SCIENCE OF THRESHING. 53 



separating device or shoe is not level; or the machine 

 is being crowded beyond its capacity. 



The shoe or cleaning mill should receive special 

 study by anyone who intends to make threshing a 

 success. Though the device appears very simple, it 

 has many features, as already stated, which are not 

 generally understood. 



The motion of the shoe varies in different 

 machines. In some it is as short as five-eighths of an 

 inch, in others it is four inches; though there is not 

 so much speed variance in the motion of the upper 

 sieve or chaffer, which generally is from three to four 

 inches. The following rule is as close to a correct 

 expression of the true principle of its working as can 

 be stated: 



"The shorter the stroke, the more vibrations per 

 minute are required; the longer the stroke, the fewer 

 vibrations per minute are required." 



The speed should be just strong and quick enough 

 to throw the straw but slightly at the upper finish of 

 the stroke; if it does more than this, it will carry or 

 throw the grain, as well as the dirt, and a part of it 

 will pass out with the chaff. If the motion is not 

 strong enough to cause the grain to leave the upper 

 surface of the sieve slightly at each stroke, the 

 meshes become filled with grain and chaff, and the 

 sieve or chaffer becomes choked to such an extent as 

 to allow but little to pass through. If the mechan- 

 ism is such that the upper part of the stroke is strong 

 and quick, while the lower part is slow and gentle, it 



