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July 2025 - May 2026 | Baja SAE
Designed internals for the transfer case gearbox for the 2025-2026 Northwestern Baja SAE car. The transfer case is a 1:1, 90 degree gearbox used to transmit power from the main gearbox to the prop shaft, enabling the car to use 4WD.
Gear Selection
Gear Selection
The transfer case for the 2024-25 car, the only one our team had made before this, was very overdesigned. With this design, one of my main goals was to reduce the weight and size of the gearbox to only what is necessary.
Since our team doesn’t have the ability to custom-manufacture gears, I chose two commercially-available spiral bevel gears with a calculated factor of safety of 1.20. This maintains a good margin of safety while not forcing us to use larger, heavier gears.
Shaft, Bearing, and Keyway Analysis
Shaft, Bearing, and Keyway Analysis
CAD of the input and output shaft assemblies.
To conduct stress analysis on the shafts and bearings, I created a spreadsheet to automatically calculate factors of safety for the shafts and bearings as parameters like shaft material, components, distances, and key dimensions change in further design iterations. This spreadsheet allowed me to rapidly iterate through designs while maintaining a high degree of confidence in the success of the part.
Manufacturing
Manufacturing
To manufacture this gearbox, I mostly used CNC and manual lathes, as well as conversational mills. I began by turning down the major dimensions of each shaft on a CNC lathe, then grooving and threading on a manual lathe. Lastly, I finished each shaft on the conversational mill, adding hexes and keyways. All gear modifications (turning down the hubs and adding counterbores) were also done on the CNC lathe.
Performance
Performance
This gearbox suffered during competition because of improper shimming. When I shimmed, I adjusted the axial positions of both gears until axial play was minimized, while allowing the shafts to freely spin. However, I neglected to perform contact checks once shimming was complete. Due to this oversight, the gears were not properly meshed, and instead had significant toe contact on the pinion and significant heel contact on the gear. This caused rapid wear during the 4 hour endurance race, which was evidenced by the "glittery" oil notable when draining and disassembling the gearbox after the competition.
After cleaning the gears and re-shimming (with contact checks), the gearbox functions normally. The primary impact of this issue was rapid aging of the gears during cycling, removing an estimated ~50% of their life. However, the gearbox is still usable for future seasons!
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