선박용 대형 저속 2행정 디젤엔진의 크랭크 축계 파손에 관한 연구

Abstract

Since the diesel engine for ship was developed and the world war broke out, the demand for the low cost transportational means has greatly increased, requiring engines having more output and strength. This kind of explosion in demand caused a lot of the unexpected engine problems. Most of the failure led to the temporary stop of operation of ships but there have been large accidents in which case all engine or shafts shall be replaced.

The biggest problem was the accidents related to the breakage of shaft in early 20th century, which led many ship to stop their operations. The accidents were later found out by a German engineer in 1900 to be caused by the torsional vibration. Then, there have been a lot of theories and empirical formula presented, showing the accurateness of more than 95% now. Recently, with the appearance of G-type engine, MDT (MAN Diesel & Turbo) has developed the output of engines ranging from Mark 5 engine to Mark 9 engine. Wärtsilä also has shown the new type engine such as X engine and has developed the mileage and output at the same time. According to the development of new engines, the calculation of torsional vibration has been also complemented.

In addition, since the oil crisis, there have been a great development in the optimal design of crankshaft which takes up a great deal in the total weight of the engines.

But as far as the crankshaft is concerned, as it is to be operated until the end of life and it is exposed to severe environment, there have been a lot of limitations in design. So, IACS established a design standard for the stress of crankshaft in 1986. It is just IACS UR M53. The crankshaft which is supposed to convert the output of engine into the rotational power shall be designed and manufactured to the standard of IACS UR M53. In other words, the crankshaft which is proper for the standard shall have no defect during its operation. But often the failure or slip of the crankshaft makes the ship owner file claims against engine makers. Though there have been intense examinations on the causes of the accident, the results have been often finished as unclear, thus causing the engine maker to lose their trust and image in the industry.

In this paper, the cases of accidents related to the crankshaft during running will be examined to prove that the accidents were not directly related to the crankshaft. Three approaches will be used for the study.

The first chapter will examine the stress calculation of the crankshaft which is specified in IASC UR M53. In addition, the calculation sheets will be prepared and tested through the analysis and measuring for comparison. In case of the allowable factor of the stress of crankshaft, there have been only a little error rate when the value from calculation is compared with the measured values. But in terms of error between the measured value and the analysis value, there have been some discrepancies. The errors are likely to be caused by the fact that the measure values do not take into account the shaft stress concentration factor and that the boundary conditions for the analysis of the crank throw is different from the actual running conditions. As shown in the results above, the error rate between the calculated value not taking into account the shaft direction stress concentration factor and the measured values is little with the rate of within 5%. As the analysis value considering the shaft stress concentration factor has the smallest value. The analysis value is 21.7% above the allowable factor, it is likely that there is no safety problem related to the crank throw.

Chapter 2 examines the stability of crankshaft from the perspective of torsional vibration and further examines the factors damaging the propeller, thus trying to improve the reliability.

The analysis of shaft natural frequencies & related amplitude of shafting system graph and vector summation components shows that the points are formed around 2nd node and 3rd node of No. 6 crank throw. In addition, the 1st node vibration is formed in the inter-shaft. The analysis of the critical revolution and the vector summation components showed that in case of 1st node the 7th order component becomes the biggest at 38.6 rpm, in case of 2nd node, 17th oder component becomes the biggest at 75.6 rpm and 18th order component becomes the biggest at 71.4rpm.

In the actual measuring, the relatively big values appeared in the 7th order component in 1st node, in the 17th order component and 18th order component in 2nd node. It was found out that the there has no error in the discrepancy between analysis values and measured values in the perspective of the torsional vibration.

It was found out that the crank shaft is less affected by the propeller damage than by the inter-shaft and propeller-shaft. It is likely that if there was the increasing torsional stress due to the damage to the propeller, it would likely to directly affect the propeller-shaft.

Chapter 3 deals with the shrinkage fitting of crankshaft. Using the formula and analysis, the values are reviewed and the torque capacity is verified for checking the stability. The theoretical calculation using the shrinkage fitting calculation sheet showed that the results are within the allowable values. In addition, as the maximum stress caused by the shrinkage fitting is larger than the stress value during the maximum torque from the torque capacity, it is confirmed that there is no problem with it.

There have been the conventional calculation methods to check the stability of the crankshaft shaft. But the data which was verified in connection with the actual cases was not easy to get. So, I think that this paper would be helpful. In the modern ship, all aspects of ship are recorded every short period that any momentous torsion and status exceeding the safety margin due to the abnormal explosion can be found and the cause of that can be inferred. If there is any accident which is hard to find out the cause, this paper would be helpful in finding out the causes and enhancing the trust of engine manufacturers if this study is used to prove the safety of the crankshaft first.