2019-2020
Bevel Gearbox
Cornell Baja
Scroll ↓
During the 2019-2020 season, the competition organizers wanted to challenge teams to bring more fresh designs to the competition. Thus, they incentivized us to design a four-wheel drive transmission in exchange for a considerable 450 point bonus. Although the change was late-breaking, the points were too significant to ignore, and it made sense to pivot our design since four-wheel drive would be mandatory in future seasons.
As a team lead that year, I played a large part in architecting the new four-wheel drive system, performing R&D on alternative methods, and analyzing the effects on other subsystems (which led us to pursue four-wheel steering as well!). One of my R&D efforts was the design of a bevel gearbox, which is commonly used with a propeller shaft to transmit load across the span of the vehicle. This was ultimately not used in the final competition vehicle, but still served as my senior design project and led to future learnings for the team.
Design Requirements:
The requirements for this design were similar to previous gearboxes we designed. It needed to withstand our vehicle loads and package within the Baja vehicle while optimizing performance. The design also featured some unique constraints, like redirecting power by 90 degrees. The full list can be seen below:
Constraints:
Withstand a specified output torque
Use a 1:1 gear ratio
Redirect power transmission by 90 degrees
Package within the full vehicle
Must complete design within truncated timeline
Objectives:
Minimize mass, inertia, and volume
Ease of manufacture
Ease of assembly
Minimize cost
Gear Selection:
To redirect power transmission by 90 degrees, there are only two viable and common options: straight-cut and spiral bevel gears. Other options like universal and constant-velocity joints can redirect power, but not at sharp 90 degree angles. Straight-cut bevel gears are simpler and more efficient due to the direction of the load, while spiral are quieter and more wear resistant due to their gradual contact. Since performance was the number one priority, straight-cut bevel gears were selected.
Since the design would need to be completed in less time and manufactured alongside many other new systems, I made the decision to bound the design to use off-the-shelf gears. This drove the material to be AISI 1045, since off-the-shelf gears are typically only available in medium carbon steels compared to low carbon alloys. Thankfully, this did not have a significant impact on mass or volume, since the material can be hardened to 40 HRC.
From here, I wrote a MATLAB script to read a spreadsheet of existing gear pairs from online component catalogs, perform AGMA strength equations for bending and contact stress, and sort the output gears by mass.
Bearing Selection:
The gearbox features two bearings for each shaft. One is straddle mounted, since this will better support the reaction moments of the gears, while the other is overhang mounted due to packaging constraints with the opposing shaft. The differences in reaction loads is reflected in the size ratio between the bearings on each shaft.
In order to properly size the bearings, I created a 3D free-body diagram and MATLAB script to calculate the reaction forces at the bearings. Compared to previous gearbox designs, which transmitted purely radial loads, bevel gears also feature an axial component that needs to be considered.
To withstand this axial load, I initially researched thrust, tapered roller, and angular contact bearings. Each of these come with their own drawbacks in weight, complexity, and serviceability. However, while deep groove ball bearings are meant to support radial loads, they can also withstand a thrust load of half of their static load capacity. The loads on my gearbox were low enough where deep groove ball bearings could be used, so I chose this option since it was lighter and simpler. A full trade table can be viewed on this page.
Housing Design:
The gears are enclosed in an Aluminum housing, which features three tabs to mount it to a vehicle interface. The housing consists of a central case and two keyed endcaps, which features bore seals and radial shaft seals. Finally, the gears operate in an oil-bath, which is filled from a dedicated inlet port. The operating temperatures on the housing did not necessitate a transmission breather or gore vent in past gearbox designs, however it would be something to consider after testing.
Alternate casing designs were considered for this project as seen on this page. The first was a two part housing split across the middle, which was eliminated since it would make the manufacturing of bearing surfaces more difficult and gave no functional or assembly benefits. The second was a single part housing with a top cover plate which would have likely weighed more and made serviceability far more difficult with the many snap rings and spacers used on the shafts.
The final housing design (left) and two alternate designs
Analysis:
Finite element analysis was conducted in order to understand the integrity of the housing. Mock shafts and were modelled to properly represent the bearing stresses on the housing, and beam elements were used on the endcap bolt patterns. Aluminum 6061-T6 resulted in a 1.6 safety factor on the housing, and allowed me to use the minimum finger guard thickness of 0.12” to minimize weight. Bolt strength calculations were also done on the fasteners, and the gap monitored on the end caps.
Lastly, the shafts themselves were analyzed in separate sub-models. These results showed a high correlation to torsion stress equations, and drove the need for AISI 4340 hardened to 38-40 HRC as the material.
Integration and Conclusion:
Many concepts were mocked up for how and where a bevel gearbox would be integrated into the final vehicle. This included trade studies for how the engine would be oriented, the number of gearboxes required on the vehicle, and where the bevel gears would be relative to the other transmission components among other aspects. This is an area that I cannot share the full details on, but generally we found the size of the bevel gears to be one of the more flexible aspects of the design compared to others. Although, if this design was in the final vehicle architecture, I would have altered the ratio from 1:1 in order to include some additional reduction in these components.
Overall, the bevel gearbox design proved to be a viable and clear path for the team. While we chose to pursue another option that year, the Baja team has since pivoted to designs like this using the foundation shown in this here.