Patent Publication Number: US-2023138292-A1

Title: Optical scanning device and image forming apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an optical scanning device which exposes a plurality of image carriers and an image forming apparatus. 
     BACKGROUND 
     In an electrophotographic image forming apparatus, an optical scanning device which exposes a plurality of photosensitive drums (image carriers) is known (Patent Document 1). The optical scanning device is provided with an adjustment part which corrects a deflection (bow) of a scanning line on the photosensitive drum. When an adjustment screw is screwed in the adjustment part to deflect a second lens in a bow shape, the bow is corrected. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Japanese Unexamined Patent Application Publication No. 2009-222863 
     SUMMARY OF THE INVENTION 
     Problems to be solved by the Invention 
     The corrected deflection (deformation) of the second lens is changed (creep change) with the elapse of time due to a so-called creep phenomenon. The deformation amount of the second lens due to the creep phenomenon increases as the adjustment amount of the second lens increases. 
     In order to prevent the creep deformation, for example, the second lens may be held by a holder having a strength capable of resisting the creep deformation. However, when the holder having a high strength is used, another problem occurs such as an increase in manufacturing cost and an increase in the size of the apparatus. 
     The present invention provides an optical scanning device capable of reducing the creep deformation and an image forming apparatus in consideration of the above circumstances. 
     Means of Solving the Problems 
     An optical scanning device on the present invention is an optical scanning device which exposes a plurality of photosensitive drums. The optical scanning device includes a housing provided with a light source; a reference light guide part which includes at least one reflection mirror and guides light emitted from the light source and passed through a reference lens to the photosensitive drum; a sub-light guide part which includes a larger number of reflection mirrors than the reference light guide part and guides light emitted from the light source and passed through a sub-lens to the photosensitive drum; a reference holding structure which holds the reference lens and includes a reference reception part configured so as to be in contact with the reference lens deflected in a sub-scanning direction perpendicular to a main scanning direction; a sub-holding structure which holds the sub-lens, and includes a sub-reception part configured so as to be in contact with the sub-lens deflected in the sub-scanning direction and a deflection adjustment mechanism which presses the sub-lens to adjust a deflection of the sub-lens, wherein the reference lens and the sub-lens are arranged such that a deflection direction of the reference lens with respect to the reference reception part coincides with a deflection direction of the sub-lens with respect to the sub-reception part, and when it is assumed that the reference lens and the sub-lens are not deflected, an absolute value of a smallest distance between the sub-reception part and the sub-lens is set to be equal to or larger than an absolute value of a smallest distance between the reference reception part and the reference lens. 
     In this case, when the reference lens and the sub-lens are deflected, the reference lens may be provided in a deflected state so as to be close to the reference reception part, and the sub-lens may be provided in a deflected state so as to be close to the sub-reception part. 
     In this case, the housing may have a bottom portion and a top portion facing each other in the sub-scanning direction, the reference reception part is provided so as to be in contact with a center portion of the reference lens in the main scanning direction on a side closer to the bottom portion, the reference holding structure includes: a pair of reference support parts which supports both end portions of the reference lens in the main scanning direction; and a pair of reference biasing members which presses the reference lens on the pair of reference support parts, the sub-reception part and the deflection adjustment part are provided so as to in contact with a center portion of the sub-lens in the main scanning direction on a side closer to the top portion, and the sub-holding mechanism includes; a pair of sub-support parts which is provided on an opposite side to the deflection adjustment mechanism across the sub-lens and supports both end portions of the sub-lens in the main-scanning direction; a pair of sub-biasing members which presses the sub-lens on the pair of sub-support parts; and a pressing member which presses the sub-lens in a direction opposite to a pressing direction of the deflection adjustment part. 
     In this case, the sub-lens may have a protrusion protruding along an optical axis, the sub-reception part may be a groove formed in a holder which holds the sub-lens, and the protrusion may be engaged with the sub-reception part in a movable manner in the scanning direction. 
     An image forming apparatus of the invention includes the optical scanning device. 
     Effects of the Invention 
     According to the present invention, the shapes of the plurality of scanning light beams can be easily adjusted by adjusting the deflection of the sub-lens by the deflection adjustment mechanism with the light beam incident on the reference light guide part as a reference. Further, since it becomes possible to deflect the sub-lens more than reference lens, the deflection of the sub-lens can be properly adjusted with the reference lens as the reference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a front view schematically showing an inner structure of an image forming apparatus according to one embodiment of the present invention. 
         FIG.  2    is a plan view showing an inner structure of an optical scanning device according to the embodiment of the present invention. 
         FIG.  3    is a sectional view showing the inner structure of then optical scanning device according to the embodiment of the present invention. 
         FIG.  4    is a perspective view showing a lens of the optical scanning device according to the embodiment of the present invention. 
         FIG.  5    is a perspective view showing a part of the inner structure of the optical scanning device according to the embodiment of the present invention. 
         FIG.  6    is a side view schematically showing a reference lens and a reference holding structure in the optical scanning device according to the embodiment of the present invention. 
         FIG.  7    is a perspective view showing a sub-lens held by a holder, in the optical scanning device according to the embodiment of the present invention. 
         FIG.  8    is a side view schematically showing the sub-lens and a sub holding structure in the optical scanning device according to the embodiment of the present invention. 
         FIG.  9    is a view schematically explaining light deflected by a polygon mirror, in the optical scanning device according to the embodiment of the present invention. 
         FIG.  10    is a view explaining deflection (bow) of scanning light on four photosensitive drums. 
         FIG.  11    is a side view schematically showing the reference lens and the reference holding structure in an modified example of the optical scanning device according to the embodiment of the present invention. 
         FIG.  12    is a side view schematically showing the sub-lens and the sub holding structure in the modified example of the optical scanning device according to the embodiment of the present invention. 
         FIG.  13    is a sectional view showing the inner structure of the optical scanning device in another modified example of the embodiment of the present invention. 
     
    
    
     EMBODIMENT FOR CARRYING OUT THE INVENTION 
     Hereinafter, with reference to the attached drawings, a preferred embodiment of the present invention will be described. The reference numerals Fr, Rr, L, R, U, and D in the drawings indicate front, rear, left, right, upper, and lower. Although the terms used in the specification refer to directions and positions, these terms are used for convenience for explanation and do not limit the scope of the invention. 
     With reference to  FIG.  1   , an image forming apparatus  1  according to a first embodiment will be described.  FIG.  1    is a front view schematically showing the inner structure of the image forming apparatus  1 . 
     The image forming apparatus  1  is a color printer which forms a full-color toner image in an electrophotographic method, transfers it to a sheet P and forms an image. The image forming apparatus  1  includes an apparatus main body  2  constituting a substantially rectangular parallelepiped external appearance. In the lower portion of the apparatus main body  2 , a sheet feeding cassette  3  in which the sheet S is stored is detachably provided, and on the upper surface of the apparatus main body  2 , a sheet discharge tray  4  on which the sheet P having the image is stacked is provided. Below the discharge tray  4 , four toner containers  5  containing toner (developer) of four colors (magenta, cyan, yellow and black) for replenishment are detachably attached. Inside the apparatus main body  2 , a conveyance path  6  along which the sheet P is conveyed from the sheet feeding cassette  3  to the discharge tray  4  is formed. 
     Inside the apparatus main body  2 , a sheet feeding part  10 , an image forming part  11 , and a fixing part  12  are provided. The sheet feeding part  10  is provided at the upstream end of the conveyance path  6 , and the fixing part  12  is provided at the downstream portion of the conveyance path  6 . The image forming part  11  is provided on the conveyance path  6  between the sheet feeding part  10  and the fixing part  12 . 
     The image forming part  11  includes an intermediate transfer belt  13 , four drum units  14 , and an optical scanning device  15 . The intermediate transfer belt  13  is provided below the toner containers  5  and travels in the direction indicated by the arrow in  FIG.  1   . The four drum units  14  are arranged side by side in the left-and-right direction below the intermediate transfer belt  13 , and the optical scanning device  15  is provided below the drum units  14 . The four drum units  14  correspond to the magenta, cyan, yellow and black toners in order from left to right. Since the four drum units  14  have the same structure, one drum unit  14  will be described below. 
     The drum unit  14  includes a photosensitive drum  20 , a charging device  21 , a developing device  22 , a primary transfer roller  23 , a cleaning device  24 , and a static eliminator  25 . The photosensitive drum  20  as an example of an image carrier is driven to be rotated around an axis while coming into contact with the lower surface of the intermediate transfer belt  13 . The charging device  21 , the developing device  22 , the primary transfer roller  23 , the cleaning device  24 , and the static eliminator  25  are arranged around the photosensitive drum  20  in the order of the image forming process. The primary transfer roller  23  faces the photosensitive drum  20  from the upper side across the intermediate transfer belt  13 . A secondary transfer roller  26  is in contact with the right end of the intermediate transfer belt  13 . 
     [Image Forming Process] The operation of the image forming apparatus  1  will be described. A controller (not shown) executes the image forming process based on image data input from an external terminal as follows. 
     The charging device  21  charges the surface of the photosensitive drum  20 . The optical scanning device  15  exposes the photosensitive drum  20  in accordance with the image data, and forms an electrostatic latent image on the surface of the photosensitive drum  20 . The developing device  22  develops the electrostatic latent image formed on the surface of the photosensitive drum  20  into a toner image by using the toner supplied from the toner container  5 . The toner images of four colors carried on the four photosensitive drums  20  are primarily transferred sequentially to the intermediate transfer belt  13  by the primary transfer rollers  23  to which a primary transfer bias is applied. Thus, a full-color toner image is formed on the surface of the intermediate transfer belt  13 . 
     The sheet feeding part  10  takes out the sheet P stored in the sheet feeding cassette  3  and feeds it to the conveyance path  6 . The secondary transfer roller  26  to which a secondary transfer bias is applied secondarily transfers the toner image on the intermediate transfer belt  13  to the sheet P. Thus, the toner image is formed on the sheet P. The fixing part  12  thermally fixes the toner image on the sheet P. The sheet P having the image is discharged to the discharge tray  4 . The cleaning device  24  removes the toner remaining on the surface of the photosensitive drum  20  after the primary transfer, and the static eliminator  25  irradiates the photosensitive drum  20  with charge elimination light to remove the charge remaining on the photosensitive drum  20 . 
     [Optical Scanning Device] Next, with reference to  FIG.  2    and  FIG.  3   , the optical scanning device  15  will be described.  FIG.  2    is a plan view showing the inner structure of the optical scanning device  15 .  FIG.  3    is a sectional view showing the inner structure of the optical scanning device  15 . 
     The optical scanning device  15  exposes the four photosensitive drums  20  while moving a plurality of light beams along a main scanning direction and a sub scanning direction. As shown in  FIG.  2    and  FIG.  3   , the optical scanning device  15  includes a housing  30 , a light source  31 , a polygon mirror  32 , an fθ lens  33 , a first sub-light guide part  40 M, a second sub-light guide part  40 C, a third sub-light guide part  40 Y, and a reference light guide part  50 . In this specification, for convenience of explanation, when the first sub-light guide part  40 M, the second sub-light guide part  40 C, and the third sub-light guide part  40 Y are described in common, they are referred to as “sub-light guide part  40 ”, and only arithmetic numerals are attached to the reference numeral. Further, when the sub-light guide part  40  and the reference light guide part  50  are described in common, they are referred to as “light guide parts  40  and  50 ”. 
     &lt;Housing&gt; The housing  30  has a low rectangular parallelepiped external appearance, and supports each member of the optical scanning device  15 . As shown in  FIG.  3   , the housing  30  has a housing body  30 A having an upper opening, and a lid part  30 B covering the opening of the housing body  30 A. The housing body  30 A has a bottom portion  30 C disposed on the lower side (one side of a perpendicular direction). The lid part  30 B constitutes a top portion disposed on the upper side (the other side of the perpendicular direction) facing the bottom portion  30 C. The lid part  30 B has a first emission port  301 , a second emission port  302 , a third emission port  303 , and a fourth emission port  304  through which laser light are emitted toward the four photosensitive drums  20 . 
     &lt;Light Source, Polygon Mirror, and fθ LENS&gt; As shown in  FIG.  2   , the light source  31  is provided in the left rear side portion of the inside (on the bottom portion  30 C) of the housing  30 . The light source  31  emits the four laser beams irradiated on the four photosensitive drums  20 . The polygon mirror  32  is supported on the left side portion of the inside (on the bottom portion  30 C) of the housing  30  in a rotatable manner around an axis. The polygon mirror  32  is formed in a polygonal shape when viewed in a plan view, and reflecting surfaces  34  (deflecting surfaces) are formed on their side surfaces. The rotating polygon mirror  32  reflects the plurality of light beams emitted from the light source  31  on the reflecting surfaces  34 . 
     As shown in  FIG.  3   , the fθ lens  33  is disposed on the downstream side of the polygon mirror  32  in the optical path, and the plurality of light beams deflected by the polygon mirror  32  pass therethrough. The fθ lens  33  makes the laser beam reflected on the polygon mirror  32  at a constant angle scan the photoreceptor drum  20  at a constant angular velocity. In this specification, the laser beams with which the four photosensitive drums  20  are irradiated are referred to as the first light beam L 1 , the second light beam L 2 , the third light beam L 3 , and the fourth light beam L 4  in order from left to right. 
     &lt;Outline of Light Guide Part&gt; As shown in  FIG.  3   , the first sub-light guide part  40 M, the second sub-light guide part  40 C, the third sub-light guide part  40 Y, and the reference light guide part  50  are arranged in this order from the vicinity of the polygon mirror  32  toward the right side. The four light guide parts  40  and  50  are arranged between the fθ lens  33  and the four photosensitive drums  20  in the optical path, and guide the light deflected by the polygon mirror  32  to the four photosensitive drums  20 . Specifically, the first sub-light guide part  40 M exposes the magenta photosensitive drum  20 , the second sub-light guide part  40 C exposes the cyan photosensitive drum  20 , the third sub-light guide part  40 Y exposes the yellow photosensitive drum  20 , and the reference light guide part  50  exposes the black photosensitive drum  20 . 
     &lt;First Sub-Light Guide Part&gt; The first sub-light guide part  40 M includes a first reflection mirror  411 , a first sub-lens  42 M, and a second reflection mirror  412 . The first reflection mirror  411  is disposed on the bottom portion  30 C of the housing body  30 A, and reflects the first light beam L 1  passing through the fθ lens  33  upward and leftward. The first sub-lens  42 M is disposed on the optical path of the first light beam L 1  reflected by the first reflection mirror  411  on the side closer to the lid part  30 B. The second reflection mirror  412  is disposed on the side closer to the lid part  30 B, and reflects the first light beam L 1  passed through the first sub-lens  42 M toward the upper photosensitive drum  20 . 
     &lt;Second Sub-Light Guide Part&gt; The second sub-light guide part  40 C includes a third reflection mirror  413 , a second sub-lens  42 C, and a fourth reflection mirror  414 . The third reflection mirror  413  is disposed on the bottom portion  30 C of the housing body  30 A, and reflects the second light beam L 2  passed through the fθ lens  33  upward and leftward. The second sub-lens  42 C is disposed on the optical path of the second light beam L 2  reflected by the third reflection mirror  413  on the side closer to the lid part  30 B. The fourth reflection mirror  414  is disposed on the side closer to the lid part  30 B, and reflects the second light beam L 2  passed through the second sub-lens  42 C toward the upper photosensitive drum  20 . 
     &lt;Third Sub-Light Guide Part&gt; The third sub-light guide part  40 Y includes a fifth reflection mirror  415 , a sixth reflection mirror  416 , a third sub-lens  42 Y, and a seventh reflection mirror  417 . The fifth reflection mirror  415  is disposed on the bottom portion  30 C of the housing body  30 A, and reflects the third light beam L 3  passed through the fθ lens  33  upward. The sixth reflection mirror  416  is disposed on the side closer to the lid part  30 B, and reflects the third light beam L 3  reflected by the fifth reflection mirror  415  leftward. The third sub-lens  42 Y is disposed on the optical path of the third light beam L 3  reflected by the sixth reflection mirror  416  on the side closer to the lid part  30 B. The seventh reflection mirror  417  is disposed on the side closer to the lid part  30 B, and reflects the third light beam L 3  passed through the third sub-lens  42 Y toward the upper photosensitive drum  20 . 
     &lt;Reference Light Guide Part&gt; The reference light guide part  50  includes a reference lens  52  and an eighth reflection mirror  51 . The reference lens  52  is disposed on the optical path of the fourth light beam L 4  passed through the fθ lens  33  on the bottom portion  30 C of the housing body  30 A. The eighth reflection mirror  51  is disposed on the bottom portion  30 C of the housing body  30 A, and reflects the fourth light beam L 4  passed through the reference lens  52  toward the upper photosensitive drum  20 . 
     In this specification, for convenience of explanation, when the first sub-lens  42 M, the second sub-lens  42 C, and the third sub-lens  42 Y are described in common, they are referred to as “sub-lens  42 ”, and only arithmetic numerals are attached to the reference numeral. Further, when the sub-lens  42  and the reference lens  52  are described in common, they are referred to as “lenses  42  and  52 ”. Further, the first to eighth reflection mirrors  411  to  417  are described in common, they are referred to as “reflection mirrors  41  and  51 ”, and the reference numerals thereof are simplified. 
     As described above, the reference light guide part  50  includes one eighth reflection mirror  51 , and each of the sub-light guide parts  40  includes two or more reflection mirrors  41  more than the reference light guide part  50 . The reference light guide part  50  guides the light emitted from the light source  31  and passed through the reference lens  52  to the photosensitive drum  20 , and the sub-light guide part  40  guides the light emitted from the light source  31  and passed through the sub-lens  42  to the photosensitive drum  20 . The first light beam L 1 , the second light beam L 2 , the third light beam L 3 , and the fourth light beam L 4  passed through the fθ lens  33  are arranged in this order from the bottom portion  30 C side of the housing  30  toward the lid part  30 B. That is, the first light beam L 1  passes through the fθ lens  33  at a position closest to the bottom portion  30 C, and the fourth light beam L 4  passes through the fθ lens  33  at a position closest to the lid part  30 B. 
     &lt;Detailed Description of Lens&gt; Next, with reference to  FIG.  4   , the lenses  42  and  52  will be described.  FIG.  4    is a perspective view showing the lenses  42  and  52 . 
     The three sub-lenses  42  and the reference lens  52  have the same shape for the purpose of cost reduction due to common use of parts. The lenses  42  and  52  are made of synthetic resin, for example. The lenses  42  and  52  are formed in a rod shape extending long in the front-and-rear direction (the main scanning direction). The lenses  42  and  52  are formed such that an incident surface F 1  on which the light beam is incident and an emission surface (not shown) from which the light beam emits are opposed to each other in the left-and-right direction. The lenses  42  and  52  are formed such that a first side portion S 1  and a second side portion S 2  are opposed to each other in the upper-and-lower direction. The lenses  42  and  52  have a protrusion V protruding along the optical axis from the center portion of the first side portion S 1  in the front-and-rear direction on the side of the incident surface F 1 . 
     Since the lenses  42  and  52  made of synthetic resin are manufactured using a mold or the like, they are often deflected (slightly curved) in the upper-and-lower direction (the sub-scanning direction crossing the main scanning direction), for example. The deflection direction (warpage direction) of the lenses  42  and  52  changes depending on the arrangement of the mold itself and the arrangement of the gate (the inlet of the molten synthetic resin) of the mold during manufacturing. Therefore, the deflection direction (the warpage direction) of the lenses  42  and  52  can be unified by unifying the arrangement of the molds and the like. 
     The sub-lens  42  and the reference lens  52  have different mounting structures to the housing  30 . Mainly, the reference lens  52  is supported on the side closer to the bottom portion  30 C of the housing  30 , and the sub-lenses  42  are supported on the side closer to the top portion of the housing  30 . 
     &lt;Reference Holding Structure&gt; First, with reference to  FIG.  3   ,  FIG.  5    and  FIG.  6   , the reference holding structure  53  for holding the reference lens  52  will be described.  FIG.  5    is a perspective view showing a part of the inner structure of the optical scanning device  15 .  FIG.  6    is a side view schematically showing the reference lens  52  and the reference holding structure  53 . 
     As shown in  FIG.  3    and  FIG.  5   , the reference holding structure  53  has a first upright wall H 1 , a second upright wall H 2 , and a central restriction portion H 3 . The first upright wall H 1  and the second upright wall H 2  protrude from the right portion of the bottom portion  30 C of the housing body  30 A. The central restriction portion H 3  is formed at the center portion of the first upright wall H 1  in the front-and-rear direction. The reference lens  52  is disposed between the first upright wall H 1  and the second upright wall H 2  in a posture in which the first side portion S 1  faces the bottom portion  30 C (a posture in which the lenses  42  and  52  in  FIG.  4    are turned upside down). The protrusion V of the reference lens  52  is engaged with the central restriction portion H 3  (see  FIG.  5   ). The housing body  30 A is provided with a pair of inner walls (not shown) coming into contact with both front and rear ends of the reference lens  52 . The moving of the reference lens  52  in the front-and-rear direction is restricted by the central restriction portion H 3  and the pair of inner walls. 
     As shown in  FIG.  6   , the reference holding structure  53  includes a reference reception part  55 , a pair of reference support parts  56 , and a pair of reference biasing members  57 . The reference reception part  55  and the pair of reference support parts  56  are provided on the side closer to the bottom portion  30 C of the housing  30 , and the pair of reference biasing members  57  are provided on the side closer to the top portion of the housing  30 . 
     &lt;Reference Reception Part and Reference Support part&gt; The reference reception part  55  and the pair of reference support parts  56  are protrusions protruding from the bottom portion  30 C between the first upright wall H 1  and the second upright wall H 2 . The reference reception part  55  is provided on the center portion of the bottom portion  30 C in the front-and-rear direction, and the pair of reference support parts  56  are provided on both side portions of the bottom portion  30 C in the front-and-rear direction. The pair of reference support parts  56  support (come in contact with) both end portions of the reference lens  52  (the first side portion S 1 ) in the front-and-rear direction (the main scanning direction). The reference reception part  55  is formed to have a height lower than each reference support part  56 . 
     (Reference Reception Part) As shown in  FIG.  6   , the reference lens  52  is disposed in a posture where the first side portion S 1  faces the bottom portion  30 C as described above, and is deflected so as to expand downward (toward the bottom portion  30 C). The reference reception part  55  is provided so as to be in contact with the center portion of the reference lens  52  when the reference lens  52  is deflected to expand downward, for example. If the reference lens  52  is not deflected and extends straight in the front-and-rear direction (see the two-dot chain line in  FIG.  6   ), the reference reception part  55  does not come in contact with the reference lens  52  (the first side portion S 1 ), and a gap is formed between the reference lens  52  (the first side portion S 1 ) and the reference reception part  55 . When the deflection amount of the reference lens  52  is large, the reference reception part  55  comes into contact with the reference lens  52  (not shown) to restrict an increase in the deflection amount. When the deflection amount of the reference lens  52  is small, the reference reception part  55  may not come into contact with the reference lens  52  (a gap is formed as shown in  FIG.  6   ). 
     (Reference Biasing Member) The pair of reference biasing members  57  are provided between both end portions of the second side portion S 2  of the reference lens  52  in the front-and-rear direction and both end portions of the housing body  30 A in the front-and-rear direction. The pair of reference biasing members  57  are formed by a plate spring, a coil spring, rubber or the like, for example, and press the reference lens  52  against the pair of reference support parts  56 . The moving of the reference lens  52  in the upper-and-lower direction is restricted by the biasing force of the reference biasing members  57 . 
     &lt;Sub-Holding Structure&gt; Next, with reference to  FIG.  7    and  FIG.  8   , the sub-holding structure  43  for holding the sub-lens  42  will be described.  FIG.  7    is a perspective view showing the sub-lens  42  held by the holder  44 .  FIG.  8    is a side view schematically showing the sub-lens  42  and the sub-holding structure  43 . Since the three sub-holding structures  43  holding the first sub-light guide part  40 M, the second sub-light guide part  40 C and the third sub-light guide part  40 Y have the same structure, one sub-holding structure  43  will be described below. 
     As shown in  FIG.  7    and  FIG.  8   , the sub-holding structure  43  includes a holder  44 , a sub-reception part  45 , a pair of sub-support parts  46 , a pair of sub-biasing members  47 , a deflection adjustment mechanism  48 , and a pressing member  49 . The holder  44  holds the sub-lens  42  at a position separated from the bottom portion  30 C to the side closer to the lid part  30 B. The sub-reception part  45 , the deflection adjustment mechanism  48  and the pair of sub-biasing members  47  are provided on the side closer to the top portion of the housing  30 . The pair of sub-support parts  46  are provided on the opposite side to the deflection adjustment mechanism  48  with respect to the sub-lens  42 . 
     (Holder) The holder  44  is formed by bending a sheet metal, for example. As shown in  FIG.  7   , the holder  44  has an upper surface portion  441 , a pressing portion  442 , a side surface portion  443 , and a bent portion  444 . The upper surface portion  441  covers the upper surface of the first side portion S 1  of the sub-lens  42 , and the pressing portion  442  extends downward from one end of the upper surface portion  441  and covers the upper portion (the first side portion S 1 ) of the incident surface Fl. The side surface portion  443  extends downward from the other end of the upper surface portion  441  and faces the emission surface of the sub-lens  42 . The side surface portion  443  has an opening (not shown) through which the emission surface is exposed. The bent portion  444  extends outward from the lower end of the side surface portion  443 . The holder  44  is fixed to the housing body  30 A by screwing the bent portion  444  to the housing body  30 A. The sub-lens  42  is held by the holder  44  in a posture in which the first side portion S 1  faces the lid part  30 B. The sub-lens  42  is supported by the housing body  30 A with the holder  44 . 
     (Sub-Reception Part) The sub-reception part  45  is a groove formed in the pressing portion  442  of the holder  44 . The sub-reception part  45  is cut out upward from the lower end of the center portion of the pressing portion  442  in the front-and-rear direction (the main scanning direction). The protrusion V of the sub-lens  42  is engaged with the sub-reception part  45  in a movable manner in the upper-and-lower direction (the sub scamming direction). By engagement of the protrusion V with the sub-reception part  45 , the moving of the sub-lens in the front-and-rear direction is restricted. 
     As shown in  FIG.  8   , the sub-lens  42  is disposed in a posture in which the first side portion S 1  faces upward, and is deflected to expand upward, as described above. The sub-reception part  45  is provided so as to be in contact with the protrusion V (the center portion in the front-and-rear direction) of the sub-lens  42  when the sub-lens  42  expands upward, for example. Specifically, the protrusion V of the deflected sub-lens  42  often comes into contact with the deepest portion  45 D of the sub-reception part  45  (the groove). If the sub-lens  42  is not deflected and extends straight in the front-and-rear direction (see the two-dot chain line in  FIG.  8   ), the deepest portion  45 D of the sub-reception part  45  does not come into contact with the protrusion V of the sub-lens  42 , and a gap is formed between the deepest portion  45 D of the sub-reception part  45  and the protrusion V. When the deflection amount of the sub-lens  42  is large, the deepest portion  45 D of the sub-reception part  45  comes into contact with the protrusion V (not shown), and an increase in the deflection amount is restricted. When the deflection amount of the sub-lens  42  is small, the deepest portion  45 D of the sub-reception part  45  sometimes does not come into contact with the protrusion V (a gap is formed as shown in  FIG.  8   ). 
     (Sub-Support Part) The pair of sub-support parts  46  protrude from a pair of inner support walls  35  provided in both side portions of the housing body  30 A in the front-and-rear direction. The pair of sub-support parts  46  support (come in contact with) both end portions of the second side portion S 2  of the sub-lens  42  in the front-and-rear direction (the main scanning direction). 
     (Sub-Biasing Member) The pair of sub-biasing members  47  are provided between both end portions of the upper surface portion  441  of the holder  44  in the front-and-rear direction and both end portions of the housing body  30 A (or the lid part  30 B) in the front-and-rear direction. The pair of sub-biasing members  47  are formed by a plate spring, a coil spring, rubber or the like, for example, and press the sub-lens  42  held by the holder  44  against the pair of sub-support parts  46 . The moving of the sub-lens  42  in the upper-and-lower direction is restricted by the biasing force of the sub-biasing members  47 . 
     (Deflection Adjustment Mechanism) As shown in  FIG.  7    and  FIG.  8   , the deflection adjustment mechanism  48  is provided on the center portion of the upper surface portion  441  of the holder  44  in the front-and-rear direction. The deflection adjustment mechanism  48  includes a screw hole (not shown) opened in the upper surface portion  441  of the holder  44 , and an adjustment screw  48 A having a male screw meshing with a female screw of the screw hole. The adjustment screw  48 A penetrates the screw hole (the upper surface portion  441 ), and is provided so as to be in contact with the center portion of the sub-lens  42  (the first side portion S 1 ) in the front-and-rear direction (the main scanning direction). When the adjustment screw  48 A is screwed in, the tip end portion of the adjustment screw  48 A presses the first side portion S 1 . When the adjustment screw  48 A is turned in the pull-out direction, the pressing force to the first side portion S 1  is reduced (released). By turning the adjustment screw  48 A in the forward and reverse directions, the deflection of the lens  42  along the main scanning direction is adjusted. The reference holding structure  53  is not provided with a mechanism for adjusting the deflection of the reference lens  52 . 
     (Pressing Member) The pressing member  49  is formed integrally with the holder  44 . The pressing member  49  is a pair of plate springs extending from the lower end of the side surface portion  443  of the holder  44  in the direction opposite to the bent portion  444  (see  FIG.  7   ). The pressing member  49  supports the lower portion (the second side portion S 2 ) of the sub-lens  42 , and restricts the detachment of the sub-lens  42  from the holder  44 . The pressing member  49  presses the sub-lens  42  in the direction (upward) opposite to the pressing force by the deflection adjustment mechanism  48 . 
     The pressing member  49  is a pair of leaf springs, but is not limited to this, and may be one (or more than three) leaf spring. Further, although the pressing member  49  is formed integrally with the holder  44 , it is not limited to this, and it may be formed of a plate spring, a coil spring, a rubber or the like which is a member different from the holder  44  (not shown). For convenience of explanation, in the specification, the sub-reception part  45  and the reference reception part  55  are described in common, they are simply referred to as “reception parts  45  and  55 ”. 
     (Gap between Lens and Reception Part) As described above, the four lenses  42  and  52  are manufactured so that the deflection directions (the warpage directions) are the same. In the optical scanning device  15  according to the present embodiment, as shown in  FIG.  6    and  FIG.  8   , when the reference lens  52  and the sub-lens  42  are deflected, the deflection direction of the reference lens  52  with respect to the reference reception part  55  coincides with the deflection direction of the sub-lens  42  with respect to the sub-reception part  45 . That is, the reference lens  52  and the three sub-lenses  42  are mounted to the housing  30  in a deflected (warped) posture in the same direction. Specifically, the reference lens  52  is provided in a deflected state so as to be close to the reference reception part  55 , and the sub-lenses  42  are provided in a deflected state so as to be close to the sub-reception part  45 . 
     As shown in  FIG.  6    and  FIG.  8   , when it is assumed that the lenses  42  and  52  are not deflected (or the deflection amount is small) (see the two-dot chain line), a gap is formed between the reception parts  45  and  55  and the lenses  42  and  52 . In this embodiment, the relationship between the size (distance) of the gap is defined between the reference lens  52  and the sub-lens  42 . Specifically, when it is assumed that the reference lens  52  and the sub-lens  42  are not deflected, the absolute value of the shortest distance (B) between the sub-reception part  45  (the deepest portion  45 D) and the sub-lens  42  (the protrusion V) is set to be larger than or equal to the absolute value of the shortest distance (A) between the reference reception part  55  and the reference lens  52  (|A|≤|B|). In this embodiment, as an example, the absolute value of the shortest distance (B) is set larger than the absolute value of the shortest distance (A). 
     [Scanning Light on Photosensitive Drum] The scanning light on the photosensitive drum  20  will be described with reference to  FIG.  9    and  FIG.  10   .  FIG.  9    is a view schematically explaining light deflected by the polygon mirror  32 .  FIG.  10    is a view explaining a bending (bow) of the scanning light in the sub-scanning direction on the four photosensitive drums  20 . 
     As shown in  FIG.  9   , the plurality of light beams emitted from the light source  31  are incident on the reflection surface  34  of the polygon mirror  32  at a predetermined acute angle (or an acute angle with respect to a surface perpendicular to the reflecting surface  34 ). The rotational locus of the outer circumferential edge of the reflection surface  34  shifts between P 1  and P 2  shown in  FIG.  9   . P 1  corresponds to a minimum outer diameter of the polygon mirror  32 , and P 2  corresponds to a ridge line between the adjacent reflection surfaces  34 . When the reflection surface  34  is located at P 1 , the incident light Q 11  is reflected to be a reflection light Q 12 , and the incident light Q 21  is reflected to be a reflection light Q 22 . On the other hand, when the reflection surface  34  is located at P 2 , the incident light Q 11  is reflected to be a reflection light Q 13 , and the incident light Q 21  is reflected to be a reflection light Q 23 . Therefore, as shown in  FIG.  10   , a bending (bow) of the scanning light in the sub-scanning direction occurs on each photosensitive drum  20 . 
     The first light beam L 1  and the second light beam L 2  reflected by the reflection surface  34  travel below the surface center CM (see  FIG.  9   ) of the reflection surface  34 , and the third light beam L 3  and the fourth light beam L 4  reflected by the reflection surface  34  travel above the surface center CM of the reflection surface  34 . Therefore, as shown in  FIG.  10   , a shape of the bow of the first light beam L 1  and the second light beam L 2  and a shape of the bow of the third light beam L 3  and the fourth light beam L 4  on the photosensitive drum  20  are linearly symmetrical with respect to the rotational axis of the photosensitive drum  20 . Since the direction of the bow of the scanning light is reversed every time when the light is reflected by the reflection mirrors, the direction changes according to the number of the reflection mirrors. 
     In this embodiment, the first sub-light guide part  40 M and the second sub-light guide part  40 C are provided with two reflection mirrors, the third sub-light guide part  40 Y is provided with three reflection mirrors, and the reference light guide part  50  is provided with one reflection mirror. In addition, the four lenses  42  and  52  have the same deflection direction (the warpage direction). Therefore, the directions of the bow of the four scanning lights are linearly symmetric as shown in  FIG.  10   . In  FIG.  9   , the larger the oblique incident angle of the incident light to the reflection surface  34 , the larger the deflection amount (the warpage amount) of the bow on the photosensitive drum  20 . 
     When the lenses  42  and  52  and the reflection mirrors  41  and  51  are fixed in an inclined posture with respect to the main scanning direction, an inclination (slew) occurs on the scanning light on the photosensitive drum  20 . The bending and inclination of the scanning light causes a defective image such as a color shift. 
     In the optical scanning device  15  according to the embodiment described above, the reference lens  52  of the reference light guide part  50  is not provided with the deflection adjustment mechanism  48  (a mechanism for adjusting the bow of the scanning light along the sub-scanning direction on the photosensitive drum  20 ), and serves as a reference for the deflection adjustment of the other sub-lenses  42 . Since the reference light guide part  50  has the smallest number of reflection mirrors compared with the other sub-light guide parts  40 , the bending (bow) and the inclination (skew) occurred in the scanning light (the fourth light beam L 4 ) on the photosensitive drum  20  owing to the weight and the inclination of the reflection mirror are smaller than those of the sub-light guide part  40 . Therefore, when the fourth light beam L 4  of the reference light guide part  50  serves as a reference and the deflection of the sub-lens  42  is adjusted by rotating the adjustment screws  48 A in the forward or reverse direction, it becomes possible to adjust the shape of the four scanning lights easily. Further, the deflection adjustment mechanism  48  of the reference light guide part  50  can be omitted, and the manufacturing cost of the optical scanning device  15  can be reduced. 
     The deflection of the sub-lens  42  adjusted by the deflection adjustment mechanism  48  changes with the elapse of time (so-called creep phenomenon). The deflection amount of the sub-lens  42  due to the creep phenomenon increases as the adjustment amount (the deflection amount) of the sub-lens  42  is increased. For the problem caused by the creep phenomenon, in the optical scanning device  15  according to the present embodiment, when the four lenses  42  and  52  are deflected, the deflection directions (the warpage directions) of the four lenses  42  and  52  are made to be the same. According to this configuration, since the warping directions of the four lenses  42  and  52  are the same, the adjustment amount of only some sub-lenses  42  is not increased, and the adjustment amount of each sub-lens  42  can be minimized. Thus, the creep deformation of the adjusted sub-lens  42  can be reduced. 
     If the shortest distance (A) between the reference reception part  55  and the reference lens  52  is larger (longer) than the shortest distance (B) between the sub-reception part  45  (the deepest portion  45 D) and the sub-lens  42  (the protrusion V), there is a possibility that the deflection of the sub-lens  42  cannot be fully adjusted by the deflection adjustment mechanism  48  with the fourth light beam L 4  (the reference lens  52 ) as a reference. For the problem, in the optical scanning device  15  according to the present embodiment, the absolute value of the above-described shortest distance (B) is set to be equal to or larger than the absolute value of the above-described shortest distance (A) (|A|≤|B|) (see  FIG.  6    and  FIG.  8   ). According to this constitution, since the sub-lens  42  can be deflected more than the deflection of the reference lens  52 , the deflection of the sub-lens  42  can be properly adjusted with the fourth light beam L 4  (the reference lens  52 ) as a reference. 
     Further, according to the optical scanning device  15  of the present embodiment, since the sub-lens  42  is provided in a deflect state so as to be close to the sub-reception part  45 , it becomes possible to restrict further increase of the deflection when the deflected sub-lens  42  come into contact against the sub-reception part  45 . The deflection adjustment mechanism  48  presses the sub-lens  42  in the direction opposite to the deflection direction to adjust the deflection of the sub-lens  42  with the fourth light beam L 4  (the reference lens  52 ) as a reference. 
     Further, in the optical scanning device  15  according to the present embodiment, since the sub-light guide part  40  is provided on the side closer to the top portion of the housing  30 , the deflection adjustment mechanism  48  (the adjustment screw  48 A) is exposed by detaching the lid part  30 B. According to this configuration, the operator can easily operate the deflection adjustment mechanism  48  (the adjustment screw  48 A). Further, since it is not necessary to adjust the deflection of the reference lens  52  (the reference holding structure  53 ) provided on the side closer to the bottom portion  30 C of the housing  30 , it is possible to easily adjust the deflection of the scanning light of the optical scanning device  15  as a whole. The adjustment hole may be formed in the lid part  30 B so that the deflection can be adjusted without detaching the lid part  30 B. 
     Further, according to the optical scanning device  15  of the present embodiment, since the protrusion V of the sub-lens  42  is engaged with the sub-reception part  45  in a movable manner in the upper-and-lower direction (the sub scanning direction), the moving of the sub-lens  42  at the time of deflection adjustment can be guided while positioning the sub-lens  42  in the main scanning direction. Since the protrusions V are disposed in the center portion of the lenses  42  and  52  in the main scanning direction, the positions of the protrusions V do not substantially change even if the lenses  42  and  52  are thermally expanded. Thereby, the deviation in the sub-scanning direction between the plurality of scanning lights can be reduced. Further, since the protrusions V are formed in the lenses  42  and  52 , it becomes possible to discriminate between the incident surface F 1  and the emission surface easily, and a mounting work of the lenses  42  and  52  can be easily carried out. 
     In the optical scanning device  15  according to the present embodiment, the lenses  42  and  52  are provided in a deflect state so as to be close to the reception parts  45  and  55 , but the present invention is not limited thereto. On the contrary, as shown in  FIG.  11    and  FIG.  12   , as the optical scanning device  15  according to a modified example, the lenses  42  and  52  may be provided in a deflect state so as to be separated from the reception parts  45  and  55 . Even in this case, when it is assumed that the lenses  42  and  52  are not deflected (see the two-dot chain line in  FIG.  11    and  FIG.  12   ), the absolute value of the above-described shortest distance (B) is set to be equal to or larger than the absolute value of the above-described shortest distance (A). Therefore, for example, as shown in  FIG.  11   , the reference reception part  55  may be higher than the reference support parts  56 . In this case, when it is assumed that the reference lens  52  is not deflected, the reference reception part  55  bites into the reference lens  52 , and the shortest distance (A) is a negative number. For example, as shown in  FIG.  12   , the deepest portion  45 D of the sub-reception part  45  may be lower than the position shown in  FIG.  8   . Even in this case, when it is assumed that the sub-lens  42  is not deflected, the deepest portion  45 D of the sub-reception part  45  may bite into the sub-lens  42 , and the shortest distance (B) may become a negative number (not shown). 
     In the optical scanning device  15  according to the present embodiment (including the modified examples, and the same shall apply hereinafter), the protrusions V are formed in all the lenses  42  and  52 , but it is not limited to this, and the protrusions V need only be formed in at least the sub-lens  42 , and may not be formed in the reference lens  52 . 
     Since the fourth light beam L 4  travels farther away from the center of the fθ lens  33  than the third light beam L 3 , the bow (deflection amount) of the fourth light beam L 4  is likely to be larger than the bow (deflection amount) of the third light beam L 3  on the photosensitive drum  20  (see  FIG.  10   ). Since the reference lens  52  is not provided with the deflection adjustment mechanism  48 , the bow of the fourth light beam L 4  is preferably made as small as possible. 
     The light guide parts  40  and  50  may be provided with a skew adjustment mechanism for adjusting the skew of the scanning light on the photosensitive drum  20 . In this case, like the deflection adjustment mechanism  48 , the skew adjustment mechanism may be provided on the sub-lens  42  but not on the reference lens  52 . The first to seventh reflection mirrors  411  to  417  except the eighth reflection mirror  51  may be provided with the skew adjustment mechanism. 
     In the optical scanning device  15  according to the present embodiment, the lenses  42  and  52  and the reflection mirrors  41  and  51  are arranged as shown in  FIG.  3   , but the present invention is not limited thereto. For example, as shown in  FIG.  13   , as the optical scanning device  15  according to another modified example, the third sub-light guide part  40 Y and the reference light guide part  50  are configured so that the fourth light beam L 4  travels below the third light beam L 3 . The fifth reflection mirror  415  is disposed above the optical path of the fourth light beam L 4 , and is formed in a trapezoidal shape with its lower end cut out so as not to block the optical path of the fourth light beam L 4 . 
     In the optical scanning device  15  according to another modified example, the first light beam L 1 , the second light beam L 2 , the fourth light beam L 4  and the third light beam L 3  passing through the fθ lens  33  are arranged in this order from the side of the bottom portion  30 C of the housing  30  toward the lid part  30 B. Since the oblique incident angle (see  FIG.  9   ) of the fourth light beam L 4  to the reflection surface  34  of the polygon mirror  32  is smallest among the four light beams, the bow (the deflection amount) of the fourth light beam L 4  on the photosensitive drum  20  can be reduced. Therefore, it can easily be used as a reference for adjusting the bow of the sub-light guide  40 . In order to further reduce the deflection amount of the fourth light beam L 4 , the thickness of the eighth reflection mirror  51  may be larger than those of the other first to seventh reflection mirrors  411  to  417 , or a reinforcing rib may be provided (not shown). In order to reduce the influence of the deflection of the eighth reflection mirror  51  on the bow of the fourth light beam L 4  on the photosensitive drum  20 , the angle of reflection of the fourth light beam L 4  to the eighth reflection mirror  51  may be set small (not shown). 
     In the description of the present embodiment (including modified examples), a case where the present invention is applied to a color printer is shown as an example, but the present invention is not limited to this and may be applied to a monochrome printer, a copying machine, a facsimile machine or a multifunction machine, for example. 
     It should be noted that the description of the above embodiments shows one aspect of the optical scanning device and the image forming apparatus according to the present invention, and the technical scope of the present invention is not limited to the above embodiments. The invention may be variously changed, substituted, or modified without departing from the spirit of the technical idea, and the claims include all embodiments that may be included within the scope of the technical idea.