Abstract:
The image forming apparatus includes a first placement member, a second placement member, a third placement member and a fourth placement member which contact with a placement surface for placing the image forming apparatus thereon, and which support the image forming apparatus, a side plate of the image forming apparatus, and supporting means as the supporting member for supporting the third placement member and the fourth placement member movably in a vertical direction of the image forming apparatus. This can prevent the deformation of the image forming apparatus without increasing the load on a bottom plate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the invention  
         [0002]     The present invention relates to a placement structure for a monochromatic image forming apparatus or a full-color image forming apparatus employing electrophotography.  
         [0003]     2. Related Background Art  
         [0004]     In the field of image forming apparatuses employing electrophotography such as full-color printers, a smaller-sized printer with a lower price and higher image quality is required by the market, in particular, as more and more personal computers are being generally used.  
         [0005]      FIGS. 5A and 5B  are schematic section diagrams of an indirect-transfer-type, full color image forming apparatus that employs electrophotography.  
         [0006]     In  FIGS. 5A and 5B , reference numeral  11  denotes paper feeding means for separating stacked paper sheets and feeding them; reference numeral  12  denotes a registration roller for detecting the leading edge of a paper sheet and conveying the paper sheet in synchronization with an image signal; reference numeral  13  denotes a transfer belt for forming an intermediate image; reference numeral  14  denotes a developing device having four photosensitive drums  19 ; reference numeral  15  denotes exposure means for forming a latent image on each photosensitive drum  19 ; reference numeral  16  denotes first-transfer means for transferring a toner image onto the transfer belt  13  by reverse biasing; reference numeral  17  denotes second-transfer means for transferring the toner image from the transfer belt  13  onto a paper sheet by reverse biasing; and reference numeral  18  denotes a fixing device for fixing the toner as a permanent image on the paper sheet by heating and pressing.  
         [0007]     In the indirect-transfer-type, full-color image forming apparatus having such a arrangement, a latent image on each photosensitive drum  19  is visualized as a toner image on the developing device  14 ; the toner image is first-transferred on the transfer belt  13 , and the toner image is then second-transferred onto a paper sheet by reverse biasing from the backside of the paper sheet, thereby forming a desired full-color image.  
         [0008]     A function unit, which is constituted of the above described paper feeding means  11 , registration roller  12 , transfer belt  13 , developing device  14 , exposure means  15 , first-transfer means, second transfer means  17 , and fixing device  18 , is fixed to a main frame  6 . The main frame  6  is then supported by four placement legs  21  (only two of them shown) provided at front and rear, left and right positions.  
         [0009]     A tandem-type, multicolor image forming apparatus, which has a plurality of photosensitive drums  19  arranged in line, may be an image forming apparatus of lateral arrangement in which a plurality of photosensitive drum  19  are arranged horizontally as shown in  FIG. 5A ; or an image forming apparatus of vertical arrangement in which a plurality of photosensitive drum  19  are arranged vertically as shown in  FIG. 5B . An image forming apparatus having a lateral arrangement shown in  FIG. 5A  has a smaller height but difficulty in replacement of consumables in the developing device  14  or the like. On the other hand, an image forming apparatus having a vertical arrangement shown in  FIG. 5B  allows easy replacement of consumables but has a larger height.  
         [0010]     Such a tandem-type multicolor image forming apparatus requires high positional accuracy between photosensitive drums that form respective colors. However, when the multicolor image forming apparatus is placed on an uneven surface, for example, on an area where it is placed with four placement points with only one point being different in a height level than the other points, the following problem arises. Since the straight line connecting a point and another point of different height levels and the straight line connecting the two other points to be in parallel with each other become not parallel with each other, it results the occurrence of force that deforms the main body of the image forming apparatus. The occurrence of deformation of the main body of the image forming apparatus change the positional relationship between the photosensitive drums and scanner as the exposure means for each color, depending on the strength of the bottom plate of the main body of the image forming apparatus, leading to misregistration in images.  
         [0011]     Accordingly, in the case where a tension is given to an endless belt to move according to the frame of the main body of the image forming apparatus, the deformation of the frame of the main body of the image forming apparatus may increase the displacement force on the endless belt. As a result, the increase in the displacement force on the endless belt may cause the deterioration or breakage of end portion of the belt, affecting the life of the endless belt. The endless belt may be, for example, an electrostatic transfer belt that contacts with a plurality of photosensitive drums as image bearing members and conveys transfer material as attracted electrostatically thereon to a fixing device consisting of a fixing roller and a pressing roller, or an intermediate transfer belt that contacts with a plurality of image bearing members, sequentially transfer and overlay images onto the belt, and transfer the all images on the belt onto the transfer material at a time.  
         [0012]     Therefore, there are methods for preventing the above problems by reducing the deformation of the main frame, as follows. A first method is to substantially increase the rigidity of the main frame so that the main frame cannot be deformed even when the main body of the image forming apparatus is placed on the surface mentioned above. A second method is to substantially increase the rigidity of the bottom plate against deformation so that the unevenness of the placement surface cannot cause the main frame to be deformed. A third method is to use only three placement points (Japanese Patent Application Laid-Open No. 2000-330351). In the case where only three placement points are used, a plane is defined by the three points, and therefore the main frame is not deformed even when the main body of the image forming apparatus is placed on an uneven surface.  
         [0013]     In the first and second methods, to increase the rigidity of components of the main frame or the bottom plate against deformation, the thickness of the bottom plate may be increased or the section area thereof may be increased with a drawing shape or the like. This results in the heavy weight of the main body and the increased cost for parts. Further, the use of a drawing shape for improving the rigidity may badly affect the flatness and the dimension accuracy, resulting in the lowered accuracy in mounted position of the photosensitive drums or scanner. Therefore, this presents a significant problem in developing a smaller-sized printer with a lower price and higher quality image, for which the market has a strong demand.  
         [0014]     Regarding the reduction of the deformation of the main frame, the most advantageous method for providing a smaller-sized printer with a lower price and higher quality image is the above-mentioned, third method that uses only three placement points, but the method has a significant problem as described below.  
         [0015]      FIGS. 7A and 7B  show side section diagrams for showing a typical arrangement of an image forming apparatus in which a paper feeding cassette  22  for stacking recording paper sheets is provided at the bottom thereof; an image forming section is provided above the paper feeding cassette  22 ; a paper discharge tray  23  is provided at the top thereof; a recording paper sheets is conveyed in the direction of shown arrow so that operations including paper feeding, image formation and paper discharge are performed. Here, the shown dimension X, which may be a depth or width of the product, is designed to be as small as possible to provide a smaller-sized image forming apparatus.  
         [0016]     To achieve such a smaller-sized printer for which the market has a strong demand, it is a common practice that recording paper sheets of A4, B5 or letter size, which are more frequently used as transfer material, are accommodated in a size within the shown dimension X as shown in  FIG. 7A , while larger-sized paper sheets such as those of A3 or legal-size, which are less frequently used as transfer material, are accommodated in the paper feeding cassette  22  that has a larger seize than the shown dimension X, as shown in  FIG. 7B . Thus, when using a recording paper of or smaller than A4, B5 or letter size which are more frequently used, the placement area can be within the shown dimension X thereby allowing effective use of space. When the placement area is increased by the shown dimension Y, larger-sized recording paper can be used. Therefore, such a type of arrangement in which larger-sized recording paper is used while exceeding the normal placement area is very advantageous for users.  
         [0017]     In order to achieve both the arrangements: the arrangement in which only three placement points as described above is defined to reduce the deformation of the main frame, and the arrangement of type in which larger-sized recording paper is used while exceeding the normal placement area, it is required that three placement points A, B and C are defined symmetrically with respect to the center of image and in particular the placement point C is defined under the paper feeding cassette  22  as shown  FIG. 8A , or that three placement points A′, B′ and C′ are defined non-symmetrically with respect to the center of image as shown in  FIG. 8B .  
         [0018]     However, even if the placement point C is provided under the paper feeding cassette  22  as shown in  FIG. 8A , using only the space under the paper feeding cassette  22  is not enough since substantial rigidity along edge line a is required for supporting the body. Therefore, to secure the rigidity along edge line a, an expensive member having large section area must be applied to increase the rigidity of the bottom plate. Moreover, expanding the space under the cassette  22  interferes with the object of using the arrangement with only three placement points to provide a smaller-sized, lower-price apparatus.  
         [0019]     When using the arrangement in which the main frame is non-symmetrically supported as shown in  FIG. 8B , placement legs cannot be provided on the front side of the apparatus, which is a side for taking out the paper feeding cassette  22 . The front side of the apparatus can be subject to a load from a user who accesses the operation section or replaces consumables. This causes the non-symmetrical deformation of the main frame, which directly leads to misregistration, the increase in displacement force on the endless belt, and inclined conveyance of recording paper. Therefore, this arrangement is not feasible.  
         [0020]     Furthermore, in the case of three-placement point, each point bears heavier load, and thus the bottom place must be reinforced more than otherwise required.  
       SUMMARY OF THE INVENTION  
       [0021]     An object of the present invention is to prevent the deformation of an image forming apparatus when placed on an uneven surface, without increasing the load on its bottom plate.  
         [0022]     The object and other objects can be achieved by an image forming apparatus comprising: a first placement member and a second placement member fixed to a body of the image forming apparatus for supporting the image forming apparatus while contacting with a placement surface when the image forming apparatus is placed thereon; a side plate disposed on a side face of the image forming apparatus; a third placement member and a fourth placement member for supporting the image forming apparatus while contacting with the placement surface when the image forming apparatus is placed thereon; and supporting means disposed on said side plate, for supporting the third placement member and the fourth placement member movably in a vertical direction of the image forming apparatus.  
         [0023]     Other objects of the present invention will be understood when reading the following description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIGS. 1A, 1B  and  1 C show schematic section diagrams of an image forming apparatus according to the present invention;  
         [0025]      FIG. 2  is a perspective diagram of an image forming apparatus according to the present invention;  
         [0026]      FIG. 3  is an illustrative diagram for showing an advantage of an image forming apparatus according to the present invention;  
         [0027]      FIGS. 4A, 4B  and  4 C show illustrative diagrams for showing an advantage of an image forming apparatus according to the present invention;  
         [0028]      FIGS. 5A and 5B  show schematic section diagrams of a conventional image forming apparatus;  
         [0029]      FIG. 6  is a diagram of an actual image forming apparatus;  
         [0030]      FIGS. 7A and 7B  show illustrative diagram of a conventional image forming apparatus; and  
         [0031]      FIGS. 8A and 8B  show illustrative diagram of a conventional image forming apparatus. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     Embodiments of the present invention will be described with reference to appended drawings.  
         [0033]      FIGS. 1A  to  1 C show an indirect-transfer-type, full-color image forming apparatus employing electrophotography:  FIG. 1A  is a diagram when viewing  FIG. 1B  in the direction of arrow D;  FIG. 1B  is a schematic section diagram; and  FIG. 1C  is a diagram when viewing  FIG. 1B  in the direction of arrow E.  
         [0034]     In  FIGS. 1A, 1B  and  1 C, reference numeral  11  denotes paper feeding means for separating stacked paper sheets and feeding them; reference numeral  12  denotes a registration roller for detecting the leading edge of a paper sheet and conveying the paper sheet in synchronization with an image signal; reference numeral  13  denotes a transfer belt for forming an intermediate image as an intermediate transfer member; reference numeral  14  denotes a developing device as developing means having four photosensitive drums  19  as image bearing members arranged substantially in horizontal direction, which forms a toner image on the image bearing members; reference numeral  15  denotes four exposure means for forming a latent image on each photosensitive drum  19 ; reference numeral  16  denotes first-transfer means for transferring a toner image onto the transfer belt  13  by reverse biasing; reference numeral  17  denotes second-transfer means for transferring the toner image from the transfer belt  13  onto a paper sheet by reverse biasing; and reference numeral  18  denotes a fixing device as fixing means for fixing the toner on the paper sheet by heating and pressing.  
         [0035]     In such indirect-transfer-type, full-color image forming apparatus, a latent image on the photosensitive drum  19  is visualized as a toner image on the developing device  14 ; the toner image is first-transferred on the transfer belt  13 , and the toner image is then second-transferred onto a paper sheet by reverse biasing from the backside of the paper sheet, thereby forming a desired full-color image.  
         [0036]     A function unit, which is constituted of the above described paper feeding means  11 , registration roller  12 , transfer belt  13 , developing device  14 , exposure means  15 , first-transfer means  16 , second transfer means  17 , and fixing device  18 , is fixed to the frame  6  of the main body. The frame  6  of the main body is then supported by flat, first and second placement legs  1  and  2  as first and second placement members) provided at left and right positions in the front lower portion, and third and fourth placement legs  3  and  4  as third and fourth placement members provided at left and right positions in the rear portion. Here, the third and fourth placement legs  3  and  4  are disposed movably in upward and downward direction, and one end of each leg (top end) is formed in a mountain-like shape as shown.  
         [0037]     On the backside of the main frame  6 , a pivoting member  5  formed in a plate shape elongated in lateral direction is pivotably supported by a pivot  5   a  at the center thereof in the width direction. The pivoting member  5  is supported at left and right positions thereof by the tips in mountain shape of the third and fourth legs.  
         [0038]      FIG. 2  is a perspective diagram when viewing the image forming apparatus from the rear side.  
         [0039]     In  FIG. 2 , reference numerals  1  and  2  denote the first and second placement legs; reference numerals  3  and  4  denotes the third and fourth placement legs; reference numeral  5  denotes the pivoting member; reference numeral  5   a  denotes the pivot of the pivoting member  5  (the center of pivoting); and reference numeral  6  denotes the frame of the main body of the image forming apparatus, while the exposure means and transfer means supported by the frame  6  of the main body are not shown. The pivot  5   a  is arranged at the side plate of the image forming apparatus to rotatably support the pivoting member  5 . The third placement member  3  and fourth placement member  4  is supported by the pivoting member  5 . The side of the image forming apparatus is constituted of the side plates.  
         [0040]     As shown in  FIG. 2 , the first and second placement legs  1  and  2  directly support the frame  6  of the main body on the placement surface, while the third and fourth placement legs  3  and  4  support the frame  6  of the main body on the placement surface, by means of the pivoting member  5 .  
         [0041]     Here, the third and fourth placement legs  3  and  4  are mounted to the main frame  6  with the legs movable rectilinearly in the direction of shown arrow (upward and downward directions), and with the lower ends of the legs contacting with the placement surface and the upper ends of the legs contacting with the pivoting member  5  to be freely separated from it. The pivoting member  5  is mounted to the main frame  6  to be pivotable about the pivot  5   a  in the direction of shown arrow.  
         [0042]     When the image forming apparatus is placed on an uneven placement surface as shown in  FIG. 3  with the frame  6  of the main body supported by the pivoting member  5  on the placement surface as described above, the third placement leg  3  moves downward causing the pivoting member  5  to pivot about the direction of the arrow about the pivot  5   a  (counter-clockwise). In this situation, while the third and fourth placement legs  3  and  4  and the pivoting member  5  are balanced at respective positions that are non-symmetrical, the frame  6  of the main body is supported substantially at three points of the first and second legs  1  and  2  and the pivot  5   a,  thereby minimizing the deformation of the main frame  6  due to the self weight thereof.  
         [0043]     Here, the mechanism in which the body is supported at three points positioned substantially symmetrically with respect to the center of image is similar to the example of conventional art shown in  FIG. 8A . However, supporting the body at a point under the paper feeding cassette  22  (shown point C) as shown in  FIG. 8A  requires that the rigidity along the edge line shown by “a” is substantially high, but using only the space under the paper feeding cassette  22  cannot achieve the rigidity, as described above.  
         [0044]     On the other hand, in the case where the pivot  5   a  is provided at a high level as shown in  FIG. 2 , it is possible to secure a substantial dimension h shown in  FIG. 2  for the pivot  5   a  for supporting pivoting member  5 , thereby easily giving the frame  6  of the main body required substantial flexural rigidity.  
         [0045]     The paper feeding cassette  22  is provided so that paper sheets are placed substantially horizontally. Thus, when viewing paper sheets of the maximum size stacked in the paper feeding cassette  22  from vertically upper side, a potion of the paper feeding cassette  22  or the stacked paper sheets is projecting from the tetragon defined by connecting the centers of the first, second, third and fourth placement legs  1 ,  2 ,  3  and  4  (see  FIG. 2 ).  
         [0046]     Similar advantages as described above can be achieved when using the arrangement in which a member  5 ′ or  5 ″ that is constituted of the pivoting member and the placement legs in an integrated manner is supported at the center in width direction by the pivot  5   a  pivotably with respect to the frame  6  of the main body, as shown in  FIGS. 4B and 4C .  
         [0047]     Alternatively, as shown in  FIG. 4A , when, the pivoting member  5  is placed outside the shown hatched region S between the left and right, third and fourth legs  3  and  4  so that the pivot  5   a  of the pivoting member  5  can be positioned at as high as possible, the region whose rigidity must be ensured is limited to the upper portion shown as h, thereby allowing openings to be provided in the region shown by “S” of the frame  6  of the main body, and thus making it easy to incorporate the exposure means, transfer means and electric boards inside the frame  6  of the main body.  
         [0048]     Here, since the pivot  5   a  is desirably configured so as to reduce the pivoting resistance as low as possible, it may be a swage pivot, a roller bearing, or a sliding bearing. If the pivoting resistance can be substantially low, a configuration may be used in which the pivoting member  5  simply abuts a vertically angled portion of a metal plate.  
         [0049]     The pivot  5   a  is desirably provided at the center of weight of the frame  6  of the main body (the center of gravity) rather than at the center in the lateral direction of the apparatus.  
         [0050]     Since the third and fourth placement legs  3  and  4  are desirably configured to reduce the friction resistance in the rectilinear movement, they may be guided by rollers to reduce the friction. If the friction resistance can be substantially lower, a configuration of simple sliding on a surface may be used.  
         [0051]     As described above, even when the image forming apparatus is placed on an uneven surface, the distortion of the frame  6  of the main body can be minimized since the action of the pivoting member  5  and the third and fourth placement legs  3  and  4  allows the frame  6  of the main body to be supported at three points: at the first placement leg  1 , at the second placement leg  2  and at the pivot  5   a  of the pivoting member  5 .  
         [0052]     Furthermore, since an unevenness absorbing mechanism is constructed with a simple arrangement of the components, an image forming apparatus can be provided with lower cost, which does not have a problem such as image misregistration or the lowered printing accuracy because mutual positional relationship between elements of the image forming system (photosensitive drum  19 , exposure means  15 ) is not badly affected even when the image forming apparatus is placed on an uneven surface.  
         [0053]     Moreover, an image forming apparatus (not shown), in which a unit in an endless belt shape is positioned along the main frame, can minimize the distortion of the frame of the main body even when the body is placed on an uneven surface, producing no displacement force on the endless belt and extending the life of the endless belt.  
         [0054]     The above embodiment has been described with illustrating an indirect-transfer-type, full-color image forming apparatus that retransfers a toner image formed on the transfer belt onto a recording paper sheet. However, similar advantage can also be obtained in the case of a direct-transfer-type, full-color image forming apparatus that directly transfers a toner image onto a recording paper sheet.  
         [0055]     Instead of a color image forming apparatus, a monochromatic image forming apparatus according to the present invention can also reduce the influence of unevenness of placement surface on the parallel accuracy of rollers or the like, thereby advantageously improving printing accuracy.  
         [0056]     Although examples of the present invention have been described, the present invention is not limited the above described examples, and rather any modification can be made within the technical concept of the present invention.  
         [0057]     This application claims priority from Japanese Patent Application No. 2003-410200 filed on Dec. 9, 2003, which is hereby incorporated by reference herein.