Quick answer
A ball-mill main bearing that runs hotter after a shutdown and restart should be diagnosed from the restart transient, not only the final temperature. Compare oil flow, oil temperature, bearing inlet/outlet temperatures, mill load, cooling-water condition, bearing lift/jacking sequence and shaft movement before and after the stop. Common causes include cold or viscous oil, delayed oil flow, cooling problems, debris after maintenance, misalignment changes, overloaded bearing contact or an abnormal restart sequence.
Problem / job
Bearing temperature climbs faster or stabilizes at a higher value after restarting the mill.
Evidence
Capture oil pressure/flow, oil inlet temperature, bearing metal temperatures, cooling-water condition, mill load, restart timing and any jacking-oil status.
Decision
Separate lubrication-supply issues from cooling loss, load/alignment changes, contamination and abnormal bearing contact.
As-found evidence capture
Record the temperature rise from the moment the lubrication system starts, through mill start and into steady production. Note oil pressure, oil flow if measured, reservoir temperature, bearing inlet and outlet temperature, cooling-water inlet/outlet condition, mill feed and drive current. Compare the trace with a healthy restart.
Check whether the temperature increase began before load was applied, immediately after rotation started or only as the mill reached production load. The timing is often more diagnostic than the peak value.
Seven-step diagnostic workflow
1. Verify lubrication before rotation
Confirm the main lubrication system achieves the required pressure and flow before the mill rotates. Where a jacking or high-pressure lift system is used, verify its correct sequence and permissives. A delayed or weak oil film at restart can create early friction and heat.
2. Check oil temperature and viscosity condition
Oil that is colder than normal can flow poorly through restrictions, while overheated oil can lose viscosity. Compare reservoir and bearing-inlet temperature with the equipment manufacturer’s operating range.
3. Inspect cooling performance
Check cooling-water valves, strainers, flow and exchanger condition. A restart after maintenance can expose a partly closed valve, air lock or fouled exchanger that was not obvious while the mill was stopped.
4. Compare load and contact condition
Review mill load, feed, drive current and bearing temperature together. If temperature rises only as load increases, investigate bearing contact, alignment and load distribution rather than only the oil circuit.
5. Check for contamination or restriction
Inspect filters, differential pressure, oil cleanliness and any recent maintenance. Debris can restrict flow or damage the bearing surface. Do not repeatedly restart a bearing with unexplained rapid temperature rise.
6. Review alignment and restart changes
Thermal contraction during shutdown, foundation movement, work on couplings or bearing housings and incorrect reassembly can alter alignment. Compare vibration and axial position where instrumentation exists.
7. Prove stable temperature under load
After correcting the identified cause, trend the next restart through full representative load. Acceptance requires a repeatable temperature rise and stable plateau consistent with the mill’s approved baseline, not simply a value below an emergency alarm.
Evidence-to-decision table
| Evidence | Likely direction | Next check |
|---|---|---|
| Temperature rises before production load | Lubrication/cooling/start sequence | Oil flow, jacking system, exchanger |
| Heat rise begins with load increase | Contact/load/alignment | Mill load, alignment, bearing condition |
| Oil pressure normal but flow/temperature abnormal | Restriction or viscosity issue | Filters, oil temperature, lines |
| Both bearings rise similarly | Common oil/cooling/process cause | Shared systems and mill load |
| One bearing alone rises sharply | Local bearing/alignment issue | Local oil path, contact and alignment |
Return-to-service acceptance criteria
Return to normal operation only when oil supply and cooling are stable, the bearing temperature curve is repeatable through full load, vibration and drive load remain acceptable and no abnormal metal or contamination evidence appears in the lubrication system. Use OEM limits for alarm and trip values.
Common mistakes
Do not judge the bearing only by one peak-temperature number. Do not bypass or shorten the lubrication/jacking sequence. Do not increase cooling blindly without checking oil flow and bearing contact. Do not keep restarting after a rapid unexplained heat rise. Finally, do not import temperature limits from another mill design.
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