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Fig. 2, showing the "standard gyratory with straight concaves," is a section through any vertical, radial plane in the crushing chamber of one of the intermediate sizes of the crusher. In order to understand the crushing action in such a chamber it is helpful to consider the process as though each step took place in an orderly and "ideal" fashion. It is hardly necessary to add that the action never does take place in just that fashion; nevertheless the concept is fundamentally a correct one, and the average performance of the crusher follows the pattern so closely that it is possible to predict, within surprisingly close limits, what any particular design of crusher will do.
We start out by visualizing the crushing chamber filled with a tractable material which will act just the way we want it to, with a head of material (choke-feed) above the receiving opening so that no up-surge of load will occur during the closing stroke of the crusher head. Now, consider any horizontal plane through this body of material as, for example, the plan e at the receiving opening, represented by line "O" in the diagram. The crusher head is at the moment in the close-side position.
As the head recedes on its opening stroke, the body of material moves downward until, at the end of the stroke, the plane has moved to position "l." Note that the length of line "1," from concave to open-side head position, is the same as that of line "O," from concave to close-side head position.
On the next closing stroke line "1" is compressed by the amount of the head movement at that level, and on the next opening stroke it moves down to position "2"-and so on down through the chamber, until it be comes short enough to pass through the open-side discharge setting.
We can just as readily visualize the process as being the movement of the trapezoidal areas enclosed by each adjacent pair of horizontal lines and the two crushing faces. Better still, we can consider it as the movement of annular volumes whose cross-sections are the areas just mentioned. This latter conception is essential in visualizing the action of non-choking con caves and flared crushing chambers.
In the diagram, the broken line through the center of the crushing chamber is the line-of-mean-diameters of the compacted areas. When the profiles of both crushing faces are straight lines, as in the case under consideration, this mean-diameter line is also straight, and its slope depends upon the relative tapers of the head and concaves. When the line approximately parallels the center-line of the crusher, which is al so the case for the diagram we are examining, the theoretical action closely approximates that of the jaw crusher of similar cross-sectional proportions. Practically, however, the gyratory will have some advantage over the jaw, as regards freedom from choking, because the spider arms of the gyratory prevent a complete filling of the crushing chamber at the top. When the line slopes away from the crusher centerline at its lower end the characteristics change quite definitely in favor of the gyratory, as will be seen.
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