Ball screw device

Abstract

The present invention provides a ball screw device that can provide sufficient strength to the circulating member at the connection between the return passage and the ball return hole, and can increase the speed of ball rolling in an infinite circulation path. [Solution] A ball screw device comprising: a screw shaft having a helical rolling groove; a nut member having a through hole into which the screw shaft is loosely fitted, and having a helical load rolling groove on the inner circumferential surface of the through hole that faces the rolling groove of the screw shaft and constitutes a load passage for the balls; a ball return hole provided in the nut member; and a pair of circulating members attached to both ends of the nut member to construct a return passage connecting the end of the load passage and the end of the ball return hole, wherein the return passage is formed in a curved shape with a continuously changing curvature, and the curvature is greatest in the center than at both ends in the longitudinal direction, and a centripetal force directed radially inward of the nut member is always acting on the balls rolling in the return passage.

Description

Technical Field

[0001] The present invention relates to a ball screw device capable of mutually converting rotational motion and linear motion.

Background Art

[0002] A ball screw device is a mechanical element capable of mutually converting rotational motion and linear motion, and is widely used in various machine tools, conveying devices, industrial robots and the like for the purpose of converting rotational motion generated by a servo motor into linear motion. The ball screw device is configured such that a screw shaft and a cylindrical nut member are screwed with each other via a large number of balls, and a spiral rolling groove on which the balls roll is formed on the outer peripheral surface of the screw shaft at a predetermined lead. Further, a spiral loaded rolling groove facing the rolling groove of the screw shaft is formed on the inner peripheral surface of the nut member, and the rolling groove and the loaded rolling groove face each other to form a spiral loaded track on which the balls roll while bearing a load.

[0003] The nut member is provided with a circulation passage that connects both ends of the loaded track to circulate the balls, and various structures of such circulation passage are known. As one of them, a structure is known in which end plates are attached to both axial end faces of the nut member to provide a ball circulation passage (for example, Patent Document 1). In this end plate type ball screw device, ball return holes extending along the axial direction are provided in the nut member, and a pair of end plates are respectively attached to both end faces of the nut member, and the end portions of the loaded track and the end portions of the ball return holes are connected by return passages formed in the end plates.

[0004] Furthermore, in the ball screw device disclosed in Patent Document 1, when the return passage formed in the end plate is viewed from the axial direction of the nut member, the return passage extends tangentially to the inner circumferential surface of the nut member and is formed in a curved shape toward the ball return hole, and its radius of curvature is set to gradually increase as it approaches the ball return hole from the load track side. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2010-025301 [Overview of the project] [Problems that the invention aims to solve]

[0006] The return passage exists along a plane perpendicular to the axial direction of the nut member, while the ball return hole formed in the nut member extends in a direction parallel to the axial direction of the nut member. Therefore, at the connection between the return passage and the ball return hole, the ball needs to change its rolling direction by approximately 90 degrees. Consequently, at the end of the return passage on the ball return hole side, the ball repeatedly collides with the end wall of the return passage. In particular, in ball screw devices where the ball is expected to circulate at high speed, sufficient strength had to be ensured in the circulating member.

[0007] However, in conventional ball screw devices, the connection direction of the return passage to the ball return hole is closer to the radial direction of the nut member than to the circumferential direction. Therefore, in order to give the circulating member sufficient strength, the outer diameter of the nut member to which the circulating member is attached naturally becomes larger, raising concerns that the ball screw device will become larger. [Means for solving the problem]

[0008] The present invention has been made in view of these problems, and its objective is to provide a ball screw device that can provide sufficient strength to the circulating member at the connection between the return passage and the ball return hole, and can increase the speed of ball rolling in the infinite circulation path.

[0009] In other words, the present invention is based on a ball screw device comprising: a number of balls; a screw shaft having a helical rolling groove on its outer circumference through which the number of balls roll; a nut member having a through hole into which the screw shaft is loosely fitted, and having a helical load rolling groove on the inner circumference of the through hole that forms a load passage for the balls opposite to the rolling groove of the screw shaft; a ball return hole provided parallel to the through hole with respect to the nut member; and a pair of circulating members attached to both axial ends of the nut member to form a return passage connecting the end of the load passage and the end of the ball return hole, wherein the return passage is formed in a curved shape with a continuously changing curvature, and the curvature is greatest in the center of the return passage than at both ends in the longitudinal direction, and a centripetal force directed radially inward of the nut member is always acting on the balls rolling in the return passage. [Effects of the Invention]

[0010] According to the present invention, since the ball return hole of the nut member and the return passage for transferring the balls are connected to the ball return hole from the circumferential direction of the nut member or from a direction adjacent thereto, even if the strength of the circulating member at the connection part between the ball return hole and the return passage is improved, the diameter of the nut member does not increase, making it possible to achieve both high-speed ball rolling in the infinite circulation path and miniaturization of the nut member. [Brief explanation of the drawing]

[0011] [Figure 1] This is a perspective view showing an example of a ball screw device to which the present invention is applied. [Figure 2] This is a schematic diagram showing the configuration of the ball infinite circulation path in the nut component. [Figure 3] This is a perspective view showing the nut component. [Figure 4] This is a perspective view showing the nut component with the circulation component removed. [Figure 5] This is a perspective view showing the circulating components. [Figure 6] This is an enlarged perspective view showing the main part of the accommodating groove for the circulating member provided in the nut member. [Modes for carrying out the invention]

[0012] The ball screw device of the present invention will be described in detail below with reference to the attached drawings.

[0013] Figures 1 and 2 show an example of a ball screw device to which the present invention is applied. This ball screw device 1 consists of a screw shaft 2 with ball rolling grooves 20 formed spirally on its outer circumferential surface, and a cylindrical nut member 3 that screws around the screw shaft 20 via a number of balls 4. By rotating the screw shaft 2 relative to the nut member 3, the nut member 3 moves in the axial direction of the screw shaft 2, or by rotating the nut member 3 relative to the screw shaft 2, the screw shaft 2 moves in the axial direction of the nut member 3.

[0014] A single rolling groove 20 is formed on the outer circumferential surface of the screw shaft 2 with a predetermined lead, and the rolling grooves 20 are present at a constant pitch along the axial direction of the screw shaft 2. Furthermore, the space between adjacent rolling grooves 20 is a threaded portion 21, and this threaded portion 21 represents the outer diameter of the screw shaft 2. The rolling groove 20 is formed by a pair of ball rolling surfaces intersecting at an angle of approximately 90 degrees, and its cross-section is a so-called Gothic arch shape. It should be noted that, for example, two or more rolling grooves 20 may be formed on the outer circumferential surface of the screw shaft 2.

[0015] The nut member 3 is formed in a substantially cylindrical shape with a through hole 22, the through hole 22 has an inner diameter slightly larger than the outer diameter of the screw shaft 2, and the screw shaft 2 is inserted therein. At one axial end of the outer peripheral surface of the nut member 3, a flange portion 30 for fixing the nut member 3 to a mechanical device is provided, and a plurality of mounting holes 31 for fixing bolts are formed in this flange portion 30. Note that the flange portion is not an essential component of the ball screw device of the present invention.

[0016] On the other hand, a spiral loaded rolling groove having the same lead as the rolling groove 20 of the screw shaft 2 is formed on the inner peripheral surface of the nut member 3. The rolling groove 20 of the screw shaft 2 and the loaded rolling groove of the nut member 3 face each other to form a loaded raceway 60 for the balls 4, and the balls 4 rolling in this loaded raceway 60 bear a load between the screw shaft 2 and the nut member 3. Similar to the rolling groove 20 of the screw shaft 2, the loaded rolling groove is formed by a pair of ball rolling surfaces intersecting at an angle of approximately 90 degrees, and its cross section has a so-called gothic arch shape.

[0017] Depending on the preload application method for the balls 4, the balls 4 may bear a load while making two-point contact with each of the rolling groove 20 of the screw shaft 2 and the loaded rolling groove of the nut member 3, or may bear a load while making one-point contact with each of the rolling groove 20 of the screw shaft 2 and the loaded rolling groove of the nut member 3.

[0018] Further, the nut member 3 is provided with an infinite circulation path for the balls 4. This infinite circulation path is composed of the loaded raceway 60 and unloaded raceways 61 connecting both ends thereof. The unloaded raceway 61 is composed of a linear ball return hole 32 penetrating the nut member 3 in the axial direction, and return passages 50 connecting the ends of the loaded raceway 60 and the ball return hole 32.

[0019] Further, a circulating member 5 is attached to each of the axial end faces of the nut member 3, and the return passage 50 is constructed in the nut member 3 by the attachment of the circulating member 5. That is, when a pair of circulating members 5 are attached to the nut member 3, the no-load raceway 61 composed of the ball return hole 32 and the return passage is completed, and accordingly, the infinite circulation path for the balls 4 composed of the loaded raceway 60 and the no-load raceway 61 is completed.

[0020] Figure 3 is a perspective view showing the nut member 3, and shows a state where the circulating member 5 is attached to the end face of the nut member 3. Further, Figure 4 is a perspective view showing the nut member 3 in a state where the circulating member 5 is removed. A spiral loaded rolling groove 33 is formed on the inner peripheral surface of the through hole 22 of the nut member 3. As described above, this loaded rolling groove 33 faces the spiral rolling groove 20 formed on the outer peripheral surface of the screw shaft 2, and constitutes the loaded raceway 60. Therefore, in a state where the screw shaft 2 is inserted into the through hole 22 of the nut member 3, the balls 4 rolling in the loaded rolling groove 33 are in contact with both the loaded rolling groove 33 and the rolling groove 20 of the screw shaft 2, and roll while bearing a load between the screw shaft 2 and the nut member 3.

[0021] The circulating member 5 is mounted in a receiving groove 34 formed in the axial end face of the nut member 3, whereby the return passage 50 that connects the loaded raceway 60 and the ball return hole 32 is completed. The split surface between the circulating member 5 and the nut member 3 is positioned along the central trajectory of the balls 4 passing through the return passage 50, and the return passage 50 is formed by being divided into the circulating member 5 and the receiving groove 34 of the nut member 3.

[0022] As shown in Figure 4, the receiving groove 34 is provided in a curved shape to connect the through hole 22 of the nut member 3 and the ball return hole 32. The receiving groove 34 consists of a retaining and fixing portion 34a with an opening for the ball return hole 32 and a ball release portion 34b facing the through hole 22 of the nut member 3. These retaining and fixing portion 34a and ball release portion 34b are covered from the axial direction of the nut member 3 by the circulation member 5 which is mounted in the receiving groove 34, thereby constructing the return passage 50.

[0023] Figure 5 is a perspective view showing the circulation member 5. This circulation member 5 consists of a direction-changing portion 51 that fits into the holding and fixing portion 34a of the housing groove 34, and an introduction portion 52 that fits into the ball release portion 34b of the housing groove 34. Furthermore, a ball guide portion 54 is provided at the tip of the direction-changing portion 51, which connects to the opening edge of the ball return hole 32. The ball 4 that has rolled through the return passage 50 has its direction of travel changed by approximately 90 degrees by the ball guide portion 54 and is sent from the return passage 50 to the ball return hole 32. In addition, when the circulation member 5 is fitted into the housing groove 34 of the nut member 3, a scooping piece 53 is provided that protrudes to the inner diameter side of the through hole 22 of the nut member 3 and loosely fits into the rolling groove 20 of the screw shaft 2. The dashed line in Figure 5 shows the return passage 50 formed between the circulation member 5 and the nut member 3.

[0024] As can be seen from Figure 4, the retaining and fixing portion 34a and the ball release portion 34b are formed in a gently curved arc shape, but the receiving groove 34 that constitutes the return passage 50 is greatly curved in the intermediate guide portion 34c that connects the retaining and fixing portion 34a and the ball release portion 34b. That is, when comparing the ball release portion 34b, the intermediate guide portion 34c and the retaining and fixing portion 34a, the curvature of the intermediate guide portion 34c is set to be greater than the curvature of the ball release portion 34b and the retaining and fixing portion 34a. As a result, the ball release portion 34b guides the ball 4 in the tangential direction to the inner circumferential surface of the through hole 22 of the nut member 3, while the retaining and fixing portion 34a can guide the ball 4 to the ball return hole 32 from the circumferential direction of the nut member 3.

[0025] As described above, the return passage is formed by dividing it into the circulation member 5 and the accommodating groove 34 of the nut member 3. It is formed in a curved shape along its length, and its curvature changes continuously, with the curvature being greatest in the center than at both ends. That is, the intermediate guide portion 34c of the accommodating groove 34 corresponds to the center of the return passage, connecting the ball release portion 34b and the retaining and fixing portion 34a located at both ends of the return passage. This intermediate guide groove 34c does not necessarily have to be formed in a curved shape with a single curvature; it may be a composite curve formed by a series of curves with different curvatures.

[0026] Furthermore, since the receiving groove 34 is formed in a curved shape that is curved toward the center of the nut member 3 in a plane perpendicular to the axial direction of the nut member 3, a centripetal force directed radially inward of the nut member 3 is always acting on the ball 4 rolling in the return passage 50, and the direction of travel of the ball 4 changes continuously. In addition, a centrifugal force acts on the ball rolling in the return passage, and the ball 4 is pressed against the inner wall of the return passage 50 radially outward of the nut member. Although the ball 4 is unloaded within the return passage 50, the constant acting of such centrifugal force on the ball 4 rolling within the return passage 50 allows the ball 4 to roll stably within the return passage 50 without meandering.

[0027] The ball release portion 34b of the receiving groove 34 extends tangentially to the inner circumferential surface of the through hole 22 of the nut member 3 and is formed in a curved shape that curves toward the center of the nut member 3. The balls 4 rolling on the load track 60 move spirally around the screw shaft 2, and a centripetal force toward the radially inward side of the nut member 3 always acts on the balls 4 rolling on the load track 60, while a centrifugal force toward the radially outward side of the nut member 3 acts on them. For this reason, if the release portion 34b, which constitutes part of the return passage 50, extends tangentially to the inner circumferential surface of the through hole 22 of the nut member 3, the balls that reach the connection between the load track 60 and the return passage 50 can smoothly enter the return passage 50 from the load track 60 due to the centrifugal force described above.

[0028] Furthermore, the retaining and fixing portion 34a of the housing groove 34 is connected to the ball return hole 32 along the circumferential direction of the nut member 3 and is arranged substantially concentrically with the inner circumferential surface of the through hole 22 of the nut member 3. Also, the ball return hole 32 does not open at the longitudinal end of the retaining and fixing portion 34a, but opens at a position slightly away from the end of the retaining and fixing portion 34a that extends circumferentially on the nut member 3. As a result, a fitting seat 35 for positioning and fixing the circulating member 5 to the nut member 3 is provided at the longitudinal end of the retaining and fixing portion 34a. That is, the fitting seat 35 is located on the opposite side of the return passage 50, with the opening of the ball return hole 32 in between. The ball guide portion 54 provided at the tip of the direction changing portion 51 of the circulating member 5 fits into this fitting seat 35.

[0029] The balls 4 that roll from the load track 60 to the ball return hole 32 within the return passage 50 proceed through the release portion 34b and the holding and fixing portion 34a that constitute the housing groove 34, and at the holding and fixing portion 34a reach the entrance of the ball return hole 32 along the circumferential direction of the nut member 3. The return passage 50 is located on the axial end face of the nut member 3, and the balls 4 that enter the ball return hole 32 from the return passage 50 change their rolling direction by approximately 90 degrees in the axial direction of the nut member 3 at the entrance of the ball return hole 32 by the ball guide portion 54 of the circulation member 5.

[0030] At this time, the ball guide portion 54 of the circulating member 5 is subjected to repeated loads from the balls 4, which change direction significantly. In particular, as the relative rotational speed of the nut member 3 and the screw shaft 2 increases, the rolling speed of the balls 4 in the return passage 50 increases, and consequently, a large load is repeatedly applied to the ball guide portion 54 by the balls 4. For this reason, the ball guide portion 54 of the circulating member 5 needs to be strengthened, and measures such as increasing the thickness of the ball guide portion 54 are necessary.

[0031] In this regard, in the ball screw device of this embodiment, as described above, the retaining and fixing groove 34a is connected to the ball return hole 32 along the circumferential direction of the nut member 3, so the fitting seat 35 is located in the circumferential direction of the nut member 3 with respect to the ball return hole 32. For this reason, by increasing the size of the fitting seat 35, it is possible to increase the size of the ball guide portion 54 of the circulating member 5 that fits into the fitting seat, thereby increasing the strength of the circulating member 5. Furthermore, since the fitting seat 35 is located in the circumferential direction of the nut member 3 with respect to the ball return hole 32, even if the size of the fitting seat 35 is increased, the outer diameter of the nut member 3 does not increase, and it is possible to accommodate a higher ball rolling speed while keeping the outer diameter of the nut member 3 under control.

[0032] Figure 6 is an enlarged view showing the area around the retaining and fixing portion 34a of the housing groove 34. The retaining and fixing portion 34a is separated from the through hole 22 of the nut member 3 by a partition wall 36, which extends along the inner circumferential surface of the through hole 22. The retaining and fixing portion 34a is provided with a pair of locking surfaces 37 on both sides of the ball return hole 32. These locking surfaces 37 are provided along the circumferential direction of the nut member, sandwiching the fitting seat 35, and are opposite each other in the radial direction of the nut member 3. In addition, one of the locking surfaces 37 is formed on the partition wall 36. The fitting seat 35 and the pair of locking surfaces 37 can be formed by cutting using a ball end mill or a square end mill.

[0033] The pair of locking surfaces 37 contact the ball guide portion 54 of the circulating member 5, which fits into the housing groove 34, from the radially inner and outer sides of the nut member 3, thereby positioning the circulating member 5, which is mounted in the housing groove 34, relative to the nut member 3. As a result, the entire circulating member 5 that fits into the housing groove 34 does not wobble radially with respect to the nut member 3, enabling smooth high-speed circulation of the balls 4. In other words, the combination of the ball guide portion 54 of the circulating member 5 and the fitting seat 35 of the housing groove 34 contributes to the proper fixing of the circulating member to the nut member and to improving the strength of the circulating member, thereby enabling the circulating member to roll at high speeds within the infinite circulation path.

[0034] As described above, in the ball screw device of this embodiment, the return passage 50 connecting the load track 60 of the nut member 3 and the ball return hole 32 is curved overall, with a continuously changing curvature, and is formed in a curved shape where the curvature is greatest in the central part than at the end on the load passage 60 side and the end on the ball return hole 32 side.

[0035] As a result, a centripetal force directed radially inward from the nut member 3 is always acting on the ball 4 rolling within the return passage 50, thereby stabilizing the trajectory of the ball 4 rolling within the return passage 50 and enabling an increase in the relative rotational speed between the nut member 3 and the screw shaft 2.

[0036] Furthermore, by forming the return passage 50 in such a curved shape, the return passage 50 can be connected to the ball return hole 32 of the nut member 3 from the circumferential direction of the nut member 3. This makes it possible to increase the design flexibility of the connection between the ball return hole 32 and the return passage 50 without increasing the outer diameter of the nut member 3. As a result, it becomes possible to increase the thickness around the return passage 50 in the circulating member 5 to increase the strength of the circulating member 5, thereby increasing the rolling speed of the balls 4 in the infinite circulation path and increasing the relative rotational speed between the nut member 3 and the screw shaft 2.

[0037] Furthermore, since the return passage 50 can be connected to the ball return hole 32 of the nut member 3 from the circumferential direction of the nut member 3, it becomes possible to provide a partition wall 36 along the circumferential direction of the nut member 3 that separates the through hole 22 from the housing groove 34 of the circulation member 5. The presence of this partition wall 36 makes it possible to firmly fix the circulation member 5 to the nut member 3 without rattling in its radial direction, and in this respect as well, it is possible to appropriately respond to the increased rolling speed of the balls 4 in the infinite circulation path. [Explanation of Symbols]

[0038] 1...Ball screw device, 2...Screw shaft, 3...Nut member, 4...Ball, 5...Circulation member, 20...Rolling groove, 33...Load rolling groove, 34...Housing groove, 35...Matching seat, 36...Separation wall, 53...Ball guide section

Claims

1. Many balls, A screw shaft having spiral rolling grooves on its outer surface through which the numerous balls roll, A nut member having a through hole into which the screw shaft is loosely fitted, and having a helical load rolling groove on the inner circumferential surface of the through hole that forms a load passage for the ball opposite to the rolling groove of the screw shaft, A ball return hole is provided parallel to the through hole in the nut member, A pair of circulating members are attached to both axial ends of the nut member to form a return passage connecting the end of the load passage and the end of the ball return hole, In a ball screw device equipped with, The ball screw device is characterized in that the return passage is formed in a curved shape with a continuously changing curvature, and the curvature is greatest in the center of the return passage than at both ends in the longitudinal direction, and a centripetal force directed radially inward of the nut member is always acting on the ball rolling in the return passage.

2. The ball screw device according to claim 1, characterized in that the nut member has a receiving groove formed therein for fitting the circulating member, the receiving groove is provided with a retaining fitting portion including the opening of the ball return hole, and the retaining fitting portion has a pair of locking surfaces that face each other in the radial direction of the nut member and contact the circulating member to position it.

3. The ball screw device according to claim 2, characterized in that a partition wall extending in the circumferential direction of the nut member is formed between the retaining fitting portion of the housing groove and the through hole of the nut member, and one of the pair of locking surfaces is provided on the partition wall.

4. The ball screw device according to claim 2, characterized in that a fitting seat is formed in the housing groove adjacent to the opening of the ball return hole, and this fitting seat is located on the opposite side of the return passage with respect to the ball return hole.

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