Читать книгу Hydraulic Fluid Power - Andrea Vacca - Страница 53

Pascal's law states that the pressure is transmitted undiminished in a confined body of fluid at rest.

The Pascal's law of fluid statics is the foundation of what is considered by most engineers the modern era of fluid power technology.

The first hydraulic machines of the nineteenth century, such as the first hydraulic presses and hydraulic lifts, are based on this law.

The use of Pascal's law in an elementary hydraulic machine is shown in Figure 3.1. At level z^*, the fluid pressure must be equal in both the vertical branches of the apparatus. Therefore:

(3.1)

It is evident that the geometrical ratio of the areas of the two pistons is related to the ratio of the force applied:

(3.2)

Figure 3.1 Basic hydraulic machine.

Equation (3.1) shows how it is possible to produce large loading forces using small geometrical areas and establishing high fluid pressures. This is based on power density, the main advantage of fluid power technology. Equation (3.2) is based on many hydraulic machines that require force multiplication, such as hydraulic brakes. For the practical applications of fluid power technology, the upper pressure limit (usually defined by the relief valve setting) is never given based on the fluid, but on the structural requirements of the components or of the machines. This also explains the current trend of increasing, where possible, the operating pressures of fluid power machines so that the power‐to‐weight ratio is reduced. A significant example for this is fluid power for aviation technology, where over the years the working pressure of the hydraulic actuation systems (flap and slat drives, landing gears, nose wheel steering, and many others) increased up to 210 bar, which is used in most commercial airliners. High performance military aircraft recently increased to 350 bar.