Ball screw device

Abstract

To provide a ball screw device that can maintain excellent lubrication by leaving a required amount of grease for lubrication on a ball rolling section without scraping off the grease and can suppress leakage of grease.SOLUTION: A ball screw device 10 includes: a screw shaft 20 having an outer peripheral screw groove 21 and a land section 22; a nut 30 having an inner peripheral screw groove; a plurality of balls 26 rolling on a ball rolling section BL formed between the outer peripheral screw groove 21 and the inner peripheral screw groove; and a seal member 50 for preventing leakage of lubricant. The seal member 50 has a body section 51 having an edge section 52 fixed to the nut 30, and a lip section 55 which is brought into elastic contact with the outer peripheral screw groove 21 and the land section 22. The lip section 55 has a sealing performance suppression section 59 which has lower sealing performance than that of other portions in the ball rolling section BL and at least a part of a range in the vicinity thereof in a contact section with the outer peripheral screw groove 21.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a ball screw device capable of stably lubricating a ball screw. [Background technology]

[0002] Ball screws for high-load drives used in injection molding machines and the like are used in harsh environments with high loads and high cycles, so they must always maintain good lubrication, and a large amount of grease is automatically supplied. As a result, contamination of the surrounding area of ​​the machine due to grease scattering and increased running costs for grease become problems. In response to this, a seal specialized for sealing grease by closely adhering to the entire circumference of the screw shaft without any gaps has been devised. By using this seal, it is expected that grease will not easily leak, and contamination due to grease scattering and the amount of grease supplied will be reduced. However, if the amount of grease supplied is significantly reduced to an insufficient amount, the closely-adhered seal may scrape off the grease on the screw shaft, causing the lubrication condition to deteriorate. Therefore, the effect of reducing grease is limited.

[0003] Patent Document 1 discloses a linear guide device in which the inner diameter surface of a lubricant-containing polymer member is provided with a protrusion that contacts only the vicinity of the contact portion between the screw groove and the ball, and the other portion of the inner diameter surface is spaced apart from the screw groove by a certain distance, and when the ball screw is driven, lubricant gradually seeps out from the protrusion of the lubricant-containing polymer part over time and is supplied to the screw groove of the screw shaft to lubricate it.

[0004] Furthermore, Patent Document 2 discloses a ball screw in which a gap smaller than the "interference" of the seal with respect to the screw shaft is provided between the raceway grooves of the nut and screw shaft and the balls, and the axial contact position of the lip of the seal with the screw shaft changes as the nut reciprocates in the axial direction, causing the lubricant sealed in the nut to be adhered to different positions on the lip, improving the lubrication condition of the contact surface between the lip and the screw shaft, and suppressing early wear of the lip, thereby improving the sealing performance of the ball screw. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 10-176713 [Patent Document 2] Patent No. 5195434 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the linear guide device described in Patent Document 1 lubricates the screw groove with a relatively small amount of lubricant that seeps out from the lubricant-containing polymer portion, and has a problem that it is difficult to apply to high-load drive ball screws used in harsh environments such as high loads and high cycles that require a large amount of grease.

[0007] Furthermore, the ball screw described in Patent Document 2 lubricates the contact surface between the lip of the seal and the screw shaft to prevent premature wear of the lip, but does not lubricate the area between the ball and the rolling path, which is the subject of the present invention, in particular the load-bearing part of the rolling path (ball rolling part) where the load from the ball is applied.

[0008] The present invention has been made in consideration of the above-mentioned problems, and its object is to provide a ball screw device that can maintain good lubrication by leaving the amount of grease necessary for lubrication on the ball rolling surface without scraping it off, while suppressing grease leakage. [Means for solving the problem]

[0009] Therefore, the above object of the present invention is achieved by the following configuration [1] relating to a ball screw device. [1] A screw shaft having a spiral outer peripheral screw groove formed on an outer peripheral surface and a land portion formed between the outer peripheral screw grooves adjacent in the axial direction; A nut having a helical inner circumferential screw groove formed on an inner circumferential surface so as to face the outer circumferential screw groove of the screw shaft; A plurality of balls rolling on a ball rolling section formed between the outer peripheral screw groove and the inner peripheral screw groove; A seal member that is attached to the nut to close an internal space between the screw shaft and the nut and prevent leakage of a lubricant sealed in the internal space; A ball screw device comprising: The seal member includes a body portion having an edge portion fixed to the nut, and a lip portion elastically contacting the outer peripheral thread groove and the land portion of the screw shaft, The lip portion has a sealing performance suppressing portion in which the sealing performance is lower than that of other portions in at least a part of the ball rolling portion and its vicinity in a contact portion with the outer peripheral thread groove. Ball screw device. Effect of the Invention

[0010] According to the ball screw device of the present invention, the amount of grease required for lubrication is left on the ball rolling surface without being scraped off, thereby maintaining good lubrication and suppressing leakage of grease. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a partially cutaway top view of a ball screw device according to a first embodiment of the present invention. [Diagram 2] FIG. 2 is a top view of the nut shown in FIG. [Diagram 3] FIG. 3 is a vertical sectional view of the screw shaft. [Figure 4] FIG. 4 is a side view of the screw shaft showing the ball rolling portion in the outer peripheral thread groove. [Diagram 5] FIG. 5 is a vertical cross-sectional view showing a range in which a ball rolling section where the outer peripheral screw groove of the ball screw and the ball come into contact is formed. [Figure 6] FIG. 6 is a front view of the seal member shown in FIG. [Figure 7] 7 is a cross-sectional view of the seal member shown in FIG. 6 taken along line AA. [Figure 8] FIG. 8 is an enlarged view of part B in FIG. [Figure 9]FIG. 9 is a front view of the seal member showing the range where the lip portion is short in length. [Figure 10] FIG. 10 is a cross-sectional view taken along line CC of FIG. [Figure 11] FIG. 11 is an enlarged view of a main portion illustrating the contact state between the lip portion and the outer peripheral thread groove. [Figure 12] FIG. 12 is a front view of a seal member of a modified example. [Figure 13] FIG. 13 is a front view of a seal member according to a second embodiment of the present invention. [Figure 14] FIG. 14 is a cross-sectional view taken along line DD of FIG. [Figure 15] FIG. 15 is a cross-sectional view taken along line E-E of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a ball screw device according to the present invention will be described in detail with reference to the drawings. (First embodiment) As shown in Fig. 1 and Fig. 2, the ball screw device 10 of this embodiment is a device that converts rotational motion into linear motion, and includes a screw shaft 20 having a helical outer peripheral screw groove 21 and a land portion 22 formed between the outer peripheral screw grooves 21, 21 adjacent to each other in the axial direction on its outer peripheral surface, a nut 30 having a helical inner peripheral screw groove (not shown) formed on its inner peripheral surface opposite to the outer peripheral screw groove 21 of the screw shaft 20, a number of balls 26 (see Fig. 5) rollably interposed in a ball rolling path (not shown) formed by the outer peripheral screw groove 21 of the screw shaft 20 and the inner peripheral screw groove of the nut 30, and a seal member 50. The ball screw device 10 of this embodiment is preferably used for a large load acting on the shaft, for example, a press shaft of a press machine or an injection shaft of an injection molding machine, and maintains a good lubricated state.

[0013] The nut 30 has a flange portion 31 at one axial end (the left side in FIG. 1), and a portion of the outer circumferential surface is cut out to form a notched plane 32. A substantially U-shaped circulation tube 40 (five in the embodiment shown in the figure), which is a circulation part for returning the balls 26 that have rolled in the ball rolling path to a predetermined position in the ball rolling path, is fixed to the notched plane 32 by a set screw 11.

[0014] The circulation tube 40 has a ball circulation passage provided therein for circulating the balls 26 rolling on the ball rolling path, and the ball circulation passage and the ball rolling path form a circuit (five in the embodiment shown in the figure) which is an infinite circulation path for infinitely circulating the balls 26.

[0015] A circular recess 33 that is recessed in the axial direction is formed in the flange portion 31 of the nut 30, and a countersunk portion 34 is recessed radially outward at the axially outer end of the circular recess 33. Furthermore, a circular recess 35 having the same inner diameter as the circular recess 33 is formed at the end opposite the flange portion 31 of the nut 30 (the right side in Figure 1).

[0016] 6 and 7, the seal member 50 includes a main body 51 and a lip portion 55. The main body 51 is made by pressing a cold-rolled steel plate or the like, has a substantially L-shaped cross section, and is generally annular. The main body 51 includes a cylindrical edge portion 52 that is fitted and fixed into the circular recesses 33, 35 at the axial end of the nut 30, and a disk-shaped ring portion 53 that is bent at a substantially right angle from the axial end of the edge portion 52 toward the inside in the radial direction.

[0017] The lip portion 55 is formed from an elastic material such as rubber or elastomer, such as acrylonitrile butadiene rubber (NBR), acrylic rubber (ACM), or fluororubber (FKM), and has a base portion 56 fixed to the circular ring portion 53, and a disk-shaped circular ring lip portion 57 extending from the base portion 56 toward the inner diameter side. At the tip portion of the circular ring lip portion 57, a seal lip 58, which is a lip portion that elastically comes into sliding contact with the outer peripheral thread groove 21 and the land portion 22 of the screw shaft 20, is formed in an arc-shaped cross section. By forming the seal lip 58 in an arc-shaped cross section, it comes into uniform contact with any part of the screw shaft 20.

[0018] 1, the seal member 50 attached to the flange portion 31 has a cylindrical edge portion 52 pressed into the circular recess 33, and is fixed in place by a washer 14 fixed by a fixing screw 13 to the bottom surface of the large-diameter countersunk portion 34. The seal member 50 attached to the end opposite the flange portion 31 has a cylindrical edge portion 52 disposed in the circular recess 35 with a gap therebetween, and is prevented from coming off by a set screw 15 that screws into a female thread formed in the inner diameter direction from the outer circumferential surface of the nut 30. Specifically, the corner between the edge portion 52 and the circular ring portion 53 is fixed by abutting against a tapered portion of the set screw 15.

[0019] In such a ball screw device 10, if the grease or lubricating oil (hereinafter referred to as grease) adhering to the screw shaft 20 is excessively scraped off by the sealing member 50 that slides against the screw shaft 20, the lubrication condition will deteriorate, which may result in a shortened lifespan of the ball screw device 10.

[0020] The ball screw device 10 of this embodiment aims to maintain a good lubrication environment by leaving a certain amount of grease unscraped at the ball rolling section BL on the screw shaft 20 where grease is particularly needed, while reliably sealing other parts to prevent grease from leaking to the outside.

[0021] The ball rolling section BL is a section where the screw shaft 20 (peripheral screw groove 21) and the balls 26 actually come into contact, and in a normal ball screw device 10 in which load acts in both axial directions as the screw shaft 20 reciprocates as shown in Figures 3 and 4, two ball rolling sections BL (BL1, BL2) are formed for one peripheral screw groove 21. The ball rolling section BL is formed at a position diagonally above the bottom 21a of the peripheral screw groove 21 depending on the magnitude of the load acting on the screw shaft 20.

[0022] FIG. 5 shows the range where the ball rolling section BL of the outer peripheral screw groove 21 is formed. The ball rolling section BL is a portion where the outer peripheral screw groove 21 and the ball 26 contact each other, and its formation position changes depending on the magnitude of the load acting on the ball screw device 10. When the center of the outer peripheral screw groove 21 and the center C1 of the ball 26 are aligned, the formation range of the ball rolling section BL is approximately in the range of 40° to 70° when the phase of the straight line connecting the center C1 of the ball 26 and the bottom 21a of the outer peripheral screw groove 21 is 0°. Therefore, it is preferable to set γ1=35° to γ2=75° with a little margin. Here, γ1 is the minimum value of the phase of the ball rolling section BL and its vicinity, and γ2 is the maximum value of the phase of the ball rolling section BL and its vicinity. The reason why the range is wide is that the position of the ball rolling section BL moves in the direction of γ2 as the load applied to the screw shaft 20 increases.

[0023] The function of leaving a certain amount of grease on the ball rolling section BL without being scraped off is achieved by the seal member 50. As shown in Fig. 6 to Fig. 8, a seal lip 58 which is a sliding contact portion of the seal member 50 elastically slides against the outer peripheral thread groove 21 and the land portion 22 of the screw shaft 20. Specifically, the seal lip 58 slides along a curve where an imaginary plane perpendicular to the central axis CL of the screw shaft 20 intersects with the outer peripheral thread groove 21 and the land portion 22 (see Fig. 4).

[0024] In other words, as shown in FIG. 6, the shape of the seal lip 58 is not circular about the center C1 of the seal member 50, but is non-circular in that the portion that comes into sliding contact with the outer peripheral thread groove 21 is formed as a curve with a small radius, and the portion that comes into sliding contact with the land portion 22 is formed as a curve with a large radius, and the radius gradually increases from the small diameter portion that comes into sliding contact with the outer peripheral thread groove 21 to the large diameter portion that comes into sliding contact with the land portion 22.

[0025] A sealing performance suppression portion 59 is provided in a portion of the seal lip 58 of the seal member 50 in the circumferential direction, corresponding to the ball rolling section BL. The radius R1 of the sealing performance suppression portion 59 is formed slightly larger than the radius R2 on both sides in the circumferential direction. That is, the sealing performance suppression portion 59 has a small radial gap S between it and the screw shaft 20. As a result, a small amount of grease flows out from the radial gap S as the ball screw device 10 operates and adheres to the ball rolling section BL in streaks.

[0026] Grease adhering to the ball rolling surface BL remains on the ball rolling surface BL and contributes to lubrication without being scraped off by the seal lip 58. In the portion other than the sealing performance suppression portion 59, the seal lip 58 is in firm contact with the screw shaft 20, and the adhering grease is scraped off by the seal lip 58, preventing leakage.

[0027] The size of the radial gap S is preferably very small, about 0 to 0.5 mm, so as not to impair the effect of sealing in the grease.

[0028] As described above, it is preferable to set the sealing performance suppression portion 59 in the range where the ball rolling portion BL is formed, that is, in the range corresponding to the range of γ1 to γ2 (see FIG. 5) of the outer peripheral thread groove 21. Specifically, as shown in FIG. 6, when the phase of a straight line connecting the center C1 of the seal member 50 and the contact point P1 between the seal lip 58 and the bottom 21a of the outer peripheral thread groove 21 is set to 0°, the range is θ1 to θ2 obtained by the following formulas (1) and (2).

[0029] θ1=Dw / 2·sinγ1·cosβ·360 / L···(1) θ2=Dw / 2·sinγ2·cosβ·360 / L···(2) Where, Dw: diameter of ball β: Lead angle of the outer peripheral thread groove L: Lead of outer screw groove γ1: The minimum phase value of the ball rolling area and its vicinity when the phase of the line connecting the center of the ball and the bottom of the outer peripheral screw groove is 0°. γ2: The maximum phase value of the ball rolling area and its vicinity when the phase of the line connecting the center of the ball and the bottom of the outer peripheral screw groove is 0°.

[0030] The radial gap S between the screw shaft 20 does not necessarily have to be provided over the entire range of θ1 to θ2, and may be provided only over a portion of θ1 to θ2. For example, when the load applied to the screw shaft 20 is large, the radial gap S may be provided only near θ2.

[0031] 9 to 11, in order to improve sealing performance, the lip portion 55 in sliding contact with the outer peripheral thread groove 21 has a tip inclined toward the inside of the nut 30. Also, the length L1 of the circular lip portion 57 (indicated by arrow F in FIG. 9) in contact with the inner side wall 21b of the outer peripheral thread groove 21 on the inside of the nut 30 is shorter than the length L2 of the circular lip portion 57 in contact with the outer side wall 21c of the outer peripheral thread groove 21 on the outside of the nut 30.

[0032] The reason is that a pressure acts on the lip portion 55 to cause the grease to flow out from the right side, which is the inside of the nut 30, toward the left in Fig. 11. When the seal lip 58 is in sliding contact with the outer side wall 21c, which is the outside of the nut 30, the seal lip 58 comes into stronger contact with the outer side wall 21c due to the pressure of the grease, so that the grease will not leak even if the length of the circular lip portion 57 is long.

[0033] On the other hand, when the seal lip 58 is in sliding contact with the inner side wall 21b on the inside side of the nut 30, the seal lip 58 is displaced in a direction away from the inner side wall 21b due to the pressure of the grease, which makes it easier for the grease to leak. Therefore, the length of the circular lip portion 57 is shortened to increase the rigidity and suppress the leakage of grease. Note that instead of shortening the length of the circular lip portion 57, the thickness of the circular lip portion 57 may be increased to increase the rigidity.

[0034] As described above, according to the ball screw device 10 of this embodiment, the amount of grease required for lubrication is left on the ball rolling section BL of the outer peripheral screw groove 21 without being scraped off by the seal member 50, thereby maintaining good lubrication and suppressing grease leakage. Furthermore, by providing a small radial gap S (sealing performance suppressing section 59) in a part between the seal lip 58 and the screw shaft 20, the grease scraped off in the part other than the sealing performance suppressing section 59 tries to flow into the radial gap S and accumulates on the ball rolling section BL, so that the effect of further enhancing the lubricating effect of the ball rolling section BL can be expected.

[0035] (Modification) 12 shows a modified seal member 50, which differs from the seal member 50 of the first embodiment in that the seal lip 58 has only one seal performance suppressing portion 59 (a portion where a small radial gap S is provided between the seal lip 58 and the screw shaft 20). The other configurations and effects are the same as those of the seal member 50 of the first embodiment, so the same and corresponding reference characters are used for the same parts and detailed descriptions are omitted.

[0036] The ball screw device 10 of this modified example can be suitably used for a ball screw device 10 in which a high load acts on one axial side. That is, when a high load acts on one axial side, a large load acts on one of the two ball rolling sections BL. Therefore, the grease on the ball rolling section BL on the side on which the large load acts is left without being swept away. This reduces the number of places where the grease can easily leak out, and is expected to have a higher sealing effect.

[0037] Second embodiment As shown in Figures 13 to 15, the sealing member 50 of the ball screw device 10 of this embodiment differs from the ball screw device 10 of the first embodiment in that, instead of the radial gap S, which is the sealing performance suppression portion 59 of the first embodiment, a low rigidity portion 57a ​​having low axial rigidity is provided in a portion corresponding to the ball rolling portion BL of the circular lip portion 57, thereby forming a sealing performance suppression portion 59.

[0038] The low rigidity portion 57a ​​is a thin-walled portion in which the axial thickness t1 (see FIG. 14) of the portion of the circular lip portion 57 that corresponds to the ball rolling surface BL is thinner than the axial thickness t2 (see FIG. 15) of the other portion. As a result, the low rigidity portion 57a ​​is easily bent in the axial direction and the contact force with the screw shaft 20 is small, making it difficult to sweep off the grease, and the grease adheres along the ball rolling surface BL.

[0039] The range in which the low rigidity portion 57a ​​is provided is the same as the sealing performance suppressing portion 59 of the lip portion 55 in the first embodiment, and is set in the range of θ1 to θ2 when the phase of a straight line connecting the center C1 of the seal member 50 and the contact point P1 between the seal lip 58 and the bottom 21a of the outer peripheral thread groove 21 is set to 0° (see FIG. 6). Note that FIG. 13 shows an example in which the low rigidity portion 57a ​​(sealing performance suppressing portion 59) is provided at two locations on the seal member 50, but as shown in FIG. 12, the low rigidity portion 57a ​​may be provided only on one axial side where a high load acts. Also, the range in which the low rigidity portion 57a ​​is provided does not necessarily have to cover the entire range of θ1 to θ2, and may be provided only in a part of θ1 to θ2.

[0040] The present invention is not limited to the above-described embodiments and modified examples, and can be modified, improved, etc., as appropriate. For example, the seal performance suppression portion 59 is not limited to forming a small radial gap S between the seal lip 58 and the screw shaft 20 by increasing the radius of a part of the seal lip 58, or reducing the axial rigidity of the seal lip 58 to reduce the contact force with the screw shaft 20, and any means is possible as long as a gap through which grease can flow out is formed between the seal lip 58 and the screw shaft 20.

[0041] As described above, the present specification discloses the following: (1) A screw shaft having a helical outer peripheral thread groove formed on an outer peripheral surface and a land portion formed between the outer peripheral thread grooves adjacent to each other in the axial direction; A nut having a helical inner circumferential screw groove formed on an inner circumferential surface so as to face the outer circumferential screw groove of the screw shaft; A plurality of balls rolling on a ball rolling section formed between the outer peripheral screw groove and the inner peripheral screw groove; A seal member that is attached to the nut to close an internal space between the screw shaft and the nut and prevent leakage of a lubricant sealed in the internal space; A ball screw device comprising: The seal member includes a body portion having an edge portion fixed to the nut, and a lip portion elastically contacting the outer peripheral thread groove and the land portion of the screw shaft, The lip portion has a sealing performance suppressing portion in which the sealing performance is lower than that of other portions in at least a part of the ball rolling portion and its vicinity in a contact portion with the outer peripheral thread groove. Ball screw device. According to this configuration, the amount of grease required for lubrication is left on the ball rolling surface without being scraped off, thereby maintaining good lubrication and suppressing leakage of grease.

[0042] (2) When the phase of a straight line connecting the center of the seal member and the contact point of the lip portion and the bottom of the outer peripheral thread groove is 0°, It is provided at least in a part between θ1 and θ2 obtained by the following formula (1) and formula (2), A ball screw device as described in (1). θ1=Dw / 2·sinγ1·cosβ·360 / L···(1) θ2=Dw / 2·sinγ2·cosβ·360 / L···(2) Where, Dw: diameter of ball β: Lead angle of the outer peripheral thread groove L: Lead of the outer peripheral thread groove γ1: The minimum value of the phase of the ball rolling section and its vicinity when the phase of the straight line connecting the center of the ball and the bottom of the outer peripheral screw groove is 0°. γ2: The maximum value of the phase of the ball rolling section and its vicinity when the phase of the straight line connecting the center of the ball and the bottom of the outer peripheral screw groove is 0°. According to this configuration, the sealing performance suppression portion can be provided at a position corresponding to the ball rolling portion, and the ball screw device can be efficiently lubricated.

[0043] (3) The sealing performance suppression portion is a radial gap provided in a part of a contact portion between the outer peripheral thread groove and the lip portion. A ball screw device as described in (1). According to this configuration, a sealing performance suppressing portion having lower sealing performance than other portions can be formed with a simple mechanism.

[0044] (4) The sealing performance suppressing portion is a low-rigidity portion in which the axial rigidity of a portion in the circumferential direction of the lip portion is lower than the axial rigidity of the other portion. A ball screw device as described in (1). According to this configuration, the sealing performance suppressing portion can be formed by the low rigidity portion.

[0045] (5) The low rigidity portion is a thin-walled portion in which a part of the lip portion has an axial thickness smaller than the axial thickness of the other part. A ball screw device as described in (4). According to this configuration, a sealing performance suppressing portion having lower sealing performance than other portions can be formed with a simple mechanism.

[0046] (6) The length of the lip portion in contact with the inner wall of the outer peripheral thread groove in the inward direction of the nut is shorter than the length of the lip portion in contact with the outer wall of the outer peripheral thread groove in the outward direction of the nut. A ball screw device as described in (1). With this configuration, even if the pressure of the grease inside the nut increases, leakage of the grease can be effectively prevented.

[0047] (7) The tip shape of the lip portion that contacts the screw shaft has a cross-sectional arc shape. A ball screw device as described in (1). According to this configuration, the lip portion comes into uniform contact with any part of the screw shaft. [Explanation of symbols]

[0048] 10. Ball screw device 20 Screw shaft 21 Outer Circumferential Thread Groove 21a Bottom of outer thread groove 21b Inner wall of outer peripheral screw groove 21c Outer wall of peripheral thread groove 22 Land Department 26 Ball 30 Nut 50 Sealing material 51 Main body 52 Edge 55 Lip 57 Circular lip 57a Low rigidity section (sealing performance suppression section, thin wall section) 58 Seal Lip 59 Seal performance suppression part C1 Center of seal material BL(BL1,BL2) Ball rolling section L1, L2 Lip length P1: Contact point between the lip and the bottom of the outer thread groove S Radial gap (seal performance suppression area) t1, t2 Axial thickness of lip

Claims

1. A screw shaft having a spiral outer peripheral screw groove formed on an outer peripheral surface and a land portion formed between the outer peripheral screw grooves adjacent to each other in the axial direction; A nut having a helical inner circumferential screw groove formed on an inner circumferential surface so as to face the outer circumferential screw groove of the screw shaft; A plurality of balls rolling on a ball rolling section formed between the outer peripheral screw groove and the inner peripheral screw groove; A seal member that is attached to the nut to close an internal space between the screw shaft and the nut and prevent leakage of a lubricant sealed in the internal space; A ball screw device comprising: The seal member includes a body portion having an edge portion fixed to the nut, and a lip portion elastically contacting the outer peripheral thread groove and the land portion of the screw shaft, The lip portion has a sealing performance suppressing portion in which the sealing performance is lower than that of other portions in at least a part of the ball rolling portion and its vicinity in a contact portion with the outer peripheral thread groove. Ball screw device.

2. When the phase of a straight line connecting the center of the seal member and the contact point of the lip portion and the bottom of the outer peripheral thread groove is 0°, Provided at least in a portion between θ1 and θ2 obtained by the following formulas (1) and (2): The ball screw device according to claim 1 . θ1=Dw / 2・sinγ1・cosβ・360 / L...(1) θ2=Dw / 2・sinγ2・cosβ・360 / L...(2) Where, Dw: diameter of the ball β: Lead angle of the outer peripheral thread groove L: Lead of the outer peripheral thread groove γ1: The minimum value of the phase of the ball rolling section and its vicinity when the phase of the straight line connecting the center of the ball and the bottom of the outer peripheral thread groove is 0°. γ2: The maximum value of the phase of the ball rolling section and its vicinity when the phase of the straight line connecting the center of the ball and the bottom of the outer peripheral thread groove is 0°.

3. The sealing performance suppression portion is a radial gap provided in a part of the contact portion between the outer peripheral thread groove and the lip portion. The ball screw device according to claim 1 .

4. The sealing performance suppressing portion is a low rigidity portion in which the axial rigidity of a part in the circumferential direction of the lip portion is lower than the axial rigidity of the other part. The ball screw device according to claim 1 .

5. The low rigidity portion is a thin-walled portion in which a part of the lip portion has an axial thickness smaller than the axial thickness of the other part. The ball screw device according to claim 4.

6. The length of the lip portion contacting the inner wall of the outer peripheral thread groove in the inward direction of the nut is shorter than the length of the lip portion contacting the outer wall of the outer peripheral thread groove in the outward direction of the nut. The ball screw device according to claim 1 .

7. The tip shape of the lip portion that contacts the screw shaft has an arc-shaped cross section. The ball screw device according to claim 1 .

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