EXAMPLES 79 



surfaces ; <o = angular velocity in radians per second ; and RI and L 

 are the outer and inner radii of the submerged portion of the rotatin 

 disc in feet. 



On integrating this gives _^p_ (Rf _ R) foot Ibs. 



With n discs the work done against the resistance per second (neglecl 

 ing the effect of the edges of the discs) is then given by 



2 n X 2 ^ ^ - E ^ foot lbs ' 



EXAMPLE. 



Brake provided with three rotating discs enclosed on both faces. Oute 

 diameter 9 inches, submerged to an inner diameter of 6 inches. Assure 

 ing/ = '004, determine the B.H.P. absorbed at 10,000 revolutions pe 

 minute. 



Here = ' 2 x 10,000 



60 



Rf = (-375) 5 = -007413 

 Itf = (;25) 5 = -000976 



12 X T x -004 X 8 7T 3 x 10 9 X '006437 



B.H.P. = 



5 X 216 X 550 

 = 405. 



EXAMPLES. 



(1) A direct acting hydraulic lift has a travel of 40 feet. The cage an 

 ram weigh four tons, and are counterbalanced to the extent of three ton 

 by hanging weights. The lift is to take a load of three tons, with 

 maximum acceleration of 2 feet per second per second. The diameter ( 

 ram is decided on as being 5 inches. Determine the working pressure i 

 the cylinder, assuming ram friction to account for 5 per cent, of this. If th 

 pressure supply is 750 Ibs. per square inch, this being reduced by mear 

 of a reversed intensifier of the ordinary type and having a stroke of 4 fee 

 determine the necessary size of this cylinder and of its ram, assumin 

 friction to cause a loss of 5 per cent. Also determine the relative gain i 

 efficiency of this system as compared with supplying the lift cylindt 

 directly with pressure water throttled down to the required pressure. 



