460 CEMENTS, LIMES, AND PLASTERS. 



"It is absolutely necessary that the tube mill should be filled up 

 to the center line with the flint balls; there is, consequently, no interior 

 shaft. The tube revolves upon stub shafts, which are firmly anchored 

 to the dome-shaped ends, the one at the inlet end being hollow, and 

 through which the material is fed. 



"The fact that flint balls in a tube mill represent an extraordinarily 

 large grinding surface, makes it possible to run the mill at a slow speed 

 so that all the power required is actually made use of in the grinding 

 process itself and nothing is wasted in maintaining a high speed of ma- 

 chinery, which effects nothing in the grinding process itself. 



"The fineness of the output is regulated by the speed at which the 

 material is fed into the machine. As every particle of the material fed 

 must pass under the grinding action of the entire charge of balls, a thor- 

 ough and uniform grinding is bound to be the result; in fact, in practice 

 it is found that the uniformity of the output is so great that it is unneces- 

 sary that sieves should be used, and no provision is made in the machine 

 by which they can be used. There is, of course, nothing to prevent the 

 materials being sieved, if it is desired, but this is absolutley useless when 

 there is regularity of feeding. At a slow rate of feeding any required 

 fineness can be obtained with any grindable material. The requirements 

 for grinding Portland-cement clinkers are ordinarily that no more than 

 12 to 15 per cent residue shall be caught on a No. 200 sieve; when the 

 tube mill is fed with coarsely ground product it will turn out from 8 to 

 16 barrels per hour of Portland-cement ground to the required fineness. 

 However, these figures do not represent fully the value of the grinding 

 capacity of the tube mill. Careful scientific investigation has proved 

 that the fine product from the tube mill, which will pass sieve No. 200, 

 contains 50 per cent more overfine particles than is the case in the out- 

 put from any other existing grinding-machine. 



"The foundation may consist of any material which will support 

 the dead weight and enable the journals to be held firmly enough to 

 withstand the tension of the driving-belt. In practice, however, it is, 

 of course, preferable that the pedestal foundations should be deep 

 enough in the earth to resist the action of frost, and should be made 

 of permanent material, such as brick, stone or concrete. 



"In the matter of repairs in this machine, as in others, the advan- 

 tage of slow speed over high speed is practically noticeable. This prin- 

 cipal wear is, of course, upon the flint balls, and by merely dropping 

 in a few at stated intervals, to maintain the charge at its original bulk, 

 the principal results of wear are made good at once. 



"The flint balls, or pebbles, are a natural product; they are found 



