Angle of nip

If crushing rolls 1 m diameter are set so that the crushing surfaces are 12.5 mm apart and the angle of nip is 31°, what is the maximum size of particle which should be fed to the rolls ... 

For steel rolls, the angle of nip is not greater than about 32°. 

Toothed rolls such as those of Table 12.8(b) can handle relatively large lumps, for example, 14 in. maximum with 24 in. rolls according to the table. To smooth rolls, the feed size is limited by the angle of nip which depends on the surface conditions but often is approximately 16° or arccos 0.961. Accordingly the relation between the diameters of the roll dr and feed df and the gap d0 between the rolls is given by... 

Fig. 5.7. Johanson s theoretical roll press design angle of nip as a function of material flow properties [13].

As the swinging jaw closes, the angle of nip becomes greater and the tendency is more and more to push the rock out of the machine, rather than to crush it. This is particularly noticeable on rocks of a slippery nature and in any machine where the limit of the angle of nip has been closely approximated by the manufacturer, which is nearly always the case. 

FIG. 21-178 Regions of compression in roll presses. Slippage and particle rearrangement occur above the angle of nip, and powder compaction at high pressure occurs in the nonslip region below the angle of nip. 

Therefore, later research concluded " that material is entrained at some other angle—the true angle of nip —which is smaller than the angle of friction and must be determined experimentally. The use of an empirical angle of entrainment makes allowances for the upward movement of material avoiding the squeeze after compaction has commenced. 

When p = tan a, the angle a is half the angle of nip. A simple relationship exists between the radius of the rolls, the size of the feed, and the gap between the rolls. Thus, from Figure 4.7,... 

Calcite rocks of 2350 kg/m density are comminuted in crushing rolls, having 1 m diameter and 40 cm length. The faces of the rollers are separated by 1.25 cm in the narrowest point and operate at a rotation speed of 75 rpm. The angle of nip has been determined as 30°. Determine the maximum permissible size of fed particles and the capacity in ton/h, considering that it only represents 12% of the theoretical. 

As in the case of Example 4.2, in order to calculate the maximum permissible size of feed, the transposed form of Equation 4.7 could be used, considering that the angle a represents half the angle of nip. Thus cos a = cos(15°) = 0.9659. On the other hand, the radius of the roller, R, is 0.5 m, while the dimension d represents half the distance between rollers, that is, 0.00625 m. Substituting, then in the transposed form of Equation 4.7,... 

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