Harmonic Drive (Strain Wave Gear)
谐波减速器
What Is a Harmonic Drive?
A harmonic drive (strain wave gear) transmits torque by elastically deforming a thin-walled flexspline against a rigid circular spline via an elliptical wave generator, achieving a 50–160:1 reduction in a single stage with near-zero backlash in a compact, lightweight package. It is one of the three precision robot gearbox families alongside cycloidal (RV) and planetary drives.
Harmonic Drive vs. Planetary Gearbox
| Dimension | Harmonic drive | Planetary gearbox |
|---|---|---|
| Single-stage ratio | 50–160:1 | 3–10:1 (stackable) |
| Backlash | Near zero | Small but present |
| Impact tolerance | Flexspline is shock-sensitive | Solid gears, robust |
| Backdrivability | Poor (high friction) | Good at low ratios |
| Typical joints | Cobot wrists/elbows, humanoid arms | Legs, high-dynamic joints |
One-line verdict: choose harmonic for precision, planetary for impact tolerance and force control.
How Robots Choose
Positioning accuracy makes harmonic drives the default in industrial and collaborative robot arm joints. But humanoid and quadruped legs must absorb landing impacts and estimate external forces from motor current, where the high friction and flexspline fatigue of a high-ratio harmonic drive fall short — so legs overwhelmingly use the quasi-direct-drive (QDD) planetary route. The BXI 85/70/50-series joint actuators use a 19.5:1 planetary design balancing torque density with backdrivability. Base joints needing extreme torque often use cycloidal (RV) gears — hence the common split: RV at the base, harmonic in the arms, planetary in the legs.