Patent Publication Number: US-8971762-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus of the electrophotographic type or electrostatic recording type, such as a copy machine, a multifunction printer and a laser beam printer, for example. 
     2. Description of the Related Art 
     The color-image forming apparatus of the electrophotographic type can be further divided into two types: one transfers a toner image directly from an image bearing member to a recording material; and the other transfers a toner image from an image bearing member to an intermediate transfer member in a primary transfer process, and then transfers the toner image from the intermediate transfer member to a recording material in a secondary transfer process. 
     The color-image forming apparatus of the type of transferring a toner image directly from an image bearing member to a recording material often uses an electrostatic adsorption belt for conveying the recording material, while the color-image forming apparatus of the type of transferring a toner image from an intermediate transfer member to a recording material in the secondary transfer process often uses an intermediate transfer belt. 
     In general, for the secondary transfer process to transfer a toner image to a recording material, the color-image forming apparatus has a configuration in which, by means of a secondary transfer roller, the recording material is conveyed while the secondary transfer process is being performed with the recording material being held between a secondary transfer opposing roller of the intermediate transfer member and the secondary transfer roller. 
     Some secondary transfer rollers have a configuration in which a secondary transfer unit that integrally unitizes conveying paths before and after the secondary transfer process together is spaced apart from the intermediate transfer member, thereby enabling the secondary transfer unit to be opened and closed for the clearance of jams of the recording material such as a sheet of paper, the easy replacement of the intermediate transfer member or the unit including the intermediate transfer member and the like. 
     The secondary transfer roller is required to be capable of applying high pressure on the intermediate transfer member and the secondary transfer opposing roller to achieve a good image quality and to prevent the slip of the recording material during its conveyance in the secondary transfer process. 
     In addition, for locking (securing) or holding the secondary transfer unit, some color-image forming apparatuses have a configuration in which one or more locking portions are respectively provided on both end sides of the secondary transfer unit outside the recording material conveying area. In some cases, not all the locking portions can be fully locked due to reaction force in response to the high pressure of the secondary transfer roller. 
     In order to avoid this, some color-image forming apparatuses have a configuration in which detection sensors are respectively provided on all the locking portions, and when incomplete locking of the locking portions is detected, a user is prompted to lock the locking portions again. 
     As in Japanese Patent Application Laid-Open No. 2010-286658, some color-image forming apparatuses have a configuration in which rolling members of locking portions are locked in synchronization with an openable and closable member with the help of separate leading guides. 
     However, the above prior techniques have the following problems. 
     It is expensive to provide detection sensors to all the locking portions for detecting incomplete locking of the locking portions and prompting a user to lock the locking portions again. Additionally, this approach is not desirable in terms of usability, because it requires excessive open-close operation by the user. 
     For the configuration as shown in Japanese Patent Application Laid-Open No. 2010-286658 in which rolling members of locking portions are locked in synchronization with an openable and closable member with the help of separate leading guides, it is difficult to save space because large space is needed for the movement of the leading guides. 
     SUMMARY OF THE INVENTION 
     In these contexts, a purpose of the present invention is to provide an image forming apparatus for reducing incomplete locking of an openable and closable transfer unit. 
     Another purpose of the present invention is to provide an image forming apparatus including an image bearing member, on a surface of which a toner image is formed, a main body unit configured to support the image bearing member, a transfer unit provided so as to be openable and closable with respect to the main body unit, the transfer unit being configured to hold a recording material between the transfer unit and the image bearing member and transfer a toner image formed on the image bearing member to the recording material, a first engagement portion provided to the transfer unit, the first engagement portion being configured to engage with the main body unit, an opposing member movable and opposed to the first engagement portion, and a regulation member provided to the main body unit, the regulation member being configured to regulate movement of the opposing member, wherein the opposing member contacts the regulation member when the transfer unit is closed with respect to the main body unit, and the opposing member moves away from the regulation member when the transfer unit is opened with respect to the main body unit. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustrative cross-sectional view describing a configuration of an image forming apparatus according to the present invention. 
         FIG. 2  is an illustrative front view describing a configuration in which an image bearing member is held by a main body unit. 
         FIG. 3  is an illustrative perspective view describing the relationship between a secondary transfer unit and the main body unit. 
         FIG. 4  is an illustrative cross-sectional view describing a configuration of major components of the image forming apparatus according to the present invention, with the secondary transfer unit being closed with respect to the main body unit. 
         FIG. 5  is an illustrative cross-sectional view describing states of respective components of the image forming apparatus according to the present invention in a contacting position where a transferring-side engagement portion contacts an opposing member during the process of closing the secondary transfer unit with respect to the main body unit. 
         FIG. 6  is an illustrative cross-sectional view describing a state of the image forming apparatus according to the present invention where the transferring-side engagement portion pushes and rotates the opposing member to move the opposing member away from the contacting position during the process of closing the secondary transfer unit with respect to the main body unit. 
         FIG. 7  is an illustrative cross-sectional view describing actions of respective components of the image forming apparatus according to the present invention when the secondary transfer unit is opened with respect to the main body unit. 
         FIG. 8  is a view describing force exerted in the direction of closing the secondary transfer unit with respect to the main body unit in the image forming apparatus according to the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. 
       FIG. 1  is an illustrative cross-sectional view describing a schematic configuration of an image forming apparatus  100  according to the present invention. In particular, the image forming apparatus  100  shown in  FIG. 1  is a four-color laser beam printer of the electrophotographic type and uses an intermediate transfer belt  10  as an image bearing member on the surface of which a toner image is formed. 
     The image forming apparatus  100  shown in  FIG. 1  includes a drum type electrophotographic photoconductor (hereinafter referred to as “photoconductive drum”)  1  as an image bearing member. The photoconductive drum  1  is rotatably supported by the image forming apparatus  100  and driven by a driving unit not shown in the drawings to rotate in the direction of the arrow “a”. 
     A charging roller  2  of the contact type for uniformly charging the surface of the photoconductive drum  1 , and an exposure unit  30  for emitting laser light  3  on the surface of the photoconductive drum  1  according to image information to form an electrostatic latent image are provided along the rotation direction around the photoconductive drum  1 . A developing unit  4  for transferring toner onto the electrostatic latent image to develop the electrostatic latent image as a toner image, and an intermediate transfer belt  10  to which the toner image on the surface of the photoconductive drum  1  is transferred in the primary transfer process are also provided. In addition, a cleaning unit  5  for removing toner left on the surface of the photoconductive drum  1  after the primary transfer process is provided. 
     A primary transfer roller  11  arranged inside the intermediate transfer belt  10  presses the intermediate transfer belt  10  against the surface of the photoconductive drum  1  to form a primary transfer nip portion N 1  between the photoconductive drum  1  and the intermediate transfer belt  10 . A primary transfer bias is applied to the primary transfer roller  11  by a power source not shown in the drawings. 
     A secondary transfer roller  12  arranged outside the intermediate transfer belt  10  forms a secondary transfer nip portion N 2  between the intermediate transfer belt  10  and the secondary transfer roller  12 . A secondary transfer bias voltage is applied to the secondary transfer roller  12  by a power source not shown in the drawings. Moreover, a roller charger  51  of the electrostatic type for cleaning the intermediate transfer belt  10  is arranged so as to face the intermediate transfer belt  10 . 
     A fixing unit  20  for heating and pressing the toner image transferred to a recording material  6  to fix the toner image to the recording material  6  is arranged on the downstream side of the secondary transfer nip portion N 2  in the conveying direction of the recording material  6  as shown by the arrow “b” in  FIG. 1 . 
     The photoconductive drum  1  is made by providing a photoconductive layer such as an Organic Photoconductor (OPC) on the outer surface of an aluminum cylinder. 
     The charging roller  2  includes a metal core and a conductive elastic member surrounding the metal core and is arranged in contact with the surface of the photoconductive drum  1  and rotated, and a charging bias is applied to the charging roller  2  by a power source not shown in the drawings. 
     The exposure unit  30  includes a laser oscillator, which is not shown in the drawings, for emitting the laser light  3  according to the image information, a polygon mirror  31  and a mirror  32 , and exposes the surface of the photoconductive drum  1  to form an electrostatic latent image according to the image information. 
     The developing unit  4  includes a rotating body  4 A and four color developing devices  4   a - 4   d  mounted on the rotating body  4 A, i.e., a yellow developing device  4   a , magenta developing device  4   b , cyan developing device  4   c , and black developing device  4   d . The rotating body  4 A is rotated by a driving unit not shown in the drawings. Thus, the developing unit  4  (the yellow developing device  4   a  in the case of  FIG. 1 ) is placed in a position for development where the developing unit  4  faces the surface of the photoconductive drum  1  so that the developing unit  4  can be used to develop the electrostatic latent image on the surface of the photoconductive drum  1 . In the four-color full-color image forming, the developing devices  4   a - 4   d  are placed in turns in the position for development. 
     The intermediate transfer belt  10  is formed as an endless belt and stretched between a driving roller  13  and tension rollers  14 ,  15 , which serve as three supporting roller arranged parallel to each other. The intermediate transfer belt  10  is conveyed in the direction of the arrow “c” in  FIG. 1  by the driving roller  13  rotated by a driving unit not shown in the drawings. The tension rollers  14 ,  15  are rotated by the intermediate transfer belt  10 . 
     The primary transfer roller  11  is arranged in a position opposite the photoconductive drum  1  and on the inner surface side of the intermediate transfer belt  10 , and presses the intermediate transfer belt  10  against the surface of the photoconductive drum  1  to form the primary transfer nip portion N 1 . 
     The secondary transfer roller  12  is arranged in a position opposite the driving roller  13  and on the outer surface side of the intermediate transfer belt  10 , and the secondary transfer nip portion N 2  is formed between this secondary transfer roller  12  and the surface of the intermediate transfer belt  10 . In addition, the cleaning unit  50  of the electrostatic type is arranged so as to face the surface of the intermediate transfer belt  10  on the downstream side of the secondary transfer nip portion N 2  in the conveying direction of the intermediate transfer belt and on the upstream side of the primary transfer nip portion N 1  in the conveying direction of the intermediate transfer belt  10 . The cleaning unit  50  includes the roller charger  51  arranged on the surface of the intermediate transfer belt  10  and a DC power source, which is not shown in the drawings, connected to the roller charger  51 . 
     A feeder  40  feeds the recording material  6  to an image forming portion, and comprises a recording material cassette  41  which holds multiple sheets of the recording material  6 , a feed roller  42  and a registration roller  43 . 
     The operation of the above-mentioned image forming apparatus  100  will be described below. 
     The surface of the photoconductive drum  1  driven to rotate in the direction of the arrow “a” in  FIG. 1  is uniformly charged by applying to the charging roller  2  a charging bias which is a combination of DC voltage and AC voltage. 
     When a yellow image signal is input to the laser oscillator not shown in the drawings, the laser light  3  is emitted onto the charged surface of the photoconductive drum  1  to form an electrostatic latent image. After that, when the photoconductive drum  1  rotates in the direction of the arrow “a” in  FIG. 1 , the yellow developing device  4   a  transfers yellow toner onto the electrostatic latent image on the surface of the photoconductive drum  1  to develop the electrostatic latent image as a toner image. 
     In the primary transfer process, the yellow toner image on the surface of the photoconductive drum  1  is transferred to the intermediate transfer belt  10  via the primary transfer nip portion N 1  due to the primary transfer bias applied to the primary transfer roller  11 . Once the toner image is transferred, toner left on the surface of the photoconductive drum  1  after the primary transfer process is removed by the cleaning unit  5  so that the photoconductive drum  1  can be used for the next image forming. 
     The above sequence of image forming processes: charging, exposure, development, primary transfer and cleaning are repeated for the other three colors, i.e., magenta, cyan and black to form a four-color toner image on the intermediate transfer belt  10 . 
     In the secondary transfer process, the four-color toner image on the intermediate transfer belt  10  is transferred to the recording material  6 , which is conveyed in the direction of the arrow “b” in  FIG. 1 , via the secondary transfer nip portion N 2  by way of the secondary transfer bias voltage which is applied to the secondary transfer roller  12  by a power source. 
     After the transfer of the toner image via the secondary transfer nip portion N 2 , the recording material  6  is conveyed to the fixing unit  20 , which in turn heats and presses the recording material  6  to fix the toner image to the recording material  6 , thereby providing a four-color full-color image on the recording material  6 . 
     Meanwhile, after the transfer of the toner image, some toner which was not transferred to the recording material  6  in the secondary transfer process is left on the intermediate transfer belt  10 . The toner left on the intermediate transfer belt  10  after the secondary transfer process is collected by the cleaning unit  5  via the photoconductive drum  1  by way of the function of the cleaning unit  50 . 
     In particular, the toner left after the secondary transfer process is given the opposite polarity, i.e., positive charges, by the cleaning unit  50  and thus transferred in the reverse direction to the surface of the photoconductive drum  1  via the primary transfer nip portion N 1 . The toner left after the secondary transfer process and transferred in the reverse direction is removed by the cleaning unit  5  together with the toner left on the surface of the photoconductive drum  1  after the primary transfer process. 
       FIG. 2  is an illustrative front view describing a configuration in which the intermediate transfer belt  10  is held by a main body unit  7 .  FIG. 3  is an illustrative perspective view describing the relationship between a secondary transfer unit  70  and the main body unit  7 .  FIG. 4  is an illustrative cross-sectional view describing a configuration of major components of the image forming apparatus according to the present invention, with the secondary transfer unit  70  being closed with respect to the main body unit  7 . Features of the present embodiment will be described below with reference to  FIGS. 2 to 8 . 
       FIG. 4  is a view describing a locked state of the secondary transfer unit  70  including the secondary transfer roller  12  and conveyance guides provided on the upstream and downstream sides of the secondary transfer roller  12  in the sheet conveying direction in the image forming apparatus  100 . 
     In particular, the secondary transfer unit  70  is a secondary transfer unit for transferring to the recording material  6  in the secondary transfer process a toner image which has been transferred from the photoconductive drum  1  to the intermediate transfer belt  10  in the primary transfer process. 
     As shown in  FIG. 3 , the secondary transfer unit  70  is provided so as to be openable and closable with respect to the main body unit  7  of the apparatus by rotating around a pivot axis  70   a . A toner image formed on the outer surface of the intermediate transfer belt  10  is transferred to the recording material  6  with the recording material  6  being held between the intermediate transfer belt  10  and the secondary transfer roller  12  provided so as to be rotatable in the secondary transfer unit  70 . 
     The secondary transfer unit  70  according to the present embodiment is configured to be openable and closable with respect to the main body unit  7  of the apparatus around the open-close fulcrum, pivot axis  70   a , wherein the open-close fulcrum is the first pivot hereinafter referred in this specification. Meanwhile, a locking member  75  is provided so as to be rotatable around a pivot point  75   a  in the main body unit  7  of the apparatus. A boss  70   b  provided on the secondary transfer unit  70  is fit in an engagement groove  75   c  of the locking member  75  provided so as to be rotatable around the pivot point  75   a  arranged in the main body unit  7  of the apparatus, whereby the secondary transfer unit  70  is held in the closed state. 
     The locking member  75  is configured to be rotatable around the pivot point  75   a  downward from the locking position shown in  FIG. 4 , and urged by an urging unit, e.g., a spring, which is not shown in the drawings, around the pivot point  75   a  in the clockwise direction in  FIG. 4 . 
     The secondary transfer roller  12  is urged by an urging unit, secondary transfer spring  73 , toward the intermediate transfer belt  10  wound around the driving roller  13 , and thus contacts the intermediate transfer belt to form the secondary transfer nip portion N 2 . The secondary transfer spring  73  is provided on the secondary transfer unit  70 . 
     The urging unit, secondary transfer spring  73 , urges a bearing member  72  which serves as a transferring-side engagement portion for rotatably supporting the secondary transfer roller  12  in such a direction as to enable the secondary transfer roller  12  of the secondary transfer unit  70  and the intermediate transfer belt  10  to hold the recording material  6  between them. 
     The bearing member  72 , which serves as a transferring-side engagement portion, is provided to the secondary transfer unit  70 , and engages with an engagement groove  74   c  of a hooking-receiving member  74 , which serves as an opposing member, provided so as to be rotatable around a pivot point  74   a  in the main body unit  7  of the apparatus, wherein the pivot point  74   a  is the second pivot hereinafter referred in this specification. (Hereinafter, the transferring-side engagement portion may be referred to as the first engagement portion and the engagement groove may be referred to as the second engagement portion.) The engagement groove  74   c  is formed in the shape of the letter “J.” The hooking-receiving member  74  is placed in a position opposite the bearing member  72 . The “J” shape of the engagement groove  74   c  has a guide portion  74   c   1  for guiding the bearing member  72  and a holding portion  74   c   2  for holding the bearing member  72 . 
     Reaction force exerted on the secondary transfer roller  12  which forms the secondary transfer nip portion N 2  is received by the pivot axis  70   a  via the secondary transfer unit  70  and by the locking member  75 . 
     The hooking-receiving member  74  is mounted so as to be rotatable around the second pivot  74   a  provided in the main body unit  7  of the apparatus. The hooking-receiving member  74  is held in such a manner that the hooking-receiving member  74 , due to its own weight, contacts an contact surface  76   a  of a rotation regulation member  76  provided to the main body unit  7  of the apparatus and the rotation of the hooking-receiving member  74  is regulated by the contact surface  76   a.    
     Furthermore, the bearing member  72  which holds each end of the secondary transfer roller  12 , and the engagement groove  74   c  of the hooking-receiving member  74  have a predetermined clearance. Accordingly, even if the position of the driving roller  13  or the position of the secondary transfer roller  12  changes because of poor accuracy or distortion of parts of them, the secondary transfer process is not affected by those changes. 
     Actions of the hooking-receiving member  74  at the time of closing the secondary transfer unit  70  will be described next with reference to  FIG. 5  and  FIG. 6 . 
     The bearing member  72  is configured so as to contact a contact surface  74   b  of the hooking-receiving member  74  on a path of rotation of the bearing member  72  when the secondary transfer unit  70  is closed by rotating it around the pivot axis  70   a .  FIG. 5  shows that the bearing member  72  contacts the contact surface  74   b  of the hooking-receiving member  74  and the hooking-receiving member  74  contacts the contact surface  76   a  of the rotation regulation member  76  when the secondary transfer unit  70  is closed with respect to the main body unit  7  of the apparatus. 
     The locking member  75  is provided so as to be rotatable around the pivot point  75   a  arranged in the main body unit  7  of the apparatus. A boss  75   b  is provided so as to protrude from the end portion of the locking member  75 . Meanwhile, a cam  70   c  is provided to the secondary transfer unit  70  in a position corresponding to the boss  75   b . The boss  75   b  contacts and slides along a cam surface  70   c   1  of the cam  70   c  moving integrally with the secondary transfer unit  70  when the secondary transfer unit  70  is closed by rotating it around the pivot axis  70   a . As a result, the boss  75   b  provided on the locking member  75  is pushed downward. In this manner, the locking member  75  is pushed downward in the direction of the arrow “d” in  FIG. 5  around the pivot point  75   a  against urging force exerted by an urging unit not shown in the drawings. 
     As shown in  FIG. 5 , the bearing member  72  contacts the contact surface  74   b  of the hooking-receiving member  74 . In this state, the second pivot  74   a  of the hooking-receiving member  74  is outside a circle “m” (circular arc) centered at the pivot axis  70   a  of the secondary transfer unit  70  and passing through a contacting position “k”. 
     In addition, the contacting position “k” where the bearing member  72  contacts the contact surface  74   b  of the hooking-receiving member  74  is arranged on the same side of a line “n” extending in the direction along which the bearing member  72  is urged by the secondary transfer spring  73  as the second pivot  74   a  of the hooking-receiving member (this side is hereinafter referred to as the pivot point side). 
     In other words, when the bearing member  72  contacts the contact surface  74   b  of the hooking-receiving member  74 , the second pivot  74   a  of the hooking-receiving member  74  is positioned above the contacting position “k” in  FIG. 5 . The second pivot  74   a  is positioned above the path of rotation of the bearing member  72  centered at the pivot axis  70   a  of the secondary transfer unit  70  and passing through the contacting position “k” in  FIG. 5 . The second pivot  74   a  is positioned above the line “n” extending in the direction of urging the bearing member  72  by the secondary transfer spring  73  in  FIG. 5 . 
     This configuration allows the secondary transfer unit  70  to be further rotated around the pivot axis  70   a  in the direction of the arrow “e” in  FIG. 5 , i.e., closing direction, from the position shown in  FIG. 5 . As a result, the contact surface  74   b  below the second pivot  74   a  of the hooking-receiving member  74  in  FIG. 5  is pushed by the bearing member  72 , which induces moment of rotation acting on the hooking-receiving member  74  in the direction of the arrow “f” in  FIG. 5  around the second pivot  74   a . Then, the hooking-receiving member  74  contacts the contact surface  76   a  of the rotation regulation member  76  and the rotation of the hooking-receiving member  74  is regulated by the contact surface  76   a , and thus the urging force of the secondary transfer spring  73  is received by the contact surface  76   a  of the rotation regulation member  76 , thereby holding the hooking-receiving member  74 . 
     When the secondary transfer unit  70  is further moved in its closing direction, the secondary transfer spring  73  is compressed and its spring length decreases from L 1  to L 2 , as shown in  FIG. 6 . Then, the bearing member  72  moves downward in  FIG. 6  along the contact surface  74   b  from the contacting position “k” shown in  FIG. 5  with the bearing member  72  contacting the contact surface  74   b  of the hooking-receiving member  74 . 
     At this point, the distance between the pivot axis  70   a  of the secondary transfer unit  70  and the bearing member  72  decreases as the spring length of the secondary transfer spring  73  become shorter. 
       FIG. 6  shows the positions of the bearing member  72  immediately before the bearing member  72  moves beyond the contact surface  74   b  of the hooking-receiving member  74 . At this point, the secondary transfer spring  73  is compressed and its spring length decreases from L 1  to L 2 . 
     When the bearing member  72  moves beyond the contact surface  74   b  of the hooking-receiving member  74 , the secondary transfer spring  73  expands and its length increases from L 2  to L 1  with its one end on the secondary transfer unit  70  side being fixed with respect to the secondary transfer unit  70 . Accordingly, the secondary transfer roller  12  and the bearing member  72  at each end of the secondary transfer roller  12  move in a direction toward the driving roller  13  which serves as a secondary transfer opposing roller, i.e., the closing direction of the secondary transfer unit  70 . 
     In practice, a difference between the spring lengths L 1  and L 2  of the compressed secondary transfer spring  73  tends to be smaller than a designed nominal value. This is because parts around the secondary transfer spring  73  having high spring force are not completely rigid bodies and elastically deform. 
     After the bearing member  72  moves beyond the contact surface  74   b  of the hooking-receiving member  74 , restoring force caused by those elastic deformations combines with the force exerted on the secondary transfer roller  12  and the bearing member  72  on each side of the secondary transfer roller  12  in the closing direction of the secondary transfer unit  70 . 
       FIG. 8  is a view describing force exerted in the direction of closing the secondary transfer unit  70  with respect to the main body unit  7  of the apparatus in the image forming apparatus  100 . As shown in  FIG. 8 , reaction force caused by the secondary transfer spring  73  in a position where the spring length of the secondary transfer spring  73  is L 2  is exerted on the secondary transfer unit  70 . A user produces moment of rotation (length L 3 ×force F 3 ) in the closing direction via some parts which moment is equal to or greater than moment of rotation (length L 4 ×force F 4 ) produced by the reaction force around the pivot axis  70   a.    
     This force (moment) is equal to or greater than the force (moment) required when the secondary transfer unit  70  is in the locked state because the secondary transfer spring  73  is compressed and its spring length decreases from L 1  to L 2 . In other words, L 3 ×F 3  is equal to or greater than L 4 ×F 4 . 
     By this force (moment), after the bearing member  72  moves beyond the contact surface  74   b  of the hooking-receiving member  74 , the secondary transfer unit  70  is pulled to the locking position. 
     When the locking member  75  is pushed downward by the boss  70   b  and the boss  70   b  reaches the engagement groove  75   c , the locking member  75  is urged by an urging unit not shown in the drawings to rotate in the direction of the arrow “i” in  FIG. 8  around the pivot point  75   a . Then, the boss  70   b  is fit in the engagement groove  74   c  of the locking member  75 , and thus the rotation of the secondary transfer unit  70  is stopped. At this point, the secondary transfer unit  70  is locked by the locking member  75  by way of the urging force of the secondary transfer spring  73 . 
     Actions of the hooking-receiving member  74  and components around the hooking-receiving member  74  at the time of opening the secondary transfer unit  70  shown in  FIG. 7  from its locked state shown in  FIG. 4  will be described next. 
     A lever  77  is moved in the direction of the arrow “g” in  FIG. 7  to open the secondary transfer unit  70  by a user or other parts not shown in the drawings. Accordingly, the locking member  75  is rotated in the direction of the arrow “d” in  FIG. 7  around the pivot point  75   a  from the locking position shown in  FIG. 4  to separate the boss  70   b  provided on the secondary transfer unit  70  from the engagement groove  75   c  of the locking member  75 , thereby unlocking the secondary transfer unit  70 . 
     When the secondary transfer unit  70  is unlocked from the locking member  75 , the secondary transfer unit  70  rotates and opens in the direction of the arrow “g” in  FIG. 7  around the pivot axis  70   a . In doing so, the bearing member  72  contacts the upper surface of the engagement groove  74   c  of the hooking-receiving member  74  on the path of rotation of the bearing member  72  around the pivot axis  70   a . After the bearing member  72  contacts the upper surface of the engagement groove  74   c  of the hooking-receiving member  74 , the hooking-receiving member  74  is rotated in the direction of the arrow “h” in  FIG. 7  around the second pivot  74   a  in synchronization with the action of the bearing member  72 , and thus the hooking-receiving member  74  does not interfere with opening of the secondary transfer unit  70 . 
     At this point, as the secondary transfer unit  70  is opened with respect to the main body unit  7  of the apparatus, the hooking-receiving member  74  rotates in the direction of the arrow “h” in  FIG. 7  around the second pivot  74   a  and separates from the rotation regulation member  76 . When the secondary transfer unit  70  is opened and the bearing member  72  is separated from the hooking-receiving member  74 , the hooking-receiving member  74 , due to its own weight, rotates around the second pivot  74   a  and returns to the locked state shown in  FIG. 4 . 
     The secondary transfer unit  70  is rotated in the direction of the arrow “e” in  FIG. 5  around the pivot axis  70   a  from the open position where the secondary transfer unit  70  is opened with respect to the main body unit  7  of the apparatus as shown in  FIG. 7 . In the end, the secondary transfer unit  70  is moved to an engaging position where the secondary transfer unit  70  is closed with respect to the main body unit  7  of the apparatus as shown in  FIG. 4 . 
     In doing so, as shown in  FIG. 5 , the bearing member  72  urged by the secondary transfer spring  73  contacts the contact surface  74   b  of the hooking-receiving member  74  and then rotates the hooking-receiving member  74  in the direction of the arrow “f” in  FIG. 5  around the second pivot  74   a . Subsequently, as shown in  FIG. 6 , the bearing member  72  moves beyond the contact surface  74   b  of the hooking-receiving member  74 , and the hooking-receiving member  74  moves out of the way of the bearing member  72 . 
     In addition, the hooking-receiving member  74 , which is provided so as to be rotatable around the second pivot  74   a , is configured to move in synchronization with the rotation of the bearing member  72  around the pivot axis  70   a  when the secondary transfer unit  70  is opened. This may reduce incomplete locking of the locking portion of the openable and closable secondary transfer unit  70  with a low-cost and small (space-saving) configuration. 
     The secondary transfer unit  70  can be moved with lesser force when it is opened. Furthermore, excessive open-close operation by a user due to incomplete locking can be reduced. 
     In the present embodiment, the pivot axis  70   a  of the secondary transfer unit  70  is configured so as to be rotatably supported in a circular hole, and the rotation of the secondary transfer unit  70  is stopped by engagement of the boss  70   b  with the engagement groove  75   c  of the locking member  75 . However, the pivot axis  70   a  may be configured so as to be rotatably supported in an elongated hole provided in the horizontal direction, for example, and the position of the secondary transfer unit  70  may be adjusted in one direction along the elongated hole, while the position in the height direction can be adjusted at a different portion. 
     The present invention is not limited to the open-close mechanism of the secondary transfer unit  70 . For example, the present invention can be implemented in a configuration in which, instead of the secondary transfer roller  12 , the primary transfer roller  11  shown in  FIG. 1  serving as a transfer unit can associate with and dissociate from the main body unit which holds the photoconductive drum  1  serving as an image bearing member. 
     In the present embodiment, the secondary transfer unit  70  is configured so as to move integrally with an openable and closable cover which is an openable and closable portion of the exterior of main body of the image forming apparatus  100  (main body of image forming apparatus). In other embodiments, the secondary transfer unit  70  may be configured so as to move in synchronization with an openable and closable portion of the exterior of main body of the image forming apparatus  100 . 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2012-091700, filed Apr. 13, 2012, which is hereby incorporated by reference herein in its entirety.