750 



HYDRAULICS AND ITS APPLICATIONS 



ART. 198. EFFICIENCY OF THE HYDRAULIC JIGGER. 



The chain, or wire rope, and pulley multiplying gear known as th 

 hydraulic jigger which is used to multiply the motion of a short strok 

 hydraulic ram for crane or hoist, as illustrated in Figs. 364 to 366 

 diminishes the mechanical efficiency of the system to a fairly large extent 



The loss thus introduced increases with the number of multiplications 

 and its magnitude varies so largely with the size and condition of sheaves 

 bearings, and rope or chain, that no definite law can be expected to cove 

 each case even approximately. 



With ordinary W 7 ell-designed gearing, having large pulleys, smal 

 bearings, and wire ropes in good condition and well lubricated, tb 

 efficiency of the jigger may be taken as being approximately givei 

 by the following formula (H. Adams) 



ri = -906 '021 m 

 where m is the number of multiplications of the stroke. 



If, then, the friction of packing leathers = 5 per cent, of the total forci 

 on the ram, the efficiency of ram and jigger is equal to 



95 { -006 - -021 m } 

 = '86 -02 m 



This gives the following values of the efficiency : 



ART. 199. HYDRAULIC CRANES. 



Where high-pressure water is available it provides a most convenient 

 means of operating power cranes, and in its safety, adaptability to suii 

 varying conditions, and steadiness of operation, offers many advantages 

 over its rivals compressed air and electricity. ' 



Such cranes are usually operated by hydraulic jiggers, the various 

 operations of lifting, racking, and slewing being often performed by 

 separate rams and cylinders, each regulated by its own separate valve. 



Where the load to be lifted may vary within wide limits, some device is 

 usually adopted to economise water at light loads. For small cranes, up 

 to about two tons, a differential or telescopic ram may be used, the smaller 



