The advantage of hydraulic technology is that you don't have to memorize a lot of complicated formulas to understand how things work. If you learn the basic principles shown in the following paragraphs, you can apply them to almost any other valve to understand how it works.
Hydraulic valves all use the same structure such as basic pistons, springs, and throttling holes, but in slightly different ways.
Piston force compresses spring
Spool displacement is controlled by the pressure at one end of the spool, which resists the spring force at the other end. As the pressure increases, the spring compresses and the spool moves.
Orifice size controls pressure or flow
With constant pressure at port P, the flow rate will be controlled by the size of the port.
In the case of constant flow at port P, the size of the orifice will produce a fixed pressure drop, which will control the pressure upstream of the orifice.
In the valve diagram shown, the orifice size is controlled by the displacement of the spool to open or close the triangular notch in the spool between the oil ports P and A.
Fluid flow determines speed
Valve dynamics are controlled by orifice restrictions in the design. In the valve shown, spool motion delays the time required for the volume of fluid held in the end cavity to flow through the opening at port T.
Smaller valves are often used to control the pilot flow of larger valves. Sometimes the valve needs to move very quickly to provide a quick response, while other times the valve switching speed may need to be slowed down to provide more stable operation.
Leakage and trapped oil
Some fluid leakage will always occur through the gap around the outside of the spool. It is this characteristic that means there is no metal contact, so hydraulic equipment requires such a high power density and load for such a long time.
Leakage may reduce overall efficiency, but it does prevent being trapped in pressure. Care must be taken to consider standby and switching conditions. For example, if port A is a sealed pipe, its pressure will always be the ratio of P and T pressures, based on the leakage at each choke edge.
The spool shown includes an O-ring seal on a guide step. This will stop the leakage, but it will also affect the spool characteristics and lag.
Lift valves are in physical contact with the seat, and while you can never guarantee a 100% seal, lift valves often become trapped in oil pressure, which creates the risk of extreme pressure caused by temperature changes. Every 15° increase produces a pressure increase of 70bar.
Fluid dynamic effect
Whenever the flow changes direction or passes through a different flow area, its local pressure changes. These pressure changes can have a significant impact on the spool or lift valve pressure balance, thereby affecting the set position. Valve manufacturers often do a lot of work to limit the impact of the internal flow forces in their valves.
The shape of the valve pressure and flow (PQ) characteristic curve is a good way to compare the quality of the valve. You should also check for lag, opening pressure, resolution, etc.
