1、Dynamics and screening characteristics of a vibrating screen with variable elliptical traceHE Xiao-mei,LIU Chu-sheng School of Mechanical and Electrical Engineering, China University of Mining Technology, Xuzhou, Jiangsu 221116，ChinaAbstract: the ideal motion character sties for the vibrating screen
2、 was presented，according to the principle of screening process with constant bed thickness. A new vibrating screen with variable elliptical trace was proposed. An accurate mechanical model was constructed according to the required structural motion features. Applying multi degree of-freedom vibratio
3、n theory, characteristics of the vibrating screen was analyzed. Kinematics parameters of the vibrating screen which motion traces were linear, circular or elliptical were obtained. The stable solutions of the dynamic equations gave the motions of the vibrating screen by means of computer simulations
4、. Technological parameters, including amplitude, movement velocity and throwing index, of five specific points along the screen surface were gained by theoretical calculation . The results show that the traces of the new designed vibrating screen follow the ideal screening motion . The screening eff
5、iciency and processing capacity may thus be effectively improved. Keywords: variable elliptical trace; screening process with constant bed thickncss;dynamic model;motion characteristic;screening characteristics1、IntroductionScreening operations are an important part of coal processing. The vibrating
6、 screen is one of the most extensively used screening tools. Vibrating screens, such as linear vibrating screen, circular vibrating screen or elliptical vibrating screen, have a simple translational motion. The motion follows the same path everywhere on the screen and so the screen has constant tran
7、sport velocity and throwing index, which leads to low screening efficiency. Augmenting the throwing index to improve breaks the exciting motors processing capacity lowers the working.In this paper，we report on the design of a new vibrating screen with variable motion traces that is based on the prin
8、ciple of screening process with constant bed thickness 3 - 4. Different parts of the vibrating screen traverse different elliptical traces and the resulting motion grees well with the ideal motion . Thus the screen processing capacity and efficiency can both be improved.2、Ideal motion for a screen s
9、urface and the proposal of a vibrating screen with variable elliptical trace2. 1 Screening characteristics of common vibrating screensVibrating screens commonly work at a fixed vibration intensity . Material on the screen surface moves by throwing, rolling or sliding motions . For common screeners ,
10、 material granularity is widely distributed at the feed end . The energy imparted to the material particles from the vibrating screen is severely dissipated . Consequently，a large number of particles become laminated only a short distance from the feed end . The material penetrates the screen within
11、 the first 1/4 to 1/2 of the screen , which affects screening and lowers processing capacity 5. The decrease of fine-grained material causes the ratio of. particles close in size to，or larger than，the mesh to increase . Thus，the screening efficiency declines dramatically . The material granularity s
12、imultaneously becomes uniform and the energy imparted from the vibrations to the material suffers little loss . Hence , the amplitude and velocity of the material particles increase . This causes the material bed depth at the feed end to be thick while at the discharge end it is Thin . This kind of
13、motion leads to an asymmetrical penetration along the screen surface, which influences the screening efficiency and processing capability 6. Common screening characteristics are shown in Fig. 12. 2 Ideal motion for screen surface and implementing schemeThe ideal motion for screen surface is describe
14、d below, according to the principle of screening process with constant bed thickness . The feed end of the screen has a bigger throwing index and a higher material del ivery velocity，which makes bulk material quickly penetrate and causes rapid de-laminating. Earlier lamination of material increases
15、the probability of fine-grained material passing through the mesh . The screen has an appropriate throwing index and a little higher material delivery velocity in its middle part .This is of benefit for stabilizing fine-grained materials and for penetrating uniformly along the screen length . A lowe
16、r throwing index and material delivery velocity near the discharge end causes the material to stay longer on the screen and encourages more complete penetration of the mesh. Two methods are currently used to improve screening efficiency 7 - 8. The first is to add material to the screen from multiple feed ports.This is troublesome in practical use especially in terms of controlling the distribution of differently granulated materials. Hence it is rarely used in
