Hydraulic Fluid Power. Andrea Vacca

Hydraulic Fluid Power - Andrea Vacca


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

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

Schematic illustration of the basic hydraulic machine.

      The Pascal's law presented in Section 3.1 is related to the basic equation of fluid statics:

      Many hydraulic machines are often referred to as “hydrostatic machines” because their basic functioning can be evaluated using only static equations (not involving fluid velocity), as described in Section 3.1. Indeed, in almost all hydraulic components, the effect of fluid pressure is significantly higher than that of fluid velocity, thus making the latter negligible.

Schematic illustration of the fluid pressure increases with depth.

      A reasonable value for h can be 3 m. Considering a fluid density of 870 kg/m3, which is typical for a hydraulic oil, the pressure difference between two points with the maximum elevation difference is

      (3.5)normal upper Delta p equals rho dot g dot h equals 870 left-bracket k g slash m cubed right-bracket dot 9.81 left-bracket m slash s squared right-bracket dot 3 left-bracket m right-bracket equals 25 600 upper P a equals 0.256 b a r

      which is considerably lower than the typical operating pressure of the system, which is easily above 100 bar.

      The elevation difference in typical hydrostatic circuits can be neglected when calculating the system pressures. The effects of elevation become critical only when in parts of the system the pressure is close to the saturation conditions, such as at the suction of hydraulic pumps.

      Before entering into more specific considerations for the hydraulic case, it is important to recall the definition of scalar product. Figure 3.4 shows the generic case of a pipe where a particle of fluid is traveling with a defined velocity along the direction of the pipe. The figure also represents a generic surface area defined by the section of the pipe with a generic plane. As shown in the figure, both are vector quantities: vector ModifyingAbove v With right-arrow denotes velocity, while vector ModifyingAbove upper A With right-arrow has the magnitude of the area A and it is perpendicular


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