726 



HYDRAULICS AND ITS APPLICATIONS 



(2) For rams of different diameters exposed to the same pressure inten- 

 sity, the friction in Ibs. is directly proportional to the diameter, and 



therefore to the square root of 

 the gross load. 



(3) The depth of the collar 

 does not affect the friction. 

 In several of the experiments 

 the depth was reduced from 

 | inch to inch without any 

 appreciable effect on the fric- 

 tion. 



The following approximate 

 formulae were deduced from 

 the results of these experi- 

 ments : 



Friction in Ibs. = C X dia- 

 meter in inches X pressure in 

 Ibs. per square inch. 



(C = '0471 for new or badly lubricated collars, 

 ( C = '0814 for collars in good condition and well lubricated. 

 The annexed table gives the frictional resistance expressed as a per- 

 centage of the total pressure on the piston, for rams from 2 inches to 20 

 inches diameter and for pressures exceeding 400 Ibs. per square inch : 



FIG. 349. Leather Collars for Hydraulic Earns and 

 Pistons. 



For lower pressures, the formulae 



F (Ibs.) = p d { -0467 - '0000139 p } 



gives more accurate results, the coefficients here applying to leathers in 

 good condition and well lubricated. 



Where the loading is eccentric, as is often the case in hydraulic 

 jacks, etc., these values may however be increased by as much as 

 100 per cent. 



Recent experiments by Prof. Martens, of Berlin, 1 on the packi 



1 Mechanical Engineer, Sept. 7th, 1907. 



ings 



I 



