Abstract:
The present invention provides a sheet supplying apparatus comprising a main frame, a sheet supporting device provided within the main frame to support a sheet, a sheet supply rotary device for feeding out the sheet supported by the sheet supporting device, a recovery device attached to the main frame to recover a sheet supplying force of the sheet supply rotary device, and a switching device for shifting the recovery device between a recovery position to be abutted against the sheet supply rotary device to effect a recovery operation and a waiting position to be spaced apart from the sheet supply rotary device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet supplying apparatus in which a sheet is separated and supplied from a plurality of stacked sheets and which is used with an image forming apparatus such as a printer, a copying machine and the like. 
     2. Related Background Art 
     In conventional image forming apparatuses (for example, printers), there is provided an auto sheet feeder (ASF) as a sheet supplying apparatus for separating and supplying a single sheet from a plurality of stacked sheets (such as paper sheets). 
     In such an auto sheet feeder, there is provided a sheet supply roller (pick-up roller) or a sheet supply belt for supplying an uppermost sheet or a lowermost sheet from a sheet stack (stacked sheets) while applying a frictional force to the sheet, and, a bank or separation pawl(s) is provided for separating a single sheet to prevent a plurality of sheets from being supplied together with the uppermost or lowermost sheet, or, pads having high coefficient of friction (separation pads) are provided in a confronting relation to the sheet supply roller to prevent the other sheets from shifting together with the uppermost or lowermost sheet. 
     However, in the above-mentioned conventional techniques, if a surface of the sheet supply roller or the sheet supply belt is degenerated or hardened (due to reduction in coefficient of friction) for long term use, since a frictional force (between the sheet supply roller and the sheet) is reduced not to achieve the proper sheet supply. Further, when paper powder (from the sheet) and/or dirt is adhered to the surface of the sheet supply roller, the frictional force is also reduced not to achieve the proper sheet supply. 
     In order to eliminate the poor sheet conveyance due to the above causes, the sheet supply roller or the sheet supply belt must be dismounted from the apparatus to clean it or to polish the surface thereof by a grinding sheet kit, the maintenance is worsened. 
     Further, in recent years, ink jet printers have been developed as compact printers. In such ink jet printers, a sheet having a coating (for example, silica) layer at its recorded surface is sometimes used for the purpose of controlling the ink fixing ability and ink blotting. In this case, the coating material is adhered to the surface of the sheet supply roller in addition to the paper powder to reduce the sheet supplying force to thereby cause the poor sheet supply. 
     SUMMARY OF THE INVENTION 
     The present invention intends to eliminate the above-mentioned conventional drawbacks, and has an object to provide a sheet supplying apparatus and an image forming apparatus, in which a sheet can be properly conveyed by restoring a reduced friction force and which can provide a high quality image and good maintenance. 
     To achieve the above object, according to the present invention, there is provided a sheet supplying apparatus comprising a main frame, a sheet supporting means provided within the main frame for supporting a sheet, a sheet supply rotary means for feeding out the sheet supported by the sheet supporting means, a recovery means attached to the main frame for recovering a sheet supplying force of the sheet supply rotary means, and a switching means for shifting the recovery means between a recovery position where the recovery means abuts against the sheet supply rotary means to effect a recovery operation, and a waiting position where the recovery means is spaced apart from the sheet supply rotary means. 
     Further, the present invention may provide a sheet supplying apparatus comprising a main frame, a sheet supporting means provided within the main frame for supporting sheets, a sheet supply rotary means for feeding out the sheet supported by the sheet supporting means, a friction member disposed on the sheet supporting means at a position confronting to the sheet supply rotary means and contacted with a lowermost sheet among the sheets supported by the sheet supporting means, a recovery means for recovering a sheet supplying force of the sheet supply rotary means, and a switching means for selectively shifting the friction member or the recovery means to a position confronting to the sheet supply rotary means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A, 1B, 1C and 1D are explanatory views for explaining an operation of a sheet supplying apparatus; 
     FIG. 2 is a flow chart showing an automatic operation of a recovery means; 
     FIG. 3 is a schematic sectional view of an ink jet image forming apparatus (image forming apparatus) having a sheet supplying apparatus; 
     FIGS. 4A and 4B are explanatory views showing a main part of a sheet supplying apparatus according to another embodiment of the present invention in which a cleaning pad and a polishing pad are shifted in parallel; 
     FIGS. 5A and 5B are explanatory views showing a main part of a sheet supplying apparatus according to a further embodiment of the present invention in which a cleaning pad and a polishing pad are rotatingly shifted; 
     FIGS. 6A and 6B are explanatory views showing a main part of a sheet supplying apparatus according to a still further embodiment of the present invention in which a cleaning pad and a polishing pad can be contacted with a pick-up roller simultaneously; 
     FIG. 7 is a schematic explanatory view of a recovery means according to another embodiment, looked at from a sheet supplying direction; 
     FIGS. 8A and 8B are explanatory views showing a sheet supplying condition and an operation of the recovery means; 
     FIGS. 9A and 9B are views of a recovery means, looked at from a sheet supplying direction, where FIG. 9A shows a schematic construction and FIG. 9B shows a sheet supplying condition; 
     FIGS. 10A and 10B are views showing an operation of the recovery means, where FIG. 10A shows a polishing operation and FIG. 10B shows a cleaning operation; 
     FIGS. 11A and 11B are views of a recovery means of a sheet supplying apparatus having two pick-up rollers, looked at from a sheet supplying direction, where FIG. 11A shows a schematic construction and FIG. 11B shows a sheet supplying condition; 
     FIGS. 12A and 12B are views showing an operation of the recovery means, where FIG. 12A shows a polishing operation and FIG. 12B shows a cleaning operation; 
     FIGS. 13A and 13B are views of the recovery means of the sheet supplying apparatus, looked at from a direction perpendicular to the sheet supplying direction, where FIG. 13A shows a schematic construction and FIG. 13B shows a sheet supplying condition; 
     FIGS. 14A and 14B are views showing an operation of the recovery means, where FIG. 14A shows a polishing operation and FIG. 14B shows a cleaning operation; 
     FIG. 15 is an enlarged view of a main part of a sheet supplying apparatus according to a further embodiment of the present invention; 
     FIGS. 16, 17 and 18 are explanatory views showing an operation of the apparatus of FIG. 15; 
     FIG. 19 is an enlarged view of a main part of a sheet supplying apparatus according to a still further embodiment of the present invention; 
     FIGS. 20, 21 and 22 are explanatory views showing an operation of the apparatus of FIG. 19; and 
     FIG. 23 is an explanatory view showing an operation of a main part of a sheet supplying apparatus according to the other embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First of all, the entire construction of an image forming apparatus will be explained, and then, the construction of a sheet supplying apparatus will be explained. 
     &lt;Image Forming Apparatus&gt; 
     As shown in FIG. 3, an image forming apparatus P according to a preferred embodiment of the present invention is embodied as an ink jet printer of ink jet recording type. After a plurality of sheets S are stacked on a sheet supply tray 1, when a recording operation is started, the sheets S are separated and supplied one by one by a sheet supplying apparatus ASF which will be described later. The supplied sheet S is conveyed, by a convey means 2, to a recording area, where a predetermined image is formed on the sheet by an image forming means 3. Thereafter, the sheet is discharged out of the printer. 
     The convey means 2 comprises a convey roller 2a for conveying the supplied sheet S to the recording area, a pinch roller 2b urged against the convey roller and driven by the rotation of the convey roller, a discharge roller 2c for discharging the recorded sheet S out of the printer, and a spur roller 2d urged against the discharge roller and driven by the rotation of the discharge roller. Incidentally, the spur roller 2d is rotated while contacting with the imaged (recorded) surface of the sheet S but has a smaller contact area with the sheet S so that, even when the spur roller is contacted with the ink imaged surface of the sheet S, the ink image is not distorted. 
     In the illustrated embodiment, an ink jet recording system is used as the image forming means 3. More specifically, the image forming means includes a platen 3a for supporting a rear surface (non-imaged surface) of the sheet S to be conveyed, and a carriage 3b disposed in a confronting relation to the platen. The carriage 3b is reciprocally shifted by a motor (not shown) in a sheet width-wise direction (perpendicular to a sheet conveying direction) along a long guide shaft 3c and a guide rail 3d, and an ink cartridge 3e integrally incorporating an ink jet recording head and an ink tank is removably mounted on the carriage 3b. By shifting the carriage 3b and by discharging ink from the recording head, a predetermined image is recorded on the sheet S. The recording head includes fine liquid discharge openings (orifices), liquid passages communicating with the respective discharge openings, energy acting portions disposed in the respective liquid passages, and energy generating means for generating liquid droplet forming energy acting on the liquid in the acting portions. 
     In the illustrated embodiment, the ink is discharged from the discharge opening of the recording head by growth and contraction of a bubble generated in the ink by film boiling of the ink caused by thermal energy produced by applying current to an electrical/thermal converter in response to a recording signal to thereby effect the recording. Further, the recording head is movable so that a distance between an ink discharge surface (including the discharge opening) of the head and the sheet S can be adjusted in accordance with a thickness of the sheet S. 
     The sheet supplying apparatus ASF, convey means 2 and image forming means 3 are driven by a battery (power source) 4, and the driving thereof is controlled by a control means provided on a control board 6 in accordance with command from an operation panel 5. 
     As shown in FIGS. 1A to 1D, 2 and 3, the sheet supplying apparatus ASF for separating and supplying the sheets S one by one includes a pick-up roller (sheet supply rotary member) 7 for applying a sheet supplying force to the sheet S, separation pads (separation members) 8 for separating the sheets S supplied by the pick-up roller 7, and a recovery means 9 for recovering reduction in the sheet supplying force of the pick-up roller 7. 
     The pick-up roller 7 has two rubber roller portions. A coefficient of friction between the pick-up roller and the sheet is greater than a coefficient of friction between the sheets. When the pick-up roller is rotated by the motor, the sheet supplying force is applied to the sheet S from the pick-up roller. The pick-up roller 7 is rotatably supported by a pivotable arm for up-and-down movement in accordance with the sheet picking-up operation. 
     The separation pads 8 are formed from material having high coefficient of friction and serve to urge the sheets S against the pick-up roller 7 so that, even if a plurality of sheets are supplied by the pick-up roller 7, the supply of the sheets other than an uppermost sheet is prevented. Thus, only the uppermost sheet is separated and supplied. 
     The recovery means 9 serves to recover the reduction in the sheet supplying force of the pick-up roller 7 and includes a cleaning means for cleaning a surface of the pick-up roller 7 when silica coated on the sheet S and/or paper powder are adhered to the surface of the pick-up roller 7, and a polishing means for polishing the surface of the pick-up roller 7 when the surface of the pick-up roller 7 is degenerated or hardened. 
     The cleaning means includes two cleaning pads 9a attached to a plate-shaped holder 10 at positions in a confronting relation to two rubber roller portions of the pick-up roller 7, and the polishing means includes two polishing pads 9b attached to the plate-shaped holder 10 at positions in a confronting relation to two rubber ruller portions of the pick-up roller 7. 
     The cleaning means may be of dry cleaning type for cleaning in a dry condition or of wet cleaning type for cleaning by using cleaning liquid, but the wet cleaning is more effective than the dry cleaning. The cleaning pad may be formed from hydrophilic non-woven fabric or plastic obtained by sintering polyorefin particles. The cleaning liquid impregnated into the cleaning pad may be ethanol or water which has high contamination removing ability and high friction force recovering ability. 
     The polishing means may be formed from a member having hardness greater than that of the roller to be polished and having a rough surface. For example, a &#34;sand paper&#34; obtained by adhering particle polishing agent to a sheet substrate, a member obtained by adhering polishing agent to nylon or polyester nonwoven fabric, a member obtained by adhering particle polishing agent to a polyester sheet, or porous member (for example, porous plastic obtained by sintering polyorefin particles of about 100 μm) may be used. 
     Incidentally, the surfaces of the cleaning pads 9a and the polishing pads 9b are positioned below a contact surface between the separation pad 8 and the pick-up roller 7 so that the pads 9a, 9b do not affect a bad influence upon the sheet S during the sheet supplying operation. 
     The holder 10 to which the cleaning pads 9a and the polishing pads 9b are attached is provided at its lower surface with a rack 10a which is engaged by a pinion gear 11 provided on a main frame of the apparatus, so that, when the pinion gear 11 is rotated by a motor (not shown), the holder are slid in a left-and-right direction in FIGS. 1A to 1D. 
     Next, an operation of the sheet supplying apparatus ASF will be explained with reference to FIGS. 1B to 1D. FIG. 1B shows a normal sheet supplying condition. In this condition, the separation pads 8 are opposed to rubber roller portions of the pick-up roller 7 to cooperate with the latter to separate the sheets S one by one. 
     When the surface of the pick-up roller 7 is degenerated or hardened to reduce the sheet supplying force for the long term use, as shown in FIG. 1C, the pinion gear 11 is rotated to slide the holder 10 to the right to thereby oppose the polishing pads 9b to the rubber roller portions of the pick-up roller 7. Then, the pick-up roller 7 is lowered to urge it against the polishing pads 9b and is rotated. Thus, the surface of the pick-up roller 7 is polished by the polishing pads 9b to thereby scrape the degenerated or hardened surface of the pick-up roller. 
     Thereafter, the pick-up roller 7 is lifted. Then, as shown in FIG. 1D, the holder 10 is slid to the left to oppose the cleaning pads 9a to the rubber roller portions of the pick-up roller 7. And, similar to the polishing pads 9b, the pick-up roller 7 is urged against the cleaning pads 9b and is rotated. In this way, paper powder and/or coating silica adhered to the surface of the pick-up roller 7 or chips scraped by the polishing pads 9b are removed. 
     By operating the recovery means in this way, the coefficient of friction of the surface of the pick-up roller 7 can be recovered to thereby restore the sheet supplying force. 
     While an example that, in the recovery operation, the polishing operation is immediately followed by the cleaning operation was explained, the polishing operation and the cleaning operation may be performed independently. 
     The recovery operation can be effected automatically in accordance with the kind or number of sheets. To this end, for example, the control in a flow chart shown in FIG. 2 is effected by the control means. That is to say, for each kind of sheet designated in the image formation on the printer driver, the number of recorded sheets is accumulate-counted (step S1), and, in dependence with the kind of sheet to be supplied, the count numbers are accumulated in accordance with a predetermined coefficient (steps S2 to S5). For example, when the sheet to be supplied is a normal sheet, the coefficient a is set to 1 (α=1), and, when the sheet to be supplied is an ink jet recording coated sheet, the coefficient β is set to 2 (β=2). In this case, when 50 normal sheets were supplied in total and 40 coated sheets were supplied in total, A=50×1=50 and B=40×2=80. Thus, total value (A+B) becomes 130. The total value is compared with a predetermined cleaning reference value C (for example, C=100) (step S6). If the total value (A+B) exceeds the reference value C, the cleaning operation is effected automatically (step S7). 
     Further, the total value (A+B) is compared with a predetermined polishing reference value D (for example, D=300) (step S8). If the total value exceeds the reference value D, the polishing operation is performed (step S9). For example, at the time when 100 normal sheets were supplied in total and 50 coated sheets were supplied in total, since the total value (A+B) 100+100=200&lt;D, the polishing operation is not effected. Whereas, at the time when 100 normal sheets were supplied in total and 101 coated sheets were supplied in total, since the total value (A+B)=100+202=302&gt;D, the polishing operation is effected. After the polishing operation is performed, the numbers of accumulate supply sheets are reset (step S10). 
     Incidentally, the kinds of sheets are not limited to the normal sheet and the coated sheet, and, thus, the coefficients α, β are not limited to the above values but may be appropriately set. Further, the recovery operation can be manually performed by the operator periodically. For example, by using a lever and the like, the operator can slide the holder 10 to the left and right to oppose the cleaning pads 9a or the polishing pads 9b to the rubber roller portions of the pick-up roller 7, and then the pick-up roller 7 may be rotatingly driven. In this case, since the holder 10 is formed from the flat plate, the drive mechanism such as the pinion gear 11, rack 10a and the like can be omitted, so that the entire apparatus is made more compact. 
     In the illustrated embodiment, while an example that the pick-up roller 7 is lowered and lifted in accordance with the picking-up operation was explained, now, sheet supplying apparatuses in which a rotary shaft of a pick-up roller is not shifted (but rotatable) will be explained with reference to FIGS. 4A, 4B, 5A, 5B, 6A and 6B. 
     In a sheet supplying apparatus shown in FIGS. 4A and 4B, the separation pads 8 are biased upwardly by a spring 12 to urge the sheet S against the pick-up roller 7 as shown in FIG. 4A. Thus, when the sheet supplying operation is started, the pick-up roller 7 is rotated to cooperate with the separation pads 8 to thereby separate the sheets one by one from an uppermost sheet to be supplied toward directions a, b. 
     A recovery means 9 comprises an inverted U-shaped holder 13, a cleaning pad 9a attached to one of opposed side walls of the holder and a polishing pad 9b attached to the other side wall, and the holder 13 is secured to a rail 14. The rail 14 is provided with a rack 14a with which a pinion 15 is meshed. When the pinion 15 is rotated, the holder 13 is shifted together with the rail 14. In the sheet supply, as shown in FIG. 4A, the holder 13 is positioned so that both the cleaning pad 9a and the polishing pad 9b are not contacted with the pick-up roller 7. Thus, in this case, the pads 9a, 9b do not disturb the sheet supply of the pick-up roller 7. 
     In order to restore the sheet supplying force of the pick-up roller 7, for example, when the polishing operation is performed, as shown in FIG. 4B, the holder 13 is shifted in a direction b to urge the polishing pad 9b against the pick-up roller 7. In this condition, when the pick-up roller 7 is rotated, the surface of the pick-up roller 7 is polished. On the other hand, when the holder 13 is shifted in the direction a, the cleaning pad 9a is urged against the pick-up roller 7 to thereby permit the cleaning operation. In this way, the same advantage as the aforementioned embodiment can be achieved. 
     Incidentally, while an example that the cleaning pad 9a and the polishing pad 9a are shifted in parallel via the holder 13 was explained, the cleaning pad and the polishing pad may be rotated as shown in FIGS. 5A and 5B. 
     A recovery means shown in FIGS. 5A and 5B comprises a cleaning pad 9a and a polishing pad 9b both attached to a flange 16a of a motor 16. As shown in FIG. 5B, by rotating the motor 16, the cleaning pad 9a or the polishing pad 9b can be urged against the pick-up roller 7 to thereby permit the cleaning operation or the polishing operation. 
     An arrangement as shown in FIGS. 6A and 6B provides similar operations. In a sheet supplying apparatus shown in FIGS. 6A and 6B, a cleaning pad 9a and a polishing pad 9b are supported on shafts 17a and 17b, respectively for rotational movements, and gears 18a and 18b are secured to the shafts 17a and 17b, respectively. The gear 18a is meshed with a gear 19a of a gear train 19a to 19d and the gear 18b is meshed with a gear 19d of the gear train. With this arrangement, when the gear train 19a to 19d is rotated, as shown in FIG. 6B, the cleaning pad 9a and the polishing pad 9b are rotated and cocked. 
     In the above arrangement, when the sheet S is supplied by the pick-up roller 7, as shown in FIG. 6A, the cleaning pad 9a and the polishing pad 9b are positioned below the sheet S to be retracted below guide plates 20. Thus, the pads do not disturb the sheet supply. Further, end faces 20a of the guide plates 20 are chamfered not to disturb the sheet supply. Thus, the end of the sheet S is hard to be entered between the guide plate 20 and the cleaning pad 9a and between the guide plate 20 and the polishing pad 9b. In this sheet supplying apparatus, if the sheet supplying force of the pick-up roller 7 is reduced, the sheet supplying force can be restored or recovered by operating the recovery means. In this recovery means, when the gear train 19a to 19d are rotated, as shown in FIG. 6B, the cleaning pad 9a and the polishing pads 9b are cocked to be urged against the pick-up roller 7. In this condition, when the pick-up roller 7 is rotated, the cleaning operation and the polishing operation are performed simultaneously. 
     In the embodiment shown in FIGS. 6A and 6B, while an example that the cleaning pad 9a and the polishing pad 9b are simultaneously cocked by the gear train 19a to 19d was explained, the cleaning pad and the polishing pad may be cocked by independent drive sources. In this case, the cleaning operation and the polishing operation can be performed independently. 
     Also in this embodiment, as is in the aforementioned embodiment, the recovering operation of the pick-up roller 7 can be automatically effected in accordance with the kind and number of sheets. 
     Next, a further example of a recovery means will be described with reference to FIGS. 7, 8A and 8B. In a sheet supplying apparatus ASF according to this embodiment, as shown in FIG. 7, a separation pad 8, a cleaning pad 9a and a polishing pad 9b are attached onto a belt 22 mounted around a drive pulley 21a and a driven pulley 21b to be rotated in directions c, d (perpendicular to a sheet supplying direction). Further, a pressure plate 23 for urging the pads 8, 9a and 9b against the pick-up roller 7 via the belt 22 is provided for movement in directions e. 
     With this arrangement, when the sheet S is supplied, as shown in FIG. 8A, the separation pad 8 is opposed to the pick-up roller 7 and is urged against the pick-up roller by lifting the pressure plate 23. In this condition, when the recording operation is started, by the cooperation between the pick-up roller 7 and the separation pad 8, the sheets S are separated one by one from an uppermost sheet. 
     When it is desired to restore the sheet supplying force of the pick-up roller 7 by the polishing operation, as shown in FIG. 8B, the belt 22 is rotated to oppose the polishing pad 9b to the pick-up roller 7, and then, the polishing pad 9b is urged against the pick-up roller 7 by lifting the pressure plate 23. In this condition, when the pick-up roller 7 is rotated, the surface of the roller 7 is polished. Incidentally, when the cleaning operation is performed, the belt 22 is rotated to oppose the cleaning pad 9a to the pick-up roller 7 and the cleaning pad is urged against the pick-up roller by lifting the pressure plate. 
     Since the belt 22 is rotated in the direction perpendicular to the sheet supplying direction, when the pad 8, 9a or 9b is urged against the pick-up roller 7, the pad is not displaced in the rotational direction of the belt due to the rotation of the pick-up roller 7. The sheet supplying force of the pick-up roller 7 can be restored by scraping the surface of the pick-up roller 7 or removing the paper powder and other foreign matters from the pick-up roller. 
     A still further example of a recovery means will be described with reference to FIGS. 9A, 9B, 10A, 10B, 11A, 11B, 12A, 12B, 13A, 13B, 14A and 14B. In a sheet supplying apparatus ASF according to this embodiment, as shown in FIGS. 9A and 9B, similar to the abovementioned embodiment, cleaning pads 9a and the like are shifted by a belt 22. Unlike to the embodiment shown in FIGS. 7, 8A and 8B, two separation pads 8, two cleaning pads 9a and two polishing pads 9b are attached to the belt 22. FIGS. 9A and 9B are views of a recovery means, looked at from a sheet supplying direction, where FIG. 9A shows a schematic construction and FIG. 9B shows a sheet supplying condition. FIGS. 10A and 10B are views showing an operation of the recovery means, where FIG. 10A shows a polishing operation and FIG. 10B shows a cleaning operation. 
     By providing two pads 8, 9a, 9b, respectively, if one of two pads 8, 9a or 9b is worn, by using the other pad, the pad itself can be refreshed. Further, by using two pads 8, 9a or 9b alternately, the entire service life of each pad can be increased. 
     Further, when a shifting amount of each cleaning pad 9a and each polishing pad 9b with respect to the pick-up roller 7 is adjustable, by shifting the pad 9a or 9b in one direction or reciprocally shifting the pad by a predetermined amount, only a desired portion of the pick-up roller 7 can be cleaned or polished. In this case, by setting so that the section of the cleaning pad 9a and the polishing pad 9b in a direction perpendicular to the shifting direction of the pad is aligned with the section of the pick-up roller 7, the cleaning efficiency and polishing efficiency can be improved. 
     Incidentally, as shown in FIGS. 11A, 11B, 12A and 12B, when two pick-up rollers 7 are provided on the same rotary shaft, the picking-up operation can be performed more positively. Also in this case, two cleaning pads 9a and two polishing pads 9b may be associated with each pick-up roller 7. 
     While an example that the belt 22 for shifting the recovery means is rotated in the direction perpendicular to the sheet supplying direction was explained, as shown in FIGS. 13A, 13B, 14A and 14B, even when the belt 22 is rotated in a direction parallel with the sheet supplying direction, similar advantage can be obtained. 
     FIGS. 15 to 18 are explanatory views (enlarged views of a pick-up roller and a separation pad 8) for explaining an operation of a friction separation means. As shown in FIG. 15, a pick-up roller 7 is supported for up-and-down movement, and a separation pad 8, a polishing pad (polishing means) 9b and a cleaning pad (cleaning means) 9a are attached, via springs (biasing means) A, B and C, to a polygonal holder 10A rotated by a drive source such as a motor. The polishing pad and the cleaning pad constitute a recovery means 9 for the pick-up roller 7. 
     Sheets S are rested on a plate 25 having an opening 25a through which the pad 8, 9a or 9b can be exposed to the pick-up roller 7. When the pad 8, 9a or 9b is aligned with the opening 25a by rotating the holder 10A, the pad are biased by the spring A, B or C to be protruded from the plate 25 toward the pick-up roller 7 through the opening 25a. 
     FIG. 16 shows a normal condition that the sheet is supplied. In this condition, the separation pad 8 is exposed through the opening 25a to be opposed to the pick-up roller 7. When the pick-up roller 7 is rotated in a direction shown by the arrow, only an uppermost sheet among the sheets S stacked on the plate 25 is supplied. If the other sheets try to move together with the uppermost sheet, the other sheets cannot be advanced by the friction force of the separation pad 8. 
     FIG. 17 shows a condition that the polishing pad 9b is exposed through the opening 25a (after rotation of the holder 10A) and is urged against the pick-up roller 7 by the spring C. In this condition, when the pick-up roller 7 is rotated, the surface of the pick-up roller 7 is polished (scraped) to thereby restore the friction force of the roller. 
     FIG. 18 shows a condition that the cleaning pad 9a is exposed through the opening 25a (after rotation of the holder 10A) and is urged against the pick-up roller 7 by the spring B. In this condition, when the pick-up roller 7 is rotated, foreign matters adhered to the surface of the pick-up roller 7 is cleaned (removed), thereby restoring the friction force of the roller. Incidentally, after the polishing operation, when the cleaning operation is performed by the cleaning pad 9a, chips generated by the polishing operation can also be removed. 
     The sheet supplying operation is normally performed in the condition shown in FIG. 16, and, by rotating the holder 10A periodically to bring the apparatus to the condition shown in FIG. 17 or 18, the polishing operation or the cleaning operation can be effected. Thus, since the friction force of the pick-up roller 7 can be maintained in a good condition, the proper sheet supply can always be performed. 
     The holder 10A may be rotated by periodically depressing a switch (or a roller refresh button) (by the operator) or may be automatically rotated at predetermined frequency in accordance with the kind and number of sheets. Alternatively, a detection device for detecting the surface condition of the pick-up roller or a detection device for detecting a sheet supplying speed is provided, and the holder may be rotated when it is judged that the polishing operation and/or cleaning operation should be performed by the detection device. 
     FIGS. 19 to 22 show embodiments in which a cylindrical holder is used. 
     FIG. 19 is an enlarged view of a pick-up roller 7 and a holder 10B. A separation pad 8, a polishing pad 9b and a cleaning pad 9a are provided on a cylindrical surface of the holder 10B so that the pad can be exposed through an opening 25a of a plate 25 without utilizing any springs. 
     FIG. 20 shows a normal condition that the sheet is supplied. In this condition, the separation pad 8 is exposed and protruded through the opening 25a. FIG. 21 shows a condition that the polishing operation is performed. In this condition, the polishing pad 9b is exposed and protruded through the opening 25a. FIG. 22 shows a condition that the cleaning operation is performed. In this condition, the cleaning pad 9a is exposed and protruded through the opening 25a. 
     FIG. 23 shows an embodiment in which a polish means for polishing the surface of the separation pad 8 is provided. 
     As shown in FIG. 23, a polish holder 27 provided at its surface with a polish pad 26 is supported for movement toward and away from the separation pad 8. In a condition that the polish pad 26 is urged against the separation pad 8, when the holder 10B is rotated, the surface of the separation pad 8 is polished, the friction force of the separation pad 8 is maintained in a good condition, so that the sheet separating ability is improved. 
     In the above-mentioned embodiments, while an example that the sheet supplying apparatus is used with the ink jet printer was explained, the image forming means is not limited to the ink jet recording means, and the present invention can be applied to other image forming apparatuses of electrophotographic type, heat transfer recording type and the like. 
     Further, in place of the image forming means, an image reading means may be used. That is to say, the sheet supplying apparatus ASF may be used with an image reading apparatus in which originals are separated and supplied one by one and image information on the supplied original is read by the image reading means.