Crossed Roller Bearings in Humanoid Robot Joints: From Bearing Design to Joint Performance

As humanoid robots continue to develop toward more compact, lightweight, and highly integrated designs, the mechanical performance of each joint becomes increasingly important.

A humanoid robot joint typically integrates a motor, reducer, encoder, housing, shaft, bearing system, and other components into a limited installation space. In this environment, the bearing is not simply a supporting component. Its precision, rigidity, friction, and load capacity can directly influence the performance of the complete joint.

For applications requiring compact dimensions, high rotational accuracy, and multi-directional load capacity, crossed roller bearings are an important bearing solution to consider.

1. What Is a Crossed Roller Bearing?

A crossed roller bearing is a type of precision roller bearing in which cylindrical rollers are arranged alternately at approximately 90-degree orientations.

This unique arrangement allows the bearing to support radial loads, axial loads, and moment loads within a relatively compact structure.

Compared with conventional bearing arrangements that may require multiple bearings to support different load directions, a crossed roller bearing can simplify the mechanical structure of certain rotary applications.

This makes crossed roller bearings particularly relevant to precision robotics, rotary tables, automation equipment, and compact robotic joint mechanisms.

For engineers looking for a suitable bearing for a humanoid robot joint, our guide How to Choose a Precision Crossed Roller Bearing for a Humanoid Robot Joint provides additional information on the main selection factors.

2. Why Are Crossed Roller Bearings Suitable for Robotic Joints?

Humanoid robot joints can experience complex loading conditions during movement.

For example, when a robot arm holds an object, an external force can generate radial and axial loads as well as a tilting moment around the joint.

A bearing used in this type of mechanism needs to maintain stable rotation while resisting deformation.

The crossed roller bearing configuration can provide several advantages for these applications:

• High rotational rigidity

• High running accuracy

• Compact radial dimensions

• Ability to accommodate combined loads

• Good moment-load capacity

• Potential for simplified joint structures

The actual performance depends on the bearing design, preload, installation conditions, housing stiffness, shaft geometry, and operating environment.

3. The Importance of Bearing Rigidity

Rigidity is particularly important in precision robot joints.

When an external force is applied to a robot arm or leg, the joint structure can experience deformation. Excessive angular displacement may affect the actual position of the robot's end effector.

The bearing is therefore one part of the overall mechanical stiffness of the joint.

A suitable crossed roller bearing can provide a rigid rotational support structure, but bearing selection alone cannot determine the final joint stiffness.

Engineers should evaluate the complete system, including:

• Bearing preload

• Raceway geometry

• Roller contact conditions

• Housing stiffness

• Shaft stiffness

• Mounting accuracy

• Fastening method

• Applied radial and axial loads

• Moment loads

The bearing and its surrounding components should be designed as one mechanical system.

4. Bearing Accuracy and Robot Joint Performance

Precision is another important consideration.

In a humanoid robot actuator, the encoder may provide highly precise position feedback. However, mechanical errors between the encoder and the output structure can still influence the actual positioning performance.

Bearing-related factors may include:

• Radial runout

• Axial runout

• Rotational accuracy

• Internal clearance

• Preload

• Raceway accuracy

• Roller dimensional accuracy

These parameters should be considered together with the accuracy requirements of the complete actuator.

International standardization of rolling bearings is handled within ISO/TC 4, which covers rolling-bearing terminology, geometrical product specifications, roller bearings, ball bearings, load ratings and life, and testing and measurement.

Engineers should refer to the applicable standards when defining bearing specifications for precision applications.

5. Preload: Finding the Right Balance

Preload can be used to reduce internal movement and improve bearing rigidity.

However, more preload does not automatically mean better performance.

Excessive preload can increase:

• Friction torque

• Operating temperature

• Energy consumption

• Wear

• Risk of reduced service life

For a humanoid robot joint, the appropriate preload should be determined according to the actual operating conditions.

A slowly rotating high-torque joint may have different requirements from a high-speed wrist or hand actuator.

Therefore, preload should be specified based on the complete application rather than selected independently.

6. Installation Accuracy Matters

Even a high-precision bearing cannot compensate for an inaccurate mounting structure.

The performance of a crossed roller bearing can be affected by:

• Housing roundness

• Mounting surface flatness

• Shaft accuracy

• Installation concentricity

• Bolt tightening

• Assembly tolerances

• Structural deformation

For this reason, bearing selection should take place together with the mechanical design of the housing and shaft.

This is particularly important for integrated humanoid robot joints, where several precision components must work together within a small installation envelope.

7. Crossed Roller Bearings vs. Other Bearing Solutions

Crossed roller bearings are not the only option for robotic applications.

Depending on the load, speed, available space, and required performance, engineers may also consider:

Angular Contact Ball Bearings

Suitable for applications requiring combined radial and axial load support and high-speed operation.

Four-Point Contact Bearings

Can be considered when combined axial and radial loads need to be accommodated within a compact structure.

Thin-Section Deep Groove Ball Bearings

Useful when reducing radial section and weight is an important design consideration.

The correct bearing type should be selected according to the actual requirements of the joint rather than based solely on the bearing name or nominal load rating.

8. Example: Compact Bearings for Robotic Applications

For compact robotic mechanisms, a thin-section crossed roller bearing can be considered when designers need to balance installation space, rigidity, and load capacity.

Beining's product portfolio includes various crossed roller bearings for robotics and automation applications, including compact and high-precision configurations.

For example, the RAU4505C0P5 is a 45 * 56 * 5 mm thin-section crossed roller bearing designed for applications where compact dimensions and rigidity are important considerations.

The specific bearing model should be selected based on the required shaft diameter, outside diameter, width, load, speed, preload, accuracy, and expected service life.

9. When Should You Consider a Customized Bearing?

Standard bearing dimensions may not always match the mechanical architecture of an integrated robot joint.

A customized bearing may be considered when the application requires:

• Non-standard dimensions

• Special mounting interfaces

• Specific preload

• Special sealing

• Customized internal geometry

• Reduced friction

• Higher accuracy

• Special lubrication requirements

For these applications, working directly with a bearing manufacturer during the design stage can help engineers optimize the bearing around the joint architecture.

For more information, see our article Custom Bearings for Robotics: When Standard Bearings Are Not Enough.

10. Choosing a Bearing Manufacturer for Robotics

For robotic applications, the bearing supplier's manufacturing capability is an important consideration in addition to the product specification.

Beining Intelligent Technology (Zhejiang) Co., Ltd., formerly Ningbo Beining Bearing Co., Ltd., was established in February 2013.

The company operates a manufacturing facility covering more than 30,000 m² and has more than 200 employees, including more than 20 engineers, senior engineers, and master's-level professionals.

Beining operates more than 100 automated bearing production lines and manufactures crossed roller bearings, four-point contact bearings, angular contact ball bearings, thin-section deep groove ball bearings, and other precision bearing products.

The company also supports custom non-standard bearing development for robotics, automation, and other demanding applications.

Conclusion

Crossed roller bearings can provide an effective combination of compact dimensions, rotational accuracy, rigidity, and multi-directional load capacity for many robotic applications.

For humanoid robot joints, however, bearing selection should not be considered independently.

Load conditions, preload, installation accuracy, housing stiffness, shaft geometry, operating speed, lubrication, and service life all need to be evaluated as part of the complete joint system.

For standard applications, an existing crossed roller bearing may provide a suitable solution. When the joint architecture requires non-standard dimensions or performance characteristics, customized bearing development can provide additional flexibility.

Beining supports both standard precision bearings and customized bearing solutions for robotics and automation applications.