Crankshaft Deflection Readings After Overhaul
A 6-cylinder medium-speed auxiliary engine has just completed a top-end overhaul. During the sea trial, the engine runs at 750 rpm with a load of 85%. The vibration monitoring system shows an increase in overall vibration from 2.1 mm/s to 4.8 mm/s, and the crankshaft deflection readings, taken before and after the overhaul, differ by up to 0.06 mm on cylinder #3. The engine maker's limit for deflection change is 0.03 mm. The engine has run for 18,000 hours since the last major overhaul. The chief engineer must decide whether to continue the voyage, which has 9 days remaining, or to stop for investigation. The vessel is carrying perishable cargo, and the next port with dry-dock facilities is 4 days away.
1. How should you interpret the deflection and vibration data, and what are the possible causes?
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Interpretation: A deflection change of 0.06 mm exceeds the maker's 0.03 mm limit, indicating possible misalignment or structural distortion. The vibration increase from 2.1 to 4.8 mm/s supports this. However, readings can be affected by temperature, engine load, and measurement technique, so repeat the readings under identical conditions and compare with baseline data.
Possible causes:
- Incorrect bearing clearance or bearing shell installation during the overhaul.
- Distorted bedplate or foundation bolts not tightened to the correct torque.
- Crankshaft or connecting rod misalignment.
- Faulty deflection gauge or measurement error.
- Thermal effects from the overhaul, requiring a settling period.
Action: Repeat the deflection readings after the engine has cooled to the same reference temperature, check bearing clearances and foundation bolt torque, and review the overhaul records. If the readings remain out of limit, do not continue at full load.
2. What decision would you make about continuing the voyage, and how would you manage the risk?
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Decision: Do not continue at 85% load with deflection beyond the maker's limit. Reduce load to a level where vibration and deflection are stable, and monitor continuously. The 4-day passage to a dry-dock port is feasible only if the engine can be operated safely at reduced load.
Risk management:
- Reduce load to 60% or lower and monitor vibration, exhaust temperatures, and deflection at intervals.
- Inform the master, charterer, and company; document the technical justification.
- Arrange for the engine maker's service engineer to attend at the next port.
- Prepare contingency plans for a further reduction or shutdown, including tug assistance and cargo care.
- If vibration increases or deflection worsens, stop the engine and proceed on the other engine if available.
Documentation: Record all readings, decisions, and communications in the engine-room log and the planned maintenance system.