148 SCIENCE OF SUCCESSFUL THRESHING. 



Screens. A screen removes particles smaller than the 

 grain or seed being threshed, such as weed seeds, sand, or 

 other foreign matter which is usually valueless. Sometimes, 

 however, a useful seed, such as timothy is screened out of 

 one of the large grains, as oats. In general, for weed seeds 

 that are approximately round, the round hole are better than 

 the oblong hole screens. However, the latter are the only 

 ones that will take out "cheat" which is often found in wheat. 

 The screen lies in the bottom of the shoe and is held in place 

 'by hooks with thumb nuts which engage castings fastened 

 on the frame of the screen. When a screen is used the re- 

 movable strip in the bottom of the shoe is taken out to allow 

 the screenings to fall on the ground. All screens are liable 

 to become clogged and in this condition are an obstruction to 

 the grain and wind. They should therefore be kept clean 

 and only used when necessary. The list of screens is given 

 on page 147 and they are illustrated on page 149. 



The Tailings Elevator returns to the cylinder for a 

 second threshing the unthreshed heads and all trash, which 

 is too coarse to fall through the sieves and too heavy to be 

 blown out by the blast. It consists of an elevator with cups 

 or flights carried on sprocket chain, into which the tailings 

 are delivered by an auger (called the tailings auger), and 

 a spout to carry the tailings from the end of the elevator to 

 the cylinder. This spout has an auger on some separators 

 and it is then called the "tailings conveyor." The tailings 

 elevator is driven from the crank-shaft with a crossed belt 



