Harmonic vs planetary vs cycloidal reducers

By Tolerance Labs engineering team · Updated 13 October 2026

Most robot joints use one of three reducers. Each makes a different trade between precision, stiffness, efficiency, shock tolerance and size, and the right one depends on the joint.

How each reducer works

Planetary. A central sun gear drives several planet gears that mesh with a fixed ring gear, and the planet carrier is the output. The planets share the load. One stage typically gives 3:1 to 10:1, and stages are stacked for higher ratios. A single low-ratio stage is the basis of most quasi-direct-drive joints.

Harmonic (strain wave). An elliptical wave generator flexes a thin toothed cup, the flexspline, so its teeth engage a rigid circular spline at two zones. The flexspline has two fewer teeth than the circular spline, so it shifts by two teeth per turn of the wave generator. One stage typically gives 30:1 to 160:1. See harmonic joint modules.

Cycloidal. An eccentric input drives a lobed disc around a ring of pins. The disc has one fewer lobe than there are pins, so it turns backward by one lobe per input turn, and output pins pick up that slow rotation. A single stage typically gives about 10:1 to 100:1. RV reducers put a planetary stage in front of a cycloidal stage for higher ratios. See high-ratio and cycloidal joints.

Comparison table

Values are typical ranges. Individual products vary with size, grade and preload.

Backlash and stiffness

Backlash is lost motion when torque reverses. It matters most at the end of a long arm. One arcmin at the base moves a point 1 m away by about 0.29 mm (1/60° = 0.00029 rad, times 1,000 mm).

Stiffness is how much the output twists under load, and it matters even with zero backlash. A harmonic drive has almost no backlash, but its flexspline winds up under torque and is softest near zero torque. Cycloidal reducers are usually the stiffest of the three at a given size. Low stiffness lowers the joint’s resonant frequency, which limits control bandwidth with heavy payloads.

Efficiency and backdrivability

A planetary stage loses little power, which is why single-stage planetary joints backdrive easily and suit torque control from motor current. Harmonic drives combine higher friction, worst at light load and low temperature, with a high ratio. They are not self-locking, but backdriving one takes significant output torque. Cycloidal reducers sit between the two.

Reflected rotor inertia scales with ratio squared for all three types, so ratio separates them as much as gear type does. The guide on QDD vs high-ratio joints works through that trade-off.

Shock loads

Legs hit the ground and arms hit things. Cycloidal reducers spread load across many lobes in contact, which gives them a high momentary peak rating relative to rated torque. Harmonic drives list a momentary peak torque; exceeding it can make the flexspline skip teeth, and repeated overloads shorten its fatigue life. Planetary gears fail by tooth bending or pitting. A low-ratio planetary also passes less impact into its teeth, because its reflected inertia is lower.

Which joints use which

• Arms and cobots. Harmonic drives are common at most joints because they are compact, light and have near-zero backlash. Larger industrial arms often use RV cycloidal reducers at the base, shoulder and elbow, where loads and shocks are highest. Browse robot arm and cobot joints.
• Wrists. Harmonic drives are the usual choice. The short axial length and hollow shaft help with cable routing and keep mass off the end of the arm.
• Legged robots. Single-stage planetary joints are common at hips and knees, where impact tolerance and backdrivability matter. Larger humanoids use a mix of higher-ratio planetary, cycloidal and harmonic reducers at high-torque leg joints, and harmonic drives are common in their arms. Browse humanoid and legged robot joints.
• Exoskeletons. Designs that must feel transparent to the wearer favor low-ratio planetary joints. Designs that deliver large assist torque use harmonic or multi-stage reducers, often with a series elastic element or a torque sensor.

Checklist

• List each joint’s peak torque, RMS torque, top speed and exposure to impact.
• Set a backlash and stiffness budget from the accuracy needed at the end effector.
• If the joint must backdrive or estimate torque from current, start with a low-ratio planetary.
• For joints that take impacts, compare momentary peak ratings, not only rated torque.
• Check axial length and hollow-shaft needs for wrists and cable routing.
• Confirm efficiency at your actual load and temperature, not only at rated conditions.