hydraulic machine

Hydrostatic Devices

Water or oil under pressure is commonly used as a source of power for many types of presses, riveting machines, capstans, winches, and other machines. The hydraulic press, or hydrostatic press, was invented by Joseph Bramah and is therefore sometimes called the Bramah press. It consists essentially of two cylinders each filled with liquid and each fitted with a piston; the cylinders are connected by a pipe also filled with the liquid. One cylinder is of small diameter, the other of large diameter. According to Pascal's law, pressure exerted on the smaller piston is transmitted undiminished through the liquid to the surface of the larger piston, which is forced upward. Although the pressure (force per unit of area) is the same for both pistons, the total upward force on the larger piston is as many times greater than the force on the smaller piston as the area of the larger piston is greater than the area of the smaller piston. If, for example, the smaller piston has an area of 2 sq in. and a force of 100 lb is exerted on it, then the force on the larger piston having an area of 50 sq in. would be 2,500 lb (100×50/2 = 2,500). However, when the pistons move, the distance the smaller piston travels is proportionately greater than the distance the larger piston travels, satisfying the law of conservation of energy. If the smaller piston moves 25 in., the larger one will only move 1 in. The hydraulic press is used, for example, to form three-dimensional objects from sheet metal and plastics and to compress large objects.

The hydraulic jack, also an application of Pascal's law, is used to exert large forces or to lift heavy loads. Like the hydraulic press it consists essentially of two different-sized pistons contained in cylinders that are connected by a pipe. When the smaller piston is moved back and forth by a handle connected to it, it pumps a liquid into the cylinder of the larger piston, forcing the larger piston to move. In this way a weak force applied to the smaller piston can raise a heavy load on the larger one. The hydraulic elevator is also an application of Pascal's law.

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The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

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