Spindle motor

A spindle motor comprises a rotational member supported by bearings for rotation about a rotational axis, a rotational body and a motor rotor mounted on the rotational member for rotation therewith, and a motor stator mounted around the motor rotor. The rotational body has an outer diameter determined so that a windage loss of the rotational body is proportional to the third power of a peripheral velocity of the rotational body and an air gap diameter of the bearings and the diameter of the motor stator are determined so that the air loss of each of the bearings and the motor rotor is proportional to the square of the peripheral velocity of each of the bearings and the motor rotor at a range of constant speed of rotation of the rotational member.

BACKGROUND OF THE INVENTION 
The present invention relates to a polygon scanner motor used for laser 
scanning-like laser beam printers and copy machines, and a spindle motor 
applicable for VTR drum spindles. 
In the prior art polygon scanner motor, ball bearings generally have been 
used for bearings and have been rotated at about 20000 rpm. 
Because of the recent acceleration of data processing, a polygon mirror 
needs accelerating revolution: more than 30000 rpm. At such revolution, a 
dynamic pressure air bearing using air as lubricating fluid is used 
because of the life of the bearing and to protect the polygon mirror from 
stains caused by scattered lubricant (see, for example, the Journal of 
Precision Society, Vol. 61, No. 9, 1995, Page 1284). 
As ambient noise of the polygon mirror becomes large by accelerating 
revolution of the above-mentioned polygon mirror, the polygon mirror 
portion tends to be shut tightly to prevent exposure to the ambient noise. 
Moreover, recent polygon scanner motors have required high rotational 
accuracy, miniaturization and acceleration. 
However, any attempt to make a polygon scanner motor having acceleration 
and high rotational accuracy(constant speed), and to miniaturize the 
polygon scanner as mentioned above results in the problems that the output 
of the motor becomes small and its heating value becomes large because the 
ratio Tloss/Tout of friction torque Tloss to output torque Tout becomes 
large, thereby worsening the efficiency of the polygon scanner motor. 
To solve the foregoing problems with the prior art, improvements are needed 
in both the generation of the output torque and the reduction of friction. 
To improve the generation of output torque, it is required to minimize the 
ratio Tout/.DELTA.T of output torque Tout to cogging torque (torque 
irregularity) .DELTA.T and to minimize rotating fluctuation. It is also 
required to minimize the ratio Wout/Wloss of output Wout to core loss 
Wloss. 
Next, to improve the reduction of friction, it is required to minimize the 
ratio Ti/Td of the drag torque Td to sudden torque Ti in order to maintain 
the rotating accuracy and the life of the bearing. It is also required to 
minimize the drag torque Td to keep the ratio Ti/Td small. 
Although it is proposed to improve the generation of the output torque to 
solve the foregoing problems with the prior art, it is difficult to 
miniaturize the motor without decreasing the output torque. Especially in 
a polygon scanner motor, the polygon mirror generally can not be 
miniaturized because the size of the polygon mirror is fixed. Thus, it 
becomes difficult to miniaturize the motor without decreasing the output 
torque. 
On the other hand, although it is proposed to miniaturize the bearing to 
decrease the generation of mechanical friction, it is undesirable because 
miniaturization of the bearing causes deterioration of life at a required 
accelerating rotation. 
Although it has not been easy to realize a motor having an accelerating 
rotation, a high rotation accuracy, and a miniaturized size, continued 
research has been undertaken by the inventors. 
As the result of the research, new knowledge has been gained. That is, it 
is generally considered that windage loss occurring at a polygon mirror 
which is loaded causes deterioration in efficiency which increases loss 
when the polygon scanner motor rotates, but even if the spindle motor is 
rotated at an accelerating rotation, miniaturization without rotation 
fluctuation is realized using windage loss occurring at the accelerating 
rotation. 
The polygon scanner motor has a problem that the polygon mirror moves at an 
accelerating rotation if the polygon mirror is not fixed tight on the 
rotational axis. Especially when the heating value of the polygon mirror 
increases because of an increase in windage loss by the accelerating 
rotation, the motor has a problem that a flange fixing the polygon mirror 
becomes loose at the rotational axis and the polygon mirror is easily 
moved when the temperature of the fixed portion falls after heating. 
In the case that the polygon mirror is set on the rotational axis through 
the flange, it needs to be machined for correction of run out in order to 
achieve squareness after the flange is assembled and fixed on the 
rotational axis. 
The present invention was born on the above-mentioned new knowledge. The 
object of the present invention is to provide a spindle motor having no 
rotation fluctuation, a miniaturized size, and a long life. 
Another object of the present invention is to provide a spindle motor 
having a load, such as a polygon mirror, which does not become loose and 
does not move after the load is fixed on the rotational axis. 
SUMMARY OF THE INVENTION 
The present invention comprises a shaft supported so as to rotate freely on 
bearings about a rotational axis, a rotational body and a motor rotor 
coaxially mounted on the shaft, and a motor stator fixed around the motor 
rotor. The outer diameter of the rotational body is determined so that 
windage loss of the rotational body is proportional to the third power of 
the peripheral velocity of the rotational body, and the air gap diameter 
of the bearings and the diameter of the motor stator are determined so 
that the air loss of each of the bearings and the motor rotor is 
proportional to the square of the peripheral velocity of each of the 
bearings and the motor rotor at the range of constant speed rotation of 
the shaft. According to the above-mentioned means, the present invention 
achieves the foregoing object. 
According to another aspect of the present invention, the rotational body 
is bored at its center and is mounted on a taper portion formed at one end 
of the shaft of the spindle motor. The rotational body is fixed to the 
taper portion of the shaft by tightening a screw in a direction opposite 
to the normal direction of rotation of the shaft. According to the 
above-mentioned means, the present invention achieves the foregoing 
object. 
In another aspect of the present invention, the rotational body comprises a 
polygon mirror covered with a case. According to the above-mentioned 
means, the present invention achieves the foregoing object. 
The present invention further achieves the above-mentioned object by the 
following means: the ratio .phi.b/.phi.p of the outer diameter .phi.p of 
the rational body to air gap diameter .phi.b of the bearings is limited to 
.phi.b/.phi.p .ltoreq.1/3, and the ratio .phi.m/.phi.p of the outer 
diameter .phi.p of the rational body to the outer diameter .phi.m of the 
motor rotor is limited to .phi.m/.phi.p .ltoreq.1/3 in the spindle motor. 
The present invention further achieves the above-mentioned object by the 
following means: the rotational body comprises a polygon mirror and a 
mounting or set portion for setting the polygon mirror, and the set 
portion is tightened by a screw to the taper portion of the shaft of the 
spindle motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The fundamental aspects of the present invention are described with 
reference to FIGS. 1-8. 
FIG. 1 is a view of the main components for explaining the fundamental 
aspects of the present invention. 
In the present invention shown in FIG. 1, a rotational member comprising a 
shaft 1 is supported by a pair of bearings 2 and 3 for undergoing free 
rotation about a rotational axis, a rotational body 4 comprising, for 
example, a circular plate or a polygon plate, and a motor rotor 5 are 
coaxially mounted on the shaft 1, a motor stator 6 is fixed around the 
motor rotor 5, and a motor 7 consists of the motor rotor 5 and the motor 
stator 6. 
By the foregoing construction of the present invention, the following 
points are observed: windage loss occurring on the rotational body 4 is 
used for its air damper; the effect of the air damper is proportional to 
the windage loss; and the windage loss suddenly increases when a 
peripheral velocity at the periphery of the rotational body 4 is greater 
than a stated value (30 m/s.) 
Therefore, in the present invention, while windage loss occurring at the 
rotational body 4 is made as large as possible, windage loss occurring at 
rotational portions other than the rotational body 4 is made as small as 
possible, and the effect of air damper is occurred intensively at the 
rotational body 4. 
It is known by experiments that windage loss at a rotating portion of the 
motor is proportional to the square of the peripheral velocity Va when the 
peripheral velocity Va of the rotating portion is less than 30 m/s, and is 
proportional to the third power of the peripheral velocity Va when the 
peripheral velocity Va of the rotating portion is more than 30 m/s. 
In the present invention, while the outer diameter of rotational body 4 is 
determined so that the velocity at the periphery of the rotational body 4 
is more than 30 m/s at a range of constant speed rotation of the shaft 1, 
the outer diameter of the motor rotor 5 is determined so that the velocity 
at the periphery of the motor rotor 5, which is another rotating portion, 
is less than 30 m/s. 
Next, based on the foregoing viewpoint, it is described how to concretely 
determine sizes of the rotational body 4. 
Defining the outer diameter of the rotational body 4 as .phi.p mm!, the 
velocity Va at the periphery of the rotational body 4 is expressed with 
expression (1) when the rotational body 4 rotates at a number of rotations 
N rpm!. 
EQU Va=.pi..multidot..phi.p10.sup.3 .times.N/60m/s! (1) 
When the rotational body 4 rotates at 10000 rpm!, the outer diameter 
.phi.p of the rotational body 4 when the velocity Va at the periphery of 
the rotational body 4 is 30 m/s! becomes 57.3 mm! by the expression (1). 
That is, the outer diameter .phi.p corresponding to a boundary where 
windage loss acting on the periphery of the rotational body 4 changes from 
the square to the third power of peripheral velocity. 
Similarly, when the number N of rotations is 20000 rpm!, 30000 rpm!, 
40000 rpm!, and 50000 rpm!, the outer diameter .phi.p of the rotational 
body 4 when the velocity Va at the periphery of the rotational body 4 is 
30 m/s! becomes 28.6 mm!, 19.1 mm!, 14.3 mm!, and 11.5 mm!, 
respectively, from the expression (1). 
By plotting the results on a figure and connecting each point, the curve 
shown in FIG. 2 is obtained. 
As shown in FIG. 2, the requirements for using windage loss occurring at 
the periphery of the rotational body 4 effectively when the rotational 
body 4 rotates steadily at some number of rotations are that the outer 
diameters .phi.p of the rotational body 4 are on the upper side of the 
curve in FIG. 2 and that the outer diameter of the rotational portions 
other than for the rotational body 4, that is, air-gap diameter .phi.b of 
the bearing 2 or 3 and the outer diameter .phi.m of the motor rotor 5 are 
on the lower side, that is, the oblique line side, of the curve in FIG. 2. 
Therefore, when the number of steady rotations is 50000 rpm!, it is 
required that the outer diameter .phi.p of the rotational body 4 is more 
than 11.5 mm! and that the outer diameter of the rotational portions 
other than the rotational body 4 is less than 11.5 mm!. 
In the present invention, it is required that outer diameters of the 
rotational body 4 and the rotational portions other than the rotational 
body 4 be on different areas of FIG. 2. In the embodiment of the present 
invention, adding such requirements, it is desirable that the relation 
between the outer diameter of the rotational body 4 and the outer diameter 
of the rotational portions other than the rotational body 4 is determined 
as mentioned below because of the required actual size of the rotational 
body 4 and torque of the motor 7. 
In relation to the diameters of FIG. 1, the ratio .phi.b/.phi.p of the 
outer diameter .phi.p of the rotational body 4 to air gap diameter .phi.b 
of the bearing 2 (or bearing 3) is limited to the expression (2), and the 
ratio .phi.m/.phi.p of the outer diameter .phi.p of the rotational body 4 
to outer diameter .phi.m of the motor rotor 5 is limited to the expression 
(3). 
EQU .phi.b/.phi.p.ltoreq.1/3 (2) 
EQU .phi.m/.phi.p.ltoreq.1/3 (3) 
Here, the outer diameter .phi.p is the volume determining the size of the 
rotation 4. If the rotational body 4 is a disk plate, the size is regarded 
as the outer diameter. If the periphery of the rotational body 4 is a 
polygon body, like a polygon mirror, the size is regarded as the mean 
outer diameter which is the mean value of an inscribed circle diameter of 
the polygon forming a flat face of the polygon mirror and the 
circumscribed circle diameter of the polygon. That is similar in the 
explanation below. 
The air gap diameter .phi.b is the volume determining the size of the 
bearing 2. The size is regarded as a pitch circle diameter of a track of 
rolling element in an anti-friction bearing. In a slide bearing, a 
hydrodynamic bearing, and a magnetic bearing, the size is regarded as the 
air gap distance between the rotational axis (or fixed axis) and the 
rotation housing (or fixed housing.) 
In the present invention having such a structure, windage loss occurring on 
the rotational body 4 at steady rotation, that is loss occurring because 
the periphery of the rotational body 4 has a friction with air, becomes 
very large and operates as an air damper because the air damping effect 
acts intensively at the periphery of the rotational body 4 by the windage 
loss. Therefore, the air damping effect prevents rotation of the 
rotational body 4 from fluctuating and decreases vibration and shock 
caused by the rotational body 4 to the bearings 2 and 3, thereby 
increasing the life of the spindle motor. 
When the rotational body 4 is arranged with a rotational body 4a comprising 
a polygon plate like a polygon mirror as shown in FIGS. 3 and 4, the 
above-mentioned air damping effect by windage loss acts at the periphery 
of the rotational body 4a. Moreover, it is supposed to act as an air 
damping effect as described below. 
As the peripheral velocity of the corner portion 4a.sub.1 becomes larger in 
comparison to the corner portion 4a.sub.1 with the plane portion 4a.sub.2 
of the rotational body 4a, it is supposed that air flow occurs as shown in 
FIGS. 3 and 4. As a result, the air flow of the corner portion 4a.sub.1 of 
the rotational body 4 becomes dense and the air flow of the plane portion 
4a.sub.2 does not become dense. 
A force acting upon the periphery of the rotational body 4a is shown in 
FIG. 5 according to the dense condition of the air. It is considered that 
the force acts as an air damper of the rotational body 4a, and that the 
air damper acts upon the rotational body 4a helping with the air damping 
effect by the above-mentioned windage loss. 
Therefore, in the rotational body 4a, the air damping effect prevents 
rotation of the rotational body 4 from fluctuating and decreases vibration 
and shock caused by the rotational body 4 on the bearings 2 and 3. 
In the above explanation, the air damping effect occurring at the periphery 
of the rotational body 4a is explained using air flow occurring around the 
rotational body 4a. It is also possible to explain using sound pressure as 
shown in FIGS. 6 and 7. 
A sound propagation state is displayed supposing that there is a sound 
source at the corner portion 4a.sub.1 of the rotational body 4a. It is 
considered that the sound pressure of each corner portion 4a.sub.1 being 
the sound source becomes large and the sound pressure of each plane 
portion 4a.sub.2 becomes small, and that the force shown in FIG. 5 acts 
upon the rotational body 4a. 
Thus it is possible to explain the foregoing by both sound pressure and air 
flow. 
Next, referring to FIG. 8, a suitable form of the embodiment of a spindle 
motor according to the present invention is described. 
FIG. 8 is a sectional view of a spindle motor applied in a polygon scanner 
motor in an embodiment of the present invention. 
The embodiment, as shown in FIG. 8, has an under side case 11 supporting a 
shaft 1 for free rotation about a rotational axis, a motor 7, a bearing 
mounting or set plate 13 blocking an open portion of the under side case 
11 and a polygon mirror case 14 entirely covering a polygon mirror 4b 
comprised of a rotational body arranged over the bearing set plate 13. The 
underside case 11 and the set plate 13 define a housing for supporting the 
shaft 1 for free rotation. 
A bearing 2 is fixed at the bottom of the under side case 11, a bearing 3 
is arranged at the center of the bearing mounting plate 13, and the shaft 
1 is supported by the bearing 2 and the bearing 3 so as to revolve freely. 
Although in this example a rolling bearing, such as a ball bearing, is 
used for the bearings 2 and 3, it is possible to use a slide bearing, a 
hydrodynamic bearing, or a magnetic bearing instead of the rolling 
bearing. 
A motor rotor 5 comprising a rotor yoke 51 and a rotor magnet 52 are 
coaxially mounted on the shaft 1 for rotation therewith. The rotor yoke 51 
comprises a tubular body fixedly and coaxially mounted at a lower portion 
of the shaft 1. The rotor magnet 52 comprises a tubular body fixedly and 
coaxially mounted on the rotor yoke 51. 
The motor stator 6 is fixedly mounted in the under side case 11 around the 
motor rotor 5 mounted on the shaft 1. The motor stator 6 comprises a 
stator core 61 and a stator coil 62. The stator core 61 is fixed by a bolt 
16 at an inner periphery of the under side case 11. The motor 7 comprises 
the motor rotor 5 and the motor stator 6. 
If the motor rotor 5 is of a permanent-magnet rotation type having two 
poles, the motor stator 6 may be of a core type or a core-less type. If 
the motor rotor 5 is of a two-pole reluctance type having no 
permanent-magnet, the motor stator 6 may be of a core type or a core-less 
type. 
A Hall sensor board 18 having a center hole 17 is arranged at the bottom of 
the under side case 11 and is mounted on the stator core 61 with a 
mounting or setting bar 19. A Hall sensor 20 is set on the Hall sensor 
board 18. 
An external thread portion 101 is formed at an end of the shaft 1, and next 
to the external thread portion 101 a taper portion 102 is formed. After a 
center hole of an under mirror cap 21 is inserted in the taper portion 102 
of the shaft 1, the under mirror cap 21 is fixed to the taper portion 102 
by tightening a screw in the direction opposite to the direction of normal 
rotation of the shaft 1. 
A cylinder portion 212 is formed integrally with the under mirror cap 21 to 
define a recess, and a lower half of the center hole of the polygon mirror 
4b is inserted in the recess defined by the mirror cap 21 and the cylinder 
portion 212. In the upper half of the center hole of the polygon mirror 4b 
is inserted a cylinder portion 241 formed at a lower side of the upper 
mirror cap 24 inserted on the external thread portion 101 of the shaft 1. 
Therefore, the polygon mirror 4b is sandwiched between the under mirror 
cap 21 and the upper mirror cap 24 at both of its surface. 
After a plain washer 25 and a spring washer 26 are inserted from the end of 
the external thread portion 101 of the shaft 1, an internal thread member 
27 is screwed on the external thread portion 101. Thus, the polygon mirror 
4b is fixed tightly on the shaft 1 through the under mirror cap 21 and the 
upper mirror cap 24. 
As described above, the internal thread member 27 is tightened to the 
external thread portion 101 of the shaft 1. The external thread portion 
101 of the shaft 1 and the internal thread member 27 are formed so that 
the tightening direction of the internal thread member 27 is opposite to 
the direction of steady rotation of the shaft 1. If the direction of 
steady rotation of the shaft 1 is counterclockwise, the screw thread of 
the internal thread member 27 is formed so as to tighten by clockwise 
rotation and the screw thread of the external thread portion 101 is formed 
according to the internal thread member 27. 
As the polygon mirror 4b is generally made of aluminum, the under mirror 
cap 21 and upper mirror cap 24 are also preferably made of aluminum. 
Although the under mirror cap 21 and the upper mirror cap 24 are used to 
set the polygon mirror 4b on the shaft 1, it is possible to set the 
polygon mirror 4b directly on the taper portion 102 of the rotational axis 
1 instead. 
A circular tubular portion 131 surrounding the bearing 3 is provided around 
an upper surface, where the bearing 3 is set on, of the bearing setting 
plate 13, and is loosely inserted in a circular guide 211, without 
contacting one another, formed at an under side of the under mirror cap 
21. Therefore, the circular tubular portion 131 and the circular guide 211 
form a labyrinth structure for preventing lubricating oil of the bearing 3 
from coming to the side of the polygon mirror 4b. 
In the form embodying the present invention, the outer diameter .phi.p of 
the rotational body 4 is in an upper area of the curve of FIG. 2, and the 
rotational portions other than the rotational body 4, which are the air 
gap diameter .phi.b of the bearing 2 (or bearing 3) and the outer diameter 
.phi.m of the motor rotor 5, are in a lower side of the curve, denoted by 
the oblique lined area. 
Therefore, in the example, the number of rotations is more than 30000 
rpm!, the mean outer diameter .phi.p of the polygon mirror 4b is 38 mm, 
the air gap diameter .phi.b of the bearings 2 and 3 is 10.5 mm, and the 
outer diameter .phi.m of the rotor magnet 52 is 10.8 mm. These numbers 
satisfy the above mentioned requirement. 
Moreover, as the mean outer diameter .phi.p of the polygon mirror 4b is 38 
mm, the air gap diameter .phi.b of the bearings 2 and 3 is 10.5 mm, and 
the outer diameter .phi.m of the rotor magnet 52 is 10.8 mm in the 
above-mentioned embodiment, .phi.b/.phi.p=10.5/38=0.286 and 
.phi.m/.phi.p=10.8/38=0.284 satisfy the requirement of the above-mentioned 
expressions (2) and (3). 
As described above, in the form embodying the present invention, windage 
loss occurring at the polygon mirror 4b becomes large and the air damping 
effect acts intensively at the periphery of the polygon mirror by the 
windage loss at a steady rotation. That acts as an air damper. At the same 
time, an air damping effect like explained in FIGS. 3 and 4 acts by air 
flow occurring at surroundings of the polygon mirror 4b. The air damping 
effect is considered to act more efficiently than in the case of no 
polygon mirror 4b because the polygon mirror 4b is disposed within a 
sealed space 9 defined by a polygon mirror case 14. The air damping effect 
and the air damping effect by the above-mentioned windage loss act upon 
the polygon mirror 4b together. 
Therefore, in the present embodiment, these air damping effects prevent 
rotation of the polygon mirror 4b from fluctuating, and vibration and 
shock caused by the polygon mirror 4b on the bearings 2 and 3 decreases. 
This increases the life of the motor. 
In the present embodiment of the invention, the taper portion 102 and the 
external thread portion 101 are formed on the shaft 1, the center hole of 
the under mirror cap 21 is positioned over the taper portion 102, and the 
under mirror cap 21 is fixed to the taper portion 102 by tightening the 
screw in the opposite direction to the normal rotation of the shaft 1. The 
center hole of the polygon mirror 4b and the center hole of the upside 
mirror cap 24 are positioned over the external thread portion 101 so that 
the polygon mirror 4b is sandwiched between under mirror cap 21 and the 
upper mirror cap 24, and the internal thread member 27 is tightened on the 
external thread portion 101. Thus, the polygon mirror 4b is set on the 
shaft 1. 
Therefore, the structure of the spindle motor according to the present 
invention prevents the polygon mirror 4b from deflecting at its plane 
because the aligning accuracy is raised, and decreases the rotation 
fluctuation. Moreover, there is no need to correct the deflection after 
setting the under mirror cap 21 on the shaft 1, and the accuracy in the 
labyrinth structure can be kept. 
In the form embodying the present invention, since the shaft 1 is fitted 
with the under mirror cap 21 in taper contact, and the under mirror cap 21 
is fixed on the taper portion 102 with the screw tightened in the opposite 
direction to the direction of steady rotation of the shaft 1, the under 
mirror cap 21 reinforces lightness cutting into the taper portion 102 
during rotation. Therefore, if a junction portion of the taper portion 102 
of the shaft 1 and the under mirror cap 21 runs hot, the junction portion 
does not become loose when the temperature of the junction falls. That 
point is effective especially when the materials and the coefficients of 
expansion are different for the taper portion 102 and the under mirror cap 
21, such as iron for the taper portion 102 and aluminum for the under 
mirror cap 21. 
As the polygon mirror 4b does not become loose from the shaft 1, windage 
loss gained by rotation of the polygon mirror 4b is used efficiently. 
As described above according to the present invention, windage loss 
occurring at the polygon mirror becomes large and the air damping effect 
acts intensively at the periphery of the rotational body by the windage 
loss during steady rotation which acts as air damper. As the air damping 
effect prevents rotation of the rotational body from fluctuating and 
decreases vibration and shock caused by the rotational body to the 
bearing, a motor having a small rotation fluctuation, a miniaturized size, 
and a long life is realized under the requirement of accelerating 
rotation. 
In the present invention, as the taper portion is formed on the shaft 1, 
the taper portion is fitted with a center hole of the rotational body, and 
the rotational body is tightly fixed to the taper portion in the direction 
opposite to the direction of steady rotation of the shaft. Thus the 
aligning accuracy when the rotational axis is fitted with the rotational 
body is good, and the rotational body does not become loose from the shaft 
at accelerating rotation. 
In the present invention, as the polygon mirror used for the rotational 
body is covered by a case, there are acting both the air damping effect 
caused by windage loss and the air damping effect caused by air flow 
occurring at surroundings of polygon mirror. By both of these air damping 
effects, the effect of the present invention is sure to be realized. 
In the present invention, a polygon mirror and a fitting portion fitting 
the polygon mirror are used for the rotational body, the fitting portion 
is tightened to the taper portion of rotational body, and the polygon 
mirror is fitted with the fitting portion. Therefore, the accuracy of 
correcting deflection when the rotational body is fitted with the fitting 
portion is good and the polygon mirror does not become loose from the 
rotational body.