CraneE.| Prospecting, Mining and Milling Machinery. 269 
ings, Fand G. This plate has a receptacle, O, for the hel- 
ical spring, Z, and is also provided with a threaded hole 
through which the hand bolt, K, passes and engages with 
the threads. The boxing, M, has a corresponding receptacle 
or seat for the other end of the spring. The arrangement of 
boxing, spring and hand screw is shown in figures 50 and 52. 
Cylinders of car-bumper rubber are occasionally used in place 
of the helical springs. 
On the shaft of the front roller, V, is keyed a large spur 
wheel, P, which is driven by the small pinion or cog wheel, 
C. This is to allow a high speed to be given to the shaft, B, 
which is, by this gearing down, reduced to a relatively slow 
speed for the rollers. The front rolleris provided with flanges 
on both ends, while the back or rear roller has none. See 
figures 50 and 51. This is to hold the material crushed be- 
tween the rollers and the rollers themselves in the same rela- 
tive position. 
The degree of fineness of the material crushed is regulated 
by the tension given to the spring, ZL, all other conditions 
being constant. 
A hopper is bolted to the supporting castings, Q. It is 
made of cast iron, and is provided with plates, Z, bolted to 
the sides to protect it from wear, and to prolong the sides of 
the hopper down between the rollers (3, figure 65). A trough, 
W, catches the crushed material and conducts it on its way 
to some other apparatus in the mill. Another trough, U, 
bears the feed material to the roll. 
When coarse material is to be crushed, star wheels (4, fig- 
ure 52), or misshapen cog wheels, are put on the roller shafts, 
X and Y, figure 51. These star wheels are to keep both 
rollers going at the same speed, and to allow for a certain 
amount of lateral movement between them. They are made 
as large and heavy as is consistent with their strength and 
durability to insure against breaking, which would be of fre- 
quent occurrence if the cogs were small. These wheels are 
seldom used except on the first set of rolls, and when the 
breaker is set to crush fine they are often dispensed with on 
this set also—friction being depended upon to drive the second 
roller. 
