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Need resource/ answer about what will bend...

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  • Need resource/ answer about what will bend...

    I built a catapult (trebuchet actually) last year for the "Punkin Chunkin" where you throw pumpkins for max distance. This year I am planning on a slightly larger one and need some advice on aluminum. We basically use a primitve crane idea to lift the weight up rather than using a hoist or whatever. See attached pic. By cranking up the cable, the beam slowly rises and brings the weight up with it, at the top, we secure the weight and crank down the beam. I used a salt-treated 4x4 last time and it was maxxed out with about 600 lbs. I was looking for a person or place to tell me what load I can use for a particular aluminum beam and a particular length lifting a weight. I know a few engineering sites but either you get no real help or about 1000 questions then no real help. I can just start buying beams and trying it, but trial -and -error can get expensive at $300+ a beam.

    I want to use a 18 foot long aluminum american standard beam 4"x.326. The load basically is pulling at a slight angle as shown in the pic, and I can provide a little more info, but I'm unsure of where to ask. The reason I'm asking here is that people who actually build things have answers, not more questions that end in a response of "it depends..." to "this might work but I'm not going to guarantee it"
    Last edited by teamtom; 11-12-2008, 10:18 AM.

  • #2
    Here's another option, and no beam needed. Why not consider taking your winch, and hooking it up at the pivot point somehow, similar to a garage door cable on the pulleys. (Think of an overhead door) If you have it wrap around a pully that can be pinned in place when needed, and unpinned when not so it can freewheel could something like that work?

    I realize with the leverage involved, you may need more pulleys or a larger cable, but it should be do-able.

    Another option might be a gear and pawl on the pivot point, with the pawl being lifted, lifting the weight. (A lever or jack may be worked in on that end to aid in lifting.)

    Back to your original question, I'm afraid I can't help you much on that one.


    • #3
      That off-axis pull is just caused by the weight of the beam right?

      I'd find the compressive yield strength of the beam's material..

      Calculate the area of half of it (get some safety factor here)... That's 4x .326 = 1.3 inches^2

      do a bit more math and you'll know if it's close to yielding under ideal conditions or not...

      I'd not use it if was less than 10 times over capable..

      Then I'd go to the supplier and pick it up by one and and make a judgement call on it... =)

      Such a long-unsupported length... small wonder engineering types won't make a call. It's not as if tebuchets are dangerous or anything...

      I'm wondering why you don't pull it down with the throwing arm of course... but it's your pumpkin chucker...



      • #4
        A heavy arm uses a lot of the energy meant for the pumpkin, the lighter the arm, the farther it throws!