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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 what types of solenoid valves you will need for your new machine there are many things to consider, what size of valve do I need? What voltage? What size ports? There are many other things to decide on.

What I want to discuss in this article is when you need to select a double solenoid valve over a single solenoid valve.

Most pneumatic applications require a standard 4-way, 5 ported directional air valve to control your double acting pneumatic actuator. A pneumatic actuator can be either a linear air cylinder or a pneumatic rotary actuator. Both items have 2 ports to either extend or retract an air cylinder or to rotate the rotary actuator clockwise or counter clockwise.

So how do I decide to either use a single solenoid or double solenoid valve?

Single and Double Air Solenoid Valve Differences

Single-Solenoid Valves

When voltage is applied to the “A” coil, the main spool inside the valve will shift which will switch the valve output ports causing the pneumatic actuator to move to a new position. When power is removed, the main spool will return to the original position by either an internal mechanical spring or by compressed air and now the pneumatic actuator will return to its original position.

Double-Solenoid Valves

When voltage is applied to the “A” coil, the main spool inside the valve will shift which will switch the valve output ports causing the pneumatic actuator to move to a new position. Power can be removed from the “A” coil and the main spool will remain in that position. Now to return the actuator to its original position, you must energize the “B” coil.

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

Pneumatic Safety Circuit

What pneumatic actuator is critical that it does not move.

Here is a perfect scenario where you would need a double solenoid valve.

Simple clamp and drilling machine.

1. Operator loads part into pneumatic clamp fixture

2. Operator depresses two hand start unit

3. Solenoid valve #1 gets energized and pneumatic clamp clamps the part

4. Pneumatic clamps makes limit switch which energized solenoid valve #2

5. Solenoid valve #2 makes pneumatic drill go down and start drilling part.

As the part is being drilled, what happens if the machine loses power?

Well if you had made the mistake and design a single solenoid valve for the pneumatic clamp instead of a double solenoid valve – the pneumatic clamp would unclamp the part while it was being drilled

Very Dangerous!!!!

That part being drilled now has the potential of injuring the operator or damaging the machine.

This is a perfect example why you should have selected a double solenoid valve.

With a double solenoid valve, the pneumatic clamp would have remained clamped with a loss of power.

So, when designing a new machine always ask yourself what pneumatic actuator do I need to hold position in case of a power loss.

I hope this article was helpful to understand the reason way in some applications you need a double solenoid valve over a single solenoid valve.

If you have questions regarding this article or have any questions regarding any new pneumatic application – Please e-mail me at rquarto@numaticengineering.com Ralph Quarto Senior Pneumatic Technical Support Engineer Certified Pneumatic Specialist  - CFPPS Direct Line 818 683-1471