Best-Practice for Choosing an Air Solenoid Valve

Knowing How to Choose an Air Solenoid Valve Can Improve Pneumatic System Safety:

As a design engineer with the task of selecting solenoid air valves for your new machine, there are many things to consider: what port size do I need? what voltage? individual valves or on a manifold? I thought I would share this important pneumatic system design tip to keep people safe around your machine.

So, how do you decide when it is crucial for safety reasons to use a double solenoid instead of a single solenoid? Let’s first break down the differences between these types of valves

Single and Double Air Solenoid Valve Differences

You need to move a load with a pneumatic actuator which can either be a linear air cylinder or a pneumatic rotary actuator. Both types have 2 ports and putting pressure into one or the other creates motion. The air cylinder will either extend or retract compared with the rotary actuator that will turn clockwise or counterclockwise.

5-port 4-way air solenoid valve

Most pneumatic applications require a standard 4-way, 5 ported directional air valve to control these double acting pneumatic actuators. When the spool in this type of valve is shifted, it sends pressure to one or the other actuator port fittings depending on the position. It is the method that causes the spool to shift that differentiates a single from a double solenoid valve.

Single-Solenoid Valves

If your valve is a single solenoid type, it means when voltage is applied to the single “A” solenoid coil, the main spool inside the valve will shift which will cause the valve ports to output air on one side causing the pneumatic actuator to extend to a new position. When power is removed, the main spool will automatically return to the original position because there is an internal spring or air forcing it back. This shifts the air output to the other port forcing the actuator to return to its original position.

Double-Solenoid Valves

If the valve is a Double solenoid type, when a voltage is applied to the “A” solenoid coil, the same will happen as above. The main spool inside will shift to one side allowing air to flow to extend the actuator. When power is removed from the “A” coil, this time the main spool will remain in the same position with a detent. No motion of the air actuator will occur. The spool will only shift back if power is applied to the “B” solenoid coil, this valve has 2 solenoids, which is how it gets its name double solenoid. In order to return the actuator to its original position, you must energize the second “B” coil. This will shift the spool back and cause air to flow out the other port, providing pressure for the actuator to retract back.

Now with that information, you need to look at each pneumatic actuator on your new machine and ask yourself- “What would happen if the machine lost electrical power?”

Pneumatic Safety Circuit

Imagine a simple clamp and drilling machine, here is a perfect example where to use a double solenoid valve.

  1. An operator loads the part into a pneumatic clamp fixture, like an open vice.
  2. The operator then presses start, which applies voltage to the “A” solenoid coil.
  3. When the “A” coil is energized, it shifts the spool so the air pressure forces the pneumatic clamp to close and hold the part.
  4. A limit switch is activated when the clamp is in the closed position, this tells the machine that it is safe to start the process.
  5. The drill gets up to speed, moves down, and starts making a hole in the part. As the part is being drilled, what happens if the machine loses power?

If you had made the mistake of specifying a single solenoid valve for the pneumatic clamp, the spring return would automatically open the clamp at power loss. The part being drilled could go flying through the air with the potential of injuring the operator or damaging the machine. VERY DANGEROUS!!

If you had correctly used a double solenoid valve, the spool will not shift with pressure remaining to keep the pneumatic clamp closed. Without power, there is nothing to energize the “B” coil so the valve will not shift and the clamp will not open.

For new machine designs always ask yourself, “If power is lost, which actuator is critical not to move?”

I hope this article was helpful to understand where some applications need a double solenoid valve over a single solenoid valve. If you have questions regarding this article or questions about your pneumatic application, please e-mail me Ralph Quarto at [email protected]

Congratulations to June’s Quiz Contest Winner: Philip McCall

June’s quiz was a contest asking readers to choose between a single or a double solenoid valve for the situation described in this article. Philip McCall was the first to answer double solenoid valve and to describe the correct reason: It stays in position if power is lost. Philip won a $25 Amazon gift card. Congratulations!

July’s $25 quiz contest asks readers what’s happening during a flow control troubleshooting condition. Please join our conversations and provide your answers. Next month’s article will share the best-practice for pneumatic system design and the quiz contest’s winner. Good luck!

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