The process of passing a group of broken materials with different particle sizes through a single or multi-layer screen surface with uniform holes for many times and dividing them into several different levels is called screening. Particles larger than the screen holes remain on the screen surface and are called the oversize of the screen surface. Particles smaller than the screen holes pass through the screen holes and are called the undersize of the screen surface. The actual screening process is: after a large amount of broken materials with different particle sizes and mixed coarse and fine particles enter the screen surface, only a part of the particles are in contact with the screen surface. Due to the vibration of the screen box, the material layer on the screen is loosened, so that the gaps that already exist in large particles are further expanded, and small particles take the opportunity to pass through the gaps and transfer to the lower layer or the conveyor. Since the gaps between small particles are small, large particles cannot pass through, so the originally disorderly arranged particle group is separated, that is, they are layered according to the particle size, forming an arrangement rule of small particles at the bottom and coarse particles at the top. The fine particles that reach the screen surface and those smaller than the screen holes pass through the screen, and finally the coarse and fine particles are separated, completing the screening process. However, there is no complete separation. During screening, a portion of the undersize material generally remains in the oversize material. When fine particles are screened, although the particles are smaller than the sieve holes, they have different degrees of difficulty in passing through the sieve. It is more difficult for particles with similar sizes to pass through the sieve, and it is even more difficult to pass through the gaps between particles below the sieve surface.