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Robotic Wrist – Yonder Dynamics Rover Arm:
The differential wrist is a compact, high-torque, dual-axis mechanism enabling both pitch and yaw actuation for the rover arm. Powered by two Cubemars AKE60-8 brushless outrunner motors, each providing 5 Nm of continuous torque, the system uses 2:1 HTD 5mm belt reductions to drive custom hypoid bevel gears. The design supports a 5 kg payload with a factor of safety (FoS) of 3:1 for applied torque and a gear FoS of 3.0, as each gear is rated for 30 Nm. Precision feedback is provided via embedded O-Drive magnetic encoders, controlled by dual ODrive motor controllers for high-resolution.
Design Process:
After three major virtual iterations, the final differential layout emerged when the initial yaw-linkage design was scrapped. The goal became a modular, self-contained wrist that could be removed or serviced independently of other rover components. The main structure is a CNC-milled 1x2” aluminum tube stock housing embedded encoders, while the motors are mounted to laser-cut ⅛” aluminum plates spaced by 1.1” standoffs.
The T-shaped shaft dead axle design enables differential control:
Both gears moving in the same direction and speed generates pitch motion
Gears moving in opposite or different speed will generate a roll motion
Manufacturing:
The tube stock's holes and cut out were manually milled, its curved elements were CNC milled using Autodesk Fusion to create the tool path. The motor mounting plates were laser cut out of 1/8" aluminum. The T shaft is made up of two 0.5" inch shafts lathed, tapped, and then milled to create a flat surface to attach them to each other. The bevel bears and pulley were printed out initially tough PLA then switched to CF-Nylon on a Mark Forge 3D printer.
Assembly:
Assembly uses standard M3 and M4 fasteners, and 0.5” ID bearings. Each bevel gears utilizes two bearings to spin freely on the shaft. The main structure uses two bearings to constrain the shaft. The Cubemars motors are fastened to the motor plates using M3 screws, the pinion pulley uses M3 screws to attach to the motor. Then HTD 5mm belts are attached to the pulleys.
Additional Photos
Hypoid Bevel Gear Design:
The wrist’s torque is transferred via custom hypoid bevel gears, selected for their ability to minimize backlash and withstand higher shear forces than traditional bevel gears. While standard bevel gears typically engage in a single plane of contact, hypoid gears maintain three simultaneous planes of contact, distributing force across a broader surface. This results in superior shear handling, especially useful for 3D printed components and contributes to longer gear life. Additional the spiral pitch pattern is made possible by the 3D printed manufacturing process.
Each gear is rated for 30 Nm, yielding a factor of safety (FoS) of 3.0—well above the wrist’s expected torque demands. The entire hypoid gear system is mounted on a dead axle embedded within a T-shaped shaft, forming a rigid backbone for dual-axis actuation (pitch and yaw).
The gears were designed in Onshape using a custom FeatureScript, allowing for precise control over the spiral geometry and offset parameters. Once the hypoid form was complete, the geometry was boolean-merged into another custom FeatureScript that generated the HTD 5mm belt profile, ensuring perfect mesh alignment between the motor pulley and gear body while maintaining a printable, monolithic structure for strength and simplicity.
MATLAB Analysis:
To validate the hypoid bevel gear design, a custom MATLAB script was developed to estimate gear stresses and confirm safety margins. The script calculates both contact stress and bending stress using standard AGMA equations, factoring in real-world variables like dynamic loading, material properties, load distribution, and gear geometry. Key modifiers such as size corrections, crown factor, curvature factor, and reliability factor were included to ensure conservative and robust results. Geometry factors for spiral bevel gears were approximated using AGMA design charts, and the analysis verified that the gear design maintained a safety factor of 3.0, supporting the move from PLA prototypes to CF Nylon production parts.
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