Shaft unit, manufacturing method therefor, and electric assist device

Abstract

Provided are a shaft unit capable of suppressing deformation of a center hole due to the formation of a caulked portion, a method for manufacturing the shaft unit, and an electric assist device equipped with the shaft unit. A shaft unit 56 has a shaft member 12 that has a coupling shaft portion 39 at one end in the axial direction and has a center hole 43a opened at one end surface of the coupling shaft portion 39 in the axial direction, and a joint member 25, which has a center hole 30 penetrating radially inward in the axial direction and in which the coupling shaft portion 39 is fitted into the center hole 30, wherein the coupling shaft portion 39 has a caulked portion 41 that is formed by plastically deforming only the radially outer part of the edge portion of the coupling shaft portion on one side in the axial direction, the caulked portion 41 being in surface contact with an opening peripheral edge 77 of the center hole 30 of the joint member 25 on one side in the axial direction.

Description

Axle unit, manufacturing method thereof, and electric assist device

[0001] The present disclosure relates to an axle unit formed by joining an axle member and a joint member with a crimped portion, a manufacturing method thereof, and an electric assist device configured to include the axle unit.

[0002] In the field of steering devices, electric power steering devices that are configured to apply auxiliary power to steering force transmission members such as a steering shaft, pinion shaft, and rack shaft in order to reduce the force required to operate the steering wheel are widely used. The electric power steering device is equipped with an electric assist device for applying the auxiliary power.

[0003] An electric assist device includes an electric motor as a power source, a worm that is driven to rotate by the electric motor, and a worm wheel that meshes with the worm. A conventional electric assist device structure includes a coupling that connects the output shaft of the electric motor and the worm to enable torque transmission while allowing for misalignment such as tilt and misalignment of the axes (see U.S. Pat. No. 1,1084,522).

[0004] The coupling includes a joint member connected to the output shaft of the electric motor and a joint member connected to one axial end of the worm gear. These joint members are connected directly or via an intermediate joint member to allow for torque transmission while allowing for misalignment between them.

[0005] U.S. Pat. No. 1,108,522 describes a specific structure of a shaft unit formed by connecting a shaft member made of a worm to a joint member. In this shaft unit, the shaft member has a connecting shaft portion at one axial end. The joint member has a central hole penetrating axially on the radially inner side, and the connecting shaft portion of the shaft member is fitted into this central hole. Furthermore, the shaft member has a center hole opening in the radial center of the end face on one axial side of the connecting shaft portion. In other words, the connecting shaft portion of the shaft member has a cylindrical end on one axial side.

[0006] An outward flange-shaped crimped portion is formed on one axial end edge of the connecting shaft portion by plastically deforming the entire end edge radially outward. The crimped portion presses down on the axially opening peripheral edge of the central hole of the joint member, thereby joining the shaft member and the joint member.

[0007] U.S. Pat. No. 1,108,4522

[0008] In the conventional shaft unit described above, when forming the crimped portion, the entire edge portion on one axial side of the connecting shaft portion of the shaft member is plastically deformed radially outward, which causes the center hole of the connecting shaft portion to deform.

[0009] The center hole is a part used when machining the outer surface of a shaft member, measuring bending and rotational runout of the shaft member, assembling the shaft member with other members, etc. Specifically, it is engaged with a center shaft for centering and / or rotation support. Therefore, if the center hole is deformed as a result of forming the crimped portion, there is a possibility that the center hole will not be able to be used effectively when performing the measurements and assembly.

[0010] This problem is not limited to the shaft unit made up of a worm and a joint member that constitutes an electric assist device, but also occurs in shaft units that constitute various types of mechanical devices.

[0011] The present disclosure aims to provide an axle unit that can suppress deformation of the center hole that occurs when a crimped portion is formed, a method for manufacturing the same, and an electric assist device that includes the axle unit.

[0012] A shaft unit according to one aspect of the present disclosure includes a shaft member having a coupling shaft portion at one axial end and a center hole opening in the radial center of the end face on one axial side of the coupling shaft portion, and a joint member having a central hole penetrating the axial direction on the radially inner side and into which the coupling shaft portion is fitted. The joint member is a member for connecting the shaft member and a mating shaft member adjacent to the shaft member so as to enable torque transmission.

[0013] In one embodiment of the shaft unit of the present disclosure, the connecting shaft portion has a crimped portion formed by plastically deforming only the radially outer portion of the end edge portion on one axial side protruding from the center hole, and the crimped portion is in surface contact with the opening peripheral portion on one axial side of the center hole of the joint member.

[0014] In one embodiment of the shaft unit of the present disclosure, at least the radially outer portion of the side surface on one axial side of the crimped portion is configured with an inclined surface portion that slopes in a direction toward the other axial side as it moves radially outward.

[0015] In one aspect of the shaft unit of the present disclosure, the opening peripheral edge of the joint member is configured with a chamfered portion that slopes radially outward as it extends toward one axial side.

[0016] In the shaft unit of one aspect of the present disclosure, the shaft member is constituted by a worm.

[0017] An electric assist device according to one aspect of the present disclosure includes a worm wheel, a worm meshed with the worm wheel, a coupling including a joint member connected to a connecting shaft portion provided at one axial end of the worm, and an electric motor transmitting torque to the worm via the coupling. The shaft unit including the worm and the joint member is configured by the shaft unit according to one aspect of the present disclosure.

[0018] In one embodiment of the method for manufacturing a shaft unit of the present disclosure, the shaft unit to be manufactured comprises a shaft member having a connecting shaft portion at one axial end and a center hole opening in the radial center of the end face on one axial end of the connecting shaft portion, and a joint member having a center hole penetrating axially on the radially inner side.

[0019] A method for manufacturing a shaft unit according to one aspect of the present disclosure includes a step of inserting the connecting shaft portion into the central hole of the joint member, and pressing the radially outer portion of the edge portion on one axial side of the connecting shaft portion protruding from the central hole toward the other axial side with the pressing surface of a pressing punch, thereby crushing the radially outer portion in the axial direction to form a crimped portion, and bringing the crimped portion into surface contact with the opening peripheral portion on one axial side of the central hole of the joint member.

[0020] A manufacturing method of a shaft unit according to one aspect of the present disclosure includes a step of press-fitting the joint member onto the outer peripheral surface of the connecting shaft portion from one axial side, wherein in the shaft unit, the inner peripheral surface of the central hole is formed by a serration portion, and the overall outer diameter of the portion of the connecting shaft portion that protrudes from the central hole of the joint member onto one axial side is equal to or smaller than the inner diameter of the serration portion, and in the step of press-fitting the joint member, the teeth of the serration portion are caused to bite into the outer peripheral surface of the connecting shaft portion, completing the press-fit of the joint member, thereby bringing the connecting shaft portion into a state of being internally fitted into the central hole of the joint member.

[0021] In one embodiment of the manufacturing method for the shaft unit of the present disclosure, in the process of forming the crimped portion, the radially outer portion of the pressing surface presses the side surface on one axial side of the crimped portion toward the other axial side and radially inward.

[0022] In one aspect of the manufacturing method for a shaft unit of the present disclosure, the shaft member is constituted by a worm.

[0023] The present disclosure can be implemented by appropriately combining the configurations of the above-described aspects within the scope of not causing any contradiction.

[0024] According to the present disclosure, it is possible to provide an axle unit that can suppress deformation of the center hole that occurs when a crimped portion is formed, a method for manufacturing the same, and an electric assist device that includes the axle unit.

[0025] FIG. 1 is a partially cutaway side view showing an electric power steering device to which an electric assist device according to a first example of an embodiment of the present disclosure is applied. FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1 , showing the electric assist device according to the first example. FIG. 3 is an enlarged view of section B in FIG. 2 , showing a connection between a worm and an output shaft of an electric motor in the electric assist device according to the first example. FIG. 4 is an exploded perspective view of a coupling comprising a worm, a worm-side joint member, an intermediate joint member, and a motor-side joint member at the connection in the electric assist device according to the first example. The shaft unit according to the first example is composed of a shaft member comprising a worm and a worm-side joint member. FIG. 5 is a perspective view showing one axial end of the shaft unit according to the first example. FIG. 6 is a cross-sectional view showing one axial end of the shaft unit according to the first example. FIG. 7 is an enlarged view of section C in FIG. 6 . FIG. 8 is a view showing one axial end of the shaft member according to the first example before the worm-side joint member is press-fitted onto the outer circumferential surface of the connecting shaft portion of the shaft member. Fig. 9 is a partial cross-sectional view showing a stage immediately before a worm side joint member is press-fitted onto the outer circumferential surface of a coupled shaft portion in a process for manufacturing the shaft unit of the first example. Fig. 10 is a partial cross-sectional view showing a stage after the worm side joint member has been press-fitted onto the outer circumferential surface of a coupled shaft portion in a process for manufacturing the shaft unit of the first example. Fig. 11 is a cross-sectional view of a pressing punch used in a method for manufacturing the shaft unit of the first example. Fig. 12 is an enlarged view of portion D in Fig. 11. Fig. 13 is a partial cross-sectional view showing a stage immediately before a crimped portion is formed in a process for manufacturing the shaft unit of the first example. Fig. 14 is a partial cross-sectional view showing a start stage of crimped portion formation in a process for manufacturing the shaft unit of the first example. Fig. 15 is an enlarged view of portion E in Fig. 14. Fig. 16 is a partial cross-sectional view showing a completion stage of crimped portion formation in a process for manufacturing the shaft unit of the first example. Fig. 17 is an enlarged view of portion F in Fig. 16. Fig. 18 is a partial cross-sectional view of one axial side portion of a shaft member (worm) constituting a shaft unit according to a second example of an embodiment of the present disclosure. Fig. 19(a) is a partial cross-sectional view showing a stage in which a worm-side joint member is press-fitted onto an outer circumferential surface of a coupled shaft portion of the shaft member in a process of manufacturing the shaft unit according to the second example, and Fig. 19(b) is a partial cross-sectional view showing a stage in which formation of a crimped portion is completed.

[0026] The present disclosure is applicable to various shaft units that are incorporated into various mechanical devices and are composed of a shaft member and a joint member. Examples of the shaft member include torque transmission shafts such as worms, sliding screw shafts, ball screw shafts, motor shafts, and steering shafts, as well as rack shafts and pinion shafts. The joint member is a member that connects the shaft member to an adjacent mating shaft member to enable torque transmission. Examples of the joint member include joint members that constitute various joint devices such as couplings, universal joints, and constant velocity joints.

[0027] 1 to 17, a first example of an embodiment of the present disclosure will be described. In the first example, the present disclosure is applied to a shaft unit including a worm and its joint member, which is incorporated into an electric assist device of an electric power steering device.

[0028] (1) Shaft Unit A first example of a shaft unit 56 is particularly shown in Figures 1 to 7. As shown in Figures 2 and 3, the shaft unit 56 of this example includes a shaft member 12 having a coupling shaft portion 39 at one axial end and a center hole 43a opening in the radial center of the end face on one axial side of the coupling shaft portion 39, and a joint member 25 having a center hole 30 penetrating axially on the radially inner side, with the coupling shaft portion 39 fitted into the center hole 30. The coupling shaft portion 39 has a crimped portion 41 formed by plastically deforming only the radially outer portion of the edge portion on one axial side protruding from the center hole 30, and the crimped portion 41 is in surface contact with an opening peripheral portion 77 on one axial side of the center hole 30 of the joint member 25.

[0029] With respect to the shaft unit 56, one axial side is the right side in FIGS. 2 and 3, and the other axial side is the left side in FIGS.

[0030] In this example, the shaft member 12 is made up of a worm, and the joint member 25 is made up of a worm-side joint member that is a part of the coupling 13.

[0031] The shaft member (worm) 12 of this example is made of metal and has worm teeth 22 on the outer peripheral surface of an axially intermediate portion. The constituent material and shape of the shaft member 12 are determined arbitrarily depending on the type and application of the shaft unit to which the present disclosure is applied. In this example, the connecting shaft portion 39 provided at one axial end of the shaft member 12 has a smaller outer diameter than the portion adjacent to it on the other axial end. That is, the shaft member 12 of this example includes, in order from one axial end, a fitting shaft portion 44 having a larger diameter than the connecting shaft portion 39 and a flange portion 45 protruding radially outward between the connecting shaft portion 39 and the worm teeth 22. The outer peripheral surfaces of the connecting shaft portion 39 and the fitting shaft portion 44 are connected by a stepped surface 40 facing one axial end.

[0032] In this example, the center hole 43a, which opens in the radial center of one axial end face of the coupling shaft portion 39, is located at the end portion on one axial side of the coupling shaft portion 39 and is arranged to overlap the crimped portion 41 in the radial direction. The portion of the end portion on one axial side of the coupling shaft portion 39 that surrounds the center hole 43a is formed by a cylindrical portion 76. The center hole 43a is a portion used when processing the outer peripheral surface of the shaft member 12, measuring bending and rotational runout of the shaft member 12, and assembling the shaft member 12 with other members. Specifically, a center shaft for centering and / or rotation support engages with the center hole 43a.

[0033] The shaft member 12 of this example also has a center hole 43b at the other axial end, which opens to the radial center of the end surface on the other axial end.

[0034] In this example, the portion of the outer circumferential surface of the connecting shaft 39 that is fitted into the central hole 30 of the joint member (worm-side joint member) 25 is configured as a cylindrical surface. In this example, as shown in Figure 8 and other figures, an escape groove 71 is provided around the entire periphery of the outer circumferential surface of the connecting shaft 39 at the other axial end that is located on the other axial side of the portion that is fitted into the central hole 30 of the joint member 25. The escape groove 71 is a portion that allows the blade of a cutting tool to escape when manufacturing the worm 12. When implementing the present disclosure, the escape groove 71 can be omitted if it is not necessary.

[0035] In this example, the joint member 25 is made of metal. The constituent material and shape of the joint member 25 are determined arbitrarily depending on the type and use of the axle unit to which the present disclosure is applied. The central hole 30 of the joint member 25 is provided radially inside a cylindrical portion 31 that constitutes the radially inner portion of the joint member 25. In this example, the inner peripheral surface of the central hole 30 is formed by a serration portion 34.

[0036] 4 to 6 , the joint member 25 of this example includes a circular flange 32 that protrudes radially outward from an axially intermediate portion of a cylindrical portion 31, and teeth 33 that protrude toward one axial side from a plurality of locations (four locations in this example) that are equally spaced circumferentially on the radially outer side of the flange 32. The radial thickness of the cylindrical portion 31 is greater in a portion located on the other axial side of the flange 32 than in a portion located on the one axial side of the flange 32.

[0037] In this example, the joint member 25 is formed by subjecting a metal material to necessary processes such as forging and cutting, and then is subjected to heat treatment on the entire surface, thereby making it harder than the worm 12. In other words, the serration portion 34 is harder than the outer circumferential surface of the connecting shaft portion 39.

[0038] The joint member 25 is press-fitted onto the outer peripheral surface of the connecting shaft portion 39 from one axial side, and the end face of the tubular portion 31 of the joint member 25 on the other axial side abuts against the stepped surface 40. In this example, as a result of the press-fitting of the joint member 25, the teeth of the serration portion 34 (tooth tips of the tooth portion 33) bite into the outer peripheral surface of the connecting shaft portion 39.

[0039] In this example, a large-diameter portion 37 having an inner diameter larger than the inner peripheral surface of the center hole 30 is provided at the opening peripheral edge 78 on the other axial side of the center hole 30 of the joint member 25. The large-diameter portion 37 forms a space between the outer peripheral surface of the coupling shaft portion 39 and the large-diameter portion 37 to collect shavings generated during the press-fitting. The large-diameter portion 37 is configured with a conical surface such as a C-chamfered portion that slopes radially outward toward the other axial side. However, when implementing the present disclosure, the large-diameter portion 37 may also be configured with a curved surface such as an R-chamfered portion, or a cylindrical surface with a diameter larger than the inner peripheral surface of the center hole.

[0040] In this example, the opening peripheral edge 77 on one axial side of the central hole 30 of the joint member 25 is configured with a chamfered portion 35 that is inclined radially outward toward one axial side, as shown in Figures 6 and 7. That is, the inner peripheral surface of the central hole 30 and the end face 36 on one axial side of the tubular portion 31 are connected via the chamfered portion 35. In this example, the chamfered portion 35 is configured with a linearly inclined C-chamfer. However, when implementing the present disclosure, the chamfered portion 35 that constitutes the opening peripheral edge 77 of the joint member 25 can also be configured with a curvedly inclined R-chamfer or a chamfer whose inclination angle gradually increases toward one axial side.

[0041] In this example, the outer diameter of the chamfered portion 35, i.e., the inner diameter of the end face 36 on one axial side of the cylindrical portion 31, is larger than the groove bottom diameter of the serration portion 34. Therefore, the end of the groove on one axial side of the serration portion 34 opens only to the chamfered portion 35, and does not open to the end face 36 on one axial side of the cylindrical portion 31.

[0042] The engagement relationship between the shaft member 12 and the joint member 25 is determined arbitrarily depending on the type and application of the shaft unit to which the present disclosure is applied. For example, when the present disclosure is applied to a shaft unit constituted by a worm and its joint member, the inner circumferential surface of the center hole of the joint member can be formed as a cylindrical surface, and the portion of the outer circumferential surface of the coupled shaft portion that is fitted into the center hole of the worm-side joint member can be formed as a serrated portion, and the hardness of the worm-side joint member can be made lower than that of the coupled shaft portion.

[0043] In addition, as for the engagement relationship between the shaft member and the joint member, depending on the constituent material, type, and application of the shaft unit, it is also possible to apply press-fitting between cylindrical surfaces, knurled engagement, key engagement, spline engagement, serration engagement, etc., as long as the relative rotation between the shaft member and the joint member can be effectively prevented.

[0044] 6 and 7 , the crimped portion 41, formed by plastically deforming only the radially outer portion of the edge portion on one axial side of the coupling shaft portion 39, is in surface contact with the chamfered portion 35 over the entire circumference. That is, the crimped portion 41 protrudes radially outward from a portion of the outer circumferential surface of the coupling shaft portion 39 adjacent to the other axial side of the crimped portion 41. The side surface on the other axial side of the crimped portion 41 is formed by an annular surface that matches the chamfered portion 35 and is inclined radially outward toward the one axial side, and is in surface contact with the chamfered portion 35 over the entire circumference. In this example, the side surface on the other axial side of the crimped portion 41 is in surface contact with the chamfered portion 35 over the entire circumference without any gaps.

[0045] In this example, the opening on one axial side of the groove of the serration portion 34 is entirely closed by the crimped portion 41 .

[0046] In this example, the joint member 25 is fixed to the outside of the connecting shaft portion 39 while being clamped from both axial sides by the stepped surface 40 and the crimping portion 41 of the shaft member 12, and the crimping portion 41 prevents the joint member 25 from falling off from the connecting shaft portion 39 to one side in the axial direction.

[0047] In particular, in this example, the crimping portion 41 is in surface contact with the opening peripheral portion 77 (chamfered portion 35) of the joint member 25 over the entire circumference, and therefore, compared to the structure of the comparative example in which the crimping portion is in only line contact with the opening peripheral portion (corner portion) of the center hole over the entire circumference, the crimping portion 41 can firmly hold the joint member 25 from one axial side. Therefore, the crimping portion 41 can effectively prevent the joint member 25 from falling off the connecting shaft portion 39 to one axial side.

[0048] In this example, the joint member 25 and the shaft member 12 can be effectively prevented from rotating relative to each other by utilizing not only the engagement force acting on the fitting portion between the serration portion 34 of the joint member 25 and the outer peripheral surface of the connecting shaft portion 39 of the shaft member 12, and the friction force acting on the abutment portion between the end face on the other axial side of the tubular portion 31 of the joint member 25 and the step surface 40 of the shaft member 12, but also the large friction force acting on the surface contact portion between the opening peripheral portion 77 of the joint member 25 and the crimped portion 41 of the shaft member 12.

[0049] In particular, in the structure of this example, the contact area between the crimped portion 41 and the opening periphery 77 (chamfered portion 35) can be made larger than in the structure of the comparative example, in which the crimped portion makes line contact. This ensures a large frictional force acting on the contact portion, effectively preventing relative rotation between the shaft member 12 and the joint member 25.

[0050] When implementing the present disclosure, it is also possible to omit the chamfered portion 35 of the opening peripheral edge portion 77 of the joint member 25. In this case, the opening peripheral edge portion with which the crimping portion 41 comes into surface contact is the radially inner end portion of the end face on one axial side of the joint member 25.

[0051] 6 and 7 , at least the radially outer portion of the side surface on one axial side of the crimped portion 41 is configured with an inclined surface portion 42 that inclines toward the other axial side as it extends radially outward. In this example, only the radially outer portion of the side surface on one axial side of the crimped portion 41 is configured with the inclined surface portion 42. However, when implementing the present disclosure, the entire side surface on one axial side of the crimped portion 41 may be configured with the inclined surface portion 42. The inclined surface portion 42 is a plastically processed surface formed by pressing a pressure punch 60 (see FIG. 17 ) to form the crimped portion 41.

[0052] That is, in this example, when forming the crimped portion 41, the radially outer portion of the side surface on one axial side of the crimped portion 41, where at least the inclined surface portion 42 is formed, is pressed toward the other axial side and radially inward by the pressing punch 60, thereby applying compressive stress to the crimped portion 41. For this reason, cracks are less likely to occur in the crimped portion 41 than when the crimped portion is formed by pressing the entire side surface on one axial side of the crimped portion only toward the other axial side with the pressing punch, or when the crimped portion is formed by expanding the entire end edge portion on one axial side of the connecting shaft portion radially outward.

[0053] In this example, the generatrix shape (cross-sectional shape) of the inclined surface portion 42 is linear. The inclination angle θ of the inclined surface portion 42 with respect to an imaginary plane perpendicular to the axial direction of the worm 12 can be set arbitrarily within a range greater than 0 degrees. However, from the viewpoint of ensuring the crack suppression effect of the crimped portion 41, the inclination angle θ is preferably set to 15 degrees or more, and from the viewpoint of suppressing the processing force required to form the crimped portion 41, the inclination angle θ is preferably set to 45 degrees or less.

[0054] When implementing the present disclosure, the generatrix shape (cross-sectional shape) of the inclined surface portion 42 that forms the side surface on one axial side of the crimped portion 41 can also be a curved shape.

[0055] When implementing the present disclosure, one axial side surface of the crimping portion 41 may be configured as a flat surface perpendicular to the axial direction of the shaft member 12 .

[0056] (2) Manufacturing Method of Shaft Unit A manufacturing method of the shaft unit 56 of the first example is particularly shown in Figures 8 to 17. The manufacturing method of the shaft unit 56 of this example is characterized in that it includes a step of, with the coupling shaft portion 39 fitted into the central hole 30 of the joint member 25, pressing the radially outer portion of the edge portion on one axial side of the coupling shaft portion 39 protruding from the central hole 30 toward the other axial side with the pressing surface 61 of a pressing punch 60 to crush the radially outer portion in the axial direction to form a crimped portion 41, and bringing the crimped portion 41 into surface contact with the opening peripheral portion 77 on one axial side of the central hole 30 of the joint member 25.

[0057] When manufacturing the shaft unit 56, optional preparatory steps can include a step of obtaining the intermediate shaft member 12a and the joint member 25, and a step of press-fitting the joint member 25 onto the outer peripheral surface of the connecting shaft portion 39a of the intermediate shaft member 12a from one axial side.

[0058] First, the intermediate shaft member 12a before the connecting shaft portion 39a is fitted into the center hole 30 of the joint member 25 is obtained by any material and processing means depending on the constituent material and shape of the shaft member 12. In this example, the intermediate shaft member 12a is obtained by performing necessary processing such as forging or cutting on a metal material.

[0059] The intermediate shaft member 12a differs from the completed shaft member 12 only in the shape of the outer circumferential surface of the coupling shaft portion 39a. Specifically, no crimped portion 41 is formed at one axial end of the outer circumferential surface of the coupling shaft portion 39a. In this example, as shown in Figure 8, the outer circumferential surface of the coupling shaft portion 39a has a cylindrical surface portion 69 in the axial middle portion, a guide surface portion 70 at one axial end adjacent to one axial side of the cylindrical surface portion 69, and an escape groove 71 running along the entire periphery at the other axial end adjacent to the other axial end of the cylindrical surface portion 69.

[0060] The cylindrical surface portion 69 is configured by a cylindrical surface whose outer diameter does not change in the axial direction. In this example, the outer circumferential surface of the connecting shaft portion 39a has an axial range L 69 is the axial range L of the part that is fitted into the center hole 30 of the joint member 25 after the shaft unit 56 is completed. 30 Including (L 69 ⊂ L30 ).

[0061] The outer diameter of the cylindrical surface portion 69 is larger than the inner diameter of the serration portion 34 of the joint member 25. More specifically, the outer diameter of the cylindrical surface portion 69 is larger than the tip diameter of the serration portion 34 and smaller than the root diameter of the serration portion 34.

[0062] The guide surface portion 70 is configured with a conical surface whose outer diameter decreases toward one axial side. The guide surface portion 70 functions as a guide when the joint member 25 begins to be press-fitted onto the outer peripheral surface of the connecting shaft portion 39a from one axial side. When implementing the present disclosure, the guide surface portion 70 may be omitted.

[0063] Furthermore, the joint member 25 can be obtained by any material and processing method depending on the constituent material and shape of the joint member 25. In this example, the joint member 25 is obtained by subjecting a metal material to necessary processing such as forging or cutting, and heat treatment. Alternatively, the joint member 25 can be obtained by sintering metal powder, provided that the necessary mechanical properties, such as hardness, can be ensured.

[0064] Next, the joint member 25 is press-fitted onto the outer circumferential surface of the connecting shaft portion 39a of the intermediate shaft member 12a from one axial side. In the process of press-fitting the joint member 25, first, as shown in Fig. 9, the intermediate shaft member 12a is supported by a support base 57 with one axial end facing upward. Specifically, the outer circumferential surface of the intermediate shaft member 12a is fitted into a support hole 58 provided in the support base, and the side surface on the other axial side of the flange portion 45 provided on the intermediate shaft member 12a is supported by a support surface 59 of the support base 57, thereby preventing radial displacement of the intermediate shaft member 12a and displacement toward the other axial side.

[0065] In this example, the worm teeth 22 of the intermediate shaft 12 a are fitted into the support holes 58 of the support base 57, and the conical convex side surface of the flange 45 of the intermediate shaft 12 a is supported by a conical concave support surface 59 provided on the peripheral edge of the opening on one axial side of the support hole 58 of the support base 57.

[0066] Then, using a cylindrical press-fitting jig 72, the joint member 25 is press-fitted onto the outer circumferential surface of the connecting shaft portion 39a of the intermediate shaft member 12a from one axial side.

[0067] In this example, the press-fitting of the joint member 25 is performed with the ball bearing 46 and the worm damper 53 on the other axial side arranged around one axial side portion of the intermediate shaft member 12a, which is located above the support base 57, and with the worm damper 53 on one axial side arranged around the end portion on the other axial side of the tubular portion 31 of the joint member 25.

[0068] More specifically, first, as shown in FIG. 9, a cylindrical center shaft 65 is inserted without any radial play into the joint member 25 having the worm damper 53 on one axial side disposed around the other axial end of the tubular portion 31, and into a press-fit jig 72 disposed adjacent to and above the joint member 25, and then a conical engaging portion 73 provided in the radial center portion of the lower end of the center shaft 65 is engaged with the center hole 43 a on one axial side of the intermediate shaft member 12 a.

[0069] As a result, the worm damper 53 on one axial side, the joint member 25, and the press-fit jig 72 are arranged above the intermediate shaft member 12a coaxially with the intermediate shaft member 12a.

[0070] 9 and 10 , the press-fitting jig 72 presses the end face 36 on one axial side of the tubular portion 31 of the joint member 25, thereby press-fitting the joint member 25 from one axial side onto the outer circumferential surface of the connecting shaft portion 39a, and the end face on the other axial side of the tubular portion 31 of the joint member 25 abuts against the stepped surface 40 of the intermediate shaft member 12a. In this example, as a result of this press-fitting, the teeth (tooth tips of the tooth portion 33) of the serration portion 34 of the worm-side joint member 25 are caused to bite into a cylindrical surface portion 69 provided in the axial middle portion of the outer circumferential surface of the connecting shaft portion 39a.

[0071] In this example, if shavings are generated on the outer peripheral surface of the connecting shaft portion 39a due to this press-fitting, the shavings will accumulate in the space between the outer peripheral surface of the connecting shaft portion 39a and the large diameter portion 37 of the joint member 25.

[0072] In this example, as shown in FIG. 10 , with the joint member 25 press-fitted onto the outer peripheral surface of the connecting shaft portion 39 a from one axial side, the end portion on one axial side of the connecting shaft portion 39 a (including the end portion on one axial side of the cylindrical surface portion 69) protrudes from the center hole 30 of the joint member 25 to one axial side.

[0073] Next, by completing the step of press-fitting the joint member 25, the connecting shaft portion 39 is fitted into the center hole 30 of the joint member 25, and then a step of forming the crimped portion 41 is carried out using a pressing punch 60 as shown in Figures 11 to 13. The pressing punch 60 is configured to be cylindrical as a whole, and is provided with a pressing surface 61 on the radially inner side of the lower end surface in the axial direction.

[0074] In this example, the radially inner portion of the pressing surface 61 is configured with a flat surface portion 62 perpendicular to the axial direction of the pressing punch 60, and the radially outer portion of the pressing surface 61 is configured with an inclined surface portion 63 that inclines downward as it extends radially outward. The radially outer end of the flat surface portion 62 and the radially inner end of the inclined surface portion 63 are smoothly connected. The central hole of the pressing punch 60 has a guide hole portion 64 with a smaller diameter than the axially lower end portion, located in a portion from the axially middle portion to the axially upper end portion.

[0075] 13, in the process of forming the crimped portion 41 using the press punch 60, first, the conical engaging portion 73a provided at the lower end of the center shaft 65a, which is inserted without radial play through the guide hole portion 64 of the press punch 60, is engaged with the center hole 43a on one axial side of the intermediate shaft member 12a. As a result, the press punch 60 is positioned coaxially with the intermediate shaft member 12a above the intermediate shaft member 12a.

[0076] Next, the pressing punch 60 is moved downward along the center axis 65a from the state shown in Figure 13 to the state shown in Figures 14 and 15, and the flat surface portion 62 of the pressing surface 61 of the pressing punch 60 is brought into contact with the radially outer portion of the end face on one axial side of the connecting shaft portion 39a.

[0077] 14 and 15 to the state shown in Figures 16 and 17, the pressing punch 60 is further moved downward along the center axis 65a, whereby the flat surface portion 62 of the pressing surface 61 of the pressing punch 60 presses the radially outer portion of the edge portion on one axial side of the connecting shaft portion 39a toward the other axial side. This causes the radially outer portion to be crushed in the axial direction, forming the crimped portion 41.

[0078] Specifically, the radially outer portion is crushed in the axial direction, and the material of the radially outer portion flows radially outward along the flat surface portion 62 and the inclined surface portion 63 of the pressing surface 61 of the pressing punch 60, thereby forming the crimped portion 41. Then, the crimped portion 41 is brought into surface contact with the chamfered portion 35 of the joint member 25 over the entire circumference.

[0079] In particular, in this example, when forming the crimped portion 41 in this manner, the inclined surface portion 63 of the pressing surface 61 of the pressing punch 60 presses the radially outer portion of the side surface on one axial side of the crimped portion 41 toward the other axial side and radially inward, thereby applying compressive stress to the crimped portion 41. The side surface on one axial side of the crimped portion 41 becomes the inclined surface portion 42, which is a plastically processed surface, formed by pressing the inclined surface portion 63 of the pressing surface 61.

[0080] In this example, when forming the crimped portion 41, only the radially outer portion of the edge portion on one axial side of the connecting shaft portion 39a is pressed and plastically deformed by the pressing surface 61 of the pressing punch 60, so deformation of the center hole 43a can be substantially prevented, unlike when forming the crimped portion by expanding the entire edge portion on one axial side of the connecting shaft portion radially outward. Therefore, after forming the crimped portion 41, the center hole 43a can be effectively used when measuring bending and rotational runout of the shaft member 12, and when assembling the shaft member 12 with other members.

[0081] In this example, when forming the crimped portion 41, the radially outer portion of the side surface on one axial side of the crimped portion 41 is pressed toward the other axial side and radially inward, thereby applying compressive stress to the crimped portion 41 and making it less likely for cracks to occur in the crimped portion 41. However, when implementing the present disclosure, when forming the crimped portion 41, the side surface on one axial side of the crimped portion 41 can also be pressed toward the other axial side by a flat portion perpendicular to the axial direction.

[0082] When implementing the present disclosure, it is also possible to employ a configuration in which the crimping portion 41 contacts only a plurality of circumferentially spaced apart locations on the opening peripheral edge 77 (e.g., the chamfered portion 35) on one axial side of the central hole 30 of the joint member 25. In this case, the processing load on the crimping portion can be reduced, making manufacturing easier.

[0083] When the present disclosure is implemented, the press working of the crimped portion 41 can be performed in multiple steps. In this case, the processing load per step can be reduced, making manufacturing easier.

[0084] (3) Electric power steering device As shown in FIG. 1, the electric power steering device 1 of this example includes a steering wheel 2, a steering shaft 3, a steering column 4, a pair of universal joints 5a, 5b, an intermediate shaft 6, a steering gear unit 7, and an electric assist device 8 to which the shaft unit 56 of this example is applied.

[0085] The steering wheel 2 is fixedly supported at the rear end of a steering shaft 3. The steering shaft 3 is rotatably supported inside a steering column 4 that is supported on the vehicle body. The front end of the steering shaft 3 is connected to a pinion shaft 9 of a steering gear unit 7 via a rear universal joint 5a, an intermediate shaft 6, and a front universal joint 5b.

[0086] Therefore, when the driver rotates the steering wheel 2, the rotation of the steering wheel 2 is transmitted to the pinion shaft 9 via the steering shaft 3, the pair of universal joints 5a, 5b, and the intermediate shaft 6. The rotation of the pinion shaft 9 is converted into linear motion of a rack shaft (not shown) of the steering gear unit 7 that meshes with the pinion shaft 9. As a result, the pair of tie rods 10 are pushed and pulled, and a steering angle corresponding to the amount of rotation of the steering wheel 2 is applied to the left and right steered wheels. The electric assist device 8 is a device that generates auxiliary power to reduce the force required for the driver to operate the steering wheel 2.

[0087] 2 and 3, the electric assist device 8 of this example includes a worm wheel 11, a worm 12, a coupling 13, and an electric motor 14. In the electric assist device 8, the worm 12 is the shaft member 12 that constitutes the shaft unit 56 of this example.

[0088] In this example, the electric assist device 8 transmits the rotational torque of the output shaft 23 of the electric motor 14 to the worm 12 via the coupling 13, and then the rotational torque is amplified by a worm reducer formed by meshing the worm 12 with the worm wheel 11 before being applied to the steering shaft 3.

[0089] The electric assist device 8 of this example further includes a housing 15 supported and fixed to the front end of the steering column 4. The housing 15 is a cast product of an iron-based alloy, a die-cast product of a light alloy such as aluminum, or an injection-molded product of a synthetic resin, and includes a wheel accommodating portion 16 and a worm accommodating portion 17 that is disposed in a twisted position with respect to the wheel accommodating portion 16 and has an axially intermediate portion that opens into the wheel accommodating portion 16.

[0090] The wheel accommodating portion 16 is supported and fixed to the front end portion of the steering column 4 so that its central axis is coaxial with the central axis of the steering column 4 .

[0091] The worm housing 17 is cylindrical and has openings at both axial ends. In the following description, with respect to the worm housing 17 and the components housed in the worm housing 17, one axial side is the right side in Figures 2 and 3, and the other axial side is the left side in Figures 2 and 3.

[0092] An opening on one axial side of the worm accommodating portion 17 is closed by the electric motor 14 supported and fixed to the housing 15. An opening on the other axial side of the worm accommodating portion 17 is closed by a lid 18.

[0093] The worm wheel 11 has wheel teeth 19 on its outer circumferential surface, and is rotatably supported inside the wheel accommodating portion 16. In this example, the worm wheel 11 is supported and fixed around the periphery of the front end of the steering shaft 3, which is rotatably supported inside the wheel accommodating portion 16, so as to rotate integrally with the steering shaft 3. The worm wheel 11 in this example is formed by connecting and fixing a gear portion 21 made of synthetic resin and having wheel teeth 19 on its outer circumferential surface to the periphery of a metal, circular disk-shaped core portion 20.

[0094] The worm 12 is rotatably supported inside the worm housing portion 17 by a pair of ball bearings 46, 54. One axial end of the worm 12 is connected to the tip of the output shaft 23 of the electric motor 14 via a coupling 13 so as to be capable of transmitting torque and swinging.

[0095] A biasing mechanism 55 including an elastic body such as a coil spring or a leaf spring is mounted between the outer peripheral surface of a ball bearing 54 that supports the other axial end of the worm 12 and the inner peripheral surface of the worm accommodating portion 17. The biasing mechanism 55 elastically biases the worm teeth 22 of the worm 12 toward the wheel teeth 19 of the worm wheel 11. This configuration suppresses backlash between the worm teeth 22 and the wheel teeth 19, thereby suppressing the generation of teeth rattle noise.

[0096] The electric assist device 8 in this example is disposed in a position where it applies auxiliary power to the steering shaft 3. However, the present disclosure can also be applied to an electric assist device disposed in a position where it applies auxiliary power to the pinion shaft or rack shaft of a steering gear unit.

[0097] The coupling 13 includes a worm-side joint member 25, a motor-side joint member 24 that is fixedly coupled to the tip of the output shaft 23 of the electric motor 14, and an intermediate joint member 26 that connects the motor-side joint member 24 and the worm-side joint member 25 to enable torque transmission while allowing for misalignment between them. The entire coupling 13 is housed inside the worm housing portion 17. In the electric assist device 8 of this example, the joint member 25 of this example is applied to the worm-side joint member 25 that constitutes the coupling 13. Therefore, the worm 12 and the worm-side joint member 25 constitute the shaft unit 56 of this example.

[0098] The motor-side joint member 24 is a metal member and includes a cylindrical tube portion 27 that is fitted and fixed to the tip end portion of the output shaft 23 of the electric motor 14, a circular ring-shaped flange portion 28 that protrudes radially outward from an axially intermediate portion of the tube portion 27, and teeth 29 that protrude toward the other axial side from a plurality of equally spaced locations (four locations in this example) on the radially outer side of the flange portion 28. The other axial end portions (tips) of these teeth 29 are located on the other axial side of the tube portion 27.

[0099] The intermediate joint member 26 is configured in an annular shape as a whole. The intermediate joint member 26 has insertion holes 38a at multiple locations (four locations in this example) on one axial side at equal circumferential intervals, into which the teeth 29 of the motor-side joint member 24 can be inserted, and has insertion holes 38b at multiple locations (four locations in this example) on the other axial side at equal circumferential intervals, into which the teeth 33 of the worm-side joint member 25 can be inserted. In this example, the intermediate joint member 26 is formed by combining an annular core material made of synthetic resin or metal with a rubber cushioning material. The portions of the inner circumferential surfaces of the insertion holes 38a, 38b that come into contact with the teeth 29, 33 when torque is not being transmitted are made of the cushioning material.

[0100] The plurality of teeth 29 of the motor-side joint member 24 are inserted into the plurality of insertion holes 38a of the intermediate joint member 26 from one axial side, and the plurality of teeth 33 of the worm-side joint member 25 are inserted into the plurality of insertion holes 38b of the intermediate joint member 26 from the other axial side. In this way, the motor-side joint member 24 and the worm-side joint member 25 are connected via the intermediate joint member 26 to enable torque transmission while allowing for misalignment between them.

[0101] In this example, the tip of the output shaft 23 of the electric motor 14 and the end of one axial side of the worm 12 are connected via a coupling 13 so that torque can be transmitted, thereby enabling the worm 12 to oscillate relative to the output shaft 23.

[0102] It should be noted that when implementing the present disclosure, the structure of the coupling is not limited to the structure of this example. For example, the coupling may be configured to directly connect the motor-side joint member and the worm-side joint member without an intermediate joint member, allowing torque transmission while allowing misalignment between them.

[0103] In this example, the fitting shaft portion 44 of the worm 12 is rotatably supported by a ball bearing 46 relative to the worm accommodating portion 17 .

[0104] The ball bearing 46 includes an outer ring 47 , an inner ring 48 , and a plurality of balls 49 .

[0105] The outer ring 47 is fitted into a cylindrical surface portion 50 provided on the inner peripheral surface of one axial end of the worm accommodating portion 17. In this state, the outer ring 47 is sandwiched from both axial sides by a stepped surface 51 facing one axial side that is present in a portion of the worm accommodating portion 17 adjacent to the other axial side of the cylindrical surface portion 50, and a retaining ring 52 engaged with the cylindrical surface portion 50.

[0106] The inner ring 48 is loosely fitted onto the mating shaft portion 44 of the worm 12. An O-ring 67 is engaged in a circumferential groove 66 formed on the outer peripheral surface of the axially intermediate portion of the mating shaft portion 44, and the O-ring 67 is elastically sandwiched between the bottom surface of the circumferential groove 66 and the inner peripheral surface of the inner ring 48. In other words, a small radial gap exists between the inner peripheral surface of the inner ring 48 and the outer peripheral surface of the mating shaft portion 44, and the worm 12 and the worm-side joint member 25 are supported by the O-ring 67 in a radially floating manner relative to the inner ring 48.

[0107] The worm 12 and the worm-side joint member 25 are allowed to move radially relative to the inner ring 48 by the amount of the minute radial gap.

[0108] The inner ring 48 is disposed with a small axial gap between a side surface on the other axial side of the cylindrical portion 31 of the worm-side joint member 25 and a stepped surface 68 facing one axial side and provided on the outer peripheral surface of the fitting shaft portion 44. The inner ring 48 is also sandwiched from both axial sides by a side surface on one axial side of the flange portion 45 of the worm 12 and a side surface on the other axial side of the flange portion 32 of the worm-side joint member 25 via a pair of worm dampers 53, each of which is elastically compressible in the axial direction.

[0109] Specifically, the side surface on the other axial side of the inner ring 48 abuts against the side surface on one axial side of the flange portion 45 via the worm damper 53 on the other axial side, and the side surface on one axial side of the inner ring 48 abuts against the side surface on the other axial side of the flange portion 32 via the worm damper 53 on one axial side.

[0110] That is, there is a small axial gap between the side surface on one axial side of the inner ring 48 and the side surface on the other axial side of the cylindrical portion 31 of the worm-side joint member 25, and between the side surface on the other axial side of the inner ring 48 and the stepped surface 68 of the worm 12, and the worm 12 and the worm-side joint member 25 are supported in an axially floating manner with respect to the inner ring 48 by a pair of worm dampers 53. The worm 12 and the worm-side joint member 25 are allowed to move relative to the inner ring 48 in both axial directions by an amount equivalent to the small axial gap.

[0111] A plurality of balls 49 are arranged between an outer ring raceway provided on the inner peripheral surface of the outer ring 47 and an inner ring raceway provided on the outer peripheral surface of the inner ring 48 .

[0112] The ball bearing 46 has radial gaps between the outer ring 47 and the inner ring 48 and between the balls 49 and the outer ring 47 and the inner ring 48 .

[0113] Therefore, the fitting shaft portion 44 of the worm 12 is supported in the worm accommodating portion 17 so as to be capable of swinging displacement based on the presence of the minute radial gap, the minute axial gap, and the radial gap.

[0114] When the electric assist device 8 of this example is operating, an axial meshing reaction force acts on the worm 12 at the meshing portion with the worm wheel 11. The direction of the axial meshing reaction force reverses depending on the rotation direction of the worm 12.

[0115] The meshing reaction force acting on one axial side of the worm 12 is supported by the retaining ring 52 via the worm damper 53 and the ball bearing 46 on the other axial side.

[0116] The meshing reaction force acting on the worm 12 on the other axial side is supported by the stepped surface 51 of the worm accommodating portion 17 via the worm-side joint member 25, the worm damper 53 on one axial side, and the ball bearing 46. In particular, when the meshing reaction force acting on the worm 12 on the other axial side becomes large, the side surface on one axial side of the inner ring 48 comes into contact with the side surface on the other axial side of the cylindrical portion 31 of the worm-side joint member 25, and a large force directed toward one axial side is applied from the inner ring 48 to the worm-side joint member 25.

[0117] Even in this case, in this example, the crimped portion 41 provided on the connecting shaft portion 39 of the worm 12 is in surface contact over the entire periphery with the chamfered portion 35 that forms the opening periphery on one axial side of the central hole 30 of the worm-side joint member 25. Therefore, the crimped portion 41 can effectively prevent the worm-side joint member 25 from falling off the connecting shaft portion 39 to one axial side.

[0118] Second Example A second example of the embodiment of the present disclosure will be described with reference to FIGS. 18 and 19. FIG.

[0119] In this example, as in the first example, as shown in Figure 19 (a) , in the shaft unit 56a, the inner surface of the central hole 30 of the joint member 25 is formed by a serration portion 34, and the teeth of the serration portion 34 (tooth tips of the tooth portion 33) bite into the outer peripheral surface of the connecting shaft portion 39 of the shaft member 12 as the joint member 25 is press-fitted onto the outer peripheral surface of the connecting shaft portion 39 from one axial side.

[0120] In the manufacturing method of the shaft unit 56a of this example, as shown in FIG. 18 , in the intermediate shaft member 12b before the joint member 25 is press-fitted onto the outer peripheral surface of the connecting shaft portion 39b from one axial side, the overall outer diameter of the portion of the connecting shaft portion 39b that protrudes from the radial inside of the joint member 25 to one axial side after the press-fitting of the joint member 25 is completed (the portion shown by axial range S in FIGS. 18 and 19(a)) is set to be equal to or smaller than the inner diameter of the serration portion 34 provided on the inner peripheral surface of the center hole 30 of the joint member 25.

[0121] More specifically, in this example, the outer peripheral surface of the coupling shaft portion 39b of the intermediate shaft member 12b has a small diameter portion 74 between the cylindrical surface portion 69 and the guide surface portion 70 in the axial direction, as shown in Figure 18. The small diameter portion 74 is formed of a cylindrical surface whose outer diameter does not change in the axial direction. One axial end of the small diameter portion 74 is directly connected to the other axial end of the guide surface portion 70. The other axial end of the small diameter portion 74 is connected to the one axial end of the cylindrical surface portion 69 via a connecting portion 75 formed of a conical surface whose outer diameter increases toward the other axial end.

[0122] 19( a), when the step of press-fitting the joint member 25 is completed and the coupling shaft portion 39b is fitted into the central hole 30 of the joint member 25, only the other axial end of the small diameter portion 74 is positioned radially inside the central hole 30 of the joint member 25. In other words, when the press-fitting operation is completed, the guide surface portion 70 and the portion of the small diameter portion 74 excluding the other axial end are positioned so as to protrude from the central hole 30 to one axial side.

[0123] In this example, the outer diameter of the small diameter portion 74 is equal to or smaller than the inner diameter of the serration portion 34. Therefore, the outer diameter of the guide surface portion 70 is smaller than the inner diameter of the serration portion 34.

[0124] In this example, by adopting the above-described positional and dimensional relationships, at a stage prior to the step of press-fitting the joint member 25 onto the outer peripheral surface of the connecting shaft portion 39b from one axial side, the outer diameter of the entire portion of the connecting shaft portion 39b that protrudes from the central hole 30 of the joint member 25 to one axial side after the press-fitting is completed (the portion indicated by axial range S in Figures 18 and 19(a)), i.e., the portion of the small-diameter portion 74 excluding the end portion on the other axial side, and the entire guide surface portion 70, is set to be equal to or smaller than the inner diameter of the serration portion 34.

[0125] In this example, the shape of the protruding portion (the portion indicated by the axial range S in FIGS. 18 and 19(a)) is a combination of a small-diameter portion 74 formed by a cylindrical surface and a guide surface portion 70 formed by a conical surface. However, when implementing the present disclosure, the protruding portion can have any shape as long as its overall outer diameter is equal to or smaller than the inner diameter of the serration portion 34. For example, the shape of the protruding portion can be a shape formed by a cylindrical surface as a whole, or a shape in which the outer diameter decreases continuously or in steps toward one axial side.

[0126] In this example, as in the first example, as shown in FIG. 19( a), by completing the press-fitting, the coupling shaft portion 39 b is fitted into the central hole 30 of the joint member 25, and then, as shown in FIGS. 19( a) and 19(b), the radially outer portion of the edge portion on one axial side of the coupling shaft portion 39 b protruding from the central hole 30 is pressed toward the other axial side by the pressing surface of the pressing punch, thereby crushing the radially outer portion in the axial direction to form a crimped portion 41, and the crimped portion 41 is brought into surface contact with the opening peripheral portion 77 (chamfered portion 35) on one axial side of the central hole 30 of the joint member 25.

[0127] In the manufacturing method of this example, the outer diameter of the entire protruding portion (the portion indicated by the axial range S in FIGS. 18 and 19(a)) is equal to or smaller than the inner diameter of the serration portion 34. Therefore, compared to a structure (such as the structure of the first example) in which the outer diameter of at least a portion of the protruding portion is larger than the inner diameter of the serration portion 34, the axial press-fitting stroke when performing the press-fitting can be reduced. Therefore, the energy required to perform the press-fitting can be reduced accordingly.

[0128] Furthermore, a wider radial gap can be secured between the outer peripheral surface of the protruding portion and the opening peripheral edge portion (chamfered portion 35 in this example) on one axial side of central hole 30. Therefore, when the radially outer portion of the protruding portion is crushed in the axial direction to form crimped portion 41, the crushed material can be made to flow more easily toward the outer diameter side, improving the formability of crimped portion 41.

[0129] Furthermore, it is possible to more effectively prevent the formation of axial passage marks (scraping marks) of the teeth of the serration portion 34 at multiple circumferential locations on the outer peripheral surface of the protruding portion. That is, in this example, axial passage marks of the teeth of the serration portion 34 are formed at multiple circumferential locations on the outer peripheral surface of the coupling shaft portion 39b, and even if burrs are formed on the edges of the passage marks, the passage marks and burrs are not present in the portion of the joint member 25 that protrudes from the central hole 30 on one axial side. The opening on one axial side inside the central hole 30, i.e., the opening on one axial side of the grooves of the serration portion 34, is blocked by the crimped portion 41. This effectively prevents the scraping marks from falling off into the surrounding area.

[0130] In a structure in which the entire coupling shaft portion 39 is larger than the inner diameter of the serration portion 34, axial passage marks of the teeth of the serration portion 34 are formed at multiple locations circumferentially on the outer circumferential surface of the protruding portion, and even if chips are formed on the edges of the passage marks, a step of removing the chips can be performed after the formation of the crimped portion 41 and before the completed shaft unit 56 is housed in the housing 15 of the worm reducer, thereby preventing the chips from falling off inside the housing 15. In this example, it is possible to omit the step of removing such chips. The other configurations and effects are the same as those of the first example.

[0131] REFERENCE SIGNS LIST 1 electric power steering device 2 steering wheel 3 steering shaft 4 steering column 5a, 5b universal joint 6 intermediate shaft 7 steering gear unit 8 electric assist device 9 pinion shaft 10 tie rod 11 worm wheel 12 shaft member (worm) 12a, 12b intermediate shaft member 13 coupling 14 electric motor 15 housing 16 wheel accommodating section 17 worm accommodating section 18 cover body 19 wheel teeth 20 core section 21 gear section 22 worm teeth 23 output shaft 24 motor side joint member 25 joint member (worm side joint member) 26 intermediate joint member 27 cylindrical section 28 flange section 29 tooth section 30 central hole 31 cylindrical section 32 flange section 33 tooth section 34 serration section 35 Chamfered portion 36 End face 37 Large diameter portion 38a, 38b Insertion hole 39, 39a, 39b Connecting shaft portion 40 Step surface 41 Crimping portion 42 Inclined surface portion 43a, 43b Center hole 44 Fitting shaft portion 45 Flange portion 46 Ball bearing 47 Outer ring 48 Inner ring 49 Ball 50 Cylindrical surface portion 51 Step surface 52 Retaining ring 53 Worm damper 54 Ball bearing 55 Biasing mechanism 56 Shaft unit 57 Support base 58 Support hole 59 Support surface 60 Pressing punch 61 Pressing surface 62 Flat surface portion 63 Inclined surface portion 64 Guide hole portion 65, 65a Center shaft 66 Circumferential groove 67 O-ring 68 Step surface 69 Cylindrical surface portion 70 Guide surface portion 71 Relief groove 72 Press-fit jig 73, 73a Engagement portion 74 Small diameter portion 75 Connection portion 76 Cylindrical portion 77 Opening peripheral portion 78 Opening peripheral portion

Claims

1. A shaft member having a coupling shaft portion at one end in the axial direction and having a center hole that opens at the radial center of one end face in the axial direction of the coupling shaft portion; A joint member having a center hole that penetrates in the axial direction on the inner side in the radial direction, and the coupling shaft portion is fitted into the center hole; Comprising: The coupling shaft portion has a caulking portion formed by plastically deforming only the radially outer portion of the axially one-side edge portion protruding from the center hole; The caulking portion is in surface contact with the circumferential edge portion of the opening on the axially one-side of the center hole of the joint member; Shaft unit.

2. The shaft unit according to claim 1, wherein at least the radially outer portion of the axially one-side surface of the caulking portion is constituted by an inclined surface portion that inclines in a direction toward the axially other side as it goes toward the radially outer side.

3. The shaft unit according to claim 2, wherein the inclined surface is a plastically processed surface.

4. The generatrix shape of the inclined surface portion is a straight line shape, The inclination angle of the inclined surface portion with respect to a virtual plane orthogonal to the axial direction of the shaft member is 15 degrees or more and 45 degrees or less. The shaft unit according to claim 2.

5. The shaft unit according to claim 1, wherein the circumferential edge portion of the opening of the joint member is constituted by a chamfered portion that inclines in a direction toward the radially outer side as it goes toward the axially one-side.

6. The inner peripheral surface of the center hole is constituted by a serration portion, The teeth of the serration portion bite into the outer peripheral surface of the coupling shaft portion. The shaft unit according to claim 1.

7. A large-diameter portion having an inner diameter larger than the inner peripheral surface of the center hole is provided at the circumferential edge portion of the opening on the axially other side of the center hole of the joint member, There is a space between the outer peripheral surface of the coupling shaft portion and the large-diameter portion. The shaft unit according to claim 6.

8. The opening on the axially one-side of the groove of the serration portion is entirely blocked by the caulking portion. The shaft unit according to claim 6.

9. A relief groove is provided over the entire circumference at the axially other end portion of the outer peripheral surface of the coupling shaft portion, which is located on the axially other side of the portion fitted into the center hole of the joint member. The shaft unit according to claim 1.

10. The center hole is present at one end of the coupling shaft portion in the axial direction, is arranged to overlap with the caulking portion in the radial direction, and at one end of the coupling shaft portion in the axial direction, the portion existing around the center hole is constituted by a cylindrical portion. The shaft unit according to Claim 1.

11. The shaft unit according to any one of Claims 1 to 10, wherein the shaft member is constituted by a worm.

12. The shaft member includes, in the portion between the coupling shaft portion and the worm teeth, in order from one side in the axial direction, a fitting shaft portion having a larger diameter than the coupling shaft portion, and a flange portion protruding radially outward. The outer peripheral surface of the coupling shaft portion and the outer peripheral surface of the fitting shaft portion are connected by a stepped surface facing one side in the axial direction. The joint member is sandwiched from both sides in the axial direction by the stepped surface of the shaft member and the caulking portion. The shaft unit according to Claim 11.

13. A worm wheel, A worm meshing with the worm wheel, A coupling including a joint member coupled to a coupling shaft portion provided at one end of the worm in the axial direction, An electric motor that transmits torque to the worm via the coupling, and The shaft unit including the worm and the joint member is constituted by the shaft unit according to Claim 11. An electric assist device.

14. A method for manufacturing a shaft unit, comprising a shaft member having a coupling shaft portion at one end in the axial direction and a center hole opening at the radial center of one end surface of the coupling shaft portion in the axial direction, and a joint member having a center hole penetrating axially inward in the radial direction. In a state where the coupling shaft portion is fitted inside the center hole of the joint member, the radially outer portion of the edge portion of one end of the coupling shaft portion protruding from the center hole is pressed toward the other side in the axial direction by the pressing surface of a pressing punch, so that the radially outer portion is axially crushed to form a caulking portion, and the caulking portion is brought into surface contact with the peripheral edge portion of the opening of the center hole of the joint member. A method for manufacturing a shaft unit.

15. In the step of forming the caulking portion using the pressing punch, a conical engaging portion provided at the lower end of a center shaft inserted through a guide hole portion of the pressing punch without radial play is engaged with the center hole. The method for manufacturing a shaft unit according to Claim 14.

16. A step of press-fitting and externally fitting the joint member onto the outer peripheral surface of the coupling shaft portion from one axial side is provided. In the shaft unit, The inner peripheral surface of the central hole is constituted by a serration portion. Of the coupling shaft portion, the overall outer diameter of the portion protruding axially from the central hole of the joint member to one axial side is equal to or less than the inner diameter of the serration portion. In the step of press-fitting and externally fitting the joint member, the teeth of the serration portion are bitten into the outer peripheral surface of the coupling shaft portion, and by completing the press-fitting and external fitting of the joint member, the coupling shaft portion is fitted inside the central hole of the joint member. The method for manufacturing a shaft unit according to claim 14.

17. A method for manufacturing a shaft unit according to claim 14, comprising a step of press-fitting and externally fitting the joint member onto the outer peripheral surface of the coupling shaft portion from one axial side using a cylindrical press-fitting jig. In this step, a cylindrical center shaft is inserted into the joint member and the press-fitting jig adjacent to the upper side of the joint member without radial play, and a conical engaging portion provided at the radially central portion of the lower end of the center shaft is engaged with the center hole.

18. In the step of forming the caulking portion, the side surface on one axial side of the caulking portion is pressed by the radially outer portion of the pressing surface toward the other axial side and radially inward. The method for manufacturing a shaft unit according to claim 14.

19. The method for manufacturing a shaft unit according to any one of claims 14 to 18, wherein the shaft member is constituted by a worm.