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Trebuchet Competetion

  1. Oct 9, 2008 #1
    Hello guys, I got a nice challenge. Myself and some other people at my school are constructing a trebuchet to compete in the Kwantlen College Science Challenge. I've already got ideas for the frame construction, however a few points need to be worked out.

    Limitations on the device

    - Must fit in a 75cm box when loaded
    - No more then a 5Kg counterweight
    - projectile can be anywhere from 10 - 100

    The frame I'm going to try and build out of the heaviest materials I can get. Thus it would sink any kick from the arm and hold transfer more energy to the projectile itself.

    The trough I'm thinking of just using some curved sheet metal, the metal would be smoothed out as much as possible, then given a coat of Armour All to eliminate as much friction as possible.

    The trigger I think would work well on the arm, a simple pin release. Reason being that putting the pin on the end of the sling might cause it to veer.

    Now I come to the two main problems.

    1. The sling release. I've taken a good look around and all the designs use a slot of some kind on the end of the arm but I have not been able to locate specific plans. How do these systems work?

    To more accurately set release angles I was thinking of a string trigger release, where once the sling reaches a certain angle relative to the arm its it's own release by pulling a string which in turn pulls something that releases the connecting band.

    2. Connecting arm to the axle. Well this sounds simple, the axle can be made out of whatever. However the arm must be as light as possible while being strong enough to withstand about 5.5Kg of mass on each ends, the other forces while firing, and while having a certain flexibility.

    Whatever axle we decide to use it will be at least one inch thick, in order to both hold the weight and stand up to the torque. Drilling enough one inch holes into the arm would to allow us to adjust the ratio as needed will compromise the integrity of the arm.

    I was thinking of using a collar of some sort. One we could loosen, slide the arm, and then re-tighten. This gives us more freedom to adjust the ratio, while not weakening the arm, and having to detach the axle constantly to do so.

    Any ideas, help, and opinions are welcome.
  2. jcsd
  3. Nov 11, 2008 #2
    A trebuchet is very difficult to model and optimise. People tried recently to build full-scale models with modern software and went back to middle-age description which were hugely more efficient.

    http://en.wikipedia.org/wiki/Trebuchet as a beginning, of course.

    First, completely forget any model that has shocks. Medieval machines had only smooth movements, this is vital for a powerful machine to last. Avoid any stops.

    The sling release was based on a bent open stub whose angle determined the time when the sling was released. Again, avoid any brutal movement.

    You wanted to build heavy: pity! A light frame on wheels achieves 40% more distance. The counterweight pulls the frame and the axle forwards, giving much more speed to the projectile. The articulated counterweight had the same kind of advantage over the one just secured on the arm.

    The more you study its medieval design, the more you admire a trebuchet. Everything is optimised in it, it's very powerful and works so smoothly at the same time. Please forget illustrations in Asterix or other comics, they simply don't make sense - these would be self-destroying machines.

    I wouldn't seek any flexibility in the arm. It is slow and works with big forces. Only the sling converts it to high speed and low mass (look at the drawing at Wiki).
    So the mass of the arm isn't a big challenge, any piece of wook or tube is fine. Even easier for a small model, as you can use tall plywood or a plywood sandwich with the axle in the middle.

    The very best option would be to find a specialist and medieval engineering descriptions. The BBC sponsored a full-scale model a few years ago, this team would be perfect. Or at least, you could buy a DVD or cassette from the BBC, as seeing the dynamics of the machine is very important to understand how the sling works - this machine isn't a lever.

    The most important adjustment isn't the lever ratio but the sling-to-arm ratio. Unfortunately, it changes if you put your frame on wheels or articulate the counterweight.
    Last edited: Nov 11, 2008
  4. Nov 11, 2008 #3
    I disagree. Vector loops and newtons second law are all you need to design a well optimized trebuchet. IIRC I actually had to do this for one of my undergraduate courses. If you want maximum efficiency then look for a floating arm trebuchet design.
  5. Nov 11, 2008 #4
    Well this took a while to get some responses. Well as an update then I've gotta have it ready and built by the 22nd for the competition.

    The restrictions on sizes limit my ability to play around with certain things.

    Essentially me and the rest of the group have decided to use 2X4's for the frame, and use the longest sling we possibly can.

    I also discovered that wheeling it lets it shoot farther. This is the schematic viewed from the back. The stick people appeared after I got bored. But basically it fits the wheels I have while allowing more vertical space for the projectile to drag in.

    The main supports have secondary triangular supports on the sides.


    Right now I'm having trouble finding a good angle to put the arm in. I'm curious would having a bend in the arm cause any issues? So that it went down at a certain angle, then bent to a less steep angle closer to the bottom of the base?
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