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  1. #171
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    @dejoha:
    The couplers/sleeves should be as tight a fit as possible, so the "bending" forces are distributed on the full length and not only on the edge at the end and the struts are kept as straight as possible. Shimming will help, but introduces point stresses wich is also a problem.

    Couplers that give both inner and outer support to the struts/poles used with a nice snug fit would be the strongest. I bet you could even get away with machining them out of a stiff plastic if you make them long enough and you kept the core support as a solid piece.

    I'm really excited about tensegrity stands and hope to experiment a little myself with some discarded Laser top-masts I think I may have laying around somewhere. They are aluminum, but may be too bendy, we'll see.

    Thanks for the inspiration and good reading in this thread everyone!

    -Frikk

  2. #172
    dejoha's Avatar
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    Quote Originally Posted by MacEntyre View Post
    My hypothesis is this: The cause wasn't the seam in the coupler. That was the weak point of the strut, but the cause of the failure was the cord used for tendons.
    Quote Originally Posted by WV View Post
    I Think Mac is right about this: tendons should not stretch. A lesser factor might be the shims. They may have taken the wobble out of the joint, but did you use enough of them to space them evenly around the couplers so the struts were perfectly straight?
    I think you're right. It's hard to say how much everything is stretching, but it is obvious the poles take some flex, even with the couplers. I put the couplers on the top part of the pole, which seemed to make the poles straight, but it isn't exact.

    Quote Originally Posted by WV View Post
    What next, Dejoha? I will send you amsteel as soon as it gets here. How are your splicing skills? I can put locked brummels in the ends and make whoopies for the side tendons if you wish, but we'll need to confer on sizes.
    I'm a whiz at splicing. No problems. Love it. Having the amsteel to work with will be perfect.

    Quote Originally Posted by WV View Post
    Can you find some conduit that fits easily but snugly inside your struts? A 2' internal ferrule would be a stronger connector. Then shim the external couplers with flashing? Then cable clamps around the couplers? (Maybe skip the cable clamps.)
    I've been day-dreaming of ideas since the last failure. I don't think I'll use the same couplers again. I'm hoping to find some longer couplers made from extruded pipe that has a tighter fit. I was considering even drilling holes through both the pipe and coupler where I can slip a quick-connect bolt or something to keep everything in place.

    Quote Originally Posted by Frikk View Post
    The couplers/sleeves should be as tight a fit as possible, so the "bending" forces are distributed on the full length and not only on the edge at the end and the struts are kept as straight as possible. Shimming will help, but introduces point stresses wich is also a problem.
    Yes, I agree. I think finding some long tight-fitting couplers is the next step.

  3. #173
    dejoha's Avatar
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    Quote Originally Posted by WV View Post
    Can you find some conduit that fits easily but snugly inside your struts?
    I made it down to the junk shop today and found some thick pipe pieces that fit perfectly over my railing. Each coupler is about 2 feet long and I feel a lot more secure with the structure, although the 550 cord is still very spongy.

    EDIT: I just watched Mac's video when he loaded 3 in his tensegrity. My tensegrity with 550 cord moves A LOT. It's hard to imagine what a difference the cord will make; I can't wait to try the amsteel.
    Last edited by dejoha; 10-14-2010 at 00:02.

  4. #174
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    Amsteel stretches less than wire rope. That's the key!
    "We must, indeed, all hang together or, most assuredly, we shall all hang separately." - Ben Franklin
    www.MollyMacGear.com

  5. #175
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    When I tightened the amsteel tendons on a tensegrity and then used it for a day or two, I noticed that they seemed to go slack a bit. It still functioned well as a hammock support, and the amount of looseness in the tendons was far less than one would get with nylon stretching. I think some of the slack came from connections at the end caps that tightened up. The rope had been wound around the struts and friction had kept it from tightening completely when it was first set up. It may also have been displaying the effects of creep, the slow elongation of fibers under continual tension. I'm not sure how much creep to expect with amsteel. It didn't affect the performance of the tensegrity, so I'm not going to rush out and buy Vectran.

  6. #176
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    Quote Originally Posted by WV View Post
    I'm not sure how much creep to expect with amsteel.
    I think there is not much. The other causes you listed are more likely, don't you think?

    I'm going to use a wrap, a hole and a toggle when I set up my 4" bamboo stand with four struts. (Maybe next week.)
    "We must, indeed, all hang together or, most assuredly, we shall all hang separately." - Ben Franklin
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  7. #177
    dejoha's Avatar
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    WV, I thought I owed you an update with the stand. Just before Thanksgiving I finally got some time to cut and splice the amsteel to create the tendons. I created one long tendon/whoopie sling for the top -- one continuous loop (I think it was 14 feet between each pole). Another long whoopie for the bottom -- one continuous loop (the bottom was 6 foot sections). For the three side tendons, I made those at 9 feet each. All are adjustable, so I can make minor adjustments.

    My first few attempts at getting the stand back up were epic fails. I'm not sure if the pipes were laying down wrong or maybe my side tendons were attached wrong, but when I pulled on the top tendon that was threaded through two pipe ends, those two pipes just slid together.

    I've come back to the thread to review your set-up video again to see if I'm missing anything simple. The pipes are currently sitting under a few inches of snow, but I'd like to get the stand up before this weekend because we're due for some more snow.

    Hopefully a refresher on the video will be the trick, but if you or MacEntyre have any suggestions, I'm all ears!

  8. #178
    WV's Avatar
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    Glad you're still working on it, but don't feel obligated to work with metal pipes in the snow. There's no rush.

    The continuous top tendon can't slide through the top end caps. I couldn't tell for sure, but it sounded like the struts could move on yours. I found that if I didn't twist the loop (see video) it didn't have enough friction to keep the strut from sliding on the continuous tendon.

    Mac uses an entirely different method of raising his tensegrities, because he doesn't use a continuous top loop, and thus had only one top tendon to pull on, instead of a pair of them attached to an end cap. I found it quite baffling when I saw his method of laying out the poles before he pulled it together. But it worked!

    In the second video I sent you, I was measuring the full length of each side tendon, but I later marked each one a yard from each end. With the winder midway between the marks, all I had to do was measure the distance between the marks as I tightened each of the three tendons. It made it easier.

    Good luck. Perseverence furthers!

  9. #179

    A couple of Questions

    I have enjoyed reading about this system. I have a couple of quick questions. First, I weigh about 250 pounds. What type of posts do think I would need to use to support that much weight?
    Second, besides portability, is there a reason the posts are cut in half? Would it be stronger if it was left in one piece?

  10. #180
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    C&PDad,
    Portability is the only reason to cut the struts in half. We have learned not to overload these structures, and it's wise to remember that a three-strut tensegrity theoretically will hold three hammocks, so don't be afraid of over-building. At one of the MAHHA group hangs a bunch a family members found that having three hammocks close together led them to relax and enjoy talking to each other for half an hour or more.

    Now the bad news: I don't know how big the struts need to be. They should probably be at least 12 feet long. I have made stands using 10 foot struts, but most hammocks need a wider spacing than these will provide. Depending on the material, they may need to be 3 or 4 inches in diameter. MacEntyre may have some suggestions. Rails for chain-link fencing are not strong enough. Antenna masts are not strong enough. Don't skimp on tendons and connections, either. Be careful.

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