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The significance of this factor is so obvious that it sometimes does not receive quite as much thought as it should. From the standpoint of minimum requirement, it is of course closely tied up with product size, or crusher setting. But the primary crusher can seldom be chosen solely on the basis of capacity; it should never be selected with a view to just meeting the average capacity required to feed the rest of the crushing plant.
Just how much the rated capacity of the primary crusher (at the required discharge setting) should exceed the average capacity of the plant depends upon how uniformly the crusher will be fed; or to put it more definitely, what percentage of the total operating period the crusher will operate at full rated capacity.
The answer to this is not always an easy one to predetermine, as it may depend upon several details of plant design and quarry operation.
In the average quarry operation, the only surge capacity between the quarry and the primary crusher consists of whatever quantity of rock may be, at the moment, loaded in cars or trucks, and usually this is not large. For that reason, any operating delays occurring in loading, transportation or primary crushing quickly affect all three of them, with the result that the feed to the balance of the crushing plant is cut off until the trouble is rectified. If the plant as a whole is to maintain its rated average output, these departments must be capable of making up for such interruptions, and they can only do this if they have reserve capacity over and above the average requirement.
Such interruptions to continuous production are not uncommon' in the primary crusher house; they may assume serious proportions if the crusher receiving opening is not large enough for the material it is expected to handle, and the largest crushers of any type will occasionally bridge or block.
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