Abstract:
An image forming apparatus is disclosed. The image forming apparatus includes a photosensitive body, a magnetic roller disposed at a predetermined distance from the photosensitive body, and a donor roller disposed between the photosensitive body and the magnetic roller. The donor roller has a diameter larger than the closest distance between the photosensitive body and the magnetic roller. The image forming apparatus includes gap rings and pressing device to maintain desired gaps between the magnetic roller and the donor roller and between the donor roller and the photosensitive body.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 2008-0006054, filed on Jan. 21, 2008 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus capable of maintaining a gap between a donor roller and a magnetic roller and a gap between a donor roller and a photosensitive body to a predetermined gap. 
         [0004]    2. Description of the Related Art 
         [0005]    An image forming apparatus refers to an apparatus, in which an electrostatic latent image corresponding to a desired image is formed on a photosensitive body by an exposure unit, such as, e.g., a laser scanning unit. The electrostatic latent image is developed into a visible image by the use of developer, and the visible image is transferred and fused onto a printing medium. The image forming apparatus includes a developing unit which supplies the developer to the photosensitive body and develops the electrostatic latent image into a visible image. 
         [0006]    For example,  FIG. 1  illustrates a developing unit generally in use in an image forming apparatus, and  FIG. 2  is a perspective view showing the relevant portions of the developing unit depicted in  FIG. 1 . As shown in  FIGS. 1 and 2 , a developing unit  200  includes a photosensitive body  220 , a magnetic roller  240  to supply a developer to the photosensitive body  220 , and a donor roller  230  mounted between the magnetic roller  240  and the photosensitive body  220 . 
         [0007]    In a one-component developing system, either using an insulating developer or a conductive developer, the donor roller  230  serves to make an image of high quality. In a two-component developing system using a magnetic carrier and a toner, the donor roller  230  serves to prevent the carrier of the magnetic roller  240  from moving to the photosensitive body  220 . 
         [0008]    To obtain a desired image quality, the developing unit  200  should maintain each of the gap between the donor roller  230  and the photosensitive body  220  (i.e., the developing gap) and the gap between the donor roller  230  and the magnetic roller  240  to be uniform. For example,  FIG. 2  illustrates a holding member  281  formed with a pair of through-holes  282  and  283 , through which the rotating shaft  231  of the donor roller and the rotating shaft  241  of the magnetic roller are respectively received and supported. This holding member  281  maintains uniform gaps between the donor roller  230  and the magnetic roller  240 . 
         [0009]    Also provided is an elastic member  285 , which elastically supports one side of the holding member  281  and pushes the holding member  281  toward the photosensitive body  220 . By the elastic bias imparted by the elastic member  285  on the holding member  281  toward the photosensitive body  220 , the gap between the donor roller  230  and the photosensitive body  220  are maintained. 
         [0010]    An image forming apparatus with the above described configuration, however, unfortunately, relies on the precisely formed through-holes and/or the rotating shafts to maintain the gaps, and thus often fails to maintain the gaps to the desired preset gaps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Various aspects and advantages of the embodiments of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, of which: 
           [0012]      FIG. 1  is a sectional view showing a developing unit of a conventional image forming apparatus; 
           [0013]      FIG. 2  is a perspective view showing the relevant portions of the developing unit of the conventional image forming apparatus depicted in  FIG. 1 ; 
           [0014]      FIG. 3  is a sectional view showing an image forming apparatus according to an embodiment of the invention; 
           [0015]      FIG. 4  is a sectional view showing a developing unit according to an embodiment of the invention usable in the image forming apparatus depicted in  FIG. 3 ; 
           [0016]      FIG. 5  is a perspective view showing a relevant portions of the developing unit depicted in  FIG. 4 ; 
           [0017]      FIG. 6  is an exploded perspective view showing the portions of the developing unit depicted in  FIG. 5 ; 
           [0018]      FIG. 7  is a view showing a coupling structure of the developing unit depicted in  FIG. 6  and a developing unit housing according to an embodiment of the invention; and 
           [0019]      FIG. 8  is a sectional view taken along line I-I in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. While the embodiments are described with detailed construction and elements to assist in a comprehensive understanding of the various applications and advantages of the embodiments, it should be apparent however that the embodiments can be carried out without those specifically detailed particulars. Also, well-known functions or constructions will not be described in detail so as to avoid obscuring the description with unnecessary detail. It should be also noted that in the drawings, the dimensions of the features are not intended to be to true scale, and may be exaggerated for the sake of allowing greater understanding. 
         [0021]    As shown in  FIG. 3 , an image forming apparatus according to an embodiment may include a main body  10 , which forms an exterior appearance, and which may support various internally mounted components. The internally mounted components within the main body  10  may vary, but for illustration of an embodiment of the invention, may include, e.g., a paper feeding unit  20 , a laser scanning unit  30 , a developing unit  100 , a transfer unit  40 , a fusing unit  50  and a paper discharge unit. 
         [0022]    A cover  11  may be coupled to the main body  10 . The cover  11  may allow user access to the interior of the main body  10 , so the user may inspect the interior of the main body  10  or replace internal components such as, e.g., the developing unit  100 . 
         [0023]    The paper feeding unit  20  may feed a printing medium, e.g., a sheet of paper S, toward the transfer unit  40 . The paper feeding unit  20  may include a paper feeding tray  21  for supporting the paper S thereon, a pickup roller  22  to pick up the paper S loaded on the paper feeding tray  21 , and a feeding roller  23  to feed the picked-up paper toward the transfer unit  40 . 
         [0024]    The developing unit  100  may supply developer, e.g., toner, to the photosensitive body  120  to develop an electrostatic latent image formed on the photosensitive body  120  into a visible image. The developing unit  100  may be provided in a cartridge form and may be removably mounted in the main body  10 . The developing unit  100  will be described in more detail later. 
         [0025]    The transfer unit  40  may include a transfer roller  41  supported in the main body  10  to oppose the photosensitive body  120 . The transfer roller  41  acts to press the paper toward the photosensitive body  120  so that the visible image formed on the surface of the photosensitive body  120  may be transferred onto the paper. 
         [0026]    The fusing unit  50  may include a heating roller  51  including a heat source, and a press roller  52  which may be pressed against the heating roller  51  with a predetermined pressure. When the paper passes between the heating roller  51  and the press roller  52 , the visible image that had been transferred may be fused to the paper by the heat from the heating roller  51  and the pressure generated between the heating roller  51  and the press roller  52 . 
         [0027]    The paper discharge unit  60  may include discharge rollers  61  and  62 , and may discharge the paper that had passed through the fusing unit  50  to the outside of the main body  10 . 
         [0028]    The developing unit of the image forming apparatus according to an embodiment of the invention is shown in  FIG. 4 . The developing unit  100  may include a developing unit housing  110 , which defines an exterior appearance of the developing unit  100 . The developing unit housing  110  may be divided into a component mounting part  111  and a developer storage part  112 . The component mounting part may house therein a photosensitive body  120 , a donor roller  130 , a magnetic roller  140  and a charging roller  150 . The developer storage part  112  may contain a supply of developer. 
         [0029]    The arrangement of the developer storage part may vary, but as an illustration, may include a supply roller  160  and an agitator  170  supported therein. The supply roller  160  may be rotatably mounted near the magnetic roller  140  so that it may supply the developer stored in the developer storage part  112  to the magnetic roller  140 . The agitator  170  may agitate the developer to prevent solidification of the developer. 
         [0030]    The photosensitive body  120  may be rotatably mounted in the component mounting part  111  such that a portion of the surface of the photosensitive body  120  is exposed to the outside of the developing unit housing  110 . The outer surface of the photosensitive body  120  may be coated, by deposition or the like, with a photoconductive material layer, and may be charged to a predetermined electric potential by the charging roller  150 . A light beam from the laser scanning unit  30  may be irradiated to the charged surface of the photosensitive body  120 , generating an electric potential difference, and thereby forming an electrostatic latent image. 
         [0031]    As shown in  FIGS. 5 to 7 , in one embodiment of the invention, the magnetic roller  140  may be disposed at a predetermined distance from the photosensitive body  120 . The magnetic roller  140  may receive the developer stored in the developer storage part  112  and supply the developer to the donor roller  130 . As shown, the magnetic roller  140  may be rotatably supported in the developing unit housing  110 , in particular, in the component mounting part  111 , to rotate about a rotating shaft  141 . The rotating shaft  141  may be fitted through a shaft hole  114  formed on the developing unit housing  110 . A third gap ring  143  may be disposed on the rotating shaft  141  to maintain a preset gap between the magnetic roller  140  and the donor roller  130 . 
         [0032]    The donor roller  130  serves to supply the developer transmitted from the magnetic roller  140  to an electrostatic latent image formed on the photosensitive body  120 , thereby developing the electrostatic latent image into a visible image. As shown in  FIGS. 5 to 8 , the donor roller  130  may be rotatably supported in the component mounting part  111  between the photosensitive body  120  and the magnetic roller  140  to rotate about a rotating shaft  131 . 
         [0033]    The rotating shaft  131  may be fitted through a shaft hole  113  formed on the developing unit housing  110 . A first gap ring  133  may be disposed on the rotating shaft  131  of the donor roller  130 . The first gap ring  133  may contact the third gap ring  143  to maintain the preset gap between the donor roller  130  and the magnetic roller  140 . Alternatively, the first gap ring  133  may contact the magnetic roller  140  or the rotating shaft  141  to maintain the preset gap between the donor roller  130  and the magnetic roller  140 . A second gap ring  135  may be disposed on the rotating shaft  131  of the donor roller  130 . The second gap ring  135  may contact the photosensitive body  120  to maintain a preset gap between the donor roller  130  and the photosensitive body  120 . The first gap ring  133  and the second gap ring  135  may be sequentially disposed on the rotating shaft  131  of the donor roller  130 , and may differ in diameter in order to prevent the first gap ring  133  from interfering with the second gap ring  135 . The first gap ring  133  may be disposed in parallel with the third gap ring  143  to allow contact with the third gap ring  143 . 
         [0034]    As shown in  FIG. 4 , according to an embodiment, the charging roller  150  may charge the outer surface of the photosensitive body  120  to a uniform electric potential. The charging roller  150  may be provided with a rotating shaft  151 , with which it may be rotatably mounted in the developing unit housing  110 . 
         [0035]    As shown in  FIGS. 5 to 8 , according to an embodiment, a pressing unit  180  may be provide to maintain contact between the first gap ring  133  and the third gap ring  143 , and to maintain contact between the second gap ring  135  and the photosensitive body  120 . 
         [0036]    The pressing unit  180  may include a holding member  181  to hold the donor roller  130 , and an elastic member  185  to elastically support the holding member  181 , and to elastically bias the donor roller  130  towards the photosensitive body  120  and the magnetic roller  140 . The holding member  181  may include a through-hole  182  through which the rotating shaft  131  of the donor roller may be fitted. One end of the holding member  181  may have a supporting protrusion  183  to support an end of the elastic member  185 . The holding member  181  may have a coupling groove  184 , and the developing unit housing  110  may have a coupling rib  115  which fits into the coupling groove  184 . The elastic member  185  may allow the holding member  181  to slide with the sliding contact between the coupling groove  184  and the coupling rib  115 . 
         [0037]    The holding member  181  may be slidably mounted to the outer surface of the developing unit housing  110  by the elastic member  185 . The holding member  181  may alternatively be slidably mounted on the inner surface of the developing unit housing  110  by the elastic member  185 . The shaft hole  113  through which the rotating shaft  131  of the donor roller  130  is fitted may be formed in a slot shape which extends in the elasticity direction of the elastic member  185 . The rotating shaft  131  fitted through the holding member  181  may move with the holding member  181  when the holding member  181  slides by the elastic member  185 . 
         [0038]    The coupling groove  184  of the holding member  181  and the coupling rib  115  of the developing unit housing  110  may extend in the elasticity direction of the elastic member  185 . While this embodiment is shown with a configuration, in which the holding member  181  is formed with the coupling groove  184  and the developing unit housing  110  is provided with the coupling rib  115 , the coupling rib  115  may alternatively be provided in the holding member  181 , in which case the coupling groove  184  may be provided on the developing unit housing  110 . 
         [0039]    In order to maintain the gap between the donor roller  130  and the photosensitive body  120  or the gap between the donor roller  130  and the magnetic roller  140  to a gap preset by the pressing unit  180 , according to a preferred aspect of the present invention, as shown in  FIG. 5 , the diameter D of the donor roller  130  disposed between the photosensitive body  120  and the magnetic roller  140  is arranged to be larger than the closest surface distance L between the photosensitive body  120  and the magnetic roller  140 . 
         [0040]    The first gap ring  133  and the second gap ring  135 , which are placed on the rotating shaft  131  of the donor roller, have different diameters, so as to be contacted to the third gap ring  143  and the photosensitive body  120 , respectively, without interfering with each other. Because the donor roller  130  has the diameter D larger than the distance L between the photosensitive body  120  and the magnetic roller  140 , the central axis of the donor roller  130  may be positioned above or below the central axis of the photosensitive body  120  and the central axis of the magnetic roller  140 . 
         [0041]    That is, according to a preferred embodiment, with the diameter D of the donor roller  130  being larger than the distance L between the photosensitive body  120  and the magnetic roller  140 , the contacts between the first gap ring  133  and the third gap ring  143 , and between the second gap ring  135  and the photosensitive body  120  may maintained without interfering each other with the simpler arrangement of the holding member  181  of the pressing unit  180  elastically biased by the elastic member  185  to press only the donor roller  130  toward the magnetic roller  140  and the photosensitive body  120 . 
         [0042]    Although embodiments of the present invention have been shown and described, those skilled in the art can appreciate that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.