Manufacturing method of screw shaft

Abstract

Provided is a screw shaft which sufficiently ensures the accuracy of the shape of a male-side screw section. A screw shaft (17) is equipped with a large-diameter shaft section (20), medium-diameter shaft sections (21), (22), and small-diameter shaft sections (23), (24). The large-diameter shaft section (20) has a male-side screw section (26) which spans the entire length of the outer-circumferential surface in the axial direction thereof. The medium-diameter shaft sections (21), (22) are positioned next to the large-diameter shaft section (20) in the axial direction, and have a smaller outer diameter than does the large-diameter shaft section (20). The small-diameter shaft sections (23), (24) are positioned next to the medium-diameter shaft sections (21), (22) on the sides thereof opposite the large-diameter shaft section (20) in the axial direction, have a smaller outer diameter than do the medium-diameter shaft sections (21), (22), and do not have spiral-shaped thread-rolling marks in the outer-circumferential surface thereof.

Description

[Technical field]

[0001] The present invention relates to a screw shaft that constitutes a feed screw mechanism incorporated in various mechanical devices and a manufacturing method thereof, as well as to an electric position adjuster for a steering wheel that includes an electric motor and a feed screw mechanism. [Background technology]

[0002] Various types of electric steering wheel position adjustment devices that use an electric motor as a drive source to adjust the fore-aft position and height position of the steering wheel have been known (see, for example, Japanese Patent Publication No. 2005-199760 (Patent Document 1), Japanese Patent Publication No. 2006-321484 (Patent Document 2), and Japanese Patent Publication No. 2015-227166 (Patent Document 3)).

[0003] A feed screw mechanism is widely used as a mechanism incorporated in various mechanical devices, including such an electric steering wheel position adjustment device, to convert the rotational motion of a drive source into linear motion. The feed screw mechanism includes a screw shaft having a male threaded portion on its outer circumferential surface, and a nut having a female threaded portion on its inner circumferential surface.

[0004] There are two types of feed screw mechanisms: a sliding screw type feed screw mechanism and a ball screw type feed screw mechanism. In a sliding screw type feed screw mechanism, the male threaded portion of the screw shaft and the female threaded portion of the nut are screwed together. In a ball screw type feed screw mechanism, the male threaded portion of the screw shaft forms a male thread groove, the female threaded portion of the nut forms a female thread groove, and multiple balls are arranged between the male thread groove and the female thread groove.

[0005] In either feed screw mechanism, the male thread of the screw shaft can be formed by rolling, in which a workpiece, which is an intermediate material for the screw shaft, is rolled between a number of rolling dies, and the outer circumferential surface of the workpiece is plastically deformed by the rolling dies to form the male thread.

[0006] There are various methods for rolling the male thread portion, such as the through-feed method described in JP 09-225573 A (Patent Document 4) and the in-feed method described in JP 2008-281142 A (Patent Document 5). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2005-199760 [Patent Document 2] Japanese Patent Publication No. 2006-321484 [Patent Document 3] Japanese Patent Application Publication No. 2015-227166 [Patent Document 4] Japanese Patent Application Publication No. 9-225573 [Patent Document 5] Japanese Patent Application Publication No. 2008-281142 Summary of the Invention [Problem to be solved by the invention]

[0008] A specific example of the through-feed rolling process will be described with reference to Figures 11 to 13. In this example, as shown in Figures 11(a) and 11(b), a pair of rolling dies 101 is used to perform the through-feed rolling process on a workpiece 100. This forms a male thread portion 102, thereby obtaining a screw shaft 103 equipped with the male thread portion 102.

[0009] In the following description of this specific example, unless otherwise specified, the axial direction refers to the axial direction of the workpiece 100 and the screw shaft 103, one axial side is the left side in Figures 11(a) and 11(b), and the other axial side is the right side in Figures 11(a) and 11(b).

[0010] The screw shaft 103 is a screw shaft constituting a ball screw type feed screw mechanism. The screw shaft 103 includes a screw shaft portion 104 and a base end side shaft portion 105. The screw shaft portion 104 exists in a portion from an end portion on one axial side of the screw shaft 103 to a portion close to an end portion on the other axial side. The screw shaft portion 104 has a helical male side screw portion (male screw groove) 102 on the outer circumferential surface. The base end side shaft portion 105 exists at an end portion on the other axial side of the screw shaft 103. That is, the base end side shaft portion 105 is disposed adjacent to the other axial side of the screw shaft portion 104. The base end side shaft portion 105 is configured in a cylindrical shape and has an outer diameter larger than the outer diameter of the screw shaft portion 104.

[0011] The workpiece 100 includes a large diameter shaft portion 106 for rolling and a small diameter shaft portion 107 for rolling, which are formed into a threaded shaft portion 104 by rolling, and a base end side shaft portion 105 that is not rolled. The large diameter shaft portion 106 for rolling is formed in a cylindrical shape and has an outer diameter smaller than the outer diameter of the base end side shaft portion 105. The small diameter shaft portion 107 for rolling is disposed adjacent to the other axial side of the large diameter shaft portion 106 for rolling. The small diameter shaft portion 107 for rolling is formed in a cylindrical shape and has an axial length significantly smaller than the axial length of the large diameter shaft portion 106 for rolling, and has an outer diameter smaller than the outer diameter of the large diameter shaft portion 106 for rolling. The base end side shaft portion 105 is disposed adjacent to the other axial side of the small diameter shaft portion 107 for rolling.

[0012] The pair of rolling dies 101 are round dies each having a short cylindrical shape, and their outer circumferential surfaces face each other. Each of the rolling dies 101 has a spiral rolling tooth 108 on its outer circumferential surface, and has a chamfered portion 109 at each of the locations adjacent to both axial sides of the rolling tooth 108. Each of the chamfered portions 109 is formed of an inclined surface (conical surface) that is inclined toward the axial center of the rolling die 101 as it goes radially outward. As shown in an enlarged view in FIG. 12, the diameter of the small diameter end of the chamfered portion 109 is approximately equal to the diameter of the tooth root circle of the rolling tooth 108. The chamfered portion 109 is provided to relieve stress concentration during rolling processing.

[0013] When performing through-feed rolling on a workpiece 100 using a pair of rolling dies 101, the pair of rolling dies 101 are rotated in the same direction while maintaining a constant distance between the outer circumferential surfaces of the pair of rolling dies 101 (the tip circles of the rolling teeth 108 of each of the rolling dies 101), as shown in Fig. 11(a). At this time, the distance between the outer circumferential surfaces of the pair of rolling dies 101 is set smaller than the outer diameters of the large diameter shaft portion 106 for rolling and the small diameter shaft portion 107 for rolling.

[0014] In this state, the workpiece 100 is inserted between the outer circumferential surfaces of the pair of rolling dies 101 from one end in the axial direction. Then, the rolling teeth 108 of the pair of rolling dies 101 are cut into the outer circumferential surface of the end of the workpiece 100 on one axial side, the workpiece 100 rotates in the opposite direction to the pair of rolling dies 101, and the rolling process for forming the male thread portion 102 begins. At this time, a step occurs, which is a phenomenon in which the workpiece 100 moves to one axial side relative to the pair of rolling dies 101, based on a lead angle error that occurs between the rolling teeth 108 of the pair of rolling dies 101 and the male thread portion formed on the workpiece 100. As a result, the rolling process automatically progresses.

[0015] That is, while the workpiece 100 moves axially to one side relative to the pair of rolling dies 101, the rolling teeth 108 of the pair of rolling dies 101 sequentially plastically deform the rolling large diameter shaft portion 106 and the rolling small diameter shaft portion 107, which are the parts to be processed. As a result, the male thread portion 102 is formed as shown in FIG. 11(b). FIG. 13(a) is a partially enlarged view of FIG. 11(b). Then, when the male thread portion 102 is thus formed, the pair of rolling dies 101 are retracted in directions away from each other in the radial direction, thereby completing the rolling process.

[0016] In this specific example, similar to the workpiece described in JP 09-225573 A, the end of the workpiece 100 on the other axial side has a small diameter shank portion 107 for rolling having an outer diameter smaller than that of the large diameter shank portion 106 for rolling. Therefore, the material of the large diameter shank portion 106 for rolling that flows due to the rolling process can be released to the small diameter shank portion 107 for rolling. As a result, the outer diameter of the end of the male thread portion 102 on the other axial side is sufficiently secured, while the end of the male thread portion 102 on the other axial side is easily prevented from swelling outward in the radial direction compared to the portion adjacent to the one axial side.

[0017] However, when performing the above-mentioned rolling process, the following problem may occur: That is, when performing the above-mentioned rolling process, the rolling process is performed on the processed portion of the workpiece 100 not only by the rolling teeth 108 of the pair of rolling dies 101 but also by the chamfered portion 109 located at the end of each of the pair of rolling dies 101 on the other axial side.

[0018] When rolling is performed, the inclined surfaces constituting the chamfered portion 109 on the other axial side of each of the pair of rolling dies 101 ride on the processed portion of the workpiece 100. A large load is applied to the portion of the processed portion of the workpiece 100 where the inclined surfaces ride on, facing inward in the radial direction and toward the other axial direction. As a reaction force to the radial component of such a load, the rolling die 101 is subjected to a load in a direction in which the rolling die 101 escapes from the processed portion, that is, a load facing outward in the radial direction. As a result of the inclined surfaces constituting the chamfered portion 109 riding on the processed portion of the workpiece 100, excessive elongation and twisting may occur in the workpiece 100 starting from the portion of the processed portion of the workpiece 100 where the inclined surfaces ride on. As a result, shape errors such as tooth profile error, tooth lead error, and thread pitch error may occur in the male thread portion 102, and the shape accuracy of the male thread portion 102 may be reduced. Furthermore, in general, in the rolling process of the male thread portion 102, problems occur in that radial burrs and bulges are formed on the outer circumferential surface and axial end of the processed part due to shaft elongation and extremely high axial thrust generated during the process. In particular, in the portion where the chamfered portion 109 of the rolling die 101 rides up, biased thread thrust is concentrated at one point, which is likely to lead to a deterioration in the shape accuracy of the male thread portion 102.

[0019] Furthermore, in the final stage of the rolling process shown in Fig. 11(b), Fig. 13(a) and Fig. 13(b), the material pushed toward the other axial direction at the end of the male thread portion 102 on the other axial direction cannot escape, and the inclined surface constituting the chamfered portion 109 rides up on a part of the material (β portion in Fig. 13(b)). As a result, the material pushed against the inclined surface strongly presses and deforms the end face on one axial direction side of the base end shaft portion 105, causing a relatively large pitch deviation in the male thread portion 102. Therefore, in the final stage of the rolling process, the shape error of the male thread portion 102 is particularly likely to worsen. In addition, if radial burrs or bulges are formed at the end of the male thread portion 102 on the other axial direction and these burrs or bulges become large enough to interfere with the female thread portion of the nut, it becomes impossible to move the nut in the axial direction to the periphery of the end of the male thread portion 102 on the other axial direction, and the operating stroke of the feed screw mechanism becomes shorter accordingly.

[0020] The above-mentioned inconvenience, i.e., the inconvenience that the shape accuracy of the male thread portion may be reduced due to the inclined surface constituting the chamfered portion of the rolling die riding on the processed portion of the workpiece when performing rolling, is not limited to through-feed type rolling, but also occurs when performing other types of rolling, such as the in-feed type.

[0021] The object of the present invention is to provide a screw shaft in which the shape precision of the male screw portion is sufficiently ensured, a manufacturing method thereof, and an electric position adjustment device for a steering wheel which is equipped with an electric motor and a feed screw mechanism and in which the operating performance of the feed screw mechanism is sufficiently ensured. [Means for solving the problem]

[0022] A screw shaft according to one aspect of the present invention has a large diameter shaft portion and a medium diameter shaft portion disposed adjacent to both axial ends of the large diameter shaft portion. The large diameter shaft portion has a male thread portion on its outer circumferential surface over the entire axial length. The medium diameter shaft portion has an outer diameter smaller than the outer diameter of the large diameter shaft portion, and has a spiral rolling mark on its outer surface that is in phase with an extension of the spiral curve that is the groove bottom line of the male thread portion.

[0023] In one aspect of the screw shaft of the present invention, the outer diameter of the medium diameter shaft portion is 0.93 to 1.07 times the groove bottom diameter (root diameter, diameter of groove bottom circle) of the male thread portion.

[0024] A screw shaft according to one aspect of the present invention includes a large diameter shaft portion, a medium diameter shaft portion, and a small diameter shaft portion. The large diameter shaft portion has a male thread portion on its outer circumferential surface over the entire axial length. The medium diameter shaft portion is disposed axially adjacent to the large diameter shaft portion and has an outer diameter smaller than an outer diameter of the large diameter shaft portion. The small diameter shaft portion is arranged adjacent to the medium diameter shaft portion on the opposite side of the large diameter shaft portion in the axial direction, has an outer diameter smaller than the outer diameter of the medium diameter shaft portion, and has no spiral rolling marks on its outer peripheral surface.

[0025] In one aspect of the threaded shaft of the present invention, the medium diameter shaft portion has on its outer peripheral surface a spiral rolling mark that is in phase with an extension of a spiral curve that is the groove bottom line of the male thread portion.

[0026] In one embodiment of the screw shaft of the present invention, an adjacent shaft portion is arranged adjacent to the small diameter shaft portion on the opposite side of the large diameter shaft portion in the axial direction, and has an outer diameter larger than the outer diameter of the medium diameter shaft portion.

[0027] In one aspect of the screw shaft of the present invention, the outer diameter of the medium diameter shaft portion is 0.93 to 1.07 times the groove bottom diameter (root diameter, diameter of groove bottom circle) of the male thread portion.

[0028] In one aspect of the screw shaft of the present invention, the outer diameter of the small diameter shaft portion is 0.9 times or more and less than 1.0 times the groove bottom diameter of the male thread portion.

[0029] In one aspect of the screw shaft of the present invention, the medium diameter shaft portion is composed of a first medium diameter shaft portion arranged on one axial side of the large diameter shaft portion and a second medium diameter shaft portion arranged on the other axial side of the large diameter shaft portion, The small diameter shaft portion is composed of a first small diameter shaft portion arranged on one axial side of the first medium diameter shaft portion, and a second small diameter shaft portion arranged on the other axial side of the second medium diameter shaft portion.

[0030] In one aspect of the present invention, the threaded shaft can be incorporated into an electric position adjustment device for a steering wheel.

[0031] A method for manufacturing a screw shaft according to one embodiment of the present invention includes a rolling process using a plurality of rolling dies to form a male thread portion over the entire axial length of the outer peripheral surface of the large diameter shaft portion for rolling, the workpiece including a large diameter shaft portion for rolling and a medium diameter shaft portion for rolling arranged adjacent to both axial ends of the large diameter shaft portion for rolling and having an outer diameter smaller than the outer diameter of the large diameter shaft portion for rolling. In the process of performing the rolling process on the workpiece, the rolling die is used to form a spiral rolling mark on the outer peripheral surface of the medium diameter shaft portion for rolling, and a rolling process is performed to form a male thread portion on the outer peripheral surface of the large diameter shaft portion for rolling.

[0032] In one embodiment of the manufacturing method for a screw shaft of the present invention, the outer diameter of the rolling medium diameter shaft portion is set to be 0.93 to 1.07 times the groove bottom diameter of the male thread portion to be formed on the outer peripheral surface of the rolling large diameter shaft portion.

[0033] A method for manufacturing a screw shaft according to one embodiment of the present invention includes a rolling process that uses a plurality of rolling dies to produce a step in the workpiece, the rolling process comprising: a large diameter shaft portion for rolling; a medium diameter shaft portion for rolling arranged axially adjacent to the large diameter shaft portion for rolling and having an outer diameter smaller than the outer diameter of the large diameter shaft portion for rolling; and a small diameter shaft portion arranged adjacent to the opposite side of the large diameter shaft portion for rolling from the medium diameter shaft portion for rolling in the axial direction and having an outer diameter smaller than the outer diameter of the medium diameter shaft portion for rolling, in order to form a male thread portion over the entire axial length on the outer peripheral surface of the large diameter shaft portion for rolling. In the step of performing the rolling process on the workpiece, the rolling die is used to perform the rolling process to form a male thread portion on the outer peripheral surface of the large diameter shaft portion for rolling, and the rolling die is not brought into contact with the outer peripheral surface of the small diameter shaft portion.

[0034] In a method for manufacturing a screw shaft according to one aspect of the present invention, in the step of performing the rolling process on the workpiece, a spiral rolling mark is formed on the outer peripheral surface of the rolling medium diameter shaft portion.

[0035] In one embodiment of the method for manufacturing a screw shaft of the present invention, a workpiece is used as the workpiece, which is arranged adjacent to the small diameter shaft portion on the opposite side of the large diameter shaft portion for rolling in the axial direction, and has an adjacent shaft portion having an outer diameter larger than the outer diameter of the medium diameter shaft portion for rolling.

[0036] In one embodiment of the manufacturing method for a screw shaft of the present invention, the outer diameter of the rolling medium diameter shaft portion is set to be 0.93 to 1.07 times the groove bottom diameter of the male thread portion to be formed on the outer peripheral surface of the rolling large diameter shaft portion, and the outer diameter of the small diameter shaft portion is set to be 0.9 to less than 1.0 times the groove bottom diameter of the male thread portion.

[0037] In a method for manufacturing a screw shaft according to one aspect of the present invention, a rolling die having a chamfered portion at an axial end portion of an outer circumferential surface is used as each of the rolling dies. In addition, in the process of performing the rolling process on the workpiece, the chamfered portion is maintained in a state facing the outer circumferential surface of the small diameter shaft portion.

[0038] In one embodiment of the method for manufacturing a screw shaft of the present invention, the workpiece used is one in which the rolling medium diameter shaft portion is composed of a first rolling medium diameter shaft portion arranged on one axial side of the rolling large diameter shaft portion and a second rolling medium diameter shaft portion arranged on the other axial side of the rolling large diameter shaft portion, and the small diameter shaft portion is composed of a first small diameter shaft portion arranged on one axial side of the first rolling medium diameter shaft portion and a second small diameter shaft portion arranged on the other axial side of the second rolling medium diameter shaft portion.

[0039] In a method for manufacturing a screw shaft according to one aspect of the present invention, a screw shaft to be incorporated in an electric position adjuster for a steering wheel is used as the screw shaft.

[0040] SUMMARY OF THE PRESENT EMBODIMENT An electric steering wheel position adjuster according to one aspect of the present invention includes an electric motor, a feed screw mechanism, and a steering component. The feed screw mechanism includes a screw shaft having a male threaded portion on its outer peripheral surface, and a nut having a female threaded portion on its inner peripheral surface that engages with the male threaded portion, and is configured so that the screw shaft and the nut can be displaced relative to each other in the axial direction based on the relative rotation between the screw shaft and the nut caused by the rotational force transmitted from the electric motor. The steering component is movable in a direction for adjusting the position of the steering wheel as the steering wheel is fixed in the use state and the screw shaft and the nut are displaced relative to each other in the axial direction. The screw shaft is constituted by the screw shaft of the present invention. Effect of the Invention

[0041] According to one aspect of the present invention, there is provided a screw shaft in which the shape precision of the male screw portion is sufficiently ensured, a manufacturing method thereof, and an electric position adjustment device for a steering wheel which is equipped with an electric motor and a feed screw mechanism and in which the operating performance of the feed screw mechanism is sufficiently ensured. [Brief description of the drawings]

[0042] [Figure 1] FIG. 1 is a partial cross-sectional view showing an electric steering wheel position adjuster according to one embodiment of the present invention. [Figure 2A] FIG. 2A is a side view of a screw shaft according to one embodiment of the present invention. [Figure 2B] FIG. 2B is a side view of a screw shaft according to another embodiment of the present invention. [Diagram 3] FIG. 3 is a side view of a workpiece that is an intermediate material for a screw shaft according to one embodiment of the present invention. [Figure 4] FIG. 4 is a diagram showing a state in which a workpiece is set in a rolling machine according to one embodiment of the present invention. [Diagram 5] FIG. 5 is a diagram showing an example of an embodiment of the present invention in which the distance between a pair of rolling dies rotating in the normal direction relative to each other is narrowed, and the rolling dies are pressed against a large diameter rolling shaft portion of a workpiece to start the rolling process. [Figure 6] FIG. 6 is a partially enlarged cross-sectional view of FIG. [Figure 7] Figure 7(a) is a diagram showing a state in which, during a mid-stage of rolling processing, the workpiece has moved from one axial end to the other axial end as a pair of rolling dies rotate in the forward direction, in accordance with one example of an embodiment of the present invention. Figure 7(b) is a diagram showing a state in which, during a mid-stage of rolling processing, the workpiece has moved from the other axial end to one axial end as a pair of rolling dies rotate in the reverse direction. [Figure 8]Figure 8(a) is a diagram showing, in one example of an embodiment of the present invention, a state in which, in the final stage of rolling processing, as a pair of rolling dies rotate in the forward direction, the workpiece moves from one axial end to the other axial end, and rolling marks are formed on the outer peripheral surface of the rolling medium diameter shaft portion on one axial side of the workpiece, and Figure 8(b) is a diagram showing, in the final stage of rolling processing, as a pair of rolling dies rotate in the reverse direction, the workpiece moves from the other axial end to the one axial end, and rolling marks are formed on the outer peripheral surface of the rolling small diameter shaft portion on the other axial side of the workpiece. [Figure 9] FIG. 9 is a partially enlarged cross-sectional view of FIG. 8(b). [Figure 10] FIG. 10 is a side view showing another example of a screw shaft to which the present invention can be applied. [Figure 11] FIG. 11(a) is a side view showing the start of a process for forming a male thread portion by rolling using a conventional through-feed method, and FIG. 11(b) is a side view showing the final stage of the process. [Figure 12] FIG. 12 is a partially enlarged view of a rolling die used in the process shown in FIG. [Figure 13] FIG. 13(a) is a partially enlarged cross-sectional view of FIG. 11(b), and FIG. 13(b) is a view of the screw shaft as seen from the arrow α in FIG. 13(a). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] [Example of implementation] An embodiment of the present invention will be described with reference to FIGS. 1 to 9. FIG.

[0044] (Steering wheel electric position adjustment device and screw shaft) Fig. 1 shows an electric steering wheel position adjustment device 1 using a screw shaft 17 (Fig. 2A) of this example. Note that the front-rear direction of the electric steering wheel position adjustment device 1 means the front-rear direction of a vehicle to which the device 1 is attached, with the front side being the left side in Figs. 1 and 2A, and the rear side being the right side in Figs. 1 and 2A. The electric steering wheel position adjustment device 1 of this example uses an electric motor (not shown) as a drive source to enable adjustment of the front-rear position of the steering wheel 2. Fig. 1 shows a state in which the steering wheel 2 is located in the middle of the front-rear position adjustment range.

[0045] The electric steering wheel position adjustment device 1 of the present invention can be composed of a steering column 3, a steering shaft 4, and an electric actuator 5. The electric steering wheel position adjustment device 1 of the present invention includes at least an electric motor (not shown) and a feed screw mechanism 12 which constitute the electric actuator 5, and an outer tube 9 which constitutes the steering shaft 4 and corresponds to a steering part.

[0046] The steering column 3 comprises a front outer column 6 and a rear inner column 7. The outer column 6 is prevented from displacing in the axial direction with respect to the vehicle body. The front portion of the inner column 7 is slidably inserted into the inner diameter side of the rear portion of the outer column 6.

[0047] The steering shaft 4 includes an inner shaft 8 on the front side and an outer tube 9 on the rear side. The inner shaft 8 and the outer tube 9 are combined by spline engagement or the like to enable torque transmission and relative displacement in the axial direction. The inner shaft 8 is rotatably supported on the inner diameter side of the outer column 6 via a bearing (not shown). The outer tube 9 is rotatably supported on the inner diameter side of the inner column 7 via a bearing 10. With this configuration, the steering shaft 4 is rotatably supported on the inner diameter side of the steering column 3. At the same time, the inner column 7 and the outer tube 9 are capable of relative displacement in the axial direction with respect to the outer column 6 and the inner shaft 8. The steering wheel 2 is supported and fixed to the rear end of the outer tube 9, which is a steering component.

[0048] The electric actuator 5 includes a housing 11, a sliding screw type feed screw mechanism 12, and an electric motor (not shown). The housing 11 is supported and fixed to the lower surface of the outer column 6.

[0049] The feed screw mechanism 12 includes a nut 13 and a rod 14. The central axis of the feed screw mechanism 12 is arranged parallel to the central axes of the steering shaft 4 and the steering column 3.

[0050] The nut 13 has a female thread portion 15 on its inner peripheral surface. The nut 13 is supported in the housing 11 so as to be rotatable but not displaceable in the axial direction. The nut 13 is driven to rotate by an electric motor via a worm reduction gear 16.

[0051] The rod 14 is formed by combining a threaded shaft 17 arranged on the front side with an extension shaft 18 arranged on the rear side.

[0052] The screw shaft 17 comprises a large diameter shaft portion 20, a first medium diameter shaft portion 21 and a second medium diameter shaft portion 22 corresponding to the medium diameter shaft portion, a first small diameter shaft portion 23 and a second small diameter shaft portion 24 corresponding to the small diameter shaft portion, and a base end side shaft portion 25 corresponding to the adjacent shaft portion.

[0053] The large diameter shaft portion 20 has a male thread portion 26 on its outer circumferential surface over the entire axial length, which screws into the female thread portion 15. The male thread portion 26 is formed by rolling. An axial intermediate portion of the male thread portion 26, excluding both axial end edge portions, is composed of a complete thread portion having a predetermined thread height. Each of the axial end edge portions of the male thread portion 26 is composed of an incomplete thread portion that does not reach the predetermined thread height.

[0054] In particular, in this example, the shape precision of the male thread portion 26 is sufficiently ensured. In other words, in this example, the entire male thread portion 26, including not only the complete thread portion but also the incomplete thread portion, is finished with high precision so as to function as a normal thread portion, i.e., to screw into the female thread portion 15 of the nut 13. That is, the flank surface of the male thread portion 26 is precisely finished not only in the axial middle portion, which is the complete thread portion, but also in both axial end edge portions, which are the incomplete thread portions. For this reason, in this example, the axial length of the entire male thread portion 26 corresponds to the effective thread length. The flank surface refers to the side surface of the thread, i.e., the tooth surface.

[0055] The first medium diameter shaft portion 21 is disposed adjacent to one axial side, which is the front side of the large diameter shaft portion 20. The first medium diameter shaft portion 21 is a cylindrical portion having an outer diameter smaller than the outer diameter of the large diameter shaft portion 20. In this embodiment, the first medium diameter shaft portion 21 has a first spiral rolling mark 27 on its outer peripheral surface. The first rolling mark 27 may be formed by a rolling die 35 for forming the male thread portion 26 in the rolling process. The first rolling mark 27 is in phase with an extension line of the spiral curve, which is the groove bottom line of the male thread portion 26. In the illustrated example, the first medium diameter shaft portion 21 has a chamfered portion 38 on the edge portion on one axial side of the outer peripheral surface.

[0056] The second medium diameter shaft portion 22 is disposed adjacent to the other axial side, which is the rear side of the large diameter shaft portion 20. The second medium diameter shaft portion 22 is a cylindrical portion having an outer diameter smaller than the outer diameter of the large diameter shaft portion 20. In this embodiment, the second medium diameter shaft portion 22 has a spiral second rolling mark 28 on its outer circumferential surface. The second rolling mark 28 may be formed by a rolling die 35 for forming the male thread portion 26 in the rolling process. The second rolling mark 28 is in phase with an extension of the spiral curve, which is the groove bottom line of the male thread portion 26.

[0057] In this example, the outer diameter D1 of the first inner diameter shaft portion 21 and the outer diameter D2 of the second inner diameter shaft portion 22 are equal to each other (D1=D2). However, when implementing the present invention, the outer diameter D1 of the first inner diameter shaft portion 21 and the outer diameter D2 of the second inner diameter shaft portion 22 may be made different from each other.

[0058] In this example, the outer diameter D1 of the first medium diameter shaft portion 21 and the outer diameter D2 of the second medium diameter shaft portion 22 are each set in a range of 0.93 to 1.07 times the groove bottom diameter (valley diameter) Da of the male thread portion 26 (a difference from the groove bottom diameter Da is within a range of ±7%) (0.93Da≦D1≦1.07Da, 0.93Da≦D2≦1.07Da). However, when implementing the present invention, the ranges of the outer diameters D1 and D2 can be set to ranges different from those of this example, as long as the first rolling mark 27 and the second rolling mark 28 can be formed in the rolling process.

[0059] In this example, the outer diameter D1 of the first medium diameter shaft portion 21 and the outer diameter D2 of the second medium diameter shaft portion 22 are each smaller than the inner diameter (the inscribed circle diameter of the thread) of the female thread portion 15 of the nut 13. Therefore, the first rolling mark 27 and the second rolling mark 28 do not screw into the female thread portion 15 of the nut 13. In other words, the first rolling mark 27 and the second rolling mark 28 do not function as a normal thread portion that screws into the female thread portion 15 of the nut 13.

[0060] The first small-diameter shaft portion 23 is disposed adjacent to one axial side, which is the front side of the first medium-diameter shaft portion 21. The first small-diameter shaft portion 23 is a columnar portion having an outer diameter smaller than the outer diameter of the first medium-diameter shaft portion 21, and has no spiral forging marks on the outer peripheral surface. In the illustrated example, the first small-diameter shaft portion 23 has a chamfered portion 39 at an edge portion on one axial side of the outer peripheral surface, and has a groove portion 40 at an edge portion on the other axial side in the axial direction of the outer peripheral surface. Among the outer peripheral surface of the first small-diameter shaft portion 23, the portion sandwiched between the chamfered portion 39 and the groove portion 40 is constituted by a cylindrical surface whose outer diameter does not change in the axial direction.

[0061] The second small-diameter shaft portion 24 is disposed adjacent to the other axial side, which is the rear side of the second medium-diameter shaft portion 22. The second small-diameter shaft portion 24 is a columnar portion having an outer diameter smaller than the outer diameter of the second medium-diameter shaft portion 22, and has no spiral forging marks on the outer peripheral surface. In the illustrated example, the outer peripheral surface of the second small-diameter shaft portion 24 is constituted by a cylindrical surface whose outer diameter does not change in the axial direction.

[0062] In this example, the outer diameter d1 of the first small-diameter shaft portion 23 is smaller than the outer diameter d2 of the second small-diameter shaft portion 24 (d1 < d2). However, when implementing the present invention, the outer diameter d1 of the first small-diameter shaft portion 23 can be made larger than the outer diameter d2 of the second small-diameter shaft portion 24, or the outer diameter d1 of the first small-diameter shaft portion 23 and the outer diameter d2 of the second small-diameter shaft portion 24 can be made equal to each other.

[0063] In this example, each of the outer diameter d1 of the first small-diameter shaft portion 23 and the outer diameter d2 of the second small-diameter shaft portion 24 is set in the range of 0.9 times or more and less than 1.0 times the groove bottom diameter Da of the male screw portion 26 (smaller than the groove bottom diameter and the difference from the groove bottom diameter Da is within -10%) (0.9Da ≦ d1 < 1.0Da, 0.9Da ≦ d2 < 1.0Da). However, when implementing the present invention, if the chamfered portion 37 of the forging die 35 does not ride on the outer peripheral surfaces of the first small-diameter shaft portion 23 and the second small-diameter shaft portion 24 in the forging process, that is, if no forging marks are formed on these outer peripheral surfaces, the ranges of the outer diameters d1 and d2 can be set to ranges different from those of this example.

[0064] The base-side shaft portion 25 is disposed adjacent to the other axial side, which is the rear side of the second small diameter shaft portion 24. The base-side shaft portion 25 is a stepped cylindrical portion having an outer diameter that is larger overall than the outer diameters of the second medium diameter shaft portion 22 and the second small diameter shaft portion 24. The base-side shaft portion 25 has a flange portion 29 that protrudes radially outward at an axially intermediate portion. In addition, a portion of the base-side shaft portion 25 located rearward of the flange portion 29 is formed by a cylindrical fitting portion 30.

[0065] The extension shaft 18 is configured in a cylindrical shape. The front end of the extension shaft 18 is externally fitted and fixed to the fitting portion 30 of the screw shaft 17. The rear end of the extension shaft 18 is connected to the rear side of the inner column 7 via an arm portion 19.

[0066] When adjusting the fore-aft position of the steering wheel 2, the nut 13 is rotationally driven by the electric motor via the worm reduction gear 16, thereby displacing the rod 14 in the axial direction relative to the nut 13. This displaces the inner column 7 connected to the rod 14 via the arm portion 19, and the outer tube 9 supported on the inner diameter side of the inner column 7, in the same direction as the rod 14, thereby adjusting the fore-aft position of the steering wheel 2.

[0067] (Manufacturing method of screw shaft) The method for manufacturing the screw shaft 17 of the present invention includes a step of performing rolling processing on the workpiece 31, as shown in Fig. 3, which includes a large-diameter shaft portion 32 for rolling, medium-diameter shaft portions 33, 34 for rolling arranged adjacent to the large-diameter shaft portion 32 in the axial direction and having an outer diameter smaller than that of the large-diameter shaft portion 32 for rolling, and small-diameter shaft portions 23, 24 arranged adjacent to the large-diameter shaft portion 32 for rolling on the opposite side of the large-diameter shaft portion 32 for rolling relative to the medium-diameter shaft portions 33, 34 in the axial direction and having an outer diameter smaller than that of the medium-diameter shaft portions 33, 34 for rolling, in order to form a male thread portion 26 on the outer peripheral surface of the large-diameter shaft portion 32 for rolling over the entire axial length, using a plurality of rolling dies 35 to generate a step in the workpiece 31. The rolling processing method in this example is an in-feed method in which a step is generated in the workpiece.

[0068] A manufacturing method of the screw shaft 17 of the present invention will be described with reference to Figs. 3 to 9. In the screw shaft 17 to be manufactured in this example, the outer diameter D1 of the first medium diameter shaft portion 21 and the outer diameter D2 of the second medium diameter shaft portion 22 are each set in a range of 0.93 to 1.07 times the groove bottom diameter Da of the male thread portion 26. In addition, the outer diameter d1 of the first small diameter shaft portion 23 and the outer diameter d2 of the second small diameter shaft portion 24 are each set in a range of 0.9 to less than 1.0 times the groove bottom diameter Da of the male thread portion 26. In the following description, unless otherwise specified, the axial direction refers to the axial direction of the workpiece 31, which is the intermediate material of the screw shaft 17, and one axial side is the left side in Figs. 3 to 9, and the other axial side is the right side in Figs. 3 to 9.

[0069] 3 shows a workpiece 31. The workpiece 31 has a shape similar to that of the screw shaft 17 (see FIG. 2A) except for the male thread portion 26, the first rolling mark 27, and the second rolling mark 28. That is, the workpiece 31 includes a large diameter shaft portion 32 for rolling having the male thread portion 26 formed on its outer circumferential surface, a first medium diameter shaft portion 33 for rolling having the first rolling mark 27 formed on its outer circumferential surface, a second medium diameter shaft portion 34 for rolling having the second rolling mark 28 formed on its outer circumferential surface, a first small diameter shaft portion 23, a second small diameter shaft portion 24, and a base end side shaft portion 25.

[0070] In the illustrated example, the outer circumferential surface of the large diameter rolling shaft portion 32 is configured as a cylindrical surface whose outside diameter does not change in the axial direction.

[0071] The first rolling medium diameter shaft portion 33 is disposed adjacent to one axial side of the rolling large diameter shaft portion 32, and has an outer diameter smaller than that of the rolling large diameter shaft portion 32. In the illustrated example, the outer peripheral surface of the first rolling medium diameter shaft portion 33 is configured as a cylindrical surface whose outer diameter does not change in the axial direction, except for a chamfered portion 38 formed on an end edge portion on one axial side. The outer diameter of the first rolling medium diameter shaft portion 33 is D1, which is the same as the outer diameter of the first medium diameter shaft portion 21 (see FIG. 2A).

[0072] The second rolling medium diameter shaft portion 34 is disposed adjacent to the other axial side of the rolling large diameter shaft portion 32, and has an outer diameter smaller than that of the rolling large diameter shaft portion 32. In the illustrated example, the outer peripheral surface of the second rolling medium diameter shaft portion 34 is configured as a cylindrical surface whose outer diameter does not change in the axial direction. The outer diameter of the second rolling medium diameter shaft portion 34 is D2, which is the same as the outer diameter of the second medium diameter shaft portion 22 (see FIG. 2A).

[0073] The first small diameter shaft portion 23 is disposed adjacent to one axial side of the first rolling medium diameter shaft portion 33 and has an outer diameter smaller than the outer diameter of the first rolling medium diameter shaft portion 33 .

[0074] The second small diameter shaft portion 24 is disposed adjacent to the other axial side of the second rolling medium diameter shaft portion 34 and has an outer diameter smaller than the outer diameter of the second rolling medium diameter shaft portion 34 .

[0075] The base end side shaft portion 25 is disposed adjacent to the other axial side of the second small diameter shaft portion 24 .

[0076] When the screw shaft 17 is obtained by rolling the workpiece 31, first, the workpiece 31 is set on a rolling machine as shown in FIG.

[0077] The rolling disk includes a pair of rolling dies 35. The pair of rolling dies 35 are round dies each having a short cylindrical shape, and are arranged parallel to each other with their outer circumferential surfaces facing each other.

[0078] Each of the rolling dies 35 has, on its outer circumferential surface, helical rolling teeth 36 (see Figs. 6 and 9; illustration of the shape is omitted in Figs. 4, 5, 7, and 8) for rolling the male thread portion 26, and has chamfered portions 37 at locations adjacent to both axial sides of the rolling teeth 36. That is, each of the rolling dies 35 has rolling teeth 36 at an axial middle portion of the outer circumferential surface, and has chamfered portions 37 at both axial end portions of the outer circumferential surface.

[0079] Each of the chamfered portions 37 is constituted by an inclined surface that inclines in a direction toward the axial center side of the forging die 35 as it goes toward the radially outer side. As shown enlarged in FIGS. 6 and 9, the diameter of the small-diameter side end portion of the chamfered portion 37 is substantially equal to the root diameter of the forged tooth 36. Note that the chamfered portion 37 is provided to relieve stress concentration during forging. Also, in the illustrated example, the cross-sectional shape of the chamfered portion 37 is a linear shape, but when implementing the present invention, the cross-sectional shape of the chamfered portion of the forging die can also be a convex arc shape.

[0080] Also, in this example, the axial dimension X1 of the forged tooth 36, in other words, the interval (X1) between a pair of chamfered portions 37, is larger than the axial dimension X2 from the edge on one axial side of the first forged intermediate-diameter shaft portion 33 of the workpiece 31 to the edge on the other axial side of the second forged intermediate-diameter shaft portion 34 (X1>X2). Also, in this example, the axial dimension X1 of the forged tooth 36 is smaller than the axial dimension X3 from the edge on one axial side of the first small-diameter shaft portion 23 of the workpiece 31 to the edge on the other axial side of the second small-diameter shaft portion 24 (X1<X3). However, when implementing the present invention, if it is possible to maintain a state in which the chamfered portions 37 on the other axial side of a pair of forging dies 35 exist at the same axial position as the second small-diameter shaft portion 24 during forging, depending on the method of supporting the workpiece 31 on the first small-diameter shaft portion 23 side, that is, by adopting a support method that avoids interference between the support member for the workpiece 31 on the first small-diameter shaft portion 23 side and a pair of forging dies 35, a configuration in which X1≧X3 can also be adopted.

[0081] As shown in FIG. 4, at the stage of setting the workpiece 31 on the forging table, the interval between the outer peripheral surfaces of a pair of forging dies 35 (the tip circle of the forged teeth 36 of each of the forging dies 35) is sufficiently larger than the outer diameter of the large-diameter shaft portion 32 for forging of the workpiece 31. With the workpiece 31 set on the forging table, the workpiece 31 is arranged parallel to a pair of forging dies 35 at the central position between the outer peripheral surfaces of a pair of forging dies 35.

[0082] In this state, the rolling teeth 36 of the pair of rolling dies 35 face the outer circumferential surfaces of the large diameter shaft portion 32 for rolling, the first medium diameter shaft portion 33 for rolling, and the second medium diameter shaft portion 34 for rolling of the workpiece 31. In addition, the chamfered portion 37 on one axial side of the pair of rolling dies 35 faces the outer circumferential surface of the first small diameter shaft portion 23 of the workpiece 31, and the chamfered portion 37 on the other axial side of the pair of rolling dies 35 faces the outer circumferential surface of the second small diameter shaft portion 24 of the workpiece 31.

[0083] Next, the pair of rolling dies 35 are rotated in the same direction while narrowing the gap between the pair of rolling dies 35. As shown in Figs. 5 and 6, cutting is started, which is a process in which the rolling teeth 36 of the pair of rolling dies 35 are caused to bite into the outer circumferential surface of the large diameter shaft portion 32 for rolling of the workpiece 31. When the cutting starts, the workpiece 31 is given a rotational force by the pair of rolling dies 35 and rotates in the opposite direction to the pair of rolling dies 35. As a result, rolling is performed on the entire outer circumferential surface of the large diameter shaft portion 32 for rolling of the workpiece 31, and the male thread portion 26 is gradually formed.

[0084] In the rolling process of this example, as the cutting progresses, a phenomenon known as slippage occurs in which the workpiece 31 moves axially relative to the pair of rolling dies 35 based on a lead angle error that occurs between the rolling teeth 36 of the pair of rolling dies 35 and the large diameter rolling shaft portion 32 of the workpiece 31.

[0085] In this example, the pair of rolling dies 35 are repeatedly rotated alternately in the forward direction, which is the direction of rotation at the start of the cutting, and in the reverse direction, which is the opposite direction, by NC control. Therefore, the workpiece 31 is subjected to rolling while moving back and forth in the axial direction between the pair of rolling dies 35. Specifically, when the pair of rolling dies 35 rotate in the forward direction, the workpiece 31 moves to the other axial side as shown in FIG. 7(a), and when the pair of rolling dies 35 rotate in the reverse direction, the workpiece 31 moves to one axial side as shown in FIG. 7(b). The workpiece 31 is subjected to rolling while such movement of the workpiece 31 to the other axial side and to one axial side is alternately repeated.

[0086] In this example, the movement of the workpiece 31 toward the other axial side is stopped at the axial position shown in Fig. 7(a) by the NC control. The axial position shown in Fig. 7(a) is an axial position where the axial center of the rolling large diameter shaft portion 32 of the workpiece 31 is located on the other axial side of the axial center of the rolling teeth 36 of the pair of rolling dies 35. In particular, in this example, even in the axial position shown in Fig. 7(a), the chamfered portion 37 on one axial side of the pair of rolling dies 35 faces the outer peripheral surface of the first small diameter shaft portion 23 of the workpiece 31, and the chamfered portion 37 on the other axial side of the pair of rolling dies 35 faces the outer peripheral surface of the second small diameter shaft portion 24 of the workpiece 31. In other words, as the workpiece 31 moves axially to the other side, the chamfered portion 37 on the other axial side of the pair of rolling dies 35 does not move axially to a position facing the outer peripheral surface of the second rolling medium diameter shaft portion 34.

[0087] Similarly, the movement of the workpiece 31 to one axial side is stopped at the axial position shown in FIG. 7(b). The axial position shown in FIG. 7(b) is an axial position where the axial center of the rolling large diameter shaft portion 32 of the workpiece 31 is located on one axial side of the axial center of the rolling teeth 36 of the pair of rolling dies 35. In particular, in this example, even at this axial position, the chamfered portion 37 on one axial side of the pair of rolling dies 35 faces the outer circumferential surface of the first small diameter shaft portion 23 of the workpiece 31, and the chamfered portion 37 on the other axial side of the pair of rolling dies 35 faces the outer circumferential surface of the second small diameter shaft portion 24 of the workpiece 31. That is, with the movement of the workpiece 31 to one axial side, the chamfered portion 37 on one axial side of the pair of rolling dies 35 does not move in the axial direction to a position facing the outer circumferential surface of the first rolling medium diameter shaft portion 33. In addition, in the axial position shown in Figure 7 (b), each of the pair of rolling dies 35 is located on one axial side of the base end shaft portion 25 of the workpiece 31, and does not collide with the base end shaft portion 25.

[0088] In this example, the cutting process is performed until, as shown in Fig. 8(a), a first spiral rolling mark 27 is formed on the outer circumferential surface of the first medium-diameter shaft portion 33 while a rolling process is performed to form the male thread portion 26 on the outer circumferential surface of the large-diameter shaft portion 32 by the rolling teeth 36 of a pair of rolling dies 35, and until, as shown in Fig. 8(b), a second spiral rolling mark 28 is formed on the outer circumferential surface of the second medium-diameter shaft portion 34 while a rolling process is performed to form the male thread portion 26 on the outer circumferential surface of the large-diameter shaft portion 32 by the rolling teeth 36 of a pair of rolling dies 35. Fig. 9 shows a partially enlarged cross-sectional view of Fig. 8(b). In this example, the first rolling mark 27 and the second rolling mark 28 are formed in this manner, and therefore each of the first rolling mark 27 and the second rolling mark 28 becomes a spiral mark in phase with the extension line of the spiral curve that is the groove bottom line of the male thread portion 26.

[0089] In this example, from the start to the end of the cutting process, the chamfered portion 37 on one axial side of the pair of rolling dies 35 faces the outer peripheral surface of the first small diameter shaft portion 23 of the workpiece 31 with a gap therebetween, or is in contact with the outer peripheral surface to the extent that it does not ride up on the outer peripheral surface, i.e., the rolling marks are not formed on the outer peripheral surface, and the chamfered portion 37 on the other axial side of the pair of rolling dies 35 faces the outer peripheral surface of the second small diameter shaft portion 24 of the workpiece 31 with a gap therebetween, or is in contact with the outer peripheral surface to the extent that it does not ride up on the outer peripheral surface, i.e., the rolling marks are not formed on the outer peripheral surface, and a state is maintained. In other words, in this example, the outer diameter d1 of the first small diameter shaft portion 23 and the outer diameter d2 of the second small diameter shaft portion 24 are each set to a range less than 1.0 times the groove bottom diameter Da of the male thread portion 26 so that such a state is maintained.

[0090] If the outer diameter D1 of the first rolling medium-diameter shaft portion 33 and the outer diameter D2 of the second rolling medium-diameter shaft portion 34 are equal to or greater than the groove bottom diameter Da of the male thread portion 26, each of the first rolling mark 27 and the second rolling mark 28 will be a spiral mark formed continuously with the male thread portion 26. In contrast, if the outer diameter D1 of the first rolling medium-diameter shaft portion 33 and the outer diameter D2 of the second rolling medium-diameter shaft portion 34 are somewhat smaller than the groove bottom diameter Da of the male thread portion 26, the first rolling medium-diameter shaft portion 33 and the second rolling medium-diameter shaft portion 34 will not sufficiently support the pair of rolling dies 35 during rolling, causing the shaft to wobble, bend, break, or the like, and the first rolling mark 27 and the second rolling mark 28 may become discontinuous with the male thread portion 26.

[0091] After the cutting process is completed, the distance between the pair of rolling dies 35 is increased, and the completed screw shaft 17 is removed from the rolling machine.

[0092] According to the manufacturing method of the screw shaft 17 of this embodiment, it is possible to sufficiently ensure the shape precision of the male thread portion 26, and it is possible to precisely finish the flank surface of the male thread portion 26 not only in the axial middle portion which is a complete thread portion, but also in both axial end edge portions which are incomplete thread portions. This point will be specifically described below.

[0093] In this example, after the cutting process is started, initially, as shown in Figures 7(a) and 7(b), rolling processing is performed to form the male thread portion 26 with the rolling teeth 36 of a pair of rolling dies 35 in contact only with the outer peripheral surface of the rolling large diameter shaft portion 32 of the workpiece 31.

[0094] In this case, as shown in Fig. 7(a), when the axial center of the large diameter shaft portion 32 for rolling of the workpiece 31 is located on the other axial side of the axial center of the rolling teeth 36 of the pair of rolling dies 35 due to the step occurring in the workpiece 31, or as shown in Fig. 7(b), when the axial center of the large diameter shaft portion 32 for rolling of the workpiece 31 is located on one axial side of the axial center of the rolling teeth 36 of the pair of rolling dies 35 due to the step occurring in the workpiece 31, a bias occurs in the axial distribution of the rolling load acting between the pair of rolling dies 35 and the workpiece 31. Then, the amount of elastic deformation of the rolling disk supporting the pair of rolling dies 35 and the workpiece 31 changes by the amount of the bias in the axial distribution of the rolling load, and a relative displacement such as inclination tends to occur between the pair of rolling dies 35 and the workpiece 31. As a result, the machining accuracy of both axial ends of the male thread portion 26 during machining is low.

[0095] However, in the final stage of the cutting process, as shown in Figures 8(a) and 8(b), the rolling teeth 36 of the pair of rolling dies 35 come into contact not only with the outer circumferential surface of the large diameter rolling shaft portion 32, but also with the outer circumferential surfaces of the first medium diameter rolling shaft portion 33 and the second medium diameter rolling shaft portion 34. At this time, one axial end of the rolling teeth 36 of the pair of rolling dies 35 is supported in contact with the outer circumferential surface of the first medium diameter rolling shaft portion 33, and the other axial end of the rolling teeth 36 of the pair of rolling dies 35 is supported in contact with the outer circumferential surface of the second medium diameter rolling shaft portion 34. This makes it possible to prevent a large deviation in the axial distribution of the rolling load acting between the pair of rolling dies 35 and the workpiece 31. Therefore, in the final stage of the cutting process, the change in the amount of elastic deformation of the rolling disk supporting the pair of rolling dies 35 and the workpiece 31 can be kept sufficiently small. This makes it possible to make it difficult for relative displacement, such as tilt, to occur between the pair of rolling dies 35 and the workpiece 31. As a result, in the final stage of the cutting process, the machining accuracy of both axial ends of the male thread portion 26 can be improved. That is, the flank surface of the male thread portion 26 can be precisely finished not only in the axial middle portion, which is the complete thread portion, but also in both axial end edges, which are incomplete thread portions.

[0096] Furthermore, in this example, from the start to the end of the cutting process, the chamfered portion 37 on one axial side of the pair of rolling dies 35 faces the outer peripheral surface of the first small diameter shaft portion 23 of the workpiece 31 through a gap or is in contact with the outer peripheral surface to the extent that it does not ride up on the outer peripheral surface, and the chamfered portion 37 on the other axial side of the pair of rolling dies 35 faces the outer peripheral surface of the second small diameter shaft portion 24 of the workpiece 31 through a gap or is in contact with the outer peripheral surface to the extent that it does not ride up on the outer peripheral surface. That is, in this example, from the start to the end of the cutting process, the inclined surface constituting the chamfered portion 37 does not ride up on the outer peripheral surface of the workpiece 31. Therefore, when the inclined surface rides up on the outer peripheral surface of the workpiece 31, a large load does not act on the riding portion in a radially inward direction and in a direction opposite to the center of the rolling die 35 in the axial direction. Therefore, it is possible to avoid inconveniences such as excessive elongation and twisting of the workpiece 31 starting from the portion of the outer circumferential surface of the workpiece 31 where the inclined surface climbs up, resulting in shape errors such as tooth profile error, tooth trace error, and thread pitch error of the male thread portion 26, and reduced shape accuracy of the male thread portion 26. It is also possible to avoid the occurrence of radial burrs and bulges at one axial end of the first medium diameter shaft portion 21 and one axial end of the second medium diameter shaft portion 22. Therefore, from this perspective as well, the shape accuracy of the male thread portion 26 can be sufficiently ensured.

[0097] In addition, when the present invention is implemented, if the outer diameter D1 of the first rolling medium diameter shaft portion 33 and the outer diameter D2 of the second rolling medium diameter shaft portion 34 are made smaller than the groove bottom diameter Da of the male thread portion 26, the relative displacement such as the tilt occurring between the pair of rolling dies 35 and the workpiece 31 in the state shown in Figures 8(a) and 8(b) cannot be sufficiently suppressed if these outer diameters D1 and D2 are made too small. In this example, in order to prevent such inconvenience from occurring, when the outer diameter D1 of the first rolling medium diameter shaft portion 33 and the outer diameter D2 of the second rolling medium diameter shaft portion 34 are made smaller than the groove bottom diameter Da of the male thread portion 26, these outer diameters D1 and D2 are set to 0.93 times or more the groove bottom diameter Da of the male thread portion 26.

[0098] In this example, when the outer diameter D1 of the first rolling medium diameter shaft portion 33 and the outer diameter D2 of the second rolling medium diameter shaft portion 34 are made larger than the groove bottom diameter Da of the male thread portion 26, the reason for making these outer diameters D1 and D2 1.07 times or less the groove bottom diameter Da of the male thread portion 26 is to prevent each of the first rolling mark 27 and the second rolling mark 28 from screwing into the female thread portion 15 of the nut 13, that is, to prevent each of them from becoming an effective thread portion. In other words, if the outer diameters D1 and D2 are made larger than 1.07 times the groove bottom diameter Da of the male thread portion 26, there is a possibility that each of the first rolling mark 27 and the second rolling mark 28, which have poor shape accuracy as a thread portion, will screw into the female thread portion 15 of the nut 13. In order to prevent such a possibility, the outer diameters D1 and D2 are set to 1.07 times the groove bottom diameter Da of the male thread portion 26 or less.

[0099] In this example, when performing rolling to form the male thread portion 26, it is necessary to reduce the outer diameter d1 of the first small diameter shaft portion 23 and the outer diameter d2 of the second small diameter shaft portion 24 to such an extent that the chamfered portion 37 of the rolling die 35 does not ride up on the outer peripheral surfaces of the first small diameter shaft portion 23 and the second small diameter shaft portion 24. However, in this example, in order to ensure the strength of the outer diameter d1 of the first small diameter shaft portion 23 and the outer diameter d2 of the second small diameter shaft portion 24, the outer diameter d1 of the first small diameter shaft portion 23 and the outer diameter d2 of the second small diameter shaft portion 24 are each set to 0.9 times or more and less than 1.0 times, preferably 0.9 times or more and less than 1.0 times, the groove bottom diameter Da of the male thread portion 26.

[0100] When carrying out the present invention, the rolling trace length of the first rolling trace 27 and the second rolling trace 28 and the axial range in which the first rolling trace 27 and the second rolling trace 28 are formed can be set to any value. For example, the rolling trace length of the first rolling trace 27 and the second rolling trace 28 (L: the axial length of each of the first rolling trace 27 and the second rolling trace 28 when the screw rotates once) can be about 0.03 to 2.5 times, preferably about 0.5 to 2 times, the thread lead of the male threaded portion 26. That is, the axial length in which the first rolling trace 27 and the second rolling trace 28 are formed (L1 = the axial length of each of the first medium diameter shaft portion 21 and the second medium diameter shaft portion 22) can be about 0.03 to 2.5 times, preferably about 0.5 to 2 times, the thread pitch of the male threaded portion 26.

[0101] As described above, when the relationship between the groove bottom diameter Da of the male thread portion 26, the outer diameter D1 of the first rolling medium diameter shank portion 33, the outer diameter D2 of the second rolling medium diameter shank portion 34, the outer diameter d1 of the first small diameter shank portion 23, and the outer diameter d2 of the second small diameter shank portion 24 is expressed by a formula, it is preferable to adopt the following dimensional relationship when performing the rolling process to form the male thread portion 26. D1: Da-(Da×0.070) or more, Da+(Da×0.070) or less D2: Da-(Da×0.070) or more, Da+(Da×0.070) or less d1: Da-(Da×0.1) or more, less than Da d2: Da-(Da×0.1) or more, less than Da

[0102] Furthermore, when implementing the present invention, it is more preferable to adopt the following dimensional relationships for the groove bottom diameter Da of the male thread portion 26, the outer diameter D1 of the first rolling medium diameter shaft portion 33, the outer diameter D2 of the second rolling medium diameter shaft portion 34, the outer diameter d1 of the first small diameter shaft portion 23, and the outer diameter d2 of the second small diameter shaft portion 24. D1: Da+(Da×0.001 to 0.070) D2:Da+(Da×0.001x~0.070x) d1: Da-(Da×0.001 to 0.070) d2:Da-(Da×0.001x~0.070x) That is, in order to sufficiently suppress the inclination of the rolling die 35 with respect to the workpiece 31 during rolling, the outer diameters D1 and D2 are preferably larger than the groove bottom diameter Da, for example, "Da+(Da×0.001 times)" or more. In addition, in order to easily prevent the chamfered portion 37 of the rolling die 35 from riding on the outer peripheral surfaces of the first small diameter shaft portion 23 and the second small diameter shaft portion 24, the outer diameters d1 and d2 are preferably smaller than the groove bottom diameter Da, for example, "Da-(Da×0.001 times)" or less. In addition, in order to sufficiently ensure the rigidity of the first small diameter shaft portion 23 and the second small diameter shaft portion 24, the outer diameters d1 and d2 are preferably, for example, "Da-(Da×0.070 times)" or more.

[0103] In this case, h: tooth height of the male thread portion 26 (diameter height from the bottom of the thread groove to the top of the thread) p: pitch of the male thread portion 26 (axial distance between adjacent thread crests in the axial direction) L2: Axial dimension of each of the first small diameter shaft portion 23 and the second small diameter shaft portion 24 In this case, it is preferable that L2 is equal to or greater than h. Specifically, the axial dimension L2 of each of the first small diameter shaft portion 23 and the second small diameter shaft portion 24 is at least about h+p×(0.05 to 4 times), but may be longer than that. The axial dimension L2 is preferably about h+p×(0.50 to 2.00 times).

[0104] Furthermore, in the electric steering wheel position adjuster 1 of this embodiment, the entire male threaded portion 26 of the screw shaft 17 constituting the feed screw mechanism 12 is finished with high precision so as to function as a normal threaded portion. Therefore, when adjusting the longitudinal position of the steering wheel 2, the rod 14 can be displaced in the axial direction relative to the nut 13 to a position where the axial end edge of the male threaded portion 26 screws into the female threaded portion 15. This makes it possible to lengthen the operating stroke of the feed screw mechanism 12, in other words, to widen the adjustment range of the longitudinal position of the steering wheel 2.

[0105] Furthermore, in this example, in the rolling process, the chamfered portion 37 of the rolling die 35 does not ride up on the end portion on one axial side of the first medium diameter shaft portion 21 and the end portion on the other axial side of the second medium diameter shaft portion 22. Therefore, in the rolling process, there are no radial burrs or bulges caused by the chamfered portion 37 riding up on the end portion on one axial side of the first medium diameter shaft portion 21 and the end portion on the other axial side of the second medium diameter shaft portion 22. Therefore, even if the screw shaft 17 is moved in the axial direction to a position where the end portion on one axial side of the first medium diameter shaft portion 21 and the end portion on the other axial side of the second medium diameter shaft portion 22 enter the radial inside of the female thread portion 15 of the nut 13, the end portion on one axial side of the first medium diameter shaft portion 21 and the end portion on the other axial side of the second medium diameter shaft portion 22 can be prevented from interfering with the female thread portion 15. Therefore, from this point of view as well, the operating stroke of the feed screw mechanism 12 can be made longer.

[0106] In the above-mentioned embodiment, there is no portion of the threaded shaft 17 on one axial side of the first medium diameter shaft portion 21 that has an outer diameter larger than that of the first medium diameter shaft portion 21. That is, there is no portion of the workpiece 31 on one axial side of the first rolling medium diameter shaft portion 33 that has an outer diameter larger than that of the first rolling medium diameter shaft portion 33. For this reason, when the present invention is implemented and a threaded shaft is manufactured in a manner similar to the above-mentioned embodiment, even if the first small diameter shaft portion 23 is omitted, that is, even if the end portion on one axial side of the first medium diameter shaft portion 21 is made a free end, it is possible to prevent the chamfered portion 37 on one axial side of the pair of rolling dies 35 from riding on the outer peripheral surface of the first rolling medium diameter shaft portion 33 (first medium diameter shaft portion 21) during rolling processing. Therefore, when the present invention is implemented and a threaded shaft is manufactured in a manner similar to the above-mentioned embodiment, the first small diameter shaft portion 23 can also be omitted.

[0107] The present invention can also be applied to a screw shaft in which, for example, an adjacent shaft portion having an outer diameter larger than the outer diameter of the first medium diameter shaft portion 21 exists only on one axial side of the first medium diameter shaft portion 21, and an adjacent shaft portion having an outer diameter larger than the outer diameter of the second medium diameter shaft portion 22 does not exist on the other axial side of the second medium diameter shaft portion 22. In this case, the first small diameter shaft portion 23 is essential, but the second small diameter shaft portion 24 may not be essential. That is, in this case, the second small diameter shaft portion 24 can be provided, or the second small diameter shaft portion 24 can be omitted.

[0108] Further, in the above-described embodiment, the base end side shaft portion 25 having an outer diameter larger than the outer diameter of the second medium diameter shaft portion 22 is present on the other axial side of the second medium diameter shaft portion 22 in the screw shaft 17. That is, the base end side shaft portion 25 having an outer diameter larger than the outer diameter of the second medium diameter shaft portion 34 for rolling is present on the other axial side of the second medium diameter shaft portion 34 in the workpiece 31. At this time, in this embodiment, when manufacturing the screw shaft by the same method as the above-described embodiment, as shown in FIG. 2B, in the screw shaft, both the first small diameter shaft portion and the second small diameter shaft portion can be omitted, and the base end side shaft portion 25 can be disposed adjacent to the other axial side of the second medium diameter shaft portion 22. That is, in the workpiece, both the first small diameter shaft portion and the second small diameter shaft portion can be omitted, and the base end side shaft portion 25 can be disposed adjacent to the other axial side of the second medium diameter shaft portion 34 for rolling. In this case, it is preferable to appropriately control the outer diameter of the second rolling medium diameter shaft portion 34 (second medium diameter shaft portion 22) and set it small within the range where a spiral rolling mark is formed. This makes it possible to suppress a decrease in processing accuracy caused by the chamfered portion 37 on the other axial side of the pair of rolling dies 35 climbing up onto the outer peripheral surface of the second rolling medium diameter shaft portion (second medium diameter shaft portion 22).

[0109] In addition, it is preferable to arrange the base end side shaft portion 25 on the other axial side of the second medium diameter shaft portion 22 via the second small diameter shaft portion 24, that is, in the work, arrange the base end side shaft portion 25 on the other axial side of the second rolling medium diameter shaft portion 34 via the second small diameter shaft portion 24. If the second small diameter shaft portion 24 is arranged between the other axial side of the second medium diameter shaft portion 22 and the base end side shaft portion 25, it is possible to reliably prevent the chamfered portion 37 on the other axial side of the pair of rolling dies 35 from climbing up onto the outer circumferential surface of the second rolling medium diameter shaft portion 34 (second medium diameter shaft portion 22) during rolling processing. Therefore, when carrying out the present invention, it is even more preferable to arrange the second small diameter shaft portion 24 at least on one end when manufacturing a screw shaft by the same method as the above-mentioned embodiment.

[0110] Furthermore, when the first rolling medium diameter shaft portion 33 (first medium diameter shaft portion 21) is disposed on one axial side of the rolling large diameter shaft portion 32 (male thread portion 26), the first small diameter shaft portion 23 is disposed on this one axial side, and the second rolling medium diameter shaft portion 34 (second medium diameter shaft portion 22) is disposed on the other axial side of the rolling large diameter shaft portion 32 (male thread portion 26), and the second small diameter shaft portion 24 is disposed on this other axial side, it is possible to judge whether rolling is being performed normally by observing the surfaces of the first small diameter shaft portion 23 and the second small diameter shaft portion 24. In other words, when the rolling marks are left on the first small diameter shaft portion 23 or the second small diameter shaft portion 24, it indicates that the material that flows due to the rolling process does not escape to the small diameter shaft portion, and it can be judged that there is a significant adverse effect on the thread precision. On the other hand, if rolling is performed normally, there will be no rolling marks on the first small diameter shaft portion 23 and the second small diameter shaft portion 24. Therefore, the arrangement of the first small diameter shaft portion 23 and the second small diameter shaft portion 24 can also be used as a guide for finding defective products when manufacturing high-precision screws. Note that when a defect occurs, a spiral mark will not necessarily remain on the first small diameter shaft portion 23 or the second small diameter shaft portion 24, and a short scratch may be formed in either position.

[0111] As described above, when the screw shaft 17 according to the present embodiment has either the first small diameter shaft portion 23 or the second small diameter shaft portion 24 and has no rolling marks, it can be determined that rolling has been performed normally. In such a case, depending on the outer diameter of the medium diameter shaft portion, even if there is a slight deviation in the axial distribution of the rolling load acting between the pair of rolling dies 35 and the workpiece 31, there are cases where the first medium diameter shaft portion 21 and the second medium diameter shaft portion 22 do not have spiral rolling marks. Specifically, scratches may be formed on parts of the surfaces of the first medium diameter shaft portion 21 and the second medium diameter shaft portion 22 due to a slight deviation. Even in such a case, it is indicated that the first medium diameter shaft portion 21 and the second medium diameter shaft portion 22 suppress the runout of the shaft, and it can be determined that the machining accuracy of both axial ends of the male thread portion 26 is good.

[0112] The screw shaft 17 of this example can also be used by being incorporated into a device having a different configuration from that of this example. In this case, for example, the base end side shaft portion 25, which is the adjacent shaft portion, can be used as a portion to be combined with a driving member such as a motor. Specifically, for example, a serration portion can be formed on the outer circumferential surface of the base end side shaft portion 25, a gear made of synthetic resin can be injection molded to cover the serration portion, and the gear can be used to configure a reduction mechanism installed between the motor and the screw shaft 17. When such a configuration is adopted, since the outer diameter of the base end side shaft portion 25 is large, the amount of synthetic resin used as the material for the gear can be reduced. Therefore, the amount of synthetic resin used, which is an expensive material, can be reduced, thereby reducing manufacturing costs, and deformation of the synthetic resin during molding and use of the gear can be effectively prevented.

[0113] The rolling method of the screw shaft in the present invention is not limited to the in-feed method in the above-mentioned embodiment, and other rolling methods such as the through-feed method and the flat plate rolling method can be adopted. Among these, the through-feed method can be adopted as a rolling method for a screw shaft that does not have a large diameter portion having an outer diameter larger than the outer diameter of the medium diameter shaft portion on both axial sides, or a screw shaft that has a large diameter portion having an outer diameter larger than the outer diameter of the medium diameter shaft portion only on one axial side. When the through-feed method is adopted, it is not essential to provide a small diameter shaft portion at the tip portion, which is the axial end portion of the workpiece that starts to be inserted between the rolling dies. However, by providing a small diameter shaft portion at the tip portion of the workpiece, the small diameter portion can also be used as a guide portion, i.e., a guide portion, when inserting the workpiece between the dies. The in-feed method can be adopted as a rolling method for the same screw shaft as the through-feed method, and can also be adopted as a rolling method for a screw shaft 17a, for example, as shown in FIG. 10. The screw shaft 17a has a large diameter portion (adjacent shaft portion) having an outer diameter larger than that of the medium diameter shaft portion on each of both axial sides. Specifically, the screw shaft 17a has a large diameter portion 41a that is disposed adjacent to the first small diameter shaft portion 23 on one axial side and corresponds to a medium diameter shaft portion having an outer diameter larger than that of the first medium diameter shaft portion 21, and a large diameter portion 41b that is disposed adjacent to the second small diameter shaft portion 24 on the other axial side and corresponds to a medium diameter shaft portion having an outer diameter larger than that of the second medium diameter shaft portion 22.

[0114] When carrying out the manufacturing method of the screw shaft of the present invention, the number of rolling dies used in the rolling process can be three or more.

[0115] The present invention can be applied not only to a screw shaft constituting a sliding screw type feed screw mechanism, but also to a screw shaft constituting a ball screw type feed screw mechanism. In this case, the male thread of the screw shaft is a male thread groove. When the present invention is applied to a screw shaft constituting a sliding screw type feed screw mechanism, the shape of the thread of the male thread can be various shapes such as a triangular tooth shape, a trapezoidal tooth shape, an involute tooth shape, a serration tooth shape, etc. The present invention is not limited by the number of threads or the number of teeth of the serration.

[0116] The electric steering wheel position adjustment device of the present invention is applicable to devices of various structures that are conventionally known and are described in Japanese Patent Application Publication No. 2005-199760, Japanese Patent Application Publication No. 2006-321484, Japanese Patent Application Publication No. 2015-227166, etc., specifically, devices that make it possible to adjust at least one of the fore-aft position and the up-down position of the steering wheel.

[0117] The feed screw mechanism equipped with the screw shaft of the present invention can be incorporated and used in various mechanical devices, such as an electric position adjustment device for a steering wheel, an electric storage device for an automobile steering wheel or headlight, and a table movement device for a machine tool.

[0118] Although various embodiments have been described above with reference to the drawings, it goes without saying that the present invention is not limited to such examples. It is clear that a person skilled in the art can come up with various modified or revised examples within the scope of the claims, and it is understood that these also naturally belong to the technical scope of the present invention. Furthermore, the components in the above-mentioned embodiments may be arbitrarily combined within the scope of the invention.

[0119] This application is based on a Japanese patent application (Patent Application No. 2020-172548) filed on October 13, 2020, the contents of which are incorporated by reference into this application. [Explanation of symbols]

[0120] 1 Electric steering wheel position adjustment device 2 Steering wheel 3. Steering column 4 Steering shaft 5 Electric Actuator 6 Outer column 7 Innacolumn 8 Inner shaft 9 Outer tube 10. Bearings 11. Housing 12 Lead screw mechanism 13 Nut 14 Rod 15 Female thread 16 Worm reducer 17, 17a Screw shaft 18 Extension shaft 19 Arm 20 Large diameter shaft 21 First medium diameter shaft 22 Second medium diameter shaft 23 First small diameter shaft portion 24 Second small diameter shaft 25 Proximal shaft part 26 Male thread 27 First rolling mark 28 Second rolling mark 29 Flange 30 Fitting part 31 Work 32 Large diameter shaft for rolling 33 First rolling medium diameter shaft part 34 Second rolling medium diameter shaft section 35 Rolling dies 36 Rolled teeth 37 Chamfer 38 Chamfer 39 Chamfer 40 Groove 41a, 41b Large diameter part 100 Work 101 Rolling dies 102 Male thread 103 Screw shaft 104 Screw shaft 105 Proximal shaft part 106 Large diameter shaft for rolling 107 Small diameter shaft for rolling 108 Rolled teeth 109 Chamfered part

Claims

1. The method includes a rolling process that uses a plurality of rolling dies to form a male thread on an outer peripheral surface of the large diameter shaft portion for rolling, the rolling process comprising: a large diameter shaft portion for rolling; a medium diameter shaft portion for rolling that is arranged adjacent to the large diameter shaft portion for rolling in the axial direction and has an outer diameter smaller than that of the large diameter shaft portion for rolling; and a small diameter shaft portion that is arranged adjacent to the medium diameter shaft portion for rolling on the opposite side of the large diameter shaft portion for rolling in the axial direction and has an outer diameter smaller than that of the medium diameter shaft portion for rolling, in order to form a male thread on the outer peripheral surface of the large diameter shaft portion for rolling over the entire axial length of the workpiece; The workpiece is a workpiece in which the rolling medium diameter shaft portion is composed of a first rolling medium diameter shaft portion arranged on one axial side of the rolling large diameter shaft portion and a second rolling medium diameter shaft portion arranged on the other axial side of the rolling large diameter shaft portion, and the small diameter shaft portion is composed of at least one of a first small diameter shaft portion arranged on one axial side of the first rolling medium diameter shaft portion and a second small diameter shaft portion arranged on the other axial side of the second rolling medium diameter shaft portion, In the step of performing the rolling process on the workpiece, the rolling die is used to perform rolling to form a male thread portion on the outer peripheral surface of the large diameter shaft portion for rolling, and a spiral rolling mark is formed on the outer peripheral surface of the medium diameter shaft portion for rolling. Furthermore, by not bringing the rolling die into contact with the outer peripheral surface of the small diameter shaft portion, the axial length of the entire male thread portion corresponds to an effective thread length, As each of the rolling dies, a rolling die having a chamfered portion at an axial end of an outer circumferential surface is used, In the step of performing the rolling process on the workpiece, the rolling teeth provided on the outer peripheral surface of the rolling die face the large diameter shaft portion and the medium diameter shaft portion for rolling, and the chamfered portion faces the outer peripheral surface of the small diameter shaft portion via a gap. Manufacturing method of screw shaft.

2. 2. The method for manufacturing a screw shaft according to claim 1, wherein the workpiece is provided with an adjacent shaft portion that is arranged adjacent to the small diameter shaft portion on the opposite side of the large diameter shaft portion for rolling in the axial direction and has an outer diameter larger than the outer diameter of the medium diameter shaft portion for rolling.

3. The outer diameter of the rolling medium diameter shaft portion is set to 0.93 times or more and 1.07 times or less of the groove bottom diameter of the male thread portion to be formed on the outer peripheral surface of the rolling large diameter shaft portion, The method for manufacturing a threaded shaft according to claim 1 or 2, wherein the outer diameter of the small diameter shaft portion is 0.9 times or more and less than 1.0 times the groove bottom diameter of the male thread portion.

4. The small diameter shaft portion is composed of the first small diameter shaft portion and the second small diameter shaft portion, and the outer diameter of the first small diameter shaft portion is smaller than the outer diameter of the second small diameter shaft portion. The manufacturing method of the screw shaft according to any one of claims 1 to 3.

5. The method for manufacturing a screw shaft according to any one of claims 1 to 4, wherein the male screw portion is formed so that L2 is equal to or greater than h, where h is the tooth height of the male screw portion formed and L2 is the axial dimension of each of the first small diameter shaft portion and the second small diameter shaft portion.

6. The method for manufacturing a screw shaft according to any one of claims 1 to 5, wherein a screw shaft to be incorporated in an electric position adjustment device for a steering wheel is used as the screw shaft.

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