The joints of humanoid robots can achieve rotational and linear movements, and the key role behind this cannot be separated from bearings. Bearings are mainly distributed in the reduction devices and joint connection parts of humanoid robots, providing support for the flexible movement of robots.

1. Speed Reducers & Bearings

The speed reducer is the core power transmission mechanism in humanoid robots, which can be compared to the human joint. The precision speed reducer is an intermediate device connecting the power source and the actuator. Its role is to reduce the high speed of the servo motor, amplify the original torque of the servo motor through the gear reduction ratio, and provide high rigidity maintenance and high-precision positioning. The speed reducers of humanoid robots mainly include harmonic speed reducers, RV speed reducers, planetary speed reducers and other types, and different types of speed reducers have different requirements for bearings:

1) Harmonic Speed Reducer Bearings

The rotating joints of robots mainly use harmonic speed reducers, whose manufacturing technology is based on harmonic drive. The harmonic speed reducer is a mechanical transmission composed of four basic components: a wave generator (flexible bearing), a crossed roller bearing (rigid bearing), a flexible gear with an outer ring (flexible wheel), and a rigid gear (rigid wheel).

A harmonic speed reducer typically uses 1 crossed roller bearing and 1 flexible bearing.

• Rigid Bearing/Crossed Roller Bearing
  The domestic crossed roller bearing market is highly concentrated. Due to the high technical requirements for manufacturing crossed roller bearings, the world's main manufacturers are mainly concentrated in Japan and Germany, including THK, IKO, NSK of Japan, as well as Harmonic Drive of Japan and INA brand of Schaeffler Group of Germany. In terms of price, there is a significant difference in the selling price of crossed roller bearings at home and abroad, with the gap exceeding 30%.

• Flexible Bearing
  Flexible bearing is an indispensable core component in harmonic speed reducer. Different from traditional bearings, the flexible bearing is a thin-walled ball bearing with a thin outer ring, which is easy to have radial deformation. Due to the large prestress and pre-deformation generated during the assembly process, the flexible bearing is circular before being installed on the elliptical cam of the wave generator, and becomes elliptical after installation. For the harmonic drive device, the elastic deformation ability of the flexible bearing is the key to achieving a high reduction ratio.

2) RV Speed Reducer Bearings

RV speed reducer, also known as rotating vector speed reducer, includes a front-stage planetary gear speed reducer and a rear-stage cycloidal pinwheel speed reducer. This transmission method is further developed on the basis of the classic pin cycloid planetary transmission technology, and it integrates many advantages, such as compact volume, light weight, wide transmission ratio, long service life, stable precision maintenance, high efficiency and smooth transmission performance.

Compared with the harmonic speed reducer, the RV speed reducer shows more excellent fatigue strength, rigidity and longer service life, and its motion precision will not decrease significantly with the increase of service time. Because of these characteristics, the RV speed reducer has the trend of gradually replacing the harmonic speed reducer in the field of high-end robot transmission.

The RV speed reducer uses three types of bearings, including thin-walled angular contact ball bearings for the main bearings (output shaft, 2 pieces), thin-walled tapered roller bearings for the eccentric shaft positioning and main body support (3×2 pieces), cylindrical roller (needle roller) (3×2 pieces) cage assemblies for the cycloid wheel support, and thin-walled deep groove ball bearings for the gear support.

3) Planetary Speed Reducer Bearings

The planetary speed reducer is the gear transmission structure with the highest transmission efficiency. The planetary gear transmission mechanism is mainly composed of a planetary gear, a planetary carrier and a sun gear. When the precision planetary speed reducer works, it is usually the prime mover such as the servo motor that drives the sun gear to rotate, and the meshing of the sun gear and the planetary gear drives the planetary gear to generate autorotation; at the same time, because the other side of the planetary gear meshes with the ring inner gear ring on the inner wall of the speed reducer housing, the planetary gear will finally roll on the ring inner gear ring in the same direction as the rotation of the sun gear under the drive of autorotation, forming a "revolution" movement around the sun gear. There is a tooth number difference between the sun gear and the ring gear, so as to achieve the purpose of speed reduction.

Planetary speed reducer bearings include planetary gear bearings, main bearings and input shaft bearings. Each planetary gear is loaded with 1 needle roller bearing, and a planetary speed reducer generally has 3 planetary gears, so 3 needle roller bearings are required. The two ends of the planetary speed reducer are equipped with main bearings and input shaft bearings, both of which are ball bearings (usually deep groove ball bearings). Therefore, a planetary speed reducer requires a total of 5 bearings, including 3 needle roller bearings and 2 ball bearings.

Taking the Tesla Optimus Gen-2 robot as an example, its planetary speed reducer part uses deep groove ball bearings and needle roller bearings. Deep groove ball bearings are used to bear small radial and axial loads, while needle roller bearings are used to bear the huge radial loads generated by the planetary gears during high-speed rotation.

2. Lead Screws & Bearings

The lead screw is a transmission component that converts rotational motion into linear motion, providing precise positioning and motion control for the joints. The types of lead screws mainly include ball screws and planetary roller screws: the ball screw is composed of a screw shaft and a nut, and the nut is composed of steel balls, a preload plate, a reverser, a dustproof device, etc.; the planetary roller screw is mainly composed of a screw, a roller, a nut, an inner gear ring, a cage and an elastic retainer ring, and relies on the thread meshing transmission between multiple rollers and the screw and nut to convert the rotational motion of the servo motor into linear motion. The rollers are evenly distributed between the screw and the nut through the cage.

Support Types Commonly Used at Both Ends of the Lead Screw

At present, the installation of the lead screw mainly includes the three methods shown in the figure above. According to different installation methods of the lead screw, two types of bearings are usually used in combination. For example, a radial bearing and a tapered bearing can be combined, or a radial bearing and a thrust bearing/angular contact bearing can be combined to ensure that the thrust bearing/angular contact bearing provides sufficient axial rigidity.

According to the information of Tesla 2022 AI DAY, its linear actuator uses a combination of deep groove ball bearings and four-point contact bearings.

Tesla Optimus Humanoid Robot Linear Actuator

• Four-point Contact Ball Bearing, also known as an angular contact ball bearing that can bear bidirectional axial loads, belongs to a type of angular contact ball bearing, with low barriers and small value. This bearing is a single-row angular contact ball bearing with a separated inner and outer ring. The inner ring is divided into two parts, which can bear axial loads from two directions at the same time. The groove curvature radius of the outer ring is very small, so that the steel balls are in uniform contact with the inner and outer rings at four contact points. The rolling elements have four force points, so it is called a four-point contact ball bearing, which has a high load-bearing capacity.

Compared with ordinary angular contact ball bearings, four-point contact ball bearings have smaller axial clearance, or even negative clearance, and can bear bidirectional axial loads. It is suitable for occasions where radial and axial loads need to be borne at the same time, especially with axial loads as the main load. If a single-row angular contact ball bearing is used to achieve the same function, two identical bearings are usually arranged in a back-to-back or face-to-face manner. The design of the four-point contact ball bearing can effectively save space and provide a more concise layout scheme.

• Deep Groove Ball Bearing: This kind of bearing can mainly bear radial load, and can also well cope with radial and composite loads. They have a small friction coefficient and are suitable for use in high-speed rotation environments. At present, many manufacturers have the ability to produce this kind of bearing, making the cost and selling price relatively low. China ranks first in the world in the production of deep groove ball bearings, which are widely used in the industrial field and have been highly localized.

3. Motors & Bearings

Since the most commonly used motors in humanoid robots are torque motors and hollow cup motors, other types of motors will not be discussed in detail in this article.

1) Hollow Cup Motor

The dexterous hands of humanoid robots usually use hollow cup motors as driving devices. The hollow cup motor belongs to a kind of micro servo motor, which generates a magnetic field through permanent magnet materials and is powered by a DC power supply. In terms of bearing selection, hollow cup motors mainly use deep groove ball bearings or sliding bearings.

2) Frameless Torque Motor

The frameless torque motor is a permanent magnet motor without a housing structure, and its performance is mainly measured by the output torque, and it does not include a bearing structure. The working principle of this motor is similar to that of a traditional permanent magnet motor, but there are differences in structural design: the frameless torque motor omits components such as the shaft, bearing, housing and end cover, and is only composed of two parts: the rotor and the stator.

Chinese humanoid robot bearing manufacturers are facing huge market opportunities and challenges: on the one hand, with the rapid development of the humanoid robot market, the demand for high-precision, long-life, low-noise bearing products is increasing; on the other hand, domestic bearing enterprises still need to continue to improve in technical research and development, production technology, quality control and other aspects to meet the market demand for high-quality bearing products. Therefore, domestic bearing enterprises need to strengthen technological innovation and market expansion, and continuously improve their own competitiveness, so as to occupy a favorable position in the fierce market competition.