Abstract:
A retard roller of an automatic document feeder provides a frictional force to separate plural documents from each other. The retard roller can provide a first damping torque and a second damping torque with different directions. The second damping torque is smaller than the first damping torque. As such, the frictional force exerted on the document is reduced for returning the document out of the automatic document feeder. A retard roller module having such a retard roller is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a divisional application of application Ser. No. 12/725,184, filed Mar. 16, 2010, which is incorporated by reference 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a retard roller, and more particularly to a retard roller for use in an automatic document feeder. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the early stage, a scanning apparatus is used to scan the image of a single document. For scanning both sides of the document, the document should be manually turned over after one side of the document has been scanned in order to sequentially scan the other side of the document. For scanning a stack of documents, after one document has been scanned, the document should be removed from the scanning apparatus and then a next document could be placed on the scanning apparatus in order to be further scanned. Since the process of manually turning over the document or manually replacing the document is very troublesome, the conventional scanning apparatus is not feasible. Recently, an automatic document feeder is usually integrated into the scanning apparatus. The automatic document feeder is suitable to perform a duplex scanning operation and successively scan plural documents without the need of manually turning over or replacing the documents. 
         [0004]    Generally, the automatic document feeder has a sheet input tray for placing a stack of documents. The automatic document feeder also has a sheet pick-up module for successively feeding the stack of documents from the sheet input tray to the internal portion of the automatic document feeder in a sheet-feeding direction. For allowing only one document to be fed into the internal portion of the automatic document feeder at each feeding time, the sheet pick-up module has a sheet separation roller and a separation pad. The separation pad is disposed under the sheet separation roller. The sheet separation roller may provide a frictional force to the document that is contacted with the separation pad. The frictional forces between the sheet pick-up module, the separation pad and the documents should be elaborately controlled. Generally, the frictional force between the sheet pick-up module and the document contacted with the sheet pick-up module is greater than the frictional force between the documents. In addition, the frictional force between the separation pad and the document contacted with the separation pad is also greater than the frictional force between the documents. As a consequence, only one document is allowed to be fed into the internal portion of the automatic document feeder at each feeding time. As the automatic document feeder is used for a long time, the separation pad is usually abraded, or even losses the function of separating documents. In this situation, the separation pad needs to be replaced with a new one. Since the separation pad is usually securely fastened on the automatic document feeder, the process of replacing the separation pad is very complicated. 
         [0005]    For solving the above drawbacks, U.S. Pat. No. 6,659,450 disclosed an automatic document feeder with an easily disassembled separation pad and retard roller. During the sheet-feeding process, the retard roller generates a damping torque in a sheet-feeding rotating direction. In response to the damping torque, the retard roller provides a frictional force to the document. Although the separation pad and the retard roller of the automatic document feeder described in U.S. Pat. No. 6,659,450 are disassembled more easily when compared with the prior art, there are still some drawbacks. For example, after the retard roller is disassembled, the spring for providing a normal force on the retard roller is still retained in the automatic document feeder. During the process of assembling the retard roller, the retard roller needs to be installed in the automatic document feeder while aligning the retard roller with the spring. In other words, the retard roller needs to be precisely combined with the spring in order to achieve a normal function of the retard roller. The process of assembling the retard roller is not user-friendly. 
         [0006]    Moreover, in a case that the documents are jammed in the internal portion of the automatic document feeder, the installation of the retard roller or separation pad incurs some drawbacks. For example, due to the frictional force between the jammed documents and the retard roller (or separation pad), the jammed documents fail to be pulled out of the automatic document feeder in a sheet-returning direction, which is opposed to the sheet-feeding direction. For releasing the jammed documents, the user needs to open the upper cover of the automatic document feeder to uplift the sheet pick-up module. As such, the sheet pick-up module is separated from the upper sides of the jammed documents and the frictional force between the jammed documents and the retard roller (or separation pad) is eliminated. Meanwhile, the jammed documents could be effectively released. 
         [0007]    Since the process of removing the document from the automatic document feeder is very troublesome, there is a need of providing a retard roller for moving the document in the sheet-feeding direction and the sheet-returning direction without the need of opening the upper cover. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the present invention provides a retard roller for moving the document in the sheet-feeding direction and the sheet-returning direction. 
         [0009]    Another object of the present invention provides an easily disassembled/assembled retard roller module. 
         [0010]    In accordance with an aspect of the present invention, there is provided a retard roller of an automatic document feeder for providing a frictional force to separate a first document and a second document from each other. The first document lies on the second document. The retard roller includes a sleeve, a separation pad, a helical spring and a rotating shaft. The sleeve has a sleeve inner wall. The separation pad is sheathed around the sleeve, and contacted with the second document. The helical spring is disposed within the sleeve, and includes a first spring segment and a second spring segment. The first spring segment has a first spring inner diameter. The second spring segment has a second spring inner diameter smaller than the first spring inner diameter. The first spring segment is contacted with the sleeve inner wall. The rotating shaft is penetrated through the helical spring and contacted with the second spring segment. When the second document is moved in a first direction, the sleeve is rotated in a first rotating direction, the first spring segment is twisted in the first rotating direction, and the first spring inner diameter of the first spring segment is widened, so that the first spring segment is fixed on the sleeve inner wall and the second spring segment is twisted with respect to the rotating shaft to generate a first damping torque. After the first spring inner diameter of the first spring segment is widened and the second document is moved in a second direction opposed to the first direction, the sleeve is rotated in a second rotating direction, so that the second spring segment is fixed on the rotating shaft and the first spring segment is twisted to generate a second damping torque. The first damping torque is greater than the second damping torque. 
         [0011]    In an embodiment, the sleeve inner wall includes a first inner wall part and a second inner wall part. The first inner wall part is near a first end of the sleeve, and has a first sleeve inner diameter. The second inner wall part is near a second end of the sleeve, and has a second sleeve inner diameter. The first sleeve inner diameter is smaller than the second sleeve inner diameter. 
         [0012]    In an embodiment, the sleeve inner wall is an inclined wall. 
         [0013]    In an embodiment, the first spring segment is contacted with the first inner wall part. The second spring segment is separated from the second inner wall part but contacted with the rotating shaft. When the sleeve is rotated in the first rotating direction, the first spring segment is twisted in the first rotating direction, and the first spring inner diameter of the first spring segment is widened, so that the first spring segment is fixed on the first inner wall part and the second spring segment is twisted with respect to the rotating shaft to generate the first damping torque. After the first spring inner diameter of the first spring segment is widened and the second document is moved in the second direction, the sleeve is rotated in the second rotating direction, so that the second spring segment is fixed on the rotating shaft and the first spring segment is twisted to generate the second damping torque. 
         [0014]    In an embodiment, the first spring segment is eccentrically connected with the second spring segment. 
         [0015]    In an embodiment, the retard roller further includes a receiving shaft inserted into the first end of the sleeve. The rotating shaft is inserted into the second end of the sleeve and received within the receiving shaft. 
         [0016]    In accordance with another aspect of the present invention, there is provided a retard roller of an automatic document feeder for providing a frictional force to separate a first document and a second document from each other. The first document lies on the second document. The retard roller includes a first sleeve, a second sleeve, a separation pad, a helical spring, a one-way clutch and a rotating shaft. The first sleeve has a sleeve inner tube. The second sleeve is accommodated within the first sleeve, and has a sleeve outer tube. The sleeve outer tube is arranged at an end of the second sleeve and contacted with an end of the sleeve inner tube. The separation pad is sheathed around the first sleeve, and contacted with the second document. The helical spring is accommodated within the first sleeve. A first end of the helical spring is sheathed around the sleeve outer tube to define a first spring segment. A second end of the helical spring is sheathed around the sleeve inner tube to define a second spring segment. The first spring segment has a first spring inner diameter. The second spring segment has a second spring inner diameter smaller than the first spring inner diameter. The one-way clutch is accommodated within the second sleeve for preventing the second sleeve from rotating in a first rotating direction. The rotating shaft is penetrated through the first sleeve, the second sleeve and the one-way clutch. When the second document is moved in a first direction, the first sleeve is rotated in the first rotating direction, and the second sleeve fails to be rotated in response to the one-way clutch, so that the first spring segment is fixed on the sleeve outer tube and the second spring segment is twisted with respect to the sleeve inner tube to generate a first damping torque. When the second document is moved in a second direction opposed to the first direction, the first sleeve is rotated in a second rotating direction, the first spring segment is fixed on the sleeve outer tube, and the second spring segment is fixed on the sleeve inner tube, so that the second sleeve is rotated with the first sleeve and the one-way clutch is rotated with respect to the rotating shaft to generate a second damping torque. The first damping torque is greater than the second damping torque. 
         [0017]    In an embodiment, a tube diameter of the sleeve inner tube is smaller than that of the sleeve outer tube, so that the interference between the first spring segment and the sleeve outer tube is greater than the interference between the second spring segment and the sleeve inner tube. 
         [0018]    In accordance with a further aspect of the present invention, there is provided a retard roller module of an automatic document feeder. The retard roller module has a retard roller for providing a frictional force to separate a first document and a second document from each other. The first document lies on the second document. The retard roller module includes a retard roller frame, the retard roller and an elastic element. The retard roller is installed on the retard roller frame. The retard roller includes a sleeve, a separation pad, a helical spring and a rotating shaft. The sleeve has a sleeve inner wall. The separation pad is sheathed around the sleeve, and contacted with the second document. The helical spring is disposed within the sleeve, and includes a first spring segment and a second spring segment. The first spring segment has a first spring inner diameter. The second spring segment has a second spring inner diameter smaller than the first spring inner diameter. The first spring segment is contacted with the sleeve inner wall. The rotating shaft is penetrated through the helical spring and contacted with the second spring segment. When the second document is moved in a first direction, the sleeve is rotated in a first rotating direction, the first spring segment is twisted in the first rotating direction, and the first spring inner diameter of the first spring segment is widened, so that the first spring segment is fixed on the sleeve inner wall and the second spring segment is twisted with respect to the rotating shaft to generate a first damping torque. After the first spring inner diameter of the first spring segment is widened and the second document is moved in a second direction opposed to the first direction, the sleeve is rotated in a second rotating direction, so that the second spring segment is fixed on the rotating shaft and the first spring segment is twisted to generate a second damping torque. The first damping torque is greater than the second damping torque. The elastic element is disposed on the retard roller frame and contacted with the retard roller for providing an elastic force on the retard roller, so that the retard roller is movable upwardly and downwardly with respect to the retard roller frame. 
         [0019]    In an embodiment, the sleeve inner wall includes a first inner wall part and a second inner wall part. The first inner wall part is near a first end of the sleeve, and has a first sleeve inner diameter. The second inner wall part is near a second end of the sleeve, and has a second sleeve inner diameter. The first sleeve inner diameter is smaller than the second sleeve inner diameter. 
         [0020]    In an embodiment, the sleeve inner wall is an inclined wall. 
         [0021]    In an embodiment, the first spring segment is contacted with the first inner wall part. The second spring segment is separated from the second inner wall part but contacted with the rotating shaft. When the sleeve is rotated in the first rotating direction, the first spring segment is twisted in the first rotating direction, and the first spring inner diameter of the first spring segment is widened, so that the first spring segment is fixed on the first inner wall part and the second spring segment is twisted with respect to the rotating shaft to generate the first damping torque. After the first spring inner diameter of the first spring segment is widened and the second document is moved in the second direction, the sleeve is rotated in the second rotating direction, so that the second spring segment is fixed on the rotating shaft and the first spring segment is twisted to generate the second damping torque. 
         [0022]    In an embodiment, the first spring segment is eccentrically connected with the second spring segment. 
         [0023]    In an embodiment, the retard roller further includes a receiving shaft inserted into the first end of the sleeve. The rotating shaft is inserted into the second end of the sleeve and received within the receiving shaft. 
         [0024]    In an embodiment, the elastic element further includes a torsion spring arm, which is contacted with the retard roller for providing the elastic force on the retard roller, so that the retard roller is movable upwardly and downwardly with respect to the retard roller frame. 
         [0025]    In an embodiment, the elastic element is a supporting torsion spring. 
         [0026]    In an embodiment, the rotating shaft further comprises a confining edge. When the confining edge is fixed on the retard roller frame, the rotating shaft is fixed and fails to be rotated. 
         [0027]    In accordance with a further aspect of the present invention, there is provided a retard roller module of an automatic document feeder. The retard roller module has a retard roller for providing a frictional force to separate a first document and a second document from each other. The first document lies on the second document. The retard roller module includes a retard roller frame, the retard roller and an elastic element. The retard roller is installed on the retard roller frame. The retard roller includes a first sleeve, a second sleeve, a separation pad, a helical spring, a one-way clutch and a rotating shaft. The first sleeve has a sleeve inner tube. The second sleeve is accommodated within the first sleeve, and has a sleeve outer tube. The sleeve outer tube is arranged at an end of the second sleeve and contacted with an end of the sleeve inner tube. The separation pad is sheathed around the first sleeve, and contacted with the second document. The helical spring is accommodated within the first sleeve. A first end of the helical spring is sheathed around the sleeve outer tube to define a first spring segment. A second end of the helical spring is sheathed around the sleeve inner tube to define a second spring segment. The first spring segment has a first spring inner diameter. The second spring segment has a second spring inner diameter smaller than the first spring inner diameter. The one-way clutch is accommodated within the second sleeve for preventing the second sleeve from rotating in a first rotating direction. The rotating shaft is penetrated through the first sleeve, the second sleeve and the one-way clutch. When the second document is moved in a first direction, the first sleeve is rotated in the first rotating direction, and the second sleeve fails to be rotated in response to the one-way clutch, so that the first spring segment is fixed on the sleeve outer tube and the second spring segment is twisted with respect to the sleeve inner tube to generate a first damping torque. When the second document is moved in a second direction opposed to the first direction, the first sleeve is rotated in a second rotating direction, the first spring segment is fixed on the sleeve outer tube, and the second spring segment is fixed on the sleeve inner tube, so that the second sleeve is rotated with the first sleeve and the one-way clutch is rotated with respect to the rotating shaft to generate a second damping torque. The first damping torque is greater than the second damping torque. The elastic element is disposed on the retard roller frame and contacted with the retard roller for providing an elastic force on the retard roller, so that the retard roller is movable upwardly and downwardly with respect to the retard roller frame. 
         [0028]    In an embodiment, a tube diameter of the sleeve inner tube is smaller than that of the sleeve outer tube, so that the interference between the first spring segment and the sleeve outer tube is greater than the interference between the second spring segment and the sleeve inner tube. 
         [0029]    In an embodiment, the rotating shaft further comprises a confining edge. When the confining edge is fixed on the retard roller frame, the rotating shaft is fixed and fails to be rotated. 
         [0030]    In an embodiment, the elastic element further includes a torsion spring arm, which is contacted with the retard roller for providing the elastic force on the retard roller, so that the retard roller is movable upwardly and downwardly with respect to the retard roller frame. 
         [0031]    In an embodiment, the elastic element is a supporting torsion spring. 
         [0032]    The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  is a schematic side view illustrating a retard roller module in a sheet-feeding status according to a first embodiment of the present invention; 
           [0034]      FIG. 2  is a schematic exploded view illustrating the retard roller module according to the first embodiment of the present invention; 
           [0035]      FIG. 3  is a schematic assembled view illustrating the retard roller module according to the first embodiment of the present invention; 
           [0036]      FIG. 4  is a schematic cross-sectional view illustrating the retard roller of the retard roller module according to the first embodiment of the present invention; 
           [0037]      FIG. 5  is a schematic side view illustrating the retard roller module in a sheet-returning status according to the first embodiment of the present invention; 
           [0038]      FIGS. 6A ,  6 B and  6 C are schematic views illustrating the process of disassembling/assembling the retard roller module according to the first embodiment of the present invention; 
           [0039]      FIG. 7  is a schematic cross-sectional view illustrating the retard roller of the retard roller module according to a second embodiment of the present invention; 
           [0040]      FIG. 8  is a schematic exploded view illustrating the retard roller module according to a third embodiment of the present invention; and 
           [0041]      FIG. 9  is a schematic cross-sectional view illustrating the retard roller module according to the third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0042]    The present invention provides a retard roller module for use in an automatic document feeder.  FIG. 1  is a schematic side view illustrating a retard roller module in a sheet-feeding status according to a first embodiment of the present invention. The retard roller module is mounted in an automatic document feeder. As shown in  FIG. 1 , the automatic document feeder  1  comprises a retard roller module  10  and a sheet pick-up mechanism  20 . The sheet pick-up mechanism  20  is used for feeding a first document S 1  and a second document S 2  into an internal portion of the automatic document feeder  1 . The retard roller module  10  is used to provide a frictional force to the second document S 2 . Due to the frictional force, the first document S 1  lying on the second document S 2  could be separated from the second document S 2 . 
         [0043]    Hereinafter, the configurations of the retard roller module  10  will be illustrated with reference to  FIGS. 2 and 3 .  FIG. 2  is a schematic exploded view illustrating the retard roller module according to the first embodiment of the present invention.  FIG. 3  is a schematic assembled view illustrating the retard roller module according to the first embodiment of the present invention. The retard roller module  10  comprises a retard roller frame  101 , a retard roller  102  and an elastic element  103 . The retard roller  102  is installed on the retard roller frame  101 . The elastic element  103  is disposed on the retard roller frame  101 . The elastic element  103  has a torsion spring arm  1031 . The torsion spring arm  1031  is contacted with the retard roller  102  for providing an elastic normal force on the retard roller  102 , so that the retard roller  102  is movable upwardly and downwardly with respect to the retard roller frame  101 . In this embodiment, the elastic element  103  is a supporting torsion spring. 
         [0044]    The configurations of the retard roller  102  will be illustrated in  FIG. 4 , which is a schematic cross-sectional view illustrating the retard roller of the retard roller module according to the first embodiment of the present invention. The retard roller  102 , which is installed on the retard roller frame  101 , comprises a sleeve  1021 , a separation pad  1022 , a helical spring  1023  and a rotating shaft  1024 . The sleeve  1021  has a sleeve inner wall  10211 . The separation pad  1022  is sheathed around the sleeve  1021 . When the separation pad  1022  is contacted with the second document S 2 , a frictional force is generated. In this embodiment, the separation pad  1022  is a rubbery wheel. The helical spring  1023  is disposed within the sleeve  1021 . The helical spring  1023  comprises a first spring segment  10231  and a second spring segment  10232 . The first spring segment  10231  has a first spring inner diameter r 1 . The second spring segment  10232  has a second spring inner diameter r 2 . The first spring segment  10231  is contacted with the sleeve inner wall  10211 . The second spring segment  10232  is separated from the sleeve inner wall  10211 . The second spring inner diameter r 2  is smaller than the first spring inner diameter r 1 . The first spring segment  10231  is eccentrically connected with the second spring segment  10232 . The rotating shaft  1024  is penetrated through the helical spring  1023  and contacted with the supporting torsion spring  103  for receiving the elastic normal force, which is provided by the supporting torsion spring  103 . The rotating shaft  1024  further comprises a confining edge  10241 . When the confining edge  10241  is fixed on the retard roller frame  101 , the rotating shaft  1024  is fixed and fails to be rotated (see  FIGS. 2 and 3 ). 
         [0045]    Please refer to  FIG. 1  again. For feeding the first document S 1  and the second document S 2  by the automatic document feeder  1 , the sheet pick-up roller and the sheet separation roller of the sheet pick-up mechanism  20  are rotated in a second rotating direction C 2  to transport the first document S 1  and the second document S 2 . As such, the first document S 1  and the second document S 2  are moved in a first direction A 1 . The separation pad  1022  of the retard roller  102  is contacted with the second document S 2 , so that the retard roller  102  is rotated in a first rotating direction C 1 . The first rotating direction C 1  is opposed to the second rotating direction C 2 . In this embodiment, the first direction A 1  is a sheet-feeding direction, the first rotating direction C 1  is an anti-clockwise direction, and the second rotating direction C 2  is a clockwise direction. When the first document S 1  and the second document S 2  are transported and moved in the first direction A 1 , the first document S 1  and the second document S 2  are sustained against the retard roller  102 , so that the retard roller  102  is moved downwardly with respect to the retard roller frame  101 . As such, the first document S 1  and the second document S 2  are allowed to be fed into the internal portion of the automatic document feeder  1  through the region between the sheet pick-up mechanism  20  and the retard roller module  10 . 
         [0046]    When the separation pad  1022  of the retard roller  102  is contacted with the second document S 2  and the retard roller  102  is rotated in the first rotating direction C 1 , the sleeve  1021  of the retard roller  102  is also rotated in the first rotating direction C 1 . Since the first spring segment  10231  is contacted with the sleeve inner wall  10211 , the first spring segment  10231  is twisted in the first rotating direction C 1  upon rotation of the sleeve inner wall  10211 . Due to the twisting direction of the helical spring  1023 , the first spring segment  10231  is stretched. As such, the inner diameter of the first spring segment  10231  is widened to be larger than the original first spring inner diameter r 1 . As such, the gap between the first spring segment  10231  and the sleeve inner wall  10211  is shortened, and the interference between the first spring segment  10231  and the sleeve inner wall  10211  is increased, so that the first spring segment  10231  is fixed (tightened) on the sleeve inner wall  10211 . At the same time, the second spring segment  10232  is twisted with respect to the rotating shaft  1024 . In addition, for overcoming an inner stress resulted from the interference between the second spring segment  10232  and the rotating shaft  1024 , the second spring segment  10232  generates a first damping torque T 1 . In response to the first damping torque T 1 , the retard roller  102  provides a first frictional force to the second document S 2 , so that the second document S 2  fails to be transported. At the same time, the first document S 1  is allowed to be transported in the first direction A 1  by the sheet pick-up mechanism  20 . 
         [0047]      FIG. 5  is a schematic side view illustrating the retard roller module in a sheet-returning status according to the first embodiment of the present invention. In a case that the first document S 1  is jammed in the internal portion of the automatic document feeder  1 , the jammed first document S 1  needs to be removed from the automatic document feeder  1 . For removing the jammed first document S 1 , the jammed first document S 1  needs to be moved in a second direction A 2 , which is opposed to the first direction A 1 . In this embodiment, the second direction A 2  is a sheet-returning direction. When the first document S 1  is moved in the second direction A 2 , the retard roller  102  is rotated in the second rotating direction C 2  because the separation pad  1022  of the retard roller  102  is contacted with the second document S 2 . As such, the sleeve  1021  of the retard roller  102  is also rotated in the second rotating direction C 2 . Due to the twisting direction of the helical spring  1022 , the second spring segment  10232  is fixed (tightened) on the rotating shaft  1024 . Since the first spring segment  10231  is contacted with the sleeve inner wall  10211 , the first spring segment  10231  is twisted in the second rotating direction C 2  upon rotation of the sleeve inner wall  10211 . In addition, for overcoming an inner stress resulted from the interference between the first spring segment  10231  and the sleeve inner wall  10211 , the first spring segment  10231  generates a second damping torque T 2 . In response to the second damping torque T 2 , the retard roller  102  provides a second frictional force to the second document S 2 . Since the interference between the first spring segment  10231  and the sleeve inner wall  10211  is very low, the second damping torque T 2  is very low. In other words, the second frictional force corresponding to the second damping torque T 2  is also very low. Without obvious obstruction, the second document S 2  could be smoothly moved in the second direction A 2  to be removed from the automatic document feeder  1 . 
         [0048]    It is noted that the damping torque is in direct proportion to the frictional force. As the first damping torque T 1  is increased, the first frictional force is increased. Whereas, as the second damping torque T 2  is decreased, the second frictional force is decreased. 
         [0049]      FIGS. 6A ,  6 B and  6 C are schematic views illustrating the process of disassembling/assembling the retard roller module according to the first embodiment of the present invention. For disassembled the retard roller module  10  from the automatic document feeder  1 , the bilateral sides of the retard roller  102  are firstly held by the user&#39;s hands, then the retard roller module  10  is turned, and finally the retard roller module  10  is detached. On the other hand, the user may assemble the retard roller module  10  in the automatic document feeder  1  in the sequence of the steps shown in  FIG. 6C ,  FIG. 6B  and  FIG. 6A . 
         [0050]    Another exemplary retard roller and another exemplary retard roller assembly will be illustrated with reference to  FIG. 7 .  FIG. 7  is a schematic cross-sectional view illustrating the retard roller of the retard roller module according to a second embodiment of the present invention. Except for the retard roller, the configurations of the other components included in the automatic document feeder of this embodiment are similar to those illustrated in the first embodiment, and are not redundantly described herein. As shown in  FIG. 7 , the retard roller  202  comprises a sleeve  2021 , a separation pad  2022 , a helical spring  2023 , a rotating shaft  2024  and a receiving shaft  2025 . The sleeve  2021  comprises a first inner wall part  20211  and a second inner wall part  20212 . The first inner wall part  20211  has a first sleeve inner diameter d 1 . The first inner wall part  20211  is near a first end of the sleeve  2021 . The second inner wall part  20212  has a second sleeve inner diameter d 2 . The second inner wall part  20212  is near a second end of the sleeve  2021 . The first sleeve inner diameter d 1  is smaller than the second sleeve inner diameter d 2 . The inner wall of the sleeve  2021  is an inclined wall. That is, the inner wall of the sleeve  2021  is cone-shaped wall. 
         [0051]    The separation pad  2022  is sheathed around the sleeve  2021 . When the separation pad  2022  is contacted with the second document S 2 , a frictional force is generated. In this embodiment, the separation pad  2022  is a rubbery wheel. The helical spring  2023  is disposed within the sleeve  2021 . The helical spring  2023  comprises a first spring segment  20231  and a second spring segment  20232 . The first spring segment  20231  has a first spring inner diameter r 1 . The second spring segment  20232  has a second spring inner diameter r 2 . The first spring segment  20231  is contacted with the sleeve inner wall  20211 . The second spring segment  20232  is separated from the sleeve inner wall  20211 . The second spring inner diameter r 2  is smaller than the first spring inner diameter r 1 . The first spring segment  20231  is eccentrically connected with the second spring segment  20232 . The rotating shaft  2024  is penetrated through the helical spring  2023  and inserted into the second end of the sleeve  2021 . The receiving shaft  2025  is inserted into the first end of the sleeve  2021  for receiving the rotating shaft  2024 . 
         [0052]    Hereinafter, the operating principles of the retard roller  202  when the automatic document feeder is in the sheet-feeding status will be illustrated in more details. Except for the retard roller, the operating principles of the other components included in the automatic document feeder of this embodiment are similar to those illustrated in the first embodiment, and are not redundantly described herein. When the first document and the second document are fed, the separation pad  2022  of the retard roller  202  is contacted with the second document. As such, the retard roller  202  is rotated in the first rotating direction, and the sleeve  2021  of the retard roller  202  is also rotated in the first rotating direction. Since the first spring segment  20231  is contacted with the first inner wall part  20211 , the first spring segment  20231  is twisted in the first rotating direction upon rotation of the first inner wall part  20211 . Due to the twisting direction of the helical spring  2022 , the first spring segment  20231  is stretched. As such, the first spring segment  20231  is fixed on the first inner wall part  20211 . At the same time, the second spring segment  20232  is twisted with respect to the rotating shaft  2024 . In addition, for overcoming an inner stress resulted from the interference between the second spring segment  20232  and the rotating shaft  2024 , the second spring segment  10232  generates a first damping torque. In response to the first damping torque, the retard roller  202  provides a first frictional force to the second document, so that the second document fails to be transported. At the same time, the first document is allowed to be transported in the first direction by the sheet pick-up mechanism  20 . 
         [0053]    In a case that the first document is jammed in the internal portion of the automatic document feeder, the jammed first document needs to be removed from the automatic document feeder. For removing the jammed first document, the jammed first document needs to be moved in a second direction, which is opposed to the first direction. When the first document is moved in the second direction, the retard roller  202  is rotated in the second rotating direction opposed to the first rotating direction because the separation pad  2022  of the retard roller  202  is contacted with the second document. As such, the sleeve  2021  of the retard roller  202  is also rotated in the second rotating direction. Due to the twisting direction of the helical spring  2022 , the second spring segment  20232  is fixed (tightened) on the rotating shaft  2024 . Since the first spring segment  20231  is contacted with the sleeve inner wall  20211 , the first spring segment  20231  is twisted in the second rotating direction upon rotation of the first inner wall part  20211 . In addition, for overcoming an inner stress resulted from the interference between the first spring segment  20231  and the first inner wall part  20211 , the first spring segment  20231  generates a second damping torque T 2 . In response to the second damping torque T 2 , the retard roller  202  provides a second frictional force to the second document. The second frictional force is nearly zero. As such, the second document could be smoothly moved in the second direction to be removed from the automatic document feeder without obvious obstruction. 
         [0054]    In this embodiment, the sleeve  2021  comprises a first inner wall part  20211  and a second inner wall part  20212 . The inner wall of the sleeve  2021  is substantially an inclined wall. Since the first inner wall part  20211  of the sleeve  2021  is gradually tapered, the interference between the first inner wall part  20211  and the first spring segment  20231  within the sleeve  2021  becomes more uniform. In other words, the damping torque is generated more smoothly and stably. 
         [0055]    A more preferred embodiment is illustrated with reference to  FIGS. 8 and 9 .  FIG. 8  is a schematic exploded view illustrating the retard roller module according to a third embodiment of the present invention.  FIG. 9  is a schematic cross-sectional view illustrating the retard roller module according to the third embodiment of the present invention. The retard roller module  30  comprises a retard roller frame  301 , a retard roller  302  and an elastic element  303 . The retard roller  302  is installed on the retard roller frame  301 . The elastic element  303  is disposed on the retard roller frame  301 , and contacted with the retard roller  302 . The elastic element  303  is used for providing an elastic normal force on the retard roller  302 , so that the retard roller  302  is movable upwardly and downwardly with respect to the retard roller frame  301 . In this embodiment, the elastic element  303  is a supporting torsion spring. The structure of the retard roller  302  will be illustrated as follows. The retard roller  302  comprises a first sleeve  3021 , a second sleeve  3022 , a separation pad  3023 , a helical spring  3024 , a one-way clutch  3025  and a rotating shaft  3026 . The first sleeve  3021  has a sleeve inner tube  30211 . The second sleeve  3022  is accommodated within the first sleeve  3021 . The second sleeve  3022  has a sleeve outer tube  30221 . The sleeve outer tube  30221  is arranged at an end of the second sleeve  3022 , and contacted with an end of the sleeve inner tube  30211 . The tube diameter d 2 ′ of the sleeve inner tube  30211  is smaller than the tube diameter d 1 ′ of the sleeve outer tube  30221 . As such, the interference between the first spring segment  30241  and the sleeve outer tube  30221  is greater than the interference between the second spring segment  30242  and the sleeve inner tube  30211 . 
         [0056]    The separation pad  3022  is sheathed around the first sleeve  3021 . When the separation pad  3022  is contacted with the second document S 2 , a frictional force is generated. In this embodiment, the separation pad  3022  is a rubbery wheel. The helical spring  3024  is disposed within the first sleeve  3021 . An end of the helical spring  3024  is sheathed around the sleeve outer tube  30221  to define a first spring segment  30241 . The other end of the helical spring  3024  is sheathed around the sleeve inner tube  30211  to define a second spring segment  30242 . The first spring segment  30241  has a first spring inner diameter r 1 ′. The second spring segment  30242  has a second spring inner diameter r 2 ′, which is smaller than the first spring inner diameter r 1 ′. The one-way clutch  3025  is accommodated within the second sleeve  3022  for preventing the second sleeve  3022  from rotating in the first rotating direction. That is, due to the one-way clutch  3025 , the second sleeve  3022  is allowed to be rotated in the second rotating direction. The rotating shaft  3026  is penetrated through the first sleeve  3021 , the second sleeve  3022  and the one-way clutch  3025 . The rotating shaft  3026  further comprises a confining edge  30261 . When the confining edge  30261  is fixed on the retard roller frame  301 , the rotating shaft  3026  is fixed and fails to be rotated. 
         [0057]    The operating principles of the retard roller module will be illustrated as follows. For feeding the first document (not shown) and the second document (not shown) by the automatic document feeder, the sheet pick-up mechanism (not shown) is rotated in a second rotating direction to transport the first document and the second document. As such, the first document and the second document are moved in a first direction (not shown). When the first document and the second document are transported and moved in the first direction, the first document and the second document are sustained against the retard roller  302 , so that the retard roller  302  is moved downwardly with respect to the retard roller frame  301 . When the separation pad  3022  of the retard roller  302  is contacted with the second document and the retard roller  302  is rotated in the first rotating direction, the first sleeve  3021  is also rotated in the first rotating direction. Due to the one-way clutch  3025 , the second sleeve  3022  fails to be rotated, so that the first spring segment  30241  is fixed on the sleeve outer tube  30221 . In addition, the second spring segment  30242  is twisted with respect to the sleeve inner tube  30211  to generate a first damping torque. In response to the first damping torque, the retard roller  302  provides a first frictional force to the second document, so that the second document fails to be moved. At the same time, the first document is allowed to be transported in the first direction. 
         [0058]    In a case that the first document is jammed in the internal portion of the automatic document feeder, the jammed first document needs to be removed from the automatic document feeder. For removing the jammed first document, the jammed first document needs to be moved in a second direction, which is opposed to the first direction. When the first document is moved in the second direction, the retard roller  302  is rotated in the second rotating direction because the separation pad  3022  of the retard roller  302  is contacted with the second document. As such, the first sleeve  3021  is also rotated in the second rotating direction. The first spring segment  30241  is fixed on the sleeve outer tube  30221 , and the second spring segment  30242  is fixed on the sleeve inner tube  30211 . The second sleeve  3022  is rotated with the first sleeve  3021 . In addition, the one-way clutch  3025  is rotated with respect to the rotating shaft  3026 , so that a second damping torque is generated. The first damping torque is greater than the second damping torque. Since the one-way clutch  3025  is smoothly rotated with respect to the rotating shaft  3026 , the interference between the one-way clutch  3025  and the rotating shaft  3026  is nearly zero. In other words, the second damping torque is nearly zero, and the second frictional force corresponding to the second damping torque is nearly zero. As such, the second document could be smoothly moved in the second direction to be removed from the automatic document feeder without obvious obstruction. 
         [0059]    From the above description, since the helical spring of the retard roller of the present invention comprises a first spring segment with a larger inner diameter and a second spring segment with a smaller inner diameter, different interference magnitudes are generated by the first spring segment and the second spring segment when the helical spring is twisted. In response to different interference magnitudes, the first damping torque and the second damping torque with different torque magnitudes are generated, wherein the first damping torque is greater than the second damping torque. By mean of the above configurations, the retard roller of the present invention can provides two damping torques with different directions and different magnitudes. In a case that the document is jammed in the automatic document feeder, the automatic document feeder is capable of returning the document in the sheet-returning direction to remove the document without the need of opening the upper cover. 
         [0060]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.