HYDRAULIC CRANES 



* With a pressure representing a column of 1,500 feet, the loss of head by friction in 

 the pipes forms so small a deduction from the entire column as to bo a matter of no 



1171 



consideration, and consequently the distance at which the engine may bo situated 

 from the points where the hydraulic machines may be placed is of little importance, 

 except as regards the cost of the pipe. It is advisable, however, if the pipe bo very 

 long, to apply an accumulator at each extremity, so as to charge the pipe from both 

 ends. 



' With regard to the mechanism of hydraulic cranes, the arrangement which I first 

 adopted, and have ever since adhered to, consists of one or more hydraulic presses, 

 with a set of sheaves, used in the inverted order of blocks and pulleys, for the purpose 

 of obtaining an extended motion in the chain from a comparatively short stroke of 

 the piston. This construction, which characterises nearly all the varieties of the 

 hoisting and hauling machines to which I have applied hydraulic pressure is ex- 

 hibited in fig. 1172, which represents one of these presses with sheaves attached, to 

 multiply the motion fourfold. In cases where the resistance to be overcome varies 

 very considerably, I generally employ three such cylinders, with rams or pistons 

 acting either separately or conjointly upon the same set of multiplying sheaves, 

 according to the amount of power required. 



1172 



'In hydraulic cranes the power is applied, not only for lifting the load, but also for 

 swinging the jib, which latter object is effected by means of a rack or chain operating 

 on the base of the moveable part of the crane, and connected cither with a cylinder 

 and piston having alternate motion, like that of a steam-engine, or with two presses 

 applied to produce the same effect by alternate action. 



