It's only been a week since my last update but man was it an eventful week. Last friday, March 30th, we needed to send NASA a picture of our final design so they could spot major changes come competition time. As we were cranking out work in the shop, we ran into issue after issue. I'll talk about those more later but, in short, we were not able to finish the build and send NASA a picture of our final design so we won't be competiting on the 13th and 14th. Really bummed out but we knew what we were getting into was ambitious and we set up our project so if we didn't make it to competition, we'd be okay from the school's perspective. So, time to keep trucking and finish strong. In addition to that, I've made some progress working in Creo Simulate to get good simulations done to validate my suspension design. Both of these will be discussed in this post.
The first design issue that we ran into was that the axle would hit the shocks in between the A-arms.
In the original design, the shocks ran between the A-arms to allow for the most compact design possible. The axle's hadn't been modeled and I didn't have time to model it so I had to estimate that the axle would fit. There looked like there would be enough clearance to make it work. Unfortunately, my teammate called me to the shop to inform me that the axles, would indeed, hit the axles. That, for obvious reasons, wouldn't work and needed to be solved immediately.
Our initial design had both drivers facing forwards. This meant that the pedal assemblies for the front driver would be over the suspension and the rear driver themselves would be over the suspension assembly. The easiest solution would be to swap the A-arms so the shock would mount to the top A-arm as apposed to the bottom A-arm. Now, the axles have plenty of space with only minimal changes to the suspension geometry.
The new issue is that there is not enough space for the operators. On one side of the rover, I physically couldn't fit and on the other I just barely fit. Also, the pedals hit the tops of the shocks. Easiest solution: move the pedals farther out.
What we decided to do is to remove the pedal assemblies from the tubes but leave the tubes in place. We can then slide a new tube inside the angled piece that the pedal assemblies will be mounted to. The tubes currently used are larger than 1 inch (1.25 inch and an ovular tube from the original bike frame). So, a 1 inch tube will be slotted into the original tube and will be fastened into place with Rosette welds. This will allow the shocks to maintain their mount, the pedal assembly will be adequately supported (if not, a brace can easily be added), and we will get the extra length that we need. The new issue that this could bring up is being able to still fit within the 5ft x 5ft x 5ft cube. It is possible that the extension will protrude from the cube. If this is the case, we will place a hinge where the current pedal assembly is. This will allow the pedal assembly to be collapsed inwards and no longer protrude from the frame. The chain can also be routed to be slightly slacked when you begin to collapse it so we can remove the chain during collapsing and place it back on when getting ready for use. This way we don't need to figure out a complicated tension system. If the hinge is used, a cradle will be made out of a larger diameter tubing to support the angled pedal assembly arm since it won't be welded to the vertical tube with the hinge.
Not being able to compete was a huge blow. That being said, we have a plan of action. We know the things that we need to get done, who can do them and when, and we are planning to have the last task completed by next Wednesday, the 11th. That will give us time to test and write the final report.
Creo has given me more headaches than I'm used to recently. We've had some very strange issues that I haven't run into before and I've had to find work-arounds for them. Currently, my issue is that it doesn't want to complete a mesh on my assemblies. I've changed every setting I can imagine, double checked all constraints and material definitions, yet it still throws errors. So, in an attempt to get something to validate my design, I decided to simulate a single part: the bent tubes that make up the side of the A-arms. These arms will be taking the majority of the load during use and under impact. Thus, they are the most likely to fail.
To my great delight, it worked! Creo let me run AutoGem and create my mesh and run my simulation! I set the load to be 150lbf. I did some rough impact calculations earlier in the semester based on an estimated speed, wheel size and obstacle height. 150lbf is right about the load it gave so if it can survive this, it will survive testing and competition.
Now, where the constraint had to be placed does not accurately represent how it will be on the rover. In the simulation, I had to constrain the rear face of the tube. This means that the section just past the face of the tube can have a bending moment about it. However, on the rover, we have an end-nut that takes up the first inch or so of the tube. This will distribute that load throughout that end of the tube and the max stress concentration shown in the results is likely to not be lower in the actual part.
The image above has the 150lbf load on the end. The maximum stress, the tiny red area by the coordinate system, has a stress of around 55,600 Psi. The ultimate tensile strength of steel ranges between 58,000 and 80,000 psi. Based on that, this little section is close to failure with the rest of the part being well away from failing. Again, since the end nut sits inside the first inch, or more, of the tube, that stress will be distributed throughout the beginning of that tube so I expect the actual stress to be less. Plus, the 150lbf load will be spread out between this tube and the other tube making up the A-arm. This shows that the A-arm will easily survive a load of 150lbf.
For fun, I upped the load to 500lbf. We see that the same area has the max stress. This time, its in the range of 150,000 Psi. This is well above the ultimate strength so this tube would fail. That being said, the stress could still be distributed through the end by way of the end nut and the overall stress could be reduced with the inclusion of the other bent tube. So, it is possible that the A-arm as a whole would survive a 500lbf load. But, I need to sort out the bugs in Simulate in order to run an analyses of the full suspension arm. I'm hoping to be able to work with it some more and have those full simulations by our presentation during the week of the 22nd.
To conclude, it's been an eventful week. Sadly, we won't be able to compete but we have a plan of action to finish our rover and still have a successful project. Our rover will also serve as a great platform for future teams to build on. Also, Simulate is starting to play nice so I might be able to get more, good data soon -fingers crossed-.
Stay tuned for more developments! Hopefully just as exciting but in a good way!