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Scratch Build – In Progress Trebuchet: Update - Construction complete!

Discussion in 'Project Logs' started by crazybob, 13 Sep 2007.

  1. crazybob

    crazybob Voice of Reason

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    With the distance the counterweight falls, we end up seeing a bit over 400 lbs of force on the frame, but most of that's downward. If we decide to weight it down, we'll do some calculations to make sure we get enough on there. We're more likely to stake it down, though. The holes are already drilled for it, and we've already got washers and one-foot-long steel spikes so we can get a solid footing without damaging the frame at all. And thanks - I agree about the dangerous look; I love having the thing sitting in my bedroom.

    Thanks. SolidWorks can draw quite well in either 2D or 3D, and can also display, print, and save any 2D view of your 3D model.
     
  2. Ryu_ookami

    Ryu_ookami I write therefore I suffer.

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    why not just make one out of steel girders and launch the kids instead :)
     
  3. mvagusta

    mvagusta Did a skid that went for two weeks.

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    So did you like the solenoid idea? Another guy suggest remote control aswell! Even corded control would be cool i think :D Unless you wanna keep it traditional or something? You can always still manually flick a solenoid if the battery is dead. If you have any questions just let us know :thumb:
     
  4. crazybob

    crazybob Voice of Reason

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    I do like the idea, though I would want to use a wired control rather than risk interference firing the trebuchet when someone is near it. However, if I were going to use electronics I'd like to have designed the base a bit differently for mounting. I think we'll stick with using a pull-cord for now, but it can always be upgraded later or done differently in a second version.
     
  5. Stuey

    Stuey You will be defenestrated!

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    <-- Wants to see the throwing arm!

    Also, I believe that some of the old threads mentioned counter-balances. When that weight swings, there might be some lateral forces acting against the 1 foot stakes.
     
  6. radodrill

    radodrill Resident EI

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    Actually with the hinge point between the arm and the counterweight lateral forces shouldn't be much of an issue, maybe just need a bit of ballast; with a fixed counterweight design you'd actually want a wheeled base to maximize performance.
     
  7. mattthegamer463

    mattthegamer463 What's a Dremel?

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    Nice job on this beast, but I'm wondering why people seem to fancy trebuchets over catapults. Surely catapults can deliver more force and whip stuff farther, no? Or is it not about how far you can throw something?
     
  8. Ryu_ookami

    Ryu_ookami I write therefore I suffer.

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    its not about distance its about style and looks after all would you rather take a mankey old catapult to the party or the nice shiney bright trebuchet. :)
     
  9. crazybob

    crazybob Voice of Reason

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    Actually, trebuchets are capable of throwing much larger projectiles many times the range of catapults. The use of a counterweight rather than a spring allows a much smoother, more fluid throwing motion and less wasted energy, as well as allowing you to add power easily. Springs have limits, especially the twisted ropes used in catapults - but with a counterweight, as long as the supports are strong enough there's really no limit to the amount of power you can add. Additionally, and perhaps most significantly, the sling on a trebuchet is enormously beneficial. Without that sling, a trebuchet would be much worse than a catapult if it worked at all. Of course, you need the smooth motion of the trebuchet to make a sling work well, so putting a sling on a catapult might help your range but it still wouldn't be able to compete with a trebuchet.

    Edit: Also, they look cooler. And trebuchets don't smack part of themselves into another part with every firing like a catapult does. I don't think that's great for durability.
     
  10. Morphine-Kitty

    Morphine-Kitty Dead account.

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    Love it. Looking great.

    I might have to make on of these in my woodwork class now. :D
     
  11. mvagusta

    mvagusta Did a skid that went for two weeks.

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    Trebuchet's are much more fancier than a catpault i reckon. A catapult is almost as simple as a slingshot, and i think it's power is kinda obvious based on the size of the elastic. A trebuchet however, looks deceptively innocent in comparison.... until you see it start moving and the amplified speed from the lever arm & sling are just like "holy $h#t". So i guess a trebuchet is a bit of a trojan horse. I must admit that i could barely remember what a trebuchet was before i saw this log.

    Using a long cord to yank a pin which releases the firing mechanism is cool, that way you wont have to be standing next to it when you fire, which leaves a better view for the spectators & video camera :thumb: The video of this thing in action is gonna be cool! Maybe you have some dead pc parts you could launch? like maybe an old hdd :D
     
  12. Cheapskate

    Cheapskate Insane? or just stupid?

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    So It's not big enough to throw a dead mongolian? -bummer!
    I guess I could settle with a dead siamese cat launch.
     
  13. crazybob

    crazybob Voice of Reason

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    It won't be big enough to launch a hard drive, but there's a chance for things like CPUs. We'll have to see how our materials testing ends up; if the factor of safety on the arm is below 1.5 (the arm is only 1.5 times as strong as it needs to be for the highest load it's supposed to see), then we're very limited in our choice of ammunition. Anything that doesn't weigh exactly the same as the projectile we're designing for will place higher stresses on the components and could potentially break something.

    Remember that this is an engineering project with specific goals, rather than just a fun toy. Because of the way we're doing things, the result will be a device that does exactly one thing but does it extremely well.
     
  14. radodrill

    radodrill Resident EI

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    What's your design SF? for this kind of a project I would have gone with 1.8-2.
     
  15. crazybob

    crazybob Voice of Reason

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    Yeah, that's the range we'd like to have on the arm. However, the stiffness in the arm also comes into play, so the factor of safety may be much higher than it needs to be in order to keep everything stiff enough.

    The factor of safety on everything else will hopefully be around 5, since we just want things to be durable and the arm is the only place where weight really matters. We should be starting to make the composites on Tuesday, so we'll be doing material testing in the next week or so. After that we should be able to get final dimensions on a few things.
     
  16. Teyber

    Teyber ******

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    may i sig- quote you? i think i will and if you have any problems pm me k?
     
  17. Alaskan Assassin

    Alaskan Assassin cooling?

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    According to tests of full scale trebuchets, the projectile goes further if the trebuchet is allowed to roll when firing. At least, this is what I learned according to physics class.
     
  18. radodrill

    radodrill Resident EI

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    This is only the case with a fixed counterweight trebuchet. The counterweight provides the most power to the arm if it can drop straight down, hence wheels when the weight is fixed to the boom; in the case of a swinging counterweight (like the design in this log) the hinge allows the weight to drop straight down in the time it's accelerating the sling before releasing the projectile.

    There was actually a Nova/PBS show on trebuchets a while back; it's the Medieval Siege episode from the Secrets of Lost Empires series.
     
  19. mvagusta

    mvagusta Did a skid that went for two weeks.

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    I just had a thought.. maybe have two arms, since it will be quick & easy to change them! The "light" one for launching objects like golf balls efficiently, and a heavier duty one for handling heavier objects!
     
  20. crazybob

    crazybob Voice of Reason

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    That's a really good idea; I'll have to bring it up with my project partner. There are a few problems, but nothing insurmountable. First, each arm takes around $15 of ball bearings and neither of us has much spare cash. Second, there's almost no way we'll be able to get enough carbon fiber to make a second arm, so it would have to be something else. Finally, the release mechanism is the most complex part to build, and part of it would be permanent on each arm. Still, I'll look into it.

    Of course, this is small enough that heavy things won't go far enough to damage them much. It'd be a lot more interesting if the trebuchet were designed to throw bowling balls (a longtime dream of mine...version 2?)
     

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