Factors affecting the screening effect of vibrating screen

        The screening effect of a vibrating screen is related to many factors, including material properties, screen surface structural parameters, and vibrating screen motion parameters.
        1. Material characteristics
        The nature of the material is an important factor affecting the screening effect. During the use of vibrating screens, enrichment phenomena often occur, such as the screen being piled up, the effective screening area becoming smaller, and the process efficiency decreasing. This is mainly related to the type of material composition, loose density of materials, and particle size of materials.
        1) Material type and particles
        Different types of materials have different physical properties. The types of materials can be divided into two categories: brittle and viscous. Viscous materials are prone to forming dense adhesion during the vibration screening process, blocking the screen and reducing the permeability. However, brittle materials are not, and the process efficiency can be guaranteed. The particle shape of the material will affect its sieving efficiency. Materials with cubic and spherical shapes are easy to pass through the sieve, while sheet materials are easy to get stuck in the sieve holes, which affects process efficiency.
        2) Loose density of materials
        The material particles are basically layered and sieved according to the size of the particle volume, that is, the loose density of the material directly affects the processing capacity of the vibrating screen. Bulk materials with higher loose density are easily screened and have higher screening efficiency; On the contrary, materials with lower loose density and powdery materials are not easily screened, resulting in lower screening efficiency.
        3) Material humidity
        The high moisture content of the material can easily cause adhesion. During the vibration process, the blocks compress each other, making the adhesion more dense, thereby increasing the resistance of the material movement at the force port, making it difficult for the material particles to layer and pass through the screen; Material adhesion also reduces the size of the sieve hole, blocks the sieve hole, and reduces the effective screening area. Materials with high water content sometimes cannot even be screened. Therefore, when the moisture content of the material is too high, the screening process should consider some remedial methods, such as taking measures such as drying the material.
        4) Material particle size composition
        The probability of material passing through the sieve during the screening process is influenced by many factors, among which the most direct and important factor is the relative size of material particle size and sieve hole size. The ratio of material particle size to sieve size is called relative particle size. The smaller the relative particle size, the higher the probability of passing through the sieve. When the relative particle size approaches 1, the probability of passing through the sieve approaches zero. Therefore, materials with a relative particle size equal to 0.7-1 are usually referred to as difficult to screen materials or critical materials. The higher the content of difficult to screen materials in the material, the lower the probability of passing through the sieve and the lower the screening efficiency; On the contrary, the higher the screening efficiency. In order to achieve higher screening efficiency, the content of difficult to screen materials in the aggregate should be minimized as much as possible. In addition, the difference in particle size index also has a certain impact on screening efficiency. Excessive differences in particle size index can cause a large production cycle, which restricts the quality and processing capacity of the product. Therefore, the grading of aggregates should be strictly controlled.

        2 Sieve surface structural parameters
        1) Screen surface 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 screening efficiency. Wide screen surface, increased effective area, and increased productivity. The longer the screen surface, the longer the material stays on the screen surface, and the more opportunities for screening, the higher the screening efficiency. However, the longer the screening efficiency, the better. The relationship between screening efficiency and the time the material experiences on the entire screen surface is complex. At the beginning, due to the large number of fine-grained materials on the screen surface, the higher the probability of screening per unit time and the higher the screening rate. After a certain period of time, the particle size of the material left on the screen surface is mostly difficult to screen material close to the size of the screen hole. Even if the material stays on the screen surface for a long time (i.e. the length of the screen surface is large), it is difficult to improve the material's screening rate, leading to a decrease in work efficiency. Therefore, selecting an appropriate screen length can effectively improve screening efficiency. In fact, the width of the screen surface has varying degrees of impact on the screening efficiency and the length of the screen surface has varying degrees of impact on the screening ability. The matching between the two is also important, and generally the width to length ratio is 1:2-1:3.
        2) Sieve shape
        Although the shape of the sieve pore mainly depends on the requirements for the particle size of the screened product and the use of the product under the sieve, it also has a certain impact on the screening efficiency. Compared with other shapes of sieve holes, circular sieve holes have smaller particle sizes when passing through the same nominal size. For example, the average maximum particle size of particles passing through circular sieve holes is only 80% to 95% of particles passing through square sieve holes of the same size. The effective area of the rectangular sieve surface is large, the weight of the sieve surface is light, and the production capacity is large. At the same time, the particle size of the material passing through the sieve hole is greater than that passing through the circular and square sieve holes with the same nominal size. Therefore, in order to achieve higher screening efficiency, it is necessary to choose different mesh shapes for different screening materials.
        3) Sieve size and sieve surface porosity
        When screening materials for a certain amount, the mesh size has a significant impact on screening efficiency. The larger the mesh size, the stronger the screening ability 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 screened material. The opening rate of the screen surface refers to the ratio of the opening area of the screen surface to the area of the screen surface (effective area coefficient). A high porosity increases the probability of material particles passing through the sieve, resulting in a higher processing capacity of the vibrating screen. On the contrary, the processing capacity of the vibrating screen is low. Therefore, in order to improve screening efficiency, a screen surface with a larger effective area coefficient should be selected.
        4) Sieve surface material
        Non metallic sieve surfaces, such as rubber grading sieve surfaces, polyurethane woven grading sieve surfaces, polyurethane slotted sieve surfaces, nylon sieve surfaces, etc., due to the characteristics of these non-metallic materials, secondary high-frequency vibration is generated during the screening process, making it difficult to block holes, which is beneficial for material screening and improves the processing efficiency of the vibrating sieve compared to metal sieve surfaces.

        3 Vibration characteristic parameters
        The vibration characteristic parameters include vibration frequency, amplitude, vibration direction angle, and screen surface inclination angle.
        1) Screen surface inclination angle α
        The angle between the screen surface and the horizontal plane is called the inclination angle of the screen surface. The size of the inclination angle is closely related to the processing capacity and screening efficiency of the screening equipment. When the inclination angle increases, it will increase the throwing strength of the material on the screen, thereby accelerating the forward movement speed of the material on the screen surface, increasing the processing capacity of the vibrating screen. However, the residence time of the material on the screen surface is shortened, reducing the opportunity for particles to pass through the screen, and thus reducing the screening efficiency. On the contrary, it will reduce the processing capacity and improve screening efficiency. In order to control the screening efficiency of the vibrating screen within an ideal range, the inclination angle of the screen surface of the circular vibrating screen is generally between 15-25 °, and the inclination angle of the screen surface of the linear vibrating screen is between 0-8 °.
        2) 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, the shorter the distance the material moves each time it is thrown, and the slower the material moves forward on the screen surface. The material can be fully screened, thus achieving greater screening efficiency. The smaller the value of the vibration direction angle, the farther the material is thrown and advanced each time. The faster the material passes through the sieve surface, the higher the processing capacity, but the material cannot be fully screened. Therefore, the vibration direction angle should be reasonably selected. For difficult to screen materials, the vibration direction angle should be taken as the larger value, while for easy to screen materials, the vibration direction angle should be taken as the smaller value; In general, the vibration direction angle of a circular vibrating screen is 90 °, while the vibration direction angle of a linear vibrating screen ranges from 30 to 60 °, often using 45 °. This value not only has good adaptability to various screening performance, but also can achieve the best motion speed and higher productivity.
        3) Amplitude A
        The increase in amplitude will greatly reduce the blockage of sieve holes and also facilitate the layering of materials. But too large an amplitude can also cause significant damage to the equipment. The selection of amplitude is based on the particle size and properties of the screened material. In general, the larger the size of the vibrating screen, the larger the amplitude selected. When a linear vibrating screen is used for grading, the amplitude is slightly larger; When used for dehydration and desliming, the amplitude should be smaller. When the particle size of the processed material is large, the amplitude should also increase accordingly; When the particle size of the processed material is small, the amplitude should be smaller. Usually, the amplitude A of a linear vibrating screen is 3.5~6mm.
        4) Vibration frequency ω
        The increase in vibration frequency can increase the number of jumps of the material on the screen surface, resulting in an increase in the probability of the material passing through the screen. This is very helpful for accelerating the screening speed and improving the screening efficiency, but too high will reduce the service life of the equipment. For materials with larger particle sizes, choose larger amplitudes and lower frequencies; For materials with finer particle sizes, choose smaller amplitudes and higher frequencies. www.jiashengsz.com