Patent Description:
A stabilizer disposed in a vehicle suspension mechanism part includes a torsion part (twisted part) extending in the width direction of the vehicle, a pair of arm parts continuous with both ends of the torsion part, and a bent part formed between the torsion part and the arm parts. In an example of the suspension mechanism part, the torsion part is supported by a vehicle body via a rubber bushing, etc. The arm parts are connected to a suspension arm or the like of the suspension mechanism part. In the stabilizer assembled to the suspension mechanism part, the arm parts, the bent part, and the torsion part are elastically deformed against the rolling behavior of the vehicle body that occurs when the vehicle travels on a curve, and function as springs. The roll rigidity of the vehicle body can be increased by such a stabilizer.

To reduce the weight of the vehicle, a hollow stabilizer formed of a metal pipe such as a steel pipe has been put into practical use. The pipe that is the material of the hollow stabilizer is a round pipe having a substantially circular cross-section in the radial direction. When such a pipe is formed by a bending machine (pipe bender), the cross section of the bent part (cross section in the pipe radial direction) becomes slightly flat.

As disclosed in, for example, Patent Literature <NUM>, a hollow stabilizer having an inner surface of a bent part having an elliptical cross section has been proposed. In addition, as disclosed in Patent Literature <NUM>, a hollow stabilizer having the pipe thickness changed in the circumferential direction has also been proposed. In these hollow stabilizers, a bent part or the like is formed by bending a metal pipe with a pipe bender.

As described in, for example, Patent Literature <NUM> and Patent Literature <NUM>, the pipe bender grips a portion of a predetermined length from the distal end of the pipe with a pipe clamp (chuck). Then, the pipe bender contacts the pipe with the roller while pulling the pipe, and thereby bends the pipe. For this reason, the pipe bender can suppress the bent part becoming flat to some extent, and can form a bent part having a relatively small flatness.

Patent Literature <NUM> forms the basis for the preambles of claims <NUM> and <NUM>.

According to the specifications of the stabilizer, the distance from the distal end of the arm part to the bent part may be required to be shorter than that in the conventional stabilizer. To form a metal pipe by a pipe bender, the end of the pipe needs to be held with a pipe clamp (chuck). In this case, a "grip" having a certain length is required at the end of the pipe. For this reason, a stabilizer having a short distance from the distal end of the arm part to the bent part is difficult to bend by the pipe bender. Therefore, forming the bent part of the stabilizer using a die, instead of the pipe bender, was considered.

However, a problem arises that when the pipe is pushed and bent in the radial direction by a conventional die, the bent part is flattened. For example, the flatness becomes larger than that when bent by a pipe bender at a part that is bent to a right angle with a relatively small radius of curvature, such as a bent part (so called a shoulder part) between the torsion part and the arm part. The allowable range of flatness is, for example, up to ±<NUM>% of the diameter of the pipe. Since the bent part formed by the conventional metal die has a large flatness, the stress of the bent part may be a problem. In addition, if the flatness of the bent part is large, the bent part may undesirably interfere with parts around the stabilizer.

Accordingly, an object of the present invention is to provide a hollow stabilizer, a stabilizer manufacturing device, and a hollow stabilizer manufacturing method, capable of suppressing an increase in the flatness of the cross section of the bent part and suppressing an increase in variation in the stress distribution in the circumferential direction of the cross section of the bent part.

One embodiment is a hollow stabilizer disposed in a vehicle suspension mechanism part, and comprises a torsion part, a bent part continuous with the torsion part, and an arm part continuous with the bent part. Regarding the cross section of the pipe radial direction of the bent part, the bent part comprises a first cross-sectional part, a second cross-sectional part, a third cross-sectional part, and a fourth cross-sectional part. When the center of the bending inside is <NUM>° and the center of the bending outside is <NUM>°, the first cross-sectional part is in the range of <NUM>° to <NUM>° centered at <NUM>°. The second cross-sectional part is formed within a range of <NUM>° to <NUM>° centered at <NUM>°, and has a smaller curvature than that of the first cross-sectional part. The third cross-sectional part is formed within a range of more than <NUM>° and less than <NUM>° centered at <NUM>°, and has a smaller curvature than that of the second cross-sectional part. The fourth cross-sectional part is formed within a range of more than <NUM>° and less than <NUM>° centered at <NUM>°, and has a smaller curvature than that of the second cross-sectional part. The flatness of the cross section of the bent part is within ±<NUM>% of the diameter of the pipe.

A stabilizer manufacturing device according to one embodiment comprises a base die, a clamp die, a pressing die, and a moving die. The base die includes a bottom wall on which a pipe is placed, a support wall with which a side surface of the pipe is in contact, and an arc-shaped forming curved surface corresponding to a curvature of a bending inside of a bent part of the pipe. The clamp die holds the pipe by sandwiching the pipe between the support wall of the base die and the clamp die in a radial direction. The pressing die is disposed to face the bottom wall of the base die, and forms a cavity which the bent part of the pipe enters between the bottom wall and the pressing die. The moving die is disposed to face the forming curved surface of the base die. The moving die moves in a direction of bending the pipe in a state in which a part closer to the distal end side than a part that is to be the bent part is held at a part of the pipe in the longitudinal direction. Furthermore, this moving die allows the part that is to be the bent part to enter the cavity and presses the part against the forming curved surface. In this embodiment, a tapered surface that increases a distance from the bottom wall toward the opening of the cavity may be formed on a surface that faces the bottom wall on a part of the pressing die. The upper surface of the pipe moves toward the forming curved surface along the tapered surface, while the pipe is being bent.

The hollow stabilizer manufacturing method according to one embodiment comprises a heating step, a placing step, and a bending step. The heating step heats the pipe that is a material of the hollow stabilizer to a warm region. The placing step places the pipe on a base die. The bending step forms the bent part by bending the pipe with a moving die in a state where crushing the part that is to be the bent part of the pipe in a flat shape is restricted by the base die, a clamp die, and a pressing die.

A hollow stabilizer according to another embodiment includes eight regions defined in the circumferential direction of the cross section when the center of the bending inside is <NUM>° and the center of the bending outside is <NUM>° in relation to a cross section in the pipe radial direction of the bent part. That is, the hollow stabilizer includes a first region including a first part located at <NUM>°, a third region including a third part located at <NUM>°, a fifth region including a fifth part located at <NUM>°, a seventh region including a seventh part located at <NUM>°, a second region including a second part between the first region and the third region, a fourth region including a fourth part between the third region and the fifth region, a sixth region including a sixth part between the fifth region and the seventh region, and an eighth region including the eighth part between the first region and the seventh region. Furthermore, the hollow stabilizer includes an outer circumferential surface on which a radius of curvature of an outer surface of each of the third part and the seventh part is larger and a radius of curvature of an outer surface of each of the second part and the sixth part is smaller than a radius of curvature of an outer surface of each of the fourth part and the fifth part. The flatness of the cross section of the bent part is within ±<NUM>% of the diameter of the pipe.

In this embodiment, the hollow stabilizer includes an inner circumferential surface on which a radius of curvature of an inner surface of each of the third part and the seventh part is larger and a radius of curvature of an inner surface of each of the second part and the sixth part is smaller than a radius of curvature of an inner surface of each of the fourth part and the fifth part.

The hollow stabilizer including a bent part according to the embodiments has a flatness smaller than that of a bent part bent by a conventional die, and the cross section of the bent part is a shape close to a perfect circle. For this reason, dispersion in the stress distribution of a bent part becoming large is suppressed. This bent part can be formed by the stabilizer manufacturing device according to the embodiments.

A hollow stabilizer <NUM> according to one of embodiments will be described hereinafter with reference to <FIG>.

<FIG> shows a part of a vehicle <NUM> provided with a hollow stabilizer <NUM>. The hollow stabilizer <NUM> is disposed in a suspension mechanism part <NUM> of the vehicle <NUM>. The hollow stabilizer <NUM> includes a torsion part <NUM> extending in the width direction of a vehicle body <NUM> (direction indicated by arrow W in <FIG>), a pair of bent parts <NUM> and <NUM> continuous with both ends of the torsion part <NUM>, and a pair of arm parts <NUM> and <NUM> continuous with the bent parts <NUM> and <NUM>.

The torsion part <NUM> is supported by, for example, a part of the vehicle body <NUM> via a pair of support parts <NUM> and <NUM> provided with rubber bushes or the like. The pair of arm parts <NUM> and <NUM> are connected to a suspension arm of the suspension mechanism part <NUM> via link members <NUM> and <NUM>, respectively. If loads of opposite phases are input to the arm parts <NUM> and <NUM> when the vehicle <NUM> travels on a curve or the like, a bending force is applied to the arm parts <NUM> and <NUM> and bending and twisting forces are applied to the bent parts <NUM> and <NUM>. Then, the torsion part <NUM> is twisted, and a repulsive load that suppresses rolling of the vehicle body <NUM> is thereby generated.

<FIG> is a plan view schematically showing the hollow stabilizer <NUM>. The material of the hollow stabilizer <NUM> is a pipe <NUM> formed of a metal (for example, spring steel) whose strength can be improved by heat treatment such as quenching. An example of an outer diameter of the pipe <NUM> is <NUM> and a thickness is <NUM>. An example of the radius of curvature (center radius of curvature r) of the bent parts <NUM> and <NUM> is <NUM>. In an endurance test (double swing test), one arm part <NUM> is fixed at fixed point A while a load in the vertical direction is applied to load point B of the other arm part <NUM>.

As shown in <FIG>, the hollow stabilizer <NUM> has a bilaterally symmetric shape with the center in the longitudinal direction used as an axis of symmetry X1. Since the shapes of the bent parts <NUM> and <NUM> are substantially common to each other, the bent part <NUM> will be explained as a representative in the following descriptions. Since the other bent part <NUM> has the same structure, its explanations will be omitted. A specific shape of the hollow stabilizer <NUM> may be a three-dimensionally bent shape or one or more bent parts may be formed in the arm parts <NUM> and <NUM>. In addition, one or more bent parts may be formed in the middle of the longitudinal direction of the torsion part <NUM>.

<FIG> shows a cross section of the bent part <NUM> of the hollow stabilizer <NUM> (the cross section in the radial direction of the pipe <NUM>). <FIG> shows a cross section at a position that an angle θ1 (shown in <FIG>) is formed from a boundary between the torsion part <NUM> and the bent part <NUM>. In this specification, in the cross section in the pipe radial direction (<FIG>), the center of the bending inside (bending center direction) is defined as <NUM>° and the center of the bending outside is defined as <NUM>°.

As shown in <FIG>, the bent part <NUM> includes a first cross-sectional part <NUM>, a second cross-sectional part <NUM>, a third cross-sectional part <NUM>, and a fourth cross-sectional part <NUM> with respect to the cross section in the pipe radial direction. The center of the bending inside is defined as <NUM>° and the center of the bending outside is defined as <NUM>°. The first cross-sectional part <NUM> is in the range from <NUM>° to <NUM>° centered at <NUM>°. The second cross-sectional part <NUM> is in the range from <NUM>° to <NUM>° centered at <NUM>°. The third cross-sectional part <NUM> is in the range of more than <NUM>° and less than <NUM>° centered at <NUM>°. The fourth cross-sectional part <NUM> is in the range of more than <NUM>° and less than <NUM>° centered at <NUM>°. A two-dot-chained line Q1 in <FIG> represents a contour of a surface of the pipe <NUM> to be bent. The cross section of the other bent part <NUM> has the same shape.

A radius of curvature r1 of the first cross-sectional part <NUM> is the distance from first center of curvature C1 (center of the pipe <NUM>) to the surface of the first cross-sectional part <NUM>. A region close to <NUM>° in the first cross-sectional part <NUM> forms a part of a circle (arc) equivalent to the surface of the pipe <NUM> to be bent.

A radius of curvature r2 of the second cross-sectional part <NUM> is the distance from second center of curvature C2 to the surface of the second cross-sectional part <NUM>. The radius of curvature r2 of the second cross-sectional part <NUM> is larger than the radius of curvature r1 of the first cross-sectional part <NUM>. That is, the curvature of the second cross-sectional part <NUM> is smaller than the curvature of the first cross-sectional part <NUM>.

The third cross-sectional part <NUM> has a region indicated by ΔS1 in <FIG>. This region ΔS1 has a nearly flat shape due to contact with a pressing wall <NUM> of a pressing die <NUM> when the bent part <NUM> is bent by the stabilizer manufacturing device <NUM>. The stabilizer manufacturing device <NUM> will be explained later in detail. A radius of curvature r3 of the third cross-sectional part <NUM> is a distance from third center of curvature C3 to the surface of the third cross-sectional part <NUM>. The radius of curvature r3 of the third cross-sectional part <NUM> is larger than the radius of curvature r2 of the second cross-sectional part <NUM>. That is, the curvature of the third cross-sectional part <NUM> is smaller than the curvature of the second cross-sectional part <NUM>. When the third cross-sectional part <NUM> is a perfect plane, the radius of curvature r3 is infinite.

The fourth cross-sectional part <NUM> is in contact a bottom wall <NUM> of a base die <NUM> of the stabilizer manufacturing device <NUM>. As a result, a region indicated by ΔS2 in <FIG> has a nearly flat shape. A radius of curvature r4 of the fourth cross-sectional part <NUM> is the distance from the fourth center of curvature C4 to the surface of the fourth cross-sectional part <NUM>. The radius of curvature r4 of the fourth cross-sectional part <NUM> is larger than the radius of curvature r2 of the second cross-sectional part <NUM>. That is, the curvature of the fourth cross-sectional part <NUM> is smaller than the curvature of the second cross-sectional part <NUM>. When the fourth cross-sectional part <NUM> is a complete plane, the radius of curvature r4 is infinite. The surface of the third cross-sectional part <NUM> and the surface of the fourth cross-sectional part <NUM> are substantially parallel to each other.

The hollow stabilizer <NUM> includes a pair of arm parts <NUM> and <NUM>. <FIG> shows a cross section of the bent part <NUM> in the pipe radial direction. <FIG> shows an example of a relationship (stress distribution) between the circumferential position of the cross section of the bent part <NUM> and the stress generated in the bent part <NUM> when loads having opposite phases are applied to the arm parts <NUM> and <NUM>. A solid line L1 in <FIG> is a stress distribution when the one arm part <NUM> is fixed while a downward load (positive load) is applied to the other arm part <NUM>. When an upward load (negative load) is applied to the arm part <NUM>, the stress distribution is symmetrical with respect to the solid line L1 while the horizontal axis being <NUM>° in <FIG> is set as an axis of symmetry X2.

Conventional dies have been used to bend solid stabilizers. When a pipe is bent using a conventional die, the bent part is flattened excessively and the flatness often exceeds ±<NUM>%. The flatness is the ratio of deformation to the diameter of the pipe. Since the conventional bent part has a large flatness, it cannot be used as a product. Moreover, the shape variation of the inner surface of the crushed part is large. For this reason, the peak of the stress often has been large as illustrated by P1 and P2 in <FIG> and the variation in stress has also been large, in the conventional bent part.

In contrast, the bent part <NUM> of the hollow stabilizer <NUM> of the present embodiment has a cross section in the pipe radial direction as shown in <FIG>. This cross section is not exactly circular, but has a shape close to a circle. The flatness of the cross section of the bent part <NUM> is within ±<NUM>% of the diameter of the pipe. The stabilizer <NUM> of the present embodiment can be formed by a stabilizer manufacturing device <NUM> (shown in <FIG>) explained below. In the cross section of the bent part <NUM> of the present embodiment, the absolute value of the flatness is less than <NUM>%. Such a bent part <NUM> was able to reduce the variation in stress distribution as compared with a bent part bent by a conventional die and having a large flatness.

A compressive residual stress effective for durability can be generated by performing shot peening on the outer surface of the hollow stabilizer <NUM>. However, performing shot peening on the inner surface of the hollow stabilizer <NUM> is actually difficult. It is not desirable that the peak of stress generated on the inner surface of the hollow stabilizer <NUM> (the inner surface of the pipe <NUM>) is high or that the shape variation of the inner surface is large. This is because if a defect such as a scratch exists on the inner surface of the pipe <NUM>, it may become the starting point of breakage. For this reason, the hollow stabilizer <NUM> is desired to particularly minimize the peak of stress on the inner surface side as much as possible. The bent part <NUM> of the hollow stabilizer <NUM> of the present embodiment is a cross section close to a circle in which the flatness is suppressed. For this reason, the peak value of the stress can be lowered as compared with the stress of the conventional bent part having a large flatness.

The stabilizer manufacturing device <NUM> according to the present embodiment will be explained hereinafter with reference to <FIG>. <FIG> is a perspective view showing a part of the stabilizer manufacturing device <NUM>. <FIG> shows a state where a part of the pipe <NUM> (bent part <NUM>) is bent by the stabilizer manufacturing device <NUM>. <FIG> shows a state where the bending step using the stabilizer manufacturing device <NUM> is finished. <FIG> are plan views schematically showing the stabilizer manufacturing device <NUM>, respectively. <FIG> is a cross-sectional view of the stabilizer manufacturing device <NUM> taken along line F11-F11 in <FIG>.

The stabilizer manufacturing device <NUM> includes a base die <NUM>, a clamp die <NUM>, a pressing die <NUM>, a moving die <NUM>, an actuator <NUM> such as a hydraulic cylinder for driving the moving die <NUM>, and the like.

As shown in <FIG>, the base die <NUM> includes a bottom wall <NUM>, a support wall <NUM>, and an arc-shaped forming curved surface <NUM>. A lower surface 40a of the pipe <NUM> is in contact with the bottom wall <NUM>. A side wall 40b of the pipe <NUM> is in contact with the support wall <NUM>. The forming curved surface <NUM> is curved in accordance with the curvature of the bending inside of the bent part <NUM>. The forming curved surface <NUM> is formed between the bottom wall <NUM> and the support wall <NUM>. The forming curved surface <NUM> forms an arc having a quarter of the curvature corresponding to the outer diameter of the pipe <NUM>.

As shown in <FIG>, the forming curved surface <NUM> forms an arc as viewed from the upper side of the base die <NUM>. The radius of curvature of the forming curved surface <NUM> corresponds to a radius of curvature r5 (shown in <FIG>) of the bending inside of the bent part <NUM>. A vertical wall <NUM> is formed continuously to the forming curved surface <NUM>. The pipe <NUM> is placed on the bottom wall <NUM> of the base die <NUM>.

The clamp die <NUM> includes a first clamp wall <NUM> (shown in <FIG>) and a second clamp wall <NUM>. The pipe <NUM> is sandwiched in the radial direction between the first clamp wall <NUM> and the bottom wall <NUM> of the base die <NUM>. The pipe <NUM> is sandwiched in the radial direction between the second clamp wall <NUM> and the support wall <NUM> of the base die <NUM>. An upper surface 40c of the pipe <NUM> is in contact with the first clamp wall <NUM>. The pipe <NUM> is fixed by the base die <NUM> and the clamp die <NUM>.

The pressing die <NUM> is disposed to face the upper side of the bottom wall <NUM> of the base die <NUM>. As shown in <FIG>, the pressing wall <NUM> is formed on the lower surface of the pressing die <NUM>. The pressing wall <NUM> faces the bottom wall <NUM> of the base die <NUM>. A cavity <NUM> into which the pipe <NUM> can enter is formed between the pressing wall <NUM> and the bottom wall <NUM>. An opening width G1 in the vertical direction of the cavity <NUM> is slightly larger than the diameter of the pipe <NUM>.

A tapered surface <NUM> is formed on a part of the pressing die <NUM> (a part of the pressing wall <NUM>). The tapered surface <NUM> faces the bottom wall <NUM> of the base die <NUM>. The opening width G1 shown in <FIG> is a distance between the tapered surface <NUM> and the bottom wall <NUM>. The tapered surface <NUM> is inclined such that the opening width G1 gradually increases toward the opening 82a of the cavity <NUM>. The inclination angle of the tapered surface <NUM>, that is, an angle α formed by the tapered surface <NUM> with respect to the line segment L4 parallel to the bottom wall <NUM> is, for example, approximately <NUM> to <NUM>°. This angle α is a value that changes according to the diameter, thickness, etc., of the pipe <NUM>.

The moving die <NUM> is disposed to face the forming curved surface <NUM> of the base die <NUM> in the horizontal direction. As shown in <FIG>, the moving die <NUM> is attached to the arm <NUM>. When the arm <NUM> is rotated by the actuator <NUM>, the moving die <NUM> moves in the direction of bending the pipe <NUM>. That is, the moving die <NUM> is reciprocally rotated from the initial position (position shown in <FIG> and <FIG>) to the bending end position (position shown in <FIG> and <FIG>) around a shaft <NUM> by the actuator <NUM>.

The moving die <NUM> includes a holding portion <NUM> that holds the pipe <NUM>. The holding portion <NUM> holds a part of the pipe <NUM>, that is, a part 40d closer to the distal end side than a part that becomes the bent part <NUM>. The part 40d on the distal end side of the pipe <NUM> is held by the holding portion <NUM>. In this state, the moving die <NUM> rotates around the shaft <NUM>. Thus, the holding portion <NUM> moves in a direction in which the pipe <NUM> is bent. Then, the part that becomes the bent part <NUM> enters the cavity <NUM> and is pressed against the forming curved surface <NUM>.

As shown in <FIG> and <FIG>, the pipe <NUM> is inserted between the base die <NUM> and the clamp die <NUM>, and the pipe <NUM> is fixed. At this time, the moving die <NUM> is retracted to a position where it does not interfere with the pipe <NUM>. The part 40d on the distal end side of the pipe <NUM> is in a state of protruding to the outside of the base die <NUM>. The pipe <NUM> is heated by a heating means in advance in a warm region of, for example, <NUM> or less (temperature lower than the temperature at which the steel is austenitized). The heated pipe <NUM> has a hardness that enables the pipe to be plastically processed more easily when bent than when it is cold (room temperature). An example of the heating means is a heating furnace, but electric heating or high-frequency induction heating may be employed.

As shown in <FIG> and <FIG>, when the actuator <NUM> is actuated, the moving die <NUM> rotates around the shaft <NUM> toward the vertical wall <NUM> of the base die <NUM>. The part that becomes the bent part <NUM> of the pipe <NUM> enters the cavity <NUM> during the rotation. At this time, the upper surface 40c of the pipe <NUM> moves toward the forming curved surface <NUM> at the back of the cavity <NUM> while contacting the tapered surface <NUM>. For this reason, it is suppressed that the upper surface 40c of the pipe <NUM> is scratched. Then, as shown in <FIG> and <FIG>, the bent part <NUM> is formed by moving the movable die <NUM> to the bending end position.

Thus, the manufacturing method of the hollow stabilizer of the present embodiment comprises the heating step, the placing step, and the bending step. The material of the hollow stabilizer <NUM> is the pipe <NUM>. In the heating step, the pipe <NUM> is heated to a warm region by the heating means. In the placing step, the pipe <NUM> is placed on the base die <NUM> of the stabilizer manufacturing device <NUM>. In the bending step, the bent part <NUM> is formed by bending the pipe <NUM> by the moving die <NUM> in a state where crushing the part that is the bent part <NUM> in a flat shape is restricted by the base die <NUM>, the clamp die <NUM>, and the pressing die <NUM> of the stabilizer manufacturing device <NUM>.

According to the stabilizer manufacturing device <NUM> of the present embodiment, the bent part <NUM> enters the cavity <NUM> while a part of the pipe <NUM> in the longitudinal direction (the bent part <NUM>) is bent. Accordingly, the bottom wall <NUM> and the pressing wall <NUM> can suppress the bent part <NUM> being flattened. The cavity <NUM> is formed between the bottom wall <NUM> and the pressing wall <NUM>. Moreover, the bent part <NUM> is restrained with the upper surface being in contact with the pressing wall <NUM>. For this reason, the third cross-sectional part <NUM> having a small curvature is formed. The bent part <NUM> is restrained with the lower surface being in contact with the bottom wall <NUM>. The fourth cross-sectional part <NUM> having a small curvature is thereby formed. If the bottom wall <NUM> and the pressing wall <NUM> are parallel to each other, the surface of the third cross-sectional part <NUM> and the surface of the fourth cross-sectional part <NUM> are parallel to each other.

When the bent part <NUM> is bent by the stabilizer manufacturing device <NUM>, the outside of the bending is stretched. For this reason, the outside of the bending becomes slightly flat. Therefore, the curvature of the outside of the bending (second cross-sectional part <NUM>) is smaller than the curvature of the inside of the bending (first cross-sectional part <NUM>). That is, the radius of curvature r2 of the second cross-sectional part <NUM> is larger than the radius of curvature r1 of the first cross-sectional part <NUM>.

The third cross-sectional part <NUM> is plastically deformed by being pressed in the radial direction by the pressing wall <NUM> of the pressing die <NUM>. For this reason, the part that is in contact with the pressing wall <NUM> becomes flat. When the pressurization is released, the shape returns slightly, but the surface of the third cross-sectional part <NUM> is nearly flat. For this reason, the curvature of the third cross-sectional part <NUM> is smaller than the curvature of the second cross-sectional part <NUM>.

The fourth cross-sectional part <NUM> is plastically deformed by being pressurized in the radial direction by the bottom wall <NUM> of the base die <NUM>. For this reason, the part that is in contact with the bottom wall <NUM> becomes flat. When the pressurization is released, the shape returns slightly, but the surface of the fourth cross-sectional part <NUM> is nearly flat. For this reason, the curvature of the fourth cross-sectional part <NUM> is smaller than the curvature of the second cross-sectional part <NUM>.

Thus, when the bent part <NUM> is formed by the stabilizer manufacturing device <NUM> of the present embodiment, the cross section of the bent part <NUM> is not a perfect circle correctly, but it is possible to suppress the flatness becoming large. In addition, the tapered surface <NUM> is formed on the lower surface (pressing wall <NUM>) of the pressing die <NUM>. The upper surface of the pipe <NUM> moves toward the forming curved surface <NUM>, along the tapered surface <NUM>, while being bent. For this reason, the upper surface of the bent part <NUM> can be prevented from contacting a side surface <NUM> of the pressing die <NUM> and being scratched.

According to the stabilizer manufacturing device <NUM> of the present embodiment, a "grip" at the end of the pipe, which is required when the pipe is bent by the pipe bender, is unnecessary. For this reason, the bent part of the stabilizer with a short distance from the distal end of the pipe to the bent part can also be bent. Moreover, it is possible to suppress the cross section of the bent part being flattened excessively, and to form a bent part that is closer to a perfect circle with the flatness suppressed. The flatness of the cross section of the bent part is within ±<NUM>% of the diameter of the pipe.

The pipe <NUM> heated to the warm region and having a low deformation resistance tends to have a large flatness at the bent part. According to the stabilizer manufacturing device <NUM> of the present embodiment, however, even if the pipe <NUM> is preheated to a warm region and has a deformation resistance lowered, the bent part <NUM> having the flatness suppressed can be formed when bending is performed.

<FIG> shows a part of a stabilizer manufacturing device 50A according to the other embodiment. In this embodiment, a minute gap ΔG of approximately several tens to several hundreds of µm is formed between an upper surface of a pipe <NUM> placed on a bottom wall <NUM> of a base die <NUM> and a pressing die <NUM>. The pipe <NUM> is allowed to move by a minute amount with respect to the base die <NUM> by the gap ΔG. Since the stabilizer manufacturing device 50A is the same as the stabilizer manufacturing device <NUM> (<FIG>) with respect to the other constituent elements, they are referred to as the same reference numerals common to both of the embodiments and their explanations will be omitted.

<FIG> shows a cross section (cross section of the pipe radial direction) of a curved part <NUM> of a hollow stabilizer <NUM> manufactured by the stabilizer manufacturing device 50A. An outer circumferential surface 40e and an inner circumferential surface 40f of the pipe <NUM> are shown in <FIG>. The outer circumferential surface 40e and an inner circumferential surface 40f of the bent part <NUM> are not perfect circles, but slightly distorted circles as explained below in detail. The flatness of the cross section of the bent part <NUM> is within ±<NUM>% of the diameter of the pipe.

As shown in <FIG>, the cross section in the radial direction of the bent part <NUM> includes eight regions S1 to S8 defined by <NUM>° in the circumferential direction. That is, the cross section includes a first region S1 centered at <NUM>°, a third region S3 centered at <NUM>°, a fifth region S5 centered at <NUM>°, and a seventh region S7 centered at <NUM>°. The first part No. <NUM> is included in the first region S1. The third part No. <NUM> is included in the third region S3. The fifth part No. <NUM> is included in the fifth region S5. The seventh part No. <NUM> is included in the seventh region S7.

Furthermore, the bent part <NUM> includes a second region S2 between the first region S1 and the third region S3, a fourth region S4 between the third region S3 and the fifth region S5, a sixth region S6 between the fifth region S5 and the seventh region S7, and an eighth region S8 between the first region S1 and the seventh region S7. The second part No. <NUM> is included in the second region S2. The fourth part No. <NUM> is included in the fourth region S4. The sixth part No. <NUM> is included in the sixth region S6. The eighth part No. <NUM> is included in the eighth region S8.

In the cross section shown in <FIG>, the first region S1 is defined in a range from <NUM>° to <NUM>°. The first part No. <NUM> exists in the first region S1 centered at <NUM>°. The third region S3 is defined in a range from <NUM>° to <NUM>°. The third part No. <NUM> exists in the third region S3 centered at <NUM>°. The fifth region S5 is defined in a range from <NUM>° to <NUM>°. The fifth part No. <NUM> exists in the fifth region S5 centered at <NUM>°. The seventh region S7 is defined in a range from <NUM>° to <NUM>°. The seventh part No. <NUM> exists in the seventh region S7 centered at <NUM>°.

In the cross section shown in <FIG>, the second region S2 is defined between the first region S1 and the third region S3. The second part No. <NUM> exists in the second region S2 centered at <NUM>°. The fourth region S4 is defined between the third region S3 and the fifth region S5. The fourth part No. <NUM> exists in the fourth region S4 centered at <NUM>°. The sixth region S6 is defined between the fifth region S5 and the seventh region S7. The sixth part No. <NUM> exists in the sixth region S6 centered at <NUM>°. The eighth region S8 is defined between the first region S1 and the seventh region S7. The eighth part No. <NUM> exists in the eighth region S8 centered at <NUM>°.

In <FIG>, R1 to R8 represent radii of curvature of outer surfaces of the first to eighth parts (No. <NUM> to No. <NUM>). In <FIG>, d1 to d8 represent radii of curvature of inner surfaces of the first to eighth parts (No. <NUM> to No. <NUM>).

<FIG> shows a relationship between the positions in the circumferential direction of the curved parts and the radii of curvature of the outer surfaces, of Example <NUM> manufactured by the stabilizer manufacturing device 50A on trial. <FIG> shows a relationship between the positions in the circumferential direction and the radii of curvature of the inner surfaces, of Example <NUM>. The outer diameter of the pipe to be bent is <NUM>, and the thickness of the pipe is <NUM>.

<FIG> shows a relationship between the positions in the circumferential direction of the curved parts and the radii of curvature of the outer surfaces, of Example <NUM> manufactured by the stabilizer manufacturing device 50A on trial. <FIG> shows a relationship between the positions in the circumferential direction and the radii of curvature of the inner surfaces, of Example <NUM>. The outer diameter and the thickness of the pipe to be bent are the same as those in Example <NUM>.

In contrast, <FIG> shows a relationship between the positions in the circumferential direction of the curved parts and the radii of curvature of the outer surfaces, of each of conventional products <NUM>, <NUM>, and <NUM> manufactured by a pipe bender. Similarly, <FIG> shows a relationship between the positions in the circumferential direction of the curved parts and the radii of curvature of the inner surfaces, of each of conventional products <NUM>, <NUM>, and <NUM>. The outer diameter and the thickness of the pipe to be bent in each of the articles are the same as those in Example <NUM>.

<FIG> shows a relationship between the positions in the circumferential direction of the curved parts and the radii of curvature of the outer surfaces, of each of conventional products <NUM>, <NUM>, and <NUM> manufactured by a pipe bender. Similarly, <FIG> shows a relationship between the positions in the circumferential direction of the curved parts and the radii of curvature of the inner surfaces, of each of conventional products <NUM>, <NUM>, and <NUM>. The outer diameter and the thickness of the pipe to be bent in each of the articles are the same as those in Example <NUM>.

Comparison between Examples <NUM> to <NUM> (<FIG>) and conventional products <NUM> to <NUM> (<FIG>) indicates that Examples <NUM> to <NUM> have the following characteristic shape on the outer circumferential surface 40e of the bent part. That is, in Examples <NUM> to <NUM>, the radii of curvature R3 and R7 of the respective outer surfaces of the third part No. <NUM> and the seventh part No. <NUM> are larger and the radii of curvature R2 and R6 of the respective outer surfaces of the second part No. <NUM> and the sixth part No. <NUM> are smaller than the radii of curvature R4 and R5 of the respective outer surfaces of the fourth part No. <NUM> and the fifth part No. <NUM>. This feature cannot be seen in the conventional products <NUM> to <NUM>.

Moreover, in Examples <NUM> to <NUM>, the inner circumferential surface 40f also has a characteristic shape. That is, in Examples <NUM> to <NUM>, the radii of curvature d3 and d7 of the respective inner surfaces of the third part No. <NUM> and the seventh part No. <NUM> are larger and the radii of curvature d2 and d6 of the respective inner surfaces of the second part No. <NUM> and the sixth part No. <NUM> are smaller than the radii of curvature d4 and d5 of the respective inner surfaces of the fourth part No. <NUM> and the fifth part No. <NUM>. This feature cannot be seen in the conventional products <NUM> to <NUM> either.

The hollow stabilizer including a bent part according to Examples <NUM> to <NUM> has a flatness smaller than that of a bent part bent by a conventional pipe bender, and the cross section of the bent part is a shape close to a perfect circle. For this reason, dispersion in the stress distribution of a bent part becoming large is suppressed. The hollow stabilizer having such a bent part can be formed by the stabilizer manufacturing device 50A according to the above-described embodiment.

The present invention can also be applied to a stabilizer of a suspension mechanism of a vehicle other than a car. In carrying out the present invention, it goes without saying that the specific shapes, dimensions, etc., of the torsion part, the arm part, and the bent part can be variously changed including the metal pipe which is the material of the hollow stabilizer.

Claim 1:
A hollow stabilizer (<NUM>) for a vehicle suspension mechanism part,
comprising:
a torsion part (<NUM>);
a bent part (<NUM>, <NUM>) continuous with the torsion part (<NUM>); and
an arm part (<NUM>, <NUM>) continuous with the bent part (<NUM>, <NUM>),
and further comprising, when a center of bending inside is <NUM>° and a center of bending outside is <NUM>° in a cross section in a pipe radial direction of the bent part (<NUM>, <NUM>),
a first cross-sectional part (<NUM>) in a range from <NUM>° to <NUM>° centered at <NUM>°;
characterized by:
a second cross-sectional part (<NUM>) formed in a range from <NUM>° to <NUM>° centered at <NUM>° and having a curvature of an outer surface smaller than that of the first cross-sectional part (<NUM>);
a third cross-sectional part (<NUM>) formed in a range of more than <NUM>° and less than <NUM>° centered at <NUM>° and having a curvature of an outer surface smaller than that of the second cross-sectional part (<NUM>); and
a fourth cross-sectional part (<NUM>) formed in a range of more than <NUM>° and less than <NUM>° centered at <NUM>° and having a curvature of an outer surface smaller than that of the second cross-sectional part (<NUM>).