Study On Stiffness Identification For Damping Springs And Dynamic Problems Of Beam Structures In A Large Vibrating Screen

Coal is the primary energy in China. The direct utilization of the poor-quality raw coal with high-content ash and sulfur may cause serious environmental pollution and waste of resources. Coal preparation is the most economical and effective method for high-quality coal utilization. The screening process as well as the corresponding widely-used vibrating screen are the key to coal preparation. Up to now, the extent of processing-before-utilization for domestic raw coal is still low, which is limited by screening technology and results in restricting large-scale coal upgrading of China. Thus, large-capacity and high-efficiency vibrating screens with a screen surface width or beam span of over 3.6 meters and a sieve area of over 20 square meters are in great demand. Actually, structural failure such as beam fracturing and plate crack occur too frequently to meet the requirement of modern large coal preparation plant.The dissertation focused on the significant reliability problems of the large vibrating screen, such as the stiffness identification for damping springs, the dynamic prosperity of exciting beam, the vibration of bearing beam during the screening process and the vibration properties and crack diagnosis of bearing beam with external crack. Main purpose of this study is to lay the foundation of the health diagnosis and structural high-reliability design of large vibrating screen to ensure the safe and reliable operation. The main research work and conclusions are as follows:Firstly, a 3-degree-of-freedom(3-dof) four-point-elastic-support rigid plate(FERP) structure to describe the spring fault was presented. With the free response matrix established, theoretical derivation for stiffness matrix calculation by free response(SMCby FR) and the method of stiffness identification by stiffness matrix disassembly(SIby SMD) were proposed. By integrating the SMCby FR and SIby SMD, procedures for stiffness identification of a FERP(SIFERP) structure were summarized. Then, a simulator for stiffness identification of FERP(SSIFRP) was developed for high signal-to-noise case. The simulated tested free response data were generated by the numerical integration as well as the digital signal processing and then were adopted for the SIFERP validation in the numerical example. Finally, validity of the developed SIFERP for stiffness identification was supported by an experimental test.Secondly, the exciting beam with an inner-welded stiffened plate instead of the traditional thicken one or the external-welded stiffening rib was proposed. According to the classic beam theory distinguishing the Euler-Bernoulli beam(EB) and the Timoshenko beam(TB) by geometric dimension, a composite beam unit(ETE) consisting of EB and TB was developed by using boundary continuity condition. Then the exciting beam with an inner-welded stiffened plate was modelled as the combination of alternately ETEs in sequence. Based on the analyzed vibration differential equation of each EB and TB in an ETE, frequency equation of the composite beam with classic boundary conditions were derived, then numerical algorithm to obtain the natural frequencies and corresponding mode shapes simultaneously were summarized. Numerical examples were presented and validated by results of the finite element method. Finally, an experimental test was carried out to demonstrate the limitation of the developed ETE model.Thirdly, the traditional dynamic model of vibrating is modified with the screening process considered. The property of ergodic prediction random vibration of the scree-box yielded by the material during the screening process was analyzed, which suggested that total motion of a bearing beam structure excited by the side-plate motion in screening process should be decomposed into the traditional followed rigid translation(FRT), bending vibration(BV) and axial linear-distributed random rigid translation(ALRRT). Moreover, the BV and the ALRRT was solved using the Rayleigh’s method and the stochastic analysis for random process, respectively. Thus expressions for the shearing force and bending-moment were updated considering BV and ALRRT. Experimental and numerical investigation demonstrates that the improved method has an advantage of the traditional one.Finally, properties of the external through-thickness crack and part-through crack on a rectangular-hollow-sectional beam were studied. Linear elastic fracture mechanics were adopted to develop the test apparatus and method of obtaining the equivalent torsion spring stiffness at the crack position. Meanwhile, the theoretical formula for computing the equivalent torsion spring stiffness of external crack was derived and then validated by the experiment above. On this basis, the transfer matrix method was introduced to obtain the frequency equations with the classic boundary conditions and followed by the numerical solution method for frequency value. Experimental tests on a clamped-free beam were used to verify the feasibility and accuracy of the method described above. Finally, relation between the natural frequencies with crack was discussed to illustrate the contour lines for crack detection.