JP7741546B2 - Spline shaft and manufacturing method thereof - Google Patents

Description

The present invention relates to a spline shaft used, for example, as an inner shaft constituting an intermediate shaft in an automobile steering device, and to a method for manufacturing the same.

Figure 10 shows an example of a steering device for an automobile. Rotation of the steering wheel 100 is transmitted to the input shaft 102 of the steering gear unit 101. As the input shaft 102 rotates, a pair of tie rods 103 (left and right) are pushed and pulled, imparting a steering angle to the front wheels. The steering wheel 100 is fixedly supported on the rear end of a steering shaft 104, which passes axially through the inside of a steering column 105 and is rotatably supported by the steering column 105. The front end of the steering shaft 104 is connected to the rear end of an intermediate shaft 107 via a universal joint 106, and the front end of the intermediate shaft 107 is connected to the input shaft 102 via another universal joint 108.

The steering device shown in the figure is equipped with a telescopic mechanism that allows the fore-aft position of the steering wheel 100 to be adjusted. To achieve this, the steering shaft 104 is constructed by combining an inner shaft 109 and an outer tube 110 through spline engagement, allowing torque transmission and extension/retraction. Furthermore, the steering column 105 is constructed by combining an inner column 111 and an outer column 112, allowing extension/retraction.

In addition, the intermediate shaft 107 incorporated into the steering device has an expandable structure, for example, to prevent vibrations input from the steered wheels during driving from being transmitted to the steering wheel 100, and/or to allow it to be incorporated into the vehicle body with its overall length shortened.

Figures 11 and 12 show the telescopic intermediate shaft 107a described in JP 2020-143766 A. The intermediate shaft 107a comprises an inner shaft 114 having a male spline portion 113 on the outer peripheral surface of the end on one axial side (left side of Figure 11), and a cylindrical outer tube 116 having a female spline portion 115 on the inner peripheral surface of the end on the other axial side (right side of Figure 11). The intermediate shaft 107a is configured by combining the inner shaft 114 and outer tube 116 so that their entire length can be extended or retracted by spline-engaging the male spline portion 113 with the female spline portion 115.

The inner shaft 114 comprises a metal shaft body (shaft body) 117 and a synthetic resin coating layer (resin layer) 118 that covers one axial end of the shaft body 117.

The shaft body 117 has a male spline core portion 121 on the outer peripheral surface at one end in the axial direction, which is made up of multiple tooth core portions 119 and groove core portions 120 arranged alternately in the circumferential direction.

The male spline core 121 has toothless portions 122a, 122b, and 122c at three axially spaced locations. The toothless portions 122a, 122b, and 122c are formed by cutting out the entire circumference of the tooth core 119 at each location. Therefore, the toothless portions 122a, 122b, and 122c have the same outer diameter as the groove bottom diameter of the male spline core 121. Furthermore, of the toothless portions 122a, 122b, and 122c, the axial length of the toothless portion 122b located in the middle is longer than the toothless portions 122a and 122c located on either side.

The coating layer 118 covers the entire circumference of the male spline core portion 121. The coating layer 118 is bonded and fixed to the male spline core portion 121 of the shaft body 117 by injection molding. The portions of the coating layer 118 that cover multiple tooth core portions 119 and groove core portions 120 form the male spline portion 113 that spline engages with the female spline portion 115 of the outer tube 116.

In the inner shaft 114 of JP 2020-143766 A, when the coating layer 118 is produced by injection molding, the synthetic resin that makes up the coating layer 118 is also filled into the toothless portions 122a, 122b, and 122c, forming filled portions 123 in these areas. Therefore, even if a force is applied that attempts to displace the shaft body 117 and the coating layer 118 relative to each other in the axial direction, the engagement between the toothless portions 122a, 122b, and 122c and the filled portions 123 reliably prevents relative axial displacement between the shaft body 117 and the coating layer 118. In other words, in the inner shaft 114 described in JP 2020-143766 A, the toothless portions 122a, 122b, and 122c ensure the axial bond strength between the shaft body 117 and the coating layer 118.

Japanese Patent Application Laid-Open No. 2020-143766

In the inner shaft 114 described in JP 2020-143766 A, in order to improve the axial bond strength between the shaft body 117 and the coating layer 118, the toothless portions 122a, 122b, and 122c are formed by cutting out the tooth core portion 119 around the entire circumference of the male spline core portion 121. This increases the number of steps required to process the shaft body 117 and reduces material yield, potentially increasing the manufacturing cost of the inner shaft 114.

In addition, with the inner shaft 114 described in JP 2020-143766 A, a mold is placed around the male spline core portion 121, and molten synthetic resin is pumped into the mold cavity from gates located around the central toothless portion 122b, where it is cooled and solidified, creating the coating layer 118. The molten synthetic resin pumped in from the gate flows circumferentially around the inside of the toothless portion 122b, merging (colliding) with molten synthetic resin pumped in from other gates within the toothless portion 122b. This can result in linear marks called weld lines forming on the surface of the synthetic resin filled in the toothless portion 122b, potentially compromising the appearance and strength of the product.

In particular, simulations conducted by the inventors have shown that when a gate for feeding molten synthetic resin is placed around the central toothless portion 122b, the meeting angle between the flow fronts becomes smaller. This is thought to be why weld lines become more clearly visible.

In consideration of the above circumstances, the present invention aims to realize a spline shaft structure that can reduce manufacturing costs while ensuring the axial bonding strength between the metal shaft body and the synthetic resin coating layer.

A spline shaft according to one aspect of the present invention comprises a metal shaft body and a synthetic resin coating layer.

The shaft body includes a spare shaft portion, a splined shaft portion, and a non-splined shaft portion.

The spare shaft portion is located at one axial end of the shaft body.

The spline shaft portion has a male spline core portion with tooth core portions and groove core portions arranged alternately in the circumferential direction on its outer peripheral surface, and is located on the other axial side of the spare shaft portion.

The non-splined shaft portion is located on the other axial side of the splined shaft portion.

The coating layer covers the outer peripheral surface of the spare shaft portion, the outer peripheral surface of the splined shaft portion, and the outer peripheral surface of one axial end of the non-splined shaft portion.

In particular, in a spline shaft according to one aspect of the present invention, the outer circumferential surface of the reserve shaft portion and the outer circumferential surface of the spline shaft portion are connected by a first stepped surface facing one side in the axial direction.
Furthermore, the radial thickness of the portion of the coating layer covering the outer peripheral surface of the spare shaft portion is greater than the radial thickness of the portion of the coating layer covering the outer peripheral surface of the splined shaft portion, and the radial thickness of the portion of the coating layer covering the outer peripheral surface of one axial end of the non-splined shaft portion is greater than the radial thickness of the portion of the coating layer covering the outer peripheral surface of the splined shaft portion.

In the spline shaft according to one aspect of the present invention, the first step surface may be configured by a flat surface that is perpendicular to the central axis of the shaft body.
Alternatively, the first step surface may be formed of a truncated cone surface inclined in a direction such that the outer diameter increases toward the other side in the axial direction.

In a spline shaft according to one aspect of the present invention, the outer peripheral surface of the splined shaft portion and the outer peripheral surface of one axial end of the non-splined shaft portion can be connected by a flat second stepped surface perpendicular to the central axis of the shaft body.

A spline shaft according to one aspect of the present invention may have notched recesses at multiple locations around the circumference of the male spline core portion, recessed radially inward relative to adjacent portions on the other axial side.

In a spline shaft according to one aspect of the present invention, it is possible to prevent the presence of weld lines in the portion of the coating layer that covers the outer surface of the spline shaft portion.

In the spline shaft according to one aspect of the present invention, the coating layer may have a gate mark in a portion covering the outer peripheral surface of one axial end of the non-spline shaft portion.
In this case, the phase of the gate mark in the circumferential direction can be made to coincide with the phase of any one of the tooth core portions in the circumferential direction.

A method for manufacturing a spline shaft according to one aspect of the present invention includes the steps of: arranging a mold around one axial end of the non-splined shaft portion, the splined shaft portion, and the spare shaft portion; and, with a gate positioned around one axial end of the non-splined shaft portion, injecting molten synthetic resin from the gate into a cavity in the mold to form the coating layer, in order to produce the splined shaft according to one aspect of the present invention.

In the method for manufacturing a spline shaft according to one aspect of the present invention, the shaft body can be set in the mold with the central axis of the shaft body oriented horizontally.
In the method for manufacturing a spline shaft according to one aspect of the present invention, the shaft body can be set in the mold with the center axis of the shaft body oriented vertically.

In a spline shaft manufacturing method according to one aspect of the present invention, the outer diameter of one axial end face of the spare shank can be made smaller than the outer diameter of an ejector pin used to press against the one axial end face of the spare shank when removing the spline shaft from the mold.

The spline shaft according to one aspect of the present invention can reduce manufacturing costs while ensuring the axial bonding strength between the metal shaft body and the synthetic resin coating layer. Furthermore, the method for manufacturing a spline shaft according to one aspect of the present invention can efficiently manufacture the spline shaft according to one aspect of the present invention industrially.

FIG. 1A is a cross-sectional view showing a spline shaft according to a first example of an embodiment, and FIG. 1B is an end view seen from the left side of FIG. 1A. FIG. 2 is an enlarged view of the left side portion of FIG. 3A is an enlarged view of the X portion of FIG. 2, and FIG. 3B is an enlarged view of the Y portion of FIG. FIG. 4 is an enlarged perspective view of a main part of a shaft body according to a first example of the embodiment. FIG. 5 is a cross-sectional view showing how the coating layer is formed by injection molding. FIG. 6(A) is an end view of the shaft body as viewed from one side in the axial direction, and FIG. 6(B) is an enlarged view of portion Z in FIG. 6(A) showing the state after the coating layer has been formed. FIG. 7 is a diagram showing a second example of the embodiment, and corresponds to FIG. 3(B). FIG. 8 is an enlarged perspective view of a main part of a shaft body according to a second example of the embodiment. FIG. 9 is a view similar to FIG. 5, showing a third example of the embodiment. FIG. 10 is a partially cutaway perspective view showing an example of a conventional steering device. FIG. 11 is a cross-sectional view showing an example of a conventional intermediate shaft. FIG. 12 is a partially cutaway perspective view showing an example of a conventional inner shaft.

[First Example of Embodiment]
A first embodiment of the present invention will be described with reference to Figures 1(A) to 6(B). The spline shaft 1 of this example has a male spline portion 2 on the outer peripheral surface of one axial side portion (the left side portion in Figures 1(A) and 2). The spline shaft 1 can be used in combination with an outer tube 116 (see Figure 11) having a female spline portion 115 on its inner peripheral surface. In other words, the spline shaft (inner shaft) 1 and the outer tube 116 can be combined by spline-engaging the male spline portion 2 with the female spline portion 115 to form a telescopic intermediate shaft 107a.

The spline shaft 1 comprises a shaft body 3 and a coating layer 4.

The shaft body 3 is constructed as a single unit, hollow cylinder, from a light alloy such as an aluminum alloy, magnesium alloy, or titanium alloy. However, when implementing the present invention, the shaft body can also be constructed from an iron-based alloy such as carbon steel and/or can be solid.

The shaft body 3 comprises, from one axial end, a spare shaft portion 5, a splined shaft portion 6, and a non-splined shaft portion 7.

The spare shank 5 is provided at one axial end of the shaft body 3. The spare shank 5 has a cylindrical outer peripheral surface whose outer diameter does not change axially, except for a chamfered portion 8a provided on the outer peripheral surface of the end on one axial end. The spare shank 5 also has an outer diameter D5 that is smaller than the groove bottom diameter (the circumscribed circle diameter of the groove core portion 12) _d12 of a male spline core portion ₁₀ provided on the outer peripheral surface of the spline shank 6 (described later). That is, the outer peripheral surfaces of the spare shank 5 and the spline shank 6 are connected by a first stepped surface 9 facing one axial side. In this example, the first stepped surface 9 is formed by a flat surface perpendicular to the central axis O of the spline shaft 1 (shaft body 3). However, the first stepped surface may also be formed by a truncated cone surface inclined in such a direction that the outer diameter increases toward the other axial side. In the illustrated example, the axial dimension of the spare shank 5 is sufficiently shorter than the axial dimension of the spline shank 6.

In this example, the outer diameter _d5 of the end face on one axial side of the spare shank 5 is larger than the outer diameter D30 of an ejector pin 30 that presses the end face on one axial side of the spare shank 5 toward the other axial side when removing the spline shaft 1 from the mold ₂₆ after the coating layer 4 has been injection molded, as will be described later.

The spline shank 6 is provided in the shaft body 3, extending from the axially intermediate portion to the portion adjacent to the other axial side of the spare shank 5 (the right side in Figure 1(A)). The spline shank 6 has a male spline core 10 on its outer surface. The male spline core 10 is composed of multiple (18 in the illustrated example) convex tooth cores 11 extending in the axial direction and concave groove cores 12 extending in the axial direction, arranged alternately in the circumferential direction.

In this example, the tooth tip surface of the tooth core portion 11 is composed of a flat surface or a partial cylindrical surface centered on the central axis O, with no axial irregularities, except for both axial end portions, i.e., the end portion on one axial side provided with the chamfered portion 13, and the end portion on the other axial side provided with the convex portion 18 described below. Furthermore, the bottom surface of the groove core portion 12 (the surface facing radially outward) is composed of a flat surface or a partial cylindrical surface centered on the central axis O, with no axial irregularities.

The non-splined shaft portion 7 is provided in a portion adjacent to the other axial side of the splined shaft portion 6. The non-splined shaft portion 7 has a connecting portion 14 located at one end in the axial direction, and a small-diameter shaft portion 15 provided in the range from the other end in the axial direction to the portion adjacent to the other axial side of the connecting portion 14.

The connecting portion 14 has an incomplete male spline core portion 16 on the outer peripheral surface of one axial end portion, and a first inclined surface portion 17a on the outer peripheral surface of the other axial end portion, which has a truncated conical surface shape with an outer diameter that decreases toward the other axial end. The other axial end of the first inclined surface portion 17a is connected to the axial end of one axial end of the small-diameter shaft portion 15, which will be described later.

The incomplete male spline core portion 16 is composed of multiple (18 in the illustrated example) convex portions 18 and concave portions 19, each of which has a right-angled triangular cross section containing the central axis O, arranged alternately in the circumferential direction. The convex portions 18 have a second inclined surface portion 17b on their outer peripheral surface, which has a partial truncated cone shape and is inclined in a direction in which the outer diameter decreases toward the other axial direction. One axial end of the second inclined surface portion 17b is connected to the other axial end of the tooth tip surface of the tooth core portion 11, and the other axial end of the second inclined surface portion 17b is connected to the outer peripheral surface of one axial end of the first inclined surface portion 17a.

The first inclined surface portion 17a and the second inclined surface portion 17b exist on the same truncated cone surface. In other words, in this example, the outer peripheral surface of the other axial end of the spline shaft portion 6 and the outer peripheral surface of one axial end of the small diameter shaft portion 15 are connected by a truncated cone-shaped inclined surface portion 17 consisting of the first inclined surface portion 17a and the second inclined surface portion 17b.

The small diameter shaft portion 15 is provided in a range from the other axial end of the shaft body 3 to a portion adjacent to the other axial side of the connecting portion 14. The small diameter shaft portion 15 has a cylindrical outer peripheral surface whose outer diameter does not change in the axial direction, except for a chamfered portion 8b provided on the outer peripheral surface of the other axial end. The small diameter shaft portion 15 also has an outer diameter D15 that is smaller than the groove bottom diameter (circumscribed circle diameter of the groove core portion 12) _d12 of the male spline core portion 10 provided on the outer peripheral surface of the spline shaft portion _6. In this example, the outer diameter _D15 of the small diameter shaft portion 15 is larger than the outer diameter _D5 of the spare shaft portion 5 ( _D15 > _D5 ).

The shaft body 3 also has a central hole 21 that passes through the center in the axial direction. The central hole 21 is configured as a stepped hole whose inner diameter gradually increases toward one axial end, except for the other axial end that is provided with a chamfered portion 8c.

The coating layer 4 is made of a synthetic resin, such as polyamide resin (PA), polyphenylene sulfide resin (PPS), or polyacetal resin (POM), which reduces sliding resistance against the female spline portion 115 of the outer tube 116 that is combined with the spline shaft 1 of this example. The coating layer 4 covers the entire outer surface of the spare shaft portion 5, the outer surface of the spline shaft portion 6, and the outer surface of one axial side portion of the non-spline shaft portion 7 (the outer surface of the connection portion 14 and the outer surface of one axial side portion of the small diameter shaft portion 15). Note that while Figure 1(B) shows a circumferential portion of the coating layer 4 omitted, in reality, the coating layer 4 is provided along the entire circumference.

For this reason, the male spline portion 2 is composed of a male spline core portion 10 provided on the outer peripheral surface of the spline shaft portion 6, and a male spline covering portion 22 of the coating layer 4 that covers the male spline core portion 10. In this example, as shown in FIG. 6(B), the male spline covering portion 22 has cutout recesses 20 at multiple locations (four locations in the illustrated example) circumferentially on the outer peripheral surface of one axial end portion (the range indicated by α in FIG. 1(A)), as shown in FIG. 6(B). Each cutout recess 20 has a semicircular cross-sectional shape and opens to the outer peripheral surface of the spline shaft 1 and to one axial end face. In this example, each cutout recess 20 is provided in a portion of the male spline portion 2 whose circumferential phase coincides with the spline groove. Note that the cutout recesses 20 are omitted from FIG. 1(B).

As described below, each notched recess 20 is formed by engaging a positioning pin 27 with a spline groove to circumferentially position the shaft body 3 relative to the mold 26 when injection molding the coating layer 4. After the spline shaft 1 is completed, when the spline shaft 1 is combined with the outer tube 116 to form the intermediate shaft 107a, each notched recess 20 can be used as a grease reservoir to hold grease applied to the spline engagement portion between the male spline portion 2 and the female spline portion 115.

The axial length of each notched recess 20 is not particularly limited, but can be, for example, between 1/20 and 1/6 of the axial length of the male spline portion 2.

The male spline covering portion 22 has a tooth covering portion 23 that covers the tooth core portion 11 and a groove covering portion 24 that covers the groove core portion 12. The radial thickness t of the male spline covering portion 22 is approximately constant in the circumferential and axial directions, except for the portion where the notched recess 20 is provided. The radial thickness t of the male spline covering portion 22 is not particularly limited, but can be, for example, 0.01 mm or more and 0.50 mm or less, and preferably 0.25 mm or more and 0.40 mm or less.

The radial thickness of the portion of the coating layer 4 where the notched recess 20 is provided is extremely thin. The radial thickness of the portion of the coating layer 4 where the notched recess 20 is provided is not particularly limited, but can be, for example, 0.01 mm or more and 0.10 mm or less.

In this example, the radial thickness _TA of the first thick portion 25a of the coating layer 4 that covers the outer peripheral surface of the spare shaft portion 5 is made thicker than the radial thickness t of the male spline covering portion 22 of the coating layer 4 that covers the male spline core portion 10 provided on the outer peripheral surface of the spline shaft portion 6 ( _TA > t).

For this reason, the outer diameter of the portion of the first thick-walled portion 25a that coincides with the tooth core portion 11 in the circumferential direction is made the same as the outer diameter of the tooth covering portion 23, and the outer diameter of the portion of the first thick-walled portion 25a that coincides with the groove core portion 12 in the circumferential direction is made the same as the outer diameter of the groove covering portion 24. In this example, the outer diameter _D5 of the spare shank 5 is smaller than the groove bottom diameter _d12 of the male spline core portion 10, and therefore the radial thickness _TA of the first thick-walled portion 25a that covers the outer peripheral surface of the spare shank 5 is thicker than the radial thickness t of the male spline covering portion 22 that covers the male spline core portion 10, regardless of the phase in the circumferential direction.

In this example, the axial length of the radially outer portion of the first thick-walled portion 25a, i.e., the axial length L _a1 from one axial end face of the first thick-walled portion 25a to the other axial end face of the chamfered portion 13, is set to be greater than the radial thickness t of the male spline covering portion 22 (L _a1 > t). Furthermore, the axial length of the radially inner portion of the first thick-walled portion 25a, i.e., the axial length L _a2 from one axial end face of the first thick-walled portion 25a to the first step surface 9, is set to be greater than the radial thickness t of the male spline covering portion 22 (L _a2 > t).

The radial thickness _TA of the first thick portion 25a is not particularly limited, but can be set to, for example, 3 to 7 times the radial thickness t of the male spline covering portion 22.

Furthermore, the radial thickness T B (the radial thickness of the thickest part) of the second thick portion 25 b of the coating layer ₄ , which covers the outer peripheral surface of one axial end of the non-spline shaft portion 7, i.e., the outer peripheral surface of the connecting portion 14 and the outer peripheral surface of one axial end of the small diameter shaft portion 15, is made thicker than the radial thickness t of the male spline covering portion 22 of the coating layer 4 which covers the male spline core portion 10 provided on the outer peripheral surface of the spline shaft portion 6 (T _B > t).

For this reason, the outer diameter of the portion of the second thick-walled portion 25b that coincides with the tooth core portion 11 in the circumferential direction is made the same as the outer diameter of the tooth covering portion 23, and the outer diameter of the portion of the second thick-walled portion 25b that coincides with the groove core portion 12 in the circumferential direction is made the same as the outer diameter of the groove covering portion 24. In this example, the outer peripheral surface of one axial end of the small diameter shaft portion ₁₅ , which has an outer diameter _D15 smaller than the groove bottom diameter D12 of the male spline core portion 10, is connected to the outer peripheral surface of the other axial end of the spline shaft portion 6 by a truncated conical inclined surface portion 17. Therefore , the radial thickness T _B of the second thick-walled portion 25b that covers the outer peripheral surface of the connecting portion 14 and the outer peripheral surface of the one axial end of the small diameter shaft portion 15 is thicker than the radial thickness t of the male spline covering portion 22 that covers the male spline core portion 10, regardless of the phase in the circumferential direction.

In this example, the axial length of the radially outer portion of the second thick-walled portion 25b, i.e., the axial length _Lb1 from the other axial end face of the second thick-walled portion 25b to one axial end of the connecting portion 14, is set to be greater than the radial thickness t of the male spline covering portion 22 ( _Lb1 > t). Furthermore, the axial length of the radially inner portion of the second thick-walled portion 25b, i.e., the axial length _Lb2 from the other axial end face of the second thick-walled portion 25b to the other axial end of the connecting portion 14, is set to be greater than the radial thickness t of the male spline covering portion 22 ( _Lb2 > t).

The coating layer 4 has gate marks (not shown) at one location or at multiple equally spaced locations in the circumferential direction on the outer peripheral surface of the thin-walled portion 32, which is located axially away from the second thick-walled portion 25b, in the portion covering the small diameter shaft portion 15. Specifically, in this example, the coating layer 4 has gate marks at two locations on radially opposite sides of the outer peripheral surface of the thin-walled portion 32. In this example, the gate marks are formed on a portion of the outer peripheral surface of the thin-walled portion 32 that is in phase with the tooth core portion 11 in the circumferential direction. Note that the gate marks refer to marks formed by removing ( cutting off ) the gate 29 after injection molding the coating layer 4, as will be described later.

The coating layer 4 is formed by injection molding and is simultaneously bonded and fixed to the shaft body 3.

Next, the manufacturing method for the spline shaft 1 of this example will be described below.

First, a metal wire or rod is cut to a predetermined length to obtain a cylindrical material. The cylindrical material is then subjected to the necessary processes, such as forging, cutting, and grinding, to obtain the shaft body 3. For example, the male spline core portion 10 can be formed by rolling or cutting.

In the subsequent process of forming the coating layer 4, first, the shaft body 3 is set in a mold 26, as shown in Figure 5. In this example, the shaft body 3 is set in the mold 26 with its central axis oriented horizontally. Note that the mold 26 can have, for example, a split mold structure consisting of multiple mold parts.

In this example, with the shaft body 3 set in the mold 26, as shown in FIG. 6(A), positioning pins 27 are engaged (inserted) into some (four in the illustrated example) of the groove core portions 12 that make up the male spline core portion 10 of the shaft body 3. This positions the shaft body 3 circumferentially relative to the mold 26. In this state, a gate 29 for feeding molten synthetic resin into a cavity 28 existing between the inner surface of the mold 26 and the outer surface of the shaft body 3 is located radially outward from one axial end of the small diameter shaft portion 15 (the position indicated by the thick arrow in FIG. 1), and is positioned in a portion that is circumferentially aligned with the tooth core portion 11. Note that the gate 29 does not need to be positioned perpendicular to the central axis of the shaft body 3; it can also be positioned in a direction inclined relative to the central axis of the shaft body 3, for example, in a direction that forms an angle of 30° to 60° with the central axis of the shaft body 3 in a cross section including the central axis of the shaft body 3.

With the mold 26 positioned around one axial end of the shaft body 3, molten synthetic resin is injected under pressure through the gate 29 into the cavity 28, and then cooled and solidified within the cavity 28. After the synthetic resin has cooled and solidified, the mold 26 is separated, and the end face of one axial end of the shaft body 3 (spare shaft portion 5) is pressed toward the other axial end with an ejector pin 30, thereby removing the spline shaft 1 from the mold 26. The gate 29 is then cut to obtain the completed spline shaft 1. Note that the synthetic resin (male spline coating portion 22) around the male spline core portion 10 where the positioning pin 27 was located is much thinner than the rest of the material, and a notched recess 20 is formed in that portion.

The spline shaft 1 of this example ensures the axial bonding strength between the metal shaft body 3 and the synthetic resin coating layer 4 while reducing manufacturing costs.

That is, in the spline shaft 1 of this example, the radial thickness _TA of the first thick-walled portion 25a of the coating layer 4 that covers the outer peripheral surface of the spare shaft portion 5 is made thicker than the radial thickness t of the male spline covering portion 22 of the coating layer 4 that covers the male spline core portion 10. Also, the radial thickness _TB of the second thick-walled portion 25b of the coating layer 4 that covers the outer peripheral surface of one axial end of the non-spline shaft portion 7 is made thicker than the radial thickness t of the male spline covering portion 22.

Therefore, because the coating layer 4 is formed by injection molding, the amount of thermal shrinkage (volume reduction) associated with cooling and solidification after filling the cavity 28 with molten synthetic resin is greater in the first thick-walled portion 25a and the second thick-walled portion 25b than in the male spline covering portion 22. As a result, the bond strength (degree of adhesion) of the first thick-walled portion 25a and the second thick-walled portion 25b, located on both axial sides of the male spline core portion 10, to the outer circumferential surface of the shaft body 3 is greater than the bond strength (degree of adhesion) of the male spline covering portion 22 covering the male spline core portion 10 to the outer circumferential surface of the shaft body 3. For this reason, the first thick-walled portion 25a and the second thick-walled portion 25b, which have a high bond strength to the shaft body 3, can be positioned on both axial sides of the male spline core portion 10 of the shaft body 3. In other words, the male spline core portion 10 can be sandwiched from both axial sides by the first thick-walled portion 25a and the second thick-walled portion 25b, which have a high bond strength to the shaft body 3. This ensures the axial bond strength between the shaft body 3 and the coating layer 4.

Furthermore, the spline shaft 1 of this example achieves a structure that ensures the axial bonding strength between the shaft body 3 and the coating layer 4 without forming missing tooth portions 122a, 122b, and 122c around the entire circumference of the male spline core portion 121 of the shaft body 117, as in the inner shaft 114 described in JP 2020-143766 A. This prevents an unnecessary increase in the number of steps required to process the shaft body 3 and a decrease in material yield, thereby reducing the manufacturing cost of the spline shaft 1.

Furthermore, in this example, the axial length L _a1 of the radially outer portion and the axial length L _a2 of the radially inner portion of the first thick-walled portion 25 a are made larger than the radial thickness t of the male spline covering portion 22, and the axial length L _b1 of the radially outer portion and the axial length L _b2 of the radially inner portion of the second thick-walled portion 25 b are made larger than the radial thickness t of the male spline covering portion 22. This makes it possible to ensure sufficient volumes for the first thick-walled portion 25 a and the second thick-walled portion 25 b located on both axial sides of the male spline core portion 10. This also ensures sufficient bonding strength in the axial direction between the shaft body 3 and the coating layer 4.

Furthermore, the manufacturing method for the spline shaft 1 in this example allows for industrially efficient manufacturing of the spline shaft 1.

In particular, in this example, when forming the coating layer 4 by injection molding, the shaft body 3 is set in the mold 26 with its central axis oriented horizontally, and the gate 29 for feeding molten synthetic resin into the cavity 28 is positioned radially outward from one axial side of the small diameter shaft portion 15. In this way, by oriented horizontally in the axial direction of the shaft body 3 and restricting the axial position of the gate 29, it is possible to prevent weld lines from forming on the surface of the male spline coating portion 22 that constitutes the male spline portion 2. The reason for this is explained below.

According to a simulation conducted by the inventors, when molten synthetic resin is fed into the cavity 28 through the gate 29, the molten synthetic resin spreads circumferentially and toward one axial side, merging (colliding) with molten synthetic resin fed from other gates 29 in the circumferential direction around one axial side of the non-splined shaft portion 7 (small-diameter shaft portion 15). After the flow fronts of molten synthetic resin fed from adjacent gates 29 merge around one axial side of the non-splined shaft portion 7, the flow toward one axial side becomes more prevalent. In this example, the molten synthetic resin is allowed to merge circumferentially around one axial side of the non-splined shaft portion 7 before being fed around the male spline core portion 10. This ensures that the meeting angle between the flow fronts is sufficiently large before the molten synthetic resin reaches the outer peripheral surface of the male spline core portion 10. This prevents weld lines from forming in the male spline coated portion 22. This prevents the aesthetic appearance of the completed spline shaft 1 from being marred and the strength of the male spline covering portion 22 that constitutes the male spline portion 2 from being reduced.

Although there is a possibility that weld lines may form on the surface of the thin-walled portion 32 of the coating layer 4 that covers the outer peripheral surface of one axial side of the non-splined shaft portion 7 (small-diameter shaft portion 15), this does not pose a particular problem in terms of ensuring the aesthetic appearance of the completed spline shaft 1 or the strength of the male spline coated portion 22.

In addition, in this example, the circumferential phase of the gate 29 is matched with the circumferential phase of the tooth core portion 11. Therefore, the allowance remaining after cutting the gate 29 can be allowed up to the height of the tooth tip surface of the tooth core portion 11. However, when implementing this invention, the circumferential phase of the gate can also be matched with the circumferential phase of the groove core portion.

In this example, when forming the coating layer 4 by injection molding, the shaft body 3 is set in the mold 26 with its central axis oriented horizontally. However, when implementing the present invention, the shaft body's central axis can also be oriented vertically. Alternatively, the shaft body can be set in the mold with its central axis tilted relative to the horizontal.

Furthermore, the spline shaft of the present invention is not limited to inner shafts that constitute intermediate shafts for steering devices, but can also be applied to various types of spline shafts. For example, the spline shaft of the present invention can be applied to inner shafts that constitute steering shafts whose rear ends support and fix a steering wheel.

[Second Example of Embodiment]
A second embodiment of the present invention will be described with reference to Figures 7 and 8. In this embodiment, the shaft body 3a includes, from one axial end, a spare shank 5, a splined shank 6, and a non-splined shank 7a. In this embodiment, the non-splined shank 7a consists only of a small-diameter shank 15. That is, unlike the shaft body 3 according to the first embodiment, the shaft body 3a does not include a connecting portion 14 having an incomplete male spline core portion 16 on its outer circumferential surface. The outer circumferential surfaces of the splined shank 6 and the small-diameter shank 15 are connected by a flat second stepped surface 31 perpendicular to the central axis O of the shaft body 3a. The second stepped surface 31 can be formed, for example, by cutting after forging.

As a result, the resistance to the assumed displacement of the coating layer 4a toward one side in the axial direction relative to the shaft body 3a can be increased compared to the spline shaft 1 according to the first example embodiment.

In this example, the volume of the second thick-walled portion 25c of the coating layer 4a, which covers one axial end of the non-spline shaft portion 7a, is larger than that of the spline shaft 1 according to the first embodiment. However, even in this case, it is possible to prevent weld lines from forming in the male spline covering portion 22 that constitutes the male spline portion 2.

Furthermore, according to a simulation performed by the present inventors, when the outer peripheral surface of the spline shaft portion 6 and the outer peripheral surface of the small diameter shaft portion 15 are connected by a flat second step surface 31 perpendicular to the central axis O of the shaft body 3a, as in the present example, the flow of molten synthetic resin toward the downward side when forming the coating layer 4a by injection molding can be made to flow at approximately the same speed regardless of the phase in the circumferential direction, compared to when the outer peripheral surface of the spline shaft portion 6 and the outer peripheral surface of the small diameter shaft portion 15 are connected by a truncated cone-shaped inclined surface portion 17, as in the first example of the embodiment.

The configuration and effects of other parts are the same as those of the first embodiment.

[Third Example of Embodiment]
A third embodiment of the present invention will be described with reference to Fig. 9. In this embodiment, the outer diameter _d5 of one axial end face of the spare shank 5a of the shaft body 3b is made smaller than the outer diameter D30 of the ejector pin _30. The ejector pin 30 is used to press one axial end face of the spare shank 5a and one axial end face of the coating layer 4 (one axial end face of the first thick-walled portion 25a) toward the other axial side when the spline shaft 1 is removed from the mold 26 after the coating layer 4 has been injection molded.

In this example, when the spline shaft 1 is removed from the mold 26, the ejector pin 30 can simultaneously press against one axial end face of the preliminary shaft portion 5a and one axial end face of the coating layer 4. This reliably prevents the shaft body 3b and the coating layer 4 from shifting axially when the spline shaft 1 is removed from the mold 26. The configurations, functions, and effects of other parts are the same as those of the first and second embodiments.

REFERENCE SIGNS LIST 1 spline shaft 2 male spline portion 3, 3a, 3b shaft body 4, 4a coating layer 5, 5a spare shaft portion 6 spline shaft portion 7, 7a non-spline shaft portion 8a, 8b, 8c chamfered portion 9 first step surface 10 male spline core portion 11 tooth core portion 12 groove core portion 13 chamfered portion 14 connection portion 15 small diameter shaft portion 16 incomplete male spline core portion 17 inclined surface portion 17a first inclined surface portion 17b second inclined surface portion 18 convex portion 19 concave portion 20 notched concave portion 21 central hole 22 male spline coated portion 23 tooth coated portion 24 groove coated portion 25a first thick portion 25b, 25c second thick portion 26 mold 27 locating pin 28 Cavity 29 Gate 30 Ejector pin 31 Second step surface 32 Thin-walled portion 100 Steering wheel 101 Steering gear unit 102 Input shaft 103 Tie rod 104 Steering shaft 105 Steering column 106 Universal joint 107, 107a Intermediate shaft 108 Universal joint 109 Inner shaft 110 Outer tube 111 Inner column 112 Outer column 113 Male spline portion 114 Inner shaft 115 Female spline portion 116 Outer tube 117 Shaft body 118 Coating layer 119 Tooth core portion 120 Groove core portion 121 Male spline core portions 122a, 122b, 122c Missing tooth portion 123 Filled portion

Claims (9)

a metal shaft body including a spare shank portion disposed at one end in the axial direction, a male spline core portion having tooth core portions and groove core portions alternately disposed on an outer circumferential surface thereof, a spline shank portion disposed on the other axial side of the spare shank portion, and a non-spline shank portion disposed on the other axial side of the spline shank portion;
A coating layer made of synthetic resin;
Equipped with
an outer circumferential surface of the spare shaft portion and an outer circumferential surface of the spline shaft portion are connected by a first step surface facing one side in the axial direction,
the coating layer has a first thick portion covering the outer peripheral surface of the spare shaft portion, a male spline covering portion covering the outer peripheral surface of the spline shaft portion, a second thick portion covering the outer peripheral surface of one end portion of the non-spline shaft portion in the axial direction, and a thin portion covering a portion of the outer peripheral surface of the non-spline shaft portion that is offset from the second thick portion to the other side in the axial direction,
a radial thickness of the first thick-walled portion is thicker than a radial thickness of the male spline covering portion , and a radial thickness of the second thick-walled portion is thicker than a radial thickness of the male spline covering portion ,
an outer diameter of the thin-walled portion is smaller than an outer diameter of a portion of the second thick-walled portion that coincides with the tooth core portion in the circumferential direction;
the coating layer has a gate mark on the outer peripheral surface of the thin-walled portion;
Splined shaft. There is no weld line on the surface of the male spline covering portion.
The spline shaft according to claim 1 . a metal shaft body including a spare shank portion disposed at one end in the axial direction, a male spline core portion having tooth core portions and groove core portions alternately disposed on an outer circumferential surface thereof, a spline shank portion disposed on the other axial side of the spare shank portion, and a non-spline shank portion disposed on the other axial side of the spline shank portion;
a synthetic resin coating layer covering an outer peripheral surface of the spare shaft portion, an outer peripheral surface of the splined shaft portion, and an outer peripheral surface of one axial end of the non-splined shaft portion;
Equipped with
an outer circumferential surface of the spare shaft portion and an outer circumferential surface of the spline shaft portion are connected by a first step surface facing one side in the axial direction,
a radial thickness of a portion of the coating layer covering the outer peripheral surface of the spare shaft portion is greater than a radial thickness of a portion of the coating layer covering the outer peripheral surface of the splined shaft portion, and a radial thickness of a portion of the coating layer covering the outer peripheral surface of one axial end of the non-splined shaft portion is greater than a radial thickness of a portion of the coating layer covering the outer peripheral surface of the splined shaft portion,
the male spline core portion has, at a plurality of circumferential locations, notched recesses recessed radially inward relative to adjacent portions on the other axial side,
Splined shaft. a weld line is not present in the coating layer in a portion that covers the outer peripheral surface of the spline shaft portion;
The spline shaft according to claim 3 . the coating layer has a gate mark in a portion covering the outer peripheral surface of one end of the non-splined shaft portion in the axial direction;
5. The spline shaft according to claim 4. a phase of the gate mark in the circumferential direction coincides with a phase of any one of the tooth core portions in the circumferential direction;
The spline shaft according to claim 5. an outer circumferential surface of the splined shaft portion and an outer circumferential surface of one axial end of the non-splined shaft portion are connected by a flat second stepped surface perpendicular to the central axis of the shaft body;
The spline shaft according to claim 1 or 3 . a metal shaft body including a spare shank portion disposed at one end in the axial direction, a male spline core portion having tooth core portions and groove core portions alternately disposed on an outer circumferential surface thereof, a spline shank portion disposed on the other axial side of the spare shank portion, and a non-spline shank portion disposed on the other axial side of the spline shank portion;
a synthetic resin coating layer covering an outer peripheral surface of the spare shaft portion, an outer peripheral surface of the splined shaft portion, and an outer peripheral surface of one axial end of the non-splined shaft portion;
Equipped with
an outer circumferential surface of the spare shaft portion and an outer circumferential surface of the spline shaft portion are connected by a first step surface facing one side in the axial direction,
a radial thickness of a portion of the coating layer covering the outer peripheral surface of the spare shaft portion is greater than a radial thickness of a portion of the coating layer covering the outer peripheral surface of the splined shaft portion, and a radial thickness of a portion of the coating layer covering the outer peripheral surface of one axial end of the non-splined shaft portion is greater than a radial thickness of a portion of the coating layer covering the outer peripheral surface of the splined shaft portion,
a method for manufacturing a spline shaft , the coating layer having a gate mark on a portion covering an outer peripheral surface of one end portion of the non-spline shaft portion in an axial direction ,
a step of disposing a mold around one axial end of the non-splined shaft portion, the splined shaft portion, and the spare shaft portion, and disposing a gate around one axial end of the non-splined shaft portion, and then feeding molten synthetic resin into a cavity in the mold through the gate, thereby forming the coating layer ;
an outer diameter of an end surface on one axial side of the spare shaft portion is smaller than an outer diameter of an ejector pin that presses the end surface on one axial side of the spare shaft portion when removing the spline shaft from the mold;
A method for manufacturing a spline shaft. The shaft body is set in the mold with the central axis of the shaft body oriented horizontally.
The method for manufacturing a spline shaft according to claim 8 .