Why are piston valves usually round?

[bigboom]

I was hoping some people here could shed some light on this because since there isn't anything rotating, only sliding why aren't the square or triangular or just more of an oval?

Ben Dennis

[Chuck(G)]

The original 1818 Stölzel valve is known as a "box valve" and is square in shape:

http://www.public.asu.edu/~jqerics/earlval.htm

However, there's a pefectly good practical reason for the cylindrical shape of valves--they're easy to machine to close tolerances that way. Besides, the extra material added by a rectangular shape would only serve to make the valve heavier.

[Joe Baker]

In theory they could be square, hexagonal, whatever -- but a relatively "easy" (compared to what it would take with those other shapes) way to get the very smooth surfaces and very close tolerances required is to use a lathe to machine and/or polish a perfectly round piston, and a hone to make a perfectly round hole.

Another problem that you'd run into with irregularly shaped pistons comes to mind after some comments someone made last week about the guide slot in pistons. Whoever it was pointed out that, because there's a helical spring at the bottom of the piston, it wants to rotate slightly when pressed. Your square piston would then find the piston applying a slight torque on it's square casing -- eventually causing undue wear, but certainly creating undesirable friction as the valve moves.

You're not the first to think "I wonder if anyone's ever tried...". So far, I've yet to see an idea that someone hasn't considered, tried, and abandoned. The craftsmen who do this stuff have been experimenting for 150 years or so, and most of the really useful inventions have already been thought of.
______________________________
Joe Baker, who notes that there WAS an Olds trombone (can't remember the model... was it the Opera?) that had an octagonal handslide. My guess: a BEAR to repair!

[Lew]

Here's a photo of some early valves, including a square valve. As already noted, I'm sure that the shape of current valves is due to the ease of manufacture and the fact that it's probably easier to maintaint he tolerances and therefore seal necessary in a sound valve than in other shapes.

[ThomasDodd]

Here's a photo of some early valves, including a square valve.

The 4th on from the left is interesting. (As is the rotor too)

I wonder what the casing looked like? Did it complete the passage, or was the passage not cylindrical through the valve?

[Chuck(G)]

Here's a photo of some early valves, including a square valve.

The 4th on from the left is interesting. (As is the rotor too)

I wonder what the casing looked like? Did it complete the passage, or was the passage not cylindrical through the valve?

By its diameter, looks like an old Stözel piston to me--the top two ports are connected at the top, the bottom are vented out the bottom. Most often, the innards of these things are made of nothing more exotic than cork.

Perhaps the original question could be answered by "Go to your local Home Depot and ask for a drill bit that will drill a hexagonal hole".

[ThomasDodd]

Joe Baker, who notes that there WAS an Olds trombone (can't remember the model... was it the Opera?) that had an octagonal handslide. My guess: a BEAR to repair!

Several modles had fluted slides at various time. The Recording model is most common.
The stocking was round though, for a proper seal.

The theroy was less friction due to less contact area. But yes, every one I've heard talk about them said repairs were basically impossible. Just replace the damaged tube. Sice the correct tubes wer unusual, many now have round inner slides, or complete replacement slides (also with round tubes).

[Chuck(G)]

The theroy was less friction due to less contact area. But yes, every one I've heard talk about them said repairs were basically impossible. Just replace the damaged tube. Sice the correct tubes wer unusual, many now have round inner slides, or complete replacement slides (also with round tubes).

..and yet those fluted-slide models are prized by some. The trombone player in our quintet still plays the fluted-slide Olds Standard he bought second-hand in 1947.

[windshieldbug]

Round valves = least area. In an era of high-friction applications, less area = more speed. For large applications, less area = rotary valves (friction space being only the top and bottom)

[ThomasDodd]

The theroy was less friction due to less contact area.

..and yet those fluted-slide models are prized by some. The trombone player in our quintet still plays the fluted-slide Olds Standard he bought second-hand in 1947.

There are plenty of 50+ year old instruments that have never had "work" done to them.
I'd love to have a fluted slide myself. But they are few and far between now days.

[Lew]

I believe that these are (from left to right) Perinet valve (like current pistons), Berliner Pumpen, Vienna valves, (not sure what this is called and don't have the book in front of me, but will look it up later), Stoetzel (square) valve, and below it is a rotary valve.

[Dean E]

I was hoping some people here could shed some light on this because since there isn't anything rotating, only sliding why aren't the square or triangular or just more of an oval? Ben Dennis

Round surfaces are a good thing. The most common metalworking machine tools (lathes and drills) spin. It is quite easy to machine round surfaces to close tolerances with machine tools that spin.

Also, external, cylindrical surfaces, such as valves, can be finished with grinders; internal cylindrical surfaces can be finished with reamers or burnishers. Even hand finishing to mate moving parts to one another, such as by lapping, is easy when everything spins.

On the other hand, machining complex, external surfaces on valves and valve bores requires more complicated, expensive tooling, such as milling machines, surface grinders, shapers, and broaches. Other-than-cylindrical valves being worked on would have to be held with precision indexing devices, in order to produce the proper angles of the external surfaces. (For example, a square or oblong would have four 90-degree angles; a hex would have six 60-degree angles.) Oval, parabolic, and similar exotic shapes are even more technically difficult to machine.

Internal surfaces of geometrically complex valve casings would take hours each to machine using shapers or shaper heads mounted to vertical milling machines. A faster method would be to use broaches for machining mass-produced valve casings, but custom made broaches can cost tens of thousands of dollars each.

Complex shaped valve parts could be cast or molded using powdered metallurgy techniques, thereby avoiding a lot of machining costs, but they would still require a finishing process before being suitable for operating in a moving, precision situation good enough to seal air.