260 ANNUAL REPORT 
for this purpose is so promising that the writer has given them first 
place in this classification, since it appears to him thac these machines 
do the work of intermediate grinding at the lowest cost. Rolls should 
not be fed with material over 11% inches in diameter. it is not an easy 
matter to give the best size for rolls intended to prepare material for fire 
grinding, since the velocity, the size of the feed, and the character of 
the material to be ground are important factors. One large manufacturer 
of rolls gives 24x14 inches, another 26x14 inches as the best stamdard 
rolls. As a whole, larger diameters are to be preferred, since the in- 
creased surface means a greater capacity and since they can make a greater 
reduction with one passage and lastly because they can be run at a greater 
speed owing to their favorable angle of nip. Yet there must be con- 
sidered the fact that two small rolls cost less, give a more uniform 
output, are cheaper to run, and repair and wear more uniformly. But 
it seems they should not be less than 24 inches in diameter for cement 
work. The width varies from! 12 to 46 inches. Narrower rolls are 
easier to keep true and by running them faster may have the same capacity 
as wider rolls. ‘The average peripheral speed of many rolls was found 
by Professor Richards to be 379 feet per minute, and the average 
velocity of rolls crushing breaker products 345.7 fect per minute. With 
a peripheral speed of 300 feet a 24-inch roll would have to run 48 revolu- 
tions per minute; with a speed of 400 feet, 64 revolutions. 
There is a tendency to advocate high speeds in rolls, as higher speeds 
make the rolls run more smoothly, and make the action more “free” 
crushing, and also to use difffferential rolls, one running faster than the 
other. Differential rolls have been used to some extent and the experi- 
ence seems to be that for hard crushing like quartz, differentiation does 
not help much, but that for limestone and clay the case seems to be 
different. In one instance, one roll running 25 per cent. faster than 
the other prevented the formation of ribbons, that is, pressed flakes, 
while the wear was slightly more, but much more uniform. 
The shells have been made of various materials, chilled cast iron, 
cast steel, chrome steel, hammered steel, manganese steel, rolled steel 
and forged steel. Chilled iron is cheap, but pits and wears rapidly, chips 
and is hard to true up. Cost, 2 to 4 cents per pound. Cast steel costs 
from 6 to 64 cents per pound, but its surface is not very reliable. 
Forged steel is the most reliable material and wears uniformly with 
some attention. Cost, from 6% to Io cents per pound. The chrome 
steel is said to be still better; cost, 10 cents per pound. It is important 
that the rolls be kept smooth and parallel. This is done by setting the 
rolls with different laps at the end of every day, say 14 on one end on 
one day and the same amount at the other end the following day, and 
turning them down or grinding the surface with emery. This can fre- 
quently be done with the rolls in place. If duplicate rolls are used, one 
can be used while the other is trued up, the changing taking but little 
