Nut for ball screw and ball screw comprising the same

Abstract

To provide a nut for a ball screw capable of improving operability of the ball screw.SOLUTION: A nut 4 for a ball screw has, on an inner peripheral surface 4a, a female screw groove 5 that, together with a male screw groove 3 provided on an outer peripheral surface 2a of a screw shaft 2, forms a spiral ball rolling path 8 on which a ball 7 rolls, and a circulation groove 11 that returns the ball 7 from an end point of the ball rolling path 8 to a start point. The circulation groove 11 has an undercut part 12 in which a groove width gradually decreases from a predetermined position in a depth direction thereof toward a groove opening part 11a.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a nut for a ball screw and a ball screw including the nut. [Background technology]

[0002] A ball screw, which is a type of motion conversion mechanism that converts rotary motion into linear motion or linear motion into rotary motion, includes a screw shaft and a nut that rotate relative to each other via a number of balls, and when the screw shaft and the nut rotate relative to each other, a number of balls interposed in a ball rolling path between a male screw groove formed on the outer peripheral surface of the screw shaft and a female screw groove formed on the inner peripheral surface of the nut roll along both screw grooves. Ball screws having such a structure are incorporated into drive mechanisms that require high operating accuracy, for example, in machine tools, conveying machines, industrial robots, automobiles, motorcycles, ships, etc.

[0003] It is essential for a ball screw to have a ball circulation path for returning the balls that have reached the end of the ball rolling path as the screw shaft and the nut rotate relative to each other to the start of the ball rolling path (to circulate the balls within the nut), and in addition to the female thread groove, the inner circumferential surface of the nut may be provided with a recess (circulation groove) that constitutes the ball circulation path. A nut having such a configuration can be obtained, for example, by plastically deforming a part of the inner circumferential surface of a cylindrical steel blank toward the outer diameter to form a circulation groove on the inner circumferential surface of the blank, and then cutting the inner circumferential surface of the blank to form a female thread groove (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6015444 Summary of the Invention [Problem to be solved by the invention]

[0005] 7(a) shows a schematic diagram of a circulation groove 102 formed on an inner peripheral surface 101 of a nut 100 by plastic working. In this case, an R-shaped sag 103 inevitably occurs at the groove opening of the circulation groove 102 (the boundary between the circulation groove 101 and the inner peripheral surface 101 of the nut 100), and the two opposing surfaces that define the groove width of the circulation groove 102 are generally formed as expanding surfaces that gradually expand the groove width of the circulation groove 101 from the groove bottom side toward the groove opening side in a manner that corresponds to the draft angle of the mold.

[0006] Incidentally, the balls present in the circulation groove (moving in the circulation groove) are in a free state without receiving any load from the nut and the screw shaft. In other words, the balls present in the circulation groove are not always in contact with the nut and the screw shaft, but are freely moved between the nut and the screw shaft by centrifugal force during the operation of the ball screw and by a force pushed by a subsequent ball. Therefore, if the circulation groove is formed in the above manner, as shown in FIG. 7(b), as the balls 104 move in the circulation groove 102 during the operation of the ball screw, the balls 104 (part or all) protrude outside the circulation groove 102 and interfere with the outer circumferential surface of the screw shaft 110. If the balls 104 moving in the circulation groove 102 interfere with the screw shaft 110, smooth ball circulation is hindered, and the operability of the ball screw is reduced.

[0007] In view of the above circumstances, an object of the present invention is to provide a nut for a ball screw that can achieve improved operability of the ball screw. [Means for solving the problem]

[0008] The present invention, which has been devised to achieve the above object, A ball screw nut having an internal thread groove that forms a spiral ball rolling path along which balls roll together with an external thread groove provided on the outer peripheral surface of a screw shaft, and a circulation groove that returns balls from an end point of the ball rolling path to a start point, the nut comprising: The circulation groove is characterized by having an undercut portion in which the groove width is gradually reduced from a predetermined position in the depth direction toward the groove opening.

[0009] When the above-mentioned undercut portion is provided in the circulation groove, it is possible to suppress the amount of protrusion of the balls moving in the circulation groove to the outside of the circulation groove, particularly if the opening width of the circulation groove (groove width at the groove opening) is made smaller than the ball diameter. This makes it difficult for the balls moving in the circulation groove to interfere with the screw shaft (it is possible to reduce the frequency of contact between the balls and the screw shaft), and the balls move smoothly in the circulation groove, improving the operability of the ball screw. In addition, when the balls moving in the circulation groove are unlikely to interfere with the screw shaft, it is also possible to suppress the generation of abnormal noise and vibrations caused by contact between the two, which can also contribute to improving the quietness of the ball screw.

[0010] Incidentally, although a circulation groove having an undercut portion can be formed by plastic processing such as forging, in the case of plastic processing, in addition to the above-mentioned problem that an R-shaped sagging inevitably occurs at the groove opening (the boundary between the circulation groove and the inner peripheral surface of the nut), there are problems such as a decrease in the dimensional and shape accuracy of the nut due to the forced removal of the mold after the groove is formed, and a rise in manufacturing costs due to the need for a complex mold. The above-mentioned circulation groove can also be formed by machining such as cutting, but the processing time is unavoidable. In particular, when forming multiple circulation grooves by cutting, the circulation grooves must be formed one by one to avoid interference between cutting tools, and the processing time is significantly increased.

[0011] Therefore, in the present invention, the circulation groove is formed by electrochemical machining, and the inner surface of the circulation groove is a surface formed by electrochemical machining. With electrochemical machining, by using an electrode (cathode) corresponding to the cross-sectional shape, groove depth, etc. of the circulation groove to be formed, a circulation groove having a predetermined cross-sectional shape, groove depth, etc. can be formed accurately and quickly without applying a large load to the nut material. Such an effect can be obtained in the same way when multiple circulation grooves are provided.

[0012] When the circulation groove is formed by electrochemical machining, the occurrence of sagging at the groove opening of the circulation groove can be prevented, and the circulation groove can be made to have a sharp cross-sectional shape. Therefore, even if the female thread groove is formed using the circulation groove as a machining reference, the female thread groove can be formed with high precision, and the female thread groove and the circulation groove can be smoothly connected. In addition, in this case, there is no need to separately provide a dedicated machining reference when forming the female thread groove, so manufacturing costs can be reduced.

[0013] A ball screw having the nut for a ball screw according to the present invention described above, a screw shaft that is inserted into the inner circumference of the nut for a ball screw and has a male thread groove formed on its outer surface which forms a spiral ball rolling path along which the balls roll together with the female thread groove, and a plurality of balls interposed in the ball rolling path, has the characteristics of being excellent in operability and being able to be manufactured at low cost. Effect of the Invention

[0014] As described above, according to the present invention, it is possible to provide a nut for a ball screw that can achieve improved operability of the ball screw. [Brief description of the drawings]

[0015] [Figure 1] 1 is a schematic longitudinal sectional view of a ball screw in which a nut for a ball screw according to an embodiment of the present invention is used as a component member. [Diagram 2] FIG. 1A is a schematic perspective view of a nut for a ball screw according to one embodiment of the present invention, and FIG. [Diagram 3] This is a cross section taken along line Y1-Y1 in FIG. [Figure 4] 2(b) is a cross-sectional view taken along line Y2-Y2 in FIG. [Diagram 5] FIG. 4(a) is a cross-sectional view taken along line ZZ in FIG. 3, and FIG. 4(b) is a schematic diagram showing a state in which balls are disposed in the circulation groove of the nut shown in FIG. 4(a). [Figure 6]Figure (a) is a cross-sectional view conceptually showing the starting stage of the electrolytic machining process, Figures (b) and (c) are cross-sectional views conceptually showing intermediate stages of the electrolytic machining process, and Figure (d) is a cross-sectional view conceptually showing the final stage of the electrolytic machining process. [Figure 7] FIG. 1(a) is a partial cross-sectional view of a conventional nut, and FIG. 1(b) is a cross-sectional view for explaining problems with the conventional nut. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0017] First, a ball screw in which a ball screw nut (hereinafter simply referred to as a "nut") according to an embodiment of the present invention is used as a constituent member will be briefly described with reference to Fig. 1. The "axial direction" used in the following description refers to the direction along the central axis X of the nut. Additionally, the "radial direction" refers to the radial direction of a circle centered on the central axis X, and the "circumferential direction" refers to the circumferential direction of a circle centered on the central axis X.

[0018] The ball screw 1 comprises a screw shaft 2 having a helical male screw groove 3 on its outer peripheral surface 2a, a nut 4 having a helical female screw groove 5 on its inner peripheral surface 4a facing the male screw groove 3, a plurality of balls 7 rollably disposed in a helical ball rolling path 8 formed by both the screw grooves 3 and 5, and a series of (endless) ball passages 10 consisting of the ball rolling path 8 and a ball circulation path 9. The ball circulation path 9 is composed of a concave circulation groove 11 formed on the inner peripheral surface of the nut 4. Therefore, the ball screw 1 does not include a separate dedicated member for forming the ball circulation path 9, such as a return tube or a top.

[0019] In the ball screw 1 having such a configuration, when the nut 4 fitted to the outer periphery of the screw shaft 2 via a plurality of balls 7 rotates relative to the screw shaft 2, the balls 7 roll in the ball rolling path 8, and the screw shaft 2 and the nut 4 move relative to each other in the axial direction X. At this time, when the balls 7 rolling in the ball rolling path 8 reach the end point of the ball rolling path 8, the balls 7 are scooped up from one end of the ball circulation path 9 (circulation groove 11) into the ball circulation path 9. The scooped up balls 7 move in the ball circulation path 9 so as to overcome the thread portion that defines the male screw groove 3, and are returned to the start point of the ball rolling path 8 from the other end of the ball circulation path 9. In this way, when the screw shaft 2 and the nut 4 rotate relative to each other, the balls 7 can circulate infinitely in the endless ball passage 10, so that the screw shaft 2 and the nut 4 can continuously move relative to each other. Although not shown, the ball screw 1 is usually provided with a stopper structure for determining the forward and backward movement limit of the linear motion member of the screw shaft 2 and the nut 4 (here, the screw shaft 2).

[0020] Hereinafter, the nut 4 according to one embodiment of the present invention will be described in detail with reference to Fig. 2 to Fig. 5. The nut 4 of the ball screw 1 is formed in a cylindrical shape from a metal material having electrical conductivity, for example, a steel material, and on its inner peripheral surface 4a, a plurality of female thread grooves 5 and a plurality of circulation grooves 11 connecting the start and end of each female thread groove 5 are formed. As shown in Fig. 3 and Fig. 4, on the inner peripheral surface 4a of the nut 4 of this embodiment, four female thread grooves 5 each consisting of a helical groove with an end slightly shorter than the circumferential length of the inner peripheral surface 4a of the nut 4 and four circulation grooves 11 are provided, and two circulation grooves 11 are arranged at intervals in each of the semi-cylindrical regions obtained by dividing the cylindrical inner peripheral surface 4a in half by a plane (axis-parallel plane) including the central axis X. Therefore, four female thread grooves 5 (5A to 5D) are formed independently of each other on the inner peripheral surface 4a of the nut 4 in this embodiment, and four ball rolling paths 8 (and further ball passages 10) formed by each female thread groove 5 and the male thread groove 3 of the screw shaft 2 are also formed independently of each other. Note that the number and positions of the circulation grooves 11 shown in Figures 3 and 4 are merely examples, and these are changed as appropriate depending on the size, application, required performance, etc. of the ball screw 1 (nut 4).

[0021] FIG. 5(a) is a cross-sectional view taken along line ZZ in FIG. 3 (a plane perpendicular to the center line of the circulation groove 11), and FIG. 5(b) is a schematic diagram showing a state in which the ball 7 is disposed in the circulation groove 11 of the nut 4 shown in FIG. 5(a). As shown in FIGS. 5(a) and 5(b), the circulation groove 11 is provided with an undercut portion 12 in which the groove width is gradually reduced from a predetermined position in the depth direction toward the groove opening (end portion on the groove opening side) 11a. Therefore, the groove width of the circulation groove 11 is maximum at a predetermined position in the groove depth direction (approximately the middle portion in the depth direction in the illustrated example) and gradually decreases toward the groove opening 11a. The groove width of the circulation groove 11 at the maximum groove width portion in the groove depth direction is larger than the ball diameter d of the ball 7, and the groove width (opening width) W at the groove opening 11a of the circulation groove 11 is smaller than the ball diameter d of the ball 7. In the illustrated example, the angle α between the two opposing surfaces that face each other in the groove width direction of the circulation groove 11 and define the undercut portion 12 is set to approximately 45°, but this angle α can be set appropriately taking into account various factors.

[0022] If the circulation groove 11 has the above-mentioned undercut portion 12 and the opening width (groove width at the groove opening 11a) W of the circulation groove 11 is set smaller than the ball diameter d of the ball 7, the amount of protrusion of the ball 7 moving in the circulation groove 11 (ball circulation path 9) outside the circulation groove 11 during operation of the ball screw 1 can be suppressed [see FIG. 5(b)]. This makes it difficult for the ball 7 moving in the circulation groove 11 to interfere with the screw shaft 2 (it is possible to reduce the frequency of contact between the ball 7 and the screw shaft 2), and the ball 7 moves smoothly in the circulation groove 11 and, by extension, through a series of ball passages 10 including the circulation groove 11, so that the operability of the ball screw 1 can be improved. In addition, if the ball 7 moving in the circulation groove 11 is unlikely to interfere with the screw shaft 2, the generation of abnormal noise and vibration due to contact between the two can be suppressed, which can contribute to the realization of a quiet ball screw 1 with low operating noise. Moreover, it is preferable that the undercut portion 12 has a length in the length direction of the circulation groove 11 (the direction in which the balls 7 pass) that is at least equal to or greater than the axial width of the shoulder portion (portion defining the male thread groove 3) 13 of the screw shaft 2. Moreover, it is more preferable that the circulation groove 11 has the undercut portion 12 at least at a position where the circulation groove 11 and the shoulder portion 13 of the screw shaft 2 overlap when viewed from the radial direction.

[0023] The nut 4 having the above configuration is obtained through an electrolytic processing process in which a blank M (see Figure 6) formed into a cylindrical shape from the above steel as a nut material is subjected to electrolytic processing to obtain a cylindrical primary intermediate product having a circulation groove 11 formed on its inner surface; a machining process in which the primary intermediate product is machined to obtain a cylindrical secondary intermediate product having a female thread groove 5 formed on its inner surface; a heat treatment process in which the secondary intermediate product is heat treated to improve its mechanical properties; and a finishing process in which the heat-treated secondary intermediate product is finished to obtain the nut 4 as a final product.

[0024] The electrochemical machining process will be described with reference to Fig. 6. First, electrochemical machining is, simply put, a machining method that uses electrolysis, in which a direct current is passed between a workpiece (anode) and an electrode (cathode) made of conductive metal that are placed facing each other with a gap in an alkaline electrolyte, thereby forming a recess corresponding to the shape of the electrode in the part of the workpiece facing the electrode.

[0025] 6(a), an electrolysis tool 20 is placed on the inner periphery of a blank M, which serves as an anode during electrolytic machining, with a gap (distance) between it and the inner periphery of the blank M. The electrolysis tool 20 used has an electrode (cathode) 21 having a shape and size corresponding to the shape (cross-sectional shape) and size of the circulation groove 11 to be formed on the inner periphery of the blank M, and an insulator 21 supporting the electrode 21 from the radially inside. The insulator 21 is provided so as to cover the inner periphery of the blank M except for the area where the circulation groove 11 is to be formed.

[0026] In this state, as shown in FIG. 6(b), an electrolyte is constantly flowed between the blank M and the electrode 21 (electrolysis tool 20), and a direct current is passed between the blank M and the electrode 21 while the electrode 21 (electrolysis tool 20) is moved toward the inner peripheral surface of the blank M. As a result, the part of the inner peripheral surface of the blank M facing the electrode 21 gradually dissolves, and a recess 11' is formed on the inner peripheral surface of the blank M. By continuously moving the electrolysis tool 20 toward the inner peripheral surface of the blank M and passing a current between the blank M and the electrode 21, the amount of dissolution of the blank M increases and the recess 11' expands [see FIG. 6(c)], and finally, as shown in FIG. 6(d), a primary intermediate product M' is obtained in which a circulation groove 11 having a shape corresponding to the contour of the surface of the electrode 21 is formed on the inner peripheral surface. This electrolytic processing may be performed under either constant voltage control or constant current control. The amount of processing (amount of dissolution) by electrolytic processing can be adjusted by controlling the current value and current passing time of the direct current flowing between the blank M and the electrode 21.

[0027] The inner surface of the circulation groove 11 formed by electrochemical machining is a surface formed by dissolving a metal material. When the inner surface of a circulation groove formed by plastic processing such as forging is observed by a microscope photograph, the fiber flow is continuous along the inner surface of the groove, but when the inner surface of the circulation groove 11 formed by electrochemical machining is observed by a microscope photograph, the fiber flow is cut by the inner surface of the groove. In addition, the inner surface of the circulation groove 11 formed by electrochemical machining has a rougher surface than the inner surface of the circulation groove formed by cutting, and is black instead of metallic in color. In this way, the circulation groove 11 (the inner surface of the groove) formed by electrochemical machining has a clear difference from the circulation groove (the inner surface of the groove) formed by plastic processing such as forging or mechanical processing such as cutting, so that it is easy to determine that the circulation groove 11 formed by electrochemical machining is formed by electrochemical machining (and is not formed by other processing methods such as plastic processing).

[0028] In the electrochemical machining process described above, a primary intermediate product M' is obtained in which a circulation groove 11 is formed on the inner peripheral surface by electrochemical machining using an electrode 20 arranged on the inner periphery of a blank M. Since the electrode 20 has a small size corresponding to the shape and size of the circulation groove 11, it is also easy to arrange a plurality of electrodes 20 on the inner periphery of the blank M. Therefore, a plurality of circulation grooves 11 of a predetermined shape can be formed at the same time.

[0029] In the machining process, the primary intermediate product M' obtained in the electrochemical machining process is subjected to cutting as machining to obtain a secondary intermediate product having a female screw groove 5 formed on its inner circumferential surface. When forming the female screw groove 5 on the inner circumferential surface of the primary intermediate product M', the circulating groove 11 (the groove opening 11a) formed in the electrochemical machining process can be used as a machining reference in the axial, radial and circumferential directions. That is, as described above, when the circulating groove 11 is formed by electrochemical machining, the formation of sagging at the groove opening 11a of the circulating groove 11 can be suppressed and the cross-sectional shape of the circulating groove 11 can be made sharp, so that even if the groove opening 11a of the circulating groove 11 is used as a cutting reference for the female screw groove 5, the female screw groove 5 can be formed with high accuracy and the circulating groove 11 and the female screw groove 5 can be smoothly connected. In this case, there is no need to provide a dedicated machining reference for forming the female screw groove 5, so the manufacturing process of the nut 4 can be simplified and the manufacturing cost can be reduced.

[0030] The secondary intermediate product obtained in the above manner is subjected to a heat treatment in a heat treatment process, and is then finished into a finished product in a subsequent finishing process. In the finishing process, finishing can be performed using the phase reference formed on the primary intermediate product M'.

[0031] As described above, when the circulation groove 11 to be provided on the inner peripheral surface 4a of the nut 4 is formed by electrochemical machining, the groove depth of the circulation groove 11 can be made deeper than when the circulation groove 11 is formed by plastic processing such as forging, because no plastic flow of the material of the nut 4 occurs. The groove depth when forming the circulation groove 11 by electrochemical machining can be adjusted by controlling the current value and the current flow time as described above. Therefore, it is easy to accommodate an increase in size of the nut 4 (ball size, etc.). In addition, when trying to reduce the size of the nut 4 when forming the circulation groove 11 by plastic processing, it is difficult to completely accommodate the reduction in size of the nut 4 because the strength of the die to be placed on the inner circumference of the blank M cannot be ensured. In contrast, with electrochemical machining, it is sufficient to place electrodes corresponding to the size of the circulation groove 11 on the inner circumference of the blank M, so that the circulation groove 11 can be formed even for a small nut 4. Therefore, the circulation groove 11 can be formed accurately and at low cost for nuts 4 of various sizes.

[0032] Furthermore, electrochemical machining does not require the complex mold structure required for plastic processing, which not only reduces the manufacturing cost of the nut 4 but also prevents problems such as foreign matter getting caught in the nut 4 (blank M) and deformation of the nut 4 (blank M) that occur during plastic processing.

[0033] In the embodiment described above, the circulation groove 11 is used as a machining reference when forming the female thread groove 5, but a machining reference may be formed in the cylindrical blank M prior to forming the female thread groove 5 and the circulation groove 11, and the circulation groove 11 and the female thread groove 5 may be formed using this machining reference. As a specific example without illustration, the outer peripheral surface and one end face of the blank are finished with high precision by grinding or the like, and a recess is formed in a partial circumferential region of the one end face of the blank. This recess can be formed by machining such as cutting and grinding using the outer peripheral surface and one end face of the blank as the machining reference. This allows the recess to be formed with high precision.

[0034] In this case, electrodes are placed on the inner circumference of the blank using one end face, the outer peripheral surface, and the recess as machining references (position references) in the axial, radial, and circumferential directions, respectively, and electrochemical machining is performed. Then, using these machining references, the female thread groove is formed by cutting or the like. In this way, both the circulation groove 11 and the female thread groove 5 can be machined based on a common machining reference, so that the relative positional accuracy of the two can be improved.

[0035] The above describes the nut 4 (and its manufacturing method) according to an embodiment of the present invention, but the embodiment of the present invention is not limited to this, and can be embodied in various other forms without departing from the gist of the present invention. [Explanation of symbols]

[0036] 1. Ball screw 2 Screw shaft 3 Male thread groove 4 Ball screw nut (nut) 5 Female thread groove 7. Ball 8 Ball Rolling Path 9 Ball Circulation Path 10 Ball Passage 11 Circulation groove 11a Groove opening 12 Undercut section 21 electrodes d ball diameter M Blank W Opening width X center axis

Claims

1. A ball screw nut having an internal thread groove that forms a spiral ball rolling path along which balls roll together with an external thread groove provided on the outer peripheral surface of a screw shaft, and a circulation groove that returns the balls from the end point of the ball rolling path to the start point, the nut comprising: The nut for a ball screw is characterized in that the circulation groove has an undercut portion in which the groove width gradually decreases from a predetermined position in the depth direction toward an opening of the groove.

2. 2. The nut for a ball screw according to claim 1, wherein an opening width of the circulation groove is smaller than a ball diameter of the ball.

3. 2. The nut for a ball screw according to claim 1, wherein an inner surface of the circulation groove is formed by electrochemical machining.

4. 2. The nut for a ball screw according to claim 1, wherein a plurality of the circulation grooves are provided.

5. 2. The nut for a ball screw according to claim 1, wherein the female screw groove is formed using the circulation groove as a machining reference.

6. 6. A ball screw comprising: the nut for a ball screw according to any one of claims 1 to 5; a screw shaft that is inserted into the inner periphery of the nut for the ball screw and has a male thread groove formed on its outer periphery that forms, together with the female thread groove, a spiral ball rolling path along which balls roll; and a plurality of balls interposed in the ball rolling path.

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