CRUSHING AND PULVERIZING MACHINERY. 463 



neither the mill nor the feeding mechanism is stopped so that an abso- 

 lutely uninterrupted grinding action takes place. The main trunnion 

 bearings are so designed as to admit of their being rebabbitted with 

 the least possible lost of time and amount of labor. 



"The mill is driven by means of a split-spur gear and pinion, the 

 former being just scant of 10 feet in outside diameter. The discharge 

 device is the Krupp patent cone discharge and includes all the advan- 

 tages claimed for what is known as the peripheral discharge and also 

 separates worn-out pebbles, and eliminates all the dust. The heads 

 of the mill are of the strongest possible type, being conical in form. 



"Every part of the mill is easily accessible, and all parts are made 

 of the strength required for mills to be used in American cement prac- 

 tice. 



"The capacity of the above mills, when working in battery, varies 

 widely with the character of the material, varying as much as from 15 

 barrels per hour on a hard Portland-cement clinker to 60 barrels on 

 a natural-cement clinker, the fineness of the product being 95 to 96 per 

 cent through 100-mesh, 75 to 80 per cent through 200-mesh. 



FIG. 104. Exterior view of Bonnot tube mill. (Bonnot & Co.) 



"The capacity on raw rock varies from 8 to 10 tons per hour, same 

 fineness as mentioned for clinker. 



"The capacity of the tube mill on coal varies from 3 to 4| tons per 

 hour, 90 to 95 per cent through a 100-mesh sieve." 



Pebbles for tube mills. For use in tube mills flint pebbles are the 

 most satisfactory grinding materials. These are obtained chiefly from 

 Greenland, Norway, Denmark, France, and England. Very little is 

 in print concerning the flint industry, two recent papers of interest 

 being listed below.* 



* Hill, R. T. Flint,- an ancient industry. Eng. and Mining Journal, Nov. 7, 



