In the development of robots, one core technology is crucial: the application of bearings in the humanoid robot field. As a core component of humanoid robots, bearings account for approximately 5.5% of the value of major components. Different types of bearings play irreplaceable roles in various parts of the robot.
Ⅰ Bearings in Reducers
The reducer is the core power transmission mechanism in a humanoid robot, comparable to a human joint. A precision reducer is an intermediate device connecting the power source and the actuator. Its function is to reduce the high speed of the servo motor, amplify the original torque of the servo motor through gear reduction ratios, and provide high rigidity holding and high-precision positioning. Humanoid robot reducers mainly include harmonic reducers, RV reducers, and planetary reducers. Different types of reducers have different requirements for bearings:
1. RV Reducer Bearings:
RV reducers, also known as rotary vector reducers, include a front-stage planetary gear reducer and a rear-stage cycloidal pinwheel reducer. This transmission method is a further development based on the classic pin-pendulum planetary transmission technology. It combines numerous advantages, such as compact size, lightweight, wide transmission ratio, long service life, stable precision maintenance, high efficiency, and smooth transmission performance.
The RV reducer uses three types of bearings: a thin-walled angular contact ball bearing for the main bearing (output shaft), a thin-walled tapered roller bearing for eccentric shaft positioning and main body support, a cylindrical roller (needle roller) for cycloidal gear support, a cage assembly, and a thin-walled deep groove ball bearing for gear support.
2. Harmonic Reducer Bearings
The rotary joints of robots primarily utilize harmonic reducers. Their manufacturing technology is based on harmonic transmission. A harmonic reducer is a mechanical transmission system composed of four basic components: a wave generator (flexible bearing), crossed roller bearings (rigid bearing), a flexible gear with an external gear ring (flexible wheel), and a rigid gear (rigid wheel).
① Crossed Roller Bearings
There are various series of crossed roller bearings for harmonic reducers, capable of withstanding multi-directional loads, and offering high precision and rigidity. Crossed roller bearings specifically designed for harmonic reducers can be divided into two main categories based on their application environment: those with a split outer ring and those with an integral inner ring. The rolling elements of these bearings are cylindrical rollers arranged at a 90° perpendicular angle in a V-shaped raceway. This design allows the bearing to simultaneously withstand loads from multiple directions, including axial, radial, and overturning moments. Furthermore, these bearings exhibit high precision, high rigidity, and excellent composite load-bearing capacity, making them an indispensable component of harmonic reducers.
② Flexible Bearings Flexible bearings specifically designed for harmonic reducers differ from traditional bearings. They are thin-walled ball bearings with a thinner outer ring, making them prone to radial deformation. Despite their thin walls, they remain highly flexible and utilize high-quality materials to ensure stable operation. These flexible bearings for harmonic reducers possess excellent stress-bearing capacity. They effectively withstand alternating bending and torque stresses, maintaining a degree of flexibility in their interaction with cams despite their relatively thin wall design. Both the inner and outer rings, as well as the rolling elements, are made of high-quality high-carbon chromium bearing steel and are equipped with an integral nylon retainer, ensuring stable and efficient operation of the bearing in the harmonic generator section of the harmonic reducer.
Harmonic reducers typically use one crossed roller bearing and one flexible bearing.
3. Planetary Gear Reducer
The planetary gear reducer is the most efficient gear transmission structure. A planetary gear transmission mechanism mainly consists of planetary gears, a planet carrier, and a sun gear. In a precision planetary reducer, a prime mover such as a servo motor typically drives the sun gear to rotate. The meshing of the sun gear with the planetary gears drives the planetary gears to rotate. Simultaneously, because the other side of the planetary gear meshes with a ring gear on the inner wall of the reducer housing, the planetary gears, driven by their own rotation, will roll on the ring gear in the same direction as the sun gear's rotation, forming a "revolutionary" motion around the sun gear. The difference in the number of teeth between the sun gear and the ring gear achieves the speed reduction purpose.
The planetary reducer converts the high-speed rotation of the motor into low-speed, high-torque output, suitable for the transmission needs of non-high-precision parts of robots, such as the lower limbs (e.g., hip and knee joints). By driving the planetary gears to rotate and revolve through the sun gear, the power is ultimately output by the planet carrier. It features simple structure, low cost, and high load-bearing capacity.
Taking the Tesla Optimus Gen-2 robot as an example, its planetary reducer uses deep groove ball bearings and needle roller bearings. Deep groove ball bearings are used to withstand smaller radial and axial loads, while needle roller bearings are used to withstand the huge radial loads generated by the planetary gears when they rotate at high speeds.
II. Other Bearings in Humanoid Robots
1. Linear Actuators: Four-Point Contact Bearings
Four-point contact bearings use ball bearings where the steel balls of the ball bearing contact the inner and outer rings at four points, achieving a two-point contact state. This allows them to withstand bidirectional axial loads and a certain proportion of radial loads. The composite inner ring design accommodates more balls, increasing load-bearing capacity; the split structure facilitates installation and is suitable for space-constrained joints. The contact angle is typically 35° or 45°, with small axial clearance, thus providing high speed limiting and stiffness.
Four-point contact bearings are commonly used in rotary joints and linear actuators, in conjunction with crossed roller bearings to achieve high-precision motion.
2. Self-lubricating Joint Bearings: Self-lubricating bearings achieve low-friction motion through bimetallic composite materials or solid lubricants, with a friction coefficient as low as 0.08, while also possessing high wear resistance (service life exceeding 10,000 hours). In humanoid robots, they are mainly used in joint modules, featuring maintenance-free operation, high load capacity, and long lifespan, meeting the requirements of high-frequency movements and micron-level precision. For example, Changsheng Bearing's self-lubricating sliding bearings have been applied to the joint modules of Unitree Robotics' humanoid robots, achieving a penetration rate of over 80%, making them the exclusive supplier.
Other applications of self-lubricating bearings in robots:
Reducer gear sets: Matching high-precision transmission requirements and ensuring motion stability.
Joint linkages: Bearing heavy loads and high-speed operation, reducing energy consumption.
Screw drive systems: Adapting to micron-level motion precision, improving overall flexibility.
Summary: In the technological pyramid of humanoid robots, bearings, though small, are a crucial core connecting the upper and lower layers. It is not merely a physical connector, but also a performance enabler, its importance manifesting in three aspects: **The cornerstone of motion precision:** High-precision, high-rigidity bearings ensure the smoothness, accuracy, and stability of every robot movement. From the micro-manipulation of dexterous hands to the powerful support of leg joints, even minute errors in the bearings are amplified, directly determining the robot's overall motion performance.
**Guarantee of load-bearing capacity and lifespan:** Humanoid robots need to withstand complex multi-dimensional loads in dynamic environments. Specialized thin-walled angular contact ball bearings, crossed roller bearings, and other lightweight designs provide superior anti-tipping capabilities and fatigue strength, forming the cornerstone of the robot's long-term, reliable operation.
**Key to energy efficiency and integration:** Low-friction bearings reduce power loss, improve energy efficiency, and indirectly extend the robot's endurance. Simultaneously, the emergence of compact bearings (such as four-point contact ball bearings) saves valuable space in robot joint design, enabling a higher degree of integration.
