Abstract:
An image forming apparatus includes a driving source, a rotary element and a coupling device. The coupling device includes an inner coupling having a cylindrical shape with a first rotation axis and a first group of claws arranged to be evenly spaced in a circumferential direction, and an outer coupling having a cylindrical shape with a second rotation axis and a second group of claws arranged to be evenly spaced in a circumference direction to alternately mesh with the first group of claws. At least one of the first and second groups of claws includes claws with pointed tips so as to effectively connect the inner coupling with the outer coupling.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This patent specification is based on Japanese patent application, No. JP2005-160006 filed on May 31, 2005 and JP2005-200143 filed on Jul. 8, 2005 in the Japanese Patent Office, the entire contents of which are incorporated by reference herein. 
   BACKGROUND 
   1. Field of Invention 
   Exemplary aspects of the present invention relate to an image forming apparatus, and more particularly to an image forming apparatus that includes a coupling device for effectively connecting a drive-side coupling and a driven-side coupling even if shaft centers of the drive-side coupling and the driven-side coupling are misaligned. 
   2. Discussion of the Background 
   A related art coupling device  500 A illustrated in  FIG. 1  includes a drive-side coupling  502 A serving as a first coupling and a driven-side coupling  501 A serving as a second coupling. The drive-side coupling  502 A is disposed on a tip surface of a drive-side base  503 A serving as a first base. A plurality of driving claws  504 A serving as first claws (one drive claw is shown in  FIG. 1 ) are integrally formed with the drive-side base  503 A. The driven-side coupling  501 A is disposed on a tip surface of a driven-side base  506 A serving as a second base. A plurality of driven claws  507 A serving as second claws are integrally formed with the driven-side base  506 A. As shown in  FIG. 1 , in a state where the drive-side coupling  502 A and the driven-side coupling  501 A are connected, each of the driving claws  504 A and the driven claws  507 A are alternately positioned, meshing each driving claw  504 A with each driven claw  507 A. The drive-side coupling  502 A is connected to a driving shaft  508 A. As the driving shaft  508 A is rotatively driven by a not-shown driving source, the drive-side coupling  502 A rotates. Accordingly, the driving claws  504 A and the driven claws  507 A are pressed with one another, and the rotation of the drive-side coupling  502 A is transmitted to the driven-side coupling  501 A, rotating a driven shaft  509 A which is secured to the driven-side coupling  501 A, thereby rotatively driving a rotation element of a driven unit  510 A (not shown). 
   The driven-side coupling  501 A is detachably connected with respect to the drive-side coupling  502 A in a shaft line direction Z 1  of the drive-side coupling  502 A. By moving the driven-side coupling  501 A together with the driven unit  510 A in an arrow A-direction, the driven-side coupling  501 A may be disengaged from the drive-side coupling  502 A. On the other hand, by pressing the driven-side coupling  501 A together with the driven unit  510 A in an arrow B-direction, the driven-side coupling  501 A and the drive-side coupling  502 A are connected. 
   The drive-side coupling  502 A is connected such that it is movable for a specific number of strokes in a shaft line direction Z 1  with respect to the driving shaft  508 A. Furthermore, a compression spring  512 A is disposed between a spring receiving part  511 A secured to the driving shaft  508 A and the drive-side coupling  502 A. 
   In a case where top faces  513 A of the driven claws  507 A collide with top faces  514 A of the driving claws  504 A when the driven-side coupling  501 A is pressed in the arrow B-direction, the drive-side coupling  502 A is pressed by the driven-side coupling  501 A. Consequently, the drive-side coupling  502 A moves in the arrow B-direction with respect to the driving shaft  508 A, and the compression spring  512 A is compressed. In this state, when the drive-side coupling  502 A rotates, the driving claws  504 A rotate with respect to the driven claws  507 A, and due to the pressure effect of the compression spring  512 A, the drive-side coupling  502 A moves in the arrow A direction. As a result, each of the driving claws  504 A advances between each of the driven claws  507 A so that each of the driving claws  504 A and each of the driven claws  507 A are alternately positioned in a peripheral direction of the coupling device  500 A. Accordingly, each of the driving claws  504 A and each of the driven claws  507 A are engaged, and the rotation of the drive-side coupling  502 A is transmitted to the driven-side coupling  501 A. 
   As described above, even if the top faces  513 A of the driven claws  507 A collide with the top faces  514 A of the driving claws  504 A when the driven-side coupling  501 A and the drive-side coupling  502 A are connected, the driven claws  507 A and the driving claws  504 A may be engaged by the rotation of the drive-side coupling  502 A. 
   However, in the above-described coupling device  500 A, the compression spring  512 A is needed so that the driven claws  507 A and the driving claws  504 A are engaged even if the top faces  513 A of the driven claws  507 A collide with the top faces  514 A of the driving claws  504 A. Consequently, it results in a growing number of parts to be used for the coupling device  500 A. Moreover, a space in which the compression spring  512 A is disposed is needed, thereby increasing the size of an entire coupling device  500 A. 
   In view of this, Japanese Laid-Open Patent Application 2005-76873, for example, proposes a coupling device in an effort to address this problem.  FIG. 2  is a schematic diagram of the coupling device  500 B proposed in Japanese Laid-Open Patent Application 2005-76873.  FIG. 3  is a perspective view of the coupling device  500 B. As shown in  FIG. 2 , the coupling device  500 B includes a drive-side coupling  502 B and a driven-side coupling  505 B. The drive-side coupling  502 B includes a cylindrical drive-side base  503 B which is secured to a driving shaft  508 B. A plurality of driving claws  504 B are disposed on an outer peripheral surface of the drive-side coupling  502 B. The driven-side coupling  505 B includes a cylindrical driven-side base  506 B, which is secured to a driven shaft  509 B. A plurality of driven claws  507 B are disposed on an inner peripheral surface of the driven-side base  506 B. As shown in  FIG. 3 , on a tip portion of each of the driving claws  504 B are formed slant faces  536 B. Tips  538 B of each of the driving claws  504 B are linearly extended along a normal line from the drive-side base  504 B. On a tip portion of each driven claws  507 B are formed the slant faces  536 B slanted in the same direction when facing each driving claw  504 B. Accordingly, tips  535 B of each of the driven claws  507 B are linearly extended from the driven-side base  506 B to a shaft center. 
   In a case where the driven claws  507 B and the driving claws  504 B come into contact when the driven-side coupling  505 B is connected to the drive-side coupling  502 B, the slant faces  535 B of the driven claws  507 B come into contact with the slant faces  536 B of the driving claws  504 B. When the slant plates  535 B of the driven claws  507 B and the slant faces  536 B of the driving claws  504 B come into contact, pressure force in which the driven-side coupling  505 B is pressed in the shaft direction is dispersed to the rotation direction and the shaft direction. Consequently, if the driven-side coupling  505 B continues to be pressed in the shaft direction, the slant faces  535 B of the driven claws  507 B and the slant faces  536 B of the driving claws  504 B slide along one another, rotating either one of the driven-side coupling  505 B or the drive-side coupling  502 B. As a result, each of the driven claws  507 B advances between each of the driving claws  504 B so that each of the driving claws  504 B and each of the driven claws  507 B are engaged. 
   In the coupling device  500 B proposed in Japanese Laid-Open Patent Application 2005-76873, without the compression spring  512 A, the driven claws  507 B and the driving claws  504 B may be engaged, even if the driven claws  507 B and the driving claws  504 B come into contact when the driven-side coupling  505 B is connected to the drive-side coupling  502 B. As a result, a number of parts and the size of the apparatus may be reduced. 
     FIG. 4  illustrates the way a driven unit  510 B, including the driven-side coupling  505 B of the coupling device  500 B, is attached to a rear plate  516 B. As illustrated in  FIG. 4 , a hole  517 B through which a shaft  511 B of the driven-side unit  510 B is inserted is provided on the rear plate  516 B. When the driven unit  510 B is mounted in the apparatus main body, the driven unit  510 B is shifted in the shaft direction so as to insert the shaft  511 B into the hole  517 B and determine the position of the driven unit  510 B. Subsequently, the driven-side coupling  505 B and the drive-side coupling  502 B are connected. In a case where the driven-side coupling  505 B and the drive-side coupling  502 B are connected after the position of the driven unit  510 B is determined, there is a possibility that the shaft center of the drive-side coupling  502 B and the shaft center of driven-side coupling  502 B are misaligned. That is, because the position of the driven unit  510 B is determined in advance, parts accuracy, accumulation of dimensional tolerance of the hole  517 B or the like, and deflection due to unit stiffness may cause misalignment of the shaft centers. 
     FIG. 5  illustrates the driven-side coupling  505 B seen from an E-direction of  FIG. 2 , and a dash-double dotted line in  FIG. 5  indicates the drive-side coupling  502 B. As illustrated in  FIG. 5 , in a case where the shaft centers of the drive-side coupling  502 B and the driven-side coupling  505 B correspond, the linear tips  538 B of the driving claws  504 B are parallel with the linear tips  540 B of the driven claws  507 B. As illustrated in  FIG. 5 , in a case where the linear tips  538 B of the driving claws  504 B are parallel with the linear tips  540 B of the driven claws  507 B, the driven-side coupling  505 B is connected to the drive-side coupling  502 B without the linear tips  538 B of the driving claws  504 B hitting the linear tips  540 B of the driven claws  507 B. 
   On the other hand, as shown in  FIG. 6 , in a case where the shaft centers of the drive-side coupling  502 B and the driven-side coupling  505 B are misaligned, as marked by the line X in  FIG. 6 , the linear tips  538 B of the driving claws  504 B and the linear tips  540 B of the driven claws  507 B will cross. In other words, because both tips of the driving claws and the driven claws are linear, when the shaft centers are misaligned, the angle of the tips  538 B of the driving claws  504 B and the angle of the tips  540 B of the driven claws  507 B will not correspond, thereby making the tips  538 B of the driving claws  504 B and the tips  540 B of the driven claws  507 B crossed. Accordingly, when the driven-side coupling  504 B is connected to the drive-side coupling  502 B while the shaft centers of the drive-side coupling  502 B and the driven-side coupling  505 B are misaligned, the linear tips  540 B of the driven claws  507 B will collide with the linear tips  538 B of the driving claws  504 B, and the driven-side coupling  505 B may not be moved to the drive-side coupling. As a result, there is a problem that the driven-side coupling  505 B and the drive-side coupling  502 B may not be connected. In addition, when forcibly trying to connect the driven-side coupling  505 B and the drive-side coupling  502 B while the driving claws and the driven claws are collided, this may cause a distortion or damage to the tips of claws or the driven-unit  510 B. 
   SUMMARY 
   In view of the foregoing, an exemplary embodiment of the present invention provides an image forming apparatus which includes a driving source, a rotary element and a coupling device configured to connect the driving source with the rotary element. 
   An exemplary embodiment of a coupling device may include an inner coupling having a first rotating shaft and a first group of claws arranged in a circumferential direction with even space therebetween, and an outer coupling having a second rotating shaft and a second group of claws arranged in a circumferential direction with even space therebetween. The coupling device is configured to alternately mesh with the first group of claws in the circumferential direction when the outer coupling is moved towards the inner coupling along in an axial direction, such that the second group of claws face the first group of claws and the second axis of the outer coupling is approximately in line with the first axis of the inner couplings. At least one of the first and second groups of claws includes claws having a pointed top. 
   An exemplary embodiment of the coupling device may further include the inner coupling including a first base mounted to one end portion of the first shaft, and on which the first group of claws are disposed in the circumferential direction. 
   An exemplary embodiment of the coupling device may further include the outer coupling including a second base mounted to one end portion of the second shaft, and on which the second group of claws are disposed in the circumferential direction. 
   In one exemplary embodiment of the image forming apparatus, one of the inner and outer couplings is connected to the driving source and another one of the inner and outer couplings is connected to the rotary element to be driven by power of the driving source through the inner and outer couplings. 
   In one exemplary embodiment of the image forming apparatus, one of the first and second groups of claws includes slant-top-surface claws having a slant top surface which faces the pointed claws. The pointed claws slide along the slant top surface of the slant-top-surface claws when the pointed claws collide with the slant-top-surface claws. The outer coupling is further moved towards the inner coupling along in the axial direction so that one of the inner and outer couplings is rotated until the first and second groups of claws are engaged with each other. 
   In one exemplary embodiment, a coupling device may include an inner coupling having a first rotating shaft and a first group of claws arranged in a circumferential direction with even space therebetween. The outer coupling may have a second rotating shaft and a second group of claws arranged in a circumferential direction with even space therebetween and configured to alternately mesh with the first group of claws in the circumferential direction when the outer coupling is moved towards the inner coupling along an axial direction such that the second group of claws faces the first group of claws and that the second axis of the outer coupling is approximately in line with the first axis of the inner couplings. At least one of the first and second groups of claws may include claws having a pointed top. 
   An exemplary embodiment of the coupling device may include the inner coupling including a first base having a cylindrical shape mounted to one end portion of the first rotating shaft, and on which the first group of claws are disposed in the circumferential direction. 
   An exemplary embodiment of the coupling device may include the outer coupling including a second base having a cylindrical shape mounted to one end portion of the second rotating shaft, and on which the second group of claws are disposed in the circumferential direction. 
   In one exemplary embodiment of the coupling device, one of the inner and outer couplings is connected to a driving source and another one of the inner and outer couplings is connected to a rotary element to be driven by power of the driving source through the inner and outer couplings. 
   In one exemplary embodiment of the coupling device, at least one of the first and second groups of claws includes semispherical tips thereof. 
   In one exemplary embodiment of the coupling device, at least one of the first and second groups of claws includes conical tips thereof. 
   In one exemplary embodiment of the coupling device, one of the first and second groups of claws includes slant-top-surface claws having a slant top surface which faces the pointed claws, and the pointed claws slide along the slant top surface of the slant-top-surface claws when the pointed claws collide with the slant-top-surface claws and the outer coupling is further moved towards the inner coupling along in the axial direction so that one of the inner and outer couplings is rotated until the first and second groups of claws are engaged with each other. 
   In one exemplary embodiment of the coupling device the pointed claws are arranged at positions different in the axial direction and the slant-top-surface claws are correspondingly arranged at positions in the axial direction so that collisions of facing pairs of the pointed claws and the slant-top-surface claws sequentially occur. 

   
     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 a schematic diagram of one example of a related art coupling device; 
       FIG. 2  is a schematic diagram of another related art coupling device; 
       FIG. 3  is a perspective view of the related art coupling device of  FIG. 2 ; 
       FIG. 4  is a schematic diagram of an example use of a related art driven unit; 
       FIG. 5  is a schematic diagram illustrating the related art coupling device of  FIG. 2  in which a drive-side and a driven-side couplings are engaged with each other when their shaft centers correspond; 
       FIG. 6  is a schematic diagram illustrating the related art coupling device of  FIG. 2  in which the drive-side and the driven-side couplings are engaged with each other when their shaft centers are misaligned; 
       FIG. 7  is a schematic diagram of a laser printer according to one exemplary embodiment of the present invention; 
       FIG. 8  is an enlarged view of a process unit for a color of yellow utilized in the laser printer of  FIG. 7 ; 
       FIG. 9  is a schematic diagram illustrating a manner in which the process unit of  FIG. 8  is mounted in the laser printer of  FIG. 7 ; 
       FIG. 10  is a schematic diagram illustrating a manner in which a fixing unit and a transfer unit are pulled out of the laser printer of  FIG. 7 ; 
       FIG. 11  is a cross sectional view of a drive-connecting section of the fixing unit of  FIG. 10 ; 
       FIG. 12  is a schematic diagram illustrating a manner in which a driven-side coupling attached to a tip of a driven shaft of the fixing unit of  FIG. 10  is connected to a drive-side coupling provided in the laser printer of  FIG. 7 ; 
       FIG. 13  is a schematic diagram of a coupling device according to one exemplary embodiment of the present invention, including the drive-side and the driven-side couplings of  FIG. 12 ; 
       FIG. 14  is a front view of the drive-side coupling of the coupling device of  FIG. 13 ; 
       FIG. 15  is a front view of the driven-side coupling of the coupling device of  FIG. 13 ; 
       FIG. 16  is a schematic diagram of one example variation of the coupling device of  FIG. 13 ; 
       FIG. 17  is a schematic diagram of another example variation of the coupling device of  FIG. 13 ; 
       FIG. 18  is a schematic diagram of another example variation of the coupling device of  FIG. 13 ; 
       FIG. 19  is a schematic diagram of another example variation of the coupling device of  FIG. 13 ; 
       FIG. 20  is a schematic diagram of another example variation of the coupling device of  FIG. 13 ; 
       FIG. 21  is a schematic diagram of another example variation of the coupling device of  FIG. 13 ; 
       FIG. 22  is a schematic diagram of a driven-side base of the example variation of  FIG. 21 , seen from a drive-side; 
       FIG. 23  is a schematic diagram of another example variation of the coupling device of  FIG. 13 ; and 
       FIG. 24  is a schematic diagram of another example variation of the coupling device of  FIG. 13 . 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   In describing exemplary 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 designate identical or corresponding parts throughout the several views, particularly to  FIG. 7 , a color laser printer according to an exemplary embodiment of the present invention is described. 
   A description will now be given of one exemplary embodiment of the present invention, which is applied to an electrophotographic-type tandem color laser printer (hereinafter referred to as printer) serving as an example of an image forming apparatus. 
   As illustrated in  FIG. 7 , the laser printer includes four process units  1 Y, M, C and BK for creating color images of yellow (Y), magenta (M), cyan (C) and black (BK), respectively. Thereafter, letters Y, M, C and BK noted after reference numerals indicate respective colors. In addition to the process units  1 Y,  1 M,  1 C and  1 K, an optical writing unit  10 , a transfer unit  11 , a pair of resist rollers  19 , three sheet feeding cassettes  20 , a fixing unit  21  and so forth are arranged in the laser printer. 
   The optical writing unit  10  includes four optical writing devices. Each optical writing device includes a light source, a polygon mirror, an f-θ lens and a reflective mirror, and so forth. Based on image data, a laser beam is emitted onto a later-described photoreceptor surface. 
     FIG. 8  is an enlarged view of a general structure of a process unit  1 Y for a color yellow among the process units  1 Y, M, C and BK. It should be noted that since the structure is the same for other process units  1 M, C and BK, a description for the process units  1 M, C and BK is omitted. The process unit  1 Y of  FIG. 8  includes a drum-shaped photoreceptor  2 Y, a charging device  30 Y, a developing apparatus  40 Y, a drum cleaning apparatus  48 Y and so forth. 
   The charging device  30 Y uniformly charges the drum surface by causing a charging roller, to which alternating current voltage is applied, to rub against the photoreceptor  2 Y. The surface of the photoreceptor  2 Y, to which a charging process has been performed, is scanned and irradiated by a laser beam which has been modulated and deflected by the optical writing unit  10 . Subsequently, an electrostatic latent image is formed on the drum surface. Then, the formed electrostatic latent image is developed by the developing apparatus  40 Y and becomes a Y toner image. 
   The developing apparatus  40 Y includes a developing roller  42 Y disposed such that a portion thereof is exposed from an opening of a casing. The developing apparatus  40 Y also includes a primary conveyance screw  43 Y, a secondary conveyance screw  44 Y, a doctor blade  45 Y, a toner density sensor (hereinafter referred to as T sensor)  46 Y and so forth. 
   In the casing, a dual-component developer including magnetic carrier particles and a Y toner for negative charging is stored. The dual-component developer is conveyed by the primary conveyance screw  43 Y and the secondary conveyance screw  44 Y, while being agitated and triboelectrically charged. Subsequently, the dual-component developer is carried onto the developing roller  42 Y. Then, the dual-component developer is conveyed to a developing area facing the photoreceptor  2 Y after the thickness of a layer of the developer is regulated by the doctor blade  45 Y, and the Y toner is adhered to the electrostatic latent image on the photoreceptor  2 Y in the developing area, thereby forming a Y toner image on the photoreceptor  2 Y. The dual-component developer from which the Y toner is consumed during the developing process is recovered to the casing along a rotating motion of the developing roller  42 Y. 
   A partition  47 Y is provided between the primary screw  43 Y and the secondary screw  44 Y. By the partition  47 Y, a primary supply part for storing the developing roller  42 Y, the primary conveyance screw  43 Y and so forth, and a secondary supply part are separated in the casing. The primary conveyance screw  43 Y is rotatively driven by a not-shown driving mechanism to supply the dual-component developer in the primary supply part to the developing roller  42 Y, conveying the developer from the front shown in  FIG. 8  to the back. The dual-component developer conveyed near the end portion of the primary supply part advances to the secondary supply part through a not-shown opening part provided in the partition  47 Y. In the secondary supply part, the secondary conveyance screw  44 Y is rotatively driven by a not-shown driving mechanism to convey the dual-component developer, which is transferred from the primary supply part in a direction opposite to the primary conveyance screw  43 Y. The dual-component developer, which is conveyed near the end of the secondary supply part by the secondary conveyance screw  44 Y, is recovered to the primary supply part through another not-shown opening part provided in the partition  47 Y. 
   The toner density sensor (T sensor)  46 Y formed of a magnetic sensor which measures magnetic permeability is provided in the bottom wall near the center of the secondary supply part, and outputs a voltage corresponding to the magnetic permeability of the dual-component developer which passes above the T sensor  46 Y. In other words, the T sensor  46 Y outputs the voltage corresponding to the density of the Y toner so as to indicate a certain correlation between the magnetic permeability of the dual-component developer and the toner density. The value of the output voltage is sent to a not-shown control unit. The control unit includes a RAM which stores a Vtref for yellow which is a target value of the output voltage from the T sensor  46 Y. In addition, the RAM stores data of a Vtref for magenta, a Vtref for cyan and a Vtref for black which are also target values of the output voltage from the not-shown T sensors mounted in other developing apparatus. The Vtref for yellow is used for drive control of a not-shown Y toner conveyance apparatus. In particular, the control unit drives and controls the not-shown Y toner conveyance apparatus to supply the Y toner to the secondary supply part  49 Y, such that the value of the output voltage from the T sensor  46 Y is brought closer to the Vtref for yellow. By this toner supply process, the toner density of the Y toner of the dual-component developer in the developing apparatus  40 Y is maintained within a predetermined range. The similar toner supply control is performed for developing apparatus of other process units. 
   The Y toner image formed on the photoreceptor  2 Y for yellow is transferred onto transfer paper which is conveyed to a later-described paper conveying belt. After the transfer, transfer residual toner is removed from the surface of the photoreceptor  2 Y by the drum cleaning apparatus  48 Y, and thereafter the surface of the photoreceptor  2 Y is discharged by a not-shown discharging device. Subsequently, the surface of the photoreceptor  2 Y is uniformly charged by the charging device  30 Y to prepare for a subsequent image forming process. The same process applies to other process units. Each of the process units  1 Y, M, C and BK is removable with respect to the printer main body, and can be replaced when the product life is reached. 
   As shown in  FIG. 7 , the transfer unit  11  includes a paper conveying belt  12 , a driving roller  13 , a pulley  14 , four transfer bias rollers  17 Y, M, C and BK, and so forth. The paper conveying belt  12  is tightly stretched by the driving roller  13  and the pulley  14  and endlessly moved counterclockwise in  FIG. 7 , by the driving roller  13  rotated by a not-shown driving system. Transfer bias is applied to the-four transfer bias rollers  17 Y, M, C and BK from a not-shown power source. Then, the paper conveying belt  12  is pressed from the backside thereof against the photoreceptors  2 Y, M, C and BK to form transfer nips, respectively. In each transfer nip, due to the transfer bias effect, a transfer electric field is formed between the photoreceptor and the transfer bias roller. The Y toner image formed on the photoreceptor  2 Y for yellow is transferred onto the transfer paper P that is conveyed on the paper conveying belt  12  due to the transfer electric field and the nip pressure. On the Y toner image, the toner images for M, C and BK formed on the photoreceptors  2 M, C and BK are sequentially transferred, overlapping one another. By the overlap transfer process, a full-color toner image as opposed to a white color of the paper is formed on the transfer paper P that is conveyed on the paper conveying belt  12 . 
   At the bottom of the transfer unit  11 , three sheet feeding cassettes which store a plurality of transfer paper P in piles are stacked, and in each cassette, the transfer paper P on the top of the pile is pressed against the paper supply roller. When the paper supply roller is rotatably driven at a predetermined timing, the transfer paper P on the top of the pile is supplied to a paper conveying path. 
   Next, a description will be given of a pair of resist rollers. The transfer paper P supplied to the paper conveying path from the sheet feeding cassette  20  is conveyed between a pair of resist rollers  19 . The pair of resist rollers  19  transfer, at a timing in which the toner images can be overlapped in each nip, the transfer paper P conveyed between the rollers. Accordingly, toner images are transferred onto the transfer paper P in each transfer nip, overlapping one another. 
   The fixing unit  21  includes a heating roller  21   a  having a heat source, such as a halogen lamp and the like inside, and a pressure roller  21   b  which is pressure-welded with the heating roller  21   a , so as to form a fixing nip. The full-color image is fixed on the surface of the transfer paper P, while the transfer paper P is conveyed in the fixing nip. The transfer paper P, which has passed the fixing unit  21 , is ejected outside the printer through a pair of not-shown paper ejecting rollers. 
   In an exemplary embodiment of the present invention, as shown in  FIG. 10 , the fixing unit  21  and the transfer unit  11  can be pulled out from the printer main body by a pull-out mechanism in the printer main body. The fixing unit  21  or the transfer unit  11  are pulled out from the inside of the printer main body and detached from the printer main body. 
   Next, a description will be given of a distinguishing structure of the laser printer of the exemplary embodiment.  FIG. 11  is a cross sectional view of a drive connecting portion of the fixing unit  21  serving as a driven unit. The fixing unit  21  of  FIG. 11  includes a case  22 , the fixing roller  21   a  disposed inside the case  22  and the not-shown pressure roller  21   b , which is pressure-welded with the fixing roller  21   a , and rotatably driven in conjunction with the fixing roller  21   a . A roller gear  24  is fixed to a shaft  23  of the fixing roller  21 , and a coupling gear  25 , which is fixed to a driven shaft  26  rotatably secured on the side of the case  22 , is meshed with the roller gear  24 . At the tip of the driven shaft  26 , a driven-side coupling  51  serving as a second coupling of a coupling device  50  is concentrically fixed. Furthermore, one end of the shaft  23  of the fixing roller  21  is rotatively secured on a faceplate  28 , which is detachably attached to a front-side panel  27  of the printer main body. The other end of the shaft  23  is rotatively supported in a hole  29   a  on the rear plate  29 . 
   The driving apparatus  60 , serving as a driving unit, is fixed to the rear plate  29  of the image forming apparatus main body and includes a support panel  61 , a drive motor  62  serving as a drive source, a transmission mechanical section  63  and a driving shaft  64 . The drive motor  62  is fixed to the support panel  61 . The transmission mechanical section  63  includes a driving gear  63   a , a driving pulley  63   c , a driven pulley  63   d  and a timing belt  63   e . The driving gear  63   a  is fixed to a rotating shaft  63   b , which is rotatively supported by the support panel  61  and the rear plate  29 , and is meshed with an output gear  62   a  extending from the drive motor  62 . The driving pulley  63   c  is fixed to the rotating shaft  63   b , and the timing belt  63   e  is tightly stretched between the driving pulley  63   c  and the driven pulley  63   d . The driven pulley  63   d  is fixed to the driving shaft  64  which is rotatively supported by the support panel  61  and the rear plate  29 . Rotation of the drive motor  62  is transmitted to the driving shaft  64  through the output gear  62   a , the driving gear  63   a , the rotating shaft  63   b , the driving pulley  63   c , the timing belt  63   e  and the driven pulley  63   d . Furthermore, on the end portion of the driving shaft  64  at the fixing roller side, a drive-side coupling  52  serving as a first coupling of a coupling is concentrically fixed. 
   As shown in  FIG. 11 , in a state where the fixing unit  21  is mounted in the printer main body, the drive-side coupling  52  is inserted into the driven-side coupling  51 . The drive-side coupling  52  is engaged with the driven-side coupling  51  in a rotation direction. Accordingly, the rotation of the driving shaft  64  is transmitted to the driven shaft  26  through the coupling device  50 , and the fixing roller  21   a  is rotatively driven. Then, the rotation of the fixing roller  21   a  is transmitted to the pressure roller  21   b  through a not-shown transmission apparatus, and the pressure roller  21   b  is rotated. 
   The driven-side coupling  51  provided at the tip of the driven shaft  26  of the fixing unit  21  is connected to the drive-side coupling  52  provided at the end of the driving shaft  64  of the driving apparatus  60 , when the fixing unit  21  is pulled into the apparatus main body as shown in  FIG. 12 . 
   Next, a description will be given of the coupling device  50 .  FIG. 13  is a plan view illustrating the coupling device  50  when the driven-side coupling device  51  is separated from the drive-side coupling  52 . The drive-side coupling  52  includes a drive-side base  52   a  serving as a first base formed in a cylindrical shape, and a plurality of driving claws  52   b  serving as first claws, which are aligned on a peripheral surface of the drive-side base  52   a  and integrally formed therewith. Three driving claws  52   b  are provided, as shown in  FIG. 14 , and disposed at each of three positions of an evenly divided circumference of the drive-side base  52   a , respectively. The tip portion of the driving claws  52   b  is in a semispherical shape, and tips  52   c  of the driving claws  52   b  are pointed. Furthermore, the tip  52   c  of one of the plurality of the driving claws  52   b  of the drive-side coupling  52  is disposed at a position more adjacent to the tips  52   d  of the drive-side base  52   a  than the tips  52   c  of other driving claws  52   b.    
   The driven-side coupling  51  includes a driven-side base  51   a  serving as a second base formed in a tubular shape having one open end, and a plurality of driven claws  51   b  serving as the second claws, which are aligned on an inner peripheral surface of the driven-side base  51   a  and integrally formed with the driven-side base  51   a . Three driven claws  51   b  are provided, as shown in  FIG. 15 , and disposed at each of three positions of an evenly divided circumference of the driven-side base  51   a , respectively. At the tip portion of the driven claws  51   b , slant faces  51   c  are formed. Tips  51   d  of a plurality of the driven claws  51   b  are linearly extended to the shaft center. Furthermore, the tips  51   d  of the plurality of the driven claws  51   b  are all located at the same position in the direction of the shaft center of the driven-side coupling  51 . The positions of all the tips  51   d  shown in  FIG. 13  correspond to the positions of a tip  51   e  of the driven-side base  51   a.    
   As shown in  FIG. 11 , when the driven-side coupling  51  and the drive-side coupling  52  are connected, one end of the cylindrical drive-side base  52   a  enters inside the tubular driven-side base  51   a  having one open end so that the driving claws  52   b  and the driven claws  51   b  are meshed with each other. 
   Next, a description will be given of a connection of the driven-side coupling  51  and the drive-side coupling  52 . First, when the fixing apparatus  21  is mounted in the printer main body, the shaft  23  of the fixing roller  21  is inserted into the hole  29   a  on the rear plate  29  so as to determine the position of the fixing apparatus  21 . Next, in order to connect the driven-side coupling  51  to the drive-side coupling  52 , the fixing apparatus  21  is slid toward the back thereof. The driven-side coupling  51  is shifted in B direction indicated by an arrow in  FIG. 13  with respect to the drive-side coupling  52 . In a case where the slant faces  51   c  of the driven claws  51   b  come into contact with the tips  52   c  of the driving claws  52   b  at this time, first, one of the tips  52   c  of the driving claws  52   b , which is more protruded than other driving claws  52   b , comes into contact with only one of the slant faces  51   c  of the driven claws  51   b . In this case, since one of the tips  52   c  of the driving claws  52   b  initially comes into contact with one of the slant faces  51   c  of the driven claws  51   b , pressure force in which the driven-side coupling is pushed against the driving shaft side is concentrated in the protruded tip  52   c  of the driving claw  52   b . Accordingly, with a little pressure force, at least either the drive-side coupling  52  or the driven-side coupling  51  rotates. The tip  52   c  of the driving claw  52   b  is guided between the driven claws  51   b , relatively moving on the slant faces  51   c  of the driven claws  51   b . Assuming that when the driven-side coupling  51  is connected to the drive-side coupling  52 , a plurality of the tips  52   c  of the driving claws  52   b  come into contact with a plurality of the slant faces  51   c  of the driven claws  51   b  all at the same time, the pressure force is dispersed across the plurality of driving claws  52   b . Consequently, unless the driven-side coupling  51  is strenuously pressed, at least either the drive-side coupling  52  or the driven-side coupling  51  will not rotate, and the tips  52   c  of the driving claws  52   b  cannot relatively move on the slant faces  51   c  of the driven claws  51   b . As a result, it is difficult to connect the driven-side coupling  51  and the drive-side coupling  52 . In the coupling device of this exemplary embodiment, when connecting, one driving claw  52   b  is connected to one driven claw  51   b  so that the driven-side coupling  51  and the drive-side coupling  52  are easily connected when compared with a coupling device in which a plurality of driving claws  52   b  and a plurality of the driven claws  51   b  come into contact all at the same time,. 
   Furthermore, since the tip portion of the driving claws  52   b  is in a semispherical shape having pointed tips  52   c , one of the tips  52   c  of the driving claws  52   b  point-contacts one of the slant faces  5   c  of the driven claws  51   b . As a result, friction between the driving claws  52   b  and the slant faces  51   c  of the driven claws  51   b  is reduced, making it possible to connect the driven-side coupling  51  to the drive-side coupling  52  with less pressure force. 
   In addition, in a case where the tips  52   c  of the driving claws  52   b  come into contact with the slant faces  51   c  of the driven claws  51   b  when connecting the drive-side coupling  52  and the driven-side coupling  51 , either the drive-side coupling  52  or the driven-side coupling  51  rotates. When the drive-side coupling  52  rotates, rotational load is generated due to inertial forces of the timing belt  63   e , each gear  62   a  and  63   b , and so forth. Consequently, in order to rotate the drive-side coupling  52 , it is necessary to strenuously press the driven-side coupling  51 . Furthermore, when connecting the drive-side coupling  52  and the driven-side coupling  51 , in a case where the driven-side coupling  51  is rotated, rotational load is generated due to inertial forces of the fixing roller  21   a  and so forth. Therefore, in a case where the driven-side coupling  51  is rotated when connecting the drive-side coupling  52  and the driven-side coupling  51 , the driven-side coupling  51  will not rotate, unless the driven-side coupling  51  is strenuously pressed further back of the apparatus. 
   Consequently, as shown in  FIG. 11 , an electromagnetic clutch  65  is provided between the driving shaft  64  and the driven pulley  63   d . When the driving force of the drive motor  62  is transmitted to the fixing roller  21   a , the electromagnetic clutch  65  is turned on so as to connect the driving shaft  64  and the driven pulley  63   d . On the other hand, when connecting driven-side coupling  51  and the drive-side coupling  52 , the electromagnetic clutch is turned off so that the driving shaft  64  can freely rotate with respect to the driven pulley  63   d . Accordingly, when connecting the driven-side coupling  51  and the drive-side coupling  52 , the only rotational load on the drive-side coupling  52  will be the inertial force of the driving shaft  64 . Therefore, when compared with a case in which no electromagnetic clutch  65  is provided, the rotational load can be reduced. As a result, when connecting, without strenuously pressing the driven-side coupling  51 , the drive-side coupling  52  rotates, making it possible to connect the driven-side coupling  51  to the drive-side coupling  52 . 
   Instead of the electromagnetic clutch  65 , a one-way clutch may be provided between the driving shaft  64  and the driven pulley  63   d  to reduce the rotational load of the drive-side coupling  52 . In this case, the rotation direction of the driving shaft  64 , when the drive-side coupling  52  and the driven-side coupling  51  are connected, is set to be different from the rotation direction of the driving shaft  64  when the driving force of drive motor  62  is transmitted to the fixing roller  21   a . Specifically, by the inclination direction of the inclination plates  51   c  of the driven claws  51   b , the rotation direction of the drive-side coupling  52  when connecting the drive-side coupling  52  and the driven-side coupling  51 , and the rotation direction of the drive-side coupling  52  when the driving force is transmitted, can be changed. Furthermore, in the rotation direction of the driving shaft  64  when connecting the drive-side coupling  52  and the driven-side coupling  51 , the driving shaft  64  is configured to freely rotate with respect to the driven pulley  63   d . In the rotation direction of the driving shaft  64  when being driven, the one-way clutch is set such that the driven pulley  63   d  and the driving shaft  64  rotate together. Accordingly, when the driven-side coupling  51  and the drive-side coupling  52  are connected, the rotational load on the drive-side coupling  52  is reduced. Without strenuously pressing the driven-side coupling  51 , the drive-side coupling  52  rotates, making it possible to connect the driven-side coupling  51  to the drive-side coupling  52 . 
   As described above, when the fixing unit  21  is mounted in the printer main body, the shaft  23  of the fixing roller  21   a  is inserted into the hole  29   a  of the rear plate  29  so as to determine the position of the fixing unit  21 . Subsequently the driven-side coupling  51  and the drive-side coupling  52  are connected. Consequently, due to accumulation of the dimensional tolerance of the hole  29   a  and the like, when connecting the driven-side coupling  51  to the drive-side coupling  52 , there is a possibility that the shaft center of the driven-side coupling  51  does not align with the shaft center of drive-side coupling  52 . However, in this exemplary embodiment, the tip portion of the driving claws  52   b  is in a semispherical shape having pointed tips  52   c . Consequently, even in a state where there is a misalignment between the shaft center of the driven-side coupling  51  and the shaft center of the drive-side coupling  52  when connecting the driven-side coupling  51  and the drive-side coupling  52 , the tips  52   c  of the driving claws  52   b  are guided to the slant faces  51   c  of the driven claws  51   b , thereby connecting the driven-side coupling  51  to the drive-side coupling  52 . In other words, since the tip portion of the driving claws  52   b  is in a semispherical shape forming pointed tips  52   c , the tips  52   c  of the driving claws  52   b  do not cross the linear tips  51   d  of the driven claws  51   b . As a result, even if there is a misalignment between the shaft center of the driven-side coupling  51  and the shaft center of the drive-side coupling  52 , the driven-side coupling  51  can be pressed to the drive-side, so that it is ensured that the driven-side coupling  51  is connected to the drive-side coupling  52 . 
   As described above, to avoid crossing of the tips  52   c  of the driving claws  52   b  and the tips  51   d  of the driven claws  51   b , either the tips  52   c  of the driving claws  52   b  or the tips  51   d  of the driven claws  51   b  may be pointed. Therefore, as shown in  FIG. 16 , the tip portion of the driving claws  152   b  may be configured to be conically shaped, or, though it is not shown, may be configured to be triangular pyramid or quadrangular pyramid in shape. By configuring the tip portion of the driving claws  152   b  to be conically shaped, such as a circular cone, it is possible to form the tips  152   c  of the driving claws  152   b  to be pointed. Thus, even if there is a misalignment between the shaft center of the driven-side coupling  151  and the shaft center of the drive-side coupling  152 , the tips  151   d  of the driven claws  151   b  and the tips  152   c  of the driving claws  152   b  will not cross, so that the driven-side coupling  151  can be connected to the drive-side coupling  152 . 
   Alternatively, as shown in  FIG. 17 , drive-side coupling  252  including slant faces  252   c  may be provided on the driving claws  252   b , and the tip portion of the driven claws  251   b  is configured to be semispherical. As shown in  FIG. 18 , it is also possible to configure the tip portion of the driven claws  351   b  of drive-side coupling  351  to be conically shaped, such as a circular cone, forming the tips  351   c  of the driven claws  351   b  to be pointed. In this case, the tips  352   d  of the driving claws  352   b  of the drive-side coupling  352  need to be aligned with the tip of the drive-side base. In the coupling device shown in  FIG. 17 , since the tips  251   c  of the driven claws  251   b  are pointed, even if there is a misalignment between the shaft center of the driven-side coupling  251  and the shaft center of the drive-side coupling  252 , the tips  251   c  of the driven claws  251   b  and the tips  252   d  of the driving claws  252   b  will not cross. In addition, as shown in  FIG. 19 , one of the plurality of the tips  451   c  of the driven claws  451   b  of the driven-side coupling  451  may be disposed at a position more adjacent to the tips  451   e  of the driven-side base than the tips  451   c  of other driven claws  451   b . Since one of the tips  451   c  of the driven claws  451   b  is more protruded toward the drive-side coupling  452  than the tips  451   c  of other driven claws  451   b , when connecting the drive-side coupling  452  and the driven-side coupling  451 , the protruded driven claw  451   b  among other driven claws  451   b  first comes into contact with one of the slant faces  452   c  of the driving claws  452   b . Consequently, pressure force in which the driven-side coupling  451  is pressed against the driving shaft side is concentrated in the tip  451   c  of the protruded driven claw  451   b  so that with a little pressure force the drive-side coupling  452  and the driven-side coupling  451  can be connected. 
   As shown in  FIG. 20 , both the tip portion of the driving claws  552   b  of drive-side coupling  552  and the tip portion of the driven claws  551   b  of driven side coupling  551  may be conically shaped, forming the tips  552   c  of the driving claws  552   b  and the tips  551   c  of the driven claws  551   b  to be pointed. In this manner, since tips of both the driving claws  552   b  and the driven claws  551   b  are pointed, even if there is a misalignment between the shaft centers, the tips  552   c  of the driving claws  552   b  and the tips  551   c  of the driven claws  551   b  will not cross. 
   As shown in  FIG. 21 , the driven-side base  651   a  of the driven-side coupling  651  and the driven claws  651   b  may be separate members. In this case, as shown in  FIG. 22 , holes  651   f  are formed at three places in a circle on the side surface of the cylindrical driven-side base  651   a  toward the drive-side coupling  652 . The driven claws  651   b  are pressed into the holes  651   f , integrating the driven-side base  651   a  with the driven claws  651   b . Alternatively, as shown in  FIG. 23 , the holes  751   f  of driven-side coupling  751  are formed on the bottom surface of the cylindrical driven-side base  751   a , and the driven claws  751   b  are pressed into the holes  751   f . Furthermore, as shown in  FIG. 24 , the drive-side base  852   a  of drive-side coupling  852  and the driving claws  852   b  may be separate members. 
   In this exemplary embodiment, a coupling device of the present invention is used to connect the fixing unit  21  and the driving apparatus  60 . However, the present invention is not limited specifically to this exemplary embodiment. For example, the coupling device of an exemplary embodiment of the present invention may be used to connect a developing unit including a developing roller and a driving apparatus for rotating the developing roller. Furthermore, the coupling device of an exemplary embodiment of the present invention may be used to connect a photoconductor and a driving apparatus for driving the photoconductor. As previously illustrated in  FIG. 9 , the coupling device of an exemplary embodiment of the present invention may also be used to connect the process units  1 Y, M, C and BK, and the photoconductor or the developing unit in the process units. In addition, the coupling device of an exemplary embodiment of the present invention may be used not only in an image forming apparatus, but also other types of devices and apparatus. 
   As described above, the drive-side coupling  52  and the driven-side coupling  51  of the coupling apparatus  50  generally are a pair of cylindrically-shaped inner coupler and outer coupler. It may also be possible to form the drive-side coupling  52  as an inner coupler, and the driven-side coupling  51  as an outer coupler and vice-versa. 
   According to the coupling device of an exemplary embodiment, at least either the driving claws serving as the first claws or the driven claws serving as the second claws are pointed in shape. Thus, when connecting the driven-side coupling which is the second coupling and the driving coupling which is the first coupling, it prevents the tips of the driven claws from colliding with the tips of the driving claws so that the driven-side coupling and the driving coupling will not be connected. 
   According to the coupling device of an exemplary embodiment, at least either the tip portion of the above-described driving claws or the tip portion of the above-described driven claws is in a semispherical shape. Therefore, the tips thereof can be formed to be pointed. 
   Furthermore, at least either the tip portion of the above-described driving claws or the tip portion of the above-described driven claws is conically shaped. Therefore, the tips thereof can be formed to be pointed. 
   According to the coupling device of an exemplary embodiment, since the tips of the driving claws are pointed, even if there is a misalignment between the shaft center of the driven-side coupling and the shaft center of the drive-side coupling, the tips of the driving claws and the tips of the driven claws will not cross. As a result, when connecting the driven-side coupling and the drive-side coupling, the tips of the driven claws are prevented from colliding with the tips of the driving claws so that the driven-side coupling and the driving coupling will not be connected. 
   Furthermore, when the tips of the driven-claws are formed to be pointed, even if there is a misalignment between the shaft center of the driven-side coupling and the shaft center of the drive-side coupling, the tips of the driving claws and the tips of the driven claws will not cross. As a result, when connecting the driven-side coupling and the drive-side coupling, the tips of the driven claws are prevented from colliding with the tips of the driving claws so that the driven-side coupling and the driving coupling will not be connected. 
   Furthermore, when both tips of the driven claws and the driving claws are formed to be pointed, even if there is a misalignment between the shaft center of the driven-side coupling and the shaft center of the drive-side coupling, the tips of the driving claws and the tips of the driven claws will not cross. As a result, when connecting the driven-side coupling and the drive-side coupling, the tips of the driven claws are prevented from colliding with the tips of the driving claws so that the driven-side coupling and the driving coupling will not be connected. 
   According to the coupling device of an exemplary embodiment, slant faces are formed on the tip portion of either the driving claws or the driven claws of which tips are not pointed. Therefore, when the driven-side coupling is pressed into the drive-side coupling in the shaft direction thereof so that the driven-side coupling is connected to the drive-side coupling, at least either the driven-side coupling or the drive-side coupling rotates. Accordingly, when connecting, even if the driven claws and the driving claws come into contact, the tips of either the driven claws or the driving claws slide along the slant faces of the other claws, so that each drive claw may be inserted into each driving claw. As a result, the driving claws and the driven claws are meshed with one another, and the driven-side coupling and the drive-side coupling are connected. 
   According to the coupling device of an exemplary embodiment, the position of each driven claw with respect to the driven-side base and the position of each driving claw with respect to the drive-side base are set such that one of the driven claws first comes into contact with one of the driving claws in a case where the driven claws and the driving claws come into contact when connecting the driven-side coupling to the drive-side coupling. When connecting, since only one of driven claws comes into contact with one of driving claws, pressure force is concentrated in this single driven claw and this single driving claw. Accordingly, without strenuously pressing the driven-side coupling to the drive-side coupling, either the driven-side coupling or the drive-side coupling can be rotated. As a result, the driven-side coupling and the drive-side coupling are easily connected. 
   According to the coupling device of an exemplary embodiment, one of the tips of the driving claws of the drive-side coupling is disposed at a position more adjacent to the tip of the drive-side base, than the tips of other driving claws. Accordingly, in a case where the driven claws and the driving claws come into contact when connecting the driven-side coupling to the drive-side coupling, one of the driven claws and one of the driving claws first come into contact. 
   Furthermore, even if one of the tips of the driven claws of the driven-side coupling may be disposed at a position more adjacent to the tip of the driven-side base, than the tips of other driven claws, one of the driven claws and one of the driving claws first come into contact in a case where the driven claws and the driving claws come into contact when connecting the driven-side coupling to the drive-side coupling. 
   According to the image forming apparatus of an exemplary embodiment, since the above-described coupling device is provided, the rotating shaft of the driven unit is easily connected to the driving shaft of the driving unit. 
   According to the image forming apparatus of an exemplary embodiment, the driven coupling is provided to the driven shaft, which is a rotating shaft of an attachable/detachable unit, and the driving coupling is provided to the driving shaft, which is a rotating shaft of the apparatus main body. Consequently, a problem, in which the driven coupling and the driving coupling are not connected when the unit is mounted so that the unit fails to be mounted in the apparatus main body, may be prevented. 
   Exemplary embodiments of this invention may be conveniently implemented using a general purpose digital computer programmed according to the teachings of the present specification. Appropriate software coding can readily be prepared by programmers based on the teachings of the present disclosure. Exemplary embodiments of the present invention may also be implemented by the preparation of application specific integrated circuits or by interconnecting an appropriate network of component circuits. 
   Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that the disclosure of this patent specification may be practiced otherwise than as specifically described herein.