阅读说明：本技术 齿条杆和转向装置 (Rack bar and steering device ) 是由 泉佳明 石见博史 鸟居功 于 2020-03-31 设计创作，主要内容包括：一种齿条(11)包括：齿条齿排(112),该齿条齿排包括与小齿轮齿(9a)啮合的多个齿条齿(111)；硬化层(K),该硬化层连续地设置在齿条齿排(112)的整个外周上；以及中央部分(S),该中央部分设置在硬化层(K)的内部且其硬度低于硬化层(K)的硬度。在从齿条杆(11)的轴向方向上观察齿条杆(11)时,来自下述位置i)、位置ii)和位置iii)的硬化层(K)的深度顺次增大：i)齿条齿(111)的齿底(111v)；ii)齿条杆(11)的相对于齿底(111v)的侧面(111s)；以及iii)齿条杆(11)的相对于齿底(111v)的背面(111b)。(A rack (11) comprising: a rack tooth row (112) including a plurality of rack teeth (111) meshing with the pinion teeth (9 a); a hardened layer (K) that is continuously provided over the entire periphery of the rack tooth row (112); and a central portion (S) which is provided inside the hardened layer (K) and has a hardness lower than that of the hardened layer (K). When the rack bar (11) is viewed in the axial direction of the rack bar (11), the depth of the hardened layer (K) from the following positions i), ii), and iii) increases in the order: i) a tooth bottom (111v) of the rack teeth (111); ii) a side surface (111s) of the rack bar (11) with respect to the tooth bottom (111 v); and iii) a back surface (111b) of the rack bar (11) opposite to the tooth bottom (111 v).)

1. A rack bar (11), characterized by comprising:

a rack tooth row (112), the rack tooth row (112) comprising a plurality of rack teeth (111) meshing with pinion gear teeth (9 a);

a hardened layer (K) that is provided continuously over the entire circumference 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), wherein,

the depth of the hardened layer (K) at the following positions i), ii), and iii) increases in order when the rack bar (11) is viewed in the axial direction of the rack bar (11):

i) a tooth bottom (111v) of the rack teeth (111);

ii) a side surface (111s) of the rack bar (11) opposite to the tooth bottom (111 v); and

iii) a back surface (111b) of the rack bar (11) opposite to the tooth bottom (111 v).

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

3. The rack bar (11) according to claim 1 or 2,

the rack bar (11) is used in a steering device for a vehicle,

the rack rod (11) has a rack diameter smaller than a standard rack diameter of a standard rack rod conforming to a required specification of the vehicle, and

the tooth bottom (111v) of the rack teeth (111) of the rack bar (11) is displaced toward the axis of the rack bar (11) with respect to the position of the axis of the rack bar (11) as compared to the position of the tooth bottom of the standard rack teeth of the standard rack bar with respect to the axis of the standard rack bar.

4. Steering device (1), characterized in that it comprises:

a housing;

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; and

a pinion shaft (9) supported by the housing so as to be rotatable about an axis of the pinion shaft (9), the pinion shaft (9) meshing with a rack tooth row (112) of the rack bar (11), the pinion shaft (9) being coupled to a steering wheel of the vehicle.

Technical Field

The present invention relates to a rack bar and a steering apparatus.

Background

A rack-and-pinion type steering apparatus for a vehicle is used to convert a rotational motion of a steering shaft into an axial linear motion of a rack bar connected to steered wheels to transmit a steering force of a steering wheel to the steered wheels. In the rack-and-pinion mechanism, the pinion shaft is supported by a ball bearing, a needle bearing, or the like. The rack bar is supported by a rack bush and a meshing portion between rack teeth formed on the rack bar and pinion teeth formed on the pinion shaft. There are various specifications regarding the meshing between the rack teeth and the pinion teeth, and the specifications of the teeth are changed according to the required specifications of the vehicle (e.g., a specific stroke, a rack stroke, etc.).

In the rack-and-pinion type steering apparatus, the meshing portion between the rack teeth and the pinion teeth receives a load due to a positive input torque applied from the steering wheel and a reverse input load applied from the tire. The reverse input load exerted by the tire tends to increase as the meshing portion between the rack teeth and the pinion teeth approaches the end of the rack stroke (either end of the rack tooth row), due to the effects of tire friction, suspension geometry, and other factors.

In addition, when the meshing portion between the rack teeth and the pinion teeth reaches either end of the rack tooth row due to the driver abruptly rotating the steering wheel for the purpose of getting the vehicle into a garage or for other reasons, so-called tip abutment occurs in which the rack bar abuts against the stopper and stops. The impact load with which the tip ends abut is applied to the meshing portion between the rack teeth and the pinion teeth. In the case where the rack-and-pinion type steering apparatus is a rack-and-pinion type steering apparatus having an assist mechanism, the impact load becomes larger. This is required to ensure the required strength (particularly axial strength) of the rack teeth of the rack bar.

In addition, when any steered wheel of the vehicle accidentally hits a hole during running, a heavy impact load is applied to the rack bar. This is required to ensure the required strength (particularly bending strength) of the rack bar. Japanese patent applications laid-open nos. 6-264992 and 2017-057442 each disclose a rack bar having a hardened layer (mainly composed of a martensite structure) continuously formed over the entire periphery of a rack tooth row including a plurality of rack teeth. The hardened layer helps ensure the required strength (particularly axial strength) of the rack teeth of the rack bar and the required strength (particularly bending strength) of the rack bar.

Disclosure of Invention

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

The invention helps to increase axial and bending strength and reduce embrittlement.

A first aspect of the invention relates to a rack bar. The rack bar includes: a rack tooth row including a plurality of rack teeth meshing with the pinion teeth; a hardened layer continuously provided on 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. The depth of the hardened layers from the following positions i), ii), and iii) increases in order when the rack bar is viewed in the axial direction of the rack bar: i) a tooth bottom of the rack teeth; ii) the side of the rack bar opposite the tooth bottom; iii) the back of the rack bar opposite the tooth bottom.

According to the above aspect, the hardened layer is continuously formed over the entire outer periphery of the rack tooth row of the rack bar. This increases the axial strength of the rack teeth and the bending strength of the rack bar. Further, the hardened layer is formed such that the depth thereof increases in the order of the tooth bottom of the rack teeth, the side face of the rack bar, and the back face of the rack bar. This reduces the reduction of the central portion formed inside the hardened layer and having a hardness lower than that of the hardened layer, thereby contributing to the reduction of the embrittlement of the rack bar.

A second aspect of the invention relates to a steering device. The steering device includes: a housing; a rack bar supported by the housing to be movable in an axial direction and coupled to a wheel of a vehicle; and a pinion shaft supported by the housing so as to be rotatable about an axis of the pinion shaft. The pinion shaft meshes with the rack tooth row of the rack bar and is coupled to a 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 provided on the entire periphery of the rack tooth row; a central part which is arranged in the hardening layer and has lower hardness than the hardening layer. The depth of the hardened layers from the following positions i), ii), and iii) increases in order when the rack bar is viewed in the axial direction of the rack bar: i) a tooth bottom of the rack teeth; ii) the side of the rack bar opposite the tooth bottom; iii) the back of the rack bar opposite the tooth bottom.

According to the above aspect, the hardened layer is continuously formed over the entire outer periphery of the rack tooth row of the rack bar. This increases the axial strength of the rack teeth and the bending strength of the rack bar. Further, the hardened layers are formed such that the depth at the tooth bottom of the rack teeth, the side surface of the rack bar, and the back surface of the rack bar increases in order. This reduces the reduction of the central portion formed inside the hardened layer and having a hardness lower than that of the hardened layer, thereby contributing to the reduction of embrittlement of the rack.

Drawings

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

fig. 1 shows a schematic configuration of a steering device of an embodiment of the present invention;

fig. 2A shows a view of a rack bar of an embodiment of the present invention as viewed in a direction perpendicular to the axis thereof;

fig. 2B is a sectional view of the rack bar of fig. 2A taken along line IIB-IIB as viewed in the axial direction thereof;

fig. 2C is a sectional view of the conventional standard rack bar as viewed in the axial direction thereof;

fig. 2D is a sectional view of the temporary rack bar as viewed in the axial direction thereof;

FIG. 3 illustrates the axial strength ratio between different rack bars;

fig. 4 shows section coefficient ratios between different rack bars;

fig. 5 is a sectional view showing a thickness distribution of a hardened layer of a rack bar when viewed in an axial direction of the rack bar according to an embodiment of the present invention;

fig. 6A shows a method for forming a hardened layer of a rack bar according to an embodiment of the present invention and the state of the hardened layer when viewed in the axial direction of the rack bar;

fig. 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

fig. 7 is a flowchart of a method for manufacturing a rack bar according to an embodiment of the present invention.

Detailed Description

A description will be given of a schematic configuration of a steering apparatus 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 having a column assist mechanism in which an electric motor transmits power to a column shaft; or a rack-and-pinion type electric power steering apparatus having a pinion assist mechanism in which an electric motor transmits power to a pinion shaft.

The steering apparatus may also be a rack-and-pinion type electric power steering apparatus having a rack assist mechanism in which an electric motor transmits power to a rack bar, or a rack-and-pinion type steering apparatus having no assist mechanism. The steering device at least comprises a steering control mechanism, a steering mechanism and an auxiliary mechanism.

As an example, the steering apparatus described below is a rack-and-pinion type electric power steering apparatus having a column assist mechanism. Therefore, as shown in fig. 1, the steering device 1 includes a steering mechanism 2 and a steering mechanism 3, and steers the steered wheels 5 in accordance with the operation of a steering wheel 4 (steering member) by the driver. The steering mechanism 2 includes an assist mechanism 6 that assists the steering operation of the driver.

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 PR 1. 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 7 c. The intermediate shaft 8 is coupled to the pinion shaft 9 via a second universal joint 8 d.

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, the first shaft 8a and the second shaft 8b being fitted to each other in a manner movable relative to each other and rotatable together, for example, by spline fitting. The first shaft 8a is coupled to a first universal joint 8c and the second shaft 8b is coupled to a second universal joint 8 d. The pinion shaft 9 is formed with pinion teeth 9 a.

The steering mechanism 3 includes a rack bar 11 and a tie rod 12 that constitute a rack-and-pinion mechanism PR 1. The rack bar 11 is formed with rack teeth 111 that mesh with the pinion teeth 9a (see fig. 2A). Each tie rod 12 is coupled at one end thereof to the rack bar 11 and at the other end thereof to the corresponding steered wheel 5. In response to rotation of the steering wheel 4 by the operation of the driver, the pinion shaft 9 is rotated via the input shaft 7a, the output shaft 7b, and the intermediate shaft 8.

The rotation of the pinion shaft 9 is converted into the axial reciprocating motion of the rack bar 11. This axial reciprocating movement of the rack bar 11 is transmitted to the steered wheels 5 via the respective tie rods 12. This changes the steering angle of the steered wheels 5, thereby changing the traveling direction of the vehicle.

The assist mechanism 6 includes a torque sensor 13, an Electronic Control Unit (ECU)14, an electric motor 15, and a worm reducer 16. The torque sensor 13 detects the amount of torsion between the input shaft 7a and the output shaft 7 b. The ECU14 determines the assist torque based on the steering torque obtained from the amount of torsion detected by the torque sensor 13 and based on the vehicle speed detected by the vehicle speed sensor 10.

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

The rack bar 11 of the present embodiment may be a rack bar having a constant gear ratio; or may be a rack bar with a variable transmission ratio in which the specifications of the rack teeth are varied according to the axial position on the rack bar. As an example, the rack bar discussed herein is a rack bar having a constant gear ratio.

As shown in fig. 2A, the rack bar 11 of the present embodiment is formed on an outer surface thereof with a rack tooth row 112 including a plurality of rack teeth 111, and each end surface of the rack bar 11 is also formed with an internal thread 113 coupled with the corresponding tie rod 12. The rack tooth row 112 is formed by pressing a solid shaft member 11R made of steel subjected to a preheating treatment (quenching and tempering) with a die so as to be plastically deformed, or by cutting the shaft member 11R. The shaft member 11R may be hollow except for a portion thereof where the rack-and-pinion row 112 is formed.

Recently, vehicles are required to be downsized to improve fuel efficiency for better environmental performance. For this reason, it is necessary to reduce the weight of the rack-and-pinion type steering apparatus. One effective way to achieve this is to reduce the rack diameter of the rack bar. Therefore, the rack diameter R of the rack bar 11 of the present embodiment as shown in fig. 2B is smaller than the rack diameter Ra of the standard rack bar 21 provided to meet the specifications (e.g., a specific stroke and a rack stroke) required for the vehicle as shown in fig. 2C. This allows reduction in the weight of the rack-and-pinion type steering apparatus.

However, as described in the summary section above, reducing the rack diameter of the rack bar involves reducing the tooth depth and the face width of the rack teeth, which often makes it difficult to ensure the required strength (particularly axial strength) of the rack teeth. Specifically, as shown in fig. 2C, the standard rack teeth 211 of the conventional standard rack bar 21 have a face width Wa and a tooth depth Ha set based on a tooth specification conforming to a required specification (e.g., a specific stroke and a rack stroke) of the vehicle. Meanwhile, the temporary rack bar 31 shown in fig. 2D is a smaller diameter version of the standard rack bar 21 (the diameter thereof is equal to the rack diameter R of the rack bar 11 of the present embodiment).

Temporary rack teeth 311 of temporary rack bar 31 are formed such that distance d between axial center Cb of temporary rack bar 31 and tooth bottom 311v of temporary rack teeth 311 is equal to the distance between Ca of standard rack bar 21 and tooth bottom 211v of standard rack teeth 211. Therefore, the tooth face width Wb and the tooth depth Hb of the temporary rack teeth 311 of the temporary rack bar 31 are smaller than the tooth face width Wa and the tooth depth Ha of the standard rack teeth 211 of the standard rack bar 21, respectively. This means that the axial strength of the temporary rack teeth 311 is smaller than that of the standard rack teeth 211.

When the face width and the depth of tooth of the temporary rack bar 31 are made equal to those of the standard rack bar 21, respectively, the distance between the axial center Cb of the temporary rack bar 31 and the bottom 311v of the temporary rack teeth 311 becomes smaller than the distance between the axial center Ca of the standard rack bar 21 and the bottom 211v of the standard rack teeth 211. This still results in a reduction in the axial strength of the temporary rack teeth 311.

Therefore, as shown in fig. 2B, the rack bar 11 of the present embodiment is cut deep in the radial direction so that the position of the tooth bottom 111v of the rack tooth 111 is displaced closer to the axial center C of the rack bar 11 than the position of the tooth bottom 311v of the temporary rack tooth 311. This enables the face width W and the tooth depth H of the rack teeth 111 to be larger than the face width Wb and the tooth depth Hb of the temporary rack bar 31, respectively, although 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 rack teeth 111 to have an axial strength similar to that of standard rack teeth 211.

Fig. 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 an axial strength 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.

Also, as described in the summary section above, reducing the rack diameter of the rack bar often makes it difficult to ensure the required strength (particularly bending strength) of the rack bar. Fig. 4 is a graph comparing section factors of the conventional standard rack bar 21, the temporary rack bar 31, and the rack bar 11 of the present embodiment. Assuming that the section modulus 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 a section modulus within about-30%.

It is an effective measure to form the hardened layer continuously over the entire outer periphery of the rack bar. As shown in fig. 5, the rack bar 11 of the present embodiment includes a hardened layer K (indicated by cross-hatching in the drawing) that is continuously formed over the entire outer 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 drawing) that is formed inside the hardened layer K, has toughness, and has hardness lower than that of 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 contribute to reduction of tooth surface wear and tooth breakage (or increase of fatigue strength and increase of breaking strength) at the region where the rack tooth 111 meshes with the pinion tooth 9 a.

Further, the hardened layer K on the tooth bottom 111v side of each rack tooth 111 contributes to suppression of fracture occurring at the tooth bottom 111v due to bending of the rack bar 11. The hardened layer K is also present on the back surface 111b of the rack bar 11 with respect to the tooth bottom 111v of the rack teeth 111; the back surface 111b is a second portion, which is second to the tooth bottom 111v and is easily broken by bending. The hardened layer K on the side surface 111s of the rack bar 11 with respect to the tooth bottom 111v of the rack teeth 111 is necessary to provide reinforcement against bending.

As shown in fig. 6B, a hardened layer Ka (indicated by cross hatching in the drawing) of the conventional standard rack bar 21 is formed by conduction (resistance heating) using the heating element 40. Each time heating is performed, the hardened layer Ka is formed only on the rack teeth 211 (rack tooth row 212) side. Therefore, heating the entire outer periphery of the rack teeth 211 (rack tooth row 212) of the standard rack bar 21 by conduction (resistance heating) requires at least two times of heating, one for the rack teeth 211 (rack tooth row 212) side and the other for the back surface 211b with respect to the rack teeth 211 (rack tooth row 212) side.

In view of this, as shown in fig. 6A, a hardened layer K (indicated by cross hatching in the drawing) is formed by induction (high-frequency induction heating) quenching and tempering at an appropriate temperature using a heating coil 50. Heating the entire outer 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 an increase in cost.

As described above, continuously forming the hardened layer K over the entire outer periphery of the rack teeth 111 (rack tooth rows 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, having toughness and having hardness lower than that of the hardened layer K.

In view of this, as shown in fig. 5, the thickness of the hardened layer K (indicated by cross hatching in the drawing) varies depending on the circumferential position on the rack bar 11. This helps prevent reduction of the central portion S and embrittlement of the rack bar 11. Specifically, when the rack bar 11 of the present embodiment is viewed in the direction of the axial center C, the depth of the hardened layer K (indicated by cross hatching in the drawing) at the tooth bottom 111v of the rack teeth 111 is smallest, next to the depth of the hardened layer at the side surface 111s of the rack bar 11 with respect to the tooth bottom 111v, and next to the depth of the hardened layer at the back surface 111b of the rack bar 11 with respect to the tooth bottom 111 v.

The depths kv, kb of the hardened layers K at the tooth bottoms 111v of the rack teeth 111 and at the back surface 111b of the rack bar 11 are respectively measured along straight lines perpendicular to the tooth bottoms 111v of the rack teeth 111 and passing through the axial center C of the rack bar 11. The depth ks of the hardened layer K from the rack bar side surface 111s is measured along a line perpendicular to the above-mentioned line and passing through the axial center C of the rack bar 11.

The reason why the depth of the hardened layer K is changed according to the circumferential position on the rack bar 11 is given below. Since the tooth bottom 111v of the rack tooth 111 is likely to be broken by bending of the rack bar 11, the hardened layer K at the tooth bottom 111v is preferably deep, but there is a problem in forming a deep hardened layer K as described below. Specifically, the hardened layer K of the rack bar 11 is formed using the heating coil 50. Therefore, placing the heating coil 50 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.

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 be deformed. 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.

In view of this, the depth kb of the hardened layer K on the back surface 111b of the rack bar 11, which is second to the tooth bottom 111v of the rack teeth 111 and is likely to break due to bending, is made the largest, whereby the bending strength of the rack bar 11 can be increased. The depth ks of the hardened layer K at the side surface 111s of the rack bar 11, which provides bending resistance enhancement, is formed to the second greatest depth next to the depth kb of the hardened layer of the back portion 111b, which also increases the bending resistance of the rack bar 11.

The depths kb, ks of the hardened layers K of the back surface 111b and the side surfaces 111S are determined, respectively, in consideration of the necessity of suppressing the decrease of the central portion S. Specifically, in order to achieve 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 layers K are formed such that the depth kv of the hardened layers from the tooth bottoms 111v of the rack teeth 111, the depth ks of the hardened layers from the side surfaces 111S of the rack bar 11, and the depth kb of the hardened layers from the back surface 111b of the rack bar 11 are sequentially increased (kv < ks < kb).

Next, a method for manufacturing the rack bar 11 will be explained. The steel solid shaft member 11R is set on a cutting device to perform cutting work so that the internal thread 113 for coupling with the tie rod 12 is cut on each end face of the shaft member 11R (step S1 in fig. 7). Then, the shaft member 11R subjected to the cutting process is set on a pressing device to be subjected to the pressing process, whereby the rack tooth row 112 is formed on the outer surface of the shaft member 11R (step S2 in fig. 7).

The shaft member 11R subjected to press working is set in a high-frequency induction heating furnace to be heated. Then, the heated shaft member 11R is quenched by rapid cooling (step S3 in fig. 7). The quenched shaft member 11R is placed in a high-frequency induction heating furnace and heated for a predetermined period of time to be tempered (step S4 of fig. 7). The hardened layer K thus formed has a minimum 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.

The quenched and tempered shaft member 11R is set in a pressing device in which the shaft member 11R is pressed to remove its residual strain (step S5 in fig. 7). The shaft member 11R from which the residual strain is removed is set in a polishing machine to be subjected to polishing processing, whereby the rack tooth row 112 and the other portions are polished (step S6 in fig. 7). The rack bar 11 is completed by completing the above steps.