Patent Publication Number: US-7210678-B2

Title: Sheet conveying device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to a sheet conveying device, for use in an image forming apparatus, that conveys sheets to a recording unit, by holding the sheets between a feed roller and a pinch roller. 
   2. Description of Related Art 
   Printing devices, such as ink jet printers, are provided with sheet conveying devices for conveying sheets mounted on a sheet cassette one by one to a recording or printing unit. The sheet conveying device includes a metal feed roller that is driven by a drive source to rotate, and a plastic pinch roller pressed against the feed roller. The sheet is held between the feed roller and the pinch roller and conveyed to the recording unit. 
   To increase a printing accuracy of the printing devices, an accuracy of a sheet conveyance with the sheet conveying device needs to be increased. Especially, the ink jet printers that perform printing in high dot density, require the higher sheet conveying accuracy. 
   To increase an accuracy of a sheet conveying speed, the pinch roller and the feed roller of the sheet conveying device need to be firmly and uniformly pressed against each other to prevent improper sheet feeding. To achieve this, for example, Japanese Laid Open Patent Publication No. 9-142691 discloses a plurality of roller holders that rotatably support the pinch roller at one end thereof and springs that urges the roller holder at rear side ends thereof. 
   Recently, a margin at an end of the sheet tends to be reduced, so that distance between a printing portion and a nip portion between the feed roller and the pinch roller need to be reduced. 
   However, reduction of the distance between the printing portion and the nip portion is physically limited. As the distance between the printing portion and the nip portion becomes shorter, the pinch roller needs to be supported to press the feed roller firmly. In the sheet conveying device having the pinch roller and the feed roller pressed firmly against each other, a rear end of the sheet being conveyed flicks or springs due to an impact applied when the rear end of the sheet passes through the nip portion between the pinch roller and the feed roller. This causes improper sheet feeding or variances in sheet feeding amounts, resulting in poor image formation. 
   SUMMARY OF THE INVENTION 
   Accordingly, one aspect of the invention is to provide a sheet conveying device that prevents a poor image formation by prevent improper sheet feeding and by minimizing variances of sheet feeding amounts. 
   A sheet conveying device for use in an image forming apparatus, according to the invention may hold a sheet and convey the sheet to a recording unit. The sheet conveying device may include a pair of rollers that includes a feed roller driven by a drive source and a pinch roller pressed against the feed roller, an elastic ring that is fitted over at least one of the feed roller and the pinch roller, partially in a width direction thereof, on an outer surface thereof, and a guide film that guides the sheet, the guide film extending from an upstream of the roller pair in a sheet conveying direction toward a nip portion between the pinch roller and the feed roller, without contacting the elastic ring fitted over the roller pair. 
   The feed roller driven by the drive source may be made of metal. The pinch roller pressed against the feed roller may be made of resin. The pinch roller may fit thereover partially in the width direction thereof the elastic ring, on the outer surface of the pinch roller. The guide film disposed on the roller holder for guiding the sheet may extend from the upstream of the pinch roller in the sheet conveying direction toward the nip portion between the pinch roller and the feed roller, without contacting the elastic ring fitted over the pinch roller. 
   According to the invention, elastic deformation of the elastic ring fitted over the roller pair may reduce or absorb load (impact) that causes a rear end of the sheet to flick or spring and the improper sheet feeding, when the rear end of the sheet passes through the nip portion between the feed roller and the pinch roller. Thus, variances in the sheet feeding amounts and the poor image formation may be prevented. 
   When the feed roller and the pinch roller are reversely rotated for sheet registration, a leading end of the sheet may be prevented from contacting the elastic ring with the leading end being lifted, and the sheet may be favorably conveyed near the nip portion between the feed roller and the pinch roller, with the guide film. Therefore, poor sheet feeding such that the sheet end is turned up by the elastic ring while the feed roller and the pinch rollers are reversely rotated, may be prevented. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will be described in detail with reference to the following figures wherein: 
       FIG. 1  is a cross sectional view of an ink jet printer including a sheet conveying device according to an embodiment of the invention; 
       FIG. 2  is a plan view of a roller holder of the sheet conveying device; 
       FIG. 3  is a cross sectional view of the roller holder taken along the line A—A of  FIG. 2 ; 
       FIG. 4  is a bottom plan view of the roller holder; 
       FIG. 5  is a half-cut view of a pinch roller; 
       FIG. 6  is a half-cut view of the pinch roller and an elastic ring; 
       FIG. 7  is a cross sectional view of the pinch roller with the elastic ring fitted thereover; 
       FIG. 8  is a cross sectional view of the pinch roller with the elastic ring fitted thereover according to a modification of the embodiment of the invention; 
       FIG. 9  is a plan view of a guide film; 
       FIG. 10  is a flowchart showing a sheet conveying operation performed for printing; and 
       FIG. 11  is a plan view of a roller holder according to a modification of the embodiment of the invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   An ink jet printer  1  including a sheet conveying device  10  according to an embodiment of the invention will be described with reference to  FIG. 1 . As shown in  FIG. 1 , the ink jet printer  1  is provided at a rear side thereof (left side in  FIG. 1 ) with a sheet feeder  2  capable of mounting a plurality of sheets P thereon. A pick-up roller  3  is disposed on a lower front side of the sheet feeder  2  at a position to contact the uppermost sheet P. The pick-up roller  3  is supported by a rotating shaft disposed along the width direction of the sheet P. 
   The sheet feeder  2  includes a tray  4  on which sheets P are mounted. The tray  4  is urged by a spring  5  toward the pick-up roller  3 . Therefore, the uppermost sheet P on the tray  4  contacts the surface of the pick-up roller  3  by the urging force of the spring  5 . The uppermost sheet P is separated by the pick-up roller  3  driven by a motor (not shown), and fed along a guide  6  in a sheet feeding direction (in a slanting downward direction). 
   The sheet conveying device  10  is disposed in a downward slanting direction of the pick-up roller  3  inside a cover frame  7 . The sheet conveying device  10  includes a feed roller  11  that is driven by a motor (not shown) to rotate, and a pinch roller  12  that is pressed against the feed roller  11 . The feed roller  11  is a metal roller having ceramic coating on the outer surface thereof. 
   The pinch roller  12  is rotatably supported at an end (lower end) of a roller holder  13 . The roller holder  13  is supported so as to pivot about a pivot  14  provided at substantially a central portion thereof. The roller holder  13  is urged in the clockwise direction in  FIG. 1  by a spring (coil spring)  15 , so that the pinch roller  12  and the feed roller  11  can be pressed against each other with a certain pressure. Accordingly, the sheet P fed by the pick-up roller  3  along the guide  6  is sandwiched at a nip portion between the feed roller  11  and the pinch roller  12  and fed in the sheet feeding direction toward the downward slanting direction. 
   A printing unit  20  is provided downstream of the sheet conveying device  10  in the sheet feeding direction. The printing unit  20  includes a print head  21  disposed so as to face a surface of the sheet P conveyed by the sheet conveying device  10 . The print head  21  includes a plurality of ink chambers attached to piezoelectric elements (not shown). Application of voltage to the piezoelectric elements causes the volumetric capacity of the ink chambers to change. Accordingly, the ink is ejected onto the sheet P from nozzles (not shown) formed with the ink chambers. The print head  21  is not limited to print heads having the piezoelectric elements in association with ink chambers, but so-called thermal ink-jet print heads having heating elements may be used. 
   The print head  21  is mounted on the carriage  22 . The carriage  22  is slidably supported by a guide shaft  23  that is disposed along a lateral direction of the printer  1 . 
   The carriage  22  is reciprocated by a drive motor (not shown) along the guide shaft  23 . The print head  21  mounted on the carriage  22  moves in the lateral direction to perform printing onto the sheet P. 
   Provided below the print head  21  are a discharge roller  24  that is driven by a motor (not shown) so as to rotate and a presser roller  25  that holds the printed sheet P in cooperation with the discharge roller  24  for discharging the sheet P. A discharge tray  27  that receives the discharged sheets P is disposed on a base frame  26 . 
   In the sheet conveying device  10 , the roller holder  13  is made up of four roller holders, that is, a left roller holder  13 L, a right roller holder  13 R, and two central roller holders  13 C, that are disposed in the width direction of the printer  1 , as shown in  FIG. 2 . The difference between the central roller holder  13 C and the right and left roller holders  13 R,  13 L is that the right and left roller holders  13 R,  13 L have an extended right end portion and an extended left end portion, respectively. As described above, the roller holder  13  made up of four holders  13 L,  13 C,  13 C,  13 R is pivotally and independently supported by the metal pivot  14  disposed at a substantially central portion of the roller holder  13 , as shown in  FIG. 1 . 
   The roller holder  13  is integrally formed of resin material into a substantially “L” shape, as shown in  FIGS. 1 and 3 . The pinch roller  12  is rotatably supported at a lower end of the roller holder  13  and disposed on the right and left side of each roller holder  13 L,  13 C,  13 R. As shown in  FIG. 2 , each roller holder  13 L,  13 C  13 R has substantially rectangular recesses  13   a  on the lower right and left sides thereof. The pinch roller  12  is rotatably supported in the recess  13   a  by a metal roller pin  16  that passes through the pinch roller  12 , as shown in  FIG. 7 . The pinch roller  12  consists of a left pinch roller  12 L for use in the left roller holder  13 L, a right pinch roller  12 R for use in the right roller holder  13 R, and a central pinch roller  12 C for use in the central roller holder  13 C. Except for the pinch roller  12 C, the right and left pinch rollers  12 R,  12 L have the same structures. The differences between the left and right pinch rollers  12 L,  12 R and the central pinch roller  12 C will be described below. 
   As shown in  FIG. 5 , the left and right pinch roller  12 L,  12 R is formed of resin material, such as polyacetal, into a substantially cylindrical shape. Similarly, the central pinch roller  12 C for use in the central roller holder  13 C is formed of resin material, such as polyacetal. However, a groove  12   a  having a width W is formed on an outer surface of the central pinch roller  12 C at a substantially central portion in the width direction thereof, as shown in  FIG. 6 . The pinch roller  12 C has a tapered surface  12   b  at an end thereof that is gradually narrowed down toward an axial direction. 
   An elastic ring  17  that is fitted over the pinch roller  12 C is made of thermoplastic fluororubber having a high wear or abrasion resistance. A tube of the thermoplastic fluororubber is cut into pieces to have the width b, as shown in  FIG. 7 . The width b is set to 1 mm or greater to provide a sufficient strength for the elastic ring  17  and to increase the durability. The thermoplastic fluororubber tube that may be used as an ink tube is used for the elastic ring  17  by cutting the tube into pieces having the width b. Accordingly, the elastic ring  17  has a high resistance to ink. Even if the ink from the print head  21  should be adhered to the elastic ring  17 , the elastic ring  17  will not be corroded by the ink, maintaining its quality over the long term. 
   As shown in  FIG. 7 , the elastic ring  17  is fixedly fitted into the groove  12   a  formed on the outer surface of the central pinch roller  12 C. The elastic ring  17  can be readily inserted over the pinch roller  12 C, if the ring  17  is inserted into the groove  12   a  along the tapered surface  12   b  serving as a guide. Thus, the ease of assembly is increased. As the ring  17  is fixedly fixed into the groove  12   a , the ring  17  does not come off the pinch roller  12 C. Accordingly, the operation of the sheet conveying device  10  is highly stabilized. 
   The friction coefficient of the pinch roller  12  and the elastic ring  17  were actually measured. The friction coefficient of the plastic pinch roller  12 L,  12 R alone on its surface was 0.3. The friction coefficient of the elastic ring  17  alone on its surface was 0.6. The average friction coefficient of the pinch roller  12 C in combination with the elastic ring  17  on their surface was 0.4 to 0.47. 
   As shown in  FIG. 8 , the pinch roller  18  may have the tapered surface  12   b  on each end of the pinch roller  12 C in the axial direction. With this structure, the elastic ring  17  can be fitted over the pinch roller  12 C from either end of the pinch roller  12 C. Thus, the assembly of the elastic ring  17  and the pinch roller  12 C can be performed more easily. 
   As shown in  FIG. 7 , the width W of the groove  12   a  of the pinch roller  12 C is wider than the width b of the elastic ring  17 , so that clearances δ 1 , δ 2  are set between an end of the groove  12   a  and an end of the elastic ring  17 . The clearances δ 1 , δ 2  will be described in more detail below. 
   The outside diameter of the elastic ring  17  is set greater than that of the pinch roller  12 C. The projection Δ of the elastic ring  17  from the outer surface of the pinch roller  12 C is set to approximately 0.2 to 0.5 mm, preferably 0.3 mm. 
   As shown in  FIG. 4 , a guide film  18  for guiding the sheets P near the nip portion between the feed roller  11  and the pinch roller  12 C from an upstream of the pinch roller  12 C in the sheet feeding direction, is attached to a bottom of the central roller holder  13 C at a position near each central pinch roller  12 C but out of contact with the elastic ring  17  fitted over the pinch roller  12 C. Disposed between the guide films  18  is another guide film  19  that functions similar to the guide films  18 . The guide film  19  will be described in more detail below. 
   As shown in  FIG. 9 , the guide film  18  is formed with a pair of protrusions  18   a  that extend from right and left downstream ends in the sheet feeding direction to a position near the nip portion between the feed roller  11  and the pinch roller  12 C, while placing the elastic ring  17  between the protrusions  18 , to guide the sheets P. The protrusions  18   a  are formed such that the end of the protrusions  18   a  is positioned on the upstream side in the sheet feeding direction, with respect to a center of the pinch roller  12 C (center of the nip portion in the sheet feeding direction), so as to overlap the pinch roller  12 C. 
   In the thus-structured sheet conveying device  10 , the roller holder  13  is urged by the spring  15 , as described above with reference to  FIG. 1 . Accordingly, the pinch roller  12  is pressed against the feed roller  11  at a certain pressure. 
   The sheet P fed by the pick-up roller  3  along the guide  6  is held at the nip portion between the feed roller  11  and the pinch roller  12  and fed in the slanting downward direction to the printing unit  20  where the printing is performed onto the sheet P with the print head  21 . 
   A sheet conveying operation performed for printing by the printer  1  will be described with reference to  FIG. 10 . 
   When the operation is started (S 1 ), the pick-up roller  3  and the feed roller  11  are rotated counterclockwise (S 2 ). Accordingly, a sheet P is picked up from the sheet feeder  2  by the pick-up roller  3  and fed toward the sheet conveying device  10 . Then, it is determined whether a register sensor (not shown) disposed upstream of the feed roller  11  in the sheet feeding direction is turned on as the sheet P passes by the register sensor (S 3 ). When it is determined that the register sensor is turned on (S 3 : YES), the sheet P is further fed by a predetermined amount to correct the diagonal positioning or alignment of the sheet P (S 4 ). At this time, the feed roller  11  and the pinch roller  12  are reversely rotated, and register the sheet P. As an edge of the sheet P is received between the feed roller  11  and the pinch roller, reverse rotation of the pick-up roller  3  and the feed roller  11  is stopped. Thus, the correction of the diagonal positioning or alignment of the sheet P completes (S 5 ). 
   As the diagonal positioning or alignment of the sheet P is corrected, the feed roller  11  is rotated in the forward direction (clockwise direction). The sheet P is held between the feed roller  11  and the pinch roller  12  and conveyed to the printing unit  20 . A predetermined image is formed on the sheet P with the print head  21  of the printing unit  20  (S 6 ). 
   As the printing operation is finished, the sheet P is discharged while being held between the discharge roller  24  and the presser roller  25  (S 7 ). Thus, the sheet conveying and printing operations are finished (S 8 ). 
   In  FIG. 2 , dot-dash line P 1  represents the width of the A4 size sheet. Dot-dash line P 2  represents the width of a postcard. As shown in  FIG. 2 , the roller holder  13  is divided in the direction of the sheet width, into a plurality of portions ( 13 L,  13 C,  13 C,  13 R), according to the sizes of the sheets P to be used for printing. 
   In the sheet conveying device  10 , the elastic ring  17  is fitted over the outer surface of each of four pinch rollers  12 C, which are rotatably supported in two roller holders  13 C disposed between the left and right roller holders  13 L,  13 R. The elastic deformation of the elastic ring  17  reduces or absorbs load (impact) that causes the rear end of the sheet P to flick or spring and the improper sheet feeding, when the rear end of the sheet P passes through the nip portion between the feed roller  11  and the pinch roller  12 . Consequently, variances in the sheet feeding amounts and the poor image formation are prevented. 
   The projection Δ, as shown in  FIG. 7 , of the elastic ring  17  from the outer surface of the pinch roller  12 C is set to approximately 0.2 mm to 0.5 mm, as described above. Therefore, 0.2 mm to 0.5 mm is ensured as a deformable range of the elastic ring  17 . Consequently, the elastic ring  17  elastically deforms by an amount, within the range, enough to reduce or absorb the load (impact) that causes the rear end of the sheet P to flick or spring when the rear end of the sheet P passes through the nip portion between the feed roller  11  and the pinch roller  12 . 
   As the elastic ring  17  is pressed by the feed roller  11 , the elastic ring  17  stretches out. The stretch of the ring  17  is absorbed in the clearances δ 1 , δ 2  shown in  FIG. 7  between an end of the groove  12   a  of the pinch roller  12  and an end of the elastic ring  17 . With this structure, the elastic ring  17  does not extend beyond the edge of the groove  12   a . Accordingly, occurrences of skew of the sheet are prevented. When the sheet registration operation is performed in which the feed roller  11  and the pinch roller  12 C are reversely rotated before the sheet P is fed to the printing unit  20 , the leading edge of the sheet P contacts the elastic ring  17  fitted over each of four pinch roller  12 C, at the nip portion between the feed roller  11  and the pinch roller  12 C, where the positioning of the sheet P is corrected. 
   In the above-described embodiment, the guide film  18  for guiding the sheets P near the nip portion between the feed roller  11  and the pinch roller  12 C, is attached to a bottom of the central roller holder  13 C at a position near each central pinch roller  12 C but out of contact with the elastic ring  17  fitted over the pinch roller  12 C. The leading end of the sheet P may be raised or lifted, due to the factors attributable to the elastic ring  17  and the pinch roller  12 , such that the elastic ring  17  is fitted over the pinch roller  12 C to project some distance from the outer surface of the pinch roller  12 C, has a relatively high friction coefficient of 0.6, and is elastically deformable, as well as that the pinch roller  12  has a relatively small diameter. When the leading end of the sheet P is raised, the leading end of the sheet P may be turned up as the elastic ring  17  is reversely rotated. The end of the sheet P turned up may go off a sheet feeding path or be folded, resulting in improper sheet feeding. With the guide film  18 , the sheet P is guided by the guide film  18  near the nip portion between the rollers  11 ,  12 , while preventing the leading end of the sheet P from contacting the elastic ring  17  in the condition such that the end of the sheet P is being lifted or is to be turned up. Accordingly, when the rollers  11 ,  12  are reversely rotated during the sheet registration, the leading end of the sheet P is prevented from being turned up by the elastic ring  17 . The another guide film  19  is disposed at a position in the width direction of the sheet P where the pinch rollers  12 C are not disposed, and between the guide films  18 . The end of the guide film  19  extends farther from the end of the guide film  18  toward the downstream side in the sheet feeding direction. With the guide films  18  and another guide film  19  functioning similar to the film  18 , the sheet P is properly conveyed. 
   The printer  1  according to the embodiment adopts a “center register” system that takes a center line as a reference for feeding the sheet P. Regardless of the sheet sizes, every sheet P passes the central portion of a sheet feeding path in the width direction thereof. Therefore, flicking of the sheet P or the improper sheet feeding caused by the application of the load (impact) to the rear end of the sheet P can be prevented by fitting the elastic rings  17  over the central pinch rollers  12 C provided for the central roller holders  13 C. Thus, variances in the sheet feeding amounts can be minimized for every sheet P and consequently, the poor image formation can be prevented. If the number of the roller holder  13  employed is three, that is, left, right, and central roller holders  13 L,  13 R,  13 C, the elastic ring  17  may be fitted over each of two central pinch roller  12 C provided for the central roller holder  13 C. 
   Even in printers that do not adopt the “center register” system, the elastic ring  17  may be fitted over every pinch roller  12 , as shown in  FIG. 11 . With this structure, variances in the sheet feeding amounts can be minimized for every sheet P, regardless of the sheet sizes, and consequently, the poor image formation can be prevented. For printers including the feed roller  11  and the pinch roller  12  that are not reversely rotated for the sheet registration, the elastic ring  17  may be fitted over every pinch roller  12 . For printers including the feed roller  11  and the pinch roller  12  that are reversely rotated for the sheet registration, the elastic rings  17  may be fitted preferably only over the pinch rollers that are disposed in the substantially central portion of the roller holder, to prevent the sheet from being improperly positioned during the sheet registration, and from being turned up, as well as to reduce costs. 
   In the sheet conveying device  10 , the sheet P is held between the metal feed roller  11  driven by a drive source and the plastic pinch roller  12  pressed against the feed roller  11 , and conveyed to the print head  21 . The elastic ring  17  having a high wear resistance is fitted over a part of the outer surface of the pinch roller  12  in the width direction thereof. Accordingly, the elastic deformation of the elastic ring  17  fitted over the pinch roller  12  reduces or absorbs load (impact) that causes the rear end of the sheet P to flick or spring and the improper sheet feeding, when the rear end of the sheet P passes through the nip portion between the feed roller  11  and the pinch roller  12 . Thus, variances in the sheet feeding amounts and the poor image formation are prevented. 
   When the feed roller  11  and the pinch roller  12  are reversely rotated during the sheet registration, the leading end of the sheet P is prevented from contacting the elastic ring  17  with the leading end being lifted, and the sheet P is favorably conveyed near the nip portion between the rollers  11 ,  12 , with the guide films  18 ,  19 . 
   While the invention has been described with reference to the embodiment, it is to be understood that the invention is not restricted to the particular forms shown in the foregoing exemplary embodiment. Various modifications and alterations can be made thereto without departing from the scope of the invention, as set forth in the appended claims. 
   For example, the elastic ring  17  may be fitted over the pinch roller  11 , as well as the feed roller  12 , though the elastic ring  17  is only fitted over the pinch roller  12  in the above-described embodiment. 
   In the above-described embodiment, one elastic ring  17  is fitted over one pinch roller  12 C. However, a plurality of the elastic rings  17  may be fitted over one pinch roller  12 C. 
   The guide films  18 ,  19  are separately disposed on the roller holder  13 . However, the guide films  18 ,  19  formed integrally with a downstream end thereof being separated into a plurality of parts, may be disposed on the roller holder  13 .