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The method of using the curves is so simple as to require little comment. The vertical axes represent material sizes, which may be taken as either square or round openings provided of course that the same shape of opening is used throughout any particular analysis. The horizontal axes represent cumulative percentages passing corresponding screen openings. If we wish to check the product to be expected from a crusher set at some predetermined discharge opening, we first refer to the table showing the approximate percentage of product which will pass an opening equivalent to the crusher setting.
This gives us a point in the group of curves which may, or may not, be exactly on one of them. In the latter case we interpolate by following an imaginary curve between the two curves on either side of our point.
We can thus tabulate cumulative percentages passing all of the product sizes in which we may be interested.
Non-cumulative percentages-which are important because they are used to determine expected amounts of specific products-are simply the difference between the upper and lower cumulative percentages for the particular product limits under consideration.
For those not familiar with the use of product gradation curves an example may be helpful. Suppose that a tentative selection of a 3 ½ in. openside discharge setting has been made for a standard gyratory primary crusher to
be used for crushing quarry-run limestone. Referring to the table which lists percentages of product passing an equivalent square opening, we find that 85 to 90 percent of the crusher product should pass a 3 ½ -in. square opening. Choosing the lower percentage, to be on the conservative side, we follow the horizontal line, denoting the 3½ -in. product size in the curve chart, over to the vertical line marking the 85 percent value. We find that the point we have established does not fall directly upon any of the group of curves, but lies so close to one of them that it may be used without introducing any appreciable error into our calculations.
Now, suppose that we wish to know how much of the product of our primary crusher will be retained on a 1½ -in. square opening screen, so that we may estimate the size and number of secondary crushers required to recrush the plus 1½ in. contingent. Following the curve down to the 1½ -in. line, we find that 43 percent of the primary crusher output may be expected to pass this screen opening; 57 percent will be retained, which means that we must provide secondary crushing capacity to take care of 57 tons for each 100 tons fed to the primary crusher.
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