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Loading Over a span and selecting appropriate materials

  1. Aug 1, 2017 #1
    Hello all and thanks in advance for the help!

    I am currently working on creating a lighting fixture that will be suspended over an aquarium. What im having trouble with is determining a safe material and thickness for the spars going between the lights (see below). My goal is to have these spars be as low profile as possible yet still safe and reliable for long term. Because of the low profile needs I would prefer to use some type of flat stock rather than angle stock or square stock. MY current thought is to use aluminum bar stock as it is easy to get and reasonably priced, but im unsure of the strength. Anyone have any suggestions on materials? How would I figure out the loading on the spars?

    Each light fixture weights aprox 5 lbs
    all dimensions are in inches
    assume no Flex in fixtures and each fixture has 4 mounting points
    Spars need to be at least 3/4 inch wide for mounting points.
    Fixture will be hung from 2 cables on either side (see attaching points below)

    overall.png
    Fixture_Dimensions.png
    Fixture_Overview.png

    As for hanging locations, I would prefer to hang the fixture from the red points for aesthetic reasons, however I assume that the green mounting points would provide a decent counterbalance to the suspended light in the center. Thoughts?
    mounting_points.png

    Once again, I really appreciate any help that can be provided with this. these are extremely expensive lights so I really want to be sure that it will hold for the long term!
     
  2. jcsd
  3. Aug 1, 2017 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF, and nice drawings! :smile:

    IMO, it's a fundamental engineering mistake to prioritize aesthetics over good engineering design. That said, you can often have both -- an aesthetically pleasing appearance that uses fundamentally good engineering design.

    So in this design, I would probably go with tubular cross-supports, instead of horizontal flat ones. The strength/weight is *much* higher in the vertical direction, and it will still have an artistic appearance (especially if you choose the colors and finishes well). My two cents...
     
  4. Aug 1, 2017 #3
    Thanks! I prefer drawing things out before buying the supplys :P saves alot of headache!

    As far as tubular goes, While I do see your point in that it is significantly stronger, It would be difficult to fasten the lights to the tube. Would probably require a hole to be drilled entirely thru the pipe and then made larger on one side to accommodate the screw head. Also, I am hesitant to use tubes simply because it wouldnt match quite as well aesthetically. I am inclined to say that the increased weight of using strong enough bar stock would be a fair trade off in this instance. However, I am curious and will see about making some drawings to try and visualize the tubes.
     
  5. Aug 1, 2017 #4

    berkeman

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    That's all I can ask. :smile:

    I was thinking that if the light bodies are a dark matte finish, then chrome tubes (about 3/4" or 1" OD) with black hole plugs on the ends would make for a nice contrast.

    Also, do you have access to the inside tops of the light bodies? If so, you can drill holes through the tops of the bodies and just drill and tap holes in the undersides of the tubes to line up with those holes. That would make for a pretty clean and strong arrangement...You might even be able to get away with running your suspension cables through the tubes and hanging the fixture from the ends (with small holes at the tops of the black hole plugs on the ends. Angle the cables outward slightly to give better stability to the fixture...
     
  6. Aug 1, 2017 #5

    Nidum

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    Gold Member

    Maybe inverted T aluminium extrusion ? You can buy this from many sources and in a wide range of sizes . Some sizes are available black anodised .

    upload_2017-8-1_23-6-13.png
     
  7. Aug 1, 2017 #6

    berkeman

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    Staff: Mentor

    BTW, I hope that the structure under your aquarium in your drawing is made from some pretty beefy welded steel rectangular tubing. If that's wood, your light fixture will be illuminating a mess on the floor pretty shortly after you fill the tank, IMO.

    https://giantaquarium.files.wordpress.com/2013/03/img_0720.jpg
    img_0720.jpg
     
    Last edited: Aug 2, 2017
  8. Aug 1, 2017 #7
    Yah, Ill look into that. It's an interesting idea. Unfortunately I really dont want to tear into the housing of the lights, and If I remember correctly the threaded holes in the lights are tapped directly into the housing.

    The issue I could see with the T is both that I would have a hard time drilling the holes to run the mounting screws through, and that it isnt quite as low profile as I would like. Wouldn't angle stock be nearly the same strength as well?

    Yes infact it is made out of some really hefty 2in square stock and really nice welds. I figure it could easily handle more than twice the weight of the tank. currently the tanks is already on it and full, and its extremely sturdy. I really didnt want that sized mess. Total system volume is about 200 gal, and that much saltwater would do a bit of damage if let loose in a house. :wink:
     
  9. Aug 2, 2017 #8

    JBA

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    Square tubing might suit you better it will be easier to bolt to your fixtures and will be stronger in bending than round tubing; and, you can get decorative end inserts.
     
  10. Aug 3, 2017 #9
    True enough. So im assuming the general consensus is that most any flat stock will be unable to support the light without flex?
     
  11. Aug 3, 2017 #10

    Nidum

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    Have you considered using the support cables more effectively ? You could design a very elegant catenary suspension which would only require the bars to space the lights .
     
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