Patent Publication Number: US-8538314-B2

Title: Duplex image forming apparatus with a single drive source

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims all benefits accruing under 35 U.S.C. §119 from Korean Patent Application No. 2007-78278, filed on Aug. 3, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Example embodiments of the present invention relate to an image forming apparatus, and more particularly, to an image forming apparatus with an improved drive system to reduce manufacturing cost. 
     2. Description of the Related Art 
     An image forming apparatus forms an image on a printing medium, and may be classified according to how an image is formed. Various ways of forming the image include an ink jet type method to form the image by discharging an ink, an electrophotographic type method to form the image by proceeding to charge, expose, develop, transfer, and clean to form the image, and a thermal transfer type method to form the image by using an ink ribbon. 
     The image forming apparatus includes an image forming unit (not shown) to form an image on the printing medium according to one of the above types of forming an image, and a discharging roller to discharge the printing medium having an image formed thereon by way of an image forming unit to the outside of the image forming apparatus. Also, for a double sided printing, that is, for printing on an opposite side of a single sided printed printing medium by passing it through the image forming unit again, some image forming apparatuses have a duplex printing path for transporting the single sided printed printing medium to the image forming unit again. Here, in case of the single sided printing, a discharging roller rotates in a clockwise or a forward direction to discharge the single sided printed printing medium to the outside of the image forming apparatus  100 , whereas in case of the double sided printing, the discharging roller rotates in a counterclockwise or a reverse direction to reversely transport the single sided printed printing medium to the duplex printing path. 
     Also, a duplex roller is disposed on the duplex printing path to transport the printing medium reversely transported by way of the discharging roller to the image forming unit. 
     However, a typical image forming apparatus includes three driving motors to perform the printing operations, i.e., a first driving motor for driving a rotating body provided in the image forming unit, a second driving motor for driving the discharging roller, and a third driving motor for driving the duplex roller. Accordingly, a manufacturing cost a driving noise increase. 
     A solution to such problems from using a plurality of driving motors is suggested in co-assigned Korean Patent Publication No. 2006-24237 and No. 1999-20249. In Korean Patent Publication No. 2006-24237, a separate driving source for driving the image forming unit is necessary, so a decrease in a number of the driving source is limited. In Korean Patent Publication No. 1999-20249, a solenoid performing a piston movement is employed, so a driving configuration thereof is complicated, and it is difficult to make a small sized product due to a limitation on space that depends on the piston movement. 
     SUMMARY OF THE INVENTION 
     Several aspects and example embodiments of the present invention provide an image forming apparatus having a reduced size. 
     Another example embodiment of the present invention is to provide an image forming apparatus to drive an image forming unit, a discharging roller, and a duplex roller by way of a single driving source. 
     Additional example embodiments of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the present invention. 
     The foregoing and/or other aspects of the present invention can be achieved by providing an image forming apparatus, including an image forming unit to form an image on a printing medium; a discharging roller capable of transporting the printing medium which passes through the image forming unit in forward and reverse directions; a driving source to supply a rotation driving force in clockwise and counterclockwise directions to the discharging roller and the image forming unit; a duplex roller to transport the printing medium toward the image forming unit, which was transported in the reverse direction by the discharging roller; and a swing gear unit to receive the rotation driving force in the clockwise and counterclockwise directions to transmit a driving force to the duplex roller in a uniform direction. 
     According to an example embodiment of the present invention, the image forming apparatus may further include a frame to support the driving source, wherein the swing gear unit may be disposed to the frame to selectively swing. 
     According to an example embodiment of the present invention, the image forming apparatus may further include: a driving gear to receive the rotation driving force, and to transmit the rotation driving force to the swing gear unit, and a duplex gear connected to a rotation shaft of the duplex roller, and to receive the rotation driving force from the swing gear unit. 
     According to an example embodiment of the present invention, the swing gear unit may include: a swing member disposed to the frame to swing to correspond to clockwise and counterclockwise rotations of the driving gear, a first swing gear unit provided in an even number, and disposed to a first side of the swing member to be engaged to the duplex gear when the swing member swings in a first direction, and a second swing gear unit provided in an odd number, and disposed to a second side of the swing member to be engaged to the duplex gear when the swing member swings in a second direction. 
     According to an example embodiment of the present invention, the image forming apparatus may further include a decelerating gear disposed between the duplex gear and the swing gear unit to receive the rotation driving force from one of the first and second swing gear units, and to transmit the rotation driving force to the duplex gear. 
     According to an example embodiment of the present invention, the image forming apparatus may further include an aligning roller to receive the rotation driving force from the driving source to align the printing medium which is transported toward the image forming unit and then to transport the printing medium toward the image forming unit. 
     According to an example embodiment of the present invention, the driving gear may be disposed to an end part of a rotation shaft of the aligning roller. 
     According to an example embodiment of the present invention, the image forming apparatus may further include an electric clutch disposed to the rotation shaft of the aligning roller to intermittently transmit the rotation driving force to the rotation shaft of the aligning roller. 
     According to an example embodiment of the present invention, the duplex roller may include a first duplex roller which includes a rotation shaft to which the duplex gear is disposed, and a second duplex roller which interlocks with the first duplex roller to rotate. 
     According to an example embodiment of the present invention, the image forming apparatus may further include a belt driven by way of the first duplex roller to rotate the second duplex roller. 
     According to an example embodiment of the present invention, the image forming apparatus may further include a main body to accommodate the image forming unit, the discharging roller, the driving source, the frame, the duplex roller and the swing gear unit, and a duplex supporting frame to rotatably support the first and second duplex rollers, and to be detachably mounted to the main body. 
     According to an example embodiment of the present invention, the duplex supporting frame may further include an aligning guide to align the printing medium which is transported in the reverse direction by way of the discharging roller. 
     According to an example embodiment of the present invention, an image forming apparatus to form an image on a medium includes a single driving source to rotate in a medium output direction and a reverse direction to output corresponding rotation driving forces; a duplex transporter to use the rotation driving forces of the single driving source to transport the medium when the image forming apparatus selectively performs two sided image forming operation on the medium; an alignment roller to selectively rotate in the medium output direction or the reverse direction based on the rotation direction of the single driving source; and a swing gear unit attached to an end of the alignment roller to swing about the alignment roller, and to transfer the corresponding rotation driving forces of the single driving source to the duplex transporter, thereby causing the duplex transporter to transport the medium in one direction regardless of a rotation direction of the single driving source. 
     In addition to the example embodiments and aspects as described above, further aspects and embodiments will be apparent by reference to the drawings and by study of the following descriptions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention will become apparent from the following detailed description of example embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the following written and illustrated disclosure focuses on disclosing example embodiments of the invention, it should be clearly understood that the same is by way of illustration and example only and that the invention is not limited thereto. The spirit and scope of the present invention are limited only by the terms of the appended claims. The following represents brief descriptions of the drawings, wherein: 
         FIG. 1  is a schematic sectional view of an image forming apparatus according to an example embodiment of the present invention; 
         FIG. 2  is a perspective view of a main portion of the image forming apparatus in  FIG. 1 ; 
         FIG. 3  is an enlarged perspective view of a main portion of the image forming apparatus in  FIG. 1 ; 
         FIGS. 4A and 4B  are schematic views illustrating in sequence an operating process of a swing gear unit depending on a clockwise or a first direction rotation of a driving gear of the image forming apparatus in  FIG. 1 ; 
         FIGS. 5A and 5B  are schematic views illustrating in sequence an operating process of the swing gear unit depending on a counterclockwise or a second direction rotation of the driving gear of the image forming apparatus in  FIG. 1 ; 
         FIG. 6  is a schematic perspective view of a duplex transporting unit of the image forming apparatus in  FIG. 1 ; 
         FIG. 7  is a schematic sectional view illustrating a single side printing process of the image forming apparatus in  FIG. 1 ; and 
         FIG. 8  is a schematic sectional view illustrating an opposite side printing process of the image forming apparatus in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The example embodiments are described below so as to explain the present invention by referring to the figures. 
     As shown in  FIGS. 1 and 2 , an image forming apparatus  100  according to an example embodiment of the present invention includes an image forming unit  130 , a discharging unit  160 , a duplex transporting unit  170 , a driving source  200 , and a swing gear unit  180 . Also, the image forming apparatus  100  includes a simplex (or a single side) printing path SP, along which a printing medium is supplied and a printing of an image is performed on the printing medium by way of the image forming unit  130 , and a duplex (or a double side) printing path DP for transporting the printing medium printed along the printing path SP to the image forming unit  130  again to print another image on the other side of the printing medium. 
     The image forming unit  130  includes a photosensitive body  132   b , an exposing unit  131  to expose the photosensitive body  132   b  to form an electrostatic latent image on a surface of the photosensitive body  132   b , and a developing roller  132   a  to develop the electrostatic latent image formed on the surface of the photosensitive body  132   b  with a developer. 
     The photosensitive body  132   b  and the developing roller  132   a  are accommodated to a developing cartridge  132 . The developing cartridge  132  includes a developer storing unit (not shown) to store the developer, and is detachably mounted to a main body  101  of the image forming apparatus  100 . 
     Also, the image forming unit  130  further includes a transferring roller  134  to transfer a visible image formed on the photosensitive body  132   b  by the developing roller  132   a  to the printing medium, and a heating roller  135  and a pressing roller  136  to fuse the transferred visible image on to the printing medium by way of heat and pressure. 
     The discharging unit  160  may move the printing medium in a forward direction to discharge the printing medium to the outside of the image forming apparatus  100  after passing the printing medium through the image forming unit  130 , and may move the printing medium in a reverse direction toward the duplex transporting unit  170 . 
     As shown in  FIG. 1 , the discharging unit  160  may include a discharging roller  163  capable of rotating in a clockwise direction (or a first direction) and a counterclockwise direction (or a second direction) by the driving source  200 , and a plurality of idle rollers  165  that are in contact with the discharging roller  163  to be driven. 
     As shown in  FIGS. 1 and 6 , the duplex transporting unit  170  may include a first duplex roller  175 , a second duplex roller  171 , and a duplex supporting frame  177  to rotatably support the first and second duplex rollers  175  and  171 . As shown therein, the second duplex roller  171  is provided in plural. Alternatively, a single second duplex roller  171  may be provided as necessary. 
     The duplex supporting frame  177  rotatably supports the first and second duplex rollers  175  and  171 . Also, the duplex supporting frame  177  may be detachably mounted to the main body  101  of the image forming apparatus  100 . An insertion unit  177   a  may be provided to opposite sides of the duplex supporting frame  177  in a lengthwise direction thereof. The duplex supporting frame  177  may be mounted and detached to and from the main body  101  by inserting and extracting the insertion unit  177   a  in and from a sliding guide N, which is shown in  FIG. 1  and formed to the main body  101  to have a reverse-U shape. When the duplex supporting frame  177  is mounted and detached to and from the main body  101 , a duplex gear  175   a  is engaged and released with and from a decelerating gear  109  (shown in  FIG. 3 ). 
     The duplex gear  175   a  is disposed to a first end part of a rotation shaft  175   c  of the first duplex roller  175 , and a first pulley  175   b  is disposed to a second end part thereof. The duplex gear  175   a  receives a rotation driving force from the driving source  200  through the swing gear unit  180 . 
     The received rotation driving force is transmitted to the second duplex roller  171  through a second pulley  171   a  disposed to the second duplex roller  171  by way of the first pulley  175   b  and a belt  178 . For convenience, a single second duplex roller  171  is illustrated. The rotation driving force may be transmitted from the first duplex roller  175  to the second duplex roller  171  by way of other types of transmitting mechanisms instead of the belt as long as the first and second duplex rollers  175  and  171  can rotate in the same directions. 
     As shown in  FIG. 6 , the duplex transporting unit  170  may further include an aligning guide  179 , a printing medium supporting plate  174 , and a printing medium skewing member  172 . The aligning guide  179  is coupled to the duplex supporting frame  177  on one side to align the printing medium P that is reversely transported by the discharging roller  163 . The printing medium supporting plate  174  extends from the aligning guide  179  to bend, and forms a printing medium moving passage B together with the duplex supporting frame  177 . 
     The printing medium skewing member  172  is disposed to the printing medium supporting plate  174  to rotate about a rotation axis that is inclined with respect to a rotation axis of the second duplex roller  171 . Accordingly, the printing medium P that is reversely transported by the discharging roller  163  proceeds slanted toward the aligning guide  179  by way of the printing medium skewing member  172 , and a side S of the printing medium P is bumped against the aligning guide  179  so that the printing medium P can be aligned. 
     As shown in  FIG. 3 , the swing gear unit  180  includes a swing member  181  disposed to the main body  101  to swing, a first swing gear unit  183 , and a second swing gear unit  185 . As shown in  FIG. 3 , the swing member  181  swings in a first direction F and a second direction G about an aligning roller shaft  121   a , which is a rotation shaft of the aligning roller  121 . A driving gear  108  may be disposed to a first end part of the aligning roller shaft  121   a , and an electric clutch  203  shown in  FIG. 2  may be disposed to a second end part thereof. 
     The aligning roller shaft  121   a  is rotatably inserted to the swing member  181  so that the swing member  181  can swing in the first direction F and the second direction G by way of a friction force between the swing member  181  and the aligning roller shaft  121   a , as the driving gear  108  and the aligning roller shaft  121   a  rotate in a clockwise direction H and a counterclockwise direction J. 
     The first swing gear unit  183  is disposed to a first side of the swing member  181  to be engaged to the decelerating gear  109  as the swing member  181  swings in the first direction F. As shown in  FIG. 4A , the decelerating gear  109  may include a first gear  109   a  and a second gear  109   b  which are coaxial, and may integrally rotate. 
     The decelerating gear  109  is for changing a rotation number due to a rotation speed difference between the aligning roller  121  and the first duplex roller  175 . The decelerating gear  109  may be omitted as necessary. The decelerating gear  109  is engaged with the duplex gear  175   a.    
     The second swing gear unit  185  is disposed to a second side of the swing member  181 , and is engaged to the decelerating gear  109  to transmit the rotation driving force to the duplex gear  175   a  as the swing member  181  swings in the second direction G. Here, to transmit a rotation force having a uniform direction to the duplex gear  175   a  irrespective of the rotation direction of the aligning roller shaft  121   a , the first swing gear unit  183  is provided in an even number of gears, and the second swing gear unit  185  is provided in an odd number of gears. 
     The driving gear  108  is disposed to the aligning roller shaft  121   a . Alternatively, the driving gear  108  may be connected to a first feeding roller shaft  105   a  shown in  FIG. 2  as necessary. However, since the distance to the first duplex roller  175  is large, a separate driving force transmitting mechanism such as a gear or a belt is necessary to transmit the rotation driving force to the duplex gear  175   a  in this case. Accordingly, it is preferable but not necessary to dispose the driving gear  108  to the aligning roller shaft  121   a  that is adjacent to the first duplex roller  175 . Also, it is optionally unnecessary to dispose the driving gear  108  to the aligning roller shaft  121   a  in case that the driving gear  108  is provided to rotate about a separate stud (not shown) paralleling the aligning roller shaft  121   a  and inserted to the swing member  181 . To connect the driving gear  108  to a rotation shaft of a rotating body existing in the image forming apparatus  100  without the separate stud, it is taken into consideration that the rotation shaft is allowed to rotate in clockwise (or first) and counterclockwise (or second) directions. For example, if the photosensitive body  132   b  rotates in a reverse direction to the rotating direction shown in  FIG. 1 , the developer accommodated in the developing cartridge  132  may leak. Also, if the heating roller  135  or the pressing roller  136  rotates in a reverse direction to the rotating direction shown in  FIG. 1 , a problem may happen to a sensor for sensing a surface temperature of the heating roller  135 . Accordingly, the driving gear  108  is preferably but not necessarily disposed to the aligning roller shaft  121   a.    
     As shown in  FIG. 1 , the image forming apparatus  100  may further include a printing medium supplying cassette  110 , and an aligning unit  120 . The printing medium supplying cassette  110  includes a cassette casing  111 , a plate  113  accommodated in the cassette casing  111  to load a printing medium, and a friction pad  115  to prevent or reduce the printing medium loaded on the plate  113  from being doubly transported (or more than one printing medium from being transported at a time). The printing medium supplying cassette  110  is detachably mounted to the main body  101  of the image forming apparatus  100 . 
     The aligning unit  120  may include a pair of aligning rollers  121  and  123 . One ( 121 ) of the pair of the aligning rollers may drive, and the other ( 123 ) thereof may be driven. The aligning unit  120  aligns a transported printing medium, and transports the aligned printing medium toward the image forming unit  130 . 
     Also, as shown in  FIG. 1 , the image forming apparatus  100  according to an example embodiment may further include a pickup roller  103 , a first feeding roller  105 , and a second feeding roller  107 . A second feeding roller gear  107   a  is disposed to an end part of a rotation shaft of the second feeding roller  107 . The pickup roller  103  picks up the printing medium loaded on the plate  113 , and the picked up printing medium is individually separated by way of the friction force against the friction pad  115  to be transported to the first feeding roller  105 . 
     The first feeding roller  105  is disposed between the pickup roller  103  and the aligning unit  120  to transport the picked up printing medium toward the aligning unit  120 . The second feeding roller  107  is disposed between the heating roller  135  and the discharging roller  163  to transport the printing medium passing through the image forming unit  130  toward the discharging roller  163 . 
     As shown in  FIG. 2 , the driving source  200  may include a single driving motor. The driving source  200  supplies a rotation driving force in clockwise (or first) and counterclockwise (or second) directions to the image forming unit  130 , the discharging roller  163 , and the duplex roller  175 , for example. More in detail, the driving source  200  supplies the rotation driving force to all rotating bodies that are inside the image forming apparatus  100 . 
     Hereinafter, a rotation driving force transmitting course to each component of the image forming apparatus  100  will be described by referring to  FIGS. 1 and 2 . At first, a first feeding roller gear  105   b  is disposed to an end part of a rotation shaft  105   a  of the first feeding roller  105 , and the rotation driving force of the driving source  200  is transmitted to the first feeding roller gear  105   b  through a pinion disposed to a driving shaft (not shown) of the driving source  200  (hereinafter, referred to as a ‘driving pinion’)→a gear  201 →a gear  202 →a gear  205 →a gear  203   a  of the electric clutch  203 →transmitting gears  204 . 
     The pickup roller  103  receives the rotation driving force by way of a gear  205 →a gear  211 →a gear  212 →a pickup roller gear  103   a . Also, the rotation driving force is intermittently transmitted to the pickup roller  103  by way of a solenoid (not shown). 
     The rotation driving force of the driving source  200  is intermittently transmitted to the aligning roller  121  by way of the electric clutch  203  disposed to the aligning roller shaft  121   a . That is, the aligning roller shaft  121   a  is driven or is idle depending on turning on/off of the electric clutch  203 . 
     The heating roller  135  receives the rotation driving force through the driving pinion→a gear  206 →a gear  209 →a gear  213 →a heating roller gear  135   a . The heating roller gear  135   a  is disposed to a rotation shaft of the heating roller  135 , and the pressing roller  136  contacts to the heating roller  135  to be driven. The gear  209  and the gear  213  are coaxially connected. The rotation driving force is transmitted from the gear  209  to the gear  213  only if the driving pinion rotates in the clockwise direction (first or forward direction), and is not transmitted if the driving pinion rotates in the counterclockwise direction (second or reverse direction). 
     The developing cartridge  132  receives the rotation driving force through a developing cartridge driving gear  132   c , and the developing roller  132   a  and the photosensitive body  132   b  accommodated therein are driven. The developing cartridge driving gear  132   c  and the gear  202  are coaxially disposed, and the rotation driving force is transmitted from the gear  202  to the developing cartridge driving gear  132   c  only if the driving pinion rotates in the clockwise direction. 
     A discharging roller gear  163   a  is disposed to a discharging roller shaft  163   b  which is a rotation shaft of the discharging roller  163  to receive the rotation driving force from the driving source  200 . The rotation driving force is transmitted to the discharging roller gear  163   a  through the driving pinion→the gear  206 →a gear  207 →a gear  208 . The duplex transporting unit  170  (the first duplex roller  175 ) receives the rotation driving force of the driving source  200  through the swing gear unit  180 . 
     As shown in  FIG. 4A , if the driving gear  108  rotates in the clockwise direction H, the first swing gear unit  183  and the second swing gear unit  185  engaged to the driving gear  108  rotate. Also, as shown in  FIG. 4B , the swing member  181  swings in the first direction F so that the first swing gear unit  183  can be engaged to the second gear  109   b  of the decelerating gear  109  to rotate the decelerating gear  109  in a counterclockwise direction K. Accordingly, the duplex gear  175   a  can rotate in a clockwise direction L. 
     On the other hand, as shown in  FIG. 5A , if the driving gear  108  rotates in the counterclockwise direction J, the first swing gear unit  183  and the second swing gear unit  185  rotate in a reverse direction to the direction shown in  FIG. 4A . Accordingly, as shown in  FIG. 5B , the swing member  181  swings in the second direction G so that the second swing gear unit  185  can be engaged to the second gear  109   b  of the decelerating gear  109 , and the decelerating gear  109  can rotate in the counterclockwise direction K which is the same direction as the direction shown in  FIG. 4B . Accordingly, the duplex gear  175   a  can rotate in the clockwise direction L as shown in  FIG. 4B . That is, the duplex gear  175  can rotate in a uniform direction irrespective of the clockwise and counterclockwise rotations of the driving gear  108 . Accordingly, as shown in  FIG. 1 , the duplex transporting unit  170  can rotate in the direction L in which the printing medium is transported toward the aligning roller  121  irrespective of the clockwise and counterclockwise rotations of the driving source  200 . 
     As described above, if the driving source  200  rotates in the clockwise direction as shown in  FIG. 1 , each rotating body  103 ,  105 ,  121 ,  132   b ,  132   a ,  134 ,  135 ,  107 , and  163  accommodated inside the image forming apparatus  100  rotates in a direction for the printing medium to be transported along the simplex printing path SP. On the other hand, if the driving source  200  rotates in the counterclockwise direction, the heating roller  135 , the pressing roller  136  and the developing cartridge  132  are idle not to rotate, while the first feeding roller  105 , the aligning roller  121 , the second feeding roller  107 , and the discharging roller  163  rotate in the reverse direction. However, the duplex transporting unit  170  still rotates in the uniform direction L irrespective of the clockwise and counterclockwise rotations of the driving source  200 . 
     Hereinafter, a single side printing process of the image forming apparatus  100  will be described by referring to  FIG. 7 . Here, P 1  to P 4  refer to a printing medium moving along the simplex printing path in sequence. A control unit (not shown) rotates the driving source  200  in the clockwise direction, and controls the solenoid (not shown) to transmit the rotation driving force to the pickup roller  103 . Accordingly, a printing medium P 1  is picked up by the pickup roller  103 , and is transported toward the aligning unit  120  through the first feeding roller  105 . 
     The control unit turns off the electric clutch  203  so that the aligning unit  120  can be idle to allow a front end part of the transported printing medium P 1  to be bumped to the aligning unit  120 . Then, after a predetermined period of time, the control unit turns on the electric clutch  203  to transport the printing medium P 1  toward the image forming unit  130 . An image developed out of the developer by the image forming unit  130  begins to be formed on the transported printing medium P 2 . 
     Then, the printing medium P 2  passes through the heating roller  135  and the pressing roller  136 , and then the developer is fused on the printing medium P 3 . Then, the printing medium P 4 , a single side of which is printed, is discharged to the outside by the discharging roller  163 . 
     Hereinafter, a double side printing process will be described by referring to  FIG. 8 , which occurs once the single side printing process of the image forming apparatus  100  is completed, but before the printing medium P 4  is discharged to the outside by the discharging roller  163 . Here, P 5  to P 8  refer to a printing medium transported in the duplex printing path in sequence during the double side printing process. That is, the control unit rotates the driving source  200  in the counterclockwise direction so that the discharging roller  163  holding the printing medium P 4 , the single side of which is printed, can rotate in a direction A. Accordingly, the printing medium P 4  is transported toward the duplex transporting unit  170 . Also, to reduce power consumption, the electric clutch  203  may be turned off until the printing medium P 4  enters the second duplex roller  171 . 
     If the printing medium P 4  begins to enter in to the second duplex roller  171 , the control unit turns on the electric clutch  203  so that the rotation driving force can be transmitted to the first duplex roller  175  and the second duplex roller  171  through the aligning roller shaft  121   a  and the swing gear unit  180 . Accordingly, the second duplex roller  171  can rotate in the direction L to hold the printing medium P 4  that is reversely transported from the discharging roller  163 . The printing medium P 5 , front and rear end parts of which are respectively held to the second duplex roller  171  and the discharging roller  163 , is separated from the discharging roller  163  to be transported toward the first duplex roller  175  as the second duplex roller  171  continuously rotates in the direction L. 
     Until the printing medium P 5  is held to the aligning unit  120 , the printing medium P 5  can be transported toward the aligning unit  120  irrespective of the clockwise and counterclockwise direction of rotation of the driving source  200 . However, the control unit converts the rotation state of the driving source  200  into a clockwise direction rotating state before the front end part of the printing medium P 5  enters the aligning unit  120 . 
     Since the driving source  200  is rotated in the counterclockwise direction to rotate the discharging roller  163  in the direction A, the aligning roller  121  rotates in a clockwise direction H as long as the driving source  200  is not under a clockwise rotation state again. Accordingly, the printing medium P 5  can be prevented from being held to the aligning unit  120 . Although the printing medium P 5  is held to the aligning unit  120 , since the aligning roller  121  rotates in the clockwise direction H and the first duplex roller  175  rotates in the direction L, the front end part of the printing medium P 5  receives a transporting force toward the first duplex roller  175 , and the rear end part of the printing medium P 5  receives a transporting force toward the aligning roller  121  so that the printing medium P 5  can be stagnated (or caught) between the first duplex roller  175  and the aligning unit  120 . To solve this problem, the driving source  200  is converted from the counterclockwise rotation state to the clockwise rotation state. 
     Since the first duplex roller  175  still rotates in direction L by way of the swing gear unit  180  in  FIG. 8 , although the driving source  200  is converted to the clockwise rotation state, front and rear end parts of a printing medium P 6  are concurrently held so that the printing medium P 6  can be transported toward the image forming unit  130 . The electric clutch  203  may be turned off after the rear end part of the printing medium P 6  passes through the aligning unit  120 . 
     An image is formed on a second side of the printing medium P 7  as the printing medium P 7  passes through the image forming unit  130 . Since the driving source  200  is under the clockwise rotation state, the discharging roller  163  rotates in the direction D to discharge a printing medium P 8 , the opposite sides of which are printed, to the outside of the image forming apparatus  100 . Accordingly, the double side printing process is completed. 
     As described above, since all rotating bodies inside the image forming apparatus  100  can be rotated by using the single driving source  200  and the swing gear unit  180 , a manufacturing cost can be reduced by approximately 15% in comparison with the typical configurations. 
     As described above, an image forming apparatus according to the present invention has following effects. First, a swing gear unit is employed to reduce the size of a product. Second, an image forming unit, a discharging roller, and a duplex roller are driven by a single driving source to reduce manufacturing cost and driving noise. 
     While there have been illustrated and described what are considered to be example embodiments of the present invention, it will be understood by those skilled in the art and as technology develops that various changes and modifications, may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. Many modifications, permutations, additions and sub-combinations may be made to adapt the teachings of the present invention to a particular situation without departing from the scope thereof. For example, various types of image forming devices include devices such as photocopiers, printers, facsimile machines, or multifunction peripherals (MFP) (which offer functions of the preceding devices in a single device) commonly have a printing function. Accordingly, it is intended, therefore, that the present invention not be limited to the various example embodiments disclosed, but that the present invention includes all embodiments falling within the scope of the appended claims. Although a few example embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these example embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.