Friday, April 24, 2020

Criteria for re use or replacement of Aux engine connecting rod

The power generated by the piston in the combustion chamber of marine engine is transferred to the crankshaft using the connecting rod. While transferring this tremendous power, the connecting rod itself is subjected to different stresses, which makes is vulnerable to damage. For this reason connecting rod bolts are used to join the connecting rod and con-rod bearing cover under the tremendous stresses generated by the running engine.
The most common type of connecting rod that is used in the ship’s auxiliary engine is oblique or cross-cut connecting rod which is made of two parts.
The connecting rod of the engine must be inspected at regular intervals of time to ensure there is no defect or problem to avoid any future accidents/emergency. It’s the knowledge and skills of the engineer that decides whether to use the same connecting rod or to renew it with the spare one.
Following points must be considered when checking the connecting rod for re-use or replacement:
1. Check the ovality of the connecting rod: Check the ovality of the connecting rod by tightening both parts at its rated torque. Inside micrometer is used to determine the correct and current ovality of the connecting rod. If the ovality is out of limits, the connecting rod is not to be reused.

2. Check connecting rod for fretting and corrosion: Connecting rod to be checked for fretting and corrosion. If fretting is small, it can be removed by using oilstone. Never use grinder or scrapper for this purpose. Con-rod with severe fretting and racks must be discarded.
3. If connecting rod with fretting is to be used, check for cracks: Minute fretting can be tolerated over the connecting for reuse only if the surface does not have any cracks. If the fretting is in small area and oil stone is used to treat the same, the complete area must be checked again by using magnaflux which detects small hairline cracks which are not visible to the naked human eye.
4. Check cracks between connecting rod and bearing cover: The serration and bore between connecting rod and bearing cover to be checked for cracks by using die penetrating
5. Renew connecting rod and connecting rod bearing cover together:Connecting rod and connecting rod bearing cover to be renewed as a whole. Never renew single part of the connecting- rod in case of damage as the serrations are machined in pair to locate the two halves relative to one another.
6. Re-machining of serrations should not be done: Re-machining of serrations of connecting rod /bearing cap or bore should not be done even if there is minute damage or indentation
crack detection test. Rod to be used only when there are no cracks detected. If there are small cracks in the connecting rod bearing cap serration, renew it with new spare one.

7. Bearing shell with fretting at its back should not be used: Bearing shell with fretting at it’s back side must not be used with connecting rod and it is to be renewed in set.
8. Ensure bolts and bearings are of same type during replacement: If the connecting rod is replaced, it should be of same type and same con-rod bolts and bearings to be used. Ensure to read the older generator data to confirm the bearing size (undersize or oversize) to be fitted while renewing the shell.
9. Replace Connecting Rod Bolts in Sets: Connecting rod bolts to be used and replaced in sets. If the connecting rod is renewed with the spare one, the con-rod bolts from the old con-rod must not be used with the new spare connecting rod.
10. Correct sequence and tightening torque: Once it is decided to reuse or replace the connecting rod, the engineer office must ensure that he knows the correct assembling sequence
(front/ back side, pair of con-rod and bearing cap etc. ) and rated tightening torque is applied on the con-rod bolt with step- by-step tightening procedure as listed in the manual.
The assessment of connecting rod for reusing/ discarding is a critical decision to make for the ship’s engineer officer. A slight mistake in making the correct choice may lead to major generator damages and sometimes even causality.
The above mentioned are some of the important basic checks which should be performed by the engineer to upkeep the generator’s long life and efficient performance

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