Patent Publication Number: US-7907886-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to Japanese Patent Application No. 2006-189199 filed on Jul. 10, 2006 whose priority is claimed under 35 USC §119 and the disclosure of which is incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of high-speed printing, which accommodates a large amount of recording sheets at a front-access type sheet feed portion. 
     2. Description of the Related Art 
     Recently, with a view to make an apparatus compact in order to reduce the occupied area thereof, an image forming apparatus provided with a front-access type sheet feed portion is under development (see Japanese Unexamined Patent Application No. 10-129856). A front-access type image forming apparatus includes a document reading section (scanner portion) arranged at an upper portion, an operation section arranged at a front of the document reading section, an image forming section arranged at a below the document reading section, and a sheet feed section having a sheet cassette, which accommodates recording sheets necessary for printing, and arranged at a lowermost part for transporting the sheet one by one to the image forming section, for example. In the image forming apparatus of this type, a sheet is accommodated and replenished with the sheet cassette drawn out of the apparatus, and after the replenishment is finished, the sheet cassette is pushed into the apparatus to be reloaded. A sheet placing plate is provided at a bottom part of the sheet cassette. An upward movement of the sheet placing plate raises an uppermost sheet to a sheet feeding point, and a pickup roller is lowered to the sheet feeding point from an upper stand-by position for transporting the sheet one by one to the image forming section by the pickup roller. 
     In some of the front-access type image forming apparatuses, a separation pawl (regulating member) for regulating a stacking amount and an uppermost surface of the accommodated sheets is provided at an end edge of the sheet cassette at the downstream side in a sheet transporting direction (see Japanese Unexamined Patent Application No. 2004-307183). 
     For the high-speed printing of the image forming apparatus, a sheet feed section with a larger capacity is now being developed. In order to provide a large capacity to the sheet feed section, since it is not acceptable to raise the positions of the document reading section and the operation section because this causes an inconvenience for a user, it is necessary to devise a way for accommodating many sheets as possible in the sheet cassette. For example, it is considered that the regulation member provided at the conventional sheet cassette is removed so that the sheets can be accommodated to a full height that is generally equal to the height of the sheet cassette, specifically the sheets can be accommodated up to the top height of a side plate that regulates a sheet setting area provided at a bottom part of the sheet cassette. 
     However, when the sheets S are fully replenished to the sheet cassette  211  drawn out of a main body  200  of the apparatus and the sheet cassette  211  is pushed strongly to be reloaded into the main body  200  with the regulating member for regulating the uppermost surface of the sheets eliminated as shown in  FIG. 7A , the uppermost sheet S might lifts up to run on to the side plate  212  as shown in  FIG. 7B . In this case, misalignment in printing or jam during transportation occur due to skew conveyance. Notably, reference numeral  220  denotes a pickup roller in  FIG. 7 . 
     As a measure to prevent the misalignment of the sheet described above, it is considered that, during the loading process of the sheet cassette, the pickup roller is moved down to press the uppermost sheet. In this case, it is necessary that a stack of sheets is moved up so that the uppermost surface of the sheet is brought to a position (e.g., sheet feeding point) higher than the sheet cassette and the side plate, so as to prevent the descending pickup roller from contacting the sheet cassette. However, this method entails problems as described below. That is, the sheet pressed by the pickup roller may be damaged or wrinkled, resulting in degradation of image quality caused by printing performed on the damaged sheet, or the movement of the sheets positioned at a position higher than the side plate is stopped before completion of the loading to cause misalignment with respect to the sheet cassette, thereby producing a misalignment in printing. 
     SUMMARY OF THE INVENTION 
     The present invention aims to solve these problems, and to provide an image forming apparatus that can accommodate a larger amount of recording sheets without increasing the size of the apparatus. 
     The present invention provides an image forming apparatus comprising: a main body having an image forming section; a sheet feed section having a sheet cassette that accommodates a recording sheet and can be drawn out and loaded to the main body; and a sheet misalignment preventing mechanism that intermittently comes in contact with an uppermost recording sheet in the sheet cassette so as to prevent misalignment of the recording sheet in a loading direction when the sheet cassette drawn out of the main body is reloaded. 
     According to the present invention, a larger amount of recording sheets can be accommodated in the sheet cassette without increasing the size of the image forming apparatus and without conducting a drastic change in design, and further, the sheet cassette into which recording sheets are replenished can be reloaded to the main body without causing sheet misalignment and a damage or wrinkle on the recording sheet (without producing factors of skew conveyance, jam during conveyance, deterioration in image quality, etc.). Accordingly, the present invention is preferable for an image forming apparatus capable of high-speed printing (e.g., more than 100 sheets per one minute) that requires accommodation of a large number of sheets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing an overall configuration of an image forming apparatus according to one embodiment of the present invention; 
         FIG. 2  is a side view for explaining a schematic configuration of a pickup roller and its driving mechanism in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3  is a plan view for explaining the schematic configuration of the pickup roller and its driving mechanism in  FIG. 2 ; 
         FIG. 4  is a view for explaining a reloading state of the sheet cassette in the image forming apparatus in  FIG. 1 ; 
         FIG. 5  is a view for explaining a vertical movement of the pickup roller in the image forming apparatus in  FIG. 1 ; 
         FIG. 6  is a block diagram showing a control system in the image forming apparatus in  FIG. 1 ; and 
         FIG. 7  is a view for explaining a reloading state of a sheet cassette in a conventional image forming apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An image forming apparatus according to the present invention comprising: a main body having an image forming section; a sheet feed section having a sheet cassette that accommodates a recording sheet and can be drawn out and loaded to the main body; and a sheet misalignment preventing mechanism that intermittently comes in contact with an uppermost recording sheet in the sheet cassette so as to prevent misalignment of the recording sheet in a loading direction when the sheet cassette drawn out of the main body is reloaded. 
     The image forming apparatus according to the present invention includes at least the main body having the image forming section, the sheet feed section having the sheet cassette, and the sheet misalignment preventing mechanism as described above. It is needless to say that the image forming apparatus may additionally be provided with other peripheral functional sections to be described later that are generally provided to an image forming apparatus. It should be noted that, in the present invention, the recording sheet is not limited to paper, but includes all recording media on which a toner image can be printed. 
     In the present invention, the image forming section has a function of directly reading a document, a function of receiving document data (image data) from an external device (e.g., a personal computer), or both of these functions. The configuration of the image forming section is not particularly limited as long as it has a function of printing a toner image onto a recording sheet on the basis of the document data, and it may either perform a monochrome printing or color printing. One example of the configuration of the image forming section is one provided with a document reading section, an operation section, a photoconductor, a latent image forming section that forms a latent image onto a surface of the photoconductor on the basis of the document data read by the document reading section, a developing section that forms a toner image on the basis of the latent image on the surface of the photoconductor, a transfer section that transfers the toner image onto a recording sheet, a fixing section that fixes the transferred toner image on the recording sheet, and a cleaner section that removes residual toner on the photoconductor after the transfer. Instead of the document reading section, a document data receiving section may be provided, or the document reading section may have a document data receiving section that receives document data from an external device. 
     In the present invention, the sheet feed section is of a front-access type in which the sheet cassette can be drawn out toward the front (at the side of the operation section) of the main body and reloaded to the main body. The sheet feed section includes, in addition to the sheet cassette, a pickup roller that sends the recording sheet in the sheet cassette one by one to the image forming section, and a pickup roller driving mechanism that drives the pickup roller in rotating as well as vertically moving manner. The pickup roller and the pickup roller driving mechanism will be described later. 
     In the present invention, the configuration of the sheet misalignment preventing mechanism is not particularly limited, as long as it has a function of preventing the misalignment of the recording sheet in the loading direction by intermittently contacting the uppermost recording sheet in the sheet cassette, when the sheet cassette is reloaded after the recording sheets are replenished to the sheet cassette drawn out of the main body. 
     The pickup roller and the pickup roller driving mechanism can be used as a part of the configuration of the sheet misalignment preventing mechanism, and the sheet misalignment preventing mechanism can be configured to include a control section for controlling the pickup roller so as to repeatedly move up and down during the reloading operation of the sheet cassette. Alternatively, the pickup roller and the pickup roller driving mechanism may not be used, and a contact member (e.g., a rod with a round tip) that intermittently contacts the uppermost sheet and a vertically moving mechanism for moving the contact member up and down may be provided, whereby the control section controls the vertically moving mechanism and the vertical movement of the contact member. However, the former configuration is desirable in term of simplification of the apparatus structure and cost for the apparatus, since it includes a less number of components. 
     When the pickup roller and the pickup roller driving mechanism are also used as a part of the configuration of the sheet misalignment preventing mechanism, the pickup roller is formed into generally a half-round shape with a part of its circumference cut to be flat, whereby the rotational moment of the pickup roller can be utilized for the vertical movement. 
     In this case, the pickup roller driving mechanism is configured to include a drive roller that is arranged at the downstream side of the pickup roller in the sheet conveying direction and parallel to the axis of the pickup roller, a link member that rotatably links the drive roller and the pickup roller, a pulley and a belt that transmit a rotational force of the drive roller to the pickup roller, a spring member that urges the pickup roller upward about the axis of the drive roller, and a descending drive section that is turned ON at the time of conveying the sheet so as to descend the pickup roller about the axis of the drive roller and does not regulate the vertical movement of the pickup roller in its OFF state. By virtue of this configuration, when the sheet cassette is reloaded, the drive roller is rotated by a signal from the control section to rotate the pickup roller, whereby the pickup roller can repeatedly move up and down by the rotational moment of the pickup roller caused by the rotation thereof and the urging force of the spring member. The pickup roller mechanism itself has conventionally been known, and a special change in design is not necessary. A conventionally known mechanism using a solenoid or a cam that pivots the pickup roller downward in its ON state can be used as the descending drive section. 
     In the image forming apparatus according to the present invention, it is preferable that the sheet feed section further includes sheet ascending mechanism that lifts the recording sheet of at least the downstream side in the sheet conveying direction so as to raise the uppermost surface of the sheet to a sheet feeding point that is higher than the sheet cassette during the reloading operation of the sheet cassette, wherein the control section of the sheet misalignment preventing mechanism synchronously drives the pickup roller drive mechanism and the sheet ascending mechanism, and controls such that the pickup roller makes a vertical movement at an upward portion from the sheet feeding point. By virtue of this configuration, it can surely be prevented that the vertically moving pickup roller and the sheet cassette collide with each other during the loading operation of the sheet cassette. Further, since the pickup roller makes a vertical movement at the portion higher than the sheet feeding point, the pickup roller lightly comes into contact with the uppermost sheet intermittently to such an extent that the pickup roller does not damage the sheet, whereby the misalignment of the sheet in the loading direction can be prevented. 
     In this case, the sheet feed section may further include a loading detection section that detects a start of the reloading operation and a completion of the loading of the sheet cassette, wherein the control section may synchronously drive the pickup roller drive mechanism and the sheet ascending mechanism on the basis of a signal from the loading detection section and controls a start and a stop of the vertical movement of the pickup roller. 
     The image forming apparatus according to the present invention may have, as a peripheral functional section, an automatic document feeding section for feeding a document to the document reading section, a sheet discharge tray that receives a printed sheet from the image forming section, a manual feeding tray that feeds a desired sheet to the image forming section, a large-capacity sheet cassette provided separately from the sheet cassette for feeding a large amount of sheets to the image forming section, and the like. 
     Embodiments of the present invention will now be described with reference to the drawings. It is to be noted that the present invention is not limited to the embodiments described below. 
     (Overall Configuration and Operation of Image Forming Apparatus) 
       FIG. 1  is a schematic view showing an overall configuration of an image forming apparatus according to the present invention. This image forming apparatus  100  forms a monochrome image on a predetermined recording sheet (sheet) in accordance with externally transmitted image data. The image forming apparatus  100  includes a document conveying section  1  mounted on a main body, an image forming section provided at an upper part of the main body, a sheet feed section that is provided at a lower part of the main body, a large-capacity sheet cassette (LCC)  25  connected to a side face of the main body at the upstream side in the sheet conveying direction, a manual feeding tray  26  provided at a side face of the main body at the upstream side in the sheet conveying direction, and a sheet discharge tray  12  provided at a side face of the main body at the downstream side in the sheet conveying direction. 
     The image forming section is composed of an image reading section  2 , an exposure unit  3 , a developing device  4 , a photoconductor  5 , a charging device  6 , a cleaner unit  7 , a transfer unit  8 , a fixing unit  9 , etc. The sheet feed section is composed of sheet cassettes  11   a  and  11   b , sheet conveying path  10 , etc. 
     The image reading section  2  is mainly composed of a light source holder  13 , mirror group  14 , and a CCD  15 . When the document fed from the document conveying section  1  is scanned, the image on the document is scanned with the light source holder  13  and the mirror group  14  in a stationary state. 
     When the document is conveyed from the document conveying section  1 , light is irradiated to the document from a light source of the light source holder  13 , and the optical path of the light reflected from the document is changed through the mirror group  14  to be focused on the CCD  15  so as to be converted into electrical image data. 
     The charging device  6  is for uniformly charging the surface of the photoconductor  5  to a predetermined potential. Although a charger-type charging device is employed in  FIG. 1 , a contact roller type charger or brush type charger may be employed. 
     As shown in  FIG. 1 , the exposure unit  3  is a system using a laser scanning unit (LSU) provided with a laser irradiating sections  16   a ,  16   b  and reflection mirrors  17   a ,  17   b . In order to conduct a high-speed printing process, the exposure unit  3  employs a technique (two-beam method) of using a plurality of laser beams to reduce an increase in irradiation timing per laser device. Apart from the LSU, the exposure unit  3  may be, for example, an EL or LED writing head where light emitting elements are arranged in array. 
     The exposure unit  3  exposes the surface of the photoconductor  5  having been charged uniformly by the charging device  6  according to the image data inputted to the exposure unit  3  so as to form an electrostatic latent image on the surface of the photoconductor  5  according to the image data. 
     The image forming apparatus  100  includes a control section (not shown) for integrally controlling the overall apparatus. 
     The control section is composed of a CPU, a ROM that stores a control program executes by the CPU, a RAM that provides a work area to the CPU, a nonvolatile memory that holds control data, an input circuit to which a signal from detection mechanisms at each section of the image forming apparatus  100  is inputted, a driver circuit that drives an actuator and a motor for operating a driving mechanism of each section in the image forming apparatus  100 , an output circuit for driving the laser irradiating sections  16   a ,  16   b , etc. This control section is also operated at the time of reloading the sheet cassette which will be described later. 
     The developing device  4  develops the latent image formed on the photoconductor  5  with a black toner. 
     The cleaner unit  7  removes and collects residual toner remaining on the surface of the photoconductor  5  after the development and the image transfer. 
     The toner image developed on the photoconductor  5  is transferred onto the conveyed sheet at the transfer unit  8  (in the present apparatus, transfer belt unit) in such a manner that an electric field having a polarity opposite from that of the charges of the latent image is applied from the transfer unit  8 . For example, when the latent image has charges of negative polarity, the applied polarity from the transfer unit  8  is positive. 
     In the transfer unit  8  of the present apparatus, a transfer belt  19  is disposed so as to be wound around a drive roller  20 , a driven roller  21 , and other rollers. The transfer belt  19  has a predetermined resistance (ranging from 1×10 9  to 1×10 13  Ωcm). Further, an elastic conductive roller  22  is disposed at a portion where the photoconductor  5  comes into contact with the transfer belt  19 . The elastic conductive roller  22  can apply a transfer electric field with electrically conductive that is different from electric conductivities of the drive roller  20  and the driven roller  21 . The elastic conductive roller  22  has elasticity, and this allows the photoconductor  5  and the transfer belt  19  to come into contact with each other at their surfaces, not in lines, having a given width (referred to as “transfer nip”). By this means, efficiency of the transfer onto a sheet being transported is enhanced. 
     Additionally, a charge removing roller is disposed on a backside of the transfer belt  19  downstream from a transfer area of the transfer belt  19 . The charge removing roller removes the electric-field (charge) having been applied to the transported sheet in the transfer area to facilitate a shift to a subsequent process. 
     Besides, the transfer unit  8  includes a cleaning unit for cleaning toner contamination on the transfer belt  19  and a plurality of charge removing mechanisms for removing a charge of the transfer belt  19 . It is to be noted that the plurality of charge removing mechanisms may employ a technique of grounding through a device or a technique of actively applying an electric-field having a polarity opposite from that of the transfer electric-field. 
     The unfixed toner image having been transferred onto the sheet by the transfer unit  8  is transported to the fixing unit  9 , so that the unfixed toner is fused and fixed onto the sheet. 
     The fixing unit  9  includes a heat roller  23  and a pressure roller  24 . Around the outer periphery of the heat roller  23 , a sheet separation pawl, a roller surface temperature detecting member (thermistor), and a roller surface cleaning member are disposed. The heat roller  23  has a heat source at the inner periphery thereof. The heat source maintains the surface of the heat roller  23  at a predetermined temperature (preset fixing temperature: generally 160 to 200° C.). 
     Meanwhile, on both ends of the pressure roller  24 , pressure members are disposed. The pressure members cause the pressure roller  24  to press the heat roller  23  at a predetermined pressure. Around the outer periphery of the pressure roller  24 , a sheet separation pawl and a roller surface cleaning member are disposed as with the outer periphery of the heat roller  23 . 
     At a portion where the heat roller  23  and the pressure roller  24  are in press-contact with each other (referred to as “fixing nip”), the unfixed toner on the sheet being transported is fixed onto the sheet through fusing at a surface temperature of the heat roller  23  and a press-contact function. 
     The sheet cassettes  11   a  and  11   b  are trays for stacking recording sheets to be used for the image formation and provided below the image forming section in the present apparatus as described above. The sheet cassettes  11   a  and  11   b  are vertically arranged in three stages. The image forming apparatus of the present embodiment aims at high speed print processing, so that each of the sheet cassettes can hold 500 to 1500 sheets of standard size. Arranged at the side face of the main body are the large-capacity sheet cassette  25  that can hold a large amount of sheets of different types, and the manual feeding tray  26  mainly used in e.g., printing on sheets of irregular size. 
     The sheet discharge tray  12  is disposed at a side surface of the main body that is opposite to the surface thereof on which the manual feeding tray  26  is disposed. Optionally, the sheet discharge tray  12  may be replaced with a post-processing device (e.g. stapler and hole-punching) for the discharged sheets or a multi-stage sheet discharge tray. 
     (Configuration and Operation of Sheet Feed Section) 
     The sheet feed section has the sheet cassettes  11   a  and  11   b  vertically arranged in three stages as described above. 
     The upper sheet cassette  11   a  has two spaces for storing sheets. A pickup roller, a separation roller and a feed roller are provided integrally with the housing of the sheet cassette at the downstream side of the spaces in the sheet conveying direction. The sheet cassette  11   a  has, at a bottom part of each storing space, a sheet placing plate on which recording sheets are placed, and an ascending mechanism that independently raises each placing plate, so as to raise the uppermost surface of the sheet to the sheet feeding point that is the position of the pickup roller. 
     The middle and lower sheet cassettes  11   b  and  11   b , which have the same configuration each other, have a capacity smaller than that of the upper sheet cassette  11   a . A sheet placing plate  111  on which recording sheets are placed is provided at a bottom part of each of the sheet cassettes  11   b  and  11   b  at the downstream side in the sheet conveying direction. Further, a side plate  112  for regulating the size of the sheet is provided to the sheet cassettes  11   b  and  11   b  (see  FIG. 4 ). The side plate  112  is set to have the same height as or slightly lower than the height of the side edge portion (front and rear) of the sheet cassette  11   b . The placing plate  111  is pivotably mounted at generally a middle position of the sheet cassette  11   b  in the sheet conveying direction by a shaft member extending in a direction orthogonal to the sheet conveying direction. The sheet cassette  11   b  further has a pivot member and a shaft section that lift up the downstream end portion of the placing plate  111  to an end portion at the downstream side in the sheet conveying direction, and the sheet feed section has at the side of the main body a driving source  111   a  (see  FIG. 6 ) that can be engaged or disengaged with the shaft section so as to pivot the shaft section at a predetermined rate. Therefore, the pivot member pivots upward by the driving source  111   a  via the shaft section, by which the downstream end portion of the placing plate  111  pivots upward, whereby the stack of sheets moves up to bring the uppermost surface thereof to the sheet feeding point. 
     As shown in  FIGS. 1 to 3 , the sheet feed section is provided with a pickup roller driving section  130  including a pickup roller  120  and a separation roller  131 , and a sheet feed roller  140  above the sheet cassettes  11   b  at the downstream side in the sheet conveying direction. In  FIG. 2 , reference symbol S denotes a recording sheet. 
     The pickup roller  120  is formed into generally a half-round shape (a shape rather close to a circle than a semi-circle) when viewed from an axial direction, formed by cutting a part of its circumference to be flat, and a band-like rubber member having a corrugated surface is adhered onto the outer peripheral surface of its arc portion. 
     The pickup roller driving mechanism  130  has the separation roller (drive roller)  131  that is arranged at the downstream side of the pickup roller  120  in the sheet conveying direction and parallel to the axis of the pickup roller  120 , a link member  132  that rotatably links the separation roller  131  and the pickup roller  120  through shafts  120   a  and  131   a , a pair of pulleys  133  and a belt  134  that transmit rotational force of the separation roller  131  to the pickup roller  120 , a spring (spring member)  135  that urges the pickup roller  120  upward about the axis of the separation roller  131 , a descending drive section  136  that is turned ON during the conveyance of the sheet to descend the pickup roller  120  about the axis of the separation roller  131  and does not regulate the vertical movement of the pickup roller  120  in its OFF state, and a motor  130   a  (see  FIG. 6 ) that drives and rotates the shaft  131   a  so as to rotate the separation roller  131  and the pickup roller  120 . The descending drive section  136  can be composed of, e.g., a solenoid and a link member that links the shaft of the solenoid and the link member  132 . 
     In the present embodiment, an electromagnetic clutch  137  and another shaft  138  are provided at one end of the shaft  120   a  of the pickup roller  120 . The pair of pulleys  133  are attached to the shaft  138  and the shaft  131   a  of the separation roller  131 . The electromagnetic clutch  137  links the shaft  120   a  of the pickup roller  120  and the shaft  138  in its ON state, and releases the link between the shaft  120   a  and the shaft  138  in its OFF state. The electromagnetic clutch  137  may be omitted, and one of the pulleys  133  may be directly attached to the shaft  120   a  of the pickup roller  120 . 
     Provided at the sheet feed section above each of the sheet cassettes  11   a  and  11   b  that are attached to the main body are a known sheet height detecting section  150  for detecting the height of the uppermost surface of the sheets in each of the sheet cassettes  11   a  and  11   b  and a loading detection section  151  for detecting that the middle and lower cassettes  11   b , which have been drawn out of the main body, are reloaded. The loading detection section  151  is provided at a position where a start and completion of the reloading operation of the sheet cassette  11   b  can be detected. For example, an optical sensor or a mechanical sensor is arranged on the main body at the near side in the drawing direction of the sheet cassette, whereby the operation (start of reloading, completion of loading) of the sheet cassette  11   b  can be detected from a detection state (ON-OFF, etc.) of the sensor. 
     As shown in  FIG. 6 , the sheet height detecting section  150  and the loading detection section  151  transmit a detection signal to the aforementioned control section  160 . The control section  160  transmits a control signal to the driving source  111   a  for lifting the placing plates of the sheet cassettes  11   a  and  11   b  and the motor  130   a  of the pickup roller driving section  130 . 
     Subsequently, the operations of the middle and lower sheet cassettes of the thus configured image forming apparatus when they are reloaded will be described with reference to  FIGS. 2 to 6 . 
     Recording sheets are replenished to the middle or lower sheet cassette drawn out of the main body. In this case, the sheets are accommodated to the top height of the side plate of the sheet cassette. 
     When the sheet cassette  11   b  is then inserted into the main body  100   a  to start the reloading as shown in  FIG. 4A , a loading start signal is transmitted to the control section  160  from the loading detection section  151  (see  FIG. 6 ). Then, a control signal is transmitted to the driving source  111   a  for lifting the placing plate and the motor  130   a  of the pickup roller driving mechanism  130  from the control section  160 , whereby the driving source  111   a  and the motor  130   a  are synchronously driven. Consequently, the placing plate  111  in the sheet cassette  11   b  moves up to raise the uppermost recording sheet S to the sheet feeding point, and simultaneously, the pickup roller  120  starts rotating and making the vertical movement. 
       FIG. 5  is a view for explaining the state in which the pickup roller  120  rotates and vertically moves. Upon reloading the sheet cassette  11   b , the descending drive section  136  (see  FIG. 3 ) of the pickup roller drive mechanism  130  is in its OFF state, so that the vertical movement of the pickup roller  120  is not regulated by the descending drive section  136 . Therefore, the pickup roller  120  vertically pivots about the shaft  131   a  due to the rotational moment generated by the rotation of the generally half-round pickup roller  120  and the upward urging force of the spring  135 . 
     Specifically, since downward centrifugal force at the time when the middle part of the arc of the pickup roller  120  reaches the upstream side in the sheet conveying direction exceeds the urging force of the spring  135 , the pickup roller  120  pivots downward with the link member  132  so as to lightly come into contact with the uppermost sheet surface instantaneously (see  FIG. 5A ). Thereafter, when the middle part of the arc of the pickup roller  120  reaches the downstream side in the sheet conveying direction, the centrifugal force directs upward, whereby the pickup roller  120  pivots upward (see  FIG. 5B ). As described above, the pickup roller  120  repeats the vertical movement while rotating. Since the rotation number of the motor  130   a  is controlled by the control section  160 , the rotational moment of the pickup roller  120  is controlled, whereby the lowermost position of the pickup roller  120  is set to the sheet feeding point (the uppermost surface of the recording sheet S). 
       FIG. 4A  shows the state in which the pickup roller  120  pivots downward to be in contact with the uppermost surface of the sheet as in the case of  FIG. 5A . Since the sheet cassette  11   b  moves in the loading direction (in the direction of arrow A), there may be the case where a deflection f is produced in the uppermost sheet S that comes into contact with the pickup roller  120 . However, since the pickup roller  120  instantaneously moves up as shown in  FIG. 4B  and  FIG. 5B , the sheet S returns to be flat and the deflection is eliminated, when the pickup roller  120  is apart from the uppermost sheet S. In this manner, the pickup roller  120  comes into contact with the uppermost sheet S three to five times during the period from the start of the reloading of the sheet cassette  11   b  to the completion of the reloading shown in  FIG. 4C , when the sheet size accommodated in the sheet cassette is A4 (210 mm×299 mm) and short sides of the sheet is located in the sheet conveying direction, for example. Owing to the instantaneous repeated contact (light strike) described above, the positional misalignment of the sheet S above the side plate  112  in the loading direction is prevented. In  FIG. 4 , reference numeral  20  denotes a cushion member for relieving an impact upon the completion of the loading of the sheet cassette  11   b.    
     After the loading of the sheet cassette  11   b  is completed, a loading completion signal is transmitted from the loading detection section  151  (see  FIG. 6 ) to the control section  160 , whereby the control signal is transmitted from the control section  160  to the motor  130   a . Accordingly, the motor  130   a  is stopped to stop the rotation and vertical movement of the pickup roller  120 . After the rotation of the pickup roller  120  is stopped, the pickup roller  120  waits at an upward stand-by position by the spring  135  (see  FIG. 2 ). The control signal is also transmitted from the control section  160  to the driving source  111   a , whereby the driving source  111   a  comes into an OFF state. Accordingly, the placing plate  111  is lowered to the lowermost position, so that the stack of sheets descends. 
     When a printing process is started, a sheet height signal is transmitted from the sheet height detecting section  150  to the control section  160 , whereby the control signal is transmitted from the control section  160  to the driving source  111   a . Accordingly, the uppermost sheet ascends to the sheet feeding point as described above. Further, the control signal is transmitted from the control section  160  to the descending drive section  136  and the motor  130   a , whereby the pickup roller  120  descends to the sheet feeding point and the pickup roller  120  rotates to feed the uppermost sheet S to the image forming section. 
     When a plurality of sheets are continuously printed by the printing process, the electromagnetic clutch  137  is turned OFF to stop the rotation of the pickup roller  120  after one sheet is fed, and the electromagnetic clutch  137  is turned ON to rotate the pickup roller  120  at the timing of feeding a next sheet, in order to provide a predetermined sheet space. Thus, a sheet is conveyed one by one. In the configuration in which the electromagnetic clutch is omitted, the pickup roller that keeps on rotating is caused to make a vertical movement so as to provide a sheet space. 
     The present invention is applicable to an electrophotographic image forming apparatus such as a copying machine, printer, and the like.