Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-08-31
2001-01-09
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S198100, C359S216100
Reexamination Certificate
active
06172786
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical deflection device which deflects a laser beam for scanning and to an image forming apparatus which forms images by means of a laser beam by the use of the optical deflection device.
In an image recording apparatus such as a laser beam printer, a laser beam is caused to strike upon a polygon mirror which rotates at high speed, based on information obtained by reading the image as a writing means, and reflected light is projected on the surface of a photoreceptor on a scanning basis for recording images.
FIG. 11
is a sectional view showing the structure of a scanning optical device in the prior art wherein a light beam is deflected for scanning when a polygon mirror is driven to rotate.
When a polygon mirror is rotated at a low speed, it is fixed directly on a rotary shaft of a driving motor to be used. When the polygon mirror is rotated at high speed, however, it is driven to rotate through an air bearing wherein the polygon mirror is fixed on an outer tube of a radial shaft which floats without touching an inner tube of a radial shaft to rotate. The inventors of the invention disclose technologies about an optical deflecting device having therein a dynamic pressure bearing, in TOKKAIHEI Nos. 7-24343, 7-259849, 8-114219 and 8-121471.
FIG. 11
is a diagram showing the sectional structure of optical deflecting unit
100
representing an optical deflecting device having dynamic pressure bearing
110
as a bearing means composed of upper thrust plate
111
, lower thrust plate
112
and radial shaft inner tube
113
. In
FIG. 11
, the dynamic pressure bearing
110
is composed of center shaft
114
of casing
101
, radial shaft inner tube
113
structured to be solid with the center shaft
114
, upper thrust plate
111
and lower thrust plate
112
. Coil
115
constituting static magnetic field of a motor is fixed on casing
101
serving as a supporting member. Ring-shaped magnet (permanent magnet)
121
for rotating magnetic field, outer ring section
122
made of aluminum, radial shaft outer ceramic tube
123
, rotary polygon mirror
124
and mirror holder
125
are assembled solidly and concentrically as rotor
120
wherein the rotary polygon mirror
124
is sandwiched between the outer ring section
122
and the mirror holder
125
. The rotor
120
is fitted to the radial shaft inner tube
113
, and the upper thrust plate
111
is fixed on the center shaft
114
. When the rotor
120
is rotating, there are formed clearances S of about 3-10 &mgr;m between the group of the radial shaft inner tube
113
, the lower thrust plate
112
and the upper thrust plate
111
and the group of the upper and lower surfaces and inner circumferential surface for fitting of the radial shaft outer tube
123
, thus, the rotor
120
can continue rotating smoothly without touching the dynamic pressure bearing
110
while floating in the air.
Namely, in company of the rotor
120
, polygon mirror
124
also rotates, and a laser beam emitted from a laser unit is deflected toward an unillustrated photoreceptor for scanning.
Casing
101
for optical deflecting unit
100
composed of the rotary polygon mirror
124
, the dynamic pressure bearing
110
and the rotor
120
is formed to be one body through an aluminum die casting, and an upper opening is covered with cover
102
made of a sheet metal or a synthetic resin plate.
When an air bearing having the structure stated above is used, it is possible to rotate a polygon mirror at a rate of tens of thousands rpm, and as a result, an image forming apparatus such as a high speed digital copying machine or laser printer has been realized.
However, when a polygon mirror is rotated at high speed, heat is generated in large quantities, and thereby the temperature of an optical deflecting device and temperature around the optical deflecting device in an image forming apparatus in which the optical deflecting device is mounted are raised.
When the amount of heat generated from the optical deflecting device is large, deterioration of surface accuracy of a polygon mirror caused by thermal deformation and fluctuation of rotation of the rotor
120
are generated, and thereby uneven scanning and image distortion are caused on outputted images to deteriorate quality of images. This phenomenon is conspicuous especially when enhancing recording density by rotating the polygon mirror
124
at high speed.
When a cooling device is provided on an optical deflecting device additionally as measures for the aforesaid problems, the number of parts in the optical deflecting device is increased, resulting in another problem that assembly man-hour is increased, cost is increased and an optical deflecting device needs to be large in size.
In addition, in the optical apparatus employing a laser scanning optical system such as an image reading device, miniaturization or cost reduction of an apparatus has been pursued.
FIG. 12
shows a conventional example of an optical detecting device which makes an optical beam to scan at prescribed angle for scanning in a laser optical unit.
In
FIG. 12
, polygon mirror J
1
is fixed on polygon mirror supporting member J
2
by holding member J
7
. The polygon mirror supporting member J
2
is supported by shaft J
3
and is supported by bearing J
6
through electromagnetic actions of coil J
5
and magnet J
4
to rotate. The magnet J
4
is fixed on vertical supporting arm J
21
extended from the polygon mirror supporting member J
2
.
The optical deflecting device mentioned above has the structure to fix polygon mirror J
1
and magnet J
4
on the polygon mirror supporting member J
2
. Since a magnet is fixed on a supporting member which is bent at right angles, therefore, the structure for supporting polygon mirror J
1
is complicated, and polygon mirror supporting member J
2
and holding member J
7
are needed, resulting in a large number of parts, cost increase and difficulty in miniaturization.
Due to an employment of the air bearing stated above, it has become possible to rotate a polygon mirror at a rate of tens of thousands rpm, resulting in realization of a high speed digital copying machine and a laser printer.
In the optical deflecting device having a rotating body which rotates at high speed, it has been found that heat is generated in large quantities with rotation, and thereby the temperature of an optical deflecting device and temperature of the apparatus portion around the optical deflecting device are raised, which is not preferable. When a cooling device is provided on an optical deflecting device additionally as measures for the aforesaid problems, the number of parts in the optical deflecting device is increased, resulting in problems that assembly man-hour is increased, cost is increased and an optical deflecting device needs to be large in size.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide an optical deflecting device wherein temperature rise can be impeded despite high speed rotation of a polygon mirror without causing problems of an increase in the number of parts of the optical deflecting device and of a large-sized device, and to provide an image forming apparatus.
The object stated above can be attained by the organization wherein radiating fins are solidly formed on a supporting member on which a coil representing a heat generating source is fixed. The object can further be attained by the organization wherein an optical deflecting device is fixed in an image forming apparatus so that an air current in the image forming apparatus may be almost in parallel with the direction of the radiating fins formed solidly on the supporting member.
REFERENCES:
patent: 5434695 (1995-07-01), Saito et al.
patent: 5606448 (1997-02-01), Suzuki et al.
Fujita Atsushi
Itoh Masao
Kamimura Naoji
Morita Shinji
Shibuya Satoshi
Bierman Jordan B.
Bierman, Muserlian and Lucas
Konica Corporation
Phan James
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