186 



HYDRAULICS AND ITS APPLICATIONS 



Power absorbed in Disc Resistance. Since the resisting moment is 



4 TT f (t) n 



given by ' Q Ri n + 3 foot Ibs., the work absorbed in overcoming this 



4 7T f to" + 1 



resistance is given by 



+ 3 foot Ibs. per second 







N 



+ 3 horse-power. 



550 (n + 3) 



The following table indicates the magnitude of the horse-power 

 absorbed at 1,500 revolutions and at 2,000 revolutions per minute, in a 

 few typical cases with a f inch side clearance : 



HORSE-POWER ABSORBED. 



Effect of Roughness in Surface of Disc. The table on p. 187 shows com- 

 parative values of the resistance at 1,500 revolutions per minute as 

 affected by disc roughness, the resistance of a polished brass disc being 

 taken as unity for each type of casing. 



From these results it appears that at this speed the resistance of a 

 polished -brass and of a painted cast-iron disc are practically identical. 

 At still higher speeds the resistance of the painted disc becomes -propor- 

 tionately greater than that of the polished disc if the interior of the 

 casing is rough, and becomes proportionately less at lower speeds. In a 

 painted casing, however, the relative resistances are the same for all 

 speeds. The relative resistances of a rough cast-iron disc and of one 

 made of polished brass depend largely on .the clearance, the 

 roughness of the casing, and the speed. In a rough cast-iron casing at 

 1,500 revolutions per minute the resistance of a rough disc varies from 

 about T12 times to 1*08 times that of a polished disc, as the clearance 

 varies from f inch to 2J inches, while in a smooth painted casing the 

 ratio varies from 1'195 to 1"22. 



