210 FARM MACHINERY 



at the rear in order that a larger opening be provided 

 for the straw to pass through as it is expanded in the 

 operation of threshing. As a rule, it is advisable to use 

 few rows of concave teeth and set them well up against 

 the cylinder, as there is little chance of the concave be- 

 coming clogged. 



286. Cylinder and concave teeth. The teeth in both 

 the cylinder and the concave are curved backward 

 slightly to prevent the straw being carried past the cylin- 

 der without being threshed. Teeth become more rounded 

 by use and reduce the capacity and interfere with the 

 proper working of the machine. It is stated that a very 

 large amount of power is required when the teeth become 

 rounded off. When worn the teeth should be replaced, 



making it necessary to bal- 

 ance the cylinder before re- 

 placing in the machine, and 

 also calling for watchfulness 

 on the part of the thresher 

 lest some of the new teeth 

 become loose and cause 

 damage. The teeth are usu- 

 ally made of a good grade of 

 mild steel, yet certain manu- 

 facturers prefer tool steel 

 with a hardened edge. No 

 doubt the latter wear better. 



FIG. I58-THE GRATE AND CONCAVE . Q{ 



parallel bars, with openings between, designed to retard 

 the straw and allow a large portion of the grain to pass 

 through to the grain conveyor before reaching the straw 

 rack (Fig. 158). 



288. The beater. After passing through the cylinder 

 and concave and over the grate the grain comes in con- 



