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It has been found for the nonsteady state conditions existing during test sieving on such machines as the standard Rotap sifter that the mechanism of sieving can be divided into two distinctly different regions with a transition region between.
Region 1 exists where there are many particles much less than the mesh size still on the sieve. An equation relating major sieving variables such as mesh size, particle size distribution, sieve loading, etc., was developed from experiments using four typical materials.
As particles much less than the mesh size are removed the mechanism changes and region 2 sieving begins. In region 2 all particles much less than the mesh size have been eliminated from the residue, and the particles remaining that can pass the sieve are very near mesh size. The cumulative percentage passing the sieve has been found to follow the log-normal law in region 2. From this fact an equation relating the major sieving variables in region 2 has been derived analytically and proven experimentally.
Using the laws developed for regions 1 and 2, the effects of material, sieve load, sieve motion, relative humidity and sieve material were investigated to a limited extent. These laws were also extended analytically to steady state sieving.
It is believed that the proposed sieving laws represent the best fit over the broadest range of conditions of any sieving laws that have been proposed to date.
Assisstant Professor of Mechanical Engineering, University of Minnesota, Minneapolis, Minn.