CHAPTER XLI 

 THRESHING MACHINES 



Development. It is a big step of progress from the simple 

 flail to the modern threshing machine. The use of the flail 

 required the time of a man for the entire winter season to 

 thresh even a very moderate crop of small grain which he had 

 grown; whereas the modern machine is able to thresh hun- 

 dreds or even thousands of bushels in a single day, delivering 

 it cleaned and ready for market. 



The Operation of Threshing. The modern threshing ma- 

 chine performs four quite distinct operations. The first is the 

 process of threshing or shelling. This is accomplished when 

 the unthreshed grain passes between the teeth of a revolving 

 cylinder and those arranged in the concave. Second, the 

 machine separates the straw from the grain and chaff. This 

 operation is performed by the grate, the beater, the check 

 board, and the straw rack. Third, the grain is separated 

 from the chaff and dirt by screens in the shoe and by a blast 

 from the fan. Fourth, by means of the stacker and the grain 

 elevator or weigher, the straw is delivered to one point and 

 the grain to another. 



Cylinder. The cylinder of a threshing machine is built 

 up with heavy bars of steel mounted on disks or spiders, into 

 which the teeth are fastened by thread ends and nuts or by 

 keys. There are two sizes of cylinders in use, known as the 

 small and the big cylinder. The big cylinder usually has 

 about 20 bars. The speed at which a cylinder revolves will 

 depend upon its size, but varies from about 800 revolutions 

 for the big cylinder to 1100 revolutions for the smaller one. 



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