THE HYDKAULIC JACK 



763 



Hand Lev er 



Lowering 

 Jcrew 



V a and V D , into the space C below the lifting ram, and raises the latter. 

 The weight to be lifted is carried either on the ram table T, or on a side 

 shoe projecting from the ram 

 casing. Screws are provided 

 for supplying the cistern A Air Screw 

 with water and for allowing of 

 the inlet of air, while a lower- 

 ing screw permits of the escape 

 of pressure water from the 

 space below the lifting ram into 

 the supply cistern when it is 

 required to lower the load. 

 The lifting ram is usually 

 packed by means of a cup 

 leather, and the pump plunger 

 by means of a single leather 

 ring. 



If m = ratio of travels of 

 lever handle and of pump 

 plunger, and if a and A are 

 the areas of plunger and ram, 

 the theoretical mechanical 



advantage of the jack = m , 



a FIG. 377. Hydraulic Jack. 



so that, neglecting friction, the 



force P to be applied at the lever handle to support a weight W on the 



ram table is given by P = W 



Thus if 





m = 



JL 



1-25 



diameter of ram 

 diameter of plunger 



if P = 50 Ibs., W = 



= 19-2. 



= 5J inches ) A 

 = I inch f a 



50 X 19'2 X 30'25 



= 18 tons. 



Actually the weight lifted is less than this because of friction losses. 

 The efficiency of the jack, or the ratio -TT- rp f weight lifted by the 



ram, can only be determined experimentally, and varies largely with the 

 condition and size of the apparatus, as well as with the magnitude and 

 position of the load, 



