Ball screw device and electric power steering device

Abstract

A ball screw nut (28) has a communication groove (53) including a pair of groove side surfaces (56) and a groove bottom surface (57). A return tube (41) has a tubular portion (73) inserted into the communication groove (53). A fixture (42) has a pair of side wall portions (91) each disposed between the tubular portion (73) and the corresponding one of the pair of groove side surface (56), a connection wall portion (92) connecting the pair of side wall portions (91) to each other, and spring pieces (93) each provided on each of the pair of side wall portions (91). Each of the pair of groove side surfaces (56) has a recess (61) including a locking surface (62) to which the corresponding spring piece (93) is locked. The outer peripheral surface of the tubular portion (73) includes a planar tube upper surface (81) configured to be in surface contact with the planar inner surface of the connection wall portion (92).

Description

Ball Screw Device and Electric Power Steering Device

[0001] The present disclosure relates to a ball screw device and an electric power steering device.

[0002] For example, as described in Patent Document 1, there is a return tube type ball screw device. Such a ball screw device includes a screw shaft having a screw groove, a ball screw nut having a screw groove, a return tube, a plurality of balls, and a fixture. In the ball screw device, the screw groove of the screw shaft defines a spiral rolling path facing the screw groove of the ball screw nut. The return tube defines a return path connecting a first connection point and a second connection point in the rolling path to each other. The balls are arranged in an endless circulation path defined by the rolling path and the return path. The fixture fixes the return tube to the ball screw nut.

[0003] Japanese Patent Application Laid-Open No. 2007-46689

[0004] The fixture used in the ball screw device of Patent Document 1 is a plate-like member bent according to the shape of the return tube. This fixture is fastened to the ball screw nut with a small screw. Therefore, many parts are required to fix the return tube to the ball screw nut, and parts management becomes complicated.

[0005] One aspect of the present disclosure provides a ball screw device. The ball screw device comprises a screw shaft having a shaft-side screw groove; a ball screw nut having a nut-side screw groove, wherein the nut-side screw groove defines a helical rolling path opposite to the shaft-side screw groove; a return tube defining a return path configured to connect a first connection point and a second connection point in the rolling path; a plurality of balls arranged in an endless circulation path defined by the rolling path and the return path; and a fastener configured to fix the return tube to the ball screw nut. The ball screw nut has a first through hole provided at a position corresponding to the first connection point; a second through hole provided at a position corresponding to the second connection point; and a communication groove provided on the outer circumferential surface of the ball screw nut and connecting the first through hole and the second through hole, wherein the communication groove includes a pair of groove side surfaces and a groove bottom surface. The return tube has a first end inserted into the first through hole, a second end inserted into the second through hole, and a tubular portion connecting the first end and the second end, the tubular portion being inserted into the communication groove. The fastener has a pair of side wall portions, each positioned between the tubular portion and the pair of groove sides, a connecting wall portion connecting the pair of side wall portions, and spring pieces provided on each of the pair of side wall portions. Each of the pair of groove sides has a recess including a locking surface facing radially inward of the ball screw nut. The pair of spring pieces are elastically deformable to move closer to each other and are configured to lock into the corresponding locking surfaces of the recesses. The inner surface of the connecting wall portion is planar. The outer circumferential surface of the tubular portion includes a planar tube top surface configured to be in surface contact with the inner surface of the connecting wall portion.

[0006] Another aspect of the present disclosure provides an electric power steering device. The electric power steering device comprises the ball screw device, a steering shaft configured to move axially integrally with the screw shaft, and a motor configured to rotate the ball screw nut.

[0007] This is a schematic diagram of an electric power steering device equipped with a ball screw device according to one embodiment. This is a cross-sectional view along the axial direction near the ball screw device in the electric power steering device of Figure 1. This is a plan view of the ball screw device of Figure 1. This is a plan view of the ball screw nut provided in the ball screw device of Figure 3. This is a cross-sectional view along the longitudinal direction of the communication groove in the ball screw device of Figure 3, and is a cross-sectional view taken along the line V-V in Figure 3. This is a cross-sectional view along the transverse direction of the communication groove in the ball screw device of Figure 3, and is a cross-sectional view taken along the line VI-VI in Figure 3. This is a perspective view of the return tube and fastener provided in the ball screw device of Figure 3.

[0008] Hereinafter, an embodiment of a ball screw device and an electric power steering device (hereinafter referred to as EPS) equipped therewith will be described with reference to the drawings. (Overall configuration) As shown in Figure 1, the EPS 1 includes a steering mechanism 4 that steers the steering wheels 3 based on the operation of the steering wheel 2, and an EPS actuator 5 that provides assist force to the steering mechanism 4 to assist the driver's operation of the steering wheel 2.

[0009] The steering mechanism 4 comprises a steering shaft 11 to which the steering wheel 2 is fixed, a rack shaft 12 which is the steering axis, and a rack housing 13 in which the rack shaft 12 is reciprocally housed. The steering shaft 11 is constructed by connecting a column shaft 14, an intermediate shaft 15, and a pinion shaft 16 in order from the side where the steering wheel 2 is located. The pinion teeth 17 of the pinion shaft 16 mesh with the rack teeth 18 of the rack shaft 12. As a result, the rotation of the steering shaft 11 is converted into axial movement of the rack shaft 12. Tie rods 19 are connected to both ends of the rack shaft 12. The tip of each tie rod 19 is connected to a knuckle (not shown) to which the steering wheel 3 is assembled.

[0010] Therefore, in EPS1, the rotation of the steering shaft 11 associated with the operation of the steering wheel 2 is converted into axial movement of the rack axis 12, and this axial movement is transmitted to the knuckle via the tie rod 19, thereby changing the steering angle of the steering wheel 3.

[0011] The EPS actuator 5 includes a motor 21 which is a drive source, a belt mechanism 22 which transmits the rotation of the motor 21, and a ball screw device 23 which converts the rotation transmitted via the belt mechanism 22 into axial movement of the rack shaft 12.

[0012] As shown in Figures 1 and 2, the motor 21 is fixed to the outer surface of the rack housing 13 such that its rotating shaft 24 is parallel to the rack shaft 12. The rotating shaft 24 is inserted into the rack housing 13. The belt mechanism 22 comprises a drive pulley 25, a driven pulley 26, and a belt 27. The drive pulley 25 and the driven pulley 26 are cylindrical in shape. The drive pulley 25 is fixed to the rotating shaft 24 of the motor 21 so as to rotate integrally with the rotating shaft 24. The driven pulley 26 is rotatably positioned on the outer circumference of the rack shaft 12. The belt 27 is made of, for example, rubber. The belt 27 is wrapped around the drive pulley 25 and the driven pulley 26 so as to generate a predetermined tension. The ball screw device 23 comprises a screw shaft which is part of the rack shaft 12, a ball screw nut 28 rotatably arranged on the outer circumference of the rack shaft 12, and a plurality of balls 29 provided between the rack shaft 12 and the ball screw nut 28, as will be described later. The ball screw nut 28 is located on the inner circumference of the driven pulley 26. The ball screw nut 28 is fixed to the driven pulley 26 so as to rotate integrally with the driven pulley 26. The rack shaft 12, the driven pulley 26, and the ball screw nut 28 are arranged coaxially.

[0013] Therefore, the rotation of the motor 21 is transmitted to the ball screw nut 28 of the ball screw device 23 via the belt mechanism 22. The EPS actuator 5 then provides an assist force to the steering mechanism 4 by converting the rotational motion of the ball screw nut 28 in the ball screw device 23 into linear motion of the rack shaft 12.

[0014] (Ball Screw Device) Next, the configuration of the ball screw device 23 will be described. The outer circumferential surface of the rack shaft 12 is provided with a spirally extending shaft-side screw groove 31. The shaft-side screw groove 31 is provided over a predetermined range spaced axially from the area on the outer circumferential surface of the rack shaft 12 where the rack teeth 18 are provided. The area on the rack shaft 12 where the shaft-side screw groove 31 is provided corresponds to the screw shaft.

[0015] As shown in Figure 2, the ball screw nut 28 has a stepped cylindrical shape in which the outer diameter changes along the axial direction. Specifically, the ball screw nut 28 has a small diameter cylindrical portion 32 and a large diameter cylindrical portion 33 provided on one end of the small diameter cylindrical portion 32. The outer diameter of the small diameter cylindrical portion 32 is smaller than the outer diameter of the large diameter cylindrical portion 33. A bearing 34 is provided between the small diameter cylindrical portion 32 and the rack housing 13. As a result, the ball screw nut 28 is rotatably supported within the rack housing 13. The outer diameter of the large diameter cylindrical portion 33 is approximately equal to the inner diameter of the driven pulley 26. The driven pulley 26 is fitted to the outer circumference of the large diameter cylindrical portion 33. The driven pulley 26 is fixed to the ball screw nut 28 by a bolt 35 so that the driven pulley 26 rotates integrally with the ball screw nut 28.

[0016] A spirally extending nut-side screw groove 36 is provided on the inner circumferential surface of the ball screw nut 28. In the illustrated example, the nut-side screw groove 36 is provided over a predetermined range that includes the entire axial range corresponding to the large-diameter cylindrical portion 33 and a part of the axial range corresponding to the small-diameter cylindrical portion 32 on the inner circumferential surface of the ball screw nut 28. A spiral rolling path R1 is defined by the axial-side screw groove 31 and the nut-side screw groove 36 facing each other.

[0017] As shown in Figures 2 and 3, the ball screw device 23 further comprises a return tube 41 and a fastener 42 for fixing the return tube 41 to a ball screw nut 28. As shown in Figures 3 to 5, the return tube 41 defines a return path R2 configured to connect a first connection point P1 and a second connection point P2 in the rolling path R1. Thus, an endless circulation path is defined in the ball screw device 23 by the rolling path R1 and the return path R2. In this embodiment, the ball screw device 23 is equipped with one return tube 41, and one circulation path is defined in the ball screw device 23. Note that in Figure 5, for the sake of explanation, the rack shaft 12 and the ball 29 are omitted from the illustration.

[0018] As shown in Figure 2, multiple balls 29 are arranged within the circulation path. The balls 29 located within the rolling path R1 are sandwiched between the inner surface of the shaft-side screw groove 31 and the inner surface of the nut-side screw groove 36. In other words, the ball screw nut 28 is screwed onto the outer circumference of the rack shaft 12 via the balls 29. As a result, the balls 29 located within the rolling path R1 roll within the rolling path R1 while receiving a load as the relative rotation between the rack shaft 12 and the ball screw nut 28 occurs. The rolling of the balls 29 causes a displacement in the axial relative position between the rack shaft 12 and the ball screw nut 28, thereby applying the torque of the motor 21 as an assisting force to the rack shaft 12.

[0019] Balls 29 that have rolled through the rolling path R1 and reached the first connection point P1 or the second connection point P2 are discharged to the second connection point P2 or the first connection point P1 by passing through the return path R2. As a result, balls 29 circulate within the circulation path by moving from the downstream side to the upstream side of the rolling path R1. Balls 29 located within the return path R2 are pushed by the adjacent balls 29 behind them as new balls 29 enter the return path R2 from the rolling path R1, causing them to move within the return path R2.

[0020] Next, the configuration for fixing the return tube 41 to the ball screw nut 28 will be described. (Ball screw nut 28) As shown in Figures 3 and 4, the ball screw nut 28 has a mounting portion 43 configured for attaching the return tube 41. The mounting portion 43 is provided, for example, on the large diameter cylindrical portion 33 of the ball screw nut 28. The return tube 41 is attached to the mounting portion 43 from a direction perpendicular to the axial direction of the ball screw nut 28. Hereinafter, the direction in which the return tube 41 is attached to the ball screw nut 28 may be referred to as the mounting direction D.

[0021] More specifically, the mounting portion 43 has a first through hole 51 and a second through hole 52 that connect the inside and outside of the ball screw nut 28, and a communication groove 53 provided on the outer circumferential surface of the ball screw nut 28. The first through hole 51 is provided at a position corresponding to the first connection point P1, and the second through hole 52 is provided at a position corresponding to the second connection point P2. The communication groove 53 connects the first through hole 51 and the second through hole 52 to each other. The mounting portion 43 of this embodiment has a shape that is point-symmetric with respect to the center of the mounting portion 43 when viewed from the mounting direction D.

[0022] More specifically, as shown in Figures 4 and 5, the first connection point P1 and the second connection point P2 are set at positions spaced apart from each other in the circumferential direction of the ball screw nut 28. In the illustrated example, the first connection point P1 is located approximately 180° circumferentially from the second connection point P2. Furthermore, the first connection point P1 and the second connection point P2 are set at positions spaced apart from each other in the axial direction of the ball screw nut 28. In the illustrated example, the first connection point P1 is set at a position with several turns of the nut-side screw groove 36 between it and the second connection point P2 in the axial direction. Therefore, the first through hole 51 and the second through hole 52 are located at positions spaced apart from each other in both the circumferential and axial directions. Both the first through hole 51 and the second through hole 52 extend linearly along the mounting direction D. In other words, the first through hole 51 and the second through hole 52 open in the same direction.

[0023] A first stepped surface 54 is provided between the inner surface of the first through hole 51 and the inner surface of the ball screw nut 28. A second stepped surface 55 is provided between the inner surface of the second through hole 52 and the inner surface of the ball screw nut 28. Both the first stepped surface 54 and the second stepped surface 55 are planes perpendicular to the mounting direction D.

[0024] As shown in Figures 4 and 6, the communication groove 53 has a pair of groove sides 56 and a groove bottom 57. The communication groove 53 extends in a straight line connecting the first through hole 51 and the second through hole 52 when viewed from the mounting direction D. The longitudinal direction of the communication groove 53 intersects with the axial direction of the ball screw nut 28. The longitudinal direction of the communication groove 53 is sometimes referred to as the direction of extension of the communication groove 53. The communication groove 53 has a wide portion 58 and a narrow portion 59 that is narrower than the wide portion 58. The width of the communication groove 53 is the dimension of the communication groove 53 along the longitudinal direction and the transverse direction perpendicular to the mounting direction D. The wide portion 58 is located in the central part of the longitudinal direction of the communication groove 53, and the narrow portion 59 is located at both ends of the longitudinal direction of the communication groove 53. In other words, the width of the communication groove 53 is wider in its central part in the longitudinal direction than at both ends in the longitudinal direction. Note that, for the sake of clarity, the rack axis 12 and ball 29 are omitted from the illustration in Figure 6.

[0025] Each groove side surface 56 has a recess 61. The recess 61 is, for example, a square hole. The recess 61 is located in the portion that constitutes the wide portion 58 of the groove side surface 56, that is, in the longitudinal central portion of the communication groove 53. Of the sides of the recess 61, the side that is located radially outward of the ball screw nut 28 is the locking surface 62. In other words, the locking surface 62 faces radially inward of the ball screw nut 28. The locking surface 62 is, for example, a plane perpendicular to the mounting direction D.

[0026] As shown in Figures 4 and 5, the groove bottom surface 57 has a flat surface 63 located in the longitudinal center of the connecting groove 53, and inclined surfaces 64 that are continuous on both longitudinal sides from both ends of the flat surface 63. The flat surface 63 is a plane perpendicular to the mounting direction D. Each inclined surface 64 is bent multiple times such that the angle of inclination with respect to the flat surface 63 increases in stages as it moves away from the flat surface 63. In the illustrated example, each inclined surface 64 is bent twice and includes three surfaces with different inclinations with respect to the flat surface 63.

[0027] (Return Tube 41) As shown in Figures 5 and 7, the return tube 41 has a first end 71 inserted into the first through hole 51, a second end 72 inserted into the second through hole 52, and a tubular portion 73 connecting the first end 71 and the second end 72. The tubular portion 73 is inserted into the communication groove 53. The return tube 41 of this embodiment is made of resin material and is capable of some elastic deformation. The return tube 41 has a shape that is, for example, a cylindrical pipe curved into a roughly C shape. The inner circumferential surface of the return tube 41 defines the return path R2. The return tube 41 of this embodiment is point-symmetric with respect to the center of the tubular portion 73 when viewed from the mounting direction D. The return path R2 has a substantially constant inner diameter over its entire longitudinal direction.

[0028] In this embodiment, the return tube 41 is formed by dividing the return tube 41 along its longitudinal direction into a radially outer portion and a radially inner portion of the ball screw nut 28. In another embodiment, the return tube 41 may be formed by dividing the return tube 41 along its longitudinal direction into an axially oriented portion and an axially oriented portion of the ball screw nut 28. In yet another embodiment, the return tube 41 may be formed as a seamless, continuous, one-piece unit.

[0029] The first end 71 and the second end 72 are generally semi-cylindrical in shape and extend in a straight line. The first end 71 and the second end 72 extend parallel to each other. The first end 71 is provided with a first tongue portion 74, and the second end 72 is provided with a second tongue portion 75.

[0030] The first tongue portion 74 is provided at the first end portion 71, in a portion located radially inward of the ball screw nut 28. The first tongue portion 74 has a semicircular shape corresponding to the shaft-side screw groove 31 of the rack shaft 12. The first tongue portion 74 protrudes into the rolling path R1 when the return tube 41 is attached to the ball screw nut 28. As a result, the first tongue portion 74 plays the role of scooping up the balls 29 rolling in the rolling path R1 into the return path R2, and discharging the balls 29 moving in the return path R2 into the rolling path R1. The second tongue portion 75 is provided at the second end portion 72, in a portion located radially inward of the ball screw nut 28. The second tongue portion 75 has the same shape as the first tongue portion 74.

[0031] A first projection 76 is provided on the outer circumferential surface of the first end portion 71. The projection height of the first projection 76 is set such that it presses against the inner circumferential surface of the first through hole 51 when the first end portion 71 is inserted into the first through hole 51. In other words, the first end portion 71 is configured to be press-fitted into the first through hole 51. A second projection 77 is provided on the outer circumferential surface of the second end portion 72. The second projection 77 has the same shape as the first projection 76. As a result, the second end portion 72 is configured to be press-fitted into the second through hole 52.

[0032] As shown in Figures 5 to 7, the tubular portion 73 is cylindrical and extends generally in the circumferential direction of the ball screw nut 28. The outer circumferential surface of the tubular portion 73 has a flat tube top surface 81 and a pair of flat tube side surfaces 82. The tube top surface 81 and the tube side surfaces 82 are located in the longitudinal central portion of the tubular portion 73. The fastener 42 is attached to the portion of the tubular portion 73 where the tube top surface 81 and the tube side surfaces 82 are provided. In other words, the longitudinal central portion of the tubular portion 73 corresponds to the attachment area, and the longitudinal ends of the tubular portion 73 correspond to the non-attachment areas.

[0033] The upper surface 81 of the tube is a plane perpendicular to the mounting direction D. The pair of tube sides 82 extend along the mounting direction D and are planes parallel to each other. The width of the tubular portion 73 is substantially constant throughout its entire longitudinal direction. The width of the tubular portion 73 is substantially equal to the width of the narrow portion 59 of the communication groove 53. Therefore, when the tubular portion 73 is inserted into the communication groove 53, both longitudinal ends of the tubular portion 73 (mounting portions) are in contact with the pair of groove sides 56, while a gap is defined between the longitudinal central portion of the tubular portion 73 (non-mounting portions) and the pair of groove sides 56.

[0034] The lengths of the first end 71 and the second end 72 are set such that, with the return tube 41 attached to the mounting portion 43, a gap 83 extending in the circumferential and radial directions of the ball screw nut 28 is defined between the mounting portion of the tubular portion 73 and the groove bottom surface 57 of the communication groove 53. In other words, with the tip surfaces of the first end 71 and the second end 72 in contact with the first stepped surface 54 and the second stepped surface 55 of the first through hole 51 and the second through hole 52, respectively, a gap 83 is defined between the mounting portion of the tubular portion 73 and the groove bottom surface 57. In this embodiment, as shown in the figure, the entire tubular portion 73 is not in contact with the groove bottom surface 57, and the gap 83 extends over the entire area of ​​the groove bottom surface 57. In other embodiments, a part of the groove bottom surface 57, for example, the corner between the flat surface 63 and the inclined surface 64, may be in contact with the tubular portion 73.

[0035] (Fixing device 42) As shown in Figures 6 and 7, the fixing device 42 has a pair of side wall portions 91 which are respectively positioned between the tubular portion 73 and the pair of groove side surfaces 56, a connecting wall portion 92 which connects the pair of side wall portions 91 to each other, and a spring piece 93 provided on each of the pair of side wall portions 91. The fixing device 42 in this embodiment is formed, for example, by bending a rectangular metal plate.

[0036] The connecting wall portion 92 is a flat plate shape perpendicular to the mounting direction D. In the illustrated example, the connecting wall portion 92 is roughly H-shaped when viewed from the mounting direction D. The connecting wall portion 92 connects the ends of the pair of side wall portions 91 opposite to the groove bottom surface 57. The inner surface of the connecting wall portion 92 facing the tubular portion 73 is planar, perpendicular to the mounting direction D, and is in surface contact with the upper surface 81 of the tube. The pair of side wall portions 91 extend along the mounting direction D and are flat plates parallel to each other. In the illustrated example, each side wall portion 91 is roughly C-shaped when viewed from the transverse direction. The inner surface of each side wall portion 91 facing the tubular portion 73 is planar, parallel to the mounting direction D, and is in surface contact with the corresponding tube side surface 82. The distance between the pair of side wall portions 91 is approximately equal to the width of the tubular portion 73. Therefore, the pair of side wall portions 91 sandwich the tubular portion 73.

[0037] The pair of spring pieces 93 are elastically deformable so as to move closer to each other. Each of the spring pieces 93 is inclined with respect to the corresponding side wall 91 so as to move away from the tubular portion 73 as it moves in the direction opposite to the mounting direction D, that is, from the radially inside to the outside of the ball screw nut 28. The tip of each spring piece 93 is engaged with the locking surface 62 of the corresponding recess 61. This restricts the movement of the fastener 42 in the direction opposite to the mounting direction D, and secures the return tube 41 to the ball screw nut 28. When the return tube 41 is secured to the ball screw nut 28 by the fastener 42, the mounting portion of the tubular portion 73 is slightly elastically deformed so as to move closer to the bottom surface 57 of the groove.

[0038] Next, the attachment of the return tube 41 to the ball screw nut 28 will be described. First, as shown in Figure 7, the fastener 42 is attached to the return tube 41. Then, the fastener 42 is grasped and the return tube 41 is moved in the attachment direction D, and the first end 71 and the second end 72 are inserted into the first through hole 51 and the second through hole 52, respectively, and the tubular portion 73 is inserted into the communication groove 53. At this time, as the tip surfaces of the first end 71 and the second end 72 approach the first stepped surface 54 and the second stepped surface 55, respectively, the pair of spring pieces 93 elastically deform so that they come closer to each other. When the tip surfaces of the first end 71 and the second end 72 contact the first stepped surface 54 and the second stepped surface 55, respectively, the tubular portion 73 has not elastically deformed, and the tip of the spring piece 93 is located radially outward of the ball screw nut 28 than the locking surface 62 of the recess 61. In other words, the spring piece 93 is not locked to the locking surface 62. Then, with the tip surfaces of the first end 71 and the second end 72 in contact with the first stepped surface 54 and the second stepped surface 55, respectively, the fastener 42 is further pushed in the mounting direction D, causing the tubular portion 73 to elastically deform so that the mounting portion is close to the bottom surface 57 of the groove. As a result, the tip of the spring piece 93 is positioned radially inward of the ball screw nut 28 than the locking surface 62 of the recess 61, causing the shape of the spring piece 93 to be restored and lock into the locking surface 62. This fixes the return tube 41 to the ball screw nut 28.

[0039] (Operation and Effects of this Embodiment) Next, the operation and effects of this embodiment will be described. (1) The fastener 42 has a pair of side wall portions 91 which are respectively positioned between the tubular portion 73 and a pair of groove side surfaces 56, a connecting wall portion 92 which connects the pair of side wall portions 91 to each other at the ends opposite to the groove bottom surface 57, and a spring piece 93 provided on each of the pair of side wall portions 91. Each of the pair of groove side surfaces 56 has a recess 61 which includes a locking surface 62 that faces radially inward of the ball screw nut 28. The pair of spring pieces 93 are elastically deformable so as to be close to each other and lock into the locking surface 62 of the corresponding recess 61.

[0040] According to the above configuration, in a state where the fixture 42 is attached to the tubular portion 73, the return tube 41 is fixed to the ball screw nut 28 by the spring piece 93 of the fixture 42 being locked to the locking surface 62. Thereby, compared with the case where the fixture 42 is fastened to the ball screw nut 28 with a small screw, for example, the number of parts of the ball screw device 23 can be reduced.

[0041] By the way, in the ball screw device 23, as the ball screw nut 28 rotates, the balls 29 located in the return tube 41 rotate around the axis of the ball screw nut 28, and thus a centrifugal force acts on these balls 29. Then, due to this centrifugal force, a radially outward force that pulls the return tube 41 out of the ball screw nut 28 acts on the return tube 41 from the balls 29. As a result, a large load may act on the return tube 41 from the fixture 42.

[0042] In this regard, the outer peripheral surface of the tubular portion 73 of the present embodiment includes a planar tube upper surface 81 that is in surface contact with the planar inner surface of the connecting wall portion 92. Therefore, it is possible to suppress a large load from acting on the return tube 41 from the fixture 42 compared to the case where the outer peripheral surface of the tubular portion 73 is in line contact with the inner surface of the connecting wall portion 92, for example.

[0043] (2) A gap 83 extending in the circumferential direction of the ball screw nut 28 is defined between the mounting portion of the tubular portion 73 and the groove bottom surface 57. Each of the pair of spring pieces 93 is locked to the corresponding locking surface 62 in a state of being elastically deformed so that the mounting portion of the tubular portion 73 approaches the groove bottom surface 57.

[0044] According to the above configuration, since the return tube 41 is pressed against the ball screw nut 28 by the fixture 42, it is possible to suppress rattling of the return tube 41. (3) The communication groove 53 has a narrow portion 59 configured such that the pair of groove side surfaces 56 contact the non-mounting portion of the tubular portion 73.

[0045] According to the above configuration, it is possible to suitably suppress the rattling caused by the displacement of the return tube 41 in the transverse direction. (4) The pair of spring pieces 93 are inclined with respect to the corresponding side wall portions 91 so as to be separated from the tubular portion 73 as they go from the inner side to the outer side in the radial direction of the ball screw nut 28.

[0046] According to the above configuration, the return tube 41 in a state where the fixture 42 is attached to the tubular portion 73 can be fixed to the ball screw nut 28 only by moving it in the attachment direction D with respect to the attachment portion 43 of the ball screw nut 28. Therefore, the assemblability of the return tube 41 can be improved.

[0047] A first protrusion 76 is provided on the outer peripheral surface of the first end portion 71, whereby the first end portion 71 is configured to be press-fitted into the first through hole 51. A second protrusion 77 is provided on the outer peripheral surface of the second end portion 72, whereby the second end portion 72 is configured to be press-fitted into the second through hole 52.

[0048] According to the above configuration, it is possible to suppress the displacement of the position of the first end portion 71 in the first through hole 51 and the position of the second end portion 72 in the second through hole 52 due to, for example, a radially outward force acting on the return tube 41 from the ball 29.

[0049] The inner surfaces of the pair of side wall portions 91 are planar. The outer peripheral surface of the tubular portion 73 further includes a pair of planar tube side surfaces 82 configured to be in surface contact with the inner surfaces of the pair of side wall portions 91, respectively.

[0050] According to the above configuration, it is possible to suppress the detachment of the fixture 42 from the tubular portion 73 due to the frictional force acting between the inner surface of the side wall portion 91 and the tube side surface 82. Therefore, in a state before the return tube 41 is fixed to the ball screw nut 28 by the fixture 42, the return tube 41 with the fixture 42 attached can be handled as a single assembly. This facilitates the parts management before the manufacture of the ball screw device 23.

[0051] This embodiment can be implemented with the following modifications. This embodiment and the following modifications can be combined with each other to the extent that they are not technically contradictory. In the above embodiment, the tube side surface 82 does not have to be in surface contact with the inner surface of the side wall portion 91. In this case, the tube side surface 82 may be, for example, an arc-shaped curved surface.

[0052] In the above embodiment, the pair of spring pieces 93 may be inclined with respect to the corresponding side wall portion 91 such that they move away from the tubular portion 73 as they move from one axial side of the ball screw nut 28 to the other side. In this case, the edges of the spring pieces 93 that are located radially outward from the ball screw nut 28 may engage with the locking surface 62.

[0053] In the above embodiment, the tubular portion 73 may contact the groove bottom surface 57 while the spring piece 93 of the fastener 42 is locked to the locking surface 62. In this case, the tubular portion 73 may be elastically deformed between the inner surface of the connecting wall portion 92 and the groove bottom surface 57 of the communication groove 53 so that its cross-sectional shape perpendicular to its longitudinal direction is compressed.

[0054] - In the above embodiment, the first projection 76 does not need to be provided on the outer circumferential surface of the first end portion 71, and the second projection 77 does not need to be provided on the outer circumferential surface of the second end portion 72. - In the above embodiment, the communication groove 53 has a wide portion 58 and a narrow portion 59, but it is not limited to this, and for example, the communication groove 53 may have only a wide portion 58.

[0055] In the above embodiment, the mounting portion of the tubular portion 73 was elastically deformed so that it was close to the bottom surface 57 of the groove, and each of the pair of spring pieces 93 was locked to the corresponding locking surface 62. However, the embodiment is not limited to this, and the mounting portion of the tubular portion 73 does not need to be elastically deformed even if each of the pair of spring pieces 93 is locked to the corresponding locking surface 62.

[0056] - In the above embodiment, the upper surface 81 of the tube may not be in surface contact with the inner surface of the connecting wall portion 92, but may be in line contact. In this case, the upper surface 81 of the tube may be, for example, an arc-shaped curved surface. - In the above embodiment, one circulation path was formed by attaching one return tube 41 to the ball screw nut 28, but the invention is not limited to this, and multiple return tubes may be attached to the ball screw nut 28 to form multiple independent circulation paths.

[0057] In the above embodiment, the ball screw device 23 was applied to the EPS1. However, it is not limited to this, and may also be applied to the steering unit of a steer-by-wire type steering system in which the power transmission path between the steering unit steered by the driver and the steering unit that steers the steering wheels 3 in response to the driver's steering is separated. Furthermore, the ball screw device may be used for purposes other than steering systems.

[0058] Next, the technical concepts that can be understood from the above embodiments and modifications are described below. (Note) A ball screw device comprising: a screw shaft having a screw groove on the shaft side; a ball screw nut having a screw groove on the nut side, wherein the screw groove on the nut side defines a spiral rolling path opposite to the screw groove on the shaft side; a return tube defining a return path configured to connect a first connection point and a second connection point in the rolling path; a plurality of balls arranged in an endless circulation path defined by the rolling path and the return path; and a fastener configured to fix the return tube to the ball screw nut, wherein the ball screw nut has a first through hole provided at a position corresponding to the first connection point; a second through hole provided at a position corresponding to the second connection point; and a communication groove provided on the outer circumferential surface of the ball screw nut and connecting the first through hole and the second through hole, wherein the communication groove includes a pair of groove side surfaces and a groove bottom surface; and the return tube is The fastener has a first end portion inserted into the first through hole, a second end portion inserted into the second through hole, and a tubular portion connecting the first end portion and the second end portion, the tubular portion being inserted into the communication groove, the fastener has a pair of side wall portions respectively disposed between the tubular portion and the pair of groove side surfaces, a connecting wall portion connecting the pair of side wall portions to each other, and a spring piece provided on each of the pair of side wall portions, each of the pair of groove side surfaces having a recess including a locking surface facing radially inward of the ball screw nut, the pair of spring pieces are elastically deformable to move closer to each other and are configured to lock into the locking surface of the corresponding recess, a gap extending in the circumferential direction of the ball screw nut is defined between the mounting portion on the tubular portion and the bottom surface of the groove, and each of the pair of spring pieces may be elastically deformed to move closer to the bottom surface of the groove and lock into the corresponding locking surface.

Claims

1. A ball screw device comprising: a screw shaft having a screw groove on the shaft side; a ball screw nut having a screw groove on the nut side, wherein the screw groove on the nut side defines a spiral rolling path opposite to the screw groove on the shaft side; a return tube defining a return path configured to connect a first connection point and a second connection point in the rolling path; a plurality of balls arranged in an endless circulation path defined by the rolling path and the return path; and a fastener configured to fix the return tube to the ball screw nut, wherein the ball screw nut has a first through hole provided at a position corresponding to the first connection point; a second through hole provided at a position corresponding to the second connection point; and a communication groove provided on the outer circumferential surface of the ball screw nut and connecting the first through hole and the second through hole, wherein the communication groove includes a pair of groove side surfaces and a groove bottom surface; and the return tube is A ball screw device comprising: a first end inserted into a first through hole; a second end inserted into a second through hole; a tubular portion connecting the first end and the second end, the tubular portion being inserted into a communication groove; a fixing device comprising: a pair of side wall portions respectively disposed between the tubular portion and the pair of groove sides; a connecting wall portion connecting the pair of side wall portions; and spring pieces provided on each of the pair of side wall portions; each of the pair of groove sides having a recess including a locking surface facing radially inward of the ball screw nut; the pair of spring pieces being elastically deformable to move closer to each other and configured to lock into the corresponding locking surfaces of the recesses; the inner surface of the connecting wall portion being planar; and the outer circumferential surface of the tubular portion including a planar tube top surface configured to be in surface contact with the inner surface of the connecting wall portion.

2. A ball screw device according to claim 1, wherein a gap extending in the circumferential direction of the ball screw nut is defined between the mounting portion on the tubular portion to which the fastener is attached and the bottom surface of the groove, and each of the pair of spring pieces is elastically deformed so that the mounting portion is close to the bottom surface of the groove and engages with the corresponding locking surface.

3. A ball screw device according to claim 1 or 2, wherein the communicating groove has a narrow portion configured such that the pair of groove sides contact a non-mounted portion of the tubular portion where the fastener is not mounted.

4. A ball screw device according to claim 1 or 2, wherein each of the pair of spring pieces is inclined with respect to the corresponding side wall portion such that it moves away from the tubular portion as it moves from the radially inner side of the ball screw nut outward.

5. A ball screw device according to claim 1 or 2, wherein a first projection is provided on the outer circumferential surface of the first end, thereby configured to press-fit the first end into the first through hole, and a second projection is provided on the outer circumferential surface of the second end, thereby configured to press-fit the second end into the second through hole.

6. A ball screw device according to claim 1 or 2, wherein the inner surfaces of the pair of sidewalls are planar, and the outer surface of the tubular portion further includes a pair of planar tube sides configured to be in surface contact with the inner surfaces of the pair of sidewalls.

7. An electric power steering device comprising: a ball screw device according to claim 1 or 2; a steering shaft configured to move axially integrally with the screw shaft; and a motor configured to rotate the ball screw nut.

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