Study On Dynamic Stability And Elastic Screening Mechanism Of Vibrating Flip-flow Screen

In the thermal coal separation process,3 mm screening of coal can reduce the amount of coal slime and simplify the washing process.In the iron ore grinding process,deep screening of the crushed products of the high-pressure roller mill can reduce the particle size of the subsequent grinding operations,achieving the purpose of "more crushing and less grinding".Fine screening of construction waste can realize resource utilization.The deep screening of fine-grained material is an effective means to achieve energy saving,emission reduction,and efficient utilization of resources.When the traditional vibrating screens screen sticky fine material,there are phenomena such as hole blocking,adhesion,and hardening on the screen surface,which hinder the screening process.The application of a vibrating flip-flow screen(VFFS)solves these problems.Due to the particularity of the structure of the VFFS and the complexity of the vibration mechanism,there is a lack of systematic understanding of the dynamic stability of VFFS,the movement and screening behavior of the particles on the elastic sieve mat in the VFFS.In this thesis,the method of combining theoretical modeling and the dynamic test is used to systematically study the effects of dynamic parameters of VFFS on its vibration response,and propose a stable working condition during loading material.The movement and the sieving behavior of particles on the elastic sieve mat are investigated by screening experiment and DEM simulation in detail,the main contents and conclusions are as follows:(1)Based on the designed pilot test VFFS with the type 0827,an experimental platform for testing VFFS’s dynamic response was built.The mathematical model and differential equations of the two-degree-of-freedom system of VFFS were established.The effects of the dynamic parameters such as damping,stiffness,mass,and external excitation parameters on VFFS’s vibration response were systematically studied through theoretical analysis and dynamic test experiments.It was obtained that the influence of material load on the system was mainly the additional stiffness force generated by the tensile deformation of the sieve mat.The contours of the three-dimensional response surface were used to evaluate the stability characteristics of the system’s dynamic response in different frequency ranges.It was proposed that the frequency range of the stable working condition is at the near anti-resonance region,and it was verified by the loading material experiments.(2)In the stable operating frequency range,the effects of mass ratio parameters on the second-order natural frequency and relative amplitude of the vibration system were analyzed.As the mass ratio increased,its effect on the second-order natural frequency and relative amplitude gradually decreased.An inflection point with a mass ratio of about 0.33 was found,which can effectively overcome the influence of the mass on the vibration stability,meanwhile does not increase the vibrating mass too much.The mass ratio parameter of the floating and main screen frame was optimized to be about 0.33.(3)Since the effect of damping on the vibration response of working conditions can be ignored,a five-degree-of-freedom mechanical model and differential equation of VFFS were established.Then,the spatial trajectories of the feeding end,the centroid,and the discharging end of two screen frames were obtained and the rationality of the mechanical model and equation was verified by analyzing the theoretical and test values of the vibration response.Furthermore,the simplified calculation formula of the amplitude at the centroid of the main and floating screen frame was obtained.A debugging method to realize the amplitude under stable working conditions of the VFFS was proposed.Adjust the shear stiffness to find a stable working frequency range,adjust the eccentric mass moment to increase or decrease the relative amplitude linearly,thus ensuring that when the VFFS reaches a reasonable amplitude,the operating point is located at a stable frequency range.It has a certain guiding significance for the application of engineering.(4)An experimental device for measuring the kinematics of the elastic sieve mat was designed,and the kinematic behavior of the elastic sieve mat under different stretched length and material loading parameters was studied.By analyzing the frequency spectrum of the displacement signal of each point on the screen surface,the implementing mechanism of the material loading on the kinematic characteristics of the elastic sieve mat was revealed.The vibration of the elastic sieve mat had obvious nonlinear characteristics in the case of-2 mm stretched length.With the increase of material load,the amplitude decreased significantly,the sub-harmonic frequency component gradually disappeared,and the vibration fundamental frequency of the sieve mat gradually became the excitation frequency.Under the +2 mm stretched length,the vibration fundamental frequency of the sieve mat was the excitation frequency.The amplitude of the high-frequency signal gradually decreased until disappeared with the increase of the material load,and the material load had little influence on the amplitude at the fundamental frequency.The material had a "filtering" effect on the kinematics of the sieve mat.(5)The distribution and sieving behavior of the material with various sizes in different sections during the screening process in the circular vibrating screen(CVS)and VFFS were studied by the method of multi-stage sampling analysis,and the elastic screening mechanism of the VFFS was revealed.The large vibration acceleration of the screen surface can effectively overcome the blocking and adhesion of particles,meanwhile,it effectively promoted the desorption,and deagglomeration of fine particles.When the coal was screened at a cut size of 3 mm,the screening efficiency of the VFFS was 77.62%,and the total misplaced material was 5.76%.Compared with the CVS,the screening efficiency was increased by 15.63%,and the total misplaced material was reduced by 4.06%.The VFFS shows obvious advantages in the screening of sticky fine material.(6)A numerical simulation method based on the Fourier series to realize the kinematics of the elastic sieve mat was proposed.The displacement of each point on the elastic sieve mat was assigned by Fourier analysis,and the vibration characteristics of each point on the screen surface are obtained.The continuous motion of the sieve mat in the simulation was realized by the motion setting of each discrete element on the elastic sieve mat.Based on the Hertz-Mindlin particle contact model,the movement and sieving behavior of particles with various sizes in the VFFS were studied,and the rationality of the discrete model was verified by the screening experiment results.Based on the JKR particle contact model,the sieving mechanism of sticky fine particles on the VFFS was revealed.The greater the surface energy between particles,the longer the time and distance the agglomerated particles need to deagglomerate.