Patent Publication Number: US-8121515-B2

Title: Image-forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 U.S.C. 119 from Japanese Patent Application No. 2009-263586, which was filed on Nov. 19, 2009. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an image-forming apparatus. 
     2. Related Art 
     In some image-forming apparatuses, a process cartridge, which is a unit including an image-holding member that holds an image, is detachably attached to a main body of the apparatus. 
     SUMMARY 
     In one aspect of the present invention, there is provided an image-forming apparatus including: a housing including a first frame member and a second frame member opposed to the first frame member; plural image-forming units each including an image-holding member on which an image is formed, the image-holding member having a rotation shaft; a supporting unit provided to the first frame member, the supporting unit having plural insertion holes each receiving and supporting an end portion of a rotation shaft of a corresponding one of the plural image-holding members inserted in an insertion direction from the second frame member to the first frame member; and plural flat elastic bodies provided to the respective insertion holes, each flat elastic body having a plate member that is bent to form: an attachment part that is attached to a surface of the supporting unit facing in a direction opposite to the insertion direction; a leg part extending from the attachment part in the direction opposite to the insertion direction; and an elastic part extending from the leg part in the insertion direction through the insertion hole to press the end portion of the rotation shaft against an edge of the insertion hole at a position spaced apart from the insertion hole in the insertion direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will now be described in detail with reference to the following figures, wherein: 
         FIG. 1  schematically shows a configuration of an image-forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  is a perspective view showing a configuration of a housing; 
         FIG. 3  is a perspective view showing an image-forming unit installed in the housing; 
         FIG. 4A  is a perspective view showing a structure of a first supporting plate and a flat spring, and  FIG. 4B  is a perspective view showing a back side of the flat spring; 
         FIG. 5  shows a flat spring attached to an insertion hole; 
         FIGS. 6A-6C  are explanatory views showing a rotation shaft being inserted into an insertion hole; and 
         FIG. 7  is a view for explaining a relationship between a flat spring and an upper edge of an insertion hole. 
     
    
    
     DETAILED DESCRIPTION 
     1. Exemplary Embodiment 
     An image-forming apparatus, such as a printer or a copy machine, is provided with a cover on a front or lateral side of a housing, for example, in such a manner that the cover can be opened and closed to facilitate maintenance or replacement of a component part, or removal of a jammed sheet. In the following, taking such an image-forming apparatus as an example, explanation will be made of an exemplary embodiment of the present invention.  FIG. 1  schematically shows a configuration of an image-forming apparatus according to the exemplary embodiment. In the following description, as indicated in the drawings, when the image-forming apparatus is viewed from its front by a user, the horizontal direction is denoted as the X-axis direction, with right/left directions from the user&#39;s perspective being indicated by X(+) and X(−), respectively; the front-back direction of the image-forming apparatus is denoted as the Y-axis direction, with back/front directions of the image-forming apparatus being indicated by Y(+) and Y(−), respectively; and the vertical direction is denoted as the Z-axis direction, with up/down directions being indicated by Z(+) and Z(−), respectively. It is to be noted that unless otherwise indicated, a term “inward” indicates a direction with respect to frames  43 ,  44 , which will be described later, toward a space in which an exposure device is contained, and a term “outward” indicates a direction with respect to frames  43 ,  44  that is opposite to the direction facing toward the space in which the exposure unit is contained. 
     &lt;Configuration of Image-Forming Apparatus&gt; 
     Image-forming apparatus  1  is adapted to constitute a full-color printer of a tandem type, and contains an image-processing unit (not shown in the drawings) that performs image-processing on image data received from a device such as a scanner or a personal computer (not shown in the drawings), or received via a telephone line (not shown in the drawings), etc. Provided inside image-forming apparatus  1  are four image-forming units  2 Y,  2 M,  2 C,  2 K for yellow (Y), magenta (M), cyan (C), and black (K), respectively. Image-forming units  2 Y,  2 M,  2 C,  2 K are arranged generally in the horizontal direction so as to be spaced apart from each other and to extend in parallel, and vertical positions of image-forming units  2 Y,  2 M,  2 C,  2 K are respectively lower in this order (thus, the vertical position of image-forming unit  2 Y is higher than that of image-forming unit  2 K), whereby a plane in which image-forming units  2 Y,  2 M,  2 C,  2 K are arranged is inclined at a certain angle (e.g., 10 degrees) with respect to the horizontal direction. By this arrangement of image-forming units  2 Y,  2 M,  2 C,  2 K in a plane inclined at a certain angle with respect to the horizontal direction, the horizontal dimension is reduced in comparison with a case where image-forming units  2 Y,  2 M,  2 C,  2 K are arranged in a horizontal plane. 
     Each of the four image-forming units  2 Y,  2 M,  2 C,  2 K has basically the same structure, and contains photosensitive drum  3  that is driven to rotate at a certain speed by a drive unit (described later) and that serves as an image-holding member, primary charging roll  4  that charges a surface of photosensitive drum  3 , developer unit  6  that develops, with toner, an electrostatic latent image formed on photosensitive drum  3  as a result of image exposure performed by image exposure unit  5  (described later), and cleaning unit  7  that cleans the surface of photosensitive drum  3 . Photosensitive drum  3  is constituted, for example, of an organic photosensitive member having a cylindrical shape with a diameter of 30 mm, and having an overcoat layer on its surface. Photosensitive drum  3  is rotated by rotation of rotation shaft  3 A (shown in  FIGS. 3 and 4 ). Charging roll  4  is, for example, a roll-shaped charger constituted of a core bar coated with a conductive layer made of a synthetic resin or rubber and having an adjusted electric resistance, and a charging bias is applied to the core bar of charging roll  4 . Further, a cleaning roll for removing foreign matters such as toner adhering to a surface of charging unit  4  is arranged to contact the surface of charging roll  4 . 
     In the following description, where it is not necessary to distinguish between image-forming units  2 Y,  2 M,  2 C,  2 K, the image-forming units will be simply referred to as image-forming unit(s)  2 . 
     Below image-forming units  2 Y,  2 M,  2 C,  2 K, exposure unit  5  is provided. Exposure unit  5  has four semiconductor laser units (not shown in the drawings) for emitting laser beams modulated in accordance with image data. The four laser beams emitted from these semiconductor laser units are deflected by a polygon mirror for scanning, and are irradiated onto photosensitive drum  3  of each image-forming unit  2 Y,  2 M,  2 C,  2 K via optical elements such as a lens and a minor (not shown in the drawings). 
     In this exemplary embodiment, exposure unit  5  extends along an underside of the four image-forming units  2 Y,  2 M,  2 C,  2 K, which are arranged in a plane inclined with respect to the horizontal direction. Thus, a length of a light path of the laser beam irradiated onto photosensitive drum  3  is the same for each of image-forming units  2 Y,  2 M,  2 C, and  2 K. 
     Image exposure unit  5 , which is provided in common to each image-forming unit  2 Y,  2 M,  2 C,  2 K, receives image data of respective colors sequentially from the image-processing unit. The laser beam emitted from image exposure unit  5  in accordance with the image data is irradiated onto a surface of corresponding photosensitive drum  3  to form an electrostatic latent image thereon. The electrostatic latent images formed on photosensitive drums  3  are developed by developer units  6 Y,  6 M,  6 C,  6 K to form toner images of respective colors. The toner images of respective colors formed sequentially on photosensitive drums  3  of image-forming units  2 Y,  2 M,  2 C,  2 K are transferred one on top of another by primary transfer rolls  11  to intermediate transfer belt  10 , which is arranged obliquely over the top of each image-forming units  2 Y,  2 M,  2 C,  2 K, and serves as an intermediate transfer member. 
     Intermediate transfer belt  10  is an endless belt-shaped member tension-supported by multiple rolls. Specifically, intermediate transfer belt  10  is wound around drive roll  12 , backup roll  13 , tension roll  14 , and idler roll  15 , such that intermediate transfer belt  10  is circulatingly moved in a direction indicated by an arrow in  FIG. 1  by drive roll  12 , which is rotated by a dedicated drive motor (not shown in the drawings) capable of maintaining a constant rotation speed. Intermediate transfer belt  10  has an upper moving section and a lower moving section, and the lower moving section is inclined with respect to the horizontal direction, with a downstream end of the lower moving section positioned lower than an upstream end of the same with respect to the direction of movement of the lower moving section. As intermediate transfer belt  10 , a flexible film made of a synthetic resin, such as polyimide, may be used, where the ends of the synthetic resin film are connected by means of welding or the like to form an endless belt member. Intermediate transfer belt  10  is arranged such that the lower moving section is in contact with photosensitive drums  3 Y,  3 M,  3 C,  3 K of image-forming units  2 Y,  2 M,  2 C,  2 K. 
     It is to be noted that intermediate transfer belt  10 , primary transfer rolls  11 , drive roll  12 , backup roll  13 , tension roll  14 , idler roll  15 , etc., are integrated into a single unit referred to as intermediate transfer unit  9 . 
     Recording sheets  18 , having a prescribed size and being made of a prescribed material, and serving as recording media, are contained in sheet container  24  disposed inside image-forming apparatus  1 , and are conveyed from sheet container  24  along conveyance path  21  formed by pairs of rollers. In this conveyance path  21 , recording sheets  18  in sheet container  24  are conveyed, one sheet at a time, by means of sheet supply roll  25  and a pair of rolls  26  for sheet separation and conveyance to registration roll  28 , and are temporarily stopped there. Then, recording sheet  18  is further conveyed to a secondary transfer position of intermediate transfer belt  10  by registration roll  28 , which is rotated at a predetermined timing. Recording sheet  18 , on which the toner images of respective colors have been transferred at the secondary transfer position, is applied with a heat and pressure by fixing unit  19  to fix the toner images. Thereafter, recording sheet  18  passes through exit roll  20  of fixing unit  19 , and is discharged by discharge roll  22  onto sheet-receiving tray  23  provided at an upper portion of image-forming apparatus  1 . 
     At a position on conveyance path  21  that is opposed to backup roll  13  across intermediate transfer belt  10  is provided secondary transfer roll  17 , which is constituted of a rotating member and is urged against intermediate transfer belt  10 . When recording sheet  18  moves between secondary transfer roll  17  and intermediate transfer belt  10 , secondary transfer roll  17  presses recording sheet  18  against intermediate transfer belt  10 , whereby the toner images of yellow (Y), magenta (M), cyan (C), and black (K), which have been overlappingly transferred onto intermediate transfer belt  10 , are transferred onto recording sheet  18  owing to pressure and electrostatic force. The position at which secondary transfer roll  17  and backup roll  13  are opposed to each other across intermediate transfer belt  10  is the above-mentioned secondary transfer position. 
     Arranged between sheet-receiving tray  23  and intermediate transfer belt  10  are toner cartridges  29 Y,  29 M,  29 C,  29 K. Toner cartridges  29 Y,  29 M,  29 C, and  29 K supply toner to developer units  6 Y,  6 M,  6 C, and  6 K, respectively. 
     &lt;Configuration of Housing&gt; 
     Next, explanation will be made of a configuration of main body (or housing)  40  of image-forming apparatus  1 , with reference to  FIG. 1  and  FIG. 2 .  FIG. 2  is a perspective view schematically showing a configuration of main body  40 , which constitutes a base of image-forming apparatus  1 . 
     As shown in  FIG. 2 , main body  40  includes four pillars  41 A- 41 D extending in the vertical direction (Z-axis direction), and plural beams  42  connecting pillars  41 A- 41 D. Further, back frame  43  serving as a first frame member is provided between upper parts of back-side (or Y(+) side) pillars  41 C,  41 D, and front frame  44  serving as a second frame member is provided between upper parts of front-side (or Y(−) side) pillars  41 A,  41 B. Front frame  44  is provided with a front cover  46  that is moveable in directions indicated by arrow “a” to open and close a space for containing image-forming units  2  (photosensitive drums  3 ) therein with respect to an outside. Image-forming units  2  are contained in the space so as to be removable in a direction opposite to a direction of insertion (Y-axis direction). Each image-forming unit  2  is inserted into the space for containment in a direction from the front side (Y(−) side) to the back side (Y(+) side), and is drawn out from the space for removal in a direction from the back side (Y(+) side) to the front side (Y(−) side). 
     Back frame  43  is provided on its inner surface (a surface facing in Y(−) direction) with first supporting plate  50  serving as a first supporting unit, and front cover  46  of front frame  44  includes second supporting plate  70 . Though not shown in the drawings, front cover  46  may have an outer plate-shaped member made of plastic or the like attached to an outer surface (or a surface facing in Y(−) direction) of second supporting plate  70 . Further, as shown in  FIG. 1 , main body  40  includes upper partition plate  47  that extends between frames  43  and  44  of main body  40  at a position above image exposure unit  5  (or a position on a Z(+) side of exposure unit  5 ), to define a space in which exposure unit  5  is accommodated, where upper partition plate  47  is secured to frames  43  and  44  by means of welding or the like. 
     &lt;Configuration of Supporting Plate&gt; 
     Explanation will now be made of a configuration of first supporting plate  50  with reference to  FIGS. 3-5 .  FIG. 3  is a perspective view showing image-forming unit  2  installed in main body  40  of image-forming apparatus  1 ,  FIG. 4  is a perspective view showing a configuration of first supporting plate  50  and flat spring  60 , and  FIG. 5  shows flat spring  60  attached to insertion hole  55 . 
     As shown in  FIGS. 3 and 4 , first supporting plate  50  is constituted of a generally rectangular plate member, which is bent along its longitudinal sides to form ribs  51 ,  51  extending outward (in Y(+) direction). First supporting plate  50  further includes four planar portions  52 , . . . ,  52 , which extend in the X-Z plane and are connected by ribs  51 ,  51 . In each planar portion  52 , insertion hole  55  for receiving rotation shaft  3 A of corresponding photosensitive drum  3  is formed. A part of each planar portion  52  positioned above insertion hole  55  (Z(+) side) serves as spring attachment part  53  where attachment part  61  of flat spring  60  is attached to a front surface (Y(−) side) of planar portion  52  in surface-to-surface contact. A coupling is formed on one end of each rotation shaft  3 A to engage with a gear of a drive unit not shown in the drawings. 
     First supporting plate  50  is secured to back frame  43  by fixing of an end of each rib  51  to an inner surface (Y(−) side) of back frame  43  by means of laser welding or the like. 
     Next, explanation will be made of a configuration of flat spring  60 , which serves as a flat elastic body. As shown in  FIG. 4 , flat spring  60  is formed by bending a plate-shaped spring member such that a portion of flat spring  60  between attachment part  61 , which serves as a base part, and a free end part has a cross-section in a shape of a chevron. More specifically, flat spring  60  is bent to form leg part  62  extending in the frontward direction (Y(−) direction) from attachment part  61 , and at bending part  63 , further bent in the opposite direction to form spring part  64 , which serves as an elastic part. Spring part  64  extends through insertion hole  55  in the backward direction (Y(+) direction), and pressing part  65  for pressing rotation shaft  3 A from radially outside is formed in spring part  64  at a position spaced apart from insertion hole  55  in the backward direction (Y(+) direction). In this flat spring  60 , spring part  64  has a larger length than leg part  62 , whereby pressing part  65  is positioned to be spaced apart from a back surface of first supporting plate  50 . 
     As shown in  FIG. 4B , which shows a back side of flat spring  60 , rib  61 A is formed on a left side (X(−) side) of attachment part  61  to limit movement in the rightward direction (X(+) direction). Flat spring  60  also has bend  61 B formed along an upper side (Z(+) side) of attachment part  61  to limit movement in the frontward/backward direction (Y-axis direction) and in the downward direction (Z(−) direction). With rib  61 A and bend  61 B engaging spring attachment part  53 , flat spring  60  is secured to first supporting plate  50  by means of a screw to be in surface-to-surface contact with spring attachment part  53 . In this way, even when image-forming unit  2  is inserted into the space with a large momentum, damage to flat spring  60  and first supporting plate  50  can be made smaller in comparison with a case where the attachment parts are not secured to each other in surface-to-surface contact. Also, when flat spring  60  is attached or when replacement of flat spring  60  becomes necessary due to damage, front cover  46  is opened to expose the space to outside, whereby the attachment or replacement can be carried out easily via the space. 
     Further, because spring attachment part  53  to which flat spring  60  is attached is provided on an inward-facing side (Y(−) side), leg part  62  and bending part  63  are positioned inside main body  40 , whereby image-forming apparatus  1  can be smaller and requires less space for installation in comparison with a case where flat spring  60  is attached to an outward-facing side. 
     Further, as shown in  FIGS. 6A-6C , an upper edge of insertion hole  55  of spring attachment part  53  is positioned lower (Z(−) side) than attachment part  61  of flat spring  60 . Thus, spring attachment part  53  also serves as a stopper that prevents spring part  64 , pushed by rotation shaft  3 A, from being bent excessively. 
     Explanation will now be made of a relationship between flat spring  60  and the upper edge of insertion hole  55 , with reference to  FIG. 7 . Range D indicates a range of elastic deformation of flat spring  60 , within which spring part  64  can flex elastically relative to leg part  62 . In other words, if the flexion exceeds the range of elastic deformation, a plastic deformation will be caused, whereby flat spring  60  will become unable to function as a spring. For these reasons, in this exemplary embodiment, where the flexion of flat spring  60  to the maximum limit of elastic deformation is represented as 100%, flat spring  60  is used in a range between 0% and 50%. Accordingly, the upper edge of insertion hole  55  is positioned within range D. 
     On the other hand, as shown in  FIGS. 2 and 3 , second supporting plate  70  is made of a substantially rectangular plate member that is larger than first supporting plate  50 . Second supporting plate  70  is formed with insertion holes  72  at positions opposed to insertion holes  55  of first supporting plate  50 , respectively, when front cover  46  is closed, such that each insertion hole  72  has the same shape as that of insertion hole  55 . 
     First supporting plate  50  and second supporting plate  70  are composed of metallic plates made of the same material and having the same thickness. When insertion holes  55 ,  72  are bored, the boring is performed on first supporting plate  50  and second supporting plate  70  stacked one over the other. In this way, burrs and distortions that may be generated as a result of the boring can be substantially the same between corresponding insertion holes  55 ,  61 . This contributes to suppressing any positional misalignment between the ends of each rotation shaft  3 A. 
     Insertion holes  55 ,  72  are formed in supporting plates  50 ,  70 , respectively, such that insertion holes  55 ,  72  are arranged at an angle equal to the angle of arrangement of image-forming units  2  with respect to the horizontal direction (e.g., 10 degrees). 
     As shown in  FIG. 5 , each insertion hole  55  ( 72 ) has a substantially pentagonal shape, such that oblique edge segments  55 A,  55 B in a lower (Z(−) side) part thereof have different angles of inclination with respect to the horizontal direction (X-axis direction). Specifically, angle of inclination α of oblique edge segment  55 A (first edge segment) is larger than angle of inclination β of oblique edge segment  55 B (second edge segment), whereby oblique edge segment  55 B is positioned lower in the direction of gravity. Consequently, an end portion of rotation shaft  3 A received in insertion hole  55  contacts oblique edge segments  55 A and  55 B such that a tangential line of the end portion of rotation shaft  3 A at a point contacting oblique edge segment  55 A is inclined with respect to the horizontal direction at an angle larger than that at which a tangential line of the end portion of rotation shaft  3 A at a point contacting oblique edge segment  55 B is inclined with respect to the horizontal direction. The reason the oblique edge segments have different angles of inclination is that a force that urges image-forming unit  2  to shift is transmitted to image-forming unit  2  from a drive unit for rotating rotation shaft  3 A. 
     Specifically, a drive unit including multiple gears is provided on the back side (Y(+) side) of back frame  43 . When a rotational force is transmitted from drive gear D 1  to driven gear D 2  of the drive unit, where driven gear D 2  may serve to rotate rotation shaft  3 A or a rotation shaft other than rotation shaft  3 A, force P 1  that urges driven gear D 2  to shift is generated in a tangential direction between gears D 1  and D 2 . This force P 1  also acts similarly on rotation shaft  3 A as force P 2 . Therefore, each rotation shaft  3 A is required to be positioned so as to be urged downward (in Z(−) direction) even when force P 2  is applied thereto. To achieve this, oblique edge segment  55 B having a smaller angle of inclination is formed upstream with respect to a direction of force P 2  (or upstream with respect to a direction of rotation of rotation shaft  3 A) compared to oblique edge segment  55 A having a larger angle of inclination. It is to be noted here that a determination as to whether one circumferential portion of insertion hole  55  is positioned upstream or downstream of another circumferential portion of insertion hole  55  is made in accordance with a region defined between the two circumferential portions that corresponds to a smaller angle of circumference determined by the two circumferential portions. In this way, owing to its own weight, rotation shaft  3 A contacts oblique edge segment  55 B with a larger force than that with which it contacts oblique edge segment  55 A. Further, because flat spring  60  is provided in an upper part (Z(+)-side part) of insertion hole  55 , the spring force generated by flat spring  60  acts on rotation shaft  3 A from radially outside via pressing part  65 , to push rotation shaft  3 A against oblique edge segment  55 B with a larger force than that exerted against oblique edge segment  55 A. Thus, each rotation shaft  3 A is pressed against oblique edge segment  55 B that is positioned lower in the direction of gravity, whereby a positional shift of each rotation shaft  3 A is suppressed. 
     Next, with reference to  FIGS. 6A-6C , explanation will be made of an operation of flat spring  60 .  FIGS. 6A-6C  are explanatory views showing an operation of flat spring  60  during an insertion of rotation shaft  3 A into insertion hole  55 . 
     As shown in  FIG. 6A , spring part  64  of flat spring  60  extends through insertion hole  55  such that pressing part  65  is positioned on a back side (Y(+) side) of first supporting plate  50 . When rotation shaft  3 A (image-forming unit  2 ) is inserted in a direction indicated by arrow b, an end of rotation shaft  3 A abuts a lower (Z(−) side) edge of insertion hole  55 . As rotation shaft  3 A is inserted further in the direction of arrow b, as shown in  FIG. 6B , rotation shaft  3 A is moved upward (in Z(+) direction) such that an upper part of rotation shaft  3 A contacts spring part  64  of flat spring  60 . Accordingly, flat spring  60  is bent at bending part  63 , and spring part  64  gradually exerts a spring force in the downward direction (Z(−) direction). In a state that rotation shaft  3 A has been inserted into insertion hole  55 , as shown in  FIG. 6C , pressing part  65  is positioned to push rotation shaft  3 A in the radial direction, and flat spring  60  applies a spring force upon rotation shaft  3 A to press it downward (in Z(−) direction). In this way, flat spring  60  serves to position rotation shaft  3 A in insertion hole  55 . 
     From experimental results, the spring force applied by flat spring  60  on rotation shaft  3 A is preferably in a range from 0.5 to 3.0 (N) for steadily holding rotation shaft  3 A in insertion hole  55  while allowing rotation shaft  3 A to be inserted into insertion hole  55  without excessive force. More preferably, the spring force applied by flat spring  60  on rotation shaft  3 A is in a range from 1.5 to 2.0 (N). 
     Thus, flat spring  60  is provided to each insertion hole  55  of first supporting plate  50  for supporting respective rotation shaft  3 A (photosensitive drum  3  or image-forming unit  2 ) in a back portion of image-forming apparatus  1  such that flat spring  60  is attached on a frontward-facing surface (Y(−) side) of first supporting plate  50  and spring part  64  extends from bending part  63  in the backward direction (Y(+) direction) to a position on a back side of first supporting plate  50 . In such a structure, when rotation shaft  3 A is inserted into insertion hole  55 , an impact generated from abutment of rotation shaft  3 A to insertion hole  55  is absorbed by flat spring  60 , and thus the impact is reduced compared to a case where flat spring  60  is absent. 
     In a state after rotation shaft  3 A has been inserted into insertion hole  55 , the spring force of flat spring  60  pushes down rotation shaft  3 A to prevent rotation shaft  3 A from floating in insertion shaft  55 , whereby a change in the support position of rotation shaft  3 A of photosensitive drum  3  is prevented, and hence misalignment of toner images of various colors is avoided. 
     2. Modified Embodiments 
     The foregoing exemplary embodiment may be modified as described in the following. 
     &lt;2-1&gt; 
     In the foregoing exemplary embodiment, flat springs  60  are provided to first supporting plate  50  that supports each rotation shaft  3 A in a back portion of main body  40  in the direction of insertion. However, flat springs  60  may be provided to insertion holes  72  of second supporting plate  70  of front cover  46 , or to each of insertion holes  55  and  72 . 
     &lt;2-2&gt; 
     In the foregoing exemplary embodiment, rotation shaft  3 A is supported by first supporting plate  50  in a back portion of main body  40 , and the forward end of rotation shaft  3 A is supported by second supporting plate  70  of front cover  46 . However, the present invention may be applied to a case where rotation shaft  3 A is supported only by first supporting plate  50  in a cantilever manner. 
     The foregoing description of the embodiments of the present invention is provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.