Small electric motor

A small electric motor includes a first stator unit including a first outside stator, a first inside stator, and a coil between the first outside stator and the first inside stator; and a second stator unit including a second outside stator, a second inside stator, and a coil between the second outside stator and the second inside stator, in such a manner that the first and second stator units are set one on another in such a manner that the first and second inside stators are positioned between the first and second outside stators. In the motor, the first stator unit has a first holding section which is adapted to position the second inside stator while the second stator unit has a second holding section which is adapted to position the first inside stator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a small electric motor such as a stepping motor, 
and more particularly to an improvement of a small electric motor which 
comprises a stack of stator units each of which is made up of an outside 
stator and an inside stator. 
2. Related Art 
A small electric motor is known in the art of which the stator structure is 
of a stack of stator units each of which is made up of an inside stator 
and an outside stator. For instance, a stepping motor has a stator 
structure as shown in FIG. 13. That is, the stator structure comprises: a 
first stator unit 34; and a second stator unit 38. The first stator unit 
34 includes: a first outside stator 31 which has pole teeth 31b and forms 
an outer periphery of the motor casing, a first inside stator 32 having 
pole teeth 32b, and a coil 33 wound on a coil bobbin held between those 
first inside and outside stators 32 and 31. On the other hand, the second 
stator unit 38 includes: a second outside stator 35 which has pole teeth 
35b and forms an outer periphery of the motor casing, a second inside 
stator 36 having pole teeth 36b, and a coil 37 wound on a coil bobbin held 
between those second inside and outside stators 35 and 36. Those first and 
second stator units 34 and 38 are stacked one on another in such a manner 
that the two inside stators 32 and 36 are located between the coils 33 and 
37. Thus, the aimed stator structure has been formed. And a rotor 39 is 
arranged at the center of the stator structure in such a manner as to 
confront with the pole teeth 31b, 32n, 35b and 36b. 
More specifically, the stator structure is designed as follows: First, two 
inside stators 32 and 36 as shown in the part (A) of FIG. 14 are prepared 
which are equal in configuration to each other, having four protrusions 
32a and four protrusions 36a which are symmetrical with respect to 
sectional line B--B passing through the center of rotation of the rotor. 
The two inside stators 32 are set back to back with their pole teeth 
extended in the opposite directions which have been bent in one and the 
same direction. Under this condition, the inside stators are fixedly 
joined together with the protrusions 32a and 36a set together which are 
symmetrical with respect to the aforementioned sectional line B--B. 
Thereafter, the coils 33 and 37 are set as was described above, and the 
inside stators 32 and 36 thus joined are held between the outside stators 
31 and 35. In this case, as shown in FIG. 15, the peripheries of the 
outside stators 31 and 35 have cuts 31a and 35a in correspondence to the 
protrusions 32a and 36a, respectively. The protrusions 32a and 36a are 
equal in configuration, and are fixedly stacked at the same position, and 
therefore the cuts 31a and 35a are also formed at equal positions. Now, 
with the protrusions 32a and 36a fitted in the cuts 31a and 35a, the 
stator structure is positioned both circumferentially and axially. In this 
connection, the inside stators 32 and 36 may be not constant in thickness. 
Hence, the depth T2 of the cuts 31a and 35a is made larger than the 
thickness T1 of the inner stators 32 and 36 (T1&lt;T2). 
As was described above, in the stator structure, T1&lt;T2, and therefore a gap 
G is formed as shown in FIG. 15. This gap G allows the protrusions 32a and 
36a to axially freely move. Hence, as shown in FIG. 16 the inside stators 
32 and 36 thus joined are shifted in the gap G (not being held middle in 
the gap G in the axial direction). Originally, the line L along which the 
outside stators 31 and 35 are abutted against each other should be equal 
to the line M along which the inside stators 32 and 36 are jointed 
together. However, as shown in FIG. 16, the line L is shifted from the 
line M. If, in the stator structure, the amount of shift between those two 
lines L and M is not constant depending on the positions of the 
protrusions; more specifically, different amounts of shift (between the 
lines M and L) are provided at four positions where the protrusions 32a 
and 36a of the inside stators joined together are engaged with the cuts 
31a and 35a of the outside stators, then the inside stators 32 and 36 are 
inclined with respect to the outside stators 31 and 35. In this operation, 
it should be noted that the joined inside stators 32 and 36 have the pole 
teeth. The pole teeth on one side are shifted inwardly (or towards the 
center) when the inside stators are inclined with respect to the outside 
stators, thus decreasing the effective inside diameter of the stator. As a 
result, the gap between the rotor 39 and the inside stators 32 and 36 is 
made smaller than the designed value, which obstructs the miniaturization 
of the electric motor. On the other hand, sometimes those lines L and M 
may be shifted as much as one and the same distance at the four positions 
where the protrusions 32a and 36a of the joined inside stators are engaged 
with the cuts 31a and 35a of the outside stators; however, since the lines 
are shifted, the resultant motor is not so high in performance as 
expected. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is to provide a small electric 
motor in which its joined inside stators 32 and 36 are held at the 
designed center with high accuracy. 
There is provided a small electric motor comprising a first stator unit 
including a first outside stator forming a part of a motor casing, a first 
inside stator provided inside said motor, and a coil between said first 
outside stator and said first inside stator; and a second stator unit 
including a second outside stator forming a part of said motor casing, a 
second inside stator provided inside said motor, and a coil between said 
second outside stator and said second inside stator, wherein said first 
and second stator units is laid one on another in such a manner that said 
first and second inside stators are positioned between said coil of said 
first stator unit and said coil of said second stator unit, and said first 
stator unit has a first holding section which is adapted to position said 
second inside stator, and said second stator unit has a second holding 
section which is adapted to position said first inside stator. 
As is apparent from the above description, in the small electric motor, the 
first holding section of the first outside stator positions the second 
inside stator, while the second holding section of the second outside 
stator positions the first inside stator. This feature makes it possible 
to hold the two inside stators at the designed center. Therefore, the 
motor thus formed is free from the difficulty that the inside stators are 
held oblique with respect to other members such as the outside stators and 
bobbins. Hence, the gap between the rotor and the inside stator has the 
designed value. The resultant small electric motor has been designed as 
planned.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will be described with reference to the accompanying drawings 
in detail. 
First Embodiment 
FIGS. 1 through 6 shows an example of a small electric motor, which 
constitutes a first embodiment of the invention. The small electric motor 
of the invention is similar to the above-described small electric motor, 
and is substantially equal in fundamental structure to the one shown in 
FIG. 13. That is; in the small electric motor of the invention, its stator 
structure comprises a first stator unit 4; and a second stator unit 8. The 
first stator unit 4 includes: a first outside stator 1 which has pole 
teeth 1b and forms an outer periphery of the motor casing; a first inside 
stator 2 having pole teeth 2b; and a coil 3 wound on a coil bobbin 10 held 
between those first inside and outside stators, whereas the second stator 
unit 8 includes: a second outside stator 5 which has pole teeth 5b and 
forms an outer periphery of the motor casing; a second inside stator 6 
having pole teeth 6b; and a coil 7 wound on a coil bobbin 11 held between 
those second inside and outside stators 35 and 36. Those first and second 
stator units 4 and 8 are laid one on another in such a manner that the two 
inside stators 2 and 6 are located between the coils 3 and 7. Thus, the 
aimed stator structure has been substantially formed. And a rotor is 
arranged at the center of the stator structure in such a manner as to 
confront with the pole teeth 1b, 2b, 5b, and 6b. 
The stator structure is formed as follows: First, as shown in the part (A) 
of FIG. 3, the inside stators 2 and 6 are formed. That is, those inside 
stators 2 and 6 have protrusions 2a and 6a which are not symmetrical with 
respect to the sectional line B--B which passes through the center of the 
rotor. The inside stators 2 and 6 have the aforementioned protrusions 2a 
and 6a, pluralities of pole teeth 2b and 6b, pluralities of engaging holes 
2c and 6c, pluralities of engaging protrusions 2d and 6d, and pluralities 
of bobbin-rotation preventing holes 2e and 6e, respectively. Thereafter, 
the inside stators 2 and 6 are set back to back with their pole teeth 2b 
and 6b extended in the opposite directions which have been bent in one and 
the same direction; and then the engaging protrusions 6d of the stator 6 
are fixedly engaged with the engaging holes 2c of the stator 2 by 
caulking. Thus, the inside stators 2 and 6 have been fixedly secured to 
each other as shown in FIG. 4. In the resultant stator structure, the 
protrusions 2a and 6a are not symmetrical with the sectional line B--B, 
and therefore the protrusions 2a and 6a have surfaces which are exposed 
being shifted as indicated at 2X and 6X in FIG. 4 (hereinafter referred to 
as "shifted (exposed) surfaces 2X and 6X", when applicable). 
The bobbins 10 and 11 have small protrusions (not shown). Those protrusions 
are inserted into the bobbin-rotation preventing holes 2e and 6e, so that 
the bobbins 10 and 11 are secured to the stator 2 and 6, respectively. As 
shown in FIG. 1, the coils 3 and 7 are wound on the bobbins 10 and 11, 
respectively, which are then held between the outside stators 1 and 5. The 
outside stator 1 has a plurality of cuts 1a corresponding to the plurality 
of protrusions 2a, while the outside stator 5 has also a plurality of cuts 
5a corresponding to the plurality of protrusions 6a. Moreover, the outside 
stators 1 and 5 have a plurality of pole teeth 1b and a plurality of pole 
teeth 5b, and large cuts 1c and 5c, respectively, with which terminals 10a 
and 11a of the bobbins 10 and 11 holding the coils 3 and 7 are engaged, 
respectively. Furthermore, as shown in FIG. 1, the cuts 1a of the outside 
stator 1 engage with the protrusions 2a of the inside stator 2, being 
substantially equal in circumferential length to the protrusions 2a of the 
inside stator 2, and being deeper as much as the gap G than the axial 
thickness of the protrusions 2a. Similarly, the cuts 5a of the outside 
stator 5 engage with the protrusions 6a of the inside stator 6, being 
substantially equal in circumferential length to the protrusions 6a of the 
inside stator 6, and being deeper as much as the gap G than the axial 
thickness of the protrusions 6a. Hence, when the two upper and lower 
stator units 4 and 8 are laid one on another with the inside stators 2 and 
6 set between the coils 3 and 7, the protrusions 2a and 6a are not 
entirely placed on each other; that is, each protrusion 2a has the 
aforementioned shifted surface 2X which is abutted against the outside 
stator 5, while each protrusions 5a has the aforementioned shifted surface 
5X which is abutted against the outside stator 1. Thus, the protrusions 
are axially positioned. In this case, the shifted surface 2X of the 
protrusion 2a is abutted against an abutting portion of the second outside 
stator 5 (hereinafter referred to as "a second holding section 5d", when 
applicable); and similarly the shifted surface 6X of the protrusion 6a is 
abutted against an abutting portion of the first outside stator 1 
(hereinafter referred to as "a second holding section 1d", when 
applicable). The two inside stators 2 and 6 are circumferentially 
positioned by the two outside stators 1 and 5 as follows: That is, the 
positioning of the inside stators 2 and 6 are achieved by engaging the 
cuts 1a and 5a of the two outside stators 1 and 5 with the protrusions 2a 
and 6a of the two inside stators 2 and 6 (as described later in more 
detail). In this connection, similarly as in the case of the 
above-described conventional art, the depth T2 of the cuts 1a and 5a is 
made larger than the thickness T1 of the inner stators 2 and 6 (T1&lt;T2), 
because the latter 2 and 6 may not be constant in thickness (T1). This 
difference is the gap G in the depth of each of the cuts 1a and 5a. 
However, with the inside stators 2 and 6 fixedly secured, the shifted 
surface 2X of each protrusion 2a is held by the second holding section 5d 
while the shifted surface 6X of each protrusion 6a is held by the first 
holding section 1d. Thus, the inside stators 2 and 6 are positioned in the 
axial position, setting between the gaps G and G. 
The protrusions 2a and 6a are positioned not only in the axial direction 
but also in the circumferential direction. And the protrusions 2a and 6a 
are substantially equal in circumferential length to the cuts 1a and 5a. 
Hence, when the protrusions 2a and 6a are engaged with the cuts 1a and 5a, 
the former scarcely play in the circumferential direction. That is, the 
inside stators 2 and 6 are not turned with respect to the outside stators 
1 and 5; that is, they are fixedly positioned in the circumferential 
direction. If the stators are circumferentially positioned at many points, 
then it may be rather difficulty to make the position of the stators high 
in dimensional accuracy. Hence, it is preferable that the number of stator 
positioning points is only one. 
In the above-described embodiment, the inside stators are axially held by 
abutting the protrusions 2a of the inside stator 2 against the outside 
stator 5 and those 6a of the inside stator 6 against the outside stator 1. 
Hence, the line L along which the two outside stators 1 and 5 are abutted 
against each other is in alignment with the line M along which the two 
inside stators 2 and 6 are abutted against each other. 
Furthermore, in the embodiment, as shown in FIG. 6, two of the plurality of 
cuts 1a of one outside stator 1 are made integral with the large cut 1c in 
such a manner that the ends of the cuts 1a which are adjacent to each 
other are continuous with the large cut 1c of the outer stator 1. 
Similarly, two of the plurality of cuts 5a of the other outside stator 5 
are made integral with the large cut 5c in such a manner that the ends of 
the cuts 5a which are adjacent to each other are continuous with the large 
cut 5c of the outer stator 5. That is, the points of engagement of the 
protrusions 2a and 6a and the cuts 1a and 5a are not uniform in 
circumferential interval. However, if, as in a second embodiment shown in 
FIGS. 7 through 10 below, the points of engagement of them are arranged 
uniform in circumferential direction, then it is possible to further 
improve the accuracy of the small electric motor. 
Second Embodiment 
The second embodiment of the invention is equal in fundamental arrangement 
to the above-described first embodiment; however, the former is different 
from the latter in the configuration and position of the protrusions 2a 
and 6a and the configuration and position of the cuts 1a and 5a. In the 
second embodiment, parts corresponding functionally to those in the 
above-described first embodiment are therefore designated by the same 
reference numerals or characters. 
In the second embodiment, as shown in FIG. 7, the inside stators 2 and 6 
have the same shape protrusions 2a and 6a at angular intervals of 
120.degree., respectively. However, it should be noted that those 
protrusions 2a and 6a are not symmetrical with respect to the sectional 
line B--B (FIG. 7) passing through the center of the rotor as is seen from 
FIG. 7. On the other hand, as shown in FIG. 9, the outside stators 1 and 5 
have the same shape cuts 1a and 5a at angular intervals of 120.degree. in 
correspondence to the positions of the protrusions 2a and 6a. In other 
words, as shown in FIG. 10, the small electric motor has three pairs of 
protrusions 2a and 6a, and accordingly three pairs of cuts 1a and 5a in 
its outer periphery at angular intervals of 120.degree.. The two inside 
stators are laid on each other as follows: First, as shown in FIG. 8, the 
inside stators 2 and 6, which are equal in configuration to each other and 
have the pole teeth which are bent and extended in the same direction, are 
set back to back in such a manner that the pole teeth are extended in the 
opposite directions. The inside stators 2 and 6 thus set are fixedly 
secured to each other. In the second embodiment, at each of the 
engagements of the protrusions 2a and 6a, the latter are shifted from each 
other, thus having shifted-portions (surfaces) 2X and 6X. Hence, all the 
protrusions 2a and 6a thus engaged are axially positioned. This feature 
improves the small electric motor in the accuracy of performance. The two 
(first and second) outside stators 1 and 5 are positioned and held in the 
same manner as in the above-described first embodiment. In the 
above-described embodiment, the number of pairs of protrusions 2a and 6a 
are three; however, the invention is not limited thereto or thereby--it 
may be two or four. For instance, if the protrusions 2a and 6a are made 
larger in width, then they are increased in mechanical strength, and 
therefore in this case, the motor may have only two pair of protrusions 2a 
and 6a. 
Third Embodiment 
Now, a third embodiment of the invention will be described with reference 
to FIG. 11. 
In the conventional art shown in FIGS. 13 through 16, the joined inside 
stators 32 and 36 are not axially held; i.e., they may be shifted in one 
direction between the gaps G. This difficulty is eliminated by the third 
embodiment as follows: that is, in the third embodiment, the second 
holding sections 5d of the outside stator 5, which are adapted to hold the 
protrusions 2a of the inner stators 2, are formed on the bobbin 11; while 
the first holding sections 1d of the outside stator 1, which are adapted 
to hold the protrusions 6a of the inner stator 6, are formed on the bobbin 
10. In FIG. 11 showing the third embodiment, parts corresponding 
functionally to those already described with reference to FIGS. 13 through 
16, showing the conventional art, are therefore designated by the same 
reference numerals or characters. The inside stators 32 and 36 are in the 
same shape. 
In the third embodiment, similarly as in the above-described conventional 
art, the inside stators 32 and 36 are fixedly set back to back. On the 
bobbin 10 of the upper (first) stator unit, three position holding 
protrusions 10b are provided which correspond to the first holding 
sections 1d of the first embodiment and are adapted to axially hold the 
inside stator; while on the bobbin 11 of the lower (second) stator unit, 
three position holding protrusions 11b are provided which correspond to 
the second holding sections 5d of the first embodiment and are adapted to 
axially hold the inside stator. The inside stator 32 has through-holes 32f 
in such a manner that the position holding protrusions 10b are inserted 
into the through-holes 32f and abutted against the other inside stator 36. 
In other words, the inside stator 36 is axially positioned by the ends of 
the position holding protrusions 10b of the bobbin 10 of the upper (first) 
stator unit. Similarly, the inside stator 36 also has through-holes 36f in 
such a manner that the position holding protrusions 11b are inserted into 
the through-holes 36f and abutted against the other inside stator 32. In 
other words, the inside stator 32 is axially positioned by the ends of the 
position holding protrusions 11b of the bobbin 11 of the lower stator 
unit. 
While there has been described in connection with the preferred embodiments 
of the invention, it will be obvious to those skilled in the art that 
various changes and modifications may be made therein without departing 
from the invention. For instance, the first and second holding section may 
be formed on positioning boards provided on the sides of the outside 
stators, instead of the outside stators or bobbins. Furthermore, as shown 
in FIG. 12, the protrusions 2a and 6a may be so arranged that they are not 
overlapped with each other all. The outside stators 1 and 5 should have at 
least one cut 1a and one cut 5a, respectively and the inside stators 2 and 
6 should have at least one protrusion 2a and one protrusion 6a, 
respectively; however, in order to assemble the motor with higher 
accuracy, it is preferable that they have a plurality of cuts and 
protrusions. In addition, in the inside and outside stators, the 
arrangement of the protrusions and cuts at equal angular intervals 
contributes greatly to the improvement in accuracy of the motor assembly 
work. 
The invention has been described with reference to the small electric 
motor; however, it goes without saying that the technical concept of the 
invention may be applied to a general small electric motor in which a 
plurality of stator units are laid one on another. 
As is apparent from the above description, in the small electric motor, the 
first holding section of the first outside stator positions the second 
inside stator, while the second holding section of the second outside 
stator positions the first inside stator. This feature makes it possible 
to hold the two inside stators at the designed center. Therefore, the 
motor thus formed is free from the difficulty that the inside stators are 
held oblique with respect to other members such as the outside stators and 
bobbins. Hence, the gap between the rotor and the inside stator has the 
designed value. The resultant small electric motor has been designed as 
planned.