Pneumatic separation of grain from straw and chaff in the combine harvester

© 1970 John Hunter ShellardThe riddle mechanism, used to clean threshed grain in harvesting machinery, has remained basically unchanged for over a century. The desired function and throughput of the cleaning mechanism in a harvester is discussed, together with the effectiveness of the conventional system. The published work on a combined harvester is reviewed; the history and development of the various components leading to their present configurations is examined together with work done on developing alternatives to the usual components in a harvester. Pneumatic separation in a vertical air stream, proposed as an alternative to the riddle system of cleaning the efflux from the threshing system of a harvester, is examined in depth. Theoretical prediction of the performance of a vertical air stream, as a separator, is shown to be intractable while predictions of separation based on terminal velocities of all the types of particles present in the mixture (which were measured fully) are shown to provide only a fair indication of likely performance. A small pneumatic separator was designed and built; the performance of this unit was investigated in detail; the effects of air velocity, feed rate, and of material composition are discussed. Likely ways of improving separation, together with the lines along which further investigations should be conducted, are discussed using the results obtained from the separation tests