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Indiana University Studies 



by the first set of weights and thrown against the retaining 

 case of the machine from which it rebounds in the way of the 

 next set of weights to receive a greater impact due to its own 

 motion as well as the motion of the swinging weight. These 

 mills are rotated at speeds from 700 to 1,500 revolutions per 

 minute. The lower walls of the mill may be made of cast-iron 

 screens so that the pulverized material can escape, while the 

 uncrushed stone is carried around again till it is reduced to a size 

 that will pass the screen. The advantage of these mills is that 

 the screening and pulverizing can be done in one machine. One 

 drawback to their use is the large amount of power used to drive 

 them. The cost of pulverizing with such a mill is about 10 

 cents per ton for limestone when the output is about 200 tons per 

 day. This figure will be increased for smaller output and de- 

 creased for increased output. The high speed of rotation of the 

 machine tends to drive out the crushed product by air pressure, 

 the swinging parts acting as fans. Many of these machines are 

 coming into use for this work and all seem to be giving satisfac- 

 tion. 



The ring-roll mill is a mill in which the rollers are placed 

 inside a ring or cylindrical case and the crushing force comes 

 between these rolls and the inside surface of the mill. The 

 rotation of the ring imparts a motion in the same direction to the 

 rollers, and since they are held firmly to the inside of the ring the 

 material in passing between the ring wall and the roller is brought 

 under great pressure. Much of the force applied is due to the 

 centrifugal force of the rotating parts, and this force keeps the 

 material confined to the outside of the ring and tends to draw it 

 under the rolls. The great advantage of the ring-roll mill lies 

 in the fact that there are few wearing parts. The mill is very 

 accessible, and the speed is slow enough so that there is little 

 vibration, and the parts can be so well balanced that elaborate 

 foundations are not necessary. Another advantage is the fact 

 that the consumption of power per unit of output is comparatively 

 small. These machines will handle about 1 ton per horse-power 

 per hour in limestone crushing. For the larger units the}^ aie 

 built in duplex, that is, with two identical machines on a single 

 shaft; this has the advantage of less cumbersome parts and less 

 vibration with less cost of repair because the smaller parts of the 

 two mills cost less than the larger parts of a large single mill. 



The cost of handling limestone thru such a machine from 2- 

 inch size or finer, down to a size to pass a 20-mesh screen, can 



