Patent Publication Number: US-7917062-B2

Title: Image forming apparatus with easily operable inner and outer covers for accessing a jam

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus provided with an opening cover, particularly with a vertical conveyance cover which is opened and closed at the time of a jam processing. 
     2. Description of the Related Art 
     In an image forming apparatus such a printer or a copier for forming an image on a sheet by an electrophotographic method, an electrostatic latent image formed on an image bearing member such as a photoconductive drum is developed into a toner image and this toner image is transferred to a sheet fed at a suitable timing by a pair of registration rollers. The sheet having the toner image transferred thereto is conveyed to a fixing unit and is discharged onto a discharge tray outside the apparatus after having the toner image fixed by heat and pressure in the fixing unit. In this way, a series of image forming operations are completed. 
     There is known an image forming apparatus provided with a vertical conveyance cover which is openable and closable and constitutes a part of a sheet conveyance path so that a jammed sheet can be easily removed in the case of a jam in the sheet conveyance path during an image forming operation. 
     A known image forming apparatus of this type is such that a vertical conveyance cover is made up of an outer cover and an inner cover, which are independently opened and closed, and the position of the inner cover is determined by being pushed by the outer cover during a closing operation (see, for example, Japanese Unexamined Patent Publications Nos. 2005-138967 and 2004-123393). 
     In such an image forming apparatus, the outer and inner covers of the vertical conveyance cover are locked onto an apparatus main body by means of hooks respectively provided on these outer and inner covers. In this case, there is also proposed an image forming apparatus provided with a link mechanism for operating a link of an inner cover as a hook of an outer cover is operated (see, for example, Japanese Unexamined Patent Publications No. 2005-055862). 
     However, if the outer and inner covers of the vertical conveyance cover are locked onto the apparatus main body by means of the hooks respectively provided on these outer and inner covers, the number of parts increases. Further, since the opening/closing operability of the outer and inner covers is deteriorated, there has been a problem of reducing the operability of a jam processing. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to promote a cost reduction by reducing the number of parts and to improve the opening/closing operability of a cover member. 
     In order to accomplish this object, one aspect of the present invention is directed to an image forming apparatus, comprising an apparatus main body; a cover member openably and closably mounted on the apparatus main body and including an outer cover and an inner cover which are independently opened and closed, the inner cover being positioned by being pushed by the outer cover when the cover member is closed; and an engaging portion provided in the apparatus main body for temporarily holding the inner cover in the apparatus main body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a color laser printer according to one embodiment of the invention, 
         FIG. 2  is a section showing the internal construction of the color laser printer, 
         FIG. 3  is a perspective view showing a vertical conveyance cover and its periphery of the color laser printer, 
         FIG. 4  is a side view showing a state where the vertical conveyance cover is opened, 
         FIG. 5  is a perspective view showing a state where an inner cover of the vertical conveyance cover is opened, 
         FIG. 6  is a front view of the inner surface of the inner cover, 
         FIG. 7  is a perspective view showing a state where the inner cover is temporarily held, 
         FIG. 8  is a side view partly in section showing an engaging portion in the state where the inner cover is temporarily held, 
         FIG. 9  is a side view partly in section showing a guiding portion in the state where the inner cover is temporarily held, 
         FIG. 10  is a perspective view showing a secondary transfer roller and its periphery of the color laser printer, 
         FIG. 11  is a diagram schematically showing an outer cover, the inner cover and a detachment mechanism, 
         FIG. 12  is a perspective view showing the detachment mechanism in detail, and 
         FIGS. 13A ,  13 B and  14 A to  14 F are diagrams showing the operation of the detachment mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the present invention is described below with reference to the accompanying drawings.  FIG. 1  is perspective view of a color laser printer  1  as one embodiment of an image forming apparatus of the present invention, and  FIG. 2  is a section showing the internal construction of the color laser printer  1 . The color laser printer  1  is provided with an apparatus main body  100  in the form of a rectangular box. A vertical conveyance cover  24  (cover member), which doubles as a front cover and is opened and closed at the time of a jam processing, is provided on the front surface of the apparatus main body  100 . A discharge tray  21  is openably and closably provided at the top of the apparatus main body  100 . 
     Four image forming units, i.e. a magenta image forming unit  1 M, a cyan image forming unit  1 C, a yellow image forming unit  1 Y and a black image forming unit  1 K are arranged at specified intervals in a tandem manner in a central part of the interior of the main body  100  of the color laser printer  1 . In addition, a laser scanner unit (LSU)  13 , a sheet cassette  14 , a fixing unit  22 , a conveyance path (first conveyance path) L and a reversing conveyance path L′ (second conveyance path) are provided in the main body  100 . 
     The respective image forming units  1 M,  1 C,  1 Y and  1 K include photoconductive drums  2   a ,  2   b ,  2   c  and  2   d , and charging rollers  3   a ,  3   b ,  3   c  and  3   d , developing devices  4   a ,  4   b ,  4   c  and  4   d , primary transfer rollers  5   a ,  5   b ,  5   c  and  5   d  and drum cleaning devices  6   a ,  6   b ,  6   c  and  6   d  are respectively arranged around the corresponding photoconductive drums  2   a  to  2   d.    
     Each of the photoconductive drums  2   a  to  2   d  is a drum-shaped photoconductor and is driven to rotate at a specified process speed in a direction of an arrow in  FIG. 2  (clockwise direction) by an unillustrated variable speed stepping motor. The charging rollers  3   a  to  3   d  uniformly charge the outer surfaces of the corresponding photoconductive drums  2   a  to  2   d  to a specified potential by charging biases applied from an unillustrated charging bias power supply. 
     The developing devices  4   a  to  4   d  respectively contain magenta (M) toner, cyan (C) toner, yellow (Y) toner and black (K) toner and develop electrostatic latent images formed on the respective photoconductive drums  2   a  to  2   d  into toner images of the respective colors by attaching the toners of the respective colors to the electrostatic latent images. 
     The primary transfer rollers  5   a  to  5   d  are so arranged as to be able to come into contact with the corresponding photoconductive drums  2   a  to  2   d  via an intermediate transfer belt  7  at corresponding primary transfer positions. Here, the intermediate transfer belt  7  is mounted between a secondary transfer facing roller  8  and a tension roller  9  and arranged to be movable in a circulating manner above the respective photoconductive drums  2   a  to  2   d . The secondary transfer facing roller  8  is so arranged as to be able to come into contact with a secondary transfer roller  10  (roller member) via the intermediate transfer belt  7  at a secondary transfer position (image transfer portion). Further, a belt cleaning device  11  is disposed near the tension roller  9 . 
     Toner containers  12   a ,  12   b ,  12   c  and  12   d  for resupplying magenta, cyan, yellow and black toners to the respective developing devices  4   a  to  4   d  are arranged in a line above the respective image forming units  1 M,  1 C,  1 Y and  1 K in the apparatus main body  100 . 
     The LSU  13  is arranged below the respective image forming units  1 M,  1 C,  1 Y and  1 K and exposes the respective photoconductive drums  2   a  to  2   d  with laser lights based on image information transmitted from a personal computer (not shown) or the like, thereby removing electric charges in exposed parts to form electrostatic latent images. 
     The sheet cassette  14  is arranged below the LSU  13  at the bottom of the apparatus main body  100  and is detachably mountable into the apparatus main body  100 . A stack of a plurality of unillustrated sheets is stored in the sheet cassette  14 . A pickup roller  15  for dispensing the sheets one by one from the sheet cassette  14  and a feed roller  16  and a retard roller  17  for feeding the dispensed sheet to the conveyance path L are disposed near this sheet cassette  14 . 
     The fixing unit  22  includes a fixing roller and a pressure roller and fixes a toner image to a sheet by heating and pressing the sheet having the toner image transferred thereto. 
     The conveyance path L is a conveyance path vertically extending at a lateral part of the apparatus main body  100  and includes a pair of conveyor rollers  18  for conveying a sheet and a pair of registration rollers  19  for feeding the sheet to the secondary transfer portion as a nip portion between the secondary transfer facing roller  8  and the secondary transfer roller  10  at a specified timing after causing the sheet to temporarily wait on standby. The conveyance path L extends up to the discharge tray  21  provided on the upper surface of the apparatus main body  100 , and the fixing unit  22  and a pair of discharge rollers  23  are arranged at intermediate positions thereof. 
     The reversing conveyance path L′ is a conveyance path arranged at an outer side of the conveyance path L and vertically extending. The reversing conveyance path L′ is used in the case of forming images on both sides of a sheet and adapted to return the sheet having passed the secondary transfer portion to a side of the conveyance path L upstream of the secondary transfer portion. A plurality of pairs of conveyor rollers  20  are arranged at appropriate intervals on this conveyance path L′. 
     Next, the image forming operation by the color laser printer  1  having the above construction is described with reference to  FIG. 2 . When an image formation start signal is given to the printer  1 , the respective photoconductive drums  2   a  to  2   d  are driven to rotate at the specified process speed in the arrow direction of  FIG. 2  (clockwise direction) in the respective image forming units  1 M,  1 C,  1 Y and  1 K, and these photoconductive drums  2   a  to  2   d  are uniformly charged by the charging rollers  3   a  to  3   d . The LSU  13  emits laser lights modulated by color image signals of the respective colors to irradiate the outer surfaces of the respective photoconductive drums  2   a  to  2   d  with these laser lights, whereby electrostatic latent images corresponding to the color image signals of the respective colors are respectively formed on the photoconductive drums  2   a  to  2   d.    
     First of all, the magenta toner is attached to the electrostatic latent image formed on the photoconductive drum  2   a  of the magenta image forming unit  1 M by the developing device  4   a  applied with a development bias of the same polarity as the charging polarity of the photoconductive drum  2   a . In this way, the electrostatic latent image on the photoconductive drum  2   a  is developed into a visible magenta toner image. This magenta toner image is primarily transferred onto the intermediate transfer belt  7  driven to rotate in an arrow direction of  FIG. 2  by the action of the transfer roller  5   a  applied with a primary transfer bias having a polarity opposite to that of the toner in a primary transfer portion (transfer nip portion) between the photoconductive drum  2   a  and the transfer roller  5   a.    
     The intermediate transfer belt  7  having the magenta toner image primarily transferred thereto as described above is moved to the next cyan image forming unit  1 C. In the cyan image forming unit  1 C as well, a cyan toner image formed on the photoconductive drum  2   b  is transferred and superimposed onto the magenta toner image on the intermediate transfer belt  7  in a primary transfer portion similar to the above. 
     Similarly, yellow and black toner images respectively formed on the photoconductive drums  2   c ,  2   d  of the yellow and black image forming units  1 Y,  1 K are successively transferred and superimposed onto the magenta and cyan toner images transferred to the intermediate transfer belt  7 . In this way, a full color toner image is formed on the intermediate transfer belt  7 . Transfer residual toners residual on the respective photoconductive drums  2   a  to  2   d  without being transferred to the intermediate transfer belt  7  are removed by the corresponding drum cleaning devices  6   a  to  6   d , and the respective photoconductive drums  2   a  to  2   d  are prepared for the next image formations. 
     In synchronism with an arrival timing of the leading end of the full color toner image on the intermediate transfer belt  7  to the secondary transfer portion (transfer nip portion) between the secondary transfer facing roller  8  and the secondary transfer roller  10 , the sheet is fed to the secondary transfer portion by the pair of registration rollers  19 . In order to realize such a sheet feed, the sheet is fed from the sheet cassette  14  to the conveyance path L by the pickup roller  15 , the feed roller  16  and the retard roller  17  and is temporarily stopped by the pair of registration rollers  19 . Thereafter, the full color toner image is secondarily transferred at once from the intermediate transfer belt  7  to the sheet conveyed to the secondary transfer portion by the secondary transfer roller  10  applied with a secondary transfer bias having a polarity opposite to that of the toner. 
     Thereafter, the sheet having the full color toner image transferred thereto is conveyed to the fixing unit  22  along the conveyance path L and the full color toner image is thermally fixed to the surface of the sheet by being heated and pressed. The sheet having the toner image fixed thereto is discharged onto the discharge tray  21  by the pair of discharge rollers  23 , whereby a series of image forming operations are completed. Transfer residual toner residual on the intermediate transfer belt  7  without being transferred to the sheet is removed by the belt cleaning device  11  and the intermediate transfer belt  7  is prepared for the next image formation. The above is a series of operations in the case of forming an image on one side of a sheet. 
     In the case of forming images on both sides of a sheet, the sheet having a toner image fixed to one side thereof by the fixing unit  22  is fed to the reversing conveyance path L′ by an unillustrated switching flapper. The sheet is conveyed along the conveyance path L′ by the plurality of pairs of conveyor rollers  20  arranged on the conveyance path L′, transferred from the conveyance path L′ to the conveyance path L after being turned upside down and fed to the pair of registration rollers  19  again. Thereafter, a toner image on the intermediate transfer belt  7  is transferred to the other side of the sheet by operations similar to the above and is fixed by the fixing unit  22 , whereby the images are formed on the both sides of the sheet. The sheet having the images formed on the both sides is discharged onto the discharge tray  21  by the pair of discharge rollers  23  similar to the above. 
     Next, the vertical conveyance cover  24  and its periphery of the color laser printer  1  are described with reference to  FIGS. 3 to 10 .  FIG. 3  is a perspective view showing the vertical conveyance cover  24 ,  FIG. 4  is a side view showing a state where the vertical conveyance cover  24  is opened,  FIG. 5  is a perspective view showing a state where an inner cover  24 B of the vertical conveyance cover  24  is opened,  FIG. 6  is a front view of the inner surface of the inner cover  24 B,  FIG. 7  is a perspective view showing a state where the inner cover  24 B is temporarily held,  FIG. 8  is a side view partly in section showing an engaging portion in the state where the inner cover  24 B is temporarily held,  FIG. 9  is a side view partly in section showing a guiding portion in the state where the inner cover  24 B is temporarily held, and  FIG. 10  is a perspective view showing the secondary transfer roller  10  and its periphery. 
     As shown in  FIGS. 3 and 4 , the vertical conveyance cover  24  (cover member) includes an outer cover  24 A and the inner cover  24 B arranged at an inner side of the outer cover  24 A. The outer cover  24 A is so supported as to be rotatable about a first supporting shaft  26  (first shaft member) and the inner cover  24 B is so supported as to be rotatable about a second supporting shaft  25  (second shaft member), so that the outer and inner covers  24 A,  24 B can be independently opened and closed. The first supporting shaft  26  is arranged at a lower position of one side surface of the apparatus main body  100 , and the second supporting shaft  25  is arranged at a position above the first supporting shaft  26 . 
     The outer and inner covers  24 A,  24 B pivot in conjunction with each other. When the conveyance cover  24  is opened, the inner cover  24 B is opened by being pulled by the outer cover  24 A. When the conveyance cover  24  is closed, the position of the inner cover  24 B is positioned by being pushed by the outer cover  24 A. The associated operations of these outer and inner covers  24 A,  24 B are described in more detail later with reference to  FIGS. 11 to 14 . 
     Here, a wall surface of the conveyance path L at one side (see  FIG. 2 ) is formed by the inner surface of the inner cover  24 B. Further, a wall surface of the reversing conveyance path L′ at one side is formed by the outer surface of the inner cover  24 B and that of the reversing conveyance path L′ at the other side is formed by the inner surface of the outer cover  24 A. As shown in  FIG. 3 , a cassette mounting portion  102  formed by a body frame  101  made of sheet metal makes an opening below the vertical conveyance cover  24 . The above sheet cassette  14  is mounted into and detached from the cassette mounting portion  102 . 
     One of each pair of conveyor rollers  20  (see  FIG. 2 ) arranged along the reversing conveyance path L′ is rotatably mounted on the inner surface of the outer cover  24 A. The other roller of each pair of conveyor rollers  20  is rotatably mounted on the outer surface of the inner cover  24 B (see  FIG. 7 ). 
     As shown in  FIGS. 4 to 6 , the secondary transfer roller  10  and one of the pair of registration rollers  19  are rotatably held on the inner surface of the inner cover  24 B. Here, as shown in  FIG. 5 , engaging portions  104  (only one is shown in  FIG. 5 ) for being engaged with the first bearings  27  (shaft receiving members) for supporting a rotary shaft of the secondary transfer roller  10  and guiding portions  105  (only one is shown in  FIG. 5 ) for guiding second bearings  29  for supporting a rotary shaft  28  of the registration roller  19  are formed in the inner surfaces of left and right plates  103  of the body frame  101 . 
     As shown in detail in  FIG. 8 , each engaging portion  104  is in the form of a groove obliquely extending upward to the back (to the left in  FIG. 8 ) of the apparatus main body  100 . The engaging portion  104  is formed with a guiding surface  104   a  in the form of a slant for guiding the first bearing  27  of the secondary transfer roller  10  to the engaging portion  104 , and a receiving surface  104   b  onto which the first bearing  27  is fitted. Here, a straight line L 1  corresponding to the inclination of an extending direction of the receiving surface  104   b  is inclined at a specified angle θ to a line L 2  connecting a shaft center  10   c  of the secondary transfer roller  10  and a shaft center  8   c  of the secondary transfer facing roller  8 . 
     As shown in detail in  FIG. 9 , each guiding portion  105  is in the form of a guide groove inclined slightly downward to the back (to the left in  FIG. 9 ) of the apparatus main body  100 . 
     The secondary transfer roller  10  is supported movably along longitudinal directions (directions of arrows X-X′ of  FIG. 10 ) and vertical directions (directions of arrows Y-Y′ of  FIG. 10 ) by unillustrated springs. Further, as shown in  FIG. 6 , the rotary shaft  28  of the registration roller  19  is rotatably supported by sleeve-shaped bearings  29  at the opposite longitudinal ends of the rotary shaft  28  and the bearings  29  are held on the inner cover  24 B by retaining members  30  made of resin. A pair of left and right bearings  29  and the registration roller  19  supported thereby are held by being pressed against the retaining members  30  by unillustrated pressing springs. 
     In the printer  1 , the image forming operation is performed with the vertical conveyance cover  24  comprised of the outer and inner covers  24 A,  24 B closed as shown in  FIGS. 1 to 3 . At this time, the first bearings  27  of the secondary transfer roller  10  held on the inner cover  24 B are engaged with the engaging portions  104  of the apparatus main body  100  as shown in  FIG. 8 , whereby the closed state of the inner cover  24 B is temporarily held (see  FIG. 7 ). Further, the bearings  29  of the registration roller  19  are fitted into the guiding portions  105  of the apparatus main body  100  to position the registration roller  19 . 
     Specifically, when the first bearings  27  of the secondary transfer roller  10  are engaged with the engaging portions  104  along the guiding surfaces  104   a , the secondary transfer roller  10  is pressed against the secondary transfer facing roller  8  with the intermediate transfer belt  7  held therebetween. Then, the secondary transfer roller  10  receives a reaction force from the secondary transfer facing roller  8 . 
     Here, the receiving surfaces  104   b  extend along the straight line L 1  inclined at the specified angle θ to the line L 2  connecting the shaft center  10   c  of the secondary transfer roller  10  and the shaft center  8   c  of the secondary transfer facing roller  8 . Thus, the first bearings  27  are received by the receiving surfaces  104   b  of the engaging portions  104  to exhibit a fixing effect, whereby the inner cover  24 B is temporarily held by the engaging portions  104 . In this way, a force of the secondary transfer facing roller  8  for pressing the secondary transfer roller  10  is utilized as a force for temporarily holding the inner cover  24 B. Here, the angle of inclination θ of the receiving surfaces  104   b  sets the reaction force received by the secondary transfer roller  10  from the secondary transfer facing roller  8  at a value sufficient to receive the rotary shaft  27 . 
     In the case of a jam of a sheet in the conveyance path L or the reversing conveyance path L′ during the image forming operation, a user operates an unillustrated lever to unlock the outer cover  24 A of the vertical conveyance cover  24 . Then, the outer cover  24 A is rotated forward about the first supporting shaft  26  to be opened. Then, the inner cover  24 B is simultaneously opened as shown in  FIG. 4  as the outer cover  24 A is opened. Thus, the user can simultaneously confirm states of the conveyance paths L, L′. 
     In the case of a sheet jam in the conveyance path L, the user can directly remove the jammed sheet from the conveyance path L. Further, in the case of a sheet jam in the reversing conveyance path L′, the user closes the inner cover  24 B and temporarily holds it in the engaging portions  104 . By this temporary holding, the bearings  29  of the registration roller  19  are also fitted into the guiding portions  105  of the apparatus main body  100  to position the registration roller  19 . In this state, the user can remove the jammed sheet in the reversing conveyance path L′. 
     When a jam processing of removing the jammed sheet in the conveyance path L is completed and the user rotates the outer cover  24 A in a counterclockwise direction in  FIG. 4  about the first supporting shaft  26  and closes it, the inner cover  24 B is pushed by the outer cover  24 A to rotate in the counterclockwise direction in  FIG. 4  about the second supporting shaft  25 . The inner cover  24 B is first temporarily held on the apparatus main body  100  as described above and, then, the outer cover  24 A is fixed by an unillustrated hook to be locked in the closed state. 
     Also when a jam processing of removing the jammed sheet in the reversing conveyance path L′ is completed, the user rotates only the outer cover  24 A in the counterclockwise direction in  FIG. 4  about the first supporting shaft  26  to close it. In this case, since the inner cover  24 B is already temporarily held in the engaging portions  104 , only the outer cover  24 A is fixed by the unillustrated hook to be locked in the closed state. 
     In the printer  1  of this embodiment, a detachment mechanism  80  is disposed between the outer and inner covers  24 A,  24 B to set and cancel a link relationship between the outer and inner covers  24 A,  24 B. This point is described in detail below. 
       FIG. 11  is a diagram schematically showing the outer cover  24 A, the inner cover  24 B and the detachment mechanism  80 . As shown in  FIG. 4 , the outer cover  24 A is rotated about the first supporting shaft  26 , the inner cover  24 B is rotated about the second supporting shaft  25 , and the upper sides of these covers can pivot substantially in the same directions. The detachment mechanism  80  includes a first hook member  81  and a second hook member  82 . 
     The first hook member  81  is arranged at the upper end of the inner cover  24 B. The second hook member  82  is arranged at a position on the inner surface of an upper side of the outer cover  24 A corresponding to the first hook member  81 . The second hook member  82  moves upward and downward in directions of arrows “a” relative to the first hook member  81  as the outer cover  24 A is closed and opened. The arrangement position of the detachment mechanism  80  is near the pair of conveyor rollers  20  in  FIG. 2 . 
       FIG. 12  is a perspective view showing the detachment mechanism  80  in detail. The first hook member  81  has a base end portion (not shown) thereof fixed to the outer cover  24 A. A plate-like first arm  81   a  extends from this base end portion toward the outer cover  24 A (backward). A first claw portion  81   b  bent to the right is provided at the leading end (rear end in  FIG. 12 ) of the first arm  81   a . The first claw portion  81   b  is a plate member substantially parallel to the outer surface of the outer cover  24 A, and a first engaging surface  81  is formed on the front side thereof. 
     This first engaging surface  81   c  is inclined such that a part located at the leading end side (right end side) of the first claw portion  81   b  is located slightly forward than a part located at the base end side (left end side). This is for reliably engaging the first engaging surface  81   c  with a second engaging surface  82   c  of the second hook member  82  to be described later. A rear surface  81   e  of the first claw portion  81   b  extends substantially in the same direction as the first engaging surface  81   c , but steeper than the first engaging surface  81   c . The entire first claw portion  81   b  has a thin wedge shape tapered toward the leading end when viewed from above. The first claw portion  81   b  is formed with a cutout  81   f  to be smoothly engaged with and disengaged from the second claw portion  82   b  to be described later. The first claw portion  81   b  is appropriately beveled. The entire first hook member  81  is formed, for example, integral to the frame of the inner cover  24 B. 
     The second hook member  82  has a base end portion  82   h  fixed to the outer cover  24 A. A plate-like second arm  82   a  extends from the base end portion  82   h  toward the inner cover  24 B (front side) via a curved portion  82   i . The second claw portion  82   b  bent to the left to reach the first engaging surface  81   c  by way of the leading end of the above first claw portion  81   b  is provided at the leading end (front side in the figures) of the second arm portion  82   a . The second claw portion  82   b  is a plate member substantially parallel to the outer surface of the outer cover  24 A in the closed state, and the second engaging surface  82   c  is formed on the rear surface thereof. 
     This second engaging surface  82   c  is inclined such that a part located at the leading end side (left end side) of the second claw portion  82   b  is located slightly backward than a part located at the base end side (right end side). In other words, the second engaging surface  82   c  is formed substantially parallel to the first engaging surface  81   c  of the above first claw portion  81   b  and, when the first and second engaging surfaces  81   c ,  82   c  are engaged, a force acts in a direction toward the base end sides of the respective engaging surfaces, i.e. in such a direction as to ensure an engaged state. 
     A front surface  82   e  of the second claw portion  82   b  is an inclined surface substantially in the same direction as the second engaging surface  82   c  and steeper than the second engaging surface  82   c , and the entire second claw portion  82   b  has a thin wedge shape tapered toward the leading end when viewed from above. The second claw portion  82   b  is formed with a cutout  82   f  to be smoothly engaged with and disengaged from the first claw portion  81   b . Recessed grooves  82   g  are formed at the inner side of a connecting portion of the second claw portion  82   b  and the above second arm  82   a  to make the second claw portion  82   b  easily deformable. The second claw portion  82   b  is appropriately beveled. The entire second hook member  82  is, for example, made of synthetic resin, and the second claw portion  82  side is laterally movable mainly with the curved portion  82   i  as a fixed point due to its elasticity. 
     With the first and second hook members  81 ,  82  respectively fixed to the inner and outer covers  24 B,  24 A, the first and second claw portions  81   b ,  82   b  can be relatively moved substantially in the vertical direction. If the outer cover  24 A is moved backward with the first claw portion  81   b  located before the second claw portion  82   b , the leading end of the first claw portion  81   b  collides with the leading end of the second claw portion  82   b . However, as described above, the second hook member  82  is elastic and deformed in a direction away from the first hook member  81  upon the collision of the first and second claw portions  81   b ,  82   b . Therefore, the first claw portion  81   b  can move over the leading end of the second claw portion  82   b  to reach the engaging surface  82   c  of the second claw portion  82   b.    
     With reference to  FIGS. 13A ,  13 B and  14 A to  14 F, the operation of the detachment mechanism  80  is described. As shown in  FIG. 14A , a vertical engagement length of the first and second claw portions  81   b ,  82   b  is called an effective engagement length R. This effective engagement length R is an engagement length which effectively acts when the second claw portion  82   b  is pulled by the first claw portion  81   b . In  FIG. 14E , R 1  denotes a distance from the first supporting shaft  26  of the outer cover  24 A to the second hook member  82  and R 2  denotes a distance from the second supporting shaft  25  of the inner cover  24 B to the first hook member  81 . 
     As shown in  FIGS. 13A and 14A , with the outer cover  24 A and the inner cover  24 B respectively located at closed positions A 1 , B 1 , the first claw portion  81   b  of the first hook member  81  and the second claw portion  82   b  of the second hook member  82  are engaged. At this time, the effective engagement length R is maximized.  FIG. 14F  is a view showing a state of  FIG. 14A  from above. 
     As shown in  FIGS. 14B and 14C , as an opening angle (angle of inclination) of the outer cover  24 A increases, the first hook member  81  moves downward relative to the second hook member  82 , whereby the effective engagement length L of these becomes gradually shorter. This results from the fact that the centers of pivotal movements of the outer and inner covers  24 A,  24 B, i.e. the positions of the first and second shafts  26 ,  25  differ and the distances R 1 , R 2  differ. 
     As the outer cover  24 A is more opened, the upper end of the first claw portion  81   b  of the first hook member  81  reaches the bottom end of the second claw portion  82   b  of the second hook member  82  as shown in  FIG. 14D . Then, the effective engagement length R becomes 0 and the first and second claw portions  81   b ,  82   b  are disengaged. In other words, the first and second hook members  81 ,  82  are disengaged before reaching specified opened positions A 2 , B 2 . 
     As the opening angle of the outer cover  24 A further increases, the outer and inner covers  24 A,  24 B respectively reach the opened positions A 2 , B 2  with the first and second hook members  81 ,  82  disengaged as shown in  FIGS. 13B ,  14 E. In this state, the inner cover  24 B can be freely pivoted without being restricted by the outer cover  24 A. Accordingly, it can be easily performed to pivot the inner cover  24 B to temporarily hold it in the engaging portions  104 , for example, upon removing a jammed sheet in the reversing claw portion L′. 
     If the outer cover  24 A is closed from the opened state of  FIG. 14E , the outer and inner covers  24 A,  24 B follow a course opposite to the one at the time of opening. In other words, in a state of  FIG. 14D , the engagement of the first and second hook members  81 ,  82  is started and the effective engagement length R gradually becomes longer as shown in  FIGS. 14C ,  14 B. When the outer cover  24 A is arranged at the closed position A 1 , the inner cover  24 B is also arranged at the closed position B 1 . In other words, it is not necessary to individually open and close the inner cover  24 B and the inner cover  24 B can be opened and closed as the outer cover  24 A is opened and closed. Regardless of at which position the inner cover  24 B is located, the first and second hook members  81 ,  82  can be engaged by closing the outer cover  24 A. 
     As described above, the inner cover  24 B can be opened and closed by opening and closing the outer cover  24 A by providing the detachment mechanism  80 . Further, when the outer cover  24 A is located at the opened position A 2 , the first and second hook members  81 ,  82  are free from the link relationship, wherefore the inner cover  24 B can be individually operated. Thus, the inner cover  24 B can be temporarily held in the engaging portions  104  without any problem to contribute to an easier jam processing. 
     As described above, according to the printer  1  of this embodiment, the apparatus main body  100  is formed with the engaging portions  104  for temporarily holding the inner cover  24 B in the apparatus main body  100 , wherefore the inner cover  24 B needs not have a hook for locking the inner cover  24 B and a cost reduction can be promoted by reducing the number of parts. Upon a jam processing, the locked state by the hook of the outer cover  24 A can be released only by operating the lever provided on the outer cover  24 A, wherefore jam processability can be improved. 
     When the inner cover  24 B is temporarily held, the registration roller  19  held on the inner cover  24 B can be simultaneously positioned by the guiding portions  105 . Thus, the registration roller  19  can be precisely positioned and a stable image forming operation can be realized. 
     In this embodiment, the registration roller  19  is positioned by the guiding portions  105  provided in the image forming apparatus main body. Instead, the secondary transfer roller  10  may be positioned by the guiding portions  105 . 
     Although the present invention is applied to the color laser printer  1  in the above embodiment, it is, of course, similarly applicable to a monochromatic image forming apparatus. 
     The above specified embodiment mainly embraces inventions having the following construction. 
     An image forming apparatus according to one aspect of the present invention comprises an apparatus main body; a cover member openably and closably mounted on the apparatus main body and including an outer cover and an inner cover which are independently opened and closed, the inner cover being positioned by being pushed by the outer cover when the cover member is closed; and an engaging portion provided in the apparatus main body for temporarily holding the inner cover in the apparatus main body. 
     According to this construction, since the engaging portion for temporarily holding the inner cover in the apparatus main body is formed in the apparatus main body, the inner cover needs not include a hook or the like for locking the inner cover. Therefore, a cost reduction can be promoted by reducing the number of parts. 
     In the above construction, it is preferable that a first shaft member arranged at a lower position of one side surface of the apparatus main body and a second shaft member arranged at a position of the one side surface of the apparatus main body above the first shaft member are further provided; that the outer cover is supported rotatably about the first shaft member; and that the inner cover is supported rotatably about the second shaft member. According to this construction, a structure for closing the inner cover by pushing it by the outer cover when the cover member is closed and for independently opening and closing the outer and inner covers can be easily built. 
     In the above construction, it is preferable that an image transfer portion for transferring an image to a sheet, a first conveyance path for conveying the sheet via the image transfer portion and a second conveyance path for returning the sheet having passed the image transfer portion to a side of the first conveyance path upstream of the image transfer portion are further provided; that a wall surface of the first conveyance path at one side is formed by the inner surface of the inner cover; and that a wall surface of the second conveyance path at one side is formed by the outer surface of the inner cover and a wall surface thereof at the other side is formed by the inner surface of the outer cover. According to this construction, a jammed sheet in the first or second conveyance path can be removed by opening the outer and inner covers. Since the inner cover can be temporarily held at this time, the operability of a jam processing can be improved. 
     In the above construction, it is preferable that a roller member held on the inner cover and a guiding portion provided in the apparatus main body for positioning the roller member are further provided; and that the guiding portion positions the roller member at a position where the inner cover is temporarily held in the apparatus main body. According to this construction, displacements of the roller member can be suppressed since the roller member held on the inner cover can be positioned by the guiding portion when the inner cover is temporarily held. 
     In the above construction, it is preferable that a guiding portion provided in the apparatus main body for positioning a roller member, a transfer roller held on the inner cover for forming a transfer nip portion in the image transfer portion and a registration roller held on the inner cover to be arranged upstream of the image transfer portion in the first conveyance path are further provided; and that the guiding portion positions the transfer roller or the registration roller at a position where the inner cover is temporarily held in the apparatus main body. According to this construction, the transfer roller or the registration roller held on the inner cover can be simultaneously positioned by the guiding portion when the inner cover is temporarily held, whereby a stable image forming operation can be realized. 
     In the above construction, it is preferable that a facing roller member provided in the apparatus main body to face the roller member with the inner cover closed is further provided; and that a force of the facing roller member pushing the roller member is utilized as a force for temporarily holding the inner cover. According to this construction, the force of the facing roller member pushing the roller member can be effectively utilized to temporarily hold the inner cover. 
     In this case, it is preferable that the roller member includes a bearing member at an end portion thereof; that the engaging portion includes a receiving surface portion, into which the bearing member is fitted; and that the receiving surface portion is inclined at a specified angle to a line connecting a shaft center of the roller member and that of the facing roller member. According to this construction, an effect of fixing the bearing member by the inclination of the receiving surface portion can be exhibited. 
     In the above construction, it is preferable that a transfer facing roller provided in the apparatus main body to form the transfer nip portion together with the transfer roller with the inner cover closed is further provided; and that a force of the transfer facing roller pushing the transfer roller is utilized as a force for temporarily holding the inner cover. According to this construction, the inner cover can be temporarily held, utilizing the force of the transfer facing roller pushing the transfer roller. 
     In this case, it is preferable that the transfer roller includes a bearing member at an end portion thereof; that the engaging portion includes a receiving surface portion, into which the bearing member is fitted; and that the receiving surface portion is inclined at a specified angle to a line connecting a shaft center of the transfer roller and that of the facing transfer roller. According to this construction, an effect of fixing the bearing member by the inclination of the receiving surface portion can be exhibited. 
     In the above construction, it is preferable to further comprise a detachment mechanism for opening the inner cover as the outer cover is opened. In this case, it is preferable that the detachment mechanism includes a first hook member arranged on the inner cover and a second hook member arranged on the outer cover and engageable with and disengageable from the first hook member; that the first and second hook members are engaged with each other and the inner cover can be opened as the outer cover is opened with the outer cover closed relative to the apparatus main body; that the first and second hook members are disengaged before the outer cover is rotated by a specified amount about the first shaft member to reach an opened position; and that the outer and inner covers are individually rotatable about the first and second shaft members when the outer cover reaches the opened position. 
     According to this construction, by disposing the detachment mechanism, the inner cover can be opened and closed as the outer cover is opened and closed. Further, when the outer cover is located at the opened position, the first and second hook members are released from a link relationship, wherefore the outer and inner covers can be individually operated. Therefore, the inner cover can be temporarily held in the engaging portion without any problem, which contributes to an easier jam processing. 
     Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein. 
     This application is based on Japanese Patent Application No. 2007-313029 filed on Dec. 4, 2007, the contents of which are hereby incorporated by reference.