Research On Vibration Control Methods Of A Propeller-shaftstern Coupled System

The low-frequency vibration of the propeller-shaft-stern coupled system induced by propeller excitation is one of the main sources of ship vibration and sound radiation.In the propeller-shaft-stern coupled system,the natural frequency of the long-span shaft is low,the bending vibration frequency of the hull is also low because of its small bending modulus.In the low frequency band,the bending mode of the shaft system is close to that of the hull which leads to coupling between the two,so the propeller-shaft-stern coupled system shows obvious low frequency characteristics.the bending mode of the shaft system is close to that of the hull at low frequency,so low frequency characteristics is obvious in the propeller-shaft-stern coupled system.At the same time,there are many supports in the system such as thrust bearings,intermediate bearings,stern tube water lubricated bearings,front stern bearings,rear stern bearings,etc.Due to the different bearing support structure,the mechanical characteristics of the coupled system are different.Therefore,the vibration transmission paths of the system excited by propeller are diverse,and the transmission characteristics are complex,which brings great challenges to the analysis of the system’s vibration sound radiation characteristics and vibration control.In this paper,the research of the propeller-shaft-stern coupled system is conducted around the system’s vibration and acoustic radiation characteristics and vibration control methods.(1)The dynamic model of a propeller-shaft-stern system is established using FEM/BEM method.According to the model,the vibration transmission characteristics and sound radiation characteristics of the system are analyzed,and the influence of bearing parameters on the vibration transmission characteristics of the system is studied.It is found that reducing the bearing stiffness can effectively reduce the transmission force,and moving the bearing position to make the mode frequencies of the front and rear shaft equal can minimize the transmission force of the bearing.Increasing the number of bearings can reduce the transmission force of the bearings at the rear of the transmission path.The conclusions above can provide a reference for the structure optimization and vibration control of the propeller-shaft-stern coupled system.(2)To reduce the vibration of the propeller-shaft-stern system excited by the propeller forces,the multi-frequency optimization design method of distributed dynamic vibration absorbers is proposed based on genetic algorithm.The distributed dynamic vibration absorbers are arranged according to mode shapes,and the frequency response synthesis method is used to calculate the vibration response of the propeller-shaft-stern coupled system under the control of the dynamic vibration absorber.Taking the mean square vibration velocity of the stern hull surface as the object function,the parameters of the dynamic vibration absorbers are optimized using the genetic algorithm.The results show that the optimized distributed dynamic vibration can suppress the vibration transimission from the propeller excited by vertical force and solves the problem of poor performance when single-frequency optimization parameters are applied to multi-frequency optimization.And the use of frequency response synthesis method can reduce the optimization calculation time of complex systems.(3)Vibration isolation devices,constrained damping layer and vibration blocking masses are used to control the vibration of the propeller-shaft-stern coupled system.The stiffness of the vibration isolation devices are obtained according to the alignment conditions,the constrained damping layer is laid on the connection between the stern frame and the stern shell,and the vibration blocking masses are installed on the stern frame connection plate.The results show that the vibration isolation devices have a good impact on reducing the low-frequency vibration of the system excited by lateral propeller forces,the constrained damping layer can effectively reduce the vibration in high frequency range,and the vibration blocking masses can limit the vibration transmission from the shaft to the hull.(4)The scale test model of the propeller-shaft-stern coupled system was built,and the lateral vibration control test of the stern structure excited by propeller force was carried out.The test results show that vibration isolation has limited effectiveness in reducing vibration in the low to medium frequency range due to the limited range of stiffness variation;vibration blocking masses are more effective in isolating vibration transmission to the stern structure;damping coating has significant effect on the attenuation of vibration in high frequency range.The acceleration level of the stern structure was reduced by at least 6dB under the comprehensive application of the three control methods.