JP7742709B2 - How to polish rack bars - Google Patents

Description

The present invention relates to a method for polishing a rack bar.

Rack bars are used as components of rack-and-pinion steering devices installed in automobiles, for example, and have a rack section formed with a rack having multiple teeth that mesh with the pinion.

When 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 the rack is heat-treated (hardened) to increase its strength.

Japanese Patent Application Laid-Open No. 2020-66020

However, in the method described in Patent Document 1, when the rack is heat-treated (quenched), an oxide film forms on the surface of the rack bar, which causes unevenness on the surface of the rack bar.

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 section in the thickness direction (described below), and rack bushings that support both ends. When the pinion rotates by operating the 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 member will be referred to as the "sliding surface").

Due to the steering device's drive mechanism described above, the bearing member may be worn due to sliding contact with the sliding surface of the rack bar. Therefore, it is preferable that the frictional resistance between the sliding surface of the rack bar and the bearing member is low.

As mentioned above, in the steering device, the rack bar moves axially 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.

In light of the above, the present invention aims to provide a method for polishing a rack bar that can reduce frictional resistance with bearing members in a steering device and that can easily mesh with the teeth of a pinion in the steering device.

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.

With this configuration, the sliding surface is polished with an abrasive as described above, reducing frictional resistance between the sliding surface and the bearing member, and enabling the rack bar to easily mesh with the pinion teeth in a steering device.

Specifically, in a steering device, 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. As a result, frictional resistance is relatively likely to occur between the support yoke and this sliding surface. Therefore, frictional resistance can be effectively reduced by polishing the sliding surface in this area.

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

Furthermore, in the rack bar polishing method according to the present invention, it is preferable that the height adjustment from the sliding surface is performed using a pin gauge.

This configuration makes it easy to polish the sliding surface with varying amounts of removal.

Furthermore, in the rack bar polishing method according to the present invention, it is preferable that the polishing direction using the abrasive material is along the axial direction of the rack bar.

With this configuration, the grinding direction is as described above, so 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 further reduces frictional resistance between the sliding contact surface of the rack bar and the bearing member.

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

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.

With this configuration, the outer peripheral surface of the grinding wheel having abrasive grains (the so-called 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 or the rack bar is moved along the axial direction. This means that both the rotation direction of the outer peripheral surface (grinding surface) of the grinding wheel (the direction in which it rotates and grinds) and the movement direction of the outer peripheral surface (the direction in which it moves and grinds) are aligned with the movement direction of the rack bar. This makes it easier for grinding marks to form on the surface of the sliding surface along the axial direction of the rack bar. This in turn reduces frictional resistance between the sliding surface of the rack bar and the bearing member.

As described above, the present invention provides a method for polishing a rack bar that can reduce frictional resistance with bearing members in a steering device and that can easily mesh with the teeth of a pinion in a steering device.

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

The following describes a rack bar polishing method according to one embodiment of the present invention, with reference to the drawings. Note that, hereinafter, the axial direction of the rack bar may be referred to as the axial direction X (longitudinal direction). The horizontal direction perpendicular to the axial direction X may 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 may be referred to as the thickness direction Z (second short-side direction). The cross section (Y-Z plane) of the rack bar perpendicular to the axial direction X may be referred to as the transverse cross section.

First, with reference to Figures 1 to 4, we will explain the rack bar 1 polished using the polishing method of this embodiment, while showing a steering device 100 that employs the rack bar 1.

As shown in Figure 1, the steering device 100 is a rack-and-pinion type steering device. The steering device 100 includes a pinion 2 connected to the steering shaft of a vehicle or the like, a rack bar 1 formed with a rack 11a that meshes with the pinion 2, and a bearing member 3 that supports the rack bar 1.

The rack bar 1 comprises 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 slides against the bearing member 3.

The rack bar 1 is manufactured from, for example, solid (cylindrical) steel material or hollow (cylindrical) steel material. Examples of such steel material include carbon steel material for machine structures as specified in JIS 4051:2016. Among these, S45C steel material is preferred.

As shown in Figures 1 and 2, the rack 11a is composed of multiple teeth 111 whose tooth tips 113 extend along the width direction Y (a direction perpendicular to the plane of the paper in Figures 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 multiple grooves 112 between them, into which the teeth 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 slides relative to the bearing member 3, and the sliding surface 13 slides against the bearing member 3.

The rack bar 1 has sliding surfaces 13, including a tooth back sliding surface 13a located on the opposite side of the rack portion 11 in the thickness direction Z, and 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 surface 131a of the tooth back sliding surface 13a has, for example, an arc shape. Furthermore, in cross section, the surface 131b of the end sliding surfaces 13b has, for example, a circular shape.

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

In the steering device 100, the support yoke 31 is formed in a shape that covers the surface 131a of the tooth back contact surface 13a from below. The support yoke 31 has a mounting surface on which the tooth back contact surface 13a rests, and supports the rack bar 1 with the mounting surface and surface 131a in contact.

In addition, 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 circumferential 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 circumferential surface and the surface 131b in contact.

3 and 4, in a cross section, the diameter of the arc described by the surface 131a of the tooth back surface 13a and the diameter of the circle described by the surfaces 131b of the end contact surfaces 13b are substantially the same. If these diameters are designated as R1, then 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 end contact surfaces 13b to determine the average circumference C1, and then using the following formula (1). The value of pi is assumed to be 3.14.
R1=C1/Pi...(1)

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

The steering device 100 is configured so 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 inner peripheral surface of the bearing member 3.

Next, the method for polishing the rack bar 1 of this embodiment will be described with reference to Figures 5 to 8.

In the polishing method of this embodiment, an untreated rack bar 1a (a workpiece after heat treatment (hardening)) on whose surface 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 Figure 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 polishing method shown in Fig. 7. That is, the polishing method may be the polishing method shown in Fig. 7, or may be polished along the axial direction X while polishing along the width direction Y by buffing.

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 center 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 both adjacent 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 part 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.

Next, when grinding the portion corresponding to the tooth portion 111, which has been set to a relatively low predetermined height from the sliding surface, a relatively large amount of material is removed from 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. For example, in the grinding method shown in Figure 7, the moving speed V of the grindstone 4 in the axial direction of the untreated rack bar 1a is reduced to relatively increase the amount of grinding.

Furthermore, when grinding the portion corresponding to the tooth portion 111, which has been set to 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 relatively small. For example, in the grinding method shown in Figure 7, the movement speed V of the untreated rack bar 1a of the grinding wheel 4 in the axial direction X is increased to relatively reduce the amount of grinding.

In the rack bar polishing method according to this embodiment, as described above, polishing marks are formed on the surface of the sliding contact surface 13. 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.

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 grinding wheel 4 is slowed down in areas where the over-pin dimension is large, and the moving speed V of the grinding wheel 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.

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 ±5° from the axial direction X toward the width direction Y or the thickness direction Z.

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 surface 131b of 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. Also, only one of the first polishing process or the second polishing process may be performed.

In the polishing method of this embodiment, it is preferable to polish 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 also 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.

The polishing method of this embodiment may polish only the surface 131a of the tooth back contact surface 13a that comes into sliding contact with the bearing member 31. However, depending on the steering device 100, a portion of the surface 131a of the tooth back contact surface 13a (particularly both end surfaces 133a, 133a intersecting the width direction Y (see FIG. 3)) 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 concentrated on a portion of the surface 131a. Therefore, to polish the tooth back contact surface 13a more efficiently, only a specified 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 specified on the surface 131a of the tooth back contact surface 13a, and only that surface that needs to be polished may be polished.

The surface requiring polishing may be set, for example, at a location (reference symbol 132a) as shown in FIG. 5. More specifically, coordinate axes are defined that include an X axis that overlaps the line segment defining the diameter R1 of the surface 131a in the cross section (Y-Z plane) (more specifically, a line segment that is parallel to the tips 113 of the teeth 111 of the rack 11a and defines the diameter R1 of the surface 131a), a Y axis that is 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 polishing 132a is set at a portion located on the positive side of the Y axis. The surface requiring polishing 132a is then set at a portion located on the positive side of the Y axis via a pair of straight lines L1 that pass through the origin O and are inclined ±65° relative to the Y axis. The surface requiring polishing 132a is also set at a portion between the pair of straight lines L1 and a pair of straight lines L2 that pass through the origin O and are inclined ±25° relative to the Y axis.

That is, the contact surface between the surface 131a of the tooth backside sliding 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° (Figure 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°, taking into account wear of the bearing member as described above, it is possible to create a rack bar 1 that can sufficiently reduce the above-mentioned wear resistance.

As shown in Figure 7, in the polishing method of this embodiment, the surface 131a of an untreated rack bar 1a, which is fixed by a fixing member (not shown), is polished with an abrasive material 4 that is movable along the axial direction X of the untreated rack bar 1a.

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 on the rack 11a face downward. The abrasive 4 is configured to be movable, for example, along the axial direction X of the untreated rack bar 1a 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 that it can be in both contact with and out of contact with the surface 131a. Alternatively, the abrasive 4 may be fixed and the untreated rack bar 1a 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.

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 direction of movement of the outer peripheral surface 41) is aligned with the axial direction X of the untreated rack bar 1a.

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 possibility of remaining polished portions during the polishing process of the curved surfaces 131a and 131b.

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

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 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.

As shown in Figure 6, the rotating grindstone 4 has, for example, an outer peripheral surface 41 formed in a concave shape corresponding to the shape (arcuate shape) of the surface 131a of the tooth backside sliding surface 13a. More specifically, in the grindstone cross section overlapping with the rotation axis CY2, the outer peripheral surface 41 has, for example, an arcuate portion 411 having a diameter the same length as the diameter R1 of the surface 131a.

In the polishing method of this embodiment, the grinding wheel 4 is rotated so that the outer peripheral surface 41 of the grinding wheel 4 as described above is aligned with the axial direction X of the untreated rack bar 1a, and the grinding wheel 4 or the untreated rack bar 1a (preferably the grinding wheel 4) is moved along the axial direction X to polish the sliding surface 13 with the outer peripheral surface 41. In other words, grinding is performed such that both the rotation direction of the grinding wheel 4 and the movement direction of the outer peripheral surface 41 of the grinding wheel 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. Therefore, it is possible to further reduce the friction resistance between the sliding contact surface 13 of the untreated rack bar 1a and the bearing member.

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 portion of the surface 131a, followed by a second single-sided grinding process to grind another portion of the surface 131a. More specifically, as shown in Figure 8, when the relative positions of the untreated rack bar 1a and the grinding wheel 4 are 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 relative to the rotation axis CY2 of the grinding wheel 4 is 45±5° (as shown in Figure 8(b)). The grinding wheel 4 is then moved downward to contact the untreated rack bar 1a, and the grinding wheel 4 is then 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 (for example, the state shown in Figure 8(a)). Next, the rack bar 1 is rotated in the other 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 Figure 8(c)). The grindstone 4 is then moved downward to contact the untreated rack bar 1a, and the grindstone 4 is then moved along the axial direction X of the untreated rack bar 1a to perform the second single-sided polishing process.

The areas polished by the first single-sided polishing process and the areas polished by the second single-sided 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 come into sliding contact between the surface 131a and the support yoke 31 can be polished relatively focused and efficiently.

The movement speed V of the grinding wheel 4 is set, for example, to 30 to 50 mm/sec (with a 10% tolerance). More specifically, first, the amount of grinding (removal amount) is determined for each portion of the surface 131a corresponding to each tooth portion 111 of the unprocessed rack bar 1a. Then, for portions where the removal amount is 10 to 20 μm, the movement speed V is set, for example, to 50±5 mm/sec, and for portions where the removal amount is 20 to 30 μm, the movement speed V is set, for example, to 30±3 mm/sec.

The second polishing process may involve polishing the entire surface 131b of the sliding contact surfaces 13b at both ends. However, for more efficient polishing, it is preferable to focus polishing on the portions of surface 131b that are likely to be in concentrated sliding contact with the inner circumferential surface of the rack bush 32 in the steering device. In other words, it is preferable to set polishing-required surfaces 132b on surface 131b that require polishing.

The surface requiring grinding 132b is set at a position in the circumferential direction CY1 of the rack bar 1 that corresponds to the position of the surface requiring grinding 132a on the tooth backside sliding surface 13a. This is efficient because it allows the surface requiring grinding 132b to be polished immediately after the surface requiring grinding 132a without changing the fixed state of the rack bar 1. Furthermore, in the steering device 100, the load of the rack bar 1 tends to cause sliding contact with the both end sliding surfaces 13b, concentrated at the bottom of the inner circumferential surface of the rack bush 32. Therefore, by setting the position of the surface requiring grinding 132b as described above, it is possible to focus on polishing the areas that are likely to slide with the rack bush 32.

When implementing the polishing method of this embodiment, the unprocessed rack bar 1a has a predetermined polishing allowance. This polishing allowance is, for example, 10 to 30 μm.

In the polishing method of this embodiment, the movement direction D of the grinding wheel 4 relative to the rotation direction B of the grinding wheel 4 may be the same direction as the axial direction X, or may be opposite directions, as long as they are along the axial direction X.

The polishing method of this embodiment may use a coolant liquid that suppresses heat generation during polishing. The coolant liquid may be either a water-soluble coolant liquid or an oil-based coolant liquid, but a water-soluble coolant liquid may be used from a cost perspective.

According to the polishing method of this embodiment, the arithmetic mean roughness Ra of the sliding contact surface 13 in the axial direction X after polishing is 0.1 μm (measurement method: needle contact measurement, evaluation length 4 mm, standard JIS 2001), 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 JIS 2001). 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.

As described above, the polishing method for polishing the sliding surface 13 of this embodiment can reduce frictional resistance with the bearing members in the steering device and 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 makes it possible to 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 fine. 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.

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 effects described above. Various modifications are possible to the rack bar grinding method according to the present invention without departing from the spirit and scope of the present invention.

For example, in the above embodiment, a rotating grinding wheel 4 is used as the abrasive material, but this is not limited to this. The abrasive material 4 may also be, for example, abrasive paper cloth or buff abrasive material.

In addition, in the above embodiment, either the rack bar 1 or the abrasive material 4 is moved, but this is not limited to this, and both the rack bar 1 and the abrasive material 4 may also be moved.

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: polishing direction (direction of movement of abrasive material), L1, L2: straight line, R1: diameter, T1, T2: height from sliding surface

Claims (4)

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. The rack bar grinding method according to claim 1, wherein the preset heights of the plurality of teeth vary in parts. The rack bar polishing method according to claim 1 or 2, wherein the sliding surface is polished with an abrasive so that the polishing direction is along only the axial direction of the rack bar. 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.