Ball screw device and assembly method of ball screw device

Abstract

To provide a ball screw device capable of using a temporary shaft while preventing a screw shaft from coming off from a nut and to provide an assembly method.SOLUTION: A ball screw device includes: a screw shaft having one end pointing in a first direction, the other end pointing in a second direction, and an outer peripheral raceway surface provided on an outer peripheral surface; a nut having an inner peripheral raceway surface provided on an inner peripheral surface; a plurality of balls; and a circulation part provided on the inner peripheral surface of the nut. The nut includes a cylindrical nut body provided with the inner peripheral raceway surface, a first opening that opens in the first direction from inside the nut body, a retaining part projecting radially inward from an end in the second direction on the inner peripheral surface of the nut body, and a second opening provided radially inward from the retaining part and opening in the second direction from inside the nut body. The retaining part includes an inner peripheral surface that faces a radially inner side. The inner peripheral surface has a diameter smaller than that of the outer peripheral surface of the screw shaft.SELECTED DRAWING: Figure 4

Description

[Technical field]

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

[0002] A ball screw device is a device that converts rotational motion into linear motion and linear motion into rotational motion. The ball screw device includes a screw shaft, a nut that is inserted through the screw shaft, a plurality of balls that are disposed between the screw shaft and the nut, and a plurality of circulation units that circulate the balls. The nut in the following patent document has a cylindrical nut body and a disk-shaped lid that closes one opening of the nut body. This lid unit prevents the screw shaft from falling off the nut during transportation of the ball screw device. In addition, the circulation unit in the following patent document uses a top that fits into the inner peripheral surface of the nut. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2019 / 277380 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, when a circulation portion is provided on the inner peripheral surface of a nut, a temporary shaft is used in assembling a ball screw device. However, according to the ball screw device of the above-mentioned patent document, even if a temporary shaft is inserted into the inside of the nut from the other opening of the nut and balls are assembled between the nut and the temporary shaft, it is not possible to insert a screw shaft into the inside of the nut from one opening of the nut. In other words, a temporary shaft cannot be used, and it takes a lot of effort to assemble the ball screw device.

[0005] The present disclosure has been made in consideration of the above, and aims to provide a ball screw device and a method for assembling a ball screw device that can use a temporary shaft while preventing the screw shaft from falling off the nut. [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 having one end pointing in a first direction and the other end pointing in a second direction, and an outer peripheral raceway surface provided on the outer peripheral surface, a nut into which the screw shaft is inserted and having an inner peripheral raceway surface provided on the inner peripheral surface, a plurality of balls arranged between the outer peripheral raceway surface and the inner peripheral raceway surface, and a circulation unit provided on the inner peripheral surface of the nut to circulate the balls. The nut has a cylindrical nut body provided with the inner peripheral raceway surface, a first opening opening in the first direction from the inside of the nut body, a retaining portion protruding radially inward from an end of the inner peripheral surface of the nut body in the second direction, and a second opening provided radially inward of the retaining portion and opening in the second direction from the inside of the nut body. The retaining portion has an inner diameter surface facing radially inward. The diameter of the inner diameter surface is smaller than the diameter of the outer peripheral surface of the screw shaft.

[0007] According to the ball screw device of the present disclosure, the diameter of the second opening is reduced by the retaining portion, and the screw shaft does not pass through the second opening. Therefore, the screw shaft is prevented from falling off from the second opening of the nut. Also, a temporary shaft can be inserted into the inside of the nut through the second opening, and the screw shaft can be inserted into the inside of the nut from the first opening. That is, the ball screw device can be assembled using the temporary shaft. Therefore, the effort required for assembling the ball screw device can be reduced.

[0008] In the ball screw device, the diameter of the inner diameter surface is larger than a diameter obtained by subtracting the diameter of two of the balls from the diameter of the inner peripheral surface of the nut body.

[0009] In a preferred embodiment of the ball screw device, the retaining portion is annular, and the inner diameter surface of the retaining portion is circular when viewed from an axial direction parallel to a center line of the screw shaft.

[0010] When the temporary shaft is inserted into the second opening, the outer circumferential surface of the temporary shaft comes into contact with the inner circumferential surface of the retaining portion. This prevents the temporary shaft from shifting outward in the radial direction. In other words, the temporary shaft is arranged coaxially with the nut, making it easy to position the temporary shaft.

[0011] In a preferred embodiment of the ball screw device, the nut has a concave surface recessed radially inward from the outer circumferential surface of the nut body. The nut body has a first nut body and a second nut body having the concave surface and an outer diameter smaller than the outer diameter of the first nut body. At least a portion of the retaining portion is provided on the inner circumferential side of the second nut body.

[0012] The second nut body has a smaller radial thickness than the first nut body. In other words, the second nut body has a lower rigidity than the first nut body. However, a retaining portion is provided on the inner peripheral side of the second nut body, and the second nut body is reinforced by the retaining portion. Therefore, deformation or damage of the second nut body is avoided.

[0013] In the ball screw device, a thread groove surface is formed on an inner peripheral surface of the nut body, the thread groove surface extending from an end portion in the first direction to an end portion in the second direction and a part of the thread groove surface constituting the inner peripheral raceway surface. The retaining portion may be provided with the thread groove surface.

[0014] In addition, in order to achieve the above-mentioned object, a method for assembling a ball screw device according to one embodiment of the present disclosure is a method for assembling the ball screw device described above, and includes a temporary shaft insertion process for inserting a temporary shaft into the second opening of the nut, a ball placement process for inserting the ball into the inside of the nut from the first opening of the nut and placing the ball between the outer peripheral surface of the temporary shaft and the inner peripheral raceway surface, a screw shaft abutting process for abutting the end face of the screw shaft in the second direction against the end face of the temporary shaft in the first direction, a screw shaft moving process for rotating the screw shaft and the nut relatively to move the screw shaft in the second direction, and a removal process for removing the temporary shaft from the nut.

[0015] According to the method for assembling the ball screw device of the present disclosure, the ball screw device can be assembled using a temporary shaft, and the labor required for assembling the ball screw device is reduced. In addition, when the screw shaft moves in the second direction after assembly, the screw shaft comes into contact with the retaining portion. This prevents the screw shaft from falling off the second opening of the nut.

[0016] In a preferred embodiment of the method for assembling the ball screw device, when viewed from an axial direction parallel to the center line of the screw shaft, an inscribed circle is an imaginary circle centered on the center line, and the circle is tangent to each of the plurality of balls arranged inside the inner circumferential raceway surface from the radially inner side, and the outer diameter of the virtual shaft is smaller than the diameter of the inscribed circle.

[0017] According to the above-mentioned configuration, the gap between the outer peripheral surface of the temporary shaft and the inner peripheral raceway surface is increased, making it easier to arrange the balls.

[0018] In a preferred embodiment of the method for assembling the ball screw device, an inscribed circle is a virtual circle centered on the center line of the screw shaft when viewed from an axial direction parallel to the center line of the screw shaft, and the circle is tangent to each of the plurality of balls arranged inside the inner peripheral raceway surface from the radially inner side. The outer diameter of the virtual shaft is the same as the diameter of the inscribed circle.

[0019] If the balls arranged between the outer peripheral surface of the temporary shaft and the inner peripheral raceway surface rattle in the radial direction, the balls will get caught on the end face of the screw shaft during the screw shaft movement process, making it difficult for the balls to enter the outer peripheral raceway surface of the screw shaft. On the other hand, according to the above configuration, the balls arranged between the outer peripheral surface of the temporary shaft and the inner peripheral raceway surface do not rattle in the radial direction. Therefore, it is possible to prevent the balls from getting caught on the end face of the screw shaft during the screw shaft movement process.

[0020] In a preferred embodiment of the method for assembling the ball screw device, the retaining portion is annular, the inner diameter surface of the retaining portion is circular when viewed from the axial direction, and the outer diameter of the temporary shaft is smaller than the diameter of the inner diameter surface of the retaining portion.

[0021] According to the above-mentioned configuration, the temporary shaft is arranged coaxially with the nut, which facilitates positioning of the temporary shaft. In addition, after the temporary shaft is inserted into the second opening, the temporary shaft can be moved smoothly in the axial direction.

[0022] In a preferred embodiment of the method for assembling the ball screw device, the temporary shaft is cylindrical. The screw shaft has a screw shaft main body on which the outer circumferential raceway surface is provided, and a protruding portion protruding from the screw shaft main body in the second direction. The outer diameter of the protruding portion is the same as the inner diameter of the temporary shaft. The screw shaft abutting step includes inserting the protruding portion into the temporary shaft.

[0023] According to the above-mentioned configuration, the screw shaft is arranged coaxially with the temporary shaft. In other words, the screw shaft is prevented from being displaced radially outward. This prevents the balls from getting caught on the end face of the screw shaft during the screw shaft movement process. Effect of the Invention

[0024] According to the present disclosure, the screw shaft does not fall off from the second opening of the nut, and the labor required for assembling the ball screw device is reduced. [Brief description of the drawings]

[0025] [Figure 1]FIG. 1 is a cross-sectional view of a ball screw device according to a first embodiment taken along an axial direction. [Diagram 2] FIG. 2 is a cross-sectional view of the nut of the first embodiment taken along an axial direction. [Diagram 3] FIG. 3 is a side view of the ball screw device of the first embodiment as viewed from a second direction. [Figure 4] FIG. 4 is an enlarged view of a part of FIG. [Diagram 5] FIG. 5 is a diagram showing a state before the temporary shaft is inserted into the nut in the temporary shaft inserting step of the first embodiment. [Figure 6] FIG. 6 is a diagram showing a state in which balls are arranged on the fourth inner circumferential raceway surface and the fourth S-shaped groove surface in the ball arrangement step of the first embodiment. [Figure 7] FIG. 7 is a diagram showing a state after the temporary shaft is moved in the first direction after the balls are arranged on the fourth inner peripheral raceway surface and the fourth S-shaped groove surface in the ball arranging step of the first embodiment. [Figure 8] FIG. 8 is a diagram showing a state after the ball placement step of the first embodiment is completed. [Figure 9] FIG. 9 is a diagram showing a state of the screw shaft abutting step of the first embodiment. [Figure 10] FIG. 10 is a diagram showing a state in the middle of a screw shaft moving process in the first embodiment. [Figure 11] FIG. 11 is a diagram showing a state after the screw shaft moving step of the first embodiment is completed (before the removal step). [Figure 12] FIG. 12 is a partially enlarged view of a cross section of the nut body with the temporary shaft inserted therein, cut in the axial direction. [Figure 13] FIG. 13 is a cross-sectional view showing a nut of a ball screw device according to the first modification. [Figure 14] FIG. 14 is a diagram showing a screw shaft abutting step using the temporary shaft of the second modification. [Figure 15] FIG. 15 is a cross-sectional view showing a nut of a ball screw device according to the first modification. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The embodiments for carrying out the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the contents described in the following description. The components described below include those that a person skilled in the art can easily imagine and those that are substantially the same. Furthermore, the components described below can be appropriately combined.

[0027] (Embodiment 1) Fig. 1 is a cross-sectional view of a ball screw device of the first embodiment cut in the axial direction. As shown in Fig. 1, the ball screw device 100 of the first embodiment includes a screw shaft 1, a nut 2, a plurality of balls 3, and a plurality of circulation units 4. Hereinafter, a direction parallel to a center line O1 of the screw shaft 1 is referred to as an axial direction. In addition, within the axial direction, a direction in which one end 5 of the screw shaft 1 points is referred to as a first direction X1, and a direction in which the other end 6 of the screw shaft 1 points is referred to as a second direction X2.

[0028] The screw shaft 1 has a screw shaft body 10 having a spiral outer circumferential raceway surface 12 formed on its outer circumferential surface 11. In other words, the screw shaft 1 of this embodiment is composed only of the screw shaft body 10. Note that although the screw shaft 1 of this embodiment is composed only of the screw shaft body 10, the screw shaft 1 of the present disclosure may have other components in addition to the screw shaft body 10, such as a power transmission shaft that transmits rotational motion to the screw shaft body 10.

[0029] One end face 5a and the other end face 6a of the screw shaft 1 are flat surfaces. The outer circumferential raceway surface 12 is formed from the end of the screw shaft body in the first direction X1 to the end in the second direction X2. Therefore, the one end face 5a and the other end face 6a of the screw shaft 1 are cut out in the axial direction to form cutout portions 5b, 6b that are continuous with the outer circumferential raceway surface 12. The outer circumferential raceway surface 12 of this embodiment has a circular arc shape. Note that the present disclosure may also be a gothic arc-shaped outer circumferential raceway surface.

[0030] The nut 2 includes a cylindrical (tubular) nut body 20 and a retaining portion 30 protruding radially inward from an inner peripheral surface 23 of the nut body 20. Hereinafter, the internal space of the nut body 20 may be referred to as the interior of the nut body 20.

[0031] The nut body 20 has a first opening 21 that opens in a first direction X1 from the inside of the nut body 20, and a second opening 22 that opens in a second direction X2 from the inside of the nut body 20. The screw shaft 1 is inserted into the inside of the nut body 20 from the first opening 21. The center line O2 of the nut body 20 coincides with the center line O1 of the screw shaft. The cross-sectional shapes of the inner peripheral surface 23 and the outer peripheral surface 24 of the nut body 20 are each circular when viewed from the axial direction.

[0032] Fig. 2 is a cross-sectional view of the nut of the first embodiment cut in the axial direction. As shown in Fig. 2, the inner peripheral surface 23 of the nut body 20 is provided with a plurality of inner peripheral raceway surfaces 25 and a plurality of circulation portions 4. The inner peripheral raceway surface 25 extends in a spiral shape and faces the outer peripheral raceway surface 12 (see Fig. 1). The length of the inner peripheral raceway surface 25 is slightly shorter than approximately one revolution (approximately one lead). Between each inner peripheral raceway surface 25 and the outer peripheral raceway surface 12 is a raceway 7 (see Fig. 1). A plurality of balls 3 (see Fig. 1) are arranged in each raceway 7.

[0033] The circulation portion 4 of this embodiment is an S-shaped groove surface 40 formed by recessing the inner peripheral surface 23 of the nut body 20 radially outward. The S-shaped groove surface 40 is formed by forging. The S-shaped groove surface 40 connects one end and the other end of the inner peripheral raceway surface 25. As a result, the ball 3 that has moved from one end of the raceway 7 to the other end is circulated to one end of the raceway 7 by the S-shaped groove surface 40. Note that the circulation portion 4 of the present disclosure is not limited to the S-shaped groove surface 40. For example, it may be a top that is fitted into a concave surface formed on the inner peripheral surface 23 of the nut body 20. Alternatively, it may be a top that fits into a hole that penetrates from the outer peripheral surface 24 to the inner peripheral surface 23 of the nut body 20. In the case of a top that fits into a through hole, the ball 3 can be assembled by passing through the through hole, but the ball 3 can also be assembled using a temporary shaft 70 described later, which provides a high degree of freedom in assembly.

[0034] In this embodiment, four inner circumferential raceway surfaces 25 and four S-shaped groove surfaces 40 are provided. That is, the number of raceways 7 is also four. Hereinafter, the four inner circumferential raceway surfaces 25 are referred to as the first inner circumferential raceway surface 25a, the second inner circumferential raceway surface 25b, the third inner circumferential raceway surface 25c, and the fourth inner circumferential raceway surface 25d in the order from the first direction X1. Also, the four S-shaped groove surfaces 40 are referred to as the first S-shaped groove surface 40a, the second S-shaped groove surface 40b, the third S-shaped groove surface 40c, and the fourth S-shaped groove surface (not shown) in the order from the first direction X1. Note that, in this embodiment, four inner circumferential raceway surfaces 25 and four S-shaped groove surfaces 40 are provided, but the present disclosure may have a number other than four, for example, one each.

[0035] Here, when a load acts on the screw shaft 1 in a direction in which the S-shaped groove surface 40 is disposed as viewed from the center line O1, the balls 3 are disposed on the S-shaped groove surface 40, and the nut 2 cannot support the screw shaft 1. In this embodiment, the four S-shaped groove surfaces 40 are disposed at 90 degree intervals. Therefore, the areas that the nut 2 cannot support are disposed so as not to overlap in the circumferential direction. In other words, the nut 2 supports the screw shaft 1 from all directions in the circumferential direction.

[0036] The retaining portion 30 is disposed at an end portion of the inner peripheral surface 23 of the nut body 20 in the second direction X2. The retaining portion 30 has an inner diameter surface 31 facing radially inward. A second opening 22 is provided on the radially inner side of this inner diameter surface 31. Therefore, the retaining portion 30 makes the second opening 22 smaller in diameter than the first opening 21.

[0037] Fig. 3 is a side view of the ball screw device of the first embodiment as viewed from a second direction. As shown in Fig. 3, the retaining portion 30 is annular. That is, the retaining portion 30 extends in the circumferential direction along the inner peripheral surface 23 of the nut body 20. The inner diameter surface 31 is formed in a circular shape centered on the center line O2 of the nut 2.

[0038] FIG. 4 is an enlarged view of a part of FIG. 1. As shown in FIG. 4, the diameter of the inner diameter surface 31 is r1. The diameter r1 of this inner diameter surface 31 is smaller than the diameter r2 of the outer peripheral surface 11 of the screw shaft 1 (r1 < r2). In other words, the diameter of the second opening 22 is smaller than the outer diameter of the screw shaft 1. Note that the diameter r2 of the outer peripheral surface 11 of the screw shaft 1 is the same as the diameter of the thread 12a of the screw shaft 1. Therefore, even when the screw shaft 1 moves in the second direction X2 during the conveyance of the ball screw device 100, the screw shaft 1 cannot pass through the second opening 22. Therefore, it is regulated that the screw shaft 1 falls off in the second direction X2 of the nut 2.

[0039] The diameter r1 of the inner diameter surface 31 is equal to or larger than the outer diameter r3 of the dummy shaft 70 (see FIG. 5) (r3 ≦ r1). That is, the dummy shaft 70 can be inserted into the second opening 22. For this reason, the ball screw device 100 can be assembled using the dummy shaft 70.

[0040] As shown in FIG. 4, the corner portion 32 where the end surface 2a in the second direction X2 of the nut 2 and the inner diameter surface 31 intersect is chamfered.

[0041] A relief groove 27 having a larger diameter than the inner peripheral surface 23 is formed adjacent to the retaining portion 30 in the first direction X of the inner peripheral surface 23 of the nut body 20. According to this, when the inner peripheral surface 23 is polished with a grindstone, the grindstone can be released into the relief groove 27. In the present embodiment, the relief groove 27 is provided, but the present disclosure may not provide the relief groove 27. Further, the present disclosure may polish the inner peripheral surface 23 regardless of the presence or absence of the relief groove 27.

[0042] Next, a method for assembling the ball screw device 100 will be described. The method for assembling the ball screw device 100 includes a dummy shaft insertion step S1, a ball arrangement step S3, a screw shaft contact step S5, a screw shaft movement step S7, and a removal step S9.

[0043] 5 is a diagram showing a state before the temporary shaft is inserted into the nut in the temporary shaft inserting step of the first embodiment. As shown in FIG. 5, the temporary shaft inserting step S1 first prepares the nut 2 and the temporary shaft 70. The temporary shaft 70 of this embodiment is a cylindrical solid part. The temporary shaft 70 is arranged in the second direction X2 of the nut 2, and the temporary shaft 70 is parallel to the axial direction. Next, the end 72 of the temporary shaft 70 in the first direction X1 is inserted into the second opening 22 (see arrow A in FIG. 5). Next, the temporary shaft 70 is further pushed in, and the end 72 of the temporary shaft 70 is arranged inside the nut 2 as shown in FIG. 6.

[0044] In this embodiment, the temporary shaft 70 has a corner 74 where the end face 73 in the first direction X1 and the outer circumferential surface 72 intersect. The corner 74 is chamfered. Furthermore, the corner 32 is chamfered. This makes it easy to insert the temporary shaft 70 into the second opening 22. The outer diameter r3 of the temporary shaft 70 is slightly smaller than the diameter r1 (see FIG. 4) of the inner diameter surface 31 of the retaining portion 30. Therefore, after the end 72 in the first direction X1 of the temporary shaft 70 is inserted into the second opening 22, the temporary shaft 70 moves smoothly in the axial direction. Furthermore, since the inner diameter surface 31 of the retaining portion 30 is circular (see FIG. 3), the temporary shaft 70 is disposed substantially coaxially with the center line O2 of the nut 2.

[0045] Fig. 6 to Fig. 8 are diagrams showing the ball placing step of the first embodiment. In detail, Fig. 6 is a diagram showing a state in which balls are placed on the fourth inner raceway surface and the fourth S-shaped groove surface in the ball placing step of the first embodiment. Fig. 7 is a diagram showing a state after the temporary shaft is moved in the first direction after the balls are placed on the fourth inner raceway surface and the fourth S-shaped groove surface in the ball placing step of the first embodiment. Fig. 8 is a diagram showing a state after the ball placing step of the first embodiment is completed.

[0046] The ball placement step S3 is a step of placing the balls 3 on the inner raceway surface 25 and the circulation portion 4. As shown in Fig. 6, the ball placement step S3 first adjusts the insertion amount of the temporary shaft 70 so that the outer peripheral surface 71 of the end portion 72 of the temporary shaft 70 and a part of the fourth inner raceway surface 25d face each other in the radial direction. As a result, the remaining part of the fourth inner raceway surface 25d (the part not facing the temporary shaft 70) is opened to the inside of the nut 2.

[0047] Next, as shown by arrow B in Fig. 6, the balls 3 are inserted into the nut 2 from the first opening 21. Then, the balls 3 are inserted into the open portion of the fourth inner raceway surface 25d. This operation of inserting the balls 3 is repeated until the fourth inner raceway surface 25d and the fourth S-shaped groove surface are filled with balls 3.

[0048] When the fourth inner raceway surface 25d and the fourth S-shaped groove surface are filled with the balls 3, the temporary shaft 70 is moved in the first direction X1. As shown in FIG. 7, the movement amount of the temporary shaft 70 is adjusted so that the outer peripheral surface 71 of the temporary shaft 70 after the movement faces a part of the third inner raceway surface 25c. This makes the remaining part of the third inner raceway surface 25c open to the inside of the nut 2. In addition, the balls 3 arranged on the fourth inner raceway surface 25d and the fourth S-shaped groove surface are restricted from moving radially inward by the temporary shaft 70. In other words, the balls 3 do not fall off the fourth inner raceway surface 25d and the fourth S-shaped groove surface. The size (the minimum value of the outer diameter r3 of the temporary shaft 70) required for the temporary shaft 70 to perform the function of preventing the balls 3 from falling off will be described later.

[0049] 7, the balls 3 are inserted into the open portion of the third inner raceway surface 25c, and the third inner raceway surface 25c and the third S-shaped groove surface 40c are filled with the balls 3. Next, the temporary shaft 70 is moved in the first direction X1, and the outer peripheral surface 71 of the temporary shaft 70 and a part of the second inner raceway surface 25b face each other. Then, the balls 3 are inserted into the open portion of the second inner raceway surface 25b, and the second inner raceway surface 25b and the second S-shaped groove surface 40b are filled with the balls 3.

[0050] Next, the temporary shaft 70 is moved in the first direction X1, and the outer peripheral surface 71 of the temporary shaft 70 faces a part of the first inner raceway surface 25a. Then, the balls 3 are inserted into the open portion of the first inner raceway surface 25a, and the first inner raceway surface 25a and the first S-groove surface 40a are filled with the balls 3. Finally, as shown in Fig. 8, the temporary shaft 70 is moved in the first direction X1, and the ball arrangement process S3 is completed.

[0051] Thus, in the ball placement step S3, the movement of the temporary shaft 70 and the placement of the balls 3 are repeated. Here, as shown in Figs. 6 to 8, when the balls 3 are placed into the inner raceway surface 25 and the S-shaped groove surface 40, it is preferable to place the balls 3 so that the second direction X2 (temporary shaft 70) is on the lower side and the first direction X1 (nut 2) is on the upper side. In this way, even if the worker drops the balls 3 after placing them inside the nut 2, the balls 3 will accumulate above the end surface 73 of the temporary shaft 70 (in the first direction X1). Then, by tilting the nut 2 from this state, the balls 3 can be guided to the open portion of the fourth inner raceway surface 25d. In other words, the effort required for putting in the balls can be reduced.

[0052] 8 passes through the first opening 21 of the nut 2 and is disposed outside the nut 2 in the first direction X1 at the end of the ball arranging step S3, but the position of the end 72 is not particularly important as long as it is possible to prevent the ball 3 from falling off. In other words, in the present disclosure, the end 72 may be disposed inside the nut 2.

[0053] Fig. 9 is a diagram showing a state in a screw shaft abutting step of embodiment 1. Fig. 10 is a diagram showing a state during a screw shaft moving step of embodiment 1. Fig. 11 is a diagram showing a state after the screw shaft moving step of embodiment 1 is completed (before a removal step).

[0054] 9, the screw shaft abutting step S5 is a step of abutting an end face 73 of the temporary shaft 70 against the other end face 6a of the screw shaft 1. In addition, the screw shaft 1 and the temporary shaft 70 are arranged so as to be coaxial with each other, in other words, the screw shaft 1 is arranged so as to be coaxial with the center line O2 of the nut 2.

[0055] In the screw shaft moving step S7, the screw shaft 1 and the nut 2 are rotated relative to each other while pushing the screw shaft 1 in the second direction X2. As a result, the temporary shaft 70 moves in the second direction X2, and the other end 6 of the screw shaft 1 enters the inside of the nut 2. Thereafter, the balls 3 arranged on the first inner peripheral raceway surface 25a enter the outer peripheral raceway surface 12 from the cutout portion 6b of the other end face 6a of the screw shaft 1.

[0056] When the multiple balls 3 enter the outer peripheral raceway surface 12 of the screw shaft 1, the action of pushing the screw shaft 1 in the second direction X2 is stopped, and only the relative rotation of the screw shaft 1 and the nut 2 is performed, so that the screw shaft 1 is moved in the second direction X2 as shown in FIG. 10.

[0057] 11, when the balls 3 arranged on each inner peripheral raceway surface 25 enter the outer peripheral raceway surface 12 of the screw shaft 1, the screw shaft moving step S7 is completed. The temporary shaft 70 is pushed out by the screw shaft 1 and moves in the second direction X2.

[0058] Here, if the screw shaft 1 is displaced radially with respect to the center line O2 of the nut 2 in the screw shaft moving step S7, the balls 3 arranged on the inner peripheral raceway surface 25 will get caught on the other end surface 6a of the screw shaft 1, making it difficult for the balls 3 to enter the outer peripheral raceway surface 12. However, in this embodiment, the screw shaft 1 is arranged coaxially with the temporary shaft 70 in the screw shaft abutting step S5. This prevents the balls 3 from getting caught on the other end surface 6a of the screw shaft 1.

[0059] The removal step S9 is a step of removing the temporary shaft 70 from the nut 2. Specifically, as shown by the arrow D in FIG. 11, the temporary shaft 70 can be removed from the nut 2 by moving the temporary shaft 70 in the second direction X2. When the removal step S9 is completed, the ball screw device 100 is completed, and all steps of the method for assembling the ball screw device 100 are completed. According to the above-mentioned method for assembling the ball screw device 100, the temporary shaft 70 is used, and the labor required for assembly is reduced. Next, the outer diameter r3 of the temporary shaft 70 will be described in detail.

[0060] FIG. 12 is a cross-sectional view of the nut body with the temporary shaft inserted therein, cut in the axial direction. As shown in FIG. 12, if the outer diameter r3 of the temporary shaft 70 is too small (see imaginary circle P70 in FIG. 12), the gap between the outer circumferential surface 71 of the temporary shaft 70 and the inner circumferential surface 23 of the nut body 20 becomes large. Therefore, the ball 3 falls off between the outer circumferential surface 71 of the temporary shaft 70 and the inner circumferential surface 23 of the nut body 20 (see ball 3A shown by the broken line in FIG. 12). In other words, the ball 3 cannot be held on the inner circumferential raceway surface 25, and the temporary shaft 70 does not function as a temporary shaft 70. For this reason, the gap amount r4 between the outer circumferential surface 71 of the temporary shaft 70 and the inner circumferential raceway surface 25 must be less than the diameter r6 of the ball 3. In other words, the outer diameter r3 of the temporary shaft 70 must be greater than the diameter r7 obtained by subtracting (subtracting) the diameter of two balls 3 (diameter r6×2) from the diameter r5 of the inner circumferential surface 23 of the nut body 20 (r7 <r3)。

[0061] In addition, since the diameter r1 of the inner diameter surface 31 is equal to or larger than the outer diameter r3 of the temporary shaft 70 (r3≦r1), it follows that the diameter r1 of the inner diameter surface 31 is also larger than the diameter r7 (r7 <r1)。

[0062] Moreover, the outer diameter r3 of the temporary shaft 70 in this embodiment is larger than the diameter r7 and smaller than the diameter r8 of the inscribed circle Q70. The inscribed circle Q70 is an imaginary circle centered on the center line O1 and is a circle that is tangent to each of the multiple balls 3B from the radially inner side in a state in which the balls 3B are in contact with the inner raceway surface 25. If the outer diameter r3 of the temporary shaft 70 were the same as the diameter r8 of the inscribed circle Q70, the balls 3 would be sandwiched between the temporary shaft 70 and the inner raceway surface 25 with no gap between them.

[0063] On the other hand, the outer diameter r3 of the temporary shaft 70 in this embodiment is smaller than the diameter r8 of the inscribed circle Q70. As a result, the gap (radial gap amount r4) in the open portion of the inner raceway surface 25 is larger than when the outer diameter r3 of the temporary shaft 70 is the same as the diameter r8. This makes it easier to insert the balls 3 into the inner raceway surface 25 in the ball arrangement step S3, improving assembly efficiency.

[0064] As described above, the ball screw device 100 of the first embodiment includes a screw shaft 1 having one end pointing in a first direction X1 and the other end pointing in a second direction X2, and an outer peripheral raceway surface 12 provided on an outer peripheral surface 11, a nut 2 inserted into the screw shaft 1 and having an inner peripheral raceway surface 25 provided on an inner peripheral surface 23, a plurality of balls 3 arranged between the outer peripheral raceway surface 12 and the inner peripheral raceway surface 25, and a circulation section 4 provided on the inner peripheral surface 23 of the nut 2 to circulate the balls 3. The nut 2 has a cylindrical nut body 20 provided with the inner peripheral raceway surface 25, a first opening 21 opening in the first direction X1 from the inside of the nut body 20, a retaining portion 30 protruding radially inward from an end of the inner peripheral surface 23 of the nut body 20 in the second direction X2, and a second opening 22 provided radially inward of the retaining portion 30 and opening in the second direction X2 from the inside of the nut body 20. The retaining portion 30 has an inner diameter surface 31 facing radially inward. A diameter r1 of the inner diameter surface 31 is smaller than a diameter r2 to the outer circumferential surface 11 of the screw shaft 1.

[0065] According to the ball screw device 100 of the first embodiment, the screw shaft 1 is prevented from falling off from the second opening 22 of the nut 2. In addition, the ball screw device 100 can be assembled using the temporary shaft 70. This reduces the labor required for assembly.

[0066] In addition, the assembly method of the ball screw device 100 of embodiment 1 includes a temporary shaft insertion process S1 of inserting a temporary shaft 70 into the second opening 22 of the nut 2, a ball placement process S3 of inserting a ball 3 into the inside of the nut 2 from the first opening 21 of the nut 2 and placing the ball 3 between the outer peripheral surface 71 of the temporary shaft 70 and the inner peripheral raceway surface 25, a screw shaft abutment process S5 of abutting the other end face 6a of the screw shaft 1 in the second direction X2 against the end face 73 of the temporary shaft 70 in the first direction X1, a screw shaft movement process S7 of rotating the screw shaft 1 and the nut 2 relatively to move the screw shaft 1 in the second direction X2, and a removal process S9 of removing the temporary shaft 70 from the nut 2.

[0067] According to the assembly method of the first embodiment, the ball screw device 100 is assembled using the temporary shaft 70, which reduces the labor required for assembly. Furthermore, in the assembled ball screw device 100, the screw shaft 1 is prevented from falling off the second opening 22 of the nut 2.

[0068] The retaining portion 30 in the first embodiment is annular. The inner diameter surface 31 of the retaining portion 30 is circular when viewed from the axial direction. The outer diameter r3 of the temporary shaft 70 is smaller than the diameter r1 of the inner diameter surface 31 of the retaining portion 30.

[0069] According to the above configuration, the temporary shaft 70 inserted into the second opening 22 is disposed coaxially with the nut 2, facilitating positioning of the temporary shaft 70. In addition, the temporary shaft 70 inserted into the second opening 22 can move smoothly in the axial direction.

[0070] Although the first embodiment has been described above, the present disclosure is not limited to the first embodiment. For example, the retaining portion 30 in the first embodiment is annular, but the present disclosure may be a retaining portion formed in a C-shape when viewed from the axial direction. That is, the retaining portion in the present disclosure may protrude from a part of the inner peripheral surface 23 in the circumferential direction. In addition, in the present disclosure, when the retaining portion is not annular, a plurality of retaining portions may be provided. In addition, the diameter r1 of the inner diameter surface 31 is constant in the axial direction, but the present disclosure is not limited to this. For example, the inner diameter surface 31 may be formed in a tapered or stepped shape.

[0071] In addition, in the embodiment, a dummy shaft 70 with an outer diameter r3 smaller than the diameter r8 of the inscribed circle Q70 is used, but the present disclosure is not limited thereto (see FIG. 10). According to the embodiment, since the outer diameter r3 of the dummy shaft 70 is smaller than the diameter r8 of the inscribed circle Q70, the ball 3 moves (rattles) radially between the dummy shaft 70 and the inner peripheral raceway surface 25. As a result, the ball 3 is likely to be caught by the other end surface 6a of the screw shaft 1 in the screw shaft movement step S7. Therefore, in the present disclosure, the outer diameter r3 of the dummy shaft 70 may be the same as the diameter r8 of the inscribed circle Q70. According to this, the ball 3 is held without moving radially between the dummy shaft 70 and the inner peripheral raceway surface 25. That is, since the ball 3 does not rattle, it is possible to avoid the ball 3 being caught by the other end surface 6a of the screw shaft 1 in the screw shaft movement step S7.

[0072] Note that the diameter r8 of the inscribed circle Q70 shown in FIG. 12 is the same (r8 = r9) as the diameter r9 (see FIG. 4) of the groove bottom of the outer peripheral raceway surface 12 of the screw shaft 1 in the present embodiment where the outer peripheral raceway surface 12 has a circular arc shape. Therefore, if the outer diameter r3 of the dummy shaft 70 and the diameter r9 of the groove bottom of the outer peripheral raceway surface 12 are the same (r3 = r9), the ball 3 is held without rattling between the dummy shaft 70 and the inner peripheral raceway surface 25. Therefore, when the outer diameter r3 of the dummy shaft 70 is smaller than the diameter r9 of the groove bottom of the outer peripheral raceway surface 12 (r3 < r9), the gap between the outer peripheral surface 71 of the dummy shaft 70 and the inner peripheral surface 23 of the nut body 20 becomes larger, and the insertion of the ball 3 becomes easier.

[0073] On the other hand, when the outer peripheral raceway surface 12 has a gothic arc shape, the diameter r9 of the groove bottom of the outer peripheral raceway surface 12 is smaller than the diameter r8 of the inscribed circle Q70 (r8 > r9). Therefore, even if the outer diameter r3 of the dummy shaft 70 and the diameter r9 of the groove bottom of the outer peripheral raceway surface 12 are the same (r3 = r9), the ball 3 rattles radially between the dummy shaft 70 and the inner peripheral raceway surface 25. That is, when suppressing the rattling of the ball 3, it is necessary to make the outer diameter r3 of the dummy shaft 70 the same as the diameter r8 of the inscribed circle Q70 (r3 = r8), and when facilitating the insertion of the ball 3, it is necessary to make the outer diameter r3 of the dummy shaft 70 smaller than the diameter r8 of the inscribed circle Q70 (r3 < r8).

[0074] The present disclosure may also be modified as described below. Note that the description of each modified example will focus on the differences from the first embodiment.

[0075] (Variation 1) Fig. 13 is a cross-sectional view showing a nut of a ball screw device of Modification 1. As shown in Fig. 13, the nut 2A of Modification 1 differs from the nut 2 of the embodiment 1 in that an annular part 60 is fitted to the nut 2A. The nut 2A of Modification 1 also differs from the nut 2 of the embodiment 1 in that a concave surface 28 is formed on the outer circumferential surface. Details will be described below.

[0076] The concave surface 28 is provided at an end of the outer circumferential surface 24 of the nut body 20 in the second direction X2. When the nut body 20 is divided by the radial thickness, the nut body 20 has a first nut body 121 in which the concave surface 28 is not formed, and a second nut body 122 in which the concave surface 28 is formed. The second nut body 122 has a smaller radial thickness than the first nut body 121. In other words, the second nut body 122 has lower rigidity than the first nut body 121.

[0077] On the other hand, the retaining portion 30 is provided on the inner circumferential side of the second nut body 122. In other words, the second nut body 122 is reinforced by the retaining portion 30. This prevents the second nut body 122 from being deformed or damaged. Note that a portion of the retaining portion 30 in the first modification is also provided on the inner circumferential side of the first nut body 121. This also reinforces a portion of the first nut body 121.

[0078] The above describes Modification 1. The retaining portion 30 of Modification 1 is provided across the second nut body 122 and the first nut body 121, but the retaining portion 30 of the present disclosure may be provided only on the inner circumferential side of the second nut body 122. Alternatively, the retaining portion 30 may be provided only on a portion of the axial direction of the second nut body 122. In other words, when at least a portion of the retaining portion 30 is disposed on the inner circumferential side of the second nut body 122, the second nut body 122 can be reinforced.

[0079] Further, examples of the type of the annular part 60 that is fitted to the nut 2A include a gear, a pulley, and a piston, but the present disclosure is not limited to these examples.

[0080] (Variation 2) Fig. 14 is a diagram showing a screw shaft abutting step using a temporary shaft of Modification 2. As shown in Fig. 14, the screw shaft 1B of Modification 2 differs from the screw shaft 1 of Embodiment 1 in that, in addition to the screw shaft body 10, it has a protruding portion 15 that protrudes in the second direction X2 from the other end face 6a of the screw shaft body 10. The protruding portion 15 is cylindrical and centered on the center line O1, and has a smaller diameter than the screw shaft body 10.

[0081] Also, the temporary shaft 70B of the second modification is different from the temporary shaft 70 of the first embodiment in that it is cylindrical (tubular). The inner diameter of the temporary shaft 70 is the same as the outer diameter of the protruding portion 15. When using this temporary shaft 70B, as shown in FIG. 14, in the screw shaft abutting step S5, the protruding portion 15 is inserted into the temporary shaft 70. As a result, the protruding portion 15 is fitted into the temporary shaft 70B, and the screw shaft 1B and the temporary shaft 70B are reliably arranged coaxially. As a result, it is possible to prevent the screw shaft 1B from being displaced radially with respect to the center line O2 of the nut 2. In other words, it is possible to prevent the ball 3 arranged on the inner peripheral raceway surface 25 from being caught on the other end surface 6a of the screw shaft 1B in the screw shaft moving step S7.

[0082] In addition, the protrusion 15 of the screw shaft 1B may have a function as a power transmission part that transmits power to the screw shaft 1B, a pressing part that presses other parts, etc., in addition to the function of a positioning part with respect to the temporary shaft 70B.

[0083] (Variation 3) FIG. 15 is a cross-sectional view showing a nut of a ball screw device of Modification 1. As shown in FIG. 15, the nut 2C of Modification 3 is different from the first embodiment in that a screw groove surface 29 extending from an end in the first direction X1 to an end in the second direction X2 is formed on the inner peripheral surface 23 of the nut body 20C. Also, the nut body 20C is different from the first embodiment in that a plurality of recesses 42 are provided on the inner peripheral surface 23, and a top (not shown) is arranged in the recesses 42. That is, the circulation section 4 of Modification 3 uses a top. Also, a part of the screw groove surface 29 constitutes the inner peripheral raceway surface 25. The screw groove surface 29 is also formed in the retaining portion 30C. Therefore, the inner diameter surface 31C is spiral. As described above, even in the retaining portion 30C of Modification 3, the second opening 22 is made small in diameter. Therefore, it is possible to prevent the screw shaft 1 from falling off from the second opening 22.

[0084] The ball screw device of the present disclosure may be used so that the nut moves linearly due to the rotational motion of the screw shaft, or so that the screw shaft moves linearly due to the rotational motion of the nut, and there is no particular restriction on the method of use. In addition, the above-mentioned "same" includes not only completely same things, but also things that are considered to be substantially "same" due to tolerances (the range of general manufacturing errors).

[0085] The present disclosure may also be implemented in the following combinations: (1) A screw shaft having one end pointing in a first direction and the other end pointing in a second direction, and an outer circumferential surface provided with an outer circumferential raceway; a nut into which the screw shaft is inserted and having an inner circumferential raceway on its inner circumferential surface; a plurality of balls disposed between the outer circumferential raceway surface and the inner circumferential raceway surface; a circulation portion provided on the inner circumferential surface of the nut to circulate the balls; Equipped with The nut is A cylindrical nut body provided with the inner circumferential raceway surface; A first opening that opens in the first direction from the inside of the nut body; A retaining portion protruding radially inward from an end portion in the second direction of the inner peripheral surface of the nut body; A second opening is provided radially inside the retaining portion and opens in the second direction from the inside of the nut body; having The retaining portion has an inner diameter surface facing radially inward, The diameter of the inner diameter surface is smaller than the diameter of the outer diameter surface of the screw shaft. Ball screw device. (2) The diameter of the inner diameter surface is larger than the diameter of the inner peripheral surface of the nut body minus the two diameters. A ball screw device as described in (1). (3) The retaining portion is annular, The inner diameter surface of the retaining portion is circular when viewed from an axial direction parallel to the center line of the screw shaft. A ball screw device as described in (1) or (2). (4) The nut has a concave surface recessed radially inward from the outer circumferential surface of the nut body, The nut body is A first nut body; A second nut body having the concave surface and an outer diameter smaller than the outer diameter of the first nut body; having At least a portion of the retaining portion is provided on the inner peripheral side of the second nut body. A ball screw device according to any one of (1) to (3). (5) The inner circumferential surface of the nut body is formed with a thread groove surface that extends from the end in the first direction to the end in the second direction and a part of which constitutes the inner circumferential raceway surface, The retaining portion is provided with the thread groove surface. A ball screw device according to any one of (1) to (4). (6) A method for assembling the ball screw device according to (1), A temporary shaft inserting step of inserting a temporary shaft into the second opening of the nut; a ball placement step of inserting the ball into the nut through the first opening of the nut and placing the ball between an outer peripheral surface of the temporary shaft and the inner peripheral raceway surface; A screw shaft abutting process in which an end surface of the screw shaft in the second direction is abutted against an end surface of the temporary shaft in the first direction; A screw shaft moving process in which the screw shaft and the nut are rotated relative to each other to move the screw shaft in the second direction; a removing step of removing the false shaft from the nut; A method for assembling a ball screw device comprising: (7) an inscribed circle is a virtual circle having a center on the center line of the screw shaft when viewed from an axial direction parallel to the center line of the screw shaft, and the circle is tangent to each of the plurality of balls arranged inside the inner circumferential raceway surface from the radially inner side; The outer diameter of the provisional shaft is smaller than the diameter of the inscribed circle. A method for assembling the ball screw device described in (6). (8) an inscribed circle is a virtual circle having a center on the center line of the screw shaft when viewed from an axial direction parallel to the center line of the screw shaft, and the circle is tangent to each of the plurality of balls arranged inside the inner circumferential raceway surface from the radially inner side; The outer diameter of the provisional shaft is the same as the diameter of the inscribed circle. A method for assembling the ball screw device described in (6). (9) The retaining portion is annular, The inner diameter surface of the retaining portion is circular when viewed in the axial direction. The outer diameter of the temporary shaft is smaller than the diameter of the inner diameter surface of the retaining portion. A method for assembling a ball screw device according to any one of (6) to (8). (10) The pseudocoil is cylindrical; The screw shaft is A screw shaft body provided with the outer circumferential raceway surface; A protruding portion protruding from the screw shaft main body in the second direction; having The outer diameter of the protruding portion is the same as the inner diameter of the temporary shaft, The screw shaft abutting step includes inserting the protrusion into the temporary shaft. A method for assembling a ball screw device according to any one of (6) to (9). [Explanation of symbols]

[0086] 100 Ball screw device 1, 1B Screw shaft 2, 2A Nut 3. Ball 4 Circulation section 5 One end 6 Other end 7 orbit 11 Outer surface 12 Outer raceway surface 15 Protrusion 20, 20C Nut body 21 First opening 22 Second Opening 23 Inner surface 24 Outer surface 25 Inner raceway surface 28 Concave 29 Thread groove surface 30, 30C Stopper 31, 31C Inner surface 40 S-shaped groove surface 60 Annular parts 70, 70B temporary shaft 121 First nut body 122 Second nut body

Claims

1. A screw shaft having one end pointing in a first direction and the other end pointing in a second direction, and an outer circumferential surface provided with an outer circumferential raceway; a nut into which the screw shaft is inserted and having an inner circumferential raceway on its inner circumferential surface; a plurality of balls disposed between the outer circumferential raceway surface and the inner circumferential raceway surface; a circulation portion provided on the inner circumferential surface of the nut to circulate the balls; Equipped with The nut is A cylindrical nut body provided with the inner circumferential raceway surface; A first opening that opens in the first direction from the inside of the nut body; A retaining portion protruding radially inward from an end portion of the inner circumferential surface of the nut body in the second direction; A second opening is provided radially inside the retaining portion and opens from the inside of the nut body in the second direction; having The retaining portion has an inner diameter surface facing radially inward, The diameter of the inner diameter surface is smaller than the diameter of the outer diameter surface of the screw shaft. Ball screw device.

2. The diameter of the inner diameter surface is larger than the diameter of the inner circumferential surface of the nut body minus the diameter of two of the balls. The ball screw device according to claim 1 .

3. The retaining portion is annular, The inner diameter surface of the retaining portion is circular when viewed from an axial direction parallel to the center line of the screw shaft. The ball screw device according to claim 1 or 2.

4. The nut has a concave surface recessed radially inward from the outer circumferential surface of the nut body, The nut body is A first nut body; A second nut body having the concave surface and an outer diameter smaller than the outer diameter of the first nut body; having At least a portion of the retaining portion is provided on the inner peripheral side of the second nut body. The ball screw device according to claim 3.

5. A thread groove surface is formed on the inner peripheral surface of the nut body, the thread groove surface extending from the end in the first direction to the end in the second direction and a portion of the thread groove surface constituting the inner peripheral raceway surface, The retaining portion is provided with the thread groove surface. The ball screw device according to claim 1 or 2.

6. A method for assembling the ball screw device according to claim 1, comprising the steps of: A temporary shaft inserting step of inserting a temporary shaft into the second opening of the nut; a ball placement step of inserting the ball into the nut through the first opening of the nut and placing the ball between an outer peripheral surface of the temporary shaft and the inner peripheral raceway surface; A screw shaft abutting process in which an end surface of the screw shaft in the second direction is abutted against an end surface of the temporary shaft in the first direction; A screw shaft moving process in which the screw shaft and the nut are rotated relative to each other to move the screw shaft in the second direction; a removing step of removing the false shaft from the nut; A method for assembling a ball screw device comprising:

7. an inscribed circle is a virtual circle having a center on the center line of the screw shaft when viewed from an axial direction parallel to the center line of the screw shaft, and the circle is tangent to each of the plurality of balls arranged inside the inner circumferential raceway surface from the radially inner side; The outer diameter of the provisional shaft is smaller than the diameter of the inscribed circle. A method for assembling the ball screw device according to claim 6.

8. an inscribed circle is a virtual circle having a center on the center line of the screw shaft when viewed from an axial direction parallel to the center line of the screw shaft, and the circle is tangent to each of the plurality of balls arranged inside the inner circumferential raceway surface from the radially inner side; The outer diameter of the provisional shaft is the same as the diameter of the inscribed circle. A method for assembling the ball screw device according to claim 6.

9. The retaining portion is annular, The inner diameter surface of the retaining portion is circular when viewed in the axial direction. The outer diameter of the temporary shaft is smaller than the diameter of the inner diameter surface of the retaining portion. A method for assembling a ball screw device according to claim 7 or 8.

10. The pseudocoil is cylindrical; The screw shaft is A screw shaft body provided with the outer circumferential raceway surface; A protruding portion protruding from the screw shaft main body in the second direction; having The outer diameter of the protruding portion is the same as the inner diameter of the temporary shaft, The screw shaft abutting step includes inserting the protrusion into the temporary shaft. A method for assembling a ball screw device according to claim 7 or 8.

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