This blog will follow the progression of using additive design and manufacturing processes to build a desktop sized trebuchet to launch M&Ms. The trebuchet will incorporate strategies learned in ME 514 at the University of Wisconsin-Madison and will compete with another group in a battle of longest toss of an M&M.
A trebuchet is a device that uses gravity to launch a projectile large distances. A famous example of this is the “Punkin Chunkin” Competition (https://www.youtube.com/watch?v=1iy7y8KgkwY). A trebuchet is composed of several important parts:
-Support Stand: This is what holds the entire assembly. It must be strong enough to support the weight and forces exerted on it throughout the throwing process.
-Counterweight: A heavy object, or group of objects that provides the energy to launch the projectile via gravity.
-Throwing arm: The arm transfers the energy of the falling counterweight to the projectile being thrown. It acts as a lever using mechanical advantages associated with levers to transfer the energy.
-Sling: This holds the projectile and controls the release. Ideal release points for maximum distance of a projectile are around 45 degrees. The sling works by being rigidly attached to the arm at one point and loosely attached at another. When the loosely attached point slides off, the projectile becomes free-flying. For this reason it is important to make sure the sling is adequately designed.
For this project the two competing groups decided on the following restrictions.
1) Projectile: peanut M&M
– This was decided as it was a fun, recognizable object that would likely be available in a desktop setting. It also is very light and would result in impressive throwing distances.
2) Max throwing arm length: 25cm
-25cm (~10in) was selected somewhat arbitrarily as it was deemed a good size for something that will sit on a desk.
3) Max counterweight: 0.5kg
-Once again, somewhat of an arbitrary number to give good throwing distance but still be suitable for sitting on a desk.
The first concern we are going to address of mechanical design for performance, is optimizing the different lengths in the throwing arm, in order to maximize the usage of the available energy. The arm works like a lever with mechanical advantage. The further the throwing portion of the arm is from the pivot, the larger the mechanical advantage, but also the larger the amount of inertia that needs to be overcome. We will attempt to optimize this.
In regards of the printing process we have to select the right parameters:
1) Given that there is a load that travels along the arm the build orientation and bead deposition orientation are crucial. The build orientation will need to be such that axial stresses will be transmitted along the beads rather than perpendicular. This will allow for maximum strength and minimum chance of bending/breaking.
2) We must account for possible stress concentrators and redesign the part in order to avoid them. Since we are using additive manufacturing this is relatively easy to account for however. Simply adding more material where the stress concentrations are likely to occur (around the holes in the arm) will provide us with the strength we need, while barely increasing the overall weight of the assembly. This is one of the biggest advantages of additive manufacturing, it allows us to optimize our design without the need for subtractive processes and wasted material.
