This invention relates to an improvement in a structure of a thin
DC brushless motor and, more particularly, to a bearing structure of a thin DC
A thin DC brushless motor has been frequently used as a rotary drive
source of a floppy disk drive for a personal computer. Fig. 4 is a sectional view
showing a conventional thin DC brushless motor illustrating the state that a rotor
is rotatably provided on a motor mounting plate formed of a printed circuit board.
In Fig. 4, numeral 1 denotes a board as a motor mounting plate, which is made of
a ferromagnetic material such as steel and on which a printed wiring circuit is
formed on the surface through a thin insulating layer, and an electronic component
such as an IC for a motor drive circuit is mounted. A circular hole 2 is perforated
at the board 1, a bearing base 3 is engaged within the circular hole 2, and clamped
at the board with screws 4. Two ball bearings 5 and 6 are secured to the bearing
Numeral 7 designates a rotating disk for rotating the floppy disk
to constitute a rotor in a thin circular dish shape. The rotating poles 9 made
of a ringlike permanent magnet is fixed to the inside of the outer periphery 8
of the rotating disk 7. A speed detecting permanent magnet 10 for detecting the
rotating speed of the rotating disk 7 is fixed to the lower part of the outer periphery
8 of the rotating disk 7. The poles 9 made of the permanent magnet is formed of
rare earth metal or the like in a ring shape as shown in Fig. 5. A magnetic shielding
plate 30 formed of soft iron similarly in a ring shape is engaged outside the
poles 9. A plurality of permanent magnets are formed at the poles 9 by magnetizing.
These permanent magnets are magnetized in the radial direction of the poles 9 in
such a manner that the polarity directions of these poles are opposite at the
adjacent poles. A rotating disk 7 assembly including the poles 9 constitutes a
A rotating shaft mounting plate 11 made of brass is fixed to the
center of the rotating disk 7. A rotating shaft 12 is fixed to the center of the
rotating shaft mounting plate 11. The rotating shaft 12 is rotatably mounted at
the ball bearings 5 and 6. In a conventional thin DC brushless motor shown in Figs.
4 and 5, the bearing base 3 for holding the ball bearings 5, 6 for rotatably supporting
the rotating shaft 12 is constructed separately from a stator frame 23 having stator
poles 21. However, such a thin DC brushless motor has been required to be reduced
in size and thickness without decreasing its output in the development of a light,
thin, short and small articles.
Therefore, to maintain the output of the conventional thin DC brushless
motor in a smaller space, a gap G between the rotating poles 9 and the stator poles
21 is reduced to its critical limit. In this respect, the squareness of the rotating
shaft 12 to the stator poles 21 becomes a problem. The squareness cannot be obtained
in a stable accuracy since the stator poles 21 and the bearing base 3 for receiving
the bearings 5, 6 are formed separately so far and respectively fixed to the board
This invention is constituted in view of the above-described circumstances,
and an object of this invention is not only to facilitate the squareness management
of the bearing mechanism for the stator poles 21 but also to reduce the cost by
the decrease in the number of components.
There has been a method of reducing a gap between a rotor and a stator
as means to need to raise the efficiency of a magnetic circuit to obtain the same
output as the conventional motor with a reduced size. However, if the gap is reduced,
the gap is further decreased from a set value due to the inclination of the rotor
shaft (rotating shaft) to the stator and the concentricity of the stator and the
rotor, and hence the contact of the stator with the rotor may occur. A motor structure
that the concentricity of the stator with the rotor without the inclination can
be manufactured by integrating both the stator and the housing of the bearing
since the inclination of the shaft and the coaxiality a determined according to
the accuracy of the stator and the housing of the bearings.
JP-A-04183287 discloses a motor structure having a stator including
armature coils wound on fixed poles of a stator core.
According to the present invention, a motor structure comprises a
stator including armature coils wound on fixed poles of a laminated stator core
comprising stator plates arranged in parallel on the surface of a metal motor
mounting board having an electric circuit thereon, the motor construction further
comprising a bearing provided at the centre of the stator for rotatably supporting
a rotor shaft which holds a rotor oppositely to the stator, wherein the stator
core is coupled with a housing for the bearing by a molded form which is integral
with the housing and is characterized in that a conductive material is provided
on the surface of the housing to electrically connect the rotor via the rotating
shaft and the bearing to either the metal part of the metal motor mounting board
or a ground circuit of the electric circuit which is insulated from the metal part
or both the metal part of the metal motor mounting board and a ground circuit of
the electric circuit which is insulated from the metal part.
The stator is integrated with the housing for constituting the bearing
of the rotating shaft thereby to reduce the number of the components. Further,
the coaxiality of the stator core with the rotor magnet of particularly important
accuracy to determine the characteristics becomes accurate, and hence the stable
motor structure having small unevenness can be obtained.
Fig. 1 is a sectional view showing an embodiment of this invention.
Fig. 2 is a partial perspective view showing a stator core.
Fig. 3 is a plane view showing the motor of the embodiment of this
invention partly in cutout section of the rotor.
Fig. 4 is a sectional view showing a conventional motor structure.
Fig. 5 is a perspective view showing a rotor magnet.
An embodiment of this invention will be described in detail with
respect to the accompanying drawings. Fig. 1 is a sectional view of a thin DC brushless
motor according to this invention. In Fig. 1, numeral 51 denotes a stator core.
The stator core 51 is formed, as shown in Fig. 2, by adhering or coating the surface
of a laminate of stator plates 51' made of soft magnetic metal thin plates with
an insulating layer 52 for insulating to an armature coil 62. Numeral 57 designates
a stator. An armature coil 62 is wound on the surface of the stator core 51. Numeral
53 indicates a housing for fixedly containing ball bearings 54 as bearings of
a rotating shaft 58. Numeral 55 depicts a stator frame material for coupling the
housing 53 and the stator 57 with synthetic resin. The detailed description will
be explained later.
The rotating shaft 58 is supported by the ball bearings 54. A hub
base 56 is fixed to the rotating shaft 58.Further, a rotor yoke 59 made of soft
iron plate in a thin disk shape is mounted on the outer periphery of the hub base
56. A rotor magnet 60 is fixed as the rotating poles similarly to the prior art
as shown in Fig. 5 to the inside of the outer periphery of the rotor yoke 59. A
gap G' exists between the rotor magnet 60 and the end of the stator core 51. Numeral
61 denotes a permanent magnet for detecting a speed similarly to the prior art.
A rotor 63 is formed of the hub base 56, the rotor yoke 59, the rotor magnet 60
and the speed detecting permanent magnet 61.
A method of manufacturing, the structure of a stator assembly 64
and a method of mounting it on a board 65 as a motor mounting plate similar to
the conventional motor having a printed wiring circuit according to the features
of this invention will be described. The stator core 51 having the insulating
layer 52 for insulating to the armature coil 62 and the housing 53 for holding
the ball bearings 54 as the bearers of the rotating shaft 58 of the rotor 63 are
integrally molded by an injection molding using synthetic resin for injection
molds made of liquid crystal polymer resin or the like.
In the molds used at the time of the integral molding, the position
of the hole for containing the ball bearings 54 is so accurately determined with
the outer periphery of the stator core 51 as a reference as to be the center of
the outer periphery of the stator core 51, and the position of the hole for containing
the bearings 54 to be provided in the housing 53 to contain the two bearings 54
is so determined that the rotating shaft 58 is held to be rotated perpendicularly
to the reference surface 66 of the stator assembly 64. At the time of molding the
stator assembly 64, bosses 67 for positioning on a board 65 are provided, and a
conductive plate 68 for discharging static electricity generated on the disk to
be rotatably driven on the rotor 63 is integrally molded on the metal plate for
constituting the ground circuit of the printed wiring circuit of the board 65
or the board by the rotating shaft 58 by disposing on the surface of the housing
53 in contact with the disk to be rotatably driven. A protrusion 53' to be engaged
with the hole provided at the board 65 is provided at the lower surface of the
To reduce the magnetic reluctance of the entire stator structure,
soft magnetic powder such as iron oxide may be mixed with the synthetic resin for
the injection molding. The above-described soft magnetic powder may also be mixed
within the insulating layer 52 for covering the surface of the stator plate 51'.
In the embodiment described above, the rotating shaft is supported to the ball
bearings 54 held in the housing.
However, the ball bearings 54 may be omitted, and a structure for
rotatably supporting the rotating shaft in a bearing hole provided in the housing
may be employed.
After the armature coil 62 is wound on the stator core 51 of the
stator assembly 64 formed as described above, the ball bearings 54 are mounted
in the hole of the housing 53 with the outer periphery of the stator core 51 as
a reference, the bosses 67 and the protrusion 53' of the housing 53 are engaged
with the positioning hole provided at the board 65, and the stator assembly 64
is mounted on the board 65 with mounting screws 69. Though not shown, at this
time, the conductive plate 68 is contacted with the metal part of the metal motor
mounting plate or a ground circuit of the printed wiring circuit of the board 65
or with the both.
In the case of assembling the rotor 63, the rotating shaft 58 is
first press-fitted in the hub base 56, the hub base 56 is inserted into the rotor
yoke 59, caulked, the coaxiality of the rotating shaft 58 with the inner periphery
of the rotor magnet 60 is obtained, and the rotor magnet 60 is then adhesively
secured to the rotor yoke 59 at the inner periphery of the rotor yoke 59 in this
Then, the rotating shaft 58 of the rotor is inserted into the ball
bearings 54 of the stator assembly 64 mounted on the board 65 as described above,
and a preload is applied to the ball bearings 54 to adhesively adhere theinner
races of the ball bearings 54 to the rotating shaft 58. Fig. 3 is a plane view
showing the rotor of the motor assembled partly in section as described above.
This invention has been described with respect to the embodiments
described above. However, the invention in its broader aspects is not limited to
the specific details, and representative devices shown and described herein.
Accordingly, various modifications may be made without departing
from the scope of the invention as defined by the appended claims. For example,
the material of the molding resin of the stator assembly may also be made of ABS
resin. The material of the conductive plate may be made of conductive rubber so
as not to incorporate the conductivity with the rotating shaft, or the shape may
be formed in a square sectional shape. Further, the ball bearings may be integrated
together with the stator assembly at the time of molding to improve the positional
accuracy of the stator assembly with the ball bearings.
According to this invention as described above, the housing for constituting
the stator and the bearing of the stator which have been heretofore separate can
Therefore, the number of the components can be deleted. The coaxiality
of the stator core with the rotor magnet of particularly important accuracy to
determined the characteristics of the motor structure is enhanced to be less uneven