Miniature motor with preassembled commutator

A miniature motor in which stators are mounted in an interior of a casing, and a preassembled commutator is provided on a rotor rotatably disposed to face the stators. The commutator has a sleeve body having a cylindrical portion provided with a flanged portion and a plurality of recesses, directed outwardly, in a side wall of the flanged portion, with the sleeve body being made of insulating material and mounted on a rotary shaft of the rotor. The commutator also has a plurality of conductive commutator pieces each having a body portion formed in an arcuate shape and mounted on an outer circumferential surface of the cylindrical portion, a terminal portion and at least one retainer portion, with each of the terminal portion and the retainer portion being received in the recesses.

BACKGROUND OF THE INVENTION 
The present invention relates to a miniature electric motor for use in 
optical precision equipment such as compact camera, audio visual equipment 
such as a headphone stereo cassette tape recorder, and automotive 
electronic equipment. In particular, the invention relates to a miniature 
electric motor provided with a preassembled commutator. 
Miniature motors have been widely used in various fields including the 
above-mentioned equipment and have been highly miniaturized to reduce 
their size and weight as well as their height. 
In the motors, permanent magnets are mounted within an interior of a 
casing, and a rotor is disposed inside the permanent magnets. A rotary 
shaft of the rotor is rotatably supported by bearings mounted on the 
casing. A commutator is provided around the rotary shaft. Brushes mounted 
on the casing are slidingly engaged with the commutator. 
Commutators which are components of the motors are classified into a mold 
type such that the entire structure is integrally formed of thermosetting 
resin and a preassembled type such that thermoplastics resin is used at 
dielectric parts. 
The mold type and the preassembled type commutators are preferably selected 
depending upon revolving speed of the rotors or the circumstances where 
the motors are to be used. In general, it is preferable to use the 
preassembled type commutator for the case of the low revolving speed and 
to use the mold type commutator, which can withstand heat, for the case of 
the high revolving speed or a large size motor. 
In the case where the revolving speed of the motor is low, since the 
current would be small to generate a small amount of heat and a low 
mechanical strength would suffice, a relatively thin metal plate would be 
used as commutator pieces of the preassembled commutator. 
On the other hand, of the mold type commutator, since a relatively thick 
plate is used as commutator pieces and thermosetting rein is used for the 
dielectric parts, even if a large amount of current is caused to flow to 
generate a large amount of heat, a sufficient heat resistivity is insured. 
Accordingly, this type of the commutator is suitable for relatively large 
motors. 
In contrast thereto, according to the preassembled commutator, there would 
be a fear that the thermoplastics resin or the commutator pieces would be 
molten by heat if a large amount of current would be caused to flow. It is 
necessary to reduce the amount of the heat. Accordingly, this type of the 
commutator is suitable for relatively small motors. Also, since the 
preassembled type commutator is small in size, a large mechanical strength 
would not be required. 
For instance, a preassembled commutator is well known which is composed of 
a sleeve having a flanged portion at one end of a cylindrical portion, a 
plurality of commutator pieces made of metal and mounted around the 
sleeve, and a ring mounted around outer peripheries of the commutator 
pieces. 
In such a commutator, means for fixing the plurality of commutator pieces 
to the sleeve is formed depending upon the pressure applied from the ring 
only. Accordingly, there is a fear that the commutator pieces would be 
offset in the circumferential direction when wirings of an armature are 
connected to terminal portions of the commutator pieces. 
Also, one terminal portion is formed on each commutator piece and proximal 
portions of commutator pieces are depressed by the ring only. Accordingly, 
the mechanical strength of the commutator piece itself is low, and the 
fastening strength of the commutator piece to the sleeve is low, too. As a 
result, it would be difficult to make the outer circumferential surface of 
the plurality of commutator pieces into a true cylindrical shape. 
Also, a distal end portion of each commutator piece would be raised 
radially outwardly, so that the true cylindrical shape of the outer 
circumferential surface would not be attained. The smaller the commutator, 
the more the assembling work would become difficult. Thus, the degradation 
of the true circle property of the outer circumferential surface would 
frequently be encountered. 
SUMMARY OF THE INVENTION 
Accordingly, in order to overcome the above-noted defects inherent in the 
prior art motors, an object of the present invention is to provide a 
miniature motor provided with a preassembled commutator which is capable 
of preventing the offset of commutator pieces by increasing a fastening 
strength of the commutator pieces to the sleeve. 
Another object of the invention is to provide a miniature motor which can 
enhance a circularity of the outer circumferential surface of the 
preassembled commutator. 
According to the invention, in order to attain these and other objects, a 
miniature electric motor is provided in which stators are mounted in an 
interior of a casing and a preassembled commutator is provided on a rotor 
rotatably disposed to face the stators, the preassembled commutator 
comprising: 
a sleeve body having a cylindrical portion provided with a flanged portion 
and a plurality of recesses, directed outwardly, in a side wall of the 
flanged portion, the sleeve body being made of insulating material and 
mounted on a rotary shaft of the rotor; and 
a plurality of conductive commutator pieces each having a body portion 
formed in an arcuate shape and mounted on an outer circumferential surface 
of the cylindrical portion, a terminal portion and at least one retainer 
portion, each of the terminal portion and the retainer portion being 
received in the recesses. 
The cylindrical portion of the sleeve body has a long structure in an axial 
direction. The flanged portion is formed at one end of the cylindrical 
portion opposite to one end face of the cylindrical portion. The 
cylindrical portion is integrally formed on one side of the flanged 
portion. A proximal portion is formed on the other side of the flanged 
portion for contacting with a rotor core of the motor and positioning the 
preassembled commutator. An outer diameter of the flanged portion is 
larger than outer diameters of the cylindrical portion and the proximal 
portion. 
Preferably, the plurality of recesses are formed radially outwardly from a 
centerline of the sleeve body. An inner surface of each of the recesses is 
rectangular in cross section. The inner surface is defined by a bottom and 
a pair of inner side walls which face each other and extend perpendicular 
to the bottom. 
A width (w) between the pair of inner side walls is substantially equal to 
or somewhat smaller than a width (w1) of the terminal portion and the 
retainer portion. As a result, the terminal portion and the retainer 
portion are press-fit to the recesses, and each back surface and each side 
surface of the terminal portion and the retainer portion are brought into 
intimate contact with the bottoms and the inner side walls of the recesses 
respectively. 
Otherwise, the width (w) between the pair of inner walls is somewhat larger 
than the width (w1) of the terminal portion and the retainer portion, 
whereby the terminal portion and the retainer portion are fixed to the 
recesses with adhesives. Each back surface and each side surface of the 
terminal portion and the retainer portion are brought into intimate 
contact with the bottoms and the inner walls of the recesses respectively. 
An inner surface of the body portion of the commutator piece has 
substantially the same radius of curvature as that of the outer 
circumferential surface, whereby the body portion comes into intimate 
contact with the outer circumferential surface. 
When the commutator piece is mounted on the sleeve body, the body portion 
has such an axial length that its distal end is substantially flush with 
an end face of the cylindrical portion and such a circumferential length 
that a slit is formed so that a uniform dimension (d) is kept between the 
adjacent commutator pieces. 
The sleeve body as a whole may be integrally made of insulating material 
such as thermoplastics resin. The commutator piece is made of conductive 
material such as copper and silver. 
The terminal portion may have an elongated structure extending outwardly 
for connection with an armature winding. The retainer portion has a 
shortened structure so as not to extend beyond an outer circumferential 
surface of the flanged portion. 
The number of the commutator pieces is the same as that of poles of the 
motor. The miniature motor may comprise a three-slot three-pole structure, 
wherein the preassembled commutator has three commutator pieces. Each 
commutator piece may have one terminal portion and one retainer portion. 
The terminal portions and the retainer portions of the three commutator 
pieces are fixedly mounted within the recesses respectively. The three 
commutator pieces are uniformly arranged at 120.degree. in the 
circumferential direction of the sleeve body. 
The commutator may further comprise an insulating ring member for being 
brought into intimate contact with the outer circumferential surface of 
the body portion of the commutator piece mounted on the sleeve body and 
for causing the terminal portion and the retainer portion to enter the 
recesses. 
The ring member is made of insulating material such as polyethylene resin. 
An inner diameter of the ring member is substantially equal to or somewhat 
larger than an outer diameter of the outer circumferential surface of the 
body portion of the commutator piece. The ring member is brought into 
contact with the terminal portion, the retainer portion and the side walls 
of the flanged portion. 
A pair of flat portions for defining surfaces which are in parallel with 
each other are formed in the casing of the miniature motor. A mounting 
position of the terminal portion is offset from a center position (E) of 
one edge of the body portion, whereby the terminal portion is interposed 
in a space between adjacent armature windings. 
Under the condition that the preassembled commutator is incorporated into 
the three-slot three-pole type motor, the terminal portions are mounted at 
60.degree. relative to a centerline of one pole of a rotor core. 
The motor may be of a cylindrical type where the casing is cylindrical, 
wherein a mounting position of the terminal portion of the commutator 
piece is the center position (E) of the one edge of the body portion, the 
terminal portion is bent outwardly at the center position (E). The 
preassembled commutator are mounted on the rotary shaft of the rotor for 
positional adjustment so that the terminal portion is interposed in a 
space between adjacent armature windings. 
A pair of the retainer portions are formed on both sides of the terminal 
portion of the commutator piece, each of the retainer portions being bent 
radially outwardly from the one edge. The recesses are formed in positions 
corresponding to the terminal portion and the retainer portions in the 
flanged portion of the sleeve body. 
The commutator piece having the terminal portion and the retainer portions 
is symmetrical with respect to the center position (E) on right and left 
sides. 
One retainer portion of the commutator piece is formed in the vicinity of 
the terminal portion and bent radially outwardly from the one edge. The 
recesses are formed in positions in correspondence with the terminal 
portion and the retainer portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preassembled commutator for a miniature electric motor will now be 
described with reference to FIGS. 13 and 14 showing a related art to the 
present invention for the purpose of well understanding the concept of the 
invention. 
As shown in FIGS. 13 and 14, the preassembled commutator is provided with a 
sleeve 3 having a flanged portion 2 at an end portion of a cylindrical 
portion 1, a plurality of three or more commutator pieces 4 made of metal 
and mounted on the sleeve 3, and a ring 5 mounted on outer surfaces of the 
commutator pieces 4. A single terminal portion 6 is formed on each of the 
commutator pieces 4 for connection with an armature winding. 
In such a commutator, a fixing means for fixing the plurality of commutator 
pieces 4 to the sleeve 3 depends upon the pressure by the ring 5 only. 
Accordingly, when the armature winding is connected to the terminal 
portions 6, as shown in FIG. 13, the commutator pieces 4 would be offset 
in position in the circumferential direction. Therefore, each distance d1 
through d3 between adjacent commutator pieces 4 would be non-uniform or 
the adjacent commutator pieces 4 would come into contact with each other. 
Also, only one terminal portion 6 is formed on each commutator piece 4 and 
the ring 5 is used to press a distal end portion of each commutator piece 
4 only. As a result, the mechanical strength of the commutator pieces 4 
per se would be low and also a fastening strength of the commutator pieces 
4 to the cylindrical portion 1 would be low. Therefore, it is difficult to 
form the outer circumferential surface 8, defined by the plurality of 
commutator pieces 4, into a true cylindrical shape. 
For this reason, as shown in FIG. 14, a distal end portion 9 of each 
commutator piece 4 would be raised radially outwardly to degrade a 
circularity of the outer circumferential surface 8. 
The present invention will now be described with reference to FIGS. 1 
through 12. 
Referring first to FIGS. 9 through 12, the overall structure of a miniature 
electric motor with a preassembled commutator in accordance with the 
invention will now be described. 
FIG. 9 is a perspective view showing an outer appearance of the miniature 
motor which is a so-called "flat type" motor in which flat portions are 
provided in a casing. FIG. 10 is a perspective view showing another type 
of the miniature motor which is a cylindrical type motor where no flat 
portions are provided in the casing. FIG. 11 is a longitudinal sectional 
view showing commonly the miniature motor shown in FIGS. 9 and 10. FIG. 12 
is a cross-sectional view showing the flat type miniature motor shown in 
FIG. 9. 
As shown in FIGS. 9 and 11, the miniature electric motor 20 is provided 
with stators 21 mounted within the casing 22 and a rotor 23 disposed 
within the casing 22 to face the stators 21. A rotary shaft 26 of the 
rotor 23 is rotatably supported by bearing members 24 and 25 provided in 
the casing 22. 
The casing 22 is provided with a housing 27 formed into a bottomed hollow 
cylinder and a cap member 29 made of insulating material and fitted on an 
opening portion 28 of the housing 27. The housing 27 is made of metal 
material such as a cold rolled steel plate made mainly of soft steel and 
the cap member 29 is made of resin material or any other insulating 
material, for example. A pair of flat portions 30 which are parallel to 
each other are formed in the casing 22. 
A thrust bearing 42 is pressingly inserted into an inner bottom portion 41 
of a bearing receiving portion 40 formed in the cap member 29. The thrust 
bearing 42 is made of a circular metal plate or a circular synthetic resin 
plate for supporting the rotating rotary shaft 26 in the thrust direction 
(i.e., axial direction). 
As shown in FIGS. 11 and 12, the stators 21 are fixed to circular inner 
circumferential surfaces 31 of an inner surface of the housing 27. The 
stators 21 are composed of, for example, a pair of permanent magnets 
formed into arcuate segments and made of magnetic material such as hard 
ferrite. 
The rotor 23 is provided with the rotary shaft 26 extending in a centerline 
direction for rotation, a rotor core 33 mounted around the rotary shaft 
26, and a preassembled commutator 34 mounted on the rotary shaft 26 and 
electrically connected to an armature winding 32. The armature winding 32 
is wound in shape of coil around the rotor core 33. The rotor core 33 is 
disposed with a predetermined air-gap relative to the stators 21 and 
inside the latter. 
A plurality of two or more sets of brushes 35, which are made of conductive 
material, are provided in the cap member 29 and come into sliding contact 
with the preassembled commutator 34 to allow a current to flow 
therebetween. A plurality of one pair or more connecting terminals 36 
electrically connected to the respective brushes 35 are mounted on the cap 
member 29. A terminal portion 37 of each connecting terminal 36 extends 
outwardly from a surface 38 of the cap member 29. 
Incidentally, the motor 20 is of a three-slot three-pole structure as shown 
in FIG. 12. 
In the motor 20 having the above-described structure, the current will flow 
from the connecting terminals 36 through the brushes 35 and the commutator 
34 to the armature winding 32. Then, a rotational torque will be applied 
to the rotor 23 which is disposed in a magnetic field formed by the 
stators 21 made of the pair of permanent magnets so that the rotor 23 will 
take a rotational motion. 
Thus, the motor 20 will drive an optical precision device (not shown) 
through an output portion 39 of the rotary shaft 26 which is rotating. 
A miniature motor 20a shown in FIG. 10 is of a cylindrical type where no 
flat portions shown in FIG. 9 are provided. A casing 22a having a circular 
shape in cross section is provided with a housing 27a and a cap member 29a 
fitted in the housing 27a. The internal structure of the motor 20a is the 
same as that shown in FIG. 11. 
The preassembled commutator will be described hereunder. 
FIGS. 1 through 3 show the first embodiment of the invention. FIG. 1 is a 
perspective view showing an outer appearance of the preassembled 
commutator. FIG. 2 is a perspective view showing a commutator piece of the 
commutator shown in FIG. 1. FIG. 3 is a perspective view showing a sleeve 
of the commutator shown in FIG. 1. 
As shown in FIG. 1, the preassembled commutator 34 is provided with an 
insulating sleeve body 50 mounted on the rotary shaft 26 of the rotor 23 
shown in FIG. 11 and a plurality of conductive commutator pieces 51 
mounted on the sleeve body 50. The sleeve body 50 is made of electrical 
insulating material such as a thermoplastic resin. Each commutator piece 
51 is made of conductive material such as copper and silver. In the 
embodiment, since the motor 20 is of the three-slot three-pole structure, 
the preassembled commutator 34 is provided with the three commutator 
pieces 51. 
As shown in FIGS. 1 and 3, the sleeve body 50 as a whole is formed 
integrally. A through-hole 52 into which the rotary shaft 26 is to be 
inserted is formed in the central portion of the sleeve body 50. 
A cylindrical portion 53 of the sleeve body 50 has a long axial length. A 
flanged portion 55 is formed at one end portion 54 of the cylindrical 
portion 53 opposite to an end face 53a thereof. A plurality (six in the 
embodiment) of recesses 57 are formed in a side wall 56 of the flanged 
portion 55. The side wall 56 is positioned to a side of the cylindrical 
portion 53. The recesses 57 are directed radially outwardly from a 
centerline G of the sleeve body 50. Incidentally, it is sufficient that 
the recesses 57 extend outwardly and may be eccentrically offset from the 
centerline G. An inner surface of each recess 57 is rectangular in cross 
section and is defined by a bottom 58 and a pair of facing inner side 
walls 59 perpendicular to the bottom 58. 
The cylindrical portion 53 is provided on one side of the flanged portion 
55, whereas a proximal portion 60 is provided on the other side of the 
flanged portion 55 for contacting with the rotor core 33 and positioning 
the preassembled commutator 34 in place. These components are formed 
integrally. An outer diameter of the flanged portion 55 is larger than 
outer diameters of the cylindrical portion 53 and the proximal portion 60. 
As shown in FIGS. 1 and 2, the commutator pieces 51 mounted on the sleeve 
body 50 are each provided with a body portion 61 formed into an arcuate 
shape in cross section, a terminal portion 63 for connection with the 
armature winding 32, and a retainer portion 64 positioned in the vicinity 
of the terminal portion 63 to thereby form an integral structure. The 
terminal portions 63 and the retainer portions 64 are bent outwardly (for 
example, radially outwardly) from arcuate first edges 62 of the body 
portions 63. 
An inner surface of each body portion 61 has substantially the same radius 
of curvature as that of an outer circumferential surface 65 of the 
cylindrical portion 53 whereby the body portion 61 may be in intimate 
contact with the outer circumferential surface 65. When the commutator 
pieces 51 are mounted on the sleeve body 50, each body portion 61 has such 
an axial length that its distal end 61a is substantially flush with the 
end face 53a of the cylindrical portion 53. Also, each body portion 61 has 
such a circumferential length that a slit 67 is formed under the condition 
that a substantially constant dimension d is kept between the adjacent 
commutator pieces 51. 
When the terminal portions 63 and the retainer portions 64 of the three 
commutator pieces 51 are received in and fixed to the recesses 57 of the 
flanged portion 55, respectively, the three commutator pieces 51 are 
uniformly arranged in the circumferential direction of the sleeve body 50 
at 120.degree.. Thus, the preassembled commutator 34 is completed. 
The means for fixing the terminal portions 63 and the retainer portions 64 
into the recesses 57 is preferably constituted as follows. A width w 
between the facing inner walls 59 of each recess 57 is equal to or 
somewhat smaller than a width w1 of each terminal portion 63 and/or each 
retainer portion 64. This dimensional relationship causes the terminal 
portion 63 and/or retainer portion 64 to pressingly fit the recess 57. 
Incidentally, the width w may be somewhat larger than the width w1. In 
this case, the terminal portion 63 and the retainer portion 64 are to be 
fixed to the recesses 57 with adhesives. 
Thus, the press-fit means or adhesives are used. Respective back surfaces 
68 and 69 of the terminal and retainer portions 63 and 64 and respective 
side surfaces 68a and 69a of the terminal and retainer portions 63 and 64 
are fixedly brought into intimate contact with the bottoms 58 and the 
inner side walls 59 respectively. As a result, the intimate contact area 
is much increased in comparison with that obtained by the conventional 
technique, thereby considerably improve the fastening strength of the 
commutator pieces 51 to the sleeve body 50. 
Since the terminal portions 63 are connected through wirings to the 
armature winding 32, it is preferable to take a long structure that the 
terminal portions 63 extends radially outwardly for easy connection. 
However, it is preferable to take a short structure for the retainer 
portions 64 so that the retainer portions do not extend beyond the outer 
circumferential surface 66 of the flanged portion 55. If the retainer 
portions 64 would extend exceeding the outer circumferential surface 66 of 
the flanged portion 55, the retainer portions 64 would be brought into 
contact with the armature winding 32 wound around the rotor core 33 so 
that there would be a short-circuit. This can be avoided to meet the 
above-described condition. 
Thus, if the terminal portions 63 and the retainer portions 64 of the three 
commutator pieces 51 are mounted in and fixed to the respective recesses 
57, the respective commutator pieces 51 are uniformly arranged in the 
circumferential direction and the displacement of the pieces 51 therein 
may be avoided. 
As a result, the slit 57 between each adjacent portions 61 may be kept 
constant at dimension d to enhance reliability. Also, even when the 
armature winding 32 is to be wound around the rotor core 33 and is 
connected to the terminal portions 63 (not shown), or when vibrations due 
to the motor rotation or any other external forces are applied, there is 
no fear that the commutator pieces 51 would be drawn in the 
circumferential direction to cause any positional displacement. Thus, it 
is possible to avoid the conventional defects. 
The retainer portions 64 are provided in addition to the terminal portions 
63 and these are fixed in the recesses 57. Accordingly, the mechanical 
strength of the commutators 51 per se is enhanced and the fastening 
strength of the commutator pieces 51 to the sleeve body 50 is also 
enhanced. With such a structure, unlike the conventional technique, it is 
possible to obviate the fear that the distal end portions 61a of the 
commutator pieces 51 would be inadvertently raised, thereby improve a 
circularity of the outer circumferential surface of the commutator 34 
which has been preassembled. 
When the commutator 34 is preassembled or fabricated, it is sufficient to 
mount the terminal portions 63 and the retainer portions 64 into the 
respective recesses 57. Accordingly, it is possible to accurately position 
the small size commutator pieces 51 in the circumferential direction. It 
is therefore possible to enhance the circularity even with the miniature 
commutator. 
The commutator pieces 51 in connection with the foregoing embodiment is 
preferably used in the flat type miniature motor 20 in which the pair of 
flat portions 30 are formed in the casing 22. 
Namely, as shown in FIG. 12, an internal space of the motor 20 is 
particularly small at positions corresponding to the flat portions 30. 
Accordingly, the pair of stator 21 and the pair of connecting terminals 36 
are juxtaposed on a centerline C extending in the longitudinal direction 
and arranged to intersect with the centerline C. The pair of brushes 35 
extending from the respective connecting terminals 36 toward the 
preassembled commutator 34 are mounted in parallel to the centerline C. 
When a centerline of one pole of the rotor core 33 is identical with the 
centerline C of the stators 21, an electrical switching of the armature 
winding 32 is performed to rotate the rotor 23. Accordingly, under this 
switching moment, it is necessary to locate one slit 67 in the direction 
of a centerline D which is perpendicular to the centerline C. 
Meanwhile, the terminal portions 63 are located between the adjacent 
armature windings 32 wound around the respective poles of the rotor core 
33. The reason for this is that, since the winding operation of the 
armature windings 32 is usually carried out after the preassembled 
commutator 34 has been preassembled on the rotary shaft 26 upon the motor 
fabrication, if the terminal portions 63 would be located in the winding 
positions, the terminal portions 63 would obstruct the winding operation 
when the armature windings 32 are wound around the rotor core 33. Also, 
the additional reason is that a sufficient amount of the armature windings 
32 could be mounted on the rotor core 33. 
Accordingly, under the condition that the preassembled commutator 34 is 
incorporated into the three-slot three-pole type motor 20, the terminal 
portions 63 is most preferably mounted at an angle of 60.degree. relative 
to the centerline of one pole of the rotor core 33. 
For those reasons, in the preferred embodiment, as shown in FIGS. 1, 2 and 
12, the mounting position of the terminal portions 63 to the body portion 
61 is offset from a center position E of one edge 62 of each body portion 
61, whereby the terminal portion 63 is interposed in a space between the 
adjacent armature windings 32. 
FIG. 4 is a perspective view showing the second embodiment of the 
invention, showing a preassembled condition of a preassembled commutator. 
FIG. 5 is a side view showing the preassembled commutator shown in FIG. 4. 
A preassembled commutator 34a in accordance with the second embodiment is 
provided with an insulating ring member 70 in addition to the sleeve 50 
and three commutator pieces 51 shown in the first embodiment. 
The ring member 70 is brought into intimate contact with an outer 
circumferential surface 71 of each body portion 61 of each commutator 
piece 51 mounted on the sleeve body 50 for fastening the terminal portions 
63 and the retainer portions 64 to the recesses 57. The ring member 70 is 
made of electrical insulating material such as polyethylene resin. 
The terminal portions 63 and the retainer portions 64 of the commutator 
pieces 51 are mounted in the recesses 57 to thereby mount the commutator 
pieces 51 onto the sleeve body 50. Thereafter, the ring member 70 is 
mounted around the outer peripheral portion of the commutator pieces 51 as 
indicated by arrows in FIG. 4. 
An inner diameter of the ring member 70 is substantially equal to or 
somewhat larger than an outer diameter of the outer circumferential 
surface 71 of the body portions 61 of the commutator pieces 51. 
Accordingly, the ring member 70 is advanced while depressing the outer 
circumferential surface 71 of the commutator pieces 51 inwardly and is 
positioned in place while coming into contact with the terminal portions 
63, the retainer portions 64 and the side walls 56 of the flanged portion 
55. Thus, the respective commutator pieces 51 are fastened to the 
cylindrical portion 53 of sleeve body 50. 
As described above, since it is unnecessary to use means such as 
press-fitting or adhesives for fastening the terminal portions 63 and the 
retainer portions 64 to the recesses 57 if the ring member 70 is used, the 
assembling work may be facilitated, and at the same time, the commutator 
pieces 51 are firmly fastened in place. 
Also, the second embodiment ensures the same effect as that of the first 
embodiment. 
The third embodiment of the invention will now be described. 
The third embodiment is drawn to a modification of the commutator pieces 
used in the first and second embodiments. FIG. 6 is a perspective view of 
a commutator piece and FIG. 7 is a side view of a preassembled commutator 
including the commutator piece shown in FIG. 6 and is similar to FIG. 5. 
The preassembled commutator 34b in accordance with the third embodiment is 
to be used in the cylindrical type miniature motor 20a as shown in FIG. 
10. In case of the motor 20a, since there are no flat portions, there is 
no restriction to the space for the case where the connecting terminals 36 
are disposed within an interior of the motor 20a. Accordingly, by rotating 
the cap member 29a relative to the housing 27a, the connecting terminals 
36 may be rotated to thereby adjust the brushes 35 at desired positions. 
Consequently, in the third embodiment, the mounting position of terminal 
portions 63a of commutator pieces 51a is not offset from the central 
position E of one edge 62 of each body portion 61. The terminal portions 
63a are bent outwardly (for example, radially outwardly) at the central 
position E. Even with such an arrangement, it is possible to set the 
preassembled commutator 34b on the rotary shaft 26 so that the terminal 
portions 63a are interposed in the spaces between the adjacent armature 
windings 32. Also, if the position of the brushes 35 is adjusted in 
correspondence with the position of the slits 67 as described above, the 
electrical switching of the armature windings 32 may be smoothly attained 
by the commutator 34b. 
A pair of retainer portions 64a are symmetrically formed on the right and 
left sides relative to the central position E on both sides of each 
terminal portion 63a. Each retainer portion 64a is bent outwardly (for 
example, radially outwardly) from one edge 62. 
Meanwhile, recesses 57a are formed in positions in correspondence with the 
terminal portions 63a and the retainer portions 64a in a flanged portion 
55a of a sleeve body 50a. Accordingly, also in accordance with the third 
embodiment, the same effect and advantage as those of the first and second 
embodiments may be insured. 
Since the two retainer portions 64a are thus provided for one commutator 
piece 51a, it is possible to enhance the fastening strength of the 
commutator pieces 51a. Also, as shown in FIGS. 6 and 7, if the commutator 
pieces 51a having the terminal portions 63a and the retainer portions 64a 
are arranged symmetrically on the right and left sides relative to the 
central positions E, the manufacture and fabrication works are facilitated 
without any working errors. 
FIG. 8 is a side view showing a preassembled commutator 34c in accordance 
with a fourth embodiment of the invention and is similar to FIG. 5. 
The preassembled commutator 34c is also used in the cylindrical type 
miniature motor 20a in the same way as in the third embodiment. The 
terminal portions 63a are mounted at the center positions E on the first 
edges of the body portions of the commutator pieces. In the third 
embodiment, the two retainer portions 64a are provided for one commutator 
piece 51a, but in the fourth embodiment, only one retainer portion 64a of 
the commutator piece 51b is provided close to the terminal portion 63a. 
The retainer portion 64a is bent radially outwardly from one edge 62. The 
recesses 57a are formed in positions in correspondence with the terminal 
portions 63a and the retainer portions 64a in the flanged portion 55a of 
the sleeve body 50b. The other structure of the fourth embodiment is the 
same as that of the third embodiment. 
Also, in the fourth embodiment, the same effect and advantage as that of 
the third embodiment may be insured. 
In the present invention, the number of the retainer portions 64, 64a 
provided in addition to the terminal portions 63, 63a may be one or more. 
Incidentally, in the case where the structure of the miniature motor is 
other type than the three-pole type, the number of the commutator pieces 
should correspond to the pole number. 
Also, as in the respective embodiments, the recesses for receiving the 
terminal portions and the recesses for receiving the retainer portions may 
take the same cross section but may take different cross sections. 
Incidentally, throughout the foregoing embodiments, the same reference 
numerals and characters are used for indicated the corresponding 
components or members. 
Various details of the invention may be changed without departing from its 
spirit nor its scope. Further, the foregoing description of the 
embodiments according to the present invention is provided for the purpose 
of illustration only, and not for the purpose of limiting the invention as 
defined by the appended claims and their equivalents.