Ball screw device

Abstract

A ball screw device 10 comprises: a screw shaft 11 that has a first spiral groove 15 on the outer peripheral surface thereof; a cylindrical nut 12 that has, on the inner peripheral surface thereof, a second spiral groove 16 which is opposite from the first spiral groove 15 and that surrounds the screw shaft 11; and a plurality of balls 13 that are disposed in a running track 17 which his formed between the first spiral groove 15 and the second spiral groove 16. The nut 12 has, partway along the second spiral groove 16, a circulation groove 23 for returning the balls 13 from a position P2 on the end point side of the running track 17 to a position P1 on the starting point side. When moving to one side in the axial direction, the nut 12 receives an axial load F that is greater than when moving to the other side in the axial direction. Groove centers X3 of the circulation groove 23 at a first introduction part 26 and a second introduction part 28 are disposed separate from the groove center X2 of the second spiral groove 16 to the one side in the axial direction.

Description

Ball screw device

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

[0002] A ball screw device is known as a device that converts rotational motion into linear motion (see, for example, Patent Document 1). The ball screw device includes a screw shaft, a cylindrical nut, and a plurality of balls. The screw shaft has a first helical groove on its outer periphery. The nut has a second helical groove on its inner periphery. The plurality of balls are disposed in a rolling path formed between the first and second helical grooves. The nut has a circulation groove in the middle of the second helical groove for returning the balls from the end point side of the rolling path to the start point side. The ball screw device disclosed in Patent Document 1 is a circulation-type device, a so-called "top type." In order to improve ball circulation performance, the ball screw device disclosed in Patent Document 1 has introduction portions at both ends of the circulation groove of the top, the introduction portions having central axes coinciding with the central axes of the rolling path (internal thread groove).

[0003] Japanese Patent Application Laid-Open No. 2006-292085

[0004] In the ball screw device disclosed in Patent Document 1, in order to improve ball circulation performance, lead-in portions having central axes coinciding with the central axis of the rolling path (internal thread groove) are provided at both ends of the circulation groove of the bearing. In the ball screw device configured in this manner, a minute gap exists between the rolling path and the balls, which is necessary for the smooth circulation of the balls within the rolling path and for manufacturing tolerances. Therefore, when an axial load is applied to the nut in the ball screw device, the nut moves axially relative to the screw shaft, causing axial misalignment between the central axes of the rolling path and the lead-in portions.

[0005] The ball screw device disclosed herein comprises a screw shaft having a first spiral groove on its outer peripheral surface, a cylindrical nut having a second spiral groove on its inner peripheral surface that faces the first spiral groove and surrounds the screw shaft, 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 through the second spiral groove for returning the balls from the end side of the rolling path to the start side, the nut being a ball screw device that receives a greater axial load when moving to one axial side than when moving to the other axial side, and the groove center of the circulation groove being arranged away from the groove center of the second spiral groove to one axial side.

[0006] According to the ball screw device of the present disclosure, when used in a state where an axial load acts on the nut, deviation in the axial position between the circulation groove and the first spiral groove can be suppressed.

[0007] 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 radially inner side. FIG. 5 is an explanatory diagram showing a circulation groove in a misaligned state. FIG. 6 is an enlarged explanatory diagram showing a circulation groove in a misaligned state. FIG. 7 is an explanatory diagram showing the positional relationship between a first helical groove and a second helical groove and a conventional circulation groove. FIG. 8 is an explanatory diagram showing the positional relationship between a first helical groove and a second helical groove and a circulation groove of this embodiment. FIG. 9A is a cross-sectional view of the A1-A1 portion of FIG. 8. FIG. 9B is a cross-sectional view of the A2-A2 portion of FIG. 8. FIG. 10 is an explanatory diagram showing another embodiment of a nut having a circulation groove in a misaligned state.

[0008] <Details of the embodiment of the present invention> Hereinafter, an embodiment of the present invention will be described with reference to Figures 1 to 10. Note that the same reference numerals denote the same configurations even in different figures.

[0009] [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 a metal such as carbon steel.

[0010] 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."

[0011] 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.

[0012] The screw shaft 11 has a long cylindrical shape. The screw shaft 11 has a first helical groove 15 on its outer periphery. The first helical groove 15 is a helical groove formed along the axial direction. The groove shape of the first helical groove 15 is an arc-shaped cross section. The first helical 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 opposing 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. The screw shaft 11 has a first helical thread 35 on its outer periphery. The first thread 35 is located between adjacent first helical grooves 15.

[0013] 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 helical groove 16 on its inner periphery. The second helical groove 16 faces the first helical groove 15 in the radial direction. The second helical groove 16 has an arc-shaped cross section. The portion of the second helical groove 16 other than the circulation groove 23 (the portion other than the link 14) has a second groove bottom 50, a second side wall 51, and a second opposing side wall 52. The second opposing side wall 52 faces the second side wall 51. The second groove bottom 50 is the portion between the second side wall 51 and the second opposing side wall 52. The nut 12 has a second helical thread 36 on its inner periphery. The second threads 36 are present between adjacent second spiral grooves 16 .

[0014] Nut 12 is formed with a plurality of through holes 18 that penetrate in the radial direction. Blocks 14 are inserted and fixed into these through holes 18. A circulation groove 23 is formed on the radial inside of block 14 to return balls 13 from the end point side of rolling path 17 to the start point side.

[0015] 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, the balls 13 circulate while rolling on the rolling path 17, and when they reach the end point, they are returned to the start point by the circulation groove 23. This circulation of the balls 13 applies an axial force to the nut 12. When the rotation direction of the screw shaft 11 changes, the circulation direction of the balls 13 in the rolling path 17 changes, and the axial movement direction of the nut 12 changes. By changing the rotation direction of the screw shaft 11, the nut 12 can be made to reciprocate in the axial direction.

[0016] As shown in FIG. 1 , the ball screw device 10 is used in, for example, a brake device 200. In addition to the ball screw device 10, the brake device 200 includes a cap-shaped piston 160 that covers one end of the nut 12, a brake pad 170 and a brake disc 180 that are arranged on one end side of the ball screw device 10, and a case 190. The brake pad 170 includes a pair of first and second pads 171 and 172. The first pad 171 is provided integrally with the nut 12 via the piston 160. The case 190 does not displace axially relative to the screw shaft 11. The second pad 172 is provided in the case 190. The brake disc 180 is arranged between the first pad 171 and the second pad 172.

[0017] In the brake device 200, the screw shaft 11 is connected to a drive device (motor or the like) not shown, and is rotated forward and backward around its own central axis C. The nut 12 is movable in the axial direction but is unable to rotate around the central axis C. The nut 12 moves linearly in the axial direction as the screw shaft 11 rotates.

[0018] By rotating the screw shaft 11 in the forward direction, the brake device 200 can displace the first pad 171 (nut 12) toward the brake disc 180 (one axial side), thereby clamping the brake disc 180 between the first pad 171 and the second pad 172. At this time, the brake device 200 is in an activated state. When the brake device 200 is in an activated state, the nut 12 is subjected to a load F acting toward the other axial side. By rotating the screw shaft 11 in the opposite direction, the brake device 200 can displace the nut 12 and the first pad 171 toward a side away from the brake disc 180 (the other axial side), thereby releasing the activated state. When the nut 12 moves toward one axial side, it is subjected to a load F acting toward the other axial side that is greater than when it moves toward the other axial side.

[0019] 2 is a side view of the appearance 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.

[0020] One piece 14 is attached to one through-hole 18. The piece 14 is attached by being positioned in the through-hole 18.

[0021] The hole for attaching the block 14 may be something other than the through-hole 18 (for example, the block 14 may be attached to a recess provided in the inner diameter surface of the nut 12). The nut 12 may have a hole and the block 14 attached to the hole, and the block 14 may have the circulation groove 23. The block 14 may be, for example, a forged metal product or a molded resin product. In the present embodiment, the component having the circulation groove 23 in the nut 12 is the block 14 that is separate from the tubular portion 21, but the circulation groove may also be formed integrally with the tubular portion 21.

[0022] [Regarding the Groove Shape of the Circulation Groove 23, etc.] FIG. 3 is a perspective view of the block 14, viewed from the radially inner side. A circulation groove 23 is formed on the inner surface of the block 14. The circulation groove 23 has an arc-shaped cross section. The circulation groove 23 has a second groove bottom 40, a second side wall 41, and a second opposing side wall 42. The second opposing side wall 42 is the portion opposing the second side wall 41. The second groove bottom 40 is the portion between the second side wall 41 and the second opposing side wall 42. The circulation groove 23 has a first introduction portion 26 and a second introduction portion 28 as introduction portions at both ends in the groove longitudinal direction, and a central portion 27 at the center in the groove longitudinal direction. 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.

[0023] When the ball 13 enters the circulation groove 23 from the first introduction portion 26, the ball 13 comes into contact with the second opposing side wall 42. After the boundary B1, the ball 13 is sandwiched between the first opposing side wall 32 and the second opposing side wall 42 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 opposing side wall 32 and the second opposing side wall 42 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.

[0024] The second opposing side wall 42 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 opposing side wall 32 and the second opposing side wall 42 and moves along the circulation groove 23. The lift-up range K includes a contact position Q, which is a position at which the ball 13 moving along the rolling path 17 (including the circulation groove 23) begins to press against the first opposing side wall 32 and the second opposing side wall 42. In other words, the ball 13 moving along the circulation groove 23 first comes into contact with the first opposing side wall 32 at the contact position Q.

[0025] The lift-up range K 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 to the boundary B2 side. The contact position Q is a position on the boundary B1.

[0026] In this embodiment, the second opposing side wall 42 has a flat portion 43 over the entire lift-up range K. In Fig. 3, the cross-hatched portion is the flat portion 43. The 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 23a of the circulation groove 23 toward the second groove bottom 40. It is not necessary to provide a flat portion in the lift-up range K, and the lift-up range K may have a curved surface similar to the conventional case.

[0027] 4 is a front view of the top 14 as viewed from the inside in the radial direction. When the rotation direction of the screw shaft 11 changes, the circulation direction of the balls 13 in the rolling path changes. For this reason, there are cases where the balls 13 enter the circulation groove 23 from the first introduction portion 26 and cases where the balls 13 enter the circulation groove 23 from the second introduction portion 28. For this reason, with respect to the center point Y of the top 14 as the reference, the first introduction portion 26 and the second introduction portion 28 have a structure that is rotationally symmetrical by 180 degrees, 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 structure that is rotationally symmetrical by 180 degrees.

[0028] The groove center X3 indicates the central axis of the first introduction portion 26 and the second introduction portion 28 in the width direction of the circulation groove 23. More specifically, the groove center X3 is a central axis passing through the center of the width direction of the range from the first introduction portion 26 to the boundary B1 and the range from the second introduction portion 28 to the boundary B2 in the circulation groove 23. Alternatively, the groove center X3 may be a central axis passing through the center of the width direction of the range from the first introduction portion 26 beyond the boundary B1 to the lift-up range K in the circulation groove 23 and the range from the second introduction portion 28 to the boundary B2 of the lift-up range K.

[0029] [Detailed Configuration of the Circulation Groove] The state in which the block 14 is inserted into the through-hole 18 of the cylindrical portion 21 will be described using Figures 5 and 6. Figure 5 is a cross-sectional view of the nut 12. The figure shows the second spiral groove 16 formed on the inner diameter surface of the cylindrical portion 21 and the circulation groove 23 formed on the radially inner side of the block 14 as viewed from the inside of the nut 12. The circulation groove 23 is located between the end point E and the start point S of the second spiral groove 16.

[0030] FIG. 6 is an enlarged view of the circulation groove 23 portion of FIG. 5 . The groove center X2 indicates the central axis passing through the center of the width direction of the second spiral groove 16. The groove center X3 of the circulation groove 23 is a distance D away from the groove center X2 of the second spiral groove 16 toward the other axial direction. The distance D corresponds to the displacement D caused by the nut 12 moving toward the other axial direction relative to the screw shaft 11 when subjected to a load F. The through hole 18 is formed at an axial position in the cylindrical portion 21 such that the groove center X3 of the circulation groove 23 of the block 14 is axially distanced from the groove center X2 of the second spiral groove 16 by the distance D. In this embodiment, the through hole 18 is formed at a position in the cylindrical portion 21 such that the groove center X3 is axially distanced from the groove center X2 by the distance D, but the present invention is not limited to this, and the distance D may be determined in another configuration.

[0031] An end of the second opposing side wall 42 in the first introduction portion 26 of the circulation groove 23 and an end of the second opposing side wall 42 in the second introduction portion 28 are at the same axial position as the second opposing side wall 52 of the second spiral groove 16. An end of the second side wall 41 in the first introduction portion 26 of the circulation groove 23 and an end of the second side wall 41 in the second introduction portion 28 are at a position on one axial side of the second side wall 51 of the second spiral groove 16.

[0032] 7 and 8, the positional relationship between the first spiral groove 15 of the screw shaft 11, the second spiral groove 16 of the nut 12, and the circulation groove 23 of the top 14, and the behavior of the balls 13 circulating in the rolling path 17 in this embodiment will be explained schematically in comparison with that in a conventional ball screw device. In Fig. 7 and Fig. 8, parts with the same configuration are designated by the same reference numerals, and parts with different configurations are designated by different reference numerals.

[0033] [Behavior of balls in a conventional rolling path] The positional relationship between the first spiral groove of the screw shaft, the second spiral groove of the nut, and the circulation groove of the top in a conventional ball screw device, and the behavior of balls circulating in the rolling path will be explained below using Figure 7. Figure 7 is a diagram schematically showing the positional relationship between the first spiral groove 15 of the screw shaft 11, the second spiral groove 16 of the nut 12, and the circulation groove 23 of the top 14 from the radial direction.

[0034] As shown in Fig. 7, in a conventional ball screw device 100, introduction portions 26, 28 having a groove center X4 that coincides with the central axes (hereinafter referred to as groove centers X1, X2) of the rolling path 17 (first spiral groove 15 and second spiral groove 16) are provided at both ends of the circulation groove 23. As shown in the left diagram of Fig. 7, in the ball screw device 100, the arrangement positions of the first spiral groove 15, the second spiral groove 16, and the circulation groove 23 are set so that the groove center X1, the groove center X2, and the groove center X4 coincide when viewed from the radial direction when the nut 12 is not receiving a load F on the other axial side.

[0035] As shown in the left diagram of FIG. 7 , in the ball screw device 100, the position where the ball 13 introduced into the circulation groove 23 first contacts the circulation groove 23 is referred to as the contact position Q. The contact position Q when no load F is applied to the other axial direction is referred to as the first contact position Q1. Of the first contact positions Q1, the first contact position Q1 on the end side (position P2 side) of the rolling path 17 is referred to as the first contact position Q1b, and the first contact position Q1 on the start side (position P1 side) of the rolling path 17 is referred to as the first contact position Q1a, to distinguish them from each other. The two first contact positions Q1 (Q1a and Q1b) are point-symmetric with respect to the center point Y of the circulation groove 23. In the ball screw device 100 shown in the left diagram of FIG. 7 , the positional relationship between the groove centers X1, X2, and X4 and the positional relationship between the first contact positions Q1a and Q1b are each an "ideal positional relationship."

[0036] 7, in the ball screw device 100, when the nut 12 is subjected to a load F acting toward the other axial direction, the nut 12 (the second spiral groove 16 and the second screw thread 36) and the top 14 (the first side wall 41 and the second side wall 42) are displaced toward the other axial direction. In this description, the amount of displacement of the nut 12 toward the other axial direction in this case is defined as a displacement amount D. In this case, the groove centers X2 and X4 are displaced toward the other axial direction by the displacement amount D relative to the groove center X1, and the groove center X1 no longer coincides with the groove center X2 and the groove center X4.

[0037] In the right diagram of Fig. 7 , the contact position Q in a state in which a load F is applied to the other axial side is referred to as a second contact position Q2. Of the second contact positions Q2, the second contact position Q2 on the end side (position P2 side) of the rolling path 17 is referred to as a second contact position Q2b, and the second contact position Q2 on the start side (position P1 side) of the rolling path 17 is referred to as a second contact position Q2a, to distinguish them from one another. The two second contact positions Q2 (Q2a and Q2b) are not point-symmetric with respect to each other with respect to the center point Y of the circulation groove 23. In the ball screw device 100 shown in the right diagram of Fig. 7 , the positional relationship between the groove centers X1, X2, and X4 and the positional relationship between the second contact positions Q2a and Q2b are not "ideal positional relationships."

[0038] In other words, when the conventional ball screw device 100 constituting the brake mechanism 200 receives a load F toward the other axial direction during operation of the brake mechanism 200, the nut 12 and the top 14 are displaced toward the other axial direction by a displacement amount D, the groove center X3 of the circulation groove 23 and the groove center X1 of the rolling path 17 of the screw shaft 11 are shifted toward the other axial direction by a distance D, and the contact position Q between the ball 13 and the circulation groove 23 is shifted circumferentially to become a second contact position Q2 (Q2a and Q2b).

[0039] [Regarding the propulsive force acting on the ball] When the ball 13 is scooped up in the circulation groove 23 and moves in the radial direction, a propulsive force acts on the ball 13. If the propulsive force acting on the ball 13 is excessively large, it can cause vibration and noise when the ball 13 is introduced into the circulation groove 23, and if it is too small, it becomes difficult for the ball 13 to be scooped up by the circulation groove 23. For this reason, the propulsive force acting on the ball 13 is required to be of an appropriate magnitude.

[0040] The propulsive force that the ball 13 receives from the circulation groove 23 varies depending on the contact position Q of the ball 13 with respect to the circulation groove 23. At the first contact positions Q1a and Q1b, the propulsive forces that the ball 13 receives from the nut 12 are approximately equal and of appropriate magnitude. On the other hand, at the second contact positions Q2a and Q2b, the propulsive forces that the ball 13 receives from the nut 12 are different, with one being excessively large and the other being insufficiently large. Therefore, when the nut 12 receives a load F toward the other axial side, the propulsive force acting on the ball 13 varies between the end point side (position P2 side) and the start point side (position P1 side) of the rolling path 17.

[0041] In this way, in the conventional ball screw device 100 used in a state where an axial load acts on the nut 12, the thrust force acting on the balls 13 fluctuates when the nut 12 receives a load F on the other axial side. As a result, the conventional ball screw device 100 has the following problems: 1) increased vibration and noise generated when the balls 13 move in the circulation groove 23, 2) a shortened life of the top 14 (circulation groove 23), and 3) increased torque fluctuation of the screw shaft 11.

[0042] The present disclosure aims to suppress fluctuations in the thrust force acting on the balls in a ball screw device used in a state where an axial load acts on the nut.

[0043] FIG. 8 is a diagram showing a schematic view of the positional relationship between the first spiral groove 15 of the screw shaft 11, the second spiral groove 16 of the nut 12, and the circulating groove 23 of the block 14 in the present invention, viewed from the radial direction.

[0044] 8, in this description, the central axis of the first spiral groove 15 is defined as a groove center X1, the central axis of the second spiral groove 16 is defined as a groove center X2, and the central axis of the introduction portions (first introduction portion 26 and second introduction portion 28) in the circulation groove 23 is defined as a groove center X3. In the ball screw device 10 of this embodiment, the amount of displacement of the nut 12 toward the other axial side when a load F (see FIG. 1) is applied is defined as a displacement amount D.

[0045] 8, in the ball screw device 10 of the present disclosure, the groove centers X3 of the first introduction portion 26 and the second introduction portion 28 are offset to one side in the axial direction with respect to the groove center X2 of the second spiral groove 16. In this embodiment, the amount of offset between the groove centers X2 and X3 is set to match the amount of displacement D of the nut 12 to the other side in the axial direction when a load F is applied (hereinafter also referred to as the amount of offset D).

[0046] The left diagram in Figure 8 shows a state in which the nut 12 is not receiving a load F (see Figure 1) toward the other axial side. In this state, the nut 12 is not displaced toward the other axial side. At this time, the groove center X2 of the second spiral groove 16 coincides with the groove center X1 of the first spiral groove 15 when viewed from the radial direction. On the other hand, the groove centers X3 of the first introduction portion 26 and the second introduction portion 28 do not coincide with the groove centers X1 and X2 when viewed from the radial direction, and are shifted toward one axial side. In this state, the groove center X3 is shifted toward one axial side by a shift amount D with respect to the groove centers X1 and X2.

[0047] The right diagram of Figure 8 shows a state in which the nut 12 is subjected to a load F (see Figure 1) toward the other axial side. When the load F is applied, the nut 12 is displaced toward the other axial side by a displacement amount D. At this time, the groove center X2 does not coincide with the groove center X1 when viewed from the radial direction. On the other hand, the groove center X3 is displaced toward the other axial side by the displacement amount D and coincides with the groove center X1 when viewed from the radial direction. In this state, the amount of deviation of the groove center X3 toward one axial side relative to the groove center X1 is "0". Note that it is preferable that the deviation amount D between the groove centers X2 and X3 coincide with the displacement amount D of the nut 12 when the load F is applied, but it does not necessarily have to coincide.

[0048] 8 , in the nut 12, a contact position Q where the ball 13 introduced into the circulation groove 23 first contacts the circulation groove 23 is referred to as a third contact position Q3. Of the third contact positions Q3, the third contact position Q3 on the end side (position P2 side) of the rolling path 17 is referred to as a third contact position Q3b, and the third contact position Q3 on the start side (position P1 side) of the rolling path 17 is referred to as a third contact position Q3a, to distinguish them from each other. The two third contact positions Q3 (Q3a and Q3b) are point-symmetric with respect to the center point Y of the circulation groove 23. In the ball screw device 10 of the present disclosure, when the nut 12 is subjected to a load F toward the other axial side, the groove center X3 of each of the first introduction portion 26 and the second introduction portion 28 can be made to substantially coincide with the groove center X1 of the screw shaft 11. At this time, the third contact position Q3 (Q3a and Q3b) between the ball 13 and the circulation groove 23 substantially coincides with the "ideal" first contact positions Q1a and Q1b (see the left diagram in FIG. 7).

[0049] In the ball screw device 10 of the present disclosure, it is most preferable that the third contact position Q3 completely coincide with the "ideal" first contact position Q1, but it may also be configured so that they roughly coincide. In the ball screw device 10 of the present disclosure, the third contact position Q3 does not vary greatly, unlike the second contact positions Q2 (Q2a and Q2b) shown in the right diagram of Figure 15. Therefore, the ball screw device 10 of the present disclosure can suppress fluctuations in the propulsive force acting on the ball 13 moving through the circulation groove 23.

[0050] FIG. 9A is a cross-sectional view of the A1-A1 portion indicated by the dashed line in FIG. 8 . FIG. 9B is a cross-sectional view of the A2-A2 portion indicated by the dashed line in FIG. 8 . The A1-A1 portion and the A2-A2 portion correspond to the third contact position Q3 (Q3b). FIG. 9A shows a state in which the nut 12 is not receiving a load F toward the other axial direction. In this state, the groove center X2 of the second spiral groove 16 coincides with the groove center X1 of the first spiral groove 15 when viewed from the radial direction. Meanwhile, the groove centers X3 of the first introduction portion 26 and the second introduction portion 28 do not coincide with the groove centers X1 and X2 when viewed from the radial direction, and are shifted to one axial side. In this state, the groove center X3 is shifted to one axial side by a shift amount D relative to the groove centers X1 and X2.

[0051] 9B shows a state in which the nut 12 is subjected to a load F toward the other axial side. When the load F is applied, the nut 12 is displaced toward the other axial side by a displacement amount D. At this time, the groove center X2 does not coincide with the groove center X1 when viewed from the radial direction. On the other hand, the groove center X3 is displaced toward the other axial side by the displacement amount D and coincides with the groove center X1 when viewed from the radial direction. In this state, the amount of deviation of the groove center X3 toward one axial side relative to the groove center X1 is "0".

[0052] [Regarding another embodiment of the nut] Figure 10 is an explanatory diagram showing another embodiment of a nut having a circulation groove in a misaligned state. The ball screw device 10 of the present disclosure may use the nut shown in the right diagram of Figure 10 instead of the nut 12.

[0053] The right diagram of Figure 10 shows another embodiment of a nut constituting the ball screw device 10 of the present disclosure. In the following description, the nut according to this another embodiment will be referred to as nut 22. As shown in the right diagram of Figure 10, the nut 22 according to this another embodiment differs from the previously described nut 12 in that it has a protrusion 19 on the inner circumferential surface of the through hole 18. Furthermore, in the nut 22, the through hole 18 is enlarged on one side in the axial direction, and the shape of the through hole 18 differs from that of the previously described nut 12.

[0054] 10 , the protrusion 19 is a convex portion formed on the inner circumferential surface of the through hole 18. The protrusion 19 is formed so that the amount of protrusion from the inner circumferential surface of the through hole 18 is T.

[0055] The nut 22 uses a block 24 (see the left diagram in FIG. 10 ) having the same shape as a conventional block (see the block 14 in FIG. 6 ) as the block inserted into the through hole 18. Therefore, the conventional block 14 (see FIG. 6 ) can be used as the block 24. The circulation groove 23 formed in the block 24 is not shifted to one side in the axial direction relative to the center point of the block 24. In the nut 22, the entire block 24 is shifted to one side in the axial direction by the protrusion 19 provided in the through hole 18. By providing the protrusion 19 in the through hole 18 and enlarging the through hole 18, the nut 22 configured in this manner can easily provide a circulation groove 23 that is shifted to one side in the axial direction while still using the conventional block 24. When using the nut 22, it is preferable to further provide a member to fill the gap formed between the block 24 and the through hole 18.

[0056] By employing a nut 22 having such a configuration, the amount of misalignment of the circulation groove 23 to one side in the axial direction can be easily adjusted by adjusting the height of the protrusion 19 and the expansion amount of the through hole 18. In this embodiment, the protrusion 19 is provided on the inner peripheral surface of the through hole 18, but the protrusion 19 may also be provided on the outer peripheral surface of the block 24. In this case, it is preferable to add the protrusion 19 to the block 24 of a different product.

[0057] [Regarding Operation and Effects of the Present Embodiment] As described above, the ball screw device 10 of the present embodiment includes the screw shaft 11 having the first helical groove 15 on its outer peripheral surface, the cylindrical nut 12 having the second helical groove 16 on its inner peripheral surface that faces the first helical groove 15 and surrounds the screw shaft 11, and a plurality of balls 13 disposed in the rolling path 17 formed between the first helical groove 15 and the second helical groove 16. The nut 12 has a circulation groove 23 midway along the second helical groove 16 for returning the balls 13 from a position P2 on the end side of the rolling path 17 to a position P1 on the start side. The nut 12 receives a larger axial load F when moving toward one side in the axial direction than when moving toward the other side in the axial direction. In the ball screw device 10, the groove centers X3 of the first introduction portion 26 and the second introduction portion 28 in the circulation groove 23 are disposed away from the groove center X2 of the second helical groove 16 to one side in the axial direction.

[0058] According to the ball screw device 10 of this embodiment, even if an axial load F is applied when the nut 12 moves to one side in the axial direction and the second helical groove 16 of the nut 12 shifts to the other side in the axial direction relative to the first helical groove 15 of the screw shaft 11, the circulation groove 23 is positioned offset to one side in the axial direction relative to the second helical groove 16, thereby suppressing axial positional deviation between the circulation groove 23 and the first helical groove 15 of the screw shaft 11. As a result, in the ball screw device 10 used in a state in which a load F acting on the other side in the axial direction acts on the nut 12, fluctuations in the thrust of the balls 13 when the balls 13 are scooped by the circulation groove 23 and move in the radial direction can be suppressed. This also suppresses vibration and noise generated when the balls 13 move in the circulation groove 23, improves the life of the circulation groove 23, and suppresses torque fluctuations of the screw shaft 11.

[0059] In the ball screw device 10 of this embodiment, the nut 12 has a through hole 18 that penetrates in the radial direction, and a top 14 that is disposed in the through hole 18. In the ball screw device 10 of this embodiment, the circulation groove 23 is formed in the top 14. According to the ball screw device 10 of this embodiment, damage to the circulation groove 23 caused by contact with the balls 13 can be reduced, thereby improving the life of the top 14.

[0060] The ball screw device 10 of this embodiment has a protrusion 19 provided on the inner peripheral surface of the through hole 18 or the outer peripheral surface of the nut 12. In the ball screw device 10 of this embodiment, the top 14 is positioned axially away from the through hole 18 on one side by the protrusion 19. According to the ball screw device 10 of this embodiment, the amount of positional deviation of the circulation groove 23 with respect to the second spiral groove 16 can be easily adjusted by adjusting the protrusion amount T of the protrusion 19.

[0061] [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.

[0062] 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 19 Projection 23 Circulation groove F Load X2 Groove center (of second spiral groove) X3 Groove center (of introduction portion of circulation groove) P1 Position on the starting point side (of the rolling path) P2 Position on the end point side (of the rolling path)

Claims

1. A ball screw device comprising: a screw shaft having a first spiral groove on its outer peripheral surface; a cylindrical nut having a second spiral groove on its inner peripheral surface that faces the first spiral groove and surrounds the screw shaft; 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 balls from the end side of the rolling path to the start side, and wherein the nut is subjected to a greater axial load when moving to one axial side than when moving to the other axial side, and the groove center of the circulation groove is arranged away from the groove center of the second spiral groove to one axial side.

2. A ball screw device according to claim 1, wherein the nut has a through hole penetrating in the radial direction and a top disposed within the through hole, and the circulation groove is formed in the top.

3. A ball screw device according to claim 2, further comprising a protrusion provided on the inner peripheral surface of the through hole or the outer peripheral surface of the nut, wherein the top is positioned away from the through hole on one side in the axial direction by the protrusion.

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