Torsional Vibration Analysis And Characteristic Simulation Research Of The 8L27/38 Diesel Engine Propulsion Shafting

The main ship classification societies have all specified that the first set diesel engine device system whose power is greater than 110kW has to be tested and calculated for the shafting torsional vibration. Additionally, unless the error between the torsional vibration calculation result and measurement result of the first order natural frequency is less than 5%,the shafting stress is allowed to deduced by the testing torsion angle according to the calculation vibration modes. However,no one has given a way to deal with the problem that once the error between calculation and test of the first order natural frequency is over standard.This paper focuses on a problem that is the comparison results between the theoretical calculation and the test results of torsional vibration of an 8-cylinder in-line diesel engine are too large to be neglected. After confirming the test instrument and operation with no problem,the rationality of the torsional vibration model and the accuracy of the parameters are checked based on the condition of the real ship and the dynamic characteristic simulation analysis has been carried out.Firstly, to solve the problem that the comparison results between the theoretical calculation and the test results of torsional vibration are large, the actual shafting has been double checked to make sure the accuracy of shaft modeling. Compared to the original torsional vibration calculation model, it has been found that the elastic coupling torsional stiffness is uncertain. After comparison of the natural frequency calculation value and measured value, the torsional stiffness of the elastic coupling is modified. In order to find the missing natural frequencies of higher order calculation values, the branch of the shafting is refined, and the possible source of the test frequency is given.Next, According to the original model and that of modified, the propulsion shaft torsional vibration calculation has been calculated. The additional stress and additional torque of the crankshaft,elastic coupling and gear are analyzed. It has been found that the stiffness of the modified elastic coupling is larger than that of the design value, and the additional torque of the elastic coupling is also increased with larger stiffness.Finally, in order to further explore the dynamic characteristics of the torsional vibration of the elastic coupling. The elastic coupling characteristics under various conditions has been analyzed in this paper, mainly including: the impact of the coupling torsional stiffness, the influence of branch system to the shafting, diesel engine combustion disorder and diesel engine single cylinder misfire effect. It has been found that the additional torque of the elastic coupling is influenced by the diesel engine combustion disorder. Secondly is influenced by its torsional stiffness. And the effect of the branch of the shaft is the minimum.