Sizing and Selecting Shovel and Crusher Sizes

It has been customary for writers on this subject to tabulate a list of shovel dipper sizes and opposite these to list sizes of gyratory and jaw crushers that are considered to be suitable for operating in conjunction with power shovels equipped with such sizes of dippers. This is a little like putting the cart before the horse. The procedure omits one very essential factor: the character of the material that is to be crushed. For example, most limestone deposits are stratified, the cleavage planes usually being roughly parallel, and ranging anywhere from horizontal to almost vertical.

Regardless of how these strata lie, their thickness not only has an important bearing upon the type and size of primary crusher that can be expected to handle the rock with a minimum amount of secondary shooting, or bridging and blocking, but it also has a bearing upon the maximum size of shovel dipper that is suited for use in conjunction with a certain size of primary crusher. This statement is particularly applicable to the gyratory crusher because of the shape of its receiving openings. Two or three different maximum dipper sizes may be indicated for any one size of gyratory, depending upon the maximum strata thickness. In the case of the jaw crusher, with its rectangular opening, maximum dipper width and maximum strata thickness are established simply by the opening dimensions, and one maximum size dipper applies for each machine.

Table I takes both dipper width and strata thickness into consideration. Opposite each listed size of gyratory crusher is tabulated the maximum advisable size of dipper, in cubic yards, for the different strata thicknesses-up to the thickest ledge for which each crusher is suitable; that is, which it can be expected to handle with a minimum amount of secondary blasting. Crushers up to and including the 42-in. machines are listed with both straight and non-choking concaves. Six sizes of jaw crushers are listed, the dipper sizes for each having been chosen as outlined in the preceding paragraph.

These dipper selections are of course predicated upon the assumption that the dipper is expected to act as a measuring stick for the crusher, which necessarily presupposes that all rock will be passed through the dipper-a procedure that is rarely followed completely in any quarry operation. The experienced shovel runner soon learns how to judge the rock he is loading, with respect to the size and shape of the receiving opening it must enter, and a certain amount of "shuffling off of the dipper teeth" goes on in most quarries.

Admittedly no tabulation of this sort can substitute for actual experience with any particular rock or ore deposit. Some deposits which are of monolithic structure, and hence might be expected to shoot out in massive form, are readily shattered into workable sizes in the primary blasting operation; on the other hand, some stratified rocks shoot out in large slabs, which require either a large primary receiving opening, or a considerable amount of secondary shooting, or both. For those cases where no operational data are available to point the way, the table wiII serve as a guide that is believed to be on the safe side.

No particular problem exists in the matching of primary crusher and dipper size in the average gravel pit operation. The primary crusher is chosen on the basis of capacity (usually arrived at by estimating as closely as possible the percentage of boulders in the deposit), and receiving opening (as large as is consistent with the size of the operation), which may or may not involve crushing all of the boulders encountered in the bed of material. Quite frequently, in small or medium gravel operations, the larger boulders are cast aside and left in the pit. The question of first cost of the crusher versus continued cost of disposing of oversize boulders is one that each gravel pit operator must settle to his own satisfaction. Most gravel pit operations involving the handling of large boulders incorporate some form of grizzly ahead of the primary crusher to "stop out" any boulders that are too large to enter the receiving opening. Therefore, the shovel dipper can generally be selected on the basis of required capacity. If the boulders are extremely large, and the plant capacity is high enough to justify the installation of a large primary crusher, the grizzly may be dispensed with, and the shovel dipper matched to the crusher receiving opening in the same manner as for the quarry.