518 CEMENTS, LIMES, AND PLASTERS. 



of the mill. The dust-casing is made in three parts, a small section 

 being easily removable for brushing the sieves. The top half may be 

 removed for repair of the drum. 



"The illustration below (Fig. 130) shows the type B machine, which 

 is the same as type A, except that the shaft, instead of going through 

 the feed-hopper, is stopped short inside the drum and there fixed in a 

 ' spider' the external part of which runs on a roller bearing. By means 

 of this arrangement lumps up to 10 inches each way can be fed into the 

 mill, and the feed opening itself at the same time is reduced in diameter, 

 which allows a heavier charge of balls to be used. 



"The very large capacity of the Davidsen tube mill as a pul- 

 verizer created the demand for a large ball mill of sufficient capacity 

 to feed the tube mill with coarse material. This demand has been 

 met by the type C ball mill, in the construction of which the through- 

 shaft of type A and the 'spider' of type B have been omitted, giving 

 a clear feeding opening of 10 inches without interference. The omis- 

 sion of the shaft and the 'spider' have made it possible to use a much 

 larger charge of steel balls and at the same time lower the cost of repairs 

 by avoiding the necessity for replacement of either shaft or 'spider'. 





FIG. 130. Smidth ball mill. (F. L. Smidth&Co.) Type B, showing roller bearing 



as used on type C. 



" Up to this time there has been but one size of the type C constructed, 

 namely, No. 7. This has become the popular form and size for Port- 

 land-cement work. One No. 7 ball mill and one No. 12 tube mill can 

 be brought together as a grinding unit through placing on the ball-mill 



