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Two questions.

Has anyone had experience in retrofitting consumer (locally-available) square aluminum tube as telescoping structural support? Ergo; with 0.125" wall thickness square tube at an outside dimension (OD) of 1.5", would minor superficial sanding and end beveling of an inner square tube with OD of 1.25" enable a telescoping pair?

Furthermore, could said pair be capable of implementing a gas spring internally for vertical assist? See image below, noted: gas spring improperly sketched upside down.

Loads and dimensions can be provided if necessary.

enter image description here

ADDITION: Is there an implied ideal mount offset for a gas spring in this circumstance relative to the load distribution (please see 2nd attached image below)?

enter image description here

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  • $\begingroup$ How heavy of a load are you supporting? $\endgroup$
    – Acumen Simulator
    Sep 3, 2015 at 23:27
  • $\begingroup$ Approx. 150 lb. maximum 4 feet above the telescoping junction $\endgroup$
    – Katalpa
    Sep 4, 2015 at 11:47

2 Answers 2

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Aluminium rubbing on aluminum generally will wear very rapidly. You generally want some sort of plastic bushing to reduce friction and wear. (Like with luggage handles there's just a little piece of plastic between the tubes) you should also have a bushing for the other end of the joint (the one inside that's not visible.)

You should do a beam analysis to check that the smaller beam won't buckle right at the joint.

The position of the piston shouldn't matter very much, but the minor benefit is probably worth the small effort to offset it.

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  • $\begingroup$ Considering the available clearance - is a reasonable bushing some thin milled plastic sheet, possibly adhesively backed to the outer walls of the inner square tube? Additionally, the plan was to significantly bevel the drop-point end of the inner tube to prevent scour/binding. Would such be sufficient, or would implementation of some plastic endcap as you iterated be superior in your opinion? $\endgroup$
    – Katalpa
    Oct 4, 2015 at 12:58
  • $\begingroup$ I think the plastic sheet wild work fine, assuming you could get it to adhere. You'd want the top set adhered to the inside of the outer tube and the bottom set adhered to the outside of the inner tube. That way they stay at the ends maximizing the lever arm to deal with the torque. If you can get the sheet plastic to work it would definitely be better than raw aluminum in aluminum. $\endgroup$
    – Rick
    Oct 4, 2015 at 23:55
  • $\begingroup$ Thanks, following up on such implementation and will update in the near future on results. $\endgroup$
    – Katalpa
    Oct 6, 2015 at 11:38
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I have experience with both of these but not for this purpose.

Telescoping tubing usually has a thickness of .105" . Also alot of square tubing has flashing left on the inside. Both of these may cause issues with fit up.

As for the gas springs, I have only seen them go up to 500# force. Also since the gas is compressible the force is not constant across the length of travel. I would definitely try to mechanically prevent the gas spring from bottoming out.

Foremost the concern here is safety. What happens when one or both of the springs fail? Or if they are overloaded? Are there any pinch points? Is the operator in danger of 300# of force hitting the bottom of his chin when a pin is released? These are issues to consider when evaluating alternatives.

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  • $\begingroup$ These are all very useful considerations. $\endgroup$
    – Katalpa
    Sep 4, 2015 at 11:48
  • $\begingroup$ Regarding prevention of the spring bottoming out - something along the lines of a internal tab or bolt through the outer tube prior to complete compression of the spring? Also - the maximum load of this apparatus is 150 lb. Do you have any suggestions from experience on means of limiting pinching between the telescoping junctions? As this is actually an aspect of a scientific display for all ages, and safety in that regard is priority. $\endgroup$
    – Katalpa
    Sep 4, 2015 at 12:04
  • $\begingroup$ Yes, a tab is what I had in mind. For the pinch point on the tube itself, you could extend the outer tube to get some distance from the pin and put a guard around the pinch point, or something along these lines. Gas springs have a dampening effect; so their motion is pretty safe. $\endgroup$
    – Acumen Simulator
    Sep 4, 2015 at 12:27
  • $\begingroup$ I have a question based on your advocated experience ... If the load at the maximum height is distributed offset from the telescoping axis (to the right in the diagram above), would there be a preferred offset in the junction of the gas spring and bottom of the telescoping tube? Possibly to compensate the lleveraged load down the length of the inner telescoping boom section (moving the bolt closer towards the right rather then the left as viewed via the diagram above)? $\endgroup$
    – Katalpa
    Sep 5, 2015 at 14:29
  • $\begingroup$ Steel tube is often welded and has flashing on the inside. Aluminum tube of this type will almost certainly be extruded and should have a clean inside. $\endgroup$
    – Ethan48
    Oct 3, 2015 at 16:44

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