Ball screw device

Abstract

To provide a ball screw device capable of realizing smooth ball circulation on an endless circulation passage while forming a circulation groove to an inner peripheral surface of a nut member by cutting work.SOLUTION: A ball screw device includes: a number of balls; a screw shaft having a ball rolling groove spirally formed on its outer peripheral surface; and a nut member having a through hole for inserting the screw shaft, further having a forwarding passage formed on an inner peripheral surface of the through hole for circulating the number of balls only by one round at the circumference of the screw shaft, and engaged with the screw shaft via the number of balls. The forwarding passage has a load rolling groove opposed to a part of the rolling groove of the screw shaft, and a circulation groove connecting both ends of the load rolling groove while crossing a screw thread of the screw shaft. The forwarding passage is provided in adjacent to an opening edge of the through hole of the nut member, and a corner portion where an inner peripheral surface of the through hole and an axial end face of the nut member cross, is provided with an inclination portion formed by cutting out the corner portion with an uneven width along a circumferential direction of the through hole.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to a ball screw device in which an endless ball circulation path is provided for one revolution around the periphery of a screw shaft in a nut member. [Background technology]

[0002] Ball screw devices are mechanical elements capable of converting rotational motion and linear motion, and are widely used in various machine tools, conveying devices, industrial robots, etc. to convert rotational motion generated by servo motors into linear motion. Such ball screw devices are comprised of a screw shaft with a spiral rolling groove, a number of balls that roll in the rolling groove, and a nut member having a through-hole through which the screw shaft is inserted and an infinite circulation path through which the balls roll. The screw shaft and the nut member are threadedly engaged with each other via the balls. Various types of conventional ball screw devices with different structures of the infinite circulation path are known, but Patent Document 1 discloses a ball screw device in which the nut member is provided with multiple infinite circulation paths, each of which runs around the entire circumference of the screw shaft.

[0003] In the ball screw device of Patent Document 1, a load rolling groove is formed on the inner circumferential surface of the through-hole of the nut member, facing a portion of the rolling groove of the screw shaft, and a circulation groove is provided connecting the start and end of the load rolling groove. The load rolling groove faces the rolling groove of the screw shaft, forming a load passage, and balls roll within the load passage while bearing a load between the screw shaft and the nut member. Meanwhile, the circulation groove is an unloaded passage through which balls roll when released from the load, connecting both ends of the load passage to form an infinite circulation path for balls. The circulation groove allows balls that reach the end of the load passage to leave the rolling groove of the screw shaft, ride over the threads, and return to the start of the load passage. In other words, the circulation groove can return balls rolling within the load passage by one lead of the rolling groove of the screw shaft, thereby forming an infinite circulation path provided in the nut member that is a circuit spanning one revolution around the screw shaft.

[0004] In the ball screw device of Patent Document 1, the load rolling groove and the circulation groove are formed directly on the inner peripheral surface of the nut member, and the latter circulation groove in particular is machined into the inner peripheral surface of the nut member using a tool such as an end mill. In this machining, the tool is inserted axially into the through hole of the nut member, and the tool cuts into the inner peripheral surface of the through hole of the nut member in the radial direction, and the cutting depth is controlled to form the circulation groove. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2015-24471 Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, the circulation groove allows the balls to disengage from the rolling groove of the screw shaft and ride over the threads of the screw shaft, and the circulation groove is formed as a passage that is open to the screw shaft, rather than as a passage that is separated from the screw shaft.

[0007] For this reason, the balls rolling in the circulation groove tend to come into irregular contact with the threads of the screw shaft, which raises concerns that smooth circulation of the balls in the endless circulation path may be hindered.

[0008] In order to prevent contact between the balls rolling in the circulation groove and the screw shaft as much as possible, and to allow the balls to progress smoothly from the rolling groove of the screw shaft into the circulation groove, it is possible to provide a holding portion on the opening edge of the circulation groove along the direction in which the balls progress, and to make the opening width of the circulation groove open toward the screw shaft narrower than the diameter of the balls.

[0009] In this regard, in Patent Document 1, when processing the nut member, a tool is inserted into the through hole of the nut member from the axial direction, and the tool is used to cut radially into the inner surface of the through hole of the nut member to form the circulation groove, which makes it difficult to form the embracing portion on the opening edge of the circulation groove. [Means for solving the problem]

[0010] The present invention has been made in consideration of such problems, and its purpose is to provide a ball screw device that can achieve smooth circulation of balls in an infinite circulation path while forming a circulation groove on the inner surface of the nut member by cutting.

[0011] That is, the present invention provides a nut member having a large number of balls, a screw shaft having rolling grooves for the balls formed in a spiral on its outer peripheral surface, and a through hole through which the screw shaft is inserted, and an inner peripheral surface of the through hole having a path for circulating the large number of balls around the screw shaft only once, and a load rolling groove facing the rolling groove formed in a spiral, and screwed onto the screw shaft via the large number of balls rolling on the path, wherein the path is provided adjacent to the opening edge of the through hole of the nut member, and at the corner where the inner peripheral surface of the through hole and the axial end face of the nut member intersect, an inclined portion is provided by cutting out the corner with an uneven width along the circumferential direction of the through hole. [Effects of the Invention]

[0012] The ball screw device of the present invention has a corner where the inner circumferential surface of a through hole formed in a nut member intersects with the axial end face of the nut member, and the corner is provided with an inclined portion cut out of the corner with a non-uniform width along the circumferential direction of the through hole. Therefore, in this ball screw device, the insertion angle of a cutting tool relative to the inner circumferential surface of the through hole is not limited by the distance from the axial end face of the circulation groove during cutting of the nut member, and cutting of the circulation groove can be performed at any insertion angle corresponding to the non-uniform width of the inclined portion. This allows sufficient support to be formed in the circulation groove, guiding the balls as they climb over the threads. Therefore, the ball screw device of the present invention can achieve smooth, infinite circulation of rolling balls while forming a circulation groove on the inner circumferential surface of the nut member by cutting. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a perspective view showing a ball screw device according to an embodiment of the present invention; [Figure 2] FIG. 2 is a cross-sectional view showing a nut member according to one embodiment of the present invention. [Figure 3] FIG. 4 is a cross-sectional view showing a circulation groove. [Figure 4] FIG. 1 is a perspective view showing a nut member according to an embodiment of the present invention. [Figure 5] FIG. 10 is a perspective view showing a state in which cutting is being performed on the nut member. [Figure 6] 1 is a view showing a state in which a nut member according to an embodiment of the present invention is observed from an axial direction. FIG. [Figure 7] FIG. 2 is an enlarged cross-sectional view of a nut member according to one embodiment of the present invention. [Figure 8] 10 is a graph showing the relationship between the notch width w of the inclined portion and the circumferential position θ. [Figure 9] 10 is a graph showing the surface depth h of the inclined portion versus the circumferential position θ. [Figure 10] 10 is a graph showing the distance s of the circulation groove versus the position θ in the circumferential direction. [Figure 11] FIG. 7 is a cross-sectional view taken along line III-III in FIG. 6. [Figure 12] FIG. 10 is a cross-sectional view showing a state in which cutting is being performed on the nut member. [Figure 13] FIG. 10 is an enlarged cross-sectional view showing the circulation groove when the nut member is machined. [Figure 14] FIG. 10 is a perspective view showing a state where cutting is performed on a conventional nut member. [Figure 15] FIG. 10 is an enlarged cross-sectional view showing a circulation groove when a conventional nut member is machined. DETAILED DESCRIPTION OF THE INVENTION

[0014] The ball screw device of the present invention will be described in detail below with reference to the accompanying drawings.

[0015] 1 is a perspective view showing a ball screw device 100 according to one embodiment of the present invention. The ball screw device 100 includes a screw shaft 110 having a ball rolling groove 111 formed in a spiral shape on its outer circumferential surface, and a nut member 120 having a through hole through which the screw shaft 110 is inserted and screwed onto the screw shaft 110 via a number of balls. The balls roll spirally between the screw shaft 110 and the nut member 120, so that when the screw shaft 110 is rotated relative to the nut member 120, the nut member 120 moves in the axial direction of the screw shaft 110, or when the nut member 120 is rotated relative to the screw shaft 110, the screw shaft 110 moves in the axial direction of the nut member 120.

[0016] FIG. 2 is a cross-sectional view of the nut member 120 taken along the axial direction. A path groove 210 is provided on the inner peripheral surface 240 of the through hole of the nut member 120, allowing the balls to circulate endlessly around the screw shaft 110. In this embodiment, a pair of path grooves 210 are provided adjacent to the opening edge of the through hole of the nut member 120, and each path groove 210 runs around the inner peripheral surface 240 of the through hole of the nut member 120. The path groove 210 is composed of a load rolling groove 121 formed on the inner peripheral surface 240 of the through hole of the nut member 120 and a circulation groove 220 connecting both ends of the load rolling groove 121. In this embodiment, the path grooves 210 are provided on the opening edges of both axial ends of the through hole, but the path groove 210 may be provided on only one of the opening edges.

[0017] The load rolling groove 121 is formed in a spiral shape facing a part of the rolling groove of the screw shaft 110, and is formed to a length that is less than one full circumference of the screw shaft. This load rolling groove 121 forms a load passage by facing the rolling groove 111 of the screw shaft 110, and the balls roll in the load passage while applying a load between the screw shaft and the nut member.

[0018] Meanwhile, the circulation groove 220 is formed on the inner peripheral surface 240 of the nut member 120, continuing from the load rolling groove 121. The circulation groove 220 is deeper than the load rolling groove 121 so that the balls can disengage from the rolling groove 111 of the screw shaft 110 and ride over the threads of the screw shaft 110. That is, the circulation groove 220 connects one end of the load rolling groove 121 to the other end, crossing the threads of the screw shaft 110. When the balls enter the circulation groove 220 from the load rolling groove 121, they are released from the load and enter an unloaded state. When the balls enter the load rolling groove 121 from the circulation groove 220, they are loaded again. That is, the circulation groove 220 is an unloaded passage for balls, and both ends of the loaded passage are connected by an unloaded passage, thereby forming an infinite circulation passage that circles around the screw shaft 110. Therefore, the nut member 120 of this embodiment has two endless circulation paths for balls.

[0019] Connecting portions (not shown) are provided at both ends of the circulation groove 220 to transfer balls between the circulation groove 220 and the load rolling groove 121. The connecting portions are gently inclined with respect to the circulation groove 220, and release the balls rolling in the load rolling groove 121 from the load and gradually release the balls from the rolling groove of the screw shaft 110. This allows the balls rolling in the load rolling groove 121 to smoothly enter the circulation groove 220 and ride over the threads of the screw shaft 110.

[0020] 3 is a cross-sectional view of the circulation groove 220. At both ends of the circulation groove 220 in the width direction, protrusions 221 are provided that are continuously formed along the length direction of the circulation groove 220. The protrusions 221 are shaped to protrude toward the inside of the circulation groove 220. The opening width of an embracing portion 222 formed by the pair of protrusions 221 is set to be narrower than the diameter of the ball. Therefore, when a ball enters the circulation groove 220 from the end of the load rolling groove 121, the ball is embraced by the embracing portion 220 when it goes over the thread of the screw shaft 110, and can go over the thread while avoiding contact with the thread.

[0021] 2 and 4, a corner 310 where an inner peripheral surface 240 of the through hole and an axial end face of the nut member 120 intersect has an inclined portion 230 formed as a curved surface along the circumferential direction of the through hole. This inclined portion 230 is used when cutting the circulation groove 220 into the nut member. The inclined portion 230 is formed by cutting out the corner 310 with a non-uniform width. The inclined portion 230 is provided at a position that is 180 degrees opposite to the position where the circulation groove 220 is formed in the circumferential direction of the through hole of the nut member 120. The non-uniform width of the inclined portion 230 includes a notch width w and a surface depth h, which will be described later.

[0022] 5 is a diagram showing a state in which a circulation groove 220 is being machined in a nut member 120 using a ball end mill 400 as a cutting tool. The ball end mill 400 used has a narrow neck, and a spherical head 410 is formed at the tip of a handle 420 of the ball end mill 400. When cutting the circulation groove 220 with the ball end mill 400, a provisional groove is formed in advance at the position where the circulation groove 220 will be formed. The cutting of the circulation groove 220 is performed by inserting the spherical head 410 of the ball end mill 400 into the provisional groove.

[0023] FIG. 6 shows the nut member 120 as viewed from the axial direction. The broken line indicates the circulation groove 220 formed in the nut member 120. The angle between one end 211 and the other end 212 of the circulation groove 220 and the center of the through hole is approximately 90 degrees. The circulation groove 220 connects both ends of the spirally formed load rolling groove, so that the one end 211 of the circulation groove is closer to the axial end face of the nut member 120 than the other end 212 (see FIG. 4). Therefore, in FIG. 6, the ball traveling from the one end 211 to the other end 212 in the circulation groove 220 gradually moves away from the axial end face of the nut member 120.

[0024] 2. As shown in FIGS. 6 and 7, the inclination angle of the inclined portion 230 from the axial end face of the nut member 120 is determined by the notch width w and surface depth h of the inclined portion 230. Therefore, the inclination angle of the inclined portion 230 is relatively small where the notch width w is wide and the surface depth h is small. The notch width w and surface depth h of the inclined portion 230 are adjusted according to the distance between the circulation groove 220 and the axial end face of the nut member 120.

[0025] In the example of FIG. 6 , the notch width w and face depth h of the inclined portion 230 at a position approximately 180 degrees away from one end 211 of the circulation groove 220 in the circumferential direction of the through hole of the nut member are wider and larger than those at other positions on the inclined portion 230 (see FIG. 7 ). That is, the notch width w and face depth h of the inclined portion 230 are maximum at a position 180 degrees away from the position where the circulation groove 220 is closest to the axial end face of the nut member 120 in the circumferential direction of the through hole of the nut member 120. Note that in this embodiment, the notch width w and face depth h of the inclined portion 230 are maximum at a position 180 degrees away from the position where the circulation groove 220 is closest to the axial end face of the circulation groove 220. However, this is not limited thereto, and any appropriate design change is possible as long as the notch width w and face depth h continuously change along the circumferential direction of the through hole of the nut member 120. However, as shown in FIG. 7, the surface depth h of the inclined portion 230 is set to a size that prevents the inclined portion 230 from interfering with the load rolling groove 121.

[0026] 8 and 9 are graphs showing the relationship between the circumferential position θ of the nut member 120 (see FIG. 6) and the notch width w and surface depth h of the inclined portion 230. FIG. 10 is a graph showing the relationship between the circumferential position θ of the nut member 120 and the distance s of the path groove 210 from the axial end face of the nut member 120. As shown in FIG. 7, the distance s indicates the formation position of the path groove 210, including the circulation groove 210, from the axial end face. The area within the dashed line in FIG. 10 indicates the load rolling groove 121, which is part of the path groove 210, and the area outside the dashed line indicates the circulation groove 220. As shown in these graphs, the notch width w and surface depth h of the inclined portion 230 are maximum at approximately the same circumferential position θ. The position where the notch width w and surface depth h of the inclined portion 230 are maximum is approximately 180 degrees opposite, in the circumferential direction of the nut member 120, to the position where the path groove 210 is closest to the axial end face of the nut member 120.

[0027] Furthermore, with respect to the circumferential position θ of the nut member 120, the inclined portion 230 is provided at a position facing the circulation groove 220 by 180°, but is not provided at a position overlapping the circulation groove 220. Furthermore, with respect to the circumferential position θ, the notch width w and face depth h of the inclined portion 230 become smaller as it approaches the position where the circulation groove 220 is formed. In other words, the notch width w and face depth h of the inclined portion 230 change continuously along the circumferential direction of the nut member 120.

[0028] 11 is a cross-sectional view taken along line III-III in FIG. 6, showing the state in which one end 211 of the circulation groove 220 is being machined. At this time, the notch width w and surface depth h of the inclined portion 230 at a position approximately 180 degrees opposite the end 211 are formed large, allowing the ball end mill 400 to stand up to a position nearly perpendicular to the inner circumferential surface 240 of the nut member 120. FIG. 12 is a view showing the state in which the intermediate position of the circulation groove 220 is being machined. The machining of the intermediate position of the circulation groove 220 is performed by inserting the spherical head 410 deeper into the inner circumferential surface of the nut member 120 than when machining one end 211 of the circulation groove 220.

[0029] 13 is an enlarged view showing the circulation groove 220 after cutting with the ball end mill 400. By cutting the inner peripheral surface of the nut member 120 with the ball end mill 400, a circulation groove 220 having the same curvature as the spherical head 410 is formed on the inner peripheral surface 240 of the nut member 120. In addition, as described above, the circulation groove 220 has an embracing portion 222 formed from a pair of protrusions 221.

[0030] In this regard, conventional nut members do not have inclined portion 230, and when cutting the nut member, shank 420 of ball end mill 400 comes into contact with corner portion 310 of the nut member (see FIG. 14). Therefore, in conventional nut members, when cutting circulation groove 220, ball end mill 400 cannot be inserted in an upright position nearly perpendicular to inner circumferential surface 240. As a result, the upright angle μ of ball end mill 400 relative to inner circumferential surface 240 cannot be sufficiently secured, and therefore embracing portion 222 cannot be formed in circulation groove 220.

[0031] On the other hand, the ball screw device 100 of the present invention has an inclined portion 230 formed by cutting out a corner 310 at an intersection between an inner circumferential surface 240 of a through hole in the nut member 120 and an axial end face of the nut member, with the corner 310 being cut out with a non-uniform width along the circumferential direction of the through hole. Therefore, in the ball screw device 100, the rising angle μ of the ball end mill 400 relative to the inner circumferential surface 240 can be set large when machining the circulation groove 220, and cutting with the ball end mill 400 can be performed in a state close to perpendicular to the inner circumferential surface 240. This allows a holding portion 222 to be formed in the circulation groove 220, and the circulation groove 220 can guide the balls that climb over the threads. Therefore, the ball screw device 100 of the present invention can realize smooth, infinite circulation of rolling balls while forming a circulation groove on the inner circumferential surface of the nut member by cutting.

[0032] 100... ball screw device, 110... screw shaft, 120... nut member, 121... load rolling groove, 210... dead path, 220... circulation groove, 230... inclined portion, 240... through hole, 310... corner portion, 222... embracing portion, 400... ball end mill

Claims

1. Many balls and a screw shaft having a rolling groove for the balls formed in a spiral shape on an outer peripheral surface thereof; The screw shaft has a through hole through which the screw shaft is inserted, and the inner circumferential surface of the through hole has a path that circulates the large number of balls around the screw shaft only once, and a nut member that is screwed onto the screw shaft via the large number of balls rolling on the path. The deadhead path is a load rolling groove facing a part of the rolling groove of the screw shaft; a circulation groove that crosses the threads of the screw shaft and connects both ends of the load rolling groove, The deadhead path is provided adjacent to an opening edge of the through hole of the nut member, A ball screw device characterized in that a corner where the inner surface of the through hole and the axial end face of the nut member intersect is provided with an inclined portion cut out of the corner with an uneven width along the circumferential direction of the through hole.

2. 2. The ball screw device according to claim 1, wherein the inclined portion is provided at a position facing at least a position where the circulation groove is formed in the circumferential direction of the through hole of the nut member.

3. 3. The ball screw device according to claim 2, wherein the notch width of the inclined portion and the inclination angle relative to the axial end face of the nut member change continuously along the circumferential direction of the through hole of the nut member.

4. 4. The ball screw device according to claim 3, wherein the width of the notch in the inclined portion is greatest in the circumferential direction of the through hole of the nut member from the position closest to the axial end face of the nut member to the position opposite the circulation groove.

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