US20080107550A1

US20080107550A1 - Eletric pump

Info

Publication number: US20080107550A1
Application number: US11/934,290
Authority: US
Inventors: Toshiro Fujii
Original assignee: Toyota Industries Corp
Current assignee: Toyota Industries Corp
Priority date: 2006-11-02
Filing date: 2007-11-02
Publication date: 2008-05-08
Also published as: DE102007052275A1; JP2008115748A; JP4784484B2

Abstract

A Roots pump has a driving shaft press fitted into a driving rotor, and a driven shaft press fitted into a driven shaft. A driving timing gear is located between an electric motor and the driving rotor. The driving rotor has a driving assist shaft that projects in a direction opposite to the driving timing gear. The specific gravity of the material of the driving assist shaft is less than the specific gravity of the material of the driving rotary shaft. A driven rotor has a driven assist shaft that projects in a direction opposite to a driven timing gear. The specific gravity of the material of the driven assist shaft is less than the specific gravity of the material of the driven rotary shaft.

Description

BACKGROUND OF THE INVENTION

The present invention relates to an electric pump having timing gears between an electric motor and rotors.
Japanese Laid-Open Patent Publication No. 2002-54587 discloses a screw pump that includes a driving shaft and a driven shaft. The driving shaft extends through and supports a driving rotor, and the driven shaft extends through and supports a driven rotor. When an electric motor rotates the driving shaft, meshing of the driving timing gear and the driven timing gear makes the driven shaft rotate synchronously with the driving shaft. The driving timing gear is located between the electric motor and the driving rotor.
To reduce the weight of the screw pump disclosed in the document, the axial dimension of the driving shaft and driven shaft may be reduced. However, if the axial dimension of the driving shaft is reduced while maintaining the structure in which the driving shaft extends through the driving rotor, the axial dimension of the driving rotor needs to be reduced. This reduces the amount of fluid that is transferred by the driving rotor, which lowers the performance of the screw pump. Likewise, if the axial dimension of the driven shaft is reduced while maintaining the structure in which the driven shaft extends through the driven rotor, the axial dimension of the driven rotor needs to be reduced.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide an electric pump that is capable of reducing the weight without changing the size and shape.
According to one aspect of the invention, an electric pump including an electric motor and a first rotary shaft driven by the electric motor is provided. A first rotor is coupled to and rotates integrally with the first rotary shaft. The first rotor is formed of a material the specific gravity of which is less than the specific gravity of the material of the first rotary shaft. A first timing gear is provided to the first rotary shaft. The first timing gear is located between the electric motor and the first rotor with respect to an axial direction of the first rotary shaft. The first rotor has a first gear facing surface, a first opposite surface, and a first assist shaft. The first gear facing surface is an end face that faces the first timing gear with respect to the axial direction. The first opposite surface is an end face that is opposite to the first gear facing surface. The first assist shaft projects from the first opposite surface. The first gear facing surface has a first recess. The first rotary shaft is press fitted into the first recess so that the first rotor is coupled to the first rotary shaft. The first assist shaft is arranged to be coaxial with the first rotary shaft. The specific gravity of the material of the first assist shaft is less than the specific gravity of the material of the first rotary shaft. A first bearing rotatably supports the first assist shaft. The electric pump includes a second rotary shaft and a second rotor that is coupled to and rotates integrally with the second rotary shaft. The second rotor is formed of a material the specific gravity of which is less than the specific gravity of the material of the second rotary shaft. A second timing gear is provided to the second rotary shaft. The first timing gear and the second timing gear cause the second rotary shaft to rotate synchronously with the first rotary shaft. The second timing gear is located between the electric motor and the second rotor with respect to an axial direction of the second rotary shaft. The second rotor has a second gear facing surface, a second opposite surface, and a second assist shaft. The second gear facing surface is an end face that faces the second timing gear with respect to the axial direction. The second opposite surface is an end face that is opposite to the second gear facing surface. The second assist shaft projects from the second opposite surface. The second gear facing surface has a second recess. The second rotary shaft is press fitted into the second recess so that the second rotor is coupled to the second rotary shaft. The second assist shaft is arranged to be coaxial with the second rotary shaft. The specific gravity of the material of the second assist shaft is less than the specific gravity of the material of the second rotary shaft. A second bearing rotatably supports the second assist shaft.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:
FIG. 1 is a cross-sectional view illustrating an electric Roots pump according to one embodiment of the present invention;
FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1; and
FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 3 show one embodiment of the present invention. FIG. 1 shows an electric pump according to the present embodiment, which is a Roots pump 10. Arrow Y of FIG. 1 represents a direction from the rear toward the front of the Roots pump 10.
As shown in FIG. 1, a pump housing 10 a, which is the housing of the Roots pump 10, includes a rotor housing member 11, a bearing housing member 12, and a motor housing member 14 arranged in this order from the rear toward the front. The bearing housing member 12 is secured to the front end of the rotor housing member 11, and the motor housing member 14 is secured to the front end of the bearing housing member 12.
The rotor housing member 11 defines a pump chamber 15 that accommodates a first rotor, which is a driving rotor 22, and a second rotor, which is a driven rotor 23, and the bearing housing member 12 covers the opening of the pump chamber 15. The motor housing member 14 defines a motor chamber 17 that accommodates an electric motor M, and a gear chamber 16 that accommodates a first timing gear, which is a driving timing gear 28, and a second timing gear, which is a driven timing gear 29. The gear chamber 16 is located at the opening of the motor housing member 14, and the bearing housing member 12 lids the gear chamber 16. The driving rotor 22 and the driven rotor 23 are each made of aluminum.
The pump housing 10 a accommodates a first rotary shaft, which is a driving shaft 20, and a second rotary shaft, which is a driven shaft 21. The driving shaft 20 and the driven shaft 21 extend parallel to each other in a direction along arrow Y. The driving shaft 20 and the driven shaft 21 are each made of an iron-based material. That is, the specific gravity of the material of the driving rotor 22 and the driven rotor 23 is less than the specific gravity of the material of the driving shaft 20 and the driven shaft 21.
The Roots pump 10 has five bearings, or a first bearing 31, a second bearing 32, a third bearing 33, a fourth bearing 34, and a fifth bearing 35. The third bearing 33 and the fifth bearing 35 rotatably support the driving shaft 20, and the fourth bearing 34 rotatably supports the driven shaft 21. The rotor housing member 11 has the first bearing 31 and the second bearing 32, the bearing housing member 12 has the third bearing 33 and the fourth bearing 34, and the motor housing member 14 has the fifth bearing 35.
The driving shaft 20 extends from the rear toward the front with connecting the driving rotor 22, the third bearing 33, the driving timing gear 28, the electric motor M, and the fifth bearing 35 in this order. That is, the driving timing gear 28 is located between the driving rotor 22 and the electric motor M with respect to an axial direction of the driving shaft 20. The driving shaft 20 has a front end 20 a supported by the fifth bearing 35 and a rear end 20 b coupled to the driving rotor 22. The driving rotor 22, the driving timing gear 28, and the rotor of the electric motor M rotate integrally with the driving shaft 20.
The driven shaft 21 extends from the rear toward the front with connecting the driven rotor 23, the fourth bearing 34, and the driven timing gear 29 in this order. That is, the driven timing gear 29 is located between the driven rotor 23 and the electric motor M with respect to an axial direction of the driven shaft 21. The driven shaft 21 has a front end 21 a coupled to the driven timing gear 29 and a rear end 21 b coupled to the driven rotor 23. The driven rotor 23 and the driven timing gear 29 rotate integrally with the driven shaft 21.
As shown in FIGS. 2 and 3, the driving rotor 22 and the driven rotor 23 are each a two-lobe type Roots rotor. A cross section of each of the rotors 22, 23 perpendicular to the axial direction is shaped like a gourd. The driving rotor 22 has a pair of driving lobes 24 extending radially outward from the driving shaft 20 in opposite directions. Also, two driving recesses 25 are formed between the driving lobes 24. Likewise, the driven rotor 23 has a pair of driven lobes 26 extending radially outward from the driven shaft 21 in opposite directions. Also, two driven recesses 27 are formed between the driven lobes 26. That is, a pair of the driving lobes 24 are arranged in the circumferential direction at an equal interval, and the driven lobes 26 are also arranged in circumferential direction at an equal interval.
The outer surface of the driving rotor 22, the outer surface of the driven rotor 23, and the inner surface of the rotor housing member 11 define the pump chamber 15. As shown in FIG. 3, the rotor housing member 11 has a suction port 18 for drawing fluid into the pump chamber 15, and a discharge port 19 for discharging fluid from the pump chamber 15.
The driving timing gear 28 and the driven timing gear 29 form pair of timing gears meshed with each other. When the electric motor M rotates the driving shaft 20, the rotation of the driving shaft 20 is transmitted from the driving timing gear 28 to the driven timing gear 29, so that the driven shaft 21 rotates synchronously with the driving shaft 20. As a result, the driving rotor 22 and the driven rotor 23 rotate in opposite directions, so that fluid is drawn into the pump chamber 15 through the suction port 18, and is discharged to the outside through the discharge port 19.
As shown in FIG. 1, the driving rotor 22 has a driving gear facing surface 22 a, which is an end face facing the driving timing gear 28 with respect to the axial direction of the driving shaft 20, and a driving opposite surface 22 b, which is an end face opposite to the driving gear facing surface 22 a. Likewise, the driven rotor 23 has a driven gear facing surface 23 a, which is an end face facing the driven timing gear 29 with respect to the axial direction of the driven shaft 21, and a driven opposite surface 23 b, which is an end face opposite to the driven gear facing surface 23 a. The driving gear facing surface 22 a, which functions as a first gear facing surface, is a front end face of the driving rotor 22, and the driving opposite surface 22 b, which functions as a first opposite surface, is a rear end face of the driving rotor 22. Likewise, the driven gear facing surface 23 a, which functions as a second gear facing surface, is a front end face of the driven rotor 23, and the driven opposite surface 23 b, which functions as a second opposite surface, is a rear end face of the driven rotor 23.
A center portion of the driving gear facing surface 22 a has a driving recess 41, which functions as a first recess. The driving recess 41 has a circular cross section, and the axis M1 of the driving recess 41 coincides with the axis L1 of the driving shaft 20. The rear end 20 b of the driving shaft 20 is press fitted into the driving recess 41, so that the driving rotor 22 is coupled to and rotates integrally with the driving shaft 20. That is, the driving shaft 20 does not extend through the driving rotor 22. Torque of the driving shaft 20 is transmitted from the circumferential surface of the driving shaft 20 to the circumferential surface of the driving recess 41.
In the present embodiment, the driving shaft 20 is inserted to the bottom of the driving recess 41. The depth, or the axial dimension, of the driving recess 41 is less than half the axial dimension (thickness) of the driving rotor 22. That is, the length of a portion of the driving shaft 20 connected to the driving rotor 22 is less than the axial dimension of the driving rotor 22. The axial dimension of a press fitted portion, which is a portion of the driving shaft 20 that is press fitted into the driving recess 41, is set to a value that allows transmission torque to be transmitted from the driving shaft 20 to the driving rotor 22, while ensuring that the press fitting strength is equal to or less than the strength of the driving shaft 20. Specifically, the length of the portion of the driving shaft 20 that is press fitted into the driving recess 41 is set such that the value obtained by multiplying the transmission torque transmitted from the driving shaft 20 to the driving rotor 22 by a safety factor is equal to the fastening force between the driving recess 41 and the driving shaft 20. That is, the depth of the driving recess 41 is set to a value that allows transmission torque to be transmitted from the driving shaft 20 to the driving rotor 22, and prevents the driving shaft 20 from slipping relative to the driving rotor 22.
Likewise, a center portion of the driven gear facing surface 23 a has a driven recess 42, which functions as a second recess. The driven recess 42 has a circular cross section, and the axis M2 of the driven recess 42 coincides with the axis L2 of the driven shaft 21. The rear end 21 b of the driven shaft 21 is press fitted into the driven recess 42, so that the driven rotor 23 is coupled to and rotates integrally with the driven shaft 21. That is, the driven shaft 21 does not extend through the driven rotor 23. Torque of the driven shaft 21 is transmitted from the circumferential surface of the driven shaft 21 to the circumferential surface of the driven recess 42.
In the present embodiment, the driven shaft 21 is inserted to the bottom of the driven recess 42. The depth, or the axial dimension, of the driven recess 42 is less than half the axial dimension (thickness) of the driven rotor 23. That is, the length of a portion of the driven shaft 21 connected to the driven rotor 23 is less than the axial dimension of the driven rotor 23. The axial dimension of a press fitted portion, which is portion of the driven shaft 21 that is press fitted into the driven recess 42, is set to a value that allows transmission torque to be transmitted from the driven shaft 21 to the driven rotor 23, while ensuring that the press fitting strength is equal to or less than the strength of the driven shaft 21. Specifically, the length of the portion of the driven shaft 21 that is press fitted into the driven recess 42 is set such that the value obtained by multiplying the transmission torque transmitted from the driven shaft 21 to the driven rotor 23 by a safety factor is equal to the fastening force between the driven recess 42 and the driven shaft 21. That is, the depth of the driven recess 42 is set to a value that allows transmission torque to be transmitted from the driven shaft 21 to the driven rotor 23, and prevents the driven shaft 21 from slipping relative to the driven rotor 23.
The driving rotor 22 has a columnar driving assist shaft 37, which functions as a first assist shaft projecting from a center portion of the driving opposite surface 22 b in the axial direction. The axis N1 of the driving assist shaft 37 is set to coincide with the axis L1 of the driving shaft 20 and the axis M1 of the driving recess 41. That is, the driving assist shaft 37, functioning as a first shaft portion, is coaxial with the driving shaft 20. The driving assist shaft 37 and the driving rotor 22 are formed integrally by molding. That is, the driving assist shaft 37 is also made of aluminum. The first bearing 31 receives the radial load of the driving rotor 22 and a small amount of transmission torque by rotatably supporting the driving assist shaft 37 to the rotor housing 11. That is, the driving rotor 22 is supported at both axial ends by the first bearing 31 and the third bearing 33.
Likewise, the driven rotor 23 has a columnar driven assist shaft 38, which functions as a second assist shaft projecting from a center portion of the driven opposite surface 23 b in the axial direction. The axis N2 of the driven assist shaft 38 is set to coincide with the axis L2 of the driven shaft 21 and the axis M2 of the driven recess 42. That is, the driven assist shaft 38, functioning as a second shaft portion, is coaxial with the driven shaft 21. The driven assist shaft 38 and the driven rotor 23 are formed integrally by molding. That is, the driven assist shaft 38 is also made of aluminum. The second bearing 32 receives the radial load of the driven rotor 23 and a small amount of transmission torque by rotatably supporting the driven assist shaft 38 to the rotor housing 11. That is, the driven rotor 23 is supported at both axial ends by the second bearing 32 and the fourth bearing 34.
The load (transmission torque) applied to the driving rotor 22 by the driving shaft 20 has the greatest value in an area in the vicinity of the driving gear facing surface 22 a close to the electric motor M, and is reduced as the distance from the electric motor M increases. Thus, the transmission torque acting on the driving assist shaft 37 is close to zero and is smaller than the transmission torque acting on the driving shaft 20. Namely, the torsion of the driving assist shaft 37 is close to zero and is smaller than the torsion of the driving shaft 20. The load applied to the driving assist shaft 37 is merely the sum of the small amount of transmission torque and the radial load of the driving rotor 22. That is, unlike the driving shaft 20, which transmits torque from the electric motor M to the driving rotor 22, the driving assist shaft 37 does not need to have a great stiffness. As a result, the specific gravity of the material of the driving assist shaft 37 is permitted to be less than the specific gravity of the material of the driving shaft 20. Therefore, the driving shaft 20 does not need to extend through the driving rotor 22, and the axial dimension of the driving shaft 20 can be reduced. By making the specific gravity of the material of the driving assist shaft 37 smaller than the specific gravity of the material of the driving shaft 20, the weight of the Roots pump 10 is reduced without changing the shape and size of the Roots pump 10.
Likewise, the load (transmission torque) applied to the driven rotor 23 by the driven shaft 21 has the greatest value in an area in the vicinity of the driven gear facing surface 23 a close to the driven timing gear 29, and is reduced as the distance from the driven timing gear 29 increases. Thus, the transmission torque acting on the driven assist shaft 38 is close to zero and is smaller than the transmission torque acting on the driven shaft 21. Namely, the torsion of the driven assist shaft 38 is close to zero and is smaller than the torsion of the driven shaft 21. The load applied to the driven assist shaft 38 is merely the sum of the small amount of transmission torque and the radial load of the driven rotor 23. That is, unlike the driven shaft 21, which transmits torque from the driven timing gear 29 to the driven rotor 23, the driven assist shaft 38 does not need to have a great stiffness. As a result, the specific gravity of the material of the driven assist shaft 38 is permitted to be less than the specific gravity of the material of the driven shaft 21. Therefore, the driven shaft 21 does not need to extend through the driven rotor 23, and the axial dimension of the driven shaft 21 can be reduced. By making the specific gravity of the material of the driven assist shaft 38 smaller than the specific gravity of the material of the driven shaft 21, the weight of the Roots pump 10 is reduced without changing the shape and size of the Roots pump 10.
The preferred embodiment has the following advantages.
(1) The driving shaft 20 is press fitted partway into the driving rotor 22 along the axial direction of the driving rotor 22. The driving rotor 22 has the driving assist shaft 37, the material of which has a specific gravity smaller than that of the material of the driving shaft 20. Therefore, for example, compared to a case where the driving shaft 20 extends through the driving rotor 22, the weight of the Roots pump 10 is reduced. That is, the weight of the Roots pump 10 can be reduced without changing the size and the shape of the Roots pump 10.
Likewise, the driven shaft 21 is press fitted partway into the driven rotor 23 along the axial direction of the driven rotor 23. The driven rotor 23 has the driven assist shaft 38, the material of which has a specific gravity smaller than that of the material of the driven shaft 21. Therefore, the weight of the Roots pump 10 can be reduced without changing the fluid transferring performance.
(2) The driving assist shaft 37 is integrally molded with the driving rotor 22. Therefore, compared to a case where the driving assist shaft 37 is formed separately from and attached to the driving rotor 22, the driving assist shaft 37 is prevented from being eccentric with respect to the driving rotor 22. This reduces vibration of the driving shaft 20. Likewise, since the driven assist shaft 38 is integrally molded with the driven rotor 23, the driven assist shaft 38 is prevented from being eccentric with respect to the driven rotor 23. This reduces vibration of the driven shaft 21.
(3) The driving assist shaft 37 and the driving rotor 22 are formed simultaneously using a common material (aluminum). Likewise, the driven assist shaft 38 and the driven rotor 23 are formed simultaneously using a common material (aluminum). Therefore, compared to, for example, a case where the driving assist shaft 37 and the driving rotor 22 are formed separately and assembled thereafter, the manufacturing costs of the Roots pump 10 are reduced, and the productivity is increased.
(4) The depth of the driving recess 41 is equal to or less than half the axial dimension of the driving rotor 22, and the depth of the driven recess 42 is equal to or less than the axial dimension of the driven rotor 23. As the axial dimensions of the driving shaft and the driven shaft 21 are reduced, the weight of the Roots pump 10 is reduced.
(5) The driving recess 41 can be formed simultaneously when the driving rotor 22 is molded. Therefore, for example, compared to a case where the driving shaft 20 extends through the driving rotor 22 and a through hole is formed in the driving rotor 22 after the driving rotor 22 is molded, the time required for forming the driving rotor 22 is reduced. This increases the productivity of the Roots pump 10. Likewise, since the driven recess 42 is formed simultaneously with the driven rotor 23 when the driven rotor 23 is molded, the time required for forming the driven rotor 23 is shortened.
The above-mentioned embodiment may be modified as follows.
The driving shaft 20 does not need to be press fitted into the driving recess 41 to the bottom of the driving recess 41. Likewise, the driven shaft 21 does not need to be press fitted into the driven recess 42 to the bottom of the driven recess 42. Also, the depth of the driving recess 41 and the depth of the driven recess 42 may be changed.
The driving assist shaft 37 and the driving rotor 22 do not need to be integrally molded, but may be formed separately and assembled thereafter. Likewise, the driven assist shaft 38 and the driven rotor 23 do not need to be integrally molded, but may be formed separately and assembled thereafter.
The material of the driving assist shaft 37 does not need to be the same as the material of the driving rotor 22. The driving assist shaft 37 may be formed of any material as long as its specific gravity is less than the specific gravity of the material of the driving shaft 20. For example, in a case where the driving shaft 20 is made of an iron-based material, the driving assist shaft 37 may be formed of titanium or a synthetic resin. Likewise, the material of the driven assist shaft 38 does not need to be the same as the material of the driven rotor 23. The driven assist shaft 38 may be formed of any material as long as its specific gravity is less than the specific gravity of the material of the driven shaft 21. For example, the material of the driven assist shaft 38 may be titanium or a synthetic resin.
The driving rotor 22 and the driven rotor 23 may be formed of synthetic resin. In this case, the driving shaft 20 and the driven shaft 21 are made of a material having a specific gravity that is grater than that of the synthetic resin.
As long as the driving rotor 22 and the driven rotor 23 are Roots rotors each having two or more lobes, the rotors 22, 23 may each have three lobes.
The driving rotor 22 and the driven rotor 23 do not need to be Roots rotors, but may be screw rotors. That is, the electric pump may be an electric screw pump.

Claims

1. An electric pump, comprising:

an electric motor;

a first rotary shaft driven by the electric motor;

a first rotor that is coupled to and rotates integrally with the first rotary shaft, the first rotor being formed of a material the specific gravity of which is less than the specific gravity of the material of the first rotary shaft;

a first timing gear provided to the first rotary shaft, the first timing gear being located between the electric motor and the first rotor with respect to an axial direction of the first rotary shaft, wherein the first rotor has a first gear facing surface, a first opposite surface, and a first assist shaft, the first gear facing surface being an end face that faces the first timing gear with respect to the axial direction, the first opposite surface being an end face that is opposite to the first gear facing surface, and the first assist shaft projecting from the first opposite surface, wherein the first gear facing surface has a first recess, wherein the first rotary shaft is press fitted into the first recess so that the first rotor is coupled to the first rotary shaft, wherein the first assist shaft is arranged to be coaxial with the first rotary shaft, and wherein the specific gravity of the material of the first assist shaft is less than the specific gravity of the material of the first rotary shaft;

a first bearing rotatably supporting the first assist shaft;

a second rotary shaft;

a second rotor that is coupled to and rotates integrally with the second rotary shaft, the second rotor being formed of a material the specific gravity of which is less than the specific gravity of the material of the second rotary shaft;

a second timing gear provided to the second rotary shaft, wherein the first timing gear and the second timing gear cause the second rotary shaft to rotate synchronously with the first rotary shaft, the second timing gear being located between the electric motor and the second rotor with respect to an axial direction of the second rotary shaft, wherein the second rotor has a second gear facing surface, a second opposite surface, and a second assist shaft, the second gear facing surface being an end face that faces the second timing gear with respect to the axial direction, the second opposite surface being an end face that is opposite to the second gear facing surface, and the second assist shaft projecting from the second opposite surface, wherein the second gear facing surface has a second recess, wherein the second rotary shaft is press fitted into the second recess so that the second rotor is coupled to the second rotary shaft, wherein the second assist shaft is arranged to be coaxial with the second rotary shaft, and wherein the specific gravity of the material of the second assist shaft is less than the specific gravity of the material of the second rotary shaft; and

a second bearing rotatably supporting the second assist shaft.

2. The electric pump according to claim 1,

wherein the axial dimension of the first recess is less than half the axial dimension of the first rotor, and

wherein the axial dimension of the second recess is less than half the axial dimension of the second rotor.

3. The electric pump according to claim 1,

wherein the first assist shaft is made of a material that is the same as the material of the first rotor, and

wherein the second assist shaft is made of a material that is the same as the material of the second rotor.

4. The electric pump according to claim 3,

wherein the first assist shaft is integrally molded with the first rotor, and

wherein the second assist shaft is integrally molded with the second rotor.

5. The electric pump according to claim 1,

wherein the length of a portion of the first rotary shaft that is press fitted into the first recess is set such that the value obtained by multiplying transmission torque transmitted from the first rotary shaft to the first rotor by a safety factor is equal to a fastening force between the first recess and the first rotary shaft,

wherein the length of a portion of the second rotary shaft that is press fitted into the second recess is set such that the value obtained by multiplying transmission torque transmitted from the second rotary shaft to the second rotor by the safety factor is equal to a fastening force between the second recess and the second rotary shaft.