Recording medium conveying device and image forming apparatus including the same

A recording medium conveying device for an image forming apparatus of the present invention includes a conveying member for conveying a sheet or similar recording medium by nipping it. After the sheet being conveyed by the conveying member has abutted against a registering member, the registering member rotates in synchronism with the rotation of an image carrier carrying an image thereon for thereby conveying the recording medium toward an image transfer position. After the recording medium has slackened on abutting against the registering member, a nip canceling mechanism cancels the nip of the conveying member acting on the recording medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus for forming an image on a paper sheet, OHP (OverHead Projector) film, intermediate image transfer body or similar recording medium and more particularly to a device included in the image forming apparatus for conveying the recording medium.

2. Description of the Background Art

Generally, in a recording medium conveying device included in an image forming apparatus, a sheet or similar recording medium is fed from a sheet cassette and conveyed along a preselected path. A roller pair nips the sheet fed from the sheet cassette and conveys it along the above path toward a registration roller pair. The registration roller pair once stops the sheet and then conveys it toward an image transfer station in synchronism with the rotation of an image carrier carrying a toner image thereon. At the image transfer station, the toner image is transferred from the image carrier to the sheet.

A problem with the conventional recording medium conveying device is that when, e.g., sheets are stacked on the sheet cassette in an inclined position, the sheet fed from the sheet cassette is apt to skew. It is a common practice to correct the skew of the sheet by causing the sheet to abut against the registration roller pair2and slacken.

Today, considering the increasing demand for the compact configuration of an image forming apparatus, it is difficult to provide a distance great enough for the sheet to sufficiently slacken on a path between the roller pair and the registration roller pair. However, if the distance between the roller pair and the registration roller pair and therefore the space available for the sheet to slacken is reduced, then heavy stress acts on the sheet and shifts it in, e.g., the widthwise direction. This brings about color shift in a color mode or brings about the bend of a straight line in a monochrome mode.

Japanese Paten Laid-Open Publication Nos. 7-261572 and 7-261573 each disclose a recording medium conveying device in which a pickup roller pays out sheets from a sheet cassette while a feed roller and a reverse roller cooperate to separate the top sheet from the underlying sheets. The sheet thus fed from the sheet cassette is conveyed by consecutive roller pairs until the leading edge of the sheet abuts against a registration roller pair, which is in a halt. Subsequently, the registration roller pair is rotated in synchronism with the rotation of a photoconductive drum carrying a toner image thereon, conveying the sheet. At this instant, an adhesion roller and an adhesion charger, which is disposed in the image transfer drum, cooperate to wrap the sheet around the drum being rotated. At a position short of an image transfer position, nip canceling means associated with the registration roller pair and roller pairs for conveyance is operated to cancel the nip acting on the sheet. The sheet is then brought to the image transfer position by the further rotation of the image transfer drum.

As stated above, the nip canceling means is associated with the registration roller pair, which conveys the sheet toward the image transfer position at preselected or reference timing, and cancels the nip of the roller pair. This, however, brings about a problem that the reference timing varies every time the nip of the registration roller is canceled, preventing the toner image from being transferred to the sheet with stable quality. Further, the charger and roller for adhesion increase the number of parts and overall size of the conveying device.

Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 8-91637.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide a recording medium conveying device capable of insuring high image quality by causing a recording medium to slacken for the removal of skew while obviating stress ascribable to the slackening, and reducing the number of parts to thereby implement a compact configuration, and an image forming apparatus including the same.

It is a second object of the present invention to provide a recording medium conveying device allowing a registering member to surely convey a recording medium free from skew to an image transfer position, and an image forming apparatus including the same.

It is a third object of the present invention to provide a recording medium conveying device capable of accurately obviating the influence of stress by accurately determining timing, and an image forming apparatus including the same.

It is a fourth object of the present invention to provide a recording medium conveying device capable of surely obviating the stress ascribable to slacking at the beginning of image transfer, and an image forming apparatus including the same.

It is a fifth object of the present invention to provide a recording medium conveying device capable of canceling nip acting on a recording medium with a simple configuration, and an image forming apparatus including the same.

It is a sixth object of the present invention to provide a recording medium conveying device capable of obviating image shift during image transfer even when a recording medium of relatively large size is used, and an image forming apparatus including the same.

It is a seventh object of the present invention to provide a recording medium conveying device capable of obviating the influence of stress in accordance with the length of a recording medium and obviating unnecessary operation of nip canceling means to thereby save power, and an image forming apparatus including the same.

It is an eighth object of the present invention to provide a recording medium conveying device configured to facilitate mounting and dismounting of the individual structural parts, and an image forming apparatus including the same.

It is a ninth object of the present invention to provide a recording medium conveying device capable of promoting the conveyance of a recording medium with a conveying member even after a nip on the recording medium has been canceled, and an image forming apparatus including the same.

A recording medium conveying device for an image forming apparatus of the present invention includes a conveying member for conveying a sheet or similar recording medium by nipping it. After the sheet being conveyed by the conveying member has abutted against a registering member, the registering member rotates in synchronism with the rotation of an image carrier carrying an image thereon for thereby conveying the recording medium toward an image transfer position. After the recording medium has slackened on abutting against the registering member, a nip canceling mechanism cancels the nip of the conveying member acting on the recording medium.

DESCRIPTION OF THE PREFERRED EMBODIMENT

To better understand the present invention, brief reference will be made to a conventional recording medium conveying device included in a copier or similar image forming apparatus, shown in FIG.1. As shown, the recording medium conveying device includes a roller pair or conveying member1and a registration roller pair or registering member2arranged on a path Y. A sheet or recording medium Z is fed from a sheet cassette, not shown, and conveyed along the path Y.

More specifically, the roller pair1nips the sheet Z fed from the sheet cassette to the path Y and conveys it along the path Y toward the registration roller pair2. The registration roller pair2once stops the sheet Z and then conveys it toward an image transfer station in synchronism with the rotation of an image carrier on which a toner image is formed. At the image transfer station, the toner image is transferred from the image carrier to the sheet Z.

Assume that the sheets stacked on the sheet cassette is inclined or that, as shown inFIG. 2, the roller pair1is inclined relative to the registration roller pair2by an angle α. Then, the sheet Z is conveyed askew. To correct the skew of the sheet Z, the registration roller pair2once stops the sheet Z and causes it to slacken.

Today, considering the increasing demand for the compact configuration of an image forming apparatus, it is difficult to provide a distance great enough for the sheet Z to sufficiently slacken on the path Y between the roller pair1and the registration roller pair2. However, if the distance between the roller pair1and the registration roller pair2and therefore the space available for the sheet Z to slacken is reduced, then heavy stress acts on the sheet Z and shifts it in, e.g., the widthwise direction. This brings about color shift in a color mode or brings about the bend of a straight line in a monochrome mode.

FIG. 3shows another conventional recording medium conveying device taught in Laid-Open Publication Nos. 7-261572 and 7-261573 mentioned earlier. As shown, the recording medium conveying device includes a path X extending from sheet cassettes3aand3bto an image transfer position T between a photoconductive drum or image carrier U and an image transfer drum V. Arranged on the path X are pickup rollers4, feed rollers5and reverse rollers6cooperating with each other, a plurality of roller pairs or conveying members7a,7b,7c,7dand7e, a registration roller pair or registering member8, and an adhesion roller9, as named from the upstream side in the direction of sheet conveyance.

In operation, the pickup roller4associated with, e.g., the sheet cassette3apays out sheets W while the feed roller5and reverse roller6cooperate to separate the top sheet W from the underlying sheets W. The sheet W thus fed from the sheet cassette3ais conveyed along the path X by the consecutive roller pairs7b,7cand7duntil the leading edge of the sheet W abuts against the registration roller pair8, which is in a halt. Subsequently, the registration roller pair8is rotated in synchronism with the rotation of the photoconductive drum Y carrying a toner image thereon, conveying the sheet W toward the image transfer position T. At this instant, the adhesion roller9and an adhesion charger, not shown, disposed in the image transfer drum V cooperate to wrap the sheet W around the drum V being rotated. At a position short of the image transfer position T, nip canceling means, not shown, associated with the registration roller pair8and roller pairs7dand7eis operated to cancel the nip acting on the sheet W. The sheet W is then brought to the image transfer position T by the further rotation of the image transfer drum V.

The recording medium conveying device with the above configuration has some problems left unsolved, as stated earlier.

Referring toFIG. 4, an image forming apparatus embodying the present invention is shown and implemented as a tandem, electrophotographic color copier by way of example. As shown, the color copier includes a copier body A in which a recording medium conveying device B is arranged for conveying a sheet or similar recording medium S. The recording medium conveying device B includes an inlet path P1, a manual feed path P2, a path P, and an outlet path P3, which will be described specifically later. The path P obliquely extends from the bottom right toward the top left, as viewed in FIG.1.

A magenta (M), a cyan (C), a yellow (Y) and a black (K) image forming means10M,10C,10Y and10K are sequentially arranged on the path P from the bottom right toward the top left in a tandem configuration. The image forming means10M, for example, includes an image carrier unit12M and a developing unit13M and is removably mounted to the copier body A. The image carrier unit12M includes a photoconductive drum or image carrier14M. Because the other image forming means10C,10Y and10K are identical in configuration with the image forming means10M, the structural elements of the former are simply distinguished by the structural elements of the latter by suffixes C, Y and K. An optical writing unit16is positioned above and inclined along the image forming means10M through10K.

An endless belt or recording medium support18is positioned below the image forming means10M through10K with the intermediary of the path P. In the illustrative embodiment, the belt18is passed over four rollers19with its upper run extending along the path P in contact with the drums14M through14K. A drive source, not shown, causes the belt18to move counterclockwise, as viewed in FIG.4.

Image transfer rollers20M,20C,20Y and20K and image transfer brushes21M,21C,21Y and21K are arranged between the opposite runs of the belt18and assigned to the drums14M,14C,14Y and14K, respectively.

A registration roller pair or registering member33and a fixing unit24are positioned on the path P upstream and downstream of the belt18, respectively. The fixing unit24includes an endless fixing belt25, a press roller26pressed against the belt25, and an outlet roller pair27. The outlet path P3contiguous with the path P extends from the outlet of the fixing unit24to an outlet roller pair31. The outlet roller pair31on the outlet path P3discharges the sheet S to a stack tray30positioned on the top of the copier body A.

Two sheet cassettes34are positioned below the belt18one above the other, and each is loaded with a stack of sheets S of a particular size. The inlet path P1extends from the right ends of the sheet cassettes34, as viewed inFIG. 4, for guiding the sheet S to the registration roller pair33positioned on the path P. Sequentially arranged on the inlet path P1are pickup rollers35, feed rollers36, back rollers37and a plurality of roller pairs or conveying members38, as named from the upstream side. The pickup roller35pays out the sheets S from one of the sheet cassette34associated therewith while the feed roller36and back roller37cooperate to separate the top sheet S from the underlying sheets S. The top sheet S fed from the sheet cassette34is conveyed by the roller pairs38.

A manual feed section43includes a foldable, manual feed tray39mounted on the right side of the copier body A, as viewed in FIG.4. The manual feed path P2extends from the left end of the manual feed tray39, as viewed inFIG. 4, to the registration roller pair33. A pickup roller40, a feed roller41and a back roller42are sequentially arranged on the manual feed path P2, as named from the upstream side.

A registration sensor44is positioned upstream of the registration roller pair33in the direction of sheet conveyance. The registration sensor14senses the leading edge of the sheet S being conveyed by the roller pair38.

In operation, when the operator of the copier pushes a start switch, not shown, the pickup roller35, feed roller36and back roller37assigned to one of the sheet cassettes34selected are driven to feed one sheet S to the inlet path P1. The roller pair38conveys the sheet along the inlet path P1. The sheet S is caused to abut against the registration roller pair33at preselected timing based on the output signal of the registration sensor44. The registration roller pair33in a halt stops the sheet S to thereby cause it to slacken. This is also true with the sheet S fed from the manual feed tray39except that the sheet S is conveyed along the path P2to the registration sensor44.

On the other hand, in the image forming means10M through10K, the drums14M through14K each are rotated to form a toner image of a particular color thereon. At the same time, a drive motor, not shown, causes one of the rollers19to rotate for thereby moving the belt18; the other rollers19are rotated by the belt18.

The registration roller pair33is caused to rotate in synchronism with the rotation of the drums14M through14K to convey the sheet S to the path P. Subsequently, the belt18in movement conveys the sheet P via consecutive image transfer positions E, F, G and H between the belt18and the image forming means10M through10K, respectively. While the belt18is in movement, the toner images of different colors formed on the drums14M through14K are sequentially transferred to the sheet S one above the other by the brushes21M through21K at the image transfer positions E through H, completing a full-color image on the sheet S. Of course, in a bicolor mode or a monochrome mode, for example, only toner images of desired colors or a toner image of a desired color will be transferred to the sheet S.

The sheet S carrying the toner image is conveyed to the fixing unit24and has the toner image fixed thereby. The sheet S coming out of the fixing unit24is further conveyed along the outlet path P3and then driven out to the stack tray30by the outlet roller pair31.

FIG. 5shows a specific configuration of nip canceling means45included in the illustrative embodiment and configured to cancel the nip of the roller pair38acting on the sheet S. As shown, the nip canceling means45includes a solenoid or drive means46and an arm50. The solenoid46is mounted on a bracket or support member47mounted on the copier body A and includes a plunger48formed with a slot49. A pin50ais studded on one end of the arm50and received in the slot49, thereby connecting the arm50to the plunger48. The other end of the arm50is bent upward, as viewed in FIG.5. The arm50is swingably mounted on a shaft51, which is supported by the bracket47, at its intermediate bent portion.

The roller pair38is made up of a driven roller38aand a drive roller38b. The driven roller38a, solenoid46that moves the driven roller38aaway from the drive roller38band a guide47afor guiding the sheet S are mounted on the bracket47, constituting a single unit. This facilitates mounting and dismounting of the individual structural elements. In the construction shown inFIG. 5, the bracket47is partly bent to form the guide plate47a. The driven roller38ais mounted on a shaft D adjoining the end of the arm50remote from the pin50a.

A drive source, not shown, causes the drive roller38bfacing the driven roller38awith the intermediary of the inlet path P1to rotate. A spring or biasing means, not shown, constantly biases the driven roller38atoward the driven roller38b. The driven roller38ais therefore usually held in contact with the drive roller38b, as shown in FIG.5.

When the sheet S arrives at the drive roller38band driven roller38a, the rollers38band38anip the sheet S and convey it further downward by the rotation of the roller38b. On abutting against the registration roller pair33, the sheet S slackens and has its skew corrected thereby. The drive roller38bis then caused to stop rotating. Subsequently, the registration roller pair33and roller pair38both are rotated in synchronism with the rotation of the drums14M through14K. As a result, the sheet S is conveyed toward the consecutive image transfer positions E through G while being nipped by the registration roller pair33.

After the registration roller pair33has nipped the sheet S, the solenoid46of the nip canceling means45is turned on before the leading edge of the sheet S arrives at the image transfer position E, retracting its plunger48downward, as viewed in FIG.5. The turn-on of the solenoid46may be effected in response to the output of the registration sensor44or a drive signal output from a CPU (Central Processing Unit)55, which drives the registration roller pair33. The plunger48so retracted pulls one end of the arm50for thereby causing the arm50to swing clockwise, as viewed inFIG. 5, about the shaft51against the action of the spring. Consequently, the other end of the arm50urges the shaft D of the driven roller38with the result that, as shownFIG. 6, the driven roller38ais moved away from the drive roller38b. This cancels the nip of the roller pair38acting on the sheet S. It is to be noted that the driven roller38adoes not have to be noticeably retracted away from the drive roller38b. The crux is that the nip acting on the sheet S be canceled only to such a degree that the sheet S can become straight with its own elasticity.

Even after the nip acting on the sheet S between the drive roller38band the driven roller38ahas been canceled, the drive roller38bshould preferably be continuously rotated so as to promote the conveyance of the sheet S, which may contact the drive roller38b, to the downstream side.

After the trailing edge of the sheet S has moved away from the roller pair38, the solenoid46is turned off to protrude its plunger48upward, as viewed inFIG. 6, thereby urging one end of the arm50. As a result, the arm50rotates counterclockwise about the shaft51while the driven roller38aagain contacts the drive roller38bunder the action of the spring, as shown in FIG.5.

As stated above, in the illustrative embodiment, the nip canceling means45cancels the nip of the roller pair38acting on the sheet S after the skew of the sheet has been corrected by slackening. This successfully obviates the influence of stress ascribable to the slackening of the sheet S and thereby allows an image to be stably formed on the sheet S without any shift. In addition, the distance between the roller pair38and the registration roller pair33can be reduced only if the nip canceling means45is added, so that the number of parts is reduced to implement a compact construction.

If desired, sheet size sensing means may be associated with each of the sheet cassettes34. In such a case, when the sheets S stacked on either one of the sheet cassettes34are of relatively large size, e.g., A3 or B4, the nip of a plurality of roller pairs38acting on the sheet S can be canceled when the registration roller pair33nips the sheet in accordance with the output of the sheet size sensing means. With this configuration, too, the advantages described above are achievable because a length for removing the slack of the sheet S is guaranteed despite the large size of the sheet S.

Further, the number of roller pairs38whose nip should be canceled may be varied in accordance with the length of the sheet S determined by the sheet size sensing means. This obviates unnecessary operation of the nip canceling means45and thereby saves power while obviating the influence of stress ascribable to the slackening of the sheet S.

While the drive member and driven member of the conveying member both are implemented as rollers in the illustrative embodiment, one or both of them may be implemented as belts, if desired. While the illustrative embodiment has concentrated on an image forming apparatus of the type directly transferring toner images from the drums14M through14K to the sheet S, the present invention is similarly applicable to an image forming apparatus of the type transferring toner images from drums to a sheet by way of an intermediate image transfer body.

In summary, it will be seen that the present invention provides a recording medium conveying device and an image forming apparatus having the following various advantages. The influence of stress ascribable to the slackening of a recording medium and therefore image shift. This frees a color image from color shift and frees a monochromatic image from, e.g., the bending of a straight line for thereby insuring high image quality. This advantage is achievable without regard to the size of the recording medium. A distance between a roller pair for conveyance and a registration roller pair can be reduced by a minimum number of parts, implementing a compact configuration.

Further, the registration roller pair can surely convey the recording medium free from skew to an image transfer position. The influence of stress ascribable to the slackening of the recording medium can be adequately obviated at correct timing. Wasteful power consumption and noise are obviated.