1、Hydraulic Machine From: The Columbia Encyclopedia, Sixth Edition Date: 2008 Hydraulic machine that derives its power from the motion or pressure of water or some other liquid. Hydraulic equipment and technology is something that we are all at least passingly familiar with. If we think about it, we k
2、now that the principles of hydraulics are applied to make many common machines work. For example hydraulics are used in agricultural equipment, giant earth moving and mining machines, they are used to steer and stabilize giant ocean liners, help airplanes climb and turn, and make the brakes in our c
3、ars work. So hydraulics can provide great force, are obviously very adaptable and used in all kinds of applications, but how do they actually work? What is this hydraulics stuff? Hydraulics is based on a very simple fact of nature - you cannot compress a liquid. You can compress a gas (think about p
4、utting more and more air into a tire, the more you put in, the higher the pressure). If youre really strong you can compress a solid mass as well. But no matter how much pressure you apply onto a liquid, it isnt possible to compress it. Now if you put that liquid into a sealed system and push on it
5、at one end, that pressure is transmitted through the liquid to the other end of the system. The pressure is not diminished. . Hydraulics is Old Stuff The basic concept of hydraulics is not new. The Greeks understood about using water to provide lift and force, and the name hydraulics come form the L
6、atin word for water - HYDRA. In the middle Ages, Leonardo da Vinci formulated the basic principle of hydraulics called continuity and Galileo experimented with hydraulics. Hydraulics were even used during the construction of the Eiffel Tower in Paris in the late 1880s. Hydraulic jacks were used to l
7、evel the tower and align the metal girders to an accuracy of 1 millimeter。 Today most hydraulic systems utilize oil rather than water but the principle is the same. You can compress a liquid, and a force that is applied at one point is transmitted to another point by that incompressible fluid. Becau
8、se of its liquid nature, hydraulic systems can transmit force through pipes of any shape and length, so the force can be applied at one central point and transmitted efficiently to another point or to multiple points far away. Think about the master cylinder in you cars brakes and how, by stepping o
9、n the brake pedal, you apply stopping pressure on the brakes on all four wheels. Physics Basic physics tells us that we can trade off force for distance in all mechanical systems. In a hydraulic system, we do this by changing the relative size of the pistons at each end of the system. For example, a
10、 small piston moving a relatively large distance (say a foot) will exert pressure on a larger piston at the other end. The force will be enough to move a heavy weight a small distance (much less than a foot). Engineers and physicists can calculate exactly how much distance needs to be traveled and t
11、he relative sizes of the pistons required to move a particular weight. Hydraulic Engines Water falling from one level to a lower one is used to drive machines like the water wheel and the turbine . The difference in height between the highest and the lowest level is called the head。 The amount of wo
12、rk produced per pound of falling water is proportional to the head. Water power can be produced in this way from many natural sources, such as waterfalls and dammed rivers.。 Where no natural sources are available, an artificial reservoir can be made. When energy is plentiful, it is used to pump wate
13、r into the reservoir； the water is then available as a power source to drive turbines when energy becomes scarce. In driving certain industrial hydraulic machines an apparatus called an accumulator is employed to supply high power for short periods of time. One type consists essentially of a cylinde
14、r enclosing a piston loaded with weights. When water is slowly pumped into the cylinder, the piston and weights are forced up to a position where they are held. When they are released, they force the water out of the cylinder rapidly, providing the machine with hydraulic power. Hydrostatic Devices W
15、ater 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 o
16、f 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 Pascals law , pressure exerted on the smaller piston is transmitted undiminished thro
17、ugh 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 greate
18、r 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 pis
19、ton 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 Pascals 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
