HYDRAULIC PRESS (from Lat. hydrauli ems, Gk. 1-dpae7dm4c, hydraulikos, pertaining to the water-organ, from fadymg.eS, hydrautis, water organ, from Midi), hythir. water + av46c, autos, pipe). An apparatus for obtaining a heavy pres sure for compacting fibrous substances, raising heavy weights, etc. by means of water under pressure. The first design of a working hy draulic press is credited to Joseph Brainah. and for this reason such machines are sometimes called Brainah presses. The operation of the hydraulic press is based upon the principle of hydrostatics that a pressure exerted on any part of the surface of a liquid is transmitted un diminished to all parts of the mass and in all directions. Thus, if we have a cylinder filled with water in one end of which works a plunger one inch square, and in the other end of which works a plunger twelve inches square, and we exert a pressure of one pound on the smaller plunger. this pressure will be transmitted un diminished by the water, and will exert a total pressure of 1 X 12 X 12=144 pounds.
The essentials of a hydraulic press are. there fore, a small cylinder containing a plunger, to which pressure is applied, connected by pipe with a large cylinder containing a plunger, which transmits the multiplied pressure to the object to be pressed or lifted. It is evident, further more, that the two cylinders may be located any desired distance apart. the only limit being that at which the pipe and cylinder friction reduces the available pressure for performing work below practicable amounts. (See POWER. TRANSMIS SION OF.) In modern practice the pressure pump commonly feeds into a hydraulic accumulator (see ACCUMULATOR) , and the press cylinder is operated from the accumulator and not directly from the pump. The construction of hydraulic
presses varies with the purposes for which they arc employed. Hydraulic presses for haling hay and cotton and expressing the oil from seeds, etc., are built substantially as follows: Four iron pillars are erected at the corners of a hori zontal square. and carry a cast-iron cap rigidly fastened to their tops. The under face of this cap forms the surface against which the material is pressed. At the bottom of the four pillars there is a similar casting which has a circular hole in the centre down through which passes the cylinder casting for the plunger. The plunger has an upward motion in the act of pressing. and carries on its top a head or platen consisting of a square casting. The material to be pressed is placed between the lower face of the top casting and the upper face of the platen. Often a boxing is placed around the four sides of the space in which the head moves to prevent the material being pressed from squeezing out laterally. The construction of a press for hydraulic forging differs from the above as follows: The cylinders and plungers are carried by the top casting or column cap and the plungers work downward in the net of pressing; the bottom casting carries the anvil; the plungers carry a die for the head; and two or more cylinders are provided for raising the plunger and die for a fresh stroke. Generally, also, two pressure plungers and cylin ders are employed instead of one. Forging presses are made of all sizes, from that required in pressing out articles like revolver cartridge cases to those used in forging steamship shafts and armor plates. Armor-plate presses having a capacity of 14.000 tons are in use.