Dynamic pressure bearing and rotary polygon mirror device with the bearing

A rotary polygon mirror device comprises: a rotor including a polygon mirror secured to one end face of a cylinder, a central rod secured inside said cylinder in such a manner that the central rod is extended along the axis of the cylinder, and a thrust bearing magnet secured to the end face of the central rod, said magnet; and a casing coaxially surrounding the cylinder with a gap therebetween. In the device, the gap provides radial bearing means, and the cylinder and the casing are made equal in thermal expansion coefficient.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to rotary polygon mirror devices, and more 
particularly to a rotary polygon mirror device which employs a dynamic 
pressure air bearing for rotating its rotary polygon mirror. 
2. Description of the Related Art 
For instance in a laser beam printer, the laser beam must be accurately 
deflected at high speed. In order to meet this requirement, a rotary 
polygon mirror is employed. A rotary polygon mirror is in the form of a 
regular polyhedron with mirrors on its sides. The rotary polygon mirror is 
fixedly mounted on the rotor of a drive motor, so that it is rotated at 
high speed. Therefore, a rotary polygon mirror rotated at a speed of lower 
than 15,000 rpm employs a ball bearing, and a rotary polygon mirror 
rotated at a speed higher than 15,000 rpm employs a fluid bearing 
utilizing air or magnetic fluid, or a fluid bearing in combination with a 
magnetic bearing (cf. Japanese Patent Application Publication No. 
6854/1978, and Japanese Patent Application (OPI) No. 164413/1984 (the term 
"OPI" as used herein means an "unexamined published application")). 
In one example of the fluid bearing, herring bone or spiral grooves are 
formed in the surface of the rotor or in the surface of a member 
confronted through a small gap with the rotor, and a fluid drawing 
phenomenon due to the rotation of the rotor or the confronted member is 
utilized to produce a high pressure. In another example, such as a movable 
pad type fluid bearing, a plurality of pads are arranged around the rotor 
which can be freely tilted, in such a manner that small gaps are locally 
formed, and high dynamic pressures produced there are utilized. 
In any of the bearing systems, the rotation in vibration cannot be obtained 
unless the gap is maintained most suitable. Of those bearing systems, the 
dynamic pressure groove system has a small range of tolerable gap 
dimension, several micro-meters (.mu.m) to several tens of micro-meters 
(.mu.m). Hence, in forming the bearing, it is necessary to use a material 
such as a ceramic or a special alloy which is not highly affected in 
dimension by thermal variation and is wear resistant. However, the use of 
such material provides another problem that it is difficult to form spiral 
grooves or the like in the component of the bearing. This increases the 
manufacturing cost. 
Furthermore, in the case where, in order to increase the bearing rigidity, 
a viscous fluid other than air is employed in the dynamic pressure groove 
system, the polygon mirror is limited in range of speeds. That is, when 
the polygon mirror is turned at high speed, a windage loss or bearing loss 
occurs to increase the temperature, as a result of which the bearing 
characteristic becomes unstable, and therefore the allowable range of 
speeds of the polygon mirror is limited. 
The movable pad type air bearing operates stably over a relatively wide 
range of temperature variations. However, it is intricate in construction 
and accordingly high in manufacturing cost. 
FIG. 1 is a sectional view of a conventional polygon mirror rotating motor. 
As shown in FIG. 1, a polygon mirror 105 and a rotor 106 are mounted on a 
rotating body 104. When current is applied to a stator 107, the rotor 106 
is rotated in the direction of the arrow A. As the rotating body turns, it 
draws the air around it, so that the rotating body 104 and upper and lower 
bearings 101 are spaced from each other. Each of the upper and lower 
bearings 101 comprises: a bearing base 102; and a wear-resisting plastic 
member 103 bonded to the bearing base 102. 
The shaft of the motor is held vertical, and its thrust end supporting the 
whole weight of the rotating body 104 is floated by the force of repulsion 
induced between two permanent magnets 108 and 109 which are set with the 
same poles. 
Thus, the motor is in a non-contact state both in the radial direction and 
in the thrust direction. Therefore, when the motor is turned at high 
speed, it should rotate smoothly; however, in practice, it vibrates. There 
are some causes for the vibration of the motor. One of the causes is the 
imbalance of the rotating body. This cause may be eliminated by detecting 
the imbalance of the rotating body 104 with a balance tester. Another 
cause of the vibration is the bearings. In this case, the vibration cannot 
be eliminated with the tester. 
More specifically, the vibration may attributed to the fact that the upper 
and lower bearings 101 are not coaxial, or to the configuration of those 
bearings 101. In the case where the upper and lower bearings 101 are not 
coaxial, the vibration occurs as follows: That is, the rotating body 104 
is turned with its rotating axis tilted because of the misalignment of the 
bearings 101, so that the gap between the rotating body 104 and the 
bearings 101 becomes non-uniform, thus causing the vibration. In order to 
eliminate the misalignment of the bearings 101, heretofore the following 
method is employed: That is, the position of one of the bearings 101 is 
adjusted with screws in three directions until it aligns with the other 
bearing 101. 
The vibration attributing to the configuration of the bearings 101 is 
called "whirl vibration". It has been considered that the whirl vibration 
can be eliminated by using a bearing 101 having an inner surface which is 
made up of a plurality of circular-arc surfaces as shown in FIG. 2 or 3. 
However, in practice, the vibration cannot be eliminated even with such a 
bearing. This will be described in more detail. 
As shown in FIG. 2 or 3, the gap between the rotating body 104 and the 
bearing 101 is gradually decreased in the direction of rotation. As the 
rotating body 104 is turned, the gas in the gap is drawn viscously by the 
relative movement of the surfaces; that is, the gas is pushed in the gap, 
thus producing a pressure (or positive pressure) to float the rotating 
body 104. Thereafter, the gap is gradually increased in the direction of 
rotation of the rotating body 104. In this case, the viscosity of the gas 
produces a pressure (or negative pressure) to pull the rotating body 104. 
As a result, while the rotating axis rotates with an angular speed 
.omega., the rotating body 104 turns around the center of the bearing 101 
in the direction of rotation of the rotating axis with a radius 
corresponding to an amount of eccentricity e and with a swirling angular 
speed .omega..sub.0. The swirling angular speed .omega..sub.0 is 1/2 to 
1/3 of the angular speed .omega.. 
Furthermore, the vibration may be caused when the gap between the rotating 
body 104 and the bearings 101 is changed with temperature. This is due to 
the fact that the rotating body 104 is different from the bearings 101 in 
thermal expansion coefficient. This will be described in more detail 
below. 
Heretofore, the bearing base 102 is made of copper or plastic material, and 
the rotating body 104 is made of iron or steel. When the rotating body 104 
is turned, the temperature is increased, and, since the thermal expansion 
coefficient of the bearing base 102 is higher, the inside diameter of the 
latter is increased by thermal expansion more than the diameter of the 
rotating body 104, so that the gap therebetween is increased. Hence, the 
dynamic pressure is decreased while the rigidity is lowered, so that 
vibration occurs. 
On the other hand, if, in the case where the wear-resisting plastic member 
on the bearing base 102 is relatively thick, the plastic member is smaller 
in thermal expansion coefficient than the bearing base 102, then the gap 
is increased similarly as in the above-described case. However, when the 
plastic member is equal to or larger than the bearing base 102 in thermal 
expansion coefficient, then the gap is decreased, so that the bearing loss 
is increased, and the temperature rises greatly. 
As was described above, the conventional dynamic pressure bearing is 
disadvantageous in that it will vibrate the rotating body 104 unstably. 
The vibration attributing to the misalignment of the two bearings 101 can 
be eliminate by adjusting the positions of the bearings 101 so that they 
are coaxial with each other. However, this method provides another problem 
that the adjustment required time and labor, and the number of components 
is increased. 
When the speed of rotation is increased, the energy loss is increased: that 
is, the temperature rises, so that the bearing gap is varied, with the 
result that the vibration is produced. Hence, it is difficult to increase 
the speed of rotation to a high value (20,000 rpm or higher). In order to 
overcome this difficulty, it is necessary to externally cool the bearing 
101, and accordingly the motor. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to eliminate the 
above-described difficulties accompanying a conventional rotary polygon 
mirror device. 
More specifically, an object of the invention is to provide a dynamic 
pressure bearing and a rotary polygon mirror device using the bearing, 
which is simple in construction, and operates stably over a wide range of 
high speeds, and is low in cost. 
The foregoing object and other objects of the invention have been achieved 
by the provision of a rotary polygon mirror device comprising: a rotor 
including a polygon mirror secured to one end face of a cylinder, a 
central rod secured inside the cylinder in such a manner that the central 
rod is extended along the axis of the cylinder, and a magnet secured to 
the end face of the central rod, the magnet forming thrust bearing means, 
and a stationary casing which substantially coaxially surrounds the 
cylinder with a gap therebetween; in which, according to the invention, 
the gap provides radial bearing means, and the cylinder and the stationary 
casing are equal in thermal expansion coefficient. 
In the device of the invention, the bearing gap is maintained unchanged 
even when the bearing means and the rotor change in temperature, and the 
bearing means and the rotor can be efficiently cooled when the polygon 
mirror is rotated at high speed. Furthermore, when the polygon mirror is 
started or stopped, the bearing surfaces are prevented from being worn out 
by being brought into contact with each other. Thus, the device is able to 
operate stably for a long period of time. 
Also, the foregoing object and other objects of the invention have been 
achieved by the provision of a dynamic pressure bearing comprising: a 
bearing base, and a wear-resisting member bonded to the inner surface of 
the bearing base which surface confronts with a rotating body, in which, 
according to the invention, the wear-resisting member has a predetermined 
bearing configuration. 
In order to prevent the misalignment of the bearing, both in the case where 
it has a plurality of bearing surfaces, and in the case where the bearing 
surface is wide along the axis of rotation, a single bearing base is 
employed, and the wear-resisting plastic members are bonded to the inner 
surface of the bearing base, and are machined simultaneously to have the 
predetermined bearing configuration. 
Therefore, the bearing surfaces thus formed are in alignment with each 
other, and the resultant bearing is free from misalignment. 
In order to suppress the occurrence of whirl vibration in the case where 
the bearing surface is made up of a plurality of circular-arc surfaces, it 
is essential to eliminate the negative pressure. For this purpose, the 
bearing is so set that the bearing surface surrounds a part of the 
rotating body substantially coaxially, and it is so shaped that the gap 
between the surface and the rotating body is decreased gradually in the 
direction of rotation of the rotating body, and changed abruptly from a 
minimum value to a maximum value at predetermined positions. Therefore, in 
the gap, the air viscosity is lost quickly, and therefore no negative 
pressure is produced. However, if the ratio of the minimum value to the 
maximum value is excessively small, then the negative pressure is liable 
to be formed when the rotating body turns at high speed. Thus, in order to 
turn the rotating body stably, it is desirable to set the ratio to at 
least six (6). 
In the bearing, the gap is increased abruptly. Therefore, the bearing is 
free from the difficulty that the gap is gradually increased to produce 
negative pressure. However, as the difference between the minimum gap and 
the maximum gap is decreased, the negative pressure is liable to be 
produced, and therefore the ratio of the maximum gap to the minimum gap 
should be at least six (6). 
Further, the foregoing object and other objects of the invention have been 
achieved by the provision of a rotary polygon mirror device comprising: a 
rotating body with a polygon mirror; a fluid dynamic pressure bearing 
which surrounds a part of the rotating body in such a manner that the 
bearing is substantially coaxial with the rotating body, and has a 
wear-resisting member formed on the inner surface thereof which is 
confronted with the rotating body, the part of the rotating body being a 
passageway for the magnetic flux of an electric motor formed therein, in 
which, according to the invention, the inner surface of the bearing, which 
is confronted with the rotating body, is such that the gap between the 
surface and the rotating body is gradually decreased in the direction of 
rotation of the rotating body, and increased abruptly at predetermined 
positions. 
The rotary polygon mirror device dispenses with a troublesome adjusting 
operation such as alignment, and is free from the whirl vibration. 
In the bearing of the device, the bearing base is made of the same material 
as the rotating body, and the wear-resisting member formed on the inner 
surface of the bearing base is smaller in thickness than the bearing base, 
which contributes to stabilization of the temperature withstanding 
characteristic of the device. 
In order to improve the temperature withstanding characteristic, it is 
desirable that the wear-resisting member is 0.5 mm or less in thickness. 
Furthermore, in the bearing, the bearing base is equal in thermal expansion 
coefficient to the rotating body, and therefore the gap is maintained 
constant regardless of temperature variation. In addition, the 
wear-resisting plastic member bonded to the bearing base is small in 
thickness, and therefore the bearing is less affected by the thermal 
expansion of the plastic member. 
The bearing is secured directly to the housing, so as to be cooled 
effectively. 
The nature, principle, and utility of the invention will be more clearly 
understood from the following detailed description of the invention when 
read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of this invention will be described with reference to 
the accompanying drawings. 
An example of a rotary polygon mirror device, which constitutes a first 
embodiment, as shown in FIG. 4, comprises a rotary cylinder 1, and a 
regular polyhedron 2 with mirrors on its sides which is secured to one end 
of the rotary cylinder 1 with a retaining screw 13. The cylinder 1 has a 
central rod 3 along its central axis. Magnets 6 forming drive motor means 
are secured to the central rod 3, and a permanent magnet 4 is fixedly 
bonded to the end face of the central rod 3. 
The rotary cylinder 1 is accommodated in a stationary external housing 
which is made up of a head cover 10 having light going-in-and-out windows 
(not shown), a side casing 11, and an end casing 12. 
An air-core type stator coil 8 is secured to the end casing 12 in such a 
manner that it is confronted with the motor magnets 6 inside the rotary 
cylinder 1, and is coaxial with the magnets 6 with a certain gap 
therebetween. The stator coil 8 forms a part of the drive motor means. 
A permanent magnet 5 is positioned in the inner surface of the end casing 
12 at the center. The permanent magnet 5 and the permanent magnet 4 of the 
central rod 3 form thrust bearing means. 
The rotary cylinder 1 is set in the side casing 11 in such a manner that 
there is a small gap of the order of 10 to 100 .mu.m between the inner 
surface 9 of the side casing 11 and the outer surface 7 of the rotary 
cylinder 1. The outer surface of the rotary cylinder 1 is cylindrical, and 
the inner surface of the side casing 11 is as shown in FIG. 5. More 
specifically, the inner surface of the side casing 11 consists of 
multiple-circular-arc surfaces 15, 15' and 15", thus forming dynamic 
pressure radial bearing means which is stable in rotation, and shows a 
high bearing rigidity. 
In formation of the dynamic pressure radial bearing means, the gap 14 
between the bearing and the bearing surface is an indispensable factor. As 
for the gap, there is a most suitable value with which the rotation is 
made stable with respect to the dimensions of the bearing and the aimed 
speed of rotation. 
When the polygon mirror is rotated at high speed, the temperatures of the 
rotary cylinder 1 and the side casing 11 are increased greatly by the 
windage loss and the bearing loss of the polygon mirror. In the device of 
the invention, in order to maintain the bearing gap unchanged against the 
above-described temperature increase, the cylindrical portion of the 
rotary cylinder 1 and the side casing 11 are made of materials which are 
equal in thermal expansion coefficient. On the other hand, in order to 
improve the performance of rotation of the motor, it is desirable that the 
rotary cylinder 1 be made of a material high in magnetic permeability. In 
addition, in the drive motor means of the rotary polygon mirror device, 
the magnets 6 are secured to the rotary cylinder 1. Therefore, with the 
efficiency of the motor taken into consideration, it is unnecessary to use 
silicon steel or electromagnetic soft iron for the rotary cylinder and the 
side casing. Hence, in the embodiment, the rotary cylinder 1 and the side 
casing 11 are made of a structural steel. 
Furthermore, when the polygon mirror is started or stopped, the rotary 
cylinder 1 may be brought into contact with the side casing 11, thus 
wearing out the bearing surfaces. In order to eliminate this difficulty, 
it is preferable to form a wear-resisting layer on the outer surface of 
the rotary cylinder 1 and/or the inner surface of the side casing 11. For 
this purpose, in the embodiment, a polyimide resin layer is formed on the 
inner surface 9 of the side casing 11. In order to prevent the rotary 
cylinder from rusting, the surface of the rotary cylinder 1 may be plated 
with nickel or chromium. 
In the above-described embodiment, the bearing means has its base on the 
casing, and therefore the bearing means can be cooled with high 
efficiency. Accordingly the temperature rise of the rotary cylinder 1 can 
be effectively prevented. 
In the above-described embodiment, the bearing surfaces, which are 
non-circular-arc surfaces, are formed in the inner surface of the side 
casing 11. However, the non-circular-arc surfaces may be formed in the 
outer surface of the rotary cylinder 1. Furthermore, the wear-resisting 
layer on the bearing surfaces may be formed by using other wear-resisting 
resin such as fluoro-resin. In addition, the wear-resisting layer may be 
formed by plating the surface with suitable metal, or may be a carbon 
film. 
FIG. 6 shows one modification of the rotary polygon mirror device, which 
constitutes a second embodiment of the invention. In the modification, in 
order to cool the bearing means more positively, cooling fins 16 are 
formed on the outer cylindrical surface of the side casing 11. Those fins 
16 act to prevent the temperature rise of the bearing means and the rotary 
cylinder 1, whereby the predetermined bearing gap is maintained, and the 
wear-resisting layer is held unchanged in characteristic. 
A dynamic pressure bearing according to a third embodiment of the invention 
is as shown in FIGS. 7(a), 7(b) and 8. 
The bearing 101, as shown in FIG. 7(a), comprises: a bearing base 102 made 
of a hard material; and a wear-resisting member, namely, a plastic member 
103 bonded to the inner surface of the bearing base 102 with adhesive. 
More than one plastic member may be employed as shown in FIG. 7(b). The 
inner surface of the plastic member 103, as shown in FIG. 8, is made up of 
a plurality of circular-arc surfaces. The inner surface of the plastic 
member 103 is formed with the bearing base held, for instance, on an NC 
machine. In the case of the bearing shown in FIG. 7(b), the two plastic 
members 103' are machined simultaneously. 
FIG. 9 shows a polygon mirror motor using the bearing shown in FIGS. 7(a), 
7(b) and 8. A polygon mirror 105 and a rotor 106 are mounted on a rotating 
body 104. A stator 107 is set around the bearing 101. When current is 
applied to the stator 107, the rotor 106 is turned in the direction of the 
arrow A. As the rotor 106 is turned, the bearing 101 and the rotating body 
104 are placed in a non-contact state, being spaced apart from each other. 
In this case, the plastic members 103 of the bearing 101 are coaxial. 
Hence, the rotating body is prevented from being vibrated by the 
misalignment of the upper and lower bearings. That is, it is unnecessary 
to adjust the bearings for alignment. 
FIG. 10 shows how the whirl vibration occurs, in which the horizontal axis 
represents ratios (=G.sub.a /G.sub.b) of minimum gaps G.sub.a to maximum 
gaps G.sub.b, and the vertical axis represents numbers of revolutions per 
minute. As is apparent from FIG. 10, when the air gap ratio is smaller 
than six (6), the whirl vibration occurs when the speed is in a particular 
range; whereas when it is equal to or larger than six (6), no whirl 
vibration occurs with any speed. In this connection, the presence or 
absence of the whirl vibration is determined as follows: A vibration meter 
is connected to a dynamic pressure bearing under test. When, under this 
condition, the meter measures a vibration the frequency of which is lower 
than that of the vibration produced in the primary rotation (a vibration 
having a frequency of 200 Hz being produced when the rotating body is 
turned at a speed of 12,000 rpm), it is determined that the whirl 
vibration is present. 
The bearing base 102 may be made of non-magnetic material such as aluminum 
or copper, or magnetic material such as steel. 
The plastic member 103 may be made of polyimide resin, polyamide resin, or 
polyacetal resin. Of those materials, the polyimide resin is most suitable 
because it is high both in hardness and in wear resistance. 
In bonding more than one plastic member 103 to the inner surface of the 
bearing base 101, those plastic members 103 should be moved along the axis 
of the bearing 101. In this case, the plastic members are set coaxial with 
each other, forming a step therebetween, and therefore the rotating body 
can be turned stably. In addition, since more than one plastic members 103 
can be machined simultaneously, the bearing is improved in manufacturing 
efficiency as much. 
FIG. 11 shows a polygon mirror rotating motor using the bearing shown in 
FIGS. 7(a), 7(b) and 8 according to a fourth embodiment of this invention. 
As shown in FIG. 11, a polygon mirror 105 and a rotor 106 are mounted on a 
rotating body 104. The rotor 106 comprises a dipole permanent magnet 111 
for generating magnetic flux, and a yoke 110 made of magnetic material 
through which the magnetic flux spreads. A coil 107 is fixedly secured 
between the permanent magnet 111 and the yoke 110. Current is applied to 
the coil 107 to turn the rotating body 104. 
The outer cylindrical surface of the yoke 110 is confronted with the 
bearing 101. As the rotating body 104 is turned in the direction of the 
arrow A, it is spaced from the bearing 101 owing to the structure of the 
latter 101. Since the bearing is shaped as shown in FIG. 8, the rotating 
body 104 can be turned smoothly even at a high speed. 
The bearing base 102, and the yoke 110 of the rotor 106 are made of one and 
the same material. Therefore, even when heat is generated by the high 
speed rotation of the rotating body, the gap between the bearing 101 and 
the rotor 106 is maintained unchanged. 
In the bearing 101, the wear-resisting plastic member 103 bonded to the 
bearing base 102 is made as small in thickness as possible, so that it may 
not be affected by the thermal expansion. 
FIG. 12 shows how the gap varies when the plastic material is changed 
thickness, in which the horizontal axis represents variations in thickness 
of the wear-resisting plastic member, and the vertical axis represents 
variations of the gap. More specifically, in FIG. 12, the curve indicated 
by the two-dot chain line is for the case where the rotating body is 
larger in thermal expansion coefficient than the bearing base, whereas the 
curve indicated by the solid line is for the case where the rotating body 
is equal in thermal expansion coefficient to the bearing base. 
As is seen from FIG. 12, in the case where the thickness of the plastic 
member 103 is set to 0.5 mm or less, the gap is less affected by heat. In 
this case, the variation of the gap is 1 .mu.m or less, and the energy 
loss is small, so that the rotating body is turned stably. The same effect 
may be obtained when, in the case where the thermal expansion coefficient 
of the bearing base 102 is set 1.7 times that of the yoke 110, the 
thickness of the plastic member 103 is set to 2.5 mm (as seen from the 
curve indicated by the two-dot chain line). However, this method is not 
preferable, because the material cost is increased. Thus, the thickness of 
the plastic member 103 should be in a range of from 0.1 to 0.5 mm. 
In the polygon mirror rotating motor shown in FIG. 11, the bearing 101 is 
secured directly to the housing 12 so as to effectively radiate the heat 
which is generated by the high speed rotation. 
As was described above, in the device of the invention, the bearing means 
and the rotor confronted with the latter are made of the same material, so 
that they are equal in thermal expansion coefficient. Further in the 
device, the bearing means, being provided by the casing, is effectively 
cooled. Thus, the bearing gap, which is one of the important factors in 
forming an air bearing, can be held stable against the variations of 
temperature, and therefore the polygon mirror is stable in performance 
over a wide range of high speeds. In addition, the material of the base of 
the bearing means may be a structural steel, and therefore a rotary 
polygon mirror low in manufacturing cost and high in performance can be 
realized according to the invention. 
As was described above, the wear-resisting members, namely, the plastic 
members bonded to the bearing base are machined simultaneously to have a 
plurality of circular arc surfaces. Therefore, the bearing is high in 
alignment; that is, it is unnecessary to adjust the bearing. Furthermore, 
since the ratio of the maximum gap to the minimum gap is set to at least 
six (6), the bearing will not vibrate the rotating body. 
In addition, in the bearing, the bearing base is made of the same material 
as the rotating body, and the plastic members bonded to the bearing base 
are smaller in thickness than the bearing base, and the bearing is secured 
directly to the housing. Hence, the bearing gap is varied less even when 
the temperature changes. 
Thus, the rotary polygon mirror device according to the invention operates 
stably over a wide range of speeds. It goes without saying that the 
technical concept of the invention can be effectively employed for 
provision of other high speed and high precision rotating structures. 
While the invention has been described in connection with the preferred 
embodiments, it will be obvious to those skilled in the art that various 
changes and modifications may be made therein without departing from the 
invention, and it is aimed, therefore, to cover in the appended claims all 
such changes and modifications as fall within the true spirit and scope of 
the invention.