Annotation Purpose. Refinement of the mathematical description for calculating the movement of particles of sunflower material in the working gap of the threshing-separating device of the plot combine. Methods. Theoretical studies were carried out on the basis of the analysis of the dynamics of the movement of particles of sunflower material relative to the surface of a closed-type drum of a selection combine. Results. A mathematical description of the movement of particles of sunflower material in the working gap of the threshing-separating device of a selection combine with a closed drum is given. The dependence of the speed of movement of material particles on the initial speed of material feeding has been established. The dependence of the particle velocity on the radius of the closed-type drum of the selection combine has been established. The dependence of the speed of movement of the mass of sunflower material under the action of a closed-type drum of a selection combine on the coefficient of friction of the material is obtained. Conclusions. The obtained theoretical dependences of the speed of movement of the mass of sunflower material in the working gap of the threshing device of the selection harvester make it possible to substantiate the rational design and technological parameters of the closed-type threshing drum. Keywords: closed drum, speed, movement, coefficient of friction, plot combine, sunflower.