What Are Rated, Intermittent, and Peak Torque in a Harmonic Actuator?

When selecting a harmonic actuator for a robot joint, engineers often focus on its maximum output torque. However, a single torque value cannot fully describe an actuator's operating capability. A robot joint may need to support a load continuously, generate higher torque during periodic movements, or withstand brief external impacts. These different operating conditions require different torque ratings.

Rated torque, intermittent torque, and peak torque represent three distinct aspects of a harmonic actuator's performance. Understanding their differences is essential for evaluating continuous load capacity, dynamic motion capability, and mechanical limits. For humanoid robots, quadruped robots, and robotic arms, selecting an actuator based only on peak torque can lead to overheating, accelerated component wear, and reduced service life.

1. Rated Torque: The Foundation of Continuous Operation

Rated torque is the output torque that a harmonic actuator can continuously deliver under the manufacturer's specified operating conditions. These conditions may include the allowable operating speed range, ambient temperature, cooling conditions, and other relevant specifications.

It represents the actuator's continuous operating capability and is closely related to its thermal design.

During operation, the frameless torque motor, harmonic reducer, bearings, encoder, and other internal components generate heat. The actuator must dissipate this heat effectively to keep component temperatures within their allowable limits.

When operating within its rated conditions, the actuator is designed to maintain stable performance over extended periods. This helps protect the motor, reducer, bearings, and other components from excessive thermal stress, supporting long-term reliability and consistent transmission performance.

Why Is Rated Torque Important for Long-Term Reliability?

If a harmonic actuator continuously operates above its rated torque, heat generation may exceed its heat dissipation capacity. As the temperature rises, the motor winding insulation, reducer lubrication, bearings, and electronic components may experience additional thermal stress.

Prolonged operation under excessive load can accelerate component aging, increase mechanical wear, and affect positioning performance over time.

Therefore, rated torque should be the primary reference when evaluating loads that must be maintained continuously.

Does Rated Torque Only Apply at One Specific Speed?

Not necessarily. Rated torque should not be interpreted as an output value available only at one fixed rated speed.

For a harmonic actuator such as the HONPINEHPJM series, continuous torque capability should be evaluated across its specified operating speed range. However, the actual allowable continuous torque may vary with rotational speed, ambient temperature, cooling conditions, motor current limits, and reducer load capacity.

Engineers should therefore refer to the manufacturer's torque-speed characteristics and operating specifications rather than assuming that identical continuous torque is available at every speed.

Typical Applications of Rated Torque

Rated torque is particularly important in applications involving relatively stable loads and extended operating periods.

For example, humanoid robots require sufficient continuous joint torque to maintain standing postures. Conveyor systems and automated machinery may need to maintain stable output under relatively constant loads. Medical rehabilitation equipment may also require continuous operation within specified load conditions.

In these applications, the continuous operating load should generally remain within the actuator's rated torque under the specified conditions.

2. Intermittent Torque: Handling Periodic High-Load Movements

Intermittent torque refers to the higher output torque that a harmonic actuator can deliver for limited periods under specified duty-cycle conditions.

Unlike rated torque, intermittent torque is not intended for continuous full-load operation. It is designed for applications in which the joint periodically experiences higher loads, followed by lower-load periods or rest intervals.

This capability is particularly relevant to dynamic robotic systems, where joint torque changes throughout each movement cycle.

For example, a product datasheet may specify an intermittent torque value under an S3 intermittent duty cycle, with a defined duty cycle such as 10% or 25%.

The duty cycle indicates the proportion of the specified operating cycle during which the actuator operates under the defined load condition. The actual permissible operating duration and rest period must follow the manufacturer's specifications.

An intermittent torque rating must always be interpreted together with its duty cycle, cycle duration, and operating conditions.

Why Can't Intermittent Torque Be Used for Continuous Operation?

Higher output torque generally requires greater motor current, resulting in increased heat generation.

During intermittent operation, lower-load periods or rest intervals provide opportunities for the actuator to dissipate heat. This helps keep the average thermal load within the specified operating limits.

However, if the actuator continuously operates at its intermittent torque level, heat may accumulate faster than it can be dissipated. This can cause excessive temperature rise and accelerate the aging of motor insulation, reducer components, and bearings.

Repeated operation beyond the specified duty cycle may also increase mechanical stress and affect long-term positioning stability.

Therefore, intermittent torque should not be treated as an alternative to rated torque. Its use must comply with the specified duty cycle and thermal limitations.

Why Is Intermittent Torque Important for Humanoid Robots?

Humanoid robot joints experience significantly changing loads during walking, running, squatting, standing up, and other dynamic movements.

Consider the leg joints during a walking cycle. Different movement phases impose different torque requirements:

Stance phase: The leg joints support body weight and maintain balance.

Push-off phase: The joints generate higher torque to propel the body forward.

Swing phase: The leg moves forward, with a different load and torque profile.

Landing and impact absorption: The joints respond to changing external forces and dynamic loads.

The push-off and other high-load phases may require torque above the continuous operating level. Whether the actuator can support these repeated movements depends on its intermittent torque capability, duty cycle, thermal performance, and actual motion profile.

Similar requirements occur in quadruped robots during climbing, acceleration, and jumping, as well as in robotic arms that repeatedly lift and reposition workpieces.

For these applications, intermittent torque is an important parameter for evaluating dynamic joint performance.

3. Peak Torque: The Short-Duration Operating Limit

Peak torque is the maximum short-duration torque that a harmonic actuator is designed to deliver or withstand under specified conditions.

It represents a transient operating limit determined by the combined capabilities of the motor, harmonic reducer, output structure, bearings, and other mechanical and electrical components.

The peak torque capability is influenced by several factors, including:

• Flexspline strength and allowable deformation in the harmonic reducer.

  
  

• Mechanical strength of the output flange and shaft.

  
  

• Motor current limits and electromagnetic characteristics.

  
  

• Bearing load capacity.

  
  

• Structural stress limits of the actuator assembly.

Unlike rated torque, peak torque is not a continuous thermal operating rating. It is generally associated with brief transient events, and its allowable duration and frequency must follow the manufacturer's specifications.

What Happens When Peak Torque Is Exceeded?

If the applied torque exceeds the actuator's specified peak torque limit, excessive mechanical or electromagnetic stress may damage internal components.

Depending on the magnitude and duration of the overload, possible consequences include permanent deformation or damage to the harmonic reducer's flexspline, excessive stress on the output flange, bearing damage, and deterioration of motor performance.

Severe overload conditions may also result in increased backlash and permanent loss of positioning accuracy.

Even if an actuator does not fail immediately, repeated overload events may cause cumulative mechanical damage and reduce its service life.

Peak torque should therefore be treated as a short-duration operating limit rather than a normal design target.

When Should Engineers Consider Peak Torque?

Peak torque is relevant when evaluating brief transient conditions, such as sudden external disturbances, short-duration impact loads, emergency deceleration, or unexpected contact.

For humanoid robots, engineers may need to consider transient loads caused by unexpected contact or impact during operation.

However, planned movements such as walking, running, jumping, and squatting should not rely on peak torque as their normal operating capability. These movements must be evaluated using the appropriate rated and intermittent torque specifications, together with the actual load profile.

4. Rated Torque vs. Intermittent Torque vs. Peak Torque: Understanding the Differences

Although all three parameters describe the torque capability of a harmonic actuator, they correspond to different operating conditions.

The fundamental difference is straightforward:

Rated torque defines the continuous output capability of the actuator.

Intermittent torque defines the higher torque available under specified duty-cycle conditions.

Peak torque defines the short-duration torque limit under specified transient conditions.

A harmonic actuator with a high peak torque rating does not necessarily have a high rated torque. Likewise, a high intermittent torque value does not mean that the actuator can maintain that output continuously.

When comparing different actuators, engineers should examine all three ratings and understand the conditions under which each value is specified.

5. How Should Engineers Select a Harmonic Actuator Based on Torque Requirements?

Selecting a harmonic actuator requires more than comparing maximum torque values. Engineers should evaluate the actual load profile of the robot joint throughout its operating cycle.

A practical evaluation involves three main considerations.

Evaluate the Continuous Load

First, calculate the torque required during long-duration operation.

For example, a humanoid robot may need to maintain a standing posture for an extended period. The leg joints must continuously support the body and maintain stability.

The calculated continuous load should be compared with the actuator's rated torque to ensure that the joint can operate within its specified thermal and mechanical limits.

Evaluate Periodic High-Load Movements

Next, identify the movement phases that require higher torque.

For humanoid robots, these may include push-off, squatting, and standing up. For robotic arms, they may include lifting and moving workpieces.

Engineers should evaluate the required torque, operating duration, cycle frequency, and duty cycle, then compare these conditions with the actuator's intermittent torque specifications.

Check Transient Loads

Finally, evaluate brief overloads caused by impacts, sudden external forces, or emergency deceleration.

The maximum transient torque should remain within the actuator's specified peak torque limit, with an appropriate engineering safety margin.

Peak torque should not be used as the basis for sizing a joint that needs to operate at high torque continuously or repeatedly.

Consider the Complete Operating Profile

Torque is only one part of harmonic actuator selection. Engineers should also consider operating speed, acceleration, motion profile, ambient temperature, cooling conditions, radial and axial loads, overturning moments, torsional stiffness, positioning accuracy, and reducer service life.

For dynamic robotic applications, thermal performance and mechanical load capacity must be evaluated together.

A suitable harmonic actuator should not only meet the joint's maximum torque requirement but also maintain reliable performance throughout the complete operating cycle.

Conclusion

Rated torque, intermittent torque, and peak torque describe three different aspects of a harmonic actuator's operating capability. Rated torque determines its continuous load capacity, intermittent torque addresses periodic high-load movements, and peak torque defines its short-duration transient limit.

For humanoid robots and other dynamic robotic systems, understanding these distinctions helps engineers avoid improper actuator sizing and excessive operating loads. Rather than selecting a harmonic actuator based solely on its maximum torque, engineers should evaluate the complete motion profile, duty cycle, thermal conditions, and mechanical requirements of the joint.