Differences Between Single and Double Air Solenoid Valves

Moving loads require a pneumatic actuator that can either be a linear air cylinder or a pneumatic rotary actuator. Both have two ports and create motion by putting pressure into each. The air cylinder will extend or retract while the rotary actuator will turn clockwise or counterclockwise.

Most pneumatic applications require standard 4-way, five-ported directional air valves 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. This method causes the spool to move and differentiates a single from a double solenoid valve.

Single Solenoid Valves

A single solenoid type valve means the voltage is applied to a single A solenoid coil, and the main spool inside the valve shifts, which causes the valve ports to output air on one side. This causes the pneumatic actuator to extend to a new position. When power is removed, the main spool will automatically return to the original position due to the 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

Double solenoid valves are drastically different. When voltage is applied to the A solenoid coil, the same process occurs as the single solenoid valve. The main spool inside will shift to one side, allowing airflow to extend the actuator. When power is removed from the A coil, the main spool will remain in the same position with a detent; no actuator motion will occur. The spool will only shift back if power is applied to the B solenoid coil. You must energize the B coil to return the actuator to its original position. This shifts the spool back and causes air to flow out of the other port, providing enough pressure for the actuator to retract.

After deciding if you want a single or double solenoid valve, you must ask yourself what would happen if the machine lost power.

The Pneumatic Safety Circuit

A single clamp and drilling machine is the perfect example of using a double solenoid valve.

An operator loads the part into a pneumatic clamp fixture.

The operator presses start, applying voltage to the A solenoid coil. Here's an example of what could happen:

1. When the A coil is energized, it shifts the spool, so the air pressure forces the pneumatic clamp to close and hold the part in place.
2. A limit switch is activated when the clamp is in the closed position. This tells the machine it is safe to start the process.
3. The drill gets up to speed, moves down, and starts making a hole in the part. As the part is drilled, what happens if the machine loses power?

If you mistakenly used a single solenoid valve for the pneumatic clamp, the spring return would automatically open the clamp when the power is lost. The part being drilled could go flying, potentially injuring the operator or damaging the machine.

Correctly using the double solenoid valve would prevent the spool from shifting with pressure remaining to keep the pneumatic clamp closed. Without power, there is nothing to energize the B coil so the valve would not shift, and the clamp would not open.

For a new machine, it is essential always to ask yourself: "if power is lost, which actuator is critical not to move?"

If you still need clarification about which solenoid valve to use, contact Motion Automation Intelligence to speak to a rep today.