Ball screw device

Abstract

To provide a ball screw device that is excellent in operational accuracy and quietness, while also being excellent in assemblability and replacement workability of a rolling bearing.SOLUTION: A ball screw device 1 comprises: a ball screw 10 having a screw shaft 11 and a nut 12, which relatively rotate via a plurality of balls 13; a rolling bearing 30 rotatably supporting the screw shaft 11; and a carrier 24 of a planetary reduction mechanism 20, which transmits rotational power of a rotation drive source to the screw shaft 11. The carrier 24 in a flange shape is integrally provided on the screw shaft 11, and an inner ring 31 of the rolling bearing 30 is fixed to an outer peripheral surface of the carrier 24.SELECTED DRAWING: Figure 3

Description

[Technical Field]

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

[0002] Ball screws, which have a screw shaft and a nut that rotate (move) relative to each other via multiple balls, are widely used as a type of motion conversion mechanism that converts rotary motion into linear motion, or linear motion into rotary motion. When this ball screw is driven, the balls roll between the screw shaft and the nut, allowing the screw shaft and the nut to move relative to each other very efficiently. Therefore, ball screws are incorporated into various mechanical devices that require good operability and high operating precision, such as automobiles, industrial robots, and machine tools.

[0003] For example, Patent Document 1 listed below discloses a so-called rotating shaft ball screw (ball screw device) in which a screw shaft is rotationally driven by the rotational power of a rotary drive source that has been reduced in speed by a planetary reduction mechanism. This ball screw device includes a screw shaft and a nut that rotate relative to each other via a plurality of balls, a carrier that constitutes the output member of the planetary reduction mechanism, and a rolling bearing that rotatably supports the carrier, and is mainly characterized in that the inner raceway surface of the rolling bearing is formed directly on the outer peripheral surface of the carrier that is fitted and fixed to the screw shaft so as not to rotate relative to it. With this configuration, the carrier also functions as the inner ring of the rolling bearing, so the inner ring can be omitted and the number of parts can be reduced, thereby realizing a ball screw device with excellent assembly ease despite the use of a planetary reduction mechanism.

[0004] In the ball screw device of Patent Document 1, the precision of the fit of the carrier to the screw shaft is important for accurately transmitting the rotational power of the rotational drive source to the screw shaft, and because the carrier and the screw shaft are connected by a spline fit or a spigot fit so that they can rotate together, some play inevitably exists at the fit between the carrier and the screw shaft due to fit clearance, etc. The presence of play at the fit appears as runout when the screw shaft rotates, so in order to improve the operational precision and quietness of the ball screw (preventing the generation of operating noise and vibration), it is preferable to suppress or prevent the occurrence of this play as much as possible.

[0005] Therefore, Patent Document 2 listed below describes a ball screw device in which a carrier portion (corresponding to the carrier of a planetary reduction mechanism) is provided integrally with the screw shaft, and the inner raceway surface of the rolling bearing is formed directly on the outer peripheral surface of the carrier portion. In this case, since there is no fitting portion between the carrier and the screw shaft, it is thought that the occurrence of rattle due to fitting clearance etc. can be prevented. [Patent Document 1] International Publication No. 2023 / 132172 [Patent Document 2] Patent No. 7338810 Summary of the Invention [Problem to be solved by the invention]

[0006] As described in Patent Document 2, when the inner raceway surface of the rolling bearing is formed directly on the outer peripheral surface of a carrier part that is integrally provided on the screw shaft, the number of parts can be reduced compared to the ball screw device disclosed in Patent Document 1. However, in this case, the member having the inner raceway surface becomes significantly larger and more difficult to handle, so the assembly of the ball screw device is not necessarily improved, and may even result in a decrease in assembly ease. Furthermore, if a malfunction occurs during use and it becomes necessary to replace the rolling bearing, the screw shaft (including the ball screw) also needs to be replaced, which requires a great deal of effort and cost.

[0007] In view of the above circumstances, an object of the present invention is to reliably improve the ease of assembly and the ease of replacing rolling bearings in a ball screw device in which the rotational power of a rotary drive source is transmitted to a screw shaft rotatably supported by rolling bearings via a carrier of a planetary reduction mechanism. [Means for solving the problem]

[0008] The present invention, which has been devised to achieve the above object, a ball screw having a screw shaft and a nut that rotate relative to each other via a plurality of balls; a rolling bearing that rotatably supports the screw shaft; a carrier of a planetary reduction mechanism that transmits rotational power of a rotational drive source to a screw shaft, A flange-shaped carrier is provided integrally with the screw shaft, The inner ring of the rolling bearing is fixed to the outer peripheral surface of the carrier.

[0009] First, in the ball screw device according to the present invention, the flange-shaped carrier of the planetary reduction mechanism is integrally provided on the screw shaft, and therefore there is no fitting portion between the screw shaft and the carrier. This prevents the occurrence of backlash due to a fitting gap that occurs when the carrier is fitted to the screw shaft, and the occurrence of axial runout during rotation of the screw shaft due to backlash, thereby realizing a ball screw device with excellent operating accuracy and quietness.

[0010] Furthermore, in the ball screw device according to the present invention, the inner ring of the rolling bearing is fixed to the outer peripheral surface of the carrier, so the entire rolling bearing can be a separate part from the screw shaft with which the carrier is integrated, making it easy to replace the rolling bearing.

[0011] Furthermore, in the present invention, in which the inner ring of the rolling bearing is fixed to the outer peripheral surface of the carrier, it is preferable to provide a cylindrical inner ring fixing surface on the outer peripheral surface of the carrier, taking into consideration ease of assembly of the inner ring. Since various dimensional accuracy and runout accuracy of rolling bearings are usually governed by standards (e.g., JIS B 1514-1, JIS B 1514-3, etc.), as long as the desired accuracy (roundness, cylindricity, etc.) is ensured for the inner ring mounting surface, runout of the screw shaft and the rolling bearing during rotational driving of the screw shaft can be effectively suppressed. Furthermore, since the inner ring mounting surface is cylindrical as described above, it is easy to ensure accuracy.

[0012] In the above configuration, it is preferable that the inner ring of the rolling bearing is press-fitted (interference-fitted) onto the outer peripheral surface of the carrier, thereby ensuring ease of replacement of the rolling bearing and effectively suppressing runout of the screw shaft and the rolling bearing when the screw shaft is rotated.

[0013] The thread groove (external thread groove) provided on the outer peripheral surface of the screw shaft can be formed by rolling, and the inner ring fixing surface provided on the outer peripheral surface of the carrier can be formed by grinding. Because the flange-shaped carrier is provided integrally with the screw shaft, the rolling of the thread groove on the outer peripheral surface of the screw shaft and the grinding of the inner ring fixing surface on the outer peripheral surface of the carrier can both be performed using the central axis of the screw shaft as a reference, so both the thread groove and the inner ring fixing surface can be formed with high precision.

[0014] In the above-described configuration, the carrier may have integrally therewith, on the axially outer side of the inner ring fixing surface provided on the outer circumferential surface thereof, an engaging portion that is engageable with the inner ring in the axial direction.

[0015] The carrier may also have, on the axially outer side of an inner ring fixing surface provided on the outer circumferential surface thereof, a fitting groove for a retaining ring that can be engaged with the inner ring in the axial direction. [Effects of the Invention]

[0016] As described above, according to the present invention, it is possible to realize a ball screw device that can suppress the runout of the screw shaft and nut during driving, has excellent operating accuracy and quietness, and is also easy to assemble and to replace the rolling bearings. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a side view of a ball screw device according to an embodiment of the present invention. [Figure 2] 1 is a perspective view of a ball screw device according to an embodiment of the present invention; [Figure 3] FIG. 2 is a schematic cross-sectional view taken along the line AA in FIG. [Figure 4] FIG. 10 is a schematic cross-sectional view of a screw shaft according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0019] Fig. 1 is a side view of a ball screw device 1 according to an embodiment of the present invention, Fig. 2 is a perspective view of the ball screw device 1, and Fig. 3 is a cross-sectional view taken along line AA in Fig. 1. 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 a screw shaft 11 that 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. In addition, the left side (the side closer to a planetary reduction mechanism 20, which will be described later) and the right side (the side farther from the planetary reduction mechanism 20) of Fig. 1 are also referred to as "one axial side" and "the other axial side," respectively.

[0020] The ball screw device 1 of this embodiment shown in Figures 1 to 3 includes a ball screw 10, a planetary reduction mechanism 20, and a rolling bearing 30 that rotatably supports a screw shaft 11, which is a rotating member of the ball screw 10, with respect to a casing 2.

[0021] The ball screw 10 has a screw shaft 11, a nut 12 fitted onto the outer periphery of the screw shaft 11 so as to be rotatable relative to the screw shaft 11, and a plurality of balls 13 arranged between the screw shaft 11 and the nut 12.

[0022] The screw shaft 11 integrally includes a shaft portion 11b having an external thread groove 11a formed on its outer peripheral surface and a flange portion 11c extending radially outward from one axial end of the shaft portion 11b. A spiral female thread groove 12a is formed on the inner peripheral surface of the nut 12, facing the male thread groove 11a of the screw shaft 11. A rolling path 14, along which balls 13 roll, is formed between the opposing male thread groove 11a and female thread groove 12a, and multiple balls 13 are arranged in a row along this rolling path 14. When the screw shaft 11 and the nut 12 rotate relative to each other (in this embodiment, the screw shaft 11 rotates), the balls 13 arranged in the rolling path 14 move along the longitudinal direction of the rolling path 14 while contacting both the male thread groove 11a and the female thread groove 12a, thereby transmitting torque between the screw shaft 11 and the nut 12. The moving direction of the balls 13 changes depending on the rotation direction of the screw shaft 11.

[0023] 1 and 2, the nut 12 has windows 12b formed on its inner and outer peripheral surfaces, and a ball member 15 is fitted into the windows 12b. The ball member 15 functions as a circulation member that allows the balls 13, which move (roll) along the longitudinal direction of the rolling path 14 as the screw shaft 11 rotates, to circulate endlessly. That is, a circulation groove 15a (see FIG. 3) that connects adjacent female thread grooves 12a in the axial direction is formed on the inner diameter surface of the ball member 15. This circulation groove 15a smoothly connects the end (one end in the longitudinal direction) and the start (the other end in the longitudinal direction) of the rolling path 14, forming a circulation path that allows the balls 13 to circulate endlessly within the nut 12, and further forming a series of ball passages consisting of the rolling path 14 and the circulation path. The groove depth of the circulation groove 15a provided in the top member 15 is set to be a predetermined amount larger than the groove depth of the female screw groove 12a so that the ball 13 moving within the circulation groove 15a (circulation path) can climb over the threads between the male screw grooves 11a.

[0024] 1 and 2, only one piece of top member 15 is shown due to the viewing direction of the ball screw device 1, but as shown in Fig. 3, a plurality of circulation paths formed by the circulation grooves 15a of the piece of top member 15 (and further a series of ball passages consisting of the rolling paths 14 and the circulation paths) are provided at intervals in the axial direction. When a plurality of piece of top members 15 are provided, the circumferential phases of two piece of top members 15 adjacent to each other in the axial direction are different from each other.

[0025] The planetary reduction mechanism 20 is configured to reduce the output (rotational power) of a rotational drive source (e.g., an electric motor) (not shown) before transmitting it to the screw shaft 11 of the ball screw 10. The presence of this planetary reduction mechanism 20 makes it possible to use a small rotational drive source. The ball screw 10 of this embodiment, in which the output of the rotational drive source is transmitted to the screw shaft 11, is an axial rotation type in which the screw shaft 11 constitutes a rotating-side member and the nut 12 constitutes a linear-motion-side member that moves linearly in the axial direction as the screw shaft 11 rotates. Although not shown, the ball screw device 1 is provided with a rotation prevention structure to prevent the nut 12, which constitutes the linear-motion-side member, from rotating together with the screw shaft 11.

[0026] The planetary reduction mechanism 20 includes an annular ring gear 21 fixed to the inner peripheral surface of the casing 2, a sun gear 22 arranged coaxially with the screw shaft 11 and driven to rotate around the central axis X of the screw shaft 11 by receiving the output of a rotary drive source, a plurality of planetary gears 23 arranged between the ring gear 21 (its internal teeth) and the sun gear 22 (its external teeth) and meshed with both gears 21, 22, a carrier 24, and a support pin 25.

[0027] 2, three planetary gears 23 are arranged at equal intervals in the circumferential direction, and each planetary gear 23 is rotatably supported by the carrier 24. In detail, with a portion of the support pin 25 protruding to one axial side of the carrier 24, the remaining portion of the support pin 25 is fitted and fixed in the pin fitting hole 24a of the carrier 24, and the planetary gear 23 is rotatably fitted around the outer periphery of the protruding portion of the support pin 25.

[0028] As shown in Fig. 3, the flange portion 11c of the screw shaft 11 that constitutes the ball screw 10 is formed by a carrier 24. That is, the carrier 24 is provided integrally with (the shaft portion 11b of) the ball screw 10. Therefore, when the sun gear 22 of the planetary reduction mechanism 20 rotates as a rotational drive source (not shown) is driven to rotate, the planet gears 23 revolve around the sun gear 22 while rotating on their own axes, and the revolution of the planet gears 23 is transmitted to the screw shaft 11 via the support pins 25 and the carrier 24. As a result, the screw shaft 11 rotates around its central axis X.

[0029] The screw shaft 11, which rotates in the above manner, is rotatably supported relative to the casing 2 by a rolling bearing 30. The rolling bearing 30 is a ball bearing including an inner ring 31 and an outer ring 32 arranged opposite each other with a radial gap between them, a plurality of rolling elements (here, balls 33) arranged between an inner raceway surface 31a formed on the outer peripheral surface of the inner ring 31 and an outer raceway surface 32a formed on the inner peripheral surface of the outer ring 32, and a cage 34 that holds the plurality of balls 33 at intervals in the circumferential direction.

[0030] An inner ring 31 of a ball bearing serving as the rolling bearing 30 is fixed to a cylindrical inner ring fixing surface 16 provided on the outer peripheral surface of a flange portion 11c of the screw shaft 11, which functions as a carrier 24. In this embodiment, the outer peripheral surface of the inner ring 31 is press-fitted into the inner ring fixing surface 16 of the screw shaft 11.

[0031] The screw groove 11a provided on the outer peripheral surface of the shaft portion 11b of the screw shaft 11 is formed by rolling, and the outer peripheral surface of the flange portion 11c (which functions as the carrier 24) (the cylindrical inner ring fixing surface 16 provided thereon) is grindingAlthough not shown in the drawings, the thread grooves 11a are rolled on the threaded shaft 11 (the shaft blank that will ultimately become the threaded shaft 11) and the inner ring fixing surface 16 is ground while the shaft blank is centered. This improves the parallelism between the thread grooves 11a (the outer peripheral surface of the shaft portion 11b on which they are formed) and the inner ring fixing surface 16, thereby making it possible to obtain a threaded shaft 11 with high rotational accuracy and reduced rotational runout. To facilitate centering of the shaft blank, a center hole for bearing may be provided in one or both of the end faces on one and the other axial sides of the shaft blank. When a center hole is provided in the shaft blank, the center hole remains in the finished threaded shaft 11 without being removed.

[0032] The formation of the thread groove 11a by rolling and the formation of the inner ring fixing surface 16 by grinding are performed on a shaft material that is a so-called green material that has not been subjected to heat treatment such as quenching. This makes it possible to easily and accurately obtain the thread groove 11a and the inner ring fixing surface 16. After the thread groove 11a and the inner ring fixing surface 16 are formed in the shaft material, the shaft material is heat treated to form a hardened layer by quenching at least on the surface layer of the shaft material (threaded shaft 11), thereby ensuring the fatigue strength and wear resistance required for the thread groove 11a (groove bottom surface) and the inner ring fixing surface 16.

[0033] In the ball screw device 1 of this embodiment described above, the flange-shaped carrier 24 of the planetary reduction mechanism 20 is provided integrally with the screw shaft 11 of the ball screw 10, so there is no fitting portion between the screw shaft 11 and the carrier 24. This prevents the occurrence of backlash resulting from a fitting gap that occurs when the carrier is fitted to the screw shaft, and the occurrence of rotational runout of the screw shaft 11 due to this backlash, making it possible to realize a ball screw device 1 that is excellent in operational accuracy and quietness.

[0034] Furthermore, in the ball screw device 1 of this embodiment, the inner ring 31 of the rolling bearing 30 is fixed to the outer peripheral surface of the carrier 24 (flange portion 11c) that is integrally provided on the screw shaft 11, so that the rolling bearing 30 that rotatably supports the screw shaft 11 is a completely separate part from the screw shaft 11 that has the carrier 24 integrally therewith. Therefore, the rolling bearing 30 can be easily replaced.

[0035] In the ball screw device 1 of this embodiment, in which the inner ring 31 of the rolling bearing 30 is fixed to the outer peripheral surface of the carrier 24 (flange portion 11c) of the screw shaft 11, the outer peripheral surface of the carrier 24 is provided with a cylindrical inner ring fixing surface 16, so that the inner ring 31 can be easily assembled to the carrier 24. Here, various dimensional accuracy and runout accuracy of the rolling bearing 30 are controlled by standards such as JIS B 1514-1 and JIS B 1514-3. Therefore, as long as the desired roundness, cylindricity, etc. are ensured for the inner ring fixing surface 16, runout of the screw shaft 11 and the rolling bearing 30 during rotational driving of the screw shaft 11 can be effectively suppressed, and since the inner ring fixing surface 16 is cylindrical as described above, it is easy to ensure this accuracy. Furthermore, in this embodiment, the inner ring 31 of the rolling bearing 30 is press-fitted and fixed to the inner ring fixing surface 16 on the outer peripheral surface of the carrier 24, thereby ensuring ease of replacement of the rolling bearing 30 and effectively suppressing vibration of the screw shaft 11 and the rolling bearing 30 when the screw shaft 11 is rotated and driven.

[0036] From the above, the ball screw device 1 of this embodiment has the characteristics of being able to suppress the vibration of the screw shaft 11 and the rolling bearing 30 during driving, thereby providing excellent operating accuracy and quietness, while also being easy to assemble and to replace the rolling bearing 30.

[0037] 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.

[0038] For example, as shown in FIG. 4, the outer peripheral surface of the carrier 24 (flange portion 11c) integrally provided with the screw shaft 11 may be integrally provided with locking portions 11d axially engageable with the inner ring 31 of the rolling bearing 30 on the axially outer side (the other axial side in the illustrated example) of the inner ring fixing surface 16. Alternatively, an annular groove 17 may be provided on the axially outer side (the one axial side in the illustrated example) of the inner ring fixing surface 16 as a "retaining ring fitting groove," and a retaining ring 18 axially engageable with the inner ring 31 may be fitted into this annular groove 17. This configuration restricts axial movement of the rolling bearing 30 (the inner ring 31 thereof) relative to the screw shaft 11, thereby minimizing the occurrence of problems such as unstable rotation accuracy of the screw shaft 11. The engaging portions 11d may be formed as a continuous annular shape in the circumferential direction, or as arc-shaped portions spaced apart in the circumferential direction.

[0039] Furthermore, in the ball screw device 1 described above, the top member 15 is used as a circulation member that forms the circulation path of the ball screw 10, but it is also possible to use a known circulation member other than the top member 15 (for example, an end cap, a return tube, etc.). In other words, the present invention is applicable not only to so-called top-type ball screws, but also to ball screw devices 1 equipped with end-cap-type or return-tube-type ball screws 10. [Explanation of symbols]

[0040] 1. Ball screw device 2 Casing 10 Ball screw 11 Screw shaft 11a male thread groove 11c Flange part (carrier) 11d Locking part 12 nuts 12a female thread groove 13 Ball 14 Rolling path 15 Top parts (circulating parts) 16 Inner ring fixing surface 17 Annular groove 18 Retaining ring 20 Planetary reduction mechanism 23 Planetary gear 24 Career 30 Rolling bearings 31 Inner circle

Claims

1. a ball screw having a screw shaft and a nut that rotate relative to each other via a plurality of balls; a rolling bearing that rotatably supports the screw shaft; a carrier of a planetary reduction mechanism that transmits rotational power of a rotational drive source to the screw shaft, The flange-shaped carrier is integrally provided on the screw shaft, A ball screw device, characterized in that an inner ring of the rolling bearing is fixed to an outer peripheral surface of the carrier.

2. 2. The ball screw device according to claim 1, wherein an inner ring of the rolling bearing is press-fitted and fixed to the outer peripheral surface of the carrier.

3. 2. The ball screw device according to claim 1, wherein the screw groove provided on the outer peripheral surface of the screw shaft is formed by rolling, and the inner ring fixing surface provided on the outer peripheral surface of the carrier is formed by grinding.

4. 2. The ball screw device according to claim 1, wherein the carrier has an engaging portion integrally formed on the outer circumferential surface of the carrier, the engaging portion being axially engageable with the inner ring.

5. 2. The ball screw device according to claim 1, wherein the carrier has an inner ring fixing surface provided on its outer circumferential surface, on the axially outer side thereof, a fitting groove for a retaining ring that can be engaged with the inner ring in the axial direction.

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