Frameless Motor For Robot Joint

A frameless robot joint motor, also known as a frameless torque motor, is a split-type direct-drive brushless motor specifically developed for robotic joints. Unlike conventional motors, it eliminates the housing, bearings, and output shaft, providing only the stator and rotor assemblies. These components are directly integrated into the joint housing and become an integral part of the robotic joint.

Many robotics projects fail not because the motor's basic specifications are inadequate, but because engineers treat a frameless joint motor like a conventional off-the-shelf motor and overlook system-level design requirements such as mechanical integration, thermal management, and encoder matching. This can result in vibration, overheating, rotor-stator rubbing, inaccurate positioning, and other failures.

This article provides a comprehensive engineering and export-oriented guide to frameless robot joint motors, comparing them with conventional solutions and covering motor selection, mechanical integration, and export considerations to help robotics manufacturers avoid common development mistakes.

What Is a Frameless Robot Joint Motor?

Basic Definition

A frameless robot joint motor is a specialized type of permanent magnet brushless torque motor optimized for robotic joint applications.

Unlike a complete motor, a standard frameless motor is supplied with only two core components:

• Stator winding assembly

• Permanent magnet rotor

It does not include a housing, bearings, output shaft, or end caps, and therefore cannot be operated directly as a standalone motor.

The stator is mounted inside the robotic joint housing, while the rotor is secured to the main output shaft through an interference fit or adhesive bonding. The joint's own cross-roller bearing provides rotational support, while the joint housing acts as both the motor housing and the primary heat dissipation structure.

Cables and wiring harnesses can pass through the motor's large central hollow bore, allowing internal cable routing and enabling a highly integrated motor-and-joint design.

Inner-Rotor vs. Outer-Rotor Frameless Joint Motors

Inner-Rotor Frameless Joint Motor

In an inner-rotor configuration, the rotor is positioned inside the stator.

Its main advantages include:

• Low rotor inertia

• Fast dynamic response

• Excellent acceleration and deceleration performance

• Good suitability for frequent start-stop movements

This configuration is particularly suitable for robotic joints that require fast and precise movement, such as the shoulder, elbow, wrist, and waist joints of humanoid robots.

Under the same outer diameter, the hollow bore is generally smaller, making inner-rotor motors more suitable for low- to medium-torque applications.

Outer-Rotor Frameless Joint Motor

In an outer-rotor configuration, the rotor surrounds the outside of the stator.

Because the rotor diameter is larger, an outer-rotor motor can deliver higher continuous torque. However, its rotor inertia is also higher.

It is therefore well suited for high-load joints such as:

• Hip joints

• Knee joints

• Heavy-duty robotic joints

• Load-bearing joints

Outer-rotor motors can also provide a larger hollow bore. Their dynamic response is generally lower than that of inner-rotor designs, but they are particularly suitable for heavy-duty collaborative robots and quadruped robots.

HOLRY provides both inner-rotor and outer-rotor frameless joint motor series. Winding configurations, lamination stack length, and magnet designs can be customized according to different joint loads, covering applications ranging from lightweight robotic grippers and dexterous hands to high-load humanoid robot joints.

Frameless Robot Joint Motors vs. Conventional Joint Drive Solutions

The mainstream robotic joint drive architectures can generally be divided into three categories:

1. Enclosed servo motor + reducer

2. Integrated joint module

3. Frameless joint motor

Each solution has its own advantages and suitable application scenarios.

Solution 1: Conventional Enclosed Servo Motor + Harmonic Drive

Advantages

• Highly standardized products

• Ready to use

• Simple assembly

• Mature supply chain

• Short prototype development cycle

Limitations

The motor comes with its own housing and bearings, increasing the overall size and weight.

Couplings may introduce installation errors and backlash. Internal cable routing can also be difficult.

In addition, multiple bearing and transmission stages increase friction losses, which can increase the overall energy consumption of the robot.

Suitable Applications

This solution is suitable for:

• General industrial collaborative robots

• Projects with limited R&D resources

• Rapid prototype validation

Solution 2: Integrated Robot Joint Module

Advantages

The motor, reducer, encoder, and brake are integrated into a single module.

Robot manufacturers can simply purchase the complete module and install it into the robot, significantly reducing development work.

Limitations

• Higher cost

• Fixed dimensions

• Limited structural customization

• Less flexibility for deep mechanical integration

• Supply chain dependency on the module manufacturer

Suitable Applications

This solution is suitable for small- and medium-volume projects where the robot manufacturer does not want to invest heavily in motor and joint development.

Solution 3: Frameless Robot Joint Motor

A frameless joint motor consists primarily of the stator and rotor assemblies, allowing the robot manufacturer to design the surrounding mechanical structure according to the specific application.

Key Advantages

1. High Torque Density and Significant Weight Reduction

By eliminating the motor housing, redundant bearings, and other unnecessary components, a frameless motor can significantly reduce joint weight.

Under comparable torque requirements, the overall joint weight can potentially be reduced by approximately 30–40%, while the mechanical package can be substantially smaller.

Reducing robot weight also helps:

• Reduce joint loads

• Lower power consumption

• Improve battery life

• Improve human-robot collaboration safety

2. Large Hollow Bore for Internal Cable Routing

The annular hollow structure allows cables, signal lines, and even tubing to pass directly through the motor center.

This helps avoid external cable bending and wear while enabling internal cable routing through continuously rotating joints.

Large hollow bores are therefore an important design feature for humanoid robot joints.

3. Fewer Transmission Components and Higher System Stiffness

The rotor can be directly connected to the joint's main shaft, eliminating the need for a coupling and reducing transmission errors.

Through optimized magnetic circuit design, cogging torque can also be reduced, resulting in smoother low-speed operation and lower vibration.

This is particularly important for:

• Dexterous robotic hands

• Surgical robots

• High-precision robotic systems

4. Customizable Thermal Management for Continuous Heavy-Duty Operation

The stator can be directly mounted against the aluminum alloy joint housing, allowing the housing itself to act as a heat sink.

For heavy-duty applications, water-cooling channels can also be integrated into the joint housing.

This provides a highly customizable thermal path and can outperform externally mounted servo motor solutions in demanding continuous-duty applications.

5. High Degree of Customization for International Applications

Key motor parameters can be customized, including:

• Outer diameter

• Lamination stack length

• Hollow bore diameter

• Winding voltage

• Temperature sensors

• Hall sensors

Winding insulation and magnet materials can also be selected according to high-temperature, low-temperature, and high-humidity operating environments, helping meet international export and certification requirements such as CE.

Inherent Limitations

1. Higher Development Requirements

The robot manufacturer must handle:

• Joint cavity design

• Bearing selection

• Rotor-stator assembly tooling

• Thermal design

• Encoder integration

• Mechanical alignment

Therefore, frameless motors are not ideal for customers without sufficient mechanical and electromechanical engineering capabilities.

2. Strict Assembly Requirements

The air gap between the stator and rotor is very small.

Poor concentricity or improper assembly can result in:

• Rotor-stator rubbing

• Magnet damage

• Excessive vibration

• Increased operating noise

• Motor failure

For overseas customers without precision assembly capabilities, these risks can be particularly significant.

3. Prototype Costs May Not Always Be Lower

Although the motor kit itself can be cost-effective, the total development cost may increase during the prototype stage because of additional requirements for:

• Bearings

• Encoders

• Assembly tooling

• Mechanical development

• Engineering labor

The cost advantage of frameless motors becomes more significant when moving into high-volume OEM production.

Main Applications of Frameless Robot Joint Motors

Humanoid Robots

Frameless joint motors can be used in:

• Shoulders

• Elbows

• Hips

• Knees

• Waist joints

• Wrists

• Dexterous hands

Inner-rotor motors are generally preferred for joints requiring fast dynamic response, while outer-rotor motors can be considered for high-load hip and knee joints.

Large hollow bores also facilitate internal cable routing, making frameless motors an important drive technology for modern humanoid robots.

Collaborative Robots

Frameless motors are used in industrial collaborative robotic arms where lightweight construction and compact dimensions are important.

They help reduce overall robot weight while improving safety and enabling applications such as:

• Assembly

• Sorting

• Grinding

• Material handling

Medical Robots

Potential applications include:

• Surgical robots

• Rehabilitation exoskeletons

• Medical inspection systems

• Precision medical rotary joints

These applications require low noise, low vibration, high precision, cleanliness, and safety.

The frameless structure also provides greater flexibility for sealed mechanical designs and can be adapted to international medical equipment requirements.

Quadruped Robots and Specialized Exoskeleton Robots

Heavy-duty exoskeletons and robotic platforms require both high torque output and overall weight reduction.

Frameless motors can provide high torque density while accommodating complex and constantly changing dynamic loads.

Precision Gimbals and Specialized Robots

Applications include:

• Airborne gimbals

• Inspection rotary mechanisms

• Precision positioning systems

• Specialized robotic equipment

These applications typically have limited installation space and require lightweight motors capable of operating under demanding conditions such as vibration and wide temperature ranges.

Applications Where Frameless Motors May Not Be Suitable

Frameless motors may not be the best choice for:

• Simple production-line robots

• Small-volume rapid prototype projects

• Projects without sufficient mechanical integration capabilities

In these cases, a complete servo motor or integrated joint module may provide a faster and more economical development path.

Five Key Considerations When Selecting a Frameless Robot Joint Motor

Distinguish Continuous Torque from Peak Torque

Robot joints may experience:

• Gravity holding loads

• Continuous motion

• Short-term impact loads

• Acceleration and deceleration loads

Continuous rated torque determines the motor's long-term operating capability, while peak torque only represents short-duration torque output.

Motor selection should therefore be based on actual motion profiles and operating conditions.

For heavy-duty applications and high-temperature export markets, it is recommended to reserve approximately 20–30% torque safety margin.

Selecting a motor based only on its peak torque rating can result in overheating and permanent magnet demagnetization during continuous operation.

Select the Right Inner-Rotor or Outer-Rotor Configuration

For joints requiring:

• Fast movement

• Frequent start-stop operation

• High dynamic response

such as wrists, elbows, and shoulders, inner-rotor motors are generally preferred.

For high-load joints requiring high torque, such as hips and knees, outer-rotor motors should be evaluated.

The required hollow bore diameter must also be checked to ensure sufficient space for cables and wiring.

Match Rotor Inertia to the Joint

Joint dynamic performance depends heavily on inertia matching.

Dexterous robotic hands require low-inertia motors for precise and delicate movements.

Heavy-load joints require an appropriate inertia match to maintain stability.

Poor inertia matching can result in:

• Excessive impact

• Oscillation

• Poor control performance

• Difficult commissioning

Thermal Management and Environmental Adaptation

This is particularly important for international exports.

A frameless motor does not have its own complete cooling structure. Heat dissipation depends heavily on the robot joint housing.

For high-temperature tropical markets, the thermal path must be strengthened.

For low-temperature environments, the low-temperature performance of the permanent magnets must be verified.

For high-humidity environments, winding insulation and protection requirements must be carefully evaluated.

A motor that performs well during domestic prototype testing may experience significant failures after being exported because environmental adaptation was not properly considered.

Electrical and Feedback Component Matching

A frameless motor typically does not include an encoder.

The robot manufacturer therefore needs to select and integrate an appropriate ring-type encoder.

The motor driver must support torque control and be configured with the motor's electrical parameters, including:

• Resistance

• Inductance

• Back EMF

• Motor constants

• Temperature sensor specifications

HOLRY can provide complete motor electrical parameter files and temperature sensor specifications to help overseas customers complete system integration and commissioning.

Common Frameless Motor Integration Mistakes

Mistake 1: Assuming a Frameless Motor Can Run Immediately After Connecting a Driver

A frameless motor consists only of the stator and rotor and does not include bearings, support structures, or feedback components.

It cannot simply be connected to a driver and powered on.

The system must first complete:

• Mechanical assembly

• Concentricity calibration

• Encoder installation

• Air-gap verification

• Control parameter configuration

Some overseas customers mistakenly treat frameless motors as complete motors and power them directly, which can cause rotor-stator rubbing and permanent magnet damage.

Mistake 2: Selecting a Motor Based Only on Peak Torque

The peak torque listed in a datasheet usually represents short-term capability.

Robot joints may continuously support gravity and loads, so continuous torque is often more important than peak torque.

Selecting a motor based only on peak torque can result in continuous overheating and torque degradation during actual operation.

Mistake 3: Ignoring Rotor-Stator Concentricity Requirements

Because the rotor-stator air gap is small, improper assembly tooling or excessive concentricity error can cause rubbing and motor damage.

Overseas repair costs can be extremely high.

Suppliers should therefore provide:

• English assembly manuals

• Air-gap specifications

• Concentricity requirements

• Installation drawings

• Recommended assembly procedures

Mistake 4: Treating Prototype Selection as Mass-Production Selection

Prototype development focuses primarily on speed and flexibility.

Mass production requires additional evaluation of:

• Batch consistency

• Production yield

• Process stability

• Quality control

• Long-term supply capability

Some suppliers can produce prototypes but cannot support stable high-volume production.

Mistake 5: Directly Exporting a Domestic Prototype Configuration

Different international markets have different:

• Ambient temperatures

• Humidity levels

• Power supply conditions

• Safety requirements

• Certification requirements

Motor voltage, insulation, magnet materials, and thermal design may therefore need to be adapted for the target market.

A domestic prototype configuration should not automatically be treated as an export-ready configuration.

FAQ

Q1: Does a Frameless Robot Joint Motor Always Need to Be Used with a Harmonic Drive?

A: Not necessarily.

Some lightweight direct-drive joints can operate without a reducer.

However, many humanoid and collaborative robot joints use harmonic drives to increase the final output torque and reduce the required motor torque.

Q2: Do Frameless Joint Motors Include Brakes and Encoders?

A: Standard frameless motors generally do not include brakes or encoders.

These are system-level components selected according to the robot's requirements.

HOLRY can provide provisions for sensor installation and integration so that robot manufacturers can select the appropriate encoder and brake components.

Q3: What Technical Documents Should Overseas Buyers Request?

A: Overseas buyers should request:

• Motor datasheets

• Torque-speed curves

• Rotor-stator air-gap specifications

• Concentricity requirements

• Winding electrical parameters

• Temperature sensor specifications

• English assembly instructions

• Installation drawings

• Relevant export certification documents

These documents can significantly reduce integration and commissioning risks.

Q4: What Can HOLRY Customize for Frameless Robot Joint Motors?

A: HOLRY supports customization of:

• Outer diameter

• Lamination stack length

• Hollow bore diameter

• Winding voltage

• Permanent magnet configuration

• Temperature sensors

• Hall sensors

HOLRY can also support:

• Prototype development

• Small-batch trial production

• Large-volume OEM production

Q5: What Are the Most Common Causes of Frameless Joint Motor Failure?

A: The most common causes include:

1. Insufficient thermal design leading to overheating and demagnetization

2. Rotor-stator misalignment causing rubbing

3. Incorrect load calculations and insufficient torque margin

4. Encoder and motor driver parameter mismatch

5. Improper assembly procedures

6. Insufficient environmental adaptation for overseas markets

Conclusion

HOLRY specializes in the R&D and manufacturing of frameless robot joint motors, offering both standard and highly customized solutions.

Our product portfolio covers inner-rotor and outer-rotor frameless motors for applications including:

• Dexterous robotic hands

• Humanoid robot joints

• Collaborative robotic arms

• Medical robots

• Quadruped robots

• Specialized robotic systems

HOLRY also provides comprehensive English technical documentation to support overseas OEM customers throughout the entire product lifecycle, from prototype development and testing to mass production.

If you are currently selecting a drive motor for a robotic joint, contact the HOLRY technical team with your joint load requirements, available installation space, motion profile, and target export market to receive a free motor selection evaluation.