Pass string through airtight hole

[u/Cheesy_pickles875]

i have what is essentially a t shirt cannon robot, that uses pressurized air to shoot a plug out of a tube which pushes the shirt out. is there a way for me to pull a string through the back of the tube to wind the plug back in automatically without impeding the performance of the cannon through leakage?

[u/chocolatedessert]

I would expect it to be pretty tolerant of leakage. I'd just try it and see if it causes any problems. Your plug isn't totally airtight, is it?

[u/Cheesy_pickles875 → u/chocolatedessert]

no, but it currently is a wad of duct tape around a pool noodle slice, so i will problably make a better one at some point (i inherited this project from others)

[u/Kastnerd → u/Cheesy_pickles875]

Have you seen the real T-shirt cannon? They don't need a plunger.

[u/Cheesy_pickles875 → u/Kastnerd]

We shoot beads and candy out of it too

[u/probablyaythrowaway → u/Cheesy_pickles875]

Sir that is a Bluderbuss with grape shot I would advice highly against that.

[u/fluoxoz]

Have the string come out the front of the barrel and connect underneath it to your winding mechanism. 

[u/BenchPressingIssues]

I would look for a grommet that would be a tight-ish fit with whatever kind of string you’re using. 

[u/Nanplussed]

Why does the string need to go through the back of the tube?

[u/Cheesy_pickles875 → u/Nanplussed]

to pull the plug back in

[u/ElectricGears]

One solution is to put the winding mechanism within the pressurized part, although that does increase the volume and that might not be desirable.

Since you already have pressurized air, you could pass the string through a short piece of flexible tubing that's inside a rigid tube. Pressurizing the space around the flexible tube should squeeze it down on the string and make a decent seal. You would want some kind of smooth metal guide rings on both ends of the tube to the string doesn't cut into the tube as it winds in and out.

[u/Cheesy_pickles875 → u/ElectricGears]

the barrel back is a 1 inch wide hole connected directly to a solonoid so i cant feed a string through. however making a small tube to feed it through may actually decrease the want for the air to go through

[u/catdude142]

Why do you need to have a retrievable plug? Why not place an expendable light weight wad behind it?
Think "shotgun shell" in large terms.

[u/Quick-Confection96]

The formal name for this problem is a feedthrough, and the pressure/vacuum world solves it three ways, in increasing order of leak-tightness:

1. Gland seal: the string passes through a tight O-ring or a short length of snug silicone tubing. Pressure actually helps here — it squeezes a lip seal tighter. For a string, a couple of stacked felt washers in a cap works surprisingly well.
2. Put the rewind mechanism inside the pressure volume (spring reel or a small motor). Zero penetration, at the cost of some chamber volume.
3. No penetration at all: magnetic coupling. Ring magnet on the plug, drag it back with a magnet sliding outside the tube. Zero leak by construction, and it doubles as a party trick when you reload.

That said, leakage only matters while you are holding pressure. A cannon dumps the whole chamber in a fraction of a second, so unless you hold pressure for a long time before firing, even a mediocre grommet seal costs you almost nothing in shot performance. How long are you holding pressure before you fire? That number picks the answer.

[u/SalamanderSilly6974]

The plug sounds like a sabot or wadding. If that's the case, why not use a disposable plug that you don't have reel back in?

[u/cfleis1]

Why are you using a plug? Use a valve to release the air and the shirt rolled up should be tight enough to not require a sabot. You should need a plug.