Pinion shaft, electric power steering device, and method for manufacturing pinion shaft

Abstract

Provided is a pinion shaft capable of suppressing damage due to diameter reduction. Specifically, the pinion shaft includes: a pinion tooth part (35) that has pinion teeth (26) formed on the outer periphery; and a first bearing fitting part (37) that is located closer to one side in the rotation axis direction than the pinion tooth part (35) and that has an inner ring of a first bearing fitted onto the outer peripheral surface. The first bearing fitting part (37) is formed in a columnar shape having a diameter smaller than the addendum circle of the pinion teeth (26) on the end surface, so that the end surface of the pinion tooth part (35) abuts the side surface of the inner ring of the first bearing. The pinion tooth part (35) also has an area-increasing part, in which the area of a cross section perpendicular to the rotation axis direction of a portion located at the addendum side of the pinion teeth (26) and protruding further outward than the outer peripheral surface of the first bearing fitting part (37) increases toward the first bearing fitting part (37) side.

Description

Pinion shaft, electric power steering device, and method for manufacturing a pinion shaft

[0001] This invention relates to a pinion shaft, an electric power steering device, and a method for manufacturing a pinion shaft.

[0002] Conventionally, for example, a dual-pinion type electric power steering system (EPS) having two pinion shafts has been proposed (see, for example, Patent Document 1). In the dual-pinion type EPS described in Patent Document 1, one of the two pinion shafts is connected to the steering shaft side, and the other pinion shaft is connected to the motor side that assists the steering force. The rack teeth of the rack shaft mesh with these two pinion shafts, thereby moving the rack shaft in the vehicle width direction.

[0003] Japanese Patent Publication No. 2010-23796

[0004] In a dual-pinion type EPS, it is desirable to reduce the size of the assist-side motor from the viewpoint of component layout. However, reducing the size of the motor may lead to a decrease in motor torque and a decrease in rack thrust. Therefore, for example, it is conceivable to increase rack thrust while miniaturizing the motor by making the assist-side pinion shaft thinner, reducing the diameter of the pitch circle of the pinion teeth, and reducing the rack displacement amount per rotation of the pinion shaft (specific stroke). However, simply making the pinion shaft thinner (reducing its diameter) can lead to a decrease in strength and the possibility of the pinion shaft breaking. The present invention aims to provide a pinion shaft that can suppress breakage associated with miniaturization, an electric power steering device, and a method for manufacturing a pinion shaft.

[0005] To achieve the above objective, a pinion shaft according to one aspect of the present invention comprises: (a) a pinion tooth portion having pinion teeth formed on its outer circumference; (b) a bearing fitting portion located on one side of the pinion tooth portion in the direction of rotation axis, with the inner ring of a bearing fitted to its outer surface; (c) the bearing fitting portion is formed in a cylindrical shape with a diameter smaller than the tip circle of the pinion teeth at the end surface, such that the end surface of the pinion tooth portion abuts against the side surface of the inner ring of the bearing; and (d) the pinion tooth portion has an area-increasing portion in which the area of ​​the cross-section perpendicular to the direction of rotation axis of the protruding portion, which is the portion on the tip side of the pinion teeth that protrudes outward from the outer surface of the bearing fitting portion, increases toward the bearing fitting portion.

[0006] Furthermore, an electric power steering device according to another aspect of the present invention includes (a) a pinion shaft having a pinion tooth portion that extends in the direction of the rotation axis and has pinion teeth formed on its outer circumference that mesh with the rack teeth of the rack shaft, and (b) The present invention comprises (c) a housing that accommodates at least a portion of the pinion shaft, (d) a first bearing fitted to one side of the pinion shaft in the direction of rotation axis beyond the pinion teeth and supporting the pinion shaft rotatably with respect to the housing, (e) an electric motor that applies rotational torque to the other side of the pinion shaft in the direction of rotation axis beyond the pinion teeth and supporting the pinion shaft rotatably with respect to the housing, (f) a bearing fitting portion of the pinion shaft into which the first bearing is fitted, which is formed in a cylindrical shape with a smaller diameter than the tip circle of the pinion teeth at the end face, such that the end face of the pinion teeth abuts against the side surface of the inner ring of the first bearing, and (g) a portion of the pinion teeth that has an area-increasing portion in which the area of ​​the cross section perpendicular to the direction of rotation axis of the protruding portion, which is the portion of the pinion teeth that protrudes outward from the outer circumferential surface of the bearing fitting portion and is on the tip side of the pinion teeth, increases as it approaches the bearing fitting portion.

[0007] Furthermore, another embodiment of the present invention provides a method for manufacturing a pinion shaft, wherein (a) the method for manufacturing a pinion shaft described above, and (b) in the manufacturing step of the pinion teeth of the area-enlarged portion, the pinion shaft is cut with the cutting edge on the outer circumference of the hobbing tool at the planned position for forming the area-enlarged portion, thereby making the shape of the tooth root of the pinion teeth of the area-enlarged portion conform to the shape of the outer circumference of the hobbing tool.

[0008] According to one aspect of the present invention, it is possible to provide a pinion shaft capable of suppressing damage associated with reducing the diameter, an electric power steering device, and a method for manufacturing a pinion shaft.

[0009] This is a schematic diagram of an electric power steering device according to an embodiment. This is a schematic cross-sectional view of the electric power steering device when it breaks on the electric motor side (assist side). This is a side view of the pinion shaft according to an embodiment, using the omission of figures in mechanical drawing. This is a side view of the pinion shaft according to an embodiment, without the omission of figures in mechanical drawing. This is a diagram showing the first incomplete gear portion when it breaks along lines A-A, B-B, and C-C in Figure 3. This is a diagram showing the area of ​​the portion of the pinion teeth that contacts the side surface of the inner ring of the first bearing. This is a side view of a pinion shaft according to a comparative example. This is a diagram showing each step of the manufacturing method of a pinion shaft, and shows the first half of the process. This is a diagram showing each step of the manufacturing method of a pinion shaft, and shows the second half of the process. This is a diagram showing an example of a method for forming the second incomplete gear portion, the complete gear portion, and the first incomplete gear portion. This is a diagram showing the area of ​​the portion of the pinion teeth of modified example (1) that contacts the side surface of the inner ring of the first bearing. This is a side view of a pinion shaft according to modified example (2). This is a side view of the pinion shaft according to modified example (3).

[0010] Below, an example of a pinion shaft, an electric power steering device, and a method for manufacturing a pinion shaft according to an embodiment of the present invention will be described with reference to the drawings. Note that the embodiments shown below are examples of devices and methods for realizing the technical concept of the present invention, and the technical concept of the present invention is not limited to the configuration and arrangement of the components described below. The technical concept of the present invention can be modified in various ways within the technical scope defined by the claims described in the patent claims. Furthermore, although the following description focuses on the application of the present invention to an electric power steering device, the present invention is not limited to application to electric power steering devices and is applicable to a variety of uses.

[0011] Embodiments of the present invention will be described in the following order: 1. Overall configuration of the electric power steering device 2. Structure of the pinion shaft 3. Effects of this embodiment 4. Method for manufacturing the pinion shaft 5. Modified examples

[0012] [1. Overall Configuration of the Electric Power Steering System] The electric power steering system 1 according to this embodiment will now be described. The electric power steering system 1 according to this embodiment is a dual-pinion type electric power steering system (EPS). Figure 1 is a schematic diagram of the electric power steering system 1 according to this embodiment. Figure 2 is a schematic cross-sectional view of the electric power steering system 1 when it breaks on the electric motor 58 side (assist side). In Figure 2, the rotation axis direction (central axis direction) of the pinion shaft 3 is referred to as the "Z direction", one side in the rotation axis direction is referred to as the "Z2 side", and the other side in the rotation axis direction is referred to as the "Z1 side". Similarly, the direction perpendicular to the rotation axis direction (Z direction) is referred to as the "Y direction", one side in the Y direction is referred to as the "Y2 side", and the other side in the Y direction is referred to as the "Y1 side".

[0013] As shown in Figure 1, the electric power steering system 1 includes, in the order in which the force applied by the operator is transmitted, a steering wheel 50, a steering shaft 51, a universal joint 52, an intermediate shaft 53, a universal joint 54, a shaft 55, a rack and pinion 56, and a tie rod 57. The rack and pinion 56 has a manual-side pinion shaft (not shown) connected to the shaft 55 connected to the steering wheel 50, an assist-side pinion shaft 3 (see Figure 3), which will be described later, and a rack shaft 6 (see Figure 3) that meshes with these two pinion shafts and is connected to the tie rod 57. The electric power steering system 1 also includes an electric motor 58, an ECU 59 (Electronic Control Unit) that controls the operation of the electric motor 58, and a torque sensor 60 that detects the steering torque transmitted to the shaft 55 and outputs the detection result to the ECU 59. The ECU 59 also acquires a vehicle speed signal from a vehicle speed sensor 61 installed in the vehicle via CAN communication.

[0014] Furthermore, as shown in Figure 2, the electric power steering device 1 has a transmission mechanism for transmitting the force output from the electric motor 58 to the rack shaft 6 of the rack and pinion 56. Specifically, the electric power steering device 1 comprises a housing 2, a pinion shaft 3, a worm wheel 4, a worm shaft 5, and a rack shaft 6. The housing 2 houses the pinion shaft 3, the worm wheel 4, the worm shaft 5, and the rack shaft 6 inside. The housing 2 has a rack housing 7 located on the Z2 side and a reduction gear housing 8 located on the Z1 side. The rack housing 7 and the reduction gear housing 8 are fixed to each other via bolts (not shown).

[0015] The rack housing 7 has a main body 9, a flange 10, a tip 11, and a rack guide housing 12. The main body 9 is formed in a cylindrical shape extending in the Z direction. The flange 10 protrudes radially from the Z1 end of the main body 9. An annular projection 13 is formed on the radially inner side of the flange 10, projecting toward the Z1 side. The inner surface 15 of the reducer housing 8 is fitted onto the outer circumferential surface 14 of the annular projection 13, and the reducer housing 8 is joined in a spigot configuration. A bearing (hereinafter also referred to as "second bearing 16") is arranged on the inner circumference side of the annular projection 13. For example, a four-point contact ball bearing or a deep groove ball bearing can be used as the second bearing 16. The outer ring of the second bearing 16 is fitted onto the inner circumferential surface 17 of the annular projection 13 and abuts against the bottom surface 18, and the inner ring of the second bearing 16 is fitted onto the outer circumferential surface of the pinion shaft 3. As a result, the Z1 side of the pinion shaft 3 is rotatably supported by the flange 10 (rack housing 7) via the second bearing 16. In other words, the second bearing 16 rotatably supports the pinion shaft 3 relative to the rack housing 7.

[0016] Furthermore, the tip portion 11 is formed at the Z2 side end of the main body portion 9. A bearing (hereinafter also referred to as the "first bearing 19") and a cap 20 located on the Z2 side relative to the first bearing 19 are arranged on the inner circumference side of the tip portion 11. For example, a four-point contact ball bearing or a deep groove ball bearing can be used as the first bearing 19. The outer ring of the first bearing 19 is fitted to the inner circumference surface 21 of the tip portion 11 and abuts against the cap 20, and the inner ring of the first bearing 19 is fitted to the outer circumference surface of the pinion shaft 3. As a result, the Z2 side of the pinion shaft 3 is rotatably supported by the tip portion 11 (rack housing 7) via the first bearing 19. That is, the first bearing 19 rotatably supports the pinion shaft 3 relative to the rack housing 7. Furthermore, a male thread is formed on the outer circumference of the cap 20. The male thread formed on the outer circumference of the cap 20 engages with a female thread formed on the inner circumference of the tip portion 11.

[0017] The rack guide housing 12 is formed in a cylindrical shape that protrudes from the side of the main body 9 toward the Y2 side. Inside the rack guide housing 12 are the rack guide 22 and the spring 23. A sealing member 24 is fitted into the opening of the rack guide housing 12 on the Y2 side. The spring 23 pushes the rack shaft 6 toward the Y1 side via the rack guide 22, thereby pressing the rack shaft 6 against the pinion shaft 3. This maintains the meshing between the rack teeth 25 of the rack shaft 6 and the pinion teeth 26 of the pinion shaft 3.

[0018] The gearbox housing 8 has a top surface 27, an inner surface 15, and a flange 28. The inner surface 15 extends from the outer peripheral end of the top surface 27 toward the Z2 side. The space enclosed by the top surface 27 and the inner surface 15 houses the worm wheel 4 and the worm shaft 5. The flange 28 is formed at the Z2 side end of the gearbox housing 8. The flange 28 is fixed to the flange 10 of the rack housing 7 via bolts. The worm wheel 4 has a core metal portion 29 and a wheel tooth portion 30. The wheel tooth portion 30 meshes with the shaft tooth portion 31 of the worm shaft 5. The worm shaft 5 is attached to the output shaft of an electric motor 58 (see Figure 1). As a result, by rotationally driving the worm shaft 5 with the electric motor 58, the rack shaft 6 can be moved in the vehicle width direction via the worm shaft 5, worm wheel 4, and pinion shaft 3. In other words, the electric motor 58 applies rotational torque to the Z1 side of the pinion shaft 3.

[0019] [2. Structure of the Pinion Shaft] Next, the structure of the pinion shaft 3 will be described in detail. As shown in Figures 3 and 4, the pinion shaft 3 has the following parts formed in this order from the Z1 side to the Z2 side: worm wheel holding portion 32, flange 33, second bearing fitting portion 34, pinion teeth portion 35, first groove portion 36, first bearing fitting portion 37 (broadly speaking, "bearing fitting portion"), second groove portion 38, and male thread portion 39. That is, the first bearing fitting portion 37 is located on the Z2 side of the pinion teeth portion 35. Also, the second bearing fitting portion 34 is located on the opposite side of the first bearing fitting portion 37, with the pinion teeth portion 35 in between. Figures 3 and 4 are side views of the pinion shaft 3 according to this embodiment. Figure 3 is a diagram drawn using the omission technique of mechanical drawing, omitting the pinion teeth 26 and male screw 45, while Figure 4 is a diagram drawn without omitting the pinion teeth 26 and male screw 45.

[0020] As shown in Figure 2, the core metal portion 29 of the worm wheel 4 is fitted onto the outer circumference of the worm wheel holding portion 32. The Z2 side surface of the core metal portion 29 is in contact with the Z1 side surface of the flange 33. As a result, the Z1 side of the pinion shaft 3 rotates integrally with the worm wheel 4, transmitting the rotation of the worm wheel 4 to the pinion shaft 3. The second bearing fitting portion 34 is formed in a cylindrical shape extending in the Z direction. The inner ring of the second bearing 16 is fitted onto the outer circumference of the second bearing fitting portion 34. As a result, the Z1 side of the pinion shaft 3 is rotatably supported relative to the rack housing 7.

[0021] The pinion tooth part 35 has pinion teeth 26 meshing with the rack teeth 25 formed on its outer circumference. The pinion teeth 26 are continuously formed from the end on the Z1 side to the end on the Z2 side of the pinion tooth part 35. As the pinion teeth 26, for example, sawtooth or flat teeth can be adopted. In this embodiment, the case of using sawtooth is illustrated. On the pinion tooth part 35, a second incomplete gear part 40, a complete gear part 41, and a first incomplete gear part 42 are formed in this order from the Z1 side to the Z2 side. The complete gear part 41 is the part where the rack teeth 25 and the pinion teeth 26 mesh, and is formed on the Z1 side of the first incomplete gear part 42 (area enlarging part), and the diameter of the root circle of the pinion teeth 26 is constant regardless of the position in the Z direction (rotation axis direction) (hereinafter, also referred to as "diameter R a "). The diameter R a of the root circle of the complete gear part 41 is smaller than the diameter of the root circle of the manual side pinion shaft connected to the shaft 55 (see FIG. 1). Also, the complete gear part 41 has a constant diameter (hereinafter, also referred to as "diameter R b ") of the tip circle of the pinion teeth 26 regardless of the position in the Z direction. The diameter R b is smaller than the diameter R c of the second bearing fitting part 34 (R b < R c ). Also, the second incomplete gear part 40 has a root circle diameter that expands from diameter R a to diameter R c (> R a . The diameter of the second bearing fitting part 34) as it goes from the complete gear part 41 side toward the Z1 side. Also, the diameter of the tip circle of the pinion teeth 26 of the second incomplete gear part 40 expands from diameter R b to diameter R c in the middle as it goes from the complete gear part 41 side toward the Z1 side. That is, the outer diameter of the second incomplete gear part 40 expands from diameter R b to diameter R c . Also, at the end on the Z1 side of the second incomplete gear part 40, the diameter of the root circle and the diameter of the tip circle of the pinion teeth 26 are the same.

[0022] Also, the first incomplete gear part 42 has a root circle diameter that expands from diameter R a to diameter Rd (>R a This is the part that expands in diameter to ). Diameter R d The diameter R of the first bearing fitting portion 37 e It is larger than that. That is, the shape of the tooth root of the pinion tooth 26 of the first incomplete gear section 42 is a tapered shape that gradually widens in diameter as it approaches the first bearing fitting section 37. As a result, the change in the shape of the pinion shaft 3 becomes gradual, stress concentration that increases locally can be suppressed, and damage to the pinion shaft due to stress concentration can be suppressed. In addition, as shown in Figure 5, the tooth thickness W of the portion of the pinion tooth 26 that protrudes outward from the outer circumferential surface of the first bearing fitting section 37 (hereinafter also referred to as the "protruding portion 43") is larger as it approaches the Z2 side (first bearing fitting section 37 side). As a result, the first incomplete gear section 42 has a portion in which the area D of the cross section perpendicular to the Z direction (rotation axis direction) of the protruding portion 43 increases as it approaches the first bearing fitting section 37. Figure 5 shows the first incomplete gear section 42 when fracture occurs along lines A-A, B-B, and C-C in Figure 3. In Figure 5, the tooth thickness W is shown as an example where the tooth thickness at the tooth tip is used.

[0023] Furthermore, the diameter of the tip circle of the pinion teeth 26 of the first incomplete gear section 42 is the diameter R as it moves from the complete gear section 41 side towards the Z1 side. b From diameter R f The diameter is enlarged. That is, the tooth tip of the portion of the pinion tooth 26 located at the Z2 end (hereinafter also referred to as the "shoulder portion 44") protrudes outward more than the tooth tip of the pinion tooth 26 of the complete gear portion 41. As a result, as shown in Figure 6, the area E of the portion of the pinion tooth portion 35 (protruding portion 43) that contacts the side surface of the inner ring of the first bearing 19 can be increased. Figure 6 is a diagram showing the area E of the contact portion. In Figure 6, the diameter R of the inner circumference of the non-chamfered region (unchamfered region) of the side surface of the inner ring of the first bearing 19 is increased due to the chamfering of the inner ring of the first bearing 19. h However, the diameter R of the first bearing fitting portion 37 eThis example illustrates a case where the value is larger than the given value. Specifically, the end face of the pinion tooth portion 35 is configured to contact the non-chamfered region of the side surface of the inner ring of the first bearing 19 in an intermittent annular manner. More specifically, the diameter R of the root circle of the portion of the pinion tooth 26 located at the end of the first incomplete gear portion 42 (area-increased portion) on the first bearing fitting portion 37 side. d The diameter R of the first bearing fitting portion 37 is such that only the end faces of the pinion teeth 26 contact the side surface of the inner ring of the first bearing 19. e Larger diameter (R d > R e ) and the inner diameter R of the non-chamfered region on the side surface of the inner ring of the first bearing 19 h Smaller diameter (R d <R h ) (See Figures 6 and 1). R d <R h This allows for a more gradual change in the diameter of the root circle of the pinion teeth 26, suppressing stress concentration and further reducing damage to the pinion shaft due to stress concentration.

[0024] The first groove 36 is formed in an annular shape along the circumferential direction of the pinion shaft 3. The widthwise cross-section of the first groove 36 has a concave shape that is recessed towards the axial center. As shown in Figure 3, the diameter R of the bottom surface of the first groove 36 g The diameter R of the first bearing fitting portion 37 e It is smaller than [the specified value]. This prevents the cutting tool from interfering with the Z2-side end face (bearing seating surface) of the pinion teeth 35 when cutting the first bearing fitting portion 37, and allows for efficient cutting of the first bearing fitting portion 37. The first bearing fitting portion 37 is formed in a cylindrical shape extending in the Z direction. The inner circumferential surface of the inner ring of the first bearing 19 is fitted to the outer circumferential surface of the first bearing fitting portion 37. Diameter R of the first bearing fitting portion 37 e The end face (bearing seating surface) of the pinion teeth 35 is in contact with the side surface of the inner ring of the first bearing 19, and the tip circle (diameter R) of the pinion teeth 26 at that end face is such that the end face (bearing seating surface) of the pinion teeth 35 is in contact with the side surface of the inner ring of the first bearing 19. f It is formed to have a smaller diameter than ). Also, the diameter R of the first bearing fitting portion 37 e This is the diameter R of the root circle of the pinion tooth 26 of the complete gear section 41. a It is formed with a larger diameter than R. f > R e > Ra That's how it is.

[0025] Furthermore, a male thread 45 is formed on the outer circumference of the male thread portion 39. The thread diameter of the male thread 45 is equal to the diameter R of the first bearing fitting portion 37. e It is smaller than that. That is, it is smaller than the inner diameter of the inner ring of the first bearing 19. This allows the pinion shaft 3 to pass through the inner ring of the first bearing 19 from the Z2 side of the male threaded portion 39. As shown in Figure 2, a nut 46 is fastened to the male threaded portion 39 with the first bearing 19 fitted into the first bearing fitting portion 37. The nut 46 presses the side surface of the inner ring of the first bearing 19 toward the Z1 side. As a result, the inner ring of the first bearing 19 is held in place by being sandwiched between the nut 46 and the protrusion 43, and its position in the Z direction is maintained.

[0026] [3. Effects of this Embodiment] In a dual-pinion type EPS, it is desirable to reduce the size of the assist-side electric motor 58 from the viewpoint of component layout. However, reducing the size of the electric motor 58 may lead to a decrease in motor torque and a decrease in rack thrust. Therefore, for example, by making the assist-side pinion shaft 3 thinner, reducing the diameter of the pitch circle of the pinion teeth 26, and reducing the rack displacement amount (specific stroke) per rotation of the pinion shaft, it is conceivable to increase the rack thrust while miniaturizing the electric motor 58. For example, as shown in Figure 7, the pinion shaft 3 is made thinner, the first incomplete gear section 42 is omitted, the complete gear section 41 is extended to the Z2 side end of the pinion tooth section 35, and furthermore, the diameter R of the tooth root circle of the complete gear section 41 is reduced. a and the diameter R of the first bearing fitting portion 37 e A configuration that is identical to (R a = R e ) is a possibility. However, the diameter R of the first bearing fitting portion 37 e If the diameter becomes too thin, it will lead to a decrease in strength, and the first bearing fitting portion 37 may break when a bending moment is applied. Therefore, compared to the configuration in Figure 7, the diameter R of the first bearing fitting portion 37 is increased in order to improve strength. e The diameter R of the root circle of the complete gear section 41 a It is possible to make it larger than (R e > R a ). However, Re > R a In this case, the area of ​​the portion of the pinion teeth 35 that contacts the inner ring of the first bearing 19 (the protruding portion 43) becomes smaller. Therefore, if the pinion shaft 3 is displaced due to, for example, road surface reaction force, vibration of the entire vehicle, vibration of the worm wheel 4, or axial force of the worm shaft 5, and a compressive force in the Z direction is generated on the protruding portion 43, the protruding portion 43 may be damaged by that compressive force.

[0027] In contrast, in the pinion shaft 3 according to this embodiment, as shown in Figures 3 and 5, the pinion tooth portion 35 has a first incomplete gear portion 42 (area-increasing portion) in which the area D of the cross-section perpendicular to the Z direction of the portion (protruding portion 43) that protrudes outward from the outer circumferential surface of the first bearing fitting portion 37 on the tooth tip side of the pinion tooth 26 increases as it approaches the Z2 side (first bearing fitting portion 37 side). This makes it possible to increase the area D of the protruding portion 43 (the portion that protrudes outward from the outer circumferential surface of the first bearing fitting portion 37) on the Z2 side end face of the pinion tooth portion 35. Therefore, as shown in Figure 6, the area E of the portion of the pinion teeth 35 that contacts the inner ring of the first bearing 19, that is, the area E of the portion that receives compressive force in the Z direction from the inner ring of the first bearing 19, can be increased, the stress applied to the first incomplete gear portion 42 can be distributed more efficiently, and damage to the protruding portion 43 due to compressive force in the Z direction (e.g., buckling) can be suppressed. Thus, according to the pinion shaft 3 of this embodiment, damage to the pinion shaft 3 due to reduction in diameter can be suppressed. In addition, the tooth tips of the portion of the pinion teeth 26 located at the Z2 side end of the first incomplete gear portion 42 (shoulder portion 44) are configured to protrude outward more than the tooth tips of the pinion teeth 26 of the complete gear portion 41. This increases the area E of the portion of the pinion teeth 35 that contacts the side surface of the inner ring of the first bearing 19, and the stress applied to the first incomplete gear portion 42 can be distributed more efficiently.

[0028] Here, for example, the diameter R of the root circle at the Z2 end of the first incomplete gear portion 42. d The diameter R of the first bearing fitting portion 37 e If the following configuration is adopted (R d ≤R e ), diameter R d As the diameter is reduced, the diameter R of the first groove 36g The diameter is also reduced, which may decrease the strength of the first groove 36. In contrast, in the pinion shaft 3 according to this embodiment, the diameter R of the root circle of the Z2 end of the first incomplete gear portion 42 d The diameter R of the first bearing fitting portion 37 e Larger than (R d > R e Therefore, diameter R d The diameter R of the first groove 36 due to the reduction in diameter g This can suppress the reduction in diameter and suppress the decrease in strength of the first groove 36. Also, the diameter R d Because of its large size, sufficient strength can be obtained against the torsional and bending stresses applied to the Z2 end of the first incomplete gear section 42.

[0029] Furthermore, in the pinion shaft 3 according to this embodiment, the diameter R of the first bearing fitting portion 37 e The diameter R of the root circle of the pinion tooth 26 of the complete gear section 41 a Larger diameter than (R e > R a ). Here, the rack thrust F ラック According to the calculation formula, as shown in equations (1) and (2) below, the specific stroke γ 比スト Since it appears in the denominator, the relative stroke γ 比スト The smaller the value, the greater the rack thrust F. ラック It can be seen that R can be obtained. Therefore, according to the pinion shaft 3 of this embodiment, e > R a As a result, the specific stroke γ 比スト As this will reduce the size of the electric motor 58, the rack thrust F will be reduced. ラック This can be increased. F ラック = 2πT ピニオン / γ 比スト ×η 効率 ... (1) T ピニオン = T モータ ×i w / wのギヤ比 ×η 効率 ... (2)

[0030] [4. Method for Manufacturing a Pinion Shaft] Next, the method for manufacturing a pinion shaft will be described. First, a first turning process is performed using an NC lathe to turn the outer diameter of the pinion shaft 3, as shown in Figures 8A and 8B. Figures 8A and 8B are diagrams showing each step of the pinion shaft manufacturing process, with Figure 8A showing the first half of the process and Figure 8B showing the second half of the process. Subsequently, a gear cutting process is performed using a hobbing machine equipped with a hobbing tool F (generally also called a "hob cutter" or "hob") to form pinion teeth 26 at the planned positions for the pinion tooth portion 35, thereby forming the pinion tooth portion 35. In the gear cutting process, the hobbing tool F (hob cutter) is moved parallel to the Z direction from the Z1 side to the Z2 side, and then moved from the Z2 side to the Z1 side (Z1 side → Z2 side → Z1 side) to form the second incomplete gear portion 40, the complete gear portion 41, and the first incomplete gear portion 42.

[0031] Specifically, first, at the planned formation position of the second incomplete gear section 40, the hobbing tool F is gradually moved from a direction perpendicular to the Z direction towards the pinion shaft 3, and the pinion shaft 3 is cut with the cutting edge on the outer circumference of the hobbing tool F. This makes the shape of the tooth roots of the pinion teeth 26 of the second incomplete gear section 40 conform to the shape of the outer circumference of the hobbing tool F, thereby forming the second incomplete gear section 40. The depth of the tooth roots of the pinion teeth 26 of the second incomplete gear section 40 becomes shallower as it approaches the Z1 side. Once the second incomplete gear section 40 is formed, the hobbing tool F is moved parallel to the Z direction from the Z1 side to the Z2 side, and the pinion shaft 3 is cut at the planned formation position of the complete gear section 41. This makes the tooth root depth of the pinion teeth 26 constant, thereby forming the complete gear section 41.

[0032] Next, when the hobbing tool F reaches the planned position for forming the first incomplete gear portion 42, machining of the first incomplete gear portion 42 (the process of forming the pinion teeth 26 in the area-increasing portion) is started, and the pinion shaft 3 is cut by the cutting edge on the outer circumference of the hobbing tool F. As a result, the shape of the tooth roots of the pinion teeth 26 of the first incomplete gear portion 42 is made to conform to the shape of the outer circumference of the hobbing tool F, thereby forming the first incomplete gear portion 42. The depth of the tooth roots of the pinion teeth 26 of the first incomplete gear portion 42 becomes shallower as it approaches the Z2 side. As the depth of the tooth roots of the pinion teeth 26 becomes shallower as it approaches the Z2 side, the entire first incomplete gear portion 42 forms an area-increasing portion (upward-cut shape) in which the tooth thickness W of the protruding portion 43 (see Figure 4) and the area D of the cross-section perpendicular to the rotation axis direction of the protruding portion 43 become larger as it approaches the Z2 side. Once the first incomplete gear portion 42 is formed and the rough machining of the entire pinion tooth portion 35 is completed, the hobbing tool F is moved parallel to the Z direction from the Z2 side to the Z1 side, and for finishing, the hobbing tool F is used to cut again the formation positions of the first incomplete gear portion 42, the complete gear portion 41, and the second incomplete gear portion 40. Although an example of moving the hobbing tool F back and forth between the Z1 side and the Z2 side has been shown, it may also be moved in only one direction without reciprocating.

[0033] In addition, other forming methods can be used for forming the second incomplete gear portion 40, the complete gear portion 41, and the first incomplete gear portion 42. For example, as shown in Figure 9, when machining the second incomplete gear portion 40, the hobbing tool F is moved parallel to the rotation axis of the pinion shaft 3 from the Z1 side to the Z2 side, and gradually moved in a direction approaching the pinion shaft 3, thereby gradually increasing the depth of the tooth roots of the pinion teeth 26 of the second incomplete gear portion 40 by cutting the pinion shaft 3 with the cutting edge of the outer circumference of the hobbing tool F. Furthermore, during the machining of the first incomplete gear section 42 (in the process of forming the pinion teeth 26 of the area-increased section), the hobbing tool F is moved parallel to the rotation axis of the pinion shaft 3 from the Z1 side to the Z2 side, and gradually moved away from the pinion shaft 3, thereby cutting the pinion shaft 3 with the cutting edge of the outer circumference of the hobbing tool F, and gradually making the tooth root depth of the pinion teeth 26 of the first incomplete gear section 42 shallower. Note that the direction of movement of the hobbing tool F may be Z2 side → Z1 side → Z2 side. In this case, during the machining of the first incomplete gear section 42, the hobbing tool F is moved parallel to the rotation axis (Z direction) of the pinion shaft 3 from the Z2 side to the Z1 side, and gradually moved towards the pinion shaft 3, thereby gradually making the tooth root depth of the pinion teeth 26 deeper and forming the area-increased section (upper cut shape).

[0034] Next, after carburizing and quenching using a carburizing furnace, the pinion shaft 3 is straightened using an automated machine. Then, a second turning process is performed using an NC lathe to turn the outer diameters of the first bearing fitting portion 37, the second groove portion 38, and the male thread portion 39. Next, a thread rolling process is performed using a thread rolling machine to form male threads 45 on the male thread portion 39. Finally, the outer diameters of the second bearing fitting portion 34 and the first bearing fitting portion 37 are ground using a grinding machine, and the tooth groove runout is measured using a tooth groove runout measuring instrument. By going through this process, the pinion shaft 3 shown in Figure 3 is manufactured.

[0035] [5. Modified Example] (1) In this embodiment, as shown in FIG. 6, an example is shown in which the end face of the pinion tooth portion 35 intermittently contacts the non-chamfered region on the side surface of the inner ring of the first bearing 19 in an annular shape, but other configurations can also be adopted. For example, as shown in FIG. 10, the diameter R of the bottom circle of the portion located at the end on the side of the first bearing fitting portion 37 of the first incomplete gear portion 42 (area increasing portion) among the pinion teeth 26 d is set to be larger than the diameter R of the first bearing fitting portion 37 such that the end face of the pinion tooth portion 35 continuously contacts the side surface of the inner ring of the first bearing 19 in an annular shape (R e > R d > R e ). More specifically, R is set such that the end face of the pinion tooth portion 35 continuously contacts the non-chamfered region on the side surface of the inner ring of the first bearing 19 in an annular shape, and R d > R e , and the diameter R of the inner circumference of the non-chamfered region on the side surface of the inner ring of the first bearing 19 is larger than the diameter R h (R d > R d > R h ). Thereby, the end face of the pinion tooth portion 35 can continuously contact the non-chamfered region on the side surface of the inner ring of the first bearing 19 in an annular shape, and the area E of the portion of the pinion tooth portion 35 that contacts the inner ring of the first bearing 19 can be increased.

[0036] (2) Also, in this embodiment, as shown in FIG. 3, an example is shown in which the entire first incomplete gear portion 42 is an area increasing portion where the area of the cross-section perpendicular to the Z direction of the protruding portion 43 increases as it goes toward the Z2 side, but other configurations can also be adopted. For example, as shown in FIG. 11, the Z1 side of the first incomplete gear portion 42 is the above-mentioned area increasing portion 47, and the Z2 side of the first incomplete gear portion 42 is the maximum diameter (diameter R dThe connection portion 48 may be configured to have a cylindrical tooth root having the same diameter as the pinion teeth 42. That is, the pinion teeth 35 (part of the first incomplete gear portion 42) is configured to have a connection portion 48 between the first bearing fitting portion 37 and the area-enhancing portion 47. This makes it possible to increase the thickness of the portion of the protruding portion 43 that receives compressive force in the Z direction from the inner ring of the first bearing 19, thereby suppressing damage to the protruding portion 43 due to compressive force in the Z direction. When a configuration with a connection portion 48 is adopted, the first incomplete gear portion 42 (area-enhancing portion 47, connection portion 48) can be processed by moving the hobbing tool F along the planned formation position of the tooth root of the pinion teeth 26 of the area-enhancing portion 47 and the connection portion 48.

[0037] (3) In this embodiment, as shown in Figure 3, an example is shown in which the corner between the end face of the pinion tooth 26 on the first bearing fitting portion 37 side and the tooth tip surface of the pinion tooth 26 is a sharp corner shape without chamfering, but other configurations can also be adopted. For example, as shown in Figure 12, a configuration may be used in which a chamfered portion 49 is provided between the end face of the pinion tooth 26 on the first bearing fitting portion 37 side (Z2 side) and the tooth tip surface of the pinion tooth 26. Figure 12 illustrates a case in which the corner of the chamfered portion 49 is machined to a 45° angle. This makes it possible to remove burrs generated during the manufacturing of the pinion shaft 3 and prevent damage caused by burrs. (4) In this embodiment, as shown in Figure 3, an example is shown in which helical teeth are used as the pinion teeth 26, but other configurations can also be adopted. For example, spur teeth may be used.

[0038] 1...Electric power steering device, 2...Housing, 3...Pinion shaft, 4...Worm wheel, 5...Worm shaft, 6...Rack shaft, 7...Rack housing, 8...Gearbox housing, 9...Main body, 10...Flange, 11...Tip, 12...Rack guide housing, 13...Annular projection, 14...Outer surface, 15...Inner surface, 16...Second bearing, 17...Inner surface, 18...Bottom surface, 19...First bearing, 20...Cap, 21...Inner surface, 22...Rack guide, 23...Spring, 24...Sealing member, 25...Rack teeth, 26...Pinion teeth, 27...Top surface, 28...Flange, 29...Core metal part, 30...Wheel teeth part, 31...Shaft teeth part, 32...Worm wheel holder 33...Flange, 34...Second bearing fitting part, 35...Pinion teeth, 36...First groove, 37...First bearing fitting part, 38...Second groove, 39...Male thread, 40...Second incomplete gear part, 41...Complete gear part, 42...First incomplete gear part, 43...Protruding part, 44...Shoulder part, 45...Male thread, 46...Nut, 47...Area enlargement part, 48...Connecting part, 49...Chamfered part, 50...Steering wheel, 51...Steering shaft, 52...Universal joint, 53...Intermediate shaft, 54...Universal joint, 55...Shaft, 56...Rack and pinion, 57...Tie rod, 58...Electric motor, 59...ECU, 60...Torque sensor, 61...Vehicle speed sensor

Claims

1. A pinion shaft comprising: a pinion tooth portion having pinion teeth formed on its outer circumference; and a bearing fitting portion located on one side in the direction of rotation axis than the pinion tooth portion, with the inner ring of a bearing fitted to its outer surface, wherein the bearing fitting portion is formed in a cylindrical shape with a smaller diameter than the tip circle of the pinion teeth at the end surface, such that the end surface of the pinion tooth portion abuts against the side surface of the inner ring of the bearing, and the pinion tooth portion has an area-increasing portion in which the area of ​​the cross-section perpendicular to the direction of rotation axis of the protruding portion, which is the portion that protrudes outward from the outer surface of the bearing fitting portion on the tip side of the pinion teeth, increases toward the bearing fitting portion.

2. The pinion shaft according to claim 1, wherein the pinion tooth portion is formed on the other side in the rotational axis direction of the area-increased portion and has a complete gear portion in which the diameter of the root circle of the pinion tooth is constant regardless of the position in the rotational axis direction, and the diameter of the bearing fitting portion is larger than the diameter of the root circle of the pinion tooth of the complete gear portion.

3. The pinion shaft according to claim 2, wherein the diameter of the root circle of the portion of the pinion tooth located at the end of the increased area portion on the bearing fitting portion side is larger than the diameter of the bearing fitting portion, such that the end face of the pinion tooth portion abuts the side surface of the inner ring of the bearing in a continuous annular manner.

4. The pinion shaft according to claim 2, wherein the diameter of the root circle of the portion of the pinion tooth located at the end of the area-enlarged portion on the bearing fitting portion side is larger than the diameter of the bearing fitting portion and smaller than the diameter of the inner circumference of the non-chamfered region on the side surface of the inner ring of the bearing, such that only the end face of the pinion tooth abuts against the side surface of the inner ring of the bearing.

5. The pinion shaft according to claim 2, wherein the shape of the tooth root of the pinion tooth in the area-enlarged portion is tapered, gradually increasing in diameter toward the bearing fitting portion, and the tooth thickness of the protruding portion increases toward the bearing fitting portion in the area-enlarged portion.

6. The pinion shaft according to claim 5, wherein the pinion tooth portion is formed between the bearing fitting portion and the area-enhancing portion and comprises a connecting portion having a cylindrical tooth root having the same diameter as the maximum diameter of the tapered shape.

7. The pinion shaft according to claim 1, wherein the pinion tooth portion is formed on the other side in the rotation axis direction of the area-increased portion and has a complete gear portion in which the diameter of the root circle of the pinion tooth is constant regardless of the position in the rotation axis direction, and the tooth tips of the portion of the pinion tooth located at the end on the bearing fitting portion side protrude outward more than the tooth tips of the pinion tooth of the complete gear portion.

8. The pinion shaft according to claim 1, having a chamfered portion formed between the end face of the pinion tooth on the bearing fitting side and the tooth tip surface of the pinion tooth.

9. The pinion shaft according to claim 1, wherein the pinion teeth are helical teeth or flat teeth.

10. A pinion shaft having a pinion tooth portion extending in the direction of rotation and having pinion teeth formed on its outer circumference that mesh with the rack teeth of a rack shaft; a housing for housing the pinion shaft; a first bearing fitted to one side of the pinion shaft in the direction of rotation beyond the pinion tooth portion and rotatably supporting the pinion shaft relative to the housing; a second bearing fitted to the other side of the pinion shaft in the direction of rotation beyond the pinion tooth portion and rotatably supporting the pinion shaft relative to the housing; and an electric motor for applying rotational torque to the other side of the pinion shaft in the direction of rotation, wherein the bearing fitting portion of the pinion shaft into which the first bearing is fitted is formed in a cylindrical shape with a diameter smaller than the tip circle of the pinion teeth on the end face, such that the end face of the pinion tooth portion abuts against the side surface of the inner ring of the first bearing. The pinion tooth portion has an area-increasing portion, where the area of ​​the cross-section perpendicular to the rotation axis of the protruding portion, which is the tooth tip side of the pinion tooth and protrudes outward from the outer circumferential surface of the bearing fitting portion, increases as it approaches the bearing fitting portion.

11. A method for manufacturing a pinion shaft according to claim 1, wherein in the step of forming the pinion teeth of the area-enlarged portion, the pinion shaft is cut with the cutting edge of the outer circumference of the hobbing tool at the planned position for forming the area-enlarged portion, thereby making the shape of the tooth root of the pinion teeth of the area-enlarged portion conform to the shape of the outer circumference of the hobbing tool.

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