852 



HYDRAULIC MACHINERY, FOR MINES 



away by tho eduction opening c. The bar valve, as will bo seen, moves into a single 

 leather d, when the water flows to the cylinder, and on flowing from the cylinder tho 

 valve leather e passes into the cylindrical part / of the valve-box. Experience has 



1184 



shown that leathers worked in this way are thoroughly reliable, and that the valve 

 itself is effective under very high pressures. Both valve bar and box are of gun- 

 metal. 



Balance and Differential Pistons. Under considerable pressures balance-valve 

 pistons are necessary. Examples of their application is shown in Darlington's 

 pressure-engine, NO, page 855 jig. 1187, and also in Trevi thick's engine, BC,/^. 1188, 

 page 856. Differential valve pistons, KI, are shown in/^. 1187, page 855. 



Cup Leathers. Cup leathers are either single or double, tho former are partly 

 clipped between two metallic surfaces, tho latter are supported on a loose ring. For 

 large plungers, leathers are costly, and apt to wear quickly. Iron surfaces are readily 

 attacked by mineral waters which produce a saw-like roughness calculated to tear the 

 edge and face of the leathers. For brass rams, leathers may bo used with the greatest 

 advantage, since in contact with that metal they will bo very durable and cause but 

 little friction. The table at tho head of p. 853 gives tho friction occasioned by tho 

 iise of leathers on rams tho surfaces of which are in good smooth working order. 



Packing. For cylinders of very largo diameter under moderately low pn - 

 hemp rope, or gasket may be employed. Brass packing rings are also reliable. For 

 valve pistons, passing ports, a close working fit between the piston and cylinder will 

 prove satisfactory. Small pistons or rams travelling under high pressures should he 

 fitted with single or double cup leathers. Iron rods above 3 inches diameter, if 

 packed with lamp-wick, will give less trouble than if worked in contact with cup 

 lea' hers. 



