The Evolution of Bearing Solutions in Humanoid Robots

What You’ll Learn:

The technical aspects of bearing specifications, touching on the evolution from standard to tailored solutions.
An overview of the bearings used in humanoid robots, including crossed roller bearings, four-point contact, spherical plain, flexible ball and bushings.

Humanoid robots require smooth, precise and reliable joint motion to emulate human movement. At the heart of this performance are bearings, critical components that reduce friction, support loads and maintain alignment within the joints.

In early development stages, engineers often rely on off‐the‐shelf bearings for rapid prototyping and testing. However, as application demands grow, requiring higher precision, better load handling and more compact designs, the need for custom bearing solutions becomes more apparent.

Humanoid robots are engineered to operate in complex, human-centered environments. To move naturally and safely, their joints must achieve exacting movement while managing varying loads and minimizing energy loss. Bearings are vital in this context because they:

Reduce friction torque. Ensuring energy-efficient and smooth rotation.

Minimize internal clearance. Preventing unwanted play for high repeatability.

Enable compact, lightweight designs. Critical for the limited space available in joints.

Allow custom integration. Enabling the embedding of mating mechanical components and seals to boost load capacity and overall performance while also reducing assembly time.

Flexible ball bearing — Used in wave generators of harmonic drive mechanisms, flexible ball bearings are characterized by low friction, high accuracy and efficiency.

Early Stages: Off-the-Shelf Components

During the initial phases of robotic design, off-the-shelf bearings are used for quick prototyping and testing. These readily available components allow for rapid assembly and early validation. However, their generic specifications often lead to issues such as:

Excessive internal clearance. Resulting in imprecise joint movements.

Limited load capacity. Inadequate for dynamic and complex tasks and often leading to premature bearing failure.

Larger sizes and weight restrictions. Making them unsuitable for compact, human-like joints.

Precision bushing — Robotics engineers can work with manufacturers to develop precision bushings and motion components that align with their application-specific requirements.

Transition to Custom Bearings

As the complexity of tasks and performance expectations increase, standard bearings prove insufficient. Engineers start using custom-bearing solutions that offer:

Enhanced precision. By reducing internal clearance and employing advanced preloading techniques, play within the bearing can be minimized—even reaching zero or negative clearance—which improves precision. At the same time, these techniques impact friction torque since reducing clearance typically increases friction, and vice-versa. Striking the right balance between these factors, informed by theoretical calculations and prototype verification, is crucial.

Additionally, optimizing bearing runout ensures concentricity and alignment, further enhancing performance. This combination of tighter clearances, precise preloading and minimal runout results in bearings with exceptional accuracy and low friction, making them ideal for demanding robotic applications.

Greater load handling. Tailored bearings deliver higher load capacities by incorporating larger rollers, transitioning to full-complement bearings and increasing the pitch circle diameter. These techniques further enhance load capacity and overall performance.

Compact and lightweight profiles. Advancements in bearing geometry and materials allow for seamless integration into highly confined spaces. For example, the demand for ultra-thin bearings has introduced new design challenges, which led CCTY to develop innovative solutions, such as:

Formulating a self-lubricating PTFE fabric liner for robotics, which allows for low friction and long life.
Introducing a thin bushing with half a millimeter wall thickness in cylindrical and flanged designs to accommodate application needs, such as those found in robotic hands.

These adaptations and many others ensure that space-saving designs maintain strength, functionality and load capacity.

Integrated functionalities. Custom bearings can be combined with mated fixtures and forged as a single unit, enhancing stiffness and load-bearing capacity while reducing assembly time. Additionally, incorporating seals directly into the bearing design helps retain lubrication and protects against dust and moisture ingress, reducing maintenance requirements and ensuring long-term reliability in demanding environments.

Precision bushings — In humanoid robotics, small alignment errors can throw off movement, so high-tolerance (precision) bushings ensure consistency and control.

Overview of Bearings Used in Humanoid Robots

Roboticists now choose from a range of bearing solutions based on the specific demands of each joint:

Crossed Roller Bearings

Used in rotary actuators, these bearings manage combined axial, radial and moment loads with exceptional stiffness and minimal friction torque. CCTY has made great innovations for this bearing, such as:

Manufacturing crossed roller bearings with inner diameters as small as 10 mm.
Enhancing stiffness through the use of negative clearance while maintaining low friction torque.
Developing an ultra-compact sealing solution that meets strict IP standards.

Four-Point Contact and Deep Groove Ball Bearings

Four-point contact bearings handle combined loads effectively, while deep groove ball bearings provide durability and ease of maintenance, particularly in linear actuators.

Spherical Plain Bearings (SPBs), Rod Ends & Linkages

Metal-to-metal SPBs provide robustness for high-impact loads but larger initial clearance when compared to self-lubricating SPBs.
Self-Lubricating SPBs (with PTFE-based liners) offer low friction, maintenance-free operation, and negative clearance capabilities.
Rod ends and linkages in humanoid robots are vital for addressing misalignment. A typical product used in robotics is the dog bone linkage, which incorporates a spherical plain bearing at both ends to accommodate misalignment.

Flexible Ball Bearings

Used in wave generators of harmonic drive mechanisms, flexible ball bearings have very thin sections and will flex slightly when assembled by a camshaft to achieve the desired function. They offer low friction, high accuracy and efficiency.

Bushings

Found in extremely confined areas (such as robotic hands/fingers), bushings are efficient solutions that can be custom-designed to achieve very tight tolerances and low friction post-assembly. For example, CCTY has developed steel-backed, PTFE-based self-lubricating bushings with an inner diameter as small as 2.5 mm. These ultra-compact components are especially vital for robotic hands, where durability and precision are key. Unlike plastic bushings, self-lubricating versions offer significantly longer life and can be custom engineered to achieve extremely tight initial clearances.

Precision in Bearing Specifications Enables Progress in Humanoid Robots

Bearing specifications such as friction torque, internal clearance, compactness, weight and custom integration are fundamental to the performance of humanoid robots. Optimizing these parameters results in smoother, more energy-efficient and highly precise joints.

As technology continues to evolve, innovations like full assembly solutions promise to push the boundaries of today’s humanoid robotics even further. Collaboration between robotic engineers and specialized manufacturers remains essential in driving these advancements and ensuring that future humanoid robots meet the rigorous demands of real-world applications.