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How to make a swing stable?

  1. May 6, 2015 #1
    Hi,

    I would like to get some help from you physics experts to solve this problem I am facing in my backyard.

    Here is the swing I have created using a hanging chair tied to a 23 feet high branch with a 1/4" aircraft cable made of steel strands.

    Problem:
    When the chair is empty it stays in one position, but when someone sits in the chair (without swinging it in any direction) the swing starts spinning slowly anticlockwise (from the person's point of view as they are looking at the ground). It keeps spinning in the same direction for a long time and never comes to a halt. I want to be able to enjoy the chair without the spinning.

    Solution attempts:
    1. First I thought it was due to the slightly tilted horizontal wooden bar of the chair. As you can see I have added a counterweight on the right to it make the bar parallel to the ground. But the spinning continued.

    2. Another reason could be the way the strands of the cable are twisted. I am not sure how to figure that out.
    Then I observed a construction crane and the load tied to the line doesn't seem to spin. How do they achieve that?

    http://imageshack.com/a/img540/9001/bqI2SE.jpg [Broken]
     

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    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. May 7, 2015 #2

    jbriggs444

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    How about using two cables, each attached to the branch at different points a few feet apart and spliced or otherwise joined together at the bottom where the attachment is made to the swing? If the cables have any natural twist, you may want to ensure that their natural twists are in opposite directions so that they have no tendency to twist together when dangling side by side.

    You could try a single cable hanging in a loop and attach a hook at the bottom of the loop, free to slide up or down. I would be concerned about the radius of curvature and the possibility of wear and weakening of the cable at the resulting kink, however. A pulley at the attachment point could alleviate that problem.
     
  4. May 7, 2015 #3

    anorlunda

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    I think jbriggs444 gave the right answer, use two cables, or a second line of any material fastened to a different place on the tree.

    Wire ropes are usually twisted. If you stretch a twisted rope, it wants to untwist. However, the untwisting should not continue in the same direction forever. I'm sure the OP didn't mean to imply that; otherwise it would be a perpetual motion machine whose discussion is forbidden on PF.
     
  5. May 7, 2015 #4

    sophiecentaur

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    Yes. When you hang a load on the end, the strands tend to unwind, which makes the seat rotate.
    If you have two lengths of wire cable, spaced by the width of the seat, that will limit the twist because rotating the chair will produce a restoring torque. But there can still be some twist. The stabilising effect would be greater if there is a big spacing between the top attachment points.
    I doubt that you can get hold of wire rope laid up in both directions.

    One way to limit the twist would be a long rope, stretched horizontally and at right angles to the direction you want the chair to swing. This would allow just fore and aft movement but it would introduce some curvature into the path - the longer the control rope, the less curvature.

    Hawser wire is double laid, which involves two components to the wire. There are large, multi stranded wires which are laid up around each other, in one direction and each of these multi strands are laid up the other way. By getting the twists 'right' in each case, the torques could cancel out. This[/PLAIN] [Broken] link shows a picture of what I am getting at. I think you'll find that the wire you are using is plain rigging wire, which has just 19 single strands so it can be expected to twist.
    (I only just found out about that and I must say I found it really interesting. I do love that sort of question!!!)
     
    Last edited by a moderator: May 7, 2017
  6. May 7, 2015 #5

    rcgldr

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    Try a web search for anti-rotation cable, and you should find a source for such cable. A very large steel anti-rotation cable is used for a tethered balloon ride at the Great Park in Irvine, California, which typically rises to 400 feet, holding up to 25 people.
     
  7. May 7, 2015 #6

    CWatters

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  8. May 7, 2015 #7

    sophiecentaur

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  9. May 7, 2015 #8
    Find some left handed cable.
     
  10. May 7, 2015 #9
    1. A swivel attached to the top and bottom. Some chair twisting due to friction in the swivel will happen due to fiction

    2. Use 2 cables, regardless of the twist, attached to the same point in tree and chair. Use 2 spacers to separate the two cables - near the branch and the other near the chair. For asthetics, a spacer width the same as the wooden bar of the chair, and a cable angle about 45 degrees. That way the chair is able to swing in all directions.
     
  11. May 7, 2015 #10

    sophiecentaur

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    AKA Hawser cable, I think. Possibly, your anti-rotation version is a high spec version but the requirement for crane cable is pretty demanding too.
     
  12. May 7, 2015 #11

    rcgldr

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    That suggested web search should be "anti rotation steel cable", or "Hawser steel cable", this should provide more hits to sites that sell the stuff. On a side note, the balloon at the Great Park in Irvine California is one of the larger ones, AERO30NG, and normally goes up to 400 feet (it could go higher, but it's near an airport and in a 400 foot ceiling zone). The stated specs for the cable is that it support 99,000 lbs of tension.
     
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