Abstract:
The image forming apparatus according to the present invention has: a unit including a transfer medium that circularly moves; an image forming section provided along a circulation direction of the transfer medium; a movement mechanism that moves the unit in a direction substantially perpendicular to a moving direction of the transfer medium, and enables the unit to move in a first direction so as to bring the unit into contact with the image forming apparatus and in a second direction so as to move the unit apart from the image forming apparatus; and a cushioning means for applying a load in directions opposite to moving directions of the unit when the unit is moved in the first and second directions, respectively, thereby to relax impact caused by motion of the unit.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims the benefit of priority from the prior Japanese Patent Application No. 2006-42770, filed on Feb. 20, 2006, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an image forming apparatus such as a printer or a MFP (Multi-Function Peripheral) which is also called a digital composite apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    A tandem type image forming apparatus has been known as an image forming apparatus such as a copier or printer. In a tandem type image forming apparatus, plural photosensitive drums are arranged in parallel. Toner images respectively formed on the photosensitive drums are transferred to a paper sheet, multi-layered on the paper sheet, to obtain a color image. An image forming apparatus of this tandem type has an intermediate transfer belt unit. 
         [0006]    Jpn. Pat. Appln. Laid-Open Publication No. 10-293514 describes an example of an image forming apparatus having such an intermediate transfer belt unit. 
         [0007]    The image forming apparatus in this example has a unit casing, a conveyor mechanism, and an elevation mechanism. The conveyor mechanism has a conveyor belt suspended between first and second rollers provided in the unit casing. The elevation mechanism moves up and down the conveyor mechanism in relation to the unit casing. When replacing the conveyor belt or the like, the conveyor belt is moved away from a photosensitive drum. The elevation mechanism has a cam and a lever, and rotates the cam in accordance with rotation of the lever, thereby moving up and down the conveyor mechanism. 
         [0008]    The conveyor belt and the photosensitive drum need to be apart from each other during maintenance services and in contact with each other during use. When the belt and drum are brought into contact with and moved apart from each other by the cam mechanism, peripheral units and members receive impact in some cases. For example, when plural gears are engaged together through such contact, tooth tips of the gears collide with each other to scratch or leave an impact scar on tooth surfaces of the gears. In this case, driving of the mechanism becomes stiff for each cycle, and causes color drifting or jitters in a formed image. 
         [0009]    Jpn. Pat. Appln. Laid-Open Publication No. 2005-91725 describes an image forming apparatus implemented with a measure for relaxing impact as described above. In an example of this apparatus, a distant state of a transfer roller in which the roller is apart from an intermediate transfer belt transits to a contact state. Timing of such transition is adjusted so that an end of the transfer roller in the lengthwise direction is brought into contact with the intermediate transfer belt prior to the other end of the transfer roller. 
         [0010]    However, the above measure for relaxing impact requires delicate adjustment for transition timing from a distant state to a contact state, and gives rise to a drawback of complex structure. 
         [0011]    The present invention provides an image forming apparatus of a tandem type, which includes a mechanism for moving a transfer unit and a photosensitive drum apart from each other and reduces impact generated when a state of the unit and drum being apart from each other transits to a contact state. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows an overall structure of an image forming apparatus according to an embodiment of the present invention; 
           [0013]      FIG. 2  is an enlarged side view of a part of an image forming section of the image forming apparatus according to the invention; 
           [0014]      FIG. 3  is a side view showing a structure of a transfer unit in the image forming apparatus according to the invention; 
           [0015]      FIG. 4  is a perspective view showing a separation mechanism of the transfer unit in the image forming apparatus according to the invention; 
           [0016]      FIG. 5A  and  FIG. 5B  are side views depicting operation of the separation mechanism in the image forming apparatus according to the invention; 
           [0017]      FIG. 6  is a perspective view showing a structure of a major part of the separation mechanismin the image forming apparatus according to the invention; 
           [0018]      FIGS. 7A and 7B  are perspective views depicting operation of a major part of the separation mechanism shown in  FIG. 6 ; and 
           [0019]      FIG. 8  is an enlarged perspective view showing a part of a rotation drive mechanism of the transfer unit in the image forming apparatus according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus of the present invention. 
         [0021]    An embodiment of the invention will now be described in detail with reference to the drawings. 
         [0022]      FIG. 1  shows an internal structure of an image forming apparatus according to an embodiment of the invention.  FIG. 2  is an enlarged side view of a part of  FIG. 1 . The following description will be made with reference to an example of a MFP (Multi-Functional Peripheral) as a composite apparatus. The invention is applicable to other image forming apparatuses such as printers, etc. 
         [0023]    In  FIG. 1 , an image forming apparatus  1  has an image forming section  2  located in a middle part of the apparatus. An image reader section  3 , an automatic document feeder (ADF)  4 , and a sheet output section  5  are provided in upper parts of the image forming apparatus  1 . The image forming apparatus  1  has an operation section and a display section in an upper part of the image forming apparatus  1  although the operation and display sections are omitted from the drawings. At a lower part of the image forming apparatus  1 , a sheet feeder section  6  is provided. 
         [0024]    The automatic document feeder  4  feeds a document to the image reader section  3 , which reads the document and generates image data. 
         [0025]    The image forming section  2  is constituted by, for example, a tandem type color laser printer, and scans a photosensitive member with a laser beam from a laser exposure device  20 , to form an image. 
         [0026]    The image forming section  2  includes image forming sections  2 Y,  2 M,  2 C, and  2 K for colors of yellow (Y), magenta (m), cyan (c), and black (K), respectively. The image forming sections  2 Y,  2 M,  2 C, and  2 K are arranged in parallel from the upstream side to the downstream side, below an intermediate transfer belt  11  as an intermediate transfer medium. 
         [0027]    In the following description, components forming the image forming sections  2 Y,  2 M,  2 C, and  2 K will be denoted at reference numerals added with Y, M, C and K, respectively. In some cases, the components will be described omitting the reference numerals Y, M, C and K. 
         [0028]    Since the image forming sections  2 Y,  2 M,  2 C and  2 K have the same structure, only the image forming section  2 Y will be described below as a representative examples of the image forming sections. The image forming section  2 Y has a photosensitive drum  12 Y. An electric charger  13 Y, a developing device  14 Y, a transfer roller  15 Y, a cleaner  16 Y, a blade  17 Y, and the like are located around the photosensitive drum  12 Y. Details of the structure of the image forming section  2 Y is shown enlarged in  FIG. 2 . 
         [0029]    The intermediate transfer belt  11  circularly moves, and semiconductive polyimide is used for the belt in view of heat resistance and abrasion resistance. The intermediate transfer belt  11  is suspended over a driving roller  22  and driven rollers  23  and  24 . The intermediate transfer belt  11  can have contact with photosensitive drums  12 Y to  12 K. To a position of the intermediate transfer belt  11  where the belt faces the photosensitive drum  12 Y, a primary transfer voltage of +1,000 V or so is applied from a primary transfer roller  15 Y so that a toner image on the photosensitive drum  12 Y is primarily transferred to the intermediate transfer belt  11 . 
         [0030]    A secondary transfer roller  25  is located so as to face the driving roller  22  suspending the intermediate transfer belt  11 . When a paper sheet P passes between the driving roller  22  and the secondary transfer roller  25 , a secondary transfer voltage of +1,000 V or so is applied from the secondary transfer roller  25 , so that toner images on the intermediate transfer belt  11  are secondarily transferred to the paper sheet P. A belt cleaner  26  is provided near the driven roller  24  for the intermediate transfer belt  11 . 
         [0031]      FIG. 2  shows enlarged one of the image forming sections  2 Y,  2 M,  2 C, and  2 K. Referring to the image forming section  2 Y as an example, an electric charger  13 Y, a developing device  14 Y, a primary transfer roller  15 Y, a cleaner  16 Y, a blade  17 Y, and the like are provided around the photosensitive drum  12 Y. To an exposure position of the photosensitive drum  12 Y, a yellow laser beam is emitted from a laser exposure device  20 , to form a latent image on the photosensitive drum  12 Y. 
         [0032]    In each of the image forming sections  2 Y to  2 K, the electric charger  13  electrically charges uniformly the whole surface of the photosensitive drum  12  to, for example, −700 V or so. The developing device  14  supplies the photosensitive drum  12  with a two-component developer by a developing roller  14   a  which is applied with a developing bias of −500 V or so. The two-component developer contains toner of one corresponding color and a carrier. The cleaner  16  removes residual toner on the surface of the photosensitive drum  12  by use of the blade  17 . 
         [0033]    Meanwhile, the laser exposure device  20  scans the photosensitive drum  12  in an axial direction of the drum with a laser beam emitted from a semiconductor laser element. The laser exposure device  20  includes a polygon mirror  20   a , an imaging lens system  20   b , a mirror  20   c , and the like. 
         [0034]    The sheet feeder section  6  has plural sheet feeder cassettes  6   a  and  6   b  to contain paper sheets of various sizes. The image forming apparatus  1  further has a manual feed tray  30  for manually feeding paper sheets. 
         [0035]    Between the sheet feeder cassettes  6   a  and  6   b  and the secondary transfer roller  25 , there are provided pickup rollers  31   a  and  32   a , separation rollers  31   b  and  32   b , conveyor rollers  33  and  34 , and a resist roller  35 . The pickup rollers  31   a  and  32   a  pick out paper sheets from inside the sheet feeder cassettes  6   a  and  6   b . Between the manual feed tray  30  and the resist roller  35 , there are provided a pickup roller  36   a  for picking up paper sheets P, and a manual sheet feed roller  37 . 
         [0036]    Further, a fixing device  39  is provided in the downstream side of the secondary transfer roller  25  along a vertical path  38  for vertically conveying paper sheets P fed from the sheet feeder cassettes  6   a  and  6   b  or the manual feed tray  30 . 
         [0037]    Between the fixing device  39  and the sheet output section  5 , there are provided a sheet output conveyor path  40  and a reverse conveyor path  41 . A gate  42  is provided on the reverse conveyor path  41  to distribute paper sheets P to the sheet output section  5  or to the reverse conveyor path  41 . The reverse conveyor path  41  reverses and guides paper sheets P in a direction toward the secondary transfer roller  25 . The reverse conveyor path  41  is used when carrying out double-sided printing. 
         [0038]    Operation of the image forming apparatus shown in  FIGS. 1 and 2  will be described next. As image forming is started, image information is inputted from a scanner, personal computer terminal, or the like. Then, photosensitive drums  12  rotate and the image forming sections  2 Y to  2 K sequentially form images. 
         [0039]    Referring to the image forming section  2 Y as an example, the photosensitive drum  12 Y is irradiated with a laser beam in accordance with image information for yellow (Y), thereby forming an electrostatic latent image. From the electrostatic latent image, a toner image for yellow (Y) is formed by the developing device  14 Y. Subsequently, the photosensitive drum  12 Y makes contact with the intermediate transfer belt  11  being rotated, thereby primarily transferring the toner image for yellow (Y) to the intermediate transfer belt  11  by the primary transfer roller  15 Y. 
         [0040]    In a similar manner to the toner image forming process for yellow (Y), toner images for magenta (M), cyan (C), and black (K) are formed by the image forming section  2 M,  2 C, and  2 K, and are sequentially transferred to the same position on the intermediate transfer belt  11  as the toner image for yellow (Y) has been formed. Thus, toner images for yellow (Y), magenta (M), cyan (C), and black (K) are transferred to the intermediate transfer belt  11 , multi-layered on one another, so that a full color toner image is obtained. 
         [0041]    Further, the intermediate transfer belt  11  secondarily transfers the full color toner image all at once to a paper sheet P by a transfer bias of the secondary transfer roller  25 . The paper sheet P is fed to the position of the secondary transfer roller  25  from the sheet feeder cassettes  6   a  or  6   b  or the manual feed tray  30 , synchronized with timing when the full color toner image on the intermediate transfer belt  11  reaches the secondary transfer roller  25 . The paper sheet P to which the toner image has been secondarily transferred reaches a fixing roller  39 , and the toner image is fixed. 
         [0042]    In case of printing an image only on one side (single-sided printing), the paper sheet P is distributed to the sheet output section  5  by the gate  42 . Otherwise, in case of double-sided printing or multi-layered printing, the paper sheet P is distributed to the reverse conveyor path  41  and conveyed again to the secondary transfer roller  25 . 
         [0043]    After completion of the secondary transfer, residual toner is cleaned from the intermediate transfer belt  11  by the belt cleaner  26 . From each photosensitive drum  12 , residual toner is cleaned by the cleaner  16  and blade  17  after primary transfer of a toner image to the intermediate transfer belt  11 , to become ready for next image forming. 
         [0044]    The blade  17  is in contact with the photosensitive drum  12 . As the photosensitive drum  12  rotates, the blade  17  finely scrapes away a coating on the photosensitive drum  12  and an edge of the blade  17  itself is abraded. The process as described above is repeated so that an amount of abrasion of the photosensitive drum  12  or blade  17  exceeds a certain amount, and desired performance cannot be achieved. In other words, lifecycle of the photosensitive drum  12  or blade  17  depends on total operation period. 
         [0045]    Therefore, the photosensitive drum  12  and primary transfer roller  15  are located away from each other (for example, at color image forming sections during monochrome character printing) in order to extend lifecycle except for some unavoidable part. 
         [0046]      FIG. 3  schematically illustrates a structure of a transfer unit  21  including a transfer belt  11 . The transfer belt  11  is driven by a driving roller  22  to travel in the direction of an arrow S. A bias is applied to primary transfer rollers  15 Y to  15 K located at positions where the rollers face the photosensitive drums  12 Y to  12 K. Toner images developed on the photosensitive drums  12 Y to  12 K are transferred to the transfer belt  11 . At this time, each of the primary transfer rollers  15  is pressed against the photosensitive drum  12  so as to form a constant nip by dead weight of the roller and pressure from a spring  43 . 
         [0047]    The same process as described above is carried out to form a toner image by each of the image forming sections  2 Y to  2 K for respective colors. Toner images for respective colors are layered on one another to form a color image. After forming the image, residual toner on the transfer belt  11  is cleaned by the belt cleaner  26 . 
         [0048]    When no color image is formed, e.g., when monochrome text information is formed, toner consumption can be reduced by developing only a latent image for black (K) In this case, the other color image forming sections  2 Y,  2 M, and  2 C than the image forming section  2 K should desirably not operated because lifecycle of each image forming section  2  depends on a total operation period. 
         [0049]    If the transfer belt  11  is rotated in contact with the photosensitive drums  12  under pressure applied by primary transfer, the photosensitive drums  12  and the transfer belt  11  are abraded or damaged. Therefore, a mechanism for moving the transfer belt  11  apart from the photosensitive drums  12  is required. 
         [0050]    A cam  44  and lifters  45 Y,  45 M,  45 C, and  45 K lift up the primary transfer rollers  15 Y to  15 K upon necessity, respectively, so as to leave the transfer belt  11 . 
         [0051]    Meanwhile, during a maintenance service for inspecting, repairing, or replacing a component, the whole transfer unit  21  needs to be moved apart from the photosensitive drums  12 . Therefore, the invention employs a separation mechanism for moving the whole transfer unit  21  apart from the photosensitive drums  12  and bringing the unit  21  into contact with the drums  12 . 
         [0052]      FIG. 4  is a perspective view showing the separation mechanism for the transfer unit  21  in the image forming apparatus according to the invention.  FIGS. 5A and 5B  are side views for explaining the structure and operation of the separation mechanism. 
         [0053]    In  FIG. 4 , the transfer unit  21  is held by a unit holder mechanism  51  configured in a laterally symmetrical structure. The unit holder mechanism  51  is moved up and down by rotating a handle  52 , so as to move the transfer unit  21  apart from and into contact with the photosensitive drums  12 . 
         [0054]    The transfer unit  21  has support pins  212  which are attached to a frame  211  and protrude outward from the frame  211 , as schematically illustrated within a circle A in  FIG. 4 . The support pins  212  are engaged in fixed rails  53  of the unit holder mechanism  51 . As the fixed rails  53  are elevated up and down by operating the handle  52 , the whole transfer unit  21  is elevated up and down. 
         [0055]    Although the fixed rails  53  are hidden behind frames  54  in  FIG. 4 , a fixed rail  53  is shown enlarged in  FIG. 6 . The following description will be made referring to  FIG. 6  along with  FIG. 4 . 
         [0056]    The unit holder mechanism  51  includes frames  54 , a handle  52 , an elevation links  55 , link rods  56 , and cams  57 . The frames  54  are attached to the body of the image forming apparatus. The elevation links  55  are attached respectively to two sides of the handle shaft  52   a  which rotates as the handle  52  rotates. The link rods  56  move in lateral directions of the drawings as the elevation links  55  rotate. The cams  57  rotate as the link rods  56  move. 
         [0057]    The fixed rails  53  each have a recessed cross-section in order to engage the support pins  212 . A container section  58  for receiving a cam  57  is formed at a ceiling section of each recessed section. More specifically, as illustrated enlarged in a circle B in  FIG. 6 , each fixed rail  53  is partially bent so as to form an S-shaped cross-section. 
         [0058]    The cams  57  each have a rotating shaft fixed to the body of the image forming apparatus  1 . The cams  57  each lift up the ceiling section of the container section  58  or push down a bottom section of the container section  58 , thereby to move up or down the whole fixed rails  53 . 
         [0059]    A spring  59  is attached to the ceiling section of each container section  58 . The spring  59  makes contact with a protrusion  60  provided on the body of the image forming apparatus  1  when the fixed rails  53  move down. When the fixed rails  53  are going to move down, tip ends of the springs  59  are brought into contact with the protrusions  60 . The fixed rails  53  are thereby energized by the springs  59  so as to lift up in an upward direction. Therefore, the cams  57  push down the fixed rails  53  against the biasing force of the springs  59 . 
         [0060]    Described next will be elevation operation of the transfer unit  21  by the unit holder mechanism  51 . When the transfer unit  21  is set down, the transfer unit  21  is in contact with the photosensitive drums  12 . When the transfer unit  21  is set up, the transfer unit  21  is apart from the photosensitive drums  12 . 
         [0061]      FIG. 5A  shows a state in which the transfer unit  21  has lifted up apart from the photosensitive drums  12  wherein the elevation links  55  have been pulled back toward a rear side as shown in  FIG. 4 . In this state, the cams  57  are rotated rightward by the link rods  56  and lift up the fixed rails  53 . 
         [0062]      FIG. 7A  shows a state in which the cams  57  have lifted up the ceiling sections of the container sections  58 . Accordingly, the transfer unit  21  is apart from the photosensitive drums  12 . 
         [0063]    The cams  57  are eccentric cams which have rotating points located on the body of the image forming apparatus as illustrated in  FIG.6 . Each cam  57  rotates with an end  571  of the cam  57  linked to a link rod  56  and with another end  572  kept in contact with a container section  58  of a fixed rail  53 . 
         [0064]      FIG. 5B  shows a state in which the transfer unit  21  has moved down making contact with the photosensitive drums  12  wherein the elevation links  55  have pushed forward the link rods  56  toward a front side as shown in  FIG. 4 . In this state, the cams  57  are rotated leftward by the link rods  56  and push down the fixed rails  53 . 
         [0065]      FIG. 7B  shows a state in which the cams  57  have pushed down the bottom sections of the container sections  58 . Accordingly, the transfer unit  21  is in contact with the photosensitive drums  12 . 
         [0066]    When to lift up the fixed rails  53 , the cams  57  lift up the fixed rails  53  against the dead weight of the fixed rails  53  (i.e., against gravity). When to move down the fixed rails  53 , the cams  57  push down the fixed rails  53  against the biasing force of the springs  59 . 
         [0067]    Therefore, the fixed rails  53  move up and down continuously kept in contact with the cams  57 . Accordingly, a cushioning effect is generated when the fixed rails  53  move up or down, and prevents the transfer unit  21  from abruptly moving apart from or into contact with the fixed rails  53 . That is, the springs  59  and the gravity of the fixed rails  53  constitute a load means which continuously applies a load to the cams in a direction opposite to an upward or downward moving direction of the fixed rails  53  when the fixed rails  53  are moved up or down by rotation of the cams  57 . 
         [0068]      FIG. 8  is a perspective view showing a gear structure of the transfer unit  21  for driving the transfer belt  11 . To rotate the transfer belt  11 , a drive gear  61  is provided on the body, and a gear  62  is provided on the transfer unit  21  so as to engage with the driver gear  61 . 
         [0069]    When the transfer unit  21  is brought into contact with the fixed rails  53  after once moving apart from the fixed rails  53 , the gears  61  and  62  are engaged with each other. If impact occurs when the transfer unit  21  is brought into contact, tooth tips of the gears collide with each other and leave impact scars on tooth surfaces. If an impact scar is left, irregular rotation is caused and results in a defect such as a jitter in a formed image. However, the present invention is capable of relaxing impact caused by moving the transfer unit  21  apart from or into contact with the fixed rails  53  by means of the cams  57  and springs  59  When the fixed rails  53  are set down completely, the cams  57  position the fixed rails  53  in the lower side of the cams  57 . Therefore, the transfer unit  21  can form images without receiving influence such as vibration. 
         [0070]    Thus, the image forming apparatus according to the present invention relaxes impact which is caused when moving the transfer unit. Accordingly, malfunctions and defective images can be prevented. 
         [0071]    Although exemplary embodiments of the present invention have been shown and described, it will be apparent to those having ordinary skill in the art that a number of changes, modifications, or alterations to the invention as described herein may be made, none of which depart from the spirit of the present invention. All such changes, modifications, and alterations should therefore be seen as within the scope of the present invention.