Study On Nonlinear Dynamic Characteristics Of Vibrating Flip-flow Screen

Dry screening of moist and fine coal is an urgent problem to be solved in the screening industry of coal.However,conventional screening equipments encounter the problem of serious blockage of the screen's apertures,which reduces screening efficiency and seriously restricts the development of the coal screening industry in our country.Vibrating flip-flow screen is a kind of screening equipment which uses the slack and tension motion to screen materials,it has the advantanges of large acceleration of screen panel,high screening efficiency and strong application,and it is difficult to block the screen's apertures,which results vibrating flip-flow screens provide an effecitive solution for screening moist fine coal materials.However,owing to researchers at home and abroad don't deeply understand the vibration mechanism of vibrating flip-flow screen,the produced vibrating flip-flow screens at present encounter the problem of poor stability,and its key component is easily damaged,such as shear spring.In view of the existing problems in the mentioned vibrating flip-flow screens,an industrial vibrating flip-flow screen is regarded as research object,and the methods of theoretical calculation,numerical simulation and test analysis are used in this paper.A method of global sensitivity analysis is applied to the vibrating flip-flow screen system to show the most important component that affects the dynamic response of an operating vibrating flip-flow screen.A novel dynamic model of shear spring and a new dynamic model of vibrating flip-flow screen are proposed,which can accurately describe the dynamic characteristics of shear spring and dynamic response of vibrating flip-flow screen,respectively.A method of multi-objective optimization is applied to the vibrating flip-flow screen system to improve the system's stability.The main work and research conclusions in this paper are summarized as follows:The dynamic model of vibrating flip-flow screen with damping is established using the method of centralized mass,and the corresponding vibration differential equation is established using the principle of D'Alembert,then the expression of the dynamic response of the system in steady state is obtained.The influence of the main parameters in the system on its dynamic response is analyzed,and the vibration mechanism of vibrating flip-flow screen is preliminarily revealed.A method of Sobol'sensitivity analysis is used to analyze the global sensitivity of the linear system of vibrating flip-flow screen,and the key parameters affecting the dynamic response of the system in different working region are obtained.Results show that when the operating angular frequency of vibrating flip-flow screen increases from the near antiresonance region to the near resonance area,the dominant parameter affecting the amplitude of main screen frame is changed from the mass of floating screen frame to the stiffness of shear spring,however,the dominant parameter affecting the relative amplitude of the main and floating screen frame is always the stiffness of shear spring.When the vibrating flip-flow screen operates in the antiresonance region,the difference between the first-order sensitivity coefficient and the total sensitivity coefficient of shear spring and the mass of floating screen frame mass to the amplitude of main screen frame is large,this means that there is an obvious interaction between these two parameters and other parameters in the region on the amplitude of the main screen frame.The experimental system of dynamic test of shear spring is established,and the dynamic characteristics of shear spring are tested using different amplitude and frequency,then the amplitude and frequency dependency of dynamic stiffness and lag angle of shear spring is analyzed.A nonlinear model of shear spring is proposed based on a superposition of elastic force,friction force and fractional derivative force,in the proposed model,the nonlinear elastic force and the friction force are used to describe the amplitude-dependent behaviour of its dynamic characteristics,and the fractional derivative force is used to describe the frequency dependency of its dynamic characteristics.The newly proposed shear spring model is compared with the widely used Sj?berg model and generalized Maxwell model based on the dynamic test results of shear spring.And the reasonability and accuracy of the newly proposed shear spring model in describing the amplitude and frequency dependency of the dynamic characteristics are revealed.The proposed model of shear spring is applied in the dynamic analysis of vibrating flip-flow screen,a nonlinear dynamic model of vibrating flip-flow screen is establised.A method of Newmark algorithm is used to theoretically calculate the dynamic response of vibrating flip-flow screen,and the dynamic response of vibrating flip-flow screen in the time region and frequency region is obtained.An experimental platform used to test the dynamic response of vibrating flip-flow screen is established,and the dynamic response of vibrating flip-flow screen under different frequencies is tested.The newly proposed nonlinear dynamic model of vibrating flip-flow screen is compared with other vibrating flip-flow screen models based on the experimental results,and the reasonability of the newly proposed model in describing its dynamic response is revealed.The influence of the main parameters of the newly proposed model on its dynamic response is analyzed,and the deficiencies of other dynamic models of vibrating flip-flow screen in describing the vibration mechanism of vibrating flip-flow screen is illustrated.A method of Sobol' sensitivity analysis is used to analyze the global sensitivity of the newly proposed nonlinear dynamic model of vibrating flip-flow screen,and the dominant parameters affecting the relative amplitude of the main and floating screen frame is obtained.The influence of the change of system parameters caused by the change of external conditions on the dynamic response of the system is discussed.A mult-objective optimization algorithm of NSGA-II is used to optimize the nonlinear dynamic system of vibrating flip-flow screen,and the Pareto optimal solution set considering both the rate of change of amplitude and the complexity of system's motion is obtained.Results show that the stability of the system caused by external conditions can be reduced when vibrating flip-flow screen equips the shear spring of hardening characteristics,however,when the hardening characteristics of shear spring increases to a certain extent,the motion of the system will be changed from simple periodic motion to quasiperiodic motion with different complexity.