Ball screw device

Abstract

This ball screw device comprises a screw shaft, a nut, and a plurality of balls. An outer circumferential raceway surface is provided to the outer circumferential surface of the screw shaft. An inner circumferential raceway surface is provided to the inner circumferential surface of the nut, and the screw shaft passes through the nut. The nut has at least three circulation units. The raceway has at least three ball circuits corresponding to each circulation unit. The at least three ball circuits have, in the axial direction parallel to the center axis of the screw shaft, a center circuit positioned in the center region of the nut, a first circuit positioned further along the first direction than the center circuit, and a second circuit positioned further along the second direction than the center circuit. The effective diameter of each thread of the outer circumferential raceway surface is equal. From among the inner circumferential raceway surfaces, the effective diameter of the center inner circumferential raceway surface of the center circuit is different from that of the first inner circumferential raceway surface of the first circuit and that of the second circumferential raceway surface of the second circuit.

Description

Technical Field

[0001] The present disclosure relates to a ball screw device.

Background Art

[0002] A ball screw device includes a screw shaft, a nut penetrated by the screw shaft, and a plurality of balls disposed between the screw shaft and the nut. When the ball screw device is used in an injection molding machine, a press machine, etc., it converts a rotational motion into a linear motion. The nut has a nut body having a cylindrical shape and an inner circumferential raceway surface on which balls roll, and a mounting portion located at an end of the nut body to which other components are attached.

[0003] When the ball screw device is driven, a load is input to the mounting portion, and an axial stress acts on the nut body. In particular, a large stress acts on the portion of the nut body close to the mounting portion. For this reason, the portion of the nut body close to the mounting portion is greatly elastically deformed (expanded and contracted) in the axial direction. Further, along with the elastic deformation of the nut body, the portion of the inner circumferential raceway surface close to the mounting portion is greatly displaced in the axial direction. As a result, the balls rolling near the mounting portion receive a larger load from the inner circumferential raceway surface than the balls rolling far from the mounting portion. Therefore, the load distribution of the balls varies in the axial direction.

[0004] On the other hand, in the inner circumferential raceway surface of the nut of Patent Document 1, the effective diameter increases as it approaches the mounting portion. In other words, the axial gap between the ball and the inner circumferential raceway surface increases as it approaches the mounting portion. For this reason, even if the nut is elastically deformed by stress, the load acting on the balls rolling near the mounting portion is suppressed from increasing.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the ball screw device of Patent Document 1 does not consider the expansion and contraction of the screw shaft. In addition, there is a limitation that there is only one ball circuit, and it cannot be applied when there are a plurality of ball circuits. Therefore, the development of a ball screw device having a plurality of ball circuits and capable of suppressing variations in the load distribution for each ball circuit is desired.

[0007] The present disclosure has been made in view of the above, and an object thereof is to provide a ball screw device capable of suppressing variations in the load distribution for each ball circuit.

Means for Solving the Problems

[0008] To achieve the above object, a ball screw device according to an aspect of the present disclosure includes a screw shaft, a nut, and a plurality of balls. One end of the screw shaft points in a first direction, the other end points in a second direction, and an outer peripheral raceway surface is provided on the outer peripheral surface. The nut has an inner peripheral raceway surface provided on the inner peripheral surface and is penetrated by the screw shaft. The plurality of balls are arranged in a track between the outer peripheral raceway surface and the inner peripheral raceway surface. The nut has at least three or more circulation components. The track has at least three or more ball circuits corresponding to each of the circulation components. At least three or more of the ball circuits include a central circuit located at the central portion of the nut, a first circuit located in the first direction from the central circuit, and a second circuit located in the second direction from the central circuit in an axial direction parallel to the central axis of the screw shaft. The outer peripheral raceway surface has the same effective diameter for each strip. The first inner peripheral raceway surface belonging to the first circuit and the second inner peripheral raceway surface belonging to the second circuit among the inner peripheral raceway surfaces have different effective diameters from the central inner peripheral raceway surface belonging to the central circuit.

[0009] If the effective diameters of the first inner circumferential raceway surface and the second inner circumferential raceway surface are made larger or smaller than the effective diameter of the central inner circumferential raceway surface, the load acting on the balls rolling in the first circuit and the second circuit increases or decreases. That is, according to the ball screw device of the present disclosure, by appropriately setting the effective diameters of the first inner circumferential raceway surface and the second inner circumferential raceway surface while considering the stress acting on the nut and the screw shaft and the influence of the expansion and contraction of the screw shaft, the load distribution for each ball circuit can be made uniform.

[0010] Further, as a desirable aspect of the ball screw device according to one aspect of the present disclosure, the nut has a cylindrical shape, and includes a nut body provided with the inner circumferential raceway surface, and a first attachment portion provided at an end portion of the nut body in the first direction. The screw shaft has a shaft shape, and includes a screw shaft body provided with the outer circumferential raceway surface, and a second attachment portion provided at an end portion of the screw shaft body in the second direction. The effective diameters of the first inner circumferential raceway surface and the second inner circumferential raceway surface are larger than the effective diameter of the central inner circumferential raceway surface.

[0011] The first inner circumferential raceway surface of the present disclosure is larger than the effective diameter of the central inner circumferential raceway surface. Therefore, even if the first inner circumferential raceway surface is largely displaced in the axial direction due to elastic deformation of the nut body, an increase in the load on the balls in the first circuit can be suppressed to a small level. Also, the second inner circumferential raceway surface is larger than the effective diameter of the central inner circumferential raceway surface. Therefore, even if the portion of the outer circumferential raceway surface facing the second inner circumferential raceway surface is largely displaced in the axial direction due to elastic deformation of the screw shaft, an increase in the load on the balls in the second circuit can be suppressed to a small level. From the above, an increase in the load acting on the balls rolling in the first circuit and the second circuit is suppressed, and the load distribution for each ball circuit becomes uniform.

[0012] Further, as a desirable aspect of the ball screw device according to one aspect of the present disclosure, the axial distance between the first attachment portion and the first circuit is shorter than the axial distance between the second attachment portion and the second circuit. The effective diameter of the first inner circumferential raceway surface is larger than the effective diameter of the second inner circumferential raceway surface.

[0013] Since the distance between the second mounting portion and the second circuit is long, the amount of displacement in the axial direction of the portion of the outer peripheral raceway surface that constitutes the second circuit is small. Therefore, the load acting on the balls that roll on the second circuit is smaller (the load is smaller) than that on the balls that roll on the first circuit. And according to the ball screw device of the present disclosure, the effective diameter of the first inner peripheral raceway surface is larger than that of the second inner peripheral raceway surface. Therefore, the load acting on the balls that roll on the first circuit is more greatly reduced than that on the balls that roll on the second circuit. For this reason, the load distribution for each ball circuit becomes uniform.

[0014] Also, as a desirable aspect of the ball screw device according to one aspect of the present disclosure, each thread of the first inner peripheral raceway surface has a larger effective diameter as it is located in the first direction.

[0015] According to this, with respect to the load distribution of the balls in the first circuit, it becomes uniform in the axial direction.

[0016] Also, as a desirable aspect of the ball screw device according to one aspect of the present disclosure, each thread of the second inner peripheral raceway surface has a larger effective diameter as it is located in the second direction.

[0017] According to this, with respect to the load distribution of the balls in the second circuit, it becomes uniform in the axial direction.

[0018] Also, as a desirable aspect of the ball screw device according to one aspect of the present disclosure, each thread of the central inner peripheral raceway surface has a larger effective diameter as it moves away from the central portion of the nut body in the axial direction.

[0019] According to this, with respect to the load distribution of the balls in the central circuit, it becomes uniform in the axial direction.

Advantages of the Invention

[0020] According to the present disclosure, the variation in the load distribution for each ball circuit is eliminated, and the long life of the ball screw device can be achieved.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

DETAILED DESCRIPTION OF THE INVENTION

[0022] The embodiments for carrying out the invention will be described in detail with reference to the drawings. The present disclosure is not limited by the content described in the following description. Also, the components described below include those that can be easily assumed by those skilled in the art and substantially identical ones. Furthermore, the components described below can be combined as appropriate.

[0023] (Embodiment 1) FIG. 1 is an overall view showing the entirety of the ball screw device according to Embodiment 1. FIG. 2 is a cross-sectional view of the nut of Embodiment 1 cut axially. As shown in FIG. 1, the ball screw device 100 of Embodiment 1 includes a screw shaft 1, a nut 2, and a plurality of balls (not shown).

[0024] The screw shaft 1 is a rod-shaped component centered on the central axis O. Hereinafter, the direction parallel to the central axis O is referred to as the axial direction. The screw shaft 1 includes a screw shaft main body 10 and a second attachment portion 11. The screw shaft main body 10 is provided with an outer peripheral raceway surface 12 on its outer peripheral surface. The groove shape of the outer peripheral raceway surface 12 is a Gothic arc, although not particularly shown. Each thread of the outer peripheral raceway surface 12 has the same lead. Each thread of the outer peripheral raceway surface 12 has the same effective diameter. The second attachment portion 11 is provided at the end of the screw shaft main body 10. The second attachment portion 11 is attached to an attachment target such as an injection molding machine or a press machine where the ball screw device 100 is used.

[0025] The nut 2 includes a nut main body 20 and a first attachment portion 21. As shown in FIG. 2, the nut main body 20 has a cylindrical shape. The outer diameter (diameter) of the nut main body 20 is r1. The nut main body 20 is provided with an inner peripheral raceway surface 22 on its inner peripheral surface. The groove shape of the inner peripheral raceway surface 22 is a Gothic arc. Each thread of the inner peripheral raceway surface 22 has the same lead as the outer peripheral raceway surface 12. Therefore, a spiral orbit is formed between the outer peripheral raceway surface 12 and the inner peripheral raceway surface 22.

[0026] The balls are steel balls, although not particularly shown in the figures. Each ball has the same diameter and is arranged in an orbit. When the ball screw device is driven, the balls roll on the outer peripheral raceway surface 12 and the inner peripheral raceway surface 22. Also, in the present embodiment, the balls have an axial clearance with respect to the outer peripheral raceway surface 12 and the inner peripheral raceway surface 22. That is, the balls are assembled in a state where no preload is applied.

[0027] As shown in FIG. 1, the first mounting portion 21 is a flange that projects radially outward from the outer peripheral surface of the nut body 20. An object that moves in the axial direction is attached to the first mounting portion 21, for example, in an injection molding machine, a press machine, or the like. The first mounting portion 21 is located at the axial end of the nut body 20. Hereinafter, in the axial direction, the side on which the first mounting portion 21 is arranged as viewed from the central portion of the nut body 20 in the axial direction is referred to as the first direction X1. Also, in the axial direction, the direction opposite to the first direction X1 is referred to as the second direction X2. Also, on the screw shaft 1, the second mounting portion 11 is arranged in the second direction X2 with respect to the screw shaft body 10.

[0028] The nut 2 includes a plurality of circulation components 23. The circulation component 23 is a tube. In the present embodiment, the circulation component 23 forms one ball circuit for three of the orbits and circulates the balls. Also, the orbit has a plurality of ball circuits corresponding to each circulation component 23. In the present embodiment, the nut 2 has three circulation components 23. Therefore, the ball circuits in the present embodiment are three (see reference numerals 3A, 3B, and 3C in FIG. 2).

[0029] Hereinafter, the one of the three circulation components 23 that is arranged most in the first direction is referred to as the first circulation component 23A. Also, regarding the remaining two circulation components, in the order of being arranged in the second direction X2 from the first circulation component 23A, they are referred to as the central circulation component 23B and the second circulation component 23C. Also, the ball circuit circulated by the first circulation component 23A is referred to as the first circuit 3A. The ball circuit circulated by the central circulation component 23B is referred to as the central circuit 3B. The ball circuit circulated by the second circulation component 23C is referred to as the second circuit 3C.

[0030] As shown in FIG. 2, the inner circumferential raceway surface 22 includes a first inner circumferential raceway surface 22A belonging to the first circuit 3A, a central inner circumferential raceway surface 22B belonging to the central circuit 3B, and a second inner circumferential raceway surface 22C belonging to the second circuit 3C. The effective diameters of the first inner circumferential raceway surface 22A and the second inner circumferential raceway surface 22C are different from the effective diameter of the central inner circumferential raceway surface 22B. Specifically, each thread of the first inner circumferential raceway surface 22A has an effective diameter D1. Each thread of the central inner circumferential raceway surface 22B has an effective diameter D2. Each thread of the second inner circumferential raceway surface 22C has an effective diameter D3. The first inner circumferential raceway surface 22A and the second inner circumferential raceway surface 22C have a larger effective diameter than the central inner circumferential raceway surface 22B (D1, D3 > D2). Therefore, the axial and radial clearances between the balls and the raceway surfaces (the outer circumferential raceway surface 12 and the inner circumferential raceway surface 22) are larger in the first circuit 3A and the second circuit 3C than in the central circuit 3B. Note that the inner diameter (diameter) of the nut 2 is the same throughout from the end in the first direction X1 to the end in the second direction X2. That is, the threads of the first inner circumferential raceway surface 22A, the threads of the central inner circumferential raceway surface 22B, and the threads of the second inner circumferential raceway surface 22C have the same height, respectively.

[0031] Next, the effects of the ball screw device of Embodiment 1 will be described. In the description of the effects, the ball screw device of Comparative Example 1 will be cited as a comparison target. The ball screw device of Comparative Example 1 is different from Embodiment 1 in that the effective diameters of the first inner circumferential raceway surface 22A, the central inner circumferential raceway surface 22B, and the second inner circumferential raceway surface 22C are the same, and the other configurations are the same.

[0032] FIG. 3 shows the load distribution of the balls during the drive of the ball screw device of Comparative Example 1. FIG. 4 shows the load distribution of the balls during the drive of the ball screw device of Embodiment 1. That is, FIGS. 3 and 4 show the magnitude of the acting load corresponding to the axial position of the balls. Further, in FIGS. 3 and 4, each plotted point (see white circles) is connected to form a graph. Also, the graph formed has a wave shape. In other words, the load acting on the balls periodically increases and decreases as the load acting corresponding to the axial displacement of the balls changes. This is because, in addition to the axial load, a radial load acts on the ball screw device, causing the ball screw device to flex. Then, when the ball screw device is viewed in cross section, the magnitude of the stress occurs at each part in the circumferential direction, and the load acting on the balls increases and decreases at the circumferential position. That is, in the line graph, one period of the waveform indicates that the ball has made one revolution in the circumferential direction. Since the number of ball circuits is three, as shown in FIGS. 3 and 4, there are three periods of increase and decrease. Also, as shown in FIGS. 3 and 4, the waveform graph is divided into three in the horizontal axis direction to correspond to each ball circuit. Hereinafter, with reference to FIGS. 3 and 4, how the axial load of the balls changes for each period will be described.

[0033] As shown in FIG. 3, regarding Comparative Example 1, in the central circuit 3B, even if the period of the waveform changes, the load acting on the balls does not change significantly. On the other hand, in the first circuit 3A, as the period changes in the first direction X1, the load on the balls increases. Similarly, in the second circuit 3C, as the period changes in the second direction X2, the load on the balls increases. Therefore, overall, as the period changes from the central portion in the axial direction of the inner peripheral raceway surface 22 toward the first direction X1 and the second direction X2, the load acting on the balls increases. Therefore, the balls rolling in the first circuit 3A and the second circuit 3C have a greater load than the balls rolling in the central circuit. Also, in Comparative Example 1, it includes the period when the load acting on the balls is the smallest, which is the central circuit 3B. It includes the period when the load acting on the balls is the largest, which is the first circuit 3A and the second circuit 3C. And, in Comparative Example 1, the distribution width of the load acting on the balls is H1.

[0034] On the one hand, as shown in FIG. 4, in the central circuit 3B of the first embodiment, as in Comparative Example 1, even when the period of the waveform changes, the load acting on the ball does not change significantly. Also, in the first circuit 3A, as in Comparative Example 1, as the period changes in the first direction X1, the load on the ball increases. The second circuit 3C, as in Comparative Example 1, has an increasing load on the ball as the period changes in the second direction X2. Therefore, the waveform shapes of the respective ball circuits (3A, 3B, 3C) in the first embodiment are substantially the same as those of the respective ball circuits (3A, 3B, 3C) in Comparative Example 1.

[0035] On the other hand, in the first circuit 3A and the second circuit 3C of the first embodiment, since the effective diameters of the first inner peripheral raceway surface 22A and the second inner peripheral raceway surface 22C are large (the axial gap is large), the maximum load is reduced compared to Comparative Example 1. In other words, the waveforms of the first circuit 3A and the second circuit 3C in FIG. 4 are shifted downward compared to the waveforms of the first circuit 3A and the second circuit 3C in FIG. 3. Therefore, the increase in the load due to the rolling surface displacement of the balls rolling in the first circuit 3A and the second circuit 3C is kept low. On the other hand, the waveform of the central circuit 3B in the first embodiment is shifted upward compared to the waveform of the central circuit 3B in FIG. 3, and the load on the balls rolling in the central circuit 3B has increased. This is because the load on the balls rolling in the first circuit 3A and the second circuit 3C has been reduced, and relatively, the load on the balls rolling in the central circuit 3B has increased. From the above, the distribution width H2 of the load acting on the balls in the ball screw device 100 of the first embodiment is smaller than the distribution width H1 of Comparative Example 1. For this reason, according to the first embodiment, the loads on the balls rolling in the first circuit, the central circuit, and the second circuit are equalized.

[0036] According to the above-described ball screw device 100, it is avoided that only some of the plurality of balls (the balls in the first circuit 3A and the second circuit 3C) wear out earlier than a predetermined period, and the life of the ball screw device is prolonged.

[0037] As described above, the ball screw device 100 of Embodiment 1 includes a screw shaft 1, a nut 2, and a plurality of balls. One end of the screw shaft 1 points in the first direction X1, the other end points in the second direction X2, and an outer peripheral raceway surface 12 is provided on the outer peripheral surface. The nut 2 has an inner peripheral raceway surface 22 provided on its inner peripheral surface and is penetrated by the screw shaft 1. The plurality of balls are arranged in a track between the outer peripheral raceway surface 12 and the inner peripheral raceway surface 22. The nut 2 has at least three or more circulation components 23. The track has at least three or more ball circuits corresponding to each circulation component 23. The at least three or more ball circuits include a central circuit 3B located at the central portion of the nut 2, a first circuit 3A located in the first direction X1 from the central circuit 3B, and a second circuit 3C located in the second direction X2 from the central circuit 3B in the axial direction parallel to the central axis O of the screw shaft 1. The effective diameter of each section of the outer peripheral raceway surface 12 is the same. The first inner peripheral raceway surface 22A belonging to the first circuit 3A and the second inner peripheral raceway surface 22C belonging to the second circuit 3C have different effective diameters from the central inner peripheral raceway surface 22B belonging to the central circuit 3B.

[0038] By appropriately setting the effective diameters of the first inner peripheral raceway surface 22A and the second inner peripheral raceway surface 22C, the load distribution for each ball circuit can be made uniform.

[0039] Also, in the ball screw device of Embodiment 1, the nut 2 has a cylindrical nut body 20 provided with an inner peripheral raceway surface 22 and a first attachment portion 21 provided at the end of the nut body 20 in the first direction X1. The screw shaft 1 has a shaft-shaped screw shaft body 10 provided with an outer peripheral raceway surface 12 and a second attachment portion 11 provided at the end of the screw shaft body 10 in the second direction X2. The first inner peripheral raceway surface 22A and the second inner peripheral raceway surface 22C have larger effective diameters than the central inner peripheral raceway surface 22B.

[0040] According to the above-described configuration, an increase in the load acting on the balls rolling in the first circuit 3A and the second circuit 3C is suppressed, and the load distribution for each ball circuit becomes uniform.

[0041] Although the above has described Embodiment 1, the present disclosure is not limited to three ball circuits (circulating components 23). Hereinafter, other embodiments with an increased number of ball circuits will be described.

[0042] (Embodiment 2) FIG. 5 is an overall view showing the entire ball screw device according to Embodiment 2. FIG. 6 is a cross-sectional view of the nut of Embodiment 2 cut in the axial direction. The ball screw device 100A of Embodiment 2 is different from the ball screw device 100 of Embodiment 1 in that the nut 102 includes four circulating components 123.

[0043] Hereinafter, among the four circulating components 123, the one arranged in the first direction X1 is referred to as the first circulating component 123A. Regarding the remaining three circulating components 123, in the order of being arranged in the second direction X2 from the first circulating component 123A, they are referred to as the central first circulating component 123B, the central second circulating component 123C, and the second circulating component 123D. The ball circuit circulated by the first circulating component 123A is referred to as the first circuit 103A, the ball circuit circulated by the central first circulating component 123B is referred to as the central first circuit 103B, the ball circuit circulated by the central second circulating component 123C is referred to as the central second circuit 103C, and the ball circuit circulated by the second circulating component 123D is referred to as the second circuit 103D.

[0044] The inner peripheral raceway surface 122 includes a first inner peripheral raceway surface 122A belonging to the first circuit 103A, a central first inner peripheral raceway surface 122B belonging to the central first circuit 103B, a central second inner peripheral raceway surface 122C belonging to the central second circuit 103C, and a second inner peripheral raceway surface 122D belonging to the second circuit 103D. The effective diameter D11 of the first inner peripheral raceway surface 122A and the effective diameter D14 of the second inner peripheral raceway surface 122D are larger than the effective diameter D12 of the central first inner peripheral raceway surface 122B and the effective diameter D13 of the central second inner peripheral raceway surface 122C. Also in such a ball screw device 100A, the load on the balls rolling in the first circuit 103A and the central second circuit 103C is reduced, and the load distribution for each ball circuit becomes uniform.

[0045] Also, as shown in FIG. 5, with respect to the circumferential position of the circulating component 123 relative to the nut 102, when viewed from the central axis O, the first circulating component 123A and the second circulating component 123D are arranged in the same direction. Also, the central first circulating component 123B and the central second circulating component 123C are arranged in the same direction when viewed from the central axis O. And the first circulating component 123A and the second circulating component 123D are positioned in the opposite direction with respect to the central first circulating component 123B and the central second circulating component 123C when viewed from the central axis O. Since the balls enter the inside of the circulating component 123, during the circulation in the circulating component 123, the load on other balls that roll in the same direction as the circulating component 123 when viewed from the central axis O increases. If the circulating components 123 are arranged in the axial direction, when the balls roll in the same direction as the circulating component when viewed from the central axis, a large load will concentrate and act. Therefore, by shifting the position of the circulating component 123 in the circumferential direction, the concentration of the load is avoided.

[0046] (Embodiment 3) FIG. 7 is an overall view showing the whole ball screw device according to Embodiment 3. FIG. 8 is a cross-sectional view of the nut of Embodiment 3 cut in the axial direction. FIG. 9 is a diagram showing the load distribution of the balls during the drive of the ball screw device of Comparative Example 2. FIG. 10 is a diagram showing the load distribution of the balls during the drive of the ball screw device of Comparative Example 3. FIG. 11 is a diagram showing the load distribution of the balls during the drive of the ball screw device of Embodiment 3.

[0047] The ball screw device 100B of Embodiment 3 differs from the ball screw device 100 of Embodiment 1 in that the nut 202 includes five circulation components 223. Hereinafter, the one of the five circulation components 223 that is disposed in the first direction X1 most is referred to as the first circulation component 223A. Regarding the remaining four circulation components 223, in the order of being disposed in the second direction X2 from the first circulation component 223A, they are referred to as the central first circulation component 223B, the central second circulation component 223C, the central third circulation component 223D, and the second circulation component 223E. Also, the ball circuit circulated by the first circulation component 223A is referred to as the first circuit 203A. The ball circuit circulated by the central first circulation component 223B is referred to as the central first circuit 203B. The ball circuit circulated by the central second circulation component 223C is referred to as the central second circuit 203C. The ball circuit circulated by the central third circulation component 223D is referred to as the central third circuit 203D. The ball circuit circulated by the second circulation component 223E is referred to as the second circuit 203E. Further, in the ball screw device 100B of Embodiment 3, the axial distance between the first attachment portion 21 and the first circuit 203A is shorter than the axial distance between the second attachment portion 11 and the second circuit 203E.

[0048] The inner peripheral raceway surface 222 includes a first inner peripheral raceway surface 222A belonging to the first circuit 203A, a central first inner peripheral raceway surface 222B belonging to the central first circuit 203B, a central second inner peripheral raceway surface 222C belonging to the central second circuit 203C, a central third inner peripheral raceway surface 222D belonging to the central third circuit 203D, and a second inner peripheral raceway surface 222E belonging to the second circuit 203E. The effective diameter of the first inner peripheral raceway surface 222A is larger than that of the second inner peripheral raceway surface 222E (D21>D25). Also, the effective diameter of the second inner peripheral raceway surface 222E is larger than that of the central second inner peripheral raceway surface 222C (D25>D23). Also, the effective diameters of the central first inner peripheral raceway surface 222B, the central second inner peripheral raceway surface 222C, and the central third inner peripheral raceway surface 222D are the same (D22=D23=D24). From the above, the effective diameter of the inner peripheral raceway surface 222 has a relationship of D21>D25>D22=D23=D24. Also, the radial thickness of the nut body 220 is thicker than that of the nut 2 of Embodiment 1, and the diameter r2 of the nut body 220 is larger than the diameter r1 of the nut 2 of Embodiment 1. Therefore, the rigidity of the nut body 220 is improved.

[0049] Next, the effects of the ball screw device of Embodiment 3 will be described. In the description of the effects, the ball screw devices of Comparative Example 2 and Comparative Example 3 are cited as comparison targets. The ball screw device of Comparative Example 2 differs from the ball screw device 100B of Embodiment 3 in that the effective diameters of the inner peripheral raceway surfaces 222 are all uniform (all D23) and the diameter of the nut body 220 is r1. The ball screw device of Comparative Example 3 differs from the ball screw device 100B of Embodiment 3 in that the diameter of the nut body 220 is r1.

[0050] As shown in FIG. 9, in the ball screw device of Comparative Example 2, the central second circuit 203C located at the central portion in the axial direction includes a period when the load acting on the ball is the smallest. As the period changes in the first direction X1 with reference to the waveform of the central second circuit 203C, the load on the ball increases (refer to the waveforms of the central first circuit 203B and the first circuit 203A). Also, as the period changes in the second direction with reference to the waveform of the central second circuit 203C, the load on the ball increases (refer to the waveforms of the central third circuit 203D and the second circuit 203E). Further, starting from the second waveform of the central second circuit 203C, the load increase rate when the waveform period changes in the first direction X1 is higher than when it changes in the second direction X2. That is, the first circuit 203A includes a period when the load acting on the ball is the largest.

[0051] Note that, as in Comparative Example 2, the reason why the load on the first circuit 203A is larger than that on the second circuit 203E among the plurality of ball circuits is that the distance from the first mounting portion 21 of the screw shaft 1 to the nut 202 is long, so the axial displacement amount of the portion of the outer peripheral raceway surface 12 facing the central third circuit 203D and the second circuit 203E is small.

[0052] As shown in FIG. 10, in the ball screw device of Comparative Example 3, since the effective diameters of the first inner circumferential raceway surface 222A and the second inner circumferential raceway surface 222E are large, the load on the balls rolling in the first circuit 203A and the second circuit 203E is reduced, and relatively, the loads on the central first circuit 203B, the central second circuit 203C, and the central third circuit 203D are increased. Note that the effective diameter of the first inner circumferential raceway surface 222A is larger than that of the second inner circumferential raceway surface 222E. Therefore, the load on the balls rolling in the first circuit 203A is reduced more than that of the balls rolling in the second circuit 203E. From the above, in Comparative Example 3, the load does not concentrate on the balls rolling in the first circuit 203A and the second circuit 203E. Therefore, the distribution width H12 of the load acting on the balls in Comparative Example 3 is smaller than the distribution width H11 in Comparative Example 2.

[0053] As shown in FIG. 11, in the ball screw device of Embodiment 3, the load on the balls in the first circuit 203A is greatly reduced. This is because the nut body 220 has a greater thickness and the rigidity of the nut body 220 is increased. In other words, this is because the elastic change amount of the stress of the nut body 20 is reduced. In particular, the displacement amount of the inner circumferential raceway surface 222 near the first attachment portion 21 is suppressed, and the load on the balls rolling in the first circuit 203A is greatly reduced. Relatively, the loads on the balls in the central first circuit 203B, the central second circuit 203C, the central third circuit 203D, and the second circuit 205E are slightly increased. Therefore, the distribution width H13 of the load on the balls in Embodiment 3 is smaller than the distribution width H12 in Comparative Example 3.

[0054] From the above, according to Embodiment 3, the effective diameter is set corresponding to the characteristics of the ball screw device 102B, and the load distribution of the ball circuits (203A, 203B, 203C, 203D, 203E) can be made uniform.

[0055] (Embodiment 4) FIG. 12 is a cross-sectional view of the nut of Embodiment 4 cut in the axial direction. FIG. 13 is a diagram showing the load distribution of the balls during driving of the ball screw device of Comparative Example 4. FIG. 14 is a diagram showing the load distribution of the balls during driving of the ball screw device of Embodiment 4.

[0056] As shown in FIG. 12, the ball screw device 100C of Embodiment 4 is different from the ball screw device 100 of Embodiment 1 in that the effective diameter of each thread of the inner peripheral raceway surface increases as it goes from the central portion in the axial direction toward the first direction X1 and the second direction (D31>D32>D33>D34>D35<D36<D37<D38<D39). Specifically, the effective diameter of each thread of the first inner peripheral raceway surface 322A increases as it is located in the first direction X1 (D31>D32>D33). The effective diameter of each thread of the central inner peripheral raceway surface 322B increases as it moves away from the central portion in the axial direction in the axial direction (D34, D36>D35). The effective diameter of each thread of the second inner peripheral raceway surface 322C increases as it is located in the second direction X2 (D39>D38>D37).

[0057] Next, the effects of the ball screw device of Embodiment 4 will be described. In the description of the effects, the ball screw device of Comparative Example 4 is cited as a comparison target. The ball screw device of Comparative Example 4 is different from the ball screw device 100C of Embodiment 4 in that the effective diameters of the inner peripheral raceway surface 322 are all uniform (all D35).

[0058] As shown in FIG. 13, in Comparative Example 4, as the period changes from the central portion in the axial direction of the inner peripheral raceway surface 322 toward the first direction X1 and the second direction X2, the load acting on the ball increases. That is, the load on the balls rolling in the first circuit 303A and the second circuit 303C is high. On the other hand, as shown in FIG. 14, in the first circuit 303A of Embodiment 3, the amount of increase in load each time the period changes in the first direction X1 is smaller than that of the first circuit of Comparative Example 4. Also, in the second circuit of Embodiment 3, the amount of increase in load each time the period changes in the second direction is smaller than that of the second circuit of Comparative Example 4. Also, in the central circuit, the amount of increase when the period of the ball changes in the axial direction is slightly reduced. From the above, according to Embodiment 4, the variation in the load distribution for each ball circuit (303A, 303B, 303C) can be suppressed, and the variation in the load distribution of the balls within each ball circuit (303A, 303B, 303C) can be suppressed to be small.

[0059] The above has described Embodiments 1 to 4, but the ball screw device of the present disclosure is not limited to the examples shown in the embodiments. For example, in the embodiments, the first mounting portion 21 is located at the end portion of the nut body 20 in the first direction, and the second mounting portion 11 is located at the end portion of the screw shaft body 10 in the second direction. However, in the present disclosure, the first mounting portion 21 may be located at the end portion of the nut body 20 in the second direction, and the second mounting portion 11 may be located at the end portion of the screw shaft body 10 in the first direction. Further, in the ball screw device of the present disclosure, the first mounting portion of the nut and the second mounting portion of the screw shaft may be arranged in the same direction in the axial direction. That is, the first mounting portion may be located at the end portion of the nut body in the first direction, and the second mounting portion may be located at the end portion of the screw shaft body in the first direction. Or, the first mounting portion may be located at the end portion of the nut body in the second direction, and the second mounting portion may be located at the end portion of the screw shaft body in the second direction. Further, the present disclosure aims to equalize the load distribution for each ball circuit due to the axial displacement of the rolling surface, and the direction of the axial load acting on the ball screw device is not limited to either a tensile load or a compressive load.

[0060] Further, the first circuit and the second circuit in each embodiment were each one, but they may be divided into a plurality. That is, the first circuit may have two circuits, and the second circuit may have two circuits. Further, when the first circuit and the second circuit each have a plurality of circuits, they do not have to be the same number. Also, the inner peripheral raceway surface in the embodiment is a gothic arc, but in the present disclosure, it may be a circular arc. The mounting portion of the present disclosure may be, in addition to a flange, a mounting portion in which one end portion of the nut body 20 in the axial direction has a rectangular outer peripheral surface when viewed from the axial direction, and there is no particular limitation on its shape. Further, the circulating component may be a tumbler. In the present disclosure, the effective diameters of the inner peripheral raceway surfaces being different (larger) means that they are different (larger) by at least 1.0 μm.

Explanation of Reference Numerals

[0061] 100, 100A, 100B, 100C Ball Screw Device 1 Screw Shaft 2, 102, 202 Nut Circuit 1 of 3A, 103A, 203A, 303A Central Circuit of 3B Circuit 2 of 3C, 103D, 203E, 303C 10 Screw Shaft Body 11 Second Mounting Portion 12 Outer Peripheral Track Surface 20, 220 Nut Body 21 First Mounting Portion 22 Inner Peripheral Track Surface First Inner Peripheral Track Surface of 22A, 122A, 222A, 322A Central Inner Peripheral Track Surface of 22B Second Inner Peripheral Track Surface of 22C, 122D, 222E, 322C Circulating Parts of 23, 123, 223 Central First Circuit of 103B, 203B Central Second Circuit of 103C, 203C Central First Inner Peripheral Track Surface of 122B, 222B Central Second Inner Peripheral Track Surface of 122C, 222C Central Third Circuit of 203D Central Third Inner Peripheral Track Surface of 222D

Claims

1. A screw shaft having one end pointing in a first direction, the other end pointing in a second direction, and an outer peripheral raceway surface provided on the outer peripheral surface, A nut having an inner peripheral raceway surface provided on the inner peripheral surface and penetrated by the screw shaft, A plurality of balls arranged in a raceway between the outer peripheral raceway surface and the inner peripheral raceway surface, and comprising, The nut has at least three or more circulation components, The raceway has at least three or more ball circuits corresponding to each of the circulation components, At least three or more of the ball circuits are, In the axial direction parallel to the central axis of the screw shaft, a central circuit located at the central portion of the nut, A first circuit located in the first direction with respect to the central circuit, A second circuit located in the second direction with respect to the central circuit, and having, The outer peripheral raceway surface has the same effective diameter for each strip, Among the inner peripheral raceway surfaces, a first inner peripheral raceway surface belonging to the first circuit and a second inner peripheral raceway surface belonging to the second circuit have different effective diameters from a central inner peripheral raceway surface belonging to the central circuit, The nut, forms a cylindrical shape, a nut body provided with the inner peripheral raceway surface, A first mounting portion provided at an end of the nut body in the first direction, and having, The screw shaft, forms an axial shape, a screw shaft body provided with the outer peripheral raceway surface, A second mounting portion provided at an end of the screw shaft body in the second direction, and having, The first inner peripheral raceway surface and the second inner peripheral raceway surface have a larger effective diameter than the central inner peripheral raceway surface, The axial distance between the first mounting portion and the first circuit is shorter than the axial distance between the second mounting portion and the second circuit, The effective diameter of the first inner circumferential raceway surface is larger than that of the second inner circumferential raceway surface. Ball screw device.

2. Each thread of the first inner circumferential raceway surface has a larger effective diameter as it is located in the first direction. The ball screw device according to claim 1.

3. Each thread of the second inner circumferential raceway surface has a larger effective diameter as it is located in the second direction. The ball screw device according to claim 1 or claim 2.

4. Each thread of the central inner circumferential raceway surface has a larger effective diameter as it moves away from the central portion of the nut body in the axial direction. The ball screw device according to any one of claims 1 to 3.

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