EOAD MODELS. 



23 



that marked A in the model. The openings of the second section of 

 screen are 2 inches in diameter. The stones which pass through these 

 openings into bin B are called No. 2 stone. The third or last section of 

 screen has 3-.inch openings, passing No. 1 stone into bin C. The tail- 

 ings, or larger stones, will be forced through the opening at the end 

 of the screen, from which they will drop into the tailings conveyor, 

 to be crushed finally or eliminated from the work. The jaws of the 

 crusher should be set so as to make as few tailings as possible. The 

 sizes of these screen openings may vary, according to the quality of 

 the stone. For instance, in crushing a hard stone, the openings of 

 each section may be of slightly smaller diameter, since it is customary 



Fig. 1.— Model of portable crushing plant in operation. 



to crush a hard stone to somewhat smaller grades than a softer 

 material. 



When soft stones are being crushed, a dust jacket having a |-inch 

 mesh may be placed over the first section to eliminate dust from the 

 screenings. The bins A, B, and C, for receiving the various sizes of 

 crushed rock, should be provided with slanting metal bottoms and 

 sliding doors, so that the material may be loaded into wagons by 

 gravity. 



Two types of crusher are commonly used, one the jaw crusher, 

 shown in figure 1, and the other the gyratory crusher. The jaw 

 crusher is generally used for portable plants. In this machine one of 

 the jaws moves backward and forward by means of a toggle joint 

 and an eccentric, and the stone descends as the jaw recedes. As the 



