Steering shaft and method of manufacturing the same

Abstract

A steering shaft comprising an inner shaft having a first toothed portion, an outer shaft having a second toothed portion, and a resin layer. The resin layer has a plurality of thick film parts and at least four thin film parts. The thick film parts are arranged with a gap with respect to the second toothed portion, and the thin film parts are in contact with both the first toothed portion and the second toothed portion. The thickness of the thin film part is smaller than the thickness of the thick film part in a cross-section perpendicular to the center axis of the inner shaft.

Description

[Technical Field]

[0001] The present disclosure relates to a steering shaft and a method for manufacturing a steering shaft. [Background technology]

[0002] A vehicle is provided with a steering device as a device for transmitting the operation of the steering wheel by an operator (driver) to wheels (see, for example, Patent Document 1). The steering device includes a steering shaft for transmitting rotational torque. The steering shaft includes an inner shaft having a plurality of first teeth on its outer periphery, an outer shaft having a plurality of second teeth on its inner periphery, and a resin layer provided on the outer periphery of the first teeth. The resin layer portion provided on the tooth surface (side surface) of the first teeth has approximately the same thickness along the circumferential direction for all first teeth. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-145945 Summary of the Invention [Problem to be solved by the invention]

[0004] Resin has a higher coefficient of linear expansion than metal. Therefore, when the steering shaft described in Patent Document 1 is heated, the resin layer portion provided on the tooth surfaces of the first teeth expands and comes into contact with both the tooth surfaces of the first teeth and the tooth surfaces of the second teeth. Then, when the steering shaft returns to room temperature, the resin layer portion contracts. This creates gaps between the resin layer portions provided on all the tooth surfaces of the first teeth and the tooth surfaces of the second teeth, which increases backlash and may cause rattle noise when the vehicle is running.

[0005] The present disclosure has been made in consideration of the above-mentioned problems, and aims to provide a steering shaft and a method for manufacturing a steering shaft that can further reduce rattle noise generated when a vehicle is running. [Means for solving the problem]

[0006] In order to achieve the above object, a steering shaft according to one aspect of the present disclosure includes an inner shaft having a plurality of first tooth portions arranged along a circumferential direction on an outer circumferential surface thereof, an outer shaft having a plurality of second tooth portions arranged along a circumferential direction on an inner circumferential surface thereof and arranged on the outer circumferential side of the first tooth portions, and a resin layer arranged between the first tooth portions and the second tooth portions, the resin layer having a plurality of thick film portions and at least four thin film portions, the thick film portions being arranged between the tooth surfaces of the first tooth portions and the tooth surfaces of the second tooth portions to form a thin film layer that is thin enough to prevent the first tooth portions and the second tooth portions from being broken. the thin film portion is disposed with a gap from one of the first tooth portion and the second tooth portion, the thin film portion is disposed between the tooth flanks of the first tooth portion and the second tooth portion and is in contact with both the first tooth portion and the second tooth portion, and in a cross section orthogonal to the central axis of the inner shaft, the thickness of the thin film portion along a third imaginary line extending circumferentially through a radial midpoint between a first imaginary line extending circumferentially through an outer circumferential end of the first tooth portion and a second imaginary line extending circumferentially through an inner circumferential end of the second tooth portion is smaller than the thickness of the thick film portion along the third imaginary line. Note that the inner shaft and outer shaft of the present disclosure are applicable to both spline fitting and serration fitting.

[0007] As described above, the thin film portion is disposed between the tooth flanks of the first tooth portion and the second tooth portion and contacts both the first tooth portion and the second tooth portion. That is, the tooth flanks of the first tooth portion and the second tooth portion adjacent to each other in the circumferential direction are in contact with the thin film portion between them without any gaps, and at least four such thin film portions are provided. In a high-temperature environment, the resin layer expands circumferentially to contact the tooth flanks of the first tooth portion and the second tooth portion without any gaps. At this time, the resin layer may press the tooth flanks of the first tooth portion and the second tooth portion in the circumferential direction. Here, because the thin film portion has a smaller circumferential thickness than the thick film portion, the thin film portion expands less than the thick film portion, and the resin in the thin film portion is less likely to move radially outward than the thick film portion. Therefore, even when returning from a high-temperature environment to a room-temperature environment, gaps are less likely to form around the thin film portion. Therefore, the steering shaft of the present disclosure exhibits less backlash after returning from a high-temperature environment to a room-temperature environment than the steering shaft of Patent Document 1, thereby enabling greater reduction in rattle noise generated during vehicle operation.

[0008] In a preferred embodiment of the steering shaft, the multiple sets of thin film portions are arranged at intervals along the circumferential direction, so that the steering shaft of the present disclosure has less backlash than the steering shaft of Patent Document 1, making it possible to further reduce rattle noise generated when the vehicle is running.

[0009] In a preferred embodiment of the steering shaft, the plurality of sets of thin film portions are arranged at equal intervals along the circumferential direction. That is, since one set of thin film portions is arranged at equal intervals along the circumferential direction, the steering shaft of the present disclosure has even smaller backlash than the steering shaft of Patent Document 1, and it is possible to further reduce rattle noise generated when the vehicle is traveling. Note that one set of thin film portions means a group of thin film portions arranged adjacent to each other in the circumferential direction.

[0010] In a preferred embodiment of the steering shaft, the number of thin film portions is two, so by reducing the number of thin film portions, it is possible to reduce the backlash of the steering shaft and further reduce the rattle noise that occurs when the vehicle is running.

[0011] In a preferred embodiment of the steering shaft, one set of thin film portions includes four of the thin film portions, which reduces the backlash of the steering shaft compared to a case where two thin film portions are used, thereby making it possible to further reduce rattle noise that occurs when the vehicle is running.

[0012] In a preferred embodiment of the steering shaft, the inner shaft is a hollow member, thereby reducing the weight of the steering shaft.

[0013] In a preferred embodiment of the steering shaft, the first tooth portions include thick teeth and thin teeth whose circumferential widths are smaller than the thick teeth, and the thin-film portion contacts both the thick teeth and the second tooth portions. In this way, by changing some of the first tooth portions to thick teeth, the thin-film portion can be easily formed.

[0014] In a preferred embodiment of the steering shaft, the second tooth portions include thick teeth and thin teeth whose circumferential widths are smaller than the thick teeth, and the thin-film portion contacts both the thick teeth and the first tooth portions. In this way, by changing some of the second tooth portions to thick teeth, the thin-film portion can be easily formed.

[0015] A method for manufacturing a steering shaft according to one embodiment of the present disclosure includes a resin layer forming process for forming a resin layer on an inner shaft having a plurality of first tooth portions arranged circumferentially on its outer peripheral surface, or an outer shaft having a plurality of second tooth portions arranged circumferentially on its inner peripheral surface and arranged on the outer peripheral side of the first tooth portions of the inner shaft; a heating process for heating the inner shaft, the outer shaft, and the resin layer while the resin layer is in contact with the tooth surfaces of all of the first tooth portions and the tooth surfaces of all of the second tooth portions; and a cooling process for cooling the inner shaft, the outer shaft, and the resin layer after the heating process, wherein the plurality of first tooth portions include thick teeth and thin teeth whose width along the circumferential direction is smaller than the width of the thick teeth along the circumferential direction.

[0016] The circumferential gap between the tooth surface of the thick tooth of the first tooth portion and the tooth surface of the second tooth portion is smaller than the circumferential gap between the tooth surface of the thin tooth of the first tooth portion and the tooth surface of the second tooth portion. Therefore, the resin layer disposed in the gap between the thick tooth and the second tooth portion becomes a thin film portion, and the resin layer disposed in the gap between the thin tooth and the second tooth portion becomes a thick film portion. During the heating process, part of the resin in the thick film portion moves circumferentially and solidifies during the cooling process, creating a gap between the solidified thick film portion and the tooth surface of the second tooth portion. However, because the thin film portion is in contact with the thick tooth and the second tooth portion during both the heating process and the cooling process, no gap is created. In this way, it is possible to form the thin film portion and the thick film portion of the resin layer with a relatively easy process.

[0017] A method for manufacturing a steering shaft according to one embodiment of the present disclosure includes a resin layer forming process for forming a resin layer on an inner shaft having a plurality of first tooth portions arranged circumferentially on its outer peripheral surface, or an outer shaft having a plurality of second tooth portions arranged circumferentially on its inner peripheral surface and arranged on the outer peripheral side of the first tooth portions of the inner shaft; a heating process for heating the inner shaft, the outer shaft, and the resin layer while the resin layer is in contact with the tooth surfaces of all of the first tooth portions and the tooth surfaces of all of the second tooth portions; and a cooling process for cooling the inner shaft, the outer shaft, and the resin layer after the heating process, wherein the plurality of second tooth portions include thick teeth and thin teeth whose width along the circumferential direction is smaller than the width of the thick teeth along the circumferential direction.

[0018] The circumferential gap between the tooth surface of the thick tooth of the second tooth portion and the tooth surface of the first tooth portion is smaller than the circumferential gap between the tooth surface of the thin tooth of the second tooth portion and the tooth surface of the first tooth portion. Therefore, the resin layer disposed in the gap between the thick tooth and the first tooth portion becomes a thin film portion, and the resin layer disposed in the gap between the thin tooth and the first tooth portion becomes a thick film portion. During the heating process, part of the resin in the thick film portion moves circumferentially and solidifies during the cooling process, resulting in a gap between the solidified thick film portion and the tooth surface of the first tooth portion. However, since the thin film portion is in contact with the thick tooth and the first tooth portion during both the heating process and the cooling process, no gap is generated. In this way, it is possible to form the thin film portion and the thick film portion of the resin layer with a relatively easy process. [Effects of the Invention]

[0019] According to the present disclosure, it is possible to provide a steering shaft and a method for manufacturing a steering shaft that can further reduce rattle noise generated when a vehicle is running. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram showing an outline of a steering device according to a first embodiment. [Figure 2]FIG. 2 is a perspective view showing an outline of the steering device according to the first embodiment. [Figure 3] FIG. 3 is a side view showing the lower shaft and the universal joint of FIG. [Figure 4] FIG. 4 is a schematic diagram showing a cross section taken along line IV-IV in FIG. [Figure 5] FIG. 5 is a schematic diagram showing a cross section of the inner shaft and the resin layer of FIG. [Figure 6] FIG. 6 is a schematic diagram showing an enlarged portion of FIG. [Figure 7] FIG. 7 is a schematic diagram showing an enlarged portion of FIG. [Figure 8] FIG. 8 is a schematic diagram showing an enlarged portion of FIG. [Figure 9] FIG. 9 is a schematic view for explaining the method for manufacturing the lower shaft according to the first embodiment, showing the state in which the lower shaft is housed inside the heating furnace after the resin layer forming step is completed. [Figure 10] FIG. 10 is a schematic view illustrating the method for manufacturing the lower shaft according to the first embodiment, showing the cooling step. [Figure 11] FIG. 11 is a schematic diagram showing the deformation behavior of the inner shaft and the outer shaft when the lower shaft of the first embodiment is exposed to a high-temperature environment. [Figure 12] FIG. 12 is a schematic view showing a cross section of a lower shaft according to the second embodiment. [Figure 13] FIG. 13 is a schematic diagram showing a cross section of the outer shaft of FIG. [Figure 14] FIG. 14 is a schematic diagram showing an enlarged portion of FIG. [Figure 15] FIG. 15 is a schematic diagram showing an enlarged portion of FIG. [Figure 16] FIG. 16 is a schematic view showing a cross section of a lower shaft according to the third embodiment. [Figure 17] FIG. 17 is a schematic diagram showing an enlarged portion of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0021] Modes (embodiments) for carrying out the invention will be described in detail with reference to the drawings. The present invention is not limited to the contents described in the following embodiments. Furthermore, the components described below include those that can be easily imagined by a person skilled in the art and those that are substantially the same. Furthermore, the components described below can be combined as appropriate.

[0022] [First embodiment] Fig. 1 is a schematic diagram showing an outline of a steering device according to a first embodiment, and Fig. 2 is a perspective view showing an outline of the steering device according to the first embodiment.

[0023] 1 and 2, the steering device 80 includes, in the order in which force applied by the operator is transmitted, a steering wheel 81, an upper shaft 82, a steering force assist mechanism 83, a universal joint 84, a lower shaft (steering shaft) 1, and a universal joint 86, which are joined to a pinion shaft 87. The steering device 80 also includes an ECU (Electronic Control Unit) 90 and a torque sensor 94. A vehicle speed sensor 95 is provided on the vehicle body and outputs a vehicle speed signal V to the ECU 90 via CAN (Controller Area Network) communication.

[0024] The upper shaft 82 includes an input shaft 82a and an output shaft 82b. One end of the input shaft 82a is connected to the steering wheel 81, and the other end of the input shaft 82a is connected to the output shaft 82b. One end of the output shaft 82b is connected to the input shaft 82a, and the other end of the output shaft 82b is connected to a universal joint 84. In this embodiment, the input shaft 82a and the output shaft 82b are formed from a general steel material such as carbon steel for machine structural use (SC material (Carbon Steel for Machine Structural Use)) or carbon steel tubes for machine structural purposes (so-called STKM material (Carbon Steel Tubes for Machine Structural Purposes)).

[0025] Lower shaft (steering shaft) 1 is a member connected to output shaft 82b via universal joint 84. One end of lower shaft 1 is connected to universal joint 84, and the other end is connected to universal joint 86. One end of pinion shaft 87 is connected to universal joint 86, and the other end of pinion shaft 87 is connected to steering gear 88.

[0026] The steering gear 88 includes a pinion 88a and a rack 88b. The pinion 88a is connected to the pinion shaft 87. The rack 88b meshes with the pinion 88a. The steering gear 88 converts the rotational motion transmitted to the pinion 88a into linear motion by the rack 88b. The rack 88b is connected to a tie rod 89. In other words, the steering device 80 is of a rack-and-pinion type.

[0027] The steering force assist mechanism 83 includes a reduction gear 92 and an electric motor 93. The electric motor 93 is, for example, a brushless motor, but may also be a motor including brushes (sliders) and a commutator. The reduction gear 92 is, for example, a worm reduction gear. Torque generated by the electric motor 93 is transmitted to a worm wheel via a worm inside the reduction gear 92, causing the worm wheel to rotate. The reduction gear 92 increases the torque generated by the electric motor 93 by means of the worm and worm wheel. The reduction gear 92 then applies an auxiliary steering torque to the output shaft 82b. In other words, the steering device 80 is of a column assist type.

[0028] The torque sensor 94 detects, as steering torque, the steering force of the operator transmitted to the input shaft 82a via the steering wheel 81. The vehicle speed sensor 95 detects the traveling speed (vehicle speed) of the vehicle body on which the steering device 80 is mounted. The electric motor 93, the torque sensor 94, and the vehicle speed sensor 95 are electrically connected to the ECU 90.

[0029] The ECU 90 controls the operation of the electric motor 93. The ECU 90 also acquires signals from a torque sensor 94 and a vehicle speed sensor 95. That is, the ECU 90 acquires a steering torque T from the torque sensor 94 and a vehicle speed signal V of the vehicle body from the vehicle speed sensor 95. When an ignition switch 98 is in the on state, the ECU 90 is supplied with power from a power supply device (for example, an on-board battery) 99. The ECU 90 calculates an assist steering command value of an assist command based on the steering torque T and the vehicle speed signal V. Then, the ECU 90 adjusts the power value X to be supplied to the electric motor 93 based on the calculated assist steering command value. The ECU 90 acquires, as operation information Y, information on an induced voltage from the electric motor 93 or information output from a resolver or the like provided in the electric motor 93.

[0030] The steering force of the operator (driver) input to the steering wheel 81 is transmitted to the reduction gear 92 of the steering force assist mechanism 83 via the input shaft 82a. At this time, the ECU 90 obtains the steering torque T input to the input shaft 82a from the torque sensor 94, and obtains the vehicle speed signal V from the vehicle speed sensor 95. Then, the ECU 90 controls the operation of the electric motor 93. The assist steering torque generated by the electric motor 93 is transmitted to the reduction gear 92.

[0031] The steering torque (including the auxiliary steering torque) output via the output shaft 82b is transmitted to the lower shaft (steering shaft) 1 via a universal joint 84, and is further transmitted to a pinion shaft 87 via a universal joint 86. The steering force transmitted to the pinion shaft 87 is transmitted to a tie rod 89 via a steering gear 88, displacing the wheels.

[0032] Figure 3 is a perspective view showing the lower shaft and universal joint of Figure 2. As shown in Figure 3, the lower shaft (steering shaft) 1 includes an inner shaft 2, an outer shaft 3, a resin layer 4 (see Figure 4), and universal joints 84, 86. The universal joints 84, 86 are coupled to the ends of the lower shaft 1. The inner shaft 2 and the outer shaft 3 extend along the axial direction of the central axis Ax.

[0033] The universal joint 84 is coupled to the end of the inner shaft 2. The universal joint 84 includes a base 841 and a yoke 842. The yoke 842 is bifurcated, with a through-hole provided in each. A through-hole is also provided in the yoke 21 of the inner shaft 2. The spider 843 is fitted into these through-holes, thereby coupling the universal joint 84 to the inner shaft 2.

[0034] The universal joint 86 is coupled to the end of the outer shaft 3. The universal joint 86 includes a base 861 and a yoke 862. The yoke 862 branches into two parts, each of which has a through-hole. A through-hole is also formed in the yoke 31 of the outer shaft 3. The spider 863 is fitted into these through-holes, thereby coupling the universal joint 86 to the outer shaft 3.

[0035] Fig. 4 is a schematic diagram showing a cross section taken along line IV-IV in Fig. 3. Fig. 5 is a schematic diagram showing a cross section of the inner shaft and resin layer in Fig. 4. Fig. 6 is a schematic diagram showing an enlarged portion of Fig. 5. As shown in Figs. 4 and 5, the inner shaft 2 includes a tubular portion 22 and a first tooth portion 23. The inner shaft 2 can be made of a wide variety of metals, such as carbon steel.

[0036] The cylindrical portion 22 is hollow and extends cylindrically in the circumferential direction around the central axis Ax. In this manner, the inner shaft 2 is a hollow member. First tooth portions 23 protrude radially outward from the outer circumferential surface of the inner shaft 2. A plurality of first tooth portions 23 are arranged at equal intervals in the circumferential direction on the outer circumferential surface of the cylindrical portion 22. Each first tooth portion 23 has a tooth tip 231, a tooth surface (side surface) 232, and a tooth bottom 233. The first tooth portion 23 has thick teeth 24 and thin teeth 25. Two thick teeth 24 are provided at the end on the Y1 side and two are provided at the end on the Y2 side as shown in FIG. 4, for a total of four thick teeth 24. In this embodiment, all of the first tooth portions 23 other than the thick teeth 24 are thin teeth 25. Details will be described later.

[0037] As shown in FIGS. 4 and 5 , the resin layer 4 is provided on the outer peripheral surface of the first tooth portion 23. Specifically, the resin layer 4 is provided on the outer peripheral sides of the tooth tip 231, tooth flank 232, and tooth bottom 233 of the first tooth portion 23. The resin layer 4 has thin film portions 41 and thick film portions 42. A total of four thin film portions 41 are provided, with one pair provided at the end on the Y1 side and another pair provided at the end on the Y2 side as shown in FIG. 4 . One pair of thin film portions 41 and the other pair of thin film portions 41 are disposed symmetrically with respect to the central axis Ax. In other words, a pair of thin film portions 41 are provided circumferentially adjacent to each other at the end on the Y1 side as shown in FIG. 4 , and another pair are provided circumferentially adjacent to each other at the end on the Y2 side as shown in FIG. 4 . In this embodiment, the portions of the resin layer 4 facing the tooth flank 232 other than the thin film portions 41 are all thick film portions 42. In this way, a total of four thin film portions 41 are provided symmetrically with respect to the central axis Ax. However, the present disclosure is not limited to this, and four or more thin film portions 41 may be provided, and three or more sets of thin film portions 41 may be arranged in the circumferential direction. Furthermore, the multiple sets of thin film portions 41 may be arranged at equal intervals in the circumferential direction, but is not limited to this, and the positions at which the multiple sets of thin film portions 41 are arranged do not have to be at equal intervals. Further details will be described later.

[0038] 4 and 6, the outer shaft 3 includes a cylindrical portion 32 and a second tooth portion 33. The outer shaft 3 can be made of a wide variety of metals, such as carbon steel.

[0039] The tubular portion 32 is hollow and extends cylindrically in the circumferential direction around the central axis Ax. Second tooth portions 33 protrude radially inward from the inner periphery of the tubular portion 32. A plurality of second tooth portions 33 are arranged at equal intervals in the circumferential direction on the inner circumferential surface of the outer shaft 3. Each second tooth portion 33 has a tooth tip 331, a tooth surface 332, and a tooth bottom 333. All of the second tooth portions 33 have the same circumferential width.

[0040] FIG. 7 is a schematic diagram showing an enlargement of a portion of FIG. 5. FIG. 8 is a schematic diagram showing an enlargement of a portion of FIG. 5. As shown in FIGS. 6 to 8, a first imaginary line 110, a second imaginary line 120, and a third imaginary line 130 are set in a cross section orthogonal to the central axis Ax of the inner shaft 2. The first imaginary line 110 is centered on the central axis Ax and extends circumferentially through the tooth tip (outer peripheral end) 231 of the first tooth portion 23. The second imaginary line 120 is centered on the central axis Ax and extends circumferentially through the tooth tip (inner peripheral end) 331 of the second tooth portion 33. The third imaginary line 130 extends circumferentially, passing through a radial midpoint between the first imaginary line 110 and the second imaginary line 120.

[0041] 6, of the first tooth portion 23, the thick tooth 24 has a width T1 along the third imaginary line 130. The thin tooth 25 has a width T2 along the third imaginary line 130. The width T2 of the thin tooth 25 is smaller than the width T1 of the thick tooth 24. The thick tooth 24 has a tooth tip 241 and a tooth surface (side surface) 242. The thin tooth 25 has a tooth tip 251 and a tooth surface (side surface) 252.

[0042] Furthermore, as shown in FIG. 7, the thin film portion 41 of the resin layer 4 is provided between the thick tooth 24 of the first tooth portion 23 and the second tooth portion 33. Specifically, the thin film portion 41 contacts both the tooth surface 242 of the thick tooth 24 and the tooth surface 332 of the second tooth portion 33. Furthermore, the number of first tooth portion 23 (thick tooth 24) disposed between a pair of circumferentially adjacent thin film portions 41 is one. The thickness of the thin film portion 41 along the third virtual line 130 is thickness t1. The multiple pairs of thin film portions 41 are disposed at intervals along the circumferential direction. Note that a pair of thin film portions refers to a group of thin film portions disposed adjacently in the circumferential direction. Furthermore, the thickness t1 of the thin film portion 41 is preferably, for example, 0.2 mm or less.

[0043] Furthermore, as shown in FIG. 8 , in this embodiment, the thick film portion 42 is a portion of the resin layer 4 that is disposed between the tooth surface 232 (see FIG. 6 ) and the tooth surface 332, other than the thin film portion 41. Specifically, the thick film portion 42 of the resin layer 4 is provided between the thin tooth 25 of the first tooth portion 23 and the second tooth portion 33. More specifically, the thick film portion 42 is provided between the tooth surface 252 of the thin tooth 25 and the tooth surface 332 of the second tooth portion 33. The thickness of the thick film portion 42 along the third imaginary line 130 is thickness t2. The thickness t1 of the thin film portion 41 is smaller than the thickness t2 of the thick film portion 42. Note that, as shown in FIG. 8 , a side surface 421 of the thick film portion 42 is separated from the tooth surface 332 of the second tooth portion 33. In other words, a gap G is provided between the side surface 421 of the thick film portion 42 and the tooth surface 332 of the second tooth portion 33. This gap G is provided when the temperature is, for example, 80 degrees or less.

[0044] Next, a method for manufacturing the lower shaft (steering shaft) 1 will be described. Fig. 9 is a schematic diagram for explaining the method for manufacturing the lower shaft of the first embodiment, showing the state in which the lower shaft 1 has been stored inside a heating furnace after the resin layer forming step has been completed. Fig. 10 is a schematic diagram for explaining the method for manufacturing the lower shaft of the first embodiment, showing the cooling step.

[0045] First, the resin layer 4 is formed on the outer periphery of the inner shaft 2, and the outer shaft 3 is fitted onto the outer periphery of the inner shaft 2 via the resin layer 4 to assemble the lower shaft 1. When the inner shaft 2 is fitted onto the outer shaft 3, a radial gap is provided between the resin layer 4 and the tooth bottoms 333 of the second tooth portions 33 (see FIG. 8). Furthermore, no gap G (see FIG. 8) is provided between the tooth flanks 332 of the second tooth portions 33 and any of the resin layers 4. That is, the resin layer 4 contacts the tooth flanks 232 of all the first tooth portions 23 and the tooth flanks 332 of all the second tooth portions 33. Therefore, a pair of thin film portions 41 is disposed on each of the Y1-side end and the Y2-side end of the resin layer 4 in FIG. 9. In this state, the lower shaft 1 is housed inside a heating furnace 140 in the heating process, as shown in FIG. 9.

[0046] The internal temperature of the heating furnace 140 is raised to, for example, 130°C. Because resin has a higher linear expansion coefficient than metal, the resin layer 4 expands more than the metal inner shaft 2 and outer shaft 3. As described above, a pair of thin film portions 41 is disposed at each of the Y1-side end and the Y2-side end. Therefore, when the resin layer 4 thermally expands, the thick film portions 42 at the X1-side end and the X2-side end expand more circumferentially than the thin film portions 41 at the Y1-side end and the Y2-side end. Furthermore, some of the resin in the thick film portions 42 moves radially outward. On the other hand, because the thin film portions 41 have a smaller circumferential thickness than the thick film portions 42, the amount of thermal expansion of the thin film portions 41 when heated is smaller than that of the thick film portions 42. Therefore, the resin in the thin film portions 41 is less likely to move radially outward during the heating process. Next, as shown in FIG. 10, during the cooling process, the inner shaft 2 and the outer shaft 3 are removed from the heating furnace 140 and cooled. As a result, part of the resin in the thick film portion 42 is solidified while remaining in a state of having moved radially outward. On the other hand, the thin film portion 41 is solidified while having hardly moved radially outward.

[0047] 10, the thin film portions 41 at the Y1-side end and the Y2-side end are in contact with both the tooth surfaces 242 of the thick teeth 24 and the tooth surfaces 332 of the second tooth portions 33. On the other hand, in the thick film portions 42, a gap G is formed between the tooth surfaces 332 of the second tooth portions 33 and the resin layer 4, as described in FIG.

[0048] Next, the deformation behavior of the lower shaft when exposed to a high-temperature environment will be described with reference to Fig. 11. Fig. 11 is a schematic diagram showing the deformation behavior of the inner shaft and the outer shaft when the lower shaft of the first embodiment is exposed to a high-temperature environment. Below, the case where the rigidity of the inner shaft 2 is higher than the rigidity of the outer shaft 3 and the case where the rigidity of the inner shaft 2 is lower than the rigidity of the outer shaft 3 will be described separately.

[0049] [When the rigidity of inner shaft 2 is higher than the rigidity of outer shaft 3] When the radial thicknesses of the outer shaft 3 and the inner shaft 2 are approximately the same, the outer shaft 3 has less radial rigidity than the inner shaft 2. When the lower shaft 1 is exposed to high temperatures, the amount of expansion (elastic deformation) of the inner shaft 2 is extremely small, and the outer shaft 3 expands (elastically deforms) more than the inner shaft 2.

[0050] Furthermore, in a high-temperature environment, the resin layer 4 expands in the circumferential direction, which may cause the side surfaces 421 (see FIG. 8 ) of the thick film portions 42 to come into contact with the tooth surfaces 332 of the second tooth portions 33, causing the thick film portions 42 to spread the tooth surfaces 332 of the second tooth portions 33 in the circumferential direction. The thin film portions 41 are in contact with the tooth surfaces 232 of the first tooth portions 23 and the tooth surfaces 332 of the second tooth portions 33 even in a room-temperature environment. However, because the thick film portions 42 are thicker than the thin film portions 41, during thermal expansion in a high-temperature environment, the force with which the thick film portions 42 spread the second tooth portions 33 in the circumferential direction becomes greater than the force with which the thin film portions 41 spread the second tooth portions 33. As described above, the thin film portions 41 are located at the Y1-side end and the Y2-side end of the inner shaft 2. Therefore, the X1-side and X2-side portions of the outer shaft 3 deform more circumferentially than the Y1-side and Y2-side portions, so the outer shaft 3 on the outer periphery stretches in the X direction and elastically deforms horizontally. Furthermore, the inner shaft 2 deforms very little, smaller than the outer shaft 3, so the inner shaft 2 exposed to high temperatures maintains, for example, a substantially circular shape. As described above, the outer shaft 3 expands in the X direction and elastically deforms into a horizontally elongated ellipse, as shown in FIG. 11, and the resin layer 4 hardly moves radially outward. When the temperature subsequently returns to room temperature, the resin layer 4 contracts, and the outer shaft 3 also returns to its original shape. As a result, the thin film portion 41 again contacts the tooth surfaces of the first tooth portion 23 and the second tooth portion 33, creating a gap G (see FIG. 8) between the thick film portion 42 and the tooth surface 332 in a room temperature environment.

[0051] [When the rigidity of inner shaft 2 is lower than the rigidity of outer shaft 3] When the radial thickness of the inner shaft 2 is smaller than the radial thickness of the outer shaft 3 and the radial rigidity of the inner shaft 2 is smaller than that of the outer shaft 3, the inner shaft 2 undergoes greater elastic deformation than the outer shaft 3 in a high-temperature environment. In a high-temperature environment, expansion of the resin layer 4 may cause the thin film portion 41 to come into contact with the tooth surfaces 232 of the first tooth portions 23 and the tooth surfaces 332 of the second tooth portions 33, and a force may be applied from the thick film portion 42 to spread the second tooth portions 33. The force with which the thick film portion 42 spreads the second tooth portions 33 is greater than the force with which the thin film portion 41 spreads the second tooth portions 33. Since the radial rigidity of the inner shaft 2 is smaller than that of the outer shaft 3, the inner shaft 2 on the inner periphery side elastically deforms and extends in the Y direction, while the outer shaft 3 has an extremely small amount of elastic deformation and maintains its approximately circular shape. As a result, the inner shaft 2 expands in the Y direction, elastically deforming into a vertically elongated ellipse, with the resin layer 4 barely moving radially outward. When the temperature subsequently returns to room temperature, the resin layer 4 contracts and the outer shaft 3 returns to its original shape. As a result, in the room temperature environment, the thin film portion 41 again comes into contact with the tooth surfaces of the first tooth portion 23 and the second tooth portion 33, and a gap G (see FIG. 8) is created between the thick film portion 42 and the tooth surface 332.

[0052] As described above, the lower shaft (steering shaft) 1 according to the first embodiment includes the inner shaft 2 having a plurality of first tooth portions 23 arranged circumferentially on its outer circumferential surface, the outer shaft 3 having a plurality of second tooth portions 33 arranged circumferentially on its inner circumferential surface and arranged on the outer peripheral side of the first tooth portions 23, and the resin layer 4 arranged between the first tooth portions 23 and the second tooth portions 33. The resin layer 4 has a plurality of thick film portions 42 and at least four thin film portions 41, and the thick film portion 42 is arranged between the tooth surfaces 232 of the first tooth portions 23 and the tooth surfaces of the second tooth portions 33 with a gap between them and the second tooth portions 33, and the thin film portion 41 is arranged between the tooth surfaces 232 of the first tooth portions 23 and the tooth surfaces 332 of the second tooth portions 33 and is in contact with both the first tooth portions 23 and the second tooth portions 33. In a cross section perpendicular to the central axis Ax of the inner shaft 2, the thickness t1 of the thin film portion 41 along a third imaginary line 130 extending circumferentially through the radial midpoint between a first imaginary line 110 extending circumferentially through the outer peripheral end of the first tooth portion 23 and a second imaginary line 120 extending circumferentially through the inner peripheral end of the second tooth portion 33 is smaller than the thickness t2 of the thick film portion 42 along the third imaginary line 130.

[0053] As described above, the thin film portion 41 is disposed between the tooth surface 232 of the first tooth portion 23 and the tooth surface 332 of the second tooth portion 33, and contacts both the first tooth portion 23 and the second tooth portion 33. That is, the tooth surface 232 of the first tooth portion 23 and the tooth surface 332 of the second tooth portion 33, which are adjacent in the circumferential direction, contact the thin film portion 41 therebetween without any gaps, and at least four thin film portions 41 are provided. In a high-temperature environment, the resin layer 4 expands in the circumferential direction to contact the tooth surface 232 of the first tooth portion 23 and the tooth surface 332 of the second tooth portion 33 without any gaps. Because the thin film portion 41 has a smaller circumferential thickness than the thick film portion 42, the amount of thermal expansion of the thin film portion 41 along the circumferential direction in a high-temperature environment is smaller than that of the thick film portion 42. Therefore, in a high-temperature environment, the resin of the thin film portion 41 is less likely to move radially outward than the thick film portion 42. For this reason, even when the temperature returns from a high-temperature environment to a normal temperature environment, gaps are less likely to occur around the thin film portion 41. Therefore, the steering shaft 1 of the first embodiment has less backlash after returning from a high-temperature environment to a normal temperature environment than the steering shaft of Patent Document 1, making it possible to further reduce rattle noise that occurs when the vehicle is running.

[0054] Furthermore, the number of second tooth portions 33 arranged between one set of thin film portions 41 is one, and the multiple sets of thin film portions 41 are arranged at equal intervals along the circumferential direction. Therefore, the steering shaft 1 of the first embodiment has even smaller backlash than the steering shaft of Patent Document 1, making it possible to further reduce rattle noise generated when the vehicle is running.

[0055] Furthermore, the inner shaft 2 is a hollow member. Therefore, the radial bending rigidity of the inner shaft 2 is low. Therefore, the steering shaft 1 of the first embodiment can further reduce backlash and transmit large torque. In addition, the weight of the steering shaft 1 is reduced.

[0056] The plurality of first tooth portions 23 include thick teeth 24 and thin teeth 25 whose circumferential width is smaller than that of the thick teeth 24, and the thin film portion 41 contacts both the thick teeth 24 and the second tooth portions 33. By changing some of the plurality of first tooth portions 23 to the thick teeth 24 in this way, the thin film portion 41 of the resin layer 4 can be easily formed.

[0057] Furthermore, since the number of sets of thin film portions 41 is two, it is possible to reduce the number of sets of thin film portions 41, thereby reducing the backlash of the steering shaft and further reducing the rattle noise that occurs when the vehicle is running.

[0058] The manufacturing method of the lower shaft (steering shaft) 1 of the first embodiment includes a resin layer forming process of forming a resin layer 4 on an inner shaft 2 having a plurality of first tooth portions 23 arranged circumferentially on its outer surface; a heating process of heating the inner shaft 2, the outer shaft 3, and the resin layer 4 while the resin layer 4 is in contact with the tooth surfaces 232 of all the first tooth portions 23 and the tooth surfaces 332 of all the second tooth portions 33; and a cooling process of cooling the inner shaft 2, the outer shaft 3, and the resin layer 4 after the heating process, wherein the plurality of first tooth portions 23 include thick teeth 24 and thin teeth 25 whose width along the circumferential direction is smaller than the width of the thick teeth 24 along the circumferential direction.

[0059] The circumferential gap between the tooth surface 242 of the thick tooth 24 of the first tooth portion 23 and the tooth surface 332 of the second tooth portion 33 is smaller than the circumferential gap between the tooth surface 252 of the thin tooth 25 of the first tooth portion 23 and the tooth surface 332 of the second tooth portion 33. Therefore, the resin layer 4 disposed in the gap between the thick tooth 24 and the second tooth portion 33 becomes a thin film portion 41, and the resin layer 4 disposed in the gap between the thin tooth 25 and the second tooth portion 33 becomes a thick film portion 42. During the heating process, part of the resin in the thick film portion 42 moves radially outward and solidifies during the cooling process, resulting in a gap between the solidified thick film portion 42 and the tooth surface 332 of the second tooth portion 33. However, during the heating process, the thin film portion 41 does not move radially outward as much as the thick film portion 42. Therefore, no gap is formed around the thin film portion 41 during the cooling process. In this way, the thin film portion 41 and the thick film portion 42 of the resin layer 4 can be easily formed. If the steering shaft of Patent Document 1 is subsequently exposed to a high-temperature environment, all of the resin layer portions of the first tooth portions may expand, potentially increasing the sliding resistance between all of the resin layer portions and the tooth surfaces of all of the second tooth portions. In contrast, in the steering shaft of the present disclosure, when exposed to a high-temperature environment (e.g., 80°C or 100°C), the thin film portion 41 contacts the tooth surfaces of the first tooth portions 23 and the second tooth portions 33. However, the thick film portion 42 other than the thin film portion 41 has a gap between itself and the tooth surfaces of the tooth portions in a room-temperature environment (e.g., 80°C or below). Therefore, the force of contact with the tooth surfaces of the tooth portions is weaker in a high-temperature environment. As a result, the sliding resistance between the resin layer 4 and the tooth surfaces of the tooth portions is lower in the steering shaft of the present disclosure than in Patent Document 1.

[0060] [Second embodiment] Next, a steering shaft according to a second embodiment will be described, but parts having the same structure as those of the steering shaft according to the first embodiment described above will be assigned the same reference numerals and description thereof will be omitted.

[0061] Fig. 12 is a schematic view showing a cross section of a lower shaft according to a second embodiment. Fig. 13 is a schematic view showing a cross section of the outer shaft of Fig. 12. Fig. 14 is a schematic view showing an enlarged portion of Fig. 12. Fig. 15 is a schematic view showing an enlarged portion of Fig. 12.

[0062] As shown in FIG. 12 , a lower shaft (steering shaft) 1A includes an inner shaft 2A, an outer shaft 3A, and a resin layer 4A. In the first embodiment, all of the second tooth portions 33 have the same width. However, in the second embodiment, the second tooth portion 33A includes a thick tooth 34 and a thin tooth 35 whose circumferential width is smaller than that of the thick tooth 34. The thick tooth 34 has a tooth tip 341 and a tooth surface (side surface) 342. The thin tooth 35 has a tooth tip 351 and a tooth surface (side surface) 352. In the first embodiment, the first tooth portion 23 includes a thick tooth 24 and a thin tooth 25. However, in the second embodiment, all of the first tooth portions 23A have the same width. The thin film portion 41A of the resin layer 4A contacts both the thick tooth 34 and the first tooth portion 23A.

[0063] 12, the inner shaft 2A includes a cylindrical portion 22A and a first tooth portion 23A. The inner shaft 2A can be made of a wide variety of metals, such as carbon steel.

[0064] The inner shaft 2A is a hollow member. A plurality of first tooth portions 23A are arranged on the outer peripheral surface of the inner shaft 2A at equal intervals along the circumferential direction. As shown in Fig. 15, each first tooth portion 23A has a tooth tip 231A, a tooth surface (side surface) 232A, and a tooth bottom 233A. The first tooth portions 23A all have the same width.

[0065] As shown in FIG. 12 , the resin layer 4 has a thin film portion 41A and a thick film portion 42A. One pair of thin film portions 41A is provided at the Y1-side end portion shown in FIG. 12 , and another pair is provided at the Y2-side end portion shown in FIG. 12 , for a total of four thin film portions 41A. In other words, a pair of thin film portions 41A is provided circumferentially adjacent to each other at the Y1-side end portion shown in FIG. 12 , and a pair of thin film portions 41A is provided circumferentially adjacent to each other at the Y2-side end portion shown in FIG. 12 . The thick film portion 42A is a portion of the resin layer 4A that is disposed between the tooth surface 232A and the tooth surface 332A, other than the thin film portion 41A. In this way, a total of four thin film portions 41A are provided at positions symmetrical about the central axis Ax. However, the present disclosure is not limited thereto, and four or more thin film portions 41A may be provided, and three or more pairs of thin film portions 41A may be disposed in the circumferential direction. Furthermore, the plurality of sets of thin film portions 41A may be arranged at equal intervals in the circumferential direction, but this is not limitative, and the positions at which the plurality of sets of thin film portions 41A are arranged do not have to be at equal intervals.

[0066] 12 and 13, the outer shaft 3A includes a cylindrical portion 32A and a second tooth portion 33A. The outer shaft 3A can be made of a wide variety of metals, such as carbon steel.

[0067] As shown in FIGS. 14 and 15, a plurality of second tooth portions 33A are arranged at equal intervals along the circumferential direction on the inner circumferential surface of the outer shaft 3A. As shown in FIG. 15, the second tooth portion 33A has a tooth tip 331A, a tooth surface (side surface) 332A, and a tooth bottom 333A. As shown in FIGS. 12 and 13, the second tooth portion 33A includes a thick tooth 34 and a thin tooth 35 whose width along the circumferential direction is smaller than the width of the thick tooth 34 along the circumferential direction. Two thick teeth 34 are provided at the end on the Y1 side and two thick teeth 34 are provided at the end on the Y2 side as shown in FIG. 12, for a total of four thick teeth 34. In this embodiment, all teeth other than the thick teeth 34 are thin teeth 35.

[0068] 14, the thin film portion 41A of the resin layer 4A is provided between the thick tooth 34 of the second tooth portion 33A and the first tooth portion 23A. Specifically, the thin film portion 41A is in contact with both the tooth surface 342 of the thick tooth 34 and the tooth surface 232A of the first tooth portion 23A. The number of first tooth portions 23A disposed between a pair of circumferentially adjacent thin film portions 41A is one. The thickness of the thin film portion 41A along the third virtual line 130 is thickness t3.

[0069] 15, the thick film portion 42A is the resin layer 4A in a portion other than the thin film portion 41A. Specifically, the thick film portion 42A is provided between the tooth surface 232A of the first tooth portion 23A and the tooth surface 332A of the second tooth portion 33A. The thickness of the thick film portion 42A along the third imaginary line 130 is thickness t4. The thickness t3 of the thin film portion 41A is smaller than the thickness t4 of the thick film portion 42A. A gap G is provided between the side surface 421A of the thick film portion 42A and the tooth surface 332A of the second tooth portion 33A.

[0070] The manufacturing method of the lower shaft (steering shaft) 1A of the second embodiment is substantially the same as the manufacturing method of the lower shaft (steering shaft) 1 of the first embodiment. Specifically, the manufacturing method of the lower shaft 1A includes a resin layer forming step of forming a resin layer 4A on an inner shaft 2A having a plurality of first tooth portions 23A, a heating step of heating the inner shaft 2A, the outer shaft 3A, and the resin layer 4A in a heating furnace 140 (see FIG. 9 ) with the resin layer 4 in contact with the tooth surfaces 232A of all the first tooth portions 23A and the tooth surfaces 332A of all the second tooth portions 33A, and a cooling step of cooling the inner shaft 2A, the outer shaft 3A, and the resin layer 4A after the heating step, wherein the plurality of second tooth portions 33A include thick-walled teeth 34 and thin-walled teeth 35 whose width along the circumferential direction is smaller than the width of the thick-walled teeth 34 along the circumferential direction.

[0071] During the heating process, when the resin layer 4A thermally expands, the thick film portions 42A at the X1-side end and the X2-side end in FIG. 12 expand more in the circumferential direction than the thin film portions 41A at the Y1-side end and the Y2-side end. Furthermore, a portion of the resin in the thick film portions 42A moves radially outward. Meanwhile, because the thin film portions 41A have a smaller circumferential thickness than the thick film portions 42A, the amount of thermal expansion of the thin film portions 41A when heated is smaller than that of the thick film portions 42A. Therefore, during the heating process, the resin in the thin film portions 41A does not easily move radially outward. Therefore, during the cooling process, a portion of the resin in the thick film portions 42A solidifies while remaining in a state of having moved radially outward. Meanwhile, the thin film portions 41A solidify without having moved radially outward at all.

[0072] As described above, in the second embodiment, the plurality of second tooth portions 33A include thick teeth 34 and thin teeth 35 whose circumferential width is smaller than that of the thick teeth 34, and the thin film portion 41A contacts both the thick teeth 34 and the first tooth portions 23A. In this way, by changing some of the plurality of second tooth portions 33A to thick teeth 34, it is possible to easily form the thin film portion 41A. Furthermore, the inner shaft 2A and outer shaft 3A of this embodiment are applicable to both spline fitting and serration fitting.

[0073] In addition, the manufacturing method of the lower shaft (steering shaft) 1A of the second embodiment includes a resin layer forming process of forming a resin layer 4A on an inner shaft 2A having a plurality of first tooth portions 23A arranged circumferentially on its outer peripheral surface, a heating process of heating the inner shaft 2A, the outer shaft 3A and the resin layer 4A while the resin layer 4A is in contact with the tooth surfaces 232A of all the first tooth portions 23A and the tooth surfaces 332A of all the second tooth portions 33A, and a cooling process of cooling the inner shaft 2A, the outer shaft 3A and the resin layer 4A after the heating process, wherein the plurality of second tooth portions 33A include thick teeth 34 and thin teeth 35 whose width along the circumferential direction is smaller than the width of the thick teeth 34 along the circumferential direction.

[0074] The circumferential gap between the tooth surface 342 of the thick tooth 34 of the second tooth portion 33A and the tooth surface 232A of the first tooth portion 23A is smaller than the circumferential gap between the tooth surface 352 of the thin tooth 35 of the second tooth portion 33A and the tooth surface 232A of the first tooth portion 23A. Therefore, the resin layer 4A disposed in the gap between the thick tooth 34 and the first tooth portion 23A becomes the thin film portion 41A, and the resin layer 4A disposed in the gap between the thin tooth 35 and the first tooth portion 23A becomes the thick film portion 42A. During the heating process, part of the resin in the thick film portion 42A moves radially and solidifies during the cooling process, resulting in a gap between the solidified thick film portion 42A and the tooth surface 232A of the first tooth portion 23A. However, because the thin film portion 41A is in contact with the thick tooth 34 and the first tooth portion 23A in both the heating process and the cooling process, no gap is generated. In this way, the thin film portion 41A and the thick film portion 42A of the resin layer 4A can be formed by a simple operation.

[0075] [Third embodiment] Next, a steering shaft according to a third embodiment will be described, but parts having the same structure as those of the steering shaft according to the first embodiment described above will be assigned the same reference numerals and description thereof will be omitted.

[0076] Fig. 16 is a schematic diagram showing a cross section of a lower shaft according to a third embodiment. Fig. 17 is a schematic diagram showing an enlarged portion of Fig. 16. As shown in Fig. 16, a lower shaft (steering shaft) 1B according to the third embodiment includes an inner shaft 2B, an outer shaft 3, and a resin layer 4B. The inner shaft 2B includes a cylindrical portion 22B and a first tooth portion 23B. The inner shaft 2B can be made of a wide variety of metals, such as carbon steel.

[0077] The cylindrical portion 22B is hollow and extends cylindrically in the circumferential direction around the central axis Ax. Thus, the inner shaft 2B is a hollow member. First tooth portions 23B protrude radially outward from the outer circumferential surface of the inner shaft 2B. A plurality of first tooth portions 23B are arranged at equal intervals in the circumferential direction on the outer circumferential surface of the cylindrical portion 22B. Each first tooth portion 23B includes a first thick-thick tooth 24B, a second thick-thick tooth 24C, and a thin-thick tooth 25. Two first thick-thick teeth 24B and one second thick-thick tooth 24C are adjacent to each other in the circumferential direction. That is, as shown in FIG. 16 , when viewed clockwise, the first thick-thick tooth 24B, the second thick-thick tooth 24C, and the first thick-thick tooth 24B are arranged in this order.

[0078] As shown in FIG. 17, the first thick tooth 24B has a tooth tip 241B and a tooth surface (side surface) 242B. The second thick tooth 24C has a tooth tip 241C and a tooth surface (side surface) 242C. The width of the second thick tooth 24C along the third imaginary line 130 is width T3. The width of the first thick tooth 24B along the third imaginary line 130 is width T4. The width of the thin tooth 25 along the third imaginary line 130 is width T2. The width T4 of the first thick tooth 24B is larger than the width T2 of the thin tooth 25. The width T3 of the second thick tooth 24C is larger than the width T4 of the first thick tooth 24B.

[0079] 16 and 17, the resin layer 4B has thin film portions 41B and thick film portions 42B. Two sets of thin film portions 41B are provided, one set at the end on the Y1 side and one set at the end on the Y2 side, as shown in FIG. 16. Four sets of thin film portions 41B are provided adjacent to each other in the circumferential direction. That is, a total of eight thin film portions 41B are provided.

[0080] 17, the thin film portion 41B of the resin layer 4B is provided between the first thick tooth 24B and the second tooth portion 33, and between the second thick tooth 24C and the second tooth portion 33. Specifically, the thin film portion 41B is in contact with both the tooth surface 242B of the first thick tooth 24B and the tooth surface 332 of the second tooth portion 33. The thin film portion 41B is also in contact with both the tooth surface 242C of the second thick tooth 24C and the tooth surface 332 of the second tooth portion 33.

[0081] The thickness of the thin film portion 41B along the third virtual line 130 is a thickness t1. In this embodiment, one set of thin film portions is four thin film portions 41B adjacent to each other in the circumferential direction. Furthermore, four thin film portions 41B are provided at the end portion on the Y1 side. The four thin film portions 41B form one set of thin film portions adjacent to each other in the circumferential direction. The set of thin film portions at the end portion on the Y2 side and the set of thin film portions at the end portion on the Y1 side are disposed symmetrically with respect to the central axis Ax.

[0082] In this way, a total of eight thin film portions 41B are provided at positions symmetrical with respect to the central axis Ax. Note that the present disclosure is not limited to this, and eight or more thin film portions 41B may be provided, and four or more sets of thin film portions 41B may be arranged in the circumferential direction. Furthermore, multiple sets of thin film portions 41B may be arranged at equal intervals in the circumferential direction, but is not limited to this, and the positions at which the multiple sets of thin film portions 41B are arranged do not have to be equal intervals.

[0083] As described above, the lower shaft 1B according to the third embodiment includes two sets of thin film portions 41B, each set consisting of four circumferentially adjacent thin film portions 41B. In this manner, in this embodiment, one set of thin film portions 41B includes four thin film portions 41B, which reduces the backlash of the steering shaft compared to two thin film portions 41B, thereby making it possible to further reduce rattle noise that occurs when the vehicle is traveling.

[0084] The present disclosure is not limited to the above-described embodiments and can be applied to a wide range of technologies. For example, in the above-described embodiments, the resin layers 4, 4A, and 4B are formed on the first tooth portions 23, 23A, and 23B of the inner shafts 2, 2A, and 2B. However, they may be formed on the second tooth portions 33, 33A of the outer shafts 3, 3A. Although the number of first tooth portions 23, 23A, and 23B of the inner shafts 2, 2A, and 2B is 18 in the above-described embodiments, this is not limiting and various numbers of teeth, such as 19 and 23, can be used. Furthermore, in the above-described embodiments, the multiple sets of thin film portions 41 are arranged at equal intervals in the circumferential direction. However, this is not limiting and the positions of the multiple sets of thin film portions 41 do not have to be equal intervals. [Explanation of symbols]

[0085] 1, 1A, 1B Lower shaft (steering shaft) 2, 2A, 2B inner shaft 21 York 22, 22A cylinder part 23, 23A, 23B 1st tooth part 231, 231A Tooth tip (periphery) 232, 232A Tooth surface (side surface) 233, 233A tooth bottom 24 Thick-walled teeth 24B 1st thick tooth 24C 2nd thick tooth 241, 241B, 241C tooth tip 242, 242B, 242C tooth surface 25 Thin-walled teeth 251 Tooth tip 252 Tooth surface 3, 3A outer shaft 31 York 32, 32A cylinder part 33, 33A 2nd tooth part 331, 331A Tooth tip (inner peripheral end) 332, 332A tooth surface (side surface) 333, 333A tooth bottom 34 Thick-walled teeth 341 Tooth tip 342 Tooth surface 35 Thin-walled teeth 351 Tooth tip 352 Tooth surface 4, 4A, 4B resin layer 41, 41A, 41B Thin film part 42, 42A, 42B Thick film part 80 Steering device 81 Steering wheel 82 Upper shaft 82a Input shaft 82b Output shaft 83 Steering force assist mechanism 87 Pinion shaft 88 Steering gear 88a Pinion 88b Rack 89 tie rod 90 ECU 92 Reducer 93 Electric Motor 94 Torque Sensor 95 Vehicle speed sensor 98 Ignition switch 99 Power supply 110 First Virtual Line 120 Second Virtual Line 130 Third Virtual Line 140 Heating Furnace Ax center axis T1, T2 width t1, t2, t3 thickness

Claims

1. an inner shaft having a plurality of first teeth arranged along a circumferential direction on an outer circumferential surface thereof; an outer shaft having a plurality of second tooth portions arranged along a circumferential direction on an inner peripheral surface thereof and arranged on an outer peripheral side of the first tooth portions; a resin layer disposed between the first tooth portion and the second tooth portion, the resin layer has a plurality of thick film portions and at least four thin film portions; the thick film portion is disposed between a tooth surface of the first tooth portion and a tooth surface of the second tooth portion, with a gap provided between the thick film portion and one of the first tooth portion and the second tooth portion; the thin film portion is disposed between a tooth surface of the first tooth portion and a tooth surface of the second tooth portion, and is in contact with both the first tooth portion and the second tooth portion; In a cross section orthogonal to the central axis of the inner shaft, a thickness of the thin film portion along a third imaginary line extending in the circumferential direction and passing through a radial midpoint between a first imaginary line extending in the circumferential direction through an outer circumferential end of the first tooth portion and a second imaginary line extending in the circumferential direction through an inner circumferential end of the second tooth portion is smaller than a thickness of the thick film portion along the third imaginary line. Steering shaft.

2. The plurality of sets of thin film portions are spaced apart along the circumferential direction. The steering shaft according to claim 1 .

3. The plurality of sets of thin film portions are arranged at equal intervals along the circumferential direction.

3. The steering shaft according to claim 1 or 2.

4. The number of the plurality of thin film portions is two.

4. The steering shaft according to claim 2 or 3.

5. The set of thin film portions includes four thin film portions. A steering shaft according to any one of claims 1 to 4.

6. The inner shaft is a hollow member. A steering shaft according to any one of claims 1 to 5.

7. the plurality of first tooth portions include thick teeth and thin teeth whose widths along the circumferential direction are smaller than the widths of the thick teeth along the circumferential direction, the thin film portion contacts both the thick tooth and the second tooth portion; A steering shaft according to any one of claims 1 to 6.

8. the second tooth portions include thick teeth and thin teeth whose widths along the circumferential direction are smaller than the widths of the thick teeth along the circumferential direction, the thin film portion contacts both the thick tooth and the first tooth portion; A steering shaft according to any one of claims 1 to 7.

9. a resin layer forming step of forming a resin layer on an inner shaft having a plurality of first tooth portions arranged along a circumferential direction on an outer peripheral surface thereof, or on an outer shaft having a plurality of second tooth portions arranged along a circumferential direction on an inner peripheral surface thereof and arranged on the outer peripheral side of the first tooth portions of the inner shaft; a heating step of heating the inner shaft, the outer shaft, and the resin layer in a state in which the resin layer is in contact with tooth surfaces of all of the first tooth portions and tooth surfaces of all of the second tooth portions; a cooling step of cooling the inner shaft, the outer shaft, and the resin layer after the heating step, The first tooth portions include thick teeth and thin teeth whose widths along the circumferential direction are smaller than the widths of the thick teeth along the circumferential direction. A method for manufacturing a steering shaft.

10. a resin layer forming step of forming a resin layer on an inner shaft having a plurality of first tooth portions arranged along a circumferential direction on an outer peripheral surface thereof, or on an outer shaft having a plurality of second tooth portions arranged along a circumferential direction on an inner peripheral surface thereof and arranged on the outer peripheral side of the first tooth portions of the inner shaft; a heating step of heating the inner shaft, the outer shaft, and the resin layer in a state in which the resin layer is in contact with tooth surfaces of all of the first tooth portions and tooth surfaces of all of the second tooth portions; a cooling step of cooling the inner shaft, the outer shaft, and the resin layer after the heating step, The second tooth portions include thick teeth and thin teeth whose widths along the circumferential direction are smaller than the widths of the thick teeth along the circumferential direction. A method for manufacturing a steering shaft.

Images