Summary

A minimalist, silent, high-torque capstan drive optimized for lightweight robotic systems such as quadrupeds. Designed for ease of printing (no supports), ease of assembly, and precise motion using a simple string-wrap mechanism. Compatible with AS5600 encoders for closed-loop position control.

Tested with 220° of motion using 1.5mm 200 lb Dacron which is overkill, but I assume should be compatible with other thin high-strength cords, such a masonry twine.

Silent operation using a 3-wrap string spool
Why Capstan Drives Beat Gears
No gear noise – ultra-quiet operation ideal for stealthy or lifelike motion

No broken teeth – eliminates one of the most common failure modes in geartrains

No grinding – smooth, continuous motion with no mechanical clash

Minimal wear and tear – string and spool contact is gentle, even over extended use

Zero backlash – inherently compliant and tight with proper tensioning

High precision – ideal for closed-loop systems using encoders

Print-friendly tolerances – far more forgiving than meshing gear teeth, making it easier to achieve reliable performance with hobby-grade printers

Maintenance-free – no lubrication, alignment, or gear meshing needed

No bearings required

Features

~220° of motion from center

Designed for M3 hardware

Mounting plate for AS5600 magnetic encoder

Works with Dacron, masonry twine, or similar line

Capstan wraps rest at spool center in neutral position if assembled correctly

Print Settings
Material: PLA+ (PETG/ABS also suitable)

Supports: None required

Infill: 15–40%

Wall lines: 5 for screw strength

Layer height: 0.2 mm

Assembly Notes
Wrap line 3 times around spool; avoid crossover

Center wrap = neutral (0°); allows travel to ±110°

String should be tight enough for tension, but not stressed

Assembly uses M3 screws and drives a 6mm threaded rod (capped at one end)

Encoder mount fits AS5600 for closed-loop control

Applications
Quadruped robot joints

Cable-driven mechanisms

Low-backlash, quiet actuators

Servo replacements or grippers

Files Included
STL files for all components

Optional encoder plate

Source files available (.f3d or .step) for remixing

License
Choose what fits your goals, but CC-BY or GPLv3 recommended for open robotics projects.