Nm/kg Comparison at a Glance: Harmonic vs Planetary vs QDD
- Torque Density (Nm/kg) compares actuator output torque against total module mass, making it critical for humanoid, cobot, exoskeleton, and mobile robot joints.
- Harmonic / strain wave actuators are strongest when compact packaging, high reduction ratio, and low backlash matter more than shock-load tolerance.
- Planetary and QDD-style actuators trade some reduction ratio for efficiency, backdrivability, and impact resistance in leg, wheel, and dynamic joints.
- Direct-drive and cycloidal options can fit special cases, but they must be checked against real joint torque, envelope, backlash, and thermal requirements before RFQ.
How to validate actuator comparison data before RFQ
Use this comparison as a shortlist, then verify each candidate with published specifications, compliance documents, and measured factory test data.
Review harmonic actuator torque, mass, encoder, and dimensional data.
Compare planetary/QDD-style modules for efficiency, backdrivability, and joint fit.
Confirm backlash, torque curve, efficiency, encoder calibration, and thermal run-in values by serial number.
Check ISO 9001:2015, CE, RoHS, and REACH files before vendor approval.
Match the actuator type to a ZHR module family
If your shortlist favors low-backlash harmonic drive joints, start with ZHR-H. If it favors efficient compact planetary or QDD-style modules, compare ZHR-P sizes and CAD downloads before sending an RFQ.
For decades, industrial automation robots (like heavy welding arms in automotive plants) didn't care much about weight. They were bolted to concrete floors. However, the paradigm shift toward Humanoid Robots, Exoskeletons, and Collaborative Robots (Cobots) has abruptly changed the engineering physics, necessitating zero-backlash robot joints to maintain precision.
Today, a robot must carry its own body weight before it can carry a payload. If an actuator is heavy, the shoulder joint must be exponentially larger to lift the elbow and wrist. The critical metric to solve this recursive weight problem is Torque Density. This guide presents a comprehensive robot actuator torque density comparison to help engineers select the optimal joint module.
1. Understanding the Metric: Mass vs. Volume Density
Torque density reveals how effectively an electromechanical system converts structural mass/volume into twisting force. It is measured in two ways:
- Gravimetric Torque Density (Nm/kg): The Rated or Peak output Torque divided by the total mass of the actuator (Motor + Reducer + Driver + Encoder). This is paramount for mobile and legged robotics.
- Volumetric Torque Density (Nm/Liter or Nm/cm³): The Torque output divided by the physical envelope size. This is crucial for collaborative robots needing slim, human-like arm profiles.
2. The Champion of Compactness: Harmonic Drive Actuators
Harmonic reducers (strain wave gears), when integrated with an ultra-thin frameless BLDC motor, represent the zenith of volumetric and gravimetric torque density in the commercial robotics sector.
How do they achieve this? By utilizing a flex spline that allows for massive gear reductions (e.g., 50:1, 80:1, 100:1, or 120:1) in a single, extremely thin axial stage. A 100:1 reduction means the output torque is theoretically multiplied by 100 times compared to the bare motor shaft (minus efficiency losses of about ~15%).
Empirical Data from ZHR-H Series:
- ZHR-H14: 46 Nm peak torque / 0.70 kg = 65.7 Nm/kg
- ZHR-H17: 75 Nm peak torque / 1.00 kg = 75.0 Nm/kg
- ZHR-H20: 111 Nm peak torque / 1.30 kg = 85.3 Nm/kg (Peak Density)
*Note: Peak density is calculated using instantaneous peak torque. Using continuous rated torque, the density of the H20 stabilizes at an impressive 42.3 Nm/kg.
3. The Rugged Contender: Planetary Gear Actuators (QDD)
Planetary gearboxes must stack multiple stages to achieve high reduction ratios. A single stage typically maxes out at roughly 10:1. To get 100:1, you need two or three planetary stages end-to-end, which rapidly increases both mass and axial length.
Therefore, modern Quasi-Direct Drive (QDD) actuators (like our ZHR-P series) pair a very high-torque, large-diameter frameless motor with a low-ratio (e.g., 6:1 or 9:1) single-stage planetary gear.
While this yields a lower overall torque density compared to harmonic drives, planetary systems compensate with superior benefits that legged robots desperately need: >95% Efficiency, Extreme Back-drivability, and High Impact Resistance (300% nominal nominal torque limit).
Empirical Data from ZHR-P Series:
- ZHR-P05: 5.5 Nm peak torque / 0.19 kg = 28.9 Nm/kg
- ZHR-P60: 60 Nm peak torque / 0.90 kg = 66.6 Nm/kg
4. Comparative Data Matrix
| Actuator Type | Typical Reduction Ratio | Average Rated Torque Density | Key Trade-off |
|---|---|---|---|
| Direct Drive (Outrunner) | 1:1 | 1-5 Nm/kg | Zero backlash, 100% efficient, but utterly massive and heavy for high torque needs. |
| Planetary (QDD / ZHR-P) | 6:1 to 10:1 | 12-28 Nm/kg | Moderate weight, but allows energy regeneration and survives harsh jumping impacts. |
| Harmonic (Strain Wave / ZHR-H) | 50:1 to 120:1 | 25-39 Nm/kg | Incredible torque-to-weight ratio and no backlash, but less efficient and vulnerable to sudden shock drops. |
Looking for actuators that actually meet these Nm/kg benchmarks?
Check out the ZHR-H Series (up to 122 Nm/kg) with <5 arcsec backlash. Available for OEM sampling.
5. Test Methodology & Measurement Conditions
All torque density values referenced in this comparison are measured under standardized laboratory conditions. Understanding the test methodology is critical because measurement conditions can change Nm/kg results by 30-50% on the same actuator.
Test Conditions (ZHR Laboratory Standard)
- Temperature: 25°C ± 2°C ambient, no forced air cooling
- Torque measurement: HBM T40B torque transducer (±0.1% accuracy)
- Mass measurement: Mettler Toledo XS6002S precision balance (±0.01 g)
- Duty cycle for rated torque: 30% on / 70% off (unless otherwise specified)
- Peak torque duration: Maximum 3 seconds, followed by 60-second cooldown
- Included mass: Motor stator + rotor, reducer, encoder (dual), housing, output flange, cabling up to 100 mm from housing
- Excluded mass: External driver/controller, power cables, mating connectors
Warning: Comparing Across Manufacturers
Some competitor datasheets quote torque density using bare motor mass only (excluding reducer and encoder), which inflates the Nm/kg figure by 40-60%. Others use peak torque at 10% duty cycle instead of rated continuous torque. Always verify which mass definition and torque rating is used before comparing specifications across manufacturers. ZHR reports all values using total integrated module mass and rated continuous torque unless explicitly labeled as "peak."
6. Application Scenario Selection Matrix
The "best" actuator technology depends entirely on your application's specific constraints. Use the matrix below to match your scenario with the optimal actuator type and ZHR series.
| Application | Key Requirement | Recommended Type | ZHR Model | Why This Choice |
|---|---|---|---|---|
| Humanoid walking | 30+ Nm/kg + precision | Harmonic | ZHR-H20 | 85.3 Nm/kg peak, zero backlash for stable gait |
| Humanoid running | Shock survival + efficiency | Planetary QDD | ZHR-P60 | 66.6 Nm/kg + 5× shock tolerance + energy regen |
| Collaborative arm | Back-drivability + safety | Planetary QDD | ZHR-P36 | Low reflected inertia for safe human interaction |
| Surgical robot | <1 arcmin precision | Harmonic | ZHR-H14 | <20 arcsec backlash, 0.7 kg ultra-compact |
| Exoskeleton | Light weight + back-drivable | Planetary QDD | ZHR-P14 | 45.1 Nm/kg at 0.31 kg, minimal user fatigue |
| AGV drive | Durability + cost | Planetary | ZHR-P120 | 84.5 Nm/kg, lowest TCO, 300% shock margin |
| Quadruped leg | High dynamics + impact | Planetary QDD | ZHR-P36 | Rapid acceleration, survives landing shock |
| Space/orbital | Max density + vacuum | Harmonic | ZHR-H25 | 122.2 Nm/kg, vacuum-rated lubrication optional |
Frequently Asked Questions (FAQ)
What is torque density in a robot actuator?
Torque density is the ratio of an actuator's rated torque output to its total mass, measured in Newton-meters per kilogram (Nm/kg). Higher torque density implies a stronger, lighter robot joint.
Why is high torque density critical for humanoid robots?
Humanoid robots must lift their own limbs before carrying paylods. A low torque density means the robot spends 80% of its battery merely keeping its heavy arms from drooping. High torque density minimizes the robot's dead-weight inertia.
Decision Framework: 5-Factor Scorecard
Each actuator technology excels on a different combination of metrics. Use the scorecard below to weigh your priorities, then apply the application matrix to find your optimal technology.
| Factor | Harmonic Drive | Planetary | QDD | Cycloidal | Direct-Drive |
|---|---|---|---|---|---|
| Torque Density (Nm/kg) | 65-122 | 11-27 | 10-38 | 40-70 | 2-8 |
| Backlash (arcmin) | <0.3 | 3-15 | 0 (direct) | <0.5 | 0 |
| Efficiency (%) | 75-85 | 90-96 | 85-92 | 70-85 | 92-97 |
| Cost Index (1-10) | 8 | 3 | 4 | 9 | 10 |
| Lifespan (hours) | 8k-20k | 20k-50k | 15k-30k | 10k-30k | 50k+ |
How to Use This Scorecard
- Weight each factor for your application (e.g., humanoid hip: torque density=50%, cost=20%, efficiency=15%, lifespan=10%, backlash=5%)
- Score each technology on a 1-5 scale per factor using the table above
- Multiply weights × scores and sum for each technology
- The highest total is your optimal technology for that specific joint
Example: For a humanoid hip with 50% weight on torque density, harmonic drive (122 Nm/kg) scores 5/5 while planetary (27 Nm/kg) scores 1/5 — a 4-point gap that cost and lifespan differences cannot close.