Ball screw device

Abstract

A ball screw device 10 comprises: a screw shaft 11 having a first spiral groove 15 on an outer periphery; a cylindrical nut 12 having, on an inner periphery, a second spiral groove 16 facing the first spiral groove 15; and a plurality of balls 13 disposed on a rolling path 17 formed between the first spiral groove 15 and the second spiral groove 16. The nut 12 has, in the middle of the second spiral groove 16, a circulation groove 23 for returning each ball 13 from an end-point side to a start-point side of the rolling path 17. The first spiral groove 15 has a first side wall 31 with which the ball 13 moving on the rolling path 17 comes into contact. The circulation groove 23 has a second side wall 41 with which the ball 13 moving on the rolling path 17 comes into contact. The second side wall 41 has a flat part 43 with which the ball 13 comes into contact, in a lift-up range K for moving the ball 13 along the circulation groove 23 in a state in which the ball 13 is held between the second side wall 41 and the first side wall 31.

Description

Ball screw device

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

[0002] Ball screw devices are known as devices that convert rotational motion into linear motion (see, for example, Patent Document 1 and Patent Document 2). The ball screw device has a screw shaft, a cylindrical nut, and multiple balls. The screw shaft has a first spiral groove on its outer periphery. The nut has a second spiral groove on its inner periphery. The multiple balls are arranged in a rolling path formed between the first spiral groove and the second spiral groove. The nut has a circulation groove in the middle of the second spiral groove for returning the balls from the end point side of the rolling path to the start point side.

[0003] JP 2006-292085 A International Publication No. 2012 / 066693

[0004] In the case of the ball screw device disclosed in Patent Document 1, the nut has a "top" that has a circulation groove on its inner circumference. Fig. 12 is an explanatory diagram of a conventional circulation groove. When a ball 100 returns from the end side of a rolling path 90 to the start side thereof by means of a circulation groove 97, the ball 100 is sandwiched between a first side wall 93 of a first spiral groove 92 of a screw shaft 91 and a second side wall 96 of a circulation groove 97 of a top 95 of a nut 99, and moves along the circulation groove 97.

[0005] At this time, the first side wall 93 and the second side wall 96 are pushed by the balls 100 and elastically deform. If the force of the balls 100 becomes large, for example, a portion of the second side wall 96 may be plastically deformed and dented. In particular, if the ball screw device is a top-type ball screw, the second side wall 96 has a thin portion 96a, which makes it prone to plastic deformation. Furthermore, even if the second side wall 96 does not plastically deform, the frictional force acting on the balls 100 increases, making it more likely for the balls 100 to become clogged, and reducing the circulation of the balls 100.

[0006] In the invention disclosed in Patent Document 1, the circulation groove has lead-in portions at both ends, and the lead-in portions have a central axis that coincides with the central axis of the rolling path. The lead-in portions make it difficult for the ball to get caught. However, the range of the lead-in portions is between points A and B shown in Figure 6 of Patent Document 1, and is not within the range where the ball 100 (see Figure 12) travels over the first side wall 93 to return from the end point side of the rolling path 90 to the start point side using the circulation groove 97.

[0007] The invention disclosed in Patent Document 2 is similar to that of Patent Document 1, and the circulation groove has linear introduction portions at both ends. Furthermore, the circulation groove has a curved portion that is continuous with the introduction portions in the middle portion located between the introduction portions at both ends. The curved portion is included in the range where the ball is sandwiched between the first side wall and the second side wall when the ball returns from the end point side of the rolling path to the start point side by the circulation groove. The curved portion is a curved groove whose angle gradually changes along the longitudinal direction of the circulation groove.

[0008] As a result, while the ball 100 moves along the curved portion, the gap between the first side wall 93 and the second side wall 96 rapidly narrows. The ball 100 presses against the first side wall 93 and the second side wall 96, which may result in, for example, a large elastic deformation of the second side wall 96 and further a possibility that the second side wall 96 may even undergo plastic deformation. Therefore, an object of the present disclosure is to provide a ball screw device that can suppress an increase in the force acting on the ball as the ball moves along the circulation groove.

[0009] A ball screw device according to an embodiment of the present invention comprises a screw shaft having a first spiral groove on its outer periphery, a cylindrical nut having a second spiral groove on its inner periphery that faces the first spiral groove, and a plurality of balls arranged in a rolling path formed between the first spiral groove and the second spiral groove, wherein the nut has a circulation groove midway along the second spiral groove for returning the ball from the end side of the rolling path to the start side, the first spiral groove having a first side wall with which the ball contacts as it moves along the rolling path, and the circulation groove has a second side wall with which the ball contacts as it moves along the rolling path, and the second side wall has a flat portion with which the ball contacts in a lift-up range in which the ball is sandwiched between the first side wall and the second side wall and moves along the circulation groove.

[0010] According to the ball screw device of the embodiment of the present invention, it is possible to prevent the force generated between the ball and the first side wall of the first spiral groove and the second side wall of the circulation groove from increasing.

[0011] FIG. 1 is a cross-sectional view of a ball screw device. FIG. 2 is a side view of a nut. FIG. 3 is a perspective view of a top. FIG. 4 is a view of the top as seen from the inside in the radial direction. FIG. 5 is an explanatory diagram of a screw shaft and a nut. FIG. 6 is an explanatory diagram of a screw shaft and a nut. FIG. 7 is an explanatory diagram of a screw shaft and a nut. FIG. 8 is an explanatory diagram of a screw shaft and a nut. FIG. 9 is an image diagram illustrating how a ball returns from a position on the end side of a rolling path to a position on the start side by a circulation groove. FIG. 10 is an explanatory diagram showing a part of the circulation groove of this embodiment. FIG. 11 is an explanatory diagram showing a part of a conventional circulation groove. FIG. 12 is an explanatory diagram of a conventional circulation groove.

[0012] <Outline of Embodiments of the Present Invention> The following is a description of outlines of embodiments of the present invention. (1) A rolling bearing device according to an embodiment of the present invention includes a screw shaft having a first spiral groove on its outer periphery, a cylindrical nut having a second spiral groove on its inner periphery that faces the first spiral groove, and a plurality of balls arranged in a rolling path formed between the first spiral groove and the second spiral groove, the nut having a circulation groove midway along the second spiral groove for returning the ball from the end point side to the start point side of the rolling path, the first spiral groove having a first side wall with which the ball contacts as it travels along the rolling path, the circulation groove having a second side wall with which the ball contacts as it travels along the rolling path, and the second side wall having a flat portion with which the ball contacts in a lift-up range in which the ball is sandwiched between the first side wall and the nut and travels along the circulation groove.

[0013] According to the above configuration, when the ball returns from the end point side of the rolling path to the start point side by the circulation groove, the ball is sandwiched between the first side wall and the second side wall and moves along the circulation groove. The circulation groove has a flat portion in the lift-up range. The flat portion makes it possible to prevent the force generated between the first side wall and the second side wall and the ball from increasing. As a result, for example, the effect of preventing damage to the second side wall is enhanced. The friction force between the first side wall and the second side wall and the ball is less likely to increase, preventing clogging of the ball and improving ball circulation.

[0014] (2) Preferably, the lift-up range is a range between a first position where the ball traveling along the rolling path starts to press against the first side wall and the second side wall, and a second position where the ball, which has been pressing against the first side wall and the second side wall, can be released from the first side wall. With this configuration, a configuration is obtained in which the ball contacts the flat portion while traveling through the lift-up range.

[0015] (3) Preferably, in the ball screw device of (1) or (2), the second side wall has the flat portion over the entire lift-up range. With this configuration, it is possible to more effectively prevent the force generated between the ball and the first and second side walls from increasing.

[0016] (4) Preferably, in the ball screw device of (1) to (3), the flat portion is a two-dimensional plane that is linearly flat along the longitudinal direction of the circulation groove and linearly flat along the direction from the groove end to the groove bottom of the circulation groove. Although there is play between the screw shaft and the nut, the function of the flat portion can be effectively obtained by the above configuration.

[0017] (5) Preferably, in the ball screw device of any one of (1) to (4), the nut has a hole and a top attached to the hole and having the circulation groove. This configuration provides a top-type ball screw device. The second side wall of a top-type ball screw device has a thin portion, making it prone to plastic deformation. However, this configuration effectively prevents damage to the second side wall of the top. Furthermore, even if a problem occurs with the circulation groove, only the top needs to be replaced.

[0018] <Details of the embodiment of the present invention> Hereinafter, an embodiment of the present invention will be described. [Overall configuration of ball screw device] Fig. 1 is a cross-sectional view of a ball screw device. The ball screw device 10 is used, for example, in a brake device of a vehicle (automobile). The ball screw device 10 has a screw shaft 11, a nut 12, and a plurality of balls 13. The screw shaft 11, the nut 12, and the balls 13 are made of metal such as carbon steel.

[0019] The ball screw device 10 of the present disclosure is a circulation type device, and the ball screw device 10 shown in Fig. 1 is a so-called "top type." The ball screw device 10 has a "top 14" attached to the cylindrical portion 21 of the nut 12. In this embodiment, the ball screw device 10 has a plurality of tops 14.

[0020] The screw shaft 11 is connected to a drive device including a motor (not shown). The drive device causes the screw shaft 11 to rotate forward and backward around its own central axis C. The nut 12 is connected to a part (movable member) of a device such as a brake device. The nut 12 is movable in the direction of the central axis C but is unable to rotate around the central axis C. Therefore, when the screw shaft 11 rotates, the nut 12 moves linearly in a direction along the central axis C.

[0021] The directions of the ball screw device 10 of the present disclosure will now be described. The direction along the central axis C of the screw shaft 11 is defined as the "axial direction." The axial direction also includes a direction parallel to the central axis C. The direction perpendicular to the central axis C is defined as the "radial direction." The direction along an imaginary circle centered on the central axis C is defined as the "circumferential direction." The rotation direction of the screw shaft 11 is the circumferential direction, and the movement direction of the nut 12 is the axial direction.

[0022] The screw shaft 11 has a long cylindrical shape. The screw shaft 11 has a first spiral groove 15 on its outer periphery. The first spiral groove 15 is a spiral groove formed along the axial direction. The groove shape of the first spiral groove 15 is an arc-shaped cross section. The nut 12 has a cylindrical shape. The nut 12 is located radially outward of the screw shaft 11 and surrounds the screw shaft 11. The central axis of the nut 12 coincides with the central axis C of the screw shaft 11. The nut 12 has a second spiral groove 16 on its inner periphery. The second spiral groove 16 faces the first spiral groove 15 in the radial direction. The groove shape of the second spiral groove 16 is an arc-shaped cross section.

[0023] A rolling path 17 through which the balls 13 pass is formed between the first spiral groove 15 and the second spiral groove 16. A plurality of balls 13 are arranged in the rolling path 17. When the screw shaft 11 rotates in a first direction, the balls 13 apply an axial force to the nut 12 while rolling on the rolling path 17. This axial force causes the nut 12 to move toward a first side in the axial direction. When the screw shaft 11 rotates in a second direction opposite to the first direction, the balls 13 apply an axial force to the nut 12 while rolling on the rolling path 17. This axial force causes the nut 12 to move toward a second side in the axial direction.

[0024] 2 is a side view of the nut 12. The nut 12 has a cylindrical tubular portion 21, and a peripheral wall 21a of the tubular portion 21 is provided with through holes 18 that penetrate in the radial direction. A plurality of through holes 18 are provided at intervals in the circumferential direction. In the present embodiment, four through holes 18 are provided at 90° intervals along the circumferential direction. These through holes 18 are arranged at different positions in the axial direction.

[0025] One top 14 is attached to one through hole 18. The top 14 is positioned and attached to the through hole 18. The top 14 attached to the through hole 18 cannot be displaced. The top 14 has a circulation groove 23 that allows the ball 13 to pass through (see FIG. 1). The circulation groove 23 is a groove that returns the ball 13 from the end point side to the start point side of the rolling path 17. With the above configuration, the nut 12 has the circulation groove 23 midway along the second spiral groove 16.

[0026] The hole for attaching the top 14 may be something other than the through-hole 18. The nut 12 may have a hole and the top 14 attached to the hole, and the top 14 may have the circulation groove 23. The top 14 may be, for example, a forged metal product. In this embodiment, the component having the circulation groove 23 in the nut 12 is the top 14, which is separate from the tubular portion 21, but the tubular portion 21 may have an integrated structure in which the circulation groove 23 is formed.

[0027] [Regarding the groove shapes of the circulation groove 23, etc.] Fig. 3 is a perspective view of the top 14, as viewed from the inside in the radial direction. Fig. 4 is a view of the top 14 as viewed from the inside in the radial direction. The circulation groove 23 has a first introduction portion 26 and a second introduction portion 28 as introduction portions at both ends in the longitudinal direction of the groove, and a central portion 27 at the center in the longitudinal direction of the groove. A boundary B1 between the first introduction portion 26 and the central portion 27 is indicated by a two-dot chain line, and a boundary B2 between the second introduction portion 28 and the central portion 27 is indicated by a two-dot chain line.

[0028] The nut 12 (see FIG. 1) may move from the first side to the second side in the axial direction, or from the second side to the first side in the axial direction. That is, the nut 12 moves back and forth in the axial direction. Therefore, the ball 13 may enter the circulation groove 23 from the first introduction portion 26, or from the second introduction portion 28. For this reason, the first introduction portion 26 and the second introduction portion 28 have a 180-degree rotationally symmetric structure with respect to the center X of the top 14 (see FIG. 4), and the portion of the central portion 27 on the first introduction portion 26 side and the portion on the second introduction portion 28 side have a 180-degree rotationally symmetric structure.

[0029] The circulation groove 23 has a depth that varies along the longitudinal direction of the groove. The groove is deeper in the central portion 27 than in the first introduction portion 26, and is deeper in the central portion 27 than in the second introduction portion 28. The depth of the circulation groove 23 gradually varies from the first introduction portion 26 to the central portion 27, and gradually varies from the central portion 27 to the second introduction portion 28.

[0030] 5 and 6 are explanatory diagrams of the screw shaft 11 and the nut 12. FIG. 5 shows a state in which the screw shaft 11 is not rotating, i.e., the nut 12 is stationary. FIG. 6 shows a state in which the screw shaft 11 is rotating, i.e., the nut 12 is moving to the first side in the axial direction (to the right in FIG. 6). Furthermore, FIG. 6 shows a state in which the ball 13 is located at the boundary B1 (see FIGS. 3 and 4) between the first introduction portion 26 and the central portion 27. Below, a case in which the ball 13 enters the circulation groove 23 from the first introduction portion 26 will be described.

[0031] 5 and 6 , the first spiral groove 15 has a first groove bottom 30, a first side wall 31, and a first opposing side wall 32. The first opposing side wall 32 is a portion facing the first side wall 31. The first groove bottom 30 is a portion between the first side wall 31 and the first opposing side wall 32. When the screw shaft 11 rotates and the nut 12 moves in the axial direction, the balls 13 come into contact with the first side wall 31 and are not in contact with the first opposing side wall 32.

[0032] The portion of the second spiral groove 16 other than the circulation groove 23 (the portion other than the top 14) has a third groove bottom 50, a third side wall 51, and a third opposing side wall 52. The third opposing side wall 52 is the portion opposing the third side wall 51. The third groove bottom 50 is the portion between the third side wall 51 and the third opposing side wall 52. When the screw shaft 11 rotates and the nut 12 moves axially (see FIG. 1 ), the ball 13 comes into contact with the third side wall 51 and is not in contact with the third opposing side wall 52. As described above, the ball 13 moving along the rolling path 17 comes into contact with the first side wall 31 of the first spiral groove 15 and the third side wall 51 of the second spiral groove 16.

[0033] The circulation groove 23 (see FIGS. 5 and 6 ) located midway along the second spiral groove 16 has a second groove bottom 40, a second side wall 41, and a second opposing side wall 42. The second opposing side wall 42 faces the second side wall 41. The second groove bottom 40 is located between the second side wall 41 and the second opposing side wall 42. The groove width of the circulation groove 23 is slightly larger than the groove width of the second spiral groove 16. When the screw shaft 11 rotates and the nut 12 moves axially, the ball 13 contacts the second side wall 41 and does not contact the second opposing side wall 42. As described above, the ball 13 traveling along the circulation groove 23 contacts the first side wall 31 of the first spiral groove 15 and the second side wall 41 of the circulation groove 23.

[0034] The screw shaft 11 has a spiral first screw thread 35. The first screw thread 35 has a first opposing side wall 32 on a first axial side thereof and a first side wall 31 on a second axial side opposite thereto. The circulation groove 23 is a groove that straddles one first screw thread 35 (see FIG. 9 ). FIG. 9 is an image diagram illustrating how the ball 13 returns from a position P2 on the end side of the rolling path 17 to a position P1 on the start side thereof by the circulation groove 23.

[0035] [Circulation Operation of Ball 13] The following describes the operation (circulation operation) of ball 13 returning from position P2 on the end side of rolling path 17 to position P1 on the start side by circulation groove 23. During this operation, a lift-up operation is performed in which ball 13 separates from first groove bottom 30 of first spiral groove 15. The circulation operation including the lift-up operation will be described below.

[0036] Figures 7 and 8 are explanatory diagrams of the screw shaft 11 and the nut 12. Figure 7 shows a state in which the screw shaft 11 rotates, the balls 13 roll, and the nut 12 moves further axially toward the first side (right side in Figure 7) from the state shown in Figure 6. Figure 8 shows a state in which the screw shaft 11 rotates, the balls 13 roll, and the nut 12 moves further axially toward the first side (right side in Figure 8) from the state shown in Figure 7.

[0037] As ball 13 returns from position P2 on the end side of rolling path 17 to position P1 on the start side (see FIG. 9 ) along circulation groove 23, ball 13 is sandwiched between first side wall 31 and second side wall 41 and moves along circulation groove 23 (see FIGS. 6 and 7 ). This operation will be further described below.

[0038] The ball 13 traveling along the rolling path 17 (circulation groove 23) is sandwiched between the first side wall 31 and the second side wall 41 (see FIG. 6 ) from the boundary B1 (see FIGS. 3 and 4 ) between the first introduction section 26 and the central section 27. Note that the ball 13 that has entered from the first introduction section 26 is not sandwiched between the first side wall 31 and the second side wall 41 until it reaches the boundary B1, and the ball 13 is in a state where it can be displaced freely.

[0039] After the boundary B1 (see FIGS. 6 and 7 ), the frictional force between the ball 13 and the first and second side walls 31, 41 gradually increases. Due to this frictional force, the ball 13 is scooped up (lifted up) by the second side wall 41. The ball 13 leaves the first groove bottom 30, rolls along the second side wall 41 while in contact with the first side wall 31, and moves toward the second groove bottom 40. During this time, the ball 13 remains in contact with both the first side wall 31 and the second side wall 41.

[0040] Eventually, the ball 13 reaches the apex 31 a of the first side wall 31 (see FIG. 7 ). After reaching the apex 31 a, the ball 13 can separate from the first side wall 31 (see FIG. 8 ). The ball 13 is pushed by an adjacent ball 13 (not shown), passes between the second groove bottom 40 and the outer peripheral surface of the first thread 35, and returns to position P1 (see FIG. 4 ) on the start side of the rolling path 17.

[0041] The distance (radial dimension) between the second groove bottom 40 (see FIG. 8 ) and the outer peripheral surface of the first screw thread 35 is larger than the diameter of the ball 13. Therefore, the position of the ball 13 that has passed the top 31 a of the first side wall 31 is not restricted by the circulation groove 23 and the first spiral groove 15. In other words, the ball 13 is not sandwiched between the first side wall 31 and the second side wall 41, and is free to move. In this way, the circulation groove 23 allows the ball 13 to circulate along the rolling path 17, returning from position P2 on the end side of the rolling path 17 to position P1 on the start side.

[0042] As described above, the circulation groove 23 has a second side wall 41, and when the ball 13 returns from position P2 on the end side of the rolling path 17 to position P1 on the start side, the ball 13 comes into contact with the second side wall 41. After the boundary B1 (see FIGS. 3 and 4 ), the ball 13 is sandwiched between the first side wall 31 and the second side wall 41 and moves along the circulation groove 23. In this way, the range after the boundary B1 where the ball 13 is sandwiched between the first side wall 31 and the second side wall 41 and can move away from the first groove bottom 30 and further away from the first spiral groove 15 (first thread 35) is the lift-up range K, which will be described later.

[0043] [Flat portion 43 of second side wall 41] The second side wall 41 has a flat portion 43 with which the ball 13 comes into contact, in the lift-up range K. The lift-up range K is a range in which the ball 13 is sandwiched between the first side wall 31 and the second side wall 41 and moves along the circulation groove 23, and is a range from a first position Q1 defined below to a second position Q2 defined below.

[0044] First position Q1: A position where the ball 13 moving along the rolling path 17 (including the circulation groove 23) begins to press against the first side wall 31 and the second side wall 41. Second position Q2: A position where the ball 13 that was pressing against the first side wall 31 and the second side wall 41 can move away from the first side wall 31.

[0045] Fig. 6 shows a state in which the ball 13 is at a first position Q1, where the ball 13 begins to press against the first side wall 31 and the second side wall 41. Fig. 7 shows a state in which the ball 13 is at a position just before the second position Q2. As the ball 13 moves further from the state shown in Fig. 7, the ball 13 can separate from the first side wall 31.

[0046] The lift-up range K (see FIGS. 3 and 4) is included in the central portion 27 of the circulation groove 23. In other words, the lift-up range K is a part of the range from the boundary B1 toward the boundary B2. The first position Q1 is a position on the boundary B1.

[0047] In this embodiment, the second side wall 41 has a first flat portion 43 over the entire lift-up range K. In FIG. 3 , the cross-hatched portion is the first flat portion 43. The first flat portion 43 is a two-dimensional plane that is linearly flat along the longitudinal direction of the circulation groove 23 and linearly flat along the direction from the groove end 24 of the circulation groove 23 toward the second groove bottom 40. In this way, the second side wall 41 has the first flat portion 43, which has a flat surface shape, in the lift-up range K.

[0048] [Regarding Each Part of the Circulation Groove 23] The following describes each part of the circulation groove 23 other than the first flat portion 43. As shown in Figures 3 and 4, the portion of the second groove bottom 40 in the lift-up range K is an arcuate surface portion 44 having an arc-shaped cross section. The portion of the second side wall 41 in the lift-up range K is the first flat portion 43, as described above. The portion of the second opposing side wall 42 in the lift-up range K is a flat portion (third flat portion 46), similar to the second side wall 41. In the lift-up range K, the first flat portion 43, the arcuate surface portion 44, and the third flat portion 46 are continuous.

[0049] The portion of the second side wall 41 in the introduction portion 26 has a second flat portion 45. The second flat portion 45 is continuous with the first flat portion 43 in the groove longitudinal direction. The first flat portion 43 and the second flat portion 45 are located along a common plane. The portion of the second side wall 41 on the opposite side of the first flat portion 43 from the second flat portion 45 is a curved portion 47 (see FIG. 4 ). Unlike the first flat portion 43, the curved portion 47 has a curved shape along the groove longitudinal direction of the circulation groove 23.

[0050] The curved surface portion 47 has a curved shape whose angle gradually changes along the longitudinal direction of the circulation groove 23. That is, the portion of the second side wall 41 at the curved surface portion 47 has a curved shape whose angle gradually changes along the longitudinal direction of the circulation groove 23. The portion of the second side wall 41 on the opposite side of the first flat portion 43 across the curved surface portion 47 has a side surface portion 48. The side surface portion 48 may be linearly flat along the longitudinal direction of the circulation groove 23, or may have a curved shape along the longitudinal direction of the circulation groove 23.

[0051] As described above, in the ball screw device 10, the nut 12 may move from the first side to the second side in the axial direction, or from the second side to the first side in the axial direction. Therefore, the second introduction portion 28 side of the second opposing side wall 42 (see FIGS. 3 and 4 ) has the same configuration as the first introduction portion 26 side of the second side wall 41. In other words, the lift-up range K is also defined on the second introduction portion 28 side, and the second opposing side wall 42 has the first flat portion 43 in the lift-up range K with which the ball 13 comes into contact.

[0052] Function of First Flat Portion 43 of Circulation Groove 23 Figure 10 is an explanatory diagram showing a portion (lift-up range K and its surroundings) of the circulation groove 23 of this embodiment. As described above, the lift-up range K is the range between the first position Q1 and the second position Q2. The first position Q1 is the position where the ball 13 traveling along the rolling path 17 (circulation groove 23) begins to press against the first side wall 31 and the second side wall 41 (see Figure 6). The second position Q2 is the position where the ball 13 pressing against the first side wall 31 and the second side wall 41 can be released from the first side wall 31 (see Figures 7 and 8).

[0053] 11 is an explanatory diagram showing a portion (lift-up range K and its surroundings) of a conventional circulation groove 123. The configuration shown in FIG. 11 corresponds to the configuration disclosed in Patent Document 2. In FIG. 11 and FIG. 10, a line parallel to the lead angle direction of the first spiral groove 15 is defined as a "reference line L."

[0054] In the case of the conventional structure (see FIG. 11 ), the lift-up range K of the circulation groove 123 is the curved portion 147. The curved portion 147 has a curved shape whose angle gradually changes along the longitudinal direction of the circulation groove 123. In other words, in the case of the conventional structure, the lift-up range K has a curved shape along the longitudinal direction of the circulation groove 23, similar to the curved portion 47 of the present embodiment (see FIG. 4 ).

[0055] 11 , in the lift-up range K having such a curved surface shape, the tangent to the ball 13 at the first position Q1 is S1, and the tangent to the ball 13 at the second position Q2 is S2. The angle of the tangent S1 with respect to the reference line L is "θ1," and the angle of the tangent S2 with respect to the reference line L is "θ2." θ2 is larger than θ1 (θ1<θ2). From the first position Q1 to the second position Q2, the angle between the tangent to the ball 13 and the reference line L gradually increases.

[0056] In this embodiment (see FIG. 10 ), the lift-up range K includes a first flat portion 43 having a flat surface shape. In the lift-up range K having the flat surface shape formed by this first flat portion 43, the tangent to the ball 13 at the first position Q1 is S11, and the tangent to the ball 13 at the second position Q2 is S12. The angle of the tangent S11 with respect to the reference line L is “θ11,” and the angle of the tangent S12 with respect to the reference line L is “θ12.” In the groove longitudinal direction, one end of the flat portion 43 is the first position Q1, and the other end of the flat portion 43 is the second position Q2. Therefore, the tangent S11 and the tangent S12 coincide, and θ11 and θ12 are the same. From the first position Q1 to the second position Q2, the angle between the tangent to the ball 13 and the reference line L does not change or increase.

[0057] Even if the angle θ1 (conventional) and the angle θ11 (present embodiment) are the same at the first position Q1, the angle θ12 (present embodiment) is smaller than the angle θ2 (conventional) at the second position Q2 (θ12<θ2). That is, in the case of the present embodiment ( FIG. 10 ), the angle of the tangent line S12 of the ball 13 with respect to the reference line L is smaller in the lift-up range K due to the first flat portion 43.

[0058] When the ball 13 reaches the first position Q1, the second side wall 41 is pushed by the ball 13 and begins to elastically deform. The second side wall 41 continues to elastically deform until the ball 13 reaches the second position Q2. Once the ball 13 passes the second position Q2, as described above, the ball 13 is no longer constrained, and the second side wall 41 does not elastically deform. Therefore, according to the first flat portion 43 of this embodiment, in the lift-up range K, the angle θ12 of the tangent S12 of the ball 13 relative to the reference line L becomes smaller, and the reaction force that the second side wall 41 receives from the ball 13 becomes weaker. In other words, the force that the ball 13 exerts on the second side wall 41 becomes smaller.

[0059] As described above, the ball screw device 10 of this embodiment includes the screw shaft 11, the nut 12, and a plurality of balls 13 disposed in the rolling path 17 formed between the first spiral groove 15 of the screw shaft 11 and the second spiral groove 16 of the nut 12 (see FIG. 1 ). The first spiral groove 15 has a first side wall 31 with which the balls 13 moving along the rolling path 17 come into contact. The second spiral groove 16 has a third side wall 51 with which the balls 13 moving along the rolling path 17 come into contact (see FIG. 6 ).

[0060] The nut 12 has a circulation groove 23 in the middle of the second spiral groove 16 for returning the ball 13 from the end point side to the start point side of the rolling path 17. The circulation groove 23 has a second side wall 41 with which the ball 13 contacts as it moves along the rolling path 17. The second side wall 41 has a first flat portion 43 with which the ball 13 contacts in a lift-up range K where the ball 13 is sandwiched between the second side wall 41 and the first side wall 31 and moves along the circulation groove 23 (see FIG. 10 ).

[0061] According to the ball screw device 10 of this embodiment, when the ball 13 returns from the end point side to the start point side of the rolling path 17 due to the circulation groove 23, the ball 13 is sandwiched between the first side wall 31 and the second side wall 41 and moves along the circulation groove 23. The circulation groove 23 has a first flat portion 43 in the lift-up range K. Therefore, as described with reference to FIGS. 10 and 11 , it is possible to prevent the force generated between the ball 13 and the first side wall 31 and the second side wall 41 from increasing.

[0062] As a result, the amount of elastic deformation of the second side wall 41 in the lift-up range K is reduced, improving the effect of preventing damage to the second side wall 41. In addition, the frictional force between the first side wall 31 and the second side wall 41 and the balls 13 is less likely to increase, preventing clogging of the balls 13 and improving circulation of the balls 13.

[0063] The first flat portion 43 is a two-dimensional plane that is linearly flat along the longitudinal direction of the circulation groove 23 and linearly flat along the direction from the groove end 24 of the circulation groove 23 toward the second groove bottom 40. Although there is some play between the screw shaft 11 and the nut 12, this configuration allows the function of the first flat portion 43 to be effectively obtained.

[0064] At the first position Q1 (see FIG. 10 ), the angle θ11 is preferably set smaller than the conventional angle θ1, which effectively reduces the frictional force between the first side wall 31 and the second side wall 41 and the ball 13. Note that because the second side wall 41 is elastically deformed by being pressed by the ball 13, the values ​​of θ12 and θ2 may actually differ slightly from the values ​​shown in the figure.

[0065] [Others] The above-described embodiments are illustrative in all respects and are not limiting. The scope of the present invention is defined by the claims, not the above-described embodiments, and includes all modifications within the scope of equivalents to the configurations described in the claims.

[0066] REFERENCE SIGNS LIST 10 ball screw device 11 screw shaft 12 nut 13 ball 14 top 15 first spiral groove 16 second spiral groove 17 rolling path 18 through hole (hole) 21 cylindrical portion 23 circulation groove 24 groove end 31 first side wall 40 second groove bottom (groove bottom) 41 second side wall 43 flat portion (first flat portion) K lift-up range Q1 first position Q2 second position

Claims

1. A ball screw device comprising: a screw shaft having a first spiral groove on its outer periphery; a cylindrical nut having a second spiral groove on its inner periphery that faces the first spiral groove; and a plurality of balls arranged in a rolling path formed between the first spiral groove and the second spiral groove, wherein the nut has a circulation groove midway along the second spiral groove for returning the ball from the end side of the rolling path to the start side, the first spiral groove having a first side wall with which the ball contacts as it advances along the rolling path, the circulation groove having a second side wall with which the ball contacts as it advances along the rolling path, and the second side wall having a flat portion with which the ball contacts, in a lift-up range where the ball is sandwiched between the first side wall and the circulation groove and advances along the circulation groove.

2. A ball screw device as described in claim 1, wherein the lift-up range is a range between a first position where the ball traveling along the rolling path begins to press against the first side wall and the second side wall, and a second position where the ball pressing against the first side wall and the second side wall can move away from the first side wall.

3. A ball screw device according to claim 1 or 2, wherein the second side wall has the flat portion over the entire lift-up range.

4. A ball screw device as described in claim 1 or claim 2, wherein the flat portion is a two-dimensional plane that is linearly flat along the longitudinal direction of the circulation groove and linearly flat along the direction from the groove end to the groove bottom of the circulation groove.

5. A ball screw device as described in claim 1 or 2, wherein the nut has a hole and a top attached to the hole and having the circulation groove.

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