Recently a marine engine has been needed a large scale to have higher power since vessels are needed to be bigger size to transfer more baggage quickly. According to this need, the marine engine has required more cylinders and bigger bore. Therefore present time, shafting system suffers from 2 node vibration that has never occurred before for the marine engines which are below 7 cylinders.
2 node vibration occur in a crankshaft and a propeller shaft. It is different with 1 node vibration which normally occur in intermediate shaft of the propulsion shafting system. The crankshaft can be mainly damaged only since the crankshaft is weaker in torsional vibration than propeller shaft so the limit of IACS is stricter for crankshaft.
Generally a solution of the problem by 2 node vibration is what a vibration damper is applied. However, if the engine has a higher power then now engine, the size of damper will be bigger to have high damping coefficient so it will be able to affect to a bearing with heavy weight. Also the cost is high to buy and to install and the time limit for delivery is needed almost an year from a decision to delivery.
By reason of these, engine maker should try to find other solutions to solve problem of 2 node vibration without the vibration damper.
According to these reasons, the other solution was reviewed on this paper. That is the method to reduce the torsional stress in a crankshaft by increasing a diameter of journal and pins synchronously at the position where the maximum torsional stress is occurred.
The maximum torsional stress can be occurred at a node but according to excitation characteristics, it can be occurred the other position. Because the position of the maximum torsional stress can be changed occasionally, so the characteristics have to be confirmed.
The node position should be defined since the node surely can affect to the crankshaft with high vibration and if there are some mistakes with choosing the position to increase the diameter of journal and pins, the maximum torsional stress can be increased.
This study is explained for two cases. One of those is the case of what the node position is different with the maximum torsional stress and the other one is what the node position and the maximum torsional stress position are the same.
For two cases, the first case is needed to apply a tuning wheel to change the position of node to have same position with the maximum torsional stress position since it is effective and easy to control 2 node vibration but in case of 12K98MC-C, the initial maximum torsional stress is too high in the crankshaft and several cranks have similar level to the maximum torsional stress. Owing to such a property, it is not able to control with only increasing the diameter of journal and pins. It should need a torsional vibration damper to reduce.
For the second case, two adapted shafting systems are all controlled with increasing the diameter and changing the mass inertia of moment of flywheel.
The effect of proposed method has been confirmed through this study and it will be able to be applied with a vibration damper if the engine which has much higher power is developed.
For the future plan, the exact position of the node should be defined through FEM to increase only a diameter of a journal or a pin and before adopting this method, it has to be confirmed the validity of manufacture at design stage and characteristics of engine structural vibration with increasing diameter of crank, etc.