Mathias Kohler

Gas Cylinder Cage

Overview

My first project as a member of Cornell ChemE Car mechanical sub team in the Fall of 2021 was to create a cage for the gas cylinder of the pressure car. The pressure-powered car had a very powerful gas cylinder that would attach to the back of the car and would tend to create such a powerful force that it would fall of the car while running. The team wanted a safer, integrated design that could slide into the compartment in the car and allow for usage of the car at higher pressures. This required many meetings with members from the other sub teams who would be using the cage for their testing, as well as design reviews with the mechanical sub team lead to ensure that it was up to the standards of the team.

Design

The first step of my design process was to hold a meeting with the Pressure sub team lead. She provided an empty gas cylinder I could use as a reference and explained the basic design requirements of the cage. In addition to the basic design requirements of safely holding the gas cylinder, I was also asked to keep the weight to a minimum as the car was already close to being overweight for competition. My first step in designing the cage was to determine the optimal height of the cage. I calculated that since the center of gravity of a cylinder is at its midpoint then making the cage be three fourths of the height of the gas cylinder would allow enough of a margin of safety to prevent it falling out. I also decided to use a dovetail joint between the sides of the cage walls as it is the strongest type of joint and it would withstand the pressure from the cylinder. You can see a closeup of these joints below in Figure 1.

gas cylinder cage closeup joints
Fig. 1: Closeup of dovetail joints

I also added two slits to the left and right sides of the cage in order to insert “arms” that would stabilize it from shaking as the pressure was released. Below is the CAD of the side and the stabilizing “arm”, both of which I made using Fusion360. Next to it is a picture highlighting both stabilizing arms attached to the cage. 

Fig. 2A: CAD model of side and stabilizing arm
Fig. 2B: Picture of both stabilizing arms attached to cage

The most challenging part of the design process was cutting out areas that would significantly reduce the weight of the cage without compromising structural integrity. It took a lot of trial and error in Fusion360 as I used the material feature to make the CAD quarter-inch acrylic and the weight calculator to see if I was making any progress. Finally, I came up with long rectangles for the sides which take less force and a small square for the front and back which take the majority of the force from the cylinder. This combination reduced a significant amount of weight, especially since we were using quarter-inch acrylic, and received the approval of the mechanical sub team lead.

Fig. 3: Final CAD design of the cage with the weight reduction areas removed

Manufacturing 

As previously mentioned, the whole cage was manufactured out of quarter-inch acrylic. Once the above CAD was ready, I took it to the Cornell Rapid Prototyping Lab along with the acrylic sheet to laser cut it. The .stl file I designed in Fusion360 and brought to the Cornell RPL to laser cut can be found below under “Resources”. Due to the thickness of the acrylic, it took several passes by the laser to cut through all of it. Once it was ready, I brought it back to the ChemE Car work room and used acrylic glue to glue the sides together. I then slipped the stabilizing arms through the slits of the cage and attached them using acrylic glue to the edges of the box where the cage would stand as shown in Figure 2B. Below is an image showing the completed gas cylinder cage attached to the car.

Fig. 4: Front view of the finish gas cylinder cage