Several basic theories of hydraulic transmission and control

1. Transfer energy through fluid

Fluid power transmits power through hydraulic oil inside the pipe, regardless of the length or number of elbows. It allows the hydraulic system to have the same pressure in every part of the circuit.

2. Conservation of energy

Conservation of energy means that energy input is always equal to energy output. Therefore, since force = P x A, small force large displacement = large force small displacement. In addition, the energy lost due to pressure drop or restriction will be converted into heat.

3. Control movement through braking

Hydraulic systems are traditionally a braking technology. This means that the actuator is controlled through a small orifice or valve clearance to limit the fluid flowing out of the cylinder. In this way, both sides of the cylinder remain under pressure, providing more firm and positive control.

4, pressure drop = heat

The pressure loss that occurs on each control orifice converts energy into heat, such as a 100 bar pressure drop that increases the temperature by about 5.5°C.

5. Thermal expansion of fluid

Fluid volume varies with temperature. With a temperature increase of 15 degrees Celsius, the volume should increase by 1%, which will increase the pressure in the sealing volume by 150 bar. Just use Boyle's law P1xV1/T1 = P2xV2/T2.

6. Compressibility of fluids

For most "steady-state" directional valve applications, hydraulic fluids are assumed to be incompressible, such that the fluid is so hard that flow at one point immediately causes movement at another point.

For most (advanced) "dynamic" servo and proportional valve applications, for example, hydraulic oil is assumed to be compressible. The compliance of the fluid means that there will be a small but significant time delay or elasticity between the accelerated flow or movement at one point and the actuator movement at another point.

7, gauge pressure or absolute pressure

Hydraulic readings are usually measured in gauge pressure. Gauge pressure is the value read by the meter and is the difference between the air pressure around the meter and the fluid pressure.

Sometimes it is important to know and measure the absolute ambient pressure. Absolute pressure includes local air pressure, which varies depending on altitude and potentially other environmental factors.
The minimum inlet pressure requirement for a pump is usually expressed in absolute pressure and is critical to achieving a long pump life.

8. Dimensional analysis or unit conversion

In hydraulic fluid calculations, converting units is a routine and risky process, so you must fully understand how it is done. Fortunately, there is a relatively simple technique that can do just that.
In this example, we show how to convert 100 L/min to US Gal/sec. We start by adding square brackets with US gallons at the top and liters at the bottom. There are 3.785 liters in a US gallon, so by adding this value to the bottom, we get this value at the top and bottom of the equation, so this value is constant. We can also add a second square bracket with minutes at the top and 60 seconds at the bottom. Again, the value remains the same.

Next, we can delete the top and bottom units so that we lose all the liters and minutes. This leaves us with a simple calculation, dividing 100 by 3.785 and 60.

The secret is to always write out all units clearly, rather than trying to cut with simple units that you know. With a little practice, you'll soon fall in love with this technique.

Source: Hydraulic drive and control