Seat structure and torsion bar

Abstract

To improve the stability of a posture when seated by improving the supporting property of the pelvis.　A seat structure (1) has a support plate (43) that is biased forward as a pelvis support mechanism (4), and supports the pelvis of the seated person. The physiological curvature of the spinal column is easily maintained, and the shoulder blades are easily brought into contact with the upper part of the backrest, allowing the stable seating posture to be easily maintained. The support plate (43) is biased forward and is configured to be displaceable in a rotation direction in a plan view, that is, in a rotation direction centered on the torso of the seated person. During the ride, the pelvis and the shoulder blades are suppressed from being significantly separated from the backrest, enhancing the stability of the posture.

Description

Seat structure and torsion bar

[0001] The present invention relates to a seat structure suitable for use in vehicles such as automobiles, aircraft, trains, and ships, and to a torsion bar suitable for use in this seat structure.

[0002] The applicant has disclosed in Patent Documents 1 and 2 a seat structure in which a pelvis support member is provided on the backrest and contacts the upper part of the occupant's pelvis. This pelvis support member is connected to a torsion bar via a link member and is biased forward with a predetermined elastic force. Input vibration causes the pelvis support member to move back and forth along with the up and down movement of the seat. This synergistic effect allows the pelvis support member to easily follow the rotational movement of the pelvis caused by the input vibration, effectively reducing vibrations that are uncomfortable for people. Furthermore, when an impact force greater than a predetermined level is input, the pelvis presses the pelvis support member in a backward rotational direction, displacing the pelvis so that it rides on the pelvis support member and absorbing the impact force.

[0003] JP 2004-209017 A JP 2004-229957 A JP 2011-239910 A

[0004] The seat structures disclosed in Patent Documents 1 and 2 utilize a pelvic support member to improve vibration and shock absorption characteristics, achieving a comfortable ride with minimal fatigue. However, the movements of a seated person while riding in a vehicle are affected by complex factors such as the vehicle's direction of travel and vertical vibrations caused by uneven road surfaces. Therefore, the seated person's body, particularly the upper body, is affected by input vibrations not only in the vertical and front-to-back directions, but also in the left-to-right direction and the rotational direction around the trunk (yaw direction). The pelvic support members disclosed in Patent Documents 1 and 2 are insufficient to accommodate such movements and leave room for improvement. Furthermore, since the front edge of the seat abuts against the back of the thighs near the back of the knees, it is desirable for the structure to provide a certain level of support while not exerting too much pressure and providing excellent leg movement compliance. From this perspective, the present applicant has proposed a mechanism, as disclosed in Patent Document 3, in which a front frame is swingably supported by a torsion bar spanning the left-to-right direction of the seat. However, because the entire front frame swings back and forth evenly in the left-right direction, movement of one leg causes the portion supporting the other leg to move in the same way. For example, if one leg sinks, the entire front frame sinks, reducing the support for the other leg. Therefore, if the front frame could follow such individual leg movements, support would be improved. Furthermore, since the torsion bar that elastically supports the pelvic support member and front frame is incorporated into the seat structure, it is necessary to prevent breakage. Conventionally, the torsion bar has been housed in a pipe. However, housing it in a pipe increases the number of parts, hindering weight reduction.

[0005] The present invention has been made in view of the above, and aims to provide a seat structure that can improve support for the pelvis and thighs and increase posture stability when seated. Another aim is to provide a torsion bar that is suitable for use in seat structures, has excellent bending properties, and can contribute to reducing the weight of the seat structure.

[0006] In order to solve the above problems, the present invention provides a seat structure having a seat portion and a backrest, in which a body support mechanism is provided on at least one of the seat portion and the backrest, the body support mechanism comprising a support plate arranged so that its longitudinal direction follows the left-right direction of the seat portion or the backrest, and an elastic support mechanism that urges the support plate in a direction to bring it closer to a body part of the seated person and supports the support plate so that it can be displaced in a rotational direction around a central position in the longitudinal direction.

[0007] The elastic support mechanism is preferably configured to include a pair of arm members provided at a predetermined interval in the left-right direction on a frame constituting the seat or backrest, and a torsion bar used as a rotation axis for rotatably supporting the base of each of the pair of arm members on the frame, and the tip of each of the pair of arm members is preferably rotatably supported on the support plate.

[0008] It is preferable that the tip ends of the pair of arm members and the support plate are connected via link plates. The body support mechanism is preferably configured such that, when the spring constants obtained from the load-deflection characteristics obtained by pressing the support plate are k1, the spring constant for the displacement range up to the equilibrium point when seated is k2, the spring constant for a predetermined displacement range from the equilibrium point is k3, and the spring constant for the displacement range beyond the predetermined displacement range from the equilibrium point is k3, the spring constant k2 is the largest among the spring constants k1, k2, and k3, and the body support mechanism is supported by a spring force of spring constant k2 at the equilibrium point when seated, and a spring force of spring constant k1 or k3 acts when vibration is input. It is preferable that the support plate is shaped so that each of the longitudinal ends is located forward of the longitudinal center.

[0009] The body support mechanism is preferably configured as a pelvis support mechanism in which the support plate is disposed in a portion of the backrest corresponding to the pelvis, and the support plate constituting the pelvis support mechanism is preferably configured so that its lower edge is below a position corresponding to at least the upper part of the sacrum of the pelvis of the seated person and does not come into contact with the seat, and its upper edge is above the position corresponding to at least the upper part of the sacrum and extends to a position corresponding to the fourth lumbar vertebra.Furthermore, the body support mechanism is preferably configured as a thigh support mechanism in which the support plate is disposed in a portion corresponding to the thighs near the front edge of the seat.

[0010] It is preferable that a seat net is hung with a predetermined tension across a cushion frame that constitutes the seat, and a backrest net is hung with a predetermined tension across a back frame that constitutes the backrest.It is preferable that the seat net and the backrest net are three-dimensional knitted fabrics knitted using multifilaments or monofilaments made of synthetic resin.

[0011] It is preferable that the torsion bar be heat-treated, have a hard layer having a relatively higher average hardness than other layers near the center of a cross section along the radial direction perpendicular to the longitudinal direction, have a flat hardness layer having a substantially constant hardness, a soft layer having a gradually decreasing hardness, and a decarburized layer from the hard layer toward the surface, and have a multi-layer structure in which the flat hardness layer, the soft layer, and the decarburized layer are formed in a ring shape along the outer periphery of the cross section along the radial direction and surround the hard layer.

[0012] The hard layer is preferably formed within a distance range of 30 to 70% of the radius of the torsion bar from the center of a cross section perpendicular to the longitudinal direction of the torsion bar. The ring width of the flat hardness layer is preferably 5 to 40% of the radius of the torsion bar, and the ring width of the soft layer is preferably 15 to 60% of the radius of the torsion bar. The ring width of the decarburized layer is preferably 1% or less of the radius of the torsion bar.

[0013] The present invention also provides a torsion bar used as a spring material incorporated into a seat or backrest in a seat structure and arranged to elastically support the seat or backrest, the torsion bar being manufactured by heat treatment, the torsion bar having a hard layer at the center of a cross section taken along a radial direction perpendicular to the longitudinal direction, the hard layer having a higher average hardness than other layers, and from the hard layer toward the surface, a flat hardness layer having a substantially constant hardness, a soft layer whose hardness gradually decreases, and a decarburized layer, the flat hardness layer, the soft layer, and the decarburized layer being formed in a ring shape along the outer periphery of the cross section taken along the radial direction, surrounding the hard layer, forming a multi-layer structure.

[0014] The hard layer is preferably formed within a distance range of 30 to 70% of the radius of the torsion bar from the center of a cross section perpendicular to the longitudinal direction of the torsion bar. The ring width of the flat hardness layer is preferably 5 to 40% of the radius of the torsion bar, and the ring width of the soft layer is preferably 15 to 60% of the radius of the torsion bar. The ring width of the decarburized layer is preferably 1% or less of the radius of the torsion bar.

[0015] The seat structure of the present invention includes a body support mechanism having a support plate arranged longitudinally along the left-right direction of the seat or backrest and biased toward the occupant's body parts. The support plate is supported by an elastic support mechanism so that it can be rotated about a longitudinal center position. When the support plate is provided as a pelvic support mechanism at a position on the backrest corresponding to the pelvis, the physiological curvature of the spine is maintained, the shoulder blades are more likely to contact the upper part of the backrest, and a stable seated posture is more likely to be maintained. Furthermore, when the trunk moves in the same direction in response to the vehicle's lateral and yaw movements, the support plate moves in the same direction, following the pelvis. As a result, the occupant's back can respond to various vehicle movements, not just up-down and forward-backward. This prevents the pelvis and shoulder blades from moving too far away from the backrest during riding, improving posture stability, reducing the effects of vibrations transmitted from the vehicle body to the occupant, and suppressing sympathetic nervous activity, contributing to maintaining a more relaxed riding posture. Furthermore, if the body support mechanism is provided as a thigh support mechanism near the front edge of the seat, the ability to follow the movement of each leg is improved, and thigh support is improved.

[0016] Furthermore, since the torsion bar of the present invention can exhibit high bending properties, there is no need to house the torsion bar inside a covering pipe as in the past, and it can be placed as is, which contributes to reducing the weight of the seat structure.

[0017] FIG. 1(a) is a perspective view showing a seat structure according to one embodiment of the present invention, and FIG. 1(b) is a perspective view showing the internal structure with the seat net and backrest net removed. FIG. 2(a) is a perspective view of the pelvis support mechanism used in the above embodiment, as seen from the rear, and FIG. 2(b) is a rear view of the pelvis support mechanism. FIG. 3 is a plan view of the seat structure, showing the movement of the pelvis support mechanism. FIG. 4 is a graph showing the load-deflection characteristics of the pelvis support mechanism. FIG. 5(a) is a diagram showing a vibration experiment, and FIGS. 5(b) and (c) are diagrams showing experiments related to the evaluation of physiological indices. FIG. 6 is a graph showing the vertical vibration transmissibility occurring in the seat in the vibration experiment. FIG. 7 is a graph showing the left-right vibration transmissibility occurring in the backrest in the vibration experiment. FIG. 8 is a graph showing the front-rear vibration transmissibility occurring in the backrest in the vibration experiment. 9(a) is a diagram showing the dynamic body pressure distribution in the backrest of the seat structure of the comparative example when vertical vibrations are input at 5 Hz using a uniaxial vibrator, FIG. 9(b) is a diagram showing the dynamic body pressure distribution in the backrest of the seat structure of the embodiment when vertical vibrations are input at 5 Hz using a uniaxial vibrator, FIG. 9(c) is a diagram showing the dynamic body pressure distribution in the backrest of the seat structure of the comparative example when left-right vibrations are input at 5 Hz using a 6-axis vibrator, and FIG. 9(d) is a diagram showing the dynamic body pressure distribution in the backrest of the seat structure of the embodiment when left-right vibrations are input at 5 Hz using a 6-axis vibrator. FIG. 10 is a graph showing the measurement results of blood pressure and heart rate. Figures 11(a) and 11(b) are graphs showing power spectra (logarithmic representation) of ECG, PPG, APW temporal data, APW back data, APW lumbar data, and APW buttocks data. Figure 11(a) is a graph analyzing measurement data from a comparative seat structure, and Figure 11(b) is a graph analyzing measurement data from a seat structure according to an embodiment. Figures 12(a) and 12(b) are graphs showing time waveforms of ECG, PPG, APW left temporal data, APW back data, APW lumbar data, and APW buttocks data. Figure 12(a) shows data measured with the comparative seat structure, and Figure 12(b) shows data measured with the seat structure according to an embodiment. Figure 13 is a diagram for explaining the posture of an anthropomorphic dummy seated in a seat structure according to an embodiment.Figures 14(a) and (b) are diagrams for explaining the fatigue test method. Figure 15 is a graph showing the hardness distribution of a cross section along the diameter when the center position of torsion bars A and B is 0 mm. Figure 16(a) shows the hardness distribution in a cross section along the diameter of torsion bar A, shown by color coding, and Figure 16(b) shows the hardness distribution in a cross section along the diameter of torsion bar B, shown by color coding. Figure 17 is a graph showing the results of the fatigue test. Figure 18 is a graph showing the results of a bending test with downward restriction. Figure 19 is a graph showing the results of a bending test with up-down restriction. Figure 20(a) is a photograph showing the appearance of torsion bar A after a bending test with up-down restriction, and Figure 20(b) is a photograph showing the appearance of torsion bar B after a bending test with up-down restriction. Figures 21(a) and (b) show the measured spring constants and calculated spring constants for different effective lengths of torsion bar A, with (a) showing data for an effective length of 150 mm and (b) showing data for an effective length of 174 mm. Figure 22(a) is a scanning electron microscope photograph showing the state of the fracture surface of torsion bar A that broke in the torsion test, and Figure 22(b) is a scanning electron microscope photograph showing the state of the fracture surface of torsion bar B that broke in the torsion test. Figure 23 is a conceptual diagram showing a high-frequency induction heating device that heat-treated the torsion bars used in the experiment. Figure 24 is a perspective view showing a frame structure of a seat structure according to another embodiment of the present invention.

[0018] The present invention will be described in further detail below with reference to the embodiment shown in the drawings. As shown in Figures 1(a) and 1(b), a seat structure 1 of this embodiment includes a seat portion 2, a backrest 3, etc. This seat structure 1 is an automobile seat, and the backrest 3 is supported relative to the seat portion 2 so as to be reclining.

[0019] The seat portion 2 has a cushion frame 20 having a pair of side frames 21, 22 arranged at a predetermined distance in the left-right direction of the seat structure 1, a front frame 23 arranged between the front portions of the pair of side frames 21, 22, and a rear frame 24 arranged between the rear portions of the pair of side frames 21, 22. A base net 2a is hung between the pair of side frames 21, 22, and a seat net 2b is arranged to cover the upper part of the base net 2a and to be hung between the pair of side frames 21, 22 and the front frame 23. The seat net 2b is hung across the cushion frame 20 in this way, and forms a tension structure arranged with a predetermined tension.

[0020] The backrest 3 has a back frame 30 which has a pair of side frames 31, 32 arranged at a predetermined distance in the left-right direction of the seat structure 1, an upper frame 33 arranged between the upper parts of the pair of side frames 31, 32, and a lower frame 34 arranged between the lower parts of the pair of side frames 31, 32. A base net 3a is hung between the pair of side frames 31, 32, and a backrest net 3b is arranged to cover the upper part of the base net 3a and to hang across the pair of side frames 31, 32 and the upper frame 33. The backrest net 3b is also hung across the back frame 30 and is a tension structure arranged with a predetermined tension.

[0021] It is preferable to use a three-dimensional knitted fabric for the seat net 2b and the backrest net 3b. The three-dimensional knitted fabric is formed by connecting a pair of ground knitted fabrics arranged at a distance from each other with a connecting yarn. The thickness of the ground yarn forming the ground knitted fabric is selected so that it can provide the necessary strength for the three-dimensional knitted fabric while not making the knitting process difficult. Furthermore, while monofilament can be used as the ground yarn, multifilament can also be used from the perspective of texture and softness of the surface feel. Although multifilament yarn can be used as the connecting yarn, monofilament yarn is preferred because it is easier to obtain the desired elasticity.

[0022] Various materials can be used for the yarns or connecting yarns that form the ground fabric of the three-dimensional knitted fabric, but preferably they are made of synthetic resin, such as polyester fibers typified by polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), polyamide fibers typified by nylon 6 and nylon 66, polyolefin fibers typified by polyethylene and polypropylene, or a combination of two or more of these fibers. Because of this structure, three-dimensional knitted fabrics have the characteristics of high surface rigidity and a low coefficient of friction.

[0023] A pelvis support mechanism 4, which is a body support mechanism, is provided and is supported by either the cushion frame 20 that constitutes the seat portion 2 or the back frame 30 that constitutes the backrest 3. As shown in Figures 2 and 3, the pelvis support mechanism 4 has an elastic support mechanism 40 that includes a pair of arm members 41, 42 and torsion bars 44, 45 that are provided at a predetermined distance in the left-right direction of the seat structure 1, and a support plate 43 that is supported by this elastic support mechanism 40.

[0024] In this embodiment, the pair of arm members 41, 42 are rotatably supported on the lower parts of the side frames 31, 32. Specifically, L-shaped brackets 35, 36 are fixed by bolts or the like to the inside of the lower parts of the side frames 31, 32. In addition to the L-shaped brackets 35, 36, connect plates 311, 321 are also fixed to the inside of the lower parts of the side frames 31, 32. Then, bases 41a, 42a of the arm members 41, 42 are disposed, via bushings 35b, 36b, between the L-shaped brackets 35, 36 and bent surfaces 31a, 32a formed by bending the lower parts of the connect plates 311, 321 inward. Insertion holes are provided in the lower surfaces 35a, 36a of the L-shaped brackets 35, 36 and the bases 41a, 42a of the arm members 41, 42, and lower portions 44a, 45a of torsion bars 44, 45, which are arranged with their longitudinal direction extending up and down, are non-rotatably inserted into these insertion holes and connected to bushes 35b, 36b. Thus, the torsion bars 44, 45 function as rotation shafts that rotatably support the bases 41a, 42a of the arm members 41, 42 on the lower portions of the side frames 31, 32. Upper portions 44b, 45b of the torsion bars 44, 45 are fixed to appropriate positions on the upper portion of the back frame 30 via fixing bushes 37, 38. As a result, when the arm members 41, 42 rotate, the torsion bars 44, 45 rotate together, twisting the torsion bars 44, 45 and generating torque.

[0025] The arm members 41, 42 are arranged so that their tip ends 41b, 42b extend toward the center of the backrest 3, and the tip ends 41b, 42b are supported on the back side of the support plate 43 via link plates 48, 49 (described below) at a predetermined interval in the left-right direction of the backrest 3. Specifically, the support plate 43 is formed in a substantially rectangular shape and is positioned so that its longitudinal direction is aligned with the left-right direction of the backrest 3 and its width direction is aligned with the up-down direction of the backrest 3. A mounting bracket 431 having a predetermined length along the longitudinal direction of the support plate 43 is fixed to the back of the support plate 43. This mounting bracket 431 has a substantially U-shaped cross section near each end, with upper and lower opposing surfaces 431a, 431b. Shaft members 432, 433 are disposed between the opposing surfaces 431a, 431b. Link plates 48, 49 are hung between the shaft members 432, 433 and connecting shafts 46, 47 provided on the distal ends 41b, 42b of the arm members 41, 42, connecting the two. Two link plates 48, 49 are used, one on top of the other, and the distal ends 41b, 42b of the arm members 41, 42 are disposed between them. This allows the support plate 43 to rotate around a longitudinal center position C in a plan view (as viewed from above the backrest 3), as shown in FIG. 3 . Therefore, when the pelvis of the seated person moves left or right or in a rotational direction around the trunk, the support plate 43 will attempt to move in the same direction, and the link plates 48, 49 will move in the rotational direction accordingly.

[0026] As shown in Fig. 3, in a plan view of the support plate 43 supported by the arm members 41 and 42, a central portion 43a extending over a predetermined length of the support plate 43 about a center position C in the longitudinal direction is flat. End portions 43b and 43c on both sides of the central portion 43a are positioned forward of the central portion 43a and have inclined surfaces 43d and 43e that slope obliquely forward from their respective boundaries with the central portion 43a toward the respective end portions 43b and 43c. This allows the central portion 43a and the inclined surfaces 43d and 43e to conform to the shape of the seated person's pelvis, thereby increasing the support area. When the support plate 43 is displaced in the rotational direction, as shown in Fig. 3, when one end portion 43b moves forward, the other end portion 43c moves backward, and when one end portion 43b moves backward, the other end portion 43c moves forward, resulting in high compliance with the movement of the seated person's pelvis.

[0027] As described above, the support plate 43 is disposed with its width oriented along the vertical direction of the backrest 3, but its lower edge 43f is positioned below a position corresponding to at least the upper part of the sacrum of the pelvis when the person is seated, without contacting the seat portion 2. The upper edge 43g, which varies depending on the position of the lower edge 43f and the width of the support plate 43 (the length between the lower edge 43f and the upper edge 43g), is positioned at least above a position corresponding to the upper part of the sacrum, preferably up to a position corresponding to the fourth lumbar vertebra. This allows the support plate 43 to support the area corresponding to the pelvis of the seated person. With the support plate 43 supporting the area corresponding to the pelvis, the physiological curvature of the spine makes it easier for the scapulae to abut against the upper part of the backrest 3, and the backrest 3 becomes a support structure centered on the pelvis and scapulae. If the position of the upper edge 43g of the support plate 43 is higher than the position corresponding to the fourth lumbar vertebra, it may become difficult to maintain the physiological curvature of the spine, so it is preferable to keep it in the above range.

[0028] As described above, the backrest net 3b is arranged to be hung across the pair of side frames 31, 32 and the upper frame 33 of the back frame 30, but in Fig. 1(a) , in order to show the pelvic support mechanism 4, the backrest net 3b is shown cut at a position above the pelvic support mechanism 4. However, in the final product, the backrest net 3b is arranged to also cover the front surface of the support plate 43 of the pelvic support mechanism 4, and the cushioning properties of the backrest net 3b reduce the feeling of contact with the support plate 43. Because the support plate 43 is biased in a direction that protrudes forward by the torsion bars 44, 45, this tension acts on the backrest net 3b covering the front surface of the support plate 43, and this also has the function of suppressing sagging and wrinkles when hung across the back frame 30.

[0029] The torsion bars 44, 45 are arranged with their torsion angles adjusted so that, in a normal state, they bias the tip ends 41 b, 42 b of the arm members 41, 42 in a direction that protrudes forward. The torsion bars 44, 45 are preferably heat-treated, and further quenched and tempered to form a four-layer structure in the hardness distribution of a cross section along the radial direction perpendicular to the longitudinal direction, as shown in Figures 15 and 16(b), in which a hard layer (T1) having a relatively higher average hardness than the other layers is formed near the center, and a flat-type hardness layer (T2), a soft layer (T3), and a decarburized layer (T4) are formed from the hard layer (T1) toward the surface. The flat-type hardness layer (T2), the soft layer (T3), and the decarburized layer (T4) are formed in a ring shape along the outer periphery of the cross section along the radial direction, surrounding the hard layer (T1).

[0030] The hard layer (T1) has a martensite structure and preferably has a Vickers hardness of 550 Hv or more, more preferably 570 Hv or more. The hard layer (T1) is preferably formed within a distance range of 30 to 70% of the radius of the torsion bars 44, 45 from the center of the torsion bars 44, 45, and more preferably within a range of 35 to 65%.

[0031] In this specification, the hardness of the flat hardness layer (T2) is generally constant within its range, and in the graph of FIG. 15, this appears as a nearly horizontal change, representing a plateau-like step between the hard layer (T1) and the soft layer (T3). Because of its nearly constant hardness, the hardness difference within the range of the flat hardness layer (T2) is preferably within 20 Hv, and more preferably within 10 Hv. The hardness of the flat hardness layer (T2) is slightly lower than that of the hard layer (T1), preferably in the range of 500 to 600 Hv in Vickers hardness, and more preferably in the range of 550 to 570 Hv. The ring width of the flat hardness layer (T2) is preferably 5 to 40% of the radius of the torsion bars 44, 45, and even more preferably 20 to 30%. By having such a flat hardness layer (T2), compressive residual stress is generated in the torsion bars 44, 45, improving fatigue strength.

[0032] The soft layer (T3) is formed so that its hardness gradually decreases toward the surface. Preferably, the hardness is in the range of 600 to 350 Hv, more preferably 570 to 420 Hv. The ring width of the soft layer (T3) is preferably 15 to 60% of the radius of the torsion bars 44, 45, and more preferably 45 to 55%. The presence of the ring-shaped soft layer (T3) contributes to durability against torsion and reduces notch sensitivity due to minor scratches on the surface.

[0033] The decarburized layer (T4) is the layer formed on the surface side and has the lowest hardness, and its ring width is equivalent to 1% or less of the radius of the torsion bar. This layer contributes to improving bending characteristics, but if it is too wide (if it is formed deep from the surface), it will affect durability, so it is preferable to make it in the range of 1 to 10 μm.

[0034] In the torsion bars 44, 45 of this embodiment, the flat hardness layer (T2), the soft layer (T3), and the decarburized layer (T4) surround the central hard layer (T1) in a ring shape, thereby achieving a balance between hardness and bending characteristics, achieving the desired durability, and suppressing breakage when an impact load is applied.

[0035] Heat treatment to impart the above-described properties to the torsion bars 44, 45 is performed as follows. The heat treatment is preferably performed by high-frequency induction heating. Figure 23 shows the high-frequency induction heating apparatus used to heat treat the torsion bars A and B in the experimental example described below. The torsion bar (workpiece) to be heat treated is placed inside the coil of the high-frequency induction heating apparatus. Torsion bars used as springs, which are incorporated into the seat 2 or backrest 3 of the automobile seat structure 1 and have one end fixed to one of the frames to elastically support the seat 2 or backrest 3, typically have a diameter of approximately 5 to 20 mm. The diameter of the torsion bars 44, 45 is 7.8 mm. To heat the torsion bars to their centers, the frequency during high-frequency induction heating is preferably 100 kHz or less, more preferably 10 to 40 kHz, and even more preferably 20 to 30 kHz.

[0036] In the quenching process, the material is first rapidly heated to a temperature higher than the A1 transformation point, followed by rapid cooling. In the tempering process, the material is rapidly heated to 200 to 400°C, preferably 220 to 350°C, followed by rapid cooling. In both processes, the rapid heating is preferably carried out at a rate of 300°C / second or more, more preferably 400°C / second or more, and the rapid cooling is preferably carried out at a rate of 300°C / second or more, more preferably 800°C / second or more. In particular, the rapid cooling process is carried out within 1 second, preferably 0.4 to 0.8 seconds. This results in a multilayer torsion bar 44, 45 having the above-mentioned properties, but it is important to not raise the tempering temperature too high and to perform the rapid cooling as quickly as possible.

[0037] According to this embodiment, when a seated person sits in the seat structure 1, the pelvis presses against the support plate 43 of the pelvis support mechanism 4. The support plate 43 is biased forward, i.e., toward the pelvis, a body part, by the torsion bars 44, 45 via the arm members 41, 42. Therefore, the support plate 43 presses against and supports the pelvis when seated. This facilitates maintaining the physiological curvature of the spine, and facilitates the shoulder blades coming into contact with the upper part of the backrest 3 when seated. As a result, a stable seated posture is easily maintained, and the transmission of vertical input vibrations to the seated person is suppressed. Furthermore, when the seated person's back moves left or right or in a rotational direction around the trunk, the support plate 43 can follow the movement (the dashed-dotted line in FIG. 3 indicates the movement of the support plate 43), thereby maintaining support for the pelvis even when such movements occur. Therefore, even when input vibrations act left or right, the transmission of vibrations to the seated person's back is suppressed. When input vibration acts on the backrest 3 in the front-rear direction, the spring forces of the torsion bars 44 and 45 act, causing a phase difference and even an opposite phase, thereby damping the vibration.

[0038] Therefore, since the seat structure 1 of this embodiment has the pelvis support mechanism 4 of the above-described structure, it stabilizes the seating posture and makes it easier to suppress the transmission of input vibrations, thereby contributing to increased comfort when riding.

[0039] (Experimental Example) Below, experiments were conducted on vibration, body pressure distribution, and physiological index evaluation for the seat structure 1 of this embodiment and a seat structure of a comparative example. The torsion bars 44, 45 were 7.8 mm in diameter and 500 mm in length, and had the four-layer structure shown in Figures 15 and 16(b). The seat structure of the comparative example did not include a pelvic support mechanism 4, but did include a lumbar support supporting the first to third lumbar vertebrae, padding on the sides, and a structure with emphasis on lateral support, with the sides slightly bulging forward compared to the seat structure 1 of this embodiment. Other structures, including the frame shape, base nets 2a, 3a, seat net 2b, and backrest net 3b, were similar to those of the seat structure 1 of this embodiment. The test subjects were five men in their 20s to 60s.

[0040] 4 is a diagram showing the static load characteristics of the pelvis support mechanism 4. This shows the load-deflection characteristics when the center of the support plate 43 of the pelvis support mechanism 4 is pressed by 50 mm with a pressure plate having a diameter of 100 mm.

[0041] From Figure 4, it can be seen that the pelvis support mechanism 4 has four different spring constants, k1 to k4. When a load is applied to the backrest 3, the spring constant k1 (0.3 kg / mm) acts due to the tension of the backrest net 3b as the support plate 43 moves rearward up to a displacement of approximately 31 mm (the displacement range from when the person sits down to the equilibrium point). At a displacement of approximately 31 mm, the spring constant switches from k1 to k2 (2 kg / mm). This is the point at which the arm members 41, 42 twist from a state in which the support plate 43 has been displaced by the rotation of the link plates 48, 49, and the spring force of the torsion bars 44, 45 begins to act. The predetermined displacement range in which the spring constant k2 acts becomes the equilibrium point when the person sits down, supporting the back of the seated person.

[0042] When the stroke of the support plate 43 of the pelvis support mechanism 4 increases or decreases due to input vibration, the weight distribution of the backrest net 3b located in the shoulder blade area and the seat net 2b under the buttocks increases. This changes the support load of the support plate 43 of the pelvis support mechanism 4, and corresponds to a spring constant k1 of 0.3 kg / mm ​​for a displacement range of 0 to 31 mm and a spring constant k3 of 0.8 kg / mm ​​for a displacement range of 35 to 45 mm. Therefore, when the stroke of the support plate 43 of the pelvis support mechanism 4 increases or decreases, support by the shoulder blades and buttocks can be increased, while the feeling of a strong reaction force from the pelvis support mechanism 4 is suppressed, which is highly effective in suppressing poor posture without reducing the seating comfort when riding.

[0043] (Vibration Experiment) Experimental Conditions A seat structure was placed on a vibration exciter, and each subject was seated on this seat structure to measure vibration transmissibility (see Figure 5(a)). A 1-axis and 6-axis vibration exciter manufactured by Delta Tooling Co., Ltd. were used. Vibration transmissibility was measured in three directions: up / down, left / right, and front / back. The vibration input was a sine wave with an amplitude displacement of 2 mmp-p in both the up / down direction (log sweep: 0.5 to 15 Hz (1-axis exciter)), and a sine wave with an amplitude displacement of 10 mmp-p in both the left / right direction (log sweep: 0.5 to 6 Hz (6-axis exciter)). The front / back vibration was determined by measuring the acceleration of the front / back vibration caused by the vibration input in the up / down direction.

[0044] The acceleration sensors used were a Rion seat acceleration pickup PV-62 and a Rion acceleration pickup PV-85. One was placed at the position corresponding to the ischial tuberosity to detect acceleration at the seat, and the other was placed at the position corresponding to the fusion of the fifth lumbar vertebra and the sacrum to detect acceleration at the backrest.

[0045] 6 to 8 show the experimental results of a male subject (in his 30s) weighing 75 kgf and standing 177 cm tall when vibrations were input in the vertical direction from a uniaxial vibrator. FIG. 6 shows the vibration transmissibility due to the vertical vibration acceleration at the seat and the vertical vibration acceleration on the platform of the vibrator. Compared to the seat structure of the comparative example, the seat structure 1 of this embodiment has reduced acceleration in the resonance frequency bands of 1 to 4.5 Hz and 6.5 to 8 Hz, and it can be seen that it reduces the vertical vibrations transmitted from the spine to the head and the vibrations that cause internal organs to resonate.

[0046] 7 shows the vibration transmissibility of the lateral vibration acceleration measured at the backrest and the vertical vibration acceleration on the platform of the vibrator. Compared to the seat structure of the comparative example, the seat structure 1 of this embodiment has a significantly improved lateral vibration transmissibility in the 1 to 5 Hz range, and also an improved lateral vibration transmissibility in the 6 to 8 Hz range, indicating that the support plate 43 of the pelvis support mechanism 4 has high lateral compliance and is effective in absorbing vibrations.

[0047] FIG. 8 shows the vibration transmissibility of the front-to-back vibration acceleration measured at the backrest and the vertical vibration acceleration on the vibration exciter platform. The seat structure 1 of this embodiment has a higher front-to-back acceleration at 4 to 6 Hz compared to the seat structure of the comparative example. This indicates that the vertical vibration acceleration is converted to the front-to-back direction by the pelvis support mechanism 4, and this phenomenon contributes to the reduction of the vertical vibration acceleration in the seat portion shown in FIG. 6. In FIG. 8, the seat structure 1 of this embodiment has a resonance frequency of 5 Hz (gain of 2.0), but enters the damping range from 6.5 Hz, and vibration is quickly suppressed by the trunk mass and phase difference (antiphase). Therefore, visceral resonance at 7 Hz does not occur, and the front-to-back rocking at 5 Hz or less reduces the floating sensation (front-to-back) and bouncy sensation (vertical) that occur at 1 to 4 Hz. In addition, the seat structure 1 of this embodiment uses a three-dimensional knitted fabric as the seat net 2b of the seat 2, and as described above, the surface of the three-dimensional knitted fabric has low friction, and is prone to producing a movement that damps vibrations converted in the front-to-back direction by a phase difference (opposite phase).

[0048] (Evaluation of body pressure dispersion) Experimental conditions Dynamic body pressure distribution was evaluated by inputting vertical vibration: sine wave with amplitude displacement of 2 mm p-p in both directions (vibration of 3 to 10 Hz input in 1 Hz increments (single-axis vibrator)), and horizontal vibration: sine wave with amplitude displacement of 10 mm p-p in both directions (vibration of 3 to 6 Hz input in 1 Hz increments (six-axis vibrator)). The same acceleration sensors as those used in the vibration experiment were placed in the same positions.

[0049] Experimental Results: Figure 9 shows the dynamic body pressure distribution when vibration was applied at 5 Hz (subject: male, in his 30s, weighing 75 kgf and standing 177 cm tall). (a) and (b) show the dynamic body pressure distribution when vertical vibration was applied using a uniaxial vibrator, while (c) and (d) show the dynamic body pressure distribution when horizontal vibration was applied using a 6-axis vibrator. Figures 9(a) and (c) show the seat structure of a comparative example, in which the support pressure is high in the first to third lumbar vertebrae. Comparing Figure 9(a) with the dynamic body pressure distribution of the seat structure 1 of this embodiment in Figure 9(b), it can be seen that the peak value of the support pressure is higher in Figure 9(a). It can be seen that the seat structure 1 of this embodiment in Figures 9(b) and (d) supports the areas near the fourth and fifth lumbar vertebrae by the pelvic support mechanism 4, resulting in high support pressure near the pelvis and scapulae.

[0050] From these findings, the dynamic body pressure distribution in the seat structure of the comparative example shows that the load is concentrated near the lumbar region and the pressure value is high at approximately 70 mm from the spine, making it more likely to experience backslap, a phenomenon in which the backrest hits the center of the back. On the other hand, the seat structure 1 of the present embodiment, which includes the pelvic support mechanism 4, provides high support pressure in the sacrum region, and as described above, the shoulder blades are more likely to contact the backrest 3, resulting in high pressure values ​​at locations 75 mm or more away from the spine. Therefore, while the seat structure of the comparative example is structured to maintain posture using the erector spinae muscles, the seat structure 1 of the present embodiment is structured to maintain posture using the skeleton. Furthermore, while the seat structure 1 of the present embodiment exhibits minimal scapular movement in response to input vibration, as shown in the vibration experiment, the vibration is converted to a forward-backward direction, making it more likely to cause an inverted pendulum motion in which the pelvis swings back and forth around the scapulae. The maximum swing in the front-to-back direction is preferably 50 mm, and even with such a swing amount, the phase difference and opposite phase can be generated as described above, allowing for rapid damping. This reduces vertical acceleration and also reduces head swing. Furthermore, as described above, it is preferable to use a three-dimensional knitted fabric on the surface of the seat net 2b to reduce friction, which makes it easier to generate phase difference and opposite phase control due to the trunk mass between 1 and 5 Hz, thereby enhancing the effect of reducing vertical acceleration.

[0051] (Evaluation of physiological indices) Experimental conditions: One male subject in his twenties was seated in the seat structure 1 of the present embodiment and the seat structure of the comparative example, and electrocardiogram (ECG), pulse wave (PPG), and biosignals (acoustic pulse wave) of the body surface of the back were measured. The subjects' pulse wave (APW), brachial blood pressure, and heart rate were measured. Figure 5(b) shows the subject seated in the comparative seat structure, and Figure 5(c) shows the subject seated in the seat structure 1 of this embodiment. Figure 5(b) shows the measurement positions for each physiological index, all of which are viewed from the front of the body. The measurement positions in Figure 5(c) are the same as those in Figure 5(b). APW was measured at four locations: the left temporal region, the back, the third and fourth lumbar vertebrae, and the ischial tuberosity. The ECG, PPG, and APW were continuously measured for six minutes at a sampling frequency of 1000 Hz while the subject was seated in each seat structure. Brachial blood pressure and heart rate were measured twice before the six-minute continuous measurement and once after the six-minute continuous measurement.

[0052] The electrocardiogram sensor used was a Vitrode (registered trademark) F150M manufactured by Nihon Kohden Corporation, the fingertip volume pulse wave sensor was a Finger Clip Probe SR-5C manufactured by Amco Corporation, and the APW sensor was an acoustic pulse wave detection sensor used in a drowsy driving warning device (Sleep Buster (registered trademark)) manufactured by Delta Tooling Co., Ltd. Data was recorded on LabChart V8 using a high-sensitivity amplifier AB-611J manufactured by Nihon Kohden Corporation and a PowerLab (registered trademark) 8 / 30 (ML870) manufactured by ADI Instruments. Blood pressure and heart rate were measured using a UM-211 manufactured by A&D MEDICAL.

[0053] Experimental Results Figure 10 shows the fluctuations in systolic blood pressure and heart rate. In the figure, "before" represents data measured before the start of 6 minutes of continuous measurement using ECC or the like, and "after" represents data measured after the end of the 6 minutes of continuous measurement. When sitting in the seat structure of the comparative example, both blood pressure and heart rate were higher than when sitting in the seat structure 1 of this embodiment.

[0054] 11 is a graph showing power spectra (logarithmic representation) of ECG, PPG, APW temporal data, APW back data, APW lumbar data, and APW buttocks data. The ECG, PPG, APW left temporal data, and APW back data were almost equivalent when measured with the seat structure of the comparative example in FIG. 11( a) and when measured with the seat structure 1 of this embodiment in FIG. 11( b).

[0055] In contrast, there was a significant difference in the data for the lumbar region of the APW and the data for the buttocks of the APW when measured with the seat structure of the comparative example in Fig. 11(a) and when measured with the seat structure 1 of this embodiment in Fig. 11(b). With the seat structure 1 of this embodiment, the power spectra of the data for the lumbar region of the APW and the data for the buttocks of the APW showed almost the same tendency, but with the seat structure of the comparative example, the power spectrum for the lumbar region was particularly small, indicating poor blood flow and pressure being generated in the lumbar or buttocks.

[0056] FIG. 12 shows the time waveforms of ECG, PPG, APW left head data, APW back data, APW lumbar data, and APW buttocks data. Comparing measurements taken with the comparative seat structure (FIG. 12(a)) and measurements taken with the seat structure 1 of the present embodiment (FIG. 12(b)), there is a large difference in the time waveform of the APW lumbar data, and this difference is also reflected in the time waveform of the APW buttocks data. That is, when measured with the seat structure 1 of the present embodiment, the APW lumbar data and APW buttocks data had approximately the same amplitude as the APW left head and back data. In contrast, with the comparative seat structure, the amplitude of the time waveform of the APW lumbar data was very small, and the time waveform of the buttocks data also showed portions with reduced amplitude due to this influence. This suggests that the comparative seat structure may increase sympathetic nerve activity and reduce blood vessel diameter due to the strong support pressure in the lumbar region.

[0057] The above experimental results indicate that the data of subjects measured using the seat structure of the comparative example showed increased sympathetic activity, while the data of subjects measured using the seat structure 1 of the present embodiment showed parasympathetic dominance. This is thought to be related to the fact that the seat structure of the comparative example provides high support for the lumbar region and sides of the body, making it easy to engage in thoracic breathing when seated, while the seat structure 1 of the present embodiment provides high support for the pelvis and shoulder blades, making it easy to engage in abdominal breathing. In fact, when the subjects' state was visually observed to confirm whether they were engaging in thoracic or abdominal breathing, it was found that thoracic breathing was predominant when seated in the seat structure of the comparative example, but abdominal breathing was predominant when seated in the seat structure 1 of the present embodiment. Abdominal breathing stimulates the celiac plexus, alternating tension in the sympathetic and parasympathetic nervous systems, and autonomic nervous control induces a state of parasympathetic dominance even in a state of tension. Therefore, the seat structure 1 of this embodiment is suitable for suppressing blood flow obstruction in the lumbar region, for taking a posture that accepts circulatory fluctuations in blood flow due to the activation of the parasympathetic nervous system, for easily coping with increased environmental stress, and is expected to be highly effective in suppressing the progression of fatigue. Therefore, the activation state of the sympathetic and parasympathetic nervous systems is kept in a balanced and antagonistic state, and the autonomic nervous system is easily maintained in a moderate state of activation, making it suitable as a driver's seat.

[0058] Figure 13 shows the seated posture of the dummy set in the seat structure 1 of this embodiment. It also shows the assumed position of the dummy when the frame of the seat structure of the comparative example is aligned in the same position as the frame of the seat structure 1 of this embodiment. Comparing the two, the torso angle is 22 degrees in the seat structure of the comparative example, while it is 30 degrees in the seat structure 1 of this embodiment.

[0059] In the seat structure 1 of this embodiment, the eye point moved 55.6 mm rearward and 64.5 mm downward compared to the seat structure of the comparative example. Since the WL and TL directions of the hip point are consistent, the difference is due to the difference in the torso angle and the spring structure of the backrest. In the seat structure 1 of this embodiment, the angle between the scythe angle and the torso angle is 115°, which facilitates abdominal breathing. Furthermore, in the seat structure 1 of this embodiment, the backrest 3 is supported by the tension structure of the base net 3a and the backrest net 3b, and the pelvis is supported by the pelvis support mechanism 4. This provides a high degree of freedom of deformation between the lower back and chest, i.e., a high degree of freedom of the bending point when bending over, effectively accommodating individual differences in physique.

[0060] (Experiments on torsion bars) - Test materials A torsion bar in which almost the entire structure of the radial cross section is martensite (hereinafter referred to as "torsion bar A") and a multi-layered torsion bar in which a hard layer (T1) is formed at the center of the radial cross section and, in that order, a flat hardness layer (T2), a soft layer (T3), and a decarburized layer (T4) are formed toward the surface (hereinafter referred to as "torsion bar B") were manufactured. Both torsion bars A and B have a diameter of 7.8 mm.

[0061] All heat treatments were performed at a frequency of 25 kHz using the high-frequency induction heating device shown in Figure 23. Torsion bar A was rapidly heated to 900°C in about 1 second so that it was hardened to the center, and then rapidly cooled to 200°C or less in about 0.5 seconds. Torsion bar B was hardened under the same conditions as torsion bar A, and then further subjected to low-temperature heat treatment in which it was rapidly heated to about 260°C in about 0.5 seconds and then rapidly cooled to 200°C or less in about 0.5 seconds.

[0062] Experimental Method (1-1) Low Load Vickers Hardness Test The indenter was pressed with a test load of HV0.5 (4.903 N), and the diagonal length of the indentation was read to measure the hardness (see JIS Z 2244-1). Measurement was performed from the center as the origin toward the surface.

[0063] (1-2) Fatigue Test A jig incorporating torsion bars A and B, which were test materials, at the center of rotation of the link mechanism shown in Figure 14(b) was placed in a fatigue testing machine (Servopulsa (registered trademark)) manufactured by Shimadzu Corporation shown in Figure 14(a), and this jig was then placed on a vibrator. Torsion bars A and B both had an effective length of 150 mm and were incorporated with an initial twist of 13.5 degrees. A predetermined position of the link mechanism was set as the load position, and a sine wave was applied by the vibrator to vibrate the entire jig, and the reaction force at the load position was measured with a load cell.

[0064] (1-3) Bending test Two types of jigs were prepared: one with a double-end support structure that restricted downward movement, and the other with a double-end support structure that restricted vertical movement by inserting each end into a bush. For the former, a load was applied to the center between the supports with a distance between the supports of 100 mm, while for the latter, measurements were taken on two types of torsion bars with different effective lengths (150 mm and 174 mm), with the distance between the supports set to the same effective lengths of 150 mm and 174 mm, and a load was applied to the center between the supports. The shape of both loading jigs was R5.

[0065] (1-4) Torsion test A static torsion test was carried out using the jig shown in Figure 14, which was used for the fatigue test. One end of the torsion bars A and B was fixed, and the other end was free to rotate. A downward load was applied to the center of the link mechanism, causing it to break. To eliminate the effect of friction, a bearing was installed between the load point and the link mechanism.

[0066] (1-5) Observation of Fracture Surface The fracture surface of the specimen fractured in the torsion test was observed using a scanning electron microscope (SEM) (manufactured by JEOL Ltd., JSM-7200F).

[0067] Experimental Results (2-1) Comparison of Surface Hardness Figure 15 is a graph showing the hardness distribution of the cross section along the diameter direction when the center position of torsion bars A and B is 0 mm, and Figures 16(a) and (b) show the hardness distribution shown by color coding on the cross section along the diameter direction. As shown in Figures 15 and 16(a), torsion bar A has a small layer with low hardness within a range of 0.8 mm from the surface, but most of the range had a hardness of 680 Hv to approximately 700 Hv. The area within a radius of 3 mm from the center was a martensitic structure with a hardness exceeding approximately 700 Hv.

[0068] 15 and 16(b), the torsion bar B had a generally circular hard layer (T1) with a diameter of about 4 mm formed in the center, a ring-shaped flat-hardness layer (T2) with a width of about 1 mm formed outside the hard layer (T1) so as to surround the hard layer (T1), a ring-shaped soft layer (T3) with a width of about 2 mm formed outside the hard layer (T2), and a decarburized layer (T4) with a width of about several μm formed on the outer surface of the soft layer (T3). The radius of the hard layer (T1) was about 51% of the radius of the torsion bar B, the width of the flat-hardness layer (T2) was about 25%, the width of the soft layer (T3) was about 51%, and the width of the decarburized layer (T4) was less than about 0.3%. The hardness of the hard layer (T1) was about 590 Hv, the flat hardness layer (T2) was about 550 to 570 Hv, and the soft layer (T3) was about 420 to 570 Hv.

[0069] (2-2) Fatigue Test Results As shown in Figure 17, torsion bar A achieved 1 million cycles at a torsion angle of 20.75 degrees, and torsion bar B achieved 1 million cycles at a torsion angle of 16.75 degrees. The difference in torsion angle is the difference in surface hardness. Note that the torsion bar B's torsion angle at a length of 500 mm is 55.83 degrees.

[0070] (2-3) Bending test results for downward restriction Figure 18 shows the bending test results for both ends supported with downward restriction. Torsion bar A broke at 2750 MPa (elastic range), as indicated by the "X" mark in the figure. Torsion bar B did not break even at 3900 MPa within the test environment.

[0071] (2-4) Bending test results for vertical restriction Figure 19 shows the results of a bending test for both ends supported with vertical restriction. Torsion bar A with an effective length of 150 mm broke at 3,300 N, as indicated by the solid "x" mark, and torsion bar A with an effective length of 174 mm broke at 2,200 N, as indicated by the dashed "x" mark (see Figure 20(a)). In contrast, torsion bar B did not break at either the 150 mm or 174 mm effective lengths, as shown in Figures 19 and 20(b).

[0072] (2-5) Spring constant by effective length of torsion bar Figures 21(a) and (b) show the spring constant by effective length for torsion bar A based on the results of the torsion test. Figure 21(a) shows data for an effective length of 150 mm, and Figure 21(b) shows data for an effective length of 174 mm. The solid lines show the measured data up to fracture, and the dashed lines show the calculated spring constant. These data were calculated to predict durability in conjunction with Figure 17, and it can be seen from this data that the torsion angle increases as the effective length increases. Torsion bar B also showed a similar trend.

[0073] (2-6) Observation results of fracture surfaces in torsion tests Figure 22(a) is a photograph of the fracture surface of torsion bar A with a martensitic structure taken with a scanning electron microscope. The observation positions are a, b, and c shown in the photograph of the fracture surface at the top left, with a being the surface side, c being the center side, and b being the position between the two. The two microscope photographs at each observation position are at the top right and bottom. Torsion bar A with a martensitic structure shows a lot of intergranular fracture near the surface, and dimples can be observed closer to the center. Brittle fracture occurred on the surface, followed by ductile fracture, resulting in a fracture morphology similar to that of carburized quenching.

[0074] Figure 22(b) is a scanning electron microscope photograph of the fracture surface of multilayer torsion bar B. The observation positions are indicated by a, b, and c in the upper left photograph of the fracture surface, with a and c being the surface side and b being the center side. The two microscope photographs at each observation position are shown in the upper right and lower left. In multilayer torsion bar B with structures of different hardness, refined ferrite structures were dominant in the surface layer (particularly at position c), while martensite structures were dominant in the center side of b. The areas between b and c and between a and b correspond to the plateau-like hardness plateau (T2) where the two phases of ferrite and martensite are in equilibrium, forming the critical phase between the two structures. It is assumed that the presence of this critical phase creates a synergistic effect, resulting in performance similar to that of dual-phase steel. Therefore, dimples caused by the presence of ferrite structure in the surface layer can be seen in the medium hardness flat layer (T2) and the central hard layer (T1), which is thought to be the reason why brittle fracture did not occur.

[0075] ・Discussion From the above experimental results, Figures 17 and 21 show that although there is a difference in hardness between the martensitic torsion bar A and the multi-layered torsion bar B, the fifth power law can be applied to the S-N curve to predict durability in both cases. Also, Figures 17 to 20 show that the torsion bar A has sufficient properties as a spring just by increasing the hardness, but the torsion bar B not only has the properties as a spring, but also has a good balance between hardness and bending properties, making it suitable as a structural member.

[0076] Therefore, it is preferable to use the multi-layered torsion bar B in this experimental example as the torsion bars 44, 45 used in the pelvis support mechanism 4, which is the body support mechanism of the above embodiment. The torsion bars 44, 45 in the above embodiment are arranged near the side frames 31, 32 of the back frame 30 that constitutes the backrest 3. For this reason, it is important that the torsion bars 44, 45 have high load-bearing strength so that they do not easily break when an impact load is input, and it is preferable to use a multi-layered torsion bar B that exhibits high bending characteristics. This eliminates the need to house the torsion bars in a covering pipe as in the conventional case, and they can be arranged as they are, which contributes to reducing the weight of the seat structure 1.

[0077] Furthermore, by using the multi-layered torsion bar B as the torsion bars 44, 45, it is possible to improve the strength of the seat structure 1 including the back frame 30, which in turn contributes to reducing the weight of the frame material constituting the back frame 30, etc. The torsion bar that has undergone the heat treatment described above is not limited to being incorporated into the back frame 30 and elastically supporting the backrest 3 as in the seat structure 1 of the above embodiment, but can also be incorporated into the seat portion 2 of the seat structure 1 and used to elastically support the frame and cushion member provided in the seat portion.

[0078] 24 shows the frame structure of a seat structure 1' according to another embodiment of the present invention. The backrest 3 is provided with a pelvis support mechanism 4 having the same structure as in the above embodiment, but the seat portion 2 is provided with a thigh support mechanism 4000, which is a body support mechanism.

[0079] In the seat portion 2 of this embodiment, a front bracket 230 and a rear bracket 240 are disposed below the front frame 23 and the rear frame 25, respectively, and torsion bars 4440 and 4450 constituting an elastic support mechanism 4400 are suspended along the front-to-rear direction of the seat portion 2 near the side frames 21 and 22 of the front bracket 230 and the rear bracket 240. The torsion bars 4440 and 4450 are provided so that rear ends 4441 and 4451 are fixed ends and front ends 4442 and 4452 are movable ends. The bases of arm members 4410 and 4420 are connected to the front ends 4442 and 4452, respectively, and the arm members 4410 and 4420 can rotate up and down around the torsion bars 4440 and 4450 as rotation axes. The arm members 4410, 4420 are arranged so that their distal ends extend to the center of the front edge of the seat 2, and are rotatably connected to link plates 4480, 4490 similar to those in the above embodiment. The link plates 4480, 4490 are provided on the back side of the support plate 4430. Therefore, the support plate 4430 is provided in a position corresponding to the area of ​​the thighs near the knees, that is, in a region corresponding to the thighs near the front edge of the seat 2.

[0080] The support plate 4430 is formed in a substantially rectangular shape and is disposed in such a position that its longitudinal direction is aligned with the left-right direction of the seat portion 2 and its width direction is aligned with the front-rear direction of the seat portion 2. The connecting structure of the arm members 4410, 4420 and the link plates 4480, 4490 is exactly the same as the structure employed in the pelvis support mechanism 4 of the above embodiment. As a result, when the load of the thighs is applied to the support plate 4430 during sitting, the support plate 4430 is displaced downward, and the torsion bars 4440, 4450 are twisted via the link plates 4480, 4490 and arm members 4410, 4420. As a result, the torsion bars 4440, 4450 urge the support plate 4430 upward, i.e., in a direction approaching the back of the thighs of the seated person. The elastic force of the torsion bars 4440, 4450 allows the support plate 4430 to follow any change in posture such as when the thighs of both legs are displaced up and down almost equally. As a result, the vibration absorption characteristics can be improved, contributing to an improvement in ride comfort.

[0081] On the other hand, for example, if a person sitting with both knees bent slightly extends one leg forward, the area of ​​the support plate 4430 corresponding to that leg is pressed downward. As a result, a rotational movement occurs in the support plate 4430, in which the longitudinal ends 4432, 4433 are displaced up and down around the longitudinal center position C. Therefore, the area corresponding to the back of the thigh of the other leg with the knee still bent does not displace downward significantly, and tends to remain close to the back of the other thigh. Therefore, the thigh support mechanism of this embodiment can follow various posture changes of the thigh and exhibit high supportability.

[0082] As in the above embodiment, the support plate 4430 preferably has a flat central portion 4431, which is a predetermined length ranging from the center position C in the longitudinal direction, and has inclined surfaces 4434, 4435 such that the end portions 4432, 4433 on both sides of the flat central portion 4431 are higher than the central portion 4431. This prevents the thighs near the backs of the knees from shifting left and right, and improves conformability by conforming to the shape of the thighs. Note that the support plate 4430 in this embodiment has the same shape as the support plate 43 in the above embodiment, but the surfaces corresponding to the body parts of the seated person are oriented approximately 90 degrees apart. However, the support plate 4430 can be shaped to fit the shape of the thighs. For example, the inclination angles of the inclined surfaces 4434, 4435 can be made steeper.

[0083] In the embodiment shown in Figure 24, elastic support mechanisms 40, 4400 are incorporated into both the backrest 3 and the seat portion 2, and are configured as a pelvis support mechanism 4 and a thigh support mechanism 4000, respectively, providing two body part support mechanisms, but it is also possible to have a mechanism that does not include the pelvis support mechanism 4 and only has the thigh support mechanism 4000 of the seat portion 2. In that case, the advantages unique to the pelvis support mechanism 4 explained in the above embodiment will be lost, but it is possible to improve the supportability of the thighs.

[0084] DESCRIPTION OF SYMBOLS 1, 1' Seat structure 2 Seat portion 20 Cushion frame 2b Seat net 3 Backrest 30 Back frame 3b Backrest net 4 Pelvis support mechanism 40 Elastic support mechanism 41, 42 Arm member 43 Support plate 43a Central portion 43b, 43c End portion 43d, 43e Inclined surface 43f Lower edge 43g Upper edge 44, 45 Torsion bar 48, 49 Link plate 4000 Thigh support mechanism 4400 Elastic support mechanism 4410, 4420 Arm member 4430 Support plate 4440, 4450 Torsion bar 4480, 4490 Link plate

Claims

1. A seat structure having a seat and a backrest, wherein a body support mechanism is provided on at least one of the seat and the backrest, the body support mechanism comprising a support plate arranged so that its longitudinal direction runs along the left-right direction of the seat or the backrest, and an elastic support mechanism which biases the support plate in a direction to bring it closer to a body part of a seated occupant and supports the support plate so that it can be displaced in a rotational direction around a central position in the longitudinal direction.

2. The seat structure according to claim 1, wherein the elastic support mechanism comprises a pair of arm members provided at a predetermined distance in the left-right direction on a frame constituting the seat or backrest, and a torsion bar used as a rotation shaft for rotatably supporting the base of each of the pair of arm members on the frame, and the tip of each of the pair of arm members is rotatably supported on the support plate.

3. The seat structure according to claim 2, wherein the tip ends of the pair of arm members are connected to the support plate via link plates.

4. The seat structure of claim 3, wherein the spring constant of the body support mechanism is determined from the load-deflection characteristics obtained by pressing the support plate, where k1 is the spring constant in the displacement range up to the equilibrium point when seated, k2 is the spring constant in a predetermined displacement range from the equilibrium point, and k3 is the spring constant in a displacement range beyond the predetermined displacement range from the equilibrium point, and the spring constant of k2 is the largest among the spring constants k1, k2, and k3, and the body support mechanism is supported by a spring force of spring constant k2 at the equilibrium point when seated, and a spring force of spring constant k1 or k3 acts when vibration is input.

5. The seat structure according to claim 1, wherein the support plate is formed in a shape such that each end in the longitudinal direction is located forward of the center portion in the longitudinal direction.

6. The seat structure of claim 1, wherein the body support mechanism is a pelvis support mechanism in which the support plate is disposed at a portion of the backrest corresponding to the pelvis, and the support plate constituting the pelvis support mechanism is disposed such that its lower edge is below a position on the pelvis of the seated person corresponding to at least the upper part of the sacrum and in a range not in contact with the seat, and its upper edge is disposed above the position corresponding to at least the upper part of the sacrum and in a range up to a position corresponding to the fourth lumbar vertebra.

7. The seat structure according to claim 1, wherein the body support mechanism is a thigh support mechanism in which the support plate is disposed at a position corresponding to the thighs near the front edge of the seat.

8. The seat structure according to claim 1, wherein a seat net is hung across the cushion frame constituting the seat with a predetermined tension, and a backrest net is hung across the back frame constituting the backrest with a predetermined tension.

9. The seat structure according to claim 8, wherein the seat net and the backrest net are three-dimensional knitted fabrics made of synthetic resin multifilament or monofilament.

10. A seat structure as claimed in claim 2, wherein the torsion bar is heat-treated, the center of a cross section taken along a radial direction perpendicular to the longitudinal direction has a hard layer having an average hardness higher than other layers, and from the hard layer towards the surface side, a flat hardness layer having a substantially constant hardness, a soft layer and a decarburized layer having a gradually decreasing hardness, the flat hardness layer, the soft layer and the decarburized layer being formed in a ring shape along the outer periphery of the cross section taken along the radial direction and surrounding the hard layer, forming a multi-layer structure.

11. The seat structure according to claim 10, wherein the hard layer is formed in a range of a distance of 30 to 70% of the radius of the torsion bar from the center of a cross section perpendicular to the longitudinal direction of the torsion bar.

12. The seat structure according to claim 10, wherein the width of the ring of the flat hardness layer is equivalent to 5 to 40% of the radius of the torsion bar, and the width of the ring of the soft layer is equivalent to 15 to 60% of the radius of the torsion bar.

13. The seat structure according to claim 10, wherein the width of the ring of the decarburized layer is equal to or less than 1% of the radius of the torsion bar.

14. A torsion bar used as a spring material incorporated into a seat or backrest in a seat structure and arranged to elastically support said seat or backrest, the torsion bar being manufactured by heat treatment, the torsion bar having a hard layer having a higher average hardness than other layers at the center of a cross section taken along a radial direction perpendicular to the longitudinal direction, the torsion bar having, from the hard layer toward the surface, a flat hardness layer having a substantially constant hardness, a soft layer and a decarburized layer having a gradually decreasing hardness, the flat hardness layer, the soft layer and the decarburized layer being formed in a ring shape along the outer periphery of the cross section taken along the radial direction and surrounding the hard layer, the torsion bar having a multi-layer structure.

15. The torsion bar according to claim 14, wherein the hard layer is formed within a distance range of 30 to 70% of the radius of the torsion bar from the center of a cross section perpendicular to the longitudinal direction of the torsion bar.

16. The torsion bar according to claim 14, wherein the width of the ring of the flat hardness layer is equivalent to 5 to 40% of the radius of the torsion bar, and the width of the ring of the soft layer is equivalent to 15 to 60% of the radius of the torsion bar.

17. The torsion bar according to claim 14, wherein the width of the ring of the decarburized layer is equal to or less than 1% of the radius of the torsion bar.

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