764 HYDRAULICS AND ITS APPLICATIONS 



Friction losses are proportionately less as the size of the machine 

 increases, and, owing to the fact that the friction is partly mechanical 

 and independent of the load, the losses diminish proportionately as the 

 load increases. 



Eccentric loading largely increases the mechanical friction, and may 

 double the friction losses. 



The efficiency of a jack in fairly good order may be taken as varying 

 from about 66 per cent, in the case of a small 3-ton jack, with eccentric 

 loading to about 93 per cent, with a 100-ton jack and central loading, and 

 is approximately constant for loads greater than one-fifth of the nominal 

 capacity of the jack. 



ART. 203. THE HYDRAULIC PRESS. 



Reference has already been made in Art. 8 to the Bramah's press. Its 

 modifications, as applied to such work as cotton baling, boiler-plate 

 flanging, cartridge-case drawing, and heavy forging, are too numerous for 

 detailed mention, and only one or two of such applications will be con- 

 sidered in detail in the present treatise. 1 Fig. 378 2 illustrates a form ol 

 press used for flanging operations, &c., and capable of exerting a pressure of 

 420 tons. In this machine two small rams are installed for lifting the 

 head on the up stroke, while the pressure is applied by one central and 

 two side rams, pressure water being supplied to these in succession as 

 additional force is required. During the idle part of the down stroke, 

 water from the exhaust is allowed to fill the space vacated by the rams. 

 The arrangement of valves by which this is rendered possible is simple 

 and worthy of notice, and is shown in detail in Fig. 379. The main 

 pressure and exhaust valves V P and V K each carry a closely fitting piston 

 of greater area than the valve itself, so that if the pressure above the 

 piston and below the valve is equalised the effect of the pressure between 

 the piston and valve is to lift the latter from its seat. A small orifice is 

 provided in each piston, so that under normal conditions the pressure 

 above and below the piston is equal, and pressure keeps the valve on its 

 seat. The auxiliary valves V\ and F 2 are worked directly from the 

 operating lever, and when open give free communication between the 

 under side of the corresponding main valve and the upper side of its 

 piston. 



1 For further applications of the press the reader is referred to Elaine's " Hydraulics." 

 By courtesy of the makers, Messrs. Henry Berry & Co., Leeds. 



