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The effect of the undersize in the feed upon the capacity of the crusher depends upon several factors. If the crusher is a primary breaker, and the discharge opening is large, undersize will usually sift readily through the voids between the large pieces of material, and discharge quickly. The material must, of course, be fee-flowing to behave in this manner. For such a condition, large quantities of undersize may materially add to the rated capacity of the crusher, but it must be admitted that such a condition is the exception rather than the rule. More often than not the discharge setting, even in primary crushers, is such that the crushed material in the lower part of the crushing chamber effectually throttles the flow of tine material to the pace of the larger pieces; hence, any increase in capacity would be due entirely to whatever increase in density of the body of material might result form the presence of the fines in the voids between the larger pieces. This is usually not large, and as has been pointed out, the presence of fine material may event be detrimental under certain conditions.
If a clean, sized feed to a reduction crusher includes a preponderance of particles smaller than the dimension between crushing surfaces at the choke point, the effect upon capacity will be beneficial. On the other hand, if there are enough larger particles to trap these smaller pieces, we get the same throttling effect mentioned in the preceding paragraph – unless the choke – point is well up in the crushing chamber, where the small pieces have a better opportunity to sift through. If capacity alone were to be considered in selecting the feed size for a reduction crusher, the ideal would be a feed having a one-way dimension not exceeding the choke-point dimension, and not smaller than the close-side discharge setting.
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