Patent Publication Number: US-9423749-B2

Title: Image forming apparatus with a fulcrum that pivotally supports an image bearer and detector

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 from Japanese Patent Application Nos. 2014-154996, filed on Jul. 30, 2014, and 2014-223325, filed on Oct. 31, 2014, both in the Japan Patent Office, which are hereby incorporated herein by reference in their entirety. 
     BACKGROUND 
     1. Technical Field 
     Exemplary aspects of the present disclosure generally relate to an image forming apparatus including a detector to detect conditions of an image bearer. 
     2. Description of the Related Art 
     Generally, known image forming apparatuses include an optical detector to detect conditions of an image bearer. The optical detector is disposed opposite to the image bearer with a certain space therebetween. The detection accuracy of the detector depends largely on the positional accuracy (distance) of the image bearer as a detection target and the detector. 
     SUMMARY 
     In view of the foregoing, in an aspect of this disclosure, there is provided an improved image forming apparatus including an image bearer, a detector, an image bearer support, and a fulcrum. The image bearer bears an image on a surface thereof. The detector detects a condition of the image bearer. The image bearer support supports the image bearer and includes a stopper to regulate movement of the detector. The fulcrum pivotally supports the image bearer and the detector. 
     According to another aspect, an image forming apparatus includes an image bearer, a detector, a first support, and a detector retainer. The image bearer bears an image on a surface thereof. The detector detects a condition of the image bearer. The first support pivotally supports the image bearer about a fulcrum. The detector retainer pivotally supports the detector about the fulcrum. The first support includes a stopper to regulate movement of the detector retainer. 
     The aforementioned and other aspects, features and advantages would be more fully apparent from the following detailed description of illustrative embodiments, the accompanying drawings and the associated claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be more readily obtained as the same becomes better understood by reference to the following detailed description of illustrative embodiments when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram illustrating a printer as an example of an image forming apparatus according to an illustrative embodiment of the present disclosure; 
         FIG. 2  is an enlarged schematic diagram illustrating an example of an image forming unit employed in the image forming apparatus of  FIG. 1 ; 
         FIG. 3  is a perspective view schematically illustrating an image bearer support that supports an image bearer when the image bearer support is installed on a main-body side support having a common fulcrum; 
         FIG. 4  is a side view schematically illustrating the image bearer support of Embodiment 1 installed on the main-body side support; 
         FIG. 5  is a side view schematically illustrating the image bearer support of Embodiment 1 separated from the main-body side support; 
         FIG. 6  is a side view schematically illustrating a contact-and-separation mechanism of the image bearer support in a separated state; 
         FIG. 7  is a side view schematically illustrating the contact-and-separation mechanism of the image bearer support in a contact state; 
         FIG. 8  is a schematic diagram illustrating the image bearer and a detector support that swingably supports the detector; 
         FIG. 9  is an enlarged schematic diagram illustrating the detector support at the fulcrum side; 
         FIG. 10  is a side view schematically illustrating an image bearer support of Embodiment 2 installed on the main-body side support; 
         FIG. 11  is a schematic diagram illustrating another example of an image forming apparatus; 
         FIG. 12  is a schematic diagram illustrating another example of an image forming apparatus; and 
         FIG. 13  is a schematic diagram illustrating another example of an image forming apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     A description is now given of illustrative embodiments of the present invention. It should be noted that although such terms as first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, it should be understood that such elements, components, regions, layers and/or sections are not limited thereby because such terms are relative, that is, used only to distinguish one element, component, region, layer or section from another region, layer or section. Thus, for example, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of this disclosure. 
     In addition, it should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this disclosure. Thus, for example, as used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Moreover, the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     In describing illustrative 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 have the same function, operate in a similar manner, and achieve a similar result. 
     In a later-described comparative example, illustrative embodiment, and alternative example, for the sake of simplicity, the same reference numerals will be given to constituent elements such as parts and materials having the same functions, and redundant descriptions thereof omitted. 
     Typically, but not necessarily, paper is the medium from which is made a sheet on which an image is to be formed. It should be noted, however, that other printable media are available in sheet form, and accordingly their use here is included. Thus, solely for simplicity, although this Detailed Description section refers to paper, sheets thereof, paper feeder, etc., it should be understood that the sheets, etc., are not limited only to paper, but include other printable media as well. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, exemplary embodiments of the present patent application are described. 
     In order to prevent fluctuations in the distance between the detector that detects a toner pattern formed on a belt as the image bearer and the belt, in one example, there is known an image forming apparatus that employs a support to support and to position the detector in place relative to the belt and a roller around which the belt is looped. 
     Since the rotary body, the support, and the detector are constituted as a single integrated unit and detachably attachable all together relative to the main body of the image forming apparatus, the detector is forced to be detached from the main body upon replacement of the unit and/or the detector is detached from the unit, and then the detector is assembled to a new unit, thereby complicating assembling operation. 
     In another example of the image forming apparatus, the detector is disposed on the main body side of the image forming apparatus and is not detachably attachable. The detector is swingably supported by a support that contacts a shaft of a rotary body around which the belt as the image bearer is looped. Accordingly, the distance between the detector and the belt is maintained. 
     In this configuration, the support that supports the detector is swingably disposed in the main body of the image forming apparatus and positioned in place by contacting the shaft of the roller so that the detector does not need to be replaced or reassembled. However, vibrations from the unit and the main body during printing cause displacement of the detector. 
     In view of the above, there is thus an unsolved need for an image forming apparatus capable of maintaining reliably a desired distance between the detector and the image bearer. 
     With reference to  FIG. 1 , a description is provided of an electrophotographic color printer as an example of an image forming apparatus  500  according to an illustrative embodiment of the present disclosure. The image forming apparatus of the present disclosure is not limited to a printer. The image forming apparatus includes, but is not limited to, a copier, a printer, a facsimile machine, and a multi-functional system including a combination thereof. A printer refers also to a plotter. 
     As illustrated in  FIG. 1 , the image forming apparatus  500  includes four image forming units  1 Y,  1 M,  1 C, and  1 K for forming toner images, one for each of the colors yellow, magenta, cyan, and black, respectively. It is to be noted that the suffixes Y, M, C, and K denote colors yellow, magenta, cyan, and black, respectively. To simplify the description, the suffixes Y, M, C, and K indicating colors may be omitted herein, unless differentiation of colors is necessary. The image forming apparatus  500  includes a transfer unit  30  serving as a transfer device, an optical writing unit  80 , a fixing device  90 , a paper cassette  100 , and a pair of registration rollers  101 . 
     The image forming units  1 Y,  1 M,  1 C, and  1 K all have the same configuration as all the others, differing only in the color of toner employed as a powder-form developing agent. The image forming units  1 Y,  1 M,  1 C, and  1 K are replaced upon reaching their product life cycles. According to the illustrative embodiment, the image forming units  1 Y,  1 M,  1 C, and  1 K are detachably attachable relative to a main body  501  of the image forming apparatus  500  and replaceable. 
       FIG. 2  is an enlarged diagram schematically illustrating one of the image forming units  1 Y,  1 M,  1 C, and  1 K as a representative example. The image forming units  1 Y,  1 M,  1 C, and  1 K all have the same configuration as all the others, differing only in the color of toner employed. Thus, the description is provided without the suffixes Y, M, C, and K indicating colors unless differentiation of the color is necessary. 
     The image forming unit  1  includes a drum-shaped photoconductor  2  serving as a latent image bearer, a photoconductor cleaner  3 , a static eliminator, a charging device  6 , a developing device  8 , and so forth. These devices are held in a common casing so that they are detachably installable and replaceable all together relative to the main body  501 , thereby constituting a process cartridge. 
     The photoconductor  2  comprises a drum-shaped base on which an organic photosensitive layer is disposed. The photoconductor  2  is rotated in a clockwise direction indicated by arrow D 1  by a driving device. The charging device  6  includes a charging roller  7  to which a charging bias is applied. The charging roller  7  contacts or approaches the photoconductor  2  to generate an electrical discharge therebetween, thereby charging uniformly the surface of the photoconductor  2 . Instead of using the charging roller  7  or the like that contacts or disposed close to the photoconductor  2 , a corona charger or the like that does not contact the photoconductor  2  may be employed. 
     The uniformly charged surface of the photoconductor  2  by the charging roller  7  is scanned by exposure light such as a light beam projected from the optical writing unit  80 , thereby forming an electrostatic latent image for black on the surface of the photoconductor  2 . The electrostatic latent image on the photoconductor  2  is developed with toner of the respective color by the developing device  8 . Accordingly, a visible image, also known as a toner image, is formed. The toner image formed on the photoconductor  2  is transferred primarily onto an intermediate transfer belt  31  formed into an endless loop. 
     The photoconductor cleaner  3  removes residual toner remaining on the surface of the photoconductor  2  after a primary transfer process, that is, after the photoconductor  2  passes through a primary transfer nip between the intermediate transfer belt  31  and the photoconductor  2 . The photoconductor cleaner  3  includes a brush roller  4  which is rotated and a cleaning blade  5 . The cleaning blade  5  is cantilevered, that is, one end thereof is fixed to a housing of the photoconductor cleaner  3 , and the other end is a free end that contacts the surface of the photoconductor  2 . The brush roller  4  rotates and brushes off the residual toner from the surface of the photoconductor  2  while the cleaning blade  5  scraping off the residual toner from the surface. 
     The static eliminator may employ a known static eliminating device and removes residual charge remaining on the photoconductor  2  after the surface thereof is cleaned by the photoconductor cleaner  3  in preparation for the subsequent imaging cycle. The surface of the photoconductor  2  is initialized in preparation for the subsequent imaging cycle. 
     The developing device  8  includes a developing portion  12  and a developer conveyer  13 . The developing portion  12  includes a developing roller  9  inside thereof. The developer conveyer  13  mixes and transports the developing agent. The developer conveyer  13  includes a first chamber equipped with a first screw  10  and a second chamber equipped with a second screw  11 . The first screw  10  and the second screw  11  are rotatably supported by a casing or the like of the developing device  8 . The first screw  10  and the second screw  11  are rotated to deliver the developing agent to the developing roller  9  while circulating the developing agent. 
     As illustrated in  FIG. 1 , the optical writing unit  80  for writing a latent image on the photoconductors  2  is disposed above the image forming units  1 Y,  1 M,  1 C, and  1 K. Based on image information received from an external device such as a personal computer (PC), the optical writing unit  80  scans optically the photoconductors  2 Y,  2 M,  2 C, and  2 K with a light beam projected from a laser diode of the optical writing unit  80 . Accordingly, the electrostatic latent images of yellow, magenta, cyan, and black are formed on the photoconductors  2 Y,  2 M,  2 C, and  2 K, respectively. 
     Referring back to  FIG. 1 , a description is provided of the transfer unit  30 . The transfer unit  30  is disposed substantially below the image forming units  1 Y,  1 M,  1 C, and  1 K. The transfer unit  30  includes the intermediate transfer belt  31  serving as an image bearer formed into an endless loop and rotated in the counterclockwise direction. The transfer unit  30  also includes a plurality of rollers: a drive roller  32 , a secondary-transfer back surface roller  33 , a cleaning auxiliary roller  34 , and four primary transfer rollers  35 Y,  35 M,  35 C, and  35 K (which may be referred to collectively as primary transfer rollers  35 ). The primary transfer rollers  35 Y,  35 M,  35 C, and  35 K are disposed opposite the photoconductors  2 Y,  2 M,  2 C, and  2 K, respectively, via the intermediate transfer belt  31 . The transfer unit  30  is detachably attachable (replaceable) relative to the main body  501 . 
     Outside the loop formed by the intermediate transfer belt  31 , a secondary transfer unit  41 , a belt cleaner  37 , a voltage detector  38  are disposed. The secondary transfer unit  41  includes a secondary transfer belt  36  serving as an image bearer and also as a secondary transfer device. The secondary-transfer back surface roller  33  can be also referred to as a secondary-transfer opposed roller. 
     The intermediate transfer belt  31  is looped around and stretched taut between the plurality of rollers, i.e., the drive roller  32 , the secondary-transfer back surface roller  33 , the cleaning auxiliary roller  34 , and the four primary transfer rollers  35 Y,  35 M,  35 C, and  35 K. The drive roller  32  is rotated in the counterclockwise direction by a motor or the like, and rotation of the drive roller  32  enables the intermediate transfer belt  31  to rotate in the same direction. In the transfer unit  30 , the intermediate transfer belt  31  is looped around the plurality of rollers, thereby delivering the recording medium P. 
     The intermediate transfer belt  31  is interposed between the primary transfer rollers  35 Y,  35 M,  35 C, and  35 K, and photoconductors  2 Y,  2 M,  2 C, and  2 K, thereby forming primary transfer nips serving as transfer sections for each color between the front surface or the image bearing surface of the intermediate transfer belt  31  and the photoconductors  2 Y,  2 M,  2 C, and  2 K. A primary transfer bias is applied to the primary transfer rollers  35 Y,  35 M,  35 C, and  35 K by a transfer bias power source. Accordingly, a primary transfer electric field is formed between the primary transfer rollers  35 Y,  35 M,  35 C, and  35 K, and the toner images of yellow, magenta, cyan, and black formed on the photoconductors  2 Y,  2 M,  2 C, and  2 K. 
     A yellow toner image formed on the photoconductor  2 Y enters the primary transfer nip for yellow as the photoconductor  2 Y rotates. Subsequently, the yellow toner image is primarily transferred from the photoconductor  2 Y to the intermediate transfer belt  31  by the transfer electric field and the nip pressure. The intermediate transfer belt  31 , on which the yellow toner image has been transferred, passes through the primary transfer nips of magenta, cyan, and black. Subsequently, a magenta toner image, a cyan toner image, and a black toner image on the photoconductors  2 M,  2 C, and  2 K, respectively, are superimposed on the yellow toner image which has been transferred on the intermediate transfer belt  31 , one atop the other in the primary transfer process. Accordingly, a composite toner image, in which the toner images of four different colors are superimposed on one atop the other, is formed on the surface of the intermediate transfer belt  31  in the primary transfer process. 
     According to the present illustrative embodiment, roller-type primary transfer devices, that is, the primary transfer rollers  35 Y,  35 M,  35 C, and  35 K, are employed as primary transfer devices. Alternatively, a transfer charger and a brush-type transfer device may be employed as the primary transfer device. 
     The secondary transfer unit  41  is disposed outside the loop of the intermediate transfer belt  31 . A nip forming roller  400  of the transfer unit  30  is disposed outside the loop formed by the intermediate transfer belt  31 , opposite to the secondary-transfer back surface roller  33 . The intermediate transfer belt  31  is interposed between the secondary-transfer back surface roller  33  and the nip forming roller  400 , thereby forming a secondary transfer nip N at which the front surface of the intermediate transfer belt  31  contacts the secondary transfer belt  36 . The secondary transfer belt  36  is grounded. 
     By contrast, a secondary transfer bias is applied to the secondary-transfer back surface roller  33  by a secondary-transfer bias power source  39 . With this configuration, a secondary-transfer electrical field is formed between the secondary-transfer back surface roller  33  and the secondary transfer belt  36  so that the toner having a negative polarity is moved electrostatically from the secondary-transfer back surface roller  33  to the secondary transfer belt  36 . 
     As illustrated in  FIG. 1 , the paper cassette  100  storing a sheaf of recording media P such as paper sheets and resin sheets is disposed below the transfer unit  30 . The paper cassette  100  is equipped with a feed roller  100   a  to contact the top sheet of recording media P in the paper cassette  100 . As the feed roller  100   a  is rotated at a predetermined speed, the feed roller  100   a  picks up and sends the top sheet of the recording media P to a delivery path. Substantially near the end of the delivery path, a pair of registration rollers  101  is disposed. The pair of registration rollers  101  temporarily stops rotating, immediately after the recording medium P delivered from the paper cassette  100  is interposed therebetween. The pair of registration rollers  101  starts to rotate again to feed the recording medium P to the secondary transfer nip N in appropriate timing such that the recording medium P is aligned with the composite toner image formed on the intermediate transfer belt  31  in the secondary transfer nip N. 
     In the transfer unit  30 , the intermediate transfer belt  31  is an endless looped belt serving as an image bearer onto which a toner image is transferred, and is looped around the plurality of rollers, i.e., the drive roller  32 , the secondary-transfer back surface roller  33 , and the cleaning auxiliary roller  34 . The toner image transferred on the intermediate transfer belt  31  is delivered to the secondary transfer nip N at which the toner image is transferred from the intermediate transfer belt  31  to the recording medium P in the secondary transfer process. 
     In the secondary transfer nip, the recording medium P tightly contacts the composite toner image on the intermediate transfer belt  31 , and the composite toner image is transferred onto the recording medium P by the secondary transfer electric field and the nip pressure applied thereto, thereby forming a color image on the surface of the recording medium P. 
     After the intermediate transfer belt  31  passes through the secondary transfer nip N, the toner residue not having been transferred onto the recording medium P remains on the intermediate transfer belt  31 . The toner residue is removed from the intermediate transfer belt  31  by the belt cleaner  37  which contacts the surface of the intermediate transfer belt  31 . The cleaning auxiliary roller  34  disposed inside the loop formed by the intermediate transfer belt  31  supports the cleaning operation performed by the belt cleaner  37 . 
     The voltage detector  38  is disposed outside the loop formed by the intermediate transfer belt  31 . More specifically, the voltage detector  38  is disposed opposite to a portion of the intermediate transfer belt  31  wound around the drive roller  32  with a predetermined gap between the voltage detector  38  and the intermediate transfer belt  31 . The surface potential of the toner image primarily transferred onto the intermediate transfer belt  31  is measured when the toner image comes to the position opposite to the voltage detector  38 . 
     The fixing device  90  is disposed on the right side in  FIG. 1 , that is, downstream from the secondary transfer nip N in the direction of conveyance of the recording medium P. The fixing device  90  may be a known fixing device. After the secondary transfer, the recording medium, onto which the composite color toner image is transferred, is transported to the fixing device  90 . The fixing device  90  includes a fixing roller  91  including a heat source inside thereof and a pressing roller  92 . The fixing roller  91  and the pressing roller  92  contact to form the fixing nip where heat and pressure are applied. The composite toner image is softened and fixed on the recording medium P as the recording medium P passes through the fixing nip. After the toner image is fixed to the recording medium P, the recording medium P is output from the fixing device  90 . Subsequently, the recording medium P is delivered outside the image forming apparatus  500  via a post-fixing medium path. 
     In the image forming apparatus  500  of the present disclosure, in a case in which a monochrome image is formed, a movable support plate supporting the primary transfer rollers  35 Y,  35 M, and  35 C of the transfer unit  30  is moved to separate the primary transfer rollers  35 Y,  35 M, and  35 C from the photoconductors  2 Y,  2 M, and  2 C. Accordingly, the front surface or the outer peripheral surface of the intermediate transfer belt  31  is separated from the photoconductors  2 Y,  2 M, and  2 C so that the intermediate transfer belt  31  contacts only the photoconductor  2 K. In this state, only the image forming unit  1 K among four image forming units is driven to form a black toner image on the photoconductor  2 K. 
     With reference to  FIGS. 3, 4, and 5 , a description is provided of a first support assembly  40  as a main-body side support that supports the secondary transfer unit  41  which serves as an image bearer support. 
     As illustrated in  FIGS. 3, 4, and 5 , the main body  501  of the image forming apparatus  500  includes the secondary transfer unit  41  and the first support assembly  40  that supports the secondary transfer unit  41 . The first support assembly  40  detachably supports the secondary transfer unit  41 . The secondary transfer unit  41  is replaceable relative to the first support assembly  40 .  FIG. 4  illustrates a state in which the secondary transfer unit  41  is installed in the first support assembly  40 .  FIG. 5  illustrates a state in which the secondary transfer unit  41  is detached from the first support assembly  40 . It is to be noted that  FIGS. 4 through 7 , and  9  and  10  are side views of the first support assembly  40  and the secondary transfer unit  41 , illustrated in an opposite orientation to the orientation shown in  FIG. 1 . 
     The secondary transfer unit  41  includes the secondary-transfer back surface roller  33 , the nip forming roller  400  disposed opposite to the secondary-transfer back surface roller  33  via the intermediate transfer belt  31 , three rollers  401 ,  402 , and  403 , and the secondary transfer belt  36  serving as an image bearer looped around the nip forming roller  400  and three rollers  401 ,  402 , and  403 . The secondary transfer unit  41  is a belt conveyor in which the secondary transfer belt  36  is an endless looped belt serving as an image bearer, and is looped around the plurality of rollers, i.e., the nip forming roller  400 , and the rollers  401 ,  402 , and  403 . The nip forming roller  400  can also be referred to as a secondary transfer roller. The roller  401  can also be referred to as a separation roller. 
     The nip forming roller  400  secondarily transfers the toner image on the intermediate transfer belt  31  onto the recording medium P. The nip forming roller  400  is disposed inside the belt loop of the secondary transfer belt  36 , facing to the secondary-transfer back surface roller  33 . The intermediate transfer belt  31  and the secondary transfer belt  36  are interposed between the nip forming roller  400  and the secondary-transfer back surface roller  33 . The nip forming roller  400  is biased against the intermediate transfer belt  31  so as to pressingly contact the intermediate transfer belt  31 , thereby forming the secondary transfer nip N between the intermediate transfer belt  31  and the secondary transfer belt  36 . 
     A bias for the secondary transfer or a secondary transfer bias is applied to the nip forming roller  400  or to the secondary-transfer back surface roller  33 . In a case in which the secondary transfer bias is applied to the nip forming roller  400 , the secondary transfer bias having a polarity opposite that of the toner is applied thereto. In a case in which the secondary transfer bias is applied to the secondary-transfer back surface roller  33 , the secondary transfer bias having the same polarity as that of the toner is applied thereto. 
     The roller  401  is to strip the recording medium P electrostatically absorbed to the secondary transfer belt  36  from the secondary transfer belt  36  by self stripping. 
     As illustrated in  FIG. 4 , the nip forming roller  400 , and the rollers  401 ,  402 , and  403  are rotatably supported by roller shafts  400 A,  401 A,  402 A, and  403 A, respectively, on a pair of lateral plates  404  and  405  disposed in a longitudinal direction of the rollers. The secondary transfer belt  36  contacting the intermediate transfer belt  31  causes the nip forming roller  400 , and the rollers  401 ,  402 , and  403  to rotate. Alternatively, a driving gear  400 B disposed on the roller shaft  400 A rotates the nip forming roller  400 , and the rollers  401 ,  402 , and  403 . According to the present illustrative embodiment, the place where the peripheral surface of the intermediate transfer belt  31  and the secondary transfer belt  36  contact is a so-called secondary transfer nip N. The secondary transfer unit  41  is contactable and separable relative to the intermediate transfer belt  31  while the secondary transfer unit  41  is installed in the first support assembly  40 . 
     The first support assembly  40  includes base plates  421  and  422 , a rotary shaft  43  disposed on the base plates  421  and  422 , a detector retainer  44  disposed on the rotary shaft  43 , a pattern detector  45  supported by the detector retainer  44 , and a coil spring  46  serving as a biasing device to bias the first support assembly  40  as a whole against the secondary-transfer back surface roller  33 . The base plates  421  and  422  are spaced apart a certain distance in the longitudinal direction of the nip forming roller  400 . The coil spring  46  is a tension spring. 
     When installed in the first support assembly  40 , the secondary transfer unit  41  and the first support assembly  40  become a single integrated unit. Therefore, as the first support assembly  40  is biased against the secondary-transfer back surface roller  33  by the coil spring  46 , the secondary transfer unit  41  is biased against the secondary-transfer back surface roller  33  (intermediate transfer belt  31 ). 
     As illustrated in  FIGS. 6 and 7 , a pair of eccentric cams  450  is disposed on a shaft  33 A of the secondary-transfer back surface roller  33 . The pair of eccentric cams  450  moves the secondary transfer belt  36  to contact and separate from the intermediate transfer belt  31 . A pair of rollers  406  as a receiver is disposed on the shaft  400 A of the nip forming roller  400  (secondary transfer roller), opposite to the pair of eccentric cams  450 . Each of the rollers  406  is rotatably disposed on each end of the shaft  400 A supporting the nip forming roller  400  in the axial direction thereof. 
     Each of the eccentric cams  450  is disposed on each end of the shaft  33 A in the axial direction thereof supporting the secondary-transfer back surface roller  33  such that the pair of eccentric cams  450  rotates integrally with the shaft  33 A. The pair of eccentric cams  450  contacts the pair of rollers  406 . As illustrated in  FIG. 6 , the shaft  33 A on which the pair of eccentric cams  450  is disposed is rotated by a drive motor M 1  serving as a contact-and-separation drive source. In this configuration, as the shaft  33 A is rotated by the drive motor M 1 , the angle of each eccentric cam  450  changes. 
     Each eccentric cam  450  includes a first cam portion  450 A and a second cam portion  450 B on an outer circumferential surface of the eccentric cam  450 . As will be later described in detail, the first cam portion  450 A forms a separating state, and the second cam portion  450 B forms a contact state. In accordance with image formation information, the drive motor M 1  is controlled to rotate the pair of eccentric cams  450 , thereby changing the portion of the pair of eccentric cams  450  that comes in contact with the pair of rollers  406 . The eccentric cams  450  have the same profile and are fixed to the shaft  33 A at the same phase. 
     That is, as illustrated in  FIG. 6 , when the first cam portion  450 A is separated from the roller  406 , the first support assembly  40  (not shown in  FIG. 6 ) is moved up pivotally about the rotary shaft  43  in a contact direction by the coil spring  46 , causing the intermediate transfer belt  31  and the secondary transfer belt  36  to come in contact with each other. By contrast, as illustrated in  FIG. 7 , when the second cam portion  450 B is in contact with the roller  406 , the second cam portion  450 B pushes the roller  406  in a separating direction, thereby pivotally moving down the first support assembly  40  (not shown in  FIG. 7 ) about the rotary shaft  43 . In this configuration, the intermediate transfer belt  31  and the secondary transfer belt  36  are separated from each other. 
     It is to be noted that the image formation information includes signals that are generated upon image formation when an image is transferred onto a recording medium P and upon forming a test toner pattern for an adjustment of an image density of the image to be transferred onto a recording medium P. 
     The image forming apparatus  500  has an image adjustment mode in which an image density is adjusted. When the image adjustment mode is set, the signals, i.e., the image formation information, are generated, and the test toner pattern for the adjustment of the image density is formed on the intermediate transfer belt  31 . The test toner pattern is transferred not onto a recording medium P, but transferred onto the secondary transfer belt  36  in the secondary transfer nip N. In the image adjustment mode, the pattern detector  45  detects the density of the test toner pattern transferred onto the secondary transfer belt  36 . In accordance with the image density information detected by the pattern detector  45 , a controller carries out feed-back control such that the image density of the test toner pattern has a predetermined value. The condition of the image bearer detected by the pattern detector is the density of the test toner pattern. 
     Embodiment 1 
     With reference to  FIGS. 3 through 8 , a description is provided of installation of the pattern detector  45  according to an illustrative embodiment of the present disclosure. 
     As illustrated in  FIGS. 3 and 8 , the pattern detector  45  detects optically the density of the test toner patterns on the secondary transfer belt  36 . For example, the pattern detector  45  is a known detector that irradiates detection light against the secondary transfer belt  36  and receives reflected light from the secondary transfer belt  36 . 
     As illustrated in  FIG. 8 , the detector retainer  44  that supports the pattern detector  45  includes arms  44 A and  44 B, a support  44 C, and contact portions  44 D and  44 E. The arms  44 A and  44 B are disposed in the axial direction of the nip forming roller  400 , which coincides with a width direction of the secondary transfer belt  36 . The support  44 C, and the contact portions  44 D and  44 E are disposed across the arms  44 A and  44 B. As illustrated in  FIGS. 3 and 4 , a first end  44 Aa of the arm  44 A and a first end  44 Ba of the arm  44 B are swingably supported by the rotary shaft  43 . Second ends  44 Ab and  44 Bb, which are on the opposite ends of the first ends  44 Aa and  44 Ba, respectively, extend to the roller shaft  402 A of the roller  402 . The support  44 C has a planar shape and is disposed at a place at which the secondary transfer belt  36  is looped around the roller  402  among the entire secondary transfer belt  36  in its circumference direction. A plurality of pattern detectors  45  is arranged on the support  44 C in the axial direction of the nip forming roller  400 . The plurality of pattern detectors  45  is disposed opposite to a surface  36   a  (illustrated in  FIG. 8 ) of the secondary transfer belt  36  with a certain gap therebetween. More specifically, each detection surface  45   a  of the plurality of pattern detectors  45  faces a surface  36   a  (illustrated in  FIG. 8 ) of the secondary transfer belt  36  looped around the roller  402 . The plurality of pattern detectors  45  is fixed to the support  44 C. 
     That is, the detector retainer  44  supports the pattern detectors  45  in such a manner that the pattern detectors  45  is pivotally movable about the rotary shaft  43  as a fulcrum. The rotary shaft  43  as the fulcrum serves also as a fulcrum to pivotally support the first support assembly  40  that supports or holds the secondary transfer unit  41 . In this configuration, the secondary transfer unit  41  including the secondary transfer belt  36  and the pattern detectors  45  are pivotally supported by a common fulcrum, that is, the rotary shaft  43 . In  FIGS. 4 and 5 , the detector retainer  44  moves up in a first direction indicated by arrow A 1 , and the detector retainer  44  moves down in a second direction indicated by arrow A 2 . The detector retainer  44  is pivotally supported by the rotary shaft  43  in the first direction A 1  and the second direction A 2 . 
     As illustrated in  FIG. 8 , the contact portions  44 D and  44 E are formed on the second ends  44 Ab and  44 Bb of the arms  44 A and  44 B, rather than on the support  44 C. When the secondary transfer unit  41  is installed in the first support assembly  40 , shaft ends  402 Aa and  402 Ab of the roller shaft  402 A come into contact with the contact portions  44 D and  44 E, respectively. More specifically, the place of the contact portions  44 D and  44 E coming in contact with the shaft ends  402 Aa and  402 Ab of the shaft  402 A is flat. In other words, the contact portions  44 D and  44 E include planar positioning surfaces  44 Da and  44 Ea, respectively, that contact the shaft ends  402 Aa and  402 Ab of the shaft  402 A. 
     With reference to  FIGS. 4 and 5 , a description is provided of the detector retainer  44  according to an illustrative embodiment of the present disclosure. As illustrated in  FIGS. 4 and 5 , the detector retainer  44  is biased by a helical torsion spring  47  wound around the rotary shaft  43  such that the detector retainer  44  can rotate towards the first direction A 1 . The first ends  44 Aa and  44 Ba of the arms  44 A and  44 B on the supporting side on which the detector retainer  44  is swingably supported include projections  44 F and  44 G serving as range controllers to determine a range of swingable movement of the detector retainer  44  in the first direction A 1 . As illustrated in  FIG. 9 , the projections  44 F and  44 G project beyond the first end  44 Aa and the first end  44 Ba, respectively, in a radial direction perpendicular to the axis of the rotary shaft  43 . As the detector retainer  44  moves in the first direction A 1 , the projections  44 F and  44 G come in contact with walls  48  surrounding the first ends  44 Aa and  44 Ba of the rotary shaft  43 . The walls  48  are disposed on the base plates  421  and  422 . 
     The arms  44 A and  44 B, the support  44 C, the contact portions  44 D and  44 E, and the projections  44 F and  44 G are obtained through monolithic molding of sheet metal as a single integrated unit. This configuration reduces assembly errors as compared with assembling individual parts into a single unit. 
     According to the present illustrative embodiment, when the projections  44 F and  44 G, and the walls  48  contact, the opening angle of the detector retainer  44  is at its maximum in the first direction A 1 . When the secondary transfer unit  41  is installed in the first support assembly  40 , this maximum opening of the detector retainer  44  is on the first direction A 1  side relative to a position pressed by the roller shaft  402 A when coming in contact with the roller shaft  402 A. 
     More specifically, among the plurality of roller shafts around which the secondary transfer belt  36  is looped, the shaft ends  402 Aa and  402 Ab of the roller shaft  402 A of the roller  402  facing the plurality of pattern detectors  45  contact the contact portions  44 D and  44 E. Thus, the roller shaft  402 A of the roller  402  functions as a stopper to regulate the swingable movement of the plurality of pattern detectors  45  in the first direction A 1 . 
     According to the present illustrative embodiment, the secondary transfer belt  36  and the plurality of pattern detectors  45  are pivotally supported by a common fulcrum, that is, the rotary shaft  43 . Furthermore, the rotary shaft  43  regulates the swingable movement of the plurality of pattern detectors  45 , that is, the swingable movement of the detector retainer  44  including the plurality of pattern detectors  45 . In this configuration, when the secondary transfer unit  41  is installed in the first support assembly  40 , the roller shaft  402 A contacts the positioning surfaces  44 Da and  44 Ea of the contact portions  44 D and  44 E, pushing down the detector retainer  44  in the second direction A 2  against a biasing force. 
     Accordingly, even when the secondary transfer unit  41  is detached from the first support assembly  40  to replace with a new secondary transfer unit, it is not necessary to replace or reassemble the plurality of pattern detectors  45 , thereby simplifying installation operation. Furthermore, since it is not necessary to replace the pattern detectors  45 , the cost can be saved and it is good for the environment. 
     Still further, when the pattern detectors  45  remain unchanged, the installation accuracy of the pattern detectors  45  can be maintained so that a technician who replaces the secondary transfer unit  41  is not required of a skill for attachment and detachment of the pattern detectors  45 . This configuration facilitates replacement of the secondary transfer unit  41 , hence increasing the operating efficiency and maintaining reliably the distance between the pattern detectors  45  and the secondary transfer belt  36 . 
     The plurality of pattern detectors  45  is mounted on the detector retainer  44  pivotally supported by the rotary shaft  43  which also serves as a fulcrum for the secondary transfer belt  36 . The plurality of pattern detectors  45  is disposed opposite to the curved surface of the surface  36   a  of the secondary transfer belt  36  wound around the roller  402 . With this configuration, attachment and detachment of the secondary transfer unit  41  hardly affect the plurality of pattern detectors  45  while reducing the assembly errors. Therefore, the plurality of pattern detectors  45  and the secondary transfer belt  36  are positioned in place accurately, and the distance between the plurality of pattern detectors  45  and the secondary transfer belt  36  can be reliably maintained. Fluctuations in detection of the density of test toner patterns by the pattern detectors  45  are reduced so that the image density can be adjusted accurately, hence forming an image with good quality. 
     The detector retainer  44  supporting the plurality of pattern detectors  45  contacts the roller shaft  402 A while the detector retainer  44  is biased by the helical torsion spring  47 . With this configuration, even when the main body  501  and the secondary transfer unit  41  vibrate, the swingable movement of the detector retainer  44  absorbs the vibration. With this configuration, the roller shaft  402 A and the contact portions  44 D and  44 E remain contacting with each other, so that fluctuations in the positional relation between the plurality of pattern detectors  45  and the secondary transfer belt  36  are insignificant. In a configuration in which the plurality of pattern detectors  45  is disposed at the main body  501  side, the detection accuracy of the plurality of pattern detectors  45  can be reliably maintained. Therefore, the distance between the plurality of pattern detectors  45  and the secondary transfer belt  36  can be reliably maintained. 
     The positioning surfaces  44 Da and  44 Ea of the contact portions  44 D and  44 E of the detector retainer  44  contact or pressingly contact the roller shaft  402 A serving as the stopper, more specifically, the shaft ends  402 Aa and  402 Ab, thereby keeping reliably the positioning surfaces  44 Da and  44 Ea in contact with the shaft ends  402 Aa and  402 Ab. 
     Examples of the shape of the positioning surfaces  44 Da and  44 Ea include a curved surface and a planar surface. Considering the fact that the detector retainer  44  is swingably movable, preferably, the shape of the positioning surfaces  44 Da and  44 Ea is planar, thereby increasing a contact area that contacts the roller shaft  402 A. 
     Embodiment 2 
     With reference to  FIG. 10 , a description is provided of another illustrative embodiment of the present disclosure. According to Embodiment 2, the configuration of the contact portions of the detector retainer  44  is different from that of EMBODIMENT 1. Except for the configurations described above, the configurations in the present illustrative embodiment are similar to or the same as in Embodiment 1. Thus, the description is provided mainly of the contact portions. 
     As illustrated in  FIG. 10 , the detector retainer  44  pivotally supported by the rotary shaft  43  serving as a common fulcrum for the secondary transfer unit  41  and the pattern detectors  45  includes contact portions  44 D 1  and  44 E 1  that contact the shaft ends  402 Aa and  402 Ab of the roller shaft  402 A. The contact portions  44 D 1  and  44 E 1  are disposed opposite to the roller shaft  402 A as a stopper that supports rotatably the roller  402  facing the plurality of pattern detectors  45 . The contact portions  44 D 1  and  44 E 1  are formed integrally with other parts of the detector retainer  44  through plate work or the like. 
     According to the present illustrative embodiment, the contact portions  44 D 1  and  44 E 1  include at least two surfaces that contact the shaft ends  402 Aa and  402 Ab of the roller shaft  402 A in different directions. More specifically, the contact portion  44 D 1  includes a pair of a first surface  44 D 1   a  and a second surface  44 D 1   b , and the contact portion  44 E 1  includes a pair of a first surface  44 E 1   a  and a second surface  44 E 1   b . The first surfaces  44 D 1   a  and  44 E 1   a  are planer surfaces that contact the shaft ends  402 Aa and  402 Ab of the roller shaft  402 A in the second direction A 2  when the secondary transfer unit  41  is installed in the first support assembly  40 . The second surfaces  44 D 1   b  and  44 E 1   b  are disposed apart from the rotary shaft  43 , more than the first surfaces  44 D 1   a  and  44 E 1   a , and are continuous with the first surfaces  44 D 1   a  and  44 E 1   a . The second surfaces  44 D 1   b  and  44 E 1   b  are planar surfaces that push the roller shaft  402 A in the first direction A 1  when the detector retainer  44  swingably moves in the first direction A 1 . 
     According to the present illustrative embodiment, each of the contact portions  44 D 1  and  44 E 1  has a V-shape with the first surface  44 D 1   a  and the second surface  44 D 1 , and the first surface  44 E 1   a  and the second surface  44 E 1   b , respectively. With this configuration, the roller shaft  402 A is supported from the first direction A 1  even when vibration or the like causes the detector retainer  44  to move swingably. More specifically, the first surfaces  44 D 1   a  and  44 E 1   a  are planar surfaces disposed on the second direction A 2  side relative to a virtual straight line X between a center of rotation X 1  of the rotary shaft  43  and a center of rotation X 2  of the rotary shaft  402 A. The second surfaces  44 D 1   b  and  44 E 1   b  are planar surfaces disposed on a tangent line X 3  touching the roller shaft  402 A on the first direction A 1  side relative to the virtual straight line X. An angle θ formed between the first surface  44 D 1   a  and the second surface  44 D 1   b , and between the first surface  44 E 1   a  and the second surface  44 E 1   b  is an obtuse angle. 
     According to the present illustrative embodiment, the first surfaces  44 D 1   a  and  44 E 1   a , and the second surfaces  44 D 1   b  and  44 E 1   b  that come in contact with the roller shaft  402 A (shaft end portions  402 Aa and  402 Ab) from different directions are formed on the respective contact portions  44 D 1  and  44 E 1 . With this configuration, the roller shaft  402 A and the contact portions  44 D 1  and  44 E 1  can contact more reliably. Therefore, the distance between the plurality of pattern detectors  45  and the secondary transfer belt  36  can be reliably maintained. 
     The illustrative embodiments of the present disclosure are employed to prevent fluctuations in the distance between the secondary transfer belt  36  serving as the image bearer and the plurality of pattern detectors  45  to detect test toner patterns transferred on the secondary transfer belt  36 . The present disclosure is employed in the secondary transfer unit  41  and the first support assembly  40 . The application of the present disclosure is not limited thereto. 
     The test toner pattern used to detect the density of an image is formed on the intermediate transfer belt  31  before being transferred onto the secondary transfer belt  36 . As illustrated in  FIG. 11 , the intermediate transfer belt  31  is considered as an image bearer, and the pattern detectors  45  are disposed opposite to the portion of the intermediate transfer belt  31  looped around the drive roller  32 . In the image forming apparatus  500  of the present disclosure, in a case in which a monochrome image is formed, a movable support plate supporting the primary transfer rollers  35 Y,  35 M, and  35 C of the transfer unit  30  is pivotally moved so as to separate the primary transfer rollers  35 Y,  35 M, and  35 C from the photoconductors  2 Y,  2 M, and  2 C. 
     For example, in the case of the transfer unit  30 , the transfer unit  30  is pivotally supported such that the transfer unit  30  is swingably movable about a roller shaft  32 A of the drive roller  32 . The roller shaft  32 A of the drive roller  32  is detachably and rotatably supported by the first support assembly  40 . The plurality of pattern detectors  45  is mounted on the detector retainer  44  which is pivotally supported by the rotary shaft  43  of the first support assembly  40 . In this configuration, the roller shaft  32 A of the drive roller  32  serves as the stopper that regulates the movement of the detector retainer  44  in the first direction A 1 . Accordingly, the present disclosure can be applied to the transfer unit  30 . 
     According to the present illustrative embodiment, the detector detects conditions of the density of the test toner pattern formed on the image bearer as the condition of the image bearer. However, the condition of the image bearer detected by the detector is not limited to the condition of the density of the test toner pattern. For example, in the image forming apparatus  500  the voltage detector  38 , which is disposed spaced apart from a surface  31   a  of the intermediate transfer belt  31 , detects (measures) the surface potential of the toner image (the image to be transferred onto the recording medium P) on the intermediate transfer belt  31 . Fluctuations in the distance between the voltage detector  38  and the surface  31   a  of the intermediate transfer belt  31  can cause detection failure. 
     In view of the above, the condition of the image bearer detected by the detector includes the potential of the surface of the intermediate transfer belt  31 , and the voltage detector  38  that detects the potential thereof is changed to the pattern detector  45  illustrated in  FIG. 11 . The pattern detector  45  is supported by the detector retainer  44 . This configuration is preferable because fluctuations in the distance between the voltage detector  38  and the intermediate transfer belt  31  can be prevented. 
     It is to be noted that in the configurations illustrated in  FIGS. 11 and 12 , instead of the secondary transfer unit  41 , a secondary transfer roller  360  serving as a secondary transfer device is disposed opposite to the secondary-transfer back surface roller  33  via the intermediate transfer belt  31  to form the secondary transfer nip N. Alternatively, in some embodiments, the secondary transfer unit  41  may be disposed independent of the first support assembly  40 . 
     In one example of the first support assembly  40  to support the secondary transfer unit  41  equipped with the secondary transfer belt  36  such as illustrated in  FIG. 1 , the roller  401  is disposed downstream from the secondary transfer nip N in the direction of paper conveyance indicated by arrow. Alternatively, in some embodiments, the roller  401  may be disposed upstream from the secondary transfer nip N such as illustrated in  FIG. 13 . 
     That is, the main body  501  of the image forming apparatus  500  includes the secondary transfer unit  41  and the first support assembly  40  that supports the secondary transfer unit  41 . The first support assembly  40  detachably supports the secondary transfer unit  41 . The secondary transfer unit  41  is replaceable independently.  FIG. 4  illustrates a state in which the secondary transfer unit  41  is installed in the first support assembly  40 .  FIG. 5  illustrates a state in which the secondary transfer unit  41  is detached from the first support assembly  40 . 
     The secondary transfer unit  41  includes the secondary-transfer back surface roller  33 , the nip forming roller  400  disposed opposite to the secondary-transfer back surface roller  33  via the intermediate transfer belt  31 , three rollers  401 ,  402 , and  403 , and the secondary transfer belt  36  serving as an image bearer looped around the nip forming roller  400  and three rollers  401 ,  402 , and  403 . The roller  401  illustrated in  FIG. 13  serves as a belt pulley around which the secondary transfer belt  36  is simply looped, and does not function as a separation roller as compared with the roller  401  of  FIG. 1  which functions as a separation roller. 
     The secondary transfer unit  41  is a belt conveyor unit in which the secondary transfer belt  36  is an endless looped belt serving as an image bearer, and is looped around the plurality of rollers, i.e., the nip forming roller  400 , and the rollers  401 ,  402 , and  403 . 
     Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the embodiments described above, but a variety of modifications can naturally be made within the scope of the present disclosure. 
     According to an aspect of this disclosure, the present invention is employed in the image forming apparatus. The image forming apparatus includes, but is not limited to, an electrophotographic image forming apparatus, a copier, a printer, a facsimile machine, and a multi-functional system. 
     Furthermore, it is to be understood that elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. In addition, the number of constituent elements, locations, shapes and so forth of the constituent elements are not limited to any of the structure for performing the methodology illustrated in the drawings. 
     Example embodiments being thus described, it will be obvious that the same may be varied in many ways. Such exemplary variations are not to be regarded as a departure from the scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.