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  1. #191
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    Quote Originally Posted by WV View Post
    Given fixed top tendon length, bottom tendon length, and strut length, you should be able to make a rigid tensegrity simply by tightening all the side tendons, BUT there is a range of possible values for those first three lengths. AND it is not all that simple to tighten it enough. I can calculate whether your given lengths will make a tensegrity if I have one more fact: how many struts you are using. I assume 3, but 4 might be okay, too. With that information I could tell you how tight your side tendons need to be and how high your hammock will hang. (I'm guessing that you might need to shorten your top tendons a bit.) Let me know how you are tightening those side tendons, too. You need to get some mechanical advantage somewhere - it's not just a matter of pulling on a whoopie sling.
    I have 3 struts. I'm interested in the model you're using? As I said, I'm having trouble thinking in terms of "if I shorten this tendon, it will effect everything else like this..."

    I have 2:1 mechanical advantage on two of the side tendons and 4:1 on the third tendon (by going back & forth between ends as in your video). Tightening them a bit didn't seem to make that much of a difference, so I didn't try to set up anything more rigorous, but if you say that's the crux I'll give it a more serious try - thanks!

  2. #192
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    Hey MacEntyre - you can get long length aluminum poles here http://www.aircraftspruce.com/catalo...ube_6061t6.php

  3. #193
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    This looks like a fun project. I am a tall guy and would like a 14 foot span to hang my hammock. What is the optimal strut length for a 14 foot span ?? I am considering some square aluminum tubing in 1 inch cross sections. Just used some to build a Turtle dog tripod stand last night. Seems very rigid and hopefully able to handle the stress of a active Boy Scout Troop.... As a Scout Master, I'll be on the turtle stand, but hope to get the older boys together for a project in building a Tensegrity
    Last edited by talldaddy; 04-04-2013 at 09:40.

  4. #194

    Tensegrity specs

    Quote Originally Posted by dejoha View Post
    I fiddled with my tensegrity stand for a few hours today and came across my own breakthroughs. First, let it be said that I'm no mathematician. I'm sure there are some very accurate computer models that could have done a better job at "I-told-you-so" than me falling on my backside time and time again.

    The main discovery I made is that the poles must have a higher angle, otherwise the downward force drags the poles to the ground, instead of directing the weight through the poles (if that makes sense). To do this, I had to shorten my top triangle length and lengthen the side tendons.

    The other discovery I made is that I needed more than just one side tendon. There are two reasons for this. First, when my weight was applied to the hammock, the two struts/poles I was attached to bent inward. Adding the second side tendon to each strut kept the top of the tensegrity from moving or compressing.

    The additional side tendon also eliminated all twisting motion from the stand, which also helped keep my backside off the ground.

    I had to stake at least two of the struts to prevent tipping with one person. I probably should do all the struts.













    How long are your poles?

  5. #195
    How long are the poles in this setup ?

  6. #196
    New Member fortbuilder's Avatar
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    Most of the components are available at your local hardware store except your hammock and probably the Amsteel which can be bought here: http://www.onlineropes.com/servlet/StoreFront
    As a Civil Engineer I'm surprised I hadn't thought of this. It's a fantastic idea. I've been hanging for many years, beginning with "hanging from rock faces" (as a rock climber) to wilderness survival and no trace left behind (which I teach now).

  7. #197
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    To compensate for bending "buckling", you might consider a shroud and spreader on the back side of your strut, opposite from the end of the apposing strut. Thus you could potentially use lighter strut materials or compensate for joints.
    As for the fire in the center, one caution would be of annealing the metal of your struts. Concentrated heat on one end of the strut could potentially affect the ductility of the struts, where as the one end would be more malleable and the other rigid, also affecting placement of your spreaders, if used.
    However, as a wilderness survival instructor I am very fond of the Dakota hole and believe the above consideration would be negligible. Specifically because the heat source is below the surface of the substratum and then dispersed evenly or at least not in direct contact or proximity with your struts and shroud.

  8. #198
    New Member fortbuilder's Avatar
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  9. #199
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    Fortbuilder, the bending moment I have observed is directed somewhat downward, so it would not be solved by a spreader and strut underneath.

    I solved the problem by lashing the struts together where they cross in the center. That improved the strength of the structure immensely, by making each strut serve as a spreader for another strut. Of course, you have to adjust the tendons so that the struts cross close to one another... the less the struts are inclined, the farther apart they cross.

    - MacEntyre
    "We must, indeed, all hang together or, most assuredly, we shall all hang separately." - Ben Franklin
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  10. #200
    New Member fortbuilder's Avatar
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    I didn't say underneath the strut, but opposite the the end of the apposing strut. And should have said opposite the bottom end of the apposing strut. Essentially the back side of where it will bend. In practicality, by lashing the struts together at the bend you are doing the same thing, supporting from the adjoining struts. Simply short shrouds and the other struts as spreaders, except without the benefit of the longer "shroud" and the tension added with a "spreader". Doing that you have the benefit of all three or however many struts supporting the center of each other, though if one buckles they all will. This way their support is only the strength of the weakest joint in the adjoining struts. By doing as I am suggesting, you would be taking advantage of the characteristics of the material your strut is made from and the tensile strength of your shroud material and isolating them from each other.

    I see by the picture how you assumed I meant underneath. And yes, that is toward the bend, it should have been drawn on the other side. I pulled the pic from the comment above and quickly drew in what I meant by a" shroud and spreader", it's comparable to rigging on a sailing vessel and should be on the outward side of the bend. However, the shroud (from one end of your strut to the other) and spreader will compensate for greater stress than just wrapping the weak point to the weak point of the next strut. My suggestion strengthens each strut, individually. Thus strengthening the overall structure. Where as your suggestion only draws on the existing strength of the other struts. "Robbing Peter to pay Paul"
    Also in a tensegrity, the point at which the struts come together is the collapsing point of that structure. To demonstrate, it's simply the point of a spring that can go no further (you are winding it up all of the way). Placing your struts in a manor that they all touch compromises the effect of having a tensegrity, limiting the ability of it to draw on the tinsel strength of your tendons.
    Last edited by fortbuilder; 06-22-2013 at 15:59.

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