FUNDAMENTALS Josh Cosford • Contributing Editor
Hydraulic Symbology 203 — Pressure valves In Hydraulic Symbology 101, I covered the basic square used for pressure valves and also showed the most stripped-down versions of the two most commonly used pressure valve symbols, the relief valve and the pressure reducing valve. In this edition of Hydraulic Symbology, I’m going to cover the four primary pressure valves; the relief valve, motion control valve, sequence valve and reducing valve. Each is based on the same square symbol but are used quite differently in both circuits and real-life function.
valve closed with 3,000 psi of force, and in this circuit, it acts as the maximum limit pump pressure can achieve before being exhausted to tank.
Shown below are the quartet referenced from the same angle as each other. Each shows the basic square with a vertical arrow, abreast of a pilot line to the left and a spring to the right. The dashed line stands for a pilot signal, which is a fluid column of pressure energy used to push or act upon other components internal to the valve. The relief valve is normally closed (nonflowing). As pressure rises in the bottom port, energy pushes around to the pilot line to the left, but the valve is still closed. As the pressure continues to increase, the force pushing against the left side of the arrow starts to overcome the spring force applied from the right. When pilot pressure creates enough force, it can overcome spring pressure to slowly open the valve.
Springs are drawn to signify force being applied inwards and in the case of these symbols that force is to the left. The relief valve spring can be set as weak or strong within its allowable range by loosening or tightening its screw adjustment. The weaker the spring compression, the easier pilot pressure can force open the valve. As previously mentioned, the diagonal arrow depicts adjustability, and most pressure valves are adjustable. The example shows a circuit with all four types of pressure valves used. It looks like a lot going on, but I’m going to break them all down one by one so they make sense. The relief valve teed into the right after the pump is drawn just as the relief valve above, and it operates under the same principle. The spring is pushing the 22
FLUID POWER WORLD
4 • 2020
Sequence valves are not much different from relief valves, and this is at once obvious by their similar appearance. This sequence valve downstream of the pump is exactly the same as the relief save for the drain line and reduced pressure setting. A sequence valve is purposed to provide a secondary flow path which occurs in sequence to a parallel function. In other words, when the cylinder in this application extends to the end of stroke, pressure will rise immediately. When pressure hits 2,000 psi, our sequence valve opens, diverting all pump flow to rotate the motor while the cylinder remains stalled and as long as its directional valve remains energized. The sequence valve drain line is required to keep the valve’s performance consistent. Because the sequence valve experiences pressure on both ports, internal leakage allows pressure buildup inside the spring chamber which is additive to spring pressure.
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