JP2008017568A - Electric motor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the vibration resistance of a motor used in an environment subject to vibration such as a motor for ETC. <P>SOLUTION: The junction between a yoke 21 and a bracket 22 is provided with a soft structure of a caulking part 23. The caulking part 23 is composed of a caulking claw 41 which is formed at the end of the yoke 21 and a caulking hole 43 which is formed at a bracket 22 corresponding to the caulking claw 41. The diametral width dimension W of the caulking claw 43 is larger than the thickness t of the caulking claw 41, and the caulking claw 41 is coupled in the caulking hole 43 having a gap G in its diametrical direction. The yoke 21 is fixed not tightly but having minute play (the gap G) in its diametrical direction to the bracket 22, and the yoke 21 and the bracket 22 are of soft structure capable of relative shifting, and vibrational energy at the resonation of an armature is dissipated by the caulking part 23, and thus the vibration of the armature is suppressed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an improvement in vibration resistance of an electric motor, and more particularly to a technique effective when applied to a motor used in an electronically controlled throttle device of an automobile.

  In recent years, with the digitization of automobile parts, a so-called electronically controlled throttle device (hereinafter abbreviated as ETC as appropriate) that drives an engine throttle valve with an electric motor is widely used. In this case, the throttle valve is controlled by an electric signal instead of the mechanical operation by the conventional accelerator wire. The accelerator depression amount is electrically detected by a potentiometer or the like, and the motor is driven in accordance with the value to open and close the throttle valve.

A motor used as a drive source of such an ETC is disposed close to the engine because of its function, and it is difficult to avoid vibrations from the engine and the vehicle. For this reason, in a conventional ETC motor, each component (armature, bracket, yoke) constituting the motor is firmly fastened to improve vibration resistance. For example, the bracket and the yoke that support the armature are fixed by welding, and the bracket is firmly fixed to the throttle body main body by bolts. That is, in the conventional ETC motor, the rigidity as a motor assembly is increased by adopting a rigid structure that firmly integrates each component, and the vibration resistance against external vibration has been improved. Further, in order to improve vibration resistance, the plate thickness and the number of bolt fastening points are also increased.
JP2003-206762 JP 2001-132495 A International Publication WO01 / 077506

  However, when the motor has a rigid structure in this way, when the armature resonates, the stress applied to the motor increases due to the high rigidity. In other words, when the armature vibrates at the natural frequency due to vibration from the outside and resonance occurs, if each part is rigidly connected, the vibration cannot be released, and the armature itself swings around. Accordingly, stress may occur in each part of the armature. Such stress may damage the armature coil, the bearing that supports the rotating shaft, and the like, and countermeasures have been demanded. Further, when the plate thickness and the number of bolt fastening points are increased, there are problems that the motor weight and the physique increase, the number of parts increases, and the product cost increases.

  An object of the present invention is to improve vibration resistance of a motor used in an environment subject to vibration, such as an ETC motor.

  A motor according to the present invention includes a stator including a cylindrical yoke, an armature rotatably disposed in the stator, and a bracket that is attached to one end of the yoke and through which the rotation shaft of the armature is inserted. A vibration buffer portion capable of attenuating vibration of the bracket is provided at a joint portion between the yoke and the bracket.

  In the motor of the present invention, the coupling form between the yoke and the bracket becomes a flexible structure by the vibration buffer, and the vibration propagated from the armature to the bracket is absorbed by the vibration buffer. For this reason, the resonance energy at the time of armature resonance diverges in the vibration buffer, and the armature vibration is suppressed. Therefore, the stress caused to each part of the motor by the vibration of the armature is reduced, and the vibration resistance of the motor is improved.

  In the motor, the yoke and the bracket may be coupled at the vibration buffer portion in a state where the yoke is movable relative to the bracket. Further, in the vibration buffer portion, the yoke may be coupled to the bracket so that the yoke can be vibrated in the radial direction, with the one end side of the yoke being a fixed end and the other end side being a free end.

  On the other hand, another motor of the present invention includes a stator having a cylindrical yoke, an armature rotatably disposed in the stator, and a bracket that is attached to one end of the yoke and through which the rotation shaft of the armature is inserted. A projecting piece projecting on the one end side of the yoke and extending in the axial direction, and penetratingly formed in the bracket corresponding to the projecting piece. It has a projecting piece insertion hole whose width dimension is wider than the plate thickness of the projecting piece, and a coupling portion in which the projecting piece is coupled to the projecting piece insertion hole with a gap in the radial direction. And

  In the motor according to the present invention, the projecting piece provided on the yoke is inserted into the projecting piece insertion hole provided on the bracket, and the coupling portion for coupling the projecting piece to the projecting piece insertion hole with a gap in the radial direction is provided. By providing, the coupling form between the yoke and the bracket becomes a flexible structure, and the vibration propagated from the armature to the bracket is absorbed by the vibration buffer. For this reason, the resonance energy at the time of armature resonance diverges in the vibration buffer, and the armature vibration is suppressed. Therefore, the stress caused to each part of the motor by the vibration of the armature is reduced, and the vibration resistance of the motor is improved.

  In the other motor, a notch formed in a V shape is provided at the center of the tip of the projecting piece, a pressing member is pressed against the notch, and the projecting piece is opened in the circumferential direction by pushing the notch by the pressing member. By deforming the projection piece, the projection piece may be coupled to the projection piece insertion hole. Further, a groove portion extending along the axial direction from the end surface of the yoke may be provided in the base portion of the projecting piece, whereby the projecting piece is easily elastically displaced, and the yoke is fixed at one end side, The other end side is a free end, and it is easy to vibrate in the radial direction. Further, a notch portion formed by notching the end surface of the yoke may be provided at the end portion of the yoke on the bracket side, and this also makes the yoke one end side as a fixed end and the other end side as a free end. It will be in the state which is easy to vibrate radially.

  On the other hand, the motor may be used as a drive source for an electronically controlled throttle device, and a throttle valve disposed in the intake passage of the engine may be driven to open and close by the rotational output of the motor. In this case, the electronically controlled throttle device is provided with a throttle body including a bore portion in which the throttle valve is accommodated and a bottomed motor accommodating portion in which the motor is accommodated, and the motor is connected to the motor accommodating portion. You may attach in the said motor accommodating part in the state which interposed the elastic member which presses the said motor to an axial direction between the bottom face and the end surface of the said yoke. By this elastic member, vibration of the motor is appropriately absorbed and excessive displacement and vibration of the yoke are suppressed.

  According to the motor of the present invention, the stator includes a cylindrical yoke, an armature that is rotatably disposed in the stator, and a bracket that is attached to one end of the yoke and through which the rotation shaft of the armature is inserted. Since the vibration buffer part that can attenuate the vibration of the bracket is provided at the joint between the yoke and the bracket, the coupling form between the yoke and the bracket has a flexible structure, and the vibration transmitted from the armature to the bracket can be reduced. It becomes possible to absorb with this vibration buffer part. For this reason, the resonance energy at the time of armature resonance can be dissipated by the vibration buffer, and the vibration of the armature can be reduced. Therefore, the stress caused to each part of the motor due to the vibration of the armature is reduced, and the vibration resistance of the motor can be improved.

  According to another motor of the present invention, there is provided a stator having a cylindrical yoke, an armature that is rotatably arranged in the stator, and a bracket that is attached to one end of the yoke and through which the rotation shaft of the armature is inserted. In this motor, a protruding piece is formed to protrude from one end of the yoke, and a protruding piece insertion hole is formed in the bracket corresponding to this protruding piece. Since the projecting piece is inserted and coupled to the projecting piece insertion hole in the state of holding, the coupling form between the yoke and the bracket becomes a flexible structure, and the vibration transmitted from the armature to the bracket can be absorbed by this coupling portion. Become. For this reason, the resonance energy at the time of armature resonance can be spread at the coupling portion, and the vibration of the armature can be reduced. Therefore, the stress caused to each part of the motor due to the vibration of the armature is reduced, and the vibration resistance of the motor can be improved.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a sectional view showing a configuration of an electronically controlled throttle device (ETC) using an electric motor according to an embodiment of the present invention. A throttle control device 1 shown in FIG. 1 is disposed in an intake passage of an engine, and controls an intake air amount of the engine by an opening degree of a throttle valve 2. The throttle valve 2 is fixed to the throttle shaft 3 and is driven by the motor 4 via the speed reduction mechanism 11.

  The throttle shaft 3 is rotatably supported by the throttle body 5. The throttle body 5 is formed by aluminum die casting, and a bore portion 12 having a circular cross section is formed. A disc-shaped throttle valve 2 is disposed inside the bore portion 12. A throttle gear 13 is fixed to the throttle shaft 3. Further, a torsion coil spring (not shown) is attached to the throttle shaft 3, and the throttle shaft 3 is urged in a predetermined rotational direction by the torsion coil spring. The throttle valve 2 automatically returns to the fully closed position by the biasing force of the coil spring.

  An aluminum die cast or synthetic resin cover 6 is attached to the upper portion of the throttle body 5. The cover 6 houses a speed reduction mechanism 11 and a sensor unit (not shown) that detects the opening degree of the throttle valve 2. The speed reduction mechanism 11 includes a throttle gear 13, an idle gear 14, and a motor gear 15. The motor gear 15 is attached to the motor shaft 27 and meshes with the large-diameter gear 16 of the idle gear 14. The idle gear 14 is rotatably supported on an idle gear shaft 17, and a large diameter gear 16 and a small diameter gear 18 are integrally formed. The small-diameter gear 18 meshes with the throttle gear 13 fixed to the throttle shaft 3, and the rotation of the motor 4 is decelerated by the speed reduction mechanism 11 and transmitted to the throttle shaft 3.

  2 is a perspective view showing an outline of the appearance of the motor 4 according to one embodiment of the present invention, FIG. 3 is a sectional view of the motor of FIG. 2, and FIG. 4 is a front view of the motor of FIG. As the motor 4, a DC brush motor is used, and a bottomed cylindrical yoke 21 is provided. The yoke 21 is formed of a magnetic material such as iron and serves also as a motor housing. The yoke 21 is joined to the bracket 22 by a caulking portion (coupling portion) 23. Two field magnets 24 are attached to the inner peripheral surface of the yoke 21. The magnets 24 are magnetized so that the polarities on the inner surfaces facing each other are different from each other, and a stator 25 is formed by the yoke 21 and the magnets 24. An armature 26 is rotatably disposed inside the magnet 24. A motor shaft 27 is attached to the armature 26, and the motor shaft 27 is rotatably supported by bearings 28a and 28b. The bearing 28 a is attached to the bottom of the yoke 21, and the bearing 28 b is attached to the bracket 22.

  The armature 26 has an armature core 31 in which a plurality of slots 29 are formed. The armature core 31 is fixed to the motor shaft 27, and an armature coil 32 is wound between the slots 29. A commutator 33 is disposed on the left side of the armature core 31 in the figure. The commutator 33 is also fixed to the motor shaft 27, and a plurality of commutator pieces 34 are fixed to the outer periphery thereof. Each commutator piece 34 is connected to the armature coil 32, and a brush 35 is in sliding contact with the outer periphery thereof. The brush 35 is accommodated in a brush holder 36 fixed to the bracket 22, and is urged toward the commutator piece 34 by a spring 37. A pigtail 38 is attached to the brush 35 and is connected to a connector 39 through a conductive portion (not shown).

  Such a motor 4 is accommodated in a motor accommodating portion 7 formed in the throttle body 5. The bracket 22 of the motor 4 is fixed to the throttle body 5 by bolts 19, and the motor 4 is firmly fixed to the throttle body 5. A wave washer 20 is mounted between the bottom 7 a of the motor housing 7 and the end surface 21 a of the yoke 21. The motor 4 is housed and fixed in the motor housing portion 7 while pressing the wave washer 20, and the vibration of the motor 4 can be absorbed by the urging force of the wave washer 20.

  Here, in the conventional motor, as described above, the yoke and the bracket are rigidly connected by welding in consideration of vibration resistance. On the other hand, in the motor 4 according to the present invention, the joint portion 10 between the yoke 21 and the bracket 22 is joined by the crimping portion 23. 5A and 5B are explanatory views showing the configuration of the crimping portion 23, where FIG. 5A is a plan view and FIG. 5B is a cross-sectional view. On the opening side (one end side) end surface 21b of the yoke 21 (outer diameter 32 mm, plate thickness t = 1.4 mm), four crimping claws (projection pieces) 41 as shown in FIG. (See FIG. 4). A V-shaped notch 42 is formed at the center of the tip of the crimping claw 41. On the other hand, the bracket 22 (plate thickness 1.6 mm) is provided with a crimping hole 43 (projection piece insertion hole: radial width dimension W = 1.6 mm, circumferential length 6.15 mm) corresponding to the crimping claw 41. The caulking claw 41 is inserted into the caulking hole 43 and fixed by caulking. “Fixed” in the caulking portion 23 means not a form that does not move completely in a fixed position or state, but a combined form that allows a minute displacement while maintaining a certain position or state.

  FIG. 6 is an explanatory view showing a caulking process of the caulking claw 41. The caulking claw 41 is fixed to the caulking hole 43 by “open buckling caulking” as shown in FIGS. Here, first, the caulking claw 41 is inserted into the caulking hole 43 as shown in FIG. Here, the caulking claw 41 is formed to have a width of 6 mm, a height of 3.6 mm, and a plate thickness of 1.4 mm, and U-shaped grooves (groove portions) 44 having a depth of 2 mm and a width of 3 mm are formed on both sides of the base portion. ing. Further, the notch 42 of the crimping claw 41 is formed with a depth of 1.2 mm and an opening angle of 75 °. As shown in FIG. 6B, the caulking claw 41 is inserted into the caulking hole 43 until the end surface 21 b of the yoke 21 contacts the back surface 22 a of the bracket 22. Thereafter, as shown in FIG. 6C, the crimping claw 41 is pressed by a V-shaped punch (pressing member) 45 and fixed in the crimping hole 43.

  The front end portion of the punch 45 is formed in a triangle according to the notch 42, and the front end angle is 80 ° which is slightly larger than the opening angle of the notch 42. Therefore, when the punch 45 is inserted into the notch 42, a contact allowance of about 0.6 mm is generated as shown in FIG. 6 (d). In this state, the punch 45 is pressed from above so as to open the upper end portion 41a of the crimping claw 41. As a result, the crimping claw upper end portion 41 a is shaped so as to buckle while opening in the left-right direction in the figure, and is fixed in a stretched manner in the circumferential direction of the crimping hole 43. Further, on the surface 22 b of the bracket 22, the side portion of the crimping claw 41 protrudes to form a bulging portion 46, whereby the crimping claw 41 is fixed in a state where it is prevented from coming off in the crimping hole 43.

  On the other hand, in the caulking portion 23 fixed by “open buckling caulking”, the radial width of the caulking hole 43 is 1.6 mm, which is slightly larger than the plate thickness of the caulking claw 41 of 1.4 mm. . Therefore, the caulking claw 41 is fixed to the caulking hole 43 with a gap G (= W−t) of about 0.2 mm in the radial direction (in FIG. 5, its width is set so that the gap G can be easily grasped). Exaggerated). That is, the yoke 21 is not completely rigidly connected to the bracket 22 but is fixed with a slight play in the radial direction, and the yoke 21 and the bracket 22 have a flexible structure that can move relatively. ing. Therefore, in the motor 4, even if the armature 26 resonates due to external vibration, the yoke 21 is appropriately displaced relative to the bracket 22 and the vibration is absorbed.

  That is, in the motor 4, the caulking portion 23 acts as a vibration buffer portion due to the minute displacement of the caulking claw 41, and the vibration propagated from the armature 26 to the bracket 22 is attenuated by the caulking portion 23 having a flexible structure. For this reason, the vibration energy generated in the armature 26 is diffused by the crimping portion 23, and the vibration of the armature 26 is suppressed accordingly. In the motor 4, preload is applied to the yoke 21 by a wave washer (elastic member) 20 so that excessive displacement does not occur in the yoke 21.

  In the motor 4, a U-shaped groove 44 is formed in the base portion of the crimping claw 41. For this reason, even when the caulking claw 41 is caulked and fixed in the caulking hole 43, the base 41b of the caulking claw 41 can be elastically displaced in the radial direction. That is, the caulking portion 23 itself has an elastic structure, and at the time of armature resonance, the yoke 21 can elastically swing in the radial direction with the caulking portion 23 side as a fixed end and the end surface 21a side as a free end. Therefore, this also absorbs the resonance vibration of the armature 26 and suppresses the vibration of the armature 26. Also in this respect, the crimping portion 23 functions as a vibration buffering portion. In this case, the caulking portion is reduced by reducing the width and thickness of the caulking claw 41 within a range where there is no problem in terms of stress (strength), or by increasing the height of the claw or the depth of the U-shaped groove 44. The elasticity of 23 can be increased, and vibration can be absorbed more effectively.

Next, regarding the motor having such a configuration, a vibration experiment performed by the inventors will be described. FIG. 7 is a graph showing the results of vibration experiments by the inventors, and shows the vibration state of the armature during motor excitation. In FIG. 7, a diagram A is an experimental result of a motor in which a yoke and a bracket are coupled by a caulking portion 23 having a flexible structure as shown in FIG. 5, and a diagram B is an experimental result of a motor in which a yoke and a bracket are welded and fixed. . The excitation conditions are: acceleration: 225.4 m / s ² (23G), excitation direction: direction perpendicular to the motor shaft (X direction in FIG. 3), vibration sweep direction, time: low frequency → high frequency, 15 minutes. As can be seen from FIG. 7, in the motor having a flexible structure, the acceleration peak value of the armature is greatly reduced (about 15% reduction). On the other hand, in the motor fixed by welding, the vibration energy of the armature is not dissipated, and the vibration behavior of the armature becomes resonance itself. That is, it has been found that when the yoke-bracket structure is a flexible structure as shown in FIG. 5, vibration energy at resonance is dissipated due to displacement and vibration of the yoke.

  FIG. 8 is an explanatory diagram showing the propagation of vibration and the state of vibration energy by comparing the motor with a flexible structure and a rigid structure. As shown in FIG. 8, when the armature 26 resonates due to external vibration, vibration energy generated there is transmitted to the yoke 21 via the bracket 22. Assuming that the generated resonance energy is stored in the vibration system at this time, as shown in FIG. 8, when the flexible structure is adopted, the energy of the armature 26 becomes relatively low. That is, in the conventional rigid structure motor, since the yoke 21 is rigidly connected, not much energy is consumed there (low energy). For this reason, vibration energy due to the resonance of the armature 26 is exclusively applied to the armature 26 (high energy). In contrast, in the flexible structure as shown in FIG. 5, vibration energy is consumed by the yoke 21 (in the energy). Therefore, the energy applied to the armature 26 is reduced by that amount (in the energy), the vibration of the armature 26 at the time of resonance is suppressed, and the effect has been confirmed by the above-described experiments by the inventors.

  As described above, in the motor 4 according to the present invention, the yoke 21 and the bracket 22 are coupled by the caulking portion 23 having a flexible structure as shown in FIG. 5, so that the resonance energy generated in the armature 26 can be diffused there. The vibration at the time of resonance of the armature 26 can be suppressed. That is, the resonance energy is positively released to other parts, so that the vibration of the armature 26 is suppressed, and the stress on the armature coil 32, the bearings 28a and 28b, and the pigtail 38 is reduced. Thereby, the vibration resistance of the motor can be improved, and the function as a motor can be maintained even in an environment where the vibration conditions are severe. Further, since it is not necessary to increase the thickness of the yoke 21 and the bracket 22 more than necessary in terms of strength, the motor can be reduced in size and weight. Furthermore, since material costs are also reduced, product costs can be reduced.

It goes without saying that the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention.
For example, the various dimensions and materials of each member in the above-described embodiments are merely examples, and the present invention is not limited to the above-described numerical values and materials. In the above-described embodiment, a motor with a brush is shown as the motor 4, but the present invention can also be applied to a brushless motor.

  On the other hand, in the above-described embodiment, the yoke end surface 21b is appropriately cut out within a range where there is no problem in strength, thereby forming a cutout portion, reducing the contact area between the yoke 21 and the bracket 22, and making the yoke 21 more likely to swing. Is also possible. Further, by providing a hole in the end portion of the yoke 21 on the bracket 22 side, a buffer portion may be provided at the end portion of the yoke 21 so that the yoke 21 can be easily shaken. Further, the preload of the wave washer 20 can be adjusted by appropriately changing the specifications of the wave washer 20, whereby the armature 26 is optimized while optimizing the stress of the yoke 21 and the caulking portion 23 according to the specifications of the motor and ETC. It is possible to adjust the vibration energy generated in In addition to the wave washer, rubber, synthetic resin, a compression coil spring, or the like can be used as the elastic member for applying preload to the motor 4.

It is sectional drawing which shows the structure of the electronically controlled throttle apparatus using the electric motor which is one Example of this invention. 1 is a perspective view showing an outline of the appearance of a motor that is an embodiment of the present invention. It is sectional drawing of the motor of FIG. It is a front view of the motor of FIG. It is explanatory drawing which shows the structure of a crimping | crimped part, (a) is the top view, (b) is sectional drawing. It is explanatory drawing which shows the crimping process of a crimping nail | claw. It is a graph which shows the result of the vibration experiment by inventors, and has shown the vibration state of the armature at the time of motor excitation. It is explanatory drawing which compared the propagation of vibration and the state of vibration energy with the motor of the flexible structure and the rigid structure.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Throttle control device 2 Throttle valve 3 Throttle shaft 4 Motor 5 Throttle body 6 Cover 7 Motor accommodating part 7a Bottom part 10 Joint part 11 Reduction mechanism 12 Bore part 13 Throttle gear 14 Idle gear 15 Motor gear 16 Large diameter gear 17 Idle gear shaft 18 Small diameter Gear 19 Bolt 20 Wave washer 21 Yoke 21a, 21b End face 22 Bracket 22a Back face 22b Front face 23 Caulking portion 24 Magnet 25 Stator 26 Armature 27 Motor shaft 28a, 28b Bearing 29 Slot 31 Armature core 32 Armature coil 33 Commutator 34 Commutator piece 35 Brush 36 Brush holder 37 Spring 38 Pigtail 39 Connector 41 Caulking claw 41a Upper end 41b Base 42 Notch 43 Caulking hole 44 U Gutter 45 Punch 46 Swelling part G Crimping part gap W Caulking hole radial width dimension t Caulking plate thickness

Claims (9)

A motor comprising a stator including a cylindrical yoke, an armature rotatably disposed in the stator, and a bracket that is attached to one end of the yoke and through which the rotation shaft of the armature is inserted. ,
A motor comprising a vibration buffering portion capable of attenuating vibration of the bracket at a joint between the yoke and the bracket.   The motor according to claim 1, wherein the yoke is coupled to the bracket in a state in which the yoke is movable relative to the bracket at the vibration buffer portion.   3. The motor according to claim 1, wherein the yoke is coupled to the bracket in the vibration damping portion so that the one end side is a fixed end and the other end side is a free end and is capable of vibrating in a radial direction. A motor characterized by that. A motor comprising a stator including a cylindrical yoke, an armature rotatably disposed in the stator, and a bracket that is attached to one end of the yoke and through which the rotation shaft of the armature is inserted. ,
A protruding piece formed on the one end side of the yoke and extending in the axial direction;
A projecting piece insertion hole formed through the bracket corresponding to the projecting piece, and having a radial width dimension wider than the plate thickness of the projecting piece,
The motor having a coupling portion coupled with the projecting piece insertion hole with a gap in a radial direction with respect to the projecting piece insertion hole.   The motor according to claim 4, wherein a notch formed in a V shape is provided at the center of the tip of the projecting piece, a pressing member is pressed against the notch, and the projecting piece is pushed and opened by the pressing member. A motor characterized in that the projecting piece is coupled to the projecting piece insertion hole by being deformed in a circumferential direction.   6. The motor according to claim 4, wherein a groove portion extending in an axial direction from an end surface of the yoke is provided at a base portion of the protruding piece.   The motor according to any one of claims 4 to 6, wherein a notch formed by notching an end surface of the yoke is provided at an end of the yoke on the bracket side.   The motor according to any one of claims 1 to 7, wherein the motor is used as a drive source of an electronically controlled throttle device, and a throttle valve disposed in an intake passage of the engine is opened and closed by a rotation output of the motor. A motor characterized in that   9. The motor according to claim 8, wherein the electronically controlled throttle device has a throttle body including a bore portion in which the throttle valve is accommodated and a bottomed motor accommodating portion in which the motor is accommodated. The motor is mounted in the motor housing portion with an elastic member interposed between the bottom surface of the motor housing portion and the end surface of the yoke in an axial direction.

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