28 SCIENCE OF THRESHING. 



Motion is imparted to the racks by means of a 

 crank. In some instances the rack is hung to links 

 so that the crank gives it a swinging motion like a 

 pendulum, as is shown in the adjacent cut, its path 

 being concave, or the rack may be supported on the 

 upper ends of rockers in which case it oscillates back 

 and forth in an upwardly curved or convex path. 



The rack has a tendency to jar and compress the 

 straw on each upward stroke, tossing it into the air 

 very slightly and moving back from under it. The 

 elasticity of the straw causes the mass to expand 

 while clear of the rack. As it falls, it meets the now 

 ascending rack and is thus given another sharp com- 

 pressing blow which tends to move the different 

 straws in the mass upon each other. The most 

 effective blow is given by the rack at the middle of 

 the stroke, as the peculiarities of the crank motion 

 cause it to travel fastest at about this point. 



It is plain that if a quantity of straw and grain 

 should lie on the rack and vibrate up and down with 

 the rack, and not move away from it, the stalks 

 would maintain their relative positions and not give 

 the kernels a chance to fall out. To be effectively 

 cleaned, each stalk must be moved as related to its 

 neighbor. This inter-motion of the stalks constitutes 

 the chief feature of effective separation. 



The motion of the rack should be such that its 

 upward stroke will move the straw onward slightly, 

 while the rack is permitted to descend from under it. 

 Gravity will then start the straw downward with 

 increasing speed as it approaches the rack. The 



