The next builder’s meeting at TheLab is Thursday, October 6 at 7P: http://www.meetup.com/TheLab-ms/events/233968782/
Using advanced wheel technology we will be able to make it mobile without too much effort. Since the energy available is mass * gravity * height, we can either double the height or double the mass of the counterweight to get roughly the same increase. Stacking lots of weights on a 2" steel bar seems easier than making something 20 feet tall.
The approximate goal is launching a 5-10 pound projectile 100+ yards
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Anyone have any guesses on how to estimate the load the bar holding the weight will have to withstand?
When the weight reaches the bottom it’s going to be decelerating very quickly, and transferring all that kinetic energy into accelerating the arm. A36 hot rolled steel rod has a yield strength of ~36,000 psi, how large diameter of a rod is needed to ensure it won’t bend when 750 pounds is dropped from a height of 7 feet? (the rod has a stickout from arm to center of weights on each side of around 12 inches)
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Probably the strongest force will be if it hits a stop to launch object. It will be a Max V and therefore with max energy. If no stop it will just decelerate. Agree with weight verses height … more weight. Easy to add bar bell weights. Plus people can loan for project and no cost involved.
Don’t use yield strength - this is what it takes to stretch the material. Bending moment and elasticity more relevant factor. Way too long since statics and dynamic to do calculation. Could barely do it 40+ years ago.
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are you sure you want the info to this solution? are you sure you don’t want the number, spacing, and type of spring to catch the bar instead? if you want the former, will the weights be falling straight onto grass or some sort of cushion? will the diameter of the bar be constrained by the type of weights employed (e.g. gym weights have a set inner diameter unless they’re milled)?
i’m glimpsed through the project rules on github specifically for constraints and didn’t see a section specifically addressing build constraints. can you please give me the rundown of what we’re going for… how tall? made of what? i’m confused if we’re going for distance or if this is some sort of battlefield thing
I could be wrong, but I’m pretty sure this is in response to the challenge from thelab.ms folks:
the rules, etc. are linked in this thread, as well.
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We are competing in this
The idea is to build a version of this: http://www.instructables.com/id/Floating-Arm-Trebuchet/
Looks like they are using a 2 to 2.25" diameter steel rod to hold the weights, as the drawings on that page show a 2.5" wide slot
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thanks, i will take my weekend to look through everything. once i’m caught up to speed, i will begin working on the original question
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It is now Wednesday, October 5. Looks like we have another moving target…
Okay Bill, are you a spy for the Plano rats dawgs or the Noble DMS imperialist?
more like a double agent…
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I’m assuming 3pm sunday is still a good time to meet tomorrow.
We need to iterate on the prototype to get it firing reliably (better weight axle mount, decrease weight fall gap, actual sling) so that we can experiment with different arm and sling lengths.
When we were kids, my brothers and I would go around singing, “Secret Asian Man.”
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I am going to busy helping to hang up 10x10 exhibits.
I’ll be at the space making stuff, and should have time for 3pm meeting.
I have a class at 5 to teach tho, but sounds like we are on a roll after building the 2x4 prototype. They really do help
Made some improvements and it now fires a grapefruit sized object (.4 kg) ~150 feet. Replaced the weight holding axle with 1/2" iron pipe (was 1"), decreased the width of drop slot, reinforced and aligned rails, used some 1/8" steel bar stock to make plates to aid in axle to arm mounting, new sling, added 3rd wheel inside rail to allow weights to be raised higher.
Precision wood working
Also the wheels being used are from casters and so they had a curve to the top, flattened them out on the lathe
Glow stick + wall wart transformer make a good night time test projectile
Video of test fire:
Next we need to start experimenting with different variables to get some data before starting the full scale build, things like projectile weight by filling water balloons to different sizes, sling length, hook angle, arm length, arm to pivot ratio, etc
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Anyone have any electric solenoids they could donate to this project? Or any ideas on a good trigger? Need something better than the 2x4s in video which takes two people to arm, the requirements would be:
- As weight bar is raised, it must automatically latch and secure the bar (so that a single person can load and arm it)
- There needs to be a mechanism on each of the uprights holding each side of the bar(to avoid twisting arm, and dont want a string on the end of the arm because then the arm is under tension from the weights)
- Each side needs to release near simultaneously
Initial idea:
The question with initial idea is how to pull out the pin that would be holding the arm up, using bicycle cable and spring could work, but mounting two solenoids seems easier
The latch/trigger on a Trebuchet is subject to a LOT of force. I suggest you’ll adhere to the KISS principal.
For this scale each latch will just need to hold ~5 pounds (the ratio of pivot to arm to latch means most of load will be held by the pivot point that is closer to the bar), these car power door lock actuators seem too cheap to not try