The article presents the theoretical study of the passage of materials trash particles through the screening surface.It indicated that the small impurities content in seeds is relatively low and their selection takes place through small holes. Because of the low probability of passage of particles in a dense layer of the seed through the holes of the screening surface, the efficiency of separation of fine impurities can not be high.It is also pointed out that the efficiency of allocation of trash increases with the length of the screening surface. With the increase in diameter of the holes of the screening surface difference probability sieving particles of various lengths through the same hole decreases.It was noted that the fine waste in the seed is relatively small and its separation takes place in small holes. Since the probability of the waste passing through the holes of the waste discharger through the dense seedbed is relatively low, the waste removal efficiency will also not be high.Also the efficiency of separation of waste mixtures depends on the length of the mesh surface.As the diameter of the discharge surface holes increases, the difference in the probabilities that different lengths of waste will pass through the same hole decreases.