The next step for the design of our desktop trebuchet is to produce an initial design and CAD files that can be converted into STL files for printing.
Using inspiration for generic trebuchets online we developed our own style for this trebuchet. Most of what we found were 10-20 foot trebuchets so we will attempt to scale this down and change what we need to. The advantage of this is that most size ratios should scale very similarly and we know the design works. Completely inventing our own design we run the risk of it not working at all. Simply copying a design we found would work, but the point of this project is to have fun, and to win a competition. Having our own design allows us to claim victory for ourselves and not need to attribute it to someone else.
Several potential designs for the trebuchet were generated, with the differences being varying arm length ratios and finger angles and we produced CAD models for each of these. We singled out the ratio of important lengths on the throwing arm as major factors in the performance of our device. We also discussed designs that would be easier to produce using additive manufacturing than standard manufacturing. We also discussed attaching the sling and various options for the “finger” that acts as the release mechanism. These discussions are explained in detail below.
Important discussion conclusions made this week:
1. We determined the optimum throwing arm-to-counterweight length ratio to fall between 3:1 and 5:1. This was found online for many large-scale trebuchet designs. Since we are simply going to scale down a large design, the ratios should fall in a similar range. An arm was produced with pivot holes located at points that give us ratios of 3:1, 4:1, and 5:1. This will allow us to test at each point and determine which is the best at the scale we are working at.
2. Instead of producing a straight rectangular arm as is seen in most large scale designs online, using additive manufacturing we can contour the arm so material is only where it needs to be. This is no more difficult than producing a straight arm using AM since all we have to do is generate a corresponding CAD file. In fact since less material would be used, it would take less time to produce a contoured arm than a straight one.
3. We decided our throwing arm would be made in two separate pieces to allow for rapid prototyping of the release mechanism: the main arm and the release piece. The main arm will be produced with multiple pivot points at ratios of 3:21, 4:1 and 5:1 to allow us to test what ratios work best at the scale we are working at. The release piece will have a small extrusion (finger) on the end of the arm that holds half of our sling in place pre-launch. Once the trebuchet swing upward, this string slides off the finger, releasing the M&M. The angle of this finger determines when the string slides off, and thus the angle of release of our M&M (45 degerees is ideal). We will produce several release pieces with various release mechanism angles. This allows us to test at not just different arm ratios but also different angles for the release of the sling. We chose 0-25 at 5 degree increments to initially produce.
3. The design work that goes in to the support structure is much simpler than the work put in to throwing arm and will be designed after we work out the mechanics of our throwing arm/sling. This will allow us to focus on the core issue relating to how far we can throw our test piece (M&M)
Below are images of our models for the arm and one of our release pieces:
The leftmost hole is for the counterweight. We added additional material to this area since it will undergo the largest stresses with the counterweight swinging freely from the hole. Moving to the right there are 3 more holes. These are our pivot points at ratios of 5:1, 4:1, and 3:1 respectively. We will test throws at each of these ratios to determine which maximizes our distance. Lastly, the end of our arm has a rectangular hole in it. This will be used for attaching various end-pieces for examining different finger designs.
This is an example end-piece that we will insert into the arm for testing. The rectangular protrusion on the left end is meant to be a snug fit into the arm. The small hole in the bottom right corner is our “fixed” point for the sling. The hole on the right end of the attachment is where we will attach our finger (probably a small piece of metal). This is what was varied on each attachment. The hole is angled differently for each part which will allow the metal finger to protrude at slightly different angles (0-25 degrees) and will allow us to test various parts. Most of what we found online showed fingers at angles near zero and this is why we chose our range. We went up to 25 because we thought friction may have a larger or smaller impact that doesn’t scale linearly with size.
References:
http://tuhsphysics.ttsd.k12.or.us/Research/IB07/CarpTorb/index.htm