Ball screw device

Abstract

To circulate a ball of a ball screw device smoothly.SOLUTION: A ball screw device 1 comprises: a nut having a nut main body 5 and a cover 6; a screw shaft 3; a rolling path 10 formed of a screw groove 5a of the nut main body 5 and a screw groove 3a of the screw shaft 3; a circulation path 20 formed of an external peripheral face of the nut main body 5 and the cover 6; a pair of connecting paths 30 for making one end of the circulation path 20 and one end of the rolling path 10 communicate with each other, and also making the other end of the circulation path 20 and the other end of the rolling path 10 communicate with each other; and a large number of balls 4 arranged at the rolling path 10, the circulation path 20 and the pair of connecting paths 30. The circulation path 20 is formed along a cylinder part 5c of the external peripheral face of the nut main body 5.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a ball screw device.

Background Art

[0002] In a ball screw device, a large number of balls are arranged so as to be freely rollable on a rolling path constituted by a screw groove formed on the inner circumference of a nut and a screw groove formed on the outer circumference of a screw shaft, and the balls that have reached the end point of the rolling path are returned to the starting point of the rolling path via a circulation path.

[0003] As a method for circulating the balls of a ball screw device, a tube type in which a circulation path is formed using a tube (for example, see Patent Document 1 below) and a guide plate type in which a circulation path is formed by a guide plate fixed to the outer circumference of a nut (for example, see Patent Document 1 below) are known.

[0004] In a tube-type ball screw device, since the tube protrudes from the outer circumference of the nut, the ball screw device is enlarged.

[0005] On the other hand, as shown in FIG. 14, a guide plate-type ball screw device 100 forms a circulation path 112 with a nut body 103 and a guide plate 105 fixed to the outer circumference thereof. In this ball screw device 100, the balls that have reached the end point of the rolling path 108 formed by the screw groove 107 of the nut body 103 and the screw groove 106 of the screw shaft 102 are returned to the starting point of the rolling path 108 via the connection path 110, the circulation path 112, and the other connection path (not shown). In this ball screw device 100, since the guide plate 105 does not protrude from the outer circumference of the nut body 103, the ball screw device is downsized compared to the above-described tube-type ball screw device.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

[0007] In a ball screw device, it is necessary to ensure that the balls circulate smoothly within the ball circulation path. For example, in the ball screw device shown in Patent Document 2 (see Figure 14), the direction in which the connecting path 110 extends is set to be the tangential direction T of the screw grooves 106 and 107, and also in the lead angle direction, thereby stabilizing the behavior of the balls 104 at the boundary between the running path 108 and the connecting path 110.

[0008] However, in this ball screw device 100, the guide plate 105 is fixed to a flat surface 103a provided on the outer circumferential surface of the nut body 103, so the circulation path 112 formed by these is formed along the flat surface 103a. In this case, a bend 120 is formed at the boundary between the circulation path 112 and the connecting path 110, so the ball 104 may get caught in this bend 120, hindering the circulation of the ball 104 and potentially preventing the screw shaft 102 and the nut body 103 from rotating smoothly relative to each other.

[0009] Therefore, the present invention aims to smoothly circulate the balls in a ball screw device, thereby enabling smooth relative rotation between the nut and the screw shaft. [Means for solving the problem]

[0010] To achieve the above objective, the present invention provides a ball screw device comprising: a nut body having a screw groove formed on its inner circumference; a nut having a circulation path forming member fixed to the outer circumferential surface of the nut body; a screw shaft having a screw groove formed on its outer circumference; a traffic path formed by the screw groove of the nut body and the screw groove of the screw shaft; a circulation path formed by the outer circumferential surface of the nut body and the circulation path forming member; a pair of connecting paths connecting one end of the circulation path to one end of the traffic path, and the other end of the circulation path to the other end of the traffic path, respectively; and a number of balls arranged in the traffic path, the circulation path, and the pair of connecting paths, characterized in that the circulation path is formed along the cylindrical portion of the outer circumferential surface of the nut body.

[0011] Thus, in the ball screw device of the present invention, the circulation path formed by the outer circumferential surface of the nut body and the circulation path forming member is formed in an arch shape along the cylindrical portion of the outer circumferential surface of the nut body. As a result, the bending angle at the boundary between the circulation path and the connecting path becomes gentler, making it less likely for the ball to get stuck at this boundary and allowing the ball to circulate smoothly.

[0012] By the way, the ball screw device shown in Figure 14 is assembled by placing the balls 104 in the circulation path 112 formed in the guide plate 105, and then fixing the guide plate 105 to the nut body 103. In this case, it is necessary to fix the guide plate 105 to the nut body 103 while being careful not to let the balls 104 fall out of the circulation path 112, which makes the workability poor.

[0013] Therefore, it is preferable to form the circulation path with a circulation groove formed in the cylindrical portion on the outer circumferential surface of the nut body and a circulation path forming member that covers the circulation groove. In this case, the ball screw device can be easily assembled by placing the ball in the circulation groove formed on the outer circumferential surface of the nut body and then covering it with the circulation path forming member from the outer circumference.

[0014] By providing a recess in a circumferential region of the outer surface of the nut body that is recessed inward compared to other circumferential regions, and housing the guide plate within the depth region of the recess, the circulation path forming member does not protrude from the outer surface of the nut body, thus reducing the outer diameter of the nut.

[0015] The circulation path forming member has a groove that forms the circulation path, and by providing an opening at the bottom of the groove that extends along the direction of the groove's extension, the outer diameter of the circulation path forming member can be reduced so that the edge of this opening becomes the outermost diameter part. This makes it easier to secure space for other members to be provided around the outer circumference of the circulation path forming member.

[0016] The circulation path forming member can be fixed to the nut body by crimping or adhesive.

[0017] The ball screw device described above includes, for example, a coil-shaped ball pick-up member that screws into the thread groove of the nut body. A guide surface provided at the end of this ball pick-up member guides the ball from the rolling path to the connecting path. At this time, if the ball collides with the guide surface of the ball pick-up member, the ball pick-up member may rotate relative to the nut, causing the position of the guide surface to shift. Therefore, it is preferable to provide an anti-rotation means to restrict the relative rotation between the ball pick-up member and the nut.

[0018] In the ball screw device described above, the nut body and the inner ring of the rolling bearing can be integrally formed as a single component. Alternatively, the nut body and the inner ring of the rolling bearing may be formed as separate components, and a mounting surface for attaching the inner ring of the rolling bearing may be provided on the outer circumferential surface of the nut body. In this case, the circulation path can be extended to the axial region of the inner ring, i.e., the axial region of the mounting surface, thereby increasing the length of the circulation path and increasing the load capacity of the ball screw device.

[0019] In a ball screw device, usually, a thread groove is formed over the entire inner peripheral surface of the nut body, and a part of it functions as a rolling path. In this case, in order to prevent the balls from entering the part of the thread groove that does not function as a rolling path, it is necessary to provide a member such as a ball lifting member. Therefore, a cylindrical portion without a thread groove may be provided in a region on one axial side of the rolling path in the inner peripheral surface of the nut body. Thus, by omitting the thread groove that does not function as a rolling path, it is possible to avoid the balls from entering the thread groove that does not function as a rolling path without providing a member such as a ball lifting member.

Advantages of the Invention

[0020] As described above, according to the present invention, the balls of the ball screw device can be smoothly circulated, and the nut and the screw shaft can be smoothly rotated relative to each other.

Brief Description of the Drawings

[0021] [Figure 1] It is a plan view of a ball screw device according to a first embodiment of the present invention. [Figure 2] It is a cross-sectional view taken along line A-A of FIG. 1. [Figure 3] It is an exploded perspective view of the ball screw device of FIG. 1. [Figure 4] It is a cross-sectional view taken along line B-B of FIG. 1. [Figure 5] It is a cross-sectional view taken along line C-C of FIG. 2. [Figure 6] It is a side view seen from direction D of FIG. 1. [Figure 7] It is a side view seen from direction E of FIG. 1. [Figure 8] It is a perspective view of a ball screw device according to a second embodiment of the present invention. [Figure 9] It is a cross-sectional view of the ball screw device of FIG. 8. [Figure 10] It is a perspective view of a ball screw device according to a third embodiment of the present invention. [Figure 11] It is a partially exploded perspective view of the ball screw device of FIG. 10. [Figure 12] Figure 10 is a plan view of a ball screw device. [Figure 13] This is a cross-sectional view of a ball screw device according to a fourth embodiment of the present invention. [Figure 14] This is a cross-sectional view of a conventional ball screw device. [Modes for carrying out the invention]

[0022] Hereinafter, embodiments of the present invention will be described based on the drawings.

[0023] As shown in Figures 1 to 3, the ball screw device 1 according to the first embodiment of the present invention mainly comprises a nut 2, a screw shaft 3, and a number of balls 4. The nut 2 has a nut body 5 and a cover 6 as a circulation path forming member. This ball screw device 1 is a nut rotation type in which the screw shaft 3 is moved axially by rotating the nut 2, and the nut 2 is rotatably supported by a rolling bearing (for example, a deep groove ball bearing). In this embodiment, the nut body 5 and the inner ring 7a of the rolling bearing are formed integrally as a single part.

[0024] A helical thread groove 5a is formed on the inner circumference of the nut body 5, and a helical thread groove 3a is formed on the outer circumference of the screw shaft 3. A portion of the thread groove 5a of the nut body 5 and a portion of the thread groove 3a of the screw shaft 3 form a track 10 for the ball 4 (see Figure 2).

[0025] As shown in Figure 4, the ball screw device 1 is provided with a circulation path 20 and a pair of connecting paths 30 for circulating the balls 4 in the circulation path 10. The circulation path 20 is formed by the outer surface of the nut body 5 and the cover 6. The pair of connecting paths 30 connect one end of the circulation path 20 to one end of the circulation path 10, and the other end of the circulation path 20 to the other end of the circulation path 10. Numerous balls 4 are arranged to roll freely in the circulation path formed by the circulation path 10, the circulation path 20, and the pair of connecting paths 30. Balls 4 that reach the end of the circulation path 10 are returned to the starting point of the circulation path 10 by passing through one connecting path 30, the circulation path 20, and the other connecting path 30.

[0026] A recess 5b is provided in a circumferential region of the outer surface of the nut body 5 (see Figure 3). A cylindrical portion 5c is provided at the bottom of the recess 5b. A large-diameter cylindrical portion 5d, which is larger in diameter than the cylindrical portion 5c, is provided in the circumferential region of the outer surface of the nut body 5 other than the recess 5b. A circulation groove 5e is formed in the cylindrical portion 5c. As shown in Figure 4, a pair of through holes 5f are formed in the nut body 5, and this pair of through holes 5f functions as a pair of connecting passages 30. One end of each through hole 5f opens to the end of the circulation groove 5e, and the other end opens to the screw groove 5a. The through holes 5f are formed in a straight line by machining with a drill or the like. In the illustrated example, two through holes 5f are connected to the screw groove 5a at positions that differ in phase by 180° + 360n° (n=1, 2...) and are provided parallel to each other.

[0027] The cover 6 is fixed to the outer circumferential surface of the nut body 5, and in the illustrated example, it is attached to the recess 5b of the nut body 5. The cover 6 in the illustrated example is a plate-shaped member with a uniform thickness, and is made of a press-formed metal plate such as a steel plate. In addition, the cover 6 may be formed from a resin molded product or a sintered body (MIM molded product). The cover 6 integrally has a cylindrical portion 6b, a raised portion 6c that rises outward from the cylindrical portion 6b, and mounting portions 6d provided at both circumferential ends of the cylindrical portion 6b (see Figures 1 and 3). The inner surface of the raised portion 6c of the cover 6 becomes a groove 6a (see Figure 2). The cylindrical portion 6b covers the cylindrical portion 5c on the outer circumferential surface of the nut body 5 from the outer diameter side, and the raised portion 6c covers the circulation groove 5e of the nut body 5 from the outer diameter side. The mounting portions 6d are fixed to the outer circumferential surface of the nut body 5 by appropriate means such as crimping or adhesive.

[0028] The cover 6 is housed within the depth region of the recess 5b on the outer circumferential surface of the nut body 5. More specifically, the outermost diameter portion of the cover 6, i.e., the top of the raised portion 6c, is positioned on the inner diameter side of the virtual cylindrical surface that extends from the large-diameter cylindrical surface 5d on the outer circumferential surface of the nut body 5. As a result, the cover 6 does not protrude from the outer circumference of the nut body 5, thereby reducing the outer diameter of the nut 2.

[0029] The circulation path 20 is formed along the cylindrical portion 5c on the outer circumferential surface of the nut body 5 (see Figure 4). In the illustrated example, the circulation path 20 is formed by the circulation groove 5e formed in the cylindrical portion 5c of the nut body 5 and the groove 6a (inner surface of the raised portion 6c) provided along the cylindrical portion 6b of the cover 6. The circulation path 20 extends in a direction inclined with respect to the axial and circumferential directions (see Figure 1). In the illustrated example, the circulation path 20 extends in a direction that forms a certain angle with respect to the circumferential direction. As shown in Figure 1, when the circulation path 20 (raised portion 6c) is viewed from the outer diameter side, the dimension of the circulation path 20 in the direction perpendicular to the axial direction (up and down direction in Figure 1) is larger than the dimension in the axial direction (left and right direction in Figure 1). In particular, in the illustrated example, the dimension of the circulation path 20 in the direction perpendicular to the axial direction is larger than the outer diameter of the screw shaft 3 (outer diameter at the top of the screw threads).

[0030] The connecting passage 30 consists of, for example, a through hole 5f provided in the nut body 5. One end of the connecting passage 30 is connected to the end of the turnaround passage 10, and the other end of the connecting passage 30 is connected to the end of the circulation passage 20. In a cross-section perpendicular to the axis (see Figure 5), the connecting passage 30 extends tangentially to the end (start or end) of the turnaround passage 10, and in an axial cross-section, it extends perpendicular to the axis. In this embodiment, as shown in Figure 4, the start and end points of the turnaround passage 10 are located at positions where the phase difference is 180° + 360n° (n=1, 2...), i.e., on a plane L that includes the axis of the nut body 5, and the connecting passages 30 extend from these start and end points, respectively. In this case, the pair of connecting passages 30 are parallel. Alternatively, the start and end points of the turnaround passage 10 may be located on the circulation passage 20 side (above Figure 4) of the plane L, and the connecting passages 30 may extend from there. In this case, the distance between the pair of connecting paths 30 gradually narrows from the turnaround path 10 towards the circulation path 20. Alternatively, the start and end points of the turnaround path 10 may be set on the opposite side of the circulation path 20 from the plane L (lower in Figure 4), and the connecting paths 30 may be extended from there. In this case, the distance between the pair of connecting paths 30 gradually widens from the turnaround path 10 towards the circulation path 20.

[0031] A bend 40 is formed at the boundary between the circulation path 20 and the connecting path 30. In this case, since the circulation path 20 is formed in an arch shape along the cylindrical portion 5c of the nut body 5, the bending angle at the boundary between the circulation path 20 and the connecting path 30 can be made gentler. As a result, the balls 4 move smoothly from the connecting path 30 to the circulation path 20, and the balls 4 circulate smoothly within the circulation path, so that the nut 2 and the screw shaft 3 can rotate relative to each other smoothly. In particular, since the ball screw device 1 of this embodiment is a nut rotation type, as described above, the circulation path 20 is formed in an arch shape along the cylindrical portion 5c, so that the centrifugal force when the nut 2 rotates promotes the movement of the balls 4 in the circulation path 20, and the balls 4 circulate even more smoothly.

[0032] Coil-shaped ball pick-up members 8 are screwed into the thread grooves 5a of the nut body 5 on both axial sides of the ball travel path 10 (see Figures 2 and 3). As shown in Figure 5, a guide surface 8a is provided at one end of each ball pick-up member 8. The guide surface 8a of the ball pick-up member 8 closes the thread groove 5a and is positioned to face the connecting path 30. Balls 4 that reach the end of the ball travel path 10 are scooped up by the guide surface 8a of the ball pick-up member 8 and guided to the connecting path 30 without entering the thread grooves 3a and 5a that do not constitute the ball travel path 10.

[0033] If the ball 4 repeatedly collides with the guide surface 8a of the ball pick-up member 8, the ball pick-up member 8 may rotate relative to the nut 2, potentially causing the guide surface 8a to shift position. In this embodiment, a rotation-preventing mechanism is provided to restrict the relative rotation between the ball pick-up member 8 and the nut 2. In this embodiment, the relative rotation between the ball pick-up member 8 and the nut 2 is restricted by engaging them in both rotational directions. Specifically, as shown in Figures 6 and 7, the rotation-preventing mechanism is composed of a locking portion 8b formed by bending the other end of the ball pick-up member 8 (the end opposite to the guide surface 8a) toward the outer diameter, and a locking groove 5g provided on the end face of the nut body 5. By fitting the locking portion 8b of the ball pick-up member 8 into the locking groove 5g of the nut body 5 and engaging them in the circumferential direction, the rotation of the ball pick-up member 8 relative to the nut 2 is restricted.

[0034] The ball screw device 1 described above can be assembled by following the procedure below. First, the screw shaft 3 is inserted into the inner circumference of the nut body 5, and the screw grooves 5a and 3a form the ball path 10. Then, the ball pick-up member 8 is screwed into the screw groove 5a of the nut body 5 from both axial sides, and the guide surface 8a is positioned in the predetermined position (see Figure 5). Then, the ball 4 is inserted through the through hole 5f of the nut body 5, filling the ball path 10 and the pair of connecting paths 30 with the ball 4. After that, the ball 4 is fitted into the circulation groove 5e of the nut body 5, and the cover 6 is placed over its outer circumference and fixed to the nut body 5, thereby completing the ball screw device 1. In this way, in the ball screw device 1 of this embodiment, the ball 4 can be easily incorporated into the circulation path 20 by placing the ball 4 in the circulation groove 5e of the nut body 5 and then covering it with the cover 6.

[0035] The present invention is not limited to the embodiments described above. Other embodiments of the present invention will be described below, but details similar to those of the embodiments described above will be omitted.

[0036] In the second embodiment shown in Figure 8, an opening 6e is provided in the raised portion 6c of the cover 6. The opening 6e is located at the top of the raised portion 6c (i.e., the bottom of the groove 6a) and extends along the direction of extension of the raised portion 6c (i.e., the direction of extension of the groove 6a). As shown in Figure 9, the width W of the opening 6e is smaller than the diameter D of the ball 4. This prevents the ball 4 from falling out of the opening 6e. In this cover 6, the edge of the opening 6e, or in the illustrated example, the end face of the raised portion 6c provided around the opening 6e, is the outermost diameter portion, and this outermost diameter portion is located on the inner diameter side than the outermost diameter portion (top of the raised portion 6c) of the cover 6 in the embodiment shown in Figure 2. This reduces the outer diameter of the cover 6, making it easier to avoid interference with other members arranged on the outer circumference of the cover 6.

[0037] In the third embodiment shown in Figure 10, the nut body 5 and the inner ring 7a of the rolling bearing 7 are formed separately and then fixed together. As shown in Figure 11, a mounting surface 5h is provided at one axial end of the outer circumferential surface of the nut body 5, and the inner ring 7a is fixed to this mounting surface 5h by press-fitting or the like. The recess 5b (cylindrical portion 5c) formed on the outer circumferential surface of the nut body 5 extends to the axial region of the inner ring 7a of the rolling bearing 7, and a part of the cover 6 attached to the recess 5b is positioned on the inner circumference of the inner ring 7a (see Figure 10). A radial gap is formed between the cylindrical portion 6b of the cover 6 and the inner circumferential surface of the inner ring 7a, and a part of the raised portion 6c of the cover 6 fits into this gap. That is, as shown in Figure 12, a part of the raised portion 6c (circulation path 20) of the cover 6 is positioned on the inner circumference of the inner ring 7a.

[0038] In the embodiment shown in Figure 1, the nut body 5 and the inner ring 7a are formed integrally, so it is not possible to form a circulation groove 5e in the axial region of the inner ring 7a on the outer circumferential surface of the nut body 5. In contrast, in the embodiments shown in Figures 10 to 12, the inner ring 7a is assembled to the nut body 5 after the circulation groove 5e and through hole 5f have been formed in the nut body 5, so the circulation groove 5e, i.e., the circulation path 20, can be extended to the axial region of the inner ring 7a, i.e., the axial region of the mounting surface 5h. By extending the circulation path 20 in the axial direction in this way, the axial dimension of the turnout path 10 can be increased, so the number of balls 4 arranged in the turnout path 10 can be increased and the load capacity can be increased.

[0039] In the fourth embodiment shown in Figure 13, the screw grooves 5a on the inner circumferential surface of the nut body 5 are provided only in the portion that functions as a traffic path 10. That is, a cylindrical surface 5i without screw grooves 5a is provided in the region of the inner circumferential surface of the nut body 5 that is axially outward from the traffic path 10. In this case, since no screw grooves that do not function as a traffic path 10 are provided, the situation in which the balls 4 become trapped in such screw grooves can be avoided. Therefore, the ball pick-up member 8 provided in the above embodiment becomes unnecessary, reducing the number of parts and lowering costs. Alternatively, the cylindrical surface 5i may be provided only in the region on one axial side of the traffic path 10 on the inner circumferential surface of the nut body 5, and screw grooves 5a may be formed in the region on the other axial side. In this case, the other axial end of the screw groove 5a reaches the axial end of the inner circumferential surface of the nut body 5, making machining easier. [Explanation of symbols]

[0040] 1. Ball screw device 2 nuts 3 Screw shaft 3a Screw groove 4 balls 5. Nut body 5a Screw groove 5c Cylindrical section 5e Circulation groove 5f through hole 5g locking groove 6. Cover (circulation path forming member) 6a groove 6b Cylindrical section 6c raised section 7 Rolling bearings 7a Inner ring 8. Ball picking mechanism 8a Guide surface 8b Locking part 10 Turning track 20 Circulation route 30 connecting routes

Claims

1. A ball screw device comprising a nut body having a screw groove formed on its inner circumference, a nut having a circulation path forming member fixed to a cylindrical portion on the outer surface of the nut body, a screw shaft having a screw groove formed on its outer circumference, a flow path formed by the screw groove of the nut body and the screw groove of the screw shaft, a circulation path formed by the outer surface of the nut body and the circulation path forming member, a pair of straight connecting paths connecting one end of the circulation path to one end of the flow path, and the other end of the flow path to the other end of the flow path, a bent portion formed at the boundary between the flow path and the connecting paths, and a number of balls arranged in the flow path, the flow path, and the pair of connecting paths, The circulation path is formed in an arch shape along the cylindrical portion on the outer surface of the nut body, from one end to the other. A ball screw device in which the circulation path is formed by a circulation groove formed in the cylindrical portion on the outer circumferential surface of the nut body from one end to the other, and a circulation path forming member that covers the circulation groove.

2. A recess is provided in a circumferential region of the outer surface of the nut body that is recessed inward more than other circumferential regions. The ball screw device according to claim 1, wherein the circulation path forming member is housed within the depth region of the recess.

3. The circulation path forming member has grooves that form the circulation path, An opening is provided at the bottom of the groove, extending along the direction in which the groove extends. The ball screw device according to claim 1 or 2, wherein the edge of the opening is the outermost diameter portion of the circulation path forming member.

4. The ball screw device according to any one of claims 1 to 3, wherein the circulation path forming member is fixed to the nut body by crimping.

5. The ball screw device according to any one of claims 1 to 3, wherein the circulation path forming member is fixed to the nut body by adhesive.

6. The nut body has a coil-shaped ball-picking member that screws into the screw groove, and the end of the ball-picking member is provided with a guide surface that guides the ball from the running path to the connecting path. A ball screw device according to any one of claims 1 to 5, further comprising a rotation-preventing means for restricting the relative rotation between the ball picking member and the nut.

7. The ball screw device according to any one of claims 1 to 6, wherein the nut body and the inner ring of the rolling bearing are integrally formed as a single component.

8. A ball screw device comprising a nut body having a screw groove formed on its inner circumference, a nut having a circulation path forming member fixed to the outer surface of the nut body, a screw shaft having a screw groove formed on its outer circumference, a flow path formed by the screw groove of the nut body and the screw groove of the screw shaft, a circulation path formed by the outer surface of the nut body and the circulation path forming member, a pair of connecting paths connecting one end of the circulation path to one end of the flow path, and the other end of the flow path to the other end of the flow path, and a number of balls arranged in the flow path, the circulation path, and the pair of connecting paths, The circulation path is formed along the cylindrical portion of the outer circumferential surface of the nut body, The outer circumferential surface of the nut body has a mounting surface on which the inner ring of the rolling bearing is attached, A ball screw device in which the circulation path extends to the axial region of the mounting surface.

9. The ball screw device according to any one of claims 1 to 8, wherein a cylindrical portion in which the screw groove is not formed is provided on the inner circumferential surface of the nut body in a region on one axial side of the running path.

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