Bearing system and brushless DC motor using such system

A bearing system for brushless DC motors is disclosed. The bearing system has an electromagnetic force generator in a bearing so as to generate an electromagnetic force or a magnetic force. The bearing system thus magnetically attracts the shaft when eccentricity of the shaft in the bearing is reduced during a high speed operation of the shaft, thus allowing the shaft to keep desired eccentricity in the bearing and increasing dynamic pressure of oil in the bearing and preventing a formation of an oil whirl, and improving dynamic characteristics of the motor such as low operational vibrations and noises during a high speed operation of the motor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates, in general, to brushless DC motors and, more 
particularly, to a bearing system capable of allowing the gap between the 
shaft and bearing of a brushless DC motor to keep desired eccentricity 
when the shaft is operated at high speeds, thus maintaining a dynamic 
pressure at a position between the shaft and bearing and preventing a 
formation of an oil whirl during a high speed operation of the motor, and 
improving a motor's dynamic characteristics, such as low operational 
vibrations and noises. The present invention also relates to a brushless 
DC motor using such a bearing system. 
2. Description of the Prior Art 
As well known to those skilled in the art, small-sized precision motors, 
typically used in office machines, are required to be designed to rotate 
at high speeds and provide dynamic characteristics of low operational 
vibrations and noises in order to meet the necessity of high speed 
operation and provide a large capacity of such office machines. Therefore, 
it is a recent trend to change bearings for such motors from ball bearings 
into hydrosintered or hydrodynamic bearings with excellent dynamic 
characteristics. 
FIG. 1 is a sectional view of a spindle motor using a conventional 
hydrodynamic bearing. As shown in the drawing, a bearing 1a is vertically 
and concentrically arranged on the base panel of a motor housing 1 through 
a fitting process, while a shaft 2 is rotatably and downwardly inserted 
into the bearing 1a. A core 1b, with a coil 1c, is arranged around the 
bearing 1a, thus forming a stator of the motor. The top end of the shaft 2 
is coupled to a cap-shaped rotor 3, thus being rotatable along with the 
rotor 3. A cylindrical magnet 3a is attached to the inner surface of the 
rotor's sidewall, thus surrounding the stator 1b. When the motor is 
started, electric power is applied to the coil 1c of the core 1b, thus 
allowing the magnet 3a to generate magnetic force. The rotor 3 is thus 
rotated along with the shaft 2 at high speeds. 
In an operation of the above spindle motor, the shaft 2 is rotated inside 
the bearing 1a at high speeds. In such a case, oil, filled in the gap 
between the bearing 1a and the shaft 2, generates a hydrodynamic pressure 
and effectively supports the shaft 2 in a radial direction during a high 
speed rotation of the shaft 2. When the shaft 2 is rotated at high speeds 
as described above, the rotor 3, carrying a disc (not shown) thereon, is 
rotated at high speeds, thus allowing data stored in the disc to be 
reproduced. 
However, such a conventional hydrodynamic bearing for spindle motors is 
problematic in that it generates an oil whirl, thus being unstable during 
an operation of the motor. Such an oil whirl is formed as bearing 
eccentricity of the gap between the bearing 1a and the shaft 2 is 
gradually reduced at a speed higher than a predetermined level. That is, 
an increase in the rotating speed of the shaft 2 causes the Sommer Felt 
number to be reduced, thus gradually reducing eccentricity of the shaft 2 
in the bearing 1a. Such a reduction in the eccentricity of the shaft 2 is 
caused when the oil, rotated along the shaft 2, has a given speed 
distribution. Such a reduction in eccentricity is typically formed in 
genuine circular hydrodynamic bearings free from dynamic pressure grooves. 
Therefore, the conventional hydrodynamic bearings reduce a motor's dynamic 
characteristics, such as low operational vibrations and noises, during a 
high speed operation of the motor. 
In an effort to overcome the above problems experienced in the above 
spindle motors, Japanese Patent Laid-open Publication No. Hei. 7-110,028 
discloses a hydrodynamic bearing. However, the above Japanese patent is 
problematic in that it is very difficult to form the dynamic pressure 
grooves on both the shaft and the bearing surface. Another disadvantage 
experienced in the above Japanese bearing is that the bearing reduces work 
efficiency while assembling the shaft with the bearing. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention has been made keeping in mind the above 
problems occurring in the prior art, and an object of the present 
invention is to provide a bearing system for brushless DC motors, which is 
capable of allowing the gap between the shaft and bearing of a motor to 
keep desired eccentricity when the motor is operated at a speed higher 
than a predetermined level, thus maintaining a dynamic pressure at a 
position between the shaft and bearing and preventing a formation of an 
oil whirl during a high speed operation of the motor, and improving a 
motor's dynamic characteristics, such as low operational vibrations and 
noises. 
Another object of the present invention is to provide a brushless DC motor 
using such a bearing system. 
In order to accomplish the above objects, the bearing system for brushless 
DC motors in accordance with the primary embodiment of the present 
invention comprises: an inner wall portion interiorly and rotatably 
holding a shaft with a variable oil gap being defined between the shaft 
and the inner wall; an outer wall surrounding the inner wall and being 
used as a holding surface of the bearing system; at least one 
electromagnetic force generator adapted for selectively generating an 
electromagnetic force so as to magnetically and radially attract the 
shaft, thus forming a dynamic pressure in oil in the oil gap, the 
electromagnetic force generator being positioned at an intermediate 
portion between the inner and outer walls; top and bottom walls 
integrating the inner and outer walls into a single structure at the top 
and bottom ends of the inner and outer walls; and a controller adapted for 
selectively turning on the electromagnetic force generator, thus allowing 
the electromagnetic force generator to magnetically attract the shaft when 
the dynamic pressure of the oil is reduced during a high speed operation 
of the shaft.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 is a sectional view of a brushless DC motor using a bearing system 
in accordance with the primary embodiment of this invention. As shown in 
the drawing, the motor is provided with a base panel 10 for holding a 
bearing 11, which forms a bearing system of the motor. A stator 14, with a 
coil 15, is arranged around the bearing 11 on the base panel 10. A shaft 
12 is rotatably fitted into the bearing 11, while a rotor holder 16 is 
fitted over the top end portion of the shaft 12, thus being rotatable 
along with the shaft 12. A rotor 17 is concentrically fitted over the 
rotor holder 16, thus being rotatable along with the rotor holder 16. The 
rotor 17 is opened downwardly and has a cylindrical sidewall which 
surrounds the stator 14. A cylindrical magnet 18 is attached to the inner 
surface of the rotor's sidewall with an air gap being formed between the 
stator 14 and the magnet 18. When the motor is started, electric power is 
applied to the coil 15 of the stator 14, thus allowing the magnet 18 to 
generate magnetic force. The rotor 17 is thus rotated along with the shaft 
12 at high speeds. 
The bearing 11 also has an electromagnetic force generator 19, which allows 
the shaft 12 to keep desired eccentricity during a high speed operation of 
the shaft 12. The above electromagnetic force generator 19 thus prevents a 
formation of an oil whirl when the shaft 12 is rotated at a speed higher 
than a predetermined level. The electromagnetic force generator 19 
attracts the shaft 12 in a direction, thus increasing eccentricity of the 
shaft 12 in the bearing 11. In the preferred embodiment, the 
electromagnetic force generator 19 is comprised of an electromagnet 20, 
which is interiorly installed in the bearing 11. FIG. 3 is an exploded 
perspective view showing the construction of the electromagnetic force 
generator, while FIG. 4 is a plan sectional view of the electromagnetic 
force generator. As shown in the drawings, the electromagnet 20 has a 
rectangular configuration and the bearing 11 has a fitting part 23 for 
carrying the electromagnet 20. 
The electromagnet 20 has four electromagnetic force generating parts 21, 
each of which extends in a direction perpendicular to the shaft 12. A coil 
22 is wound around each of the electromagnetic force generating parts 21 
and is selectively turned on in accordance with a rotating speed of the 
shaft 12. That is, when the shaft 12 is rotated at high speeds and results 
in a reduction in eccentricity of the shaft 12, the coils 22 of the parts 
21 are turned on, while the parts 21 generate electromagnetic force at the 
same time, thus attracting the shaft 12 to the electromagnet 20. In the 
drawings, the reference numeral 24 denotes a controller, which checks the 
rotating speed of the shaft 12 and selectively outputs a start signal for 
the electromagnet 20 when the rotating speed of the shaft 12 exceeds a 
predetermined level. 
In a brief description, the bearing system according to the primary 
embodiment has an electromagnetic force generator 19, which allows the 
shaft 12 to keep desired eccentricity during a high speed operation of the 
shaft 12. The electromagnetic force generator 19 thus prevents a reduction 
in eccentricity of the shaft 12 in the bearing during a high speed 
operation of the shaft 12, thus preventing a reduction in the dynamic 
pressure of the gap between the bearing 11 and the shaft 12 and removing 
an oil whirl from the gap. The electromagnetic force generator 19 is 
comprised of an electromagnet 20, which is fitted into the fitting part 23 
of the bearing 11. The electromagnet 20 has four electromagnetic force 
generating parts 21, each of which extends in a direction perpendicular to 
the shaft 12. A coil 22 is wound around each of the electromagnetic force 
generating parts 21 and is selectively turned on in accordance with a 
rotating speed of the shaft 12. That is, when the shaft 12 is rotated at 
high speeds and results in a reduction in eccentricity of the shaft 12, 
the coils 22 of the parts 21 are turned on, while the parts 21 generate 
electromagnetic force. 
When the shaft 12 is rotated at high speeds, the eccentricity of the shaft 
may be reduced. In such a case, the rotating speed of the shaft 12 is 
sensed by the controller 24 and the controller 24 selectively outputs a 
start signal to the coils 22, thus turning on the coils 22. When the coils 
22 are turned on, the electromagnetic force generating parts 21 of the 
electromagnet 20 generate an electromagnetic force, thus attracting the 
shaft 12 to the electromagnet 20. Therefore, the shaft 12 is forcibly and 
eccentrically positioned in the bearing 11, thus effectively increasing 
the dynamic pressure of oil 13 and removing the oil whirl, and improving 
dynamic characteristics of the motor during a high speed operation of the 
motor. 
FIGS. 5 to 7 show a brushless DC motor using a bearing system in accordance 
with the second embodiment of this invention. In the second embodiment, 
most of the elements are common with those of the primary embodiment. 
Those elements common to both the primary and second embodiments will thus 
carry the same reference numerals. In the bearing system according to the 
second embodiment, an electromagnetic force generator 19 is interiorly 
provided on the bearing 11 for electromagnetically attracting the shaft 
12. The electromagnetic force generator 19 thus effectively prevents a 
formation of an oil whirl when the shaft 12 is rotated at a speed higher 
than a predetermined level. The electromagnetic force generator 19 
electromagnetically attracts the shaft 12 in a desired direction, thus 
increasing eccentricity of the shaft 12 in the bearing 11. 
In the second embodiment, the electromagnetic force generator 19 is 
comprised of a plurality of electromagnets 20, which are radially arranged 
in the bearing 11. Each of the electromagnets 20 has a rectangular 
configuration, while the bearing 11 has a plurality of fitting parts 23 
for carrying the electromagnets 20 respectively. Each of the 
electromagnets 20 has two electromagnetic force generating parts or upper 
and lower parts 21, each of which extends in a direction perpendicular to 
the shaft 12. A coil 22 is wound around each of the electromagnetic force 
generating parts 21 and is selectively turned on in accordance with a 
rotating speed of the shaft 12. That is, when the shaft 12 is rotated at 
high speeds and results in a reduction in eccentricity of the shaft 12, 
the coils 22 of the parts 21 are turned on. Therefore, the electromagnetic 
force generating parts 21 generate electromagnetic force at the same time, 
thus attracting the shaft 12 to the electromagnet 20. 
In addition, a sensor coil 22a is provided at each of the electromagnetic 
force generating parts 21 for generating an induction current. The above 
sensor coil 22a generates a variable induction current in accordance with 
an interval between the shaft 12 and the electromagnet 20. The variable 
induction current of each sensor coil 22a is output to a controller 24 
through an amplifier 25, thus allowing the electromagnets 20 to be 
controlled under the control of the controller 24. That is, the sensor 
coils 22a of the electromagnets 20 are selectively turned on in response 
to a control signal from the controller, thus allowing the electromagnetic 
force generating parts 21 to generate an electromagnetic force so as to 
electromagnetically attract the shaft 12 in a desired direction. 
Therefore, eccentricity of the shaft 12 in the bearing 11 is increased. 
For example, when the shaft 12 is rotated at a high speed, the shaft 12 is 
electromagnetically attracted in a direction by the electromagnets 20, 
thereby being rotated while being eccentrically positioned in the bearing 
11. The shaft 12 is also rotated in the bearing 11 while being 
continuously and eccentrically positioned relative to the electromagnets 
20. That is, the shaft 12 can be rotatable in the bearing 11 through a 
scroll-type rotation suitable for improving compressibility of oil 13. 
In a brief description, the electromagnetic force generator 19 according to 
the second embodiment is comprised of a plurality of electromagnets 20, 
which are radially arranged in the bearing 11. The electromagnets 20 are 
fitted into the fitting parts 23 of the bearing 11. Each of the 
electromagnets 20 has two electromagnetic force generating parts 21, each 
of which extends in a direction perpendicular to the shaft 12 and has a 
coil 22. When the coils 22 of the parts 21 are turned on, the 
electromagnetic force generating parts 21 generate electromagnetic force 
at the same time, thus attracting the shaft 12 to the electromagnets 20. 
In addition, a sensor coil 22a is provided at each of the electromagnetic 
force generating parts 21 for generating an induction current. The above 
sensor coil 22a generates a variable induction current in accordance with 
an interval between the shaft 12 and the electromagnet 20. The variable 
induction current of each sensor coil 22a is output to the controller 24 
through the amplifier 25. Therefore, each of the electromagnets 20 is 
appropriately controlled by the controller 24 in response to an induction 
current of associated sensor coils 22a. 
The shaft 12 is controlled as follows. That is, when the shaft 12 is 
rotated at high speeds, the eccentricity of the shaft may be reduced. In 
such a case, the sensor coils 22a of the electromagnets 20 generate 
variable induction currents in accordance with both the distances between 
the shaft 12 and the coils 22a and a rotating speed of the shaft 12. The 
variable induction current of each of the sensor coils 22a is output to 
the controller 24 through the amplifier 25. Upon receiving the induction 
current from the sensor coils 22a, the controller 24 outputs a start 
signal to the coils 22 of the electromagnetic force generating parts 21, 
thus turning on the coils 22. When the coils 22 are turned on, the 
electromagnetic force generating parts 21 of the electromagnet 20 generate 
an electromagnetic force, thereby attracting the shaft 12 to the 
electromagnet 20. Therefore, the shaft 12 is forcibly and eccentrically 
positioned in the bearing 11 and effectively increases the dynamic 
pressure of oil 13 and removes the oil whirl, and improving dynamic 
characteristics of the motor during a high speed operation of the motor. 
The shaft 12 is also rotatable in the bearing 11 through a scroll-type 
rotation suitable for improving compressibility of oil 13. 
FIGS. 8 to 10 show a brushless DC motor using a bearing system in 
accordance with the third embodiment of this invention. In the third 
embodiment, most of the elements are common with those of the primary 
embodiment. Those elements common to both the primary and third 
embodiments will thus carry the same reference numerals. In the bearing 
system according to the third embodiment, an electromagnetic force 
generator 19 is interiorly provided on the bearing 11 for 
electromagnetically attracting the shaft 12. The electromagnetic force 
generator 19 thus effectively prevents a formation of an oil whirl when 
the shaft 12 is rotated at a speed higher than a predetermined level. The 
electromagnetic force generator 19 electromagnetically attracts the shaft 
12 in a desired direction, thus increasing eccentricity of the shaft 12 in 
the bearing 11. In the third embodiment, the electromagnetic force 
generator 19 is comprised of a plurality of electromagnets 20, which are 
radially arranged in the bearing 11 while being spaced apart from each 
other in an axial direction of the shaft 12. Each of the electromagnets 20 
has a rectangular configuration, while the bearing 11 has a plurality of 
upper and lower fitting parts 23 for carrying the electromagnets 20 
respectively. 
Each of the electromagnets 20 has two electromagnetic force generating 
parts 21, each of which extends in a direction perpendicular to the shaft 
12. A coil 22 is wound around each of the electromagnetic force generating 
parts 21 and is selectively turned on in accordance with a rotating speed 
of the shaft 12. That is, when the shaft 12 is rotated at high speeds and 
results in a reduction in eccentricity of the shaft 12, the coils 22 of 
the parts 21 are turned on. Therefore, the electromagnetic force 
generating parts 21 generate electromagnetic force at the same time, thus 
attracting the shaft 12 to the electromagnet 20. In addition, a sensor 
coil 22a is provided at each of the electromagnetic force generating parts 
21 for generating an induction current. 
The above sensor coil 22a generates a variable induction current in 
accordance with an interval between the shaft 12 and the electromagnet 20. 
The variable induction current of each sensor coil 22a is output to a 
controller 24 through an amplifier 25, thus allowing the electromagnets 20 
to be controlled under the control of the controller 24. That is, the 
sensor coils 22a of the electromagnets 20 are selectively turned on in 
response to a control signal from the controller 24, thus allowing the 
electromagnetic force generating parts 21 to generate an electromagnetic 
force so as to electromagnetically attract the shaft 12 in a desired 
direction. For example, when the shaft 12 is rotated at a high speed, the 
shaft 12 is electromagnetically attracted in a direction by the 
electromagnets 20, thereby being rotated while being eccentrically 
positioned in the bearing 11. The shaft 12 is also rotated in the bearing 
11 while being continuously and eccentrically positioned relative to the 
electromagnets 20. That is, the shaft 12 can be rotatable in the bearing 
11 through a scroll-type rotation suitable for improving compressibility 
of oil 13. When the electromagnets 20, which are placed at the upper and 
lower positions in the axial direction of the shaft 12, are simultaneously 
or alternately operated, the shaft 12 may be selectively operated in a 
parallel vibration mode as shown in FIG. 11a or a conical vibration mode 
as shown in FIG. 11b. 
In a brief description, the electromagnetic force generator 19 according to 
the third embodiment is comprised of a plurality of electromagnets 20, 
which are radially arranged in the bearing 11 at upper and lower positions 
in the axial direction of the shaft 12. The electromagnets 20 are fitted 
into the upper and lower fitting parts 23 of the bearing 11. Each of the 
electromagnets 20 has two electromagnetic force generating parts 21, each 
of which extends in a direction perpendicular to the shaft 12 and has a 
coil 22. When the coils 22 of the parts 21 are turned on, the 
electromagnetic force generating parts 21 generate electromagnetic force 
at the same time. 
In addition, a sensor coil 22a is provided at each of the electromagnetic 
force generating parts 21 for generating an induction current. The above 
sensor coil 22a generates a variable induction current in accordance with 
an interval between the shaft 12 and the electromagnet 20. The variable 
induction current of each sensor coil 22a is output to a controller 24 
through an amplifier 25. Therefore, each of the electromagnets 20 is 
appropriately controlled by the controller 24 in response to an induction 
current of associated sensor coils 22a. 
FIG. 11a shows the shaft 12 in a parallel vibration mode. In such a 
parallel vibration mode, the electromagnets 20, which are placed at the 
upper and lower positions in the axial direction of the shaft 12, are 
simultaneously operated. Meanwhile, FIG. 11b shows the shaft 12 in a 
conical vibration mode. In such a conical vibration mode, the 
electromagnets 20, placed at the upper and lower positions in the axial 
direction of the shaft 12, are alternately operated. Therefore, the shaft 
12 is forcibly and eccentrically positioned in the bearing 11 and 
effectively increases the dynamic pressure of oil 13 and removes the oil 
whirl, and improving dynamic characteristics of the motor during a high 
speed operation of the motor. The shaft 12 is also rotatable in the 
bearing 11 through a scroll-type rotation suitable for improving 
compressibility of oil 13. 
FIGS. 12 and 13 show a brushless DC motor using a bearing system in 
accordance with the fourth embodiment of this invention. In the fourth 
embodiment, most of the elements are common with those of the primary 
embodiment. Those elements common to both the primary and fourth 
embodiments will thus carry the same reference numerals. In the bearing 
system according to the fourth embodiment, an electromagnetic force 
generator 19 is interiorly provided on the bearing 11 for 
electromagnetically attracting the shaft 12. In the third embodiment, the 
electromagnetic force generator 19 is comprised of a permanent magnet 20, 
which is provided in the bearing 11. The magnet 20 has a rectangular 
configuration, while the bearing 11 has a fitting part 23 for carrying the 
magnet 20. The magnet 20 has a plurality of magnetic force generating 
parts 21, each of which extends in a direction perpendicular to the shaft 
12. Each of the magnetic force generating parts 21 is magnetized so as to 
effectively keep a desired magnetic flux density capable of magnetically 
attracting the shaft 12 to the magnet 20. 
In a brief description, the bearing system according to the fourth 
embodiment has an electromagnetic force generator 19, which allows the 
shaft 12 to be forcibly and eccentrically positioned in the bearing 11. 
That is, the electromagnetic force generator 19 magnetically attracts the 
shaft 12 during a high speed operation of the shaft 12, thus making the 
shaft 12 rotate while being eccentrically positioned in the bearing 11. 
The bearing system thus effectively prevents any reduction in the dynamic 
pressure of oil 13 and removes the oil whirl. In the third embodiment, the 
electromagnetic force generator 19 is comprised of the permanent magnet 
20, which is fitted into the fitting part 23 of the bearing 11. The magnet 
20 has a plurality of magnetic force generating parts 21, each of which 
extends in a direction perpendicular to the shaft 12. Each of the magnetic 
force generating parts 21 is magnetized so as to effectively keep a 
desired magnetic flux density capable of magnetically attracting the shaft 
12 to the magnet 20. In the operation of the motor, the magnetic force 
generating parts 21 generate a magnetic force, thus magnetically 
attracting the shaft 12 in a direction. Therefore, during a high speed 
operation of the shaft 12, the shaft 12 is rotated while being 
eccentrically positioned in the bearing 11. The bearing system thus 
effectively prevents any reduction in the dynamic pressure of oil 13 and 
removes the oil whirl, and improves dynamic characteristics of the motor. 
As described above, the present invention provides a bearing system for 
brushless DC motors. The bearing system has an electromagnetic force 
generator in a bearing so as to generate an electromagnetic force or a 
magnetic force. The bearing system thus magnetically attracts the shaft 
when eccentricity of the shaft in the bearing is reduced during a high 
speed operation of the shaft, thus allowing the shaft to keep desired 
eccentricity in the bearing and increasing dynamic pressure of oil in the 
bearing and improving dynamic characteristics of the motor such as low 
operational vibrations and noises during a high speed operation of the 
motor. 
Although the preferred embodiments of the present invention have been 
disclosed for illustrative purposes, those skilled in the art will 
appreciate that various modifications, additions and substitutions are 
possible, without departing from the scope and spirit of the invention as 
disclosed in the accompanying claims.