Air Seals?

[cnczoner]

Anyone know what I can use for an air seal on a small shaft, with rotary-only motion? Say about a 1/8" dia shaft. OD of the seal I'm flexible on.

I am not sure how an O-ring will hold up to rotary motion (as I've always felt they were for static operation), but I could be wrong there. Will an oil seal work for air? I've even thought I could use a double-sealed or double-shielded ball bearing. I did find "cup seals" at McMaster but they don't come in smaller sizes.

I don't need a 100% seal, because I'll have air flowing through it.

Thoughts appreciated.

Cheers,
-Neil.

[RICHARD ZASTROW]

Beware of blowing the grease out of a sealed bearing. They really aren't intended to hold back external pressure. However, you could probably route the air flow around the bearing so the pressure is equalized on both sides.

The bearing itself would probably neutralize internally with the external pressure on both sides.

I really don't know the pressure differential or rate of flow the air is at.

Just a thought.

Dick Z

[cnczoner]

I thought that may have been a problem with shielded bearings, but figured sealed would be, well, sealed. I don't have a spec for airflow, but it will be very low (perhaps a bit more than a $5 aquarium pump). Pressure will be maybe 10 PSI.

FWIW, I've just been told by one company that oil seals (the standard garter-spring type) don't work for vacuum.

Cheers,
-Neil.

[RICHARD ZASTROW]

Because you said "air flowing through it", I ASSumed it was pressurized air. Now you mentioned vacuum.

Is the bearing facing pressurized air or a vacuum?

You might want to nose around rotary unions or rotary couplings. They use a lot of air/gas seals in their products. Might give you some ideas.

Dick Z

[Geof]

Originally Posted by cnczoner ......FWIW, I've just been told by one company that oil seals (the standard garter-spring type) don't work for vacuum. Cheers, -Neil.

Those oil seals only seal in one direction so for a vacuum application they need to be installed 'backwards'.

[cnczoner]

Well, I was trying to narrow down the problem to simplify it, but here's the situation in more detail...

I will have a shaft (~1/4"-3/16" dia) with a hole axially through it, and a vacuum will suck air through the tube. But the tube needs to rotate (via a stepper motor), so to create an air-tight swivel joint, my plan is to drill a few holes on the side of the tube and create a small "chamber" around that part of the tube. Sealing the ends of the chamber is what I need these seals for.

The chamber will have some vacuum in it, which should essentially the same as there being positive pressure on the other side of the seal (or bearing or o-ring). The seal will only see a difference in the pressure from one side to the other. I'm thinking -20 inHg (though I should measure that), which is roughly 10 PSI.

FWIW, I tried a test with just a shielded bearing yesterday, and it was *okay*, but I'm sure I can do better with a seal or o-ring.

Geof, which side of a seal is the higher pressure side? The side with the garter spring?

One company I spoke with told me to get a "spring energized teflon seal" and pointed me to balseals.com as a source. I've yet to contact them.

I'm going to run a couple tests with some O-rings also.

Cheers,
-Neil.

[Dropout]

Teflon rings.

O rings will work but they need lubrication. You can get square o rings that might seal better.

On a lip seal, the pressure side is the spring side. Lip seals don't like to see vacuum on the spring side.

The small pressure differential that you're working with is harder than high pressure.

[Kipper]

Originally Posted by cnczoner Well, I was trying to narrow down the problem to simplify it, but here's the situation in more detail... I will have a shaft (~1/4"-3/16" dia) with a hole axially through it, and a vacuum will suck air through the tube. But the tube needs to rotate (via a stepper motor), so to create an air-tight swivel joint, my plan is to drill a few holes on the side of the tube and create a small "chamber" around that part of the tube. Sealing the ends of the chamber is what I need these seals for. The chamber will have some vacuum in it, which should essentially the same as there being positive pressure on the other side of the seal (or bearing or o-ring). The seal will only see a difference in the pressure from one side to the other. I'm thinking -20 inHg (though I should measure that), which is roughly 10 PSI. FWIW, I tried a test with just a shielded bearing yesterday, and it was *okay*, but I'm sure I can do better with a seal or o-ring. Geof, which side of a seal is the higher pressure side? The side with the garter spring? One company I spoke with told me to get a "spring energized teflon seal" and pointed me to balseals.com as a source. I've yet to contact them. I'm going to run a couple tests with some O-rings also. Cheers, -Neil.

Possibly that is a "mechanical seal" Good for use up to 28" HG and good RPM rating too....Personally the 1/8" would be a hard one for me.....Need to put my specs on to see it Try a search for "gland packings" I used to rebuild Roper pumps with ptfe/teflon seals and they stood up to -2 bar to + 2 bar easily...wore out much faster than the mechanical conversion models. I have some carbide/ceramic one laying around I can take a picture of? hth

[insanedragons]

it is a lot of code i know....not the most ive seen though. so youre saying theres a more appropriate forum for as3 in air?

[cnczoner]

Some updates -- I've spoken with various people/companies about this, and the summary is that teflon is indeed the way to go. However there is also graphite-impregnated teflon, which is slipperier, but would make the shaft wear faster.

For the seal design, the best I've found are the ones from balseal, which are all custom items, except that McMaster carries a handful of sizes of these (see the bottom of page 3469). The thing about these seals though is that they are designed to rotate both around the shaft and within the bore, so my original intention of making the bore out of polycarbonate would be an issue. Plus, I'd need to machine from both ends of the bore to make a "seat" that would prevent the seal from walking within the bore.

In the meanwhile, a friend made me realize that I can simplify the application so that the shaft only has to rotate +/- 90 deg, (rather than +/-180 deg), so I'm simplifying the design for a bit right now, to eliminate any need for seals, and using a flexible coiled hose to allow for the rotation. I'm most probably have to make the version with the seals eventually, but for now I can get a working version so I can run other tests.

Cheers,
-Neil.

[Dropout]

Teflon needs a binder for the seal to hold it's shape. Glass is commonly used but it will wear the bore over time. Graphite is the best.

If you can manage it, change your design to use seals about 1.125 inches. The find a local steering gear rebuilder and buy the pinion rings. You should be able to get them for less than $0.25 each (free if you ask nicely).

There is also a teflon seal used for the cylinder lines on the TRW rack that might work. It's for a 1/4 line so the ID might be fine.

http://www.partcraft.com/Rack.html will have what you need as will Precision Remanufacturing in Chicago.

If you are going to build something, I'd find the best, least expensive seal and build to that.