Research On Crankshaft Crack Detection For Marine Diesel Engines At Early Stage

The Crankshaft is one of the important parts of marine diesel engine.Under the combined action of cylinder pressure,cyclic impact of reciprocating inertia force and torsional load,crankshaft is prone to crack dues to bending and torsional fatigue,which results in significant safety hazards.If the crankshaft crack fault can be effectively monitored and the existence of cracks can be found in time,the crankshaft can be repaired or replaced in time,to avoid casualties and reduce economic losses.In this paper,the Xichai 4120SG diesel engine is taken as the research object.Firstly,the modals of the normal crankshaft and cracked crankshaft are calculated and analyzed to study the free-mode frequency and the corresponding vibration mode shapes of the crankshaft before and after cracks appear.Besides the influence of crack on the inherent characteristics of the crankshaft is explored.On this basis,the multi-body dynamic calculation model of diesel engine crankshaft system is established.Taking the peak value of signal and crank rotation angle as characteristic parameters in a cycle of diesel engine,and the influence rule of crack on characteristic parameters of diesel engine crankshaft system is studied.Besides,the factors that interfere with crankshaft crack identification are analyzed.Finally,the experimental verification is carried out on 4120 SG diesel engine bench.The main works and conclusions are as follows:1.The modal simulation calculation and the corresponding modal experiments show that the appearance of cracks has little influence on modal modes,and the amplitude will changes only when the cracks are deeper and the number of cracks is larger.The cracks in the middle of the crankshaft affect the modal frequency greatly,compared with the cracks at the two ends of the crankshaft.The deeper the cracks are,the more the number of cracks is,the greater the change of the modal frequency is.The appearance of cracks changes the original stiffness and damping of crankshaft structure.The deeper the cracks are,the more the number of cracks is,the more the rigidity of crankshaft decreases.Therefore,if the change of torsional vibration resonance frequency of the shaft can be monitored,it is possible to detect the shaft crack at early stage.2.The multi-body dynamics simulation calculation of 4120SG marine diesel engine shows that the crankshaft instantaneous speed is insensitive to cracks,while the longitudinal vibration is sensitive to cracks.The appearance of cracks will cause the longitudinal displacement of crankshaft where the crack located to increase at the ignition time,while the non-crack locations will remain basically unchanged.On the whole,the deeper and more cracks,the greater the change of crankshaft longitudinal displacement.Therefore,the purpose of early identification for crankshaft crack failure can be achieved by monitoring the longitudinal vibration of diesel crankshaft in real time.3.The analysis of the factors influencing the identification of crankshaft crack fault shows that although variable speed and load can also cause the sudden change of the axial displacement of the crankshaft,it is different from the longitudinal vibration of the crack.When the speed changes,the longitudinal vibration signal changes greatly,and the change trend is obvious;when the load changes,the longitudinal vibration signal increases or decreases,and the whole change trend is a dynamic equilibrium process;but when the crankshaft cracks,the longitudinal vibration signal changes locally near the ignition point.Therefore,the change of engine speed and load will not interfere with the identification of crankshaft cracks.4.The influence of crankshaft flexibility on the longitudinal vibration of6L16/24 diesel engine bench with long shafting system in the laboratory was studied.The results of multi-body dynamics simulation show that the longer the crankshaft is,the smaller the stiffness is,and the longitudinal vibration is more sensitive to cracks.5.Experiments were carried out on 4120 SG diesel engine.The results of those experiments show that the simulation model can better reflect the vibration characteristics of diesel engine,and the conclusions obtained by simulation are credible.That is to say,if the longitudinal vibration signal changes obviously locally near the ignition point of the diesel engine,it shows that the crankshaft has cracks.If the amplitude of the longitudinal vibration signal changes dramatically and the longitudinal displacement near the ignition point exceeds 0.01mm compared with the normal working condition shows that the crankshaft has deeper cracks or more cracks,which should be paid enough attention.The magnitude of crankshaft longitudinal amplitude varies in the order of 10^-3-10^-2 mm when cracks occur.Therefore,accurate monitoring of small changes in the longitudinal vibration of the crankshaft is a difficult problem for early identification of diesel engine crankshaft cracks.The crankshaft crack fault has a great influence on the reliability of the diesel engine.Although the analog test of the crankshaft crack fault could not be carried out due to the limitation of the test conditions,the feasibility of early fault identification of crankshaft crack has been found and the difficulties of cracks identification have been pointed out mainly through the calculation and analysis of simulation model in this paper.This paper can provide reference for the engineering application of early crankshaft crack identification in diesel engines.