A Review of Research on Precision Rotary Motion Systems and Driving Methods

Abstract

As the core component of modern mechanical transmission, the precision rotary motion mechanism and its drive system have wide applications in aerospace, robotics, and other fields. This article systematically reviews the design principles, performance characteristics, and research progress of various rotational motion mechanisms and their driving technologies. The working principles, advantages, disadvantages, and applicable scenarios of gears, drive belts, sprockets, camshafts, ratchet claw mechanisms, and linkage mechanisms were analyzed in terms of traditional mechanisms. In terms of new mechanisms, we focused on exploring the innovative design and application potential of intermittent indexing mechanisms, magnetic gears, 3D-printed spherical gears, and multi-link mechanisms. In addition, the paper compared the performance differences of electric, hydraulic, pneumatic, and piezoelectric drive methods. Research has shown that through material innovation, structural optimization, and intelligent control, there is still significant room for improvement in the load capacity, accuracy, and reliability of precision rotary motion mechanisms, providing theoretical support and practical reference for innovative design and engineering applications of future mechanical transmission technologies.

1. Introduction

The rotating motion mechanism is a key component for achieving power transmission and motion conversion, and its performance directly affects the efficiency and stability of mechanical systems [1,2,3,4,5]. With the rapid development of industrial automation, precision manufacturing, and robotics technology, the demand for high load, low noise, and high reliability rotating mechanisms is becoming increasingly urgent. Although traditional mechanisms such as gears and belt pulleys are relatively mature, their vibration and wear problems under high dynamic conditions still need to be solved [6,7,8,9,10]. In recent years, the proposal of new mechanisms such as noncircular gears, magnetic gears, and spherical joints, as well as the application of technologies such as 3D printing and intelligent materials, have injected new vitality into the design of rotary motion mechanisms [11,12,13,14,15,16,17].

The driving method of the rotating motion mechanism is also a key factor determining its performance and application range [18,19,20]. The traditional driving methods mainly include motor drive, hydraulic drive, and cylinder drive. However, with the continuous improvement of performance requirements for rotating mechanisms, piezoelectric drives have made significant progress in the driving of micro-rotating mechanisms in microelectromechanical systems due to their fast response and high resolution, providing a new solution for micro- and nano-scale rotational motion control [21,22]. Electro-hydraulic servo drive achieves precise control of load position, speed, force, and other parameters, significantly improving the accuracy and stability of traditional hydraulic drive [23,24].

This article aims to systematically review existing rotating mechanisms’ research progress and driving methods, analyze their technical bottlenecks, and look forward to future development directions. By integrating mechanical design, materials science, and control theory, this article provides a comprehensive perspective for innovative design and optimization of rotary motion mechanisms to meet diverse needs in complex engineering scenarios. Due to the impossibility of publishing all articles on rotational motion mechanisms and driving methods, this article reviews as much research as possible and mainly analyzes representative mechanisms. The structure of this article is as follows: Section 2 will review the classification and development status of typical rotary motion mechanisms, and Section 3 will introduce some new rotary motion mechanisms. The fourth section provides an overview of the research on the transmission system of rotary motion mechanisms. Finally, based on existing research, a systematic examination of relevant research prospects is significant for the innovative development of rotary motion mechanisms. It can provide a reference for the design work of mechanical engineers.

2. The Development of Typical Rotating Motion Mechanisms

2.1. Gear Drive

The gear drive is one of modern machinery’s most widely used transmission devices. A gear is a mechanical transmission device that uses the meshing of gears to transmit motion and power. It has an extensive transmission power range, high efficiency, an accurate transmission ratio, a long service life, and safe and reliable operation [25]. In addition to high manufacturing costs and noise generation [26], gear transmission has significant advantages in operational characteristics compared to other mechanical transmissions. Gear drives are particularly evident regarding operational safety, durability, efficiency, small size, and reliability, accounting for about 80% compared to other mechanical transmissions [27]. The classification of gears is shown in Figure 1, adapted from [28].

2.1.1. Traditional Metal Gears

• Spur gear

Spur gears are an essential transmission component widely used in automobiles and various mechanical equipment [29]. In an ideal state, where the relative sliding between tooth surfaces during meshing is low, spur gears can effectively transmit power, achieving high transmission efficiency [30]. In the power system of machine tools and the transmission chain of simple automatic transmission systems, multi-stage spur gear transmission can effectively change speed and torque [31].

Adjusting parameters such as the gear displacement coefficient and tooth crest coefficient can increase the gear contact ratio to 2 or above. This type of spur gear is called a high coincidence ratio (HCR) spur gear [32], as shown in Figure 2. Compared with traditional spur gears with contact ratios between 1 and 2, HCR spur gears have more tooth pairs that mesh simultaneously during operation. This feature enhances their load-bearing capacity and reduces vibration and noise, making them particularly suitable for high speed and heavy-duty applications.

2.

Helical gear

The teeth of the inclined gear are spiral-shaped, and its gear profile is shown in Figure 3 [33]. They form a specific angle between the inclined gear’s thread and the gear shaft. Spiral gears overlap more than spur gears, meaning more load distribution teeth are in the transmission process. Therefore, helical gears have good gear performance and long fatigue life and are widely used in mechanical power transmission [34].

Face gears can be used in cross-axis transmission systems. Compared with bevel gearboxes, face gears have unique advantages in rotary transmission, with little or no axial force [36]. The helical surface gear pair is a new pair of noncircular and helical surface gears. Figure 4 [37] shows that it can achieve a variable transmission ratio between gear intersecting axes.

The worm gear consists of a turbine and a worm [38]. Due to their unique characteristics, including rapid deceleration in the smallest space [39], low noise [40], high motion stability [41], and high load capacity, worm gears are widely used in various industries [42,43]. Worm gearboxes in fields such as smart cars and smart homes can significantly reduce the weight of products [44].

Zhang XC et al. [45] designed an innovative biased worm gear transmission mechanism, as shown in Figure 5. The dual lead offset worm drive system can achieve linear contact conjugation between tooth surfaces, ensuring the accuracy of the instantaneous transmission ratio. Its transmission principle is simple and easy to implement. This transmission method not only inherits the advantages of traditional worm gear transmission (including offset worm gear transmission) but is also more flexible in adjusting the axial position. Whether it is the worm gear or worm wheel adjustment, it can effectively change the meshing clearance without interfering with the contact state of the conjugate tooth surface, simplifying the assembly process and providing adjustability of the meshing clearance.

Mu SB et al. [46] proposed a novel linear contact surface worm gear transmission mechanism consisting of a hardened cylindrical and surface worm gear, as shown in Figure 6. The worm gear is formed by grinding with a conical grinding wheel, and after hardening treatment, the surface wear resistance is significantly improved, extending its service life. The worm rotates around its axis, driving the surface worm wheel to rotate around the vertical axis, achieving non-orthogonal shaft transmission. The number of worm heads and worm teeth determines the transmission ratio. By replacing the traditional spiral-driven Archimedes worm with a conical enveloping worm, the problem of non-developable surfaces that cannot be precisely ground has been solved, improving hardness and accuracy. Determine the installation range of the worm gear through the meshing limit line function to ensure meshing stability. The wire contact design improves the load-bearing capacity and transmission efficiency. Numerical analysis shows excellent lubrication performance, with a sliding angle close to 90°, conducive to oil film formation. However, the asymmetry of the tooth surface may lead to vibration and noise, and cone grinding equipment requires precision and high manufacturing costs. The axial installation distance of the worm gear needs to be strictly adjusted. Otherwise, it will affect the meshing performance.

3.

Conical gear

Straight bevel gears are widely used in the mechanical field for transmitting motion between orthogonal axes. According to the different inclined curves, it can be divided into circular spur bevel gears (CSB-gear) and noncircular spur bevel gears (NCSB-gear). Cone gears generate axial force during transmission, producing significant vibration and noise during high speed operation. They are usually used under conditions of low transmission speed and light load. A noncircular spur bevel gear is a special type of spur bevel gear that can be used for gear shifting. It can perform special functions, such as the anti-slip function of the car differential, which circular spiral bevel gears cannot achieve. Its model [47] is shown in Figure 7.

Due to their high load-bearing capacity, high sealing coefficient, and low noise, spiral bevel gears are often used in automotive drives and reducers. At present, spiral limestone wheels are divided into two gear systems: a circular bevel gear with a circular tooth profile shape, and the tooth profile height decreases along the tooth length. Another type of cycloidal bevel gear has a tooth profile that extends outward, with the height of the tooth profile equal to the length of the tooth. The cycloidal bevel gear [48] is shown in Figure 8.

4.

Hyperbolic gear

The hyperbolic gear is widely used in various automotive drive systems. The main difference between them and spiral bevel gears is that the former has two spatial displacement axes, and the distance between the two axes is called hyperbolic bias. Introducing a slight gear bias transforms a planar helical gear into a spatial hyperbolic gear, which has significant advantages such as improving the strength of the small gear, improving the contact ratio, and flexibly adjusting the height of the car chassis. However, the biggest drawback is that the sliding ratio will increase [49], as shown in Figure 9.

5.

Noncircular gear

Noncircular gears are typical gears that can achieve non-uniform transmission and have been widely used. They have the advantages of high transmission accuracy and compact size [51]. Mundo D. [52] proposed a new concept of nonlinear planetary transmission, as shown in Figure 10. The planetary gear system consists of three noncircular gears (sun gear, planetary gear, and ring gear), which achieve variable transmission ratios through the geometric design of noncircular pitch curves. The degrees of freedom of the planetary gear system are achieved by fixing the sun gear or ring gear to achieve different input/output configurations, supporting four transmission modes, and flexibly adjusting the relationship between torque and speed. Combining the variable transmission ratio characteristics of noncircular gears with the compactness of planetary gear systems to achieve high dynamic transmission requirements. By defining the transmission ratio function through the Fourier series and combining it with numerical integration, interference-free tooth profiles are generated to ensure meshing stability.

The schematic diagram of the noncircular gear proposed by Yu GH et al. [53] is shown in Figure 11. This conjugate concave-convex noncircular gear consists of a driving wheel and a driven wheel and achieves variable speed transmission through the meshing of noncircular pitch curves. When the driving wheel rotates, the conjugate tooth profile generated by the generation method ensures smooth meshing, where the gear teeth adopt an involute tooth profile, and the concave part adopts a transition curve similar to a cam. Use the Gaussian function superposition method to define the transmission ratio and flexibly adjust the position and size of two unequal peaks. The transmission ratio has an extensive dynamic range and can meet special motion requirements. There may be slight errors in the transmission ratio within specific ranges.

6.

Double cycloidal gear

Shin JH. [54] proposed a new prestressed gear, as shown in Figure 12. The double cycloidal gear adopts a cycloidal profile design instead of traditional involute gears. It utilises the low sliding speed during cycloidal meshing to achieve smooth transmission under impact loads. Gear design is based on the principle of double cycloid meshing, which optimises the contact trajectory by adjusting parameters such as tooth number difference and modulus, reducing tooth surface interference and deformation. The double cycloidal gear has strong impact resistance and small tooth surface deformation, making it suitable for high dynamic load applications.

Table 1. Comparison of transmission performance of different types of traditional metal gears.

Types	Advantages	Disadvantages	Applications
Spur gear [28,29,30]	High transmission efficiency, simple structure, and low tooth surface sliding.	High noise level, significant vibration under high load	Machine tool power system, multi-stage transmission device
HCR spur gear [32]	High load-bearing capacity, low vibration, and noise	Need to adjust gear parameters (displacement coefficient)	Industrial machinery, heavy-duty transmission systems
Helical gear [33,34,35]	Uniform load distribution, long lifespan, smooth transmission	Generate axial force, requiring additional bearing support	Reducer, General Industrial Equipment
Face gear [36,37]	No/low axial force, variable transmission ratio	An asymmetric tooth surface may cause vibration	Robot joints, non-orthogonal axis transmission system
Worm gear [38,39,40,41,42,43,44,45,46]	High deceleration ratio, low noise, high stability, auto-lock	High sliding friction, low efficiency, and lubrication required	Elevators, car steering systems, and heavy machinery
Straight bevel gear [47]	Reliable and straightforward, noncircular spur bevel gears can achieve special functions.	High speed noise is loud and requires axial fixation	Low speed light-load transmission
Hypoid gear [49,50]	The chassis height can be flexibly adjusted, with a high contact rate.	Large sliding ratio, easy to wear, requires lubrication	Automotive drive systems, high offset transmission
Non- circular gear [51,52,53]	Non-uniform variable speed, high transmission accuracy, compact structure, fast dynamic response	Complex processing and high cost, uneven stress on the tooth surface, prone to local wear	Printing machines, textile machinery, variable transmission bicycles
Double cycloidal gear [54]	Strong impact resistance and minimal tooth surface deformation, smooth transmission, and low noise	The dynamic response needs to be optimised; there may be a delay in the initial meshing.	Car seat belt tensioner, high dynamic load machinery

summarizes and compares the transmission performance of different traditional metal gears mentioned above.

2.1.2. New Gears Based on 3D Printing Technology

• Spherical gear

Liang GQ 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 bevel 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·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.

Tanaka 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° to 74°). 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°). 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.

2.

Linear gear

Linear gears are a new transmission method suitable for compact space transmission based on the theory of conjugate space curves. Chen YG 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. Segmented curve control is suitable for high temperature, low temperature, or vacuum environments, ensuring no impact and smooth movement during transmission. Figure 17 [58] also shows a typical linear gear transmission structure.

3.

Nonrelative sliding gear

Chen Z et al. [59] proposed a design of non-relative sliding gears for transmitting parallel axes. Figure 18 is a physical photo of the gear, and Figure 19 shows its working characteristics. This non-relative sliding gear is designed based on the functional positioning of the meshing line. Pure rolling contact of parallel axis transmission was achieved, significantly reducing friction and temperature rise. Using helical motion to generate cylindrical helical surfaces with equal or variable pitch simplifies the design process and improves flexibility. The concave-convex arc tooth surface design significantly reduces contact stress compared to involute gears. The non-relative sliding gear reduces sliding friction, improves transmission efficiency and service life, has low contact stress, is suitable for high load scenarios, is flexible in design, and can adapt to different motion requirements by adjusting the meshing line function.

Table 2. Comparison of transmission performance of different types of 3D-printed gears.

Types	Advantages	Disadvantages	Applications
Spherical gear [55,56]	Multi-degree-of-freedom, lightweight structure	Low output torque; efficiency decreases with increasing shaft angle	Robot flexible joints, drone gimbal, and medical minimally invasive surgical equipment
Linear gear [57,58]	Small size, suitable for micro machinery, wide speed ratio range, zero slip design	High load performance to be verified	High temperature/vacuum environment transmission, micro-robots, lightweight equipment
No relative sliding gear [59]	Pure rolling engagement, low friction, low contact stress, and long lifespan	The manufacturing accuracy requirements, as well as the installation error sensitivity, are incredibly high.	High load precision transmission

summarizes and compares the transmission performance of the different 3D printed gears mentioned above.

2.1.3. Magnetic Gear

Traditional mechanical gears have contact surfaces between gear teeth, which can lead to energy consumption, noise, and vibration [60]. Yin X 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 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.

Jing LB 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.

Superconducting materials can increase torque density compared to permanent magnets [63]. Dong K et al. [64] proposed a novel axial flux modulated superconducting magnetic gear (AFMSMG), as shown in Figure 22. Axial flux modulation superconducting magnetic gear (AFMSMG) uses superconducting block magnets (SBM) instead of traditional permanent magnets, utilising the high critical current density and ideal diamagnetism of superconducting materials to form a strong magnetic field at low temperatures. Through axial magnetic flux design, the interaction between the inner and outer rotor magnetic fields and the fixed modulation ring generates harmonic magnetic fields, achieving non-contact torque transmission with a torque density more than twice that of traditional magnetic gears. The non-contact transmission reduces wear, has a long lifespan, and is suitable for high precision applications. Superconducting properties are stable at low temperatures and ideal for unique environments. Cooling conditions below 77 K are required, which increases costs and system complexity. Compared to traditional solutions, the cost of superconducting materials and cooling systems is much higher.

Huang PW et al. [65] proposed an Axial Radial Cone Magnetic Gear (ARCMG) based on 3D printing technology, as shown in Figure 23. Using FeSiCr as the rotor and pole piece material, we explored three manufacturing methods: wire cutting, selective laser melting (SLM) hybrid manufacturing, and particle deposition modelling (PDM). Among them, PDM technology achieves the integrated moulding of complex conical magnets by compounding 95% MQP-S magnetic particles with 5% PA12 adhesive. Compared to traditional axial radial magnetic gears (ARMG), ARCMG shortens the magnetic flux path through a conical magnet structure, reduces volume by 52%, increases torque density by 28%, and simplifies assembly processes. The experiment showed that its gear ratio remained stable at 6.5, verifying its efficient transmission characteristics. However, introducing adhesives in 3D printing technology decreases magnetic flux density, and the magnetic barrier structure manufactured by SLM hybrid may increase magnetic resistance and leakage. This design is suitable for high precision, low vibration scenarios such as servo motors, robot joints, and customised industrial drive systems, especially for small batch production needs that require rapid prototyping or complex geometric structures. It provides a new solution for applying magnetic gears in precision engineering and green manufacturing.

Table 3. Comparison of transmission performance of different types of magnetic gears.

Types	Advantages	Disadvantages	Applications
Single-layer permanent magnet coaxial magnetic gear [61]	Noncontact, low wear/noise Low magnetic leakage, high magnetic efficiency High torque (1.73 times that of traditional gears)	Low temperature cooling is required (77 K) High cost (HTS/cooling) System complexity	High precision equipment/medical Low noise industrial environment
Improved magnetic gear [62]	High air gap magnetic density Lightweight, high torque density Simple structure, efficient coupling	Traditional permanent magnet with a low torque upper limit The modulation loop is complex and difficult to manufacture	Electric vehicles, wind power, etc Lightweight industrial transmission
Axial superconducting magnetic gear [64]	High torque density Noncontact, long lifespan Superconducting low temperature stability	Need superconducting state (<77 K) High cooling cost System complexity	Extreme environment, Ultra high torque
3D-printed conical magnetic gear [65]	Small size, high torque Assembly simplicity	3D printing adhesive reduces magnetic flux density	Robot joints, servo motors, and green manufacturing scenarios

summarizes and compares the transmission performance of the different magnetic gears mentioned above.

Traditional metal gears continue to improve performance through tooth profile optimisation and parameter control. For example, high coincidence spur gears achieve high load and low vibration through multi-tooth meshing, but their tooth profile modification coefficients need to be precisely matched. The new biased worm gear transmission system adopts a line contact conjugate tooth surface design, which inherits the advantages of traditional worm self-locking while achieving flexible adjustment of axial position and simplified assembly; Noncircular gears exhibit unique advantages in dynamic variable speed scenarios by defining the gear ratio function through Fourier series and combining it 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.

2.2. Belt Drive

2.2.1. V-Belt

Since their introduction in the late 1970s, V-belt drive systems have become increasingly important in the automotive industry [66]. V-belt transmission has the advantages of flexible design, strong output stability, strong absorption capacity, large transmission ratio, overload protection, lubrication, pollution, and high cost-effectiveness. It is widely used in advanced mechanical transmission systems such as automobiles, ship engines, and industrial robots [8,67,68].

Ruan SH et al. [69] combined a belt drive with a triboelectric nanogenerator (TENG) and proposed an intelligent triboelectric V-belt drive (TVB) system, as shown in Figure 24. TVB combines a V-belt drive with TENG, which is embedded in an independent mode at the bottom of the pulley. During operation, the V-belt periodically contacts and separates from the copper electrode at the bottom of the pulley, generating an AC signal through frictional charging. The signal output characteristics are related to load resistance, speed, and preload distance and can be used for real-time monitoring of the transmission system status. TENG is integrated at the bottom of the pulley, eliminating the need for additional space and avoiding direct contact with the working surface of the V-belt, significantly improving reliability and lifespan.

2.2.2. Synchronous Belt

Chen JN 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.

Hu FW 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. Comparison of the performance of different types of belt drives.

Types	Advantages	Disadvantages	Applications
Traditional V-belt drive [8,66,67,68]	Vibration absorption, high transmission ratio, overload protection	Low reliability, short lifespan	Automobiles, ship engines, industrial robots, etc.
V-belt drive (TVB system) [69]	Real-time monitoring and fault diagnosis, high reliability, and long lifespan	Affected by a humid or high temperature environment	Conveyor belts and small mechanical transmissions that require long term monitoring
Synchronous belt drive (traditional noncircular) [70]	Specific non-uniform motion or velocity variation	The belt is loose, and the transmission stability is poor	Machinery requiring a non-uniform transmission
Synchronous belt drive (new nonlinear) [70]	Maintain tension on the belt. It has high stability and is suitable for environments with poor lubrication.	Extra noncircular tensioning wheels are required, with a complex structure.	Machinery with precise speed changes for long-distance transmission
3D Printing Synchronous Belt [71]	Direct molding without molds, a high degree of design freedom, support for rapid customisation, and low cost.	Poor durability, low load capacity, and limited material mechanical properties are also factors.	Low power, simple transmission devices, and other non-high-intensity scenarios

summarizes and compares the transmission performance of the different belts mentioned above.

Traditional V-belt transmission has advantages such as strong vibration absorption capacity, large transmission ratio, overload protection, and high cost-effectiveness. However, it has disadvantages such as low reliability and a short lifespan. It is widely used in high load scenarios such as automotive and marine engines. The improved intelligent friction electric V-belt (TVB) achieves real-time status monitoring by integrating friction nanogenerators, significantly improving reliability and lifespan. However, it is limited by humid and hot environments and is suitable for conveyor belts and small transmission systems that require long term monitoring. In synchronous belt transmission, traditional noncircular synchronous belts can achieve non-uniform motion, but they are prone to relaxation and have poor stability, making them suitable for non-uniform transmission machinery. The new type of nonlinear synchronous belt improves the relaxation problem through a tension wheel compensation device, with high transmission stability and resistance to low lubrication environments. However, its complex structure requires additional tension wheels, making it suitable for long-distance, precise variable speed transmission. The 3D printing synchronous belt adopts moldless moulding technology, with high design freedom and support for rapid customisation. However, due to material strength and process defects (such as insufficient durability of PLA materials), it is only suitable for low power, non-high-strength scenarios such as educational robots. Belt drive has significant advantages in cost-effectiveness, flexible design, and environmental adaptability. However, it must still combine material and structural innovation to improve performance boundaries under high dynamic loads and long term, high-intensity working conditions.

2.3. Chain Drive

2.3.1. Single-Row Chain Drive

• Single-row chain drive

The chain drive transmits the motor’s driving force to the rotating actuator. The basic structure of modern chains was first designed and proposed by the great scientist and artist Leonardo da Vinci during the European Renaissance. The sleeve roller chain adds sleeves and rollers, reduces wear, and dramatically extends the chain’s service life [72]. As shown in Figure 27 [73], the chain consists of alternating inner and outer rings. The chain pitch is the distance between two consecutive sleeve shafts.

Figure 28 [73] shows the typical chain drive. The drive wheel is on the right side, and the drive wheel is on the left. The drive wheel drives the gear to rotate clockwise. Therefore, the tight chain is used as a rising chain to transfer the load, and the bottom is relaxed. The end of the chain represents the transition between the chain teeth and the sprocket.

2.

Multi-row chain drive

Cheng WJ et al. [74] mentioned multi-row sprockets with flanges, as shown in Figure 29. The core of this article is the cold semi-precision forging process of 5052 aluminium alloy multi-row sprockets. By designing a new type of sprocket tooth profile and using a circular arc transition instead of traditional sharp tooth tips, stress concentration and crack risk in the mould cavity have been reduced. Propose a three-step forging process, pre-forming teeth, machining flange grooves, and final forging, to simplify operations and improve mould filling efficiency. Cold forging technology reduces material waste and heat treatment requirements, lowering production costs. The new tooth profile and segmented forging have improved dimensional accuracy.

3.

Silent chain drive

The silent chain transmissions in the specific automotive engine shown in Figure 30 [75] have the advantages of low noise, high transmission accuracy, high transmission efficiency, and good durability. They are widely used in automotive engines, transmissions, machine tools, and other high speed transmission devices.

4.

Bidirectional chain drive

Cheng YB et al. [76] proposed a novel dual-phase single-toothed chain plate bidirectional chain transmission system. Based on the dynamic model shown in Figure 31. Adopting dual chain and dual phase sprockets, the polygonal effect is suppressed by phase difference, vibration is reduced, and transmission stability is improved. The dual-chain design enhances wear resistance and load capacity. The chain plate only has a single-tooth meshing, breaking through the limitations of traditional double tooth design, simplifying the structure and reducing weight, and verifying transmission stability through dynamic simulation. Optimising the shape of the chain plate and pin shaft reduces system weight (by 23.3%) and meets lightweight requirements.

2.3.2. Composite Material Chain Drive

Krithikaa D et al. [77] proposed an E-glass fibre-reinforced interpenetrating polymer network (IPN) composite chain plate for transmitting low loads, as shown in Figure 32. Using E-glass fibre reinforced IPN resin instead of traditional iron chain plates, it is 60% lighter than iron chain plates and reduces system energy consumption. Adjusting the vinyl ester to polyurethane ratio allows a balance between stiffness and elasticity, which is superior to metal chain plates in noise reduction and vibration reduction. Verify the fatigue life of chain plates under different resin ratios. IPN resin is resistant to chemical corrosion and suitable for harsh environments. Adjust mechanical properties through the resin ratio to meet various load requirements. The increase in PU ratio leads to an increase in elasticity, a decrease in fatigue resistance, and complex processing techniques that require manual layering and precise resin mixing. The manufacturing cost of composite materials is higher than that of traditional metals.

2.3.3. Polymer Material Chain Links Based on 3D Printing

R ă Rădulescu B et al. [78] investigated the mechanical properties of four materials, including ABS, ABS Kevlar composite, PLA, and PETG, under tensile load through systematic experiments and finite element analysis, as shown in Figure 33. They also constructed an empirical mathematical model to quantify the influence of printing parameters such as link diameter, printing direction, and filling density on strength. The innovation lies in combining the optimisation of 3D printing process parameters with the comparison of multi-material properties, revealing the significant impact of printing direction on mechanical properties—horizontally printed chain links have nearly 9 times higher strength than vertically printed links due to the continuous distribution of material layers along the principal stress direction. Research has found that PETG material exhibits optimal performance in horizontal printing, with a fracture force of 300.8 N, far exceeding the 4.70 N of PLA. The positive control effect of extrusion temperature, filling density, and other parameters on strength has been verified through mathematical models. Compared to traditional metal chain links, polymer chain links have advantages such as lightweight, corrosion resistance, and insulation, but their mechanical strength is still limited to low-to medium-load scenarios, such as decorative chains, pet traction chains, or religious product suspension devices. However, their applicability under extreme loads is insufficient, and the strength of ABS Kevlar composite materials decreases due to weakened bonding between fibers and matrix interface, exposing material compatibility issues.

Table 5. Comparison of the performance of different types of chain drives.

Types	Advantages	Disadvantages	Applications
Single-row chain drive [72,73]	Simple structure, easy maintenance, long lifespan, and low cost	High noise, lubrication required, low precision, and prone to vibration at high speeds	Bicycles, motorcycles, and other low load scenarios
Multi-row chain transmission [74]	Strong load-bearing capacity, high material utilisation rate, and reduced mould stress concentration.	High requirements for processing equipment and precise control of parameters	Industrial machinery, long-distance or high load scenarios.
Silent chain drive [75]	Low noise	Torque and speed are limited	Engines, machine tools, and other high speed transmission devices
Bidirectional chain drive [76]	High transmission accuracy and efficiency, good durability	High installation accuracy, heavy weight, and complex structure	High speed, high precision, and lightweight transmission system
Composite material chain drive [77]	Lightweight, energy-saving, corrosion-resistant, noise-reducing, and vibration-reducing	The proportion of PU affects the anti-fatigue ability, resulting in a high cost	Corrosion-resistant, lightweight, and low load scenarios
3D printing polymer chain links [78]	Insulation, customisable design	Low mechanical strength and poor material compatibility	Non-load-bearing scenarios or short-term low load applications

summarizes and compares the transmission performance of the different chains mentioned above.

Single-row chain transmission (such as a sleeve roller chain) has a simple structure, easy maintenance, and low cost. However, it has problems such as high noise, regular lubrication, and significant high speed vibration. It is suitable for low load scenarios such as bicycles and motorcycles; multi-row chain transmission optimises tooth profile through multi-tooth collaborative load-bearing and cold forging technology, significantly improving load capacity and material utilisation. However, it has strict requirements for machining accuracy and mold control and is commonly used in heavy load conditions of industrial machinery. Silent chain transmission achieves low noise and high efficiency through meshing optimisation, but is limited by the torque-carrying range and is specifically designed for high precision, high speed transmission in automotive engines, machine tools, and other applications. The bidirectional chain transmission adopts a phase difference dual chain design to suppress the polygonal effect, which has the advantages of high stability and lightweight. However, the complex structure requires high installation accuracy, making it suitable for precision transmission systems. Composite material chains (such as E-glass fibre-reinforced resin) break through the limitations of traditional metals with their lightweight, corrosion-resistant, and vibration-reducing properties. However, their high cost and fatigue resistance are limited by material ratios, making them suitable for corrosive, light-load environments such as chemical and marine industries; 3D-printed polymer chain links (such as PETG) achieve lightweight and insulation properties through parameter optimisation, but their mechanical strength is only suitable for non-load-bearing fields such as decoration and pet traction, exposing the shortcomings of insufficient material compatibility and extreme load adaptability. Overall, various types of chain drives form a gradient complementarity between load capacity, noise control, environmental adaptability, and cost efficiency and need to be selected based on a comprehensive consideration of operating conditions.

2.4. Cam Mechanism

The cam profile can be accurately designed and processed based on the expected motion law of the driven component, and high precision motion output can be achieved by moving the driven component through the rotation of the active element. Therefore, cam mechanisms also belong to the precision rotary motion mechanisms research scope. Cam mechanisms are essential in production and handling technology [79]. A disc-shaped cam mechanism typically consists of three components: a cam plate, a follower, and a connecting frame. The input motion that drives the cam disc has a constant speed [80].

2.4.1. Cam Roller

A cam roller is a ball bearing with a thick outer ring wall. These pre-greased and ready-to-install units are used for various cam drives and conveying systems [81], as shown in Figure 34. Cam rollers achieve power transmission through frictional rolling, and compared to other transmission methods, they have the characteristics of high efficiency and low energy consumption [82].

2.4.2. Conjugate Cam

The disc cam mechanism is a simple and reliable mechanism that can generate monotonic reciprocating motion on a machine. The conjugate cam mechanism, composed of a frame, a pair of conjugate disc cams, and a swinging or translating follower, can simultaneously form two pairs of cams and directly drive the cam mechanism with the follower [83], which is very suitable for high speed applications [84]. Chang WT et al. [85] conceptually designed a pair of combined conjugate cams, as shown in Figure 35. The conjugate cam mechanism consists of a pair of conjugate disc-shaped cams (cam A and cam B) and a swinging or translating follower, which is driven to move by synchronous rotation of the two cams. By combining the arc groove design with the axial outer integral weight block, static and dynamic balance can be achieved, thereby reducing the vibration of the mechanism. Adopting an arc-shaped cam to minimise mass and balance the remaining imbalance through an axially formed counterweight block on the outer side avoids the space occupation problem of traditional eccentric counterweights. The conjugate cam mechanism achieves static and dynamic balance, significantly reducing vibration and bearing load.

2.4.3. Cylindrical Cam

The relative motion between the cam and the follower in the cylindrical cam mechanism is spatial motion; therefore, it belongs to the spatial cam mechanism. The cylindrical cam mechanism occupies a small space and is suitable for use when space is limited. The cam profile can be designed according to the follower’s needs to achieve complex motion laws [86].

Kamali SH et al. [87] developed a cylindrical cam mechanism, as shown in Figure 36. The cylindrical cam mechanism converts linear motion into rotational motion through spiral grooves. Specifically, linear displacement drives the axial movement of the cam, and the coupling slides in the spiral groove, driving the motor shaft to rotate, thereby driving the electromagnetic machine to generate electricity or a damping force. Compared with ball screws and gear mechanisms, the stroke-to-maximum length ratio of cylindrical cam mechanisms is significantly increased. The motor and cam are coaxial, avoiding the size limitations of complex gears or vertical layouts.

2.4.4. 3D-Printed Cam Formed by Melt Deposition

Zayas Figueras et al. [88] compared the performance differences in dimensional accuracy and surface quality between fused deposition forming (FFF) 3D printing and CNC milling manufacturing of fixed-width camshafts, as shown in Figure 37. They proposed a kinematic design method based on Bézier curves to meet the innovative design framework of geometric confinement. Research has shown that the cam made of polylactic acid (PLA) material for 3D printing has a radial dimension error (0.12%) similar to that of a traditional aluminium CNC cam (0.07%). Still, due to the layer-by-layer stacking characteristics, its axial dimension error (1.64%) is significantly higher than that of the milling process (0.54%). The surface roughness (Ra = 10.41 μm) is about 20 times worse than that of CNC-machined parts (Ra = 0.50 μm). The main advantages of this technology are reflected in rapid prototyping, low cost, and lightweight characteristics, making it suitable for low power transmission scenarios such as educational models; however, its insufficient interlayer bonding strength and surface quality limitations make it difficult to withstand high load and high-wear industrial scenarios, such as automotive camshafts. The research innovatively verified the feasibility of plastic cams in non-power-intensive mechanical systems, providing an experimental basis for applying additive manufacturing in customised motion mechanisms, especially suitable for engineering scenarios with low dynamic performance requirements but requiring rapid iteration.

Table 6. Comparison of different types of cam mechanisms.

Types	Advantages	Disadvantages	Applications
Cam roller [81,82]	High efficiency and low energy consumption, low edge stress, and low maintenance cost	Under high load, the lifespan may be shortened due to friction	Conveyor system, low energy consumption demand
Conjugate cam [83,84,85]	Smooth movement, minimal vibration, high speed operation, compact structure	High cost, requiring precision machining and weight design	High speed weaving machine, high speed sorting, precision instruments
Cylindrical cam [86,87]	Small space occupation, long-distance movement, adjustable damping	Easy to wear and tear, high cost, poor stability	Equipment vibration suppression, long stroke, and compact requirements
3D-Printed Cam [88]	Low cost, lightweight, suitable for geometric innovative design	Significant axial dimension error, rough surface, low interlayer bonding strength, and inability to withstand high loads	Low power transmission scenarios, customised motion mechanisms that require rapid iteration

summarizes and compares the transmission performance of the different cam mechanisms mentioned above.

Single-row chain transmission (such as a sleeve roller chain) has a simple structure, easy maintenance, low cost, and long service life. However, it has problems such as high noise, lubrication, low transmission accuracy, and high speed vibration. It is suitable for low load scenarios such as bicycles and motorcycles; multi-row chain transmission improves load-bearing capacity and material utilisation through multi-row tooth design, and cold forging technology is used to reduce stress concentration. However, the processing equipment requires high precision parameter control, making it suitable for industrial machinery, high load, or long-distance transmission. Silent chain transmission is characterized by low noise, high precision, and high efficiency but is limited by torque and speed and is commonly used in high speed precision equipment such as automotive engines and machine tools. The bidirectional chain drive suppresses the polygonal effect through phase difference, enhances stability and wear resistance, and has a lightweight structure but high installation accuracy requirements, making it suitable for high speed and high precision transmission systems. Composite material chain drives (such as glass fiber reinforced IPN resin) have the advantages of being lightweight, corrosion resistant, and vibration and noise reducing, but their fatigue resistance is affected by the resin ratio, and the cost is high, making them suitable for corrosive environments or light-load energy-saving scenarios. 3D-printed polymer chain links (such as PETG materials) have customizable insulation, are lightweight and corrosion-resistant, but have low mechanical strength and poor material compatibility. They are only suitable for non-load-bearing or short-term light-load scenarios, such as decoration and pet traction chains. Various chain drives complement traditional machinery and emerging material technologies, covering diverse applications from low load to high precision, from conventional environments to special needs.

2.5. Ratchet and Pawl Mechanism

The ratchet and pawl mechanisms convert intermittent rotational motion into continuous rotational motion or connect and release shafts of different speeds. Its working cycle includes closure, tight closure, opening, and free movement. Closure can be achieved through friction or engagement. The ratchet mechanism transmits torque through the mutual locking of the pawl and ratchet teeth. When the ratchet teeth move freely, the pawl and ratchet teeth separate. The pawl and locking mechanism are classic designs of rotary motion mechanisms [89], as shown in Figure 38. The classic ratchet mechanism has been applied to various daily items such as bicycle conveyors, zippers, and keys, as well as more precise and complex systems such as ultra-high speed clutches and micro-drive systems [90,91].

2.5.1. Flexible Ratchet and Pawl Mechanism

Refer to Roach G M’s summary of the types of flexible ratchet and pawl mechanisms [92], as shown in

Table 7. Comparison of different types of flexible ratchet teeth and claws.

Types	Core Innovation	Advantages	Disadvantages	Applications
Bending loading	The cantilever beam replaces the spring hinge	Low cost, few parts	Low torque, prone to fatigue	Light machinery, low cost equipment
Tension loading	Short-length flexible pivots enhance stiffness	High torque ratio	High material strength requirements	Electric tools, industrial transmission
Compressive loading	Rigid tooth and flexible segment separation design	Ultra-high torque ratio, low friction	Manufacturing complexity	Heavy machinery, automotive components
MEMS applications	Silicon-based integrated microstructure	Miniaturisation, no wear and tear	Minimal output, prone to failure	Micromechanical systems, sensors

.

2.5.2. Ratchet and Pawl Mechanism for High Speed Transmission

Referring to Bondaletov VP summarised the ratchet mechanisms used for high speed transmission [93], as shown in

Table 8. Comparison of ratchet mechanisms for high speed transmission.

Types	Core Innovation	Advantages	Disadvantages	Applications
Traditional ratchet mechanism	Single pawl, fixed tooth pitch	Simple structure and low cost	High noise and poor high speed performance	Low speed and low load scenarios
Modular ratchet mechanism	Multi-disc, multi-pawl, and elastic tooth design	High load capacity, low noise	Manufacturing is complex and costly	High speed pulse transmission, heavy machinery
Micro ratchet mechanism	Miniaturisation and elastic rod design	Miniaturisation and low friction	Low load capacity and easy failure	Micromechanical system

.

2.5.3. New Ratchet Mechanism for Multi-Material Additive Manufacturing Technology

Sachai AH et al. [89] proposed a novel flexible ratchet mechanism that utilises additive manufacturing techniques for various materials, as shown in Figure 39. This design eliminates the general movement of springs, claws, or gears in traditional ratchet and pawl mechanisms. The organisation uses 3D printing technology to replace conventional mechanisms with multi-material mechanisms that combine the principles of flexible mechanisms and classic ratchet mechanisms, allowing parts to move in one direction while preventing movement in the opposite direction. This behaviour is obtained through elastic deformation, which transfers the displacement of the component to the flexible area during insertion. In contrast, the geometric shape of the ring allows for limiting the displacement of the element in the opposite direction.

Rizescu CI et al. [94] conducted innovative research on the design optimisation of rotary motion mechanisms. They proposed a dual-mode ratchet mechanism with 12 and 24 teeth based on the modular combination principle, as shown in Figure 40. The mechanism has constructed a motion transmission system with self-locking characteristics through the coordinated cooperation of the driving unit, compression spring, and locking pawl. The spring element achieves dynamic meshing control between the pawl and ratchet, effectively reducing motion impact through an elastic energy storage mechanism. The rapid prototyping of the mechanism was achieved using fused deposition modelling (FDM) additive manufacturing technology, with polylactic acid (PLA) and acrylonitrile butadiene styrene copolymer (ABS) as forming materials. The experimental results show that this design combines lightweight and low noise characteristics and reduces manufacturing costs by about 68% compared to traditional machining processes. Through the collaborative optimisation of tooth profile parameterisation modelling and material mechanical properties, researchers have established a quantitative relationship model between tooth number, contact stress, and fatigue life, verifying the trade-off law that increasing the number of teeth can improve transmission stability but sacrifice compactness. This technical solution is particularly suitable for rapid concept validation of low torque applications such as small electromechanical equipment and automation instruments. Its modular design concept provides an effective technical path for the iterative development of customised motion mechanisms.

Table 9. Comparison of different types of multi-material 3D-printed new ratchet wheels.

Types	Advantages	Disadvantages	Applications
Elastic deformation [94]	3D printing multi-material integrated molding, no spring, compact space	Easy to fatigue, limited load capacity	Small load scenario
Gear optimisation [95]	Adjustable number of teeth, low noise, lightweight, low cost	PLA/ABS materials have low strength and poor durability	Low torque scenarios, such as small electromechanical equipment and automation instruments

summarizes and compares the transmission performance of different 3D printed ratchet and pawl mechanisms mentioned above.

The ratchet and pawl transmission mechanism achieves one-way intermittent transmission through meshing or friction between teeth, which has the advantages of compact structure, reliable directional locking, and flexible modular design. However, it also has common problems such as noise, wear, and limited load. The traditional ratchet adopts a single-claw fixed-tooth-pitch design, which is simple in structure and low in cost but has poor high speed performance and evident noise, mainly suitable for low speed and low load scenarios. Flexible ratchet wheels can be divided into three categories based on the loading method: bending loading type, which replaces spring hinges with cantilever beams to achieve low cost and lightweight but has weak fatigue resistance and is mainly used in light machinery and low-end equipment; Stretch loading type enhances stiffness through short, flexible pivots to achieve high torque ratios, but requires strict material strength requirements, commonly found in electric tools and industrial transmissions. The compression loading type adopts a rigid tooth and flexible segment separation design, which reduces friction while achieving an ultra-high torque ratio. However, the manufacturing process is complex and suitable for heavy machinery and automotive components. In addition, the MEMS micro ratchet, which is based on a silicon-based integrated microstructure, breaks through the limitation of miniaturisation. It has no wear, but due to its minimal output and easy failure, it specialises in microelectromechanical systems and sensors. Modular ratchet gears are designed with multiple discs, claws, and elastic teeth to achieve high load capacity and low noise in response to the demand for high speed transmission. However, their complex structure leads to increased costs, and they are mainly used for pulse transmission in heavy machinery. The micro high speed ratchet adopts an elastic rod design to achieve low friction transmission, but due to low load and insufficient reliability, it is limited to micro-mechanical systems. The elastic deformation type ratchet wheel generated by the new multi-material 3D printing technology replaces springs and gears with an integrated structure, which is compact in space but prone to fatigue and suitable for low load scenarios. The gear optimisation type achieves lightweight and low noise through parameterised tooth profile modeling and PLA/ABS material printing. Although limited by material strength and durability, it still demonstrates cost advantages in low torque scenarios such as small electromechanical equipment and automation instruments. This technology system covers cross-scale applications from micrometer-level sensors to heavy machinery, and its innovative direction is focusing on material composites, structural flexibility, and the integration of additive manufacturing technology.

2.6. Linkage Mechanism

2.6.1. Four-Bar Linkage

• Crank-rocker mechanism

The planar crank rocker linkage mechanism is widely used to convert continuous rotational motion into oscillatory motion [95]. The crank rocker mechanism mainly comprises four components: crank, connecting rod, rocker, and frame [96]. The crank can also rotate under certain conditions when the joystick is used as the active component, as shown in Figure 41. The crank rocker mechanism has the advantages of a simple structure, strong load-bearing capacity, and high reliability in converting rotational motion. It is widely used in fields such as internal combustion engines [97,98], stamping machines [99], and biomimetic machinery [100]. However, its disadvantages of discontinuous motion, vibration noise, and ample space occupation limit its application in high speed and high precision scenarios.

Joshi R et al. [102] designed and constructed a novel crank rocker mechanism based on a coil spring, which utilises a long free-rotating arm with one end fixed and the other end mounted with an airfoil and provides the necessary sine drive through a crank rocker mechanism based on a coil spring. Figure 42 [102] is a schematic diagram of the mechanism, and Figure 43 [102] is the experimental platform of the mechanism. The mechanism is based on a four-bar crank rocker mechanism, where the crank is connected to the rocker (oscillating arm) through a non-extendable steel rope, and external torque is provided through a coil spring. The rotation of the crank is transmitted to the rocker through a steel rope, causing the rocker to produce an approximately sinusoidal oscillation motion. The spiral spring design ensures the mechanism’s light weight and reliability during long-distance transmission. By using spiral springs and carbon fibre materials, the inertia of the mechanism is reduced, and the response speed is improved. Capable of precise control of oscillation frequency and amplitude, suitable for various experimental conditions.

2.

Hyperbolic handle mechanism

The double crank mechanism consists of two cranks, two connecting rods, and a frame. Both cranks can perform a full rotation motion. When the driving crank rotates at a constant speed, the driven crank rotates at a variable speed. The motion condition is that the shortest rod is the frame, and both connecting rods (cranks) can make a full rotation [103]. The structural diagram is shown in Figure 44.

3.

Elastic inside link

Radaelli G. [105] proposed a new concept of a flexible rotary joint with low axial drift, high support stiffness, and an extensive range of motion, as shown in Figure 45. This concept is based on a spiral shell, with a portion rotating along its rotational direction. The opposite region gradually increases, resulting in a constant reaction torque. The prototype of this concept has been used to demonstrate the ability of various neutral and stable flexible linkages that can exhibit a wide range of motion with extremely low driving forces.

2.6.2. Double Spherical Linkage Mechanism

Liu WQ et al. [106] were inspired by Kirigami and proposed a super-constrained double ball linkage mechanism, as shown in Figure 46. The 6R over-limit linkage mechanism can be composed of a crank or a hyperbolic crank linkage. The connecting rod and rotating joint replace the panel and the crease increase mode.

Table 10. Comparison of different types of linkage mechanisms.

Types	Advantages	Disadvantages	Applications
Crank-rocker mechanism [101,102]	Simple structure, high reliability, strong load capacity	High speed is prone to vibration and occupies ample space	Internal combustion engine, stamping press, biomimetic machinery
Hyperbolic handle mechanism [103,104]	Full rotation, uniform input, variable output	The speed of the driven crankshaft is unstable	Scenarios of bidirectional rotation or variable speed transmission
Elastic inside link [105]	High stiffness, wide range of motion, and low driving force	Dependency on the preloading design of the snail shell	Precision instruments, flexible joints
Double spherical 6R linkage [106]	Deformable, with multiple degrees of freedom	High design complexity and manufacturing cost	Expandable structure, biomimetic structure, robotic arm

summarizes and compares the transmission performance of the different linkage mechanisms mentioned above.

The crank rocker mechanism has a simple structure, high reliability, and strong load capacity. It is widely used in internal combustion engines, stamping machines, and biomimetic machinery through the conversion of rotation and swing. However, its motion is discontinuous, high speed vibration is prone to occur, and it occupies a large amount of space, which limits its application in high precision and high speed scenarios. The hyperbolic handle mechanism (hyperbolic handle mechanism) can achieve full rotation of the hyperbolic handle, with constant input speed and variable output speed, suitable for bidirectional rotation or variable speed transmission scenarios. However, the speed fluctuation of the driven shaft is significant, and the stability is limited. The elastic inner connecting rod achieves high stiffness, an extensive range of motion, and a low driving force through a spiral shell preloading design, which is suitable for precision instruments and flexible joints, but its performance depends on the preloading optimisation of the snail shell structure; The double spherical 6R linkage mechanism is based on super constraint design, with multiple degrees of freedom and deformation capability, strong scalability, and suitability for biomimetic structures and robotic arms, but the high complexity of design and manufacturing raises costs. Overall, various linkage mechanisms complement each other in simple transmission, variable speed requirements, precision control, and spatial deformation and require comprehensive selection based on motion characteristics, load conditions, and costs.

3. Novel Rotating Motion Mechanism

3.1. Intermittent Indexing Mechanism

Indexing cam and Geneva mechanisms are commonly used in engineering applications [107,108]. Their common transmission characteristic is that the output shaft undergoes intermittent motion for every input shaft rotation, as shown in Figure 47.

3.1.1. Coaxial Indexing Mechanism

Yang YH et al. [109] proposed a new design method for a coaxial cam connecting rod indexing mechanism, as shown in Figure 48. The mechanism comprises a conjugate cam and a parallelogram linkage, with input and output shafts arranged coaxially. When the input shaft rotates, the conjugate cam pushes the parallelogram linkage, causing the output shaft to achieve intermittent motion. Specifically, during the active period, the input shaft rotates a certain angle, the cam pushes the linkage, and the output shaft completes one indexing motion. During the stationary phase, the input shaft rotates the remaining angle while the output shaft remains stationary. This mechanism has a more compact mechanical structure than traditional indexing mechanisms. The output shaft can achieve more divisions at the same input shaft speed. The combination of conjugate cam and parallelogram linkage improves the motion accuracy and stability of the mechanism.

3.1.2. New Geneva Mechanisms

The continuous circular motion designed by mechanical designers from the internet has been transformed into an intermittent circular motion mechanism [110]. The side of the driving wheel of this mechanism has a rounded triangular groove, and one of the connecting rods matches the groove, while the other is connected to the piston. The driven wheel is connected to the other end of the piston through two connecting rods. When the driving wheel rotates, it drives the piston to make a reciprocating linear motion, and then the driven wheel swings [110], as shown in Figure 49.

A mechanical designer from the internet has designed the crank and groove wheel drive mechanism, consisting of a crank and groove wheel. The driving wheel of the mechanism is a protruding circular crank. When the crank rotates to the position where it meshes with the grooved wheel, it can drive the driven grooved wheel. However, since only the circular crank has a single protrusion, the grooved wheel can only rotate at a certain angle every time the crank spins. Therefore, the groove wheel undergoes intermittent circular motion when the circular crank rotates at a constant speed [110], as shown in Figure 50.

The eccentric spiral intermittent mechanism designed by a mechanical designer from the internet [111] has an active wheel with a straight-end eccentric wheel, which rotates through a drive shaft. When the eccentric wheel rotates to the outside, it briefly contacts a step of the driven turntable, which drives it to rotate during the contact process. Then, when the eccentric wheel rotates to the inside, the turntable loses contact with it and stops rotating until the next contact. The mechanism can achieve intermittent rotational motion of the turntable [111], as shown in Figure 51.

Table 11. Comparison of different types of intermittent indexing mechanisms.

Types	Advantages	Disadvantages	Applications
Coaxial indexing mechanism [109]	Compact structure, high precision, and good stability	Complex structure, low load	Packaging, printing machinery, machine tool changing system
Circular groove wheel drive mechanism [110]	Simple structure, high reliability, and stable movement	High instantaneous impact upon contact, low speed	Low precision, light-load, low speed scenarios
Eccentric spiral intermittent mechanism [111]	Compact structure, adjustable intermittent motion	The eccentric wheel is prone to wear when in contact with the turntable.	Intermittent drive for lightweight rotary table

summarizes and compares the transmission performance of the different intermittent indexing mechanisms mentioned above.

The above content shows that traditional indexing cams and Geneva mechanisms are still widely used in packaging machinery and machine tool changing systems due to their simple structure and reliable operation advantages. However, their shortcomings, such as vibration suppression, instantaneous impact, and insufficient load capacity under high dynamic working conditions, limit their application in high speed and high precision fields. In recent years, new mechanisms have gradually broken through traditional limitations through structural innovation and performance optimisation. Akin to the camshaft linkage indexing mechanism, the coaxial layout and indexing frequency of the input/output shaft have been significantly improved through the synergistic effect of the cam and parallelogram linkage. However, its complex structure leads to high assembly accuracy requirements and insufficient adaptability to dynamic loads. The eccentric spiral intermittent mechanism adopts a contact-type drive to simplify the spatial layout, but problems include contact surface wear and impact vibration. The current research trend focuses on the deep integration of intelligence and high performance materials, such as introducing active damping control algorithms to suppress dynamic impacts, developing wear-resistant coatings or self-lubricating materials to extend the life of key components, and balancing the compactness and dynamic performance of mechanisms through topology optimisation and multi physics field simulation technology. In the future, intermittent indexing transmission mechanisms will evolve towards high dynamic accuracy, low wear, and intelligence. Combining mechatronics design and modular manufacturing processes will promote them for high-end applications in precision automation equipment, micro-mechanical systems, and high speed indexing scenarios. At the same time, key challenges such as complex structural error accumulation, dynamic response delay, and cost-effectiveness balance must be addressed.

3.2. Linear Motion to a Rotational Motion Mechanism

3.2.1. Ball Screw

The ball screw transmission module consists of sliding bearings, screws, nuts, columns, and tapered roller bearings [112], as shown in Figure 52. By applying pressure to the active column from the outside, this mechanism can convert reciprocating vertical motion into reciprocating rotational motion.

• Planetary ball screw

A planetary ball screw can convert linear motion into rotational motion [113]. Compared to traditional ball screw mechanisms, it can achieve higher loads and have a longer expected lifespan. The structure of the planetary ball screw is shown in Figure 53, which establishes three different coordinate systems containing the motion states of each component [114].

2.

Nut-driven static pressure screw

Liu YD et al. [115] used an innovative nut-driven static pressure screw to achieve heavy-duty, high rigidity, and ultra-precision feed at extremely low speeds (differential synthesis of the synchronous drive of screw and nut), as shown in Figure 54. The nut-driven static pressure screw system includes components such as a drive motor, bearings, static pressure screw, and sealing device.

3.2.2. EHSA Based on the Slider Crank Mechanism and Ratchet Pawl Mechanism

Wang SX et al. [116] proposed an Energy Harvesting Shock Absorber (EHSA) based on a crank slider mechanism and a ratchet pawl mechanism, as shown in Figure 55. The vertical vibration of the suspension drives the slider to move up and down, and the crank rotates through the connecting rod, converting linear vibration into rotational mechanical energy. By alternately engaging the ratchet with two claws (push claw and claw-shaped claw), bidirectional rotation is converted into unidirectional rotation, eliminating reverse inertia loss. By combining the vibration capture of the crank slider with the unidirectional rectification of the ratchet pawl, the energy conversion efficiency is significantly improved, with a mechanical efficiency of 67.75%, and the ratchet mechanism avoids reverse impact.

3.2.3. A New Type of Reverse-Pole Magnetic Suspension System

Magnetic lead screw (MLS) is used for wave energy converter (WEC), which converts the slow linear motion of the float into high speed rotational motion. Then it drives the rotating motor to generate current. Zhu LX et al. [117] proposed a new reverse magnetic pole magnetic levitation system, which uses a reverse magnetic structure to save permanent magnet losses. Meanwhile, the rotor adopts a traditional bipolar structure to increase the maximum traction force [117], as shown in Figure 56. Suitable for point absorption wave energy converters (point absorption WECs), effectively converting the low speed linear motion of buoys into the high speed rotation required by generators.

3.2.4. Series Coupling Rack Mechanism

Zhang TS et al. [118] proposed a series-coupled gear rack rotary transmission mechanism, as shown in Figure 57. The mechanism includes track clamps, gear racks, gearboxes, and generators. As the blue arrow indicates, the lower gear rotates clockwise under the corresponding motion, and the output shaft rotates clockwise because the small gear is equipped with disposable bearings inside. Similar to the red arrow, the upper gear rotates counterclockwise. A pair of large gears drives the output shaft clockwise using disposable bearings inside the small gear. Therefore, regardless of the track’s displacement direction, the output shaft can always maintain continuous clockwise rotation. The maximum efficiency of energy conversion in mechanical structures is 64.31%.

3.2.5. Screw Gear Ratchet Combination Mechanism

Zou HX et al. [119] proposed a bidirectional energy harvesting floor with a slow-release regulation mechanism. The working principle of this mechanism is shown in Figure 58 [119]. When pedestrians step on the floor, the torsion bar moves downward, driving the driven gear to rotate counterclockwise. Through the gear set, the speed increases, and the direction changes, causing the ratchet wheel I to drive the ratchet wheel to rotate counterclockwise. After the pedestrian leaves, the reset spring pushes the torsion bar to reset, driving the driven gear to rotate clockwise. The gear set accelerates again, causing the ratchet wheel II to drive the ratchet wheel counterclockwise. Ratchet discs I and II are driven unidirectionally by the pawl, ensuring that the ratchet constantly rotates at high speed in one direction regardless of whether it is stepped on or reset.

Table 12. Comparison of linear motion conversion to rotary motion mechanisms.

Types	Advantages	Disadvantages	Applications
Planetary ball screw [114]	3 times the load	High-cost	Stamping equipment
Static pressure screw [115]	High precision, up to sub-micron level	Low load High cost	Heavy-duty precision machinery
Slider crank ratchet mechanism [116]	The conversion efficiency of vibration recycling Machinery can reach 67.75%	There is a material fatigue issue	Train shock absorption
Magnetic guide screw [117]	zero friction	Difficult to Maintain	Wave Power Generation
Coupling rack mechanism [118]	Flywheel stabilisation, the mechanical conversion Efficiency can reach 64.31%	Difficulty in maintenance and limited power	Track vibration
Screw gear ratchet combination mechanism [119]	High-frequency conversion, high energy harvesting efficiency	Easy to wear and tear	Energy recovery in densely populated areas

summarizes and compares the transmission performance of the above different mechanisms for converting linear motion into rotational motion.

The research on linear to rotational motion transmission mechanisms presents a trend of coordinated development of multiple technological paths, achieving significant breakthroughs in precision, efficiency, and environmental adaptability. The traditional ball screw mechanism achieves a triple load increase through planetary structure innovation, while the static pressure screw achieves submicron precision through differential synchronous drive but is limited by high cost and low load characteristics; The EHSA system based on the slider-crank pawl mechanism improves the vibration energy conversion efficiency to 67.75% through a bidirectional rectification mechanism, but its material fatigue problem urgently needs to be solved by new composite materials. The new reverse magnetic pole maglev system adopts a polarised magnetic circuit design to achieve zero-friction transmission in wave energy conversion. Still, the dependence on low temperature superconductivity leads to a significant increase in system complexity. The series-coupled rack mechanism achieves displacement direction-independent continuous output through the innovation of one-way bearings, and its mechanical conversion efficiency of 64.31% shows potential in the field of track vibration recovery. However, the bottleneck of power density needs to be overcome through topology optimisation. The high-frequency energy harvesting characteristics of the helical gear ratchet combination mechanism are outstanding in energy recovery in densely populated areas. Still, the wear problem reveals the urgent need for surface strengthening technology. The future development trend will focus on three aspects: firstly, the design of hybrid transmission systems based on multi physics field coupling, integrating the non-contact advantages of magnetic levitation with the reliability of mechanical transmission; The second is the deep integration of intelligent materials and adaptive control technology, such as the application of shape memory alloy driven variable topology screws and self-healing coatings; The third is the optimisation of energy transfer links under dynamic loads, which establishes a multi-objective optimisation model of transmission efficiency durability through digital twin technology to achieve cross scale prediction of mechanism performance. These breakthroughs will drive the field towards ultra-precision, long lifespan, and adaptability, providing core transmission solutions for new energy equipment and intelligent infrastructure.

3.3. Joint Transmission Mechanism

3.3.1. Twisted Polymer-Driven Series-Parallel Hybrid Finger Mechanism

He J et al. [120] proposed a twisted polymer-driven series-parallel hybrid finger mechanism, as shown in Figure 59. The parallel section adopts 1-UP (universal prismatic joint) and 3-SPS (spherical prismatic joint) configurations, with twisted and coiled polymers (TCP) as the SPS limb-driven parallel platform and linear springs maintaining tension. Two degrees of freedom rotation and redundant translation were achieved using TCP’s contraction-driven platform to rotate around orthogonal axes. The serial part comprises R-R rotary joints driven by TCP to the distal joint and provides restoring force through passive torsion springs. Redundant translational degrees of freedom in parallel segments can accelerate opening and closing actions.

3.3.2. New Type of 2-DOF Ball Joint Hydraulic Spherical Motion Mechanism

Bian B et al. [121] proposed a new type of two-degree-of-freedom ball joint hydraulic spherical motion mechanism (SMM) for robots, aimed at solving the problems of complex transmission systems, large volume, and insufficient dynamic performance of traditional serial or parallel mechanisms in multi-degree-of-freedom rotation, and achieving smooth spherical motion in all directions. The proposed SMM can generate continuous 2-DOF rotation in a single joint without an intermediate transmission mechanism. The proposed SMM has a compact structure, low inertia, and high stiffness. The SMM prototype [121], is shown in Figure 60.

3.3.3. Hook Joint in Stewart Platform

On the Stewart platform, when a screw drives the mobile platform, the screw is passively rotated relative to the nut through Hook’s joint, and the driven branch is connected to the joint shaft [122], as shown in Figure 61. Each Hu Ke joint has two degrees of freedom of rotation. When the moving platform rotates relative to the base platform, the two Hooke joints of the branch rotate around jc_i (jf_i).

3.3.4. Three Degrees of Freedom Tensioned Integral Structure

Li LX et al. [123] proposed a three-degree-of-freedom tensioned integral structure, which can simulate the complex movements of the human shoulder. The design of the cable-driven action mechanism is shown in Figure 62 [123]. The mechanism consists of three rigid bodies and 16 steel wires and belongs to the I-level tensioned integral structure. It combines stiffness and flexibility to resist external impacts and ensure safe human-machine interaction. Simulate the complex range of human shoulder movements, such as flexion and extension, adduction and abduction, and internal and external rotation, through cable drive and motor control of three orthogonal rotation axes. Adjust the power transmission direction using pulley blocks and bevel gears to ensure balanced cable tension. Reduce cable slack caused by rotation by offsetting uneven tension through the reverse axis and complementary cables.

3.3.5. New Offset Slider Crank, Crank, and Connecting Rod Combination Hybrid Mechanism

Datta B et al. [124] proposed a hybrid biased slider crank mechanism for the biomimetic bending motion of prosthetic hands. This mechanism combines a linkage mechanism with a tendon drive to simulate human fingers’ joint angle trajectory by optimising the crank’s length parameters and linkage. The design includes three phalanges and one metacarpal bone, which are synchronously pulled by a linear actuator to drive a slider with multiple tendons, allowing the angle of each joint to change independently. The naming convention of phalanges and interphalangeal joints describes human fingers [124], as shown in Figure 63a. Figure 63b shows the corresponding part of the mechanism with human fingers. This mechanism integrates multi-level connections within the size limitations of human fingers, balancing biomimetic functionality and appearance. The design of a single driver reduces power consumption and cost. 3D printing (PLA material) has achieved a compact structure that adapts to the size of the human body.

3.3.6. A New RCM Mechanism

Tang HY et al. [125] derived a new RCM mechanism based on a parallelogram, as shown in Figure 64. The fixed rod is fixed to the base, and the movable rod moves in a circular motion around the remote rotation centre. Therefore, the relative motion between the fixed and movable rod is purely rotational. The mechanism not only has a remote rotation centre but also has a relatively small scale. The RCM mechanism has been applied for the first time in the design of sagittal exoskeletons, achieving human-machine motion axis alignment through a virtual rotation centre to avoid physical interference. This mechanism has high motor coordination and is significantly superior to traditional tendon-driven mechanisms. The sagittal layout conforms to the human body curve, and 3D printing (made of PLA material) achieves a lightweight design. The linkage mechanism absorbs the start-stop impact of the motor and reduces the instantaneous joint load.

3.3.7. Noncircular Gear Five-Bar Mechanism

Wang GB et al. [126] proposed an exoskeleton knee joint robot based on differential noncircular gears and a five-bar mechanism. By constraining the degrees of freedom of the mechanism through the arbitrary transmission ratio characteristics of noncircular gears, the complex structure of traditional multi-bar mechanisms is simplified, as shown in the three-dimensional model in Figure 65 [126]. The noncircular gear system consists of a sun gear and two planetary gears, which drive a five-bar mechanism through the variable transmission ratio characteristics of the noncircular gear to achieve complex motion trajectories. The noncircular gear drives the planetary gear to rotate, driving the linkage mechanism to move, and the end effector reproduces the flexion and extension trajectory of the knee joint. By changing the transmission ratio of noncircular gears, the degrees of freedom of the five-bar mechanism are constrained to ensure that the trajectory matches the natural motion of the human body. This mechanism has a compact structure and better transmission stability than traditional linkage mechanisms.

Table 13. Comparison of joint transmission mechanisms.

Types	Advantages	Disadvantages	Applications
Series-parallel hybrid finger mechanism [120]	High response speed, balanced stiffness and flexibility, and adaptability to various grasping modes	Complex structure, limited material properties	Robot hands that require quick response and flexible grasping
Hydraulic spherical motion mechanism [121]	Compact structure, high stiffness, 2-degree-of-freedom spherical motion	High energy consumption	In robot joints and spherical motion scenes
Stewart Platform Hook Joint [122]	They adapt to complex movements, have a simple structure, and are easy to integrate	Limited carrying capacity, long term wear and tear	High precision positioning platforms
Three degrees of freedom tensioned integral structure [123]	Combining rigidity and flexibility with strong impact resistance	Control complexity: Cable tension balance requires precise adjustment	Lightweight robotic arms
Offset slider crank connecting rod mechanism [124]	Multi-joint single-drive synchronous drive, lightweight structure	Low load capacity, dependent on linear actuators, limited travel	Functional pseudo-bionic robot fingers
New RCM mechanism [125]	Lightweight, shock-absorbing, and highly coordinated in motion	Restricted range of motion	Knee exoskeleton, human-machine motion axis alignment scene
Noncircular gear five-bar Mechanism [126]	Simplify the structure; have a high degree of freedom constraint and strong motion controllability	Complex processing and gear meshing accuracy affect transmission efficiency	Biomimetic joint with variable transmission ratio characteristics

summarizes and compares the transmission performance of the different joint transmission mechanisms mentioned above.

The research status of joint rotation transmission mechanisms presents a trend of deep integration of biomimetic design, intelligent driving, and structural innovation. The current research focuses on improving joints’ flexibility, dynamic response, and environmental adaptability, such as the series-parallel hybrid finger mechanism proposed by He J et al., [120] which achieves multi-degree of freedom rotation and redundant translation through TCP drive. Combining rigid and flexible coupling design balances response speed and grasping stability but is limited by material properties and structural complexity. The hydraulic spherical motion mechanism developed by Bian B et al. [121] adopts a compact hydraulic drive to achieve continuous dual-degree-of-freedom spherical motion, significantly reducing the inertia of traditional multi-degree-of-freedom mechanisms, but high energy consumption remains a bottleneck. Li LX et al. [123] three-degree-of-freedom tensioned integral structure simulates the complex movement of the human shoulder through the coordination of cable drive and rigid components, demonstrating the potential of rigid-flexible integrated design in impact resistance and human-machine interaction safety. However, the precise control of cable tension balance must still be optimised. In addition, the noncircular gear five-bar mechanism simplifies the complexity of traditional multi-bar structures by constraining degrees of freedom through variable transmission ratios. However, the impact of machining accuracy and meshing stability on transmission efficiency urgently needs to be addressed. The future development trend will focus on the deep integration of intelligent materials (such as self-healing composite materials) and driving technologies (dielectric elastomers, magnetostrictive materials), combined with topology optimisation and dynamic mechanical modeling, to break through the contradiction between structural lightweighting and high load; At the same time, the popularisation of real-time closed-loop control, multi physics field collaborative simulation, and high precision additive manufacturing processes based on digital twin technology will promote the evolution of joint mechanisms towards adaptability, low power consumption, and high reliability, further expanding their application boundaries in fields such as medical exoskeletons, flexible robots, and precision assembly.

3.4. Multi-Link Rotating Mechanism

3.4.1. Double Four-Bar Rotary Transmission Mechanism

Kim JW et al. [127] proposed a novel servo-free automatic tool changing mechanism based on a dual four-bar mechanism rotary transmission, as shown in Figure 66, and its working principle is shown in Figure 67. By designing a double four-bar mechanism to replace traditional single four-bar or gear mechanisms, the intermittent motion problem caused by the limited spindle rotation angle in existing non-servo motor ATC has been solved. The working principle of this mechanism is that the connection point of two four-bar mechanisms moves along a predetermined “crescent” trajectory, alternately contacting the grooves on the output disk. When the connection point of the four-bar mechanism is disconnected from the output disc, another mechanism immediately takes over the driver to ensure continuous rotation of the output disc. When the input shaft rotates 180°, the output disc completes continuous rotation at a specific angle through trajectory speed ratio control. This mechanism has a simple structure and few parts and is easy to manufacture and maintain. It has a compact space and is suitable for narrow installation environments.

3.4.2. Cam Five-Link Mechanism

The cam linkage mechanism, as a combination of cam and linkage mechanisms, combines the advantages of both mechanisms, enabling the cam linkage mechanism to maintain high reliability and compact structure while having superior motion performance. Zhu GD et al. [128] developed a cam five-bar mechanism for transverse devices, as shown in Figure 68. As shown in Figure 69 [128], the mechanism can change the position of the workpiece from horizontal to vertical during motion. The working principle of this mechanism is that the input shaft (rod 1) rotates at a constant angular velocity, and the angular velocity of the output shaft (rod 4) is controlled by the cam profile and geometric constraints of the connecting rod to achieve specific motion laws. Solved the problem of insufficient accuracy of traditional low speed design methods in high dynamic scenarios, significantly increased the transmission angle, reduced the pressure angle, and improved the smoothness of motion. The reliability under the influence of errors was verified through Monte Carlo analysis, with an average angle error of less than 0.003°. Practical application has proven that the production efficiency has doubled, which has engineering promotional value.

3.4.3. Multi-Link Crank Slider Rotating Mechanism

Liu Y et al. [129] designed a spacecraft module with a heavy-duty seven-degree freedom assembly robot, as shown in Figure 70. Propose a redundant configuration consisting of 3 mobile joints (prismatic joints) and four rotating joints, which combine high load, ample workspace, and flexibility. The working principle of this mechanism is as follows: the second and fourth rotating joints adopt a multi-link parallel crank slider mechanism, and the servo motor drives the ball screw to convert linear motion into rotational motion. The power is transmitted through a gear set and a rotary bearing to ensure high precision and stiffness. Adopting a redundant design of four rotating joints improves the ability to avoid obstacles and posture adjustment flexibility. This mechanism can achieve high load and high precision and is suitable for the precision assembly of heavy equipment. It has a large working range in a narrow space, and its foldable guide rail design supports movement inside and outside the cabin. This mechanism is suitable for precision assembly and handling large equipment in narrow spaces such as spacecraft cabins and submarines.

Table 14. Comparison of different multi-link rotary mechanisms.

Types	Advantages	Disadvantages	Applications
Double four-bar rotary transmission mechanism [127]	The structure is simple, the space is compact, the cost is low, and the output disk rotates continuously	Limited motion accuracy, speed, and rotation angle	Automatic tool changing systems for machining centers and tapping machines
Cam five-link mechanism [128]	High reliability, compact structure, high precision, and stability	The design is complex, the cost is high, and the ability to dynamically adjust is limited	High-dynamic industrial scenarios such as packaging machinery and medical production lines
Multi-link crank slider rotating mechanism [129]	High carrying capacity, high precision, and flexibility	Complex structure, low speed, and accumulated errors	Gyroscope, low speed, high precision, redundant degree of freedom scene

summarizes and compares the transmission performance of the different multi link rotating mechanisms mentioned above.

The research status of the multi-link rotary transmission mechanism reflects the multidimensional integration of structural innovation and performance optimisation. Significant progress has been made in motion accuracy, load capacity, and spatial adaptability. Taking the dual four-bar rotary transmission mechanism as an example, it addresses the intermittent motion problem of traditional non-servo tool changing devices through the “dual four-bar alternating drive” strategy and the “crescent-shaped” trajectory design, achieving continuous rotation output in a compact space. However, due to the limitations of trajectory speed ratio control, there remains room for improvement in its motion accuracy and speed. The cam five-bar mechanism, combined with cam profile and geometric constraints, has been validated for error robustness through Monte Carlo analysis in high speed dynamic scenarios (average angle error < 0.003°), significantly enhancing transmission smoothness and production efficiency. Nevertheless, the lack of complex cam design and dynamic adjustment capabilities limits its widespread application. The multi-link crank slider mechanism achieves high load (heavy equipment assembly) and high precision coordination in the narrow space of the spacecraft cabin through a redundant configuration (3 moving joints + 4 rotating joints) and a folding guide rail design. However, the accumulated errors and low speed characteristics caused by structural complexity still require further optimisation. Current research focuses on institutional lightweighting, dynamic performance improvement, and intelligent control, but it continues to face challenges in balancing error accumulation, dynamic response delay, and manufacturing costs. Future development trends will reconstruct the configuration of asymmetric variable pitch sprockets through topology optimisation algorithms, combined with laser cladding additive manufacturing technology, to achieve precise control of the dynamic characteristics of the transmission system. At the same time, by introducing embedded fiber optic sensors and active damping control, a dynamic load self-sensing and vibration phase compensation system will be constructed to promote the evolution of multi-link mechanisms towards high dynamic accuracy, low wear, and modularity, further expanding their application boundaries in high-end scenarios such as intelligent manufacturing and aerospace equipment.

3.5. Planetary Rotary Transmission Mechanism

3.5.1. Cam Connecting Rod and Planetary Gear Combination Mechanism

Tong ZP et al. [130] designed a planetary gear system vegetable potted seedling picking mechanism, which adopts a four-speed two-stage transmission planetary gear system mechanism. Xue XL et al. [131] developed a transplantation mechanism based on planetary gear systems. Jin X et al. [132] proposed a cam link planetary gear system seedling picking mechanism for dryland vegetable transplanters, as shown in Figure 71 and Figure 72. The working principle of this mechanism is that the input shaft drives the cam and housing to rotate, and the cam profile adjusts the swing of the sun gear through a linkage mechanism. The cam is divided into ascending and descending stages and a returning stage. By changing the speed of the planetary gears through gear meshing, the seedling picking arm is driven to complete variable speed rotation, achieving an optimised seedling picking trajectory. The seedling picking needle is controlled to open and close by an internal cam rocker, which maintains contact with the cam through a spring to ensure smooth movement.

3.5.2. A New Propeller System Based on Planetary Gears and Crank Rocker Mechanism

Zhu B et al. [133] designed a new propeller system based on planetary gears and a crank rocker mechanism, which achieves real-time pitch variation motion. The inspiration comes from the ability of American lizards to dynamically adjust the angle of their feet while running on the water surface [133], as shown in Figure 73. The sun gear of this mechanism is fixed, with four driven gears fixed on the cross, rotating both around its centre and around the system centre of the cross. The edge of the driven gear drives the rocker arm through the connecting rod, and the rocker arm drives the blade to swing periodically, achieving dynamic adjustment of the blade angle. Its comprehensive performance is superior to that of traditional fixed-pitch propellers.

3.5.3. Cycloid Reducer

• Universal RV reducer

The RV reducer is a two-stage transmission mechanism [134], as shown in Figure 74. The motion of the RV reducer can be divided into two stages: at high speed, the sun gear one is connected to the drive shaft and meshes with the planetary gears 2, rigidly connected to the crankshaft 3. At low speeds, swing gear 6 is driven by the crankshaft and locked with pin gear 4 located in pin housing 5. Due to the two-stage gear pair, namely (1) sun gear and planetary gear, and (2) cycloid gear and pin gear, the crankshaft rotates around its axis while also rotating around the centerline of the RV reducer. The rotation of the crankshaft is then transmitted to output disc 8, thereby achieving a large transmission ratio.

2.

New two-stage cycloidal reducer

Blagojevic M et al. [135] proposed a new design scheme for a two-stage cycloidal reducer, as shown in Figure 75. The reducer mechanism only uses one cycloidal disc per stage, and a central disc roller connects the two-stage cycloidal discs. The first-stage cycloidal disc meshes with the fixed ring gear, while the second-stage cycloidal disc meshes with the rotating ring gear. The central disc roller synchronously transmits motion, and the final output shaft rotates in the same direction as the input shaft. The number of cycloid discs in this mechanism has been reduced by half, resulting in a smaller volume. Using a two-stage cycloidal disc phase difference to counteract centrifugal vibration requires no additional balancing mechanism.

3.

New single-stage precision cycloidal pinwheel reducer

Xu LX et al. [136] designed and developed a new type of single-stage precision cycloidal pinwheel reducer, as shown in Figure 76. Replacing the traditional fixed output pin mechanism with a rotatable output pin mechanism and introducing bearings to convert the sliding friction between the output pin and the output hole into rolling friction significantly improves transmission efficiency. The performance of three different tolerance design schemes for the reducer was verified through simulation and experimentation. The results show that under reasonable tolerance design, the new reducer can achieve high transmission accuracy, and the transmission efficiency under rated load can reach 83.39%, which is better than the traditional design. The new reducer adopts a rotatable output pin mechanism, and the output pin is connected to the flange through a needle roller bearing. When the cycloidal gear rotates, the sliding friction between the output hole and the output pin is converted into rolling friction. At the same time, the flange adopts a fan-shaped connection mechanism to improve torsional stiffness.

Table 15. Comparison of planetary rotary transmission mechanisms.

Types	Advantages	Disadvantages	Applications
Cam connecting rod and planetary gear combination mechanism [130,131,132]	High stability, compact structure, high speed operation, and reliability	The cost is high, and the connecting rod requires high manufacturing accuracy	High speed, low-damage vegetable transplanter and automated agricultural equipment for seedling extraction
Planetary gear and crank rocker propeller mechanism [133]	Real-time pitch control simplifies the transmission control of the system	Easy to wear and tear, poor vibration reduction	Ship propulsion, uncrewed aerial vehicles
Universal RV reducer [134]	High transmission ratio and strong ability to withstand torque, high precision, and low recoil	High cost and difficult maintenance	Industrial robot joints and other high transmission-ratio, high precision-demand scenarios
New two-stage cycloidal reducer [135]	Small size, phase difference offsets centrifugal vibration, and single-tooth stress safety	Complex design and assembly, low efficiency	Compact and high transmission ratio scenarios, such as robot joints
New single-stage precision cycloidal reducer [136]	High transmission efficiency, strong torsional stiffness, and simplified structure	High cost: the manufacturing process requires high standards	High efficiency and low friction loss scenarios, such as precision instruments

summarizes and compares the transmission performance of the different planetary rotation mechanisms mentioned above.

The research status of planetary rotary transmission mechanisms presents characteristics of multidimensional innovation and engineering application expansion. Its core advantages lie in high transmission ratio, compact structure, and controllable dynamic performance. However, it faces cost, wear suppression, and manufacturing accuracy challenges. In agricultural automation, the combination mechanism of cam linkage and planetary gear (such as a vegetable transplanter) adjusts the swing of the sun gear through the cam profile to optimise the variable speed rotation trajectory, significantly improving the stability and efficiency of the operation. However, the lack of adaptability to high precision linkage manufacturing and dynamic loads limits its large-scale application. In biomimetic propulsion systems, the collaborative design of planetary gears and crank rocker arms achieves superior biomimetic motion performance through dynamic pitch adjustment. However, the physical coupling mechanism between friction, wear, and vibration suppression must be further modelled. In the field of reducers, traditional RV reducers dominate in industrial robot joints with their two-stage gear pair design (planetary cycloid), but their complex assembly and maintenance costs have become bottlenecks for industrial promotion. The new two-stage cycloidal reducer improves compactness by offsetting centrifugal vibration through phase difference and integrating components, but the nonlinear characteristics of tooth contact still limit transmission efficiency and dynamic response. The precision single-stage cycloidal reducer replaces sliding friction with rolling friction, increasing transmission efficiency to 83.39%, revealing the importance of material interface optimisation and tolerance collaborative design. The future development trend will focus on interdisciplinary technology integration: firstly, combining topology optimisation and additive manufacturing technology to develop lightweight and fatigue resistant composite planetary architectures, such as the integrated design of ceramic-based planetary carriers and gradient-coated gears; Secondly, introduce embedded sensors and real-time impedance matching algorithms to construct a dynamic error compensation system to address transmission stability issues under high variable load conditions. Thirdly, explore the collaborative design paradigm of multiple physical fields, such as the coupling of non-contact transmission of magnetic planetary gears and fluid-lubricated cycloidal reducers, to break through the traditional limitations of friction loss and noise. These directions will drive the evolution of planetary transmission mechanisms towards high energy efficiency, intelligence, and extreme environmental adaptability, providing core power solutions for precision robots, new energy equipment, and space exploration fields.

3.6. Rotary Actuator

3.6.1. A Novel Nonlinear Series Elastic Actuator Based on Conjugate Cylindrical Cam (N3CSEA)

Sun YX et al. [137] proposed a novel nonlinear series elastic actuator based on a conjugate cylindrical cam, as shown in Figure 77. This actuator uses two sets of conjugate cylindrical cams to drive the corresponding coil springs, and the cams rotate unidirectionally at a constant speed by a motor. (1) Standing stage: Cam 1 actively compresses spring 1, providing high torque; (2) Swing stage: Cam 2 drives spring 2, providing low torque and achieving seamless switching between stages. By customising the cam profile, the knee joint’s nonlinear load motion is converted into the motor’s uniform motion, reducing energy loss under non-rated operating conditions. The motor must only rotate uniformly in one direction through periodic cam design, simplifying control and reducing inertial energy loss. The motor can operate near its rated speed by combining a nonlinear transmission, significantly reducing Joule heat loss. The cylindrical cam can be integrated into the prosthetic knee joint cavity, making it suitable for wearable devices.

3.6.2. Bistable Rotating Mechanism

Liu YD et al. [138] designed a new bistable rotating mechanism using dielectric elastomer actuators, as shown in Figure 78. Combining symmetrically arranged uniaxial fibre-constrained dielectric elastomer actuators (FCDEA) with mechanical locks has solved the problem of small actuation range and non-repeatability of existing bistable actuators. Mechanical locks fix two symmetric stable states (90° and 150°). The dual FCDEA adversarial layout ensures the repeatability of actions and precise angular output. Negative stiffness is introduced through structural design to expand the actuation range (45° rotation angle). After switching, there is no need to maintain a continuous voltage state, reducing energy consumption.

3.6.3. Twisted-Spring Connected Nonlinear Stiffness Actuator

Qu XX et al. [139] proposed a torsion spring-connected nonlinear stiffness actuator (TSNSA) principle, as shown in Figure 79. This structure adopts a symmetrical layout of torsion springs, assembled sleeves, and cam mechanisms, combined with disc springs for energy storage to achieve nonlinear stiffness characteristics. The external torque causes the average coil diameter of the torsion spring to decrease, thereby pressing the coil spring in contact with the joint sleeve, generating a nonlinear reaction force. The stiffness increases with the increase of load, thus achieving passive adjustment. By driving the slider to adjust the spacing and change the contact force between the cam and the joint sleeve, the compression amount of the coil spring can be actively controlled, thereby altering the stiffness and achieving active adjustment. The cam profile design determines the force transmission path. Through the sliding of rollers on the cam, the radial deformation of the torsion spring is converted into axial compression, achieving nonlinear changes in stiffness. The actuator adopts a symmetrical layout and a split sleeve design to reduce volume, adapt to the axis of the rotary joint, and is lighter than traditional NSA. The biomechanical characteristics of “low stiffness for small loads and high stiffness for large loads” have been achieved by optimising the cam profile, improving safety, and adaptability.

3.6.4. Compact and Reconfigurable Disc Spring Variable Stiffness Actuator

Ji C et al. [140] proposed a compact and reconfigurable disc spring variable stiffness actuator named SDS-VSA (Symmetric Disc Spring Variable Stiffness actuator), as shown in Figure 80. This actuator adopts a cam roller spring mechanism with symmetrical compression springs, replacing traditional spring designs. Based on the cam roller spring mechanism, output torque is generated by symmetrically compressing the axial displacement of the disc spring. The connecting rod side motor drives the roller to rotate, driving the cam to move axially, compressing the spring, and generating anti-load torque. The stiffness motor adjusts the relative angle of the two side cams, changes the spring preload force, and thus adjusts the joint output stiffness. The equivalent stiffness coefficient can be flexibly configured without changing the structural space by series, parallel, or hybrid combination of disc springs with different thicknesses/stiffnesses. The actuator adopts a disc spring, which, combined with its high stiffness and compact characteristics, significantly improves the torque density and stiffness range. Compared to single-sided tension spring schemes, the output torque increases by 30%, and the structure is more compact.

Table 16. Comparison of rotary actuators.

Types	Advantages	Disadvantages	Applications
Based on the conjugate cylindrical cam [137]	Reduce motor inertia energy loss, compact structure	Depending on the cam design, high cost, and limited adaptability	Artificial knee joint, a low power wearable assistive device
Bistable rotating mechanism [138]	Low energy consumption, repeatability, high precision, and large load capacity	Relying on mechanical locks, the range of action is limited	Biomimetic robots or soft robots, medical, and surgical instruments
Twisted spring connected nonlinear stiffness actuator [139]	Wide adaptability, lightweight, improved safety through biomechanical properties, and low cost	Complex structure, relying on cam profile optimisation, limited real-time performance	Human-robot collaborative robots, highly dynamic environment walking robots
Disc spring variable stiffness actuator [140]	Flexible configuration of equivalent stiffness, high torque density, fast and wide range stiffness adjustment	Multiple motors need to be coordinated for control, and fatigue of the disc spring may affect long term reliability	Humanoid/quadruped robots, exoskeleton/rehabilitation robots, robot arm joint

summarizes and compares the transmission performance of the different rotary actuators mentioned above.

3.6.5. Rotary Piezoelectric Stick-Slip Actuators

Referring to Zhong BW [141], the current status of rotary piezoelectric stick-slip actuators was summarised, as shown in

Table 17. Comparison table of classification of rotary piezoelectric visco sliding actuators [141].

Types	Advantages	Disadvantages	Applications
Piezoelectric tube spiral electrode type	Ultra compact, high angular velocity, silicon carbide-coated steel shaft reduces wear rate.	High driving voltage (high dielectric loss of PZT-5A), poor low temperature performance	Micro-optical focusing system
Double piezoelectric tube inertia type	Large step angle (reduced adhesion of alumina ceramic shaft), fast response (0.1 ms), low threshold voltage (less than 50 V)	Large volume (up 120% compared to traditional design), weak axial load (<10 N, limited preload force of beryllium bronze spring)	Precision rotating platform
Dual-mode incentive type	Inhibit rollback (stable friction with silicon carbide coating), high speed (elastic amplification strain of titanium alloy hinge 1.8 times), high temperature resistance (150 °C)	Complex structure (requiring dual PZT-4 stack), humidity sensitive (fluctuation of silicon carbide friction coefficient greater than 20% when greater than 80%)	Space agency pointing control
Umbrella-shaped flexible hinge type	Anti-impact (hard alloy driven foot), low power consumption (less than 5 W, beryllium bronze hinge with low hysteresis loss), bidirectional consistency greater than 95%	Low rotational speed (0.85 rad/s, insufficient damping of aluminium alloy substrate), demanding machining accuracy (±1 μm)	Field robot joints
Bionic snake-like rotating type	High torque (12 mN · m), smooth motion (microstructured copper alloy oil storage reduces viscosity and slip mutation), radiation resistance	Extremely high cost, high humidity failure (copper alloy texture blockage above 80% humidity)	Biomimetic robots

.

The rotary actuator discussed in this section demonstrates diverse innovative designs for different application requirements. The nonlinear series elastic actuator based on a conjugate cylindrical cam transforms the joint nonlinear load into uniform motor motion by customising the cam profile. Combined with a unidirectional driving strategy, it significantly reduces inertia and Joule heat loss. Its high integration is particularly suitable for wearable devices such as prosthetic knee joints, but the dependence on cam design limits its universality. The bistable rotating mechanism utilises fibre-constrained dielectric elastomer actuators and mechanical locks with symmetrical adversarial layout to achieve high repeatability angle positioning (90°/150°) and zero power consumption after switching. The negative stiffness design extends the actuation range (45°), but mechanical locks and finite angles are its main constraints. The torsion spring-connected nonlinear stiffness actuator adopts a symmetrical torsion spring sleeve cam mechanism, which converts the radial deformation of the torsion spring into axial compression through rollers, achieving biomimetic passive stiffness adjustment. Its lightweight design improves the safety of human-machine cooperation, but the structural complexity and real-time performance need to be optimised. The compact and reconfigurable disc spring variable stiffness actuator innovatively utilises symmetrical disc spring groups and dual cam angle adjustment to achieve flexible configuration of equivalent stiffness and high torque density (30% higher than single-sided schemes), significantly expanding the range and speed of stiffness adjustment. However, multi-motor collaborative control and disc spring fatigue are reliability challenges. At the same time, the piezoelectric stick-slip rotary actuator (Table 17) represents progress in the field of precision micro drives, which achieves stepwise rotation through the inverse piezoelectric effect of piezoelectric elements. The ultra compact and high speed piezoelectric tube spiral electrode type is suitable for micro optical focusing; The double piezoelectric tube inertia type with large step size and fast response is ideal for precision turntables; Dual mode incentive type anti rollback and high temperature resistance meet the requirements of aerospace orientation; Umbrella shaped flexible hinge type for impact resistance and low power adaptation of field robot joints; The high torque and radiation resistance characteristics of the biomimetic snake-like rotating type are aimed at special biomimetic applications. However, piezoelectric stick-slip actuators face high driving voltage, humidity sensitivity, extreme machining accuracy requirements, or high cost. Overall, these actuators have made breakthroughs in high-efficiency compact driving, low power precise positioning, biomimetic safe interaction, efficient adjustable stiffness, and precision micro-nano driving, jointly promoting the development of rotary actuation technology towards high performance, intelligence, and scene adaptability.

3.7. Universal Joints Drive

3.7.1. Steel Flexible Universal Joint

Tank CM et al. [142] proposed a steel-compliant universal joint, which, for the first time, combines a compliant mechanism with a traditional universal joint and is made of steel, as shown in Figure 81. By transmitting torque through elastic deformation, the axial angular deviation is allowed. The universal joint consists of two symmetrical steel components assembled vertically, with a flexible hinge of spring steel at the core and a stainless-steel clamp plate on the outside. When bent, the hinge absorbs deflection through elastic deformation while transmitting rotational motion. Only two symmetrical parts are needed for assembly, with a minimal number of parts, reducing manufacturing complexity.

3.7.2. New Type of Anti-Buckling Flexible Universal Joint

Li SY et al. [143] proposed a new anti-buckling flexible universal joint, which uses two reverse symmetric cross-spring pivot points to form the universal joint, as shown in Figure 82. Compressive loads are converted into tensile stresses by reversing the arrangement of long stretch sheets, avoiding the buckling problem of traditional compressed sheets and allowing rotation around two orthogonal axes. The middle ring is designed in series with a motion platform to maintain a compact structure.

3.7.3. New Type of Fully Compliant Universal Joint

Karakuş R 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 prefabricated flexible section. The flexible part transmits torque while achieving angular displacement (up to 24°) 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. Comparison of universal joints.

Types	Advantages	Disadvantages	Applications
Steel flexible universal joint [142]	Balancing elasticity and strength, no need for lubrication, lightweight and compact, long lifespan	Elastic deformation may lead to a decrease in transmission accuracy	Compact transmission, high reliability joint without lubrication
Anti-buckling flexible universal joint [143]	High stability, high precision of dual-axis rotation, no need for lubrication	Complex structure, high cost, and limited load-bearing capacity by tension plates	Optical positioning platform/surgical robot joint, continuum robot flexible arm segment
Fully compliant universal joint [144]	A single-piece structure does not require assembly, has an approximate constant transmission speed, and is flexible.	High torque/large angle can easily cause stress concentration, and the polypropylene material is weak	Small robot/drone joints, lightweight transmission of medical equipment, and noise-sensitive scenes

summarizes and compares the transmission performance of the different universal joints mentioned above.

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. In terms of material selection, developing new high-strength, high toughness, and self-healing materials can effectively improve the load-bearing capacity, fatigue resistance, and service life of universal joints; In terms of design methods, the use of topology optimisation techniques can further optimise the structure of universal joints, reduce stress concentration, and improve transmission efficiency and accuracy; In terms of manufacturing processes, high precision additive manufacturing technology and multi material composite manufacturing technology will continue to develop, achieving more complex and precise structural manufacturing to meet the needs of miniaturisation and integration; The integration of intelligent technology, such as sensors and smart control algorithms, can achieve real-time monitoring and thoughtful regulation of the operating status of universal joints, improving their adaptability and reliability in complex working conditions.

4. Common Driving Methods for Rotating Motion Mechanisms

As the core functional unit of a precision rotary motion mechanism, the drive system selection directly determines the equipment’s structural design, dynamic performance, and applicable scenarios. By systematically comparing and analysing the key technical parameters of mainstream driving modes, including electromagnetic drive, hydraulic transmission, servo motor, and piezoelectric drive, the focus is on studying the impact of different power transmission methods on core performance indicators such as motion accuracy, response speed, load capacity, and energy efficiency ratio. Based on typical application scenarios such as industrial automation, precision instruments, and aerospace, a multidimensional evaluation system is established to assess the technical differences in output torque stability, environmental adaptability, maintenance costs, and energy conversion efficiency among various driving schemes, providing reasonable driving schemes for the design of precision rotary motion mechanisms.

4.1. Electric Drive

4.1.1. Motor Drive

Electric motors generate rotational power to control various machines and equipment. The motor drive system is highly efficient in energy conversion. According to their intended use, electric motors can be divided into power and control motors. Power motors can be divided into rotary motors and linear motors. As shown in Figure 84 [145], control motors can be divided into stepper motors, servo motors, speed measuring motors, and torque motors.

• Superconducting rotating motor

Superconducting rotating motors are more efficient, smaller, and lighter than traditional motors. Compared to low temperature superconductors, using high temperature superconductors (HTS) in machines simplifies cooling design. HTS motors have advantages in improving efficiency and reducing mass and volume, Ref. [146] as shown in

Table 19. Comparison of high temperature superconducting rotating electrical machines.

Types	Working Principle	Advantages	Disadvantages	Applications
Synchronous motor [148,149,150]	The rotor uses superconducting coils or blocks to generate a direct current magnetic field, while the stator is made of conventional conductors or superconducting materials	High efficiency, lightweight, high magnetic field, low synchronous reactance	The cooling system is complex, with communication losses, requiring rotational cooling coupling	Wind turbines, ship propulsion, aerospace, industrial motors
Induction motor [151,152]	The rotor adopts superconducting squirrel cage bars, and the stator is a conventional conductor. The superconducting material loses its superconducting state during startup and returns to its superconducting state during operation	High starting torque, low slip operation, high efficiency	In synchronous mode, the rotor magnetic field is limited, and precise cooling control is required	Electric vehicles, industrial motors, and low temperature fluid pumps
Claw pole motor [153,154,155]	The stator superconducting coil guides the magnetic field through a claw pole structure, and the rotor is made of conventional materials	Compact structure, non-rotating superconducting components, suitable for high speed applications	The magnetic field distribution is uneven, and high mechanical strength is required for the claw pole	Aircraft generators, high power density motors
Unipolar motor [156,157,158]	The DC magnetic field is generated by superconducting coils, and the rotor is a conductive disk or offset magnetic pole structure	High torque, no gear requirements, suitable for low speed and high torque scenarios	Sliding contact is prone to wear and requires a liquid metal current collector	Ship propulsion, flywheel energy storage, and aerospace high power generators
Magnetic flux modulation motor [159,160]	Superconducting blocks or stacked tapes shield or concentrate magnetic fields, and the stator is a conventional conductor	High power density, low magnetic leakage, and adaptability to complex shapes	Pulse magnetic field pre-magnetisation is required, and demagnetisation may occur during dynamic operation	Aerospace and high power-density propulsion systems
Hysteresis motor [161,162]	Superconducting bulk rotors generate hysteresis effects in alternating magnetic fields to drive rotation	Simple structure, no friction loss, suitable for high speed applications	Hysteresis loss cannot be ignored, resulting in low output power	Micro-motors, specialised equipment for low temperature environments
Fully superconducting motor [163,164,165]	The stator and rotor are superconducting materials (wires or blocks)	Theoretical efficiency is close to 100%, with a small size and extremely light-weight	High communication loss, complex cooling system, and extremely high cost	Future aviation propulsion and ultra-efficient power generation systems

. Figure 85 [146,147] is a schematic diagram of the structure of a fully superconducting motor.

2.

Spherical motor

Öğülmüş AS et al. [166] designed a novel 3-degree-of-freedom non-integrated rotor permanent magnet spherical motor (NR-PMSM) that separates tilting (X-Y axis) and rotating (Z-axis) motions, simplifying the control algorithm. The main components are shown in Figure 86 [166]. The motor adopts a modular design, with a stator arranged in layers and 18 coils (12 inclined coils and 6 rotating coils) for easy assembly and maintenance. The motor is equipped with a multi-ball bearing mechanism, and the rotor is supported by upper and lower cover plates, which reduces friction and improves motion stability without affecting the electromagnetic field. Using ABS material to reduce weight, 24 cubic neodymium magnets are embedded on the rotor surface. The motor is driven by electromagnetic force, and the stator coil is energised to generate a magnetic field, which interacts with the rotor’s permanent magnet. Drive the tilting and rotating motion of the motor by separately controlling the phase and amplitude of the current.

3.

Coreless Motor

The outer rotor of the hollow cup permanent magnet motor is equipped with permanent magnets, and the inner and outer rotors rotate synchronously. This structure adopts a coreless cup-shaped stator, which can overcome the iron loss and cogging torque pulsation caused by the iron core and has the advantage of low power consumption [167]. Sun JJ et al. [168] and Zhang L et al. [169] reduced the torque ripple of the motor and further improved the performance of the hollow cup motor through a new design of the inner rotor.

Szelag W et al. [170] proposed a low cost hollow cup permanent magnet synchronous motor (PMSM) magnetic circuit structure suitable for large-scale production using a 16-pole 12-coil configuration, as shown in Figure 87. The motor’s stator has no iron core and is only composed of copper windings. The rotor adopts a double ferromagnetic ring structure, with radially magnetised permanent magnets attached to the surface. After the stator three-phase winding is energised, a rotating magnetic field is generated, which interacts with the magnetic field of the rotor permanent magnet to drive the rotor to rotate synchronously. Due to the coreless structure, the cogging effect of traditional motors is eliminated, resulting in smoother operation. The dual rotor structure of the motor is combined with sintered neodymium magnets to optimise the magnetic flux path and improve torque density. Despite the high cost, the first attempt to use injection moulded magnets instead of sintered magnets provides direction for future process improvements.

4.1.2. Piezoelectric Drive

The basic principle of piezoelectric driving is to utilise the inverse piezoelectric effect of piezoelectric ceramic materials, which means that under the action of an electric field, the piezoelectric material will deform, resulting in mechanical displacement [171]. This effect enables piezoelectric materials to convert between electrical and mechanical energy, generating rotation by controlling their mechanical deformation. Resonant piezoelectric actuators operate in a resonant state and are typically designed in the ultrasonic frequency range; hence, they are also known as ultrasonic motors. Piezoelectric ultrasonic motors (USMs) have received more attention due to their unique qualities compared to traditional magnetic coil-based motors, such as miniaturisation, high precision, speed, non-magnetic, silent operation, simple structure, and adaptability [172].

Zhao L et al. [173] proposed a novel rotating ultrasonic motor that only uses longitudinal vibration mode, as shown in Figure 88. The motor consists of two symmetrically arranged longitudinal sensors, a rotor, a base, and an adjustable mounting base. The motor utilises the longitudinal vibration mode of piezoelectric ceramics to generate a frictional force through the longitudinal displacement of the foot driven by sensors, driving the rotor to rotate. Only a single-frequency excitation voltage is required, and directional control can be achieved by adjusting the voltage phase. The relative position between the sensor and the rotor can be adjusted, providing an additional means of changing the output characteristics. The dual-sensor synchronous drive enhances the stability of torque output. However, it requires a high-voltage drive, which may limit low power applications, and relies on friction transmission. Long term operation may cause contact surface wear and require regular maintenance.

Shi MH et al. [174] proposed a novel non-contact ultrasonic motor that can generate levitation force and acoustic drive torque, as shown in Figure 88. The motor includes a stator and a rotor. The stator structure is fixed on the mounting plate, and the entire structure is fixed on the support frame. The composition of the stator is shown in Figure 89b [174]. Figure 89c [174] shows the stator’s main dimensions. The high-frequency longitudinal vibration of the motor stator compresses the air film, forming a high-pressure air film, generating axial and radial suspension forces, overcoming the gravity of the rotor, and maintaining stable suspension. The gradual deepening of the design of artificial grooves leads to uneven distribution of gas film pressure in the circumferential direction, forming a pressure gradient, driving the airflow, and generating circumferential shear stress, thereby generating driving torque. The air film completely isolates the stator and rotor, avoiding friction and wear and significantly reducing heat.

Pan QS et al. [175] proposed a resonant piezoelectric rotary motor using a parallel moving gear mechanism, as shown in Figure 90. The piezoelectric transducer generates linear displacement under a sinusoidal voltage and synthesises the planar circular translational motion of the stator through a hinge connection. The stator (internal gear) and rotor (external gear) undergo periodic misalignment meshing due to the difference in tooth count, driving the rotor to decelerate and rotate. By adjusting the phase difference between the excitation voltages of the two transducers, the direction of rotor rotation can be changed. Replacing traditional friction transmission with gear meshing reduces wear and energy loss and improves efficiency. Utilising the resonance characteristics of vibrators to enhance energy transfer efficiency and support high power output. The output speed and torque can be flexibly regulated by changing the transmission ratio (TR), excitation frequency, and voltage amplitude. The motor integrates gear transmission and piezoelectric drive, which can achieve miniaturisation and high power density.

Xun MX et al. [176] proposed a rotary piezoelectric actuator (RPA) based on a spatial spiral compliance mechanism (SSCM). The proposed RPA structure [176] is shown in Figure 91. A single PZT (lead zirconate titanate) stack generates linear displacement through the inverse piezoelectric effect. SSCM converts the linear displacement of the P stack into pure rotational motion. SSCM consists of spiral rods and parallel platforms. The symmetrical layout of the spiral rod counteracts lateral forces and avoids axis deviation. By constraining the design of thrust bearings and bolt connections, the longitudinal degrees of freedom are limited to ensure that the output is a single rotational motion. This actuator can achieve high precision and fast response.

Table 20. Comparison of piezoelectric rotary motors.

Types	Advantages	Disadvantages	Applications
Longitudinal vibration mode rotating ultrasonic motor [173]	Simple control, adjustable output characteristics, stable torque, and no magnetic interference	High voltage power dependence, high power consumption, low output torque and speed	Optical instrument focusing system, micro robot joint drive, low temperature/vacuum environment
Non-contact ultrasonic motor [174]	No contact friction, long lifespan, no wear and heat accumulation, fast operating speed	Air film stiffness limits load-bearing capacity	Medical devices that require silent operation in high precision cleaning scenarios, such as semiconductor manufacturing
Gear mesh resonant piezoelectric motor [175]	High efficiency, high power output, flexible speed regulation, and resonance design improve energy efficiency	Risk of brittle fracture in piezoelectric ceramics	Scanning probe microscope for nano-positioning, joint drive for surgical robots, high torque velocimeter in precision manufacturing
Space Spiral Flexible Mechanism (SSCM) RPA [176]	High precision and fast response, high motion stability, low cost, noaxis offset	Polarisation attenuation of the piezoelectric stack	Biological microscope, nano-positioning platform, and other micro-nano operation scenarios, dynamic vibration compensation system

summarizes and compares the driving performance of the different piezoelectric motors mentioned above.

The current research on motor drive in precision rotary motion mechanisms presents a pattern of diversified technological routes and parallel development, focusing on three dimensions: efficiency improvement, structural optimisation, and precision control. Applying high temperature superconducting (HTS) materials has significantly improved system energy efficiency and power density in superconducting motors. Synchronous HTS motors reduce thermal management complexity through a non-rotating cooling coupling design. However, the physical limits of critical current characteristics and eddy current losses of superconducting materials still limit their magnetic field modulation stability and cooling system integration under dynamic conditions. The highly efficient topology structure represented by the fully superconducting motor is theoretically close to energy-lossless transmission. Still, the dynamic mismatch problem caused by the magnetic flux pinning effect between the dual superconducting circuits leads to torque fluctuations and transient response hysteresis in practical applications. The spherical motor breaks through the traditional single-axis rotation limitation through a non-integrated rotor permanent magnet topology and multi-degree of freedom decoupling control. Its modular electromagnetic field design and ABS lightweight rotor effectively reduce motion inertia. However, the suppression of magnetic field distortion and three-dimensional attitude accuracy feedback under multi-physical field coupling still needs to be further optimised based on magnetic, mechanical, and electrical collaborative simulation.

The hollow cup motor eliminates the cogging effect with a coreless structure, and its dual rotor magnetic circuit configuration achieves magnetic flux path refocusing through a hybrid topology of injection moulded magnets and sintered neodymium magnets. However, the uniformity of the air gap magnetic field and the balance of axial magnetic pulling force still rely on precision mechanical tolerance control. The piezoelectric drive technology expands the boundaries of traditional friction transmission through non-contact ultrasonic excitation and a spatial spiral compliant mechanism (SSCM). The gear meshing piezoelectric motor uses the resonance enhancement effect to improve energy conversion efficiency to 68%. However, the nonlinearity of hysteresis and high-frequency vibration noise caused by the relaxation of the preload force of the piezoelectric stack are still key bottlenecks restricting its industrial application. The non-contact ultrasonic suspension drive generates tangential driving force through the pressure gradient of the gas film, which avoids contact wear but faces the physical upper limit of gas film stiffness and load-bearing capacity. It requires a micro groove flow field control to strengthen the circumferential shear stress.

The future development trend will present four major technological fusion features: firstly, superconducting motors will explore multi physics field collaborative design, and solve the problems of rotational quench and dynamic stability through topology optimization and adaptive cooling systems; Secondly, precision drive mechanisms will develop towards heterogeneous integration, combining the multi degree of freedom characteristics of spherical motors with the low inertia advantages of hollow cup motors to construct a composite motion platform; Thirdly, piezoelectric driving will deepen resonance mode control and intelligent material applications, utilizing the high strain coefficient of relaxor ferroelectric single crystal (PMN-PT) to enhance power density, and developing a self sensing self correcting integrated driver; Fourthly, digital twins and real-time impedance matching algorithms will drive the evolution of the driving system towards dynamic reconstruction, achieving nonlinear compensation under load disturbances through online parameter identification, and ultimately forming a cross scale, multimodal precision driving technology system.

4.2. Hydraulic Drive

A hydraulic drive generates force and motion by utilising liquid to transmit pressure within a closed system, achieving control and operation of mechanical devices [177]. The hydraulic system can carry large loads and achieve precise motion control. It can achieve seamless transmission, effortless operation, and fast response, and it can start at high speed and change direction frequently.

Electro-Hydraulic Servo Drive

Zhu DM et al. [178] designed and validated an integrated electro-hydraulic pump (EHP) based on an axial piston pump, as shown in Figure 92. The electro-hydraulic pump is driven by a brushless DC motor, which generates electromagnetic torque through three-phase back electromotive force to drive the rotor to rotate. The rotor drives the plunger to reciprocate and adjusts the plunger stroke through the inclined plate and valve plate to achieve periodic oil suction and discharge. The oil is distributed to the high-pressure chamber and low pressure chamber through the valve plate, and the movement of the plunger produces high-pressure oil output. The motor and pump share the same housing, eliminating the need for a drive shaft, dynamic sealing, and external leaks. Using hydraulic oil to cool the motor eliminates the need for traditional cooling fans and reduces noise and vibration.

Du R et al. [179] proposed a novel energy regenerative hybrid drive system (ERHD), as shown in Figure 93. By integrating the motor, hydraulic, and control modules, the recovery and reuse of braking energy can be achieved and applied to the roll transmission system of reversible rolling mills. During the start-up phase, the electric motor and hydraulic motor jointly drive the load, and the accumulator releases the stored hydraulic energy to assist in acceleration and reduce the load on the electric motor. The electric motor is driven separately in the constant speed stage, and the hydraulic module exits. During the braking phase, the hydraulic motor converts the kinetic energy of the load into hydraulic energy and stores it in an accumulator to achieve energy regeneration.

Table 21. Hydraulic drive comparison.

Types	Advantages	Disadvantages	Applications
Traditional hydraulic drive [177]	High load, precise movement, smooth operation, seamless transmission, and rapid response	Potential leakage risk, high maintenance cost, and loud noise	Scenarios that require high load and precise control
Electrohydraulic servo drive [178]	Small size, lightweight, high power density, low noise, good dispersion performance, high reliability	Insufficient oil absorption during high speed operation leads to reduced efficiency and vibration	Aircraft hydraulic systems require equipment in environments with sustained high-pressure or low noise levels
Electrohydraulic servo drive [179]	Energy regeneration, reducing the peak power and energy consumption of the motor, and cooling down	The system requires precise control, additional energy storage devices, and pipelines	High-inertia, frequent start-stop scenarios, green, and low-carbon industrial applications

summarizes and compares the driving performance of the different hydraulic drives mentioned above.

In precision rotary motion mechanisms, hydraulic drive technology has become essential in high precision motion control due to its core advantages of high load capacity, seamless transmission, and fast dynamic response. In recent years, innovation in electro-hydraulic servo drive technology has significantly improved system integration and energy efficiency: Zhu DM et al.’s [178] axial piston integrated electro-hydraulic pump (EHP) eliminates the need for dynamic sealing through an integrated structure design of brushless motor and hydraulic pump, replacing traditional air cooling with hydraulic oil self circulation cooling, achieving high power density output while reducing noise and vibration; The Energy Regeneration Hybrid Drive System (ERHD) proposed by Du R et al. [179] utilises hydraulic accumulator and motor coordinated control to convert load inertia energy into hydraulic energy storage, effectively reducing the peak power demand of the motor and providing a low-carbon path for high inertia frequent start stop scenarios. However, current technology still faces bottlenecks such as efficiency degradation caused by insufficient oil absorption of hydraulic media under high speed conditions, nonlinear control problems of multi-domain coupled systems, and spatial constraints caused by complex pipelines and energy storage components. Future research needs to focus on three dimensions: firstly, developing new high temperature resistant, low viscosity hydraulic media and adaptive sealing materials to enhance high speed dynamic performance. Secondly, integrating deep learning and model predictive control (MPC) algorithms to achieve multi-objective collaborative optimisation of hydraulic motor composite drive systems; Thirdly, reducing system inertia through topology reconstruction and miniaturisation design promotes the development of hydraulic drive towards deep integration of electromechanical and hydraulic systems.

4.3. Pneumatic Drive

The pneumatic drive system uses compressed air as the working medium to drive the movement of robots or other machinery through the pressure difference generated by the airflow [180]. The exhaust gas treatment is simple and does not pollute the environment [181]. The system can quickly respond to control signals, achieve automatic control, and is easy to maintain. The startup driver can operate safely in harsh environments such as flammable, explosive, dusty, strong magnetic, radiation, and vibration [182]. The pneumatic drive is widely used in industrial robots, functional equipment of technical systems, transportation systems, and any equipment that executes discrete movements of actuators [183].

4.3.1. Multi-Mode Pneumatic Motor

Yang Y et al. [184] designed a multi-mode small pneumatic motor based on a rigid, flexible coupling structure, as shown in Figure 94. Pneumatic motors have continuous rotation and step modes and can quickly switch working modes by adjusting the input air pressure signal. The cylinder stator adopts a circular structure, and the inner wall is fixed with a deformable silicone tube. The rotor consists of rollers, brackets, and pin shafts, and the rollers squeeze the silicone tube to generate thrust. When compressed air is input, the silicone tube expands and deforms locally, pushing the roller to rotate. Multiple ports supply gas simultaneously in continuous mode, and the roller continuously pushes the silicone tube to achieve high speed rotation. In step mode, pulse air pressure is sequentially inputted to a single port, and the roller is gradually moved to achieve a fixed step angle. However, the accuracy of the motor is affected by the elastic deformation and inertia of the silicone tube in step mode. The output torque of the step mode is lower than that of the continuous mode, and the lifespan and pressure resistance of the silicone tube limit its long term high-pressure application.

4.3.2. Pneumatic Artificial Muscles

Stoll JT et al. [185] proposed a rotational drive unit that combines compliance and high precision by combining pneumatic artificial muscles (PAMs) with inclined plate structures, as shown in Figure 95. The linear contraction force of PAMs is converted into rotational torque through the inclined plate structure, and the normal force is transmitted through the four pivot bearings, avoiding the stick-slip phenomenon of traditional aerodynamics. By independently adjusting the pressure of each PAM and controlling the total torque and system stiffness, high precision positioning and flexible motion can be achieved. Dynamic adjustment of system stiffness is achieved through pressure regulation to adapt to different load requirements. A 16-bit encoder and proportional valve control are used to achieve fine motion. However, the rotary drive unit has a large volume and weight, requiring lightweight improvements and independent control of multiple proportional valves. The system is complex and costly, with a maximum pressure limit of 600 kPa, which may limit torque output.

Table 22. Comparison of pneumatic drive.

Types	Advantages	Disadvantages	Applications
Traditional pneumatic drive [180,181,182,183]	Environmental protection, strong adaptability, fast response, and easy to automatically control	Low torque, accuracy affected by pressure fluctuations and elastic components, high noise	Industrial robots and automation scenarios in harsh environments
Multi-mode pneumatic motor [184]	Supports continuous rotation and step mode switching, with good output performance and low cost	Accuracy is affected by the elastic deformation and inertia of silicone tubing	Light load scenarios of pipeline robots, modular robotic arms, and fast mode switching
Pneumatic artificial muscles [185]	Flexibility and high precision, no stick-slip phenomenon, high precision positioning	The maximum pressure limit restricts the torque output	Flexible robot joints, precision assembly, and medical equipment

summarizes and compares the driving performance of the different pneumatic drives mentioned above.

In precision rotary motion mechanisms, pneumatic drive technology has demonstrated unique advantages due to its environmental compatibility, fast response, and adaptability to harsh working conditions. However, the coordinated control of high precision and high torque output still faces challenges due to core issues such as nonlinear hysteresis effects, material elastic deformation, and system inertia. The current research focuses on multimodal drive architecture and biomimetic structural innovation: a multi-mode pneumatic motor based on rigid-flexible coupling achieves rapid switching between continuous rotation and step mode through a dynamic coupling mechanism of silicone tube deformation and roller thrust. However, its step accuracy is constrained by the viscoelastic hysteresis and dynamic inertia disturbance of the silicone tube, and the fatigue resistance of silicone material limits its lifespan under high pressure conditions; The integrated design of pneumatic artificial muscles (PAMs) and inclined plate structures breaks through the bottleneck of stick slip effect in traditional pneumatic systems through a geometric conversion mechanism of linear contraction force rotational torque and a dynamic stiffness adjustment strategy. However, the system complexity brought by multi-valve independent control, the torque output threshold caused by material pressure limit, and the lightweight requirements of modular structures still need to be urgently addressed. The future development trend will revolve around multi-level optimisation of materials structure control: developing composite flexible materials with super elasticity and low creep characteristics to enhance deformation controllability; Using topology optimisation and additive manufacturing technology to achieve high power to weight ratio design of driving units; Integrating pressure flow collaborative compensation algorithm with high-resolution encoder feedback, a nonlinear hysteresis feedforward disturbance observation closed-loop control system is constructed. In addition, pneumatic electro-hydraulic hybrid drive, variable stiffness joints based on pneumatic impedance adjustment, and collaborative control of distributed pneumatic arrays will become important directions to break through the bottleneck of accuracy torque response speed trade-off, promoting the deep application of pneumatic drive technology in precision assembly, flexible surgical instruments, and adaptive robots.

4.4. The Influence of Different Driving Modes on Rotating Motion Mechanisms—Taking Cam Mechanisms as an Example

The above content demonstrates the impact of different types of drivers on the performance of the studied system. Taking the cam mechanism as an example, a comparative analysis is conducted on the performance impact of different driving modes on the cam mechanism, as shown in Figure 96 and

Table 23. The influence of different driving modes on cam mechanisms.

Types	Advantages	Disadvantages	Applications
Motor drive	High precision, high response speed, and high energy conversion efficiency	The load capacity is limited, and the volume and cost increase when high power demand is required	Precision instruments, automation equipment, and cam mechanisms require high dynamic response
Piezoelectric drive	Nonmagnetic, noise-free, compact structure, and extremely high resolution	Low output force, requiring high voltage drive, long term use may require maintenance due to friction and wear	Micro-nano positioning systems, cam fine-tuning in optical devices, vacuum, or low temperature environments
Hydraulic drive	High load capacity, seamless transmission, fast dynamic response, and strong impact resistance	The system is complex, with a risk of leakage, high maintenance costs, and loud noise	Heavy machinery, aerospace high load cam mechanisms, and high-inertia scenarios require frequent start-stop operations
Electro-hydraulic servo drive	High power density, low noise, and high integration	Insufficient oil absorption at high speeds leads to decreased efficiency, which requires precise control	Cam control in aircraft hydraulic systems and high precision industrial equipment
Pneumatic drive	Environmentally friendly, adaptable to harsh environments, fast response, and easy maintenance	Low output torque, accuracy affected by air pressure fluctuations, and high noise	Industrial robots, lightweight cam mechanisms in automated production lines, and flammable and explosive environments
Multi-mode pneumatic motor	Low cost, flexible mode switching, and simple structure	The deformation of elastic components affects accuracy, and the lifespan of silicone tubing is limited for high-pressure applications	Pipeline robots, modular robotic arms, and scenarios that require quick switching of action modes
Pneumatic artificial muscle	High flexibility, no stick-slip phenomenon, high precision positioning	Large volume, high weight, complex independent control of multiple valves, and limited torque output due to material pressure	Cam drives in flexible robot joints, precision assembly, and medical equipment

.

Through the above comparative analysis, the core performance requirements and adaptive driving methods of different rotary motion mechanisms need to be analysed case-by-case. For example, cam mechanisms need to balance dynamic response and accuracy, and motor or electro-hydraulic servo drives are preferred, which are typically applied in automation equipment and aviation hydraulic systems; Gear mechanisms focus on torque bearing and transmission efficiency, while hydraulic drives or high power motors are suitable for construction machinery and heavy-duty gearboxes; The linkage mechanism needs to coordinate motion and adapt to the environment, and the pneumatic drive or servo motor is ideal for industrial robots and automotive suspension systems; Ratchet and pawl emphasize the reliability of one-way locking, and pneumatic drive or spring mechanical are often used for lifting equipment and safety braking; Ball screws rely on high positioning accuracy and repeatability, and piezoelectric drive or closed-loop servo motors are suitable for CNC machine tools and semiconductor lithography machines; Universal joints require multi-directional torque transmission capability, and electro-hydraulic hybrid drive is ideal for vehicle transmission shafts and ship propulsion systems; Planetary reducers require high reduction ratios and compact structures, and integrated servo motors are commonly used for robot joints and precision speed regulation; Pneumatic artificial muscles focus on flexible movement and safety, requiring multi valve collaborative pneumatic control, and are mainly used in medical exoskeletons and bionic robots. The driving mode selection must comprehensively balance the mechanism’s characteristics and the scenario’s requirements.

5. Conclusions and Future Research Direction

5.1. Conclusions

This article systematically reviews the research progress of precision rotary motion mechanisms and their driving technologies, revealing the core differences in performance, efficiency, and applicable scenarios between traditional mechanisms and new designs. Traditional gear mechanisms dominate industrial transmissions due to their high efficiency and structural stability. However, vibration noise and tooth surface wear issues under high dynamic conditions, such as contact stress concentration in spur gears and nonorthogonal shaft friction losses in worm gear transmissions, remain key bottlenecks. Traditional mechanisms have improved load capacity and service life through tooth profile correction of multi-tooth meshing, such as HCR gears, and surface treatment material strengthening, such as hardened worm gears. However, due to the fatigue limit and manufacturing accuracy of metal materials, it isn’t easy to meet the requirements of ultra-high speed or extreme environments.

The new mechanism has achieved performance breakthroughs through asymmetric topology design and multi-physics field coupling. The noncircular gear planetary system, combined with the gear ratio characteristics defined by the Fourier series, exhibits unique advantages in dynamic variable speed scenarios. However, the local wear caused by uneven stress distribution on the tooth surface must be optimised. Magnetic gears utilise the ideal diamagnetism of high temperature superconductors to achieve non-contact transmission, with torque density up to 1.73 times that of traditional structures; however, low temperature cooling systems’ high cost and complexity limit industrial applications. The intermittent indexing mechanism improves the indexing frequency and motion stability through the cam connecting rod collaborative design. However, the accumulation of assembly errors and dynamic impact suppression in complex structures still needs to be solved through real-time damping control algorithms.

The diversified driving technology development provides a new path for optimising institutional performance. In the electric drive system, the hollow cup motor eliminates the cogging effect through a coreless structure, but its dual rotor magnetic circuit design requires strict uniformity of the air gap magnetic field; Piezoelectric driving utilises the inverse piezoelectric effect to achieve micro nano motion control, but the hysteresis nonlinearity and preload relaxation problems under high-frequency excitation urgently require material control collaborative optimisation. Hydraulic and pneumatic drives perform outstandingly in high load and flexible scenarios, respectively. Still, the energy loss of hydraulic systems and the accuracy fluctuations of pneumatic transmission need to be balanced through a hybrid drive strategy.

By analysing the impact of different driving methods on the performance of cam mechanisms through case studies, the comparative analysis results also apply to other precision rotary motion systems, providing engineers and researchers in various fields with more intuitive selection guidance.

5.2. Future Research Direction

Future research on precision rotational motion mechanisms and their driving methods can be explored in depth in the following directions:

(1)

Develop high-strength composite materials such as carbon nanotube-reinforced TPU and self-healing coatings such as ceramic-based gradient coatings, combined with laser melting additive manufacturing to achieve a lightweight and fatigue-resistant design of key components such as gears and cams. Regarding the tooth surface wear of planetary cycloidal reducers, super-lubricating coating technology can be explored. Breakthroughs are needed in room-temperature superconducting materials or magnetocaloric cooling technology to address the bottleneck of superconducting cooling for magnetic gears.

(2)

Embedding fibre optic or piezoelectric sensors in the mechanism to monitor real-time tooth contact stress and bearing temperature rise, and predicting wear trends through deep learning algorithms to achieve adaptive adjustment of the lubrication system. For piezoelectric ultrasonic motors, it is necessary to develop resonance frequency self-tracking technology to suppress efficiency degradation caused by load disturbances.

(3)

Combining magnetic levitation non-contact transmission with piezoelectric energy recovery to construct a low friction, high-efficiency composite drive system. For example, in a wave energy converter, a magnetic guide screw is used to efficiently convert linear motion into rotational kinetic energy, while vibration energy is recovered through piezoelectric materials.

(4)

Inspired by biological joints, design a flexible multilink mechanism that utilises the phase transition characteristics of shape memory alloys to achieve variable stiffness adjustment. Promote the standardisation of modular driving units and support rapid reconstruction and fault redundancy of robot joints.

(5)

A multi-scale dynamic model of the mechanism is constructed using artificial intelligence and topology optimisation algorithms. The impact of assembly errors on motion accuracy is quantified through digital twin technology, and design parameters are iteratively optimised by combining actual working condition data.