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This factor is intimately connected with that of capacity, and it has been discussed to some extent under that heading. Usually the operator wants to get as much reduction as is economically feasible out of his primary crushing stage; therefore, the primary crusher will, more often than not, be set at or near the minimum permissible setting. If his operation involves shovel-Ioading of quarry-run rock, he will generally want as much receiving opening as he can get for whatever expenditure he feels is justifiable for primary crushing equipment; in the case of the standard gyratory and jaw crushers this means that straight con caves or jaw plates will usually be chosen, and hence the minimum product size must be predicated upon minimum permissible settings with such liners.
There are cases; however, where the choice of a crusher fitted with straight concaves or jaw plates does not work out for the most economical over-alI installation. Considering the standard gyratory line as an example-if we compare almost any two contiguous sizes, we find that the larger machine if fitted with non-choking concaves can be set to a closer discharge opening than can the smaller one when fitted with straight concaves. Also we find that, although there is so me sacrifice in receiving opening because of the non-choking concaves, the larger machine will still exceed the smaller one in this respect. And we find that our larger machine, even at the reduced discharge setting, has a marked advantage in capacity over the smaller crusher with its straight concaves. In certain cases the reduction in size of product obtainable in this fashion may enable the plant designer to eliminate one stage of reduction.
Where this can be done, the greater cost of the larger primary crusher will usually be more than offset by the saving in cost of recrushing equipment.
In choosing the setting for a primary crusher it should be borne in mind that it is difficult, if not impossible, to maintain a given setting over a long period of time. This is especially true when the rock is abrasive. Even though the crusher is provided with means for adjusting the discharge opening, it is the exception rather than the rule to find operations where the machine is maintained strictly at any one setting; certainly it cannot be so maintained when the only means of adjustment is to reset or renew concaves.
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