Introduction to Valve Symbol Reading

Objectives

  • Relate symbol elements to actual valve components
  • Explain valve port labels
  • Describe valve flow path positions

The Problem

Hydraulic valves have a tendency to be the most complex components of a hydraulic system, and their schematic symbols are just as complex.

The Solution

If we break down the symbols, you'll see they are very straightforward. Every part of a valve symbol corresponds with a part of the real valve. Let's do a side by side comparison of a valve schematic symbol with an actual valve, and point out each part.

A Relief Valve

A direct acting relief valve is one of the simplest valves.

Relief valve cross-section.

But there are still many different parts to be represented by the schematic.

Relief valve schematic symbol.

Ports

The ports of a valve are the places on the valve body where flow enters or leaves the valve. Relief valves have an inlet and outlet port. The inlet is sometimes labelled as port 1, and the outlet labelled as port 2.

Inlet
Outlet
Inlet
Outlet

Valve Body

The dashed manifold line represents the valve body. It's not always added to a relief valve schematic symbol, since many relief valves are cartridge valves and do not have a separate manifold.

Poppet

This arrow represents the relief valve poppet, which allows the valve to crack open when the pressure on the inlet port gets high enough.

An arrow that is in-line with the inlet and outlet ports indicates that the valve is normally open.

An arrow that is not in-line with the inlet and outlet ports indicates that the valve is normally closed.

Pilot Line

The dashed line represents the valve pilot line. In this style of relief valve, the inlet port and the pilot passage are one and the same. This is not necessarily the case in more complex valves.

Spring

In the schematic, the zig-zag line is the spring. In the cutaway drawing, the spring is shown as a series of dots. (In other cutaway graphics, springs are sometimes drawn with diagonal lines connecting the dots, representing the coil of the spring.)

Adjustment Screw

The angled arrow across the spring indicates that the spring setting is variable. In this example, the relief valve has an adjustable knob that compresses the spring, increasing the relief setting.

Directional Control Valves

Let's take a look at something a little more complex: a four port, three position directional control valve.

Directional control valves, as the name would imply, control the direction that hydraulic oil flows as it travels to and from an actuator.

Directional control valve cross-section.
Directional control valve schematic symbol.

When referring to a directional control valve, it is best to first describe the number of ports and positions.

Ports

The ports of a directional control valve are the places on the valve body where hydraulic lines can be attached.

A
B
P
T
A
B
P
T

Rather then simply being referred to by a number, the ports on a directional control valve are labelled to indicate the purpose of the port. A and B are the work ports that connect to the actuator, P comes from the pump and T returns to tank.

Positions

Each square section in a directional control valve schematic symbol — called an envelope — represents a position that the valve spool can be in. The arrangement of symbols and arrows inside each envelope tells you how the ports are interconnected when the valve is in that position.

Let's look at each of the 3 positions in this valve, in both schematic and graphic cutaway.

In our graphics, red indicates a pressurized passage, and blue indicates a return/tank passage.

A
B
P
T
A
B
P
T
In the center position, all four ports are blocked.
A
B
P
T
A
B
P
T
In the right position, the pump port P is connected to the A work port, and the B work port is connected back to the tank port T.
A
B
P
T
A
B
P
T
The left position is the reverse of the right position, with the pump port P connected to the B work port, and the A work port connected back to tank T.

As you saw, our example valve is a 4 port, 3 position valve.

Centers

Take special note of the center envelope. It tells you how this valve works when in the initial, resting position. There are four primary types of valve center to be aware of.

A
B
P
T
A
B
P
T

This is an open center. All ports are connected to the tank, depressurizing the circuit.

A
B
P
T
A
B
P
T

This is a closed center. All ports are blocked. Any pressure in the A or B lines remains trapped.

A
B
P
T
A
B
P
T

With the float center, the P port is blocked but both the A and B work ports are open to tank.

A
B
P
T
A
B
P
T

Finally, the tandem center. The P port is open to tank, but the A and B work ports are blocked.

Operators

The operators are little symbols at the left and right sides of the valve symbol, which indicate how the spool is moved.

The solenoid operators indicate that the valve can be operated electrically. The envelope that a solenoid operator is attached to is the position that the valve spool will move to when that solenoid is energized.

The handle operator means that the valve can be manually operated.

Spring operators indicate that the valve spool will return to its center position if there is nothing actively driving it to a different position.

Remember, in our graphic cutaway the spring is drawn as a series of dots.

Here's a different valve. Instead of having solenoids, this one is pilot operated. When present in a hydraulic schematic, you will see dashed pilot lines routed to the pilot operators.

Review

Click and drag the valve center to its matching label.

Tandem Center
Open Center
Closed Center
Float Center

Which statement best describes this valve?

Which statement best describes this valve?

In Conclusion

Armed with this knowledge, you now have the basics for deciphering valve symbols!

We hope you enjoyed Introduction to Valve Symbol Reading

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High Pressure
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