Ball screw device

Abstract

The ball screw device of the present disclosure, which aims to reduce the amount of grease leaking, includes a screw shaft, a nut having a through hole penetrating in an axial direction parallel to the screw shaft and into which the screw shaft is inserted, and a number of balls arranged between the nut and the screw shaft. The inner circumferential surface of the nut includes a main body inner circumferential surface provided with an inner circumferential raceway surface on which the balls roll, and a reduced diameter inner circumferential surface arranged closer to the axial end of the nut than the main body inner circumferential surface. The diameter of the reduced diameter inner circumferential surface is smaller than the diameter of the main body inner circumferential surface.

Description

[Technical field]

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

[0002] The ball screw device includes a nut, a screw shaft that passes through the nut, and a plurality of balls arranged between the nut and the screw shaft. To explain how to use the ball screw device, in the following Patent Document 1, the nut is supported by a bearing. Also, a gear is fitted to the nut. Thus, a rotational motion is transmitted to the nut, and the screw shaft performs linear motion. On the other hand, in the following Patent Document 2, a gear is fitted to the screw shaft. Thus, a rotational motion is transmitted to the screw shaft, and the nut performs linear motion. Also in the following Patent Document 2, the nut presses against a brake pad. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2021-131124 A [Patent Document 2] JP 2016-035322 A Summary of the Invention [Problem to be solved by the invention]

[0004] Grease is applied to the inner peripheral surface of the nut to facilitate the rolling of the balls. When the screw shaft moves axially relative to the nut, the grease on the inner peripheral side of the nut follows the movement of the screw shaft and leaks out to the outside of the nut. Therefore, it is desirable to reduce the amount of grease that leaks.

[0005] The present disclosure has been made in consideration of the above, and has an object to provide a ball screw device that can reduce the amount of grease leaking. [Means for solving the problem]

[0006] In order to achieve the above object, a ball screw device according to one aspect of the present disclosure includes a screw shaft, a nut having a through hole penetrating in an axial direction parallel to the screw shaft and into which the screw shaft is inserted, and a plurality of balls arranged between the nut and the screw shaft. The inner circumferential surface of the nut includes a main body inner circumferential surface provided with an inner circumferential raceway surface on which the balls roll, and a reduced diameter inner circumferential surface arranged closer to an end of the nut in the axial direction than the main body inner circumferential surface. The diameter of the reduced diameter inner circumferential surface is smaller than the diameter of the main body inner circumferential surface.

[0007] According to the present disclosure, the gap between the reduced diameter inner peripheral surface and the screw shaft is smaller than the gap between the main body inner peripheral surface and the screw shaft. Therefore, when the screw shaft moves axially relative to the nut, the grease adhering to the outer peripheral surface of the screw shaft is scraped off by the reduced diameter inner peripheral surface. This reduces the amount of grease leaking out of the nut.

[0008] In a preferred embodiment of the ball screw device, the axial portion of the nut on the inner side of which the reduced diameter inner circumferential surface is disposed is a thick-walled portion having a greater radial thickness than the inner circumferential portion of the nut on the inner side of which the main body inner circumferential surface is disposed. At least a portion of the outer circumferential surface of the thick-walled portion serves as a fitting surface with which another component is fitted.

[0009] According to the above-mentioned configuration, the thick-walled portion has a large radial thickness and is less likely to deform due to temperature changes, which prevents a gap from occurring between the fitting surface and other components, or prevents the tightening margin with respect to other components from becoming larger than a predetermined amount.

[0010] In a preferred embodiment of the ball screw device, the axial length of the fitting surface is smaller than the axial length of the reduced diameter inner circumferential surface. The outer circumferential surface of the thick-walled portion has an exposed surface that is not fitted with the other component and is exposed, and the fitting surface has a smaller diameter than the exposed surface.

[0011] According to the above configuration, when a small-diameter fitting surface is formed, the strength of the thick-walled portion is reduced. On the other hand, the fitting surface has a smaller axial length than the reduced-diameter inner circumferential surface. Therefore, even if the fitting surface has a small diameter, the reduction in strength of the thick-walled portion is kept small.

[0012] In a preferred embodiment of the ball screw device, the portion of the nut in the axial direction where the reduced diameter inner peripheral surface is located on the inner peripheral side is a thick-walled portion having a larger radial thickness than the portion of the nut in the axial direction where the main body inner peripheral surface is located on the inner peripheral side. The thick-walled portion is located at an end of the nut. The end face of the thick-walled portion serves as a pressing surface.

[0013] The thick-walled portion has a large radial thickness, and therefore has improved strength, so that even if the end face of the thick-walled portion serves as a pressing surface, the thick-walled portion is unlikely to deform.

[0014] In a preferred embodiment of the ball screw device, a corner where the reduced diameter inner circumferential surface and the pressing surface intersect is chamfered.

[0015] According to the above configuration, stress is unlikely to be concentrated on the corners, and the corners are unlikely to be damaged.

[0016] In addition, as a preferred embodiment of the ball screw device described above, the inner circumferential surface of the nut is disposed between the inner circumferential surface of the main body and the reduced diameter inner circumferential surface, and has a clearance groove surface having a larger diameter than the diameter of the inner circumferential surface of the main body.

[0017] According to the above-mentioned configuration, the grindstone can be released into the clearance groove surface after grinding the end of the inner circumferential surface of the main body, so that the end of the inner circumferential surface of the main body can be ground reliably.

[0018] In a preferred embodiment of the ball screw device, the reduced diameter inner circumferential surface and the relief groove surface are provided on both axial sides of the inner circumferential surface of the main body.

[0019] According to the above-mentioned configuration, it is difficult for grease to leak from both ends of the nut, not just one end of the nut. Also, since the relief groove surfaces are provided on both sides of the inner circumferential surface of the main body, both ends of the inner circumferential surface of the main body can be reliably polished.

[0020] In addition, as a preferred embodiment of the ball screw device described above, the inner surface of the nut has a first tapered surface that is arranged between the main body inner surface and the escape groove surface and gradually increases in diameter toward the axial end of the nut, and a second tapered surface that is arranged between the escape groove surface and the reduced diameter inner surface and gradually decreases in diameter toward the axial end of the nut.

[0021] According to this configuration, stress is unlikely to concentrate on the first tapered surface and the second tapered surface, and the nut is unlikely to be damaged. Effect of the Invention

[0022] According to the ball screw device of the present disclosure, the amount of grease leaking to the outside of the nut is reduced. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a cross-sectional view of a ball screw device according to a first embodiment, taken along the center line of a screw shaft. [Diagram 2] FIG. 2 is a cross-sectional view of the nut of the first embodiment taken along the center line. [Diagram 3] FIG. 3 is an enlarged view of the vicinity of an end portion of a nut in a first direction in the ball screw device of FIG. [Figure 4] FIG. 4 is an enlarged view of a part of the ball screw device of the second embodiment. [Diagram 5] FIG. 5 is a cross-sectional view of the ball screw device of the third embodiment taken along the center line of the screw shaft. [Figure 6] FIG. 6 is an enlarged view of a part of the ball screw device of the fourth embodiment. [Figure 7] FIG. 7 is an enlarged cross-sectional view of a nut of a ball screw device according to a fifth embodiment. [Figure 8]FIG. 8 is an exploded view of the nut of the first modified example cut in the axial direction. [Figure 9] FIG. 9 is an exploded view of the nut of the second modification cut in the axial direction. [Figure 10] FIG. 10 is an exploded view of the nut of the third modified example cut in the axial direction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the following modes for carrying out the invention (hereinafter, referred to as embodiments). Furthermore, the components in the following embodiments include those that a person skilled in the art can easily imagine, those that are substantially the same, and those that are within the so-called equivalent range. Furthermore, the components disclosed in the following embodiments can be appropriately combined.

[0025] (Embodiment 1) Fig. 1 is a cross-sectional view of a ball screw device of embodiment 1 taken along the center line of a screw shaft. As shown in Fig. 1, a ball screw device 100 includes a screw shaft 1, a nut 2 inserted into the screw shaft 1, and a plurality of balls (not shown) disposed between the screw shaft 1 and the nut 2. Hereinafter, a direction parallel to the center line O1 of the screw shaft 1 will be referred to as the axial direction.

[0026] An outer circumferential raceway surface 5 is provided on the outer circumferential surface of the screw shaft 1. The outer circumferential raceway surface 5 extends from one end 6 to the other end 7 of the screw shaft 1. Hereinafter, the axial direction in which the one end 6 points is referred to as a first direction X1. The axial direction in which the other end 7 points is referred to as a second direction X2.

[0027] The nut 2 is formed in a cylindrical shape centered on a center line O1. Thus, the nut 2 has a through hole 10 penetrating in the axial direction. The screw shaft 1 is inserted into the through hole 10. The diameter of the outer circumferential surface 14 of the nut 2 is constant from the end in the first direction X1 to the end in the second direction X2. The nut 2 has a first end face 11 facing the first direction X1 and a second end face 12 facing the second direction X2.

[0028] Fig. 2 is a cross-sectional view of the nut of embodiment 1 cut along the center line. As shown in Fig. 2, the inner peripheral surface 20 of the nut 2 has a main body inner peripheral surface 21 arranged in the center of the inner peripheral surface 20, a first reduced diameter inner peripheral surface 22a arranged at the end of the inner peripheral surface 20 in the first direction X1, a first clearance groove surface 23a arranged between the main body inner peripheral surface 21 and the first reduced diameter inner peripheral surface 22a, a second reduced diameter inner peripheral surface 22b arranged at the end of the inner peripheral surface 20 in the second direction X2, and a second clearance groove surface 23b arranged between the main body inner peripheral surface 21 and the second reduced diameter inner peripheral surface 22b.

[0029] The diameter of the main body inner circumferential surface 21 is r1. The main body inner circumferential surface 21 is provided with a thread groove surface 30 and four circulation portions 31 (one circulation portion 31 is not shown in FIG. 2). Note that the imaginary line H1 in FIG. 2 is an extension line from the main body inner circumferential surface 21 in the second direction X2.

[0030] The thread groove surface 30 faces the outer circumferential raceway surface 5 of the screw shaft 1 and is a groove surface that extends in the same helical direction as the outer circumferential raceway surface 5. The thread groove surface 30 in this embodiment is composed of four inner circumferential raceway surfaces 32. The inner circumferential raceway surfaces 32 are formed by cutting. The four inner circumferential raceway surfaces 32 are not connected to each other.

[0031] The space between the inner raceway surface 32 and the outer raceway surface 5 forms a raceway. A plurality of balls (not shown) are arranged in this raceway. Grease 80 (not shown in FIG. 2; see grease 80 in FIG. 3) is supplied to the balls (not shown). Thus, a large amount of grease adheres to the main body inner raceway surface 21 (mainly the inner raceway surface 32 and the circulation portion 31) of the inner raceway surface 20 of the nut 2.

[0032] The circulation portion 31 is an S-shaped groove surface formed by forging on the inner peripheral surface 20 of the nut 2. The S-shaped groove surface connects one end and the other end of the inner peripheral raceway surface 32. As a result, the balls that move from one end of the raceway to the other end are circulated to one end of the raceway by the S-shaped groove surface.

[0033] Next, the first reduced diameter inner circumferential surface 22a and the second reduced diameter inner circumferential surface 22b will be described. The first reduced diameter inner circumferential surface 22a and the second reduced diameter inner circumferential surface 22b are symmetrical with respect to a virtual plane P passing through the axial center of the nut 2. Therefore, in the following description, the first reduced diameter inner circumferential surface 22a is used as a representative example, and a description of the second reduced diameter inner circumferential surface 22b will be omitted. The first reduced diameter inner circumferential surface 22a and the second reduced diameter inner circumferential surface 22b will be collectively referred to as the reduced diameter inner circumferential surface 22.

[0034] The first reduced diameter inner circumferential surface 22a is annular about the center line O1. The diameter r2 of the first reduced diameter inner circumferential surface 22a is smaller than the diameter r1 of the main body inner circumferential surface 21 (r1>r2). Therefore, the first reduced diameter inner circumferential surface 22a protrudes radially inward from the main body inner circumferential surface 21 (see imaginary line H1 in FIG. 2). Therefore, the part of the nut 2 in the axial direction where the first reduced diameter inner circumferential surface 22a is provided on the inner circumferential side has a larger radial thickness than the part of the nut 2 in the axial direction where the main body inner circumferential surface 21 is provided on the inner circumferential side. Hereinafter, the part of the nut 2 in the axial direction where the first reduced diameter inner circumferential surface 22a is provided on the inner circumferential side is referred to as the first thick portion 24a. Also, the part of the nut 2 in the axial direction where the second reduced diameter inner circumferential surface 22b is provided on the inner circumferential side is referred to as the second thick portion 24b. The first thick portion 24a and the second thick portion 24b are collectively referred to as the thick portion 24.

[0035] Next, the first clearance groove surface 23a and the second clearance groove surface 23b will be described. The first clearance groove surface 23a and the second clearance groove surface 23b are symmetrical with respect to the virtual plane P. Therefore, in the following description, the first clearance groove surface 23a is used as a representative example, and a description of the second clearance groove surface 23b will be omitted. The first clearance groove surface 23a and the second clearance groove surface 23b will be collectively referred to as the clearance groove surface 23.

[0036] The first clearance groove surface 23a is annular about the center line O1. The diameter r3 of the first clearance groove surface 23a is larger than the diameter r2 of the first reduced-diameter inner circumferential surface 22a (r3>r2). The diameter r3 of the first clearance groove surface 23a is also larger than the diameter r1 of the main body inner circumferential surface 21 (r3>r1). Therefore, the first reduced-diameter inner circumferential surface 22a is a concave surface that is recessed radially outward from the main body inner circumferential surface 21.

[0037] Fig. 3 is an enlarged view of the vicinity of the end of the nut in the first direction in the ball screw device of Fig. 1. As shown in Fig. 3, a corner 27 where the first reduced-diameter inner circumferential surface 22a (reduced-diameter inner circumferential surface 22) and the first end face 11 intersect is chamfered. Therefore, when a load acts on the nut 2, the structure is such that stress is unlikely to concentrate on the corner 27. Note that the corner 27 of the present disclosure may be rounded instead of chamfered.

[0038] A first tapered surface 28 is provided between the main body inner circumferential surface 21 and the first clearance groove surface 23a. The first tapered surface 28 gradually expands in diameter in the first direction X1. A second tapered surface 29 is provided between the first clearance groove surface 23a and the first reduced diameter inner circumferential surface 22a. The second tapered surface 29 gradually reduces in diameter in the first direction X1. Therefore, when a load is applied to the nut 2, stress is less likely to concentrate on the first tapered surface 28 and the second tapered surface 29.

[0039] Next, the operation and effect of the ball screw device 100 of the first embodiment will be described. As shown in Fig. 3, a gap S1 is provided between the thread 4 of the screw shaft 1 and the main body inner peripheral surface 21. Also, a gap S2 is provided between the thread 4 of the screw shaft 1 and the reduced-diameter inner peripheral surface 22. Note that the imaginary line H2 in Fig. 3 is an extension line from the reduced-diameter inner peripheral surface 22 in the second direction X2.

[0040] Furthermore, grease 80 adhering to the thread 4 of the screw shaft 1 may be present in the gap S1. When the screw shaft 1 moves relatively to the nut 2 in the first direction X1 (see arrow A in FIG. 3), the grease 80 adhering to the thread 4 passes through the gap S2 and leaks out of the nut 2.

[0041] In this embodiment, the diameter r2 of the reduced diameter inner circumferential surface 22 is smaller than the diameter r1 of the main body inner circumferential surface 21. In other words, the size L2 of the gap S2 is smaller than the size L1 of the gap S1. Therefore, when the screw shaft 1 moves in the first direction X1, a portion of the grease 80 attached to the thread 4 that is disposed radially outward of the imaginary line H2 is scraped off by the reduced diameter inner circumferential surface 22. Therefore, the amount of grease leaking out to the outside of the nut 2 is reduced.

[0042] Further, the reduced diameter inner circumferential surface 22 is provided on both sides in the axial direction of the main body inner circumferential surface 21. Therefore, not only when the screw shaft 1 moves in the first direction X1, but also when it moves in the second direction X2, the grease 80 is scraped off by the reduced diameter inner circumferential surface 22 (the second reduced diameter inner circumferential surface 22b). Therefore, the amount of grease leaking out of the nut 2 is greatly reduced.

[0043] In addition, after forging the S-shaped groove surface (circulation portion 31) on the main body inner peripheral surface 21 and forming the thread groove surface 30 (inner peripheral raceway surface 32), the main body inner peripheral surface 21 may be ground with a grindstone to set the diameter of the main body inner peripheral surface 21 to r1. In addition, the grindstone is moved axially along the main body inner peripheral surface 21 to change the grinding location in the axial direction. If the reduced diameter inner peripheral surface 22 is adjacent to the main body inner peripheral surface 21, the grindstone will come into contact with the reduced diameter inner peripheral surface 22 and the end of the main body inner peripheral surface 21 cannot be ground. On the other hand, according to this embodiment, the grindstone can be moved axially outward from the end of the main body inner peripheral surface 21, that is, it can be made to escape (move) to the inside of the escape groove surface 23. Therefore, the end of the main body inner peripheral surface 21 can be ground reliably. In the present disclosure, the grindstone of the main body inner peripheral surface 21 is not limited to being ground after the thread groove surface 30 (inner peripheral raceway surface 32) of the main body inner peripheral surface 21 is formed.

[0044] As described above, the ball screw device 100 of the first embodiment includes the screw shaft 1, the nut 2 having a through hole 10 penetrating in an axial direction parallel to the screw shaft 1 and in which the screw shaft 1 is inserted into the through hole 10, and a plurality of balls arranged between the nut 2 and the screw shaft 1. The inner circumferential surface 20 of the nut 2 includes a main body inner circumferential surface 21 provided with an inner circumferential raceway surface 32 on which the balls roll, and a reduced-diameter inner circumferential surface 22 arranged closer to the axial end of the nut 2 than the main body inner circumferential surface 21. The diameter r2 of the reduced-diameter inner circumferential surface 22 is smaller than the diameter r1 of the main body inner circumferential surface 21.

[0045] According to this embodiment, the grease 80 that moves to the outside of the nut 2 is scraped off by the reduced-diameter inner circumferential surface 22. Therefore, the amount of grease leaking to the outside of the nut 2 is reduced.

[0046] In the ball screw device 100 of the first embodiment, a corner 27 where the reduced diameter inner circumferential surface 22 and the end faces (first end face 11 and second end face 12) of the nut 2 intersect is chamfered. In the ball screw device 100 of the first embodiment, the inner circumferential surface 20 of the nut 2 has a first tapered surface 28 that is disposed between the main body inner circumferential surface 21 and the relief groove surface 23 and gradually increases in diameter toward the axial end of the nut 2, and a second tapered surface 29 that is disposed between the relief groove surface 23 and the reduced diameter inner circumferential surface 22 and gradually decreases in diameter toward the axial end of the nut 2.

[0047] As a result, stress is less likely to concentrate on the corner portion 27, the first tapered surface 28, and the second tapered surface 29, making them less likely to break.

[0048] In addition, in the ball screw device 100 of embodiment 1, the inner surface 20 of the nut 2 is arranged between the main body inner surface 21 and the reduced diameter inner surface 22, and has a clearance groove surface 23 with a larger diameter than the diameter of the main body inner surface 21.

[0049] This allows the end of the main body inner circumferential surface 21 to be polished reliably.

[0050] In the ball screw device 100 of the first embodiment, the reduced diameter inner circumferential surface 22 and the relief groove surface 23 are provided on both axial sides of the main body inner circumferential surface 21, respectively.

[0051] This greatly reduces the amount of grease leaking out of the nut 2. In addition, both ends of the main body inner circumferential surface 21 can be polished reliably.

[0052] The first embodiment has been described above. In the first embodiment, the reduced diameter inner circumferential surface 22 and the clearance groove surface 23 are provided on both sides of the main body inner circumferential surface 21 in the axial direction. However, in the present disclosure, the reduced diameter inner circumferential surface 22 and the clearance groove surface 23 may be provided only on one side of the main body inner circumferential surface 21 in the axial direction. In addition, the corner portion 27 of the present disclosure may not be chamfered. In addition, in the present disclosure, the first tapered surface 28 and the second tapered surface 29 may be a surface extending in the radial direction (a surface perpendicular to the first clearance groove surface 23a). In addition, the nut of the present disclosure may not have the clearance groove surface 23 (the first clearance groove surface 23a, the second clearance groove surface 23b). In other words, the nut of the present disclosure may not have the main body inner circumferential surface 21 ground. Alternatively, the nut of the present disclosure may have the main body inner circumferential surface 21 ground regardless of the presence or absence of the clearance groove surface 23 (the first clearance groove surface 23a, the second clearance groove surface 23b).

[0053] Next, another embodiment of the ball screw device of the present disclosure will be described. Note that in the following description, the differences from the ball screw device 100 of the first embodiment will be focused on.

[0054] (Embodiment 2) 4 is an enlarged view of a portion of the ball screw device of the embodiment 2. The ball screw device 100A of the embodiment 2 differs from the embodiment 1 in that an inner ring 91 of a bearing 90 is fitted to an outer peripheral surface 14 of a nut 2. An outer ring 92 of the bearing 90 is fitted to a housing 93. That is, the nut 2 is rotatably supported by the housing 93. Details will be described below.

[0055] The inner ring 91 is fitted to the end of the nut 2 in the first direction X1. In other words, the inner ring 91 is fitted to the first thick portion 24a. Hereinafter, the portion of the outer circumferential surface of the first thick portion 24a where the inner ring 91 is fitted is referred to as the fitting surface 15. The diameter of the fitting surface 15 is smaller than the outer circumferential surface 14 of the nut 2. Also, the axial length L3 of the fitting surface 15 is smaller than the axial length of the first thick portion 24a (the axial length L4 of the first reduced diameter inner circumferential surface 22a). Thus, the outer circumferential surface of the first thick portion 24a has the small diameter fitting surface 15, the exposed surface 16 that is disposed in the second direction X2 from the fitting surface 15 and is exposed without fitting the inner ring 91 (another part), and the stepped surface 17 that extends radially between the fitting surface 15 and the exposed surface 16 and faces the first direction X1.

[0056] According to the second embodiment, the inner ring 91 abuts against the step surface 17 from the first direction. Thus, the positional deviation in the first direction X1 by the nut 2 is restricted. Meanwhile, the first thick portion 24a is formed with a small-diameter fitting surface 15, and its strength is reduced. However, the fitting surface 15 has a smaller axial length than the first thick portion 24a (first reduced-diameter inner circumferential surface 22a). Thus, the reduction in strength of the first thick portion 24a due to the formation of the fitting surface 15 is kept small.

[0057] Furthermore, the fitting surface 15 is provided on the first thick portion 24a (thick portion 24). The thick portion 24 has a large radial thickness and is less likely to change shape due to temperature changes. In other words, the diameter of the fitting surface 15 is less likely to deform. This prevents a gap from occurring between the fitting surface 15 and the inner ring 91, or prevents the interference with the inner ring 91 from becoming larger than a predetermined amount.

[0058] Although the second embodiment has been described above, in the present disclosure, the axial length L3 of the fitting surface 15 may be the same as the axial length L4 of the first reduced diameter inner circumferential surface 22a. Alternatively, the axial length L3 of the fitting surface 15 may be greater than the axial length L4 of the first reduced diameter inner circumferential surface 22a. Even in such a modified example, the reduced diameter inner circumferential surface 22 is provided on the inner circumferential side of the thick-walled portion 24, and leakage of the grease 80 is suppressed to a small extent as in the first embodiment. In addition, in the present disclosure, the fitting surface 15 may have the same diameter as the outer circumferential surface 14 of the nut 2.

[0059] (Embodiment 3) 5 is a cross-sectional view of a ball screw device of embodiment 3 cut along the center line of the screw shaft. A ball screw device 100B of embodiment 3 differs from embodiment 1 in that it is used in a brake caliper 200. The brake caliper 200 is a device for braking the rotational motion of a wheel (not shown) by sandwiching a brake disc 201, which rotates together with the wheel (not shown), between two brake pads 202 (only one brake pad 202 is shown in FIG. 5). Details will be described below.

[0060] In the ball screw device 100B of the third embodiment, the outer peripheral surface 14 of the nut 2 is slidably supported by the sliding surface 221 of the housing 220. A notch 18 is provided at the end of the outer peripheral surface 14 in the first direction X1, and a rotation prevention protrusion 19 is fitted therein. The rotation prevention protrusion 19 is slidably inserted into a groove 222 extending in the axial direction. Thus, the nut 2 is supported in the housing 220 so as to be movable in the axial direction and not to be rotatable around the center line O1. The notch 18 is formed by cutting out a part of the first thick portion 24a in the circumferential direction. In addition, the second end surface 12 of the second thick portion 24b (nut 2) serves as a pressing surface 12A that abuts against the brake pad 202 and presses the brake pad 202 in the second direction X2.

[0061] A connecting shaft 8 protruding in the first direction X1 is provided at one end 6 of the screw shaft 1. The connecting shaft 8 is fitted with a power transmission shaft 210. The power transmission shaft 210 is rotatably supported by a housing 220 via a bearing 211. A rotational motion generated by a motor (not shown) is transmitted to the power transmission shaft 210. When the motor is driven, the screw shaft 1 rotates together with the power transmission shaft 210. Then, the nut 2 moves in the second direction X2, and the pressing surface 12A presses the brake pad 202 in the second direction X2. As a result, the brake disc 201 is sandwiched between the two brake pads 202, and a braking force is applied to the wheel.

[0062] According to the third embodiment, the end of the nut 2 in the second direction X2 presses the brake pad 202, and a large load acts on it. On the other hand, the end of the nut 2 in the second direction X2 is the second thick portion 24b, and has improved strength. Therefore, the end of the nut 2 in the second direction X2 is less likely to deform, such as buckling. If the nut 2 is deformed, the smooth rolling of the balls is hindered, and the operability of the ball screw device 100 is reduced. Therefore, by preventing the deformation of the second thick portion 24b (nut 2), the operability of the ball screw device 100 is also ensured.

[0063] A corner 27 where the second end face 12 (pressing surface 12a) and the second reduced diameter inner circumferential surface 22b intersect is chamfered. A second tapered surface 29 (see FIG. 3) is formed between the second reduced diameter inner circumferential surface 22b and the second clearance groove surface 23b, and no corner is provided. A first tapered surface 28 (see FIG. 3) is formed between the second clearance groove surface 23b and the main body inner circumferential surface 21, and no corner is provided. From the above, the nut 2 does not have corners where stress is likely to concentrate, and damage to the nut 2 is suppressed.

[0064] (Embodiment 4) Fig. 6 is an enlarged view of a portion of the ball screw device of embodiment 4. As shown in Fig. 6, a ball screw device 100C of embodiment 4 differs from embodiment 1 in that the reduced diameter inner circumferential surface 22 is not disposed at the end in the axial direction (first direction X1) of the nut 2. In other words, embodiment 4 differs from embodiment 1 in that the inner circumferential surface 20 of the nut 2 has an end inner circumferential surface 25 provided axially outwardly of the reduced diameter inner circumferential surface 22 (first direction X1 in Fig. 6).

[0065] Diameter r4 of end inner circumferential surface 25 is the same as diameter r1 (see FIG. 2) of main body inner circumferential surface 21. Thus, end inner circumferential surface 25 has a larger diameter than reduced-diameter inner circumferential surface 22. Therefore, reduced-diameter inner circumferential surface 22 in embodiment 4 is not at the axial end of inner circumferential surface 20, but is closer to the axial end than main body inner circumferential surface 21. Note that diameter r4 of end inner circumferential surface 25 does not have to be the same as diameter r1 of main body inner circumferential surface 21.

[0066] As described above, even in the fourth embodiment, the size of the gap S2 between the reduced diameter inner circumferential surface 22 and the thread 4 is L2 (see FIG. 3), which is the same as in the first embodiment. Therefore, similar to the first embodiment, the portion of the grease 80 adhering to the thread 4 that is disposed radially outward of the imaginary line H2 is scraped off by the reduced diameter inner circumferential surface 22. This reduces the amount of grease leaking.

[0067] (Embodiment 5) 7 is an enlarged view of a cross section of a nut of a ball screw device of embodiment 5. A ball screw device 100D of embodiment 5 differs from embodiment 1 in that a circulation portion 31 of a nut 2 is a top (not shown) instead of an S-shaped groove surface. Therefore, a plurality of rectangular through holes 33 for accommodating the tops (not shown) are formed in the nut 2 of embodiment 5.

[0068] Even in this embodiment 5, the nut 2 has a reduced diameter inner circumferential surface 22. Therefore, the amount of grease leaking is reduced. In this manner, the present disclosure does not particularly limit the type of the circulation part 31. In other words, the present disclosure may be a circulation part such as a tube.

[0069] The nut 2 of the fifth embodiment also differs from the first embodiment in that the thread groove surface 30 is formed in a portion other than the main body inner peripheral surface 21. Specifically, the thread groove surface 30 of the fifth embodiment is formed from one end to the other end of the inner peripheral surface 20 of the nut 2. Such a thread groove surface 30 can be formed by rolling or cutting. In this embodiment, the thread groove surface 30 is continuous in the spiral direction.

[0070] In addition, the reduced diameter inner circumferential surface 22 (22a, 22b) of the fifth embodiment is cut out by the thread groove surface 30. Therefore, the reduced diameter inner circumferential surface 22 (22a, 22b) of the fifth embodiment is spiral (see the area shaded with dots in FIG. 7). In addition, the thread groove surface 30 faces the outer circumferential raceway surface 5 of the screw shaft 1. In other words, the reduced diameter inner circumferential surface 22 (22a, 22b) faces the screw thread 4 of the screw shaft 1.

[0071] As described above, even in the fifth embodiment, the size of the gap S2 between the reduced diameter inner circumferential surface 22 (22a, 22b) and the thread 4 remains the same as L2 (see FIG. 3). Therefore, the amount of grease leaking is reduced.

[0072] In addition, in the fifth embodiment, the same clearance groove surface 23 (23a, 23b) as in the first embodiment is formed (see FIG. 2). However, the clearance groove surface 23 (23a, 23b) is cut out by the thread groove surface 30. The clearance groove surface 23 (23a, 23b) is indicated by the dashed line Q in FIG. 7. In this manner, in the fifth embodiment, a part of the clearance groove surface 23 (23a, 23b) remains on the inner peripheral surface 20. Therefore, also in this embodiment, when grinding the main body inner peripheral surface 21, the grindstone can be allowed to escape to the clearance groove surface 23 (23a, 23b), and the end of the main body inner peripheral surface 21 can be reliably ground.

[0073] Although each embodiment has been described above, the present disclosure is not limited to the above examples. For example, in the first embodiment, the thread groove surface 30 formed on the inner peripheral surface 20 of the nut 2 is formed only on the main body inner peripheral surface 21. In addition, the thread groove surface 30 in the fifth embodiment extends from one end to the other end of the inner peripheral surface 20 of the nut 2. However, the present disclosure is not limited to this. For example, when forming the thread groove surface 30 by cutting, cutting may be started from one end of the inner peripheral surface 20 of the nut 2 and stopped before reaching the other end. When cutting is stopped halfway in this way, a trace (cut-up) of the cutting tool gradually escaping radially inward is formed on the inner peripheral surface 20. The position of this cut-up will be described in detail below. Note that cutting will be described by taking an example in which cutting is started from the end of the inner peripheral surface 20 of the nut 2 in the second direction X2.

[0074] 8 is an axially cut development of the nut of the first modification. As shown in FIG. 8, in the nut 2E of the first modification, cutting is performed up to the second tapered surface 29. The width of the thread groove surface 30 is greater than the second tapered surface 29. Therefore, the undercut 36 spans the clearance groove surface 23 (22a) and the second tapered surface 29.

[0075] Fig. 9 is an axially cut development of the nut of the modified example 2. As shown in Fig. 9, in the nut 2F of the modified example 2, cutting is performed up to the first clearance groove surface 23a (clearance groove surface 23). Therefore, the cut-out 36 is provided on the clearance groove surface 23 (23a).

[0076] Fig. 10 is an axially cut development of the nut of the modified example 3. As shown in Fig. 9, in the nut 2G of the modified example 2, cutting is performed up to the first tapered surface 28. The width of the thread groove surface 30 is greater than the first tapered surface 28. Therefore, the cut-out 36 spans the second tapered surface 29 and the main body inner peripheral surface 21.

[0077] Although the modified examples have been described above, the present disclosure may be modified in such a way that the raised portion 36 is provided on the main body inner circumferential surface 21 .

[0078] The present disclosure may also be implemented in the following combinations: (1) A screw shaft, A nut having a through hole penetrating in an axial direction parallel to the screw shaft, and the screw shaft is inserted into the through hole; A plurality of balls disposed between the nut and the screw shaft; Equipped with The inner peripheral surface of the nut is a main body inner circumferential surface provided with an inner circumferential raceway surface along which the balls roll; a reduced diameter inner circumferential surface disposed closer to an axial end of the nut than the main body inner circumferential surface; having The diameter of the reduced diameter inner peripheral surface is smaller than the diameter of the main body inner peripheral surface. Ball screw device. (2) In the axial direction of the nut, a portion in which the reduced diameter inner circumferential surface is disposed on an inner circumferential side is a thick portion having a radial thickness greater than a portion in which the main body inner circumferential surface is disposed on an inner circumferential side, At least a part of the outer circumferential surface of the thick portion serves as a fitting surface with which another component fits. A ball screw device as described in (1). (3) an axial length of the fitting surface is smaller than an axial length of the reduced diameter inner circumferential surface; The outer circumferential surface of the thick portion is an exposed surface to which the other component is not fitted and which is exposed; the fitting surface having a smaller diameter than the exposed surface; Possesses A ball screw device as described in (2). (4) In the axial direction of the nut, a portion in which the reduced diameter inner circumferential surface is disposed on an inner circumferential side is a thick portion having a radial thickness greater than a portion in which the main body inner circumferential surface is disposed on an inner circumferential side, The thickened portion is disposed at an end of the nut, The end surface of the thick portion serves as a pressing surface. A ball screw device according to any one of (1) to (3). (5) The corner where the reduced diameter inner circumferential surface and the pressing surface intersect is chamfered. A ball screw device as described in (4). (6) The inner peripheral surface of the nut is disposed between the inner peripheral surface of the main body and the reduced diameter inner peripheral surface and has a relief surface having a diameter larger than the diameter of the inner peripheral surface of the main body. A ball screw device according to any one of (1) to (5). (7) The reduced diameter inner circumferential surface and the relief groove surface are provided on both axial sides of the main body inner circumferential surface, respectively. A ball screw device as described in (6). (8) The inner circumferential surface of the nut is a first tapered surface disposed between the main body inner circumferential surface and the relief groove surface, the first tapered surface gradually increasing in diameter toward the end of the axial direction of the nut; a second tapered surface disposed between the relief groove surface and the reduced diameter inner circumferential surface, the second tapered surface gradually reducing in diameter toward the end of the axial direction of the nut; Possesses A ball screw device as described in (6) or (7). [Explanation of symbols]

[0079] 1 Screw shaft 2, 2E, 2F, 2G Nuts 4 Threads 5 Outer raceway surface 14 Outer surface 15 Mating surface 16 Exposed surface 17 Step surface 20 Inner surface 21 Inner surface of main body 22 Reduced diameter inner circumferential surface 23 Undercut surface 24 Thick part 25 End inner circumferential surface 27 Corner 28 First tapered surface 29 Second tapered surface 30 Thread groove surface 31 Circulation section 32 Inner raceway surface 36 Round up 80 Grease 100, 100A, 100B, 100D Ball screw device 200 Brake caliper S1, S2 gap

Claims

1. A screw shaft, A nut having a through hole penetrating in an axial direction parallel to the screw shaft, and the screw shaft is inserted into the through hole; A plurality of balls disposed between the nut and the screw shaft; Equipped with The inner peripheral surface of the nut is a main body inner circumferential surface provided with an inner circumferential raceway surface along which the balls roll; a reduced diameter inner circumferential surface disposed closer to an axial end of the nut than the main body inner circumferential surface; having The diameter of the reduced diameter inner circumferential surface is smaller than the diameter of the main body inner circumferential surface, The inner peripheral surface of the nut is disposed between the inner peripheral surface of the main body and the reduced diameter inner peripheral surface and has a relief surface having a diameter larger than the diameter of the inner peripheral surface of the main body. Ball screw device.

2. In the axial direction of the nut, a portion in which the reduced diameter inner circumferential surface is disposed on an inner circumferential side is a thick portion having a radial thickness greater than a portion in which the main body inner circumferential surface is disposed on an inner circumferential side, At least a part of the outer circumferential surface of the thick portion serves as a fitting surface with which another component fits. The ball screw device according to claim 1 .

3. an axial length of the fitting surface is smaller than an axial length of the reduced diameter inner circumferential surface; The outer circumferential surface of the thick portion is an exposed surface to which the other component is not fitted and which is exposed; the fitting surface having a smaller diameter than the exposed surface; Possesses The ball screw device according to claim 2 .

4. In the axial direction of the nut, a portion in which the reduced diameter inner circumferential surface is disposed on an inner circumferential side is a thick portion having a radial thickness greater than a portion in which the main body inner circumferential surface is disposed on an inner circumferential side, The thickened portion is disposed at an end of the nut, The end surface of the thick portion serves as a pressing surface. The ball screw device according to any one of claims 1 to 3.

5. The corner where the reduced diameter inner circumferential surface and the pressing surface intersect is chamfered. The ball screw device according to claim 4.

6. The reduced diameter inner circumferential surface and the relief groove surface are provided on both axial sides of the main body inner circumferential surface, respectively. The ball screw device according to claim 1 .

7. The inner circumferential surface of the nut is a first tapered surface disposed between the main body inner circumferential surface and the relief groove surface, the first tapered surface gradually increasing in diameter toward the end of the nut in the axial direction; a second tapered surface disposed between the relief groove surface and the reduced diameter inner circumferential surface, the second tapered surface gradually reducing in diameter toward the end of the axial direction of the nut; Possesses The ball screw device according to claim 1 or 6.

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