Abstract:
A sheet supply device includes a sheet storing section on which sheets are stacked and are stored; a sheet feed unit controlled by a controller that feeds the sheets stored in the sheet storing section to a next process; and an air blower controlled by the controller that blows air to the sheets stored in the sheet storing section, the air blower has an air nozzle and a mechanism that can adjust a height of the air nozzle in an up-down direction, and the mechanism increases or decreases the air flow rate of the air being blown to the sheets, wherein a force provided by the air blower to push up the sheets is exerted on the sheets, such that a less force is exerted when the sheets are being fed compared to when the sheets are not being fed.

Description:
This is a divisional of application Ser. No. 11/052,761, filed on Feb. 9, 2005 now abandoned, which is hereby incorporated by reference, and claims a benefit of Japanese Patent Applications No. 2004-191549 filed Jun. 29, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet supply device. 
     2. Description of the Related Art 
     Conventionally, image forming apparatuses such as printers and copying machines generally use cut sheets (copying paper) as a medium on which an image is formed, which cut sheets enable continuous supply. Hitherto, plain paper and woodfree paper specified by manufacturers of copying machines have been used as these sheets (paper). Plain paper and woodfree paper have low smoothness, and thus have weak adhesion. It is thus relatively easily to prevent so-called double feed wherein when sheets are to be supplied one by one from a sheet stacking section such as a sheet feed tray, a plurality of sheets are fed in an adhered state. 
     In recent years, however, as a result of the diversification of recording media, it has become necessary for various types of sheets including sheets having high surface smoothness to be conveyed. In particular, as coloring techniques are developed, versatile apparatuses that can also or alternatively convey media other than the conventional copying paper, such as enamel paper, the degree of whiteness of which is heightened and to which a glaze is applied (e.g., coated paper, and composite sheet where coating color is applied as one type of a coating material to both surfaces or one surface of a sheet in order to improve suitability for printing), film sheet or tracing paper, are in high demand. 
     Since these types of coated paper, film sheet and tracing paper have high surface smoothness, the adhesive force between sheets is strong, and thus it is difficult to prevent double feed thereof. For this reason, special measures are necessary for feeding such sheets. For example, the case of coated paper is explained below. As the quality of the coated paper becomes higher, the amount of coating increases, as does the optical property of white glossness. Further, unevenness on the surface of the paper is reduced so that the smoothness of the surface is increased. On the other hand, when the surface smoothness becomes high, the gap between contacting sheets becomes narrow, so that air does not pass through the gap. 
     As a result, negative pressure is generated and maintained, resulting in strong adhesion between sheets of coated paper. Particularly in cases where coated paper is stacked in a high-humidity environment, sheets of coated paper adsorb to each other so that the degree of adhesion becomes higher. Film sheet and tracing paper have high surface smoothness in themselves, and thus adhesive force between sheets thereof is naturally high and they exhibit the same characteristics in this regard as coated paper. 
     When these types of smooth sheets with strong adhesion between sheets are to be fed one sheet at a time from a sheet feed tray on which the sheets are stacked, it is very difficult to convey the sheets separately in a device that normally feeds normal paper. In a sheet supply device that uses an engaging roll which contacts with a surface of a sheet at a predetermined pressure and rotates so as to take-in the sheet and a system for pressurizing a supply roll and a separating roll at a constant pressure so as to separate the sheets when the sheets are sorted, a pressurizing force which is about 30 times as strong as that needed to separately convey normal paper is required for conveying smooth sheets separately. 
     As a method of eliminating the adhesion between the smooth sheets in advance, a technique for blowing air to side surfaces of the stacked sheets has been proposed (see, for example, Japanese Patent Application Laid-Open Nos. 3-211136 (1991) and 11-5643 (1999)). 
       FIG. 13A , for example, shows a case where air is constantly blown during feeding of the sheets, and  FIG. 13B  shows a case where an air blowing device is actuated intermittently in synchronous with the feeding of the sheets. In such cases, as shown in  FIG. 13C , a sheet P is fed while rubbing against members above the sheet P, such as lift regulating members  564 , a nudger roller  556  which has moved upward and a chute member  510 , in a state in which an upward-pressing force for pushing up the sheet P is at a maximum. That is to say, the sheet P is fed while the surface thereof is rubbing hard against the lift regulating members  564 , the nudger roller  556  and the chute member  510 . For this reason, the surface of the sheet P is occasionally damaged. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised in view of the above circumstances and provides a sheet supply device that does not damage a sheet surface even when it blows air to ease adhesion between sheets and feed the sheets to a next process. 
     According to a first aspect of the invention, a sheet supply device includes: a sheet storing section on which sheets are stacked and are stored; a sheet feed unit that feeds the sheets stored in the sheet storing section to a next process; and an air blower that blows air to the sheets stored in the sheet storing section. A force for pushing up the sheets which is exerted on the sheets due to the air blower blowing the air is made to be weaker when the sheets are being fed than when the sheets are not being fed. 
     According to this aspect, the air blower constantly blows air to the sheets stacked and stored on the sheet storing section, so that adhesion between the sheets is released. The feed unit feeds the sheets to the next process in a state in which the adhesion between the sheets has been released. The force for pushing up the sheets which is exerted on the sheets due to the blowing of the air is weaker when the sheets are being fed than when the sheets are not being fed. Therefore, even when a sheet is fed with the surface thereof rubbing against members above the sheets such as a regulating member that regulates the lift of the sheets, the force pressing the sheet against the regulating member is weak. For this reason, damage to the surfaces of the sheets is minimized, and thus the surfaces of the sheets are unlikely to be damaged. 
     According to a second aspect of the invention, a sheet supply device includes: a sheet storing section on which sheets are stacked and stored; a feed unit that feeds the sheets stored on the sheet storing section to a next process; an air blower that blows air to the sheets stored on the sheet storing section; and a flow channel moving unit that moves a flow channel for the air blown by the air blower, wherein a height of the flow channel in a vertical direction is set to be higher when the sheets are being fed than when the sheets are not being fed. 
     According to this aspect, the sheet supply device has a flow channel moving unit that moves a flow channel for the air blown by the air blower, wherein a height of the flow channel in a vertical direction is set to be higher when the sheets are being fed than when the sheets are not being fed. Therefore, even when the sheets are fed with the surfaces thereof rubbing against the members above the sheets such as the regulating members that regulate the lift of the sheets, the force pressing the sheets against the regulating members is weak. For this reason, damage to the surfaces of the sheets is minimized, and thus the surfaces of the sheets are unlikely to be damaged. 
     According to a third aspect of the invention, a sheet supply device includes: a sheet storing section on which sheets are stacked and stored; a feed unit that feeds the sheets stored on the sheet storing section to a next process; an air blower that blows air to the sheets stored on the sheet storing section; and a flow channel moving unit having an air blocking member that moves in a stacking direction of the sheets so as to block the air, and the air blocking member is moved so that a height of an upper end of the air blocking member is higher when the sheets are being fed than when the sheets are not being fed. According to this aspect, the height of the upper end of the air blocking member is set to be higher when the sheets are being fed than when the sheets are not being fed. As a result, the function of the fifth aspect is effectively achieved. 
     In short, according to the present invention, even when the air is blown so as to release the adhesion between the sheets and the sheets are fed to a next process, the force pushing up the sheets or the height of the flow channel for the air in the vertical direction is smaller when the sheets are being fed than they are not being fed. Damage to the surfaces of the sheets is, therefore, minimized. Accordingly, the surfaces of the sheets are unlikely to be damaged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic structural diagram illustrating an image forming apparatus having a sheet supply device according to a first embodiment of the present invention; 
         FIG. 2  is a perspective view illustrating the sheet supply device according to the first embodiment of the invention; 
         FIG. 3  is a plan view illustrating the sheet supply device according to the first embodiment of the invention; 
         FIG. 4  is a perspective view illustrating a feed mechanism of the sheet supply device according to the first embodiment of the invention; 
         FIGS. 5A to 5C  are pattern diagrams illustrating states in which air is blown out of a nozzle of the sheet supply device according to the first embodiment of the invention and sheets are lifted; 
         FIG. 6  is a diagram illustrating operation timing between an air blowing device and a feed mechanism of the sheet supply device according to the first embodiment of the invention; 
         FIG. 7  is a pattern diagram illustrating a state in which a sheet is supplied from the sheet supply device; 
         FIGS. 8A to 8C  are pattern diagrams illustrating the sheet supply device according to a second embodiment of the invention and states in which a shutter descends and sheets are lifted; 
         FIG. 9  is a diagram illustrating a position of the shutter in the sheet supply device according to the second embodiment of the invention, a position of an air nozzle of the air supplying device according to a third embodiment, and operation timing of the feed mechanism; 
         FIGS. 10A to 10C  are pattern diagrams illustrating the sheet supply device according to the third embodiment of the invention and states in which the nozzle descends and sheets are lifted; 
         FIG. 11  is a diagram illustrating the feed mechanism that feeds a sheet using a semilunar roller, as a modified example of the sheet supply apparatus of the invention; 
         FIG. 12  is a diagram illustrating operation timing between a number of revolutions of a fan in the air blowing device and the feed mechanism of the sheet supply device according to a fourth embodiment of the invention; and 
         FIGS. 13A to 13C  are diagrams illustrating a conventional sheet supply device, in which  FIGS. 13A and 13B  illustrate operation timing between an air blowing device and a feed device, and  FIG. 13C  is a pattern diagram illustrating a state in which a sheet is fed to a next process with the upper surface thereof rubbing hard against a lift regulating member, a nudger roller in a rest position, and a chute member. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An image forming apparatus having a sheet supply device according to embodiments of the present invention will be explained below with reference to the drawings. As an image forming system in the image forming apparatus, a known electrophotographic process is used. 
     A structure of the image forming apparatus  10  and a summary of image formation will be described first, and a main section of the invention will be explained thereafter. As sheets P on which an image is formed, sheets with a smooth surface are used such as normal paper or coated paper, the surface of which undergoes a coating process in order to provide whiteness and gloss. 
     As shown in  FIG. 1 , the image forming apparatus  10  has a control unit  24  that controls the entire image forming apparatus  10  and stores various information therein. An operation panel  11  is provided on an upper portion of the apparatus. When a user operates the operation panel  11 , the control unit  24  controls the apparatus in accordance with the contents of the peration. The image forming apparatus  10  is further provided with an image forming unit  12  that forms an image using a known electrophotographic process. The image forming unit  12  has a photosensitive drum  14 . A charging unit  16 , a developing device  18  and a cleaner  20  are disposed along a circumferential direction of the photosensitive drum  14 . An image writing device  22  is provided so as to emit a laser beam L to the surface of the photosensitive drum  14  between the charging unit  16  and the developing device  18 . A transfer roller  34  is provided at a side of the photosensitive drum  14  opposite to the image writing device  22 . 
     The image writing device  22  is connected to the control unit  24 , and the control unit  24  is connected to a receiving unit  26 . The receiving unit  26  is connected to external devices such as an image reading device  28  and a personal computer  30  via a communication line  32 , and image information is transmitted to the receiving unit  26  from the image reading unit  28  and the personal computer  30 . The image information is transmitted from the receiving unit  26  to the control unit  24 , and the control unit  24  controls the image writing device  22  based on the image information so that the image writing device  22  emits the laser beam L. 
     The photosensitive drum  14  is charged by the charging unit  16  so that the surface thereof has a predetermined electric potential. The image writing device  22  emits the laser beam L so that the surface of the photosensitive drum  14  is exposed and an electrostatic latent image is formed thereon. The developing device  18  develops the electrostatic latent image so that a toner image is formed on the surface of the photosensitive drum  14 . 
     A sheet P is transported from a sheet feed unit  40 , described below, via a sheet transport unit  44  having plural transport rollers  46  to a nip portion between the photosensitive drum  14  and the transfer roller  34 . The transfer roller  34  transfers the toner image on the photosensitive drum  14  to the sheet P, and the sheet P is sent to a fixing device  36  installed downstream in a transport direction, so that the toner image is fixed to the sheet P. A pair of discharge rollers  38  are provided downstream of the fixing device  36  in the transport direction, which discharge the sheet P to which the toner image is fixed onto a discharge tray  39 . 
     The cleaner  20  collects the toner which is not transferred to the sheet P and remains on the surface of the photosensitive drum  14 . The sheet feed unit  40 , in which plural (four in this embodiment) sheet supply devices  50  are aligned in an up-down direction, is disposed at a lower portion of the image forming apparatus  10 . Each of the sheet supply devices  50  has a sheet feed tray  52  on which the sheets P are stacked and stored. A bottom plate (not shown) is provided in the sheet feed trays  52 , and is raised and lowered by a driving mechanism (not shown). Due to the raising/lowering of the bottom plate, the stacked sheets P are raised and lowered. 
     As shown in  FIG. 3 , an end guide  64 , which can be moved according to the size of the sheets P and regulates a rearward end surface of the sheets P, is provided at an upstream side of the sheet feed tray  52  in a feeding direction S of the sheets P. A side surface fixed guide  66  is disposed at one side surface in a direction perpendicular to the feeding direction S of the sheets P. A side surface movable guide  68 , which can be moved according to the size of the sheets P, is disposed at the side surface opposite to the side surface fixed guide  66 . 
     As shown in  FIG. 1 , a feed mechanism  90 , which has a nudger roller (drawing-in roller)  56  and sequentially feeds the sheets P to the nip portion between the photosensitive drum  14  and the transfer roller  34  via the sheet transport unit  44 , is provided downstream of the sheet feed tray  52  in the feeding direction S. As shown in  FIGS. 2 and 3 , the nudger roller  56  frictionally contacts with an upper surface of a top sheet TP at the top position of the stacked sheets P so as to sequentially feed the sheets P. The nudger roller  56 , as shown in  FIG. 7 , can be moved to a feeding position A where it contacts with the upper surface of the top sheet TP so as to feed it and to a rest position B where it rests in an upper position. As shown in  FIGS. 2 and 3 , a feed roller (transport roller)  58  and a retard roller (sorting roller)  60  pressurized by the feed roller  58  are provided downstream side of the nudger roller  56  in the feeding direction S. 
     The nudger roller  56 , the feed roller  58  and the retard roller  60  are composed of rollers having the same shape and size, which frictionally contact with the sheets P so as to transport the sheets P. Specifically, the sheets P fed by the nudger roller  56  are sorted into separate sheets by the feed roller  58  and the retard roller  60  and conveyed downstream one sheet at a time. As shown in  FIG. 4 , the retard roller  60  is pressed against the feed roller  58  with weak pressure by a spring  96  via a support which moves rotationally about a pivot  92 . The retard roller  60  is connected to a first gear  100 , which moves rotationally via a torque limiter  98  provided on a shaft  60 A, and to a fixed second gear  102 . 
     A driving force is transmitted from a feed motor, not shown, to the second gear  102 , and the retard roller  60  receives the driving force in the direction of the arrow shown. The nudger roller  56  is structured so as to move rotationally about a shaft  58 A of the feed roller  58  via an arm  82 , and is rotated by a gear group  84  in conjunction with driving of the feed roller  58 . When a plunger  86 A moves inward and outward due to an operation of a solenoid  86  which operates on the basis of a driving signal received from the control unit  24  (see  FIG. 1 ), a link  88  rotates around a shaft  89  so as to raise and lower a protrusion  88 A. 
     A pin  82 A of the arm  82  is mounted on the protrusion  88 A, and moves upward and downward in conjunction with the protrusion  88 A. As a result, the arm  82  rotationally moves upward and downward so that the nudger roller  56  moves between the rest position B and the feeding position A (see  FIG. 7 ). In other words, when the protrusion  88 A of the link  88  is lowered, the nudger roller  56  descends to the feeding position A where it contacts with the upper surface of the top sheet TP and feeds the top sheet TP by rotatably driving it with a predetermined pressurizing force. 
     The movement of the arm  82  (nudger roller  56 ) in the feeding position A is controlled in the following manner. A photosensor  85  detects a protrusion  83  of the arm  82 , and the control unit  24  (see  FIG. 1 ) controls raising and lowering of the bottom plate (not shown) so that the height of the top sheet TP falls within a constant range. As shown in  FIG. 7 , a chute member  110  is provided downstream of the nudger roller  56  to guide the sheets P to the feed roller  58  and the retard roller  60 . As shown in  FIG. 3 , each of the sheet supply devices  50  has an air blowing device  54 , which blows air at the side surface of the sheets stacked on the sheet feed tray  52 , in a vicinity of the side surface fixed guide  66 . 
     The air blowing device  54  has a fan  55  which rotates in a direction shown by an arrow K, and high-pressure air is blown from a nozzle  70  of the air blowing device  54  and through a nozzle  71  formed at the side surface fixed guide  66 . When the air is blown to the sheets P, the air is sent between the stacked sheets P so that the sheets P lift, and the adhesion between the sheets P is released. The air blowing device  54  is controlled by the control unit  24  so that an operation and a non-operation are repeated for predetermined periods, for example, as shown in  FIG. 6 . The nozzles  70  and  71  have approximately the same size, and their positions are approximately the same. 
     More specifically, the number of revolutions of the fan  55  can be changed and is set so as to be lower when the sheets are being fed than when the sheets are not being fed. That is to say, the air flow rate of the air blower  54  is set to be lower when the sheets are being fed than when the sheets are not being fed, and the force pushing up the sheets is made weaker when the sheets are being fed than when the sheets are not being fed. 
     This feature that an air flow rate of the air blower is set at a lower rate when the sheets are being fed than when the sheets are not being fed, of the present embodiment, may be applied to the other embodiments described below. As shown in  FIGS. 2 ,  3  and  7 , two lift regulating members  62  are provided upstream of the nudger roller in the feeding direction S of the sheets P, which contact against the top sheet TP when it lifts due to the air and thus regulate the lift of the sheets P. The function of the sheet supply device  50  according to the invention is explained below. When feeding the sheets P, as shown in  FIGS. 4 and 5A  to  5 C, the solenoid  86  is operated based on the driving signal from the control unit  24  (see  FIG. 1 ), so that the arm  82  is lowered. As a result, the nudger roller  56  is moved from the rest position B to the feeding position A. 
     In the sheet feed tray  52 , the stacked sheets P are raised by the raising of the bottom plate (not shown). When the top sheet TP contacts with the nudger roller  56  and the nudger roller  56  is raised, the photosensor  85  detects the protrusion  83  of the arm  82  and the control unit  24  stops the raising of the bottom plate. As the sheets P are sequentially fed, the position of the top sheet TP descends and thus the nudger roller  56  descends. As a result, the photosensor  85  does not detect the protrusion  83  of the arm  82 , and the control unit  24  raises the bottom plate. In such a manner, the height of the top sheet TP is controlled so as to fall within the constant range. 
     When the air blowing device  54  is driven and operated based on the driving signal from the control unit  24 , air is blown at the side surface of the stacked sheets P. When the air is blown the sheets P lift gradually as shown in  FIGS. 5A to 5C . When the operation is stopped (becoming inoperative), the sheets P descend gradually (from the state shown in  FIG. 5C  to the state in  FIG. 5A ). Further, the air blowing device  54  repeats operation and non-operation for predetermined periods by means of the control unit  24  as shown in  FIG. 6 . The sheets P repeatedly change from the states shown in  FIG. 5A  to  FIG. 5C  and from  FIG. 5C  to  FIG. 5A , so that the adhesion between the sheets P is released more effectively. 
     As shown in  FIG. 4 , when the nudger roller  56  rotates in the direction of the arrow shown, it frictionally contacts with the upper surface of the top sheet TP so as to feed the sheets P. The sheets P fed by the nudger roller  56  are held between the feed roller  58  and the retard roller  60 , and are sorted into separate sheets to be conveyed downstream one sheet at a time. Since the adhesion between the stacked sheets P is released by the air, double feed wherein plural sheets P are fed in an adhered manner can be prevented. 
     The solenoid  86  is operated with a timing corresponding to when a sheet P reaches the feed roller  58 , whereby the arm  82  is raised so that the nudger roller  56  is moved to the rest position B. As shown in  FIG. 6 , the feed mechanism  90  feeds the sheets P in synchronization with the operation/non-operation of the air blowing device  54 . Specifically, after the air blowing device  54  is changed from an operational state to a non-operational state, the feed mechanism  90  feeds the sheets P. That is to say, the upward-pressing force of the air blowing device  54  pushing up the sheets P is weaker when the feed mechanism  90  is operated (sheet is fed) than when it is not operated (sheet is not fed). 
     As shown in  FIG. 7 , therefore, the top sheet TP is fed in a state in which the members above the top sheet TP such as the rise regulating members  62 , the nudger roller  56  in the rest position B and the chute member  110  do not rub the upper surface of the top sheet TP, or rub in response to a weak upward-pressing force. Damage to the surface of the top sheet TP is, therefore, minimized, and thus the surface of the top sheet TP is not damaged. 
     In the present embodiment and other embodiments described below, a time when the air blower is operated may correspond to a time when the sheets are not fed, and a time when the air blower is not operated may correspond to a time when the sheets are fed, as typically shown in  FIG. 6 . According to this feature, the sheets are fed when the air blower is not operated. That is, the air flow rate is made remarkably low when the sheets are being fed, whereby the function of the second aspect is achieved more effectively. 
     The sheet supply device according to a second embodiment of the invention will be explained below with reference to the drawings. Members identical to those in the first embodiment are designated by the same reference numerals, and explanations thereof will be omitted. As shown in  FIGS. 8A to 8C , in a sheet supply device  51 , the air blowing device  154  is provided with a shutter  150  between the nozzle  70  and the nozzle  71  of the side surface fixed guide  66 . A rack (flat plate gear)  150 A is formed on a side of the shutter  150 , and is engaged with a gear  152  attached to a shaft of the driving mechanism, not shown. The shutter  150  is supported by a guide rail, not shown, so that it can freely move in a vertical direction. 
     The gear  152  can rotate in either direction based on a signal from the control unit  24  (see  FIG. 1 ), so as to move the shutter  150  upward and downward. The control unit  24  controls the shutter  150  so that the shutter  150  is moved to an upward or downward position for predetermined periods and is thus repeatedly closed and opened. The function of the sheet supply device  51  according to the invention will be explained below. 
       FIGS. 8A to 8C  illustrate subsequent states in which the shutter  150  is being opened and a boundary delimiting the air blown to the sheets P (in this embodiment, an upper end  150 B of the shutter  150 ) gradually descends so that the sheets P lift accordingly. Conversely, the process of the shutter being closed is approximately illustrated in the reverse order of  FIG. 8C  to  FIG. 8A . 
     The shutter  150  moves up and down for predetermined periods and the state of the sheets P repeatedly changes from that of  FIG. 8A  to that of  FIG. 8C  and from that of  FIG. 8C  to that of  FIG. 8A . As a result, the adhesion between the sheets P is released more effectively. As shown in  FIG. 9 , the feed mechanism  90  feeds the sheets P in synchronization with the up-down movement of the shutter  150 . Specifically, when the shutter  150  reaches the upper position, namely, when the upward-pressing force pushing up the sheets P is at a minimum, the feed mechanism  90  feeds a sheet P. 
     As shown in  FIG. 7 , the top sheet TP is fed either without the upper surface thereof rubbing against the members above the top sheet TP such as the lift regulating members  62 , the nudger roller  56  in the rest position B and the chute member  110 , or while rubbing thereagainst due to a weak upward-pressing force. Damage to the surface of the top sheet TP is, therefore, minimized, and thus the surface of the top sheet TP is not damaged. 
     The sheets are not necessarily fed when the shutter  150  reaches the uppermost position. It is sufficient to feed the sheets in synchronization with the up-down movement of the shutter  150  such that the height of the shutter  150  is higher when the feed mechanism  90  is operated (feed) than when it is not operated (non-feed). This is because the upward-pressing force pushing up the sheets P becomes weaker when the feed mechanism  90  is operated (supply) than when it is not operated (non-supply). The sheet supply device according to a third embodiment of the invention will be explained below with reference to the drawings. Members identical to those in the first and the second embodiments are designated by the same reference numerals, and explanations thereof will be omitted. 
     As shown in  FIGS. 10A to 10C , in a sheet supply device  53 , the dimensions of a nozzle  271  formed on the side surface fixed guide  66  are larger than the dimensions of a nozzle  270  of an air blowing device  254 . A protruded contact plate  250  is formed below the nozzle  270  of the air blowing device  254 . A rack (flat plate gear)  250 A is formed on a side of the protruded contact plate  250 , and the rack  250 A is engaged with a gear  252  attached to the shaft of a driving mechanism, not shown. The air blowing device  254  is supported by a guide rail, not shown, so that it can freely move in a vertical direction. 
     The gear  252  can rotate in both directions based on a signal from the control unit  24  (see  FIG. 1 ), such that the air blowing device  254  moves up and down; namely, the nozzle  270  moves up and down. The control unit  24  controls the nozzle  270  (air blowing device  254 ) such that it moves up and down for predetermined periods. When the nozzle  270  is in the lower position, the upward-pressing force pushing up the sheets P is at a maximum, and when it is in the upper position, the upward-pressing force reaches a minimum. The function of the sheet supply device  53  according to the invention is explained below. 
       FIGS. 10A to 10C  illustrate subsequent states in which the nozzle  270  descends and the boundary delimiting the air blown to the sheets P (in this embodiment, the lower end  270 A of the nozzle  270 ) gradually descends so that the sheets P lift. Conversely, a state of the nozzle  270  ascending is approximately illustrated in the reverse order of  FIG. 10C  to  FIG. 10A . When the nozzle  270  moves up and down for predetermined periods, the air is sequentially blown to the sheets P from the stacked upper sheets P to the lower sheets P, and from the lower sheets P to the upper sheets P. As a result, since, for example, sheets above sheets with weak adhesion can be prevented from being lifted in a bundled state, the adhesion between the stacked sheets P can be released more effectively. Double feed or the like can, therefore, be reliably prevented. 
     As shown in  FIG. 9 , the feed mechanism  90  feeds the sheets P in synchronization with the upward-downward movement of the nozzle  270 . Specifically, when the nozzle  270  reaches the upper position, namely, when the upward-pressing force pushing up the sheets P is at a minimum, the feed mechanism  90  feeds the sheets P. As shown in  FIG. 7 , therefore, the sheets P are fed either without the upper surface of the top sheet TP rubbing against the members above the top sheet TP such as the lift regulating members  62 , the nudger roller  56  in the rest position B and the chute member  110 , or while rubbing thereagainst due to a weak pushing-up force. Damage to the surface of the top sheet TP is, therefore, minimized, and the surface of the top sheet TP is not damaged. 
     The sheets do not always have to be fed when the nozzle  70  reaches the upper point. That is to say, it is sufficient to feed the sheets P in synchronization with the up-down movement of the nozzle  270  so that the height of the nozzle  270  is higher when the feed mechanism  90  is operated (feed) than when it is not operated (non-feed), because then the upward-pressing force becomes weaker when the feed mechanism  90  is operated (supply) than when it is not operated (non-supply). The sheet supply device according to a fourth embodiment of the invention will be explained below with reference to the drawings. Members identical to those in the first to the third embodiments are designated by the same reference numerals, and explanations thereof will be omitted. 
     The structure of the sheet supply device is similar to that in the first embodiment, but the control unit  24  (see  FIG. 1 ) controls the driving voltage of the fan  55  (see  FIG. 3 ) so that the number of revolutions of the fan  55  is controlled. Specifically, as shown in  FIG. 12 , the number of revolutions is controlled such that fast rotation and slow rotation are repeated for predetermined periods. That is to say, the air flow rate is repeatedly increased and decreased for the predetermined periods. The function of the sheet supply device according to the invention is explained below. The fan  55  repeatedly switches between fast rotation (large air flow rate) and the slow rotation (small air flow rate) so that the adhesion between the sheets P can be released more effectively, similarly to in the first embodiment. 
     As shown in  FIG. 12 , the feed mechanism  90  feeds the sheets P in synchronization with fluctuations in the number of revolutions of the fan  55 . Specifically, when the number of revolutions of the fan  55  is at its lowest (the air flow rate is the lowest), namely, when the upward-pressing force pushing up the sheets P is weakest, the feed mechanism  90  feeds the sheets P. As shown in  FIG. 7 , therefore, the sheets P are fed either without the surface of the top sheet TP rubbing against the members above the top sheet TP such as the lift regulating members  62 , the nudger roller  56  in the rest position B and the chute member  110 , or while rubbing thereagainst due to a weak upward-pressing force. Damage to the surface of the top sheet TP is, therefore, minimized, and the surface of the top sheet TP is not damaged. 
     The sheets P do not have to be fed when the number of revolutions of the fan  55  is at its lowest. The sheets P may be fed in synchronization with a fluctuation or variation in the number of revolutions of the fan  55  so that the number of revolutions is lower when the feed mechanism  90  is operated (feed) than when it is not operated (non-feed), since then the upward-pressing force pushing up the sheets P is weaker when the feed mechanism is operated (feed) than when the feed mechanism is not operated (non-feed). The invention is not limited to the above embodiments. For example, in the embodiments, the nudger roller  56  moves between the feeding position A and the rest position B, but the invention is not limited to this. 
     For example as shown in  FIG. 11 , the sheets P may be fed by a semilunar roller  356  the fixed section of which has a D shape. In this case, an arc portion  356 B contacts with the top sheet so as to feed the top sheet TP and, thereafter, a flat portion  356 A faces downward so that the semilunar roller  356  is disjoined from the top sheet TP and disposed thereabove. The nudger roller may be provided with a rotation allowance mechanism (a mechanism according to which when the nudger roller contacts with the fed sheet, it rotates accordingly) that allows the rotation of a one-way clutch, an electromagnetic clutch, a torque limiter or the like in the sheet feeding direction. 
     The above embodiments use the well-known electrophotographic process as the image forming system, but the system is not limited to this. For example, the image forming system may be a conventionally-known ink jet recording system or another image forming system. 
     Although the above-described embodiments of the present invention are those regarding the image forming apparatus, the present invention is not limited to the image forming apparatus. The invention can also be applied to other devices which transport sheets such as cutting machines or press machines.