Abstract:
A sheet skew correcting apparatus, incorporated in a printer or the like to correct a phenomenon of obliquely feeding a sheet called skewing. The apparatus includes a transporting path in which the sheet is transported, a pair of registration rollers, a pair of timing rollers disposed upstream from the registration rollers, and a paper guide having a first and a second curved portion and constituting the transporting path. When a sheet fed into the transporting path abuts against a contact point of a registration roller pair, the sheet is further transported by an arbitrary distance in the sheet transporting direction. First and second curves are then formed in the sheet in the first and second curved portions. Then the registration roller pair is rotated in the direction opposite to the sheet transporting direction, the front end of the sheet uniformly abuts against the contact point of the registration roller pair to correct skew.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a sheet skew correcting apparatus for a copier, a printer, or the like, and also to an image forming apparatus using it. 
   2. Background Art 
   Conventionally, a sheet transporting apparatus which is incorporated in a printer or the like is often provided with a sheet skew correcting apparatus in order to correct a phenomenon of obliquely feeding a sheet, or a phenomenon which is so-called skewing. 
     FIG. 6  is a front view of a conventional sheet skew correcting apparatus which is incorporated in a printer or the like. The apparatus is configured by: a driving roller (hereinafter, referred to as registration drive roller  1 ) which, in a sheet transporting path connecting a sheet feeding apparatus to a transferring apparatus, causes the front end of a sheet transported in a sheet transporting path disposed upstream from the transferring apparatus, to abut against the roller, thereby registering the front end of the sheet on the nip line of the roller; a driven roller (hereinafter, referred to as registration idler roller  2 ) which is similarly disposed; a driving roller (hereinafter, referred to as timing drive roller  4 ) which is used for transporting the sheet in a sheet transporting path disposed upstream from the registration rollers; a driven roller (hereinafter, referred to as timing idler roller  5 ) which is similarly disposed; and a sensor  20  which detects the passing of a sheet  22  over a contact point  27  between the timing drive roller  4  and the timing idler roller  5 . 
   The sheet skew correcting apparatus corrects skew of the sheet  22  in the following manner. The timing drive roller  4  and the timing idler roller  5  are rotated in the directions of the arrows shown in  FIG. 6 , to cause the sheet  22  fed out from the sheet feeding apparatus to abut against a contact point  23  between the registration drive roller  1  and the registration idler roller  2  which are stopping. Thereafter, the timing drive roller  4  and the timing idler roller  5  are rotated in the directions of the arrows shown in  FIG. 7  to feed the sheet  22  by an arbitrary distance, whereby the sheet is curved in a range between the registration drive roller  1  and the registration idler roller  2 , and the timing drive roller  4  and the timing idler roller  5 . The registration drive roller  1  and the registration idler roller  2  are rotated in the directions of the arrows shown in FIG.  7 . Thereafter, the curved portion of the sheet is pressed by a buffer paper guide  28  in a direction along which the front end of the sheet  22  is aligned with the contact point  23 , whereby the front end of the sheet  22  is aligned with the contact point  23  to correct skew of the sheet  22 . 
     FIG. 8  is a diagram showing the curved state of the sheet  22  in the conventional sheet skew correcting apparatus shown in  FIGS. 6 and 7 . When the skewed sheet  22  is aligned with the nip line of the registration drive roller  1  and the registration idler roller  2 , the curve of the side which first butts against the contact point  23  is larger as indicated by B, and that of the side which lately butts against the contact point is smaller as indicated by C. As a result, the buffer paper guide  28  which presses the curved portion of the sheet  22  presses mainly only the side of the larger curve, so that the force of pressing the front end of the sheet  22  on the side which lately butts is reduced. Therefore, there occurs a phenomenon that the sheet  22  in the range from the curved portion to the contact point  23  is rotated generally by the pressing on the curved portion of the sheet  22  by the buffer paper guide  28 , about an intersection  0  of a ridge line a-a′ on the side of the registration drive roller  1 , and a ridge line b-b′ connecting the starting points of the sheet  22  on the side of the timing drive roller  4 . This phenomenon remarkably occurs particularly in a sheet of high rigidity, such as cardboard. 
   In the above-mentioned conventional sheet skew correcting apparatus, when, in order to align a sheet with the nip portion of the registration roller pair, the sheet is sent to the registration roller pair by a distance which is longer than the length of the sheet transporting path between the timing roller pair and the registration roller pair to form a curve in the sheet, the force of pressing the front end of the sheet on the side which lately butts against the registration roller pair, against the registration roller pair is reduced. Therefore, a portion which fails to abut against the nip line of the registration roller pair is produced in the front end of the sheet. This causes a problem in that a necessary skew correcting ability cannot be obtained. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a sheet skew correcting apparatus which can solve the problem of the conventional art, and which causes the front end of a sheet to abut against a nip line of a registration roller pair to exhibit an excellent skew correcting ability. It is another object of the invention to provide an image forming apparatus which causes the front end of a sheet to abut against a nip line of a registration roller pair to exhibit an excellent skew correcting ability. 
   The objects can be attained by a sheet skew correcting apparatus in which the apparatus comprises: a pair of registration rollers which are disposed in a transporting path connecting a sheet feeding apparatus to a transferring apparatus and upstream from the transferring apparatus; and a pair of timing rollers which are disposed upstream from the pair of registration rollers, detecting means for detecting passing of a sheet through the pair of timing rollers is disposed, the apparatus further comprises: driving means configured by a stepping motor for driving the pair of registration rollers in a forwardly and reversely rotatable manner; driving means configured by a stepping motor for the pair of timing rollers; and a control circuit which controls operations of the driving means of the pair of registration rollers and the pair of timing rollers, a first curved portion is disposed in a paper guide forming a transporting path between the pair of registration rollers and the pair of timing rollers, and a second curved portion is disposed downstream from the first curved portion, wherein a position of a front end of a sheet is detected by the sheet detecting means to know arrival of the sheet front end position at the registration rollers, and, when the sheet is transported by a distance which is longer than a length of a sheet transporting path between the pair of timing rollers and the pair of registration rollers to uniformly align the front end of the sheet with a nip line of the pair of registration rollers, a difference of a curving amount of the sheet due to skew of the sheet is absorbed by the second curved portion to uniformalize curving of the sheet in the first curved portion, whereby a force of pressing the front end of the sheet against the pair of registration rollers can be uniformalized to exhibit an excellent skew correcting ability. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of the sheet skew correcting apparatus of the invention. 
       FIG. 2  is a diagram of the sheet skew correcting apparatus of the invention. 
       FIG. 3  is a diagram showing the operation of the sheet skew correcting apparatus of the invention. 
       FIG. 4  is a diagram showing the operation of the sheet skew correcting apparatus of the invention. 
       FIG. 5  is a block diagram illustrating a controlling section of the sheet skew correcting apparatus of the invention. 
       FIG. 6  is a diagram showing the operation of a conventional sheet skew correcting apparatus which is incorporated in a printer or the like. 
       FIG. 7  is a diagram showing the operation of the conventional sheet skew correcting apparatus which is incorporated in a printer or the like. 
       FIG. 8  is a plan view illustrating a skewing state of a sheet in a conventional printer or the like. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, the invention will be described with reference to the drawings showing an embodiment of the invention.  FIG. 1  is a perspective view showing an example in which an image forming apparatus having a sheet skew correcting apparatus of the invention is embodied. Referring to  FIG. 1 , a registration idler roller  2  is made of a material such as a plastic or a metal, which is harder, which has a smoother surface, and which is less deformed than that of a registration drive roller  1 . The registration idler roller  2  is pressed by springs  3  against a registration drive roller  1  configured by an elastic member of rubber hardness of 85° or higher. When the registration drive roller  1  is rotated, the registration idler roller  2  is rotated in the direction opposite to that of the registration drive roller  1 . A timing idler roller  5  is pressed by springs  6  against a timing drive roller  4 . When the timing drive roller  4  is rotated, the timing idler roller  5  is rotated in the direction opposite to that of the timing drive roller  4 . In an end portion of the registration drive roller  1 , disposed are a pulley  7 , a pulley  8 , and a timing belt  9  which serve as a mechanism for transmitting a rotation power supplied from a stepping motor  10  functioning as a power source for rotating the registration drive roller  1 . In an end portion of the timing drive roller  4 , disposed are a pulley  11 , a pulley  12 , and a timing belt  13  which serve as a mechanism for transmitting a rotation power supplied from a stepping motor  14  functioning as a power source for rotating the timing drive roller. 
     FIG. 2  is a side view schematically showing the embodiment. Referring to  FIG. 2 , a buffer paper guide  15  is pressed by a torsion coil spring  16  against a paper guide  17  while being swung about the point A. A paper guide  18  is disposed above the buffer paper guide  15 . The paper guide  18  and buffer paper guide constitutes a first curved portion  50  of the transporting path. 
   The paper guide  17  has a shape which is expanded toward the registration idler roller  2  upstream from the registration drive roller  1 . The registration drive roller  1  and the registration idler roller  2  are in mutual contact at a contact point  23 . The paper guide  17  is configured so that the front end of a sheet  22  is first in contact with the registration idler roller  2  and then arrives the contact point  23  while sliding over the surface of the registration idler roller  2 . The paper guide  18  has a first inclining portion  100  and a second inclining portion  101 . The second inclining portion  100  abuts on the first inclining portion  101  from up stream. The first inclining portion  100  tapers the transporting path gradually in the transporting direction. The second inclining portion  101  tapers the transporting path gradually towards upstream. When an imaginary plane  200  being tangent to the surfaces of both of the registration drive roller  1  and the registration idler roller  2  and passing through the contact point  23  is taken as a reference, an angle  201  between the first inclining portion  100  and the imaginary plane  200  is 45° or less, and an angle  202  between the second inclining portion  101  and the imaginary plane  200  is 45° or more. 
   The first inclining portion  100  and the second inclining portion  101  constitute a side of a second curved portion  51  of the transporting path which is disposed downstream to the first curved portion  50 . 
   In order to detect the sheet  22  transported from a sheet feeding apparatus  21 , a sensor  20  which detects the front end of a sheet and the presence of a sheet is disposed downstream from the timing drive roller  4 . 
     FIG. 5  is a block diagram illustrating a controlling section of the sheet skew correcting apparatus of the invention. In  FIG. 5 , numeral  25  denotes a calculating circuit. A counting circuit  26  which counts the numbers of steps respectively supplied to the stepping motors  10  and  14  is connected to the calculating circuit  25 . A controlling circuit  24  which controls the operations of the stepping motors  10  and  14  in accordance with instructions from the calculating circuit  25  is connected to the calculating circuit  25 . The stepping motors  10  and  14  are connected to the controlling circuit  24 . The sensor  20  which detects the front end of a sheet in accordance with instructions from the calculating circuit  25  is connected to the calculating circuit  25 . 
     FIGS. 3 and 4  are side views illustrating the operation of the sheet skew correcting apparatus. Hereinafter, the manner of the operation of the sheet skew correcting apparatus will be described with reference to  FIGS. 3 and 4 . First, the timing drive roller  4  and the timing idler roller  5  are rotated in the directions shown in FIG.  3 . At this time, the sheet  22  is transported in the direction to the registration drive roller  1  and the registration idler roller  2 , by the rotations of the timing drive roller  4  and the timing idler roller  5 . When the front end of the sheet  22  is detected by the sensor  20 , the number of rotations of the stepping motor  14  which is required for the front end of the sheet  22  to arrive the contact point  23  can be known because relationships between the number of rotation steps of the stepping motor  14  and the distance of a sheet transporting path from the sensor  20  to the contact point  23  between the registration drive roller  1  and the registration idler roller  2  are already apparent. 
   After the front end of the sheet  22  arrives the contact point  23  in this way, the stepping motor  14  is further rotated to transport the sheet by a distance which is larger than the distance of the sheet transporting path to the contact point, thereby forming a curve in the sheet  22  as shown in FIG.  4 . Thereafter, the registration drive roller  1  and the registration idler roller  2  are rotated by an arbitrary angle in the directions of the arrows shown in  FIG. 4 , whereby the front end of the sheet  22  is vibrated to uniformly align the front end of the sheet  22  with the contact point  23 . The sheet  22  which is sent out from the timing drive roller  4  and the timing idler roller  5  is smoothly introduced to the registration idler roller  2  in the vicinity of the contact point  23  by the expansion of the paper guide  17  upstream from the registration drive roller  1  and the registration idler roller  2 , and also by the inclination of the first inclining portion  100 . The sheet transporting path is expandingly bent toward the registration idler roller  2  by the expansion of the paper guide  17 . After the front end of the sheet  22  abuts against the contact point  23 , therefore, the sheet  22  forms a curve in the second curved portion  51 . In the second curved portion  51 , the inclination on the second inclining portion  101  is larger than that on the first inclining portion  100 . Consequently, the repulsive force of the sheet  22  which is generated by the curving in the second curved portion  51  acts in a direction along which the front end of the sheet  22  is pressed against the contact point  23 . Furthermore, the difference of the curving amount of the sheet  22  due to skew can be absorbed by the curve of the sheet  22  formed on the side of the paper guide  18 . Therefore, an influence on the curve of the sheet  22  formed on the side of the buffer paper guide  15  can be reduced, and pressing can be performed while the curve of the sheet  22  produced on the side of the buffer paper guide  15  is made uniform in the width direction of the sheet  22 . 
   Since the second curved portion  51  and the first curved portion  50  are alternately curved with respect to the sheet transporting path, the curving of the sheet  22  can form an S-like shape and the force of pressing the sheet  22  by the buffer paper guide  15  can be efficiently transmitted to the curve formed in the second curved portion  51 . Therefore, the front end of the sheet  22  is aligned with the contact point  23 , so that skew can be corrected. As a result, the state where the front end of the sheet  22  uniformly abuts against the contact point  23  can be maintained, and hence skew occurring in the sheet  22  can be easily corrected. 
   Since the second curved portion  51  is curved in a position opposed to the registration drive roller  1 , the front end of the sheet  22  is pressed against the registration drive roller  1  by the repulsive force of the sheet  22  due to the curving in the second curved portion  51 . During a process of reversely rotating the registration drive roller  1  to register the front end of the sheet  22 , however, the front end of the sheet  22  is not rolled in the contact point  23  because the registration drive roller  1  is made of an elastic member of rubber hardness of 85° or higher and hence is less deformed. Therefore, it is possible to prevent damage such as breakage or deformation of the front end of the sheet  22  from occurring. 
   It is also possible to provide an image forming apparatus comprising: a cassette  21  of a sheet feeding apparatus and for housing sheets; the sheet skew correcting apparatus of this invention for correcting skew of a sheet fed out from the cassette; a developing/transferring section  300  (See  FIG. 2 ) which forms an image on the sheet fed out from the cassette; a fixing section (not shown) which fixes the image transferred to the sheet in the developing/transferring section; a finisher (not shown) having a post processing apparatus including a stacker on which printed sheets are piled up, or a stapler; and the like. This image forming apparatus is capable of transporting sheets stably. 
   As described above, according to the invention, skew of even a sheet which is high in rigidity depending on the thickness and the kind can be effectively corrected.