Process of clearing and fractionating of grains is considered. The new device containing the distributor and the chamber of separation is developed. Movement of the particles of the grain mixture taking off the distributor for the purpose of substantiation of parameters of the device, by criterion fractionating of the particles of the grain mixture on the weight is investigated. The equation of movement of any particle of the grain mixture is worked out and mathematical expressions for definition of the distance from the beginning of coordinates to the point when any particle of the grain mixture will rise on the maximum height are deduced. It is established, that the height of lifting of the grain changes in direct ratio initial speed, weight and in inverse proportion to the factor of resistance of air. The received expressions allow to investigate and define parameters of separator of division of particles on weight depending on initial speed, a corner of a start and factor of resistance of air.