Another strangled flute

[weedie]

Jeez,who'd have thought that a length of string could generate twenty pages of discussion ..
Will there ever be definitave answer I wonder ??

[Terry McGee]

Jeez,who'd have thought that a length of string could generate twenty pages of discussion ..

You've heard the expression "How long is a piece of string?", I imagine. Now we know. Too long!

Will there ever be definitave answer I wonder ??

Possibly not one that satisfies everyone. Perhaps when I wrap up the experiment and discussion has died down, we should have a poll to determine whether people accept the findings or not?

Terry

[Denny]

I was thinking about goin' all bamboo....

da drama dun did me dude

[George]

I believe your on the right track. It doesn't matter if the 150 wraps (w/13 layers) covers 9.42cm^2 or 1 wrap (w/13 layers) covers .0628cm^2. The only real question is how to properly determine if the layers add, multiply, or even only add a factor of force to the inner most PSI.

Thanks a lot for that, seriously! I'm glad I'm not the only person this makes sense to.

This discussion started with an assertion that thread wraps in general are applying massive amounts of force and pressure on tenons (just shy of 3,000lb of force) strangling them. If that starting figure is off by over 2,980lb it makes the discussion very weird and strained.

Just like to say again that I have loads of respect for Terry, there's nothing personal going on here. I just think his conclusions are being influenced by numbers that are wildly incorrect.

[benhall.1]

Just a thought, Terry: would it be easy to make two more 'in extremis' tenons, as similar as possible to the one you've thread-wrapped? One could be wrapped in cork and the third could be left unwrapped. Then the other two should be subjected to the same moisture/drying cycles as the wrapped tenon. I'd particularly be interested in what happens at the 'shoulders', and whether all of them, or only or, or two, of them display that swelling at the shoulders that you mention with the thread-wrapped one . Part of the problem all the way through, which I don't think anyone would deny, is that the thread trough is the thinnest part of the tenon and is, presumably, therefore more prone to damage of some sort. But, I am also wondering if the thicker wood at the shoulders will absorb more moisture and therefore, irrespective of wrapping, may swell more than the thinner wood in the trough.

[Arch_Angel]

It is quite remarkable that a thread about thread has managed an average of one page per day for the past 21 days.

I keep getting lost on who is debating which sides of the argument(s).

What all do we have know?
Three or more forms of PSI
How Psi spread over the length of the tenon.
How Psi increases per layer.
How the force is calculated around the tenon.
Moisture fXs
The effect of moisture in decreasing the woods integrity.
If the psi remains constant as the wood accumulates moisture.
Is there any on the possibility the wood becoming petrified?
Oh, and my personal favorite the FXs of color (and/or if it is spelled Colour) of the thread or wood used. --Ah, you say you want to play western country, I'm sorry those are the red ones over there, these white ones are for bluegrass.--

[paddler]

One important issue that I think has been ignored in all this analysis is the effect of friction. If there was zero friction I suspect you would not be able to apply any more total force to the tenon than the force required to break the thread, no matter how many times you wrapped it around or over what area. (I'm wondering if this is where some of George's questions might be coming from). But clearly, there is some friction involved in our example. The question is how much, and is it sufficient to allow you to apply anything close to the breaking strain of the thread in a single loop? In other words, would a single wrap around the tenon be sufficient to hold the thread and allow it to be broken. I suspect not, and if this is the case then we can't assume that each loop around the tenon applies the force required to break the thread, even in the worst case. In this case the friction would be the limiting factor. So it might be worth doing a small experiment to see how many times you have to wrap the thread around before you can pull on it and break the thread. Whatever this number of wraps is, we would at least have to divide Terry's numbers by this in order to get a worst case number.

Once a thread has been wrapped around this many times the force it applies to the tenon can equal the breaking strain of the thread, and that force is distributed over the area taken up by that many wraps. Subsequent wraps (lets assume the same number again) contribute additional force to the tenon. This additional force does not break the thread because the friction of the first set of wraps is sufficient to confine the force applied during those wraps to the section of thread involved in those wraps. In other words, the next set of wraps is starting again.

Jon

[Terry McGee]

The friction seems to be quite high, Jon. You can wrap a tenon hard, let go of the end, and it doesn't come unravelling like a spring. It just sort of sits there.

I am a bit tempted to do the test I mentioned, with a thread-wrapped poly tenon slit down the middle, so I can then load it up with weights until the thread breaks, or conceivably comes undone. I'm struggling a bit trying to think of what weights I can load it up with that I can lift, and where to hang it from. Children and the tyre swing are my best ideas so far. ....

Terry

[Terry McGee]

Much more to think about at:

http://www.mcgee-flutes.com/Effects_of_ ... apping.htm

Terry

[jemtheflute]

I am a bit tempted to do the test I mentioned, with a thread-wrapped poly tenon slit down the middle, so I can then load it up with weights until the thread breaks, or conceivably comes undone. I'm struggling a bit trying to think of what weights I can load it up with that I can lift, and where to hang it from. Children and the tyre swing are my best ideas so far. ....

Terry

Terry, I do think this one is really off-target: ALL it can demonstrate is the breaking strain of the complete multi-thread lapping - which I'm sure would be pretty high. That has no bearing whatever on the inward forces applied to the tenon by the thread, however it may be wrapped. We know from (historic) experience that even the most hygroscopic boxwood tenon can't swell with sufficient force to rupture a thread lapping, even one applied with low tension in a loose-spun thread whose strands may pull apart fairly readily, even if that lapping has become dessicated and the threads friable. The breaking strain of the lapping is simply not a relevant matter.

More-or-less any thread lapping will withstand the outward pressure of a swelling tenon, and yes, that may well contribute to the ultimate deformation of the tenon in repeated cycles of wetting/drying & swelling/contracting.

Trying to work a conceptual example from scratch: if we start with a new-made tenon in timber we will assume is well seasoned and selected, as stable as wood can be, and apply a lapping to it with moderate tension, there will be some constrictive force on the tenon from the lapping. If play-induced moisture causes the tenon to swell, that outward pressure will be resisted by the lapping, increasing the total pressure at least temporarily. OK so far?

Almost any kind (- material, organic or synthetic) of spun thread will have some "give" in it such that, when tensioned more than the existing tension in it, it will stretch a little. Most threads will not be elastic in the sense that they will re-contract after such stretching. Even the initially applied tension will not be completely sustained/permanent due to this same fact. (e.g. wrap a steel rod with thread pulled as tight as possible short of breaking, keep it in a stable environment.... and I doubt whether after some time has elapsed the full individual thread or overall tension would be completely retained, even in a tight-spun synthetic thread, let alone in loose-spun cotton, silk, hemp or linen, though I wouldn't expect it to "give" very much..... not enough to become loose on the rod.) So, on our wooden tenon, if the wood swells, yes, the pressure between it and the lapping will increase, but the lapping will probably also "give" somewhat, and will not subsequently be as tight (on the original tenon dimensions) as before. Then the wood dries and shrinks....... and clearly the standing, at rest tension/pressure will be less than originally, even though the lapping may not become loose on the tenon.

Then we have also to consider the behaviour of the material from which the lapping thread is spun (This was considered, or at least mentioned, way up-thread, but not fully explored and has since been ignored.) On historic flutes, old lappings will always have been organic - cotton, hemp, linen or silk. Even if thoroughly imbued with a grease, they will not be fully water-proof and all (as previously mentioned) have some kind of swell/shrink behaviour when wetted and dried which can affect both their simple bulk (in terms of fitting a socket) and also their tension, at any given moment in particular conditions, but also affecting their residual, at-rest tension. I do not know whether an old organic thread lapping that has been multiply wetted and dried will have permanently shrunk (on a stable rod) and so become tighter/more tense, have permanently stretched, or have partially rotted and become less capable of restrictive tension in any case. In considering what has happened to period flutes, this issue is surely material to understanding the processes involved in any deformation of the wood.

Subjective experience suggests that most organic lappings' bulk swells with wetting (or is it just the wood swelling and taking the lapping with it???), but I suspect the wetness also increases elasticity (the swelling will lengthen the thread as well as fatten it, which must relieve tension??? or does the pressure increase due to the fattening??? or is there a balance or not with any tension-reduction??? or does swelling of the fibres cause the twist to "lock" the thread harder against stretching??? VERY complicated and pretty much unknown!) - on balance, in moistened condition we probably have some extra inward pressure from the lapping as well as the outward pressure from the swelling wood. Lots of extra load, wherever the starting point is!

When everything dries out, does the wood shrink more than the lapping, or less, or do they fairly evenly go back to their neutral starting point with no significant pressure changes? Also, does the wood dry faster or slower than the lapping? Here I am thinking, wood is more pliable while damp and swollen; if the thread dries and shrinks faster than the wood, that will be a temporary big increase in strangulation, and the wood will still be quite responsive until it dries and shrinks itself, so may be deformed and will then "set" in that deformation as it dries: but if the wood dries faster than the thread, it will presumably shrink inwards away from it, thus reducing strangulation effects until the thread catches up - any deformation will then be primarily due to the wood's own structural behaviour.

Of course, in modern terms, using a presumably moisture impervious and therefore non-swelling/stretching/shrinking synthetic thread (save that even such will have some mechanical "give" in it from first application, as mentioned above - it won't retain its original tension 100%), the interaction between thread and tenon will presumably be simpler. That would be easier to conduct experimental observations upon, but may well not give an accurate simulation and understanding of what has happened to old instruments!

[benhall.1]

Ooh! I like that, Jem.

Not like me to approve of certain people's loquaciousness ...

[jemtheflute]

A further thought on Terry's proposed (but IMO pointless) total-lapping-breaking-strain test..... Surely, the breaking-strain will have nothing to do with any tension in the wound lapping or its geometric composition (wound around a cylinder). Logically, you can do the experiment simply by testing the breaking-strain of a hank of your chosen threads containing the same number of strands as the number of turns in the lapping x2. The load-bearing element in Terry's split-tenon set-up is the number of threads crossing the cut on each side. Terry, no need for a fancy gadget: you could do it by suspending a rod horizontally, holding another below it at any distance, wrapping the same number of turns of your thread around the two as in a lapping, then suspend your weights from the lower rod...... And I bet even that is unnecessary - there's probably a known formula for the increase in breaking-strain achieved by (loose) multiplication of the same line (i.e. without any bonding together by spinning). I'm fairly sure it won't be simple multiplication! I don't think two lengths of the same strength line stressed together have either the same breaking-strain as one alone nor double that, but somewhere between.

Going back to lapping on a cylinder, as part of how a thread breaks is by pulling apart of constituent spin-locked fibres, not just the molecular shear- resistance of the actual material, I suppose the friction against the surface and the directional displacement of force entailed in the threads being wrapped around a cylinder may somewhat increase their composite contextual strength, as may the tight layering (extra locking), but I doubt that is very significant.

[highwood]

I think that in fact friction can potentially reduce the applied force - my reasoning is that the top layers could compress the lower layers thereby reducing their tension the friction would prevent the the tension from becoming equal over the whole length of the string.

This whole discussion reminds me of teaching pulleys and vector forces to students, in the end the only way to get most people to understand (and note I did not say all) was to provide hands on demonstrations - such as the one where I had a couple of the smaller students pull a pickup truck with its hand brake on across a dry paved parking lot, just using a rope (a big rope) and a tree.

Even a solid well commented math proof will probably not convince some since it seems that many do not see that each turn of a continuous thread applies a force that adds up with each turn.

Unfortunately I do not have time to come up a demo and video at present, perhaps by the time Terry's experiment is done - till then over and out for me

Bill

[Rob Sharer]

Even a solid well commented math proof will probably not convince some since it seems that many do not see that each turn of a continuous thread applies a force that adds up with each turn.

This one little observation neatly sums up my own unceasing wonder at certain responses to this topic. None so blind...



Rob

[Terry McGee]

Ah, I'm pleased (and relieved!) that people don't feel I need to do the "dangling children from the wrapped tenon" test. The children ask me to thank you all too, and have mostly dried their eyes and dusted off their knees.

My logic in proposing this test was based on George's resistance to the idea that more turns increases the force applied to the tenon. Since that force is limited by the breaking strain of the thread, it seemed likely to me that showing the breaking strain of a mass of threads is far higher than one thread, we could extrapolate that to the force exerted by a mass of threads is far higher than one thread.

Now, something else occurred to me in the night, and I've added it to the web page. I'll reproduce it here for convenience, but if you haven't seen the web page since the addition of the bore distortions data, you'll definitely want to read the lot again:

http://www.mcgee-flutes.com/Effects_of_ ... apping.htm

Here's the new observation:

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Serial Strangulation?
Now, let's just imagine that the tenon on our flute had suffered the kind of distortions we've seen above. It had started out with a thread wrap of 20.6mm, which would have been exactly what it needed for a nice snug fit in the socket. But that wrap has now reduced to 20.25mm in diameter, 0.35mm too small for a snug wrap. Indeed, well before this stage, the joint would be sliding about uncontrollably, and even leaking. So what would we do about that? Put on more thread of course, or perhaps even remove the old thread and start again. Either way, we now have a tight grip on the tenon again, and an even stronger band of thread. The game starts over!
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Terry