Over-pressure, is prevented by safety-valve shown in front of the pump A. [Fig. 146] shows an enlarged section of the pump used in connection with this press. When the plunger P rises a partial vacuum is formed below it and the suction valve O rises allowing the pump barrel to fill with water through the strainer and suction pipe in the cistern.
When the plunger descends the valve O closes and the water passes through the discharge valve h into the pipe K, thence into the cylinder B of the press where it acts upon the ram. When the press has done its work the ram may be lowered by opening the relief valve r. The safety valve is shown at i. By removing the plug h the discharge valve can be reached to grind it in when necessary.
Note.—Hydraulic Pressure Transmission. Water under high pressure—500 to 3000 lbs. per square inch and upwards—affords a very satisfactory method of transmitting power to a distance, especially for the movement of heavy loads at small velocities, as by cranes and elevators. The system consists usually of one or more pumps capable of developing the required pressure; 2, accumulators, described on the [next page]; 3, the distributing pipes, and 4, the presses, cranes, or other machinery to be operated. This property of fluids invests us with a power of increasing the intensity of a pressure exerted by a comparatively small force, without any other limit than that of the strength of the materials of which the engine itself is constructed. It also enables us with great facility to transmit the motion and force of one machine to another, in cases where local circumstances preclude the possibility of instituting any ordinary mechanical connection between the two machines. Thus, merely by means of water-pipes, the force of a machine may be transmitted to any distance, and over inequalities of ground, or through any other obstructions.
THE HYDRAULIC ACCUMULATOR.
This useful and indispensable apparatus was designed by Sir William Armstrong. Its use was to secure a uniform pressure of water in a reservoir by weight so that however much or little of this water was used the pressure would remain constant.
ELEVATION.
Fig. 147.
In the first accumulator which is still in use the ram was attached to the foundation while the cylinder rose and fell as the pressure was utilized. The weights were annular in shape and were hung upon the outside of cylinder. In the modern types of accumulators the cylinder is stationary and the ram supporting the weights is made to rise and fall.
By means of a hydraulic accumulator a uniform pressure can be established and maintained on all parts of a hydraulic main or system.