262 - ANNUAL REPORT 
half of which is called the angle of nip. If the lump is as large as 
the space between the rolls, the angle is, of course, zero, increasing until 
the angle is so large that the rolls cannot nip the rock. The angle 
is a function of the diameter of the rolls, the diameter of the lump and 
Fig. 35. Diagram illustrating the angle of nip of rolls. 
the distance between the rolls. It is diminished by increasing the diam- 
eter of the rolls, by increasing the space between the rolls and by decreas- 
ing the size of the lumps. 
If b=the radius of the sphere to be crushed, 
a=, the space between the rolls, 
n=the angle of nip, 
and r=radius of the roll=¥% diameter, 
rta 
rb. 
In practice the angles of nip are found to be from 4° 16’ up to 
24° 5°, with, according to Professor Richards, an average of about 
LR? ROY 
The standard angle of nip is given as 16°, which should not be 
exceeded. Angles smaller than this will have still more favorable action, 
which, however, is unnecessary. In the following tables the relations 
between the diameter of rolls, the size of feed and the space between the 
rolls when the angle of nip is 16°, as given by Richards, are quoted in the 
table on page 2632. 
In preparing for the tube mills, the material coming from the 
breaker should at least make two passages through the rolls, which 
should not be less than 24 inches in diameter and 12 inches wide. In 
the accompanying cut, figure 36, rolls are shown which prepare material 
for the tube mill, taking the crushings of a breaker, but requiring 
that these should not be more than 34 of an inch in size. This machine 
has two feed rolls, which distribute the material uniformly 
over the surface of the roll. Two curved sheets of iron deliver the 
cos N= 
