Ball screw

Abstract

This ball screw includes a circulation member 5 which is attached to a fitting hole 6 provided in a nut 3 and which circulates a rolling ball. A pair of gaps 15 for inserting a jig 20 for positioning the circulation member 5 with respect to the fitting hole 6 are provided between the fitting hole 6 and the circulation member 5. The pair of gaps 15 are positioned respectively on one side and the other side in the extension direction of a screw shaft.

Description

ball screw

[0001] The present invention relates to a ball screw having a circulating member for circulating balls.

[0002] A known ball screw used in general industrial machine tools or automotive actuators includes a nut having a helical thread groove on its inner peripheral surface, a screw shaft having a helical thread groove on its outer peripheral surface, a plurality of balls rollably housed between the thread grooves of the nut and the screw shaft inserted into the nut, and a circulation member for circulating the balls.

[0003] For example, Patent Document 1 (Japanese Patent No. 4756645) discloses a method for fixing a circulation member in a fitting hole provided in a nut. In this method, the circulation member is inserted into the fitting hole, and then a convex engaging portion provided on the circulation member is bent outward to engage (crimp) with the edge of the fitting hole. This fixes the circulation member in the fitting hole and prevents the circulation member from falling off the nut. Furthermore, when the circulation member is fixed in the fitting hole, adjacent thread grooves of the nut are connected by connecting grooves in the circulation member, so that balls are guided along the connecting grooves and circulate through the thread grooves.

[0004] Patent No. 4756645

[0005] In the above-described configuration in which the circulation member is attached to the fitting hole of the nut, the fitting hole is formed slightly larger than the circulation member to make it easier to insert the circulation member into the fitting hole. As a result, when the circulation member is attached to the fitting hole, a gap is created between the connecting groove of the circulation member and the thread groove of the nut.

[0006] Since such gaps become steps when the balls roll, it is preferable that the gaps be uniform in order for the balls to roll smoothly. However, if the positioning accuracy of the circulation member relative to the fitting hole is poor, variations in the installation position of the circulation member will occur, and variations in the gaps will also occur. In such cases, the steps will also become large, especially in areas where the gaps are large, which may hinder the smooth rolling of the balls.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a ball screw that can accurately position the circulation member relative to the fitting hole and ensure smooth rolling of the balls.

[0008] In order to solve the above problems, the present invention provides a ball screw comprising a nut having a spiral thread groove on its inner surface, a screw shaft having a spiral thread groove on its outer surface, a plurality of balls that are rotatably accommodated between the thread groove of the nut and the thread groove of the screw shaft inserted into the nut, and a circulation member that is attached to a fitting hole provided in the nut and circulates the rolling balls, wherein a pair of gaps are provided between the fitting hole and the circulation member for inserting a jig to position the circulation member relative to the fitting hole, and the pair of gaps are located on one side and the other side in the extension direction of the screw shaft, respectively.

[0009] In this way, in the ball screw according to this embodiment, a pair of gaps are provided between the fitting hole and the circulation member, into which a jig for positioning the circulation member can be inserted. By inserting the jig into this gap, the circulation member can be positioned with high precision. As a result, the variation in the gap between the circulation member and the nut can be reduced, thereby ensuring smooth rolling of the balls. Here, the one side and the other side in the extending direction of the screw shaft refer to one side and the other side when the direction in which the axis of the screw shaft extends is defined as the axial direction.

[0010] It is preferable that the center of the circulation member is positioned so that it coincides with the center of the fitting hole. By attaching the circulation member so that the center of the circulation member coincides with the center of the fitting hole, the gap between the circulation member and the nut becomes uniform, allowing the balls to roll more smoothly.

[0011] The circulation member may also have a non-flat contact surface that comes into contact with the portion of the jig that is inserted into the pair of gaps.

[0012] According to the present invention, the circulation member can be positioned with high precision relative to the fitting hole, and smooth rolling of the balls can be ensured.

[0013] 8 is a plan view of a ball screw according to one embodiment of the present invention. FIG. 9 is a longitudinal sectional view of the ball screw according to the present embodiment. FIG. 10 is a longitudinal sectional view of the nut according to the present embodiment. FIG. 11 is a sectional perspective view showing a state before a circulation member is attached to the nut according to the present embodiment. FIG. 12 is a view showing a state in which the center of the circulation member coincides with the center of the fitting hole. FIG. 13 is a view showing a state in which the center of the circulation member is deviated from the center of the fitting hole. FIG. 14 is a perspective view of a jig according to the present embodiment. FIG. 15 is a side view of a jig according to the present embodiment. FIG. 16 is a sectional view of the jig according to the present embodiment, cut along line A-A in FIG. 8. FIG. 17 is a view for explaining a method of fixing the circulation member. FIG. 18 is a view for explaining a method of fixing the circulation member. FIG. 19 is a perspective view of a circulation member and a jig according to another embodiment of the present invention. FIG. 19 is a view showing a state in which the circulation member is positioned by a jig according to another embodiment of the present invention.

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. In each drawing for explaining the present invention, components such as members or components having the same function or shape are designated by the same reference numerals as far as they can be distinguished. Therefore, a description of a component that has already been described will be omitted.

[0015] The overall configuration of a ball screw according to one embodiment of the present invention will be described with reference to Figures 1 to 4. Figure 1 is a plan view of the ball screw according to this embodiment, and Figure 2 is a longitudinal cross-sectional view of the ball screw. Figure 3 is a longitudinal cross-sectional view of a nut provided in the ball screw, and Figure 4 is a cross-sectional perspective view showing the nut in a state before a circulation member is attached thereto.

[0016] As shown in FIGS. 1 and 2 , a ball screw 1 according to this embodiment includes a screw shaft 2 , a nut 3 , a plurality of balls 4 , and a circulation member 5 .

[0017] The screw shaft 2 has a helical thread groove 2a on its outer circumferential surface and is inserted into a cylindrical nut 3. On the other hand, the nut 3 has a helical thread groove 3a on its inner circumferential surface that faces the thread groove 2a of the screw shaft 2. A plurality of balls 4 are housed between these thread grooves 2a, 3a in a rollable manner.

[0018] The cross-sectional shape of each of the thread grooves 2a, 3a may be a circular arc shape or a gothic arc shape. In this embodiment, each of the thread grooves 2a, 3a is formed in a gothic arc shape, which allows a large contact angle with the ball 4 and a small axial clearance. This increases the rigidity of each of the thread grooves 2a, 3a against axial loads and suppresses the occurrence of vibration.

[0019] The nut 3 is provided with a circular fitting hole 6 that penetrates from its inner peripheral surface to its outer peripheral surface and cuts out a part of the thread groove 3a. A circulation member 5 having a substantially circular cross section is inserted into this fitting hole 6.

[0020] The circulation member 5 has a connecting groove 5a that connects adjacent thread grooves 3a of the nut 3 for one revolution. Therefore, as shown in FIG. 3 , when the circulation member 5 is inserted into the fitting hole 6, the S-shaped connecting grooves 5a connect adjacent thread grooves 3a of the nut 3, and the connecting grooves 5a and approximately one revolution of the thread grooves 3a form a rolling path for the balls 4. As a result, the multiple balls 4 are guided by the connecting grooves 5a of the circulation member 5, ride over the threads of the screw shaft 2, and return to the adjacent thread grooves 3a. The multiple balls 4 then circulate by rolling again along the thread grooves 2a, 3a. The depth of the connecting grooves 5a is set to a depth that allows the balls 4 to ride over the threads of the screw shaft 2 within the connecting grooves 5a.

[0021] 3, the circulation member 5 is provided with a pair of arms 9 that protrude in opposite directions. When the circulation member 5 is inserted into the fitting hole 6 of the nut 3, each arm 9 of the circulation member 5 fits into the thread groove 3a of the nut 3, thereby positioning the circulation member 5 in the axial direction with respect to the nut 3 and preventing the circulation member 5 from falling off in the radial direction of the nut. Note that each arm 9 engages with the thread groove 3a where the balls 4 do not roll so as not to interfere with the rolling of the balls 4.

[0022] 1 , a pair of engagement portions 12 are provided on the outer diameter side of the nut of the circulation member 5 so as to protrude in the outer diameter direction. When the circulation member 5 is inserted into the fitting hole 6 of the nut 3, the engagement portions 12 are pressed outward (away from each other) and tightened, whereby the engagement portions 12 engage with the edges of the fitting hole 6. This fixes the circulation member 5 to the nut 3.

[0023] In the above-described embodiment, in a configuration in which the circulation member 5 is mounted in the fitting hole 6 of the nut 3, the positioning accuracy of the circulation member 5 relative to the fitting hole 6 affects the rolling properties of the balls 4. Specifically, when the circulation member 5 is mounted so that the center O of the circulation member 5 coincides with the center P of the fitting hole 6, as in the example shown in FIG. 5 , the gaps S1 and S2 between the connecting groove 5 a of the circulation member 5 and the thread groove 3 a of the nut 3 can be made uniform. However, when the center O of the circulation member 5 is offset from the center P of the fitting hole 6 and the circulation member 5 is mounted offset, as in the example shown in FIG. 6 , the gaps S1 and S2 between the connecting groove 5 a of the circulation member 5 and the thread groove 3 a of the nut 3 vary (S1 > S2). When the gaps S1 and S2 vary, the difference in level between the connecting groove 5 a of the circulation member 5 and the thread groove 3 a of the nut 3 increases, particularly in a location where the gap is large (gap S1 in FIG. 6 ), which may hinder the smooth rolling of the balls 4.

[0024] Therefore, in the ball screw according to this embodiment, the circulation member 5 is fixed using the following jig in order to accurately position the circulation member 5 relative to the fitting hole 6. The configuration of the jig used in this embodiment and the method for fixing the circulation member 5 using the jig (the manufacturing method of the ball screw) will be described below.

[0025] 7 is a perspective view of a jig used in this embodiment, FIG. 8 is a side view of the jig, and FIG. 9 is a cross-sectional view of the jig taken along line AA in FIG.

[0026] 7 to 9, the jig 20 has a cylindrical main body 21, a pair of positioning portions 22, and a pair of crimping portions 23. The pair of positioning portions 22 and the pair of crimping portions 23 are each provided to protrude in the longitudinal direction from one end of the main body 21 in the longitudinal direction. The pair of positioning portions 22 protrude more than the crimping portions 23 and are arranged at a distance from each other. On the other hand, the pair of crimping portions 23 protrude less than the positioning portions 22 and are arranged at a distance from each other in a direction intersecting or perpendicular to the direction in which the positioning portions 22 face each other.

[0027] Each crimping portion 23 has an inclined surface 23a that is inclined toward the center line L (see FIG. 9) of the main body portion 21 toward its tip. This inclined surface 23a functions as a pressing portion that presses the engaging portion 12 when the engaging portion 12 of the circulation member 5 is crimped.

[0028] On the other hand, the pair of positioning portions 22 are formed to have a D-shaped cross section consisting of an arc portion and a straight portion in a cross section perpendicular to the center line L of the main body portion 21. That is, each positioning portion 22 has a flat portion 22a arranged to face the center line L side (inside) of the main body portion 21, and an arc surface portion 22b arranged to face the opposite side from the center line L side (outside).

[0029] To attach the circulation member 5 to the nut 3 using the jig 20 of the above embodiment, first, the circulation member 5 is inserted into the fitting hole 6 from the inner diameter side of the nut 3, and then, as shown in Figure 10, the positioning portion 22 of the jig 20 is inserted into the gap 15 between the circulation member 5 and the fitting hole 6.

[0030] 1 and 3, in this embodiment, the circulation member 5 has a pair of flat surfaces 5b facing in opposite directions, and therefore, when the circulation member 5 is inserted into the fitting hole 6, a gap 15 with a D-shaped cross section is formed between these flat surfaces 5b and the inner circumferential surface of the fitting hole 6. Then, a positioning portion 22 with the same D-shaped cross section is inserted into this D-shaped gap 15.

[0031] As described above, in this embodiment, D-shaped gaps 15 are provided on one side and the other side in the extension direction of the screw shaft 2 between the fitting hole 6 and the circulation member 5, and the circulation member 5 can be positioned in the fitting hole 6 by inserting a pair of positioning portions 22 into these gaps 15. That is, as shown in FIG. 10 , when the positioning portions 22 are inserted into the gaps 15, the flat surface portions 22a of the positioning portions 22 come into contact with the flat surface portions 5b of the circulation member 5, and the arcuate surface portions 22b of each positioning portion 22 come into contact with the inner circumferential surface of the fitting hole 6, so that displacement (misalignment) of the circulation member 5 in the fitting hole 6 is restricted by the pair of positioning portions 22. As a result, the circulation member 5 is positioned with high precision with respect to the fitting hole 6.

[0032] Then, with the circulation member 5 positioned, by further pushing the jig 20 into the fitting hole 6, the pair of engaging portions 12 of the circulation member 5 are pressed by the inclined surfaces 23a of the pair of crimping portions 23, and each engaging portion 12 is pressed outward (crimped), as shown by the two-dot chain line in Figure 11. As a result, each engaging portion 12 engages with the edge of the fitting hole 6, and the circulation member 5 is fixed to the nut 3.

[0033] As described above, in this embodiment, the gap 15 between the fitting hole 6 and the circulation member 5 is used as a gap for inserting the jig 20 (positioning portion 22) for positioning the circulation member 5, thereby enabling the circulation member 5 to be accurately positioned with respect to the fitting hole 6. As a result, as shown in Fig. 5, the center O of the circulation member 5 and the center P of the fitting hole 6 can be aligned, and the gaps S1 and S2 between the connecting groove 5a of the circulation member 5 and the thread groove 3a of the nut 3 can be made equal, ensuring smooth rolling of the balls.

[0034] The position of the center O of the circulation member 5 does not necessarily have to coincide with the center P of the fitting hole 6. Even in such a case, by using the jig 20 according to this embodiment, it is possible to reduce the positional deviation of the circulation member 5 with respect to the fitting hole 6, thereby improving the positioning accuracy and ensuring smooth rolling of the balls.

[0035] The pair of flat surfaces 5b of the circulation member 5 are preferably machined surfaces such as by cutting or grinding. In this case, each flat surface 5b can be more accurately aligned with the surface of the positioning portion 22, thereby improving positioning accuracy. Furthermore, since each flat surface 5b is a surface formed in a direction substantially perpendicular to the axial direction of the screw shaft 2, the presence or absence and degree of misalignment of the circulation member 5 can be easily confirmed by appearance.

[0036] Furthermore, the jig 20 is preferably a jig having the positioning portion 22 and the crimping portion 23 integrally therewith, as in the above embodiment. By having the positioning portion 22 and the crimping portion 23 integrally therewith, the positioning and fixing (crimping) of the circulation member 5 relative to the fitting hole 6 can be performed using a single jig 20, improving work efficiency. Furthermore, in the above embodiment, each engaging portion 12 is formed at a position perpendicular to the position of the gap 15 (flat portion 5b) so that the positioning and fixing (crimping) of the circulation member 5 can be performed simultaneously using a single jig 20.

[0037] In addition, in the above-described method, the circulation member 5 is inserted into the fitting hole 6, and then the positioning portion 22 of the jig 20 is inserted, but the procedure for positioning the circulation member 5 is not limited to the above-described method. For example, the positioning portion 22 may be inserted into the fitting hole 6 before the circulation member 5, and then the circulation member 5 may be inserted into the fitting hole 6 to be positioned.

[0038] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and can be modified as appropriate within the scope of the gist of the invention.

[0039] 12 , a convex portion 220 having a semicircular cross section may be provided on the flat surface 22 a of each positioning portion 22 of the jig 20, and a concave portion 50 having a semicircular cross section may be provided on each flat surface 5 b of the circulation member 5. In this case, as shown in FIG. 13 , when the positioning portions 22 of the jig 20 are inserted on both sides of the circulation member 5, the convex portions 220 of the positioning portions 22 engage with the concave portions 50 of the circulation member 5, thereby positioning the circulation member 5 within the engagement hole 6. In this way, the convex portions 220 and the concave portions 50 that engage with each other may be provided on the flat surface 22 a of the positioning portion 22 and the flat surface 5 b of the circulation member 5, thereby positioning the circulation member 5. That is, the circulation member 5 and the jig 20 may have non-planar contact surfaces (convex portion 220 and concave portion 50) that come into contact with each other between the circulation member 5 and a portion of the jig 20 (positioning portion 22) that is inserted into a gap located on one side and the other side in the extension direction of the screw shaft 2 between the fitting hole 6 and the circulation member 5. Furthermore, the shapes of the convex portion 220 and the concave portion 50 may be shapes other than semicircular in cross section, or, contrary to the examples of Figures 12 and 13, a concave portion may be provided in the jig 20 and a convex portion that fits into the concave portion may be provided in the circulation member 5.

[0040] REFERENCE SIGNS LIST 1 ball screw 2 screw shaft 2a screw groove 3 nut 3a screw groove 4 ball 5 circulation member 5a connecting groove 6 fitting hole 15 gap 20 jig 21 main body 22 positioning portion 23 crimping portion

Claims

1. A ball screw comprising: a nut having a helical thread groove on its inner peripheral surface; a screw shaft having a helical thread groove on its outer peripheral surface; a plurality of balls rollably accommodated between the thread groove of the nut and the thread groove of the screw shaft inserted into the nut; and a circulation member attached to a fitting hole provided in the nut and circulating the rolling balls, wherein a pair of gaps are provided between the fitting hole and the circulation member, into which a jig for positioning the circulation member with respect to the fitting hole can be inserted, and the pair of gaps are located on one side and the other side in the extension direction of the screw shaft.

2. A ball screw as set forth in claim 1, wherein the center of said circulation member is arranged to coincide with the center of said fitting hole.

3. A ball screw as claimed in claim 1 or 2, wherein said circulation member has a non-flat contact surface which contacts the portion of said jig which is inserted into said pair of gaps.

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