320

MODERN STEAM PRACTICE.

To meet the Variation of load it was formerly the practice tocombine three of the pressure cylinders so as to act either sepa-rately or collectively upon the lifting chain; but a Variation of poweris now obtained with a single-bored cylinder, fitted with a combinedpiston and ram, as follows:—A (Fig. 204) is the cylinder, fitted withthe piston E and ram B; the water from the accumulator enters thevalve ehest F through the pressure pipe J and the inlet valve H.For the lower power the water is admitted to both sides of thepiston E by opening the valve L, in which case the power exertedand the water expended are proportionate to the area of the ram B.

For the higher power the valve L isclosed and the valve M opened, sothat the front side of the piston E isthrown open to the exhaust K, andthe result, both as regards powerand expenditure, is then propor-tionate to the full area of the pis-ton E. It is seldom necessary tohave more than these lower andhigher powers; but where a thirdor less power is required, a smallerram is used with the other. For lowering the load the valves H andM are closed and the outlet valve I opened, allowing the water toescape from the cylinder A into the exhaust pipe IC; at the sametime the valve L is opened to allow the water to follow up the piston

in the inward stroke. The packing ofthe solid piston A is held in position bymeans of a ring of metal B, and secured tothe piston by stud bolts and nuts, andconsists of a cupped leather washer C,which is pressed against the side of thecylinder by the hydraulic pressure.

In hydraulic cranes the power is appliednot only for lifting the load, but alsofor swinging the jib, which is effected by means of a rack or chainacting on the base of the movable part of the crane, connectedeither with a cylinder and piston, or with two single-acting cylindersapplied to produce the same effect by alternate action; as shown,the two cylinders N and O (Fig. 203) are fitted with rams, workingby a chain passing round the base of the crane post F. The motion

Fig. 205.—Detail of Valves for Cylinder of Double-power Hydraulic Crane. — F, Valve ehest .h, Inlet valve. 1, Outlet valve. l, Valve forhigher power. m, Valve . N, Relief valve.

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Fig. 20fi.—Piston and Cupped Leatherfor Cylinder of Double-power Hy-draulic Crane.— a, Piston, b, Ringwith bolts. c, Cupped leather.