Dynamic pressure bearing

In a dynamic pressure bearing, a dynamic pressure-generating groove is provided to one of a thrust bearing surface and a part of a rotator facing the thrust bearing surface on one of a first and second thrust bearings, and a thrust bearing surface and a part of the rotator on the other one of the first and second thrust bearings which is not provided with the dynamic pressure-generating groove are smoothed so that the smoothed surfaces are formed.

BACKGROUND OF THE INVENTION 
The present invention relates to a dynamic pressure bearing of a rotating 
machine in which an air gap is formed between a rotator and a stator which 
is not rotated and the rotator can be rotated at a high speed by an 
uniform air layer formed in the air gap by the rotation of the rotator. 
In a conventional dynamic pressure bearing, an air current generated by a 
high speed rotation of a rotator is introduced to a dynamic 
pressure-generating groove provided on either the rotator or a stator, 
whereby the dynamic pressure-generating groove blows a strong wind 
pressure onto the surfaces of the rotator and the stator. As a result, an 
air gap of several .mu.m in thickness is formed between the rotator and 
the stator. A technique to make it possible to rotate smoothly the rotator 
at a high speed by reducing a resistance between the rotator and the 
stator with the air gap is disclosed in Japanese Patent Application Open 
To Public Nos. 63-87162 and 63-173014. A light deflecting apparatus with a 
polygonal mirror uses a dynamic pressure bearing in which a rotator as 
mentioned above is floated with an air gap of several .mu.m in thickness 
by the work of a dynamic pressure-generating groove provided on a fixed 
radial bearing and a fixed thrust bearing and then is rotated at a high 
speed not lower than 3000 rpm. 
In a conventional dynamic pressure bearing, as shown in FIG. 4, a 
rotation-supporting device 1 for a polygonal mirror is provided with a 
dynamic bearing 11 which comprises a plate-like thrust bearings 2, 3 on 
its upper and lower portions and a pillar-like radial bearing 4 fixed 
between the thrust bearings 2, 3. Dynamic pressure-generating grooves 22, 
32, 42 are formed on the bearing surfaces 21, 31 of the thrust bearings 2, 
3 and the bearing surface 41 of the radial bearing 4 respectively. A 
rotator 5 is arranged so as to have its rotation axis in the radial 
bearing 4 and to form supported surfaces 51, 52, 53 which are rotatable 
with regard to the bearing surfaces 21, 31, 41. A polygonal mirror 7 is 
fixed on an attaching member 6 which is integrally formed on the outer 
periphery of the rotator 5 as a single unit. On the lower portion of the 
attaching member 6 is provided a magnet 6A which is shaped in a ring or 
separated into several pieces in the rotation direction, and on the 
rotation-supporting device 1 is provided the stator coil 8 so as to locate 
opposite to the magnet 6A. The magnet 6A and stator coil 8 are constructed 
in such a manner that a high rotation of the rotator 5 is induced when the 
stator coil 8 is activated. With the thus induced rotation, a very thin 
air gap of 1 .mu.m to 7 .mu.m is formed between the bearing surfaces 21, 
31 of the thrust bearings and the supported surfaces 51, 52 of the rotator 
5 and between the bearing surface 41 of the radial bearing 4 and the 
supported surface 53 of the rotator 5, whereby the rotator can be rotated 
at a high speed. 
In the above mentioned dynamic pressure bearing, for the rotating surfaces 
of the rotator 5, the dynamic pressure-generating grooves 22, 32, 42 are 
formed on the bearing surfaces of the radial bearing 4 and the thrust 
bearings 2, 3 respectively. In order to form the very thin air gap, the 
dynamic pressure-generating grooves 22, 32, 42 are to be a shallow groove 
of several .mu.m in depth which is required to a high precision 
processing. In the case that all or some of the rotator 5, the thrust 
bearings 2, 3, and the radial bearing 4 are made of a hard ceramic 
material, the processing to make the dynamic pressure-generating groove is 
required a special processing technique and needs a long processing time. 
As a result, a cost for parts of the bearing becomes very expensive. 
On the other hand, in the case that the dynamic pressure-generating groove 
is provided on one side of the thrust bearings, there is a fear that the 
rotator may contact with the bearing. Further, since the a force works in 
one direction onto the rotator, there is a fear that a rotating attitude 
of the rotator becomes unstable. 
SUMMARY OF THE INVENTION 
The present invention has been conceived in order to improve above 
mentioned drawbacks. That is, in a dynamic pressure bearing constructed by 
a rotator, a radial bearing and thrust bearings, an air gap is formed by 
the enhancement of the true roundness (deviation from circularity) and the 
surface roughness with a high precision treatment on a radial bearing 
surface which dose not receive the weight of the rotator and a magnetic 
force of a magnet and on an upper thrust bearing surface, and a dynamic 
pressure-generating groove is provided only on a lower thrust bearing 
surface which receives the weight of the rotator, whereby an object of the 
present invention is to reduce a number of the dynamic pressure-generating 
grooves which needs a high degree processing-technique and to provide a 
low price dynamic pressure bearing. 
The above object can be achieved by the following structure. In a dynamic 
pressure bearing comprising a rotator, a radial bearing to regulate a 
rotation axis of the rotator, and a first and second thrust bearings 
disposed so as to sandwich the rotator therebetween so that a movement of 
the rotator in the axial direction is regulated by the first and second 
thrust bearings, a dynamic pressure-generating groove is provided to one 
of a bearing surface and a part of the rotator on one of the first and 
second thrust bearings and a bearing surface and a part of the rotator on 
the other one of the first and second thrust bearings which is not 
provided with the dynamic pressure-generating groove are smoothed so that 
the smoothed surfaces are formed. 
The smoothed surface has a roughness not greater than 0.5 .mu.m. 
The first and second bearings are disposed on an upper portion and lower 
portion of the radial bearing and the dynamic pressure-generating groove 
is provided on the thrust bearing disposed on the lower portion. The 
smoothed surfaces are formed on both of the first and second thrust 
bearings. 
The true roundness of the surfaces facing each other between the radial 
bearing and the rotator is not greater than 5 .mu.m. 
In a dynamic pressure bearing comprising a radial bearing, thrust bearings 
disposed on both upper and lower ends of the radial bearing, and a rotator 
supported rotatably by the radial bearing and the thrust bearings, a 
bearing surface of the radial bearing facing a part of the rotator and a 
bearing surface of the upper thrust bearing are smoothed so that smoothed 
surfaces are formed, and one of a bearing surface of the lower thrust 
bearing and a part of the rotator facing each other is provided with a 
dynamic pressure-generating groove.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is an apparatus in which a dynamic pressure bearing is used in a 
light-deflecting device. A reference number 1 represents a 
rotation-supporting device for a polygonal mirror. On an upper portion and 
a lower portion of the rotation-supporting device are provided an upper 
thrust bearing 3 and a lower thrust bearing 2 which are made of a low 
thermal expansion coefficient material, such as a ceramic. A column-shaped 
radial bearing 4 is integrally fixed between the upper thrust bearing 3 
and the lower thrust bearing 4 as a single unit. Bearing surfaces 21, 31 
of the thrust bearings 2, 3 are applied with a high precision surface 
treatment so that the bearing surfaces 21, 31 have a surface roughness 
(hereinafter refer to Ra) not greater than 0.5 .mu.m. Any conventional 
method of high precision surface treatment may be used as the surface 
treatment in this embodiment. For measuring Ra, for example, a device of 
SARFU COADER SE-30H (manufactured by Kosaka Kenkyusho), may be used. 
Bearing surface 41 of the radial bearing 4 is also applied with the high 
precision surface treatment so that the bearing surface 41 has a true 
roundness with a deviation not greater than 5 .mu.m between the maximum 
diameter and the minimum diameter. As a processing method for the bearing 
surface 41, for example, a grinding treatment with a rotating grinder may 
be used. For measuring the true roundness, a high precision type true 
roundness measuring device EC-10D (manufactured by Kosaka Kenkyusho) may 
be used. On the bearing surface 21 of the lower thrust bearing 2, a 
dynamic pressure-generating groove 22 is provided. 
A rotator 5 made of a low thermal expansion coefficient material such as a 
ceramic is supported by the bearing surface 41 of the radial bearing 4 so 
that the rotator 5 has its rotation axis in the radial bearing 4. The 
rotator 5 forms supported surfaces 51, 52, 53 so as to face the bearing 
surfaces 21, 31, 41 respectively. The supported surfaces 51, 52, 53 of the 
rotator 5 are applied with the similar surface treatment of the bearing 
surfaces 21, 31, 41 so that the surfaces 51, 52, 53 have the roughness Ra 
not greater than 0.5 .mu.m. The supported surface 53 facing the bearing 
surface 41 of the radial bearing 4 is also applied with the similar 
precision treatment of the bearing surface 41 so that the supported 
surface has a true roundness having a deviation not greater than 5 .mu.m. 
With regard to the treating method and the measuring method, the above 
mentioned devices may be used. 
A polygonal mirror 7 is fixed with a fixing member 61 onto an attaching 
member 6 formed integrally onto the outer periphery of the rotator 5. 
Along the rotation direction on the lower portions of the rotator is 
provided a plurality of magnets 64 or a round-shaped magnet 64. On the 
rotation-supporting device is provided a stator coil 8 so as to locate 
opposite to the magnet 64, whereby a high rotation speed of the rotator 5 
is induced when the electrical circuit of the stator coil 8 is activated. 
In the dynamic pressure bearing 11 constructed as mentioned above, a 
rotation force is induced on the magnets 64 provided on the rotator 5 so 
that the rotator 5 and the polygonal mirror 7 are rotated at a high speed. 
At this time, the supported surface 51 of the rotator 5 receives the 
dynamic pressure generated by the dynamic pressure-generating groove 22 
formed on the bearing surface 21 of the lower thrust bearing 2. 
Accordingly, as the rotation speed of the rotator increases, the rotator 
is floated in spite of the weight of both the rotator 5 and the polygon 
mirror 7 and a gap of 1 to 7 .mu.m is formed between the supported surface 
51 and the bearing surface 21. Further, with the high rotation speed, a 
gap of 2 to 20 .mu.m is formed between the supported surface 52 of the 
rotator 5 and the bearing surface 31 of the upper thrust bearing 3 and 
between the supported surface 53 of the rotator 5 and the bearing surface 
41 of the radial bearing 4 by the dynamic pressure. With the bearing 
surface of the thrust bearing applied with the surface treatment as 
mentioned above, the dynamic pressure is generated between the bearing 
surface of the bearing and the supported surface of the rotator because of 
the following reasons. As shown in FIG. 5, when one of two surfaces which 
are opposite to each other is rotated at a high speed, the smaller the 
distance between the opposite surfaces is, the higher the generated 
dynamic pressure is. As shown in FIG. 6, if the surface roughness of the 
opposite surfaces could be made smaller, the distance between the opposite 
surfaces could be made substantially smaller. Therefore, if the surface 
roughness between the opposite surfaces is made smaller so as to form a 
smoothed surface, the higher dynamic pressure can be generated. 
Accordingly, the rotator 5 and the polygonal mirror 7 are floated by the 
work of the dynamic pressure generating-groove 22, and the high dynamic 
pressure is generated on the upper thrust bearing by the smoothed surface 
even if the upper thrust bearing is not provided with a dynamic 
pressure-generating groove. As a result, the rotator can be rotated 
smoothly at a high speed between the lower thrust bearing and the upper 
thrust bearing, keeping a balance on a stable condition without contacting 
the bearing surfaces. In this embodiment, the above object can be 
accomplished by the surface roughness not greater than 0.5 .mu.m. 
Consequently, the rotator 5 and the polygonal mirror 7 can be rotated 
smoothly at a high speed with the gap created by the floating with the 
work of the dynamic pressure-generating groove. 
FIG. 2 is an apparatus in which a dynamic pressure bearing is used in a 
light-deflecting device, likewise with FIG. 1. A reference number 1 
represents a rotation-supporting device for a polygonal mirror. On an 
upper portion and a lower portion of the rotation-supporting device are 
provided an upper thrust bearing 3 and a lower thrust bearing 2 which are 
made of a low thermal expansion coefficient material, such as a ceramic. A 
column-shaped radial bearing 4 is integrally fixed between the upper 
thrust bearing 3 and the lower thrust bearing 4 as a single unit. Bearing 
surface 31 of the thrust bearings 2, 3 are applied with a high precision 
surface treatment so that the bearing surface 31 have a surface roughness 
Ra not greater than 0.5 .mu.m. Bearing surface 41 of the radial bearing 4 
is also applied with the high precision surface treatment as mentioned 
above so that the bearing surface 41 has a true roundness with a deviation 
not greater than 5 .mu.m. On the bearing surface 21 of the lower thrust 
bearing 2, a dynamic pressure-generating groove 22 is provided. 
In this embodiment, firstly, the upper thrust bearing and the radial 
bearing are integrally formed in a single unit. Then, the lower thrust 
bearing on which the dynamic pressure generating groove 22 is provided is 
formed as a separate unit from the radial bearing 4, and an attaching hole 
formed on the lower thrust bearing 2 is fixed in engagement with an 
attaching shaft 42 provided integrally to the radial bearing 4. With this 
construction, the bearing surface 31 of the upper thrust bearing 3 and 
bearing surface 41 of the radial bearing 4 can be applied with a more 
precise cutting process and a more precise polishing process. The cutting 
process and the polishing process can be conducted in the similar manner 
mentioned above. Further, the processing for the dynamic 
pressure-generating groove on the bearing surface 21 of the lower thrust 
bearing 2 can be conducted more simply. 
FIG. 3 is an apparatus in which a dynamic pressure bearing is used in a 
light-deflecting device, likewise with FIGS. 1 and 2. A reference number 1 
represents a rotation-supporting device for a polygonal mirror. On an 
upper portion and a lower portion of the rotation-supporting device are 
provided an upper thrust bearing 3 and a lower thrust bearing 2 which are 
made of a low thermal expansion coefficient material, such as a ceramic. A 
column-shaped radial bearing 4 is integrally fixed between the upper 
thrust bearing 3 and the lower thrust bearing 2 as a single unit. Bearing 
surfaces 21, 31 of the thrust bearings 2, 3 are applied with a high 
precision surface treatment so that the bearing surfaces 21, 31 have a 
surface roughness Ra not greater than 0.5 .mu.m. Bearing surface 41 of the 
radial bearing 4 is also applied with the high precision surface treatment 
as mentioned above so that the bearing surface 41 has a true roundness 
with a deviation not greater than 5 .mu.m. On the bearing surface 21 of 
the lower thrust bearing 2, a dynamic pressure-generating groove 22 is 
provided. 
In this embodiment, also, the upper thrust bearing and the radial bearing 
are integrally formed in a single unit. Then, the lower thrust bearing on 
which the dynamic pressure generating groove 22 is provided is formed as a 
separate unit from the radial bearing 4, and an attaching hole formed on 
the lower thrust bearing 2 is fixed in engagement with an attaching shaft 
42 provided integrally to the radial bearing 4. With this construction, 
the bearing surface 31 of the upper thrust bearing 3 and bearing surface 
41 of the radial bearing 4 can be applied with a more precise cutting 
process and a more precise polishing process. The cutting process and the 
polishing process can be conducted in the similar manner mentioned above. 
Further, the processing for the dynamic pressure-generating groove on the 
bearing surface 21 of the lower thrust bearing 2 can be conducted more 
simply. Furthermore, in this embodiment, the relation between the outer 
diameter L2 of the upper thrust bearing 3 and the outer diameter L1 of the 
rotator 5 is constituted so as to satisfy the following inequality: L2&gt;L1 
, whereby dynamic air pressure generated between the supported surface 52 
of the rotator 5 and the bearing surface 31 of the upper thrust bearing 3 
and between the supported surface 53 of the rotator 5 and the bearing 
surface 41 of the radial bearing 4 by the high speed rotation of the 
rotator 5 is blocked so as not to leak to the outside by the bearing 
surface 3 of the upper thrust bearing 3, thereby keeping stably the 
dynamic pressure condition. 
Incidentally, in FIGS. 2 and 3, the rotator 5 and the polygonal mirror 7 
can be smoothly rotated at a high speed with the air gap on a floating 
condition by the high speed rotation of the rotator 5, similarly with FIG. 
1. 
Further, in accordance with a requirement, it may be possible to make the 
rotator 5, the radial bearing 4, the upper thrust bearing 3 and the lower 
thrust bearing from a resin material. 
As mentioned above, in the dynamic pressure bearing of the present 
invention in which the rotator is rotatably supported by the radial 
bearing which supports the rotation shaft of the rotator, an upper thrust 
bearing and a lower thrust bearing, a dynamic pressure-generating groove 
is formed on the bearing surface of the lower thrust bearing, and the 
bearing surfaces of the radial bearing and the upper thrust bearing and 
the supported surface of the rotator are applied with the high precision 
surface treatment or polishing instead of the dynamic pressure-generating 
groove, thereby obtaining the dynamic pressure action. As a result, a 
number of processes to make the dynamic pressure-generating groove can be 
reduced to a great extent on comparison with the conventional dynamic 
pressure bearing. Consequently, the dynamic pressure bearing of the 
present invention can be manufactured easily at a low cost.