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Fig. 2 shows a diagram of a standard type of Blake crusher with regular or straight jaw plates. This type of plate has been the standard from the time of its inception, emulating, in this respect, the straight concaves in the gyratory machine. And it will be noted that the theoretical action in this straight-plate jaw chamber follows quite closely the pattern of the action in the straight-concave gyratory chamber.
Calculations for the jaw crusher chamber are somewhat simpler than in the case of the gyratory because the volumes included between the successive pairs of horizontal lines are rectangular in plan, whereas, in the gyratory we have to deal with annular volumes (actually, a descending spiral, rather than flat rings). But, when the line-of-mean-diameters in the gyratory chamber parallels-to a close approximation, at least-the centerline of the crusher, the general characteristics of the two crushing chambers, with respect to ratio-of-volume reduction and concomitant reduction.
In percentage of voids are similar. Crushing angles and throws would of course have to be the same for the comparison to be exact.
There is one readily discernible difference between the two diagrams we are comparing; the drop per stroke in the jaw crusher diagram is noticeably smaller. This difference is most pronounced in the upper part of the chamber. The reason for the difference is two-fold. In the particular machines selected for these diagrams, there is a difference O) several degrees of crushing angle in favor of the gyratory; this accounts for the average difference over the full depth of the chambers. Secondly, the motion of the swing jaw, at the top of the crushing chamber is, proportionately, smaller than the movement at the top of the gyratory chamber. The reason for this is that the distance from the
top of the crushing chamber to the fulcrum-point-as compared to the total length from discharge-point to fulcrum-point-is smaller in the case of the jaw crusher. This is generally true of Blake-type crushers as compared to gyratories except for the vertical swing jaw type, in which the swing jaw is carried up some distance above the top of the stationary jaw.
The close proximity of the fulcrum-point to the receiving opening gives the type of crusher we are considering enormous leverage on large blocks of stone, and the small motion tends to decrease the shocks incidental to gripping and shattering such blocks. From the capacity standpoint, the smaller drop per stroke is compensated for by the fact that the jaw crusher speed, for comparable sizes, is higher than the eccentric-speed of the gyratory.
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