Linkage conversion

[Rick Denney]

I wonder if using that brass tubing to cover the threaded part of the linkage helps in the longevity and durability of DIY linkage.

It actually might help the strength, slightly. If the ball ends are screwed tightly down onto the brass tubing, the threaded rod underneath will be under tension where it is strongest, with the brass under compression where it is strongest.

But strength isn't really the issue. Even without the brass tubing, the threaded rod would be fine. Those linkages on the Alex that Dan showed were bent up on purpose. Sometimes you have to put a bend in the linkage so that the ball end clears the stop arm throughout the range of motion.

Here's a pic of the linkages I put on my Miraphone at least 15 years ago:



The original had those white plastic disasters, with the result that the stop arms were not ideal for my modification. I have always intended to replace these stop arms with something a little more elegant, but these have been reliable and I've never gotten around to it. I used a short brass tube to space the ball link out away from the stop arm enough to provide clearance, and the soldered it in place. Not pretty at all. I still had to put a slight bend in the linkage to provide clearance.

Rick "whose later attempts look better" Denney

[choisy]

take a look on this also
http://www.cablenet.ne.jp/~seanf/isleriche/modify.html

[Rick Denney]

Rick,

I disagree - I think that covering the thread with brass provides substantial support.

First off, the quote you included was Dan Schultz, not me. In my own response, I said that it does improve strength, and explained why (my Dad is also a civil engineer, but so am I). When you talk of force vectors, realize that the shap of the rod has nothing to do with any of it. But, as I explained, tightening the ball ends down tightly on the tube will put the threaded rod in tension so that it won't buckle under compression. It works the same way as a spoked bicycle wheel, where the design puts tensile elements always in tension where they are strongest.

Yes, the tube itself resists buckling.

In practice, though, a 4-40 rod should be plenty strong enough not to bend. If someone bends that rod, even without the brass tube, then I fear for the longevity of everything else in their linkage train. That's just too much force. So, I'm not buying that the linkages on that Alex were bent up from normal use, unless somebody annealed that threaded rod to ruin its strength.

Rick "who has seen linkages made with 2-56 rod that were strong enough" Denney

[Dan Schultz]

Rick, I disagree - I think that covering the thread with brass provides substantial support.

First off, the quote you included was Dan Schultz,

HUH! Wasn't me. I didn't comment one way or the other except to say I often use brass tubing over the all-thread to 'clean things up'. Otherwise, I think the 4-40 rod (steel, stainless steel, or brass) is stong enough as it is.

[tubeast]

JohnH,
what are people doing to their horns ? Just try bending a rod that short by pressing on the ends. What kind of force do those kids have in their fingers ?
Your impression is correct, a threaded rod WILL be weaker than an unthreaded one. For one thing, you have to consider the inner diameter of the threads only, the threads themselves don´t contribute to the part´s strength. In addition, there is a phenomenon called "Kerbwirkung" in German. That´s a factor by which the local stress that a force will create in a given structure is raised by sharp edges. (This is what helps you rip open the wrap of a candy bar that has frizzied ends, and this is why modern bike frames tend to have very smooth, rounded structures where two pipes are connected).
For the brass or aluminum piping to do any good, though, it needs to be clamped very tight between the two ball joints.

[Chuck(G)]

It seems to me that you could get some solid brass rod and a 4-40 die for about the same price as a bunch of brass tubing and stainless threaded rod.

[Rick Denney]

It seems to me that you could get some solid brass rod and a 4-40 die for about the same price as a bunch of brass tubing and stainless threaded rod.

Maybe. But if you started with brass rod of the same diameter as the tubing, you'd have to turn it down on the ends and then thread them. Fun if you have a lathe, and by all rights I should have one, but I don't. And the amount of rod I would ruin figuring out how to use it would amount to another tidy sum.

Also, the tubing and the threaded rod are both available at the same kind of store.

Rick "who thinks this is all about taking the easy way out" Denney

[Rick Denney]

If you take a good look at a linkeage as force is applied, the vector is not always in a perfectly straight line. When it hits the rotor stop, most paddles will start to push the paddle side of the rod up slightly, making it bend in the middle. It does on my horns, and without a huge amount of pressure. Check and see if I'm not right.

Hmmmm. I'm trying to visualize how there could be any non-axial force vectors in a linkage like that. There are only two possibilities: 1.) the socket has bottomed out so that the edge of the socket opening is resting on the spacer or the bolt rather than the surface of the ball itself, or 2.) something in the linkage is hitting the stop arm. I just conducted the experiment, and pressed the first-valve button on my B&S sufficiently to cause some deflection. Yes, the linkage moved up away from the instrument. But the deflection was not in the rod, it was in the frame of the finger paddles. There was no deflection in the rod.

If you had a student who bent them, and aluminum tubing stopped the damage, then the conclusion I draw is that before he was trying to bend them, and after he was not. I'm reasonably sure that I would bend the finger-paddle, the frame holding it, the stop-arm pin, or the bolt holding one of the balls (which is loaded in bending, not in compression) before the rod buckled. Maybe without the tube it would become the weak link, but it is still many times stronger than even a ham-fisted player should require.

Students won't break the anvil if you hit them over the head with it. Okay, maybe not.

Rick "whose web page was for grownups, in any case" Denney

[Tubaryan12]

I'm a band director, not an engineer

you forgot to say "Dammit, Rick"

[Rick Denney]

My rods bend slightly in the middle when I do this, on my MW25 which has a ball joint on the rotor and pin&tube on the paddle., and on my MW182 which has ball joints both ends (all original factory parts.) Why do mine bend and yours don't? I dunno. I'm a band director, not an engineer (imagine DeForest Kelly impersonation here.)
...
It wasn't my student but a friend's, and she is a very petite 6th grader who doesn't strike me as the destructive type. She thanked me very graciously for making her horn work better.

The pin and tube does offset the load slightly, and will favor buckling in a particular direction.

If you compress any column with enough force, it will buckle. The question is, how much is enough? Were your 4-40 rods steel? Did you have to put a bend in them (as I did with my Miraphone)? That would provide the buckling path at the outset. Where the linkages really long? The longer they are, the easier they will buckle.

But I still say that unless there's something wrong--some interference you didn't notice--it should take considerable force to cause them to buckle, with or without a tube. Do you have pictures you could post?

Rick "seeing a potentially untreated symptom" Denney

[Dan Schultz]

Tomorrow... I'll be putting DuBro links with 4-40 stainless threaded rod and 1/8" ID brass tubing on a customer's Sanders tuba. It's guaranteed that he will be pleased with the performance of the links for years to come.

No wise cracks about the Sanders tuba. This one, although it IS one of the Chinese horns... is actually a very good player.

[Dan Schultz]

I can loan them one of my horns since both of my jr. high tuba players were suspended today for pulling the emergency shower in the lab (idiots.)

Are these the same clowns that bent the linkages Sorry... had to ask

[Tubaryan12]

but why don't people use linkages similar to those found on most French Horns i.e. cord.

We hashed that one out before and if I'm not mistaken, we could come up with no real reason for the switch except for people must have wanted less maintenance. Bloke brought up the point that high end "french" horns still use string. (btw...I'm lucky....I have a Marzan with strings)