Straight Jaw Plates: A 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 of 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.
Non-choking Jaw Plates: When the non-choking concave demonstrated its capabilities in the gyratory crusher, it was only natural that the principle should be applied to the jaw crusher. But, inasmuch as the Blake-type jaw is used very largely for primary breaking, where maximum receiving opening is, more often than not, a controlling factor, the application of non-choking plates to standard machines of this type has one unfavorable aspect, as will be evident from an examination of B “Standard Blake jaw crusher non-choking swing jaw plate.” This diagram shows the same machine as the last one considered, except that the swing jaw is fitted with a curved jaw plate.
These non-choking jaw plates have been developed in two styles, reversible and non-reversible. The advantage of the reversible design has been outlined in connection with its application to the gyratory reduction crusher. For both types of jaw plate, the effective receiving opening is reduced, as compared to the standard, straight-plate setup. For the non-reversible plate, this reduction is determined by the actual increase in depth of the plate, resulting from the introduction of the curve. For the reversible plate, it is established by the point where the tangent to the curved surface coincides with the maximum angle of nip for the material being crushed. Similar restrictions were outlined in our discussion of non-choking concaves in gyratory crushers.
Regarding the reversible plate, it should be noted that, while that portion of the chamber above the point of nip does no crushing, it does constitute an “active” receiving hopper and, as such, is of definite value in minimizing bridging across the receiving opening. For applications which do not require the full rated receiving opening of the crusher, non-choking jaw plates offer a substantial improvement in performance, an improvement comparable to that resulting from the substitution of non-choking concaves in the gyratory crusher. While this is readily apparent from a comparison of the two diagrams we have presented, more concrete evidence is contained in the table of capacity ratings, which gives ratings for both types of jaw plates, for sizes up to, and including, the 60×48″ machine.
