Ball screw device

Abstract

To provide a ball screw device that has a high degree of freedom in utilizing the outer peripheral surface of a nut and is compact overall.SOLUTION: A ball screw device 1 comprises: a nut 2 and a screw shaft 3, which make relative rotation (relative movement) via a plurality of balls 4 arranged in a rolling path 7; a piece member 5 as a circulation member that forms a circulation path 8 for circulating the balls 4; and a position holding mechanism 20 that holds the nut 2 at a predetermined axial position. The position holding mechanism 20 is configured to include the piece member 5, and a support shaft 21 as moving means for moving the piece member 5 radially between an allowable position that allows the balls 4 to circulate and a restriction position that restricts the circulation of the balls 4, and the support shaft 21 is arranged in a hollow portion 3e of the screw shaft 3 that is formed in a hollow shaft.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] A ball screw, a type of motion conversion mechanism that converts rotational motion into linear motion or linear motion into rotational motion, includes a screw shaft and a nut that rotate (move) relative to each other via multiple balls. During relative movement between the screw shaft and the nut, multiple balls located in a rolling path between a male thread groove formed on the outer peripheral surface of the screw shaft and a female thread groove formed on the inner peripheral surface of the nut roll along both thread grooves. The ball screw also has a circulation path that allows the balls to circulate. As the screw shaft and the nut move relative to each other, balls that reach the end (one end in the longitudinal direction) of the rolling path return to the beginning (the other end in the longitudinal direction) of the rolling path via the circulation path. This configuration realizes a ball screw that allows continuous relative movement between the screw shaft and the nut. The circulation path is formed, for example, by a ball member (groove provided in) that serves as a circulation member fixed to the nut (see, for example, Patent Document 1).

[0003] Ball screws are sometimes used in applications where it is necessary to temporarily restrict the relative movement between the screw shaft and the nut (temporarily holding the linearly moving members of the screw shaft and the nut at a predetermined axial position.) In such cases, a ball screw device equipped with a position holding mechanism such as that described in Patent Document 1 is used.

[0004] The position-retaining mechanism described in Patent Document 1 includes an arc-shaped groove extending circumferentially along the outer peripheral surface of the nut, a pressing member housed in the groove via an elastic member, a radial through-hole opening at the bottom of the groove and the bottom of the female thread groove of the nut, and a protruding member inserted into the through-hole so as to be able to move forward and backward (radially) (see Figures 1 to 4 of the document). In this position-retaining mechanism, when the rotation of the rotating component (the nut) of the nut and the screw shaft stops, the pressing member presses the protruding member radially inward, and the tip of the protruding member is positioned within the rolling path. This restricts the rolling of the balls within the rolling path, thereby maintaining the screw shaft, which is the linear-acting component, at a predetermined axial position. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2017-32109 Summary of the Invention [Problem to be solved by the invention]

[0006] The position retention mechanism of Patent Document 1 is configured so that a portion thereof is exposed and protrudes from the outer periphery of the nut, which raises concerns that it may be difficult to utilize the outer periphery of the nut as a fitting surface for fitting other members (for example, a member for transmitting the output of the ball screw to an object to be operated).Furthermore, in the ball screw device of Patent Document 1, the nut is provided with a position retention mechanism that is separate and independent from the top member that serves as a circulating member fitted into the nut, which raises concerns that the nut may become larger and the entire ball screw device may become more complex.

[0007] In view of the above circumstances, the main object of the present invention is to provide a ball screw device equipped with a position holding mechanism that holds the screw shaft or nut at a predetermined axial position, which has a high degree of freedom in utilizing the outer peripheral surface of the nut and is compact overall. [Means for solving the problem]

[0008] The present invention, which has been devised to achieve the above object, a nut having an internal thread groove formed on its inner circumferential surface; a screw shaft that is inserted into the inner periphery of the nut and has a male screw groove formed on its outer periphery; a plurality of balls disposed in a rolling path formed between the female screw groove and the male screw groove; a circulation member that forms a circulation path for allowing the balls to circulate; a position holding mechanism that holds the nut or the screw shaft at a predetermined axial position, The position holding mechanism is a top member as the circulation member fitted onto the screw shaft; The ball bearing is characterized by being provided with a moving means for moving the ball member radially between an allowable position that allows the balls to circulate and a restricting position that restricts the balls to circulate, and the moving means is disposed in the hollow portion of the screw shaft formed in the hollow shaft.

[0009] As described above, in the ball screw device according to the present invention, the position-retaining mechanism that holds the nut or screw shaft at a predetermined axial position is achieved by utilizing a top member as a circulation member essential for realizing ball circulation, and the top member is fitted to the screw shaft, rather than the nut as in the conventional technology. In addition, a moving means for radially moving the top member between a permissive position that allows ball circulation and a restrictive position that restricts ball circulation is located in a hollow portion of the screw shaft formed in the hollow shaft, and the components of the position-retaining mechanism are integrated into the screw shaft rather than provided in the nut. This increases the degree of flexibility in using the nut (its outer peripheral surface) while avoiding the nut, and therefore the ball screw device, becoming larger and more complex.

[0010] In the above configuration, the moving means can be configured as a support shaft that is movable in the axial direction relative to the screw shaft and the top member and that supports the top member on a tapered support surface that is inclined with respect to the axial direction. If the moving means is configured as a support shaft as described above, it is possible to avoid an increase in the size of the screw shaft, and also to adjust the radial position of the top member according to the axial position of the support shaft relative to the screw shaft and the top member, making it easy to change the position of the top member.

[0011] The support shaft may have an engagement surface that is radially inward of the tapered support surface and that can radially engage with the top member. This prevents the radial position of the top member from changing due to factors other than the axial movement of the support shaft relative to the screw shaft (and the top member) (for example, the weight of the top member, centrifugal force acting on the top member as the screw shaft rotates, etc.), thereby realizing a highly reliable position-retaining mechanism.

[0012] In the ball screw device according to the present invention, the nut can be the linear motion side, and the screw shaft can be the rotation side. [Effects of the Invention]

[0013] As described above, according to the present invention, it is possible to provide a ball screw device equipped with a position holding mechanism that holds the screw shaft or nut at a predetermined axial position, which has a high degree of freedom in utilizing the outer peripheral surface of the nut, and is compact overall. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic vertical cross-sectional view of a ball screw device according to an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view taken along the line AA in FIG. [Figure 3] FIG. 2 is a partially enlarged view of the outer circumferential surface of the screw shaft. [Figure 4] FIG. 2 is a schematic perspective view of a nut and an output member connected to the nut. [Figure 5]FIG. 2 is a schematic perspective view of a support shaft serving as a moving means. [Figure 6] 1A is an enlarged view showing a state in which the circulation member is positioned at a permissive position, and FIG. 1B is an enlarged view showing a state in which the circulation member is positioned at a restrictive position. [Figure 7] 1. FIG. 1(a) is a schematic perspective view of a support shaft according to a modified example, and FIG. 1(b) is a partially enlarged cross-sectional view taken along line AA in FIG. 1 when the support shaft shown in FIG. 1(a) is used. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings (FIGS. 1 to 7).

[0016] Fig. 1 is a schematic longitudinal sectional view of a ball screw device 1 according to an embodiment of the present invention, and Fig. 2 is a sectional view taken along line AA in Fig. 1 (and a partially enlarged view of the sectional view). Unless otherwise specified, the terms "axial direction," "radial direction," and "circumferential direction" used in the following description refer to the direction along the central axis X of the screw shaft 3, which is a component of the ball screw device 1, the radial direction of a circle centered on the central axis X, and the circumferential direction of a circle centered on the central axis X, respectively. The left and right sides of Fig. 1 are also referred to as "one axial side" and "other axial side," respectively.

[0017] The ball screw device 1 of this embodiment shown in Figure 1 comprises a nut 2 and a screw shaft 3 which rotate (move relative to each other) via a plurality of balls 4, a top member 5, a power transmission mechanism 10, a position holding mechanism 20 which can hold the linearly acting member of the nut 2 and the screw shaft 3 (here, the nut 2) at a predetermined axial position, and a housing 6 which accommodates these.

[0018] A spiral female screw groove 2a is formed in the nut 2, and a spiral male screw groove 3a opposing the female screw groove 2a is formed on the outer circumferential surface of the screw shaft 3. A rolling path 7 along which balls 4 roll is formed between the opposing female screw groove 2a and male screw groove 3a, and multiple balls 4 are arranged in a row along this rolling path 7. When the nut 2 and the screw shaft 3 rotate relative to each other, the balls 4 arranged in the rolling path 7 move along the longitudinal direction of the rolling path 7 while making contact with both the female screw groove 2a and the male screw groove 3a, thereby transmitting torque between the nut 2 and the screw shaft 3. The moving direction of the balls 4 changes depending on the relative rotation direction of the nut 2 and the screw shaft 3. Note that the ball screw device 1 of this embodiment is provided with two rolling paths 7 spaced apart in the axial direction.

[0019] The power transmission mechanism 10 has a drive gear 11 that is rotationally driven by the output of a rotary drive source such as an electric motor (not shown), and a driven gear 12 that meshes with the drive gear 11, and the driven gear 12 is attached to the outer periphery of the screw shaft 3 so as to be rotatable integrally with the screw shaft 3. Therefore, the ball screw device 1 employs an axially rotating ball screw in which the screw shaft 3 is on the rotating side and the nut 2 is on the linear acting side that moves linearly in the axial direction as the screw shaft 3 rotates.

[0020] 2 and 4, the nut 2 of this embodiment integrally comprises a cylindrical nut portion 2b having an inner circumferential surface on which a female thread groove 2a (omitted in FIG. 4) is formed, and a guide portion 2c having an axial hole 2d into which a detent shaft 13 is inserted so as to be relatively slidable, for preventing the nut 2, which is the linear motion side, from rotating relative to the housing 6. The detent shaft 13 is inserted into the axial hole 2d of the guide portion 2c via a bushing 14 made of a material (e.g., a resin material, an oil-impregnated sintered metal, etc.) that has better sliding properties than the nut 2, and when the screw shaft 3 rotates, the nut 2 moves linearly in the axial direction, with the guide portion 2c (the bushing 14 fitted in the axial hole 2d) being guided by the detent shaft 13.

[0021] As shown in Fig. 2, two pin-shaped protrusions 2e are integrally formed on the nut portion 2b at 180° intervals and protrude radially outward from the outer circumferential surface of the nut portion 2b. As shown in Fig. 4, an output member 15 is connected to the protrusions 2e to transmit the output (linear motion) of the nut 2 to an operation target (not shown).

[0022] The screw shaft 3 is rotatably supported relative to the housing 6 via rolling bearings 16, 17 mounted on the outer peripheries of the ends on one and the other axial sides, respectively. For this reason, cylindrical bearing mounting surfaces 3b, 3c are provided on the outer periphery of the screw shaft 3 in the illustrated example at the ends on one and the other axial sides. Furthermore, a gear mounting surface 3d is provided on the outer periphery of the screw shaft 3 between the bearing mounting surface 3b on one axial side and the male thread groove 3a, and a driven gear 12 constituting a power transmission mechanism 10 is mounted on this gear mounting surface 3d so as to be rotatable integrally with the screw shaft 3.

[0023] The screw shaft 3, which has a male screw groove 3a formed on its outer peripheral surface, is formed as a hollow shaft having a hollow portion (shaft hole) 3e that opens to both axial end surfaces. In the axial region of the hollow screw shaft 3 where the male screw groove 3a is formed, window portions 3f that open to the outer and inner peripheral surfaces of the screw shaft 3 are formed, and a top member 5 is fitted into these window portions 3f. In this embodiment, two window portions 3f (a first window portion 3f1 and a second window portion 3f2) are formed in the screw shaft 3, and the axial positions and circumferential positions (circumferential phases) of the two window portions 3f1, 3f2 are different from each other.

[0024] The top members 5 (5A, 5B) fitted into the two windows 3f (the first window 3f1 and the second window 3f2) function as circulation members that allow the balls 4 to roll along the longitudinal direction of the rolling path 7 in response to the relative rotation between the nut 2 and the screw shaft 3 (here, the rotation of the screw shaft 3). Specifically, as shown in FIG. 3, an S-shaped groove 5a is formed on the outer diameter surface of the top member 5, connecting one end and the other end of the male screw groove 3a for approximately one turn. This groove 5a smoothly connects the end (one longitudinal end) and the start (the other longitudinal end) of the rolling path 7, forming a circulation path 8 that allows the balls 4 to circulate. The groove depth of the groove 5a formed in the top member 5 is greater than the groove depth of the male screw groove 3a. Therefore, when the balls 4 move in the circulation path 8, they do not come into contact with (the bottom surface of) the female screw groove 2a of the nut 2 and move in a so-called unloaded state.

[0025] The top member 5 fitted into the window 3f is prevented from rotating relative to (the window 3f of) the screw shaft 3. Here, the top member 5 is prevented from rotating relative to the window 3f by employing a top member 5 having two arms 5b and a screw shaft 3 having a fitting groove into which the arms 5b fit.

[0026] Of the two pieces 5 (5A, 5B) fitted into the two windows 3f (3f1, 3f2), the radial dimension of the piece 5 (5A) fitted into the first window 3f1 is smaller than the radial dimension of the first window 3f1 (the thickness of the portion of the screw shaft 3 where the external thread groove 3a is formed). Here, the piece 5A is fitted into the first window 3f1 so that the inner diameter surface of the piece 5A is positioned radially outward from the inner circumferential surface of the screw shaft 3, which is a hollow shaft. The piece 5A fitted into the first window 3f1 is fixed to the screw shaft 3 by an appropriate method (e.g., crimping, welding, adhesive, etc.) that can prevent it from coming off the first window 3f1. This configuration prevents interference between the piece 5A fitted into the first window 3f1 and the support shaft 21, which will be described later.

[0027] In contrast, the radial dimension of the piece 5 (5B) fitted into the second window 3f2 is larger than the radial dimension of the second window 3f2, and here, the piece 5B is fitted into the second window 3f2 so that the inner diameter surface of the piece 5B is always positioned radially inward of the inner circumferential surface of the screw shaft 3. Furthermore, the piece 5B is fitted into the second window 3f2 in a state in which it can move radially along the inner wall surface of the second window 3f2.

[0028] The position holding mechanism 20 temporarily holds the nut 2, which is the linear motion side of the nut 2 and the screw shaft 3, in a predetermined axial position by restricting the circulation of the balls 4 arranged in the rolling path 7 (the entry of the balls 4 into the circulation path 8). Its components are a top member 5B fitted radially movably into the second window portion 3f2 of the screw shaft 3, and a moving means for moving the top member 5B radially between an allowable position that allows the circulation of the balls 4 via the circulation path 8 and a restrictive position that restricts the circulation of the balls 4. The moving means in this embodiment is composed of a support shaft 21 arranged in the hollow portion 3e of the screw shaft 3 so as to be slidable in the axial direction relative to the screw shaft 3 and the top members 5 (5A, 5B).

[0029] The support shaft 21 of the position holding mechanism 20 is movable in the axial direction (forward and backward movement) by being connected to, for example, the output shaft of a linear actuator (not shown). The linear actuator is provided with a position sensor capable of detecting the axial position of the output shaft, and by using a signal from this position sensor to control the axial position of the output shaft, the axial position of the support shaft 21 connected to the output shaft, and further the radial position of a top member 5B (described later), are controlled.

[0030] As shown in Figures 1, 2, and 5, one axial end of the support shaft 21 is provided with a support portion 22 that supports the top member 5B fitted into the second window portion 3f2. This support portion 22 has a tapered support surface 23 that gradually shifts radially outward from one axial end to the other axial end. The support shaft 21 shown in Figure 1 supports the top member 5B so that it is positioned at the "allowable position." As described above, the "allowable position" is a position where the circulation of balls 4 through the circulation path 8 is permitted. To allow the circulation of balls 4, the end and beginning of the rolling path 7 must be smoothly connected by the circulation path 8 (the groove portion 5a of the top member 5B). Therefore, as shown in Figure 1 and Figure 6(a), which is a partial enlarged view of Figure 1, the allowable position corresponds to a position where the groove bottom of the groove portion 5a of the top member 5B is radially inward by a predetermined amount (e.g., approximately the radius of the balls 4) from the groove bottom of the male thread groove 3a of the screw shaft 3.

[0031] When the support shaft 21 supporting the top member 5B at the permissible position (the support shaft 21 in the state shown in FIG. 1) moves to one side in the axial direction, the top member 5B moves from the permissible position shown in FIGS. 1 and 6(a) to the restricted position shown in FIG. 6(b). On the other hand, when the support shaft 21 supporting the top member 5B at the restricted position moves to the other side in the axial direction, the top member 5B moves from the restricted position to the permissible position. As described above, the restricted position is a position where the circulation of the balls 4 through the circulation path 8 is restricted.

[0032] In this embodiment, because the support surface 23 of the support shaft 21 that supports the piece 5B is formed into a tapered surface as described above, when the support shaft 21 in the state shown in Fig. 1 moves a predetermined amount in one axial direction, the piece 5B moves radially outward so as to approach the inner peripheral surface of the nut 2, making the inlet opening of the circulation path 8 smaller than the diameter of the balls (see Fig. 6(b)). This restricts the balls 4 that have reached the end of the rolling path 7 from entering the circulation path 8 (groove portion 5a of the piece 5B), and more specifically, restricts the circulation of the balls 4, so the nut 2 is held in a predetermined axial position.

[0033] Note that even if the support shaft 21 is moved axially to the other side from the state shown in FIG. 1 to position the top member 5B radially inward from the state shown in FIG. 1, the top member 5B can still be considered to be in the restricted position. However, in this case, the radially inward movement of the top member 5B creates a recess at the location of the window portion 3f2 of the screw shaft 3, causing the balls 4 that reach the end of the rolling path 7 to fall into the recess. In this case, even if the top member 5B is returned to the allowable position by moving the support shaft 21 axially to the other side again, it becomes difficult to smoothly circulate the balls 7. Therefore, in the ball screw device 1 of this embodiment, the restricted position of the top member 5B is a position shifted radially outward from the allowable position.

[0034] As described above, in the ball screw device 1 of this embodiment, the position holding mechanism 20 that holds the nut 2, which is the linear-acting component, at a predetermined axial position is composed of a top member 5 (5B) as a circulation member fitted into the screw shaft 3, and a moving means that moves this top member 5B radially between an allowable position that allows the circulation of the balls 4 and a restricting position that restricts the circulation of the balls 4, and the above-mentioned moving means is arranged in the hollow portion 3e of the screw shaft 3 formed in the hollow shaft.

[0035] As described above, the position-retaining mechanism 20 of the ball screw device 1 of this embodiment is obtained by utilizing the top member 5 (5B) as a circulation member that is essential for realizing the circulation of the balls 4, and the top member 5 (5B) is fitted into the screw shaft 3, not into the nut 2 as in the past. In addition, a moving means for radially moving the top member 5B between an allowable position that allows the circulation of the balls 4 and a restricting position that restricts the circulation of the balls 4 is disposed in the hollow portion 3e of the screw shaft 3 formed in the hollow shaft, and the components of the position-retaining mechanism 20 are concentrated on the screw shaft 3 rather than provided in the nut 2. This increases the degree of freedom in utilizing the nut 2 (its outer peripheral surface), while preventing the nut 2, and therefore the ball screw device 1, from becoming larger and more complex.

[0036] If the degree of freedom in use of the nut 2 (its outer peripheral surface) can be increased, the nut 2 of this embodiment as shown in FIGS. 1, 2 and 4 can be easily realized.

[0037] The moving means is configured with a support shaft 21 that is movable in the axial direction relative to the screw shaft 3 and the top member 5 (5B) and that supports the top member 5B with a tapered support surface 23 that is inclined with respect to the axial direction. This makes it possible to avoid an increase in the size of the screw shaft 3, and in turn the ball screw device 1. In addition, the radial position of the top member 5B can be adjusted according to the axial position of the support shaft 21 relative to the screw shaft 3 and the top member 5B fitted into its second window portion 3f2, making it easy to change the position of the top member 5B.

[0038] In addition, the support portion 22 of the support shaft 21 in this embodiment is provided with an engagement surface 24 that extends parallel to the support surface 23 and is radially engageable with the top member 5B radially inward of the support surface 23 (the side closer to the center axis X of the screw shaft 3). In this embodiment, as shown in FIG. 2 , the support portion 22 has a generally cross-shaped cross section, thereby providing the support surface 23 and the engagement surface 24 on the support shaft 21. The provision of the engagement surface 24 prevents the top member 5B from moving radially outward (moving from the allowable position to the restricted position) due to factors other than the axial movement of the support shaft 21 (e.g., the weight of the top member 5B or centrifugal force acting on the top member 5B as the screw shaft 3 rotates). This allows for a highly reliable position-retaining mechanism 20 in which the top member 5B moves from the allowable position to the restricted position (only) when the support shaft 21 moves forward in one axial direction from the state shown in FIG. 1 .

[0039] The above describes a ball screw device 1 according to one embodiment of the present invention, but the embodiment of the present invention is not limited to this, and appropriate modifications can be made within the scope that does not deviate from the gist of the present invention.

[0040] For example, the support shaft 21 shown in Figures 7(a) and 7(b) can be used as the support shaft 21 having both a tapered support surface 23 that supports the top member 5B fitted in the window portion 3f (3f2) of the screw shaft 3 and an engagement surface 24 that is radially inward of the support surface 23 and can radially engage with the top member 5B. The support shaft 21 shown in Figure 7 has a pair of shaft-shaped support portions 22 that are spaced apart, and each support portion 22 is provided with the support surface 23 and the engagement surface 24. In this case, it is necessary to use a top member 5B whose shape has been changed in accordance with the change in the form (shape) of the support portions 22.

[0041] Furthermore, in the ball screw device 1 described above, two rolling paths 7 and two top members 5 serving as circulation members forming the circulation path 8 are arranged spaced apart in the axial direction, but it is also possible to arrange three or more rolling paths 7 and three or more top members 5 spaced apart in the axial direction. In that case, if any one of the top members 5 is made movable in the radial direction and this movable top member 5 is supported by a support surface 23 provided on a support shaft 21 serving as a moving means, a ball screw device 1 similar to the above can be realized. [Explanation of symbols]

[0042] 1. Ball screw device 2 nuts 2a female thread groove 3 Screw shaft 3a external thread groove 3e Hollow part 3f, 3f1, 3f2 window section 4 balls 5, 5A, 5B Top parts (circulation parts) 5a Groove 6. Housing 7 Rolling Path 8 Circulation route 20 Position holding mechanism 21 Support shaft (transportation means) 23 Support surface 24 Engagement surface

Claims

1. a nut having an internal thread groove formed on its inner circumferential surface; A screw shaft inserted into the inner periphery of the nut and having a male screw groove formed on its outer periphery; a plurality of balls disposed in a rolling path formed between the female screw groove and the male screw groove; a circulation member that forms a circulation path for allowing the balls to circulate; a position holding mechanism that holds the nut or the screw shaft at a predetermined axial position, The position holding mechanism includes: a top member as the circulation member fitted onto the screw shaft; a moving means for moving the top member in a radial direction between two positions, a permitting position for permitting the circulation of the balls and a restricting position for restricting the circulation of the balls, A ball screw device, characterized in that the moving means is disposed in a hollow portion of the screw shaft formed in a hollow shaft.

2. 2. The ball screw device according to claim 1, wherein the moving means is configured by a support shaft that is movable axially relative to the screw shaft and the top member and that supports the top member on a tapered support surface that is inclined with respect to the axial direction.

3. 3. The ball screw device according to claim 2, wherein the support shaft has an engaging surface that is radially engageable with the top member and located radially inward of the support surface.

4. 4. The ball screw device according to claim 1, wherein the nut is a linear motion side and the screw shaft is a rotation side.

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