Motor, a printer having such a motor and a disk drive system having such a motor

A motor has a rotor mounted on a stator to rotate about a rotation axis. The rotor is driven due to the interaction of a drive coil on the stator and a permanent magnet on the rotor. A magnetic thrust bearing, having permanent magnets on the stator and rotor, applies a radial thrust force. The radially outer surface of the permanent magnet on the rotor abuts a part of the rotor, so as to resist forces on the permanent magnet at high speed rotation or due to thermal deformation. The permanent magnets on the stator and the rotor may be axially offset by an offset of not more than 1/4 the axial length of the permanent magnets, or the axial length of the longer of the permanent magnets if they have different axial lengths. This generates an axial thrust at the thrust bearing, which counteracts the attractive force between the permanent magnet and the drive coil, and also the weight of the rotor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a motor, and in particularly to a motor 
having a stator and a rotor mounted on the stator so as to be rotatable 
about a rotation axis. The present invention also relates to a printer 
having such a motor, and a disk drive system having such a motor. 
2. Summary of the Prior Art 
It is well known to provide a motor with a stator and rotor, and to drive 
the rotor about a rotation axis using suitable drive means. That drive 
means may, for example, be a coil mounted on the stator which receives a 
current which interacts with the magnetic field of a permanent magnet on 
the rotor to generate a force to rotate the rotor. 
In such a motor, it is important that radial movement of the axis of 
rotation of the rotor is prevented, or at least restrained, and therefore 
it is known to provide a magnetic thrust bearing which applies a thrust 
force in a direction radial of the rotor axis between the rotor and the 
stator. Such a magnetic thrust bearing may comprise a first permanent 
magnet on the rotor and a second permanent magnet on the stator, with the 
first and second permanent magnets being concentrically arranged around 
the rotation axis, and having a radial gap therebetween. By suitable 
arrangement of the polarity of the first and second permanent magnets, a 
thrust force may be generated therebetween which stabilizes the rotation 
of the rotor. A motor incorporating such a magnetic thrust bearing is 
disclosed in, for example, JP-A-4-150753 (corresponding to U.S. patent 
application No. 07/629462, now U.S. Pat. No. 5,325,006). Other examples of 
magnetic thrust bearings are disclosed in JP-A-61-201916, JP-A-62-85216 
and JP-A-63-88314. 
Motors of the type discussed above are used in printers and disk drives. In 
a laser printer, the laser beam is caused to scan the surface on which 
printing is to occur. That scanning is achieved by directing the beam onto 
a rotating polygonal mirror, so the speed of scanning is determined by the 
speed of rotation of the polygonal mirror. A motor described above may 
then be used to rotate the polygonal mirror. Similarly, in a disk drive, a 
disc is caused to rotate and that rotation may be achieved by a motor 
described above. To achieve high density of data recordal on the disk, it 
is necessary to use a high speed of rotation. 
SUMMARY OF THE PRESENT INVENTION 
For both the printer and the disk drive discussed above, there is a desire 
to increase the speed of rotation of the motor. This then permits more 
rapid scanning in the printer, and a greater data density in the disk 
drive system. In particular, there is a desire to obtain a motor with a 
rotation speed of greater than 10,000 rotations per minute, preferably 32 
to 40,000 rotations per minute. 
However, the inventors of the present application have found that such high 
rotation speeds cause strong forces to be applied to the permanent magnet 
of the magnetic thrust bearing on the rotor, and these forces can cause 
the permanent magnet on the rotor to deform in an axially outward 
direction. Furthermore, the high speeds of rotation may generate 
considerable amounts of heat, which can cause a rise in temperature and so 
cause thermal expansion of the permanent magnet on the rotor. 
Both of these effects cause the surface of the permanent magnet of the 
thrust bearing on the rotor to approach closely the opposed surface of the 
permanent magnet of the thrust bearing on the stator. There is then the 
risk that those surfaces may contact due to any slight radial movement of 
the axial rotation of the rotor, which would then damage the thrust 
bearing. In addition, since the distance between the permanent magnets of 
the thrust bearing is changed, the force therebetween is also changed and 
this may affect the performance of the thrust bearing. 
Therefore, the present invention proposes, in a first aspect, that the 
radially outer surface of the permanent magnet on the rotor abuts a part 
of that rotor. Thus, that abutment prevents radially outward movement of 
the permanent magnet, either due to the forces caused by rotation or due 
to thermal expansion, thereby preventing damage to the permanent magnet on 
the rotor, and preventing thermal expansion in the radially outward 
direction. 
In the prior art documents referred to previously, which employ a magnetic 
thrust bearing, the permanent magnet on the rotor is then mounted on a 
radially outer surface of the rotor, and is radially inward of the 
permanent magnet on the stator. In order to achieve the present invention, 
it is preferable that the permanent magnet on the rotor is radially 
outward of the permanent magnet on the stator. This may be achieved by 
suitable shaping of the rotor and stator, with the permanent magnet on the 
rotor then being on a radially inner surface of the rotor. Preferably, in 
this case, the rotor has a slot in that radially inner surface, and the 
permanent magnet on the rotor is then mounted in that slot. The permanent 
magnet may then abut an axial wall, (i.e. a wall extending axially) of the 
slot, to prevent radial outward movement. 
However, it is also possible for the first permanent magnet to be radially 
inward of the second permanent magnet, if the rotor is shaped so as to 
provide an abutting part on the radially outer surface of the permanent 
magnet on the rotor. This may be achieved, for example, by providing a 
slot extending axially in the rotor, and locating the permanent magnet in 
that slot. A radially outer wall of the slot then may abut the first 
permanent magnet to prevent axially outward movement. 
In either of these two alternatives, adhesive may be used to mount the 
permanent magnet in the slot, that adhesive then being considered part of 
the rotor. 
Normally, either or both of the permanent magnets on the rotor and stator 
extend continuously around the rotation axis, and may be, for example, in 
the form of annuli. It is also possible for either or both of the 
permanent magnets of the thrust bearing to be formed by a plurality of 
magnet sections. 
The first aspect of the present invention, discussed above, seeks to 
prevent, or at least to reduce, radial movement of the axis rotation of 
the rotor. It is also important, however, that axial movement of the rotor 
is limited. Such axial movement may occur, for example, due to the weight 
of the rotor or due to the magnetic drive force exerted between a drive 
coil on the stator and a permanent magnet on the rotor, particularly in 
arrangements of the motor in which that magnetic drive force is axial. The 
present inventors have realized that these forces may be counteracted if 
the permanent magnets on the rotor and stator are not axially aligned, but 
have a small axial displacement therebetween. Such axial displacement, 
e.g. of axially outer end surfaces generates a force between the permanent 
magnets which has an axial component, and that axial component may then be 
arranged to counteract the weight and/or other magnetic forces on the 
rotor. 
The present inventors have found that axial displacement should not be 
greater than a quarter of the length of the permanent magnets. For 
displacements up to quarter of length of the permanent magnets, the 
arrangement is stable in that increasing displacement (due to weight or 
other magnetic forces) tends to cause an increase in the force between the 
permanent magnets of the thrust bearing, that any movement of the rotor is 
resisted. If, however, the axial displacement is greater than a quarter of 
the length of the permanent magnets, the arrangement is unstable in that 
increasing displacement reduces the force. This relative axial 
displacement of the permanent magnets of the thrust bearing is thus a 
second, independent, aspect of the present invention. 
In the above discussion of the second aspect of the present invention, it 
is assumed that the permanent magnets have the same axial length. This is 
not necessary, however, and they may have different axial lengths. Then, 
the axial displacement should not be more than a quarter of the axial 
length of the axially longer of the permanent magnets. Furthermore, where 
the permanent magnets have different lengths, the axial length of the 
shorter should not be less than 3/4 of the axial length of the longer. 
In JP-A-4-150753 referred to above, embodiments were disclosed in which 
there was axial displacement of end surfaces of the permanent magnets on 
the rotor stated, but there was no discussion of the extent or effect of 
such axial displacement. Furthermore, the permanent magnets had very 
different axial lengths. Similarly, in JP-A-63-88314, there was axial 
displacement of end surfaces of the permanent magnets of the thrust 
bearing, but again there was no disclosure of the extent or effect of that 
displacement. 
Preferably, when the motor is to be used with the rotor uppermost, the 
axially outer end surface of the permanent magnets which are displaced in 
axial direction are upper surfaces, and may then be surfaces remote from 
the stator. However, it is also possible to arrange the motor so that the 
corresponding axially outer end surfaces of the permanent magnets are 
remote from the motor. 
It should be noted that, although the second aspect of the present 
invention seeks to counteract axial movement of the rotor due to the 
weight of the rotor, and due to magnetic forces between the stator and 
rotor due to the drive thereof, the present invention is not limited to 
arrangements in which a motor has particular orientation, and at least 
some of those forces will exist irrespective of the orientation of the 
motor. Furthermore, although the first and second aspects of the present 
invention are independent, they may be used in conjunction. 
A motor incorporating the first and/or second aspects of the present 
invention finds particular advantage as the motor of a printer or a disk 
drive system. However, the present invention is not limited to such uses 
of the motor. 
BRIEF DESCRIPTION OF THE DRAWINGS

DETAILED DESCRIPTION 
FIG. 1 is a section showing a motor for rotating a polygon mirror of a 
printer, the motor being a first embodiment of the present invention, and 
FIGS. 2 to 4 shown the construction of magnetic bearings which are applied 
to the motor of this embodiment. The embodiment shown in FIG. 1 is a 
face-opposed type motor, in which there is a multipole flat permanent 
magnet 10 mounted on a rotor 2 to establish driving magnetic fields in the 
axial direction. The rotor 2 is fixed to a magnetically permeable shift or 
spindle 1 and carries the permanent magnet 10, a magnetic plate 11 and a 
polygon mirror 3. A bearing housing or stator 4 contains radial bearings 
5, a magnetic fluid seal 13 using a permanent magnet, and a dynamic 
viscous seal 14 having a helical groove. Between the bearings 5 and a 
viscous seal 14, there is a space which is filled with a magnetic fluid 
6a. The radial bearings may be made of a sintered alloy, which may be an 
"oil-containing metal" or an "oilless metal" requiring no oiling. The 
magnetic fluid seal 13 is shaped so as to extend around the permeable 
spindle 1. A housing 8 is fixed on the bearing housing or stator 4, to 
which is attached a motor substrate 12 carrying a stator coil 9 for 
generating a magnetic field. 
The stator 4 and the rotor 2 respectively support ring-shaped permanent 
magnets 7a and 7b which are magnetized in the axial direction. Thus, the 
magnetic attraction of the permanent magnets 7a and 7b form a magnetic 
bearing for the shaft 1. This magnetic bearing is arranged between the 
radial bearings 5 and the polygon mirror 3, and overhands the radial 
bearings 5. 
As shown in FIGS. 1 and 2a, the axially magnetized, ring-shaped permanent 
magnets 7a and 7b have a common axis with a predetermined clearance 
therebetween. They thus form a magnetic circuit in the directions of the 
arrows in FIG. 2a, so that the magnetic flux is concentrated at the N-S 
poles of the opposed portions to establish a magnetic attraction. This 
magnetic attraction provides the bearing action, as it establishes forces 
in the axial direction and in the radial direction so that the load 
bearing capacity is determined in dependence upon the sizes of the axial 
displacement or offset d and the radial clearance C. 
If the permanent magnets are displaced in the axial direction, the magnetic 
flux of the poles is concentrated to enhance the load bearing capacity of 
the bearing. 
In particular, the axial thrust force F between the permanent magnets 7a 
and 7b varies with the displacement d. FIG. 2b shows the variation in that 
thrust force F with displacement or offset d, and also shows the variation 
in bearing stiffness K. 
It can be seen from FIG. 2b that the axial thrust force F increases as the 
axial displacement or offset d increases. When d is less than or equal to 
1/4.multidot.L, where L is the axial length of the permanent magnets 7a, 
7b, the increase in thrust force F is approximately proportional to the 
axial displacement or offset d. Within this region, therefore, the bearing 
stiffness is approximately constant. However, for axial displacement or 
offset d greater than 1/4.multidot.L, the increase in thrust force is not 
proportional to the distance d. Therefore, the bearing stiffness K 
decreases as shown in FIG. 2b. 
Hence, in order to ensure that the magnetic bearing formed by the permanent 
magnets 7a and 7b operates correctly, it is necessary that the axial 
displacement or offset d be less than or equal to 1/4.multidot.L. For 
displacements or offset above this, the decrease in bearing stiffness 
means that the change in rotor dimensions in axial direction for a given 
thrust force is larger than compared with the case when the displacement 
or offset is less than or equal to 1/4.multidot.L. When the motor is used 
for driving a polygon mirror, significant changes in the axial dimension 
of the rotor affects the scanning accuracy, and therefore it is important 
that the motor is operated in the region for which the bearing stiffness 
is approximately constant. 
The above discussion assumes that the permanent magnets 7a and 7b have the 
same axial length L. FIG. 3 shows an arrangement in which the permanent 
magnet 7a and 7b have different axial lengths L.sub.1 and L.sub.2. There 
are then two factors that have to be taken into account when assuring the 
axial thrust force F is satisfactory. Firstly, the axial displacement or 
offset d must be less than or equal to a quarter of the axial length of 
whichever is longer of the permanent magnet 7b and 7a. Thus, in FIG. 3 the 
axial displacement or offset d must be less than or equal to 
1/4.multidot.L.sub.2. In addition, however, the axial force will be 
affected by the relative lengths of the permanent magnets 7a and 7b. It 
has been found that the axial lengths of the shorter of the permanent 
magnets must not be less than 3/4 of the axial length of the permanent 
magnet. Thus, in FIG. 3, L1 must be greater than or equal to 
3/4.multidot.L.sub.2. 
The action of the magnetic bearing can be enhanced if the permanent magnets 
7a and 7b each have magnetic plates 15 on the axially outer surfaces 
thereof. This concentrates the magnetic flux in the regions of the opposed 
ends of the magnetic plates 15, as shown in FIG. 4. A magnetic fluid seal 
13 may be arranged in the vicinity of the magnetic plates 15. When the 
magnetic fluid seal 13 is arranged in the vicinity of the magnetic 
bearings, as shown in FIG. 1, its magnetic fluid 6b may be attracted by 
the permanent magnet 7a. This makes it necessary to eliminate leakage of 
the magnetic flux of the magnetic bearing 7. The use of magnetic plates 15 
in the shape of letter "L", as shown in FIG. 4, solves that problem. The 
magnetic flux is concentrated in the magnetic plates 15, so substantially 
to eliminate the leakage magnetic flux. If the magnetic fluid seal 13 is 
not significantly influenced by the leaking magnetic flux of the magnetic 
bearing 7, the magnetic plates 15 may be flat. 
FIG. 1 shows a further feature of this embodiment of the present invention. 
A radially inner surface 30 of the rotor 2 (i.e. a surface which faces the 
shaft 1), has a slot 31 therein, and the permanent magnet 7b on the rotor 
is mounted in that slot 31. Thus, the radially outer surface of the 
permanent magnet 7b on the rotor abuts a part of the rotor. The effect of 
this is to protect the rotor against deformation. Such deformation can 
arise, for example, due to the forces exerted on the permanent magnet 7b 
when the rotor rotates, or due to thermal effects on the permanent magnet 
7b. The slot 31 supports the permanent magnet in the radially outer 
direction, and thus prevents damage to the permanent magnet 7b. The 
permanent magnet 7b may be secured in the slot 31 by adhesive. 
Thus, for a high speed rotation such as several tens of thousands r.p.m. of 
the polygon mirror motor, the permanent magnet 7b may be broken by the 
centrifugal force or by the thermal stress resulting from a rise in 
temperature of the motor. This breakage is prevented in the embodiment by 
adopting a structure in which the permanent magnet 7b has its radially 
outer surface abutting the rotor 2. 
The operation and effects of the motor and this first embodiment will now 
be described. If the stator coil 9 is energized in the embodiment of FIG. 
1, a rotating magnetic field is established in the axial direction from 
the coil 9 so that the polygon mirror 3 is rotated by the interaction of 
the coil 9 with the magnetic field of the permanent magnet 10. An 
attractive force is established by the interaction of the magnetic field 
of the coil 9 and the magnetic field of the permanent magnet 10. The rotor 
2 is thus attracted toward the coil 9 so that the permanent magnets 7a, 7b 
of the magnetic bearing are axially displaced downwardly in FIG. 1 which 
displacement is resisted by the upward axial force generated between the 
permanent magnets 7a, 7b. The magnetic attraction between the coil 9 and 
the permanent magnet 10 is within a predetermined range, and force in the 
thrust direction generated by the permanent magnets 7a, 7b increases as 
described above, for larger displacements of the permanent magnets 7a,7b 
of the magnetic bearing. This feature is employed in the present invention 
to enhance the load bearing capacity of the magnetic bearing. 
Thus, during operation, the rotor 2 is attracted toward the coil 9 by the 
magnetic attraction so that the displacement between the magnets 7a and 7b 
will increase. If this displacement increases, the magnetic bearing has 
axial attraction increased to return the rotor 2 to the initial position. 
Thus, the rotor 2 is held in the position in which its weight of the 
magnetic attraction between the coil 9 and the rotor, and the attraction 
generated by the magnetic bearing are balanced, so that its position is 
not extremely dispersed. This position balance is similar even when the 
shaft 1 is horizontal. 
For vertical rotation, as shown in FIG. 1, the axial load of the rotor is 
supported by the magnetic attraction of the magnetic bearing, and the load 
in the radial direction is mainly borne by the fluid bearings 5 which are 
lubricated by the magnetic fluid. The magnetic bearing also has a radial 
load bearing capacity so that accurate rotation can be maintained. 
For a horizontal rotation, the polygon mirror 3 is carried as a load in the 
radial direction by the magnetic bearing formed by permanent magnets 7a, 
7b. If the embodiment of FIG. 1 is used for horizontal rotation, the rotor 
has most of its weight supported by the magnetic attraction of the 
magnetic bearing in the radial direction. Little or no radial load is 
exerted upon the radial bearings 5 so that sliding contact between the 
shaft 1 and the bearings 5 is avoided at the starting and stopping of the 
motor, This eliminates or reduces the problem of the wear of the bearings. 
Since, the axial position is accurately determined by the magnetic 
bearing, the rotor is carried wholly out of contact at high speed rotation 
by the fluid bearings 5 and the magnetic bearing. Thus, ensuring a highly 
accurate rotation. It can be noted that the present invention can be 
applied to a motor with an air bearing if the bearings 5 of FIG. 1 are 
replaced by an air bearing having no seal. 
FIG. 5 illustrates an embodiment in which the rotor 2 is mounted below the 
stator 4. With one exception, this embodiment is thus the inverted 
arrangement of the embodiment of FIG. 1, and corresponding components are 
indicated by the same reference numerals. In the embodiment of FIG. 5, 
however, the coil 9 and the permanent magnet 10 exert an upward force on 
the rotor. Since the weight of the rotor is constant, the main fluctuation 
in the forces between the stator 4 and the rotor 2 is in the upward 
direction. Therefore, the permanent magnet 7a and 7b of the magnetic 
bearing are modified, so that the permanent magnet 7b on the rotor has its 
upper edge displaced downwardly relative to the upper edge of the 
permanent magnet 7a on the stator. Thus, the relative displacement between 
the magnet 7a and 7b is in the opposite direction to the embodiment of 
FIG. 1. Thus, an upwardly directed force is generated by the magnetic 
bearing on the rotor 2. In the embodiment of FIG. 1, the axially outer 
surfaces of the permanent magnets are displaced. In FIG. 1, the important 
displacement is of the axially outer end surfaces which are remote from 
the stator (although the end surfaces closest to the stator are also 
displaced) in the embodiment of FIG. 5, on the other hand, the important 
displacement is the axial displacement of the outer end surfaces which is 
closest to the stator. (Although again, the other surfaces are also 
displaced.) FIG. 3 shows that, particularly where the lengths of the 
permanent magnet 7a and 7b are unequal, the displacement is of the upper 
axial end surfaces, and the lower axial end surfaces may be aligned. 
Known motors using an air bearing cannot avoid some sliding contact between 
the shaft and the bearing at the starting and stopping of the rotor so 
that they suffer from the problem of wear. In the present invention, 
however, the spindle or shaft 1 is kept away from contact with the bearing 
even while rotating in the horizontal direction, so that a highly accurate 
rotation can be maintained without the problem of wear of the spindle 1 
and the bearing. Thus, it is possible to provide a highly reliable motor. 
In the embodiment shown in FIG. 1, leakage of the magnetic fluid when the 
motor is stationary is prevented by the magnetic fluid seal 13, and the 
magnetic fluid cannot overflow during rotation due to the viscous seal 14 
having a helical groove therein. For high speed rotation, the magnetic 
fluid in the bearing housing expands in volume, so increasing its internal 
pressure, due to the heat generated by the bearing. The magnetic fluid 6b 
in the magnetic fluid seal 13 is sucked to the bearing, by making the 
pressure of the viscous seal 14 higher than the seal breakdown pressure of 
the magnetic fluid seal 13 so that the fluid 6b is never scattered outside 
of the magnetic fluid seal 13. For both vertical rotation and horizontal 
rotation, that viscous seal 14 can break the magnetic fluid seal instantly 
to drop the internal pressure of the bearing housing 4 to atmospheric 
pressure, thereby ensuring reliable sealing. This reliable seal can also 
be achieved for the reverse rotation of the motor. 
FIG. 6 shows a third embodiment of a spindle motor, which embodiment is for 
driving a magnetic disk. The rotor 2 supports a magnetic disk 16 as a 
load, via a spacer ring 17 and a disc clamp 19. The rest of the 
construction of the embodiment is similar to that of the polygon mirror 
motor shown in FIG. 1. For a magnetic disk however, the magnetic disc 16 
acting as the load has a weight greater by several times or more that of 
the polygon mirror 3. Moreover, if the rotating magnetic field of the 
motor or the magnetic field of the magnetic bearing interacts with the 
magnetic disk 16, the data written on the disk will be erased. Thus, the 
load bearing force for the disk must be increased, and leakage of the 
magnetic flux is eliminated or at least reduced sufficiently by adopting 
the bearing construction in which there is a magnetic shield plate 18 and 
in which the magnetic bearing has magnetic plates 15, as shown in FIG. 4. 
Existing magnetic disk driving spindle motors are rotated at about 3,600 
r.p.m. by using the ball bearing. However, the demand for high speed 
recording and dense recording means that there is a demand for more 
accurate and high speed rotation. In this respect, the present invention 
permits such a demand to be satisfied, to give faster and more accurate 
rotation by employing a motor having the bearing construction described 
above, and in particular due to the operation and effect of the magnetic 
fluid bearing and the magnetic bearing. Similar operations and effects can 
be achieved if the present invention is applied to an optical disk driving 
spindle motor, because this motor can have the same construction as that 
of the magnetic disk driving spindle motor and is required to have 
substantially the same accuracy. 
FIG. 7 shows a fourth embodiment of the present invention, for driving a 
polygon mirror motor. In the third embodiment, the permanent magnets 7a 
and 7b of the magnetic bearing are supported in the motor housing 8 and 
the rotor 2. In the third embodiment the magnetic bearing has a higher 
load bearing capacity because it has a larger diameter than in the first 
embodiment. Thus, this motor is suitable for a heavy load such as a 
large-diameter polygon mirror or magnetic disk. In the third embodiment, 
the permanent magnet 7b in the rotor 2 has its outer circumference fixed 
in a slot 40 in the rotor 2, so that the separation from the other magnet 
7b is to drop the bearing action. In order to concentrate the magnetic 
flux magnetic plates 15 are used (as in FIG. 4) to improve the action of 
the bearing. 
Thus, in FIG. 7, the slot 40 extends axially into the rotor 2, and the 
permanent magnet 7b on the rotor is then mounted in the slot. It may be 
secured in place by adhesive. Thus, a wall 41 of the slot 40 abuts the 
radially outer surface of the permanent magnet 7b, thereby preventing 
outward deformation of the permanent magnet 7b due to the forces applied 
thereto during rotation, or during thermal expansion. Although the wall 41 
increases the clearance C (see FIG. 2) between the permanent magnet 7a and 
7b, the wall 41 ensures that clearance C is maintained with little or no 
variation as the rotor 2 rotates. It can be appreciated that, in this 
embodiment, housing 8 is considered part of the stator. The third 
embodiment shown in FIG. 7 is otherwise similar to the first embodiment 
shown in FIG. 1, corresponding parts are indicated by the same reference 
numerals. 
In each of the embodiments of FIGS. 1, 5, 6 and 7, the coil 9 and the 
permanent magnet 10 generate axial forces therebetween for driving the 
rotor 2 about the shaft 1. However, the present invention may also be 
applied to an inner rotor motor type motor, shown in FIG. 8. In FIG. 8, 
the permanent magnet 10 extends around the shaft 1, and is arranged 
axially parallel to the plane of the coil 9, which coil 9 has a core 20 
which is radially outward of the permanent magnet 10. Thus, in the 
embodiment of FIG. 8, the forces between the permanent magnet 10 and the 
coil 9 are axial. FIG. 8 also shows that the permanent magnet 10 is held 
in place by a wall 21. 
In the embodiment of FIG. 8, the permanent magnet 7b on the rotor 2 is 
mounted in a slot 40, as in the embodiment of FIG. 7. Thus, outward 
movement thereof is prevented by the wall 41 of the rotor 2, which extends 
over the axially outer surface of the permanent magnet 7b between that 
permanent magnet 7b and the permanent magnet 7a on the stator. Apart from 
this, this fifth embodiment of the present invention is similar to the 
fourth embodiment shown in FIG. 7, and corresponding parts are indicated 
by the same reference numerals. 
As has previously been mentioned, a motor according to the present 
invention may be used in a printer, and in particular a laser beam 
printer. FIG. 9 shows an embodiment of such a printer. In the printer, a 
photosensitive drum 101 is driven by a motor 102 so that the drum 101 
rotates about a longitudinal axis. A light emitting diode 103 emits light 
towards the edge of a polygon mirror 104, and the light is reflected from 
one of the surfaces of the polygon through a lens 106 onto the 
photosensitive drum 101. The polygon mirror 104 is driven by a motor shown 
schematically at 105. That motor 105 may then be according to one of the 
embodiments previously described. 
When the polygon mirror 104 is rotated by the motor 105, it causes the 
light from the light emitting diode 103 to scan the photosensitive drum 
101. By suitable modulation of the light from the light emitting diode 
103, and due to the scanning of that light due to the rotation of the 
polygon mirror 104, the photosensitive drum 101 is exposed to light in a 
suitable pattern, thereby forming a latent image. 
The rotation of the mirror 105, and hence the polygon mirror 104, the 
modulation of light from the light emitting diode 103, and the rotation of 
the photosensitive drum 101 by the motor 102 are controlled by a 
controlling unit 107, to control the timing of the components so that the 
latent image on the photosensitive drum 101 has the desired configuration. 
Then, toner may be adhered to the photosensitive drum 101, due to the 
static electricity on the photosensitive drum 101 due to exposure by the 
light from the light emitting diode, and that toner can be transferred to 
a paper sheet 108. 
FIG. 10 shows another embodiment of the present invention, when applied to 
a magnetic disk apparatus. That magnetic disk apparatus has a head unit 
200 A and a motor unit 200 B. The head unit 200 A comprises a plurality of 
data read-out heads 201, which are supported on a support 202. That 
support 202 is movable, with that movement being controlled by a drive 
mechanism 203. In the embodiment of FIG. 10, the motor unit 200 B is a 
housing 211, which supports a shaft 214 for rotation about a vertical 
axis, with that shaft 214 carrying a hub 217 which, in turn carries 
magnetic disks 218. This embodiment differs from that of FIG. 6 in that 
the shaft 214 is supported by the housing 211 on both sides of the 
magnetic disk 218. 
In this embodiment, the housing 211 forms a stator, and the shaft 214 forms 
a rotor. The stator coil 212 is mounted on the housing 211 at the lower 
end of the shaft 214, and a permanent magnet 213, being the rotor 
permanent magnet is mounted on the shaft 214. Thus, the shaft 214 is 
driven by the interaction of the coil 212 and permanent 213. FIG. 10 also 
shows a magnetic fluid seal 215 and a first bearing 216. That thrust 
bearing 216 is not shown in detail, but corresponds to the permanent 
magnet 7a and 7b shown in e.g. FIG. 2, FIG. 3 or FIG. 4. 
The magnetic disks 218 carry data, and rotation of the disks 218 due to the 
motor 200 B combined with radial movement of the heads 201, causes the 
heads 201 to scan the disks 218, to enable the data to be read. 
In the discussion of all the embodiments described above, it is assumed 
that the permanent magnets 7a and 7b extend continuously around the shaft, 
so that they are in the form of annuli. It is also possible to form the 
permanent magnets by having a plurality of curved magnet sections, then 
there are practical limits to the gaps between such sections if the thrust 
bearing is to operate satisfactorily. 
Thus, in embodiments of the present invention: 
1) In a motor in which the rotor carrying the load such as a polygon mirror 
or a magnetic disk is rotatably borne by the fluid lubricated bearing, the 
load may be carried so that it overhangs the fluid-lubricated bearing. A 
magnetic bearing having excellent radial and thrust load bearing 
capacities is arranged between that load and the fluid-lubricated bearing, 
thereby maintaining accurate rotation of the motor and shortening the 
length of the fluid bearing. 
2) In the magnetic bearing, ring-shaped permanent magnets may be arranged 
to face each other at the station side and the rotor side, which are 
arranged coaxially with a radial clearance therebetween, and with an axial 
displacement, providing radial and thrust bearing actions. 
3) The magnetic bearing may be arranged with permeable magnetic disks at 
the end of the faces of the ring-shaped permanent magnets so that the 
magnetic flux may be concentrated at opposed parts of the disks. 
4) The magnetic disk may have a structure in which an L-shaped magnetic 
disc is fixed at the end faces of the ring-shaped permanent magnets so as 
to concentrate the magnetic flux. 
5) Because the magnetic bearing may have opposed faces of the ring-shaped 
permanent magnets axially displaced, the load bearing capacity is 
enchanced. 
6) Since the opposed faces of the two permanent magnets are axially 
displaced, the magnetic attraction of the motor may be concentrated. 
7) The ring-shaped permanent magnets are arranged to face each other at the 
station side and at the rotation side with a radial clearance 
therebetween. The permanent magnets may have difference thicknesses at the 
station side and at the rotor side. 
8) The magnetic bearing may have permanent magnets of a rare earth metal 
having a high magnetic energy product. 
9) The magnetic bearing may be constructed such that its components at the 
station side are fixed to the bearing housing or the motor housing and the 
permanent magnet arranged at the rotor side has its outer circumference 
abutting the rotor, so that its position is fixed. 
10) A magnetic fluid bearing may be used in which a magnetic fluid is used 
for the bearing lubrication and sealing, in which the sealing portion is a 
composite seal having a dynamic type viscous seal combined with a magnetic 
fluid seal and in which the bearing unit is partially filled with magnetic 
fluid. 
Thus, according to the present invention, high speed and accurate rotation 
of the motor can be maintained by arranging a magnetic bearing having 
excellent radial and thrust load bearing capacity between the load of the 
rotor and the fluid bearing. Since the load of the rotor can be supported 
by the magnetic attraction of the magnetic bearing in the radial 
direction, no wear is caused due to the contact of the fluid bearing even 
at the starting and stopping of the motor. Thus, it is possible to provide 
a highly reliable and accurate motor for rotations in any direction. 
Since the magnetic bearing has satisfactory effects in the radial and 
thrust directions, the load of the rotor upon the fluid bearing can be 
reduced so as to shorten the length of the radial bearing thereby to 
reduce the size of the motor. 
If the present motor is applied to a polygon mirror motor or a spindle 
motor for driving the magnetic disk or an optical disk, highly accurate 
rotation can be maintained up to a high speed range for a long time. Thus, 
the lifetime of the motor is increased and the reliability improved. In 
addition, the motor can have its size reduced for miniaturization and 
enhance the performance of the system.