How to polish rack bars

Abstract

To provide a rack bar polishing method for manufacturing a rack bar that can reduce friction resistance with a bearing member in a steering device and is easy to engage with a tooth part of a pinion in the steering device.SOLUTION: In a rack bar polishing method, a rack bar can slide with respect to a bearing member that supports the rack bar when being incorporated into a steering device, and includes a shaft member. The shaft member includes a rack part formed with a rack engaging with a pinion and a slide surface that is in slide contact with the bearing member. The rack part includes a plurality of tooth parts. The slide surface is polished with a polishing member by varying an allowance amount of the slide surface located on the other side facing a thickness direction of the rack part such that a height of each of the tooth parts from the slide surface is a predetermined height.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to a method for polishing a rack bar. [Background technology]

[0002] The rack bar is used as a component of a rack-and-pinion steering device mounted on, for example, an automobile, and includes a rack portion on which a rack having a plurality of teeth that mesh with a pinion is formed.

[0003] In manufacturing rack bars used in such rack-and-pinion steering devices, the rack is typically formed by pressing a die against a cylindrical or columnar steel material, and then heat-treated (hardened) to increase the strength of the rack. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2020-66020 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the method described in Patent Document 1, when the rack is heat-treated (quenched), an oxide film is formed on the surface of the rack bar due to the heat treatment, which causes unevenness on the surface of the rack bar.

[0006] Typically, in a steering device, the rack bar is supported by bearing members such as a support yoke that supports the opposite side of the rack portion in the thickness direction (described below) and rack bushings that support both ends. When the pinion rotates by operating a steering shaft connected to the pinion, the rack bar moves along its axial direction. At this time, the rack bar moves while sliding against the bearing members (hereinafter, the surface of the rack bar that is supported by the bearing members will be referred to as the "sliding surface").

[0007] The driving mechanism of the steering device as described above may cause the bearing member to wear due to sliding contact with the sliding surface of the rack bar, so it is preferable that the friction resistance between the sliding surface of the rack bar and the bearing member is small.

[0008] In addition, as described above, in the steering device, the rack bar moves in the axial direction due to the rotation of the pinion. At this time, it is preferable that the teeth of the rack bar that mesh with the teeth of the pinion also have a predetermined height so that they can easily mesh with the teeth of the pinion in the steering device.

[0009] In view of the above circumstances, an object of the present invention is to provide a method for polishing a rack bar that can reduce frictional resistance with a bearing member in a steering device and that easily meshes with the teeth of a pinion in the steering device. [Means for solving the problem]

[0010] In the rack bar grinding method according to the present invention, the rack bar is slidable relative to a bearing member that supports the rack bar when the rack bar is incorporated into the steering device, and includes a shaft member, the shaft member having a rack portion on which a rack that meshes with a pinion is formed and a sliding contact surface that comes into sliding contact with the bearing member, the rack portion having a plurality of teeth, so that the height of each of the plurality of tooth portions from the sliding contact surface is a predetermined height, The sliding surfaces located on opposite sides of the rack in the thickness direction are polished with an abrasive material while varying the amount of removal of the sliding surfaces.

[0011] According to this configuration, the sliding surface is polished with an abrasive as described above, thereby reducing the frictional resistance between the sliding surface and the bearing member, and making it possible to provide a rack bar that easily engages with the teeth of a pinion in a steering device.

[0012] Specifically, in a steering system, the rack bar is supported by a bearing member called a support yoke at a sliding surface located on the opposite side of the rack section in the thickness direction. This support yoke is usually biased toward the rack bar by a spring member or the like. For this reason, frictional resistance is relatively likely to occur between the support yoke and the sliding surface. Therefore, polishing the sliding surface in this area can effectively reduce frictional resistance.

[0013] Furthermore, since the polishing is performed by polishing the sliding surface with an abrasive so that the height of each of the multiple tooth portions of the rack portion from the sliding surface becomes a predetermined height, it is possible to manufacture a rack bar having tooth portions that easily mesh with the tooth portions of the pinion in a steering device.

[0014] In the rack bar grinding method according to the present invention, it is preferable that the height adjustment from the sliding contact surface is performed using a pin gauge.

[0015] According to this configuration, it is possible to easily perform polishing with varying amounts of removal of the sliding contact surface.

[0016] In the rack bar polishing method according to the present invention, it is preferable that the polishing direction of the polishing material is along the axial direction of the rack bar.

[0017] With this configuration, because of the grinding direction as described above, grinding marks are formed on the surface of the sliding contact surface along the axial direction of the rack bar. In other words, the direction of the grinding marks is along the direction of movement of the rack bar. This makes it possible to further reduce the friction resistance between the sliding contact surface of the rack bar and the bearing member.

[0018] More specifically, as described above, in a steering device, the rack bar is supported by a bearing member called a support yoke at a sliding contact surface located on the opposite side of the rack section in the thickness direction. This support yoke is usually biased toward the rack bar by a spring member or the like. For this reason, frictional resistance is relatively likely to occur between the support yoke and the sliding contact surface. In contrast, with the above-described polishing method, the direction of the polishing marks formed on the surface of the sliding contact surface that makes sliding contact with the support yoke is aligned with the direction of movement of the rack bar, thereby more effectively reducing frictional resistance.

[0019] In addition, in the rack bar grinding method according to the present invention, It is preferable to use a rotating grindstone as the abrasive material, which is disk-shaped, rotatable in the circumferential direction, and has abrasive grains on its rotating outer peripheral surface, rotate the rotating grindstone so that the outer peripheral surface is aligned with the axial direction of the rack bar, and polish the sliding surface with the outer peripheral surface while moving at least one of the rotating grindstone or the rack bar along the axial direction.

[0020] According to this configuration, the outer peripheral surface of the grinding wheel having abrasive grains (the grinding surface of the grinding wheel) is rotated along the axial direction of the rack bar, and the sliding surface is polished with the outer peripheral surface while at least one of the grinding wheel and the rack bar is moved along the axial direction. This causes both the rotation direction (the direction of rotation and polishing) of the outer peripheral surface (polishing surface) of the grinding wheel and the movement direction (the direction of movement and polishing) of the outer peripheral surface to be aligned with the movement direction of the rack bar. This makes it easier for polishing marks to be formed on the surface of the sliding surface along the axial direction of the rack bar. This further reduces frictional resistance between the sliding surface of the rack bar and the bearing member. [Effects of the Invention]

[0021] As described above, the present invention can provide a method for polishing a rack bar that can reduce frictional resistance with bearing members in a steering device and that easily meshes with the teeth of a pinion in a steering device. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a schematic diagram showing a steering device. [Figure 2] FIG. 2 is a schematic side view of the rack bar. [Figure 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. [Figure 5] FIG. 5 is a diagram showing a surface to be polished located on the opposite side in the thickness direction of the rack portion. [Figure 6] FIG. 6 is a schematic diagram showing the outer peripheral surface of a grinding wheel. [Figure 7] FIG. 7 is a diagram showing the positional relationship between the rack bar and the abrasive in the rack bar abrasive grinding method of one embodiment, as viewed from a direction perpendicular to the axial direction of the rack bar. [Figure 8] FIG. 8 is a view of FIG. 7 as viewed from one axial side of the rack bar. [Figure 9] FIG. 9 is a conceptual diagram for explaining the polishing method according to this embodiment. [Figure 10] FIG. 10 is a diagram showing a polishing method using a pin gauge. DETAILED DESCRIPTION OF THE INVENTION

[0023] A rack bar grinding method according to one embodiment of the present invention will be described below with reference to the drawings. Hereinafter, the axial direction of the rack bar will be referred to as the axial direction X (longitudinal direction). The horizontal direction perpendicular to the axial direction X will be referred to as the width direction Y (first short-side direction). The vertical direction perpendicular to both the axial direction X and the width direction Y will be referred to as the thickness direction Z (second short-side direction). The cross section (YZ plane) of the rack bar perpendicular to the axial direction X will be referred to as the transverse cross section.

[0024] First, with reference to FIGS. 1 to 4, a rack bar 1 polished by the polishing method of the present embodiment will be described while showing a steering device 100 in which the rack bar 1 is used.

[0025] 1, the steering device 100 is a rack-and-pinion type steering device 100. The steering device 100 includes a pinion 2 connected to a steering shaft of a vehicle or the like, a rack bar 1 on which a rack 11a meshing with the pinion 2 is formed, and a bearing member 3 that supports the rack bar 1.

[0026] The rack bar 1 includes a shaft member 10. The shaft member 10 has a rack portion 11 on which a rack 11a is formed, and a sliding surface 13 on the surface of the shaft member 10 that comes into sliding contact with the bearing member 3.

[0027] The rack bar 1 is made of, for example, a solid (cylindrical) steel material or a hollow (cylindrical) steel material. Examples of such steel materials include carbon steel materials for machine structures specified in JIS 4051:2016. Among these, S45C steel material is preferred.

[0028] As shown in FIGS. 1 and 2, the rack 11a is configured with a plurality of tooth portions 111 formed so that tooth tips 113 extend along the width direction Y (a direction perpendicular to the plane of the paper in FIGS. 1 and 2). The tooth portions 111 are arranged, for example, at equal intervals along the axial direction X. The tooth portions 111 also form a plurality of grooves 112 between them, into which the tooth portions of the pinion 2 can be inserted and removed. With this configuration, the rack bar 1 in the steering device 100 moves along the axial direction X as the pinion 2 meshes with the rack 11a rotates. At this time, the rack bar 1 moves so as to slide relative to the bearing member 3, and the sliding surface 13 slides against the bearing member 3.

[0029] The rack bar 1 has, as sliding surfaces 13, a tooth back sliding surface 13a located on the opposite side of the rack portion 11 in the thickness direction Z, and both end sliding surfaces 13b located at both end portions where the rack 11a is not yet formed. As shown in Figures 3 and 4, in cross section, the shape of the surface 131a of the tooth back sliding surface 13a is, for example, an arc. In addition, in cross section, the shape of the surface 131b of the both end sliding surfaces 13b is, for example, a circle.

[0030] In the steering device 100, the rack bar 1 is supported at the tooth back sliding surface 13a by a bearing member 31 called a support yoke, and at both end sliding surfaces 13b by bearing members 32 called rack bushes. The bearing members 3 are usually made of a resin material.

[0031] In the steering device 100, the support yoke 31 is formed in a shape that covers the surface 131a of the tooth back-side sliding surface 13a from below. The support yoke 31 has a mounting surface on which the tooth back-side sliding surface 13a is placed, and supports the rack bar 1 with the mounting surface and the surface 131a in contact with each other.

[0032] In the steering device 100, the rack bush 32 is formed in a shape that covers the surfaces 131b of the sliding contact surfaces 13b at both ends in the circumferential direction CY1 of the rack bar 1. The rack bush 32 has an inner peripheral surface that covers the sliding contact surfaces 13b at both ends in the circumferential direction CY1, and supports the rack bar 1 with the inner peripheral surface and the surface 131b in contact with each other.

[0033] 3 and 4, in a cross section, the diameter of the arc described by the surface 131a of the tooth back contact surface 13a and the diameter of the circle described by the surface 131b of the both end contact surfaces 13b are substantially the same. If these diameters are designated as R1, the diameter R1 is, for example, 22 mm to 37 mm. The diameter R1 can be calculated by measuring the circumference at any five points on the both end contact surfaces 13b to find the average circumference C1, and then using the following formula (1). The constant π is assumed to be 3.14. R1=C1 / Pi...(1)

[0034] The tooth back surface 13a has an arc whose central angle θ1 is, for example, 170° on the cross section (YZ plane) of the surface 131a. The central angle θ1 can be calculated by measuring the circumference at any five points on the tooth back surface 13a to determine the average circumference C2, and then using the following formula (2): θ1=C2 / C1×360° (2)

[0035] The steering device 100 is configured such that the rack bar 1 moves along the axial direction X while sliding against the bearing member 3, and as a result, the sliding surface 13 comes into sliding contact with the mounting surface and the inner surface of the bearing member 3.

[0036] Next, a method for grinding the rack bar 1 of this embodiment will be described with reference to FIGS.

[0037] In the polishing method of this embodiment, an untreated rack bar 1a (a workpiece after heat treatment (hardening)) on which an oxide film has been formed through a heat treatment such as high-frequency induction heating is polished with an abrasive 4 to obtain a treated rack bar 1b. At this time, as shown in Fig. 7, the sliding surface 13 is polished so that the polishing direction D is along the axial direction X (the polishing direction D is approximately parallel to the axial direction X). In this embodiment, the polishing method is not limited to the one shown in Fig. 7. That is, the polishing method may be the one shown in Fig. 7, or may be buffed to polish along the axial direction X while polishing along the width direction Y.

[0038] FIG. 9 is a conceptual diagram for explaining the polishing method according to this embodiment. In the polishing method of this embodiment, the tooth back sliding surface 13a located on the opposite side facing the thickness direction Z of the rack portion 11 is polished with an abrasive by changing the amount of removal of the tooth back sliding surface 13a so that the height of each of the multiple tooth portions 111 of the rack portion 11 from the sliding surface becomes a predetermined respective height. For example, as shown in Fig. 9, the height T1 from the sliding contact surface of the central portion of the rack portion 11 is set in advance to a relatively low predetermined height, and the heights T2, T2 from the sliding contact surface of adjacent portions on both sides of the rack portion 11 in the axial direction X are set in advance to relatively high predetermined heights. In the steering device 100 shown in Fig. 1, these predetermined heights are set as appropriate to achieve an optimal meshing state, taking into account factors such as movement of the rack bar in the axial direction X at the distance between the deep portion of the tooth portion of the pinion 2 and the upper surface of a bearing member 31 called a support yoke. Note that the predetermined height of the plurality of tooth portions 111 themselves of the rack portion 11 is set as appropriate in accordance with the predetermined height of the tooth portions themselves of the pinion 2, and therefore description thereof will be omitted.

[0039] Next, when grinding the portion corresponding to the tooth portion 111, which has been set in advance to have a relatively low predetermined height from the sliding surface, a relatively large amount of removal is made on the tooth back sliding surface 13a located on the opposite side of the corresponding portion facing the thickness direction Z of the rack portion 11. For example, in the grinding method shown in Fig. 7, the moving speed V of the rotary grindstone 4 in the axial direction of the untreated rack bar 1a is reduced to relatively increase the grinding amount.

[0040] Furthermore, when grinding the portion corresponding to the tooth portion 111, which has been set in advance to have a relatively high predetermined height from the sliding surface, the amount of removal of the tooth back sliding surface 13a located on the opposite side of the corresponding portion in the thickness direction Z of the rack portion 11 is made relatively small. For example, in the grinding method shown in Fig. 7, the moving speed V of the untreated rack bar 1a of the grinding wheel 4 in the axial direction X is increased to make the grinding amount relatively small.

[0041] In the rack bar grinding method according to this embodiment, grinding marks are formed on the surface of the sliding contact surface 13 as described above. This reduces frictional resistance between the surface of the sliding contact surface 13 and the mounting surface or inner peripheral surface of the bearing member 3. Specifically, the rack bar 1 has reduced frictional resistance with the bearing member 3. This also reduces wear on the bearing member 3. Furthermore, in the rack bar grinding method according to this embodiment, the tooth back-side sliding contact surface located on the opposite side facing the thickness direction Z of the rack portion is ground with an abrasive while changing the amount of removal allowance so that the height of each of the multiple tooth portions of the rack portion from the sliding contact surface becomes a predetermined respective specified height, thereby making it possible to achieve an optimal meshing state between the tooth portions of the pinion 2 and the tooth portions of the rack bar 1. In other words, the rack bar grinding method according to this embodiment makes it possible to manufacture a rack bar that easily meshes with the tooth portions of the pinion in a steering device.

[0042] Specifically, a pin gauge G as shown in FIG. 9 may be used to adjust the height of the tooth portion 111. The pin gauge G is cylindrical and has a length equal to or greater than the width of the tooth portion 111 and a constant outer diameter along its length. With such a pin gauge G placed in the groove 112 and in contact with the surfaces of adjacent tooth portions 111, the over-pin dimension in each groove 112 is measured. Then, the moving speed V of the grindstone 4 is slowed down in areas where the over-pin dimension is large, and the moving speed V of the grindstone 4 is increased in areas where the over-pin dimension is small. By using such a pin gauge G, it is possible to easily perform polishing with varying amounts of removal of the sliding contact surface.

[0043] In the polishing method, it is preferable to polish the sliding surface 13 so that the polishing direction D is along the axial direction X (so that the polishing direction D is approximately parallel to the axial direction X) as shown in FIG. By such a polishing direction D, polishing marks are formed on the surface of the sliding contact surface 13 so as to extend along the axial direction X. In other words, the polishing marks are formed along the direction in which the sliding contact surface 13 and the bearing member 3 slide against each other. This reduces frictional resistance between the surface of the sliding contact surface 13 and the mounting surface or the inner peripheral surface of the bearing member 3. More specifically, compared to a rack bar polished along the width direction Y, for example, the rack bar 1 has reduced frictional resistance with the bearing member 3 in the axial direction X. This also reduces wear on the bearing member 3. Note that the polishing direction D includes directions tilted by ±5° from the axial direction X toward the width direction Y or the thickness direction Z.

[0044] The polishing method of this embodiment may include a first polishing process for polishing the surface 131a of the tooth back contact surface 13a, and a second polishing process for polishing the surfaces 131b of the both end contact surfaces 13b. The first polishing process and the second polishing process may be performed simultaneously or separately. When performed separately, either the first polishing process or the second polishing process may be performed first, and the order may be changed as appropriate. Only one of the first polishing process and the second polishing process may be performed.

[0045] In the grinding method of this embodiment, it is preferable to grind the tooth back sliding surface 13a located on the opposite side facing the thickness direction Z of the rack portion 11. As described above, in the steering device 100, the rack bar 1 is supported by a bearing member 31 called a support yoke 31 at a sliding surface (tooth back sliding surface 13a) located on the opposite side of the rack portion 11 in the thickness direction Z. The support yoke 31 is usually biased toward the rack bar 1 by a spring member or the like. The tooth back sliding surface 13a is a sliding surface located on the opposite side of the rack portion 11 on which the rack 11a that meshes with the pinion 2 is formed, and therefore is a location to which stress is likely to be directly applied when the rack bar 1 meshes with the pinion. For this reason, frictional resistance is relatively likely to occur between the support yoke 31 and the sliding surface 13a. In contrast, with the rack bar 1 obtained by the above-described polishing method, the polishing marks formed on the surface of the sliding surface 13a that comes into sliding contact with the support yoke 31 are formed along the movement direction of the rack bar 1, thereby more effectively reducing frictional resistance.

[0046] In the polishing method of this embodiment, only the surface 131a of the tooth back-side sliding contact surface 13a that comes into sliding contact with the bearing member 31 may be polished. Here, depending on the steering device 100, a portion of the surface 131a of the tooth back-side sliding contact surface 13a (particularly both end surfaces 133a, 133a (see FIG. 3) intersecting with the width direction Y) may not be in contact with the support yoke 31. In other words, in some steering devices 100, sliding contact with the support yoke 31 may occur concentratedly on a portion of the surface 131a. Therefore, in order to polish the tooth back-side sliding contact surface 13a more efficiently, only a predetermined portion of the surface 131a may be polished. In other words, a surface that needs to be polished (a surface that needs to be polished) may be set on the surface 131a of the tooth back-side sliding contact surface 13a, and only the surface that needs to be polished may be polished.

[0047] The surface requiring grinding may be set, for example, at a location (reference numeral 132a) as shown in FIG. 5. More specifically, in a cross section (YZ plane), coordinate axes are defined, including an X axis overlapping a line segment defining the diameter R1 of the surface 131a (more specifically, a line segment parallel to the tips 113 of the teeth 111 of the rack 11a and defining the diameter R1 of the surface 131a), a Y axis perpendicular to the X axis so as to bisect the rack bar 1, and an origin O of the X and Y axes. The rack 11 is positioned on the negative side of the Y axis, and the surface requiring grinding 132a is set at a portion located on the positive side of the Y axis. Then, the surface requiring grinding 132a is set at a portion located on the positive side of the Y axis via a pair of straight lines L1 passing through the origin O and inclined at ±65° with respect to the Y axis. The surface requiring grinding 132a is also set at a portion between the pair of straight lines L1 and a pair of straight lines L2 passing through the origin O and inclined at ±25° with respect to the Y axis.

[0048] That is, the contact surface between the surface 131a of the tooth back surface 13a and the support yoke 31 is set, for example, at a portion of the surface 131a corresponding to a central angle θ1 of 25 to 65° and a portion corresponding to a central angle θ1 of 115 to 155°, with the positive direction of the X-axis being 0° (FIG. 5). Furthermore, as wear of the support yoke 31 progresses, the contact surface becomes, for example, a portion of the surface 131a corresponding to a central angle θ1 of 25 to 155°, with the positive side of the X-axis being 0°. Therefore, by setting the surface to be polished 132a to the portion corresponding to 25 to 155° in consideration of wear of the bearing member as described above, it is possible to obtain a rack bar 1 in which the above-mentioned wear resistance can be sufficiently reduced.

[0049] As shown in FIG. 7, in the polishing method of this embodiment, the surface 131a of the untreated rack bar 1a fixed by a fixing member (not shown) is polished by an abrasive material 4 configured to be movable along the axial direction X of the untreated rack bar 1a.

[0050] The untreated rack bar 1a is fixed, for example, so that the surface 131a faces upward in the vertical direction (thickness direction Z) and the tooth tips 113 of the tooth portions 111 of the rack 11a face downward. The abrasive 4 is configured to be movable along the axial direction X of the untreated rack bar 1a, for example, while in contact with the surface 131a. Furthermore, the abrasive 4 may be configured to be movable along the vertical direction (thickness direction Z) so as to be in both a contacting state and a non-contacting state with the surface 131a. Alternatively, the abrasive 4 may be fixed, and the untreated rack bar 1a may be moved along the axial direction X. To reduce the footprint of the polishing apparatus, it is preferable to move the abrasive 4 rather than moving the long untreated rack bar 1a.

[0051] As shown in Figures 6 to 8, the abrasive material 4 in this embodiment is a rotary grindstone 4. The rotary grindstone 4 is formed in a disk shape and is rotatable around a rotation axis CY2. The rotary grindstone 4 has abrasive grains on its rotating outer peripheral surface 41. The rotary grindstone 4 is arranged so that its rotation direction B (the movement direction of the outer peripheral surface 41) is aligned with the axial direction X of the untreated rack bar 1a.

[0052] The rotary grindstone 4 is composed of the abrasive grains and a resin as a binder that binds the abrasive grains together. From the viewpoint of cost, alumina-based abrasive grains with relatively low hardness, such as white alumina (WA), are used as the abrasive grains. Furthermore, a ceramic-based binder, such as vitrified ceramic, is used as the binder. Furthermore, a resin-based binder may also be used as the binder, taking into consideration the amount of polishing residue remaining after polishing the curved surfaces 131a and 131b.

[0053] The grit size of the abrasive grains used is #60 to #800, preferably #120 to #220. When polishing an untreated rack bar 1a that requires a polishing allowance (e.g., 10 to 30 μm), a grindstone with such a grit size can sufficiently remove the oxide film formed during quenching before the polishing process, thereby achieving a surface roughness sufficient to reduce the above-mentioned wear resistance. Furthermore, when the rack bar 1 is incorporated into the steering device 100, a lubricant such as grease is usually applied to the surfaces 131a and 131b to improve lubrication with the bearing member 3. Therefore, a grindstone with the above grit size can also achieve a surface roughness that allows the lubricant to be appropriately retained on the surfaces 131a and 131b.

[0054] The outer diameter of the grinding wheel 4 is, for example, 200 mm or more and 360 mm or less. The contact pressure of the rotating outer peripheral surface 41 of the grinding wheel 4 against the sliding surface 13 during the grinding process is, for example, 0.1 MPa or more and 1 MPa or less. The peripheral speed of the grinding wheel 4 during the grinding process is, for example, 25 m / s or more and 30 m / s or less.

[0055] 6, the grindstone 4 has an outer peripheral surface 41 formed in a concave shape corresponding to the shape (arcuate shape) of the surface 131a of the tooth back-side sliding surface 13a. More specifically, in the grindstone cross section overlapping with the rotation axis CY2, the outer peripheral surface 41 has an arcuate portion 411 having a diameter the same length as the diameter R1 of the surface 131a.

[0056] In the grinding method of this embodiment, the grindstone 4 is rotated so that the outer peripheral surface 41 of the grindstone 4 is aligned with the axial direction X of the untreated rack bar 1a, and the grindstone 4 or the untreated rack bar 1a (preferably the grindstone 4) is moved along the axial direction X to grind the sliding surface 13 with the outer peripheral surface 41. That is, grinding is performed such that both the rotation direction of the grindstone 4 and the movement direction of the outer peripheral surface 41 of the grindstone 4 are aligned with the axial direction X of the untreated rack bar 1a. This makes it easier to form polishing marks along the axial direction X of the untreated rack bar 1a on the surface of the sliding contact surface 13. This makes it possible to further reduce the friction resistance between the sliding contact surface 13 of the untreated rack bar 1a and the bearing member.

[0057] When a grinding wheel 4 having a concave outer peripheral surface 41 is used in the first grinding process, it is preferable to perform a first single-sided grinding process to grind a partial region of the surface 131a, and then a second single-sided grinding process to grind another partial region of the surface 131a. More specifically, as shown in Fig. 8, when the positional relationship between the untreated rack bar 1a and the grinding wheel 4 is viewed from one side in the axial direction X of the untreated rack bar 1a, the rack bar 1 is rotated in one direction about the axial direction X (e.g., counterclockwise) and fixed so that the inclination angle θ2 of the tooth tips 113 of the tooth portions 111 with respect to the rotation axis CY2 of the grinding wheel 4 is 45±5° (the state shown in Fig. 8(b)). The grinding wheel 4 is then moved downward to contact the untreated rack bar 1a, and the grinding wheel 4 is rotated and moved along the axial direction X of the untreated rack bar 1a, thereby performing the first single-sided grinding process. After the first single-sided polishing process is completed, the grindstone 4 is moved upward so that the surface 131a and the grindstone 4 are not in contact with each other (for example, the state shown in FIG. 8(a)). Next, the rack bar 1 is rotated in the other direction around the axial direction (for example, clockwise) so that the inclination angle θ2 is 45±5°, and fixed in this state (for example, the state shown in FIG. 8(c)). The grindstone 4 is then moved downward to contact the untreated rack bar 1a, and the grindstone 4 is moved along the axial direction X of the untreated rack bar 1a, thereby performing the second single-sided polishing process.

[0058] The areas polished by the first single-side polishing process and the areas polished by the second single-side polishing process may partially overlap. This allows the polishing of the surface 132a that requires polishing to be focused. In other words, the areas that are likely to slide between the surface 131a and the support yoke 31 can be polished relatively focused and efficiently.

[0059] The moving speed V of the grindstone 4 is set to, for example, 30 to 50 mm / sec (including a 10% error). More specifically, first, the amount of grinding (machining allowance) is determined for each portion of the surface 131a corresponding to each tooth portion 111 of the untreated rack bar 1a. Then, for example, the moving speed V is set to 50±5 mm / sec in the portion where the machining allowance is 10 to 20 μm, and for example, the moving speed V is set to 30±3 mm / sec in the portion where the machining allowance is 20 to 30 μm.

[0060] In the second polishing process, the entire surface 131b of the sliding contact surfaces 13b at both ends may be polished. However, for more efficient polishing, it is preferable to focus on polishing portions of the surface 131b that may be in concentrated sliding contact with the inner peripheral surface of the rack bush 32 in the steering device. In other words, it is preferable to set a polishing-required surface 132b on the surface 131b that needs to be polished.

[0061] The surface needing polishing 132b is set at a position corresponding to the position of the surface needing polishing 132a of the tooth backside sliding contact surface 13a in the circumferential direction CY1 of the rack bar 1. This is efficient because the surface needing polishing 132b can be polished following the polishing of the surface needing polishing 132a without changing the fixed state of the rack bar 1. Also, in the steering device 100, the load of the rack bar 1 tends to cause sliding contact with the both end sliding contact surfaces 13b to be concentrated at the bottom part of the inner circumferential surface of the rack bush 32. Therefore, by setting the position of the surface needing polishing 132b as described above, it is possible to focus on polishing the parts that are likely to cause sliding contact with the rack bush 32.

[0062] In carrying out the polishing method of this embodiment, the unprocessed rack bar 1a has a predetermined polishing allowance, which is, for example, 10 to 30 μm.

[0063] In the polishing method of this embodiment, the moving direction D of the grindstone 4 relative to the rotation direction B of the grindstone 4 may be the same direction as the axial direction X, or may be opposite directions.

[0064] In the polishing method of this embodiment, a coolant liquid may be used to suppress heat generation during polishing. The coolant liquid may be a water-soluble coolant liquid or an oil-based coolant liquid, but a water-soluble coolant liquid may be used from the viewpoint of cost.

[0065] According to the polishing method of this embodiment, the arithmetic mean roughness Ra of the sliding surface 13 in the axial direction X after polishing is 0.1 μm (measurement method: needle contact measurement, evaluation length 4 mm, standard JIS2001), and the arithmetic mean roughness Ra in the circumferential direction CY1 is 0.4 μm (measurement method: needle contact measurement, evaluation length 4 mm, standard JIS2001). That is, the rack bar polished by the polishing method of this embodiment has different roughness Ra in the axial direction X and the circumferential direction CY1. Specifically, the roughness Ra in the axial direction X is small, and the roughness Ra in the circumferential direction CY1 is large. Here, the bearing member 3 (particularly the rack bush 32) also has the function of preventing rotation of the rack bar 1 in the circumferential direction CY1 in the steering device 100. Therefore, the rack bar 1 having the sliding surface 13 with a relatively large arithmetic mean roughness Ra in the circumferential direction CY1 can enhance the above function of the bearing member 3, and can more effectively prevent rotation of the rack bar 1 in the circumferential direction CY1 in the steering device.

[0066] As described above, the polishing method for polishing the sliding contact surface 13 of this embodiment can reduce frictional resistance with the bearing member in the steering device and can obtain a rack bar that easily meshes with the teeth of the pinion in the steering device. To obtain a rack bar that easily meshes with the teeth of the pinion, it is possible to adjust the height of each tooth by grinding the tooth portion itself. However, the polishing method of this embodiment can eliminate the need for such height adjustment by grinding the tooth portion itself. Furthermore, even if it is necessary to grind the tooth portion itself, the resulting height adjustment can be a fine adjustment. In other words, the polishing method of this embodiment is an efficient method that can simultaneously reduce frictional resistance and adjust the height of the tooth portion, as described above.

[0067] As described above, one embodiment has been shown as an example, but the rack bar grinding method according to the present invention is not limited to the configuration of the above embodiment. Furthermore, the rack bar grinding method according to the present invention is not limited by the above-described effects. The rack bar grinding method according to the present invention can be modified in various ways without departing from the gist of the present invention.

[0068] For example, in the above embodiment, the grindstone 4 is used as the abrasive material, but the present invention is not limited to this, and the abrasive material 4 may be, for example, abrasive cloth or buff abrasive material.

[0069] Furthermore, in the above embodiment, either the rack bar 1 or the abrasive material 4 is moved, but this is not limiting, and both the rack bar 1 and the abrasive material 4 may be moved. [Explanation of symbols]

[0070] 1: rack bar, 10: shaft material, 11: rack portion, 11a: rack, 111: tooth portion, 112: groove, 113: tooth tip, 13: sliding surface, 13a: tooth back sliding surface, 131a: surface, 132a: surface to be polished, 13b: both end sliding surfaces, 131b: surface, 132b: surface to be polished, 133a: end surface, 2: pinion, 3: bearing member, 31: support yoke, 32: rack bush, 4: abrasive material, 41: outer peripheral surface, 411: arc-shaped portion, X: Axial direction, Y: Width direction, Z: Thickness direction, CY1: Circumferential direction, CY2: Rotation axis, θ1: Central angle, θ2: Tilt angle, B: Rotation direction, D: Grinding direction (direction of movement of abrasive material), L1, L2: Linear, R1: Diameter, T1, T2: Height from sliding surface

Claims

1. A method for grinding a rack bar, comprising: the rack bar is slidable relative to a bearing member that supports the rack bar while meshing with the pinion when incorporated into the steering device, and includes a rack portion formed with a plurality of teeth that mesh with the pinion, and a sliding surface that is in sliding contact with the bearing member, a height of each tooth portion from the sliding contact surface is preset in a cross section passing through the axis of the rack bar and parallel to the thickness direction; In the polishing method, an untreated rack bar that has undergone heat treatment to form an oxide film on its surface is polished with an abrasive material while removing the oxide film and changing the amount of removal to achieve the height.

2. The rack bar grinding method according to claim 1 , wherein the preset heights of the plurality of teeth are different in parts.

3. 3. The rack bar polishing method according to claim 1, wherein the sliding contact surface is polished with an abrasive so that the polishing direction is along only the axial direction of the rack bar.

4. As the abrasive material, a rotating grindstone is used which is disk-shaped, can rotate in a circumferential direction, and has abrasive grains on a concave outer peripheral surface corresponding to the sliding contact surface, 4. The rack bar grinding method according to claim 3, wherein the grinding wheel is rotated so that the outer peripheral surface follows only an axial direction of the rack bar, and the sliding surface is ground with the outer peripheral surface while moving at least one of the grinding wheel and the rack bar so as to follow only the axial direction.

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