## 2 The Development of Typical Rotating Motion Mechanisms
### 2.1. Gear Drive
#### 2.1.3 Magnetic Gear  
Traditional mechanical gears have contact surfaces between gear teeth, which can lead to energy consumption, noise, and vibration [60]. [PERSON] et al. [61] proposed a novel coaxial magnetic small gear, which only fixes one layer of permanent magnet, as shown in Figure 20. The coaxial magnetic gear adopts a single-layer static, permanent magnet array combined with magnetic flux modulation teeth made of high temperature superconducting (HTS) material. The external magnetic field is repelled through the ideal diamagnetism of high temperature superconductivity, reducing magnetic leakage and concentrating magnetic \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Types** & **Advantages** & **Disadvantages** & **Applications** \\ \hline Spherical gear & Multi-degree-of-freedom, & Low output torque; efficiency & Robot flexible joints, drone \\ [55; 56] & lightweight structure & decreases with increasing & gimbal, and medical \\  & & shaft angle & minimally invasive \\  & & & surgical equipment \\ Linear gear & Small size, suitable for & & High temperature/vacuum \\  & micro machinery, wide & High load performance to & environment transmission, \\  & speed ratio range, zero & be verified & micro-robots, lightweight \\  & slip design & & equipment \\ No relative sliding & Pure rolling engagement, & The manufacturing accuracy & \\ gear [59] & low friction, low contact & requirements, as well as the & High load precision \\  & & installation error sensitivity, are & transmission \\  & & incredibly high. & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of transmission performance of different types of 3D-printed gears. Figure 19: Kinematic performance test of the mechanism: (**a**) uniform motion of the meshing point, (**b**) uniform acceleration motion of the meshing point. flux. The relative motion between the inner and outer rotors generates a harmonic magnetic field through magnetic flux modulation, achieving non-contact torque transmission. After optimising parameters such as tooth thickness and permanent magnet thickness, the torque density was significantly improved, and the maximum output torque reached 1.73 times that of traditional ferromagnetic tooth structures. HTS teeth replace ferromagnetic materials, utilising their ideal diamagnetism to reduce end effects and leakage, improve magnetic flux utilisation, eliminate mechanical contact, reduce wear and noise, and are suitable for high precision scenarios. High temperature superconducting materials require low temperatures (77 K) to maintain superconductivity, rely on cooling systems, and have high costs. [PERSON] et al. [62] provide a new structure to increase the torque of magnetic gears, as shown in Figure 21. The magnetic gear comprises an inner and outer rotor and a magnetic modulation ring. The magnetic modulation circuit regulates the internal and external air gap magnetic fields to couple the internal and external rotor magnetic fields and achieve torque transmission. The inner rotor adopts radial magnetisation surface-mounted permanent magnets. In contrast, the outer rotor adopts tangential magnetisation, a spoke-type permanent magnet that improves the air gap magnetic density through the magnetic flux focusing effect. To ensure torque transmission, the slotted outer rotor reduces the number of iron cores, reduces weight, and improves torque density. By improving the design of the magnetic modulation circuit, the coupling efficiency of harmonic magnetic fields has been significantly improved, and the torque output capability has also been enhanced. Figure 21: Improved magnetic gear model. Figure 20: Schematic diagram of a single permanent magnet array coaxial magnetic gear. Superconducting materials can increase torque density compared to permanent magnets [63]. [PERSON] et al.