Rotating device and light beam deflecting apparatus

In a rotating device which comprises a rotor; a radial bearing; a lower thrust bearing; a dynamic pressure generating means, a radial bearing area (mm.sup.2) which is the area of the rotor facing the radial bearing, a thrust bearing area (mm.sup.2) which is the area of the rotor facing the lower thrust bearing, and the rotor weight (g) are determined so as to satisfy the following formula: EQU (a radial bearing area)/(thrust bearing area).times.(rotor weight)<300.

BACKGROUND OF THE INVENTION 
The present invention relates to a rotating device provided with a dynamic 
pressure bearing in which a space is formed between a rotor and a lower 
thrust bearing and between the rotor and a radial bearing by the action of 
a dynamic pressure generating groove with rotation of the rotor. Further, 
the present invention relates to a light beam deflecting apparatus using 
the rotating device. 
In comparison with conventional rotating devices using ball bearings, a 
rotating device using a dynamic pressure bearing is superior in that it 
makes extremely high speed rotation possible. As a result, recently, 
further development has been conducted for it. Generally, the rotating 
device using the dynamic pressure bearing is composed of upper and lower 
thrust bearings, a radial bearing, and a rotor rotatable on the radial 
bearing. With the rotation of the rotor, an air gap of several microns is 
formed between the rotor and the bearings by the action of dynamic 
pressure generating grooves provided on each of the bearings, thereby 
reducing the resistance between the rotor and the bearings. As a result, 
it allows the rotor to rotate at very high speeds. In such rotating device 
using the dynamic pressure bearing, since it is necessary to maintain the 
air gap of several microns between the rotor and the bearings, it is 
requested for the rotor to be extremely well balanced so as to have high 
grade balance. Taking an example of a rotor rotating in 20,000 rpm, its 
balance grade has to be not lower than G1 grade as defined in JIS 
BO905-1978. If its balance grade is G2 grade, vibration on the rotor 
become excessive. The vibration affects not only the accuracy of light 
beam deflection, but also causes galling and burning. 
Further, ambient temperature surrounding the rotor is raised by heat 
generated from a motor coil during rotation of the rotor. Especially, in 
the case that a rotor is rotated in 20,000 rpm or more in a light beam 
deflecting device to deflect a laser beam in an image forming apparatus, 
the ambient temperature rise becomes approximate 40.degree. C. and a 
temperature of the rotor is also raised. Since the rotating device using 
the dynamic pressure bearing is required the high grade balance as 
discussed above, if the temperature is raised, the rotor becomes 
unbalance, resulting in galling and burning so that a required performance 
of the rotating device may not be achieved. For example, if the ambient 
temperature rise becomes 40 degrees, the balance grade may happen to lower 
from G1 grade to G2 grade, increasing vibration to an extent of ten times 
heavier. As a result, in the light beam deflecting device, the deflected 
light beam is so deviated that high quality image can not be formed. 
As factors which cause the unbalance of the rotor due to temperature 
change, thermal expansion in the structural material of the rotor, thermal 
expansion in plural structural members when the rotor is composed of the 
plural structural members, influence of adhesive to joint the plural 
structural members may be listed, and they are considered to cause various 
thermal expansions differing in extent or location in the rotor. 
Then, the objective of the present invention is to provide a rotating 
device or a light deflecting apparatus comprising a dynamic bearing with a 
structure which causes less balance fluctuation and is resistant to 
balance fluctuation even when the balance degree of the rotor becomes poor 
due to various causes, 
The above objective can be attained by the following structures of the 
present invention. 
In a rotating device provided with a dynamic pressure bearing in which a 
space is formed between a rotor and a lower thrust bearing and between the 
rotor and a radial bearing by an action of dynamic pressure generating 
grooves with rotation of the rotor, the rotating device satisfies the 
following formula: 
EQU (a radial bearing area)/(thrust bearing area).times.(rotor weight)&lt;300 
wherein the radial bearing area (mm.sup.2) is an area of the rotor facing 
the radial bearing, the thrust bearing area (mm.sup.2) is an area of the 
rotor facing the lower thrust bearing, and the rotor weight (g) is the 
weight of the rotor. 
In the above rotating device, on an upper section of the radial bearing is 
provided a preventing means for preventing the rotor from slipping out. 
In the above rotating device, on the thrust bearing is formed a dynamic 
pressure generating groove. 
In the above rotating device, on a lower section of the rotor is provided a 
magnet, and on a position facing the magnet is provided a coil so that the 
rotor is rotated together with the magnet by switching on an electric 
circuit for the coil. 
In the above rotating device, on the rotor is provided a polygonal mirror.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a perspective view showing an embodiment of a unit of an optical 
light beam scanning system in which a rotating device provided with a 
dynamic pressure bearing is used as a light beam deflecting device. 
In FIG. 1, reference numeral 100 is a base on which a rotating device is 
mounted, 2 is a collimator (a optical system for correcting a shape of a 
beam), 5 is a cylindrical lens, 116 is a polygonal mirror, 7 is a f.theta. 
lens, 8 is a second cylindrical lens, 9 is a reflecting mirror, 10 is a 
photoreceptor drum. Further, 11 is a timing detecting mirror, 12 is a 
synchronization detecting means, 13 is a driving motor for the polygonal 
mirror 116. A beam emitted from a semiconductor type laser emitting device 
1A is shaped in parallel light beam by the collimator 2. The beam is 
brought through the first cylindrical lens to incidence on the rotating 
polygonal mirror and then is reflected from it. The reflected beam passes 
through the second image forming optical system composed of the f.theta. 
lens 7 and the second cylindrical lens 8 and passes through the reflecting 
mirror 9 so as to scan in the form of a spot of a predetermined diameter 
in the main scanning direction on the photoreceptor drum 10. Incidentally, 
the synchronization detection for each scanning line is conducted by 
bringing the beam through the mirror 11 to incident on the synchronization 
detector 12 before starting the main scanning. 
FIG. 2 is an enlarged view of the rotating device shown in FIG. 1 and is a 
sectional view showing the entire construction of a rotating device 101 
provided with a dynamic pressure device. 
The rotating device 101 comprises a lower thrust bearing 103, a radial 
bearing 105, a retaining sheet plate 118 used as the preventing means for 
preventing the rotor from slipping out, a screw 119 and the rotor 104. The 
lower thrust bearing is provided with a dynamic pressure bearing. 
On the base 100 is fixed vertically a core shaft 102 for supporting the 
rotating device 101. A disk-shaped lower thrust bearing 103 made of a 
ceramic material and a cylindrical radial bearing 105 are fitted around 
the core shaft 102 and are mounted in such order on the base 100. With the 
retaining sheet plate 118 used as the slipping-out preventing means and 
the screw 119, the lower thrust bearing 103 and the radial bearing 105 are 
fixed on the base 100. Incidentally, the lower thrust bearing 103 and the 
radial bearing 105 may be made in one body. 
Further, the rotor 104 is fitted around the radial bearing 105 and is 
located between the retaining sheet plate 118 and the lower thrust bearing 
103 in such a manner that the rotor 104 is rotatable around the radial 
bearing 105. Each of the radial bearing 105 and a rotor core 107 are 
subjected to a high precision surface treatment so that a gap of 1 to 7 
.mu.m is formed between the radial bearing 105 and the rotor core 107. 
A support section 114 is fixed on the outer periphery of the rotor core 107 
and the polygonal mirror 116 on which plural reflecting mirror surfaces 
115 are formed is fixed on the support section 114 with a fixing member 
117, thereby forming the rotor unit 104 rotatable in one body around the 
radial bearing 105. 
In the present invention, the rotor 104 means all members actually rotating 
as one body. In this embodiment, the rotor 104 comprises the rotor core 
107, the support section 117, the polygonal mirror 116 on which plural 
reflecting mirror surfaces 115, the fixing member 117 and magnet 125 which 
is mentioned later on. 
In this embodiment, as the slipping-out preventing means, the retaining 
sheet plate 118 and the screw 119 are used without using the upper thrust 
bearing. The reason is to provide a function to fix the lower thrust 
bearing 103 and the radial bearing 105 in addition to the function to 
prevent the slipping-out. Since the retaining sheet plate 118 and the 
screw 119 do not need manufacturing precision and achieve the above two 
functions, the number of machinery parts can be reduced and the device can 
be made at lower cost. Needless to say, it may be possible to provide the 
upper thrust bearing between the retaining sheet plate 118 and the radial 
bearing 105. 
The dynamic pressure generating groove is provided not only on the lower 
thrust bearing, but also can be provided on the upper thrust bearing. 
However, as demonstrated in the this embodiment, in the case that the 
dynamic pressure generating groove 121 is formed only on the lower thrust 
bearing 103, the rotation precision is not affected. Since the dynamic 
pressure generating groove 121 is not required to be formed on the upper 
thrust bearing, the number of processes to form the dynamic pressure 
generating groove can be reduced and the device can be manufactured at 
lower cost. 
As a driving source to rotate the rotor 104, an axial type driving motor 13 
is used. For this driving motor 13, a stator coil 124 is provided together 
with an insulating material 123 on the base 100 and magnets 125 are 
provided at a position facing the stator coil 124 on a lower section of 
the supporting section 114 of the rotor 104. When the stator coil 124 is 
activated, the rotation of the rotor 104 is induced. Subsequently, under 
the rotation of the rotor 104, an air gap is formed between the rotor 104 
and the lower thrust bearing 103 and between the rotor 104 and the radial 
bearing 105 by the action of dynamic pressure generated by the dynamic 
pressure generating grooves 121 formed on the lower thrust bearing 103, 
whereby the polygonal mirror 116 can be smoothly rotated at a high speed. 
The rotating device 101 of the present invention is constructed as 
explained above and make it possible to attain extremely high speed 
operation. 
Now, an inventive example of the present invention and a comparative 
example will be explained. 
In FIG. 3 in which a part of the rotor corresponding in position to the 
dynamic pressure bearing of the rotating device 101 shown in FIG. 2 is 
enlarged, test devices were prepared by varying the diameter D1 (mm) of 
the thrust bearing, the inside diameter D2 (mm) of the rotor, the height H 
(mm) of the rotor, the weight W(g) of the rotor as shown in Table 1 and 
anti-unbalance capability test (balance retaining capability test) was 
conducted on the test devices. 
TABLE 1 
______________________________________ 
Anti- 
unbalance 
H(mm) D1(mm) D2(mm) W(g) Index 
capability 
______________________________________ 
Inventive 
6 19 10 20 18 A 
Example 1 
Inventive 
6 19 10 25 23 A 
Example 2 
Inventive 
10 22 10 35 36 A 
Example 3 
Inventive 
15 22 10 40 63 B 
Example 4 
Inventive 
6 19 10 120 110 C 
Example 5 
Inventive 
15 19 10 50 115 C 
Example 6 
Inventive 
16 22 16 38 171 C 
Example 7 
Inventive 
15 22 10 100 156 C 
Example 8 
Inventive 
15 22 10 160 250 C 
Example 9 
Comparative 
16 19 16 38 371 D 
Example 1 
Comparative 
16 26 16 180 439 D 
Example 2 
Comparative 
16 26 16 200 488 D 
Example 3 
______________________________________ 
In this test, occurrence of burning in each test device was observed while 
ambient temperature surrounding the test devices was changed. In Table 1, 
in an example indicated with A in anti-unbalance capability, burning was 
not observed in the test device of the example even when the ambient 
temperature change exceeded over 50.degree. C. In examples indicated with 
B, burning was not observed in the test device of the examples even when 
the ambient temperature change exceeded over 40.degree. C. In examples 
indicated with C, burning was not observed in the test device of the 
examples even when the ambient temperature change exceeded over 30 
.degree. C. In examples indicated with D, burning was observed in the test 
device of the examples before the ambient temperature change exceeded over 
30.degree. C. 
In Table 1, Index is calculated by the following formula: 
EQU Index=(a radial bearing area)/(thrust bearing area).times.(rotor weight) 
wherein the radial bearing area (mm.sup.2) is the area of the rotor 104 
facing the radial bearing 105, the thrust bearing area (mm.sup.2) is the 
area of the rotor 104 facing the lower thrust bearing 103, and the rotor 
weight (g) is the weight of the rotor 104. 
More concretely explaining with reference to FIG. 3, since (a radial 
bearing area), (thrust bearing area), and (thrust bearing area) are 
represented as follows, 
EQU (a radial bearing area)=.pi..times.D2.times.H (mm.sup.2), 
EQU (thrust bearing area)=.pi..times.(D1.sup.2 -D2.sup.2)/4 (mm.sup.2), 
EQU (rotor weight)=W(g) 
Index can be obtained by the following formula: 
EQU Index=(.pi..times.D2.times.H)/(.pi..times.(D1.sup.2 -D2.sup.2)/4).times.W 
As can be seen from the test results of Inventive Examples and Comparative 
Examples indicated in Table 1, in the case that Index is not larger than 
300, the test device became tough against balance fluctuation caused by 
the ambient temperature change. It is preferable that Index is not larger 
than 100. It is more preferable that Index is not larger than 50. 
Incidentally, during manufacture of the rotating device, while the rotation 
speed of the rotating device is increased stepwise, balance adjustment is 
conducted for each step. Finally, it is necessary to conduct the balance 
adjustment at a normal rotation. Because, since at the initial 
manufacturing stage of the rotating device, a rotor may be unbalanced, it 
may be impossible to directly increase the rotation speed of the rotating 
device to the normal rotation speed. Accordingly, in conventional rotating 
devices, after the balance adjustment is conducted several times at 
reduced rotation speeds, the final balance adjustment is conducted at the 
normal rotation speed. 
Then, while conducting the balance adjustment for the test devices of 
Inventive Examples and Comparative Examples in Table 1 during the 
manufacturing stage, an initial rotation speed at which no contact or no 
galling was observed was investigated. In the test devices of the examples 
marked with "A" in anti-unbalance capability, no galling was observed up 
to about 16,000 rpm. In the test devices of the examples marked with "B", 
no galling was observed up to about 12,000 rpm. In the test devices of the 
examples marked with "C", no galling was observed up to about 8,000 rpm. 
However, in the test devices of the examples marked with "C", galling was 
observed at a lower rotation speed than 8,000 rpm. 
Therefore, by satisfying the index defined by the present invention, since 
the initial rotation speed at which the balance adjustment is initially 
conducted becomes high, the number of the balance adjustment conducted 
while increasing a rotation speed stepwise to the normal rotation speed 
can be reduced. As a result, as accompanying effects, manufacturing cost 
can be greatly reduced and a manufacturing time period can also be greatly 
shortened. 
Incidentally, not only the shape of the rotor 104 shown in FIG. 3, with a 
shape shown in FIG. 4, the same test result as stated above may be 
obtained. 
In the rotor 104 shown in FIG. 4, chamfering is applied on corners 104A, 
104B of the surfaces of the rotor 104 facing the radial bearing 105, and 
cut-out portion 104 is provided on the outside of the surface of the rotor 
104 facing the lower thrust bearing 103. 
In this case, since the radial bearing surface (mm.sup.2) is the area of 
the surface of the rotor 104 facing the radial bearing 105, the radial 
bearing surface is represented in FIG. 4 as (radial bearing 
area)=.pi..times.D2.times.H' (mm.sup.2). In the case that the chamfering 
is not applied on corners 104A, 104B as same as in FIG. 3, since H'=H, the 
radial bearing surface is represented as (radial bearing 
area)=.pi..times.D2.times.H (mm.sup.2) Incidentally, if the width applied 
with the chamfering is not larger than 0.5 mm, the chamfering is deemed as 
not being applied, H' is deemed as being equal to H. Further, if the 
surface of the rotor 104 facing the radial bearing 105 is slanted in 
positional relation to the surface of the radial bearing 105, when the 
width of the slant in the radial direction is not larger than several 
millimeters, the slant is negligible and the average inner diameter of the 
slanted rotor is used as D2. 
Also, since the thrust bearing surface (mm.sup.2) is the area of the 
surface of the rotor 104 facing the thrust bearing 105, the thrust bearing 
surface is represented in FIG. 4 as (a thrust bearing 
area)=.pi..times.(D1'.sup.2 -D2'.sup.2)/4 (mm.sup.2). In the case that the 
chamfering and the cut-out portion are not applied as same as in FIG. 3, 
since D2'=D2, the thrust bearing surface is represented as (a thrust 
bearing area)=.pi..times.(D1.sup.2 -D2.sup.2)/4 (mm.sup.2). If the width 
applied with the chamfering on the rotor 104 is not larger than 1 mm, the 
chamfering is deemed as not being applied, D2' is deemed as being equal to 
D2. If a width of the cut-out portion in the radial direction is not 
larger than several millimeters, the width of the cut-out portion is 
negligible and D1' is deemed as being equal to D1. 
The rotor weight (g) is the weight of the rotor, that is, a total weight of 
actually rotating members. For example, in the case that the rotor 104 is 
provided with a magnet to rotate the rotor 104 and a polygonal mirror, the 
total weight includes a weight of the magnet and the polygonal mirror. 
From the abovementioned points, in the case of the rotating device shown in 
FIG. 4, assuming that (rotor weight) is W(g), Index of the present 
invention calculated by (a radial bearing area)/(thrust bearing 
area).times.(rotor weight) can be obtained by the following formula: 
EQU Index=(.pi..times.D2.times.H)/(.pi..times.(D1'.sup.2 -D2'.sup.2)/4).times.W 
In the rotating device shown in FIG. 4, by making Index calculated by (a 
radial bearing area)/(thrust bearing area).times.(rotor weight) not larger 
than 300, the abovementioned effects can be obtained. It may be preferable 
that Index is not larger than 100. It may be more preferable that Index is 
not larger than 50. 
In the present invention, in rotating devices provided with a dynamic 
pressure bearing in which a space is formed between a rotor and a lower 
thrust bearing and between the rotor and a radial bearing by action of 
dynamic pressure generating grooves with rotation of the rotor, by 
satisfying the following formula among the radial bearing area (mm.sup.2) 
which is the area of the rotor 104 facing the radial bearing 105, the 
thrust bearing area (mm.sup.2) which is the area of the rotor 104 facing 
the lower thrust bearing 103, and the rotor weight (g) which is the weight 
of the rotor 104, regardless of the shape of the rotor, 
EQU (a radial bearing area)/(thrust bearing area).times.(rotor weight)&lt;300, 
even if unbalance takes place on the rotor 104 due to various factors, the 
present invention can provide a dynamic pressure bearing having a 
structure which causes less balance fluctuation and is resistant to 
balance fluctuation. 
Further, on the upper portion of the radial bearing, since the above 
rotating device comprises the slipping-out preventing means which do not 
need a relatively high processing accuracy, the rotating device can be 
manufactured at low cost. 
Since the dynamic pressure generating groove is provided only on the lower 
thrust bearing, the number of processes to make the pressure generating 
groove which needs relatively high processing accuracy can be reduced. As 
a result, the rotating device can be manufactured at lower cost. 
Since magnets are provided on the lower section of the rotor and a coil is 
provided on a position facing the magnet, by switching on an electric 
circuit for the coil, the rotor can be easily rotated at a high speed. 
When a laser beam is reflected on the polygonal mirror provided on the 
rotor, the reflected laser beam does not deviate.