Mance: Quarry Industry of Southern Indiana 141 



It will be seen by a comparison of the two tables that as soon 

 as the output passes the 200-ton per day mark it is cheaper to 

 use a gyratory crusher; and that the 10x4-inch jaw crusher is 

 only slightly cheaper in operation than the crusher of the gyratory 

 type with the same output. It must also be remembered that the 

 higher cost has been included in the calculations, or at least that 

 the interest on the larger outlay has been accounted for. 



The Sturtevant Mill Company, of Boston, and Ferrel and Bacon, 

 of New York, deal in the jaw breakers while the Power and Min- 

 ing Machine Company, of Milwaukee, Wis., handle the McCully 

 type of gyratory crushers. All these companies have furnished 

 figures on the cost of crushing with their special type of ma- 

 chinery. 



Following the coarse crushing the product is fed to some sort 

 of pulverizer. The most common form in use is what is known 

 as rolls. These are heavy metal cylinders held together by 

 powerful springs or by gravity and rotated at such high speed 

 that their centrifugal force tends to hold them together and at 

 the same time to impart a heavy blow to the stone as it passes 

 between them. The closeness of approach of the rolls is regulated 

 by shims or compression bolts. The larger the roll the greater 

 its capacity since with the same speed of rotation its peripheral 

 velocity is greatly increased. The cost of rolling 100 tons per 

 24 hours would be as follows, on the basis of ten horse-power: 



Power 1.30 cents per ton; attendance, 1.50 cents per ton; wear on roll 

 shells 0.02 to 4.00 cents per ton; repairs, oil, babbitt, etc., 0.37 to 0.60 cents 

 per ton; total 3.19 to 7.40 cents per ton. 



The above calculation is given by Richards (op. cit., pp. 69, 

 70). 



Several special types of machinery are in use for the final 

 reduction of the product after it has passed the crusher, and 

 probably better results can be obtained from a hammer-bar 

 pulverizer or a set of ring rolls than can be secured from the 

 common type of rolls. The hammer-bar pulverizer depends on a 

 blow struck in space to effect the crushing. The harder the 

 impact the finer will be the product. The mill works on the 

 principle that a weight placed to swing freely on a revolving 

 shaft will stand at right angles to the shaft when the shaft rotates 

 rapidly, and that the faster the rotation of the shaft the harder 

 will be the blow that can be struck by the weight before it will 

 be forced back. The stone is fed into this machine and struck 



