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  • Looking for info on a simple design principle (used in tent poles!)

    Discussion in 'The main mechanical design forum' started by JCombs, Sep 18, 2015.

    1. JCombs

      JCombs New Member

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      Hi all,

      I started looking into this as a result of a poor design I encountered at work. The application involved a shaft that slides axially, but is very poorly guided by only one small bushing, rather than two spaced apart. To make matters worse, the shaft constantly experiences alternating side-loads, causing it to lean and angle within its guide, predictably causing poor operation and many complications.

      There is also a seal placed around the shaft near the guide bushing and only retained by a spring washer, so it is able to float laterally with the shaft as it moves. The clearance between the shaft and seal is very tight and early testing showed that the seal had a tendency to grab onto the shaft, creating extremely high resistance to motion.

      At one point while staring at the thing, it suddenly clicked that I had seen this type of effect before, on tent poles! Certain tents, especially older ones that use telescoping poles, have a similar design that is specifically used to prevent motion entirely. The narrower pole of the telescoping set also slides through a flat "tab" of metal with a hole through it. When you lift up on the sliding pole, it moves through the tab freely, but when you release it and gravity pulls it down, the tab with the hole gets pulled down with it, into an angled position, and its ID binds against the OD of the sliding pole, completely stopping from sliding through the tab any further.

      It's a very simple mechanism and since this realization I have noticed it all over the place. However, being as this principle has seemingly been in common usage for decades (and probably a lot longer than that) I was kind of surprised that I have never seen details on how to design for it or how to avoid it. For example: What kind of clearances give you plenty of clearance in one direction, and good lockup in the other? How much force can this lockup withstand before failing? With increasing force will the pole eventually slide or does the tab have to fail first? How dependent is the effect on material, finish, lack of radii or break-edge, etc?

      Does anyone know any name for this type of device? Or any way to find design guidelines for it, etc?
       
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    3. maniacal_engineer

      maniacal_engineer Well-Known Member

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      its also used in extensible clamps, and in pipe clamps. The analysis is similar to the analysis used to determine the maximum overhang of a load in a linear motion system.
       
    4. JCombs

      JCombs New Member

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      Just to be clear, this is the mechanism I'm trying to describe.
      [​IMG]
       
    5. jagengrg

      jagengrg New Member

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      http://www.ebay.com/bhp/adjustable-tent-pole
      Tent pole. You will have to make assumptions about friction. If you know the materials, handbooks will have a range of values for various material on material coefficients. I would start with the flat piece the narrower rod goes thru. If you have an application you should have some idea of what load will be on the tip. Solve for the angle the flat plate needs to be at to bind with the narrow rod. If you are using a standard rod see if there are published tolerance for dia. See how the angle changes as the clearance between the plate and the rod changes. There may be many approaches.
       

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