What Is Frameless Motor

Many purchasing managers and engineers tend to confuse frameless motors with conventional servo torque motors, or even assume that a frameless motor is simply a standard motor with its housing removed.

In fact, a frameless motor is not a complete motor that can be installed and operated directly. Instead, it is a set of core electromagnetic motor components consisting primarily of a stator and rotor. It does not include the housing, bearings, output shaft, end covers, or other mechanical structures. Engineers need to integrate the motor directly into the equipment itself.

Basic Definition and Core Structure

What Is a Frameless Motor?

A conventional servo motor is a complete, self-contained motor. Its housing, bearings, shaft, and windings are fully integrated. Users simply mount the motor and connect the load through a coupling before operation.

A frameless motor, by contrast, is supplied as two core components:

Stator (laminated core with windings) + Rotor (with permanent magnets)

It does not include a housing, bearings, output shaft, or end covers.

The equipment housing itself serves as the motor housing, while the equipment's own bearings support the rotor. The machine's main shaft passes directly through the rotor bore, allowing the motor to become an integrated part of the complete machine.

Analysis of the Two Core Components

Stator:
The stator consists of laminated silicon-steel sheets and copper windings. It is securely mounted inside the equipment housing and generates a rotating magnetic field when energized.

Some models include built-in temperature sensors for overheating protection. Hall sensors and encoder mounting bases can also be configured according to customer requirements.

Rotor:
The rotor contains or is bonded with high-performance permanent magnets and is directly mounted on the machine's rotating main shaft, rotating together with the load.

Frameless motors are generally available in two structural configurations: inner-rotor and outer-rotor designs.

• Inner-rotor motors feature lower inertia and faster response.

• Outer-rotor motors have a larger rotor diameter and provide higher torque at low speeds, making them suitable for heavy-duty joint applications.

How Does a Frameless Motor Work?

The operating principle is essentially the same as that of a brushless servo motor.

The servo drive supplies three-phase current to the stator windings, generating a rotating magnetic field. This magnetic field drives the permanent-magnet rotor to rotate synchronously.

An external encoder is typically used to achieve closed-loop control of position, speed, and torque.

The key difference is that all mechanical support structures are provided by the machine itself, while the motor components are primarily responsible for generating torque and rotational speed.

Frameless Motor vs. Conventional Enclosed Servo Motor

Key Advantages of Frameless Motors

Extremely High Torque Density, Smaller Size and Lower Weight

By eliminating the motor housing and unnecessary mechanical structures, frameless motors can significantly reduce the axial length while maintaining the same torque output.

The overall equipment weight can be reduced by approximately 20–40%, making frameless motors particularly suitable for space-constrained and weight-sensitive applications such as robotic joints, handheld medical devices, and airborne equipment.

Eliminates Intermediate Transmission, Zero Backlash and Higher Positioning Accuracy

The rotor can be directly mounted onto the load shaft, eliminating intermediate transmission components such as couplings, timing belts, and gears.

This helps eliminate transmission backlash, increase system stiffness, improve response speed, and achieve extremely high repeat positioning accuracy.

As a result, frameless motors are particularly suitable for precision motion applications.

High Integration and Greater Design Flexibility

Many frameless motors feature a large hollow-ring design. Cables, tubing, and other components can pass directly through the motor's central hollow bore.

This greatly simplifies internal cable routing and is especially useful in applications such as robotic joints and rotary tables.

Flexible Thermal Management

Because the stator can be mounted directly against the equipment housing, the machine housing itself can serve as a heat-dissipation base.

The system can use natural cooling or incorporate water-cooling or oil-cooling channels, making frameless motors suitable for continuous high-torque operation.

Suitable for Special Environments and Customized Export Applications

The motor components can be specially protected for demanding operating environments, including vacuum, high- and low-temperature, and high-humidity applications.

This makes them suitable for medical equipment, aerospace systems, semiconductor equipment, and other applications requiring international certifications for export markets.

Inherent Limitations of Frameless Motors

Higher System-Level Engineering Requirements

A frameless motor is not a plug-and-play solution. The equipment engineering team must handle mechanical structure design, bearing selection, stator-rotor concentricity, encoder integration, and thermal management.

The development workload is therefore significantly higher than that of a conventional servo motor, making frameless motors less suitable for small projects or rapid prototyping.

Thermal Management Becomes the Responsibility of the Equipment Design

A frameless motor does not have its own integrated cooling structure.

If the equipment does not provide sufficient heat dissipation, continuous operation can easily result in overheating alarms or permanent-magnet demagnetization.

This is one of the most common failure points in overseas equipment projects.

Strict Assembly Requirements

The air gap between the stator and rotor is very small. Misalignment or eccentric assembly can cause abnormal current, excessive vibration, and permanent-magnet rubbing or damage.

Therefore, strict requirements are placed on factory assembly processes, fixtures, and tooling.

Total Cost May Not Be Lower for Small-Batch Projects

The motor kit itself may offer a cost advantage. However, once mechanical development, tooling, encoders, bearings, and other components are included, the total cost of a small-batch project can actually be higher than purchasing a complete servo motor.

The cost advantages of frameless motors are generally more apparent in large-volume OEM production.

When Should You Choose a Conventional Enclosed Servo Motor?

For standard automation lines, conventional machine tools, conveyor systems, projects with limited engineering resources, rapid deployment requirements, or small-batch prototypes, a complete servo motor is generally the better choice.

It reduces development complexity and overall project risk.

Major Application Industries for Frameless Motors

Robotics — The Largest Application Market

Collaborative robots, humanoid robot joints, quadruped robots, and exoskeleton robots are major application areas.

Hollow frameless motors have become a mainstream solution for robotic joints because they can deliver high torque within limited installation space while providing convenient internal cable routing.

Global OEM demand continues to grow.

Medical Precision Equipment

Applications include surgical robots, rehabilitation equipment, and medical rotary tables.

Medical equipment places strict requirements on size, weight, cleanliness, and safety.

Frameless motors can be integrated directly into the equipment, minimizing exposed transmission components and helping meet overseas medical equipment certification requirements.

Semiconductor and Precision Automation

Applications include wafer rotary stages and precision inspection rotary mechanisms.

These systems prioritize zero backlash and high stiffness. Directly driving the load reduces errors introduced by conventional transmission systems.

Aerospace and Special-Purpose Equipment

Applications include airborne actuators and electro-optical or sensor gimbals.

These applications require strict weight control and reliable operation under demanding high- and low-temperature conditions.

High-End Machine Tools and Direct-Drive Rotary Tables

Frameless motors can be integrated directly into rotary tables to provide direct-drive rotation.

This eliminates traditional worm-and-wheel transmission mechanisms while improving rotational speed and positioning accuracy.

Applications Not Recommended:
For conventional production lines, standard engraving equipment, and simple transmission mechanisms, using a frameless motor may only increase development complexity without providing meaningful performance benefits.

Five Key Points for Frameless Motor Selection and Integration

Distinguish Continuous Torque from Peak Torque

Motor selection must be based on the actual operating conditions of the equipment.

Engineers need to distinguish between continuous RMS torque and short-term peak torque.

Many customers select motors based only on peak torque while ignoring continuous torque requirements during long-term operation. This can cause overheating and motor failure.

For equipment exported to different overseas markets and operating environments, sufficient torque safety margin should always be reserved.

Mechanical Structure and Concentricity Control

The stator must be securely fixed to the machine frame, while the rotor must be firmly connected to the main shaft.

The stator-rotor air gap must be strictly maintained.

Improper concentricity can result in vibration, noise, and permanent-magnet wear.

For equipment intended for export, transportation vibration during ocean freight should also be considered, and appropriate anti-loosening structural designs should be implemented.

Thermal Management Is Critical

Frameless motors do not come with an integrated cooling system.

Under continuous heavy-load conditions, heat dissipation typically relies on the aluminum housing or other machine structures. Water cooling may be required when necessary.

A common issue among overseas customers is directly powering the motor at full load without designing an appropriate thermal path. The motor may overheat within a short period of time.

For equipment exported to tropical and high-temperature markets, additional thermal margin should be considered.

Match the Feedback Encoder with the Servo Drive

A frameless motor itself normally does not include an encoder.

The equipment manufacturer needs to select and integrate an encoder according to the application.

The encoder's accuracy, communication protocol, and mounting dimensions must be fully compatible with the servo drive.

The drive should support brushless torque motor / servo operation. A standard general-purpose drive may not be able to fully utilize the performance of the frameless motor.

Evaluate the Engineering Capabilities of the Target Customer

If an overseas end customer does not have sufficient mechanical integration capabilities, supplying a frameless motor as loose components is generally not recommended.

Incorrect assembly can easily lead to equipment failure.

For overseas OEM customers, HOLRY can provide standardized frameless motor kits together with complete assembly instructions and technical documentation, helping reduce integration risks for international customers.

Common Misunderstandings About Frameless Motors

Misconception 1: A Frameless Motor Is Simply a Conventional Servo Motor Without the Housing

A frameless motor is essentially an electromagnetic motor assembly.

Bearings, thermal management, feedback systems, and mechanical support structures all need to be designed externally.

It cannot simply replace a complete servo motor. A direct replacement without redesigning the mechanical system will inevitably create integration problems.

Misconception 2: Motor Selection Should Be Based Only on Peak Torque

Peak torque is only available for short-term operation.

For long-term operation, continuous torque and thermal management are much more important.

Even a high-performance motor can overheat and suffer permanent-magnet demagnetization if the cooling system is inadequate.

Misconception 3: Frameless Motors Always Reduce Costs for Small-Batch Prototypes

For small-batch projects, development, tooling, encoder, bearing, and other supporting costs can significantly increase the total investment.

The full cost advantages of frameless motors are generally realized in high-volume OEM production.

Misconception 4: Concentricity Does Not Need to Be Strictly Controlled

Because the stator-rotor air gap is extremely small, assembly misalignment can cause rotor-stator contact, permanent-magnet damage, excessive vibration, and abnormal noise.

For overseas equipment, on-site maintenance and replacement costs can be extremely high.

Misconception 5: Export Equipment Can Use the Same Design Regardless of the Environment

Equipment shipped to regions with high temperature, high humidity, or high dust levels requires additional consideration of winding insulation and permanent-magnet protection.

For low-temperature markets, the low-temperature performance of the permanent magnets must also be evaluated.

Domestic prototype designs should not simply be transferred to overseas markets without environmental adaptation.

FAQ

Q1: Can I use a frameless motor immediately after purchasing it?

A: No.

A frameless motor is supplied as a stator + rotor kit. The complete machine must provide the housing, bearings, main shaft, encoder, and servo drive.

Mechanical assembly and electrical commissioning must be completed before the motor can operate properly.

Q2: What Is the Relationship Between a Frameless Motor and a Direct-Drive Torque Motor?

A: A frameless motor is one form of direct-drive torque motor.

Direct-drive motors are available both as complete motors with housings and as frameless motor kits.

The frameless version requires integration into the customer's equipment.

Q3: What Types of Projects Are Suitable for Frameless Motors?

A: Frameless motors are particularly suitable for applications that:

• Have strict space constraints

• Require lightweight design

• Demand zero backlash and high positioning accuracy

• Require high-volume OEM production

• Have engineering teams capable of mechanical and servo integration

For projects with limited R&D resources or small-batch prototypes, a complete servo motor is generally a better option.

Q4: Can Frameless Motors Be Exported? What Supporting Components Are Required?

A: Yes.

HOLRY frameless motors support export requirements such as CE certification.

For overseas customers, suppliers should provide assembly manuals, torque curves, temperature protection parameters, and technical guidance for cooling systems, encoders, and bearing selection.

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

A: Common failure causes include:

• Insufficient thermal management resulting in overheating and permanent-magnet demagnetization

• Stator-rotor misalignment causing rotor-stator contact

• Encoder and servo drive incompatibility

• Improper bearing selection

• Incorrect load calculations and insufficient torque margin

Conclusion

A frameless motor is not a “higher-performance version of a conventional complete servo motor.”

Instead, it is a core electromagnetic motor assembly designed for highly integrated and customized equipment.

Its key advantages come from its high level of integration, but this also creates higher requirements for system-level mechanical and electrical engineering.

Motor selection should not be based solely on torque and speed specifications listed in a product catalog.

Engineers should consider the complete application, including:

• Available installation space

• Weight-reduction requirements

• Production volume

• R&D and engineering capabilities

• Target export market

• Operating temperature and environmental conditions

• Thermal management requirements

• Encoder and servo drive compatibility

For the right application, a frameless motor can help achieve smaller equipment size, higher precision, lower weight, and better system integration.

However, selecting a frameless motor without sufficient engineering preparation can result in development delays, higher failure rates, and significantly increased overseas after-sales costs.

HOLRY specializes in the R&D and manufacturing of frameless motors and servo torque motors.

With 15 years of practical experience in motion-control engineering and 10 years of experience serving global international trade projects, HOLRY provides standardized frameless motor kits while also supporting customized windings, hollow-bore dimensions, and optional sensors.

These solutions are designed to meet the requirements of robotics, medical equipment, precision automation, and other high-end OEM applications.

If you are evaluating whether a frameless motor is suitable for your project, contact the HOLRY technical team and provide your load conditions, available installation space, and target market.

HOLRY can provide a free motor selection evaluation and technical support to help you identify the right frameless motor solution for your application.