Precision Reducers for Industrial Robots: From Conventional Transmission to Self-Compensating Backlash Technology

Hello everyone, I’m Theodore Li, Technical Director at HONPINE. Today, I would like to talk about precision reducers for industrial robots.

What Is a Precision Reducer?

According to commonly used industry classifications, precision reducers are generally characterized by low transmission error and backlash, with precision requirements typically reaching the arc-minute level or better.

Backlash is one of the key factors affecting positioning performance. Clearance between mating gears can create lost motion between the input and output shafts, directly affecting positioning accuracy, repeatability, and motion control performance.

Based on the primary transmission mechanism, precision reducers can generally be divided into four major categories.

Precision Cycloidal Reducers

RV reducers and cycloidal pinwheel reducers are representative examples. They offer high load capacity and good resistance to impact loads, but their structures are relatively complex and manufacturing costs can be high.

Harmonic Reducers

Harmonic transmission relies on the elastic deformation of the flexspline to transmit motion and torque. This architecture provides high reduction ratios, compact dimensions, and high transmission precision, but its torsional rigidity and load capacity can be lower than those of some rigid transmission architectures, making application selection particularly important for heavy-load conditions.

Precision Planetary Reducers

Planetary transmission offers high efficiency, high torque density, and a relatively rigid transmission structure. However, conventional planetary architectures generally face limitations in single-stage reduction ratio and backlash when compared with specialized precision transmission technologies.

Why Are Industrial Robots Placing Higher Demands on Precision Reducers?

Industrial robot applications have developed into highly specialized segments. Different payload levels, processes, motion profiles, and joint configurations place different requirements on transmission precision, torque capacity, rigidity, and service life.

The appropriate precision reducer can therefore vary not only between different types of industrial robots, but also between different joints within the same robot.

Six-Axis Robots Below 10 kg Payload

For lightweight six-axis robots, compact dimensions and precision are often prioritized.

All six joints may use harmonic reducers, particularly where the robot requires a compact transmission with a high reduction ratio and precise motion control.

The compact size, low weight, high reduction ratio, and precision of harmonic transmission make it suitable for lightweight robots operating within constrained mechanical envelopes.

Six-Axis Robots with 10–50 kg Payload

Medium-payload six-axis robots are commonly used for applications such as automotive component handling, metal processing, and other industrial operations requiring a balance between payload capacity, motion accuracy, and stability.

These robots may combine harmonic and RV reducers according to the requirements of different joints.

For example, harmonic transmission can be used in wrist and end-effector joints where compact dimensions, high reduction ratios, and precise motion are important, while RV transmission can be used in higher-load joints requiring greater rigidity and load capacity.

Six-Axis Robots Above 50 kg Payload

Heavy-payload six-axis robots require high load capacity and stable precision under demanding operating conditions.

RV reducers are widely used in high-load robot joints because their transmission architecture provides high rigidity and can withstand substantial radial and axial loads.

These characteristics are particularly important when heavy-payload robots are used for material handling, welding, machining, and other operations involving large workpieces.

SCARA Robots Below 20 kg Payload

SCARA robots place particularly high demands on positioning accuracy and repeatability.

The rotational joints therefore require transmission systems with good accuracy and dynamic response. Harmonic transmission can be used in rotary joints, while the vertical Z-axis mechanism may use a planetary transmission, belt drive, or another suitable linear-motion architecture.

Parallel and Delta Robots

Parallel and Delta robots are characterized by high speed and high structural rigidity, while their joint transmission requirements are strongly influenced by the robot's architecture and motion profile.

Many parallel robots use relatively low-ratio planetary gearboxes in the base joints to support high-speed movement and maintain low reflected inertia.

Depending on the design, cycloidal or harmonic transmission may also be used in base joints.

For additional axes or end-of-arm mechanisms, planetary gearboxes are commonly used. However, in high-precision assembly applications, a harmonic reducer may also be selected for the end joint to improve positioning performance.

Why Conventional Planetary Reducers Face a Precision Challenge?

Across many precision manufacturing applications, harmonic and RV transmission systems have traditionally occupied important positions in industrial robotics.

Conventional planetary reducers, despite their high efficiency, rigidity, and torque density, have historically faced challenges in achieving the same level of precision and compact integration as specialized harmonic or RV transmission systems.

High-end precision planetary gearboxes can still achieve very low backlash and remain suitable for demanding applications. Some precision planetary products can achieve backlash levels below 60 arcsec, providing an alternative for applications requiring high positioning accuracy.

However, high precision can come with higher product costs and longer lead times depending on the supplier and configuration.

As the industrial robotics market continues to expand and robot manufacturers require faster development cycles and more flexible supply chains, this creates an opportunity for new precision planetary transmission architectures.

HONPINE's Approach to Precision Planetary Transmission

At HONPINE, we have been working on a different approach to planetary transmission.

By introducing self-compensating backlash technology, HONPINE aims to improve the precision, rigidity, and precision retention of planetary transmission systems.

The technology is designed for applications such as industrial robots and machine tools, where maintaining transmission accuracy over long-term operation is particularly important.

The objective is not simply to reduce initial backlash, but to address one of the fundamental challenges of precision transmission: maintaining accuracy as mechanical components wear over time.

What Technical Challenges Can Self-Compensating Planetary Actuators Address?

Traditional precision transmission systems face an unavoidable issue: mechanical wear can gradually affect transmission accuracy.

During long-term operation, continuous gear meshing and repeated load cycles can change the effective clearance between mating components. As backlash increases, lost motion can increase and positioning accuracy can gradually deteriorate.

For equipment such as industrial robots and machine tools, this loss of precision can eventually affect process quality, repeatability, and service life.

Self-compensating backlash technology provides a different approach to this problem.

Automatic Backlash Compensation During Wear

Unlike conventional passive backlash-reduction structures, a self-compensating transmission incorporates an adaptive compensation mechanism within the transmission system.

As the transmission components experience wear, the mechanism can compensate for changes in effective gear clearance and maintain a more stable transmission relationship.

The purpose is to reduce the rate at which transmission accuracy deteriorates during long-term operation and extend the effective precision life of the reducer.

This distinction is important.

Reducing initial backlash and maintaining low backlash throughout the service life are two different engineering objectives. A precision transmission should therefore be evaluated not only by its initial backlash specification, but also by how its transmission accuracy changes after prolonged operation.

Self-Compensating Backlash Technology for Harmonic Transmission

HONPINE's self-compensating approach is not limited to planetary transmission.

We have also developed a dual-flexspline self-compensating harmonic transmission architecture designed to maintain low effective backlash over long-term operation.

Dual-Flexspline Self-Compensating Harmonic Transmission

The HONPINE dual-flexspline harmonic transmission uses a structural arrangement involving inner and outer flexsplines together with a limiting ring.

This architecture is designed to continuously suppress effective backlash and maintain stable transmission performance during operation.

According to HONPINE's internal product testing and comparison data, the lost motion of this architecture can be approximately half that of comparable overseas harmonic reducers under the specified test conditions.

The actual performance depends on the specific product size, test methodology, load condition, and measurement system, so direct comparisons should always be made using equivalent test conditions.

From Initial Precision to Precision Life

For precision transmission systems, initial accuracy is only one part of the engineering equation.

A reducer may achieve excellent backlash when new, but if backlash increases significantly as the transmission wears, the practical precision life of the component may be limited.

This is why HONPINE's development focus extends from initial transmission precision to precision retention over the operating life.

For industrial robots, machine tools, and humanoid robots, this means evaluating not only initial backlash and positioning performance, but also how transmission characteristics evolve under repeated motion, load, and wear.

Self-compensating backlash technology is intended to address this long-term performance challenge by actively compensating for changes in transmission clearance rather than relying solely on the initial mechanical tolerance of the gear system.

Conclusion

The development of precision reducers is moving beyond the simple pursuit of lower initial backlash.

Industrial robots, machine tools, and humanoid robots require transmission systems that combine precision, rigidity, compact dimensions, torque density, and long-term accuracy retention.

Harmonic and RV transmission systems remain important technologies for precision robotics, while conventional planetary transmission continues to offer advantages in efficiency, rigidity, and torque density.

The opportunity for self-compensating planetary and harmonic transmission is to address the gap between these performance characteristics by introducing a mechanism that can compensate for transmission clearance changes during long-term operation.

At HONPINE, we are continuing to develop precision transmission technologies around this concept, from self-compensating planetary actuators to dual-flexspline harmonic transmission and lightweight PEEK-based harmonic transmission for humanoid robots.

For us, the goal is not simply to achieve high precision when a transmission leaves the factory. The more important question is how much of that precision can be retained throughout the service life of the robot or machine.