Daren SchwenkeFor building up the C1 motion platform this time, I ordered some 110lb test fluorocarbon fishing line. The thought was that the higher break strength would equate to less stretching of the line.
I was wrong.
I made a quick jig and tested the two. The jig was simply a bar with a pivot with both lines tied at equal lengths from the pivot point. I then put both lines under as much load as I could easily manage and measured the resulting angle. This methodology doesn't give me any actual numbers, but it did give me a clear result.
The 50lb test 'SpiderWire' line I had previously used exhibited less length distortion under load than my brand new 110lb test line.
I then bought some brand new Spiderwire and repeated the test, just to eliminate the possibility of my existing line having been 'stretched to limit' and settling in. Very nearly the same result. The 50lb line still stretched less.
My current theory is that the PTFE coating which is smoothing out the outer contour for the Spiderwire has somehow locked the braid into its current shape, which then serves to help prevent length distortion. The 110lb test line has a much more pronounced braid to it and does not seem to be coated, whereas the 50lb is smooth and slick to be point of creeping if you use a standard fishing line knot. Up until this point I've just mitigated this by putting a drop of super glue on the knot, but there is a special knot you are supposed to use with this stuff to eliminate that issue. I'll have to look it up.
In any case, I'll be using the Spiderwire now. I will need to see if they sell it in higher break strength..
EDIT: Apparently what I called a 'standard fishing line knot', and what I've been using for 40 years, is actually a hangman's knot. It's also used for tying fish hooks apparently but it is by far not the easiest version to use for that. Not that I'm a big fisherman, but growing up in WI this was just one of those things you knew how to do. I can also get air on a snowmobile, and field dress and skin a deer... :)
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I found that the Davy knot works well with slippery lines like dyneema. I was using it to make a corexy mechanism for a sand table project. I found restringing the cables so onerous that I decided to convert the project to belt drive.
Polymer lines creep. That means they will lose tension after sitting for a while, which is why you have to keep retuning guitars with nylon strings. Obviously, creep can't go on forever, and what will happen is that when you're retensioning the line for the 20th time, it will fail.
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Very useful information! Thank you!
I will now test more carefully my cables that extend the GT2 belts!
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The line on the right was significantly cheaper too. I'm sure that has something to do with it.
There may actually be another variable here now that I think about it. I'm operating the 50lb line at about half of its capacity, and the 110lb line at less than 1/4 of its. Perhaps if I fully loaded the 110lb line it would settle in. For now, I'm moving on with the 50lb.
I think they actually make a fluorocarbon line specifically for our task (motion control) but I have not tried it yet.
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What I'd imagine is going on is that the 110lb test line will have a higher UTS as that's what that rating would encompass. However, either out of necessity or coincidence the 110lb line has a lower modulus leading to higher strain at equal loading.
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