Flywheel power storage device

Abstract

The present invention provides a flywheel power storage device that can suppress flywheel breakage while keeping manufacturing costs down. [Solution] The flywheel power storage device 1 comprises a flywheel 5 made of carburized steel and a rotating shaft 3 inserted concentrically and integrally into the flywheel 5, and stores electricity as rotational kinetic energy by rotating the flywheel 5 together with the rotating shaft 3. The flywheel 5 has a through hole 51 through which the rotating shaft 3 is inserted, and the radially outer region 52 of the flywheel 5 beyond the through hole 51 has a structure that substantially does not contain nonmetallic inclusions 10, and the inner circumference of the flywheel 5 around the through hole 51 consists of a surface layer 53 which is a carburized layer or carbonitride layer, and residual compressive stress is generated in the surface layer 53.

Description

Technical Field

[0004] , , , , , ,

[0003] , , ,

[0001] The present disclosure relates to a flywheel power storage device.

Background Art

[0002] For example, the flywheel energy storage system shown in Patent Document 1 includes a housing, a rotating shaft rotatably supported with respect to the housing, a disk-shaped flywheel that rotates integrally with the rotating shaft, and a generator / motor. This system stores electric power by converting it into rotational kinetic energy by rotating the flywheel at high speed with the generator / motor. The stored rotational kinetic energy is converted into electric power by the generator / motor and taken out.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] <00)0025>Generally, a flywheel is formed in a disk shape from a steel material made of an atmospheric melting steel ingot. However, in the manufacturing process, the central region of the disk shape tends to contain a large amount of non-metallic inclusions. When such non-metallic inclusions are contained, the fatigue strength of the central region of the flywheel decreases, and there is a problem that the flywheel is likely to break starting from the non-metallic inclusions due to the centrifugal force accompanying high-speed rotation. On the other hand, although there is a method of subjecting the material to bainite (ausforming) heat treatment to make it easier to deform and thus blunt the inclusion sensitivity, the problem that the flywheel is likely to break starting from the non-metallic inclusions in the central region remains. Therefore, nonmetallic inclusions are removed by subjecting the aforementioned material to vacuum arc remelting (see Patent Document 1). However, this method has the problem of poor productivity and high manufacturing costs.

[0005] In view of these challenges, this disclosure aims to provide a flywheel power storage device that can suppress flywheel breakage while keeping manufacturing costs down. [Means for solving the problem]

[0006] (1) The flywheel power storage device of the present disclosure comprises a flywheel made of carburized steel and a rotating shaft inserted concentrically and integrally with the flywheel, and stores power as rotational kinetic energy by rotating the flywheel together with the rotating shaft, wherein a through hole is formed in the flywheel through which the rotating shaft is inserted, the region of the flywheel radially outward from the through hole has a structure that substantially does not contain the nonmetallic inclusions, the inner circumference of the flywheel around the through hole consists of a carburized layer or a carbonitride layer, and residual compressive stress is generated in the carburized layer or carbonitride layer.

[0007] According to the flywheel power storage device described above, the outer-radial region radially outward from the through-hole of the flywheel has a structure that is substantially free of non-metallic inclusions, thus suppressing fracture of the flywheel caused by non-metallic inclusions. Furthermore, the inner circumference around the through-hole of the flywheel consists of a carburized or carbonitride layer where residual compressive stress is present, making it less likely for fine cracks to occur in the inner circumference due to insufficient fatigue strength of the flywheel. As a result, even if a through-hole is formed in the flywheel, cracks originating from the inner circumference due to insufficient fatigue strength of the flywheel can be suppressed. In addition, since there is no need to perform vacuum arc remelting to remove non-metallic inclusions from the flywheel material, manufacturing costs can be reduced.

[0008] (2) In the flywheel power storage device described in (1) above, it is preferable that the diameter of the through hole is 15% or more and 20% or less of the outer diameter of the flywheel. In this case, the outer diameter region of the flywheel can be made to contain even fewer non-metallic inclusions, and the fatigue strength of the flywheel can be ensured, thus further suppressing flywheel fracture.

[0009] (3) In the flywheel power storage device of (1) or (2) above, the circumferential surface of the through hole is preferably tapered, gradually increasing in diameter from one axial side to the other axial side, and the insertion portion of the through hole on the outer circumferential surface of the rotating shaft is preferably tapered, formed along the circumferential surface of the through hole. In this case, the other axial side of the through-hole in the flywheel is enlarged, so the outer diameter region of the flywheel can be made to contain even fewer non-metallic inclusions. This further suppresses the risk of the flywheel fracturing due to non-metallic inclusions. In addition, by inserting the tapered insertion portion of the rotating shaft through the tapered circumferential surface of the through-hole, the rotating shaft can be inserted into the flywheel's through-hole without any gaps. This suppresses rattling of the flywheel relative to the rotating shaft during rotation.

[0010] (4) The flywheel power storage device described in (3) is preferably further provided with a biasing member that biases the flywheel in the other axial direction with respect to the rotating shaft inserted through the through hole. In this case, even if the flywheel deforms so that the through-hole expands in diameter due to the centrifugal force accompanying its rotation, the biasing member biases the rotating shaft toward the expanded diameter side (the other axial side), thus preventing a gap from forming between the rotating shaft and the through-hole during the rotation of the flywheel. This maintains the concentricity between the rotating shaft and the flywheel, thereby preventing the flywheel from rattling relative to the rotating shaft during rotation.

[0011] (5) In the flywheel power storage device described in (4) above, the biasing member is preferably a disc spring. In this case, the entire device can be made compact in the axial direction while suppressing rattling of the flywheel relative to the rotating shaft during rotation.

[0012] (6) In any of the flywheel power storage devices described in (1) to (5) above, the carburized steel is preferably SNCM439. In this case, the risk of flywheel fracture can be further suppressed. [Effects of the Invention]

[0013] The flywheel power storage device of this disclosure can suppress flywheel breakage while keeping manufacturing costs down. [Brief explanation of the drawing]

[0014] [Figure 1] This is a cross-sectional view showing a flywheel power storage device according to an embodiment of the present disclosure. [Figure 2] This is a plan view showing a disc-shaped steel material during the manufacturing process of a flywheel. [Figure 3] This graph shows the residual compressive stress distribution in the thickness direction in the surface layer of the flywheel through-hole. [Figure 4] This graph shows the hardness distribution in the thickness direction of the surface layer of the through-hole of a flywheel. [Modes for carrying out the invention]

[0015] Preferred embodiments of this disclosure will be described below with reference to the drawings. [Overall Configuration of a Flywheel Power Storage System] FIG. 1 is a cross-sectional view showing a flywheel power storage device 1 according to an embodiment of the present disclosure. The flywheel power storage device 1 is mounted on a vehicle such as an automobile, for example. In FIG. 1, the flywheel power storage device 1 includes a housing 2, a rotating shaft 3, a generator / motor 4, and a flywheel 5. The rotating shaft 3, the generator / motor 4, and the flywheel 5 are all disposed within the housing 2. The entire flywheel power storage device 1 including the housing 2 is maintained in an environment with a vacuum gauge pressure of -0.1 MPa. The outer diameter of the flywheel 5 is, for example, 800 mm. The generator / motor 4 is of a switched reluctance type that does not use permanent magnets.

[0016] During charging, the flywheel power storage device 1 converts electric power (electrical energy) into rotational kinetic energy and stores it by rotating the flywheel 5 together with the rotating shaft 3 at a high speed (for example, 6500 rpm) by the generator / motor 4. During discharging, the flywheel power storage device 1 converts the stored rotational kinetic energy into electric power by rotating the generator / motor 4.

[0017] Hereinafter, in this specification, the "axial direction" is the direction along the axis X of the rotating shaft 3. The "radial direction" is the direction orthogonal to the axis X of the rotating shaft 3.

[0018] The housing 2 is formed in a cylindrical shape. The rotating shaft 3 is disposed at the central portion within the housing 2. The housing 2 and the rotating shaft 3 are disposed with the axial direction oriented in the vertical direction. The rotating shaft 3 has, in order from the upper side in the axial direction, a first shaft portion 31, a second shaft portion 32, a third shaft portion 33, a fourth shaft portion 34, and a fifth shaft portion 35.

[0019] The outer peripheral surfaces of the first shaft portion 31, the second shaft portion 32, the third shaft portion 33, and the fifth shaft portion 35 are circumferential surfaces with a constant outer diameter over the entire axial direction, and each of these outer peripheral surfaces is subjected to polishing. The outer diameter of the first shaft portion 31 and the outer diameter of the fifth shaft portion 35 are the same. The outer diameter of the second shaft portion 32 is larger than the outer diameter of the first shaft portion 31. The outer diameter of the third shaft portion 33 is larger than the outer diameter of the second shaft portion 32. The fourth shaft portion 34 will be described later.

[0020] The first shaft portion 31 of the rotating shaft 3 is supported by a plurality of rolling bearings 8 at the upper axial end portion of the housing 2. The fifth shaft portion 35 of the rotating shaft 3 is supported by a plurality of rolling bearings 9 at the lower axial end portion of the housing 2. Thereby, the rotating shaft 3 can rotate around the axis X with respect to the housing 2. The rolling bearings 8 and 9 are grease-type ceramic bearings that can be used in a vacuum environment.

[0021] The power generation and motor 4 is disposed between the second shaft portion 32 of the rotating shaft 3 and the housing 2. The power generation and motor 4 has a stator 41 provided on the inner peripheral surface of the housing 2 and a rotor 42 provided on the outer peripheral surface of the second shaft portion 32 facing the stator 41 in the radial direction. The power generation and motor 4 functions as an electric motor that rotationally drives the rotating shaft 3 and the flywheel 5 during charging, and functions as a generator during discharging. As described above, the power generation and motor 4 that does not use a permanent magnet avoids heat generation due to the rotation of the permanent magnet during standby without power supply or free-running operation, resulting in low loss.

[0022] The fourth shaft portion 34 of the rotating shaft 3 is inserted into the flywheel 5 so as to be concentric and rotatable integrally. The flywheel 5 is made of carburized steel. The flywheel 5 of the present embodiment is made of, for example, SNCM439. When manufacturing the flywheel 5, the raw material is melted in the atmosphere, subjected to electric furnace refining and degassing treatment, cast into an ingot (material), then formed into a disk-shaped steel material by forging, and cooled.

[0023] FIG. 2 is a plan view showing a disk-shaped steel material 50 during the manufacture of the flywheel 5. As shown in FIG. 2, in the manufacturing process of the flywheel 5 described above, non-metallic inclusions 10 are likely to be contained on the center side of the disk-shaped steel material 50. The non-metallic inclusions 10 are defined in JIS G 0555:2020. The non-metallic inclusions 10 include elongated oxide-based A-type inclusions 10a, alumina-based B-type inclusions 10b arranged discontinuously in a granular shape in the processing direction, and silicate-based C-type inclusions 10c that are irregularly scattered.

[0024] During the manufacture of the flywheel 5, the central region 50a in the steel material 50, which is prone to containing non-metallic inclusions 10, is removed along its entire axial direction by forging or the like. The central region 50a is a circular region concentric with the steel material 50. The steel material 50 from which the central region 50a has been removed is then subjected to machining using a lathe or the like, and heat treatment.

[0025] In Figure 1, a through-hole 51 is formed in the region of the flywheel 5 manufactured as described above, which is prone to containing non-metallic inclusions 10 (the central region 50a of the steel material 50), and which penetrates axially. Therefore, the outer-radial region 52, which is the region radially outward from the through-hole 51 in the flywheel 5, has a structure that is substantially free of non-metallic inclusions 10. Here, "substantially free of non-metallic inclusions 10" means not only a structure that is completely free of non-metallic inclusions 10, but also a structure that contains non-metallic inclusions 10 to an extent that does not affect the fatigue strength of the flywheel 5.

[0026] Preferably, the diameter of the through hole 51 is 15% or more and 20% or less of the outer diameter of the flywheel 5. If the diameter of the through hole 51 is less than 15% of the outer diameter of the flywheel 5, non-metallic inclusions 10 are more likely to be included in the outer diameter region 52 of the flywheel 5, increasing the likelihood of the flywheel 5 fracturing starting from the non-metallic inclusions 10. Also, if the diameter of the through hole 51 exceeds 20% of the outer diameter of the flywheel 5, the fatigue strength of the flywheel 5 cannot be ensured, and the insufficient fatigue strength of the flywheel 5 increases the likelihood of fine cracks occurring in the inner circumference around the through hole 51 of the flywheel 5.

[0027] The inner circumference of the flywheel 5 around the through hole 51 consists of a surface layer 53 hardened by the heat treatment. In this embodiment, the surface layer 53 is a carburized layer or a carbonitriding layer. The carburized layer is formed in the heat treatment by allowing carbon to penetrate from the surface of the steel material 50 through a carburizing treatment, followed by quenching and tempering treatments. The carbonitriding layer is formed in the heat treatment by allowing carbon and nitrogen to penetrate simultaneously from the surface of the steel material 50 through a carbonitriding treatment, followed by quenching and tempering treatments.

[0028] After the heat treatment is performed on the steel material 50, the circumferential surface 54 of the through hole 51 (the surface of the surface layer 53) is polished. Other parts of the steel material 50 are also polished as needed. The radial thickness of the surface layer 53 after polishing is preferably 0.5 mm or more and 1.0 mm or less (preferably 0.8 mm). Residual compressive stress is present in the surface layer 53.

[0029] Figure 3 is a graph showing the residual compressive stress distribution in the thickness direction in the surface layer 53 of the through hole 51 of the flywheel 5. As shown in Figure 3, the residual compressive stress is high, at approximately 200 MPa or more, near the surface of the surface layer 53, from 0 mm to 1 mm from the surface.

[0030] Figure 4 is a graph showing the hardness distribution in the thickness direction of the surface layer 53 of the through hole 51 of the flywheel 5. As shown in Figure 4, the Vickers hardness of the surface layer 53 is highest at approximately 750 HV at a distance of 0.1 mm from the surface.

[0031] Therefore, near the surface of the surface layer 53, both residual compressive stress and hardness are high, indicating that the fatigue strength of the inner circumference of the flywheel 5 (the area around the through hole 51) is high. As a result, when the flywheel 5 is rotated at high speed, the tensile stress generated in the inner circumference of the flywheel 5 due to the centrifugal force associated with the high-speed rotation can be offset by the residual compressive stress near the surface of the surface layer 53. As a result, even if a through hole 51 is formed in the flywheel 5, cracks originating from the inner circumference can be suppressed due to insufficient fatigue strength in that area.

[0032] Furthermore, as shown in Figure 4, the Vickers hardness of the surface layer 53 decreases to approximately 350 HV when the distance from the surface is 0.8 mm or more. As a result, the radially outer portion of the flywheel 5 remains at the hardness of the base material (steel 50) compared to the inner circumference, making it more prone to deformation rather than fracture when tensile stress occurs. This also helps to suppress fracture of the radially outer portion of the flywheel 5.

[0033] In Figure 1, the fourth shaft portion 34 of the rotating shaft 3 is inserted through the through hole 51 of the flywheel 5. The fourth shaft portion 34 is longer in the axial direction than the through hole 51. The circumferential surface 54 of the through hole 51 has a tapered shape, gradually increasing in diameter from the axial upper side (one axial side) to the axial lower side (the other axial side). The minimum outer diameter of the fourth shaft portion 34 is larger than the outer diameter of the third shaft portion 33.

[0034] The outer circumferential surface 34a of the fourth shaft portion 34, which is the insertion portion of the through hole 51 in the rotating shaft 3, has a tapered shape formed along the circumferential surface 54 of the through hole 51. In other words, the outer circumferential surface 34a of the fourth shaft portion 34 has a tapered shape that gradually widens in diameter from the axial upper side to the axial lower side. The fourth shaft portion 34 is inserted into the through hole 51 of the flywheel 5 from the axial lower side to the axial upper side.

[0035] As described above, the diameter of the through hole 51 is preferably 15% or more and 20% or less of the outer diameter of the flywheel 5. Therefore, the minimum diameter at the axial upper end of the through hole 51 is preferably 15% or more of the outer diameter of the flywheel 5, and the maximum diameter at the axial lower end of the through hole 51 is preferably 20% or less of the outer diameter of the flywheel 5. The surface layer 53 is formed in a tapered shape along the circumferential surface 54 of the through hole 51.

[0036] The circumferential surface 54 of the through hole 51 and the circumferential surface 34a of the fourth shaft portion 34 may have a tapered shape that gradually increases in diameter from the lower axial side to the upper axial side. Alternatively, the circumferential surface 54 of the through hole 51 and the circumferential surface 34a of the fourth shaft portion 34 may be a circular surface with a constant diameter throughout the entire axial direction.

[0037] The flywheel power storage device 1 further comprises a nut 6 provided on the third shaft portion 33 of the rotating shaft 3, and a biasing member 7 provided between the nut 6 and the flywheel 5. The nut 6 is tightened from the axial upper side to the axial lower side onto a male thread 33a formed on the outer circumference of the third shaft portion 33.

[0038] The biasing member 7 is, for example, an annular disc spring. The inner circumference of the biasing member 7 abuts against the axially lower end face of the nut 6. The outer circumference of the biasing member 7 abuts against the axially upper end face of the flywheel 5. The biasing member 7 biases the flywheel 5 axially downward (towards the enlarged diameter side of the fourth shaft portion 34) relative to the fourth shaft portion 34 inserted through the through hole 51.

[0039] The biasing force of the biasing member 7 is adjusted by the amount the nut 6 is tightened against the male thread 33a of the third shaft portion 33. Specifically, the more the nut 6 is tightened axially downward against the male thread 33a of the third shaft portion 33, the greater the biasing force of the biasing member 7 can be increased.

[0040] The disc spring of the biasing member 7 may have its inner circumference in contact with the flywheel 5 and its outer circumference in contact with the nut 6. Alternatively, multiple disc springs may be provided axially between the nut 6 and the flywheel 5. The biasing member 7 may also be an elastic member other than a disc spring.

[0041] [Effects and Effects] According to the flywheel power storage device 1 of this embodiment, the radially outer region 52 of the flywheel 5, which is radially outward from the through hole 51, has a structure that substantially does not contain non-metallic inclusions 10, thus suppressing fracture of the flywheel 5 starting from the non-metallic inclusions 10. Furthermore, the inner circumference of the flywheel 5 around the through hole 51 consists of a carburized layer or carbonitrided layer, which is a surface layer 53 where residual compressive stress is generated, so fine cracks are less likely to occur in the inner circumference due to insufficient fatigue strength of the flywheel 5. As a result, even if a through hole 51 is formed in the flywheel 5, cracks starting from the inner circumference due to insufficient fatigue strength of the flywheel 5 can be suppressed. In addition, since it is not necessary to perform vacuum arc remelting to remove non-metallic inclusions 10 from the material (steel ingot) of the flywheel 5, manufacturing costs can be reduced.

[0042] Since the diameter of the through-hole 51 is 15% or more and 20% or less of the outer diameter of the flywheel 5, the outer diameter region 52 of the flywheel 5 can be made to have a structure that further does not contain non-metallic inclusions 10, and the fatigue strength of the flywheel 5 can be ensured. As a result, fracture of the flywheel 5 can be further suppressed.

[0043] The circumferential surface 54 of the through hole 51 of the flywheel 5 has a tapered shape that gradually widens in diameter from the axial upper side to the axial lower side. As a result, the axial lower side of the through hole 51 is widened, which allows the outer diameter region 52 of the flywheel 5 to have a structure that is even less free of nonmetallic inclusions 10. This further suppresses fracture of the flywheel 5 starting from the nonmetallic inclusions 10.

[0044] Since both the circumferential surface 54 of the through hole 51 of the flywheel 5 and the circumferential surface 34a of the fourth shaft portion 34 are tapered, the circumferential surface 34a of the fourth shaft portion 34 can be inserted through the circumferential surface 54 of the through hole 51 of the flywheel 5 without any gap. This makes it possible to suppress rattling of the flywheel 5 relative to the fourth shaft portion 34 during rotation.

[0045] The flywheel 5 is biased axially downward with respect to the rotating shaft 3 inserted through the through hole 51 by the biasing member 7. As a result, even if the through hole 51 deforms due to the centrifugal force accompanying the rotation of the flywheel 5, the biasing member 7 biases the fourth shaft portion 34 toward the expanded diameter side (the other axial side), thereby suppressing the formation of a gap between the outer circumferential surface 34a of the fourth shaft portion 34 and the circumferential surface 54 of the through hole 51 during the rotation of the flywheel 5. Therefore, the concentricity between the fourth shaft portion 34 and the flywheel 5 can be maintained, and rattling of the flywheel 5 relative to the fourth shaft portion 34 can be further suppressed.

[0046] Since the biasing member 7 is a disc spring, the entire flywheel power storage device 1 can be configured compactly in the axial direction, while suppressing rattling of the flywheel 5 relative to the fourth shaft portion 34 during rotation. Since the flywheel 5 is made of SNCM439, the risk of the flywheel 5 breaking can be further suppressed.

[0047] [others] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the claims, not in the sense described above, and is intended to include all modifications in the sense and scope equivalent to the claims. [Explanation of symbols]

[0048] 1. Flywheel power storage device 3 rotation axes 5 Flywheel 7. Biasing member 10 Non-metallic media 34a Outer Peripheral Surface 51 Through Hole 52. Outside the Circumference 53 Surface layer (carburized layer, carburized annealing layer) 54 Weeks

Claims

1. A flywheel power storage device comprising a flywheel made of carburized steel and a rotating shaft inserted concentrically and integrally through the flywheel, wherein power is stored as rotational kinetic energy by rotating the flywheel together with the rotating shaft, The flywheel has a through hole through which the rotating shaft is inserted, and the outer-radial region of the flywheel, radially outward from the through hole, has a structure that is substantially free of non-metallic inclusions. A flywheel power storage device wherein the inner circumference of the flywheel around the through hole is made of a carburized layer or a carbonitride layer, and residual compressive stress is generated in the carburized layer or the carbonitride layer.

2. The flywheel power storage device according to claim 1, wherein the diameter of the through hole is 15% or more and 20% or less of the outer diameter of the flywheel.

3. The circumferential surface of the through hole has a tapered shape that gradually widens in diameter from one axial side to the other axial side. The flywheel power storage device according to claim 1 or claim 2, wherein the insertion portion of the through hole on the outer circumferential surface of the rotating shaft is tapered in shape along the circumferential surface of the through hole.

4. The flywheel power storage device according to claim 3, further comprising a biasing member that biases the flywheel in the other axial direction with respect to the rotating shaft inserted through the through hole.

5. The flywheel power storage device according to claim 4, wherein the biasing member is a disc spring.

6. The flywheel power storage device according to claim 1 or claim 2, wherein the carburized steel is SNCM439.

Images