Guide
Planetary vs harmonic vs cycloidal vs RV: how to choose a gearbox for robotics
A data-backed comparison of the four precision reducer types used in robots and automation: ratio, backlash, stiffness, efficiency and shock capacity, with a decision path.
Key takeaways
- Planetary: best value and efficiency for ratios up to about 100:1 (two stages); 3–8 arcmin backlash.
- Harmonic: 30–160:1 in one light stage with practically zero backlash; lowest stiffness of the four.
- Cycloidal: 11–119:1 in one short stage, ≤ 1 arcmin, excellent shock capacity.
- RV: the stiffest, with integrated main bearings for heavy robot joints.
Four reducer types dominate robotics and precision automation. They all reduce speed and multiply torque, but trade backlash, stiffness, size, efficiency and cost very differently. This guide puts the numbers side by side and ends with a simple way to choose. You can also watch each one work in the ratio explorer on its segment page.
The four at a glance
| Property | Planetary | Harmonic | Cycloidal | RV |
|---|---|---|---|---|
| Ratio per stage | 3–10:1 | 30–160:1 | 11–119:1 | 30–185:1 (2 stages built in) |
| Backlash | 1–8 arcmin | ≈ 0 (hysteresis < 1 arcmin) | ≤ 1 arcmin | ≤ 1 arcmin |
| Efficiency | 94–97% per stage | 70–85% | 85–92% | 80–90% |
| Torsional stiffness | Medium | Low–medium | High | Very high |
| Momentary / rated torque | about 2× | about 3–4× | about 5× | about 5× |
| Weight for torque | Medium | Lightest | Medium | Heavier |
| Relative cost | Lowest | High | Medium–high | Highest |
Where each design came from
Planetary gearing is centuries old and made with the same involute gear processes as any gearbox. The cycloidal drive was invented by Lorenz Braren in Germany in the 1920s. Strain wave gearing — the harmonic drive — was patented by C. Walton Musser in the United States in 1959 (US 2,906,143) and first used in aerospace. The RV reducer was developed in Japan in the 1980s specifically for industrial robot joints, combining a spur stage with a two-disc cycloidal stage.
Backlash and accuracy
Harmonic drives have no tooth clearance because the flexspline is preloaded into the circular spline; what remains is a small hysteresis loop. Cycloidal and RV reducers reach 1 arcmin or less because many lobes are in contact at once. Precision planetary gearboxes are typically 3 arcmin in one stage and 5 arcmin in two. At a 500 mm radius, 1 arcmin is 0.15 mm and 5 arcmin is 0.73 mm of movement; see backlash explained for the full table.
Stiffness and shock
RV reducers are the stiffest, then cycloidal, then planetary, then harmonic. Stiffness decides how far the output twists under load, which matters for long robot arms and machine-tool axes. The lobe designs also tolerate impact well: many rolling contacts share the load, which is why their momentary torque ratings are around five times rated.
Worked comparison: one joint, four answers
A 3,000 rpm servo must drive a joint at 30 rpm, so the ratio is 100:1.
- Planetary: two stages (10:1 × 10:1). Longest, about 94% efficient, around 5 arcmin. Fine for a gantry rotary axis, weak for a robot joint.
- Harmonic: one stage, 200-tooth flexspline. Shortest and lightest, near-zero backlash, about 80% efficient. Ideal for a cobot joint.
- Cycloidal: nearest single-stage standard ratios are 87:1 and 119:1. Short, stiff, ≤ 1 arcmin. Good for a heavy indexing joint.
- RV: 105:1 or 121:1. Stiffest, with its own main bearing. The standard answer for an industrial robot shoulder.
A simple decision path
- Heavy robot joint or turntable with large tilting moments? Choose RV.
- High ratio in a light, compact joint with near-zero backlash? Choose harmonic.
- High ratio, low backlash and shock tolerance in a short package? Choose cycloidal.
- Otherwise, for servo axes up to 100:1, choose planetary.
References
- C. W. Musser, Strain wave gearing, US Patent 2,906,143 (1959).
- ISO 6336, Calculation of load capacity of spur and helical gears.
- ANSI/AGMA 6123, Design manual for enclosed epicyclic gear drives.
Frequently asked questions
Which is better, a harmonic drive or a cycloidal reducer?
Neither is better in general. Harmonic drives are lighter and more compact with practically zero backlash; cycloidal reducers are stiffer and handle shock loads better. Cobot and wrist joints favour harmonic drives; rotary tables and heavy joints favour cycloidal or RV reducers.
Is an RV reducer the same as a cycloidal reducer?
An RV reducer contains a cycloidal stage, but adds a spur-gear first stage and crankshafts. That gives higher ratios and higher stiffness than a single-stage cycloidal reducer in a similar size.
Why are planetary gearboxes cheaper?
They use conventional involute gears made on standard hobbing and grinding machines, while the other three types need specialised profiles (cycloid discs, flexsplines) and tighter assembly.