Ball screw and method for manufacturing nut for ball screw

Abstract

This ball screw is provided with a screw shaft, a nut (1), and a plurality of balls disposed between the nut (1) and the screw shaft. The nut (1) has: a helical groove (20) which is formed in a spiral shape on the inner peripheral surface (11) of the nut (1) and in which a plurality of balls roll; and a return groove (30) which is formed on the inner peripheral surface (11) and returns a ball from a first end (21) of the helical groove (20) to a second end of the helical groove (20). The return groove (30) is formed to have a depth dimension larger than the depth dimension of the helical groove (20). The return groove (30) has, at a boundary between the helical groove (20) and the return groove (30), a connection end part (33) formed so as to have, as a cross section, at least one of an inclined surface shape and a curved surface shape that extends from the bottom surface of the return groove (30) to the bottom surface of the helical groove (20).

Description

Ball screw and method of manufacturing ball screw nut

[0001] The present invention relates to a ball screw and a method for manufacturing a nut for the ball screw.This application claims priority to Japanese Patent Application No. 2024-130755, filed August 7, 2024, the contents of which are incorporated herein by reference.

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

[0003] For example, Patent Document 1 discloses a method for manufacturing a nut having a spiral groove (ball rolling path) along which multiple balls roll and a circulation groove (ball circulation path) that circulates the balls from the end point of the spiral groove back to the start point. In this nut manufacturing method, the circulation groove is first formed on the inner peripheral surface of the nut by pressing a die against it to perform plastic processing, and then the spiral groove is formed by cutting. According to the technology described in Patent Document 1, the pressing load when forming the circulation groove is reduced by increasing the radius of curvature of the end of the die that forms the circulation groove compared to conventional methods and shortening the length of the die end. As a result, the die that forms the circulation groove is less susceptible to wear and deformation, thereby extending the die's lifespan. Furthermore, because a ball circulation system is adopted in which the circulation groove is formed directly in the nut, there is no need to attach other components (return tubes and bearings) that form the circulation path to the nut, which is said to improve design flexibility in nut formation.

[0004] Patent No. 5549331

[0005] However, with the technology described in Patent Document 1, there is a risk that the cross section of the boundary between the circulation groove and the spiral groove will have an edged shape. Here, FIG. 12 is a perspective view of a circulation groove forming die 950 according to the prior art. FIG. 13 is a perspective view of the boundary between the spiral groove 920 and the circulation groove 930 formed in a nut 901 according to the prior art, viewed from the inner diameter side. As shown in FIG. 12 , a circulation groove forming die 950 according to an example of the prior art uses a punch 955 having, for example, a semicircular upper end and a shape that is constant in cross section perpendicular to the length direction of the punch 955 (i.e., the length direction of the circulation groove 930 formed by transfer). The punch 955 is pressed against the inner circumferential surface 911 of the nut 901 for plastic processing, thereby forming the circulation groove 930 as shown in FIG. 13 . After the circulation groove 930 is formed, the spiral groove 920 is formed by cutting or the like. In this case, the spiral groove 920 is formed so that its inner shape is smaller (or its recess depth is shallower) than the circulation groove 930. Furthermore, the longitudinal end surfaces of the punch 955 are flat. Therefore, a step 945 having a surface substantially perpendicular to the ball movement direction is formed at the boundary between the circulation groove 930 and the spiral groove 920, which have different depths. This raises the risk of ball clogging or poor circulation due to the step 945 when the balls are returned from the circulation groove 930 to the spiral groove 920 or when they are introduced from the spiral groove 920 to the circulation groove 930. As a result, vibration spikes may occur during operation of the ball screw, and the ball circulation efficiency may be reduced.

[0006] Therefore, in the prior art described in Patent Document 1 and the like, in a ball circulation type ball screw in which a spiral groove and a circulation groove are integrally formed in the nut, there was room for improvement compared to the prior art in terms of suppressing ball clogging and poor circulation caused by the step at the boundary between the spiral groove and the circulation groove.

[0007] Therefore, an object of the present invention is to provide a ball screw and a method for manufacturing a nut for this ball screw that can suppress the occurrence of ball clogging and poor circulation compared to conventional techniques.

[0008] In order to solve the above problems, the present invention proposes the following means: A ball screw according to a first aspect of the present invention includes a screw shaft, a nut, and a plurality of balls arranged between the nut and the screw shaft, the nut having a helical groove formed in a spiral shape on an inner peripheral surface of the nut and in which the plurality of balls roll, and a circulation groove formed on the inner peripheral surface and in which the balls return from one end of the helical groove to the other end, the depth of the circulation groove being greater than the depth of the spiral groove, and the circulation groove having a connection end portion at a boundary between the spiral groove and the circulation groove, the connection end portion being formed to have at least one of an inclined cross section and a curved cross section that connects from a bottom surface of the circulation groove to a bottom surface of the spiral groove.

[0009] A first aspect of the present invention relates to a method for manufacturing a nut for a ball screw having a spiral groove in which multiple balls roll and a circulation groove that returns the balls from one end of the spiral groove to the other end, and includes a circulation groove forming step of forming the circulation groove on the inner peripheral surface of the nut by forging, and a spiral groove forming step of forming the spiral groove on the inner peripheral surface by subtractive processing so that the inner shape is smaller than that of the circulation groove, wherein the mold that forms the circulation groove in the circulation groove forming step has a first portion corresponding to a main groove portion of the circulation groove and a second portion corresponding to an end portion of the circulation groove, and the second portion forms at least one of a sloped cross-sectional shape and a curved cross-sectional shape that connects from the bottom surface of the circulation groove to the bottom surface of the spiral groove at the boundary between the circulation groove and the spiral groove in the nut.

[0010] According to the method for manufacturing a ball screw and a nut for a ball screw of the present invention, it is possible to provide a ball screw and a method for manufacturing a nut for this ball screw that can reduce the occurrence of ball clogging and poor circulation compared to conventional techniques.

[0011] 10 is a schematic cross-sectional view showing a ball screw according to the first embodiment. A cross-sectional view of a nut for a ball screw according to the first embodiment. A perspective view of a boundary between a spiral groove and a circulation groove formed in the nut according to the first embodiment, viewed from the inner diameter side. A cross-sectional view taken along line IV-IV in FIG. 3. A flow diagram showing the flow of a method for manufacturing a nut for a ball screw according to the first embodiment. A cross-sectional view of the nut, illustrating the circulation groove forming step. A perspective view of a mold used in the circulation groove forming step of the first embodiment. A cross-sectional view of the nut, illustrating the spiral groove forming step. A perspective view of a mold used in the circulation groove forming step of the second embodiment. A perspective view of the boundary between a spiral groove and a circulation groove formed in a nut according to the second embodiment, viewed from the inner diameter side. A cross-sectional view taken along line XI-XI in FIG. 10. A perspective view of a mold for forming a circulation groove according to the prior art. A perspective view of the boundary between a spiral groove and a circulation groove formed in a nut according to the prior art, viewed from the inner diameter side.

[0012] An embodiment of the present invention will be described with reference to the drawings. In one embodiment, a ball screw 10 is incorporated into various mechanical devices, such as an electric brake device for a vehicle, an automatic manual transmission (AMT), or a positioning device for a machine tool, and is used to convert the rotational motion of a drive source, such as an electric motor, into linear motion to operate a driven part (actuating part). Various types of electric brake devices are applicable, such as an electro-mechanical brake (EMB) that applies braking force via a ball screw driven by a motor, and an electro-hydraulic brake (EHB) that controls the hydraulic pressure of a hydraulic brake via a ball screw driven by a motor. The ball screw 10 can also be applied to mechanical devices other than those described above.

[0013] In the following description, unless otherwise specified, the axial direction, radial direction, and circumferential direction refer to the direction along the central axis C of the ball screw 10 (nut 1), the radial direction of the ball screw 10 (nut 1), and the direction around the central axis C of the ball screw 10 (nut 1), respectively.

[0014] (First embodiment) (Ball screw) Fig. 1 is a schematic cross-sectional view showing a ball screw 10 according to a first embodiment. The ball screw 10 is a device that converts rotational motion into linear motion. As shown in Fig. 1, the ball screw 10 includes a screw shaft 7, a nut 1, and a plurality of balls 8 (rolling elements) arranged between the screw shaft 7 and the nut 1.

[0015] The screw shaft 7 has a shaft body 70 and a spiral outer peripheral spiral groove 71 (outer peripheral rolling groove) provided on the outer peripheral surface of the shaft body 70. The shaft body 70 is formed in a cylindrical shape centered on the central axis C. At least a portion of the screw shaft 7 is made of metal. The outer peripheral spiral groove 71 is formed by applying cutting or plastic processing (such as rolling) to the outer peripheral surface of the shaft body 70. In forming the outer peripheral spiral groove 71, grinding can be additionally performed. The cross-sectional shape (groove bottom shape) of the outer peripheral spiral groove 71 on the screw shaft 7 is, for example, a Gothic arch or a circular arc. The number of threads of the outer peripheral spiral groove 71 is set to one, two, or more.

[0016] The nut 1 has a cylindrical nut body 3 centered on a central axis C, and a spiral groove 20 (inner circumferential rolling groove) and a circulation groove 30 provided on an inner circumferential surface 11 of the nut body 3. The screw shaft 7 is inserted and disposed radially inside the nut 1. At least a portion of the nut 1 is made of metal. As will be described in detail later, in this embodiment, the spiral groove 20 of the nut 1 is formed by cutting. Note that the spiral groove 20 may be formed by plastic processing or the like instead of or in addition to cutting. A grinding process may also be performed to form the spiral groove 20. The cross-sectional shape (groove bottom shape) of the spiral groove 20 of the nut 1 corresponds to the groove shape of the screw shaft 7, such as a Gothic arch or a circular arc. The number of threads of the spiral groove 20 of the nut 1 corresponds to the number of threads of the outer circumferential spiral groove 71 of the screw shaft 7, and is set to one, two, or more. When the nut 1 and the screw shaft 7 are combined, a spiral rolling path 75 is formed by the spiral groove 20 formed in the nut 1 and the outer spiral groove 71 formed in the screw shaft 7.

[0017] A plurality of balls 8 are disposed between the screw shaft 7 and the nut 1. In FIG. 1 , two balls 8 are indicated by two-dot chain lines. In reality, the ball screw 10 includes a large number of balls 8. The plurality of balls 8 are made of, for example, metal (such as steel) or ceramics. The plurality of balls 8 roll in the rolling paths 75 as the screw shaft 7 and the nut 1 rotate relative to each other.

[0018] FIG. 2 is a cross-sectional view of a ball screw nut 1 (hereinafter, sometimes simply referred to as the nut 1) according to the first embodiment. As shown in FIG. 2 , in addition to the helical groove 20, a circulation groove 30 is formed on the inner peripheral surface 11 of the nut 1. The circulation groove 30 forms a return path that returns the balls 8 from one end of the helical groove 20 to the other end. In other words, the balls 8 return from the end point of the helical groove 20 (main path) to the starting point via the circulation groove 30 (return path) provided in the nut 1. In this embodiment, the circulation groove 30 is formed in an S-shape. The maximum depth of the circulation groove 30 is set to be larger than the maximum depth of the helical groove 20. In this embodiment, the inner diameter dimension of the circulation groove 30 is set to be larger than the inner diameter dimension of the helical groove 20 (see also FIGS. 3 and 4 ). For example, the depth of the circulation groove 30 is set to a degree that allows the balls 8 to ride over the threads of the screw shaft 7 when the screw shaft 7 and the nut 1 are assembled.

[0019] Multiple circuits are defined on the inner peripheral surface 11 of the nut body 3, and each circuit includes a main path formed based on the spiral groove 20 and a return path formed based on the circulation groove 30. In each circuit, the circulation groove 30 (return path) spatially connects a first end 21 (one end of the claim) and a second end 22 (the other end of the claim) of the spiral groove 20 (main path). The circulation groove 30 connects one end and the other end of the spiral groove 20 to form an infinite circulation circuit. When multiple balls 8 are filled in this infinite circulation circuit, the balls 8 circulate infinitely within the infinite circulation circuit. The ball screw 10 of this embodiment has multiple infinite circulation circuits each consisting of a spiral groove 20 (main path) and a circulation groove 30 (return path). In other words, the ball screw 10 of this embodiment is a so-called ball circulation type ball screw.

[0020] In one example, the ball 8 moves from the first end 21 (starting point) of the spiral groove 20 to the second end 22 (ending point). The ball 8 that moves to the second end 22 of the spiral groove 20 is guided by the circulation groove 30. The ball 8 arranged in the circulation groove 30 is pushed by the following ball 8 and moves from the second end 39 to the first end 38 of the circulation groove 30. That is, the ball 8 that enters the circulation groove 30 from the second end 22 of the spiral groove 20 returns to the first end 21 of the spiral groove 20 via the circulation groove 30. The starting point and ending point of the spiral groove 20 are reversed depending on the relative rotation direction between the screw shaft 7 and the nut 1. During relative rotation in the reverse direction, the ball 8 moves from the second end 22 (starting point) of the spiral groove 20 to the first end 21 (ending point). The ball 8 that moves to the first end 21 of the spiral groove 20 is guided by the circulation groove 30 and moves from the first end 38 to the second end 39 of the circulation groove 30. That is, the balls 8 that enter the circulation groove 30 from the first end 21 of the spiral groove 20 return to the second end 22 of the spiral groove 20 via the circulation groove 30. In this way, in the ball screw 10, as the screw shaft 7 and the nut 1 rotate relative to each other, the balls 8 can continuously circulate through a circulation path (circulation circuit) that includes the spiral groove 20 and the return circulation groove 30.

[0021] FIG. 3 is a perspective view of the boundary between the spiral groove 20 and the circulation groove 30 formed in the nut 1 according to the first embodiment, viewed from the inner diameter side. FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. 3. As shown in FIGS. 2 to 4, each circulation groove 30 formed in the nut 1 has a main groove portion 31 and a pair of connecting ends 33 located at positions corresponding to both ends of the main groove portion 31. Since the main groove portion 31 and the connecting ends 33 in each circulation groove 30 have the same shape, the following description will be given using one circulation groove 30 as an example. The main groove portion 31 is located in the center of the extension direction of the circulation groove 30 (the direction along the movement direction of the ball 8). The inner shape of the main groove portion 31 is larger than the inner shape of the spiral groove 20. In other words, the main groove portion 31 is recessed radially outward from the inner circumferential surface 11 of the nut 1 more than the spiral groove 20.

[0022] A pair of connection ends 33 are provided at both ends of the main groove portion 31 in the extension direction of the circulation groove 30. The connection ends 33 are provided at the boundary between the spiral groove 20 and the circulation groove 30. The connection ends 33 are formed in a shape that allows a smooth connection between the spiral groove 20 and the main groove portion 31 of the circulation groove 30. Specifically, as shown in FIGS. 3 and 4 , the connection ends 33 are formed at the boundary between the spiral groove 20 and the circulation groove 30 so as to have a curved cross-sectional shape that connects from the bottom surface of the circulation groove 30 to the bottom surface of the spiral groove 20. In this embodiment, the connection ends 33 are formed in an overall spherical shape. As a result, the connection ends 33 are formed in an arc-shaped cross-section when viewed in a cross-sectional view along the movement direction of the ball 8.

[0023] As another example, the connection end 33 may be formed into an ellipsoidal shape as a whole. In this case, the connection end 33 is formed into an elliptical cross section when viewed in a cross section along the moving direction of the ball 8. Alternatively, the connection end 33 may be formed into a quadratic cross section when viewed in a cross section along the moving direction of the ball 8. The connection end 33 may be formed into a curved shape with a combination of multiple radii of curvature when viewed in a cross section along the moving direction of the ball 8.

[0024] As shown in Figure 4, the nut 1 of this embodiment further has a chamfered portion 40. The chamfered portion 40 is formed between the connection end portion 33 of the circulation groove 30 and the spiral groove 20. The chamfered portion 40 is formed, for example, by crowning or cutting. The chamfered portion 40 may be, for example, an R-chamfer. The chamfered portion 40 may also be omitted.

[0025] (Method of Manufacturing a Nut for a Ball Screw) Next, a method of manufacturing the nut 1 for the above-described ball screw 10 will be described. Fig. 5 is a flow chart showing the flow of a method of manufacturing a nut for a ball screw according to the first embodiment. The method of manufacturing the nut for a ball screw 1 (nut 1) is a method of manufacturing a nut 1 for a ball screw 10 having a spiral groove 20 in which a plurality of balls 8 roll, and a circulation groove 30 that returns the balls 8 from one end (first end 21) of the spiral groove 20 to the other end (second end 22). As shown in Fig. 5, the method of manufacturing the nut 1 includes a circulation groove forming step ST01, a spiral groove forming step ST02, and a chamfering step ST03.

[0026] FIG. 6 is a cross-sectional view of the nut 1 showing the circulation groove forming step ST01. FIG. 7 is a perspective view of a die 50 used in the circulation groove forming step ST01 of the first embodiment. As shown in FIGS. 6 and 7 , in the circulation groove forming step ST01, multiple circulation grooves 30 are formed in the inner circumferential surface 11 of the nut 1 by plastic processing such as forging. The multiple circulation grooves 30 are formed, for example, by plastic processing (forging) using a die 50 shown in FIG. 7 and a cam mechanism (not shown). In the plastic processing using the die 50 and cam mechanism, the die 50 and a die holder (not shown) are first inserted axially into the nut body 3 with respect to the fixedly held nut 1, and the die 50 is set on the nut body 3. The die 50 is attached to the die holder in a manner that allows it to move radially relative to the nut 1. The die holder has a hole extending axially, the shape of which corresponds to the outer diameter shape of the mandrel. Next, when the mandrill is inserted axially into the hole in the die holder, the die 50 is pushed against the tapered surface of the mandrill and moves radially outward, being forced into the nut body 3. As a result, a circulation groove 30 of a predetermined shape is formed on the inner circumferential surface 11 of the nut body 3 (see FIG. 6). When the mandrill retracts axially, the die 50 moves radially inward and separates from the nut body 3. The die 50 then moves axially and is removed from the nut body 3. The circulation groove 30 may also be formed by plastic processing using another method.

[0027] As shown in FIG. 7 , the mold 50 used in the circulation groove forming step ST01 has a punch 55 having a shape corresponding to the circulation groove 30. The mold 50 (punch 55) that forms the circulation groove 30 has a first portion 51 that corresponds to the main groove portion 31 of the circulation groove 30 and a second portion 52 that corresponds to the connection end portion 33 of the circulation groove 30. The first portion 51 is formed to have a semicircular or Gothic arch cross section. In this embodiment, the second portion 52 of the mold 50 is formed to have a spherical cross section. By providing the second portion 52, a connection end portion 33 (see FIG. 6 ) having a curved cross section that connects from the bottom surface of the circulation groove 30 to the bottom surface of the spiral groove 20 is formed at the boundary between the circulation groove 30 and the spiral groove 20 in the nut 1.

[0028] FIG. 8 is a cross-sectional view of the nut 1 showing the spiral groove forming step ST02. As shown in FIG. 8 , the spiral groove forming step ST02 is performed after the circulation groove forming step ST01. In the spiral groove forming step ST02, a removal process is performed to form the spiral groove 20 on the inner peripheral surface 11 of the nut 1 so that the inner diameter is smaller than that of the circulation groove 30. In this embodiment, the spiral groove 20 is formed by cutting. In the spiral groove forming step ST02, the spiral groove 20 is formed so that at least a portion of the connection end 33 remains on the nut 1 when the spiral groove 20 is formed. In other words, in the spiral groove forming step ST02, not all of the connection end 33 in the circulation groove 30 is removed. Note that the spiral groove 20 may be formed by a separate removal process. In the spiral groove forming process ST02, the spiral groove 20 is formed so that the first end 38 of the circulation groove 30 is connected to the first end 21 of the spiral groove 20, and the second end 39 of the circulation groove 30 is connected to the second end 22 of the spiral groove 20.

[0029] After the circulation groove forming step ST01 and the spiral groove forming step ST02 are completed, the chamfering step ST03 is performed. In the chamfering step ST03, a chamfered portion 40 is formed between the circulation groove 30 and the spiral groove 20 in the nut 1 by removal processing. More specifically, as described above, the chamfered portion 40 is formed between the connection end 33 of the circulation groove 30 and the spiral groove 20 (see FIG. 4 ). Through these steps, a nut 1 having the spiral groove 20 and the circulation groove 30 on its inner circumferential surface 11 is formed. Although not described in the above embodiment, for example, a rough forming step may be performed before the circulation groove forming step ST01, in which the outer shape of the nut 1 is roughly formed from a base material to obtain a blank for the nut 1. Furthermore, an outer periphery finishing step may be performed, in which a gear, a flange, or the like is formed on the outer periphery of the blank for the nut 1 by performing plastic processing, removal processing, or the like on the outer periphery of the nut 1. Furthermore, a separate heat treatment step, such as carburizing, carbonitriding, quenching, or tempering, may be performed after the chamfering step ST03.

[0030] (Operations and Effects) The ball screw 10 of this embodiment includes a screw shaft 7, a nut 1, and a plurality of balls 8. The nut 1 has a helical groove 20 and a circulation groove 30 formed deeper than the helical groove 20. The circulation groove 30 has a connection end 33 formed in a spherical shape (curved cross-section) that connects from the bottom surface of the circulation groove 30 to the bottom surface of the helical groove 20 at the boundary between the helical groove 20 and the circulation groove 30. This smooths the connection between the helical groove 20 and the circulation groove 30 at the boundary, preventing ball clogging and poor circulation caused by a step at the boundary. Because the connection end 33 is formed in a curved cross-section, robustness at the connection end 33 (the boundary between the helical groove 20 and the circulation groove 30) can be improved compared to, for example, conventional techniques in which the helical groove 20 and the circulation groove 30 without the connection end 33 are formed and then the step is removed by crowning, cutting, or the like. This improves the connection between the spiral groove 20 and the circulation groove 30 compared to the prior art. This provides a ball screw 10 that can suppress ball clogging and poor circulation compared to the prior art. As a result, it is possible to suppress the occurrence of vibration spikes and a decrease in the circulation efficiency of the balls 8 during operation of the ball screw 10 compared to the prior art. Furthermore, the improved robustness can ease the positioning accuracy required when chamfering the boundary by crowning or the like. This improves the workability of the spiral groove 20 and the circulation groove 30.

[0031] The connection end 33 is formed to have an arc-shaped cross section in a cross-sectional view along the movement direction of the ball 8. By forming the connection end 33 in this manner, the connection between the spiral groove 20 and the circulation groove 30 is smoothed, and ball clogging and poor circulation caused by a step at the boundary between the spiral groove 20 and the circulation groove 30 can be suppressed. In addition, the versatility of the shape of the connection end 33 can be improved.

[0032] In a cross-sectional view along the movement direction of the ball 8, the connection end 33 may be formed in the shape of a quadratic curve. Even when the connection end 33 is formed in this manner, the connection between the spiral groove 20 and the circulation groove 30 can be made smooth, and the occurrence of ball clogging and poor circulation due to the step at the boundary between the spiral groove 20 and the circulation groove 30 can be suppressed. Therefore, the versatility of the shape of the connection end 33 can be improved.

[0033] A chamfered portion 40 is formed between the connection end 33 and the spiral groove 20. The chamfered portion 40 is formed by, for example, crowning or cutting. This allows for a smoother connection between the spiral groove 20 and the circulation groove 30. This further reduces ball clogging and poor circulation caused by steps.

[0034] The method for manufacturing a ball screw nut according to this embodiment includes a circulation groove forming step ST01 in which a circulation groove 30 is formed on the inner circumferential surface 11 of the nut 1 by forging, and a spiral groove forming step ST02 in which a helical groove 20 having an inner diameter smaller than that of the circulation groove 30 is formed on the inner circumferential surface 11 of the nut 1 by removal processing. Through the circulation groove forming step ST01 and the spiral groove forming step ST02, a nut 1 for a ball screw 10 is manufactured, which has a helical groove 20 in which a plurality of balls 8 roll and a circulation groove 30 that returns the balls 8 from one end of the helical groove 20 to the other end. A mold 50 that forms the circulation groove 30 has a first portion 51 corresponding to the main groove portion 31 of the circulation groove 30 and a second portion 52 corresponding to an end of the circulation groove 30. The second portion 52 is formed, for example, in a spherical shape. By forming the second portion 52 in the mold 50, a cross-sectionally curved portion (connection end 33) is formed at the boundary between the circulation groove 30 and the spiral groove 20 in the nut 1, connecting the bottom surface of the circulation groove 30 to the bottom surface of the spiral groove 20. Furthermore, by including the second portion 52 in the mold 50, the connection between the spiral groove 20 and the circulation groove 30 at the boundary is smoothed, thereby preventing ball clogging and poor circulation due to a step at the boundary. The portion of the circulation groove 30 formed using this mold 50 that corresponds to the second portion 52 of the mold 50 (connection end 33) is formed with a curved surface. This improves robustness at the connection end 33 (the boundary between the spiral groove 20 and the circulation groove 30) compared to, for example, conventional techniques in which the circulation groove 30 and the spiral groove 20 are formed without the connection end 33 and then the step is removed by crowning, cutting, or the like. Therefore, a nut 1 can be manufactured that has an improved connection between the spiral groove 20 and the circulation groove 30 compared to conventional techniques. Therefore, a method for manufacturing a nut for a ball screw can be provided that can suppress ball clogging and poor circulation compared to conventional techniques. Furthermore, by applying the nut 1 manufactured by this manufacturing method to the ball screw 10, it is possible to suppress the occurrence of vibration spikes and a decrease in the circulation efficiency of the balls 8 during operation of the ball screw 10 compared to conventional techniques. Furthermore, by forming the second portions 52, which are both ends of the die 50, into a spherical shape, the forged excess material on the nut 1 side flows smoothly during the forging process.This reduces the stress acting on the die 50, thereby suppressing damage to the die 50 and shortening of the die life compared to conventional techniques. Furthermore, by rectifying the forged excess material during forging, it is possible to suppress the forged excess material from protruding toward the inner periphery of the nut 1 more than conventional techniques. This makes it possible to forge a thick nut 1, which was difficult to forge using conventional techniques. Therefore, the versatility of the nut 1 that can be manufactured using this method for manufacturing a ball screw nut can be improved.

[0035] The second portion 52 is formed in a spherical shape. As a result, the shape of the connection end portion 33 of the nut 1 corresponding to the second portion 52 is formed in an arc-shaped cross section in a cross section along the movement direction of the balls 8. This makes it possible to manufacture a nut 1 that smooths the connection between the spiral groove 20 and the circulation groove 30 and can prevent ball clogging and poor circulation caused by a step at the boundary between the spiral groove 20 and the circulation groove 30. In addition, the versatility of the shape of the connection end portion 33 of the nut 1 can be improved.

[0036] The method for manufacturing a ball screw nut further includes a chamfering step ST03, after the circulation groove forming step ST01 and the spiral groove forming step ST02, in which a chamfered portion 40 is formed between the circulation groove 30 and the spiral groove 20 by removal processing. The chamfered portion 40 is formed by, for example, crowning processing or cutting processing. This allows for a smoother connection between the spiral groove 20 and the circulation groove 30. This makes it possible to manufacture a high-performance ball screw nut 1 that further suppresses ball clogging and poor circulation caused by steps.

[0037] Second Embodiment Next, a second embodiment of the present invention will be described. In the following description, components similar to those in the first embodiment described above will be assigned the same reference numerals and will not be described again. The specific configuration is not limited to these embodiments and can be modified as appropriate without departing from the spirit and scope of the present invention. FIG. 9 is a perspective view of a mold 250 used in the circulation groove forming step ST01 of the second embodiment. FIG. 10 is a perspective view of the boundary between the spiral groove 20 and the circulation groove 230 formed in the nut 201 according to the second embodiment, viewed from the inner diameter side. FIG. 11 is a cross-sectional view taken along line XI-XI in FIG. 10. The second embodiment differs from the first embodiment described above in that the connection end 233 of the circulation groove 230 is formed with an inclined cross section.

[0038] As shown in FIG. 9 , the mold 250 for forming the circulation groove in the second embodiment has a punch 255 having a shape corresponding to the circulation groove 230. The mold 250 (punch 255) for forming the circulation groove 230 has a first portion 251 corresponding to the main groove portion 31 of the circulation groove 230 and a second portion 252 corresponding to the connecting end portion 233 of the circulation groove 230. As in the first embodiment, the first portion 251 is formed with a semicircular or Gothic arch cross section. In the second embodiment, the second portion 252 of the mold 250 is formed with a conical surface shape whose diameter decreases from the first portion 251 side toward the end side. In the circulation groove forming step ST01, the mold 250 having the second portion 252 thus formed is used to form the circulation groove 230 on the inner circumferential surface 11 of the nut 201. Then, as in the first embodiment, the spiral groove 20 is formed by cutting in the spiral groove forming step ST02. As a result, as shown in Figures 10 and 11, a connection end 233 having a cross-sectional sloped shape that connects from the bottom surface of the circulation groove 230 to the bottom surface of the spiral groove 20 is formed at the boundary between the circulation groove 230 and the spiral groove 20 in the nut 201.

[0039] As shown in Figure 11, the connection end 233 of the second embodiment has a slope that is inclined at approximately 45° in a cross-sectional view along the moving direction of the ball 8 (see Figure 1). Note that the angle of inclination when the connection end 233 is formed with a sloped cross section is not limited to the above-mentioned angle. Furthermore, as in the first embodiment, after the circulation groove 230 and the spiral groove 20 are formed, a chamfered portion (see Figure 4; not shown in Figure 11) may be formed between the connection end 233 and the spiral groove 20 by performing crowning processing, cutting processing, or the like.

[0040] According to the ball screw 10 and the method for manufacturing a ball screw nut of the second embodiment, the second portion 252 of the mold 250 is formed into a conical surface shape that decreases in diameter from the first portion 251 toward the end. Therefore, the connection end 233 of the nut 201 corresponding to the second portion 252 is formed into a sloped shape in a cross-sectional view along the movement direction of the balls 8. In other words, the circulation groove 230 formed using this mold 250 has a connection end 233 formed into a conical surface shape (a sloped cross-section) that connects from the bottom surface of the circulation groove 230 to the bottom surface of the spiral groove 20 at the boundary between the spiral groove 20 and the circulation groove 230. This achieves the same effects as the first embodiment. That is, the connection between the spiral groove 20 and the circulation groove 230 is smooth, and ball clogging and poor circulation caused by a step at the boundary between the spiral groove 20 and the circulation groove 230 can be suppressed. Therefore, it is possible to provide a ball screw 10 and a method for manufacturing a nut 201 for this ball screw 10 that can suppress the occurrence of ball clogging and poor circulation compared to the prior art. In addition, it is possible to increase the versatility of the shape of the connection end 233 of the nut 201.

[0041] The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, in the first embodiment, the second portion 52 of the punch 55 is described as being spherical. However, the shape of the second portion 52 is not limited to the above-described embodiment. For example, the second portion 52 of the punch 55 may be ellipsoidal. In this case, the shape of the connection end 33 of the nut 1 corresponding to the second portion 52 is curved in a cross-sectional view along the movement direction of the ball 8. This allows for a smooth connection between the spiral groove 20 and the circulation groove 30, thereby manufacturing a nut 1 that can prevent ball clogging and poor circulation due to a step at the boundary between the spiral groove 20 and the circulation groove 30. Furthermore, the shape of the connection end 33 of the nut 1 can be made more versatile.

[0042] The shape of the connection end 33 may be formed to have at least one of a sloped cross-section and a curved cross-section that connects the bottom surface of the circulation groove 30 to the bottom surface of the spiral groove 20. For example, the connection end 33 may have both a sloped cross-section and a curved cross-section. Alternatively, the connection end 33 may have multiple curved surfaces with different curvatures. In this case, however, it is preferable that the multiple curved surfaces with different curvatures are smoothly connected to each other. In the circulation groove forming step ST01, the circulation groove 30, 230 may be formed by plastic processing other than plastic processing (forging) using a mold 50 and a cam mechanism (not shown). In addition to the circulation groove forming step ST01 and the spiral groove forming step ST02, another step may be added in which removal processing, plastic processing, or both are performed on the inner peripheral surface 11 of the nut 1, 201.

[0043] The present disclosure may also be applied to combinations of the following configurations: (1) A ball screw comprising: a screw shaft; a nut; and a plurality of balls arranged between the nut and the screw shaft, wherein the nut has: a helical groove formed in a spiral shape on an inner peripheral surface of the nut and in which the plurality of balls roll; and a circulation groove formed on the inner peripheral surface and returning the balls from one end of the helical groove to the other end, wherein the depth dimension of the circulation groove is formed greater than the depth dimension of the helical groove, and the circulation groove has a connection end portion at a boundary between the helical groove and the circulation groove, the connection end portion being formed to have at least one of a sloped cross-section and a curved cross-section that connects from a bottom surface of the circulation groove to a bottom surface of the helical groove. (2) The ball screw according to (1), wherein the connection end portion has a circular arc cross-section when viewed in a cross section along the movement direction of the balls at the connection end portion. (3) The ball screw according to (1) or (2), wherein, in a cross-sectional view along the moving direction of the ball at the connection end, the connection end is formed in a shape of a quadratic curve in cross section. (4) The ball screw according to (1), wherein the connection end is formed in a conical surface shape whose diameter decreases from the circulation groove side toward the spiral groove side in the moving direction of the ball. (5) The ball screw according to any one of (1) to (4), wherein a chamfered portion is further formed between the connection end and the spiral groove. (6) A method for manufacturing a nut for a ball screw having a helical groove in which a plurality of balls roll and a circulation groove that returns the balls from one end of the helical groove to the other end, the method comprising: a circulation groove forming step of forming the circulation groove on an inner peripheral surface of the nut by forging; and a helical groove forming step of forming the helical groove on the inner peripheral surface by subtractive machining so that the inner shape is smaller than that of the circulation groove, wherein a mold that forms the circulation groove in the circulation groove forming step is formed with a first portion corresponding to a main groove portion of the circulation groove and a second portion corresponding to an end portion of the circulation groove, and the second portion forms at least one of a sloped cross-sectional shape and a curved cross-sectional shape that connects from the bottom surface of the circulation groove to the bottom surface of the helical groove at a boundary portion in the nut between the circulation groove and the helical groove.(7) The method for manufacturing a nut for a ball screw according to (6), wherein the second portion is formed in a spherical shape. (8) The method for manufacturing a nut for a ball screw according to (6) or (7), wherein the second portion is formed in an ellipsoidal shape. (9) The method for manufacturing a nut for a ball screw according to any one of (6) to (8), wherein the second portion is formed in a conical shape whose diameter decreases from the first portion side toward the end side. (10) The method for manufacturing a nut for a ball screw according to any one of (6) to (9), further comprising, after the circulation groove forming step and the spiral groove forming step, a chamfering step of forming a chamfered portion in the nut between the circulation groove and the spiral groove by removal processing.

[0044] 1, 201 Nut 7 Screw shaft 8 Ball 10 Ball screw 11 Inner peripheral surface (of nut) 20, 920 Spiral groove 21 First end (one end of spiral groove) 22 Second end (other end of spiral groove) 30, 230, 930 Circulation groove 33, 233 Connection end 40 Chamfered portion 50, 250, 950 Mold 51, 251 First portion 52, 252 Second portion ST01 Circulation groove forming process ST02 Spiral groove forming process ST03 Chamfering process

Claims

1. A ball screw comprising: a screw shaft; a nut; and a plurality of balls arranged between the nut and the screw shaft, wherein the nut has: a helical groove formed in a spiral shape on the inner peripheral surface of the nut and in which the plurality of balls roll; and a circulation groove formed on the inner peripheral surface and which returns the balls from one end of the helical groove to the other end, wherein the depth dimension of the circulation groove is formed to be greater than the depth dimension of the spiral groove, and wherein the circulation groove has a connection end portion at the boundary between the spiral groove and the circulation groove, which is formed to have at least one of a sloped cross-sectional shape and a curved cross-sectional shape that connects from the bottom surface of the circulation groove to the bottom surface of the spiral groove.

2. The ball screw according to claim 1, wherein the connection end is formed to have an arc-shaped cross section in a cross-sectional view along the direction of movement of the ball at the connection end.

3. A ball screw according to claim 1 or 2, wherein, in a cross-sectional view along the direction of movement of the ball at the connection end, the connection end is formed in the shape of a quadratic curve in cross section.

4. A ball screw according to claim 1, wherein the connection end is formed in a conical surface shape whose diameter decreases from the circulation groove side toward the spiral groove side in the direction of movement of the balls.

5. A ball screw according to any one of claims 1 to 4, wherein a chamfered portion is further formed between the connection end and the spiral groove.

6. A method for manufacturing a nut for a ball screw having a helical groove in which a plurality of balls roll and a circulation groove that returns the balls from one end of the helical groove to the other, comprising: a circulation groove forming step of forming the circulation groove on the inner peripheral surface of the nut by forging; and a helical groove forming step of forming the helical groove on the inner peripheral surface by subtractive machining so that the inner shape is smaller than that of the circulation groove, wherein the mold that forms the circulation groove in the circulation groove forming step has a first portion corresponding to a main groove portion of the circulation groove and a second portion corresponding to an end of the circulation groove, and wherein the second portion forms at least one of a sloped cross-sectional shape and a curved cross-sectional shape that connects the bottom surface of the circulation groove to the bottom surface of the helical groove at the boundary between the circulation groove and the helical groove in the nut.

7. The method for manufacturing a nut for a ball screw according to claim 6, wherein the second portion is formed in a spherical shape.

8. A method for manufacturing a nut for a ball screw according to claim 6 or 7, wherein the second portion is formed in an ellipsoidal shape.

9. A method for manufacturing a nut for a ball screw according to any one of claims 6 to 8, wherein the second portion is formed in a conical surface shape whose diameter decreases from the first portion side toward the end side.

10. A method for manufacturing a nut for a ball screw as described in any one of claims 6 to 9, further comprising a chamfering process for forming a chamfered portion between the circulation groove and the spiral groove in the nut by removal processing after the circulation groove forming process and the spiral groove forming process.

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