Balanced UnBalanced Piston

[airatomic]

Hello,

Does anyone know, in this scenario, will the piston move to the right, or remain stationary (as in QDV)?

balanced_unbalanced_piston.jpg (17.43 KiB) Viewed 5104 times

[jackssmirkingrevenge]

It will move towards the right so quickly that limp wristed Antifa "activisits" will show up to riot in protest

[farcticox1]

I think right is the right answer

[airatomic]

I had my suspicion about it before posting, but just wanted confirmation from the experts here.

How about this one? The o-rings are not on the piston but on the piston valve body (what you call this type of piston + grooves)? Will the piston move to the right?

balanced_unbalanced_piston2.jpg (18.02 KiB) Viewed 4976 times

[jackssmirkingrevenge]

This will also move to the right for the same reasons - the force around the o-rings is balanced, but there is nothing to oppose the force on the base of the piston:

unbalancedvalve.png (15.39 KiB) Viewed 4969 times

[farcticox1]

Use the force

[airatomic]

And so, its Plan B then, since the previous diagrams logic ain't working.

Plan B:

3way_slide.jpg (37.6 KiB) Viewed 4761 times

To make it, obviously you need to make "rod seal" type, which is head-scratching, since "piston seal" is easier and more common. Materials: solid rod (got to make 2 inline holes with exits at appropriate distances), matching o-rings, and the body (I used ABS tube and knowing for sure, it wouldn't crack - as the fiberglass body would have to crack first) with fiberglass casting.


1) Rod with o-rings, ready for fiberglass casting deployment. Be sure to oil the rod's body first.

2) 24 hours later, and poking the rod out, revealing the fiberglass body, with o-rings exactly where they supposed to be:

3) Epoxied (JSR gonna like this) the threads to the rod's body. Since all the parts of my launcher are "serviceable", had to create ways to make this slide valve serviceable too, esp the o-rings, if need replacements.

4) Ready to test:

Comments: Sadly, there are leaks. I wondered if the o-rings were misplaced, or the cast body got leaks, but no, the problems was the o-rings area, where the edge/sides covering the o-rings have some "casting imperfections of fiberglass". I was genuinely !@#$%^&*

5) A day later, decided to make another one, with better precisions in the casting. Ex: wiping off any greese from the o-rings before casting:

Conclusion: Same leaking issues as the first one. No matter how careful, the casted edges of the o-rings always have some sort of imperfections, creating paths for leaks. I am beat.

Anyone with better ideas of casting, so that the edges surrounding the o-rings are "perfect"?

[Anatine Duo]

interesting problem! I would try lower viscosity epoxy, maybe no fiberglass (is it the glass flocking you are using or longer strands?)

Also if you can improvise a vacuum chamber maybe you can pull some bubbles out before the epoxy cures.

[jackssmirkingrevenge]

I recognize that construction method are the o-rings tight around the rod though? If you don't have to stretch them to fit them, you will never have a good seal under pressure. They look pretty loose and I think that is your main issue.

[airatomic]

JSR. a day after the second attempt, i had that thought exactly. The orings only had "nice fit" on the rod. Not loose, nor too tight, but just nice. Even the rod slides in and out rather easy. Interestingly, if i rotate the body around, at some point, the leaking just stops.

And so, this gives another incentive to try the third attempt, this time with tighter orings around the rod. I have this question though: if i use tighter orings around the rod (having to stretch them on the rod), what will happen to the orings and their sockets after casting is done, and the rod poked out? Will the orings return to their smaller dia (pre-stretch) and come off from their sockets? Or the sockets would be tight and hold on to the orings' exaggerated stretched dia?

A.Duo, the fiberglass resin + MEKP mixtures have very low viscosity. I used these mixtures before to replicate gun's body shape from hollow toy guns, and details were amazing. I am sure JSR can testify to its viscosity.

[jackssmirkingrevenge]

I have this question though: if i use tighter orings around the rod (having to stretch them on the rod), what will happen to the orings and their sockets after casting is done, and the rod poked out? Will the orings return to their smaller dia (pre-stretch) and come off from their sockets? Or the sockets would be tight and hold on to the orings' exaggerated stretched dia?

As long as there isn't too much of a difference in diameter between rod and unstretched o-ring - I would say less than 1mm - they should remain in their groove.

[mrfoo]

Pulling a vacuum on your epoxy will almost certainly improve matters. For your construction method, I would look at casting the inner portion, with the o-rings, as pure epoxy with no reinforcement, and then casting the reinforced bit over the top.

Or, if you have access to a lathe, turn it from metal.

[airatomic]

MrFoo, can you pull out some diagrams to explain your methods?

All these years on spudding, i only used a drill press, grinder and lots of epoxy (yah!). Coz that's all i have.

[mrfoo]

I'm shit at drawing

Shaft seals are hard to do without the proper tools, "the proper tools" being a (preferably collet) lathe and an internal groove cutting tool. Still, I think you should be able to get an acceptable seal as long as you're willing to live with a certain amount of "stiction" (I can't see an easy way of casting a groove for a floating O-ring) as long as you're careful.

The only bits that really need to be "clean" are the groove edges, that's where air pressure is going to leak around your o-rings. For that, you need epoxy with no bubbles or other surface imperfections. Glass fibre will give you surface imperfections (and bubbles). Even the mechanical process of mixing resin will put bubbles in, and you need to get them out before using the resin. Mix resin in a container / cup, put it in a vacuum vessel, pull a vacuum on it, wait a few minutes for the bubbles to pop out, use resin. Easy as. My vacuum vessel is an old pressure cooker, but you should be able to get away with anything relatively strong that you can get a hose onto and seal the rest. For pulling the vacuum, I use the "wrong" end of a fridge compressor. Google "fridge compressor vacuum pump", you'll get loads of hits, although for outgassing resin you don't need any valves or vacuum accumulators, just a hose from the pump to the container.

As you only need the bits around your o-rings to be "clean" cast, mix up just enough resin to cover your o-rings, pull the bubbles, put it over your o-rings and leave it to set for a while. Once it's gelled, cast the rest with the fibre reinforcement.

Personally, I'd try and work out a way of casting grooves for floating o-rings, but that's not going to be easy.

Alternatively, with a couple more o-rings and you can put the seals on the rod and port the barrel, which might be easier to deal with overall, it'll certainly be easier to change the seals if you need to.

[Gippeto]

Any chance of finding the right size tube to use between the o-rings? Short sections of tube sanded square using the drill press....

Have yet to buy a vacuum pump for the garage, but I did have an old piston type compressor kicking around....couple fittings and a 1/2" needle valve from the junk bin threaded into the inlet and.....