Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2006-95763 filed on Mar. 30, 2006, the entire contents of which are incorporated herein by reference. 
   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus and a sheet conveyor and more particularly to a sheet conveyor, when transferring a developer image onto a sheet, regardless of a thickness of the sheet, for preventing a displacement of the developer image, an image forming apparatus having the sheet conveyor, and a sheet conveying method. 
   2. Description of the Related Art 
   In an image forming apparatus such as a color copy machine or a printer, an image forming apparatus of a type of arranging a plurality of image forming units along a photoconductor in a belt shape or sheet shape, superimposing developer images formed by the respective image forming units on the photoconductor in the belt shape or sheet shape, transferring them onto a sheet in a batch, thereby obtaining a color image is known. 
   In an image forming apparatus for transferring developer images onto a sheet, thereby obtaining image output, a method for stopping once the sheet guided toward the image transfer position before the image transfer position and aligning the sheet position to the developer image position is used widely. Further, it is known that an error in the stop position when the sheet is stopped once before the image transfer position causes a displacement of the developer image (from the sheet) transferred to the sheet, that is, a variation in the size of the marginal portion. 
   Therefore, as disclosed in Japanese Patent Application Publication No. 2000-95394, it is proposed to install a brake clutch in at least one of a pair of rollers called an aligning roller pair for temporarily stopping a sheet and prevent a shock at start time of rotation of the aligning roller pair. 
   On the other hand, it is known that when a sheet is thicker than a fixed thickness and the rear end of the sheet is separated from the rollers of the aligning roller pair (gets out of the position pressurized between the rollers), the respective rollers are applied with force by which the rollers intend to closely contact to each other by the pressure applied to the rollers, thereby are pressed out (the speed for conveying the sheet is increased temporarily). This varies the speed of the sheet being conveyed at the image transfer position, thus the transferred developer image is displaced. 
   Further, by the art disclosed in Japanese Patent Application Publication No. 2000-95394 aforementioned, the aforementioned temporary increase in the conveying speed caused when the sheet is thicker than the fixed thickness and the displacement of the transferred developer image due to a change in the speed of the sheet conveyed at the image transfer position are not canceled. 
   SUMMARY 
   An object of the present invention is to provide an image forming apparatus and a sheet conveyor, when transferring a toner image onto a sheet, even if the sheet is thicker than a fixed thickness, for preventing a displacement of the toner image. 
   According to the embodiments of the present invention, there is provided a sheet conveyor comprising a first roller; a second roller pressurized to the first roller to hold and convey a sheet, wherein the first roller and the second roller are separated at a space equivalent to a thickness of the sheet when the sheet is held between the first roller and the second roller; and a pressurizing delay mechanism configured to bring controllably the first roller and the second roller into contact with each other after the sheet is conveyed from the first roller and the second roller. 
   According to the embodiments of the present invention, there is provided an image forming apparatus comprising a transfer unit configured to transfer a toner image onto a sheet; and a sheet conveyor configured to send the sheet toward the transfer unit at predetermined timing, wherein the sheet conveyor includes a first roller; a second roller pressurized to the first roller to hold and convey the sheet, wherein the first roller and the second roller are separated at a space equivalent to a thickness of the sheet when the sheet is held between the first roller and the second roller; and a pressurizing delay mechanism configured to bring controllably the first roller and second roller into contact with each other after the sheet is conveyed from the first roller and second roller. 
   According to the embodiments of the present invention, there is provided a sheet conveying method comprising separating, when a sheet is held between a first roller and a second roller pressurized to the first roller to hold and convey the sheet, the first roller and the second roller at a space equivalent to a thickness of the sheet; and bringing controllably the first roller and second roller into contact with each other after the sheet is conveyed from the first roller and second roller. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view showing an example of the image forming apparatus to which an embodiment of the present invention is applied; 
       FIG. 2A  is a perspective view showing the constitution of the aligning roller pair (aligning roller pressurizing delay mechanism) incorporated in the color copy machine shown in  FIG. 1 ; 
       FIG. 2B  is a front view showing the space setting cam used in the aligning roller pressurizing delay mechanism shown in  FIG. 2 ; 
       FIGS. 3A to 3C  are schematic views for explaining the pressurizing delay mechanism of the aligning roller pair shown in  FIG. 2 ; 
       FIGS. 4A to 4C  are schematic views for explaining another embodiment of the pressurizing delay mechanism of the aligning roller pair; 
       FIG. 5  is a graph for explaining an example of changes of the sheet conveying speed when the aligning roller pressurizing delay mechanism shown in  FIG. 2  is used; and 
       FIG. 6  is a schematic view showing the image forming apparatus in which a cover is opened. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Hereinafter, the embodiments of the present invention will be explained with reference to the accompanying drawings. Further, to the same parts in the drawings, the same numerals are assigned and duplicate explanation will be omitted. 
     FIG. 1  schematically shows an example of the image forming apparatus to which an embodiment of the present invention is applied. 
   As shown in  FIG. 1 , the image forming apparatus, that is, a color copy machine  1  includes a scanner unit  2  and an image forming unit  3 . The scanner unit  2  grasps image information of a copying object as light and shade and forms an image signal on the basis of the light and shade. The image forming unit  3 , on the basis of the image signal, forms a developer image (hereinafter, referred to as a toner image) on a sheet P as an output image. 
   Under the image forming unit  3 , the image forming apparatus has sheet cassettes  4  for taking out sheets P one by one. In correspondence to the timing of forming an output image, the sheets P are supplied one by one from the sheet cassettes  4  to the image forming unit  3 . 
   Between the sheet cassettes  4  and the image forming unit  3 , a conveying path  5  for guiding the sheets P from the sheet cassettes  4  to the image forming unit  3  is provided. The conveying path  5 , via an image transfer position  6 A where a toner image formed by the image forming unit  3  is transferred on the sheet P, guides the sheet P to a fixing device  6  for fixing the toner image transferred to the sheet P on the sheet P. The sheet P on which the toner image is fixed, is discharged on a sheet discharge unit  1   a  from the conveying path  5 . 
   The image forming unit  3  has, for example, an intermediate transfer belt  11  of an insulating film formed in a belt shape. The intermediate transfer belt  11  can use a sheet-shaped thin metal whose surface is protected by resin. 
   The intermediate transfer belt  11  is given a predetermined tension by a driving roller  12 , a first tension roller  13 , and a second tension roller  14 . The intermediate transfer belt  11  is moved in the direction of an arrow A by rotation of the driving roller  12 . The surface of the intermediate transfer belt  11  is circulated in one direction at the moving speed of the outer peripheral surface of the driving roller  12 . 
   In the section where the surface of the intermediate transfer belt  11  moves substantially in a plane shape, first to fourth image forming units  21 ,  22 ,  23  and  24  are arranged at predetermined spaces. Further, in the example shown in  FIG. 1 , in the section where the intermediate transfer belt  11  moves substantially in the plane shape between the driving roller  12  and the first tension roller  13 , the first image forming unit  21  is positioned on the side of the driving roller  12  and the fourth image forming unit  24  is positioned on the side of the first tension roller  13 . 
   The first to fourth image forming units  21  to  24  respectively include at least the developing devices for storing toners of colors of C (cyan), M (magenta), Y (yellow) and BK (black) and photoconductors for holding electrostatic images to be developed by the respective developing devices. On the photoconductors of the respective image forming units, electrostatic images of the colors to be developed by the developing devices in the image forming units are formed by image light from an exposure unit  31  and then toner images are formed by the corresponding developing devices. 
   Transferring rollers  41  to  44  for transferring the toner images formed on the respective photoconductors of the image forming units  21  to  24  to the intermediate transfer belt  11  are arranged on the rear side of the intermediate transfer belt  11 . In the respective image forming units  21  to  24 , so that the toner images transferred sequentially are mutually superimposed on the intermediate transfer belt  11 , electrostatic images are formed on the intermediate transfer belt at predetermined timing and are developed by the developing devices. 
   By the intermediate transfer belt  11  and a transfer roller  51  pressurized to the intermediate transfer belt  11 , the image transfer position  5 A is formed on the conveying path  5 . The toner images superimposed on the intermediate transfer belt  11  are guided by the image transfer position  5 A and are transferred onto the sheet P. The transferring roller  51 , when no toner images are transferred onto the sheet P, by a roller retreating mechanism not drawn, is retreated to a position far from the intermediate transfer belt  11 . 
   At a predetermined position on the conveying path  5  between the sheet cassette  4  and the image transfer position  5 A, an aligning roller pair  61  is installed. The aligning roller pair  61  temporarily stops the sheet P guided from the sheet cassette  4  to the image transfer position  5 A. With respect to the aligning roller pair  61 , one side of the roller is rotated in a predetermined direction and the other of the roller is pressed to one side of the roller at a predetermined pressure via a pressurizing mechanism not drawn. 
   The sheet P conveyed from the sheet cassette  4  to the image transfer position  5 A is stopped once by the aligning roller pair  61 , thus an inclination (in the conveying direction) which may be caused during conveying on the conveying path  5  from the sheet cassette  4  is corrected. 
   In correspondence to the timing of rerotation of the aligning roller pair  61 , the timing of approaching of the toner image conveyed toward the image transfer position  5 A by the intermediate transfer belt  11  to the image transfer position  5 A and the timing of arrival of the sheet P at the image transfer position  5 A are set. By this setting, the position of the toner image to the sheet P is set (the position of the toner image on the sheet P can be set optionally). 
   When the sheets P are thicker than the predetermined thickness, the moment the rear ends of the sheets P conveyed toward the fixing device  6 , while the toner image is transferred at the image transfer position  5 A, come out of the rollers of the aligning roller pair  61 , macroscopically, the thick portion of the rear ends of the sheets P is given pressure from the rollers intending to closely contact to each other. 
   At this time, the conveying speed of the sheets P is increased temporarily, and although the toner image is transferred at the image transfer position  5 A, the sheets P are pressed out rapidly toward the fixing device  6 . This causes a displacement, that is, a transfer variation in the toner image transferred onto the sheets P at the image transfer position  5 A. Further, when the image output is continued two times or more, at time of an occurrence of a transfer variation, the intermediate transfer belt, photosensitive drum, and laser scanner unit also vibrate and a primary transfer vibration and an exposure displacement may be also caused. Further, a mechanism of reducing the pressure between the rollers of the aligning roller pair before complete separation of the rear ends of the sheets P from the rollers (after the sheets P pass between the rollers, the rollers are prevented from closely contacting to each other for a predetermined period of time) is installed, thus the aforementioned phenomenon of a temporary increase in the conveying speed of the sheets P can be prevented. However, the mechanism of maintaining the distance between the rollers for the predetermined period of time until the next sheet P is guided in order to reduce the pressure between the rollers is complicated. 
     FIG. 2A  shows the constitution of the aligning roller pair used in the color copy machine shown in  FIG. 1 . 
   As shown in  FIG. 2A , the aligning roller pair  61  are composed of a first roller  62  and a second roller  63  which are closely contacted to each other by a predetermined pressure. Further, in the example shown in  FIG. 2 , the first roller  62  is rotated in the direction of an arrow B and the second roller  63  is rotated in the direction of an arrow C in the opposite direction of the arrow B. Therefore, the moving directions of the roller surfaces of the respective rollers are the same direction at a position  61 A (shown in  FIG. 3A ) where both rollers make contact with each other. 
   At a predetermined position of a rotary shaft  62   b  of the first roller  62 , a space setting cam  62   a  which is a pressurizing delay mechanism is provided slidably. The rotary shaft of the second roller  63  functions as a space referring roller  63   a . Further, the space referring roller  63   a  may be formed by forming the rotary shaft in a predetermined thickness. 
   The space setting cam  62   a  is given a predetermined pressure in the direction of an arrow D, for example, by an extension spring  64 . By this pressure, the space setting cam  62   a  is rotated up to a position, balanced between the space setting cam  62   a  and the space reference roller  63   a  of the second roller  63 , having a predetermined rotational angle. As shown in  FIG. 2B , the space setting cam  62   a , in the size from the rotational center thereof to the cam surface  62   c , is given different outside diameters of a dimension Df on the side of an arrow F and a dimension De on the side of an arrow E. Namely, the dimension Df on the side of the arrow F is specified to be larger than the dimension De on the side of the arrow E. Separately from this, it is possible to give, for example, a predetermined weight for enabling a predetermined amount of rotation in the direction of the arrow D to the space setting cam  62   a , thereby balance the rotational angle between the space setting cam  62   a  and the space reference roller  63   a  of the second roller  63 . For example, the space setting cam  62   a  is weighted on the position thereof on the arrow E, thereby can be given a predetermined weight. 
   Further, the difference between the outside diameter on the side of the arrow E and the outside diameter on the side of the arrow F, in the copy machine  1 , is specified on the basis of the thickness of a thickest sheet which may be used for image output. For example, the difference between the outside diameter De on the side of the arrow E of the space setting cam  62   a  and the outside diameter Df on the side of the arrow F, in the color copy machine  1 , when the thickness of the thickest sheet which may be used for image forming is 250 μm, is specified to be at least 250 μm. 
   The space setting cam  62   a  is made of abrasion resistant resin such as polyacetal and the cam surface  62   c  thereof is always in contact with the space reference roller  63   a . Further, the space reference roller  63   a  is a roller having a surface composed of resin or rubber given a friction coefficient in a fixed relationship to the friction coefficient of the surface of a sheet using the friction coefficient of a metal or the roller surface for image output. 
     FIGS. 3A to 3C  show the relationship between the space between the rollers of the aligning roller pair shown in  FIG. 2  and the aligning roller pressurizing delay mechanism, that is, the rotational angle of the space setting cam. 
   The force in the opposite direction (the direction of the arrow C) to the can surface  62   c  of the space setting cam  62   a  given a predetermined pressure in the direction of the arrow D is given to the space reference roller  63   a . Therefore, when there are no sheets P between the roller  62  of the aligning roller pair  61  and the roller  63  (the sheets P do not reach between the rollers of the aligning roller pair), as shown in  FIG. 3A , the space setting can  62   a , at the position at a predetermined rotational angle balanced with the rotation in the direction of the arrow C of the space reference roller  63   a , is in the state against the rotation of the space reference roller  63   a.    
   On the other hand, as shown in  FIG. 3B , since there is a sheet P between the rollers  62  and  63  of the aligning roller pair  61  (the sheet P is guided between the rollers of the aligning roller pair), the roller  62  and roller  63  are separated at a space corresponding to the thickness of the sheet P. By this separation, the space setting cam  62   a , at the position rotated at a predetermined angle related to the thickness of the sheet P, is moved into the state against the rotation of the space reference roller  63   a . Namely, the space setting cam  62   a , due to the thickness of the sheet P, in correspondence to the space between the roller  62  and the roller  63  in the state that the space to the roller  62  is spread, by sliding of the surface, is rotated up to the position balanced with the rotary power of the roller  63 . 
   On the other hand, as shown in  FIG. 3C , when the sheet P existing between the roller  62  and the roller  63  of the aligning roller pair  61  is completely separated from the rollers (the sheet P gets out of the rollers of the aligning roller pair), by the rotation of the space reference roller  63   a  rotating in the direction of the arrow C, the space setting cam  62   a  is pressed back in the direction of the arrow F. 
   At this time, to the space setting cam  62   a , a predetermined force in the direction of the arrow D by the extension spring  64  is given. By the given predetermined force, the space setting cam  62   a , after the sheet P is completely separated from the rollers, for a predetermined period of time specified on the basis of the difference between the rotary power of the roller  63  in the direction of the arrow C and the force by the spring  64  in the direction of the arrow D, due to the rolling contact between the surface of the space setting cam  62   a  and the surface of the space reference roller  63   a , to the position where there is no sheet P as shown in  FIG. 3A , is returned comparatively controllably. Therefore, the roller  62  of the aligning roller pair  61  and the roller  63  are not closely contacted to each other the moment the sheet P passes through the rollers but are closely contacted after a lapse of a fixed period time. 
   As mentioned above, on either of the rollers  62  and  63  of the aligning roller pair  61 , the space setting cam is installed and on another, a stopper (a space reference unit with a large outside diameter) is installed, thus immediately after passing of the sheet P through the rollers, the space between the two rollers of the aligning roller pair is prevented from suddenly returning (the rollers of the aligning roller pair are closely contacted to each other). 
   By doing this, the sheet P to which the toner image is transferred at the image transfer position  6 A is prevented from suddenly pressed out toward the fixing device  6  and the toner image transferred at the image transfer position  5 A can be prevented from an occurrence of a displacement, that is, a transfer variation. Furthermore, a primary transfer variation and an exposure variation can be prevented. 
   Further, as shown by a curve a in  FIG. 5 , by use of the aligning roller pair pressurizing delay mechanism (the cam  62   a  and reference roller  63   a ) explained in  FIGS. 2 and 3A  to  3 C, compared with a curve b showing an example of a case that the pressurizing delay mechanism is not used, it is confirmed that the conveying speed of the sheet P is prevented from changing. 
   As explained above, the moment the rear end of the sheet conveyed toward the fixing device by transferring the toner image at the image transfer position gets out of the rollers of the aligning roller pair, the conveying speed of the sheet is prevented from increasing suddenly. By doing this, the toner image transferred can be prevented from an occurrence of a displacement, that is, a transfer variation. Furthermore, a primary transfer variation and an exposure variation can be prevented. 
   Further, in the pressurizing delay mechanism explained in  FIGS. 2 and 3A  to  3 C, it is recommended just to give the rotational force (driving force) to at least either of the rollers of the aligning roller pair. 
   Next, another embodiment of the pressurizing delay mechanism of the aligning roller pair will be explained by referring to  FIGS. 4A to 4C . 
   As shown in  FIG. 4A , an aligning roller pair  72  are composed of a first roller  73  and a second roller  74  which are closely contacted to each other by a predetermined pressure. Further, in the example shown in  FIG. 4A , the first roller  73  is rotated in the direction of an arrow B and the second roller  74  is rotated in the direction of an arrow C in the opposite direction of the arrow B. Therefore, the moving directions of the roller surfaces of the respective rollers are the same direction at a position  75 A where both rollers make contact with each other. 
   Between a rotary shaft  73   b  of the first roller  73  and a rotary shaft  74   a  of the second roller  74 , a space setting wedge  70  in a wedge shape is arranged. A right slant portion  70   a  of the space setting wedge  70  is in contact with the rotary shaft  74   a . Similarly, a left slant portion  70   b  of the space setting wedge  70  is in contact with the rotary shaft  73   b . To one end of the space setting wedge  70 , an extension spring  76  is connected and the space setting wedge  70  is always extended in the direction of an arrow G. As a result, the right slant portion  70   a  of the space setting wedge  70  is pressurized to the rotary shaft  74   a  and the left slant portion  70   b  is pressurized to the rotary shaft  73   b . In other words, the first roller  73  and second roller  74  are applied with force in the direction of mutual separation. 
   When there are no sheets P between the first roller  73  of the aligning roller pair  72  and the second roller  74  (the sheets P do not reach between the rollers of the aligning roller pair  72 ), as shown in  FIG. 4A , the space setting wedge  70  stands still at the position where the rotary power of the rotary shaft  74   a  in the direction of the arrow C, the rotary power of the rotary shaft  73   b  in the direction of the arrow B, and the extension force of the extension spring  76  in the direction of the arrow F are balanced. 
   On the other hand, as shown in  FIG. 4B , since there is a sheet P between the rollers  73  and  74  of the aligning roller pair  72  (the sheet P is guided between the rollers of the aligning roller pair  72 ), the roller  73  and roller  74  are separated at a distance corresponding to the thickness of the sheet P. By this separation, the space setting wedge  70  is moved at a predetermined distance related to the thickness of the sheet P in the direction of the arrow F. Namely, the space setting wedge  70 , due to the thickness of the sheet P, in correspondence to the space between the roller  73  and the roller  74  in the state that the space between them is spread, is moved up to the position balanced with the rotary power of the roller  73  and roller  74 . 
   On the other hand, as shown in  FIG. 4C , when the sheet P existing between the roller  73  of the aligning roller pair  72  and the roller  74  is completely separated from the rollers (the sheet P gets out of the rollers of the aligning roller pair  72 ), by the rotary force of the rotary shaft  74   a  in the direction of the arrow C and the rotary force of the rotary shaft  73   b  in the direction of the arrow B, the space setting wedge  70  is pressed back in the direction of the arrow H. 
   At this time, to the space setting wedge  70 , a predetermined force in the direction of the arrow F by the extension spring  76  is given. By the given predetermined force, in the space setting wedge  70 , after the sheet P is completely separated from the rollers, for a predetermined period of time specified on the basis of the difference of force between the force in the direction of the arrow D caused by the rotary power of the rotary shaft  74   a  in the direction of the arrow C and the rotary power of the rotary shaft  73   b  in the direction of the arrow B and a predetermined force by the extension spring  76  in the direction of the arrow G, as shown in  FIG. 4A , the first roller  73  and second roller  74  are returned comparatively controllably to the position where there are no sheets P. Therefore, the roller  73  of the aligning roller pair  72  and the roller  74  are not closely contacted to each other the moment the sheet P passes through the rollers but are closely contacted after a lapse of a fixed period time. 
   As mentioned above, the space setting wedge is installed between the roller  73  of the aligning roller pair  72  and the roller  74 , thus immediately after passing of the sheet P through the rollers, the space between the two rollers of the aligning roller pair is prevented from suddenly returning (the rollers of the aligning roller pair are closely contacted to each other). 
   By doing this, the sheet P to which the toner image is transferred at the image transfer position  5 A is prevented from suddenly pressed out toward the fixing device  6  and the toner image transferred at the image transfer position  5 A can be prevented from an occurrence of a displacement, that is, a transfer variation. Furthermore, a primary transfer variation and an exposure variation can be prevented. 
   As shown in  FIG. 6 , on a part of the main body  1 , an openable and closable cover  1   b  is formed. In the cover  1   b , the sheet reversing unit  7  is provided and furthermore, the transferring roller  51 , the second roller  63  among the aligning roller pair  61  and the aligning roller pair  71  in the sheet reversing unit  7  are arranged. When the sheet P is jammed at the position of the aligning roller pair  61 , the cover  1   b  is opened, thus the second roller  63  is separated from the first roller  62 , so that the jammed sheet P can be removed. After removal of the sheet P, when the cover  1   b  is closed again, the aligning roller pair  61 , that is, the first roller  62  and second roller  63  are rotated in a predetermined direction for a predetermined period of time. By this rotation, the space setting cam or space setting wedge is placed at a predetermined position. 
   The outside diameter of the space setting cam and the tension of the space reference roller and extension spring are set optionally on the basis of various parameters such as the number of image output sheets in the unit time, the diameters of the respective rollers of the aligning roller pair, and the pressure given between the rollers of the aligning roller pair. 
   The present invention is not limited to the embodiments aforementioned and within the scope of the present invention described in the claims, can be modified variously, and needless to say, those modifications are included in the scope of the present invention. 
   For example, in the embodiments, the aligning rollers are illustrated by examples. However, the sheet conveyor of the present invention can be applied to an aligning roller pair  71  in a sheet reversing unit  7  of the color copy machine  1 , an automatic document feeder (ADF) integrally installed in the scanner unit  2 , and a printer not drawn.

Technology Category: b