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hydraulic press[hi′drȯ·lik ′pres]
a machine powered by high-pressure fluid and used for working of materials by pressure. The first hydraulic presses were used at the end of the 18th century and the beginning of the 19th for such purposes as baling hay, pressing grapes, and squeezing butter. By the middle of the 19th century hydraulic presses were being widely used in metalworking for forging of billets, sheet metal stamping, die stamping, bending and straightening, volumetric stamping, extrusion of piping and structural shapes, packaging and briquetting of waste products, compressing of powdered materials, metal sheathing of cables, and so on. Hydraulic presses have also found use in making plastic and rubber articles, wood veneers, plywood, textolite, and the like. They are also employed in syntheses of new materials (such as artificial diamonds).
The operation of the hydraulic press is based on Pascal’s law. Its force is developed by a piston of a working cylinder to which fluid (water or oil) under high pressure is admitted. The piston in turn is connected to the working tool. A hydraulic press may be driven by a pump or by a pump-accumulator station with steam, air, hydraulic, or electromechanical actuation. The working cylinder may be either vertical or horizontal. The pressure of the working fluid is usually 20-32 meganewtons/m2 (200-203 kilograms-force/ cm2), but in special cases (such as diamond syntheses) it may reach pressures of 450 meganewtons/m2 (4,500 kilograms-force/cm2). The cost of metalworking on the hydraulic press is lower than the cost of hammering and the efficiency is higher. The hydraulic press does not require a heavy foundation and does not cause great vibrations and noise such as is inevitable during hammer operation.
The most powerful hydraulic presses for die stamping were built in the 1960’s in the USSR and develop a force of 735 meganewtons (approximately 75,000 tons-force). Constructing hydraulic presses with a significantly larger force is possible.
REFERENCEMashinostroenie: Entsiklopedicheskii spravochnik, vol. 8. Moscow, 1948.
Moshchnye gidravlicheskie pressy. Edited by B. V. Rozanov. Moscow, 1959.
B. V. ROZANOV and V. P. LINTS
A combination of a large and a small cylinder connected by a pipe and filled with a fluid so that the pressure created in the fluid by a small force acting on the piston in the small cylinder will result in a large force on the large piston. The operation depends upon Pascal's principle, which states that when a liquid is at rest the addition of a pressure (force per unit area) at one point results in an identical increase in pressure at all points.
The principle of the hydraulic press is used in lift jacks, earth-moving machines, and metal-forming presses (see illustration). A comparatively small supply pump creates pressure in the hydraulic fluid. The fluid then acts on a substantially larger piston to produce the action force. Heavy objects are accurately weighed on hydraulic scales in which precision-ground pistons introduce negligible friction. See Mechanical advantage, Simple machine