Sheet feeding device

A member for feeding sheets is provided with a core and a cover mounted on the core. The cover is made of a conductive material or one whose coefficient of friction is lower than that of the sheets.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a sheet feeding device including a sheet 
feed member. It is based upon Japanese Application no. 10-23480 filed on 
Feb. 4, 1998 and Japanese Application no. 98-6873 filed on Dec. 3, 1998, 
both of which are hereby incorporated by reference. 
2. Discussion of the Background 
An image forming apparatus (e.g., copiers, facsimiles, printers or similar 
image forming apparatus) uses various kinds of sheet feeding devices. For 
example, a sheet feed member may cooperate with a friction pad feed system 
to feed sheets by using a frictional force between the sheet feed member 
and the friction pad. Japanese Laid-Open Patent Publication No. 3-116325 
discloses such a sheet feed mechanism having a plurality of cams which are 
coaxially fixed to both ends of a sheet feed roller. However, that 
configuration requires the presence of a part devoted exclusively thereto, 
and so the cost of the apparatus inevitably increases. 
In the above sheet feed mechanism, the cams separate the sheet feed roller 
from a friction pad after a part of a top sheet fed from a stack and 
contacts the friction pad. A trailing edge of the top sheet is held 
between the friction pad and the sheet feed roller. However, it is 
possible that the sheet may be improperly fed because of a change in a 
state of contact against the sheet. 
Moreover, as shown in FIG. 11 and FIG. 12, it is possible that the sheet P, 
which is fed by the sheet feed roller 1 and the friction pad 3, may be 
frictionally charged by the cams 2, and a toner image may be improperly 
transferred (e.g., transfer hollow) on the sheet P. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of such problems and addresses 
efficiencies in known systems. 
Accordingly, it is an object of the present invention to provide a novel 
sheet feeding device having a stable sheet feeding, image transferring and 
inexpensive configuration. 
According to a feature of the invention, the above and other objects are 
achieved by a member for feeding sheets and comprising a core, a surface 
portion formed on the core so as to be rotatable, at least one portion of 
the circumferential extent of the surface portion being made of a material 
whose coefficient of friction is lower than that of sheets to be fed and 
at least another portion of the circumferential extent the surface portion 
being made of a material whose coefficient of friction is higher than that 
of the sheets. 
According to another feature of the invention, the above and other objects 
are achieved by a member for feeding sheets comprising a core having a 
surface portion, at least a portion of the surface portion being made of a 
conductive material. 
According to yet another feature of the invention, the above and other 
objects are achieved by a sheet feeding device comprising a cassette to 
hold a stack of sheets; a sheet feed member mounted so as to feed sheets 
one by one from said cassette, said sheet feed member including a core and 
a surface portion formed on said core so as to be rotatable, said surface 
portion having at least one circumferential portion being made of a 
material whose coefficient of friction is lower than that of sheets; and a 
friction member to pressed against said sheet feed member. 
According to yet another feature of the invention, the above and other 
objects are achieved by a sheet feeding device comprising cassette to hold 
a stack of sheets; a sheet feed member mounted so as to feed sheets one by 
one from said cassette, said sheet feed member including a core and a 
surface portion formed on said core so as to be rotatable, said surface 
portion having at least one circumferential portion being made of a 
conductive material; and a friction member mounted so as to be pressed 
against said sheet feed member.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is explained in detail hereinafter by the embodiments 
shown in the accompanied drawings. 
FIG. 1 shows a laser printer of an image forming apparatus in an embodiment 
of the present invention. As shown, the laser printer 11 as the image 
forming apparatus includes a photoconductive member 12 (e.g., a drum or a 
belt) which is located in the middle of the laser printer 11. A charge 
device 13 (e.g., a charge roller, a blade, a belt or a brush) charges the 
surface of the photoconductive member 12. A writing device 14 forms an 
electrostatic latent image. A developing device 15 develops the 
electrostatic latent image. 
A transfer device 17 (e.g., a transfer roller, a blade, a belt or a brush) 
transfers a toner image from the photoconductive member 12 to a sheet P. A 
cleaning device 18 cleans residual toner from the surface of the 
photosensitive member. A sheet-feeding device 10 or a by-pass-feeding 
device 16 feed the sheet P past the photoconductive member to form an 
image thereon. Sheets stored in the sheet-feeding device 10 may be fed 
thereby. Alternatively, non-standard paper sheets may be fed from the 
by-pass-feeding device 16. 
The sheet-feeding device 10 is provided with a cassette 21 that holds a 
stack of the sheets P, a sheet feed member 22 (called a roller 22) that 
feeds the sheets P from the cassette 21, a bottom plate 23 that is 
rotatably mounted and presses the leading edge of the sheet stack against 
the sheet feed roller 22 with a preselected pressure, and a friction 
member 24 (called a friction pad 24) that presses against the sheet feed 
roller 22. The sheet feed member described above is implemented as the 
roller 22, but it may instead be implemented as a belt or similar member 
if desired. The friction member described above is implemented as the pad 
24, but it may be implemented as a belt, a roller or similar member if 
desired. 
A pair of registration rollers 28 and a registration sensor 25 which are 
located between the sheet feed roller 22 and the transfer device 17 set 
feeding timing of the sheet P to a transfer area in which the 
photoconductive member 12 and the transfer device 17 are facing each other 
in synchronism with the image on the photoconductive member 12. 
The by-pass-feeding device 16 is provided with a by-pass feed table 29, a 
by-pass sheet feed roller 26, and a by-pass friction pad 27. The by-pass 
sheet feed roller 26 has the same structure as the sheet feed roller 22. 
A fixing device 31, which is located above the photoconductive member 12, 
fixes the toner image on the sheet P. A pair of discharge rollers 32 
discharges the sheet P fixed the image into a stack tray 33 which is 
located at an upper part of a cover 11a. 
A control device 35 which is located below the stack tray 33 is provided 
with a control board 36. Various switches and control units, not shown, 
are arranged on the control board 36. A power source 37, the writing 
device 14 and a case 39 storing a circuit board 38 are located below a 
case 34 holding the control board 36. 
The image forming operation will be described hereinafter. 
A laser beam which issues from the writing device 13 scans the charged 
surface of the photoconductive member 12, thus forming an electrostatic 
latent image in accordance with image data. The developing device 15 
develops the latent image with toner, thus forming a toner image on the 
photoconductive member 12. 
When the sheet feed roller 22 starts rotating, it grips a top sheet P of 
the sheet stack, and sends out the sheets P contained the cassette 21 to 
the registration rollers 28, one by one. The registration rollers 28 
control when the sheet P is to be fed to the transfer area, and sends it 
out. 
When the sheet P passes through the transfer area, the transfer device 17 
transfers the toner image from the photoconductive member 12 to the sheet 
P by using an electric field which is formed between the photoconductive 
member 12 and the transfer device 17. The fixing device 31 fixes the toner 
image on the sheet P with heat and pressure. The discharge rollers 32 
discharge the sheet P fixed with the toner image into the stack tray 33. 
Further, the by-pass sheet feed roller 26 and the by-pass friction pad 27 
also send out a top sheet P of the sheet stack held at the-by-pass feed 
table 29 to the registration rollers 28 one by one. 
FIG. 2 shows the roller structure of the sheet-feeding device 10 or the 
by-pass-feeding device 16. FIGS. 3(a) and (b) are end and side views 
showing the sheet feed roller 22 or the by-pass-feed roller 26 with the 
covering member 43 removed, from the front and side. FIGS. 4(a) and (b) 
show the covering member 43 from the front and side. 
The sheet-feeding device will be described hereinafter. 
The sheet feed roller 22 or the by-pass sheet feed roller 26 has a shaft 40 
made of resin or similar material, a core 41 formed on the shaft 40 and 
made of the same material as one body, a rotary body 42 which is a 
columnar roll formed on the core 41, and the covering member 43 (called 
the cover 43) which is fixed on a part of a surface of the rotary body 42. 
The rotary body 42 is made of rubber or similar material having a high 
friction coefficient (e.g., a coefficient of friction .mu.1 of 0.8 to 3.0) 
against the sheet P (e.g., having a coefficient of friction .mu.2 of 0.6 
to 0.7) in order to surely feed the sheet P. The cover 43 is made of 
metal, resin, ceramics, glass or similar material having a low friction 
coefficient (e.g., a coefficient of friction .mu.3 of 0.01 to 0.5) against 
the sheet P in order to smoothly feed the sheet P. Namely, the sheet feed 
roller 22 or roller 26 has at least one portion whose coefficient of 
friction is lower than that of the sheet and at least one portion whose 
coefficient of friction is higher than that of the sheet (e.g., 
.mu.3&lt;.mu.2&lt;.mu.1). Thus, at least one portion of the circumferential 
extent of the surface portion is made of a material whose coefficient of 
friction is lower than that of sheets to be fed and at least another 
portion of the circumferential extent the surface portion is made of a 
material whose coefficient of friction is higher than that of the sheets. 
The cover 43 has been described as being applied to either the sheet feed 
roller 22 or the by-pass sheet feed roller 26, but it is preferably 
applied to only the by-pass sheet feed roller 26. 
Moreover, the rotary body described above has a high frictional portion and 
a low frictional portion, but it may have more than two portions, if 
desired. 
The cover 43 may be made of an electrically conductive material, e.g., 
metal, resin, ceramics or similar material having a low electric 
resistance (e.g., having an electric resistance of 1.times.10.sup.12 
.OMEGA. or less (JISK6911)) in order to prevent the sheet P from being 
frictionally charged by the rotary body 42. The surface resistance is 
determined according to "resistivity" defined in JIS K 6911. Namely, the 
sheet feed roller 22 or roller 26 has at least one conductive surface 
portion. Moreover, the rotary body described above has a conductive 
portion, but it may have more than two conductive portions, if desired. 
The cover 43 has a fan-shaped portion 43d extending along the surface of 
the roller portion 42 and two planar pieces 43a having an edge curved in 
correspondence with the shape of the fan-shape portion 43d. The planar 
pieces 43a are each formed with two-forked portions, which have 
projections 43b at each end. 
Two engaging portions 41a, that engage with the projections 43b, are formed 
on each end of the core 41. The two engaging portions 41a engage with the 
projections 43b. The elastic rotary body 42 thus presses against the cover 
43 in a direction indicated by an arrow B (see FIG. 2). Consequently, the 
cover 43 is fixed on the surface of the rotary body 42. Further, the cover 
43 may be detached from the rotary body 42, if desired by unhooking the 
projections 43b from the engaging portions 41a. 
FIG. 5, FIGS. 6(a), (b), FIGS. 7(a) and (b) show a sheet-feeding device of 
the second embodiment. 
The sheet feed roller 50 has a shaft 51 made of conductive material, a core 
52 formed on the shaft 51, a rotary body 53 which is a columnar roll 
formed on the core 52 and the cover 54 which is fixed on a surface of the 
rotary body 53. 
The cover 54 has a fan-shaped portion 54d extending along the circular 
surface of the roller portion 53 and planar pieces 54a having an edge 
curved in correspondence with the shape of the fan-shape portion 54d. The 
planar pieces 54a are each formed with three-forked portions. Both outer 
ones of the three-forked portions have a ridge 54b on the inner surface of 
its end. The middle side portion of three-forked portions has a projection 
54c in its end. 
Two holes 52a that engage with the ridges 54b are formed on the core 52. 
The projections 54c each engages with the holes 52a. The elastic body 53 
presses against the cover 54 in a direction indicated by an arrow E (see 
FIG. 5). Consequently, the cover 54 is fixed on the surface of the rotary 
body 53. Further, the projection 54c engages the shaft 51, thereby 
electrically connecting the two. 
As stated above, in the illustrated embodiments, a conventional sheet feed 
roller is fitted with the cover 43. This decreases the number of the 
portions of the sheet feed device and accordingly construction can be 
simplified to accomplish a reduction of the cost of the apparatus. 
The sheet feeding operation will be described hereinafter. 
When the cassette 21 is set in the laser printer 11, the friction pad 24 
engages the surface of the fan-shape portion 43d of the sheet feed roller 
22. The friction pad 24 pressure against the sheet feed roller 22 is 
determined by a pressure spring (not shown) and is applied evenly to the 
sheet feed roller 22. The bottom plate 23 is also lifted up when the 
cassette 21 is set, and the leading edge of the sheet stack engages and is 
pressed against the sheet feed roller 22 by a preselected pressure. 
When a print start key (not shown) is turned on, the sheet feed roller 22 
starts rotating in the direction A. When the surface of the fan-shape 
portion 43d of the sheet feed roller 22 is held in contact with the top 
sheet P of the sheet stack under the preselected pressure, the top sheet P 
moves little because the friction force between the fan-shape portion 43d 
and the top sheet of the stack is low. But when the roller 22 has rotated 
such that the rotary body 42 is in contact with the top sheet P, the top 
sheet P is driven by the friction between the roller portion 42 made of 
the high friction material and the sheet P. Accordingly, the top sheet P 
can be surely fed toward the registration rollers 28. 
Sometimes sheets tend to stick together due to static electricity or poorly 
cut edges. When this happens, two or more sheets may feed between the 
rotary body 42 and the friction pad 24. However, the friction between the 
friction pad 24 and the lower sheet is much greater than that between the 
two sheets, so the top sheet slips through while the pad 24 holds the 
lower sheet. Therefore, during the sheet feed, the top sheet is separated 
from the stack and fed to the registration rollers. 
The sheet feed roller 42 rotates for one revolution, at which time the top 
sheet P sent out from the cassette 21 has reached the registration rollers 
28. A trailing edge of the top sheet P is then held between the friction 
pad 24 and the fan-shape portion 43d of the sheet feed roller 22 by the 
preselected pressure, in preparation for feeding to the transfer area. 
While the rotary body is described as making one rotation, it may instead 
make two rotations or more, if desired. 
The registration rollers 28 then start rotating in synchronism with the 
moving toner image on the photoconductive member 12, and send the top 
sheet P thus held to the transfer area. At that time, the trailing edge of 
the top sheet P is held between the friction pad 24 and the fan-shape 
portion 43d of the sheet feed roller 22 by the preselected pressure. 
However, since the friction force between the moving trailing edge of the 
top sheet P and the fan-shape portion 43d made of the low friction 
material is low, the top sheet P can be smoothly fed toward the transfer 
area by the registration rollers 28. 
Further, the pressure between the friction pad 24 and the sheet feed roller 
22 is prevented from changing during the sheet feeding operation because 
the contact between them is held stable. This prevents the two or more top 
sheets P to be sent out toward the registration rollers 28 from sticking 
together. Consequently, in the illustrated embodiment, the top sheets P 
can be surely fed one by one toward the transfer area. 
Further, the trailing edge of the top sheet P is held in contact with the 
cover 43 under the preselected pressure and moved toward the transfer area 
by the registration rollers 28 after the sheet feed roller 42 stops 
rotating. However, at that time the trailing edge of the top sheet P is 
prevented from charging due to friction because the cover 43 is made of 
the conductive material. Consequently, the illustrated embodiment is 
capable of producing attractive transfer images. 
FIG. 8 shows a state in which there is a difference in level C between a 
surface of the rotary body 42 and the cover 43 on the surface of the sheet 
feed roller 22. 
FIG. 9 shows a state in which there is no difference in level between the 
surface of the rotary body 42 and the cover 43 on the surface of the sheet 
feed roller 22. FIG. 10 is an enlarged drawing illustrating a portion D of 
FIG. 9. 
As shown in FIG. 8, if there is an extremely noticeable difference in level 
C on the surface of the sheet feed roller 22, there exists a possibility 
that the sheet feed roller 22 will fail to sufficiently feed the sheet, 
resulting in a defective sheet separation. Accordingly, as shown in FIG. 9 
and FIG. 10, a leading edge or a trailing edge of the fan-shape portion 
43d is bent radially inward from a circular arc, and so bites the surface 
of the rotary body 42. Thus the sheet feed roller 22 achieves a 
substantially smooth outer circumferential surface. As an alternative, a 
substantially smooth outer circumferential surface can be achieved without 
bending the leading or trailing edge of the fan-shape portion 43d, by 
pressing the entire fan-shape portion 43d onto the rotary body 42 with a 
higher pressure. 
Various modifications will become possible for those skilled in the art 
after receiving the teachings of the present disclosure without departing 
from the scope thereof. 
The above-mentioned embodiment has been explained with a structure and an 
arrangement of the sheet feed roller 22, but may also be suitably selected 
in matching relation to various condition including sheet feeding 
conditions. 
The above-mentioned values of coefficient of friction of the cover 43 or 
the rotary body 42 may also be suitably selected in matching relation to 
various condition including sheet feeding conditions. 
The above-mentioned values of electric resistance of the cover 43 or the 
rotary body 42 may also be suitably selected in matching relation to 
various condition including sheet feeding conditions. 
The above-mentioned arrangement of the cover 43 may also be suitably 
selected in matching relation to various condition including sheet feeding 
conditions. 
The covering member described above is implemented as the cover 43, but it 
may be implemented as seals made of a conductive material or a low 
friction material, tapes made of a conductive material or a low friction 
material or similar members, if desired. 
The sheet-feeding device have been shown and described as being used for 
copiers, facsimiles, printers or similar image forming apparatus. However, 
the invention is similarly applicable to any kind of sheet feeding device 
for various apparatus (e.g., printing presses, cash dispensers or similar 
apparatus).