Every humanoid build reaches the same fork: harmonic drive or planetary gearbox? Both are compact, both are used in shipping robots, and the datasheets rarely tell you which one is right for your axis. This guide is the short version we walk customers through.
What each reducer is good at
Harmonic drives use a flexspline deforming under load. They reach high reduction in one stage, near-zero backlash and very high torque density — which is why they dominate humanoid arms and legs.
Planetary gearboxes spread load across several planet gears. They tolerate shock loads and reversing torque better, cost less at the same size, and are the usual choice for lifting axes, mobile platforms and grippers.
Side by side
| Factor | Harmonic | Planetary |
|---|---|---|
| Torque density | Higher | Moderate |
| Backlash | ≤1 arcmin typical | 5–15 arcmin typical |
| Shock / impact load | Poor | Good |
| Efficiency | Moderate | Higher |
| Cost at same size | Higher | Lower |
| Typical use | Arm / leg / waist axes | Lifting, wheels, grippers |
Figures are typical industry ranges; check the specific datasheet before ordering.
Rule of thumb by axis
- Shoulder, elbow, knee, waist — harmonic: you need torque per gram and precise positioning.
- Vertical lift and wheels — planetary: shock loads and sustained duty eat harmonic flexsplines.
- Hands and wrists — either miniature planetary joints or direct-drive micro actuators, depending on force target.
- Mixed build — the standard answer: harmonic where precision matters, planetary where abuse happens.
Specs that actually decide it
Rated torque is necessary but not sufficient. Ask for peak torque and the allowed moment load, the rated duty cycle, the reduction ratio, control protocol (CANopen, EtherCAT, MIT mode), and whether a holding brake and torque sensor are integrated. Two modules with identical rated torque can differ by 30% in thermal capability.
FAQ
Can a planetary module replace a harmonic one in the same envelope?
Rarely at the same torque. To match harmonic output torque from a planetary stage you usually need a bigger frame or an extra stage, and you accept more backlash.
Do I need a torque sensor on every axis?
No. Force control is worth it on contact-heavy axes (arms, hands); position-controlled legs and waists usually do without.
Which protocols should I standardise on?
EtherCAT for high axis counts and synchronised motion; CANopen or MIT-mode CAN for prototypes and lower axis counts.
Specs quoted from manufacturer datasheets; final figures confirmed at quotation.