Screw shaft for ball screw device and method for manufacturing same

Abstract

Provided are a screw shaft for a ball screw device, the screw shaft comprising a hardened layer and enabling suppression of manufacturing cost, and a method for manufacturing the same. A screw shaft (1) includes: a screw shaft part (2) having a spiral groove (4) on the outer peripheral surface thereof; an induction hardened layer (5) formed on the outer peripheral surface of the screw shaft part (2); and a laser hardened layer formed on at least a part of a portion of the surface outside the outer peripheral surface of the screw shaft part (2), the portion being in contact with another member during use.

Description

Screw shaft for ball screw device and method for manufacturing the same

[0001] This disclosure relates to a screw shaft applied to a ball screw device, which is a mechanical element for converting linear motion to rotational motion or rotational motion to linear motion, and to a method for manufacturing the screw shaft.

[0002] Ball screw devices achieve higher efficiency compared to sliding screw devices, which directly contact the screw shaft and nut, because they use a ball that rolls between the screw shaft and nut. For this reason, ball screw devices are incorporated into various mechanical devices, such as electric brake systems and automatic manual transmissions (AMTs) in automobiles, and positioning devices in machine tools, to convert the rotational motion of a drive source, such as an electric motor, into linear motion.

[0003] The ball screw device comprises a screw shaft having a shaft-side helical groove on its outer circumference, a nut having a nut-side helical groove on its inner circumference, and a plurality of balls arranged to roll freely between the shaft-side helical groove and the nut-side helical groove.

[0004] The shaft-side helical groove and the nut-side helical groove are arranged to face each other radially, forming a helical load path. The start and end points of the load path are connected by a return path. The return path returns the ball that has reached the end point of the load path back to the start point of the load path, causing the ball to circulate indefinitely. The start and end points of the load path are swapped depending on the direction of the relative axial displacement between the screw shaft and the nut, that is, the relative rotation direction between the screw shaft and the nut.

[0005] In ball screw devices, depending on the application, one of the screw shaft or nut is used as a rotational motion element, and the other as a linear motion element. For example, Japanese Patent Publication No. 2024-104913 describes a ball screw device in which the nut is used as the rotational motion element and the screw shaft is used as the linear motion element. Also, Japanese Patent Publication No. 2024-066857 describes a ball screw device in which the screw shaft is used as the rotational motion element and the nut is used as the linear motion element.

[0006] In either case, the screw shaft typically comprises a screw shaft portion having a helical groove on its outer circumference, and also has at least a portion of the surface of the screw shaft portion that is not on the outer circumference, which comes into contact with another component during use.

[0007] For example, the screw shaft described in Japanese Patent Publication No. 2024-104913 has a non-screwed shaft portion with a smaller diameter than the screw shaft portion at an end that is axially offset from the screw shaft portion. An engaging member for restricting the amount of axial movement of the screw shaft relative to the nut is fitted and fixed to the non-screwed shaft portion. In addition, the end face of the non-screwed shaft portion is in contact with the bottom surface of a recess provided in the lid portion of the pressing member.

[0008] The screw shaft described in Japanese Patent Publication No. 2024-066857 has a non-screwed shaft portion with a smaller diameter than the screw shaft portion at an end that is axially offset from the screw shaft portion. An inner ring constituting a rolling bearing for rotatably supporting the screw shaft relative to the housing, and a carrier fitted inside the inner ring, are externally fitted and fixed to the non-screwed shaft portion.

[0009] Japanese Patent Publication No. 2024-104913 Japanese Patent Publication No. 2024-066857

[0010] In conventional ball screw devices, such as screw shafts described in Japanese Patent Publication No. 2024-104913 and Japanese Patent Publication No. 2024-066857, a hardened layer is formed by heat treatment on the outer circumferential surface of the screw shaft portion having a shaft-side helical groove, and on the contact portion of the surface away from the outer circumferential surface of the screw shaft portion that comes into contact with another component during use, for the purpose of improving mechanical performance such as hardness and wear resistance. Specifically, a hardened layer is formed on the outer circumferential surface of the screw shaft portion to improve the rolling fatigue life of the shaft-side helical groove. In addition, a hardened layer is formed on the contact portion to prevent damage such as fretting wear and deformation.

[0011] Generally, in the screw shafts that constitute ball screw devices, the hardened layer is formed by high-frequency induction hardening. In high-frequency induction hardening, only the portion where the hardened layer is to be formed is heated using an induction heating coil during the heating process, so the heat treatment deformation of the screw shaft can be kept to a minimum.

[0012] In the screw shafts described in Japanese Patent Publication No. 2024-104913 and Japanese Patent Publication No. 2024-066857, the outer diameter of the non-screw shaft portion is smaller than the outer diameter of the screw shaft portion. Therefore, when forming a hardened layer on the outer circumferential surface of the screw shaft portion and the outer circumferential surface of the non-screw shaft portion by high-frequency induction hardening, it is necessary to prepare at least two types of induction heating coils with different diameters, which may increase manufacturing costs.

[0013] In the screw shaft described in Japanese Patent Publication No. 2024-104913, as the direction of axial displacement of the screw shaft changes, the end face of the non-screw shaft portion and the bottom surface of the recess provided in the cover portion repeatedly come into contact with each other, causing friction and potentially resulting in fretting wear in that portion. For this reason, it is desirable to form a hardened layer on the end face of the non-screw shaft portion by heat treatment. In this case, an induction heating coil for heating the end face of the non-screw shaft portion would be required, which could further increase manufacturing costs.

[0014] Furthermore, since the outer circumferential surface and end face of the non-threaded shaft portion do not come into contact with multiple rolling balls, the same level of hardness and wear resistance as the shaft-side helical groove is not required. For this reason, forming a hardened layer on the outer circumferential surface and end face of the non-threaded shaft portion by high-frequency induction hardening may result in over-engineering, which could be disadvantageous in terms of cost reduction.

[0015] This disclosure aims to provide a screw shaft for a ball screw device and a method for manufacturing the same, which can reduce manufacturing costs while having the required hardened layer on its surface.

[0016] A method for manufacturing a screw shaft for a ball screw device according to one aspect of the present disclosure comprises the steps of: preparing a screw shaft having a screw shaft portion with helical grooves on its outer circumferential surface and the surface thereof being an unheat-treated surface; forming a high-frequency induction hardened layer on the outer circumferential surface of the screw shaft portion by high-frequency induction hardening; and forming a laser hardened layer on at least a portion of the surface of the screw shaft that is separate from the outer circumferential surface of the screw shaft portion and that comes into contact with another component during use by laser hardening.

[0017] In a method for manufacturing a screw shaft for a ball screw device according to one aspect of the present disclosure, the screw shaft has a non-screwed portion in the portion axially separated from the screw shaft portion, and the laser hardening layer is formed on at least a part of the non-screwed portion.

[0018] In a method for manufacturing a screw shaft for a ball screw device according to one aspect of the present disclosure, the laser hardening layer is formed on at least a portion of the outer circumferential surface of the non-screw shaft portion.

[0019] In a method for manufacturing a screw shaft for a ball screw device according to one aspect of the present disclosure, the laser hardening layer is formed on at least a portion of the end face of the non-screwed shaft portion.

[0020] In a method for manufacturing a screw shaft for a ball screw device according to one aspect of the present disclosure, the non-screwed shaft portion has an outer diameter smaller than the outer diameter of the screw shaft portion.

[0021] In a method for manufacturing a screw shaft for a ball screw device according to one aspect of the present disclosure, the laser hardening layer is formed on at least a portion of the end face of the screw shaft portion.

[0022] A screw shaft for a ball screw device according to one aspect of the present disclosure comprises: a screw shaft portion having a helical groove on its outer circumferential surface; a high-frequency induction hardened layer formed on the outer circumferential surface of the screw shaft portion; and a laser induction hardened layer formed on at least a portion of the surface of the screw shaft portion that is not on the outer circumferential surface and comes into contact with another component during use.

[0023] In a screw shaft for a ball screw device according to one aspect of the present disclosure, the laser-hardened layer has an effective hardened layer depth that is shallower than the effective hardened layer depth of the high-frequency induction hardened layer.

[0024] In a screw shaft for a ball screw device according to one aspect of the present disclosure, the effective hardened layer depth of the laser hardened layer is less than 1 mm, and the effective hardened layer depth of the high-frequency hardened layer is 1 mm or more.

[0025] In a screw shaft for a ball screw device according to one aspect of the present disclosure, a non-screwed shaft portion is provided in a portion that is axially offset from the screwed shaft portion. The non-screwed shaft portion includes the portion on which the laser hardened layer is formed.

[0026] In a screw shaft for a ball screw device according to one aspect of the present disclosure, the outer circumferential surface of the non-screwed shaft portion includes the portion on which the laser hardened layer is formed.

[0027] In a screw shaft for a ball screw device according to one aspect of the present disclosure, the end face of the non-screwed shaft portion includes the portion on which the laser hardened layer is formed.

[0028] In one embodiment of the present disclosure, the screw shaft for a ball screw device has a non-screwed portion having an outer diameter smaller than the outer diameter of the screw shaft portion.

[0029] In a screw shaft for a ball screw device according to one aspect of the present disclosure, the end face of the screw shaft portion includes the portion on which the laser hardened layer is formed.

[0030] According to one aspect of the present disclosure, a screw shaft for a ball screw device and a method for manufacturing the same can be provided, while keeping manufacturing costs down, that has a hardened layer for improving rolling fatigue life and a hardened layer for preventing damage such as fretting wear and deformation.

[0031] FIG. 1 is a side view showing a usage state of a screw shaft for a ball screw device according to a first example of an embodiment of the present disclosure. FIG. 2 is a cross-sectional view schematically showing the screw shaft for the ball screw device of the first example. FIG. 3A is a cross-sectional view schematically showing a step of forming a hardened layer on a screw shaft portion by high-frequency quenching treatment in the first example. FIG. 3B is a cross-sectional view schematically showing a step of forming a hardened layer on an outer peripheral surface of a non-screw shaft portion by laser quenching treatment in the first example. FIG. 3C is a cross-sectional view schematically showing a step of forming a hardened layer on an end surface of a non-screw shaft portion by laser quenching treatment in the first example. FIG. 4 is a view corresponding to FIG. 2 for a screw shaft for a ball screw device according to a second example of an embodiment of the present disclosure. FIG. 5 is a view corresponding to FIG. 2 for a screw shaft for a ball screw device according to a third example of an embodiment of the present disclosure. FIG. 6A is a view corresponding to FIG. 2 for a screw shaft for a ball screw device according to a fourth example of an embodiment of the present disclosure. FIG. 6B is a view schematically showing a step of forming a hardened layer on an outer peripheral surface of the screw shaft of the fourth example by high-frequency quenching treatment. FIG. 7A is a view corresponding to FIG. 2 for a screw shaft for a ball screw device according to a fifth example of an embodiment of the present disclosure. FIG. 7B is a view schematically showing a step of forming a hardened layer on an outer peripheral surface of the screw shaft of the fifth example by high-frequency quenching treatment. FIG. 8 is a view corresponding to FIG. 2 for a screw shaft for a ball screw device according to a sixth example of an embodiment of the present disclosure.

[0032] [First Example] The first example of the embodiment of the present disclosure will be described with reference to FIGS. 1 to 3C.

[0033] The screw shaft 1 of this example is applied to the screw shaft 1 that constitutes a ball screw device 15 that converts linear motion into rotational motion or rotational motion into linear motion. In a state where the ball screw device 15 is assembled, the screw shaft 1 is inserted radially inside a cylindrical nut 16. The screw shaft 1 moves linearly or rotationally when the ball screw device 15 is in use. That is, the screw shaft 1 is used as a linear motion element or a rotational motion element.

[0034] Regarding the ball screw device 15, unless otherwise specified, the axial direction, radial direction, and circumferential direction refer to the axial direction, radial direction, and circumferential direction of the screw shaft 1. The axial direction, radial direction, and circumferential direction of the screw shaft 1 coincide with the axial direction, radial direction, and circumferential direction of the nut 16. Also, one side in the axial direction refers to the left side in FIG. 1, and the other side in the axial direction refers to the right side in FIG. 1.

[0035] <Screw shaft configuration> The screw shaft 1 is entirely made of a steel material such as high-carbon steel or medium-carbon steel and is configured in a substantially cylindrical or substantially cylindrical shape.

[0036] The screw shaft 1 includes a screw shaft portion 2 having a spiral groove 4 on the outer peripheral surface 3, a high-frequency induction hardened layer 5 formed on the outer peripheral surface 3 of the screw shaft portion 2, and a laser induction hardened layer 6 formed on at least a part of the surface of the outer peripheral surface 3 of the screw shaft portion 2 that is separated from the outer peripheral surface 3 and contacts another member during use.

[0037] The screw shaft 1 has at least the screw shaft portion 2. The screw shaft portion 2 constitutes the axial range in which the spiral groove 4 is formed in the screw shaft 1. The screw shaft 1 can also be composed only of the screw shaft portion 2 over the entire axial direction. In FIGS. 2 and 3A to 3C, the illustration of the shape of the spiral groove 4 is omitted.

[0038] The spiral groove 4 forms a load path in which a plurality of balls 18 constituting the ball screw device 15 are arranged between the nut-side spiral groove 17 provided on the inner peripheral surface of the cylindrical nut 16 constituting the ball screw device 15. The start point and the end point of the load path are connected by a return path (not shown) provided on the outer peripheral surface 3 of the screw shaft portion 2 or the nut 16. The return path returns the ball 18 that has reached the end point of the load path to the start point of the load path and circulates the ball 18 infinitely. The start point and the end point of the load path are switched according to the direction of the relative displacement in the axial direction between the screw shaft 1 and the nut 16, that is, the relative rotation direction between the screw shaft 1 and the nut 16.

[0039] The number of the spiral grooves 4 is arbitrary. The number of the spiral grooves 4 may be one as in this example, or may be multiple, i.e., two or more.

[0040] The groove shape of the cross-section of the helical groove 4 is arbitrary. The groove shape of the cross-section of the helical groove 4 can be, for example, a Gothic arch groove shape or a circular arc groove shape.

[0041] The screw shaft 1 may have an optional non-screwed shaft portion 7. Depending on the application of the screw shaft 1, the non-screwed shaft portion 7 may be provided adjacent to one or both sides of the screw shaft portion 2 in the axial direction. The outer diameter of the non-screwed shaft portion 7 may be smaller than the outer diameter of the screw shaft portion 2. In this case, the outer circumferential surface 3 of the screw shaft portion 2 and the outer circumferential surface 8 of the non-screwed shaft portion 7 are connected by an axially oriented stepped surface 9. The outer circumferential surface 8 of the non-screwed shaft portion 7 may be a simple cylindrical surface, or it may be a non-cylindrical surface such as serrations, splines, or knurling.

[0042] The surface of the screw shaft 1 includes the outer circumferential surface 3 of the screw shaft portion 2 and the surface separated from the outer circumferential surface 3 of the screw shaft portion 2. Of the surface of the screw shaft 1, the surface separated from the outer circumferential surface 3 of the screw shaft portion 2 includes, depending on the structure of the screw shaft 1, the outer circumferential surface 8 of the non-screw shaft portion 7, the end face 24 of the screw shaft portion 2 or the end face 10 of the non-screw shaft portion 7 which constitute the end face of the screw shaft 1, and an axially oriented stepped surface 9 that exists between the outer circumferential surface 3 of the screw shaft portion 2 and the outer circumferential surface 8 of the non-screw shaft portion 7.

[0043] The high-frequency induction hardened layer 5 is an element that improves the rolling fatigue life of the helical groove 4 by improving the hardness of the outer surface 3 of the screw shaft portion 2, particularly the helical groove 4.

[0044] The high-frequency induction hardened layer 5 is formed by high-frequency induction hardening treatment. Effective hardened layer depth d of the high-frequency induction hardened layer 5 ５ It is preferable that the effective cured layer depth d is 1 mm or more. ５ This refers to the distance from the surface of the hardened layer to the point of critical hardness, Hv550, and can be measured by the measurement method specified in JIS (Japanese Industrial Standards) G0557:2019.

[0045] The laser-hardened layer 6 is an element that improves the mechanical properties such as hardness and wear resistance of the contact portion that comes into contact with another component during use, thereby preventing damage such as fretting wear and deformation from occurring in the contact portion. The laser-hardened layer 6 is formed on at least a part of the contact portion, preferably the entire contact portion, but it can also be formed on the entire surface away from the outer circumferential surface 3 of the screw shaft portion 2. However, it is preferable to selectively form it on the contact portion, or on the parts of the contact portion where mechanical properties such as wear resistance are required.

[0046] The laser-hardened layer 6 has an effective hardened layer depth d that is shallower than the effective hardened layer depth of the high-frequency induction hardened layer 5. ６ It has the following characteristics. Specifically, the laser-hardened layer 6 is formed by laser hardening treatment, and the effective hardened layer depth is preferably less than 1 mm. Since the contact portion with the other member does not require the same mechanical properties such as hardness and wear resistance as the helical groove 4 in which the multiple balls 18 roll and make contact, sufficient wear resistance can be ensured even if the effective hardened layer depth of the laser-hardened layer 6 is less than 1 mm. Effective hardened layer depth d of the laser-hardened layer 6 ６ If the effective hardened layer depth d of the laser hardened layer 6 is increased to more than 1 mm, it will result in over-quality and may be disadvantageous in terms of cost reduction. ６ The effective hardened layer depth d of the high-frequency induction hardened layer is ５ It can be measured using a similar measurement method.

[0047] The portion of the surface of the screw shaft 1 that is separate from the outer surface of the screw shaft portion 2, where the laser hardened layer 6 is formed, is determined by the manner in which the screw shaft 1 is used and its intended application.

[0048] For example, when the screw shaft 1 is used to axially press a pressing member, such as a piston 19, which is part of an electric brake system (EMB) or electric brake booster system (EHB) of an automobile, for pressing a pad against a disc, a laser-hardened layer 6 can be formed on the end face or stepped surface of the screw shaft 1 that comes into contact with the pressing member, such as the piston 19, and corresponds to the contact portion. The end face of the screw shaft 1 includes the end face 24 of the screw shaft portion 2 or the end face 10 of the non-screw shaft portion 7.

[0049] In this case, the laser-hardened layer 6 can be formed on the end face of the screw shaft 1 corresponding to the contact portion, that is, the entire end face 24 of the screw shaft portion 2 or the end face 10 of the non-screw shaft portion 7, or on the entire stepped surface of the screw shaft 1. Alternatively, if another component comes into contact with only a portion of these surfaces during use, the laser-hardened layer 6 can be formed only on that portion.

[0050] If, additionally or alternatively, another member is fitted onto the outer circumferential surface 8 of the non-threaded shaft portion 7, a laser-hardened layer 6 can be formed on the outer circumferential surface 8 of the non-threaded shaft portion 7 corresponding to the contact portion. For example, the other member may consist of a stopper member 20 for restricting excessive relative rotation between the screw shaft 1 and the nut 16. When the screw shaft 1 is used as a linear motion element, the other member may consist of a restricting member for preventing the screw shaft 1 from rotating relative to a stationary member such as a housing. When the screw shaft 1 is used as a rotational motion element, the other member may consist of a bearing device component for rotatably supporting the screw shaft 1 relative to a stationary member such as a housing, gears constituting a torque transmission mechanism for transmitting torque to the screw shaft 1, drive members such as pulleys, etc.

[0051] In this case, the laser-hardened layer 6 can be formed on the entire outer surface 8 of the non-threaded shaft portion 7, which is the contact area. Alternatively, if another component comes into contact with only a portion of the axial range of the outer surface 8 of the non-threaded shaft portion 7 during use, the laser-hardened layer 6 can be formed only on that portion of the axial range.

[0052] In this example, the screw shaft 1 is applied to a ball screw device 15 incorporated into an electric brake system of an automobile, and is used to press the axial side surface of a piston 19. The screw shaft 1 has a non-screw shaft portion 7 in the portion axially offset from the screw shaft portion 2, and the end face 10 of the non-screw shaft portion 7 is configured to press the piston 19 in the axial direction. The non-screw shaft portion 7 has an outer diameter smaller than the outer diameter of the screw shaft portion 2. Furthermore, the outer circumferential surface 8 of the non-screw shaft portion 7 is made up of serrations, and the inner circumferential surface of a cylindrical stopper member 20, which constitutes a stopper mechanism for restricting the relative axial displacement between the screw shaft 1 and the nut 16, is press-fitted and fixed (serrated fitting) to the outer circumferential surface 8. For this reason, a laser-hardened layer 6 is formed on the end face 10 and the outer circumferential surface 8 of the non-screw shaft portion 7, which are contact parts with other members.

[0053] Furthermore, in order to constitute the stopper mechanism, the nut 16 has a stopper projection 21 that protrudes in the axial direction from one circumferential point on one axial side surface. Correspondingly, the stopper member 20 fitted onto the outer circumferential surface 8 of the non-threaded shaft portion 7 is provided with a claw portion 22 that protrudes radially outward from one circumferential point on the other axial end of the outer circumferential surface. The screw shaft 1 can move linearly in the axial direction relative to the nut 16 only within the axial range where the claw portion 22 is located in the axial direction relative to the stopper projection 21. When the screw shaft 1 moves in the axial direction to the other side to an axial position where the claw portion 22 circumferentially overlaps the stopper projection 21, the stopper projection 21 circumferentially strikes the claw portion 22, thereby preventing the rotation of the nut 16 relative to the screw shaft 1, and thus preventing further movement in the axial direction.

[0054] <Manufacturing Method for Screw Shafts> Next, we will explain the manufacturing method for screw shaft 1.

[0055] The manufacturing method for the screw shaft 1 includes the steps of: preparing a screw shaft 1 having a screw shaft portion 2 with a helical groove 4 on its outer circumferential surface 3 and the surface thereof being an unheat-treated surface; forming a high-frequency induction hardened layer 5 on the outer circumferential surface 3 of the screw shaft portion 2 by high-frequency induction hardening; and forming a laser hardened layer 6 on at least a portion of the surface of the screw shaft 1 that is not on the outer circumferential surface 3 of the screw shaft portion 2 and that comes into contact with another component during use by laser hardening.

[0056] When manufacturing the screw shaft 1, a screw shaft portion 2 having a helical groove 4 on its outer circumference, but whose surface is not heat-treated, is prepared (hereinafter also referred to as "screw shaft intermediate body 11"). The means of obtaining the screw shaft intermediate body 11 is arbitrary. If the preparation process includes a process for manufacturing the screw shaft intermediate body 11, the rough shape of the screw shaft 1 is formed by forging, casting, etc., of the steel material that will be the material for the screw shaft 1, and then the screw shaft intermediate body 11 having substantially the same shape as the finished screw shaft 1 is obtained by machining, plastic deformation, etc. to refine the shape (see Figures 3A to 3C).

[0057] The process involves sequentially performing the following steps on the screw shaft intermediate body 11: first, forming a high-frequency induction hardened layer 5 on the outer circumferential surface of the screw shaft portion 2 by high-frequency induction hardening; and second, forming a laser hardened layer 6 on necessary areas of the surface of the screw shaft intermediate body 11 other than the outer circumferential surface of the screw shaft portion 2 by laser hardening. The order of these steps does not matter.

[0058] In the process of forming a high-frequency induction hardened layer 5 on the outer circumferential surface 3 of the screw shaft portion 2, as shown in Figure 3A, a cylindrical high-frequency heating coil 12 is coaxially arranged around the screw shaft portion 2 that constitutes the screw shaft intermediate body 11, and the outer circumferential surface 3 of the screw shaft portion 2 is heated by high-frequency induction by energizing the high-frequency heating coil 12. This high-frequency induction heating can be performed with the screw shaft intermediate body 11 not rotating, or while the screw shaft intermediate body 11 is rotated around its own central axis. After that, the screw shaft intermediate body 11 is rapidly cooled to form a high-frequency induction hardened layer 5 on the outer circumferential surface 3 of the screw shaft portion 2. After the high-frequency induction hardening treatment, a tempering treatment is performed.

[0059] In the step of forming a laser-hardened layer 6 on the surface of the screw shaft 1 in the portion that is in contact with the other member, a laser beam 14 is irradiated onto the contact portion by a laser irradiation device 13. The laser hardening treatment causes localized rapid heating and cooling of the contact portion, thereby forming the laser-hardened layer 6.

[0060] Typically, laser hardening has lower equipment costs and shorter processing times than high-frequency induction hardening, and the resulting hardened layer is thinner. Therefore, it is possible to ensure that the necessary mechanical properties such as hardness and wear resistance are at least the minimum required level in the contact portion of the surface other than the outer circumferential surface 3 of the screw shaft portion 2, while preventing excessive mechanical performance from unnecessarily increasing costs.

[0061] When a screw shaft 1 has a non-threaded shaft portion 7, and the non-threaded shaft portion 7 has an outer diameter smaller than the outer diameter of the screw shaft portion 2, and high-carbon steel with high fatigue resistance is used as the material for the screw shaft 1, if a hardened layer is to be formed on the end face 10 or outer circumferential surface 8 of the non-threaded shaft portion 7, which has a smaller volume than the screw shaft portion 2, by high-frequency induction hardening, the heat treatment strain will be large, making it easy for damage such as quench cracks to occur. In contrast, by forming a laser-hardened layer 6 on the end face 10 and outer circumferential surface 8 of the non-threaded shaft portion 7 by laser hardening, even when high-carbon steel is used as the material for the screw shaft 1, the heat treatment strain can be kept small, and damage such as quench cracks can be effectively prevented.

[0062] Furthermore, if a high-frequency induction hardening hardened layer 5 is formed on the outer circumferential surface 3 of the screw shaft portion 2, and then an attempt is made to form a hardened layer on the outer circumferential surface 8 of the non-screw shaft portion 7 adjacent to the screw shaft portion 2 by high-frequency induction hardening, the heat from heating may extend to the high-frequency induction hardened hardened layer 5 on the outer circumferential surface 3 of the screw shaft portion 2, potentially causing the high-frequency induction hardened hardened layer 5 to temper and reduce its hardness. In contrast, by forming a laser hardened hardened layer 6 on the outer circumferential surface 8 of the non-screw shaft portion 7 by laser hardening, the heat influence on the outer circumferential surface 3 of the screw shaft portion 2 can be minimized when forming the laser hardened hardened layer 6 on the outer circumferential surface 8, effectively preventing the high-frequency induction hardened hardened layer 5 on the outer circumferential surface 3 from tempering and reducing its hardness. This effectively improves the rolling fatigue life of the helical groove 4.

[0063] As in this example, when the non-threaded shaft portion 7 has an outer diameter smaller than the outer diameter of the threaded shaft portion 2, and a laser hardening layer 6 is formed on the end face 10 and the outer circumferential surface 8 of the non-threaded shaft portion 7, in the step of forming the laser hardening layer 6 on the outer circumferential surface 8 of the non-threaded shaft portion 7, as shown in Figure 3B, a laser irradiation device 13 irradiates the outer circumferential surface 8 of the non-threaded shaft portion 7 with laser light 14.

[0064] The range over which the laser beam 14 can be irradiated onto the outer surface 8 at once is limited to a portion of the outer surface 8 in both the circumferential and axial directions. Therefore, when forming a laser-hardened layer 6 on the outer surface 8, the irradiation position of the laser beam 14 on the outer surface 8 can be moved in the circumferential direction by rotating the screw shaft intermediate body 11 around its own central axis, or by moving the laser irradiation device 13 in the axial direction of the screw shaft intermediate body 11, or by performing these movements simultaneously. This allows the irradiation position of the laser beam 14 on the outer surface 8 to be moved in the circumferential, axial, or both directions, thereby forming a laser-hardened layer 6 over the entire or a portion of the outer surface 8.

[0065] In the process of forming a laser-hardened layer 6 on the end face 10 of the non-threaded shaft portion 7, as shown in Figure 3C, a laser beam 14 is irradiated onto the end face 10 of the non-threaded shaft portion 7 by a laser irradiation device 13.

[0066] Even when irradiating the end face 10 with laser light 14, the irradiation range of the laser light 14 is limited to only a portion of the end face 10 in both the circumferential and radial directions. Therefore, when forming a laser-hardened layer 6 on the end face 10, depending on the irradiation range of the laser light 14, the screw shaft intermediate body 11 can be rotated around its own central axis to move the irradiation position of the laser light 14 to the end face 10 in the circumferential direction, or the laser irradiation device 13 can be moved in the radial direction of the end face 10, or both can be moved simultaneously to move the irradiation position of the laser light 14 in the circumferential, radial, or both directions, thereby forming a laser-hardened layer 6 on the entire end face 10 or a portion thereof. If the irradiation range of the laser light 14 covers the entire end face 10, the screw shaft intermediate body 11 can be kept stationary, and the laser-hardened layer 6 can be formed on the entire end face 10.

[0067] In this example, a laser-hardened layer 6 is formed on the end face 10 of the non-threaded shaft portion 7 that contacts the piston 19, and on the outer circumferential surface 8 of the non-threaded shaft portion 7 that contacts the inner circumferential surface of the stopper member 20, by laser hardening treatment.

[0068] [Second Example] A second example of the embodiment of the present disclosure will be described with reference to Figure 4. In the screw shaft 1a of this example, a laser hardened layer 6 is formed on the end face 10 of the non-screw shaft portion 7, but no laser hardened layer is formed on the outer circumferential surface 8 of the non-screw shaft portion 7. That is, the outer circumferential surface 8 of the non-screw shaft portion 7 is made of an unheated surface, thereby reducing manufacturing costs.

[0069] The screw shaft 1a in this example can be applied in applications where mechanical properties such as wear resistance are not particularly required on the outer circumferential surface 8 of the non-screw shaft portion 7, for example, when the claw portion 22 of the stopper member 20 fitted onto the non-screw shaft portion 7 and the stopper projection 21 provided on the nut 16 hardly ever collide during use. The other configurations and effects of the second example are the same as those of the first example.

[0070] [Third Example] A third example of the embodiment of the present disclosure will be described with reference to Figure 5. In the screw shaft 1b of this example, a laser hardened layer 6 is formed on the outer circumferential surface 8 of the non-screw shaft portion 7, but no laser hardened layer is formed on the end face 10 of the non-screw shaft portion 7. That is, the end face 10 of the non-screw shaft portion 7 is made of an untreated surface, thereby reducing manufacturing costs.

[0071] The screw shaft 1b in this example can be applied in applications where mechanical properties such as wear resistance are not particularly required for the end face 10 of the non-screw shaft portion 7, such as when the end face 10 of the non-screw shaft portion 7 does not press another component such as a piston 19 in the axial direction during use. The other configurations and effects of the third example are the same as those of the first example.

[0072] [Fourth Example] A fourth example of the embodiments of the present disclosure will be described with reference to Figures 6A and 6B.

[0073] In this example, as shown in Figure 6A, the end face 10 of the non-threaded shaft portion 7 of the screw shaft 1c has a laser-hardened layer 6 formed by laser hardening treatment. However, the outer circumferential surface 8 of the non-threaded shaft portion 7 has a high-frequency induction hardened layer 23 formed by high-frequency induction hardening treatment instead of the laser-hardened layer. The effective hardened layer depth d of the high-frequency induction hardened layer 23. ２３ The effective hardened layer depth d of the high-frequency induction hardened hardened layer 5 is 1 mm or more. ５ It is almost the same as this.

[0074] In the process of forming the high-frequency induction hardened layer 23 by high-frequency induction hardening, as shown in Figure 6B, a cylindrical high-frequency heating coil 12a is coaxially arranged around the outer surface 8 of the non-threaded shaft portion 7, similar to the process of forming the high-frequency induction hardened layer 5 on the outer surface 3 of the threaded shaft portion 2. By energizing the high-frequency heating coil 12a, the outer surface 8 of the non-threaded shaft portion 7 is heated by high-frequency induction. Subsequently, the threaded shaft intermediate body 11 is rapidly cooled to form the high-frequency induction hardened layer 23 on the outer surface 8 of the non-threaded shaft portion 7.

[0075] The outer diameter of the outer peripheral surface 8 of the non-threaded shaft portion 7 where the high-frequency induction hardened layer 23 is formed is smaller than the outer diameter of the outer peripheral surface 3 of the threaded shaft portion 2 where the high-frequency induction hardened layer 5 is formed. Therefore, the inner diameter of the high-frequency heating coil 12a for forming the high-frequency induction hardened layer 23 is made smaller than the inner diameter of the high-frequency heating coil 12 for forming the high-frequency induction hardened layer 5.

[0076] The operation of forming the high-frequency induction hardened layer 5 and the operation of forming the high-frequency induction hardened layer 23 can be performed simultaneously, or can be performed successively in terms of time. Further, the tempering process for the high-frequency induction hardened layer 5 and the high-frequency induction hardened layer 23 can also be performed simultaneously.

[0077] In the threaded shaft 1c of this example, since the high-frequency induction hardened layer 23 is formed on the outer peripheral surface 8 of the non-threaded shaft portion 7, the mechanical properties such as the hardness and wear resistance of the outer peripheral surface 8 of the non-threaded shaft portion 7 can be further improved. Other configurations and operational effects of the fourth example are the same as those of the first example.

[0078] [Fifth Example] The fifth example of the embodiment of the present disclosure will be described with reference to FIGS. 7A and 7B.

[0079] In the threaded shaft 1d of this example, the outer diameter D of the outer peripheral surface 8a of the non-threaded shaft portion ８ａ ８ａ of the fourth example the outer diameter D of the outer peripheral surface 8 of the non-threaded shaft portion ８ ８ is larger than (D ８ａ ８ａ > D ８ ８ ), and the difference between the outer diameter D of the outer peripheral surface 3 of the threaded shaft portion 2 and the outer diameter D of the outer peripheral surface 8a of the non-threaded shaft portion ３ ３ ８ａ ８ａ (D ３ ３ - D ８ａ ８ａ ), that is, twice the radial width dimension of the stepped surface 9 is smaller than that of the threaded shaft 1c of the fourth example, which is different from the fourth example.

[0080] In this example, the outer diameter D of the outer peripheral surface 3 of the threaded shaft portion 2 ３ ３ and the outer diameter D of the outer peripheral surface 8a of the non-threaded shaft portion ８ａ ８ａSince the difference is smaller than that of the screw shaft 1c in the fourth example, high-frequency induction heating when forming the high-frequency hardened layer 5 on the outer peripheral surface 3 and the high-frequency hardened layer 23a on the outer peripheral surface 8a can be performed using a single high-frequency heating coil 12b whose inner diameter does not change in the axial direction.

[0081] In this example, the radial distance from the high-frequency heating coil 12b to the outer surface 8a of the non-threaded shaft portion 7a is greater than the radial distance from the high-frequency heating coil 12b to the outer surface 3 of the threaded shaft portion 2. Therefore, the heating temperature of the outer surface 8a is lower than that of the outer surface 3. Consequently, the effective hardened layer depth d of the high-frequency hardened layer 23a on the outer surface 8a of the non-threaded shaft portion 7a is reduced accordingly. ２３ａ This refers to the effective hardened layer depth d of the high-frequency induction hardened hardened layer 5 on the outer circumferential surface 3 of the screw shaft portion 2. ５ It is shallower than (d ２３ａ <d ５ ).

[0082] In this example, the screw shaft 1d has high-frequency induction heating that forms high-frequency induction hardened layers 5 and 23a on the outer surface 3 of the screw shaft portion 2 and the outer surface 8a of the non-screw shaft portion 7a, thereby reducing manufacturing costs. The other configurations and effects of the fifth example are the same as those of the first and fourth examples.

[0083] [Sixth Example] A sixth example of the embodiment of the present disclosure will be described with reference to Figure 8. In this example, the screw shaft 1e consists only of a screw shaft portion 2. That is, the screw shaft 1e in this example does not have a non-screw shaft portion. In this example, a laser-hardened layer 6 is formed on the end face 10a of the screw shaft portion 2 by laser hardening treatment. The other configurations and effects of the sixth example are the same as those of the first example.

[0084] 1, 1a, 1b, 1c, 1d, 1e Screw shaft 2 Screw shaft portion 3 Outer surface 4 Helical groove 5 High-frequency induction hardened layer 6 Laser hardened layer 7, 7a Non-screw shaft portion 8, 8a Outer surface 9 Stepped surface 10, 10a End face 11 Screw shaft intermediate body 12, 12a, 12b High-frequency heating coil 13 Laser irradiation device 14 Laser beam 15 Ball screw device 16 Nut 17 Nut-side helical groove 18 Ball 19 Piston 20 Stopper member 21 Stopper projection 22 Claw portion 23, 23a High-frequency induction hardened layer 24 End face

Claims

1. A method for manufacturing a screw shaft for a ball screw device, comprising the steps of: preparing a screw shaft having a screw shaft portion with a helical groove on its outer circumference and the surface thereof being an unheat-treated surface; forming a high-frequency induction hardened layer on the outer circumference of the screw shaft portion by high-frequency induction hardening; and forming a laser hardened layer on at least a portion of the surface of the screw shaft that is separate from the outer circumference of the screw shaft portion and that comes into contact with another component during use by laser hardening.

2. The method for manufacturing a screw shaft for a ball screw device according to claim 1, wherein the screw shaft has a non-screw shaft portion in a portion that is axially offset from the screw shaft portion, and the laser hardening layer is formed on at least a part of the non-screw shaft portion.

3. The method for manufacturing a screw shaft for a ball screw device according to claim 2, wherein the laser hardening layer is formed on at least a portion of the outer circumferential surface of the non-screwed shaft portion.

4. The method for manufacturing a screw shaft for a ball screw device according to claim 2 or 3, wherein the laser hardening layer is formed on at least a portion of the end face of the non-screwed shaft portion.

5. The method for manufacturing a screw shaft for a ball screw device according to any one of claims 2 to 4, wherein the non-screwed shaft portion has an outer diameter smaller than the outer diameter of the screwed shaft portion.

6. A method for manufacturing a screw shaft for a ball screw device according to any one of claims 1 to 5, wherein the laser hardening layer is formed on at least a portion of the end face of the screw shaft portion.

7. A screw shaft for a ball screw device, comprising: a screw shaft portion having a helical groove on its outer circumferential surface; a high-frequency induction hardening layer formed on the outer circumferential surface of the screw shaft portion; and a laser hardening layer formed on at least a portion of the surface of the screw shaft portion that is not on the outer circumferential surface and comes into contact with another component during use, wherein the laser hardening layer has an effective hardening layer depth shallower than the effective hardening layer depth of the high-frequency induction hardening layer.

8. The screw shaft for a ball screw device according to claim 7, wherein the effective hardened layer depth of the laser-hardened layer is less than 1 mm, and the effective hardened layer depth of the high-frequency induction hardened layer is 1 mm or more.

9. The screw shaft for a ball screw device according to claim 7 or 8, wherein the screw shaft portion has a non-screwed portion in the axial direction away from the screw shaft portion, and the non-screwed portion includes the portion on which the laser hardened layer is formed.

10. The screw shaft for a ball screw device according to claim 9, wherein the outer circumferential surface of the non-screwed shaft portion includes the portion on which the laser hardened layer is formed.

11. The screw shaft for a ball screw device according to claim 9 or 10, wherein the end face of the non-screwed shaft portion includes the portion on which the laser hardened layer is formed.

12. The screw shaft for a ball screw device according to any one of claims 9 to 11, wherein the non-screwed shaft portion has an outer diameter smaller than the outer diameter of the screwed shaft portion.

13. The portion on which the laser-hardened layer is formed includes the end face of the screw shaft portion, as described in any one of claims 7 to 12, for a screw shaft for a ball screw device.

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