Abstract:
A fuser allows a sheet medium on which a toner image has formed to path through while heating the sheet medium, thereby fusing the toner image on the sheet medium. A plurality of first rollers are arrayed in a first direction. A plurality of second rollers are arrayed in the first direction and respectively coming in contact with the first rollers with a first pressure. The second rollers are adapted to eject the sheet medium nipped between the first rollers and the second rollers to the outside of the apparatus in a second direction perpendicular to the first direction. A plurality of third rollers are arrayed in the first direction so that each of the third rollers is disposed between adjacent ones of the first rollers, the third roller adapted to be brought into contact with the sheet medium with a second pressure which is smaller than the first pressure.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an image forming apparatus capable of forming a toner image on a sheet-shaped recording medium (hereinafter referred to simply as “sheet medium”) such as plain paper, thick paper, a postcard, an envelope, or an OHP sheet. 
   An image forming apparatus is known which is equipped with a fuser for fusing a toner image on a sheet medium by causing the sheet medium to which the toner image has been transferred to pass through the fuser while heating it and a sheet ejecting roller pair for ejecting the sheet medium from the apparatus. 
   In this type of image forming apparatus, a sheet medium tends to curl when it is heated in passing through the fuser. If the sheet medium is ejected as it is, it may be rounded at an ejecting section (e.g., sheet ejection tray). Or the leading end of a curled sheet medium may hit an already ejected sheet medium and push out the latter from the ejecting section (e.g., sheet medium ejection tray). 
   One conventional countermeasure against the above phenomenon is disclosed in Japanese Patent Publication No. 60-171963A. The sheet ejecting roller pair is formed by plural drive rollers that are spaced from each other and fixed to a drive shaft and follower rollers that are the same in number as the drive rollers and are brought into pressure contact with the drive rollers. Each roller of one of the two sets of rollers is provided with a large-diameter flange at both ends and a corresponding one the other set of rollers goes into the space between the flanges. As a result, a sheet medium being ejected is waved when viewed from the ejecting direction and thereby made stiffer (rollers for waving a sheet medium when viewed from its ejecting direction in this manner are called corrugation rollers). The sheet medium is thus prevented from being curled. 
   In the above apparatus, the sheet ejecting roller pair itself is given the function of waving a sheet medium; that is, the ejecting roller pair is configured so as to wave a sheet medium by its pressure contact force. Since the purpose of the sheet medium ejection pair is to eject a sheet medium, the pressure contact force of the rollers constituting the sheet ejecting roller pair needs to be strong enough to eject a sheet medium reliably irrespective of its type. This results in a problem that when a synthetic resin sheet medium is ejected after being heated by the fuser, the waved state may be permanent. 
   Japanese Patent Publication No. 5-289564A discloses an image forming apparatus in which a door cover is equipped with a roller pair which is disposed downstream of a fuser and transports a sheet medium coming from the fuser. With this configuration, the replacement of the fuser is facilitated and a sheet medium that is jammed in the fuser or a portion downstream thereof can be removed easily. 
   In this apparatus, the door cover is configured so as to be able to be opened and closed merely by a shaft. Therefore, the accuracy of positioning, with respect to the fuser, of the roller pair which is provided in the door cover tends to be low, which results in a problem that a sheet medium jam is prone to occur. 
   E-rings are known as stoppers to be attached to a shaft. Generally, the end face of a shaft to which the E-ring is attached is exposed. The exposed end face may be touched by a human hand, for example. Therefore, a state that the temperature of the shaft is high and its end face is exposed is not desirable. For example, the temperature of a shaft close to a fuser of an image forming apparatus becomes high and hence a state that its end face is exposed is not desirable. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide an image forming apparatus capable of ejecting a sheet medium without waving it even if it is made of a synthetic resin. 
   It is also an object of the present invention to provide an image forming apparatus capable of making a sheet medium jam less likely by increasing the accuracy of positioning, with respect to a fuser, of a roller pair that is provided in a door cover. 
   It is also an object of the present invention to provide an E-ring with a cover which does not expose the end face of a shaft. 
   In order to achieve at least one of the above objects, according to the invention, there is provided an image forming apparatus, comprising: 
   a fuser, allowing a sheet medium on which a toner image has formed to path through while heating the sheet medium, thereby fusing the toner image on the sheet medium; 
   a plurality of first rollers, arrayed in a first direction; 
   a plurality of second rollers, arrayed in the first direction and respectively coming in contact with the first rollers with a first pressure, the second rollers adapted to eject the sheet medium nipped between the first rollers and the second rollers to the outside of the apparatus in a second direction perpendicular to the first direction; and 
   a plurality of third rollers, arrayed in the first direction so that each of the third rollers is disposed between adjacent ones of the first rollers, the third roller adapted to be brought into contact with the sheet medium with a second pressure which is smaller than the first pressure. 
   The second pressure is such an extent that a sheet medium made of synthetic resin is not waved when viewed from the second direction. 
   With this configuration, if the sheet medium is made of a synthetic resin, it can be ejected without being waved. As a result, the phenomenon can be prevented that a synthetic resin sheet medium that has been ejected after being heated by the fuser is set while remaining in a waved state. 
   On the other hand, if the sheet medium to be ejected is a relatively weak sheet medium such as plain paper, the third rollers are brought into pressure contact with the sheet medium and the sheet medium is thereby ejected in a waved state when viewed from the second direction. 
   The third rollers may be brought into contact with the sheet medium in a direction as same as a direction that the second rollers are coming in contact with the first rollers. 
   With this configuration, there does not occur a phenomenon that the pressure contact force exerted on the sheet medium by the third rollers weakens the pressure contact force exerted on the first rollers by the second rollers. This makes it possible to eject the sheet medium reliably without the need for setting the original pressure contact force exerted on the first rollers by the second rollers unduly strong. 
   The third rollers may be disposed an upstream side of the second rollers relative to the second direction. 
   If the third rollers were disposed downstream of the second rollers, a trailing end of a sheet medium being ejected by the first and second rollers might be caught on the third rollers and prevented from being ejected smoothly from the apparatus. Disposing the third rollers as described the above, such an accident can be avoided. 
   In order to achieve at least one of the above objects, according to the invention, there is also provided an image forming apparatus, comprising: 
   a fuser, allowing a sheet medium on which a toner image has formed to path through while heating the sheet medium, thereby fusing the toner image on the sheet medium; 
   a first door cover, pivotable about a first pivot center formed by slots extending in a first direction and a shaft fitted into the slots slidably in the first direction; 
   a plurality of rollers, provided on the first door cover and adapted to transport the sheet medium which has passed through the fuser; 
   a first engagement member, provided on the first door cover; 
   a second engagement member, provided on the fuser and adapted to engage with the first engagement member when the first door cover is closed; 
   a second door cover, pivotable about second pivot center and adapted to be coupled to the first door cover; and 
   an urging member, provided on the second door cover and adapted to urge the first door cover in the first direction when the second door cover is coupled to the first door cover. 
   With this configuration, the engagement between the first and second engagement members can be secured, whereby the first door cover is positioned with respect to the fuser with high accuracy. As a result, the accuracy of the positioning of the rollers provided on the first door cover with respect to the fuser is increased, which makes a sheet jamming less likely. 
   In addition, since the urging member is provided in the second door cover, it is not necessary to provide an individual urging member on the first door cover. 
   In order to achieve at least one of the above objects, according to the invention, there is also provided an E-ring, adapted to be attached to an end portion of a shaft member, comprising: 
   an E-ring body, made of synthetic resin and adapted to be fitted with the shaft member; and 
   a cover portion, made of synthetic resin and adapted to cover an end face of the shaft member when the E-ring body is fitted with the shaft member. 
   With this configuration, the end portion of the shaft is never touched by a human hand, for example. Being made of a synthetic resin, the E-ring is superior in heat insulation. Therefore, even if the temperature of the shaft member is made high, the temperature of the cover portion is kept low; no problems arise even if the cover portion is touched by a human hand, for example. 
   The E-ring body and the cover portion may be monolithic. 
   The shaft member may be a shaft of a roller adapted to be disposed in the vicinity of a fuser in an image forming apparatus to transport a sheet medium. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein: 
       FIG. 1  is a section view of an image forming apparatus according to one embodiment of the invention; 
       FIG. 2  is a schematic side view of a sheet ejector in the image forming apparatus; 
       FIG. 3  is a perspective view of the sheet ejector; 
       FIG. 4A  is a schematic side view of a top cover and a fuser in the image forming apparatus; 
       FIG. 4B  is an enlarged side view of a positioning member of the top cover with respect to the fuser; 
       FIG. 5  is a section view of the image forming apparatus showing a state that the top cover and a side cover are opened; 
       FIG. 6  is a schematic side view of the top cover provided with the sheet ejector; 
       FIG. 7  is a section view of the image forming apparatus showing a state that the top cover and the side cover are closed; 
       FIG. 8  is a section view of the image forming apparatus showing a state that the top cover is closed while the side cover is opened; 
       FIG. 9  is a top plan view of a coupling mechanism of the top cover and the side cover; 
       FIG. 10A  is a perspective view of an end portion of a sheet transporting roller in the image forming apparatus; 
       FIG. 10B  is a side view of the end portion of the sheet transporting roller; 
       FIG. 10C  is a perspective view of an E-ring attached to the end portion of the sheet transporting roller; 
       FIG. 10D  is a front view of the E-ring; and 
       FIG. 10E  is a side view of the E-ring. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   An image forming apparatus according to one embodiment of the present invention will be hereinafter described with reference to the accompanying drawings. 
   As shown in  FIG. 1 , this image forming apparatus is a color image forming apparatus capable of forming a monochrome image or a full-color image on both surfaces of an A4-size sheet medium (including a letter-size sheet medium) by feeding it in its longitudinal direction. The image forming apparatus comprises a casing  11 ; an image carrier unit  20 , an exposing unit  30  and a developing device  40  that are housed in the casing  11  and constitute an image forming section. The image forming apparatus further comprises an intermediate transfer unit  50  and a fusing unit (fuser)  60 . The casing  11  is provided with a frame (not shown) of an apparatus main body  10  and the individual units etc. are attached to the frame. 
   The image carrier unit  20  has a photosensitive body  21  having a photosensitive layer to serve as its outer circumferential surface and a corona charger (scorotron charger)  22  for charging the outer circumferential surface of the photosensitive body  21  uniformly. An electrostatic latent image is formed by selectively exposing the outer circumferential surface of the photosensitive body  21  that has been charged uniformly by the corona charger  22  to laser light L coming from the exposing unit  30 . A visible image (toner image) is formed by applying toner (developer) to the electrostatic latent image by the developing device  40 . The toner image is transferred primarily to an intermediate transfer belt  51  of the intermediate transfer unit  50  at a primary transfer portion T 1 , and then transferred secondarily to a sheet medium at a secondary transfer portion T 2 . 
   A transporting path  16  for transporting the sheet medium on whose one surface the image has been formed at the secondary transfer portion T 2  toward a sheet ejector (ejection tray)  15  located at the top of the casing  11  and a sheet returning path  17  for causing a switchback of the sheet medium (that has been transported toward the sheet ejector  15  along the transporting path  16 ) and returning it toward the secondary transfer portion T 2  so that an image will also be formed on the other surface of the sheet medium are provided inside the casing  11 . 
   Reference numeral  70  denotes a double-side printing unit which is configured so as to be able to be attached to and detached from the apparatus main body. The sheet returning path  17  is completed when the double-side printing unit  70  is attached. 
   Reference numeral  71  denotes a driving motor for returning a sheet medium, and reference numeral  72  denotes a sheet returning roller which is driven by the motor  71  via a driving mechanism (not shown) such as a timing belt. 
   A sheet feeding cassette  18  for holding plural sheet media in a stacked manner and a sheet feeding roller  19  for feeding sheet media one by one toward the secondary transfer portion T 2  are disposed in a lower portion of the casing  11 . 
   A multi-purpose tray  81  as a manual feeding section  80  is disposed under the double-side printing unit  70 , and the apparatus main body is equipped with a sheet feeding roller  82  for feeding, one by one, sheet media that are set on the multi-purpose tray  81 . 
   The developing device  40  is a rotary developing device. Toner cartridges (not shown) of the respective colors containing a yellow toner, a cyan toner, a magenta toner, and a black toner are attached to a rotary body  41  in a detachable manner. As the rotary body  41  is rotated in a direction indicated by arrow R with a pitch of 90°, development rollers (not shown) of the respective developing device cartridges are selectively brought into contact with the photosensitive body  21 , whereby the surface of the photosensitive body  21  can be developed selectively. 
   The exposing unit  30  emits laser light L toward the photosensitive body  21 . 
   The intermediate transfer unit  50  is equipped with a unit frame (not shown) and the intermediate transfer belt  51  which is stretched by a drive roller  54  and plural follower rollers and which is supported rotatably by the unit frame. The intermediate transfer belt  51  is driven so as to circulate in a direction indicated by an arrow in  FIG. 1 . The primary transfer portion T 1  is formed at the contact position of the photosensitive body  21  and the intermediate transfer belt  51 , and the secondary transfer portion T 2  is formed at the pressure contact position of the drive roller  54  and a secondary transfer roller  10   b  which is provided on the main body side. 
   The secondary transfer roller  10   b  can come into contact with and be separated from the drive roller  54  (i.e., intermediate transfer belt  51 ). The secondary transfer portion T 2  is formed when the secondary transfer roller  10   b  comes into contact with the drive roller  54 . 
   Therefore, to form a color image, an image of one color is formed on the intermediate transfer belt  51  as it makes one rotation in a state that the secondary transfer roller  10   b  is separated from the intermediate transfer belt  51 . Images of plural colors are formed on the intermediate transfer belt  51  in a superimposed manner as it rotates plural times, whereby a color image is formed on the intermediate transfer belt  51 . Then, the secondary transfer roller  10   b  is brought into contact with the intermediate transfer belt  51  and a sheet medium is supplied to the contact position (secondary transfer portion T 2 ), whereby the color image (toner image) is transferred from the intermediate transfer belt  51  to the sheet medium (secondary transfer). 
   The sheet medium to which the toner image has been transferred is heated as it passes through the fusing unit (fuser)  60 , whereby the toner image is heated and fused. The sheet medium is ejected onto the ejection tray  15  by a sheet ejector  90 . 
   As shown in  FIGS. 2 and 3 , the sheet ejector  90  of the image forming apparatus is equipped with a sheet ejecting roller pair  91 ,  92  for ejecting a sheet medium that has passed through the fuser  60  onto the ejection tray  15  (i.e., ejecting the sheet medium from the apparatus), a switchback roller pair  93  which is provided between the fuser  60  and the sheet ejecting roller pair  91 ,  92  and causes a switchback of a sheet medium that has passed through the fuser  60  and returns it the image forming section consisting of the photosensitive body  21  etc., and corrugation rollers  94 . 
   The switchback roller pair  93  is disposed in a sheet ejecting path  16   a  which goes from the fuser  60  to the sheet ejecting roller pair  91 ,  92 . A switchback of a sheet medium is done in such a manner that the sheet ejecting roller pair  91 ,  92  and the switchback roller pair  93  are rotated reversely immediately before the tail of the sheet medium passes through the nip portion of the switchback roller pair  93  and the sheet medium is thereby supplied to the sheet returning path  17 . 
   The sheet medium that has been supplied to the sheet returning path  17  is transported by the return roller  72  and supplied to the secondary transfer portion T 2  via a gate roller pair  10   g  which determines timing of supply of the sheet medium to the secondary transfer portion T 2 . 
   As shown in  FIG. 3 , the sheet ejecting roller pair  91 ,  92  has plural (in the illustrated example, four) drive rollers  91   b  which are spaced from each other and fixed to a drive shaft  91   a  and follower rollers  92   b  which are the same in number as the drive rollers  91   a  and are brought into pressure contact with the drive rollers  91   b.    
   The corrugation rollers  94  are disposed between the drive rollers  91   b  in the axial direction of the drive shaft  91   a , and are brought into pressure contact with a sheet medium (see  FIG. 2 ) passing through the sheet ejecting roller pair  91 ,  92  with weaker pressure contact force than the pressure contact force exerted on the drive rollers  91   b  by the follower rollers  92   b.    
   The drive shaft  91   a  is supported rotatably by a frame (not shown) and driven (rotated) by a driving mechanism (not shown). 
   As shown in  FIGS. 2 and 3 , the follower rollers  92   b  are supported rotatably by a sheet guide  95 . The sheet guide  95  is pivotably supported on the frame by a support shaft  95   a , and urged by an urging member (not shown) so that the follower rollers  92   b  come into pressure contact with the drive rollers  91   b.    
   As shown in  FIG. 3 , each corrugation roller  94  is supported rotatably by an arm  96  which is generally bracket-shaped in a plan view. The arm  96  is pivotably supported by the sheet guide  95  via a support shaft  96   a . An urging member (in this case, a torsion spring)  97  is disposed between the arm  96  and the sheet guide  95 . In a free state, as shown in  FIG. 2 , part of an outer circumferential surface  94   a  of the corrugation roller  94  projects to the drive rollers  91   b  side past a traveling path (see an imaginary line S in  FIG. 2 ) of the recording media. 
   Therefore, when the sheet medium is ejected by the sheet ejecting roller pair  91 ,  92 , the corrugation rollers  94  are brought into pressure contact with the sheet medium and make follower rotations but the urging force (pressure contact force) of the urging members  97  is weaker than the pressure contact force exerted on the drive rollers  91   b  by the follower rollers  92   b.    
   The pressure contact force exerted on the sheet medium by the corrugation rollers  94  is set so as to cause almost no bend in the sheet medium when viewed from its ejecting direction in the case where the sheet medium is made of a synthetic resin (e.g., an OHP sheet medium). 
   Therefore, where the recording medium is a relatively weak sheet medium such as plain paper and hence tends to curl, the corrugation rollers  94 , which are located between the drive rollers  91   b , operate so as to push the sheet medium toward the spaces between the drive rollers  91   b  and to deform the sheet medium so that it is waved when viewed from the ejecting direction and is thereby ejected after being made stiffer. On the other hand, where the recording medium is a sheet medium made of a synthetic resin, the corrugation rollers  94  operate so as to cause almost no bend in the sheet medium when viewed from its ejecting direction and to eject it in a generally flat state. 
   The direction in which the corrugation rollers  94  are brought into pressure contact with the sheet medium is the same as the direction in which the follower rollers  92   b  are brought into pressure contact with the driver rollers  91   b  (upward in  FIG. 2 ). 
   The corrugation rollers  94  are disposed upstream of (in  FIG. 2 , on the right of) the follower rollers  92   b  relative to the ejecting direction of the sheet media. 
   Therefore, if the sheet medium is made of a synthetic resin, it can be ejected without being waved. As a result, the phenomenon can be prevented that a synthetic resin sheet medium that has been ejected after being heated by the fuser  60  is set while remaining in a waved state. 
   On the other hand, if the sheet medium to be ejected is a relatively weak sheet medium such as plain paper, the corrugation rollers  94 , which are located between the drive rollers  91   b  in the axial direction of the drive shaft  91   a , are brought into pressure contact with the sheet medium and the sheet medium is thereby ejected in a waved state when viewed from the ejecting direction. 
   The direction in which the corrugation rollers  94  are brought into pressure contact with the sheet medium is the same as the direction in which the follower rollers  92   b  are brought into pressure contact with the drive rollers  91   b . This prevents a phenomenon that the pressure contact force exerted on the sheet medium by the corrugation rollers  94  weakens, via the recording medium, the pressure contact force exerted on the driver rollers  91   b  by the follower rollers  92   b.    
   This makes it possible to eject the sheet medium reliably without the need for setting the original pressure contact force exerted on the driver rollers  91   b  by the follower rollers  92   b  unduly strong. 
   The corrugation rollers  94  are disposed upstream of the follower rollers  92   b  relative to the ejecting direction of the sheet medium. This prevents interference between the drive shaft  91   a  and the corrugation rollers  94 . If the corrugation rollers  94  were disposed downstream of the follower rollers  92   b , a trailing end of a recording medium being ejected by the sheet ejecting roller pair  91 ,  92  might be caught on the corrugation rollers  94  and prevented from being ejected smoothly from the apparatus (i.e., onto the ejection tray  15 ). Disposing the corrugation rollers  94  upstream of the follower rollers  92   b  relative to the ejecting direction of the sheet medium can prevent such an event. 
   Both of the sheet ejecting roller pair  91 ,  92  and the switchback roller pair  93  are provided in a door cover (In this embodiment, a top cover)  10   c.    
   As shown in  FIGS. 4A and 5 , the door cover  10   c  is configured so as to be pivotable about a pivot center  10   d . Therefore, the accuracy of positioning of the sheet ejecting roller pair  91 ,  92  and the switchback roller pair  93  with respect to the fuser  60 , in particular, the accuracy of positioning of the roller pair immediately downstream of the fuser  60  (in this case, the switchback roller pair  93 ) with respect to the fuser  60 , is important. Particularly in this embodiment, since the roller pair immediately downstream of the fuser  60  is the switchback roller pair  93  for causing a switchback of a sheet medium with prescribed timing, the accuracy of positioning of the switchback roller pair  93  with respect to the fuser  60  is very important. 
   In view of the above, in this embodiment, as shown in  FIGS. 5 and 6 , each pivot center  10   d  of the top cover  10   c  which is provided with the switchback roller pair  93  is formed by a shaft  10   c  and a slot  10   e  which is fitted with the shaft  10   c   1 . As shown in  FIGS. 4A and 4B , a positioning member  61  for positioning between the top cover  10   c  and the fuser  60  is formed by a projection  10   c   2  and a recess  62  to be fitted with the projection  10   c   2 . The projection  10   c   2  and the recess  62  are provided in the top cover  10   c  and the fuser  60 , respectively. 
   As described later in detail, an urging member for urging the top cover  10   c  in the longitudinal direction of the slots  10   e  is provided. The urging member is provided in another door cover (in this embodiment, a side cover  10   f  as shown in  FIGS. 1 and 5 ) which is adapted to separately couple with the top cover  10   c.    
   In this embodiment, each shaft  10   c   1  is integrated with an arm portion  10   c   3  (see  FIG. 9 ) of the top cover  10   c  and the slots  10   e  are provided in the casing  11  of the image forming apparatus. Alternatively, the shafts  10   c   1  and the slots  10   e  may be provided in the casing  11  and the top cover  10   c , respectively. 
   In either case, the top cover  10   c  is pivotable about the pivot centers  10   d  and is movable in the longitudinal direction (indicated by arrows X 1  and X 2  in  FIG. 6 ) of the slots  10   e  within such a range that the shafts  10   c   1  are movable in the longitudinal direction. 
   Alternatively, the top cover  10   c  and the fuser  60  may be provided with a recess and a projection, respectively. 
   As shown in  FIGS. 7 and 8 , the side cover  10   f  is pivotably attached to the main body of the image forming apparatus with a shaft  10   h.    
   The side cover  10   f  is provided with a slidable projection  10   f   1  at a middle position in the vertical direction. The slidable projection  10   f   1  is provided slidably in a cylinder  10   f   2 , and a coil spring  10   f   3  for urging the slidable projection  10   f   1  inward of the apparatus body is accommodated in the cylinder  10   f   2 . Therefore, the slidable projection  10   f   1  is always urged by the coil spring  10   f   3  but its projecting length is restricted by a stopper (not shown). 
   On the other hand, as shown in  FIG. 9 , the side cover  10   f  is provided with a slider  10   f   4  at a top position. The slider  10   f   4  can be slide-manipulated in a direction indicated by arrows Y 1  and Y 2  by holding its knob  10   f   5 . 
   The slider  10   f   4  is formed with hooks  10   f   6  at both ends (top and bottom ends in  FIG. 9 ). 
   The above-mentioned top cover  10   c  is provided with, at positions corresponding to the respective hooks  10   f   6 , engagement portions  10   c   6  which is adapted to be separatably engaged with the respective hooks  10   f   6 . The hooks  10   f   6  and the engagement portions  10   c   6  are disengaged from each other when the slider  10   f   4  is slid in the direction of arrow Y 1 , and are engaged with each other (and the side cover  10   f  and the door cover  10   c  are coupled to each other) when the slider  10   f   4  is slid in the direction of arrow Y 2 . 
   As shown in  FIG. 7 , the image forming apparatus is used in a state that the top cover  10   c  and the side cover  10   f  are closed and the hooks  10   f   6  of the slider  10   f   4  and the engagement portions  10   c   6  of the top cover  10   c  are engaged with each other. 
   In this state, the tip of the slidable projection  10   f   1  which is provided in the side cover  10   f  is in contact with a counter portion  10   i  of the image forming apparatus main body opposing the slidable projection  10   f   1 , whereby the side cover  10   f  is urged in the opening direction. However, the opening of the side cover  10   f  is prohibited because the hooks  10   f   1  are engaged with the engagement portions  10   c   6  of the top cover  10   c.    
   In other words, the top cover  10   c  is urged in the direction indicated by arrow X 1  (i.e., in the longitudinal direction of the slots  10   e ) by the side cover  10   f , that is, by the coil spring  10   f   3  (above-mentioned urging member), via the slidable projection  10   f   1 . 
   As a result, as shown in  FIG. 4B , the portions  10   c   21  and  62   a  (positioning members) opposed to each other of the above-mentioned projection  10   c   2  and recess  62  come into contact with each other reliably, whereby the door cover  10   c  is positioned with respect to the fuser  60  with high accuracy. 
   A slight gap C exists between the projection  10   c   2  and the recess  62 , because without the gap C the top cover  10   c  could not be opened or closed smoothly. 
   When the slider  10   f   4  of the side cover  10   f  is slid in the direction of arrow Y 1  and the hooks  10   f   6  are thereby disengaged from the engagement portions  10   c   6 , the side cover  10   f  is opened as shown in  FIG. 8  by the thrust of the slidable projection  10   f   1 . As shown in  FIG. 5 , the top cover  10   c  can be opened from this state. The slidable projection  10   f   1  serves to open the side cover  10   f  automatically to some extent at the initial stage (see  FIG. 8 ) when the slider  10   f   4  is slid in the direction of arrow Y 1  and the hooks  10   f   6  are thereby disengaged from the engagement portions  10   c   6 . The side cover  10   f  can further be opened thereafter manually. 
   With the above configuration, the door cover  10   c  is positioned with respect to the fuser  60  with high accuracy. As a result, the accuracy of the positioning of the roller pair  93  provided in the door cover  10   c  with respect to the fuser  60  is increased, which makes a sheet jamming less likely. 
   In addition, since the urging member is provided in the door cover  10   f , it is not necessary to provide an individual urging member on the door cover  10   c.    
   As shown in  FIGS. 10A to 10E , an E-ring  110  in this embodiment comprises: an E-ring body  111  which is made of synthetic resin and attached to an end portion  101   a  of a shaft  101 ; and a cover portion  112  which is made of synthetic resin and integrated with the E-ring body  111 . The cover portion  112  is adapted to cover an end face  101   b  of the shaft  101 . 
   The shaft  101  shown in  FIGS. 10A and 10B  is a metal shaft of a sheet transporting roller  101   c  that is disposed close to the fuser  60  (e.g., the switchback roller pair  93 ). The E-ring  110  serves as a stopper for fixing a gear  101   d  to the end portion  101   a.    
   The E-ring  110  is a monolithic product made of a synthetic resin, and the E-ring body  111  and the cover portion  112  are connected to each other by a link portion  113 . 
   To attach the E-ring  110  to the end portion  101   a  of the shaft  101 , the E-ring body  111  is attached to the end portion  101   a  like an ordinary E-ring. In an attached state, the end face  101   b  of the shaft  101  is covered with the cover portion  112 . 
   According to the E-ring  110 , the end face  101   b  of the shaft  101  is covered with the cover portion  112  when the E-ring  110  is attached to the end portion  101   a  of the shaft  101 . Therefore, the end face  101   b  of the shaft  101  is never touched by a human hand, for example. 
   Being made of a synthetic resin, the E-ring  110  is superior in heat insulation. Therefore, even if the temperature of the shaft  101  is made high, the temperature of the cover portion  112  is kept low; no problems arise even if the cover portion  112  is touched by a human hand, for example. 
   Although the present invention has been shown and described with reference to specific preferred embodiments, various changes and modifications will be apparent to those skilled in the art from the teachings herein. Such changes and modifications as are obvious are deemed to come within the spirit, scope and contemplation of the invention as defined in the appended claims.