Rack and pinion steering gear

Abstract

A rack (11) includes: a rack tooth row (112) including a plurality of rack teeth (111) meshing with pinion teeth (9a); a hardened layer (K) continuously disposed on the entire periphery of the rack tooth row (112); and a central portion (S) disposed inside the hardened layer (K) and having a lower hardness than the hardened layer (K). When the rack bar (11) is observed in the axial direction of the rack bar (11), the depth of the hardened layer (K) increases in sequence from the following positions i), ii), and iii): i) a tooth bottom (111v) of the rack teeth (111); ii) a side surface (111s) of the rack bar (11) relative to the tooth bottom (111v); and iii) a back surface (111b) of the rack bar (11) relative to the tooth bottom (111v).

Description

Technical Field

[0001] The invention relates to a rack rod and a steering device. Background Art

[0002] A rack-and-pinion steering device for a vehicle is used to convert the rotational motion of a steering shaft into an axial linear motion of a rack rod connected to a steering wheel, so as to transmit the steering control force of the steering wheel to the steering wheel. In a rack-and-pinion mechanism, a pinion shaft is supported by a ball bearing, a needle bearing, etc. The rack rod is supported by a meshing portion between rack teeth formed on the rack rod and pinion teeth formed on the pinion shaft, and a rack bushing. There are various specifications regarding the meshing between the rack teeth and the pinion teeth, and the specifications of the teeth vary according to the required specifications of the vehicle (e.g., a specific stroke, a rack stroke, etc.).

[0003] In a rack-and-pinion steering gear, the meshing portion between the rack teeth and the pinion teeth is loaded due to the positive input torque applied from the steering wheel and the reverse input load applied by the tires. Due to the influence of tire friction, suspension geometry and other factors, the reverse input load applied by the tires tends to increase as the meshing portion between the rack teeth and the pinion teeth approaches the end of the rack travel (either end of the rack teeth row).

[0004] In addition, when the meshing portion between the rack teeth and the pinion teeth reaches either end of the rack gear row due to the driver's sharp turning of the steering wheel in order to put the vehicle into a garage or for other reasons, a so-called end abutment occurs, in which the rack rod abuts against a stopper and stops. The impact load of this end abutment is applied to the meshing portion between the rack teeth and the pinion teeth. In the case where the rack-and-pinion steering device is a rack-and-pinion steering device with an auxiliary mechanism, the impact load becomes larger. This requires ensuring the required strength (particularly axial strength) of the rack teeth of the rack rod.

[0005] In addition, when any steering wheel of the vehicle accidentally hits a hole during driving, a heavy impact load is applied to the rack bar. This requires ensuring the required strength (especially bending strength) of the rack bar. Japanese patent applications with publication numbers 6-264992 and 2017-057442 each disclose a rack bar having a hardened layer (mainly composed of a martensitic structure) continuously formed on the entire periphery of a rack tooth row including a plurality of rack teeth. The hardened layer helps to ensure the required strength (especially axial strength) of the rack teeth of the rack bar and the required strength (especially bending strength) of the rack bar. Summary of the invention

[0006] However, it has been found that forming a hardened layer (mainly composed of martensite structure) deeply in the rack may make the rack bar embrittled because the formation of such a deep hardened layer reduces the central portion of the rack bar formed inside the hardened layer, which has toughness and lower hardness than the hardened layer.

[0007] The present invention helps to increase axial strength and bending strength and reduce embrittlement.

[0008] A first aspect of the present invention relates to a rack bar. The rack bar includes: a rack tooth row including a plurality of rack teeth meshing with pinion teeth; a hardened layer continuously provided over the entire periphery of the rack tooth row; and a central portion provided inside the hardened layer and having a hardness lower than that of the hardened layer. When the rack bar is observed in the axial direction of the rack bar, the depth of the hardened layer increases sequentially from the following positions i), ii), and iii): i) the tooth bottom of the rack teeth; ii) the side of the rack bar relative to the tooth bottom; iii) the back side of the rack bar relative to the tooth bottom.

[0009] According to the above aspect, a hardened layer is continuously formed on the entire outer periphery of the rack teeth row of the rack bar. This increases the axial strength of the rack teeth and the bending strength of the rack bar. In addition, the hardened layer is formed so that its depth increases sequentially at the tooth bottom of the rack teeth, the side of the rack bar, and the back of the rack bar. This reduces the reduction of the central portion formed inside the hardened layer and having a lower hardness than the hardened layer, thereby helping to reduce the embrittlement of the rack bar.

[0010] A second aspect of the present invention relates to a steering device. The steering device includes: a housing; a rack bar supported by the housing so as to be movable in an axial direction and connected to a wheel of a vehicle; and a pinion shaft supported by the housing so as to be rotatable around the axis of the pinion shaft. The pinion shaft meshes with the rack teeth of the rack bar and is connected to the steering wheel of the vehicle. The rack bar includes: a rack tooth row including a plurality of rack teeth meshing with the pinion teeth; a hardened layer continuously arranged on the entire periphery of the rack tooth row; and a central portion arranged inside the hardened layer and having a lower hardness than the hardened layer. When the rack bar is observed in the axial direction of the rack bar, the depth of the hardened layer from the following positions i), ii) and iii) increases sequentially: i) the tooth bottom of the rack teeth; ii) the side of the rack bar relative to the tooth bottom; iii) the back side of the rack bar relative to the tooth bottom.

[0011] According to the above aspect, a hardened layer is continuously formed on the entire outer periphery of the rack teeth row of the rack bar. This increases the axial strength of the rack teeth and the bending strength of the rack bar. In addition, the hardened layer is formed so that the depth increases sequentially at the tooth bottom of the rack teeth, the side of the rack bar, and the back of the rack bar. This reduces the reduction in the central portion formed inside the hardened layer and having a lower hardness than the hardened layer, thereby helping to reduce the embrittlement of the rack. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, in which like reference numerals represent like elements, and in which:

[0013] Figure 1 shows a schematic configuration of a steering device according to an embodiment of the present invention;

[0014] Figure 2A A view showing a rack bar according to an embodiment of the present invention when viewed in a direction perpendicular to its axis;

[0015] Figure 2B yes Figure 2A A cross-sectional view of the rack rod taken along line IIB-IIB when viewed in its axial direction;

[0016] Figure 2C is a cross-sectional view of a conventional standard rack rod when viewed along its axial direction;

[0017] Figure 2D is a cross-sectional view of the temporary rack rod when viewed along its axial direction;

[0018] Figure 3 The axial strength ratio between different rack bars is shown;

[0019] Figure 4 The section coefficient ratios between different rack bars are shown;

[0020] Figure 5 is a cross-sectional view showing a thickness distribution of a hardened layer of a rack bar when viewed in the axial direction of the rack bar according to an embodiment of the present invention;

[0021] Fig. 6A A method for forming a hardened layer of a rack bar according to an embodiment of the present invention and a state of the hardened layer when viewed in the axial direction of the rack bar are shown;

[0022] Figure 6B shows a method for forming a hardened layer of a conventional standard rack bar and the state of the hardened layer when viewed in the axial direction of the standard rack bar; and

[0023] Figure 71 is a flowchart of a method for manufacturing a rack bar according to an embodiment of the present invention. DETAILED DESCRIPTION

[0024] A description will be given of a schematic configuration of a steering device including a rack bar of an embodiment of the present invention. The steering device may be a rack-and-pinion type electric power steering device with a column assist mechanism, in which an electric motor transmits power to a column shaft, or a rack-and-pinion type electric power steering device with a pinion assist mechanism, in which an electric motor transmits power to a pinion shaft.

[0025] The steering device may also be a rack-and-pinion electric power steering device with a rack-and-pinion assist mechanism, in which an electric motor transmits power to a rack bar, or a rack-and-pinion steering device without an assist mechanism. The steering device comprises at least a steering control mechanism, a steering mechanism and an assist mechanism.

[0026] As an example, the steering device described below is a rack-and-pinion electric steering device with a column assist mechanism. Figure 1 As shown, the steering device 1 includes a steering mechanism 2 and a steering mechanism 3, and steers a steering wheel 5 according to the driver's operation of a steering wheel 4 (steering member). The steering mechanism 2 includes an assist mechanism 6 that assists the driver's steering operation.

[0027] The steering mechanism 2 includes an input shaft 7a, an output shaft 7b, an intermediate shaft 8, and a pinion shaft 9 constituting a rack-and-pinion mechanism PR1. The input shaft 7a is coupled to the steering wheel 4. The output shaft 7b is coupled to the input shaft 7a via a torsion bar 7c. The intermediate shaft 8 is coupled to the pinion shaft 9 via a second universal joint 8d.

[0028] The intermediate shaft 8 is configured to extend and retract in the axial direction of the intermediate shaft 8, and includes a first shaft 8a and a second shaft 8b, which are matched with each other in a manner that can move relative to each other and rotate together, for example, by spline fitting. The first shaft 8a is connected to a first universal joint 8c, and the second shaft 8b is connected to a second universal joint 8d. The pinion shaft 9 is formed with pinion teeth 9a.

[0029] The steering mechanism 3 includes a rack bar 11 and a tie rod 12 constituting a rack-and-pinion mechanism PR1. The rack bar 11 is formed with rack teeth 111 (see FIG. 1 ) that mesh with the pinion teeth 9a. Figure 2A ). Each tie rod 12 is coupled to the rack bar 11 at one end thereof and is coupled to the corresponding steering wheel 5 at the other end thereof. In response to the rotation of the steering wheel 4 due to the operation of the driver, the pinion shaft 9 rotates via the input shaft 7a, the output shaft 7b and the intermediate shaft 8.

[0030] The rotation of the pinion shaft 9 is converted into the axial reciprocating motion of the rack bar 11. This axial reciprocating motion of the rack bar 11 is transmitted to the steered wheel 5 via the corresponding tie rod 12. This changes the steering angle of the steered wheel 5, thereby changing the driving direction of the vehicle.

[0031] The assist mechanism 6 includes a torque sensor 13, an electronic control unit (ECU) 14, an electric motor 15, and a worm gear reducer 16. The torque sensor 13 detects the amount of twist between the input shaft 7a and the output shaft 7b. The ECU 14 determines the assist torque based on the steering torque obtained from the amount of twist detected by the torque sensor 13 and based on the vehicle speed detected by the vehicle speed sensor 10.

[0032] The electric motor 15 is driven and controlled by the ECU 14. The worm gear reducer 16 transmits the rotational force of the electric motor 15 to the output shaft 7b. Therefore, assist torque is applied to the output shaft 7b, thereby assisting the steering operation of the driver.

[0033] The rack bar 11 of this embodiment can be a rack bar with a constant transmission ratio; or can be a rack bar with a variable transmission ratio, wherein the size of the rack teeth varies according to the axial position on the rack bar. As an example, the rack bar discussed herein is a rack bar with a constant transmission ratio.

[0034] like Figure 2A As shown, the rack bar 11 of the present embodiment is formed with a rack tooth row 112 including a plurality of rack teeth 111 on its outer surface, and an internal thread 113 connected to the corresponding tie rod 12 is also formed on each end surface of the rack bar 11. The rack tooth row 112 is formed by pressing a solid shaft member 11R made of preheat-treated (quenched and tempered) steel with a die to plastically deform it, or by cutting the shaft member 11R. The shaft member 11R may be hollow except for the portion where the rack tooth row 112 is formed.

[0035] Recently, vehicles are required to be downsized to improve fuel efficiency and achieve better environmental performance. To this end, it is necessary to reduce the weight of the rack-and-pinion steering device. One effective way to achieve this is to reduce the rack diameter of the rack bar. Therefore, as Figure 2B The rack diameter R of the rack bar 11 of the embodiment shown is smaller than Figure 2C The arrangement shown is for a rack diameter Ra of a standard rack bar 21 that meets the required specifications of the vehicle (eg, a certain stroke and rack travel). This allows a reduction in the weight of the rack-and-pinion steering device.

[0036] However, as described in the above summary of the invention, reducing the rack diameter of the rack bar involves reducing the tooth depth and tooth face width of the rack teeth, which generally makes it difficult to ensure the required strength (especially axial strength) of the rack teeth. Figure 2C As shown in , the standard rack teeth 211 of the conventional standard rack bar 21 have a tooth surface width Wa and a tooth depth Ha set based on the tooth specifications that match the required specifications of the vehicle (eg, a specific stroke and rack stroke). Figure 2D The temporary rack bar 31 shown in FIG. 1 is a smaller diameter version of the standard rack bar 21 (whose diameter is equal to the rack diameter R of the rack bar 11 of the present embodiment).

[0037] The temporary rack teeth 311 of the temporary rack bar 31 are formed so that the distance d between the axis Cb of the temporary rack bar 31 and the tooth bottom 311v of the temporary rack teeth 311 is equal to the distance between the Ca of the standard rack bar 21 and the tooth bottom 211v of the standard rack teeth 211. Therefore, the tooth surface width Wb and the tooth depth Hb of the temporary rack teeth 311 of the temporary rack bar 31 are respectively smaller than the tooth surface width Wa and the tooth depth Ha of the standard rack teeth 211 of the standard rack bar 21. This means that the axial strength of the temporary rack teeth 311 is smaller than the axial strength of the standard rack teeth 211.

[0038] When the tooth surface width and tooth depth of the temporary rack bar 31 are made equal to the tooth surface width and tooth depth of the standard rack bar 21, respectively, the distance between the axis Cb of the temporary rack bar 31 and the tooth bottom 311v of the temporary rack teeth 311 becomes smaller than the distance between the axis Ca of the standard rack bar 21 and the tooth bottom 211v of the standard rack teeth 211. This still results in a decrease in the axial strength of the temporary rack teeth 311.

[0039] Therefore, if Figure 2B As shown, the rack bar 11 of the present embodiment is cut out teeth more deeply in the radial direction so that the position of the tooth bottom 111v of the rack teeth 111 is shifted to be closer to the axis C of the rack bar 11 than the position of the tooth bottom 311v of the temporary rack teeth 311. This enables the tooth face width W and the tooth depth H of the rack teeth 111 to be larger than the tooth face width Wb and the tooth depth Hb of the temporary rack bar 31, respectively, even though the rack diameter R of the rack bar 11 is formed smaller than the rack diameter Ra of the standard rack bar 21. This allows the rack teeth 111 to have an axial strength similar to that of the standard rack teeth 211.

[0040] Figure 3: is a graph comparing the axial strength ratios of the conventional standard rack bar 21, the temporary rack bar 31, and the rack bar 11 of the present embodiment. Assuming that the axial strength of the conventional standard rack bar 21 is 100%, both the temporary rack bar 31 and the rack bar 11 of the present embodiment have axial strengths within about -15%. That is, the axial strength of the rack teeth 111 of the rack bar 11 of the present embodiment can be set within the allowable axial strength range of the standard rack teeth 211 of the standard rack bar 21. This ensures the required strength (particularly axial strength) of the rack teeth 111 of the rack bar 11 having a smaller rack diameter.

[0041] Also, as described in the above Summary of the Invention section, reducing the rack diameter of the rack bar generally makes it difficult to ensure the required strength (particularly bending strength) of the rack bar. Figure 4 The figure compares the section moduli of the conventional standard rack bar 21, the provisional rack bar 31 and the rack bar 11 of the present embodiment. Assuming that the section moduli of the conventional standard rack bar 21 is 100%, both the provisional rack bar 31 and the rack bar 11 of the present embodiment have section moduli within about -30%.

[0042] It is an effective measure to continuously form a hardened layer on the entire periphery of the rack bar. Figure 5 As shown, the rack bar 11 of the present embodiment includes a hardened layer K (indicated by cross hatching in the figure) which is continuously formed on the entire periphery of each rack tooth 111 (rack tooth row 112) and is mainly formed of a martensite structure, and a central portion S (indicated by single hatching in the figure), which is formed inside the hardened layer K, has toughness, and has a lower hardness than the hardened layer K. This is because the hardened layer K existing from the tooth tip 111f to the tooth bottom 111v of each rack tooth 111 can help reduce tooth surface wear and tooth fracture (or increase fatigue strength and increase fracture strength) at the meshing area of ​​the rack tooth 111 and the pinion tooth 9a.

[0043] Furthermore, the hardened layer K on the tooth bottom 111v side of each rack tooth 111 helps suppress fracture at the tooth bottom 111v due to bending of the rack bar 11. The hardened layer K also exists on the back surface 111b of the rack bar 11 relative to the tooth bottom 111v of the rack teeth 111; the back surface 111b is the second most susceptible portion to fracture due to bending after the tooth bottom 111v. The hardened layer K on the side surface 111s of the rack bar 11 relative to the tooth bottom 111v of the rack teeth 111 is necessary to provide reinforcement against bending.

[0044] like Figure 6BAs shown, the hardened layer Ka (indicated by cross-hatching in the figure) of the conventional standard rack bar 21 is formed by conduction (resistance heating) using the heating element 40. Each time of heating, the hardened layer Ka is formed only on the rack teeth 211 (rack teeth row 212) side. Therefore, heating the entire periphery of the rack teeth 211 (rack teeth row 212) of the standard rack bar 21 by conduction (resistance heating) requires at least two heatings, one for the rack teeth 211 (rack teeth row 212) side and another for the back side 211b relative to the rack teeth 211 (rack teeth row 212) side.

[0045] In view of this, if Fig. 6A As shown, the hardened layer K (indicated by cross-hatching in the figure) is formed by quenching and tempering at an appropriate temperature by induction (high-frequency induction heating) using a heating coil 50. Heating the entire periphery of the rack teeth 111 (rack tooth row 112) of the rack bar 11 by induction (induction heating) requires heating only once, which helps to avoid cost increases.

[0046] As described above, continuously forming the hardened layer K over the entire periphery of the rack teeth 111 (rack tooth row 112) ensures the required strength (particularly bending strength) of the rack bar 11. However, the inventors have found that forming the hardened layer K deeply in the rack bar 11 may increase the risk of embrittlement of the rack bar 11. This is because forming such a deep hardened layer K (mainly composed of a martensite structure) in the rack bar 11 reduces the central portion S formed inside the hardened layer K and having toughness and a lower hardness than the hardened layer K.

[0047] In view of this, if Figure 5 As shown, the thickness of the hardened layer K (indicated by cross-hatching in the figure) varies depending on the circumferential position on the rack bar 11. This helps prevent the reduction of the central portion S and prevents the embrittlement of the rack bar 11. Specifically, when the rack bar 11 of the present embodiment is observed in the direction of the axis C, the depth of the hardened layer K (indicated by cross-hatching in the figure) at the tooth bottom 111v of the rack tooth 111 is the smallest, followed by the depth of the hardened layer at the side 111s of the rack bar 11 relative to the tooth bottom 111v, and then the depth of the hardened layer at the back 111b of the rack bar 11 relative to the tooth bottom 111v.

[0048] The depths kv and kb of the hardened layer K at the tooth bottom 111v of the rack tooth 111 and at the back surface 111b of the rack bar 11 are respectively measured along a straight line perpendicular to the tooth bottom 111v of the rack tooth 111 and passing through the axis C of the rack bar 11. The depth ks of the hardened layer K from the side surface 111s of the rack bar is measured along a straight line perpendicular to the above straight line and passing through the axis C of the rack bar 11.

[0049] The reason for changing the depth of the hardened layer K according to the circumferential position on the rack bar 11 is given below. Since the tooth bottom 111v of the rack teeth 111 is easily broken by bending of the rack bar 11, the hardened layer K at the tooth bottom 111v is preferably deep, but there are the following problems in forming the deep hardened layer K. Specifically, the hardened layer K of the rack bar 11 is formed using the heating coil 50. Therefore, placing the heating coil 50 so as to be offset toward the rack teeth 111 allows a deep hardened layer K to be formed at the tooth bottom 111v of the rack teeth 111.

[0050] However, if the tooth tip 111f of the rack tooth 111 is too close to the heating coil 50, the tooth tip 111f may melt or the rack tooth 111 may deform. In order to avoid these situations, it is necessary to maintain an appropriate gap between the heating coil 50 and the tooth tip 111f of the rack tooth 111. This means that the maximum depth kv of the hardened layer K from the tooth bottom 111v of the rack tooth 111 is limited, which may result in insufficient bending strength of the rack bar 11.

[0051] In view of this, the depth kb of the hardened layer K on the back side 111b of the rack bar 11, which is the second most likely to be broken by bending, is made the largest after the tooth bottom 111v of the rack teeth 111, thereby increasing the bending strength of the rack bar 11. The depth ks of the hardened layer K on the side 111s of the rack bar 11, which provides bending reinforcement, is formed to be the second largest depth after the depth kb of the hardened layer on the back side 111b, which also increases the bending strength of the rack bar 11.

[0052] The depths kb and ks of the hardened layer K of the back surface 111b and the side surface 111s are determined respectively in consideration of the necessity of suppressing the reduction of the central portion S. Specifically, in order to strike a balance between the bending strength of the rack bar 11 and the radial cross-sectional area of ​​the central portion S of the rack bar 11, the hardened layer K is formed so that the depth kv of the hardened layer from the tooth bottom 111v of the rack tooth 111, the depth ks of the hardened layer from the side surface 111s of the rack bar 11, and the depth kb of the hardened layer from the back surface 111b of the rack bar 11 increase in sequence (kv<ks<kb).

[0053] Next, a method for manufacturing the rack bar 11 will be described. A steel solid shaft member 11R is set on a cutting device to perform cutting processing, so that an internal thread 113 ( 114 ) for coupling with the tie rod 12 is cut on each end face of the shaft member 11R. Figure 7 Then, the shaft member 11R subjected to the cutting process is placed on a pressing device to perform a pressing process, thereby forming a rack tooth row 112 ( Figure 7 Step S2 in ).

[0054] The shaft member 11R that has undergone the pressing process is placed in a high-frequency induction heating furnace for heating. Then, the heated shaft member 11R is quenched by rapid cooling ( Figure 7 The quenched shaft member 11R is placed in a high-frequency induction heating furnace and heated for a predetermined period of time to perform tempering ( Figure 7 The hardened layer K thus formed has the smallest depth at the tooth bottom 111v of the rack tooth 111, followed by its depth at the side surface 111s of the rack bar 11, and then its depth at the back surface 111b of the rack bar 11.

[0055] The quenched and tempered shaft member 11R is placed in a pressing device, where the shaft member 11R is pressed to remove its residual strain ( Figure 7 The shaft member 11R from which the residual strain has been removed is placed in a polishing machine for polishing, thereby polishing the rack teeth row 112 and other parts ( Figure 7 The rack bar 11 can be completed by completing the above steps.

Claims

1. A rack rod (11), characterized in that: include: A rack tooth row (112), wherein the rack tooth row (112) includes a plurality of rack teeth (111) meshing with the pinion teeth (9a); a hardened layer (K), the hardened layer (K) being continuously provided over the entire circumference of the rack teeth row (112); and a central portion (S), the central portion (S) being arranged inside the hardened layer (K) and having a lower hardness than the hardened layer (K), wherein When the rack rod (11) is observed along the axial direction of the rack rod (11), the depths of the hardened layer (K) at the following positions i), ii) and iii) increase sequentially: i) a tooth bottom (111v) of the rack teeth (111); ii) a side surface (111s) of the rack bar (11) relative to the tooth bottom (111v); and iii) the back side (111b) of the rack bar (11) opposite to the tooth bottom (111v).

2. The rack rod (11) according to claim 1, characterized in that: The hardened layer (K) is provided by induction heating the entire circumference of the rack teeth row (112).

3. The rack rod (11) according to claim 1 or 2, characterized in that: The rack rod (11) is used in a steering device of a vehicle. The rack diameter of the rack bar (11) is smaller than the standard rack diameter of a standard rack bar that meets the required specifications of the vehicle, and Compared with the position of the tooth bottom of the standard rack teeth of the standard rack rod relative to the axis of the standard rack rod, the position of the tooth bottom (111v) of the rack teeth (111) of the rack rod (11) relative to the axis of the rack rod (11) is displaced toward the axis of the rack rod (11).

4. A steering device (1), characterized in that: include: case; The rack bar (11) according to claim 1 or 2, the rack bar (11) being supported by the housing so as to be movable in an axial direction, the rack bar (11) being coupled to a wheel of a vehicle; as well as A pinion shaft (9) is supported by the housing so as to be rotatable about the axis of the pinion shaft (9), the pinion shaft (9) is meshed with a rack tooth row (112) of the rack bar (11), and the pinion shaft (9) is connected to the steering wheel of the vehicle.

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