Introduction to Bearings in Humanoid Robots: What Roboticists Need to Know

In the world of humanoid robotics, precision, durability and efficiency are key to creating robots that move like humans. A crucial component enabling this movement is bearings. Bearings support rotational and linear motion of actuators and linkages, allowing robots to perform tasks smoothly and accurately. Understanding the different bearings used in humanoid robots is key to enhancing the overall performance and functionality of the robot.

Types of Bearings in Humanoid Robots

In the design and functionality of humanoid robots, bearings play a critical role in ensuring precise and efficient movement across various actuators and linkages. These bearings, including crossed roller, flexible ball and four-point contact ball bearings, are vital for both rotary and linear motion systems. Additionally, spherical plain bearings are essential for managing misalignment, while bushings are used in tighter spaces where bearings can’t fit.

In humanoid robot design, bushings are used in tight spaces where bearings can’t be used.

Bearings for Rotary Actuators

Rotary actuators are responsible for powering the robot’s joints, enabling rotational movement. The most common bearings used in these actuators are crossed roller bearings, four-point contact bearings, angular contact bearings and flexible ball bearings.

Cross roller bearings are used as the output bearing in rotary actuators. The crisscrossed arrangement of cylindrical rollers allow them to handle high loads, including axial loads from both directions, radial loads, tilting moment loads and any combination of loads. This design provides high rigidity, minimal clearance and low friction, making them ideal for precise, controlled motion in robotic joints and other rotational systems.

Flexible ball bearings can handle radial loads and a limited axial load. They are used in wave generators of harmonic drive mechanisms. They have very thin sections and will transform into an oval shape when assembled by a camshaft to achieve the desired function.

Four-point contact ball bearings are designed to handle a combination of radial and axial loads in both directions, making them a versatile, compact and cost-effective solution. Some of them have split inner rings or split outer rings to allow for a larger contact angle and the ability to take more axial load and moment load. Four-point contact bearings are used as the input bearing in rotary actuators.

Angular contact ball bearings excel in handling high axial loads and are typically used in pairs to allow for bidirectional axial load capabilities. As the contact angle increases, the axial load capability also increases.

These bearings support the movement of rotary actuators and improve/ensure their accuracy and repeatability.

Flexible bearings characteristically have very thin sections. These bearings transform into an oval shape when assembled by a camshaft to achieve the desired function.

Bearings for Linear Actuators

Linear actuators enable linear movements and are used in various locations in humanoid robots for many different purposes. These actuators often rely on four-point contact ball bearings and deep groove ball bearings to support the angular motion that transfers into linear motion within the actuator. Sometimes, a pair of angular contact ball bearings are used in place of a four-point contact bearing to accommodate certain application requirements such as higher loads.

In addition to four-point contact ball bearings and angular contact ball bearings, linear actuators utilize deep groove ball bearings.

Deep groove ball bearings are versatile, have a simple design, are durable in operation and are easy to maintain. They are designed to handle radial loads and a limited axial load.

The smooth and precise motion of linear actuators, essential for robots to perform intricate tasks with accuracy, is made possible by these bearings.

Bearings for Linkages

Bearings are primarily used in actuators within humanoid robots, but they also play a key role in managing misalignment in linkages of robotic systems. For instance, spherical plain bearings are indispensable components in humanoid robots, commonly found in rod ends and various linkage assemblies. A widely used linkage in this field is the dog-bone linkage, which incorporates a spherical plain bearing at each end to allow for misalignment.

Bushings in Humanoid Robots

Bushings are key components in humanoid robots, particularly in tight spaces where bearings can’t be used. They reduce friction, enable smooth movement and are commonly found in the fingers/hands of humanoid robots. Bushings improve precision, minimize wear and extend the lifespan of parts. Additionally, self-lubricating bushings help dampen noise, making robots quieter and more suitable for certain applications.

Rod ends are characterized by an eye-shaped head and an integral shank for housing a spherical plain bearing.

Customization Benefits

While standard bearings are suitable for general use, humanoid robots often require custom bearings to meet specific performance needs. Custom bearings can be designed to handle unique load profiles, reduce friction and minimize clearance. For instance, managing clearance in bearings is a critical process. While some clearance is necessary to help reduce friction torque, excessive clearance can lead to inaccurate motion and increased wear. By carefully managing clearance, engineers can improve the precision and lifespan of bearings, contributing to the robot’s overall performance. A custom crossed roller bearing, for example, can improve energy efficiency, extend the actuator’s life and enhance the robot’s overall performance.

Customization can also improve bushing performance. Self-lubricating bushings do not require regular lubrication, reducing maintenance while offering low-friction and long life.

The dog-bone linkage accommodates a spherical plain bearing at each end to compensate for misalignment.

Material Selection

Components used in humanoid robots must be durable and long-lasting to minimize the need for frequent maintenance/component replacement. Various materials provide distinct benefits, depending on their placement within the robot and their specific function.

For example, bushings with self-lubricating materials like PTFE composites or fabrics eliminate any maintenance needs while improving performance. High-strength alloys that are heat treated to the right hardness are ideal for bearings that endure high loads. Material selection is crucial to ensuring that bearings can withstand the robot’s operational demands while maintaining accuracy and smooth motion.

Flexible bearings are designed to accommodate misalignment and vibration.

The Role of Seals

For robots operating in harsh environments, bearings can be designed with seals to prevent contaminants from damaging internal components. Seals protect bearings from dirt, dust and moisture, which can lead to premature wear and failure. Seals also play the important role of keeping the lubrication inside the bearing and eliminating or reducing the need for maintenance. However, seals introduce friction, which adds to the bearing’s friction torque. Advances in seal technology have helped minimize this impact, allowing for better performance without sacrificing protection.

Cross roller bearings are typically used as the output bearing in rotary actuators.

Select the Right Bearing for the Humanoid Robot’s Design

Bearings play a pivotal role in the performance of humanoid robots, enabling smooth, precise movement that mimics human actions. Understanding the types of bearings, material choices and the importance of custom solutions is essential for engineers designing robots with advanced capabilities. By selecting the right bearings—whether off-the-shelf or custom—robotic engineers can ensure that their designs achieve fluidity, accuracy and reliability.

As the field of humanoid robotics continues to evolve, bearings will remain a critical component in ensuring robots can move with the precision and agility needed for real-world tasks. By collaborating with bearing manufacturers and tailoring designs to specific needs, engineers can develop robots that push the boundaries of what’s possible.