SCIENCE OF THRESHING. 33 



A properly constructed sieve permits the grain to 

 fall through it, passing the chaff and refuse over it, 

 without allowing the straw and sticks to be retained 

 in the meshes, and directs the blast in the proper 

 direction. In practice, a sieve adapted to each kind 

 of work is used, though some combinations of sieves 

 work very well on several different grains. The 

 usual plan is to permanently secure the upper sieve, 

 called the chaffer, it being of sufficiently large mesh 

 to adapt it to the coarsest grains. The lower sieve 

 or sieves are interchangeably secured so that they can 

 be varied according to the work to be done. 



Adjustable sieves 'are constructed to change the 

 size of the opening of the meshes to suit different 

 kinds of grain, the usual construction being a frame in 

 which are hinged cross slats. The adjusting of the 

 slats changes the opening, also changes the angle 

 of the blast through the sieve. When the slats are 

 tipped back the openings are reduced in size, and the 

 blast directed in a more backward direction. 



The openings in the sieve must be large enough to 

 permit the free passage of the clean kernels, and 

 should be sufficient in number not to retard the blast. 

 If the openings are too small or too few in number, 

 the grain accumulates on the sieve and passes over 

 into the tailings elevator, by which it is carried back 

 to the cylinder. This greatly reduces the capacity of 

 the machine as it needlessly increases the amount of 

 grain it is required to separate. If the openings, on 

 the other hand are too large, they permit the sticks 



