SCIENCE OF THRESHING. 19 



to yield a little without becoming loose. A twisted 

 or spiral spring bar has been used for the same office. 

 Teeth do not loosen so badly in double as in single 

 bar cylinders, as the extra length of shank holds 

 them from sidewise movement when an excessive 

 amount of straw is encountered. A spring steel 

 split washer is sometimes used under the nut to keep 

 it fast. 



It is needless to add that worn teeth retard the 

 proper work of a separator and should be replaced by 

 new ones. 



THE CONCAVE. 



The concave, so called because of its dished or 

 hollow shape, consists of a rack or grate constructed 

 to conform to the cylinder under which it is adjust- 

 ably hung. It is provided with teeth which interact 

 with the cylinder teeth. It is adjustable to and from 

 the cylinder. The concave holds the concave teeth 

 in position between the cylinder teeth and forms a 

 floor or grate over which the straw passes while 

 being acted upon by the cylinder. The open space 

 between the teeth permits the shelled grain to fall 

 through as soon as dislodged from the heads, thus 



