34 SCIENCE OF THRESHING. 



and straw to pass out with the grain, causing it to be 

 dirty and poorly cleaned. 



The openings should be so shaped as to direct the 

 flow of the blast in the proper direction, which is 

 approximately at right angles to the sieve surface. 

 This prevents the blast from blowing the grain along 

 the sieve surface. If there are too many openings, 

 and this is the case in a wire sieve too much of a 

 blast passes through; this causes so strong a current 

 above the sieve that it prevents kernels which are 

 lifted from the surface, from falling again until the 

 rear of the sieve is reached. 



The sieves should be stiff and rigid so as not to 

 spring at the center; otherwise the motion will be too 

 violent there and throw the grain so high as to keep 

 it from falling through the sieve. A sieve frame 

 which travels two inches at each stroke and makes 

 two hundred and fifty strokes a minute, goes five hun- 

 dred inches every minute ; if its center springs an inch, 

 it goes three inches at every stroke, or seven hundred 

 and fifty feet a minute, a motion which may be alto- 

 gether too violent for that part of the grain which 

 falls on the center of the sieve. 



THE FAN. 



A fan is a device for producing a blast or current 

 of air. The usual form used in separators consists of 

 a central shaft from which radiate arms which carry 

 blades or wings. These revolve in a casing, and the 

 rapid motion of the outer ends of the blades forces 

 the air to rush out of an opening in the side of the 



