Solenoid Valves: The Air Switches
A solenoid valve is an electrically controlled valve that routes working-pressure air to a cylinder. Two types matter, and the difference is crucial for both wiring and behavior.
Single Solenoid (single-acting valve)
Has one electrical connection. Energize it and air flows one way; remove power and a spring returns it to its default state. Because it returns to default when power is lost, a single solenoid is safe-by-default — when the robot is disabled or powered off, the valve (and its cylinder) returns to the spring position. Use this when a fail-safe resting state matters.
Double Solenoid (double-acting valve)
Has two electrical connections and holds its last commanded state even with no power — it needs an active signal to switch direction. A double solenoid drives a double-acting cylinder forward or back on command and stays put when disabled. Use this when the mechanism should hold position through a disable.
Valves are limited by rule to a 1/8 inch NPT maximum port size (R804). Many teams use SMC valves; the Pneumatics Manual shows a typical valve with ports 1, 2, and 4 that you plumb to supply and the two cylinder ports.
Cylinders (Pneumatic Actuators)
A cylinder is a piston in a tube; air pressure on one side pushes the rod out or in.
Single-Acting (spring-return)
Air extends the rod; an internal spring retracts it (or vice versa). Needs only one air line and pairs with a single solenoid. Simpler plumbing, but the spring eats some force and the retract direction is not powered.
Double-Acting
Has two air ports; air drives the rod both directions with full force. Pairs with a double solenoid. This is the most common FRC choice because both strokes are powered and strong. Popular options include Bimba and SMC cylinders sold through AndyMark — for example a Bimba double-acting cylinder with a ~1.06 in bore and a 6 in stroke.
Sizing Preview
Bore (piston diameter) sets force; stroke sets travel distance. A bigger bore = more force but more air per stroke. You will calculate exact force and air usage in the next module. For now, remember: single solenoid + single-acting cylinder for fail-safe binary jobs, double solenoid + double-acting cylinder for powered-both-ways, hold-on-disable jobs.
Key takeaways
- Single solenoids have one wire and spring back to a default state when unpowered (fail-safe); double solenoids have two wires and hold their last state.
- Single-acting cylinders use a spring return and one air line; double-acting cylinders are powered both directions and use two lines.
- Pair single solenoid with single-acting cylinder, and double solenoid with double-acting cylinder.
- Valve ports are limited to 1/8 in NPT (R804); double-acting Bimba/SMC cylinders are the common FRC default.
Keep going
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Sources & corrections
This lesson is AI-assisted: drafted from primary sources, then reviewed and edited by hand. Errors still get through. When one is reported we fix it and write down what changed — publicly, in the corrections log.
Sources and further reading
- AndyMark - Pneumatic Cylindersandymark.com
- WPILib Docs - Operating pneumatic cylinders (Solenoids)docs.wpilib.org
- FIRST Pneumatics Manual (Working Pressure Side)firstinspires.org
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Lesson quiz
RequiredAll 3 right completes the lesson. Miss one and only that question comes back — anything you already answered correctly stays banked.
0 of 3 answered
01.What happens to a single solenoid valve when power is removed (e.g., the robot is disabled)?
02.What is the key behavioral advantage of a double solenoid valve over a single solenoid valve?
03.What distinguishes a double-acting cylinder from a single-acting (spring-return) cylinder?
Answer every question to submit.
All 47 lessons in Mechanical, Build & Pneumatics
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