Wear in a Main Bearing During Planned Maintenance
Your vessel is in dry dock for its five-year special survey. The main engine is a six-cylinder four-stroke medium-speed diesel. During inspection, No.4 main bearing shell shows scoring and a wear depth of 0.08 mm, against a maximum allowable of 0.10 mm. The crankshaft journal shows slight scoring but is within tolerance. The classification society surveyor is on board. The spare bearing shell on board is of the correct size but has been in storage for four years. The owner wants to avoid the cost of a new shell and the delay of ordering one.
1. What is your assessment of the bearing condition and the spare shell?
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Bearing condition:
- The wear depth of 0.08 mm is within the 0.10 mm limit, but it is close to the limit and the shell is scored. Scoring indicates contamination or inadequate lubrication. The cause must be found, or a new shell will wear rapidly.
- The crankshaft journal scoring is within tolerance, but it should be polished to remove high spots that could damage a new shell.
Spare shell:
- A shell stored for four years may have deteriorated. Check for corrosion, distortion, and contamination. Check that the shell is the correct size and part number. Measure the wall thickness and check for ovality.
- The shell should be inspected by the surveyor before fitting. If there is any doubt, it should not be used.
Recommendation:
- The wear is within limits, so the existing shell could remain in service if the cause of scoring is corrected and the wear rate is monitored. However, the owner's desire to avoid cost must not override safety.
- If the spare is sound, fit it and keep the worn shell as a spare. If not, order a new shell. The surveyor's acceptance is required.
2. The owner instructs you to refit the worn shell and continue. How do you respond?
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You must not accept an instruction that compromises safety.
- State the facts. The shell is within wear limits but is scored, and the cause of scoring has not been identified. Refitting it without finding the cause risks rapid deterioration and possible bearing failure at sea.
- Explain the consequences. A main bearing failure can cause crankshaft damage, engine seizure, and loss of propulsion. The cost of a new shell is small compared with the cost of a failure.
- Involve the surveyor. The classification society surveyor has the authority to reject the shell. Refer the matter to the surveyor and to the chief engineer.
- Document. Record the owner's instruction, your advice, and the surveyor's decision in writing.
- Escalate. If the owner persists, report through the company's safety management system. The master and the designated person ashore must be informed. The engineer's professional obligation is to safety of life and the vessel, not to cost saving.