Shaft unit

Abstract

To provide a shaft unit in which first teeth on the outer periphery of a shaft or second teeth on the inner periphery of a shaft coupling are hardly damaged.SOLUTION: A shaft unit 15 comprises a shaft coupling 10 that connects a first shaft 11 and a second shaft 12. The shaft coupling 10 has a main body 4, a pair of first fastening portion 51 and second fastening portion 52, and a fastening member 6. The strength of first teeth 2 is set to be equal to or less than the strength of second teeth 3. In the state that the first fastening portion 51 and the second fastening portion 52 are fastened together by the fastening member 6, a first end portion 24 on the tooth tip 21 side of a tooth surface 22 of the first tooth 2 abuts against a tooth surface 32 of the second tooth 3, and other portions of the tooth surface 22 of the first tooth 2 other than the first end portion are separated from the tooth surface 32 of the second tooth 3.SELECTED DRAWING: Figure 5A

Description

[Technical Field]

[0001] The present disclosure relates to a shaft unit. [Background technology]

[0002] The steering device described in Patent Document 1 includes a shaft unit. The shaft unit has two steering shafts and a shaft coupling that connects the two steering shafts. The shaft coupling has two fastening pieces that are fastened together via bolts.

[0003] In addition, first teeth are provided on the outer periphery of the steering shaft, and second teeth that mesh with the first teeth are provided on the inner periphery of the shaft coupling. When the first teeth are meshed with the second teeth, the entire tooth surfaces of the first teeth and the entire tooth surfaces of the second teeth abut against each other. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6596958 Summary of the Invention [Problem to be solved by the invention]

[0005] When the steering shaft rotates, torque is applied from the first tooth to the second tooth. Because the entire tooth surface of the first tooth and the entire tooth surface of the second tooth are in contact with each other, a pressing force is applied from the entire tooth surface of the first tooth to the entire tooth surface of the second tooth from the early stage of the steering shaft rotation. That is, when the steering shaft rotates, a shear force is suddenly applied to the root of the first tooth or the second tooth along the circumferential direction. In particular, when the steering wheel is suddenly turned while the vehicle is cornering, a large force may be suddenly applied to the root of the first tooth or the second tooth. This may make the first tooth or the second tooth more susceptible to damage.

[0006] The present disclosure has been made in view of the above-mentioned problems, and aims to provide a shaft unit in which the first teeth on the outer periphery of the shaft or the second teeth on the inner periphery of the shaft coupling are less likely to be damaged. [Means for solving the problem]

[0007] In order to achieve the above-mentioned object, a shaft unit according to one embodiment of the present disclosure is a shaft unit comprising: a shaft having first teeth on its outer periphery; and a shaft coupling extending in the axial direction of a central axis and having second teeth on its inner periphery that mesh with the first teeth, the shaft coupling having a cylindrical main body portion on whose inner periphery the second teeth are provided, the strength of the first teeth is set to be equal to or less than the strength of the second teeth, a first end portion on the tooth tip side of the tooth flank of the first tooth abuts against the tooth flank of the second tooth, and other portions of the tooth flank of the first tooth other than the first end portion are spaced apart from the tooth flank of the second tooth.

[0008] As described above, in Patent Document 1, the entire tooth flank of the first tooth on the shaft and the entire tooth flank of the second tooth on the shaft coupling are in contact with each other, so that a pressing force is applied from the entire tooth flank of the first tooth to the entire tooth flank of the second tooth from the early stage of rotation of the shaft. In other words, when the shaft rotates, a shearing force is suddenly applied in the circumferential direction to the root of the first tooth or the second tooth, which may make the first tooth or the second tooth more susceptible to damage.

[0009] In contrast, in the present disclosure, a first end portion on the tooth tip side of the tooth flank of the first tooth abuts against the tooth flank of the second tooth, and the strength of the first tooth is set to be equal to or less than the strength of the second tooth. Therefore, in the initial stage of rotating the shaft, the first end portion first presses against the tooth flank of the second tooth, and the first end portion elastically deforms, recessing toward the center in the thickness direction (center in the circumferential direction) of the first tooth, and assumes a shape that conforms to the tooth flank of the second tooth. Thereafter, the entire portion of the tooth flank of the first tooth that corresponds to the tooth flank of the second tooth elastically deforms, recessing toward the center in the thickness direction of the first tooth, and assumes a shape that conforms to the tooth flank of the second tooth, and torque is applied from the first tooth to the second tooth.

[0010] In this manner, in the present disclosure, when the first shaft rotates, the tip of the first tooth elastically deforms relative to the base of the first tooth, bending in the circumferential direction. This reduces the likelihood of circumferential shear force being applied to the base of the first tooth or the second tooth. Therefore, when the first shaft rotates, the first tooth or the second tooth is less likely to be damaged. Methods for reducing the strength include, for example, using a material with low hardness or reducing the tooth thickness. Furthermore, the engagement between the first tooth and the second tooth in the present disclosure can also be achieved by, for example, serration engagement or spline engagement.

[0011] In a preferred aspect, the shaft coupling further includes a pair of fastening portions that protrude radially outward from the main body and are arranged opposite each other with a slit extending in the axial direction therebetween, and a fastening member that penetrates the pair of fastening portions and fastens the pair of fastening portions together. This makes it possible to easily fasten the pair of fastening portions together via the fastening member.

[0012] A shaft unit according to one embodiment of the present disclosure is a shaft unit comprising: a shaft having first teeth on its outer periphery; and a shaft coupling extending in the axial direction of a central axis and having second teeth on its inner periphery that mesh with the first teeth, the shaft coupling having a cylindrical main body portion on whose inner periphery the second teeth are provided, the strength of the second teeth being set lower than the strength of the first teeth, a second end portion on the tooth surface of the second tooth on the tooth tip side abutting against the tooth surface of the first tooth, and other portions of the tooth surface of the second tooth other than the second end portion being spaced apart from the tooth surface of the first tooth.

[0013] In the initial stage of rotating the shaft, first, the tooth flank of the first tooth presses the second end, and the second end elastically deforms, recessing toward the center in the thickness direction (circumferential center) of the second tooth, and assumes a shape that matches the tooth flank of the first tooth. After that, the entire tooth flank of the second tooth, which corresponds to the tooth flank of the first tooth, elastically deforms, recessing toward the center in the thickness direction of the second tooth, and assumes a shape that matches the tooth flank of the first tooth, and torque is applied from the first tooth to the second tooth.

[0014] In this manner, in the present disclosure, when the first shaft rotates, the tip of the second tooth elastically deforms relative to the base of the second tooth in the circumferential direction, making it difficult for a shear force to be applied to the base of the first tooth or the second tooth in the circumferential direction, and therefore the first tooth or the second tooth is less likely to be damaged when the first shaft rotates.

[0015] In a preferred aspect, the shaft coupling further includes a pair of fastening portions that protrude radially outward from the main body and are arranged opposite each other with a slit extending in the axial direction therebetween, and a fastening member that penetrates the pair of fastening portions and fastens the pair of fastening portions together. This makes it possible to easily fasten the pair of fastening portions together via the fastening member.

[0016] In a preferred embodiment, the first teeth are arranged at the same pitch around the entire circumference, and the second teeth have a pitch that increases from the part of the main body that is farthest from the pair of fastening portions in the circumferential direction toward the pair of fastening portions.

[0017] The meshing between the second teeth and the first teeth located in a position close to the first fastening portion and the second fastening portion is compared with the meshing between the second teeth and the first teeth located in a position far from the first fastening portion and the second fastening portion.

[0018] When the distance between the first fastening portion and the second fastening portion decreases due to fastening of the fastening member, the meshing force between the first tooth and the second tooth located near the first fastening portion and the second fastening portion is greater than the meshing force between the first tooth and the second tooth located farther from the first fastening portion and the second fastening portion. Here, because the pitch of the second tooth located near the first fastening portion and the second fastening portion is larger than the pitch of the second tooth located farther from the first fastening portion and the second fastening portion, even if a greater meshing force acts on the second tooth and the first tooth, damage to the second tooth or the first tooth due to the meshing force is suppressed. [Effects of the Invention]

[0019] According to the present disclosure, it is possible to provide a shaft unit in which the first teeth on the outer periphery of the shaft or the second teeth on the inner periphery of the shaft coupling are less likely to be damaged. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram of a steering device according to a first embodiment. [Figure 2] FIG. 2 is a perspective view of the steering device of the first embodiment. [Figure 3] FIG. 3 is a cross-sectional view of the shaft unit of the first embodiment. [Figure 4] FIG. 4 is an enlarged cross-sectional view of a portion A1 in FIG. [Figure 5A] FIG. 5A is an enlarged schematic diagram of part A2 in FIG. [Figure 5B] FIG. 5B is a schematic diagram showing a state in which the shaft is rotating and the first teeth are pressing against the second teeth. [Figure 5C] FIG. 5C is a schematic diagram showing a state in which the shaft is rotating and the first teeth are pressing against the second teeth. [Figure 6] FIG. 6 is an enlarged cross-sectional view of a part of the shaft unit of the second embodiment. [Figure 7A] FIG. 7A is an enlarged schematic diagram of part A3 in FIG. [Figure 7B] FIG. 7B is a schematic diagram showing a state in which the shaft is rotating and the first teeth are pressing against the second teeth. [Figure 7C] FIG. 7C is a schematic diagram showing a state in which the shaft is rotating and the first teeth are pressing against the second teeth. [Figure 8A] FIG. 8A is a cross-sectional view of a shaft unit according to a third embodiment. [Figure 8B] FIG. 8B is a cross-sectional view of the shaft unit of the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention will be described in detail below with reference to the drawings. Note that the present invention is not limited to the following modes for carrying out the invention (hereinafter referred to as embodiments). Furthermore, the components in the following embodiments include those that can be easily imagined by a person skilled in the art, those that are substantially the same, and those that are within the so-called equivalent range. Furthermore, the components disclosed in the following embodiments can be combined as appropriate.

[0022] [First embodiment] First, a first embodiment will be described. Fig. 1 is a schematic diagram of a steering device of the first embodiment. As shown in Fig. 1, a steering device 80 includes a steering wheel 81, a steering shaft 82, a steering force assist mechanism 83, a first universal joint 84, an intermediate shaft 85, and a second universal joint 86.

[0023] 1, the steering 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. 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. The other end of the output shaft 82b is connected to the first universal joint 84.

[0024] As shown in FIG. 1 , one end of the intermediate shaft 85 is connected to a first universal joint 84. The other end of the intermediate shaft 85 is connected to a second universal joint 86. One end of the pinion shaft 87 is connected to the second universal joint 86. The other end of the pinion shaft 87 is connected to a steering gear 88. The first universal joint 84 and the second universal joint 86 are, for example, Cardan joints. The rotation of the steering shaft 82 is transmitted to the pinion shaft 87 via the intermediate shaft 85. The second universal joint 86 is connected to the pinion shaft 87.

[0025] As shown in FIG. 1, the steering gear 88 includes a pinion 88a and a rack 88b. The pinion 88a is connected to a 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. The angle of the wheels changes as the rack 88b moves.

[0026] As shown in FIG. 1, the steering force assist mechanism 83 includes a reduction gear 92 and an electric motor 93. 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 applies an auxiliary steering torque to the output shaft 82b. In other words, the steering device 80 is of a column assist type.

[0027] As shown in Fig. 1, the steering device 80 includes an ECU (Electronic Control Unit) 90, a torque sensor 94, and a vehicle speed sensor 95. The electric motor 93, the torque sensor 94, and the vehicle speed sensor 95 are electrically connected to the ECU 90. The torque sensor 94 outputs the steering torque transmitted to the input shaft 82a to the ECU 90 via CAN (Controller Area Network) communication. The vehicle speed sensor 95 detects the traveling speed (vehicle speed) of the vehicle body on which the steering device 80 is mounted. The vehicle speed sensor 95 is provided on the vehicle body, and outputs the vehicle speed to the ECU 90 via CAN communication.

[0028] The ECU 90 controls the operation of the electric motor 93. The ECU 90 acquires signals from a torque sensor 94 and a vehicle speed sensor 95. When an ignition switch 98 is on, the ECU 90 is supplied with power from a power supply device 99 (for example, an on-board battery). The ECU 90 calculates an auxiliary steering command value based on the steering torque and the vehicle speed. The ECU 90 adjusts the value of power supplied to the electric motor 93 based on the auxiliary steering command value. The ECU 90 acquires information on the induced voltage from the electric motor 93 or information output from a resolver or the like provided in the electric motor 93. When the ECU 90 controls the electric motor 93, the force required to operate the steering wheel 81 is reduced.

[0029] FIG. 2 is a perspective view of the steering device of the first embodiment. As shown in FIG. 2, intermediate shaft 85 penetrates dash panel 100. Dash panel 100 is a partition plate that separates the passenger compartment from the engine compartment. Intermediate shaft 85 includes upper shaft 851 and lower shaft 852. Upper shaft 851 is disposed closer to the passenger compartment than dash panel 100 and is connected to first universal joint 84. Lower shaft 852 penetrates dash panel 100. The end of lower shaft 852 facing the passenger compartment is inserted inside upper shaft 851. When an excessive force acts on intermediate shaft 85 due to a vehicle collision or the like, upper shaft 851 and lower shaft 852 can move relative to each other in the axial direction.

[0030] As shown in FIG. 2 , the lower shaft 852 includes a first shaft 11 and a second shaft 12. The first shaft 11 passes through a dash panel 100. The steering device 80 includes a dust cover 101 to close the gap between the first shaft 11 and the dash panel 100. The second shaft 12 is disposed closer to the engine compartment than the dash panel 100 and is connected to a second universal joint 86. The first shaft 11 and the second shaft 12 are connected by a shaft coupling 10. In this embodiment, the intermediate shaft 85 includes a shaft unit 15. The shaft unit 15 includes the first shaft 11, the second shaft 12, and the shaft coupling 10. The shaft unit 15 can also be applied to configurations other than the intermediate shaft 85.

[0031] FIG. 3 is a cross-sectional view of the shaft unit of the first embodiment. FIG. 4 is an enlarged cross-sectional view of portion A1 of FIG. 3. As described above, the shaft unit 15 includes a first shaft 11, a second shaft 12, and a shaft coupling 10. The shaft coupling 10 connects two shafts, the first shaft 11 and the second shaft 12. While FIG. 3 shows a cross-section of the first shaft 11, the cross-section of the second shaft 12 also has the same shape as the cross-section of the first shaft 11. That is, first teeth 2 (see FIG. 4) that protrude radially outward are provided on the outer peripheries of the first shaft 11 and the second shaft 12. As shown in FIG. 3, the first teeth 2 are arranged at equal intervals (equal pitch) around the entire circumferential direction of the central axis AX1.

[0032] As shown in FIG. 3, the shaft coupling 10 includes a main body 4, a pair of a first fastening portion 51 and a second fastening portion 52, and a fastening member 6.

[0033] The main body 4 has a cylindrical shape that extends in the circumferential direction around the central axis AX1. Second teeth 3 that protrude radially inward are provided on the inner periphery of the main body 4. The second teeth 3 are arranged at equal intervals (equal pitch) in the circumferential direction. The second teeth 3 mesh with the first teeth 2. The first teeth 2 and second teeth 3 extend in the axial direction.

[0034] 3, the first fastening portion 51 and the second fastening portion 52 protrude radially outward from the main body portion 4. Here, with respect to a line L1 passing through the central axis AX1, the first fastening portion 51 is located on the X1 side, and the second fastening portion 52 is located on the X2 side. A slit 53 extending in the axial direction is disposed between the first fastening portion 51 and the second fastening portion 52. That is, the first fastening portion 51 and the second fastening portion 52 are disposed opposite each other with the slit 53 sandwiched therebetween.

[0035] A first through hole 511 penetrates the first fastening portion 51 in the axial direction of the central axis AX2. A female thread portion 512 is formed on the edge of the first through hole 511. A second through hole 521 penetrates the second fastening portion 52 in the axial direction of the central axis AX2.

[0036] The fastening member 6 includes a head 61, a shaft 62, and an externally threaded portion 63. The externally threaded portion 63 engages with the internally threaded portion 512 of the first fastening portion 51. Therefore, as shown by the two-dot chain line in FIG. 3 , the externally threaded portion 63 is inserted into the second through-hole 521 of the second fastening portion 52, and then the externally threaded portion 63 is engaged with the internally threaded portion 512 of the first fastening portion 51, thereby fastening the first fastening portion 51 and the second fastening portion 52 together with the fastening member 6. In this way, the fastening member 6 passes through the first fastening portion 51 and the second fastening portion 52 and fastens the first fastening portion 51 and the second fastening portion 52 together.

[0037] Fig. 5A is a schematic diagram showing an enlargement of part A2 in Fig. 4. The strength of the first teeth 2 is set to be equal to or less than the strength of the second teeth 3. Possible methods for reducing the strength include using a material with low hardness or reducing the tooth thickness.

[0038] As shown in FIG. 5A, the first tooth 2 protrudes radially outward from the tooth bottom 25. The first tooth 2 has a shape symmetrical with respect to the line L1. The first tooth 2 has a tooth tip 21 and a tooth flank 22. The circumferential thickness of the first tooth 2 decreases radially outward. In other words, the left-right distance between the tooth flank 22 on the X1 side of the line L1 and the tooth flank 22 on the X2 side of the line L1 decreases radially outward (toward the upper side in FIG. 5A). The end 21a of the tooth tip 21 is the boundary between the tooth tip 21 and the tooth flank 22. The tooth flank 22 consists of a first end 24, which is the end on the tooth tip 21 side, and a portion other than the first end 24. The first end 24 includes the end 21a.

[0039] As shown in FIG. 5A, the second tooth 3 protrudes radially inward from the tooth bottom 34. The second tooth 3 has a tooth tip 31 and a tooth flank 32. The circumferential thickness of the second tooth 3 decreases radially inward. An end 31a of the tooth tip 31 is the boundary between the tooth tip 31 and the tooth flank 32. The tooth tip 31 and the tooth bottom 34 are spaced apart radially, and the tooth tip 31 and the tooth bottom 25 are also spaced apart radially.

[0040] Referring to FIG. 4, a straight line L1 passes through the circumferential center of the first tooth 2. A straight line L11 passes through the end 21a (see FIG. 5A) and is parallel to the straight line L1. A straight line L2 passes through the circumferential center of the second tooth 3. A straight line L21 passes through the end 31a (see FIG. 5A) and is parallel to the straight line L2. As shown in FIG. 4, the intersection angle between the tooth flank 22 and the straight line L11 is angle θ1. The intersection angle between the tooth flank 32 and the straight line L21 is angle θ2. The angle θ2 is greater than the angle θ1. Therefore, as shown in FIG. 5A, the first end 24 of the first tooth 2 abuts against the tooth flank 32 of the second tooth 3, and the rest of the tooth flank 22 of the first tooth 2 other than the first end is spaced apart from the tooth flank 32 of the second tooth 3 in the circumferential direction.

[0041] Next, we will explain in order how the first shaft 11 rotates relative to the shaft coupling 10 and torque is applied from the first shaft 11 to the shaft coupling 10. Figures 5B and 5C are schematic diagrams showing a state in which the shaft is rotating and the first teeth are pressing against the second teeth.

[0042] When the steering wheel 81 (see FIG. 2) is rotated, and the first shaft 11 and the first tooth 2 rotate counterclockwise (leftward) in FIG. 5B, the first end 24 of the first tooth 2 shown in FIG. 5B on the X1 side (left side in FIG. 5B) with respect to the straight line L1 presses against the tooth flank 32 of the second tooth 3. Due to this pressing, as shown in FIG. 5B, the first end 24 elastically deforms, recessing toward the center in the thickness direction of the first tooth 2 (toward the circumferential center, toward the X2 side), and assumes a shape that conforms to the tooth flank 32 of the second tooth 3. In this state, the portion of the tooth flank 22 of the first tooth 2 that is on the tooth base 23 side is spaced apart from the tooth flank 32 of the second tooth 3 in the circumferential direction. Although not shown in FIGS. 5B and 5C, the first tooth 2 as a whole elastically bends (flexes) slightly in the clockwise direction (rightward) around the tooth base 23. That is, the circumferential position of the tooth base 23 remains unchanged, and the portion on the tooth tip 21 side undergoes elastic deformation, tilting slightly clockwise (to the right in FIG. 5B) relative to the tooth base 23. Then, a torque P2 is applied from the first tooth 2 to the second tooth 3, and as a reaction force, a torque P1 is applied from the second tooth 3 to the first tooth 2.

[0043] Next, as shown in FIG. 5C , when the first shaft 11 and the first tooth 2 further rotate counterclockwise, the entire portion of the tooth flank 22 of the first tooth 2 corresponding to the tooth flank 32 of the second tooth 3 elastically deforms and recesses toward the center in the thickness direction of the first tooth 2, conforming to the tooth flank 32 of the second tooth 3. In this state, a torque P4 is applied from the first tooth 2 to the second tooth 3, and as a reaction force, a torque P3 is applied from the second tooth 3 to the first tooth 2. Torque P4 is greater than torque P2, and torque P3 is greater than torque P1. Note that, for example, torque P2 causes the shaft coupling 10 and the second shaft 12 (see FIG. 2 ) to start rotating.

[0044] As described above, the shaft unit 15 according to the first embodiment includes the shaft coupling 10 that connects the first shaft 11 and the second shaft 12. The shaft coupling 10 has a main body 4, a pair of first and second fastening portions 51 and 52, and a fastening member 6. The strength of the first tooth 2 is set to be equal to or less than the strength of the second tooth 3. When the first fastening portion 51 and the second fastening portion 52 are fastened together by the fastening member 6, the first end portion 24 on the tooth tip 21 side of the tooth flank 22 of the first tooth 2 abuts against the tooth flank 32 of the second tooth 3, and the rest of the tooth flank 22 of the first tooth 2 other than the first end portion is separated from the tooth flank 32 of the second tooth 3.

[0045] As described above, in Patent Document 1, the entire tooth flank of the first tooth on the shaft and the entire tooth flank of the second tooth on the shaft coupling are in contact with each other, so that a pressing force is applied from the entire tooth flank of the first tooth to the entire tooth flank of the second tooth from the early stage of rotation of the shaft. In other words, when the shaft rotates, a shearing force is suddenly applied in the circumferential direction to the root of the first tooth or the second tooth, which may make the first tooth or the second tooth more susceptible to damage.

[0046] In contrast, in this embodiment, the first end 24 on the tooth tip 21 side of the tooth flank 22 of the first tooth 2 abuts against the tooth flank 32 of the second tooth 3, and the strength of the first tooth 2 is set to be equal to or less than the strength of the second tooth 3. Therefore, as described with reference to FIGS. 5A to 5C , in the initial stage of rotation of the first shaft 11, the first end 24 first presses against the tooth flank 32 of the second tooth 3, and the first end 24 elastically deforms, recessing toward the center in the thickness direction of the first tooth 2 (toward the center in the circumferential direction, toward the X2 side), and assumes a shape that conforms to the tooth flank 32 of the second tooth 3. Thereafter, the entire portion of the tooth flank 22 of the first tooth 2 that corresponds to the tooth flank 32 of the second tooth 3 elastically deforms, recessing toward the center in the thickness direction of the first tooth 2, and assumes a shape that conforms to the tooth flank 32 of the second tooth 3, and torque is applied from the first tooth 2 to the second tooth 3.

[0047] In this manner, in this embodiment, when the first shaft 11 rotates, the tooth tip 21 portion of the first tooth 2 undergoes elastic deformation such that it bends in the circumferential direction relative to the tooth base 23, and therefore, a shear force along the circumferential direction is less likely to be applied to the tooth base of the first tooth 2 or the second tooth 3. Therefore, when the first shaft 11 rotates, the first tooth or the second tooth is less likely to be damaged.

[0048] [Second embodiment] Next, a second embodiment will be described. Fig. 6 is an enlarged cross-sectional view of a portion of a shaft unit of the second embodiment. As shown in Fig. 6, a shaft unit 15A includes a shaft coupling 10A, and the shaft coupling 10A has a main body 4A. Second teeth 3A that protrude radially inward are provided on the inner periphery of the main body 4A. The second teeth 3A are arranged at equal intervals (equal pitch) in the circumferential direction. The second teeth 3A mesh with the first teeth 2A. The first teeth 2A and second teeth 3A extend in the axial direction.

[0049] FIG. 7A is a schematic diagram enlarging a portion A3 in FIG. 6. First, the strength of the second tooth 3A is set lower than the strength of the first tooth 2A. In FIG. 6, a line L1 passes through the circumferential center of the first tooth 2A. A line L11 is parallel to the line L1. A line L2 passes through the circumferential center of the second tooth 3A. A line L21 is parallel to the line L2. As shown in FIG. 7A, the second tooth 3A protrudes radially inward from the tooth bottom 34. The second tooth 3A has a tooth tip 31 and a tooth flank 32A. The circumferential tooth thickness of the second tooth 3A decreases radially inward. An end 31a of the tooth tip 31 is the boundary between the tooth tip 31 and the tooth flank 32A. The tooth tip 31 and the tooth bottom 34 are spaced apart radially, and the tooth tip 31 and the tooth bottom 25 are spaced apart radially.

[0050] The circumferential tooth thickness of the second tooth 3A decreases toward the radially inner side. In other words, the circumferential distance between the left and right tooth flanks 32A decreases toward the radially inner side (the lower side in FIG. 7A). The tooth flank 32A is composed of a second end 35, which is the end on the tooth tip 31 side, and a portion other than the second end 35. The second end 35 includes the end 31a.

[0051] 6, the intersection angle between the tooth flank 22A and the straight line L11 is angle θ3. The intersection angle between the tooth flank 32A and the straight line L21 is angle θ4. Angle θ3 is larger than angle θ4. Therefore, the second end portion 35 of the second tooth 3A abuts against the tooth flank 22A of the first tooth 2A, and the other portion of the tooth flank 32A of the second tooth 3A other than the second end portion is spaced apart in the circumferential direction from the tooth flank 22A of the first tooth 2A.

[0052] Next, we will explain in order how the first shaft 11 rotates relative to the shaft coupling 10 and torque is applied from the first shaft 11 to the shaft coupling 10. Figures 7B and 7C are schematic diagrams showing a state in which the shaft is rotating and the first teeth are pressing against the second teeth.

[0053] When the steering wheel 81 (see FIG. 2) is rotated, causing the first shaft 11 and the first tooth 2A to rotate counterclockwise (leftward) in FIG. 7B, the second end 35 on the right side of FIG. 7B of the two second ends 35 of each second tooth 3A shown in FIG. 7B presses against the tooth flank 22A of the first tooth 2A. Due to this pressure, as shown in FIG. 7B, the second end 35 elastically deforms, recessing toward the center in the thickness direction (center in the circumferential direction) of the second tooth 3A, and assumes a shape that conforms to the tooth flank 22A of the first tooth 2A. In this state, the portion of the tooth flank 32A of the second tooth 3A on the tooth base 33 side is spaced apart from the tooth flank 22A of the first tooth 2A in the circumferential direction. Although not shown in FIGS. 7B and 7C, the second tooth 3A as a whole elastically deforms, bending (flexing) slightly to the left in FIG. 7B around the tooth base 33. That is, the circumferential position of the tooth base 33 remains unchanged, but the portion on the tooth tip 21 side undergoes elastic deformation, tilting slightly to the left in Figure 7B relative to the tooth base 33. Then, torque P2 is applied from the first tooth 2A to the second tooth 3A, and as a reaction force, torque P1 is applied from the second tooth 3A to the first tooth 2A.

[0054] Next, as shown in FIG. 7C , when the first shaft 11 and the first tooth 2A further rotate counterclockwise, the entire portion of the tooth flank 32A of the second tooth 3A corresponding to the tooth flank 22A of the first tooth 2A elastically deforms, recessing toward the center in the thickness direction of the second tooth 3A and conforming to the tooth flank 22A of the first tooth 2A. In this state, a torque P4 is applied from the first tooth 2A to the second tooth 3A, and a torque P3 is applied from the second tooth 3A to the first tooth 2A as a reaction force. Torque P4 is greater than torque P2, and torque P3 is greater than torque P1. Note that, for example, torque P2 causes the shaft coupling 10 and the second shaft 12 (see FIG. 2 ) to start rotating.

[0055] As described above, in the second embodiment, the strength of the second tooth 3A is set lower than the strength of the first tooth 2A. When the pair of fastening portions is fastened together by the fastening member 6, the second end 35 on the tooth tip side of the tooth flank 32A of the second tooth 3A abuts against the tooth flank 22A of the first tooth 2A, and the rest of the tooth flank 32A of the second tooth 3A other than the second end is separated from the tooth flank 22A of the first tooth 2A.

[0056] 7A to 7C, in the second embodiment, in the initial stage of rotation of the first shaft 11, first, the tooth flank 22A of the first tooth 2A presses the second end 35, and the second end 35 elastically deforms and recesses toward the center in the thickness direction (toward the center in the circumferential direction) of the second tooth 3A, and assumes a shape that conforms to the tooth flank 22A of the first tooth 2A. Thereafter, the entire portion of the tooth flank 32A of the second tooth 3A that corresponds to the tooth flank 22A of the first tooth 2A elastically deforms and recesses toward the center in the thickness direction of the second tooth 3A, and assumes a shape that conforms to the tooth flank 22A of the first tooth 2A, and torque is applied from the first tooth 2A to the second tooth 3A.

[0057] In this manner, in this embodiment, when the first shaft 11 rotates, the tooth tips 31 of the second teeth 3A undergo elastic deformation such that they bend in the circumferential direction relative to the tooth bases 33, making it difficult for shear forces to be applied in the circumferential direction to the tooth bases of the first teeth 2A or the second teeth 3A. Therefore, when the first shaft 11 rotates, the first teeth 2A or the second teeth 3A are less likely to be damaged.

[0058] [Third embodiment] Next, a third embodiment will be described. Figures 8A and 8B are cross-sectional views of a shaft unit according to the third embodiment. In detail, Figure 8A shows the state before fastening member 6 is fastened to first fastening portion 51 and second fastening portion 52, and Figure 8B shows the state after fastening.

[0059] In the first and second embodiments described above, the first teeth and second teeth are arranged at equal intervals. In contrast, in the third embodiment, the second teeth are not arranged at equal intervals. Also, as in the second embodiment, the second end 35 of the second tooth 3B abuts against the tooth surface 22A of the first tooth 2A. This will be explained in detail below.

[0060] 8A and 8B, the shaft unit 15B includes a first shaft 11B and a shaft coupling 10B. First teeth 2A are provided on the outer periphery of the first shaft 11B. The first teeth 2A are arranged at a uniform pitch over the entire circumference.

[0061] The shaft coupling 10B has a main body 4B. Second teeth 3B are provided on the inner circumference of the main body 4B. The strength of the second teeth 3B is lower than the strength of the first teeth 2A. The main body 4B has an apex P100. The apex P100 is the part of the main body 4B that is farthest from the first fastening portion 51 and the second fastening portion 52. In other words, the part of the main body 4B that intersects with the straight line L1 is referred to as the apex P100. The pitch of the second teeth 3B is smallest at the apex P100, and the pitch of the second teeth 3B gradually increases with increasing distance from the apex P100, and is largest near the first fastening portion 51 and the second fastening portion 52.

[0062] Here, as shown in the enlarged view of portion A4 in Figure 8A, the second end 35 on the tooth tip side of the tooth flank 32A of the second tooth 3B abuts against the tooth flank 22A of the first tooth 2A, and the other portions of the tooth flank 32A of the second tooth 3B other than the second end are separated from the tooth flank 22A of the first tooth 2A.

[0063] As described above, in the third embodiment, the first teeth 2A are arranged at the same pitch around the entire circumference, and the pitch of the second teeth 3B increases as they approach the first fastening portion 51 and the second fastening portion 52 from the apex P100 of the main body portion 4B, which is farthest from the first fastening portion 51 and the second fastening portion 52 in the circumferential direction.

[0064] The third embodiment has the following advantages in addition to the advantages of the second embodiment.

[0065] The meshing between the second tooth 3B and the first tooth 2A, which are located in a position close to the first fastening portion 51 and the second fastening portion 52, is compared with the meshing between the second tooth 3B and the first tooth 2A, which are located in a position far from the first fastening portion 51 and the second fastening portion 52 (for example, the apex P100).

[0066] 8B , when the distance between the first fastening portion 51 and the second fastening portion 52 is reduced by fastening the fastening member 6, a greater meshing force is exerted on the meshing between the first tooth 2A and the second tooth 3B located near the first fastening portion 51 and the second fastening portion 52. Here, the pitch of the second tooth 3B located near the first fastening portion 51 and the second fastening portion 52 is greater than that of the second tooth 3B located at the apex P100. Therefore, even if a greater meshing force acts on the second tooth 3B and the first tooth 2A, interference between the second tooth 3B and the first tooth 2A due to the meshing force is suppressed.

[0067] The present invention is not limited to the above-described embodiments. For example, the shaft unit of the present invention may be a universal joint including a shaft member and a yoke connected to the end of the shaft member, as described in Patent Document 1 (Japanese Patent No. 6596958). Furthermore, the engagement between the first teeth and the second teeth can be, for example, a serration fit or a spline fit. [Explanation of symbols]

[0068] 10, 10A, 10B shaft coupling 11, 11B First shaft (shaft) 12 Second shaft (shaft) 15, 15A, 15B shaft unit 2, 2A 1st tooth 21 Tooth tip 21a end 22, 22A tooth surface 23 Tooth dedendum 24 First end 25 Root 3, 3A, 3B 2nd tooth 31 Tooth tip 31a end 32, 32A tooth surface 33 Tooth dedendum 34 Root of tooth 35 Second end 4, 4A, 4B Main body 51 First fastening portion (pair of fastening portions) 511 First through hole 512 Female thread 52 Second fastening portion (pair of fastening portions) 521 Second Through Hole 53 Slit 6 Fastening members 61 Head 62 Shaft 63 Male thread 80 Steering device 81 Steering wheel 82 Steering shaft 82a Input shaft 82b Output shaft 83 Steering force assist mechanism 85 intermediate shaft 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 100 dash panel 101 Dust cover 851 Upper shaft 852 lower shaft AX1, AX2 center axis P100 Top

Claims

1. a shaft having a first tooth on an outer periphery thereof; a shaft coupling that extends in the axial direction of a central axis, has second teeth on an inner periphery that mesh with the first teeth, and connects the shaft; A shaft unit comprising: The shaft coupling comprises: a cylindrical main body portion having the second teeth provided on an inner periphery thereof; The strength of the first tooth is set to be equal to or less than the strength of the second tooth, a first end portion on the tooth tip side of the tooth flank of the first tooth abuts against the tooth flank of the second tooth, and a portion of the tooth flank of the first tooth other than the first end portion is spaced apart from the tooth flank of the second tooth; Shaft unit.

2. The shaft coupling comprises: a pair of fastening portions that protrude radially outward from the main body portion and are arranged opposite each other with a slit that extends in the axial direction therebetween; a fastening member that passes through the pair of fastening portions and fastens the pair of fastening portions together, The shaft unit according to claim 1 .

3. a shaft having a first tooth on an outer periphery thereof; a shaft coupling that extends in the axial direction of a central axis, has second teeth on an inner periphery that mesh with the first teeth, and connects the shaft; A shaft unit comprising: The shaft coupling comprises: a cylindrical main body portion having the second teeth provided on an inner periphery thereof; The strength of the second teeth is set lower than the strength of the first teeth, a second end portion of the tooth flank of the second tooth on the tooth tip side abuts against the tooth flank of the first tooth, and a portion of the tooth flank of the second tooth other than the second end portion is spaced apart from the tooth flank of the first tooth; Shaft unit.

4. The shaft coupling comprises: a pair of fastening portions that protrude radially outward from the main body portion and are arranged opposite each other with a slit that extends in the axial direction therebetween; a fastening member that passes through the pair of fastening portions and fastens the pair of fastening portions together, The shaft unit according to claim 3 .

5. The first teeth are arranged at a uniform pitch around the entire circumference, the second teeth have a pitch that increases from a portion of the main body farthest from the pair of fastening portions in the circumferential direction toward the pair of fastening portions; The shaft unit according to claim 4.

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