Ball screw device and electric power steering device

Abstract

A ball screw device (23) is provided with a ball screw shaft (12), a ball nut (31), a plurality of balls (32), and a return tube (33). The return tube has an intermediate part (41), a first insertion end part (42), a second insertion end part (43), a first curved part (44), and a second curved part (45). The ball nut has an attachment part (34) to which the return tube is attached. The attachment part has an intermediate part facing part (51), a first insertion hole (52), a second insertion hole (53), a first curved part facing part (54), and a second curved part facing part (55). The intermediate part facing part is constituted of a single flat surface parallel to the axis of the ball nut. Each of the first curved part facing part and the second curved part facing part is constituted of a plurality of flat surfaces (54A, 54B, 54C, 55A, 55B, 55C) parallel to the axis of the ball nut.

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] Electric power steering devices having a ball screw device have been known in the past. For example, the ball screw device disclosed in Patent Document 1 includes a steering shaft for steering a steered wheel, a nut rotatably provided on the steering shaft, and a plurality of balls interposed between the steering shaft and the nut. A first helical ball screw groove is provided on the outer peripheral surface of the steering shaft. A second helical ball screw groove is provided on the inner peripheral surface of the nut. The balls roll in a helical rolling path formed by the first ball screw groove and the second ball screw groove.

[0003] The nut has a connecting member that short-circuits two points set in the rolling path. The connecting member is a U-shaped tube. A first end of the connecting member is inserted into a first connecting passage that opens on the outer peripheral surface of the nut. A second end of the connecting member is inserted into a second connecting passage that opens on the outer peripheral surface of the nut. The balls rolling in the rolling path move from downstream to upstream between the two points set in the rolling path by passing through the connecting member. As a result, the balls circulate endlessly within the rolling path.

[0004] The connecting member has a first curved portion and a second curved portion. The first curved portion is provided between the first end and the intermediate portion of the connecting member and is formed so that the traveling direction of the ball changes in a curved manner. The second curved portion is provided between the second end and the intermediate portion of the connecting member and is formed so that the traveling direction of the ball changes in a curved manner.

[0005] The first connecting passage of the nut has a first curved surface portion. The first curved surface portion faces the first curved surface portion of the connecting member. A gap is formed between the first curved surface portion and the first curved surface portion. The second connecting passage of the nut has a second curved surface portion. The second curved surface portion faces the second curved surface portion of the connecting member. A gap is formed between the second curved surface portion and the first curved surface portion.

[0006] JP 2015-160497 A

[0007] In the ball screw device of Prior Art Document 1, it is necessary to strictly control the gap between the first curved surface portion of the nut and the first curved surface portion of the connecting member, and the gap between the second curved surface portion of the nut and the second curved surface portion of the connecting member. Therefore, it is necessary to ensure machining accuracy of the first curved surface portion and the second curved surface portion. However, it is difficult to ensure machining accuracy over the entire area of ​​the first curved surface portion and the second curved surface portion.

[0008] A ball screw device according to one aspect of the present disclosure includes a ball screw shaft having a first spiral rolling groove on its outer circumferential surface, a ball nut having a second spiral rolling groove on its inner circumferential surface facing the first rolling groove, a plurality of balls rolling on a spiral rolling path formed by the first rolling groove and the second rolling groove, and a return tube configured to short-circuit two points on the rolling path and return the balls from the downstream side to the upstream side of the rolling path. The return tube has an intermediate portion that is a central portion in the axial direction of the return tube, a first insertion end portion extending in a direction intersecting the intermediate portion, a second insertion end portion extending in the same direction as the first insertion end, a first curved portion provided between the intermediate portion and the first insertion end, and a second curved portion provided between the intermediate portion and the second insertion end. The ball nut has an attachment portion configured to allow the return tube to be attached from the radially outer side of the ball nut. The mounting portion has a middle portion facing portion that faces the middle portion, a first insertion hole into which the first insertion end is inserted, a second insertion hole into which the second insertion end is inserted, a first curved portion facing portion that faces the first curved portion, and a second curved portion facing portion that faces the second curved portion. The middle portion facing portion is formed on a single plane parallel to the axis of the ball nut. The first curved portion facing portion and the second curved portion facing portion each are formed on a plurality of planes parallel to the axis of the ball nut.

[0009] Fig. 1 is a cross-sectional view of an electric power steering device equipped with a ball screw device according to one embodiment. Fig. 2 is a plan view of the ball screw device according to one embodiment. Fig. 3 is a cross-sectional view of a ball nut taken along line 3-3 in Fig. 2. Fig. 4 is an enlarged cross-sectional view of a main portion of the ball nut in Fig. 3. Fig. 5 is a cross-sectional view showing a main portion of the return tube in Fig. 3.

[0010] An electric power steering device according to one embodiment will be described. <Configuration of electric power steering device 1> As shown in Fig. 1, electric power steering device 1 includes a steering mechanism 2 and a steering actuator 3. Steering mechanism 2 steers steerable wheels 5 of a vehicle in response to operation of a steering wheel 4 by a driver. Steering actuator 3 applies an assist force to steering mechanism 2. The assist force is a force for assisting operation of steering wheel 4.

[0011] The steering mechanism 2 has a steering shaft 11, a steered shaft 12, and a housing 13. A first end of the steering shaft 11 is connected to the steering wheel 4. A second end of the steering shaft 11 is connected to the steered shaft 12. The steered shaft 12 extends in the left-right direction relative to the direction of travel of the vehicle. Both ends of the steered shaft 12 are connected to the steered wheels 5 via tie rods 14. The housing 13 accommodates the steered shaft 12 so that it can move back and forth. The steered shaft 12 has rack teeth 12A.

[0012] The steering shaft 11 is formed by connecting a column shaft 11A, an intermediate shaft 11B, and a pinion shaft 11C. The pinion shaft 11C has pinion teeth 11C1. The pinion teeth 11C1 mesh with rack teeth 12A. Rotation of the steering shaft 11 caused by operation of the steering wheel 4 is converted into axial movement of the steered shaft 12 via the meshing of the pinion teeth 11C1 and the rack teeth 12A. The axial movement of the steered shaft 12 changes the steering angle of the steered wheels 5.

[0013] The steering actuator 3 has a motor 21, a belt transmission device 22, and a ball screw device 23. The belt transmission device 22 transmits the rotation of the motor 21 to the ball screw device 23. The ball screw device 23 converts the rotation transmitted via the belt transmission device 22 into axial movement of the steering shaft 12. In other words, the rotation of the motor 21 is imparted to the steering shaft 12 via the belt transmission device 22 and the ball screw device 23. This assists the axial movement of the steering shaft 12. The motor 21 is a drive source for the electric power steering device 1.

[0014] <Configuration of Ball Screw Device 23> Next, the configuration of the ball screw device 23 will be described. As shown in Fig. 2, the ball screw device 23 has a steered shaft 12, a ball nut 31, and a plurality of balls 32. The ball nut 31 is screwed onto the outer periphery of the steered shaft 12 via the balls 32. In other words, the balls 32 are provided between the steered shaft 12 and the ball nut 31.

[0015] The steered shaft 12 has a first rolling groove 12B. The first rolling groove 12B is a spiral groove provided on the outer peripheral surface of the steered shaft 12. The first rolling groove 12B is provided within a predetermined range in the axial direction of the steered shaft 12. The steered shaft 12 also functions as a ball screw shaft.

[0016] The ball nut 31 is a cylindrical body with a circular cross section, and is arranged coaxially on the outer periphery of the steered shaft 12. The ball nut 31 has a second rolling groove 31A. The second rolling groove 31A is a spiral groove provided on the inner circumferential surface of the ball nut 31. The second rolling groove 31A is provided over the entire axial length of the ball nut 31.

[0017] The first rolling groove 12B and the second rolling groove 31A face each other in the radial direction of the ball nut 31. The first rolling groove 12B and the second rolling groove 31A form a spiral rolling path R1. The rolling path R1 is a raceway for the balls 32. The balls 32 are accommodated inside the rolling path R1 so as to be able to roll.

[0018] The rotation of motor 21 is transmitted to ball nut 31 via belt transmission device 22. For ease of explanation, illustration of belt transmission device 22 is omitted in Figure 2. Ball nut 31 rotates in conjunction with motor 21. Due to the relative rotation between steered shaft 12 and ball nut 31, balls 32 roll inside rolling path R1 while receiving a load. The rolling of balls 32 causes the relative positions of steered shaft 12 and ball nut 31 to be displaced in the axial direction. As a result, the torque of motor 21 is applied to steered shaft 12 as an assist force.

[0019] The ball nut 31 has a return tube 33. The return tube 33 short-circuits two points set on the rolling path R1. The two points are a first connection point and a second connection point. The first connection point and the second connection point are separated by a predetermined distance in the axial direction of the ball nut 31 and also by a predetermined distance in the circumferential direction of the ball nut 31. The balls 32 circulate endlessly by moving from the downstream side to the upstream side of the rolling path R1 via the return tube 33. The balls 32 sequentially enter the interior of the return tube 33. The front balls 32 in the circulation direction move inside the return tube 33 by being pushed by the rear balls 32.

[0020] The return tube 33 is attached to an attachment portion 34 of the ball nut 31. The attachment portion 34 is a recessed or grooved portion provided on the outer peripheral surface of the ball nut 31. The return tube 33 is attached to the attachment portion 34 from the radially outer side of the ball nut 31. The return tube 33 is maintained in a state where it is pressed radially inward of the ball nut 31 by a holder 35. This secures the return tube 33 to the ball nut 31. The holder 35 is, for example, a rectangular plate-like body. The central portion of the holder 35 is curved so as to cover the straight portion of the return tube 33. Both ends of the holder 35 are secured to the outer peripheral surface of the ball nut 31 by set screws 36.

[0021] <Configuration of Return Tube 33> Next, the configuration of the return tube 33 will be described. As shown in Fig. 3, the return tube 33 is a U-shaped cylindrical body with a circular cross section. The return tube 33 is curved so as to be convex outward in the radial direction of the ball nut 31. The return tube 33 has an intermediate portion 41, a first insertion end portion 42, a second insertion end portion 43, a first curved portion 44, and a second curved portion 45.

[0022] The intermediate portion 41 is the axial center portion of the return tube 33. When viewed in the axial direction of the ball nut 31, the intermediate portion 41 is gently curved in the circumferential direction of the ball nut 31. However, when viewed in the radial direction of the ball nut 31, the intermediate portion 41 extends linearly (see FIG. 2).

[0023] The first insertion end 42 is connected to the first end of the intermediate portion 41 via the first curved portion 44. That is, the first curved portion 33B is provided between the intermediate portion 41 and the first insertion end 42. The first insertion end 42 extends in a direction intersecting with the intermediate portion 41.

[0024] The second insertion end 43 is connected to the second end of the intermediate portion 41 via the second curved portion 45. The second curved portion 45 is provided between the intermediate portion 41 and the second insertion end 43. The second insertion end 43 extends in the same direction as the first insertion end 42. The second insertion end 43 is parallel to the first insertion end 42.

[0025] For the sake of convenience, the ball 32 and the holder 35 are not shown in Fig. 3. <Configuration of Mounting Portion 34> Next, the configuration of the mounting portion 34 will be described.

[0026] 3 , the mounting portion 34 has a middle portion facing portion 51, a first insertion hole 52, a second insertion hole 53, a first curved portion facing portion 54, and a second curved portion facing portion 55. The middle portion facing portion 51 is a portion of the mounting portion 34 that faces the middle portion 41 of the return tube 33 in the radial direction of the ball nut 31. The middle portion facing portion 51 is made of a single plane that is parallel to the axis of the ball nut 31. The middle portion facing portion 51 is a plane that faces the middle portion 41 of the return tube 33 in the radial direction of the ball nut 31.

[0027] The first insertion hole 52 is a portion of the mounting portion 34 into which the first insertion end portion 42 of the return tube 33 is inserted. The first insertion hole 52 penetrates the peripheral wall of the ball nut 31 in a direction perpendicular to the axis of the ball nut 31. The interior of the first insertion hole 52 communicates with the rolling path R1.

[0028] The second insertion hole 53 is a portion of the mounting portion 34 into which the second insertion end portion 43 of the return tube 33 is inserted. The second insertion hole 53 penetrates the peripheral wall of the ball nut 31 in a direction perpendicular to the axis of the ball nut 31. The second insertion hole 53 is parallel to the first insertion hole 52. The interior of the second insertion hole 53 communicates with the rolling path R1.

[0029] The first curved portion opposing portion 54 is a portion of the mounting portion 34 that faces the first curved portion 44 of the return tube 33 in the radial direction of the ball nut 31. The first curved portion opposing portion 54 is disposed between the intermediate portion opposing portion 51 and the first insertion hole 52.

[0030] The first curved portion opposing portion 54 has a first flat surface 54A, a second flat surface 54B, and a third flat surface 54C. The first flat surface 54A, the second flat surface 54B, and the third flat surface 54C are parallel to the axis of the ball nut 31. The first flat surface 54A is adjacent to the middle portion opposing portion 51 in the circumferential direction of the ball nut 31. The second flat surface 54B is disposed between the first flat surface 54A and the third flat surface 54C in the circumferential direction of the ball nut 31.

[0031] The first plane 54A, the second plane 54B, and the third plane 54C are each inclined at a different angle relative to the flat intermediate portion facing portion 51. The inclination angles relative to the intermediate portion facing portion 51 increase in the order of the inclination angle of the first plane 54A relative to the intermediate portion facing portion 51, the inclination angle of the second plane 54B relative to the intermediate portion facing portion 51, and the inclination angle of the third plane 54C relative to the intermediate portion facing portion 51.

[0032] The first curved portion facing portion 54 has a first corner C1, a second corner C2, and a third corner C3. The first corner C1 is provided at the connection point between the intermediate portion facing portion 51 and the first curved portion facing portion 54, i.e., the connection point between the intermediate portion facing portion 51 and the first flat surface 54A. The first corner C1 is a corner formed by the intermediate portion facing portion 51 and the first flat surface 54A. The second corner C2 is provided at the connection point between the first flat surface 54A and the second flat surface 54B. The second corner C2 is a corner formed by the first flat surface 54A and the second flat surface 54B. The third corner C3 is provided at the connection point between the second flat surface 54B and the third flat surface 54C. The third corner C3 is a corner formed by the second flat surface 54B and the third flat surface 54C.

[0033] The second curved portion opposing portion 55 is a portion of the mounting portion 34 that faces the second curved portion 45 of the return tube 33 in the radial direction of the ball nut 31. The second curved portion opposing portion 55 is disposed between the intermediate portion opposing portion 51 and the second insertion hole 53.

[0034] The second curved portion opposing portion 55 has a fourth flat surface 55A, a fifth flat surface 55B, and a sixth flat surface 55C. The fourth flat surface 55A is located on the opposite side of the intermediate portion opposing portion 51 from the first flat surface 54A in the circumferential direction of the ball nut 31. The fourth flat surface 55A is adjacent to the intermediate portion opposing portion 51 in the circumferential direction of the ball nut 31. The fifth flat surface 55B is located between the fourth flat surface 55A and the sixth flat surface 55C in the circumferential direction of the ball nut 31.

[0035] The fourth plane 55A, the fifth plane 55B, and the sixth plane 55C are each inclined at a different angle relative to the intermediate portion facing portion 51. The inclination angles relative to the intermediate portion facing portion 51 increase in the order of the inclination angle of the fourth plane 55A relative to the intermediate portion facing portion 51, the inclination angle of the fifth plane 55B relative to the intermediate portion facing portion 51, and the inclination angle of the sixth plane 55C relative to the intermediate portion facing portion 51.

[0036] The second curved portion facing portion 55 has a fourth corner C4, a fifth corner C5, and a sixth corner C6. The fourth corner C4 is provided at the connection point between the intermediate portion facing portion 51 and the second curved portion facing portion 55, i.e., the connection point between the intermediate portion facing portion 51 and the fourth flat surface 55A. The fourth corner C4 is a corner formed by the intermediate portion facing portion 51 and the fourth flat surface 55A. The fifth corner C5 is provided at the connection point between the fourth flat surface 55A and the fifth flat surface 55B. The fifth corner C5 is a corner formed by the fourth flat surface 55A and the fifth flat surface 55B. The sixth corner C6 is provided at the connection point between the fifth flat surface 55B and the sixth flat surface 55C. The sixth corner C6 is a corner formed by the fifth flat surface 55B and the sixth flat surface 55C.

[0037] When attaching the return tube 33 to the attachment portion 34, the first insertion portion 42 is inserted into the first insertion hole 52, and the second insertion portion 43 is inserted into the second insertion hole 53. Movement of the first insertion portion 42 in the insertion direction is restricted by the tip of the first insertion portion 42 abutting against the first abutment portion 31B in the insertion direction. The first abutment portion 31B is a part of the ball nut 31. Movement of the second insertion portion 43 in the insertion direction is restricted by the tip of the second insertion portion 43 abutting against the second abutment portion 31C in the insertion direction. The second abutment portion 31C is a part of the ball nut 31.

[0038] The return tube 33 is fixed to the ball nut 31 by the holder 35. The return tube 33 is pressed by the holder 35 via the intermediate portion 41 in the insertion direction relative to the mounting portion 34. Therefore, the tip of the first insertion portion 42 is maintained in a state pressed against the first abutment portion 31B. Also, the tip of the second insertion portion 43 is maintained in a state pressed against the second abutment portion 31C.

[0039] As shown in Fig. 4, when the return tube 33 is attached to the attachment portion 34, a first gap δ1 is formed between the intermediate portion opposing portion 51 and the intermediate portion 41 of the return tube 33. In addition, a second gap δ2 is formed between each of the first to sixth corners C1 to C6 and the return tube 33. The second gap δ2 is smaller than the first gap δ1. For ease of explanation, the fourth to sixth corners C4 to C6 are not shown in Fig. 4.

[0040] The second gap δ2 is provided between the first curved portion 44 and the first curved portion opposing portion 54, and corresponds to the connection point between the intermediate portion opposing portion 51 and the first plane 54A, the connection point between the first plane 54A and the second plane 54B, and the connection point between the second plane 54B and the third plane 54C.

[0041] In addition, the second gap δ2 is provided between the second curved portion 45 and the second curved portion opposing portion 55, corresponding to the connection point between the intermediate portion opposing portion 51 and the fourth plane 55A, the connection point between the fourth plane 55A and the fifth plane 55B, and the connection point between the fifth plane 55B and the sixth plane 55C.

[0042] By attaching the return tube 33 to the mounting portion 34, the first connection point and the second connection point on the rolling path R1 are connected to each other by the return tube 33. This forms an endless track for the balls 32. When the ball nut 31 rotates as the motor 21 is driven, the balls 32 roll and the steered shaft 12 moves in the axial direction. Depending on the rotation direction of the ball nut 31, the balls 32 are pushed into the return tube 33 from the first connection point or the second connection point on the rolling path R1. The balls 32 pass through the inside of the return tube 33 and are discharged to the second connection point or the first connection point.

[0043] <Operation of this embodiment> Next, the operation of this embodiment will be described. As shown in Fig. 5 , the balls 32 sequentially enter the interior of the return tube 33. The front balls 32 in the circulation direction DC are pushed by the rear balls 32 and move inside the return tube 33. At this time, there is a concern that the meandering of the balls 32 may cause a so-called staggered state inside the return tube 32. The staggered state is a state in which the balls 32 are alternately shifted relative to the axis O1 of the return tube 32. In particular, the trajectories of the balls 32 are curved inside the first curved portion 44 and the second curved portion 45, making the staggered state more likely to occur.

[0044] For example, when the front ball 32 is located radially inside the ball nut 31 and the rear ball 32 is located radially outside the ball nut 31 inside the first curved portion 44, the front ball 32 receives a force F0 from the rear ball 32. The direction of the force F0 is, for example, a direction inclined by a stagger angle θ with respect to the axis O1 of the return tube 32. The stagger angle θ is the angle between the axis O1 of the return tube 33 and a center line O2 connecting the centers of the front and rear balls 32.

[0045] The force F0 can be resolved into a first force F1 and a second force F2. The direction of the first force F1 is the same as the circulation direction DC. The direction of the second force F2 is a direction toward the radially inward side of the ball nut 31. In other words, the second force F2 is a force that acts in a direction that expands the inner diameter of the return tube 33.

[0046] The second force F2 hinders the smooth movement of the ball 32. Furthermore, when the inner diameter of the return tube 33 expands due to the second force F2, the zigzag angle θ becomes even larger. As the zigzag angle θ becomes larger, the second force F2 becomes even larger. However, when the inner diameter of the return tube 33 expands, the outer peripheral surface of the return tube 33 comes into contact with the first corner C1, the second corner C2, or the third corner C3. This prevents the inner diameter of the return tube 33 from expanding. Since the increase in the zigzag angle θ and the second force F2 is prevented, the smooth movement of the ball 32 is ensured.

[0047] Note that even when a staggered state occurs inside the second curved portion 45, the same effect as when a staggered state occurs inside the first curved portion 44 is achieved. That is, when the inner diameter of the return tube 33 expands, the outer peripheral surface of the return tube 33 comes into contact with the fourth corner C4, the fifth corner C5, or the sixth corner C6. This suppresses the expansion of the inner diameter of the return tube 33. Since the increase in the staggered angle θ and the second force F2 is suppressed, the smooth movement of the ball 32 is ensured.

[0048] <Advantages of this embodiment> This embodiment has the following advantages. (1) The intermediate portion opposing portion 51 is formed of a single plane parallel to the axis of the ball nut 31. The first curved portion opposing portion 54 is formed of a plurality of planes parallel to the axis of the ball nut 31. The planes include a first plane 54A, a second plane 54B, and a third plane 54C. The second curved portion opposing portion 55 is formed of a plurality of planes parallel to the axis of the ball nut 31. The planes include a fourth plane 55A, a fifth plane 55B, and a sixth plane 55C.

[0049] Compared to when the first curved portion opposing portion 54 and the second curved portion opposing portion 55 are configured with curved surfaces, the first curved portion opposing portion 54 and the second curved portion opposing portion 55 can be processed more easily. This improves the processing accuracy of the first curved portion opposing portion 54 and the second curved portion opposing portion 55. Furthermore, it is easy to ensure the accuracy of the gap between the first curved portion opposing portion 54 and the return tube 33, and the accuracy of the gap between the second curved portion opposing portion 55 and the return tube 33. This makes it easy to manage the gap. Furthermore, it becomes possible to set the gap narrower.

[0050] (2) The first curved portion opposing portion 54 and the second curved portion opposing portion 55, which are made up of multiple flat surfaces, are easy to process. This allows for simplification of the processing machine. Furthermore, the processing speed of the first curved portion opposing portion 54 and the second curved portion opposing portion 55 is improved, allowing for reduction in product costs.

[0051] (3) A second gap δ2 smaller than the first gap δ1 is provided between the first curved portion 44 and the first curved portion opposing portion 54, and between the second curved portion 45 and the second curved portion opposing portion 55. The second gap δ2 is provided corresponding to the following connection points P1 to P4.

[0052] P1. The connection point between the intermediate portion facing portion 51 and the first curved portion facing portion 54. The connection point is the connection point between the intermediate portion facing portion 51, which consists of a single plane, and the first plane 54A. P2. The connection point between adjacent planes among the multiple planes that make up the first curved portion facing portion 54. The multiple planes include the first plane 54A, the second plane 54B, and the third plane 54C. The connection point includes the connection point between the first plane 54A and the second plane 54B, and the connection point between the second plane 54B and the third plane 54C.

[0053] P3. The connection point between the intermediate portion facing portion 51 and the second curved portion facing portion 55. The connection point is the connection point between the intermediate portion facing portion 51, which is made up of a single plane, and the fourth plane 55A. P4. The connection point between adjacent planes among the multiple planes that make up the second curved portion facing portion 55. The multiple planes include the fourth plane 55A, the fifth plane 55B, and the sixth plane 55C. The connection point includes the connection point between the fourth plane 55A and the fifth plane 55B, and the connection point between the fifth plane 55B and the sixth plane 55C.

[0054] According to this configuration, each of the first curved portion opposing portion 54 and the second curved portion opposing portion 55 can be appropriately configured with a plurality of flat surfaces. (4) The connection points P1 to P4 between the above-mentioned flat surfaces have corners formed by two flat surfaces adjacent to each other in the circumferential direction of the ball nut 31. The corners include first to sixth corners C1 to C6. The first to sixth corners C1 to C6 have the following effects.

[0055] For example, if a staggered pattern occurs inside the first curved portion 44, a force acts on the first curved portion 44 in a direction that expands the inner diameter. When the inner diameter of the first curved portion 44 expands, the outer peripheral surface of the first curved portion 44 comes into contact with the first corner C1, the second corner C2, or the third corner C3. This suppresses the expansion of the inner diameter of the first curved portion 44. Furthermore, similar to the first curved portion 44, even if a staggered pattern occurs inside the second curved portion 45, the expansion of the inner diameter of the second curved portion 45 is also suppressed. Since the increase in the staggered angle θ and the second force F2 is suppressed, smooth movement of the ball 32 is ensured. Furthermore, since fluctuations in the assist force are suppressed, the driver of the vehicle can smoothly operate the steering wheel.

[0056] (5) The number of flat surfaces that make up the first curved portion facing portion 54 is three. The flat surfaces include a first flat surface 54A, a second flat surface 54B, and a third flat surface 54C. Therefore, the first curved portion facing portion 54 has three corners. The corners include first to third corners C1 to C3. The number of flat surfaces that make up the second curved portion facing portion 55 is three. The flat surfaces include a fourth flat surface 55A, a fifth flat surface 55B, and a sixth flat surface 55C. Therefore, the second curved portion facing portion 55 has three corners. The corners include fourth to sixth corners C4 to C6.

[0057] This configuration makes it easier to ensure the thickness of ball nut 31 compared to when the number of flat surfaces constituting first curved portion opposing portion 54 and the number of flat surfaces constituting second curved portion opposing portion 55 are each two. The thickness is the radial thickness of the peripheral wall of ball nut 31 at the portions corresponding to first curved portion opposing portion 54 and second curved portion opposing portion 55.

[0058] For example, it is possible to adopt a configuration in which the second corner C2 is omitted from the first curved portion opposing portion 54. In this case, a single flat surface is provided between the intermediate portion opposing portion 51 and the third flat surface 54C. However, the thickness of the portion of the ball nut 31 corresponding to the first curved portion opposing portion 54 is thinner than when the first curved portion opposing portion 54 has the first to third corners C3. However, as long as the thickness required by the product specifications can be ensured, a configuration in which the second corner C2 or the third corner C3 is omitted from the first curved portion opposing portion 54 may be adopted. Furthermore, a configuration in which the fifth corner C5 or the sixth corner C6 is omitted from the second curved portion opposing portion 55 may be adopted.

[0059] (6) When the return tube 33 is fixed to the mounting portion 34, a radially inward force of the ball nut 31 may be applied to the intermediate portion 41 of the return tube 33. In this case, the intermediate portion 41 bends radially inward of the ball nut 31. Furthermore, portions of the return tube 33 near both ends of the intermediate portion 41 contact the first corner C1 and the fourth corner C4. However, a first gap δ1 larger than the second gap δ2 is formed between the intermediate portion facing portion 51 and the intermediate portion 41. Therefore, even if portions of the return tube 33 near both ends of the intermediate portion 41 contact the first corner C1 and the fourth corner C4, a gap is formed between the intermediate portion facing portion 51 and the intermediate portion 41. Therefore, the intermediate portion 41 is prevented from being pressed against the intermediate portion facing portion 51 and being locally distorted. The intermediate portion facing portion 51 functions as a relief portion that relieves elastic deformation of the return tube 33.

[0060] (7) Even if a so-called staggered state occurs inside the return tube 32, smooth movement of the balls 32 and, therefore, smooth operation of the ball screw device 23 are ensured. For this reason, the ball screw device 23 of this embodiment is suitable for the electric power steering device 1.

[0061] Other Embodiments This embodiment may be modified as follows: Intermediate portion 41 does not have to be gently curved in the circumferential direction of ball nut 31 when viewed in the axial direction of ball nut 31. For example, intermediate portion 41 may extend linearly in parallel to intermediate portion opposing portion 51 of ball nut 31.

[0062] The number of flat surfaces constituting the first curved portion facing portion 54 and the second curved portion facing portion 55 is not limited to three. The first curved portion facing portion 54 and the second curved portion facing portion 55 may be composed of four or more flat surfaces. Furthermore, the first curved portion facing portion 54 and the second curved portion facing portion 55 may be composed of two flat surfaces.

[0063] The ball screw device 23 is not limited to the electric power steering device 1 and can be applied to various mechanical devices.

Claims

1. A ball screw shaft having a first spiral rolling groove on its outer circumferential surface; a ball nut having a second spiral rolling groove on its inner circumferential surface facing the first rolling groove; a plurality of balls rolling on a spiral rolling path formed by the first rolling groove and the second rolling groove; and a return tube configured to short-circuit two points on the rolling path and return the balls from the downstream side to the upstream side of the rolling path, wherein the return tube has: a middle section which is the axial center of the return tube; a first insertion end section which extends in a direction intersecting with the middle section; a second insertion end section which extends in the same direction as the first insertion end section; a first curved section which is provided between the middle section and the first insertion end section; and a second curved section which is provided between the middle section and the second insertion end section, wherein the ball nut has an attachment section which is configured so that the return tube can be attached from the radial outside of the ball nut, and the attachment section is a first insertion hole into which the first insertion end is inserted; a second insertion hole into which the second insertion end is inserted; a first curved portion opposing portion facing the first curved portion; and a second curved portion opposing portion facing the second curved portion, wherein the intermediate portion opposing portion is formed by a single plane parallel to the axis of the ball nut, and each of the first curved portion opposing portion and the second curved portion opposing portion is formed by a plurality of planes parallel to the axis of the ball nut.

2. A ball screw device according to claim 1, wherein a first gap is provided between the intermediate portion and the intermediate portion opposing portion, and a plurality of flat surfaces constituting each of the first curved portion opposing portion and the second curved portion opposing portion are adjacent to each other in the circumferential direction of the ball nut and are inclined at different angles relative to the plane constituting the intermediate portion opposing portion, and second gaps smaller than the first gap are provided between the first curved portion and the first curved portion opposing portion and between the second curved portion and the second curved portion opposing portion, and the second gaps are provided corresponding to: a connection point between the intermediate portion opposing portion and the first curved portion opposing portion; a connection point between adjacent flat surfaces among the plurality of flat surfaces constituting the first curved portion opposing portion; a connection point between the intermediate portion opposing portion and the second curved portion opposing portion; and a connection point between adjacent flat surfaces among the plurality of flat surfaces constituting the second curved portion opposing portion.

3. A ball screw device according to claim 2, wherein each of said connection points has a corner formed by two flat surfaces adjacent in the circumferential direction of said ball nut.

4. A ball screw device according to claim 1, wherein the number of flat surfaces constituting the first curved portion opposing portion and the number of flat surfaces constituting the second curved portion opposing portion are each three.

5. An electric power steering device having the ball screw device according to any one of claims 1 to 4.

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