JP2001151380A - Sheet conveying device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sheet conveying device that can prevent deformation or conveying failure of a recording sheet independently of the kind of the recording sheet, prevent abrasion of an endless belt or damage or adhesion of it to a driving system, and more certainly meandering of the endless belt. SOLUTION: The sheet conveying device comprises a plurality of endless belts having an intake hole on the surface, a pair of rolls for tightly stretching and rotating each endless belt, a housing member provided between the endless belts, and a suction means for depressing the area formed with the endless belts and the housing member, and sucks and holds a sheet-like material on the surfaces of the endless belts for conveyance. A rib is disposed on the surface of the housing member contactable with the sheet-like material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sheet conveying apparatus for conveying a sheet while sucking and holding a sheet on an endless belt surface.

[0002]

2. Description of the Related Art Heretofore, there has been widely known a sheet conveying apparatus which conveys a sheet while suction-holding a sheet on an endless belt surface. For example, in an electrophotographic image forming apparatus, to transport the recording sheet from a transfer unit that transfers a toner image onto a recording sheet such as copy paper to a fixing unit that fixes the toner image on the recording sheet,
Such a sheet conveying device may be used.

FIG. 6 is a perspective view showing an example of a conventional sheet conveying apparatus. This sheet conveying device has five endless belts each stretched over a pair of rolls, and a housing exists between the respective endless belts. Each endless belt is formed with an infinite number of intake holes, and a fan (not shown) for sucking a surrounding atmosphere is provided below through the intake holes. Then, the recording sheet is sucked toward the surface formed by the endless belt and the housing, and is conveyed on the surface as the endless belt rotates.

[0004]

However, such a sheet conveying apparatus has conventionally had the following problems.

[0005] The first problem is that, depending on the type of the recording sheet, the recording sheet may be deformed or a conveyance failure may occur. FIG. 7 illustrates such a problem. FIG. 7A illustrates a state in which a recording sheet with a weak stiffness (low rigidity) is conveyed. At the time of conveyance, the endless belt is warped up and down to some extent by being sucked, and a step is generated in the height between the housing surface and the endless belt. The weak recording sheet (low rigidity) adheres to the surface of the endless belt and the surface of the housing by being sucked, so that the recording sheet may be deformed at the step portion and wrinkles may be formed in some cases.

On the other hand, FIG. 7B shows a state in which a recording sheet with a strong stiffness (high rigidity) is conveyed. At the time of conveyance, the endless belt is warped up and down to some extent by being sucked, but a strong stiff (highly stiff) recording sheet does not adhere to the housing surface or the endless belt surface, and the housing or the endless belt And a gap is created between them. When this gap is generated, it becomes impossible to exert a sufficient suction force on the recording sheet, and in some cases, the recording sheet may not be conveyed.

The second problem is that the endless belt comes into contact with the housing, and the endless belt is worn during transportation and damage to a drive system for rotating the endless belt.
There is a problem such as sticking of the endless belt to the housing when stopped.

Third, there is a problem that the endless belt cannot be prevented from meandering at the time of conveyance due to the relationship between the endless belt and the pair of rolls that stretch the belt.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object the first object of the present invention is to provide a sheet conveying apparatus capable of preventing deformation of a recording sheet and defective conveyance regardless of the type of the recording sheet. Providing the device, and secondly,
It is an object of the present invention to provide a sheet conveying device capable of preventing abrasion of an endless belt, damage to a drive system, and sticking. Third, a sheet conveying device capable of more reliably preventing meandering of the endless belt. It is to provide a device.

[0010]

SUMMARY OF THE INVENTION First, the present invention provides a plurality of endless belts having suction holes on the surface, a pair of rolls for stretching and rotating each of the endless belts, And a suction means for reducing the pressure in a region formed by the endless belt and the housing member, and a sheet conveying apparatus for suction-holding and conveying a sheet-like material on the surface of the endless belt. A sheet conveying device in which a rib is provided on a surface of the housing member with which a sheet-like material can contact (claim 1).

With this configuration of the sheet conveying device, even if the recording sheet having relatively low stiffness (low rigidity) is sucked by the suction means, the recording sheet is kept in contact with the endless belt surface and the ribs on the housing surface. Due to the close contact, the step is small, the deformation due to the step is hard to occur, and there is no possibility of wrinkling.

On the other hand, the recording sheet to be conveyed comes into contact with the ribs on the surface of the housing. Therefore, in order to reduce the conveyance resistance of the recording sheet, the ribs have a linear contact portion with the sheet-like material. Preferably (claim 2). In order to further reduce the conveyance resistance of the recording sheet, it is preferable that the direction of the linear contact portion between the rib and the sheet is parallel to the rotation direction of the endless belt.

According to the present invention, there are provided the first to third aspects of the present invention.
The height of the rib is also lower than the height of the endless belt (claim 4). By configuring the sheet conveying device in this manner, even if a relatively strong (highly rigid) recording sheet is sucked by the suction means, a gap is hardly generated between the recording sheet and the endless belt. , A sufficient suction force can be applied to the sheet, and transport failure can be prevented.

Second, the present invention provides a plurality of endless belts having suction holes on the surface, a pair of rolls for stretching and rotating each of the endless belts, and a housing member provided between the endless belts. And a suction means for reducing the pressure in an area formed by the endless belt and the housing member, wherein the endless belt comes into contact with a sheet conveying device for suction-holding and conveying the sheet-like material on the surface of the endless belt. The present invention is a sheet conveying device in which a rib is provided on a surface of the housing member which can be provided (claim 5).

Since the endless belt comes into contact with the ribs on the surface of the housing by configuring the sheet conveying device as described above, the friction during the conveyance is small, and the endless belt is worn and the driving system for rotating the endless belt is rotated. Damage,
When stopped, the contact area is small, and sticking of the endless belt to the housing can be prevented.

On the other hand, in order to reduce the friction at the time of conveyance and the contact area at the time of stop, it is preferable that the rib has a shape in which the contact portion with the endless belt is linear. Further, the direction of the linear contact portion between the rib and the endless belt is preferably parallel to the rotation direction of the endless belt. Further, even in the configuration of the endless belt, it is preferable that the endless belt surface that comes into contact with the rib is subjected to an uneven treatment.

Third, the present invention provides a plurality of endless belts having suction holes on the surface, a pair of rolls for stretching and rotating each endless belt, and a housing member provided between the endless belts. And a suction means for reducing the pressure in the area formed by the endless belt and the housing member, and form a pair of rolls in a sheet conveying apparatus for suction-holding and conveying the sheet-like material on the surface of the endless belt. Each roll is a sheet conveying device whose diameter gradually decreases from the center to the end (Claim 7).

By configuring the sheet conveying device in this way, a meandering correction action acts not only on one roll but also on the other roll, so that the endless belt meandering during conveyance can be more effectively prevented. Can be.

[0019]

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of an electrophotographic multifunction machine (image forming apparatus) to which the sheet conveying device according to the present embodiment is applied. Note that this multifunction peripheral has an image reading unit (not shown) and has both a function as a copier and a function as a laser beam printer. The configuration and operation of the laser beam printer will be described below.

This laser beam printer has four process units 10Y, 10M, 10C, which form toner images for each color of black, yellow, magenta and cyan.
10K, and an intermediate transfer belt 20 on which a toner image is primarily transferred from each process unit. The toner image multiplex-transferred onto the intermediate transfer belt 20 is secondarily transferred onto a recording sheet P to form a full-color image. It is configured as follows.

Each of the process units 10Y, 10M, 10M
C and 10Bk are arranged in parallel so that a toner image formed according to image information of each color is primarily transferred to the intermediate transfer belt 20. These four process units are arranged along the rotation direction of the intermediate transfer belt 20 so that yellow 10Y, magenta 10M, cyan 10C, and black 1Y.
They are arranged in the order of 0Bk.

Each of the process units 10Bk, 10Y, 1
0M and 10C denote a photosensitive drum 11 as an image carrier;
A charging device 12 for charging the photosensitive drum 11 to a uniform background potential; and a ROS unit 1 for forming an electrostatic latent image on the photosensitive drum 11 by exposure to a laser beam Bm.
3, a developing device 14 for developing the electrostatic latent image on the photosensitive drum 11 to form a toner image, and electrostatically transferring the toner image on the photosensitive drum 11 onto the intermediate point copying belt 20 A primary transfer roll 15 and a photoconductor cleaner 16 for cleaning the surface of the photoconductor drum 11 after the transfer are provided.

The photoreceptor drum 11 has a structure in which an organic photosensitive layer is formed on a thin metal cylindrical drum. The charging device 12 includes a scorotron charger. The ROS unit 13 includes a semiconductor laser that emits a laser beam Bm modulated according to image information of each color, and a polygon mirror that rotates at a high speed and scans the laser beam Bm along the axial direction of the photosensitive drum 11. , Its laser light B
After traveling along a predetermined path while being reflected by the mirror, m exposes the photosensitive drum 11 through the scanning window.

Each developing device 14 is filled with a mixture of toner of each color of black, yellow, magenta, and cyan and a carrier obtained by coating a magnetic material with a semiconductive material. By friction between the toner and the carrier, the toner is negatively charged. The toner image is formed on the photosensitive drum 11 in accordance with the image information of each color.

The primary transfer roll 15 is provided on the photosensitive drum 11
The intermediate transfer belt 20 is disposed so as to sandwich the intermediate transfer belt 20 therebetween, and a predetermined transfer bias current is applied to the transfer roll 15 so that the photosensitive drum 11 and the transfer roll An electric field is formed between the intermediate transfer belt 15 and the toner image charged on the photosensitive drum 11 by the Coulomb force. Each of the primary transfer rolls 15 is configured by covering a metal shaft with a conductive foam rubber.

The intermediate transfer belt 20 is formed endless and has a pair of belt transport rolls 21, 22a to 22a.
c, 23, and while rotating in the direction of the arrow, the respective color process units 10Y, 10M, 10C, 10B
It is configured to receive the primary transfer of the toner image formed by k. The material of the intermediate transfer belt 20 is polyimide, and is imparted with conductivity by containing an appropriate amount of carbon black. It is necessary that the size is larger than the maximum print image size, and the circumference is preferably an integral multiple of the circumference of the photoconductor drum 11 from the viewpoint of preventing color misregistration of each color toner image.

A secondary transfer roll 30 is provided at a position facing one of the belt transport rolls 21 with the intermediate transfer belt 20 interposed therebetween, and the recording sheet P is pressed against the transfer roll 30 and the intermediate transfer belt. 20 to receive the secondary transfer of the toner image from the intermediate transfer belt 20. That is, the belt transport roll 21 functions as a backup roll of the transfer roll 30. Note that the transport roll 22b is also a drive roll, and the transport roll 23 is also a steering roll.

A cleaner 24 for the intermediate transfer belt 30 is provided at a position facing the belt transport roll 22b so that toner remaining on the intermediate transfer belt 20 after the secondary transfer is removed from the intermediate transfer belt 20. It is configured.

On the other hand, as a conveyance system for the recording sheets P, a recording sheet tray 40 for accommodating a plurality of recording sheets P, a pickup roll 41 for taking out the recording sheets P one by one from the recording sheet tray 40, A pair of transport rolls 42a to 42c for sequentially transporting, a pair of registration rolls 43 for starting transport at a predetermined timing, a sheet chute 44 for regulating a transport path of the recording sheet P, and a sheet transporting device for transporting the transferred recording sheet P to the fixing device 5. 6a, b
It has. Further, the fixing device 5 includes a heating roll 51 and a pressure roll 52.

In forming a full-color image by the multi-functional apparatus having the above-described configuration, first, each ROS unit 10 causes the photoconductor drums 11 of the process units 10Y, 10M, 10C, and 10Bk to correspond to image information of each color.
Is exposed at a predetermined timing to form an electrostatic latent image, and the latent image is developed, whereby each process unit 1
On the photoconductor drums 11 of 0Y, 10M, 10C, and 10Bk, toner images corresponding to image information are formed.

Each of the process units 10Bk, 10Y, 1
The toner images formed at 0M and 10C are sequentially transferred to the rotating intermediate transfer belt 20, and a multi-toner image in which toner images of respective colors overlap is formed on the intermediate transfer belt 20.

On the other hand, the recording sheet P is sent out from the paper feed cassette 40 at a predetermined timing, passes through a pickup roll 41 and a pair of transport rolls 42a to 42c, and the toner image primarily transferred onto the intermediate transfer belt 20 is secondarily transferred. At the timing of reaching the position, the registration roll pair 43 starts rotating and is inserted between the secondary transfer roll 30 and the intermediate transfer belt 20.

Thus, the multiple toner image on the intermediate transfer belt 20 is secondarily transferred to the recording sheet P. Then, the recording sheet P on which the secondary transfer has been performed is transferred to the sheet conveying device 6a.
b, the toner image is conveyed to the fixing device 5, and the fixing device 5 fixes the toner image.
A full color image is completed on top.

Embodiments The present invention relates to the sheet conveying devices 6a and 6b. Hereinafter, a more detailed configuration and operation of the sheet devices 6 (a and b) will be described as examples. .

FIG. 2 shows a sheet conveying device 6 according to this embodiment.
It is a perspective view which shows the outline of a structure of. As shown in the figure,
The sheet conveying device 6 includes a housing main body 6A serving as a skeleton of the apparatus, roll shafts 64 and 65 attached at predetermined intervals in the sheet conveying direction of the housing main body 6A, and a roll shaft 65 on the upstream side in the conveying direction at predetermined intervals. Five driven rolls 62 (a to e) and five drive rolls 61 provided at predetermined intervals on a roll shaft 64 on the downstream side in the transport direction.
(A) to (e), the driven roll 62 and the drive roll 6
1 and an endless belt 60 (a to e) stretched by the belt 1.

The surface of each endless belt 60 (a to e) has countless air intake holes. In addition, each belt 60 (a to
The housing surface 63 is exposed between e). Furthermore, housing body 6A (including housing surface 63)
A vacuum area is formed by the endless belts 60 (a to e), and a fan unit (suction means) not shown in FIG. It has a duct (not shown) that discharges outside the printer.

Hereinafter, AA ', BB' and CC 'in FIG.
The characteristic configuration and operation of the sheet conveying device 6 shown in FIG. 2 will be described with reference to a sectional view.

FIG. 3 schematically shows the configuration of the cross section taken along line AA 'in FIG. 2 as a state in which the recording sheet P is being conveyed. As shown in the figure, a housing surface 63 is present at each end of the endless belt 60. Each housing surface 63 includes a surface portion 63 (U) existing outside the endless belt 60 and a surface portion 63 (L) existing inside the endless belt 60. The difference Δ (UL) between the heights of both surface portions 63 (U) (L) is 0 [mm] <Δ (UL) <1 [m
m].

Two ribs 66 are formed on each surface portion 63 (U) in parallel with the rotation direction of the endless belt 60. The tip of the rib 66 is formed in a round shape, and the contact portion with the conveyed recording sheet P is linear. On the other hand, one rib 67 is formed on each surface portion 63 (L) in parallel with the rotation direction of the endless belt 60. The tip of the rib 67 is formed in a round shape, and the contact portion with the endless belt 60 is linear. Further, the portion of the endless belt 60 that comes into contact with the rib 67 is subjected to a texture process by embossing.

Here, the endless belt 6 in the transport state
The height for 0 surface portions 63 (U) B, and the height relative to the surface portion 63 of the rib 66 (U) and L _1, 0.
5 [mm] <B <1.5 [mm], and 0 [m
m] <L ₁ <1 [mm]. Further, these heights are determined so as to satisfy the relationship of L ₁ <B. Also when the height relative to the surface portion 63 of the rib 67 (L) and L _2, 0 (mm) <L ₂ <about 1 mm.

Next, the operation achieved by such a configuration will be described. First, due to the two ribs 66 provided on the front surface portion 63 (L), it is possible to prevent the deformation of the recording sheet and the conveyance failure regardless of the type of the recording sheet. That is, as shown in FIG. 3A, when the stiffness of the conveyed recording sheet P is low (the stiffness is low), the recording sheet P tries to stick downward while being sucked, but the surface portion 63 ( It is supported by the rib 66 provided in L) and does not significantly deform. Further, as shown in FIG. 3B, when the stiffness of the P of the recording sheet to be conveyed is high (the stiffness is high), the recording sheet P has the height L ₁ of the rib 66 <the height B of the endless belt 60. Since the relationship is satisfied, a gap generated between the endless belt 60 and the recording sheet P is small, and it is possible to prevent a conveyance failure of the recording sheet P.

Next, due to the rib 67 provided on the surface portion 63 (U), the endless belt 60 and the housing 63
Since the contact area with the endless belt 60 is reduced, it is possible to prevent wear of the endless belt 60 at the time of conveyance, damage to the drive system, and sticking at the time of stopping. Similarly, due to the unevenness processing performed on the endless belt 60, the endless belt 6
Since the contact area between the housing 0 and the housing 63 is reduced, it is possible to prevent wear of the endless belt 60 during transport, damage to the drive system, and sticking during stoppage.

FIG. 4 schematically shows the configuration of the cross section taken along the line BB 'in FIG. 2 with the endless belt 60 omitted. Although only the drive roll 61 is shown in the figure, the driven roll 62 has basically the same configuration. As shown in the figure, the diameter of the drive roll 61 gradually decreases from the center to the end (in the axial direction). Further, the diameter is configured to decrease more rapidly toward the end. Here, assuming that the maximum diameter at the center of the drive roll 61 is D ₁ and the minimum diameter at the end is D ₂ , 20.9 [mm]
<D ₁ <21.1 [mm], 19.4 [mm] <
D ₂ is about 19.8 [mm].

Next, the operation achieved by such a configuration will be described. Due to the formation of both rolls 61 and 62 in such a shape, a force acts to correct the meandering of the rotating endless belt 60 in any of the rolls 61 and 62, and the meandering of the endless belt 60 is reduced. It can be prevented more reliably.

FIG. 5 schematically shows the structure of the cross section taken along the line CC 'in FIG. FIG. 5A shows one embodiment. In this embodiment, the power supply 69 is attached to the roll shaft 64.
DC voltage is applied from a. A current (+ in this embodiment) having the same polarity as the polarity of the toner on the recording sheet P being conveyed is applied. And the roll shaft 6
4. Each of the drive roll 61 and the endless belt 60 is conductive or semiconductive. Note that the voltage supplied from the power supply unit 69a may be only a DC voltage as in this embodiment, or may be a voltage in which an AC voltage is superimposed on the DC voltage. Alternatively, a voltage may be applied to the roll shaft 65. Note that a fan unit 68 is provided below the endless belt 60.

FIG. 5B shows another embodiment. In this embodiment, a DC voltage is applied to the fixed brush 69b from the power supply unit 69a. The fixed brush 69b includes a metal pedestal and a brush implanted on the pedestal. This brush is formed by mixing insulating fibers made of a resin such as polypropylene and fibers having a predetermined conductivity by mixing such a resin with carbon black or the like. As for the polarity of the voltage applied to the fixed brush 69b, a current (+ in this embodiment) having the same polarity as the polarity of the toner on the conveyed recording sheet P is applied. The endless belt 60 is semiconductive. Note that the voltage supplied from the power supply unit 69a may be only a DC voltage as in this embodiment, or may be a voltage in which an AC voltage is superimposed on the DC voltage.

With such a configuration, it is possible to prevent the toner from adhering to the endless belt 60.

Lastly, the operation of the sheet conveying device 6 configured as described above will be briefly described. First, fan unit 68
The inside fan rotates, and the pressure in the vacuum area including the endless belts 60 (a to e) and the housing surface 63 is reduced. When the pressure in the vacuum region is reduced, an air flow is generated from the intake holes 600 of each endless belt 60 to the duct (not shown) through the vacuum region and the fan unit 68. Due to the suction effect by this air flow, the recording sheet P holding the unfixed full-color toner image on the surface of the endless belt 60 and the housing surface 63 is held by suction.
On the other hand, the endless belt 60 rotates with the rotation of the rolls 61 and 62. As a result, with the rotation of the endless belt 60, the recording sheet P is conveyed while being suction-held on the surface of the endless belt 60.

[0049]

As described above in detail, according to the present invention, first, there is provided a sheet conveying apparatus capable of preventing deformation of a recording sheet and defective conveyance regardless of the type of the recording sheet. Secondly, it is to provide a sheet conveying device capable of preventing wear of the endless belt, damage to the drive system, and sticking, and thirdly, to more surely meander the meandering of the endless belt. Thus, a sheet conveying apparatus that can prevent the above problem can be provided.

[Brief description of the drawings]

FIG. 1 illustrates the configuration of an image forming apparatus to which a sheet conveying device according to the present invention can be applied.

FIG. 2 is a perspective view of the sheet conveying apparatus according to the embodiment.

FIG. 3 schematically illustrates the configuration of a cross-sectional view taken along line AA ′ in FIG. 2 as a state in which a recording sheet is being conveyed.

FIG. 4 is a schematic view of the configuration of a cross-sectional view taken along line BB ′ in FIG. 2, omitting an endless belt.

FIG. 5 schematically shows the configuration of a cross-sectional view taken along the line CC ′ in FIG. 2;

FIG. 6 is a perspective view of a conventional sheet conveying device.

FIG. 7 illustrates a problem of a conventional sheet conveying device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 30 ... Transfer roll, 5 ... Fixing device, 6a, 6b ... Sheet conveyance device, 6A ... Housing main body, 60 ... Endless belt, 600 ... Suction hole, 61 ... Drive roll, 62 ... Driving roll, 63 ... Housing surface, 66 ... Rib, 67 ... Rib, 68 ... Fan unit,

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masaaki Takahashi 2274 Hongo, Ebina-shi, Kanagawa Prefecture, Fuji Xerox Co., Ltd. (72) Inventor Minoru Niwa 2274 Hongo, Ebina-shi, Kanagawa Prefecture, Fuji Xerox Co., Ltd. Ebina Works (72) Inventor Naoto Yoshino 2274 Hongo, Ebina City, Kanagawa Prefecture, Fuji Xerox Co., Ltd. (72) Inventor Yukio Hayashi 2274 Hongo, Ebina City, Kanagawa Prefecture, Fuji Xerox Co., Ltd. 72) Inventor Takashi Kawabata 2274 Hongo, Ebina-shi, Kanagawa, Fuji Xerox Co., Ltd.Ebina Office (72) Inventor Shuji Iseki 2274 Hongo, Ebina-shi, Kanagawa Prefecture, Fuji Xerox Co., Ltd. Person Masao Okubo Ebina City, Kanagawa Prefecture 2274 Hongo, Fuji Xerox Co., Ltd. Ebina Works F term (reference) 3F049 AA02 BA04 BB06 BB11 DA05 LA01 LB03 3F101 FB15 FC05 LA01 LB03

Claims (7)

[Claims] 1. A plurality of endless belts having suction holes on the surface, a pair of rolls for stretching and rotating each endless belt, a housing member provided between each endless belt, and the endless belt And a suction means for reducing the pressure in an area formed by the housing member, wherein the housing member capable of contacting the sheet material is provided in the sheet conveying device for suction-holding and conveying the sheet material on the surface of the endless belt. A sheet conveying device characterized in that a rib is provided on a surface of the sheet. 2. The sheet conveying apparatus according to claim 1, wherein the rib has a shape in which a contact portion with the sheet-like material is linear. 3. The direction of the linear contact portion between the rib and the sheet-like material is parallel to the rotation direction of the endless belt.
3. The sheet conveying device according to 1. 4. The sheet conveying apparatus according to claim 1, wherein a height of the rib is lower than a height of the endless belt. 5. A plurality of endless belts having suction holes on the surface, a pair of rolls for stretching and rotating each endless belt, a housing member provided between each endless belt, and the endless belt. And a suction means for decompressing the area formed by the housing member, wherein the housing member capable of coming into contact with the endless belt in a sheet conveying apparatus for suction-holding and conveying the sheet-like material on the surface of the endless belt A sheet conveying device characterized in that a rib is provided on a surface of the sheet. 6. The sheet conveying device according to claim 5, wherein the endless belt surface that comes into contact with the rib is subjected to an unevenness treatment. 7. A plurality of endless belts having suction holes on the surface, a pair of rolls for stretching and rotating each endless belt, a housing member provided between each endless belt, and the endless belt. And a suction means for reducing the pressure in the area formed by the housing member, and in a sheet conveying device for suction-holding and conveying the sheet-like material on the surface of the endless belt, each roll constituting a roll pair has A sheet conveying device characterized in that the diameter gradually decreases from the center to the end.