Patent Publication Number: US-8977186-B2

Title: Drive transmission system, post-processing device, and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2011-263253 filed Dec. 1, 2011. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a drive transmission system, a post-processing device, and an image forming apparatus. 
     SUMMARY 
     According to an aspect of the invention, there is provided a drive transmission system circuit including a drive source and a gear. The drive source includes a rotating shaft, a magnet supported by the rotating shaft, and plural electromagnets. The plural electromagnets are arranged in a circumferential direction of the rotating shaft, and surround the magnet. The drive source drives the rotating shaft to rotate by a predetermined rotation angle by exciting at least one of the plural electromagnets in accordance with an input of an input signal and by periodically changing a magnetic pole to which each of the plural electromagnets is excited in response to an input of the input signal. The gear is supported by the rotating shaft. The least common multiple of a second frequency and a third frequency exceeds a threshold value that is a predetermined value based on an audible frequency range audible to the human ear. In this case, the second frequency is a value obtained by multiplying the number of rotations of the drive source per unit time by the number of teeth of the gear. The number of rotations of the drive source per unit time is a value obtained by dividing a first frequency by a total number of input signals required for the rotating shaft to rotate one turn. The first frequency is a value representing the number of input signals input to the drive source per unit time. Further, the third frequency is a value obtained by dividing the first frequency by the number of steps per cycle. The number of steps per cycle is a total number of input signals required for the periodically changing of the magnetic pole to complete one cycle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  illustrates an overall view of an image forming apparatus according to a first exemplary embodiment; 
         FIG. 2  is an enlarged view of a substantial part of the image forming apparatus according to the first exemplary embodiment; 
         FIG. 3  is an enlarged view of a post-processing device according to the first exemplary embodiment, and illustrates the upward and downward movement of a clamp roller used for exit; 
         FIG. 4  is an enlarged view of the post-processing device according to the first exemplary embodiment, and illustrates the upward and downward movement of a sub-paddle; 
         FIG. 5  is an enlarged view of a substantial part of the post-processing device according to the first exemplary embodiment; 
         FIG. 6  illustrates a substantial part of the rear end of a compile tray according to the first exemplary embodiment; 
         FIG. 7  is a cross-sectional view taken along line VII-VII in  FIG. 6 ; 
         FIGS. 8A and 8B  are diagrams of tampers according to the first exemplary embodiment when viewed from the top and the bottom, respectively; 
         FIGS. 9A and 9B  illustrate a drive transmission system according to the first exemplary embodiment, in which  FIG. 9A  illustrates a substantial part of the drive transmission system when the post-processing device is viewed from rear to front, and  FIG. 9B  illustrates a substantial part of a stacker exit motor, a gear, and a timing belt according to the first exemplary embodiment; 
         FIGS. 10A to 10D  illustrate a stacker exit motor according to the first exemplary embodiment, in which  FIG. 10A  is a cross-sectional view of a motor body,  FIG. 10B  is an enlarged perspective view of the teeth of rotors,  FIG. 10C  is a cross-sectional view taken along line XC-XC in  FIG. 10A , and  FIG. 10D  illustrates a substantial part of a stator unit in which coils and a power supply are removed from the configuration illustrated in  FIG. 10C ; 
         FIGS. 11A to 11C  illustrate relationships between rotor teeth and stator teeth when the right direction is the rotation direction, in which  FIG. 11A  illustrates a relationship between the rotor teeth and the stator teeth when only the A +  phase coils are energized,  FIG. 11B  illustrates a relationship between the rotor teeth and the stator teeth when the energization of the A +  phase coils is disconnected after the state illustrated in  FIG. 11A  and the B +  phase coils are energized, and  FIG. 11C  illustrates a relationship between the rotor teeth and the stator teeth when the B +  phase coils are energized after the state illustrated in  FIG. 11A ; 
         FIG. 12  illustrates the turning on and off of energization to each lead for each step when the electromagnets of the stacker exit motor according to the first exemplary embodiment are excited using the one-two phase excitation method; 
         FIG. 13  illustrates changes in the states of the magnetic poles in the respective steps illustrated in  FIG. 12 ; 
         FIG. 14  is a graph illustrating results obtained by the frequency analysis of noise generated by driving a stepping motor in a conventional printer, with noise level in decibels (dB) plotted on the y axis and frequency in hertz (Hz) plotted on the x axis; 
         FIG. 15  illustrates peak levels measured in an experimental example; and 
         FIG. 16  is a graph illustrating the operation of the first exemplary embodiment, and illustrates a relationship between peak levels obtained in Experimental Example 1 and Comparative Examples 1 and 2, with peak level in decibels (dB) plotted on the y axis and drive frequency in pulses per second (pps) (i.e., in hertz (Hz)) plotted on the x axis. 
     
    
    
     DETAILED DESCRIPTION 
     A specific example of an exemplary embodiment of the present invention (hereinafter referred to as an “exemplary embodiment”) will be described hereinafter with reference to the drawings. It is to be understood that the present invention is not limited to the following exemplary embodiment. 
     For ease of understanding of the following description, in the drawings, the front-rear direction is defined as an X-axis direction, the left-right direction as a Y-axis direction, and the up-down direction as a Z-axis direction. Also, directions indicated by arrows X, −X, Y, −Y, Z, and −Z are defined as “forward”, “rearward”, “rightward”, “leftward”, “upward”, and “downward”, respectively. In addition, sides indicated by arrows X, −X, Y, −Y, Z, and −Z are defined as “front” or “front side”, “rear” or “rear side”, “right” or “right side”, “left” or “left side”, “upper” or “upper side”, and “lower” or “lower side”, respectively. 
     Further, in the drawings, a dot in a circle represents an arrow pointing from the back to the front of the paper, and a cross in a circle represents an arrow pointing from the front to the back of the paper. 
     In the following description taken in conjunction with the drawings, illustration of members other than those necessary for the description is properly omitted for ease of understanding. 
     First Exemplary Embodiment 
       FIG. 1  illustrates the overall structure of an image forming apparatus according to a first exemplary embodiment. 
     In  FIG. 1 , a printer U, which may be an example of the image forming apparatus according to the first exemplary embodiment of the present invention, includes a printer body U 1 , which may be an example of a body of the image forming apparatus. Image information transmitted from an information processing device PC electrically connected to the printer U, which may be an example of an image information transmitting device, is input to a controller C. The image information input to the controller C is converted at a predetermined timing into image information on yellow (Y), magenta (M), cyan (C), and black (K) for forming latent images, and is output to a latent image forming circuit DL. 
     If a document image is a single-color image, or monochrome image, the image information on only black (K) is input to the latent image forming circuit DL. 
     The latent image forming circuit DL includes drive circuits (not illustrated) for the respective colors of Y, M, C, and K, and outputs signals corresponding to the input image information to latent image forming devices LHy, LHm, LHc, and LHk disposed for the respective colors at a predetermined timing. 
       FIG. 2  is an enlarged view of a substantial part of the image forming apparatus according to the first exemplary embodiment. 
     In  FIGS. 1 and 2 , the latent image writing light beams of the respective colors of Y, M, C, and K, which are emitted from latent image writing light sources of the latent image forming devices LHy, LHm, LHc, and LHk, enter rotating photoconductors PRy, PRm, PRc, and PRk, respectively. The rotating photoconductors PRy, PRm, PRc, and PRk may be examples of image holding members. In the first exemplary embodiment, each of the latent image forming devices LHy to LHk may be a light emitting diode (LED) array having LEDs arranged linearly along the width of an image. The LEDs may be examples of light emitting elements. 
     Around the photoconductors PRy, PRm, PRc, and PRk, chargers CRy, CRm, CRc, and CRk, the latent image forming devices LHy, LHm, LHc, and LHk, developing devices Gy, Gm, Gc, and Gk, first transfer devices T 1   y , T 1   m , T 1   c , and T 1   k , and photoconductor cleaners CLy, CLm, CLc, and CLk, which may be examples of cleaning devices, are disposed in the direction of rotation of the photoconductors PRy, PRm, PRc, and PRk. 
     In  FIGS. 1 and 2 , the photoconductors PRy, PRm, PRc, and PRk are charged by the chargers CRy, CRm, CRc, and CRk, respectively, and then electrostatic latent images are formed on the surfaces of the photoconductors PRy, PRm, PRc, and PRk at image writing positions Q 1   y , Q 1   m , Q 1   c , and Q 1   k , respectively, by the respective latent image writing light beams. The electrostatic latent images on the surfaces of the photoconductors PRy, PRm, PRc, and PRk are developed into toner images in developing regions Q 2   y , Q 2   m , Q 2   c , and Q 2   k  by developers held on developing rollers GRy, GRm, GRc, and GRk of developing devices Gy, Gm, Gc, and Gk, respectively. The toner images may be examples of visible images, and the developing rollers GRy, GRm, GRc, and GRk may be examples of developer holding members. 
     The developed toner images are transported to first transfer regions Q 3   y , Q 3   m , Q 3   c , and Q 3   k  that are in contact with an intermediate transfer belt B. The intermediate transfer belt B may be an example of an intermediate transfer body. In the first transfer regions Q 3   y  to Q 3   k , a first-transfer voltage having a polarity opposite to the polarity of the electric charge of toner is applied to the first transfer devices T 1   y  to T 1   k  disposed on the back side of the intermediate transfer belt B at a predetermined timing from a power supply circuit E controlled by the controller C. 
     The toner images on the photoconductors PRy to PRk are transferred (first transfer) onto the intermediate transfer belt B by the first transfer devices T 1   y  to T 1   k , respectively. The residues and debris on the surfaces of the photoconductors PRy to PRk after the first transfer has been completed are cleaned by the photoconductor cleaners CLy to CLk, respectively. The cleaned surfaces of the photoconductors PRy to PRk are recharged by the chargers CRy to CRk, respectively. 
     A visible image forming device Uy of the color of Y according to the first exemplary embodiment that forms a toner image, which may be an example of a visible image, includes the photoconductor PRy, the charger CRy, the latent image forming device LHy, the developing device Gy, the first transfer device T 1   y , and the photoconductor cleaner CLy of the color of Y. Similarly, visible image forming devices Um, Uc, and Uk of the colors of M, C, and K include the photoconductors PRm, PRc, and PRk, the chargers CRm, CRc, and CRk, the latent image forming devices LHm, LHc, and LHk, the developing devices Gm, Gc, and Gk, the first transfer devices T 1   m , T 1   c , and T 1   k , and the photoconductor cleaners CLm, CLc, and CLk, respectively. 
     A belt module BM capable of moving up and down and being pulled out forward is disposed above the photoconductors PRy to PRk. The belt module BM may be an example of an intermediate transfer device. The belt module BM includes the intermediate transfer belt B, a belt drive roller Rd, a tension roller Rt, a walking roller Rw, an idler roller Rf, a backup roller T 2   a , and the first transfer devices T 1   y  to T 1   k . The belt drive roller Rd may be an example of a drive member, the tension roller Rt may be an example of a stretching member, and the walking roller Rw may be an example of a meandering prevention member. The idler roller Rf may be an example of a driven member, and the backup roller T 2   a  may be an example of a second-transfer opposite member. The intermediate transfer belt B is supported by the rollers Rd, Rt, Rw, Rf, and T 2   a  so as to be rotatably movable. 
     A second transfer roller T 2   b , which may be an example of a second transfer member, is disposed at a position opposite the backup roller T 2   a  with the intermediate transfer belt B interposed between the backup roller T 2   a  and the second transfer roller T 2   b . A second transfer device T 2  according to the first exemplary embodiment includes the backup roller T 2   a  and the second transfer roller T 2   b . Further, a second transfer region Q 4  is a region where the second transfer roller T 2   b  and the intermediate transfer belt B are in contact with each other. 
     A single-color toner image or multiple-color toner images that are sequentially transferred so as to be superimposed on top of one another, which are transferred onto the intermediate transfer belt B in the first transfer regions Q 3   y  to Q 3   k  by the first transfer devices T 1   y  to T 1   k , are transported to the second transfer region Q 4 . 
     The first transfer devices T 1   y  to T 1   k , the intermediate transfer belt B, the second transfer device T 2 , etc., constitute a transfer device (T 1 +T 2 +B) according to the first exemplary embodiment. Further, the visible image forming devices Uy to Uk and the transfer device (T 1 +T 2 +B) constitute an image recording unit (Uy to Uk+T 1 +T 2 +B) according to the first exemplary embodiment. 
     In  FIG. 1 , four pairs of right and left guide rails GR are provided downward from the visible image forming devices Uy to Uk, and paper feed trays TR 1  to TR 4  are supported by the pairs of guide rails GR so as to be insertable into and removable from the printer body U 1  in the front-rear direction. The guide rails GR may be examples of guide members, and the paper feed trays TR 1  to TR 4  may be examples of paper feed containers. Sheets S received in the paper feed trays TR 1  to TR 4 , which may be examples of media, are picked up by pickup rollers Rp, and are separated one by one by pairs of separation rollers Rs. The pickup rollers Rp may be examples of a transport member and examples of pickup members, and the pairs of separation rollers Rs may be examples of separation members. A sheet S is transported along a paper feed path SH 1  by plural pairs of transport rollers Ra, and is fed to a pair of registration rollers Rr disposed upstream of the second transfer region Q 4  in a sheet transport direction. The paper feed path SH 1  may be an example of a media transport path, the pairs of transport rollers Ra may be examples of a transport member, and the pair of registration rollers Rr may be an example of a member for adjusting the timing at which a medium is to be transported. 
     The pickup rollers Rp, the separation rollers Rs, etc., constitute a paper feeding device (Rp+Rs) according to the first exemplary embodiment. 
     A manual feed tray TR 0 , which may be an example of a manual paper feeding unit, is disposed rightward of the top paper feed tray TR 1 . A sheet S supported by the manual feed tray TR 0  is fed by a pair of manual paper feed rollers Rp 0 , which may be an example of a manual paper feeding member, and is transported along a manual feed transport path SH 0  to the pair of registration rollers Rr. 
     The pair of registration rollers Rr transports the sheet S to a principal transport path SH 2 , which may be an example of a transport path, downstream of the paper feed path SH 1  in synchronization with the transporting of the toner image or images formed on the intermediate transfer belt B to the second transfer region Q 4 , and transports the sheet S to the second transfer region Q 4 . When the sheet S passes the second transfer region Q 4 , the backup roller T 2   a  is grounded, and a second-transfer voltage having a polarity opposite to the polarity of the electric charge of toner is applied to the second transfer device T 2   b  from the power supply circuit E controlled by the controller C. The toner image or images on the intermediate transfer belt B are transferred onto the sheet S from the intermediate transfer belt B. 
     After the second transfer has been completed, the intermediate transfer belt B is cleaned by a belt cleaner CLb, which may be an example of an intermediate transfer body cleaning device. 
     The sheet S onto which the toner image or images have been transferred (second transfer) is transported to a fixing region Q 5  that is a region where a heating roller Fh and a pressure roller Fp are in contact with each other, and is heated and fixed when passing the fixing region Q 5 . The heating roller Fh and the pressure roller Fp may be an example of a heat fixing member and a pressure fixing member of a fixing device F, respectively. A release agent is applied to the surface of the heating roller Fh by a release agent applying device Fa in order to help the sheet S release from the heating roller Fh. 
     A paper output path SH 3 , which may be an example of a transport path, along which the sheet S is transported toward a paper output tray TRh is disposed upward, or downstream of the fixing device F in the transport direction. The paper output tray TRh may be an example of a unit in which media output from the printer body U 1  are stacked. Therefore, in a case where the sheet S is transported toward the paper output tray TRh, the sheet S onto which the toner image or images have been fixed is transported along the paper output path SH 3 , and is output from a sheet output port SH 3   a  by a pair of paper output rollers Rh. The sheet output port SH 3   a  may be an example of a media output port, and the pair of paper output rollers Rh may be an example of an exiting member of the printer body U 1 . 
     In  FIG. 1 , in the first exemplary embodiment, a lower cover U 1   a , which may be an example of an upstream-side opening member, is supported at a position to the right of the three lower paper feed trays TR 2  to TR 4  so as to be openable and closable between a normal position indicated by a solid line in  FIG. 1  and an open position indicated by a broken line in  FIG. 1 . The right guide of the paper feed path SH 1  disposed on the right side of the paper feed trays TR 2  to TR 4 , and the outer rollers of the respective pairs of transport rollers Ra are supported by the lower cover U 1   a . Therefore, moving the lower cover U 1   a  to the open position allows a lower portion of the paper feed path SH 1 , that is, an upstream-side paper feed path SH 1   a  that is located on the upstream side of the paper feed path SH 1  in the transport direction, to be made open to remove jammed media. 
     The transport paths SH 0  to SH 3  constitute a transport path SH according to the first exemplary embodiment. Further, the transport path SH, the paper feeding device (Rp+Rs), the sheet transport rollers Ra, the registration rollers Rr, the paper output rollers Rh, etc., constitute a media transport system (SH+Ra to Rh). 
     Sheet Transport Unit U 2  in First Exemplary Embodiment 
     In  FIG. 1 , the printer U according to the first exemplary embodiment includes a sheet transport unit U 2  that is removably attached to the paper output tray TRh. The sheet transport unit U 2  may be an example of a media transport unit. The sheet transport unit U 2  has a side surface to be connected to the sheet output port SH 3   a  in the printer body U 1 , and an input port  1  through which the sheet S output from the pair of paper output rollers Rh enters is formed in the side surface. The sheet S that has entered through the input port  1  is transported along a communicating transport path SH 5  through pairs of communicating transport rollers Ra 2  disposed in the sheet transport unit U 2 . The communicating transport path SH 5  may be an example of a transport path, and the pairs of communicating transport rollers Ra 2  may be examples of a transport member. The sheet S transported along the communicating transport path SH 5  is output from an output port  2  that is formed in another side surface of the sheet transport unit U 2  and that is directed toward the post-processing device U 3 . 
     Post-Processing Device U 3  in First Exemplary Embodiment 
       FIG. 3  is an enlarged view of a post-processing device U 3  according to the first exemplary embodiment, and illustrates the upward and downward movement of a clamp roller  21  used for exit. 
       FIG. 4  is an enlarged view of the post-processing device U 3  according to the first exemplary embodiment, and illustrates the upward and downward movement of sub-paddles  23 . 
       FIG. 5  is an enlarged view of a substantial part of the post-processing device U 3  according to the first exemplary embodiment. 
     In  FIGS. 1 ,  3 , and  4 , the printer U according to the first exemplary embodiment includes the post-processing device U 3 . The post-processing device U 3  is removably supported by a side surface of the printer body U 1 , and is also connected to the sheet transport unit U 2  to perform post-processing, such as stapling, which may be an example of edge binding, and alignment, on the sheet S output from the sheet output port  2 . 
     In  FIGS. 1 and 3  to  5 , the post-processing device U 3  according to the first exemplary embodiment has a right side wall U 3   a  disposed opposite a left side wall U 1   b  of the printer body U 1 . The right side wall U 3   a  may be an example of an image-forming-apparatus-body-side wall surface. A sheet input port  3  to be connected to the sheet output port  2  is formed in an upper portion of the right side wall U 3   a . The sheet input port  3  may be an example of an input port of the post-processing device U 3 . Further, a pair of front and rear hook units U 3   a   1  projecting rightward and extending downward is formed in a central portion in the up-down direction of the right side wall U 3   a . The hook units U 3   a   1  are fitted into support holes U 1   b   1  formed in the left side wall U 1   b  of the printer body U 1 , and are hung on the printer body U 1 . Therefore, the post-processing device U 3  is supported by the printer body U 1 , and the right side wall U 3   a  of the post-processing device U 3  is held to extend along the left side wall U 1   b  of the printer body U 1 . The sheet input port  3  is held to be connected to the sheet output port  2  in the sheet transport unit U 2 . 
     Thus, the sheet S output from the sheet output port  2  of the sheet transport unit U 2  enters or is transported into the post-processing device U 3  through the sheet input port  3 . 
     Compile Exit Roller  4  in First Exemplary Embodiment 
     In  FIG. 1 , the sheet S that has entered the post-processing device U 3  through the sheet input port  3  is transported along a post-processing transport path SH 6  in the post-processing device U 3  by a pair of post-processing inlet rollers Ra 3  provided downstream of the sheet input port  3 . The pair of post-processing inlet rollers Ra 3  may be an example of a transport member in the post-processing device U 3 . The sheet S transported along the post-processing transport path SH 6  is output onto a compile tray  6  by a compile exit roller  4  provided at a downstream end of the post-processing transport path SH 6 . The compile tray  6  may be an example of a first stacking unit, and the compile exit roller  4  may be an example of a first exiting member. The compile exit roller  4  according to the first exemplary embodiment is rotated and stopped in response to transmission of the drive from a roller drive motor MA 1 , which may be an example of an exit drive source. 
     A compile exit sensor SN 1 , which may be an example of a media detecting member, is disposed near and upstream of the compile exit roller  4 , and detects a sheet S traveling along the post-processing transport path SH 6 . 
     Compile Tray  6  in First Exemplary Embodiment 
     In  FIGS. 1 and 3  to  5 , the compile tray  6  has a compile tray body  7 , which may be an example of a body of the first stacking unit. In  FIG. 1 , the compile tray body  7  is disposed so as to be inclined to the horizontal so that the left side is higher than the right side. 
     In  FIGS. 3 to 5 , an end wall  8  extending upward is supported by the right end of the compile tray body  7 . The end wall  8  may be an example of an edge aligning member. Edges, namely, the right edges, of the sheets S output from the compile exit roller  4  and stacked on the compile tray body  7  are caused to abut against the end wall  8 , thereby causing the right edges of the bundle of sheets S to be aligned with one another. 
     A guide wall  9  is formed at an upper end of the end wall  8  in such a manner that the distance between the guide wall  9  and a stacking surface  7   a  of the compile tray body  7  increases as the guide wall  9  extends away from the end wall  8 . The guide wall  9  may be an example of a guide unit. The guide wall  9  guides the right edge of a sheet S traveling toward the end wall  8 , that is, the upstream edge of the sheet S in a media output direction that is a direction in which media are output, to the end wall  8  when the upstream edge of the sheet S curves or curls. 
     Main Paddles  11  in First Exemplary Embodiment 
     Main paddles  11  are rotatably supported at a position diagonally to the front and the left of the guide wall  9 . The main paddles  11  may be examples of a second alignment transport member. The main paddles  11  have a rotating shaft  11   a  to which drive is transmitted from a paddle drive motor MA 6 , and plural cylindrical roller units  11   b  arranged at predetermined intervals along the rotating shaft  11   a . The paddle drive motor MA 6  may be an example of an alignment drive source, and the cylindrical roller units  11   b  may be examples of rotating bodies. 
     Three flexible plate-shaped paddle bodies  11   c  are supported at predetermined phase intervals on an outer peripheral surface of each of the roller units  11   b . The paddle bodies  11   c  may be examples of a body of the second alignment transport member. The paddle bodies  11   c  according to the first exemplary embodiment extend in tangential directions extending upstream of the outer peripheral surface of the roller units  11   b  with respect to a direction in which sheets S travel toward the end wall  8 , and the outer end of each of the paddle bodies  11   c  has such a length as to be capable of coming into contact with the stacking surface  7   a  of the compile tray body  7 . 
     The rotation of the main paddles  11  enables the paddle bodies  11   c  to be brought into contact with the top surface of the stack of sheets S on the compile tray  6 . Therefore, the stack of sheets S is transported toward the end wall  8  by the main paddles  11 , and is aligned by causing the right edges of the sheets S to abut against the end wall  8 . 
     Tamper  12  in First Exemplary Embodiment 
     A pair of front and rear tampers  12  is disposed in a left portion of the compile tray  6  in order to align the edges in the width direction of the sheets S stacked on the compile tray  6  while coming into contact with the edges in the width direction of the sheets S. The tampers  12  may be examples of a widthwise edge alignment member. 
     The configuration of the tampers  12  will be described in detail below. 
     Stapler  13  in First Exemplary Embodiment 
     In  FIGS. 3 to 5 , a stapler  13 , which may be an example of a binding member, is disposed at a position diagonally downward and to the right of the compile tray  6 . The stapler  13  binds a bundle of sheets S stacked and aligned on the compile tray  6 , with staples. The staples may be examples of binding needles. 
     The configuration of the stapler  13  will be described in detail below. 
     Stacker Exit Roller  16  in First Exemplary Embodiment 
     In  FIGS. 3 to 5 , a stacker exit roller  16  is disposed downstream of the compile tray body  7  in the media output direction, or leftward. The stacker exit roller  16  may be an example of a transport member and also an example of a second exiting member. The stacker exit roller  16  has a rotating shaft  16   a  to which drive is transmitted from a forward and reverse rotatable stacker exit motor MA 2 , and roller bodies  16   b  supported at predetermined intervals along the rotating shaft  16   a . The stacker exit motor MA 2  may be an example of a drive source, and the roller bodies  16   b  may be examples of rotation units. The stacker exit roller  16  rotates in the forward and reverse directions in accordance with the forward and reverse rotation of the stacker exit motor MA 2 . The stacker exit motor MA 2  that drives the stacker exit roller  16  according to the first exemplary embodiment may be a stepping motor that rotates at a predetermined rotation angle each time a pulse signal, which may be an example of a predetermined input signal, is input. 
     During the reverse rotation, the stacker exit roller  16  according to the first exemplary embodiment causes sheets S stacked on the compile tray  6  and subjected to post-processing such as alignment and stapling to exit to a stacker tray TH 1 , which may be an example of a second stacking unit. In addition, during the forward rotation, the stacker exit roller  16  causes a sheet S output onto the compile tray  6  to move toward the end wall  8 . 
     Shelf  17  in First Exemplary Embodiment 
     In  FIG. 5 , a shelf  17 , which may be an example of an extending member, is disposed near the stacker exit roller  16  between the rotating shaft  16   a  of the stacker exit roller  16  and the lower surface of the compile tray body  7 . 
     In  FIG. 5 , the shelf  17  has a plate-shaped shelf body  17   a  that curves in an arc shape, and an arc-shaped rack gear  17   b  formed on a lower surface of the shelf body  17   a . The shelf body  17   a  may be an example of a body of the extending member, and the rack gear  17   b  may be an example of a drive receiving unit. The rack gear  17   b  meshes with a shelf drive gear  18  disposed downward from the rotating shaft  16   a  of the stacker exit roller  16 . Drive is transmitted to the shelf drive gear  18  from a forward and reverse rotatable shelf drive motor MA 3 , which may be an example of an extending drive source. In accordance with the forward and reverse rotation of the motor MA 3 , the shelf  17  moves between an extending position indicated by a solid line in  FIG. 5  at which the bottom surface of a sheet S is supportable and an accommodation position indicated by a broken line in  FIG. 5  at which the shelf  17  is accommodated in the post-processing device U 3 . 
     The stacker exit roller  16  and the shelf  17  are known in the art, and may have any of various known configurations described in, for example, Japanese Unexamined Patent Application Publications No. 2006-69746, No. 2006-69749, No. 2011-88682, and No. 2011-88683, the detailed description of which is omitted. 
     Clamp Roller  21  in First Exemplary Embodiment 
     In  FIG. 3 , a clamp roller  21 , which may be an example of an exit driven member, is disposed upward of the compile tray body  7  so as to correspond to the stacker exit roller  16 . The clamp roller  21  is supported by a leading end of a clamp arm  22  supported so as to be rotatable about a rotating shaft  22   a . The clamp arm  22  may be an example of an arm member. In accordance with the rotation of the clamp arm  22 , the clamp roller  21  is supported so as to be movable between an up position indicated by a solid line in  FIG. 3  and a down position indicated by a broken line in  FIG. 3 . The up position may be an example of a spaced apart position at which the clamp roller  21  is spaced apart from the stacker exit roller  16 . The down position may be an example of a contact position at which, as a result of approaching the stacker exit roller  16 , the clamp roller  21  is in contact with the sheet S so that the sheet S is held between the clamp roller  21  and the stacker exit roller  16 . 
     Sub-Paddles  23  in First Exemplary Embodiment 
     In  FIG. 4 , the sub-paddles  23  are disposed at positions shifted in the front-rear direction of the clamp roller  21 . The sub-paddles  23  may be examples of a first alignment transport member. In the first exemplary embodiment, plural sub-paddles  23  are arranged at predetermined intervals in the front-rear direction, and each of the sub-paddles  23  has a configuration similar to that of each of the main paddles  11 , the detailed description of which is omitted. The sub-paddles  23  are supported by a leading end of a paddle arm  24  that is supported so as to be rotatable about a rotating shaft  24   a . The paddle arm  24  may be an example of an arm member. Each of the sub-paddles  23  is supported so as to be movable between a wait position indicated by a solid line in  FIG. 4  and a retracted position indicated by a broken line in  FIG. 4  in accordance with the rotation of the paddle arm  24 . At the wait position, the sub-paddle  23  is spaced apart from the stacking surface  7   a  of the compile tray  6  as a result of upward movement. At the retracted position, the sub-paddle  23  is close to the stacking surface  7   a  of the compile tray  6  as a result of downward movement, and the sheet S on the compile tray  6  is retracted into the end wall  8 . 
     A mechanism for moving up and down the clamp roller  21  and the sub-paddles  23  and a mechanism for driving the sub-paddles  23  are known in the art, and may have any of various known configurations described in, for example, Japanese Unexamined Patent Application Publications No. 2006-69727, No. 2006-69746, and No. 2006-69749, the detailed description of which is omitted. While in the first exemplary embodiment, the paddle drive motor MA 6  that is a drive source for the main paddles  11  is also used as a drive source for the sub-paddles  23 , an independent drive source for the sub-paddles  23  may be provided. 
     Stacker Tray TH 1  in First Exemplary Embodiment 
     In  FIGS. 1 and 3  to  5 , the stacker tray TH 1  onto which the sheets S stacked on the compile tray  6  are output is supported by a left side wall U 3   b  of the post-processing device U 3 . The stacker tray TH 1  may be an example of a second stacking unit. The stacker tray TH 1  has a tray guide  26  extending in the up-down direction along the left side wall U 3   b  of the post-processing device U 3 . The tray guide  26  may be an example of an upward and downward movement guide unit. The tray guide  26  has a slider  27  supported thereon so as to be capable of moving up and down along the tray guide  26 . The slider  27  may be an example of an exit movement unit. A stacker tray body  28 , which may be an example of a body of the second stacking unit, is fixedly supported by the slider  27 . 
     The stacker tray TH 1  is configured to move down in accordance with the height of the top surface of the stack of sheets S on the upper surface of the stacker tray body  28 . A mechanism for moving up and down the stacker tray TH 1  is known in the art, and may have any of various configurations, such as moving up and down mechanisms described in, for example, Japanese Unexamined Patent Application Publications No. 7-300270 and No. 2003-089463, the detailed description of which is omitted. 
     Details of Stapler  13  in First Exemplary Embodiment 
       FIG. 6  illustrates a substantial part of the rear end of the compile tray  6  according to the first exemplary embodiment. 
     In  FIGS. 5 and 6 , a stapler support member  61 , which may be an example of a support member of a binding device, is supported downward and to the right of the end wall  8  according to the first exemplary embodiment. The stapler support member  61  according to the first exemplary embodiment extends along the end wall  8  in the front-rear direction, which is the width direction of a sheet S, and is formed in a plate shape that is inclined so that the right side is lower than the left side, like the compile tray body  7 . 
     The stapler support member  61  has a stapler guide  62  formed thereon so as to project upward therefrom. The stapler guide  62  extends in the front-rear direction and curves inward in the front-rear direction so as to form arcs at both front and rear ends of the stapler guide  62 . The stapler guide  62  may be an example of a guide member of the binding device. The stapler guide  62  has a stapler guide groove  62   a  formed in a center portion thereof in the left-right direction so as to extend along the stapler guide  62  and extend through the stapler guide  62  in the up-down direction. The stapler guide groove  62   a  may be an example of a body of the guide member of the binding device. Rack teeth  62   b , which may be examples of flat-plate-shaped gear teeth, are formed on the right inner surface of the stapler guide groove  62   a.    
     In  FIGS. 5 and 6 , the stapler support member  61  has plate-shaped light-shielding ribs  63  disposed to the right of the stapler guide  62 . The light-shielding ribs  63  extend upward, and may be examples of detected units. In  FIG. 6 , the light-shielding ribs  63  according to the first exemplary embodiment are disposed in accordance with positions at which the stapler  13  is to stop, and are located at four positions at which the stapler  13  according to the first exemplary embodiment is to bind a bundle of sheets S, that is, at the front edge corner, the front center, the rear center, and the rear edge corner. That is, the stapler  13  according to the first exemplary embodiment may have capabilities of “front edge corner binding” for binding sheets S at the front edge corner, “side edge binding” for binding sheets S at the front center and rear center, and “rear edge corner binding” for binding sheets S at the rear edge corner. 
     As illustrated in  FIG. 6 , binding cutout portions  6   a ,  6   b , and  6   c  are formed in the front edge, center, and rear of the right edge of the compile tray body  7  and the end wall  8  so as to correspond to positions where the stapler  13  is to perform binding processing, that is, stapling processing. 
     Movable Stapling Unit  66  in First Exemplary Embodiment 
     In  FIGS. 5 and 6 , a movable stapling unit  66 , which may be an example of a movable binding device, is supported by the stapler support member  61 . In  FIG. 5 , the movable stapling unit  66  according to the first exemplary embodiment has a plate-shaped carriage  67  as an example of a moving body. The carriage  67  is disposed above the stapler guide  62  so as to straddle the stapler guide  62 . The carriage  67  has roller support units  68  and  69  formed at both right and left ends thereof, respectively. The roller support units  68  and  69  extend downward, and may be examples of a guided member support unit. A drive coupling unit  68   a  extending leftward is formed on the lower end of the left roller support unit  68 . 
     Rollers  71 , which may be examples of a guided member, are rotatably supported by the roller support units  68  and  69 . The rollers  71  come into contact with the upper surface of the stapler support member  61 . In  FIG. 6 , one roller  71  according to the first exemplary embodiment is supported by the left roller support unit  68 , and a pair of rollers  71  are supported by the right roller support unit  69  at an interval in the front-rear direction. 
     In  FIG. 5 , the upper end of a shaft  72  extending downward so as to be received in the stapler guide groove  62   a  is rotatably supported by the carriage  67 . The shaft  72  may be an example of a drive shaft. A stapler moving gear  73  whose teeth mesh with the rack teeth  62   b  is supported by the shaft  72 . The stapler moving gear  73  may be an example of a drive member of the binding device. 
     Drive is transmitted to the lower end of the shaft  72  from a stapler moving motor  74 . The stapler moving motor  74  may be an example of a binding drive source. 
     The stapler moving motor  74  is supported by a plate-shaped motor support plate  76 , which may be an example of a drive source support member, and the motor support plate  76  is supported by the drive coupling unit  68   a  through a coupling shaft  77  supported by the left end of the motor support plate  76 . The coupling shaft  77  may be an example of a coupling member. Therefore, the stapler moving motor  74  is supported so as to be movable integrally with the carriage  67  through the motor support plate  76  and the coupling shaft  77 . When the stapler moving motor  74  is driven to rotate in the forward and reverse directions, the stapler moving gear  73  whose teeth mesh with the rack teeth  62   b  rotates in the forward and reverse directions, and the carriage  67  moves along the stapler guide groove  62   a.    
     In  FIG. 5 , an optical sensor  78 , which may be an example of a detection member, is supported by the lower surface of the carriage  67  so as to correspond to the positions of the light-shielding ribs  63 . The optical sensor  78  according to the first exemplary embodiment includes a light emitting unit  78   a  that outputs light, and a light receiving unit  78   b  that receives light such that the light emitting unit  78   a  and the light receiving unit  78   b  face each other and such that the light-shielding ribs  63  are allowed to enter between the light emitting unit  78   a  and the light receiving unit  78   b . In accordance with the movement of the carriage  67 , one of the light-shielding ribs  63  enters between the light emitting unit  78   a  and the light receiving unit  78   b  and light is blocked. At this time, the movement of the movable stapling unit  66  to a binding position is detectable. 
     A stapler motor unit  81 , which may be an example of a binding operation device, is supported by the upper surface of the carriage  67 , and the stapler  13  is supported by the upper surface of the stapler motor unit  81 . 
     The stapler  13  according to the first exemplary embodiment includes, a needle shooting unit  82   a  that shoots staples, which may be examples of binding needles, and a needle bending unit  82   b  disposed opposite the needle shooting unit  82   a . The needle bending unit  82   b  bends a staple shot from the needle shooting unit  82   a  and inserted through a bundle of sheets S at a leading end of the staple. The needle shooting unit  82   a  is supported so as to be rotatable about a center of rotation  82   c  with respect to the needle bending unit  82   b.    
     A stapler operating member  83 , which may be an example of a binding operation member, is supported between the needle shooting unit  82   a  and the needle bending unit  82   b . The stapler operating member  83  has an end  83   a  coupled to the needle shooting unit  82   a , and another end on which an annular operated unit  83   b  is formed. 
     An eccentric cam  84 , which may be an example of an eccentric member, is rotatably supported by the operated unit  83   b . The eccentric cam  84  has a rotating shaft  84   a  on which a drive receiving gear  86  (not illustrated) is supported as an example of a gear, and drive is transmitted to the drive receiving gear  86  from an output gear  88  supported by an output shaft  81   a  of the stapler motor unit  81  through an intermediate gear  87 . The intermediate gear  87  may be an example of an intermediate gear, and the output gear  88  may be an example of an output gear. 
     When the stapler motor unit  81  operates, the eccentric cam  84  rotates through the gears  86  to  88  and the end  83   a  of the stapler operating member  83  moves in the up-down direction. Therefore, the needle shooting unit  82   a  is brought into proximity to the needle bending unit  82   b  to hold the bundle of sheets S between the needle shooting unit  82   a  and the needle bending unit  82   b , and a staple or staples are shot to bind the bundle of sheets S. 
     The stapler  13 , the members  67  to  88 , etc., constitute the movable stapling unit  66  according to the first exemplary embodiment. 
     In the movable stapling unit  66  according to the first exemplary embodiment, the stapler  13 , the stapler motor unit  81 , etc. are disposed above the carriage  67  disposed upward of the stapler support member  61 , and the center of gravity of the overall movable stapling unit  66  is higher than the stapler support member  61  in the direction of gravity. 
     Details of Tamper  12  in First Exemplary Embodiment 
       FIG. 7  is a cross-sectional view taken along line VII-VII in  FIG. 6 . 
       FIGS. 8A and 8B  illustrate the tampers  12  according to the first exemplary embodiment.  FIG. 8A  is a diagram of the tampers  12  when viewed from the top, and  FIG. 8B  is a diagram of the tampers  12  when viewed from the bottom. 
     In  FIGS. 6 and 7 , each of the tampers  12  according to the first exemplary embodiment is supported so as to be movable along a tamper guide groove  91  formed in the compile tray body  7  so as to extend in the front-rear direction. The tamper guide grooves  91  may be examples of a guide unit of an alignment member. In  FIGS. 6 ,  7 ,  8 A, and  8 B, each of the tampers  12  according to the first exemplary embodiment has a plate-shaped bottom board portion  92  extending along the stacking surface  7   a  of the compile tray body  7 . A plate-shaped tamper body  93  extending upward, which may be an example of a body of the alignment member, is formed on the outer edges of the bottom board portion  92  in the front-rear direction. 
     A guided rod  94  is supported by the bottom portion of the bottom board portion  92  as an example of a guided member of the alignment member. The guided rod  94  is formed in a plate shape extending in the front-rear direction, and is received in the tamper guide groove  91 . In  FIG. 8B , a pair of roller-shaped guided rollers  96  are formed at both ends of the guided rod  94  in the front-rear direction. The roller-shaped guided rollers  96  may be examples of guided units, and are rotatably supported in contact with the inner surface of the tamper guide groove  91 . Tamper rack teeth  97 , which may be an example of a drive receiving unit, are formed on a side surface of the guided rod  94  opposite to the surface on which the guided rollers  96  are formed, so as to extend along the side surface of the guided rod  94 . 
     In  FIGS. 6 and 7 , a pair of front and rear tamper drive motors  98 , which may be examples of a drive source of the alignment member, are disposed on a lower surface of the compile tray body  7  in a center portion thereof in the front-rear direction so as to correspond to the respective tampers  12 . Similarly to the stacker exit motor MA 2 , the tamper drive motors  98  according to the first exemplary embodiment may be formed of stepping motors, and are configured to be rotatable in the forward and reverse directions. 
     Each of the tamper drive motors  98  has a rotating shaft  98   a  on which a tamper drive gear  99  whose teeth mesh with the tamper rack teeth  97  is supported as an example of a drive transmitting member. The forward and reverse rotation of the tamper drive motors  98  allows the tampers  12  to move in the sheet width direction through the tamper drive gears  99  and the tamper rack teeth  97  and to come into contact with the edges in the width direction of the sheets S at which the tamper bodies  93  are mounted. Then, alignment is performed. 
     The members  7  and  93  to  99  constitute a tamper drive transmission system ( 7 + 93  to  99 ) according to the first exemplary embodiment. 
     Drive Transmission Systems  101  to  113  in First Exemplary Embodiment 
       FIGS. 9A and 9B  illustrate a drive transmission system according to the first exemplary embodiment.  FIG. 9A  illustrates a substantial part of the drive transmission system when the post-processing device U 3  is viewed from rear to front, and  FIG. 9B  illustrates a substantial part of the stacker exit motor MA 2 , a gear, and a timing belt according to the first exemplary embodiment. 
     In  FIG. 9A , the post-processing device U 3  according to the first exemplary embodiment has a rear frame  101  that rotatably supports the rear end of the rotating shaft  16   a  of the stacker exit roller  16 . The rear frame  101  may be an example of a support member. A first driven timing pulley  102  is fixedly supported by the rear end of the rotating shaft  16   a . The first driven timing pulley  102  may be an example of a first gear member and also an example of a first driven member. Further, a second driven timing pulley  103  extending rearward from and rotatably supported by the rear frame  101  is disposed at a position diagonally downward and to the right, or, in  FIG. 9A , diagonally downward and to the left, of the first driven timing pulley  102  at a position that is not related to the compile tray  6  or the movable stapling unit  66 . The second driven timing pulley  103  may be an example of a second gear member and also an example of a second driven member. 
     Further, a third driven pulley  104  and a fourth driven pulley  106 , which extend rearward from and are rotatably supported by the rear frame  101 , are disposed at a position diagonally downward and to the left, or, in  FIG. 9A , diagonally downward and to the right, of the second driven timing pulley  103  and disposed at a position diagonally downward and to the right of the first driven timing pulley  102 , respectively. The third driven pulley  104  and the fourth driven pulley  106  may be an example of third and fourth driven members, respectively. Further, the stacker exit motor MA 2  is disposed downward from the pulleys  102 ,  103 ,  104 , and  106 . 
     In  FIG. 9B , the stacker exit motor MA 2  has a motor body  107 , and a shaft  108  extending rearward from and rotatably supported by the motor body  107 . The motor body  107  may be an example of a drive source body, and the shaft  108  may be an example of a rotating shaft. A pinion gear  109 , which may be an example of a gear, is fixedly supported by the rear end of the shaft  108 . The number of teeth g 1  of the pinion gear  109  according to the first exemplary embodiment is prime, for example, 23, and the 23 teeth are arranged at intervals of about 15.7°. 
     Further, a motor bracket  111 , which may be an example of an attachment member, is supported by the front surface of the motor body  107 , and the rear end of the motor bracket  111  is supported by the rear frame  101  through a vibration absorbing member  112  composed of urethane. The vibration absorbing member  112  may be an example of an elastic member. 
     A timing belt  113 , which may be an example of a meshing member, is stretched across the pulleys  102 ,  103 ,  104 , and  106  and the pinion gear  109 . The timing belt  113  according to the first exemplary embodiment has inner teeth (not illustrated) that mesh with the timing pulleys  102  and  103  and the teeth of the pinion gear  109 , and is stretched while the outer surface of the timing belt  113  is in contact with the outer peripheral surfaces of the pulleys  104  and  106 . Therefore, the wrap angle of the timing belt  113  around the timing pulleys  102  and  103  and the pinion gear  109  is larger than that obtained in a configuration in which the pulleys  104  and  106  are not provided, and the range in which the teeth mesh with each other is larger. This facilitates stable transmission of drive caused by the driving of the rotation of the pinion gear  109 . 
     The members  101  to  113  constitute drive transmission systems  101  to  113  according to the first exemplary embodiment. 
     Details of Stacker Exit Motor MA 2  in First Exemplary Embodiment 
       FIGS. 10A to 10D  illustrate a stacker exit motor according to the first exemplary embodiment.  FIG. 10A  is a cross-sectional view of the motor body  107 ,  FIG. 10B  is an enlarged perspective view of the teeth of a rotor,  FIG. 10C  is a cross-sectional view taken along line XC-XC in  FIG. 10A , and  FIG. 10D  illustrates the substantial part of a stator unit in which coils and a power supply are removed from the configuration illustrated in  FIG. 10C . 
     Since the stacker exit motor MA 2  and the tamper drive motors  98  included in the post-processing device U 3  have similar stepping motor configurations, only the stacker exit motor MA 2  will be described. 
     In  FIGS. 10A to 10D , the stacker exit motor MA 2  according to the first exemplary embodiment may be formed of a two-phase hybrid (HB) stepping motor, which may be an example of a drive source that performs driving in accordance with the input of a pulse signal. The motor body  107  includes a rotor unit  121 , a stator unit  122 , and a housing  123 . The rotor unit  121  may be an example of a rotor disposed on the front end of the shaft  108 , and the stator unit  122  may be an example of a stator that surrounds the outer periphery of the rotor unit  121 . The housing  123  may be an example of a frame structure that fixedly supports the stator unit  122  and that rotatably supports the rotor unit  121 . 
     The rotor unit  121  according to the first exemplary embodiment includes a cylindrical permanent magnet  131 , which may be an example of a magnet (hereinafter referred to as the “magnet  131 ”). The magnet  131  is supported by the outer peripheral surface of the shaft  108  and extends in the front-rear direction. As illustrated in  FIG. 10A , the magnet  131  according to the first exemplary embodiment is disposed so that the N pole is directed rearward and the S pole is directed forward. A tubular first rotor  132  that surrounds the rear N-pole portion of the magnet  131  and that is magnetized to the N pole, and a tubular second rotor  133  that surrounds the front S-pole portion of the magnet  131  and that is magnetized to the S pole are supported by the magnet  131 . The first rotor  132  may be an example of a first rotor, and the second rotor  133  may be an example of a second rotor. The first rotor  132  according to the first exemplary embodiment has teeth  132   a  formed on the outer peripheral surface thereof, and the second rotor  133  according to the first exemplary embodiment has teeth  133   a  formed on the outer peripheral surface thereof. In the first exemplary embodiment, as illustrated in  FIG. 10B , the first rotor  132  and the second rotor  133  are arranged such that the teeth  132   a  of the first rotor  132  and the teeth  132   a  of the second rotor  133  are shifted by a ½ pitch with respect to each other, where one pitch represents a center interval between adjacent teeth  132   a  of the first rotor  132  and represents a center interval between adjacent center teeth  133   a  of the second rotor  133 . In the first exemplary embodiment, the first rotor  132  has 50 teeth  132   a  formed at intervals of 7.2° and the second rotor  133  has 50 teeth  133   a  formed at intervals of 7.2°. 
     The stator unit  122  according to the first exemplary embodiment includes eight electromagnets  141 ,  142 ,  143 ,  144 ,  145 ,  146 ,  147 , and  148  arranged radially about the shaft  108  at intervals of 45°. In  FIG. 10D , the electromagnets  141  to  148  have cores  141   a  to  148   a , respectively, and each of the cores  141   a  to  148   a  has a proximal end supported by the housing  123  and a free end extending radially toward the rotor unit  121 . The free ends of the cores  141   a  to  148   a  according to the first exemplary embodiment have facing walls  141   b  to  148   b , respectively, which face the outer peripheral surfaces of the rotors  132  and  133  and that extend in the circumferential direction of the rotors  132  and  133 . The facing walls  141   b  to  148   b  have teeth  141   c  to  148   c , respectively, which are arranged spaced apart from the teeth  132   a  and  133   a  of the rotors  132  and  133 . In the first exemplary embodiment, the facing walls  141   b  to  148   b  each have five teeth  141   c  to  148   c  formed at intervals of 7.2°. 
     In  FIG. 10C , an A +  phase lead  151 , which may be an example of a positive lead having a first phase, and an A −  phase lead  152 , which may be an example of a negative lead having the first phase, are wound around the first, third, fifth, and seventh cores  141   a ,  143   a ,  145   a , and  147   a . That is, the first, third, fifth, and seventh electromagnets  141 ,  143 ,  145 , and  147  have A +  phase coils  141   d ,  143   d ,  145   d , and  147   d , which may be examples of a positive winding having the first phase, and A −  phase coils  141   e ,  143   e ,  145   e , and  147   e , which may be examples of a negative winding having the first phase, respectively. In the first exemplary embodiment, the A +  phase coils  141   d ,  143   d ,  145   d , and  147   d  are connected to one another using the A +  phase lead  151 , and the A −  phase coils  141   e ,  143   e ,  145   e , and  147   e  are connected to one another using the A −  phase lead  152 . 
     In the first and fifth electromagnets  141  and  145  according to the first exemplary embodiment, the coils  141   d + 141   e  and  145   d + 145   e  are wound around the cores  141   a  and  145   a , respectively, in a predetermined first winding direction. In the third and seventh electromagnets  143  and  147 , the coils  143   d + 143   e  and  147   d + 147   e  are wound around the cores  143   a  and  147   a , respectively, in a second winding direction opposite to the first winding direction. 
     In the first exemplary embodiment, furthermore, the A +  phase lead  151  is wound around the first, third, fifth, and seventh cores  141   a ,  143   a ,  145   a , and  147   a  in this order by a predetermined number of turns N 1 , and the A −  phase lead  152  is wound around the third, fifth, seventh, and first cores  143   a ,  145   a ,  147   a , and  141   a  in this order by the same number of turns as the number of turns N 1  for the A +  phase lead  151 . 
     The leads  151  and  152  are configured to be connectable to a first power supply  154  via a first switch  153 , which may be an example of a first switching member. In the first exemplary embodiment, an end  151   a  of the A +  phase lead  151  on the first electromagnet  141  side and an end  152   a  of the A −  phase lead  152  on the third electromagnet  143  side, which may be examples of a first connecting portion, are connected to the positive (+) side of the first power supply  154 . An end  151   b  of the A +  phase lead  151  on the seventh electromagnet  147  side and an end  152   b  of the A −  phase lead  152  on the first electromagnet  141  side, which may be examples of a second connecting portion, are configured to be connectable to the negative (−) side of the first power supply  154  through the first switch  153 . 
     The first switch  153  according to the first exemplary embodiment is configured to be movable between a first position to be connected to the A +  phase lead  151 , a second position to be connected to the A −  phase lead  152 , and a third position where the first switch  153  disconnects the connection to the leads  151  and  152 . In the first exemplary embodiment, therefore, the first switch  153  may be controlled to enable one of the leads  151  and  152  to be energized or none of the leads  151  and  152  to be energized. 
     In the first exemplary embodiment, in the electromagnets  141 ,  143 ,  145 , and  147 , the direction of a current flowing through the A −  phase lead  152  when the first switch  153  is closed (connection is made) is opposite to the direction of a current flowing through the A +  phase lead  151  when the first switch  153  is closed (connection is made) because the directions of turns in the electromagnets  141 ,  143 ,  145 , and  147  are opposite. Therefore, the magnetic poles to which the teeth  141   c  to  148   c  are excited by the A −  phase lead  152  are opposite to the magnetic poles to which the teeth  141   c  to  148   c  are excited by the A +  phase lead  151 . 
     In the first exemplary embodiment, when the A +  phase lead  151  is energized, the teeth  141   c  of the first electromagnet  141  and the teeth  145   c  of the fifth electromagnet  145  are excited to the N pole, and the teeth  143   c  of the third electromagnet  143  and the teeth  147   c  of the seventh electromagnet  147  are excited to the S pole. When the A −  phase lead  152  is energized, the teeth  141   c  of the first electromagnet  141  and the teeth  145   c  of the fifth electromagnet  145  are excited to the S pole, and the teeth  143   c  of the third electromagnet  143  of the seventh electromagnet  147  are excited to the N pole. 
     The second, fourth, sixth, and eighth electromagnets  142 ,  144 ,  146 , and  148  have B +  phase coils  142   d ,  144   d ,  146   d , and  148   d , which may be examples of a positive winding having a second phase, and B −  phase coils  142   e ,  144   e ,  146   e , and  148   e , which may be examples of a negative winding having the second phase, respectively, in a manner similar to the first, third, fifth, and seventh electromagnets  141 ,  143 ,  145 , and  147 . In the first exemplary embodiment, a B +  phase lead  161  forming the B +  phase coils  142   d ,  144   d ,  146   d , and  148   d , which may be an example of a positive lead having the second phase, is wound around the sixth, eighth, second, and fourth cores  146   a ,  148   a ,  142   a , and  144   a  in this order by the same number of turns as the number of turns N 1  for the A +  phase lead  151  and the A −  phase lead  152 . Further, a B −  phase lead  162  forming the B −  phase coils  142   e ,  144   e ,  146   e , and  148   e , which may be an example of a negative lead having the second phase, is wound around the fourth, second, eighth, and sixth cores  144   a ,  142   a ,  148   a , and  146   a  in this order by the same number of turns as the number of turns N 1  for the B +  phase lead  161 . 
     The leads  161  and  162  are configured to be connectable to a second power supply  164  via a second switch  163 , which may be an example of a second switching member. In the first exemplary embodiment, an end  161   a  of the B +  phase lead  161  on the sixth electromagnet  146  side and an end  162   a  of the B −  phase lead  162  on the fourth electromagnet  144  side, which may be examples of a first connecting portion, are connected to the positive (+) side of the second power supply  164 . An end  161   b  of the B +  phase lead  161  on the fourth electromagnet  144  side and an other end  162   b  of the B −  phase lead  162  on the sixth electromagnet  146  side, which may be examples of a second connecting portion, are configured to be connectable to the negative (−) side of the second power supply  164  through the second switch  163 . 
     Further, the second switch  163  according to the first exemplary embodiment is configured in a manner similar to the first switch  153 , and is movable between the first, second, and third positions to enable one of the leads  161  and  162  to be energized or none of the leads  161  and  162  to be energized. 
     In the first exemplary embodiment, therefore, when the B +  phase lead  161  is energized, the teeth  142   c  of the second electromagnet  142  and the teeth  146   c  of the sixth electromagnet  146  are excited to the N pole, and the teeth  144   c  of the fourth electromagnet  144  and the teeth  148   c  of the eighth electromagnet  148  are excited to the S pole. When the B −  phase lead  162  is energized, the teeth  142   c  of the second electromagnet  142  and the teeth  146   c  of the sixth electromagnet  146  are excited to the S pole, and the teeth  144   c  of the fourth electromagnet  144  and the teeth  148   c  of the eighth electromagnet  148  are excited to the N pole. 
     In addition, the housing  123  according to the first exemplary embodiment has a stator support unit  171  that supports the stator unit  122  while surrounding the electromagnets  141  to  148 , and ball bearings  172  that rotatably support the shaft  108 , which may be examples of bearings, are supported by both front and rear ends of the housing  123 . 
       FIGS. 11A to 11C  illustrate relationships between rotor teeth and stator teeth when the right direction is the rotation direction.  FIG. 11A  illustrates a relationship between the rotor teeth and the stator teeth when only the A +  phase coils are energized,  FIG. 11B  illustrates a relationship between the rotor teeth and the stator teeth when the energization of the A +  phase coils is disconnected after the state illustrated in  FIG. 11A  and the B +  phase coils are energized, and  FIG. 11C  illustrates a relationship between the rotor teeth and the stator teeth when the B +  phase coils are energized after the state illustrated in  FIG. 11A . 
     Here, the facing walls  141   b  to  148   b  of the electromagnets  141  to  148  according to the first exemplary embodiment are configured such that the angle defined between adjacent facing walls is given by 45−(7.2×5)=9.0°. 
     For instance, if the right direction in  FIGS. 11A  to  11 C is the direction of rotation of the shaft  108 , the angle defined between a tooth  181  at the downstream end of the first teeth  141   c  in the rotation direction and a tooth  182  at the upstream end of the second teeth  142   c  in the rotation direction is 9.0°. 
     Therefore, the electromagnets  141  to  148  are arranged such that the teeth  142   c  to  148   c  and  141   c  of the downstream electromagnets  142  to  148  and  141  among the adjacent electromagnets  141  to  148  are shifted from the teeth  132   a  and  133   a  of the rotors  132  and  133  by 9.0−7.2=1.8°, or a ¼ pitch, with respect to the teeth  141   c  to  148   c  of the upstream adjacent electromagnets  141  to  148 . Therefore, for example, the third electromagnet  143  is arranged such that the teeth  143   c  of the third electromagnet  143  are shifted downstream from the teeth  132   a  and  133   a  of the rotors  132  and  133  by 1.8×2=3.6°, or a ½ pitch, with respect to the teeth  141   c  of the first electromagnet  141  that is disposed two electromagnets upstream from the third electromagnet  143 . 
     The electromagnets  141  to  148  according to the first exemplary embodiment are configured such that the coils ( 141   d + 141   e ) to ( 148   d + 148   e ) are wound around the cores  141   a  to  148   a , respectively, by the same number of coil turns, and the N pole or the S pole having the same magnetic force is generated when the leads  151 ,  152 ,  161 , and  162  are energized. 
     As a result, when the A +  phase lead  151  is energized, the S pole teeth  133   a  of the second rotor  133  are attracted by a magnetic force towards the first and fifth teeth  141   c  and  145   c  which are excited to the N pole, and are made to face the first and fifth teeth  141   c  and  145   c . At this time, the N pole teeth  132   a  of the first rotor  132  are attracted by a magnetic force towards the third and seventh teeth  143   c  and  147   c  which are excited to the S pole. Therefore, the teeth  132   a  and  133   a  of the rotors  132  and  133  become stable in the state illustrated in  FIG. 11A  where the teeth  132   a  and  133   a  face the teeth ( 143   c + 147   c ) and ( 141   c + 145   c ) which are excited to a magnetic pole. In this case, as illustrated in  FIG. 11A , the second rotor  133  is arranged such that the S pole teeth  133   a  of the second rotor  133  are shifted a ¼ pitch upstream from and a ¾ pitch downstream from the second and sixth teeth  142   c  and  146   c  having no magnetic pole. In addition, the first rotor  132  is arranged such that the N pole teeth  132   a  of the first rotor  132  are shifted a ¼ pitch upstream from and a ¾ pitch downstream from the fourth and eighth teeth  144   c  and  148   c  having no magnetic pole. 
     When the energization of the A +  phase lead  151  is disconnected after the state illustrated in  FIG. 11A  and the B +  phase lead  161  is energized, the second rotor  133  is arranged such that S pole teeth  133   a  of the second rotor  133  on the upstream side are closer to the second and sixth teeth  142   c  and  146   c  which are excited to the N pole than S pole teeth  133   a  of the second rotor  133  on the downstream side by a ½ pitch. Therefore, the S pole teeth  133   a  on the downstream side are attracted towards the N pole teeth  142   c  and  146   c  on the upstream side by a magnetic force without the S pole teeth  133   a  on the upstream side being attracted towards the N pole teeth  142   c  and  146   c  on the downstream side, and are made to face the N pole teeth  142   c  and  146   c  on the upstream side. Further, the first rotor  132  is arranged such that, similarly to the S pole teeth  133   a , N pole teeth  132   a  of the first rotor  132  on the upstream side are closer to the fourth and eighth teeth  144   c  and  148   c  which are excited to the S pole than N pole teeth  132   a  of the first rotor  132  on the downstream side by a ½ pitch. Therefore, the N pole teeth  132   a  on the downstream side are attracted towards the S pole teeth  144   c  and  148   c  on the upstream side by a magnetic force, and are made to face the S pole teeth  144   c  and  148   c . As a result, the rotors  132  and  133  become stable, without reversely rotating, in the state illustrated in  FIG. 11B  where the rotors  132  and  133  move downstream in the rotation direction by a ¼ pitch. 
     When the B +  phase lead  161  is energized without the energization of the A +  phase lead  151  being disconnected after the state illustrated in  FIG. 11A , the S pole teeth  133   a  of the second rotor  133  are also attracted by the same magnetic force as the N pole teeth  141   c  and  145   c  towards the second and sixth teeth  142   c  and  146   c  which are newly excited to the N pole. Therefore, the magnetic force of the N pole teeth  142   c  and  146   c  attracts the S pole teeth  133   a  to intermediate positions between the positions at which the S pole teeth  133   a  are shifted upstream from the N pole teeth  142   c  and  146   c  by a ¼ pitch and the positions at which the S pole teeth  133   a  face the N pole teeth  142   c  and  146   c.    
     Further, similarly to the S pole teeth  133   a , the N pole teeth  132   a  of the first rotor  132  are also attracted by the same magnetic force as the S pole teeth  143   c  and  147   c  towards the fourth and eighth teeth  144   c  and  148   c  which are newly excited to the S pole. Therefore, the magnetic force of the S pole teeth  144   c  and  148   c  attracts the N pole teeth  132   a  to intermediate positions between the positions at which the N pole teeth  132   a  are shifted upstream from the S pole teeth  144   c  and  148   c  by a ¼ pitch and the positions at which the N pole teeth  132   a  face the S pole teeth  144   c  and  148   c.    
     Consequently, the rotors  132  and  133  rotate and move only half the rotation and movement in the state illustrated in  FIG. 11B , and become stable in the state illustrated in  FIG. 11C  where the rotors  132  and  133  are moved downstream in the rotation direction by a ⅛ pitch. 
     When the energization of the A +  phase lead  151  is disconnected after the state illustrated in  FIG. 11C  and only the B +  phase lead  161  is energized, the rotors  132  and  133  become stable, without reversely rotating, in the state illustrated in  FIG. 11B  where the rotors  132  and  133  are moved downstream in the rotation direction by a ⅛ pitch. 
     In addition, when the energization of the B +  phase lead  161  is disconnected after the state illustrated in  FIG. 11B  and only the A −  phase lead  152  is energized, similarly to when the state illustrated in  FIG. 11A  is changed to the state illustrated in  FIG. 11B , the rotors  132  and  133  become stable, without reversely rotating, in the state where the rotors  132  and  133  are moved downstream in the rotation direction by a ¼ pitch. Additionally, when the A −  phase lead  152  is energized without the energization of the B +  phase lead  161  being disconnected after the state illustrated in  FIG. 11B , similarly to when the state illustrated in  FIG. 11A  is changed to the state illustrated in  FIG. 11C , the rotors  132  and  133  become stable, without reversely rotating, in the state where the rotors  132  and  133  are moved downstream in the rotation direction by a ⅛ pitch. 
     When the energization of the A +  phase lead  151  is disconnected while the B +  phase lead  161  is being energized after the state illustrated in  FIG. 11C  and the A −  phase lead  152  is energized, the rotors  132  and  133  become stable, without reversely rotating, in the state where the rotors  132  and  133  are moved downstream in the rotation direction by a ¼ pitch. 
     In the first exemplary embodiment, therefore, in a one-phase excitation method in which the leads  151 ,  152 ,  161 , and  162  are periodically energized in the order of only the A +  phase lead  151 , only the B +  phase lead  161 , only the A −  phase lead  152 , and only the B −  phase lead  162  in accordance with a pulse signal, the shaft  108  rotates in the rotation direction by a ¼ pitch for each pulse. Also in a two-phase excitation method in which the leads  151 ,  152 ,  161 , and  162  are periodically energized in the order of a set of the A +  phase lead  151  and the B +  phase lead  161 , a set of the B +  phase lead  161  and the A −  phase lead  152 , and a set of the A −  phase lead  152  and the B −  phase lead  162 , the shaft  108  rotates in the rotation direction by a ¼ pitch for each pulse. 
     That is, in one-phase excitation or two-phase excitation, four steps of energization control are executed for each pulse, and the shaft  108  rotates by a ¼ pitch with the magnetic poles of the teeth  141   c  to  148   c  being changed by 45° in the rotation direction by one step. 
       FIG. 12  illustrates the turning on and off of energization to each lead for each step when the electromagnets  141  to  148  of the stacker exit motor MA 2  according to the first exemplary embodiment are excited using a one-two phase excitation method. 
       FIG. 13  illustrates changes in the states of the magnetic poles in the respective steps illustrated in  FIG. 12 . 
     In a one-two phase excitation method in which the leads  151 ,  152 ,  161 , and  162  are periodically energized in the order of only the A +  phase lead  151 , a set of the A +  phase lead  151  and the B +  phase lead  161 , only the B +  phase lead  161 , a set of the B +  phase lead  161  and the A −  phase lead  152 , only the A −  phase lead  152 , a set of the A −  phase lead  152  and the B −  phase lead  162 , only the B −  phase lead  162 , and a set of the B −  phase lead  162  and the A +  phase lead  151 , the shaft  108  rotates by a ⅛ pitch in the rotation direction for each pulse. 
     That is, as illustrated in  FIGS. 12 and 13 , the shaft  108  rotates by a ⅛ pitch while the number of magnetic poles of each type is alternately changed to two and four in eight steps ST 1  to ST 8  for the individual pulses and while each magnetic pole is shifted by 45° in the rotation direction by two steps. 
     In the first exemplary embodiment, a controller of the post-processing device U 3  is predetermined so as to control the driving of the stacker exit motor MA 2  using the one-two phase excitation method so that the shaft  108  rotates by a ⅛ pitch in the rotation direction. 
     In the first exemplary embodiment, therefore, the number of steps s 1  per cycle representing the number of steps required for a change in magnetic pole to complete one cycle is preset to 8, and the rotation angle θ 1  of the shaft  108  per step is preset to 0.9°. That is, a cycle angle θs, which is an angle obtained by multiplying the rotation angle θ 1  by the number of steps s 1  per cycle, is preset to θs=θ 1 ×s 1 =0.9×8=7.2°. 
     In the first exemplary embodiment, furthermore, the total number p 1  of pulses required for one rotation of the shaft  108  is preset to p 1 =360/θ 1 =360/0.9=400 [step/rotation], and the number of divisions d 1  obtained by dividing one rotation of the shaft  108  by the cycle angle θs is preset to d 1 =360/θs=360/7.2=50 [8 steps/rotation]. 
     In the first exemplary embodiment, furthermore, a drive frequency f 1 , which may be an example of a first frequency that represents the number of pulse signals input to the stacker exit motor MA 2  per unit time, is preset to 2424 pps. Therefore, the number of rotations r 1  per unit time that is a value obtained by dividing the drive frequency f 1  by the total number p 1  is preset to r 1 =f 1 /p 1 =2424/400=6.06 rotations/sec (Hz). 
     Further, if a meshing frequency f 2  of the pinion gear  109 , which may be an example of a second frequency, is a value obtained by multiplying the number of rotations r 1  per unit time by the number of teeth g 1  of the pinion gear  109 , the meshing frequency f 2  is preset to f 2 =r 1 ×g 1 =6.06×23=139.38≈139 Hz. In addition, if an excitation fundamental frequency f 3  of the stacker exit motor MA 2 , which may be an example of a third frequency, is a value obtained by dividing the drive frequency f 1  by the number of steps s 1  per cycle, the excitation fundamental frequency f 3  is preset to f 3 =f 1 /s 1 =2424/8=303 Hz. 
     In the first exemplary embodiment, therefore, the least common multiple f 23  of the meshing frequency f 2  and the excitation fundamental frequency f 3  is equal to f 23 =LCM(f 2 , f 3 )=f 2 ×f 3 ≈139×303=41978 Hz, and, as an example, a threshold value fs is preset to greatly exceed 4000 Hz, which is the threshold of hearing in the audible frequency range audible to the human ear. 
     In the first exemplary embodiment, furthermore, the natural frequencies fa, fb, and fc of the timing belt  113 , the motor bracket  111 , and the rear frame  101  are predetermined so that the least common multiples f 2   a , f 2   b , and f 2   c  of the natural frequencies fa, fb, and fc and the meshing frequency f 2 , respectively, or the least common multiples f 3   a , f 3   b , and f 3   c  of the natural frequencies fa, fb, and fc and the excitation fundamental frequency f 3 , respectively, exceed the threshold value fs. For example, if the natural frequencies fa, fb, and fc are set to fa=151 Hz, fb=401 Hz, and fc=503 Hz, respectively, which may be examples of a prime frequency having a value different from the frequencies f 2  and f 3 , it may be possible to set the least common multiples f 2   a  to f 2   c  and f 3   a  to f 3   c  to exceed the threshold value fs. 
     The tamper drive motors  98  and the tamper drive gears  99  according to the first exemplary embodiment may also be configured in a manner similar to the stacker exit motor MA 2  and the pinion gear  109 , and the following settings are preset: g 1 =23 teeth, s 1 =8 steps, θ 1 =0.9°, θs=7.2°, p 1 =400 [step/rotation], d 1 =50 [8 steps/rotation], f 1 =2424 pps, r 1 =6.06 rotations/sec, f 2 ≈139 Hz, f 3 =303 Hz, and f 23 =41978 Hz. 
     Similarly to the natural frequencies fa and fb, the natural frequencies of the guided rod  94  having the tamper rack teeth  97 , the tamper body  93 , the compile tray body  7 , and the brackets and support members of the tamper drive motors  98  are also preset to a divisor of the least common multiple f 23 . 
     Operation of First Exemplary Embodiment 
     In the printer U according to the first exemplary embodiment having the above configuration, the controller of the post-processing device U 3  controls the stacker exit motor MA 2 , which may be formed of a stepping motor, so that the stacker exit roller  16  is rotated in the forward and reverse directions through the drive transmission systems  101  to  113 . When the stacker exit roller  16  is rotated in the forward direction, the trailing ends of sheets S are caused to abut against the end wall  8  so that the sheets S are aligned with one another. When the stacker exit roller  16  is rotated in the reverse direction, the sheets S on the compile tray  6  are output onto the stacker tray TH 1 . The stacker exit motor MA 2  according to the first exemplary embodiment may be formed of, as with the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2000-310893 (Abstract, paragraphs [0023] to [0037], FIGS. 1 to 6), a two-phase HB stepping motor using the one-two phase excitation method, and noise generated from the stepping motor may be reduced. 
     As described in Japanese Unexamined Patent Application Publication No. 05-127441 (paragraphs [0011] to [0016], FIGS. 2 to 4), Japanese Unexamined Patent Application Publication No. 05-323684 (paragraphs [0002], [0029], and [0030], FIG. 4), Japanese Unexamined Patent Application Publication No. 2000-310893 (Abstract, paragraphs [0023] to [0037], FIGS. 1 to 6), etc., when the stepping motor is driven, vibration of the stepping motor resonates through the bracket, the frame, and the drive transmission systems depending on conditions such as the total number of pulses per second, that is, the drive frequency f 1  of the stepping motor, and the natural frequencies fa to fc of the bracket, the frame, and the drive transmission systems, and noise may be generated. The human ear is particularly sensitive to noise of high frequencies from 1 kHz to 4 kHz, and such noise may be perceived as noise that is uncomfortable for users. 
       FIG. 14  is a graph illustrating results obtained by the frequency analysis of noise generated by driving a stepping motor in a conventional printer, and noise levels are represented in frequencies, with noise level in decibels (dB) plotted on the y axis and frequency in hertz (Hz) plotted on the x axis. 
     In an example of the conventional printer, a two-phase HB stepping motor may have a drive frequency f 1  of 2230 Hz and may be driven using the one-two phase excitation method, and the pinion gear may have 25 teeth, which is most commonly used, as the number of teeth g 1 . In this case, the frequency analysis of noise generated from the printer shows that, as illustrated in  FIG. 14 , a noise level pn is especially as high as approximately 34 dB at a frequency of 1115 Hz, which may cause noise that is uncomfortable for users. 
     A peak frequency fn of 1115 Hz, which is a frequency at which the generated noise level pn exhibits a peak, and a drive frequency f 1  of 2230 Hz have a relationship of fn:f 1 =1:2, and it is considered that there is a close relationship between the peak frequency fn of noise and the drive frequency f 1 . 
     If the center of the rotating shaft of the stepping motor is eccentric from an actual center of rotation due to individual differences in manufacturing error, assembling error, or the like, a periodic oscillation occurs in accordance with the rotation of the rotating shaft, and the entire stepping motor may vibrate. 
     Vibration of the rotating shaft may be caused not only by eccentricity between the center of the bearing and the center of the rotating shaft but also by, for example, a change in the orientation and magnitude of the magnetic force which may be caused by a change in the number of magnetic poles based on the resonant frequency of a rotor, individual differences between cores or coils of electromagnets, and excitation pattern of one-two phase excitation. 
     Since the rotation of the stepping motor is basically based on small repetitions of operations of “starting” and “stopping”, the rotor may vibrate or pulsation of magnetic force may weaken the rigidity of the teeth of the stator and may cause the stator to vibrate. In this case, due to variation in the magnetic force or position in the respective excitation patterns, a vibration occurs in accordance with the period of the excitation patterns, and the waveform of the vibration of the entire stepping motor has a period corresponding to the time period required for one cycle of using the excitation patterns once. The frequency of a fundamental wave component of vibration based on the excitation patterns is considered to depend on a value obtained by dividing the drive frequency f 1  by the number of steps s 1  per cycle, and is defined herein as the excitation fundamental frequency f 3 . Thus, the excitation fundamental frequency f 3  of a two-phase HB stepping motor based on the one-two phase excitation method is given by f 3 =f 1 /s 1 =2230/8=278.75 Hz. 
     The vibration of the rotating shaft may also be caused when the teeth of the pinion gear supported by the rotating shaft mesh with the teeth of gears and the like of the drive transmission systems, due to variation of depth of mesh, time during which the teeth mesh with each other, etc., depending on individual differences in teeth shapes etc. In this case, the waveform of the vibration described above has a period corresponding to the time period during which the pinion gear rotates one turn, that is, the time period during which the rotating shaft rotates one turn. Therefore, the frequency of a fundamental wave component of vibration based on mesh patterns is considered to depend on a value obtained by multiplying the number of teeth g 1  of the pinion gear and the number of rotations r 1  of the rotating shaft per second, and is defined herein as the meshing frequency f 2 . Thus, the meshing frequency f 2  of a two-phase HB stepping motor based on the one-two phase excitation method is given by f 2 =g 1 ×r 1 =25×(2230/400)=25×5.575=139.375 Hz. 
     Accordingly, there is a relationship of fn:f 3 :f 2 =1115:278.75:139.375=8:2:1 between the noise peak frequency fn=1115 Hz, the excitation fundamental frequency f 3 =278.75 Hz, and the meshing frequency f 2 =139.375 Hz. That is, in a two-phase HB stepping motor based on the one-two phase excitation method, the relationship fn=4×f 3 =8×f 2  is established, and the frequency (4×f 3 ) of a fourth harmonic component of vibration having a frequency equal to the excitation fundamental frequency f 3  or the frequency (8×f 2 ) of an eighth harmonic component of vibration having a frequency equal to the meshing frequency f 2  matches the peak frequency fn of noise. 
     Consequently, in the conventional printer, the noise is considered to have a high noise level pn because superimposition of a fourth harmonic component of vibration having a frequency equal to the excitation fundamental frequency f 3  and an eighth harmonic component of vibration having a frequency equal to the meshing frequency f 2  resonates through the bracket, the gear, the timing belt, etc. That is, the peak frequency fn of the noise may be any of resonant frequencies fa′ to fc′ having values that are integer multiples α, β, and γ of the natural frequencies fa to fc of the bracket, etc., that is, fa′=α×fa [Hz], fb′=β×fb [Hz], and fc′=γ×fc [Hz]. 
     In contrast, the stacker exit motor MA 2  according to the first exemplary embodiment has a relationship of f 2 :f 3 =139.375:303≈139:303 between the meshing frequency f 2  and the excitation fundamental frequency f 3 . In addition, for the least common multiple f 23  of the meshing frequency f 2  and the excitation fundamental frequency f 3 , the relationship f 23 =f 2 ×f 3  is established, and the least common multiple f 23  is set to exceed the threshold value fs=4 [kHz], which may be perceived as uncomfortable noise. 
     In the first exemplary embodiment, therefore, even if the timing belt  113 , the motor bracket  111 , the rear frame  101 , etc., resonate in accordance with the resonance of the n-th harmonic component of vibration having a frequency equal to the excitation fundamental frequency f 3  and the m-th harmonic component of vibration having a frequency equal to the meshing frequency f 2 , where n and m are natural numbers, the relationship fn=n×f 3 =m×f 2  is established, where fn&gt;fs. The resonant frequencies fa′ to fc′, which may become the peak frequency fn, exceed the threshold value fs, and the noise level pn of the frequency band to which the human ear is less sensitive becomes high. 
     In the printer U according to the first exemplary embodiment, the peak frequency fn at which superimposition of harmonic components of vibration having frequencies equal to the frequencies f 2  and f 3  increases the noise level pn exceeds the threshold value fs. Therefore, it may be difficult for users to hear sound having the peak frequency fn. 
     As a result, the printer U according to the first exemplary embodiment may reduce noise that is uncomfortable for users, compared to the configuration in which the least common multiple f 23 , which becomes equal to the peak frequency fn, does not exceed the threshold value fs. 
     In addition, for example, even if 8×139.375=1115 is established and an eighth harmonic component of vibration having a frequency equal to the meshing frequency f 2  has a frequency equal to the resonant frequency of 1115 Hz of the bracket etc., an n-th harmonic component of vibration having a frequency equal to the excitation fundamental frequency f 3  does not have a frequency of 1115 Hz. Therefore, the printer U according to the first exemplary embodiment may prevent the motor bracket  111  etc., from resonating in accordance with resonance of harmonic components of vibration having frequencies equal to the frequencies f 2  and f 3 . As a result, the printer U according to the first exemplary embodiment may reduce an increase in the noise level of high frequencies to which the human ear is more sensitive, compared to the configuration in which the least common multiple f 23 , which becomes equal to the peak frequency fn, does not exceed the threshold value fs. 
     Experimental Example 
       FIG. 15  illustrates peak levels measured in an experimental example. 
     Following experiments are performed in order to determine whether or not it is possible to reduce noise of the stacker exit motor MA 2  when the least common multiple f 23 , which becomes equal to the peak frequency fn, exceeds the threshold value fs. 
     Experimental Conditions 
     In the experimental examples, a configuration in which an n-th harmonic component (n×f 2 ) of vibration having a frequency equal to the meshing frequency f 2  causes the bracket etc., to resonate at a frequency less than or equal to the threshold value fs is used to measure the noise levels pn (in dB) of the printer U in a case where the least common multiple f 23  exceeds the threshold value fs and in a case where the least common multiple f 23  is less than or equal to the threshold value fs. 
     Specifically, a noise level pn at each frequency is measured as illustrated in  FIG. 15  in a case where f 23 =f 2 ×f 3 &gt;fs is obtained by adjusting the number of teeth g 1  and the drive frequency f 1  and in a case where f 23 =f 3 =2×f 2 ≦fs is obtained by adjusting the number of teeth g 1  and the drive frequency f 1 , and a peak level pn 1  that is a local maximum among the noise levels pn obtained in a range from 1 kHz to 4 kHz both inclusive is detected. 
     Experimental Example 1 
     In Experimental Example 1, the drive frequency f 1  (in pps (Hz)) is adjusted so that the meshing frequency f 2  becomes equal to 139.375 Hz when the number of teeth g 1  of the pinion gear  109  is 27, 26, and 24 to 22, and the peak level pn 1  obtained when f 23 =f 2 ×f 3 &gt;fs is established is detected. 
     In Experimental Example 1-1, a peak level pn 1  is detected under the conditions of g 1 =27 teeth and f 1 =2065 pps. In this case, the relationships f 2 =139.3875 Hz, f 3 =258.125 Hz, and f 3 ≠2×f 2  are established, where f 23 &gt;fs. 
     In Experimental Example 1-2, a peak level pn 1  is detected under the conditions of g 1 =26 teeth and f 1 =2144 pps. In this case, the relationships f 2 =139.36 Hz, and f 3 =268 Hz, f 3 ≠2×f 2  are established, where f 23 &gt;fs. 
     In Experimental Example 1-3, a peak level pn 1  is detected under the conditions of g 1 =24 teeth and f 1 =2323 pps. In this case, the relationships f 2 =139.38 Hz, f 3 =290.375 Hz, and f 3 ≠2×f 2  are established, where f 23 &gt;fs. 
     In Experimental Example 1-4, a peak level pn 1  is detected under the conditions of g 1 =23 teeth and f 1 =2424 pps. In this case, the relationships f 2 =139.38 Hz, f 3 =303 Hz, and f 3 ≠2×f 2  are established, where f 23 &gt;fs. 
     In Experimental Example 1-5, a peak level pn 1  is detected under the conditions of g 1 =22 teeth and f 1 =2534 pps. In this case, the relationships f 2 =139.37 Hz, f 3 =316.75 Hz, and f 3 ≠2×f 2  are established, where f 23 &gt;fs. 
     Comparative Example 1 
     In Comparative Example 1, a peak level pn 1  is detected when the stacker exit motor MA 2  is driven at the drive frequencies f 1  given in Experimental Examples 1-1 to 1-5 under conditions where the number of teeth g 1  of the pinion gear  109  is 25 and the relationship f 3 =2×f 2 ≦fs is always established. 
     In Comparative Example 1-1 corresponding to Experimental Example 1-1, a peak level pn 1  is detected under the conditions of g 1 =25 teeth and f 1 =2065 pps. In this case, the relationships f 2 =129.0625 Hz and f 3 =2×f 2 ≦fs are established. 
     In Comparative Example 1-2 corresponding to Experimental Example 1-2, a peak level pn 1  is detected under the conditions of g 1 =25 teeth and f 1 =2144 pps. In this case, the relationships f 2 =134 Hz and f 3 =2×f 2 ≦fs are established. 
     In Comparative Example 1-3 corresponding to Experimental Example 1-3, a peak level pn 1  is detected under the conditions of g 1 =25 teeth and f 1 =2323 pps. In this case, the relationships f 2 =145.1875 Hz and f 3 =2×f 2 ≦fs are established. 
     In Comparative Example 1-4 corresponding to Experimental Example 1-4, a peak level pn 1  is detected under the conditions of g 1 =25 teeth and f 1 =2424 pps. In this case, the relationships f 2 =151.5 Hz and f 3 =2×f 2 ≦fs are established. 
     In Comparative Example 1-5 corresponding to Experimental Example 1-5, a peak level pn 1  is detected under the conditions of g 1 =25 teeth and f 1 =2534 pps. In this case, the relationships f 2 =158.375 Hz and f 3 =2×f 2 ≦fs are established. 
     Comparative Example 2 
     In Comparative Example 2, a peak level pn 1  is detected under the conditions of g 1 =25 teeth and f 1 =2230 pps. In this case, the relationships f 2 =139.375 Hz, f 3 =278.75 Hz, and f 23 =f 3 =2×f 2 ≦fs are established. 
     Experimental Results 
       FIG. 16  is a graph illustrating the operation of the first exemplary embodiment, and illustrates a relationship between peak levels obtained in Experimental Example 1 and Comparative Examples 1 and 2, with peak level in dB plotted on the y axis and drive frequency in pps (Hz) plotted on the x axis. 
     The results are as follows: As indicated by a solid line in  FIG. 16 , peak levels pn 1  detected in Experimental Example 1 are approximately 41 dB for Experimental Example 1-1, approximately 37 dB for Experimental Example 1-2, approximately 26 dB for Experimental Example 1-3, approximately 26 dB for Experimental Example 1-4, and approximately 30 dB for Experimental Example 1-5. Further, as indicated by a dotted line in  FIG. 16 , peak levels pn 1  detected in Comparative Example 1 are approximately 44 dB for Comparative Example 1-1, approximately 48 dB for Comparative Example 1-2, approximately 28 dB for Comparative Example 1-3, approximately 33 dB for Comparative Example 1-4, and approximately 34 dB for Comparative Example 1-5, and a peak level pn 1  detected in Comparative Example 2 is approximately 34 dB. 
     Therefore, it is found that the peak levels pn 1  obtained in Experimental Examples 1-1, 1-2, 1-3, 1-4, and 1-5 are reduced by approximately 3 dB, approximately 11 dB, approximately 2 dB, approximately 7 dB, and approximately 4 dB with respect to those obtained in Comparative Examples 1-1, 1-2, 1-3, 1-4, and 1-5, respectively. 
     Thus, it is found that Experimental Example 1 in which the least common multiple f 23  exceeds the threshold value fs exhibits a reduction of the peak levels pn 1  at the respective drive frequencies f 1 , compared to those in Comparative Example 1 in which the least common multiple f 23  is less than or equal to the threshold value fs. 
     Consequently, the printer U according to the first exemplary embodiment may reduce the peak level pn 1  of uncomfortable noise generated by the stacker exit motor MA 2 , compared to a configuration in which the least common multiple f 23  is less than or equal to the threshold value fs. 
     Here, an approximation function F(g 1 , f 1 ) indicated by a broken line in  FIG. 16  may be set for the peak levels pn 1  in Experimental Example 1. That is, if the meshing frequency f 2  corresponding to the number of teeth g 1  of the pinion gear  109  and the drive frequency f 1  has been predetermined, the approximation function F(g 1 , f 1 ) of a peak level pn 1  for which the relationship pn 1 =F(g 1 , f 1 ) is established may be set. The approximation function F(g 1 , f 1 ) may be considered to be the transfer function of vibration of the drive transmission systems that is set in accordance with relationships such as the relationships between the excitation fundamental frequency f 3  and the resonant frequencies fa′ to fc′ of the bracket etc. 
     In the printer U according to the first exemplary embodiment, therefore, if a meshing frequency f 2  has been predetermined, an approximation function F(g 1 , f 1 ) may be set on the basis of the results of the experiment, and the number of teeth g 1  of the pinion gear  109  that minimizes the peak level pn 1  may be set. 
     If, in printer design, an integer multiple of the number of teeth g 1  of the pinion gear  109  is equal to the total number p 1  of pulses [step/rotation] required for one rotation of the stepping motor, that is, if the total number p 1  is divisible by the number of teeth g 1 , the designer may easily control positioning of the pinion gear. 
     In commercially available stepping motors, the total number p 1  of pulses required for one rotation is generally a multiple of 5 in order to make it easy for the designer to calculate the number of pulses corresponding to the desired number of rotations. For example, in a standard two-phase stepping motor similar to the two-phase stepping motor according to the first exemplary embodiment, p 1 =400 [8 steps/rotation] for one-two phase excitation, and p 1 =200 [8 steps/rotation] for one-phase excitation or two-phase excitation. 
     For this reason, in many cases, the number of teeth g 1  of the pinion gear  109  mounted in the stepping motor is generally 10, 20, 25, or the like by which the total number p 1 , namely, 400 or 200, is divisible. 
     Thus, conventional printers, such as those disclosed in Japanese Unexamined Patent Application Publication No. 05-127441 (paragraphs [0011] to [0016], FIGS. 2 to 4), Japanese Unexamined Patent Application Publication No. 05-323684 (paragraphs [0002], [0029], and [0030], FIG. 4), and Japanese Unexamined Patent Application Publication No. 2000-310893 (Abstract, paragraphs [0023] to [0037], FIGS. 1 to 6), generally include, in combination, a two-phase stepping motor and a pinion gear having 25 teeth, which are the most widely distributed and commonly used among commercially available stepping motors and pinion gears. A pinion gear having 25 teeth may provide easier calculation of positioning than pinion gears having 21 to 24 teeth or pinion gears having 26 to 29 teeth. 
     In this case, in addition to the total number p 1  of pulses per rotation, the number of divisions d 1  per number of steps s 1  per cycle of excitation patterns is also divisible by the number of teeth g 1 . That is, for the number of divisions d 1 , the relationship d 1 =50 [8 steps/rotation] is established for one-two phase excitation, and the relationship d 1 =25 [8 steps/rotation] is established for one-phase excitation or two-phase excitation. Each of the number of teeth g 1  and the number of divisions d 1  is a multiple of 25, and the number of divisions d 1  is divisible by the number of teeth g 1 . 
     Here, the meshing frequency f 2  and the excitation fundamental frequency f 3  are represented by the following equations (1) and (2), respectively, using the respective values representing the number of teeth g 1  of the pinion gear  109 , the drive frequency f 1  of the stepping motor, the number of steps s 1  per cycle, and the number of divisions d 1 .
 
 f 2 =g 1 ×f 1/( s 1 ×d 1)  Equation (1)
 
 f 3 =f 1 /s 1  Equation (2)
 
     Therefore, f 3 /f 2  may be represented using the following equation (3).
 
 f 3 /f 2=( f 1 /s 1)/{ g 1 ×f 1/( s 1 ×d 1)}= d 1 /g 1  Equation (3)
 
     Therefore, if the number of divisions d 1  is divisible by the number of teeth g 1 , that is, if the number of teeth g 1  is a divisor of the number of divisions d 1 , as in Comparative Examples 1 and 2, the least common multiple f 23  becomes equal to the excitation fundamental frequency f 3 . In addition, if the number of teeth g 1  is divisible by the number of divisions d 1 , that is, if the number of divisions d 1  is a divisor of the number of teeth g 1 , the least common multiple f 23  becomes equal to the meshing frequency f 2 . Thus, if the frequencies f 2  and f 3 , which become equal to the least common multiple f 23 , do not exceed 4 kHz, the peak level pn 1  may increase due to vibration of the frequencies f 2  and f 3 . 
     In order to make the frequencies f 2  and f 3 , which become equal to the least common multiple f 23 , exceed 4 kHz, it may be required to satisfy f 1 &gt;16000 if, for example, the one-phase excitation method is used and the number of steps s 1  per cycle is 4. In this case, the drive frequency f 1  may be too high, and a torque for transmitting a driving force to a drive receiving member may be insufficient, resulting in a loss of synchronization being likely to occur. In addition, an expensive motor may have to be used. It is therefore difficult in practice to make the frequencies f 2  and f 3 , which become equal to the least common multiple f 23 , higher than 4 kHz by increasing the drive frequency f 1 . 
     Consequently, in a conventional printer in which each of the number of teeth g 1  and the number of divisions d 1  is a multiple of 25, the least common multiple f 23  is likely to be equal to an excitation fundamental frequency f 3  less than or equal to 4 kHz, and the peak level pn 1  is likely to become high due to vibration of the frequencies f 2  and f 3 . 
     In the first exemplary embodiment, in contrast, the pinion gear  109  has teeth, the number g 1  of which is not 25, by which the value representing the number of divisions d 1 , namely, 50 [8 steps/rotation], is not divisible. 
     Consequently, the printer U according to the first exemplary embodiment may reduce the peak level pn 1  of uncomfortable noise, compared to a configuration in which the number of divisions d 1  of the rotating shaft is an integer multiple of the number of teeth g 1  and in which the least common multiple f 23  is less than or equal to the threshold value fs. 
     In the first exemplary embodiment, furthermore, the combination of the number of teeth g 1  being 23 [teeth] and the drive frequency f 1  being 2424 [pps], which is expected to minimize the peak level pn 1 , is set from the approximation function F(g 1 , f 1 ) corresponding to the predetermined meshing frequency f 2 . 
     Therefore, the printer U according to the first exemplary embodiment may reduce the peak level pn 1  of uncomfortable noise, compared to a configuration in which the combination of the number of teeth g 1  and the drive frequency f 1  is not set from the approximation function F(g 1 , f 1 ). 
     In the printer U according to the first exemplary embodiment having the above configuration, furthermore, the natural frequencies fa to fc of the timing belt  113  etc. are set to prime numbers different from the meshing frequency f 2  or the excitation fundamental frequency f 3 , and the least common multiples f 2   a  to f 2   c  and f 3   a  to f 3   c  of the natural frequencies fa to fc and the frequencies f 2  and f 3  are set to values that exceed the threshold value fs. Thus, in the first exemplary embodiment, the resonant frequencies fa′ to fc′ that are integer multiples of the natural frequencies fa to fc and that are less than or equal to 4 kHz are set to be different from the frequencies f 2  and f 3  of a fundamental wave component of vibration of the stacker exit motor MA 2  or the frequencies (2×f 2 ,  3 ×f 2 , . . . ) and (2×f 3 , 3×f 3 , . . . ) of second and higher harmonic components. 
     Consequently, in the printer U according to the first exemplary embodiment, the timing belt  113  etc. may be prevented from resonating due to the vibration having the frequencies f 2  and f 3 , and the peak level pn 1  of uncomfortable noise may be reduced, compared to a configuration in which the least common multiples f 2   a  to f 2   c  and f 3   a  to f 3   c  are less than or equal to the threshold value fs. 
     In the printer U according to the first exemplary embodiment, furthermore, the drive transmission systems ( 7 + 93  to  99 ) of the tamper drive motors  98  may also achieve operation and effect similar to those of the drive transmission systems  101  to  113  of the stacker exit motor MA 2 . 
     Modifications 
     While an exemplary embodiment of the present invention has been described in detail, the present invention is not limited to the foregoing exemplary embodiment, and a variety of modifications may be made within the scope of the present invention defined in the appended claims. First to seventh modifications of the present invention are disclosed for the purpose of illustration. 
     First Modification 
     In the foregoing exemplary embodiment, the printer U is used as an example of an image forming apparatus for the purpose illustration. Any other image forming apparatus such as a copier, a facsimile (fax) machine, or a multifunction peripheral having plural functions of such devices may also be used. 
     Second Modification 
     In the foregoing exemplary embodiment, a configuration according to an exemplary embodiment of the present invention is applied to the drive transmission systems ( 7 + 93  to  99 ,  101  to  113 ) of the stacker exit motor MA 2  and the tamper drive motors  98  in the post-processing device U 3 . Alternatively, for example, if the other motors of the post-processing device U 3 , namely, the roller drive motor MA 1 , the shelf drive motor MA 3 , and the paddle drive motor MA 6 , and the stapler moving motor  74  are implemented by stepping motors, a configuration according to an exemplary embodiment of the present invention may also be applied to the drive transmission systems of the motors MA 1  to MA 6  and  74 . In addition, for example, if the main motor of the printer body U 1  is implemented by a stepping motor, a configuration according to an exemplary embodiment of the present invention may also be applied to the drive transmission system of the main motor. 
     Third Modification 
     In the foregoing exemplary embodiment, the stacker exit motor MA 2  and the tamper drive motors  98  are implemented by a two-phase HB motor. The type of motor is not limited to the HB type, and any other type of motor such as a permanent magnet (PM) motor or a gear-shaped iron core motor serving as a variable reluctance (VR) motor may also be used. In addition, the number of phases is not limited to two, and a motor having any other number of phases, such as a three-phase motor or a five-phase motor, may also be used. 
     Fourth Modification 
     As in the first exemplary embodiment, it may be desirable that each of the stacker exit motor MA 2  and the tamper drive motors  98  be a unipolar stepping motor of the type in which current flows through two coils in one direction. However, the present invention is not limited to this exemplary embodiment, and a bipolar stepping motor of the type in which current flows through one coil in two directions may also be used in order to add a function for short-circuit current prevention or reduction, although the complexity of the structure of a driving device may increase. 
     Fifth Modification 
     As in the foregoing exemplary embodiment, it may be desirable that the electromagnets  141  to  148  be excited using the one-two phase excitation method in order to reduce noise generated by the stacker exit motor MA 2  and the tamper drive motors  98 . However, the present invention is not limited to this exemplary embodiment, and the electromagnets  141  to  148  may also be excited using the one-phase excitation method or the two-phase excitation method. If the one-phase excitation method or the two-phase excitation method is used instead, the number of steps s 1  per cycle becomes (½) times that described above, and the meshing frequency f 2  and the excitation fundamental frequency f 3  become two times those described above. In this case, if one of the number of teeth g 1  and the number of divisions d 1  is divisible by the other, for example, if d 1 =g 1 =25, the least common multiple f 23  does not change and is less than or equal to the threshold value fs, whereas, if one of the number of teeth g 1  and the number of divisions d 1  is not divisible by the other, for example, if d 1 =25 and g 1 =23, the least common multiple f 23  becomes two times that described above, and thus more easily exceeds the threshold value fs. 
     Sixth Modification 
     In the foregoing exemplary embodiment, the vibration absorbing member  112  is supported between the rear frame  101  and the motor bracket  111 . Alternatively, for example, a member composed of urethane or a similar material, which is similar to the vibration absorbing member  112 , may also be disposed between the stacker exit motor MA 2  and the motor bracket  111  so that vibration of the stacker exit motor MA 2  may be absorbed through elastic deformation to reduce vibration of the motor bracket  111 . 
     Seventh Modification 
     The specific values in the foregoing exemplary embodiment (g 1 =23, s 1 =8, d 1 =50, f 1 =2424, p 1 =400, r 1 =6.06, f 2 ≈139, f 3 =303, f 23 ≈41978, fs=4000, fa=151, fb=401, fc=503, etc.) are not limited to the illustrated values, and may be changed as desired within a range without departing from the scope of the invention claimed herein. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.