I’ll start with the last 3 questions then get into the design process.
What design tools did you use?
The whole gearbox was completely modeled in Inventor and then individual aspects were analysed in Solidworks. The reason for the two programs is because the team models in Inventor, however Inventor struggles with FEA while Solidworks does not. Additionally I used the gates technical manuals a lot as well as their belt theory manuals to get a strong understanding of how the belts preform.
How were various iterations evaluated?
The competition gearbox is actually about the 8th iteration of the gearbox, the 7th iteration was the one that was built in the fall as the prototype. Several of the earlier iterations were improved on so quickly in CAD that they weren’t fully completed before moving onto a updated design. Most of the evaluation of these early iterations was done almost entirely by myself with a little input from Joey Milia. Once the 6th iteration was completed in CAD it was more formally reviewed by Joey and a few other mentors and members of the team. The design was heavily assessed for, manufacturability, ease of assembly, ease of maintenance, reliability, cost, size, and weight.
After the corrections, parts of this 7th iteration were tested in solidworks using FEA and the gearbox was manufactured to do physical testing. The changes between the prototype and final are subtle, mainly changes in motor placement, gearing, and the shifting shaft profile. The shifting shaft continued to be tweaked nearing the end of build as I saw how they performed.
What recommendations would you give other teams interested in designing their own gearboxes?
My main piece of advice would be to have a clear goal of what you want the gearbox to do and know what having this custom gearbox would let you achieve. If the taxing on the team’s resources outweighs the benefits, don’t waste your time; there are a lot of good gearboxes you can just buy and I’d suggest just buying one of those.
If you do decide that a custom gearbox is beneficial I’d recommend you make really a good layout sketch that has all the parts in your gearbox. Having a sketch with every element of the gearbox on it makes transitioning into 3D, and quick adjustments to the entire design, very easy, speeding up the iterative process.
From here you can base all of your parts off of these one or two layout sketches. That way, if you make any changes in the sketch, all the parts update so you don’t have to remake the entire part.
For example, here is the layout sketch of the build gearbox:
I’d like to hear more about the design process that you went through.
The best place to start would be the goals for the Drivetrain. For the past couple years 192’s main goal has been space efficiency, and sometimes that was at the cost of power efficiency. For this year I wanted the gearbox to both, have a smaller footprint than it has had in the past, and not have the inefficient right angle stages we’d used in the past. To achieve this I looked to combine techniques used by others and that we had used previously. I drew on, 971’s gearboxes that place the motors over the wheels, the VEX ball shifter that reduced the size of a two speed gearbox, and the use of belts and placing the motors on top of the gearbox that we used in 2013.
While I was deciding what options to pursue to reach the goals I made the below matrix of the possible ideas. I highlighted the possibilities I liked in red and added +1J to the options that Joey approved of.
I used some of the highlighted/+1J ideas as a goal for the design I wanted to prototype. (Note these were only things I wanted to do, and it was completely acceptable to cut some if they proved to make the design too bulky or raised other problems.)
Once the general aspects of the gearbox had been established, a modified version of JVN’s design calculator was used to determine the final gearing. I was careful to use only gears available in aluminum from WCP or VEX and belts and pulleys available from SDP-SI.
The next step is the actual design of the gearbox. Moving into inventor and laying out all the parts of the gearbox and playing with geometry. This sketch included everything, gears shafts, bearings; everything that would affect geometry. From here the design process moved along like I described in the question about iterations. I found a layout that worked and made lots improvements from there.