## 2 The Development of Typical Rotating Motion Mechanisms
### Belt Drive
#### 2.2.2 Synchronous Belt  
[PERSON] et al. [70] proposed a novel nonlinear drive, which includes an active circular pulley, a driven eccentric pulley, and a noncircular tension pulley, as shown in Figure 25. The new noncircular synchronous belt transmission device consists of an active circular wheel, a driven eccentric wheel, and a noncircular tensioner wheel. Traditional noncircular synchronous belts suffer from severe belt slack due to the noncircular shape of the pulleys. The tension wheel compensation device significantly reduces belt slack, improves transmission stability, and is smoother than noncircular chain transmission, making it suitable for environments with poor lubrication conditions. However, additional tensioning wheels are required, increasing the installation and maintenance difficulty. Figure 24: Structure of the TVB: (**a**) a V-belt drive system with the TVB, (**b**) A-A sectional view, (**c**) an exploded view, (**d**,**e**) SEM images of the cross-section and surface of the V-belt, and (**f**) photograph of the fabricated TVB. Figure 25: Schematic diagram of a new type of noncircular pulley transmission. [PERSON] et al. [71] proposed an innovative synchronous belt transmission design based on additive manufacturing (AM), which adopts a matrix-free continuous trapezoidal tooth structure and improves the flexibility of the transmission system through shape innovation, as shown in Figure 26. The study used the fused deposition modeling (FDM) process and flexible polylactic acid (PLA) material, combined with the nonlinear finite element method, to analyse the influence of parameters such as tooth height, tooth width, wedge angle, and belt thickness on the bearing capacity under pretension state, and verified its stability through driving experiments. This transmission belt relies more on rubber substrates and multi-process manufacturing than traditional synchronous belts. The advantages of 3D-printed synchronous belts include direct molding without molds, high design freedom, support for rapid iteration and customised production, and suitability for low speed, low power scenarios such as educational robots and simple transmission devices. However, due to the mechanical properties of PLA materials and the interlayer bonding strength of FDM technology, its durability and load-bearing capacity still have a gap compared to traditional industrial synchronous belts, and it is not suitable for high dynamic loads or long term high-strength working conditions. Table 4 summarizes and compares the transmission performance of the different belts mentioned above. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Types** & **Advantages** & **Disadvantages** & **Applications** \\ \hline Traditional V-belt & Vibration absorption, high & Low reliability, & Automobiles, ship engines, \\ drive [8; 66; 67; 68] & overload protection & short lifespan & industrial robots, etc. \\ V-belt drive (TVB system) [69] & Real-time monitoring and fault & Affected by a humid or & Conveyor belts and small \\  & diagnosis, high reliability, and & high temperature & mechanical transmissions \\  & long lifespan & environment & that require long term \\ Synchronous belt & Specific non-uniform motion or & The belt is loose, and the & \\ drive (traditional & velocity variation & transmission stability & monitoring \\ noncircular) [70] & Maintain tension on the belt. It & Extra noncircular & non-uniform transmission \\ Synchronous belt drive (new & has high stability and is suitable & transioning wheels are & \\  & for environments with poor & required, with a complex & \\ nonlinear) [70] & lubrication. & structure. & long-distance transmission \\
3D Printing & Direct molding without molds, a & Poor durability, low load & Low power, simple \\ Synchronous Belt & high degree of design freedom, & capacity, and limited & transmission devices, and \\  & support for rapid customisation, & material mechanical & other non-high-intensity \\  & and low cost. & properties are also factors. & scenarios \\ \hline \hline \end{tabular}
\end{table}