Coil spring and method for manufacturing same

Abstract

A coil spring according to one embodiment is made of a wire wound in a spiral shape, and at least a portion of the wire has a hardness profile that varies circumferentially about a wire axis.

Description

Technical Field

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

[0002] For example, as described in Patent Document 1, a technique for changing the hardness distribution of a helical spring wire according to the depth from the surface is known. Specifically, in the method described in Patent Document 1, the wire (spring wire) is passed through a high-frequency heating coil, and only its surface layer is heated to a temperature higher than the austenite transformation point. Then, in the next process, after performing a quenching process of cooling its center from a temperature lower than the tempering temperature, a tempering process is performed to heat the entire wire. As a result, a layer with a hardness lower than that of the surface layer and the center is formed inside the wire.

[0003] References

[0004] Patent Document 1 Japanese Patent: Publication No. 6053916. Summary of the Invention

[0005] The required characteristics of the spring may vary depending on the circumferential position of the wire. For example, different mechanical characteristics may be required on the inner diameter side facing the spring axis of the helical spring and the outer diameter side opposite thereto. In the typical manufacturing process of a helical spring, shot peening is performed to apply compressive residual stress to the wire, but the compressive residual stress applied by this shot peening may be unevenly distributed in the circumferential direction of the wire. In the present invention, by considering such a change in the compressive residual stress distribution and adjusting other characteristics, a helical spring with better performance can be achieved. Based on these circumstances and considerations, one object of the present invention is to provide a helical spring and a method for manufacturing the same that can improve the circumferential characteristics of the wire.

[0006] To achieve this object, according to one embodiment, the present invention provides a helical spring made of a wire wound in a spiral shape, and at least a part of the wire has a hardness distribution that varies circumferentially along the axis of the wire.

[0007] For example, the wire has a first layer, a second layer located inside the first layer, and a third layer located inside the second layer. In this case, the hardness of the second layer may be less than that of the first layer and the third layer. The surface of the wire includes a first position and a second position spaced apart from the first position in the circumferential direction. In addition, the first hardness distribution along a first line segment connecting the first position and the axis is different from the second hardness distribution along a second line segment connecting the second position and the axis.

[0008] For example, the first hardness distribution and the second hardness distribution are different in at least one of the width of the first layer, the width of the second layer, the width of the third layer, the minimum value of the hardness of the second layer, and the depth of the minimum value position from the surface.

[0009] Compressive residual stress can be applied to a first range along a first line segment and a second range along a second line segment respectively. In this case, the second range can extend to a deeper surface than the first range, and a part of the second layer along the second line segment can be formed at a deeper surface than a part of the second layer along the first line segment.

[0010] 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.

[0011] The wire can have a first layer and a second layer inside, and the hardness of the first layer is less than that of the second layer. In this case, the first hardness distribution and the second hardness distribution can be different. For example, the difference between the first hardness distribution and the second hardness distribution is caused by at least one of the width of the first layer, the width of the second layer, and the minimum value of the hardness of the first layer being different.

[0012] According to one embodiment, a method for manufacturing a helical spring includes: forming a wire into a helical shape; installing a first terminal and a second terminal, which are connected to a power source capable of supplying alternating current to the wire; passing alternating current through the wire between the first terminal and the second terminal to heat the wire, so as to form a circumferentially varying hardness distribution on at least a part of the wire.

[0013] The manufacturing method can further include: before passing alternating current through the wire, arranging a conductor in a non-grounded state at a position where proximity effect occurs when alternating current passes through the wire.

[0014] The manufacturing method can further include: shot peening the wire formed into a helical shape to impart residual compressive stress to the wire.

[0015] According to the present invention, a helical spring with improved circumferential characteristics of the wire and a manufacturing method thereof can be provided. Description of the Drawings

[0016] Figure 1 It is a schematic perspective view of the helical spring of this embodiment.

[0017] Figure 2 It is a schematic cross-sectional view of a structural example applicable to a helical spring.

[0018] Figure 3 It is a graph of an example of the first hardness distribution along the first line segment.

[0019] Figure 4 It is a graph of an example of the second hardness distribution along the second line segment.

[0020] Figure 5 It is a graph of an example of the first hardness distribution and the first residual stress distribution along the first line segment.

[0021] Figure 6 Graph showing examples of the second hardness distribution and the second residual stress distribution along the second line segment.

[0022] Figure 7 Graph showing the hardness distribution and the residual stress distribution according to the comparative example.

[0023] Figure 8 Flowchart showing an example of a method for manufacturing a helical spring.

[0024] Figure 9 Shows a schematic structure of an alternating current heating device that can be used for surface hardening.

[0025] Figure 10 Is as Figure 9 Schematic side view of a wire, a conductor, and a ferromagnetic body assembled as shown.

[0026] Figure 11 Is a schematic diagram for explaining the proximity effect.

[0027] Figure 12 Graph showing another example of the hardness distribution that can be imparted to the wire. Detailed Description

[0028] An embodiment will be described with reference to the accompanying drawings. The use of the helical spring disclosed in this embodiment is not particularly limited, but in one example, it can be used in the suspension system of a vehicle.

[0029] Figure 1 Is a schematic perspective view of the helical spring 1 according to this embodiment. The helical spring 1 has a wire 2 wound in a helical shape around the spring axis X1. For example, the wire 2 is made of spring steel, and its surface 20 is completely covered with a coating 21. In the following description, an axial direction DX parallel to the spring axis X1 and a radial direction DR centered on the spring axis X1 are defined.

[0030] The helical spring 1 has an effective portion 10, a first end turn portion 11, and a second end turn portion 12. The effective portion 10 is located between the first end turn portion 11 and the second end turn portion 12. For example, the first end turn portion 11 is located within a range of about one turn from the first terminal 2a of the wire 2, and the second end turn portion 12 is located within a range of about one turn from the second terminal 2b of the wire 2. In the effective portion 10, the wire 2 is wound in multiple turns.

[0031] Figure 2 Is a schematic cross-sectional view showing an example of the structure that can be applied to the helical spring 1. This cross-section corresponds to a cross-section perpendicular to the axis X2 of the wire 2. As shown, a circumferential direction Dθ is defined centered on the axis X2. In this embodiment, at least a part of the wire 2 has a hardness distribution that varies along the circumferential direction Dθ. Examples of such a structure will be described below in conjunction with Figure 2 Describe examples of such structures.

[0032] In Figure 2 the example of, the wire 2 has a first layer L1, a second layer L2 located inside the first layer L1, and a third layer L3 located inside the second layer L2. The hardness of the second layer L2 is less than the hardness 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, which will be described in combination with the following Figure 3 and so on.

[0033] The above-mentioned surface 20 corresponds to the outer surface of the first layer L1. The first layer L1 and the second layer L2 are, for example, annular as shown in the figure, but are not limited to this example. That is to say, the first layer L1 and the second layer L2 can be provided on a part of the circumferential direction Dθ.

[0034] The surface 20 of the wire 2 is, for example, a perfect circle centered on the axis X2. On the other hand, in Figure 2 the example of, 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, and their centers are offset from the axis X2. As another example, these boundaries can be perfect circles offset from the axis X2.

[0035] Generally speaking, in order to ensure the anti-sagging property of the helical spring, it is necessary to increase the hardness of the wire. On the other hand, if the hardness of the wire is too high, when corrosion pits appear near the surface, cracks will develop rapidly, increasing the risk of early fracture of the helical spring.

[0036] In contrast, in Figure 2 the structure shown, the hard first layer L1 and the third layer L3 ensure the anti-sagging property of the helical spring 1. In addition, the soft second layer L2 reduces the fracture risk and improves the anti-corrosion fatigue property of the helical spring 1.

[0037] In Figure 2 the structure shown, the hardness distribution of at least a part inside the wire 2 changes along the circumferential direction Dθ. To explain this change in hardness distribution, a first line segment V1 and a second line segment V2 are defined in Figure 2 .

[0038] The first line segment V1 is a straight line connecting a first position Q1 on the inner diameter side of the wire 2 on the surface 20 and the axis X2. The second line segment V2 is a straight line connecting a second position Q2 on the outer diameter side of the wire 2 on the surface 20 and the axis X2. For example, the first position Q1 is the part on the surface 20 closest to the spring axis X1. The second position Q2 is the part on the surface 20 farthest from the spring axis X1. In Figure 2 the example of, the first position Q1, the axis X2, and the second position Q2 are aligned in the radial direction DR.

[0039] Figure 3A graph showing an example of a first hardness distribution H1 along a first line segment V1. Figure 4 A graph showing an example of a second hardness distribution H2 along a second line segment V2. In these figures, the vertical axis represents hardness (e.g., Vickers hardness HV), and the horizontal axis represents the depth from the surface 20 of the wire (distance from the surface 20).

[0040] In the first hardness distribution H1 and the second hardness distribution H2, the hardness gradually decreases at the second layer L2. For example, the hardness of the first layer L1 and the third layer L3 is the same. However, the hardness of the first layer L1 and the third layer L3 can also be different.

[0041] In Figure 3 and Figure 4 example, the hardness distribution of the second layer L2 is V-shaped. This example is not limited to this, and the hardness distribution of the second layer L2 can also change in a smooth curve. In addition, the hardness distribution of the second layer L2 can include a range where the hardness is lower than that of the first layer L1 and the third layer L3 and is substantially constant.

[0042] Here, as Figure 3 shown, in the first hardness distribution H1, the width of the first layer L1 is defined as a1, the width of the second layer L2 is defined as b1, the width of the third layer L3 is defined as c1, the minimum hardness value of the second layer L2 is defined as d1, and the depth of the minimum value position from the surface 20 (the first position Q1) is defined as e1.

[0043] As Figure 4 shown, in the second hardness distribution H2, the width of the first layer L1 is defined as a2, the width of the second layer L2 is defined as b2, the width of the third layer L3 is defined as c2, the minimum hardness value of the second layer L2 is defined as d2, and the depth of the minimum value position from the surface 20 (the second position Q2) is defined as e2.

[0044] By Figure 3 and Figure 4 comparison, it can be seen that in this embodiment, the first hardness distribution H1 and the second hardness distribution H2 are different. For example, this difference between the hardness distributions H1 and H2 is caused by at least one of the widths a1, a2, widths b1, b2, widths c1, c2, minimum values d1, d2, and depths e1, e2 being different.

[0045] In Figure 3 and Figure 4 example, the width a1 is less than the width a2 (a1 < a2), the width b1 is less than the width b2 (b1 < b2), and the width c1 is greater than the width c2 (c1 > c2). In addition, the depth e1 is less than the depth e2 (e1 < e2). The minimum values d1, d2 are, for example, equal, but can also be different.

[0046] Therefore, inFigures 2 to 4 In the example shown, the hardness distribution inside the wire 2 varies according to the position in the circumferential direction Dθ. For example, other characteristics required for each part in the circumferential direction Dθ can be considered to determine the hardness distribution of the wire 2. For example, compressive residual stress imparted to the wire 2 by shot peening or the like can be used. The relationship between the residual stress distribution and the hardness distribution of the wire 2 will be illustrated by examples below.

[0047] Figure 5 FIG. is an example diagram of the first hardness distribution H1 and the first residual stress distribution σ1 along the first line segment V1. Figure 6 FIG. is an example diagram of the second hardness distribution H2 and the second residual stress distribution σ2 along the second line segment V2. In these diagrams, 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 the wire 2. In Figure 5 and Figure 6 In the example of, the position residual stress corresponding to the hardness of the first layer L1 and the third layer L3 is zero. Figure 5 and Figure 6 The hardness distributions H1 and H2 shown in Figure 3 and Figure 4 are the same as those shown in

[0048] For example, when shot peening is performed on the helically wound wire 2, the projectile material is likely to hit the outer diameter side of the surface 20, but is less likely to hit the inner diameter side of the surface 20 or the portion between adjacent wires 2 in the axial direction DX. In this case, compressive residual stress is applied to a farther position on the outer diameter side portion, while compressive residual stress is only applied to a closer position on the inner diameter side portion, etc. Thus, the compressive residual stress applied by shot peening may be non-uniform in the circumferential direction Dθ.

[0049] In Figure 5 In the example of, compressive residual stress is applied from the surface 20 (first position Q1) to the first range f1. In Figure 6 In the example of, compressive residual stress is applied from the surface 20 (second position Q2) to the second range f2. The second range f2 extends to a deeper position than the first range f1.

[0050] In Figure 5 In the example of, the first range f1 to which compressive residual stress is applied overlaps with the entire first layer L1 and extends to a part of the second layer L2. The first range f1 does not extend to the third layer L3. Another example is that the first range f1 can extend to a part of the third layer L3. The peak of the compressive residual stress in the first range f1 is closer to the surface 20 (closer to the first position Q1) than the position where the hardness is the smallest in the first hardness distribution H1.

[0051] Similarly in Figure 6In the example, the second range f2 of the applied compressive residual stress overlaps with the entire first layer L1 and extends to a part of the second layer L2. The second range f2 does not extend to the third layer L3. Another example is that the second range f2 can extend to a part of the third layer L3. The peak of the compressive residual stress in the second range f2 is closer to the surface 20 (closer to the second position Q2) than the position with the minimum hardness in the second hardness distribution H2.

[0052] Thus, in Figure 5 and Figure 6 In the example, the hardness distributions H1 and H2 are formed according to the residual stress distributions σ1 and σ2. Specifically, the second range f2 of the compressive residual stress extends to a deeper position than the first range f1, and the part of the second layer L2 along the second line segment V2 is deeper from the surface 20 than the part of the second layer L2 along the first line segment V1. This results in substantially similar degrees of overlap between the first range f1 and the second layer L2 and between the second range f2 and the second layer L2.

[0053] Figure 7 It is a graph of the hardness distribution Hx and the residual stress distribution σx in the comparative example. In this comparative example, the compressive residual stress is applied from the surface 20 to the 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.

[0054] Generally, in the region where the compressive residual stress is applied and the region where the hardness is reduced, the risk of fracture due to inclusions is low. Thus, in Figure 7 In the comparative example, a region with a relatively low compressive residual stress and a relatively high hardness appears near the boundary between the first layer L1 and the second layer L2. In this region, the risk of fracture due to inclusions increases.

[0055] For example, when the wire 2 has a uniform hardness distribution in the circumferential direction Dθ, the relationship between the hardness distribution Hx and the residual stress distribution σx as shown in Figure 7 the comparative example appears. That is, as described above, the range of the compressive residual stress applied by shot peening is shallower on the inner diameter side and deeper on the outer diameter side. If the hardness distribution of the wire 2 is uniform in the circumferential direction Dθ, the range where the hardness is reduced at any position in the circumferential direction Dθ may not properly overlap with the range where the compressive residual stress is applied.

[0056] If the width of the second layer L2 is widened at each position in the circumferential direction Dθ, the region with a relatively low compressive residual stress will overlap with the region with a relatively low hardness. However, if the proportion of the soft second layer L2 is increased in this way, the anti-sag property of the helical spring 1 can be reduced.

[0057] On the contrary, as in Figure 5 and Figure 6As shown, when forming the hardness distributions H1 and H2 according to the residual stress distributions σ1 and σ2, the risk of fracture caused by inclusions can be suppressed while ensuring anti-sag performance, thereby improving the corrosion fatigue resistance.

[0058] Figures 2 to 6 The hardness distribution and the residual stress distribution described in can be applied to any one of the effective part 10, the first winding part 11, and the second winding part 12. The hardness distributions and the residual stress distributions in the effective part 10, the first winding part 11, and the second winding part 12 can be substantially the same or different from each other.

[0059] The second layer L2 can be provided on a part of the wire 2 in the length direction along the axis X2. The wire 2 can be composed of two layers, namely a hard layer in the center and a soft layer around it, or can be composed of four or more layers with different hardnesses in adjacent layers.

[0060] The hardness distribution of the wire 2 in the circumferential direction Dθ does not necessarily need to be adjusted according to the compressive residual stress, and can also be adjusted according to other characteristics (such as the structure) of the wire 2. As Figures 3 to 6 shown, the hardness distribution of the wire 2 does not necessarily need to be different at all positions in the circumferential direction Dθ. For example, in addition to Figure 2 the first line segment V1 and the second line segment V2 shown, if it is assumed that there is a third line segment connecting the third position on the surface 20 and the axis X2, the hardness distribution along this third line segment can be the same as the first hardness distribution H1 or the second hardness distribution H2.

[0061] Next, a manufacturing method of the helical spring 1 will be described. Here, as an example, it is assumed that the case of manufacturing the helical spring 1 having the Figures 2 to 6 shown structure is considered. Figure 8 is a flowchart of an example of the manufacturing method of the helical spring 1. This example corresponds to so-called hot forming. First, the straight wire 2 is heated (process P1). Then, the wire 2 heated to a high temperature in step P1 is formed into a helical shape using a winding machine (process P2). In processes P1 and P2, the wire 2 is quenched.

[0062] After step P2, surface hardening is performed to reduce the hardness near the surface 20 inside the wire 2 (process P3). In step P3, as Figures 3 to 6 shown, the second layer L2 with reduced hardness is formed. Then, the wire 2 is tempered (process P4).

[0063] After step P4, hot setting is performed, and an overload is applied to the wire 2 while heating it (process P5). Then, shot peening is performed on the wire 2 (process P6). As Figure 5 and Figure 6 shown, in this shot peening process, the wire 2 is given compressive residual stress.

[0064] After step P6, the wire 2 is pre-treated (step P7). In addition, a coating film 21 is formed on the entire wire 2 (step P8). At this step of process P8, the manufacture of the spiral spring 1 is completed.

[0065] Figure 9 FIG. 6 is a schematic structural diagram of an alternating current heating device 100 (hereinafter simply referred to as the heating device 100) that can be used for surface hardening in process P3. The heating device 100 includes a conductor 3, a first terminal 4A, a second terminal 4B, and a control device 5.

[0066] The conductor 3 is, for example, cylindrical and made of a metal material with excellent electrical conductivity, such as copper or aluminum. The conductor 3 can adopt a laminated structure of a conductive layer made of a metal material and an insulating layer made of resin or the like.

[0067] The control device 5 includes a power supply 51 for providing an alternating current. The first terminal 4A and the second terminal 4B are connected to the power supply 51 through a circuit. The frequency of the alternating current provided by the power supply 51 is not particularly limited. For example, a high frequency of 1 kHz or higher can be used.

[0068] In Figure 5 the example of Figure 9 As shown in FIG. 19, the first terminal 4A and the second terminal 4B are each divided into a lower part 41 and an upper part 42. By sandwiching a part of the wire 2 between the lower part 41 and the upper part 42, the first terminal 4A and the second terminal 4B can be fixed to the wire 2. However, the structure for fixing the first terminal 4A and the second terminal 4B to the wire 2 is not limited to this example.

[0069] When heating is performed by the heating device 100, the first terminal 4A and the second terminal 4B are fixed to the wire 2 formed in a spiral shape, and the wire 2 is placed inside the conductor 3. The order of the process of fixing the first terminal 4A and the second terminal 4B to the wire 2 and the process of placing the wire 2 inside the conductor 3 is not particularly limited.

[0070] In Figure 9 the example of Figure 9 As shown in FIG. 20, the first end 2a and the second end 2b (at least partially the winding portions 11, 12) of the wire 2 protrude from both ends of the conductor 3. This example is not limited thereto, and the entire wire 2 can also be wrapped by the conductor 3.

[0071] In Figure 9In the example, the first end 2a of the wire 2 is clamped between the lower part 41 and the upper part 42 of the first terminal 4A. The second end 2b of the wire 2 is clamped between the lower part 41 and the upper part 42 of the second terminal 4B. When the first terminal 4A and the second terminal 4B are connected to the wire 2, a circuit is formed, and these components are connected in series with the power supply 51. The control device 5 starts to energize the wire 2 in response to the switch operation of the operator or the control signal received from the outside. In Figure 9 In. Figure 9 In, the solid arrow indicates an example of the current flow direction. This direction is switched periodically according to the frequency of the power supply 51.

[0072] This current heats at least a part of the wire 2. At this time, the proximity effect (described later) occurs between the conductor 3 and the wire 2. The conductor 3 is located at the position where this proximity effect occurs.

[0073] The frequency, amplitude, and energization time of the alternating current can be appropriately determined according to the shape of the wire 2 (for example, wire diameter, cross-sectional shape, coil diameter, pitch, number of turns, coil length, material), the part to be heated, the heating target temperature, etc. When it reaches the time to stop heating, the control device 5 stops the current supply of the power supply 51.

[0074] Then, the wire 2 is cooled. This cooling can be natural cooling, or when rapid cooling is required, it can be carried out by spraying a fluid such as water or air onto the wire 2. In Figure 9 the example, the heating device 100 includes a cooling mechanism 6 for spraying such a fluid.

[0075] For example, the cooling mechanism 6 includes a plurality of nozzles 61 provided on the inner surface of the conductor 3, a fluid supply source 62 provided in the control device 5, and a pipe 63 connecting each nozzle 61 and the fluid supply source 62. The fluid supply source 62 supplies the fluid to each nozzle 61 through the pipe 63 under the control of the control device 5, for example. At this time, the fluid is sprayed from each nozzle 61 onto the wire 2. It should be noted that the nozzles 61 do not necessarily have to be provided on the conductor 3, and can also be provided on a component different from the conductor 3.

[0076] The heating device 100 may further include a ferromagnetic body 7 provided near the wire 2. The ferromagnetic body 7 can be made of, for example, ferrite, but is not limited to this example. In Figure 9 the example, the ferromagnetic body 7 is inserted inside the wire 2 formed in a spiral shape.

[0077] Figure 10 is as Figure 9Schematic side view of the assembled wire 2, conductor 3, and ferromagnetic body 7 shown. The conductor 3 is, for example, cylindrical and centered on the spring axis X1. The conductor 3 is in a non-grounded state and insulated from other conductive elements such as the wire 2. The conductor 3 is supported by an insulating member (not shown in the figure, for example).

[0078] A gap G1 is formed between the conductor 3 and the wire 2. That is, the inner surface of the conductor 3 faces the outer diameter side portion (including the portion at the second position Q2 described above) of the surface 20 of the wire 2 through the gap G1.

[0079] The ferromagnetic body 7 is, for example, cylindrical with the spring axis X1. The ferromagnetic body 7 can also be other shapes, such as cylindrical with the spring axis X1. The ferromagnetic body 7 is also in an electrically floating state and insulated from other conductive elements such as the wire 2 and the conductor 3. The ferromagnetic body 7 is supported by an insulating member (not shown in the figure, for example).

[0080] A gap G2 is formed between the ferromagnetic body 7 and the wire 2. That is, the outer surface of the ferromagnetic body 7 faces the inner diameter side portion (including the portion at the first position Q1 described above) of the surface 20 of the wire 2 through the gap G2.

[0081] Next, the function of the conductor 3 will be described. When an electric current flows through a workpiece such as the wire 2, the so-called proximity effect occurs if a non-grounded conductor is placed nearby. In this embodiment, this proximity effect is used to control the current density distribution (heating temperature distribution) of the wire 2.

[0082] Figure 11 It is a schematic diagram for explaining the proximity effect, showing a rod-shaped workpiece Ws and a conductor 3s placed nearby. When a power supply current IA flows through the workpiece Ws, a magnetic field HIA (Ampere's law) is generated around the workpiece Ws.

[0083] In the conductor 3s, an eddy current IE1 is generated by this magnetic field HIA (Lenz's law). In addition, a magnetic field HIE caused by the eddy current IE1 is also generated around the conductor 3s. When this magnetic field HIE acts on the workpiece Ws, an eddy current IE2 is generated in the workpiece Ws.

[0084] The directions of the current IA, eddy current IE1, and eddy current IE2 are as shown by the arrows in the figure. That is, in the workpiece Ws, near the side far from the conductor 3s, the directions of the current IA and the eddy current IE2 are opposite. On the other hand, near the side close to the conductor 3s, the direction of the current IA is the same as the direction of the eddy current IE2. Therefore, the current density of the workpiece Ws is higher near the side close to the conductor 3s.

[0085] Using this proximity effect, the current density distribution and heating temperature distribution of the workpiece Ws can be controlled. For example, as Figure 11As shown, when the conductor 3s is arranged to face a part of the outer surface of the workpiece Ws, a current density distribution and a heating temperature distribution that vary according to the circumferential position on the surface and inside of the workpiece Ws can be obtained. These distributions can be appropriately adjusted, for example, by the distance between the conductor 3s and the workpiece Ws.

[0086] In addition, when the wire 3s faces only a part of the longitudinal direction of the workpiece Ws, a current density distribution and a heating temperature distribution that vary with the longitudinal position can be obtained on the surface and inside of the workpiece Ws.

[0087] When an alternating current flows through the wire 2, due to the skin effect, the current density near the surface 20 increases. Utilizing this, the portion near the surface 20 can be intensively heated, and a second layer L2 as shown in Figure 2 is formed in the heated portion. The depth of the second layer L2 from the surface 20 can be adjusted, for example, by the current flow time (heating time), the frequency of the alternating current, and the gap G1 between the wire 2 and the wire 3.

[0088] For the wire 2 formed in a spiral shape, since the current path through the inner diameter portion of the surface 20 is shorter, the current density near the inner diameter portion tends to increase. The heating device 100 controls the current density distribution by using the proximity effect of the conductor 3, thereby imparting a desired hardness distribution that varies circumferentially along at least a part of the wire 2.

[0089] The current density distribution and the heating temperature distribution can also be controlled by the material of the ferromagnetic body 7 and the gap G2 between the wire 2 and the ferromagnetic body 7. The ferromagnetic body 7 has the function of affecting the magnetic flux generated when current passes through the wire 2 and causing the current density to spread toward the outer diameter side. In addition to the proximity effect of the conductor 3, by utilizing the magnetic flux induction function of the ferromagnetic body 7, the current density distribution of the wire 2 can be controlled more precisely, thereby obtaining a suitable hardness distribution.

[0090] It should be noted that the hardness distribution imparted to the wire 2 by the heating device 100 is not limited to the Figures 3 to 6 shown distribution. Figure 12 is a diagram showing another example of the hardness distribution that can be imparted to the wire 2. In the example of Figure 12 , the hardness near the surface 20 is low, and the hardness increases steadily as approaching the axis X2.

[0091] That is to say, in the example of Figure 12 , the wire 2 has a first layer L1 and a second layer L2 located inside the first layer L1, and the hardness of the first layer L1 is less than the hardness of the second layer L2. In the wire 2 having such a hardness distribution in the radial direction DR, the hardness distribution in the circumferential direction Dθ may change as in the example described in reference Figures 2 to 6 .

[0092] Specifically, as Figure 12 and shown in FIG. 13, the wire 2 has a first layer L1 and a second layer L2, and the hardness of the first layer L1 is less than the hardness of the second layer L2. In the wire 2 having such a hardness distribution in the radial direction DR, the hardness distribution in the circumferential direction Dθ may change as in the reference Figures 2 to 6 example described.

[0093] Specifically, as Figures 2 to 6 shown, 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 diameter side and the outer diameter side of the surface 20 of the wire 2 respectively (as Figure 2 shown), or may also be located on other parts of the surface 20.

[0094] The difference between the first hardness distribution H1 and the second hardness distribution H2 may be caused by Figure 12 at least one of the difference in the width a of the first layer L1, the width b of the second layer L2, and the minimum hardness d of the first layer L1 as shown.

[0095] By using the heating device 100, a hardness distribution as in the Figure 12 example can be formed. In addition, the hardness distribution of the shape in the circumferential direction Dθ can also be changed. In addition, according to a manufacturing method combining heat treatment using the heating device 100, a helical spring 1 can be obtained, in which the characteristics of the wire 2 in the circumferential direction Dθ are improved in various ways.

[0096] The above embodiments do not limit the scope of the present invention to the structures disclosed in the embodiments. The present invention can be implemented by making various modifications to the structures disclosed in the embodiments.

[0097] The reference numerals in the accompanying drawings of the specification include:

[0098] 1, helical spring; 2, wire; X1, spring axis; X2, wire axis; 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 helical spring made of a helically wound wire, characterized in that at least a part of the wire has a hardness distribution that varies circumferentially along the axis of the wire.

2. In the helical spring according to claim 1, characterized in that the wire has a first layer, a second layer located inside the first layer, and a third layer located inside the second layer, wherein the hardness of the second layer is less than the hardness of the first layer and the third layer.

3. In the helical spring according to claim 2, characterized in that the surface of the wire includes a first position and a second position circumferentially spaced from the first position, wherein a first hardness distribution along a first line segment connecting the first position and the axis is different from a second hardness distribution along a second line segment connecting the second position and the axis.

4. In the helical spring according to claim 3, characterized in that the first hardness distribution and the second hardness distribution are different in 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 of the second layer, and the depth of the minimum value position from the surface.

5. In the helical spring according to claim 3, characterized in that a first range along the first line segment and a second range along the second line segment are respectively imparted with compressive residual stress, the second range extends to a position deeper from the surface than the first range, and a part of the second layer along the second line segment is formed at a position deeper from the surface than a part of the second layer along the first line segment.

6. In the helical spring according to claim 5, characterized in that 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.

7. In the helical spring according to claim 1, characterized in that the wire has a first layer and a second layer located inside the first layer, and the hardness of the first layer is less than the hardness of the second layer.

8. In the helical spring according to claim 7, characterized in that the surface of the wire includes a first position and a second position circumferentially spaced from the first position, and a first hardness distribution along a first line segment connecting the first position and the axis is different from a second hardness distribution along a second line segment connecting the second position and the axis.

9. In the helical spring according to claim 8, characterized in that the first hardness distribution and the second hardness distribution are different in at least one of the width of the first layer, the width of the second layer, and the minimum hardness value of the first layer.

10. A method for manufacturing a helical spring according to any one of claims 1 to 9, comprising: forming the wire into a helical shape; installing and connecting a first terminal and a second terminal to a power source capable of supplying alternating current to the wire; passing an alternating current through the wire via the first terminal and the second terminal to heat the wire, thereby forming a hardness distribution that varies circumferentially in at least a part of the wire.

11. In the method for manufacturing a helical spring according to claim 10, further comprising: Before allowing an alternating current to flow through a wire, a conductor in a non-grounded state is arranged at a position where a proximity effect is generated when the alternating current flows through the wire.

12. In the method for manufacturing a helical spring according to claim 10, the method further includes: performing shot peening on the wire formed in a helical shape to impart compressive residual stress to the wire.

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