Sheet conveyance device and image forming device

Abstract

To reduce mutual pulling of sheets or bending after sheet reversal in a conveying direction.SOLUTION: This sheet conveyance device comprises: a first conveyance part; a second conveyance part; a drive source; and a drive transmission part. When the first conveyance part conveys a sheet in a second conveying direction while the second conveyance part does not convey a sheet, the drive transmission part is set in a second transmission state so as to convey the sheet by the first conveyance part at a first conveying speed by means of a drive force. The sheet conveying speed of the second conveyance part is a second conveying speed higher than the first conveying speed. The drive transmission part is set in a non-transmission state so as to permit the first conveyance part to operate following the sheet conveyed at the second conveying speed from when a contact state occurs where the sheed conveyed in the second conveying direction comes into contact with both the first and second conveyance parts until releasing of the contact state.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a sheet conveying device that conveys a sheet, and an image forming apparatus that forms an image on a sheet. [Background technology]

[0002] Patent document 1 describes an image forming device that is equipped with a switchback roller pair that reverses the conveying direction of a sheet and an intermediate re-conveying roller pair that conveys the sheet together with the switchback roller pair, and is capable of forming images on both sides of a sheet. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-127223 Summary of the Invention [Problem to be solved by the invention]

[0004] When the sheet is transported by two transport sections, one on the upstream side and one on the downstream side, after the transport direction of the sheet is reversed, the transport speed of one transport section may be set faster than the transport speed of the other transport section. In such a case, the sheet may be pulled or bent between the two transport sections.

[0005] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS In view of the above, an object of the present invention is to provide a sheet conveying device and an image forming apparatus capable of reducing the occurrence of pulling or bending of a sheet after the conveying direction of the sheet is reversed. [Means for solving the problem]

[0006] One aspect of the present invention is a sheet transport device including a first transport section that transports a sheet in a first transport direction and then transports the sheet in a second transport direction opposite to the first transport direction, a second transport section that transports the sheet in the second transport direction, a drive source that drives the first transport section, and a drive transmission section that can be in a first transmission state in which a drive force of the drive source is transmitted to the first transport section so as to transport the sheet in the first transport direction, a second transmission state in which the drive force is transmitted to the first transport section so as to transport the sheet in the second transport direction, and a non-transmission state in which the drive force is not transmitted to the first transport section, when the second conveying unit is not conveying the sheet, the drive transmission unit is in the second transmission state so that the first conveying unit conveys the sheet at a first conveying speed by the driving force, the conveying speed of the sheet by the second conveying unit is a second conveying speed that is faster than the first conveying speed, and after a contact state occurs in which the sheet conveyed in the second conveying direction contacts both the first conveying unit and the second conveying unit, the drive transmission unit is in the non-transmission state so as to allow the first conveying unit to operate in accordance with the sheet conveyed at the second conveying speed until the contact state is released. Effect of the Invention

[0007] According to the present invention, it is possible to provide a sheet conveying device and an image forming apparatus that can reduce the occurrence of pulling or bending of a sheet after the conveying direction of the sheet is reversed. [Brief description of the drawings]

[0008] [Figure 1] 1 is a schematic diagram of an image forming apparatus according to a first embodiment. [Diagram 2] 1A to 1E are diagrams illustrating double-sided printing according to a first embodiment. [Diagram 3] 1A to 1D are diagrams illustrating a drive transmission mechanism according to a first embodiment. [Figure 4] 3A to 3E are views showing a coupling portion according to the first embodiment. [Diagram 5] 5A to 5D are views for explaining the operation of the coupling portion according to the first embodiment. [Figure 6] 5A to 5D are views for explaining the operation of the coupling portion according to the first embodiment. [Figure 7] 13(a) to (e) are views showing a coupling portion according to a second embodiment. [Figure 8] 13A to 13D are diagrams illustrating the operation of a coupling portion according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0010] First Embodiment A first embodiment will be described. First, the overall configuration of an image forming apparatus 1 according to the first embodiment will be described with reference to Fig. 1. Fig. 1 is a schematic diagram showing a cross section of the image forming apparatus 1 cut along a plane perpendicular to the main scanning direction (the direction of the rotation axis of a photosensitive drum 22a) during image formation.

[0011] The image forming apparatus 1 of this embodiment is a color laser beam printer that forms a color image on a recording material by an electrophotographic process using toners of multiple colors. As the sheet S, which is the recording material (recording medium), various sheet materials of different sizes and materials can be used, including paper such as plain paper and thick paper, sheet materials with surface treatments such as coated paper, sheet materials with special shapes such as envelopes and index paper, plastic films, cloth, etc.

[0012] 1, the image forming apparatus 1 includes a housing 2, a sheet feeding section 10, an image forming section 20, a cartridge tray 21, a transfer conveying unit 30, a fixing unit 40, a scanner 50, a sheet conveying section 60, and a power supply unit 70. The image forming apparatus 1 also includes a belt cleaner unit 34 and a plurality of motors as driving sources. The image forming apparatus 1 of this embodiment includes six motors, a first motor 81 to a sixth motor 86.

[0013] The housing 2 accommodates a sheet feeding section 10, an image forming section 20, a transfer transport unit 30, a fixing unit 40, a scanner 50, a sheet transport section 60, a power supply unit 70, a belt cleaner unit 34, and motors (81 to 86).

[0014] The housing 2 has an opening 2A provided on the front side of the housing 2, and a front cover 2B that can open and close the opening 2A. Note that the front side is the downstream side in a sheet discharge direction in which a sheet S with an image formed thereon is discharged by a pair of discharge rollers 65, which will be described later. In addition, a discharge tray 2E is provided on the top surface of the housing 2.

[0015] The front cover 2B is configured to be rotatable with respect to other parts of the housing 2, centering on a rotation axis 2b that passes through the lower end of the front cover 2B. By rotating about the rotation axis 2b, the front cover 2B is movable between a closed position (position shown by a solid line in FIG. 1) in which the opening 2A is closed, and an open position (position shown by a dotted line in FIG. 1) in which the opening 2A is opened. By moving the front cover 2B to the open position to open the opening 2A, it becomes possible to pull out the cartridge tray 21 from the housing 2 through the opening 2A, and to insert the cartridge tray 21 into the housing 2.

[0016] The image forming section 20 of this embodiment includes four cartridges 22 (process units, process cartridges). Each cartridge 22 is attachable to and detachable from the main body of the image forming apparatus 1 (hereinafter, collectively referred to as detachable). The main body of the apparatus refers to the portion of the image forming apparatus 1 excluding the four cartridges 22 and the cartridge tray 21. Each cartridge 22 of this embodiment is detachable from the cartridge tray 21, and is housed in the housing 2 in a state where it is attached to the cartridge tray 21.

[0017] The four cartridges 22 are arranged in a line along the rotation direction of the transfer transport belt 31 of the transfer transport unit 30. In this embodiment, the four cartridges 22 are arranged in a line along the front-rear direction of the housing 2. The four cartridges 22 use black, yellow, magenta, and cyan toners as developers, respectively, to create toner images of each color on the sheet S. Each cartridge 22 includes a photosensitive drum 22a as an image carrier, a charger 22b as a charging means, a developing roller 22c as a developing means, and a cleaning roller 23 as a cleaning means. The photosensitive drum 22a is a photosensitive body having a photosensitive layer formed of an organic photosensitive material or the like on the outer periphery of a cylindrical base body.

[0018] The scanner 50 is disposed above the image forming unit 20. The scanner 50 includes a laser oscillator as a light source and an exposure optical system for scanning the surface of each photosensitive drum 22a with laser light from the laser oscillator.

[0019] The transfer conveying unit 30 is disposed below the image forming section 20. The transfer conveying unit 30 includes a transfer conveying belt 31, a driving roller 32a, a driven roller 32b, and four transfer rollers 33. The transfer conveying belt 31 is stretched across the driving roller 32a and the driven roller 32b. The transfer conveying belt 31 is driven to rotate by the driving roller 32a, which is rotated by a driving force supplied from a driving source. The four transfer rollers 33 are disposed corresponding to the four photosensitive drums 22a. Each transfer roller 33 is disposed in contact with the inner surface of the transfer conveying belt 31 so as to face the corresponding photosensitive drum 22a across the transfer conveying belt 31. A transfer section where a toner image is transferred to the sheet S is formed between each transfer roller 33 and the corresponding photosensitive drum 22a.

[0020] The sheet feeding section 10 is disposed at the bottom of the image forming apparatus 1. The sheet feeding section 10 includes a storage section 11 for storing the sheets S, a feeding roller 12 as a feeding means for feeding the sheets S, and a separation roller 13 and a separation pad 13a as a separation means for separating and conveying the sheets S. The separation roller 13 and the separation pad 13a are an example of the separation means, and for example, a roller pair including the separation roller 13 and a rubber roller that contacts the separation roller 13 may be used. A conveying roller pair 14 and a registration roller pair 15 are disposed downstream of the separation roller 13 and upstream of the transfer conveying unit 30 in the conveying direction of the sheets S.

[0021] The belt cleaner unit 34 is disposed below the transfer transport unit 30 and above the sheet feeding section 10. The belt cleaner unit 34 is disposed in front of the power supply unit 70. The belt cleaner unit 34 includes a cleaning member 35 that collects foreign matter such as waste toner on the transfer transport belt 31, and a container that contains the collected foreign matter.

[0022] The fixing unit 40 is disposed downstream of the image forming section 20 in the sheet conveying direction along the transfer conveying unit 30. The fixing unit 40 includes a heating roller 41 as a heating member, a pressure roller 42 as a pressure member, and a heating means for heating the heating roller 41. The heating means is, for example, a halogen lamp or a coil unit for induction heating. A nip portion (fixing nip) is formed between the heating roller 41 and the pressure roller 42, where the sheet S is sandwiched and conveyed.

[0023] In this embodiment, the conveying path from the conveying roller pair 14 through the registration roller pair 15, four transfer portions, and the fixing nip to the exit of the fixing unit 40 is called an image forming path P1. The image forming path P1 is a conveying path (first conveying path) on which an image is formed on the sheet S.

[0024] The sheet conveying section 60 is a conveying mechanism that conveys the sheet S that has passed through the fixing unit 40 toward the discharge tray 2E, or re-feeds the sheet S toward the image forming section 20. The sheet conveying section 60 is an example of a sheet conveying device.

[0025] The sheet conveying section 60 of this embodiment includes an intermediate discharge roller pair 64, a discharge roller pair 65, a reversing roller pair 61, an intermediate re-feed roller pair 62, a re-feed roller pair 63, and a switching flap 67. The reversing roller pair 61 is an example of a first conveying section that conveys a sheet in a first conveying direction and then conveys the sheet in a second conveying direction opposite to the first conveying direction. The intermediate re-feed roller pair 62 is an example of a second conveying section that conveys the sheet in the second conveying direction. The reversing roller pair 61 is an example of a first roller pair having a first nip portion that sandwiches the sheet, and the intermediate re-feed roller pair 62 is an example of a second roller pair having a second nip portion that sandwiches the sheet.

[0026] Further, the sheet conveying section 60 includes three conveying paths, a reversing path P2, a re-feeding path P3, and a discharge path P4, as conveying paths through which the sheet S is conveyed. Each conveying path (P2 to P4) has a conveying guide provided in the housing 2, and a conveying space through which the sheet S passes is formed between the conveying guides facing each other. The intermediate discharge roller pair 64 and the discharge roller pair 65 are disposed in the discharge path P4. The reversing roller pair 61 is disposed in the reversing path P2. The intermediate re-feed roller pair 62 and the re-feed roller pair 63 are disposed in the re-feeding path P3.

[0027] The reversing path P2 is a conveying path (second conveying path) along which the sheet S is reversed and conveyed in order to form an image on the second side of the sheet S, which has had an image formed on its first side while passing through the image forming path P1. The re-feeding path P3 is a conveying path (third conveying path) along which the reversed and conveyed sheet S is conveyed again toward the image forming unit 20. The discharge path P4 is a conveying path along which the sheet S on which the image has been formed is conveyed toward the discharge tray 2E.

[0028] In this embodiment, the reversing path P2 and the discharge path P4 branch off from the image forming path P1 downstream of the fixing unit 40 in the sheet conveying direction, but one conveying path may serve as both the reversing path P2 and the discharge path P4. In this case, the reversing roller pair 61 is disposed on the one conveying path so as to reverse and convey the sheet S and the sheet S from the housing 2.

[0029] In this embodiment, the discharge path P4 includes a range from the space between the tip of the switching flap 67 at the discharge position and the corresponding transport guide to the opening (discharge port) of the housing 2 where the discharge roller pair 65 is arranged. The reversing path P2 includes a range from the space between the tip of the switching flap 67 at the reversing position and the transport guide facing the tip to the opening of the housing 2 (the opening above the discharge roller pair 65 in FIG. 1). The re-feeding path P3 includes a range from the space between the tip of the switching flap 67 at the reversing position and the transport guide facing the tip to the nip portion of the transport roller pair 14, via the intermediate re-feed roller pair 62 and the re-feed roller pair 63.

[0030] The switching flap 67 is configured to be able to switch the transport path of the sheet S that has passed through the fixing unit 40 between a reverse path P2 and a discharge path P4. Specifically, the switching flap 67 is able to rotate around a rotation shaft 67a between a discharge position (position indicated by a solid line in FIG. 1) where the sheet S is guided to the discharge path P4, and a reverse position (position indicated by a dashed line in FIG. 1) where the sheet S is guided to the reverse path P2.

[0031] The power supply unit 70 is disposed above the sheet feeding unit 10 and below the transfer / transport unit 30 and the fixing unit 40. The power supply unit 70 supplies power to each component of the image forming apparatus 1. For example, the first motor 81 to the sixth motor 86 each receive power supply from the power supply unit 70 to rotate.

[0032] The first motor 81 drives the reversing roller pair 61 and the intermediate re-feed roller pair 62. The first motor 81 is an example of a drive source that drives the first conveying section. The second motor 82 drives the discharge roller pair 65 and the intermediate discharge roller pair 64. The third motor 83 drives the developing roller 22c. The fourth motor 84 drives the photosensitive drum 22a. The fifth motor 85 drives the fixing unit 40. The sixth motor 86 drives the feed roller 12, the separation roller 13, the conveying roller pair 14, the registration roller pair 15, and the re-feed roller pair 63.

[0033] (single-sided printing) We will now explain the image forming operation (hereinafter referred to as single-sided printing) in which the image forming apparatus 1 forms an image on only one side of the sheet S. The control unit of the image forming apparatus 1 starts the image forming operation when it receives an instruction to perform the image forming operation (print command) together with image information from an external host computer or the like.

[0034] In the image forming operation, a single-color toner image is formed on the surface of each photosensitive drum 22a in the image forming unit 20. First, the rotation of the photosensitive drum 22a is started, and the charger 22b uniformly charges the surface of the photosensitive drum 22a. The scanner 50 irradiates the photosensitive drum 22a with a laser beam in response to a video signal generated based on image information, and forms an electrostatic latent image on the surface of the photosensitive drum 22a. The developing roller 22c carries and supplies toner to the photosensitive drum 22a, and develops the electrostatic latent image into a toner image.

[0035] In parallel with the creation of the toner image, a sheet S is fed from the sheet feeding section 10 to the image forming section 20. Specifically, the feeding roller 12 starts rotating in a state in which it abuts against the top sheet S of the sheet stack stored in the storage section 11, and the top sheet S is fed from the storage section 11 and passes through a nip portion (separation nip) between the separation roller 13 and the separation pad 13a. At this time, when multiple sheets S enter the separation nip, the frictional force with the separation pad 13a prevents sheets S other than the top sheet S from passing through the separation nip. Therefore, the sheets S are separated one by one and conveyed to the image forming path P1.

[0036] The sheet S is conveyed to the registration roller pair 15 via the conveying roller pair 14. The registration roller pair 15 corrects the skew of the sheet S and then delivers the sheet S to the transfer conveying belt 31. The sheet S is conveyed by the transfer conveying belt 31 while passing through four transfer units in order. In each transfer unit, the toner is electrostatically attracted by the transfer roller 33, so that a single-color toner image is transferred from the photosensitive drum 22a to the sheet S. The four single-color toner images are transferred so as to be superimposed on each other, so that a full-color toner image is formed on the sheet S. Note that any deposits such as transfer residual toner that remain on the photosensitive drum 22a without being transferred to the sheet S in the transfer unit are removed from the photosensitive drum 22a by the cleaning roller 23.

[0037] The sheet S onto which the toner image has been transferred is transported to the fixing unit 40. The fixing unit 40 heats and pressurizes the toner image on the sheet S while nipping and transporting the sheet S between a heating roller 41 and a pressure roller 42 in a fixing nip. This fixes the image onto the sheet S. In this manner, a toner image is created in the image forming section 20, and the image is formed on one side of the sheet S while the sheet S fed from the sheet feeding section 10 passes through an image forming path P1.

[0038] The sheet S that has passed through the fixing unit 40 is guided to a discharge path P4 by a switching flap 67 that is pre-positioned at a discharge position (position indicated by a solid line in FIG. 1). Then, the sheet S is transported through the discharge path P4 by a pair of intermediate discharge rollers 64, and is discharged to the outside of the housing 2 by a pair of discharge rollers 65, and is stacked on a discharge tray 2E.

[0039] (Double-sided printing) An image forming operation in which the image forming apparatus 1 forms images on both sides of a sheet S (hereinafter referred to as double-sided printing) will be described with reference to Figs. 2(a) to (e). Figs. 2(a) to (e) show cross sections of the image forming apparatus 1 at various times during double-sided printing. In double-sided printing, a toner image is created in the image forming section 20, and the process in which an image is formed on one side (first side) of the sheet S fed from the sheet feeding section 10 while the sheet S passes through the image forming path P1 is the same as in single-sided printing, and therefore will not be described here.

[0040] 2A, in double-sided printing, the sheet S that passes through the fixing unit 40 for the first time (the sheet S with an image formed on its first side) is guided to a reversing path P2 by a switching flap 67 that is pre-positioned at a reversing position. When the leading edge of the sheet S reaches the reversing roller pair 61, the reversing roller pair 61 conveys the sheet S in the first conveying direction D1. During this process, the leading edge of the sheet S in the first conveying direction D1 temporarily protrudes outside the housing 2.

[0041] As shown in FIG. 2B, after the rear end of the sheet S in the first transport direction D1 passes through the fixing unit 40 and before the reverse roller pair 61 passes through the reverse roller pair 61, the reverse roller pair 61 switches the transport direction of the sheet S to a second transport direction D2 opposite to the first transport direction D1. The second transport direction D2 is a direction in which the sheet S is transported from the reverse roller pair 61 side to the intermediate re-feed roller pair 62 side along the reverse path P2 and the re-feed path P3. Specifically, the rotation direction of the first motor 81 that rotates and drives the reverse roller pair 61 is switched, and the rotation direction of the reverse roller pair 61 is reversed. As a result, the reverse roller pair 61 transports the sheet S toward the re-feed path P3. Also, the rear end of the sheet S in the first transport direction D1 is drawn into the housing 2 again. The operation in which the reverse roller pair 61 transports the sheet S in the first transport direction D1 and then in the second transport direction D2 is called reverse transport.

[0042] As shown in Figure 2 (c), the sheet S inverted and transported by the reversing roller pair 61 enters the re-feeding path P3 and is transported within the re-feeding path P3 while being sandwiched between both the reversing roller pair 61 and the intermediate re-feeding roller pair 62.

[0043] 2(d), after the rear end of the sheet S in the second transport direction D2 leaves the reversing roller pair 61, the sheet S is transported to the transport roller pair 14 by the intermediate re-feed roller pair 62 and the re-feed roller pair 63. As a result, the sheet S is transported again to the image forming path P1 with the first and second sides switched. After this, the process of forming an image on the second side of the sheet S while passing through the image forming path P1 is omitted because it is the same as the process of forming an image on the first side.

[0044] 2(e), the sheet S that has passed through the fixing unit 40 for the second time (the sheet S with images formed on the first and second sides) is guided to the discharge path P4 by the switching flap 67 that is positioned in advance at the discharge position. Then, the sheet S is transported through the discharge path P4 by the intermediate discharge roller pair 64, and is discharged to the outside of the housing 2 by the discharge roller pair 65, and is stacked on the discharge tray 2E.

[0045] (Protruding part of the transport guide) Here, as shown in the enlarged view on the right side of FIG. 2(c), a protrusion 68a provided on the conveying guide 68 at the entrance of the re-feeding path P3 will be described. The entrance of the re-feeding path P3 is composed of the conveying guide 68 and the conveying guide 69. The entrance of the re-feeding path P3 is a portion upstream of the intermediate re-feeding roller pair 62 in the sheet conveying direction in the re-feeding path P3. The conveying guide 68 is disposed between the reversing roller pair 61 and the intermediate re-feeding roller pair 62 in the second conveying direction D2, and is an example of a first guide that guides the sheet S. The conveying guide 69 is an example of a second guide that faces the first guide across a space through which the sheet S passes.

[0046] The conveying guide 68 is disposed so as to face the second side S2 of the sheet S that has been inverted and conveyed by the reversing roller pair 61. The conveying guide 69 is disposed so as to face the first side S1 of the sheet S that has been inverted and conveyed by the reversing roller pair 61. In other words, the conveying guide 69 is disposed so as to face the first side S1 (image side) of the sheet S that is conveyed from the reversing roller pair 61 toward the intermediate re-feed roller pair 62 after an image is formed on the first side S1 in double-sided printing. The conveying guide 68 is also disposed so as to face the second side S2 (non-image side) opposite to the first side S1 of the sheet S that is conveyed from the reversing roller pair 61 toward the intermediate re-feed roller pair 62.

[0047] The conveying guide 68 has a protrusion 68a that protrudes toward the opposing conveying guide 69 with respect to an imaginary line Lv that connects the nip portion of the reversing roller pair 61 and the nip portion of the intermediate re-feed roller pair 62. In other words, the first guide has a portion (protrusion 68a) that protrudes toward the second guide with respect to an imaginary line that connects the first nip portion and the second nip portion when viewed in the rotation axis direction of the first roller pair.

[0048] The protruding portion 68a has a function of adjusting the entering posture when the sheet S enters the nip portion of the intermediate re-feed roller pair 62. In other words, if the protruding portion 68a is not provided, the leading edge of the sheet S in the second conveying direction D2 may enter the nip portion of the intermediate re-feed roller pair 62 at an oblique angle with respect to the nip line Ln of the intermediate re-feed roller pair 62. The nip line Ln is a virtual line that passes through the nip portion of the intermediate re-feed roller pair 62 and perpendicularly intersects with a line connecting the rotation axes of the two rollers (62a, 62b) of the intermediate re-feed roller pair 62. When the leading edge of the sheet S enters the nip portion at an oblique angle with respect to the nip line Ln, the leading edge of the sheet S may come into contact with the rollers (62a, 62b) at a position away from the nip portion, which may lead to conveyance failure such as jamming or skewing. In contrast, in this embodiment, the leading edge of the sheet S is guided to pass through a position closer to the nip line Ln by providing the protruding portion 68a. Therefore, the leading edge of the sheet S can be more reliably introduced into the nip portion, reducing the possibility of conveyance failure.

[0049] (Transport speed of the reversing roller pair and the intermediate re-feed roller pair) In this embodiment, the conveying speed Vs1 when the reversing roller pair 61 conveys the sheet S in the second conveying direction D2 is set to be smaller than the conveying speed Vs1 when the intermediate re-feed roller pair 62 conveys the sheet S (FIG. 2(c)). The conveying speeds Vs1 and Vs2 represent the peripheral speeds of the reversing roller pair 61 or the intermediate re-feed roller pair 62 when the pair is rotationally driven by the driving force of the driving source (first motor 81).

[0050] By setting the conveyance speed Vs1 of the inversion roller pair 61 to be lower than the conveyance speed Vs2 of the intermediate refeeding roller pair 62, it is possible to reduce the possibility that the first surface (image surface) of the sheet S rubs against the conveyance guide 69 between the inversion roller pair 61 and the intermediate refeeding roller pair 62. If Vs1 is greater than Vs2, during the period when both the inversion roller pair 61 and the intermediate refeeding roller pair 62 are sandwiching and conveying the sheet S, deflection of the sheet S occurs between the inversion roller pair 61 and the intermediate refeeding roller pair 62. When the image on the first surface (image surface) of the sheet S rubs against the conveyance guide 69 due to the deflection of the sheet S, rubbing marks may remain on the image. By setting the speeds as Vs1 < Vs2 as in this embodiment, it is possible to reduce the possibility of rubbing marks occurring on the image.

[0051] On the other hand, when setting the speeds as Vs1 < Vs2, tensile stress on the sheet S may occur between the inversion roller pair 61 and the intermediate refeeding roller pair 62 due to the speed difference. In this case, due to the friction with the sheet S, the wear rate of the inversion roller pair 61 or the intermediate refeeding roller pair 62 may increase.

[0052] Also, as described above, when providing the protruding portion 68a of the conveyance guide 68 between the inversion roller pair 61 and the intermediate refeeding roller pair 62, when setting the speeds as Vs1 < Vs2, the sheet S may strongly rub against the protruding portion 68a. That is, when tensile stress occurs on the sheet S between the inversion roller pair 61 and the intermediate refeeding roller pair 62, the sheet S tries to approach the virtual line Lv connecting the nip portions of the inversion roller pair 61 and the intermediate refeeding roller pair 62, and the second surface of the sheet S strongly rubs against the protruding portion 68a. As a result, wear of the conveyance guide 68 may occur. Also, for example, rubbing marks may remain on the second surface of the sheet S due to the sheet S strongly rubbing against the conveyance guide 68.

[0053] Therefore, in this embodiment, as will be described later, a configuration is adopted in which the inversion roller pair 61 is allowed to rotate at a peripheral surface speed higher than the conveyance speed Vs1 following the sheet S conveyed at the conveyance speed Vs2 by the intermediate refeeding roller pair 62.

[0054] (Drive transmission mechanism) The drive transmission mechanism DT for driving the reversing roller pair 61 and the intermediate re-feed roller pair 62 will be described with reference to Figures 3(a) to (d). Figure 3(a) is a schematic diagram showing the drive transmission mechanism DT as viewed from one side in the rotation axis direction of the reversing roller pair 61. Figure 3(b) is a schematic diagram showing the drive transmission mechanism DT as viewed from the other side in the rotation axis direction of the reversing roller pair 61. Figures 3(c) and (d) are enlarged views of a portion of Figure 3(b).

[0055] 3(a) and (b), the reversing roller pair 61 of this embodiment includes a driving roller 61a connected to the first motor 81 via the drive transmission mechanism DT, and a driven roller 61b (non-driven roller) not connected to the first motor 81 via the drive transmission mechanism DT. The intermediate re-feed roller pair 62 includes a driving roller 62a connected to the first motor 81 via the drive transmission mechanism DT, and a driven roller 62b (non-driven roller) not connected to the first motor 81 via the drive transmission mechanism DT. The driving rollers 61a and 62a are driven to rotate by a driving force transmitted from the first motor 81 via the drive transmission mechanism DT. The driven rollers 61b and 62b rotate following the driving rollers 61a and 61a due to the frictional force received from the corresponding driving rollers 61a and 62a at the nip portion.

[0056] The first motor 81 is rotatable in a counterclockwise direction in FIG. 3(a) (hereinafter referred to as a forward rotation direction Rm1) and in a rotation direction opposite to the forward rotation direction Rm1 (hereinafter referred to as a reverse rotation direction Rm2).

[0057] As shown in Fig. 3(a) and (b), the drive transmission mechanism DT includes a first belt drive unit 100, a second belt drive unit 110, a first gear train 120, and a second gear train 130. The first belt drive unit 100 transmits a drive force from the first motor 81 to the first intermediate shaft Ax1. The second belt drive unit 110 transmits a drive force from the first intermediate shaft Ax1 to the second intermediate shaft Ax2. The first gear train 120 transmits a drive force from the first intermediate shaft Ax1 to the drive roller 62a of the intermediate re-feed roller pair 62. The second gear train 130 transmits a drive force from the second intermediate shaft Ax2 to the drive roller 61a of the reversing roller pair 61.

[0058] The first belt driving unit 100 includes an output pulley 81a, a first pulley 102, a first driving belt 101, and a first tensioner 103. The output pulley 81a is attached to an output shaft of the first motor 81. The first pulley 102 rotates around a first intermediate shaft Ax1. The first driving belt 101 is stretched across the output pulley 81a and the first pulley 102. The first tensioner 103 includes a support shaft 103b, a support arm 103a that rotates around the support shaft 103b, and a roller 103c that is rotatably supported by the support arm 103a. The first tensioner 103 also includes a biasing member 103d having one end connected to the support arm 103a, and a fixed portion 103e to which the other end of the biasing member 103d is connected. The first tensioner 103 applies tension to the first drive belt 101 by pressing the roller 103c against the inner surface of the first drive belt 101 due to the biasing force of the biasing member 103d. The first pulley 102 receives the driving force via the first drive belt 101 and rotates in a rotation direction corresponding to the rotation direction of the first motor 81.

[0059] The first gear train 120 includes a distribution gear 121, a pendulum gear 122, a first gear 123a, a second gear 123b, a third gear 123c, and a fourth gear 123d. The distribution gear 121 rotates integrally with the first pulley 102 around the first intermediate shaft Ax1. The distribution gear 121 and the pendulum gear 122 mesh with each other. The pendulum gear 122 is rotatably supported by a swing arm (not shown). The swing arm is in frictional contact with the distribution gear 121, for example, and swings in a direction along the rotation direction of the distribution gear 121.

[0060] By the swinging of the swing arm, the pendulum gear 122 can move between a position where it meshes with the first gear 123a (FIG. 3(c)) and a position where it meshes with the fourth gear 123d (FIG. 3(d)). The first gear 123a meshes with the second gear 123b, the second gear 123b meshes with the third gear 123c, and the third gear 123c meshes with the fourth gear 123d. The fourth gear 123d is disposed coaxially with the drive roller 62a of the intermediate re-feed roller pair 62, and rotates integrally with the drive roller 62a.

[0061] 3(c), when the first motor 81 rotates in the forward direction Rm1, the distribution gear 121 rotates in the clockwise direction in the figure by the driving force transmitted via the first belt driving unit 100, and the pendulum gear 122 meshes with the first gear 123a. The rotation of the first gear 123a is transmitted to the fourth gear 123d via the second gear 123b and the third gear 123c. Then, the driving roller 62a of the intermediate re-feed roller pair 62 rotates in the counterclockwise direction in the figure together with the fourth gear 123d.

[0062] 3(d), when the first motor 81 rotates in the reverse direction Rm2, the distribution gear 121 rotates counterclockwise in the figure by the driving force transmitted via the first belt driving unit 100, and the pendulum gear 122 meshes with the fourth gear 123d. Then, the driving roller 62a of the intermediate re-feed roller pair 62 rotates counterclockwise in the figure integrally with the fourth gear 123d.

[0063] In this way, in both cases where the first motor 81 rotates in the forward rotation direction Rm1 and where it rotates in the reverse direction Rm2, the drive roller 62a of the intermediate re-feed roller pair 62 rotates in a predetermined rotation direction by the transmission of the driving force via the first gear train 120. The predetermined rotation direction is the direction in which the intermediate re-feed roller pair 62 conveys the sheet S in the second conveying direction D2. In this embodiment, the gear train from the distribution gear 121 to the fourth gear 123d is configured to switch between the odd-numbered train and the even-numbered train depending on the rotation direction of the first motor 81, so that the drive roller 62a rotates in the predetermined rotation direction.

[0064] As shown in Fig. 3(a) and (b), the second belt drive unit 110 includes a second drive belt 111, a second pulley 112, a third pulley 113, and an idler pulley 114. The second pulley 112 is disposed coaxially with the first pulley 102 and the distribution gear 121, and rotates integrally with the first pulley 102 and the distribution gear 121 around the first intermediate shaft Ax1. The third pulley 113 rotates around the second intermediate shaft Ax2. The second drive belt 111 is stretched between the second pulley 112 and the third pulley 113. The idler pulley 114 is in contact with the inner surface of the second drive belt 111 between the second pulley 112 and the third pulley 113. The second tensioner 115 includes a support shaft 115b, a support arm 115a that rotates around the support shaft 115b, and a roller 115c that is rotatably supported by the support arm 115a. The second tensioner 115 also includes a biasing member 115d having one end connected to the support arm 115a, and a fixed part 115e to which the other end of the biasing member 115d is connected. The second tensioner 115 applies tension to the second drive belt 111 by the roller 115c pressing against the outer surface of the second drive belt 111 by the biasing force of the biasing member 115d.

[0065] The second gear train 130 includes a reverse input gear 131 and a reverse output gear 132. The reverse input gear 131 is disposed coaxially with the third pulley 113 and rotates integrally with the third pulley 113 around the second intermediate shaft Ax2. The reverse output gear 132 is disposed coaxially with the drive roller 61a of the reverse roller pair 61. The reverse output gear 132 meshes with the reverse input gear 131.

[0066] When the first motor 81 rotates in the forward rotation direction Rm1, the driving force is transmitted from the second pulley 112 to the third pulley 113 via the second drive belt 111, and further transmitted to the drive roller 61a via the meshing of the reversal input gear 131 and the reversal output gear 132. As a result, the drive roller 61a of the reversal roller pair 61 rotates in the first rotation direction R1 when the reversal roller pair 61 conveys the sheet S in the first conveying direction D1.

[0067] When the first motor 81 rotates in the reverse direction Rm2, the driving force is also transmitted to the drive roller 61a through the same path. That is, the driving force is transmitted from the second pulley 112 to the third pulley 113 via the second drive belt 111, and further transmitted to the drive roller 61a via the meshing of the reversal input gear 131 and the reversal output gear 132. As a result, the drive roller 61a of the reversal roller pair 61 rotates in the second rotation direction R2 when the reversal roller pair 61 conveys the sheet S in the second conveying direction D2.

[0068] (Coupling part) The drive transmission configuration to the reversing roller pair 61 will be described with reference to Figs. 4(a) to (e). Fig. 4(a) is a perspective view showing a part of the drive transmission mechanism DT and the drive roller 61a of the reversing roller pair 61. Figs. 4(b) and (c) are exploded views of the coupling part CP. Fig. 4(d) is a view showing a part of the drive transmission mechanism DT and the drive roller 61a of the reversing roller pair 61, as viewed from a direction perpendicular to the rotation axis direction X described below (top view). Fig. 4(e) is a cross-sectional view of the coupling part CP taken along line AA in Fig. 4(d).

[0069] As shown in FIGS. 4(a) and 4(d), the driving roller 61a of the reversing roller pair 61 has a roller shaft 612 and multiple roller bodies 611. The direction along the rotation axis Ax of the driving roller 61a is defined as the rotation axis direction X. The roller shaft 612 is a shaft-shaped member extending in the rotation axis direction X. The multiple roller bodies 611 form a nip portion where the reversing roller pair 61 holds the sheet S together with the roller body of the driven roller 61b (FIG. 1). The multiple roller bodies are attached to the roller shaft 612 and rotate integrally with the roller shaft 612 around the rotation axis Ax.

[0070] The drive transmission mechanism DT has a coupling portion CP as an example of a drive transmission portion for transmitting a drive force to the drive roller 61a.

[0071] The coupling unit CP can be in a transmission state (connected state, coupled state) in which the driving force from the first motor 81 is transmitted to the reversing roller pair 61, and a non-transmission state (drive transmission interrupted state, disconnected state) in which the driving force from the first motor 81 is not transmitted to the reversing roller pair 61. The transmission states that the coupling unit CP can be in include a first transmission state in which the coupling unit CP transmits a driving force to the reversing roller pair 61 to transport the sheet S in the first transport direction D1, and a second transmission state in which the coupling unit CP transmits a driving force to the reversing roller pair 61 to transport the sheet S in the second transport direction D2.

[0072] The coupling part CP in this embodiment includes a reverse output gear 132 and a reverse coupling 613. The reverse output gear 132 is an example of a first member (driving member, input member) connected to a driving source (first motor 81). The reverse coupling 613 is an example of a second member (driven member, output member) connected to a driven object (here, the pair of reverse rollers 61).

[0073] The reversing coupling 613 is provided at an end of the driving roller 61a in the rotation axis direction X. The reversing coupling 613 rotates integrally with the driving roller 61a around the rotation axis Ax.

[0074] The reverse output gear 132 (see also FIGS. 3(a) and 3(b)) is connected to a reverse coupling 613. The driving force of the first motor 81 is input to the pair of reverse rollers 61 via the connection between the reverse output gear 132 and the reverse coupling 613.

[0075] The coupling part CP includes a reverse output gear 132 and a reverse coupling 613. The reverse output gear 132 is an example of a first member (driving member, input member) connected to a driving source (first motor 81). The reverse coupling 613 is an example of a second member (driven member, output member) connected to a driven object (here, the pair of reverse rollers 61).

[0076] The reverse output gear 132 and the reverse coupling 613 are arranged coaxially. That is, the reverse output gear 132 and the reverse coupling 613 can rotate around the rotation axis Ax of the drive roller 61a in a first rotation direction R1 and a second rotation direction R2. The first rotation direction R1 is the rotation direction of the reverse output gear 132 and the reverse coupling 613 when the reverse roller pair 61 conveys the sheet S in the first conveying direction D1. The second rotation direction R2 is the rotation direction of the reverse output gear 132 and the reverse coupling 613 when the reverse roller pair 61 conveys the sheet S in the second conveying direction D2. In this embodiment, the radial direction of a virtual circle centered on the rotation axis Ax when viewed in the rotation axis direction X is referred to as the radial direction of the coupling part CP or simply the "radial direction".

[0077] 4(b) and (c), the reverse output gear 132 has an engagement shaft 132a as an engagement portion. The reverse coupling 613 has an engagement hole 613a as an engaged portion that is engaged with the engagement shaft 132a.

[0078] More specifically, the reverse output gear 132 has a hole 132h through which the roller shaft 612 is inserted, and a side surface 132f extending radially outward from the hole 132h. The reverse coupling 613 has a side surface 613f extending radially outward from the roller shaft 612. The side surface 132f of the reverse output gear 132 and the side surface 613f of the reverse coupling 613 face each other in the rotation axis direction X.

[0079] The engagement shaft 132a of the reverse output gear 132 is a protrusion that protrudes from a side surface 132f of the reverse output gear 132 toward the reverse coupling 613 in the rotation axis direction X. The engagement shaft 132a in this embodiment is formed in a substantially cylindrical shape extending in the rotation axis direction X, but may have a shape other than a cylindrical shape (for example, a rectangular prism shape).

[0080] The engagement shaft 132a has a first end 132b which is an end of the engagement shaft 132a on the first rotation direction R1 side, and a second end 132c which is an end of the engagement shaft 132a on the second rotation direction R2 side. The first end 132b is an example of a first contact portion, and the second end 132c is an example of a second contact portion.

[0081] The engagement hole 613a of the reversal coupling 613 is a recess formed in the side surface 132f of the reversal coupling 613. This recess may have a bottom, or may be a through hole that penetrates the reversal coupling 613 in the rotation axis direction X. The engagement hole 613a in this embodiment is formed in a substantially sector shape (a shape obtained by cutting a part of a ring at a predetermined central angle) along a virtual arc centered on the rotation axis Ax.

[0082] 4(e), the engagement hole 613a has a first end 613b which is an end of the engagement hole 613a on the first rotation direction R1 side and a second end 613c which is an end of the engagement hole 613a on the second rotation direction R2 side. The first end 613b is a first abutted portion that abuts against the first end 132b of the engagement shaft 132a of the reverse output gear 132. The second end 613c is a second abutted portion that abuts against the second end 132c of the engagement shaft 132a of the reverse output gear 132.

[0083] The width of the engagement hole 613a is set wider than the width of the engagement shaft 132a. That is, when viewed in the rotation axis direction X, the distance from the first end 613b to the second end 613c measured along a virtual arc centered on the rotation axis Ax and passing through the center (face center) of the engagement shaft 132a is wider than the distance from the first end 132b to the second end 132c of the engagement shaft 132a measured along the virtual arc. In other words, the central angle between the first end 613b and the second end 613c based on the rotation axis Ax is wider than the central angle between the first end 132b and the second end 132c of the engagement shaft 132a based on the rotation axis Ax. In further words, the central angle between the first contact portion and the second contact portion with respect to the rotation axis of the first member and the second member is smaller than the central angle between the first contacted portion and the second contacted portion with respect to the rotation axis.

[0084] Since the width of the engagement hole 613a is wider than the width of the engagement shaft 132a, the reverse output gear 132 and the reverse coupling 613 are connected to be relatively rotatable (freely spinning) within a predetermined range. When the reverse coupling 613 is used as a reference, the reverse output gear 132 is rotatable between a position where the first end 132b of the engagement shaft 132a abuts against the first end 613b of the engagement hole 613a (see the solid line in FIG. 4(e)) and a position where the second end 132c of the engagement shaft 132a abuts against the second end 613c of the engagement hole 613a (see the dashed line in FIG. 4(e)).

[0085] In addition, "idling" of the reverse output gear 132 and the reverse coupling 613 means that the reverse output gear 132 and the reverse coupling 613 rotate relative to each other in a state in which the first end 132b of the engagement shaft 132a is not in contact with the first end 613b of the engagement hole 613a and the second end 132c of the engagement shaft 132a is not in contact with the second end 613c of the engagement hole 613a.

[0086] The first transmission state in this embodiment is a state in which the first end 132b of the engagement shaft 132a of the reverse output gear 132 abuts against the first end 613b of the engagement hole 613a of the reverse coupling 613. When the coupling part CP is in the first transmission state, the first end 132b of the engagement shaft 132a presses the first end 613b of the engagement hole 613a, thereby making it possible to transmit torque in the first rotational direction R1 from the reverse output gear 132 to the reverse coupling 613.

[0087] The second transmission state in this embodiment is a state in which the second end 132c of the engagement shaft 132a of the reverse output gear 132 abuts against the second end 613c of the engagement hole 613a of the reverse coupling 613. When the coupling part CP is in the second transmission state, the second end 132c of the engagement shaft 132a presses the second end 613c of the engagement hole 613a, thereby making it possible to transmit torque in the second rotational direction R2 from the reverse output gear 132 to the reverse coupling 613.

[0088] The non-transmitting state in this embodiment is a state in which the first end 132b of the engagement shaft 132a is not in contact with the first end 613b of the engagement hole 613a and the second end 132c of the engagement shaft 132a is not in contact with the second end 613c of the engagement hole 613a. When the coupling part CP is in the non-transmitting state, the torque in the first rotational direction R1 and the torque in the second rotational direction R2 are not transmitted between the reverse output gear 132 and the reverse coupling 613.

[0089] The angle by which the reverse output gear 132 rotates in the second rotation direction R2 relative to the reverse coupling 613 is expressed as θ [°], with the state in which the first end 132b of the engagement shaft 132a abuts against the first end 613b of the engagement hole 613a being taken as the reference (θ=0°). The maximum value of the angle θ in this embodiment is defined as the allowable idling angle θ L [°]. The first transmission state of the coupling part CP corresponds to θ=0, and the second transmission state corresponds to θ=θ L and the non-transmitting state corresponds to 0<θ<θ L In this embodiment, the range in which relative rotation (idling) between the reverse output gear 132 and the reverse coupling 613 is permitted (hereinafter, referred to as the allowable idling range) is 0<θ<θL The range is.

[0090] Normally, when the first motor 81 rotates continuously in the forward direction Rm1, the coupling part CP is in the first transmission state, and the reverse output gear 132 and the reverse coupling 613 rotate integrally in the first rotation direction R1. When the first motor 81 rotates continuously in the reverse direction Rm2, the coupling part CP is in the second transmission state, and the reverse output gear 132 and the reverse coupling 613 rotate integrally in the first rotation direction R2. When the rotation direction of the first motor 81 is switched, the reverse output gear 132 and the reverse coupling 613 rotate relative to each other, and the coupling part CP transitions from either the first transmission state or the second transmission state to the other of the first transmission state or the second transmission state via the non-transmission state. However, as described below, even if the rotation direction of the first motor 81 is not switched, the coupling part CP may be in the non-transmission state when the pair of reverse rollers 61 is rotated by the force received from the sheet.

[0091] (Coupling operation) The operation of the coupling part CP during double-sided printing will be described using Figures 5(a)-(d) and 6(a)-(d). The left side of each of Figures 5(a)-(d) and 6(a)-(d) is a cross-sectional view of the coupling part CP at the same position as line AA in Figure 4(d) at each point in double-sided printing. The right side of each of Figures 5(a)-(d) and 6(a)-(d) is a schematic diagram showing a cross-section of the image forming apparatus 1 at each point in double-sided printing. In each of Figures 5(a)-(d) and 6(a)-(d), the left and right sides show the same point in double-sided printing.

[0092] In this embodiment, double-sided printing is performed under the following conditions, as an example. Length of sheet S in the sheet transport direction (hereinafter referred to as sheet length Ls): 297 mm. Path length Lp from the reversing roller pair 61 to the intermediate re-feed roller pair 62: 144 mm. · The conveying speed Vs1 of the sheet S by the reversing roller pair 61: 264.1 mm / s. The conveying speed Vs2 of the sheet S by the intermediate re-feed roller pair 62: 266.6 mm / s. Outer diameter D of the drive roller 61a of the reversing roller pair 61: 11 mm. Allowable rotation angle θ of the reverse output gear 132 and the reverse coupling 613 L :30°.

[0093] The above-mentioned conveying speed Vs1 represents the peripheral speed of the drive roller 61a when the reversing roller pair 61 is rotationally driven by the driving force transmitted from the first motor 81, and may differ from the actual conveying speed of the sheet S. In the above example, the conveying speed when the reversing roller pair 61 conveys the sheet S in the first conveying direction D1 and the conveying speed when the reversing roller pair 61 conveys the sheet S in the second conveying direction D2 are both Vs1. The roller diameter of the reversing roller pair 61 means the outer diameter of the roller body 611 (FIG. 4(a)) of the drive roller 61a.

[0094] The right side of FIG. 5(a) shows a state in which the sheet S with an image formed on its first side is conveyed in the first conveying direction D1 by the pair of reversing rollers 61. The sheet S with an image formed on its first side and discharged from the fixing unit 40 is guided to the reversing path P2 by the switching flap 67, and is sandwiched between the pair of reversing rollers 61 and conveyed in the first conveying direction D1. At this time, as shown on the left side of FIG. 5(a), the reversing output gear 132 is rotated in the first rotating direction R1 at a rotation speed V1 (first angular speed) corresponding to the conveying speed Vs1. In addition, the first end 132b of the engagement shaft 132a of the reversing output gear 132 abuts against the first end 613b of the engagement hole 613a of the reversing coupling 613. In other words, the coupling part CP is in the first transmission state. The first end 613b of the reversing coupling 613 is pressed against the first end 132b of the engagement shaft 132a, so that the reversing coupling 613 rotates in the first rotation direction R1 at the same rotation speed V1 as the reversing output gear 132. As a result, the drive roller 61a of the reversing roller pair 61 rotates in the first rotation direction R1 at the rotation speed V1, and the reversing roller pair 61 conveys the sheet S in the first conveying direction D1 at a conveying speed Vs1.

[0095] In other words, when the first conveying section is conveying the sheet in the second conveying direction and the second conveying section is not conveying the sheet, the drive transmission section is in the second transmission state so that the first conveying section conveys the sheet at a first conveying speed by the driving force.

[0096] When the reversing roller pair 61 transports the sheet S in the first transport direction D1 at the transport speed Vs1, the drive roller 62a of the intermediate re-feed roller pair 62 is rotationally driven at a rotation speed V2 such that the peripheral speed becomes the transport speed Vs2.

[0097] As shown on the right side of Fig. 5(b), after the rear end of the sheet S in the first transport direction D1 passes through the fixing unit 40, the driving of the reversing roller pair 61 and the intermediate re-feed roller pair 62 is temporarily stopped. That is, the rotation of the first motor 81 in the forward rotation direction Rm1 (Fig. 3(a)) is temporarily stopped. Also, as shown on the left side of Fig. 5(b), at this point in time, the coupling part CP is in the first transmission state.

[0098] Next, when the first motor 81 starts to rotate in the reverse direction Rm2 (FIG. 3(a)), as shown in the left side of FIG. 5(c), the engagement shaft 132a of the reverse output gear 132 moves away from the first end 613b of the engagement hole 613a of the reverse coupling 613 and moves inside the engagement hole 613a. That is, the coupling part CP changes from the first transmission state to the non-transmission state. When the coupling part CP becomes the non-transmission state, the drive transmission from the first motor 81 to the reverse roller pair 61 is interrupted, and the stopped state of the reverse roller pair 61 is maintained. Note that, when the first motor 81 starts to rotate in the reverse direction Rm2, the drive roller 62a of the intermediate re-feed roller pair 62 is rotationally driven at the rotation speed V2.

[0099] When the first motor 81 further rotates in the reverse direction Rm2 (FIG. 3(a)), as shown in the left side of FIG. 5(d), the second end 132c of the engagement shaft 132a of the reversing output gear 132 comes into contact with the second end 613c of the engagement hole 613a of the reversing coupling 613. That is, the coupling part CP changes from the non-transmitting state to the second transmitting state. The second end 613c of the reversing coupling 613 is pressed against the second end 132c of the engagement shaft 132a, so that the reversing coupling 613 rotates in the second rotation direction R2 at the same rotation speed V1 as the reversing output gear 132. As a result, the driving roller 61a of the reversing roller pair 61 rotates in the second rotation direction R2 at the rotation speed V1, and the reversing roller pair 61 conveys the sheet S in the second conveying direction D2 at the conveying speed Vs1. That is, the conveying direction of the sheet S is reversed, and the leading end of the sheet in the first conveying direction D1, which had been protruding outside the housing 2, is pulled back inside the housing 2.

[0100] The right side of FIG. 6(a) shows a state when the leading edge of the sheet S in the second conveying direction D2, whose conveying direction has been reversed by the reversing roller pair 61, reaches the nip portion of the intermediate re-feed roller pair 62. When the leading edge of the sheet S reaches the nip portion of the intermediate re-feed roller pair 62, the sheet S is sandwiched between both the reversing roller pair 61 and the intermediate re-feed roller pair 62 and conveyed in the second conveying direction D2. A state in which the sheet S conveyed in the second conveying direction D2 contacts both the reversing roller pair 61 and the intermediate re-feed roller pair 62 can be called a contact state. When the leading edge of the sheet S reaches the nip portion of the intermediate re-feed roller pair 62, a contact state occurs. In the contact state, the sheet S is sandwiched between the reversing roller pair 61 at the nip portion of the reversing roller pair 61, and is sandwiched between the intermediate re-feed roller pair 62 at the nip portion of the intermediate re-feed roller pair 62. The contact state is released when the trailing edge of the sheet S passes through the nip portion of the reversing roller pair 61.

[0101] In this embodiment, the conveying speed Vs2 of the intermediate re-feed roller pair 62 is set to be faster than the conveying speed Vs1 of the reversing roller pair 61 (Vs2>Vs1). This reduces the occurrence of scratches on the image due to the bending of the sheet S, as described above.

[0102] The intermediate re-feed roller pair 62 conveys the sheet S while accelerating the conveying speed to a conveying speed Vs2 so as to pull the sheet S out of the reverse roller pair 61. Meanwhile, the drive roller 61a of the reverse roller pair 61 receives torque in the second rotation direction R2 due to frictional force from the sheet S conveyed at the conveying speed Vs2, and rotates in the second rotation direction R2 at a rotation speed V3 (second angular velocity) faster than the rotation speed V1 (first angular velocity). At this time, as shown on the left side of FIG. 6(a), there is a gap between the first end 132b of the engagement shaft 132a and the first end 613b of the engagement hole 613a, which allows the reverse coupling 613 to rotate relative to the reverse output gear 132 in the second rotation direction R2.

[0103] When the leading edge of the sheet S conveyed by the reversing roller pair 61 in the second conveying direction D2 reaches the intermediate re-feed roller pair 62, a contact state occurs in which the sheet S contacts both the reversing roller pair 61 and the intermediate re-feed roller pair 62. The contact state is released (ended) when the trailing edge of the sheet S conveyed in the second conveying direction D2 leaves the reversing roller pair 61. As shown on the right side of FIG. 6B, while the sheet S is in the nip portion of the reversing roller pair 61, the sheet S is conveyed by the intermediate re-feed roller pair 62 at a conveying speed Vs2. Until the trailing edge of the sheet S in the second conveying direction D2 leaves the nip portion of the reversing roller pair 61, the drive roller 61a of the reversing roller pair 61 follows the sheet S conveyed at the conveying speed Vs2 and rotates ahead of the sheet S at a rotation speed V3 faster than the rotation speed V1. That is, after the contact state occurs, until the contact state is released, the drive roller 61a of the pair of reversing rollers 61 rotates ahead of the sheet S at a rotation speed V3 faster than the rotation speed V1, following the sheet S transported at the transport speed Vs2. Here, the "precedent rotation" of the drive roller 61a means that the drive roller 61a rotates at an angular velocity (rotation speed V3) faster than the angular velocity (rotation speed V1) at which the drive roller 61a rotates due to the driving force transmitted from the first motor 81 via the drive transmission mechanism DT.

[0104] When the contact state occurs, the second end 613c of the engagement hole 613a of the reverse coupling 613 moves away from the second end 132c of the engagement shaft 132a of the reverse output gear 132 toward the second rotation direction R2. As shown on the left side of FIG. 6(b), the reverse coupling 613 rotates together with the driving roller 61a in the second rotation direction R2 at a rotation speed V3 that is faster than the rotation speed V1 of the reverse output gear 132. For this reason, relative rotation (idling) occurs between the reverse output gear 132 and the reverse coupling 613 so that the engagement shaft 132a of the reverse output gear 132 moves away from the second end 613c of the engagement hole 613a of the reverse coupling 613. That is, the coupling part CP changes from the second transmission state to the non-transmission state.

[0105] In other words, after a contact state occurs in which the sheet transported in the second transport direction contacts both the first transport section and the second transport section, the drive transmission section is in the non-transmitting state to allow the first transport section to operate following the sheet transported at the second transport speed. More specifically, after the leading edge of the sheet in the second transport direction reaches the second transport section, the drive transmission section changes from the second transmitting state to the non-transmitting state to allow the first transport section to operate following the sheet transported at the second transport speed. This allows the drive roller 61a of the reversing roller pair 61 to rotate ahead at the rotation speed V3 following the sheet S transported at the transport speed Vs2. It is preferable that this non-transmitting state continues until the contact state is released.

[0106] 6(c), even after the rear end of the sheet S in the second transport direction D2 leaves the nip portion of the reversing roller pair 61, the sheet S is transported at the transport speed Vs2 by the intermediate re-feed roller pair 62. Meanwhile, as the rear end of the sheet S leaves the nip portion of the reversing roller pair 61, the reversing roller pair 61 is released from the sheet S, and the rotation of the drive roller 61a is decelerated.

[0107] As shown in the left side of FIG. 6(c), even after the sheet S has passed through the pair of reverse rollers 61, the reverse output gear 132 continues to rotate at the rotation speed V1, so that the second end 132c of the engagement shaft 132a abuts against the second end 613c of the engagement hole 613a again. That is, the coupling portion CP returns from the non-transmitting state to the second transmitting state. Then, the drive roller 61a of the pair of reverse rollers 61 rotates in the second rotation direction R2 at the rotation speed V1.

[0108] Thereafter, when the following sheet needs to be reversed and conveyed by the reversing roller pair 61, the rotation direction of the first motor 81 is switched to reverse the rotation direction of the reversing roller pair 61, as shown on the right side of FIG. 6(d). When the rotation direction of the first motor 81 is switched, as shown on the left side of FIG. 6(d), the second end 132c of the engagement shaft 132a is separated from the second end 613c of the engagement hole 613a, and the first end 132b of the engagement shaft 132a abuts against the first end 613b. That is, the coupling part CP changes from the second transmission state to the non-transmission state and then to the first transmission state. As a result, the reversing roller pair 61 rotates in a rotation direction for conveying the sheet in the first conveying direction D1.

[0109] As shown on the right side of Fig. 6(d), the rotation direction of the first motor 81 can be switched before the rear end of the sheet S leaves the nip portion of the intermediate re-feed roller pair 62. As described above, due to the mechanism using the pendulum gear 122 (Figs. 3(c) and (d)), the intermediate re-feed roller pair 62 is rotationally driven in a rotation direction that conveys the sheet S in the second conveying direction D2 regardless of the rotation direction of the first motor 81. For this reason, the sheet conveying interval in the reversing roller pair 61 can be shortened.

[0110] (Relationship between leading rotation angle and allowable slip angle) In this embodiment, it will be described that the coupling portion CP is in a non-transmitting state until the trailing end of the sheet S passes through at least the pair of intermediate re-feed rollers 62, and the pair of reversing rollers 61 is allowed to rotate in advance.

[0111] In this embodiment, the aforementioned slip allowable angle θ LThe pre-rotation angle θ of the reverse coupling 613 is calculated as follows: S In other words, the amount of relative rotation between the first member and the second member required to change the drive transmission unit from the second transmission state to the first transmission state (θ L ) is the amount of relative rotation (θ) between the first member and the second member that occurs during a period in which both the first conveying unit and the second conveying unit convey the sheet in the second conveying direction. S ) is greater than

[0112] Pre-rotation angle θ of the inverted coupling 613 S The angle θ [°] is an angle of extra rotation of the reverse coupling 613 relative to the reverse output gear 132 rotating at the rotation speed V1, which is caused by the reverse coupling 613 rotating at a rotation speed V3 that is faster than the rotation speed V1 due to the preceding rotation of the drive roller 61a. S The standard (θ S =0°) is the rotation angle of the reverse coupling 613 with respect to the reverse output gear 132 in a state in which the second end 132c of the engagement shaft 132a abuts against the second end 613c of the engagement hole 613a (that is, the second transmission state).

[0113] 1. The length from the reversing roller pair 61 to the rear end of the sheet S at the time when the leading end of the sheet S enters the nip portion of the intermediate re-feed roller pair 62: (Sheet length Ls)-(path length Lp from the reversing roller pair 61 to the intermediate re-feed roller pair 62)=297 [mm]-144 [mm]=153 [mm]

[0114] 2. The time from when the leading edge of the sheet S enters the nip portion of the intermediate re-feed roller pair 62 to when the trailing edge of the sheet S exits the nip portion of the reversing roller pair 61: (Length of 1.) / (Conveying speed Vs2 of intermediate re-feed roller pair 62)=153 [mm] / 266.6 [mm / s]=0.57 [s]

[0115] 3. Speed ​​difference between the reversing roller pair 61 and the intermediate re-feed roller pair 62: (Transport speed Vs2 of intermediate re-feed roller pair 62)-(Transport speed Vs1 of reversing roller pair 61)=266.6 [mm / s]-264.1 [mm / s]=2.5 [mm / s]

[0116] 4. The distance that the outer circumferential surface of the drive roller 61a moves extra due to the drive roller 61a rotating at a rotation speed V3 that is faster than the rotation speed V1 during the period of 2. above: (Time of 2.) x (Speed ​​difference of 3.) = 0.57 [s] x 2.5 [mm / s] = 1.45 [mm]

[0117] 5. Circumferential length of outer circumferential surface of driving roller 61a: (Outer diameter D of the driving roller 61a)×(Pi)=34.6 [mm]

[0118] 6. Pre-rotation angle of the driving roller 61a: (Distance of travel of 4.) / (Circumference of 5.)×360°=1.45[mm] / 34.6[mm]×360°=15°

[0119] 7. Pre-rotation angle θ of the inverted coupling 613 S : In this embodiment, since the reversing coupling 613 rotates integrally with the driving roller 61a, the advance rotation angle θ of the reversing coupling 613 S is the same as the advance rotation angle (6.) of the driving roller 61a. That is, θ S =15°.

[0120] As described above, the advance rotation angle θ of the reversing coupling 613 that occurs during the period in which the sheet S is held by both the reversing roller pair 61 and the intermediate re-feed roller pair 62 is S is about 15°. Therefore, the allowable slip angle θ L It is preferable that the pre-rotation angle θ is set to a value larger than 15°. This allows the trailing edge of the sheet S to reach the pre-rotation angle θ before passing through the nip portion of the pair of reversing rollers 61. S is the allowable spin angle θ L can be prevented from reaching

[0121] Suppose that the trailing edge of the sheet S is rotated by a leading rotation angle θ before passing through the nip portion of the pair of reversing rollers 61. S is the allowable spin angle θ L Assume that the sheet S reaches the reversing output gear 132. In this case, the first end 132b of the engagement shaft 132a of the reversing output gear 132 comes into contact with the first end 613b of the engagement hole 613a of the reversing coupling 613. In this case, the reversing coupling 613 cannot rotate further in the second rotation direction R2 relative to the reversing output gear 132. In this case, the driving roller 61a of the reversing roller pair 61 tries to rotate at the rotation speed V1, which is limited by the reversing output gear 132, so that there is a possibility that the sheet S will be pulled by the reversing roller pair 61 and the intermediate re-feed roller pair 62.

[0122] In this embodiment, the allowable spin angle θ L The leading rotation angle θ of the inverted coupling 613 S Set it to a value larger than θ L = 30°. This makes it possible to prevent the first end 132b of the engagement shaft 132a from contacting the first end 613b of the engagement hole 613a before the rear end of the sheet S passes through the nip portion of the reversing roller pair 61. In other words, until the rear end of the sheet S passes through at least the intermediate re-feed roller pair 62, the coupling part CP is in the non-transmitting state, and the advance rotation of the reversing roller pair 61 is permitted.

[0123] (Advantages of this embodiment) As described above, the drive mechanism according to the present embodiment uses the coupling unit CP having an idling range. After the leading edge of the sheet S reaches the intermediate re-feed roller pair 62 during double-sided printing, the coupling unit CP changes from the second transmission state to the non-transmission state so as to allow the reversing roller pair 61 to operate following the sheet S transported at the transport speed Vs2. The coupling unit CP is in the non-transmission state at least until the trailing edge of the sheet S leaves the nip portion of the reversing roller pair 61. In other words, after a contact state occurs in which the sheet transported in the second transport direction comes into contact with both the first transport unit and the second transport unit, the drive transmission unit enters the non-transmission state so as to allow the first transport unit to operate following the sheet transported at the second transport speed. More specifically, after the drive transmission unit changes from the second transmission state to the non-transmission state, the drive transmission unit is in the non-transmission state until the trailing edge of the sheet in the second transport direction leaves the first transport unit.

[0124] This configuration reduces the possibility of the sheet S being pulled by the reversing roller pair 61 and the intermediate re-feed roller pair 62 when both are transporting the sheet S.

[0125] As a result, it is possible to suppress accelerated wear of the pair of reversing rollers 61 or the pair of intermediate re-feed rollers 62 due to friction with the sheet S. In addition, it is possible to reduce wear of the conveying guide 68 caused by rubbing between the second surface of the sheet S and the protruding portion 68a of the conveying guide 68.

[0126] (Other configurations) In this embodiment, the coupling part CP having the idling range is arranged coaxially with the drive roller 61a of the reversing roller pair 61, but it may be arranged in another part of the drive transmission mechanism DT. Even in this case, the coupling part CP goes into a non-transmitting state after the leading edge of the sheet S reaches the intermediate re-feed roller pair 62, thereby obtaining the same advantage as in this embodiment.

[0127] In addition, in this embodiment, the reversing roller pair 61 and the intermediate re-feed roller pair 62 are driven by the same driving source (first motor 81), but the reversing roller pair 61 and the intermediate re-feed roller pair 62 may be configured to be independently supplied with driving force from different driving sources.

[0128] In the present embodiment, the rotation direction of the first motor 81 is switched to switch the rotation direction of the pair of reversing rollers 61, but a motor that rotates in only one direction may be used as the drive source. In this case, the drive transmission mechanism DT may be provided with a switching mechanism that can switch the direction of rotation input to the drive roller 61a of the pair of reversing rollers 61.

[0129] In addition, in this embodiment, a configuration having one protrusion (engagement shaft 132a) and one recess (engagement hole 613a) is exemplified as an example of a coupling part CP having an idling range, but a configuration having multiple protrusions and multiple recesses may also be used.

[0130] In addition, in the present embodiment, the reverse output gear 132 (first member) has a protruding portion, and the reverse coupling 613 (second member) has a recessed portion that receives the protruding portion, but the arrangement of the protruding portion and the recessed portion may be reversed. In other words, the reverse coupling 613 (second member) may have an engagement shaft (protruding portion) that protrudes toward the reverse output gear 132 in the rotation axis direction X, and the reverse output gear 132 (first member) may have an engagement hole (recessed portion) that receives the engagement shaft. Even in this case, the same advantages as in the present embodiment can be obtained by setting the idling range by making the width of the engagement hole wider than the width of the engagement shaft.

[0131] In other words, the second member may have a protruding portion protruding in a rotation axis direction of the first member and the second member, the first abutted portion being an end of the protruding portion in the first rotation direction, and the second abutted portion being an end of the protruding portion in the second rotation direction. Also, the first member may have a recess that receives the protruding portion, the first abutting portion being an end of the recess in the first rotation direction, and the second abutting portion being an end of the recess in the second rotation direction.

[0132] In the present embodiment, the rotation speed of the pair of reversing rollers 61 is set to a constant rotation speed V1 regardless of the direction of rotation, but may be set to a different rotation speed depending on the direction of rotation. In other words, the conveying speed before the pair of reversing rollers 61 reverses the sheet S may be different from the conveying speed after the pair of reversing rollers 61 reverses the sheet S.

[0133] In the present embodiment, the second conveying section (intermediate re-feed roller pair 62) that conveys the sheet S at a conveying speed Vs2 faster than that of the first conveying section (reversal roller pair 61) is disposed downstream of the first conveying section in the second conveying direction D2. Alternatively, the second conveying section that conveys the sheet at a conveying speed faster than that of the first conveying section may be disposed upstream of the first conveying section in the second conveying direction D2. In this case, it is possible to reduce adverse effects (such as friction marks and conveying failures due to friction with the conveying guide) caused by bending of the sheet S between the first conveying section and the second conveying section. In other words, the positional relationship between the first conveying section and the second conveying section can be changed according to the specific configuration of the sheet conveying device.

[0134] More specifically, when the second conveying section is disposed upstream of the first conveying section in the second conveying direction D2, the sheet S is conveyed in the second conveying direction D2 at a conveying speed Vs2 by the second conveying section until the leading edge of the sheet S conveyed in the second conveying direction D2 reaches the first conveying section. When the leading edge of the sheet S reaches the first conveying section and a contact state occurs, the drive transmission section (coupling section) is set to a non-transmitting state so that the first conveying section follows the sheet S conveyed at the conveying speed Vs2. When the trailing edge of the sheet S leaves the second conveying section and the contact state is released, the drive transmission section is set to the second transmitting state, and the sheet S is conveyed by the first conveying section at a conveying speed Vs1.

[0135] Furthermore, the first and second conveying sections are not limited to the illustrated roller pairs. For example, the first conveying section may be a triple roller set in which the discharge roller pair and the reversing roller pair share one roller with each other.

[0136] Second Embodiment Next, a second embodiment of the present disclosure will be described. In the first embodiment, an example in which the coupling part CP is arranged coaxially with the drive roller 61a has been described, but in the second embodiment, an example in which the coupling part CP2 is arranged on an axis different from that of the drive roller 61a will be described. Hereinafter, elements with common reference symbols to the first embodiment will have substantially the same configurations and functions as those described in the first embodiment unless otherwise specified, and the following mainly describes the parts that are different from the first embodiment.

[0137] (Coupling part) The drive transmission configuration of the reversing roller pair 61 according to the second embodiment will be described with reference to Figs. 7(a) to (e). Fig. 7(a) is a perspective view showing a part of the drive transmission mechanism DT and the drive roller 61a of the reversing roller pair 61. Figs. 7(b) and (c) are exploded views of the drive transmission mechanism DT and the coupling part CP2 of the drive roller 61a. Fig. 7(d) is a view showing a part of the drive transmission mechanism DT and the drive roller 61a of the reversing roller pair 61, as viewed from a direction perpendicular to the rotation axis direction X described below (top view). Fig. 7(e) is a cross-sectional view of the coupling part CP2 taken along line AA in Fig. 7(d).

[0138] As shown in Figures 7(a) and (d), the drive transmission mechanism DT of this embodiment has a coupling pulley 116, a coupling gear 133, and a reversing roller gear 134 instead of the third pulley 113, the reversing input gear 131, and the reversing output gear 132 (Figure 3(a)) of the first embodiment.

[0139] The coupling part CP2 includes a coupling gear 133 and a coupling pulley 116. That is, the drive transmission mechanism DT of this embodiment has the coupling part CP2 as an example of a drive transmission part. The coupling gear 133 is an example of a first member (driving member, input member) connected to a drive source (first motor 81). The coupling pulley 116 is an example of a second member (driven member, output member) connected to a driven object (here, the reversing roller pair 61).

[0140] The coupling pulley 116 rotates around a rotation axis Ax3. The rotation axis Ax3 is an axis different from the rotation axis Ax of the drive roller 61a of the reversing roller pair 61. The rotation axis Ax3 of this embodiment is substantially parallel to the rotation axis Ax of the drive roller 61a. The rotation axis Ax3 corresponds to the second intermediate shaft Ax2 (FIG. 3(a)) of the first embodiment. The second drive belt 111 is stretched between the second pulley 112 and the coupling pulley 116.

[0141] The coupling gear 133 is disposed coaxially with the coupling pulley 116. The coupling gear 133 and the coupling pulley 116 are rotatable around the rotation axis Ax3 in rotation directions R3 and R4. In this embodiment, the radial direction of a virtual circle centered on the rotation axis Ax3 when viewed in the rotation axis direction X is referred to as the radial direction of the coupling part CP2 or simply as the "radial direction."

[0142] The reverse roller gear 134 is disposed coaxially with the drive roller 61a of the reverse roller pair 61, and rotates integrally with the drive roller 61a. The reverse roller gear 134 meshes with the coupling gear 133. The driving force from the first motor 81 is input to the drive roller 61a via the coupling pulley 116, the coupling gear 133, and the reverse roller gear 134, whereby the reverse roller pair 61 is driven to rotate.

[0143] 7(b) and (c), the coupling pulley 116 has a plurality of engaging claws 116b as an engaging portion, and the coupling gear 133 has a plurality of engaging recesses 133b as an engaged portion.

[0144] More specifically, the coupling pulley 116 has a shaft portion 116a extending along the rotation axis Ax3 and a side surface 116f extending radially outward from the shaft portion 116a. The coupling gear 133 has a hole portion 133a through which the shaft portion 116a is inserted and a side surface 133f extending radially outward from the hole portion 133a. The side surface 116f of the coupling pulley 116 and the side surface 133f of the coupling gear 133 face each other in the rotation axis direction X.

[0145] The multiple engagement claws 116b of the coupling pulley 116 are protrusions that protrude from a side surface 116f of the coupling pulley 116 toward the coupling gear 133 in the rotation axis direction X. In this embodiment, the multiple engagement claws 116b are arranged along an arc centered on the rotation axis Ax3.

[0146] The multiple engagement recesses 133b of the coupling gear 133 are provided on a side surface 133f of the coupling gear 133, and have a concave shape recessed toward a side away from the coupling pulley 116 in the rotation axis direction X. In this embodiment, the multiple engagement recesses 133b are arranged along an arc centered on the rotation axis Ax3, which is the same arc on which the multiple engagement claws 116b are arranged. Moreover, the multiple engagement recesses 133b are arranged in a number and at intervals corresponding to the multiple engagement claws 116b.

[0147] The multiple engagement claws 116b and the multiple engagement recesses 133b are an example of a dog clutch that transmits rotation by meshing with the projections and recesses provided on the two rotating members. Therefore, it may be said that the coupling pulley 116 has multiple engagement recesses formed between adjacent engagement claws 116b. Also, it may be said that the coupling gear 133 has multiple engagement claws formed between adjacent engagement recesses 133b, and the engagement claws of the coupling gear 133 engage with the engagement recesses of the coupling pulley 116.

[0148] 7(e), each engagement claw 116b has a first end 116c which is an end of the engagement claw 116b on the rotational direction R3 side, and a second end 116d which is an end of the engagement claw 116b on the rotational direction R4 side. The first end 132b is an example of a first contact portion, and the second end 132c is an example of a second contact portion.

[0149] Each engaging recess 133b has a first end 133c which is an end of the engaging recess 133b on the rotational direction R3 side, and a second end 133d which is an end of the engaging recess 133b on the rotational direction R4 side. The first end 133c is a first abutted portion that abuts against the first end 116c of the engaging claw 116b of the coupling pulley 116. The second end 133d is a second abutted portion that abuts against the second end 116d of the engaging claw 116b of the coupling pulley 116.

[0150] The width of each engagement recess 133b is set wider than the width of the corresponding engagement claw 116b. That is, when viewed in the rotation axis direction X, the distance from the first end 133c to the second end 133d measured along a virtual arc centered on the rotation axis Ax and passing through the engagement claw 116b is wider than the distance from the first end 116c to the second end 116d of the engagement claw 116b measured along the virtual arc. In other words, the central angle between the first end 133c and the second end 133d based on the rotation axis Ax is wider than the central angle between the end 116c on the rotation direction R3 side and the end 116d on the rotation direction R4 side of the engagement claw 116b based on the rotation axis Ax.

[0151] Since the width of the engagement recess 133b is wider than the width of the engagement claw 116b, the coupling pulley 116 and the coupling gear 133 are connected to be relatively rotatable (idleable) within a predetermined range (idle range). When the coupling gear 133 is used as a reference, the coupling pulley 116 is rotatable between a position where the first end 116c of the engagement claw 116b abuts against the first end 133c of the engagement recess 133b and a position where the second end 116d of the engagement claw 116b abuts against the second end 133d of the engagement recess 133b.

[0152] The state where the first end 116c of each engaging claw 116b abuts against the first end 133c of the corresponding engaging recess 133b is taken as a reference (θ=0°), and the angle by which the coupling pulley 116 rotates in the rotation direction R4 relative to the coupling gear 133 is expressed as θ [°]. The maximum value of the angle θ in this embodiment is the allowable idling angle θ t [°]. The first transmission state of the coupling part CP2 corresponds to θ=0, and the second transmission state corresponds to θ=θ tand the non-transmitting state corresponds to 0<θ<θ t In this embodiment, the range in which relative rotation (idling) between the coupling pulley 116 and the coupling gear 133 is permitted (hereinafter referred to as the allowable idling range) is 0<θ<θ t The range is.

[0153] (Coupling operation) The operation of the coupling part CP2 during double-sided printing will be described using Figures 8(a) to (d). The left side of each of Figures 8(a) to (d) is a cross-sectional view of the coupling part CP2 at the same position as line AA in Figure 7(d) at each point in double-sided printing. The right side of each of Figures 8(a) to (d) is a schematic diagram showing a cross-section of the image forming apparatus 1 at each point in double-sided printing. In each of Figures 8(a) to (d), the left and right sides show the same point in double-sided printing.

[0154] In this embodiment, double-sided printing is performed under the following conditions, as an example. -Seat length Ls:297mm. Path length Lp from the reversing roller pair 61 to the intermediate re-feed roller pair 62: 144 mm. · The conveying speed Vs1 of the sheet S by the pair of reversing rollers 61: 264.1 mm / s. The conveying speed Vs2 of the sheet S by the intermediate re-feed roller pair 62: 266.6 mm / s. Outer diameter D of the drive roller 61a of the reversing roller pair 61: 11 mm. Number of teeth on reversing roller gear 134: 14. Coupling gear 133 has 28 teeth. · Module of reversing roller gear 134 and coupling gear 133: 1. ·Backlash Lb between reversing roller gear 134 and coupling gear 133: 0.15 mm. The allowable slip angle θ between the coupling pulley 116 and the coupling gear 133 t :10°.

[0155] 8(a) to (d) show a process in which the conveying direction of the sheet S is reversed by the reversing roller pair 61 and the sheet S is conveyed by the reversing roller pair 61 and the intermediate re-feed roller pair 62. In double-sided printing, the operation of the reversing roller pair 61 from when the sheet S with an image formed on its first side is guided to the reversing path P2 and conveyed by the reversing roller pair 61 in the first conveying direction D1 until the conveying direction is reversed to the second conveying direction D2 is the same as in the first embodiment.

[0156] When the reversing roller pair 61 conveys the sheet S in the first conveying direction D1, the first end 116c of the engagement claw 116b of the coupling part CP2 comes into contact with the first end 133c of the engagement recess 133b as shown in FIG. 7(e). That is, the coupling part CP2 is in the first transmission state. As a result, the torque in the rotation direction R3 is transmitted from the coupling pulley 116 to the coupling gear 133. Furthermore, the torque in the first rotation direction R1 (FIG. 7(a)) is transmitted to the drive roller 61a of the reversing roller pair 61 via the meshing of the coupling gear 133 and the reversing roller gear 134, and the drive roller 61a rotates in the first rotation direction R1.

[0157] After that, by switching the rotation direction of the first motor 81, the coupling pulley 116 rotates in the rotation direction R3. Then, as shown in the left side of FIG. 8(a), the second end 116d of the engagement claw 116b abuts on the second end 133d of the engagement recess 133b. That is, the coupling part CP2 is in the second transmission state. As a result, the torque in the rotation direction R4 is transmitted from the coupling pulley 116 to the coupling gear 133. Furthermore, through the meshing between the coupling gear 133 and the reversing roller gear 134, the torque in the second rotation direction R2 is transmitted to the driving roller 61a of the reversing roller pair 61, and the driving roller 61a rotates in the second rotation direction R2. As a result, the conveying direction of the sheet S is reversed to the second conveying direction D2.

[0158] As shown on the right side of Fig. 8(a), the drive roller 61a of the reversing roller pair 61 rotates at a rotation speed V1 until the leading edge of the sheet S in the second transport direction D2 reaches the intermediate re-feed roller pair 62. Also, as shown on the left side of Fig. 8(a), the coupling pulley 116 and the coupling gear 133 rotate in the rotation direction R4 at a rotation speed V1' corresponding to the rotation speed V1 of the reversing output gear 132. The sheet S is transported by the reversing roller pair 61 at a transport speed Vs1.

[0159] 8(b) shows a state when the leading edge of the sheet S in the second transport direction D2 reaches the nip portion of the intermediate re-feed roller pair 62. When the leading edge of the sheet S reaches the nip portion of the intermediate re-feed roller pair 62, the sheet S is nipped by both the reversing roller pair 61 and the intermediate re-feed roller pair 62 and transported in the second transport direction D2.

[0160] Also in this embodiment, the conveying speed Vs2 of the intermediate re-feed roller pair 62 is set to be faster than the conveying speed Vs1 of the reversing roller pair 61 (Vs2>Vs1). This reduces the occurrence of scratches on the image due to the bending of the sheet S, as described above.

[0161] The intermediate re-feed roller pair 62 transports the sheet S at a transport speed Vs2 so as to pull the sheet S out from the reversing roller pair 61. Meanwhile, the drive roller 61a of the reversing roller pair 61 receives torque in a second rotation direction R2 due to a frictional force from the sheet S transported at the transport speed Vs2, and tries to rotate ahead in the second rotation direction R2 at a rotation speed V3 (angular velocity) faster than the rotation speed V1.

[0162] In this embodiment, the drive roller 61a is allowed to rotate ahead due to the idling range of the coupling pulley 116 and the coupling gear 133 and the backlash Lb between the coupling gear 133 and the reverse roller gear 134, as described below.

[0163] First, as shown on the left side of Fig. 8(b), the drive roller 61a rotates ahead at a rotational speed V3 by an amount corresponding to the backlash Lb between the coupling gear 133 and the reverse roller gear 134. At this time, the reverse roller gear 134 rotates integrally with the drive roller 61a at the rotational speed V3. Then, the state changes from one in which the teeth of the reverse roller gear 134 abut against the upstream tooth surface of the teeth of the coupling gear 133 in the rotational direction R4 (the state of Fig. 8(a)) to one in which the teeth of the reverse roller gear 134 abut against the downstream tooth surface of the teeth of the coupling gear 133 in the rotational direction R4 (the state of Fig. 8(b)).

[0164] Next, as shown in the left side of FIG. 8(c), when the driving roller 61a rotates ahead of the driving roller 61a by more than the backlash Lb, the reverse roller gear 134 presses the coupling gear 133 in the rotation direction R4. As a result, the coupling gear 133 rotates ahead of the coupling pulley 116 rotating at the rotation speed V1' in the rotation direction R4 at the rotation speed V3' corresponding to the rotation speed V3 of the reverse output gear 132. Due to the relative rotation between the coupling gear 133 and the coupling pulley 116, the second end 133d of the engagement recess 133b moves away from the second end 116d of the engagement claw 116b. In other words, the coupling part CP2 changes from the second transmission state to the non-transmission state.

[0165] 8(d), even after the rear end of the sheet S in the second transport direction D2 leaves the nip portion of the reversing roller pair 61, the sheet S is transported at the transport speed Vs2 by the intermediate re-feed roller pair 62. Meanwhile, as the rear end of the sheet S leaves the nip portion of the reversing roller pair 61, the reversing roller pair 61 is released from the sheet S, and the rotation of the drive roller 61a is decelerated.

[0166] As shown in the left side of FIG. 8(d), even after the sheet S has passed through the pair of reversing rollers 61, the coupling gear 133 continues to rotate at the rotation speed V1', so the second end 133d of the engagement recess 133b abuts against the second end 116d of the engagement claw 116b again. Also, the reversing roller gear 134 returns to a state in which the teeth of the reversing roller gear 134 abut against the upstream tooth surface of the teeth of the coupling gear 133 in the rotation direction R4. That is, the coupling part CP2 returns from the non-transmitting state to the second transmitting state. Then, as shown in the right side of FIG. 8(d), the driving roller 61a of the pair of reversing rollers 61 rotates in the second rotation direction R2 at the rotation speed V1.

[0167] (Relationship between leading rotation angle and allowable slip angle) In this embodiment, it will be described that until the trailing end of the sheet S passes at least through the pair of intermediate re-feed rollers 62, the coupling part CP2 is in the non-transmitting state and the pair of reversing rollers 61 is allowed to rotate in advance.

[0168] Pre-rotation angle θ of the reverse roller gear 134 S1 [°] and the leading rotation angle θ of the coupling gear 133 S2 The angle θ of the reverse roller gear 134 is defined as follows: S1 The preceding rotation angle θ of the coupling gear 133 is an angle at which the reverse roller gear 134 rotates at a rotation speed V3 that is faster than the rotation speed V1 due to the preceding rotation of the driving roller 61a, compared to the case where the rotation speed is maintained at the rotation speed V1. S2 is an angle of extra rotation of the coupling gear 133 relative to the coupling pulley 116 rotating at the rotation speed V1', as a result of the coupling gear 133 rotating at a rotation speed V3' which is faster than the rotation speed V1'.

[0169] In this embodiment, the sheet length Ls, the path length Lp, the conveying speed Vs1, the conveying speed Vs2, and the outer diameter D of the drive roller 61a are the same as those in the first embodiment. In this case, as described in the first embodiment, the advance rotation angle (θ S ) is 15°. In addition, since the reverse roller gear 134 rotates integrally with the driving roller 61a, the advance rotation angle θ S1 is 15°.

[0170] As shown on the left side of FIG. 8(b), the advance rotation angle when the reverse roller gear 134 advances by the amount of backlash Lb between the reverse roller gear 134 and the coupling gear 133 is defined as θ b Let θ b is calculated as follows:

[0171] 1. Pitch circumference of reversing roller gear 134: (Module [mm]) x (Number of teeth) x (Pi) = 1 x 14 x π = 43.9 [mm]

[0172] 2. Pre-rotation angle θ for backlash b : (Backlash Lb [mm]) / (1. pitch circumference [mm]) x 360° = 0.15 / 43.9 x 360° = 1.22°

[0173] As shown on the left side of FIG. 8(c), even after the reverse roller gear 134 has rotated ahead of the other roller 134 by the backlash Lb, the reverse roller gear 134 continues to rotate ahead of the other roller 134 at the ahead rotation angle θ S1 At this time, the coupling gear 133 is pressed by the reverse roller gear 134 to rotate ahead of the coupling pulley 116 at a pre-rotation angle θ S2 is calculated as follows:

[0174] 3. Pre-rotation angle θ of the reverse roller gear 134 S1 The leading rotation angle θ for the backlash Lb bAngles Excluding: θ S1 -θ b =15°-1.22°=13.78°

[0175] 4. Reduction ratio of coupling gear 133 and reversing roller gear 134: (Number of teeth of coupling gear 133) / (Number of teeth of reversing roller gear 134)=28 / 14=2

[0176] 5. Pre-rotation angle θ of the coupling gear 133 S2 : (3. Pre-rotation angle) / (4. Reduction ratio)=13.78° / 2=6.89°

[0177] Therefore, during the period from when the leading edge of the sheet S enters the nip portion of the intermediate re-feed roller pair 62 to when the trailing edge of the sheet S leaves the nip portion of the reversing roller pair 61, the coupling gear 133 rotates ahead of the coupling pulley 116 by 6.89°. That is, the coupling gear 133 rotates relative to the coupling pulley 116 by 6.89° in the direction in which the second end 133d of the engagement recess 133b moves away from the second end 133d of the engagement claw 116b.

[0178] In this embodiment, the allowable rotation angle θ of the coupling pulley 116 and the coupling gear 133 t is set to a value larger than the above relative rotation amount (6.89°), for example, θ t = 10°. This makes it possible to prevent the first end 133c of the engagement recess 133b from contacting the first end 133c of the engagement claw 116b before the rear end of the sheet S passes through the nip portion of the reversing roller pair 61. In other words, the coupling part CP2 is in the non-transmitting state until the rear end of the sheet S passes through at least the intermediate re-feed roller pair 62, and the advance rotation of the reversing roller pair 61 is permitted.

[0179] (Advantages of this embodiment) As described above, the drive mechanism according to the present embodiment uses the coupling unit CP2 having an idling range. After the leading edge of the sheet S reaches the intermediate re-feed roller pair 62 during double-sided printing, the coupling unit CP2 changes from the second transmission state to the non-transmission state so as to allow the reversing roller pair 61 to operate following the sheet S transported at the transport speed Vs2. The coupling unit CP2 is in the non-transmission state at least until the trailing edge of the sheet S leaves the nip portion of the reversing roller pair 61. In other words, after a contact state occurs in which the sheet transported in the second transport direction comes into contact with both the first transport unit and the second transport unit, the drive transmission unit enters the non-transmission state so as to allow the first transport unit to operate following the sheet transported at the second transport speed. More specifically, after the drive transmission unit changes from the second transmission state to the non-transmission state, the drive transmission unit is in the non-transmission state until the trailing edge of the sheet in the second transport direction leaves the first transport unit.

[0180] This configuration reduces the possibility of the sheet S being pulled by the reversing roller pair 61 and the intermediate re-feed roller pair 62 when both are transporting the sheet S.

[0181] As a result, it is possible to suppress accelerated wear of the pair of reversing rollers 61 or the pair of intermediate re-feed rollers 62 due to friction with the sheet S. In addition, it is possible to reduce wear of the conveying guide 68 caused by rubbing between the second surface of the sheet S and the protruding portion 68a of the conveying guide 68.

[0182] In this embodiment, the coupling part CP2 having the idling range is disposed on an axis different from that of the driving roller 61a of the pair of reversing rollers 61. Therefore, the amount of relative rotation (θ S2 ) is an angle (θ b) is reduced. The backlash in this embodiment refers to the backlash at the meshing portion between the coupling gear 133 and the reverse roller gear 134. If the amount of relative rotation is reduced, the width of the engagement recess 133b can be reduced, and the number of engagement claws 116b and engagement recesses 133b can be increased. As a result, the load on one claw can be reduced, and the durability of the device can be improved.

[0183] (Other configurations) In this embodiment, the configuration in which the coupling pulley 116 (first member) has a protruding portion and the coupling gear 133 (second member) has a recessed portion that receives the protruding portion has been exemplified, but the arrangement of the protruding portion and the recessed portion may be reversed. In other words, the coupling gear 133 (second member) may have an engaging claw (protruding portion) that protrudes toward the coupling pulley 116 in the rotation axis direction X, and the coupling pulley 116 (first member) may have an engaging recessed portion (recessed portion) that receives the engaging claw.

[0184] In the present embodiment, the number of the engaging claws 116b and the number of the engaging recesses 133b are five, but the number of the engaging claws 116b and the number of the engaging recesses 133b may be other than five. Also, the coupling part CP2 may include only one engaging claw 116b and one engaging recess 133b.

[0185] In addition, the modified examples described as (other configurations) in the first embodiment may be applied to this embodiment.

[0186] Other Embodiments In the above embodiment, the image forming unit 20 of the direct transfer type is exemplified as the image forming means, but the image forming means may be of an intermediate transfer type in which a toner image is primarily transferred from an image carrier to an intermediate transfer body such as an intermediate transfer belt, and then the toner image is secondarily transferred from the intermediate transfer body to a recording material. Furthermore, the image forming means is not limited to the electrophotographic type, and may be, for example, an inkjet type in which an image is formed by ejecting ink liquid from a nozzle.

[0187] In addition, in the above-described embodiment, a color printer that forms a color image using multiple color toners is exemplified as the image forming device, but the image forming device may also be a monochrome printer that forms a monochrome image using a single color toner.

[0188] In the above-described embodiment, the sheet transport unit 60 that transports the sheet S during double-sided printing in the image forming apparatus 1 has been described as an example of the sheet transport device. The sheet transport device is not limited to this, and may be a device that transports sheets in other devices included in the image forming apparatus (image forming system). For example, the sheet transport device may be an automatic document feeder (ADF) capable of automatically feeding a document in an image reading device that reads an image from a document. In addition, the sheet transport device may be a sheet transport mechanism in a sheet processing device (finisher) that performs processing such as binding on sheets on which images have been formed in the image forming apparatus main body.

[0189] Summary of the Disclosure The present disclosure includes at least the following configurations or methods.

[0190] (Configuration 1) a first conveying section that conveys the sheet in a first conveying direction and then conveys the sheet in a second conveying direction opposite to the first conveying direction; a second conveying section that conveys the sheet in the second conveying direction; A drive source that drives the first transport unit; a drive transmission section that can be in a first transmission state in which a drive force of the drive source is transmitted to the first transport section so as to transport the sheet in the first transport direction, a second transmission state in which the drive force is transmitted to the first transport section so as to transport the sheet in the second transport direction, and a non-transmission state in which the drive force is not transmitted to the first transport section; A sheet conveying device comprising: when the first transport unit transports the sheet in the second transport direction and the second transport unit does not transport the sheet, the drive transmission unit is in the second transmission state so that the first transport unit transports the sheet at a first transport speed by the driving force; a conveying speed of the sheet by the second conveying unit is a second conveying speed that is faster than the first conveying speed; after a contact state occurs in which the sheet transported in the second transport direction contacts both the first transport section and the second transport section, the drive transmission section is brought into the non-transmission state to allow the first transport section to operate in accordance with the sheet transported at the second transport speed until the contact state is released; A sheet conveying device comprising:

[0191] (Configuration 2) the drive transmission unit includes a first member connected to the drive source, and a second member arranged coaxially with the first member and connected to the first transport unit, The first member has a first contact portion and a second contact portion, the second member has a first contacted portion that contacts the first contact portion and a second contacted portion that contacts the second contact portion, The drive transmission portion is configured to be in the first transmitting state when the first contact portion and the first contacted portion come into contact with each other, to be in the second transmitting state when the second contact portion and the second contacted portion come into contact with each other, and to be in the non-transmitting state when the first contact portion and the first contacted portion separate and when the second contact portion and the second contacted portion separate. 2. The sheet conveying device according to configuration 1,

[0192] (Configuration 3) an amount of relative rotation between the first member and the second member required to change the drive transmission unit from the second transmission state to the first transmission state is greater than an amount of relative rotation between the first member and the second member that occurs during a period in which both the first conveying unit and the second conveying unit convey the sheet in the second conveying direction. 3. The sheet conveying device according to configuration 2.

[0193] (Configuration 4) a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the first conveying direction is defined as a first rotation direction, and a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the second conveying direction is defined as a second rotation direction, the first member has a protruding portion protruding in a rotation axis direction of the first member and the second member, the first contact portion being an end portion of the protruding portion in the first rotation direction, and the second contact portion being an end portion of the protruding portion in the second rotation direction, The second member has a recess that receives the protrusion, the first abutted portion is an end of the recess in the first rotation direction, and the second abutted portion is an end of the recess in the second rotation direction. 4. The sheet conveying device according to configuration 2 or 3.

[0194] (Configuration 5) A central angle between the first contact portion and the second contact portion with respect to a rotation axis of the first member and the second member is smaller than a central angle between the first contacted portion and the second contacted portion with respect to the rotation axis. 5. The sheet conveying device according to configuration 4.

[0195] (Configuration 6) the protrusion is one of a plurality of protrusions arranged along an arc centered on a rotation axis of the first member and the second member; the recess is one of a plurality of recesses disposed along the arc to engage with each of the plurality of protrusions; 6. The sheet conveying device according to configuration 4 or 5,

[0196] (Configuration 7) a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the first conveying direction is defined as a first rotation direction, and a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the second conveying direction is defined as a second rotation direction, the second member has a protruding portion protruding in a rotation axis direction of the first member and the second member, the first abutted portion being an end of the protruding portion in the first rotation direction, and the second abutted portion being an end of the protruding portion in the second rotation direction, The first member has a recess that receives the protrusion, the first contact portion is an end of the recess in the first rotation direction, and the second contact portion is an end of the recess in the second rotation direction. 7. The sheet conveying device according to any one of configurations 2 to 6.

[0197] (Configuration 8) a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the first conveying direction is defined as a first rotation direction, and a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the second conveying direction is defined as a second rotation direction, When the contact state occurs, the second contacted portion moves away from the second contact portion in the second rotation direction. 8. The sheet conveying device according to any one of configurations 2 to 7.

[0198] (Configuration 9) the first transport unit has a roller that rotates by receiving the driving force from the drive transmission unit, the roller rotates at a first angular velocity when the first transport section transports the sheet at the first transport speed; When the contact state occurs, the roller rotates at a second angular velocity faster than the first angular velocity in response to the sheet being conveyed by the second conveying unit at the second conveying speed, When the roller rotates at the second angular velocity, the second member rotates relative to the first member, and the drive transmission portion changes from the second transmission state to the non-transmission state. 9. The sheet conveying device according to any one of configurations 2 to 8.

[0199] (Configuration 10) The first member and the second member rotate around a rotation axis of the roller. 10. The sheet conveying device according to configuration 9,

[0200] (Configuration 11) The first member and the second member rotate about a rotation axis different from a rotation axis of the roller. 10. The sheet conveying device according to configuration 9,

[0201] (Configuration 12) The second transport section is disposed downstream of the first transport section in the second transport direction. 12. The sheet conveying device according to any one of configurations 1 to 11.

[0202] (Configuration 13) a first guide disposed between the first conveying section and the second conveying section in the second conveying direction and configured to guide the sheet; a second guide facing the first guide across a space through which the sheet passes; Further equipped with the first conveying section is a first roller pair having a first nip that holds the sheet, the second conveying portion is a second roller pair having a second nip that holds the sheet, the first guide has a portion that protrudes toward the second guide with respect to a virtual line connecting the first nip and the second nip when viewed in a direction of a rotation axis of the first roller pair; 13. The sheet conveying device according to configuration 12.

[0203] (Configuration 14) the first conveying section is disposed in a second conveying path along which the sheet is reversed and conveyed in order to form an image on a second side of the sheet having an image formed on a first side thereof while passing through the first conveying path; the second conveying unit is disposed on a third conveying path along which the sheet, which has been reversely conveyed by the first conveying unit, is conveyed again toward the first conveying path; the first guide is disposed to face the second surface of the sheet transported from the first transport section to the second transport section. 14. The sheet conveying device according to configuration 13,

[0204] (Configuration 15) The second transport section is disposed upstream of the first transport section in the second transport direction. 15. The sheet conveying device according to any one of configurations 1 to 14.

[0205] (Configuration 16) A sheet conveying device according to any one of configurations 1 to 15, an image forming unit for forming an image on the sheet conveyed by the sheet conveying device; Equipped with 1. An image forming apparatus comprising: [Explanation of symbols]

[0206] 61...first conveying section (reverse roller pair) / 62...second conveying section (intermediate re-feed roller pair) / 81...driving source (first motor) / 116, 132...first member (coupling pulley, reversing output gear) / 133, 613...second member (coupling gear, reversing coupling) / CP, CP2...drive transmission section (coupling section) / D1...first conveying direction / D2...second conveying direction

Claims

1. a first conveying section that conveys the sheet in a first conveying direction and then conveys the sheet in a second conveying direction opposite to the first conveying direction; a second conveying section that conveys the sheet in the second conveying direction; A drive source that drives the first transport unit; a drive transmission section that can be in a first transmission state in which a drive force of the drive source is transmitted to the first transport section so as to transport the sheet in the first transport direction, a second transmission state in which the drive force is transmitted to the first transport section so as to transport the sheet in the second transport direction, and a non-transmission state in which the drive force is not transmitted to the first transport section; A sheet conveying device comprising: when the first transport unit transports the sheet in the second transport direction and the second transport unit does not transport the sheet, the drive transmission unit is in the second transmission state so that the first transport unit transports the sheet at a first transport speed by the driving force; a conveying speed of the sheet by the second conveying unit is a second conveying speed that is faster than the first conveying speed, after a contact state occurs in which the sheet transported in the second transport direction contacts both the first transport section and the second transport section, the drive transmission section is brought into the non-transmission state to allow the first transport section to operate in accordance with the sheet transported at the second transport speed until the contact state is released; A sheet conveying device comprising:

2. the drive transmission unit includes a first member connected to the drive source, and a second member arranged coaxially with the first member and connected to the first transport unit, The first member has a first contact portion and a second contact portion, the second member has a first contacted portion that is in contact with the first contact portion and a second contacted portion that is in contact with the second contact portion, The drive transmission portion is configured to be in the first transmitting state when the first contact portion and the first abutted portion come into contact with each other, to be in the second transmitting state when the second contact portion and the second abutted portion come into contact with each other, and to be in the non-transmitting state when the first contact portion and the first abutted portion separate and the second contact portion and the second abutted portion separate.

2. The sheet transport device according to claim 1.

3. an amount of relative rotation between the first member and the second member required to change the drive transmission unit from the second transmission state to the first transmission state is greater than an amount of relative rotation between the first member and the second member that occurs during a period in which both the first conveying unit and the second conveying unit convey the sheet in the second conveying direction; 3. The sheet transport device according to claim 2.

4. a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the first conveying direction is defined as a first rotation direction, and a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the second conveying direction is defined as a second rotation direction, the first member has a protruding portion protruding in a rotation axis direction of the first member and the second member, the first contact portion being an end portion of the protruding portion in the first rotation direction, and the second contact portion being an end portion of the protruding portion in the second rotation direction, the second member has a recess that receives the protrusion, the first abutted portion is an end of the recess in the first rotation direction, and the second abutted portion is an end of the recess in the second rotation direction.

3. The sheet transport device according to claim 2.

5. a central angle between the first contact portion and the second contact portion with respect to a rotation axis of the first member and the second member is smaller than a central angle between the first contacted portion and the second contacted portion with respect to the rotation axis; 5. The sheet transport device according to claim 4.

6. the protrusion is one of a plurality of protrusions arranged along an arc centered on a rotation axis of the first member and the second member, the recess is one of a plurality of recesses disposed along the arc to engage with each of the plurality of protrusions; 5. The sheet transport device according to claim 4.

7. a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the first conveying direction is defined as a first rotation direction, and a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the second conveying direction is defined as a second rotation direction, the second member has a protruding portion protruding in a rotation axis direction of the first member and the second member, the first abutted portion being an end of the protruding portion in the first rotation direction, and the second abutted portion being an end of the protruding portion in the second rotation direction, the first member has a recess that receives the protrusion, the first contact portion being an end of the recess in the first rotation direction, and the second contact portion being an end of the recess in the second rotation direction.

3. The sheet transport device according to claim 2.

8. a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the first conveying direction is defined as a first rotation direction, and a rotation direction of the first member and the second member when the first conveying unit conveys the sheet in the second conveying direction is defined as a second rotation direction, When the contact state occurs, the second contacted portion moves away from the second contact portion in the second rotation direction.

3. The sheet transport device according to claim 2.

9. the first transport unit has a roller that rotates by receiving the driving force from the drive transmission unit, the roller rotates at a first angular velocity when the first transport section transports the sheet at the first transport speed, When the contact state occurs, the roller rotates at a second angular velocity faster than the first angular velocity in response to the sheet being conveyed by the second conveying unit at the second conveying speed, When the roller rotates at the second angular velocity, the second member rotates relative to the first member, and the drive transmission portion changes from the second transmission state to the non-transmission state.

3. The sheet transport device according to claim 2.

10. The first member and the second member rotate about a rotation axis of the roller.

10. The sheet transport device according to claim 9.

11. The first member and the second member rotate about a rotation axis different from a rotation axis of the roller.

10. The sheet transport device according to claim 9.

12. The second transport section is disposed downstream of the first transport section in the second transport direction.

2. The sheet transport device according to claim 1.

13. a first guide disposed between the first transport section and the second transport section in the second transport direction and configured to guide the sheet; a second guide facing the first guide across a space through which the sheet passes; Further equipped with the first conveying portion is a first roller pair having a first nip that holds the sheet, the second conveying portion is a second roller pair having a second nip that holds the sheet, the first guide has a portion that protrudes toward the second guide with respect to a virtual line connecting the first nip and the second nip when viewed in a direction of a rotation axis of the first roller pair; The sheet transport device according to claim 12 .

14. the first conveying section is disposed on a second conveying path along which the sheet is reversed and conveyed in order to form an image on a second surface of the sheet having an image formed on a first surface thereof while passing through the first conveying path; the second conveying unit is disposed on a third conveying path along which the sheet conveyed by the first conveying unit is conveyed again toward the first conveying path, the first guide is disposed to face the second surface of the sheet transported from the first transport section to the second transport section. The sheet transport device according to claim 13 .

15. The second transport section is disposed upstream of the first transport section in the second transport direction.

2. The sheet transport device according to claim 1.

16. A sheet conveying device according to any one of claims 1 to 15, an image forming unit for forming an image on the sheet conveyed by the sheet conveying device; Equipped with 1. An image forming apparatus comprising:

Images