Yonder Robotics · 2025

Differential Wrist

A compact dual-axis wrist giving the rover arm pitch and roll from two motors sharing a single dead axle. Two Cubemars AKE60-8 outrunners, 5 Nm continuous each, through 2:1 HTD belts into custom hypoid bevel gears.

Axes

Pitch + roll (differential)

Motors

2 × Cubemars AKE60-8, 5 Nm continuous

Reduction

2:1 HTD 5 mm belt

Gear rating

30 Nm each, FoS 3.0

Payload

5 kg, FoS 3:1 applied torque

Feedback

Embedded ODrive magnetic encoders

Housing

CNC-milled 1×2" aluminum tube stock

Role

Design & manufacture

Onshape FeatureScriptMATLABFEAAutodesk FusionMarkforged

Bench test · clip 1 Bench test · clip 2

The differential

The T-shaped dead axle is what makes it a differential rather than two stacked joints:

Same direction, same speed → pitch.
Opposite directions, or different speeds → roll.

Differential wrist: hypoid bevel gears, static 0.5 in T-shaft, HTD 5 mm belts, CubeMars AKE60-8

Wrist assembly: gears, belts, and the static T-shaft

Three iterations to get here

The final differential layout only emerged after three major virtual iterations, when the initial yaw-linkage design was scrapped outright. What replaced it had a different goal: a modular, self-contained wrist that could be removed and serviced without touching the rest of the rover.

Onshape tree: static wrist, differential wrist, motors, bevel gears, belts

Hypoid bevel gears

A standard bevel gear engages in one plane of contact. A hypoid maintains three, spreading load across a far broader surface, which matters disproportionately for printed parts, and extends gear life.

I designed the gears in Onshape with a custom FeatureScript for spiral geometry and offset control, then boolean-merged the hypoid form into a second FeatureScript generating the HTD 5 mm belt profile. Mesh alignment between pulley and gear body is guaranteed, and the part stays monolithic and printable.

Printed hypoid gear with its HTD pulley as one body

The same part in CAD: gear and pulley boolean-merged

MATLAB validation

Contact and bending stress from standard AGMA equations, accounting for dynamic loading, material, load distribution, and geometry. Size correction, crown, curvature, and reliability factors all included and kept conservative; spiral bevel geometry factors approximated from AGMA charts.

Result: safety factor 3.0. That is what justified moving from PLA prototypes to CF-Nylon production parts.

Manufacturing

Tube stock holes and cutouts manually milled; curved features CNC milled with tool paths from Autodesk Fusion. Motor plates laser cut from 1/8" aluminum. The T-shaft is two 0.5" shafts lathed, tapped, and milled flat to join.

Gears and pulleys printed in tough PLA for prototyping, CF-Nylon on a Markforged for production.

1×2 in tube stock in the vise, cutout milled

Assembly

Standard M3 and M4 fasteners and 0.5" ID bearings throughout. Each bevel gear rides on two bearings so it spins free on the shaft; the main structure uses two more to constrain the shaft itself. Motors fasten to the plates with M3 screws, the pinion pulley bolts to the motor, and the HTD 5 mm belts go on last.

Motor plates sit on 1.1" standoffs, the spacing that sets belt tension and keeps the two drives from interfering.