Torsional Vibration Analysis Of A Large Marine Generating Set Shaft System Based On FEM Method

Marine diesel generating-set is main source of main power and emergency power supply on the ship.It is an important factor that affects safety of ship sailing.With the increase of speed and reinforcement index of generating-auxiliary engine,shaft system under and transmitted more load,the torsional vibration state of shafting system is more complicated.This makes the calculation and research of generating-set shafting system to be more important.Periodic exciting torque cause torsional vibration of shafting system.When the running frequency is close to its natural frequency,there will be resonance of shafting system.The amplitude of vibration increases greatly.In light,this will lead to large noise,increasing the wear of gear system which is connected to the crankshaft.In heavy,this can break the shaft system,so it is especially important to avoid torsional resonance.This paper researches about torsional vibration of marine diesel generating-set.Take 6210 generating-set as research object.Based on actual structure,cerate 3D model of generating set shaft system,carry out modal analysis based on ANSYS software.Using AVL Excite Designer software,torsional vibration equivalent system was established.The free vibration and forced vibration were calculated to obtain the natural frequencies,model shapes,critical speeds,harmonic amplitudes and shear stresses of each shaft.Compare the ideal firing order with actual order of diesel engine about forced vibration.The result showed that these two firing order have little influence on the forced vibration.But in face,the choose firing order of diesel engine have to take many factors in consideration,for example heat load of the engine during work.Using AVL Excite Designer software to calculate the torsional vibration under single cylinder misfire,analyze the torsional amplitude and shear stress with different cylinder misfiring.The results showed,with the cylinder misfire which is near at flywheel,the impact on the torsional amplitude system of single cylinder misfiring state would be greater.The cylinder at the flywheel end misfire would impact the torsional amplitude larger than any other cylinder;its torsional rotation angle would be larger.Using Excite Designer,analyze coupling\damper\flywheel\generator rotor influence to shaft system.The results showed that the stiffness of coupling mainly affect the natural frequency and amplitude of free end,while damping of coupling mainly affect the some speed,other speeds would be influenced little.The stiffness and moment of inertia can affect the natural frequency of shaft system effectively.With the increment of coupling stiffness,2\3\4 node of natural frequency would raise while 1 node keep steady.With increment of coupling damping,the vibration amplitude would be reduced at speed 500rpm and 650rpm.But at the other speed,the influence would be little.During the torsional vibration calculation,the inside and outside parts of the moment of inertia added to flywheel and generator shaft.The infection of coupling about moment of inertia,refer to flywheel and generator shaft part.Flywheel is important to the torsional oscillation of shaft system.It can directly change the natural frequency of system,reduce the peak stress and move the peak stress to other speed.With the moment inertia increase of flywheel,2 and 4 nodes of natural frequency reduce particularly while 1 and 3 has little change.The wave crest of crankshaft stress will be reduced and move to low speed area.The moment of inertia of generator shaft can affect the natural frequency;also influence the amplitude of free end.With the increase of generator shaft moment inertia,2 node natural frequencies reduce particularly.The amplitude of crankshaft free end reduced markedly,while the stress of crankshaft has little change.The moment of inertia of damper can affect the amplitude of free end while damping can reduce the peak stress of system effectively.With the increase of damper moment of inertia,1 node natural frequency would be promoted while other nodes not,the amplitude of crankshaft will be reduced.It has little influence to the stress of crankshaft.With damping increase of silicon damper,the wave crest of crankshaft stress will reduce markedly.The generator shaft torsional vibration best bench was built;get the torsional vibration amplitude under different speed and analysis of data.Compare the actual data with simulated ata from AVL Excite Designer and check the result with classification rules.