外文翻译--液压系统和气压系统

中文3467字

附录:

Hydraulic system and Peumatic System

Hui-xiong wan1,Jun Fan2

The history of hydraulic power is a long one, dating from man’s prehistoric efforts to harness the energy in the world around him. The only source readily available were the water and the wind—two free and moving streams.

The watermill, the first hydraulic motor, was an early invention. One is pictured on a mosatic at the Great Palace in Byzantium, dating from the early fifth century. The mill had been built by the Romans. But the first record of a watermill goes back even further, to around 100BC, and the origins may indeed have been much earlier. The domestication of grain began some 5000 years before and some enterprising farmer is bound to have become tired of pounding or grinding the grain by hand. Perhaps, in fact, the inventor were some farmer’s wives. Since the often drew the heavy jobs.

Fluid is a substance which may flow; that is, its constituent particles may continuously change their positions relative to one another. Moreover, it offers no lasting resistance to the displacement, however great, of one layer over another. This means that, if the fluid is at rest, no shear force (that is a force tangential to the surface on which it acts) can exist in it.

Fluid may be classified as Newtonian or non--Newtonian. In Newtonian fluid there is a linear relation between the magnitude of applied shear stresses and the resulting rate of angular deformation. In non—Newtonian fluid there is a nonlinear relation between the magnitude of applied shear stress and the rate of angular deformation.

The flow of fluids may be classified in many ways, such as steady or non steady, rotational or irrotational, compressible or incompressible, and viscous or no viscous.

All hydraulic systems depend on Pascal’s law, such as steady or pipeexerts equal force on all of the surfaces of the container.

In actual hydraulic systems, Pascal’s law defines the basis of results which are obtained from the system. Thus, a pump moves the liquid in the system. The intake of the pump is connected to a liquid source, usually called the tank or reservoir. Atmospheric pressure, pressing on the liquid in the reservoir, forces the liquid into the pump. When the pump operates, it forces liquid from the tank into the discharge pipe

at a suitable pressure.

The flow of the pressurized liquid discharged by the pump is controlled by valves. Three control functions are used in most hydraulic systems: (1) control of the liquid pressure, （2）control of the liquid flow rate, and (3) control of the direction of flow of the liquid.

Hydraulic drives are used in preference to mechanical systems when(1) powers is to be transmitted between point too far apart for chains or belts; (2) high torque at low speed in required; (3) a very compact unit is needed; (4) a smooth transmission, free of vibration, is required;(5) easy control of speed and direction is necessary; and (6) output speed is varied steplessly.

Fig. 1 gives a diagrammatic presentation of the components of a hydraulic installation. Electrically driven oil pressure pumps establish an oil flow for energy transmission, which is fed to hydraulic motors or hydraulic cylinders, converting it into mechanical energy. The control of the oil flow is by means of valves. The pressurized oil flow produces linear or rotary mechanical motion. The kinetic energy of the oil flow is comparatively low, and therefore the term hydrostatic driver is sometimes used. There is little constructional difference between hydraulic motors and pumps. Any pump may be used as a motor. The quantity of oil flowing at any given time may be varied by means of regulating valves( as shown in Fig.7.1) or the use of variable-delivery pumps.

The application of hydraulic power to the operation of machine tools is by no means new, though its adoption on such a wide scale as exists at present is comparatively recent. It was in fact in development of the modern self-contained pump unit that stimulated the growth of this form of machine tool operation.

Hydraulic machine tool drive offers a great many advantages. One of them is that it can give infinitely-variable speed control over wide ranges. In addition, they can change the direction of drive as easily as they can vary the speed. As in many other types of machine, many complex mechanical linkages can be simplified or even wholly eliminated by the use of hydraulics.

The flexibility and resilience of hydraulic power is another great virtue of this form of drive. Apart from the smoothness of operation thus obtained, a great improvement is usually found in the surface finish on the work and the tool can make heavier cuts without detriment and will last considerably longer without regrinding.

Hydraulic and pneumatic system

There are only three basic methods of transmitting