Coil spring and method for manufacturing the same

Abstract

A coil spring according to one embodiment is formed by strands wound in a spiral manner, wherein at least a portion of the strands have a hardness distribution which changes in the circumferential direction centered on the axis of the strands.

Description

[Technical Field]

[0001] The present invention relates to a coil spring and a method for manufacturing the same. [Background technology]

[0002] For example, as described in Patent Document 1, a technique is known for changing the hardness distribution of the wires of a coil spring according to the depth from the surface. Specifically, in the method described in Patent Document 1, a hardening process is performed in which the wire (spring steel wire) is passed through a high-frequency heating coil and heated to a temperature higher than the austenite transformation point, while the center is cooled from a temperature lower than the tempering temperature in the next process. Furthermore, a tempering process is performed to heat the entire wire. As a result, a layer with lower hardness is formed inside the wire than the surface and the area near the center. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 6053916 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] For example, the required properties of a coil spring can differ depending on the position of the wire strands in the circumferential direction, such as the inner diameter side facing the coil axis and the outer diameter side opposite it. For instance, in the general manufacturing process of a coil spring, shot peening is performed to impart compressive residual stress to the wire strands, but the distribution of this compressive residual stress can be non-uniform in the circumferential direction of the wire strands. By considering such changes in the distribution of compressive residual stress and adjusting other properties, it is possible to realize a coil spring that exhibits superior performance.

[0005] This invention was made based on these circumstances, and one of its objectives is to provide a coil spring with improved circumferential characteristics of the wire strands and a method for manufacturing the same. [Means for solving the problem]

[0006] A coil spring according to one embodiment is formed from spirally wound wires, and at least a portion of the wires has a hardness distribution that changes in the circumferential direction around the axis of the wires.

[0007] For example, the wire comprises a first layer, a second layer inside the first layer, and a third layer inside the second layer. In this case, the hardness of the second layer may be less than the hardness of the first and third layers.

[0008] The surface of the wire includes a first position and a second position spaced apart from the first position in the circumferential direction. Furthermore, the first hardness distribution along the first line segment connecting the first position and the axis is different from the second hardness distribution along the second line segment connecting the second position and the axis.

[0009] For example, in the first hardness distribution and the second hardness distribution, at least one of the width of the first layer, the width of the second layer, the width of the third layer, the minimum hardness value in the second layer, and the depth from the surface at the location of the minimum hardness value are different.

[0010] Compressive residual stress may be applied to each of the first range along the first line segment and the second range along the second line segment. In this case, the second range may extend to a position deeper from the surface than the first range, and the portion of the second layer along the second line segment may be formed at a position deeper from the surface than the portion of the second layer along the first line segment.

[0011] In one example, the first position is located on the inner diameter side of the wire, and the second position is located on the outer diameter side of the wire.

[0012] The element wire has a first layer and a second layer inside the first layer, and the hardness of the first layer may be lower than that of the second layer. In this case, the first hardness distribution and the second hardness distribution may be different. For example, the difference between the first hardness distribution and the second hardness distribution is caused by a difference in at least one of the width of the first layer, the width of the second layer, and the minimum value of hardness in the first layer.

[0013] A method of manufacturing a coil spring according to an embodiment includes forming the element wire into a spiral shape, attaching a first terminal and a second terminal connected to a power source capable of supplying alternating current to the element wire, and passing an alternating current through the element wire via the first terminal and the second terminal to heat the element wire, thereby forming a hardness distribution varying in the circumferential direction in at least a portion of the element wire.

[0014] The manufacturing method may further include, before passing the alternating current through the element wire, arranging an electrically floating conductor at a position where a proximity effect is generated when the alternating current is passed through the element wire.

[0015] The manufacturing method may further include performing shot peening on the spirally formed element wire to impart compressive residual stress to the element wire. Effects of the Invention

[0016] According to the present invention, a coil spring with improved characteristics in the circumferential direction of the element wire and a method for manufacturing the same can be provided. Brief Description of the Drawings

[0017] [Figure 1] FIG. 1 is a schematic perspective view of a coil spring according to an embodiment. [Figure 2] FIG. 2 is a schematic cross-sectional view showing an example of a structure applicable to a coil spring. [Figure 3] FIG. 3 is a graph showing an example of a first hardness distribution along a first line segment. [Figure 4]FIG. 4 is a graph showing an example of a second hardness distribution along a second line segment. [Figure 5] FIG. 5 is a graph showing an example of a first hardness distribution and a first residual stress distribution along a first line segment. [Figure 6] FIG. 6 is a graph showing an example of a second hardness distribution and a second residual stress distribution along a second line segment. [Figure 7] FIG. 7 is a graph showing a hardness distribution and a residual stress distribution according to a comparative example. [Figure 8] FIG. 8 is a flowchart showing an example of a method for manufacturing a coil spring. [Figure 9] FIG. 9 is a diagram showing a schematic configuration of an alternating current energization heating apparatus that can be used in surface quenching. [Figure 10] FIG. 10 is a schematic side view of a wire, a conductor and a ferromagnetic member assembled as shown in FIG. 9. [Figure 11] FIG. 11 is a schematic diagram for explaining the proximity effect. [Figure 12] FIG. 12 is a graph showing another example of a hardness distribution that can be imparted to a wire. DESCRIPTION OF EMBODIMENTS

[0018] An embodiment will be described with reference to the drawings. Although the application of the coil spring disclosed in the present embodiment is not particularly limited, it can be used, for example, in a suspension device for a vehicle.

[0019] FIG. 1 is a schematic perspective view of a coil spring 1 according to the present embodiment. The coil spring 1 has a wire 2 helically wound around a coil axis X1. For example, the wire 2 is formed of spring steel, and the entire surface 20 thereof is covered with a coating film 21. In the following description, an axial direction DX parallel to the coil axis X1 and a radial direction DR centered on the coil axis X1 are defined.

[0020] The coil spring 1 has an effective portion 10, a first winding portion 11, and a second winding portion 12. The effective portion 10 is located between the first winding portion 11 and the second winding portion 12. For example, the first winding portion 11 is approximately one turn from the first terminal 2a of the wire 2, and the second winding portion 12 is approximately one turn from the second terminal 2b of the wire 2. The wire 2 is wound multiple times in the effective portion 10.

[0021] Figure 2 is a schematic cross-sectional view showing an example of a structure applicable to the coil spring 1. This cross-section corresponds to a cross-section perpendicular to the axis X2 of the wire strand 2. As shown in the figure, the circumferential direction Dθ is defined around the axis X2. In this embodiment, at least a portion of the wire strand 2 has a hardness distribution that changes in the circumferential direction Dθ. An example of such a structure will be described below based on Figure 2.

[0022] In the example shown in Figure 2, the wire strand 2 has a first layer L1, a second layer L2 inside the first layer L1, and a third layer L3 inside the second layer L2. The hardness of the second layer L2 is lower than that of the first layer L1 and the third layer L3. The hardness of the second layer L2 has a gradient in the radial direction DR, as will be described later with reference to Figure 3 and other figures.

[0023] The aforementioned surface 20 corresponds to the outer surface of the first layer L1. The first layer L1 and the second layer L2 are annular in shape, as shown in the figure, but are not limited to this example. That is, the first layer L1 and the second layer L2 may be provided in a part of the circumferential direction Dθ.

[0024] The surface 20 of the wire strand 2 is, for example, a perfect circle centered on axis X2. On the other hand, in the example in Figure 2, the boundary between the first layer L1 and the second layer L2, and the boundary between the second layer L2 and the third layer L3 are elliptical, with their centers offset from axis X2. As another example, these boundaries may also be perfect circles offset from axis X2.

[0025] Generally, to ensure the resistance of coil springs to sagging, it is necessary to increase the hardness of the wire strands. On the other hand, if the wire strands are hard, the risk of premature breakage of the coil spring increases because the crack propagation will be faster if corrosion pits occur near the surface.

[0026] In contrast, in the configuration shown in Figure 2, the rigid first layer L1 and third layer L3 ensure the resistance to sagging of the coil spring 1. Furthermore, the soft second layer L2 reduces the risk of breakage and improves the corrosion fatigue resistance of the coil spring 1.

[0027] In the configuration shown in Figure 2, the hardness distribution changes in the circumferential direction Dθ in at least a portion of the interior of the wire 2. To explain this change in hardness distribution, the first line segment V1 and the second line segment V2 shown in Figure 2 are defined.

[0028] The first line segment V1 is a straight line connecting the first position Q1, located on the inner diameter side of the wire strand 2 on the surface 20, to the axis X2. The second line segment V2 is a straight line connecting the second position Q2, located on the outer diameter side of the wire strand 2 on the surface 20, to the axis X2. For example, the first position Q1 is the part of the surface 20 closest to the coil axis X1. The second position Q2 is the part of the surface 20 furthest from the coil axis X1. In the example in Figure 2, the first position Q1, axis X2, and second position Q2 are aligned in the radial direction DR.

[0029] Figure 3 is a graph showing an example of the first hardness distribution H1 along the first line segment V1. Figure 4 is a graph showing an example of the second hardness distribution H2 along the second line segment V2. In these graphs, the vertical axis represents hardness (e.g., Vickers hardness HV), and the horizontal axis represents the depth from the surface 20 of the wire 2 (distance from the surface 20).

[0030] In both the first hardness distribution H1 and the second hardness distribution H2, the hardness decreases in the second layer L2. For example, the hardness of the first layer L1 and the third layer L3 are the same. However, the hardness of the first layer L1 and the third layer L3 may be different.

[0031] In the examples of Fig. 3 and Fig. 4, the hardness distribution of the second layer L2 is V-shaped. The hardness distribution of the second layer L2 is not limited to this example, and may change in a smooth curved shape. Further, the hardness distribution of the second layer L2 may include a range where the hardness is generally constant at a value lower than that of the first layer L1 and the third layer L3.

[0032] Here, as shown in Fig. 3, define the width of the first layer L1 in the first hardness distribution H1 as a1, the width of the second layer L2 as b1, the width of the third layer L3 as c1, the minimum hardness value in the second layer L2 as d1, and the depth of the position of the minimum value from the surface 20 (the first position Q1) as e1.

[0033] Further, as shown in Fig. 4, define the width of the first layer L1 in the second hardness distribution H2 as a2, the width of the second layer L2 as b2, the width of the third layer L3 as c2, the minimum hardness value in the second layer L2 as d2, and the depth of the position of the minimum value from the surface 20 (the second position Q2) as e2.

[0034] As can be seen from a comparison between Fig. 3 and Fig. 4, in the present embodiment, the first hardness distribution H1 and the second hardness distribution H2 are different. For example, such a difference between the hardness distributions H1 and H2 is caused by a difference in at least one of the widths a1 and a2, the widths b1 and b2, the widths c1 and c2, the minimum values d1 and d2, and the depths e1 and e2.

[0035] In the examples of Fig. 3 and Fig. 4, the width a1 is smaller than the width a2 (a1<a2), the width b1 is smaller than the width b2 (b1<b2), the width c1 is larger than the width c2 (c1>c2). Further, the depth e1 is smaller than the depth e2 (e1<e2). The minimum values d1 and d2 are, for example, equal, but may be different.

[0036] As described above, in the examples shown in Fig. 2 to Fig. 4, the hardness distribution inside the wire 2 varies depending on the position in the circumferential direction Dθ. The hardness distribution of the wire 2 can be determined, for example, in consideration of other characteristics required for each part in the circumferential direction Dθ. An example of the other characteristics is compressive residual stress applied to the wire 2 by shot peening or the like. An example of the relationship between the residual stress distribution and the hardness distribution of the wire 2 will be described below.

[0037] Figure 5 is a graph showing an example of the first hardness distribution H1 and the first residual stress distribution σ1 along the first line segment V1. Figure 6 is a graph showing an example of the second hardness distribution H2 and the second residual stress distribution σ2 along the second line segment V2. In these graphs, the left vertical axis represents hardness, the right vertical axis represents residual stress, and the horizontal axis represents the depth from the surface 20 of wire 2. In the examples in Figures 5 and 6, the position where the residual stress is zero coincides with the hardness of the first layer L1 and the third layer L3. The hardness distributions H1 and H2 shown in Figures 5 and 6 are the same as those shown in Figures 3 and 4.

[0038] For example, when shot peening is applied to a spirally wound wire 2, the projectile material is more likely to hit the outer diameter portion of the surface 20, but less likely to hit the inner diameter portion of the surface 20 or the portion located between adjacent wires 2 in the axial direction DX. In this case, compressive residual stress is applied to a deep position on the outer diameter portion, but only to a shallow position on the inner diameter portion. Thus, the compressive residual stress applied by shot peening can be non-uniform in the circumferential direction Dθ.

[0039] In the example in Figure 5, compressive residual stress is applied from surface 20 (first position Q1) to the first range f1. In the example in Figure 6, compressive residual stress is applied from surface 20 (second position Q2) to the second range f2. The second range f2 extends to a deeper position than the first range f1.

[0040] In the example in Figure 5, the first range f1 to which compressive residual stress is applied overlaps with the entire first layer L1 and extends to a portion of the second layer L2. The first range f1 does not extend to the third layer L3. In another example, the first range f1 may extend to a portion of the third layer L3. The peak of the compressive residual stress in the first range f1 is located on the surface 20 side (first position Q1 side) of the position where the hardness is minimum in the first hardness distribution H1.

[0041] Furthermore, in the example in Figure 6, the second range f2 to which compressive residual stress is applied overlaps with the entire first layer L1 and extends to a portion of the second layer L2. The second range f2 does not extend to the third layer L3. In another example, the second range f2 may extend to a portion of the third layer L3. The peak of compressive residual stress in the second range f2 is located on the surface 20 side (second position Q2 side) of the position where the hardness is minimum in the second hardness distribution H2.

[0042] As shown above, in the examples in Figures 5 and 6, hardness distributions H1 and H2 are formed according to the residual stress distributions σ1 and σ2. Specifically, the second range f2 of compressive residual stress extends to a deeper position than the first range f1, and the portion of the second layer L2 along the second line segment V2 is formed at a deeper position from the surface 20 than the portion of the second layer L2 along the first line segment V1. As a result, the overlap between the first range f1 and the second layer L2, and the overlap between the second range f2 and the second layer L2, are substantially the same.

[0043] Figure 7 is a graph showing the hardness distribution Hx and residual stress distribution σx for the comparative example. In this comparative example, compressive residual stress is applied from surface 20 over a range fx. The range fx overlaps with the first layer L1, but does not overlap with the second layer L2 and the third layer L3.

[0044] Generally, the risk of fracture due to inclusions is reduced in areas where compressive residual stress is applied and hardness is reduced. In this regard, in the comparative example shown in Figure 7, an area with low compressive residual stress and high hardness occurs near the boundary between the first layer L1 and the second layer L2. In this area, the risk of fracture due to inclusions increases.

[0045] The relationship between the hardness distribution Hx and the residual stress distribution σx, as shown in the comparative example in Figure 7, can occur, for example, when a uniform hardness distribution is applied to the wire 2 in the circumferential direction Dθ. That is, as described above, the range in which compressive residual stress is applied by shot peening is shallower on the inner diameter side and deeper on the outer diameter side. If the hardness distribution in the wire 2 is uniform in the circumferential direction Dθ, there is a possibility that the range in which hardness is reduced and the range in which compressive residual stress is applied do not suitably overlap at any point in the circumferential direction Dθ.

[0046] If the width of the second layer L2 is increased at each position in the circumferential direction Dθ, the region with low compressive residual stress will overlap with the region with low hardness. However, increasing the proportion of the soft second layer L2 in this way may reduce the sag resistance of the coil spring 1.

[0047] In contrast, when hardness distributions H1 and H2 are formed according to residual stress distributions σ1 and σ2, as shown in the examples in Figures 5 and 6, corrosion fatigue resistance can be improved while ensuring resistance to deformation and suppressing the risk of fracture caused by inclusions.

[0048] The hardness distribution and residual stress distribution described using Figures 2 to 6 can be applied to any of the effective portion 10, the first seat winding portion 11, and the second seat winding portion 12. The hardness distribution and residual stress distribution in the effective portion 10, the first seat winding portion 11, and the second seat winding portion 12 may be substantially the same or may be different from each other.

[0049] The second layer L2 may be provided on a portion of the wire 2 in the longitudinal direction along the axis X2. The wire 2 may also be formed of two layers: a hard central layer and a softer layer surrounding it, or it may be formed of four or more layers with different hardness levels in adjacent layers.

[0050] The hardness distribution of wire 2 in the circumferential direction Dθ does not necessarily need to be adjusted according to the compressive residual stress, but may be adjusted according to other properties of wire 2, such as its microstructure.

[0051] The hardness distribution of the wire 2, as illustrated in Figures 3 to 6, does not necessarily have to be different at all positions in the circumferential direction Dθ. For example, if we consider a third line segment connecting a third position on the surface 20 and axis X2, in addition to the first line segment V1 and second line segment V2 shown in Figure 2, the hardness distribution along the third line segment may be the same as either the first hardness distribution H1 or the second hardness distribution H2.

[0052] Next, we will explain the manufacturing method of the coil spring 1. Here, as an example, we will assume the manufacturing of a coil spring 1 having the configuration shown in Figures 2 to 6.

[0053] Figure 8 is a flowchart showing an example of a method for manufacturing a coil spring 1. This example corresponds to so-called hot forming, in which a straight wire 2 is first heated (step P1). Furthermore, the wire 2, which has become hot due to the heating in step P1, is formed into a spiral shape by a coiling machine (step P2). In these steps P1 and P2, the wire 2 is hardened.

[0054] After step P2, surface hardening is performed on the wire 2 near the surface 20 to reduce its internal hardness (step P3). Step P3 forms a second layer L2 with reduced hardness, as shown in Figures 3 to 6. Subsequently, the wire 2 is tempered (step P4).

[0055] After step P4, the wire strands 2 are heated and subjected to hot setting, which applies an overload to the wire strands 2 (step P5). Furthermore, the wire strands 2 are subjected to shot peening (step P6). In this shot peening, compressive residual stress is applied to the wire strands 2, for example, as shown in Figures 5 and 6.

[0056] After step P6, presetting is applied to the wire strands 2 (step P7). Furthermore, a coating 21 is formed on the entire surface of the wire strands 2 (step P8). With step P8, the coil spring 1 is completed.

[0057] Figure 9 shows a schematic configuration of an AC current heating device 100 (hereinafter referred to as the heating device 100) that can be used in the surface hardening process P3. The heating device 100 comprises a conductor 3, a first terminal 4A, a second terminal 4B, and a control device 5.

[0058] The conductor 3 is, for example, cylindrical and made of a highly conductive metal material such as copper or aluminum. The conductor 3 may have a laminated structure of a conductive layer made of a metal material and an insulating layer made of resin or the like.

[0059] The control device 5 is equipped with a power supply 51 that supplies alternating current. The first terminal 4A and the second terminal 4B are connected to the power supply 51 via wiring. The frequency of the alternating current supplied by the power supply 51 is not particularly limited, but in one example, a high frequency of 1 kHz or higher may be used.

[0060] In the example shown in Figure 9, the first terminal 4A and the second terminal 4B are each divided into a lower part 41 and an upper part 42. The first terminal 4A and the second terminal 4B can be attached to the wire 2 by sandwiching a portion of the wire 2 between the lower part 41 and the upper part 42. However, the structure for attaching the first terminal 4A and the second terminal 4B to the wire 2 is not limited to this example.

[0061] When the heating device 100 performs the heat treatment, the first terminal 4A and the second terminal 4B are attached to the spirally formed strand 2, and the strand 2 is positioned inside the conductor 3. The order in which the steps of attaching the first terminal 4A and the second terminal 4B to the strand 2 and positioning the strand 2 inside the conductor 3 are performed is not particularly limited.

[0062] In the example shown in Figure 9, the vicinity of the first terminal 2a and the second terminal 2b of the strand 2 (at least a portion of the winding portions 11 and 12) protrudes from both ends of the conductor 3. This example is not limited to this one, and the entire strand 2 may be enclosed by the conductor 3.

[0063] In the example shown in Figure 9, the area near the first terminal 2a of the wire strand 2 is held between the lower part 41 and the upper part 42 of the first terminal 4A. Furthermore, the area near the second terminal 2b of the wire strand 2 is held between the lower part 41 and the upper part 42 of the second terminal 4B.

[0064] When the first terminal 4A and the second terminal 4B are attached to the wire 2, a circuit is formed in which these elements and the power supply 51 are connected in series. The control device 5 starts supplying power to the wire 2 in response to the operation of a switch by a worker or the reception of an external control signal. In Figure 9, a solid arrow shows an example of the direction of current flow. This direction switches periodically according to the frequency of the power supply 51.

[0065] This current conduction heats at least a portion of the wire strand 2. At this time, a proximity effect, described later, occurs between the conductor 3 and the wire strand 2. The conductor 3 is positioned in such a location that this proximity effect occurs.

[0066] The frequency, amplitude, and energizing time of the alternating current can be appropriately determined according to the shape of the wire 2 (e.g., wire diameter, cross-sectional shape, coil diameter, pitch, number of turns, coil length, material), the part to be heated, and the target heating temperature. When it is time to stop heating, the control device 5 stops supplying current from the power supply 51.

[0067] Subsequently, the wire strands 2 are cooled. This cooling may be done naturally, or if rapid cooling is required, it may be done by blowing a fluid such as water or air onto the wire strands 2. In the example shown in Figure 9, the heating device 100 is equipped with a cooling mechanism 6 for blowing such a fluid.

[0068] For example, the cooling mechanism 6 includes a number of nozzles 61 arranged on the inner surface of the conductor 3, a fluid supply source 62 provided in the control device 5, and piping 63 connecting each nozzle 61 to the fluid supply source 62. The fluid supply source 62 supplies fluid to each nozzle 61 via the piping 63, for example, under the control of the control device 5. At this time, fluid is ejected from each nozzle 61 toward the strands 2. Note that the nozzles 61 do not necessarily have to be provided on the conductor 3, and may be provided on a member other than the conductor 3.

[0069] The heating device 100 may further include a ferromagnetic material 7 that can be placed near the wire strand 2. The ferromagnetic material 7 can be formed from, for example, ferrite, but is not limited to this example. In the example in Figure 9, the ferromagnetic material 7 is inserted inside the spirally shaped wire strand 2.

[0070] Figure 10 is a schematic side view of the wire strands 2, conductor 3, and ferromagnetic material 7 assembled as shown in Figure 9. The conductor 3 is cylindrical, for example, centered on the coil axis X1. The conductor 3 is electrically floating and insulated from other conductive elements such as the wire strands 2. The conductor 3 is supported, for example, by an insulating member (not shown).

[0071] A gap G1 is formed between the conductor 3 and the strand 2. That is, the inner surface of the conductor 3 faces the outer diameter portion of the surface 20 of the strand 2 (the portion including the second position Q2 mentioned above) via the gap G1.

[0072] The ferromagnetic material 7 is, for example, cylindrical around the coil axis X1. The ferromagnetic material 7 may also have other shapes, such as cylindrical around the coil axis X1. The ferromagnetic material 7 is also electrically floating and is insulated from other conductive elements such as the strands 2 and conductors 3. The ferromagnetic material 7 is supported, for example, by an insulating member (not shown).

[0073] A gap G2 is formed between the ferromagnetic material 7 and the wire 2. That is, the outer surface of the ferromagnetic material 7 faces the inner diameter portion of the surface 20 of the wire 2 (the portion including the first position Q1 described above) via the gap G2.

[0074] Next, the role of the conductor 3 will be described. When a current flows through a workpiece such as the element wire 2, if an electrically floating conductor is disposed in the vicinity thereof, the so-called proximity effect occurs. In the present embodiment, the current density distribution (heating temperature distribution) of the element wire 2 is controlled by utilizing this proximity effect.

[0075] FIG. 11 is a schematic diagram for explaining the proximity effect, showing a rod-shaped workpiece Ws and a conductor 3s disposed in the vicinity thereof. A current I from a power supply is supplied to the workpiece Ws A flows, a magnetic field H is generated around the workpiece Ws IA is generated (Ampère's law).

[0076] In the conductor 3s, this magnetic field H IA causes an eddy current I E1 is generated (Lenz's law). Further, the eddy current I E1 causes a magnetic field H IE is generated around the conductor 3s. When this magnetic field H IE acts on the workpiece Ws, an eddy current I is generated in the workpiece Ws E2 is generated.

[0077] The current I A , the eddy current I E1 and the eddy current I E2 flow in the directions as indicated by the arrows in the figure. That is, in the workpiece Ws, in the vicinity of the side surface far from the conductor 3s, the direction in which the current I A flows and the direction in which the eddy current I E2 flows are opposite to each other. On the other hand, in the vicinity of the side surface close to the conductor 3s, the direction in which the current I A flows and the direction in which the eddy current I E2 flows match each other. As a result, the current density of the workpiece Ws becomes higher in the vicinity of the side surface on the side close to the conductor 3s.

[0078] By utilizing this proximity effect, it is possible to control the current density distribution and heating temperature distribution of the workpiece Ws. For example, if the conductor 3s is positioned so as to face a portion of the outer surface of the workpiece Ws as shown in Figure 11, a current density distribution and heating temperature distribution that vary depending on the circumferential position can be obtained on the surface and inside the workpiece Ws. These distributions can be appropriately adjusted, for example, by the distance between the conductor 3s and the workpiece Ws.

[0079] Furthermore, if the conductor 3s is positioned so as to face only a portion of the workpiece Ws along its length, it is possible to obtain a current density distribution and heating temperature distribution that vary depending on the position along the length of the workpiece Ws on its surface and inside.

[0080] When alternating current flows through the wire 2, the current density increases near the surface 20 due to the skin effect. By utilizing this, the portion near the surface 20 can be heated intensively, and a second layer L2, as shown in Figure 2, can be formed in the heated portion. The depth of the second layer L2 from the surface 20 can be adjusted by, for example, the energizing time (heating time), the frequency of the alternating current, and the gap G1 between the wire 2 and the conductor 3.

[0081] In the spirally formed strand 2, the current path passing through the inner diameter portion of the surface 20 is short, so the current density tends to be high near that inner diameter portion. The heating device 100 controls this current density distribution by the proximity effect of the conductor 3, thereby imparting a desired hardness distribution that changes in the circumferential direction Dθ to at least a portion of the strand 2.

[0082] The current density distribution and heating temperature distribution can also be controlled by the material of the ferromagnetic material 7 and the gap G2 between the wire 2 and the ferromagnetic material 7. The ferromagnetic material 7 has the function of influencing the magnetic flux generated when current is passed through the wire 2, thereby spreading the current density towards the outer diameter. By using the magnetic flux induction function of the ferromagnetic material 7 in addition to the proximity effect of the conductor 3, the current density distribution of the wire 2 can be controlled with greater precision, resulting in a suitable hardness distribution.

[0083] The hardness distribution imparted to the wire strand 2 by the heating device 100 is not limited to those shown in Figures 3 to 6. Figure 12 is a graph showing another example of a hardness distribution that can be imparted to the wire strand 2. In the example in Figure 12, the hardness is low near the surface 20 and increases smoothly as it approaches the axis X2.

[0084] In other words, in the example shown in Figure 12, the wire 2 has a first layer L1 and a second layer L2 inside it, and the hardness of the first layer L1 is less than the hardness of the second layer L2. In a wire 2 having such a hardness distribution in the radial direction DR, the hardness distribution in the circumferential direction Dθ may change, similar to the example described using Figures 2 to 6.

[0085] Specifically, as in the examples in Figures 2 to 6, the first hardness distribution H1 along the first line segment V1 connecting the first position Q1 and the axis X2 may be different from the second hardness distribution H2 along the second line segment V2 connecting the second position Q2 and the axis X2. The first position Q1 and the second position Q2 may be located on the inner and outer diameter sides of the surface 20 of the wire 2, respectively, as shown in Figure 2, or they may be located on other parts of the surface 20.

[0086] The difference between the first hardness distribution H1 and the second hardness distribution H2 may result from differences in at least one of the width a of the first layer L1, the width b of the second layer L2, and the minimum hardness d in the first layer L1, as shown in Figure 12.

[0087] By using the heating device 100, a hardness distribution like the example in Figure 12 can be formed. Furthermore, it is possible to change the hardness distribution of this shape in the circumferential direction Dθ. In addition, by using a manufacturing method that incorporates heat treatment with the heating device 100, a coil spring 1 can be obtained in which the characteristics of the wire strands 2 in the circumferential direction Dθ are improved in various ways.

[0088] The above embodiments do not limit the scope of the present invention to the configurations disclosed in those embodiments. The present invention can be implemented by modifying the configurations disclosed in those embodiments in various ways. [Explanation of Symbols]

[0089] 1...coil spring, 2...wire, X1...coil shaft, X2...wire shaft, DX...axial direction, DR...radial direction, Dθ...circumferential direction, L1...first layer, L2...second layer, L3...third layer, H1...first hardness distribution, H2...second hardness distribution, σ1...first residual stress distribution, σ2...second residual stress distribution, 100...heating device.

Claims

1. A coil spring formed by spirally wound wires, At least a portion of the aforementioned strand has a first layer and a second layer inside the first layer, The hardness of the second layer is less than the hardness of the first layer. The surface of at least a portion of the wire includes a first position and a second position spaced apart from the first position in the circumferential direction about the axis of the wire. The first hardness distribution along the first line segment connecting the first position and the axis is different from the second hardness distribution along the second line segment connecting the second position and the axis. Compressive residual stress is applied to each of the following: a first range along the first line segment and a second range along the second line segment. The second range extends to a position deeper from the surface than the first range. The width of the first layer along the second line segment is greater than the width of the first layer along the first line segment. The first range overlaps with the entirety of the first layer along the first line segment and extends to a part of the second layer. The second range overlaps with the entirety of the first layer along the second line segment and extends to a portion of the second layer. Coil spring.

2. A coil spring formed by spirally wound wires, At least a portion of the aforementioned strand has a first layer, a second layer inside the first layer, and a third layer inside the second layer. The hardness of the second layer is less than the hardness of the first and third layers. The surface of at least a portion of the wire includes a first position and a second position spaced apart from the first position in the circumferential direction about the axis of the wire. The first hardness distribution along the first line segment connecting the first position and the axis is different from the second hardness distribution along the second line segment connecting the second position and the axis. Compressive residual stress is applied to each of the following: a first range along the first line segment and a second range along the second line segment. The second range extends to a position deeper from the surface than the first range. The width of the first layer along the second line segment is greater than the width of the first layer along the first line segment. Coil spring.

3. In the first hardness distribution and the second hardness distribution, at least one of the width of the second layer, the width of the third layer, the minimum hardness value in the second layer, and the depth from the surface at the location of the minimum hardness value is different. The coil spring according to claim 2.

4. The portion of the second layer along the second line segment is formed at a deeper position from the surface than the portion of the second layer along the first line segment. The coil spring according to claim 2.

5. A coil spring formed by spirally wound wires, At least a portion of the aforementioned strand has a first layer and a second layer inside the first layer, The hardness of the second layer is less than the hardness of the first layer. The surface of at least a portion of the wire includes a first position and a second position spaced apart from the first position in the circumferential direction about the axis of the wire. The first hardness distribution along the first line segment connecting the first position and the axis is different from the second hardness distribution along the second line segment connecting the second position and the axis. Compressive residual stress is applied to each of the following: a first range along the first line segment and a second range along the second line segment. The second range extends to a position deeper from the surface than the first range. The width of the first layer along the second line segment is greater than the width of the first layer along the first line segment. The first position is located on the inner diameter side of the wire, The second position is located on the outer diameter side of the wire. Coil spring.

6. A method for manufacturing a coil spring, The wire is formed into a spiral shape, The first and second terminals, connected to a power source capable of supplying alternating current, are attached to the strand. Before passing the alternating current through the strand, an electrically floating conductor is placed in a position that produces a proximity effect when the alternating current is passed through the strand. By passing the alternating current through the first and second terminals to the wire and heating the wire, a hardness distribution that changes in the circumferential direction around the axis of the wire is formed in at least a portion of the wire. A method for manufacturing a coil spring, including the following.

7. The method further includes applying shot peening to the spirally formed wire to impart compressive residual stress to the wire. The method for manufacturing a coil spring according to claim 6.

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