Are harmonic drives backdrivable?
Usually, yes. Check the exact model’s output breakaway torque; ratio-only estimates do not set the backdrive threshold.
Check measured backdrive torqueCheck your measured backdrive torque
Compare output-side breakaway torque for your exact gearbox or joint with the maximum your application can tolerate.
The short answer
What the evidence says
The useful published quantity is output-side backdriving torque: the torque at which reverse motion begins. Keep it separate from forward efficiency, no-load running torque, and holding torque.
| Evidence | What it supports | Boundary |
|---|---|---|
| Harmonic Drive CSG-GH catalog, pp. 2–3 | Defines backdriving torque at the output when the input first starts rotating, and gives maximum values by model and ratio. | The catalog’s figures are tied to its stated test setup; it warns against using them to hold an external load and calls for a brake when back-driving is not allowed. |
| Liu et al., Sensors, 2020 | In one exoskeleton study, the tested harmonic drive required 1.17 N·m to backdrive and had 54.6% backdrive efficiency. | This is one reducer in one test setup, not a rating for every harmonic drive or a default for this checker. |
| Application test | Measures the reducer or complete joint that will actually be used. | Record load, direction, speed, temperature, lubrication, and whether the motor and brake are connected. |
Evidence reviewed 2026-09-29. The 2020 study is included as a configuration-specific example, not as current product data.
Need the backdrive torque or test conditions for a specific joint configuration?
Request a model reviewHow to screen a real joint
- 1 · Get the exact value
Find the manufacturer’s backdriving torque for the exact size, ratio, variant, and test condition—or measure the output breakaway torque.
- 2 · Compare to the need
Set an application-specific maximum and use the checker above. A pass means only that the supplied value is under that limit.
- 3 · Validate the assembly
Repeat under expected load, direction, speed, and temperature. Test impact and holding behavior separately.
Limits and design trade-offs
If a harmonic-drive assembly exceeds the torque limit for a compliant or manually movable joint, compare a lower-ratio or direct-drive architecture and re-check motor size, heat, package, and output torque. This is a design trade-off to evaluate, not a guarantee that another gearbox family will pass.
Related engineering guides
Frequently asked questions
Are harmonic drives backdrivable?
Some can be backdriven when output torque exceeds their breakaway resistance. The required torque is model- and condition-specific, so check the exact catalog value or test the assembled joint.
Are harmonic drives self-locking?
Do not assume that they are self-locking or that they will hold a load. Use the manufacturer’s backdrive data for screening and a rated brake whenever load holding is required.
Can I use forward efficiency or a 50% rule to decide?
Not by itself. Forward efficiency is not a substitute for the model’s output-side backdriving torque or a test of the complete actuator. The checker intentionally asks for those application data instead.
Can a catalog value be used as a safety margin?
No. Harmonic Drive’s catalog explicitly warns against relying on the listed values to hold external loads and recommends a brake where back-driving is not permissible.
What should I ask a supplier for?
Ask for output-side starting/backdriving torque for your exact model and ratio, the test load and temperature, lubrication condition, and whether the value includes the motor, bearings, and brake. Request a measured result for the complete joint if those parts affect your requirement.
Sources and review notes
- Harmonic Drive LLC, CSG-GH gearhead catalog, performance definitions and brake warning, pp. 2–3.
- Chang Liu et al., “Functional Evaluation of a Force Sensor-Controlled Upper-Limb Power-Assisted Exoskeleton with High Backdrivability”, Sensors 20(21), 6379 (2020). The 1.17 N·m and 54.6% figures refer to the paper’s tested gear and setup.
First published 2026-09-25; evidence reviewed 2026-09-29. Use the latest catalog for the specific model before design release.
Need a backdrivability review for a joint?
Share the reducer model, ratio, measured output torque, load, and brake requirements so the architecture can be screened against the actual application.