What are the things to check in connecting rod at the time of unit overhaul?

The connecting rod consists of an eye at the small end to accommodate the piston pin, a long shank and a big end opening split into two parts to accommodate the crank pin. The construction of connecting rod is illustrated in Fig. 1. The basic function of the connecting rod is to transmit the push and pull forces from the piston pin to the crank pin. The connecting rod transmits the reciprocating motion of the piston to the rotary motion of the crankshaft. It also transfers lubricating oil from the crank pin to the piston pin and provides a splash or jet of oil to the piston assembly. The connecting rod of an IC engine is made by the drop forging process and the outer surfaces are left unfinished. Most internal combustion engines have a conventional two-piece connecting rod. The whole rod is forged in one piece; the bearing cap is cut off, faced and bolted in place for final machining of the big end. The small end of the rod is generally made as a solid eye and then machined.

 2 stroke and 4 stroke connecting rod differences.

2 stroke connecting rod –

1. 2 stroke connecting rod is of split type (means two halves for each small and big end bearing)

2. Round mid section changes to a rectangular palm section at the bearing ends by means of the elliptical fillet shape

3. A round section is cheaper to manufacture

4. 2 stroke connecting rod have telescopic pipe

5. Only compressive forces are acting on 2 stroke connecting rod

6. Connecting rod connects to the piston rod through the cross head pin.

4 stroke connecting rod –

1. Only the big end bearing are of split type.

2. Oblique cut is provided in the crank pin side so that it can be taken out easily with the piston from the cylinder head side because in previous design it was not possible to take out the connecting rod directly from the upward side because it width is more that was the reason we provide oblique cut

3. The top end bearing may be of bush type bearing.

4. It is subjected to high compressive and low tensile stresses of bending as well as axial type.

5. It connects the crank pin directly to the piston gudgeon pin.

Note – Surveyor mostly ask that why serration is been given in the bottom end of connecting rod in 4 stroke engine, the reason to give serration at that place so that they can take the shear stress because function of cut and bolt is to join the two surface not to take the stress so that could be taken care by serration.

What are the things to check in connecting rod at the time of unit overhaul?

Ans – This question is very frequently asked by the surveyor and there could be a number of things that you can check in connecting rod but the surveyor wants to listen the three main points which I covered below –

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.

Note – Max ovality allowed = 0.05 mm (Standard Value, may change depending upon your engine specification)

2. Bend and Twist:

Second thing that you need to check in the connecting rod is the bending of the connecting rod for that you need to take a very fine rod and put that rod in the drill passage which was given for the flow of oil. If it move inside the passage without any restriction or it goes freely then everything is okay

3.Serration Crack Test:

Third and the most important thing that you need to check in the connecting rod is the serration which can take the shear stress, to check the serration you need to perform the dye penetrant test

Note – The ovality of big end bearing is negative because of the serration

Other Important Checking Points Are :

4.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.

5.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.

6.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 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.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.

8.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.

9. 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.

10.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.

11.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.

12. 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.