Patent Application
20080116626
1. Field of the Invention
The present invention relates to a sheet feeder and an image forming apparatus, and more particularly, to a mechanism for feeding sheets one sheet by one sheet.
2. Description of the Related Art
A conventional image forming apparatus such as a printer and a copier includes a sheet feeder for feeding sheets to an image forming portion. Some of sheet feeders include a paper feed roller for feeding sheets set in a sheet tray, a sheet separator for separating for separating the fed sheets one sheet by one sheet, and a convey roller which is provided downstream of the sheet separator and which conveys the separated sheet. The sheet separator brings the separation pad into contact with the paper feed roller under pressure, and the sheets are separated from each other between the paper feed roller and the separation pad. In the separation pad type sheet separator, the separated sheet is conveyed by the convey roller and the sheet is sandwiched between the paper feed roller and the separation pad. Therefore, this sheet separator has a problem that a back tension (load) when the sheet is conveyed is high. Hence, there is a technique in which the separation pad is separated from the paper feed roller after the sheet is separated, the sheet is applied a force toward the separation pad by a rotatable roller, thereby reducing the back tension when a sheet is fed. This technique is disclosed in Japanese Patent Application Laid-open No. 2001-7221.
A conventional sheet feeder which applies a force to a rotatable roller toward the separation pad will be described with reference to
According to a sheet feeder shown in
If the sheet feeding operation is started, as shown in
Next, the paper feed roller 103 and the middle plate 102 are pressed through the sheet P as shown in
If an image formation on the sheet P is carried out and the discharging operation is completed, the return lever acting cam 110 rotates to act on the return lever 10 as shown in
When the above-described series of sheet feeding operation is completed and the next sheet P is to be fed, the above-described operation is repeated.
The sheet feeder described in the description of the related art includes the rockable middle plate, and the loaded sheets are fed by moving the middle plate. The paper feed roller and the separation pad are separated from each other during the paper feeding operation in association with movement of the middle plate. Therefore, back tension generated by them is reduced.
In the mechanism for reducing the back tension as described above, however, there is a problem that the application of the mechanism is limited to a sheet feeder using the middle plate. Further, in the mechanism for reducing the back tension, a relatively large member, i.e., the middle plate must move. Thus, it is difficult to simplify and reduce the size of the mechanism itself and thus, the entire sheet feeder.
If the sheet feeder is reduced in size, a guide space through which a sheet is conveyed becomes narrow, and the conveying direction of a sheet is changed many times, the curve becomes acute. Thus, when the feeding direction is abruptly changed immediately after the sheet passes through the paper feed roller, there is a problem that a rear end of the sheet jumps and an impact noise is generated.
The present invention has been accomplished in view of the above circumstances, and it is an object of the invention to provide a sheet feeder and an image forming apparatus capable of reducing back tension inexpensively, and stably feeding a sheet.
The present invention provides a sheet feeder including a sheet tray which supports sheets, a paper feed roller which feeds the sheets supported by the sheet tray, and a sheet separator having a separation roller and a separation member which separates a sheet fed from the paper feed roller one by one, the sheet feeder comprising;
a holding member which holds the separation member and which is applied a force in a direction where the separation member comes into contact with the separation roller,
a rockable arm member which is rockable provided an holds the paper feed roller on an upstream side in a sheet feeding direction, and which has a pressing portion capable of pressing the holding member on a downstream side in the sheet feeding direction, and
an operation mechanism which rocks the arm member in a direction where the paper feed roller is separated from the sheets supported by the sheet tray and the pressing portion presses the holding member.
Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).
The best mode for carrying out the present invention will be described in detail using the drawings.
That is, the image forming apparatus 1 is provided at its lower portion with a sheet feeder 100 for feeding sheets, and with an image forming portion 40 for forming an image on a sheet S fed from the sheet feeder 100. As shown in
The image forming portion 40 includes a process cartridge 41 having a photosensitive member drum 41a, a developing sleeve 41c, a charging roller 41b and a toner container 41d. The image forming portion 40 includes a laser scanner 42 for exposing a surface of the photosensitive member drum 41a to the light and forming an electrostatic latent image on the photosensitive member drum 41a. The laser scanner 42 includes a laser light-emitting portion (not shown), a rotating polygon mirror 42a and a folded mirror 42b.
The sheet feeder 100 includes a sheet tray 11 in which sheets are accommodated, a paper feed roller 5 for feeding sheets on the sheet tray 11, and a separation roller 7 for separating the sheet fed from the paper feed roller 5 between the separation pad 9 shown in
The transfer portion 51 includes a photosensitive member drum 41a, and a transfer roller 52 which comes into contact with the photosensitive member drum 41a under pressure to form a transfer nip. When a sheet passes through the transfer nip, the transfer roller 52 transfers a toner image on the photosensitive member drum 41a to the sheet. The fixing portion 61 includes a heating roller 61a and a pressure roller 61b which comes into contact with the heating roller 61a under pressure.
Next, an image forming operation of the image forming apparatus 1 having such a structure will be described.
If the image forming operation is started, the photosensitive member drum 41a first rotates in the direction of the arrow (see
Toner in the toner container 41d is appropriately charged by rotation of the developing sleeve 41c, and the toner is supplied onto the photosensitive member drum 41a and adhered to the electrostatic latent image the image is developed and made visible as a toner image.
With the toner image forming operation, sheets accommodated in the sheet tray 11 are sent to the paper feed roller 5, the sheets are separated and conveyed by the separation pad 9 and the separation roller 7, and conveyed to the transfer portion 51. A toner image formed on the photosensitive member drum 41a by the transfer portion 51 is transferred onto a predetermined position on the sheet S by the transfer roller 52.
Next, the sheet S on which the toner image is transferred is conveyed to a fixing nip comprising a heating roller 61a and a pressure roller 61b provided on the fixing portion 61. Here, non-fixed toner image is heated, pressurized and fixed to a sheet surface. The sheet after the toner image is fixed thereto is discharged onto a discharge tray 13 by a pair of discharge rollers 62 and 63. Residual toner which is not transferred and remains on the photosensitive member drum 41a is accommodated in a toner waste container 41f by a cleaning blade 41e. The photosensitive member drum 41a whose surface is cleaned is repeatedly brought into next image forming process.
Next, the sheet feeder 10 will be described in detail with reference to
The sheet feeder 100 includes a conveying inclined surface 15 which is continuous from the sheet tray 11 in a downstream direction of the sheet feeding direction in the sheet tray 11 fixed to the apparatus main body. A pick arm (arm member) pivotally supported by a separation roller shaft (rocking shaft) 6 is disposed above a conveying inclined surface 15 such that the pick arm 4 can rock around the separation roller shaft 6 by an operation mechanism D such as a solenoid or motor in directions of arrows R1 and R2.
The paper feed roller 5 which comes into contact with a sheet which is accommodated in the sheet tray 11 and which conveys the sheet is disposed at an end 16 of the pick arm 4 on the upstream side in the sheet feeding direction. The separation roller 7 is disposed on the separation roller shaft 6 on which the pick arm 4 is pivotally supported such that the separation roller 7 is coaxial with the separation roller shaft 6. The separation roller shaft 6 which is a rotation shaft of the separation roller 7 and the rotation shaft 5a of the paper feed roller 5 are disposed in parallel to each other.
A pressure roller (pressure member) 2 is rotatably supported at end of the pick arm 4 on the downstream side in the sheet feeding direction. The separation pad 9 is in contact with the separation roller 7 under pressure. A sheet separator 17 comprises the separation roller 7 and the separation pad 9. The separation pad 9 is disposed in the separation pad holder (separation pad holding member) 8, and a pressure receiving roller (pressure receiving member) 3 is rotatably opposed to the pressure roller 2 of the pick arm 4 at an end of the separation pad holder 8 on the downstream side in the sheet feeding direction. These pressure roller 2 and pressure receiving roller 3 constitute a roller portion 19 which guides a sheet separated in the sheet separator 17 in a sending out direction from the sheet separator 17.
The separation pad holder 8 is rockably pivotally supported by a support shaft 8a. The separation pad 9 can come into contact and separate from the separation roller 7. The separation pad holder 8 is applied a force by a spring 20 which is a resilient member such that the separation pad 9 is in contact with the separation roller 7 under pressure.
After a sheet sent from the paper feed roller 5 reaches the sheet separator 17 and separates therefrom, the paper feed roller 5 separates the pick arm 4 from the sheet, and the operation mechanism D rocks in a direction in which pressure roller 2 presses the pressure receiving roller 3 of the separation pad holder 8.
The present invention is characterized in the operation when a sheet is fed in the sheet feeder 100. Next, this operation will be described with reference to
In a feeding state of sheet shown in
At that time, since the sheet S which is being fed is conveyed while being sandwiched between the separation roller 7 and the separation pad 9, a back tension is applied to the sheet S. A reason why the back tension is applied to the sheet S is to reliably separate the batch of sheets by the separation roller 7 and the separation pad 9 (separate a sheet S from subsequent sheets).
After a tip end of the sheet S is fed to the sheet separator 17 constituted by the separation roller 7 and the separation pad 9 and the sheet is separated, the pick arm 4 rocks around the separation roller shaft 6 in a direction of the arrow R1 (see
If the paper feed roller 5 retreats, the pressure roller 2 disposed at the rocking end of the pick arm 4 on the downstream side in the sheet feeding direction comes into contact with the pressure receiving roller 3 under pressure through the sheet S. As show in
Then, the separation pad holder 8 which supports the pressure receiving roller 3 is pushed down by a pressing force from the pressure roller 2, and a sheet separating pressure in the sheet separator 17 (pressure welding pressure between the separation roller 7 and the separation pad 9) is reduced. Therefore, the separation pad pressure is reduced, and the back tension in the sheet separator 17 is reduced. Next, the sheet separator 17 and the back tension in the roller portion 19 will be described with reference to
As shown in
back tension of the sheet separator 17=μp×P+μr×P×(R′/R)+T/R,
wherein,
μr: friction coefficient between separation roller shaft 6 and bearing,
μp: friction coefficient between sheet S and separation pad 9,
As shown in
back tension of roller portion 19=2×μ2×P2×(r′/r),
wherein,
μ2: friction coefficient between roller shaft 2 and bearing
r: radius of pressure roller 2, and
r′: radius of pressure roller 2.
Therefore, from the above-described relationships, the following equations can be obtained:
back tension amount to be reduced={back tension of sheet separator 17 (at the time of pick up)}
−{back tension of sheet separator 17}(at the time of reduction operation)
−{back tension of roller portion 19}
={μp×P+μr×P×(R′/R)+T/R}
−{μp×P′+μr×P′×(R′/R)+T/R}
−{2×μ2×P2×(r′/r)}
={μp+(R′/R)×μr}×(P−P′)
−{2×μ2×P2×(r′/r)}, wherein
Therefore, although a back tension is generated also in the roller portion 19, its amount is very small, and the amount of back tension generated as a whole is reduced as a result.
In the above-described embodiment, the pick arm 4 is rocked by the operation mechanism D in the direction of the arrow R1. At that time, the pressure roller 2 presses the pressure receiving roller 3 and the separation pad holder 8 through a sheet S separated by the separation roller 7 and the separation pad 9, and the separation pad pressure between the separation roller 7 and the separation pad 9 is reduced. With this, it is possible to reduce the back tension generated by the separation roller 7 and the separation pad 9 with a small and simple mechanism. That is, after a sheet reaches the sheet separator 17 and the sheet is separated, the separation pad holder 8 is pressed the separation pad 9 is separated from the separation roller 7, and the back tension at the time of feeding operation of sheets can be reduced inexpensively.
Since the pressure roller 2 and the pressure receiving roller 3 presses a sheet S which is being conveyed near a downstream side of the sheet feeding direction in the sheet separator 17, it is possible to prevent a rear end of the sheet from jumping when the sheet S passes through the sheet separator 17, and the sheet can fed stably.
Next, a second embodiment of the present invention will be described.
A sheet feeder 100 of this embodiment is characterized in that the roller of the separation pad holder 8 is eliminated and only the pick arm 4 has the pressure roller 2. Therefore, when the pick arm 4 is rocked in the direction of the arrow R1, the pressure roller 2 presses a surface 21 of the separation pad holder 8 through the sheet S. The surface 21 is smoothly formed, and a friction resistance is small. Therefore, even if a sheet S is sent between the pressure roller 2 and the surface 21 in a state where the separation pad pressure is reduced, a friction load acting as a resistance at the time of paper feeding operation is low. Thus, if the structure shown in this embodiment is employed, a mechanism for reducing the back tension in the sheet feeder 100 can be made compact, simple and inexpensive, and the assembling performance can be enhanced.
Although the separation pad pressure is reduced to reduce the back tension in this embodiment, the separation roller 7 and the separation pad 9 may be separated from each other and the separation pad pressure may be set to 0.
The sheet feeder and the image forming apparatus of the present invention are effective for a sheet feeder having no middle plate, and are suitable for a sheet feeder and an image forming apparatus which must be small and simple.
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. 2006-310816, filed Nov. 16, 2006, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-310816 | Nov 2006 | JP | national |
