Abstract:
An optical deflection device includes: a rotor having reflection surfaces on a cirumferential surface thereof; a bearing for rotatably supporting the rotor; a magnet fixed on the rotor; a supporting member for supporting the bearing, on which a radiating fin is integrally formed; and a coil fixed on the supporting member at a position where the coil faces the magnet for forming a magnetic field.

Description:
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 μm 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general structure diagram of a digital image forming apparatus related to the invention. 
     FIG. 2 is a perspective view showing an embodiment of a plural beam scanning optical device. 
     FIG. 3 is a top view of a plural beam scanning optical device. 
     FIG. 4 is a sectional view of the aforesaid scanning optical device. 
     FIG. 5 is an enlarged sectional view of the vicinity of an optical deflecting unit of a scanning optical device. 
     FIG.  6 ( a ) is a top view of an optical deflecting unit with its cover removed, FIG.  6 ( b ) is a side view of the optical deflecting unit viewed in the direction shown with arrow mark A, and FIG.  6 ( c ) is an enlarged sectional view showing the layer structure of a double-sided adhesive tape. 
     FIG. 7 is an enlarged top view of the optical deflecting unit and an optical system. 
     FIGS.  8 ( a ) and  8 ( b ) are respectively a top view and a sectional view of an optical deflecting unit. 
     FIGS.  9 ( a ),  9 ( b ) and  9 ( c ) are respectively a side view, a rear view and a sectional view of an optical deflecting unit. 
     FIG. 10 is a side sectional structure diagram of an image forming apparatus main body. 
     FIG. 11 is a sectional structure diagram of an optical deflecting device having a dynamic pressure bearing in the prior art. 
     FIG. 12 is a diagram showing an optical deflecting device in the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of a scanning optical device and an image forming apparatus of the invention will be explained as follows with reference to the drawings attached herewith. 
     FIG. 1 is a diagram showing general structure of a digital image forming apparatus related to the invention. 
     Image forming apparatus main body  1  is composed of image reading section A, image processing section B, image storing section C, image writing section D, image forming section E and sheet-feeding section F. 
     In the image reading section A, document “d” placed on document stand glass (platen glass)  11  is illuminated by halogen lamp  12  provided on a carriage which moves on a slide rail. Light reflected on document “d” is reflected on first mirror  13 , second mirror  14  and third mirror  15 , then passes through image-forming lens  16  and thereby linear optical images are converted photoelectrically into electric signals in succession by CCD image sensor  17 . 
     Analog signals converted photoelectrically by the CCD image sensor  17  are subjected to analog processing in an image processing section, then are subjected to A/D conversion, shading correction, luminance/density conversion, EE processing, character/halftone-dot discrimination, filter/variable-power processing, copy γ correction, writing density correction, 2-beam control, error diffusion processing and data compression processing, and are outputted to image writing section D through image storing section C. 
     In the image writing section D, image data after image processing are outputted by a semiconductor laser. This output from the semiconductor laser is subjected to rotary scanning conducted by rotary polygon mirror  22  which is rotated by driving motor  21 , then it passes through fθ lens  23 , first mirror  24 , second mirror  25 , cylindrical lens  26  and third mirror  27 , and emerges out of cover glass  28  to be projected on photoreceptor drum  31 . 
     The image forming section E is composed of charger  32 , developing unit  33 , transfer unit  34 , separating unit  35  and cleaning unit  36  all arranged around photoreceptor drum  31 . There are further arranged conveyance section  37 , fixing section  38  and sheet ejecting section  39  at the downstream side of the separating unit  35 . 
     Sheet-feeding section F is composed of sheet-feeding cassette  41  in which transfer sheets “p” are loaded and of sheet-feeding means  42  which separates transfer sheet “p” in the sheet-feeding cassette  41  and feeds it. 
     FIG. 2 is a perspective view showing an embodiment of a plural-beam scanning optical device, and FIG. 3 is a top view of the plural-beam scanning optical device. 
     In these drawings, a semiconductor laser is represented by  200 A and  200 B, a collimator lens is represented by  201 A and  201 B, a compression prism for height adjustment is represented by  202 A and  202 B, a pair of prism set for fine adjustment in the primary scanning direction is represented by  203 A, a pair of prism set for fine adjustment of pitch in the secondary scanning direction is represented by  203 B, a beam-composing prism for composing 2 beams is represented by  204 , first cylindrical lens is represented by  205 , a polygon mirror is represented by  22 , an fθ lens is represented by  23 A and  23 B, second cylindrical lens is represented by  26 , third mirror is represented by  27 , a cover glass is represented by  28 , and a photoreceptor drum is represented by  31 . Incidentally, the numeral  29  represents an index mirror for timing detection,  29 S represents an index sensor for synchronization detection, and  21  represents a driving motor for the polygon mirror  22 . 
     A beam emerged from semiconductor laser  200 A is collimated by collimator lens  201 A to be collimated light, and then enters beam-composing prism  204 . A beam emerged from semiconductor laser  200 B arranged to be perpendicular to the semiconductor laser  200 A is also collimated by collimator lens  201 B to be collimated light, and then enters beam-composing prism  204 . Incidentally, the beam emerged from the semiconductor laser  200 B is arranged to be staggered by prescribed pitches from the beam emerged from the semiconductor laser  200 A in the secondary scanning direction. Both beams mentioned above enter rotary polygon mirror through first cylindrical lens  205  of the first image-forming optical system. This reflected light is transmitted through a second image-forming optical system composed of fθ lenses  23 A and  23 B and second cylindrical lens  26 , and scans on the circumferential surface of photoreceptor drum  31  through third mirror  27  and cover glass  28  on a 2-line simultaneous basis under the condition of stagger by prescribed pitches in the secondary scanning direction. With regard to the primary scanning direction, rough adjustment has already been made by an unillustrated adjusting mechanism. For synchronization detection for each line, a light flux before the start of scanning is caused to enter index sensor  29 S through index mirror  29 . 
     Synchronization for each plane and stagger of the primary scanning between two beams are subjected to fine adjustment. 
     FIG. 4 is a sectional view of a scanning optical device related to the invention, and FIG. 5 is an enlarged sectional view of the vicinity of an optical deflecting unit  100  of the scanning optical device. With regard to symbols used in this drawing, parts or components having the same functions as those in FIG. 11 are given the same symbols. 
     Driving motor  21  which drives rotary polygon mirror  22  to rotate is composed of coil (stator)  115  on the part of casing  101  and magnet (rotor)  121  on the part of the rotary polygon mirror  22 . Plural coils  115  are fixed on insulating base board  114 . The plural coils  115  are wired in series and are connected to an unillustrated power supply unit through connector  116  and lead wire  117 . On the surface of casing  101  under the insulating base board  114 , there is fixed stator yoke  118  which is made of silicon steel plate. 
     The top surface of the plural coils  115  is close to the bottom surface of ring-shaped magnet  121 , keeping a prescribed clearance. The top surface of the magnet  121  is directly fixed and glued on the rotary polygon mirror  22  through magnet yoke  126  made of silicon steel plate. A part of the inner circumferential surface of the rotary polygon mirror  22  is brought into contact with an outer circumferential surface of radial shaft outer tube  123  to be positioned, and is fixed with adhesives poured in a recessed portion. Due to this, it is possible to achieve miniaturization of an optical deflecting device without increasing the number of parts. The magnet  121 , magnet yoke  126  and rotary polygon mirror  22  are united with radial shaft outer tube  123  to be structured rotatable around dynamic pressure bearing  110 . 
     There is provided a recess on the rotary polygon mirror  22 , in which magnet  121  and magnet yoke  126  are buried to be fixed with adhesives, whereby a rotary member including the rotary polygon mirror  22  is made to be thin like a flat plate. This has made improvement in rotating accuracy of the rotary polygon mirror  22  and miniaturization of an optical deflecting device to be possible. An octagonal circumferential surface, namely, a circumferential surface forming a polygon of the rotary polygon mirror  22  is subjected to mirror finish as a reflecting surface. 
     An upper end face of casing  101  of optical deflecting unit  100  is pressed with cover (inner cover)  102  and elastic sealing member  103  so that an upper open space of casing  101  is sealed. The elastic sealing member  103  is formed with foam resin or rubber sheet, and is pasted on the inner side of cover  102  to be effective for prevention of noise. 
     Upper thrust plate  111  of dynamic pressure bearing  110  is protruded from the top surface of rotary polygon mirror  22 , and screw  119  which fixes the upper thrust plate  111 , lower thrust plate  112  and radial shaft inner tube  113  is further protruded from the top surface of the upper thrust plate  111 . The cover  102  is made of aluminum alloy sheet, and its central portion is formed to be convex through the drawing. Convex portion  102 A processed through the drawing is formed to be a narrow space having a height and a sloped conical surface so that the space may cover the upper thrust plate  111  and screw  119  both protruded from the top surface of the rotary polygon mirror  22  with an in between clearance which is mostly constant for the entire space. By forming the narrow space to keep the clearance between the cover  102  and a group of the upper thrust plate  111  and the rotary polygon mirror  22  to be almost constant as explained above, it is possible to make a volume of air in the casing  101  of the optical deflecting unit  100  to be appropriate, and thereby to reduce occurrence of windage loss, generation of heat and air-cutting noise. 
     In the optical device main body (casing of an image writing section)  20  which houses optical members of the scanning optical device, the upper end surface of a wall surface of wall body  20 A on which the optical deflecting unit  100  is fixed is in pressure contact with top cover  206  which seals the upper space of the wall body  20 A and with elastic sealing member  207 , and the upper open space of the wall body  20 A is sealed. The top cover  2 - 6  is made of resins having vibration damping characteristics such as ABS resin. 
     The elastic sealing member  207  is made of damping material composed of expandable resins such as foaming urethane rubber or foaming ethylene propylene rubber (EPDM), and is stuck on an inner surface of the top cover  206  to be effective for prevention of a noise. 
     A central portion and its vicinities of the top cover  206  are formed to be protruded in a convex form, and a narrow space is formed between an inner surface of the top cover (outer cover)  206  and an outer surface of the cover (inner cover)  102  to keep the distance between the inner surface and the outer surface to be mostly constant. 
     The elastic sealing member  207  stuck on the inner surface of the top cover  206  is brought into pressure contact with the outer surface of the cover  102  for sealing. The elastic sealing member  207  interposed and filled in a clearance between the cover  102  and the top cover  206  prevents air-cutting noise caused by the rotary polygon mirror  22  from leaking out, and is effective for vibration damping. 
     The numeral  208  is a cover which covers the upper space of the optical device main body  20 , and has on its inner surface elastic sealing member  208 A which is effective for dust-proofing and sound-proofing. 
     FIG.  6 ( a ) is a top view of optical deflecting unit  100  from which cover  102  is removed, FIG.  6 ( b ) is a side view viewed from arrow A in the optical deflecting unit  100 , and FIG. 7 is an enlarged top view of the optical deflecting unit  100  and an optical system. 
     A part of a side wall of casing  101  is cut off so that opening  101 A is provided. This opening  101 A is an outlet through which beam L is emerged out when rotary polygon mirror  22  is rotated. On the outer side of the opening  101 A, transparent light-transmitting member (glass of an aperture for light to enter and emerge)  291  is glued through double-sided adhesive tape  292 . 
     A member which is excellent in adhesive force, sealing characteristics, durability and damping properties is used as the double-sided adhesive tape  292 . For example, structural VHB adhesive tape Y-4905J or Y-4920 (both are made by SUMITOMO 3M CO.) was used, and it proved to be favorable. 
     FIG.  6 ( c ) is an enlarged sectional view showing the layer structure of double-sided adhesive tape  292 . Any of adhesive tape Y-4905J or Y-4920 is one wherein acrylic adhesives “b” are laminated on both sides of acrylic foam base material “a” having elasticity, and before it is used, release film “c” is stuck on one side of the acrylic adhesive “a” to protect it. By using this double-sided adhesive tape  292 , there have been attained improvement in adhesiveness and in easy sticking operations, while conventional adhesives (adhesives of a silicone rubber type or of an epoxy resin type) require much time to be hardened. 
     FIG.  8 ( a ) is a top view of optical deflecting unit  100  related to the invention, and FIG.  8 ( b ) is a sectional view of the optical deflecting unit  100 . 
     A part of cover  102  which covers the upper open space of casing  101  of optical deflecting unit  100  is protruded from a side wall surface in the vicinity of the opening  101 A of the casing  101  to form pent roof  102 B. This pent roof  102 B is a protecting section which widely covers the upper portion of light-transmitting member  291 , and it prevents that tools for working such as screwdrivers touch the light-transmitting member  291  to damage it in the course of operations of the optical deflecting unit  100 , or a fingertip touches the light-transmitting member  291  to leave thereon stains such as fingerprints. 
     FIG.  9 ( a ) is a side view of optical deflecting unit  100  related to the invention, FIG.  9 ( b ) is a rear view of the optical deflecting unit  100 , FIG.  9 ( c ) is a side view of casing  101 , and FIG. 10 is a side sectional structure diagram of image forming apparatus main body  1 . 
     Under the casing  101  of optical deflecting unit  100 , there are provided plural radiating fins  101 B which are arranged solidly in parallel. Due to this, heat generated by rotation of rotor  120  at high speed is irradiated. The outer surface of the casing  101  including radiating fins  101 B made of aluminum is treated with black anodized aluminum plating, by which the radiating effect is further enhanced. Further, the top surface of cover  102  made of aluminum which covers the upper open space of casing  101  is also treated with black anodized aluminum plating, and radiating effect is further enhanced. 
     Comparisons were made in terms of temperature rise in the course of rotation at high speed for the optical deflecting unit of the invention, an optical deflecting unit having no cooling means of a radiating fin and an optical deflecting unit provided with a separate radiating fin. As a result, it was proved that the optical deflecting unit of the invention which had temperature rise of only 20° C. at 16500 rpm and that of 30° C. at 25000 rpm can be used sufficiently even at high speed rotation. On the contrary, the optical deflecting unit having no radiating fin had temperature rise of 40° C. at 16500 rpm. The optical deflecting unit provided with a separate radiating fin had temperature rise of 35° C. at 16500 rpm. Therefore, it was clearly proved that the optical deflecting unit of the invention has high radiating effect. 
     As shown in FIG. 10, in the image forming apparatus main body  1  which is equipped with the optical deflecting unit of the invention, there is provided air-blowing means  30  for lowering temperature in the apparatus. The optical deflecting unit is mounted in the image forming apparatus main body  1  so that the direction of radiating fin  101 B and the air-blowing direction of the air-blowing means  30  are mostly in parallel. Air blown in the image forming apparatus main body  1  from the air-blowing means  30  hits and passes through the radiating fin  101 B, and then is ejected out of the image forming apparatus  1  from an air ejecting hole (not shown) provided on the side facing the image forming apparatus  1  or from each clearance on the image forming apparatus  1 . Owing to this, it is possible to eject the heat irradiated from fins  101 B out of an image forming apparatus effectively, and thereby to further enhance an effect of radiation conducted by the radiating fins  101 B. As a means to form an air flow in image forming apparatus  1 , the invention is not naturally limited to an air-blowing means, any means such as an air exhausting means or the like can be used, provided that an air flow of whichever type can be formed. 
     As stated in detail above, the optical deflecting device of the invention makes it possible to eject the heat generated when a rotor having reflecting surfaces on its circumferential surface is rotated at high speed out of the optical deflecting device effectively, since radiating fins are formed solidly on a supporting member for the optical deflecting device. Therefore, it is possible to prevent temperature rise in the optical deflecting device without having an increase of the number of parts, cost increase and a large-sized optical deflecting device or an image forming apparatus. Namely, it is possible to prevent deterioration of surface accuracy of a rotary polygon mirror caused by thermal deformation and to prevent occurrence of rotation fluctuation of rotor  120 . 
     Further, an image forming apparatus of the invention further makes it possible to prevent temperature rise of an optical deflecting device and temperature rise in a peripheral image forming apparatus equipped with the optical deflecting device, because the optical deflecting device is provided so that the direction of radiating fins formed solidly with a supporting member for the optical deflecting device may be in parallel mostly with the air flow direction in the image forming apparatus. It is therefore possible to continue outputting images which are free from scanning unevenness and image distortion and have excellent image quality for a long time, without causing an increase in the number of parts, cost increase and large-sized optical deflecting device and image forming apparatus.