Double valve guides, top and bottom

[timothy42b]

Is this an idea that's been tried? It would seem an obvious experiment, but I couldn't find any mention of it on the various historic brass instrument sites.

It would require a stem extending down from the piston, and an additional valve guide below. But it would allow a lighter shorter piston, one which only had to seal for air flow, not index for alignment.

[Art Hovey]

Could you explain that in more detail, with an illustration?
I don't understand how that would make the piston shorter.

[timothy42b]

I don't understand how that would make the piston shorter.

Ah, that's because I didn't think it through carefully enough. (in my defense, I play mostly trombone, some euph)

I just pulled the valves on my Martin euph and measured. The pistons are roughly 3 1/4 inches long, the air passage section 2 1/8, and I don't see any obvious way to shorten this.

What I was thinking is if I put a valve stem and a guide on both top and bottom of the piston, it would index more positively and bind less from any sideways pressure. You might even be able to reduce diameter of the piston except at the points where it seals. Or, you might be able to increase diameter for a better seal without hurting the action, because now there's no piston to cylinder pressure. All the force is taken up at the guide.

I guess it's a dumb idea or somebody would be doing it.

[tubaguy9]

I just thought about it...there wouldn't bee much need for it unless there was a loose valve, really. And a valve that has that much play should have some plating done to get the air leaks out

[timothy42b]

I just thought about it...there wouldn't bee much need for it unless there was a loose valve, really. And a valve that has that much play should have some plating done to get the air leaks out

I don't know much about valves, except for how frustrating it is to finally master a technical section and then have first valve stick down half way through it. Some of that may be my fault. Due to years of sports played with more enthusiasm than skill, none of my fingers are straight. Perfectly vertical pressure is unlikely at whatever degree of curve I may use.

So let me ask. If a valve doesn't have play, does it need a top guide either? If the cylinder walls were completely controlling the vertical motion, then the top guide would do nothing. (and on my Martin, they do nothing; the guide itself is very loose)

But conversely, if the top guide does something, then I think it already uses two points of control, top and bottom of the top guide. They aren't separated very far, so it takes a lot of force at each to apply a small torque. If you separated the two points of control to top and bottom of the piston, then you'd have a moment arm 3 1/2 inches long instead of 1/8 inches.

My theory is that the primary purpose of the cylinder wall should be sealing the air, and not controlling the motion. Requiring it to do both jobs leaves each less than optimal. Well, it's a theory, anyway; I have an inexhaustible supply.

[timothy42b]

Yes, rotation is a problem not solved by my idea. Does it contribute significantly to slow piston valve action? On my horn it doesn't look like the valve guide controls rotation, it looks like there is a groove in the cylinder wall and an insert in the piston.

But I may be using the wrong terms, and just confused.

Rotation would be easily solved with a square piston of course, it could still have round air passages through it. Hee, hee. Like I said, no shortage of theories.

[Art Hovey]

The "insert" on the piston (which runs in the groove in the cylinder) is what we call a valve guide. I think your idea is to put valve stems on each end of the piston and have them fit snugly into the holes in the top and bottom valve caps, to hold the piston surface just a tiny distance from the cylinder wall, much like the situation in a rotary valve. That would not prevent rotation of the piston (which is the valve guide's job) but it might reduce friction.
I would not try it for two reasons: First, it is too easy to knock a valve stem out of line. On a conventional piston a small misalignment is not a problem, but on the system described above it would be a disaster. Second, when a conventional piston is clean, well-fit, and lubricated the friction is imperceptible.
I have also thought about square pistons, but decided that it would be too difficult to make them fit just right. With cylindrical pistons you can often get rid of a little "schmutz" by spinning the piston around; you would not have that option with square pistons.
Don't be discouraged; keep thinking up new ideas!

[timothy42b]

I think I misunderstood how the terms were used.

I was judging from the Baer alignment site http://www.baertracksmusic.com/index.php?page=alignment" target="_blank
and from some experience with the valve guides on automobiles.

I guess I went down the wrong track thinking there was a valve guide kit available for the top stem, and so could easily be adapted to a new bottom stem.

Back to the drawing board!

But thanks all!