Within the framework of this article, the parameters of the air flow in the cavity of the cleaning chamber of a combine harvester with the use of experimental and standard sieves are analyzed. The task in this case was to determine the nature of the dependence of the air flow on changes in the gap between the combs of the upper louvre sieve. The study evaluated the magnitude and direction of the key vector of the air flow, as well as its elements. In the process of testing the cleanings in combines, measurements were made of the air flow speed over the sieves. Dynamic head was measured over the sieve with the help of Pitot tubes at 6 points in terms of length, within each section at a distance of 50 mm from the axes of fixing the louvre. In the process of cleaning with a standard sieve with the value of the gap between the combs of the upper louvre sieve of Svr = 7...13 mm, the speed index is practically constant, and in the case of Svr > 13 mm, the speed starts to constantly decrease from 4 to 2,50 m/s. The indicated regularity in the change of the speed index for standard sieves, undoubtedly, has a negative impact on the allocation of grain. Indeed, in the case of increasing the delivery volume of the straw heap, the size of the gap Svr between the blinds is expanded, in order to reduce the static resistance of the network. Nevertheless, with an increase in Svp, the airflow velocity and its loosening effect on the heap decrease. Accordingly, one of the reasons for the active increase in grain losses in chaff, in the case of increased supply of the straw heap is a decrease in the function ν (Svr). Under the influence of the vertical component of the velocity and oscillations of the sieves, the heap layer is transformed into a fluidized state, the value of the frictional force between the particles decreases, so that the process of grain passage through the layer is facilitated. The increase in Svr between the combs of the standard sieves causes a reduction in the velocity at the exit from it, and due to the latter there is insufficient loosening of the heap.