Non-Static Seals 1

[markfh11q]

Alright, of all things, this is going to be used on a high-flow, quick opening outlet sealing valve, made for a recreational "toy"...

O-rings are, first off, the most obvious choice for a moving, (non-static), seal. But they usually require an accurately machined O-ring groove for them to actually work.

I have tried, (quite a few times), to cut grooves by hand, using guides and other techniques, and I haven't been able to get a good groove. I wasn't really suprised.

So I have been trying to find other ways of creating a moving seal. So far, I've got a few ideas, but I'd like to get some more input.

The seal would be on a moving cylindrical piston. This piston is a 1-1/4" coupling filled with fiberglass resin, which is housed inside of a 2" SCH-40 carriage, (tube). The wiggle room was originally pretty bad but I built up the O.D. of the piston by using more fiberglass.

My first idea is to just to use some 1/8" neoprene sheeting past the edges slightly to seal against the wall of the 2" carriage.

My second idea is really a question. Is there some sort of O-ring cutting tool that can be used without a lathe? You would have to buy one for every size O-ring you wanted, but I'd only need one. 1/8".

Thanks in advance for any of you help. I can't see any other ways then an O-ring or the gasket seal idea.

 

[gr2vessels]

Mark,
It seems that you are tempting the faith, eventually you'll come up with a drill and atempt to scoop the groove (on the piston?)with a knife or similar. Unless you use a precision cutting / machining tool for the groove, all you can achieve is a bicycle pump, at best.
Safer is to take your piston to the nearest workshop and pay a few bucks to have a proper O-ring groove cut (you'll save on medical bills).
Cheers,

gr2vessels

 

[markfh11q]

Nearest workshop for me is a CNC shop called Acme Machining and Welding that work on stuff for oil rigs, so if I took the piston there they wouldn't even take it.

I have tried to find a gunsmith who would let me do it but again no dice.

The piston seal doesn't have too be 100% airtight. The seal is to improve the reliability of the equalization between the pilot valve, (behind the piston), and the pressurized inlet, which will allow the valve I'm building to be more easily piloted, (by a 1/4" Q.E.V).

Do you think the neoprene gasket seals at front and back of the piston would help in this sense?

The best method I've come up with for cutting grooves was to push a 2x1-1/2" bushing over the 1-1/4" coupling to the place I wanted the groove, use a small saw to cut to a depth of about 1mm, then use a triangle file to widen that, THEN use a normal edge of a sunflower bastard file to widen it and square it for the O-ring. I came pretty close, but it leaked at two locations when I pumped it with water.

Also, pressures in this device will not exceed about 80 PSIG. All components, (except the piston, which will be inside of a component that IS), are pressure rated by their manufacturer to far above this. I will, of course, hydrotest if I can find somebody to do it. If not, I know a way to do it with a grease gun, (of all things). You DEFINITELY want to test things with water before you pump compressed air into them.

Thanks for your help,
Mark

 

[SnTMan]

Can't quite picture your geometry, but two suggestions:

Cup seal, like in a bike pump,

Gland type packed joint

Or: buy a cheap hobby lathe?

Regards,

Mike

 

[nogginthenog]

I am currently in the early stages of building a single cylinder stirling engine to run from my domestic wood stove. I have the same desisions to make regarding the seal on my cylinder, which will need to be both low-friction and as airtight as practically possible. I have experimented a little with oversized neoprene disks with better than expected results, however my concern is for the service life of such a seal. I do not yet have a functioning model to test on due to low budget and would be very interested if anyone has any experience with this sort of seal or any better alternatives. Another material I have been playing with is polyethene, but although I have a simple solution to keep the cylinder walls cool enough for most soft materials, I believe the actual air temperature in the cylinder will be enough to distort the plastic. A colleague has suggested thin brass/bronze, in the manner of a rifle cartridge case, but this would be difficult for me to manufacture. Also in theory as it wears against the (stainless steel) cylinder, the seal should stretch under the pressure to maintain integrity, but with the low pressures I am expecting I am unsure as to whether this would be workable. Apparently these type of seals were used in very early diving helmets and compressors.

 

[nogginthenog]

The same colleague just also suggested what sounds like a workable idea - cast a gasket or cup-type seal out of silicon or similar liquid sealant (possibly with some finely shredded teflon mixed in). I guess just lightly oiling the cylinder and piston face would allow you to cast several so you have spares. I like the sound of this and intend to try it out soon.

 

[MikeHalloran]

Both noggin and mark would be well advised to visit:

http://www.gingerybooks.com/

http://www.lindsaybks.com/prod/index.html

and buy Dave Gingery's book on making your own lathe.



Mike Halloran
Pembroke Pines, FL, USA