Main Bearing Temperature Rise After an Overhaul
After a bearing replacement during a planned outage, a 500 kW reciprocating engine shows a main bearing temperature of 78 C after two hours at full load, against a normal 55-60 C. Oil pressure is normal at 2.5 bar, oil flow to the bearing is confirmed, and the oil cooler outlet is 42 C. The bearing was scraped and bedded by a contractor; the crankshaft journal was measured within tolerance but with 0.03 mm taper. Clearance was set at the upper limit of the manual range. Vibration readings are unchanged from before the overhaul. The engine has run 6 hours since restart.
1. Rank the plausible causes of the temperature rise and justify your ranking.
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Ranked most to least likely:
- Bearing fit and contact pattern. A hand-scraped bearing can have high spots and insufficient contact area, concentrating load and raising local temperature. This is the most common post-overhaul cause and fits the timing.
- Journal taper. 0.03 mm taper across the journal means the bearing clearance varies along its length; the tight end runs hotter and can carry disproportionate load. Within tolerance does not mean ideal, especially combined with upper-limit clearance.
- Clearance at upper limit. Maximum clearance reduces oil-film pressure and can allow the journal to run with less effective hydrodynamic support, raising temperature rather than lowering it.
- Oil supply. Pressure and cooler outlet are normal, so bulk supply is probably adequate, but flow to that specific bearing should still be verified at the bearing, not only at the header.
- Alignment and vibration. Unchanged vibration argues against a new misalignment, but a hot bearing can be an early alignment symptom that vibration has not yet picked up.
Vibration being unchanged makes gross misalignment or a damaged journal less likely as the primary cause.
2. State the immediate operating decision and the inspection plan, including acceptance criteria.
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Immediate decision: 78 C is a significant rise but may be within the bearing's alarm limit; check the manual. If the alarm is, say, 80 C, the engine may continue at reduced load with enhanced monitoring, trending temperature every 15 minutes and stopping on a rising trend or on reaching the alarm. Do not simply run to failure; a bearing failure can damage the journal and crankcase.
Inspection plan:
- Reduce load and record temperature vs load to see whether the rise is load-proportional (fit/clearance) or present even at light load (oil supply or alignment).
- Verify oil flow at the bearing and check oil condition and filter differential pressure.
- On shutdown, remove the bearing cap and inspect contact pattern with bluing; check for high spots, wiping, and edge loading.
- Re-measure journal taper and bearing clearance in place; check bearing shell crush and cap bolt torque.
- Check alignment with the driven machine if the temperature does not fall after bearing correction.
Acceptance criteria: stable temperature within the normal band at full load, even contact pattern over the specified area, clearance within the manual range, and no rising trend over a sustained run.