Harmonic actuator
Harmonic Drive RSF servo actuators (17A/20A/25A/32A)
A compact solid-shaft servo actuator that pairs a strain wave gear with an AC servo motor and incremental encoder, for precise rotary motion without a hollow shaft.
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What are you specifying?
Select everything you need. A motor, drive and gearbox can be specified together as one axis.
Key specifications
| Rated torque (max) | 98 Nm |
|---|---|
| Peak torque (max) | 330 Nm |
| Supply | 200 VAC (via RTL-230-18 / REL-230-18 / HA-800-3B-200 drives) |
| Ratio | 50–100:1 |
| Backlash (max) | 0 arcmin |
| Sizes | 17, 20, 25, 32 |
Summarised in our own words from Harmonic Drive SE / Harmonic Drive LLC's published data; series maxima, individual models vary. Last checked 2026-10-29. Source 1
Notes on this data
Maxima are for RSF-32A-100: rated 98 Nm and maximum momentary 330 Nm. The page lists only rated speed (60 rpm at 50:1, 30 rpm at 100:1), so speed_rpm_max is null. 'Zero backlash' is stated; one-way accuracy is 90–120 arcsec, although the key-features text claims <1 arcmin. RSF Mini (sizes 3–14) and RSF Supermini are separate series.
Specify Motion is not affiliated with Harmonic Drive SE / Harmonic Drive LLC. Harmonic Drive and RSF servo actuators (17A/20A/25A/32A) are used only to identify the product. Comparisons use published figures; always confirm against the current datasheet.
Harmonic Drive RSF servo actuators (17A/20A/25A/32A) replacement and alternative options
Use these supply-network options as a starting point for an engineering check. We can also help source the original Harmonic Drive part. Published series figures do not establish an exact-model or drop-in replacement: confirm ratings, fit, interfaces and approvals for your application.
Hollow-shaft harmonic servo actuator: strain-wave gearing, motor, encoder and brake in one unit, run from a separate servo drive. Six frame sizes, from 48 VDC to 220 VAC.
| Published maximum | RSF servo actuators (17A/20A/25A/32A) | MAH |
|---|---|---|
| Peak torque | 330 Nm | 800 Nm meets or exceeds |
| Rated torque | 98 Nm | 557 Nm meets or exceeds |
| Speed | — | 125 rpm |
Large-bore version of the harmonic actuator with a frameless 16-pole motor, giving a much bigger through-hole for cabling. Two sizes.
| Published maximum | RSF servo actuators (17A/20A/25A/32A) | MAT |
|---|---|---|
| Peak torque | 330 Nm | 800 Nm meets or exceeds |
| Rated torque | 98 Nm | 557 Nm meets or exceeds |
| Speed | — | 102 rpm |
Harmonic actuator with an integrated German-made Synapticon Circulo servo drive on EtherCAT, with STO and optional FSoE functional safety.
| Published maximum | RSF servo actuators (17A/20A/25A/32A) | MGM |
|---|---|---|
| Peak torque | 330 Nm | 459 Nm meets or exceeds |
| Rated torque | 98 Nm | 267 Nm meets or exceeds |
| Speed | — | 108 rpm |
Figures are the highest published for each series and are not always measured the same way. A higher figure is not automatically a better choice: mounting, duty cycle, feedback, connectors and approvals matter too. An engineer will check the fit against your application before recommending anything.
Typical applications
What a harmonic actuator is
A harmonic actuator is a rotary servo actuator that combines a strain-wave gear, a servo motor, a position encoder and often a brake in one compact housing. Many have a hollow shaft for cables, and some include the servo drive with a fieldbus interface. The strain-wave gear gives a high ratio with near-zero backlash, while a precision output bearing carries the load directly. The actuator is ready to mount as a robot joint or precision rotary axis.
When engineers choose a harmonic actuator
Choose a harmonic actuator for robot joints, positioners and precision rotary axes where compact size, zero backlash and fast integration matter. It saves the design effort of combining motor, gear, encoder and bearings. The trade-offs are cost and flexibility: the motor, gear ratio and feedback are fixed by the range, and torque limits are set by the strain-wave gear. For heavy shock loads, a cycloidal solution may be more robust.
How to specify the Harmonic Drive RSF servo actuators (17A/20A/25A/32A)
Send these with your enquiry and an engineer can confirm the right RSF servo actuators (17A/20A/25A/32A) variant or an equivalent:
- Rated and peak output torque (Nm)
- Output speed (rpm)
- Ratio (:1)
- Hollow shaft diameter (mm)
- Feedback: motor-side and output-side encoders
- Brake and integrated drive options
- Supply voltage and interface
Common mistakes to avoid
- Exceeding momentary torque in a collision
- Ignoring output bearing moment load
- Mismatching drive and encoder interface
- Forgetting heat in continuous duty
Harmonic Drive RSF servo actuators (17A/20A/25A/32A): frequently asked questions
Can you help replace Harmonic Drive RSF servo actuators (17A/20A/25A/32A) or source the original?
Send the full model code, quantity and required date. Our engineers can check original-part availability and assess suitable alternatives, including supply-network options where appropriate. We confirm compatibility and any changes needed before recommending a replacement.
What is the Harmonic Drive RSF servo actuators (17A/20A/25A/32A)?
A compact solid-shaft servo actuator that pairs a strain wave gear with an AC servo motor and incremental encoder, for precise rotary motion without a hollow shaft. Published series figures include rated torque 98 Nm, peak torque 330 Nm, ratio 50:1, ratio 100:1. It is made by Harmonic Drive SE / Harmonic Drive LLC; Specify Motion can help source it or propose an equivalent.
Who makes the Harmonic Drive RSF servo actuators (17A/20A/25A/32A)?
The Harmonic Drive RSF servo actuators (17A/20A/25A/32A) is made by Harmonic Drive (Harmonic Drive SE / Harmonic Drive LLC) and is listed here for engineers comparing options. Specify Motion is not affiliated with Harmonic Drive SE / Harmonic Drive LLC; brand names are used only to identify the product.
What is the rated torque of the Harmonic Drive RSF servo actuators (17A/20A/25A/32A)?
The published series maximum rated torque is 98 Nm. Individual models and sizes in the RSF servo actuators (17A/20A/25A/32A) range vary, so check the exact variant against your duty cycle, mounting and environment, or ask our engineers to confirm the right size for you.
Is there an alternative to the Harmonic Drive RSF servo actuators (17A/20A/25A/32A)?
We can assess suitable replacement options. On published figures, Motion Control Products MAH and Motion Control Products MAT are options to assess from our supply network. An engineer confirms the fit against your application before recommending anything.
What is the benefit of dual encoders on a harmonic actuator?
A motor-side encoder controls the motor, while an output-side encoder measures the joint position directly after the gear. This compensates for gear deflection and gives true absolute output position at power-up. Dual encoders improve accuracy and support safety and collision detection.
Can a harmonic actuator include its own drive?
Yes, some versions integrate the servo drive and communicate over EtherCAT, CANopen or similar networks. This reduces cabinet space and wiring. Other versions connect to a separate servo drive. Choose by machine architecture, cabinet space and the safety functions needed, such as safe torque off or functional safety over the network.
Further reading: How to select a harmonic actuator · Harmonic vs planetary vs cycloidal · Gearboxes and harmonic drives
Other harmonic actuator series
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- 01Tell us what you needSend a part number, or configure the application, quantities and timing.
- 02An engineer reviews itWe check the specification and find the original part, a checked equivalent or a complete axis.
- 03Receive your quotationProducts, pricing, lead time and compatibility notes in one reply.
- 04Order and deliveryWhen you are happy, the order is placed and delivery and after-sales support are arranged.