HYDRAULIC MACHINERY, FOR MIKES 



853 



Charging Plungers and Pistons. For charging accumulators or effecting the trans- 

 fer of power, plungers are usually employed. The advantage which a plunger offers 

 over a piston is that any leakage is instantly seen and can almost immediately be 

 prevented. When it is required to force water under a high pressure the valves should 

 not only be large, but set so that air cannot pass into the plunger case. A precaution- 

 ary method is to gravitate the water to the bottom valve. In addition, it will in some 

 cases be judicious to provide the ram cases with small pet valves for discharging 

 any intrusion of air. Unless the air be entirely excluded from the plunger cases, 

 the cubic inches of water delivered to the accumulator will by no means correspond to 

 the cubic inches of the stroke. A simple and effective force pump is shown in 



1185 



fig. 1185. In this arrangement the outstroke of the pump causes the water contained 

 in the annular space surrounding the rod E to be forced into the air vessel or accumu- 

 lator whilst a further supply of water enters behind the piston through the inlet G. In 

 the other half of the stroke the water behind the piston is discharged through the 

 valve D and half of it into the annular space on the other side of the piston. In this 

 pump the area of the rod E is exactly half that of the piston F. Both the inlet and 

 delivery valves are of easy access. 



Accumulators and Air Vessels. The accumulator is shown in figs. 1 170 and 1189, and 

 its use is fully explained at page 833. Air vessels may to some extent be used as a 

 substitute for the accumulator, but this class of apparatus impose a varying resistance 

 on the charging pumps, whilst the resistance offered by the accumulator is a constant 

 one. Moreover, the air vessel has the serious defect of requiring frequent replenishing, 

 owing to the absorption of air by the water. This especially occurs under high pres- 

 sures, as will be gathered from the following particulars. (See Table, p. 854.) 



The effective capacity of air vessels is governed by the pressure of water to which 

 they are to be subjected. Long and capacious vessels will afford more satisfactory 

 results than short and small ones. In most cases it will be desirable to fit the vessel 

 with a pressure-gauge, and a gauge-glass. An example of an air vessel fitted in this 

 way and applied for the purpose of relieving pressure and preventing shock is shown iu 

 fig. 1194, page 863, where D is the vessel, 



