STATE GEOLOGIST. 261 
time. This is a considerable advantage over ball-mills or more compli- 
cated apparatus. Also when the rolls used for finer crushing have been 
worn they may be placed in position as coarse rolls. The rolls are held 
in position by car-wheel springssor rubber blocks, the latter being, 
however, not suitable for heavy use. The pressures exerted by these 
springs are indicated by the following table :* 
{ 
WPresetre exerted by the two sets of springs 
Length of spring in |Amount of compres- In pounds. 
inches. sion, in inches. 
| On 26 inch roll. On 24 inch roll. 
7.9375 0.0 0 @) 
7.4375 0.5 | 15,000 22,500 
6.9375 1.0 30,000 37,506 
6.4375 1.5 41,250 52,500 
5.9375 2.0 1 A3780 Peer 250 
The rolls are driven either by direct belting or by belts and gears. 
lor slow speeds the gears are preferred, for higher speeds the belts. 
In working with rolls it is absolutely essential that the material to be 
ground be fed uniformly, and hence the rolls must be provided with 
simple but efficient feeding devices. The quality of crushing depends 
to a great extent on the way they are run. When run slowly the material 
may be crowded so that the fine particles cannot separate from the 
coarser, and hence are reduced some more by abrasion upon each other. 
This is called “choke” crushing. 
When the speed is increased the fine matter is removed as fast as 
formed; this is called “free” crushing. This is the usual and most 
satisfactory method. The power required differs, of course, with the 
size and work done. On an average according to Richards it is Io 
horsepower for 100 tons in 24 hours. The cost of crushing is from 
3 to 6 cents a ton for limestone, not including the truing of the shells. 
In regard to the space between the rolls, which may be from 
nothing up to 34 inch, the angle of nip plays an important part. For 
coarse rolls, a rough rule says that the distance should be one-half the 
diameter of the largest lumps fed. 
The angle of nip is one-half of the angle made by the tangents to 
the rolls, at the points of contact aa, between the lump of rock, e, assumed 
to be a sphere, and the rolls. These tangents form the angle 2n, 
*R. H. Richards, Ore Dressing, p. 74. 
