The particles with different particle sizes are divided into several different levels by means of a single layer or multilayer sieve with uniform holes. The particles larger than the sieve holes are left on the sieve surface, which is called the sieve surface of the sieve, and the particle is less than the sieve hole through the sieve, which is called the sieve of the sieve surface. The actual screening process is: a large number of particle size is different, thick and thin mixed material into the sieve surface, only a part of the particles and sieve surface contact, due to the vibration of the sieve box, the sieve material layer is loose, so that large particles of the existing gap is further expanded, small particles passing through the gap, transfer to the lower or conveyor. Because small particles clearance small, large particles can not pass through, so the original disorderly arrangement of the particle group has been separated, that is, according to the size of the particle stratified, forming a small particle in the lower, coarse particles on the ranking of the rules. The fine particles which reach the sieve surface are smaller than the sieve, and finally the coarse and fine particles are separated to complete the sieving process. However, the full separation is not, in the screening, there is usually a part of the sieve to stay in the sieve. Fine-grained sieve, although the particles are smaller than the sieve, but they are difficult to penetrate the degree of different, material and sieve size of similar particles, through the sieve is more difficult, through the surface of the lower layer of the particle gap is more difficult.