SHEET CONVEYANCE APPARATUS AND IMAGE FORMING APPARATUS

Abstract

A sheet conveyance apparatus includes a first conveyance unit, a second conveyance unit, a drive source, and a drive transmission unit. When the first conveyance unit conveys a sheet in a second conveyance direction and the second conveyance unit does not convey the sheet, the drive transmission unit enters a second transmission state in such a manner that the first conveyance unit conveys the sheet at a first conveyance speed. The second conveyance unit conveys the sheet at a second conveyance speed higher than the first conveyance speed. From a contact state in which the sheet conveyed in the second conveyance direction is in contact with both the first conveyance unit and the second conveyance unit is caused, until the contact state is released, the drive transmission unit is kept in a non-transmission state.

Claims

1. A sheet conveyance apparatus comprising: a first conveyance unit configured to convey a sheet in a first conveyance direction and a second conveyance direction opposite to the first conveyance direction, the first conveyance unit being configured to convey the sheet in the second conveyance direction after conveying the sheet in the first conveyance direction; a second conveyance unit configured to convey the sheet in the second conveyance direction; a drive source configured to drive the first conveyance unit; and a drive transmission unit configured to enter a first transmission state of transmitting drive force of the drive source in such a manner that the first conveyance unit conveys the sheet in the first conveyance direction, a second transmission state of transmitting the drive force in such a manner that the first conveyance unit conveys the sheet in the second conveyance direction, and a non-transmission state in which the drive force is not transmitted to the first conveyance unit, wherein, when the first conveyance unit conveys the sheet in the second conveyance direction and the second conveyance unit does not convey the sheet, the drive transmission unit enters the second transmission state in such a manner that the first conveyance unit conveys the sheet at a first conveyance speed with the drive force, wherein the second conveyance unit conveys the sheet at a second conveyance speed higher than the first conveyance speed, and wherein, from a contact state in which the sheet conveyed in the second conveyance direction is in contact with both the first conveyance unit and the second conveyance unit is caused until the contact state is released, the drive transmission unit is kept in the non-transmission state in such a manner that the first conveyance unit is allowed to be rotated by the sheet conveyed at the second conveyance speed.

2. The sheet conveyance apparatus according to claim 1, wherein 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 conveyance unit, wherein the first member includes a first to-contact portion and a second to-contact portion, wherein the second member includes a first to-be-contacted portion to be contacted by the first to-contact portion, and a second to-be-contacted portion to be contacted by the second to-contact portion, and wherein the drive transmission unit is configured to enter the first transmission state by the first to-contact portion and the first to-be-contacted portion coming into contact with each other, to enter the second transmission state by the second to-contact portion and the second to-be-contacted portion coming into contact with each other, and to enter the non-transmission state by the first to-contact portion and the first to-be-contacted portion being separated from each other and the second top-contact portion and the second to-be-contacted portion being separated from each other.

3. The sheet conveyance apparatus according to claim 2, wherein an amount of relative rotation of the first member and the second member for the drive transmission unit to transition from the second transmission state to the first transmission state is larger than an amount of relative rotation of the first member and the second member in a period in which both the first conveyance unit and the second conveyance unit convey the sheet in the second conveyance direction.

4. The sheet conveyance apparatus according to claim 2, wherein a rotational direction of the first member and the second member that is set when the first conveyance unit conveys the sheet in the first conveyance direction is a first rotational direction, and a rotational direction of the first member and the second member that is set when the first conveyance unit conveys the sheet in the second conveyance direction is a second rotational direction, wherein the first member includes a protruding portion protruding in a rotational axis direction of the first member and the second member, the first to-contact portion is an end portion of the protruding portion in the first rotational direction, and the second to-contact portion is an end portion of the protruding portion in the second rotational direction, wherein the second member includes a recess portion that accepts the protruding portion, the first to-be-contacted portion is an end portion of the recess portion in the first rotational direction, and the second to-be-contacted portion is an end portion of the recess portion in the second rotational direction.

5. The sheet conveyance apparatus according to claim 4, wherein a central angle between the first to-contact portion and the second to-contact portion with respect to a rotational axis line of the first member and the second member is smaller than a central angle between the first to-be-contacted portion and the second to-be-contacted portion with respect to the rotational axis line.

6. The sheet conveyance apparatus according to claim 4, wherein the protruding portion is one of a plurality of protruding portions arranged along a circular arc centered on a rotational axis line of the first member and the second member, and wherein the recess portion is one of a plurality of recess portions arranged along the circular arc each to respectively engage with a protruding portion of the plurality of protruding portions.

7. The sheet conveyance apparatus according to claim 2, wherein a rotational direction of the first member and the second member that is set when the first conveyance unit conveys the sheet in the first conveyance direction is a first rotational direction, and a rotational direction of the first member and the second member that is set when the first conveyance unit conveys the sheet in the second conveyance direction is a second rotational direction, wherein the second member includes a protruding portion protruding in a rotational axis direction of the first member and the second member, the first to-be-contacted portion is an end portion of the protruding portion in the first rotational direction, and the second to-be-contacted portion is an end portion of the protruding portion in the second rotational direction, and wherein the first member includes a recess portion that accepts the protruding portion, the first to-contact portion is an end portion of the recess portion in the first rotational direction, and the second to-contact portion is an end portion of the recess portion in the second rotational direction.

8. The sheet conveyance apparatus according to claim 2, wherein a rotational direction of the first member and the second member that is set when the first conveyance unit conveys the sheet in the first conveyance direction is a first rotational direction, and a rotational direction of the first member and the second member that is set when the first conveyance unit conveys the sheet in the second conveyance direction is a second rotational direction, and wherein, when the contact state is caused, the second to-be-contacted portion gets away from the second to-contact portion toward the second rotational direction.

9. The sheet conveyance apparatus according to claim 2, wherein the first conveyance unit includes a roller configured to rotate by receiving the drive force from the drive transmission unit, wherein the roller rotates at a first angular speed when the first conveyance unit conveys the sheet at the first conveyance speed, wherein, when the contact state is caused, the roller rotates at a second angular speed higher than the first angular speed following the sheet conveyed by the second conveyance unit at the second conveyance speed, and wherein, the roller rotating at the second angular speed rotates the second member relatively relative to the first member, and changes a state of the drive transmission unit from the second transmission state to the non-transmission state.

10. The sheet conveyance apparatus according to claim 9, wherein the first member and the second member rotate around a rotational axis line of the roller.

11. The sheet conveyance apparatus according to claim 9, wherein the first member and the second member rotate around a rotational axis line different from a rotational axis line of the roller.

12. The sheet conveyance apparatus according to claim 1, wherein the second conveyance unit is arranged downstream of the first conveyance unit in the second conveyance direction.

13. The sheet conveyance apparatus according to claim 12, further comprising: a first guide that is arranged between the first conveyance unit and the second conveyance unit in the second conveyance direction, and configured to guide the sheet; and a second guide facing the first guide across space through which the sheet passes, wherein the first conveyance unit is a first roller pair having a first nip at which the sheet is nipped, wherein the second conveyance unit is a second roller pair having a second nip at which the sheet is nipped, and wherein the first guide includes a portion protruding toward the second guide from an imaginary straight line connecting the first nip and the second nip when viewed in a rotational axis direction of the first roller pair.

14. The sheet conveyance apparatus according to claim 13, wherein the first conveyance unit is arranged on a second conveyance path on which the sheet is reversely conveyed to form an image on a second surface of the sheet with an image formed on a first surface while the sheet passes through a first conveyance path, wherein the second conveyance unit is arranged on a third conveyance path on which the sheet reversely conveyed by the first conveyance unit is conveyed again toward the first conveyance path, and wherein the first guide is arranged to face the second surface of the sheet conveyed from the first conveyance unit toward the second conveyance unit.

15. The sheet conveyance apparatus according to claim 1, wherein the second conveyance unit is arranged upstream of the first conveyance unit in the second conveyance direction.

16. An image forming apparatus comprising: the sheet conveyance apparatus according to claim 1; and an image forming unit configured to form an image on the sheet conveyed by the sheet conveyance apparatus.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 is a schematic diagram of an image forming apparatus according to a first exemplary embodiment.

[0008] FIGS. 2A to 2E are diagrams illustrating duplex printing according to the first exemplary embodiment.

[0009] FIGS. 3A to 3D are diagrams illustrating a drive transmission mechanism according to the first exemplary embodiment.

[0010] FIGS. 4A to 4E are diagrams illustrating a coupling unit according to the first exemplary embodiment.

[0011] FIGS. 5A to 5D are diagrams illustrating an operation of the coupling unit according to the first exemplary embodiment.

[0012] FIGS. 6A to 6D are diagrams illustrating an operation of the coupling unit according to the first exemplary embodiment.

[0013] FIGS. 7A to 7E are diagrams illustrating a coupling unit according to a second exemplary embodiment.

[0014] FIGS. 8A to 8D are diagrams illustrating an operation of the coupling unit according to the second exemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

[0015] Hereinafter, various exemplary embodiments, features, and aspects of the present disclosure will be described with reference to the drawings.

[0016] A first exemplary embodiment will be described. First of all, the general configuration of an image forming apparatus 1 according to the first exemplary embodiment will be described with reference to FIG. 1. FIG. 1 is a schematic diagram illustrating a cross-section cut along a plane vertical to its main scanning direction in image formation executed by the image forming apparatus 1 (the rotational axis direction of a photosensitive drum 22a).

[0017] The image forming apparatus 1 according to the present exemplary embodiment is a color laser beam printer that forms a color image on a recording material by an electrophotographic process that uses toner of a plurality of colors. A variety of types of sheet materials with different sizes and materials, such as paper including plain paper and thick paper, surface-processed sheet materials like coated paper, specially-shaped sheet materials including envelopes and index paper, plastic films, and cloth, can be used as a sheet S serving as a recording material (recording medium).

[0018] As illustrated in FIG. 1, the image forming apparatus 1 includes a housing 2, a sheet feeding unit 10, an image forming unit 20, a cartridge tray 21, a transfer conveyance unit 30, a fixing unit 40, a scanner 50, a sheet conveyance unit 60, and a power unit 70. The image forming apparatus 1 also includes a belt cleaner unit 34, and a plurality of motors serving as drive sources. The image forming apparatus 1 according to the present exemplary embodiment includes six motors corresponding to first to sixth motors 81 to 86.

[0019] The sheet feeding unit 10, the image forming unit 20, the transfer conveyance unit 30, the fixing unit 40, the scanner 50, the sheet conveyance unit 60, the power unit 70, the belt cleaner unit 34, and the first to sixth motors 81 to 86 are accommodated in the housing 2.

[0020] The housing 2 includes an opening portion 2A provided on the front side of the housing 2, and a front cover 2B that can open or close the opening portion 2A. The front side is located downstream in the direction of sheet discharge that is a direction in which the sheet S with a formed image on it is discharged by a discharge roller pair 65 to be described below. In addition, a discharge tray 2E is provided in the top surface portion of the housing 2.

[0021] The front cover 2B is pivotable around a pivotal line 2b along the lower end portion of the front cover 2B with respect to other portions of the housing 2. The front cover 2B pivoting around the pivotal line 2b allows the front cover 2B to move between the closed position at which the front cover 2B closes the opening portion 2A (position indicated by a solid line in FIG. 1), and the opened position at which the front cover 2B opens the opening portion 2A (position indicated by a broken line in FIG. 1). Moving the front cover 2B to the opened position and opening the opening portion 2A allows the cartridge tray 21 to be drawn and inserted out of and into the housing 2 via the opening portion 2A.

[0022] The image forming unit 20 according to the present exemplary embodiment includes four cartridges 22 (process units, or process cartridges). Each of the cartridges 22 is attachable and removable to and from the main body of the image forming apparatus 1 (hereinafter, will be collectively referred to as detachable). The apparatus main body refers to the part of the image forming apparatus 1 that excludes the four cartridges 22 and the cartridge tray 21. Each of the cartridges 22 according to the present exemplary embodiment is detachable from the cartridge tray 21, and stored in the housing 2 being attached to the cartridge tray 21.

[0023] The four cartridges 22 are adjacently arranged in the rotational direction of a transfer conveyance belt 31 included in the transfer conveyance unit 30. In the present exemplary embodiment, the four cartridges 22 are adjacently arranged in a front-back direction of the housing 2. The four cartridges 22 respectively form black, yellow, magenta, and cyan toner images on the sheet S using the respective color toners as developers. Each of the cartridges 22 includes the photosensitive drum 22a serving as an image bearing member, a charging device 22b serving as a charging unit, a development roller 22c serving as a developing unit, and a cleaning roller 23 serving as a cleaning unit. The photosensitive drum 22a is a photosensitive member in which a photosensitive layer made of an organic photosensitive material or other materials is formed over the outer circumferential potion of a columnar base member.

[0024] The scanner 50 is arranged over the image forming unit 20. The scanner 50 includes a laser oscillator serving as a light source, and an exposure optical system for scanning the surfaces of the photosensitive drums 22a using laser beams from the laser oscillator.

[0025] The transfer conveyance unit 30 is arranged under the image forming unit 20. The transfer conveyance unit 30 includes the transfer conveyance belt 31, a drive roller 32a, a driven roller 32b, and four transfer rollers 33. The transfer conveyance belt 31 is stretched over the drive roller 32a and the driven roller 32b. The transfer conveyance belt 31 is rotationally driven by the drive roller 32a rotated by the drive force supplied from a drive source. The four transfer rollers 33 are arranged respectively corresponding to the four photosensitive drums 22a. The transfer rollers 33 are arranged in contact with the inner surface of the transfer conveyance belt 31, facing the respective photosensitive drum 22a across the transfer conveyance belt 31. A transfer portion at which a toner image is transferred to the sheet S is formed between each of the transfer rollers 33 and the corresponding photosensitive drum of the photosensitive drums 22a.

[0026] The sheet feeding unit 10 is arranged in a lower portion of the image forming apparatus 1. The sheet feeding unit 10 includes a storage unit 11 that stores the sheet S, a feeding roller 12 serving as a feeding unit that feeds the sheet S, and a separation roller 13 and a separation pad 13a serving as separation unit that convey the sheets S while separating the sheets S from each other. The separation roller 13 and the separation pad 13a serve as an example of the separation unit. For example, a roller pair including the separation roller 13 and a rubber roller contacting the separation roller 13 may be used.

[0027] A conveyance roller pair 14 and a registration roller pair 15 are arranged downstream of the separation roller 13 and upstream of the transfer conveyance unit 30 in the conveyance direction of the sheet S.

[0028] The belt cleaner unit 34 is arranged under the transfer conveyance unit 30 and above the sheet feeding unit 10. In addition, the belt cleaner unit 34 is arranged ahead of the power unit 70. The belt cleaner unit 34 includes a cleaning member 35 that collects foreign matter, such as waste toner on the transfer conveyance belt 31, and a container storing the collected foreign matter.

[0029] The fixing unit 40 is arranged downstream of the image forming unit 20 in the sheet conveyance direction along the transfer conveyance unit 30. The fixing unit 40 includes a heating roller 41 serving as a heating member, a pressing roller 42 serving as a pressing member, and a heating unit that heats the heating roller 41.

[0030] The heating unit is a halogen lamp or a coil unit for induction heating, for example. A nip portion (fixing nip) through which the sheet S is conveyed while being nipped is formed between the heating roller 41 and the pressing roller 42.

[0031] In the present exemplary embodiment, the conveyance path from the conveyance roller pair 14 to the outlet port of the fixing unit 40 via the registration roller pair 15, the four transfer portions, and the fixing nip will be referred to as an image formation path P1. The image formation path P1 is a conveyance path (first conveyance path) on which an image is formed on the sheet S.

[0032] The sheet conveyance unit 60 is a conveyance mechanism that conveys the sheet S that has passed through the fixing unit 40 toward the discharge tray 2E, or refeeds the sheet S toward the image forming unit 20. The sheet conveyance unit 60 is an example of a sheet conveyance apparatus.

[0033] The sheet conveyance unit 60 according to the present exemplary embodiment includes an intermediate discharge roller pair 64, the discharge roller pair 65, a reversing roller pair 61, an intermediate refeeding roller pair 62, a refeeding roller pair 63, and a switching flap 67. The reversing roller pair 61 is an example of a first conveyance unit that conveys a sheet in a first conveyance direction, and then conveys the sheet in a second conveyance direction opposite to the first conveyance direction. The intermediate refeeding roller pair 62 is an example of a second conveyance unit that conveys a sheet in the second conveyance direction. In addition, the reversing roller pair 61 is an example of a first roller pair having a first nip portion through which a sheet is nipped, and the intermediate refeeding roller pair 62 is an example of a second roller pair having a second nip portion through which a sheet is nipped.

[0034] As conveyance paths on which the sheet S is conveyed, the sheet conveyance unit 60 also includes three conveyance paths: a reversing path P2, a refeeding path P3, and a discharge path P4. Each of the conveyance paths P2 to P4 includes conveyance guides provided in the housing 2, and a conveyance space through which the sheet S passes is formed between the conveyance guides facing each other. The intermediate discharge roller pair 64 and the discharge roller pair 65 are arranged on the discharge path P4. The reversing roller pair 61 is arranged on the reversing path P2. The intermediate refeeding roller pair 62 and the refeeding roller pair 63 are arranged on the refeeding path P3.

[0035] The reversing path P2 is a conveyance path (second conveyance path) on which the sheet S is reversely conveyed to form an image on a second surface of the sheet S with an image formed on a first surface while the sheet S passes through the image formation path P1. The refeeding path P3 is a conveyance path (third conveyance path) on which the reversely-conveyed sheet S is conveyed again toward the image forming unit 20. The discharge path P4 is a conveyance path on which the sheet S with the formed image on it is conveyed toward the discharge tray 2E.

[0036] In the present exemplary embodiment, the reversing path P2 and the discharge path P4 branch from the image formation path P1 downstream of the fixing unit 40 in the sheet conveyance direction, but one conveyance path may function as both the reversing path P2 and the discharge path P4. In this case, the reversing roller pair 61 is arranged on the above-described one conveyance path, and arranged to reversely convey the sheet S and convey the sheet S from the housing 2.

[0037] In the present exemplary embodiment, the discharge path P4 covers the range from the space between the end of the switching flap 67 arranged at a discharge position and the conveyance guides corresponding thereto to an opening (discharge port) of the housing 2 at which the discharge roller pair 65 is arranged. The reversing path P2 covers the range from the space between the end of the switching flap 67 arranged at a reversing position and the conveyance guides facing the end of the switching flap 67 to an opening of the housing 2 (an opening over the discharge roller pair 65 in FIG. 1). The refeeding path P3 covers the range from the space between the end of the switching flap 67 arranged at the reversing position, and the conveyance guide facing the end of the switching flap 67 to the nip portion of the conveyance roller pair 14 via the intermediate refeeding roller pair 62 and the refeeding roller pair 63.

[0038] The switching flap 67 can switch the conveyance path of the sheet S that has passed through the fixing unit 40 between the reversing path P2 and the discharge path P4. Specifically, the switching flap 67 can pivot around a pivot 67a between the discharge position at which the switching flap 67 guides the sheet S to the discharge path P4 (solid line position in FIG. 1) and the reversing position at which the switching flap 67 guides the sheet S to the reversing path P2 (broken line position in FIG. 1).

[0039] The power unit 70 is arranged above the sheet feeding unit 10 and below the transfer conveyance unit 30 and the fixing unit 40. The power unit 70 supplies power to components of the image forming apparatus 1. For example, the first to sixth motors 81 to 86 rotate by receiving power supplied from the power unit 70.

[0040] The first motor 81 drives the reversing roller pair 61 and the intermediate refeeding roller pair 62. The first motor 81 is an example of the drive source that drives the first conveyance unit. The second motor 82 drives the discharge roller pair 65 and the intermediate discharge roller pair 64. The third motor 83 drives the development rollers 22c. The fourth motor 84 drives the photosensitive drums 22a. The fifth motor 85 drives the fixing unit 40. The sixth motor 86 drives the feeding roller 12, the separation roller 13, the conveyance roller pair 14, the registration roller pair 15, and the refeeding roller pair 63.

(Simplex Printing)

[0041] An image forming operation in which the image forming apparatus 1 forms an image only on one side of the sheet S (hereinafter, will be referred to as simplex printing) will be described. A control unit of the image forming apparatus 1 may include one or more processors and starts the image forming operation when an execution instruction of the image forming operation (print command) is received from an external host computer or the like together with image information.

[0042] In the image forming operation, a single-color toner image is formed on the surface of each of the photosensitive drums 22a in the image forming unit 20. First of all, the rotational driving of the photosensitive drums 22a is started, and the charging devices 22b uniformly charges the surfaces of the photosensitive drums 22a. The scanner 50 performs exposure by emitting laser beams to the photosensitive drums 22a in accordance with video signals generated based on the image information, and forms electrostatic latent images on the surfaces of the photosensitive drums 22a. The development rollers 22c each bear and supply the toner to the corresponding photosensitive drum of the photosensitive drums 22a, and develop the corresponding electrostatic latent image into a toner image.

[0043] Concurrently with the formation of the above-described toner images, the sheet S is fed from the sheet feeding unit 10 toward the image forming unit 20. Specifically, the feeding roller 12 starts to rotate in contact with the uppermost sheet S of sheets stored in the storage unit 11, and the uppermost sheet S is fed from the storage unit 11 and passes through the nip portion (separation nip) between the separation roller 13 and the separation pad 13a. At this time, if a plurality of sheets S enters the separation nip, the frictional force generated between the sheets S and the separation pad 13a prevents the sheets S other than the uppermost sheet S from passing through the separation nip. Thus, the sheets S are conveyed one by one to the image formation path P1 while being separated from each other.

[0044] The sheet S is conveyed to the registration roller pair 15 via the conveyance roller pair 14. After correcting the skew of the sheet S, the registration roller pair 15 delivers the sheet S to the transfer conveyance belt 31. The sheet S is conveyed by the transfer conveyance belt 31 while passing through the four transfer portions in order. The respective color toners are electrostatically attracted by the transfer roller 33 at each of the transfer portions, each single-color toner image is transferred to the sheet S from the corresponding photosensitive drum 22a.

[0045] The four single-color toner images are transferred in an overlaid manner, forming a full-color toner image on the sheet S. The matter including the toner remaining on the photosensitive drums 22a not transferred to the sheet S at the transfer portion is removed from the photosensitive drum 22a by the cleaning roller 23.

[0046] The sheet S with the transferred toner image on it is conveyed to the fixing unit 40. The fixing unit 40 heats and presses the toner image on the sheet S while nipping and conveying the sheet S through the fixing nip using the heating roller 41 and the pressing roller 42. This fixes the image to the sheet S. In this manner, a toner image is formed in the image forming unit 20, and while the sheet S fed from the sheet feeding unit 10 passes through the image formation path P1, the image is formed on one side of the sheet S.

[0047] The sheet S that has passed through the fixing unit 40 is guided to the discharge path P4 by the switching flap 67 positioned in advance at the discharge position (solid line position in FIG. 1). Then, the sheet S is conveyed by the intermediate discharge roller pair 64 on the discharge path P4, discharged by the discharge roller pair 65 outside the housing 2, and put on the discharge tray 2E.

(Duplex Printing)

[0048] An image forming operation in which the image forming apparatus 1 forms images on both sides of the sheet S (hereinafter, will be referred to as duplex printing) will be described with reference to FIGS. 2A to 2E. FIGS. 2A to 2E each illustrate a cross-section of the image forming apparatus 1 at a corresponding time point of duplex printing. In the duplex printing, a process in which while the sheet S fed from the sheet feeding unit 10 passes through the image formation path P1, an image is formed on one side (first surface) of the sheet S via a toner image formed in the image forming unit 20 is the same as that in the simplex printing, and the description will be omitted.

[0049] As illustrated in FIG. 2A, in the duplex printing, the sheet S that has passed through the fixing unit 40 for the first time (the sheet S with an image formed on its first surface) is guided to the reversing path P2 by the switching flap 67 positioned in advance at the reversing position. If the leading edge of the sheet S reaches the reversing roller pair 61, the reversing roller pair 61 conveys the sheet S in a first conveyance direction D1. In this process, the leading edge portion of the sheet S in the first conveyance direction D1 temporarily is stuck out of the housing 2.

[0050] As illustrated in FIG. 2B, after the trailing edge of the sheet S in the first conveyance direction D1 passes through the fixing unit 40 and before the trailing edge passes through the reversing roller pair 61, the reversing roller pair 61 switches the conveyance direction of the sheet S to a second conveyance direction D2 opposite to the first conveyance direction D1. The second conveyance direction D2 is a direction in which the sheet S is conveyed along the reversing path P2 and the refeeding path P3 in the direction from the reversing roller pair 61 to the intermediate refeeding roller pair 62. Specifically, the rotational direction of the first motor 81 that rotationally drives the reversing roller pair 61 is switched, and the rotational direction of the reversing roller pair 61 is reversed. The reversing roller pair 61 accordingly conveys the sheet S toward the refeeding path P3. In addition, the trailing edge of the sheet S in the first conveyance direction D1 is drawn again into the housing 2. An operation in which the reversing roller pair 61 conveys the sheet S in the first conveyance direction D1 and then conveys the sheet S in the second conveyance direction D2 will be referred to as reverse conveyance.

[0051] As illustrated in FIG. 2C, the sheet S reversely conveyed by the reversing roller pair 61 enters the refeeding path P3, and is conveyed on the refeeding path P3 while being nipped by both the reversing roller pair 61 and the intermediate refeeding roller pair 62.

[0052] As illustrated in FIG. 2D, after the trailing edge of the sheet S in the second conveyance direction D2 passes through the reversing roller pair 61, the sheet S is conveyed by the intermediate refeeding roller pair 62 and the refeeding roller pair 63 to the conveyance roller pair 14. The sheet S is accordingly conveyed again to the image formation path P1 with the first surface and the second surface replaced. A subsequent process in which, while the sheet S passes through the image formation path P1, an image is formed on the second surface of the sheet S is the same as the process in which an image is formed on the first surface, and the description will be omitted.

[0053] As illustrated in FIG. 2E, the sheet S that has passed through the fixing unit 40 for the second time (the sheet S with images formed on its first and second surfaces) is guided to the discharge path P4 by the switching flap 67 positioned in advance at the discharge position. Then, the sheet S is conveyed by the intermediate discharge roller pair 64 on the discharge path P4, discharged by the discharge roller pair 65 outside the housing 2, and put on the discharge tray 2E.

(Protruding Portion of Conveyance Guide)

[0054] As illustrated in an enlarged view at the right of FIG. 2C, a protruding portion 68aprovided on a conveyance guide 68 at the inlet portion of the refeeding path P3 will be described. The inlet portion of the refeeding path P3 includes the conveyance guide 68 and a conveyance guide 69. The inlet portion of the refeeding path P3 is upstream of the intermediate refeeding roller pair 62 in the sheet conveyance direction on the refeeding path P3. The conveyance guide 68 is an example of a first guide that is arranged between the reversing roller pair 61 and the intermediate refeeding roller pair 62 in the second conveyance direction D2 to guide the sheet S. The conveyance guide 69 is an example of a second guide facing the first guide across the space through which the sheet S passes.

[0055] The conveyance guide 68 is arranged facing a second surface S2 of the sheet S reversely conveyed by the reversing roller pair 61. The conveyance guide 69 is arranged facing a first surface S1 of the sheet S reversely conveyed by the reversing roller pair 61. That is to say, the conveyance guide 69 is arranged facing the first surface S1 (image surface) of the sheet S conveyed from the reversing roller pair 61 toward the intermediate refeeding roller pair 62 after an image is formed on the first surface S1 in the duplex printing. The conveyance guide 68 is also arranged facing the second surface S2 (non-image surface) on the opposite side of the first surface S1 of the sheet S conveyed from the reversing roller pair 61 toward the intermediate refeeding roller pair 62.

[0056] The conveyance guide 68 includes the protruding portion 68a protruding toward the conveyance guide 69 that the conveyance guide 68 faces, from an imaginary line Lv connecting the nip portion of the reversing roller pair 61 and the nip portion of the intermediate refeeding roller pair 62. In other words, the first guide includes a portion (the protruding portion 68a) protruding toward the second guide from the imaginary straight line connecting the first nip portion and the second nip portion when viewed in the rotational axis direction of the first roller pair.

[0057] The protruding portion 68a has a function of adjusting the entrance orientation of the sheet S in entering the nip portion of the intermediate refeeding roller pair 62. That is to say, without the protruding portion 68a, the leading edge of the sheet S in the second conveyance direction D2 could enter the nip portion of the intermediate refeeding roller pair 62 from a slant angle with respect to a nip line Ln of the intermediate refeeding roller pair 62. The nip line Ln is an imaginary straight line that passes through the nip portion of the intermediate refeeding roller pair 62, and orthogonally intersects with the straight line connecting rotational axial lines of two rollers 62a and 62b of the intermediate refeeding roller pair 62. If the leading edge of the sheet S enters the nip portion from the slant angle with respect to the nip line Ln, the leading edge of the sheet S could come into contact with the rollers 62a and 62b at a position distant from the nip portion, which leads to a conveyance failure, such as jam or skew. In contrast to this, the protruding portion 68a provided in the present exemplary embodiment guides the leading edge of the sheet S to pass through a position closer to the nip line Ln. This ensures that the leading edge of the sheet S reliably enters the nip portion, reducing the possibility of the conveyance failure.

(Conveyance Speeds of Reversing Roller Pair and Intermediate Refeeding Roller Pair)

[0058] On the other hand, in the present exemplary embodiment, a conveyance speed Vs1 when the reversing roller pair 61 conveys the sheet S in the second conveyance direction D2 is set to a conveyance speed smaller than a conveyance speed Vs2 when the intermediate refeeding roller pair 62 conveys the sheet S (FIG. 2C). The conveyance speeds Vs1 and Vs2 each indicate a circumferential speed of the corresponding roller pair that is set when the corresponding roller pair from between the reversing roller pair 61 and the intermediate refeeding roller pair 62 is rotationally driven by the drive force of the drive source (the first motor 81).

[0059] The conveyance speed Vs1 of the reversing roller pair 61 set to a conveyance speed smaller than the conveyance speed Vs2 of the intermediate refeeding roller pair 62 reduces the possibility that the first surface (image surface) of the sheet S slides on the conveyance guide 69 between the reversing roller pair 61 and the intermediate refeeding roller pair 62. If the conveyance speed Vs1 is larger than the conveyance speed Vs2, while both the reversing roller pair 61 and the intermediate refeeding roller pair 62 nip and convey the sheet S, the sheet S is warped between the reversing roller pair 61 and the intermediate refeeding roller pair 62. If an image on the first surface (image surface) of the sheet S slides on the conveyance guide 69 due to the warping of the sheet S, a slid trace can remain in the image. The conveyance speeds set in such a manner that the conveyance speed Vs1 is smaller than the conveyance speed Vs2 as in the present exemplary embodiment can reduce the possibility that a slid trace remains in the image.

[0060] On the other hand, when the conveyance speeds are set in such a manner that the conveyance speed Vs1 is smaller than the conveyance speed Vs2, the sheet S can be pulled by both the reversing roller pair 61 and the intermediate refeeding roller pair 62 due to a speed difference. In this case, the friction between the sheet S and the roller pairs makes faster the wear speed of the reversing roller pair 61 or the intermediate refeeding roller pair 62.

[0061] With the protruding portion 68a of the conveyance guide 68 provided between the reversing roller pair 61 and the intermediate refeeding roller pair 62 as described above, the conveyance speeds set in such a manner that the conveyance speed Vs1 is smaller than the conveyance speed Vs2 can cause the sheet S to strongly slide the protruding portion 68a. That is to say, the sheet S being pulled by both the reversing roller pair 61 and the intermediate refeeding roller pair 62 gets closer to the imaginary line Lv connecting the nip portion of the reversing roller pair 61 and the nip portion of the intermediate refeeding roller pair 62, causing the second surface of the sheet S to strongly slide on the protruding portion 68a. This can cause scraping of the conveyance guide 68. In addition, for example, the sheet S strongly sliding on the conveyance guide 68 can cause a slid trace to remain on the second surface of the sheet S.

[0062] In view of the foregoing, in the present exemplary embodiment, as described below, a configuration is used in which the reversing roller pair 61 rotates at a circumferential speed higher than the conveyance speed Vs1 following the sheet S being conveyed by the intermediate refeeding roller pair 62 at the conveyance speed Vs2.

(Drive Transmission Mechanism)

[0063] A drive transmission mechanism DT for driving the reversing roller pair 61 and the intermediate refeeding roller pair 62 will be described with reference to FIGS. 3A to 3D. FIG. 3A is a schematic diagram illustrating the drive transmission mechanism DT viewed from one side in the rotational axis direction of the reversing roller pair 61. FIG. 3B is a schematic diagram illustrating the drive transmission mechanism DT viewed from the other side in the rotational axis direction of the reversing roller pair 61. FIGS. 3C and 3D are diagrams each illustrating a part of the drive transmission mechanism DT in FIG. 3B in an enlarged manner.

[0064] As illustrated in FIGS. 3A and 3B, the reversing roller pair 61 according to the present exemplary embodiment includes a drive roller 61a connected to the first motor 81 via the drive transmission mechanism DT, and a driven roller 61b (non-drive roller) not connected to the first motor 81 via the drive transmission mechanism DT. The intermediate refeeding roller pair 62 includes a drive roller 62a connected to the first motor 81 via the drive transmission mechanism DT, and a driven roller 62b (non-drive roller) not connected to the first motor 81 via the drive transmission mechanism DT. The drive rollers 61a and 62a are rotationally driven by the drive force transmitted from the first motor 81 via the drive transmission mechanism DT. The driven rollers 61b and 62b rotate driven by the respective drive rollers 61a and 62a with the frictional force received from the respective drive rollers 61a and 62a at the nip portions.

[0065] The first motor 81 can rotate in a counterclockwise direction in FIG. 3A (hereinafter, will be referred to as a normal rotation direction Rm1), and in a rotational direction opposite to the normal rotation direction Rm1 (hereinafter, will be referred to as a reverse rotation direction Rm2).

[0066] As illustrated in FIGS. 3A and 3B, 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 drive force from the first motor 81 to a first intermediate axis Ax1. The second belt drive unit 110 transmits drive force from the first intermediate axis Ax1 to a second intermediate axis Ax2. The first gear train 120 transmits drive force from the first intermediate axis Ax1 to the drive roller 62a of the intermediate refeeding roller pair 62. The second gear train 130 transmits drive force from the second intermediate axis Ax2 to the drive roller 61a of the reversing roller pair 61.

[0067] The first belt drive unit 100 includes an output pulley 81a, a first pulley 102, a first drive belt 101, and a first tensioner 103. The output pulley 81a is attached to the output shaft of the first motor 81. The first pulley 102 rotates around the first intermediate axis Ax1.

[0068] The first drive belt 101 is stretched over the output pulley 81a and the first pulley 102. The first tensioner 103 includes a spindle 103b, a support arm 103a that rotates around the spindle 103b, and a press roller 103c rotationally supported by the support arm 103a. The first tensioner 103 also includes an urging member 103d having one end connected to the support arm 103a, and a fixed portion 103e to which the other end of the urging member 103d is connected. By the press roller 103c pressing the inner surface of the first drive belt 101 with the urging force of the urging member 103d, the first tensioner 103 gives tensile force to the first drive belt 101. The drive force being transmitted to the first pulley 102 via the first drive belt 101 rotates the first pulley 102 in a rotational direction corresponding to the rotational direction of the first motor 81.

[0069] The first gear train 120 includes a distribution gear 121, a swing 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 axis Ax1. The distribution gear 121 and the swing gear 122 engage with each other. The swing gear 122 is rotationally supported by a swing arm (not illustrated). The swing arm is in frictional contact with the distribution gear 121, for example, and swings in the direction extending in the rotational direction of the distribution gear 121.

[0070] The swinging of the swing arm can move the swing gear 122 to a position at which the swing gear 122 engages with the first gear 123a (FIG. 3C), and a position at which the swing gear 122 engages with the fourth gear 123d (FIG. 3D). The first gear 123a and the second gear 123b engage with each other, the second gear 123b and the third gear 123c engage with each other, and the third gear 123c and the fourth gear 123d engage with each other. The fourth gear 123d is arranged coaxially with the drive roller 62a of the intermediate refeed/ing roller pair 62, and rotates integrally with the drive roller 62a.

[0071] As illustrated in FIG. 3C, when the first motor 81 rotates in the normal rotation direction Rm1, the drive force transmitted via the first belt drive unit 100 rotates the distribution gear 121 in a clockwise direction in FIG. 3C, causing the swing gear 122 to engage 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. The drive roller 62a of the intermediate refeeding roller pair 62 accordingly rotates in a counterclockwise direction in FIG. 3C integrally with the fourth gear 123d.

[0072] As illustrated in FIG. 3D, when the first motor 81 rotates in the reverse rotation direction Rm2, the drive force transmitted via the first belt drive unit 100 rotates the distribution gear 121 in a counterclockwise direction in FIG. 3D, causing the swing gear 122 to engage with the fourth gear 123d. The drive roller 62a of the intermediate refeeding roller pair 62 accordingly rotates in a counterclockwise direction in FIG. 3D integrally with the fourth gear 123d.

[0073] In this manner, either when the first motor 81 rotates in the normal rotation direction Rm1 and when the first motor 81 rotates in the reverse rotation direction Rm2, the transmission of drive force via the first gear train 120 rotates the drive roller 62a of the intermediate refeeding roller pair 62 in a predetermined rotational direction. The predetermined rotational direction is a direction in which the intermediate refeeding roller pair 62 conveys the sheet S in the second conveyance direction D2. In the present exemplary embodiment, the configuration in which gear trains from the distribution gear 121 to the fourth gear 123d are switched between an odd number gear and an even number gear in accordance with the rotational direction of the first motor 81 rotates the drive roller 62a in the predetermined rotational direction.

[0074] As illustrated in FIGS. 3A and 3B, 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 arranged 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 axis Ax1. The third pulley 113 rotates around the second intermediate axis Ax2. The second drive belt 111 is stretched over the second pulley 112 and the third pulley 113. The idler pulley 114 is contact with the inner surface of the second drive belt 111 between the second pulley 112 and the third pulley 113. A second tensioner 115 includes a spindle 115b, a support arm 115a that rotates around the spindle 115b, and a press roller 115c rotationally supported by the support arm 115a. The second tensioner 115 also includes an urging member 115d having one end connected to the support arm 115a, and a fixed portion 115e to which the other end of the urging member 115d is connected. By the press roller 115c pressing the outer surface of the second drive belt 111 with the urging force of the urging member 115d, the second tensioner 115 gives tensile force to the second drive belt 111.

[0075] The second gear train 130 includes an inverting input gear 131 and an inverting output gear 132. The inverting input gear 131 is arranged coaxially with the third pulley 113, and rotates integrally with the third pulley 113 around the second intermediate axis Ax2. The inverting output gear 132 is arranged coaxially with the drive roller 61a of the reversing roller pair 61. The inverting output gear 132 engages with the inverting input gear 131.

[0076] When the first motor 81 rotates in the normal rotation direction Rm1, drive force is transmitted via the second drive belt 111 from the second pulley 112 to the third pulley 113, and the drive force is further transmitted to the drive roller 61a via the engagement between the inverting input gear 131 and the inverting output gear 132. The drive roller 61a of the reversing roller pair 61 accordingly rotates in a first rotational direction R1 when the reversing roller pair 61 conveys the sheet S in the first conveyance direction D1.

[0077] Also when the first motor 81 rotates in the reverse rotation direction Rm2, drive force is transmitted to the drive roller 61a via the same route. More specifically, drive force is transmitted via the second drive belt 111 from the second pulley 112 to the third pulley 113, and the drive force is further transmitted to the drive roller 61a via the engagement between the inverting input gear 131 and the inverting output gear 132. The drive roller 61a of the reversing roller pair 61 accordingly rotates in a second rotational direction R2 when the reversing roller pair 61 conveys the sheet S in the second conveyance direction D2.

(Coupling Unit)

[0078] A configuration of drive transmission to the reversing roller pair 61 will be described with reference to FIGS. 4A to 4E.

[0079] FIG. 4A is a perspective view illustrating a part of the drive transmission mechanism DT and the drive roller 61a of the reversing roller pair 61. FIGS. 4B and 4C are exploded views of a coupling unit CP. FIG. 4D is a diagram (top view) illustrating a part of the drive transmission mechanism DT and the drive roller 61a of the reversing roller pair 61 that are viewed from a direction orthogonal to a rotational axis direction X to be described below. FIG. 4E is a cross-sectional view of the coupling unit CP cut along an A-A line in FIG. 4D.

[0080] As illustrated in FIGS. 4A and 4D, the drive roller 61a of the reversing roller pair 61 includes a roller shaft 612 and a plurality of roller main bodies 611. A direction extending along a rotational axis line Ax of the drive roller 61a is regarded as the rotational axis direction X. The roller shaft 612 is an axial member extending in the rotational axis direction X. The plurality of roller main bodies 611 each forms a nip portion at which the reversing roller pair 61 nips the sheet S together with roller main bodies of the driven roller 61b (FIG. 1). The plurality of roller main bodies is attached to the roller shaft 612, and rotates integrally with the roller shaft 612 around the rotational axis line Ax.

[0081] The drive transmission mechanism DT includes the coupling unit CP serving as an example of a drive transmission unit for transmitting drive force to the drive roller 61a.

[0082] The coupling unit CP can enter a transmission state (connected state, coupled state) in which the coupling unit CP transmits drive force from the first motor 81 to the reversing roller pair 61, and a non-transmission state (drive transmission blocked state, disconnected state) in which the coupling unit CP does not transmit drive force from the first motor 81 to the reversing roller pair 61. The transmission state that the coupling unit CP can enter includes a first transmission state of transmitting drive force to cause the reversing roller pair 61 to convey the sheet S in the first conveyance direction DI, and a second transmission state of transmitting drive force to cause the reversing roller pair 61 to convey the sheet S in the second conveyance direction D2.

[0083] The coupling unit CP according to the present exemplary embodiment includes the inverting output gear 132 and a reverse coupling 613. The inverting output gear 132 is an example of a first member (drive member, input member) connected to the drive source (the first motor 81). The reverse coupling 613 is an example of a second member (driven member, output member) connected to a drive target (the reversing roller pair 61 in this example).

[0084] The reverse coupling 613 is provided at an end portion of the drive roller 61a in the rotational axis direction X. The reverse coupling 613 rotates integrally with the drive roller 61a around the rotational axis line Ax.

[0085] The inverting output gear 132 (refer also to FIGS. 3A and 3B) is coupled to the reverse coupling 613. The drive force of the first motor 81 is input to the reversing roller pair 61 via the coupling between the inverting output gear 132 and the reverse coupling 613.

[0086] The inverting output gear 132 is an example of a first member (drive member, input member) connected to the drive source (the first motor 81). The reverse coupling 613 is an example of a second member (driven member, output member) connected to a drive target (the reversing roller pair 61 in this example).

[0087] The inverting output gear 132 and the reverse coupling 613 are coaxially arranged. That is to say, the inverting output gear 132 and the reverse coupling 613 can rotate around the rotational axis line Ax of the drive roller 61a in the first rotational direction R1 and the second rotational direction R2. The first rotational direction R1 is a rotational direction of the inverting output gear 132 and the reverse coupling 613 when the reversing roller pair 61 conveys the sheet S in the first conveyance direction D1. The second rotational direction R2 is a rotational direction of the inverting output gear 132 and the reverse coupling 613 when the reversing roller pair 61 conveys the sheet S in the second conveyance direction D2. In the present exemplary embodiment, the radius direction of an imaginary circle centered on the rotational axis line Ax when viewed in the rotational axis direction X will be referred to as the radial direction of the coupling unit CP or will be simply referred to as a radial direction.

[0088] As illustrated in FIGS. 4B and 4C, the inverting output gear 132 includes an engagement shaft 132a serving as a to-engage portion. The reverse coupling 613 has an engagement hole 613a serving as a to-be-engaged portion that is to be engaged with the engagement shaft 132a.

[0089] More specifically, the inverting output gear 132 has a hole portion 132h into which the roller shaft 612 is inserted, and a side surface 132f spreading from the hole portion 132h outward in the radial direction. The reverse coupling 613 has a side surface 613f spreading from the roller shaft 612 outward in the radial direction. The side surface 132f of the inverting output gear 132 and the side surface 613f of the reverse coupling 613 face each other in the rotational axis direction X.

[0090] The engagement shaft 132a of the inverting output gear 132 is a protruding portion protruding from the side surface 132f of the inverting output gear 132 toward the reverse coupling 613 in the rotational axis direction X. The engagement shaft 132a according to the present exemplary embodiment is formed in a substantially columnar shape extending in the rotational axis direction X, but the engagement shaft 132a can be formed in a shape other than the columnar shape (for example, quadrangular prism shape).

[0091] The engagement shaft 132a includes a first end portion 132b being an end portion of the engagement shaft 132a in the first rotational direction R1, and a second end portion 132c being an end portion of the engagement shaft 132a in the second rotational direction R2. The first end portion 132b is an example of a first to-contact portion, and the second end portion 132c is an example of a second to-contact portion.

[0092] The engagement hole 613a of the reverse coupling 613 is a recess portion formed in the side surface 613f of the reverse coupling 613. This recess portion can have a bottomed shape, or be a through hole penetrating through the reverse coupling 613 in the rotational axis direction X. The engagement hole 613a according to the present exemplary embodiment is formed in a substantially sector form extending along an imaginary circular arc centered on the rotational axis line Ax (shape obtained by cutting a part of a circle at a predetermined central angle).

[0093] As illustrated in FIG. 4E, the engagement hole 613a includes a first end portion 613b being an end portion of the engagement hole 613a in the first rotational direction R1, and a second end portion 613c being an end portion of the engagement hole 613a in the second rotational direction R2. The first end portion 613b is an example of a first to-be-contacted portion to be contacted by the first end portion 132b of the engagement shaft 132a of the inverting output gear 132. The second end portion 613c is an example of a second to-be-contacted portion to be contacted by the second end portion 132c of the engagement shaft 132a of the inverting output gear 132.

[0094] The width of the engagement hole 613a is set to a width wider than the width of the engagement shaft 132a. That is to say, the distance from the first end portion 613b to the second end portion 613c that is measured along the imaginary circular arc centered on the rotational axis line Ax and passing through the center (face center) of the engagement shaft 132a when viewed in the rotational axis direction X is longer than the distance from the first end portion 132b to the second end portion 132c of the engagement shaft 132a that is measured along the imaginary circular arc. In other words, the central angle between the first end portion 613b and the second end portion 613c with respect to the rotational axis line Ax is wider than the central angle between the first end portion 132b and the second end portion 132c of the engagement shaft 132a with respect to the rotational axis line Ax. Furthermore, in other words, the central angle between the first to-contact portion and the second to-contact portion with respect to the rotational axis line of the first member and the second member is smaller than the central angle between the first to-be-contacted portion and the second to-be-contacted portion with respect to the rotational axis line.

[0095] The width of the engagement hole 613a set to a width wider than the width of the engagement shaft 132a allows the inverting output gear 132 and the reverse coupling 613 to be coupled to be relatively rotatable (able to execute idling) within a predetermined range. With respect to the reverse coupling 613, the inverting output gear 132 can rotate between the position at which the first end portion 132b of the engagement shaft 132a in contact with the first end portion 613b of the engagement hole 613a (refer to a solid line in FIG. 4E), and the position at which the second end portion 132c of the engagement shaft 132a is in contact with the second end portion 613c of the engagement hole 613a (refer to a broken line in FIG. 4E).

[0096] The idling of the inverting output gear 132 and the reverse coupling 613 means the inverting output gear 132 and the reverse coupling 613 relatively rotating with the first end portion 132b of the engagement shaft 132a out of contact with the first end portion 613b of the engagement hole 613a and with the second end portion 132c of the engagement shaft 132a out of contact with the second end portion 613c of the engagement hole 613a.

[0097] The first transmission state in the present exemplary embodiment is a state in which the first end portion 132b of the engagement shaft 132a of the inverting output gear 132 is in contact with the first end portion 613b of the engagement hole 613a of the reverse coupling 613. With the coupling unit CP being in the first transmission state, the first end portion 132b of the engagement shaft 132a pressing the first end portion 613b of the engagement hole 613a allows the torque in the first rotational direction R1 to be transmitted from the inverting output gear 132 to the reverse coupling 613.

[0098] The second transmission state in the present exemplary embodiment is a state in which the second end portion 132c of the engagement shaft 132a of the inverting output gear 132 is in contact with the second end portion 613c of the engagement hole 613a of the reverse coupling 613. With the coupling unit CP being in the second transmission state, the second end portion 132c of the engagement shaft 132a pressing the second end portion 613c of the engagement hole 613a allows the torque in the second rotational direction R2 to be transmitted from the inverting output gear 132 to the reverse coupling 613.

[0099] The non-transmission state in the present exemplary embodiment is a state in which the first end portion 132b of the engagement shaft 132a is out of contact with the first end portion 613b of the engagement hole 613a and the second end portion 132c of the engagement shaft 132a is out of contact with the second end portion 613c of the engagement hole 613a. With the coupling unit CP being in the non-transmission state, the torque in the first rotational direction R1 and the torque in the second rotational direction R2 are not transmitted between the inverting output gear 132 and the reverse coupling 613.

[0100] With respect to the state in which the first end portion 132b of the engagement shaft 132a is in contact with the first end portion 613b of the engagement hole 613a (=0), the angle by which the inverting output gear 132 rotates in the second rotational direction R2 relative to the reverse coupling 613 is indicated by []. The maximum value of the angle in the present exemplary embodiment is regarded as an idling allowable angle .sub.L []. The first transmission state of the coupling unit CP corresponds to =0, the second transmission state corresponds to =.sub.L, and the non-transmission state corresponds to the range of 0<<.sub.L. In the present exemplary embodiment, the range in which the relative rotation (idling) of the inverting output gear 132 and the reverse coupling 613 is allowed (hereinafter, will be referred to as an idling allowable range) is the range of 0<<.sub.L.

[0101] Normally, when the first motor 81 continuously rotates in the normal rotation direction Rm1, the coupling unit CP enters the first transmission state, and the inverting output gear 132 and the reverse coupling 613 integrally rotate in the first rotational direction R1. When the first motor 81 continuously rotates in the reverse rotation direction Rm2, the coupling unit CP enters the second transmission state, and the inverting output gear 132 and the reverse coupling 613 integrally rotates in the second rotational direction R2. When the rotational direction of the first motor 81 is switched, a relative rotation of the inverting output gear 132 and the reverse coupling 613 occurs, and the coupling unit CP transitions from either the first transmission state or the second transmission state to the other state after the non-transmission state. Even when the rotational direction of the first motor 81 is not switched, as be described below, the coupling unit CP may enter the non-transmission state by the reversing roller pair 61 being rotated by the force received from the sheet.

(Operation of Coupling Unit)

[0102] An operation of the coupling unit CP in the duplex printing will be described with reference to FIGS. 5A to 5D and 6A to 6D. FIGS. 5A to 5D and 6A to 6D are each a cross-sectional view illustrating the coupling unit CP at the same position as the A-A line in FIG. 4D at a corresponding time point of duplex printing at its left and are each a schematic diagram illustrating the cross-section of the image forming apparatus 1 at the corresponding time point of duplex printing at its right. In each drawing of FIGS. 5A to 5D and 6A to 6D, the left and the right illustrate a time point in duplex printing.

[0103] In the present exemplary embodiment, as an example, duplex printing is performed under the following conditions. [0104] The length of the sheet S in the sheet conveyance direction (hereinafter, will be referred to as a sheet length Ls): 297 mm. [0105] A path length Lp from the reversing roller pair 61 to the intermediate refeeding roller pair 62: 144 mm. [0106] The conveyance speed Vs1 of the sheet S conveyed by the reversing roller pair 61: 264.1 mm/s. [0107] The conveyance speed Vs2 of the sheet S conveyed by the intermediate refeeding roller pair 62: 266.6 mm/s (millimeters per second). [0108] An external diameter D of the drive roller 61a of the reversing roller pair 61: 11 mm. [0109] The idling allowable angle .sub.L of the inverting output gear 132 and the reverse coupling 613: 30.

[0110] The above-described conveyance speed Vs1 indicates the circumferential speed of the drive roller 61a when the reversing roller pair 61 is rotationally driven by the drive force transmitted from the first motor 81, and may differ from the actual conveyance speed of the sheet S. In the above-described example, both the conveyance speed when the reversing roller pair 61 conveys the sheet S in the first conveyance direction D1 and the conveyance speed when the reversing roller pair 61 conveys the sheet S in the second conveyance direction D2 correspond to the conveyance speed Vs1. The roller diameter of the reversing roller pair 61 refers to the external diameter of the roller main body 611 (FIG. 4A) of the drive roller 61a.

[0111] The right of FIG. 5A illustrates a state in which the sheet S with an image formed on its first surface is conveyed by the reversing roller pair 61 in the first conveyance direction D1. The sheet S with the image formed on its first surface is discharged from the fixing unit 40, guided by the switching flap 67 to the reversing path P2, and conveyed in the first conveyance direction DI while being nipped by the reversing roller pair 61. At this time, as illustrated at the left of FIG. 5A, the inverting output gear 132 is rotationally driven in the first rotational direction R1 at a rotational speed V1 (first angular speed) corresponding to the conveyance speed Vs1. In addition, the first end portion 132b of the engagement shaft 132a of the inverting output gear 132 is in contact with the first end portion 613b of the engagement hole 613a of the reverse coupling 613. That is to say, the coupling unit CP is in the first transmission state. The first end portion 613b being pressed by the first end portion 132b of the engagement shaft 132a rotates the reverse coupling 613 in the first rotational direction R1 at the same rotational speed V1 as the inverting output gear 132. The drive roller 61a of the reversing roller pair 61 accordingly rotates in the first rotational direction R1 at the rotational speed V1, and the reversing roller pair 61 conveys the sheet S in the first conveyance direction DI at the conveyance speed Vs1.

[0112] In other words, when the first conveyance unit conveys the sheet in the second conveyance direction and the second conveyance unit does not convey the sheet, the drive transmission unit enters the second transmission state in such a manner that the first conveyance unit conveys the sheet at a first conveyance speed by the drive force.

[0113] When the reversing roller pair 61 conveys the sheet S in the first conveyance direction DI at the conveyance speed Vs1, the drive roller 62a of the intermediate refeeding roller pair 62 is rotationally driven at a rotational speed V2 at which the circumferential speed is the conveyance speed Vs2.

[0114] As illustrated at the right of FIG. 5B, after the trailing edge of the sheet S in the first conveyance direction D1 passes through the fixing unit 40, the driving of the reversing roller pair 61 and the intermediate refeeding roller pair 62 is paused. That is to say, the rotation of the first motor 81 in the normal rotation direction Rm1 (FIG. 3A) is paused.

[0115] As illustrated at the left of FIG. 5B, at this time point, the coupling unit CP is in the first transmission state.

[0116] Subsequently, when the first motor 81 starts to rotate in the reverse rotation direction Rm2 (FIG. 3A), as illustrated at the left of FIG. 5C, the engagement shaft 132a of the inverting output gear 132 gets away from the first end portion 613b of the engagement hole 613a of the reverse coupling 613, and moves inside the engagement hole 613a. That is to say, the state of the coupling unit CP changes from the first transmission state to the non-transmission state. The coupling unit CP entering the non-transmission state blocks drive transmission from the first motor 81 to the reversing roller pair 61, maintaining a stopped state of the reversing roller pair 61. The first motor 81 starting to rotate in the reverse rotation direction Rm2 drives to rotate the drive roller 62a of the intermediate refeeding roller pair 62 at the rotational speed V2.

[0117] When the first motor 81 further rotates in the reverse rotation direction Rm2 (FIG. 3A), as illustrated at the left of FIG. 5D, the second end portion 132c of the engagement shaft 132a of the inverting output gear 132 comes into contact with the second end portion 613c of the engagement hole 613a of the reverse coupling 613. That is to say, the state of the coupling unit CP changes from the non-transmission state to the second transmission state. The second end portion 613c being pressed by the second end portion 132c of the engagement shaft 132a rotates the reverse coupling 613 in the second rotational direction R2 at the same rotational speed V1 as the inverting output gear 132. The drive roller 61a of the reversing roller pair 61 accordingly rotates in the second rotational direction R2 at the rotational speed V1, and the reversing roller pair 61 conveys the sheet S in the second conveyance direction D2 at the conveyance speed Vs1. That is to say, the conveyance direction of the sheet S is reversed, pulling the sheet leading edge portion in the first conveyance direction DI that is stuck out of the housing 2 back into the housing 2.

[0118] The right of FIG. 6A illustrates a state in which the leading edge of the sheet S in the second conveyance direction D2 that is conveyed in the conveyance direction reversed by the reversing roller pair 61 has reached the nip portion of the intermediate refeeding roller pair 62. When the leading edge of the sheet S reaches the nip portion of the intermediate refeeding roller pair 62, the sheet S is conveyed in the second conveyance direction D2 while being nipped by both the reversing roller pair 61 and the intermediate refeeding roller pair 62. A state in which the sheet S conveyed in the second conveyance direction D2 is in contact with both the reversing roller pair 61 and the intermediate refeeding roller pair 62 can be referred to as a contact state. The leading edge of the sheet S reaches the nip portion of the intermediate refeeding roller pair 62, causing the contact state. In the contact state, the sheet S is nipped by the reversing roller pair 61 at the nip portion of the reversing roller pair 61, and nipped by the intermediate refeeding roller pair 62 at the nip portion of the intermediate refeeding 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.

[0119] In the present exemplary embodiment, conveyance speeds are set in such a manner that the conveyance speed Vs2 of the intermediate refeeding roller pair 62 is higher than the conveyance speed Vs1 of the reversing roller pair 61 (Vs2>Vs1). As described above, this configuration reduces the generation of a slid trace on an image that is attributed to the warping of the sheet S.

[0120] The intermediate refeeding roller pair 62 conveys the sheet S while accelerating the conveyance speed to the conveyance speed Vs2 as if to draw the sheet S out of the reversing roller pair 61. On the other hand, the drive roller 61a of the reversing roller pair 61 receives the torque in the second rotational direction R2 with the frictional force received from the sheet S conveyed at the conveyance speed Vs2 to rotate in the second rotational direction R2 at a rotational speed V3 (second angular speed) higher than the rotational speed V1 (first angular speed). At this time, as illustrated at the left of FIG. 6A, a clearance gap that allows the reverse coupling 613 to relatively rotate in the second rotational direction R2 relative to the inverting output gear 132 exists between the first end portion 132b of the engagement shaft 132a and the first end portion 613b of the engagement hole 613a.

[0121] When the leading edge of the sheet S conveyed by the reversing roller pair 61 in the second conveyance direction D2 reaches the intermediate refeeding roller pair 62, causing the contact state in which the sheet S is in contact with both the reversing roller pair 61 and the intermediate refeeding roller pair 62. The contact state is released (ended) by the trailing edge of the sheet S conveyed in the second conveyance direction D2 getting away from the reversing roller pair 61. As illustrated at the right of FIG. 6B, the sheet S being in the nip portion of the reversing roller pair 61 is conveyed by the intermediate refeeding roller pair 62 at the conveyance speed Vs2. Until the trailing edge of the sheet S in the second conveyance direction D2 passes through the nip portion of the reversing roller pair 61, the drive roller 61a of the reversing roller pair 61 preliminarily rotates at the rotational speed V3 higher than the rotational speed V1 following the sheet S conveyed at the conveyance speed Vs2. That is to say, until the contact state is released after the contact state is caused, the drive roller 61a of the reversing roller pair 61 preliminarily rotates at the rotational speed V3 higher than the rotational speed V1 following the sheet S conveyed at the conveyance speed Vs2. The preliminary rotation of the drive roller 61a refers to the drive roller 61a rotating at an angular speed (the rotational speed V3) higher than the angular speed (the rotational speed V1) when the drive roller 61a rotates with the drive force transmitted from the first motor 81 via the drive transmission mechanism DT.

[0122] When the contact state is caused, the second end portion 613c of the engagement hole 613a of the reverse coupling 613 gets away from the second end portion 132c of the engagement shaft 132a of the inverting output gear 132 in the second rotational direction R2. As illustrated at the left of FIG. 6B, the reverse coupling 613 rotates together with the drive roller 61a in the second rotational direction R2 at the rotational speed V3 higher than the rotational speed V1 of the inverting output gear 132. Thus, the relative rotation (idling) of the inverting output gear 132 and the reverse coupling 613 occurs in such a manner that the engagement shaft 132a of the inverting output gear 132 gets away from the second end portion 613c of the engagement hole 613a of the reverse coupling 613. That is to say, the state of the coupling unit CP changes from the second transmission state to the non-transmission state.

[0123] In other words, after the contact state in which the sheet conveyed in the second conveyance direction is in contact with both the first conveyance unit and the second conveyance unit is caused, the drive transmission unit enters the non-transmission state to allow the first conveyance unit to operate following the sheet conveyed at the second conveyance speed. In other words, the first conveyance unit is allowed to be rotated by the sheet conveyed at the second conveyance speed. More specifically, after the leading edge of the sheet in the second conveyance direction reaches the second conveyance unit, the drive transmission unit transitions from the second transmission state to the non-transmission state to allow the first conveyance unit to operate following the sheet conveyed at the second conveyance speed. This allows the drive roller 61a of the reversing roller pair 61 to preliminarily rotate at the rotational speed V3 following the sheet S conveyed at the conveyance speed Vs2. It is desirable that this non-transmission state is continued until the contact state is released.

[0124] As illustrated at the right of FIG. 6C, even after the trailing edge of the sheet S in the second conveyance direction D2 passes through the nip portion of the reversing roller pair 61, the sheet S is conveyed by the intermediate refeeding roller pair 62 at the conveyance speed Vs2. On the other hand, by the trailing edge of the sheet S passing through the nip portion of the reversing roller pair 61, the reversing roller pair 61 is released from the sheet S, reducing the rotation of the drive roller 61a.

[0125] As illustrated at the left of FIG. 6C, even after the sheet S passes through the reversing roller pair 61, the inverting output gear 132 continues to rotate at the rotational speed V1. Thus, the second end portion 132c of the engagement shaft 132a comes into contact with the second end portion 613c of the engagement hole 613a again. That is to say, the coupling unit CP returns from the non-transmission state to the second transmission state. Then, the drive roller 61a of the reversing roller pair 61 rotates in the second rotational direction R2 at the rotational speed V1.

[0126] After that, if the subsequent sheet is to be reversely conveyed by the reversing roller pair 61, as illustrated at the right of FIG. 6D, switching the rotational direction of the first motor 81 reverses the rotational direction of the reversing roller pair 61. As illustrated at the left of FIG. 6D, when the rotational direction of the first motor 81 is switched, the second end portion 132c of the engagement shaft 132a gets away from the second end portion 613c of the engagement hole 613a, and the first end portion 132b of the engagement shaft 132a comes into contact with the first end portion 613b. That is to say, the coupling unit CP transitions from the second transmission state to the first transmission state via the non-transmission state. Thus, the reversing roller pair 61 rotates in the rotational direction for conveying the sheet in the first conveyance direction D1.

[0127] As illustrated at the right of FIG. 6D, the rotational direction of the first motor 81 can be switched before the trailing edge of the sheet S passes through the nip portion of the intermediate refeeding roller pair 62. This is because, as described above, the mechanism that uses the swing gear 122 (FIGS. 3C and 3D) rotationally drives the intermediate refeeding roller pair 62 in the rotational direction for conveying the sheet S in the second conveyance direction D2, irrespective of the rotational direction of the first motor 81. This can shorten the sheet conveyance interval of the reversing roller pair 61.

(Relationship Between Preliminary Rotation Angle and Idling Allowable Angle)

[0128] In the present exemplary embodiment, the description will be given of a configuration in which, at least until the trailing edge of the sheet S passes through the intermediate refeeding roller pair 62, the coupling unit CP is kept in the non-transmission state, and the preliminary rotation of the reversing roller pair 61 is allowed.

[0129] In the present exemplary embodiment, the above-described idling allowable angle .sub.L is made larger than a preliminary rotation angle .sub.S of the reverse coupling 613 to be calculated as described below. In other words, the amount (.sub.L) of relative rotation of the first member and the second member that is used for the drive transmission unit to transition from the second transmission state to the first transmission state is larger than the amount (.sub.S) of relative rotation of the first member and the second member in a period in which both the first conveyance unit and the second conveyance unit convey the sheet in the second conveyance direction.

[0130] The preliminary rotation angle .sub.S [] of the reverse coupling 613 is an angle by which the reverse coupling 613 rotates excessively relative to the inverting output gear 132 rotating at the rotational speed V1, by the reverse coupling 613 rotating at the rotational speed V3 accompanying the preliminary rotation of the drive roller 61a higher than the rotational speed V1. The reference of the preliminary rotation angle .sub.S (.sub.S=0) is a rotational angle of the reverse coupling 613 relative to the inverting output gear 132 with the second end portion 132c of the engagement shaft 132a in contact with the second end portion 613c of the engagement hole 613a (i.e., second transmission state). [0131] 1. The length from the reversing roller pair 61 to the trailing edge of the sheet S at the time point at which the leading edge of the sheet S enters the nip portion of the intermediate refeeding roller pair 62:

[00001]$\left(\mathrm{the}\mathrm{sheet}\mathrm{length}\mathrm{Ls}\right)-\left(\mathrm{the}\mathrm{path}\mathrm{length}\mathrm{Lp}\mathrm{from}\mathrm{the}\mathrm{reversing}\mathrm{roller}\mathrm{pair}61\mathrm{to}\mathrm{the}\mathrm{intermediate}\mathrm{refeeding}\mathrm{roller}\mathrm{pair}62\right)=297\left[\mathrm{mm}\right]-144\left[\mathrm{mm}\right]=153\left[\mathrm{mm}\right]$ [0132] 2. The time until the trailing edge of the sheet S passes through the nip portion of the reversing roller pair 61 from the time point at which the leading edge of the sheet S enters the nip portion of the intermediate refeeding roller pair 62:

[00002]$\left(\mathrm{the}\mathrm{length}\mathrm{in}1.\right)/\left(\mathrm{the}\mathrm{conveyance}\mathrm{speed}\mathrm{Vs}2\mathrm{of}\mathrm{the}\mathrm{intermediate}\mathrm{refeeding}\mathrm{roller}\mathrm{pair}62\right)=153\left[\mathrm{mm}\right]/266.6\left[\mathrm{mm}/s\right]=0.57\left[s\right]$ [0133] 3. The speed difference between the reversing roller pair 61 and the intermediate refeeding roller pair 62:

[00003]$\left(\mathrm{the}\mathrm{conveyance}\mathrm{speed}\mathrm{Vs}2\mathrm{of}\mathrm{the}\mathrm{intermediate}\mathrm{refeeding}\mathrm{roller}\mathrm{pair}62\right)-\left(\mathrm{the}\mathrm{conveyance}\mathrm{speed}\mathrm{Vs}1\mathrm{of}\mathrm{the}\mathrm{reversing}\mathrm{roller}\mathrm{pair}61\right)=266.6\left[\mathrm{mm}/s\right]-264.1\left[\mathrm{mm}/s\right]=2.5\left[\mathrm{mm}/s\right]$ [0134] 4. The distance by which the outer circumferential surface of the drive roller 61a excessively moves by the drive roller 61a rotating at the rotational speed V3 higher than the rotational speed V1 during the above-described period in 2.:

[00004]$\left(\mathrm{the}\mathrm{time}\mathrm{in}2.\right)\left(\mathrm{the}\mathrm{speed}\mathrm{difference}\mathrm{in}3.\right)=0.57\left[s\right]2.5\left[\mathrm{mm}/s\right]=1.45\left[\mathrm{mm}\right]$ [0135] 5. The circumferential length of the outer circumferential surface of the drive roller 61a:

[00005]$\left(\mathrm{the}\mathrm{external}\mathrm{diameter}D\mathrm{of}\mathrm{the}\mathrm{drive}\mathrm{roller}61a\right)\left(\mathrm{circular}\mathrm{constant}\right)=34.6\left[\mathrm{mm}\right]$ [0136] 6. The preliminary rotation angle of the drive roller 61a:

[00006]$\left(\mathrm{the}\mathrm{moving}\mathrm{distance}\mathrm{in}4.\right)/\left(\mathrm{the}\mathrm{circumferential}\mathrm{length}\mathrm{in}5.\right)360=1.45\left[\mathrm{mm}\right]/34.6\left[\mathrm{mm}\right]360=15$ [0137] 7. The preliminary rotation angle .sub.S of the reverse coupling 613:
In the present exemplary embodiment, the reverse coupling 613 rotates integrally with the drive roller 61a, so that the preliminary rotation angle .sub.S of the reverse coupling 613 is the same as the preliminary rotation angle of the drive roller 61a (6.). More specifically, the preliminary rotation angle .sub.S is 15.

[0138] As described above, the preliminary rotation angle .sub.S of the reverse coupling 613 that is generated in the period in which the sheet S is nipped by both the reversing roller pair 61 and the intermediate refeeding roller pair 62 is about 15. It is thus desirable that the idling allowable angle .sub.L is set to a value larger than 15. This can prevent the preliminary rotation angle .sub.S from reaching the idling allowable angle .sub.L before the trailing edge of the sheet S passes through the nip portion of the reversing roller pair 61.

[0139] Suppose the preliminary rotation angle .sub.S reaches the idling allowable angle .sub.L before the trailing edge of the sheet S passes through the nip portion of the reversing roller pair 61. In this case, the first end portion 132b of the engagement shaft 132a of the inverting output gear 132 comes into contact with the first end portion 613b of the engagement hole 613a of the reverse coupling 613. This prevents the reverse coupling 613 from relatively rotating any more in the second rotational direction R2 relative to the inverting output gear 132. The drive roller 61a of the reversing roller pair 61 has a momentum to rotate due to the inverting output gear 132 at the rotational speed V1, causing a possibility that the sheet S is mutually pulled by the reversing roller pair 61 and the intermediate refeeding roller pair 62.

[0140] In the present exemplary embodiment, the idling allowable angle .sub.L is set to a value larger than the preliminary rotation angle .sub.S of the reverse coupling 613, such as .sub.L=30, for example. This can prevent the first end portion 132b of the engagement shaft 132a from coming into contact with the first end portion 613b of the engagement hole 613a before the trailing edge of the sheet S passes through the nip portion of the reversing roller pair 61. That is to say, at least until the trailing edge of the sheet S passes through the intermediate refeeding roller pair 62, the coupling unit CP is kept in the non-transmission state, and the preliminary rotation of the reversing roller pair 61 is allowed.

(Advantages of Present Exemplary Embodiment)

[0141] As described above, the drive mechanism according to the present exemplary embodiment uses the coupling unit CP with an idling range. After the leading edge of the sheet S reaches the intermediate refeeding roller pair 62 in duplex printing, the coupling unit CP transitions from the second transmission state to the non-transmission state to allow the reversing roller pair 61 to operate following the sheet S conveyed at the conveyance speed Vs2. Then, at least until the trailing edge of the sheet S passes through the nip portion of the reversing roller pair 61, the coupling unit CP is kept in the non-transmission state. In other words, after a contact state in which the sheet conveyed in the second conveyance direction is in contact with both the first conveyance unit and the second conveyance unit, the drive transmission unit enters the non-transmission state to allow the first conveyance unit to operate following the sheet conveyed at the second conveyance speed. More specifically, after the drive transmission unit transitions from the second transmission state to the non-transmission state, until the trailing edge of the sheet in the second conveyance direction passes through the first conveyance unit, the drive transmission unit is kept in the non-transmission state.

[0142] This configuration allows reduction of the possibility that, when both the reversing roller pair 61 and the intermediate refeeding roller pair 62 convey the sheet S, the sheet S is mutually pulled by the reversing roller pair 61 and the intermediate refeeding roller pair 62.

[0143] Consequently, this configuration prevents the wear of the reversing roller pair 61 or the intermediate refeeding roller pair 62 from accelerating due to the friction with the sheet S. This also reduces the scraping of the conveyance guide 68 that is caused by the second surface of the sheet S and the protruding portion 68a of the conveyance guide 68 rubbing each other.

(Other Configurations)

[0144] In the present exemplary embodiment, the coupling unit CP with an idling range is arranged coaxially with the drive roller 61a of the reversing roller pair 61, but can be arranged at another portion in the drive transmission mechanism DT. Also in this case, the coupling unit CP entering the non-transmission state after the leading edge of the sheet S reaches the intermediate refeeding roller pair 62 brings an advantage similar to that of the present exemplary embodiment.

[0145] In the present exemplary embodiment, the reversing roller pair 61 and the intermediate refeeding roller pair 62 are driven by a drive source (the first motor 81), but the reversing roller pair 61 and the intermediate refeeding roller pair 62 can be independently supplied with drive force from different drive sources.

[0146] In the present exemplary embodiment, the configuration is used of switching the rotational direction of the reversing roller pair 61 by switching the rotational direction of the first motor 81, but a motor rotating only in one direction can be used as a drive source. In this case, it is sufficient that the drive transmission mechanism DT includes a switching mechanism that can switch the rotational direction to be input to the drive roller 61a of the reversing roller pair 61.

[0147] In the present exemplary embodiment, the configuration including one protruding portion (the engagement shaft 132a) and one recess portion (the engagement hole 613a) is exemplified as an example of the coupling unit CP with the idling range, but a configuration can be employed that includes a plurality of protruding portions and a plurality of recess portions.

[0148] In the present exemplary embodiment, the configuration is exemplified in which the inverting output gear 132 (first member) includes the protruding portion, and the reverse coupling 613 (second member) includes the recess portion that accepts the protruding portion, but the arrangement of the protruding portion and the recess portion can be opposite. More specifically, the reverse coupling 613 (second member) can include an engagement shaft (protruding portion) protruding toward the inverting output gear 132 in the rotational axis direction X, and the inverting output gear 132 (first member) can include an engagement hole (recess portion) that accepts the engagement shaft.

[0149] Also in this case, an idling range set with the width of the engagement hole set to a width wider than the width of the engagement shaft brings an advantage similar to that of the present exemplary embodiment.

[0150] In other words, the second member can include a protruding portion protruding in the rotational axis direction of the first member and the second member, the first to-be-contacted portion can be an end portion of the protruding portion in the first rotational direction, and the second to-be-contacted portion an end portion of the protruding portion in the second rotational direction. In addition, the first member can include a recess portion that accepts the protruding portion, the first to-contact portion can be an end portion of the recess portion in the first rotational direction, and the second to-contact portion an end portion of the recess portion in the second rotational direction.

[0151] In the present exemplary embodiment, the rotational speed of the reversing roller pair 61 is set to the constant rotational speed VI irrespective of the rotational direction, but can be set to different rotational speeds depending on the rotational direction. That is to say, the conveyance speed before the reversing roller pair 61 reverses the sheet S and the conveyance speed after the reversing roller pair 61 reverses the sheet S can be different.

[0152] In the present exemplary embodiment, the configuration is exemplified in which the second conveyance unit (the intermediate refeeding roller pair 62) that conveys the sheet S at the conveyance speed Vs2 higher than that of the first conveyance unit (the reversing roller pair 61) is arranged downstream of the first conveyance unit in the second conveyance direction D2. In place of this, the second conveyance unit that conveys the sheet at a conveyance speed higher than that of the first conveyance unit can be arranged upstream of the first conveyance unit in the second conveyance direction D2. This allows reduction of a harmful effect (slid trace attributed to sliding with the conveyance guide, a conveyance failure, etc.) that is attributed to the warping of the sheet S that occurs between the first conveyance unit and the second conveyance unit. That is to say, the positional relationship between the first conveyance unit and the second conveyance unit can be changed in accordance with a specific configuration of a sheet conveyance apparatus.

[0153] More specifically, with the second conveyance unit arranged upstream of the first conveyance unit in the second conveyance direction D2, until the leading edge of the sheet S conveyed in the second conveyance direction D2 reaches the first conveyance unit, the sheet S is conveyed by second conveyance unit in the second conveyance direction D2 at the conveyance speed Vs2. If the leading edge of the sheet S reaches the first conveyance unit, which causes the contact state, the drive transmission unit (coupling unit) enters the non-transmission state in such a manner that the first conveyance unit follows the sheet S conveyed at the conveyance speed Vs2. When the trailing edge of the sheet S passes through the second conveyance unit, which releases the contact state, the drive transmission unit enters the second transmission state, and the sheet S is conveyed by the first conveyance unit at the conveyance speed Vs1.

[0154] The first conveyance unit and the second conveyance unit are not limited to the roller pairs illustrated in the drawings. For example, the first conveyance unit can include triple rollers in which a discharge roller pair and a reversing roller pair each include one roller shared with each other.

[0155] Next, a second exemplary embodiment of the present disclosure will be described. In the first exemplary embodiment, the example has been described in which the coupling unit CP is arranged coaxially with the drive roller 61a. In the second exemplary embodiment, an example will be described in which a coupling unit CP2 is arranged on an axis line different from that of the drive roller 61a. Hereinafter, the components assigned the same reference numerals as those in the first exemplary embodiment have substantially the same configurations and functions as those described in the first exemplary embodiment, unless otherwise a specific description is provided, and differences from the first exemplary embodiment will be mainly described.

(Coupling Unit)

[0156] The drive transmission configuration of a reversing roller pair 61 according to the second exemplary embodiment will be described with reference to FIGS. 7A to 7E. FIG. 7A is a perspective view illustrating a part of the drive transmission mechanism DT and the drive roller 61a of the reversing roller pair 61. FIGS. 7B and 7C are exploded views of the drive transmission mechanism DT and the coupling unit CP2 of the drive roller 61a. FIG. 7D is a diagram (top view) illustrating a part of the drive transmission mechanism DT and the drive roller 61a of the reversing roller pair 61 that are viewed from a direction orthogonal to a rotational axis direction X to be described below. FIG. 7E is a cross-sectional view of the coupling unit CP2 cut along an A-A line in FIG. 7D.

[0157] As illustrated in FIGS. 7A and 7D, the drive transmission mechanism DT according to the present exemplary embodiment includes a coupling pulley 116, a coupling gear 133, and a reverse roller gear 134 in place of the third pulley 113, the inverting input gear 131, and the inverting output gear 132 (FIG. 3A) according to the first exemplary embodiment.

[0158] The coupling unit CP2 includes the coupling gear 133 and the coupling pulley 116. That is to say, the drive transmission mechanism DT according to the present exemplary embodiment includes the coupling unit CP2 serving as an example of the drive transmission unit. The coupling gear 133 is an example of a first member (drive member, input member) connected to the drive source (the first motor 81). The coupling pulley 116 is an example of a second member (driven member, output member) connected to a drive target (the reversing roller pair 61 in this example).

[0159] The coupling pulley 116 rotates around a rotational axis line Ax3. The rotational axis line Ax3 is an axis line different from the rotational axis line Ax of the drive roller 61a of the reversing roller pair 61. The rotational axis line Ax3 according to the present exemplary embodiment is substantially parallel to the rotational axis line Ax of the drive roller 61a. In addition, the rotational axis line Ax3 corresponds to the second intermediate axis Ax2 (FIG. 3A) according to the first exemplary embodiment. The second drive belt 111 is stretched over the second pulley 112 and the coupling pulley 116.

[0160] The coupling gear 133 is arranged coaxially with the coupling pulley 116. The coupling gear 133 and the coupling pulley 116 can rotate around the rotational axis line Ax3 in a rotational direction R3 and a rotational direction R4. In the present exemplary embodiment, the radius direction of an imaginary circle centered on the rotational axis line Ax3 when viewed in the rotational axis direction X will be referred to as the radial direction of the coupling unit CP2 or will be simply referred to as a radial direction.

[0161] The reverse roller gear 134 is arranged coaxially with the drive roller 61a of the reversing roller pair 61, and rotates integrally with the drive roller 61a. The reverse roller gear 134 engages with the coupling gear 133. The drive force input from the first motor 81 to the drive roller 61a via the coupling pulley 116, the coupling gear 133, and the reverse roller gear 134 rotates the reversing roller pair 61.

[0162] As illustrated in FIGS. 7B and 7C, the coupling pulley 116 includes a plurality of engagement tabs 116b serving as a to-engage unit. The coupling gear 133 includes a plurality of engagement recess portions 133b serving as a to-be-engaged unit.

[0163] More specifically, the coupling pulley 116 includes a shaft portion 116a extending along the rotational axis line Ax3, and a side surface 116f spreading from the shaft portion 116a outward in the radial direction. The coupling gear 133 includes a hole portion 133a into which the shaft portion 116a is inserted, and a side surface 133f spreading from the hole portion 133a outward in the radial direction. The side surface 116f of the coupling pulley 116 and the side surface 133f of the coupling gear 133 face each other in the rotational axis direction X.

[0164] The plurality of engagement tabs 116b of the coupling pulley 116 serves as protruding portions protruding from the side surface 116f of the coupling pulley 116 toward the coupling gear 133 in the rotational axis direction X. In the present exemplary embodiment, the plurality of engagement tabs 116b is arranged along a circular arc centered on the rotational axis line Ax3.

[0165] The plurality of engagement recess portion 133b of the coupling gear 133 is provided on the side surface 133f of the coupling gear 133, and has a recessed shape recessed to get away from the coupling pulley 116 in the rotational axis direction X. In the present exemplary embodiment, the plurality of engagement recess portions 133b is arranged along a circular arc that is a circular arc centered on the rotational axis line Ax3, and is the same circular arc as the circular arc on which the plurality of engagement tabs 116b is arranged. The number of engagement recess portions 133b arranged is the same as the number of engagement tabs 116b, and the plurality of engagement recess portions 133b is arranged at intervals that correspond to those of the plurality of engagement tabs 116b.

[0166] The plurality of engagement tabs 116b and the plurality of engagement recess portions 133b serve as an example of a dog clutch that transmits rotation by irregularity portions provided on two rotational members engaging with each other. It can be thus said that the coupling pulley 116 includes a plurality of engagement recess portions each formed between the adjacent engagement tabs 116b. It can also be said that the coupling gear 133 includes a plurality of engagement tabs each formed between the adjacent engagement recess portions 133b, and each engagement tab of the coupling gear 133 engages with the engagement recess portion of the coupling pulley 116.

[0167] As illustrated in FIG. 7E, each of the engagement tabs 116b includes a first end portion 116c being an end portion of the engagement tab 116b in the rotational direction R3, and a second end portion 116d being an end portion of the engagement tab 116b on the rotational direction R4 side. The first end portion 132b is an example of the first to-contact portion, and the second end portion 132c is an example of the second to-contact portion.

[0168] Each of engagement recess portions 133b includes a first end portion 133c being an end portion of the engagement recess portion 133b in the rotational direction R3, and a second end portion 133d being an end portion of the engagement recess portion 133b in the rotational direction R4. The first end portion 133c is an example of a first to-be-contacted portion to be contacted by the first end portion 116c of the engagement tab 116b of the coupling pulley 116. The second end portion 133d is an example of a second to-be-contacted portion to be contacted by the second end portion 116d of the engagement tab 116b of the coupling pulley 116.

[0169] The width of each of the engagement recess portions 133b is set to a width wider than the width of the corresponding tab of the engagement tabs 116b. That is to say, the distance from the first end portion 133c to the second end portion 133d that is measured along an imaginary circular arc centered on the rotational axis line Ax and passing through the engagement tab 116b when viewed in the rotational axis direction X is wider than the distance from the first end portion 116c to the second end portion 116d of the engagement tab 116b that is measured along the imaginary circular arc. In other words, the central angle between the first end portion 133c and the second end portion 133d with respect to the rotational axis line Ax is wider than the central angle between the first end portion 116c of the engagement tab 116b in the rotational direction R3 and the second end portion 116d in the rotational direction R4 with respect to the rotational axis line Ax.

[0170] With the width of the engagement recess portion 133b set to a width wider than the width of the engagement tab 116b, the coupling pulley 116 and the coupling gear 133 are coupled to be relatively rotatable (able to execute idling) within a predetermined range (within an idling range). With respect to the coupling gear 133, the coupling pulley 116 can rotate between the position at which the first end portion 116c of each engagement tab 116b is in contact with the first end portion 133c of the engagement recess portion 133b, and the position at which the second end portion 116d of the engagement tab 116b is in contact with the second end portion 133d of the engagement recess portion 133b.

[0171] With respect to the state in which the first end portion 116c of each of the engagement tabs 116b is in contact with the first end portion 133c of the corresponding engagement recess portion of the engagement recess portions 133b (=0), the angle by which the coupling pulley 116 rotates in the rotational direction R4 relative to the coupling gear 133 is indicated by []. The maximum value of the angle in the present exemplary embodiment is regarded as an idling allowable angle .sub.t []. The first transmission state of the coupling unit CP2 corresponds to =0, the second transmission state corresponds to =.sub.t, and the non-transmission state corresponds to the range of 0<<.sub.t. In the present exemplary embodiment, the range in which the relative rotation (idling) of the coupling pulley 116 and the coupling gear 133 is allowed (hereinafter, will be referred to as an idling allowable range) is 0<<.sub.t.

(Operation of Coupling Unit)

[0172] An operation of the coupling unit CP2 in duplex printing will be described with reference to FIGS. 8A to 8D. The left of each of FIGS. 8A to 8D illustrates a cross-sectional view of the coupling unit CP2 at the same position as the A-A line in FIG. 7D at a corresponding time point of duplex printing. The right of each of FIGS. 8A to 8D illustrates a schematic diagram illustrating the cross-section of the image forming apparatus 1 at a corresponding time point of duplex printing. In each drawing of FIGS. 8A to 8D, the left and the right illustrate the same time point in duplex printing.

[0173] In the present exemplary embodiment, as an example, duplex printing is performed under the following conditions. [0174] The sheet length Ls: 297 mm. [0175] The path length Lp from the reversing roller pair 61 to the intermediate refeeding roller pair 62: 144 mm. [0176] The conveyance speed Vs1 of the sheet S conveyed by the reversing roller pair 61: 264.1 mm/s. [0177] The conveyance speed Vs2 of the sheet S conveyed by the intermediate refeeding roller pair 62: 266.6 mm/s. [0178] The external diameter D of the drive roller 61a of the reversing roller pair 61: 11 mm. [0179] The number of gear teeth of the reverse roller gear 134: 14. [0180] The number of gear teeth of the coupling gear 133: 28. [0181] The module of the reverse roller gear 134 and the coupling gear 133: 1. [0182] A backlash Lb between the reverse roller gear 134 and the coupling gear 133: 0.15 mm. [0183] The idling allowable angle .sub.t between the coupling pulley 116 and the coupling gear 133: 10.

[0184] FIGS. 8A to 8D illustrate a process in which the sheet S is conveyed by the reversing roller pair 61 and the intermediate refeeding roller pair 62 after the conveyance direction of the sheet S is reversed by the reversing roller pair 61. In the duplex printing, an operation of the reversing roller pair 61 until the conveyance direction is reversed to the second conveyance direction D2 after the sheet S with an image formed on its first surface is guided to the reversing path P2 and conveyed by the reversing roller pair 61 in the first conveyance direction D1 is similar to that in the first exemplary embodiment.

[0185] When the reversing roller pair 61 conveys the sheet S in the first conveyance direction D1, in the coupling unit CP2, the first end portion 116c of each engagement tab 116b contacts the first end portion 133c of the engagement recess portion 133b as illustrated in FIG. 7E. That is to say, the coupling unit CP2 enters the first transmission state. The torque in the rotational direction R3 is accordingly transmitted from the coupling pulley 116 to the coupling gear 133. Furthermore, the torque in the first rotational direction R1 (FIG. 7A) is transmitted to the drive roller 61a of the reversing roller pair 61 via the engagement between the coupling gear 133 and the reverse roller gear 134, and the drive roller 61a rotates in the first rotational direction R1.

[0186] After that, by switching the rotational direction of the first motor 81, the coupling pulley 116 rotates in the rotational direction R3. The second end portion 116d of each engagement tab 116b accordingly contacts the second end portion 133d of the engagement recess portion 133b as illustrated at the left of FIG. 8A. That is to say, the coupling unit CP2 enters the second transmission state. The torque in the rotational direction R4 is accordingly transmitted from the coupling pulley 116 to the coupling gear 133. Furthermore, the torque in the second rotational direction R2 is transmitted to the drive roller 61a of the reversing roller pair 61 via the engagement between the coupling gear 133 and the reverse roller gear 134, and the drive roller 61a rotates in the second rotational direction R2. The conveyance direction of the sheet S is accordingly reversed to the second conveyance direction D2.

[0187] As illustrated at the right of FIG. 8A, until the leading edge of the sheet S in the second conveyance direction D2 reaches the intermediate refeeding roller pair 62, the drive roller 61a of the reversing roller pair 61 rotates at the rotational speed V1. As illustrated at the left of FIG. 8A, the coupling pulley 116 and the coupling gear 133 rotate in the rotational direction R4 at a rotational speed V1 corresponding to the rotational speed V1 of the inverting output gear 132. The sheet S is conveyed by the reversing roller pair 61 at the conveyance speed Vs1.

[0188] The right of FIG. 8B illustrates a state in which the leading edge of the sheet S in the second conveyance direction D2 has reached the nip portion of the intermediate refeeding roller pair 62. When the leading edge of the sheet S reaches the nip portion of the intermediate refeeding roller pair 62, the sheet S is conveyed in the second conveyance direction D2 while being nipped by both the reversing roller pair 61 and the intermediate refeeding roller pair 62.

[0189] Also in the present exemplary embodiment, conveyance speeds are set in such a manner that the conveyance speed Vs2 of the intermediate refeeding roller pair 62 is higher than the conveyance speed Vs1 of the reversing roller pair 61 (Vs2>Vs1). As described above, this configuration reduces the generation of a slid trace on an image that is attributed to the warping of the sheet S.

[0190] The intermediate refeeding roller pair 62 conveys the sheet S at the conveyance speed Vs2 as if to draw the sheet S out of the reversing roller pair 61. On the other hand, the drive roller 61a of the reversing roller pair 61 receives the torque in the second rotational direction R2 with the frictional force received from the sheet S conveyed at the conveyance speed Vs2, and has a momentum to preliminarily rotate in the second rotational direction R2 at the rotational speed V3 (angular speed) higher than the rotational speed V1.

[0191] In the present exemplary embodiment, as described below, the preliminary rotation of the drive roller 61a is allowed with 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.

[0192] First of all, as illustrated at the left of FIG. 8B, the drive roller 61a preliminarily rotates at the 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. Consequently, the state changes from the state (state in FIG. 8A) in which a gear tooth of the reverse roller gear 134 is in contact with a gear tooth surface upstream in the rotational direction R4 of the gear teeth of the coupling gear 133, to the state (state in FIG. 8B) in which a gear tooth of the reverse roller gear 134 is in contact with a gear tooth surface downstream in the rotational direction R4 of the gear teeth of the coupling gear 133.

[0193] Subsequently, as illustrated at the left of FIG. 8C, if the drive roller 61a preliminarily rotates exceeding the amount corresponding to the backlash Lb, the reverse roller gear 134 presses the coupling gear 133 in the rotational direction R4. This rotates the coupling gear 133 in the rotational direction R4 at a rotational speed V3 corresponding to the rotational speed V3 of the inverting output gear 132, prior to the coupling pulley 116 rotating at the rotational speed V1. The relative rotation of the coupling gear 133 and the coupling pulley 116 separates the second end portion 133d of the engagement recess portion 133b from the second end portion 116d of the engagement tab 116b. That is to say, the coupling unit CP2 transitions from the second transmission state to the non-transmission state.

[0194] As illustrated at the right of FIG. 8D, even after the trailing edge of the sheet S in the second conveyance direction D2 passes through the nip portion of the reversing roller pair 61, the sheet S is conveyed by the intermediate refeeding roller pair 62 at the conveyance speed Vs2. On the other hand, by the trailing edge of the sheet S passing through the nip portion of the reversing roller pair 61, the reversing roller pair 61 is released from the sheet S, reducing the rotation of the drive roller 61a.

[0195] As illustrated at the left of FIG. 8D, even after the sheet S passes through the reversing roller pair 61, the coupling gear 133 continues to rotate at the rotational speed V1. The second end portion 133d of the engagement recess portion 133b thus comes into contact with the second end portion 116d of the engagement tab 116b again. The state of the reverse roller gear 134 returns to the state in which a gear tooth of the reverse roller gear 134 is in contact with a gear tooth surface upstream in the rotational direction R4 of the gear teeth of the coupling gear 133. That is to say, the state of the coupling unit CP2 returns from the non-transmission state to the second transmission state. Then, as illustrated at the right of FIG. 8D, the drive roller 61a of the reversing roller pair 61 rotates in the second rotational direction R2 at the rotational speed V1.

(Relationship Between Preliminary Rotation Angle and Idling Allowable Angle)

[0196] In the present exemplary embodiment, the description will be given of a configuration in which, at least until the trailing edge of the sheet S passes through the intermediate refeeding roller pair 62, the coupling unit CP2 is kept in the non-transmission state and the preliminary rotation of the reversing roller pair 61 is allowed.

[0197] The preliminary rotation angle .sub.S1[] of the reverse roller gear 134 and the preliminary rotation angle .sub.S2[] of the coupling gear 133 are defined as follows. The preliminary rotation angle .sub.S1 of the reverse roller gear 134 is an angle by which the reverse roller gear 134 rotates excessively as compared with a case where the rotational speed is maintained at the rotational speed V1, by the reverse roller gear 134 rotating at the rotational speed V3 accompanying the preliminary rotation of the drive roller 61a higher than the rotational speed V1. The preliminary rotation angle .sub.S2 of the coupling gear 133 is an angle by which the coupling gear 133 rotates excessively relative to the coupling pulley 116 rotating at the rotational speed V1, by the coupling gear 133 rotating at the rotational speed V3 higher than the rotational speed V1.

[0198] In an example of the present exemplary embodiment, the sheet length Ls, the path length Lp, the conveyance speed Vs1, the conveyance speed Vs2, and the external diameter D of the drive roller 61a, all of which have been described above, are similar to those in the first exemplary embodiment. In this case, as described in the first exemplary embodiment, the preliminary rotation angle (.sub.S) of the drive roller 61a in the period until the trailing edge of the sheet S passes through the nip portion of the reversing roller pair 61 from the time point at which the leading edge of the sheet S enters the nip portion of the intermediate refeeding roller pair 62 is 15. In addition, the reverse roller gear 134 rotates integrally with the drive roller 61a, so that the preliminary rotation angle .sub.S1 of the reverse roller gear 134 in the same period is 15.

[0199] As illustrated at the left of FIG. 8B, a preliminary rotation angle when the reverse roller gear 134 preliminarily rotates by an amount corresponding to the backlash Lb between the reverse roller gear 134 and the coupling gear 133 is denoted by .sub.b. The preliminary rotation angle .sub.b is calculated as follows. [0200] 1. The pitch circumferential length of the reverse roller gear 134:

[00007]$\left(\mathrm{module}\left[\mathrm{mm}\right]\right)\left(\mathrm{the}\mathrm{number}\mathrm{of}\mathrm{gear}\mathrm{teeth}\right)\left(\mathrm{circular}\mathrm{constant}\right)=114=43.9\left[\mathrm{mm}\right]$ [0201] 2. The preliminary rotation angle .sub.b corresponding to the backlash:

[00008]$\left(\mathrm{the}\mathrm{backlash}\mathrm{Lb}\left[\mathrm{mm}\right]\right)/\left(\mathrm{the}\mathrm{pitch}\mathrm{circumferential}\mathrm{length}\left[\mathrm{mm}\right]\mathrm{in}1.\right)360=0.15/43.9360=1.22$

[0202] As illustrated at the left of FIG. 8C, even after the reverse roller gear 134 preliminarily rotates by an amount corresponding to the backlash Lb, the reverse roller gear 134 preliminarily rotates until the preliminary rotation angle .sub.S1 becomes 15. At this time, the preliminary rotation angle .sub.S2 by which the coupling gear 133 preliminarily is rotated relative to the coupling pulley 116 by the reverse roller gear 134 pressing the coupling gear 133 is calculated as follows. [0203] 3. The angle obtained by subtracting the preliminary rotation angle .sub.b corresponding to the backlash Lb, from the preliminary rotation angle .sub.S1 of the reverse roller gear 134: .sub.S1.sub.b=151.22=13.78 [0204] 4. The reduction gear ratio of the coupling gear 133 and the reverse roller gear 134:

[00009]$\left(\mathrm{the}\mathrm{number}\mathrm{of}\mathrm{gear}\mathrm{teeth}\mathrm{of}\mathrm{the}\mathrm{coupling}\mathrm{gear}133\right)/\left(\mathrm{the}\mathrm{number}\mathrm{of}\mathrm{gear}\mathrm{teeth}\mathrm{of}\mathrm{the}\mathrm{reverse}\mathrm{roller}\mathrm{gear}134\right)=28/14=2$ [0205] 5. The preliminary rotation angle .sub.S2 of the coupling gear 133:

[00010]$\left(\mathrm{the}\mathrm{preliminary}\mathrm{rotation}\mathrm{angle}\mathrm{in}3.\right)/\left(\mathrm{the}\mathrm{reduction}\mathrm{gear}\mathrm{ratio}\mathrm{in}4.\right)=13.78/2=6.89$

[0206] As the result, in the period until the trailing edge of the sheet S passes through the nip portion of the reversing roller pair 61 from the time point at which the leading edge of the sheet S enters the nip portion of the intermediate refeeding roller pair 62, the coupling gear 133 preliminarily rotates by 6.89 relative to the coupling pulley 116. That is to say, the coupling gear 133 relatively rotates by 6.89 relative to the coupling pulley 116 in the direction in which the second end portion 133d of the engagement recess portion 133b gets away from the second end portion 116d of the engagement tab 116b.

[0207] In the present exemplary embodiment, the idling allowable angle .sub.t of the coupling pulley 116 and the coupling gear 133 is set to a value larger than the above-described relative rotation amount (6.89), such as .sub.t=10, for example. This can prevent the first end portion 133c of an engagement recess portion 133b from coming into contact with the first end portion 116c of an engagement tab 116b before the trailing edge of the sheet S passes through the nip portion of the reversing roller pair 61. That is to say, until the trailing edge of the sheet S passes through at least the intermediate refeeding roller pair 62, the coupling unit CP2 is kept in the non-transmission state, and the preliminary rotation of the reversing roller pair 61 is allowed.

(Advantage of Present Exemplary Embodiment)

[0208] As described above, the drive mechanism according to the present exemplary embodiment uses the coupling unit CP2 with the idling range. After the leading edge of the sheet S reaches the intermediate refeeding roller pair 62 in duplex printing, the coupling unit CP2 transitions from the second transmission state to the non-transmission state to allow the reversing roller pair 61 to operate following the sheet S conveyed at the conveyance speed Vs2. Then, at least until the trailing edge of the sheet S passes through the nip portion of the reversing roller pair 61, the coupling unit CP2 is kept in the non-transmission state. In other words, after a contact state in which the sheet conveyed in the second conveyance direction is in contact with both the first conveyance unit and the second conveyance unit, the drive transmission unit enters the non-transmission state to allow the first conveyance unit to operate following the sheet conveyed at the second conveyance speed. More specifically, after the drive transmission unit transitions from the second transmission state to the non-transmission state, until the trailing edge of the sheet in the second conveyance direction passes through the first conveyance unit, the drive transmission unit is kept in the non-transmission state.

[0209] This configuration allows reduction of the possibility that, when both the reversing roller pair 61 and the intermediate refeeding roller pair 62 convey the sheet S, the sheet S is mutually pulled by the reversing roller pair 61 and the intermediate refeeding roller pair 62.

[0210] Consequently, the wear of the reversing roller pair 61 or the intermediate refeeding roller pair 62 can be prevented from accelerating due to the friction with the sheet S. The scraping of the conveyance guide 68 that is caused by the second surface of the sheet S and the protruding portion 68a of the conveyance guide 68 rubbing each other can be also reduced.

[0211] In the present exemplary embodiment, the coupling unit CP2 with the idling range is arranged on an axis line different from the drive roller 61a of the reversing roller pair 61. Thus, the amount (.sub.S2) of relative rotation that occurs with the coupling unit CP2 by the reversing roller pair 61 operating following the sheet S is smaller by the angle (.sub.b) corresponding to the backlash on a drive transmission path from the coupling unit CP2 to the drive roller 61a. The backlash in the present exemplary embodiment is a backlash in the engagement portion of the coupling gear 133 and the reverse roller gear 134. A smaller rotation amount described above leads to a smaller width of the engagement recess portion 133b, allowing increase of the numbers of the engagement tabs 116b and the engagement recess portions 133b. Consequently, the load applied to one tab can be reduced, improving the durability of the apparatus.

(Other Configurations)

[0212] In the present exemplary embodiment, the configuration has been exemplified in which the coupling pulley 116 (first member) includes a protruding portion, and the coupling gear 133 (second member) includes a recess portion that accepts the protruding portion, but the arrangement of the protruding portion and the recess portion can be opposite arrangement. More specifically, the coupling gear 133 (second member) can include an engagement tab (protruding portion) protruding toward the coupling pulley 116 in the rotational axis direction X, and the coupling pulley 116 (first member) can include an engagement recess portion (recess portion) that accepts the engagement tab.

[0213] In the present exemplary embodiment, an example has been described in which the number of the plurality of engagement tabs 116b and the number of the plurality of the engagement recess portions 133b are each five, but the number of the plurality of engagement tabs 116b and the number of the plurality of the engagement recess portions 133b can be a number other than this. In addition, the coupling unit CP2 can include only one engagement tab 116b and one engagement recess portion 133b.

[0214] Moreover, modified examples described as the other configurations in the first exemplary embodiment can be applied to the present exemplary embodiment.

OTHER EXEMPLARY EMBODIMENTS

[0215] In the above-described exemplary embodiments, the image forming unit 20 of a direct transfer type has been exemplified as an image forming unit, but the image forming unit can be an image forming unit of an intermediate transfer type that primarily transfers a toner image from an image bearing member to an intermediate transfer member, such as an intermediate transfer belt, and secondarily transfers the toner image from the intermediate transfer member to a recording material. Furthermore, the image forming unit is not limited to an electrophotographic image forming unit. For example, the image forming unit can be an inkjet image forming unit that forms an image by ejecting ink liquid from nozzles.

[0216] In the above-described exemplary embodiments, a color printer that forms a color image using a plurality of color toners has been exemplified as an image forming apparatus, but the image forming apparatus can be a monochrome printer that forms a monochrome image using single-color toner.

[0217] In the above-described exemplary embodiments, the sheet conveyance unit 60 that conveys the sheet S at the time of duplex printing in the image forming apparatus 1 has been described as an example of the sheet conveyance apparatus. The sheet conveyance apparatus is not limited to this, and can be an apparatus that conveys a sheet in another apparatus included in an image forming apparatus (image forming system). For example, the sheet conveyance apparatus can be an automatic document feeder (ADF) that can automatically feed a document in an image reading apparatus that reads an image from the document. Furthermore, the sheet conveyance apparatus can be a sheet conveyance mechanism in a sheet processing apparatus (finisher) that performs processing, such as binding processing on sheets on which images are formed in the image forming apparatus main body.

[0218] According to an exemplary embodiment of the present disclosure, a sheet conveyance apparatus and an image forming apparatus can be provided that prevent a sheet from being pulled or warped after a sheet conveyance direction is reversed.

[0219] While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

[0220] This application claims the benefit of priority from Japanese Patent Application No. 2023-114773, filed Jul. 12, 2023, which is hereby incorporated by reference herein in its entirety.