Ball screw device and electric power steering device

Abstract

In the present invention, a return tube (41) has a tubular part (73) which is inserted into a communication groove (53) of a ball screw nut (28). A fixing tool (42) has a pair of side wall parts (91), a connection wall part (92), and spring pieces (93) which are respectively provided to the pair of side wall parts (91). A pair of groove side surfaces (56) of the communication groove (53) each have a recess (61) which includes a locking surface (62). The return tube (41) is fixed to the ball screw nut (28) by the fixing piece (42) in a state where a mounting portion is deformed elastically so as to approach a groove bottom surface (57). The fixing piece (42) receives the restoring force of the return tube (41) as a result of each of the tips of the pair of spring pieces (93) locking onto the locking surfaces (62) of the corresponding recesses (61).

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 opposes the screw groove of the ball screw nut to define a spiral rolling path. The return tube defines a return path that connects 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 by 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 locking surfaces of the corresponding recesses. A gap extending circumferentially around the ball screw nut is defined between the mounting portion on the tubular portion and the groove bottom surface. The return tube is fixed to the ball screw nut by the fastener in a state where the mounting portion is elastically deformed to move closer to the groove bottom surface. The fastener receives a restoring force that causes the return tube to return to its original shape, as the tips of each of the pair of spring pieces engage with the locking surfaces of the corresponding recesses.

[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 of the first 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 equipped with the ball screw device of Figure 3. This is a cross-sectional view of the ball screw device of Figure 3 along the longitudinal direction of the communication groove, and is a cross-sectional view taken along the line V-V in Figure 3. This is a cross-sectional view of the ball screw device of Figure 3 along the transverse direction of the communication groove, 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 equipped with the ball screw device of Figure 3. This is a plan view of a ball screw device of the second embodiment. This is a cross-sectional view of the ball screw device of Figure 8 along the longitudinal direction of the communication groove, and is a cross-sectional view taken along the line IX-IX in Figure 8. This is a cross-sectional view of the ball screw device of Figure 8 along the transverse direction of the communication groove, and is a cross-sectional view taken along the line X-X in Figure 8. This is a perspective view of the return tube and fastener equipped with the ball screw device of Figure 8.

[0008] (First Embodiment) Hereinafter, a first 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 Figures 1 to 7.

[0009] (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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] (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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] (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 is manufactured, for example, by injection molding. 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. In other words, the cross-section perpendicular to the longitudinal direction of the return path R2 is approximately circular over almost the entire longitudinal direction of the return path R2.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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 (non-mounted 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 (mounted portion) and the pair of groove sides 56.

[0035] As shown in Figure 7, the outer surfaces of both longitudinal ends of the tubular portion 73 are approximately circular. Therefore, the outer surface of the tubular portion 73 is partially concave such that only the central longitudinal portion is a plane perpendicular to the mounting direction D, that is, the tube upper surface 81 is formed only in the central longitudinal portion. In other words, the tube upper surface 81 is located only in the central longitudinal portion of the tubular portion 73. As described above, the cross-section perpendicular to the longitudinal direction of the return path R2 is also approximately circular, so the thickness along the radial direction of both longitudinal ends of the tubular portion 73 is approximately uniform around the entire circumference of the tubular portion 73.

[0036] 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.

[0037] (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.

[0038] 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.

[0039] The pair of spring pieces 93 are elastically deformable so as to be close to each other. Each of the spring pieces 93 is inclined with respect to the corresponding side wall 91 so as to be moved 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 fixes the return tube 41 to the ball screw nut 28. When the return tube 41 is fixed 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 be close to the groove bottom surface 57. In other words, a restoring force is generated in the return tube 41 that attempts to return it to its original shape, and the fastener 42 receives this restoring force from the return tube 41 by the engagement of the tips of the pair of spring pieces 93 with the locking surface 62.

[0040] 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.

[0041] (Operation and Effects of this Embodiment) Next, the operation and effects of this embodiment will be described. (1-1) 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.

[0042] According to the above configuration, with the fixture 42 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, for example, compared with the case where the fixture 42 is fastened to the ball screw nut 28 with a small screw, the number of parts of the ball screw device 23 can be reduced.

[0043] (1-2) 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. The return tube 41 is fixed to the ball screw nut 28 by the fixture 42 in a state of being elastically deformed so that the mounting portion of the tubular portion 73 is close to the groove bottom surface 57. Therefore, for example, rattling of the return tube 41 associated with the operation of the ball screw device 23 can be preferably suppressed. And the fixture 42 prevents the return tube 41 from restoring to its original shape by receiving the restoring force of the return tube 41 when the tip of each of the pair of spring pieces 93 is locked to the locking surface 62. Therefore, for example, unlike the case where the return tube 41 is pressed against the groove bottom surface 57 by the elastic force of a spring or the like, the return tube 41 can be firmly fixed in an elastically deformed state.

[0044] (1-3) In the ball screw device 23, as the ball 29 located in the return tube 41 rotates around the axis of the ball screw nut 28 as the ball screw nut 28 rotates, a centrifugal force acts on these balls 29. Then, 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 ball 29 due to this centrifugal force. As a result, there is a possibility that a large load acts on the return tube 41 from the fixture 42.

[0045] 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, for example, compared with 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, it is possible to suppress a large load from acting on the return tube 41 from the fixture 42.

[0046] (1-4) The communication groove 53 has a narrow portion 59 configured such that a pair of groove side surfaces 56 contact non-mounted portions of the tubular portion 73. According to the above configuration, it is possible to suitably suppress the displacement and rattling of the return tube 41 in the transverse direction.

[0047] (1-5) The pair of spring pieces 93 incline with respect to the corresponding side wall portions 91 so as to separate 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. According to the above configuration, by simply moving the return tube 41 in the mounting direction D with respect to the mounting portion 43 of the ball screw nut 28 in a state where the fixture 42 is mounted on the tubular portion 73, the return tube 41 can be fixed to the ball screw nut 28. Therefore, the assemblability of the return tube 41 can be improved.

[0048] (1-6) 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.

[0049] According to the above configuration, for example, 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 a radially outward force or the like acting on the return tube 41 from the ball 29.

[0050] (1-7) 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.

[0051] 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 mounted can be handled as a single assembly. This facilitates the parts management before the manufacture of the ball screw device 23.

[0052] (1-8) The thickness of the tubular portion 73 at both longitudinal ends is substantially uniform around the entire circumference of the tubular portion 73. Therefore, even if shrinkage occurs when the return tube 41 is manufactured by injection molding, it is possible to suppress the tubular portion 73 from becoming distorted in shape.

[0053] (Second Embodiment) Next, a second embodiment of the ball screw device will be described with reference to Figures 8 to 11. For the sake of convenience, identical components will be given the same reference numerals as in the first embodiment and their descriptions will be omitted.

[0054] In the following explanation, the configuration of the fastener 42 will be described based on the orientation of the fastener 42 when it is fixed to the ball screw nut 28. In other words, the direction that coincides with the longitudinal direction of the communication groove 53 is called the longitudinal direction of the fastener 42, and the direction that coincides with the transverse direction of the communication groove 53 is called the transverse direction of the fastener 42.

[0055] As shown in Figures 8 to 11, each side wall portion 91 of the fastener 42 is generally C-shaped when viewed from the transverse direction. The side wall portion 91 has a top end portion 101 which is the end connected to the connecting wall portion 92, and a bottom end portion 102 which is the end located on the opposite side of the top end portion 101. The connecting wall portion 92 is generally H-shaped when viewed from the mounting direction D. The longitudinal central portion of both transverse ends of the connecting wall portion 92 is cut out. Therefore, in the illustrated example, the side wall portion 91 has two top ends portion 101 which are spaced apart in the longitudinal direction. The bottom end portion 102 is a long, flat plate that extends in the longitudinal direction. The side wall portion 91 further has two intermediate portions 103 which extend from both longitudinal ends of the bottom end portion 102 toward the two top ends portion 101. Each intermediate portion 103 is a long, flat plate that extends in the mounting direction D. The two intermediate sections 103 extend parallel to each other. In other words, the length of the side wall section 91 in this embodiment along the longitudinal direction is substantially constant over the entire area of ​​the mounting direction D.

[0056] The spring piece 104 is positioned between the top end 101 and the bottom end 102. The spring piece 104 extends from the longitudinal center of the bottom end 102 in the direction opposite to the mounting direction D. Specifically, after extending along the direction opposite to the mounting direction D, the spring piece 104 is inclined with respect to the corresponding side wall portion 91 so as it moves in that opposite direction, it moves away from the tubular portion 73.

[0057] As shown in Figures 8 and 9, each groove side surface 56 has a recess 111. The recess 111 is, for example, a square hole extending in the longitudinal direction of the communication groove 53. The length of the recess 111 along the longitudinal direction is shorter than the length of the base end 102 along the longitudinal direction. The recess 111 is located in the longitudinal central portion of the communication groove 53.

[0058] As shown in Figure 10, the recess 111 has a first locking surface 112, which is a side surface located radially outward of the ball screw nut 28, and a second locking surface 113, which is the bottom surface of the recess 111. The first locking surface 112 faces radially inward of the ball screw nut 28. The first locking surface 112 is, for example, a plane perpendicular to the mounting direction D. The second locking surface 113 is connected to the first locking surface 112. The second locking surface 113 is, for example, a plane parallel to the mounting direction D. In other words, the second locking surface 113 extends in a direction intersecting the first locking surface 112. The length (depth) of the recess 111 in this embodiment along the transverse direction is shallower than the depth of the recess 61 in the first embodiment.

[0059] Each end of the pair of spring pieces 104 engages with both the first locking surface 112 and the second locking surface 113 of the corresponding recess 111. 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 section 73 is slightly elastically deformed to be close to the bottom surface 57 of the groove. Note that, for the sake of explanation, the ball 29 is not shown in Figure 10.

[0060] As shown in Figures 8 and 11, the tubular portion 73 is provided with protrusions 121 adjacent to both longitudinal ends of the fastener 42 to suppress displacement of the fastener 42. The protrusions 121 project from the outer circumferential surface of the tubular portion 73 in the transverse direction on both sides. The protrusions 121 face the top end 101 of the fastener 42 in the longitudinal direction. The side surface of the protrusion 121 that is located radially outward of the ball screw nut 28 is flush with the tube top surface 81. In the first embodiment described above, the stepped portion between the outer circumferential surface of both longitudinal ends of the tubular portion 73 and the tube top surface 81 is used to suppress displacement of the fastener 42.

[0061] In this embodiment, in addition to the same functions and effects as those of (1-1) to (1-8) of the first embodiment described above, the following functions and effects are achieved. (2-1) Each of the pair of side wall portions 91 has a top end portion 101 which is the end connected to the connecting wall portion 92, and a bottom end portion 102 which is the end located on the opposite side from the top end portion 101. The spring piece 104 is positioned between the top end portion 101 and the bottom end portion 102. The length of the communication groove 53 in the bottom end portion 102 along the longitudinal direction is longer than the length of the communication groove 53 in the recess 111 along the longitudinal direction.

[0062] When the return tube 41 is attached to the ball screw nut 28, the pair of spring pieces 93 elastically deform so that they come closer to each other. The elastic force generated in the spring piece 104 causes the fastener 42 to elastically deform so that the lower ends 102 of the pair of side wall portions 91 move apart from each other. Therefore, if the length of the lower end 102 is less than or equal to the length of the recess 111, the lower end 102 may fall into the recess 111. As a result, the lower end 102 comes into contact with the radially inner side surface of the ball screw nut 28 in the recess 111, preventing the return tube 41 and the fastener 42 from being inserted further into the communication groove 53.

[0063] In this respect, since the length of the hem end 102 in this embodiment is longer than the length of the recess 111, it is possible to prevent the hem end 102 from entering the recess 111. Therefore, the ease of assembly of the return tube 41 can be improved.

[0064] (2-2) The recess 111 has a first locking surface 112 facing radially inward of the ball screw nut 28, and a second locking surface 113 connected to the first locking surface 112 and extending in a direction intersecting the first locking surface 112. The spring piece 104 locks onto both the first locking surface 112 and the second locking surface 113.

[0065] The return tube 41 may become hot due to the operation of the EPS1 equipped with the ball screw device 23 and changes in ambient temperature. When the return tube 41 becomes hot, it expands due to thermal expansion, causing a force to act on the fixing device 42 in the opposite direction to the mounting direction D. In addition, the centrifugal force of the ball 29 generated by the rotation of the ball screw nut 28 acts on the fixing device 42 via the return tube 41. Therefore, if the spring piece 104 is locked only to the first locking surface 112, the force in the opposite direction to the mounting direction D as described above may cause the pair of spring pieces 104 to deform significantly so that they separate from each other, resulting in an excessive load on the base end of the spring piece 104, which may cause plastic deformation of the base end of the spring piece 104. As a result, for example, when the temperature of the return tube 41 decreases, the tip of the spring piece 104 may separate from the first locking surface 112, causing the fixing device 42 to rattle.

[0066] In this respect, since the spring piece 104 of this embodiment engages with both the first locking surface 112 and the second locking surface 113, even if a force acting on the fastener 42 in the opposite direction to the mounting direction D, it is possible to suppress deformation that causes the pair of spring pieces 104 to separate from each other. This prevents excessive load from acting on the base end of the spring piece 104 and causing plastic deformation.

[0067] (2-3) The tubular portion 73 has protrusions 121 that are positioned adjacent to both longitudinal ends of the fastener 42. Therefore, it is possible to prevent the fastener 42 attached to the return tube 41 from shifting position before the return tube 41 is fixed to the ball screw nut 28.

[0068] Each of the above embodiments can be implemented with the following modifications. Each of the above embodiments and the following modifications can be combined with each other to the extent that they are not technically contradictory. In the second embodiment, the second locking surface 113 does not have to be parallel to the mounting direction D, as long as it extends in a direction intersecting the first locking surface 112. The first locking surface 112 does not have to be perpendicular to the mounting direction D, as long as it faces radially inward of the ball screw nut 28. The first locking surface 112 and the second locking surface 113 do not have to be flat, but curved surfaces. The same applies to the locking surface 62 in the first embodiment.

[0069] - In the second embodiment described above, the tip of the spring piece 104 may be engaged only with the first locking surface 112 and separated from the second locking surface 113. - In the second embodiment described above, the length of the hem end portion 102 along the longitudinal direction may be less than or equal to the length of the recess 111 along the longitudinal direction.

[0070] - In the first embodiment described above, the protruding portion 121 of the second embodiment may be provided on the tubular portion 73. In this case, the stepped portion between the upper surface 81 of the tube and the outer circumferential surface of the tubular portion 73 may be eliminated. In the second embodiment described above, in addition to or instead of the protruding portion 121, stepped portions may be provided between the upper surface 81 of the tube and the outer circumferential surfaces of both ends in the longitudinal direction of the tubular portion 73.

[0071] In the second embodiment described above, the length of the side wall portion 91 along its longitudinal direction does not have to be substantially constant over the entire area of ​​the mounting direction D. For example, the length of the side wall portion 91 along its longitudinal direction may become shorter as it moves in the direction opposite to the mounting direction D.

[0072] - In each of the above embodiments, 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. - In the first embodiment, the pair of spring pieces 93 may extend from the middle portion of the side wall portion 91 in the longitudinal direction of the communication groove 53 and be inclined with respect to the corresponding side wall portion 91 so as they move away from the tubular portion 73 from one side to the other. In this case, the edge of the spring piece 93 located radially outward of the ball screw nut 28 may engage with the locking surface 62. Similarly, in the second embodiment, the pair of spring pieces 104 may extend from the middle portion 103 of the side wall portion 91 in the longitudinal direction of the communication groove 53 and be inclined with respect to the corresponding side wall portion 91 so as they move away from the tubular portion 73 from one side to the other.

[0073] In the first embodiment described above, 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 the cross-sectional shape perpendicular to its longitudinal direction is compressed. Similarly, in the second embodiment described above, the tubular portion 73 may contact the groove bottom surface 57 while the spring piece 104 of the fastener 42 is locked to the first locking surface 112 and the second locking surface 113.

[0074] - In each of the above embodiments, the first projection 76 is not required on the outer circumferential surface of the first end portion 71, and the second projection 77 is not required on the outer circumferential surface of the second end portion 72. - In each of the above embodiments, 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.

[0075] In each of the above embodiments, 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.

[0076] In each of the above embodiments, one circulation path is formed by attaching one return tube 41 to the ball screw nut 28. However, 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.

[0077] In each of the above embodiments, 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.

[0078] Next, the technical concepts that can be understood from each of the above embodiments and modifications are described below. (Note) The tubular portion may have protrusions positioned adjacent to both longitudinal ends of the fixing device.

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 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 positioned 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 spring pieces provided on each of the pair of side wall portions, each of the pair of groove side surfaces 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 locking surfaces of the corresponding recesses, 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, the return tube is fixed to the ball screw nut by the fastener in a state in which the mounting portion is elastically deformed to move closer to the bottom surface of the groove, The fastener is a ball screw device in which the tip of each of the pair of spring pieces engages with the locking surface of the corresponding recess, thereby receiving a restoring force that causes the return tube to return to its original shape.

2. A ball screw device according to claim 1, wherein the inner surface of the connecting wall portion is planar, and the outer circumferential surface of the tubular portion includes a planar tube upper surface configured to be in surface contact with the inner surface of the connecting wall portion.

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. A ball screw device according to claim 1 or 2, wherein each of the pair of side wall portions has a top end which is an end connected to the connecting wall portion, and a bottom end which is an end located opposite to the top end, the spring piece is disposed between the top end and the bottom end, and the length of the communication groove in the bottom end along the longitudinal direction is longer than the length of the communication groove in the recess along the longitudinal direction.

8. A ball screw device according to claim 1 or 2, wherein the locking surface is a first locking surface, the recess is connected to the first locking surface and further includes a second locking surface extending in a direction intersecting the first locking surface, and the spring piece locks to both the first locking surface and the second locking surface.

9. 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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