Custom Bearings for Humanoid Robot Joints: Key Design Considerations

As humanoid robots continue to evolve toward more compact, lightweight, and highly integrated designs, the bearing system inside each joint has become increasingly important. A robot joint is no longer simply a motor combined with a gearbox. Modern integrated joint modules often combine the motor, reducer, encoder, brake, and bearing system within a limited space.

For bearing manufacturers and robot developers, this creates a different set of requirements compared with conventional industrial machinery.

1. Why Bearings Matter in Humanoid Robot Joints

A humanoid robot may contain dozens of rotational joints, including the shoulder, elbow, wrist, hip, knee, and ankle. Each joint needs to provide controlled and repeatable rotational movement while supporting radial, axial, and moment loads.

The bearing therefore needs to do more than simply reduce friction.

It can directly affect:

• Joint positioning accuracy
• Rotational stiffness
• Backlash and repeatability
• Overall joint size
• Robot weight
• Operating noise and vibration
• Service life

For high-precision humanoid applications, bearing selection becomes part of the overall joint design rather than an isolated component decision.

2. Compactness Is a Major Design Requirement

Integrated actuator modules typically have strict dimensional limitations. Designers want to maximize torque density while keeping the joint as compact and lightweight as possible.

A bearing with a large outside diameter and small cross-section can provide a useful combination of load capacity and compactness.

This is one reason crossed roller bearings are frequently considered for robotic joint applications.

Their rollers are arranged alternately at right angles, allowing the bearing to accommodate radial loads, axial loads, and moment loads with a single bearing arrangement.

This can simplify the structure of an integrated joint.

3. High Rotational Stiffness

Humanoid robot joints need to maintain precise positioning even when external forces are applied.

Bearing stiffness is therefore an important consideration.

A bearing with appropriate internal geometry, preload, and manufacturing accuracy can help minimize unwanted deformation and angular displacement under load.

However, excessive preload can increase friction and heat generation.

The goal is to achieve the appropriate balance between stiffness, rotational torque, temperature rise, and service life.

4. Accuracy and Runout

In a precision robot joint, bearing runout can influence the accuracy of the entire actuator.

Important parameters may include:

• Radial runout
• Axial runout
• Rotational accuracy
• Internal clearance or preload
• Raceway geometry
• Roller dimensional accuracy

These parameters need to be considered together rather than evaluated individually.

For example, improving dimensional accuracy without properly controlling preload may not produce the expected improvement in joint performance.

5. Bearing Selection Should Follow the Joint Architecture

There is no single bearing type that is suitable for every humanoid joint.

Depending on the joint architecture, designers may consider:

• Crossed roller bearings
• Angular contact bearings
• Four-point contact bearings
• Thin-section ball bearings
• Customized bearing configurations

The correct choice depends on load direction, available space, required stiffness, rotation speed, expected service life, and integration requirements.

6. When Standard Bearings Are Not Enough

During prototype development, engineers may find that an existing standard bearing does not perfectly match the joint design.

Typical problems include:

• Limited installation space
• Special mounting dimensions
• Required preload
• Higher stiffness requirements
• Special sealing requirements
• Low-friction requirements
• Integration with a reducer or encoder

In these situations, a customized bearing can allow the bearing geometry to be designed around the joint rather than forcing the joint to adapt to an existing bearing.

Conclusion

Bearings are an important part of humanoid robot joint architecture. Compact dimensions, high stiffness, precision rotation, low friction, and long service life all need to be considered simultaneously.

For integrated actuator modules, the most suitable bearing is not necessarily the bearing with the highest load rating. It is the bearing that provides the right combination of precision, stiffness, size, friction, and reliability for the complete joint system.