Ball screw

Abstract

To provide a ball screw that can restrain a decrease in efficiency of the ball screw associated with internal pressure fluctuations in an internal space between a nut and a screw shaft, and improve machinability, in comparison with a conventional technique.SOLUTION: A ball screw comprises a nut comprising an inner peripheral rolling groove, a screw shaft 3 comprising an outer peripheral rolling groove 35, and a plurality of rolling bodies arranged between the inner peripheral rolling groove and the outer peripheral rolling groove 35. The screw shaft 3 comprises a screw shaft body 31 in which the outer peripheral rolling groove 35 is formed in an outer peripheral surface, a small-diameter part 32 having a smaller outside diameter than that of the screw shaft body 31, and a communication hole 33. The communication hole 33 is formed in the screw shaft body 31 so as to establish communication between a first opening part 11 provided so as to open in an end part on one side in an axial direction out of the screw shaft body 31, and a second opening part 12 provided closer to the other side in the axial direction than the first opening part 11 out of the screw shaft body 31. At least a portion of the communication hole 33 is provided radially outside an outer peripheral part of the small-diameter part 32 in a radial direction of the screw shaft 3.SELECTED DRAWING: Figure 2

Description

[Technical field]

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

[0002] A ball screw is known that includes a nut, a screw shaft that passes through the nut, and a plurality of rolling elements disposed between the nut and the screw shaft. For example, Patent Document 1 discloses a configuration that includes a nut (so-called cap nut) formed in a bottomed cylindrical shape, a screw shaft that passes through the nut, and a plurality of rolling elements, and the screw shaft has a circulation groove that guides the rolling elements from one end of the rolling element raceway to the other end. According to the technology described in Patent Document 1, by forming a circulation groove on the screw shaft instead of the nut, it is said that easy assembly is possible even when a cap nut with one end in the axial direction closed is used. It is also said that the nut can be made smaller than when a circulation groove is formed on the nut side. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-116825 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the conventional technology in which one axial end of the nut is blocked, as in Patent Document 1 mentioned above, there was a risk that the internal pressure in the internal space between the nut and the screw shaft would change during operation of the ball screw.

[0005] Figures 7 and 8 are cross-sectional views of a ball screw 901 according to the prior art. Figure 7 shows a case where a screw shaft 903 is located at the furthest position on one side in the axial direction relative to a nut 902. Figure 8 shows a case where a screw shaft 903 is located at the furthest position on the other side in the axial direction relative to a nut 902. Figure 9 is a cross-sectional view taken along line IX-IX in Figure 7. Figure 10 is an enlarged view of a portion X in Figure 9. As shown in Figs. 7 and 8, in the conventional technology having a nut 902 with one end closed, the volume of the internal space 920 between the nut 902 and the screw shaft 903 fluctuates during the operation of the ball screw 901. At this time, the movement of air occurs through the gaps S (see Fig. 10) between the multiple rolling elements 904 arranged in the rolling element raceway. However, as shown in Figs. 9 and 10 (particularly Fig. 10), the gaps S between the multiple rolling elements are very small, so there is a risk of the internal pressure of the internal space 920 fluctuating. When the internal pressure of the internal space 920 fluctuates, the grease applied to the rolling element raceway may leak out when the air is pushed out through the gaps S between the rolling elements arranged in the rolling element raceway. As a result, the sliding resistance of the screw shaft increases, and the efficiency of the ball screw may decrease. In addition, the load generated by the fluctuation in the internal pressure of the internal space 920 becomes a loss, so there is a risk of the efficiency of the ball screw decreasing further.

[0006] On the other hand, as a measure against the above-mentioned internal pressure fluctuation, an air hole 910 may be provided in the center of the screw shaft, as shown by an imaginary line in Fig. 7. The air hole 910 is provided coaxially with the central axis C of the screw shaft, and penetrates the entire screw shaft from one end to the other end in the axial direction of the screw shaft. This allows the internal space 920 to communicate with the outside of the ball screw 901 via the air hole 910, and it is said that this makes it possible to suppress internal pressure fluctuation in the internal space 920. However, in the conventional technology having the above-mentioned air hole 910, when it is applied to a long screw shaft in particular, there is a risk that workability will deteriorate due to the long axial length of air hole 910.

[0007] Therefore, in the prior art, particularly in ball screws having a nut with one end closed, there was room for improvement in terms of suppressing the decrease in efficiency of the ball screw caused by internal pressure fluctuations in the internal space between the nut and the screw shaft, and in improving processability.

[0008] Therefore, an object of the present invention is to provide a ball screw that can suppress a decrease in efficiency of the ball screw caused by internal pressure fluctuations in the internal space between the nut and the screw shaft and improve processability, compared to conventional technology. [Means for solving the problem]

[0009] In order to solve the above problems, the present invention proposes the following means. A ball screw according to a first aspect of the present invention comprises a nut having a helical inner circumferential rolling groove on its inner circumferential surface, a threaded shaft having a helical outer circumferential rolling groove on its outer circumferential surface, and a plurality of rolling elements arranged between the inner circumferential rolling groove of the nut and the outer circumferential rolling groove of the threaded shaft, wherein the threaded shaft has a threaded shaft main body provided on one axial side of the threaded shaft and having the outer circumferential rolling groove formed on its outer circumferential surface, a small diameter section provided on the other axial side of the threaded shaft main body and having a smaller outer diameter than the threaded shaft main body, and a first opening provided to open at an end of the threaded shaft main body on one side in the axial direction, and a second opening provided in the threaded shaft main body on the other axial side of the first opening, and at least a portion of the communicating hole is provided radially outward of the outer circumferential portion of the small diameter section in the radial direction of the threaded shaft. Effect of the Invention

[0010] According to the ball screw of the present invention, compared to the conventional technology, it is possible to provide a ball screw that can suppress a decrease in efficiency of the ball screw caused by internal pressure fluctuations in the internal space between the nut and the screw shaft and can improve processability. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view of a ball screw according to a first embodiment. [Diagram 2] FIG. 2 is a cross-sectional view of a screw shaft according to the first embodiment. [Diagram 3] FIG. 11 is a perspective view of a screw shaft according to a second embodiment. [Figure 4] FIG. 11 is a cross-sectional view of a screw shaft according to a third embodiment. [Diagram 5] FIG. 11 is a cross-sectional view of a screw shaft according to a fourth embodiment. [Figure 6] FIG. 13 is a cross-sectional view of a screw shaft according to a fifth embodiment. [Figure 7]FIG. 1 is a cross-sectional view of a ball screw according to a conventional technique. [Figure 8] FIG. 1 is a cross-sectional view of a ball screw according to a conventional technique. [Figure 9] FIG. 8 is a cross-sectional view taken along line IX-IX in FIG. [Figure 10] Enlarged view of part X in Figure 9. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the following description, the axial direction, radial direction, and circumferential direction refer to the axial direction, radial direction, and circumferential direction of the center axis C of the ball screw 1, unless otherwise specified.

[0013] (First embodiment) Fig. 1 is a cross-sectional view of a ball screw 1 according to the first embodiment. Fig. 2 is a cross-sectional view of a screw shaft 3 according to the first embodiment. The ball screw 1 is a device that converts rotational motion into linear motion. As shown in FIG. 1, the ball screw 1 of this embodiment includes a nut 2, a screw shaft 3, a plurality of rolling elements 4, and a circulating part 5.

[0014] In the ball screw 1, the screw shaft 3 rotates about a central axis C. The screw shaft 3 is rotatable about the central axis C, but does not move in a direction along the central axis C. The nut 2 does not rotate about the central axis C, but is movable in a direction along the central axis C. When the screw shaft 3 rotates, the nut 2 moves in a direction along the central axis C. The ball screw 1 is used, for example, in an electric brake device for moving brake pads (not shown) of a vehicle.

[0015] The nut 2 includes a nut body 21 and a pressing portion 22. The nut body 21 is formed in a cylindrical shape. The outer peripheral surface of the nut body 21 is a cylindrical surface over the entire axial length. That is, the outer peripheral surface of the nut body 21 is a smooth curved surface without grooves or the like over the entire axial and circumferential lengths. The nut body 21 has a spiral inner peripheral rolling groove 25 on its inner peripheral surface. For example, the cross-sectional shape of the inner peripheral rolling groove 25 is a Gothic arch including two arcs.

[0016] The pressing portion 22 is a plate-like member disposed so as to close one axial end of the nut body 21. The pressing portion 22 is formed in a disk shape. An end face of the pressing portion 22 is a contact surface that comes into contact with a pressed member. The pressed member is, for example, a brake pad. Therefore, the nut 2 can be said to be a piston that presses the pressed member.

[0017] The inner circumferential rolling groove 25 is formed in a part of the inner circumferential surface of the nut body 21. The inner circumferential rolling groove 25 extends from an end opposite to the end where the pressing portion 22 is arranged, toward the pressing portion 22. In other words, the end of the inner circumferential rolling groove 25 on the pressing portion 22 side terminates at a position that does not overlap with the pressing portion 22 in a side view seen from the radial direction.

[0018] 1 and 2, a screw shaft 3 is inserted into a nut 2 and is screwed with the nut 2 via a plurality of rolling elements 4. The screw shaft 3 includes a screw shaft main body 31, a small diameter portion 32, and a communication hole 33. The screw shaft body 31 is inserted into the nut 2. The screw shaft body 31 is disposed inside the nut body 21. The screw shaft body 31 has a spiral outer peripheral rolling groove 35 on its outer peripheral surface. The cross-sectional shape of the outer peripheral rolling groove 35 is a Gothic arch including two arcs. The outer peripheral rolling groove 35 is formed over almost the entire screw shaft body 31 in the axial direction. In this embodiment, the outer peripheral rolling groove 35 formed in the screw shaft body 31 has a passage area A1, which is an area through which the rolling elements 4 pass when the ball screw 1 is used, and a non-passage area A2, which is an area through which the rolling elements 4 do not pass when the ball screw 1 is used. On the outer periphery of the screw shaft body 31, the non-passage area A2 for the rolling elements 4 is provided on the opposite side of the passage area A1 to the pressing portion 22 of the nut 2 in the axial direction. In the following description, the "non-passage area A2" refers to the general area through which the rolling elements 4 do not pass when the ball screw 1 is in use, including, for example, areas other than the outer periphery of the screw shaft body 31 through which the rolling elements 4 do not pass.

[0019] An end face of the screw shaft body 31 located on the pressing portion 22 side of the nut 2 in the axial direction faces the pressing portion 22 of the nut 2. An opening of the nut 2 on the opposite side to the pressing portion 22 in the axial direction is covered by the screw shaft body 31. Thus, an internal space 20 surrounded by the screw shaft 3 and the nut 2 is provided between the end face of the screw shaft body 31 located on the pressing portion 22 side of the nut 2 in the axial direction and the pressing portion 22 of the nut 2. The size (capacity or volume) of the internal space 20 changes as the screw shaft 3 moves axially relative to the nut 2.

[0020] The small diameter portion 32 is provided on the opposite side of the screw shaft body 31 to the pressing portion 22 of the nut 2 in the axial direction. The small diameter portion 32 is provided coaxially with the screw shaft body 31. The small diameter portion 32 is connected to the screw shaft body 31. The small diameter portion 32 is formed, for example, in a cylindrical shape having an outer diameter smaller than that of the screw shaft body 31. Note that the small diameter portion 32 may be formed in a shape other than a cylindrical shape as long as at least a part of its outer shape is formed smaller than the outer diameter of the screw shaft body 31.

[0021] Here, the outer peripheral rolling groove 35 of the screw shaft 3 may be formed by, for example, rolling. Specifically, when manufacturing the screw shaft 3, first, the outer peripheral rolling groove 35 is formed on the entire outer peripheral surface of a cylindrical base material by rolling. Next, the cylindrical base material with the outer peripheral rolling groove 35 formed therein is cut to a predetermined length. Next, a portion of the cut base material corresponding to the small diameter portion 32 is cut. In this way, the screw shaft 3 may be formed, which has the screw shaft main body 31 with the outer peripheral rolling groove 35 formed on the entire outer peripheral surface, and the small diameter portion 32 having an outer diameter smaller than that of the screw shaft main body 31.

[0022] The communication hole 33 is formed in the screw shaft 3 so as to communicate between a first opening 11 provided so as to open at an end of the screw shaft body 31 on the nut 2 side in the axial direction, and a second opening 12 provided in a portion of the screw shaft body 31 located on the opposite side to the nut 2 in the axial direction (on the small diameter portion 32 side) from the first opening 11. The communication hole 33 will be described in detail later.

[0023] 1, the multiple rolling elements 4 are disposed between the nut body 21 and the screw shaft body 31. The rolling elements 4 are, for example, balls. The multiple rolling elements 4 move in a spiral rolling element track 45 formed by an inner peripheral rolling groove 25 formed in the nut body 21 and an outer peripheral rolling groove 35 formed in the screw shaft body 31.

[0024] Fig. 3 is a perspective view of a screw shaft 3 according to a second embodiment. Here, the configuration of the circulating part 5 in the first embodiment is the same as the configuration of the circulating part 5 in the second embodiment. Therefore, in the following description of the circulating part 5, Fig. 3 will also be referred to. As shown in FIG. 2 and FIG. 3, the circulation part 5 is a circulation top integrally provided on the outer circumferential surface of the screw shaft body 31. The circulation part 5 is formed separately from the screw shaft body 31, and is attached to the screw shaft 3 by being accommodated in a recess provided in the screw shaft body 31. For example, the circulation part 5 is attached so that the height (height along the radial direction) of the outer circumferential surface of the circulation part 5 and the outer circumferential surface of the screw shaft body 31 are equal. The height of the outer circumferential surface of the circulation part 5 and the outer circumferential surface of the screw shaft body 31 may be different from each other. The bypass passage 51 provided in the circulation part 5 connects both ends of the spiral rolling element raceway 45 formed by the outer circumferential rolling groove 35 of the screw shaft 3 and the inner circumferential rolling groove 25 of the nut 2 facing each other, thereby forming an infinite circulation circuit 55. Then, a plurality of rolling elements 4 are filled in the infinite circulation circuit 55, so that the rolling elements 4 circulate infinitely in the infinite circulation circuit 55. In other words, the ball screw 1 of this embodiment is a so-called screw circulation type ball screw 1.

[0025] 3, the surface of the circulation part 5 is curved with a curvature corresponding to the curved surface forming the outer peripheral surface of the screw shaft body 31, and is provided at the same height as the threads on the outer peripheral surface of the screw shaft body 31. A groove forming a bypass path 51 is formed in the surface of this circulation part 5, and continues from an inlet / outlet 52a at one end of the circulation part 5 to an inlet / outlet 52b at the other end. The cross section of the bypass path 51 is formed in a U-shape. The bottom of the bypass path 51 is formed in a curved shape corresponding to the spherical surface of the rolling element 4.

[0026] The bypass path 51 is largely curved on the surface of the circulation part 5, and has, for example, an inverted S-shape. An inlet 52a at one end of the bypass path 51 is connected to one end of the outer circumferential rolling groove 35. An inlet 52b at the other end of the bypass path 51 is connected to the other end of the outer circumferential rolling groove 35. The outer circumferential rolling groove 35 makes approximately one revolution along the outer circumferential surface of the screw shaft main body 31 from one end to the other end, and one thread of the screw shaft main body 31 and one thread of the nut 2 are present between the inlet 52a and the inlet 52b. In other words, the bypass path 51 connects a pair of outer circumferential rolling grooves 35 adjacent to each other in the axial direction. The bypass path 51 is gently curved in the axial and circumferential directions of the screw shaft 3 so as to overcome the thread of the nut 2 between the inlet 52a and the inlet 52b.

[0027] As a result, the rolling element 4 moves to one end of the outer circumferential rolling groove 35, changes its direction of travel through the entrance 52a, and enters the bypass path 51. The rolling element 4 that entered the bypass path 51 moves along the bypass path 51, crosses one screw thread, changes its direction of travel, and moves to the other end of the outer circumferential rolling groove 35 through the entrance 52b. The rolling element 4 that moved to the other end of the outer circumferential rolling groove 35 makes one revolution around the outer circumferential surface of the screw shaft body 31 along the outer circumferential rolling groove 35, and then returns to one end of the outer circumferential rolling groove 35 again. The ball screw 1 of this embodiment has a plurality of infinite circulation circuits 55 consisting of such outer circumferential rolling grooves 35 (i.e., the rolling element raceways 45) and the bypass paths 51 in the circulation part 5.

[0028] Next, mainly with reference to FIG. 2, the communication hole 33 provided in the screw shaft 3 will be described in detail. As described above, the communicating hole 33 is provided inside the screw shaft body 31. At least a portion of the communicating hole 33 is provided radially outward from the outer circumferential surface of the small diameter portion 32 within the screw shaft body 31. The communicating hole 33 has an axial hole 13 and a plurality of radial holes 14.

[0029] The axial hole 13 is formed inside the screw shaft body 31 from an end of the screw shaft body 31 opposite the small diameter portion 32 in the axial direction. The axial hole 13 is a hole with a circular cross section extending along the axial direction. The axial hole 13 is formed coaxially with the central axis C of the screw shaft body 31. The axial hole 13 is provided over almost the entire screw shaft body 31 in the axial direction. One axial end side (the end opposite the small diameter portion 32) of the axial hole 13 opens from the screw shaft body 31. This opened portion is defined as a first opening 11. As shown in FIG. 1, the first opening 11 opens toward the internal space 20 between the nut 2 and the screw shaft 3. The first opening 11 is provided in an area where the rolling elements 4 do not pass when the ball screw 1 is used. In other words, the first opening 11 is provided in the non-passing area A2 of the rolling elements 4. The end of the axial hole 13 opposite the first opening 11 in the axial direction terminates inside the screw shaft body 31.

[0030] The radial hole 14 is provided in the screw shaft body 31 at a position close to the small diameter portion 32 in the axial direction. The radial hole 14 is a hole extending along the radial direction and having a circular cross section. The radially inner end of the radial hole 14 communicates with the axial hole 13. The radially outer end of the radial hole 14 opens at a position corresponding to the groove bottom 36 of the outer circumferential rolling groove 35 of the screw shaft body 31. This opened portion is the second opening 12. Therefore, in this embodiment, the radial hole 14 of the communication hole 33 is provided radially outward from the outer circumferential surface of the small diameter portion 32 in the screw shaft body 31. A plurality of radial holes 14 are provided in the screw shaft body 31 at equal intervals in the circumferential direction. Moreover, the plurality of radial holes 14 are provided at equal positions in the axial direction. In other words, a plurality of second openings 12, which are openings of the radial holes 14, are provided. The second openings 12 are provided at equal intervals from one another in the circumferential direction and at equal positions in the axial direction.

[0031] The first opening 11 and the second opening 12 of the communication hole 33 thus formed are provided in the non-passage area A2 of the rolling elements 4. Specifically, in this embodiment, the first opening 11 is provided in a region other than the rolling element raceway 45 (specifically, the axial end of the screw shaft body 31). The second opening 12 is provided in a region (i.e., the non-passage area A2 of the rolling elements 4) of the outer circumferential rolling groove 35 of the screw shaft 3 constituting the rolling element raceway 45, where the rolling elements 4 do not roll when the ball screw 1 is used. More specifically, the second opening 12 is provided in the groove bottom 36 of the outer circumferential rolling groove 35 located in the non-passage area A2 of the rolling elements 4, of the outer circumferential rolling grooves 35 provided on the screw shaft 3. The hole diameters of the radial hole 14 and the second opening 12 are larger than ¼ of the diameter of the rolling elements 4. More preferably, the diameter of the radial hole 14 and the second opening 12 is larger than half the diameter of the rolling element 4 .

[0032] (Action, effect) According to the ball screw 1 of this embodiment, the screw shaft 3 has a screw shaft body 31, a small diameter portion 32, and a communication hole 33. The communication hole 33 communicates between a first opening 11 that opens at one end of the screw shaft body 31 in the axial direction and a second opening 12 that is located on the other axial side of the screw shaft body 31 relative to the first opening 11. The first opening 11 opens toward an internal space 20 between the nut 2 and the screw shaft 3. This allows the internal space 20 between the nut 2 and the screw shaft 3 to communicate with the outside of the ball screw 1 through the communication hole 33. This makes it possible to suppress the occurrence of internal pressure fluctuations in the internal space 20. Since air passes through the communication hole 33, it is possible to suppress leakage of grease provided in the rolling element raceway 45, compared to the conventional technology in which air passes through the gaps S (see FIG. 10) between multiple rolling elements 4. This makes it possible to maintain good sliding resistance of the screw shaft 3 and suppress a decrease in efficiency of the ball screw 1. Furthermore, since the generation of loads associated with fluctuations in the internal pressure of the internal space 20 is suppressed, the generation of losses can be suppressed. Thus, the decrease in efficiency of the ball screw 1 can be suppressed. Furthermore, in the conventional technology in which fluctuations in internal pressure occur in the internal space 20, the loads associated with the internal pressure fluctuations are covered by, for example, increasing the output of the motor. According to the ball screw 1 of this embodiment, since it is not necessary to cover the loads associated with internal pressure fluctuations with the output of the motor, the increase in motor output can be suppressed compared to the conventional technology. Thus, the decrease in efficiency of the entire ball screw mechanism including the motor can also be suppressed. Furthermore, at least a part of the communication hole 33 is provided radially outward of the outer periphery of the small diameter portion 32 in the radial direction of the screw shaft 3. This allows, for example, the second opening 12 of the communication hole 33 to be provided in the screw shaft body 31. Therefore, the length of the communication hole 33 can be shortened compared to the conventional technology in which a hole (e.g., air hole 910 in FIG. 7) is provided so as to penetrate the entire screw shaft 3. This makes it easier to form the communication hole 33, and improves the workability of the communication hole 33 and the screw shaft 3 having this communication hole 33. Furthermore, since the screw shaft 3 can be formed without providing the communication hole 33 in the small diameter portion 32, the small diameter portion 32 can be made smaller in diameter compared to the conventional technology in which the communication hole 33 is also formed inside the small diameter portion 32. Therefore, the screw shaft 3 can be made smaller in size. Therefore, compared to the conventional technology, it is possible to provide a ball screw 1 that can suppress a decrease in efficiency of the ball screw 1 caused by internal pressure fluctuations in the internal space 20 between the nut 2 and the screw shaft 3 and improve processability.

[0033] The first opening 11 and the second opening 12 are provided in the non-passage area A2 of the rolling element 4. This makes it possible to suppress the effect on the rolling element 4 caused by providing the first opening 11 and the second opening 12. Specifically, compared with the case where the first opening 11 and the second opening 12 are provided in the passage area A1 of the rolling element 4, damage to the rolling element 4 and the screw shaft 3 caused by the rolling element 4 riding on the first opening 11 or the second opening 12 can be suppressed. In addition, it is possible to suppress a decrease in the fluidity of the rolling element 4 caused by the rolling element 4 riding on the first opening 11 or the second opening 12. Furthermore, since the rolling element 4 is not arranged in the non-passage area A2, the size (hole diameter) of the first opening 11 and the second opening 12 can be increased. Therefore, it is possible to make it easier for air to circulate. As a result, the pressure fluctuation in the internal space 20 can be reduced. In this embodiment, the second opening 12 is provided at the groove bottom 36 of the outer peripheral rolling groove 35 located in the non-passage area A2 of the rolling elements 4. Here, the non-passage area A2 of the rolling elements 4 is formed in the same process as the outer peripheral rolling groove 35 by forming the outer peripheral rolling groove 35 on the entire outer peripheral surface of the screw shaft 3 by, for example, rolling. Therefore, according to the ball screw 1 of this embodiment in which the second opening 12 is provided at the groove bottom 36 of the outer peripheral rolling groove 35, there is no need to perform additional processing to form the non-passage area A2, which is an air flow passage. In other words, the non-passage area A2 formed without increasing the number of processes or the amount of processing can be used as an air flow passage. Therefore, pressure fluctuations in the internal space 20 of the ball screw 1 can be suppressed without deteriorating the processability compared to the conventional technology. As a result, the decrease in efficiency of the ball screw 1 due to internal pressure fluctuations in the internal space 20 can be suppressed.

[0034] The second opening 12 is provided at the groove bottom 36 of the outer circumferential rolling groove 35 provided in the screw shaft 3. This allows air to flow smoothly through the outer circumferential rolling groove 35. This makes it possible to suppress the leakage of grease caused by air flowing through the gaps between the rolling elements 4 and the occurrence of internal pressure fluctuations in the internal space 20. This makes it possible to effectively suppress a decrease in the efficiency of the ball screw 1.

[0035] The hole diameter of the second opening 12 provided in the groove bottom 36 is larger than ¼ of the diameter of the rolling element 4. This allows air to circulate well through the communication hole 33. Therefore, the decrease in efficiency of the ball screw 1 can be suppressed compared to the conventional technology.

[0036] The second opening 12 is provided in the groove bottom 36 of the outer circumferential rolling groove 35 located in the non-passage area A2 of the rolling element 4. By providing the second opening 12 in the groove bottom 36, air can be smoothly circulated through the outer circumferential rolling groove 35. By providing the second opening 12 at a position corresponding to the non-passage area A2 of the rolling element 4, the influence on the rolling element 4 can be suppressed. Furthermore, compared with the case where the second opening 12 is provided in the passage area A1 of the rolling element 4, the size of the second opening 12 can be made larger even when the second opening 12 is provided in the groove bottom 36. Therefore, it is possible to make it easier for air to circulate. Therefore, the decrease in efficiency of the ball screw 1 can be more effectively suppressed compared to the conventional technology.

[0037] The communication hole 33 has a radial hole 14 extending along the radial direction. This allows the second opening 12 to be provided at any location on the screw shaft 3. For example, the second opening 12 can be provided on the outer circumferential surface of the screw shaft body 31, not on the small diameter portion 32. This allows the length of the communication hole 33 to be shortened compared to the conventional technology in which an air hole 910 (see FIG. 7) is formed along the axial direction over the entire screw shaft 3. As a result, the communication hole 33 can be easily formed, and therefore the workability of the communication hole 33 and the screw shaft 3 having this communication hole 33 can be improved. Furthermore, since the screw shaft 3 can be formed without providing the communication hole 33 in the small diameter portion 32, the small diameter portion 32 can be made smaller in diameter.

[0038] A plurality of second openings 12 are provided. This allows air to flow more easily through the communication holes 33, effectively suppressing a decrease in efficiency of the ball screw 1 due to internal pressure fluctuations and the like. Furthermore, the plurality of second openings 12 are provided at equal intervals from one another in the circumferential direction of the screw shaft 3. This makes it possible to suppress unbalance of the screw shaft 3 caused by the formation of the second openings 12. Therefore, even when a plurality of second openings 12 are provided, a decrease in efficiency of the ball screw 1 due to unbalance can be suppressed.

[0039] The second openings 12 are provided at equal positions in the axial direction. This allows the second openings 12 to be machined simultaneously, as compared to a case in which the second openings 12 are provided at different positions in the axial direction, and therefore reduces the number of machining steps required to form the second openings 12. This improves the processability of the screw shaft 3.

[0040] Next, the second to fifth embodiments of the present invention will be described. In the following description of the second to fifth embodiments, the same components as those in the first embodiment will be denoted by the same reference numerals and will not be described. Note that the specific components are not limited to these embodiments and can be modified as appropriate without departing from the scope of the present invention.

[0041] Second embodiment First, a second embodiment of the present invention will be described. Fig. 3 is a perspective view of a screw shaft 203 according to the second embodiment. The second embodiment differs from the first embodiment in that a second opening (inter-circulation circuit second opening 212) is provided between a plurality of infinite circulation circuits 55.

[0042] In the second embodiment, the communication hole 33 has a plurality of inter-circuit second openings 212 in addition to the second opening 12 in the first embodiment. The inter-circuit second openings 212 are provided between adjacent infinite circulation circuits 55 among the plurality of infinite circulation circuits 55 formed by the above-mentioned circulation part 5. Here, between the adjacent infinite circulation circuits 55, that is, between the circulation parts 5 adjacent in the circumferential direction in one outer circumferential rolling groove 35 in this embodiment shown in FIG. 3, there is a non-passage area A2 in which the rolling elements 4 do not circulate. The inter-circuit second openings 212 are provided in the groove bottom 36 of the outer circumferential rolling groove 35 corresponding to the non-passage area A2 provided between the adjacent infinite circulation circuits 55. Furthermore, in this embodiment, a non-passage region A2 for the rolling elements 4 is also provided at the end of the outer circumferential rolling groove 35 of the threaded shaft body 31 opposite the small diameter portion 32 in the axial direction. That is, a portion of the outer circumferential rolling groove 35 located at the center in the axial direction is made into a passage region A1, and non-passage regions A2 are provided on both sides of the passage region A1 in the axial direction. A second opening 12 may also be provided in the non-passage region A2 opposite the small diameter portion 32 in the axial direction.

[0043] According to the ball screw of the second embodiment, in a ball screw of a circulation type having a plurality of infinite circulation circuits 55, the second openings 212 between the circulation circuits can be provided between the plurality of infinite circulation circuits 55. This allows the number of the second openings 12, 212 to be increased, so that air can more easily flow through the communication holes 33 between the internal space 20 between the nut 2 and the screw shaft 3 and the outside of the ball screw 1. This effectively suppresses a decrease in efficiency of the ball screw 1 due to internal pressure fluctuations and the like. In addition, since the second openings 212 between the circulation circuits are provided at the groove bottom 36 of the outer peripheral rolling groove 35 that is not used (the rolling elements 4 do not pass through) by disposing the circulation part 5, the unused outer peripheral rolling groove 35 can be utilized. Therefore, there is no need to perform additional processing to form the non-passage area A2, and the second openings 12, 212 can be provided without deteriorating the workability compared to the conventional technology. Therefore, compared to the conventional technology, it is possible to provide a ball screw that can suppress a decrease in efficiency of the ball screw caused by internal pressure fluctuations in the internal space 20 between the nut 2 and the screw shaft 3 and can improve processability.

[0044] Third embodiment Next, a third embodiment of the present invention will be described. Fig. 4 is a cross-sectional view of a screw shaft 303 according to the third embodiment. The third embodiment differs from the first embodiment in that the communication hole 33 has a plurality of axial holes 13, 316.

[0045] In the third embodiment, the communication hole 33 further has a second axial hole 316 in addition to the axial hole 13 similar to that of the first embodiment. Moreover, the communication hole 33 is formed without having a radial hole 14 (see FIG. 2). As in the first embodiment, the axial hole 13 is formed inside the screw shaft body 31 from the end of the screw shaft body 31 opposite to the small diameter portion 32 in the axial direction. The axial hole 13 is formed coaxially with the central axis C of the screw shaft body 31. The end of the axial hole 13 on the small diameter portion 32 side in the axial direction terminates at a position closer to the first opening 11 than in the first embodiment. The second axial hole 316 extends from the end of the axial hole 13 on the small diameter portion 32 side toward the small diameter portion 32 in the axial direction. The second axial hole 316 communicates with the axial hole 13. The second axial hole 316 is located radially outward from the outer circumferential surface of the small diameter portion 32 in the radial direction. In other words, the second axial hole 316 is provided at a position eccentric to the central axis C. The end of the second axial hole 316 on the small diameter portion 32 side is opened on the end face of the screw shaft body 31 on the small diameter portion 32 side, that is, on the end face of the screw shaft body 31 corresponding to the step between the screw shaft body 31 and the small diameter portion 32. This opened portion is the second opening 312. Note that the second axial hole 316 may be provided in a plurality of positions aligned in the circumferential direction, for example.

[0046] According to the ball screw 1 of the third embodiment, air flows between the internal space 20 between the nut 2 and the screw shaft 3 and the outside of the ball screw 1 through the axial hole 13 and the second axial hole 316. Since the second opening is not provided in the outer peripheral rolling groove 35, the second opening 312 can be provided even when, for example, the non-passage area A2 is not provided in the outer peripheral rolling groove 35 of the screw shaft 3 or the non-passage area A2 is small. This improves the versatility of the ball screw 1. In addition, the length of the communication hole 33 can be shortened compared to the conventional technology in which a hole (for example, the air hole 910 in FIG. 7) is provided so as to penetrate the entire screw shaft 3. This makes it easier to form the communication hole 33, and improves the workability of the communication hole 33 and the screw shaft 303 having this communication hole 33.

[0047] (Fourth embodiment) Next, a fourth embodiment of the present invention will be described. Fig. 5 is a cross-sectional view of a screw shaft 403 according to the fourth embodiment. The fourth embodiment differs from the first embodiment in that the communication hole 33 has only an axial hole 413.

[0048] In the fourth embodiment, the communication hole 33 has only the axial hole 413. In other words, the communication hole 33 is formed without having the radial hole 14 (see FIG. 2). The axial hole 413 is provided in parallel with the central axis C and at a position not overlapping with the central axis C in the radial direction. The axial hole 413 is located radially outward of the outer circumferential surface of the small diameter portion 32 in the radial direction. The axial hole 413 extends along the axial direction and penetrates the screw shaft body 31 in the axial direction. One end of the axial hole 413 is opened at the end face of the screw shaft body 31 opposite to the small diameter portion 32 in the axial direction. This opened portion is defined as the first opening 411. The other end of the axial hole 413 is opened at the end face of the screw shaft body 31 on the small diameter portion 32 side, i.e., at the end face of the screw shaft body 31 corresponding to the step between the screw shaft body 31 and the small diameter portion 32. This opened portion is defined as the second opening 412. In addition, a plurality of axial holes 413 may be provided, for example, aligned in the circumferential direction.

[0049] According to the ball screw of the fourth embodiment, it is possible to obtain the same effects as those of the third embodiment, and also to increase the versatility of the ball screw.

[0050] Fifth embodiment Next, a fifth embodiment of the present invention will be described. Fig. 6 is a cross-sectional view of a screw shaft 503 according to the fifth embodiment. The fifth embodiment differs from the first embodiment in that the radial hole 514 is inclined.

[0051] In the fifth embodiment, the communication hole 33 has an axial hole 513 and a radial hole 514. The axial hole 513 is provided in a position that is parallel to the central axis C and does not overlap with the central axis C in the radial direction. The axial hole 513 is located radially outward from the outer circumferential surface of the small diameter portion 32 in the radial direction. The axial hole 513 extends along the axial direction. One end of the axial hole 513 is opened at the end face of the threaded shaft body 31 opposite to the small diameter portion 32 in the axial direction. This opened portion is defined as a first opening 511. The other end of the axial hole 513 terminates inside the threaded shaft body 31. The radial hole 514 is provided at a position corresponding to the other end of the axial hole 513 in the axial direction. The radial hole 514 extends from the other end of the axial hole 513 in the radial and axial directions. The radially inner end of the radial hole 514 communicates with the axial hole 513. The radial hole 514 is inclined so as to be located from one side to the other side in the axial direction as it moves from the radially inner side to the radially outer side. The radially outer end of the radial hole 514 is opened at the groove bottom 36 of the outer circumferential rolling groove 35 of the threaded shaft body 31, which corresponds to the non-passage area A2 of the rolling element 4. This opened portion is defined as the second opening 512. The communication holes 33 (axial hole 513 and radial hole 514) thus formed are arranged in a plurality at equal intervals in the circumferential direction of the screw shaft body 31.

[0052] According to the ball screw of the fifth embodiment, it is possible to obtain the same effects as those of the first embodiment, and also to increase the versatility of the ball screw. In the fifth embodiment, the axial hole 513 may be formed coaxially with the central axis C, similar to the axial hole 13 in the first embodiment. In this case, the radial hole 514 may be formed so as to incline from the axial hole formed coaxially with the central axis C toward the groove bottom 36 of the outer circumferential rolling groove 35.

[0053] The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, a configuration in which the circulation part 5 is provided on the screw shaft 3 has been described, but the circulation part 5 may be provided on the nut 2. In a ball screw configured in such a way that the circulation part 5 is provided on the nut 2, the above-described configuration of the communication hole 33 may be applied.

[0054] The outer circumferential rolling groove 35 may be provided in a part of the outer circumferential part of the screw shaft body 31. In this case, for example, the second opening 12 may be provided in the outer circumferential surface of the screw shaft body 31 where the outer circumferential rolling groove 35 is not provided. The second opening 12 may be provided at a position corresponding to the thread of the outer circumferential rolling groove 35. In this case, it is possible to improve the degree of freedom in the location where the second opening 12 is provided. However, the configuration of this embodiment in which the second opening 12 is provided at the groove bottom 36 is advantageous in that air can easily circulate through the thread groove. The screw shaft 3 may be formed without a circulating piece. As an example, the outer peripheral rolling groove 35 and the bypass passage 51 of the screw shaft 3 may be formed on the screw shaft 3 by cutting.

[0055] In the above embodiment, the nut 2 is configured to be tubular with a bottom, but this is not limited to this. For example, the configuration of the communication hole 33 of this embodiment may be applied to a ball screw configured such that one end of the cylindrical nut is press-fitted or fixed to another component.

[0056] The present disclosure may also be implemented in the following combinations: (1) A nut having a spiral inner peripheral rolling groove on an inner peripheral surface thereof; A screw shaft having a spiral outer circumferential rolling groove on an outer circumferential surface thereof; A plurality of rolling elements arranged between the inner peripheral rolling groove of the nut and the outer peripheral rolling groove of the screw shaft; Equipped with The screw shaft is a threaded shaft body provided on one side in the axial direction of the threaded shaft, the outer circumferential surface of which is formed with the outer circumferential rolling groove; A small diameter portion is provided on the other side of the threaded shaft body in the axial direction and has an outer diameter smaller than that of the threaded shaft body; A communication hole formed in the screw shaft so as to communicate between a first opening provided to open at one end of the screw shaft body in the axial direction and a second opening provided on the other side of the screw shaft body in the axial direction relative to the first opening; having At least a portion of the communication hole is provided radially outward from an outer circumferential portion of the small diameter portion in the radial direction of the screw shaft. Ball screw. (2) The first opening and the second opening are provided in a non-passage area of ​​the rolling element. A ball screw as described in (1). (3) The second opening is provided at the groove bottom of the outer circumferential rolling groove provided on the screw shaft. A ball screw as described in (1) or (2). (4) The hole diameter of the second opening provided at the bottom of the groove is greater than 1 / 4 of the diameter of the rolling element. (3) A ball screw as described in (1). (5) The second opening is provided at the groove bottom of the outer circumferential rolling groove located in a non-passage area of ​​the rolling element among the outer circumferential rolling grooves provided on the screw shaft, A ball screw according to any one of (1) to (4). (6) The communication hole has a radial hole extending along the radial direction. A ball screw according to any one of (1) to (5). (7) The second opening is provided in plurality, The second openings are arranged at equal intervals in the circumferential direction of the screw shaft. A ball screw according to any one of (1) to (6). (8) The second opening is provided in plurality, The second openings are provided at equal positions in the axial direction. A ball screw according to any one of (1) to (7). [Explanation of symbols]

[0057] 1,901 Ball screws 2,902 Nuts 3,203,303,403,503,903 Screw shaft 4,904 rolling elements 11,411,511 First opening 12,412,512 Second opening 14,514 radial holes 25 Inner circumference rolling groove 31 Screw shaft body 32 Small diameter section 33 Communication hole 35 Outer periphery rolling groove 36 (Bottom of outer peripheral rolling groove) 212 Second opening between circulation circuits (second opening) A2 Closed area

Claims

1. A nut having a spiral inner peripheral rolling groove on an inner peripheral surface thereof; A screw shaft having a spiral outer circumferential rolling groove on an outer circumferential surface thereof; A plurality of rolling elements arranged between the inner peripheral rolling groove of the nut and the outer peripheral rolling groove of the screw shaft; Equipped with The screw shaft is a threaded shaft body provided on one side in the axial direction of the threaded shaft, the outer circumferential surface of which is formed with the outer circumferential rolling groove; A small diameter portion is provided on the other side of the threaded shaft body in the axial direction and has an outer diameter smaller than that of the threaded shaft body; A communication hole formed in the screw shaft so as to communicate between a first opening provided to open at one end of the screw shaft body in the axial direction and a second opening provided on the other side of the screw shaft body in the axial direction relative to the first opening; having At least a portion of the communication hole is provided radially outward from an outer circumferential portion of the small diameter portion in the radial direction of the screw shaft. Ball screw.

2. The first opening and the second opening are provided in a non-passage area of ​​the rolling element.

2. The ball screw according to claim 1.

3. The second opening is provided at the groove bottom of the outer circumferential rolling groove provided on the screw shaft.

2. The ball screw according to claim 1.

4. The hole diameter of the second opening provided at the bottom of the groove is greater than 1 / 4 of the diameter of the rolling element.

4. The ball screw according to claim 3.

5. The second opening is provided at the groove bottom of the outer circumferential rolling groove located in a non-passage area of ​​the rolling element among the outer circumferential rolling grooves provided on the screw shaft, 4. The ball screw according to claim 3.

6. The communication hole has a radial hole extending along the radial direction. The ball screw according to any one of claims 1 to 5.

7. The second opening is provided in plurality, The second openings are arranged at equal intervals in the circumferential direction of the screw shaft. The ball screw according to any one of claims 1 to 5.

8. The second opening is provided in plurality, The second openings are provided at equal positions in the axial direction. The ball screw according to any one of claims 1 to 5.

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