790 HYDRA rLICS AND ITS AI'PUCATK' 



A = 105 = -728 square foot; W = 221,000 Ibs. 

 144 



Equation (5) now becomes 



2 _ 65-7 X 4 X -386 /- _ 



224,000 V 4/ 



= -000453 (l - *) 



/. a = -02127 \/ fl 4 j square feet. 



= 3'06 V I* "" l) s ^l uare inches. 



Thus when a; = 0, i.e., at the beginning of the stroke, a = 3'06 squar 

 inches. This gives the effective area of the orifice, and, when the entrap 

 c:d#;s are well rounded so as to prevent the formation of a vena contract 

 this will be the true area. With sharp-edged orifices, however, the orific 



area will need to be greater than this in the ratio - ~. 



coefficient of contractor 



!iii(] this will depend upon the form and situation of the orifice. 



To avoid errors in the preliminary calculation it is advisable to forr 

 1 1: , orifices with rounded entrant edges. 



Assuming, in the example above, that C c = '65, we get the followin, 

 values for ai t where C e a\ = a : 



It is worthy of note that since both the hydraulic resistance and th 

 Kinetic energy of the moving body vary as the velocity squared, thei 

 ivl.itive value is independent of the velocity, so that the piston travel i 

 approximately independent of the initial velocity of the moving bod} 

 ;n nl depends only on its weight. 1 



1 In a buffer stop described by Mr. P. W. Shaw (" Proc. Tnst. C. E., :; 1905-6, Tart iii., vol. 16 

 j. L".MX tin- piston came to dead slow at about 9 inches from the end of its stroke, wlic 

 ivsistin^a mass of 11 tons, for all speeds of collision from 4 to 12 miles per hour. Th 

 details of t!n> brake are as follow: Cylinder, 10 inches diameter, 3 feet 6 inches stroV 

 piston, 9}? ; ,' inrlu's diameter, with t\vo rectangular sluts 3 inches wide x 1 inches dee 

 \u>rkiii-,' over strips a incites wide X 1J inches tapering to i inch. 



