Planetary, Harmonic or Cycloidal? Choosing a Reducer for Your Axis
Backlash, torque density and cost decide most reducer choices. A short comparison of the three types and the specification points that actually matter.
Three reducer types cover most industrial motion axes. The choice usually comes down to backlash, torque density and cost rather than brand.
Planetary gearboxes
The default for general positioning. Efficiency is typically above 90% per stage, and backlash sits in the 3–10 arcmin range for standard units, lower for precision grades. They are compact, available in ratios from about 3:1 upward, and mount directly to a standard servo or stepper flange.
Harmonic (strain wave) reducers
A wave generator flexes a thin-walled flexspline inside a rigid circular spline, producing very high ratios in a small package with near-zero backlash. Torque density and repeatability are excellent, which is why they dominate robot joints and semiconductor axes. They are less tolerant of shock loads and cost more per unit of torque than a planetary unit.
Cycloidal reducers
A cycloidal disc rolls inside a ring of pins, so load is shared across many contact points at once. That gives very high shock resistance and high ratios in a single stage, at the cost of size and weight. They are common on heavy-duty axes, positioning tables and larger robot joints.
What to specify
- Continuous and peak torque — size on continuous, then verify peak including acceleration.
- Ratio — the reducer multiplies torque and divides speed, and reflected load inertia falls with the square of the ratio.
- Backlash — matters for bidirectional positioning, much less for unidirectional indexing.
- Radial and axial load — a pulley or pinion on the output shaft loads the bearings far more than a coupled shaft.
- Duty cycle and life — continuous running at rated torque is not the same as intermittent indexing.
Common mistakes
Sizing on peak torque alone, ignoring the radial load from a belt pulley, and accepting more backlash than the positioning accuracy requires. Each of these shows up later as either a damaged gearbox or an axis that will not hold position.
Send us the axis requirements and we will size the reducer against your duty cycle, including the reflected inertia seen by the motor.