Ball screw

Abstract

A nut of this ball screw includes a nut body and a deflector (12) attached to the nut body. The deflector (12) is disposed in an accommodation hole recessed outward in a radial direction from an inner circumferential surface of the nut. The deflector (12) includes a deflector body (30) having a circulation path (33), a protruding portion (31), and a relief portion (32). The protruding portion (31) protrudes in a direction intersecting one side surface (41) of the deflector body (30), and is locked to the nut body to restrict the deflector (12) from falling out from the accommodation hole in the radial direction. The relief portion (32) is provided at a position corresponding to a proximal end (39) of the protruding portion (31), and is recessed further toward the deflector body (30) side than the one side surface (41) of the deflector body (30). A chamfered portion (34) is provided between the proximal end (39) of the protruding portion (31) and the relief portion (32).

Description

Ball screw

[0001] The present invention relates to a ball screw. This application claims priority based on Japanese Patent Application No. 2024-160114 filed on September 17, 2024, and incorporates its content herein.

[0002] Conventionally, in a ball screw having a nut, a screw shaft, and a plurality of balls (rolling elements), as one of the circuit structures for circulating the plurality of balls, a structure is known that includes a groove for a return path (circulation path) formed on the inner peripheral surface of the nut separately from the groove for the main path (rolling path).

[0003] For example, Patent Document 1 discloses a ball screw including a screw shaft, a plurality of rolling elements, and a nut having a cam in which a circulation path for returning the rolling elements from the end point to the start point of the rolling path is formed. The cam is inserted and disposed from the inner diameter side of the nut into a through hole penetrating the nut in the radial direction. Further, the cam has a protrusion protruding in the width direction of the circulation path, and this protrusion is disposed in a recess formed on the inner peripheral surface of the nut. Furthermore, an outer fitting member is attached to the outer peripheral portion of the nut so as to cover the through hole, and the outer fitting member and the above-described protrusion suppress the cam from coming off radially outward. According to the technique described in Patent Document 1, a gap is formed between the protrusion of the cam and the recess of the nut such that the back portion (the portion facing the outer side in the radial direction) of the cam contacts the outer fitting member, so that the force for pushing up the cam by the rolling elements in the circulation path is not input to the protrusion of the cam. Thereby, it is said that compared with the conventional cam-type ball screw, damage is less likely to occur in the protrusion which is the retaining portion of the cam, and the cam can be made less likely to fall out of the nut.

[0004] Japanese Unexamined Patent Application Publication No. 2015-137743

[0005] However, in the technology described in Patent Document 1, the ball bearing alone cannot withstand the force exerted by the rolling elements in the circulation path to push the ball bearing upwards, so an external fitting member is provided as a backup. For this reason, the installation of the external fitting member is essential, which could increase the number of parts. An increase in the number of parts could increase the manufacturing cost. Furthermore, in conventional ball bearing type ball screws that do not have an external fitting member, interference between the ball bearing and the nut body due to the rolling elements pushing the ball bearing upwards could cause the projection on the ball bearing to break and the ball bearing to fall off the nut.

[0006] Therefore, in the prior art described in Patent Document 1, etc., there was room for improvement in a ball screw equipped with a nut having a spool in which a circulation path is formed, in order to suppress the increase in the number of parts and thus the increase in cost, and in suppressing the detachment of the spool compared to the prior art.

[0007] Therefore, the present invention aims to provide a ball screw that suppresses cost increases by limiting the number of parts, and that can suppress the detachment of the bearings compared to the conventional technology.

[0008] To solve the above problems, this invention proposes the following means. A ball screw according to a first aspect of the present invention comprises a screw shaft having a helical outer circumferential rolling groove on its outer circumferential surface, a nut having a helical inner circumferential rolling groove on its inner circumferential surface, and a plurality of rolling elements arranged in a rolling path formed by the inner circumferential rolling groove of the nut and the outer circumferential rolling groove of the screw shaft, wherein the nut comprises a nut body and a spool attached to the nut body that forms a circulation path returning the rolling elements from one end to the other of the rolling path, wherein the spool is arranged in a housing hole recessed radially outward from the inner circumferential surface of the nut, and the spool comprises a spool body having the circulation path, a projection that protrudes in a direction intersecting one side surface of the spool body and is locked to the nut body to prevent it from falling out radially outward from the housing hole, and a relief portion provided at least at a position corresponding to the base end of the projection and recessed toward the spool body side than the one side surface, wherein a chamfered portion is provided between the base end of the projection and the relief portion.

[0009] The present invention provides a ball screw that suppresses cost increases by limiting the number of parts, and also suppresses the detachment of the bearings compared to the conventional technology.

[0010] Cross-sectional view of a ball screw according to the first embodiment. Cross-sectional perspective view of a nut according to the first embodiment. Perspective view of a spindle according to the first embodiment. Front view of a spindle according to the first embodiment. View of the nut with the spindle attached, seen from the radial outside. Enlarged view of part VI in Figure 2. Enlarged view showing the projection of a spindle according to the first modification of the first embodiment. Enlarged view showing the projection of a spindle according to the second modification of the first embodiment. Perspective view of a spindle according to the third modification of the first embodiment. Perspective view of a spindle according to the second embodiment. Enlarged view of the projection of a spindle according to the second embodiment, seen from a different angle.

[0011] Embodiments of the present invention will be described below with reference to the drawings. In the following description, unless otherwise specified, the axial, radial, and circumferential directions refer to the axial, radial, and circumferential directions with respect to the central axis C of the ball screw 1.

[0012] (First Embodiment) Figure 1 is a cross-sectional view of a ball screw 1 according to the first embodiment. Figure 2 is a cross-sectional perspective view of a nut 3 according to the first embodiment. In Figure 1, a part of the inner circumferential rolling groove 7 formed in the nut 3 is omitted from the illustration, but in reality, in this embodiment, the inner circumferential rolling groove 7 is formed over the entire axial direction of the nut 3. The inner circumferential rolling groove 7 may be formed over the entire axial direction of the nut 3 as in this embodiment, or it may be formed only in predetermined locations as shown in Figure 1. The ball screw 1 is a device that converts rotational motion into linear motion. As shown in Figure 1, the ball screw 1 comprises a screw shaft 2, a nut 3, and a plurality of rolling elements 4.

[0013] In this embodiment, the ball screw 1, for example, has a nut 3 that rotates about a central axis C. The nut 3 is rotatable about the central axis C but does not move in the direction along the central axis C. The screw shaft 2, which is connected to a driven member (not shown), does not rotate about the central axis C but is movable in the direction along the central axis C. When the nut 3 rotates, the screw shaft 2 moves in the direction along the central axis C. The ball screw 1 is used, for example, as an electric brake device for moving a brake pad (not shown) of a vehicle, which is an example of a driven member.

[0014] The screw shaft 2 is formed in a cylindrical shape with a central axis C at its center. The screw shaft 2 is positioned inside the nut 3, which will be described later. A helical outer rolling groove 6 is formed on the outer circumference of the screw shaft 2. The cross-sectional shape of the outer rolling groove 6 is a Gothic arch containing two arcs.

[0015] The nut 3 is formed in a cylindrical shape with a central axis C. The screw shaft 2 is inserted into the nut 3. A helical inner circumferential rolling groove 7 is formed on the inner circumferential surface of the nut 3. The cross-sectional shape of the inner circumferential rolling groove 7 is a Gothic arch containing two circular arcs. The inner circumferential rolling groove 7 is formed over almost the entire length of the nut 3 in the axial direction. The detailed structure of the nut 3 will be described later.

[0016] Multiple rolling elements 4 are arranged between the nut 3 and the screw shaft 2. The rolling elements 4 are, for example, balls. When the nut 3 and the screw shaft 2 are assembled, a spiral rolling path 9 is formed by the inner circumferential rolling groove 7 formed in the nut 3 and the outer circumferential rolling groove 6 formed in the screw shaft 2. The rolling elements 4 move within this spiral rolling path 9. In Figure 1, one rolling element 4 is shown by a dashed line. In reality, the ball screw 1 has multiple rolling elements 4.

[0017] Next, the structure of the nut 3 will be described. As shown in Figures 1 and 2, the nut 3 has a cylindrical nut body 11 and a plurality of links 12 attached to the nut body 11. As described above, a spiral inner circumferential rolling groove 7 is formed on the inner circumferential surface of the nut body 11. Furthermore, the nut body 11 has a plurality of receiving holes 21 for accommodating the plurality of links 12, which will be described in more detail later. The receiving holes 21 are arranged at equal intervals in the axial and circumferential directions. Each receiving hole 21 accommodates a link 12.

[0018] The housing hole 21 is formed to be recessed radially outward from the inner circumferential surface of the nut body 11. In this embodiment, the housing hole 21 penetrates the nut body 11 radially. The housing hole 21 is formed so that its outer shape and size become constant from the radial inside to the outside. This makes it easy to form the housing hole 21 in the nut body 11. In this embodiment, the housing hole 21 is formed in a rectangular shape with sides along the circumferential and axial directions. The outer shape and size of the housing hole 21 are approximately the same as the outer shape and size of the nut 12 that is housed in the housing hole 21. In this embodiment, the housing hole 21 is formed to be the same size as or slightly smaller than the nut 12. Therefore, the nut 12 is attached to the housing hole 21 by light press-fitting. The housing hole 21 may be formed smaller than the nut 12. In this case, the nut 12 may be attached to the housing hole 21 by press-fitting. Alternatively, the housing hole 21 may be formed larger than the nut 12. In this case, the spool 12 may be fitted into the housing hole 21 by a gap fit. When a gap fit is used, the spool 12 can move radially inward, but after assembly, the presence of the rolling element 4 prevents the spool 12 from falling out. In the case of a gap fit, grease or the like may be applied between the housing hole 21 and the spool 12 during assembly to prevent it from coming loose.

[0019] Figure 3 is a perspective view of the link 12 according to the first embodiment. Figure 4 is a front view of the link 12 according to the first embodiment, viewed from the radially outside. Figure 5 is a view of the nut 3 with the link 12 attached, viewed from the radially outside. In the figures, "axial direction," "radial direction," and "circumferential direction" correspond to the axial direction, radial direction, and circumferential direction of the nut 3 when the link 12 is attached to the nut body 11. Since multiple links 12 are formed in the same shape, one link 12 will be described below. As shown in Figures 2 to 5, the link 12 has a link body 30, a circulation path 33, a relief portion 32, a projection 31, and a chamfered portion 34. The link 12 is made of, for example, synthetic resin.

[0020] The nut body 30 is positioned within the housing hole 21 of the nut body 11. In particular, as shown in Figures 4 and 5, when attached to the nut body 11, the nut body 30 is formed in a rectangular shape, having a pair of side walls 41, 41 (one side of the claim) that face each other in the circumferential direction and a pair of side walls 42, 42 that face each other in the axial direction, when viewed from the radially outside. In this embodiment, the nut body 30 is formed in a rectangular shape in which the distance between the pair of side walls 41, 41 that face each other in the circumferential direction is greater than the distance between the pair of side walls 42, 42 that face each other in the axial direction. Note that the shape of the nut body 30 is not limited to the shape of the embodiment described above.

[0021] As shown in Figure 2, a circulation path 33 is formed on the radially inward-facing surface (inner circumferential surface) of the ball body 30. The circulation path 33 connects one end 17 and the other end 18 of the spiral rolling path 9 formed by the nut body 11 and the screw shaft 2 (see Figure 1), forming an infinite circulation circuit. When multiple rolling elements 4 are filled into this infinite circulation circuit, the rolling elements 4 circulate infinitely within the infinite circulation circuit. In other words, the ball screw 1 of this embodiment is a so-called ball screw 1 of the ball type. More specifically, the inner circumferential surface of the ball 12 is curved with a curvature corresponding to the curved surface forming the inner circumferential surface of the nut body 11, and is provided at the same height as the threads of the inner circumferential rolling groove 7. Note that the inner circumferential surface of the ball 12 and the threads of the inner circumferential rolling groove 7 may be provided at different heights. Furthermore, it is preferable that the center of the nut 3 (central axis C) and the center of the radius of curvature of the inner circumferential surface of the ball 12 coincide. A groove-shaped circulation path 33 is formed on the inner circumferential surface of the nut 12. The circulation path 33 is formed in a curved shape with a U-shaped cross-section corresponding to the spherical surface of the rolling element 4. The circulation path 33 is greatly curved on the inner circumferential surface of the nut 12, for example, in an S-shape. An inlet / outlet 33a at one end of the circulation path 33 is connected to one end 17 of the inner circumferential rolling groove 7. An inlet / outlet 33b at the other end of the circulation path 33 is connected to the other end 18 of the inner circumferential rolling groove 7. From one end 17 to the other end 18 of the inner circumferential rolling groove 7 it makes approximately one turn along the inner circumferential surface of the nut 3, and there is one thread of the screw shaft 2 between the inlet / outlet 33a and the inlet / outlet 33b. The circulation path 33 is gently curved in the axial and circumferential directions of the nut 3 so as to overcome the thread of the screw shaft 2 between the inlet / outlet 33a and the inlet / outlet 33b. Furthermore, in order to make it easier for the rolling elements 4 to overcome the threads of the screw shaft 2, the circulation path 33 is formed such that the amount of radial outward recess increases as it approaches the center in the circulation direction of the rolling elements 4.

[0022] As a result, the rolling element 4 moves to one end 17 of the inner circumferential rolling groove 7, then changes direction of travel through the entrance / exit 33a and enters the circulation path 33. Once inside the circulation path 33, the rolling element 4 moves along the circulation path 33, crossing one screw thread and changing direction of travel, then moves to the other end 18 of the inner circumferential rolling groove 7 through the entrance / exit 33b. After moving to the other end 18 of the inner circumferential rolling groove 7, the rolling element 4 travels around the inner surface of the nut body 11 along the inner circumferential rolling groove 7 once, and then returns to one end 17 of the inner circumferential rolling groove 7 again. The ball screw 1 of this embodiment has multiple infinite circulation circuits consisting of such inner circumferential rolling grooves 7 (i.e., rolling paths 9) and circulation paths 33 of the ball screw 12.

[0023] As shown in Figures 2 to 4, the projections 31 protrude outward from a pair of circumferentially opposing side walls (one side surface) 41, 41 of the nut body 30 toward the outside of the nut body 30 (outside in the circumferential direction). A pair of projections 31 are provided for each nut 12. Each projection 31 is formed at a position close to one end of the side wall 41, i.e., the corner with the perpendicular side wall 42, when viewed from the outside in the radial direction. The projections 31 are formed on the inner circumferential surface side (inside in the radial direction) of the nut body 30 in the radial direction. As shown in Figure 2, when the nut 12 is attached to the nut body 11, the projections 31 are locked into the inner circumferential rolling grooves 7 of the nut body 11. The projections 31 are locked into the inner circumferential rolling grooves 7 (one end 17 and the other end 18 of the inner circumferential rolling groove 7) that are connected to the circulation path 33 and the adjacent inner circumferential rolling grooves 7. The projection 31 engages with the inner circumferential rolling groove 7, thereby preventing the ball screw 12 from coming loose (falling out) radially outward from the housing hole 21 of the nut body 11. Therefore, the ball screw 1 of this embodiment is formed without a sleeve or the like on the outer circumference of the nut 3 to prevent the ball screw 12 from coming loose radially outward.

[0024] As shown in Figure 3, at least a portion of the projection 31 that faces the inner circumferential rolling groove 7 of the nut 3 (the outer surface of the projection 31) is formed in a curved shape that follows the shape of the inner circumferential rolling groove 7. In this embodiment, the entire outer surface of the projection 31 is formed in an arc shape that follows the shape of the inner circumferential rolling groove 7. Also, as shown in Figure 2, the inner surface of the projection 31 is formed in a planar shape that is flush with the inner circumferential surface of the top body 30. Therefore, in this embodiment, the projection 31 is formed in a semicircular shape in a cross-sectional view perpendicular to the direction of projection 31.

[0025] Figure 6 is an enlarged view of section VI in Figure 2. As shown in Figures 3 to 6, the relief portion 32 is provided at a position corresponding to at least the base end 39 of the projection 31 among the side walls 41, 41 of the spinning top body 30. The relief portion 32 is formed to be recessed inward from the side walls 41, 41 of the spinning top body 30. In this embodiment, the relief portion 32 is provided over the entire spinning top body 30 in the radial direction. "Provided over the entire spinning top body 30 in the radial direction" means, for example, that the relief portion 32 (recess) is provided over the entire area above the projection 31 (outside in the radial direction) in Figure 3. The relief portion 32 is provided to have a plane 45 that intersects with the protruding direction of the projection 31. Furthermore, as shown in detail in Figures 4 and 5, in this embodiment, the relief portion 32 is formed to have a surface that is inclined to avoid the corner 22 of the housing hole 21 when viewed from the outside in the radial direction. In other words, when the socket 12 is attached to the nut body 11, the plane 45 constituting the relief portion 32 is located at a position corresponding to the corner 22 of the housing hole 21 of the nut 3, and is inclined with respect to the side wall 41 and the side wall 42 when viewed from the radially outer side.

[0026] As shown in Figure 6, by providing a relief portion 32 at a position corresponding to the base end portion 39 of the projection 31, interference between the projection 31 and the inner circumferential surface of the nut 3 (more specifically, the corner portion 25 located at the boundary between the threads between the inner circumferential rolling grooves 7 and the inner circumferential rolling grooves 7) is suppressed at the base end portion 39 of the projection 31. In other words, the relief portion 32 suppresses interference between the projection 31 of the socket 12 and the inner circumferential surface of the nut 3.

[0027] As shown in Figure 3, the chamfered portion 34 is provided between the base end 39 of the projection 31 and the relief portion 32. In this embodiment, the chamfered portion 34 is an R-chamfer. The chamfered portion 34 may also be formed to have a flat inclined surface. The chamfered portion 34 is formed to have a uniform radius of curvature R over the entire circumference between the base end 39 of the projection 31 and the relief portion 32. The chamfered portion 34 may also be formed such that the radius of curvature R is largest in the portion of the base end 39 of the projection 31 that faces radially outward. In other words, the radius of curvature R of the chamfered portion 34 may differ depending on the position. By providing the relief portion 32 at the base end 39 of the projection 31, it is possible to secure a larger radius of curvature R of the chamfered portion 34 compared to the case where there is no relief portion 32.

[0028] (Function, Effect) In the ball screw 1 of this embodiment, the ball screw 1 is equipped with a nut 3 having a spool 12 in which a circulation path 33 is formed, and the spool 12 has a projection 31. The projection 31 is locked to the nut body 11, which prevents the spool 12 from falling off the nut body 11. As a result, the spool 12 can be prevented from falling off without providing a sleeve or the like on the outside of the nut 3, and the number of parts can be reduced compared to the conventional technology which has a sleeve to prevent the spool 12 from falling off. The spool 12 has a relief portion 32 that is recessed toward the spool body 30 side than one side surface 41 of the spool body 30, and a chamfered portion 34 is provided between the base end 39 of the projection 31 and the relief portion 32. By providing a relief portion 32 that is recessed inward from one side surface 41 at a position corresponding to the base end 39 of the projection 31, interference between the projection 31 and the nut body 11 (more specifically, the edge portion and corner portion 25 etc. formed by the housing hole 21 provided in the nut body 11) can be suppressed. This suppresses wear and damage to the link 12 caused by interference with the nut body 11. Therefore, compared to conventional technology in which the link 12 is prone to damage due to interference with the nut body 11, the projection 31 can be made strong enough to support the projection 31 on its own. As a result, even without a sleeve, the projection 31 alone can prevent the link 12 from falling off. Therefore, compared to conventional technology with a sleeve, the number of parts can be reduced and manufacturing costs can be reduced. Furthermore, by providing the relief portion 32 that is recessed inward from one side surface 41, the chamfered portion 34 between the base end 39 of the projection 31 and the relief portion 32 can be made larger. For example, if the chamfered portion 34 is an R chamfer, the radius of curvature R in the R chamfer can be made larger. As a result, stress concentration on the base end 39 of the projection 31 when the rolling element 4 pushes up the link 12 during the circulation of the rolling element 4 is alleviated, and the link 12 can withstand the pushing force even without a sleeve. Therefore, the lifespan of the link 12 is extended, the necessary strength of the projection 31 is maintained, and the detachment of the link 12 due to damage to the link 12 can be further suppressed. Thus, a ball screw 1 can be provided that suppresses the increase in the number of parts and thus the increase in cost, while suppressing the detachment of the link 12 compared to conventional technology.Furthermore, compared to conventional technology, interference between the nut 12 and the nut body 11 can be avoided, thus suppressing the generation of burrs and cutting chips caused by the wear of the nut 12. Therefore, a high-quality ball screw 1 can be produced with suppressed wear of the nut 12 and the inclusion of impurities.

[0029] The relief portion 32 is provided to have a plane 45 that intersects the protruding direction of the projection portion 31. Because the relief portion 32 is formed in a planar shape, it is easier to form a larger chamfered portion 34 between the base end portion 39 of the projection portion 31 and the relief portion 32. Therefore, stress concentration at the base end portion 39 of the projection portion 31 is further reduced, and the required strength of the projection portion 31 can be maintained even if a sleeve is not provided. Therefore, the detachment of the piece 12 due to damage to the piece 12 can be further suppressed.

[0030] When the socket 12 is attached to the nut body 11, the relief portion 32 is provided radially across the entire socket body 30. By forming the relief portion 32 in this way, the relief portion 32 can be easily formed, especially when the socket 12 is formed by resin molding. Therefore, an increase in the manufacturing cost of the socket 12 can be suppressed.

[0031] The chamfered portion 34 is R-shaped, and the chamfered portion 34 is formed such that the radius of curvature R is largest in the portion of the base end 39 of the projection 31 that faces the insertion direction of the link 12 (outward in the radial direction). This makes it possible to increase the chamfer R at the location where stress is most likely to concentrate when the rolling element 4 pushes up the link 12. Therefore, compared to the case where the chamfer R is increased over the entire circumference of the base end 39 of the projection 31, it is possible to ensure the necessary strength of the projection 31 while suppressing an increase in the size of the link 12, and effectively suppress the detachment of the link 12 due to damage to the link 12, etc.

[0032] When the nut 3 is viewed from the radially outer side, the relief portion 32 is formed to have an inclined surface that avoids the corner portion 22 of the housing hole 21 of the nut 3. This further suppresses interference between the edge of the nut 3 and the block 12 at the corner portion 22 of the housing hole 21.

[0033] (First Modification of the First Embodiment) Next, the first to third modifications of the first embodiment will be described. In the descriptions of the first to third modifications, components similar to those in the first embodiment described above will be denoted by the same reference numerals and their descriptions will be omitted as appropriate. Note that the specific configurations are not limited to these embodiments and can be modified as appropriate without departing from the spirit of the present invention. Figure 7 is an enlarged view showing the projection 31 of the frame 212 according to the first modification of the first embodiment. The first modification of the first embodiment differs from the first embodiment described above in that a chamfered portion 234 is provided so as to extend continuously from the side wall 41 to the projection 31.

[0034] In the first modified example, the relief portion 232 is formed to have a plane 45 that is inclined with respect to the side wall 41 and the side wall 42 when viewed from the radially outside, similar to the first embodiment described above. In the first modified example, the chamfered portion 234 is an R-chamfer having an uneven radius of curvature at the base end 39 of the projection 31. Specifically, in the base end 39 of the projection 31, the portion located near the side wall 41 (for example, the portion indicated by arrow A) has a chamfered portion 234 formed on a second plane 245 that is provided to protrude toward the side wall 41 side than the plane 45 of the relief portion 232. On the other hand, in the central portion of the base end 39 of the projection 31 (for example, the portion indicated by arrow B), a chamfered portion 234 is formed between the plane 45 of the relief portion 232 and the projection 31. As a result, the radius of curvature R3 of the chamfer in the portion indicated by arrow B is larger than the radius of curvature R4 of the chamfer in the portion indicated by arrow A (R3 > R4).

[0035] In the first modified ball screw 1, the radius of curvature R3 of the R chamfer at the central part of the base end 39 of the projection 31 (indicated by arrow B) is greater than the radius of curvature R4 of the R chamfer at the end part of the base end 39 of the projection 31 (indicated by arrow A). Here, when the rolling element 4 pushes up the link 212 in the assembled ball screw 1, a stress is generated at the base end 39 of the projection 31 that mainly pushes the link body 30 outward in the radial direction. For this reason, the largest stress acts on the part of the base end 39 of the projection 31 that faces the radially outward direction (i.e., the central part of the entire circumference of the base end 39 (indicated by arrow B)). In the first modified ball screw 1, a chamfer 234 with a larger radius of curvature can be provided at the part of the base end 39 of the projection 31 where the largest stress acts, so that the link 212 can withstand the pushing force. Therefore, the lifespan of the link 212 is extended, the necessary strength of the projection 31 is maintained, and the detachment of the link 212 due to damage to the link 212 can be further suppressed. In addition, compared to the case in which a chamfered portion 234 with a large radius of curvature is provided around the entire circumference of the base end portion 39 of the projection 31, the area in which the chamfered portion 234 and the relief portion 232 are formed can be kept small. Therefore, excessive enlargement of the link 212 can be suppressed, and a link 212 with a high degree of freedom in shape can be formed.

[0036] (Second Modification of the First Embodiment) Next, a second modification of the first embodiment will be described. Figure 8 is an enlarged view showing the projection 31 of the frame 312 according to the second modification of the first embodiment. The second modification of the first embodiment differs from the first embodiment described above in that the relief portion 332 is formed in a planar shape parallel to the side wall 41 rather than being a slope.

[0037] In the second modified example, the relief portion 332 is formed to be recessed inward from the side walls 41, 41 of the spinning top body 30. The relief portion 332 is provided to have a plane 45 that is substantially perpendicular to the protruding direction of the projection portion 31. That is, in the second modified example, the relief portion 332 is formed to have a plane 45 that is parallel to the side walls 41.

[0038] According to the second modified ball screw 1, even when the relief portion 332 is formed to have a plane 45 parallel to the side wall 41, the same effects and advantages as those of the first embodiment can be achieved. Therefore, the degree of freedom in the shape of the relief portion 332 can be improved.

[0039] (Third Modification of the First Embodiment) Next, a third modification of the first embodiment will be described. Figure 9 is a perspective view of the spinning top 412 according to the third modification of the first embodiment. The third modification of the first embodiment differs from the first embodiment described above in that the relief portion 432 in the radial direction is provided only on a part of the spinning top body 30.

[0040] In the third modified example, the relief portion 432 is provided on the side walls 41, 41 of the spinning top body 30 at a position corresponding to at least the base end portion 39 of the projection 31. The relief portion 432 is formed to be recessed inward from the side walls 41, 41 of the spinning top body 30. In the third modified example, the relief portion 432 is provided on a part of the inner circumferential surface of the spinning top body 30 in the radial direction. The radially outer portion of the side wall 41 of the spinning top body 30 where the relief portion 432 is not formed becomes a roof portion 449 formed by the extension of the side walls 41 and 42, respectively.

[0041] In the third modified ball screw 1, the radially outer end of the relief portion 432 is covered by the roof portion 449, so when the ball 412 is attached to the nut body 11, the entire housing hole 21 is covered by the ball body 30 (see also Figure 5). Therefore, it is possible to suppress foreign matter generated on the outer circumference of the nut 3 from entering the rolling path 9 through the housing hole 21. Furthermore, even when the roof portion 449 is provided, the same effects as in the first embodiment described above can be achieved. That is, the radius of curvature R in the chamfered portion 34 can be made larger than in the conventional technology, so stress concentration when the rolling element 4 pushes up the ball 412 is reduced, and the ball 412 can withstand the pushing force. In addition, by providing the relief portion 432, interference between the base end 39 of the projection 31 and the inner circumferential surface of the nut 3 can be suppressed. Therefore, the necessary strength of the projection 31 of the ball 412 can be maintained, and the detachment of the ball 412 due to damage to the ball 412 can be suppressed.

[0042] (Second Embodiment) Next, a second embodiment of the present invention will be described. In the description of the second embodiment, components similar to those in the first embodiment described above will be denoted by the same reference numerals and their descriptions will be omitted as appropriate. Note that the specific configuration is not limited to these embodiments and can be modified as appropriate without departing from the spirit of the present invention. Figure 10 is a perspective view of the spinning top 512 according to the second embodiment. Figure 11 is an enlarged view of the projection 531 of the spinning top 512 according to the second embodiment, viewed from a different angle. In the second embodiment, the shape of the projection 531 of the spinning top 512 differs from that of the first embodiment described above.

[0043] As shown in Figures 10 and 11, the relief portion 32 of the second embodiment is formed in a planar shape inclined with respect to the side walls 41 and 42, similar to the relief portion 32 of the first embodiment described above. The projection 531 protrudes from the relief portion 32 in a direction substantially perpendicular to the side wall 41. In the second embodiment, the projection 531 is formed in a rectangular shape in a cross-sectional view perpendicular to the direction of projection of the projection 531. The projection 531 has a top surface 561 facing radially outward when the nut body 512 is attached to the nut body 11, a bottom surface 563 facing radially inward, and a pair of side surfaces 562, 562 connecting the top surface 561 and the bottom surface 563. The bottom surface 563 is flush with the inner circumferential surface of the nut body 30. The pair of side surfaces 562, 562 are in contact with the inner circumferential rolling groove 7 when the nut body 512 is attached to the nut body 11. The surface of the side surface 562 may be formed in a curved shape that follows the groove shape of the inner circumferential rolling groove 7. A first chamfered portion 565 is formed between the top surface 561 and the relief portion 32. Second chamfered portions 566, 566 (see Figure 11) are formed between the pair of side surfaces 562, 562 and the relief portion 32. The radius of curvature R5 of the first chamfered portion 565 is greater than the radius of curvature R6 of the second chamfered portion 566 (R5 > R6).

[0044] According to the ball screw 1 of the second embodiment, the protrusion 531 is formed in a rectangular shape in a cross-sectional view orthogonal to the protruding direction of the protrusion 531. By forming the portion of the protrusion 531 facing the outer side in the radial direction (for example, the top surface 561 in the present embodiment) in a planar shape, a chamfer portion (first chamfer portion 565) at the stress concentration portion of the roller 512 when the rolling element 4 pushes up the roller 512 can be formed larger. Therefore, the stress concentration at the base end portion 39 of the protrusion 531 can be further alleviated, and the dropout of the roller 512 due to breakage or the like of the roller 512 can be further suppressed. In addition, the degree of freedom in the shape of the protrusion 531 can be improved.

[0045] The protrusion 531 has a top surface 561 facing the outer side in the radial direction and a pair of side surfaces 562, 562 provided at both ends of the top surface 561. The radius of curvature R5 of the first chamfer portion 565 provided between the top surface 561 and the relief portion 32 is larger than the radius of curvature R6 of the second chamfer portion 566 provided between the side surface 562 and the relief portion 32. As described above, by forming the planar top surface 561 on the protrusion 531, the radius of curvature of the first chamfer portion 565 at the stress concentration portion of the roller 512 when the rolling element 4 pushes up the roller 512 can be formed larger. Therefore, the stress concentration at the base end portion 39 of the protrusion 531 can be alleviated, and the dropout of the roller 512 due to breakage or the like of the roller 512 can be suppressed. On the other hand, for the portion of the protrusion 531 other than the portion facing the outer side in the radial direction (that is, the base end portions of the pair of side surfaces 562, 562 of the protrusion 531), since the pushing force by the rolling element 4 hardly acts, the radius of curvature R6 of the corresponding second chamfer portion 566 can be made relatively small. Therefore, while suppressing an excessive increase in the size of the roller 512, the necessary strength of the protrusion 531 can be ensured, and the dropout of the roller 512 due to breakage or the like of the roller 512 can be effectively suppressed.

[0046] It should be noted that the technical scope of the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention. For example, in the embodiments described above, the projections 31 and 531 of the links 12, 212, 312, 412, and 512 were described as protruding along the circumferential direction of the nut 3 when the links 12, 212, 312, 412, and 512 are attached to the nut body 11, but the invention is not limited to this. When the links 12, 212, 312, 412, and 512 are attached to the nut body 11, the projections 31 and 531 may protrude in the axial direction of the nut 3. In this case, recesses or holes for engaging the projections 31 and 531 may be separately formed on the inner circumferential surface of the nut body 11. However, the configuration of this embodiment in which the projections 31 and 531 protrude along the circumferential direction has the advantage of reducing the effort required for processing by engaging the projections 31 and 531 with the already formed inner circumferential rolling grooves 7.

[0047] The pieces 12, 212, 312, 412, 512 may be made of metal. However, according to the configuration of the present embodiment, even if the pieces 12, 212, 312, 412, 512 made of synthetic resin are used, the projections 31, 531 are unlikely to be damaged. Therefore, the configuration of the present embodiment using the pieces 12, 212, 312, 412, 512 made of synthetic resin is advantageous in that cost reduction and weight reduction can be achieved by using the pieces 12, 212, 312, 412, 512 made of synthetic resin. In the first embodiment, the first modification of the first embodiment, the third modification, and the second embodiment, the inclination angles of the relief portions 32, 232, 432 viewed from the outer side in the radial direction are not limited to the above-described angles or the illustrated angles. The shape of the relief portions 32, 232, 432 viewed from the outer side in the radial direction may be such that it does not interfere with the circulation path 33, and is not limited to the above-described shape. In each of the above-described embodiments and each modification, the projection 31 may protrude from any position of the relief portions 32, 232, 332, 432 formed in the piece body 30. For example, taking the projection 31 located at the lower right in FIG. 4 as an example, the projection 31 may be formed such that the chamfered portion 34 fits inside the region AR surrounded by the virtual line L1 extending the side wall 41 to the right, the virtual line L2 extending the side wall 42 downward, and the plane 45 forming the relief portion 32. The protruding position of the projection 31 with respect to the relief portion 32, the shape of the chamfered portion 34, etc. are not limited to the configuration of the above-described embodiment.

[0048] The accommodation hole 21 does not have to penetrate the nut body 11. For example, the accommodation hole 21 may be a hole (recess) that depresses radially outward from the inner peripheral surface of the nut body 11. In each of the above-described embodiments, the ball screw 1 in which the nut 3 rotates and the screw shaft 2 moves in the direction along the central axis C has been described, but the present invention is not limited to this. In the ball screw 1 in which the screw shaft 2 rotates and the nut 3 moves in the direction along the central axis C, the configurations of the above-described embodiments may be applied.

[0049] Furthermore, this disclosure may also be a combination of the following configurations. (1) A ball screw comprising: a screw shaft having a helical outer circumferential rolling groove on its outer circumferential surface; a nut having a helical inner circumferential rolling groove on its inner circumferential surface; and a plurality of rolling elements arranged in a rolling path formed by the inner circumferential rolling groove of the nut and the outer circumferential rolling groove of the screw shaft, wherein the nut comprises a nut body and a spool attached to the nut body that forms a circulation path returning the rolling elements from one end to the other of the rolling path, the spool is arranged in a housing hole recessed radially outward from the inner circumferential surface of the nut, the spool having a spool body having the circulation path, a projection that protrudes in a direction intersecting one side surface of the spool body and is locked to the nut body to prevent it from falling out radially outward from the housing hole, and a relief portion provided at least at a position corresponding to the base end of the projection and recessed toward the spool body side than the one side surface, and a chamfered portion is provided between the base end of the projection and the relief portion. (2) The relief portion is provided to have a plane intersecting the protruding direction of the projection, the ball screw according to (1). (3) The relief portion is provided over the entire body of the ball screw in the radial direction when the ball is attached to the nut body, the ball screw according to (1) or (2). (4) The chamfered portion is R-chamfered, and the chamfered portion is formed such that the radius of curvature is largest at the base end of the projection that faces the direction of insertion of the ball into the nut body, the ball screw according to any one of (1) to (3). (5) The relief portion has a surface that is inclined to avoid the corner of the housing hole when viewed from the radial direction, the ball screw according to any one of (1) to (4). (6) The projection is formed in a rectangular shape in a cross-sectional view perpendicular to the protruding direction of the projection, the ball screw according to any one of (1) to (5).(7) The ball screw according to (6), wherein the projection has a top surface facing outward in the radial direction and a pair of side surfaces connected to the top surface and facing at least one of the axial and circumferential directions of the nut, and the radius of curvature of the chamfer provided between the top surface and the relief portion is greater than the radius of curvature of the chamfer provided between the side surfaces and the relief portion.

[0050] 1 Ball screw 2 Screw shaft 3 Nut 4 Rolling element 6 Outer circumference rolling groove 7 Inner circumference rolling groove 9 Rolling path 11 Nut body 12, 212, 312, 412, 512 Piece 17 One end (of the rolling path) 18 The other end (of the rolling path) 21 Housing hole 22 Corner 30 Piece body 31, 531 Projection 33 Circulation path 32, 232, 332, 432 Relief section 34, 234, 334 Chamfered section 39 Base end (of the projection) 41 Side wall (one side) 45 Plane 561 Top surface 562 Pair of sides 565 First chamfered section (chamfered section) 566 Second chamfered section (chamfered section) R, R3, R4, R5, R6 Radius of curvature

Claims

1. A ball screw comprising: a screw shaft having a helical outer circumferential rolling groove on its outer circumferential surface; a nut having a helical inner circumferential rolling groove on its inner circumferential surface; and a plurality of rolling elements arranged in a rolling path formed by the inner circumferential rolling groove of the nut and the outer circumferential rolling groove of the screw shaft, wherein the nut comprises a nut body and a spool attached to the nut body that forms a circulation path returning the rolling elements from one end to the other of the rolling path, the spool is arranged in a housing hole recessed radially outward from the inner circumferential surface of the nut, the spool having a spool body having the circulation path, a projection that protrudes in a direction intersecting one side surface of the spool body and is locked to the nut body to prevent it from falling out radially outward from the housing hole, and a relief portion provided at least at a position corresponding to the base end of the projection and recessed toward the spool body side than the one side surface, and a chamfered portion is provided between the base end of the projection and the relief portion.

2. The relief portion is provided to have a plane that intersects with the protruding direction of the projection, as described in claim 1.

3. In a state in which the ball is attached to the nut body, the relief portion is provided over the entire length of the ball body in the radial direction, as described in claim 1 or 2.

4. The ball screw according to any one of claims 1 to 3, wherein the chamfered portion is R-chamfered, and the chamfered portion is formed such that the radius of curvature is largest at the portion of the base end of the projection that faces the direction of insertion of the socket into the nut body.

5. The relief portion has a surface that is inclined to avoid the corner of the housing hole when viewed from the radial direction, according to any one of claims 1 to 4.

6. The ball screw according to any one of claims 1 to 5, wherein the projection is formed in a rectangular shape in a cross-sectional view perpendicular to the direction of projection of the projection.

7. The projection has a top surface facing radially outward and a pair of side surfaces connected to the top surface and facing at least one of the axial and circumferential directions of the nut, and the radius of curvature of the chamfered portion provided between the top surface and the relief portion is greater than the radius of curvature of the chamfered portion provided between the side surfaces and the relief portion, the ball screw according to claim 6.

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