722 HYDRAULICS AND ITS APPLICATIONS 



and thus stops pumping until the ram descends. The steam port is 

 usually designed so that steam may be cut off gradually and the speed of 

 the pumps gradually reduced as the piston approaches the upper limit of 

 its travel. 



A drain pipe is arranged to carry away any water or steam leaking past 

 the piston. The storage capacity is P A s L foot Ibs., and this type has 

 the advantage that the ram may be placed either horizontally or vertically. 



Effect of Accumulator on Working Pressure in Motor Cylinder. If the 

 motor, piston-area a square inches, derives its supply from the accumu- 

 lator cylinder alone, then if A square inches = area of ram ; a = 

 acceleration of motor piston ; a' = acceleration of ram, we have a' = 

 a 



T 



The force necessary to produce this acceleration is given by 



W , W a 

 F = a' = . . . a, Ibs. 



9 9 A 



.*. Equivalent pressure in Ibs. per square inch on ram = . -^ . a. 



.*. Pressure at entrance to delivery pipe = -7 j 1 ^ a h Ibs. per 

 square inch. 



If a s = area of this pipe, then a $ = a , and if I is its length, we have 



a 3 



putting v = velocity of piston : 

 Pressure on piston 



a } 62-4 la 62'4 a 2 f I v* 



_ 

 A I g A ) 144 g a s " 144 * a s 2 2 g m 



W a a ( W , 62-4 I } 62-4 a 2 flv 2 .. 



= r- -jo + -T-T T a T\ Ibs. per square inch. 



A g ( A* 144 a s } 144 a* 2gm 



With a steam accumulator this becomes : 



PA, aafW. 62-4 l\ 62'4 a? f I v 2 ., 



i "Ta + TT r ~a Ti~ ~ Ibs. per square inch, 



A g I A 2 144 a s J 144 a s 2 2 # m 



and since W is now comparatively small, the term . ra , which repre- 



^/ - 



sents the effect of the inertia of the ram, becomes negligible. For this 

 reason the steam accumulator is not subject to the shocks and jars to 

 which the weighted accumulator is subjected in virtue of the great inertia 

 of its moving parts. 



To prevent inertia shocks becoming dangerous, a relief valve is some- 

 times placed on the outlet pipe, this being set to blow off at 10 per cent, 

 above normal pressure. The loss due to the leakage which this necessi- 



