Quick answer
When a cement‑plant fan shows a steady increase in bearing temperature while vibration remains within normal limits, the first step is to confirm that the temperature rise is not a symptom of a hidden mechanical issue. Follow a systematic isolation, inspection, and verification process that checks lubrication quality, bearing alignment, cooling airflow, and electrical drive health before deciding on corrective action. Safe lock‑out/tag‑out (LOTO) must be applied for any physical inspection.
Problem / job
A large axial or centrifugal fan in a cement grinding or kiln exhaust system is reporting a gradual bearing temperature increase (e.g., 5 °C per hour) while the vibration spectrum stays within the plant’s normal alarm thresholds. The maintenance team must determine whether the temperature trend signals an imminent bearing failure, a reversible condition, or a monitoring artifact, and must decide on the appropriate corrective path without unnecessary downtime.
Evidence
- Temperature trend: steady rise over several operating hours, recorded by the plant’s condition‑monitoring system.
- Vibration: RMS and FFT values remain stable; no new frequency components appear.
- Lubrication: oil pressure and temperature within normal ranges, but oil analysis may be pending.
- Operating conditions: fan load, inlet pressure, and ambient temperature unchanged during the observation period.
- Electrical drive: motor current and power factor nominal; no fault codes logged.
Decision
Proceed with a seven‑step diagnostic workflow that isolates the fan, validates sensor integrity, checks lubrication and cooling pathways, and evaluates mechanical alignment. If the investigation confirms a reversible cause (e.g., inadequate cooling airflow or degraded oil), schedule corrective maintenance during the next planned outage. If the cause points to bearing wear, mis‑alignment, or shaft damage, plan a controlled shutdown and bearing replacement.
Workflow
- Verify data integrity. Cross‑check the temperature reading against a handheld infrared thermometer or a calibrated thermocouple placed on the bearing housing. Confirm that the vibration data are from the correct sensor and that the trend line has not been corrupted by a logging glitch.
- Apply safe isolation. Follow the plant’s LOTO procedure: de‑energize the motor, lock the power circuit breaker, tag the isolation point, and verify zero voltage with a test meter before proceeding.
- Inspect lubrication system. Drain a sample of bearing oil for analysis (viscosity, water content, metal particles). Check oil level, pump pressure, and filter condition. Replace oil or filter if any parameter deviates from the OEM‑specified range.
- Examine cooling airflow. Remove any access panels and verify that the fan’s internal cooling ducts are free of dust, clinker debris, or mis‑routed ductwork. Measure airflow with a calibrated anemometer; compare to the design flow rate documented by the OEM.
- Check bearing alignment and shaft runout. With the shaft stationary, use a dial‑indicator or laser alignment tool to measure radial and axial runout. Compare readings to the alignment tolerances provided by the bearing manufacturer. Document any deviation.
- Assess electrical drive health. Review motor current, voltage, and power factor logs for subtle trends. Perform a motor insulation resistance test and a bearing‑current monitoring check if the drive supports it. Record any anomalies.
- Determine corrective action. Based on the findings, decide whether to:
- Refresh lubrication and clean cooling passages (re‑commission after LOTO release);
- Adjust alignment or replace worn shims (requires disassembly and re‑assembly);
- Plan a bearing replacement if wear particles exceed the OEM’s recommended limit or if runout exceeds alignment tolerances.
Document the decision, schedule the work, and communicate the plan to operations.
Diagnostic matrix
| Evidence | Likely direction | Next check |
|---|---|---|
| Steady temperature rise, vibration stable | Thermal issue (lubrication, cooling, heat‑transfer blockage) | Validate sensor accuracy; inspect oil condition and cooling airflow. |
| Oil temperature higher than bearing temperature | Lubricant degradation or insufficient flow | Measure pump pressure; sample oil for viscosity and contamination. |
| Cooling air velocity below design value | Obstructed ductwork or fan‑blade fouling | Clean ducts, verify blade condition, re‑measure airflow. |
| Runout exceeds OEM alignment tolerance | Mis‑alignment causing localized heating | Realign shaft, re‑measure runout, monitor temperature post‑alignment. |
| Metal particles in oil sample | Early bearing wear despite stable vibration | Schedule bearing inspection/replacement; monitor vibration for emerging frequencies. |
| Motor current trending upward | Increased mechanical load or bearing friction | Confirm load conditions; check for shaft binding or bearing preload issues. |
| No abnormal findings after checks | Transient thermal drift or sensor drift | Continue trend monitoring; set a tighter alarm threshold if appropriate. |
Acceptance criteria
After corrective action, the fan bearing must meet the following measurable conditions before the unit is returned to service:
- Temperature: Bearing temperature stabilizes within the OEM‑specified operating range for at least three consecutive operating hours. Verify with both the plant’s monitoring system and an independent handheld probe.
- Vibration: RMS and peak‑to‑peak values remain below the alarm threshold defined in the plant’s vibration‑monitoring plan. No new frequency components appear in the FFT.
- Lubrication: Oil analysis shows viscosity, water content, and particle count within the limits set by the bearing manufacturer. Oil pressure stays within the pump’s rated range.
- Cooling airflow: Measured airflow at the bearing housing matches or exceeds the minimum flow rate documented by the OEM for the given fan speed.
- Alignment: Shaft runout and bearing preload are within the tolerances specified by the bearing supplier. Document the final alignment readings.
- Electrical: Motor current, voltage, and power factor are consistent with baseline values recorded during normal operation. Insulation resistance meets the minimum value required by the motor manufacturer.
Only when all criteria are satisfied should the LOTO be removed, the lockout devices released, and the fan placed back into production.
Common mistakes and repeat‑failure traps
- Ignoring sensor drift. Relying on a single temperature probe without cross‑checking can mask a faulty sensor. Always validate with a secondary measurement device.
- Skipping lubrication analysis. Replacing oil without confirming contamination or viscosity can allow wear particles to continue damaging the bearing.
- Overlooking airflow blockage. Dust and clinker buildup in cooling ducts is a frequent cause of hidden heat buildup; cleaning must be part of every temperature‑rise investigation.
- Adjusting alignment without proper measurement tools. Using visual cues instead of calibrated alignment equipment can introduce new mis‑alignment, leading to recurring temperature spikes.
- Failing to re‑establish LOTO after partial work. Performing a quick visual check and then resuming operation without re‑locking out the motor can expose personnel to moving parts and cause premature bearing damage.
- Assuming stable vibration eliminates mechanical issues. Bearing wear can progress without generating detectable vibration until a critical stage; metal‑particle analysis is essential.
- Delaying corrective action. A modest temperature rise can quickly become a severe overheating event if the root cause (e.g., cooling blockage) is not addressed promptly.
By systematically avoiding these pitfalls, maintenance teams reduce repeat failures and extend bearing service life.
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