Basic hydraulics :
how to create pressure
There are several ways to create a hydraulic pressure.
A simple acting cylinder filled with oil is in a static position. A load of one ton is placed on the rod, the piston has a surface of 10 cm2.
A pipe is clogged with a manometer at its end.
There is a pressure of 100 bars on the manometers. This pressure applies at all points of the circuit perpendicular to all surfaces. (Pascal's theorem).
The pressure is determined by the load in relation to the application surface according to the formula :
P : pressure in bar.
F : force or load in daN or Kg.
S : area in cm2.
2- The addition of oil
A 200 liter tank is filled with oil using a hand pump (item 1). The valve (item 3) is closed after filling the tank. An anti-return valve (item 2) ensures the tightness of the circuit. A manometer is installed at the top of the tank.
It is found that to increase the pressure from 0 to 300 bar it is necessary to add oil with the pump. The oil is compressible, it is necessary to add 6 liters of additional oil.
?v = ?p * Vi / 10000
?v: difference in volume.
?p: pressure difference.
VI : initial volume.
10000 : oil compressibility coefficient (46 cSt) (varies depending on the oil).
A 200 liter tank is filled with oil using a hand pump (item 1). The valve (item 3) is closed after filling the tank. A no-return valve (item 2) ensures the tightness of the circuit. A manometer is installed at the top of the tank.
The tank is subjected to a temperature rise of 20 ° to 60 °.
It is found that the volume of oil in the tank increases (+6 liters), which causes an increase in pressure from 0 to 300 bar.
?v = ?T * Vi *0.00075
?v : difference in volume.
?T : temperature difference.
VI : initial volume.
0.00075 : coefficient of expansion of the oil (46 cSt) (varies according to the oil).
4- The oil column
A tank filled with oil is placed 10 meters high. A valve (item 1) is closed.
There is a pressure on the pressure gauge of 0.9 bar created by the oil column.
The density of the oil is 0.9.
5- The different pressures
In a hydraulic circuit, there are three types of pressure:
Relative pressure : the pressure read on the pressure gauges in the hydraulic circuits.
Atmospheric pressure : Pressure exerted by the air on the earth's surface (about 1 bar).
Absolute pressure: It is the addition of the relative pressure and the atmospheric pressure (total pressures).