Patent Publication Number: US-7720415-B2

Title: Image forming apparatus including fixing unit, and fixing unit support method and fixing unit position adjustment method therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This patent specification is based on and claims priority from Japanese Patent Application No. 2007-084767, filed on Mar. 28, 2007 in the Japan Patent Office, the entire contents of which are hereby incorporated by reference herein. 
   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus including a fixing unit, and a fixing unit support method and a fixing unit position adjustment method for the image forming apparatus. 
   2. Description of the Related Art 
   In an electrophotographic image forming apparatus, an image is formed on a recording medium such as a sheet of paper or an OHP (overhead projector) film by charging a drum-like or belt-like rotating photosensitive element; irradiating the photosensitive element with light to form a latent electrostatic image thereon; attaching toner to the latent electrostatic image with a development device to make the latent electrostatic image visible as a toner image; transferring the toner image to the recording medium directly or indirectly via a belt-like intermediate transfer unit; and fixing the toner image on the recording medium. 
   Such an image forming apparatus includes a frame for reinforcing a body of the image forming apparatus against distortion or twisting. The frame is generally made of steel plate and includes a base member, a pair of opposing side plates set up on the base member, and stays or brackets laid between the side plates. In the frame, detachably attachable components such as a photosensitive device, a charging device, an optical writing device, a development device, a transfer device, a fixing device, an intermediate transfer device, and a recording medium feed device are usually provided in the form of a modular unit or a process cartridge including such units. 
   The fixing device, which is typically unitized for ease of maintenance, generally fixes an image on a recording medium when the recording medium passes through a fixing nip formed by a fixing member and a pressure member. The fixing nip is set parallel to the transfer rollers, etc., in recording medium feeders and transferring devices that transfer a recording medium to each transfer point and fixing point. When and if the fixing nip is angled with the transfer roller, etc., the transfer directions of a recording medium at the fixing nip and the transfer roller, etc., do not match, resulting in skewing of the recording medium and production of abnormal images transformed in a trapezoidal manner, etc. 
   As a method of preventing such skewing, pursuing component parts tolerances to perfection is practically difficult in light of cost increase as well as from a technical point of view. Furthermore, since there are a number of parts in an image forming apparatus, the tolerances of individual parts such as a fixing unit and a recording medium feeder are cumulative and directly affect the relative positions of these devices, easily displacing them from their parallel positions. In addition, there are part or unit assembly errors to be considered which are difficult to eliminate completely. 
   In addition, an assembly jig is required for assembling an image forming apparatus, and naturally, highly accurate design and manufacturing are demanded therefor. However, even with such a highly accurate jig, there is a limit to improvement in the accuracy with which devices are positioned parallel to each other. 
   Although the situation is as described above, to meet current demand for high definition quality images it is desired that an image forming apparatus has recording medium transfer devices, including a fixing unit; that are positioned parallel to each other with a high degree of precision. 
   SUMMARY 
   This patent specification describes a novel image forming apparatus that includes a frame having a first side plate and a second side plate opposing the first side plate; a first unit holding member engaged with the first side plate by a combination of a guide groove and a guide protrusion; a second unit holding member attached to the second side plate; a detachable unit located between the first side plate and the second side plate and held by the first unit holding member and the second unit holding member; and a position adjustment member to move the first unit holding member in a longitudinal direction of the guide groove to adjust a position of the detachable unit, the position adjustment member engaging the first unit holding member at a position vertically aligned with a center of gravity G of the detachable unit. 
   This patent specification further describes a novel fixing unit support method for supporting a fixing unit in an image forming apparatus, including engaging a unit holding member to hold a fixing unit with a frame of a main unit of the image forming apparatus by a combination of a guide groove and a guide protrusion, and providing a position adjustment member having an eccentric cam including a rotary center shaft portion rotatably engaging the frame and an eccentric rotary shaft portion eccentrically positioned relative to the rotary center shaft portion and rotatably engaging the unit holding member at a position vertically aligned with a center of gravity G of the fixing unit. 
   This patent specification further describes a novel fixing unit position adjustment method for an image forming apparatus, the image forming apparatus including a frame; a unit holding member engaged with the frame by a combination of a guide groove and a guide protrusion to hold a fixing unit; and a position adjustment member having an eccentric cam including a rotary center shaft portion rotatably engaging the frame and an eccentric rotary shaft portion eccentrically positioned relative to the rotary center shaft portion and rotatably engaging the unit holding member at a position vertically aligned with a center of gravity G of the fixing unit, the fixing unit position adjustment method for the image forming apparatus including rotating the position adjustment member and moving the unit holding member in the longitudinal direction of the guide groove to adjust a position of the fixing unit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is an external view illustrating a color copier as an example electrophotographic image forming apparatus according to embodiments of the present invention; 
       FIG. 2  is a diagram illustrating an inner configuration of the color copier; 
       FIG. 3  is a perspective view illustrating a frame of a main unit of the color copier; 
       FIG. 4  is an enlarged perspective view illustrating an attaching portion of a front holding member as viewed from outside the main unit; 
       FIG. 5  is an enlarged perspective view illustrating the attaching portion of the front holding member as viewed from inside the main unit; 
       FIGS. 6A ,  6 B, and  6 C are diagrams illustrating rotation of an eccentric cam that moves the front holding member; 
       FIG. 7  is a perspective view illustrating the front holding member as viewed from inside; 
       FIG. 8  is an enlarged perspective view illustrating an attaching portion of a rear holding member as viewed from inside the main unit; 
       FIG. 9  is an external perspective view illustrating a fixing unit according to a first embodiment of the present invention that is included in the color copier of  FIG. 1 ; 
       FIG. 10  is a view illustrating the fixing unit according to the first embodiment supported by first and second side plates of the frame via the front and rear holding members; 
       FIG. 11  is a view illustrating a fixing unit of electromagnetic induction heating type according to a second embodiment of the present invention; 
       FIG. 12  is an external perspective view illustrating the fixing unit according to the second embodiment; 
       FIG. 13  is a perspective view illustrating a connection member for the fixing unit according to the second embodiment and assembly thereof; 
       FIG. 14  is a perspective view illustrating an induction heating device included in the fixing unit according to the second embodiment; 
       FIG. 15  is a perspective view illustrating a front holding member for the fixing unit according to the second embodiment as viewed from inside; 
       FIG. 16  is a perspective view illustrating the front holding member of  FIG. 15  as viewed from outside; 
       FIG. 17  is a perspective view illustrating the fixing unit according to the second embodiment mounted between the front holding member and a rear holding member; 
       FIG. 18  is a perspective view illustrating a cooling fan mounted on a fan holder included in the front holding member; 
       FIG. 19  is a diagram illustrating a schematic configuration of a cooling device using the cooling fan; 
       FIG. 20  is a perspective view illustrating an example embodiment in which a sirocco fan is used as the cooling fan; and 
       FIG. 21  is a perspective view illustrating another example embodiment in which a sirocco fan is used as the cooling fan. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
   Referring now to the drawings, wherein like reference numerals and reference characters designate identical or corresponding parts throughout the several views thereof, particularly to  FIG. 10 , image forming apparatuses according to exemplary embodiments of the present invention are described. 
     FIG. 1  is an external view illustrating a color copier as an example electrophotographic image forming apparatus according to embodiments of the present invention. It should be noted that the color copier also functions as a printer, a scanner, or a facsimile by establishing a LAN (local area network) cable connection or a telephone line connection. 
   A main unit  1  of the color copier, which is the main unit of the image forming apparatus, includes a face cover and a frame  40  covered therewith. The frame  40  serves as the frame of the main unit  1 . In the main unit  1 , an image forming unit  2  that forms and transfers an image to a recording medium is located in the central part of the main unit  1  and a recording medium storage unit  3  that stores and sequentially feeds recording media to the image forming unit  2  is located below the image forming unit  2 . The main unit  1  also includes an internal discharge unit  4  that outputs a recorded recording medium, an image reading unit  5  that reads the image of an original, an operating unit  6  situated in front (indicated by arrow A) of the image reading unit  5 , and a duplex unit  7  that reverses the side of a recording medium and returns the recording medium to the image forming unit  2  for duplex printing. 
   The operating unit  6  includes an input unit (including various keys such as a start key, a numeric keypad, a function setting key, a reset key, a clear/stop key) to control a plurality of functions of the main unit  1  and a display unit (such as a liquid crystal display panel or a liquid crystal touch panel also functioning as an input unit) that displays various pieces of input information and an apparatus status. 
   In  FIG. 1 , arrow A indicates the front of the main unit  1  as described above, which is the operator side, arrow B indicates the rear, arrow C indicates the left side, and arrow D indicates the right side of the main unit  1 . 
   The color copier also includes a manual feed table  8 , a side cover  9 , and a front cover  10 , all of which can be opened with respect to the main unit  1 . 
     FIG. 2  is a diagram illustrating the inner configuration of the color copier of  FIG. 1 . 
   The image forming unit  2  includes four image forming stations in a tandem arrangement along an intermediate transfer unit  17  including an endless intermediate transfer belt  17   a . The four image forming stations form yellow (Y), cyan (C), magenta (M), and black (B) images, respectively. Below the four image forming stations, an optical writing device  13  is located. 
   The four image forming stations have the same configuration and include photosensitive drums  11 Y,  11 C,  11 M, and  11 B serving as image bearing members, around which are located charging devices  12 Y,  12 C,  12 M, and  12 B, development devices  14 Y,  14 C,  14 M, and  14 B, primary transfer rollers  15 Y,  15 C,  15 M, and  15 B, and cleaning devices  16 Y,  16 C,  16 M, and  16 B, respectively. 
   The optical writing device  13  faces the four image forming stations and is formed of four light sources using respective laser diodes (LD) corresponding to the four colors; an optical system that collimates a laser beam emitted from the light sources; a polygon scanner (deflector) that includes a polygon mirror (rotating multifaceted mirror) and a polygon motor; and another optical system including scanning and imaging lenses such as an fθ lens provided on each light path for the light sources, correcting lenses, and mirrors. The laser diodes emit beams of light according to image information of each color and the polygon scanner deflects the beams of light and scans the surfaces of the four photosensitive drums  11 Y,  11 C,  11 M, and  11 B with the beams of light to write a latent electrostatic image thereon. 
   In the image forming unit  2 , toner bottles  32 Y,  32 C,  32 M, and  32 B are located below the internal discharge unit  4  from left to right in  FIG. 2  to supply toner to the development devices  14 Y,  14 C,  14 M, and  14 B in the image forming stations. The toner bottles  32 Y,  32 C,  32 M, and  32 B are filled with yellow (Y) toner, cyan (C) toner, magenta (M) toner, and black (B) toner, respectively. The toners are supplied from the toner bottles  32 Y,  32 C,  32 M, and  32 B to the development devices  14 Y,  14 C,  14 M and  14 B in a given amount through conveyance paths, not shown. 
   The intermediate transfer belt  17   a  in the intermediate transfer unit  17  is supported by a drive roller, a driven roller, etc., and rotates in a direction indicated by an arrow shown in  FIG. 2 . On the right of the intermediate transfer belt  17   a , a secondary transfer roller  22  is located. On the left of the intermediate transfer belt  17   a , an intermediate transfer belt cleaning device  18  is located. 
   The recording medium storage unit  3 , which is situated at the bottom of the main unit  1 , includes two paper feed cassettes  3   a  and  3   b  storing a recording medium S. The recording medium S is fed from one of the paper feed cassettes  3   a  and  3   b  by a paper feed device  19   a  or  19   b  and conveyed by a recording medium feeder. Specifically, the recording medium S is fed by conveyance rollers  20   a  and  20   b  to a registration roller  21  and the registration roller  21  feeds the recording medium S at an appropriate timing to a secondary transfer nip formed between the secondary transfer roller  22  and the intermediate transfer belt  17   a.    
   According to a first embodiment of the present invention, a fixing unit  35  is located above the secondary transfer roller  22 . In the fixing unit  35 , a fixing roller  36  and a heat roller  37  support a fixing belt  38  serving as a fixing member. In addition, a pressure roller  39  serving as a pressure member is provided to form a fixing nip with the fixing belt  38 . 
   Above the fixing unit  35 , a conveyance roller  23  and a discharge roller  24  that convey and discharge the recording medium S to the internal discharge unit  4 , and a switching member  25  that switches the conveyance path for duplex printing are located. Above the conveyance roller  23 , the discharge roller  24 , and the switching member  25 , a reverse roller  26  and a reverse path  27  are located to reverse the conveyance direction of the recording medium S like a switchback. In the case of duplex printing, the recording medium S is temporarily stacked in the reverse path  27  and the conveyance direction thereof is reversed by the reverse roller  26 . The recording medium S is then conveyed by conveyance rollers  28  and  29  through a conveyance path for duplex printing and fed to the registration roller  21  again. 
   The image reading unit  5  is located in the upper part of the main unit  1  and includes a contact glass  5   a  serving as an original table on which an original is placed, an illumination source  5   b  that illuminates the original, a first mirror  5   c  that reflects light reflected from the original, a second mirror  5   d , a third mirror  5   e , an imaging lens  5   f  that images the light reflected from the original, and an image sensor  5   g  such as a CCD (charge-coupled device) that is located at an image location and serves as a reading unit for reading the image of the original. 
   On the image reading unit  5 , there is provided a cover  33  that presses the original placed on the contact glass  5   a  against the contact glass  5   a  to keep it in place or an automatic document feeder (ADF) that automatically feeds the original to the contact glass  5   a.    
   A description is now given of a copying process executed by the color copier. 
   The copying process is started by opening the cover  33  (with respect to the main unit  1 ) and placing an original on the contact glass  5   a  in the image reading unit  5 . When the ADF is used instead of the cover  33 , the original is placed on the original table of the ADF. 
   By pressing the start switch of the operating unit  6 , the image reading unit  5  is instantly driven when the original is placed on the contact glass  5   a . When the original is placed on the original table of the ADF, the original reading unit  5  is driven after the original is transferred onto the contact glass  5   a . Then, a first traveling body including the light source  5   b  and the first mirror  5   c  and a second traveling body including the second mirror  5   d  and the third mirror  5   e  begin to travel. The light emitted from the light source  5   b  is directed to the surface of the original. The first mirror  5   c  in the first traveling body reflects and directs the light reflected from the original to the second traveling body. The second and third mirrors  5   d  and  5   e  in the second traveling body reflect and direct the light through the imaging lens  5   f  to the image sensor  5   g  such as a CCD serving as a reading unit, where the image of the original is read. Then, an image forming operation starts in the full color mode or the monochrome mode. The mode is determined depending on the selection including automatic mode at the operating unit  6 . 
   In the image forming unit  2 , the charging devices  12 Y,  12 C,  12 M, and  12 B uniformly charge the photosensitive drums  11 Y,  11 C,  11 M, and  11 B, which are then irradiated with laser beams emitted from the optical writing device  13  including four laser light sources, the deflector, and the four scanning optical systems to form latent electrostatic images thereon. 
   The latent electrostatic images are developed by attaching toner of different colors thereto by the four development devices  14 Y,  14 C,  14 M, and  14 B so that yellow, cyan, magenta, and black toner images are formed on the photosensitive drums  11 Y,  11 C,  11 M, and  11 B. 
   Subsequently, a primary transfer voltage is applied to the primary transfer rollers  15 Y,  15 C,  15 M, and  15 B at different timings, thereby transferring the toner images sequentially to the intermediate transfer belt  17   a  starting from the upstream side and superimposing the toner images one atop another thereon (a process that is herein referred to as primary transfer). 
   The recording medium S is supplied to the main unit  1  in synchronization with the primary transfer from the recording medium feeder  3   a  or  3   b  in the recording medium storage unit  3  by the recording medium feed device  19   a  and  19   b  or the manual feed table  8  by a recording medium feed roller  30  and a conveyance roller  31 . When the leading edge of the recording medium S reaches the registration roller  21 , the recording medium S is held there and detected by a sensor, not shown. Then, the registration roller  21  timely feeds the recording medium S to the secondary transfer nip formed between the secondary transfer roller  22  and the intermediate transfer belt  17   a  based on a detection signal generated by the sensor. 
   The images formed on the intermediate transfer belt  17   a  are conveyed to the secondary transfer nip and transferred to the recording medium S all at one time. The recording medium S is then conveyed to the fixing unit  35  and passes through the fixing nip, where the transferred image is fixed onto the recording medium S by application of heat and pressure. Thereafter, the recording medium S is conveyed by the conveyance roller  23  and discharged by the discharge roller  24  to the internal discharge unit  4 . Thus, a color image is formed on the recording medium S. 
   When the duplex mode is selected at the operating unit  6 , the conveyance path of the recording medium S is switched by the switching member  25 . The recording medium S of which the image is already fixed on one side is temporarily stacked in the reverse path  27  and the conveyance direction thereof is reversed by the reverse roller  26  like a switchback. The recording medium S is then conveyed by the conveyance rollers  28  and  29  through the conveyance path for duplex printing to the registration roller  21  again in synchronization with the image forming operation. 
   The registration roller  21  feeds the recording medium S to the secondary transfer nip, where the image formed on the intermediate transfer belt  17   a  is transferred to the back side of the recording medium S. Then, the recording medium S is conveyed to the fixing unit  35  and the transferred image on the back side thereof is fixed onto the recording medium S by application of heat and pressure. The recording medium S is conveyed by the conveyance roller  23  and discharged by the discharge roller  24  to the internal discharge unit  4 . Thus, color images are formed on both sides of the recording medium S. 
   Toner remaining on the photosensitive drums  11 Y,  11 C,  11 M, and  11 B is removed by the cleaning devices  16 Y,  16 C,  16 M, and  16 B, respectively. Then, the photosensitive drums  11 Y,  11 C,  11 M, and  11 B are simultaneously discharged and charged by the charging devices  12 Y,  12 C,  12 M, and  12 B to which a direct-current bias overlapped with an alternating-current component is applied to prepare for the next image formation. Toner remaining on the intermediate transfer belt  17   a  is also removed by the intermediate transfer belt cleaning device  18  to prepare for the next image formation. 
     FIG. 3  illustrates the frame  40  of the main unit  1 . 
   The frame  40  is made of steel plate and includes a base member  41 , opposing first and second side plates  42  and  43  set up on the base member  41 , and stays or brackets  44  laid between the first and second side plates  42  and  43 . The first side plate  42  is the front side plate provided on the operator side (indicated by arrow A) of the main unit  1 . The second side plate  43  is the rear side plate provided opposite to the first side plate  42 , from where a driving force is transmitted to, for example, the fixing unit  35 . 
   In the frame  40 , there are detachably attached the photosensitive drums  11 Y,  11 C,  11 M, and  11 B, the charging devices  12 Y,  12 M,  12 C, and  12 B, the optical writing device  13 , the development devices  14 Y,  14 C,  14 M, and  14 B, the primary transfer rollers  15 Y,  15 C,  15 M, and  15 B, the cleaning devices  16 Y,  16 C,  16 M, and  16 B, the secondary transfer roller  22 , the fixing unit  35 , the intermediate transfer unit  17 , and the recording medium feed device in the form of a modular unit or a process cartridge including the units. 
   The fixing unit  35  is positioned and held by a front holding member  45  attached to the first side plate  42  and a rear holding member  46  attached to the second side plate  43 , and supported between the first and second side plates  42  and  43 . In  FIG. 3 , arrow A indicates the front of the main unit  1 , which is the operator side, arrow B indicates the rear, arrow C indicates the left side, and arrow D indicates the right side of the main unit  1 . 
     FIG. 4  is an enlarged perspective view illustrating the attaching portion of the front holding member  45  as viewed from outside the main unit  1 .  FIG. 5  is an enlarged perspective view illustrating the attaching portion of the front holding member  45  as viewed from inside the main unit  1 . 
   As illustrated in  FIG. 5 , the front holding member  45  is formed with a long, straight main reference engagement slot  47  and, above it, a shorter sub-reference engagement slot  48  that are parallel to each other. As illustrated in  FIG. 4 , the unit holding member  45  includes a substantially circular yet elongated hole  50  in the upper part thereof. The hole  50  is elongated latitudinally in a direction parallel to the direction in which the main reference engagement slot  47  and the sub-reference engagement slot  48  extend. At upper and lower points in the front holding member  45  long guide grooves  51  or elongated holes are formed along a straight line perpendicular to the main reference engagement slot  47 , the sub-reference engagement slot  48 , and the hole  50 , although the lower guide groove  51  is not seen in  FIG. 4 . 
   An eccentric cam  52  serving as a position adjustment member is inserted into the elongated hole  50  via an eccentric shaft portion  53 . As illustrated in  FIGS. 6A ,  6 B, and  6 C, the eccentric cam  52  includes a circular disk cam  54 , a lever handle  55  extending radially from the circumferential surface of the circular disk cam  54 , a center shaft portion  56  of a large circle provided to one side of the circular disk cam  54 , the eccentric shaft portion  53  of a small circle provided to the other side of the circular disk cam  54  in an eccentric position relative to the center shaft portion  56 , and a curved slot  57  curved in an arc relative to the shaft portion  56 . The eccentric shaft portion  53  has a diameter substantially equal to a height of the elongated hole  50  so as to tightly engage the hole  50  without rattling in the height direction of the hole  50 . 
   As illustrated in  FIG. 5 , the center shaft portion  56  is rotatably engaged with a fitting hole  58  in the first (front) side plate  42 . As illustrated in  FIG. 4 , the circular disk cam  54  of the eccentric cam  52  is fixed to the first side plate  42  by attaching the circular disk cam  54  to the external surface of the first side plate  42  with a fixing screw  59  through the curved slot  57 . The end of the handle  55  of the eccentric cam  52  is set to indicate a scale  60  marked on the first side plate  42 . 
   The eccentric shaft portion  53  is fitted into the elongated hole  50 . The front holding member  45  is attached to the first side plate  42  by inserting two upper and lower guide protrusions  61  provided on the first side plate  42  into the respective guide grooves  51 . The front holding member  45  is attached to the first side plate  42  by screwing attaching screws  63  into screw holes in the first side plate  42  through four screw elongated holes in the front holding member  45 . 
   The two guide protrusions  61  in this example embodiment are integrally formed with the first side plate  42  by a drawing process such as burring or embossing. The guide protrusions  61  are on a vertical line L that is parallel to a vertical direction of passing the recording medium S through the fixing nip. Therefore, the two guide grooves  51  of the front holding member  45  extend in the direction of the vertical line L. The main reference engagement slot  47 , the sub-reference engagement slot  48 , and the elongated hole  50  extend in the direction perpendicular to the vertical line L. The four screw elongated holes in the front holding member  45  extend vertically. The vertical line L and the line connecting the center of the center shaft portion  56  and the center of the eccentric shaft portion  53  are at right angles to each other. 
   Therefore, when the eccentric cam  52  with the handle  55  is rotated, the front holding member  45  is moved in the direction guided by the two upper and lower guide protrusions  61 , that is, the direction parallel to the (vertical) direction of the recording medium S passing through the fixing nip. The guide groove  51  has a width substantially equal to the diameter of the guide protrusion  61  so that the guide protrusion  61  engages the guide groove  51  snugly, with substantially no allowance in the width direction of the guide groove  51 . Such an arrangement is preferable because it prevents rattling in the direction perpendicular to the vertical line L. 
   It should be noted that the guide groove  51  (or a elongated hole) and the guide protrusion  61  constitute a combination pair and can be formed on the front holding member  45  and the first side plate  42 , respectively, as in the embodiment described above, or vice versa. 
     FIG. 7  illustrates the front holding member  45  as viewed from inside. 
   In  FIG. 7 , a stopper lever  73  having a leading end rotating upward as indicated by an arrow about its base end is illustrated at the sub-reference engagement slot  48 , although not illustrated in  FIG. 5 . 
     FIG. 8  is an enlarged perspective view illustrating the attaching portion of the rear holding member  46  as viewed from inside the main unit  1 . 
   As illustrated in  FIG. 8 , similar to the front holding member  45 , the rear holding member  46  is formed with a long, straight main reference engagement slot  65  and a shorter sub-reference engagement slot  66  that are parallel to each other. The rear holding member  46  is fixed to the second side plate  43  by attaching the rear holding member  46  to the external surface of the second side plate  43  with attaching screws that are screwed into screw holes in the second side plate  43  through a plurality of screw elongated holes in the rear holding member  46 . The main reference engagement slot  65  is placed opposite to the main reference engagement slot  47  in the front holding member  45  and the sub-reference engagement slot  66  is placed opposite to the sub-reference engagement slot  48  in the front holding member  45 . Although not illustrated in  FIG. 8 , a stopper lever  73  having a leading end rotating upward about its base end is provided at the sub-reference engagement slot  66 . 
     FIG. 9  is an external view illustrating the fixing unit  35  according to the first embodiment as viewed obliquely from upper left. 
   As illustrated in  FIG. 9 , on each of front and rear surfaces of the fixing unit  35 , a main reference protrusion  70  and an axial sub-reference protrusion  71  that protrude forward on the front surface and backward on the rear surface are provided, spaced a certain distance apart. Handles  72  for handling the fixing unit  35  are located on a top surface of the fixing unit  35 . 
   A description is now given of insertion of the fixing unit  35  in the main unit  1  of the image forming apparatus. 
   The fixing unit  35  is inserted between the front and rear holding members  45  and  46  from the right side of the main unit  1  by opening the side cover  9  illustrated in  FIG. 1 ; holding the handles  72 ; fitting the main reference protrusions  70  to the main reference engagement slots  47  and  65  in the front and rear holding members  45  and  46 ; and fitting the sub-reference protrusions  71  to the sub-reference engagement slots  48  and  66  in the front and rear holding members  45  and  46 . 
   Subsequently, the sub-reference protrusions  71  contact curved surfaces  73   a  (illustrated in  FIG. 7 ) of the stopper levers  73  provided on the front and rear holding members  45  and  46 . The stopper levers  73  are pushed open upward against gravity and the fixing unit  35  is further forced into the holding members  45  and  46 . When the main reference protrusions  70  contact the ends of the main reference engagement slots  47  and  65 , the stopper levers  73  return to normal positions by gravity and engage the sub-reference protrusions  71  so that the sub-reference protrusions  71  are prevented from slipping. Thus, the fixing unit  35  is installed in the main unit  1 , supported by the first and second side plates  42  and  43 . As noted above, the fixing unit  35  is driven by a driving force transmitted from the rear of the main unit  1 . 
   To detach the fixing unit  35  from the main unit  1 , the stopper levers  73  are manually rotated upward to release the sub-reference protrusions  71 . Then, using the handles  72 , the fixing unit  35  is pulled out by guiding the main reference protrusions  70  along the main reference engagement slots  47  and  65  and guiding the sub-reference protrusions  71  along the sub-reference engagement slots  48  and  66 . 
   A description is now given of adjusting a skew or slant of the fixing unit  35 . 
     FIG. 10  illustrates the fixing unit  35  supported by the first and second side plates  42  and  43  of the frame  40  via the front and rear holding members  45  and  46 . 
   As illustrated in  FIG. 10 , a center of gravity G of the fixing unit  35  is positioned between the two upper and lower protrusions  61  provided to the first side plate  42  of the frame  40 . The guide grooves  51  and the guide protrusions  61  are located on the vertical line L passing through the center of gravity G of the fixing unit  35 . The position of the eccentric shaft portion  53 , where the eccentric cam  52  engages the front holding member  45 , is on the vertical line L. The vertical line L and a line M connecting the center of the center shaft portion  56  and the center of the eccentric shaft portion  53  are at right angles to each other. 
   A slant of the fixing unit  35  is adjusted by rotating the eccentric cam  52  about the center shaft portion  56  with the handle  55  and referring to the scale  60  with the fixing screw  59  and the attaching screws  63  loosened. The eccentric shaft portion  53  moves upward or downward in the direction of the vertical line L by a distance y 1  or y 2  as illustrated in  FIG. 6B  or  6 C when the eccentric cam  52  illustrated in  FIG. 6A  is rotated clockwise or counterclockwise, thereby moving the front holding member  45  upward or downward along the vertical line L. After the adjustment, the fixing screw and then the attaching screws  63  are tightened. The accuracy with which the fixing unit  35  and the registration roller  21  are positioned parallel to each other can then be checked by forming images and checking the alignment of the images. 
   When the aforementioned check indicates that the fixing unit  35  and the registration roller  21  are not parallel and therefore that their relative positions need to be adjusted, the attaching screws  63  and the fixing screw  59  are again loosened and the eccentric cam  52  is rotated to move the front holding member  45 . Then, the fixing screw  59  and then the attaching screws  63  are once again tightened. Images are then formed again to check the accuracy with which the fixing unit  35  and the registration roller  21  are positioned parallel to each other. This process is repeated until the fixing unit  35  and the registration roller  21  are correctly positioned parallel to each other. It should be noted that the front holding member  45  moves smoothly in the direction of the vertical line L by moving the eccentric shaft portion  53  in the direction of the vertical line L. 
     FIG. 11  illustrates a fixing unit  35  of electromagnetic induction heating type according to a second embodiment of the present invention. 
   This fixing unit  35  includes a unit body  75  and an induction heating device  76 . In the unit body  75 , a fixing roller  77  serving as a fixing member and a pressure roller  78  serving as a pressure member form a fixing nip. The image on the recording medium S is fixed while the recording medium S passes through the fixing nip. On the circumferential surface of the fixing roller  77 , an induction heating layer is provided. Although the fixing roller  77  is used as a fixing member in the present example, a fixing belt  38  as illustrated in  FIG. 2 , which has an induction heating layer on its surface, can alternatively be used as the fixing member. Also, the pressure roller  78  used as a pressure member can be replaced by a pressure pad that does not rotate. It should be noted that the same reference numerals designate corresponding components in  FIGS. 10 and 11 . 
   The center of gravity G of the fixing unit  35  of an electromagnetic induction heating type is also located between the two upper and lower protrusions  61  provided on the first side plate  42  of the frame  40 . The guide grooves  51  and the guide protrusions  61  are located on the vertical line L passing through the center of gravity G of the fixing unit  35 . 
     FIG. 12  is an external view of the fixing unit  35  of an electromagnetic induction heating type according to the second embodiment. 
   On each of the front and rear surfaces of the unit body  75  of the fixing unit  35 , the main reference protrusions  70  and the axial sub-reference protrusions  71  that protrude forward on the front surface and backward on the rear surface, separated by a certain distance. On the top surface of the fixing unit  35 , handles  72  are formed upward. By holding the handles  72 , the fixing unit  35  is inserted between the front and rear holding members  45  and  46  as described above. Thus, the fixing unit  35  is installed in the main unit  1 , supported by the first and second side plates  42  and  43  of the frame  40 . 
   In the fixing unit  35 , the unit body  75  and the induction heating device  76  are connected to each other using a connection member  80 . 
     FIG. 13  illustrates the connection member  80  and assembly thereof. 
   The connection member  80  includes a bent plate  79 , a positioning pin  81  protruding from the bent plate  79 , and a screw hole  82 , and is fixed to the unit body  75  by inserting an attaching screw  83  into the screw hole  82 . The leading end of the positioning pin  81  is inserted to a positioning hole  85  (illustrated in  FIG. 14 ) in a housing  84  of the induction heating device  76  to fix the induction heating device  76  to the unit body  75  such that the induction heating device  76  is pivotable about the positioning pin  81 . The induction heating device  76  is pressed against the unit body  75  by a biasing member, not shown. 
     FIG. 14  is an external view of the induction heating device  76 . 
   The induction heating device  76  includes an induction coil  93  (refer to  FIG. 19 ) in the housing  84 . A current is applied to the induction coil  93 , causing the heated induction coil  93  to generate heat in the induction heating layer of the fixing roller  77 . 
   An air intake port  86  is provided on the front of the housing  84  and the positioning hole  85  is located above the air intake port  86 . A positioning pin  87  is provided on the rear of the housing  84  at the position corresponding to the positioning hole  85 . The positioning pin  87  is inserted into a positioning hole in the rear holding member  46 . Thus, the induction heating device  76  is supported pivotably about the positioning pin  81  and the positioning pin  87 . 
     FIGS. 15 and 16  illustrate inside and outside views, respectively, of the front holding member  45  for the fixing unit according to the second embodiment. 
   Similar to the front holding member  45  illustrated in  FIGS. 4 and 5 , the front holding member  45  according to the second embodiment is formed with the long, straight main reference engagement slot  47  and the short sub-reference engagement slot  48  that are parallel to each other. At the sub-reference engagement slot  48 , the stopper lever  73  having a leading end that is rotatable upward against gravity about its base end as illustrated in  FIG. 16  is provided. 
   Unlike the front holding member  45  illustrated in  FIGS. 4 and 5 , in the present embodiment the front holding member  45  is formed with an air intake hole  88  and a fan holder  89  having a rectangular frame around the intake hole  88 . 
     FIG. 17  illustrates the fixing unit  35  mounted between the front holding member  45  and the rear holding member  46 . 
     FIG. 18  is a diagram in which a cooling fan  90  is mounted on the fan holder  89  in the front holding member  45 . 
     FIG. 19  illustrates a schematic configuration of a cooling device  91  using the cooling fan  90 . 
   As illustrated in  FIG. 19 , the front holding member  45  is mounted on the front of the first (front) side plate  42  and the cooling fan  90  is mounted on the fan holder  89 . The front surface of the induction heating device  76  is pressed against the internal surface of the first side plate  42  via a sealing member  92 . In the induction heating device  76 , the induction coil  93  is provided and an air path  94  is formed. The rear surface of the induction heating device  76  is pressed against the second (rear) side plate  43  via a sealing member  95 . On the second side plate  43 , the rear holding member  46  is mounted, and an exhaust duct  96  is connected thereto. 
   When the induction heating device  76  is in operation, the cooling fan  90  is driven to feed air to a space between the front holding member  45  and the first side plate  42  through the intake hole  88  in the front holding member  45 . The air flows from an intake opening  97  in the first side plate  42  to the air path  94  through the front air intake port  86  in the housing  84  of the induction heating device  76  and cools the induction coil  93 . The air heated by the induction coil  93  is discharged from the air path  94  to the exhaust duct  96  by way of a rear air discharge  98  in the housing  84 , an exhaust opening  99  in the second side plate  43  and an exhaust hole  100  in the rear holding member  46 . Thus, the induction coil  93  of the induction heating device  76  can maintain the thermal efficiency of the fixing roller  77 . 
   It should be noted that although the cooling fan  90  in the example embodiment described above is an intake fan that draws in air to the air path  94  in the induction heating device  76 , alternatively, an exhaust fan that exhausts air from the air path  94  can be used, or both an intake fan and an exhaust fan can be used. In addition, although an axial fan is used in the example embodiment, alternatively a sirocco fan as illustrated in  FIG. 20  or a sirocco fan as illustrated in  FIG. 21  can be used as the cooling fan  90 . 
   As can be understood by those skilled in the art, numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. 
   Further, elements and/or features of different example embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 
   Example embodiments being thus described, it will be apparent that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.