Transfer unit and image forming apparatus equipped therewith

Abstract

The present invention provides a transfer unit capable of improving the positional accuracy and smoothness of the switching operation of two transfer rollers that are selectively pressed against an image carrier, and an image forming apparatus equipped therewith. [Solution] The transfer unit comprises a switching cam and a fixed cam. The fixed cam has a second guide hole, a positioning groove, and a cam positioning recess. The second guide hole is formed to overlap the first guide hole, and the first engaging portion and the second engaging portion engage with it. The positioning groove is formed on the radially outer peripheral edge of the second guide hole, and the first engaging portion engages when the first roller is positioned opposite the image carrier, and the second engaging portion engages when the second roller is positioned opposite the image carrier. The cam positioning recess is formed on the radially outer peripheral edge of the second guide hole, and the second engaging portion engages when the first engaging portion engages with the positioning groove.

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 photoreceptor drum or an intermediate transfer belt to a recording medium, and an image forming apparatus including the same, and particularly relates to a mechanism for switching the arrangement of a plurality of transfer members.

Background Art

[0002] Conventionally, an intermediate transfer type image forming apparatus is known that includes an endless intermediate transfer belt that rotates in a predetermined direction and a plurality of image forming units provided along the intermediate transfer belt. After sequentially superimposing and primarily transferring toner images of respective colors on the intermediate transfer belt by each image forming unit, the toner image is secondarily transferred onto a recording medium such as paper by a secondary transfer roller.

[0003] In such an intermediate transfer type image forming apparatus, toner adhesion to the surface of the secondary transfer roller progresses due to durable printing. In particular, in order to improve color development and color reproducibility, it is necessary to perform calibration for correcting image density and color shift at a predetermined timing. However, the patch image formed on the intermediate transfer belt during the execution of calibration is removed by a belt cleaning device without being transferred to the paper. Therefore, a part of the toner transferred onto the intermediate transfer belt adheres to the secondary transfer roller when the patch image passes through the secondary transfer roller.

[0004] Conventionally, a method has been used to clean the secondary transfer roller by applying a reverse transfer voltage (a voltage having the same polarity as the toner) to the secondary transfer roller during non-image forming to return the toner adhering to the secondary transfer roller to the intermediate transfer belt. However, this method has a problem in that it takes time to clean the secondary transfer roller, resulting in a long printing waiting time.

[0005] Therefore, a method has been proposed to improve productivity by making the secondary transfer rollers switchable to a size suitable for the recording medium. For example, Patent Document 1 discloses an image forming apparatus comprising a plurality of secondary transfer rollers having different axial lengths, a rotating body having a support part that rotatably supports the plurality of secondary transfer rollers and is rotatable around an axis parallel to the axial direction, and a control unit that selects one roller from the plurality of secondary transfer rollers according to the width of the recording medium and rotates the support part to bring the one roller facing the intermediate transfer belt.

[0006] Patent Document 2 discloses a transfer unit comprising 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 supply, 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 select between a reference position where the first roller or the second roller facing the image carrier is pressed against the image carrier to form a transfer nip, and a separated position where it is separated from the image carrier. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2017-156653 [Patent Document 2] Japanese Patent Publication No. 2022-112901 [Overview of the project] [Problems that the invention aims to solve]

[0008] In the configuration described in Patent Document 1, the specific configuration of the separation mechanism for separating the secondary transfer roller from the intermediate transfer belt was not described, and there was a risk of damage to the secondary transfer roller or intermediate transfer belt when switching or replacing the secondary transfer roller. In the configuration described in Patent Document 2, there was room for further improvement in the positional accuracy of the transfer roller and the smoothness of the 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 and an image forming apparatus equipped therewith that can improve the positional accuracy of two transfer rollers that are selectively pressed against an image carrier, as well as the smoothness of the switching operation when switching between them. [Means for solving the problem]

[0010] To achieve the above objective, the first configuration of the present invention is a transfer unit that transfers a toner image formed on an image carrier to a recording medium through which the transfer nip portion passes, comprising a transfer roller having a core metal and an elastic layer laminated on the outer circumferential surface of the core metal, and pressing the elastic layer against an image carrier to form a transfer nip portion. The transfer unit comprises a transfer roller composed 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 positioned above the first roller and is different from 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 holding portion and a second bearing holding portion 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 positioned between the first bearing holder and the first bearing member and biases the first bearing member toward the image carrier. The second biasing member is positioned between the second bearing holder and the second bearing member and biases the second bearing member toward the image carrier. The switching cam has a first guide hole into which a first engaging portion formed in the first bearing member and a second engaging 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 facing 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 facing the image carrier is selected to be positioned at a reference position where it is pressed against the image carrier to form a transfer nip portion, and at a separated position away from the image carrier. The fixing cam has a second guide hole, a positioning groove, and a cam positioning recess. The second guide hole is formed to overlap the first guide hole, and the first engagement portion and the second engagement portion engage with it.A positioning groove is formed on the radially outer peripheral edge of the second guide hole, and the first engaging portion engages when the first roller is positioned opposite the image carrier, and the second engaging portion engages when the second roller is positioned opposite the image carrier. A cam positioning recess is formed on the radially outer peripheral edge of the second guide hole, and the second engaging portion engages when the first engaging portion engages with the positioning groove. [Effects of the Invention]

[0011] According to the first configuration of the present invention, when the first roller is positioned opposite the image carrier, the first engaging portion of the first bearing member engages with the positioning groove of the fixed cam, and the second engaging portion of the second bearing member engages with the cam positioning recess of the fixed cam. Therefore, it is possible to prevent the first roller from shifting circumferentially due to the rotation of the switching cam and to improve the positional accuracy when positioning the first roller at the reference position. [Brief explanation of the drawing]

[0012] [Figure 1] A schematic diagram showing the internal configuration of an image forming apparatus 100 equipped with the secondary transfer unit 9 of the present invention. [Figure 2] Figure 1: Enlarged view of the area near the image forming section Pa. [Figure 3] Side cross-sectional view of the intermediate transfer unit 30 mounted on the image forming apparatus 100. [Figure 4] Perspective view of a secondary transfer unit 9 according to one embodiment of the present invention, mounted on an image forming apparatus 100. [Figure 5] Enlarged perspective view showing the configuration of the roller holder 47 of the secondary transfer unit 9 of this embodiment. [Figure 6] Perspective view of the area around the roller holder 47 of the secondary transfer unit 9, viewed from the axial inner side. [Figure 7] Perspective view showing the drive mechanism of the secondary transfer unit 9 of this embodiment. [Figure 8] Block diagram showing an example of the control path of an image forming apparatus 100 equipped with the secondary transfer unit 9 of this embodiment. [Figure 9]A side cross-sectional view showing the periphery of the switching cam 50 of the secondary transfer unit 9 of the present embodiment, as viewed from the inner side in the axial direction with the first roller 40 disposed at the reference position forming the secondary transfer nip portion N [Figure 10] A view showing a state in which the switching cam 50 is removed from the state of FIG. 9 to expose the fixed cam 52 [Figure 11] A view showing a separated state of the first roller 40 in which the switching cam 50 is rotated counterclockwise by a predetermined angle from the state of FIG. 9 [Figure 12] A view showing a state in which the switching cam 50 is further rotated counterclockwise by a predetermined angle from the state of FIG. 11 to oppose the second roller 41 to the driving roller 10 [Figure 13] A view showing a state in which the switching cam 50 is rotated clockwise by a predetermined angle from the state of FIG. 12 so that the second roller 41 is disposed at the reference position forming the secondary transfer nip portion N [Figure 14] A view showing a separated state of the second roller 41 in which the switching cam 50 is further rotated clockwise by a predetermined angle from the state of FIG. 13 [Figure 15] A side view showing a state in which the shaft core 40a of the first roller 40 disposed at the reference position forming the secondary transfer nip portion N is fitted into the shaft holding portion 37 of the intermediate transfer unit 30 [Figure 16] A side view showing a state in which the first engaging portion 43a of the first bearing member 43 is not engaged with the positioning groove 66 of the fixed cam 52 and the shaft core 40a of the first roller 40 is displaced from the shaft holding portion 37 [Figure 17] A perspective view of the fixed cam 52 used in the secondary transfer unit 9 of the present embodiment as viewed from the side facing the roller holder 47 [Figure 18] A perspective view of the roller holder 47 used in the secondary transfer unit 9 of the present embodiment as viewed from the side facing the fixed cam 52 [Figure 19] A perspective view as viewed from the inner side in the axial direction of a state in which the holder positioning convex portion 67 of the fixed cam 52 is engaged with the holder positioning concave portion 68 of the roller holder 47

Embodiments for Carrying Out the Invention

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

[0014] The image forming apparatus 100 shown in Figure 1 is a so-called tandem color printer and has the following configuration. Inside the main body of the image forming apparatus 100, four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side in the transport direction (left side in Figure 1). These image forming units Pa to Pd are provided to correspond to images of four different colors (magenta, cyan, yellow, and black), and each sequentially forms magenta, cyan, yellow, and black images through the processes of charging, exposure, development, and transfer, respectively.

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

[0016] The paper S on which the toner image is transferred is housed 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 the paper feed roller 12a and the registration roller pair 12b. A seamless belt is mainly used for the intermediate transfer belt 8.

[0017] Next, the image forming sections Pa to Pd will be described. The image forming section Pa will be described in detail below, but the image forming sections Pb to Pd have basically the same configuration, so their description will be omitted. As shown in Figure 2, a charging device 2a, a developing device 3a, and a cleaning device 7a are arranged around the photoreceptor drum 1a along the drum rotation direction (clockwise direction in Figure 2), and a primary transfer roller 6a is positioned with the intermediate transfer belt 8 in between. Also, a belt cleaning unit 19 is positioned upstream of the intermediate transfer belt 8 in the rotation direction relative to the photoreceptor drum 1a, facing the tension roller 11 with the intermediate transfer belt 8 in between.

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

[0019] Each developing unit 3a to 3d is filled with a predetermined amount of magenta, cyan, yellow, and black toner, respectively. If the proportion of toner in the two-component developer filled in each developing unit 3a to 3d falls below a specified value due to the formation of the toner image described later, toner is replenished from the toner containers 4a to 4d to each developing unit 3a to 3d. This toner in the developer is supplied onto the photoreceptor drums 1a to 1d by the developing rollers 21 of the developing units 3a to 3d and adheres electrostatically. This forms a toner image corresponding to the electrostatic latent image formed by exposure from the exposure unit 5.

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

[0021] As the drive roller 10 rotates due to the belt-driven motor 61 (see Figure 8), the intermediate transfer belt 8 begins to rotate counterclockwise. The paper S is then transported from the registration roller pair 12b to the secondary transfer unit 9, which is located adjacent to the intermediate transfer belt 8, at a predetermined timing, and a full-color image is transferred. The paper S, on which the toner image has been transferred, is then transported to the fuser unit 13. Any toner remaining on the surface of the intermediate transfer belt 8 is removed by the belt cleaning unit 19.

[0022] The paper S transported to the fuser unit 13 is heated and pressurized by the fuser roller pair 13a, fixing the toner image to the surface of the paper S and forming a predetermined full-color image. The paper S on which the full-color image has been formed is then transported in a direction determined by the branching unit 14 which branches in multiple directions, and is then discharged into the discharge tray 17 by the discharge roller pair 15 (or after being sent to the double-sided transport path 18 for double-sided printing).

[0023] An image density sensor 25 is positioned downstream of the image forming unit Pd, facing the intermediate transfer belt 8. Generally, an optical sensor is used as the image density sensor 25, which includes a light-emitting element such as an LED and a light-receiving element such as a photodiode. When measuring the amount of toner deposited on the intermediate transfer belt 8, measurement light is irradiated from the light-emitting element onto each patch image (reference image) formed on the intermediate transfer belt 8. The measurement light is then incident on the light-receiving element as light reflected by the toner and light reflected by the belt surface.

[0024] The light reflected from the toner and belt surface includes specular and diffuse reflection. This specular and diffuse reflection is separated by a polarization separation prism and then incident on separate photodetectors. Each photodetector converts the received specular and diffuse reflections into photoelectric signals and outputs them to the control unit 90 (see Figure 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 specular and diffuse reflection. By comparing these with 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., density correction and color shift correction (calibration) are performed for each color.

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

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

[0028] The intermediate transfer unit 30 includes a roller contact / separation mechanism 35 which has a pair of support members (not shown) that rotatably support both ends of the rotation axes of the primary transfer rollers 6a to 6d and the pressure switching roller 34 and move perpendicular to the direction of travel of the intermediate transfer belt 8 (up and down direction in Figure 3), and a driving means (not shown) that reciprocates the primary transfer rollers 6a to 6d and the pressure switching roller 34 in the up and down direction. The roller contact / separation mechanism 35 can be switched between a color mode in which the four primary transfer rollers 6a to 6d are each pressed against the photoreceptor drums 1a to 1d (see Figure 1) via the intermediate transfer belt 8, a monochrome mode in which only the primary transfer roller 6d is pressed against the photoreceptor drum 1d via the intermediate transfer belt 8, and a retraction mode in which all four primary transfer rollers 6a to 6d are separated from the photoreceptor drums 1a to 1d.

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

[0030] As shown in Figures 4 to 7, the secondary transfer unit 9 comprises 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 and 41b are laminated on the outer surfaces of core metals 40a and 41a, respectively. As the material for the elastic layers 40b and 41b, for example, an ion-conductive rubber such as ECO (epichlorohydrin rubber) is used.

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

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

[0034] A pair of roller holders 47 are positioned at both axial ends of the first roller 40 and the second roller 41. The roller holders 47 are approximately V-shaped in side view and have a first bearing holder 47a, a second bearing holder 47b, and a through 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 through hole 47c is formed at the apex of the V-shape, through which the shaft 51 is rotatably inserted. The roller holders 47 are made of an insulating material such as synthetic resin.

[0035] As shown in Figure 5, a first coil spring 48 is positioned between the first bearing holder 47a and the first bearing member 43. A second coil spring 49 is positioned 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 it is pressed against the drive roller 10).

[0036] As shown in Figure 4, a first light-shielding plate 51a is attached to the shaft 51, and by shielding the detection part of the first position detection sensor S1 (see Figure 9), the rotation angle of the shaft 51 can be detected. Also, as shown in Figure 6, a second light-shielding plate 47d is formed on one side of the roller holder 47 in the direction of rotation. The second light-shielding plate 47d is formed in a position that can shield the detection part of the second position detection sensor S2 which is located on the unit frame 9a.

[0037] The positions of the first roller 40 and the second roller 41 supported by the roller holder 47 can be detected by turning on or off the first position detection sensor S1 and the second position detection sensor S2 in accordance with the rotation angle of the roller holder 47 (shaft 51) via the first light-shielding plate 51a and the second light-shielding plate 47d. The position detection control of the first roller 40 and the second roller 41 will be described later.

[0038] The switching cams 50 are arranged in pairs inside the roller holders 47 at both axial ends of the first roller 40 and the second roller 41. The switching cams 50 have a fan shape with a portion cut out when viewed from the side, and the key part of the fan shape (the vertex where the two radii intersect) is fixed to the shaft 51.

[0039] As shown in Figure 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 switching control of the first roller 40 and the second roller 41 will be described later.

[0040] Figure 8 is a block diagram showing an example of the control path of an image forming apparatus 100 equipped with the secondary transfer unit 9 of this embodiment. Since various controls are performed on different parts of the image forming apparatus 100 during its operation, the overall control path of the image forming apparatus 100 is complex. Therefore, this section will focus on explaining 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 memory, a RAM (Random Access Memory) 93 as a read-write memory, a temporary memory 94 for temporarily storing image data, etc., a counter 95, and multiple (in this case, two) I / F (interfaces) 96 for transmitting control signals to each device within the image forming apparatus 100 and receiving input signals from the operation unit 80. Furthermore, the control unit 90 can be placed anywhere inside the main body of the image forming apparatus 100.

[0042] ROM 92 stores control programs for the image forming apparatus 100, necessary control values, and other data that should not be changed during use of the image forming apparatus 100. RAM 93 stores necessary data generated during the control of the image forming apparatus 100, as well as data temporarily required for the control of the image forming apparatus 100. RAM 93 (or ROM 92) also stores density correction tables used for calibration, and the relationship between the on / off states of the first position detection sensor S1 and the second position detection sensor S2 used for roller switching control (described later) and the rotation angles of the first roller 40 and the second roller 41. Counter 95 counts the number of printed sheets by accumulating them.

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

[0044] The image input unit 70 is a receiving unit that receives image data transmitted to the image forming apparatus 100 from a higher-level device such as a personal computer. 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 by output signals from the control unit 90. Each of these power supplies applies a predetermined voltage according to the control signal from the voltage control circuit 71: the charging voltage power supply 72 applies a predetermined voltage to the charging rollers 20 in the charging devices 2a to 2d, the developing voltage power supply 73 applies a predetermined voltage to the developing rollers 21 in the developing devices 3a to 3d, and the transfer voltage power supply 74 applies a predetermined voltage to the primary transfer rollers 6a to 6d and the first roller 40 and second roller 41 in the secondary transfer unit 9.

[0046] The control unit 80 is equipped with a liquid crystal display 81 and LEDs 82 that indicate various statuses. The user can stop image formation by operating the stop / clear button on the control unit 80, and reset the various settings of the image forming apparatus 100 to their default state by operating the reset button. The liquid crystal display 81 is designed to show the status of the image forming apparatus 100, as well as the image formation status and the number of copies to be printed. Various settings of the image forming apparatus 100 are made from the printer driver on a personal computer.

[0047] Figure 9 is a side cross-sectional view showing the area around the switching cam 50 of the secondary transfer unit 9 of this embodiment, and is a view from the axial inside with the first roller 40 positioned to form the secondary transfer nip portion N. Figure 10 is a diagram showing the state in which the switching cam 50 has been removed from the state in Figure 9, exposing the fixed cam 52.

[0048] As shown in Figure 9, the switching cam 50 has a fan shape in plan view. The switching cam 50 has an arc-shaped first guide hole 63. The first guide hole 63 engages with a first engaging portion 43a formed in the first bearing member 43 and a second engaging portion 45a formed in the second bearing member 45.

[0049] The first guide hole 63 has a recess 64 and an engaging recess 64b. The recess 64 is formed by further recessing the radially outer peripheral edge of the first guide hole 63 radially outward, and spans from the circumferential center of the radially outer peripheral edge of the first guide hole 63 to one side in the circumferential direction (left side in Figure 9).

[0050] In this embodiment, the recess 64 is formed in a V-shape in a cross-section perpendicular to the axial direction. The bottom 64a of the recess 64 is curved in a semicircular arc. By rotating the switching cam 50, the first engaging portion 43a and the second engaging portion 45a are positioned in or separated from the recess 64. This makes it possible to switch the contact state of the first roller 40 and the second roller 41 with respect to the intermediate transfer belt 8. The switching control and position detection control of the first roller 40 and the second roller 41 will be described in detail later.

[0051] The engaging recess 64b is formed at the other circumferential end of the first guide hole 63 (right side in Figure 9) by further recessing the radially outer peripheral edge of the first guide hole 63 radially outward. The second engaging portion 45a engages with the engaging recess 64b.

[0052] The fixed cam 52 is positioned between the roller holder 47 and the switching cam 50. The fixed cam 52 is screw-fixed to the unit frame 9a of the secondary transfer unit 9 (see Figure 10).

[0053] The fixed cam 52 has a through hole 52a, an arc-shaped second guide hole 65, a positioning groove 66, and a cam positioning recess 69. A shaft 51 is rotatably inserted through the through hole 52a.

[0054] The second guide hole 65 is formed in a position that overlaps with the first guide hole 63 of the switching cam 50, and the first engaging portion 43a and the second engaging portion 45a engage with it.

[0055] The positioning groove 66 engages with the first engaging portion 43a when the first roller 40 is positioned opposite the drive roller (image carrier) 10, and with the second engaging portion 45a when the second roller 41 is positioned opposite the drive roller (image carrier) 10.

[0056] In this embodiment, the positioning groove 66 is formed as a recess extending radially outward from the circumferential center at the radially outer peripheral edge of the second guide hole 65. Furthermore, the circumferential dimension (groove width) of the positioning groove 66 is slightly larger than the outer diameter of the first engaging portion 43a and the second engaging portion 45a.

[0057] The cam positioning recess 69 is formed on the radially outer peripheral edge of the second guide hole 65 and is positioned circumferentially alongside the positioning groove 66. Furthermore, when the first engaging portion 43a of the cam positioning recess 69 engages with the positioning groove 66, the second engaging portion 45a engages with the cam positioning recess 69.

[0058] In this embodiment, the cam positioning recess 69 is formed as a recess extending radially outward from the other circumferential side (right side in Figure 10) of the radially outer peripheral edge of the second guide hole 65. Furthermore, the cam positioning recess 69 is formed to be shallower radially than the positioning groove 66, and the circumferential dimension (width) of the cam positioning recess 69 is slightly larger than the outer diameter of the second engagement portion 45a.

[0059] In the state shown in Figure 9, the first engaging 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 drive roller 10 via the intermediate transfer belt 8 by the biasing force of the first coil spring 48 (see Figure 5), forming a secondary transfer nip portion N. As a result, the first roller 40 rotates in conjunction with the drive roller 10. A transfer voltage opposite in polarity to the toner (in this case, negative polarity) is applied to the first roller 40 by the transfer voltage power supply 74 (see Figure 8). Specifically, when the first roller 40 is positioned in the position shown in Figure 9, the transfer voltage is applied via the first bearing member 43, which is electrically connected to the transfer voltage power supply 74.

[0060] Furthermore, the first light-shielding plate 51a of the shaft 51 (see Figure 4) shields (turns on) the detection part of the first position detection sensor S1, and the first light-shielding plate 47d of the roller holder 47 (see Figure 6) shields (turns on) the detection part of the second position detection sensor S2. This state (S1 / S2 on) is set as 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, thereby controlling the arrangement and spacing of the first roller 40.

[0061] Furthermore, when the first engaging portion 43a engages with the positioning groove 66 of the fixed cam 52, the second engaging portion 45a engages with the cam positioning recess 69. This prevents the first roller from shifting circumferentially due to the rotation of the switching cam, and improves the positional accuracy when positioning the first roller at the reference position.

[0062] Next, referring to Figures 4 to 10 as needed, the 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 explained using Figures 11 to 15.

[0063] Figure 11 shows the state after the switching cam 50 has been rotated counterclockwise by a predetermined angle from the state shown in Figure 9. When the shaft 51 is rotated counterclockwise, the switching cam 50 rotates together with the shaft 51. At this time, the first engaging portion 43a moves from the bottom 64a of the recess 64 along the inclined surface of the recess 64. The first bearing member 43 also moves within the positioning groove 66 toward the shaft 51 against the biasing force of the first coil spring 48 (see Figure 5). Furthermore, when the first engaging portion 43a moves to the outside of the recess 64, the engagement with the positioning groove 66 is released. At this time, the first roller 40 is separated from the intermediate transfer belt 8 (separated state).

[0064] Furthermore, the second engaging portion 45a moves along the inclined surface of the recess 64 due to the rotation of the switching cam 50. At this time, the second bearing member 45 moves within the cam positioning recess 69 toward the shaft 51 against the biasing force of the second coil spring 49 (see Figure 5). When the second engaging portion 45a moves to the outside of the recess 64, the engagement with the cam positioning recess 69 is released. This releases the engagement between the roller holder 47 and the fixed cam 52.

[0065] If the first roller 40 is pressed against the drive roller 10 for an extended period, there is a risk that the first roller 40 will bend and deform in the axial direction. Therefore, it is necessary to separate the first roller 40 from the intermediate transfer belt 8 (drive roller 10) after the job is completed. At this time, the separated state is as shown in Figure 11.

[0066] Furthermore, the first light-shielding plate 51a of the shaft 51 is retracted (turned off) from the detection unit of the first position detection sensor S1, while the second light-shielding plate 47d of the roller holder 47 continues to shield (turn on) the detection unit of the second position detection sensor S2. That is, when transitioning from the detection state in Figure 9 (S1 / S2 on) to the detection state in Figure 11 (S1 off / S2 on), it is possible to detect the movement of the first roller 40 away from its reference position.

[0067] Furthermore, when returning the first roller 40 from the separated state to the reference position, the roller holder 47 and the switching cam 50 are rotated clockwise. At this time, the rotation of the roller holder 47 in the clockwise direction is restricted by the regulating rib 9b (see Figure 5). As a result, the first engaging portion 43a and the second engaging portion 45a move into the recess 64 and engage with the positioning groove 66 and the cam positioning recess 69, respectively.

[0068] Next, the procedure for switching the roller that forms 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 separated state shown in Figure 11, the switching cam 50 also rotates counterclockwise along with the shaft 51. At this time, the first bearing member 43 is biased to move away from the shaft 51 by the biasing force of the first coil spring 48 (see Figure 5). Also, the second bearing member 45 is biased to move away from the shaft 51 by the biasing force of the second coil spring 49 (see Figure 5). As a result, the first engaging portion 43a and the second engaging portion 45a are pressed against the radially outer peripheral edge of the first guide hole 63 of the switching cam 50. Consequently, the roller holder 47 also rotates counterclockwise along with the switching cam 50.

[0069] Then, when the roller holder 47 rotates until it contacts the regulating rib 9c (see Figure 5), the second roller 41 is positioned opposite the drive roller 10, as shown in Figure 12. At this time, a part of the second engaging portion 45a enters the open end of the positioning groove 66. In the state shown in Figure 12, the first light-shielding plate 51a of the shaft 51 is retracted (off) from the detection unit 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 unit of the second position detection sensor S2. That is, when transitioning from the detection state in Figure 11 (S1 off / S2 on) to the detection state in Figure 12 (S1 / S2 off), the movement of the second roller 41 to a position opposite the drive roller 10 can be detected.

[0070] Figure 13 shows the state after the switching cam 50 has been rotated clockwise by a predetermined angle from the state shown in Figure 12. When the shaft 51 is rotated clockwise, the switching cam 50 rotates together with the shaft 51. On the other hand, since a part of the second engaging portion 45a enters and engages with the open end of the positioning groove 66, the rotation of the roller holder 47 in the clockwise direction is restricted. Therefore, only the switching cam 50 rotates clockwise. The second bearing member 45 is subjected to a force acting in a direction that separates it from the shaft 51 due to the biasing force of the second coil spring 49 (see Figure 5). As a result, the second engaging portion 45a slides along the radially outer peripheral edge of the first guide hole 63 and gradually moves towards the back of the positioning groove 66.

[0071] Furthermore, when the second engaging portion 45a reaches the bottom 64a of the recess 64, the biasing force of the second coil spring 49 (see Figure 5) presses the second roller 41 against the drive roller 10 via the intermediate transfer belt 8, forming a secondary transfer nip portion N. As a result, the second roller 41 rotates in conjunction with the drive roller 10. A transfer voltage opposite in polarity (negative in this case) to the toner is applied to the second roller 41 by the transfer voltage power supply 74 (see Figure 8). Specifically, when the second roller 41 is positioned as shown in Figure 13, the transfer voltage is applied via the second bearing member 45, which is electrically connected to the transfer voltage power supply 74.

[0072] Furthermore, the first light-shielding plate 51a of the shaft 51 shields (turns on) the detection part of the first position detection sensor S1, and the second light-shielding plate 47d of the roller holder 47 is retracted (turns off) from the detection part 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 transitioning from the detection state in Figure 12 (S1 / S2 off) to the detection state in Figure 13 (S1 on / S2 off), the movement of the second roller 41 to the reference position can be detected. Based on the rotation time of the switching cam 50 from this reference position, the rotation angle of the switching cam 50 is regulated to control the position and separation state of the second roller 41.

[0073] If the second roller 41 is pressed against the drive roller 10 for an extended period, there is a risk that 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 shaft 51 is rotated counterclockwise to achieve the separated state shown in Figure 12.

[0074] Furthermore, when performing calibration while the second roller 41 is in use, the second roller 41 is separated to prevent the reference image formed on the intermediate transfer belt 8 from adhering to the second roller 41. Note that when performing calibration with the second roller 41 separated, the reference image can also be formed in the center of the intermediate transfer belt 8 in the width direction.

[0075] At this time, the first light-shielding plate 51a of the shaft 51 is retracted (turned off) from the detection unit of the first position detection sensor S1, and the second light-shielding plate 47d of the roller holder 47 continues to be retracted (turned off) from the detection unit of the second position detection sensor S2. That is, when transitioning from the detection state in Figure 13 (S1 on / S2 off) to the detection state in Figure 12 (S1 / S2 off), it is possible to detect the movement of the second roller 41 away from its reference position.

[0076] Next, the procedure for switching the roller that forms the secondary transfer nip portion N from the second roller 41 to the first roller 40 will be described. Figure 14 shows the state in which the switching cam 50 has been rotated by a predetermined angle in the counterclockwise direction from the state in Figure 13. When the shaft 51 is rotated in the counterclockwise direction, the switching cam 50 also rotates in the counterclockwise direction along with the shaft 51. On the other hand, the rotation of the roller holder 47 in the counterclockwise direction is restricted by the regulating rib 9c (see Figure 5). As a result, the second engaging portion 45a moves from the bottom 64a of the recess 64 to the other end in the circumferential direction of the first guide hole 63 (right side in Figure 13) and engages with the engaging recess 64b.

[0077] At this point, the first engaging portion 43a has moved to the outside of the recess 64, and its engagement with the positioning groove 66 has been released. Similarly, the second engaging portion 45a has also moved to the outside of the recess 64, and its engagement with the cam positioning recess 69 has been released.

[0078] Next, when the shaft 51 is rotated clockwise, the switching cam 50 also rotates clockwise along with the shaft 51. At this time, the second engaging portion 45a engages with the engaging recess 64b, and the roller holder 47 also rotates clockwise by a predetermined angle. When the shaft 51 is rotated further clockwise, the roller holder 47 comes into contact with the regulating rib 9b (see Figure 5), and its clockwise rotation is restricted. As a result, the first engaging portion 43a and the second engaging portion 45a move into the recess 64 and engage with the positioning groove 66 and the cam positioning recess 69, respectively. This results in the state shown in Figure 9, where the first roller 40 is positioned in the reference position. The first roller 40 and the second roller 41 are then switched by repeating the above procedure.

[0079] Figure 15 is a side view showing the state in which the core metal 40a of the first roller 40, positioned at the reference position for forming the secondary transfer nip portion N, is fitted into the shaft holding portion 37 of the intermediate transfer unit 30. A pair of side frames 30a that support both ends of the drive roller 10 and primary transfer rollers 6a to 6d of the intermediate transfer unit 30 each have a shaft holding portion 37 formed therein. Note that only one side frame 30a and shaft holding portion 37 are shown in Figure 15.

[0080] The shaft holding portion 37 holds both ends of the core metal 40a of the first roller 40 or the core metal 41a of the second roller 41, which are positioned at the reference position. This allows the first roller 40 or the second roller 41 to be accurately positioned at the reference position.

[0081] Figure 16 is a side view showing a state in which the first engaging portion 43a of the first bearing member 43 does not engage with the positioning groove 66 of the fixed cam 52, and the core metal 40a of the first roller 40 is misaligned from the shaft holding portion 37. When switching the roller positioned at the reference position to the first roller 40, as shown in Figure 16, the position of the roller holder 47 may shift downward due to the influence of the weight of the first roller 40 and the roller holder 47, and the core metal 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 at the reference position, and the secondary transfer nip portion N may not be formed properly.

[0082] Figure 17 is a perspective view of the fixed cam 52 used in the secondary transfer unit 9 of this embodiment, viewed from the side facing the roller holder 47. The fixed cam 52 is made of a resin material, and as shown in Figure 17, a holder positioning projection 67 is integrally formed on the surface facing the roller holder 47.

[0083] The holder positioning projection 67 is formed between the through hole 52a and the second guide hole 65. The holder positioning projection 67 has a trapezoidal shape in side view, having a pair of inclined surfaces 67a that are inclined along the rotational direction of the roller holder 47 (left-right direction in Figure 17), and a hemispherical projection 67b is formed between the pair of inclined surfaces 67a (the top of the holder positioning projection 67).

[0084] Figure 18 is a perspective view of the roller holder 47 used in the secondary transfer unit 9 of this embodiment, viewed from the side facing the fixed cam 52. As shown in Figure 18, the roller holder 47 has a holder positioning recess 68 formed on the surface facing the fixed cam 52. The holder positioning recess 68 has an elongated elliptical shape in the radial direction (up and down direction in Figure 18) perpendicular to the rotational direction of the roller holder 47. The outer diameter of the projection 67b of the holder positioning protrusion 67 (see Figure 17) is slightly larger than the inner diameter of the holder positioning recess 68 in the rotational direction of the roller holder 47 (horizontal direction in Figure 18).

[0085] When positioning the first roller 40 in the reference position, the shaft 51 is rotated clockwise from the state shown in Figure 14 to the state shown in Figure 9, where the first roller 40 faces the drive roller 10. At this time, the roller holder 47 moves while riding up along the inclined surface 67a of the holder positioning projection 67 formed on the fixed cam 52.

[0086] Figure 19 is a perspective view from the axial inside of the state in which the holder positioning projection 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 9, the first engaging portion 43a engages with the positioning groove 66 of the fixed cam 52. The second engaging portion 45a also engages with the cam positioning recess 69 of the fixed cam 52. In addition, the projection 67b of the holder positioning projection 67 engages with the holder positioning recess 68 of the roller holder 47. At this time, since the outer diameter of the projection 67b is slightly larger than the inner diameter of the holder positioning recess 68, the projection 67b is held in a state where it is slightly bitten into the holder positioning recess 68.

[0087] Specifically, the roller holder 47 is positioned relative to the fixed cam 52 at three points: a positioning groove 66, a cam positioning recess 69, and a holder positioning protrusion 67. This ensures that the first roller 40 is precisely positioned opposite the drive roller 10.

[0088] 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 selected to be positioned at a reference position for forming the secondary transfer nip portion N and at a separated position away from the intermediate transfer belt 8.

[0089] For example, when the paper size S is less than or equal to a predetermined size (in this case, A3 size), the first roller 40 having an elastic layer 40b with a small axial length is positioned at the reference position. This ensures that when calibration is performed by forming a reference image outside the image area in the width direction of the intermediate transfer belt 8 (outside the axial direction of the first roller 40) during image formation, 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, improving image quality without reducing image processing efficiency (productivity).

[0090] Furthermore, it effectively suppresses smudging of the back of the paper S caused by toner adhering to the first roller 40 adhering to the paper S. In addition, since there is no need to perform a cleaning operation to return the toner adhering to the first roller 40 onto the intermediate transfer belt 8, the printing waiting time can also be shortened.

[0091] On the other hand, if the paper S is larger than a predetermined size (in this case, 13 inches), a second roller 41 having an elastic layer 41a with a larger axial length is positioned at the reference position. This ensures reliable secondary transfer of the toner image to both ends of the large-sized paper S in the width direction.

[0092] In this embodiment, in addition to the switching cam 50, a fixed cam 52 is provided, which has a second guide hole 65, a positioning groove 66, and a cam positioning recess 69. As a result, when the first roller 40 is positioned opposite the drive roller 10, the first engaging portion 43a of the first bearing member 43 engages with the positioning groove 66 to position it. Also, the second engaging portion 45a of the second bearing member 45 engages with the cam positioning recess 69 to position it.

[0093] Therefore, there is no risk of the first roller 40 being misaligned in the circumferential direction due to the rotation of the switching cam 50, and the positional accuracy when switching between the pressed and separated states of the first roller 40 and the second roller 41 can be improved. In addition, the switching between the pressed and separated states of the first roller 40 can be performed smoothly, and the generation of shocks, vibrations, and abnormal noises during switching can be suppressed.

[0094] In this embodiment, a holder positioning projection 67 is formed on the surface of the fixed cam 52 facing the roller holder 47, and a holder positioning recess 68 is formed on the surface of the roller holder 47 facing the fixed cam 52. This prevents the roller holder 47 from shifting downward due to the weight of the first roller 40 when the first roller 40 is placed in the reference position, and prevents improper fitting of the core metal 40a to the shaft holding portion 37 of the intermediate transfer unit 30.

[0095] Furthermore, the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention. For example, the shapes, dimensions, etc., of the first roller 40, second roller 41, roller holder 47, switching cam 50, fixed cam 52, etc., which constitute the secondary transfer unit 9 are examples and can be arbitrarily changed without hindering the effects of the present invention.

[0096] Furthermore, although the above embodiment illustrates an intermediate transfer type image forming apparatus 100 equipped with a secondary transfer unit 9 that secondarily transfers the toner image primary transferred onto the intermediate transfer belt 8 to the paper S, the same can be applied to a transfer unit mounted on a direct transfer type image forming apparatus that directly transfers the toner image formed on the photoreceptor drum to the paper. [Industrial applicability]

[0097] The present invention is applicable to an image forming apparatus equipped with a transfer unit that transfers a toner image formed on an image carrier to a recording medium. By utilizing the present invention, it is possible to provide a transfer unit and an image forming apparatus equipped therewith 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. [Explanation of symbols]

[0098] Pa~Pd Image Forming Unit 1a~1d Photoreceptor 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 64a bottom 64b Engaging recess 65 Second guide hole 66 Positioning grooves 67 Holder positioning protrusion 67a Slope 67b Protrusion 68 Holder positioning recess 69 Cam positioning recess 90 Control Unit 100 Image forming apparatus N Secondary transfer nip section S Paper (recording medium) S1 First position detection sensor S2 Second position detection sensor S3 Third position detection sensor

Claims

1. A transfer unit comprising a core metal and an elastic layer laminated on the outer surface of the core metal, a transfer roller that presses the elastic layer against an image carrier to form a transfer nip, and a transfer unit that transfers a toner image formed on the image carrier to a recording medium through which the transfer nip passes, The transfer roller comprises a first roller and a second roller positioned above the first roller and different from 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 holder and a second bearing holder 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, A first biasing member is disposed between the first bearing holding portion and the first bearing member and biases the first bearing member in a direction approaching the image carrier, A second biasing member is positioned between the second bearing holding portion and the second bearing member, and biases the second bearing member in a direction approaching the image carrier. A switching cam having a first guide hole into which a first engaging portion formed in the first bearing member and a second engaging portion formed in 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 aforementioned unit frame, Equipped with, By rotating the roller holder, either the first roller or the second roller is positioned opposite the image carrier, 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, which is positioned opposite the image carrier, is selected to be positioned at a reference position where it is pressed against the image carrier to form a transfer nip portion, and at a separated position where it is spaced away from the image carrier. The aforementioned fixed cam is A second guide hole is formed so as to overlap the first guide hole, and the first engaging portion and the second engaging portion engage with it, A positioning groove is formed on the radially outer peripheral edge of the second guide hole, such that the first engaging portion engages when the first roller is positioned opposite the image carrier, and the second engaging portion engages when the second roller is positioned opposite the image carrier. A cam positioning recess is formed on the radially outer peripheral edge of the second guide hole, and when the first engaging portion engages with the positioning groove, the second engaging portion engages with the cam positioning recess, A transfer unit having the following features.

2. When the aforementioned switching cam is rotated, The first engaging portion moves along the periphery of the first guide hole, and the first bearing member moves within the positioning groove against the biasing force of the first biasing member. The transfer unit according to claim 1, wherein the second engaging portion moves along the periphery of the first guide hole, and the second bearing member moves the cam positioning recess against the biasing force of the second biasing member.

3. The aforementioned fixed cam is The transfer unit according to claim 1 or claim 2, further comprising a holder positioning projection formed on the surface facing the roller holder, which engages with a holder positioning recess of the roller holder when the first roller is positioned facing the image carrier.

4. Multiple image forming units that form toner images of different colors, An endless intermediate transfer belt, which serves as the image carrier, moves along the image forming section, A plurality of primary transfer members are arranged opposite to the photoreceptor drums, which are positioned in each of the image forming sections with the intermediate transfer belt in between, and which primary transfer the toner image formed on the photoreceptor drum onto the intermediate transfer belt. An image forming apparatus comprising a secondary transfer unit as the transfer unit according to claim 1 or 2, which transfers the toner image primary transferred onto the intermediate transfer belt onto the recording medium.

Images