The Nuts and Bolts:

To the right, the basis for our winch is seen. We changed this model in two ways. First we used the old boxy ones without gearing inside. Also we made it so that the axel through the middle of the winch was extra long so it could go through Lego pieces on either side and provide more support for the winch, as seen in our photo of the Winch box.

Mechanically, the most interesting thing we created was the pinch. The pinch is what we use to hold the tension on the string once it has been winched back so that the winch may be unwound and then the pinch may be released and the trebuchet may swing freely. We did this by using two sets of two gear racks coming together to cinch the string between their teeth. The 3D design is shown on the right, as you can see one of the gear racks is fixed to the main body of the Trebuchet. The gear rack on top, however, is lowered and raised by a two motors on either end by a gear train involving two worm gears, three 24-toothed gears, a 24-bit crown gear, and one 8-toothed gear. These allow for the pinch mechanism to be engaged (i.e. lowered down till it meshes with the bottom fixed gear rack) and disengaged (i.e. raising the gear rack so that the string may run freely) without creating too much tension so that gravity cannot work with the counterweight (as would happen with the winch unrolling). The one problem that we have run into is that the two motors seem to raise or lower the gear rack at different rates, so one side moves faster than the other.

Our final problems that seem to be insurmountable are that the Lego Trebuchet requires human intervention in two parts. When the pinch goes down, due to something (we believe it is the design) it does not go down all the way, so one side must be manually wheeled down about 3 rotations. The other problem is that when the winch unreels, while the string is pinched, unless someone is providing some tension on the unreeling string, the string does not unwind truly (it just becomes looser around the winch, but it is still around the winch and thus provides tension against gravity when the trebuchet tries to work). Other than these two problems, it works quite well.

For further Lego pictures, click here.