The design process consisted of five steps. The first step involved using the PMKS toolbox to perform quick linkage analysis, which proved very helpful in designing for a specific desired motion path. Based on the analysis, a general framework for the design was created. It was decided that a four-bar linkage would be used, and a ratio of arm lengths was selected such that the input link would have continuous rotation that would then force the output link to have a quick outward stroke for throwing, followed by a smooth and controlled backstroke to ready the arm for another throw.
Once this general framework was chosen, the second step was kinematic analysis to determine the specific link lengths that would produce the furthest throw.
The third step of the design process involved creating a proof of concept build for the catapult. This was achieved by creating a CAD model of the links and their connections, along with a base plate, with an emphasis on manufacturability. The parts were then laser cut and 3D printed before being assembled to create the first prototype and proof of concept.
Figure 1: Proof of concept build
Following the success of the proof of concept, the final design process began. It started with designing the launcher around the motor specifications, which stated that the operating speed was 1600 rpm. To achieve an input link speed of 10 rpm, a gearing of 160:1 was required. After considering various options, the group decided to use a planetary gearbox due to its ability to provide large gear ratios, while also having a planar output for the arm portion of the gearbox. To implement this, the group decided to use the arm of the planetary as the input link for the mechanism. To achieve a 10:1 ratio, a planetary gearbox was designed using an online tool shown below.
Figure 2: Planetary gear toolbox (https://www.thecatalystis.com/gears/)
After finalizing the design of the planetary gearbox, a modular two-step gear train was chosen to transmit power from the motor to the input of the planetary gearbox. This modular design enables the entire system to have a gear ratio ranging from 10:1 to 160:1, depending on the gears placed into the gear train. This modularity proved to be extremely valuable when it was discovered that the motor actually operates at 400 rpm, rather than the previously assumed 1600 rpm. Once all the gears and arm lengths were selected, the design process moved to CAD where the mechanism was fleshed out and prepared for manufacturing.
Figure 3: CAD model of final design gearbox