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Making a 900 metre tape measure - or similar

  1. May 2, 2009 #1
    Hello all,

    I am looking for some guidance on making a mechanism that will retract a 900 metre length of fishing wire by itself.

    The idea is that the wire can be pulled up to 900 metres from its source and once let go it will return to the spool like a tape measure (without a crank or motor).

    I was wondering if it would be possible to use the mechanics of a spring-based tape measure as a starting point, and use a system of gears to increase the extension of the spring? But this is as far as I've got and not sure if/how it would work.

    Any guidance or suggestions on how to realize this is much appreciated.

    Thanks in advance
     
  2. jcsd
  3. May 2, 2009 #2

    Danger

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    I foresee some serious issues with efficiency losses. Every part of a gear train will add frictional and inertial drag. Also, the more line gets reeled up, the shallower your gear ratio becomes. I have to go think on this a bit. I'm inclined toward an explosive retraction method, but it's not something that I recommend if you don't know exactly what you're doing.
     
  4. May 2, 2009 #3

    MATLABdude

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    I'm not an ME, but if you're actually looking to measure out 1 km, I'd recommend using a laser range finder, or one of those measuring wheel devices (you put the wheel on the ground, and roll it from point A to point B and it measures the distance that it's rolled).

    If you're looking for something to wind up a few hundred feet of wiring / rope / whatever, you probably need a big winch (or to find a big, slow, high torque motor--a winch!)
     
  5. May 3, 2009 #4

    brewnog

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    A really big winch drum is the way to do this. Why can't you use a motor? What weight is the line? What space constraints have you got? What force do you need to retract the line in with?
     
  6. May 3, 2009 #5
    Thank you for the replies.

    The idea is that the origin point is fixed and the whole setup is located outdoors. Its not meant to be an accurate measuring device as such, its simply a device that allows you to hold the end of a line and pull it up to 900 meters for other reasons (metaphoric).

    Lets say you've pulled the line 900 meters, when you let go it should automatically go back to the source by itself. The reasons for not using a motor are: the setup will be located outside and it should be as simple and self sufficient as possible, preferably with no electronics.

    There are no space constraints on the site, however, I would like to aim for a modest sized mechanism for practical reasons (it will be fabricated in a small workshop). I don't have values for the weight of the line or the force of retraction - I'm afraid I haven't got into this detail yet. All I know at this stage is that fishing line is available in 900m lengths and is typically around 0.4mm thick, and the line should return by itself to its source.
     
  7. May 3, 2009 #6
    If the line is trailing a moving boat, then the total drag is proportional to length and becomes much larger as the boat speed (knots) increases, so it is very difficult to calculate a specific force. The problem is even more difficult if a slack line sinks (.i.e., with sinkers or steel cable).
     
  8. May 4, 2009 #7

    minger

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    If you have a drum 3 meters high, then by my back of hand calcs, you'll only need 127 revs around the drum to retract the entire line. Assuming the line is a centimeter in diameter or so, then the width of the drum is well under 2 meters. However, the springs that would be needed to pull a line back of that length would be impressive.

    The practical engineer at this point should note that applying that kind of force to a wire of that length can be potentially deadly. If that line breaks on anything and a portion of that line comes whipping back...who knows?
     
  9. May 4, 2009 #8

    brewnog

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    Vague requirements give vague solutions. Here you go, this will do the job perfectly:

    th_untitled.jpg
     
  10. May 10, 2009 #9

    Danger

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    I had me a thought here, but I don't know if it's practical.
    The basic idea is like an open-face fishing reel, where you have an axially-aligned drum with a follower running around it to feed the line onto it. I'm wondering if you could use a very strong coil spring (not sure if that's the proper term; the kind found in mechanical clocks) and manage sufficient gearing (maybe planetary) to operate it. I suppose that it all comes down to how much drag there is on the line vs. how strong your spring is. That is one hell of a distance to reel something in with a small device.
    Whoa, I just had another one. (Bloody hell! Two thoughts in one day! A new record.) If self-sufficiency rules out electricity, how about using an air motor with a compressed air cylinder?
     
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