## 3 Novel Rotating Motion Mechanism
### Universal joints Drive
#### 3.7.3 New Type of Fully Compliant Universal Joint  
[PERSON] et al. [144] proposed a novel, fully flexible universal joint, as shown in Figure 83. Its core feature is an integrated structure that transmits motion through pre-formed flexible segments. The universal joint comprises a rigid shaft section and a pre-fabricated flexible section. The flexible part transmits torque while achieving angular displacement (up to 24\({}^{\circ}\)) between the input shaft and output shaft through elastic deformation. Unlike traditional universal joints, this design offsets velocity fluctuations through symmetrical deformation of flexible segments. This universal joint does not require assembly and can be produced directly through additive manufacturing or injection moulding, simplifying the manufacturing process and reducing costs. However, repeated deformation of the flexible part may lead to material fatigue and limit its service life. When high torque or large bending angles require sacrificing some performance, stress concentration is prone to occur. The low yield strength of materials such as polypropylene limits high load application scenarios. Table 18 summarizes and compares the transmission performance of the different universal joints mentioned above. Figure 82: New anti-buckling universal joint: (**a**) top view, (**b**) front view. Figure 83: A new type of fully compliant universal joint. Regarding research status, the current precision universal joint rotary transmission mechanism presents a diversified and innovative development trend. The steel flexible universal joint innovatively combines the compliant mechanism with traditional universal joints, using a flexible hinge made of spring steel to transmit torque through elastic deformation, allowing axial angular deviation. It has the advantages of balancing elasticity and strength, no need for lubrication, compact structure, and long service life. It is suitable for compact transmission and high reliability lubrication free connection scenarios, but its elastic deformation will reduce transmission accuracy; The new type of anti buckling flexible universal joint is designed with reverse symmetrical cross spring pivot points, which converts compressive loads into tensile stresses, effectively avoiding the buckling problem of traditional compression plates, achieving dual axis high precision rotation, compact structure and no need for lubrication. It is commonly used in optical positioning platforms and surgical robot joints requiring high stability and accuracy. However, its structure is complex, expensive, and limited by tension plates, resulting in limited load-bearing capacity. The new fully compliant universal joint adopts an integrated structure and relies on prefabricated flexible segments to achieve motion transmission and angular displacement, which can offset speed fluctuations. It can be directly produced through additive manufacturing or injection moulding, simplifying the manufacturing process and reducing costs. It is widely used in lightweight transmission scenarios such as miniature robots, drone joints, and medical equipment. However, the flexible part is prone to fatigue and stress concentration under high torque or large bending angles, and the low yield strength of materials such as polypropylene limits their application in high load scenarios. Looking ahead to the future, research on precision universal joint rotary transmission mechanisms will move towards high performance, intelligence, and miniaturisation.