JP2010057294A - Electric motor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the operation reliability of an electric motor by preventing foreign matters from entering through slits formed on a yoke for forming a flange. <P>SOLUTION: A pair of slits 82 formed so as to extend from an opening end toward a bottom is formed on a yoke 24, a pair of flanges 81 are formed at a position retracted to the side of the bottom from the opening end of the yoke 24 by bending a portion between the slits 82, and a gear case 36 is fixed to the opening end of the yoke 24 by a bolt 34 inserted through a through-hole 83 of the flange 81. A pair of walls 91 sandwiching the flanges 81 are provided on the gear case 36, the slits 82 are covered by the walls 91 to prevent foreign matters from entering the inside of the yoke 24 and the gear case 36 through the slits 82. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electric motor in which a flange portion is formed on a bottomed cylindrical yoke that rotatably accommodates an amateur, and an end case is fixed to the open end of the yoke by the flange portion.

  2. Description of the Related Art Conventionally, an electric motor is often used as a drive source for vehicle electrical components such as a sunroof motor.

  A yoke used for such an electric motor is usually formed in a bottomed cylindrical shape by deep drawing using a press device, and a flange portion protruding radially outward from the opening end. Are integrally formed. An end case such as a gear case for accommodating a speed reduction mechanism is attached to the open end of the yoke, and this end case is fixed to the yoke by a bolt inserted into an insertion hole formed in the flange portion. .

  As described above, in the structure in which the flange portion is provided at the opening end of the yoke and the end case is fixed to the opening end of the yoke by the flange portion, the end case is disposed so that the opening portion abuts the opening end of the yoke. As a result, there is a problem that the entire length of the electric motor becomes long.

Thus, for example, in the electric motor shown in Patent Document 1, a pair of slits extending from the opening end toward the bottom is formed in the yoke, and a portion between these slits is bent outward in the radial direction so as to be more than the opening end. A flange portion is formed at a position retracted to the bottom side. As a result, it is possible to fix the end case to the yoke by the flange portion while accommodating a part of the opening end side of the yoke in the engaging recess provided in the end case, and shorten the overall length of the electric motor. Yes.
JP 2008-86110 A

  However, in the structure shown in Patent Document 1, a pair of slits for forming the flange portion is provided in the yoke, and when the flange portion is bent, the end portion of the slit remains on the yoke wall surface due to the processing. Therefore, the slit is exposed to the outside in a state after the end case is fixed to the yoke, and there is a possibility that foreign matters such as dust may enter the yoke and the end case from the slit.

  An object of the present invention is to improve the operational reliability of an electric motor by preventing foreign matter from entering through a slit provided in a yoke to form a flange portion.

  An electric motor of the present invention is formed in a bottomed cylindrical shape, and a portion between a yoke that rotatably accommodates an armature and a pair of slits that are formed to extend from the opening end of the yoke toward the bottom is radially A flange portion formed by bending outward; an end case attached to the opening end of the yoke and fixed to the yoke by a fastening member; and a wall portion provided on the end case and covering the slit. It is characterized by.

  The electric motor of the present invention is characterized in that the wall portion is formed to project in the axial direction of the armature along the outer peripheral surface of the yoke from both side portions sandwiching the flange portion of the end case.

  The electric motor according to the present invention is characterized in that the wall portion is formed along the outer peripheral surface of the yoke on one surface and is formed thinner toward the tip side.

  According to the present invention, the wall portion is provided on the end case fixed to the yoke, and the slit provided in the yoke is covered with the wall portion in order to form the flange portion. Intrusion of foreign matter into the wall can be prevented by the wall portion, and the operation reliability of the electric motor can be improved.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is an explanatory view showing an outline of a sunroof device provided on a roof of a vehicle. The sunroof device 11 includes a roof panel 12, and an opening formed in a roof 13 a of the vehicle 13 by the roof panel 12. 14 is opened and closed. A pair of shoes 15a and 15b are fixed to both sides of the roof panel 12, while guide rails 16 extending in the vehicle front-rear direction are fixed to both sides of the opening 14 of the roof 13a. , 15b are guided by the corresponding guide rails 16, so that the roof panel 12 can be moved in the vehicle front-rear direction, that is, can be opened and closed.

  One end of geared drive cables 17a, 17b is connected to each shoe 15b on the vehicle rear side, and the other ends of these drive cables 17a, 17b are respectively routed to the vehicle front side of the opening 14. Yes. A sunroof motor 21 as an electric motor is disposed on the front side of the vehicle with respect to the opening 14 and inside the roof 13a between the windshield 13b and the drive cables 17a and 17b are connected to the sunroof motor 21. Is engaged with a drive gear 22 provided in When the sunroof motor 21 is operated, the drive cables 17a and 17b are driven in the axial direction in opposite directions by the sunroof motor 21, whereby the roof panel 12 is automatically opened and closed by being pushed and pulled by the drive cables 17a and 17b. It is like that.

  2A and 2B are perspective views showing details of the sunroof motor shown in FIG. 1, FIG. 3 is an exploded perspective view of the sunroof motor shown in FIG. 2, and FIG. 4 is a fixed portion between the yoke and the gear case. FIG.

  As shown in FIGS. 2A and 2B, the sunroof motor 21 includes a motor body 23 that is a motor with a brush. The yoke 24 of the motor body 23 includes a pair of arc portions 24a that are curved around an axis, a pair of parallel flat plate portions 24b that connect the arc portions 24a, and a bottom portion 24c that closes one end in the axial direction. As shown in FIG. 3, an armature 25 is accommodated inside the yoke 24. The armature 25 includes an armature shaft 26, and one end of the armature shaft 26 is supported by a bearing (not shown) provided on the bottom 24 c of the yoke 24, so that the armature 25 is rotatable inside the yoke 24. ing. An amateur core 27 is fixed to the amateur shaft 26, and a plurality of amateur coils 28 are wound around the amateur core 27. Further, a commutator 29 is fixed to the armature shaft 26 adjacent to the armature core 27, and the coil ends of each armature coil 28 are connected to the commutator 29, respectively.

  A brush device 31 is attached to the open end of the yoke 24 to supply power to the armature 25, that is, the armature coil 28. As shown in FIG. 4, the brush device 31 has a structure in which a pair of brushes 33 a and 33 b are mounted on a brush holder 32. The brushes 33 a and 33 b are in sliding contact with the outer peripheral surface of the commutator 29, and the brush The drive current commutated at a predetermined timing via the 33a, 33b and the commutator 29 is supplied to the amateur coil 28.

  A gear case 36 as an end case is fixed to the open end of the yoke 24 by a pair of bolts 34 and nuts 35 as fastening members. The gear case 36 is formed in a bathtub shape from a resin material, and the armature shaft 26 of the motor body 23 protrudes from the yoke 24 to the inside of the gear case 36 and is accommodated in the gear case 36. The distal end portion and the intermediate portion of the armature shaft 26 are supported by a bearing (not shown) provided inside the gear case 36. A cover body 37 is attached to the gear case 36 by five claw engagements, and the open end of the gear case 36 is closed by the cover body 37.

  The details of the fixing structure between the yoke 24 and the gear case 36 will be described later.

  FIG. 5 is a cross-sectional view taken along the line AA in FIG. 2B. As shown in FIG. 5, a worm gear mechanism 41 is accommodated in the gear case 36 closed by the cover body 37. The worm gear mechanism 41 includes a worm 41a and a worm wheel 41b. The worm 41a is integrally formed on an outer peripheral surface of a portion of the armature shaft 26 protruding into the gear case 36, and the worm wheel 41b is fixed to the output shaft 42. The output shaft 42 is rotatably accommodated in a cylindrical gear accommodating portion 36a provided inside the gear case 36. The worm 41a and the worm wheel 41b are meshed with each other, whereby the rotation of the armature shaft 26 is decelerated to a predetermined rotational speed via the worm gear mechanism 41 and output from the output shaft 42.

  In the illustrated case, the worm 41a is integrally formed on the outer peripheral surface of the armature shaft 26. However, the present invention is not limited thereto, and the armature shaft 26 is connected to a shaft provided with the worm 41a. Other structures may be used as long as the worm 41a is rotationally driven by the armature shaft 26.

  A gear cover 43 is attached to the open end of the gear housing portion 36a. The gear cover 43 isolates the gear housing portion 36a in a gear case 36 from a control board housing portion 36b described later. Thereby, lubricating oil such as grease applied to the worm gear mechanism 41 is prevented from leaking to a portion other than the gear housing portion 36a of the gear case 36.

  As shown in FIG. 5, the output shaft 42 protrudes from the bottom surface of the gear case 36 to the outside of the case 36 (upward in the figure), and the drive gear 22 is fixed to the tip thereof. Further, a cable guide 36c is provided on the outside of the bottom surface of the gear case 36 (upward in the figure), and a guide plate 44 is mounted on the cable guide 36c. A pair of guide walls 44 a facing each other are provided, and the drive cables 17 a and 17 b are arranged so as to be sandwiched between the guide walls 44 a and the drive gear 22.

  With such a configuration, when the motor main body 23 is operated, the rotation of the armature shaft 26 is transmitted to the output shaft 42 via the worm gear mechanism 41, and the drive cables 17a and 17b are predetermined by the drive gear 22 rotating together with the output shaft 42. The roof panel 12 automatically opens and closes.

  As shown in FIG. 5, a tool hole 42a that can be engaged with a tool such as a hexagon wrench is formed at the other end (downward in the figure) of the output shaft 42, and the motor main body 23 has failed. In this case, the tool is engaged with the tool hole 42a through the through hole 37a formed in the cover body 37, the output shaft 42 is rotationally driven by the tool, and the roof panel 12 is manually opened and closed. Can be done.

  The sunroof motor 21 is provided with a control board 51 for controlling the operation of the motor body 23. The control board 51 has a structure in which a control circuit 51b for controlling the drive current supplied to the amateur 25 (the amateur coil 28) is provided on the board 51a, and three clips provided integrally with the cover body 37. 52 is fixed to the inner surface of the cover body 37. The clip 52 is a well-known fixing means including a cylindrical main body 52a, a clip piece 52b provided on the upper part thereof, and a cross-shaped slit 52c extending from the clip piece 52b to the main body 52a. Then, by attaching the cover body 37 to the gear case 36, the control board 51 is accommodated in a control board accommodation portion 36 b that is integrally provided inside the gear case 36.

  As shown in FIGS. 3 and 4, the brush device 31 is provided with a pair of power terminals 53 and 54, and the control board 51 is provided with a pair of connection pieces 55 and 56, and the control board 51 is fixed. When the cover body 37 is attached to the gear case 36, the connection pieces 55 and 56 are connected to the corresponding power terminals 53 and 54, and the control board 51 (control circuit 51b) and the brushes 33a and 33b are electrically connected. It has come to be.

  The gear case 36 is provided with a ground terminal (ground terminal) 57 whose one end is electrically connected to the yoke 24 via the bolt 34 and the nut 35, and the control board 51 is provided with a ground connection piece 58. When the cover body 37 to which the control board 51 is fixed is attached to the gear case 36, the connecting piece 58 is connected to the ground terminal 57, and the control circuit 51b is grounded (grounded) to the yoke 24. ing.

  A connector 59 for external connection is provided at one end of the control board 51. As shown in FIG. 2, the connector 59 protrudes from the gear case 36, and the control circuit 51b is connected to a power source (not shown) such as a battery mounted on the vehicle 13 via the connector 59 or a sunroof switch (not shown) provided in the vehicle interior. (Not shown). When the sunroof switch is operated, a drive current is supplied from the power source to the control circuit 51b via the connector 59, and this drive current is controlled to a predetermined current by the control circuit 51b. The power is supplied to the armature 25 (the amateur coil 28) through the power terminals 53 and 54, the brushes 33a and 33b, and the commutator 29.

  Reference numeral 61 denotes a positioning portion provided in the cover body 37. The positioning portion 61 is inserted into a positioning hole 62 formed in the control board 51 and engages with a positioning protrusion 63 provided in the gear case 36. Thus, the control board 51 can be positioned at a predetermined position of the gear case 36.

  In order to detect the rotation of the amateur shaft 26, the sunroof motor 21 is provided with a rotation sensor. The rotation sensor includes a sensor magnet 72 fixed to the armature shaft 26. The sensor magnet 72 is a multipolar magnetized magnet in which a plurality of magnetic poles are magnetized in the circumferential direction. It is designed to rotate with it.

  On the other hand, as shown in FIG. 3, a pair of Hall ICs 73 a and 73 b are arranged on the control board 51 so as to face the sensor magnet 72 so as to form an angle of 90 degrees with respect to the axis of the armature shaft 26. The change in the magnetic field output from the sensor magnet 72 is detected with a phase difference of 45 degrees. Each Hall IC 73a, 73b detects a change in the magnetic field generated by the sensor magnet 72 and outputs it as a pulse signal having a period inversely proportional to the rotational speed of the armature shaft 26. Detection signals, that is, pulse signals of the Hall ICs 73a and 73b are input to the control circuit 51b, and the control circuit 51b receives the period of the pulse signals input from the Hall ICs 73a and 73b and the pulse signals input from the Hall ICs 73a and 73b. Control of the drive current supplied to the amateur coil 28 is executed based on the appearance timing and the like.

  Next, a fixing structure between the yoke 24 and the gear case 36 will be described.

  6 is a front view of the yoke shown in FIG. 3 as viewed from the opening end side, FIG. 7 (a) is a side view showing a state before bending of a flange portion provided on the yoke, and FIG. 7 (b) is a yoke. The side view which shows the state after bending of the flange part provided in FIG. 7, FIG.7 (c) is sectional drawing which follows the AA line in the same figure (b). 8A is a perspective view showing a state before the gear case is fixed to the yoke, FIG. 8B is a perspective view showing a state where the gear case is fixed to the yoke, and FIG. 9 is a view showing the flange portion and the drive cable. It is explanatory drawing which shows the positional relationship with these.

  In order to fix the gear case 36 to the yoke 24, the yoke 24 is provided with a pair of flange portions 81. As shown in FIG. 6, these flange portions 81 are point-symmetrical with respect to the center of the axis of the yoke 24 at positions offset toward the flat plate portion 24 b with respect to the center position in the circumferential direction of the arc portion 24 a. It is provided to become. Since these flange portions 81 have the same shape, only one flange portion 81 will be described below.

  The flange portion 81 is formed to protrude radially outward at a position retracted toward the bottom portion 24c with respect to the opening end of the yoke 24 by cutting and raising the arc portion 24a of the yoke 24 from the opening end side. That is, as shown in FIG. 7A, the arc portion 24a of the yoke 24 has a pair of slits 82 extending from the opening end of the yoke 24 toward the bottom portion 24c in parallel with the axial direction of the yoke 24. They are formed in parallel at a predetermined interval. Then, as shown in FIGS. 7B and 7C, by bending the portion of the arc portion 24a between the slits 82 toward the radially outer side in the axial direction of the yoke 24, A flange portion 81 is formed at a position retracted toward the bottom portion 24 c with respect to the opening end of the yoke 24. Further, the flange portion 81 is provided with a through hole 83 through which the bolt 34 as a fastening member is inserted.

  On the other hand, as shown in FIG. 8A, the gear case 36 is provided with a seating surface 84 serving as a mounting position of the open end of the yoke 24, and a fastening surface serving as a seating surface of the flange portion 81 outside the seating surface 84. A portion 85 is provided. Further, a plurality of positioning convex portions 84 a are erected on the seat surface 84 in the axial direction, and the positioning convex portions 84 a are positioned on the inner surface side of the yoke 24. As shown in FIG. 4, the gear case 36 is provided with a pair of fastening portions 85 corresponding to the pair of flange portions 81, but the fixing structure of the flange portion 81 with respect to these fastening portions 85 is basically the same. Therefore, only one fastening portion 85 will be described below.

  The fastening portion 85 is formed in a block shape that protrudes radially outward from the seat surface 84, and an insertion hole 86 through which the bolt 34 is inserted is formed in the axial end surface 85a. The fastening portion 85 is formed with a nut mounting hole 87 that communicates with the insertion hole 86, and the nut 35 described above is mounted in the nut mounting hole 87. Then, as shown in FIG. 8B, the through hole 83 with the open end of the yoke 24 butted against the seating surface 84 of the gear case 36 and the flange portion 81 butted against the axial end surface 85 a of the fastening portion 85. By inserting the bolt 34 through the insertion hole 86 and screwing the bolt 34 to the nut 35, the gear case 36 is fixed to the yoke 24 by the bolt 34 and the nut 35 at the flange portion 81.

  Thus, in the sunroof motor 21, the flange portion 81 of the yoke 24 is provided at a position retracted toward the bottom portion 24 c with respect to the opening end, so that a part of the opening end side of the yoke 24 is part of the gear case 36. The gear case 36 can be fixed to the yoke 24 by the flange portion 81 while being accommodated in the seat surface 84 provided on the seat surface 84, and the overall length of the sunroof motor 21 can be shortened.

  As shown in FIG. 6, a cutout portion 81 a that is cut out linearly is provided at the tip portion of the flange portion 81 on the flat plate portion 24 b side. 9, the gear case 36 is fixed to the yoke 24 and the drive cable 17a is attached to the sunroof motor 21, so that the flange portion 81 protrudes to the outer surface side of the gear case 36 and is driven. Interference with the cable 17a can be prevented. In addition, since the cutout portions 81a having the same shape are provided at the tip portions of both flange portions 81, no matter which flange portion 81 is on the drive cable 17a side and the gear case 36 is fixed to the yoke 24, Interference with the drive cable 17a of the said flange part 81 can be prevented. Therefore, the assembling work with the gear case 36 can be facilitated by eliminating the directionality in assembling the yoke 24.

  The fastening portion 85 of the gear case 36 is provided with a pair of wall portions 91 in order to prevent foreign matters such as dust from entering the inside of the yoke 24 and the gear case 36 from the slits 82 formed in the yoke 24. Yes. As shown in FIG. 4, the wall portions 91 are provided in the respective fastening portions 85, but since these wall portions 91 have the same structure, the wall portions 91 provided in one fastening portion 85 will be described below. Only explained.

  As shown in FIG. 8A, these wall portions 91 are formed integrally with the fastening portion 85 at the inner peripheral edge portion of the axial end surface 85a of the fastening portion 85, and are centered on the axis of the yoke 24. Are spaced apart from each other by the width of the flange portion 81 and project toward the bottom 24c side of the yoke 24 in parallel with the axial direction of the armature shaft 26, that is, the yoke 24, from the end face 85a. ing. As shown in FIG. 8B, when the gear case 36 is fixed to the yoke 24, the wall portions 91 sandwich the flange portion 81 from the circumferential direction centering on the axis of the yoke 24. It is arranged on both side parts. Further, the front end portion of each wall portion 91 is formed to have a length reaching the bottom portion 24c side of the yoke 24 from the flange portion 81, that is, a length reaching the bottom portion 24c side from the end portion of the slit 82.

  FIG. 10 is a cross-sectional view of the wall portions shown in FIG. 8B, and one surface of the wall portions 91 facing the yoke 24 side, that is, the inner surface is formed in a flat shape, and the gear case 36 is fixed to the yoke 24. Then, it arrange | positions along the outer peripheral surface of the circular arc part 24a of the yoke 24. FIG. Further, the outer surface of the wall portion 91 facing the side opposite to the yoke 24 is formed in an inclined surface shape that gradually inclines in a direction approaching the yoke 24 from the proximal end side toward the distal end side, that is, each wall portion 91. Is formed in a substantially wedge-shaped cross-section that is thinner at the tip side.

  When the gear case 36 having such a wall portion 91 is fixed to the yoke 24, as shown in FIG. 8 (b), each slit 82 formed in the yoke 24 to provide the flange portion 81 has a corresponding wall. Covered from the outside by the portion 91. That is, each slit 82 formed in the yoke 24 for providing the flange portion 81 is closed by the corresponding wall portion 91 when the gear case 36 is fixed to the yoke 24. Thereby, the intrusion of foreign matter such as dust from the slit 82 into the yoke 24 and the gear case 36 is prevented by the wall portion 91, and the operation reliability of the sunroof motor 21 is improved. Further, since the outer surface of the wall portion 91 facing the side opposite to the yoke 24 is formed in an inclined surface shape that is gradually inclined toward the yoke 24 from the proximal end side toward the distal end side, the minimum necessary resin The wall portion 91 can be made of a material, and the fluidity of the resin can be ensured when the gear case 36 is resin-molded, and the moldability of the gear case can be improved.

  As described above, in the sunroof motor 21, the pair of wall portions 91 are provided in the fastening portion 85 of the gear case 36 fixed to the yoke 24, and the slits 82 provided in the yoke 24 to form the flange portion 81 are provided on these walls. Since it is covered with the portion 91, foreign matter can be prevented from entering the yoke 24 and the gear case 36 from the slit 82, and the operation reliability of the sunroof motor 21 can be improved.

  It goes without saying that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the scope of the invention. For example, in the above-described embodiment, the present invention is applied to the sunroof motor 21 used in the sunroof device 11 of the vehicle 13. However, the present invention is not limited to this, for example, a slide door device or a power window device provided in the vehicle. You may make it apply to the electric motor used for.

  Moreover, in the said embodiment, although a pair of flange part 81 is provided in the yoke 24, it does not ask | require not only this but the number.

  Further, in the above-described embodiment, the wall portion 91 is integrally formed with the fastening portion 85 of the gear case 36. However, the present invention is not limited thereto, and the wall portion 91 formed separately from the gear case 36 is used as the fastening portion 85. It is good also as a structure attached to.

It is explanatory drawing which shows the outline of the sunroof apparatus provided in the roof of the vehicle. (A), (b) is a perspective view which shows the detail of the sunroof motor shown in FIG. 1, respectively. FIG. 3 is an exploded perspective view of the sunroof motor shown in FIG. 2. It is a top view which shows the fixed part of a yoke and a gear case. It is sectional drawing which follows the AA line in FIG.2 (b). It is the front view which looked at the yoke shown in FIG. 3 from the opening end side. (A) is a side view showing a state before bending of the flange portion provided in the yoke, (b) is a side view showing a state after bending of the flange portion provided in the yoke, and (c) is in FIG. It is sectional drawing which follows the AA line. (A) is a perspective view which shows the state before fixing a gear case to a yoke, (b) is a perspective view which shows the state which fixed the gear case to the yoke. It is explanatory drawing which shows the positional relationship of a flange part and a drive cable. It is sectional drawing of the wall part shown in FIG.8 (b).

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Sunroof apparatus 12 Roof panel 13 Vehicle 13a Roof 13b Windshield 14 Opening part 15a, 15b Shoe 16 Guide rail 17a, 17b Drive cable 21 Sunroof motor (electric motor)
DESCRIPTION OF SYMBOLS 22 Drive gear 23 Motor main body 24 Yoke 24a Arc part 24b Flat plate part 24c Bottom part 25 Amateur 26 Amateur shaft 27 Amateur core 28 Amateur coil 29 Commutator 31 Brush device 32 Brush holder 33a, 33b Brush 34 Bolt 35 Nut 36 Gear case (end case)
36a Gear housing portion 36b Control board housing portion 36c Cable guide 37 Cover body 37a Through hole 41 Worm gear mechanism 41a Worm 41b Worm wheel 42 Output shaft 42a Tool hole 43 Gear cover 44 Guide plate 44a Guide wall 51 Control board 51a Board 51b Control circuit 52 Clip 52a Main body 52b Clip piece 52c Slit 53, 54 Power supply terminal 55, 56 Connection piece 57 Ground terminal 58 Connection piece 59 Connector 61 Positioning part 62 Positioning hole 63 Positioning protrusion 72 Sensor magnet 73a, 73b Hall IC
81 Flange portion 81a Notch portion 82 Slit 83 Through hole 84 Seat surface 84a Positioning convex portion 85 Fastening portion 85a Axial end surface 86 Insertion hole 87 Nut mounting hole 91 Wall portion

Claims (3)

A yoke that is formed in a bottomed cylindrical shape and rotatably accommodates an amateur;
A flange portion formed by bending a portion between a pair of slits formed extending from the open end of the yoke toward the bottom portion radially outward;
An end case attached to the open end of the yoke and fixed to the yoke by a fastening member;
An electric motor comprising a wall portion provided on the end case and covering the slit.   2. The electric motor according to claim 1, wherein the wall portion is formed to protrude in an axial direction of the armature along an outer peripheral surface of the yoke from both side portions of the end case sandwiching the flange portion. Electric motor.   3. The electric motor according to claim 1, wherein the wall portion is formed so as to be along the outer peripheral surface of the yoke on one side and to be thinner toward the tip side. 4.

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