How Does a Harmonic Drive Actuator Achieve High Torque Density?

Introduction

For a robotic joint, high torque alone is not enough.

An actuator may produce a large output torque, but if it is too heavy or too large, the robot may lose much of the performance that the actuator was intended to provide. This is particularly important in humanoid robots, collaborative robots, lightweight robotic arms, and other systems where every kilogram and every millimeter of installation space matters.

This is why torque density has become an important parameter when evaluating a modern robot actuator.

A harmonic drive actuator can achieve a high torque-to-size or torque-to-weight ratio by combining several elements rather than relying on the harmonic transmission alone. The final result depends on the interaction between the motor, reduction ratio, transmission geometry, bearings, housing, encoder arrangement, thermal design, and control strategy.

In other words:

High torque density is an actuator-level result, not simply a gearbox specification.

For a robot joint, the more useful question is therefore not:

How much torque can the harmonic reducer produce?

It is:

How much usable joint torque can the complete harmonic drive actuator deliver within a given mass, volume, speed, and thermal envelope?

This distinction becomes particularly important when designing humanoid robot joints, where the actuator has to balance torque, weight, speed, efficiency, stiffness, sensing, and continuous operating capability at the same time.

What Determines the Torque Density of a Harmonic Drive Actuator?

Torque density is commonly discussed as the relationship between output torque and actuator size or mass.

Depending on the application, engineers may evaluate it using different metrics, such as:

• Torque per kilogram

• Torque per unit volume

• Rated torque versus actuator mass

• Peak torque versus actuator mass

• Continuous torque versus actuator volume

• These metrics are not interchangeable.

For example, an actuator may have a very high peak torque but a much lower continuous torque because of motor heating. Another actuator may be slightly heavier but provide substantially higher continuous torque during repetitive operation.

For robotic applications, it is therefore useful to distinguish between:

Peak torque density and Continuous torque density

A humanoid robot may need high peak torque during acceleration, jumping, or sudden changes in motion, while the same joint also needs sufficient continuous torque for standing, walking, or repetitive movements.

The actuator architecture has to support both.

The Motor Sets the Starting Point for Torque Density

The harmonic reducer does not generate torque by itself.

The motor provides the initial torque, while the transmission converts motor speed and torque into the required joint output.

A simplified relationship is:

Motor torque → Reduction → Joint output torque

For an ideal transmission:

Output Torque ≈ Motor Torque × Reduction Ratio

In a real actuator, transmission losses must also be considered.

This means that selecting a motor with high torque density is the first step toward achieving a high torque-density actuator.

However, simply installing a larger motor is not always the best solution.

A larger motor can increase:

• Actuator mass

• Housing size

• Thermal load

• Rotor inertia

• Power consumption

Instead, actuator designers need to find an appropriate operating point where the motor can deliver the required torque without occupying excessive space or adding unnecessary weight.

This is one reason integrated actuator design is important.

The motor and harmonic transmission should be considered as one system rather than as completely independent components.

High Reduction Ratio Helps Multiply Output Torque

One of the most important contributors to harmonic actuator torque density is the reduction ratio.

A high reduction ratio allows a relatively small high-speed motor to produce a much larger output torque.

For example, consider a simplified transmission:

Motor → Harmonic Reduction → Joint Output

If the motor produces a certain input torque, the reduction mechanism can multiply the available output torque according to its reduction ratio and transmission efficiency.

This allows the motor to operate at a speed range where it can deliver power efficiently while the gearbox converts that speed into the lower-speed, higher-torque output required by the robot joint.

However, a higher reduction ratio is not automatically better.

Increasing the reduction ratio can affect:

Output speed

• Efficiency

• Backdrivability

• Dynamic response

• Motor operating point

• Thermal performance

Therefore, the reduction ratio should be selected according to the joint's actual torque and speed requirements.

Why High Reduction Ratio Does Not Automatically Mean High Torque Density?

This is an important point that is often overlooked.

Suppose two actuators use different reduction ratios.

The actuator with the higher ratio may produce more output torque from the same motor torque.

But if achieving that ratio requires:

• Additional transmission stages

• Larger components

• Heavier bearings

• A larger housing

• More cooling capacity

the overall actuator may not have better torque density.

The relevant metric is the performance of the complete actuator, not the reduction ratio alone.

This is why a good harmonic drive actuator needs to balance:

Motor Torque + Reduction Ratio + Efficiency + Mass + Volume

rather than maximizing any single parameter.

Harmonic Transmission Geometry Contributes to Compact Torque Capacity

The mechanical architecture of a harmonic transmission is another reason it can achieve substantial torque capacity within a relatively compact package.

Unlike a conventional spur gear arrangement where torque is transmitted through a relatively simple gear pair, harmonic transmission uses multiple tooth engagements between the flexspline and circular spline.

This allows the transmitted load to be distributed across multiple engagement points.

The result is a transmission architecture capable of delivering significant torque within a compact diameter.

For a robot joint, this matters because the actuator often has to fit inside a limited mechanical envelope.

The available space may already be occupied by:

• Structural components

• Bearings

• Wiring

• Sensors

• Motor components

• Cooling paths

A transmission that can provide the required torque without significantly increasing the joint diameter can therefore contribute directly to actuator-level torque density.

The Output Bearing Is Part of the Torque-Density Equation

A common mistake when comparing robot actuators is to focus only on the motor and reducer.

The output bearing can also have a significant influence on the final actuator size and mass.

A robot joint does not only transmit rotational torque.

It may also experience:

• Radial loads

• Axial loads

• Overturning moments

• External forces

• Dynamic impacts

The actuator therefore needs an appropriate output bearing arrangement.

If the bearing is oversized, the actuator becomes heavier.

If it is undersized, the actuator may not provide sufficient mechanical durability or stiffness.

A well-designed harmonic drive actuator integrates the transmission and bearing structure so that the joint can carry the expected external loads without unnecessarily increasing the overall package.

This is particularly important for humanoid robot joints.

For example, a knee or ankle actuator may experience significant external moments that are not represented by the motor torque alone.

Housing Design Can Increase or Reduce Torque Density

The actuator housing is another part of the torque-density equation.

A housing has to provide:

• Structural stiffness

• Bearing support

• Gear alignment

• Heat dissipation

• Protection for internal components

But excessive housing material increases weight.

The challenge is therefore to achieve sufficient rigidity with minimal unnecessary mass.

This is especially relevant in humanoid robots.

A heavy actuator installed near the hip or knee does not only increase the robot's total mass. It can also increase the load that downstream joints need to move.

This creates a cascading effect:

Heavier actuator → Higher joint load → Higher required torque → Larger actuator → Higher total mass

Breaking this cycle is one of the reasons lightweight, high-torque-density actuators are important for humanoid robotics.

Torque Density Depends on Continuous Torque, Not Just Peak Torque

When comparing harmonic drive actuators, peak torque can look impressive on a specification sheet.

However, peak torque is usually associated with short-duration operation.

For many robots, continuous torque is equally important.

Consider a humanoid robot standing upright.

The leg joints may need to produce torque continuously rather than only for a few seconds.

Similarly, a robotic arm performing repetitive assembly may operate for hours.

In these situations, the useful metric is closer to:

Continuous Output Torque / Actuator Mass

rather than:

Peak Output Torque / Actuator Mass

A well-designed actuator therefore needs to manage:

• Motor copper losses

• Iron losses

• Bearing losses

• Transmission losses

• Heat transfer

• Ambient temperature

• Duty cycle

High peak torque is useful, but high continuous torque density is often more relevant to real-world robot performance.

Thermal Management Sets a Practical Limit on Torque Density

Torque generates heat.

This creates an important relationship between torque density and thermal design.

As actuator size decreases, the available surface area for heat dissipation can also decrease.

At the same time, a compact actuator may have less internal space for:

• Heat sinks

• Cooling channels

• Fans

• Thermal interfaces

The actuator therefore has to manage heat within a smaller physical envelope.

For this reason, increasing torque density indefinitely is not realistic.

The actuator designer has to find a balance between:

Torque → Current → Heat → Temperature → Continuous Operating Capability

An actuator that achieves extremely high torque for a few seconds but overheats during normal operation may not be suitable for a production robot.

This is why thermal performance should be evaluated together with torque specifications.

Efficiency Directly Influences Usable Torque Density

Harmonic drive efficiency is another factor that affects actuator performance.

Not all of the input mechanical power becomes useful output power.

Some energy is lost through:

• Gear deformation

• Friction

• Bearing losses

• Lubrication

• Sealing

• Other mechanical effects

• These losses become heat.

The relationship can be simplified as:

Input Power → Useful Output Power + Losses

Higher transmission losses mean more heat must be removed from the actuator.

This can reduce the amount of torque that the actuator can continuously deliver.

Therefore, when comparing two harmonic drive actuators with similar peak torque, it is useful to examine:

• Rated torque

• Continuous torque

• Efficiency

• Input speed

• Duty cycle

• Thermal rating

• rather than relying on peak torque alone.

Why Actuator Weight Matters More Than Reducer Weight?

For a stationary industrial machine, adding several hundred grams may have little effect on system performance.

For a humanoid robot, the situation is different.

Every actuator contributes to the total moving mass of the robot.

If an actuator is installed on the upper leg, for example, its mass becomes part of the load that other joints need to move.

This means actuator mass can influence:

• Acceleration

• Energy consumption

• Walking efficiency

• Dynamic balance

• Payload

• Joint torque requirements

Therefore, a lightweight actuator with slightly lower absolute torque may sometimes be more useful than a heavier actuator with a larger peak torque rating.

This is why torque-to-weight ratio is increasingly important in humanoid robot actuator design.

Motor Size and Reduction Ratio Must Be Matched

A harmonic drive actuator cannot be optimized by selecting the reducer independently from the motor.

The motor and transmission need to operate as a matched pair.

For example, a designer may have two possible approaches:

Option A

Large motor + lower reduction ratio

Option B

Smaller motor + higher reduction ratio

Both approaches may produce similar output torque.

However, they can have very different:

• Mass

• Volume

• Efficiency

• Speed

• Thermal behavior

• Dynamic response

• Cost

The better solution depends on the target joint.

For a compact humanoid wrist, minimizing mass and diameter may be critical.

For a high-load industrial joint, continuous torque and rigidity may be more important.

This is why actuator optimization should begin with the joint's actual operating profile rather than starting with a predefined gearbox size.

Peak Torque, Rated Torque and Continuous Torque Should Be Evaluated Separately

When evaluating a harmonic drive actuator, three torque values should not be treated as interchangeable.

Peak Torque

Peak torque represents the maximum torque the actuator can deliver under specified conditions for a limited duration.

It is important for:

Acceleration

Short-term dynamic movement

Impact response

Rapid changes in direction

Rated or Continuous Torque

Rated torque represents the torque the actuator can deliver under defined operating conditions for sustained operation.

This is more relevant for:

• Walking

• Repetitive motion

• Continuous industrial operation

• Sustained load

Overload Capacity

Some applications also require short-term overload capability.

This can be important when a robot experiences:

• Sudden acceleration

• External impact

• Emergency braking

• Unexpected load changes

The relationship between these values gives a much clearer picture of actual actuator capability than a single torque number.

Why Torque Density Is Especially Important for Humanoid Robot Joints?

Humanoid robots place unusually strict constraints on actuator design.

A humanoid joint often has to fit inside a body structure designed around approximate human dimensions.

At the same time, it needs to provide enough torque to move the robot's mass.

This creates competing requirements:

Small Size

↓

Low Weight

↓

High Torque

↓

High Speed

↓

High Efficiency

↓

Thermal Stability

The actuator has to balance all of them.

This is particularly challenging in:

• Hip joints

• Knee joints

• Ankle joints

• Shoulder joints

• Elbow joints

The best actuator for each joint may therefore be different.

A high-torque leg joint may prioritize continuous torque and thermal performance, while a wrist actuator may prioritize low mass, compact dimensions, and low inertia.

Torque Density Is Not the Same as Power Density

These two terms are sometimes confused.

Torque density describes how much torque an actuator can provide relative to its mass or volume.

Power density also considers rotational speed.

A simplified relationship is:

Power = Torque × Angular Speed

This means that an actuator with extremely high torque but relatively low speed may not have the same power density as a faster actuator.

For dynamic robots, both metrics can matter.

For example:

A knee actuator may require high torque at moderate speed.

A wrist actuator may require lower torque but high speed.

A mobile robot wheel drive may require continuous torque and relatively high rotational speed.

Therefore, actuator selection should consider the complete torque-speed operating envelope rather than a single torque-density figure.

How Encoder Integration Affects the Actuator Architecture?

Sensors do not directly create torque, but they affect how effectively the actuator can use its available torque.

A harmonic drive actuator may incorporate:

• Motor-side encoder

• Output-side encoder

• Dual encoders

• Torque sensor

• Temperature sensing

A motor-side encoder can provide feedback for motor control.

An output-side encoder can measure the actual joint output position after the reduction mechanism.

For applications requiring highly accurate joint control, measuring the output shaft can provide information that cannot be obtained from the motor encoder alone.

This is especially relevant when the actuator needs to compensate for:

• Transmission errors

• Elastic deformation

• External loads

• Position deviations

Therefore, sensor integration should be considered as part of the complete actuator architecture.

Does a Torque Sensor Improve the Effective Performance of a High-Torque-Density Actuator?

A torque sensor does not increase the mechanical torque capacity of the actuator.

Its value is different.

It gives the controller information about the torque being transmitted through the joint.

This can support:

• Torque control

• Force control

• Collision detection

• Compliance control

• Contact detection

For humanoid robots, this information can be particularly useful during:

• Walking

• Object manipulation

• Physical interaction

• Balance control

• Contact-rich tasks

However, adding a torque sensor also adds:

• Mass

• Volume

• Cost

• Mechanical integration requirements

Therefore, sensor selection becomes another part of the torque-density optimization problem.

The goal is not simply to integrate as many sensors as possible.

The goal is to achieve the required sensing capability without unnecessarily increasing actuator mass and volume.

Harmonic Drive Actuator Torque Density Is a System-Level Optimization

The previous factors show why torque density cannot be attributed to the harmonic reducer alone.

A complete actuator includes multiple elements:

• Motor

• Harmonic Transmission

• Bearings

• Housing

• Encoder

• Driver

• Sensors

• Thermal Structure

The final torque density is determined by the complete package.

For example, an actuator may use a very high-performance harmonic reducer but still have poor torque-to-weight performance if:

• The motor is oversized

• The housing is unnecessarily heavy

• The bearing arrangement is inefficient

• The cooling system adds excessive mass

• The driver is physically separated from the actuator

• The sensor architecture is poorly integrated

This is why integrated actuator design can provide an advantage over simply assembling individual components.

How Integrated Design Can Improve Torque-to-Weight Ratio?

Integration allows designers to optimize components around a common mechanical envelope.

Instead of designing:

Motor → Separate Gearbox → Separate Encoder → Separate Housing → Separate Driver

the system can be designed as:

Integrated Motor + Harmonic Reducer + Encoder + Electronics + Joint Output

This can reduce duplicated:

• Housings

• Mounting structures

• Connectors

• Shafts

• Couplings

• Mechanical interfaces

The result can be a more compact actuator with a lower overall system mass.

For robotic applications, the important comparison is therefore not:

Which reducer is lighter?

but:

Which complete actuator delivers the required joint performance with the lowest practical mass and volume?

Why Mechanical Integration Matters in High-Torque Robot Joints?

High torque creates mechanical loads throughout the actuator.

The motor, reducer, bearings, housing, shaft, and mounting structure all need to handle these loads.

If the actuator is designed as a collection of unrelated components, the mechanical interfaces can become unnecessarily large.

An integrated architecture allows the designer to optimize:

• Bearing placement

• Load paths

• Housing thickness

• Shaft geometry

• Motor position

• Encoder position

• Heat paths

This can improve the ratio between structural mass and usable output torque.

For a robot joint, this matters because the actuator is not simply a transmission component.

It is part of the robot's structural system.

What Limits the Torque Density of a Harmonic Drive Actuator?

There are several practical limits.

 Motor Thermal Limits

The motor cannot continuously produce unlimited torque because current creates heat.

Transmission Load Limits

The harmonic transmission has defined torque and service-life limits.

Bearing Capacity

The output bearing must handle both torque-related loads and external joint forces.

Housing Stiffness

The housing must remain sufficiently rigid under load.

Thermal Dissipation

Heat generated inside the actuator must be transferred to the surrounding structure.

Speed Requirements

High torque at high speed creates substantially greater power and thermal demands.

Weight Constraints

Adding structural material can increase load capacity, but excessive mass reduces actuator-level torque density.

These constraints interact with one another.

Improving one parameter may negatively affect another.

That is why actuator design is fundamentally an optimization problem.

How to Compare the Torque Density of Two Harmonic Drive Actuators？

When comparing two actuators, avoid comparing only their maximum torque.

A better comparison should include:

1. Continuous Output Torque

How much torque can the actuator deliver during sustained operation?

2. Peak Output Torque

How much torque can it provide during short dynamic events?

3. Actuator Mass

Does the torque rating include the complete actuator?

4. Overall Dimensions

What is the diameter, length, and installation volume?

5. Output Speed

Can the actuator provide the required torque at the required speed?

6. Efficiency

How much input power is converted into useful mechanical output?

7. Thermal Rating

Can the actuator maintain its rated torque without exceeding its allowable temperature?

8. Encoder Configuration

Is feedback measured at the motor, output shaft, or both?

9. External Load Capacity

Can the output bearing handle the required radial, axial, and moment loads?

10. Duty Cycle

Is the actuator intended for intermittent or continuous operation?

This approach produces a much more meaningful comparison than simply selecting the actuator with the largest torque number.

How to Improve Torque Density in a Harmonic Drive Actuator？

If an actuator needs higher torque density, engineers can look at several areas simultaneously.

Optimize Motor Operating Point

Select a motor that provides the required torque without excessive mass or thermal overhead.

Optimize Reduction Ratio

Choose a reduction ratio that balances torque multiplication, speed, efficiency, and dynamic response.

Reduce Structural Mass

Use an appropriately designed housing and load path rather than simply increasing material thickness.

Optimize Bearing Arrangement

Select bearing dimensions according to actual radial, axial, and moment loads.

Improve Heat Dissipation

Better thermal paths can allow the actuator to maintain higher continuous torque without excessive temperature rise.

Integrate Electronics

Where appropriate, integrated drive electronics can reduce external hardware and wiring.

Optimize Sensor Integration

Use the feedback architecture required by the application without adding unnecessary components.

The best results usually come from optimizing several parameters together rather than maximizing one component's performance.

Harmonic Drive Actuator Torque Density for Different Robot Applications

The required torque density is highly dependent on the application.

Humanoid Robot Legs

Leg actuators typically require:

• High continuous torque

• High peak torque

• Low mass

• High dynamic response

• Thermal stability

Torque-to-weight ratio becomes particularly important because actuator mass directly influences the robot's total energy requirements.

Humanoid Robot Arms

Arm joints generally require a different balance between:

• Torque

• Speed

• Weight

• Range of motion

• Precision

Robot Wrists

Wrist actuators often place greater emphasis on:

• Compact size

• Low inertia

• Low weight

• High positioning accuracy

Collaborative Robot Joints

Cobots may require:

• Compact packaging

• Low backlash

• Smooth motion

• Torque sensing

• Controlled interaction with external loads

Industrial Robot Joints

Industrial robots may prioritize:

• Rated torque

• Repeatability

• Rigidity

• Service life

• Continuous operation

This demonstrates why there is no single torque-density target that applies to every robot.

Why Continuous Torque Density May Matter More Than Peak Torque Density？

For production robots, the most impressive specification is not always the most useful one.

A high peak torque rating may be valuable for dynamic motion, but the actuator also needs to operate reliably during the majority of its duty cycle.

For example, a humanoid robot may spend considerable time:

• Standing

• Walking

• Maintaining balance

• Holding an object

These activities require sustained actuator output.

Similarly, an industrial robot may repeat the same movement thousands of times.

For these applications, continuous torque density can be a more useful engineering metric than peak torque density.

A Practical Way to Evaluate High-Torque-Density Actuators

Before selecting a harmonic drive actuator, define the actual joint operating profile.

At minimum, determine:

• Required continuous torque

• Required peak torque

• Required speed

• Acceleration

• Duty cycle

• External load

• Installation space

• Maximum actuator mass

• Ambient temperature

• Cooling conditions

• Required position accuracy

• Required torque sensing

• Communication interface

Then compare the complete actuator against those requirements.

This prevents a common mistake:

Choosing an actuator based on a single headline specification.

A 100 Nm actuator is not necessarily better than an 80 Nm actuator if the 100 Nm model is significantly heavier, larger, less efficient, or unable to sustain the required torque under the actual duty cycle.

The Future of High-Torque-Density Harmonic Actuators

As robots become more compact and dynamic, actuator development is moving beyond simply increasing maximum torque.

Future actuator designs are likely to focus on the combined optimization of:

• Torque Density

• Power Density

• Efficiency

• Thermal Performance

• Sensing

• Control Integration

• Weight

• Mechanical Rigidity

This is particularly relevant to humanoid robots.

A high-performance humanoid actuator may need to combine a precision harmonic transmission with:

• High-resolution encoders

• Integrated motor control

• Torque sensing

• Compact bearings

• Lightweight housing

• Efficient thermal paths

• High-speed communication

The result is no longer simply a motor with a gearbox.

It becomes a complete robotic joint actuator designed around the mechanical and control requirements of the robot.

How HONPINE Approaches High-Torque-Density Harmonic Actuator Design？

For robotic applications, HONPINE develops integrated harmonic actuator and robot joint solutions that combine precision transmission with motor, sensing, and control technologies.

Depending on the actuator architecture, the integrated system can incorporate:

• Harmonic transmission

• Servo motor

• High-resolution encoder

• Output-side position feedback

• Integrated driver

• Torque sensing

• Industrial communication interfaces

The objective is not simply to maximize the torque rating of one component.

Instead, the actuator needs to provide an appropriate balance between:

Output Torque + Weight + Size + Speed + Precision + Thermal Performance

This approach is particularly relevant to humanoid robot joints, collaborative robots, robotic arms, and other applications where actuator mass and installation space directly affect system performance.

For applications requiring force feedback, an integrated torque-sensing architecture can further support torque control, compliant motion, and interaction with external loads.

Frequently Asked Questions About Harmonic Drive Actuator Torque Density

What makes a harmonic drive actuator achieve high torque density?

High torque density comes from the combined design of the motor, harmonic transmission, reduction ratio, bearings, housing, thermal system, and control architecture. The harmonic transmission provides high torque multiplication within a compact package, but actuator-level torque density depends on the complete system.

Does a higher harmonic reduction ratio always increase torque density?

No. A higher reduction ratio can increase output torque from a given motor, but it can also affect output speed, efficiency, backdrivability, and thermal performance. The complete actuator must be evaluated rather than focusing on reduction ratio alone.

Is torque density more important than peak torque?

For many robotic applications, yes. A high peak torque rating does not necessarily mean that an actuator can deliver high torque continuously. Torque-to-weight ratio, continuous torque, duty cycle, and thermal performance should all be considered.

Why is torque density important for humanoid robots?

Humanoid robots have strict weight and space constraints. A lightweight actuator with high usable torque can reduce overall robot mass and help improve acceleration, energy efficiency, balance, and dynamic performance.

How does actuator weight affect humanoid robot performance?

Actuator mass becomes part of the robot's moving load. Heavier actuators can increase the torque required from other joints, particularly in leg mechanisms. Reducing actuator mass while maintaining sufficient torque can therefore improve overall system efficiency.

Can a harmonic actuator provide both high torque and high speed?

It depends on the actuator design and operating conditions. High reduction ratios increase output torque but reduce output speed. The motor, reduction ratio, thermal design, and transmission efficiency must be matched to the required torque-speed operating range.

What is more important: peak torque or continuous torque?

It depends on the application. Peak torque is important for acceleration and short dynamic events, while continuous torque is critical for sustained loads and repetitive operation. For production robots, both should be evaluated.

Does adding a torque sensor reduce torque density?

A torque sensor can add some mass, volume, and mechanical complexity. However, if the sensor is integrated into the actuator architecture efficiently, the additional weight can be minimized while providing valuable torque feedback for force control and interaction applications.

Conclusion

A harmonic drive actuator achieves high torque density through the combined optimization of the motor, reduction ratio, harmonic transmission, bearings, housing, thermal design, sensing, and control system.

The harmonic transmission is an important part of this equation because it can provide substantial torque multiplication within a compact mechanical package. But the final torque density of the actuator depends on much more than the reducer itself.

For practical robot design, engineers should evaluate:

Continuous Torque

→ Peak Torque

→ Speed

→ Reduction Ratio

→ Efficiency

→ Thermal Performance

→ Actuator Mass

→ Installation Volume

→ External Load Capacity

→ Feedback and Sensing

rather than relying on a single torque specification.

This becomes particularly important in humanoid robotics, where actuator weight, torque, speed, and thermal performance are closely interconnected.

The goal is therefore not simply to build an actuator with the highest possible torque.

The goal is to develop an actuator that delivers the required usable torque within the smallest practical mass and volume, while maintaining the speed, precision, efficiency, thermal stability, and sensing capability required by the robot.

That is the real engineering meaning of high torque density in a harmonic drive actuator.