Patent Publication Number: US-6700291-B2

Title: Brush holding device, motor having the same, and method for assembling motor

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-394464 filed on Dec. 26, 2001. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for assembling a motor having an armature, which is rotated through engagement of brushes with a commutator of the armature, and also relates to a brush holding device and a motor having the same. 
     2. Description of Related Art 
     In general, a direct current motor includes brushes and a commutator to mechanically commutate externally supplied direct current and to supply it to an armature. Each brush is supported by a corresponding brush holding device. The brush is assembled such that the brush slidably engages the commutator secured to the armature. During manufacturing, each brush holding device is assembled to a housing of the motor before installation of the armature. In the brush holding device, the brush is temporarily held in a retracted position in a brush holder against urging force of a spring before installation of the armature in a housing of the motor. Then, when the armature is installed in a predetermined position in the housing of the motor, the brush is released from the retracted position, so that the temporarily held brush is urged against the commutator to slidably engage the commutator. 
     As a first example, the following brush holding device has been proposed to achieve the above assembling operation. Each of brushes, which are urged by springs, is engaged with an outer peripheral surface of a ring member and is temporarily held in a retracted position in a brush holder of a corresponding brush holding device. After each brush holding device is assembled to a housing of the motor, the armature is installed to a predetermined position in the housing of the motor. At this time, the ring member is pushed downward by an end surface of the commutator secured to the distal end side of the armature. Thus, each brush, which has been engaged with the outer peripheral surface of the ring member, is now engaged with the commutator instead of engaging with the ring member. 
     As a second example, the following brush holding device has been proposed to achieve the above assembling operation. A brush holder of each brush holding device, which holds a corresponding brush in a manner that allows protrusion and retraction of the brush relative to the brush holder, is provided with a tongue-shaped hook that extends toward a commutator side. The hook restrains movement of the brush, which is urged toward the commutator side by a spring, to temporarily hold the brush in a retracted position in the brush holder. When the armature is installed in a predetermined position in a housing of the motor, the tongue-shaped hook is bent, so that the temporarily held brush is released. Thus, the brush, which has been restrained to move toward the commutator side (i.e., restrained to protrude from the brush holder) by the hook, now protrudes from the brush holder and engages the commutator. 
     In the first example, the additional component, i.e., the ring member, is used to temporarily hold the brush at the time of assembly, resulting in an increase in a manufacturing cost. Furthermore, the motor needs to have a space for accommodating the ring member, which is used only for temporarily holding the brushes. 
     In the second example, similar to the first example, the temporarily held brush in the brush holder is released by the insertion of the armature. Thus, the freedom of the design has been limited in terms of assembling timing of the armature, a positional relationship between each brush and the commutator and the like. Furthermore, when the brushes do not simultaneously engage the commutator, the shaft of the armature can be displaced by the urging force, which radially urges the corresponding brush. Thus, during assembly of the armature, it could disturb the insertion of the shaft distal end of the armature into a corresponding shaft receiving hole. Furthermore, the armature moves continuously after a moment of releasing the temporarily held brushes. In such a case, the released brushes could form an axially extending scratch on a surface of the distal end side of the commutator of the armature. Since the commutator is engaged with the brushes in a circumferential direction during operation of the motor, the axially extending scratch can disturb smooth commutation. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above disadvantage. Thus, it is an objective of the present invention to provide a method for assembling a motor achieving engagement of each brush to a commutator without using insertion movement of an armature. It is another objective of the present invention to provided a brush holding device, which allows engagement of a brush with a commutator without using insertion movement of an armature at assembly of the armature and also allows a reduction in a manufacturing cost of the brush holding device. It is another objective of the present invention to provide a motor having such a brush holding device. 
     To achieve the objectives of the present invention, there is provided a method for assembling a motor. In the method, a brush is temporarily held in a first position where the brush is retracted away from an installation path of an armature into a brush holder and is urged against a side wall of the brush holder by an urging means in a direction perpendicular to a sliding direction of the brush, which substantially coincides with a radial direction of a commutator of the armature, so that the brush is temporarily held in the first position by a static frictional force generated between the side wall of the brush holder and the brush. This static frictional force is greater than an urging force of the urging means applied to the brush in the sliding direction. Then, the armature, which includes the commutator, is installed in a predetermined position within a housing of the motor along the installation pass of the armature while the brush is temporarily held in the first position. Next, the brush held in the first position is released by applying an external force to the brush in the sliding direction to overcome the static frictional force, so that the brush is slid to a second position, in which the brush is urged against the commutator in the sliding direction, by an urging force of the urging means that is greater than a frictional force between the side wall of the brush holder and the brush. 
     To achieve the objectives of the present invention, there is also provided a brush holding device for a motor that includes an armature having a commutator. The brush holding device includes a brush, a brush holder and an urging means for urging the brush and also for selectively positioning the brush between a first position and a second position. The brush holder includes a side wall. Furthermore, the brush holder slidably receives the brush such that the brush is slidable in a sliding direction, which substantially coincides with a radial direction of the commutator of the motor. In the first position, the brush is retracted away from the commutator into the brush holder and is urged against the side wall of the brush holder by the urging means in a direction perpendicular to the sliding direction, so that the brush is temporarily held in the first position by a static frictional force generated between the side wall of the brush holder and the brush. This static frictional force is greater than an urging force of the urging means applied to the brush in the sliding direction. In the second position, the brush is urged against the commutator by the urging means in the sliding direction. 
     To achieve the objective of the present invention, there is also provided a motor including a motor assembly and a speed reducing gear assembly. The motor assembly includes a motor housing, an armature and a rotatable shaft. The motor housing has an open end. The armature is at least partially received in the motor housing and includes a commutator. The rotatable shaft extends along the axis of the armature and is rotated integrally with the armature. The speed reducing gear assembly includes a gear housing, a worm and at least one above described brush holding device. The gear housing has an open end. The worm is received in the gear housing and is integrally formed with the rotatable shaft. Each brush holder is spaced apart from the open end of the gear housing to be completely received within the gear housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which: 
     FIG. 1 is a partially cut away view showing a construction of a motor unit according to an embodiment of the present invention; 
     FIG. 2 is a plan view of a speed reducing gear assembly of the motor unit showing a structure on a brush base according to the embodiment; 
     FIG. 3 is a partial enlarged perspective view showing a brush holding device according to the embodiment; 
     FIG. 4A is a top plan view of a brush according to the embodiment; 
     FIG. 4B is a side view of the brush shown in FIG. 4A; 
     FIG. 5 is a schematic plan view showing a torsion spring of the brush holding device according to the embodiment; 
     FIG. 6 is a schematic side view depicting a way of releasing the trapped brush according to the present embodiment; 
     FIG. 7A is a schematic view showing a state before release of the trapped brush; 
     FIG. 7B is a schematic view similar to FIG. 7A showing a state during the release of the trapped brush; 
     FIG. 7C is a schematic view similar to FIGS. 7A and 7B showing a state after the release of the trapped brush; 
     FIG. 8 is a schematic view showing forces present in the state shown in FIG. 7A; 
     FIG. 9 is a schematic view showing installation movement of an armature of the motor unit along its installation path according to the present embodiment; 
     FIG. 10A is a schematic partial top plan view showing a modification of the embodiment; 
     FIG. 10B is a schematic side view of FIG. 10A; 
     FIG. 11A is a top plan view of a brush shown in FIGS. 10A and 10B; 
     FIG. 11B is a side view of the brush shown in FIG. 11A; 
     FIG. 12A is a top plan view showing a modification of the brush; 
     FIG. 12B is a side view of the brush shown in FIG. 12A; 
     FIG. 13A is a top plan view showing another modification of the brush; 
     FIG. 13B is a side view of the brush shown in FIG. 13A; 
     FIG. 14 is a partial plan view showing another modification of the embodiment; 
     FIG. 15 is an enlarged perspective view of a brush spring shown in FIG. 14; 
     FIG. 16 is a top plan view showing another modification of the brush; and 
     FIG. 17 is a top plan view showing a further modification of the brush. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of the present invention implemented as a motor unit (i.e., a motor or dynamo-electric machine) used in a vehicle wiper system will be described with reference to FIGS. 1 to  8 . 
     As shown in FIG. 1, the motor unit  10  includes a motor assembly  11  and a speed reducing gear assembly  12 . The motor assembly  11  is a direct current motor and includes a motor housing  13 , which constitutes a part of a housing of the motor unit  10 . Field magnets  14  and an armature  15  are received in the motor housing  13 . The armature  15  is supported by a rotatable shaft  17 . A base end  17   a  of the rotatable shaft  17  is rotatably supported by bearings  18 ,  28 , which are secured within the motor housing  13 . The bearing  18  serves as a radial bearing, and the bearing  28  serves as a thrust bearing. The bearing  18  is secured to a bearing support  19  arranged at a base end of the motor housing  13 . The bearing  28  includes a thrust ball  28   a  and two thrust plates  28   b.  The thrust plates  28   b  rotatably hold the thrust ball  28   a  therebetween. The bearing  28  is received between a recess  17   c,  which is formed in the base end  17   a  of the rotatable shaft  17 , and the motor housing  13 . 
     A distal end  17   b  side of the rotatable shaft  17  extends from the motor housing  13  into the speed reducing gear assembly  12 . The speed reducing gear assembly  12  includes a gear housing  20 , which constitutes a part of the housing of the motor unit  10 . Brush holding devices  30  and a worm wheel (not shown) are arranged in the gear housing  20 . The gear housing  20  includes an engaging portion (open end)  20   a.  The engaging portion  20   a  is engaged with an engaging portion (open end)  13   a  of the motor housing  13  and is secured to the engaging portion  13   a,  for example, with screws (not shown), so that the gear housing  20  is connected to the motor housing  13 . 
     The worm wheel is rotatably supported in the gear housing  20 . One end of the worm wheel is supported by an output shaft  22 , which extends outwardly from the gear housing  20 . A worm  23  is integrally formed at the distal end  17   b  of the rotatable shaft  17 . The worm  23  is meshed with the worm wheel. The worm  23  is rotatably supported by a bearing  24 , which serves as a radial bearing. The bearing  24  is secured by a bearing support  29  arranged in the gear housing  20 . 
     A commutator  27  is arranged between the armature  15  and the worm  23 . The commutator  27  is shaped as a generally cylindrical body and is rotatably supported to rotate integrally with the rotatable shaft  17 . A brush base  25 , which includes the brush holding devices  30 , is connected to the gear housing  20  through floating rubbers  26  near the commutator  27 . 
     As shown in FIG. 2, a generally circular through hole (also serving as a part of an installation path of the armature  15  during assembly)  35  extends through the center of the gear housing  20  and the center of the brush base  25 . The three brush holding devices  30 , which are provided to support a LOW speed brush, a HIGH speed brush and a COM brush (i.e., a common brush), respectively, are directed to the center of the through hole  35  on the brush base  25 . The brush base  25  is made of dielectric thermosetting resin, such as phenolic resin. A dot-dot-dash line A shown in FIG. 2 depicts an outer periphery of the commutator  27  connected to the armature  15 . 
     A more specific structure of each brush holding device  30  according to the present embodiment will be described with reference to FIGS. 3-5. As shown in FIG. 3, each brush holding device  30  includes a brush  31 , a brush holder  32 , a torsion spring (serving as an urging means)  33  and a support pin  34 . 
     The brush holder  32  is made from a metal plate, such as a brass metal plate. A corresponding portion of the metal plate is blanked out from the metal plate using, for example, a press machine and is bent to a generally tubular body. As shown in FIG. 3, a recess  43  is formed in a side wall  32   b  of the brush holder  32 . A length of the recess  43  corresponds to a movable range of an engaging projection  33   a  of the torsion spring  33 . Another recess  42  is formed in a top wall  32   a  of the brush holder  32 . A length of the recess  42  corresponds to a movable range of a pigtail  36 . A side plate  40  is provided in the other side wall  32   e,  which is opposed to the above-described side wall  32   b,  such that the side plate  40  projects adjacent to a rear end surface (radially outer end surface)  31   f  side of the brush  31 . The side plate  40  has a securing portion  40   b,  to which one end  33   b  of the torsion spring  33  is securely engaged. 
     The brush holder  32  is secured to the brush base  25  by a pair of securing claws  32   c  and a protrusion  32   d.  The securing claws  32   c  are arranged at opposed open ends, respectively, of the brush holder  32 , such that each securing claw  32   c  extends from a corresponding end edge of a wall of the brush holder  32 , which contacts the brush base  25 . The protrusion  32   d  protrudes from a side edge of the side wall  32   b,  which contacts the brush holder  32 . When each securing claw  32   c  is bent and is held against a corresponding peripheral edge of the brush base  25 , the brush holder  32  is secured to the brush base  25 . When the protrusion  32   d  is inserted into and is engaged with a through hole  25   d,  which extends through the brush base  25 , the brush holder  32  made from the single metal plate is secured to the brush base  25 , and the tubular body of the brush holder  32  is maintained. The recesses  42 ,  43 , the securing portion  40   b,  the side plate  40 , the securing claws  32   c  and the protrusion  32   d  are formed in advance when the corresponding portion is blanked out from the metal plate. 
     As shown in FIGS. 4A and 4B, each brush  31  is shaped as a generally rectangular parallelopiped. A generally arcuate recess  31   d  is formed in a distal end of the brush  31 . The recess  31   d  makes surface contact with a sliding surface of the commutator  27 . The pigtail  36  is connected to a top wall  31   a  of the brush  31 . An engaging recess  37 , which serves as an engaging portion, is formed in a side wall  31   b  of the brush  31 . The engaging projection  33   a  of the torsion spring  33  engages the engaging recess  37 . The engaging recess  37  of the brush  31  is provided adjacent to a rear end surface  31   f  of the brush  31  and is arcuately recessed in an urging direction (direction of an arrow B in FIG.  4 A), in which the brush  31  is urged against the side wall  32   e  of the brush holder  32 . A tapered surface  31   c  is arranged adjacent to the rear end surface  31   f  of the brush  31 . The tapered surface  31   c  is tapered relative to a sliding direction (direction of an arrow C in FIG.  4 B). In the present embodiment, although the tapered surface  31   c  is formed in the rear end surface  31   f  of the brush  31  at the position adjacent to the top wall  31   a,  the tapered surface  31   c  can be alternatively formed at a position adjacent to the side wall  31   b.  Furthermore, as shown in FIGS. 4A and 4B, a chamfered portion  31   e  extends from the engaging recess  37  to the rear end surface  31   f  of the brush  31 . The chamfered portion  31   e  allows smooth movement of the engaging projection  33   a  of the torsion spring  33  when the engaging projection  33   a  is disengaged from the engaging recess  37  of the brush  31  and moves toward the rear end surface  31   f  of the brush  31 . 
     As shown in FIG. 5, the torsion spring  33  has a securing portion  33   b  at one end and the engaging projection  33   a  at the other end. The securing portion  33   b  is secured to the securing portion  40   b  of the brush holder  32 . The engaging projection  33   a  engages the engaging recess  37  of the brush  31 . The engaging projection  33   a  is generally arcuately curved in an imaginary plane that is substantially perpendicular to the axis of the motor unit  10 . Alternatively, the engaging projection  33   a  can be curved in any other appropriate shape. Thus, the engaging projection  33   a  of the torsion spring  33  has a curved engaging surface, and the curved engaging surface of the engaging projection  33   a  substantially coincides with a curved engaging surface of the engaging recess  37  of the brush  31 . A coiled supporting portion  33   c  is provided between the securing portion  33   b  and the engaging projection  33   a  of the torsion spring  33 . An inner diameter of the supporting portion  33   c  is substantially the same as an outer diameter of the support pin  34 . 
     As shown in FIG. 3, the torsion spring  33  is supported such that the supporting portion  33   c  is received around the support pin  34 , which extends from the brush base  25 . The securing portion  33   b  of the torsion spring  33  is secured to the securing portion  40   b  of the brush holder  32 . The engaging projection  33   a  of the torsion spring  33  engages the engaging recess  37  of the brush  31 . As described above, the torsion spring  33  temporarily holds the brush  31  such that the brush  31  is retracted away from the armature  15  into the brush holder  32  by spring force of the torsion spring  33  exerted around the support pin  34 . 
     A way of assembling the armature  15  to a main body (housing) of the motor  10  will be described with reference to FIGS. 6-9. As shown in FIG. 6, a jig  41 , which urges the rear end surface  31   f  of the brush  31 , has a tapered surface  41   c  at a distal end side of the jig  41 . The tapered surface  31   c  of the brush  31  is engaged with the tapered surface  41   c  of the jig  41  when the temporarily held brush (or trapped brush)  31  is released. When the jig  41  is urged downward (in a downward direction parallel to the axis of the motor unit  10 ), the urging force of the jig  41  is divided into force components P 1 , P 2  at the rear end surface  31   f  of the brush  31 . The force component P 1  presses the brush base  25 . The force component P 2  urges the brush  31  in the sliding direction toward the commutator  27 . 
     FIG. 7A shows the brush  31  that is temporarily held or trapped in the brush holder  32 . In this state, the engaging projection  33   a  of the torsion spring  33  is engaged with the engaging recess  37  of the brush  31 . The engaging projection  33   a  of the torsion spring  33  and the engaging recess  37  of the brush  31  are engaged together at an engaging point, which is located radially outward of the central axis of the support pin  34 . Thus, the torsion spring  33  applies side pressure against the brush  31  in a direction (direction of an arrow B in FIG. 7A) perpendicular to the sliding direction of the brush  31 . As a result, the brush  31  is urged against the side wall  32   e  of the brush holder  32  and is temporarily held such that the brush  31  is retracted away from an installation path of the armature  15  into the brush holder  32  in a retracted position (first position) In this state, as shown in FIG. 8, the force F applied from the engaging projection  33   a  of the torsion spring  33  to the engaging recess  37  of the brush  31  is divided into a first force component F 1  and a second force component F 2 . The first force component F 1  is applied in the direction (indicated by the arrow B in FIG. 7A) perpendicular to the sliding direction from the engaging projection  33   a  of the torsion spring  33  to the engaging recess  37  to generate the side pressure, and the second force component F 2  is applied in the sliding direction toward the commutator  27 . At this time, a static frictional force F 0  greater than the second force component F 2  is generated between the side wall  32   e  of the brush holder  32  and the brush  31 , so that brush  31  is held in the retracted position. 
     Next, as shown in FIG. 9, the armature  15  is installed along the installation path in a direction indicated by an arrow in FIG. 9 such that the armature  15  is received in the through hole  35 , which is formed through the center of the gear housing  20  and the center of the brush base  25 . In this case, the distal end  17   b  of the rotatable shaft  17  is inserted in the through hole  35  from the top side of the through hole  35 . When the sliding surface of the commutator  27  reaches the brush holding devices  30  on the brush base  25 , the insertion movement of the armature  15  is stopped. After the armature  15  is properly installed, the trapped brush  31  of each brush holding device  30  is released from the retracted position. 
     FIG. 7B shows a state where the trapped brush  31  is released partway. When the jig  41  is engaged with the tapered surface  31   c  of the rear end surface  31   f  of the brush  31 , and the jig  41  is urged downward, the brush  31  slightly moves in the urging direction (direction of an arrow C in FIG.  7 B), in which the brush  31  is urged against the commutator  27 . Since the securing portion  33   b  of the torsion spring  33  is secured to the securing portion  40   b  of the brush holder  32 , this will cause the engaging projection  33   a,  which is located at the other end of the torsion spring  33 , to be released from the engaging recess  37 . In this way, after the trapped brush  31  is released, the engaging projection  33   a  and the engaging recess  37  move relative to each other. Then, the engaging projection  33   a  reaches the rear end surface  31   f  of the brush  31 . 
     FIG. 7C shows a state after the engaging projection  33   a  reaches the rear end surface  31   f  of the brush  31 . The torsion spring  33  applies the urging force, which has been applied to the brush  31  as the side pressure, to the brush  31  as the urging force for urging the brush  31  toward the commutator  27 . Then, the brush  31 , which is urged by the torsion spring  33 , protrudes from the brush holder  32 . Thereafter, the distal end of the brush  31  is engaged with the sliding surface of the commutator  27  in a protruded position (second position). In this way, the brush  31  is engaged with the sliding surface of the commutator  27  without using the insertion movement of the armature  15  at the assembly of the armature  15 . 
     The above embodiment provides the following advantages. 
     (1) The brush  31  is released from the temporarily held state in the brush holder  32  without using the insertion movement of the armature  15  at the assembly of the armature  15 . In this case, the brush  31  is engaged with the commutator  27  after the armature  15  is properly assembled. Thus, each brush holding device  30  is free of design limitations with respect to the assembling timing of the armature  15 . As a result, the design of each brush holding device  30  is eased. 
     (2) Each brush holding device  30  uses the torsion spring  33 , which is initially provided to urge the brush  31  toward the commutator  27 . In this case, the torsion spring  33  temporarily holds the brush  31  in the brush holder  32  through engagement between the engaging projection  33   a  of the torsion spring  33  and the engaging recess  37  of the brush  31  while applying the side pressure to the brush  31 . Thus, there is no need to provide a new component that temporarily holds the brush  31 . As a result, the brush holding devices  30  can be manufactured at a reduced cost. 
     (3) The engaging recess  37  of the brush  31  is the curved recess that is recessed in the urging direction, in which the brush  31  is urged against the side wall  32   e  of the brush holder  32 . Also, the engaging projection  33   a  of the torsion spring  33  is curved to correspond with the shape of the engaging recess  37 . Thus, during release of the trapped brush  31 , the engaging projection  33   a  can be quickly, reliably released from the engaging recess  37 . As a result, the brush  31  in the temporarily held state in the brush holder  32  can be quickly, reliably released. 
     (4) The tapered surface  31   c , which is tapered in the sliding direction, is formed in the rear end surface  31   f  of the brush  31 . In this case, when the jig  41  is engaged with the rear end surface  31   f  of the brush  31 , and force is applied to the jig  41 , the force component P 2  can be generated in the sliding direction (direction of an arrow C in FIG. 6) at the rear end surface  31   f  of the brush  31 . Thus, the brush  31  can be reliably moved in the sliding direction by the force component (external force) P 2 . Furthermore, even though the housing is arranged adjacent to the rear end side of the brush  31 , and thus the brush  31  cannot be directly urged in the sliding direction, the brush  31  can be released from the temporarily held state in the brush holder  32 . 
     (5) The torsion spring  33  is supported by the support pin  34 , which extends at the position adjacent to the brush holder  32  on the brush base  25 . Thus, the torsion spring  33  can reliably apply the side pressure to the brush  31  by the spring force exerted around the support pin  34 . Furthermore, after the engaging projection  33   a  of the torsion spring  33  is released from the engaging recess  37  of the brush  31 , the engaging projection  33   a  of the torsion spring  33  moves to the rear end surface  31   f  of the brush  31 . Thus, the torsion spring  33  can apply the side pressure to the brush  31  and can also urge the brush  31  toward the commutator  27 . 
     (6) The engaging point of the torsion spring  33  and the brush  31  is located radially outward of the central axis of the support pin  34 . Thus, while the brush  31  is temporarily held in the brush holder  32 , the spring force of the torsion spring  33  is divided into the side pressure for urging the brush  31  against the side inner wall of the brush holder  32  and the urging force for urging the brush  31  toward the commutator  27 . As a result, when the side pressure applied to the brush  31  from the torsion spring  33  is relieved, the torsion spring  33  can reliably urges the brush  31  toward the commutator  27 . 
     (7) The engaging recess  37  of the brush  31  is formed adjacent to the rear end (radially outer end) of the brush  31 . Thus, the engaging projection  33   a  of the torsion spring  33  can be quickly moved to the rear end surface  31   f  of the brush  31  when the trapped brush  31  is released. As a result, the spring force of the torsion spring  33  can immediately urge the brush  31  toward the commutator  27  when the side pressure applied from the torsion spring  33  to the brush  31  is relieved. 
     (8) The brush holder  32  is secured to the brush base  25  by the securing claws  32   c  and the protrusion  32   d.  In this case, in the brush holder  32 , which is made from the single metal plate, the protrusion  32   d  is inserted in the through hole  25   d  of the brush base  25 , so that a space between the side wall  32   b  and the wall, which contacts the brush base  25 , is not increased. As a result, the brush holder  32 , which is shaped as the tubular body, is not easily deformed. In this way, the brush holder  32  can maintain the predetermined tubular shape even when the thickness of the metal plate is reduced. The brush  31  can be held while jittering of the brush  31  is restrained. Furthermore, the manufacturing cost of the brush retaining devices  30  can be reduced. 
     The above embodiment can be modified as follows. 
     In the above embodiment, the tapered surface  31   c,  which is tapered in the sliding direction, is formed in the rear end surface  31   f  of the brush  31 . When the jig  41  is engaged with the tapered surface  31   c  and is urged downward, the temporarily held state of the brush  31  is relieved. Alternatively, as shown in FIGS. 11A and 11B, a tapered surface  51   c  can be formed in a side surface of a brush  51 . In this case, as shown in FIGS. 10A and 10B, a jig  61  is engaged with the tapered surface  51   c  and is urged in a rotational direction, so that a temporarily held state of the brush  51  (i.e., engagement between an engaging recess  57  formed in a side wall  51   b  of the brush  51  and the engaging projection  33   a  of the spring  33 ) is relieved. 
     In the above embodiment, the engaging recess  37  of the brush  31  is shaped as the curved recess. Alternative to the curved engaging recess  37 , as shown in FIGS. 12A and 12B, an engaging recess  77  of a brush  71  formed in a side wall  71   b  can be shaped as a rectangular recess. Further alternative to the curved engaging recess  37 , as shown in FIGS. 13A and 13B, two protrusions  82   a,    82   b  can be formed in a side wall  81   b  of a brush  81 , and an engaging recess  87  can be formed between the protrusions  82   a,    82   b.    
     In the above embodiment, the rear end surface  31   f  of the brush  31 , to which the jig  41  is engaged, is arranged radially inward of the outer peripheral edge of the brush base  25 . The rear end surface  31   f  of the brush  31  can be arranged radially outward of the outer peripheral edge of the brush base  25 . 
     In the above embodiment, the torsion spring  33 , which serves as the urging means, is shaped as the coil spring. Alternatively, as shown in FIGS. 14 and 15, a torsion spring  83  can be made of a constant pressure spiral spring. The torsion spring  83  has an engaging projection  83   a  at one end and a securing portion  83   b  at the other end. In this case, the engaging projection  83   a  is formed by winding one end of the constant pressure spring in a thickness direction. 
     In the above embodiment, the metal brush holder  32  and the resin brush base  25  are made separately from each other. Alternatively, the brush holder  32  and the brush base  25  can be integrally made form a dielectric resin material. In this case, the number of components and the number of assembling steps of the brush holding devices  30  can be reduced. Thus, the brush holding devices  30  can be manufactured at reduced costs. 
     Furthermore, as shown in FIG. 16, the brush  31  can be replaced with a brush  91  shown in FIG.  16 . An engaging portion  97  of the brush  91 , which engages the engaging projection  33   a  of the torsion spring  33  is formed as a stepped portion that is stepped into a side wall  91   b  of the brush  91 , which faces the urging means  33 , and the stepped portion  97  extends to a rear end surface (radially outer end surface)  91   f  of the brush  91  and is generally parallel to a longitudinal axis of the brush  91 . In this case, unlike the engaging recess  37  of the brush  31 , the stepped portion  97  of the brush  91  does not have a resisting projection that resists movement of the engaging projection  33   a  of the torsion spring  33  to the rear end surface  31   f  when the brush  31  held in the retracted position is forced to slide in the sliding direction toward the commutator  27 , so that the brush  91  can be more easily released from the retracted position in comparison to the brush  31 . Furthermore, similar to the engaging recess  31   e  of the brush  31 , the stepped portion  97  allows easy positioning of the brush  91  relative to the engaging projection  33   a  of the torsion spring  33 . That is, when the brush  91  is installed in the brush holder  32 , the brush  91  is inserted into the brush holder  32  from the radially outer end of the brush holder  32 , and then the brush  91  is slightly retuned toward the radially outer end of the brush holder  32  to engage a wall  98  of the brush  91 , which is provided at a radially inner end of the stepped portion  97 , with the engaging projection  33   a  of the torsion spring  33 . With this engagement, an operator can notice that the brush  91  is appropriately position relative to the engaging projection  33   a  of the torsion spring  33 . It should be noted although the wall  98  is slanted relative to the engaging portion  97 , the wall  98  can extend perpendicular to the engaging portion  97 . 
     With reference to FIG. 17, the brush  31  can be replaced with a brush  101 . In this case, an engaging portion  107  of the brush  101  is formed in a side wall  101   b  near a rear end surface (radially outer end surface)  101   f  of the brush  101  and is flush with the rest of the side wall  101   b  of the brush  31 . Even with this arrangement, the brush  101  can be appropriately held in the retracted position upon engagement with the engaging projection  33   a  of the torsion spring  33 . However, it should be noted that the brush  91  is preferred over the brush  101  due to the following reason. That is, the engaging point between the stepped portion  97  of the brush  91  and the engaging projection  33   a  of the torsion spring  33  is located away from the engaging point between the engaging portion  107  of the brush  101  and the engaging projection  33   a  of the torsion spring  33  in a return direction (urging direction) of the engaging projection  33   a  of the torsion spring  33 . Thus, a first force component (similar to the first force component F 1  shown in FIG. 8) of the force applied from the engaging projection  33   a  of the torsion spring  33  to the stepped portion  97  of the brush  91  in the direction perpendicular to the sliding direction is smaller than that of the engaging portion  107  of the brush  101 . As a result, the static frictional force generated between the side wall  32   e  of the brush holder  32  and the brush  91  is reduced in comparison to the static frictional force generated between the side wall  32   e  of the brush holder  32  and the brush  101 , so that the brush  91  can be more easily released from the retracted position in comparison to the brush  101 . 
     Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore, not limited to the specific details, representative apparatus, and illustrative examples shown and described.