Dynamic-pressure gas bearing structure and optical deflection scanning apparatus

A dynamic-pressure gas bearing structure includes a columnar shaft made of a silicon-nitride-based ceramic sintered body, a hollow cylindrical sleeve opposed to the shaft as keeping a clearance in a radial direction, the sleeve being made of a silicon-nitride-based ceramic sintered body, and at least three flat face portions located on a peripheral surface of the shaft and at equal intervals to the circumference along the peripheral surface. The flat face portion includes a plurality of unit planes continuously formed at predetermined angles to a direction of the circumference on the peripheral surface of the shaft. The unit planes are formed so as to extend substantially in parallel with an axial direction of the shaft.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to a dynamic-pressure gas bearing 
structure, and more particularly, to a bearing structure in a rotation 
drive part of an optical deflection scanning apparatus used for example in 
laser beam printers etc. 
2. Related Background Art 
Generally, members for constituting the rotation drive part of the optical 
deflection scanning apparatus used in the laser beam printer apparatus are 
required to be resistant to high-speed rotation. For example, with an 
increase of print speed a drive portion of a rotary polygon mirror for 
scanning with laser light is required to rotate at the rotational speed of 
20000 or more rpm. 
Ball bearings have been used heretofore in a bearing portion of this 
rotation drive part. However, sliding portions easily got into a lack of 
grease in the high-speed range of not less than about 12000 rpm, which 
increased a risk to cause seizure of ball bearing due to lubrication 
failure. It was thus difficult to form the rotation drive part to meet the 
demand of high-speed rotation as described above as long as the ball 
bearings were employed for the bearing portion of the rotation drive part. 
In order to solve the above problem, a fluid dynamic-pressure bearing 
structure for supporting a rotor in a non-contact manner is employed for 
the rotation drive part. In the dynamic-pressure bearing structure of this 
type, herringbone or spiral grooves are formed on the peripheral surface 
of a shaft. Oil or grease is charged between the shaft and the sleeve, and 
a dynamic pressure occurs when the fluid is caught up into the above 
grooves during rotation, whereby, for example, the sleeve as a rotor can 
rotate without contact to the peripheral surface of shaft. 
The fluid dynamic-pressure bearing structure as described above, however, 
has a defect that a drive torque is large because of high viscosity of the 
lubricating oil. Particularly, because the oil comes to have heat under 
high-speed rotation, it is necessary to prepare a mechanism for cooling 
the lubricating oil. This makes the structure of the optical deflection 
scanning apparatus itself complex and makes compactification of apparatus 
difficult. Further, complexity of apparatus raises the problem of an 
increase of manufacturing cost. 
It has been considered to employ an air bearing using air as a fluid in the 
fluid dynamic-pressure bearing structure of the above type. Since this 
arrangement is free of occurrence of the problem due to the lubricating 
oil, it can be used under higher-speed rotation than the above bearing 
structure using the oil as a fluid can. It is considered that this 
structure can simplify the structure of apparatus itself because it does 
not need the mechanism for cooling the lubricating oil. 
Even if employing the air bearing, there is, however, a possibility of 
direct sliding between opposed surfaces of the shaft and the sleeve 
because of disturbance or the like during rotation. This raised the 
problem that the risk to cause seizure in the sliding portions became 
greater than in the fluid dynamic-pressure bearing structure employing the 
oil as a fluid. 
Then, Japanese Laid-open Patent Application No. 5-106635 discloses 
employing ceramics, particularly a silicon-nitride-based ceramic sintered 
body, as a material for the bearing portion. Since this can improve 
properties of wear resistance and shock resistance, bearing members 
obtained have high reliability against damage during high-speed rotation. 
The shaft as shown in FIG. 1 is known as a dynamic-pressure air bearing 
member made of a ceramic material, applicable to the optical deflection 
scanning apparatus. As shown in FIG. 1, dynamic-pressure generating 
grooves 51 are formed on the peripheral surface 50 of the shaft. 
It is, however, difficult to form by ordinary machining the groove shape 
shown in FIG. 1 on the surface of the hard-to-process material such as 
ceramics. Such groove shape is formed by etching, blasting, or the like. 
Against it, Japanese Utility Model Publication No. 1-7849 discloses the 
dynamic-pressure gas bearing apparatus having the grooves easily formed by 
machining and having improved bearing accuracy. In this dynamic-pressure 
gas bearing apparatus there are a plurality of grooves on the peripheral 
surface of a cylindrical shaft, each groove having a lateral cross section 
which is arcuate in mirror symmetry and being parallel to the axial 
direction. 
It is, however, very difficult to form the arcuate grooves in mirror 
symmetry as disclosed in the official gazette of Japanese Utility Model 
Publication No. 1-7849 in the hard-to-process material such as ceramics, 
particularly in the silicon-nitride-based ceramic sintered body. The depth 
of the grooves, according to the embodiment described in the official 
gazette, is determined within the range of some ten .mu.m to some hundred 
.mu.m. It has been practically impossible to put the formation of the 
grooves of such depth in the hard-to-process ceramic material into mass 
production on an industrial and economical basis in respect of processing 
accuracy, processing efficiency, and manufacturing cost. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to simplify the shape 
of the peripheral surface of the shaft in the rotation drive part in order 
to realize a dynamic-pressure gas bearing structure capable of stably 
rotating at higher speed and an optical deflection scanning apparatus 
provided therewith. 
The dynamic-pressure gas bearing structure according to the present 
invention has a rotation drive part consisting a columnar shaft and a 
hollow cylindrical sleeve opposed to the shaft as keeping a clearance in 
the radial direction. The shaft and sleeve are made of a 
silicon-nitride-based ceramic sintered body. The peripheral surface of the 
shaft comprises at least three flat face portions located at equal 
intervals to the circumference along the peripheral surface. Each flat 
face portion is composed of a plurality of unit planes continuously formed 
in the peripheral surface of the shaft at predetermined angles with 
respect to the circumferential direction. Each unit plane is formed as 
extending substantially in parallel with the axial direction of the shaft. 
From extensive and intensive study and research, the inventors found out 
that the arrangement wherein the at least three flat face portions each 
composed of a plurality of unit planes were located on the peripheral 
surface of the shaft was able to meet specified performance of the optical 
deflection scanning apparatus under high-speed rotation of not more than 
25000 rpm. Specifically, as shown in FIGS. 2A and 2B, the peripheral 
surface 101 of the shaft 1 is ground in the depth of some .mu.m to some 
ten .mu.m to form planes parallel to the axial direction. Defining the 
planes as unit planes 105a, 105b, 105c, plural (three in FIGS. 2A and 2B) 
planes are continuously formed in the peripheral surface 101 of the shaft 
1 at predetermined angles with respect to the circumferential direction. 
Flat face portions 105, each formed in the above arrangement, are located 
at equal intervals at at least three positions 102, 103, 104 with respect 
to the circumference along the peripheral surface 101 of the shaft. 
Formation of the above flat face portions in the peripheral surface of 
shaft can be realized by removing parts from the peripheral surface of 
shaft being a nearly cylindrical surface by surface grinding. Processing 
is thus easy on hard-to-process ceramics such as the silicon-nitride-based 
sintered body. Accordingly, in forming a dynamic-pressure generating 
portion in the clearance between the shaft and the sleeve, the processing 
efficiency can be enhanced to thereby decrease manufacturing cost as 
compared with the conventional structure in which the grooves of specific 
shape are formed on the peripheral surface of shaft. 
Further, excellent bearing stability can be secured under high-speed 
rotation of not less than 25000 rpm simply by providing the peripheral 
surface of the shaft with the flat face portions defined as described 
according to the present invention. The optical deflection scanning 
apparatus provided with the dynamic-pressure gas bearing structure of the 
present invention can achieve high accuracy as to inclination of laser 
beam reflecting faces relative to the vertical direction of the polygon 
mirror, that is, as to face inclination. 
Further, the silicon-nitride-based ceramic sintered body is employed as a 
material for the shaft and the sleeve constituting the dynamic-pressure 
gas bearing structure of the present invention. Since the members made of 
this silicon-nitride-based ceramic sintered body are lighter than the 
conventional metal members, the inertial mass in motor load can be 
decreased. This permits reduction of drive torque, which enables the 
optical deflection scanning apparatus to be operated in lower dissipation 
power. 
The inventors found out that, regarding a flat face portion composed of a 
combination of plural unit planes as a groove, stability under high-speed 
rotation was able to be achieved by forming the above grooves having the 
limited dimensions in the peripheral surface of the shaft. Specifically, 
regarding such flat face portion as the groove, three or more grooves C, 
each being formed, as shown in FIG. 2B, so that the groove depth d of the 
flat face portion may be not more than 0.020 mm and a center angle .theta. 
of the circumference (arc) along the peripheral surface 101 of the shaft, 
corresponding to the width of groove C, may be not less than 10.degree., 
are formed at equal intervals to the circumference along the peripheral 
surface 101 of the shaft. Further, a difference between the outer diameter 
of the shaft and the inner diameter of the sleeve is defined below 0.010 
mm. The inventors found out that the bearing structure was able to 
demonstrate excellent bearing stability during high-speed rotation by 
forming the grooves C having the thus defined dimensions in the peripheral 
surface of shaft and limiting the clearance between the shaft and the 
sleeve as described above. 
This can conceivably be achieved by the fact that a support membrane formed 
by a flow of air between the shaft and the sleeve can be formed with 
efficiency within the range of the limited dimensions as described above. 
It is also considered that the dynamically stable support structure can be 
provided by the feature that the support is simultaneously effected in 
directions equally distributed with respect to the circumference at three 
or more portions on the peripheral surface of shaft. 
In contrast with it, unstable behavior is recognized as discussed above 
when the grooves having the depth of some 10 to some 100 .mu.m are formed 
as described in Japanese Utility Model Publication No. 1-7849. A 
conceivable reason is as follows. As the grooves become deeper, 
disturbance occurs in the flow of air under high-speed rotation. The 
disturbance of the flow of air interrupts formation of an adequate support 
membrane between the shaft and the sleeve, thus causing unstable behavior. 
The shape of the grooves can be arbitrarily selected within the range of 
the dimensions even in a case other than the combination of a plurality of 
unit planes described above as long as the combination of the depth and 
width of grooves and the difference between the outer diameter of the 
shaft and the inner diameter of the sleeve satisfy the above dimensional 
conditions. Specifically, the shape of the grooves is determined by 
requirements of manufacturing steps. 
Applying the dynamic-pressure gas bearing structure of the present 
invention to the rotation drive part of the optical deflection scanning 
apparatus rotating at high speed and at high accuracy, a laser beam 
printer apparatus can be formed with higher print quality and with 
capability of higher-speed printing than those of the conventional 
apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be explained in detail, based on the embodiments 
shown in FIG. 2A to FIG. 6. 
Embodiment 1 
FIG. 3 shows the details of the rotation drive part (the drive part of 
polygon mirror) of the optical deflection scanning apparatus provided with 
the dynamic-pressure gas bearing structure according to an embodiment of 
the present invention. The rotation drive part shown in FIG. 3 is 
incorporated in a laser beam printer apparatus using the optical 
deflection scanning apparatus shown in FIG. 4. 
In FIG. 3, a stationary shaft 1, which is a shaft made of a ceramic 
material, is fixed to a housing 3 of a drive motor 500. A rotary sleeve 2, 
which is a sleeve made of the ceramic material, is rotatably fit over the 
stationary shaft 1. In the peripheral surface 101 of the stationary shaft 
1 there are at least three flat face portions 105 (unit processed 
portions) composed of plural unit planes 105a, 105b, 105c, . . . as shown 
in FIG. 2B. A flange 4 made of aluminum, brass, or the like is fixed by 
shrinkage fit or the like on the outer periphery of the rotary sleeve 2. A 
drive magnet 5 is fixed by adhesion or the like to the outer periphery of 
the flange 4. Further, a base 6 is fixed on the housing 3. A stator 7 is 
placed on the base 6 as opposed to the drive magnet 5, thereby forming the 
drive motor 500 for rotating the rotary sleeve 2. 
On the other hand, a second permanent magnet 9 is fixed to the lower end of 
the rotary sleeve 2 so that one magnetic pole of the second permanent 
magnet 9 may be vertically opposed to the same magnetic pole of a first 
permanent magnet 8 placed on the stationary shaft 1. A third permanent 
magnet 10 is placed near the rotary sleeve 2 on housing 3 so as to exert a 
repulsive force in a direction to urge the second permanent magnet 9 fixed 
at the lower end of the rotary sleeve 2 toward the first permanent magnet 
8 provided on the stationary shaft. 
A lid 19 for covering the stationary shaft 1 is provided at the upper end 
of the rotary sleeve 2. This forms an air reservoir 20 between the rotary 
sleeve 2 and the stationary shaft 1. This lid 19 is provided with an air 
vent hole 32 to facilitate assembling of the rotary sleeve 2 and the 
stationary shaft 1. After the assembling a seal member 33 is fixed to the 
lid in order to seal the air vent hole 32. 
The rotary polygon mirror 11 is fixed to the flange 4 by a plate spring 12 
or the like. The drive motor 500 thus constructed is incorporated into an 
optical box 14 of the optical deflection scanning apparatus, as shown in 
FIG. 4. The rotary polygon mirror 11 is rotated by the drive motor 500. In 
FIG. 4, a laser unit 15 is disposed in the optical box 14. A laser beam L 
emitted from the laser unit 15 is condensed by lenses 16, 17, and 
deflection-scans a photosensitive member 18, which is a recording medium, 
with rotation of the rotary polygon mirror 11 by the drive motor 500. 
The shaft 1 and sleeve 2 as shown in FIG. 3 are made of the 
silicon-nitride-based ceramic sintered body. The silicon-nitride-based 
sintered body is produced specifically as follows. 
Prepared is raw powder of Si.sub.3 N.sub.4 having the average grain 
diameter of 0.3 .mu.m, the particle size distribution of 3.sigma.=0.20 
.mu.m, the rate of .alpha. crystallinity of 96.5%, and the oxygen content 
of 1.4% by weight. Wet mixing using a polyamide ball mill is carried out 
in ethanol for 100 hours at the ratio of 90% by weight of the above raw 
powder of Si.sub.3 N.sub.4, 4% by weight of Y.sub.2 O.sub.3 powder with 
the average grain diameter of 0.8 .mu.m, 3% by weight of Al.sub.2 O.sub.3 
powder with the average grain diameter of 0.5 .mu.m, 1% by weight of AlN 
powder with the average grain diameter of 1.0 .mu.m, and 2% by weight of 
MgO powder with the average grain diameter of 0.5 .mu.m. After that, the 
mixture powder obtained after dried is subjected to CIP (cold isostatic 
pressing) under the pressure of 3000 kgf/cm.sup.2. A formed body thus 
obtained is held at the temperature of 1450.degree. C. under a nitrogen 
gas atmosphere of one atmospheric pressure for six hours. Further, it is 
primarily sintered at the temperature 1550.degree. C. for three hours. A 
sintered body obtained is then secondarily sintered at the temperature of 
1600.degree. C. under a nitrogen gas atmosphere of 1000 atmospheric 
pressures for one hour. 
The silicon-nitride-based sintered body thus obtained contains crystal 
particles in the linear density of 35 or more particles per length 30 
.mu.m, and the volume percentage of its grain boundary phase is not more 
than 15% by volume. Further, the silicon-nitride-based sintered body 
contains pores with the maximum diameter of not more than 20 .mu.m, and 
the porosity of the pores is not more than 3%. 
The shaft 1 and sleeve 2 are made of the silicon-nitride-based sintered 
body obtained in the above-described manner. 
The shaft 1 is processed in the following method. 
The surface of the shaft is ground using a grinding tool of diamond 
abrasive grains by an amount of the depth 10 .mu.m, thus forming a plane 
on the peripheral surface of shaft. After processing of the above plane, 
the shaft is rotated and the same surface grinding as above is repeated so 
as to continuously connect the plane with planes at predetermined angles 
to the circumference along the peripheral surface of shaft. The flat face 
portion composed of a plurality of planes thus formed will be called as a 
unit processed portion. After completion of processing of this unit 
processed portion, further unit processed portions are formed in a 
necessary number on the peripheral surface of shaft. In this case, these 
unit processed portions are arranged at equal intervals to the 
circumference along the peripheral surface of shaft, depending upon the 
number of portions. 
Embodiment 2 
The sleeve and shaft made of the silicon-nitride-based sintered body were 
produced according to Embodiment 1. The flat face portions (unit processed 
portions) 105 as shown in FIG. 2B are formed at three positions 102, 103, 
104, as shown in FIG. 2A, on the peripheral surface of shaft as equally 
distributed to the circumference. The sleeve 2 and shaft 1 thus prepared 
are incorporated in the rotation drive part of the optical deflection 
scanning apparatus as shown in FIG. 3. 
As explained above, the dynamic-pressure gas bearing structure of the 
present invention has the rotation drive part including the columnar shaft 
1 and the hollow cylindrical sleeve 2 opposed to the shaft as keeping a 
clearance in the radial direction. The shaft 1 and sleeve 2 are made of 
the silicon-nitride-based ceramic sintered body. The peripheral surface 
101 of shaft includes the flat face portions 105 arranged at at least 
three positions 102, 103, 104 located at equal intervals to the 
circumference along the peripheral surface. Each flat face portion 105 is 
composed of a plurality of unit planes 105a, 105b, 105c continuously 
formed on the peripheral surface 101 of shaft at predetermined angles to 
the circumferential direction. The plurality of unit planes 105a, 105b, 
105c are formed so as to extend substantially in parallel with the axial 
direction of shaft. 
Stability of the dynamic-pressure gas bearing structure was next evaluated 
with the shaft and sleeve produced as described above. 
The rotation drive part of the optical deflection scanning apparatus in the 
structure shown in FIG. 3 was set in an evaluation apparatus shown in FIG. 
5, and the rotation drive part was rotated at high speed of the number of 
revolutions of 25000 rpm. Vibration generated by the rotor during 
operation of the drive motor 500 was detected by a vibration pickup 200, 
was measured by a vibration meter 300, and was frequency-analyzed by FFT 
(fast Fourier transform apparatus) 400. A sample with no resonance 
appearing in the low-frequency region was determined as "stable". 
In the following description, one flat face portion 105 composed of a 
combination of plural unit planes 105a, 105b, 105c is regarded as a groove 
C, as shown in FIG. 2B. 
First evaluated was an effect of the number of grooves C comprised of the 
flat face portions 105 formed on the peripheral surface of shaft, on 
stability of bearing. As shown in Table 1, four shafts are prepared each 
with grooves in the same depth and width on the peripheral surface of 
shaft. Another shaft without grooves was also prepared. The number of 
grooves C was changed from shaft to shaft, and the grooves were arranged 
at equal intervals to the circumference along the peripheral surface of 
shaft. The shafts were coupled with respective sleeves to form scanner 
motors (the rotation drive parts of the optical deflection scanning 
apparatus) 500 so that differences (diameter differences) might become 
constant as shown in Table 1 between the outer diameter of the peripheral 
surface of shaft thus processed where no grooves were formed, and the 
inner diameter of the sleeve. Here, the width of grooves C is expressed by 
the center angle .theta. corresponding to a circular arc along the 
peripheral surface 101 of the shaft as shown in FIG. 2. 
Regarding the flat face portion 10b as the groove, the groove depth d of 
the flat face portions 105 is an average value of differences between the 
arc part (the dashed line part) along the peripheral surface 101 of shaft 
and the plural unit planes 105a, 105b, 105c. 
TABLE 1 
______________________________________ 
No. of Groove Groove Diameter 
No. grooves width depth difference 
Judge 
______________________________________ 
*1-1 1 15.degree. 
5 .mu.m 7 .mu.m 
unstable 
*1-2 2 15.degree. 
5 .mu.m 7 .mu.m 
unstable 
1-3 3 15.degree. 
5 .mu.m 7 .mu.m 
stable 
1-4 6 15.degree. 
5 .mu.m 7 .mu.m 
stable 
*1-5 0 0 0 7 .mu.m 
unstable 
______________________________________ 
*comparative examples 
As apparent from Table 1, stability of bearing was recognized even under 
high-speed rotation of 25000 rpm with the samples having three or more 
grooves C. 
Next evaluated was an effect of the depth of grooves of the flat face 
portions on the stability of bearing. Six shafts were prepared each with 
three grooves equally formed on the peripheral surface of shaft, but only 
the depth of grooves of the flat face portions was changed. The shafts 
were coupled with sleeves to form scanner motors as keeping constant the 
width of the grooves C formed on the peripheral surface of shaft and the 
difference between the outer diameter of the peripheral surface of shaft 
where no grooves were formed, and the inner diameter of sleeve. Stability 
of bearing was evaluated using the evaluation apparatus shown in FIG. 5. 
TABLE 2 
______________________________________ 
Groove Groove Diameter 
No. depth width difference 
Judge 
______________________________________ 
2-1 2 .mu.m 15.degree. 7 .mu.m 
stable 
2-2 5 .mu.m 15.degree. 7 .mu.m 
stable 
2-3 10 .mu.m 15.degree. 7 .mu.m 
stable 
2-4 20 .mu.m 15.degree. 7 .mu.m 
stable 
*2-5 50 .mu.m 15.degree. 7 .mu.m 
unstable 
*2-6 200 .mu.m 
15.degree. 7 .mu.m 
unstable 
______________________________________ 
*comparative examples 
As apparent from Table 2, it was recognized that the bearing became 
stabilized under high-speed rotation of 25000 rpm if the depth of grooves 
of the flat face portions was not more than 20 .mu.m. 
Next evaluated was an effect of the width of grooves C formed on the 
peripheral surface of shaft on the stability of bearing. Six shafts were 
prepared each with three grooves C formed on the peripheral surface of 
shaft as changing only the width of grooves C. As keeping constant the 
depth of the grooves formed on the peripheral surface of shaft and the 
difference between the outer diameter of the peripheral surface of shaft 
where no grooves were formed, and the inner diameter of sleeve, scanner 
motors were formed and evaluated as to the stability of bearing, using the 
evaluation apparatus shown in FIG. 5. 
TABLE 3 
______________________________________ 
Groove Groove Diameter 
No. width depth difference 
Judge 
______________________________________ 
*3-1 3.degree. 
5 .mu.m 7 .mu.m 
unstable 
*3-2 7.5.degree. 
5 .mu.m 7 .mu.m 
unstable 
3-3 10.degree. 
5 .mu.m 7 .mu.m 
stable 
3-4 20.degree. 
5 .mu.m 7 .mu.m 
stable 
3-5 40.degree. 
5 .mu.m 7 .mu.m 
stable 
3-6 80.degree. 
5 .mu.m 7 .mu.m 
stable 
______________________________________ 
*comparative examples 
As apparent from Table 3, it was recognized that the bearing became 
stabilized under high-speed rotation of 25000 rpm if the center angle 
corresponding to the width of the grooves C comprised of the flat face 
portions 105 was not less than 10.degree.. 
Next evaluated was an effect of the diameter difference (the difference 
between the outer diameter of the peripheral surface of shaft where no 
grooves were formed and the inner diameter of sleeve) determined by the 
combination of shaft with sleeve on the stability of bearing. Five shafts 
were prepared each with three grooves formed on the peripheral surface of 
shaft as keeping constant the width and depth of grooves. Scanner motors 
were constructed by combining the shafts with sleeves as changing the 
difference between the outer diameter of the peripheral surface of shaft 
where no grooves were formed, and the inner diameter of sleeve, and 
evaluation of stability of bearing was carried out using the evaluation 
apparatus shown in FIG. 5. 
TABLE 4 
______________________________________ 
Diameter Groove Groove 
No. difference 
width depth Judge 
______________________________________ 
4-1 3 .mu.m 15.degree. 5 .mu.m 
stable 
4-2 5 .mu.m 15.degree. 5 .mu.m 
stable 
4-3 7 .mu.m 15.degree. 5 .mu.m 
stable 
*4-4 10 .mu.m 15.degree. 5 .mu.m 
unstable 
*4-5 15 .mu.m 15.degree. 5 .mu.m 
unstable 
______________________________________ 
*comparative examples 
As apparent from Table 4, it was recognized that the bearing performance 
became stable even under high-speed rotation of 25000 rpm if the diameter 
difference is not more than 7 .mu.m, namely, below 10 .mu.m. 
As explained above, the dynamic-pressure gas bearing structure of the 
present invention is constructed in such an arrangement that, regarding 
one flat face portion 105 composed of a combination of plural unit planes 
105a, 105b, 105c as a groove C, the groove depth of the flat face portions 
105 is not more than 0.020 mm and the flat face portions 105 have the 
groove width corresponding to the center angle of not less than 10.degree. 
to the circumference along the peripheral surface 101 of the shaft 1. 
It is also constructed in such a manner that the difference between the 
outer diameter of the shaft 1 and the inner diameter of the sleeve 2 is 
less than 0.010 mm. 
It should be noted that the shape of the grooves can be arbitrarily 
selected even in a case other than the combination of plural unit planes 
described above as long as the combination of the depth and width of 
grooves and the difference between the outer diameter of the shaft and the 
inner diameter of the sleeve satisfy the above dimensional conditions. For 
example, the shape of a groove shown in FIG. 6 is conceivable. 
Specifically, the shape of the grooves is determined by requirements of 
manufacturing steps. 
Next, an overspeed test of motor was carried out in the optical deflection 
scanning apparatus constructed as described above. Changing the number of 
revolutions of motor, steady-state currents were measured at various 
numbers of revolutions. The steady-state current shows a measured value of 
current of the drive motor when the polygon mirror reaches steady-state 
rotation, which is a value corresponding to the drive torque. The face 
inclination was measured to evaluate the performance of the rotation drive 
part (scanner motor) of the optical deflection scanning apparatus. These 
measurement results are shown in Table 5. 
TABLE 5 
______________________________________ 
10000 20000 30000 50000 
rpm rpm rpm rpm rpm 
______________________________________ 
steady-state current 
0.20 A 0.21 A 0.31 A 
0.52 A 
face inclination 
38 sec 42 sec 56 sec 
65 sec 
______________________________________ 
As apparent from Table 5, the optical deflection scanning apparatus 
provided with the dynamic-pressure gas bearing structure according to the 
present invention can be obtained as an apparatus low in drive torque and 
high in rotation accuracy even under high-speed rotation. 
Using the dynamic-pressure gas bearing of the present invention, the 
optical deflection scanning apparatus capable of scanning at higher speed 
can be realized with the shaft constituting the rotary drive part in the 
easily processable shape. The manufacturing cost of the optical deflection 
scanning apparatus can be reduced accordingly.