Ball screw device and method for manufacturing a ball screw device

Abstract

We provide a ball screw device equipped with a nut that has high machining precision. [Solution] The ball screw device of this disclosure comprises a screw shaft with an outer peripheral raceway surface formed on its outer peripheral surface, a nut with an inner peripheral raceway surface formed on its inner peripheral surface and into which the screw shaft is inserted, and a plurality of balls arranged in the raceway between the screw shaft and the nut. At least one of the outer peripheral side of the nut and the end face side of the nut has a molded portion formed by cold forging. An S-shaped groove surface formed by cutting is formed on the inner peripheral surface of the nut.

Description

Technical Field

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[0001] The present disclosure relates to a ball screw device and a method for manufacturing a ball screw device.

Background Art

[0002] A ball screw device is a device that converts rotational motion into linear motion or converts linear motion into rotational motion. The ball screw device includes a screw shaft, a nut penetrated by the screw shaft, a plurality of balls disposed between the screw shaft and the nut, and a circulation part for circulating the balls. There are various types of circulation parts, and as one of the circulation parts, an S-shaped groove surface integrally formed on the inner peripheral surface of the nut as shown in the following patent document can be mentioned.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, gears, flanges, etc. may be formed on the outer peripheral surface of the nut. From the viewpoint of mass production, cold forging is often adopted as a method for forming gears and the like. Also, the above-described S-shaped groove surface can be formed by cold forging. Hereinafter, cold forging may be simply referred to as "forging" in some cases.

[0005] However, if the outer circumference or end face of a nut is forged after forming an S-shaped groove surface, the S-shaped groove surface may collapse (deform). Furthermore, if the inner circumference of a nut is forged to form an S-shaped groove surface, a portion of the nut may bulge outwards on the outer circumference or end face. As a result, it may not be possible to machine the outer circumference or end face of a nut with high precision even if it is forged. On the other hand, if a gear or the like is formed by forging the outer circumference or end face of a nut, and then the inner circumference of the nut is forged, the gear or the like may collapse (deform). Therefore, it is desirable to be able to machine nuts with high precision.

[0006] This disclosure has been made in view of the above, and aims to provide a ball screw device equipped with a nut with high processing accuracy and a method for manufacturing a ball screw device. [Means for solving the problem]

[0007] To achieve the above objective, the ball screw device according to this disclosure comprises a screw shaft having an outer peripheral raceway surface formed on its outer peripheral surface, a nut having an inner peripheral raceway surface formed on its inner peripheral surface and into which the screw shaft is inserted, and a plurality of balls arranged in a raceway between the screw shaft and the nut. At least one of the outer peripheral side and the end face side of the nut has a molded portion formed by cold forging. The inner peripheral surface of the nut has an S-shaped groove surface formed by cutting.

[0008] According to this disclosure, if the S-shaped groove surface is formed after the molded portion is formed, deformation of the S-shaped groove surface can be avoided. Therefore, the processing accuracy of the nut is high.

[0009] Furthermore, in order to achieve the above objectives, the method for manufacturing a ball screw device according to this disclosure includes a nut manufacturing step for manufacturing a nut. The nut manufacturing step includes a cold forging step of cold forging a cylindrical blank material to form a molded portion on the outer circumference side or the end face side of the blank material, and a cutting step of cutting the inner circumference surface of the blank material to form an S-shaped groove surface. The cutting step is performed after the cold forging.

[0010] According to this disclosure, the nut is cold-forged before the cutting process (before forming the S-shaped groove surface). Therefore, deformation of the S-shaped groove surface due to cold forging does not occur. Also, since the S-shaped groove surface is formed by cutting, deformation of the formed part does not occur. As a result, the S-shaped groove surface and the formed part are formed in a predetermined shape, and the processing accuracy of the nut is high.

[0011] Furthermore, the molded portion may be any one of the following: gear teeth, flange, spline, serration, stopper, key, keyway, and two-sided width shape. [Effects of the Invention]

[0012] According to the ball screw device and the method for manufacturing the ball screw device of this disclosure, the processing accuracy of the nut is high. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is a cross-sectional view of the ball screw device of Embodiment 1, cut in the axial direction. [Figure 2] Figure 2 shows the nut manufacturing process according to Embodiment 1. [Figure 3] Figure 3 is a perspective view of the nut of Modified Example 1, viewed from the axial direction. [Figure 4] Figure 4 shows the nut of the modified example 2 viewed from the axial direction. [Figure 5] Figure 5 is a cross-sectional view of the nut of the modified example 3, cut in the axial direction. [Modes for carrying out the invention]

[0014] The forms for implementing this disclosure will be described in detail with reference to the drawings. This disclosure is not limited by the contents described below. Furthermore, the components described below include those that are readily conceivable to a person skilled in the art, and those that are substantially the same. In addition, the components described below can be combined as appropriate.

[0015] (Embodiment 1) Figure 1 is a cross-sectional view of the ball screw device of Embodiment 1, cut in the axial direction. As shown in Figure 1, the ball screw device 100 is a device that converts rotational motion into linear motion or linear motion into rotational motion. The ball screw device 100 is mounted on, for example, an actuator. The actuator on which the ball screw device 100 is mounted is used in electric brakes such as EMB (Electro Mechanical Brake) and EHB (Electro-Hydraulic Brake). Note that the ball screw device 100 of this disclosure may be used in applications other than actuators.

[0016] The ball screw device 100 of Embodiment 1 comprises a screw shaft 1, a nut 2, and a ball (not shown) positioned between the screw shaft 1 and the nut 2. Hereinafter, the direction parallel to the central axis O1 of the screw shaft 1 will be referred to as the axial direction. The direction perpendicular to the central axis O1 will be referred to as the radial direction. Within the radial direction, the direction toward the central axis O1 will be referred to as the radially inward direction, and the direction opposite to the radially inward direction will be referred to as the radially outward direction.

[0017] The outer circumferential surface of the screw shaft 1 has an outer circumferential raceway surface 10 that is recessed radially inward and extends in the helical direction. The nut 2 is formed in a cylindrical shape. The nut 2 also has a first end face 20 facing one direction in the axial direction and a second end face 21 facing the other direction in the axial direction. The inner circumferential surface 22 of the nut 2 has a helical inner circumferential raceway surface 23 and an S-shaped S-groove surface 24.

[0018] The inner circumferential raceway surface 23 faces the outer circumferential raceway surface 10. The space between the inner circumferential raceway surface 23 and the outer circumferential raceway surface 10 constitutes the raceway. Multiple balls 3 are arranged on the raceway. The length of the inner circumferential raceway surface 23 is slightly shorter than one rotation (approximately 1 lead). Therefore, the length of the raceway is also slightly shorter than one rotation (approximately 1 lead).

[0019] The S-shaped groove surface 24 is formed by cutting the inner peripheral surface 22 of the nut 2. The S-shaped groove surface 24 connects one end and the other end of the inner peripheral raceway surface 23. Therefore, the ball 3 that has moved from one end to the other end of the raceway moves along the S-shaped groove surface 24 and circulates to one end of the raceway.

[0020] In addition, four inner peripheral raceway surfaces 23 and four S-shaped groove surfaces 24 are provided in the first embodiment. That is, there are also four raceways. In this embodiment, four inner peripheral raceway surfaces 23 and four S-shaped groove surfaces 24 are provided, but the present disclosure may have a number other than four.

[0021] A gear 26 is formed on the outer peripheral surface 25 of the nut 2. The gear 26 has a disk-shaped annular portion 27 that protrudes radially outward from the outer peripheral surface 25 of the nut 2, and a plurality of tooth portions 28 formed on the outer peripheral surface of the annular portion 27. Therefore, in the ball screw device 100 of the first embodiment, rotational motion is transmitted to the nut 2, and the screw shaft 1 is designed to perform linear motion.

[0022] Next, a manufacturing method of the ball screw device 100 will be described. The manufacturing method of the ball screw device 100 includes a nut manufacturing process for manufacturing the nut 2, and an assembly process for completing the ball screw device 100 by arranging a plurality of balls (not shown) between the screw shaft 1 and the nut 2.

[0023] The assembly process will be briefly described. A temporary shaft (not shown) is inserted into the nut 2, and balls (not shown) are arranged between the nut 2 and the temporary shaft. Then, the screw shaft 1 is inserted into the nut 2 so as to push out the temporary shaft, thereby completing the ball screw device 100. The present disclosure may complete the ball screw device 100 without using a temporary shaft. That is, the present disclosure is not particularly limited to the method of the assembly process. Hereinafter, the details of the nut manufacturing process will be described.

[0024] Figure 2 shows the nut manufacturing process of Embodiment 1. As shown in Figure 2, the nut manufacturing process includes a preparation step S1, an outer shape forming step S3, a cold forging step S5, a cutting step S7, and an inner circumferential raceway surface forming step S9. Hereinafter, the part formed by cold forging in the cold forging step S5 may be referred to as the molded part.

[0025] Preparation step S1 is a step of preparing a cylindrical blank material 5. The inner circumferential surface 50 and outer circumferential surface 51 of the blank material 5 are formed in a circular shape when viewed from the axial direction. As the material of the blank material 5, for example, steel can be used, but in this disclosure, it may be formed from a metal material other than steel. In addition, in preparation step S1, the blank material 5 may be prepared by hot forging or warm forging.

[0026] The outer shape forming step S3 is a step in which the rough outer shape of the nut 2 is formed. In this embodiment, the outer circumferential surface 51 of the blank material 5 is cut to form the annular portion 27. In this embodiment, the annular portion 27 is formed by cutting, but the annular portion 27 may also be formed on the outer circumferential surface 51 of the blank material 5 by cold forging or hot forging. In other words, this disclosure is not particularly limited to the processing method of the outer shape forming step S3. Furthermore, in this disclosure, for example, if the blank material 5 is manufactured by hot forging in the preparation step S1, the hot forging may be performed in the preparation step S1 to impart the shape that will be formed in the outer shape forming step S3. In other words, the blank material 5 prepared in the preparation step S1 will have the shape that will be formed in the outer shape forming step S3. And, if the outer shape of the blank material 50 prepared in the preparation step S1 is almost complete, this disclosure can omit the outer shape forming step S3.

[0027] Cold forging process S5 is a process in which the annular portion 27 of the blank material 5 is cold forged to form multiple teeth 28 (formed portions) on the outer surface of the annular portion 27. The gear 26 is completed by this process. Furthermore, due to the fiber flow (because the fibrous metal structure is not cut), the strength of the annular portion 27 is improved by cold forging.

[0028] The cutting process S7 is a process of cutting the inner circumferential surface 50 of the blank material 5 to form an S-shaped groove surface 24. Examples of cutting tools used in this process include helical cutters and ball end mills, but this disclosure does not particularly limit the type of cutting tool used.

[0029] The inner circumferential raceway surface formation process S9 is a process in which the inner circumferential surface 50 of the blank material 5 is cut to form the inner circumferential raceway surface 23. Upon completion of this process, the nut 2 is completed.

[0030] As described above, according to the manufacturing method of Embodiment 1, the cold forging process S5 is performed before the S-shaped groove surface 24 is formed. Therefore, deformation of the S-shaped groove surface 24 due to cold forging does not occur. Also, since the S-shaped groove surface 24 is formed by cutting, deformation of the formed part (tooth portion 28) does not occur. Therefore, the S-shaped groove surface 24 and the S-shaped groove surface 24 can be formed in a predetermined shape, resulting in high processing accuracy for the nut 2. Furthermore, in this embodiment, the tooth portion 28 is formed by cold forging. Therefore, compared to the case where the tooth portion 28 is formed with a hob or shaver, the manufacturing effort can be reduced, and costs can be lowered.

[0031] Although Embodiment 1 has been described above, the invention is not limited to the example described in Embodiment 1. For example, the invention may perform heat treatment after the inner circumferential raceway surface forming step S9. The heat treatment step is a process of quenching and tempering the inner circumferential surface 22 of the nut 2. The invention may also perform carburizing treatment before quenching. The quenching method may also be high-frequency induction hardening. This will give the inner circumferential surface 22 of the nut 2 the desired hardness and toughness. Furthermore, the invention is not limited to any heat treatment method other than those described above.

[0032] Furthermore, the present disclosure may also include finishing processes to the outer circumference and end faces of the nut 2, including the formed portion (tooth portion 28), after the cold forging process S5. Examples of finishing processes include cutting, grinding, and polishing, but the present disclosure is not particularly limited.

[0033] Furthermore, this disclosure may also include a grinding step to grind the inner circumferential raceway surface 23 and the S-shaped groove surface 24 after the inner circumferential raceway surface forming step S9. Moreover, the range to be ground is not limited to the inner circumferential raceway surface 23 and the S-shaped groove surface 24, and the first end face 20, the second end face 21 and the outer circumferential surface 25 may also be ground.

[0034] Furthermore, according to Embodiment 1 described above, the inner circumferential surface 22 of the nut 2 may collapse during the cold forging process S5. Therefore, the present disclosure may include a step of cutting the inner circumferential surface 22 of the nut 2 to a predetermined inner diameter after the cold forging process S5 and before the cutting process S7.

[0035] Furthermore, in Embodiment 1, cutting is performed in the outer shape forming step S3, but in this disclosure, cold forging may be performed in the outer shape forming step S3. In this case, cold forging will be performed a total of two times, in the outer shape forming step S3 and the cold forging step S5, but if the cold forging is performed before the cutting step S7, the S-shaped groove surface 24 will not be deformed. Therefore, in this disclosure, as long as it is performed before the cutting step S7, the number of cold forging steps may be one or three or more, and the number of cold forging steps is not a requirement.

[0036] Furthermore, in the external shaping process S3, when the annular portion 27 is formed by cold forging, the annular portion 27 also becomes a formed portion, and the entire gear 26 becomes a formed portion. In other words, this disclosure is not limited to the gear 26 being a formed portion, but also to the gear 28 being a formed portion. Moreover, the formed portion of this disclosure is not limited to the gear 26 (tooth portion 28, annular portion 27). Other formed portions will be described below.

[0037] (Variation 1) Figure 3 is a perspective view of the nut of Modified Example 1, viewed from the axial direction. As shown in Figure 3, a stopper 6 protruding in the axial direction is formed on the second end face 21 of the nut 2A. This stopper 6 is designed to contact a projection of a rotating body (not shown) that rotates integrally with the screw shaft 1. The rotation of the screw shaft 1 is stopped by this contact, and the nut 2A is positioned in the axial direction. In the cold forging process S5 of Modified Example 1, the stopper 6 is formed by cold forging the end face (second end face 21) side of the blank material 5, and the stopper 6 corresponds to the formed part.

[0038] (Modification 2) Figure 4 is a view of the nut of Modified Example 2 from the axial direction. As shown in Figure 4, a pair of parallel planes 70 are formed on the outer circumferential surface 25 of the nut 2B of Modified Example 2. In other words, a two-sided width shape 7 is formed on the outer circumferential surface 25 of the nut 2B. In Modified Example 2, this two-sided width shape 7 is a molded part formed by the cold forging process S5.

[0039] Furthermore, this disclosure is not limited to the two-sided width shape 7, and may also be a multi-sided width shape in which multiple pairs of planes 70 are formed. Alternatively, the nut 2 may have one plane (molded portion) formed on its outer peripheral surface 25, and the shape of the outer peripheral surface 25 of the nut 2 when viewed from the axial direction is D-shaped.

[0040] (Variation 3) Figure 5 is a cross-sectional view of the nut of Modified Example 3, cut in the axial direction. As shown in Figure 5, the outer circumferential surface 25 of the nut 2C has an inner ring portion 80 and an outer circumferential raceway surface 81 formed on the outer circumferential surface of the inner ring portion 80. The inner ring portion 80 constitutes the inner ring of a bearing device (not shown) and is integrated with the nut 2. In Modified Example 3, the inner ring portion 80 is formed in the outer shape forming process S3. The outer circumferential raceway surface 81 is the surface on which the balls of the bearing device roll. In Modified Example 3, the outer circumferential raceway surface 81 is formed in the cold forging process S5 and corresponds to the formed portion. In this disclosure, the outer circumferential raceway surface 81 may be formed to a certain extent in the cold forging process S5 and then the outer circumferential raceway surface 81 may be finished thereafter.

[0041] In addition, in the outer shape forming step S3 of the modified example 3, either cutting or cold forging may be used, but if the inner ring portion 80 is formed by cold forging, the inner ring portion 80 also becomes a formed part.

[0042] The molded parts of Modifications 1 to 3 have been described above, but the molded parts are not limited to the examples described above. The molded parts may be flanges or keys that protrude radially outward from the outer circumferential surface 25 of the nut 2, or splines or serrations formed on the outer circumferential surface 25 of the nut 2. Alternatively, they may be keyways that are recessed radially inward from the outer circumferential surface 25 of the nut 2. In other words, there are no particular limitations on the type of molded part. Keys may be projections with a small axial length or ridges with a large axial length. Keyways may be recesses with a small axial length or grooves with a large axial length.

[0043] Furthermore, in Embodiment 1 and the like, rotational motion is transmitted to the nut 2 and the screw shaft 1 performs linear motion. However, the present disclosure may also be a ball screw device in which rotational motion is transmitted to the screw shaft 1 and the nut 2 performs linear motion. Also, regarding the molded portion, in Embodiment 1 and the like, it is formed on the outer circumference side or the end face side of the nut. However, the present disclosure may be formed on at least one of the outer circumference side and the end face side of the nut. In other words, the present disclosure may have the molded portion formed on both the outer circumference side and the end face side of the nut. [Explanation of Symbols]

[0044] 1 Screw shaft 2, 2A, 2B, 2C nuts 3 balls 5. Blank material 6 Stopper 7 Width across flats shape 10 Outer raceway surface 20 1st end face 21 Second end face 22 Inner surface 23 Inner raceway surface 24 S-groove surface 25 Outer surface 26 gears 27 Ring section 28 Teeth 70 plane 80 Inner ring section 81 Outer raceway surface 100 Ball screw device

Claims

1. A screw shaft having an outer raceway surface formed on its outer surface, A nut having an inner circumferential raceway surface formed on its inner surface, with the screw shaft inserted inside, Multiple balls arranged in the track between the screw shaft and the nut, It has, A molded portion formed by cold forging is formed on at least one of the outer circumference side and the end face side of the nut. The inner circumferential surface of the nut has an S-shaped groove formed by cutting. Ball screw device.

2. The molded part is one of the following shapes: gear, flange, spline, serration, stopper, key, keyway, and two-sided width shape. The ball screw device according to claim 1.

3. The process includes a nut manufacturing step for manufacturing the nut described in claim 1 or claim 2, The aforementioned nut manufacturing process is: A cold forging process in which a cylindrical blank material is cold forged to form a molded portion on the outer circumference side or the end face side of the blank material, A cutting process to cut the inner circumferential surface of the blank material to form an S-shaped groove surface, including A method for manufacturing a ball screw device.

4. The aforementioned cutting process is performed after the cold forging. A method for manufacturing a ball screw device according to claim 3.

5. The molded part is one of the following shapes: gear, flange, spline, serration, stopper, key, keyway, and two-sided width shape. A method for manufacturing a ball screw device according to claim 3 or claim 4.

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