Ball screw

Abstract

To provide a ball screw that is relatively easy to manufacture while having a high-precision circulation path. [Solution] The ball screw comprises a screw shaft having a helical external screw groove on its outer circumference, a nut having a helical internal screw groove on its inner circumference and fitted onto the screw shaft, and a plurality of balls rotatably housed in a rolling path formed by the external screw groove and the internal screw groove, wherein the screw shaft has a plurality of annular bodies on its outer circumference, each having a helical groove constituting the external screw groove and an S-shaped groove connecting both ends of the helical groove, and at least two of the S-shaped grooves are out of phase when viewed in the axial direction.

Description

Technical Field

[0001] The present invention relates to ball screws.

Background Art

[0002] A general ball screw includes a nut having an internal thread groove, a screw shaft having an external thread groove and penetrating the nut, a plurality of balls disposed between the internal thread groove and the external thread groove, and a circulation component for circulating the balls. As the ball circulation method, there are a return tube type, an end cap type, a disk type, and a guide plate type. For designing the outer diameter of the nut to be compact, the most advantageous circulation method is the disk type, and the disk type is often adopted for applications where the outer diameter is designed to be small.

[0003] In a disk type ball screw, fitting holes are formed in the cylindrical body portion of the nut so as to penetrate the inner and outer circumferences, and a disk member as a circulation component is fitted into the fitting holes. The disk member is a component that forms a connecting groove for connecting adjacent one-turn portions of the internal thread groove, and a closed circulation path in which the balls circulate infinitely is formed by attaching it to the fitting hole of the nut. A large number of balls intervening between the internal thread groove and the external thread groove in this circulation path roll along the thread groove and move to the end of the thread groove, and then are guided by the connecting groove of the disk member and return to the adjacent thread groove after overcoming the thread crest.

[0004] However, even in a disk type ball screw in which the outer diameter of the nut is relatively small, there is a demand for further compactification.

[0005] In contrast, Patent Document 1 discloses a ball screw device that constructs an infinite circulation path for balls without using circulating parts. In such a ball screw device, an S-shaped closed external screw groove is formed on the screw shaft. Furthermore, the screw shaft is constructed by combining multiple elements, and by changing only the number of intermediate elements sandwiched between a pair of end elements, it is possible to arbitrarily change the length of the load trajectory on which the balls roll while bearing a load, and it is said that the number of balls rolling along the load trajectory can be easily increased or decreased according to the required load bearing capacity. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 6928535 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, according to the ball screw device described in Patent Document 1, the mating surfaces of the end and intermediate elements have complex three-dimensional shapes, making them difficult to manufacture. In addition, because the mating surfaces are located at the bottom of the external screw groove, the accuracy of the circulating rolling path is easily affected by the accuracy of the mating surfaces, and there is a possibility that the contact angles between the balls and the left and right flanks of the screw groove will not be uniform. Therefore, there is a concern that it will be difficult to ensure the lead accuracy and operability of the ball screw.

[0008] This invention has been made in view of the above problems, and aims to provide a ball screw that is relatively easy to manufacture while having a high-precision circulation path. [Means for solving the problem]

[0009] The ball screw of the present invention is A screw shaft having a helical external screw groove on its outer surface, A nut having a helical internal thread groove on its inner circumferential surface and fitted onto the screw shaft, A ball screw comprising a plurality of balls rotatably housed in a rolling path formed by the external screw groove and the internal screw groove, The screw shaft has a plurality of annular bodies on its outer circumference, each having a helical groove that constitutes the external screw groove and an S-shaped groove that connects both ends of the helical groove. The present invention is characterized in that, when viewed in the axial direction, at least two of the S-shaped grooves are out of phase. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a ball screw that is relatively easy to manufacture while having a highly accurate circulation path. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a cross-sectional view showing a ball screw according to the first embodiment. [Figure 2] Figure 2 is a side view showing the screw shaft. [Figure 3] Figure 3 is a side view showing the nut. [Figure 4] Figure 4 is a perspective view showing the main parts of the screw shaft in an exploded view. [Figure 5] Figure 5 is a perspective view showing an enlarged portion of Figure 4. [Figure 6] Figure 6 is a side view of the ring-shaped body. [Figure 7] Figure 7 is a perspective view showing the main part of the screw shaft according to the second embodiment in an exploded view. [Figure 8] Figure 8 is a perspective view showing the main parts of the screw shaft according to the second embodiment in an assembled state. [Figure 9] Figure 9(a) is a front view of the annular body according to the second embodiment, and Figure 9(b) is a side view of the annular body. [Figure 10] Figure 10 is a perspective view showing the main part of the screw shaft according to the third embodiment in an exploded view. [Figure 11] Figure 11(a) is a front view of the screw shaft to which the annular body of the third embodiment is assembled, viewed in the axial direction, and Figure 11(b) is a side view of the three annular bodies.

Embodiment for Carrying out the Invention

[0012] (First Embodiment) Hereinafter, an embodiment of the present invention will be described in detail based on the drawings. FIG. 1 is a cross-sectional view showing a ball screw of the first embodiment. FIG. 2 is a side view showing the screw shaft 20. FIG. 3 is a side view showing the nut 10.

[0013] As shown in FIG. 1, this ball screw is composed of a nut 10, a screw shaft 20, and a plurality of balls (not shown). A continuous spiral groove (internal thread groove) 11 is formed on the inner peripheral surface of the nut 10 that is externally fitted to the screw shaft 20. Seal structures, lubrication holes, etc. are omitted here.

[0014] FIG. 4 is a perspective view showing the screw shaft 20 disassembled. FIG. 5 is a perspective view showing a part of FIG. 4 enlarged. In FIGS. 1, 2, 4, and 5, the screw shaft 20 has a base shaft 21, four annular bodies 22, 23, 24, 25, and a retaining pin 26.

[0015] The base shaft 21 has a large cylindrical portion 21a with a diameter smaller than the inner diameter of the nut 10, and a shaft 21b extending axially from the center of the end of the large cylindrical portion 21a. On the outer periphery of the cylindrical shaft 21b, four straight grooves 21c, 21d, 21e, 21f extending to the end along its axial direction are formed at equal intervals in the circumferential direction, and between the circumferentially adjacent straight grooves is a cylindrical surface with an outer diameter of φA. The cross-sectional shape of the straight grooves 21c, 21d, 21e, 21f in the direction orthogonal to the axis is a common rectangular cross-sectional shape.

[0016] In the vicinity of the end of the shaft 21b, a through hole 21g with a circular cross-section penetrating the shaft 21b in a direction orthogonal to the axis is formed. The inner diameter of the through hole 21g is φB. Also, the distance from the end of the large cylindrical portion 21a to the through hole 21g is Δ1.

[0017] The retaining pin 26 has a cylindrical shape, and if its outer diameter is φC, then φC = φB. The total length of the retaining pin 26 is longer than the outer diameter of the shaft 21b, but shorter than the outer diameter of the annular body 22, etc., which will be described later.

[0018] Since ring bodies 22, 23, 24, and 25 share a common shape, only ring body 22 will be described here, and the descriptions of the other ring bodies will be omitted.

[0019] Figure 6 is a side view of the annular body 22. The annular body 22, which has a cylindrical inner and outer circumference, has a helical groove 22a on its outer circumference and an S-shaped groove 22b connecting both ends of the helical groove 22a. The helical groove 22a constitutes the external screw groove. The S-shaped groove 22b has a three-dimensional shape in which it gradually moves away from the outer circumference of the annular body 22 as it moves away from one end of the helical groove 22a, approaches the inner circumference of the annular body 22 closest at the center P of the S-shaped groove 22b, and then gradually approaches the outer circumference of the annular body 22 as it approaches the other end of the helical groove 22a. The S-shaped groove 22b is formed to a depth that allows it to overcome the land portion of the internal screw groove 11 at and around the center P. The cross-sectional shapes of the helical groove 22a and the S-shaped groove 22b are approximately semicircular, the same as the cross-sectional shape of the internal screw groove 11.

[0020] Here, the center P of the S-shaped groove 22b corresponds to the center of the corresponding helical groove 22a across the axis, and the axial center of the annular body 22. The helical groove 22a has the same lead and pitch as the internal thread groove 11 of the nut 10. Furthermore, by offsetting the position of the helical groove 22a relative to the annular body 22 from the center (i.e., shifting the center P from a position T / 2 away from the end face of the annular body 22), the helical groove 22a is shifted axially relative to the internal thread groove 11 of the nut 10, thereby providing an offset preload.

[0021] When the lead of the helical groove 22a is L, the axial thickness T of the annular body 22 is set by (an integer multiple of L) + (an integer multiple of L) / (number of annular bodies). Here, since four annular bodies are used, the thickness T is (an integer multiple of L) + (an integer multiple of L) / 4.

[0022] Furthermore, the annular body 22 has a central opening 22c. The inner circumference of the central opening 22c has a convex portion 22d that protrudes radially at one point in the circumferential direction (see Figure 5). The shape of the convex portion 22d as viewed from the axial direction is approximately equal to the cross-sectional shape of the straight groove 21c. If the inner diameter of the cylindrical inner surface other than the convex portion 22d is φD, then φD = φA.

[0023] The annular bodies 23, 24, and 25, like the annular body 22, each have helical grooves 23a, 24a, and 25a that constitute an external screw groove, S-shaped grooves 23b, 24b, and 25b, a central opening 23c, 24c, and 25c, and protrusions 23d, 24d, and 25d, respectively.

[0024] The assembly method for the screw shaft 20 will now be explained. First, the end of the shaft 21b of the base shaft 21 is sequentially inserted into the central openings 22c, 23c, 24c, and 25c of the annular bodies 22, 23, 24, and 25. Specifically, the convex portion 22d of the annular body 22 is engaged with the straight groove 21c of the shaft 21b, the central opening 22c is fitted onto the outer surface of the shaft 21b, and the annular body 22 is slid along the shaft 21b until it contacts the base shaft 21.

[0025] Next, the convex portion 23d of the annular body 23 is engaged with the straight groove 21d of the shaft 21b, the central opening 23c is fitted onto the outer circumferential surface of the shaft 21b, and the annular body 23 is slid along the shaft 21b until it contacts the annular body 22.

[0026] Next, the convex portion 24d of the annular body 24 is engaged with the straight groove 21e of the shaft 21b, the central opening 24c is fitted onto the outer circumferential surface of the shaft 21b, and the annular body 24 is slid along the shaft 21b until it contacts the annular body 23.

[0027] Next, the convex portion 25d of the annular body 25 is engaged with the straight groove 21f of the shaft 21b, the central opening 25c is fitted onto the outer circumferential surface of the shaft 21b, and the annular body 25 is slid along the shaft 21b until it contacts the annular body 24.

[0028] Since the straight grooves 21c, 21d, 21e, and 21f are arranged on the outer circumference of the shaft 21b with a 90-degree phase in a counterclockwise direction when viewed from the left in Figure 5, the annular bodies 22, 23, 24, and 25 are arranged such that the positions of the S-shaped grooves 22b, 23b, 24b, and 25b are sequentially shifted by a 90-degree phase around the axis of the screw shaft 20.

[0029] During the operation of the ball screw, in the annular bodies 22, 23, 24, and 25, the screw shaft 20 receives a load via the balls in the helical grooves 22a, 23a, 24a, and 25a, while the S-shaped grooves 22b, 23b, 24b, and 25b are unloaded areas.

[0030] According to this embodiment, as described above, by arranging the annular bodies 22, 23, 24, and 25 in a circular equidistant arrangement with their phases shifted by 90 degrees each, it is possible to avoid the alignment of the unloaded zone (a bias of the unloaded zone relative to the loaded zone when viewed from the perspective of the entire ball screw 20) that may occur if the phases were aligned, thereby improving the overall load balance and operability during ball screw operation.

[0031] Furthermore, since the inner diameter φD of the central openings 22c, 23c, 24c, and 25c is equal to the outer diameter φA of the shaft 21b, there is no play between the central openings 22c, 23c, 24c, and 25c and the shaft 21b when fitted together.

[0032] When the annular body 25 is assembled to the shaft 21b, the distance from the end face of the base shaft 21 to the end face of the annular body 25, which is spaced apart from it, is 4 × T = Δ1. Therefore, the end face of the annular body 25 coincides with the edge of the through hole 21g. A retaining pin 26 is then press-fitted into the through hole 21g. The inner diameter φB of the through hole 21g and the outer diameter φC of the retaining pin 26 are equal, and after the retaining pin 26 is press-fitted into the through hole 21g, it is fixed in place by an interference fit. The retaining pin 26 protruding from the through hole 21g prevents the annular body 25 from coming loose.

[0033] Subsequently, when the screw shaft 20 is assembled to the nut 10, since the thickness T of each of the four annular bodies is (an integer multiple of L) + (an integer multiple of L) / 4, the internal screw groove 11 and the helical grooves 22a, 23a, 24a, and 25a can be positioned opposite the internal screw groove 11. At this time, a first circulation path is formed by the threads of the nut 10 and the S-shaped groove 22b, and the ends of the first rolling path formed by the helical groove 22a and the internal screw groove 11 are connected by the first circulation path. Furthermore, a second circulation path is formed by the threads of the nut 10 and the S-shaped groove 23b, and the ends of the second rolling path formed by the helical groove 23a and the internal screw groove 11 are connected by the second circulation path. In addition, a third circulation path is formed by the threads of the nut 10 and the S-shaped groove 24b, and the ends of the third rolling path formed by the helical groove 24a and the internal screw groove 11 are connected by the third circulation path. Furthermore, the threads of the nut 10 and the S-shaped groove 25b form a fourth circulation path, and the ends of the fifth rolling path, formed by the helical groove 25a and the internal thread groove 11, are connected by the fourth circulation path. As for the assembly of the balls, this can be done by loading them into each circulation path through a hole in the nut 10 (not shown) and then sealing the hole.

[0034] According to this embodiment, the rotational constraint of each annular body 22, 23, 24, 25 with respect to the base shaft 21 is achieved by the engagement of the straight grooves 21c, 21d, 21e, 21f with the protrusions 22d, 23d, 24d, 25d, and the axial constraint is achieved by a retaining pin 26 fitted into the through hole 21g. However, the rotational and axial constraints are not limited to this method and can also be applied by shrink-fitting the annular bodies or by providing a lock nut at the shaft end.

[0035] When the screw shaft 20 and the nut 10 are rotated relative to each other, the balls that have rolled in each helical groove enter the S-shaped groove from the other end of the helical groove in each circulation path, and are returned to the other end of each helical groove via these grooves, thereby enabling relative rotation between the screw shaft 20 and the nut 10 with low friction.

[0036] According to this embodiment, a separate circulating member is not required, and an independent circulating path can be constructed by attaching annular bodies 22, 23, 24, and 25, each having an S-shaped groove, to the screw shaft 20. Such a ball screw is simple because it does not require shielding parts or the like. Furthermore, because it uses annular bodies as a single block with a high-precision circulating path integrally formed on the outer circumference (without divisions), it has high reliability as a rolling path.

[0037] While directly forming each circulation path on the axis is one option, creating S-shaped grooves with phase shifts is difficult to process. In contrast, by assembling annular bodies with common S-shaped grooves with phase shifts to form a screw shaft, a ball screw with excellent machinability can be realized. In this embodiment, since four annular bodies are used, the thickness T of each annular body is set to (integer multiple of L) + (integer multiple of L) / 4. However, if, for example, three annular bodies are used, the thickness T of each annular body will be (integer multiple of L) + (integer multiple of L) / 3.

[0038] According to this embodiment, by changing the number of annular bodies with a common shape, it is possible to accommodate ball screw specifications and realize a multi-row ball screw with a highly reliable rolling path. Furthermore, since a single annular body is produced and used as a common part, it offers excellent low cost and mass-producibility.

[0039] In other words, this embodiment realizes a ball screw with a simple structure that does not require circulating parts and has a highly reliable circulation path, while also reducing the cost of the ball screw by using annular bodies as common parts and ensuring high mass production efficiency.

[0040] Using annular bodies as common parts is preferable due to the significant cost advantages. However, it is also acceptable to use annular bodies with varying lengths and number of rows, rather than using completely common parts. Processing methods for these annular bodies include forging, pressing, sintering, and 3D printing; any method that allows for precise and mass production is acceptable. Such processing methods eliminate the need for post-polishing of the annular bodies, maximizing the cost advantages of ball screws. While annular bodies can also be machined using a lathe or machining center, it is important to produce them precisely and in large quantities using specialized machinery.

[0041] This embodiment and the following embodiments are applicable to applications in which the nut 10 is fixed and rotatable in the axial direction, and the screw shaft 20 is moved relative to it, or to applications in which the screw shaft 20 is fixed and rotatable in the axial direction, and the nut 10 is moved relative to it.

[0042] (Second embodiment) The ball screw of the second embodiment will be described below. Figure 7 is a perspective view showing the main parts of the screw shaft 20A according to the second embodiment in an exploded view. Figure 8 is a perspective view showing the main parts of the screw shaft 20A according to the second embodiment in an assembled state. In the second embodiment, only the screw shaft 20A is different, and the other components are the same as in the first embodiment, so their explanation is omitted.

[0043] The screw shaft 20A has a base shaft 21A, four annular bodies 22A, 23A, 24A, and 25A, and a retaining pin 26 similar to that of the first embodiment.

[0044] The base shaft 21A has a large cylindrical portion 21a similar to that of the first embodiment, and a shaft 21Ab extending axially from the center of the end of the large cylindrical portion 21a. The outer circumference of the shaft 21Ab has a first surface 21Ah and a second surface 21Ai that are symmetrical with respect to its axis and parallel to each other, and the rest is a cylindrical surface with an outer diameter φA.

[0045] Furthermore, a circular through-hole 21g is formed near the end of the shaft 21Ab, penetrating the shaft 21Ab in a direction perpendicular to the axis.

[0046] The annular bodies 22A, 23A, 24A, and 25A differ only in the shape of their central openings 22Ac, 23Ac, 24Ac, and 25Ac; their other configurations are the same as those of the first embodiment, so a detailed explanation is omitted. The shapes of the annular bodies 22A, 23A, 24A, and 25A are common to each other.

[0047] The cross-sectional shape of the central openings 22Ac, 23Ac, 24Ac, and 25Ac in the direction perpendicular to the axis has a so-called D-shape, with a straight line connecting them in a chordal manner at one point in the circumferential direction. This connected portion is designated as the connecting surface 22Ad, 23Ad, 24Ad, and 25Ad, while the inner circumference of the remaining central openings is cylindrical with an inner diameter of φD, where φD = φA.

[0048] The assembly method for the screw shaft 20A will now be explained. First, the end of the shaft 21Ab of the base shaft 21A is sequentially inserted into the central openings 22Ac, 23Ac, 24Ac, and 25Ac of the annular bodies 22A, 23A, 24A, and 25A. Specifically, the connecting surface 22Ad of the annular body 22A is engaged with the first surface 21Ah of the shaft 21Ab, the central opening 22Ac is fitted onto the outer surface of the shaft 21Ab, and the annular body 22A is slid along the shaft 21Ab until it contacts the base shaft 21.

[0049] Next, the connecting surface 23Ad of the annular body 23A is engaged with the second surface 21Ai of the shaft 21Ab, the central opening 23Ac is fitted onto the outer circumferential surface of the shaft 21Ab, and the annular body 23A is slid along the shaft 21Ab until it comes into contact with the annular body 22A.

[0050] Next, the connecting surface 24Ad of the annular body 24A is engaged with the first surface 21Ah of the shaft 21Ab, the central opening 24Ac is fitted onto the outer circumferential surface of the shaft 21Ab, and the annular body 24A is slid along the shaft 21Ab until it comes into contact with the annular body 23A.

[0051] Next, the connecting surface 25Ad of the annular body 25A is engaged with the second surface 21Ai of the shaft 21Ab, the central opening 25Ac is fitted onto the outer surface of the shaft 21Ab, and the annular body 25A is slid along the shaft 21Ab until it contacts the annular body 24A. After that, the retaining pin 26 is pressed into the through hole 21g to complete the screw shaft 20A.

[0052] (Regarding loaded and unloaded areas) Figure 9(a) is a front view of the annular body 22A of the second embodiment, and Figure 9(b) is a side view of the annular body 22A. The load zone and unload zone will be explained below using the annular body 22A as an example, but the same applies to the other annular bodies 23A, 24A, 25A, and the annular bodies 22, 23, 24, 25 of the first embodiment.

[0053] The annular body 22A is formed by creating a helical groove 22Aa and an S-shaped groove 22Ab on its outer circumference. In Figure 9(a), the ends of the S-shaped groove 22Ab connected to both ends of the helical groove 22Aa are denoted as E1 and E2, and the two straight lines connecting the center O of the annular body 22A to the ends E1 and E2 are denoted as L1 and L2. The angle between the straight lines L1 and L2 that encloses the helical groove 22Aa is denoted as θ1, and the angle between the straight lines L1 and L2 that encloses the S-shaped groove 22Ab is denoted as θ2.

[0054] As is clear from Figure 9(a), the depth from the outer circumference of the annular body 22A to the bottom of the helical groove 22Aa is greater than the depth from the bottom of the helical groove 22Aa to the bottom of the S-shaped groove 22Ab. Therefore, the balls rolling in the helical groove 22Aa are subjected to a load from the internal thread groove 11 of the nut 10, while the balls rolling in the S-shaped groove 22Ab are not subjected to a load from the internal thread groove 11 of the nut 10. Thus, in this example, the load area is within the range of angle θ1, and the unloaded area is within the range of angle θ2. The shapes of the helical groove and the S-shaped groove are the same in other embodiments.

[0055] According to this embodiment, the phase of the S-shaped grooves of the annular bodies 22A and 24A and the phase of the S-shaped grooves of the annular bodies 23A and 25A are shifted by 180 degrees around the axis of the screw shaft 20A, thereby suppressing bias in the unloaded area.

[0056] By arranging the S-shaped grooves with a 180-degree phase shift, the number of circulation paths for the screw shaft can be freely adjusted. Furthermore, by using multiple identical annular bodies, a ball screw with any number of circulation paths can be obtained, enabling design standardization. In addition, to reduce lead errors caused by the assembly of the annular bodies, the ball screw's lead can be broken in during shrink-fitting by placing it on a nut with a standard lead or a V-block with balls matching the lead pitch. This improves lead accuracy and enhances the fit with the nut.

[0057] (Third embodiment) The following describes a ball screw according to a third embodiment. Figure 10 is a perspective view showing the main parts of the screw shaft 20B according to the third embodiment in an exploded view. In the third embodiment, only the screw shaft 20B is different; the other components are the same as in the first embodiment, so their explanation is omitted.

[0058] The screw shaft 20B has a base shaft 21B, three annular bodies 22B, 23B, and 24B, and a retaining pin (not shown) similar to that of the first embodiment.

[0059] The base shaft 21B has a large cylindrical portion 21a similar to that of the first embodiment, and a shaft 21Bb extending axially from the center of the end of the large cylindrical portion 21a. On the outer circumference of the shaft 21Bb, a first surface 21Bh, a second surface 21Bi, and a third surface 21Bj are formed at equal intervals in the circumferential direction, and the rest is a cylindrical surface with an outer diameter φA.

[0060] Furthermore, a circular through-hole 21g is formed near the end of the shaft 21Bb, penetrating the shaft 21Bb in a direction perpendicular to the axis.

[0061] The annular bodies 22B, 23B, and 24B differ only in the shape of their central openings 22Bc, 23Bc, and 24Bc, and their other configurations are the same as those of the first embodiment, so their description is omitted. Furthermore, the shapes of the annular bodies 22B, 23B, and 24B are mutually identical.

[0062] The cross-sectional shape of the central openings 22Bc, 23Bc, and 24Bc is a so-called D-shape, with a straight, chordal connection at one point in the circumferential direction. This connected portion is designated as the connecting surface 22Bd, 23Bd, and 24Bd, while the inner circumference of the remaining central openings is cylindrical with an inner diameter of φD, where φD = φA.

[0063] In this embodiment, since three annular bodies are used, the thickness T of the annular bodies 22B, 23B, and 24B is (an integer multiple of L) + (an integer multiple of L) / 3. Also, when the annular body 24B is assembled to the shaft 21Bb, the distance from the end face of the base shaft 21 to the end face of the annular body 24B that is spaced apart from it is 3 × T = Δ1.

[0064] The assembly method for the screw shaft 20B will now be explained. First, the end of the shaft 21Bb of the base shaft 21B is sequentially inserted into the central openings 22Bc, 23Bc, and 24B of the annular bodies 22B, 23B, and 24B. Specifically, the connecting surface 22Bd of the annular body 22B is engaged with the first surface 21Bh of the shaft 21Bb, the central opening 22Bc is fitted onto the outer surface of the shaft 21Bb, and the annular body 22B is slid along the shaft 21Bb until it contacts the base shaft 21B.

[0065] Next, the connecting surface 23Bd of the annular body 23B is engaged with the second surface 21Bi of the shaft 21Bb, the central opening 23Bc is fitted onto the outer circumferential surface of the shaft 21Bb, and the annular body 23B is slid along the shaft 21Bb until it comes into contact with the annular body 22B.

[0066] Next, the connecting surface 24Bd of the annular body 24B is engaged with the third surface 21Bj of the shaft 21Bb, the central opening 24Bc is fitted onto the outer surface of the shaft 21Bb, and the annular body 24B is slid along the shaft 21Bb until it contacts the annular body 23B. After that, the retaining pin is pressed into the through hole 21g to complete the screw shaft 20B.

[0067] According to this embodiment, the phases of the S-shaped grooves of the annular bodies 22B, 23B, and 24B are shifted by 120 degrees each around the axis of the screw shaft 20B, thereby suppressing bias in the unloaded area.

[0068] (Regarding loaded and unloaded areas) Figure 11(a) is a front view of the screw shaft 20B, to which the annular bodies 22B, 23B, and 24B of the third embodiment are assembled, viewed in the axial direction, and Figure 11(b) is a side view of the annular bodies 22B, 23B, and 24B. The following explanation will use the annular body 22B as an example to describe the load zone and the unload zone, but the same applies to the other annular bodies 23B and 24B.

[0069] The unloaded areas corresponding to the ranges of the S-shaped grooves 22Bb, 23Bb, and 24Bb each correspond to the range of angle θ2, and the ranges other than angle θ2 constitute the loaded areas. As in this embodiment, by attaching the three annular bodies 22B, 23B, and 24B to the shaft 21Bb with a phase difference, the S-shaped grooves 22Bb, 23Bb, and 24Bb can be equally spaced in a circle with a 120-degree shift in each. In this case, by setting the angle θ2 to less than 120 degrees, it is possible to set it so that none of the unloaded areas overlap when viewed in the axial direction, as shown in Figure 11(a). That is, in annular bodies of a number that is a multiple of 3 (where n is an integer of 1 or more), when S-shaped grooves are evenly spaced in the circumferential direction, it is desirable that the angle θ2 that constitutes the range of the S-shaped grooves is less than 120 degrees.

[0070] With this configuration, when an axial external load F acts between the nut 10 and the screw shaft 20B, the axial external load F can be evenly and balanced by the three helical grooves 22Ba, 23Ba, and 24Ba via the ball. This suppresses the tilting of the nut 10 relative to the screw shaft 20B, and as a result, premature wear and damage due to uneven loading can be prevented, leading to improved operability and extended lifespan of the ball screw.

[0071] Following a similar principle, in the first embodiment, when S-shaped grooves are evenly distributed circumferentially in annular bodies of a number that is a multiple of 4 (where n is an integer greater than or equal to 1), it is desirable that the angle θ2 that constitutes the S-shaped groove be less than 90 degrees. Also, in the second embodiment, when annular bodies of a number that is a multiple of 2 (where n is an integer greater than or equal to 1) are evenly distributed alternately with a phase shift of 180 degrees, it is desirable that the angle θ2 that constitutes the S-shaped groove be less than 180 degrees.

[0072] The present invention is not limited to the embodiments described above. Within the scope of the present invention, any component of the embodiments described above can be modified. Furthermore, any component can be added to or omitted in the embodiments described above. [Explanation of Symbols]

[0073] 10: Nut 20, 20A, 20B: Screw shaft 21, 21A, 21B: Base axis 21b, 21Ab, 21Bb: Shaft 22, 22A, 22B: Ring bodies 23, 23A, 23B: Ring bodies 24, 24A, 24B: Ring bodies 25,25A: Ring-shaped body 26: Retaining pin

Claims

1. A screw shaft having a helical external screw groove on its outer surface, A nut having a helical internal thread groove on its inner circumferential surface and fitted onto the screw shaft, A ball screw comprising a plurality of balls rotatably housed in a rolling path formed by the external screw groove and the internal screw groove, The screw shaft has a plurality of annular bodies on its outer circumference, each having a helical groove that constitutes the external screw groove and an S-shaped groove that connects both ends of the helical groove. Viewed in the axial direction, at least two of the S-shaped grooves are out of phase. A ball screw characterized by the following features.

2. When the thickness of the annular body in the axial direction of the screw shaft is T, and the pitch of the internal screw groove is L, T = (an integer multiple of L) + (an integer multiple of L) / (the number of ring bodies) It can be expressed as, The ball screw according to feature 1.

3. The screw shaft has a number of common annular bodies that are n times 4 (where n is an integer greater than or equal to 1), and adjacent annular bodies are arranged with a 90-degree phase shift. Viewed in the axial direction, the angle between the two lines connecting the center of the screw shaft and both ends of the S-shaped groove, which straddle the S-shaped groove, is less than 90 degrees. The ball screw according to feature 1.

4. The screw shaft has a number of common annular bodies that are n times 2 (where n is an integer greater than or equal to 1), and adjacent annular bodies are arranged with a 180-degree phase shift. Viewed in the axial direction, the angle between the two lines connecting the center of the screw shaft and both ends of the S-shaped groove, which straddle the S-shaped groove, is less than 180 degrees. The ball screw according to feature 1.

5. The screw shaft has a number of common annular bodies that are n times 3 (where n is an integer greater than or equal to 1), and adjacent annular bodies are arranged with a phase shift of 120 degrees. Viewed in the axial direction, the angle between the two lines connecting the center of the screw shaft and both ends of the S-shaped groove, and the S-shaped groove itself, is less than 120 degrees. The ball screw according to feature 1.

6. The screw shaft has a base shaft and a shaft extending axially from the base shaft. The aforementioned shaft has a plurality of straight grooves on its outer circumference, Each annular body has a protrusion on its inner circumference that engages with the straight groove, When the annular body is superimposed and fitted onto the shaft, the protrusions of adjacent annular bodies engage with different linear grooves. A ball screw according to any one of claims 1 to 5.

7. The axial center position of the annular body and the axial center position of the helical groove coincide. The ball screw according to feature 6.

8. The axial center position of the helical groove is different from the axial center position of the annular body. The ball screw according to feature 6.

9. Viewed in the axial direction, the straight grooves are equally spaced around the axis. The ball screw according to feature 6.

Images