Abstract:
A sheet feeder stackable in multiple levels and attachable to an image forming apparatus. The sheet feeder includes a drive source, a drive transmission unit, a first drive coupling unit, a second drive coupling unit, a sheet feed roller, and a grip roller. During operation of multiple sheet feeders stacked in the multiple levels, the drive transmission unit and the first drive coupling unit, both provided to a given sheet feeder of the multiple sheet feeders, are coupled to each other, and the first drive coupling unit of the given sheet feeder is coupled to the grip roller of an adjacent sheet feeder disposed immediately below the given sheet feeder to drive the grip roller of each of the multiple sheet feeders substantially in synchrony.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application No. 2012-028813, filed on Feb. 13, 2012, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Exemplary aspects of the present invention generally relate to an image forming apparatus, and more particularly to a sheet feeder in which a drive source is installed and an image forming apparatus including the sheet feeder. 
     2. Description of the Related Art 
     Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image bearing member (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet of recording media; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus. 
     These image forming apparatuses often include multiple sheet feeders disposed one above the other at the bottom of the image forming apparatus. In a case in which a drive source is provided to each of the multiple sheet feeders, drive sources for all the multiple sheet feeders need to be driven when a recording medium is fed from a bottommost sheet feeder disposed at the bottom of the multiple sheet feeders, generating large noise. 
     Because a low-end type image forming apparatus tends to be installed near a user, there is an increasing demand for reducing noise generated by the drive sources, and various techniques for solving such a problem have been proposed. However, in the related-art techniques, reduction of noise generated by the drive sources becomes more difficult as the number of drives sources respectively provided for the multiple sheet feeders increases. 
     Specifically, in the related art, grip rollers are driven by the drive sources provided for the multiple sheet feeders via multiple clutches, respectively. Consequently, in a case in which a recording medium is fed from the bottommost sheet feeder, all the drive sources and the clutches must be driven to drive the grip rollers. 
     To reduce the size and the production cost of the image forming apparatus, a configuration in which both a clutch and an idler gear are provided to a shaft of a sheet feed roller to transmit a drive force from a motor to the sheet feed roller via the clutch and to a manual sheet feed roller via the idler gear is possible. In other words, the single drive source and clutch are used for driving the two separate rollers. However, in a case in which the image forming apparatus includes multiple sheet feeders disposed one above the other, multiple drive sources are still necessary for the multiple sheet feeders, respectively, and thus the problem of the noise generated by the multiple drive sources still remains unsolved. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, embodiments of the present invention provide a novel sheet feeder including a drive source. In a case in which multiple sheet feeders, each having a drive source, are stacked in multiple levels, only a drive source provided for a given sheet feeder, from which a recording medium is fed, is driven during feeding of the recording medium to reduce noise generated by the drive source. 
     Illustrative embodiments of the present invention also provide a novel image forming apparatus including the sheet feeder. 
     In one illustrative embodiment, a sheet feeder stackable in multiple levels and attachable to an image forming apparatus includes a drive source, a drive transmission unit, a first drive coupling unit connected to the drive source via the drive transmission unit, a second drive coupling unit connected to the drive source via the drive transmission unit and the first drive coupling unit, a sheet feed roller coaxial with the second drive coupling unit to feed a recording medium from the sheet feeder, and a grip roller provided downstream from the sheet feed roller in a sheet feeding direction. The sheet feed roller is driven by a drive force of the drive source transmitted to the second drive coupling unit via the drive transmission unit and the first drive coupling unit. The grip roller is driven by a drive force of the drive source transmitted to the first drive coupling unit via the drive transmission unit. During operation of multiple sheet feeders stacked in the multiple levels, the drive transmission unit and the first drive coupling unit, both provided to a given sheet feeder of the multiple sheet feeders, are coupled to each other, and the first drive coupling unit of the given sheet feeder is coupled to the grip roller of an adjacent sheet feeder disposed immediately below the given sheet feeder to drive the grip roller of each of the multiple sheet feeders substantially in synchrony. 
     In another illustrative embodiment, an image forming apparatus includes an image forming unit to form an image on a recording medium and the sheet feeder described above to feed the recording medium to the image forming unit. 
     Additional features and advantages of the present disclosure will become more fully apparent from the following detailed description of illustrative embodiments, the accompanying drawings, and the associated claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be more readily obtained as the same becomes better understood by reference to the following detailed description of illustrative embodiments when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a vertical cross-sectional view illustrating an example of a configuration of an image forming apparatus according to a first illustrative embodiment; 
         FIG. 2  is a vertical cross-sectional view illustrating an example of a configuration of a second sheet feed unit included in the image forming apparatus illustrated in  FIG. 1 ; 
         FIG. 3  is a vertical cross-sectional view illustrating an example of a configuration of an image forming apparatus according to a second illustrative embodiment; and 
         FIG. 4  is a vertical cross-sectional view illustrating an example of a configuration of a second sheet feed unit included in the image forming apparatus illustrated in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In describing illustrative embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Illustrative embodiments of the present invention are now described below with reference to the accompanying drawings. In a later-described comparative example, illustrative embodiment, and exemplary variation, for the sake of simplicity the same reference numerals will be given to identical constituent elements such as parts and materials having the same functions, and redundant descriptions thereof omitted unless otherwise required. 
     A configuration and operation of an image forming apparatus  100  according to a first illustrative embodiment are described in detail below, with reference to  FIG. 1 . 
       FIG. 1  is a vertical cross-sectional view illustrating an example of a configuration of the image forming apparatus  100  according to the first illustrative embodiment. The image forming apparatus  100  includes an image forming unit  150  at the top thereof, a first sheet feed unit, which, in the present illustrative embodiment, is a sheet tray  1  disposed below the image forming unit  150 , and a second sheet feed unit  200  additionally provided to the image forming apparatus  100  below the first sheet feed unit. The second sheet feed unit  200  includes multiple sheet feeders, which, in the present illustrative embodiments, are sheet trays  2 ,  3 , and  4  disposed, in that order, from the top to the bottom below the sheet tray  1 . It is to be noted that each bold solid arrow in  FIG. 1  and subsequent drawings indicates a drive train, and each bold broken line indicates a conveyance path of a recording medium such as a sheet of paper. 
     The image forming unit  150  includes photoconductor units  160 Y,  160 M,  160 C, and  160 K (hereinafter collectively referred to as photoconductor units  160 ) that form a toner image of a specific color, that is, yellow (Y), magenta (M), cyan (C), or black (K). 
     Each of the photoconductor units  160  has the same basic configuration, differing only in the color of toner used. Therefore, suffixes Y, M, C, and K, each representing the color of toner, are hereinafter omitted unless otherwise necessary. It is to be noted that only the reference numerals each denoting a component of the photoconductor unit  160 K are shown in  FIGS. 1 and 3 . Each of the photoconductor units  160  includes an image carrier, which, in the present illustrative embodiment, is a drum-type photoconductor  161 , a charger  162  that charges the photoconductor  161 , a developing device  163  that develops an electrostatic latent image formed on the photoconductor  161  with toner, and a cleaning device  164  that removes residual toner from the photoconductor  161 . The photoconductor  161 , the charger  162 , the developing device  163 , and the cleaning device  164  of each of the photoconductor units  160  are formed together as a single integrated process cartridge detachably attachable to the image forming unit  150 . A light-emitting element  165  that irradiates the photoconductor  161  with light to form the electrostatic latent image on the photoconductor  161  is disposed above each of the respective photoconductor units  160 . 
     An intermediate transfer belt  180  onto which a toner image formed by each of the photoconductor units  160  is primarily transferred is disposed below the photoconductor units  160 . The intermediate transfer belt  180  is wound around multiple rollers, and toner images of the specified colors respectively formed on the photoconductors  161  of the photoconductor units  160  are sequentially transferred onto the intermediate transfer belt  180  one atop the other to form a single full-color toner image on the intermediate transfer belt  180 . 
     A belt cleaning device, not shown, that removes residual untransferred toner from the intermediate transfer belt  180  is disposed around the intermediate transfer belt  180 . The image forming unit  150  further includes a secondary transfer roller  185  that secondarily transfer the full-color toner image formed on the intermediate transfer belt  180  onto a recording medium, and a fixing unit  190  that fixes the toner image on the recording medium. Toner cartridges  170 Y,  170 C,  170 M, and  170 K (hereinafter collectively referred to as toner cartridges  170 ) that supply toner to the respective developing devices  163  are disposed above the photoconductor units  160 . 
     Full-color image formation performed by the image forming apparatus  100  is described in detail below. In each of the photoconductor units  160 , the charger  162  evenly charges the photoconductor  161 . Next, the light-emitting element  165  irradiates the photoconductor  161  with light based on image data to form an electrostatic latent image on the photoconductor  161 . 
     The electrostatic latent image formed on the photoconductor  161  is then developed with toner of the specified color borne by a developing roller included in the developing device  163  so that a toner image is formed on the photoconductor  161 . The above-described sequence of toner image formation is performed in each of the photoconductor units  160 . Toner images formed on each of the photoconductors  161  are sequentially transferred one atop the other onto the intermediate transfer belt  180  rotated in a clockwise direction in  FIG. 1  so that a single full-color toner image is formed on the intermediate transfer belt  180 . 
     After primary transfer of the toner images from the photoconductors  161  onto the intermediate transfer belt  180 , the cleaning device  164  cleans the photoconductor  161  to be ready for the next sequence of image formation on the photoconductor  161 . In the mean time, a recording medium such as a sheet of paper is conveyed by a pair of registration rollers  186  at a predetermined timing to a secondary transfer area formed between the intermediate transfer belt  180  and the secondary transfer roller  185 . At the secondary transfer area in which the secondary transfer roller  185  and the intermediate transfer belt  180  contact each other, the full-color toner image formed on the intermediate transfer belt  180  is secondarily transferred onto the recording medium. The recording medium having the full-color toner image thereon is then conveyed to the fixing unit  190  so that the toner image is fixed onto the recording medium by the fixing unit  190 . Thereafter, the recording medium having the fixed image thereon is discharged from the image forming apparatus  100 . After secondary transfer of the toner image from the intermediate transfer belt  180  onto the recording medium, the belt cleaning device that contacts the intermediate transfer belt  180  removes untransferred toner remaining on the intermediate transfer belt  180 . 
     The sheet tray  1  disposed immediately below the image forming unit  150  includes a recording medium container  111  that accommodates a stack of recording media, a bottom plate  112  on which the stack of recording media is placed, a pickup roller  113  that picks up a sheet of recording medium placed at the top of the stack of recording media on the bottom plate  112 , and a pair of conveyance rollers  114  that conveys the recording medium thus picked up by the pickup roller  113 . The bottom plate  112  is provided to the recording medium container  111 . One end of the bottom plate  112  is hinged about a shaft, and the bottom plate  112  is pressed upward. A drive source such as a motor, not shown, that drives the pickup roller  113  and the pair of conveyance rollers  114  is provided to the sheet tray  1  individually from drive sources for the second sheet feed unit  200 . 
     As described previously, the second sheet feed unit  200  is additionally provided to the image forming apparatus  1  below the sheet tray  1 . The second sheet feed unit  200  includes the sheet trays  2  to  4 . The topmost sheet tray  2  of the second sheet feed unit  200  is disposed below the sheet tray  1 , the sheet tray  3  is disposed below the sheet tray  2 , and the sheet tray  4  is disposed below the sheet tray  3  at the bottom of the second sheet feed unit  200 . Each of the sheet trays  1  to  4  can accommodate a stack of recording media of predetermined type and size. 
       FIG. 2  is a vertical cross-sectional view illustrating an example of a configuration of the second sheet feed unit  200  according to the first illustrative embodiment. 
     The topmost sheet tray  2  of the second sheet feed unit  200  includes a drive source  5 , an electromagnetic clutch B, and a drive transmission unit  201  that transmits a drive force from an output gear  5 ′ of the drive source  5  to an input gear  10  of the electromagnetic clutch B via a reduction gear  6  and idler gears  7 ,  8 , and  9 . Grip rollers  16  and  16 ′, each of which conveys a recording medium to the image forming unit  150 , are provided opposite each other in the sheet tray  2 , and a grip roller drive gear  17  is provided coaxially with the grip roller  16 . The sheet tray  2  also includes idler gears  12  and  18 , both of which are coupled to an output gear  11  of the electromagnetic clutch B. In the present illustrative embodiment, the electromagnetic clutch B, the idler gear  12 , the grip roller drive gear  17 , and the idler gear  18  together form a first drive coupling unit  202  of the sheet tray  2 . The sheet tray  2  further includes a second drive coupling unit, which, in the present illustrative embodiment, is an electromagnetic clutch A. An input gear  13  of the electromagnetic clutch A is coupled to the idler gear  12 . The idler gear  18  is also coupled to a grip roller drive gear  31  coaxially provided with a grip roller  30  included in the sheet tray  3  described below. 
     The sheet tray  3  includes a drive source  19 , an electromagnetic clutch D, and a drive transmission unit  203  that transmits a drive force from an output gear  19 ′ of the drive source  19  to an input gear  24  of the electromagnetic clutch D via a reduction gear  20  and idler gears  21 ,  22 , and  23 . Grip rollers  30  and  30 ′, each of which conveys a recording medium to the image forming unit  150 , are provided opposite each other in the sheet tray  3 , and the grip roller drive gear  31  is coaxially provided with the grip roller  30 . The sheet tray  3  also includes idler gears  26  and  32 , both of which are coupled to an output gear  25  of the electromagnetic clutch D. In the present illustrative embodiment, the electromagnetic clutch D, the idler gear  26 , the grip roller drive gear  31 , and the idler gear  32  together form a first drive coupling unit  204  of the sheet tray  3 . The sheet tray  3  further includes a second drive coupling unit, which, in the present illustrative embodiment, is an electromagnetic clutch C. An input gear  27  of the electromagnetic clutch C is coupled to the idler gear  26 . The idler gear  32  is also coupled to a grip roller drive gear  45  coaxially provided with a grip roller  44  included in the sheet tray  4  described below. 
     The sheet tray  4  includes a drive source  33 , an electromagnetic clutch F, and a drive transmission unit  205  that transmits a drive force from an output gear  33 ′ of the drive source  33  to an input gear  38  of the electromagnetic clutch F via a reduction gear  34  and idler gears  35 ,  36 , and  37 . Grip rollers  44  and  44 ′, each of which conveys a recording medium to the image forming unit  150 , are provided opposite each other in the sheet tray  4 , and the grip roller drive gear  45  is coaxially provided with the grip roller  44 . The sheet tray  4  also includes idler gears  40  and  46 , both of which are coupled to an output gear  39  of the electromagnetic clutch F. In the present illustrative embodiment, the electromagnetic clutch F, the idler gear  40 , the grip roller drive gear  45 , and the idler gear  46  together form a first drive coupling unit  206  of the sheet tray  4 . The sheet tray  4  further includes a second drive coupling unit, which, in the present illustrative embodiment, is an electromagnetic clutch E. An input gear  41  of the electromagnetic clutch E is coupled to the idler gear  40 . 
     A method for driving the sheet tray  2 ,  3 , or  4  to feed the recording medium to the image forming unit  150  is described in detail below with reference to  FIGS. 1 and 2 . It should be noted that the speed of the drive sources  5 ,  19 , and  33  need not be fixed and may instead be variable, and moreover the drive sources  5 ,  19 , and  33  may operate in reverse as well as forward. 
     In a case of feeding a recording medium from the sheet tray  2 , the drive source  5  is driven and thus the output gear  5 ′ of the drive source  5  is rotated in a clockwise direction in  FIGS. 1 and 2 . The drive force of the drive source  5  is transmitted from the output gear  5 ′ to the input gear  10  of the electromagnetic clutch B via the reduction gear  6  and the idler gears  7 ,  8 , and  9  and is further transmitted to the input gear  13  of the electromagnetic clutch A via the idler gear  12 , so that both the electromagnetic clutches A and B are engaged. Accordingly, the input gear  10  and the output gear  11  of the electromagnetic clutch B are coupled to each other and the input gear  13  and an output gear  14  of the electromagnetic clutch A are coupled to each other. As a result, a sheet feed roller  15  coaxially provided with the output gear  14  of the electromagnetic clutch A is rotated in a counterclockwise direction in  FIGS. 1 and 2  to feed a recording medium from the sheet tray  2 . The drive force is further transmitted to the grip roller drive gear  17  via the idler gear  12  coupled to the output gear  11  of the electromagnetic clutch B so that the grip roller  16  is rotated in the counterclockwise direction to convey the recording medium to the image forming unit  150 . While the electromagnetic clutch B is engaged, the drive force is also transmitted to the grip roller drive gear  31  of the sheet tray  3  disposed below the sheet tray  2  and the grip roller drive gear  45  of the bottommost sheet tray  4  disposed below the sheet tray  3  via the first drive coupling units  202 ,  204 , and  206  coupled to one another. Therefore, all the grip rollers  16 ,  30 , and  44  of the sheet trays  2 ,  3 , and  4  are rotated while the electromagnetic clutch B is engaged. However, because the electromagnetic clutches D and F of the sheet trays  3  and  4  are disengaged, the output gears  25  and  39  of the electromagnetic clutches D and F are idly rotated and no drive force is transmitted to the input gears  24  and  38  of the electromagnetic clutches D and F, respectively. As a result, no drive force is transmitted to the output gears  19 ′ and  33 ′ of the drive sources  19  and  33  of the sheet trays  3  and  4 . When the electromagnetic clutch B is disengaged, rotation of each of the grip rollers  16 ,  30 , and  44  is stopped. 
     In a case of feeding a recording medium from the sheet tray  3 , the drive source  19  is driven and thus the output gear  19 ′ of the drive source  19  is rotated in a clockwise direction in  FIGS. 1 and 2 . The drive force of the drive source  19  is transmitted from the output gear  19 ′ to the input gear  24  of the electromagnetic clutch D and is further transmitted to the input gear  27  of the electromagnetic clutch C so that both the electromagnetic clutches D and C are engaged. Accordingly, the input gear  24  and the output gear  25  of the electromagnetic clutch D are coupled to each other and the input gear  27  and an output gear  28  of the electromagnetic clutch C are coupled to each other. As a result, a sheet feed roller  29  coaxially provided with the output gear  28  of the electromagnetic clutch C is rotated in a counterclockwise direction in  FIGS. 1 and 2  to feed a recording medium from the sheet tray  3 . The drive force is further transmitted to the grip roller drive gear  31  via the idler gear  26  coupled to the output gear  25  of the electromagnetic clutch D so that the grip roller  30  is rotated in the counterclockwise direction to convey the recording medium to the image forming unit  150 . While the electromagnetic clutch D is engaged, the drive force is also transmitted to the grip roller drive gear  45  of the bottommost sheet tray  4  disposed below the sheet tray  3  and the grip roller drive gear  17  of the topmost sheet tray  2  disposed above the sheet tray  3  via the first drive coupling units  202 ,  204 , and  206  coupled to one another. Therefore, all the grip rollers  16 ,  30 , and  44  of the sheet trays  2 ,  3 , and  4  are rotated while the electromagnetic clutch D is engaged. However, because the electromagnetic clutches B and F of the sheet trays  2  and  4  are disengaged, the output gears  11  and  39  of the electromagnetic clutches B and F are idly rotated and no drive force is transmitted to the input gears  10  and  38  of the electromagnetic clutches B and F. As a result, no drive force is transmitted to the output gears  5 ′ and  33 ′ of the drive sources  5  and  33  of the sheet trays  2  and  4 . When the electromagnetic clutch D is disengaged, rotation of each of the grip rollers  16 ,  30 , and  44  is stopped. 
     In a case of feeding a recording medium from the sheet tray  4 , the drive source  33  is driven and thus the output gear  33 ′ of the drive source  33  is rotated in a clockwise direction in  FIGS. 1 and 2 . The drive force of the drive source  33  is transmitted from the output gear  33 ′ to the input gear  38  of the electromagnetic clutch F and is further transmitted to the input gear  41  of the electromagnetic clutch E so that both the electromagnetic clutches F and E are engaged. Accordingly, the input gear  38  and the output gear  39  of the electromagnetic clutch F are coupled to each other and the input gear  41  and an output gear  42  of the electromagnetic clutch E are coupled to each other. As a result, a sheet feed roller  43  coaxially provided to the output gear  42  of the electromagnetic clutch E is rotated in a counterclockwise direction in  FIGS. 1 and 2  to feed a recording medium from the sheet tray  4 . The drive force is further transmitted to the grip roller drive gear  45  via the idler gear  40  coupled to the output gear  39  of the electromagnetic clutch F so that the grip roller  44  is rotated in the counterclockwise direction to convey the recording medium to the image forming unit  150 . While the electromagnetic clutch F is engaged, the drive force is also transmitted to the grip roller drive gear  31  of the sheet tray  3  disposed above the sheet tray  4  and the grip roller drive gear  17  of the topmost sheet tray  2  disposed above the sheet tray  3  via the first drive coupling units  202 ,  204 , and  206  coupled to one another. Therefore, all the grip rollers  16 ,  30 , and  44  of the sheet trays  2 ,  3 , and  4  are rotated while the electromagnetic clutch F is engaged. However, because the electromagnetic clutches B and D are disengaged, the output gears  11  and  25  of the electromagnetic clutches B and D are idly rotated and no drive force is transmitted to the input gears  10  and  24  of the electromagnetic clutches B and D. As a result, no drive force is transmitted to the output gears  5 ′ and  19 ′ of the drive sources  5  and  19  of the sheet trays  2  and  3 . When the electromagnetic clutch F is disengaged, rotation of each of the grip rollers  16 ,  30 , and  44  is stopped. 
     A description is now given of a second illustrative embodiment of the present invention. 
       FIG. 3  is vertical cross-sectional view illustrating an example of a configuration of the image forming apparatus  100  according to the second illustrative embodiment.  FIG. 4  is a vertical cross-sectional view illustrating an example of a configuration of the second sheet feed unit  200  according to the second illustrative embodiment. It is to be noted that, in the second illustrative embodiment, the same reference numerals are used for the same components as those of the first illustrative embodiment, and a description of such components is omitted. 
     In the second illustrative embodiment, the first sheet feed unit, which in the present illustrative embodiment, is the sheet tray  1 , further includes an idler gear  47 , auxiliary rollers  48  and  48 ′, and a gear  49  coaxially provided with the auxiliary roller  48 . 
     A method for driving the sheet tray  2 ,  3 , or  4  to feed a recording medium to the image forming unit  150  according to the second illustrative embodiment is described in detail below with reference to  FIGS. 3 and 4 . 
     In a case of feeding a recording medium from the sheet tray  2 , the drive source  5  is driven and thus the output gear  5 ′ of the drive source  5  is rotated in a clockwise direction in  FIGS. 3 and 4 . The drive force is transmitted from the output gear  5 ′ to the input gear  10  of the electromagnetic clutch B and is further transmitted to the input gear  13  of the electromagnetic clutch A, so that both the electromagnetic clutches A and B are engaged. Accordingly, the input gear  10  and the output gear  11  of the electromagnetic clutch B are coupled to each other and the input gear  13  and the output gear  14  of the electromagnetic clutch A are coupled to each other. As a result, the sheet feed roller  15  coaxially provided with the output gear  14  of the electromagnetic clutch A is rotated in a counterclockwise direction in  FIGS. 3 and 4  to feed a recording medium from the sheet tray  2 . The drive force is further transmitted to the grip roller drive gear  17  via the idler gear  12  coupled to the output gear  11  of the electromagnetic clutch B so that the grip roller  16  is rotated in the counterclockwise direction to convey the recording medium to the image forming unit  150 . At this time, the grip roller drive gear  17  is also connected to the gear  49  coaxially provided with the auxiliary roller  48  via the idler gear  47  provided to the sheet tray  1 . In a case of feeding a recording medium of a size smaller than a length from the grip roller  16  of the sheet tray  2  to the pair of registration rollers  186  provided to the image forming unit  150 , the auxiliary roller  48  conveys the recording medium to the pair of registration rollers  186  after a trailing edge of the recording medium has passed through the grip roller  16 . While the electromagnetic clutch B is engaged, the drive force is transmitted to the grip roller drive gear  31  of the sheet tray  3  disposed below the sheet tray  2  and the grip roller drive gear  45  of the bottommost sheet tray  4  disposed below the sheet tray  3  via the first drive coupling units  202 ,  204 , and  206  coupled to one another, thereby rotating all the grip rollers  16 ,  30 , and  44  provided to the sheet trays  2 ,  3 , and  4 , respectively. However, because the electromagnetic clutches D and F of the sheet trays  3  and  4  are disengaged, the output gears  25  and  39  of the electromagnetic clutches D and F are idly rotated and no drive force is transmitted to the input gears  24  and  38  of the electromagnetic clutches D and F. As a result, no drive force is transmitted to the output gears  19 ′ and  33 ′ of the drive sources  19  and  33  of the sheet trays  3  and  4 . When the electromagnetic clutch B is disengaged, rotation of each of the grip rollers  16 ,  30 , and  44  is stopped. 
     In a case of feeding a recording medium from the sheet tray  3 , the drive source  19  is driven and thus the output gear  19 ′ of the drive source  19  is rotated in a clockwise direction in  FIGS. 3 and 4 . The drive force is transmitted from the output gear  19 ′ to the input gear  24  of the electromagnetic clutch D and is further transmitted to the input gear  27  of the electromagnetic clutch C, so that both the electromagnetic clutches D and C are engaged. Accordingly, the input gear  24  and the output gear  25  of the electromagnetic clutch D are coupled to each other and the input gear  27  and the output gear  28  of the electromagnetic clutch C are coupled to each other. As a result, the sheet feed roller  29  coaxially provided with the output gear  28  of the electromagnetic clutch C is rotated in a counterclockwise direction in  FIGS. 3 and 4  to feed a recording medium from the sheet tray  3 . The drive force is further transmitted to the grip roller drive gear  31  via the idler gear  26  coupled to the output gear  25  of the electromagnetic clutch D so that the grip roller  30  is rotated in the counterclockwise direction to convey the recording medium to the image forming unit  150 . 
     While the electromagnetic clutch D is engaged, the drive force is also transmitted to the grip roller drive gear  45  of the bottommost sheet tray  4  disposed below the sheet tray  3  and the grip roller drive gear  17  of the topmost sheet tray  2  disposed above the sheet tray  3  via the first drive coupling units  202 ,  204 , and  206  coupled to one another, thereby rotating all the grip rollers  16 ,  30 , and  44  of the sheet trays  2 ,  3 , and  4 , respectively. However, because the electromagnetic clutches B and F are disengaged, the output gears  11  and  39  of the electromagnetic clutches B and F are idly rotated and no drive force is transmitted to the input gears  10  and  38  of the electromagnetic clutches B and F. As a result, no drive force is transmitted to the output gears  5 ′ and  33 ′ of the drive sources  5  and  33  of the sheet trays  2  and  4 . At this time, the grip roller drive gear  17  of the topmost sheet tray  2  is also connected to the gear  49  coaxially provided with the auxiliary roller  48  via the idler gear  47  provided to the sheet tray  1 . In a case of feeding a recording medium of a size smaller than the length from the grip roller  16  of the sheet tray  2  to the pair of registration rollers  186  provided to the image forming unit  150 , the auxiliary roller  48  conveys the recording medium to the pair of registration rollers  186  after the trailing edge of the recording medium has passed through the grip roller  16 . When the electromagnetic clutch D is disengaged, rotation of each of the grip rollers  16 ,  30 , and  44  is stopped. 
     In a case of feeding a recording medium from the bottommost sheet tray  4 , the drive source  33  is driven and thus the output gear  33 ′ of the drive source  33  is rotated in a clockwise direction in  FIGS. 3 and 4 . The drive force is transmitted from the output gear  33 ′ to the input gear  38  of the electromagnetic clutch F and is further transmitted to the input gear  41  of the electromagnetic clutch E, so that both the electromagnetic clutches F and E are engaged. Accordingly, the input gear  38  and the output gear  39  of the electromagnetic clutch F are coupled to each other and the input gear  41  and the output gear  42  of the electromagnetic clutch E are coupled to each other. As a result, the sheet feed roller  43  coaxially provided with the output gear  42  of the electromagnetic clutch E is rotated in a counterclockwise direction in  FIGS. 3 and 4  to feed a recording medium from the sheet tray  4 . The drive force is further transmitted to the grip roller drive gear  45  via the idler gear  40  coupled to the output gear  39  of the electromagnetic clutch F so that the grip roller  44  is rotated in the counterclockwise direction to convey the recording medium to the image forming unit  150 . 
     While the electromagnetic clutch F is engaged, the drive force is also transmitted to the grip roller drive gear  31  of the sheet tray  3  disposed above the sheet tray  4  and the grip roller drive gear  17  of the topmost sheet tray  2  disposed above the sheet tray  3  via the first drive coupling units  202 ,  204 , and  206  coupled to one another, thereby rotating all the grip rollers  16 ,  30 , and  44  of the sheet trays  2 ,  3 , and  4 , respectively. However, because the electromagnetic clutches B and D are disengaged, the output gears  11  and  25  of the electromagnetic clutches B and D are idly rotated and no drive force is transmitted to the input gears  10  and  24  of the electromagnetic clutches B and D. As a result, the drive force is not transmitted to the output gears  5 ′ and  19 ′ of the drive sources  5  and  19  of the sheet trays  2  and  3 . At this time, the grip roller drive gear  17  of the sheet tray  2  is also connected to the gear  49  coaxially provided with the auxiliary roller  48  via the idler gear  47  provided to the sheet tray  1 . In a case of feeding a recording medium of a size smaller than the length from the grip roller  16  of the sheet tray  2  to the pair of registration rollers  186  provided to the image forming unit  150 , the auxiliary roller  48  conveys the recording medium to the pair of registration rollers  186  after the trailing edge of the recording medium has passed through the grip roller  16 . When the electromagnetic clutch F is disengaged, rotation of each of the grip rollers  16 ,  30 , and  44  is stopped. 
     In the foregoing illustrative embodiments, all the grip rollers  16 ,  30 , and  44  respectively provided to the sheet trays  2 ,  3 , and  4  are connected to one another via the output gears  11 ,  25 , and  39  of the electromagnetic clutches B, D and F. As a result, the single drive source  5 ,  19 , or  33  of the sheet tray  2 ,  3 , or  4 , from which the recording medium is fed, can drive all the grip rollers  16 ,  30 , and  44  using the single electromagnetic clutch B, D, or F, thereby reducing noise. In addition, provision of blocking members and a rigid housing for reducing noise is not needed, thereby reducing production costs. 
     Elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 
     Illustrative embodiments being thus described, it will be apparent that the same may be varied in many ways. Such exemplary variations are not to be regarded as a departure from the scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 
     The number of constituent elements and their locations, shapes, and so forth are not limited to any of the structure for performing the methodology illustrated in the drawings.