SCIENCE OF THRESHING. 37 



made to pass through the chaff, as soon as it enters 

 the upper sieve it will lift and scatter it, the lightest 

 flying out first, thus giving the kernel an opportunity 

 to fall through the sieve much more quickly and 

 freely than if the blast were not strong enough to 

 keep the meshes open and clear of chaff. Besides, if 

 the blast ceases to flow through any of the meshes, 

 the fine dust and chaff will fall through and cause the 

 second sieve to be overloaded, and thus the grain may 

 retain a part of the chaff and dirt. 



SIEVE MOTION. 



The motion of the shoe should be sufficiently 

 strong and rapid to move the grain and chaff on the 

 surface of the sieves. Too strong a motion will 

 interfere with the kernels falling through the meshes 

 properly, and cause the grain to be carried over, either 

 with the chaff or into the tailings spout. It is found in 

 practice that the upper sieve or chaffer requires a 

 more violent and positive motion than the lower 

 sieves, on account of the great quantities of chaff 

 and cut straw to be carried. For this reason the 

 chaffer is usually placed in a different frame and 

 given a longer stroke while the remaining sieves have 

 a short and rapid motion. 



The coarser and looser the material to be handled, 

 the longer and more vigorous the motion should be. 

 The straw rack which is loaded with loose, fluffy 

 straw, requires quite a long stroke to be effective, 

 while the conveyor and chaffer sieves need less 

 motion and the shoe sieves still less. 



