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Two sets of maximum-nip dimensions are listed for the Hydrocone crushers; one full abrasive and one for non-abrasive material. An examination of the cross section through the crushing chamber of any flared-top shell crusher will make it clear that the effective receiving opening decreases as the head is raised. When crushing abrasive material, mantle and concave wear is relatively rapid as compared to the wear when crushing non-abrasive rock; furthermore, the wear is, comparatively, more rapid in the lower part of the crushing chamber, in contrast to the more even wear on non-abrasives. Hence, as the head is brought to its top position in adjusting for this weal', the effective receiving opening will be somewhat less for the crusher operating on abrasive rock. The effective openings listed are for the top position of the head; at lower positions the opening wiII be slightly more.
In making our check of receiving opening vs. discharge opening of the preceding stage, we should observe the same precaution suggested for the primary-secondary combination: the discharge setting against which we check the nip-dimension of our crusher should be the maximum opening at which we have reason to believe the preceding crusher will ever be operated. Usually secondary crushers can be, and are, maintained more closely to the established setting than are the larger primary machines, some of which are without means of adjustment other than resetting of concaves or replacement of mantles; nevertheless it is extremely difficult in actual practice to maintain crusher settings within narrow limits with a few exceptions, and some margin should be allowed for this.
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