170 SCIENCE OF SUCCESSFUL THRESHING 



carry over grain when the machine runs empty. Steady 

 feeding is therefore important on this account and the 

 separator tender should let the pitchers understand that 

 he cannot produce the best results without their aid, in 

 keeping an even and continuous stream of grain going into 

 the cylinder. 



The Wind-Board is placed in the machine so that the 

 blast from the fan will strike the conveyor sieve about half 

 way back. The strongest part of the blast will then pass 

 through the shoe sieve near the front end which gives it a 

 cleaning capacity its entire length. If the wind board be- 

 comes bent or sagged so that it stands but little above the 

 floor of the shoe, the grain will slide over it into the fan, 

 and then be thrown clear out of the machine. To prevent 

 the liability of this, belts or "traps" should not be kept in 

 the fan drum. 



Fan Speed. The speed of the fan for 12-bar separator 

 should be about 470 and for the 2O-bar about 485 revolu- 

 tions. 



Sieves. The function of all sieves is to assist the fan in 

 separating the grain from the chaff and in preventing heads 

 and other heavy objects larger in size than the grain from 

 mingling with the clean grain. Sieves are distinguished 

 from screens in that the grain being cleaned passes through 

 them while it passes over a screen. 



Adjustable Sieves. To obviate the delay and trouble 

 of changing sieves each time the machine threshes a dif- 



