In the ever-evolving robotics landscape, achieving precise and seamless motion is paramount. While articulating joints enable a robot’s range of motion, bearings serve as the backbone, ensuring smooth and accurate movement. Particularly in humanoid robots where precision is non-negotiable, the selection of bearings becomes a critical engineering decision.
Design engineers must consider the diverse range of robotic bearings, how backlash affects precision, and when to use seals to protect bearings from harsh environments.
Bearings in Humanoid Robots
Powering artificial intelligence with the right bearing will enable proper motion, and a longer bearing and articulating joint life span. Bearings used in robotic applications come in several designs, each with its own advantages. The choice of bearing depends on the specific robotic requirements with regard to load capacity, speed, precision, and rigidity.
Many of the following designs are available with self-lubricating materials or greased-for-life options which remove the need to regrease the bearing at specified intervals. Work with a bearing manufacturer to specify your application’s needs and how to best support the overall mechanical movement.
Bearings in Rotary Actuators
Cross roller bearings offer high load capacity, precision, and a compact design. They support axial loads from both directions, radial loads, tilting moment loads, and any combination of loads. Found in rotary actuators as the output bearing, cross roller bearings are known for having low friction torque and clearance, high durability and accuracy, and unique load capabilities.
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 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 bigger contact angle and take more axial load and moment load. Four-point contact bearings are used as the input bearing in rotary actuators.
Angular-contact ball bearings can take axial loads in one direction and are generally used in pairs to allow for bidirectional axial load capabilities. As the contact angle increases, the load capability also increases.
RNN bearings, also known as full complement cylindrical roller bearings, are designed to handle heavy loads with a very compact design. They reduce oscillation in robotic actuators and improve control and rigidity. In addition to having high durability and uniform contact stress, RNNs have low noise and vibration. The attributes make them a good fit for rotary actuators and linkage assemblies.
Bearings in Linear Actuators
Linear actuators have two main bearings, four-point contact and deep groove ball bearings. In this application, these bearings are used to support the shaft under load.
Four-point contact 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 bigger contact angle and the ability to take more axial load and moment load.
Deep groove ball bearings are versatile, have a simple design, durable in operation, and are easy to maintain. They are designed to handle radial loads and a limited axial load.
Ball bearings have a number of features when used in linear actuators, including high precision, low clearance and toque and high load capacity. They can also be manufactured with seals if required by the application.
Bearings and Linkages
While bearings are found mostly in actuators in humanoid robots, they are also used to manage misalignment in other robotic functions. For example, spherical plain bearings are critical components for humanoid robots and can be found in rod end and linkage assemblies.
Linkages also play a pivotal role in ensuring that the robot is reliable. One of the more popular linkages that is used in this market is the Dog Bone linkage. It has one spherical plain bearing at each end which permits misalignment at both connection points.
Clearance and Torque in Robotic Bearings
The clearance or play between components poses a significant challenge in robotic systems. Left unchecked, it can lead to motion transfer delays, imprecise positioning, and reduced accuracy. This reduced accuracy causes shaking/jerky motion in the robot’s movement, as seen in demo videos from early versions of new robotic designs. Too much clearance also introduces accelerated component wear and rattling noise due to vibration, ultimately reducing the life expectancy of the bearing.
Torque is the rotational force applied to a component. Like clearance, torque must be carefully managed. High torque leads to excessive power consumption, while too little torque may result in inadequate performance due to an increased clearance.
The right balance between torque and clearance is imperative for accurate motion control. Excessive torque can hinder movement, and too much clearance compromises precision. Therefore, minimizing both clearance and torque through innovative designs and advanced manufacturing techniques is essential.
The Repeatability of Bearings
A bearing’s precision is significantly influenced by its repeatability. A highly precise bearing can move to a specified position accurately. Meanwhile, a bearing with high precision and excellent repeatability can consistently reach that specified position throughout its lifespan.
However, excessive wear can compromise the bearing’s repeatability. This wear increases the clearance between the bearing components, leading to a lower repeatability score. Therefore, it’s vital to manage the end-of-life wear on all bearings effectively.
The Role of Seals
Seals serve as the first line of defense protecting bearings from contaminants and preserving their functionality in harsh environments. They prevent dirt, dust, water, and abrasive particles from ingress. They are also critical in keeping lubrication inside the bearings. Design engineers should consider working conditions when selecting bearings. For instance, in warehouse environments dust can penetrate the bearings — particularly in bipedal applications— leading to a shortened lifespan or even a complete loss of functionality. Also, if grease exits the bearings, it can lead to premature failure.
Seal Impact on Torque
While seals generate friction due to increased contact, advancements in seal design and materials mitigate these effects, ensuring optimal performance without compromising rotational efficiency.
Tighter seals may introduce higher initial torque but offer better contamination protection. Seals can have single, double, or even triple lip designs to help achieve application sealing requirements. Seals can also be molded over steel reinforcements to strengthen their structure. Controlling the contact between the seal lip and the bearing surface aids in limiting the introduced torque and can be achieved by modifying the interference fit between the two surfaces.
Seal Material Composition
Seals can be made from various materials, such as rubber or synthetic polymers, each with unique properties and applications. Choosing the right seal based on specific application requirements, such as using silicone rubber for higher temperatures, will prolong the life of the bearing by ensuring consistent seal performance (Table 1).
The Impact of Bearing Failure in Critical Applications
When a bearing fails, it can have cascading effects on the system.
Complete Failure: It can cause misalignment and imbalance in the system. This can lead to a loss of positioning accuracy, increasing the risk of maloperation.
Increased Friction: Failed bearings often lead to increased friction, which can cause overheating and reduce energy efficacy.
Contamination: Damaged bearings can generate metal debris which circulate outside the bearing and cause further damage to other components within the system.
Vibration: Excessive vibration is a common result of bearing failure. Vibration can be transmitted through the system, leading to wear and potential failure of other components.
Bearing failure is not isolated and can have serious effects on the entire system. Proper maintenance and monitoring of bearings are essential to prevent failure.
Collaborating with an experienced bearing manufacturer during the initial stages of a robotic bearing design can establish a robust basis for the application’s performance and longevity.
This article was written by Yaman Obaid, Mechanical Engineer specializing in robotics for CCTY (Lake Zurich, IL). For more information, visit here .