20 BULLETIN 343, U. S. DEPARTMENT OP AGRICULTURE. 



GRIT OF STONE. 



If a stone of coarse grit is used considerably less dressing with the 

 burr is necessary to attain the conditions of a sharp stone. In fact, 

 it is difficult to produce a pulp that is not coarse and full of shives. 

 Although a somewhat lower consumption of power per ton of pulp 

 is easily obtained, on account of its coarseness the pulp is not satis 

 factory from a news-print standpoint. The yield of screenings is 

 much greater, as one would anticipate. As indicated by the Mullen 

 tester, the strength of pulp ground on a coarse grit stone is a trifle less 

 than when made on a finer grit stone and consuming the same amount 

 of power per ton; but when compared as to their respective breaking 

 lengths, there is no difference. However, when the wood is steamed 

 prior to grinding, a strong pulp may be obtained at lower horse- 

 power consumption on the coarse grit stone than on a finer one. A 

 comparison of the grits of the stones may be obtained by referring to 

 Plate III, figures 1 and 2. 



In general, a stone of fine grit yields a fine-fibered pulp and a coarse 

 stone a shivy and coarse pulp. 



PRESSURE OP GRINDING. 



In any commercial grinder the pressure at which the wood is 

 forced upon the revolving grindstone — that is, the pressure per square 

 inch of wood surface in contact — varies greatly. The variations 

 in pressure are brought about by the binding of the wood in the 

 pockets ; the grinding of wood of different lengths and diameters ; the 

 variation of pressure on the grinder cylinder, due to the removal of 

 pressure from one or more of the cylinders, and the use of varying 

 amounts of split wopd. For any cylinder pressure, however, it is rea- 

 sonable to assume that the pressure per square inch on the stone varies 

 between certain limits. 



Figure 1 1 shows the relation of the pressure on the grinder cylinder, 

 the horsepower consumption per ton, power to grinder, and production 

 in 24 hours, the three curves being obtained on surfaces of different 

 degrees of sharpness. It will be noted that with increasing pressure 

 the horsepower consumption per ton decreases, and that the horse- 

 power to the grinder and production in 24 hours increases at a fairly 

 definite rate. 



The yield and quality of pulp produced vary to a considerable 

 degree with the pressure of grinding. Finer, although shorter- 

 fibered, pulps are obtained when grinding at higher pressure, the 

 advantage of the use of higher pressure being that it is possible to 

 use a duller stone and obtain finer-fibered pulp with a consumption 

 of the same amount of power as would be used at lower pressure and 

 sharper condition of the stone. 



