Motor cooling structure

Abstract

In this motor cooling structure for cooling coils inserted into slots of a stator core: the slots are disposed so as to have a closed shape with respect to the inner peripheral surface and the outer peripheral surface of the stator core; coils are disposed so as to be separated from inner walls on the inner peripheral surface side of the slots, whereby a refrigerant circulation path extending in the axial direction is formed between inner walls and the coils; and a plurality of first groove parts, which extend in the axial direction and for which the direction from the inner walls on the inner peripheral surface side of the slots toward the inside in the radial direction or the direction from the inner peripheral surface toward the outside in the radial direction serves as the depth direction, are formed so as to be aligned in the circumferential direction at positions on the inner peripheral surface side of the stator core and facing the circulation path from the radial direction, whereby beam parts projecting in the radial direction and forming ridge lines in the axial direction are formed between the first groove parts adjacent to each other.

Description

Motor cooling structure

[0001] The present invention relates to a cooling structure for a motor.

[0002] JP2716286B discloses a configuration in which a coil is incorporated into a slot of a stator, the opening of the slot arranged on the inner peripheral surface of the stator is closed with a resin part, the coil end is sealed with a resin cover, and a refrigerant supplied into the resin cover flows through the slot to directly cool the coil.

[0003] However, in JP2716286B, the resin cover extends to cover the inner circumferential surface and support the resin parts, so the air gap between the stator and rotor needs to be set wider by the thickness of the resin cover, which reduces the torque accordingly.

[0004] Therefore, an object of the present invention is to provide a motor cooling structure that can cool the coils in the slots while suppressing a decrease in torque.

[0005] According to one aspect of the present invention, in a cooling structure for a motor that cools coils inserted into slots in a stator core, the slots are arranged so as to form a closed shape relative to the inner and outer surfaces of the stator core, and the coils are arranged so as to be spaced apart from the inner wall on the inner surface side of the slot, thereby forming a refrigerant circulation path extending in the axial direction between the inner wall and the coil, and on the inner surface side of the stator core at a position facing the circulation path from the radial direction, multiple first groove portions are formed lined up circumferentially, extending axially, with the depth direction being from the inner wall on the inner surface side of the slot toward the radially inward direction or from the inner surface toward the radially outward direction, thereby forming beam portions that protrude radially between adjacent first groove portions and form ridges in the axial direction.

[0006] Fig. 1 is a plan view of the cooling structure for a motor according to a first embodiment. Fig. 2 is a partially enlarged view of the cooling structure for a motor according to the first embodiment. Fig. 3 is a partially enlarged view of the cooling structure for a motor according to a second embodiment. Fig. 4 is a partially enlarged view of the cooling structure for a motor according to a third embodiment. Fig. 5 is a partially enlarged view of the cooling structure for a motor according to comparative examples. Fig. 5(A) shows a first comparative example (A) in which the slots open to the inner peripheral surface of the stator core. Fig. 5(B) shows a second comparative example (B) in which the air gap between the stator core and rotor core is enlarged compared to the first comparative example. Fig. 5(C) shows a third comparative example (C) in which the slots are closed relative to the inner peripheral surface of the stator core. Fig. 6 is a diagram showing the torque ratios of the second comparative example (B), the third comparative example (C), the first embodiment (D), the second embodiment (E), and the third embodiment (F) when the torque of the first comparative example (A) is used as a reference, when the motors of the first comparative example (A), the second comparative example (B), the third comparative example (C), the first embodiment (D), the second embodiment (E), and the third embodiment (F) are driven under the same conditions. Fig. 7 is a partially enlarged view of the cooling structure of a motor of a modified example of the first embodiment. Fig. 8 is a partially enlarged view of the cooling structure of a motor of a modified example of the second embodiment.

[0007] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0008] [First embodiment] Fig. 1 is a plan view of a cooling structure for a motor according to a first embodiment. Fig. 2 is a partially enlarged view of the cooling structure for a stator of a motor according to the first embodiment. The motor according to the first embodiment is applied to, for example, an electric vehicle or a hybrid vehicle. The motor is composed of a stator and a rotor.

[0009] The rotor is formed by a rotor shaft 31 and a rotor core 32 that is journaled on the rotor shaft 31. A magnet (permanent magnet) is embedded in the rotor core 32.

[0010] The stator includes a cylindrical stator core 1 that houses the rotor, and coils 2 that are inserted into a plurality of slots 13 formed in the circumferential direction of the stator core 1. An air gap AG having a predetermined distance is formed between an inner peripheral surface 11 of the stator core 1 and an outer peripheral surface 321 of the rotor core 32.

[0011] The slots 13 are arranged so as to be closed relative to the inner circumferential surface 11 and the outer circumferential surface 12 of the stator core 1 and open to the axial end of the stator core 1 .

[0012] For example, a rectangular wire having a rectangular cross section is suitable for the coil 2, but other cross sections such as a circular one are also applicable. The coils 2 are divided into a U-phase coil 2, a V-phase coil 2, and a W-phase coil 2, and the arrangement of the coils 2 in the stator core 1 (slots 13) can be set arbitrarily.

[0013] A plurality of coils 2 are inserted into one slot 13 and arranged in a row in the radial direction. Of the plurality of coils 2 inserted into the slot 13, the coil 2 arranged closest to the outer peripheral surface 12 is arranged adjacent to or in contact with the inner wall of the slot 13 on the outer peripheral surface 12 side. On the other hand, the coil 2 arranged closest to the inner peripheral surface 11 side is spaced apart from the inner wall of the slot 13 on the inner peripheral surface 11 side (the radially outer end of a beam portion 15 described below), and a flow path 131 through which a coolant (oil) flows is formed between the coil 2 and the inner wall. The use of oil as a coolant achieves insulation between the coil 2 and other parts, sealing between the stacked electromagnetic steel sheets (stator core 1) (oil particles are larger than water), and rust prevention.

[0014] Here, the stator core 1 of this embodiment is incorporated into a first circulation path (not shown) that has a first pump (not shown) that circulates oil and a chiller (not shown) that cools the refrigerant. For this reason, covers (not shown) that accommodate coil ends (not shown) that protrude axially from the stator core 1 are disposed at both axial ends of the stator core 1. The two covers (not shown) form a closed space together with the axial ends of the stator core 1. One of the two covers (not shown) is connected to the refrigerant discharge side of the pump (not shown), and the other is connected to the refrigerant suction side of the pump (not shown).

[0015] Therefore, when refrigerant is supplied to one cover (not shown), the refrigerant is supplied to all slots 13 (circulation paths 131) arranged in the stator core 1, and the refrigerant that passes through the slots 13 (circulation paths 131) joins together at the other cover (not shown) and is supplied to the chiller (not shown). A housing (not shown) that accommodates the stator core 1 is incorporated into a second circulation path (not shown) that has a second pump (not shown) that circulates water and a radiator (not shown) that cools the water, and the housing (not shown) is cooled by the water circulating through the second circulation path (not shown). A chiller (not shown) is also incorporated into the second circulation path (not shown), and cools the oil circulating through the first circulation path (not shown) by performing heat exchange between the oil circulating through the first circulation path (not shown) and the water circulating through the second circulation path (not shown).

[0016] A plurality of first grooves 14 extending in the axial direction with the depth direction being the direction from the inner wall toward the radially inward side (toward the inner circumferential surface 11) are arranged in a line in the circumferential direction on the inner wall of the slot 13 on the inner circumferential surface 11 side. Therefore, beam portions 15 extending toward the coil 2 and forming ridges in the axial direction are formed between adjacent first grooves 14, and a plurality of beam portions 15 are also arranged in a comb-like pattern in the circumferential direction.

[0017] 1 and 2, five first groove portions 14 are arranged for one slot 13, and therefore four beam portions 15 are arranged for one slot 13. In addition, the portion on the inner peripheral surface 11 side of the stator core 1 that supports the beam portions 15 is a support portion 16.

[0018] The stator core 1 is formed by forming multiple through holes having the outer shape of the slots 13 as viewed from the axial direction in a ring-shaped electromagnetic steel plate that forms the outer shape of the stator core 1 as viewed from the axial direction, cutting out the outer shape (recess) of the first groove portion 14 as viewed from the axial direction on the inner surface of the through holes to form the outer shapes of the beam portion 15 and the support portion 16, and stacking multiple electromagnetic steel plates on which the outer shapes of the beam portion 15 and the support portion 16 have been formed.

[0019] Typically, between the rotor core 32 and the stator core 1, slots 13 are arranged so as to open to the inner surface 11 of the stator core 1 in order to reduce magnetic flux leakage between the magnets arranged in the rotor core 32 and the coils 2 arranged in the stator core 1.

[0020] On the other hand, if the slots 13 are open to the inner circumferential surface 11 and refrigerant is supplied into the slots 13, the refrigerant will flow out from the openings, resulting in poor circulation of the refrigerant and poor rotation of the rotor.

[0021] Therefore, by forming the slots 13 in a closed shape relative to the inner peripheral surface 11 and the outer peripheral surface 12 of the stator core 1, a refrigerant flow path 131 extending in the axial direction is formed within the slots 13.

[0022] In the stator core 1 in which the slots 13 are closed relative to the inner peripheral surface 11 of the stator core 1, the portions sandwiched between the slots 13 and the rotor core 32 (see FIG. 5C ) are regions where magnetic flux leaks in the circumferential direction. This leakage of magnetic flux in the circumferential direction reduces the torque of the motor.

[0023] However, in this embodiment, a plurality of first groove portions 14 and beam portions 15 are arranged with a predetermined gap in the circumferential direction, whereby the first groove portions 14 and beam portions 15 form flux barriers 17 that reduce leakage of magnetic flux in the circumferential direction, and since the magnetic flux is less likely to pass through the flux barriers 17, the leakage of magnetic flux is reduced accordingly, and a decrease in torque can be suppressed.

[0024] Note that circumferential magnetic flux leakage occurs in the support portion 16, which can cause iron loss (heat generation). However, in this embodiment, the beam portion 15 is a heat dissipation fin with a large surface area, and the heat generated in the support portion 16 can be dissipated in the beam portion 15. In particular, in the first embodiment, the beam portion 15 and the support portion 16 are in direct contact with the refrigerant, so the heat generated in the support portion 16 can be efficiently dissipated.

[0025] 3 is a partial enlarged view of a cooling structure for a motor according to a second embodiment. In the second embodiment, three first grooves 14 are provided, and two beams 15 (beams 15A and 15B) are provided.

[0026] Meanwhile, a second groove 18A extending in the axial direction and with its depth direction directed toward the flow path 131 (radially outward) is formed on the side surface of the support portion 16 opposite to the side surface from which the beam portion 15A extends, i.e., on the radially inner side surface of the support portion 16 at a position facing the beam portion 15A. Similarly, a second groove 18B extending in the axial direction and with its depth direction directed toward the radially outward is formed on the radially inner side surface of the support portion 16 at a position facing the beam portion 15B.

[0027] The second groove portion 18A has a circumferential width set to approximately 1 / 3 of the circumferential width of the beam portion 15A, and is positioned so that the circumferential center position of the second groove portion 18A approximately coincides with the circumferential center position of the beam portion 15A, the inside of the beam portion 15A is cut out by the second groove portion 18A, and a groove extending in the axial direction is formed on the inside of the beam portion 15A.

[0028] As a result, the circumferential widths of the first beam portion 151A and the second beam portion 152A, which are the remaining parts of the beam portion 15A cut out by the second groove portion 18A and extend parallel to the radial direction, are each approximately 1 / 3 of the circumferential width of the original beam portion 15A.

[0029] In addition, the depth of the second groove portion 18A is set so that the radial width of the third beam portion 153A, which is the remaining part of the beam portion 15A cut out by the second groove portion 18A and is the radially outer end portion, is approximately 1 / 3 of the circumferential width of the original beam portion 15A.

[0030] The second groove portion 18B is arranged in the same manner as the second groove portion 18A, and the beam portion 15B becomes a first beam portion 151B, a second beam portion 152B, and a third beam portion 153B.

[0031] The support portion 16 has a first support portion 161 connected to the stator core 1 and having a circumferential end formed by the second groove portion 18A, a second support portion 162 connected to the stator core 1 and having a circumferential end formed by the second groove portion 18B, and a third support portion 163 separated from the first support portion 161 by the second groove portion 18A and separated from the second support portion 162 by the second groove portion 18B.

[0032] As a result, a zigzag structure is formed in the stator core 1, in which the first support portion 161, the first beam portion 151A, the third beam portion 153A, the second beam portion 152A, the third support portion 163, the first beam portion 151B, the third beam portion 153B, the second beam portion 152B, and the second support portion 162 are continuously connected in this order.

[0033] In the second embodiment, gaps (second grooves 18A and 18B) are also formed between the first support portion 161 and the second support portion 162, and between the second support portion 162 and the third support portion 163. Therefore, the circumferential path along which magnetic flux leaks in the portion between the inner circumferential surface 11 of the stator core 1 and the slots 13 always passes through the first groove portion 14 or the second groove portion 18, that is, always passes through a region where the magnetic resistance is higher than that of the material (electromagnetic steel sheet) of the stator core 1. Therefore, the circumferential magnetic flux leakage is further reduced, thereby further suppressing the reduction in torque.

[0034] 4 is a partially enlarged view of a motor cooling structure according to a third embodiment. The motor cooling structure of the third embodiment differs from the second embodiment in that the radial width of the tip end portion (third beam portion 153) of the beam portion 15, which is radially outward of the second groove portion 18, is set to be wider than the radial width of the portion (support portion 16) of the stator core 1 to which the beam portion 15 is connected. In other words, the radial length of the first groove portion 14 is set to be longer than the radial length of the second groove portion 18.

[0035] As a result, the first groove portion 14 through which the coolant flows is set to be long in the radial direction, so that the surface area of ​​the beam portion 15 increases, and the heat removal performance of the flux barrier 17 can be improved.

[0036] [Comparison of Torque with Comparative Examples] Figure 5 is a partial enlarged view of the cooling structure of a motor of a comparative example, in which Figure 5(A) shows a first comparative example (A) in which the slots 13 open to the inner circumferential surface 11 of the stator core 1, Figure 5(B) shows a second comparative example (B) in which the air gap AG between the stator core 1 and the rotor core 32 is enlarged compared to that of the first comparative example, and Figure 5(C) shows a third comparative example (C) in which the slots 13 are closed relative to the inner circumferential surface 11 of the stator core 1. Figure 6 is a diagram showing the torque ratios of the second comparative example (B), the third comparative example (C), the first embodiment (D), the second embodiment (E), and the third embodiment (F) when the torque of the first comparative example (A) is used as a reference, when the motors of the first comparative example (A), the second comparative example (B), the third comparative example (C), the first embodiment (D), the second embodiment (E), and the third embodiment (F) are driven under the same conditions.

[0037] 5A, in the first comparative example (A), the slots 13 open to the inner peripheral surface 11 of the stator core 1, and the refrigerant cannot pass through the slots 13. However, the leakage of magnetic flux between the coils 2 inserted in the slots 13 and the magnets arranged in the rotor core 32 is relatively small.

[0038] In the second comparative example (B) shown in FIG. 5(B), the width of the air gap AG is wider than in the first comparative example, and as a result, the leakage of magnetic flux between the coil 2 and the magnet arranged in the rotor core 32 is relatively large.

[0039] 5C , the slots 13 are closed relative to the inner circumferential surface 11, allowing the refrigerant to flow through the slots 13 (flow paths 131). However, magnetic flux leaks in the circumferential direction in a region 19 between the slots 13 of the stator core 1 and the rotor core 32, and the magnetic flux leakage between the coil 2 and the magnets arranged in the rotor core 32 increases accordingly.

[0040] 6, if the torque of the motor in the first comparative example (A) is 100%, the torque of the motor in the second comparative example (B) is about 88% and the torque of the motor in the third comparative example (C) is about 91%, so the rate of decrease in torque of the motor in the second comparative example (B) is greater than that of the third comparative example (C). Therefore, in the case of a configuration in which the inner peripheral surface 11 of the stator core 1 is covered with a resin cover as in the above-mentioned JP2716286B, it is thought that the decrease in motor torque will be significant.

[0041] Furthermore, although the torque of the motor in the third comparative example (C) is higher than that of the motor in the second comparative example, it is about 9% lower than that of the first comparative example, which is thought to affect the driving performance of the electric vehicle.

[0042] On the other hand, the torque of the motor in the first embodiment (D) (FIGS. 1 and 2) is 98.7%; the torque of the motor in the second embodiment (E) (FIG. 3) is 98.9%; and the torque of the motor in the third embodiment (F) (FIG. 4) is 98%. All of these can be said to have good torque characteristics.

[0043] The torque of the motor of the second embodiment (E) is improved compared to the first embodiment (D). This is because, as described above, in the first embodiment (D) (FIG. 2), the flux barrier 17 is formed by the first groove portions 14 and the beam portions 15, while in the second embodiment (E) (FIG. 3), the flux barrier 17 is formed not only by the first groove portions 14 and the beam portions 15 but also by the second groove portions 18 and the support portions 16. For this reason, as described above, in the portion between the inner circumferential surface 11 of the stator core 1 and the slots 13, the circumferential path along which magnetic flux leaks always passes through an area where the magnetic resistance is higher than that of the material of the stator core 1 (electromagnetic steel sheet), and thus magnetic flux leakage is reduced accordingly.

[0044] On the other hand, in the third embodiment (F), the beam portion 15 extends radially outward more than in the second embodiment (E), and the radial width of the third beam portion 153 is wider, and the circumferential magnetic resistance of the wider portion is smaller, so the circumferential magnetic flux leakage is larger, and as a result, the torque is lower than in the second embodiment (E). However, the third embodiment (F) has better heat removal performance for the stator core 1 than the first embodiment (D) and the second embodiment (E), and therefore the motor torque is more stable.

[0045] [Modifications] Fig. 7 is a partial enlarged view of a motor cooling structure according to a modification of the first embodiment. Fig. 8 is a partial enlarged view of a motor cooling structure according to a modification of the second embodiment.

[0046] In the modification of the first embodiment shown in FIG. 7, the beam portions 15 and the support portions 16 are arranged on the stator core 1 in a manner that is inverted with respect to the radial direction compared to the first embodiment.

[0047] More specifically, the first groove portions 14 are formed so that they extend in the axial direction and are lined up in the circumferential direction in a number (five portions), with the depth direction being the direction radially outward from a position facing the slot 13 on the inner circumferential surface 11. Four beam portions 15 are arranged between adjacent first groove portions 14. The beam portions 15 extend radially inward and form ridges in the axial direction.

[0048] In the modification of the first embodiment, the flux barrier 17 is also formed by the first groove portion 14 and the beam portion 15, but the beam portion 15 does not communicate with the flow path 131. However, the beam portion 15 communicates with the air gap AG and functions as a heat dissipation fin that dissipates heat to the air in the air gap AG.

[0049] 8, the beam portions 15 and the support portions 16 are arranged on the stator core 1 in a manner that is inverted with respect to the radial direction compared to the second embodiment. More specifically, the first groove portions 14 are formed so that they extend in the axial direction and are lined up in the circumferential direction in a plurality (three) with their depth direction extending radially outward from a position facing the slots 13 on the inner circumferential surface 11. Two beam portions 15 are arranged between adjacent first groove portions 14.

[0050] Furthermore, a second groove 18 is disposed on the inner wall of the slot 13 on the inner circumferential surface 11 side at a position facing the beam portion 15 in the radial direction. The second groove 18 extends in the axial direction and has a depth direction extending from the inner wall toward the radially outer side (beam portion 15). The second groove 18 forms a groove extending in the axial direction on the inside of the beam portion 15.

[0051] In the modified example of the second embodiment, the second groove portion 18 communicates with the flow path 131, which allows the beam portion 15A and the beam portion 15B to be cooled by the refrigerant. In addition, the outside of the beam portion 15A and the beam portion 15B communicates with the air gap AG, which allows them to be cooled by the air in the air gap AG.

[0052] [Effects of this embodiment] According to the motor cooling structure of this embodiment, in the motor cooling structure that cools the coils 2 inserted in the slots 13 of the stator core 1, the slots 13 are arranged to form a closed shape with respect to the inner circumferential surface 11 and the outer circumferential surface 12 of the stator core 1, and the coils 2 are arranged so as to be spaced apart from the inner wall of the slots 13 on the inner circumferential surface 11 side, thereby forming a refrigerant circulation path 131 extending in the axial direction between the inner wall and the coils 2, and at a position on the inner circumferential surface 11 side of the stator core 1 that faces the circulation path 131 from the radial direction, a plurality of first groove portions 14 that extend axially with their depth direction being the direction from the inner wall on the inner circumferential surface 11 side of the slots 13 toward the radially inward direction or the direction from the inner circumferential surface 11 toward the radially outward direction are formed so as to be lined up in the circumferential direction, thereby forming beam portions 15 that protrude radially between adjacent first groove portions 14 and form ridge lines in the axial direction.

[0053] With the above configuration, the coil 2 can be directly cooled by supplying a refrigerant to the flow path 131. Furthermore, the first groove portions 14 and the beam portions 15 form flux barriers 17 that reduce circumferential magnetic flux leakage, thereby reducing magnetic flux leakage when the slots 13 are closed, thereby suppressing a decrease in torque. Furthermore, the first groove portions 14 and the beam portions 15 form an uneven shape, increasing the surface area and improving heat removal performance.

[0054] In this embodiment, when the first groove portion 14 is formed with its depth direction extending radially outward from the inner wall on the inner circumferential surface 11 side of the slot 13, the beam portion 15 is connected to the flow path 131.

[0055] With the above-described configuration, the beam portion 15 comes into direct contact with the refrigerant, thereby further improving the heat removal performance.

[0056] In this embodiment, a second groove portion 18 is arranged on the inner surface 11 of the stator core 1 at a position radially opposite the beam portion 15, with the depth direction being from the inner surface 11 toward the radially outward direction and extending in the axial direction, and the second groove portion 18 forms a groove on the inside of the beam portion 15.

[0057] With the above configuration, the entire portion between the inner circumferential surface 11 of the stator core 1 and the slots 13 has a zigzag structure, and the circumferential path along which magnetic flux leaks in the portion between the inner circumferential surface 11 of the stator core 1 and the slots 13 always passes through the first groove portion 14 or the second groove portion 18, i.e., always passes through an area where the magnetic resistance is higher than that of the material (electromagnetic steel sheet) of the stator core 1. Therefore, the circumferential magnetic flux leakage is further reduced, thereby further suppressing the reduction in torque.

[0058] In this embodiment, the radial length of the first groove portion 14 is set to be longer than the radial length of the second groove portion 18 .

[0059] With the above configuration, the first grooves 14 through which the refrigerant flows are set to be long in the radial direction, thereby increasing the surface area of ​​the beam portions 15 and improving the heat removal performance of the flux barrier 17. Note that with the above configuration, the radial width of the tip portions (third beam portions 153) of the beam portions 15, which are radially outward from the second grooves 18, is set to be wider than the radial width of the portion (support portion 16) to which the beam portions 15 of the stator core 1 are connected. As a result, the beam portions 15 are set to be longer radially outward, and the beam portions 15 are further inserted into the flow path 131. As a result, the radial width of the tip portions (third beam portions 153) is increased, and the surface area of ​​the tip portions (third beam portions 153) in contact with the refrigerant can be set accordingly larger, improving the heat removal performance of the flux barrier 17.

[0060] Although the embodiments of the present invention have been described above, the above embodiments merely illustrate some of the application examples of the present invention, and it is not intended that the technical scope of the present invention be limited to the specific configurations of the above embodiments.

Claims

1. A motor cooling structure for cooling a coil inserted in a slot of a stator core, wherein the slot is arranged to form a closed shape with respect to the inner and outer circumferential surfaces of the stator core, the coil is arranged to be spaced away from an inner wall on the inner circumferential surface side of the slot, thereby forming a coolant circulation path extending in the axial direction between the inner wall and the coil, and a plurality of first groove portions are formed circumferentially lined up at a position on the inner circumferential surface side of the stator core facing the circulation path from the radial direction, the first groove portions extending in the axial direction having a depth direction that is a direction radially inward from the inner wall on the inner circumferential surface side of the slot or a direction radially outward from the inner circumferential surface, so that beam portions protruding in the radial direction and forming ridges in the axial direction are formed between adjacent first groove portions.

2. A motor cooling structure as described in claim 1, wherein the first groove portion is formed with a depth direction extending from the inner wall on the inner circumferential surface side of the slot toward the radially outward direction, and the beam portion is in communication with the circulation path.

3. A motor cooling structure as described in claim 2, wherein a second groove portion is disposed on the inner circumferential surface of the stator core at a position radially opposite the beam portion, the second groove portion extending in the axial direction and with its depth direction being in the radially outward direction from the inner circumferential surface, the second groove portion forming a groove on the inside of the beam portion.

4. A motor cooling structure as set forth in claim 3, wherein the radial length of the first groove portion is set longer than the radial length of the second groove portion.

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