The term hydro-static pressure is common in Physics. It is the pressure which acts on the base of an open container filled with fluid, and which is dependent on the height of the head of liquid inside the container. A hydraulic paradox occurs here, which is that the shape of the container is irrelevant, and only the height of the head of liquid determines the pressure. Hence, this also means that the pressure at the bottom of the container is higher than at the top of the container. This fact is well-known, if you consider the pressure of water deep down in the open sea. The behaviour is the same in a “sea of air”.

In statics, care must be taken that the forces are balanced. This is also true for analogue forces in hydrostatics. At the base of a container, at the bottom of the sea, or at a particular height in the place to be measured, the pressure present does not create any changes in the existing relationships.

If the fluid is enclosed in a closed container, as for example, in a hydraulic cylinder in fluid power, and if much higher pressures are needed than exist due to gravity at a certain height in a fluid, then these pressures are created via appropriate technical measures, e.g. by a hydraulic pump. Fluid is pumped into the closed container at a pressure produced by the hydraulic pump, and this pressure exerts itself equally on all sides of the container. This fact may be made use of, by making the base of the container movable. The base then moves,when pressure is applied, and providing that the hydraulic pump continues to supply fluid under pressure, a head of liquid is moved.

If the hydraulic cylinder (also under pressure) is at rest, e.g. in clamping hydraulics the forces are in equilibrium. This effect may be described as hydro-static. However, if the piston in the cylinder is moved by a supply of flow under pressure, then not only is the pressure produced from potential energy effective, but a boost pressure is also effective which is created by the kinetic energy. This pressure must be and is taken into account in fluid power systems. The relationships in this process or system may not really be described wholly as hydro-static, but the hydro-static relationships predominate.

Systems of this type, where hydro-static relationships are predominant and the transfer of pressure is most important, operate at relatively high pressures and low flow velocities in order to keep the influence of hydro-kinetics1′ as low as possible.

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