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IT IS ENTIRELY POSSIBLE for a crusher to choke at some other point in the crushing chamber than the theoretical choke-point. When the setting of a fine-reduction crusher is too close for its eccentric throw and for the general proportions of its crushing chamber, there is a tendency toward building up a choke in the zone immediately above discharge opening, regardless of where the theoretical choke-point is located. The reason for this is that the excessive movement of the head (in relation to the discharge setting) mashes the material against the concave so tightly that the individual particles are shattered, or pulverized.
Such a crushing action builds up fines very rapidly, and consequently brings about a rapid reduction in the percentage of voids, even though the crushing chamber may have a progressive volume-expansion characteristic. Many materials, when mashed in the manner described, tend to cake, and to cling to the crushing surfaces; the movement becomes sluggish; more material crowds down from above, aggravating the packed condition, and, if the action progresses to the point where all voids are eliminated, a choke results.
Characteristics or condition of the material have a decided influence upon the action just described. Soft, friable rock is more apt to pack than hard, clean-breaking rock; damp material is more prone to cause trouble than dry, particularly so if the feed contains many fines (e.g., unscreened feed).
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