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Concentric Tube Movement

  1. Jun 21, 2012 #1
    Hi everyone,

    I'm new to this site, so if I'm breaking any rules let me know. In particular because this is for my job. The forum rules seem very clear on the guidelines for homework for school and one can assume that personal hobbyist type questions aren't an issue but i don't see anything when it comes to work related queries.

    I've run into a problem in which i have a few (3) concentric tubes with an irregular cross section (though the same as each other). Due to machining tolerances we're stuck with a small gap, mechanically it doesn't matter but perspective isn't so good as someone can easily shake the central tube making it look unstable.

    I've tried finding a material to fit into the gap (Teflon, cork, PTFE, various tapes, cabinet bumpers etc.) but every item i've tried either creates too much friction or will not endure the movement of the tubes (often the adhesion gives).

    I feel that i must not be alone in this problem as there is so many items out there with concentric tubing. How do they solve the problem?
  2. jcsd
  3. Jun 21, 2012 #2


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    Gold Member

    From your wording, I assume that this is a telescoping device. O-rings would work if the tubes were cylindrical, so I'm wondering if you could machine grooves into the inner surfaces of the tubes and lay in beads of silicone or or similar caulking material. You mentioned that your adhesives failed, so maybe having the material physically restrained would help.
    Just a basic thought.
  4. Jun 21, 2012 #3
    If this is a telescoping type arrangement, and if the intent is for stability--not just to fill the gap for aesthetics or a seal of some sort--you might consider a method similar to most telescoping antennas like on a basic shortwave radio or such. In between sections there is a ring fastened to end of each inside section, and from this ring radiate 3 or 4 flat pieces of flexible spring steel or phospher-bronze. These slightly curved pieces of metal wedge in the narrow gap between the walls of each section and the next outermost section, and although they slide easily the tension of these slightly curved 'springs' hold everything in alignment. I'm sure I'm not explaining it well, but if you take apart any old telescoping antenna such as TV 'rabbit ears' you will quickly see what I'm refering to. Just a thought...
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