Compact Humanoid Arm Actuator Sizing & Selection Guide
Screen one-link torque, compare actuator architectures, and prepare joint details for a quote.
Request a compact arm actuator quoteTorque Calculator Tool
Key Findings for Arm Actuators
1. Size each joint from its load and geometry
Payload, reach, link mass, posture, and acceleration set different requirements at each axis. The calculator above screens one horizontal link only.
2. Separate peak from continuous capability
Peak torque covers brief motion events. Continuous or RMS torque depends on the full motion cycle, speed, cooling, and the selected motor and drive. See the effective-load torque method.
3. Select a transmission by requirements, not a torque band
Compare package size, backlash, stiffness, speed, service life, bearing loads, and how much integration work the design can support.
4. Validate exact ratings before release
Check the selected configuration's torque-speed curve, peak duration, duty cycle, thermal limits, controller, and mounting interface with the manufacturer.
Sizing Methodology & Evidence
A compact humanoid arm actuator must meet the load of a specific joint over its full motion cycle and fit the arm's envelope. A torque estimate alone cannot select a motor or reducer: output speed, acceleration, inertia, duty cycle, mounting loads, control needs, and thermal limits also matter. This follows standard motor-sizing guidance to calculate inertia, torque, and speed for the application rather than choosing by one headline rating. Oriental Motor sizing calculations.
A screening calculator covers only step 2 for one link. A motion profile and exact component documentation are needed for final selection.
Reproducible one-link example
With the calculator's default inputs, the screening model produces these values:
| Input or result | Value |
|---|---|
| Payload / link | 2.0 kg / 1.5 kg |
| Link length / acceleration | 0.40 m / 10 rad/s² |
| Gravity holding torque | 10.8 Nm |
| Acceleration torque | 4.0 Nm |
| Conservative peak estimate | 14.8 Nm |
These are reproducible model outputs, not measured actuator performance or a final component rating.
| Architecture example | What the source confirms | Check for your joint |
|---|---|---|
| Strain-wave gearhead: Harmonic Drive CSD-2UF | The manufacturer describes this gearhead as ultra-flat and zero-backlash. CSD-2UF product specifications | Exact ratio and size ratings, speed, average and peak torque, stiffness, output bearing loads, lubrication, and mounting. |
| Integrated planetary module: T-Motor AK family | The AK manual describes a module combining a motor, planetary reducer, encoder, and driver. AK series manual (PDF) | Exact model torque-speed limits, peak duration, thermal behavior, backlash, communication, dimensions, and controller limits. |
| Custom frameless motor and reducer | Performance depends on the chosen motor, reducer, bearings, encoder, housing, and drive configuration; there is no universal rating for this combination. | Request the assembled unit's torque-speed curve, RMS/continuous rating, peak duration, thermal conditions, mass, envelope, and interface drawing. |
Source check: 2026-10-02. Manufacturer ratings and product revisions are configuration-specific; verify the current controlled datasheet before design release.
Risks, Limitations & Trade-offs
When selecting an actuator for humanoid arms, engineers face strict physical boundaries. Optimizing for one variable typically degrades another.
Misuse Risk: Thermal Runaway
The Risk: Treating a peak torque number or this calculator's static holding estimate as a continuous thermal rating. Neither describes motor temperature across a real operating cycle.
Mitigation: Define the motion and dwell cycle, calculate effective/RMS torque, then validate it against the motor and drive ratings under the intended cooling and ambient conditions.
Cost and Fit Risk: Choosing by One Feature
The Risk: Requiring zero backlash or a specific reducer family for every axis can add cost and constrain packaging without proving that the joint meets its actual accuracy, stiffness, or compliance target.
Mitigation: Set measurable limits for backlash, torsional stiffness, mass, envelope, and output error, then compare complete configurations against the same motion cycle.
Scenario Mismatch: High-Impact Operations
The Risk: Selecting any reducer from its nominal torque alone when the arm may brace, collide, or stop abruptly. Shock loads and emergency-stop loads depend on the mechanism and event, not just the architecture name.
Mitigation: Define the credible impact and stopping cases, request the selected unit's allowable peak and momentary loads, and validate the complete load path.
Have a joint envelope, torque-speed target, and duty cycle? Share them for a configuration review.
Discuss your actuator requirementsThermal & Mechanical Constraints
Thermal capability belongs to the configured motor, reducer, drive, housing, mounting structure, and duty cycle. A compact enclosure can limit heat rejection, but no single temperature threshold applies to every arm actuator. Confirm the allowed temperatures and lubrication conditions in the selected manufacturer's current documentation.
- Heat path: Include the mounting structure and housing in the thermal model; validate the assembled heat path under the intended ambient and airflow.
- Reducer limits: Check the exact unit's average and peak torque, speed, lubrication, and operating temperature limits. Do not transfer one model's limits to an entire reducer family.
- Structure and sensing: Verify housing and bearing deflection at the design load so the encoder and control system see acceptable joint motion and compliance.
For example, the Harmonic Drive CSD/SHD catalog documents model-specific ratings and lubrication guidance. Use the matching catalog and controlled drawing for the chosen reducer.
Frequently Asked Questions
What is the difference between peak and continuous torque?
Peak torque is allowed only for the duration and repetition specified for that motor, reducer, and drive. Continuous or effective torque must fit the thermal rating for the actual speed, duty cycle, cooling, and ambient conditions. Use the supplier's curves for the exact configuration.
Does this calculator select a motor or reducer?
No. It estimates gravity and acceleration torque for one horizontal link. Selection also needs the full motion profile, output speed, thermal conditions, package limits, control requirements, and exact component ratings.
What data should I send for a quote?
Include the joint load and geometry, position and speed profile, acceleration, peak duration, cycle and dwell time, ambient/cooling conditions, envelope, mass target, encoder and control needs, and any power-loss holding or braking requirement. These inputs let a supplier check more than the torque estimate.
When is a brake required?
Decide from the load, safe state, and behavior required after power loss. Confirm the holding/braking duty, release method, and added mass and length with the actuator supplier and the system safety design.
Request Actuator Quote
Ready to source actuators based on your calculations? Provide your peak torque and speed requirements to get tailored options.
Inquiry Email
Include target torque/speed, quantity, and delivery location.
Application Engineer
+8618857971991
Talk directly about drawings, torque-speed targets, and RFQ data gaps.