US20190062093A1

US20190062093A1 - Sheet conveyance apparatus and image forming apparatus

Info

Publication number: US20190062093A1
Application number: US16/109,943
Authority: US
Inventors: Hiroshige Inoue
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2017-08-30
Filing date: 2018-08-23
Publication date: 2019-02-28
Also published as: JP2019043701A

Abstract

A sheet conveyance apparatus includes a first rotary member pair comprising a first nip portion and configured to convey a sheet, a second rotary member pair comprising a second nip portion and configured to convey the sheet being conveyed by the first rotary member pair, and a conveyance guide including a first conveyance guide portion forming a loop space and a second conveyance guide portion arranged downstream of the first conveyance guide portion in the sheet conveyance direction and forming a curved conveyance path. A first intersection at which a first straight line and a third straight line intersect is positioned within the loop space, and a second intersection at which the first straight line and a second straight line intersect is positioned outside the conveyance path.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a sheet conveyance apparatus for conveying sheets and an image forming apparatus equipped with the same.

Description of the Related Art

Generally, in image forming apparatuses such as printers, a sheet fed from a sheet feeding unit is abutted against a nip of a registration roller pair whose rotation is stopped, to thereby form a loop on the sheet and correct skewing of the sheet. Thereafter, the registration roller pair rotates to convey the sheet to an image forming portion, and an image is formed on the sheet at the image forming portion. Conventionally, Japanese Patent Laid-Open Publication No. 2000-118801 proposes an image forming apparatus in which an amount of loop formed on the sheet is adjusted based on sheet type information. For example, the image forming apparatus controls the registration roller pair to form a greater loop in a normal paper having relatively weak stiffness compared to a sheet having greater stiffness such as envelope, postcard, thick paper and OHP sheet.
According to the printer taught in Japanese Patent Laid-Open Publication No. 2000-118801, in a state where the sheet subjected to skew correction is conveyed by the registration roller pair, a trailing edge side of the sheet is still skewed. Therefore, torsion occurs in the loop formed in a space between the registration roller pair and an upstream roller pair arranged upstream in a conveyance direction thereof. If the sheet is conveyed by the registration roller pair and the upstream roller pair in a state where there is torsion in the loop, shearing force acting on the sheet increases gradually. If the shearing force acting on the sheet exceeds the rigidity of the sheet, the sheet is buckled and creases are formed on the sheet when the sheet passes the registration roller pair. Especially if a distance between the registration roller pair and the upstream roller pair is narrow, the sheet tends to be buckled.
Recently, there are demands for image forming apparatuses that correspond to various types of media such as a long-size sheet, a small-size sheet, thin paper and coated paper. In order to convey a sheet having a high rigidity, it is necessary to set the registration roller pair and the upstream roller pair to have a high nip pressure, and in order to convey a small-size sheet, it is necessary to set the distance between the registration roller pair and the upstream roller pair to be narrow. If a thin paper having low rigidity or a long-sized sheet is conveyed by the registration roller pair and the upstream roller pair, the shearing force applied on the sheet may exceed the rigidity of the sheet, and the sheet may be buckled.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a sheet conveyance apparatus includes a first rotary member pair comprising a first nip portion and configured to convey a sheet, a second rotary member pair arranged downstream of the first rotary member pair in a sheet conveyance direction, comprising a second nip portion and configured to convey the sheet being conveyed by the first rotary member pair, and a conveyance guide configured to guide the sheet having passed through the first nip portion to the second nip portion, and forming a curved conveyance path, the conveyance guide including a first conveyance guide portion forming a loop space and a second conveyance guide portion arranged downstream of the first conveyance guide portion in the sheet conveyance direction, the conveyance path including the loop space in which a sheet abutted against the second nip portion is capable of forming a loop. In the conveyance path, the first rotary member pair and the second rotary member pair are arranged adjacently. The second nip portion is positioned upward than the first nip portion in a vertical direction and positioned at a position different from the first nip portion in a horizontal direction. The conveyance guide is arranged on an outer side of the curved conveyance path between the first nip portion and the second nip portion. The sheet conveyed by the first rotary member pair is abutted against the second nip portion so as to correct skewing of the sheet. If a common tangent of the first rotary member pair at the first nip portion is referred to as a first straight line, a common tangent of the second rotary member pair at the second nip portion is referred to as a second straight line, and a straight line that passes an intersection of the second straight line and the conveyance guide and has an inclination corresponding to the second conveyance guide portion is referred to as a third straight line, the second straight line and the third straight line intersect each other, the loop space is arranged upstream of the intersection of the second straight line and the conveyance guide in the sheet conveyance direction, a first intersection at which the first straight line and the third straight line intersect is positioned within the loop space, and a second intersection at which the first straight line and the second straight line intersect is positioned outside the conveyance path.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall schematic view illustrating a printer according to a present embodiment.

FIG. 2 is a schematic diagram illustrating a manual sheet feed apparatus and a skewing correction apparatus.

FIG. 3 is a control block diagram illustrating a controller.

FIG. 4 is a flowchart illustrating a sheet conveyance control.

FIG. 5 is a graph showing a positional relationship between respective rollers and the sheet.

FIG. 6A is a view illustrating a state in which a leading edge of the sheet is abutted against a nip of a registration roller pair.

FIG. 6B is a view illustrating a state in which the sheet has formed a loop.

FIG. 6C is a view illustrating a state in which a sheet is conveyed by the registration roller pair and a pre-registration roller pair.

FIG. 7A is a view illustrating a state in which a shearing force acting on the sheet escapes.

FIG. 7B illustrates a state in which a trailing edge of the sheet has passed through the pre-registration roller pair.

FIG. 8A is a view illustrating a conveyance guide according to a first modified example.

FIG. 8B is a view illustrating a conveyance guide according to a second modified example.

FIG. 9 is a view illustrating a skewing correction apparatus according to a comparative example.

FIG. 10 is a view illustrating torsion acting on the sheet.

DESCRIPTION OF THE EMBODIMENTS

Now, a preferred embodiment of the present embodiment will be described with reference to the drawings. In the following description, the positional relationship of the image forming apparatus in up-down, right-left and front-rear directions will be described based on a state in which the image forming apparatus is viewed from the front side, that is, viewpoint of FIG. 1.

Image Forming Apparatus

A printer 201 serving as an image forming apparatus according to a present embodiment is an electro-photographic full-color laser beam printer. The printer 201 includes, as illustrated in FIG. 1, a printer body 201A serving as an apparatus body, and a reading unit disposed above the printer body 201A for reading image data on a document.
The printer body 201A includes an image forming portion 201B for forming an image on a sheet P, a fixing portion 220 for fixing an image on the sheet P, and so on. A sheet discharge space to which the sheet P is discharged is formed between the reading unit 202 and the printer body 201A, and a sheet discharge tray 230 supporting the discharged sheets P is arranged in the sheet discharge space. Further, a sheet feeding unit 201E for feeding sheets P to the image forming portion 201B is provided on the printer body 201A. The sheet feeding unit 201E includes sheet feed cassettes 100A, 100B, 100C and 100D arranged at a lower portion of the printer body 201A and storing sheets P in cassettes 103, and a manual sheet feed apparatus 100M arranged on a right side portion of the printer body 201A. Each of the sheet feed cassettes 100A, 100B, 100C and 100D and the manual sheet feed apparatus 100M includes a pickup roller 2 for feeding sheets P, and a feed roller 3 and a retard roller 4 for separating the sheet P one by one and conveying the same.
The image forming portion 201B is a so-called four-drum full color image forming portion that includes a laser scanner 210, four process cartridges 211, and an intermediate transfer unit 201C. The process cartridges respectively form toner images of different colors, which are yellow (Y), magenta (M), cyan (C) and black (K). Each process cartridge 211 includes a photosensitive drum 212, a charger 213, a developer 214, a cleaner not shown and so on. A toner cartridge 215 storing toner of respective colors is removably attached to the printer body 201A at an area above the image forming portion 201B.
The intermediate transfer unit 201C includes an intermediate transfer belt 216 wound around a drive roller 216 a, a tension roller 216 b and so on, and the intermediate transfer belt 216 is arranged above the four process cartridges 211. The intermediate transfer belt 216 is arranged to contact the photosensitive drums 212 of the respective process cartridges 211, and driven to rotate in a counterclockwise direction, that is, direction of arrow Q, by the drive roller 216 a which is driven by a driving unit not shown. The intermediate transfer unit 201C includes primary transfer rollers 219 that contact an inner circumferential surface of the intermediate transfer belt 216 at positions opposed to respective photosensitive drums 212, and a primary transfer portion TP1 is formed as a nip portion between the intermediate transfer belt 216 and the photosensitive drum 212. Further, the image forming portion 201B includes a secondary transfer roller 217 that contacts an outer circumference surface of the intermediate transfer belt 216 at a position opposed to the drive roller 216 a. A secondary transfer portion TP2 is formed as a nip portion between the secondary transfer roller 217 and the intermediate transfer belt 216 where a toner image borne on the intermediate transfer belt 216 is transferred onto a sheet P.
In the respective process cartridges 211 configured as described above, an electrostatic latent image is formed on the surface of the photosensitive drum 212 by the laser scanner 210, and thereafter, toner is supplied from the developer 214 so that toner images of respective colors charged to negative polarity are formed. The toner images are sequentially transferred in multilayers on the intermediate transfer belt 216 at the respective primary transfer portions TP1 by having transfer bias voltage of positive polarity applied to the primary transfer roller 219, and a full color toner image is formed on the intermediate transfer belt 216.
In parallel with the above-described image forming process, the sheet P fed from the sheet feeding unit 201E is conveyed toward a registration roller pair 15 and skewing of the sheet P is corrected by the registration roller pair 15. The registration roller pair 15 conveys the sheet P to the secondary transfer portion TP2 at a matched timing with the transfer timing of the full color toner image formed on the intermediate transfer belt 216. The toner image borne on the intermediate transfer belt 216 is secondarily transferred to the sheet P at the secondary transfer portion TP2 by having transfer bias voltage of positive polarity applied to the secondary transfer roller 217.
The sheet P to which the toner image has been transferred is heated and pressed at the fixing portion 220, and the color image is fixed to the sheet P. The sheet P on which the image has been fixed is discharged by a sheet discharge roller pair 225 to the sheet discharge tray 230 and supported thereon. If it is necessary to form images on both sides of the sheet P, the sheet P having passed the fixing portion 220 is switched back by a reverse conveyance roller pair 222 that is capable of rotating in normal and reverse directions provided in a reverse conveyance portion 201D. Thereafter, the sheet P is conveyed again to the secondary transfer portion TP2 through a re-conveyance path R, and an image is formed on the rear side of the sheet P.

Manual Sheet Feed Apparatus and Skewing Correction Apparatus

Next, with reference to FIG. 2, we will describe the manual sheet feed apparatus 100M, and a skewing correction apparatus 100R including the registration roller pair 15. The pickup roller 2, the feed roller 3 and the retard roller 4 of the manual sheet feed apparatus 100M are connected to a sheet feed motor M1, and a torque limiter not shown is arranged between the sheet feed motor M1 and the retard roller 4. The pickup roller 2 and the feed roller 3 receive drive force from the sheet feed motor M1 that rotates the rollers in a direction conveying the sheet P toward the skewing correction apparatus 100R, and the retard roller 4 receives drive force toward an opposite direction from the sheet feed motor M1. In order to feed the sheet P supported on a manual sheet feed tray 7, the sheet feed motor M1 is driven in a state where the pickup roller 2 contacts the sheet P with a predetermined pressure.
The sheet P fed by the pickup roller 2 is conveyed to a separation nip 3 a formed by the feed roller 3 and the retard roller 4, and the sheet is separated one by one at the separation nip 3 a. That is, if no sheet P or one sheet P enters the separation nip 3 a, the torque limiter rotates idly, and the retard roller 4 rotates following the rotation of the feed roller 3, conveying the sheet P toward the skewing correction apparatus 100R. In a state where two or more sheets P enter the separation nip 3 a, the friction between the plural sheets P is small, such that the retard roller 4 rotates toward a direction returning the sheet P toward the manual sheet feed tray 7 and separates the multiple-transferred sheet P one by one.
The sheet P separated one by one at the separation nip 3 a is conveyed to a pre-registration roller pair 8. The pre-registration roller pair 8 serving as a first rotary member pair is connected to a pre-registration motor M2, and the pre-registration roller pair 8 is rotated by driving the pre-registration motor M2. The sheet P is conveyed toward a conveyance guide 18 by a nip 8 a serving as a first nip portion of the pre-registration roller pair 8 and guided by the conveyance guide 18 to the registration roller pair 15. A registration sensor 12 is arranged upstream in a sheet conveyance direction of a nip 15 a serving as a second nip portion of the registration roller pair 15, and the registration sensor 12 detects a leading edge position of the sheet P.
The registration roller pair 15 serving as a second rotary member pair is connected to a registration motor M3, and in a state where the leading edge of the sheet P reaches the nip 15 a of the registration roller pair 15, the registration motor M3 is stopped. Therefore, the leading edge of the sheet P is abutted against the nip 15 a of the registration roller pair 15 in the stopped state. In this state, the sheet P is further conveyed by the pre-registration roller pair 8, such that the sheet P forms a loop and skewing is corrected. The registration roller pair 15 starts to rotate after being stopped for a predetermined time, and the sheet P is conveyed by the registration roller pair 15 and the pre-registration roller pair 8 toward the secondary transfer portion TP2. Since skewing of the sheet P is corrected by the skewing correction apparatus 100R, the sheet P and the image transferred to the sheet P at the secondary transfer portion TP2 will not be skewed or laterally dislocated, and a good product can be obtained.

Controller

As illustrated in FIG. 3, a size detection sensor 11 and the registration sensor 12 are connected to an input side of a controller 9 of the printer 201. The size detection sensor 11 detects the size of the sheet P supported on the manual sheet feed tray 7 or a cassette 103. The sheet feed motor M1, the pre-registration motor M2 and the registration motor M3 are connected to an output side of the controller 9. Further, a timer 13 is connected to the controller 9.

Conveyance Control of Sheet

Next, conveyance control of the sheet P according to the present embodiment will be described. FIG. 4 is a flowchart illustrating conveyance control of the sheet P FIG. 5 is a graph illustrating theoretical positions of a leading edge and a trailing edge of the sheet at the pickup roller 2, the pre-registration roller pair 8, the registration sensor 12, the registration roller pair 15 and the secondary transfer roller 217. Line A illustrates a theoretical line indicating the tip position of the sheet P, and line B illustrates a theoretical line indicating a trailing edge position of the sheet P. Lines D, E and F respectively illustrate peripheral drive speeds of the pickup roller 2, the pre-registration roller pair 8 and the registration roller pair 15. The theoretical line B illustrating the trailing edge position of the sheet P can be calculated based on the theoretical line A of the tip position of the sheet P and a length L of the sheet P detected by the size detection sensor 11.
As illustrated in FIG. 4, if the sheet P is supported on the manual sheet feed tray 7, the controller 9 detects the size of the fed sheet P by the size detection sensor 11 (step S1). If the size of the sheet P is detected (step S1: YES), the controller 9 determines a length L of the sheet P (step S2). The controller 9 drives the sheet feed motor M1 and the pre-registration motor M2 (step S3). At this time, the peripheral drive speed of the pickup roller 2 and the pre-registration roller pair 8 are set to a sheet feed speed V1.
If the leading edge of the sheet P reaches the nip 8 a of the pre-registration roller pair 8, the controller 9 stops the sheet feed motor M1 and stops the pickup roller 2. In this state, the pickup roller 2 can be separated from the sheet P by a swing arm not shown. According to another example, an intermediate plate for supporting a sheet can be provided on the manual sheet feed tray 7, and the intermediate plate can be moved downward. According to another example, a one-way clutch can be provided to the pickup roller 2.
Then, the sheet P is conveyed toward the registration sensor 12 by the pre-registration roller pair 8. The controller 9 measures a control time, which is an elapsed time from when drive of the pickup roller 2 has been started, using a timer 13, and determines whether a leading edge of the sheet P has been detected by the registration sensor 12 within a control time T1 calculated in advance (step S4). If the leading edge of the sheet P is not detected within the control time T1 (step S4: NO), the controller 9 determines that delay jam has occurred (step S5). If it is determined that delay jam has occurred, the user is notified that jam has occurred by an alarm sound and a display on an operation panel.
If the leading edge of the sheet P is detected within the control time T1 by the registration sensor 12 (step S4: YES), the controller 9 stops the pre-registration motor M2 at control time T2 (step S6). The leading edge of the sheet P reaches the nip 8 a of the pre-registration roller pair 8 between control time T1 and control time T2, and the pre-registration motor M2 is continued to be driven even after the leading edge has reached the nip 8 a, so that the sheet P forms a loop. The controller 9 sets the control time T2 so that an amount of loop calculated in advance is formed on the sheet P.
After stopping the pre-registration motor M2 for a predetermined time so as to synchronize with the transfer timing at the secondary transfer portion TP2, the controller 9 drives the pre-registration motor M2 and the registration motor M3 at control time T3 (step S7). Thereby, the pre-registration roller pair 8 and the registration roller pair 15 are rotated at image forming speed V2, and the sheet P is conveyed toward the secondary transfer portion TP2. The image forming speed V2 is equal to a rotational speed of the intermediate transfer belt 216.
The pre-registration roller pair 8 does not have to be driven at the same timing as the registration roller pair 15 at control time T3, and driving of the pre-registration roller pair 8 can be started at a later timing than the registration roller pair 15 as long as it is started before the loop formed on the sheet P is dissolved. That is, driving of the pre-registration roller pair 8 can be started on or before control time T4, which is a time having added time X1 to control time T3, time X1 being the time when the registration roller pair 15 dissolves the loop formed on the sheet Pin a state where the pre-registration roller pair 8 is stopped.
Then, the controller 9 stops the pre-registration motor M2 at control time T5 when the trailing edge of the sheet P passes the pre-registration roller pair 8 and stops the rotation of the pre-registration roller pair 8 (step S8). Further, the controller 9 stops the registration motor M3 at control time T6 when the trailing edge of the sheet P passes the registration roller pair 15 and stops the rotation of the registration roller pair 15 (step S9). Thereby, conveyance control of the sheet P is completed.
The control times T5 and T6 are not restricted to the times at which the trailing edge of the sheet passes the pre-registration roller pair 8 and the registration roller pair 15, and the times can be somewhat varied within a range that does not affect the conveyance of the sheet P Further, conveyance control of the sheet P according to the present embodiment illustrates a case where the sheet feed speed V1 is approximately 300 mm/sec and the image forming speed V2 is approximately 250 mm/sec, but the sheet feed speed V1 and the image forming speed V2 can be varied arbitrarily.

Comparative Example

Now, a skewing correction apparatus 101R according to a comparative example will be described with reference to FIGS. 9 and 10. As illustrated in FIG. 9, the skewing correction apparatus 101R includes a pre-registration roller pair 8, a registration roller pair 15 and a conveyance guide 118 provided between the pre-registration roller pair 8 and the registration roller pair 15.
The conveyance guide 118 includes a guide surface 118 a that extends in parallel with a nip line NL2, which is a common tangent of the registration roller pair 15 at the nip 15 a, and guides the sheet P to the nip 15 a. Further, the conveyance guide 118 includes a bulged portion 118 c that curves from an upstream end 118 b of the guide surface 118 a in a sheet conveyance direction, and bulges toward a direction away from a nip line NL1, which is a common tangent of the pre-registration roller pair 8 at the nip 8 a. The bulged portion 118 c forms a loop space 119 in which the sheet P is capable of forming a loop. Further, a first opposing guide 20 and a second opposing guide 23 are arranged to oppose to the conveyance guide 118, and a curved conveyance path CP1, including the loop space 119, is formed by the conveyance guide 118, the first opposing guide 20 and the second opposing guide 23.
The nip line NL1 and the nip line NL2 intersect at an intersection 132 on the guide surface 118 a of the conveyance guide 118. That is, the nip line NL1 intersects with the guide surface 118 a. Thereby, the sheet P conveyed to a direction parallel to the nip line NL1 by the pre-registration roller pair 8 contacts the guide surface 118 a of the conveyance guide 118 near the intersection 132 and changes its path to a direction parallel to the nip line NL2 along the guide surface 118 a. In FIG. 9, the nip line NL2 and the guide surface 118 a are illustrated as if a slight gap is formed therebetween, but the gap is merely illustrated for the sake of explanation, and actually, the nip line NL2 is overlapped with the guide surface 118 a.
The sheet P forms a loop within the loop space 119 by the leading edge of the sheet P abutting against the nip 15 a of the registration roller pair 15, and by contact force of the sheet P and the conveyance guide 118, sagging of the sheet by gravity can be prevented even if the rigidity of the sheet P is low. Therefore, conveyance force of the pre-registration roller pair 8 is transmitted to the leading edge of the sheet P, and the leading edge of the sheet P is pressed against the nip 15 a, by which skewing is corrected reliably.
In FIG. 10, the sheet P having its leading edge abutted against the nip 15 a of the registration roller pair 15 and forming a loop is illustrated by a broken line, and the sheet P whose skewing is corrected and conveyed by the registration roller pair 15 and the pre-registration roller pair 8 is illustrated by a solid line. The sheet P illustrated by the broken line is subjected to skew feed control at the nip 15 a at the leading edge side, but the trailing edge side of the sheet P is still skewed, so that torsion is generated in the loop formed between the registration roller pair 15 and the pre-registration roller pair 8. Therefore, if the sheet P is continued to be conveyed by the registration roller pair 15 and the pre-registration roller pair 8, shearing force acting on the sheet P is gradually increased.
Since the shearing force applied on the sheet P is especially applied near the intersection 132, if the shearing force exceeds the rigidity of the sheet P, the sheet P is buckled from the area close to the intersection 132, and when the sheet P passes through the nip 15 a of the registration roller pair 15, creases PW tend to be formed on the sheet P. Creases PW tend to be formed on a sheet having a long size or having a low rigidity. Further, creases PW are significantly formed in a case where the distance between two sets of rollers for forming a loop on the sheet is set short or in a case where conveyance force of the two sets of rollers is set high.

Skewing Correction Apparatus

Next, the skewing correction apparatus 100R according to the present embodiment will be described with reference to FIG. 2. The skewing correction apparatus 100R includes the pre-registration roller pair 8, the registration roller pair 15, and the conveyance guide 18 provided between the pre-registration roller pair 8 and the registration roller pair 15. The pre-registration roller pair 8 is composed of a pair of rubber rollers, and nip pressure of the pre-registration roller pair 8 is set high so as to convey a sheet having a relatively high grammage, such as thick paper. The conveyance guide 18 has a different shape as the conveyance guide 118 according to the above-described comparative example.
The nip 15 a of the registration roller pair 15 is positioned upward than the nip 8 a of the pre-registration roller pair 8 in the vertical direction and at a position that differs from the nip 8 a in the horizontal direction. The conveyance guide 18 is provided on an outer side of a curved conveyance path CP between the nip 8 a and the nip 15 a. The nip line NL2 serving as a second straight line, which is a common tangent of the registration roller pair 15 at the nip 15 a intersects with a guide surface 18 a serving as a second conveyance guide portion at an intersection 31. The guide surface 18 a guides the sheet P to the nip 15 a. The intersection 31 is separated by distance h2 from the nip 15 a in a direction parallel to the nip line NL2. Further, the conveyance guide 18 includes a bulged portion 18 c bulging toward a direction away from the nip line NL1 as a first straight line which is a common tangent of the pre-registration roller pair 8 at the nip 8 a. The bulged portion 18 c, serving as a first conveyance guide portion, is formed in a curved shape and formed continuously with an upstream end 18 b, in the sheet conveyance direction, of the guide surface 18 a. It is noted that the bulged portion 18 c is provided so as to intersect with the nip line NL1. The guide surface 18 a is a flat surface which is formed linearly and non-parallelly with the nip lines NL1 and NL2 between the upstream end 18 b and a downstream end 18 d. The bulged portion 18 c forms a loop space 19 in which the sheet P can form a loop. Further, the first opposing guide 20 and the second opposing guide 23 are arranged to oppose to the conveyance guide 18, and a conveyance path CP including the loop space 19 is formed by the conveyance guide 18, the first opposing guide 20 and the second opposing guide 23. The pre-registration roller pair 8 and the registration roller pair 15 are arranged adjacently in the conveyance path CP.
If a straight line that passes the intersection 31 and has an inclination corresponding to the guide surface 18 a, that is, if a tangent which is tangent to the conveyance guide 18 at the intersection 31 is referred to as a straight line TL2, the straight line TL2 serving as a third straight line is inclined by minor angle θ₁with respect to the nip line NL2. That is, the nip line NL2 and the straight line TL2 intersect. A minor angle is a smaller angle of angles formed by two straight lines, and it is equal to or smaller than 90 degrees. Further, the straight line TL2 is inclined by minor angle θ₂, that is greater than minor angle θ₁, with respect to the nip line NL1 of the pre-registration roller pair 8. That is, the minor angle θ₂formed by the nip line NL1 serving as the first straight line and the straight line TL2 is greater than the minor angle θ₁formed by the nip line NL2 serving as the second straight line and the straight line TL2 (θ₂>θ₁). Further, an inclined surface 19 a downstream of the bulged portion 18 c formed from the upstream end 18 b of the guide surface 18 a extends along the straight line TL1, and the straight line TL2 is inclined by minor angle θ₃with respect to the straight line TL1. The loop space 19 is a space that is sufficiently greater than a pre-registration conveyance space 21 serving as conveyance space between the conveyance guide 18 and the registration roller pair 15. The nip 8 a and 15 a are positioned on one side with respect to the straight line TL2.
It is preferable to set the minor angle θ₁to approximately 10°, the minor angle θ₂to approximately 20°, the minor angle θ₃to approximately 15°, the distance h2 to approximately 10 mm and the length of the guide surface 18 a to approximately 15 to 20 mm, but the numerical values are not restricted thereto and can be varied arbitrarily.
An intersection 32 serving as a first intersection between the nip line NL1 and the straight line TL2 is separated by a distance h1 in a direction parallel with the straight line TL2 from the upstream end 18 b of the guide surface 18 a, and it is positioned within the loop space 19. That is, the intersection 32 is positioned within the conveyance path CP. Further, the nip line NL1 intersects with the inclined surface 19 a of the bulged portion 18 c, and an intersection 33 serving as a second intersection in which the nip line NL1 and the nip line NL2 intersect is positioned outside the conveyance path CP through which the sheet passes.

Skewing Correction Operation

Next, skewing correction operation of the sheet P by a skewing correction apparatus 100R according to the present embodiment will be described with reference to FIGS. 6A through 7B. The sheet P conveyed by the pre-registration roller pair 8 is conveyed to a direction parallel to the nip line NL1 and contacts the inclined surface 19 a of the bulged portion 18 c, and changes its course along the inclined surface 19 a. Further, the sheet P is transferred from the inclined surface 19 a to the guide surface 18 a, and the sheet P is guided along the guide surface 18 a to a direction parallel to the straight line TL2. That is, the leading edge of the sheet conveyed by the pre-registration roller pair 8 is guided to the guide surface 18 a by the inclined surface 19 a of the bulged portion 18 c, and thereafter, guided by the guide surface 18 a to the nip 15 a of the registration roller pair 15.
If the sheet P is further conveyed by the pre-registration roller pair 8, as illustrated in FIG. 6A, the leading edge of the sheet P is abutted against the nip 15 a of the registration roller pair 15 and the leading edge of the sheet P faces a direction parallel to the nip line NL2. In this state, a reaction force F1 acting on the leading edge of the sheet P from the nip 15 a and a contact force F2 between the sheet P and the guide surface 18 a are balanced. Further, the guide surface 18 a extends in a direction inclined by angle θ₄with respect to a gravity direction (refer to FIG. 2), contacts the sheet sagged by its own weight, and receives load from the sheet.
As illustrated in FIG. 6B, if the sheet P is conveyed further by the pre-registration roller pair 8, the sheet P forms a loop that originates from an area close to the intersection 32. In this state, a reaction force F3 acting on the leading edge of the sheet P from the nip 15 a and a contact force F4 between the sheet P and the guide surface 18 a are balanced. Further, contact force between the sheet P and the guide surface 18 a of the conveyance guide 18 prevents sagging of the sheet by gravity even if the rigidity of the sheet P is low. Therefore, conveyance force of the pre-registration roller pair 8 can be transmitted to the leading edge of the sheet P, and the leading edge of the sheet P can be pressed against the nip 15 a, by which skewing can be corrected reliably.
As illustrated in FIG. 6C, in a state where the registration roller pair 15 starts to rotate and the sheet P is conveyed by the registration roller pair 15 and the pre-registration roller pair 8, the leading edge side of the sheet is conveyed in a state where skewing is corrected along the nip 15 a. However, the trailing edge side of the sheet is still skewed, such that torsion is generated in the loop formed between the registration roller pair 15 and the pre-registration roller pair 8. In this state, a reaction force F5 applied on the sheet P from the nip 15 a and a contact force F6 between the sheet P and the guide surface 18 a are still balanced.
As illustrated in FIG. 7A, if the sheet P is conveyed further by the registration roller pair 15 and the pre-registration roller pair 8, the shearing force acting on the sheet P is increased gradually. If reaction force F7 acting on the sheet P from the nip 15 a exceeds a contact force F8 between the sheet P and the guide surface 18 a, the sheet P slips on the guide surface 18 a. Thereby, shearing force F9 which is an excess of the reaction force F7 having exceeded the contact force F8 can be released to the loop formed in the loop space 19 originating from the intersection 32, and creasing of the sheet P can be prevented. As illustrated in FIG. 7B, if the trailing edge of the sheet P passes through the nip 8 a of the pre-registration roller pair 8, the shearing force acting on the sheet P and the torsion of the loop are released.
As described, as illustrated in FIG. 2, according to the present embodiment, the nip line NL2 of the registration roller pair 15 and the guide surface 18 a of the conveyance guide 18 intersect at the intersection 31. Therefore, in a state where the sheet abuts against the nip 8 a and forms a loop, the contact force of the sheet P and the guide surface 18 a enables to press the leading edge of the sheet against the nip 8 a, and skewing of even a sheet having low rigidity, such as thin paper, can be corrected reliably.
Further, the straight line TL2 and the nip line NL1 of the pre-registration roller pair 8 intersect at the intersection 32 within the loop space 19 distant from the conveyance guide 18. Therefore, in a state where the sheet in which a loop is formed is conveyed by the registration roller pair 15 and the pre-registration roller pair 8, even if the shearing force acting on the sheet is increased, the shearing force can be released to the loop originated from the intersection 32. Thus, buckling and creasing of the sheet can be prevented.
Since the loop space 19 is sufficiently greater than the pre-registration conveyance space 21 formed between the conveyance guide 18 and the registration roller pair 15, the loop of the sheet is mainly formed within the loop space 19, and shearing force can be released reliably within the loop space 19.
As described, the embodiment is configured such that during conveyance of the sheet P, the sheet P is bent in two steps, one originating from the intersection 31 and the other originating from the intersection 32, so that even if the distance between the registration roller pair 15 and the pre-registration roller pair 8 is narrow, skewing of the sheet can be corrected reliably and creasing can be prevented. Especially in a sheet having low rigidity and long size where the conveyance distance by the pre-registration roller pair 8 is long, the shearing force can be released to the loop space 19 before the sheet is buckled, so that creasing of the sheet can be prevented. Therefore, the present embodiment is capable of corresponding to various media, such as long-size sheet, small-size sheet, thin paper, thick paper and coated paper.

Modified Example

In the above-described embodiment, the guide surface 18 a of the conveyance guide 18 and the inclined surface 19 a of the bulged portion 18 c are connected in a curved manner at the upstream end 18 b, but instead of the conveyance guide 18, conveyance guides 128 and 138 may be formed as described below.
That is, as illustrated in FIG. 8A, the conveyance guide 128 composes a contact portion 128 b between the guide surface 18 a and the bulged portion 18 c by a curved line of radius J, so that the guide surface 18 a and the inclined surface 19 a are connected smoothly. Further, as illustrated in FIG. 8B, the conveyance guide 138 includes a convex portion 138 e that protrudes inward of the conveyance path CP and a concave portion 138 d that protrudes outward of the conveyance path CP, and the inclined surface 19 a is formed continuously from the convex portion 138 e.
Even if conveyance guides 128 and 138 are formed in this manner, the intersection 32 is positioned within the loop space 19 and the intersection 33 is positioned outside the conveyance path CP. Thus, even if shearing force acting on the sheet is increased, the shearing force can be released to the loop originating from the intersection 32 and buckling and creasing of the sheet can be prevented.
According to the present embodiment, the pre-registration roller pair 8 and the registration roller pair 15 are both composed of a pair of rollers, but the present embodiment is not restricted thereto, and one or the other of the rollers may be formed of a rotary member such as a belt. Further, the conveyance guides 18, 128 and 138 may each be composed of a guide member divided into multiple parts.
All embodiments described above have been illustrated based on the electro-photographic printer 201, but the present invention is not restricted thereto. For example, the present invention can be applied to an inkjet image forming apparatus in which inks are ejected from nozzles to form images on sheets.

Other Embodiments

Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention 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.
This application claims the benefit of Japanese Patent Application No. 2017-166166, filed Aug. 30, 2017, which is hereby incorporated by reference herein in its entirety.

Claims

What is claimed is:

1. A sheet conveyance apparatus comprising:

a first rotary member pair comprising a first nip portion and configured to convey a sheet;

a second rotary member pair arranged downstream of the first rotary member pair in a sheet conveyance direction, comprising a second nip portion and configured to convey the sheet being conveyed by the first rotary member pair; and

a conveyance guide configured to guide the sheet having passed through the first nip portion to the second nip portion, and forming a curved conveyance path, the conveyance guide comprising a first conveyance guide portion and a second conveyance guide portion arranged downstream of the first conveyance guide portion in the sheet conveyance direction, the conveyance path including a loop space in which a sheet abutted against the second nip portion is capable of forming a loop,

wherein in the conveyance path, the first rotary member pair and the second rotary member pair are arranged adjacently,

the second nip portion is positioned upward than the first nip portion in a vertical direction and positioned at a position different from the first nip portion in a horizontal direction,

the conveyance guide is arranged on an outer side of the curved conveyance path between the first nip portion and the second nip portion,

the sheet conveyed by the first rotary member pair is abutted against the second nip portion so as to correct skewing of the sheet,

if a common tangent of the first rotary member pair at the first nip portion is referred to as a first straight line, a common tangent of the second rotary member pair at the second nip portion is referred to as a second straight line, and a straight line that passes an intersection of the second straight line and the conveyance guide and has an inclination corresponding to the second conveyance guide portion is referred to as a third straight line,

the second straight line and the third straight line intersect each other,

the loop space is arranged upstream of the intersection of the second straight line and the conveyance guide in the sheet conveyance direction,

a first intersection at which the first straight line and the third straight line intersect is positioned within the loop space, and

a second intersection at which the first straight line and the second straight line intersect is positioned outside the conveyance path.

2. The sheet conveyance apparatus according to claim 1, wherein a minor angle formed by the first straight line and the third straight line is greater than a minor angle formed by the second straight line and the third straight line.

3. The sheet conveyance apparatus according to claim 1, wherein the first conveyance guide portion is bulged toward a direction away from the first straight line so as to form the loop space, and

the first straight line intersects with the second conveyance guide portion.

4. The sheet conveyance apparatus according to claim 3, wherein the second conveyance guide portion is configured to contact the sheet and receive load from the sheet in a state where the sheet is abutted against the second nip portion and sagged by its own weight.

5. The sheet conveyance apparatus according to claim 3, wherein a leading edge of the sheet conveyed by the first rotary member pair is guided by the first conveyance guide portion to the second conveyance guide portion, and then, guided by the second conveyance guide portion to the second nip portion.

6. The sheet conveyance apparatus according to claim 3, wherein the first conveyance guide portion is formed in a curved shape and formed continuously with an upstream end, in the sheet conveyance direction, of the second conveyance guide portion, and

the first intersection is arranged upstream than the upstream end of the second conveyance guide portion in the sheet conveyance direction.

7. The sheet conveyance apparatus according to claim 6, wherein the second conveyance guide portion is a flat surface.

8. The sheet conveyance apparatus according to claim 7, wherein the second conveyance guide portion extends not to parallel with the first and second straight lines, and

the first and second nips are positioned on one side with respect to the third straight line.

9. The sheet conveyance apparatus according to claim 1, wherein the loop space is greater than a conveyance space formed between the conveyance guide and the second rotary member pair.

10. An image forming apparatus comprising:

the sheet conveyance apparatus according to claim 1; and

an image forming unit configured to form an image on a sheet conveyed by the sheet conveyance apparatus.