Ball screw device and steering device

Abstract

A ball screw device (21) comprises a screw shaft (31) having a screw groove (43) on the outer periphery, a ball screw nut (32) having a screw groove on the inner periphery, a plurality of return tubes, and a plurality of balls (33). The plurality of return tubes includes at least a first return tube (34) and a second return tube. The position of a first end part of the second return tube, relative to the position of a first end part (91) of the first return tube (34) in the circumferential direction of the ball screw nut (32), is a first relative position, and the position of a second end part of the second return tube, relative to the position of a second end part (92) of the first return tube (34) in the circumferential direction, is a second relative position. At least one of the first relative position and the second relative position includes circumferential misalignment.

Description

[Technical field]

[0001] The present disclosure relates to a ball screw device and a steering device. [Background technology]

[0002] For example, as described in Patent Document 1, there is a steering device equipped with an RBS (recirculating ball screw) type gearbox having a ball screw device. Such a ball screw device includes a screw shaft having a screw groove on its outer periphery, a ball screw nut having a screw groove on its inner periphery, a return tube, and a plurality of balls. In the ball screw device, the screw groove of the screw shaft faces the screw groove of the ball screw nut to form a spiral rolling path. The return tube has a return path that short-circuits two points of the rolling path. The rolling path and the return path form a circulation path in which the balls are arranged. The balls circulate in the circulation path in response to the operation of the ball screw device. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-156082 A Summary of the Invention [Problem to be solved by the invention]

[0004] There is a demand for ensuring smooth operation of the above-mentioned ball screw device while extending the life of the ball screw device. [Means for solving the problem]

[0005] According to one aspect of the present disclosure, there is provided a ball screw device comprising a screw shaft having a screw groove on an outer periphery, a ball screw nut having a screw groove on an inner periphery, a plurality of return tubes, and a plurality of balls. The screw groove of the screw shaft faces the screw groove of the ball screw nut to form a helical rolling path. Each of the plurality of return tubes has a return path configured to short-circuit two points of the rolling path, the return path and the rolling path forming a circulation path in which the balls are arranged, a first end portion which is an entrance for the balls to enter the return path when the screw shaft or the ball screw nut rotates clockwise and an exit for the balls to be discharged from the return path when the screw shaft or the ball screw nut rotates counterclockwise, and a second end portion which is an exit for the balls to be discharged from the return path when the screw shaft or the ball screw nut rotates clockwise and an entrance for the balls to enter the return path when the screw shaft or the ball screw nut rotates counterclockwise. The ball screw nut has a plurality of mounting portions configured so that the plurality of return tubes are mounted from the same mounting direction. The plurality of return tubes include at least a first return tube and a second return tube. A position of the first end of the second return tube relative to a position of the first end of the first return tube in a circumferential direction of the ball screw nut is a first relative position, and a position of the second end of the second return tube relative to a position of the second end of the first return tube in the circumferential direction is a second relative position. At least one of the first relative position and the second relative position includes a deviation in the circumferential direction.

[0006] According to another aspect of the present disclosure, there is provided a steering device comprising: the ball screw device; and a steering shaft connected to the screw shaft. [Brief description of the drawings]

[0007] [Figure 1] 1 is a schematic configuration diagram of a steering device according to a first embodiment. [Diagram 2] FIG. 2 is a cross-sectional view of a gear box provided in the steering device of FIG. [Diagram 3] FIG. 3 is a side view of a ball screw device provided in the gear box of FIG. 2. [Figure 4A] 4 is a cross-sectional view taken along a connection portion of a first return tube in the ball screw device of FIG. 3, and is a cross-sectional view taken along line IVA-IVA of FIG. 3. [Figure 4B] 4 is a cross-sectional view taken along a connection portion of a second return tube in the ball screw device of FIG. 3, and is a cross-sectional view taken along line IVB-IVB of FIG. 3. [Figure 5A] 4 is a front view of a first return tube included in the ball screw device of FIG. 3. [Figure 5B] 4 is a front view of a second return tube included in the ball screw device of FIG. 3. [Figure 6A] 4 is a graph showing the relationship between steering angle and steering torque in a steering device of a comparative example. [Figure 6B] 2 is a graph showing the relationship between steering angle and steering torque in the steering device of FIG. 1. [Figure 7A] 13 is a cross-sectional view taken along a connection portion of a first return tube in a ball screw device of a second embodiment. FIG. [Figure 7B] 13 is a cross-sectional view taken along a connection portion of a second return tube in a ball screw device of a second embodiment. FIG. [Figure 8] FIG. 11 is a side view of a ball screw device according to a third embodiment. [Figure 9A] 9 is a cross-sectional view taken along a connection portion of a first return tube in the ball screw device of FIG. 8, and is a cross-sectional view of the ball screw device taken along line IXA-IXA in FIG. 8. [Figure 9B] 9 is a cross-sectional view taken along a connection portion of a second return tube in the ball screw device of FIG. 8, and is a cross-sectional view of the ball screw device taken along line IXB-IXB in FIG. 8. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] (First embodiment) Hereinafter, a first embodiment of a ball screw device and a steering device will be described with reference to the drawings. (Overall composition) 1 steers steered wheels 3 based on the operation of a steering wheel 2 by a driver. The steering device 1 includes a steering shaft 11, a gear box 12, a pitman arm 13, and a link mechanism 14. One end of the steering shaft 11 is connected to the steering wheel 2, and the other end of the steering shaft 11 is connected to the gear box 12. The gear box 12 converts the rotation of the steering shaft 11 into the swing of the pitman arm 13. The swing of the pitman arm 13 is then transmitted to the steered wheels 3 via the link mechanism 14, thereby turning the steered wheels 3.

[0009] The steering device 1 also includes an electric actuator 15 that generates an assist force to assist the steering operation by the driver. The electric actuator 15 includes a motor 16 and a reduction gear 17. The reduction gear 17 is, for example, a worm and wheel. The motor 16 is connected to the steering shaft 11 via the reduction gear 17. The electric actuator 15 applies an assist force to the steering shaft 11 by reducing the rotation of the motor 16 using the reduction gear 17 and transmitting the rotation to the steering shaft 11. That is, the steering device 1 of this embodiment is an electric power steering device.

[0010] (Gearbox 12) 2, the gearbox 12 is an RBS type gearbox having a ball screw device 21. In addition to the ball screw device 21, the gearbox 12 includes a housing 22 and a sector shaft 23 having a sector portion 24.

[0011] The housing 22 has, for example, a main body portion 25 and a bulging portion 26. The main body portion 25 is, for example, cylindrical with one end closed. The bulging portion 26 is provided on the peripheral wall of the main body portion 25, and the inside of the bulging portion 26 is connected to the inside of the main body portion 25. The bulging portion 26 is, for example, cylindrical and extends in a direction perpendicular to the axial direction of the main body portion 25. In the following, for convenience of explanation, the closed side of the housing 22 is referred to as the lower side, and the open side of the housing 22 is referred to as the upper side. A housing (not shown) of the reducer 17 is fixed to the upper side of the housing 22.

[0012] In this specification, "cylindrical" means that the shape can be regarded as cylindrical as a whole, and includes shapes formed by combining multiple parts and shapes with a cutout such as a C-shape. "Cylindrical" shapes include, but are not limited to, circles, ellipses, and polygons with sharp or rounded corners.

[0013] The sector shaft 23 is housed in the bulge 26. The sector shaft 23 is rotatably supported via a bearing (not shown). The sector shaft 23 has an outer end exposed to the outside of the housing 22. The base end of the pitman arm 13 is fixed to the outer end of the sector shaft 23 so as to be rotatable together with the pitman arm 13.

[0014] The sector portion 24 is disposed within the bulging portion 26. The sector portion 24 has, for example, a fan shape when viewed in the axial direction of the sector shaft 23. A plurality of external teeth 27 are provided on the outer circumferential surface of the sector portion 24.

[0015] The ball screw device 21 includes a screw shaft 31 connected to the steering shaft 11, a ball screw nut 32 that meshes with the external teeth 27 of the sector portion 24, and a plurality of balls 33. The ball screw device 21 converts the rotation of the screw shaft 31, which corresponds to the rotation of the steering shaft 11, into a linear movement of the ball screw nut 32. As a result, the sector shaft 23 rotates, causing the pitman arm 13 to swing.

[0016] (Ball screw device 21) As shown in Figures 2 and 3, the ball screw device 21 has a plurality of return tubes, and thus has a plurality of independent circulation paths through which the balls 33 circulate. Specifically, the ball screw device 21 has a first return tube 34, a second return tube 35, and a holder 36 in addition to a screw shaft 31, a ball screw nut 32, and a plurality of balls 33. The ball screw device 21 has two circulation paths, a first circulation path and a second circulation path. The return tube may be referred to as a circulator. Each component of the ball screw device 21 will be described in detail below.

[0017] (Screw shaft 31 and ball screw nut 32) 2, the screw shaft 31 is rotatably housed in the main body 25. The screw shaft 31 is rotatably supported, for example, via bearings 41, 42 provided at the upper and lower ends of the main body 25. The upper end of the screw shaft 31 is connected to the steering shaft 11 so as to be rotatable together with the steering shaft 11. A helical screw groove 43 is provided on the outer circumferential surface of the screw shaft 31. The upper side and the lower side refer to the upper side and the lower side, respectively, in the vertical direction of the vehicle, for example.

[0018] As shown in Figures 2, 3, 4A and 4B, the ball screw nut 32 is cylindrical. The ball screw nut 32 of this embodiment has a rectangular outer peripheral shape and a circular inner peripheral shape when viewed in the axial direction. Rack teeth 44 are provided on one of the outer surfaces of the ball screw nut 32. The rack teeth 44 mesh with the external teeth 27 of the sector portion 24. A helical screw groove 45 is provided on the inner peripheral surface of the ball screw nut 32.

[0019] A spiral rolling path Rc is formed by opposing the screw groove 43 of the screw shaft 31 to the screw groove 45 of the ball screw nut 32. A plurality of balls 33 are arranged in the rolling path Rc, sandwiched between the screw grooves 43, 45. In other words, the ball screw nut 32 is fitted onto the outer periphery of the screw shaft 31 via the balls 33.

[0020] The ball screw nut 32 has a mounting surface 46 to which the first return tube 34 and the second return tube 35 are attached. The mounting surface 46 is, for example, a surface of the outer surface of the ball screw nut 32 opposite to the surface on which the rack teeth 44 are provided. The mounting surface 46 is, for example, flat. The first return tube 34 and the second return tube 35 are arranged side by side on the mounting surface 46 so that the first return tube 34 is located above the second return tube 35. The mounting surface 46 is configured so that the first return tube 34 and the second return tube 35 are attached from the same mounting direction. The mounting direction is, for example, a direction perpendicular to the axial direction of the ball screw nut 32, that is, a direction perpendicular to the mounting surface 46. The mounting surface 46 is provided with a first mounting portion 47 for mounting the first return tube 34 and a second mounting portion 48 for mounting the second return tube 35. The configurations of the first mounting portion 47 and the second mounting portion 48 will be described later.

[0021] (First return tube 34 and second return tube 35) As shown in Figures 4A, 4B, 5A and 5B, the first return tube 34 has a return path R1 that short-circuits two points on the rolling path Rc. The second return tube 35 has a return path R2 that short-circuits two points on the rolling path Rc that are different from the above two points. The section on the rolling path Rc that is short-circuited by the first return tube 34 does not overlap with the section on the rolling path Rc that is short-circuited by the second return tube 35. The return path R1 of the first return tube 34 and the rolling path Rc form a first circulation path, and the return path R2 of the second return tube 35 and the rolling path Rc form a second circulation path.

[0022] 4A and 5A, the first return tube 34 has a first leg 51, a second leg 52, and a connecting portion 53 that connects the first leg 51 and the second leg 52 to each other. The inner circumferential surfaces of the first leg 51, the second leg 52, and the connecting portion 53 form a return path R1. The first return tube 34 has a shape, for example, of a cylindrical pipe bent into a roughly C-shape.

[0023] Specifically, the first leg 51 has a cylindrical shape extending linearly along the mounting direction. The second leg 52 has a cylindrical shape extending linearly along the mounting direction. That is, the second leg 52 extends parallel to the first leg 51. The connecting portion 53 has a cylindrical shape. The connecting portion 53 extends, for example, in a direction perpendicular to the mounting direction and intersects with the axial direction of the ball screw nut 32. In addition, both ends of the connecting portion 53 are bent so as to be connected to the first leg 51 and the second leg 52.

[0024] A tongue portion 54 and a fitting portion 55 are provided at the tip of the first leg portion 51. The tongue portion 54 protrudes in the mounting direction from an inner portion of the tip of the first leg portion 51. The tongue portion 54 has, for example, a semicircular shape when viewed in the longitudinal direction of the connecting portion 53. The fitting portion 55 protrudes in the mounting direction from an outer portion of the tip of the first leg portion 51. The fitting portion 55 has, for example, a semicylindrical shape. The fitting portion 55 protrudes more than the tongue portion 54.

[0025] The second leg 52 has a shape symmetrical to the first leg 51 with respect to a line extending in the mounting direction. That is, the tip of the second leg 52 is provided with a tongue portion 56 and a fitting portion 57. Also, the length L11 of the first leg 51 in the mounting direction, i.e., the distance from the connecting portion 53 to the tip of the fitting portion 55, is equal to the length L12 of the second leg 52 in the mounting direction, i.e., the distance from the connecting portion 53 to the tip of the fitting portion 57.

[0026] 4B and 5B, the second return tube 35 has a similar configuration to the first return tube 34, except for its dimensions, as described below. That is, the second return tube 35 has a first leg 61, a second leg 62, and a connecting portion 63 that connects the first leg 61 and the second leg 62 to each other. The first leg 61 has a tongue portion 64 and a fitting portion 65. The second leg 62 has a tongue portion 66 and a fitting portion 67.

[0027] (First mounting portion 47 and second mounting portion 48) As shown in FIG. 3, FIG. 4A, and FIG. 4B, the first mounting portion 47 has a first through hole 71 into which the first leg portion 51 of the first return tube 34 is inserted, and a second through hole 72 into which the second leg portion 52 of the first return tube 34 is inserted. The second mounting portion 48 has a first through hole 73 into which the first leg portion 61 of the second return tube 35 is inserted, and a second through hole 74 into which the second leg portion 62 of the second return tube 35 is inserted. The first through holes 71, 73 and the second through holes 72, 74 are all open in the opposite direction to the mounting direction. That is, the first through holes 71, 73 and the second through holes 72, 74 are open in the same direction. As a result, the first return tube 34 and the second return tube 35 are attached to the ball screw nut 32 from the same mounting direction as described above.

[0028] The first through hole 71 and the second through hole 72 of the first mounting portion 47 are provided at positions corresponding to the above two points of the rolling path Rc short-circuited by the first return tube 34 as viewed in the mounting direction. The first through hole 71 and the second through hole 72 are, for example, round holes as viewed in the mounting direction. The inner diameter of the first through hole 71 is set to be larger than the width of the screw groove 45 along the axial direction. As a result, an arc-shaped step is provided between the inner peripheral surface of the first through hole 71 and the inner surface of the screw groove 45 as viewed in the mounting direction. In other words, the first through hole 71 has an arc-shaped bottom surface as viewed in the mounting direction. The depth of the first through hole 71, i.e., the distance between the mounting surface 46 and the bottom surface (step) of the first through hole 71, is equal to the length L11 of the first leg portion 51 of the first return tube 34.

[0029] The second through hole 72 has a shape symmetrical to the first through hole 71 with respect to a straight line extending in the mounting direction. The depth of the second through hole 72, i.e., the distance between the mounting surface 46 and the bottom surface (step) of the second through hole 72, is equal to the length L12 of the second leg portion 52 of the first return tube 34.

[0030] When the first return tube 34 is attached to the first mounting portion 47, the tip of the fitting portion 55 of the first leg portion 51 abuts against the bottom surface of the first through hole 71, and the tip of the fitting portion 57 of the second leg portion 52 abuts against the bottom surface of the second through hole 72. As a result, the return path R1 of the first return tube 34 short-circuits two points on the rolling path Rc, forming a first circulation path.

[0031] The first through hole 73 and the second through hole 74 of the second mounting portion 48 are provided at positions corresponding to the other two points of the rolling path Rc short-circuited by the second return tube 35 as viewed in the mounting direction. The first through hole 73 and the second through hole 74 are, for example, round holes as viewed in the mounting direction. The inner diameter of the first through hole 73 is set to be larger than the width of the screw groove 45 along the axial direction. As a result, an arc-shaped step is provided between the inner peripheral surface of the first through hole 73 and the inner surface of the screw groove 45 as viewed in the mounting direction. In other words, the first through hole 73 has an arc-shaped bottom surface as viewed in the mounting direction. The depth of the first through hole 73, i.e., the distance between the mounting surface 46 and the bottom surface (step) of the first through hole 73, is equal to the length L21 of the first leg portion 61 of the second return tube 35.

[0032] The second through hole 74 has a shape symmetrical to the first through hole 73 with respect to a straight line extending in the mounting direction. The depth of the second through hole 74, i.e., the distance between the mounting surface 46 and the bottom surface (step) of the second through hole 74, is equal to the length L22 of the second leg portion 62 of the second return tube 35.

[0033] When the second return tube 35 is attached to the second mounting portion 48, the tip of the fitting portion 65 of the first leg portion 61 abuts against the bottom surface of the first through hole 73, and the tip of the fitting portion 67 of the second leg portion 62 abuts against the bottom surface of the second through hole 74. As a result, the return path R2 of the second return tube 35 short-circuits the other two points of the rolling path Rc, forming a second circulation path.

[0034] As shown in FIG. 3, the distances from the axis AX of the ball screw nut 32 to the first through holes 71, 73 are equal to each other when viewed in the mounting direction. Also, the distances from the axis AX of the ball screw nut 32 to the second through holes 72, 74 are equal to each other when viewed in the mounting direction. For convenience of explanation, a straight line passing through the centers of the first through holes 71, 73 and parallel to the axis AX, and a straight line passing through the centers of the second through holes 72, 74 and parallel to the axis AX are shown by two-dot chain lines in FIG. 3. The straight line connecting the center of the first through hole 71 and the center of the second through hole 72 in the first mounting portion 47 is parallel to the straight line connecting the center of the first through hole 73 and the center of the second through hole 74 in the second mounting portion 48. As a result, the connecting portion 53 of the first return tube 34 attached to the ball screw nut 32 is parallel to the connecting portion 63 of the second return tube 35.

[0035] (Holder 36) As shown in FIGS. 2 and 3, the holder 36 fixes the first return tube 34 and the second return tube 35 to the ball screw nut 32. The holder 36 is, for example, in the shape of a long plate. The holder 36 has a first recess 81 and a second recess 82. The connecting portion 53 of the first return tube 34 is fitted into the first recess 81. The connecting portion 63 of the second return tube 35 is fitted into the second recess 82. The holder 36 is fixed to the mounting surface 46 of the ball screw nut 32 by a bolt 83. As a result, the holder 36 presses and fixes the first return tube 34 and the second return tube 35 against the mounting surface 46 of the ball screw nut 32.

[0036] (Operation of ball screw device 21) In the ball screw device 21 thus configured, the ball 33 receives frictional force from the screw shaft 31 and the ball screw nut 32 in response to the rotation of the screw shaft 31 and rolls in the rolling path Rc. When the ball 33 rolling in the first circulation path reaches one of two points short-circuited by the first return tube 34 after rolling in the rolling path Rc, it passes through the return path R1 and is discharged to the other. As a result, the ball 33 moves from the downstream side to the upstream side on the rolling path Rc and circulates in the first circulation path. In the return path R1, the ball 33 moves in the return path R1 by being pushed by the adjacent ball 33 located behind in the circulation direction as a new ball 33 enters the return path R1 from the rolling path Rc.

[0037] Of the two ends of the first return tube 34, an end that becomes an entrance for the ball 33 to enter the return path R1 when the screw shaft 31 rotates clockwise in FIG. 4A and an exit for the ball 33 to be discharged from the return path R1 when the screw shaft 31 rotates counterclockwise is defined as a first end 91. Also, of the two ends of the first return tube 34, an end that becomes an exit for the ball 33 to be discharged from the return path R1 when the screw shaft 31 rotates clockwise and an end that becomes an entrance for the ball 33 to enter the return path R1 when the screw shaft 31 rotates counterclockwise is defined as a second end 92. When the first leg 51 is inserted into the first through hole 71 and the second leg 52 is inserted into the second through hole 72 as in this embodiment, the first leg 51 includes the first end 91 and the second leg 52 includes the second end 92.

[0038] Similarly, when the ball 33 rolling in the second circulation path rolls in the rolling path Rc and reaches one of the two points short-circuited by the second return tube 35, it passes through the return path R2 and is discharged to the other. As a result, the ball 33 moves from the downstream side to the upstream side of the rolling path Rc and circulates in the second circulation path. As described above, the section in the rolling path Rc short-circuited by the first return tube 34 does not overlap with the section short-circuited by the second return tube 35. Therefore, the ball 33 circulating in the first circulation path does not pass through the second circulation path and circulates only in the first circulation path. Similarly, the ball 33 circulating in the second circulation path does not pass through the first circulation path and circulates only in the second circulation path.

[0039] Of the two ends of the second return tube 35, an end that becomes an entrance for the ball 33 to enter the return path R2 when the screw shaft 31 rotates clockwise in FIG. 4B and an exit for the ball 33 to be discharged from the return path R2 when the screw shaft 31 rotates counterclockwise is defined as a first end 93. Also, of the two ends of the second return tube 35, an end that becomes an exit for the ball 33 to be discharged from the return path R2 when the screw shaft 31 rotates clockwise and an end that becomes an entrance for the ball 33 to enter the return path R2 when the screw shaft 31 rotates counterclockwise is defined as a second end 94. When the first leg 61 is inserted into the first through hole 73 and the second leg 62 is inserted into the second through hole 74 as in this embodiment, the first leg 61 includes the first end 93 and the second leg 62 includes the second end 94.

[0040] As described above, the ball screw device 21 has a plurality of circulation paths. Therefore, the length of the rolling path Rc in the entire ball screw device 21 can be increased while preventing the length of the rolling path Rc in one circulation path from becoming too long. This increases the number of balls 33 rolling in the rolling path Rc, thereby reducing the load (i.e., frictional force) applied to each of the plurality of balls 33 from the screw shaft 31 and the ball screw nut 32. This can extend the life of the balls 33, and therefore the life of the ball screw device 21. In addition, since the length of the rolling path Rc in one circulation path is not too long, for example, contact between adjacent balls 33 in the rolling path Rc can be prevented, and the ball screw device 21 can be operated smoothly.

[0041] Here, as a result of intensive research, the inventors have discovered that the torque required to rotate the screw shaft 31, i.e., the steering torque, varies with the timing at which the ball 33 enters the return tube and the timing at which the ball 33 is discharged from the return tube. The timing at which the ball 33 enters the return tube is the timing at which at least a portion of the ball 33 comes into contact with the return tube, and the timing at which the ball 33 does not come into contact with the return tube is the timing at which the ball 33 is discharged from the return tube.

[0042] Therefore, in the ball screw device 21 having a plurality of circulation paths as in this embodiment, if the timing at which the ball 33 enters the first return tube 34 and the timing at which the ball 33 enters the second return tube 35 coincide with each other, the fluctuation in steering torque becomes large. Also, if the timing at which the ball 33 is discharged from the first return tube 34 and the timing at which the ball 33 is discharged from the second return tube 35 coincide with each other, the fluctuation in steering torque becomes large.

[0043] In consideration of this, in the ball screw device 21, the timing of the balls 33 entering and exiting the first return tube 34 and the second return tube 35 is actively shifted. Here, the position of the first end 93 of the second return tube 35 relative to the position of the first end 91 of the first return tube 34 in the circumferential direction of the ball screw nut 32 is defined as a first relative position. Also, the position of the second end 94 of the second return tube 35 relative to the position of the second end 92 of the first return tube 34 in the circumferential direction of the ball screw nut 32 is defined as a second relative position. When defined in this way, both the first relative position and the second relative position include a circumferential deviation. This can be understood by the fact that when Figures 4A and 4B are superimposed, the position of the first end 91 of the first return tube 34 and the position of the first end 93 of the second return tube 35 do not coincide with each other, but are shifted circumferentially about the ball screw nut 32, and that the position of the second end 92 of the first return tube 34 and the position of the second end 94 of the second return tube 35 do not coincide with each other, but are shifted circumferentially about the ball screw nut 32.

[0044] 4A and 4B, in this embodiment, the length L11 of the first leg 51 and the length L12 of the second leg 52 of the first return tube 34 are equal to each other. The length L21 of the first leg 61 and the length L22 of the second leg 62 of the second return tube 35 are equal to each other. The length L11 of the first leg 51 and the length L12 of the second leg 52 of the first return tube 34 are longer than the length L21 of the first leg 61 and the length L22 of the second leg 62 of the second return tube 35. The length L13 of the connecting portion 53 of the first return tube 34 and the length L23 of the connecting portion 63 of the second return tube 35 are equal to each other.

[0045] As a result, the first end 93 and the second end 94 of the second return tube 35 are located closer to the mounting surface 46 than the first end 91 and the second end 92 of the first return tube 34. In other words, both the first relative position and the second relative position include a circumferential deviation of the ball screw nut 32. In addition, the sum of the circumferential deviation amount of the first relative position and the circumferential deviation amount of the second relative position is different from an integer multiple of the diameter of the ball 33. The sum of the deviation amounts in this embodiment is, for example, smaller than the diameter of the ball 33, but may be larger than the diameter of the ball 33.

[0046] Next, the operation and effects of this embodiment will be described. (1-1) As described above, the ball screw device 21 has multiple circulation paths, and therefore it is possible to lengthen the length of the rolling path Rc in the entire ball screw device 21 while preventing the length of the rolling path Rc in one circulation path from becoming too long. Therefore, by reducing the load applied to each of the multiple balls 33, it is possible to extend the life of the balls 33 and, ultimately, the life of the ball screw device 21.

[0047] Here, as a comparative example, a ball screw device configured similarly to ball screw device 21 is assumed, except that neither the first relative position nor the second relative position includes a circumferential misalignment. That is, in the comparative example, the position of the first end of the first return tube in the circumferential direction of the ball screw nut coincides with the position of the first end of the second return tube, and the position of the second end of the first return tube in the circumferential direction of the ball screw nut coincides with the position of the second end of the second return tube.

[0048] Therefore, the timing at which the ball enters the first return tube and the second return tube, and the timing at which the ball is discharged from the first return tube and the second return tube, tend to coincide with each other. As a result, as shown in Fig. 6A, for example, in the ball screw device of the comparative example, when steering at a constant steering speed, the rotation angle of the screw shaft 31, i.e., the steering torque with respect to the steering angle, fluctuates greatly.

[0049] In this regard, in the ball screw device 21 of this embodiment, both the first relative position and the second relative position include a circumferential deviation. Therefore, at least one of the timing at which the ball 33 enters the first return tube 34 and the timing at which the ball 33 enters the second return tube 35, and the timing at which the ball 33 is discharged from the first return tube 34 and the timing at which the ball 33 is discharged from the second return tube 35, is likely to deviate from each other. As a result, as shown in Fig. 6B, for example, in the ball screw device 21 of this embodiment, the fluctuation in steering torque with respect to the steering angle is small when steering at a constant steering speed. In this way, the smooth operation of the ball screw device 21 can be ensured.

[0050] In the comparative example, the arrangement of the balls in the return path in the first return tube and the arrangement of the balls in the return path in the first return tube do not necessarily match each other. For example, the balls may be arranged in a zigzag (staggered) pattern in the first return tube and in a straight line in the second return tube. As a result, the timing at which the balls are discharged from the first return tube and the timing at which the balls are discharged from the second return tube may differ. However, such a timing difference occurs accidentally. Therefore, in the ball screw device of the comparative example, the timing at which the balls enter the first return tube and the timing at which the balls enter the second return tube, and the timing at which the balls are discharged from the first return tube and the timing at which the balls are discharged from the second return tube tend to match each other.

[0051] (1-2) The steering device 1 includes a ball screw device 21 and a steering shaft 11 connected to a screw shaft 31. According to the above configuration, fluctuations in steering torque are suppressed, and therefore deterioration of steering feeling can be suppressed.

[0052] (1-3) The sum of the circumferential deviation amount of the first relative position and the circumferential deviation amount of the second relative position is different from an integer multiple of the diameter of the ball 33. This makes it easier for at least one of the timing at which the ball 33 enters the first return tube 34 and the timing at which the ball 33 enters the second return tube 35, and the timing at which the ball 33 is discharged from the first return tube 34 and the timing at which the ball 33 is discharged from the second return tube 35 to deviate from each other in a suitable manner.

[0053] (1-4) The return tubes 34, 35 each have a first leg 51, 61 having a length along the mounting direction, and a second leg 52, 62 having a length along the mounting direction. The length L11 of the first leg 51 of the first return tube 34 is equal to the length L12 of the second leg 52. The length L21 of the first leg 61 of the second return tube 35 is equal to the length L22 of the second leg 62.

[0054] According to the above configuration, the first leg 51 of the first return tube 34 can be inserted into both the first through hole 71 and the second through hole 72, and the second leg 52 can be inserted into both the first through hole 71 and the second through hole 72. Therefore, when attaching the first return tube 34, it is not necessary to consider the combination of the first leg 51 and the first through hole 71 and the combination of the second leg 52 and the second through hole 72. This makes it easier to attach the first return tube 34. Similarly, the first leg 61 of the second return tube 35 can be inserted into both the first through hole 73 and the second through hole 74, and the second leg 62 can be inserted into both the first through hole 73 and the second through hole 74. This makes it easier to attach the second return tube 35.

[0055] Second embodiment Next, a second embodiment of the ball screw device and steering device will be described with reference to the drawings. For convenience of explanation, the same components as those in the first embodiment will be denoted by the same reference numerals and the description thereof will be omitted.

[0056] As shown in Figures 7A and 7B, in this embodiment, the length L11 of the first leg 51 of the first return tube 34 is longer than the length L12 of the second leg 52. The length L21 of the first leg 61 of the second return tube 35 is shorter than the length L22 of the second leg 62. The length L11 of the first leg 51 of the first return tube 34 and the length L22 of the second leg 62 of the second return tube 35 are equal to each other. The length L12 of the second leg 52 of the first return tube 34 and the length L21 of the first leg 61 of the second return tube 35 are equal to each other. The length L13 of the connecting portion 53 of the first return tube 34 and the length L23 of the connecting portion 63 of the second return tube 35 are equal to each other.

[0057] As a result, the first end 93 of the second return tube 35 is located closer to the mounting surface 46 than the first end 91 of the first return tube 34. Also, the second end 92 of the first return tube 34 is located closer to the mounting surface 46 than the second end 94 of the second return tube 35. That is, both the first relative position and the second relative position include a circumferential deviation of the ball screw nut 32. Also, the sum of the circumferential deviation amount of the first relative position and the circumferential deviation amount of the second relative position is different from an integer multiple of the diameter of the ball 33. The sum of the deviation amounts in this embodiment is, for example, smaller than the diameter of the ball 33, but may be larger than the diameter of the ball 33.

[0058] As described above, in this embodiment, in addition to the same actions and effects as those (1-1) to (1-3) of the first embodiment, the following actions and effects are achieved. (2-1) The return tubes 34, 35 each have a first leg 51, 61 having a length along the mounting direction, a second leg 52, 62 having a length along the mounting direction, and a connecting portion 53, 63 that connects the first leg 51, 61 and the second leg 52, 62 to each other and has a length along a direction intersecting the mounting direction. The length L11 of the first leg 51 of the first return tube 34 is different from the length L12 of the second leg 52 of the first return tube 34. The length L21 of the first leg 61 of the second return tube 35 is different from the length L22 of the second leg 62 of the second return tube 35. The length L11 of the first leg 51 of the first return tube 34 and the length L22 of the second leg 62 of the second return tube 35 are equal to each other. The length L12 of the second leg 52 of the first return tube 34 and the length L21 of the first leg 61 of the second return tube 35 are equal to each other. The length L13 of the connecting portion 53 of the first return tube 34 and the length L13 of the connecting portion 63 of the second return tube 35 are equal to each other. Therefore, the first return tube 34 can be made to have the same shape as the second return tube 35. In other words, the first return tube 34 can be used as the second return tube 35, and the second return tube 35 can be used as the first return tube 34. This allows, for example, reduced manufacturing costs compared to when the shape of the first return tube 34 is different from the shape of the second return tube 35.

[0059] Third embodiment Next, a third embodiment of the ball screw device and steering device will be described with reference to the drawings. For convenience of explanation, the same components as those in the first embodiment will be denoted by the same reference numerals and the description thereof will be omitted.

[0060] 8, 9A and 9B, in this embodiment, the length L11 of the first leg 51 and the length L12 of the second leg 52 of the first return tube 34 are equal to each other. The length L21 of the first leg 61 and the length L22 of the second leg 62 of the second return tube 35 are equal to each other. The length L11 of the first leg 51 and the length L12 of the second leg 52 of the first return tube 34 are longer than the length L21 of the first leg 61 and the length L22 of the second leg 62 of the second return tube 35. The length L13 of the connecting portion 53 of the first return tube 34 is longer than the length L23 of the connecting portion 63 of the second return tube 35.

[0061] As shown in FIG. 8, the first through hole 73 of the second mounting portion 48 is provided closer to the axis AX of the ball screw nut 32 than the first through hole 71 of the first mounting portion 47 when viewed in the mounting direction. The second through hole 74 of the second mounting portion 48 is provided closer to the axis AX of the ball screw nut 32 than the second through hole 72 of the first mounting portion 47. For convenience of explanation, a straight line passing through the center of the first through hole 73 and parallel to the axis AX, and a straight line passing through the center of the second through hole 74 and parallel to the axis AX are shown by two-dot chain lines in FIG. 8. The straight line connecting the center of the first through hole 71 and the center of the second through hole 72 in the first mounting portion 47 intersects with the straight line connecting the center of the first through hole 73 and the center of the second through hole 74 in the second mounting portion 48. As a result, the connecting portion 53 of the first return tube 34 and the connecting portion 63 of the second return tube 35 are not parallel to each other.

[0062] As a result, the first end 93 and the second end 94 of the second return tube 35 are located closer to the mounting surface 46 than the first end 91 and the second end 92 of the first return tube 34. In other words, both the first relative position and the second relative position include a circumferential deviation of the ball screw nut 32. In addition, the sum of the circumferential deviation amount of the first relative position and the circumferential deviation amount of the second relative position is different from an integer multiple of the diameter of the ball 33. The sum of the deviation amounts in this embodiment is, for example, smaller than the diameter of the ball 33, but may be larger than the diameter of the ball 33.

[0063] As described above, in this embodiment, in addition to the same actions and effects as those (1-1) to (1-4) of the first embodiment, the following actions and effects are achieved. (3-1) The return tubes 34, 35 each have a first leg 51, 61 having a length along the mounting direction, a second leg 52, 62 having a length along the mounting direction, and a connecting portion 53, 63 that connects the first leg 51, 61 and the second leg 52, 62 to each other and has a length along a direction intersecting the mounting direction. The length L13 of the connecting portion 53 of the first return tube 34 is different from the length L23 of the connecting portion 63 of the second return tube 35. Therefore, for example, the second return tube 35 cannot be attached to the first mounting portion 47. This can prevent the first return tube 34 and the second return tube 35 from being erroneously assembled.

[0064] The above-described embodiments may be modified as follows: The above-described embodiments and the following modifications may be combined with each other to the extent that no technical contradiction occurs. The first return tube 34 and the second return tube 35 are fixed to the ball screw nut 32 by a single holder 36. However, this is not limiting, and for example, the first return tube 34 and the second return tube 35 may be fixed to the ball screw nut 32 by individual holders. Also, the first return tube 34 and the second return tube 35 may be fixed to the ball screw nut 32 by, for example, an adhesive without using a holder, and the fixing method can be changed as appropriate.

[0065] The sum of the amount of deviation of the first relative position in the circumferential direction and the amount of deviation of the second relative position in the circumferential direction may be equal to an integer multiple of the diameter of the ball 33. · As long as at least one of the first relative position and the second relative position includes a circumferential deviation of the ball screw nut 32, the shapes of the first return tube 34 and the second return tube 35 can be appropriately changed. For example, in the first embodiment, the length L11 of the first leg 51 of the first return tube 34 may be equal to the length L21 of the first leg 61 of the second return tube 35. That is, the first relative position may not include a circumferential deviation, and only the second relative position may include a circumferential deviation. Also, for example, in the first embodiment, the length L12 of the second leg 52 of the first return tube 34 may be equal to the length L22 of the second leg 62 of the second return tube 35. That is, only the first relative position may include a circumferential deviation, and the second relative position may not include a circumferential deviation. Also, for example, in the second embodiment, the length L11 of the first leg 51 of the first return tube 34 may be different from the length L22 of the second leg 62 of the second return tube 35. Also, for example, in the third embodiment, the length L11 of the first leg 51 of the first return tube 34 may be made different from the length L12 of the second leg 52.

[0066] The ball screw device 21 may include three or more return tubes. In this case, the relative positions of the first end 91 of the first return tube 34 and the first end 93 of the second return tube 35 with respect to the first ends of the other return tubes may or may not include a circumferential misalignment of the ball screw nut 32. Also, the relative positions of the second end 92 of the first return tube 34 and the second end 94 of the second return tube 35 with respect to the second ends of the other return tubes may or may not include a circumferential misalignment of the ball screw nut 32.

[0067] The steering device 1 may include a hydraulic actuator that generates an assist force using hydraulic pressure in addition to or instead of the electric actuator 15. Also, the steering device 1 does not necessarily need to include an actuator that generates an assist force.

[0068] Although the ball screw device 21 is used so that the ball screw nut 32 is moved linearly by the rotation of the screw shaft 31, the ball screw device 21 may be used so that the ball screw nut 32 is moved linearly by the rotation of the screw shaft 31.

[0069] The ball screw device 21 may be applied to devices other than the steering device 1. Next, the technical ideas that can be understood from the above-described embodiments and modifications will be described below. (Additional Note 1) In one aspect of the present disclosure, each of the multiple mounting portions may have a first through hole that opens in the mounting direction, into which the first end is inserted, and a second through hole that opens in the mounting direction, into which the second end is inserted.

[0070] (Supplementary Note 2) In one aspect of the present disclosure, the ball screw nut may have a planar mounting surface, and the multiple return tubes may be arranged side by side on the mounting surface.

Claims

1. A screw shaft having a screw groove on an outer periphery thereof; A ball screw nut having a screw groove on its inner periphery; A plurality of return tubes; A plurality of balls; the screw shaft has a screw groove that faces the screw groove of the ball screw nut to form a spiral rolling path; Each of the plurality of return tubes a return path configured to short-circuit two points of the rolling path, the return path and the rolling path forming a circulation path in which the balls are disposed; a first end portion which serves as an entrance portion for the balls to enter the return path when the screw shaft or the ball screw nut rotates clockwise, and which serves as an exit portion for the balls to be discharged from the return path when the screw shaft or the ball screw nut rotates counterclockwise; a second end portion that serves as an outlet through which the balls are discharged from the return path when the screw shaft or the ball screw nut rotates clockwise, and serves as an inlet through which the balls enter the return path when the screw shaft or the ball screw nut rotates counterclockwise, the ball screw nut has a plurality of mounting portions configured so that the plurality of return tubes are mounted from the same mounting direction, the plurality of return tubes includes at least a first return tube and a second return tube; a position of the first end of the second return tube relative to a position of the first end of the first return tube in a circumferential direction of the ball screw nut is a first relative position, and a position of the second end of the second return tube relative to a position of the second end of the first return tube in the circumferential direction is a second relative position, At least one of the first relative position and the second relative position includes a circumferential deviation.

2. 2. The ball screw device according to claim 1, a sum of the circumferential deviation amount of the first relative position and the circumferential deviation amount of the second relative position is different from an integer multiple of a diameter of the ball.

3. 3. The ball screw device according to claim 1, Each of the plurality of return tubes A first leg portion having a length along the mounting direction; a second leg portion having a length along the mounting direction, a length of the first leg of the first return tube and a length of the second leg of the first return tube are equal to each other; A ball screw arrangement, wherein the length of the first leg of the second return tube and the length of the second leg of the second return tube are equal to one another.

4. The ball screw device according to any one of claims 1 to 3, Each of the plurality of return tubes A first leg portion having a length along the mounting direction; A second leg portion having a length along the mounting direction; a connecting portion that connects the first leg portion and the second leg portion to each other, the connecting portion having a length along a direction intersecting the mounting direction, A ball screw arrangement, wherein a length of the connection portion of the first return tube is different from a length of the connection portion of the second return tube.

5. 3. The ball screw device according to claim 1, Each of the plurality of return tubes A first leg portion having a length along the mounting direction; A second leg portion having a length along the mounting direction; a connecting portion that connects the first leg portion and the second leg portion to each other, the connecting portion having a length along a direction intersecting the mounting direction, a length of the first leg of the first return tube is different from a length of the second leg of the first return tube; a length of the first leg of the second return tube is different from a length of the second leg of the second return tube; a length of the first leg of the first return tube and a length of the second leg of the second return tube are equal to each other; a length of the second leg of the first return tube and a length of the first leg of the second return tube are equal to each other; A ball screw device, wherein a length of the connecting portion of the first return tube and a length of the connecting portion of the second return tube are equal to each other.

6. A ball screw device according to any one of claims 1 to 5, a steering shaft connected to the screw shaft.

Images