Keeper Bars I was asked.

[Joe Gwinn]

Isn't the tension only while it is being applied?

No, it is locked in, and will remain unless something gives.

We tend to think of metals as perfectly rigid, unlike rubber, but actually it's only a matter of degree. With rubber, the distortion is visible to the eye. But with metals, which are orders of magnitude more stiff, the distortion is not usually visible to the eye.

Think of it this way: If metals were perfectly rigid, springs could not exist. But they do exist.

Once the coil is wound, you can take a strat coil apart and slide the rods out of the coil.
IME There Doesn't appear To Be any tension present.

Yes. The forbon did the giving. After all, forbon is paper, while copper is metal. Which one is going to win the fight?

[big_teee]

Thanks Joe for You Valuable Insight!
I still think something is missing with this formula.
Instead of an acculative tension in inch pounds, or how you want to relate it, how about this.
It is more of a compression per turn.
after the turn is made the tension is no longer there.
Example. If you take string and wind it around a empty paper towel paper roll.
Each turn compresses the empty roll.
If measured after 20 turns the roll would measure smaller than the original roll.
If you keep winding each turn will add more compression.
However, if you cut the string there is very little recoil like a metal spring would have.
If you wind the same string around a metal pipe there is some tension, but no compression.
The metal will not allow any collapse.
so if you cut the string from the paper towel roll.
I think you would find the first layers to be loose because the paper roll is getting smaller.
So I think the cumulative tension is on the paper roll or pickup bobbin not the string or wire.
Anyway thanks for the interesting Discussion.
Terry

[Rj777]

Yes. The forbon did the giving. After all, forbon is paper, while copper is metal. Which one is going to win the fight?

If that's true then it would be easy enough to prove, take an unwound forbon type pick up and measure the outside distance of the magnets/pole-piece and lock the calliper. Then wind the pick up and remeasure, if there's as much tension as you say then the distance should be shorter. and the theory confirmed

Noted coper wire is elastic but then it yields and goes plastic and then if fails. There's probably some math that could be done to actually answer how much tension and whether it varies. The other way to put this one to bed is to put a load cell on each end of the pickup and start winding, test it every 500 turns and see if its cumulative or constant, then leave it a week or so to see if it changes over time.

[David Schwab]

Put some kind of strain gauge on a bobbin and wind it.

Clearly winding a bobbin puts pressure on it. I'm just not so sure that it's cumulative past a certain point. Otherwise some bobbins would fail when would with a lot of wire.

Wrap some wire tightly around your finger. Once you get past the first layer it doesn't really get much tighter. I've done this when unwinding a coil.

[Joe Gwinn]

I still think something is missing with this formula.
Instead of an accumulative tension in inch pounds, or how you want to relate it, how about this.
It is more of a compression per turn.
after the turn is made the tension is no longer there.
Example. If you take string and wind it around a empty paper towel paper roll.
Each turn compresses the empty roll.
If measured after 20 turns the roll would measure smaller than the original roll.
If you keep winding each turn will add more compression.
However, if you cut the string there is very little recoil like a metal spring would have.
If you wind the same string around a metal pipe there is some tension, but no compression.
The metal will not allow any collapse.
so if you cut the string from the paper towel roll.
I think you would find the first layers to be loose because the paper roll is getting smaller.
So I think the cumulative tension is on the paper roll or pickup bobbin not the string or wire.
Anyway thanks for the interesting Discussion.
Terry

I think you are almost there. When winding, the forbon (or nylon or whatever) does yield incrementally as you wind. One can model this mathematically, but it's a whole lot of work, so I use the simple nothing-yields example to show that one can generate immense forces by the simple act of winding 5 or 10 thousand turns of very thin wire. But forbon is far stronger than a toilet roll, so there will be significant residual stress. Over time, this stress will relax as the forbon yields and the wire slides around a bit. It was this relaxation in the few days after winding that started the original discussion.

[FunkyKikuchiyo]

Wrap some wire tightly around your finger. Once you get past the first layer it doesn't really get much tighter. I've done this when unwinding a coil.

I've done similar things. On the outer winds you're winding over your finger and layers of copper, and the "compression" of your finger (depending how soft your skin is... we don't need to know ) changes the more you wind; your finger is softer before you're winding on there, it is bound up and harder once you've put a few layers of copper on there. So, the quality of the "bobbin" changes with each wind because the make up of it changes. I would think that a finger is a more extreme example than a plastic/metal/paper bobbin, all of which are more rigid. In fact, layers of copper wire could be softer than the bobbin in many cases, so I'm not sure that you're going to see the same set of patterns. This is purely a guess, though.

And purely a question, not an argument concealed as one: If something is under tension, do you necessarily have to prove that it can whip back like a spring or rubber band? That seems like a specific property of a material rather than evidence of forces being exerted.

Joe's post is good, but reading "toilet paper roll" "immense forces" and "relaxation" in one post distracted me a bit.

[Rj777]

And purely a question, not an argument concealed as one: If something is under tension, do you necessarily have to prove that it can whip back like a spring or rubber band? That seems like a specific property of a material rather than evidence of forces being exerted

If something is under tension then a force is being applied, the material will stretch to accommodative it and providing the elastic limit isn't exceeded when the force is removed it will return to its original state, depending on the size of the force and the elasticity of the material it will ether ping back like an elastic band or just a couple of microns. Given that we are talking about very thin wire that stretches and apparently 600kgs of force i'd expect it to ping a little

I understand what is being said but Im have a hard time getting my head around the magnitude of these changes, if the bobbin was to crush then it would lead to visible cresent shaped gaps by the magnets or a buldging in the middle as the the top and bottom flatwork are slender then they would buckle.

Is it more that the winding tension is non-linear so it dose increase marginally and the subsequent settling is the the wire yielding and entering a plastic state? that would explain the change in sound as the wire would be slightly varying thickness and molecular make up. also it wouldnt spring back if cut

Just a thought