Nut structure in planetary roller screw device

Abstract

The invention relates to a nut structure in a planetary roller screw. A nut structure in a planetary roller screw device comprises a plurality of nuts which are coaxially arranged, the radiuses of the nuts are the same, a plurality of protrusions are arranged on the end face of one of the adjacent nuts, and a plurality of grooves matched with the protrusions are formed in the end face of the other nut. Adjacent nuts are welded after being positioned and connected through the matching structures of the grooves and the protrusions, the matching density of the protrusions and the grooves of the inner ring is high, and the matching density of the protrusions and the grooves of the outer ring is low. According to the nut structure in the planetary roller screw device, segmented manufacturing of a long nut can be met, the overall strength of the nut after segmented welding can be high, the anti-shearing capacity is close to that of base metal, stress concentration can be effectively eliminated, and stress distribution can be optimized; the technical problems that in a roller lead screw structure in the prior art, a long nut is inconvenient to manufacture, and the long nut subjected to segmented welding is unstable in performance, reduced in strength and concentrated in stress are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to a nut structure in a planetary roller screw. BACKGROUND

[0002] The standard planetary roller screw contains a screw, several rollers, a nut, an inner ring, a retainer and a circlip. The reverse planetary roller screw is becoming an ideal linear actuator for humanoid robot linear joint design due to its high rigidity, high power density, easy integration and high reliability. However, the long nut machining of the reverse planetary roller screw is very difficult due to the existing internal thread machining technology. The traditional integral long nut is difficult to achieve due to the limitation of the machining process, and the conventional segmented welding is prone to deformation, strength reduction and precision loss due to rigid contact, stress concentration and inaccurate positioning. SUMMARY

[0003] The present application provides a nut structure in a planetary roller screw device that can not only meet the segmented manufacturing of the long nut, but also achieve high overall strength of the nut after segmented welding, close to the shear capacity of the base material, and effectively eliminate stress concentration and optimize stress distribution. The technical problems of long nut manufacturing inconvenience, unstable performance of long nut after segmented welding, and strength reduction in the existing roller screw structure are solved.

[0004] The above technical problems of the present application are solved by the following technical scheme: a nut structure in a planetary roller screw device, characterized by: comprising a plurality of coaxially arranged nuts, each nut having the same radius, one of the adjacent nuts having a plurality of protrusions on the end face, and the other nut having a plurality of grooves matched with the protrusions on the end face. The adjacent nuts are connected by the matching structure of the grooves and the protrusions, and then welded. The uneven temperature field and the circumferential shrinkage stress field during the matching and welding process of the protrusions and the grooves, the matching density of the protrusions and the grooves of the inner ring, the matching of the protrusions and the grooves promoting the flow and exhaust of the molten pool, the matching density of the protrusions and the grooves of the outer ring being small, and the matching of the protrusions and the grooves as a stress buffer zone releasing the shrinkage stress in order. The matching of the grooves and the protrusions is not only for positioning or connection, but also for actively controlling welding deformation, optimizing stress distribution and realizing metallurgical-mechanical composite reinforcement through the design of the structure and size of the grooves and the protrusions. The micro-groove texture of the nut butt joint surface increases from inside to outside, which can more accurately match the uneven temperature field and stress field during welding. The high-density matching of the grooves and the protrusions of the inner ring can promote the flow and exhaust of the molten pool and prevent defects; the low-density matching of the grooves and the protrusions of the outer ring as a stress buffer zone releases the shrinkage stress in order, thereby actively inhibiting welding deformation and protecting the geometric integrity of the internal precision thread.

[0005] Preferably, the distance between the protrusions gradually increases from inside to outside, and the distance between the grooves gradually increases from inside to outside.

[0006] Preferably, the protrusions are closed protrusion rings, and the protrusion rings are parallel to each other and coaxial with the nut.

[0007] Preferably, the grooves are closed groove rings, and the groove rings are parallel to each other and coaxial with the nut.

[0008] The convenient processing can also make the connected end faces be connected and positioned more firmly.

[0009] Preferably, the protrusions have a U-shaped cross section. The protrusions (and the corresponding grooves) have a U-shaped design combining a vertical side surface and a large-radius circular arc bottom. The vertical side surface forms a large-area bonding surface perpendicular to the direction of the main shear force after welding, thereby maximizing the resistance to shear stress from the transmission torque and radial force. The large-radius circular arc bottom completely eliminates the stress concentration at the sharp corner, thereby fundamentally inhibiting the initiation of fatigue cracks under high-frequency alternating loads. This configuration is a special mechanical solution for the multi-directional composite load and high-cycle cyclic operation of the transmission nut.

[0010] Preferably, the protrusions and the grooves have a gap of 2-6 μm at room temperature. In the assembly stage, the self-guiding effect of the multiple layers of microstructures is used to realize rapid and high-precision self-centering pre-positioning, thereby ensuring the micro-alignment of the threads at both ends. In the welding stage, the protrusions are first expanded to eliminate the gap, thereby realizing the synchronous close contact and metallurgical bonding of the components, ensuring the assembly convenience and the strength of the joint close to the base material.

[0011] Therefore, the nut structure in the planetary roller screw device has the following advantages: the welding thermal stress and deformation are actively managed by the gradient micro-texture, thereby minimizing the precision loss of the inner thread raceway after welding; the unique interlocking cross-sectional shape and micro-gap thermal expansion bonding mechanism form a high-strength and high-rigidity metallurgical-mechanical composite joint, and the shear resistance is close to that of the base material; the fatigue life of the welded joint under high-frequency and variable-direction impact loads is greatly improved by the stress concentration-eliminating geometric design and optimized stress distribution, thereby meeting the reliability requirements of the robot joint in terms of ultra-long service life and maintenance-free. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 FIG. 1 is a schematic view of a nut structure in a planetary roller screw device.

[0013] Figure 2 FIG. 2 is an enlarged view of an end face of a first nut in the planetary roller screw device. Figure 1

[0014] Figure 3 FIG. 3 is an enlarged view of an end face of a second nut in the planetary roller screw device.​Figure 2 A schematic diagram of a single groove.

[0015] Figure 4 yes Figure 2 A schematic diagram of the end face.

[0016] Figure 5 yes Figure 1 Enlarged view of the end face of the second nut inside.

[0017] Figure 6 yes Figure 5 A schematic diagram of a single protrusion.

[0018] Figure 7 This is a schematic diagram of a nut used in a planetary roller screw assembly. Detailed Implementation

[0019] The technical solution of the invention will be further described in detail below through embodiments and in conjunction with the accompanying drawings.

[0020] Example: like Figure 1 As shown, a nut structure within a planetary roller screw device is used in a linear actuator for a humanoid robot joint that requires high rigidity and long lifespan to withstand high-frequency reciprocating motion. The total length of the nut in the planetary roller screw required for this actuator is 125mm. The nut includes a first nut 1 and a second nut 2. Both nut sections are of equal length and are made of high-strength alloy steel (such as GCr15) forgings. After rough turning, quenching and tempering heat treatment, and finish turning of the outer diameter and end face, internal thread raceways are machined on a precision CNC grinding machine, ensuring that the thread parameters of the two raceways are completely consistent.

[0021] like Figure 2 and 3 As shown in Figure 4, the first nut segment has six concentric circular grooves 3 machined on its mating end face. The innermost groove 3 is 0.5mm from the center hole wall, and the spacing between the rings (i.e., the groove density) from the inside to the outside is: 0.22mm, 0.25mm, 0.28mm, 0.31mm, and 0.34mm. A welding bevel 4 is machined on the upper end of the end face.

[0022] Gradient spacing sequence (0.22, 0.25, 0.28, 0.31, 0.34 mm): achieves "impedance matching" of the stress field.

[0023] Design principle: During welding, heat is conducted from the inside to the outside, forming a radial temperature gradient, which leads to uneven thermal expansion and cooling contraction, generating circumferential tensile stress. The intensity of this stress field increases from the inside to the outside.

[0024] Parameter association: Innermost ring spacing (0.22mm): This spacing is smaller than the width of the molten pool of a single laser scan (approximately 0.3mm), which allows this area to form a near-continuous metallurgical bond after welding, with the highest local stiffness and geometric stability, providing rigid support for the centering reference of the core thread area.

[0025] Gradient increment (0.03 mm): Determined through thermo-mechanical coupled finite element simulation optimization. This increment ensures smooth stiffness changes between adjacent rings, avoids secondary stress concentration caused by abrupt stiffness changes, and realizes the gradual and smooth attenuation and transmission of circumferential contraction stress from the outside to the inside.

[0026] The outermost spacing (0.34 mm): This spacing is greater than the characteristic shrinkage length of the material determined by thermo-mechanical coupling simulation under thermal cycling in this region, approximately 0.28 mm. The wider spacing forms a low-stiffness flexible region, like a "microscopic spring array," which can effectively absorb and dissipate the largest outer edge shrinkage strain energy, converting it into controllable plastic deformation in multiple micro-regions, thereby preventing macroscopic deformation.

[0027] The groove 3 has a U-shaped cross-section, a depth of 0.08mm, an opening width of 0.08mm, and a bottom arc with a radius of R0.04mm.

[0028] Achieving an optimal balance between strength and heat-affected zone (HAZ) is crucial. The cross-sectional dimensions determine the load-bearing area of ​​the mechanical interlock and the heat input required for welding. It is necessary to minimize the damage to the base material properties caused by the HAZ while ensuring sufficient connection strength. A tiny 0.08mm dimension controls the depth of the HAZ in laser processing within an extremely narrow range, maximizing the protection of the original metallographic structure and mechanical properties of the base material near the mating surfaces, and avoiding softening and deformation due to overheating.

[0029] like Figure 5 and 6 As shown, the second nut segment has six concentric circular protrusions 5 machined on its mating end face, perfectly complementing the groove. The cross-sectional design of the protrusions 5 is mirror-symmetrical to the groove, but its design dimensions at room temperature are 3μm smaller in one direction than the corresponding dimensions of the groove. That is, the height of the protrusions 5 is 3μm less than the depth of the groove 3, and the width of the protrusions is 3μm less than the width of the groove opening. This design ensures a uniform micro-clearance fit between the protrusions 5 and the groove 3 during assembly. A welding bevel is machined at the upper end of the end face of the second nut segment.

[0030] Calculations show that for protrusion 5 (GCr15 steel), the instantaneous expansion coefficient α≈11.5×10⁻ 6When the temperature rises from 20°C to near the melting point of approximately 1300°C, the radial thermal expansion on one side Δr ≈ α × r × ΔT ≈ 7.5 μm (where r is the effective radius of the protrusion). This expansion is greater than the initial gap δ (3 μm), thus achieving an automatic transition from "clearance fit" to "micro-interference contact" just before the molten pool forms. This dynamic tight contact ensures that the liquid metal can completely wet the entire interlocking interface, forming a defect-free metallurgical bond, which is a key prerequisite for obtaining a high-strength joint.

[0031] During assembly, in a clean assembly environment, the protrusion of the second nut segment is aligned with the groove of the first nut segment. Due to the 3μm micro-gap, the protrusion can easily and smoothly embed into the groove. Thanks to the self-centering effect of the multi-ring structure, the internal thread raceways of the two nut segments automatically align within seconds. Measurement with a coordinate measuring machine shows that the coaxiality error of the raceways of the two nut segments is less than 0.004mm, meeting the pre-assembly accuracy requirements.

[0032] Vacuum electron beam welding: The pre-positioned components are placed in a vacuum welding chamber. An electron beam welding machine is used to perform circumferential welding along the outermost circular path of the mating end faces.

[0033] Welding mechanism: The high energy input during welding causes the metal in the joint area to melt instantly. Due to its relatively small size, the protrusion expands first due to heat, quickly eliminating the 3μm micro-gap between it and the groove, achieving tight contact between the interfaces.

[0034] The role of microtexture: The dense grooves in the inner ring provide ample flow channels for molten metal and escape paths for gas, effectively preventing the formation of porosity and incomplete fusion defects. The sparse grooves in the outer ring act as stress buffer zones, orderly absorbing and releasing the circumferential shrinkage stress generated during welding cooling, significantly suppressing welding deformation.

[0035] Advantages of vertical sides: The vertical sides of the protrusions / grooves form a large metallurgical bonding surface perpendicular to the load direction after fusion, providing the joint with extremely high shear strength.

[0036] like Figure 7 As shown, when the nut of the present invention is applied to a planetary roller screw structure, firstly, multiple rollers 8 are precisely combined with the screw 6 in a circular array, and positioned and connected by a cage 7 to form a preliminary roller screw assembly. The cage 8 not only ensures that the rollers maintain a uniform spacing, but also guides the rollers to roll along the correct trajectory during movement.

[0037] Next, a retaining ring 9 (elastic retaining ring) is installed at the corresponding position of the lead screw 6 to achieve axial limiting, prevent relative displacement of the cage 7 in the axial direction, and ensure the stability of the component in subsequent assembly and operation.

[0038] Then, the assembled lead screw 6 and roller 8 sub-assemblies are screwed into the nut as a whole. The nut usually has threaded raceways that match the rollers. During the screwing process, it is necessary to keep it centered and steady to ensure that the rollers and nut raceways form a good meshing relationship.

[0039] After final assembly, the lead screw, rollers, and nut constitute a complete planetary roller screw transmission system, which can achieve high-precision, high-load conversion between rotary and linear motion.

[0040] The specific embodiments described herein are merely illustrative of the concept of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to replace them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A nut structure within a planetary roller screw assembly, characterized in that: It includes several coaxially arranged nuts, all with the same radius. One of the adjacent nuts has multiple protrusions on its end face, and the other nut has multiple grooves on its end face that mate with the protrusions. The adjacent nuts are positioned and connected by the mating structure of the grooves and protrusions before welding. The mating of the protrusions and grooves matches the uneven temperature field and circumferential shrinkage stress field during the welding process. The inner ring has a high mating density of protrusions and grooves, which promotes the flow of the molten pool and the venting of air. The outer ring has a low mating density of protrusions and grooves, which acts as a stress buffer zone to release shrinkage stress in an orderly manner.

2. The nut structure in a planetary roller screw device according to claim 1, characterized in that: The spacing between the protrusions gradually increases from the inside out, and the spacing between the grooves also gradually increases from the inside out.

3. The nut structure in a planetary roller screw device according to claim 1, characterized in that: The protrusion is a closed protrusion ring, with multiple protrusion rings parallel to each other, and all protrusion rings are coaxial with the nut.

4. The nut structure in a planetary roller screw device according to claim 1, characterized in that: The groove is a closed groove ring, with multiple groove rings parallel to each other, and all groove rings are coaxial with the nut.

5. A nut structure in a planetary roller screw device according to any one of claims 1 to 4, characterized in that: The cross-section of the protrusion is U-shaped.

6. A nut structure in a planetary roller screw device according to any one of claims 1 to 4, characterized in that: The protrusions and grooves maintain a clearance fit of 2-6 μm at room temperature.

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