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| Beginning |
We started by naming our Lego trebuchet, Grond, after the Uruk hai's fire-breathing siege machine in the battle of Minas Tirith. Unfortunately, our Grond does not breath fire, it does, however, sport a "made in Mordor" tag. We started with a basic A-frame for the trebuchet. Originally, it was very weak. But then it was reinforced to the present level. The A-frame has to take the lateral force of the arm swinging, thus it was reinforced to not collapse together. The main weakness of the A-frame is the connecting axel on the top that must bear the weight, recoil, and the two opposing A-frames. There was, however, no other way to build with Legos the Trebuchet. Next we worked on the base to hold the A-frame. This base is rather basic, but our main problem came with securing the A-Frame into the base. Since Legos are designed to fit at 90° , an A-frame has inherent issues, it does not connect at 90°. We got around this by building up around the base of the A-Frame so it cannot move much. The basic design was now complete. |
| The Winch |
First Try: Ahh the winch, for which there is no true Lego equivalent. A winch consists of two motions, one in which tension is slowly increased by winding up the rope, but is unable to slip as a locking mechanism is engaged and two in which said locking mechanism is disengaged and the rope runs free. Unfortunately, while we are easily able to simulate motion one, the coiling of the rope, motion two is quite impossible. Gearing: First we tried four motors in conjuction with a gear ratio of 1:5. This worked great for coiling in the rope, but not for releasing. Basically, the winch was unable to release fast enough for gravity to take over the counterweight and hence there was not enough force to launch the projectile from the sling. We then fixed this problem by adding a mechanism that pinches the string after it has been coiled. Then the winch can unreel without the tension being lost. Once it has been unrealed, then the mechanism can unpinch and the trebuchet can swing freely. See Mechanics for a more detailed explanation. |
| The Sling |
Sling:We tried creating slings out of various materials. First, we used a lightweight fabric which we cut into various shapes. This was tied by a string to the front of the arm and the idea was that it would be launched once the arm reached it's full height. Unfortunately, the sling was too heavy. Finally we settled on using Legos attached to the arm by the grey technic conectors. The only problem with these is the projectile gets caught sometimes. See Mechanics for a more detailed explanation. |
| The Projectile |
Weights:We have tried several different weights for the projectile. First off, the projectile must have enough mass to fly, but not too mass that the counterweight cannot throw it (the idea is that the counterweight should be 100 times the mass of the projectile). Also, the projectile must be small enough to fit in our slings and as frictionless as possible so it doesn't get caught while being thrown. We settled on as small ball that has enough mass to fly but not too much and since it is spherical, it has minimum friction. |
