Linear Dynamic Seal (water-tight)

[Satchmo]

I'm trying to conceptualize a design for a hydro generator that basically operates by moving an airfoil up an down along a straight path. Most of it is all worked out, except for the issue of water sealing. If you can make sense of my not-so-great drawings, the airfoil is attached to the "block", which has a linear bearing that allows it to move up and down along the guide rod. The "block" is also attached to a belt that will drive pulleys (and then gears and eventually to a generator, not drawn.)

The issue is allowing the airfoil to be exposed to the water flow while sealing the guide rod, pulley, belt, etc. One idea I had was to use a bellows/accordion type system (shown in the bottom half of 'sketch.pdf'.) The main concern with this idea is that the two bellows might act as springs and restrict the motion of the airfoil too much.

Another idea (that I have less faith in) is to use some type of linear seal (I'm picturing something like the weather sealing used around door frames, but something a little more suitable.) This idea is shown in 'linear seal.pdf'. I think for this to have any chance of working, the part that moves through the seal needs to be long and sharp to minimize any water leaking through. I have a feeling that even if this is possible to make work, the friction due to the seal would be very large.

This is where I'm at now. I would think that something like this HAS to have been done in the past, but I haven't been able to think of any applications. (Anyone with a lot of familiarity with either industrial underwater processes or satellites know of anything similar?)

Let me know if anything here is unclear!

 

[FredGarvin]

Just like what the folks at University of Michigan are doing.

Why not simply use two pistons and then use simple o-rings? I know that would complicate the mechanics for you, but it would most like likely make this more robust simce something like this would have to live under water for very long periods of time.

If you use a bellows, which I think is the way to go, you would have to make the length of the bellows much larger than the stroke in an effort to minimize the spring effect. I don't see how your "linear seal" would give you a water tight seal.

 

[Satchmo]

Just like what the folks at University of Michigan are doing.

Why not simply use two pistons and then use simple o-rings? I know that would complicate the mechanics for you, but it would most like likely make this more robust simce something like this would have to live under water for very long periods of time.

If you use a bellows, which I think is the way to go, you would have to make the length of the bellows much larger than the stroke in an effort to minimize the spring effect. I don't see how your "linear seal" would give you a water tight seal.

I promise we've been working on this since before seeing theirs, but the idea isn't anything too new :).

Regarding the "linear seal", I didn't think that would work either, but I thought I'd include it anyways so that it could maybe help spark an idea in somebody else.

I'm a little weary of using pistons and o-rings, and I would think that the robustness gained in the shafts would be lost due to the life of the o-rings (though I'm saying this with no working knowledge, whatsoever, of o-rings.) Were you thinking of a specific underwater application where that design is used?