Trombone Slide Design

[CharlieB]

A question about trombone slide design....
All the trombones I have owned have had slides with stockings that were prominently larger than the main slide tubes. The exception is my Shires. It has stockings that are invisible to the naked eye. A micrometer shows that their diameters are only 0.004" greater than the diameters of the inner tubes. That means that with a perfectly aligned slide, there is only 0.002" clearance between the inner and outer tubes. More conventionally sized stockings, especially barrel shaped stockings, would be more forgiving of slight imperfections in alignment.
What is the advantage to designing the slide with such a tight fit?

[hornbuilder]

The clearance is determined by the OD of the stocking of the inner tube, and the ID of the outer tube. Not the difference in OD of the body and stocking of the inner tube. Measure the ID of the outer tube, and compare that to the OD of the stocking.

[CharlieB]

Thanks Matt, for the correction.
I did forget to allow for the design clearance between the stocking and the outer.
I'm still wondering, though, what the advantage is to having such shallow stockings. ??

[Burgerbob]

Just musing, not like I know the answer here... If you have shallow stockings (but nice tight clearances to the outer), you can make narrower ID outers, which can be smaller overall and save on weight and materials.

FWIW, my Shires dual bore has visible stockings, they just aren't as prominent as some other slides.

[GabrielRice]

Shires does barrel-shaped stockings.

[Kbiggs]

Shires does barrel-shaped stockings.

Anything you (or others) would be willing to share about the advantages and disadvantages of barrel vs. straight stockings?

[hornbuilder]

The question is...

Is the body of the slide tube heavier, to make it closer to the OD of the stocking, or, is the OD of the stocking smaller, and closer in diameter to the inner slide tube OD?

[GabrielRice]

Shires does barrel-shaped stockings.

Anything you (or others) would be willing to share about the advantages and disadvantages of barrel vs. straight stockings?

Well...I'm really not a trombone designer or technician, but as it was explained to me, barrel-shaped stockings allow you to have smaller clearance - and therefore better response - between the inner and outer slides while keeping drag at a minimum.

[hornbuilder]

A barrel shaped stocking has a smaller contact area, as compared to a more cylindrical stocking.

This means that there is some degree of alignment leeway possible with a barrel shaped stocking, but since it is a smaller bearing surface, potentially a shorter life expectancy from the tube..

A cylindrical stocking has a larger surface area, making alignment more critical, but also a potentially longer life expectancy as a,result of the larger bearing surface.

[Burgerbob]

This means that there is some degree of alignment leeway possible with a barrel shaped stocking, but since it is a smaller bearing surface, potentially a shorter life expectancy from the tube..

And as we've seen with Bach slides, when done badly makes for leaky slides.

[CharlieB]

Original post...........Why very thin stockings?
Answered today on another forum.
Optimum slide design is for the "flow" of sound waves to be through a smooth tube of constant diameter.
That is impossible because when the slide is extended, the waves exiting the small upper tube are then confined only by the larger outer tube, then must enter the small lower tube. Keeping the diameter of the outer tubes as close as possible to that of the inner tubes necessitates small diameter stockings. The theory is that this improves tonal stability. {Not my theory. Please don't shoot the messenger.}

[robcat2075]

My understanding is that "stockings" are a newish thing... like not until the 20th Century?

Who sold the first trombone with stockings?

[BGuttman]

Stockings were soldered on most of my pre-1930 instruments. I don't have any trombones earlier than 1910, though. It's possible they were soldered on earlier as well.

My guess is the next development was an expanded stocking, where the tube was pressed out to make it bigger. They probably kept the cylindrical profile if for no other reason that it's similar to a soldered on stocking. At some point someone figured out how to draw a stocking on the tube when it was formed.

Drawing cylindrical stockings is much easier than drawing "barrel" stockings. Some drawn stockings are constricted at the free end of the inner slide to make it easier to put the outer on (and also make it harder to measure bore).

[hyperbolica]

If you look at where the wear happens on stockings, it happens at both ends, rarely in the middle. I have to say I don't think I've ever seen a Shires slide with the plating worn through, although I'm sure they exist. I think the barrel shape (slightly expanded in the middle) is there to distribute the wear along the length of the stocking. The outer slide does slightly angle down off the inner slide, especially in the outer positions. Most of the ideas we have about things are extremely idealized or exaggerated. Things are messy in the real world.

[Crazy4Tbone86]

I have never taken a micrometer up and down the length of a barrel-shaped stocking. I always assumed that the middle 60% or so was cylindrical and had the same outside diameter. In other words, it was only tapered for about .75 - 1.0 inch on each end. My rationale was that a truly "barrel-shaped" stocking would produce too much wear in one spot because the "high point" would be too small.

This thread has me very curious about the dimensions of some barrel-shaped stockings. I guess I will be pulling out the micrometer to investigate this!

[Doug Elliott]

Straight stockings wear at the ends (usually on opposite sides) because they're often not totally parallel with the rest of the tube. I think the drawing process leaves stresses that cause it to warp. Creating a barrel shape makes that less of an issue.

[LeTromboniste]

My understanding is that "stockings" are a newish thing... like not until the 20th Century?

Who sold the first trombone with stockings?

Not sure who first used them, but you see them already on 19th century instruments. I would say they probably started having stockings around the middle of the century but that's just an educated guess. That's when both the German and French trombone designs crystalised into the form they would keep for several decades, into the 20th. The stockings could have been added later, but certainly before 1880.

[robcat2075]

Has it been demonstrated that the stocking improves things?

Yes, it reduces the area of contact between the inner and the outer slide but... the weight of the slide is now concentrated in a smaller area.

I find most discussions of friction say that these changes will cancel each other out.

Or was the stocking for some non-friction reason?

[Doug Elliott]

Theoretically if a straight stocking was perfectly parallel with the rest of the tube, everything would be perfect. But in practice they're often not, and that adds more friction and wear.

[atopper333]

Just complete conjecture on my part…

The intent may be to limit the contact area while maintaining compression. Having less surface area would possibly allow for a greater plus or minus ‘tolerance’ in the manufacturing process so that a manufacturer would have to spend less time in fitting the outer and inner slides ultimately increasing turnout and profit. Again, I’m an amateur and just speculating that the overall goal would be to give a greater margin for error to increase production numbers over added expenses of creating slides with a better fit, better harmonics, etc…

[robcat2075]

I wonder how Renaissance people were making these long tubes in the stone-knives and cod-pieces era.

I suppose they were pounding out a big sheet of brass and then wrapping it around a cylindrical form?

Youtube videos show a modern hydraulic process of drawing a tube through an opening but that seems very industrial revolution-ish.

[LeTromboniste]

I wonder how Renaissance people were making these long tubes in the stone-knives and cod-pieces era.

I suppose they were pounding out a big sheet of brass and then wrapping it around a cylindrical form?

Youtube videos show a modern hydraulic process of drawing a tube through an opening but that seems very industrial revolution-ish.

Yes, sheet of brass, cut to the right width, rolled into a tube (over a cylindrical form), and brazed at the seam to form a pipe. Then pressed down onto the form using various tools (and annealing when the brass becomes too hard) until you have the right inner diameter. They likely had mechanical draw benches (the most complex part is a simple crank) among those tools. And you can alternatively do it manually by moving drawing rings around and along the tube instead of pulling the tube through, it just takes a lot longer and must be done more gradually, with smaller and smaller rings. There isn't a huge difference in the basic process really, it's just that modern hydraulic machines will draw tubes much faster and at a more consistent thickness, and most modern tubes are made from extruded brass tubes rather than sheet brass.

Not just in the Renaissance by the way. I have 20th century German trombones that are entirely made of rolled sheet brass. You can see a seam on every part of the instruments, even ferrules.