load, fuel rate).
- Perform a visual and tactile inspection. Remove the bearing guard, inspect the raceway for scoring, pitting, or oil leakage. Feel the bearing housing for hot spots with a calibrated infrared thermometer. Note any abnormal oil film thickness.
- Sample and analyse the bearing oil. Drain a small oil sample into a clean container, label it, and send it for laboratory analysis (viscosity, water content, wear metal particles). Compare results with the OEM‑specified oil condition limits.
- Check alignment and preload. Using a dial‑indicator or laser alignment tool, verify that the support roller axis is within the tolerance band specified by the kiln manufacturer. Adjust shims or bearing mounts if mis‑alignment is detected.
- Conduct a controlled restart test. With the bearing isolated, re‑apply the correct lubricant (type and quantity per OEM data). Re‑energise the drive under LOTO supervision, run the kiln at low speed for 5 min, and monitor temperature, vibration, and noise. Record the temperature trend at 30‑second intervals.
- Evaluate results and decide on corrective action. If temperature remains below the alarm point and vibration stays within baseline limits, clear the LOTO and return the kiln to normal operation. If temperature exceeds the alarm point or vibration increases, re‑apply LOTO, remove the bearing for disassembly, and replace damaged components.
Diagnostic matrix
| Evidence | Likely direction | Next check / action |
|---|---|---|
| Temperature spikes within 5 min of restart | Insufficient lubrication or oil film breakdown | Oil analysis; re‑lubricate with OEM‑approved grade; repeat test |
| Thin oil film observed visually | Oil leakage or inadequate oil supply | Inspect oil pump, seals, and supply lines; verify oil pressure |
| Increased vibration amplitude post‑restart | Mis‑alignment or bearing damage | Perform alignment check; run a bearing run‑out test; consider bearing inspection |
| Oil sample shows high water content or metal particles | Contaminated oil or early bearing wear | Replace oil; schedule bearing inspection or replacement if wear particles exceed OEM limit |
| Hot spot detected on bearing housing | Localized overheating due to friction | Disassemble bearing; inspect raceway for scoring; verify proper bearing clearance |
| Alignment measurement outside tolerance band | Improper bearing preload or support frame shift | Adjust shims; re‑measure; verify after adjustment |
| Temperature remains stable after corrective steps | Issue resolved | Document findings; remove LOTO; return to production |
Acceptance criteria
Before the bearing can be cleared for normal operation, the following measurable conditions must be satisfied. All limits should be cross‑checked with the equipment OEM or the plant’s approved maintenance specification.
- Temperature stability: Bearing temperature must remain at least 10 °C below the alarm set‑point for a continuous 30‑minute observation period at normal kiln speed.
- Vibration level: RMS vibration measured at the bearing housing must be within the baseline range recorded during normal operation (typically within ±10 % of the historical average). Any increase beyond the OEM‑defined vibration limit requires re‑evaluation.
- Lubricant condition: Oil viscosity must be within the OEM‑specified range for the operating temperature, and water content must be below the maximum allowable level indicated in the oil data sheet.
- Alignment tolerance: Axial and radial alignment must be within the tolerance band specified by the kiln manufacturer (e.g., ±0.2 mm over the bearing length). Verify with a calibrated alignment tool.
- Seal integrity: No observable oil leakage from the bearing seal after a 15‑minute run at full speed.
- Documentation: All test data, oil analysis reports, and alignment records must be logged in the maintenance management system and signed off by the responsible maintenance engineer.
Common mistakes and repeat‑failure traps
- Skipping LOTO: Performing a restart test without proper lock‑out can expose personnel to unexpected motion and may mask the true temperature trend due to intermittent power cuts.
- Assuming sensor fault: Dismissing the alarm without confirming sensor accuracy can lead to missed overheating. Always verify temperature with an independent handheld infrared thermometer.
- Re‑using old oil: Adding fresh oil to a contaminated or degraded oil bath does not restore proper lubrication. Drain and replace the oil when analysis shows abnormal wear metals or water.
- Neglecting alignment after mechanical work: Any removal of the bearing, support frame, or gearbox can shift alignment. Re‑check alignment after each mechanical intervention.
- Over‑tightening bearing preload: Excessive preload raises bearing temperature and reduces bearing life. Follow OEM torque specifications for bearing caps and retainers.
- Relying on a single temperature reading: Temperature spikes can be transient. Record a temperature profile over time to distinguish a true trend from a momentary anomaly.
- Ignoring vibration trends: Vibration often precedes temperature rise. Failure to monitor vibration can delay detection of mis‑alignment or bearing damage.
By systematically applying the workflow, using the diagnostic matrix, and meeting the acceptance criteria, maintenance teams can reliably identify the cause of post‑restart temperature rise, implement the correct remedy, and prevent recurrence.
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