Miniature planetary roller screw device

Abstract

The invention discloses a miniature planetary roller screw device, and belongs to the technical field of precision transmission. The device comprises a miniature planetary screw rod, a miniature planetary nut arranged on the screw rod in a sleeving mode and a transmission assembly arranged in the nut. The transmission assembly is composed of a plurality of miniature planetary rollers, a retainer with a groove formed in the circumferential direction and two cam glands. The retainer is used for containing the pin rollers, two ends of the retainer are fixed through glands, and limiting protrusions on the outer edges of the glands are matched with corresponding grooves in the inner walls of the nuts to achieve integral assembly and fixation. Threads of the pin roller are meshed with threads of the lead screw to form line contact transmission, and rotary motion of the lead screw is converted into axial linear motion of the nut. The invention has the advantages that: the structure is compact, the assembly is simple and convenient, the transmission precision, rigidity and bearing capacity are obviously improved while microminiaturization is realized through line contact transmission and integrated design, and the mechanism is particularly suitable for occasions with strict requirements on space and precision, such as robot finger joints.

Description

Technical Field

[0001] This invention relates to a miniature planetary roller screw device, belonging to the field of precision transmission mechanisms. Background Technology

[0002] Planetary roller screws, as precision components that convert rotary motion into linear motion, are widely used in industrial automation, aerospace, and other fields due to their high transmission efficiency, high rigidity, and long lifespan. Their traditional structure typically uses balls as the transmission medium.

[0003] With the development of cutting-edge fields such as robotic dexterity hands and micro-precision devices, more stringent requirements have been placed on the miniaturization, high precision, and high reliability of transmission components. Currently, planetary ball screws are the main type of ball screw used in micro-applications. However, during the miniaturization process, this solution has revealed several inherent defects: First, the miniaturization of size leads to a sharp reduction in the diameter of the balls, which not only greatly increases the difficulty and cost of assembly but also makes the machining of the "reverse mechanism" (used to realize ball circulation), a key component, extremely difficult, and its precision is hard to guarantee; second, the point contact between the balls and the screw / nut results in stress concentration, making the load-bearing capacity, stiffness, and impact resistance of the miniature ball screw insufficient, and unable to meet the stability requirements of high-load, high-precision motion; in addition, the friction and wear of point contact are more severe than those of line contact, affecting the service life and accuracy retention of the screw.

[0004] Therefore, there is an urgent need for a transmission device designed for micro applications that can overcome the shortcomings of existing planetary ball screws in terms of structure, strength, precision and manufacturability while achieving extremely small size, and provide a more reliable, more precise and easier-to-assemble micro linear transmission solution. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention provides a miniature planetary roller screw device. The technical solution of this invention is as follows: A miniature planetary roller screw device includes a miniature planetary screw (5) and a miniature planetary nut (2) sleeved on the miniature planetary screw (5), and also includes a transmission assembly, which includes a miniature planetary roller (1), a cage (6) and two cam caps (3). The retainer (6) is located inside the miniature planetary nut (2), and has a groove around its circumference for accommodating the miniature planetary roller (1); The two cam caps (3) are respectively installed at both ends of the retainer (6), and their outer edges are provided with limiting protrusions, which cooperate with the corresponding grooves on the inner wall of the micro planetary nut (2) to fix the transmission assembly inside the micro planetary nut (2); The planetary thread of the micro planetary roller (1) meshes with the planetary thread on the micro planetary screw (5), converting the rotational motion of the micro planetary screw (5) into the linear motion of the micro planetary nut (2) along its axial direction.

[0006] The transmission assembly is installed inside the miniature planetary nut (2) and is fixed to the miniature planetary nut (2) by the limiting protrusion of the cam cover (3).

[0007] The retainer (6) and the cam cover (3) are formed by 3D printing in one piece or in segments.

[0008] The diameter of the micro planetary roller (1) ranges from 0.9 mm to 1.5 mm.

[0009] The micro planetary screw (5), micro planetary nut (2) and micro planetary roller (1) are all subjected to a composite heat treatment process that combines tempering and high-frequency induction hardening.

[0010] The planetary threads of the micro planetary screw (5) are processed by a combination of grinding and rolling processes.

[0011] This device is suitable for the finger joints of humanoid robots, with a transmission error of no more than 0.02mm.

[0012] The micro planetary roller (1) and the micro planetary lead screw (5) are connected by linear contact transmission. The advantages of this invention are: 1. The integrated design of planetary roller structure, cage, and cam cap eliminates the difficult-to-machine and assemble reverser found in miniature ball screws. The cam cap achieves reliable fixation and axial positioning of the internal transmission components through a simple engagement between the limiting protrusion and the groove on the inner wall of the nut, greatly reducing the assembly difficulty and complexity at the miniature scale.

[0013] 2. The planetary rollers and the lead screw employ linear contact transmission, which significantly increases the contact area and reduces contact stress compared to the point contact of traditional miniature ball screws. This allows the device to maintain higher transmission rigidity, load-bearing capacity, and impact resistance while achieving miniaturization, resulting in smoother and more reliable operation.

[0014] 3. Key structural components (such as cages and cam covers) are integrally or segmentally formed using 3D printing technology, which enables the creation of complex micro-cavity structures that are difficult to achieve with traditional machining. This ensures the overall structural strength and dimensional consistency of the parts, thereby improving the precision and reliability of the device from the manufacturing source.

[0015] 4. Applying a composite heat treatment process of "quenching and tempering + high-frequency induction hardening" to load-bearing components such as lead screws, nuts, and rollers gives the parts both good comprehensive mechanical properties (strength and toughness) in the core and extremely high hardness and wear resistance in the threaded meshing area on the surface, which significantly improves the service life and accuracy retention of the device.

[0016] 5. The planetary screw thread is machined using a combination of grinding and rolling processes, ensuring high thread precision and excellent surface quality. Combined with a precise linear contact transmission method and a stable component structure, it can achieve an extremely low transmission error of no more than 0.02mm, meeting the precision control requirements for micro-movements in applications such as humanoid robot finger joints.

[0017] 6. This device combines the advantages of high precision, high rigidity, high load-bearing capacity, long life, and easy miniaturization assembly. It is particularly suitable for applications with limited space and requiring precise force and position control, such as precision medical devices, optical instrument adjustment mechanisms, and aerospace micro-actuators. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the main structure of the present invention.

[0019] Figure 2 yes Figure 1 Side view.

[0020] Figure 3 yes Figure 1 A sectional view. Detailed Implementation

[0021] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer as a result. However, these embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope of the present invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0022] See Figures 1 to 3This invention relates to a miniature planetary roller screw device, comprising a miniature planetary screw 5 and a miniature planetary nut 2 sleeved on the miniature planetary screw 5, and a transmission assembly. The transmission assembly includes miniature planetary rollers 1, a cage 6, and two cam caps 3. The cage 6 is disposed inside the miniature planetary nut 2, and has a groove around its circumference for accommodating the miniature planetary rollers 1. The two cam caps 3 are respectively installed at both ends of the cage 6, and have limiting protrusions on their outer edges that cooperate with corresponding grooves on the inner wall of the miniature planetary nut 2 to fix the transmission assembly inside the miniature planetary nut 2. The planetary threads of the miniature planetary rollers 1 mesh with the planetary threads on the miniature planetary screw 5, converting the rotational motion of the miniature planetary screw 5 into linear motion of the miniature planetary nut 2 along its axial direction.

[0023] By engaging the limiting protrusion on the outer edge of the cam cover 3 with the corresponding groove on the inner wall of the miniature planetary nut 2, the transmission component is quickly positioned and fixed inside the nut. This design eliminates the complex reversing device structure found in traditional miniature ball screws, achieving self-centering and reliable constraint of parts within an extremely limited space, significantly reducing the assembly difficulty and complexity at the miniature scale.

[0024] The miniature planetary roller 1 and the miniature planetary screw 5 are engaged by planetary threads to form a line contact transmission. Compared with the point contact of the miniature ball screw, this structure increases the contact area and disperses the contact stress, thus maintaining good transmission rigidity, load-bearing capacity and smooth movement under miniaturization conditions, laying the structural foundation for achieving a transmission error of no more than 0.02mm.

[0025] The cage 6 and the two cam caps 3 together form an integrated transmission unit. The circumferential grooves of the cage regularly accommodate multiple rollers, while the end caps provide axial restraint and radial support, ensuring that all rollers are evenly distributed and move synchronously within the nut. This integrated design enhances the structural integrity and motion consistency of the internal components, improving the reliability and anti-interference capability of the device in its small size.

[0026] The transmission assembly is integrally installed within the miniature planetary nut 2 and secured to it via the limiting protrusions of the cam cap 3. This design integrates the cage 6, the miniature planetary rollers 1, and the cam caps 3 at both ends into an independent transmission module. After being installed into the miniature planetary nut 2, it is secured solely by the locking protrusions on the cam caps engaging with the corresponding grooves on the inner wall of the nut, eliminating the need for additional connectors or complex tooling. This modular, snap-fit ​​assembly method greatly simplifies the assembly process at a miniature scale, improves assembly efficiency and consistency, and ensures stable alignment and reliable constraint of the internal transmission structure.

[0027] The retainer 6 and the cam cover 3 are integrally formed or segmented formed by 3D printing. This avoids the process limitations of traditional machining in achieving multi-part assembly structures and internal cavity features at a microscale, ensuring not only the dimensional accuracy and shape consistency of the parts, but also enhancing the structural integrity and assembly precision of the components.

[0028] The diameter of the micro planetary roller 1 ranges from 0.9 mm to 1.5 mm. This diameter range (0.9 mm to 1.5 mm) optimizes the cross-sectional area and inertia of the roller while meeting the extreme space constraints of robot finger joints. This avoids the problems of easy bending and damage caused by excessive small size, and ensures a sufficient and stable linear contact area with the lead screw thread. Thus, it can maintain reliable load-bearing capacity, transmission stiffness and motion accuracy at a microscale.

[0029] The micro planetary screw 5, micro planetary nut 2, and micro planetary roller 1 are all subjected to a composite heat treatment process that combines tempering and high-frequency induction hardening.

[0030] The advantages of this composite heat treatment process are as follows: At the microscale, this process imparts excellent comprehensive mechanical properties (high strength and high toughness) to the core of the part through "tempering and quenching," and then forms an extremely high-hardness wear-resistant layer on key friction surfaces such as threads through "high-frequency induction hardening." This gradient performance combination of "intrinsic toughness and external hardness" effectively solves the common problems of brittle fracture, rapid wear, or loss of precision in micro-transmission parts due to their small size, thus ensuring the load-bearing reliability, motion accuracy, and service life of the device under extreme working conditions.

[0031] The specific process of heat treatment is as follows: First, the part is heated as a whole to the austenitizing temperature (e.g., 850-880℃), then oil quenched to obtain a martensitic structure, and then tempered at a higher temperature (e.g., 550-600℃) to finally obtain a tempered sorbite structure with excellent strength and toughness.

[0032] High-frequency induction hardening specifically involves using a high-frequency induced current to selectively and rapidly heat only the working surface of a part (such as the threaded meshing surface) to the quenching temperature (e.g., 900-950℃), followed by immediate and rapid cooling (e.g., water spraying or oil immersion), which transforms the surface layer into a high-hardness martensitic structure. Afterward, tempering is usually carried out at a lower temperature (e.g., 180-220℃) to stabilize the structure and reduce internal stress.

[0033] The planetary threads of the miniature planetary screw 5 are manufactured using a combined grinding and rolling process. This process combines the dimensional accuracy of grinding with the surface strengthening advantages of rolling to meet the extremely high precision and performance requirements of miniature planetary threads. First, precision grinding ensures the accurate tooth profile, lead, and clearance of the thread, meeting the requirements for micron-level transmission errors. Then, rolling causes plastic deformation of the thread surface, forming a dense hardened layer and residual compressive stress. This combined process significantly improves the surface hardness, fatigue strength, and wear resistance of the thread while ensuring its geometric accuracy, thus overcoming the technical contradiction that a single process cannot simultaneously achieve precision and durability at the microscale.

[0034] The specific process of grinding and rolling composite process is as follows: 1. Using a thread grinder and matching miniature forming grinding wheels, the thread surface of the miniature planetary screw 5 is subjected to final finishing. By precisely controlling the motion trajectory of the machine tool and the grinding parameters, a thread geometry profile with accurate tooth shape and smooth surface is obtained.

[0035] 2. After grinding, a precision rolling roller or rolling head matching the thread parameters is used to perform chip-free rolling on the thread surface under controlled pressure. The rolling roller rolls along the thread surface, causing the surface metal material to flow and densify through plastic deformation, thereby correcting micro-irregularities, improving surface hardness, and forming a beneficial residual compressive stress layer on the surface.

[0036] This device is suitable for the finger joints of humanoid robots, with a transmission error of no more than 0.02mm.

[0037] The miniature planetary roller 1 and the miniature planetary screw 5 are connected by linear contact transmission. Compared to point contact, linear contact significantly increases the actual contact area of ​​the thread meshing region, thereby effectively dispersing contact stress and avoiding stress concentration. This allows the device to withstand higher loads and maintain smoother, more precise linear motion output despite its extremely small size, fundamentally overcoming the inherent defects of miniature ball screws such as insufficient rigidity, easy wear, and unstable motion caused by point contact.

[0038] The working principle of this invention, in accordance with the sequence of motion transmission, is explained as follows: Step 1: Rotary motion input and primary engagement The miniature planetary screw 5 serves as the input end and rotates around its axis under external drive. Its outer surface is machined with precision planetary threads. At this point, the planetary threads of multiple miniature planetary rollers 1 mesh with the planetary threads of the screw 5. Since the rollers 1 are circumferentially and evenly constrained within the slots of the cage 6, the rotation of the screw 5 drives each roller 1 to rotate around its own axis through the meshing of the threaded pair.

[0039] Step Two: Planetary Motion Transmission and Internal Constraints As each miniature planetary roller 1 rotates, its thread engages with the internal thread of the miniature planetary nut 2. Since the nut 2 is fixed by the limiting protrusion of the cam cap 3 or only restricted by the external structure and does not rotate freely, the roller 1, under the reaction force of the nut 2's internal thread, will generate a tendency to revolve. This tendency is guided and coordinated by the cage 6, causing all rollers 1 to synchronously and uniformly rotate around the axis of the lead screw 5, forming a standard planetary motion pattern. The integrated unit formed by the cam caps 3 at both ends and the cage 6 ensures the precise positioning of the roller assembly in the axial and radial directions, preventing offset or jamming, and providing a rigid support frame for smooth planetary motion.

[0040] Step 3: Linear Motion Output During the aforementioned motion, the internal thread of the miniature planetary nut 2 engages with the external thread of the roller 1, which in turn engages with the external thread of the lead screw 5. Because the nut 2 is fixed or constrained circumferentially, the rotational motion of the lead screw 5 cannot directly cause the nut 2 to rotate. According to the principle of planetary thread transmission, the rotational motion is forcibly converted into precise linear motion of the nut 2 along the axis of the lead screw 5. The direction of motion of the nut 2 depends on the rotational direction of the lead screw 5, and the distance of motion is precisely proportional to the number of rotations and the lead of the lead screw 5.

[0041] This invention achieves efficient and precise conversion of rotary motion to linear motion through a core transmission chain: "screw rotation input, driving roller rotation and revolution (planetary motion), and pushing nut axial linear output." Throughout the process, the linear contact threaded pair ensures high rigidity and high load-bearing capacity; the integrated cage and gland assembly achieves a compact and reliable structure without the need for a reverser; and the modular assembly method (the entire transmission assembly is installed into the nut and secured by the gland) solves the assembly challenges under miniaturization. Ultimately, all these factors ensure that the device can still achieve an extremely high transmission accuracy of ≤0.02mm even under extremely small size (e.g., roller diameter 0.9-1.5mm).

[0042] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A miniature planetary roller screw device, comprising a miniature planetary screw (5) and a miniature planetary nut (2) sleeved on the miniature planetary screw (5), characterized in that, It also includes a transmission assembly comprising a miniature planetary roller (1), a cage (6), and two cam caps (3). The retainer (6) is located inside the miniature planetary nut (2), and has a groove around its circumference for accommodating the miniature planetary roller (1); The two cam caps (3) are respectively installed at both ends of the retainer (6), and their outer edges are provided with limiting protrusions, which cooperate with the corresponding grooves on the inner wall of the micro planetary nut (2) to fix the transmission assembly inside the micro planetary nut (2); The planetary thread of the micro planetary roller (1) meshes with the planetary thread on the micro planetary screw (5), converting the rotational motion of the micro planetary screw (5) into the linear motion of the micro planetary nut (2) along its axial direction.

2. The miniature planetary roller screw device according to claim 1, characterized in that, The transmission assembly is installed inside the miniature planetary nut (2) and is fixed to the miniature planetary nut (2) by the limiting protrusion of the cam cover (3).

3. The miniature planetary roller screw device according to claim 1 or 2, characterized in that, The retainer (6) and the cam cover (3) are formed by 3D printing in one piece or in segments.

4. The miniature planetary roller screw device according to claim 3, characterized in that, The diameter of the micro planetary roller (1) ranges from 0.9 mm to 1.5 mm.

5. The miniature planetary roller screw device according to claim 4, characterized in that, The micro planetary screw (5), micro planetary nut (2) and micro planetary roller (1) are all subjected to a composite heat treatment process that combines tempering and high-frequency induction hardening.

6. The miniature planetary roller screw device according to claim 5, characterized in that, The planetary threads of the micro planetary screw (5) are processed by a combination of grinding and rolling processes.

7. The miniature planetary roller screw device according to claim 1, characterized in that, This device is suitable for the finger joints of humanoid robots, with a transmission error of no more than 0.02mm.

8. The miniature planetary roller screw device according to claim 1, characterized in that, The micro planetary roller (1) and the micro planetary lead screw (5) are connected by linear contact transmission.

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