General Pneumatic Advice and Help

Hello! Prompted by a recent forum thread, I thought I’d make a general questions thread, as I’ve gotten some very interesting ones I’m a former member of FRC 225, and still an active FTC mentor.

Through FIRST, I landed a job with a hydraulics, pneumatic, and automation company. If you have any questions on parts, terms, setups, connectors, anything, I’d be happy to work with you to get your questions answered. If you don’t want to ask them publicly because of a secret strategy, feel free to PM me.

I have personal experience, and access to experts with the following brands: Festo, Humphrey, Norgren, Bimba, SMC, Mead, Pneufit, Miller, Clippard, Koganei, Versa, TRD, and several others.

Some suggestions:

  1. Buy a tubing cutter. For the fittings used in FRC (Push-to-connect), the ends of cuts have to be perfectly square to seal.

  2. Buy quality tubing, and parts. Freelin-Wade, the default supplier for FIRST tubing is a really solid company. I’d also recommend Norgren for fittings.

  3. Understand your terms. You have air cylinders, where the force is decided by their bore (diameter) and the distance they work over, called the stroke. You can have a Double Acting cylinder that needs air to move it both ways, or a Single Acting that will retract if the air pressure is removed. You can even have a Reverse Single Acting, which will extend with air pressure removed.

  4. Understand when and where to use pneumatics. It’s not always the right choice. Air actuators are smaller, lighter, and faster than electric in most cases, but you only get 2 (Sometimes 3) positions. You get better granularity with electric devices

  5. Understand your solenoids. Use Single-Acting solenoids when you want an item to revert to a default state on power-off. Use a Double Acting if it’s important to stay in the last state on power off. Solenoids are also rated in switching speed, and air transmission volume, rated in Cubic Feet Per Minute (CFM)

Above all, stay safe. Air pressure can be extremely violent and dangerous if you’re not expecting it, and sometimes even when you are. Loose air hoses can whip through the air, air tanks can explode if damaged, cylinders can move faster than you can see with enough force to break bones.

If you have ANY questions, please ask! There are no stupid questions.

I’m mentoring our electrical/pneumatic group this year with no prior pneumatic experience. What would you say are some of the most common mistakes? If there’s anything to watch out for so we don’t have to route the tubing twice, what should we make sure to quadruple check?

The warnings/advice I’ve gotten so far are 1. cut the tube square and 2. teflon tape your connections prior to stuffing the tubing on.

Good morning!

I absolutely agree with point 1. It’s THE most common mistake. Cut it square and neat, and don’t reuse ends after you insert them. Buy a tubing cutter, they’re cheap, simple, and accurate.

For point 2, I really hope you mean taping the threads of the fitting. You should never, ever tape the tube itself. You’ll foul the fitting, and send little bits of teflon tape through your air system.

Teflon tape is a bit of a polarizing question. If you do it right, it’s fast, easy, and clean. However, if you don’t, you can reduce airflow, CREATE leaks, and foul your tubing. Instead, I’d suggest a sealant. Similar to loctite, you drip a little onto the threads, and snug them down. Permatex is the brand I’d suggest. It’s a lovely purple color.

On the subject of tightening fittings, be careful. Never use an adjustable wrench. Most air fittings are brass, or softer metal, and you just strip them out. Use a correct wrench, and just snug them down. You’ll never get them all the way to the bottom because the threads widen (NPT), and are intended to jam.

I’d also be careful mounting your air tanks. Don’t use hose clamps. Make a bracket, or use plastic plumber’s tape. Hose clamps will cause the tank to fail.

The other tip that few people know is that some fittings (Norgren, SMC, and a few others) have an INTERNAL hex. You can put an allen key inside the fitting, and snug it down that way, which is a lifesaver in tight spots.

I just wanted to thank you for posting this, I checked on the fittings we use and they do have internal hex. We’ve used pneumatics for years and somehow no one on our team has caught this! We’ve been tightening fittings onto our solenoids incorrectly for years! :rolleyes:

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Not all fittings have these, but they’re so convenient to have! I’d suggest always buying Norgren fittings if possible, they’re the only brand that consistently broaches the inside. IIRC, they have the original patent on doing so.

Thanks! I did mean taping the threads of the fitting. I’m still getting a hang of all the vocab.

I like the idea of the Permatex. However, out of curiosity, what is the right way to do Teflon tape?

Three tight wraps, with the narrowest tape you can find. Start at least two threads up from the bottom. Wrap in the same direction as the fitting will be screwed in, so that tightening the fitting doesn’t just strip the tape off.

The sealant I’ve had good experiences with is linked below.

https://www.permatex.com/products/thread-compounds/thread-sealants/permatex-pneumatichydraulic-sealant/

Wren,
Theoretical question: Given that the sealing surface of a push to connect fitting is the OD of the tube, well away from the end, why is the end of tube geometry critical to the sealing of the fitting? In other words, WHY does the end of the tube need to be perfectly square to seal properly?

fitting.jpg


fitting.jpg

Good question! I’ve not seen that particular diagram before, but it does give you a bit of the wrong idea.

The seal isn’t accomplished along a length of the tube, but instead at a very narrow band right near the end. If the tube isn’t square, it may not sit entirely inside the band. The smaller and more compact the fitting, the narrower the sealing band is, and the more critical the square.

The other reason you want tubes to end square is the same reason that you cut stranded wires flush, instead of leaving whiskers. The gripper ring can be fouled by small slivers of tube that stick out, or they can impede airflow inside a fitting itself.

Speculation: When you push a beveled-end tube in all the way, it induces a torque on the tube, pulling the OD of the tube away from the ID of the fitting on that side.

Aside from the nit below, great post!

I like this diagram to explain how the fittings work:

The packing seals against the outside of the tube near the end. It is quite possible to cut the end of the tube at an angle and achieve a sealed joint. I have personally run this experiment to satisfy my own curiosity.

However, I strongly agree that tubes should be cut with a tubing cutter only. It is an inexpensive and easy way to make sure the tubing is cut without burrs or excessive deformation. Tubing that gets mashed flat with wire cutters doesn’t seal well…

Thank you! There seems to always be surprisingly little general knowledge about air systems, especially the where/why/which of using them in FIRST.

I’ve watched several teams cut their tubing with a dull knife, while just sorta vaguely holding it. Then they’re surprised when it leaks. It’s much easier just to say “Always cut tubing square with this tool”.

That’s also a much better diagram, thanks for the better explanation.

So we were plumbing our pneumatics today and are using a manifold block for all our solenoids to save space. However, it has three ports on the front: EB, P, and EA. From what I gathered from a few Google searches the P is for supply. However, I could find nothing about EB or EA. Can someone tell me if/what we hook those up to or if we just plug them?

These stand for Exhaust. Exhaust A and Exhaust B. These can be left empty, or you can put a muffler in them, or a flow control to change the actuation speed. When the cylinder moves, the air in the side it’s moving towards has to go somewhere. It goes out the exhaust ports.

Ah! Makes sense. Can we still plug one side? Our design has the back up against a tube.

You can’t. Depending on how you have it piped, A will exhaust on the retract, B will exhaust on the extend, or vice versa. With one side plugged, the cylinder will not move, as there’s still air pressure on both sides of the piston.

Even if it’s against a part, a small gap should be fine. Just don’t seal it off.

Am I forgetting something? Stroke has nothing to do with the force output. Force is equivalent to Pressure x Area. The stroke only comes in when we look at the work out put of the cylinder.

I missed a word in there, thanks. I was trying for something closer to "You have air cylinders, which are defined by two factors; their force (decided by their bore (diameter)) and the distance they work over, called the stroke.

Hi I need help with my SMC double solenoid, it pulls to one direction and then stays there. Ive switched pcm and tested like 30 diffrent solenoids. then i checked code and it was perfect so i dont know what could be the problem.

How are the solenoid output LEDs on the Pneumatics Control Module (PCM) behaving when you try to use the solenoid?

1st LED on, the 2nd LED off
Then
1st LED off, the 2nd LED on
?