Ball screw device

Abstract

To provide a ball screw device that can have an elongated life.SOLUTION: A ball screw device of the present disclosure includes: a screw shaft; a nut; a plurality of balls; and a circulation part provided in the nut. A rotation motion is transferred to the screw shaft to cause the nut to move in an axial direction parallel with the screw shaft. An inner peripheral raceway surface includes a plurality of non-supported regions which cannot receive a load from the screws shaft. An external load acts on an end part in a first direction of the screw shaft or an end part in the first direction of the nut, inclining the screw shaft relatively with respect to the center axis of the nut. When viewed from the axial direction, a direction in which the end part in the first direction of the screw shaft is defined as one-end displacement direction relatively with respect to the center axis of the nut. A first inner peripheral raceway surface provided closest to the first direction among the plurality of inner peripheral raceway surface includes: a one-end high load region provided in a one-end displacement direction from the center axis of the nut; and a one-end non-supported region which is a non-supported region provided in a direction opposite to the one-end displacement direction from the center axis of the nut.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present disclosure relates to a ball screw device.

Background Art

[0002] A ball screw device is a device that converts rotational motion into linear motion or linear motion into rotational motion. In the ball screw device of the following patent document, rotational motion is transmitted to the screw shaft, and the nut performs linear motion. The ball screw device of the following patent document is provided with a circulation part for circulating the balls in the nut. Further, examples of the type of the circulation part include an S-shaped groove surface, a bobbin, a deflector, and a tube.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, an external load in the radial direction may act on one end portion in the axial direction of the screw shaft or one end portion in the axial direction of the nut. As a result, the screw shaft tilts relative to the central axis of the nut. Then, the inner peripheral raceway surface of the nut receives a moment load from the screw shaft.

[0005] If, for example, an S-groove surface is arranged in the direction in which the screw shaft tilts as viewed from the central axis of the nut, the S-shaped groove surface cannot receive the load from the screw shaft, so the surface pressure in the vicinity of the connection portion connecting to the S-shaped groove surface of the inner peripheral raceway surface increases. As a result, any one or more of the vicinity of the connection portion of the inner peripheral raceway surface, the portion of the screw shaft facing the vicinity of the connection portion of the inner peripheral raceway surface, and the balls contacting the vicinity of the connection portion of the inner peripheral raceway surface among the plurality of balls may be damaged. Hereinafter, it may be collectively referred to as damage such as the vicinity of the connection portion of the inner peripheral raceway surface. Therefore, the long life of the ball screw device cannot be achieved.

[0006] The present disclosure has been made in view of the above, and an object thereof is to provide a ball screw device capable of achieving a longer service life.

Means for Solving the Problems

[0007] To achieve the above object, a ball screw device according to the present disclosure includes a screw shaft having one end pointing in a first direction and the other end pointing in a second direction, a nut having the screw shaft inserted therein, a plurality of balls disposed between the screw shaft and the nut, and one or more circulation portions provided in the nut. Rotational motion is transmitted to the screw shaft, and the nut moves in an axial direction parallel to the screw shaft. A plurality of unsupported regions are provided on the inner peripheral surface of the nut where the circulation portions are disposed and which cannot receive the load from the screw shaft. The plurality of unsupported regions include an end-side unsupported region disposed most in the first direction among the plurality of unsupported regions. An external load acts on an end portion of the screw shaft in the first direction or an end portion of the nut in the first direction, and the screw shaft is inclined relative to the central axis of the nut. When viewed from the axial direction, a direction in which an end portion of the screw shaft in the first direction is displaced relative to the central axis of the nut is defined as an end-side displacement direction. On an end portion in the first direction of the inner peripheral surface of the nut, an end-side high-load region disposed from the central axis of the nut in the end-side displacement direction and the end-side unsupported region disposed from the central axis of the nut in a direction opposite to the end-side displacement direction are provided.

[0008] The end-side unsupported region of the present disclosure is disposed on the opposite side of the end-side high-load region when viewed from the central axis of the nut. For this reason, the surface pressure in the vicinity of the connection portion connecting to the end-side unsupported region among the inner peripheral raceway surfaces is extremely small. Therefore, breakage in the vicinity of the connection portion of the inner peripheral raceway surface and the like is avoided, and the service life of the ball screw device is prolonged.

[0009] Further, in the ball screw device described above, when viewed from the axial direction, an angle formed by the end-side unsupported region and the end-side high-load region with the central axis of the nut as the apex is 180°.

[0010] According to the above configuration, the surface pressure in the vicinity of the connection portion connecting to the one-end-side unsupported region among the inner peripheral raceway surfaces becomes even smaller. Therefore, the vicinity of the connection portion of the inner peripheral raceway surface and the like are less likely to be damaged, and the life of the ball screw device becomes even longer.

[0011] Also, in the ball screw device described above, among the plurality of the unsupported regions, the other-end-side unsupported region arranged most in the second direction among the plurality of the unsupported regions is included. When viewed from the axial direction, the direction in which the end portion of the screw shaft in the second direction is displaced relative to the central axis of the nut is defined as the other-end-side displacement direction. At the end portion of the inner peripheral surface of the nut in the second direction, there are provided an other-end-side high load region arranged from the central axis of the nut in the other-end-side displacement direction, and the other-end-side unsupported region arranged circumferentially displaced from the other-end-side high load region when viewed from the axial direction.

[0012] According to the above configuration, the other-end-side unsupported region and the other-end-side high load region do not overlap. Therefore, the surface pressure in the vicinity of the connection portion connecting to the other-end-side unsupported region among the inner peripheral raceway surfaces is relatively small. For this reason, the vicinity of the connection portion of the inner peripheral raceway surface and the like are less likely to be damaged, and the life of the ball screw device becomes longer.

[0013] Also, in the ball screw device described above, the other-end-side high load region and the other-end-side unsupported region are arranged with a 180° shift when viewed from the axial direction.

[0014] According to the above configuration, the surface pressure in the vicinity of the connection portion connecting to the other-end-side unsupported region among the inner peripheral raceway surfaces is extremely small. Therefore, the vicinity of the connection portion of the inner peripheral raceway surface and the like are less likely to be damaged, and the life of the ball screw device becomes longer.

[0015] Also, in the ball screw device described above, the order in which the plurality of the unsupported regions are arranged in the second direction starting from the one-end-side unsupported region and the order in which the plurality of the unsupported regions are arranged in one direction in the circumferential direction starting from the one-end-side unsupported region may coincide.

[0016] Further, in the ball screw device described above, the plurality of non-support regions may be arranged at equal intervals in the circumferential direction.

[0017] Further, in the ball screw device described above, the order in which the plurality of non-support regions are arranged in the second direction starting from the one-end-side non-support region and the order in which they are arranged in one direction in the circumferential direction starting from the one-end-side non-support region may not match.

[0018] Further, in the ball screw device described above, one end portion of the nut may be a pressing portion that presses other components.

Advantages of the Invention

[0019] According to the ball screw device of the present disclosure, the service life can be extended.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

[0021] The embodiments for carrying out the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited by the content described in the following description. Also, the components described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the components described below can be combined as appropriate.

[0022] (Embodiment 1) FIG. 1 is a cross-sectional view of the ball screw device according to Embodiment 1 taken axially. As shown in FIG. 1, the ball screw device 100 according to Embodiment 1 includes a screw shaft 1, a nut 2, balls 3, and a circulation part 4. Hereinafter, the direction parallel to the central axis O1 of the screw shaft 1 is referred to as the axial direction. Also, when the ball screw device 100 of the present embodiment is equipped in another device such as an actuator, for example, a rotational motion is transmitted to the screw shaft 1 and the nut 2 moves in the axial direction.

[0023] The circulation part 4 of the present embodiment is an S-shaped groove surface 22 formed by forging. Note that although the circulation part 4 of the present embodiment is an S-shaped groove surface 22, the present disclosure may be a comma, a deflector (middle deflector and end deflector), a tube, or the like, and is not limited to the example shown in the embodiment.

[0024] The screw shaft 1 includes a screw shaft main body 10 and a power transmission portion 11. An outer peripheral raceway surface 12 extending in the spiral direction is formed on the outer peripheral surface of the screw shaft main body 10. The power transmission portion 11 is formed in a columnar shape and extends axially from the end surface of the screw shaft main body 10. The power transmission portion 11 is rotatably supported by other components (not shown). Also, a rotational motion is input to the power transmission portion 11. When a rotational motion is input to the power transmission portion 11, the screw shaft main body 10 rotates about the central axis O1. Hereinafter, the direction in which the screw shaft main body 10 is arranged as viewed from the power transmission portion 11 in the axial direction is referred to as the first direction X1, and the opposite direction is referred to as the second direction X2.

[0025] The nut 2 is a cylindrical component and is penetrated by the screw shaft 1. When the nut 2 is equipped on other devices, it is supported so as not to be rotatable about the central axis O1 and to be movable in the axial direction. Also, when no external load F1 (see FIG. 4) acts on the ball screw device 100, the central axis O2 of the nut 2 is arranged coaxially with the central axis O1 of the screw shaft 1. The nut 2 has a first end surface 2a facing the first direction X1 and a second end surface 2b facing the second direction X2.

[0026] FIG. 2 is a cross-sectional view of the nut of Embodiment 1 cut in the axial direction. As shown in FIG. 2, a plurality of inner peripheral raceway surfaces 21 and a plurality of S-shaped groove surfaces 22 are formed on the inner peripheral surface 20 of the nut 2. The inner peripheral raceway surfaces 21 and the S-shaped groove surfaces 22 are recessed radially outward from the inner peripheral surface 20 of the nut 2. The inner peripheral raceway surface 21 faces the outer peripheral raceway surface 12 (see FIG. 1) of the screw shaft 1 and extends in the spiral direction. The inner peripheral raceway surface 21 extends for about one turn (substantially one lead) in the spiral direction. A track is formed between each inner peripheral raceway surface 21 and the outer peripheral raceway surface 12. And a plurality of balls 3 (see FIG. 1) are arranged in each track.

[0027] The S-shaped groove surface 22 connects one end and the other end of the inner peripheral raceway surface 21 in the spiral direction. Thereby, the ball 3 that has moved from one end to the other end of the track circulates to one end of the track by the S-shaped groove surface 22. Also, in the present embodiment, four inner peripheral raceway surfaces 21 and four S-shaped groove surfaces 22 are provided.

[0028] Hereinafter, with respect to the four inner circumferential track surfaces 21, in order from the first direction X1 to the second direction X2, they are referred to as the first inner circumferential track surface 21a, the second inner circumferential track surface 21b, the third inner circumferential track surface 21c, and the fourth inner circumferential track surface 21d. Also, with respect to the four S-shaped groove surfaces 22, in order from the first direction X1 to the second direction X2, they are referred to as the first S-shaped groove surface 22a, the second S-shaped groove surface 22b, the third S-shaped groove surface 22c, and the fourth S-shaped groove surface 22d (see FIG. 3).

[0029] FIG. 3 is a view of the nut of Embodiment 1 as viewed from the second direction. Hereinafter, in the description of the positions of the respective parts, angles are used for the description, and this angle is the angle when the central axis O2 of the nut 2 is the vertex. Also, regarding the circumferential direction, the description is based on the case where the nut 2 is viewed from the second direction X2. As shown in FIG. 3, when viewed from the second direction X2, the first S-shaped groove surface 22a, the second S-shaped groove surface 22b, the third S-shaped groove surface 22c, and the fourth S-shaped groove surface 22d are arranged in order in one direction (the counterclockwise direction Y) in the circumferential direction starting from the first S-shaped groove surface 22a. Therefore, in Embodiment 1, the order in which the four S-shaped groove surfaces 22 are arranged in the second direction X2 starting from the first S-shaped groove surface 22a and the order in which they are arranged in one direction (the counterclockwise direction Y) in the circumferential direction starting from the first S-shaped groove surface 22a are the same.

[0030] Also, regarding the circumferential positions where the respective S-shaped groove surfaces 22 are arranged, the second S-shaped groove surface 22b is arranged with a 90° shift in the counterclockwise direction Y from the first S-shaped groove surface 22a. The third S-shaped groove surface 22c is arranged with a 90° shift in the counterclockwise direction Y from the second S-shaped groove surface 22b. The fourth S-shaped groove surface 22d is arranged with a 90° shift in the counterclockwise direction Y from the third S-shaped groove surface 22c. From the above, the four S-shaped groove surfaces 22 are arranged at equal intervals in the circumferential direction.

[0031] Note that the portion of the inner peripheral surface 20 of the nut 2 where the S-shaped groove surface 22 is disposed cannot receive the load from the screw shaft 1. Hereinafter, the region where the S-shaped groove surface 22 is disposed is referred to as an unsupported region 30. Also, the first S-shaped groove surface 22a, the second S-shaped groove surface 22b, the third S-shaped groove surface 22c, and the fourth S-shaped groove surface 22d are respectively referred to as a first unsupported region (one-end-side unsupported region) 31, a second unsupported region 32, a third unsupported region 33, and a fourth unsupported region (the other-end-side unsupported region) 34.

[0032] FIG. 4 is a cross-sectional view schematically showing the ball screw device of FIG. 1 and is a cross-sectional view in a state where an external load is acting. Next, a case where the ball screw device 100 is equipped in another device and an external load F1 is acting will be described. As shown in FIG. 4, an external load (radial load) F1 is acting on one end portion 2c of the nut 2 in the first direction X1. As a result, the nut 2 is tilted about the central O3 in the axial direction of the nut 2 (see arrow F2). Then, when the nut 2 tilts, the nut 2 receives a moment load from the screw shaft 1. In the following description, the portion of the screw shaft 1 that radially faces the first inner peripheral raceway surface 21a of the nut 2 is referred to as a one-end-side screw portion 1c. Also, the portion of the screw shaft 1 that radially faces the fourth inner peripheral raceway surface 21d of the nut 2 is referred to as an other-end-side screw portion 1d.

[0033] FIG. 5 is a cross-sectional view of the ball screw device of FIG. 4 cut along the first inner peripheral raceway surface. As shown in FIG. 5, when viewed from the axial direction, the direction of the external load F1 is the direction in which the first unsupported region 31 is disposed from the central axis O2 of the nut 2. Therefore, one end portion 2c of the nut 2 moves in the direction in which the first unsupported region 31 is disposed from the central axis O2. Here, relatively speaking, it can be said that the one-end-side screw portion 1c of the screw shaft 1 is displaced toward the third unsupported region 33 from the central axis O2 of the nut 2. Hereinafter, the direction in which the one-end-side screw portion 1c of the screw shaft 1 is relatively displaced from the central axis O2 of the nut 2 is referred to as a one-end-side displacement direction F3. For this reason, the region of the first inner peripheral raceway surface 21a that is disposed in the one-end-side displacement direction F3 from the central axis O2 (hereinafter, referred to as a first high-load region (one-end-side high-load region) 41) receives a moment load from the screw shaft 1 and has a relatively large surface pressure.

[0034] On the other hand, the first non-support area 31 is arranged in the direction opposite to the one-end displacement direction F3 when viewed from the central axis O2. That is, the first non-support area 31 is arranged on the opposite side of the first high-load area 41 with respect to the boundary line L passing through the central axis O2. Note that the boundary line L passes through the central axis O2 when viewed from the axial direction, and is orthogonal to the virtual line K1 passing through the central axis O2 and parallel to the one-end displacement direction F3. Therefore, the angle θ1 formed by the first non-support area 31 and the first high-load area 41 exceeds 90°, and the first non-support area 31 is separated from the first high-load area 41. More specifically, the angle θ1 formed by the first non-support area 31 and the first high-load area 41 is 180°. Therefore, the first non-support area 31 is greatly separated from the first high-load area 41.

[0035] From the above, the surface pressure in the vicinity of the connection part connecting to the first non-support area 31 in the first inner peripheral raceway surface 21a is extremely small, and damage such as in the vicinity of the connection part of the first inner peripheral raceway surface 21a is avoided. Thereby, the life of the ball screw device 100 is prolonged.

[0036] FIG. 6 is a cross-sectional view of the ball screw device of FIG. 4 cut along the fourth inner peripheral raceway surface. Further, due to the inclination of the nut 2, as shown in FIG. 6, when viewed from the axial direction, the other end 2d in the second direction X2 of the nut 2 moves in the direction in which the third non-support area 33 is arranged from the central axis O2 (see the arrow F4 in FIG. 6). Here, relatively speaking, it can be said that the other-end screw part 1d of the screw shaft 1 is displaced toward the first non-support area 31 from the central axis O2 of the nut 2. Hereinafter, the direction in which the other-end screw part 1d of the screw shaft 1 is relatively displaced from the central axis O2 of the nut 2 is referred to as the other-end displacement direction F5. For this reason, the area of the fourth inner peripheral raceway surface 21d arranged in the other-end displacement direction F5 from the central axis O2 (hereinafter referred to as the other-end high-load area 42) receives a moment load from the screw shaft 1, and the surface pressure is relatively large.

[0037] The fourth unsupported region 34 of the present embodiment is arranged to be displaced circumferentially from the other-end high-load region 42. Specifically, the angle θ2 formed by the fourth unsupported region 34 and the other-end high-load region 42 is 90°. Therefore, the surface pressure in the vicinity of the connection portion connecting to the fourth unsupported region 34 on the fourth inner circumferential raceway surface 21d is relatively small. Thus, breakage in the vicinity of the connection portion of the fourth inner circumferential raceway surface 21d and the like is avoided, and the life of the ball screw device 100 is further extended.

[0038] As described above, the ball screw device 100 of Embodiment 1 includes a screw shaft 1 having one end pointing in the first direction and the other end pointing in the second direction, a nut 2 into which the screw shaft 1 is inserted, a plurality of balls 3 arranged between the screw shaft 1 and the nut 2, and one or more circulation portions 4 provided in the nut 2. Rotational motion is transmitted to the screw shaft 1, and the nut 2 moves in the axial direction parallel to the screw shaft 1. On the inner circumferential surface 20 of the nut 2, a plurality of unsupported regions 30 are provided where the circulation portions 4 are arranged and cannot receive the load from the screw shaft 1. An external load F1 acts on an end portion of the screw shaft 1 in the first direction X1 or one end portion 2c of the nut 2, and the screw shaft 1 relatively tilts with respect to the central axis O2 of the nut 2. When viewed from the axial direction, the direction in which the end portion of the screw shaft 1 in the first direction X1 is displaced relative to the central axis O2 of the nut 2 is defined as the one-end displacement direction F3. At one end portion 2c of the inner circumferential surface 20 of the nut 2, a first high-load region 41 arranged in the one-end displacement direction F3 from the central axis O2 of the nut 2 and a first unsupported region 31 which is an unsupported region 30 arranged in the direction opposite to the one-end displacement direction F3 from the central axis O2 of the nut 2 are provided. According to this Embodiment 1, breakage in the vicinity of the connection portion of the first inner circumferential raceway surface 21a and the like is avoided, and the life of the ball screw device 100 is extended.

[0039] Next, other embodiments will be described. In the following description, the description will be focused on the differences from Embodiment 1.

[0040] (Embodiment 2) FIG. 7 is a view of the nut of Embodiment 2 as viewed from the second direction. FIG. 8 is a cross-sectional view taken along the fourth inner circumferential track surface of the nut of Embodiment 2. As shown in FIG. 7, the ball screw device 100A of Embodiment 2 is different from Embodiment 1 in that the third S-shaped groove surface 22c (third non-support region 33) is displaced 180° in the counterclockwise direction Y from the second S-shaped groove surface 22b. Further, the ball screw device 100A of Embodiment 2 is different from Embodiment 1 in that the fourth S-shaped groove surface 22d (fourth non-support region 34) is displaced 90° in the clockwise direction Z from the third S-shaped groove surface 22c.

[0041] That is, regarding the plurality of non-support regions 30 of the nut 2A of Embodiment 2, the order of arrangement in the second direction X2 starting from the first non-support region 31 (the order of the second non-support region 32, the third non-support region 33, and the fourth non-support region 34) and the order of arrangement in one direction (counterclockwise direction Y) in the circumferential direction starting from the first non-support region 31 as viewed from the axial direction (the order of the second non-support region 32, the fourth non-support region 34, and the third non-support region 33) do not match.

[0042] According to Embodiment 2 described above, as shown in FIG. 8, the fourth non-support region 34 is arranged in a direction opposite to the displacement direction F5 toward the other end side from the central axis O2. Further, the angle θ3 formed by the fourth non-support region 34 and the high load region 42 on the other end side is 180°. Therefore, the fourth non-support region 34 is farther away from the high load region 42 on the other end side than in Embodiment 1. That is, the surface pressure in the vicinity of the connection portion connecting to the fourth non-support region 34 in the fourth inner circumferential track surface 21d is extremely small, and damage such as in the vicinity of the connection portion of the fourth inner circumferential track surface 21d is avoided. Therefore, according to Embodiment 2, the life of the ball screw device 100A can be made longer than in Embodiment 1.

[0043] (Embodiment 3) FIG. 9 is a view of the nut according to Embodiment 3 as viewed from the second direction. FIG. 10 is a cross-sectional view taken along the fourth inner circumferential track surface of the nut according to Embodiment 3. As shown in FIG. 9, the ball screw device 100B according to Embodiment 3 is different from Embodiment 1 in that it does not have the fourth inner circumferential track surface 21d (see FIG. 2) and the fourth S-shaped groove surface 22d (the fourth non-support region 34). According to this Embodiment 3, the angle formed by the third S-shaped groove surface 22c and the first S-shaped groove surface 22a is 180°, and the three S-shaped groove surfaces 22 are not arranged at equal intervals. Further, among the S-shaped groove surfaces 22, the third S-shaped groove surface 22c is arranged most in the second direction X2. Therefore, the other-end high-load region 42 is generated in the third S-shaped groove surface 22c.

[0044] As shown in FIG. 10, the other-end high-load region 42 is arranged in the direction in which the other-end displacement direction F5 points from the central axis O2 of the nut 2B when viewed from the axial direction. That is, the other-end high-load region 42 is generated in the direction in which the first non-support region 31 is arranged from the central axis O2 of the nut 2. On the other hand, the third non-support region 33 is arranged in the direction opposite to the other-end displacement direction F5 from the central axis O2. Further, the angle θ4 formed by the third non-support region 33 and the other-end high-load region 42 is 180°. That is, the third non-support region 33 is far away from the other-end high-load region 42.

[0045] Therefore, according to Embodiment 3 described above, the surface pressure near the connection portion connecting to the third non-support region 33 in the third S-shaped groove surface 22c is smaller than that in Embodiment 1. Therefore, according to Embodiment 3, the life of the ball screw device 100 can be made longer than that in Embodiment 1.

[0046] Next, Embodiment 4 in which the ball screw device 100 of Embodiment 1 is mounted on an electric brake of a vehicle will be described.

[0047] (Embodiment 4) FIG. 11 is a cross-sectional view of the electric brake shown in Embodiment 4 taken axially. FIG. 12 is a schematic view of the electric brake as seen from the direction of arrow XI in FIG. 11. As shown in FIG. 11, the electric brake 500 includes an actuator 501, a pair of brake pads 502 and 503, and a brake disk 504 that rotates with a wheel (not shown). The actuator 501 includes a motor (not shown), a speed reduction device (not shown), a ball screw device 100, and a piston 510 that fits into one end portion 2c of the nut 2. Although details will be described below, as shown in FIG. 12, the brake pad 502 receives a load in the direction indicated by arrow E2 from the brake disk 504. In the ball screw device 100 of the present embodiment, a first non-support region 31 is arranged in the direction indicated by arrow E2 from the central axis O2 of the nut 2. Hereinafter, details of the load indicated by arrow E2 will be described.

[0048] As shown in FIG. 12, when the vehicle is moving forward, the brake disk 504 rotates in the direction indicated by arrow E1 together with the wheel (not shown). Here, as shown in FIG. 11, when the electric brake 500 is actuated, the nut 2 moves in the first direction X1, and one end portion 2c of the nut 2 presses the piston 510 in the first direction X1. That is, in the nut 2 of the present embodiment, one end portion 2c is a pressing portion that presses other components, and one end surface 2a serves as a pressing surface. Subsequently, the piston 510 presses the brake pad 502. Then, the brake disk 504 is sandwiched between the pair of brake pads 502 and 503, and a braking force is applied to the brake disk 504.

[0049] As shown in FIG. 12, when the brake pad 502 is pressed against the brake disk 504, the brake pad 502 receives a load in the direction indicated by arrow E2 from the brake disk 504. As a result, the brake pad 502 moves in the direction of arrow E2. Further, due to the frictional force between the brake pad 502 and the piston 510, the piston 510 also moves in the direction indicated by arrow E2. For this reason, as shown in FIG. 11, a radial load (external load E3) in the same direction as arrow E2 is input to one end portion 2c of the nut 2 into which the piston 510 fits. As a result, the nut 2 tilts about the central O3 in the axial direction of the nut 2 (see arrow E4). When the electric brake 500 is driven in this way, a radial load (external load E3) is input to one end portion 2c of the nut 2. And the direction of this radial load (external load E3) is always the same direction. That is, the nut 2 always tilts in the same direction.

[0050] As shown in FIG. 12, when the nut 2 tilts, one end portion 2c of the nut 2 moves in the direction in which the first non-support region 31 is arranged from the central axis O2. Relatively, one end side screw portion 1c of the screw shaft 1 is displaced toward the third non-support region 33 from the central axis O2 of the nut 2. Therefore, the one end side displacement direction E5 of the screw shaft 1 in the fourth embodiment is the direction in which the third non-support region 33 is arranged when viewed from the central axis O2. That is, the region of the first inner peripheral raceway surface 21a arranged in the one end side displacement direction E5 when viewed from the central axis O2 becomes the first high load region 41.

[0051] Further, the angle θ1 formed by the first non-support region 31 and the first high load region 41 is 180°. That is, the first non-support region 31 is far away from the first high load region 41. Therefore, also in the fourth embodiment, as in the other embodiments, the surface pressure in the vicinity of the connection portion of the first inner peripheral raceway surface 21a is extremely small, breakage is avoided, and the life of the ball screw device 100 can be extended.

[0052] Also, in the above description, the load acting on the brake pad 502 and the piston 510 has been explained by focusing on the radial load (external load E3). However, when the piston 510 presses the brake pad 502, an axial load (reaction force) in the second direction X2 acts on the nut 2. On the other hand, no axial load in the first direction X1 acts on the nut 2. Thus, only the load in one axial direction (the second direction X2) acts on the nut 2 of the present embodiment.

[0053] FIG. 13 is a diagram showing the axial load distribution of balls when an axial load acts on a conventional ball screw device (tube type). In FIG. 13, since the ball screw device uses a tube for the circulation part, the curve shown in FIG. 13 is continuous. However, when an S-shaped groove or a bobbin is used as the circulation part, the plurality of balls 4 are divided for each circuit. Therefore, when an S-shaped groove or a bobbin is used as the circulation part, the curve shown in FIG. 13 becomes intermittent. As shown in FIG. 13, in the ball screw device 100, the axial load distribution of the balls 3 is such that the load (ball load) acting on the balls 3 arranged closer to one end 2c (closer to the pressing part) is larger. That is, also in the present embodiment, the load on the ball 3 that rolls on the first inner peripheral raceway surface 21a arranged closest to one end 2c among the plurality of inner peripheral raceway surfaces 21 is the largest. For this reason, a larger load is applied to the first inner peripheral raceway surface 21a than to the other inner peripheral raceway surfaces 21.

[0054] Thus, a large load acts on the first inner peripheral raceway surface 21a due to the axial load. Also, a larger load acts on the first inner peripheral raceway surface 21a due to the above-described radial load (external load E3; see FIG. 11). Therefore, a larger load acts on the first inner peripheral raceway surface 21a than on the other inner peripheral raceway surfaces 21. In contrast, in the present embodiment, considering both the axial load and the radial load, the circumferential position of the first non-support region 31 is appropriately managed. That is, even when both the axial load and the radial load (external load E3; see FIG. 11) act on the nut 2, breakage near the connection part of the first inner peripheral raceway surface 21a is avoided.

[0055] The above has described Embodiment 4. In the electric brake 500 of Embodiment 4, an example is given where when the brake pad 502 moves in the direction indicated by arrow E2, a radial load (external load E3) in the same direction as arrow E2 acts on one end 2c of the nut 2. However, depending on the support structures of the piston 510 and the nut 2, there are also electric brakes in which when the brake pad 502 moves in the direction indicated by arrow E2, a radial load in the direction opposite to arrow E2 acts on one end 2c of the nut 2. Therefore, the present disclosure is not limited to the electric brake shown in Embodiment 4, and may also be applied to an electric brake in which the moving direction of the brake pad 502 and the load direction acting on one end 2c of the nut 2 are opposite to each other.

[0056] The above has described each embodiment, but the present disclosure is not limited thereto. For example, in the embodiment, an example where an external load acts on the nut has been described, but an external load may act on the screw shaft and the screw shaft may be inclined with respect to the central axis of the nut. Further, in Embodiment 4, an external load E3 is input from the piston 510 to the nut 2, but the present disclosure may also be applied to an example where an external load acts on the screw shaft from a pulley or a gear fitted to the screw shaft. Further, although three or four examples of the non-support region 30 have been described, the present disclosure may be five or more and is not limited thereto. Alternatively, for example, when the circulation part 4 is of a tube type, each of the inlets and outlets of the tube type becomes a non-support region, and thus the number of non-support regions may be two. Further, in each embodiment, the external load acting on the ball screw device has been described by limiting it to a radial load. For example, in Embodiment 4, when the piston 510 presses the brake pad 502, a reaction force (axial load) in the direction opposite to the pressing direction acts on the nut 2. Thus, in addition to the radial load, an axial load also acts on the ball screw device. In addition, in Embodiment 4, the piston 510 of the actuator 501 directly presses the brake pad 502, but the present disclosure may also be applied to an electro-hydraulic type electric brake in which the brake pad 502 is driven by the hydraulic pressure generated by the pressing of the piston 510 of the actuator 501.

[0057] In addition, in the nut 2 of Embodiment 1 (see FIG. 4) and the nut 2 of Embodiment 4 (see FIG. 11), the starting point of inclination is the center O3 in the axial direction of the nut 2, but the present disclosure is not limited to this. Hereinafter, a modification in which the starting point of inclination is different from that of Embodiment 1 will be described.

[0058] FIG. 14 is a cross-sectional view of a state in which an external load is acting on the ball screw device of the modification. As shown in FIG. 14, the ball screw device 100C of the modification is different from Embodiment 1 in that the starting point Q of the inclination of the nut 2 is closer to the other end portion 2d of the nut 2. According to this modification, a portion of the first inner peripheral raceway surface 21a (not shown in FIG. 14; see FIG. 5) that is displaced from the central axis O2 in the one-end side displacement direction F3 becomes the first high load region 41. Further, similar to Embodiment 1, in the ball screw device 100C of the modification, the first non-support region 31 (not shown in FIG. 14; see FIG. 5) is arranged in the direction opposite to the one-end side displacement direction F3 when viewed from the central axis O2. Therefore, also in the modification, similar to Embodiment 1, breakage in the vicinity of the connection portion of the first inner peripheral raceway surface 21a is avoided. In the modification, an example in which the starting point Q of the inclination of the nut 2 is closer to the other end portion 2d of the nut 2 is given, but in the present disclosure, the starting point Q may be in the second direction X2 from the second end surface 2b of the nut 2.

[0059] Note that the present disclosure may also be a combination of the following configurations. (1) A screw shaft having one end pointing in the first direction and the other end pointing in the second direction, A nut having the screw shaft inserted therein, A plurality of balls arranged between the screw shaft and the nut, One or more circulation portions provided in the nut, having, Rotational motion is transmitted to the screw shaft, and the nut moves in the axial direction parallel to the screw shaft, A plurality of non-support regions that cannot receive the load from the screw shaft are provided on the inner peripheral surface of the nut, and the circulation portions are arranged in the non-support regions, Among the plurality of non-support regions, the one-end side non-support region arranged most in the first direction is included, An external load acts on the end of the screw shaft in the first direction or the end of the nut in the first direction, and the screw shaft is inclined relative to the central axis of the nut. When viewed from the axial direction, the direction in which the end of the screw shaft in the first direction is displaced relative to the central axis of the nut is defined as the one-end-side displacement direction. At the end of the inner peripheral surface of the nut in the first direction, a one-end-side high-load region arranged in the one-end-side displacement direction from the central axis of the nut, and a one-end-side non-support region arranged in the direction opposite to the one-end-side displacement direction from the central axis of the nut are provided. Ball screw device. (2) When viewed from the axial direction, the angle formed by the one-end-side non-support region and the one-end-side high-load region with the central axis of the nut as the vertex is 180°. The ball screw device according to (1). (3) Among the plurality of non-support regions, the other-end-side non-support region arranged most in the second direction among the plurality of non-support regions is included. When viewed from the axial direction, the direction in which the end of the screw shaft in the second direction is displaced relative to the central axis of the nut is defined as the other-end-side displacement direction. At the end of the inner peripheral surface of the nut in the second direction, an other-end-side high-load region arranged in the other-end-side displacement direction from the central axis of the nut, and an other-end-side non-support region arranged circumferentially displaced from the other-end-side high-load region when viewed from the axial direction are provided. The ball screw device according to (1) or (2). (4) When viewed from the axial direction, the angle formed by the other-end-side high-load region and the other-end-side non-support region with the central axis of the nut as the vertex is 180°. The ball screw device according to (3). (5) The plurality of said unsupported regions are arranged in the same order in the second direction starting from the one-end-side unsupported region and in the same order in one direction of the circumferential direction starting from the one-end-side unsupported region. The ball screw device according to any one of (1) to (4). (6) The plurality of said unsupported regions are arranged at equal intervals in the circumferential direction. The ball screw device according to (5). (7) The order in which the plurality of said unsupported regions are arranged in the second direction starting from the one-end-side unsupported region and the order in which they are arranged in one direction of the circumferential direction starting from the one-end-side unsupported region do not match. The ball screw device according to any one of (1) to (4). (8) One end portion of the nut serves as a pressing portion for pressing other components. The ball screw device according to any one of (1) to (7).

Explanation of Signs

[0060] 1 Screw shaft 2, 2A, 2B Nuts 3 Balls 4 Circulation portion 12 Outer circumferential raceway surface 20 Inner circumferential surface 21 Inner circumferential raceway surface 22 S-shaped groove surface 21a First inner circumferential raceway surface 21b Second inner circumferential raceway surface 21c Third inner circumferential raceway surface 21d Fourth inner circumferential raceway surface 22a First S-shaped groove surface 22b Second S-shaped groove surface 22c Third S-shaped groove surface 22d Fourth S-shaped groove surface 30 Unsupported region 31 First unsupported region 32 Second unsupported region 33 Third unsupported region 34 Fourth unsupported region 100, 100A, 100B, 100C Ball Screw Device 500 Electric Brake 501 Actuator 502, 503 Brake Pads 504 Brake Disc

Claims

1. A screw shaft having one end pointing in a first direction and the other end pointing in a second direction, a nut having the screw shaft inserted therein, a plurality of balls disposed between the screw shaft and the nut, one or more circulation parts provided on the nut, and having rotational motion is transmitted to the screw shaft, the nut moves in an axial direction parallel to the screw shaft, on the inner peripheral surface of the nut, a plurality of non-support regions are provided where the circulation parts are disposed and which cannot receive the load from the screw shaft, among the plurality of non-support regions, an end-side non-support region disposed most in the first direction among the plurality of non-support regions is included, an external load acts on an end portion of the screw shaft in the first direction or an end portion of the nut in the first direction, and the screw shaft is inclined relative to the central axis of the nut, when viewed from the axial direction, a direction in which an end portion of the screw shaft in the first direction is displaced relative to the central axis of the nut is defined as an end-side displacement direction, at an end portion of the inner peripheral surface of the nut in the first direction, an end-side high-load region disposed from the central axis of the nut in the end-side displacement direction, and the end-side non-support region disposed from the central axis of the nut in a direction opposite to the end-side displacement direction, are provided. A ball screw device.

2. When viewed from the axial direction, an angle formed by the end-side non-support region and the end-side high-load region with the central axis of the nut as the vertex is 180°. The ball screw device according to claim 1.

3. Among the plurality of non-support regions, an other-end-side non-support region disposed most in the second direction among the plurality of non-support regions is included. When viewed from the axial direction, the direction in which the end portion of the screw shaft in the second direction is displaced relative to the central axis of the nut is defined as the other-end-side displacement direction. At the end portion in the second direction of the inner peripheral surface of the nut, there is provided an other-end-side high-load region arranged in the other-end-side displacement direction from the central axis of the nut, and an other-end-side unsupported region arranged circumferentially offset from the other-end-side high-load region when viewed from the axial direction. are provided. The ball screw device according to claim 1.

4. When viewed from the axial direction, the angle formed by the other-end-side high-load region and the other-end-side unsupported region with the central axis of the nut as the vertex is 180°. The ball screw device according to claim 3.

5. The plurality of the unsupported regions are arranged in the same order in the second direction starting from the one-end-side unsupported region and in the same order in one direction of the circumferential direction starting from the one-end-side unsupported region. The ball screw device according to any one of claims 1 to 4.

6. The plurality of the unsupported regions are arranged at equal intervals in the circumferential direction. The ball screw device according to claim 5.

7. The plurality of the unsupported regions are not arranged in the same order in the second direction starting from the one-end-side unsupported region and in the same order in one direction of the circumferential direction starting from the one-end-side unsupported region. The ball screw device according to any one of claims 1 to 4.

8. One end portion of the nut serves as a pressing portion for pressing other components. The ball screw device according to any one of claims 1 to 4.

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