Coaxial electric drive motor with cooling structure

Abstract

The utility model discloses a coaxial electric drive motor with a cooling structure, which comprises a rotating assembly, the rotating assembly comprises a rotor and a transmission shaft arranged on the inner side surface of the rotor along the axis, and a gap with an opening end is arranged between the inner side surface of the rotor and the outer side surface of the transmission shaft. The coaxial electric drive motor further comprises a sealing assembly arranged at the open end of the gap, and the sealing assembly comprises an oil storage cavity communicated with the gap. And the cooling oil way is formed in the rotating assembly and the sealing assembly, and the oil storage cavity is communicated with the outside through the cooling oil way. According to the utility model, the oil storage cavity is arranged at the open end of the gap between the rotor and the transmission shaft, so that the oil storage cavity can store cooling oil flowing from the outside, and then the cooling oil flows into the gap between the rotor and the transmission shaft, and sufficient cooling oil in the oil storage cavity can continuously enter the gap and cool the rotor and the transmission shaft; the contact area of cooling oil and the rotating assembly is increased, and then the cooling effect is improved.

Description

Technical Field

[0001] This utility model relates to the field of new energy motor technology, specifically to a coaxial electric drive motor with a cooling structure. Background Technology

[0002] A coaxial electric drive motor is characterized by its rotating shaft being coaxial with the output shaft of the transmission system. Its main structure includes a motor, a reducer, a drive shaft, and a cooling system. The motor and drive shaft are coaxially connected. When AC power is applied, the motor outputs torque, which is transmitted to the reducer. The reducer then transmits the torque and speed to the drive shaft, and finally to the wheels. During operation, losses between the motor's stator and rotor generate heat, affecting the motor's power. The stator mainly experiences copper losses and iron losses, while the rotor mainly experiences iron losses. Copper losses refer to the losses caused by the DC resistance of the copper wire converting electrical energy into heat. Iron losses mainly include hysteresis losses and eddy current losses. Hysteresis losses refer to the losses caused by the magnetic hysteresis phenomenon in the core material, which converts magnetic field energy into heat. Eddy current losses refer to the losses caused by the induced electromotive force in the core when the magnetic field changes, resulting in eddy currents. Because these losses lead to increased stator and rotor temperatures, designing an effective cooling scheme for the motor's stator and rotor is essential.

[0003] The published invention patent CN115528862A discloses a motor cooling structure, a coaxial electric drive motor, and a new energy vehicle. The motor cooling structure is used for a coaxial electric drive motor. The motor cooling structure has a cooling oil delivery channel in the shell wall, a cooling oil passage in the end cover, and a first oil spray hole connected to the cooling oil on the side of the end cover near the rotor. The cooling oil flows through the cooling oil delivery channel, the cooling oil passage, and the first oil spray hole and is sprayed onto the end of the rotor, thereby cooling the rotor.

[0004] However, when the inventors implemented this embodiment, they found that this embodiment could only cool the surface of the rotor core, and the cooling oil flowed through the shaft of the rotor to cool the inside of the rotor, resulting in poor cooling effect on the rotor. Utility Model Content

[0005] To address the problem that existing embodiments cannot fully cool the rotor, this utility model provides a coaxial electric drive motor with a cooling structure. The specific technical solution is as follows:

[0006] This utility model includes a rotating assembly, which includes a rotor and a drive shaft disposed along the inner side of the rotor along the axis. There is a gap with an open end between the inner side of the rotor and the outer side of the drive shaft. The coaxial electric drive motor further includes: a sealing assembly disposed at the open end of the gap, the sealing assembly including an oil storage chamber communicating with the gap; and a cooling oil passage formed inside the rotating assembly and the sealing assembly, the cooling oil passage communicating the oil storage chamber with the outside.

[0007] Furthermore, the rotating assembly also includes a housing with an internal cavity, one end of which is connected to the rotor and the drive shaft respectively via bearings, and there is an open end with a gap between the connection points where the rotor and the drive shaft are respectively connected to the left end of the housing, wherein the oil storage cavity is located at the connection points where the left end of the housing is connected to the rotor and the drive shaft respectively; the cooling oil passage includes a third branched oil passage formed in the sealing assembly, which is radially distributed along the sealing assembly and connects the oil storage cavity to the outside.

[0008] Preferably, the rotor includes a hollow shaft that forms a gap with the drive shaft, and the sealing assembly further includes: an oil seal disposed between the rotor and the drive shaft and the connection points with the housing respectively; a first sealing lip in the shape of an annulus, the inner side of which is connected to the outer side of the drive shaft and the outer side of which is connected to the oil seal; and a second sealing lip in the shape of an annulus, the inner side of which is connected to the outer side of the hollow shaft and the outer side of which is connected to the oil seal. The first sealing lip, the second sealing lip and the oil seal can prevent the accommodating cavity from communicating with the outside through the connection points, and the first sealing lip, the second sealing lip and the oil seal form an oil storage cavity.

[0009] Preferably, the housing further includes a shell and an end cap, which together form a receiving cavity for placing the rotor. The bottom of the shell forms a bottom oil trough for placing the oil flowing out of the receiving cavity. The receiving cavity is in communication with the bottom oil trough. The bottom oil trough is in communication with an oil pump placed on the outer side of the shell. The oil pump is in communication with an oil cooler disposed on the outer side of the shell. The oil cooler is in communication with the receiving cavity.

[0010] Preferably, the cooling oil passage includes: a first axial oil passage formed axially on the sidewalls of the housing and the end cover respectively; a second radial oil passage formed radially on the bottom end of the end cover, the second radial oil passage communicating with the first axial oil passage; a third branch oil passage and a fourth branch oil passage radially penetrating the sealing assembly, the third branch oil passage communicating with the second radial oil passage, the third branch oil passage communicating with the oil storage cavity and the opening end of the gap, and the fourth branch oil passage communicating with the receiving cavity; a fifth axial oil passage formed in the gap, one end of the fifth axial oil passage being the opening end of the gap, and the other end of the fifth axial oil passage being closed; and a sixth spray oil passage radially penetrating the rotor, the sixth spray oil passage communicating with the fifth axial oil passage and the receiving cavity.

[0011] Preferably, an annular groove is formed on the inner side of the housing, and the annular groove is connected to the first axial oil passage; the cooling oil passage further includes: a seventh return oil passage formed on the housing, which is connected to the bottom oil tank, oil pump, oil cooler and annular groove; and an eighth return oil passage connecting the first axial oil passage and the annular groove.

[0012] As can be seen from the above technical solution, this utility model has the following beneficial effects:

[0013] This invention provides an oil storage chamber at the open end of the gap between the rotor and the drive shaft, allowing it to store cooling oil flowing in from the outside. This oil then flows into the gap between the rotor and the drive shaft, ensuring that sufficient cooling oil in the storage chamber continuously enters the gap and cools the rotor and the drive shaft. This increases the contact area between the cooling oil and the rotating components, thereby improving the cooling effect. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the structure of an embodiment of the present utility model;

[0015] Figure 2 This is a longitudinal cross-sectional view of an embodiment of the present utility model;

[0016] Figure 3 for Figure 1 Sectional view of AA;

[0017] Figure 4 for Figure 2 Enlarged view of the structure at point C in the image;

[0018] Figure 5 for Figure 1 BB section view;

[0019] Figure 6 for Figure 5 Enlarged view of the structure at point E in the image;

[0020] Figure 7 for Figure 2 Enlarged view of the structure at point D in the image.

[0021] In the diagram: 1. Rotating assembly; 11. Outer shell; 111. Housing; 112. End cap; 113. Receptacle; 114. Bottom oil trough; 115. Oil pump; 116. Oil cooler; 117. Annular groove; 12. Rotor; 121. Hollow shaft; 122. Rotor core; 13. Stator; 131. Winding; 132. Stator core; 14. Drive shaft; 2. Sealing assembly; 21. Oil seal; 22. First sealing lip; 23. Second sealing lip; 24. Oil buffer chamber; 3. Cooling oil passage; 31. First axial oil passage; 32. Second radial oil passage; 33. Third branched oil passage; 34. Fourth branched oil passage; 35. Fifth axial oil passage; 36. Sixth spray oil passage; 37. Seventh return oil passage; 38. Eighth return oil passage. Detailed Implementation

[0022] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present utility model.

[0023] In the description of the embodiments of this utility model, it should be noted that the terms "inner", "outer", "upper", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the utility model product is usually placed when in use. They are only for the convenience of describing this utility model and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this utility model.

[0024] like Figures 1 to 4 As shown, this embodiment of the utility model includes a rotating assembly 1, which includes a housing 11. The left end of the housing 11 is an end cap 112, and the right end is a shell 111. The shell 111 forms a receiving cavity 113 with an open end. The end cap 112 is fixedly connected to the open end of the receiving cavity 113 by bolts, so that the receiving cavity 113 forms a sealed cavity, which then houses the rotor 12 and the stator 13. The rotor 12, the stator 13, and the housing 11 are all coaxial. The rotor 12 passes through both ends of the housing 11 axially to output torque. The stator 13 is fixedly connected to the housing 11 and rotatably connected to the rotor 12. When an alternating current is applied to the winding 131, the alternating magnetic field generated by the iron core of the stator 13 interacts with the fixed magnetic field generated by the permanent magnet on the iron core of the rotor 12, thereby driving the rotor 12 to output torque from the right end of the shell 111. The rotor 12 extends out of the right end of the shell 111 and outputs torque to the reducer through a spline.

[0025] Secondly, the rotating assembly 1 also includes a bottom oil trough 114 disposed at the bottom of the housing 111. After cooling the rotor 12 and stator 13 inside the accommodating cavity 113, the cooling oil flows into the bottom oil trough 114 under the action of gravity. The bottom oil trough 114 is connected to an oil pump 115 fixedly connected to the outside of the housing 111. The oil pump 115 draws out the cooling oil in the bottom oil trough 114 and then introduces it into an oil cooler 116 fixedly connected to the outside of the housing 111. The oil cooler 116 cools down the delivered cooling oil and then sends it back into the accommodating cavity 113, so that the cooling oil can be recycled in this embodiment, thereby improving its utilization rate.

[0026] like Figure 2 As shown, the rotor 12 also includes a hollow rotating shaft 121 with a through hole in the middle. The rotating assembly 1 also includes a transmission shaft 14 coaxial with the hollow rotating shaft 121. The outer diameter of the transmission shaft 14 is smaller than the inner diameter of the hollow rotating shaft 121, so that there is a gap between the transmission shaft 14 and the hollow rotating shaft 121. The left end of the transmission shaft 14 and the left end of the hollow rotating shaft 121 are rotatably connected to the end cover 112 through bearings, so that the left ends of the two do not contact each other, thus forming an open end of the gap. In addition, a drum-shaped protrusion is formed on the outer side of the right end of the transmission shaft 14. The gap between the axial side of the drum-shaped protrusion and the inner side of the right end of the hollow rotating shaft 121 is extremely small, so that the gap has only one open end and is located at one end of the end cover 112.

[0027] Furthermore, this embodiment also includes: a sealing assembly 2 disposed at the open end of the gap, the sealing assembly 2 including an oil storage cavity communicating with the gap; and a cooling oil passage 3 formed inside the rotating assembly 1 and the sealing assembly 2, the cooling oil passage 3 communicating the oil storage cavity with the outside.

[0028] Specifically, the left ends of the hollow rotating shaft 121 and the drive shaft 14 are rotatably connected to the end cover 112 via bearings. The cooling oil in the accommodating cavity 113 can easily flow out from the end cover 112 through the bearing gap. Then, the sealing component 2 is fixed between the two bearings to cover the bearing gap so that the cooling oil cannot flow out. Secondly, the oil storage cavity contacts the left ends of the drive shaft 14 and the hollow rotating shaft 121 respectively, and then contacts the gap opening end between the left ends of the two, so that the oil storage cavity is connected to the gap.

[0029] Secondly, the cooling oil circuit 3 is used to transport external cooling oil to the rotating component 1 and the sealing component 2 for cooling. The oil storage chamber is connected to the outside through the cooling oil circuit 3, so that the cooling oil enters the oil storage chamber for storage, and the cooling oil stored in the oil storage chamber can flow into the gap through the open end, thereby cooling the hollow rotating shaft 121 and the drive shaft 14, increasing the contact area between the cooling oil and the rotation, and thus improving the cooling efficiency of the rotating component 1.

[0030] like Figure 4As shown, the sealing assembly 2 further includes: an oil seal 21 disposed between the rotor 12 and the drive shaft 14 and the housing 11 respectively; a first sealing lip 22 in the shape of an annulus, the inner side of which is connected to the outer side of the drive shaft 14 and the outer side of which is connected to the oil seal 21; and a second sealing lip 23 in the shape of an annulus, the inner side of which is connected to the outer side of the hollow shaft 121 and the outer side of which is connected to the oil seal 21. The first sealing lip 22, the second sealing lip 23 and the oil seal 21 can prevent the accommodating cavity 113 from communicating with the outside through the connection part. The first sealing lip 22, the second sealing lip 23 and the oil seal 21 form an oil storage cavity.

[0031] Specifically, the oil seal 21 is fixedly installed on the inner side of the end cover 112, with its two axial end faces facing the sides of the two bearings respectively; secondly, the first sealing lip 22 is a rubber ring, with its outer side embedded in the oil seal 21 and fixedly connected to it to form a sealing structure. Its thin wall bends under external force, making it tightly contact the outer side of the drive shaft 14, thereby preventing the cooling oil from flowing out from the connection between the first sealing lip 22 and the drive shaft 14; secondly, the second sealing lip 23 is a rubber ring, with its outer side embedded in the oil seal 21 and fixedly connected to it to form a sealing structure. Its thin wall bends under external force, making it tightly contact the outer side of the hollow rotating shaft 121, thereby preventing the cooling oil from flowing out from the connection between the second sealing lip 23 and the drive shaft 14.

[0032] The oil seal 21, the first sealing lip 22, and the second sealing lip 23 together form a sealing structure that seals the bearing clearance, preventing the accommodating cavity 113 from communicating with the outside through the two left-end bearings fixedly connected to the end cover 112. This prevents the cooling oil inside the accommodating cavity 113 from flowing out and causing pollution and waste. The oil seal 21, the first sealing lip 22, and the second sealing lip 23 form an oil storage cavity, which, together with the drive shaft 14 and the hollow rotating shaft 121, forms a sealed cavity communicating with the clearance, allowing the cooling oil inside to flow into the clearance to cool the rotating shaft and the hollow rotating shaft 121.

[0033] Furthermore, the cooling oil passage 3 includes a third branched oil passage 33 formed in the sealing assembly 2, the third branched oil passage 33 being radially distributed along the sealing assembly 2, the third branched oil passage 33 connecting the oil storage chamber and the outside.

[0034] Specifically, the oil seal 21 forms a through hole in the radial direction, which is a third branched oil passage 33. One end of the third branched oil passage 33 is connected to the outside for injecting cooling oil, and the other end is connected to the oil storage chamber, so that the external cooling oil can flow into the oil storage chamber for storage under the action of gravity. Then, the stored cooling oil enters the gap to cool the drive shaft 14 and the hollow rotating shaft 121. The sufficient amount of cooling oil in the oil storage chamber ensures that the cooling of the drive shaft 14 and the hollow rotating shaft 121 will not be interrupted due to the narrow oil passage, thus reducing the stability of cooling.

[0035] Combination Figure 2 As shown, the cooling oil passage 3 includes: a first axial oil passage 31 formed axially on the side walls of the housing 111 and the end cover 112 respectively; a second radial oil passage 32 formed radially on the bottom end of the end cover 112, which communicates with the first axial oil passage 31; a third branch oil passage 33 and a fourth branch oil passage 34 that radially penetrate the sealing assembly 2, which communicate with the second radial oil passage 32, the third branch oil passage 33 communicates with the oil storage cavity and the opening end of the gap, and the fourth branch oil passage 34 communicates with the accommodating cavity 113; a fifth axial oil passage 35 formed in the gap, one end of which is the opening end of the gap, and the other end of which is closed; and a sixth spray oil passage 36 that radially penetrates the rotor 12, which communicates with the fifth axial oil passage 35 and the accommodating cavity 113.

[0036] Specifically, the housing 111 and the end cap 112 are fixedly connected by bolts. The sidewalls of both form a first axial oil passage 31 for conveying cooling oil. The upper half of the bottom of the end cap 112 forms a second radial oil passage 32 for conveying the cooling oil from the first axial oil passage 31. The bearing mounting portion of the end cap 112 forms a third branched oil passage 33 and a fourth branched oil passage 34, respectively. Both the third branched oil passage 33 and the fourth branched oil passage 34 are connected to the second radial oil passage 32, so that the cooling oil in the second radial oil passage 32 is divided into two parts. One part flows into the oil storage chamber through the third branched oil passage 33, and the other part flows into the bearing connected to the receiving cavity 113 through the fourth branched oil passage 34, thereby cooling the bearing and improving its service life. The gap is the fifth axial oil passage 35. The cooling oil in the oil storage chamber enters the fifth axial oil passage 35 through the open end and flows from the left end to the right end. The right end of the drive shaft 14 and the hollow rotating shaft 121 is a gap. The gap cannot pass through the cooling oil but does not affect the relative rotation of the drive shaft 14 and the hollow rotating shaft 121. The hollow rotating shaft 121 forms a sixth spray oil passage 36 in the radial direction. The cooling oil in the fifth axial oil passage 35 enters the accommodating cavity 113 through the sixth spray oil passage 36 under the action of centrifugal force to cool the rotor 12 and stator 13. The sixth spray oil passage 36 can penetrate the hollow rotating shaft 121 or penetrate the iron core of the rotor 12 in the radial direction, thereby increasing the contact area between the cooling oil and the rotor 12 and thus improving its cooling effect.

[0037] Secondly, the cooling oil flows through the first axial oil passage 31 to the second radial oil passage 32, and then splits into two from the second radial oil passage 32, flowing into the third radial oil passage and the fourth radial oil passage respectively. From the third radial oil passage, it flows to the oil storage chamber to store the cooling oil. Under the action of gravity and pressure, the stored cooling oil flows to the fifth axial oil passage 35, and under the action of centrifugal force, it enters the accommodating cavity 113 from the sixth spray oil passage 36 to cool the rotor 12 and the stator 13.

[0038] like Figures 5 to 7 As shown, an annular groove 117 is formed on the inner side of the housing 111. The annular groove 117 is connected to the first axial oil passage 31. The cooling oil passage 3 also includes: a seventh return oil passage 37 formed on the housing 111, which is connected to the bottom oil tank 114, the oil pump 115, the oil cooler 116 and the annular groove 117; and an eighth return oil passage 38 connecting the first axial oil passage 31 and the annular groove 117.

[0039] Specifically, the width of the annular groove 117 is less than the axial length of the stator core 13, and the top of the annular groove 117 is closed by the outer side of the stator core 13, so that the annular groove 117 forms an annular cavity. This annular cavity is connected to the first axial oil passage 31, so that the cooling oil inside can enter the first axial oil passage 31.

[0040] Secondly, a seventh oil return passage 37 is formed between the shell walls of the housing 111, which connects the bottom oil tank 114 and the oil pump 115, the oil pump 115 and the oil cooler 116, and the oil cooler 116 and the annular groove 117. This allows the oil pump 115 to pump the cooling oil inside the bottom oil tank 114 to the oil cooler 116 through the seventh oil return passage 37. The oil cooler 116 cools the cooling oil and then sends the processed cooling oil into the annular groove 117 through the seventh oil return passage 37. Secondly, an eighth oil return passage 38 is formed radially in the housing 111. This eighth oil return passage 38 guides the cooling oil in the annular groove 117 to the first axial oil passage 31, thereby realizing the recycling of the cooling oil in the bottom oil tank 114 and improving the utilization rate of the cooling oil.

[0041] Secondly, the oil cooler 116 is connected to the outside, so that the external cooling oil is cooled by the oil cooler 116 and then enters the annular groove 117, and then enters the first axial oil passage 31.

[0042] Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the present invention, the scope of which is defined by the appended claims and their equivalents.

[0043] The technologies, shapes, and structures not described in detail in this utility model are all known technologies.

Claims

1. A coaxial electric drive motor with a cooling structure, comprising a rotating assembly (1), the rotating assembly (1) including a rotor (12) and a drive shaft (14) disposed along an axis on the inner side of the rotor (12), wherein a gap with an open end exists between the inner side of the rotor (12) and the outer side of the drive shaft (14), characterized in that, The coaxial electric drive motor also includes: A sealing assembly (2) is disposed at the open end of the gap, the sealing assembly (2) including an oil storage chamber communicating with the gap; and A cooling oil passage (3) is formed inside the rotating assembly (1) and the sealing assembly (2), which connects the oil storage chamber to the outside.

2. The coaxial electric drive motor according to claim 1, characterized in that: The rotating assembly (1) further includes a housing (11) with an internal cavity (113). One end of the housing (11) is connected to the rotor (12) and the drive shaft (14) respectively via bearings. There is an opening of the gap between the connection points where the rotor (12) and the drive shaft (14) are respectively connected to the left end of the housing (11). The oil storage cavity is provided at the connection points where the left end of the housing (11) is respectively connected to the rotor (12) and the drive shaft (14). The cooling oil passage (3) includes a third branched oil passage (33) formed in the sealing assembly (2), the third branched oil passage (33) being radially distributed along the sealing assembly (2), and the third branched oil passage (33) connecting the oil storage chamber to the outside.

3. The coaxial electric drive motor according to claim 2, characterized in that: The rotor (12) includes a hollow shaft (121) that forms the gap with the drive shaft (14), and the sealing assembly (2) further includes: Oil seals (21) are disposed between the rotor (12) and the drive shaft (14) and the housing (11), respectively. A first sealing lip (22) in the shape of an annulus, the inner side of which is connected to the outer side of the drive shaft (14), and the outer side of which is connected to the oil seal (21); and A second sealing lip (23) in the shape of an annulus is connected to the outer side of the hollow rotating shaft (121) on its inner side and to the oil seal (21) on its outer side. The first sealing lip (22), the second sealing lip (23) and the oil seal (21) can prevent the accommodating cavity (113) from communicating with the outside through the connection part. The first sealing lip (22), the second sealing lip (23) and the oil seal (21) form the oil storage cavity.

4. The coaxial electric drive motor according to claim 2, characterized in that: The outer casing (11) further includes a housing (111) and an end cap (112), which together form a receiving cavity (113) for placing the rotor (12). The bottom of the housing (111) forms a bottom oil trough (114) for placing the oil flowing out of the receiving cavity (113). The receiving cavity (113) is in communication with the bottom oil trough (114). The bottom oil trough (114) is in communication with an oil pump (115) placed on the outer side of the housing (111). The oil pump (115) is in communication with an oil cooler (116) disposed on the outer side of the housing (111). The oil cooler (116) is in communication with the receiving cavity (113).

5. The coaxial electric drive motor according to claim 4, characterized in that: The cooling oil circuit (3) includes: A first axial oil passage (31) is formed along the axial direction on the sidewalls of the housing (111) and the end cap (112), respectively; A second radial oil passage (32) is formed radially at the bottom end of the end cap (112), and the second radial oil passage (32) is connected to the first axial oil passage (31); A third branch oil passage (33) and a fourth branch oil passage (34) are radially penetrating the sealing assembly (2). The third branch oil passage (33) is connected to the second radial oil passage (32). The third branch oil passage (33) is connected to the oil storage cavity and the opening end of the gap. The fourth branch oil passage (34) is connected to the receiving cavity (113). A fifth axial oil passage (35) is formed in the gap, one end of which is the open end of the gap, and the other end of which is closed; A sixth spray oil passage (36) runs radially through the rotor (12), and the sixth spray oil passage (36) is connected to the fifth axial oil passage (35) and the accommodating cavity (113).

6. The coaxial electric drive motor according to claim 5, characterized in that: An annular groove (117) is formed on the inner side of the housing (111), and the annular groove (117) is connected to the first axial oil passage (31); The cooling oil circuit (3) also includes: A seventh return oil passage (37) is formed on the housing (111), which connects the bottom oil tank (114), the oil pump (115), the oil cooler (116) and the annular groove (117); The eighth return oil passage (38) connects the first axial oil passage (31) and the annular groove (117).

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