Sieve classification is one of the most important mechanical separation processes. Therefore, this process was investigated by modeling. The grade efficiency is calculated from the measured particle size distributions and mass of the individual fractions. The models depend on three or four parameters. These include the bypass parameter a, the parameter xcut, which defines the median of the grade efficiency curve, as well as the parameter α for the sharpness of the classification. Additionally, a third sharpness parameter β is introduced. A parametric study was carried out with two tumbling sieving machines.