Ball screw

Abstract

To provide a ball screw capable pf suppressing the occurrence of jamming in balls when moving from a circulation groove of a circulation piece to a rolling path between a screw shaft and a nut.SOLUTION: The ball screw includes a screw shaft 2 having a screw groove 23 and an opening 24, a circulation piece 3 arranged in the opening 24 and having a circulation groove 33, a nut having a screw groove, and a plurality of balls for rolling in the rolling path consisting of the screw groove 23 of the screw shaft 2 and the screw groove of the nut and in the circulation groove 33. A side face 25 of the opening 24 includes a pair of regions 25a where one end 23a and the other end 23b of the screw groove 23 are opened. The side face 31 of the circulation piece 3 includes a pair of regions 31a corresponding to the pair of regions 25a, where one end 33a and the other end 33b of the circulation groove 33 are opened. A distance D1 between the pair of regions 25a is longer as extending apart from a center line CL of the screw shaft 2 to the outside in the radial direction.SELECTED DRAWING: Figure 4

Description

[Technical field]

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

[0002] In a ball screw, a circulating top is sometimes used to circulate multiple balls in the rolling path between the screw shaft and the nut. The circulating top is generally provided on the nut, but may be provided on the screw shaft when a large thrust is required or when the screw shaft is rotated to linearly move the nut (for example, see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6302301 Summary of the Invention [Problem to be solved by the invention]

[0004] In a ball screw as described above, when a ball moves from the circulation groove of the circulating top to the rolling path between the screw shaft and the nut, the ball becomes pinched between the portion of the screw shaft near the groove shoulder and the portion of the nut near the groove shoulder, causing a blockage. As a result, noise and vibrations may be generated and the smooth circulation of the balls may be hindered.

[0005] Therefore, an object of the present invention is to provide a ball screw that can prevent clogging when balls move from the circulation groove of the circulating top to the rolling path between the screw shaft and the nut. [Means for solving the problem]

[0006] The ball screw of the present invention is [1] "a ball screw comprising: a threaded shaft having a first thread groove, and an opening between one end of the first thread groove and the other end of the first thread groove that is open at least radially outward; a circulating top disposed within the opening and having a circulation groove connected to each of the one end and the other end of the first thread groove; a nut having a second thread groove; a rolling path formed by the first thread groove and the second thread groove; and a plurality of balls rolling in the circulation groove, wherein an inner surface of the opening includes a pair of first regions where the one end and the other end of the first thread groove are open, an outer surface of the circulating top includes a pair of second regions corresponding to the pair of first regions and where the one end and the other end of the circulation groove are open, and a distance between the pair of first regions increases with increasing distance from the center line of the threaded shaft to the outside in the radial direction."

[0007] In the ball screw described in [1] above, the circulating top is disposed in the opening of the screw shaft, and a pair of second regions on the outer surface of the circulating top, where one end and the other end of the circulating groove are open, correspond to a pair of first regions on the inner surface of the opening, where one end and the other end of the first screw groove of the screw shaft are open. The distance between the pair of first regions increases as it moves radially outward from the center line of the screw shaft. As a result, when the ball moves from the circulating groove of the circulating top to the rolling path between the screw shaft and the nut, the ball is likely to first come into contact with a portion near the groove bottom of the first screw groove of the screw shaft, so that the ball is less likely to be pinched between a portion near the groove shoulder of the screw shaft and a portion near the groove shoulder of the nut. Therefore, according to the ball screw described in [1] above, clogging can be suppressed when the ball moves from the circulating groove of the circulating top to the rolling path between the screw shaft and the nut.

[0008] The ball screw of the present invention may be [2] "the ball screw according to the above [1], in which, when viewed from the axial direction, an intersection of a straight line tangent to one of the pair of first regions at the one end of the first thread groove and a straight line tangent to the other of the pair of first regions at the other end of the first thread groove is located between the center line of the screw shaft and the circulating top." According to the ball screw according to [2], when the ball moves from the circulation groove of the circulating top to the rolling path between the screw shaft and the nut, it is possible to reliably realize a movement in which the ball first comes into contact with a portion of the first thread groove of the screw shaft near the groove bottom.

[0009] The ball screw of the present invention may be [3] "the ball screw according to the above [2], in which the distance between the center line of the screw shaft and the intersection point when viewed from the axial direction is 1 / 3 or more of the radius of the screw shaft." According to the ball screw according to [3], when the ball moves from the circulation groove of the circulating top to the rolling path between the screw shaft and the nut, it is possible to more reliably realize the movement in which the ball first comes into contact with a portion of the screw shaft near the groove bottom of the first screw groove.

[0010] The ball screw of the present invention may be [4] "the ball screw according to the above [2] or [3], in which each of the pair of first regions is a flat surface with which the straight line makes line contact." Alternatively, the ball screw of the present invention may be [5] "the ball screw according to the above [2] or [3], in which each of the pair of first regions is a curved surface with which the straight line makes point contact."

[0011] The ball screw of the present invention may be [6] "the ball screw according to any one of the above [1] to [5], in which, when viewed from the helical direction of the first screw groove, the inner surface of the one end of the first screw groove is located inside the inner surface of the one end of the circulation groove, and when viewed from the helical direction, the inner surface of the other end of the first screw groove is located inside the inner surface of the other end of the circulation groove." According to the ball screw according to [6], it is possible to prevent one end and the other end of the circulation groove from becoming narrower than one end and the other end of the first screw groove due to a difference in thermal expansion coefficient between the circulation top and the screw shaft, and therefore it is possible to suppress clogging of balls between the circulation groove and the rolling path. In particular, when the configuration described in [6] is adopted in a conventional ball screw, the corner between the inner surface of one end of the first screw groove and one of the first regions of the inner surface of the opening is exposed to one end of the circulation groove, and the corner between the inner surface of the other end of the first screw groove and the other of the first regions of the inner surface of the opening is exposed to the other end of the circulation groove, so that when the ball moves from the circulation groove of the circulation top to the rolling path between the screw shaft and the nut, the ball is likely to be pinched between the part near the groove shoulder of the screw shaft and the part near the groove shoulder of the nut. Therefore, the configuration described in [1] above is extremely effective in suppressing clogging of the ball. Effect of the Invention

[0012] According to the present invention, it is possible to provide a ball screw that can suppress clogging when balls move from the circulation groove of the circulating top to the rolling path between the screw shaft and the nut. [Brief description of the drawings]

[0013] [Figure 1] FIG. 2 is a side view of a ball screw according to an embodiment. [Diagram 2] 2 is a side view of a portion of the screw shaft and the circulating top shown in FIG. 1, and a side view of a portion of the screw shaft shown in FIG. [Diagram 3] 3A is a cross-sectional view of a portion of the screw shaft and the circulating top taken along line aa in FIG. 2A, and a cross-sectional view of a portion of the screw shaft and the circulating top taken along line bb in FIG. 2A. [Figure 4] 2 is a front view of a portion of the screw shaft and the circulating top shown in FIG. 1. [Diagram 5] FIG. 2 is a perspective view of the circulating top shown in FIG. [Figure 6] 5A is a schematic diagram of a cross section of a portion of a ball screw of a comparative example cut along the groove bottom of a screw groove of a screw shaft, and FIG. 5B is a cross-sectional view of a portion of a ball screw of a comparative example taken along line cc shown in FIG. [Figure 7] 2A is a schematic diagram of a cross section of a portion of the ball screw of the embodiment shown in FIG. 1 cut along the groove bottom of the screw groove of the screw shaft, and FIG. 2B is a cross-sectional view of a portion of the ball screw of the embodiment taken along line dd shown in FIG. [Figure 8] FIG. 13 is a front view of a screw shaft and a portion of a circulating top of a modified ball screw. [Figure 9] FIG. 13 is a front view of a screw shaft and a portion of a circulating top of a modified ball screw. [Figure 10] FIG. 13 is a side view of a portion of a screw shaft and a circulating top of a modified ball screw. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and duplicated explanations will be omitted.

[0015] As shown in Fig. 1, the ball screw 1 includes a screw shaft 2, a plurality of circulating pieces 3, a nut 4, and a plurality of balls 5. In the following description, the axial direction is a direction parallel to the center line CL of the screw shaft 2, the radial direction is a direction perpendicular to the center line CL, and the circumferential direction is a direction along a circumference centered on the center line CL when viewed from a direction parallel to the center line CL. Note that only the nut 4 is shown in cross section in Fig. 1.

[0016] The screw shaft 2 has a shaft portion 21 and an expanded diameter portion 22. For example, a spline or serration is formed on the end of the shaft portion 21 opposite to the expanded diameter portion 22. A plurality of screw grooves (first screw grooves) 23 are formed on the outer circumferential surface of the expanded diameter portion 22. The plurality of screw grooves 23 extend in a spiral shape with the same pitch as a screw groove (second screw groove) 41 of the nut 4 described later. Each screw groove 23 has a length of, for example, approximately one lead. Strictly speaking, the length of each screw groove 23 is shorter than the length of one lead by the length of an opening 24 described later. A screw groove 41 is formed on the inner circumferential surface of the nut 4. The screw groove 41 extends in a spiral shape. The plurality of screw grooves 23 of the screw shaft 2 and the screw groove 41 of the nut 4 constitute a plurality of rolling paths 10.

[0017] A plurality of openings 24 are further formed on the outer peripheral surface of the enlarged diameter portion 22. The plurality of openings 24 correspond to the plurality of screw grooves 23. When viewed from the axial direction, the plurality of openings 24 are arranged at different positions in the circumferential direction (e.g., at equal angular intervals). Each circulating top 3 is disposed within each opening 24. Therefore, when viewed from the axial direction, the plurality of circulating tops 3 are arranged at different positions in the circumferential direction (e.g., at equal angular intervals). In this embodiment, the screw shaft 2 is a solid shaft, and each opening 24 is a recess that opens to the outside in the radial direction.

[0018] As shown in (a) and (b) of FIG. 2, when focusing on the corresponding screw groove 23 and the opening 24, the opening 24 opens radially outward between one end 23a of the screw groove 23 and the other end 23b of the screw groove 23. As shown in (a) of FIG. 2, a circulation groove 33 is formed on the outer circumferential surface of the circulating top 3 arranged in the opening 24. The circulation groove 33 is connected to each of the one end 23a and the other end 23b of the screw groove 23. More specifically, one end 33a of the circulation groove 33 is connected to the one end 23a of the screw groove 23, and the other end 33b of the circulation groove 33 is connected to the other end 23b of the screw groove 23. A pair of protrusions 34 is provided on the outer circumferential surface of the circulating top 3 so as to sandwich the circulation groove 33. The pair of protrusions 34 move relatively along the screw groove 41 in the screw groove 41 of the nut 4.

[0019] As shown in (a) of FIG. 3, when viewed from the helical direction A, the inner surface of one end 23a of the screw groove 23 is located inside the inner surface of one end 33a of the circulation groove 33. That is, the width of one end 23a of the screw groove 23 is smaller than the width of one end 33a of the circulation groove 33, and the depth of one end 23a of the screw groove 23 is smaller than the depth of one end 33a of the circulation groove 33. As shown in (b) of FIG. 3, when viewed from the helical direction A, the inner surface of the other end 23b of the screw groove 23 is located inside the inner surface of the other end 33b of the circulation groove 33. That is, the width of the other end 23b of the screw groove 23 is smaller than the width of the other end 33b of the circulation groove 33, and the depth of the other end 23b of the screw groove 23 is smaller than the depth of the other end 33b of the circulation groove 33. The helical direction A is the helical direction of the screw groove 23, and is the direction in which the screw groove 23 extends. In (a) and (b) of FIG. 3, the spiral direction A is perpendicular to the plane of the paper.

[0020] As shown in Fig. 1, the plurality of balls 5 roll in the corresponding rolling paths 10 and circulation grooves 33. When passing through the circulation grooves 33, the plurality of balls 5 pass over the threads (land portions) on the inner peripheral surface of the nut 4, and thereby circulate infinitely in each rolling path 10. Note that while only some of the balls 5 are shown in Fig. 1, in reality, the plurality of balls 5 are lined up along the rolling paths 10 and circulation grooves 33 indicated by the two-dot chain lines.

[0021] As shown in FIG. 2B, the opening 24 has a side surface (inner surface) 25 and a bottom surface 26. In this embodiment, the bottom surface 26 is a plane perpendicular to a "straight line L passing through the center of the opening 24 and perpendicular to the center line CL." The side surface 25 includes a pair of regions (first regions) 25a and a pair of regions 25b. The pair of regions 25a face each other in the circumferential direction. The pair of regions 25b face each other in the axial direction. In the pair of regions 25a, one end 23a and the other end 23b of the screw groove 23 open in the helical direction A. In this embodiment, each of the pair of regions 25a is a plane parallel to the axial direction, and each of the pair of regions 25b is a plane perpendicular to the axial direction. In FIG. 2B, the straight line L is a straight line perpendicular to the paper surface.

[0022] As shown in FIG. 4, the distance D1 between the pair of regions 25a increases as it moves away from the center line CL of the screw shaft 2 in the radial direction. In this embodiment, one of the pair of regions 25a is a plane inclined to one side with respect to the straight line L and parallel to the axial direction, and the other of the pair of regions 25a is a plane inclined to the other side with respect to the straight line L and parallel to the axial direction. The inclination angles of the pair of regions 25a with respect to the straight line L are equal to each other. The distance D1 between the pair of regions 25a means the "distance between the pair of regions 25a" when viewed from the axial direction, and means the "distance between the pair of regions 25a" parallel to the straight line connecting the center of one end 23a of the screw groove 23 and the center of the other end 23b of the screw groove 23.

[0023] When viewed from the axial direction, an intersection P between a straight line TL tangent to one of the pair of regions 25a at one end 23a of the screw groove 23 and a straight line TL tangent to the other of the pair of regions 25a at the other end 23b of the screw groove 23 is located between the center line CL and the circulating top 3. When viewed from the axial direction, a distance D2 between the center line CL and the intersection P is equal to or greater than 1 / 3 of the radius R of the screw shaft 2 (the distance from the center line CL to the outer circumferential surface of the thread of the screw shaft 2 when viewed from the axial direction). In this embodiment, the inclination angles of the pair of regions 25a with respect to the straight line L are equal to each other (in other words, the inclination angles of the pair of straight lines TL with respect to the straight line L are equal to each other), so the intersection P is located on the straight line L.

[0024] The straight line TL tangent to the region 25a at one end 23a of the screw groove 23 means a straight line tangent to the line indicating the region 25a at the center of one end 23a of the screw groove 23 in the "cross section passing through the center of the one end 23a of the screw groove 23 and perpendicular to the axial direction". In this embodiment, the line indicating the region 25a in the "cross section passing through the center of the one end 23a of the screw groove 23 and perpendicular to the axial direction" is a straight line, so the straight line TL includes the straight line (coincides with the straight line). In addition, the straight line TL tangent to the region 25a at the other end 23b of the screw groove 23 means a straight line tangent to the line indicating the region 25a at the center of the other end 23b of the screw groove 23 in the "cross section passing through the center of the other end 23b of the screw groove 23 and perpendicular to the axial direction". In this embodiment, the line indicating the region 25a in the "cross section passing through the center of the other end 23b of the screw groove 23 and perpendicular to the axial direction" is a straight line, so the straight line TL includes the straight line (coincides with the straight line). In other words, in this embodiment, each of the pair of regions 25a is a plane with which the straight line TL makes line contact.

[0025] As shown in FIG. 4 and FIG. 5, the circulation top 3 has a side surface (outer surface) 31 and a bottom surface 32. The side surface 31 includes a pair of regions (second regions) 31a and a pair of regions 31b. In the pair of regions 31a, one end 33a and the other end 33b of the circulation groove 33 are open in the spiral direction A. As shown in FIG. 4, the pair of regions 31a correspond to the pair of regions 25a. Note that, "one region (or surface) corresponds to another region (or surface)" means that the one region (or surface) is substantially in contact with the other region (or surface), that the one region (or surface) faces the other region (or surface) through a small gap, that the one region (or surface) faces the other region (or surface) through an adhesive, etc. In this embodiment, the bottom surface 32 of the circulation top 3 corresponds to the bottom surface 26 of the opening 24, and the side surface 31 of the circulation top 3 corresponds to the side surface 25 of the opening 24. As an example, the circulating top 3 is an integrally molded product made of resin or metal.

[0026] Here, the movement of the ball 5 in the ball screw 100 of the comparative example and the movement of the ball 5 in the ball screw 1 of the embodiment will be described in detail with reference to Fig. 6 and Fig. 7. Hereinafter, the movement of the ball 5 when the ball 5 moves from the other end 33b of the circulation groove 33 to the other end 23b of the screw groove 23 will be described as the movement of the ball 5 when the ball 5 moves from the circulation groove 33 of the circulation top 3 to the rolling path 10 between the screw shaft 2 and the nut 4, but the movement of the ball 5 when the ball 5 moves from one end 33a of the circulation groove 33 to one end 23a of the screw groove 23 is similar to that. Note that in Fig. 6(a) and Fig. 7(a), only one ball 5 is illustrated by a two-dot chain line, but in reality, a plurality of balls 5 are lined up along the rolling path 10 and the circulation groove 33.

[0027] As shown in (a) of FIG. 6, the ball screw 100 of the comparative example differs from the ball screw 1 of the embodiment, which is the ball screw 1 described above, in that the distance D1 between the pair of regions 25a is constant even when they are separated radially outward. In the ball screw 100 of the comparative example, since the distance D1 between the pair of regions 25a is constant, in the helical direction A, the groove shoulder 232 of the other end 23b of the screw groove 23 is located on the side of the ball 5 when the entire ball 5 is located in the circulation groove 33, with respect to the groove bottom 231 of the other end 23b of the screw groove 23. Therefore, the ball 5 moving from the other end 33b of the circulation groove 33 to the other end 23b of the screw groove 23 is likely to come into contact with a portion near the groove shoulder 232 of the other end 23b of the screw groove 23 before coming into contact with a portion near the groove bottom 231 of the other end 23b of the screw groove 23. 6(a) shows a state in which the ball 5 first comes into contact with a portion of the other end 23b of the thread groove 23 near the groove shoulder 232. In this state, the ball 5 is away from a portion of the other end 23b of the thread groove 23 near the groove bottom 231.

[0028] As shown in (b) of Figure 6, when a thrust force acts on the nut 4 and the relative position between the screw groove 41 of the nut 4 and the screw groove 23 of the screw shaft 2 is misaligned (here, as shown by the white arrow), when the ball 5 moving from the other end 33b of the circulation groove 33 to the other end 23b of the screw groove 23 contacts a portion near the groove shoulder 232 before contacting a portion near the groove bottom 231 at the other end 23b of the screw groove 23, the directions of the load acting on the ball 5 from the portion near the groove shoulder 232 of the screw shaft 2 and the portion near the groove shoulder 411 of the nut 4 (here, the direction shown by the black arrow) become approximately the same, so that the ball 5 is less likely to escape into the space on the opposite side of the inner region of the screw groove 41 from the portion near the groove shoulder 411 with which the ball 5 is in contact. Therefore, the ball 5 is more likely to be pinched by the portion near the groove shoulder 232 of the screw shaft 2 and the portion near the groove shoulder 411 of the nut 4, which are located diagonally across the rolling path 10, and the movement of the ball 5 is more likely to be clogged. In this case, if the groove shoulder 411 of the thread groove 41 of the nut 4 does not have a chamfered surface, the ball 5 is more likely to be pinched by the portion near the groove shoulder 232 of the screw shaft 2 and the portion near the groove shoulder 411 of the nut 4, which are located diagonally across the rolling path 10, and the movement of the ball 5 is more likely to be clogged. Note that the portion near the groove shoulder 232 of the screw shaft 2 means the groove shoulder 232 and the inner surface (groove raceway surface) of the thread groove 23 close to the groove shoulder 232, and the portion near the groove shoulder 411 of the nut 4 means the groove shoulder 411 and the inner surface (groove raceway surface) of the thread groove 41 close to the groove shoulder 411.

[0029] In contrast, in the ball screw 1 of the embodiment, as shown in FIG. 7(a), the distance D1 between the pair of regions 25a increases as they move outward in the radial direction. Therefore, in the helical direction A, the groove bottom 231 of the other end 23b of the screw groove 23 is located on the side of the ball 5 when the entire ball 5 is located in the circulation groove 33, with respect to the groove shoulder 232 of the other end 23b of the screw groove 23. Therefore, the ball 5 moving from the other end 33b of the circulation groove 33 to the other end 23b of the screw groove 23 is likely to contact a portion near the groove bottom 231 of the other end 23b of the screw groove 23 before contacting a portion near the groove shoulder 232 of the other end 23b of the screw groove 23. FIG. 7(a) shows a state in which the ball 5 first contacts a portion near the groove bottom 231 of the other end 23b of the screw groove 23. In this state, the ball 5 is away from a portion near the groove shoulder 232 of the other end 23b of the screw groove 23.

[0030] As shown in (b) of Figure 7, when a thrust force acts on the nut 4 and the relative positions of the screw groove 41 of the nut 4 and the screw groove 23 of the screw shaft 2 are misaligned (here, as shown by the white arrow), when the ball 5 moving from the other end 33b of the circulation groove 33 to the other end 23b of the screw groove 23 contacts a portion near the groove bottom 231 before contacting a portion near the groove shoulder 232 at the other end 23b of the screw groove 23, the directions of the loads acting instantaneously on the ball 5 from the portion near the groove bottom 231 of the screw shaft 2 and the portion near the groove shoulder 411 of the nut 4 are different (here, the directions shown by the black arrows), and the resultant force of these loads makes it easier for the ball 5 to escape to the space on the opposite side of the inner region of the screw groove 41 from the portion near the groove shoulder 411 with which the ball 5 is in contact. Therefore, the ball 5 is less likely to be pinched by the portion near the groove shoulder 232 of the screw shaft 2 and the portion near the groove shoulder 411 of the nut 4, which are located diagonally across the rolling path 10, and the movement of the ball 5 is less likely to be clogged. At this time, even if a chamfered surface is not formed on the groove shoulder 411 of the thread groove 41 of the nut 4, the state in which the ball 5 is less likely to be pinched by the portion near the groove shoulder 232 of the screw shaft 2 and the portion near the groove shoulder 411 of the nut 4, which are located diagonally across the rolling path 10, and the movement of the ball 5 is less likely to be clogged is maintained.

[0031] As described above, in the ball screw 1, the circulation top 3 is disposed in the opening 24 of the screw shaft 2, and a pair of regions 31a on the side surface 31 of the circulation top 3, where one end 33a and the other end 33b of the circulation groove 33 are open, correspond to a pair of regions 25a on the side surface 25 of the opening 24, where one end 23a and the other end 23b of the screw groove 23 of the screw shaft 2 are open. The distance D1 between the pair of regions 25a becomes larger as it moves radially outward from the center line CL of the screw shaft 2. As a result, when the ball 5 moves from the circulation groove 33 of the circulation top 3 to the rolling path 10 between the screw shaft 2 and the nut 4, the ball 5 is likely to first come into contact with the portion near the groove bottom 231 of the screw groove 23 of the screw shaft 2, and therefore the ball 5 is less likely to be pinched between the portion near the groove shoulder 232 of the screw shaft 2 and the portion near the groove shoulder 411 of the nut 4. Therefore, according to the ball screw 1, when the balls 5 move from the circulation groove 33 of the circulating top 3 to the rolling path 10 between the screw shaft 2 and the nut 4, clogging can be suppressed.

[0032] Even if the chamfered surface formed on the groove shoulder 411 of the thread groove 41 of the nut 4 is small or no chamfered surface is formed on the groove shoulder 411 of the thread groove 41 of the nut 4, in the ball screw 1, when the ball 5 moves from the circulation groove 33 of the circulating top 3 to the rolling path 10 between the screw shaft 2 and the nut 4, the ball 5 is unlikely to be pinched between the portion near the groove shoulder 232 of the screw shaft 2 and the portion near the groove shoulder 411 of the nut 4. Therefore, according to the ball screw 1, the degree of freedom in designing the groove shoulder 411 of the thread groove 41 of the nut 4 can be improved.

[0033] In the ball screw 1, when viewed from the axial direction, an intersection P between a straight line TL tangent to one of the pair of regions 25a at one end 23a of the screw groove 23 and a straight line TL tangent to the other of the pair of regions 25a at the other end 23b of the screw groove 23 is located between the center line CL of the screw shaft 2 and the circulating top 3. This ensures that when the balls 5 move from the circulation groove 33 of the circulating top 3 to the rolling path 10 between the screw shaft 2 and the nut 4, the balls 5 first come into contact with a portion near the groove bottom 231 of the screw groove 23 of the screw shaft 2.

[0034] In the ball screw 1, when viewed from the axial direction, the distance D2 between the center line CL of the screw shaft 2 and the intersection point P is 1 / 3 or more of the radius R of the screw shaft 2. This makes it possible to more reliably realize the movement of the ball 5 first contacting a portion near the groove bottom 231 of the screw groove 23 of the screw shaft 2 when moving from the circulation groove 33 of the circulating top 3 to the rolling path 10 between the screw shaft 2 and the nut 4. Note that, in order to realize the movement of the ball 5 first contacting a portion near the groove bottom 231 of the screw groove 23 of the screw shaft 2, it is more preferable that the distance D2 between the center line CL of the screw shaft 2 and the intersection point P is 1 / 2 or more of the radius R of the screw shaft 2 when viewed from the axial direction.

[0035] In the ball screw 1, when viewed from the helical direction A, the inner surface of one end 23a of the screw groove 23 is located inside the inner surface of one end 33a of the circulation groove 33, and the inner surface of the other end 23b of the screw groove 23 is located inside the inner surface of the other end 33b of the circulation groove 33. This makes it possible to prevent the one end 33a and the other end 33b of the circulation groove 33 from becoming narrower than the one end 23a and the other end 23b of the screw groove 23 due to the difference in thermal expansion coefficient between the circulation top 3 and the screw shaft 2, thereby making it possible to suppress clogging of the balls 5 between the circulation groove 33 and the rolling path 10. In particular, when this configuration is adopted in the ball screw 100 of the comparative example, a corner between the inner surface of one end 23a of the screw groove 23 and one region 25a is exposed to one end 33a of the circulation groove 33, and a corner between the inner surface of the other end 23b of the screw groove 23 and the other region 25a is exposed to the other end 33b of the circulation groove 33, so that when the ball 5 moves from the circulation groove 33 of the circulation top 3 to the rolling path 10 between the screw shaft 2 and the nut 4, the ball 5 is likely to be pinched between a portion near the groove shoulder 232 of the screw shaft 2 and a portion near the groove shoulder 411 of the nut 4. Therefore, the configuration of the ball screw 1 in which the distance D1 between the pair of regions 25a increases as it moves away from the center line CL of the screw shaft 2 toward the outside in the radial direction is extremely effective in suppressing clogging of the ball 5.

[0036] The present invention is not limited to the above-mentioned embodiment. For example, as long as the distance D1 between the pair of regions 25a in the opening 24 increases as it moves away from the center line CL of the screw shaft 2 toward the outside in the radial direction, and the pair of regions 31a in the circulating top 3 corresponds to the pair of regions 25a in the opening 24, the shapes of the opening 24 and the circulating top 3 are not limited to those described above. As an example, as shown in FIG. 8, the pair of regions 25a in the opening 24 may be formed continuously as part of a cylindrical surface. In this case, the pair of regions 31a in the circulating top 3 may also be formed continuously as part of a cylindrical surface. In other words, each of the pair of regions 25a may be a curved surface with which the straight line TL makes point contact. In addition, in the opening 24 having the above-mentioned bottom surface 26, each of the pair of regions 25a may be formed as part of a cylindrical surface. In addition, as shown in FIG. 9, the pair of regions 25a in the opening 24 may directly intersect. In this case, the pair of regions 31a in the circulating top 3 may also directly intersect.

[0037] Moreover, in the opening 24, each of the pair of regions 25a may not be a plane parallel to the axial direction. As an example, as shown in FIG. 10, each of the pair of regions 25a may be a plane perpendicular to the tangent direction of the spiral at the center of the opening 24 when viewed from the outside in the radial direction. Also, in the opening 24, each of the pair of regions 25b may not be a plane perpendicular to the axial direction. As an example, as shown in FIG. 10, each of the pair of regions 25b may be a plane parallel to the tangent direction of the spiral at the center of the opening 24 when viewed from the outside in the radial direction. In that case, the thickness T of the thin-walled portion 20a formed by cutting out the thread 20 of the screw shaft 2 facing each other across the opening 24 by the opening 24 becomes approximately constant along each of the pair of regions 25b. As a result, there is no region in the thin-walled portion 20a that is relatively weak in strength, so that deformation of the thin-walled portion 20a can be suppressed.

[0038] In addition, when viewed from the axial direction, the distance D2 between the center line CL of the screw shaft 2 and the intersection point P may be less than 1 / 3 of the radius R of the screw shaft 2. Furthermore, when viewed from the axial direction, the intersection point P between a straight line TL tangent to one of the pair of regions 25a at one end 23a of the screw groove 23 and a straight line TL tangent to the other of the pair of regions 25a at the other end 23b of the screw groove 23 may be located in a region other than between the center line CL of the screw shaft 2 and the circulating top 3. In addition, the inclination angles of the pair of regions 25a with respect to the straight line L may be different from each other. In other words, the inclination angles of the pair of straight lines TL with respect to the straight line L may be different from each other. In addition, at each of the one end 23a and the other end 23b of the screw groove 23, the inner surface of the screw groove 23 may substantially coincide with the inner surface of the circulation groove 33 when viewed from the helical direction A.

[0039] The screw shaft 2 may be a hollow shaft. In this case, the opening 24 may be a recessed portion that opens radially outward, or may be a through portion that opens radially inward and outward. That is, the opening 24 only needs to open at least radially outward. The circulating top 3 may not be provided with a pair of protruding portions 34. In this case, the outer peripheral surface of the circulating top 3 may be located on a cylindrical surface that is substantially the same as the outer peripheral surface of the thread of the screw shaft 2. The groove shoulder 411 of the screw groove 41 of the nut 4 may or may not have a chamfered surface. [Explanation of symbols]

[0040] 1...ball screw, 2...screw shaft, 3...circulating piece, 4...nut, 5...ball, 10...rolling path, 23...screw groove (first screw groove), 23a...one end, 23b...other end, 24...opening, 25...side (inner surface), 25a...area (first area), 31...side (outer surface), 31a...area (second area), 33...circulating groove, 33a...one end, 33b...other end, 41...screw groove (second screw groove), A...spiral direction, CL...center line, D1, D2...distance, P...intersection, R...radius, TL...straight line.

Claims

1. A threaded shaft having a first thread groove and an opening portion that is open at least radially outward between one end of the first thread groove and the other end of the first thread groove; a circulation top disposed within the opening and having a circulation groove connected to each of the one end and the other end of the first screw groove; A nut having a second thread groove; A rolling path formed by the first screw groove and the second screw groove, and a plurality of balls rolling in the circulation groove, The inner surface of the opening includes a pair of first regions in which the one end and the other end of the first thread groove are open, an outer surface of the circulating top includes a pair of second regions corresponding to the pair of first regions and having one end and the other end of the circulating groove open; A ball screw, wherein the distance between the pair of first regions increases as the distance between the pair of first regions increases radially outward from the center line of the screw shaft.

2. 2. The ball screw according to claim 1, wherein, when viewed from the axial direction, an intersection of a straight line tangent to one of the pair of first regions at the one end of the first thread groove and a straight line tangent to the other of the pair of first regions at the other end of the first thread groove is located between the center line of the screw shaft and the circulating top.

3. 3. The ball screw according to claim 2, wherein a distance between the center line of the screw shaft and the intersection point is equal to or greater than ⅓ of a radius of the screw shaft when viewed from the axial direction.

4. The ball screw according to claim 2 , wherein each of the pair of first regions is a plane with which the straight line makes line contact.

5. The ball screw according to claim 2 , wherein each of the pair of first regions is a curved surface with which the straight line makes point contact.

6. When viewed from the helical direction of the first screw groove, an inner surface of the one end of the first screw groove is located inside an inner surface of the one end of the circulation groove, 2. The ball screw according to claim 1, wherein, when viewed from the helical direction, an inner surface of the other end of the first screw groove is located inside an inner surface of the other end of the circulation groove.

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