Coil springs and how they are manufactured.

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

Coil spring and manufacturing method thereof

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

[0002] For example, a technique for varying the hardness distribution of a coil spring wire depending on the depth from the surface is known, as described in Patent Document 1. Specifically, in the method described in Patent Document 1, a wire (spring steel wire) is passed through a high-frequency heating coil to heat only the surface layer to a temperature higher than the austenite transformation point, and the central part is cooled from a temperature lower than the tempering temperature in the subsequent step in a quenching process, followed by a tempering process in which the entire wire is heated. As a result, a layer is formed inside the wire that is lower in hardness than the surface layer and the central part.

[0003] Patent No. 6053916

[0004] For example, the required properties may differ depending on the position in the circumferential direction of the wire, such as on the inner diameter side facing the coil axis of the coil spring and the outer diameter side opposite it. For example, in the manufacturing process of a typical coil spring, shot peening is performed to impart compressive residual stress to the wire, but the distribution of the compressive residual stress imparted by this shot peening may be uneven in the circumferential direction of the wire. By taking into account such changes in the distribution of compressive residual stress and adjusting other properties, it is possible to achieve a coil spring that exhibits better performance.

[0005] The present invention has been made in light of the above circumstances, and one of its objects is to provide a coil spring having improved characteristics in the circumferential direction of the wire, and a method for manufacturing the same.

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

[0007] For example, the wire may have a first layer, a second layer inside the first layer, and a third layer inside the second layer, and in this case, the hardness of the second layer may be smaller than the hardness of the first layer and the third layer.

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

[0009] For example, the first hardness distribution and the second hardness distribution differ 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 hardness in the second layer, and the depth from the surface of the position of the minimum value.

[0010] A compressive residual stress may be imparted to each of a first range along the first line segment and a 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 a portion of the second layer along the second line segment may be formed at a position deeper from the surface than a portion of the second layer along the first line segment.

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

[0012] The wire may have a first layer and a second layer inside the first layer, and the hardness of the first layer may be smaller than the hardness 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 the hardness of the first layer.

[0013] A method for manufacturing a coil spring according to one embodiment includes forming the 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 wire, and passing an alternating current through the wire through the first terminal and the second terminal to heat the wire, thereby forming a hardness distribution that varies in the circumferential direction in at least a portion of the wire.

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

[0015] The manufacturing method may further include subjecting the wire formed into a spiral shape to shot peening to impart compressive residual stress to the wire.

[0016] According to the present invention, it is possible to provide a coil spring having improved characteristics in the circumferential direction of the wire, and a method for manufacturing the same.

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

[0018] An embodiment will be described with reference to the drawings. The use of the coil spring disclosed in this embodiment is not particularly limited, but as an example, it can be used in a suspension system for a vehicle.

[0019] 1 is a schematic perspective view of a coil spring 1 according to this embodiment. The coil spring 1 has a wire 2 wound in a spiral shape around a coil axis X1. For example, the wire 2 is formed from spring steel, and its surface 20 is entirely covered with a coating 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 end winding portion 11, and a second end winding portion 12. The effective portion 10 is located between the first end winding portion 11 and the second end winding portion 12. For example, the first end winding portion 11 is in a range of approximately one turn from the first terminal 2a of the wire 2, and the second end winding portion 12 is in a range of 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] 2 is a schematic cross-sectional view showing an example of a structure that can be applied to the coil spring 1. This cross-section corresponds to a transverse cross-section of the wire 2 that intersects perpendicularly with the axis X2. As shown in the figure, a circumferential direction Dθ is defined with the axis X2 as the center. In this embodiment, at least a portion of the wire 2 has a hardness distribution that varies in the circumferential direction Dθ. An example of such a structure will now be described with reference to FIG. 2.

[0022] In the example shown in Fig. 2, the wire 2 includes 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 lower than the hardnesses 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 Fig. 3 and other figures.

[0023] The 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, 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 2 is, for example, a perfect circle centered on the axis X2. On the other hand, in the example of Fig. 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, and the centers of these elliptical ellipses are offset from the axis X2. As another example, these boundaries may be perfect circles offset from the axis X2.

[0025] Generally, to ensure the sag resistance of a coil spring, it is necessary to increase the hardness of the wire.However, if the wire is too hard, corrosion pits that occur near the surface will cause cracks to grow more quickly, increasing the risk of early breakage of the coil spring.

[0026] 2, the hard first layer L1 and third layer L3 ensure the settling resistance of the coil spring 1. Furthermore, the soft second layer L2 reduces the risk of breakage and also increases the corrosion fatigue resistance of the coil spring 1.

[0027] 2, the hardness distribution changes in the circumferential direction Dθ in at least a part of the inside of the wire 2. In order to explain such a change in hardness distribution, a first line segment V1 and a second line segment V2 shown in FIG.

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

[0029] Fig. 3 is a graph showing an example of a first hardness distribution H1 along a first line segment V1. Fig. 4 is a graph showing an example of a second hardness distribution H2 along a second line segment V2. In these graphs, the vertical axis represents hardness (e.g., Vickers hardness HV), and the horizontal axis represents 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 is the same as that of the third layer L3. However, the hardness of the first layer L1 may be different from that of the third layer L3.

[0031] 3 and 4, the hardness distribution of the second layer L2 is V-shaped. However, the hardness distribution of the second layer L2 may vary along a smooth curve. Furthermore, the hardness distribution of the second layer L2 may include a range in which the hardness is generally constant and lower than the hardness of the first layer L1 and the third layer L3.

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

[0033] Furthermore, as shown in FIG. 4, 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 value of hardness in the second layer L2 is defined as d2, and the depth from the surface 20 (second position Q2) at the position of the minimum value is defined as e2.

[0034] 3 and 4, in this embodiment, the first hardness distribution H1 and the second hardness distribution H2 are different from each other. For example, the difference between the hardness distributions H1 and H2 is caused by a difference in at least one of the widths a1 and a2, widths b1 and b2, widths c1 and c2, minimum values ​​d1 and d2, and depths e1 and e2.

[0035] 3 and 4, width a1 is smaller than width a2 (a1<a2), width b1 is smaller than width b2 (b1<b2), width c1 is larger than width c2 (c1>c2), and depth e1 is smaller than depth e2 (e1<e2). The minimum values ​​d1 and d2 are, for example, equal to each other, but may be different.

[0036] 2 to 4, the hardness distribution within 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, by taking into consideration other characteristics required for each portion in the circumferential direction Dθ. One example of such other characteristics is compressive residual stress imparted to the wire 2 by shot peening or the like. An example of the relationship between the residual stress distribution and hardness distribution of the wire 2 will be described below.

[0037] Fig. 5 is a graph showing an example of a first hardness distribution H1 and a first residual stress distribution σ1 along a first line segment V1. Fig. 6 is a graph showing an example of a second hardness distribution H2 and a second residual stress distribution σ2 along a 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 depth from the surface 20 of the wire 2. In the examples of Figs. 5 and 6, the positions where residual stress is zero coincide with the hardness of the first layer L1 and the third layer L3. The hardness distributions H1 and H2 shown in Figs. 5 and 6 are similar to those shown in Figs. 3 and 4.

[0038] For example, when shot peening is performed on a spirally wound wire 2, the shot material is likely to hit the outer diameter side portion of the surface 20, but is unlikely to hit the inner diameter side 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 imparted to a deep position in the outer diameter side portion, but is imparted only to a shallow position in the inner diameter side portion, etc. In this way, the compressive residual stress imparted by shot peening can be non-uniform in the circumferential direction Dθ.

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

[0040] 5 , the first range f1 to which compressive residual stress is imparted overlaps 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. As 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 closer to the surface 20 (the first position Q1) than the position where the hardness is at its minimum in the first hardness distribution H1.

[0041] 6, the second range f2 to which compressive residual stress is imparted overlaps 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. As another example, the second range f2 may extend to a portion of the third layer L3. The peak of the compressive residual stress in the second range f2 is located closer to the surface 20 (the second position Q2) than the position where the hardness is at a minimum in the second hardness distribution H2.

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

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

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

[0045] 7 can occur, for example, when a uniform hardness distribution in the circumferential direction Dθ is imparted to the wire 2. That is, the range in which compressive residual stress is imparted by shot peening is shallow on the inner diameter side and deep on the outer diameter side, as described above. However, if the hardness distribution in the wire 2 is uniform in the circumferential direction Dθ, the range in which hardness is reduced and the range in which compressive residual stress is imparted may not suitably overlap at any position 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, if the proportion of the soft second layer L2 is increased in this way, the sag resistance of the coil spring 1 may decrease.

[0047] In contrast, when hardness distributions H1 and H2 are formed according to the residual stress distributions σ1 and σ2 as in the examples of Figures 5 and 6, it is possible to improve corrosion fatigue resistance by reducing the risk of breakage due to inclusions while ensuring sag resistance.

[0048] 2 to 6 can be applied to any of the effective portion 10, the first end winding portion 11, and the second end winding portion 12. The hardness distribution and residual stress distribution in the effective portion 10, the first end winding portion 11, and the second end winding portion 12 may be substantially the same or 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 be formed of two layers, a hard central layer and soft surrounding layers, or may be formed of four or more layers in which adjacent layers have different hardnesses.

[0050] The hardness distribution of the wire 2 in the circumferential direction Dθ does not necessarily need to be adjusted in accordance with the compressive residual stress, but may be adjusted in accordance with other characteristics of the wire 2, such as the structure thereof.

[0051] 3 to 6 do not necessarily have to be different at all positions in the circumferential direction Dθ. For example, assuming a third line segment connecting a third position on the surface 20 to the axis X2 in addition to the first line segment V1 and the second line segment V2 shown in FIG. 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, a description will be given of a method for manufacturing the coil spring 1. Here, as an example, it is assumed that the coil spring 1 having the configuration shown in Figs. 2 to 6 is manufactured.

[0053] 8 is a flowchart showing an example of a method for manufacturing the coil spring 1. This example corresponds to so-called hot forming, in which a straight wire 2 is first heated (process P1). Then, the wire 2, which has been heated to a high temperature in process P1, is formed into a spiral shape by a coiling machine (process P2). In these processes P1 and P2, the wire 2 is quenched.

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

[0055] After step P4, the wire 2 is subjected to hot setting, in which an overload is applied to the wire 2 while it is heated (step P5). Furthermore, the wire 2 is subjected to shot peening (step P6). In this shot peening, compressive residual stresses such as those shown in Figs. 5 and 6 are imparted to the wire 2.

[0056] After step P6, the wire 2 is pre-set (step P7). Furthermore, a coating film 21 is formed on the entire wire 2 (step P8). The coil spring 1 is completed in step P8.

[0057] 9 is a diagram showing a schematic configuration of an AC current heating device 100 (hereinafter referred to as heating device 100) that can be used in the surface hardening of process P3. The heating device 100 includes 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 metal material with excellent conductivity 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 a resin or the like.

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

[0060] 9, each of the first terminal 4A and the second terminal 4B is divided into a lower portion 41 and an upper portion 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 portion 41 and the upper portion 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 process is performed by the heating device 100, the first terminal 4A and the second terminal 4B are attached to the strand 2 formed into a spiral shape, and the strand 2 is placed inside the conductor 3. The order of the steps of attaching the first terminal 4A and the second terminal 4B to the strand 2 and placing the strand 2 inside the conductor 3 is not particularly limited.

[0062] 9 , the first terminal 2 a and the second terminal 2 b of the wire 2 (at least a part of the end winding portions 11 and 12) protrude from both ends of the conductor 3. However, this is not limiting, and the entire wire 2 may be surrounded by the conductor 3.

[0063] 9, the wire 2 is clamped near the first end 2a between the lower portion 41 and upper portion 42 of the first terminal 4A, and the wire 2 is clamped near the second end 2b between the lower portion 41 and upper portion 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 are connected in series with the power source 51. The control device 5 starts to energize the wire 2 in response to an operator's switch operation or an external control signal. In Figure 9, solid arrows indicate an example of the direction of current flow. This direction switches periodically depending on the frequency of the power source 51.

[0065] This current flow heats at least a portion of the wire 2. At this time, a proximity effect, which will be described later, occurs between the conductor 3 and the wire 2. The conductor 3 is positioned at a position where this proximity effect occurs.

[0066] The frequency, amplitude, and energization time of the AC current can be determined appropriately depending on 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, the target heating temperature, etc. When it is time to stop heating, the control device 5 stops the current supply from the power source 51.

[0067] Thereafter, the wires 2 are cooled. This cooling may be natural cooling, or, if rapid cooling is required, may be performed by spraying a fluid such as water or air onto the wires 2. In the example of Fig. 9, the heating device 100 includes a cooling mechanism 6 for spraying 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 under the control of the control device 5, for example. At this time, the fluid is sprayed from each nozzle 61 toward the strand 2. Note that the nozzles 61 do not necessarily have to be provided on the conductor 3, and may be provided on a member different from the conductor 3.

[0069] The heating device 100 may further include a ferromagnetic body 7 that can be arranged near the wire 2. The ferromagnetic body 7 may be made of, for example, ferrite, but is not limited to this example. In the example of Fig. 9, the ferromagnetic body 7 is inserted inside the wire 2 that is formed into a spiral shape.

[0070] 10 is a schematic side view of the strands 2, conductor 3, and ferromagnetic body 7 assembled as shown in FIG. 9. The conductor 3 is, for example, cylindrical and centered on the coil axis X1. The conductor 3 is in an electrically floating state and is insulated from other conductive elements such as the 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 wire 2. That is, the inner surface of the conductor 3 faces the outer diameter side portion of the surface 20 of the wire 2 (the portion including the second position Q2 described above) via the gap G1.

[0072] The ferromagnetic body 7 has, for example, a cylindrical shape centered on the coil axis X1. The ferromagnetic body 7 may have other shapes, such as a cylindrical shape centered on the coil axis X1. The ferromagnetic body 7 is also in an electrically floating state and is insulated from other conductive elements such as the wires 2 and the conductors 3. The ferromagnetic body 7 is supported, for example, by an insulating member (not shown).

[0073] 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 an inner diameter side portion of the surface 20 of the wire 2 (a 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 wire 2, if an electrically floating conductor is placed nearby, a so-called proximity effect occurs. In this embodiment, this proximity effect is utilized to control the current density distribution (heating temperature distribution) of the wire 2.

[0075] 11 is a schematic diagram for explaining the proximity effect, showing a rod-shaped workpiece Ws and a conductor 3s arranged in the vicinity thereof. A current I from a power source flows into the workpiece Ws. Ａ When the magnetic field H flows, a magnetic field H ＩＡ occurs (Ampere's law).

[0076] In the conductor 3s, this magnetic field H ＩＡ The eddy current I caused by Ｅ１ is generated (Lenz's law). Furthermore, the eddy current I Ｅ１ The magnetic field H caused by ＩＥ is generated around the conductor 3s. This magnetic field H ＩＥ acts on the workpiece Ws, an eddy current I Ｅ２ occurs.

[0077] current I Ａ , eddy current I Ｅ１ and eddy current I Ｅ２ The direction of the current I flows is as shown by the arrow in the figure. That is, in the workpiece Ws, the current I flows in the vicinity of the side surface far from the conductor 3s. Ａ The direction of the flow and the eddy current I Ｅ２On the other hand, in the vicinity of the side surface closer to the conductor 3s, the current I Ａ The direction of the flow and the eddy current I Ｅ２ As a result, the current density of the workpiece Ws becomes higher near the side surface closer 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 a conductor 3s is positioned so as to face a portion of the outer surface of the workpiece Ws as shown in Figure 11, it is possible to obtain a current density distribution and heating temperature distribution that change depending on the circumferential position on the surface and inside of the workpiece Ws. These distributions can be appropriately adjusted, for example, by changing the distance between the conductor 3s and the workpiece Ws.

[0079] Furthermore, when the conductor 3s is positioned so as to face only a portion of the workpiece Ws in the longitudinal direction, a current density distribution and a heating temperature distribution that change depending on the longitudinal position on the surface and inside of the workpiece Ws can be obtained.

[0080] When an AC current flows through the wire 2, the current density increases near the surface 20 due to the skin effect. By utilizing this effect, it is possible to intensively heat the area close to the surface 20 and form the second layer L2 in the heated area as shown in Fig. 2. The depth of the second layer L2 from the surface 20 can be adjusted by, for example, the current flow time (heating time), the frequency of the AC current, the gap G1 between the wire 2 and the conductor 3, etc.

[0081] In the wire 2 formed into a spiral shape, the current path passing through the inner diameter portion of the surface 20 is short, and therefore the current density tends to be high near the inner diameter portion. The heating device 100 controls such a current density distribution by the proximity effect of the conductor 3, thereby imparting a desired hardness distribution that varies in the circumferential direction Dθ to at least a portion of the wire 2.

[0082] The current density distribution and 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 influencing the magnetic flux generated when current is passed through the wire 2, spreading the current density toward the outer diameter. By using the magnetic flux induction function of the ferromagnetic body 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 favorable hardness distribution.

[0083] The hardness distribution imparted to the wire 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 2. In the example of Figure 12, the hardness is low near the surface 20 and increases smoothly toward the axis X2.

[0084] 12, the wire 2 has a first layer L1 and a second layer L2 inside the first layer L1, and the hardness of the first layer L1 is smaller 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 vary as in the examples described with reference to FIGS.

[0085] 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 diameter side and the outer diameter side, respectively, of the surface 20 of the wire 2 as shown in FIG. 2, or may be located in another portion of the surface 20.

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

[0087] By using the heating device 100, it is also possible to form a hardness distribution such as the example shown in Fig. 12. Furthermore, it is also possible to change the hardness distribution of this shape in the circumferential direction Dθ. In addition, according to a manufacturing method that incorporates heat treatment using the heating device 100, it is possible to obtain a coil spring 1 in which the characteristics of the wire 2 in the circumferential direction Dθ are improved in various ways.

[0088] The scope of the present invention is not limited to the configurations disclosed in the above-described embodiments. The present invention can be implemented by modifying the configurations disclosed in the embodiments in various ways.

[0089] 1...coil spring, 2...wire, X1...coil 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

DEPCT681. A coil spring constructed from wire wound into a helical shape, where at least a portion of the wire exhibits a varying stiffness distribution in the circumferential direction around the wire's axis.

2. A coil spring of claim 1, where the wire comprises a first layer, a second layer inside the first layer, and a third layer inside the second layer, and the stiffness of the second layer is lower than that of the first and third layers.

3. A coil spring of claim 2, where the surface of the wire has a first and a second position at a distance from the first position in the circumferential direction, and the stiffness distribution at the first layer along the segment connecting the first position to the axis and the stiffness distribution at the second layer along the segment connecting the second position to the axis differ.

4. A coil spring of claim 3, where the stiffness distribution at the first and second layers differ by at least one aspect of the width of the first layer, the width of the second layer, the width of the third layer, the minimum stiffness value in the second layer, and the depth of the position with the minimum stiffness value from the surface. 5.

6. The coil spring of claim 3, where compressive residual stress is applied to each segment along the first segment and the second segment along the second segment, the second segment extends deeper than the first segment from the surface, and a portion of the second layer along the second segment is located deeper than a portion of the second layer along the first segment from the surface.

7. The coil spring of claim 5, where position one is on the inner diameter side of the wire, and position two is on the outer diameter side of the wire.

8. The coil spring of claim 1, where the wire includes the first and second layers inside the first layer, and the stiffness of the first layer is lower than that of the second layer.

9. The coil spring of claim 7, where the surface of the wire has positions one and two that are radially oriented away from position one, and the stiffness distribution at position one along the first segment connecting position one to the axis and the stiffness distribution at position two along the second segment connecting position two to the axis are different.The coil spring of claim 8, where the first and second stiffness distributions differ by at least one aspect in the width of the first layer, the width of the second layer, and the minimum stiffness value in the first layer.

10. The method of fabricating a coil spring of any of claims 1 through 9 includes: forming a helical shape of the wire; welding the first and second terminals connected to a power source, capable of supplying alternating current to the wire; and creating a stiffness distribution that changes in the circumferential direction on at least a portion of the wire by heating the wire with alternating current through the first and second terminals.

11. The method of fabricating a coil spring of claim 10 also includes: preparing an electrically buoyant conductor at a position that creates a proximity effect before supplying alternating current to the wire.

12. The method of fabricating a coil spring of claim 10 also includes: performing a granular abrasion on the helical wire to apply compressive residual stress to the wire.

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