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EP-4471289-B1 - BALL SCREW DEVICE AND STEERING DEVICE

EP4471289B1EP 4471289 B1EP4471289 B1EP 4471289B1EP-4471289-B1

Inventors

  • YAMAMOTO, RYUTA

Dates

Publication Date
20260506
Application Date
20220124

Claims (6)

  1. A ball screw device (21), comprising: a screw shaft (31) having a groove (43) on an outer periphery of the screw shaft (31); a ball screw nut (32) having a groove (45) on an inner periphery of the ball screw nut (32); a plurality of return tubes (34, 35); and a plurality of balls (33), wherein the groove (43) of the screw shaft (31) faces the groove (45) of the ball screw nut (32) to form a helical rolling path (Rc), each of the plurality of return tubes (34, 35) includes a return path (R1, R2) configured to create a connection between two points of the rolling path (Rc), the return path (R1, R2) and the rolling path (Rc) forming a circulation path in which the balls (33) are disposed, a first end portion (91, 93) that serves as an entrance for the balls (33) to enter the return path (R1, R2) when the screw shaft (31) or the ball screw nut (32) rotates clockwise, and that serves as an exit for the balls (33) to be discharged from the return path (R1, R2) when the screw shaft (31) or the ball screw nut (32) rotates counterclockwise, and a second end portion (92, 94) that serves as an exit for the balls (33) to be discharged from the return path (R1, R2) when the screw shaft (31) or the ball screw nut (32) rotates clockwise, and that serves as an entrance for the balls (33) to enter the return path (R1, R2) when the screw shaft (31) or the ball screw nut (32) rotates counterclockwise, the ball screw nut (32) has a plurality of attaching portions (47, 48) configured such that the plurality of return tubes (34, 35) is attached from a same attaching direction, the plurality of return tubes (34, 35) includes at least a first return tube (34) and a second return tube (35), a position of the first end portion (93) of the second return tube (35) relative to a position of the first end portion(91) of the first return tube (34) in a circumferential direction of the ball screw nut (32) is a first relative position, and a position of the second end portion (94) of the second return tube (35) relative to a position of the second end portion (92) of the first return tube (34) in the circumferential direction is a second relative position, and characterized in that at least one of the first relative position and the second relative position includes offset in the circumferential direction.
  2. The ball screw device (21) according to claim 1, wherein a sum of an offset amount in the circumferential direction of the first relative position and an offset amount in the circumferential direction of the second relative position is different from an integer multiple of a diameter of each of the balls (33).
  3. The ball screw device (21) according to claim 1 or 2, wherein each of the plurality of return tubes (34, 35) includes a first leg portion (51, 61) having a length along the attaching direction, and a second leg portion (52, 62) having a length along the attaching direction, the length of the first leg portion (51) of the first return tube (34) and the length of the second leg portion (52) of the first return tube (34) are equal to each other, and the length of the first leg portion (61) of the second return tube (35) and the length of the second leg portion (62) of the second return tube (35) are equal to each other.
  4. The ball screw device (21) according to any one of claims 1 to 3, wherein each of the plurality of return tubes (34, 35) includes a first leg portion (51, 61) having a length along the attaching direction, a second leg portion (52, 62) having a length along the attaching direction, and a linking portion (53, 63) that links the first leg portion (51, 61) and the second leg portion (52, 62) to each other, the linking portion (53, 63) having a length along a direction intersecting the attaching direction, and the length of the linking portion (53) of the first return tube (34) is different from the length of the linking portion (63) of the second return tube (35).
  5. The ball screw device (21) according to claim 1 or 2, wherein each of the plurality of return tubes (34, 35) includes a first leg portion (51, 61) having a length along the attaching direction, a second leg portion (52, 62) having a length along the attaching direction, and a linking portion (53, 63) that links the first leg portion (51, 61) and the second leg portion (52, 62) to each other, the linking portion (53, 63) having a length along a direction intersecting the attaching direction, the length of the first leg portion (51) of the first return tube (34) is different from the length of the second leg portion (52) of the first return tube (34), the length of the first leg portion (61) of the second return tube (35) is different from the length of the second leg portion (62) of the second return tube (35), the length of the first leg portion (51) of the first return tube (34) and the length of the second leg portion (62) of the second return tube (35) are equal to each other, the length of the second leg portion (52) of the first return tube (34) and the length of the first leg portion (61) of the second return tube (35) are equal to each other, and the length of the linking portion (53) of the first return tube (34) and the length of the linking portion (63) of the second return tube (35) are equal to each other.
  6. A steering system (1), comprising: the ball screw device (21) according to any one of claims 1 to 5; and a steering shaft (11) linked to the screw shaft (31).

Description

TECHNICAL FIELD The present invention relates to a ball screw device and a steering system comprising a ball screw device. BACKGROUND ART There is a steering system equipped with an RBS (recirculating ball screw) type gearbox that has a ball screw device, such as described in JP 2019-156082 A, for example. Such a ball screw device includes a screw shaft having a groove on an outer periphery thereof, a ball screw nut having a groove on an inner periphery thereof, a return tube, and a plurality of balls. In the ball screw device, the groove of the screw shaft faces the groove of the ball screw nut to form a rolling path that is helical. The return tube has a return path that creates a connection between two points on the rolling path. The rolling path and the return path form a circulation path in which the balls are disposed. The balls circulate through the circulation path in accordance with operation of the ball screw device. JP 2018 105 397 A discloses a ball screw device according to the preamble of claim 1. SUMMARY OF THE INVENTION Problem to be Solved by the Invention There is a demand for ensuring smooth operation of the ball screw device such as described above, while extending the life of the ball screw device. Means for Solving the Problem The objective is achieved by a ball screw device according to claim 1. The ball screw device includes a screw shaft having a groove on an outer periphery of the screw shaft, a ball screw nut having a groove on an inner periphery of the ball screw nut, a plurality of return tubes, and a plurality of balls. The groove of the screw shaft faces the groove of the ball screw nut to form a helical rolling path. Each of the plurality of return tubes includes a return path configured to create a connection between 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 that serves as an entrance for the balls to enter the return path when the screw shaft or the ball screw nut rotates clockwise, and that serves as 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 that serves as an exit for the balls to be discharged from the return path when the screw shaft or the ball screw nut rotates clockwise, and that serves as 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 attaching portions configured such that the plurality of return tubes is attached from a same attaching direction. The plurality of return tubes includes at least a first return tube and a second return tube. A position of the first end portion of the second return tube relative to a position of the first end portion 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 portion of the second return tube relative to a position of the second end portion 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 offset in the circumferential direction. According to a preferred embodiment of the invention, a steering system is provided. The steering system includes the above-described ball screw device, and a steering shaft linked to the screw shaft. BRIEF DESCRIPTION OF THE DRAWINGS [FIG. 1] FIG. 1 is a schematic diagram of a steering system according to a first embodiment.[FIG. 2] FIG. 2 is a cross-sectional view of a gearbox that the steering system in FIG. 1 is equipped with.[FIG. 3] FIG. 3 is a side view of a ball screw device that the gearbox in FIG. 2 is equipped with.[FIG. 4A] FIG. 4A is a cross-sectional view along a linking portion of a first return tube in the ball screw device in FIG. 3, taken along line IVA-IVA in FIG. 3.[FIG. 4B] FIG. 4B is a cross-sectional view along a linking portion of a second return tube in the ball screw device in FIG. 3, taken along line IVB-IVB in FIG. 3.[FIG. 5A] FIG. 5A is a front view of the first return tube provided in the ball screw device in FIG. 3.[FIG. 5B] FIG. 5B is a front view of the second return tube provided in the ball screw device in FIG. 3.[FIG. 6A] FIG. 6A is a graph showing a relation between a steering angle and steering torque in a steering system according to a comparative example.[FIG. 6B] FIG. 6B is a graph showing a relation between a steering angle and steering torque in the steering system in FIG. 1.[FIG. 7A] FIG. 7A is a cross-sectional view taken along a linking portion of a first return tube in a ball screw device according to a second embodiment.[FIG. 7B] FIG. 7B is a cross-sectional view taken along a linking portion of a second return tube in the ball screw device according to the second embodiment.[FIG. 8] FIG. 8 is a side view of a ball screw