Rotor with improved cooling structure

Abstract

The invention relates to a rotor with an improved cooling structure. A rotor is provided, and more particularly, a rotor with improved cooling efficiency is provided. The rotor with an improved cooling structure of the present disclosure may change a motor rotor structure without additional components, and increase a transmission speed of a cooling fluid by forming blades on a surface thereof using a magnetic fixing adhesive, thereby increasing a convective heat transfer rate, can be applied to a water-cooled or air-cooled motor, and can be used in a water-cooled or air-cooled motor. Therefore, the performance of the motor is improved, and a relatively simple structure can be adopted, for example, an existing cooling component such as an oil pipe is eliminated, so that the material and process cost is reduced.

Description

Technical Field

[0001] The following disclosure relates to a rotor, and more particularly to a rotor having improved cooling efficiency. Background Art

[0002] Heat sources of an electric vehicle drive motor can be coils through which current flows and electrical steel cores through which magnetic flux flows. When the motor operates, the temperature of related components may increase, and the motor may malfunction when the temperature rises excessively. To prevent such malfunctions, it may be important to cool the heat sources of the motor. In this regard, there are an oil cooling method of directly injecting oil onto the heat sources of the motor and a water cooling method of indirectly cooling the heat sources by allowing cooling water to flow through a water passage portion of a housing.

[0003] Here, an end coil portion (or each of both ends of the coil, that is, the outermost portion protruding from the core of the coil) shows a relatively high heat generation rate in the motor compared to other components, and when local cooling is not performed on the end coils, the motor performance may be affected thereby. Therefore, the prior art uses additional components such as oil pipes or cooling plates to cool the end coils, which inevitably results in additional costs. In addition, when the motor is of a water-cooled or air-cooled type rather than a fluid contact cooling type, it may be difficult to cool a stator core or a rotor core, and it is not conducive to end coil cooling because the end coils are shielded by plates, the stator core, or coils. Summary of the Invention

[0004] Embodiments of the present disclosure aim to provide a rotor having an improved cooling structure, which can maximize its cooling efficiency in such a way that the structure of the motor rotor is changed without additional components, and blades are formed on its surface by using a magnetic fixing adhesive, thereby forming blades for increasing the flow rate by changing the surface shape of the rotor components, and forming blades for increasing the flow rate by using a rotor magnetic adhesive.

[0005] In one general aspect, there is provided a rotor having an improved cooling structure, the rotor including: a rotor core formed in a cylindrical shape and defining a hollow hole passing through a central axis of the rotor core; a rotor shaft positioned within the hollow hole of the rotor core, the rotor shaft being fixed along the central axis of the rotor core and being rotatable about the central axis of the rotor core; and a flow control unit provided on at least one of the rotor core and the rotor shaft, the flow control unit being configured to control the flow of cooling fluid injected onto ends of coils protruding axially from each of both ends of the rotor core.

[0006] The rotor shaft may include: a support step that protrudes in the radial direction and contacts one end of the rotor core in the axial direction; and a support surface that contacts the other end of the rotor core in the axial direction and extends from the other end of the rotor core in the axial direction to a predetermined distance, and the flow control unit may include a first groove that has a predetermined depth on one side in the circumferential direction of the support step or the support surface.

[0007] The rotor shaft may include: a support step that protrudes in the radial direction and contacts one end of the rotor core in the axial direction; and a support surface that contacts the other end of the rotor core in the axial direction and extends from the other end of the rotor core in the axial direction to a predetermined distance, and the flow control unit may include a second groove that has a predetermined depth on one side in the radial direction of the support step or the support surface.

[0008] The flow control unit may further include a third groove that is provided in the axial direction on the side where the rotor shaft and the rotor core contact each other and has a predetermined depth at a position corresponding to the second groove in the radial direction.

[0009] The rotor may further include a rotor plate formed in a disk shape, the disk having one surface that contacts the other end of the rotor core in the axial direction, and the rotor shaft may be positioned within the center of the rotor plate. The flow control unit may further include a fourth groove that has a predetermined depth in the other surface of the rotor plate in the axial direction and extends in the radial direction, and the fourth groove may be inclined such that the depth on the opposite side in the axial direction is shallower than the depth on the side of the fourth groove that contacts the rotor shaft in the axial direction.

[0010] The flow control unit may further include a fifth groove that has a predetermined depth in one end face of the rotor core in the axial direction and extends in the radial direction, and the fifth groove may be inclined such that the depth on the opposite side in the axial direction is shallower than the depth on the side of the fifth groove that contacts the rotor shaft in the axial direction.

[0011] The rotor core may further include: a magnet bonding portion that fixes the position of a magnet within a magnet insertion hole, the magnet being positioned within the magnet insertion hole and the magnet insertion hole extending through the rotor core in the axial direction; and an insertion fixing portion that is inserted into a gap between the magnet insertion hole and the magnet, the magnet bonding portion may include a plate-shaped bonding plate that is integrated with the insertion fixing portion and has a surface that contacts one end face of the rotor core, and the flow control unit may include a blade portion that protrudes from the other surface of the bonding plate.

[0012] The blade portion may include a first blade that includes two steps spaced apart from each other by a predetermined distance, the first blade may be disposed at a position corresponding to the second groove, and a gap between respective steps of the first blade may be wider on an opposite side of the first blade than on a side of the first blade adjacent to the hollow hole of the rotor core.

[0013] The blade portion may include a second blade that is formed in a V shape and has a shielded end and an open end, and the one end of the second blade may be adjacent to the hollow hole of the rotor core, and the other end of the second blade may be adjacent to the wound coil.

[0014] The blade portion may include a sixth groove that is spaced apart from the second blade by a predetermined distance in the circumferential direction, the sixth groove may have a predetermined depth starting from an outermost end of the bonding plate in the radial direction in the radial direction, and the sixth groove may be disposed at a position corresponding to the second groove.

[0015] Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a cross-sectional view of a rotor having an improved cooling structure in the present disclosure.

[0017] Figure 2 is a perspective view of a rotor shaft according to a first embodiment of a flow control unit in the present disclosure.

[0018] Figure 3 is a schematic diagram showing the flow of a cooling fluid when a first embodiment of the flow control unit in the present disclosure is applied.

[0019] Figure 4 is a perspective view of a rotor shaft according to a second embodiment of the flow control unit in the present disclosure.

[0020] Figure 5 FIG. Figure 5 is a schematic view showing the flow of the cooling fluid when the second embodiment of the flow control unit in the present disclosure is applied.

[0021] Figure 6 FIG. Figure 6 is a perspective view of a rotor shaft according to a third embodiment of the flow control unit in the present disclosure.

[0022] Figure 7 FIG. Figure 7 is a perspective view of a rotor plate according to a third embodiment of the flow control unit in the present disclosure.

[0023] Figure 8 FIG. Figure 8 is a partial perspective view of a rotor core according to a third embodiment of the flow control unit in the present disclosure.

[0024] Figure 9 FIG. Figure 9 is a schematic view showing the flow of the cooling fluid when the third embodiment of the flow control unit in the present disclosure is applied.

[0025] Figure 10 FIG. Figure 10 is a partial perspective view of a rotor core according to a 4-1 embodiment of the flow control unit in the present disclosure.

[0026] Figure 11 FIG. Figure 11 is a schematic view showing the flow of the cooling fluid when the 4-1 embodiment of the flow control unit in the present disclosure is applied.

[0027] Figure 12 FIG. Figure 12 is a partial perspective view of a rotor core according to a 4-2 embodiment of the flow control unit in the present disclosure.

[0028] Figure 13 FIG. Figure 13 is a schematic view showing the flow of the cooling fluid when the 4-2 embodiment of the flow control unit in the present disclosure is applied. DETAILED DESCRIPTION

[0029] Hereinafter, the technical spirit of the present disclosure will be described in more detail with reference to the drawings. Before the description, the terms and words used in the specification and claims should not be construed as general or dictionary meanings, but should be construed as meanings and concepts consistent with the spirit of the present disclosure, which is based on the concept that the inventor can appropriately define the terms in order to describe the principle of his invention in the best way.

[0030] Hereinafter, the description refers to Figure 1 describes the basic structure of the rotor 1000 having an improved cooling structure of the present disclosure.

[0031] As Figure 1As shown, the rotor 1000 with an improved cooling structure in the present disclosure may include a rotor core 100 and a rotor shaft 200. More specifically, the rotor core 100 may be formed in a cylindrical shape with a hollow hole passing through its center, and a stator S wound with a coil C may be disposed on its outer side. Here, the rotor shaft 200, the rotor core 100, and the stator S may preferably be spaced apart from each other by a predetermined distance in the radial direction. In addition, the rotor core 100 may include a magnet that generates an electric field therein. Here, the magnet may have one or more layers in the radial direction. In addition, the rotor shaft 200 may be inserted into the hollow hole of the rotor core 100 and fixed to the hollow hole of the rotor core 100 along the central axis A of the rotor core, and can rotate around the central axis of the rotor core 100.

[0032] In addition, the rotor 1000 with an improved cooling structure in the present disclosure may include a flow control unit 300. The flow control unit 300 may be a structure disposed on the rotor core 100 or the rotor shaft 200, and controls the flow of the cooling fluid sprayed onto the ends of the coil C that protrudes axially from each of the two ends of the stator S that is in contact with the outside of the rotor core 100. More clearly, the flow control unit 300 may increase the flow rate of the cooling fluid flowing on the surface of the rotor core 100 or the rotor shaft 200.

[0033] As described above, the rotor 1000 with an improved cooling structure in the present disclosure may include a flow control unit 300, thereby increasing the transfer speed of the cooling fluid, thereby increasing the convective heat transfer rate, and may be applied to a water-cooled or air-cooled motor, thereby improving the motor performance. In addition, the rotor 1000 may adopt a specific shape in which the flow control unit 300 is integrated with the rotor core 100 and the rotor shaft 200, thereby eliminating existing cooling components such as oil pipes, and improving the cooling efficiency through a relatively simple structure, thereby reducing its material and process costs.

[0034] Hereinafter, the description refers to Figures 2 to 5 Describe the first embodiment and the second embodiment of the flow control unit 300 of the present disclosure in more detail.

[0035] The rotor shaft 200 of the present disclosure may include a support step 210 that protrudes in a radial direction to contact one end of the rotor core 100 in an axial direction. The rotor shaft 200 may include the support step 210 to support the position of the rotor core 100 disposed on its side surface. In addition, the rotor shaft 200 of the present disclosure may include a support surface 220 that contacts the other end of the rotor core 100 in the axial direction and extends a predetermined distance from the other end of the rotor core 100 in the axial direction. The support surface 220 may extend from the side of the rotor shaft 200 that contacts the rotor core 100 in the axial direction without protruding or recessing in the radial direction. A rotor plate 230 that supports the position of the rotor core 100 may be inserted into the support surface 220. Details of the rotor plate 230 are described below.

[0036] In Figure 2 In the first embodiment of the flow control unit 300 of the present disclosure shown, the flow control unit 300 may include a first groove 310 having a predetermined depth on one side in the circumferential direction of the support step 210 or the support surface 220. Two or more first grooves 310 may be provided, and each first groove 310 may be spaced apart from each other by a predetermined distance in the axial direction. In addition, the first groove 310 may be provided only in a part of the circumference of the support step 210 or the support surface 220, rather than in its entire circumference. Therefore, more first grooves 310 may be provided in a part where the cooling fluid is concentrated, considering the expected movement path of the cooling fluid.

[0037] The first groove 310 may be provided in the side surface in the circumferential direction of the support step 210 or the support surface 220, and as Figure 3 shown, when the rotor shaft 200 rotates, each first groove 310 may thus serve as a blade for generating a flow velocity in the cooling fluid on the surface of the rotor shaft 200 or the rotor core 100, thereby increasing the flow velocity of the cooling fluid flowing on the surface of the rotor shaft 200 or the rotor core 100, and thereby allowing the cooling fluid to be smoothly conveyed toward the end coils C protruding from each of the two ends of the stator S.

[0038] In addition, in Figure 4In the second embodiment of the flow control unit 300 of the present disclosure as shown, the flow control unit 300 may include a second groove 320 having a predetermined depth on one side in the radial direction of the support step 210 or the support surface 220. The second groove 320 may have a predetermined width in the circumferential direction. In addition, two or more second grooves 320 may be provided, and the respective second grooves 320 may be spaced apart from each other by a predetermined distance in the circumferential direction. In addition, considering the expected movement path of the cooling fluid, the second grooves 320 may be more densely provided in the portion where the cooling fluid is concentrated.

[0039] The second groove 320 may be provided in the side surface in the radial direction of the support step 210 or the support surface 220, and as Figure 5 shown, when the rotor shaft 200 rotates, each second groove 320 may thus serve as a blade for generating the flow velocity of the cooling fluid on the surface of the rotor shaft 200 or the rotor core 100, thereby increasing the flow velocity of the cooling fluid flowing on this surface, and thus allowing the cooling fluid to be smoothly conveyed toward the end coils C protruding from each of the two ends of the stator S.

[0040] Hereinafter, this description refers to Figures 6 to 9 describe in more detail the third embodiment of the flow control unit 300 of the present disclosure.

[0041] In Figure 6 In the third embodiment of the flow control unit 300 of the present disclosure as shown, the flow control unit 300 may further include a third groove 330 provided in the axial direction on the side where the rotor shaft 200 and the rotor core 100 are in contact with each other, and having a predetermined depth at a position corresponding to the second groove 320 in the radial direction. That is, the number of the third grooves 330 may be the same as the number of the second grooves 320, and the width of each third groove 330 in the circumferential direction may be proportional to the width of the corresponding second groove 320 in the circumferential direction. The third groove 330 may be formed as a straight line from the side surface of the rotor shaft 200 to the support surface 220 where the rotor shaft 200 and the rotor core 100 are in contact with each other.

[0042] In addition, as Figure 7As shown, the rotor shaft 200 may further include a rotor plate 230 formed in a disk shape (e.g., a substantially disk shape), the disk having one surface that contacts the other end of the rotor core 100 in the axial direction, and having the rotor shaft 200 inserted therein. The rotor plate 230 may be inserted into the support surface 220 described above. Here, the flow control unit 300 may further include a fourth groove 340 that has a predetermined depth in the other surface of the rotor plate 230 in the axial direction and extends in the radial direction. The fourth groove 340 may also be provided at a position corresponding to the third groove 330 described above.

[0043] In addition, the fourth groove 340 may be inclined such that the depth of the opposite side in the axial direction (i.e., the side adjacent to the end of the coil C) is shallower than the depth of the side in contact with the rotor shaft 200 in the axial direction. Thus, the cooling fluid transferred to the rotor core 100 along the third groove 330 in the surface of the rotor shaft 200 can be radiated and transferred along the fourth groove 340. Therefore, the cooling fluid can be transferred more uniformly to the upper part of the end coil C provided at one end of the stator S, thereby improving the cooling performance of the end coil C.

[0044] In addition, as Figure 8 shown, the flow control unit 300 may further include a fifth groove 350 that has a predetermined depth in one end face of the rotor core 100 in the axial direction and extends in the radial direction. The fifth groove 350 may also be provided at a position corresponding to the second groove 320 described above. Here, the fifth groove 350 may be inclined such that the depth of the opposite side in the axial direction is shallower than the depth of the side in contact with the rotor shaft 200 in the axial direction. Thus, the cooling fluid transferred to the rotor core 100 along the second groove 320 in the rotor surface can be radiated and transferred along the fifth groove 350. Therefore, the cooling fluid can be transferred more uniformly to the upper part of the end coil C provided at one end of the stator S, thereby improving the cooling performance of the end coil C.

[0045] The third embodiment of the flow control unit 300 may be applied as described above, and as Figure 9 shown, a predetermined flow path may be formed between the third groove 330 and the rotor core 100, and the cooling fluid may be transferred along the third groove 330 to the support step 210 or the support surface 220. As a result, the inner surface of the rotor core 100 or the rotor shaft 200 can also be cooled, thereby maximizing the cooling efficiency.

[0046] In addition, the cooling fluid can be ejected to the outside of the rotor core 100 through the fourth groove 340 and the fifth groove 350, and even if the end coil C has a high protruding height, the cooling fluid can be blocked by the end coil C close to the rotor shaft 200, thereby preventing the cooling failure of the end coil C provided on the relatively outer side in the radial direction of the rotor core 100. That is, the end coil C and the rotor can be cooled more effectively and more uniformly.

[0047] Hereinafter, Figures 10 to 13 a fourth embodiment of the flow control unit 300 in the present disclosure will be described in more detail with reference to

[0048] The rotor core 100 may further include a magnet bonding portion 110 and an insertion fixing portion. The magnet bonding portion 110 fixes the position of the magnet through a magnet insertion hole. The magnet is inserted into the magnet insertion hole and the magnet insertion hole passes through the rotor core in the axial direction. The insertion fixing portion is inserted into the gap between the magnet insertion hole and the magnet. Here, the magnet bonding portion 110 may include a plate-shaped bonding plate 111, which is integrated with the insertion fixing portion and has a surface in contact with one end face of the rotor core 100. More specifically, the magnet bonding portion 110 may be an adhesive such as epoxy resin, or may be a casting product such as a mold.

[0049] Here, in the fourth embodiment of the flow control unit 300 of the present disclosure, the flow control unit 300 may be integrated with the magnet bonding portion 110. More specifically, the flow control unit 300 may include a blade portion 360 protruding from the other surface of the bonding plate 111.

[0050] In Figure 10 the 4-1 embodiment of the shown flow control unit 300, the blade portion 360 may include a first blade 361, and the first blade 361 includes two steps spaced apart from each other at a predetermined distance. That is to say, the first blade 361 may have steps formed in the shape of 11 characters, and the cooling fluid can flow between them. Here, two or more first blades 361 may be provided on the other surface of the bonding plate 111 in the circumferential direction, and each first blade 361 has steps formed in the shape of 11 characters. More clearly, the first blade 361 may be provided at a position corresponding to the second groove 320. Therefore, the cooling fluid flowing along the surface of the second groove 320 can be transmitted to the first blade 361.

[0051] In addition, the gap between the corresponding steps of the first vane 361 may be wider on the opposite side of the first vane 361 (i.e., the side adjacent to the end coil C) than on the side of the first vane 361 adjacent to the hollow hole of the rotor core 100. When the cooling fluid is transferred from the first vane 361 toward the end coil C, the hydraulic pressure can be reduced compared to when the cooling fluid is transferred toward the first vane 361, and thus the cooling fluid can be ejected onto the end coil C.

[0052] Therefore, as Figure 11 shown, when the rotor shaft 200 rotates, the first vane 361 can function as a vane that generates a flow velocity in the cooling fluid on the surface of the rotor shaft 200 or the rotor core 100, thereby increasing the flow velocity of the cooling fluid flowing on the surface, allowing the cooling fluid to be smoothly conveyed toward the end coil C protruding from each of the two ends of the stator S provided outside the rotor core 100.

[0053] In addition, in Figure 12 the 4-2 embodiment of the flow control unit 300 of the present disclosure shown, the vane portion 360 may include a second vane 362 formed in a V shape, with one end of the second vane 362 being shielded and the other end being open. Here, one end of the second vane 362 (i.e., the portion corresponding to the lower end of the V shape) may be adjacent to the hollow hole of the rotor core 100, and the other end of the second vane 362 (i.e., the portion corresponding to the upper end of the V shape) may be adjacent to the wound coil C. Thus, the cooling fluid can flow along the surface of the second vane 362. Here, two or more second vanes 362 each formed in a V shape may be provided on the other surface of the bonding plate 111 in the circumferential direction.

[0054] In addition, in the 4-2 embodiment of the flow control unit 300, the vane portion 360 may include a sixth groove 363 that is spaced apart from the second vane 362 by a predetermined distance in the circumferential direction. The sixth groove 363 may have a predetermined depth starting from the outermost end of the bonding plate 111 in the radial direction and penetrate the bonding plate 111 in the axial direction. In addition, the sixth groove 363 may be provided at a position corresponding to the second groove 320.

[0055] Therefore, as Figure 13 shown, when the rotor shaft 200 rotates, the first vane 361 can function as a vane that generates a flow velocity in the cooling fluid on the surface of the rotor shaft 200 or the rotor core 100, thereby increasing the flow velocity of the cooling fluid flowing on the surface, allowing the cooling fluid to be smoothly conveyed toward the end coil C protruding from each of the two ends of the stator S provided outside the rotor core 100.

[0056] As described above, the rotor with an improved cooling structure in the present disclosure can change the motor rotor structure without additional components, and form blades on its surface by using a magnetic fixing adhesive, thereby increasing the transfer speed of the cooling fluid, thereby increasing the convective heat transfer rate, and can be applied to water-cooled or air-cooled motors, thereby improving the motor performance, and can adopt a relatively simple structure, such as eliminating existing cooling components such as oil pipes, thereby reducing its material and process costs.

[0057] The spirit of the present disclosure should not be construed as limited to the above embodiments. The present disclosure can be applied to various fields, and various modifications can be made by those skilled in the art without departing from the scope of the present disclosure claimed by the claims. Therefore, it is obvious to those skilled in the art that these changes and modifications fall within the scope of the present disclosure.

Claims

1. A rotor with an improved cooling structure, the rotor comprising: a rotor core formed in a cylindrical shape and defining a hollow hole passing through a central axis of the rotor core; a rotor shaft positioned within the hollow bore of the rotor core, the rotor shaft being fixed along the central axis of the rotor core and rotatable about the central axis of the rotor core; as well as A flow control unit is disposed on at least one of the rotor core and the rotor shaft, the flow control unit being configured to control the flow of a cooling fluid sprayed to an end of a coil protruding from each of two ends of the rotor core in an axial direction.

2. The rotor according to claim 1, wherein: The rotor shaft further comprises: a supporting step protruding in a radial direction and contacting one end of the rotor core in the axial direction; and a support surface that contacts the other end of the rotor core in the axial direction and extends to a predetermined distance from the other end of the rotor core in the axial direction, and The flow control unit includes a first groove having a predetermined depth on one side of the support step or the support surface in the circumferential direction.

3. The rotor according to claim 1, wherein: The rotor shaft further comprises: a supporting step protruding in a radial direction and contacting one end of the rotor core in the axial direction; and a support surface that contacts the other end of the rotor core in the axial direction and extends to a predetermined distance from the other end of the rotor core in the axial direction, and Wherein, the flow control unit includes a second groove having a predetermined depth at one side of the support step or the support surface in the radial direction.

4. The rotor according to claim 3, wherein: The flow control unit further includes a third groove provided at a side where the rotor shaft and the rotor core contact each other in the axial direction and having a predetermined depth in a position corresponding to the second groove in the radial direction.

5. The rotor according to claim 4, further comprising a rotor plate formed in a disk shape, the disk having one surface in contact with the other end of the rotor core in the axial direction, and wherein, The rotor shaft is positioned in the center of the rotor plate, The flow control unit further includes a fourth groove having a predetermined depth in the other surface of the rotor plate in the axial direction and extending in the radial direction, and The fourth groove is inclined such that a depth of an opposite side in the axial direction is shallower than a depth of a side of the fourth groove in contact with the rotor shaft in the axial direction.

6. The rotor according to claim 4, wherein: The flow control unit further includes a fifth groove having a predetermined depth in one end surface of the rotor core in the axial direction and extending in the radial direction, and The fifth groove is inclined such that a depth of an opposite side in the axial direction is shallower than a depth of a side of the fifth groove contacting the rotor shaft in the axial direction.

7. The rotor according to claim 3, wherein: The rotor core further includes: a magnet bonding portion that fixes a position of a magnet within a magnet insertion hole in which the magnet is positioned and which passes through the rotor core in the axial direction; and an insertion fixing portion, the insertion fixing portion being inserted into a gap between the magnet insertion hole and the magnet, The magnet bonding portion includes a plate-shaped bonding plate that is integrated with the insertion fixing portion and has one surface that contacts one end surface of the rotor core, and The flow control unit includes a blade portion protruding from the other surface of the bonding plate.

8. The rotor according to claim 7, wherein: The blade portion includes a first blade including two steps spaced apart from each other by a predetermined distance, The first blade is disposed at a position corresponding to the second groove, and A gap between the steps of the first blade is wider at an opposite side of the first blade than at a side of the first blade adjacent to the hollow hole of the rotor core.

9. The rotor according to claim 7, wherein: The blade portion includes a second blade formed in a V-shape and having one end that is shielded and the other end that is open, and The one end of the second blade is adjacent to the hollow hole of the rotor core, and the other end of the second blade is adjacent to the wound coil.

10. The rotor according to claim 9, wherein: The blade portion includes a sixth groove that is spaced apart from the second blade by a predetermined distance in a circumferential direction, The sixth groove has a predetermined depth in the radial direction from the outermost end of the bonding plate in the radial direction, and The sixth groove is disposed at a position corresponding to the second groove.

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