## 2 The Development of Typical Rotating Motion Mechanisms
### 2.1. Gear Drive
#### 2.1.2 New Gears Based on 3D Printing Technology  
1. Spherical gear  
[PERSON] et al. [55] proposed a driving integrated spherical gear (DISG), as illustrated in Figure 13. The DISG comprises spherical gears and an omnidirectional internal drive. It projects the combined profile of traditional planar gears' circular involute and level gears onto a spherical surface to enable global meshing and driving. Magnetic force links active driving and passive following magnets across the ball. The active magnet shifts within the ball to drive the passive magnet, causing the two spherical gears to roll freely. These gears can rotate in any direction, creating a multi-degree-of-freedom joint that allows full-range motion without singularities by modifying the contact point's position. Integrating active and passive magnets eliminates mechanical connections, facilitating arbitrary switching of contact points, thus removing the constraints imposed by external frames and enhancing the degree of freedom in motion. Utilising 3D-printed spherical gears and magnetic drive systems decreases overall weight and volume, making it ideal for high-density integration scenarios. However, the output torque of the magnetic drive system is relatively low (measured at 0.39 N\(\cdot\)m), necessitating enhancements to the magnet or optimisation of the tooth profile to improve load capacity. Additionally, 3D-printed gears require high precision, and wear on the tooth surface may lead to uneven transmission. [PERSON] T et al. [56] introduced and manufactured a new type of spherical gear, as shown in Figure 14. This spherical gear achieves multi-degree-of-freedom motion through a specially designed tooth profile (imitating an involute tooth profile), allowing the drive shaft and driven shaft to mesh and transmit power at different shaft angles (0\({}^{\circ}\) to 74\({}^{\circ}\)). Its tooth profile is uniformly distributed along the spherical surface, and the tooth thickness gradually decreases with the increase of contact angle, ensuring stable meshing at different angles. The hollow design inside the gear reduces weight. Compared with traditional complex spherical gears, this design uses a handle similar to a spur gear, simplifying manufacturing and assembly and improving versatility. Supports multi-axis angles (up to 74\({}^{\circ}\)). Made using 3D printing technology (ABS resin), it reduces the production difficulty of complex-shaped gears and shortens the cycle time (about 50 h). However, there are also some drawbacks, such as a decrease in transmission efficiency and a significant increase in clearance when the shaft angle increases. 3D printing can lead to substantial differences in tooth thickness errors and surface roughness. Need to rely on fixtures to adjust the axis angle, with limited flexibility in practical applications. Figure 13: The dexterous joint structure of DISGs. Linear gears are a new transmission method suitable for compact space transmission based on the theory of conjugate space curves. [PERSON] et al. [57] proposed a novel noncircular linear gear, whose structure is shown in Figure 15, and the effect is shown in Figure 16. A new design method for variable speed noncircular gears (VLG) based on spatial curve meshing theory, combined with constant speed ratio (CSR) and variable speed ratio (VSR) linear teeth, is proposed to achieve smooth transmission ratio conversion within one rotation cycle. A cylindrical or conical spiral is the contact curve to ensure a constant speed ratio. A smooth transition of speed ratio is achieved through noncircular teeth. Segmented quartic curves ensure continuous angular acceleration and impact during transmission, avoiding separation or effects. Simple structure, small size, suitable for micromechanical and lightweight applications, with a wide range of speed ratio changes and zero sliding design, which can reduce wear and extend service life.