Patent Application
20110156336
1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
As a conventional image forming apparatus such as a copying machine, a laser beam printer, a facsimile machine, a word processor and a multi functional machine thereof, there is one which forms an image on a sheet using an electrophotographic system. In such an image forming apparatus, after an image is formed on one side (first side) of a sheet by an image forming portion, the sheet is again conveyed to the image forming portion, and an image is formed on the other side (second side) of the sheet.
To form an image on the second side of the sheet, the image forming apparatus which forms an image on both sides of a sheet includes a turn-over apparatus, a re-conveying path and a re-conveying roller to switch back a sheet having an image formed on its first side and to turn over the sheet. For example, an image forming apparatus such as a small printer includes, as the turn-over apparatus, a reversely-rotatable roller which can rotate normally and reversely, and when an image is formed on the second side of the sheet, the reversely-rotatable roller is rotated normally and reversely, and the sheet is conveyed to the re-conveying path while switching back the sheet (see Japanese Patent Application Laid-open No. 2000-109275). As the conventional image forming apparatus, to enhance the productivity when a sheet is turned over, there is an image forming apparatus in which the reversely-rotatable roller is driven by a motor capable of rotating normally and reversely, and a pulling velocity is changed by switch back according to a receiving velocity of the sheet (see Japanese Patent Application Laid-open No. 2006-56682).
In the case of an image forming apparatus such as a small printer, generally, a sheet is conveyed by a fixing device having a stable conveying force so that an image can be formed stably without depending on an image pattern and types of sheets. To suppress a curl of a sheet generated by heat applied to the sheet at the time of a fixing operation, the reversely-rotatable roller is made to convey the sheet at a faster velocity than the fixing device, the sheet is pulled and the curl is corrected (see Japanese Patent Application Laid-open No. 61-75369). In the case of such an image forming apparatus, a sheet conveying velocity is set such that the following relation is established: conveying roller velocity (before forming an image)≈(substantially equal) image forming velocity≦fixing velocity<reversely-rotatable roller velocity, so that an image can be formed stably and a curl can be corrected.
In the conventional image forming apparatus such as the small printer, when an image is formed on a first side, non-fixed toner adheres to a sheet in some cases. To form an image on a second side, if the sheet to which toner adheres is conveyed to the re-conveying path, the toner on the sheet adheres to the re-conveying roller provided on the re-conveying path, and an amount of toner adhering to the re-conveying roller is increased according to the number of sheets which pass.
As described above, the sheet conveying velocity of each sheet conveying portion has the relation of a conveying roller velocity (before forming an image)≈image forming velocity fixing≦velocity<reversely-rotatable roller velocity. A velocity relation between the reversely-rotatable roller and the conveying roller which are driven by the normally and reversely rotatable motor and which carry out switch back of a sheet is reversely-rotatable roller>>conveying roller. Here, if a velocity of the re-conveying roller which conveys a sheet which is switched back by the reversely-rotatable roller to the conveying roller is set to satisfy a relation of reversely-rotatable roller>re-conveying roller>conveying roller, a sheet forms a loop between the re-conveying roller and the conveying roller by a velocity difference.
If a sheet forms a loop, even after a rear end of the sheet passes through the re-conveying roller, the rear end of the sheet abuts against a surface of the re-conveying roller and slips for a while until the loop is eliminated. At that time, if toner of a certain level or more adheres to the re-conveying roller, toner adheres to the rear end of the sheet from the re-conveying roller due to this slip. As a result, a stain is generated on the rear end of the sheet during the re-feeding operation of the sheet, i.e., a tip end of the first side at a location which is contact with a roller outer periphery of the re-conveying roller. For example, a band-shaped stain having a roller width of 2 to 4 mm of the re-conveying roller is generated on the tip end of the first side.
If the velocity is set to satisfy a relation of re-conveying roller velocity<conveying roller velocity so that a sheet does not form a loop between the re-conveying roller and the conveying roller, a relation of reversely-rotatable roller velocity>>>re-conveying roller velocity is established, and an excessively large loop is generated between the reversely-rotatable roller and the re-conveying roller. If the excessively large loop is generated, the sheet is formed into an accordion shape at a location of the re-conveying path upstream of the re-conveying roller in the sheet conveying direction, a printing side of the sheet rubs against an inner surface of a conveying guide constituting the re-conveying path, and an image stain is generated. Since it is difficult to control a loop in the conveying guide, a sheet cannot be delivered to the re-conveying roller in an orderly fashion due to a curl generated at the fixing portion, and this can cause a paper jam.
The present invention has been accomplished in view of such circumstances, and the invention provides an image forming apparatus capable of forming images on both sides of a sheet without generating an image stain.
The invention has been accomplished in view of such circumstances, and the invention provides an image forming apparatus capable of forming images on both sides of a sheet in a state where a stain caused by toner is little.
The present invention provides an image forming apparatus including an image forming portion, a conveying roller which is provided upstream of the image forming portion in a sheet conveying direction, and which conveys a sheet to the image forming portion, a re-conveying path which conveys, to the image forming portion, a sheet having an image formed on its one side by the image forming portion, a normally and reversely rotatable reversely-rotatable roller which is provided downstream of the image forming portion in the sheet conveying direction, which normally rotates and conveys the sheet having the image formed on its one side at a sheet conveying velocity that is faster than that of the conveying roller and then, which reversely rotates and conveys the sheet to the re-conveying path, and a re-conveying roller which is provided on the re-conveying path, and which conveys the sheet to the conveying roller, wherein the sheet conveying velocity of the re-conveying roller is set substantially equal to or slower than the sheet conveying velocity of the conveying roller, and the sheet conveying velocity of the reversely-rotatable roller when it reversely rotates is slower than the sheet conveying velocity of the reversely-rotatable roller when it normally rotates so that the sheet conveying velocity of the reversely-rotatable roller when it reversely rotates becomes substantially equal to or slower than the sheet conveying velocity of the re-conveying roller.
By setting the sheet conveying velocity of the reversely-rotatable roller when the reversely-rotatable roller is reversely rotated slower than the sheet conveying velocity when the reversely-rotatable roller is normally rotated as in this invention, the sheet conveying velocity of the re-conveying roller can be substantially equal or slower than the sheet conveying velocity of the conveying roller. According to this, it is possible to form images on both sides of a sheet in a state where a strain caused by toner is little.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Embodiments of the present invention will be described in detail using the drawings.
The image forming portion 100B includes a detachable process cartridge 10 integrally provided with process members such as a photosensitive drum 10a which is an image bearing member, a development sleeve 10d, a charging roller 10c, and a cleaning blade 10e. The image forming portion 100B includes a laser exposure apparatus 17 which exposes a surface of the photosensitive drum 10a to form an electrostatic latent image on the photosensitive drum.
The sheet feeding portion 100C includes a sheet tray 1 which can open and close and on which sheets P are stacked, a restricting plate 2 which restricts a position of a sheet P in a width direction intersecting with the sheet conveying direction, and a feeding roller 4 which feeds the sheets P on a sheet tray sheet by sheet. After the feeding roller 4 receives a sheet feeding-operation starting signal from a controlling portion (not illustrated) by a one-rotation controlling portion (not illustrated), the feeding roller 4 makes one rotation and feeds a sheet P toward the image forming portion 100B.
The transfer portion 100D includes the photosensitive drum 10a, and a transfer roller 11 which presses the photosensitive drum 10a to form a transfer nip, and which transfers a toner image on the photosensitive drum 10a to a sheet P when the sheet P passes through the transfer nip. The re-conveying portion 100E includes a re-conveying path 21 which turns over a sheet P and conveys the sheet P to the image forming portion 100B, a re-conveying roller 22 provided on the re-conveying path 21, and a merging conveying path 23 which conveys a sheet conveyed by the re-conveying roller 22 to a conveying roller 8. In
Next, an image forming operation in the laser beam printer 100 having the above-described configuration will be described.
When the image forming operation is started, the main motor 16 is rotated by a sheet feeding-operation starting signal from the controlling portion (not illustrated), and the feeding roller 4 rotates in a direction of the arrow. With this, a sheet feeding cam (not illustrated) which is coaxial with the feeding roller 4 also rotates, a sheet feeding plate 3 turns upward in association with a cam follower (not illustrated) which is engaged with the sheet feeding cam, and pushes a sheet P against the feeding roller 4. The feeding roller 4 sends out the sheet P by friction between the feeding roller 4 and the sheet P.
Meanwhile, when the feeding roller 4 rotates, the sheets P are separated at the same time by a separating pad 5 which is pressed against a separating pad spring 7. As a result, one sheet P is fed. A sheet feeding cam (not illustrated) which is coaxial with the feeding roller 4 pushes down the sheet feeding plate 3 to a sheet feeding standby position immediately before one rotation of the feeding roller 4 is completed. The sheet P which is fed by one rotation is conveyed by the conveying roller 8 provided upstream of the image forming portion 100B in the sheet conveying direction, and the sheet P turns a sheet tip end sensor 9. If the sheet tip end sensor 9 is turned in this manner, a photo sensor (not illustrated) is turned ON, and the controlling portion detects a tip end position of the sheet P. Thereafter, if a predetermined time is elapsed, the laser exposure apparatus 17 irradiates the photosensitive drum 10a with a laser beam based on image information.
If the image forming operation is started, the photosensitive drum 10a rotates in a direction of the arrow, and is uniformly charged by the charging roller 10c to a predetermined polarity and to predetermined potential. If the photosensitive drum 10a after its surface is charged as described above is irradiated with a laser beam, an electrostatic latent image is formed on the photosensitive drum 10a. Next, as the development sleeve 10d rotates, toner in the toner container 10b appropriately charged is supplied onto the photosensitive drum 10a, and the electrostatic latent image is developed and visualized as a toner image.
Next, the visualized toner image on the photosensitive drum is transferred to a sheet P by the transfer roller 11. Transfer residual toner not transferred and remained on the photosensitive drum is accommodated in a waste toner container 10f by the cleaning blade 10e, and the photosensitive drum 10a whose surface is cleaned is repeatedly used for a next image forming process. Next, the sheet P on which the toner image is formed is heated and pressed by a fixing portion 12 including a fixing heating member 12a and a fixing pressure roller 12b, and the toner image is permanently fixed on the sheet. Thereafter, the sheet P on which the toner image is fixed is discharged out from the printer body from a discharge opening 20 by a discharge roller 14 provided downstream of the image forming portion 100B in the sheet conveying direction and by a discharge rolling element 14a which follows the discharge roller 14. The sheet discharged from the discharge opening 20 is stacked on a discharge tray 15.
When images are formed on both sides of a sheet P, a reverse-rotation sensor 18 provided on a discharge conveying path 19 located between the discharge roller 14 and the fixing portion 12 detects a rear end of a sheet P and thereafter, the controlling portion (not illustrated) rotates the discharge roller in a direction opposite from the arrow at predetermined timing. According to this, the sheet P is switched back and reversely conveyed on the re-conveying path 21. Then, the sheet P is conveyed in the merging conveying path 23 by the re-conveying roller 22 and a re-conveying rolling element 22a which follows the re-conveying roller 22. The sheet conveyed by the re-conveying roller 22 is again guided to the conveying roller 8, the sheet is subjected to the image forming process which is the same as the one-sided printing, and the sheet is stacked on the discharge tray 15 by the discharge roller 14.
Next, the drive transmitting system which transmits a driving force from the main motor 16 to the feeding roller 4, the conveying roller 8, the re-conveying roller 22, the discharge roller 14 and the photosensitive drum 10a will be described using
The main motor 16 which is a first driving source fixed to a printer body 100A includes a motor pulley 31 which outputs rotation driving force of the main motor 16, and the motor pulley 31 is drive-connected to a deceleration pulley 33 through a drive belt 32. A deceleration gear 33a which transmits rotation transmitted from the drive belt 32 to a drive train located downstream is integrally formed on the deceleration pulley 33. The deceleration gear 33a is connected to a first idler gear 50, and the first idler gear 50 is drive-connected to a three-system drive train including a drum drive gear 40, a second idler gear 42 and a third idler gear 46. The drum drive gear is coaxially drive-connected to the photosensitive drum 10a through a coupling (not illustrated), and rotates the photosensitive drum 10a.
The second idler gear 42 transmits a driving force to the sheet feeding drive gear 34 through a fourth idler gear 43. The sheet feeding drive gear 34 is biased in a direction of the arrow by a biasing spring (not illustrated), and an engaging projection 34a of a solenoid 41 is engaged with the sheet feeding drive gear 34 against this biasing force. In a state where the engaging projection 34a is engaged, several teeth of the sheet feeding drive gear 34 are lost so that the sheet feeding drive gear 34 does not mesh with the fourth idler gear 43.
The solenoid 41 is operated when the sheet feeding operation is started, and engagement between the engaging projection 34a and the sheet feeding drive gear 34 is released, and if the engagement between the engaging projection 34a and the sheet feeding drive gear 34 is released, the sheet feeding drive gear 34 starts to rotate in the direction of the arrow by the biasing spring. As a result, the sheet feeding drive gear 34 is drive-connected to the fourth idler gear 43. Thereafter, if the sheet feeding drive gear 34 makes one rotation, the sheet feeding drive gear 34 and the engaging projection 34a of the solenoid 41 are again engaged with each other, and the sheet feeding drive gear 34 stops rotation. It is possible to intermittently drive the feeding roller 4 at any timing by such control of the solenoid 41.
The fourth idler gear 43 transmits a driving force to a conveying roller drive gear 35 fixed to the conveying roller 8 through a fifth idler gear 44. The conveying roller drive gear 35 transmits a driving force to a re-conveying roller drive gear 37 fixed to the re-conveying roller 22 through a sixth idler gear 45. The third idler gear 46 transmits a driving force to a pressure roller drive gear 36 which rotates the fixing pressure roller 12b of the fixing portion 12 through seventh and eighth idler gears 47 and 48.
In
The driving operation of the discharge roller 14 is independent from the main motor 16 in this manner, the discharge roller 14, the re-conveying roller 22 and the conveying roller 8 can be driven independently. As a result, a sheet conveying velocity when the discharge roller 14 reversely rotates can be made slower than a sheet conveying velocity when the discharge roller 14 normally rotates irrespective of sheet conveying velocities of the re-conveying roller 22 and the conveying roller 8. In other words, the sheet conveying velocities of the re-conveying roller 22 and the conveying roller 8 can be set without any relation to the sheet conveying velocity of the discharge roller 14, and the sheet conveying velocity of the re-conveying roller 22 can be set slower than that of the conveying roller 8.
Next, the drive transmitting system from the discharge driving motor 16a will be described. The discharge driving motor 16a includes a motor pinion 38 which outputs a rotation driving force of the discharge driving motor 16a, and the motor pinion 38 is connected to a discharge roller drive gear 39 fixed to the discharge roller 14 through a ninth idler gear 49.
Since no clutch mechanism is provided between the discharge driving motor 16a and the discharge roller 14, switch between normal rotation and reverse rotation of the discharge driving motor 16a uniquely determines the rotation direction of the discharge roller 14. In this embodiment, the discharge driving motor 16a is drive-controlled such that the discharge driving motor 16a rotates in a direction of the arrow CW (clockwise direction) when a sheet is discharged of one-sided printing and duplex printing, and rotates in a direction CCW (counterclockwise direction) when a sheet is turned over of duplex printing.
Next, sheet conveying velocities of the conveying rollers of the printer body 100A concerning conveyance of a sheet will be described. The number of teeth of the motor pulley 31 is defined as ZMM, the number of teeth of the drum drive gear 40 is defined as ZDR, the number of teeth of the sheet feeding drive gear is defined as ZPU, the number of teeth of the conveying roller drive gear 35 is defined as ZFP the number of teeth of the re-conveying roller drive gear is defined as ZRF, and the number of teeth of the pressure roller drive gear 36 is defined as ZPR. The number of teeth of the motor pinion 38 is defined as ZDP, and the number of teeth of the discharge roller drive gear 39 is defined as ZFD.
Outer diameters of rollers of the photosensitive drum 10a, the feeding roller 4, the conveying roller 8, the re-conveying roller 22, the pressure roller 12b and the discharge roller 14 are defined as DDR, DPU, DFP, DRF, DPR and DFD. A deceleration ratio of the deceleration pulley 33 is defined as FRD, and the number of rotations of the motor pulley 31 of the main motor 16 and the motor pinion 38 of the discharge driving motor 16a are defined as RMM and RFD. The number of rotations and the sheet conveying velocities of the rollers are as shown in Table 1.
The sheet conveying velocities of the photosensitive drum 10a, the feeding roller 4, the conveying roller 8, the re-conveying roller 22 and the pressure roller 12b are defined as VDR, VPU, VFP, VRF and VPR. The sheet conveying velocity of the discharge roller 14 in the discharge direction is defined as VFD1 and the sheet conveying velocity thereof in the reverse rotation direction is defined as VFD2. In this embodiment, a relation of velocities of the rollers is
VPU≈VFP≈VDR<VPR<VFD1 (1)
VFD2≈VRF≦VFP (2)
Examples of setting of the outer diameters of the rollers and the number of teeth of the gears which can obtain the above-described sheet conveying velocities are shown in Table 2. In Table 2, calculation is carried out while setting the deceleration ratio of the deceleration pulley 33 to 4.
By setting the sheet conveying velocity VRF of the re-conveying roller 22 slower than the sheet conveying velocity VFP of the conveying roller 8, a sheet does not form a loop between the rollers. The sheet conveying velocity VRF of the re-conveying roller and the sheet conveying velocity VFD2 of the discharge roller 14 in the reverse rotation direction are set substantially equal to each other, or the sheet conveying velocity VFD2 is set slower than the sheet conveying velocity VRF. According to this, an excessively large loop is not generated between the discharge roller 14 and the re-conveying roller 22.
As described above, in this embodiment, the discharge roller 14 is driven by the discharge driving motor 16a which can normally and reversely rotate and which can change the rotation velocity. According to this, the sheet conveying velocity VFD2 of the discharge roller 14 in the reverse rotation direction can be made slower than the sheet conveying velocity VFD1 of the discharge roller 14 in the discharge direction. As a result, the sheet conveying velocity VRF of the re-conveying roller 22 can be made substantially equal to or slower than the sheet conveying velocity VFP of the conveying roller 8.
By setting the sheet conveying velocity of the re-conveying roller 22 in this manner, a sheet does not form a loop between the re-conveying roller 22 and the conveying roller 8. As a result, it is possible to avoid a case where a sheet slips in a state in which a rear end of the sheet is in abutment against the re-conveying roller 22 after the sheet passes through the re-conveying roller 22. According to this, even if a stain of the re-conveying roller 22 caused by fog toner at the time of printing of a first side develops, it is possible to carry out the duplex printing without generating a mark of the roller on the sheet.
By making the driving operation of the discharge roller 14 independent from the main motor 16, the sheet conveying velocity VRF of the re-conveying roller 22 and the sheet conveying velocity VFD2 of the discharge roller 14 in the reverse rotation direction can be made substantially equal to each other. According to this, a printing side of a sheet does not rub against an inner surface of the conveying guide constituting the re-conveying path 21, and generation of an image stain can be avoided.
In this embodiment, to simplify the description, only the deceleration pulley 33 is described as a mechanism which decelerates the driving from the main motor 16 and transmits the same, but in addition to the deceleration pulley 33, finely adjusting deceleration gears may be added to front stages of the rollers. In the embodiment of the invention, although the driving of the motor pinion 38 and the driving of the discharge roller 14 are connected through the ninth idler gear 49, the ninth idler gear 49 may be omitted. Further, the motor pinion 38 and the discharge roller 14 may be directly connected to each other through a coupling, of course. On the contrary, a gear for deceleration may be added between the motor pinion 38 and the discharge roller 14. Although rotation of the main motor 16 is transmitted through the drive belt 32 in the embodiment, this may be changed to a gear train, of course.
Next, a second embodiment of the invention will be described.
Next, a configuration of the second deceleration gear 54 will be described using
A one-way clutch 54d is fixed to an inner diameter of the output gear 54c. A needle (not illustrated) of the one-way clutch 54d is engaged with the rotation shaft 54a only when the input gear 54b rotates in the direction CW illustrated in
Since the image forming apparatus includes the first and second deceleration gears 54 and 55 having this configuration, rotation output from the motor pinion 38 is transmitted to the tenth idler gear 53, the number of rotations of the tenth idler gear 53 is adjusted by the second deceleration gear 54 at the time of CCW. The number of rotations of the tenth idler gear 53 is adjusted by the first deceleration gear 55 at the time of CW. Drive of the discharge driving motor 16a whose number of rotations is adjusted through the second deceleration gear 54 or the first deceleration gear 55 is transmitted to eleven to fourteenth idler gears 56 to 59, and the discharge roller drive gear 39 fixed to the discharge roller 14 is driven ultimately.
In this embodiment, the discharge roller 14 is rotated in the discharge direction by the first deceleration gear 55 which constitutes a first gear mechanism having the input gear 54b and the one-way clutch 54d. The discharge roller 14 is rotated in the reverse rotation direction by the second deceleration gear 54 constituting a second gear mechanism having the same configuration.
If the number of rotations of the motor pinion 38 (discharge driving motor 16a) is the same in the discharge direction and the reverse rotation direction, the sheet conveying velocity of the discharge roller 14 can freely be set in the discharge direction and the reverse rotation direction by setting the number of teeth of the first and second deceleration gears 54 and 55. This embodiment is constituted such that if a deceleration ratio of the first deceleration gear 55 is set to R3 and deceleration ratios of the second deceleration gear 54 are set to R3 and R4, the deceleration radio becomes R3>R4.
By setting the deceleration ratios of the first and second deceleration gears 54 and 55 in this manner, the sheet conveying velocity of the discharge roller 14 in the discharge direction can be made faster than that of the conveying roller 8. Further, the sheet conveying velocity of the discharge roller 14 in the reverse rotation direction can be made slower than the sheet conveying velocity of the discharge roller 14 in the discharge direction. As a result, the sheet conveying velocity of the re-conveying roller 22 can be made substantially equal to the sheet conveying velocity of the discharge roller 14 in the reverse rotation direction.
Also in this embodiment, since the driving operation of the discharge roller 14 is independent from the main motor 16, the sheet conveying velocity of the re-conveying roller 22 can be made slower than the sheet conveying velocity of the conveying roller 8. According to this, a sheet does not form a loop between the re-conveying roller 22 and the conveying roller 8.
That is, since a transmission mechanism having different deceleration ratio of the motor pinion 38 according to normal and reverse rotation having the first and second deceleration gears 54 and 55 is provided in the drive system train which drives the discharge roller 14 as in this embodiment, the same effect as that of the first embodiment can be obtained. Further, since it is unnecessary to adjust the number of rotations of the motor pinion 38 in each of normal rotation and reverse rotation, it is easy to control. A DC motor which can only rotate normally and reversely can be used as the discharge driving motor instead of the stepping motor, it is possible to inexpensively provide a laser beam printer (image forming apparatus) capable of printing on both sides having little image disturbance.
Next, a third embodiment of the invention will be described.
In this embodiment, rotation output from the motor pinion 38 is transmitted to the re-conveying roller gear 74 through twentieth to twenty-second idler gears 71 to 73. In this embodiment, a driving force of the discharge driving motor 16a is transmitted to the discharge roller 14 and the re-conveying roller 22 by a second drive transmitting portion including the idler gears. That is, in this embodiment, the discharge roller 14 and the re-conveying roller 22 are driven by the discharge driving motor 16a through the second drive transmitting portion.
This re-conveying roller gear 74 is connected to the re-conveying roller 22 through a one-way clutch gear (not illustrated) coaxially fixed to the re-conveying roller gear 74, and transmits a rotation driving force only in the direction CW shown with the arrow in the drawing. In this embodiment, when the discharge driving motor 16a CCW-rotates the motor pinion 38, the motor pinion 38 is rotated at two velocities. The two velocities concerning CCW rotation of the motor pinion 38 are defined as CCW1 and CCW2, respectively. Here, CCW1 is set equal to or faster than CW, and CCW2 is set to such a velocity that when the sheet conveying velocity of the re-conveying roller 22 is defined as V1 and the sheet conveying velocity of the conveying roller 8 is defined V2, a relation V1≦V2 is established.
Next, operation sequence at the time of duplex printing of the laser beam printer having the above-described configuration will be described using a timing chart which illustrates a state of time series of the solenoid 41, the sheet tip end sensor 9, the reverse-rotation sensor 18 and the motor pinion 38 illustrated in
If the printer body 100A starts the image forming operation, the solenoid 41 is energized and turned ON (t0) and a sheet feeding operation by the feeding roller 4 is started. Next, if a sheet tip end passes through the sheet tip end sensor 9 (t1), then a toner image is formed on the photosensitive drum 10a, and this toner image is transferred to the sheet. The toner image is heated and pressed in the fixing portion 12 constituted by the fixing heating member 12a and the fixing pressure roller 12b and the toner image is permanently fixed onto the sheet. Next, if the sheet P passes through the reverse-rotation sensor 18 (t2), a tip end of the sheet P is once discharged from the printer body 100A through the discharge opening 20.
Next, the reverse-rotation sensor 18 does not detect existence of a sheet (t3) and then, if a given time is elapsed (t4), CW rotation of the motor pinion (discharge driving motor 16a) is switched to the CCW direction. The rotation velocity of the motor pinion at that time is set to CCW1. If the rotation velocity is set to CCW1, since the discharge roller 14 and the re-conveying roller 22 rotate at a velocity equal to or faster than CW, the productivity at the time of the turning over operation of a sheet can be enhanced.
Thereafter, if the sheet tip end passes through the re-conveying roller 22 and predetermined timing before the sheet tip end reaches the conveying roller 8 comes (t5), the number of rotations of the motor pinion 38 is switched from CCW1 to CCW2. If the number of rotations is set to CCW2, since the relation between the sheet conveying velocity V1 of the re-conveying roller 22 and the sheet conveying velocity V2 of the conveying roller 8 become V1≦V2, a sheet does not form a loop between the re-conveying roller 22 and the conveying roller 8.
Thereafter, the sheet P conveyed by the conveying roller 8 again reaches the sheet tip end sensor 9 (t6), rotation of the motor pinion 38 again returns to the CW rotation. At that time, if a long sheet whose rear end is nipped by the re-conveying roller 22 is fed, a state where a velocity difference is generated between the re-conveying roller 22 and the conveying roller 8 continues for a long time in some cases. However, the re-conveying roller 22 idles by the effect of the one-way clutch incorporated in the re-conveying roller gear 74. Therefore, a case where a rear end of a sheet and the re-conveying roller 22 pull each other is not generated by changing the number of rotations of the motor pinion 38. Then, if the reverse-rotation sensor 18 does not detect the existence of sheets again (t7), the motor pinion 38 rotates in the CW direction until the sheet rear end is completely discharged from the discharge opening 20 and then, the motor pinion 38 stops (t8).
As described above, in this embodiment, the discharge roller 14 is driven by the discharge driving motor 16a which can normally and reversely rotate and which can change the rotation velocity. According to this, the sheet conveying velocity of the discharge roller 14 in the reverse rotation direction can be made slower than the sheet conveying velocity of the discharge roller 14 in the discharge direction. Further, the discharge driving motor 16a drives not only the discharge roller 14 but also the re-conveying roller 22. According to this, the sheet conveying velocity of the re-conveying roller 22 can be made slower than the sheet conveying velocity of the conveying roller 8 immediately before the re-conveying roller 22 reaches the conveying roller 8, and a sheet does not form a loop between the re-conveying roller 22 and the conveying roller 8. When images are formed on both sides, the sheet can be conveyed at a high velocity until the sheet reaches the conveying roller 8 and as a result, it is possible to provide a laser beam printer (image forming apparatus) having high productivity.
Next, a fourth embodiment of the invention will be described.
In
The first planetary gear unit 88 is also drive-connected to the twenty-fourth idler gear 83. An engaging projection 86 controls rotation operations of first and second planetary gear units 85 and 88, the engaging projection 86 is rotatably axially supported on the side plate 101, and is driven by a reversely rotatable solenoid 87 which is a switching member fixed to the side plate 101. The twenty-fourth idler gear 83 drives the discharge roller 14 through a twenty-fifth idler gear 84 and the discharge roller drive gear 39.
Next, detailed configurations of the first and second planetary gear units 85 and 88 will be described using
The second planetary gear unit 85 includes a carrier 85a which is axially supported by a shaft 85A between a drive side plate 89 and the side plate 101, and which rotatably axially supports two planetary gears 85c. The planetary gear 85c, the sun gear 85b and the ring gear 85d which mesh with the planetary gear 85c are rotatably axially supported by the carrier 85a coaxially. A ratchet 85g which is an engaging portion is formed on the sun gear 85b. An input gear 85e is integrally formed on an outer periphery of the carrier 85a, and an output gear 85f is integrally formed on an outer periphery of the ring gear 85d.
If the ratchet 85g of the second planetary gear unit 85 is engaged with the engaging projection 86, the sun gear 85b is locked, and the second planetary gear unit 85 functions as a planetary gear mechanism which is classified into a solar type. Since the first planetary gear unit 88 whose engagement with the engaging projection 86 is released is not fixed to any of a sun gear 88b, a carrier 88a and a ring gear 88d, the first planetary gear unit 88 functions as an idler gear which does not transmit a driving force.
Next, a clutch mechanism by the engaging projection 86 will be described using
First, the operation when the discharge roller 14 rotates in the discharge direction will be described. The reversely rotatable solenoid 87 is not energized, and an actuator 87a is in a home position illustrated in
According to this, a rotation driving force shown with the arrow from the twenty-third idler gear 80 which is a drive input gear is transmitted from the input gear 85e of the carrier 85a of the second planetary gear unit 85 to an input gear 88e of the carrier 88a of the first planetary gear unit 88. If the carrier 88a rotates, this rotation is transmitted as shown with the arrow illustrated in
Next, if the reversely rotatable solenoid 87 is energized, an actuator 87b is sucked in a direction of the arrow illustrated in
According to this, a rotation driving force from the twenty-third idler gear 80 is transmitted to the input gear 85e of the carrier 85a of the second planetary gear unit 85, and the carrier 85a rotates. The rotation of the carrier 85a is transmitted as indicated by the arrow illustrated in
In this embodiment, when the deceleration ratios of the first and second planetary gear units 85 and 88 are defined as R5 and R6, a relation R5>R6 is established. Table 3 shows examples of the settings of the number of teeth and the like of the first and second planetary gear units 85 and 88 and the setting of the number of teeth of the gear of the drive system.
By adjusting the gear ratios of the input gears 85e and 88e and the output gears 85f and 88f in this manner, the sheet conveying velocity of the discharge roller 14 at the time of discharge can be made greater than that at the time of reverse rotation. Even if the number of teeth of the sun gears 85b and 88b and the planetary gears 85c and 88c of the first and second planetary gear units 85 and 88 are set as shown in Table 4, the sheet conveying velocity of the discharge roller 14 at the time of discharge can be made greater than that at the time of reverse rotation.
By such setting, sheet conveying velocities of the rollers of the printer body 100A are as shown in Table 5 when the number of rotations of the main motor is set to 9.10 RPS and the deceleration ratio of the deceleration pulley 33 is set to 4, for example.
As described above, in this embodiment, a driving force of the main motor 16 which rotates in one direction is transmitted to the discharge roller 14 through the first and second planetary gear units 85 and 88 having different deceleration ratios. According to this, the sheet conveying velocity of the discharge roller 14 in the reverse rotation direction can be made slower than the sheet conveying velocity of the discharge roller 14 in the discharge direction. As a result, the sheet conveying velocity of the re-conveying roller 22 can be made slower than that of the conveying roller 8, and it is possible to inexpensively provide a laser beam printer (image forming apparatus) having little image disturbance.
In the above description, when images are formed on both sides, the discharge roller is described as one example of the reversely-rotatable roller which normally rotates to convey a sheet by a predetermined distance at the sheet conveying velocity which is faster than the sheet conveying velocity of the conveying roller and then, the reversely-rotatable roller reversely rotates. However, the invention is not limited to this, and when the image forming apparatus includes a reversely-rotatable roller having the above-described function in addition to the discharge roller, the invention can be applied to this reversely-rotatable roller.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2009-295161, filed Dec. 25, 2009, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-295161 | Dec 2009 | JP | national |
