25:1 Cycloidal Gearbox
Project · 2026
- Machine Design / Manufacturing
- Onshape
- FDM 3D Printing
Overview
A compact 25:1 cycloidal reducer designed for NEMA 17 stepper motors to increase torque at robotic arm joints while maintaining low backlash and backdrivability. The reducer was optimized for FDM printing and integrated directly into the arm's joint assemblies.
Design
The reducer uses an eccentric input shaft driven by the NEMA 17 motor to move two cycloidal disks that are phased 180° apart. As the disks move in opposite phase against the gearbox geometry, they create the 25:1 reduction while helping balance internal loads and smooth the motion. In this design, the outer ring serves as the rotating output, transferring the reduced motion directly to the robotic arm joint. Central fasteners and bearings keep the gearbox constrained and aligned with the motor assembly.
Design Iterations
The reducer went through five design iterations to resolve issues caused by FDM manufacturing tolerances. Small amounts of material shrinkage could introduce slip, while over-extrusion could create interference and cause the gearbox to bind. I tuned XY contour compensation and XY hole compensation until the gearbox rotated smoothly with minimal backlash while still remaining backdrivable. Backdrivability was an important requirement because the robotic arm includes a learn mode, where the joints need to be moved manually while the motors are unpowered. Another issue appeared when the gearbox was fully bolted together: tightening the fasteners caused the assembly to seize. Through iteration, I traced the problem to printed features contacting both the inner and outer bearing races, which applied unwanted axial load to the bearings. Adjusting those interfaces and clearances eliminated the binding while preserving alignment and structural rigidity.
Performance
The final reducer achieved a 25:1 reduction ratio, approximately 0.67° of backlash, and 3.15 N·m of measured output torque in a lever-arm load test. After five design iterations, the gearbox operated without binding and remained backdrivable, allowing the robotic arm joints to be repositioned manually when the motors were unpowered.
Integration
Details to follow.
Technologies Used
- Onshape
- FDM 3D Printing