Transfer unit and image forming apparatus including the same

Abstract

To provide a transfer unit that can improve positional accuracy of and smoothness of an operation to switch between two transfer rollers selectively brought into pressure contact with an image carrier, and an image forming apparatus including the same.SOLUTION: A transfer unit comprises: a first roller and a second roller as a transfer roller; a first bearing member; a second bearing member; a roller holder; a switching cam; a driving mechanism; a unit frame; and a stationary cam. The transfer unit rotates the roller holder to arrange the first roller or the second roller opposite to the image carrier. The transfer unit rotates the switching cam to arrange the first roller or the second roller arranged opposite to the image carrier to a reference position and a separation position. The stationary cam has a positioning groove that performs positioning of the first roller or the second roller arranged opposite to the image carrier, and a holder positioning projection that is engaged with a holder positioning recess of the roller holder when the first roller is arranged opposite to the image carrier.SELECTED DRAWING: Figure 22

Description

[Technical field]

[0001] The present invention relates to a transfer unit that transfers a toner image formed on an image carrier such as a photosensitive drum or an intermediate transfer belt onto a recording medium, and an image forming apparatus equipped with the same, and in particular to a mechanism for switching the arrangement of multiple transfer members. [Background technology]

[0002] Conventionally, there has been known an intermediate transfer type image forming device that includes an endless intermediate transfer belt that rotates in a predetermined direction and a plurality of image forming units arranged along the intermediate transfer belt, in which each image forming unit sequentially superimposes a toner image of each color onto the intermediate transfer belt to perform primary transfer, and then a secondary transfer roller secondarily transfers the toner image onto a recording medium such as paper.

[0003] In such an intermediate transfer type image forming apparatus, toner adhesion to the surface of the secondary transfer roller progresses due to endurance printing. In particular, in order to improve color development and color reproducibility, it is necessary to perform calibration to correct image density and color shift at a predetermined timing, but the patch image formed on the intermediate transfer belt during calibration is not transferred to paper but is removed by a belt cleaning device. Therefore, when the patch image passes through the secondary transfer roller, part of the toner transferred onto the intermediate transfer belt adheres to the secondary transfer roller.

[0004] Conventionally, the secondary transfer roller has been cleaned by applying a transfer reverse voltage (a voltage with the same polarity as the toner) to the secondary transfer roller during non-image formation, returning the toner adhering to the secondary transfer roller to the intermediate transfer belt. However, this method requires time to clean the secondary transfer roller, which causes the problem of long print waiting times.

[0005] Therefore, a method has been proposed to improve productivity by making it possible to switch the secondary transfer roller to a size suitable for the recording medium. For example, Patent Document 1 discloses an image forming apparatus that includes a plurality of secondary transfer rollers each having a different axial length, a rotating body that rotatably supports the plurality of secondary transfer rollers and has a support part that can rotate around an axis parallel to the axial direction, and a control unit that selects one roller from the plurality of secondary transfer rollers in accordance with the width of the recording medium and rotates the support part to cause the one roller to face an intermediate transfer belt.

[0006] Patent Document 2 discloses a transfer unit including a first roller and a second roller as transfer rollers, a first bearing member, a second bearing member, a roller holder, a first biasing member, a second biasing member, a switching cam, a transfer voltage power source, and a drive mechanism. The transfer unit rotates the roller holder to position either the first roller or the second roller facing the image carrier, and rotates the switching cam to selectively position the first roller or the second roller facing the image carrier at a reference position where it is pressed against the image carrier to form a transfer nip portion, or at a spaced position spaced from the image carrier. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2017-156653 A [Patent Document 2] Patent Publication No. 2022-112901 Summary of the Invention [Problem to be solved by the invention]

[0008] The configuration of Patent Document 1 does not disclose a specific configuration of a separation mechanism that separates the secondary transfer roller from the intermediate transfer belt, and there is a risk of the secondary transfer roller or the intermediate transfer belt being damaged when switching or replacing the secondary transfer roller. The configuration of Patent Document 2 leaves room for further improvement in the positional accuracy of the transfer roller and the smoothness of switching when switching the transfer roller from a moving state to a pressing state.

[0009] In view of the above problems, the present invention aims to provide a transfer unit that can improve the positional accuracy of the rollers and the smoothness of the switching operation when switching between two transfer rollers that are selectively pressed against an image carrier, and an image forming apparatus equipped with the same. [Means for solving the problem]

[0010] In order to achieve the above object, the first configuration of the present invention is a transfer unit that includes a transfer roller having a core and an elastic layer laminated on the outer peripheral surface of the core, and pressing the elastic layer against an image carrier to form a transfer nip portion, and transfers a toner image formed on the image carrier to a recording medium passing through the transfer nip portion. The transfer unit includes a transfer roller consisting of a first roller and a second roller, a first bearing member, a second bearing member, a roller holder, a first biasing member, a second biasing member, a switching cam, a drive mechanism, a unit frame, and a fixed cam. The second roller is disposed above the first roller, and the axial length of the elastic layer is different from that of the first roller. The first bearing member rotatably supports the first roller. The second bearing member rotatably supports the second roller. The roller holder has a first bearing holder and a second bearing holder that slidably hold the first bearing member and the second bearing member in a direction approaching or moving away from the image carrier, respectively. The first biasing member is disposed between the first bearing holder and the first bearing member, and biases the first bearing member in a direction approaching the image carrier. The second biasing member is disposed between the second bearing holder and the second bearing member, and biases the second bearing member in a direction approaching the image carrier. The switching cam has a first guide hole with which a first engagement portion formed in the first bearing member and a second engagement portion formed in the second bearing member engage. The drive mechanism rotationally drives the roller holder and the switching cam. The unit frame rotatably supports the roller holder and the switching cam. The fixed cam is fixed to the unit frame. By rotating the roller holder, either the first roller or the second roller is positioned opposite the image carrier, and by rotating the switching cam to change the engagement position of the first engagement portion or the second engagement portion in the first guide hole, the first roller or the second roller positioned opposite the image carrier is selectively positioned at a reference position where it is pressed against the image carrier to form a transfer nip portion, or at a spaced position spaced from the image carrier.The fixed cam has a second guide hole formed to overlap the first guide hole and with which the first engagement portion and the second engagement portion engage, a positioning groove formed on the radially outer peripheral portion of the second guide hole and with which the first engagement portion engages when the first roller is placed opposite the image carrier and with which the second engagement portion engages when the second roller is placed opposite the image carrier, and a holder positioning protrusion formed on the surface facing the roller holder and which engages with the holder positioning recess of the roller holder. When the first roller is placed opposite the image carrier, the holder positioning protrusion engages with the holder positioning recess. Effect of the Invention

[0011] According to the first configuration of the present invention, when the first roller is disposed facing the image carrier, the first engagement portion of the first bearing member engages with the positioning recess of the fixed cam, and the holder positioning protrusion of the fixed cam engages with the holder positioning recess of the roller holder. Therefore, there is no risk of the roller holder being misaligned downward due to the weight of the first roller and the roller holder, and it is possible to improve the positioning accuracy when locating the first roller in the reference position. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram showing an internal configuration of an image forming apparatus 100 equipped with a secondary transfer unit 9 according to the present invention. [Diagram 2] An enlarged view of the image forming section Pa in FIG. [Diagram 3] A side cross-sectional view of an intermediate transfer unit 30 mounted in an image forming apparatus 100. [Figure 4] FIG. 1 is a perspective view of a secondary transfer unit 9 according to an embodiment of the present invention that is mounted in an image forming apparatus 100. [Diagram 5] FIG. 4 is an enlarged perspective view showing the configuration of a roller holder 47 of the secondary transfer unit 9 according to the embodiment; [Figure 6] FIG. 13 is a perspective view of the periphery of a roller holder 47 of a secondary transfer unit 9, as viewed from the inside in the axial direction. [Figure 7] FIG. 2 is a perspective view showing a drive mechanism of the secondary transfer unit 9 according to the embodiment. [Figure 8] FIG. 1 is a block diagram showing an example of a control path of an image forming apparatus 100 equipped with a secondary transfer unit 9 according to an embodiment of the present invention. [Figure 9] FIG. 2 is a side cross-sectional view including a switching cam 50 of the secondary transfer unit 9 of the present embodiment, as viewed from the axially inner side in a state where the first roller 40 is disposed at a reference position for forming the secondary transfer nip portion N. [Figure 10] FIG. 10 is a diagram showing a state in which the switching cam 50 is removed from the state shown in FIG. 9 to expose the fixed cam 52. [Figure 11] FIG. 10 is a diagram showing a first separated state of the first roller 40 when the switching cam 50 is rotated clockwise by a predetermined angle from the state shown in FIG. 9 . [Figure 12] FIG. 12 is a diagram showing a second separated state of the first roller 40 when the switching cam 50 is further rotated a predetermined angle in the clockwise direction from the state shown in FIG. 11 . [Figure 13] FIG. 13 shows a state in which the shaft 51 is rotated counterclockwise from the state in FIG. 12 to bring the second roller 41 into opposition to the drive roller 10. [Figure 14] FIG. 14 shows a state where the switching cam 50 is rotated counterclockwise by a predetermined angle from the state of FIG. 13 and the second roller 41 is positioned at a reference position for forming the secondary transfer nip portion N. [Figure 15] FIG. 15 is a diagram showing the first separated state of the second roller 41 when the switching cam 50 is further rotated counterclockwise by a predetermined angle from the state of FIG. 14. [Figure 16] FIG. 16 is a diagram showing a second separated state of the second roller 41 when the switching cam 50 is further rotated counterclockwise by a predetermined angle from the state of FIG. 15 . [Figure 17] FIG. 17 is a diagram showing a state in which the switching cam 50 is rotated clockwise by a predetermined angle from the state shown in FIG. 16 so that the first roller 40 faces the drive roller 10. [Figure 18] FIG. 11 is a side view showing a state in which the core metal 40a of the first roller 40 arranged at a reference position for forming the secondary transfer nip portion N is fitted into the shaft holding portion 37 of the intermediate transfer unit 30. [Figure 19]FIG. 11 is a side view showing a state in which the first engagement portion 43a of the first bearing portion 43 is not engaged with the positioning groove 66 of the fixed cam 52 and the core metal 40a of the first roller 40 is displaced from the shaft holding portion 37. [Figure 20] FIG. 1 is a perspective view of a fixed cam 52 used in a secondary transfer unit 9 according to an embodiment of the present invention, as viewed from the side facing a roller holder 47. [Figure 21] FIG. 4 is a perspective view of a roller holder 47 used in the secondary transfer unit 9 of the present embodiment, as viewed from the side facing a fixed cam 52. [Figure 22] 11 is a perspective view showing a state in which the holder positioning protrusion 67 of the fixed cam 52 is engaged with the holder positioning recess 68 of the roller holder 47, as viewed from the axially inner side. [Diagram 23] FIG. 13 is a perspective view showing a state in which a holder positioning protrusion 67 formed of a leaf spring is engaged with a holder positioning recess 68 of a roller holder 47. [Figure 24] FIG. 13 is a side cross-sectional view including a switching cam 50 of the secondary transfer unit 9 of the present embodiment, illustrating an example in which the reference position of the second roller 41 is detected by a third position detection sensor S3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic diagram showing the configuration of an image forming apparatus 100 equipped with a secondary transfer unit 9 of the present invention, and Fig. 2 is an enlarged view of the vicinity of an image forming section Pa in Fig. 1.

[0014] The image forming apparatus 100 shown in Fig. 1 is a so-called tandem type color printer, and has the following configuration: Within the main body of the image forming apparatus 100, four image forming sections Pa, Pb, Pc, and Pd are arranged in this order from the upstream side in the transport direction (the left side in Fig. 1). These image forming sections Pa to Pd are provided corresponding to images of four different colors (magenta, cyan, yellow, and black), and sequentially form images of magenta, cyan, yellow, and black through the processes of charging, exposing, developing, and transferring, respectively.

[0015] These image forming units Pa to Pd are provided with photoconductor drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of each color. Furthermore, an intermediate transfer belt 8 that rotates counterclockwise in FIG. 1 is provided adjacent to each image forming unit Pa to Pd. The toner images formed on these photoconductor drums 1a to 1d are transferred sequentially onto the intermediate transfer belt 8 that moves while contacting each of the photoconductor drums 1a to 1d, and then transferred all at once onto a sheet S, which is an example of a recording medium, in a secondary transfer unit 9. Furthermore, after the toner image is fixed onto the sheet S in a fixing unit 13, the sheet S is discharged from the main body of the image forming apparatus 100. While the photoconductor drums 1a to 1d are rotated clockwise in FIG. 1, an image forming process is performed for each of the photoconductor drums 1a to 1d.

[0016] The paper S onto which the toner image is transferred is stored in a paper storage cassette 16 at the bottom of the main body of the image forming apparatus 100, and is transported to the secondary transfer unit 9 via a paper feed roller 12a and a pair of registration rollers 12b. A seamless belt is generally used as the intermediate transfer belt 8.

[0017] Next, the image forming units Pa to Pd will be described. The image forming unit Pa will be described in detail below, but the image forming units Pb to Pd will not be described because they are basically configured in the same way. As shown in FIG. 2, a charging device 2a, a developing device 3a, and a cleaning device 7a are arranged around the photosensitive drum 1a along the drum rotation direction (clockwise direction in FIG. 2), and a primary transfer roller 6a is arranged with an intermediate transfer belt 8 sandwiched between them. In addition, a belt cleaning unit 19 is arranged upstream of the photosensitive drum 1a in the rotation direction of the intermediate transfer belt 8, facing a tension roller 11 with the intermediate transfer belt 8 sandwiched between them.

[0018] Next, an image forming procedure in the image forming apparatus 100 will be described. When a user inputs a command to start image formation, first, the main motor 60 (see FIG. 8) starts rotating the photoconductor drums 1a-1d, and the surfaces of the photoconductor drums 1a-1d are uniformly charged by the charging rollers 20 of the charging devices 2a-2d. Next, the surfaces of the photoconductor drums 1a-1d are irradiated with a beam of light (laser light) emitted from the exposure device 5, and an electrostatic latent image corresponding to an image signal is formed on each of the photoconductor drums 1a-1d.

[0019] The developing devices 3a to 3d are filled with a predetermined amount of toner of each color, magenta, cyan, yellow, and black. When the ratio of toner in the two-component developer filled in each developing device 3a to 3d falls below a specified value due to the formation of a toner image described later, toner is replenished from the toner containers 4a to 4d to each developing device 3a to 3d. The toner in the developer is supplied onto the photoconductor drums 1a to 1d by the developing rollers 21 of the developing devices 3a to 3d and electrostatically adheres to the photoconductor drums 1a to 1d. As a result, a toner image is formed according to the electrostatic latent image formed by exposure from the exposure device 5.

[0020] Then, the primary transfer rollers 6a-6d apply an electric field with a predetermined transfer voltage between the primary transfer rollers 6a-6d and the photosensitive drums 1a-1d, and the magenta, cyan, yellow and black toner images on the photosensitive drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. These four color images are formed with a predetermined positional relationship for forming a predetermined full-color image. Thereafter, in preparation for the subsequent formation of a new electrostatic latent image, the toner remaining on the surfaces of the photosensitive drums 1a-1d is removed by the cleaning blades 22 and rubbing rollers 23 of the cleaning devices 7a-7d.

[0021] When the intermediate transfer belt 8 starts to rotate counterclockwise with the rotation of the drive roller 10 by the belt drive motor 61 (see FIG. 8), the paper S is transported from the registration roller pair 12b to a secondary transfer unit 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and a full-color image is transferred onto the paper S. The paper S onto which the toner image has been transferred is transported to the fixing unit 13. The toner remaining on the surface of the intermediate transfer belt 8 is removed by a belt cleaning unit 19.

[0022] The paper S conveyed to the fixing section 13 is heated and pressurized by the fixing roller pair 13a, so that the toner image is fixed to the surface of the paper S, and a predetermined full-color image is formed. The paper S on which the full-color image has been formed is then directed to a different conveying direction by the branching section 14, which branches into multiple directions, and is discharged directly (or after being sent to the double-sided conveying path 18 and printed on both sides) onto the discharge tray 17 by the discharge roller pair 15.

[0023] An image density sensor 25 is disposed downstream of the image forming unit Pd at a position facing the intermediate transfer belt 8. An optical sensor equipped with a light-emitting element such as an LED and a light-receiving element such as a photodiode is generally used as the image density sensor 25. When measuring the amount of toner adhesion on the intermediate transfer belt 8, when the light-emitting element irradiates each patch image (reference image) formed on the intermediate transfer belt 8 with measurement light, the measurement light is reflected by the toner and the belt surface and enters the light-receiving element.

[0024] The light reflected from the toner and the belt surface includes specular and diffuse reflections. The specular and diffuse reflections are separated by a polarizing separation prism and then enter separate light receiving elements. Each light receiving element photoelectrically converts the received specular and diffuse reflections and outputs an output signal to the control unit 90 (see FIG. 8).

[0025] Then, the image density (toner amount) and image position of the patch image are detected from the characteristic changes of the output signals of the specularly reflected light and the diffusely reflected light, and by comparing them with a predetermined reference density and reference position, the characteristic value of the development voltage, the exposure start position and timing of the exposure device 5, etc. are adjusted, thereby performing density correction and color shift correction (calibration) for each color.

[0026] Fig. 3 is a side cross-sectional view of intermediate transfer unit 30 mounted on image forming apparatus 100. As shown in Fig. 3, intermediate transfer unit 30 has intermediate transfer belt 8 stretched between downstream drive roller 10 and upstream tension roller 11, primary transfer rollers 6a-6d in contact with photoconductor drums 1a-1d via intermediate transfer belt 8, and pressure switching roller 34.

[0027] A belt cleaning unit 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed in a position facing the tension roller 11. A secondary transfer unit 9 is disposed on the drive roller 10 via the intermediate transfer belt 8, forming a secondary transfer nip N. A detailed configuration of the secondary transfer unit 9 will be described later.

[0028] The intermediate transfer unit 30 includes a pair of support members (not shown) that rotatably support both ends of the rotation shafts of the primary transfer rollers 6a to 6d and the pressure switching roller 34 and move vertically (vertical direction in FIG. 3) to the traveling direction of the intermediate transfer belt 8, and a roller contact / separation mechanism 35 that includes a drive means (not shown) that moves the primary transfer rollers 6a to 6d and the pressure switching roller 34 back and forth in the vertical direction. The roller contact / separation mechanism 35 is switchable between a color mode in which the four primary transfer rollers 6a to 6d are pressed against the photosensitive drums 1a to 1d (see FIG. 1) via the intermediate transfer belt 8, a monochrome mode in which only the primary transfer roller 6d is pressed against the photosensitive drum 1d via the intermediate transfer belt 8, and a retraction mode in which all of the four primary transfer rollers 6a to 6d are separated from the photosensitive drums 1a to 1d.

[0029] FIG. 4 is a perspective view of a secondary transfer unit 9 according to an embodiment of the present invention mounted on an image forming apparatus 100. FIG. 5 is an enlarged perspective view showing a configuration of one end side of the secondary transfer unit 9 of this embodiment. FIG. 6 is a perspective view of the periphery of the roller holder 47 of the secondary transfer unit 9 as viewed from the inside in the axial direction. FIG. 7 is a perspective view showing a drive mechanism of the secondary transfer unit 9 of this embodiment. Note that the unit frame 9a is omitted in FIGS. 4 and 7, and the unit frame 9a is shown in a see-through state in FIG. 5. Also, the switching cam 50 and the fixed cam 52 are omitted in FIGS. 5 and 6, and the fixed cam 52 is omitted in FIGS. 4 and 7.

[0030] As shown in Figures 4 to 7, the secondary transfer unit 9 includes a first roller 40 and a second roller 41 as secondary transfer rollers, a first bearing member 43, a second bearing member 45, a roller holder 47, a switching cam 50, a fixed cam 52 (see Figures 9 and 10), and a roller switching motor 55.

[0031] The first roller 40 and the second roller 41 are elastic rollers in which conductive elastic layers 40b, 41b are laminated on the outer circumferential surfaces of core metals 40a, 41a, respectively. The elastic layers 40b, 41b are made of an ionically conductive rubber such as ECO (epichlorohydrin rubber).

[0032] The first roller 40 has an axial length of the elastic layer 40b of 311 mm, which is compatible with A3 size paper. The second roller 41 has an axial length of the elastic layer 41b that is greater than the elastic layer 40b of the first roller 40. More specifically, the axial length of the elastic layer 41b is 325 mm, which is compatible with 13-inch size paper.

[0033] The first bearing members 43 are arranged in pair at both axial ends of the first roller 40, and rotatably support the core metal 40a. The second bearing members 45 are arranged in pair at both axial ends of the second roller 41, and rotatably support the core metal 41a.

[0034] A pair of roller holders 47 are disposed at both axial ends of the first roller 40 and the second roller 41. The roller holder 47 is generally V-shaped in side view, and has a first bearing holder 47a, a second bearing holder 47b, and an insertion hole 47c. The first bearing holder 47a and the second bearing holder 47b slidably hold the first bearing member 43 and the second bearing member 45, respectively. The insertion hole 47c is formed at the apex of the V-shape, and the shaft 51 is rotatably inserted therethrough. The roller holder 47 is formed of an insulating material such as synthetic resin.

[0035] 5, a first coil spring 48 is disposed between the first bearing holder 47a and the first bearing member 43. A second coil spring 49 is disposed between the second bearing holder 47b and the second bearing member 45. The first roller 40 is biased by the first coil spring 48, and the second roller 41 is biased by the second coil spring 49, in a direction away from the shaft 51 (in a direction in which they are pressed against the drive roller 10).

[0036] As shown in Fig. 4, a first light-shielding plate 51a is attached to the shaft 51, and by shading the detection portion of the first position detection sensor S1 (see Fig. 9), it is possible to detect the rotation angle of the shaft 51. Also, as shown in Fig. 6, a second light-shielding plate 47d is formed on one side surface in the rotation direction of the roller holder 47. The second light-shielding plate 47d is formed at a position where it can block light from the detection portion of the second position detection sensor S2 arranged on the unit frame 9a.

[0037] The first light-shielding plate 51a and the second light-shielding plate 47d turn on or off the first position detection sensor S1 and the second position detection sensor S2 according to the rotation angle of the roller holder 47 (shaft 51), thereby making it possible to detect the positions of the first roller 40 and the second roller 41 supported by the roller holder 47. Position detection control of the first roller 40 and the second roller 41 will be described later.

[0038] A pair of switching cams 50 are disposed inside roller holder 47 at both axial ends of first roller 40 and second roller 41. Switching cam 50 is fan-shaped with a portion cut out in side view, and a key portion of the fan shape (apex portion where two radii intersect) is fixed to shaft 51.

[0039] 7, a roller switching motor 55 is connected to the shaft 51 via gears 53 and 54. The arrangement of the first roller 40 and the second roller 41 is switched by rotating the switching cam 50 together with the shaft 51. The control of switching between the first roller 40 and the second roller 41 will be described later.

[0040] 8 is a block diagram showing an example of a control path of an image forming apparatus 100 equipped with a secondary transfer unit 9 of this embodiment. Note that, since various controls are performed for each part of the image forming apparatus 100 when the image forming apparatus 100 is used, the control path of the entire image forming apparatus 100 becomes complicated. Therefore, the following description will focus on the parts of the control path that are necessary for implementing the present invention.

[0041] The control unit 90 includes at least a CPU (Central Processing Unit) 91 as a central processing unit, a ROM (Read Only Memory) 92 as a read-only storage unit, a RAM (Random Access Memory) 93 as a readable and writable storage unit, a temporary storage unit 94 that temporarily stores image data and the like, a counter 95, and a plurality of (here, two) I / Fs (interfaces) 96 that transmit control signals to each device in the image forming apparatus 100 and receive input signals from the operation unit 80. The control unit 90 can be disposed at any location inside the main body of the image forming apparatus 100.

[0042] ROM 92 stores data such as a control program for image forming apparatus 100, numerical values ​​necessary for control, and data that will not be changed while image forming apparatus 100 is in use. RAM 93 stores necessary data generated during the control of image forming apparatus 100, data temporarily required for the control of image forming apparatus 100, and the like. RAM 93 (or ROM 92) also stores a density correction table used for calibration, and the relationship between the on / off state of first position detection sensor S1 and second position detection sensor S2 used for roller switching control described later and the rotation angles of first roller 40 and second roller 41, and the like. Counter 95 accumulates and counts the number of printed sheets.

[0043] Furthermore, the control unit 90 transmits control signals from the CPU 91 through the I / F 96 to each portion and device in the image forming apparatus 100. Furthermore, signals indicating the state of each portion and device and input signals are transmitted to the CPU 91 through the I / F 96. Examples of each portion and device controlled by the control unit 90 include image forming units Pa to Pd, exposure device 5, primary transfer rollers 6a to 6d, secondary transfer unit 9, roller contact / separation mechanism 35, main motor 60, belt drive motor 61, image input unit 70, voltage control circuit 71, operation unit 80, etc.

[0044] The image input unit 70 is a receiving unit that receives image data transmitted from a host device such as a personal computer to the image forming apparatus 100. The image signal input from the image input unit 70 is converted into a digital signal and then sent to the temporary storage unit 94.

[0045] The voltage control circuit 71 is connected to the charging voltage power supply 72, the developing voltage power supply 73, and the transfer voltage power supply 74, and operates each of these power supplies in response to an output signal from the control unit 90. In response to a control signal from the voltage control circuit 71, each of these power supplies applies a predetermined voltage to the charging rollers 20 in the charging devices 2a-2d, the developing voltage power supply 73 applies a predetermined voltage to the developing rollers 21 in the developing devices 3a-3d, and the transfer voltage power supply 74 applies a predetermined voltage to the primary transfer rollers 6a-6d and the first roller 40 and the second roller 41 in the secondary transfer unit 9.

[0046] The operation unit 80 is provided with a liquid crystal display unit 81 and LEDs 82 that indicate various states, and the user operates the stop / clear button of the operation unit 80 to stop image formation, and operates the reset button to reset various settings of the image forming apparatus 100 to their default states. The liquid crystal display unit 81 indicates the state of the image forming apparatus 100, and displays the image formation status and the number of copies to be printed. Various settings of the image forming apparatus 100 are set from a printer driver of the personal computer.

[0047] Fig. 9 is a side cross-sectional view including the switching cam 50 of the secondary transfer unit 9 of this embodiment, as viewed from the inside in the axial direction, showing a state in which the first roller 40 is disposed at a position to form the secondary transfer nip portion N. Fig. 10 is a diagram showing a state in which the switching cam 50 is removed from the state in Fig. 9 to expose the fixed cam 52.

[0048] 9, the switching cam 50 has a sector shape in a plan view. An arc-shaped first guide hole 63 is formed in the switching cam 50. A recess 64 is formed in the center of the radially outer peripheral edge of the first guide hole 63. A first engagement portion 43a and a second engagement portion 45a that engage with the first guide hole 63 are formed in the first bearing member 43 and the second bearing member 45, respectively.

[0049] The recess 64 has a bottom portion 64a recessed to the outermost side in the radial direction and an inclined portion 64b inclined from the bottom portion 64a toward the inner side in the radial direction. By rotation of the switching cam 50, the first engagement portion 43a of the first bearing member 43 and the second engagement portion 45a of the second bearing member 45 engage with the bottom portion 64a or the inclined portion 64b of the recess 64 or move away from the recess 64, thereby making it possible to switch the contact state of the first roller 40 and the second roller 41 with the intermediate transfer belt 8 as described later.

[0050] 10, the fixed cam 52 is disposed between the roller holder 47 and the switching cam 50. The fixed cam 52 is fixed to the unit frame 9a of the secondary transfer unit 9 with screws.

[0051] The fixed cam 52 is formed with a through hole 52a and a second guide hole 65. The shaft 51 is rotatably inserted into the through hole 52a. The second guide hole 65 is formed at a position overlapping with the first guide hole 64 of the switching cam 50, and the first engagement portion 43a and the second engagement portion 45a are engaged with the second guide hole 65. A groove-like positioning groove 66 recessed radially outward is formed in the center of the radially outer peripheral edge of the second guide hole 65. The circumferential dimension (groove width) of the positioning groove 66 is slightly larger than the outer diameters of the first engagement portion 43a and the second engagement portion 45a.

[0052] In the state of FIG. 9, the first engagement portion 43a of the first bearing member 43 engages with the bottom portion 64a of the recess 64. As a result, the first roller 40 is pressed against the driving roller 10 via the intermediate transfer belt 8 by the biasing force of the first coil spring 48 (see FIG. 5), forming a secondary transfer nip portion N, and the first roller 40 rotates following the drive roller 10. A transfer voltage of the opposite polarity (negative polarity here) to that of the toner is applied to the first roller 40 by a transfer voltage power source 74 (see FIG. 8). Specifically, when the first roller 40 is disposed at the position of FIG. 9, the transfer voltage is applied via the first bearing member 43 electrically connected to the transfer voltage power source 74.

[0053] Additionally, the first light shielding plate 51a (see FIG. 4) of the shaft 51 shields (ON) the detection portion of the first position detection sensor S1, and the first light shielding plate 47d (see FIG. 6) of the roller holder 47 shields (ON) the detection portion of the second position detection sensor S2. This state (S1 / S2 ON) is the reference position (home position) of the first roller 40. The rotation angle of the switching cam 50 is regulated based on the rotation time of the switching cam 50 from this reference position, and the position and separation state of the first roller 40 are controlled.

[0054] Additionally, the first engagement portion 43a engages with the positioning groove 66 of the fixed cam 52. This allows the first roller 40 to be positioned at a reference position with high accuracy.

[0055] Next, switching control and position detection control of the first roller 40 and the second roller 41 in the secondary transfer unit 9 of this embodiment will be described using Figures 11 to 18, while also referring to Figures 4 to 10 as necessary. Note that the fixed cam 52 is omitted in Figures 11 to 18.

[0056] FIG. 11 is a diagram showing a state in which the switching cam 50 is rotated clockwise by a predetermined angle (here, 10.6° from the reference position in FIG. 9) from the state in FIG. 9. When the shaft 51 is rotated clockwise, the switching cam 50 also rotates together with the shaft 51. Meanwhile, the roller holder 47 is restricted from rotating in the clockwise direction by the restricting rib 9b (see FIG. 5). As a result, the first engaging portion 43a of the first bearing member 43 moves from the bottom portion 64a of the recess 64 to the inclined portion 64b, and the first bearing member 43 moves in the positioning groove 66 in a direction approaching the shaft 51 against the biasing force of the first coil spring 48 (see FIG. 5). As a result, the first roller 40 is in a state (first separated state) in which it is slightly (2 mm) separated from the intermediate transfer belt 8.

[0057] If the first roller 40 is kept in pressure contact with the driving roller 10 for a long period of time, the first roller 40 may bend and deform in the axial direction. Therefore, it is necessary to separate the first roller 40 from the intermediate transfer belt 8 (driving roller 10) after the job is completed. At this time, the first roller 40 is in the first separated state shown in FIG.

[0058] Also, the first light shielding plate 51a of the shaft 51 is retracted (OFF) from the detection portion of the first position detection sensor S1, and the second light shielding plate 47d of the roller holder 47 continues to shield (ON) the detection portion of the second position detection sensor S2. That is, when the detection state shifts from Fig. 9 (S1 / S2 ON) to the detection state of Fig. 11 (S1 OFF / S2 ON), it is possible to detect the movement of the first roller 40 from the reference position to the first separated state.

[0059] FIG. 12 is a diagram showing a state in which the switching cam 50 is further rotated in the clockwise direction by a predetermined angle (here, 46.4° from the reference position in FIG. 9) from the state in FIG. 11. When the shaft 51 is further rotated in the clockwise direction, the switching cam 50 also rotates further in the clockwise direction together with the shaft 51. On the other hand, the clockwise rotation of the roller holder 47 is restricted by the restricting rib 9b (see FIG. 5). As a result, the first engaging portion 43a of the first bearing member 43 moves from the recess 64, and the first bearing member 43 moves further in a direction approaching the shaft 51 against the biasing force of the first coil spring 48 (see FIG. 5), and the engagement with the positioning groove 66 is released. As a result, the first roller 40 is in a state (second separated state) in which it is completely separated (6.5 mm) from the intermediate transfer belt 8. This second separated state is used only when switching from the first roller 40 to the second roller 41.

[0060] The detection states of the first position detection sensor S1 and the second position detection sensor S2 in Fig. 12 are the same as the first separated state shown in Fig. 11 (S1 OFF / S2 ON). Therefore, when the S1 OFF / S2 ON state is set at the start of image forming apparatus 100, in order to distinguish between the first separated state and the second separated state, roller holder 47 is rotated toward the main body of image forming apparatus 100 (counterclockwise direction) for a certain period of time. If the S1 / S2 ON state is set, the state is determined to be the first separated state, and if the S1 / S2 ON state is not set, the state is determined to be the second separated state.

[0061] Furthermore, when returning the first roller 40 to the reference position from the second separated state, the roller holder 47 and the switching cam 50 must first be rotated counterclockwise to switch to the reference position of the second roller 41 (see Figure 13), and then returned to the reference position of the first roller 40 (see Figure 9).

[0062] Next, a procedure for switching the roller forming the secondary transfer nip portion N from the first roller 40 to the second roller 41 will be described. When the shaft 51 is rotated counterclockwise from the second separated state shown in FIG. 12, the switching cam 50 also rotates counterclockwise together with the shaft 51. The first bearing member 43 is biased in a direction away from the shaft 51 by the biasing force of the first coil spring 48 (see FIG. 5), and the second bearing member 45 is biased by the biasing force of the second coil spring 49 (see FIG. 5). Therefore, the first engagement portion 43a and the second engagement portion 45a are pressed against the radially outer peripheral portion of the first guide hole 63 of the switching cam 50. As a result, the roller holder 47 also rotates counterclockwise together with the switching cam 50.

[0063] Then, when the roller holder 47 rotates until it abuts against the restricting rib 9c (see FIG. 5), the second roller 41 is disposed in a position facing the drive roller 10 as shown in FIG. 13. In the state of FIG. 13, the first light shielding plate 51a of the shaft 51 is retracted (OFF) from the detection portion of the first position detection sensor S1, and the second light shielding plate 47d of the roller holder 47 is retracted (OFF) from the detection portion of the second position detection sensor S2. That is, when the detection state of FIG. 12 (S1 OFF / S2 ON) is shifted to the detection state of FIG. 13 (S1 / S2 OFF), the movement of the second roller 41 to the position facing the drive roller 10 can be detected.

[0064] Fig. 14 is a diagram showing a state in which the switching cam 50 has been rotated a predetermined angle counterclockwise from the state shown in Fig. 13. When the shaft 51 is rotated counterclockwise, the switching cam 50 also rotates together with the shaft 51. Meanwhile, the roller holder 47 is restricted from rotating in the counterclockwise direction by the restricting rib 9c (see Fig. 5). As a result, the second engaging portion 45a of the second bearing member 45 moves to the bottom portion 64a of the recess 64, and the second bearing member 45 moves in a direction away from the shaft 51 due to the biasing force of the second coil spring 49 (see Fig. 5).

[0065] As a result, the second roller 41 is pressed against the drive roller 10 via the intermediate transfer belt 8 to form a secondary transfer nip portion N, and the second roller 41 rotates following the drive roller 10. A transfer voltage of the opposite polarity to the toner (negative polarity here) is applied to the second roller 41 by a transfer voltage power source 74 (see FIG. 8). Specifically, when the second roller 41 is disposed at the position shown in FIG. 14, the transfer voltage is applied via a second bearing member 45 electrically connected to the transfer voltage power source 74.

[0066] Also, the first light shielding plate 51a of the shaft 51 shields (ON) the detection portion of the first position detection sensor S1, and the second light shielding plate 47d of the roller holder 47 is retracted (OFF) from the detection portion of the second position detection sensor S2. This state (S1 ON / S2 OFF) is set as the reference position (home position) of the second roller 41. That is, when the detection state of FIG. 13 (S1 OFF / S2 ON) is shifted to the detection state of FIG. 14 (S1 ON / S2 OFF), the movement of the second roller 41 to the reference position can be detected. The rotation angle of the switching cam 50 is regulated based on the rotation time of the switching cam 50 from this reference position, and the arrangement and separation state of the second roller 41 are controlled.

[0067] FIG. 15 is a diagram showing a state in which the switching cam 50 is rotated counterclockwise by a predetermined angle (here, 10.6° from the reference position in FIG. 14) from the state in FIG. 14. When the shaft 51 is rotated counterclockwise, the switching cam 50 also rotates counterclockwise together with the shaft 51. On the other hand, the roller holder 47 is restricted from rotating in the counterclockwise direction by the restricting rib 9c (see FIG. 5). As a result, the second engaging portion 45a of the second bearing member 45 moves from the bottom portion 64a of the recess 64 to the inclined portion 64b, and the second bearing member 45 moves in a direction approaching the shaft 51 against the biasing force of the second coil spring 49 (see FIG. 5). As a result, the second roller 41 is in a state (first separated state) in which it is slightly (2 mm) separated from the intermediate transfer belt 8.

[0068] If the second roller 41 is kept in pressure contact with the drive roller 10 for a long period of time, the second roller 41 may bend and deform in the axial direction. Therefore, it is necessary to separate the second roller 41 from the intermediate transfer belt 8 (drive roller 10) after the job is completed. At this time, the second roller 41 is in the first separated state shown in FIG. 15. When performing calibration while the second roller 41 is in use, the second roller 41 is in the first separated state so that the reference image formed on the intermediate transfer belt 8 does not adhere to the second roller 41. When performing calibration with the second roller 41 in the first separated state, the reference image can also be formed in the center of the intermediate transfer belt 8 in the width direction.

[0069] Also, the first light shielding plate 51a of the shaft 51 is retracted (OFF) from the detection portion of the first position detection sensor S1, and the second light shielding plate 47d of the roller holder 47 continues to be retracted (OFF) from the detection portion of the second position detection sensor S2. That is, when the detection state transitions from Fig. 14 (S1 ON / S2 OFF) to the detection state transitions from Fig. 15 (S1 / S2 OFF), it is possible to detect the movement of the second roller 41 from the reference position to the first separated state.

[0070] FIG. 16 is a diagram showing a state in which the switching cam 50 is further rotated counterclockwise by a predetermined angle (here, 46.4° from the reference position in FIG. 14) from the state in FIG. 15. When the shaft 51 is further rotated counterclockwise, the switching cam 50 also rotates counterclockwise together with the shaft 51. On the other hand, the roller holder 47 is restricted from rotating counterclockwise by the restricting rib 9c (see FIG. 5). As a result, the second engaging portion 45a of the second bearing member 45 moves from the recess 64, and the second bearing member 45 moves further toward the shaft 51 against the biasing force of the second coil spring 49 (see FIG. 5). This brings the second roller 41 into a state (second separated state) in which it is completely separated (6.5 mm) from the intermediate transfer belt 8. This second separated state is used only when switching from the second roller 41 to the first roller 40.

[0071] The detection states of the first position detection sensor S1 and the second position detection sensor S2 in Fig. 16 are the same as the first separation state (S1 / S2 off) shown in Fig. 15. Therefore, when the S1 / S2 off state is detected at the start of the image forming apparatus 100, the roller holder 47 is rotated toward the double-sided conveying path 18 (clockwise direction) for a certain period of time in order to distinguish between the first separation state and the second separation state. Then, if the S1 on / S2 off state is detected, the state is determined to be the first separation state, and if the S1 on / S2 off state is not detected, the state is determined to be the second separation state.

[0072] Furthermore, when returning the second roller 41 to the reference position from the second separated state, the roller holder 47 and the switching cam 50 must first be rotated clockwise to switch to the reference position of the first roller 40 (see Figure 9), and then returned to the reference position of the second roller 41 (see Figure 14).

[0073] When the roller forming the secondary transfer nip portion N is switched from the second roller 41 to the first roller 40, the shaft 51 is rotated a predetermined angle in the clockwise direction from the second separated state shown in FIG. 16. As a result, the switching cam 50 and the roller holder 47 also rotate a predetermined angle in the clockwise direction, and when the roller holder 47 rotates until it abuts against the regulating rib 9b, the first roller 40 faces the drive roller 10 as shown in FIG. 17. When the switching cam 50 is further rotated a predetermined angle in the clockwise direction from the state shown in FIG. 17, the first roller 40 is placed at the reference position as shown in FIG. 9. Thereafter, the above procedure is repeated to switch between the first roller 40 and the second roller 41.

[0074] Fig. 18 is a side view showing a state in which the core metal 40a of the first roller 40 arranged at the reference position that forms the secondary transfer nip portion N is fitted into the shaft holder 37 of the intermediate transfer unit 30. The shaft holder 37 is formed on each of a pair of side frames 30a that support both ends of the drive roller 10 and the primary transfer rollers 6a to 6d of the intermediate transfer unit 30. Note that Fig. 18 shows only one of the side frames 30a and the shaft holder 37.

[0075] The shaft holder 37 holds both ends of the core 40a of the first roller 40 or the core 41a of the second roller 41, which are arranged at the reference position. This makes it possible to position the first roller 40 or the second roller 41 at the reference position with high accuracy.

[0076] 19 is a side view showing a state in which the first engagement portion 43a of the first bearing portion 43 does not engage with the positioning groove 66 of the fixed cam 52, and the metal core 40a of the first roller 40 is misaligned from the shaft holding portion 37. When switching the roller arranged in the reference position to the first roller 40, as shown in FIG. 19, the position of the roller holder 47 may be shifted downward due to the influence of the weight of the first roller 40 and the roller holder 47, and the metal core 40a may not fit into the shaft holding portion 37 of the intermediate transfer unit 30. As a result, the first roller 40 may not be accurately positioned in the reference position, and the secondary transfer nip N may not be formed well.

[0077] 20 is a perspective view of the fixed cam 52 used in the secondary transfer unit 9 of this embodiment, seen from the side facing the roller holder 47. The fixed cam 52 is made of a resin material, and as shown in FIG. 20, a holder positioning protrusion 67 is integrally formed on the surface facing the roller holder 47.

[0078] The holder positioning protrusion 67 is formed between the through hole 52a and the second guide hole 65. The holder positioning protrusion 67 has a trapezoidal shape in a side view having a pair of inclined surfaces 67a that are inclined along the rotation direction of the roller holder 47 (the left-right direction in FIG. 20), and a hemispherical protrusion 67b is formed between the pair of inclined surfaces 67a (at the top of the holder positioning protrusion 67).

[0079] 21 is a perspective view of roller holder 47 used in secondary transfer unit 9 of this embodiment, seen from the side of the surface facing fixed cam 52. As shown in FIG. 21, roller holder 47 has holder positioning recess 68 formed on the surface facing fixed cam 52. Holder positioning recess 68 has an elliptical shape that is long in the radial direction (the up-down direction in FIG. 21) perpendicular to the rotation direction of roller holder 47. The outer diameter of protrusion 67b (see FIG. 20) of holder positioning protrusion 67 is slightly larger than the inner diameter of holder positioning recess 68 in the rotation direction of roller holder 47 (the horizontal direction in FIG. 21).

[0080] When placing first roller 40 in the reference position, shaft 51 is rotated clockwise by a predetermined angle from the state in Fig. 16 to bring first roller 40 into the state in Fig. 17 where it faces drive roller 10. At this time, roller holder 47 moves from the state in Fig. 16 to the state in Fig. 17 while riding up along inclined surface 67a of holder positioning protrusion 67 formed on fixed cam 52.

[0081] Figure 22 is a perspective view, seen from the inside in the axial direction, of the state in which the holder positioning protrusion 67 of the fixed cam 52 is engaged with the holder positioning recess 68 of the roller holder 47. When the roller holder 47 moves to the state shown in Figure 17, as shown in Figure 18, the first engagement portion 43a engages with the positioning groove 66 of the fixed cam 52. Also, as shown in Figure 22, the protrusion 67b of the holder positioning protrusion 67 engages with the holder positioning recess 68 of the roller holder 47. At this time, since the outer diameter of the protrusion 67b is slightly larger than the inner diameter of the holder positioning recess 68, the protrusion 67b is held in a state in which it is slightly biting into the holder positioning recess 68.

[0082] That is, roller holder 47 is positioned relative to fixed cam 52 at two points, namely, positioning groove 66 and holder positioning protrusion 67. As a result, first roller 40 is positioned opposite drive roller 10 with high precision.

[0083] By rotating the switching cam 50 clockwise by a predetermined angle from the state shown in Fig. 17, the first engagement portion 43a of the first bearing portion 43 moves to the recess 64 of the switching cam 50, and the first roller 40 is placed in the reference position as shown in Fig. 9. As a result, the core metal 40a of the first roller 40 fits into the shaft holding portion 37 of the intermediate transfer unit 30 as shown in Fig. 18.

[0084] Fig. 23 is a perspective view showing a state in which holder positioning protrusion 67 formed of a leaf spring is engaged with holder positioning recess 68 of roller holder 47. Note that Fig. 23 shows only holder positioning protrusion 67 attached to fixed cam 52, and omits the illustration of fixed cam 52 itself.

[0085] The holder positioning protrusion 67 is formed of a leaf spring, and has a main body 67c and a connecting portion 67d. The main body 67c is formed with an insertion hole 67e into which the shaft 51 (see FIG. 22) is inserted, and a screw hole 67f for fixing to the fixed cam 52 with a screw. The connecting portion 67d protrudes from the main body 67c in a tongue-like shape, and a semispherical protrusion 67b is formed at its tip. The connecting portion 67d is elastically deformable in a direction approaching or moving away from the roller holder 47.

[0086] In the example shown in Fig. 23, when roller holder 47 moves from the state in Fig. 16 to the state in Fig. 17, the elasticity of the leaf spring causes connecting portion 67d of holder positioning protrusion 67 to elastically deform in a direction away from roller holder 47. Then, when roller holder 47 moves to the state in Fig. 17, the restoring force of the leaf spring causes connecting portion 67d to elastically deform in a direction approaching roller holder 47. As shown in Fig. 23, protrusion 67b formed on connecting portion 67d engages with holder positioning recess 68 of roller holder 47.

[0087] As a result, roller holder 47 is positioned relative to fixed cam 52 at two points, namely, positioning groove 66 and holder positioning protrusion 67. Therefore, first roller 40 is positioned at the reference position with high accuracy.

[0088] Furthermore, when the holder positioning protrusions 67 are formed with leaf springs, the connecting portions 67d engage with the holder positioning recesses 68 while elastically deforming. Therefore, compared to when the holder positioning protrusions 67 are integrally formed with the resin fixed cam 52, it is possible to reduce the rotational load of the roller holder 47 when the positioning protrusions 67 fit into the holder positioning recesses 68. Furthermore, wear of the holder positioning protrusions 67 can also be prevented.

[0089] 14, when the roller arranged in the reference position is switched to the second roller 41, the roller holder 47 is in contact with the unit frame 9a. Therefore, there is no risk of the position of the roller holder 47 being shifted downward due to the weight of the first roller 40 and the second roller 41. Therefore, it is only necessary for the holder positioning convex portion 67 (protrusion 67b) to engage with the holder positioning concave portion 68 of the roller holder 47 when the roller arranged in the reference position is switched to the first roller 40.

[0090] 23, a third light-shielding plate 47e is provided in addition to the second light-shielding plate 47d on the roller holder 47. Furthermore, as shown in Fig. 24, a third position detection sensor S3 is provided on the unit frame 9a in addition to the second position detection sensor S2. With this configuration, as the roller holder 47 rotates, the third light-shielding plate 47e blocks (turns on) the detection portion of the third position detection sensor S3, making it possible to easily detect the reference position of the second roller 41.

[0091] According to the configuration of this embodiment, with a simple configuration using a roller holder 47 and a switching cam 50, either the first roller 40 or the second roller 41 can be positioned opposite the drive roller 10, and the first roller 40 or the second roller 41 positioned opposite the drive roller 10 can be selectively positioned between a reference position that forms the secondary transfer nip portion N and a spaced position spaced away from the intermediate transfer belt 8.

[0092] For example, when the paper S is equal to or smaller than a predetermined size (here, A3 size), the first roller 40 having an elastic layer 40a with a small axial length is placed at the reference position. As a result, when a reference image is formed outside the image area in the width direction of the intermediate transfer belt 8 (outside the first roller 40 in the axial direction) during image formation and calibration is performed, the reference image formed on the intermediate transfer belt 8 does not come into contact with the first roller 40. Therefore, calibration can be performed during image formation, and image quality can be improved without reducing image processing efficiency (productivity).

[0093] Also, it is possible to effectively prevent the back side of the paper S from being soiled due to the toner adhering to the first roller 40 adhering to the paper S. Furthermore, since there is no need to perform a cleaning operation for returning the toner adhering to the first roller 40 onto the intermediate transfer belt 8, the print waiting time can also be shortened.

[0094] On the other hand, when the paper S is larger than the predetermined size (here, 13 inches), the second roller 41 having the elastic layer 41a with a large axial length is placed at the reference position. This ensures that the secondary transfer of the toner image can be performed to both ends of the large-sized paper S in the width direction.

[0095] In this embodiment, in addition to the switching cam 50, a fixed cam 52 in which a second guide hole 65 and a positioning groove 66 are formed is disposed. As a result, when the first roller 40 is disposed in a position facing the driving roller 10, the first engagement portion 43a of the first bearing member 43 engages with the positioning groove 66 and is positioned. Also, when the second roller 41 is disposed in a position facing the driving roller 10, the second engagement portion 45a of the second bearing member 45 engages with the positioning groove 66 and is positioned. Then, when the first roller 40 and the second roller 41 are moved between the reference position, the first separated state, and the second separated state, the first engagement portion 43a and the second engagement portion 45a move along the positioning groove 66.

[0096] Therefore, there is no risk of the first roller 40 and the second roller 41 being displaced in the circumferential direction due to the rotation of the switching cam 50, and it is possible to improve the positional accuracy when switching between the pressed state and the separated state of the first roller 40 and the second roller 41. Also, the pressed state and the separated state of the first roller 40 and the second roller 41 can be switched smoothly, and the occurrence of shocks, vibrations, abnormal noise, etc. during switching can be suppressed.

[0097] Furthermore, in this embodiment, a holder positioning protrusion 67 is formed on the surface of fixed cam 52 facing roller holder 47, and a holder positioning recess 68 is formed on the surface of roller holder 47 facing fixed cam 52. This eliminates the risk of roller holder 47 being displaced downward by the weight of first roller 40 when first roller 40 is placed in the reference position, and makes it possible to prevent poor fitting of core metal 40a with shaft holding portion 37 of intermediate transfer unit 30.

[0098] In this embodiment, the separation positions of the first roller 40 and the second roller 41 can be switched between a first separation state in which the separation distance from the intermediate transfer belt 8 is small, and a second separation state in which the separation distance is large. This allows the first roller 40 and the second roller 41 to be separated from the drive roller 10 at the end of a job to prevent deformation of the first roller 40 and the second roller 41, and when calibration is performed while the second roller 41 is in use, by setting the first roller 40 and the second roller 41 to the first separation state, it is possible to shorten the time until the first roller 40 and the second roller 41 are positioned at the reference position for forming the secondary transfer nip N. This allows the reduction in image processing efficiency (productivity) associated with the movement of the first roller 40 and the second roller 41 to be minimized.

[0099] Furthermore, in this embodiment, the roller holder 47 and the switching cam 50 can be driven using one roller switching motor 55. This makes it possible to simplify the drive mechanism and drive control compared to the case where the roller holder 47 and the switching cam 50 are driven using separate motors, contributing to reducing the cost and size of the image forming apparatus 100.

[0100] The present invention is not limited to the above embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, the shapes, dimensions, etc. of the first roller 40, the second roller 41, the roller holder 47, the switching cam 50, the fixed cam 52, etc., which constitute the secondary transfer unit 9, are merely examples, and can be modified as desired without impairing the effects of the present invention.

[0101] In addition, in the above embodiment, an intermediate transfer type image forming device 100 is exemplified, which is equipped with a secondary transfer unit 9 that performs a second transfer of the toner image that has been primarily transferred onto the intermediate transfer belt 8 onto the paper S. However, the present invention can be similarly applied to a transfer unit mounted on a direct transfer type image forming device that directly transfers a toner image formed on a photosensitive drum onto the paper. [Industrial Applicability]

[0102] The present invention can be used in an image forming apparatus equipped with a transfer unit that transfers a toner image formed on an image carrier to a recording medium. By using the present invention, it is possible to provide a transfer unit that can improve the positional accuracy of the rollers and the smoothness of the switching operation when switching between two transfer rollers that are selectively pressed against the image carrier, and an image forming apparatus equipped with the same. [Explanation of symbols]

[0103] Pa~Pd Image forming section 1a to 1d Photoconductor drum (image carrier) 6a~6d Primary transfer roller 8 Intermediate transfer belt (image carrier) 9 Secondary transfer unit (transfer unit) 9a Unit Frame 25 Image density sensor 30 Intermediate transfer unit 30a Side Frame 37 Shaft holding part 40 First roller (transfer roller) 41 Second roller (transfer roller) 43 First bearing member 43a first engagement portion 45 Second bearing member 45a Second engaging part 47 Roller holder 48 First coil spring 49 Second coil spring 50 Switching Cam 51 Shaft (drive mechanism) 52 Fixed Cam 55 Roller switching motor (drive mechanism) 63 First guide hole 64 Recess 65 Second guide hole 66 Positioning groove 67 Holder positioning protrusion 67a Slope 67b Protrusion 68 Holder positioning recess 90 Control section 100 Image forming device N Secondary transfer nip S Paper (recording medium) S1 1st position detection sensor S2 Second position detection sensor S3 3rd position detection sensor

Claims

1. A transfer unit includes a transfer roller having a core bar and an elastic layer laminated on an outer peripheral surface of the core bar, the elastic layer being pressed against an image carrier to form a transfer nip portion, the transfer unit transferring a toner image formed on the image carrier to a recording medium passing through the transfer nip portion, the transfer roller including a first roller and a second roller disposed above the first roller and having an axial length of the elastic layer different from that of the first roller; a first bearing member that rotatably supports the first roller; a second bearing member that rotatably supports the second roller; a roller holder having a first bearing holding portion and a second bearing holding portion that hold the first bearing member and the second bearing member so as to be slidable in a direction approaching or moving away from the image carrier, respectively; a first biasing member disposed between the first bearing holder and the first bearing member and biasing the first bearing member in a direction approaching the image carrier; a second biasing member disposed between the second bearing holder and the second bearing member and biasing the second bearing member in a direction approaching the image carrier; a switching cam having a first guide hole with which a first engaging portion formed on the first bearing member and a second engaging portion formed on the second bearing member engage; a drive mechanism that rotates the roller holder and the switching cam; a unit frame that rotatably supports the roller holder and the switching cam; A fixed cam fixed to the unit frame; Equipped with By rotating the roller holder, one of the first roller and the second roller is disposed opposite to the image carrier, and the switching cam is rotated to change an engagement position of the first engagement portion or the second engagement portion in the first guide hole, thereby selectively disposing the first roller or the second roller disposed opposite the image carrier at a reference position where the roller is pressed against the image carrier to form a transfer nip portion, or at a spaced position spaced from the image carrier; The fixed cam is a second guide hole formed to overlap the first guide hole and into which the first engagement portion and the second engagement portion engage; a positioning groove formed on a radially outer peripheral edge of the second guide hole, the positioning groove being engaged with the first engaging portion when the first roller is disposed opposite the image carrier, and the positioning groove being engaged with the second engaging portion when the second roller is disposed opposite the image carrier; a holder positioning protrusion formed on a surface facing the roller holder and adapted to engage with a holder positioning recess of the roller holder; having a holder positioning recess that engages with the holder positioning protrusion when the first roller is disposed opposite to the image carrier;

2. The fixed cam is made of a resin material, 2. The transfer unit according to claim 1, wherein the holder positioning protrusion is formed integrally with the surface of the fixed cam facing the roller holder.

3. 3. The transfer unit according to claim 2, wherein the holder positioning protrusion has a trapezoidal shape in a side view and has a pair of inclined surfaces that are inclined along the rotation direction of the roller holder.

4. 2. The transfer unit according to claim 1, wherein the holder positioning protrusion is a leaf spring member attached to a surface of the fixed cam facing the roller holder and elastically deformable in a direction toward or away from the roller holder.

5. The holder positioning recess has an elliptical shape that is elongated in a radial direction perpendicular to a rotation direction of the roller holder, 2. The transfer unit according to claim 1, wherein the holder positioning protrusion has a semi-spherical projection that engages with the holder positioning recess.

6. 6. The transfer unit according to claim 5, wherein an outer diameter of the protrusion is slightly larger than an inner diameter of the holder positioning recess in the rotation direction.

7. A plurality of image forming units for forming the toner images of different colors; an endless intermediate transfer belt as the image carrier that moves along the image forming unit; a plurality of primary transfer members disposed opposite the photosensitive drums disposed in the image forming units with the intermediate transfer belt interposed therebetween, and which primarily transfer the toner images formed on the photosensitive drums onto the intermediate transfer belt; 7. An image forming apparatus comprising: a secondary transfer unit as the transfer unit according to claim 1, which secondarily transfers the toner image, which has been primarily transferred onto the intermediate transfer belt, onto the recording medium.

8. a pair of shaft holders for holding both ends of the core of the first roller or the second roller disposed opposite the image carrier; 8. The image forming apparatus according to claim 7, wherein the core metal of the first roller is held by the shaft holding portion when the holder positioning protrusion engages with the holder positioning recess.

Images