| In recent years,more attention has been paid to the coal washing process in the coal production process in China.The modern coal washery has been gradually popularized in the coal enterprises.Vibrating screen is an important screening equipment in coal washery.It works with the excitation produced by exciter and the main function is to classify raw coal,medium recovery,dehydration,desliming and so on.Vibrating screen is one of the key large equipment in raw coal washing process,and its reliability is directly related to the normal and efficient operation of coal washing process.As the bad working environment of vibrating screen and long continuous working time in the operation process,vibrating screen is not only bearing alternating excitation force,but also bearing the gravity and the impact force of screening material.Malfunction like beam fracture,plate cracking,screen plate loosening and bearing damage would be appeared in use in the process of vibrating screen if maintenance is not timely.Beam fracture is one of the common faults of the vibration sieve,and the crack is a manifestation of early fracture.So discovering the crack in the early stage of its development timely and taking the appropriate measures can avoid the occurrence of beam fracture failure effectively.At present,maintenance for key equipment is regular maintenance or maintenance after malfunction in coal mining enterprises.But the traditional maintenance mode has become increasingly unable to meet the requirements for modern coal mine equipment maintenance.Predictive maintenance is carried out with the equipment maintenance management mode of the inevitable trend of development.In recent years,the project team put forward a new model for mine equipment maintenance management: Large mine electromechanical equipment health diagnosis and predictive maintenance system.Because of the lack of relevant data for the diagnosis and prediction of typical faults in the field,the malfunction simulation method is used to solve this problem.Malfunction simulation refers to the use of mathematical models or computer simulation models to model these typical faults.Through simulation analysis,the characteristic indexes of typical faults are established,which can be used as the judgment basis for malfunction diagnosis and trend prediction.According to the beam crack fault.this article will complete the simulation of fault,and get the reasonable fault features of girder used for crack detection judgment.At the same time crack propagation research for the cracked girder structure will be carried out,and that analyzed the influence of different crack parameters and working conditions on crack propagation rate.Around the above problems,this paper uses the theoretical deduction,simulation analysis and experimental verification to study the system.The specific research work is mainly reflected in the following aspects:The performance evaluation of vibrating screen based on finite element dynamics analysis is completed.Two different types of vibrating screen are selected,and the 3D model of vibrating screen is established by modeling software.Considering the complexity of the structure of vibrating screen,finite element method is used to compute the natural frequency and the modal shape.Vibration test and the analysis of test data of vibrating screen in no-load condition is completed by the use of portable vibration equipment,and the actual work frequency of vibrating screen in no-load condition.The comparison of natural frequency and work frequency of vibrating screen in no-load condition indicates that the vibrating screen's working frequency are closed to the resonance frequency calculated by the simulation.The corresponding natural frequency's vibration mode is closed to the beam and the reinforced beam with displacement maximum state.That means the beam and reinforced beam structure is in a potential threat state of resonance destruction.Therefore the vibrating screen should avoid the no-load continuously work in no-load condition.The crack modeling research based on local flexibility theory is completed.In this paper,the establishment of crack model is based on the basic theory of linear elastic fracture mechanics,and introduce the idea of calculus.It will calculate the strain energy release by taking small micro crack area calculation of the strain energy release,then can get the strain release of the crack region by integral.Subjected to bending,According to the relationship between strain energy and load the beam structure for two different cross sections,local flexibility coefficient introduced by crack will be calculated respectively.The local flexibility curve calculated by numerical calculation describes the relationship between the local flexibility and crack depth.By comparing with previous experimental data,It verified the validity and rationality of the derivation process,and provide the basis for the subsequent crack structure vibration analysis.The analytical analysis and numerical analysis of free vibration of cracked beam are completed.Based on the continuum theory of vibration,the crack can be equivalent to torsional spring model of beam structure on both sides of the crack as Euler-Bernoulli beam,the analytical equation of free vibration are established,combined with the corresponding boundary conditions and the continuity conditions,obtained the natural frequency relation between the crack depth and crack position changes exist.At the same time finite element equation with free vibration of cracked beams is carried out,the key is to determine the mass matrix of the cracked structure and stiffness matrix.Finally taking YK3052 vibrating screen beam structure as an example,the comparison of the results between using analytical method and numerical method indicated that the finite element method is feasible.To some extent,a large number of analog data samples are often needed,so the numerical calculation method with higher computational efficiency has important guiding significance for theanalysis of the crack fault simulation of the vibration screen beam.The simulation study of crack fault simulation is completed.The model is modeled by solid element at first,and the the model can be added with crack of definite size and different crack conditions are established.The modal frequencies and mode shapes at different positions and depths are obtained by simulation.Through the analysis,it is found that the fault simulation based on the solid element can simulate the crack of definite size,but the steps are complex and the simulation efficiency is low.At the same time,the simulation model of the numerical example is established by the beam element.The crack damage of the structure is simulated by reducing the elastic modulus of the element,and different crack damage conditions are established.The simulation analysis of the beam model,the crack in different conditions of the natural frequencies and mode shapes,and were found to have poor sensitivity to simulate the damage displacement mode,and it is difficult to achieve the judgement and the localization degree of injury.Based on this view of strain modal,simulation results show that the strain mode for local damage of structure is more sensitive,and it changes with the changes of location and extent of damage simulation,the strain modal index is more ideal to achieve the damage severity and location.Aiming at the problem of no response of strain mode at the modal node,the previous three order strain modes is proposed as an evaluation index.The comparison shows that the index can effectively avoid the blind spot of the node position.Through the above analysis,it is found that the simulation method using beam element modeling and reducing the elastic modulus simulation crack has high simulation efficiency,but it can not add the crack of definite size.Considering the advantages and disadvantages of the above two kinds of simulation methods,the establishment of the relationship between the elastic modulus and the change of the crack depth can combine the advantages of both methods.It can provides the the basis for the quantitative identification of crack.In this paper,a laboratory simulation experiment is built,and the crack fault simulation experiment of beam structure is completed.The steel structure simulate vibrating screen beam structure and boundary conditions.Crack simulation by wire cutting with different positions and depths on the steel pipe.Through the experimental modal analysis with hammering pulse excitation method,response signal can be get from the vibration sensor.Then modal parameter identification can get the experimental modal frequency and shape.Compared to the data obtained by simulation results,the experimental results can verify the validity of the simulation results.The influence of four different factors on the crack propagation rate of vibrating screen beam is studied.By means of simulation in ABAQUS software,a simplified beam crack propagation analysis model is created which prefabricated crack is added.Then compute the crack propagation rate by the amplitude of stress intensity factors of crack front nodes based on Paris formula.The factors which have influence on thecrack growth rate: the initial position,depth and angle of crack,and the stiffness change of support spring.Then change the parameters of different factors and finish the simulation.From the results of analysis,the vibrating screen support spring failure is a more obvious factor on the beam crack growth rate compared to the effects of other factors like parameters of crack,and it has a certain guiding significance to the maintenance of the equipment on the spotTo sum up,research of vibrating screen beam crack malfunction simulation and crack propagation characteristics can not only establish the characteristics indicators which reflect the crack malfunction information,but also can predict the development trends of the beam crack.It is of significance in the safety,security and efficient operation of equipment. |
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