DISC FRICTION 



187 



It may be taken that in a rough cast-iron casing a rough cast-iron disc 

 gives about 10 per cent, greater resistance at this speed than one which 

 is polished, and that in a painted casing this is increased to about 20 per 

 cent, greater resistance. 



Owing, however, to the higher value of n in the case of a rough disc in 

 a rough casing, its resistance increases more rapidly with an increase in 



COMPARATIVE RESISTANCES WITH VARIOUS Discs. 



speed than does that of a polished disc. In a smooth casing their 

 relative resistances are practically independent of the speed. 



Effect of Roughness in the Surface of the Casing. The comparative 

 results obtained with the three surfaces at 1,500 revolutions per minute 

 are tabulated on p. 188, the resistance in a painted and varnished casing 

 being taken as unity for each type of disc. 



From these results it appears that with such clearances as are found in 

 practice the effect of a roughness of the casing in increasing the resist- 

 ance is as great as that of the roughness of the disc in fact, a painted 

 or polished disc in a rough casing gives, within about 2 per cent., the 

 same resistance as a rough disc in a painted casing. The relative effect 

 of the roughness of the casing is more pronounced with a smooth than 

 with a rough disc. 



Effect of a Variation in the Clearance of the Casing. The comparative 

 results as affected by the side clearance between the disc and casing are 

 given in the Table on p. 189, the resistance in each case being expressed 

 as a multiple of that obtained with the minimum clearance adopted with 



