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Thinking caps on : telescoping mechanism

  1. Mar 1, 2007 #1

    DaveC426913

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    I'd like some help from builders - modellers or mechanical tinkerers.

    I'm trying to build the skeletons of some polyhedra - the vertices are intersections of rods - no faces. Like this.

    I want to be able to telescope the rods by about 3x. So, I could slide them anywhere between, say, 1" long and 3" inches long at will.

    One way to do it is to use a rod-and-sleeve or piston design with brass rod and hollow brass tubing that you can buy at a hobby store. Trouble with this idea is that it won't compress to one third of its length - only half at best. To do a factor of three I'd need one rod surrounded by a sleeve surrounded by another sleeve. see attached diagram

    It gets way more complicated when you add in mechanisms to stop the pistons from simply falling apart at full extension - they need slots and pegs, etc.

    I'm looking for fresh ideas on how I can get the effect I want with less building and tinkering and with more reliability so that it doesn't fall apart. Less work would be nice too. If I can start with materials that I don't have to completely craft, that'd be great. I have to make 32 of em.

    The rods should smoothly expand and contract by a factor of about three, yet remain rigid.

    It doesn't have to be overly strong, I've been toying with plastic tubing and elastic bands and stuff.

    Ideas?
     

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    Last edited: Mar 1, 2007
  2. jcsd
  3. Mar 1, 2007 #2

    Moonbear

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    How about something like old TV/radio antennas? Would that be sturdy enough or the right size? If not, at least you might be able to dissect one to see how it's held together and create something similar out of more sturdy tubing.

    Edit: Oops...just noticed you did give sizes you wanted. Well, the dissection of an antenna might be informative, even if it's too long.
     
  4. Mar 2, 2007 #3

    DaveC426913

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    Actually, that's a good idea. While it's largely the same mechanism, it might yield a somewhat less intensive construction method than my slot-and-peg design. Thanks.

    But the rest of you aren't off the hook. I'm still looking for out-of-the-box thinking. Coiled springs is another idea.
     
  5. Mar 2, 2007 #4
    Regarding telescoping devices, if you must have 3X expansion then you need more than 3 sections since a fraction of each section must remain unextended just to hold the device together. So you would be looking at 4 telescoping sections. Depending on the size and sturdiness you require, you can even make these by rolling paper into tubes of appropriate sizes. It can work well enough if it's just for a model. Just don't sit on it...

    For a very small model, I'm also thinking of bending drinking straws. You know, those with a sort of corrugated section where the straw bends. This section can also extend and contract to various lengths as you push or pull. I have on in my hands right now and the section in question measures 7 mm compressed, 22 mm extended, just about the 3X ratio you need. But it is really small and it also bends easily so here again, you have to decide how big and how sturdy your polyhedra needs to be. Commonly, the more stretchable something is the flimsier it is.

    Then, there is also the "stretching hand" toy approach. I don't know the actual name. The device consists of a series of sticks that are hinged or articulated so that as you close one end the toy hand at the other end reaches out. I'm not sure exactly how to describe it but you may know what I'm talking about. This gives you far more than 3X extensibility. The difficulty here is to set it to a fixed length. You would probably need a bolt at one joint of each segment to tighten it into position. I have seen something like this used precisely in an extensible polyhedra at a science toy store a while back. You would pull on a string to make the form grow and shrink, cool device.
     
  6. Mar 2, 2007 #5

    DaveC426913

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    Yup. I was diagramming an ideal situation. 4 would be required.


    It owuld have to be sturdy enough to telescope on its own without my helping it i.e. as I bend the polyhedron, all the rods will move. Paper won't hold up to that.


    Yes but it has no lateral rigidity.


    Yes, I know what you mean. The name escapes me too. [ EDIT: Pantograph! although I suppose "scissors" works nicely too]

    They tend to be pretty weak laterally. Also, they tend to have soem pretty serious problem with breadth when they retract. You can get around this by increasing the number of links, but that further reduces the lateral rigidity.
     
    Last edited: Mar 2, 2007
  7. Mar 2, 2007 #6

    DaveC426913

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    Ideas I've got so far.
     

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  8. Mar 2, 2007 #7
    Yes! That's the word. And also yes, they are thin laterally.

    But behold: the 3D pantograph! Expertly manufactured from authentic cardboard pieces professionally fastened using high quality stainless steel pins, the 3D pantograph is exhibited here atop a rich oak surface. This inspiring design will lend elegance to any decor or polyhedron.

    [​IMG]

    It does run counter to your "less work would be nice" condition though... But it is rigid and it extends easily.
     

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    Last edited: Mar 2, 2007
  9. Mar 2, 2007 #8

    DaveC426913

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    1] Well. Wow. That's cool. (BTW, where are the two end points?)

    2] As far as less work, it is likely less so than the slot/peg method.

    3] Also. I wish to hire you as my personal marketing manager.
     
    Last edited: Mar 2, 2007
  10. Mar 2, 2007 #9
    Uhh... Here's one possibility.

    [​IMG]

    Either connect three edges with a wire, or your vertice balls can connect to three of them separately, or if your pantograph is made up of many segments then you can connect only one of the three ends to the vertex and it still works, although a bit unevenly... You will have to get your popsicle sticks together to work out the details. The end joints have to remain a little loose for example because the angles vary as the pantogram contracts and expands. Rounded ends would help.

    :rofl:


    PS: How do I upload an image to the forum? I won't keep these pictures online forever...

    EDIT: Never mind, I found it.
     

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    Last edited: Mar 2, 2007
  11. Mar 2, 2007 #10

    DaveC426913

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    Man that's hard to build. I'm trying to construct it from wood skewers and elastics.

    Oh, I see why. Yours is simply 3 pantographs attached at their outer vertices. What I was building was 3 pantographs attached at their inner intersections.
     
    Last edited: Mar 2, 2007
  12. Mar 4, 2007 #11

    DaveC426913

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    OK, I've got an idea I can live with.

    No one said I could only use one axis. So I've simplified the piston construction and glued two side by side. See diagram labelled PF070303telescope2.



    Also, I've calcuated that the max telecope extension does not need to be quite 3x, it needs to be only 1+2(root 2) = 2.818.


    Sigh. I can't describe the problem easily without tipping my hand. So see attached diagram. I'm trying to see how long each segment (all of which are constructed identically) must be able to telescope by.

    MY solution is 2.818 (or 1 + 2*(2^-1) ).
     

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    Last edited: Mar 4, 2007
  13. Apr 21, 2010 #12
    any body one know how is the mechanism of telescopic using wire rope?
    i.e tower lamp.
     
  14. Apr 21, 2010 #13
    Dave, at a maximum extention of 3" I would try screws within screws, like a common machinists screw jack. You'd need a lathe, or at least a drill press with a good jig, to drill a true through hole in the two larger pieces, though. I'm sure you know how to tap.

    This will have all the rigidity you need, for sure. The drawback might be slowness of length adjustment.
     
  15. Apr 24, 2010 #14
    Damn! I only just noticed the start date for this thread. After just finishing designing the 'scope. look...

    [​IMG]


    :blushing::uhh:
     
  16. Apr 24, 2010 #15
    @Thetom
    So you have essentially designed a childrens extendable light sabre?

    [​IMG]
     
  17. Apr 24, 2010 #16
    noo, is a the interstice for a four-dimensional hypercube, silly. The lightsaber uses live flanges.
     
  18. Apr 24, 2010 #17

    drizzle

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    [​IMG]


    ... But in your case, you might need a more complicated spiral, something like this...


    [​IMG]

    :biggrin:
     
  19. Apr 24, 2010 #18
    No, spirals wont work. I can't draw them in photoshop :tongue:
     
  20. Apr 24, 2010 #19

    lisab

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    Lol...

    Sorry I can't help with this group-design project -I lost my thinking cap. All I have is my tin foil hat.
     
  21. Apr 24, 2010 #20
    You can be incharge of safety then :smile:
     
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