276 ANNUAL REPORT 
The velocity of the drum must be great enough so that the curvilinear 
motion can be well developed. Harder materials require a higher drop 
and heavier pebbles than softer, and steel balls can hence do the same 
work in smaller mills as flint pebbles in larger ones. ‘The greater the 
number of pebbles, the greater is, of course, the grinding effect. It was 
also found that the exit of the material being ground was at a much 
higher point than the inlet, and hence it is not necessary to incline the 
axis of the mill toward the discharge end. - 
What causes the material to forward is now the question. As the 
pebbles descend from their highest point and strike the material to be 
ground the latter is splashed quite a distance, and if there is much 
material present a good deal will thus be distributed, while places where 
there is little throw off but a small amount. In this max:ner the material 
is distributed uniformly. This equalization takes place rapidly, as the 
velocity of the pebbles is quite great, being often as great as the circum- 
ferential velocity of the mill. At the exit the ground material is thrown 
out through the grating, while the pebbles are retained. 
The pebbles naturally wear, more or less rapidly according to the 
conditions under which they work. The statement was made by a builder 
of cement machinery that one pound of pebbles is used up for 30 barrels 
of cement clinker. This is certainly too small a figure. The writer was 
told by others that the cost of pebbles is 14 cent per barrel. 
In regard to the capacity of the tube-mill, it can be said that for 
grinding raw stock it frequently shows a capacity of © tons per hour, 
the size of the mill being 22 feet by 5 feet. For grinding cement this 
machine has a capacity of from 14 to 20 barrels per hour. The power 
consumed is 70 to 75 horsepower, though to start the mill a momentary 
power of about 120 horsepower is necessary. An actual test with five 
tube-mills, 22 feet by 5 feet, showed a power consumption of 333 horse- 
power, making 66.6 horsepower for each mill. 
The Griffin Mill.—The Griffin mill, shown in figure 45, consists 
essentially in a horizontal driving pulley (17) from which the shaft (1) 
is suspended by means of a universal bearing (9). To the lower extremity 
of the shaft the crushing roll (31) is secured, which is thus free to swing 
in any direction within the case. ‘Phe latter consists of the base or pan 
(24) containing the ring, or die (70), against which the roll works and 
upon the inner surface of which the pulverizing is done. In dry pulver- 
izing this pan has a number of openings through it downward, outside 
of the ring, which lead into a receptacle from which the material is taken 
by a conveyor. | 
Upon this base the screen (44) is secured which is surrounded 
with a sheet iron cover (45) and which is not used in wet grinding. 
