The vibrating screen is a screening machine that loosens, stratifies, sifts and transports materials through the vibration applied to the screen body to achieve material screening. The main function is to classify materials, de-sludge, mass or particle size. Sorting is an indispensable auxiliary production equipment for feed production. Common types of vibrating screens are: elliptical vibrating screen, circular vibrating screen, linear vibrating screen, etc. Among them, linear vibrating screen is widely used in the feed industry due to its simple structure and reliable work, such as the use of corn, soybean and other raw materials. Stone, ash removal, screening; screening and grading after granulation. The linear vibrating screen is synchronously reversely rotated by two vibrating motors to generate a reverse exciting force, forcing the box to drive the screen for vertical and linear reciprocating motion, and the material is continuously inclined and throwing motion on the screen surface. The material is loosened when thrown up, and the small particles are sifted through when colliding with the screen surface, thereby realizing grading and conveying of the material. The screening effect of the vibrating screen not only has a great impact on the value of the product, but also has an important impact on the efficiency of the next step. Therefore, it is of great practical significance to study the factors affecting the efficiency of vibrating screen screening and to find measures to improve the working efficiency of vibrating screen.
1 Factors affecting the efficiency of vibrating screen
The screening effect of the vibrating screen is related to many factors, including material properties, screen surface structure parameters, and vibrating screen motion parameters.
1.1 Material characteristics
In the production process, the vibrating screen tends to be enriched, that is, the screen is piled up, the effective screening area is reduced, and the process efficiency is reduced. This is mainly related to the material type and particle, material loose density, material humidity, material particle size. The relationship is the biggest.
1.1.1 Material Type and Particles
The material types are different and the physical properties are different. The types of materials can be divided into two categories: brittleness and viscosity. In the process of vibrating screening, viscous materials are easy to form dense adhesion of materials, block the screen, and reduce the permeability, while brittle materials are not, the process efficiency can be guaranteed. . The particle shape of the material will also affect its permeable rate. The cube shape and the material of the sphere are easy to pass through, while the sheet material is easy to get stuck in the sieve hole, which reduces the efficiency of the screening process.
1.1.2 Material bulk density
The material particles are basically layered and sieved according to the volume of the particles, that is, the loose density of the material directly affects the processing capacity of the vibrating screen. The loose bulk material is easy to pass through the sieve, and the screening efficiency is also high; on the contrary, the loose bulk material and the powdery material are not easily permeable and the screening efficiency is low.
1.1.3 Material humidity
The moisture content of the material is too high, and it is easy to form adhesion. During the vibration process, the blocks are squeezed each other to make the adhesion group more compact, which increases the resistance of the material movement, makes the material particles stratified and sieved difficult; the material adhesion also makes the mesh hole The size is reduced, the screen openings are blocked, the effective screening area is reduced, and sometimes it is impossible to perform screening. Therefore, when the moisture content of the material is too high, the screening process should consider some remedies, such as taking measures to dry the material.
1.1.4 Material particle composition
The permeability of the material during the screening process is affected by many factors, the most direct and important of which is the relative size of the material size and the size of the mesh. The ratio of material size to mesh size is referred to as relative particle size. The smaller the relative particle size, the higher the permeability probability. When the relative particle size is close to 1, the permeability probability approaches zero. Therefore, materials with a relative particle size equal to 0.7~1.0 are usually referred to as difficult or critical materials. The composition of the difficult-to-screen material in the material also has an influence on the permeability probability. The larger the content of the difficult-to-screen material in the material, the smaller the permeability probability and the lower the screening efficiency. In order to obtain higher screening efficiency, the content of difficult-to-screen materials in the aggregate should be minimized. In addition, the difference in particle size index also has a certain impact on the screening efficiency. If the difference in particle size index is too large, the production cycle will be large, which will restrict the quality and processing ability of the product. Therefore, the grading of the aggregate should be strictly controlled.
1.2 Screen surface structure parameters
1.2.1 Screen length and width
It is generally believed that the width of the screen surface directly affects productivity, while the length of the screen surface directly affects the efficiency of screening. The sieve surface is wide, the effective area is increased, the productivity is improved; the sieve surface is long, the material stays on the sieve surface for a long time, the sieve has many opportunities, and the screening efficiency is high. But it is not as long as possible, because the relationship between the screening efficiency and the time the material is going through the entire screen surface is complicated. At the beginning, due to the large number of fine-grained materials on the sieve surface, the unit time The probability of sifting is large. After a certain period of time, the particle size of the material remaining on the sieve surface is mostly difficult to sieve material close to the mesh size, even if the material stays on the sieve surface for a long time (ie the length of the sieve surface is large) ), it is also difficult to increase the permeability of the material, resulting in reduced efficiency of the screen machine. In fact, the screen width is different for the screening efficiency, and the screen length has a different degree of influence on the screening ability. The matching of the two is also very important. Generally, the aspect ratio is 1:2 to 1:3.
1.2.2 mesh shape
Although the shape of the mesh depends mainly on the size of the sieved product and the requirements for the use of the undersize product, it also has a certain influence on the screening efficiency. Compared with other shapes of sieve holes, the circular sieve holes have smaller particle size through the sieve holes under the same nominal size. For example, the average maximum particle size of the particles passing through the circular sieve holes only passes through the same size. The square mesh has 80%~85% of the mesh particles; while the rectangular mesh has a large effective area, the screen surface is light in weight, and the production capacity is large, and the material size passing through the mesh is larger than the circular shape passing through the same nominal size. The particle size of the square mesh. Therefore, in order to achieve higher screening efficiency, different mesh shapes must be selected for different screening materials, which can improve the processing capacity of the vibrating screen.
1.2.3 Screen size and opening ratio of the screen surface
In the case of a certain amount of screening material, the size of the mesh has a great influence on the screening efficiency. The larger the mesh size, the stronger the permeability of the material particles and the greater the processing capacity of the vibrating screen. Of course, the mesh size is mainly determined by the process requirements of the material to be screened. The opening ratio of the sieve surface refers to the ratio of the opening area of ​​the screen surface to the area of ​​the screen surface (also referred to as the effective area factor). The high opening ratio increases the probability of the material particles being permeable, and the processing capacity of the vibrating screen is high. On the contrary, the processing capacity of the vibrating screen is low. Therefore, in order to improve the screening efficiency, a screen surface with a large effective area factor should be selected.
1.2.4 Screen material
Non-metallic screen surface, such as rubber graded screen surface, polyurethane-type grading screen surface, polyurethane strip screen surface, nylon screen surface, etc., due to the characteristics of these non-metallic materials, the second high in the screening process The frequency vibration makes the plugging phenomenon difficult to occur, which is beneficial to the screening of the material, and the processing efficiency of the metal sieve vibrating screen is improved.
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1.3 Vibration characteristics parameters
Vibration characteristics include vibration frequency, amplitude, vibration direction angle, and screen inclination.
1.3.1 throwing index
The four vibration characteristic parameters have their own influence on the vibrating screen, and their common influence can be summarized into one parameter - the throwing strength D (or the throwing index), and the relationship is:
D=ω2Asinβ/gcosα=ksinβ/cosα
Where: A - amplitude (mm);
Ω—the circular frequency of vibration (rad/s);
G——gravitational acceleration;
Β——vibration direction angle (°);
Α——the inclination of the screen surface (°);
K——vibration intensity, k=ω2A/g.
According to experience, when the throwing strength D=3~3.3, one vibration period of the screen surface is exactly equal to the period of material jumping, that is, every time the screen surface vibrates, the material will have a throwing motion, and the material is most easily permeable. The state of motion is beneficial for reducing unnecessary energy consumption and increasing the screening efficiency of the vibrating screen. However, this is based on the parameters obtained on the basis of the pulsation of individual material particles. In fact, the material appears on the screen surface in the state of the material group, and friction often occurs between the material particles, thus requiring greater throwing strength, so in practice Higher throw strength should be chosen for the application.
1.3.2 Screen inclination angle α
The angle between the screen surface and the horizontal plane is called the screen inclination angle. The size of the dip is closely related to the throughput of the screening equipment and the screening efficiency. When the inclination angle is increased, the throwing strength of the material on the sieve will be increased, so that the forward movement speed of the material on the sieve surface is accelerated, so that the treatment amount of the vibrating screen is increased, but the residence time of the material on the sieve surface is shortened, and the opportunity for screening the sieve is reduced. , thereby reducing the screening efficiency. On the contrary, the throughput is reduced and the screening efficiency is improved. In order to control the screening efficiency of the vibrating screen to a relatively ideal range, the screen inclination angle of the circular vibrating screen is generally 15° to 25°, and the screen inclination angle of the linear vibrating screen is 0° to 8°.
1.3.3 Vibration direction angle β
The angle between the vibration direction line and the upper screen surface is called the vibration direction angle. The larger the value of the vibration direction angle is, the shorter the distance moved by the material is, and the slower the movement speed of the material on the screen surface, the material can be fully sieved, thereby obtaining a larger screening efficiency. The smaller the value of the vibration direction angle is, the farther the material throws and advances each time. The faster the material passes through the screen surface, the higher the processing capacity, but the material cannot be fully sieved. Therefore, the vibration direction angle should be reasonably selected. For difficult-to-screen materials, the vibration direction angle should be taken as a large value. For easy-screen materials, the vibration direction angle should be small; in general, the vibration direction angle of the circular vibrating screen is 90°. The vibrating direction angle of the linear vibrating screen ranges from 30° to 60°, often 45°. This value not only has good performance for various screening, but also obtains the best moving speed and high productivity. .
1.3.4 Amplitude A
As the amplitude increases, the clogging of the mesh hole will be greatly reduced, and it will also facilitate the stratification of the material. However, too large amplitude is also destructive to the device. The choice of amplitude is determined by the particle size and nature of the material being screened. In general, the larger the vibrating screen size, the larger the amplitude selected. When the linear vibrating screen is used for classification, the amplitude is slightly larger; when the particle size of the treated material is large, the amplitude should be correspondingly increased; when the particle size of the treated material is small, the amplitude should be smaller. Usually, the amplitude of the linear vibrating screen is A=3.5~6 mm.
1.3.5 Vibration frequency ω
The increase of the vibration frequency can increase the number of beats of the material on the screen surface, which increases the permeability of the material. This is helpful for speeding up the screening speed and improving the screening efficiency, but too high will reduce the service life of the equipment. . Usually the selection of the vibration frequency is determined by the vibration intensity k and the amplitude A. The relationship between them is:
Ω2=3 600k·g·cosα/(A·sinβ)
For larger particle sizes, use larger amplitudes and lower frequencies; for finer particles, use smaller amplitudes and higher frequencies.
2 Measures to improve the efficiency of vibrating screen
2.1 Choosing the right type of vibrating screen
Although the screening effect depends mainly on the nature of the material to be screened, different screening effects can be obtained with different types of screening equipment for the same material. For example, the screening efficiency of the fixed sieve is low. For the moving sieve, the screening efficiency is related to the movement form of the sieve surface. On the sieve surface of the sieve, the particles are shaken in the direction close to the vertical sieve hole, and the vibration frequency is higher. High, the better the screening effect; on the moving sieve surface, the particles mainly slide along the screen surface. Because the shaking frequency of the shaking sieve is lower than the frequency of the vibrating screen, the screening effect of the shaking sieve is poor; The screen surface is easy to block, so the screening efficiency is low. Therefore, the type of vibrating screen should be reasonably selected according to the nature of the material to maximize work efficiency. For pre-screening and screening of materials, circular vibrating screens are generally used; probabilistic sieves, equal-thickness sieves and large vibrating screens are used for classifying the crushed materials; sand cleaning and de-mudging of materials are used, and the thickness of the prosthetic sieve is used. The sieve effect is better.
In order to improve the working efficiency of the vibrating screen, it is necessary to select a larger mesh size, a larger effective screening area, and a higher opening ratio of the mesh surface as much as possible according to the actual conditions and the product particle size requirements. Metal screen surface. And select the appropriate mesh shape to improve the permeability of the material particles and work efficiency.
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2.2 Reasonable selection of vibration motor and adjustment of exciting force
Reasonable selection of vibration motor is one of the key factors affecting the performance of vibrating screen, and the magnitude of excitation force is the core factor affecting the productivity of vibrating screen.
2.2.1 Selection of vibration motor
As the vibration source of the vibrating screen, the vibrating motor should have the advantages of reasonable design, simple structure, compact, high excitation efficiency, energy saving, convenient installation and debugging, etc. The selection of the vibration motor includes parameters such as working frequency, maximum excitation force and power. The first is to select the operating frequency and the exciting force. The speed of the vibrating motor is close to the operating frequency; the maximum exciting force must be within the range of the combined excitation force of the selected motor. Then select the power of the vibration motor according to the operating frequency and the maximum exciting force.
2.2.2 Adjustment of excitation force
The productivity of the vibrating screen is exponentially related to the exciting force, and the increase in the exciting force causes a rapid increase in productivity, and the blocking rate rapidly decreases as the exciting force increases. The increase of the excitation force increases the vibration intensity, the force of the screen facing the material increases, the upward movement speed of the material increases, the productivity is improved, and the clogging rate decreases. The exciting force also has a certain influence on the passing rate and breaking rate of the sieve, and the variation law is wave-shaped. When the excitation force is too small, the pass rate and the breakage rate are inferior; when the excitation force is too large, the friction of the eccentric block at both ends of the vibration motor shaft is increased. In the case of high-speed rotation, the motor is easily damaged and the motor is reduced. The service life, so it is important to adjust the magnitude of the exciting force. The exciting force of the vibration motor is the centrifugal inertial force generated by the eccentric mass that rotates at a high speed. By changing the eccentricity and thus changing the amplitude of the exciting force, the effect of adjusting the exciting force can be achieved.
2.3 Improve the way of screen movement
The movement of the screen surface has a great influence on the working efficiency of the vibrating screen. The ideal screen movement should be: 1 The amplitude of the vertical direction of the feed end of the screen should be greater than the vertical amplitude of the discharge end. This is because the feed end has a large vertical amplitude, which can effectively stratify the thicker material at the end, and at the same time, the excess material at the end is quickly spread to the middle of the screen surface by the inclination angle, so that the fine material is relatively relatively The layered permeable screen in the thin material layer improves the actual use area of ​​the screen surface; when the material reaches the discharge end, the material has been divided into layers, and only a small vertical amplitude is needed to ensure the fine material. There is a good permeable condition, and too much vertical amplitude amplitude disturbs the permeable environment of the fine material. 2 along the length of the screen surface, starting from the feed end, the material movement speed should be in a decreasing state. This is because the moving speed of the material is reduced, so that the material layer can maintain a certain thickness on the entire screen surface, so that the fine material is layered and sieved on the relatively longer sieve surface to increase the actual use area of ​​the sieve. The sifting amount of the screen surface along the length of the screen tends to be uniform, and the sieving potential of the screen surface is fully utilized.
The equal-thickness screen and the double-frequency vibrating screen developed in recent years overcome the shortcomings of the common screen amplitude of the common vibrating screen and the low per unit permeable screen, so that the feeding end has a larger amplitude, and the discharge end has The same amplitude as a normal vibrating screen improves work efficiency. Recently, some scholars have obtained a new type of vibrating screen-complex motion track vibrating screen through the transformation of circular vibrating screen or linear vibrating screen. It has simple structure, low power consumption and high work efficiency. The sieve moves in a circular motion at the feeding end, the middle sieve surface performs an elliptical motion, and the discharge end performs an approximately linear arc movement, and the trajectory of the sieve surface along the sieve length direction changes point by point, along the sieve length The direction of material movement is decreasing. This not only improves the screening efficiency and work efficiency, but also reduces the production cost, which is an ideal screen movement mode.
2.4 using non-metallic mesh
The use of a thin elastic screen as a screen base is very advantageous for improving the working efficiency of the vibrating screen. The screen is generally made of rubber or polyurethane material, which can reduce the adhesion between the screen surface and the material to be screened, so that the material can generate secondary high-frequency vibration, avoid the blockage of the screen hole, and enhance the permeability of the material. And can alleviate the mechanical effect of the sieved material on the screen surface, and can withstand the large amplitude vibration more than the steel screen. Non-metallic mesh has the following advantages: 1 Improve screening efficiency. It can increase the screening efficiency by about 15% compared to metal screens. 2 good wear resistance and long service life. Its average life is more than 2.5 times that of metal mesh. 3 Reduce installation time and increase equipment operation rate. Since the service life of the non-metallic screen is greatly extended, the number of replacements of the screen surface is reduced, and the operation rate of the equipment is generally increased by 15% compared with the metal screen. 4 Reduce noise and improve the working environment. In addition to resonating with the screen box during operation, the wire mesh also produces some chattering. This phenomenon is more obvious after wear, and the steel material colliding on the box surface and the vibration of other components produce higher noise. Non-metallic mesh screen The whole sieve plate is a whole, which has a certain buffering effect and can reduce the noise by about 20 dB(A).
2.5 Adopt multi-feed mode
The vibrating screen generally uses one way feeding material. After the material is supplied to the screen surface, most of the materials smaller than the separating particle size pass through the sieve hole at the feeding end and become the undersize product, and 1/3~1/ from the discharge end. The screen surface of 2, in addition to continuing to play a certain screening role, mainly plays a role in transportation, so the utilization rate of the screen surface is not high. If multi-feeding is used, it is equivalent to increasing the width of the screen surface, reducing the thickness of the layer supplied to the screen surface, which is advantageous for the fine-grained material to quickly contact the screen surface through the sieve hole, and at the same time, fully utilize the screen surface. , reducing the unnecessary transportation distance of coarse particles, thereby improving the efficiency of screening.
2.6 Strengthening operation management
Operation and maintenance also have an impact on the performance of the vibrating screen. In order to make the vibrating screen operate at high efficiency, it must be carefully operated in strict accordance with the operating procedures. For example, the feeding should be uniform, continuous and appropriate, and the material should be evenly distributed along the entire width of the screen to facilitate the screening of fine particles and obtain high processing capacity. And screening efficiency. In addition, it is necessary to strengthen the maintenance and maintenance of the screen machine, such as timely cleaning the screen surface and repairing, replacing the damaged screen surface to ensure the integrity of the equipment.
3 Summary
The vibrating screen is the main equipment of the screening process. The design, manufacture, process operation, maintenance and operation management of the vibrating screen have a great influence on the normal and reliable operation of the equipment, which in turn affects the efficiency of the production and the quality of the product. Mastering the influencing factors and rules of the screening efficiency of vibrating screens and improving the screening efficiency of vibrating screens is of great significance to ensure the stable production and high yield of screening process engineering. There are many factors affecting the efficiency of vibrating screen screening. To improve the working efficiency of vibrating screen, it is necessary to take appropriate measures according to the actual situation and comprehensive measures to maximize the screening efficiency under the premise of ensuring the processing volume. At present, vibrating screen products are developing in the direction of large-scale, heavy-duty, spatialization, standardization, serialization, generalization, etc., seeking new forms of vibration to improve processing capacity and screening efficiency, and to optimize the system of screening machinery. Solving the problems of reliability, fatigue life, vibration and noise pollution caused by vibration working methods, and researching the screening technology of difficult-to-screen materials are issues that need to be continuously explored in the development of vibrating screens.
references
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[3] Li Yufeng, Li Yongzhi, Pan Dongming, et al. Determination of kinematic parameters of linear vibrating screen[J]. Coal Mine Machinery, 2008(3): 33-34.
[4] Wang Hongbing, Zhang Luxia. Analysis of Several Factors Affecting the Screening Effect of Linear Vibrating Screen[J].Modern Manufacturing Technology and Equipment,2008(4):29-31.
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