JPH011602A - Method for improving fatigue strength of automobile wheels - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION FIELD OF INDUSTRIAL APPLICATION This invention relates to wheels used in automobiles (including two-wheeled vehicles), and more particularly to a method for improving the fatigue strength of air vents formed in discs.

Technical Background Automobile wheels are important parts for safe driving of automobiles, and they need to have sufficient structural strength and also need to fulfill the following roles and functions.

■ Durability Wheels must constantly support the weight of the vehicle and loaded weight, but as these act as repetitive loads as the wheel rotates, the wheels must be durable against loads that act in the radial direction. Also, since the centrifugal force that acts on the car body during cornering acts as a bending moment on the wheels, durability under rotational bending is required.

■ Lightweight The ride comfort of a car when driving on rough roads is related to the weight of the unsprung mass, and making the wheels lighter helps improve ride comfort. It is also needed to reduce fuel costs.

■ Cooling function The brakes attached to the wheels of automobiles generate a large amount of friction heat due to intermittent braking. For example, in the case of disc brakes, if the rotor becomes hot, the braking performance will drop significantly and the It's dangerous. Therefore, the wheels are provided with air holes or fins to provide a cooling function. Further, the wheel needs to have a space for accommodating such brake components (for example, a space that can secure a sufficient rotor diameter for a disc brake).

■ Design Wheels are often seen by the general public because they are mounted on the outside of the car body, so there is a strong tendency for the market to favor wheels with excellent design. Therefore, design is taken into consideration not only in the shape of the wheel but also in the shape and number of air holes.

■ Corrosion Resistance Since automobiles are often used in salty environments or parked outdoors, their wheels need to be corrosion resistant.

In order to satisfy these conditions, wheels are being manufactured using new materials such as high-tensile steel, aluminum alloy, magnesium alloy, and FRP, but there are problems with strength, durability, weldability, cost, etc. However, it has not yet reached the point of widespread use.

By the way, as shown in Figure 6, the general structure of a steel wheel is a rim (1) that holds the tire.
) and a disk (2) for attachment to the hub, both of which are integrally joined by arc welding, spot welding, rivets, etc.

The rim (1) has a rim flange part (1-1), a bead seat part (1-2), a well part (1-3) and a valve hole (1-
4) is formed, and the disk (2) has a center hole (2-
1), a bolt hole (2-2), an air hole (2-3), and a hat portion (2-4) extending in the axial direction are formed.

The hat functions to prevent the disc from buckling and to increase its structural strength.

Further, an offset distance is provided between the inner contact surface of the disk (2) and the rim center line.

This serves to secure space inside for storing brake-related parts and to weaken the propagation of the impact of the ground force when driving on rough roads.

Because the disc is made of thin sheets with such an asymmetrical shape, it will undergo bending deformation and stress under most loads applied to the wheel.

In addition, stress is likely to be concentrated in the hat portion, the air hole, and the center hole, and in particular, stress is concentrated around the air hole, and fatigue cracks are likely to occur as the wheel rotates.

Therefore, it is necessary for the wheel disc to have a structure that suppresses the stress generated and has excellent fatigue strength.

BACKGROUND ART The following methods are known as methods for improving the fatigue strength of wheel discs.

(1) Specifying the disc profile and numerically limiting the variation in disc thickness so that the variation in disc thickness is at least 20% (Japanese Patent Laid-Open No. 55-8993).

(1) Strength is imparted to the disc by fixing a circular rigid continuous element, that is, a separate member, concentrically with respect to the axis of rotation of the wheel (Japanese Patent Application Laid-Open No. 140301/1983).

■ A method in which a groove is provided on the outside of the equally thick part at the center of the disk, and the outer periphery is tapered using a rolling roll (Japanese Patent Application Laid-open No. 56
-50742).

■ For the purpose of improving the strength of the opening, the method of installing the disc is to apply coining to the 1-seat of the hole (Japanese Patent Laid-Open No. 56
-120401) and a method of coining the outer peripheral edge corresponding to the point where the air hole is punched (Japanese Patent Laid-Open No. 58-1
16944) etc.

Problems with conventional technology Among the technologies proposed to improve the fatigue strength of disks, conventional disks that apply method There is a drawback that fatigue cracks are likely to occur near the opening as the wheel rotates.

In addition, with the method (2) of attaching another part to the disk, it is possible to fix and reinforce the part where stress is concentrated, but it is necessary to prepare a new part, which is expensive. This has the disadvantage that the weight of the disc increases, which leads to an increase in the weight of the wheel, which is undesirable.

In addition, although the method (■) of coining the nut seat in the hole for disk installation is effective as a measure to improve the strength of the area around the nut seat, it does not have the effect of reducing stress particularly around the air hole where stress is concentrated. The disadvantage is that fatigue cracks are likely to occur in some parts. Roughly speaking, the method of coining the area where the air hole is to be punched is aimed at preventing the occurrence of cracks during the hole punching, and does not take into account at all the prevention of fatigue cracks caused by the rotation of the wheel.

Incidentally, there is generally a surface treatment as a method of improving the fatigue strength of a material, and it is known that this treatment can increase the fatigue limit as well as increase the hardness. Known surface treatment techniques include surface hardening methods by spraying metal strips (shot peening method, shot blasting method) and surface hardening methods by applying pressure (roll rolling). A method used to improve fatigue strength has also been proposed.

For example, ■ A method of applying compressive residual stress to the rim side of the weld bead applied at the joint between the disc and the rim by shot peening to reduce the occurrence of cracks on the rim side and improve durability (special Kaisho 59-2901
),■In manufacturing the rim, the outer roll is made relatively large in diameter, and the ring-shaped material formed into a relatively small diameter is rolled with both rolls by moving the inner roll in the radial direction, resulting in large work hardening of the rim. There is a method of improving durability by adding

However, all of the above-mentioned surface processing techniques are aimed at improving the strength of the rim, and are not intended for wheel discs. Therefore, these techniques do not disclose any methods for improving the fatigue strength of openings such as air holes in wheel disks by employing surface processing techniques.

The present invention was developed in view of the above-mentioned conventional situation, and the fatigue strength of the wheel disc is improved by surface-processing the air holes, which are areas where stress is concentrated and where fatigue cracks are likely to occur as the wheel rotates. The aim is to propose an improved automobile wheel.

Means for Solving the Problems This invention improves fatigue strength by cold rolling the area around the air hole where stress is concentrated in a wheel disc, but if the cold rolling becomes excessive, the material thickness will decrease. Although this actually reduces the fatigue strength of the entire wheel, this invention was completed by focusing on the fact that it is thought that appropriate conditions always exist.

That is, this invention employs the cold rolling method, which is known as a method for improving the fatigue strength of the material itself, as a means for improving the fatigue strength of the air hole portion of the wheel disc. From the edge of the air hole to be drilled radially inward and outward over a range of 1 to 5 times the thickness of the material, in a concentric circle from the center of the disk, at a Hertzian pressure of 500 to 700 ksfJ, the amount of reduction is equal to the thickness of the material. 5
This is a method of improving the fatigue strength of the air holes by cold rolling the steel to 15%.

The reason why the cold rolling area is made concentrically from the center of the disk radially inward and outward from the edge of the air hole of the disk is to improve the fatigue strength around the air hole. This is because all the air holes in the disk are cold-rolled at once and continuously using a roll or the like, and all the air holes can be processed uniformly and at low cost. The reason for setting the range to 1 to 5 times the material thickness is that if it is less than 1 time, the fatigue strength will not be sufficiently improved, but if it is 1 time or more, the fatigue strength can be sufficiently improved, but taking into account the fluctuation of work. Then 5
It is safe within a range of twice that.

When cold rolling the disc, the Hertzian pressure is set at 500 to 7
The reason why it is set at 00 ksfJ is that if it is less than 500 ksfJ, a sufficient effect of improving fatigue strength cannot be obtained, and on the other hand, if it exceeds 700 ksfJ, the once improved fatigue strength will decrease again.

In addition, the reduction layer should be 5% to 15% of the material thickness.
This is because if the amount of processing is less than that, the fatigue strength will not be improved sufficiently, while if the reduction exceeds 15%, the thickness of the material will become thinner, which will actually reduce the fatigue strength of the entire disk.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic diagram showing an embodiment of the present invention, and illustrates a method of rolling an air hole portion before press forming a wheel disk.

In the figure, (11) is the disk material, (11-1) is the air hole, (11-2) is the center hole, (11-3) is the bolt hole, (12) is the mold, and (13) is the rolling roll. , (14) indicate roll bearings, respectively.

In other words, there is a wind hole (ii-1) in the disc material (11).
, center hole (11-2) and bolt hole (11-3)
After drilling, the disk material (11) is placed on the mold (12), and while applying a rolling load to the roll bearing (14), the rolling roll is rolled around the central axis (14-1) of the bearing. (
13) is rotated to roll the area around the air hole (11-1).

FIG. 2(a) is a plan view in which the rolling width when the periphery of the air hole is rolled is indicated by diagonal lines, and FIG. 2(b) is an enlarged plan view showing a part of FIG. 2(a).

When rolled by the method shown in FIG. 1, the shaded portions are rolled. This roll rolling area is the second
As shown in an enlarged view in Figure (b), it is preferable that the area is at least wider than the inner surface of the air hole (11-1) corresponding to the thickness of the material (an area that completely includes the part of the day of remembrance).

When the air holes are rolled, the thickness of the disk material naturally decreases. Here, the wall thickness before rolling is h,
If the reduction amount is Δh, the material thickness after rolling is (h−Δh
). Therefore, the amount of increase in fatigue strength σW of the material before rolling (α times).

It is effective to adjust the roll reduction amount of the roll so that α>1) and the wall thickness reduction amount Δh satisfy the relationship shown below, in order to increase the durability of the disk.

In other words, not only when a bending moment is applied to the wheel, but even when a load is applied in the radial direction, almost all of the load acts on the disc as a bending load. The generated stress σ before and after rolling can be expressed by the following equations (1) and (2).

Here, b is the board width in the target range, but it may be considered as a unit width.

In addition, the fatigue strength before and after rolling is as follows (3):
It can be expressed by equation (4).

Before roll rolling: σW ... (3) After roll rolling: ασW <α> 1) ... (4) α: Fatigue strength increase factor In order to increase the durability of the wheel, the generated stress and fatigue strength are It is necessary to satisfy the relationship shown in equation (5) below, and by substituting equations (1) and (2) above into equation (5), it is preferable to suppress the rate of decrease in wall thickness within the range of equation (6) below. .

σr ασW −〈□=α ・・・・・・(
5) σ0 σW For example, in the case of steel whose tensile strength σB is about 60ksf4,
When the reduction amount Δhtfio, i is ~0.2 mm and the Hertzian pressure is in the range of 8 to 12 times the tensile strength σB, the fatigue strength increase coefficient α is 1.4, so (6) According to the formula, it is preferable to suppress the wall thickness reduction rate Δh/h to about 15% or less.

The air hole portion can be rolled with a roll after press forming or after welding with the rim, but at this stage, since it is formed into a disk shape, rolls and molds that follow the disk shape are required. Therefore, it is advantageous to carry out roll rolling at a stage after perforating the air holes and before press forming.

Fig. 3 is a vertical sectional view showing a part of a wheel formed by integrating a disc and a rim, where (21) is a disc, (22)
) is the rim, and the cross-hatched area in the figure is the roll rolling area (30) of the air hole.

Example: From the base material of material 5HA60B, dimensions 385 x 385 m
We produced two types of disc materials for 14-inch wheels with a wall thickness of 41TIwl and 3.5mm, each with a wall thickness of 35mm.
A rectangular air hole of 30 mm in diameter was punched and then rolled using a roll having a diameter of 30 mm to cover a 5 mm wide area around the air hole using the method shown in FIG. At that time, the pressurizing force is applied in the range of 500 to 700 ksf in Hertz pressure,
The amount of change in wall thickness △h is 0.2 to 0.3 + yun, the reduction rate △
h/h was in the range of 5 to 9%.

As a result, the hardness of the base material (disc material) Hv (5) 23
0, the hardness around the air hole after roll rolling is H
v (5) increased to 320.

Next, in order to investigate the generated stress, the thickness of the above material was 3.5 mm.
Press-molded disc and material size 210 x 10
A 14-inch diameter wheel was manufactured using a rim molded using rim material with a diameter of 60 mm and a wall thickness of 2.9 mm, the bolt holes of the disc were fixed, and a bending moment of M-120 ksf was applied to the rim flange. Table 1 shows the actual measured values of the meridional stress and circumferential stress generated in the disk portion when the wheel is rolled, compared with a wheel using a disk made of the same material thickness but not rolled.

In addition, FIG. 4 is an explanatory view showing the above-mentioned stress measurement position (31), and the stress was measured at a position on a line at the center of the air hole and 3 mm away from the edge of the air hole.

From Table 1, the stress generated in the roll-rolled specimen is greater than that in the unrolled specimen due to the decrease in wall thickness. However, when evaluating the increase in generated stress due to a decrease in wall thickness, the generated stress is proportional to the square of the reciprocal of the wall thickness, so it becomes as follows. In other words, an increase in stress of 20% can be estimated.

On the other hand, the ratio of the actually measured stress generated around the wind hole is the meridional stress (σ[') and circumferential stress (σr) components, respectively, which is an increase of about 20% in stress, and is different from the predicted value. Almost match.

This increase in generated stress is due to the fact that the increase in fatigue strength due to roll rolling is about 40% (in the case of steel with σB = 60 klfJ, the rolling reduction Δtl = about 0.1+nm or more, the Hertzian pressure 8σB to 12σB), and the This poses no problem for durability.

Next, we conducted a rotational bending durability test on disks with wall thicknesses of 3.5 mm and 4 mm by fixing the bolt holes and rotating the wheels with a bending moment applied to the rim flange. and the test results are shown in Table 2.
The relationship between the amount of rotation N until fatigue failure and the bending moment M is shown in FIG. 5, with and without roll rolling, for comparison.

In this example, two types of bending moments were tested: 240 ksf -m and 180 kqf -In.

From Table 2 and FIG. 5, it can be seen that the durability is considerably improved by rolling the air holes of the disk.

In addition, there are roll-rolled ones with a wall thickness of 3.5 mm, and roll-rolled ones with a wall thickness of 4 mm.
Since the durability is equivalent to that of a wheel without roll rolling in mm, it is possible to reduce the thickness of the disc material in the design, and the weight of the entire wheel can be reduced to 9°0 as shown in Table 2. The weight can be reduced from - to 8.51.

Table 3 shows the results of rotary bending tests similar to those described above with various roll widths and Hertzian pressures.

From Table 3, the Hertzian pressure is 500~7001if,! range, i.e. 8 to 12 of tensile strength σe = 60ksfJ
It turns out that twice the range is effective. Furthermore, it can be seen that the durability is stabilized by setting the minimum rolling width to 3.5 mm, that is, a width equal to or more than the thickness of the material.

As described in detail below, the wheel according to the method of this invention is constructed of a disk in which the fatigue strength around the air hole is improved by rolling the area around the air hole of the wheel disk where stress is particularly concentrated. This has the effect of increasing the durability of the wheel, making it possible to obtain a high-quality wheel with high safety.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an embodiment of the present invention, and is a diagram illustrating a method of rolling an air hole portion before press forming a wheel disk. FIG. 2(a) is a plan view of a disk showing the rolling width when the circumference of the air hole is rolled, and FIG. 2(b) is a plan view showing a part of FIG. 2(a) in an enlarged manner. FIG. 3 is a longitudinal sectional view showing a portion of a wheel formed by integrating the disc of the present invention and a conventional rim. FIG. 4 is an explanatory diagram showing a stress measuring method in an embodiment of the present invention. FIG. 5 is a diagram showing the relationship between the number of rotations until fatigue failure and the bending moment in the above-mentioned example with the highest performance. FIG. 6 shows a conventional general steel wheel, in which (a) is a front view, and (b) is a longitudinal side view. 11...Material for disk 11-1...Wind hole 11
-2...Center hole 11-3...Bolt hole 1
2... Mold 13... Roll roll 14
...Roll bearing 21...Disk 22...
・Rim applicant: Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] In an automobile wheel having an air hole in the disk, a Hertzian pressure of 500 to 700 Kgf/ is applied concentrically from the center of the disk radially inward and outward from the edge of the air hole over a range of 1 to 5 times the thickness of the material. A method for improving fatigue strength of an automobile wheel, characterized by cold rolling to a rolling reduction of 5 to 15% of the material thickness at mm^2.