## 2 The Development of Typical Rotating Motion Mechanisms
### 2.1. Gear Drive
#### 2.1.3 Magnetic Gear
\begin{table}
\begin{tabular}{c c c c} \hline
**Types** & **Advantages** & **Disadvantages** & **Applications** \\ \hline \multirow{4}{*}{Single-layer permanent magnet coaxial magnetic gear [61]} & Noncontact, low wear/noise & Low temperature cooling is required (77 K) & High precision equipment/medical \\  & Low magnetic leakage, high magnetic efficiency & High cost (HTS/cooling) & Low noise industrial \\  & High torque (1.73 times & System complexity & environment \\  & that of traditional gears) & High air gap & Traditional permanent \\  & magnetic density & magnet with a low torque & Electric vehicles, wind \\  & Lightweight, high & upper limit & power, etc \\  [62] & torque density & The modulation loop is & Lightweight industrial \\  & Simple structure, & complex and difficult to & transmission \\  & efficient coupling & manufacture & \\  & High torque density & Need superconducting & \\  & Noncontact, long lifespan & state (\textless{}77 K) & Extreme environment, \\  & Superconducting low & High cooling cost & Ultra high torque \\  & temperature stability & System complexity & \\  & & 3D printing adhesive & Robot joints, servo motors, \\  & & reduces magnetic & and green manufacturing \\  & & flux density & scenarios \\ \hline \end{tabular}
\end{table} 
Table 3: Comparison of transmission performance of different types of magnetic gears. Figure 23: Structural sectional view of ARCMG. with the compactness of planetary gears. However, the problem of local wear caused by uneven stress distribution on the tooth surface has not been fully resolved. A new type of gear based on additive manufacturing breaks through traditional geometric constraints, such as spherical gears, achieving multi-degree-of-freedom motion through magnetic drive coupling. However, the insufficient 3D printing accuracy leads to torque attenuation and surface wear, which restricts their engineering applications. Linear gears use spatial curve meshing theory to achieve zero sliding transmission, which has significant potential in micromechanics. However, the fatigue life under dynamic loads still needs to be verified. Magnetic gear technology overturns the traditional contact transmission paradigm. High temperature superconducting magnetic gears utilise ideal diamagnetism to increase torque density to 1.73 times that of conventional structures, and axial magnetic flux modulation design doubles torque density. However, the cost and complexity of low temperature cooling systems have become bottlenecks for industrialisation. The future development trend will focus on four major technological dimensions: (1) Material structure process collaborative innovation, breaking through the strength weight ratio limit of existing gears by combining topology optimized tooth profile design with high-strength composite materials, such as ceramic based composite materials; (2) Intelligent dynamic control technology, integrating embedded sensors and real-time status monitoring algorithms, achieves tooth wear prediction and online compensation of transmission errors; (3) Improve adaptability to extreme working conditions, develop fully ceramic gears that are resistant to high temperature/vacuum environments and have asymmetric tooth profile designs that resist impact, to meet the needs of special fields such as aerospace; (4) Green and efficient transmission paradigm innovation, focusing on the development of multi physics field coupled magnetic gear systems, combined with breakthroughs in high temperature superconducting materials and efficient refrigeration technology, gradually replacing traditional lubrication dependent transmission.