Application and Quantity of Rolling Bearings in Humanoid Robot Core Components

With the rapid development of science and technology, humanoid robots are no longer the preserve of science fiction films; they are gradually entering real life, with their applications expanding from industrial production to domestic services. Among the many complex and sophisticated components of humanoid robots, several are considered core and crucial to their performance. Like the "bones and muscles" of humans, they support and drive the robots to perform various complex movements.

Actuators are key components of a robot's motion system, primarily including linear actuators, rotary actuators, and dexterous hands. They play a central role in humanoid robot motion control.

Linear actuators (14 joints): Convert rotational motion into linear motion. They are used for joints such as the wrist, elbow, knee, and ankle, as well as components requiring linear motion. They primarily simulate the contraction and relaxation of muscles like the biceps and thigh muscles. They consist of a frameless torque motor, a roller screw, a torque sensor, an encoder, and bearings.

Rotary actuators (14 joints): Rotate joints. They are used for joints such as the shoulder and hip, as well as components requiring rotation. They primarily simulate the rotational motion of some human joints. They consist of a frameless torque motor, a reducer, a torque sensor, an encoder, and bearings.

Dexterous hands (two units, containing 12 joints): Serving as end effectors, they mimic the structure and function of the human hand. They consist of a coreless motor, a reducer, a lead screw, a tendon, and a tactile sensor.

Actuator Assembly (Joint Module): Integrated "Motion Engine"

Humanoid robots rely on bearings for joint connections. Linear joints often use deep groove ball bearings and four-point contact bearings, while rotary joints often use angular contact bearings.

According to Tesla, its linear actuators use a combination of deep groove ball bearings and four-point contact bearings.

Rotary actuators use a combination of crossed roller bearings and angular contact ball bearings.

Dexterous Hand: The "Anthropomorphic Craftsman" at the End

Integrating a coreless motor, reducer, and lead screw drive (or tendon and sensor, worm gear drive, or connecting rod drive), it achieves multi-degree-of-freedom operation, high grasping precision, and minimal force control error, allowing the robot to hold a pen and write like a skilled craftsman.

Micro planetary roller screws are highly efficient and compact, making them ideal for applications in dexterous hands. In some cases, both a speed reducer and a lead screw can be configured simultaneously. The speed reducer primarily reduces motor speed and increases torque, while the lead screw primarily converts the motor's rotational motion into linear motion.

1. Planetary Roller Screws: The Robot's "Muscle Fibers"

In the Tesla Optimus's load-bearing joints, such as its legs and waist, are metal rods as thick as fingers. Like muscle fibers, they convert the motor's rotational motion into precise linear displacement, with an error of less than 0.01 mm. These precision actuators are used to support heavy-load movements.

I once promoted roller screw products at SKF (now Ewellix, a subsidiary of FAG). Although SKF has divested its linear products business unit, I still retain the display samples.

In the planetary roller screw sector, European, American, and Japanese companies dominate 70% of the global ball screw market, while high-end products rely on imports. Rollvis, GSA, Ewellix, and Rexroth hold a combined market share of approximately 78% in my country.

Domestic breakthroughs: Hengli Hydraulics has entered Tesla's supply chain, and Wuzhou Xinchun's micro-screws enable robot fingers to grasp eggs without damage.

II. Motor: The Source of Powerful Power

1. Frameless Torque Motor: The "Silent Engine" at the Core of Power

This housingless motor is directly integrated into the joint, providing high torque and fast response, with a power density exceeding 8kW/kg and a response time of less than 1ms. It acts as a silent engine for the robot, driving rapid movements. A single unit requires 28 units (such as the Optimus joint). Domestic substitution: Buke Co., Ltd. and Leisai Intelligent are gradually replacing foreign brands such as Maxon.

Frameless torque motors do not include bearings.

Frameless torque motors omit components such as shafts, bearings, housings, and end caps, and consist solely of a rotor and stator.

2. Coreless Motors: "Fingertip Ballet" for Dexterous Hands

Powering a robot's thumb for rapid pinching, with a response time of less than 0.1 seconds and a power density 50% higher than traditional motors, this motor, smaller than a button, enables finger movements with ballet-like precision. Technical Barriers: Only three companies worldwide mass-produce this motor, with Ming's Electric holding a 15% market share.

Coreless motors primarily use deep groove ball bearings or plain bearings.

Brushed motors typically use two plain bearings, which makes them relatively cost-effective and technically challenging.

Brushless motors typically use two deep groove ball bearings. These bearings offer high stability, high precision, and low friction, and the technology is mature in China.

3. Reducers: The Key to Precision Transmission

A reducer is an intermediary connecting the power source and actuators. It reduces the rotational speed to the required speed for each joint and increases torque to meet power output requirements. It is primarily used in rotary joints and dexterous hands. High-quality reducers provide stable transmission and high-precision positioning, thereby ensuring the stability and accuracy of the robot's motion. Humanoid robots primarily use three types of reducers: harmonic reducers, RV reducers, and planetary reducers.

1. RV Reducer: A Heavy-Duty "Steel Skeleton"

Installed at the hip joint to bear the full body weight, it utilizes a cycloidal pinwheel meshing mechanism as its core transmission component. The RV reducer offers high precision, high rigidity, and a strong load capacity, making it as strong as a steel skeleton. A domestic breakthrough: The dual-ring transmission breaks Japan's decade-long monopoly.

RV reducers use three types of bearings:

Thin-wall angular contact ball bearings (2) for the main bearing,

Thin-wall tapered roller bearings (3 x 2) for eccentric shaft positioning and main body support,

Cylindrical roller (needle) rollers (3 x 2) with a cage assembly for cycloidal gear support.

Angular contact ball bearings: Angular contact ball bearings are the most difficult bearings used in humanoid robots to manufacture, requiring extremely stringent precision and technical requirements. The market is primarily dominated by eight major bearing companies, including SKF and Timken. In China, Renben Group and Wuzhou Xinchun are relatively leading.

Tapered roller bearings: There is still a significant gap between domestic and international tapered roller bearing manufacturing. Well-known overseas brands such as Timken, FAG, and NSK have advantages in product quality, equipment precision, and price. Domestic companies such as Dongfeng Bearing, Guangyang Bearing, and Xiangyang Bearing have performed well.

Needle Roller Bearings: The global high-end needle roller bearing market is primarily controlled by foreign companies, particularly leading companies such as Germany's Schaeffler Group, NSK, and SKF. Together, they hold approximately 40% of the market share, demonstrating a high level of industry concentration. In China, Suzhou Axle Co., Ltd. and Southern Bearing are leading players in the needle roller bearing market.

2. Harmonic Reducers: The "Articular Cartilage" of Precision Transmission

Like articular cartilage, these bowl-sized metal rings achieve deceleration through elastic deformation of the flexspline, enabling flexible rotation of the robot's elbow and wrist. They boast a transmission accuracy of less than 1 arc minute and a lifespan exceeding 8,000 hours. Domestically dominated: Green Harmonic has a 25% global market share.

Harmonic reducers typically use one crossed roller bearing and one flexible bearing.

Rigid Bearings/Cross Roller Bearings: The domestic crossed roller bearing market is highly concentrated, with Luoyang Hongyuan being the leader. Major global manufacturers are primarily concentrated in Japan and Germany, including Japan's THK, IKO, and NSK, as well as Japan's Harmonic and Germany's Schaeffler Group's INA brand.

Flexible Bearings: Flexible bearings are an essential core component of harmonic reducers. Unlike traditional bearings, flexible bearings are thin-walled ball bearings with thin outer rings that are prone to radial deformation. For harmonic drive systems, the flexible bearing's elastic deformation capability is key to achieving high reduction ratios.

3. Precision Planetary Reducers: The Hub of Efficient Transmission

Planetary reducers consist of a sun gear, planet gears, a planet carrier, and a frame. They offer advantages such as a simple structure, compact size, light weight, high load capacity, maximum transmission efficiency, highest transmission precision, longest life, smooth motion, low noise, unique features of power splitting and multi-tooth meshing, and high output torque. At present, most humanoid robots produced in China adopt a combination of harmonic and planetary reducers.

Planetary reducer bearings include planetary gear bearings, main bearings, and input shaft bearings.

Three needle roller bearings are used for the three planetary gears.

Two deep groove ball bearings are typically used for the main bearings and input shaft bearings of the planetary reducer.