Axially pre-tensioning device for holding members of roller screw

Abstract

An axial preload device for a roller screw cage includes: a screw, a nut, a roller assembly, a retainer assembly, and a preload assembly. The roller assembly has a plurality of staggered first and second rollers, each roller having annular teeth on its outer circumference that mesh with a threaded structure on the screw and an annular groove section within the nut. The retainer assembly is located at both ends of the roller assembly, and utilizes the axial length difference between the ends of the first and second rollers to form staggered first and second clearances. The preload assembly passes axially through the retainer assembly and applies an adjustable tightening force, causing the retainer assembly to clamp the roller assembly, thereby applying opposite and adjustable axial preloads to the first and second rollers, creating a staggered bidirectional preload effect. This design eliminates backlash, ensures pure rolling contact, improves transmission stability and lifespan, and eliminates the need for retainers, saving space and avoiding friction problems.

Description

Technical Field

[0001] This invention relates to a transmission assembly, and more particularly to an axial preload device for a roller screw cage that can generate adjustable axial preload on the rollers to improve rolling stability. Prior Technology

[0002] Planetary roller screws utilize multiple rollers that synchronously roll and mesh between the screw and nut to efficiently convert rotary motion into linear motion. They possess high load capacity, high rigidity, and high positioning accuracy, and are widely used in machine tools, servo actuators, and aerospace control systems.

[0003] As shown in Figure 1A, a conventional planetary roller screw drive includes a screw 11, a nut 12, a plurality of rollers 13, and a cage assembly 14. The screw 11 has an external thread structure, and the nut 12 has an inner hole 120 to accommodate the rollers 13, and an annular tooth structure 12t on its inner circumference to form rolling contact with the annular teeth of the rollers 13. The cage assembly 14 is disposed at both ends of the rollers 13 to limit their circumferential spacing and angular distribution and maintain their positioning. To prevent axial displacement of the cage assembly 14 during movement, the nut 12 has two internal grooves 121 to accommodate a retainer 15 (such as a C-shaped retainer or annular retainer) in each groove. The retainer 15 is disposed on the outside of the cage assembly 14 and contacts it to fix its axial position and prevent it from dislodging.

[0004] However, traditional designs still have several problems. As shown in Figures 1B and 1C, the geometric fit between the roller 13, screw 11, and nut 12 is extremely precise. However, due to manufacturing tolerances and assembly errors, a small gap G is generated between the roller 13 and the screw 11 / nut 12, causing the actual contact position of the roller 13 to deviate from the designed pitch circle radius. If this gap G is not effectively compensated, there is insufficient friction between the roller 13 and the nut 12 during operation, which easily leads to slippage or freewheeling, making it difficult to maintain pure rolling contact and generating sliding friction. This results in greater wear on the roller 13, reducing the overall structural lifespan. Similarly, the roller 13 and screw 11 may also slip due to insufficient contact, thereby disrupting the deceleration and propulsion effects of planetary motion, causing a decrease in linear propulsion efficiency and positioning accuracy.

[0005] Although the cage assembly 14 can control the angle and spacing of the rollers 13, it only has a mechanical positioning function and cannot provide preload compensation or guide the preload direction of the rollers. It also cannot effectively improve the sliding offset problem caused by the clearance G. More importantly, the actual contact between the cage assembly 14 and the retaining ring 15 generates frictional resistance. When the cage assembly 14 rotates with the rollers 13, relative sliding occurs between it and the retaining ring 15, resulting in wear and resistance, affecting the consistency and smoothness of the rollers 13's movement, causing a decrease in transmission efficiency and a deterioration in accuracy. Furthermore, the embedded groove 121 structure required for the retaining ring 15 occupies valuable axial space in the nut 12, limiting the miniaturization and compact design of the overall transmission assembly, thus restricting applications requiring minimal space or high structural integration.

[0006] Therefore, how to effectively improve the stability of the contact between the roller 13 and the nut 12 without increasing structural complexity and manufacturing cost, and strengthen the rolling engagement between the roller 13 and the screw 11, so as to ensure that the screw 11, roller 13 and nut 12 maintain stable rolling motion, is an important issue that needs to be overcome in this field. Summary of the Invention

[0007] The present invention provides an axial preload device for the retainer of a roller screw that effectively solves the aforementioned technical problems. This device compensates for the fit clearance caused by machining errors and applies axial preload in opposite directions with adjustable preload to the staggered first and second rollers to avoid sliding friction between the roller assembly and the screw / nut, ensuring rolling contact and thus improving overall lifespan. It also effectively eliminates backlash, enhances transmission stability and positioning accuracy, and eliminates the need for the traditional design of fixing the axial position of the retainer with a retainer ring, simplifying the structure, saving axial space, and avoiding additional friction caused by retainer ring contact.

[0008] To achieve the above objectives, the present invention provides an axial preload device for a roller screw retainer, comprising: a screw, a nut, a roller assembly, a retainer assembly, and a preload assembly. The screw has a threaded structure on its outer periphery. The nut is fitted onto the screw and has at least one annular groove section on its inner side for bilateral rolling engagement with the roller assembly. The roller assembly is disposed between the screw and the nut, comprising a plurality of staggered first and second rollers. Each roller has annular teeth on its outer periphery, forming rolling contact with the threaded structure of the screw and the annular groove section of the nut, respectively.

[0009] The retainer assembly is located at both ends of the roller assembly and includes a first retainer and a second retainer, each having a plurality of positioning grooves for accommodating the two ends of the rollers. By having different axial lengths at opposite ends of the first and second rollers, staggered first and second gaps are formed between the retainers and the roller ends, thereby guiding the direction of preload transmission and establishing a selective force application effect.

[0010] The preload assembly, for example, a preload screw, is disposed between the first and second retainers and is arranged parallel to the axial direction of the roller assembly. This preload assembly can be adjusted by loosening or tightening the screw to generate a controllable tightening force, bringing the two retainers closer together, thereby clamping the roller assembly and generating axial pressure. Through the aforementioned staggered first and second gaps, the first and second rollers are subjected to axial preloads in opposite directions, establishing a staggered bidirectional axial preload structure at both ends of the roller assembly.

[0011] This invention effectively compensates for the clearance between the screw and nut, and guides the preload transmission direction through a staggered clearance design, ensuring that the rollers stably conform to the rolling tooth surfaces of the screw and nut, preventing slippage and freewheeling, and improving rolling stability and service life. Furthermore, the adjustable preload screw assembly allows for the application of a controllable tightening force between the two retainers, applying axial pressure in opposite directions with adjustable preload to different rollers, creating a bidirectional preload effect. The overall design eliminates the need for retaining rings to limit the axial position of the retainers, simplifying the structure, saving space, reducing frictional losses, and achieving a high-precision, high-efficiency, and high-reliability transmission system. Simple Explanation of the Diagram

[0012] Figure 1A is a schematic diagram of the structure of an existing planetary roller screw drive device; Figure 1B is a schematic diagram of the meshing contact relationship between the roller and the nut in Figure 1A; Figure 1C is a schematic diagram of the meshing contact relationship between the roller and the screw in Figure 1A; Figure 2 is a three-dimensional exploded view of an embodiment of the present invention; Figure 3A is an axial sectional view of an embodiment of the present invention; Figure 3B is a magnified view of a portion of the meshing shown in Figure 3A; Figure 3C is a cross-sectional view of Figure 3A showing the configuration of the rollers and retainers; Figure 4 is a schematic diagram of the axial length difference between the first roller and the second roller at opposite ends in an embodiment of the present invention; Figure 5 is a schematic diagram of the pre-tightening assembly applying force to the retaining assembly to clamp the roller assembly in an embodiment of the present invention; Figure 6 is a cross-sectional view of the pre-pressure transmission direction of the staggered first and second gaps in an embodiment of the present invention; Figure 7A is a magnified view of a portion of the first gap in Figure 6; Figure 7B is a magnified view of the second gap in Figure 6. Figure 8A is a magnified view of a portion of the preload contact formed by the first roller towards the screw. Figure 8B is a magnified view of a portion of the preload contact formed by the first roller toward the nut side; Figure 9A is a magnified view of the preload contact formed by the second roller towards the screw. Figure 9B is a magnified view of the preload contact between the second roller and the nut. Figure 10 is a schematic diagram of the preload screw applying axial clamping force from opposite directions. Implementation

[0013] The construction and functional characteristics of the bidirectional axial preload structure of the roller screw of the present invention will be described with reference to the preferred embodiment shown in the accompanying drawings.

[0014] Please refer to Figures 2, 3A-3C, 4 and 5. The axial preload device of the retainer of the roller screw in a preferred embodiment of the present invention includes: a screw 21, a nut 22, a roller assembly R, a retainer assembly K and a preload assembly P.

[0015] Nut 22 is a hollow cylindrical component with an axially extending inner bore 220. The inner circumferential surface of the inner bore of nut 22 has two annular groove sections 221 for rolling engagement with the roller assembly R. These annular groove sections 221 provide circumferential guidance and contact restraint, allowing the roller assembly R to roll stably within nut 22 in a planetary manner, preventing axial disengagement. The annular groove sections 221 have a plurality of annular grooves 221g distributed circumferentially (as shown in Figure 3B).

[0016] In addition, between the two annular groove sections 221, the inner circumferential surface of the inner hole of the nut 22 is provided with a radial clearance groove section 223. The inner diameter of the clearance groove section 223 is larger than that of other parts to provide sufficient space to avoid interference between the roller assembly R and the inner wall. The specific function will be described later.

[0017] The screw 21 is a long rod-shaped component, coaxially arranged within the inner hole 220 of the nut 22 along the axial direction, and extends to the outside of the nut 22 to facilitate connection to a drive device or installation on other external mechanisms. The outer circumferential surface of the screw 21 has a threaded structure 211 (also called a helical groove or helical teeth), which can be a multi-start thread. In actual operation, the screw 21 can be driven to rotate by an external power source (not shown in the figure), and through the double-sided meshing between the threaded structure 211 and the roller assembly R, the roller assembly R simultaneously rotates and revolves between the screw 21 and the nut 22, forming planetary rolling. This motion mechanism converts the rotational motion of the screw 21 into axial linear motion.

[0018] Referring to Figures 2, 3A-3C, and 4, the roller assembly R is disposed in the inner hole 220 of the nut 22 and located between the nut 22 and the screw 21. It includes a plurality of staggered first rollers 23 and second rollers 24. These first rollers 23 and second rollers 24 are arranged around the outer periphery of the screw 21, staggered and spaced apart circumferentially. In this embodiment, the number of first rollers 23 and second rollers 24 is even, and they are symmetrically arranged at different circumferential angular positions on the outer periphery of the screw 21, forming rolling engagement with the corresponding thread structure 211.

[0019] The first roller 23 and the second roller 24 are cylindrical members extending axially, and have a plurality of annular teeth 23t and 24t on their outer circumferential surfaces for meshing with corresponding structures on the screw 21 and the nut 22. Specifically, each of the first roller 23 and the second roller 24 is divided axially into at least one first meshing section 231 and 241 and at least one second meshing section 232 and 242, wherein the diameter of the first meshing section 231 and 241 is larger than the diameter of the second meshing section 232 and 242. The annular teeth 23t and 24t of the first meshing sections 231 and 241 are used for rolling meshing with the thread structure 211 of the screw 21, and the annular teeth 23t and 24t of the second meshing sections 232 and 242 are used for rolling meshing with the annular groove 221g of the annular groove section 221 of the nut 22 (as shown in Figure 3B).

[0020] As shown in Figures 3A and 4, the first meshing sections 231 and 241 are located in the axial middle section of the first roller 23 and the second roller 24, respectively, and each end of them is provided with a second meshing section 232 and 242, thereby forming a double-sided rolling meshing relationship with the screw 21 and the nut 22. Furthermore, in order to avoid interference between the first meshing sections 231 and 241 and the inner wall of the nut 22, these sections are correspondingly arranged in the clearance groove section 223 of the nut 22, providing sufficient radial space so that it only contacts the screw 21 and participates in rolling.

[0021] The ends 233, 234, 243, and 244 of the first roller 23 and the second roller 24 do not have annular teeth 23t and 24t, and are fitted with the retainer assembly K. The axial length L1 of one end 233 of the first roller 23 is less than the axial length L2 of the other end 234. The axial length L4 of one end 244 of the second roller 24 is less than the axial length L3 of its corresponding end 243. Thus, an axial length difference is formed between the ends 233, 234, 243, and 244 of the first roller 23 and the second roller 24. The purpose of this design will be further explained below.

[0022] Referring again to Figures 2, 3A, and 6, the retainer assembly K includes a first retainer 25 and a second retainer 26, respectively disposed at both ends of the roller assembly R and fixed inside the nut 22. The first retainer 25 has a plurality of first positioning grooves 251 and at least one first mating portion 252, the first positioning grooves 251 being used to accommodate one end 233, 243 of the first rollers 23 and the second rollers 24. The second retainer 26 has a plurality of second positioning grooves 261 and at least one second mating portion 262, the second positioning grooves 261 being used to accommodate corresponding ends 234, 244. The first mating portions 252 and the second mating portions 262 are used to engage with the preload assembly P described later. The groove depths of the first and second positioning grooves 251 and 261 can be set to be the same, to accommodate the differences in axial lengths of the two ends 233, 234, 243, 244 of the first and second rollers 23 and 24, thereby forming a selective preload configuration. In other implementations, the difference in depth between the first and second positioning grooves 251 and 261 can be used to form the first and second gaps 31 and 32 by matching the same axial length at both ends 233, 234, 243 and 244 of the first roller 23 and the second roller 24, thereby achieving selective preload configuration.

[0023] As shown in Figures 4, 6, 7A, and 7B, this embodiment utilizes the difference in axial length between the two ends of the first and second rollers 23 and 24 to form a first gap 31 and a second gap 32 between the first and second retainers 25 and 26, respectively. Specifically, the axial length L1 of one end 233 of the first roller 23, which is pivotally connected to the first positioning groove 251, is less than the groove depth of the first positioning groove 251 and less than the axial length L3 of one end 243 of the second roller 24 (as shown in Figure 4). That is, one end 233 of the first roller 23 cannot be inserted to the bottom of the first positioning groove 251, and a first gap 31 is formed between it and the first retainer 25 (as shown in Figures 6 and 7A). Furthermore, the axial length L4 of one end 244 of the second roller 24, which is pivotally connected to the second positioning groove 261, is less than the groove depth of the second positioning groove 261 and less than the axial length L2 of the other end 234 of the first roller 23 on the same side (as shown in Figure 4). That is, one end 244 of the second roller 24 cannot be inserted into the bottom of the second positioning groove 261, and a second gap 32 is formed between it and the second retainer 26 (as shown in Figures 6 and 7B). The two gaps 31 and 32 are staggered at opposite ends of the roller group R, so that the different rollers are subjected to opposite forces, and a guideable preload transmission direction is established (as shown in Figure 6).

[0024] Referring to Figures 2 and 5, the preload assembly P includes a plurality of preload screws 27, for example, at least two, respectively disposed between the first retainer 25 and the second retainer 26, and at an angle offset from the roller assembly R. Each preload screw 27 has a threaded end 271 and a head end 272 at both ends. The threaded end 271 passes through the second mating portion 262 (e.g., through hole) of the second retainer 26 and is threaded to the first mating portion 252 of the first retainer 25, which has an internal thread that engages with the threaded end 271. The head end 272 abuts against the outside of the second mating portion 262 of the second retainer 26. By tightening the preload screws 27, the axial clamping force is adjusted to bring the first and second retainers 25 and 26 closer together, thereby clamping the roller assembly R.

[0025] As shown in Figures 5, 6, 7A, and 7B, tightening the preload screw 27 generates an axial clamping force, bringing the first retainer 25 and the second retainer 26 closer together and applying a clamping force to the roller assembly R. Due to the presence of the first gap 31 and the second gap 32, the axial preload generated by each preload screw 27 is selectively transmitted to the non-gap ends of the first and second rollers 23 and 24. Specifically, there is a first gap 31 between the first roller 23 and the first retainer 25 at one end 233, so the preload of the first retainer 25 will not act on the first roller 23, but will be directly transmitted from the second retainer 26, which is in close contact with the first roller 23, thereby pushing the first roller 23 to the left in the figure (towards the first retainer 25). The second roller 24 has a second gap 32 between its end 244 and the second retainer 26. Therefore, the preload of the second retainer 26 will not act on the second roller 24, but will be directly transmitted from the first retainer 25, which is in close contact with the second roller 24, thereby pushing the second roller 24 to the right in the figure (towards the second retainer 26). In this way, the different rollers are subjected to opposite forces, achieving a bidirectional axial preload effect, ensuring that each of the first and second rollers 23 and 24 can stably and tightly adhere between the screw 21 and the nut 22, maintaining pure rolling contact and avoiding slippage and free rotation.

[0026] Therefore, as shown in Figures 6, 8A and 8B, the first roller 23, which is pushed to the left (towards the first retainer 25), has its annular teeth 23t abutting against the right side of the threaded structure 211 of the screw 21 and the right side of the annular groove 221g of the nut 22, thus creating a stable meshing contact.

[0027] Similarly, as shown in Figures 6, 9A, and 9B, the second roller 24, pushed to the right (towards the second retainer 26), has its annular teeth 24t facing the left side of the threaded structure 211 of the screw 21 and the left side of the annular groove 221g of the nut 22, achieving stable contact on both sides with the screw 21 and the nut 22. This forms a stable rolling engagement, effectively preventing slippage and freewheeling. Thus, it not only compensates for the clearance G caused by manufacturing tolerances (see Figures 1B and 1C), but also suppresses any loosening that may exist during initial assembly, ensuring that the roller movement reaches a stable state from the start.

[0028] Although the foregoing embodiment indicates that the two preload screws 27 are locked from the second retainer 26 toward the first retainer 25, it is not limited thereto. Referring further to Figure 10, the two preload screws 27 can also apply axial clamping forces to the first and second retainers 25 and 26 from opposite directions. Specifically, the threaded end 271 of one preload screw 27 passes through the second mating portion 262 (e.g., a through hole) of the second retainer 26 and is threaded to the first mating portion 252 of the first retainer 25, which has an internal thread that engages with the threaded end 271. The head end 272 abuts against the outside of the second mating portion 262 of the second retainer 26. The threaded end 271 of the other preload screw 27 passes through the first mating portion 252 (e.g., a through hole) of the first retainer 25 and is threaded to the second mating portion 262 of the second retainer 26, which also has an internal thread that engages with the threaded end 271. The head end 272 abuts against the outer side of the first mating portion 252 of the first retainer 25. By tightening the preload screw 27 in the opposite direction to adjust its axial clamping force, the first and second retainers 25 and 26 are brought closer to each other, thereby clamping the roller assembly R, thereby achieving the above-mentioned function and effect.

[0029] In summary, this invention, by directly applying the preload assembly P between the two retainers 25 and 26, uses the preload screw 27 to generate an adjustable tightening force instead of traditional elastic elements or retainers. This not only allows for precise adjustment of the applied pressure but also simplifies component configuration and axial structural design. The structure of this invention eliminates the need for additional retainers to fix the axial position of the retainer assembly K, thus eliminating friction between the retainer assembly K and the retainer, avoiding efficiency losses and roller instability caused by friction interference, and contributing to improved system reliability and lifespan. Furthermore, the design of axial length differences between the opposite ends 233, 234, 243, 244 of the first and second rollers 23 and 24, or the design of groove depth differences between the first and second positioning grooves 251, 261 of the first and second retainers 25 and 26, creates staggered first and second gaps 31 and 32 between the first and second rollers 23 and 24 and the first and second retainers 25 and 26. This guides the axial preload to selectively act on specific rollers, thereby establishing a staggered bidirectional force application mechanism. This structure can not only effectively compensate for the gaps caused by machining tolerances and eliminate initial backlash, but also stabilize the movement trajectory of the first and second rollers 23 and 24, so that they always maintain a pure rolling state, effectively suppress sliding friction and idling, and improve rolling stability and overall transmission accuracy.

[0030] The above is a detailed description of the preferred embodiments of the present invention. Any equivalent or similar modifications made in accordance with the teachings disclosed in the present invention are naturally included within the protection scope of the present invention patent.

[0031] 21: Screw 211: Threaded structure 22: Nut 220:Inner hole 221: Annular Trough Section 221g: Annular groove 223: Avoidance section R: Roller assembly 23: First roller 231: First meshing section 232: Second meshing section 233: End 234: End 23t: Ring-shaped teeth 24: Second roller 241: First meshing section 242: Second meshing section 243: End 244: End 24t: Ring-shaped teeth K: Retaining component assembly 25: First retainer 251: First positioning slot 252: First Coordination Unit 26: Second retainer 261: Second positioning slot 262: Second Coordination Unit P: Preload assembly 27: Pre-tightening screws 271:Screw terminal 272: Head end 31: First gap 32: Second gap L1~L4: Axial length

Claims

1. An axial preload device for a roller screw cage, comprising: A screw, the outer circumferential surface of which is provided with a threaded structure; A nut has an inner hole fitted onto a screw, the inner side of which has at least one annular groove section; a roller assembly is disposed between the screw and the nut, having a plurality of staggered first rollers and second rollers, each of the first and second rollers having a plurality of annular teeth on its outer circumferential surface, respectively engaging with the thread structure and the annular groove section; a retainer assembly is disposed within the nut and located at both ends of the roller assembly, having a first retainer and a second retainer, the first retainer being connected to one end of the first and second rollers, the second retainer being connected to the opposite end of the first and second rollers, and the first rollers being positioned such that one end of the first roller is not directly connected to the first retainer. A first gap is provided, and a second gap is provided between one end of the second rollers and the second retainer. The first gap and the second gap are arranged at opposite ends of the roller assembly and are staggered. A preload assembly is provided between the first retainer and the second retainer to generate an adjustable clamping force to bring the first retainer and the second retainer closer to each other and clamp the roller assembly. In conjunction with the staggered first gap and the second gap, axial preload with opposite directions and adjustable preload is applied to the first rollers and the second rollers respectively, so that the roller assembly forms a staggered bidirectional axial preload to reduce the backlash between the screw and the nut and maintain pure rolling contact of the rollers.

2. The axial preload device for the retainer of the roller screw as claimed in claim 1, wherein the preload assembly includes at least two preload screws respectively disposed between the first retainer and the second retainer and parallel to the first roller and the second roller, each preload screw having a threaded end threaded to one of the first retainer and the second retainer, and a head end abutting against the other of the first retainer and the second retainer.

3. The axial preload device for the cage of the roller screw as described in claim 2, wherein: The first retainer is provided with a plurality of first positioning grooves and at least one first mating part. The first positioning grooves are used to accommodate one end of the first rollers and the second rollers. The at least one first mating part is used to accommodate the threaded end of the preload screw and is provided with an internal thread that mates with the threaded end. The second retainer is provided with a plurality of second positioning grooves and at least one second mating part. The second positioning grooves are used to accommodate the opposite end of the first rollers and the second rollers. The at least one second mating part corresponds to the head end of the preload screw.

4. The axial preload device for the cage of the roller screw as described in claim 3, wherein: The first roller, which is housed in the first positioning groove, has an axial length that is less than the length of its opposite end and does not contact the bottom of the first positioning groove to form the first gap; and the second roller, which is housed in the second positioning groove, has an axial length that is less than the length of its opposite end and does not contact the bottom of the second positioning groove to form the second gap.

5. The axial preload device for the retainer of the roller screw as described in claim 2, wherein the first retainer is provided with a plurality of first positioning grooves and at least one first mating portion, the second retainer is provided with a plurality of second positioning grooves and at least one second mating portion; and the threaded end of one preload screw passes through the second mating portion of the second retainer and is threaded to the first mating portion of the first retainer, and the head end abuts against the outside of the second mating portion of the second retainer; the threaded end of another preload screw passes through the first mating portion of the first retainer and is threaded to the second mating portion of the second retainer, and the head end abuts against the outside of the first mating portion of the first retainer.

6. The axial preload device for the retainer of the roller screw as described in claim 5, wherein the first retainer and the second retainer are respectively provided with an internal thread engaging the screw end at the first mating portion and the second mating portion corresponding to the screw end.

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