Hydraulic Spool Valve Diagram: A Complete Guide

Spool valves are directional valves that we consider as the basic building block of any pneumatic system. We can represent this valve using an engineering drawing for easier understanding and interpretation.

How to Read A Spool Valve Schematic Drawing

Hydraulic spool valve diagrams contain pneumatic symbols that can help us know how the valve functions. The information we can get from them include the number of ports contained in the valve and the flow paths.

Additionally, they tell us the valve actuation method together with the number of operating positions. The diagram usually is made up of boxes, their total number shows the total positions of the valve.

We can take an example using different position spool diagrams such as:

· 2-Position Spool Valve Diagram

2-Position Spool Valve Diagram
2-Position Spool Valve Diagram

From the diagram, we can see that the control mechanism for this valve is a manual lever.

This spool valve type normally has a spring that resets the valve to its initial position when the lever is dormant.

That spring from our diagram is located on the right side of the valve. In the dormant condition, we mainly look at the box next to the spring and ignore the one next to the lever.

When you actuate the lever, we consider the flow path of the box next to it. We ignore the box next to the spring.

For this example, our valve has two boxes meaning it can be in two positions. However, the valve can only be in one position at a time and that is why we consider one box and ignore the other.

· 3-Position Spool Valve Diagram

The design of this spool valve is in such a way that you have two solenoids on either side of the valve. In addition, you will also find two actuator springs on either side of these valves.

The function of the spring returns is to return the valve to the initial middle position. Remember that this action can only take place when none of the two solenoids is powered.

Let us have a look at what its spool valve diagram looks like:

3-Position Spool Valve Diagram
3-Position Spool Valve Diagram

If you want to know the number of ports a valve diagram has, just look at the end points of each box in the diagram. Remember that we can only count each symbol in a given box as a single port once.

Let’s take an example of this three-position valve above. We can see that it has three boxes meaning it can operate in three different positions.

But when you look at the ports in the valve, you can count five. From the diagram, we can describe this valve as a 5/3 solenoid valve.

Hydraulic Spool Valve Diagram In Detail

There are situations where a manufacturer may decide to identify the valve parts with letters instead of numbers. But if you have a clear understanding of the diagram interpretation, then you’ll have an easy time translating such schematics.

For the following diagrams, we are going to represent the ports using the letters A, B, P, and T. Here are the diagrams of how we can classify hydraulic spool valves using diagrams:

· Closed Center

Closed Center
Closed Center

The function you get from this diagram is a neutral cylinder position while the pump pressure is kept constant. You will find that all the ports are blocked if you are dealing with a 2-position spool valve.

· Open-Center

 Open Center
Open Center

With such a diagram, the function of your valve is to unload the pump. It does this by taking the actuator and placing it in the neutral position.

If you are dealing with a 2-position hydraulic spool valve, then you will realize that all ports have a direct connection to the tank. This greatly reduces the system’s shock.

 ABT Connected
ABT Connected

This graphic valve symbol shows that its function is to keep the pump pressure constant with the actuator being neutral.

· PAT Connected

PAT Connected
PAT Connected

We can interpret the function of this hydraulic valve as unloading the pump while being in the neutral stance. It can only allow the flow of fluid in one direction therefore stopping the actuator.

· PT Is Connected But Closes During Transition

PT Is Connected But Closes During Transition
PT Is Connected But Closes During Transition

This type of diagram shows you that the valve is capable of unloading the pump while keeping the actuator neutral. You can connect your valves in serial formation.

· PT Is Connected But Opens During Transition

PT Is Connected But Opens During Transition
PT Is Connected But Opens During Transition

The spool hydraulic diagram tells you that all the ports are connected to the tank when the spool moves. This helps in reducing any shock in the hydraulic system.

· Center Opened (With Throttle)

Center Opened With Throttle
Center Opened With Throttle

You can find this type of spool diagram mostly in the 2-position type of spool. It shows that you can get a reduction in shock when the spool transitions.

· Two-Way

Two Way
Two Way

· PAB Connected

PAB Connected
PAB Connected

This diagram shows that your valve has created a differential circuit while positioned in the neutral form.

· BT Connected

BT Connected
BT Connected

The function of this spool valve from the schematic is to prevent the actuator from having a one-way slide. This may normally occur if your valve is at a neutral position but has a leak at port P.

· PA Connected

PA Connected
PA Connected

This schematic tells you that the function of your valve is blocking flow from one of its ends and diverting it to the other. This action ensures that the actuator is stopped permanently in the neutral position.

· AT Connected

AT Connected
AT Connected

This type of diagram shows you that your valve can stop the actuator from having small one-way sliding. As usual, this may be caused by any leakages from port P.

Conclusion

You now have a better understanding of hydraulic valve spool diagrams and how to interpret them. This is basically how these valves are represented by manufacturers and engineers in terms of schematics.

You can now easily translate them into how your valve operates in the real-world situation. Be sure to remember that depending on the manufacturer, the ports may be identified using either numbers or letters.

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