Bearings – Rotary bearing – Fluid bearing
Reexamination Certificate
2000-04-20
2002-04-23
Lorence, Richard M. (Department: 3681)
Bearings
Rotary bearing
Fluid bearing
C384S100000, C359S200700
Reexamination Certificate
active
06375358
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hydrodynamic bearing structure having hydrodynamic (dynamic-pressure) grooves and to a rotating apparatus having the hydrodynamic bearing structure, a deflection scanning apparatus, a laser beam printer, an image forming apparatus, and a rotating apparatus of hard disk. More particularly, the invention concerns a hydrodynamic bearing apparatus for rotationally supporting a rotary polygon mirror or the like for high-speed scanning with a light beam in laser beam printers, bar code readers, etc., a method of producing the hydrodynamic bearing apparatus, and a deflection scanning apparatus using the hydrodynamic bearing apparatus.
2. Related Background Art
The deflection scanning apparatus used in the laser beam printers, bar code readers, etc. is constructed to implement deflection scanning with a light beam such as a laser beam or the like by use of a rotary polygon mirror rotating at a high speed. In the image forming apparatus such as the laser beam printers or the like, the scanning light yielded by the rotary polygon mirror is focused on a photosensitive body on a rotary drum to form an electrostatic latent image thereon, the electrostatic latent image on the photosensitive body is developed into a toner image by a developing device, the toner image is transferred onto a recording medium of a recording sheet, and the recording medium with the toner image is transferred to a fixing device to heat the toner on the recording medium to fix it, thereby performing printing.
The deflection scanning apparatus of this structure has been increasing its operation speed and accuracy more and more in recent years and, in response thereto, the hydrodynamic bearing apparatus of a non-contact type to permit low-noise and high-accuracy rotation has been and is employed in the bearing part of the rotary polygon mirror.
FIG. 1
is a schematic, cross-sectional view showing a hydrodynamic bearing unit according to a conventional example. The bearing unit has a shaft
102
, which is arranged to rotate integrally with a rotary polygon mirror
101
having a plurality of reflecting facets
101
a,
and a sleeve
103
, in which the shaft
102
is fitted so as to be rotatable. The sleeve
103
is integral with a bearing housing
104
. Fixed at the lower end of the sleeve
103
is a thrust pad
106
provided with a spherical portion
106
a
for supporting the lower end of the shaft
102
in the thrust direction. A flange
107
is fixed to the shaft
102
at the upper part thereof. The rotary polygon mirror
101
is pressed against the upper surface of the flange
107
by an elastic press mechanism
108
including a presser spring, etc. to form an integral structure therewith, so as to rotate together with the shaft
102
.
A yoke
109
a
holding rotor magnets
109
is fixed to the peripheral part of the flange
107
and the rotor magnets
109
are opposed to al stator coil
110
on a base plate
105
fixed to the bearing housing
104
. When the stator coil
110
is energized by a driving current supplied from a driving circuit (not shown), the rotor magnets
109
rotate at about 10,000 or 20,000 rpm together with the shaft
102
and rotary polygon mirror
101
.
A fluid membrane is created between the sleeve
103
and the shaft
102
with rotation thereof, thereby constituting a hydrodynamic bearing which rotationally supports the shaft
102
in a non-contact state by the dynamic pressure of the fluid membrane. First dynamic-pressure generating grooves
102
a
and second dynamic-pressure generating grooves
102
b
are cut with some spacing in between and in the stated order in the upward direction from the lower end of the shaft
102
in the peripheral surface of the shaft
102
. Shallow grooves (not shown) forming a hydrodynamic thrust bearing are also provided at the position facing the lower end of the shaft
102
, in the upper surface of the thrust pad
106
.
With rotation of the shaft
102
, a fluid
111
such as oil or the like present in a bearing clearance between the shaft
102
and the sleeve
103
is pulled into the central part of each dynamic-pressure generating groove
102
a,
102
b
to generate a high-pressure region there. Such high-pressure regions work to support the shaft
102
while maintaining the non-contact state in the radial direction between the shaft
102
and the sleeve
103
. Because of this non-contact rotation, the above bearing structure has advantages of capability of yielding properties such as lower noise, higher rotation accuracy, etc. than sliding bearings accompanied by metal contact and capability of reducing the size and cost in terms of the number of assembled parts as compared with, for example, rolling bearings, etc.
Used heretofore as a base oil of a lubricant for such hydrodynamic bearing apparatus were oils having a molecular weight distribution, such as mineral oils, synthetic hydrocarbon oils, silicone oils, perfluoro oils, etc., single-component oils such as diester oils, polyol ester oils, and so on, and mixed oils thereof.
For rotating the hydrodynamic bearing apparatus, there were desires for decrease in viscosity of the mixed oils, and the components of the mixed oils and percentages of the components were determined for the purpose of adjusting the viscosity. Describing in more detail, because the load carrying capacity and bearing rigidity of the hydrodynamic bearing apparatus are proportional to the viscosity of the lubricant used, it is necessary to select the lubricant of the viscosity matching with the required characteristics of an equipment provided with the hydrodynamic bearing apparatus. In addition, it is necessary that the change in the viscosity is small for the selected lubricant during long-term use.
It is relatively easy to adjust the viscosity to an arbitrary value for the oils such as the mineral oils, synthetic hydrocarbon oils, silicone oils, perfluoro oils, etc. used heretofore as base oils of lubricants. Since these base oils themselves have a broad molecular weight distribution, low-molecular-weight components thereof will volatilize during use over an extended period of time as a lubricant of the hydrodynamic bearing apparatus, so as to vary the viscosity gradually. More specifically, the percentages of the low-molecular-weight components in the lubricant will lower under the long-term use of the hydrodynamic bearing apparatus, so that the viscosity of the lubricant will become higher. Because of such a change in viscosity with the elapse of time, it is difficult to keep the load carrying capacity and bearing rigidity constant throughout the long-term use.
In cases where the so-called single-component oil such as diester oils, polyol ester oils, purified and separated synthetic hydrocarbon oils, for example, PAO (poly-&agr;-olefins), etc. is used, without carrying out the step of adjusting the viscosity by addition of another oil, because the viscosity of the base oil is perfectly dependent upon the molecular structure of the single-component oil itself, molecular design is required for adjusting the base oil to an arbitrary viscosity matching with the required characteristics of the equipment provided with the bearing apparatus and it is thus hard to obtain a desired single-component oil.
As to the conventional mixed oils, it is relatively easy to adjust the viscosity thereof to an arbitrary value by combining a low-viscosity-component oil with a high-viscosity-component oil, as compared with the single-component oils. Since the conventional mixed oils were prepared while focusing on the adjustment of viscosity, they demonstrated variation in the composition of the base oil because of volatilization of highly volatile components or low-molecular-weight oils during long-term use as a lubricant of the hydrodynamic bearing apparatus, as described above. As a consequence, a change in viscosity occurred with the elapse of time.
SUMMARY OF THE INVENTION
The present invention has been accomplished in view of the unsolved problems of the prior art
Maekawa Ichiro
Takeuchi Kazutaka
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