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Lab Question - Pulling wire under tension

  1. Jan 15, 2012 #1
    I have to create a test harness for a lab. Part of the procedure is the pulling of a wire in tension. That is, the wire, when pulled, exerts resistance and thus will revert back to its original position when there is insufficient force pulling on it. I have to find a way to keep the wire in place when I pull it without the use of my hands so as to conduct experiments on the tension of the wire.

    There are a couple of ways that I found I could tackle this. Firstly, there could be a track that could pull the wire and the track could have locks to keep the wire at certain positions when needed. Secondly, I could pull the wire by hand and keep it at a certain position somehow.

    For the track option, I'm lost as to what could do this. Could anyone please provide some reference that would solve this issue? The method must be really simple, so if the method is not complicated, I would create it.

    For the manual pulling and stopping option, I was considering laying a rubber base on the platform and then pulling the wire on top of it. Then, I would have velcro going over the wire tightly to hold it in place (the rubber would provide friction, preventing it from going back to its original position). Although the method is simple, a possible issue is that the tension will overcome the velcro and rubber base. Does anyone have any other idea as to how to keep the wire at a certain position when pulled?

    Thanks a lot for the inputs.
  2. jcsd
  3. Jan 16, 2012 #2

    Simon Bridge

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    you can run the wire over a pulley and hang a weight from it?
    to pull harder, add more weights.
  4. Jan 16, 2012 #3
    Yes, I could, but the restriction with a pulley system is that the pulling distance is dependent upon the amount of weight I use. I would prefer to have a system in which I could pull however much I wanted/independently.
  5. Jan 16, 2012 #4

    Simon Bridge

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    It does not matter what system you use, there will always be some limit to the pulling distance and you were talking about manual pulling and stopping with some sort of ratchet device, possibly velcro??? These are much more limiting than using weights!

    With the weights, you can change the amount of pull by changing the weight - and this allows you very precise and independent control over the strength of the pull.
    If you want a lot more pull, add heavier weights ... they don't have to be laboratory weights - you can use bits of scrap metal, an old anvil, a car, a truck, there does not seem to be a practical upper limit to the amount of pull by this method (you are actually restricted by the strength of the wire). Similarly in the other direction, just use light weights like metal washers from a hardware store, bb-pellets, grains of salt. Your control is very fine compared with the other methods you have suggested.

    Perhaps if you described the purpose and aim of the experiment?
    What you've said is:
    The weight system is the standard way to do these experiments for a reason.

    Still, you want something overcomplicated ... be my guest: have fun.
    Last edited: Jan 16, 2012
  6. Jan 16, 2012 #5

    Simon Bridge

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    Lets examine these then:
    ... a hands-free holding system, and it has to hold the wire under tension, preferably at a known tension?

    A ratchet system? This would involve a toothed track and hooks which may get in the way of the testing you want to do.

    eg. with the above ratchet system ... is is not a "secondly" ... you don't really care how the wire achieves it's tension, you just want to hold it there while you do your tests.

    You are thinking of a toothed track - like those used on a winch crossbow or ballista.
    I used to make these out of lego ... you'd have to build it, wood would probably be good enough but you will need metal hooks.
    (You could modify a car-seat track - get one from an auto-wreckers.)

    An alternative would be to use a modern take on the same system:

    You know those ratchet-belt systems that are used to hold loads to trailers? These use the same principle as the track, only the "track" is circular so you don't need a lot of it and it stays out of the way of your experiment. You could modify one of those (a smaller one I'd guess) to hold you wire then you can crank it up to tension. The downside is that you don't know the tension.

    You could use a simple winch - attach the wire to a bobbin which you turn with a torque-wrench. When it is the correct tension, you clamp the bobbin in place.

    You have a bigger problem - the rubber no-slip pad (assuming it holds at all) will distort against the tension and so distorting your results. On top of which, you have little control over the amount of tension in the wire.

    Anyway - velcro and rubber just won't have the sticking power to do this. You are better to attach the end of the wire to a clamp and run it over a solid track - move the end into position so it has the desired tension, then tighten the clamp onto the track.

    By comparison, the weights system requires only three cheap items, allows fine-grain control over the tension in the wire, holds the wire in place while you do your experiments, holds the tension constant if you want when the wire is manipulated (plucked, say), and generally fulfills all your stated goals.

    I know that complicated machinery looks cooler - but you should always conduct experiments with the simplest most primitive equipment that will still isolate the phenomena under investigation.
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