Patent Publication Number: US-10308456-B2

Title: Sheet supplier

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2017-067534, which was filed on Mar. 30, 2017, the disclosure of which is herein incorporated by reference in its entirety. 
     BACKGROUND 
     Technical Field 
     The following disclosure relates to a sheet supplier configured to supply sheets. 
     Description of Related Art 
     There is known a sheet supplier including: a sheet supply roller (pickup roller) configured to contact an uppermost one of sheets stacked on a tray and to rotate; and a separation pad disposed so as to be opposed to the sheet supply roller. The sheet supplier enables the uppermost sheet to be separated from other sheets thereunder and to supply only the uppermost sheet when the sheet supply roller is rotated, by adjusting a frictional force between the separation pad and a lowermost one of the sheets and a frictional force between the sheet supply roller and the uppermost sheet. 
     SUMMARY 
     In a case where a plurality of sheets are placed on the tray of the sheet supplier constructed as described above, a static frictional force between the separation pad and the lowermost sheet needs to be larger than a static frictional force between the sheets, for separating the uppermost sheet from the other sheets thereunder. In a case where only a single sheet is placed on the tray, however, an increase in the static frictional force between the separation pad and the lowermost sheet causes a risk that the sheet fails to be supplied due to the static frictional force between the sheet and the separation pad. For instance, a sheet which is glossy on its surface, such as a sheet used for photo printing, a transfer seal or the like, has a relatively large friction coefficient on the surface, so that such a risk tends to be caused. 
     Accordingly, one aspect of the present disclosure relates to a sheet supplier capable of appropriately supplying sheets even in a situation in which only a single sheet is placed on a tray while preventing an occurrence of multiple feeding of sheets. 
     In one aspect of the disclosure, a sheet supplier includes: a tray including a support surface configured to support a plurality of sheets; a sheet supply roller configured to supply, in a sheet supply direction, a first sheet which is an uppermost one of the plurality of sheets supported on the support surface by rotating about a roller shaft while the sheet supply roller is held in contact with the first sheet; a rotational member configured to be rotatable and to be held in contact with a second sheet which is a lowermost one of the plurality of sheets supported on the support surface in a state in which the plurality of sheets are interposed between the rotational member and the sheet supply roller; a presser configured to press the sheet supply roller relative to the rotational member; and a supporter including a contact portion contacting the rotational member, the supporter being configured to support the rotational member by the contact portion such that the rotational member is rotated by a movement, in the sheet supply direction, of the second sheet with which the rotational member is held in contact and to limit a movement of the rotational member caused by a force of the second sheet in the sheet supply direction; wherein, where a static friction coefficient between the sheet supply roller and the first sheet is defined as μ1, a static friction coefficient between the plurality of sheets supported on the support surface is defined as μ2, a static friction coefficient between the rotational member and the second sheet is defined as μ3, and a static friction coefficient between the rotational member and the contact portion of the supporter is defined as μ4, the following expressions are satisfied: μ1&gt;μ4&gt;μ2, μ3&gt;μ4. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features, advantages, and technical and industrial significance of the present disclosure will be better understood by reading the following detailed description of embodiments, when considered in connection with the accompanying drawings, in which: 
         FIG. 1  is an external perspective view of an ink-jet printer according to one embodiment; 
         FIG. 2  is a schematic vertical cross-sectional view of the ink-jet printer; 
         FIG. 3  is a fragmentary sectional view of a sheet supplier; 
         FIG. 4A  is a view for explaining a sheet supplying condition in a known sheet supplier; 
         FIG. 4B  is a view for explaining the sheet supplying condition in the known sheet supplier; 
         FIG. 5A  is a view for explaining a sheet supplying condition in the sheet supplier according to the embodiment; 
         FIG. 5B  is a view for explaining the sheet supplying condition in the sheet supplier according to the embodiment; 
         FIG. 6A  is a fragmentary sectional view of a sheet supplier according to a modified embodiment; 
         FIG. 6B  is a fragmentary sectional view of a sheet supplier according to a modified embodiment; 
         FIG. 7A  is a fragmentary sectional view of a sheet supplier according to a modified embodiment; 
         FIG. 7B  is a fragmentary sectional view of a sheet supplier according to a modified embodiment; 
         FIG. 8A  is a fragmentary sectional view of a sheet supplier according to a modified embodiment; 
         FIG. 8B  is a view for explaining a biting force; 
         FIG. 8C  is a view for explaining the biting force; and 
         FIG. 9  is a fragmentary sectional view of a sheet supplier according to a modified embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, there will be described a printer  1  having a sheet supplier according to one embodiment. The printer  1  is normally used in a state shown in  FIG. 1 . In the following explanation, an upper side and a lower side are defined in this state. A front side and a rear side are defined by regarding a surface of the printer  1  on which an opening  11  is formed as a front surface, and a right side and a left side are defined in a state in which the printer  1  is seen from the front side. A front-rear direction and a right-left direction are parallel to a horizontal plane, and an up-down direction is a vertical direction perpendicular to the horizontal plane. 
     As shown in  FIG. 1 , the printer  1  includes a housing  1   a  shaped like a generally rectangular parallelepiped. The opening  11  is formed on a front wall of the housing  1   a  so as to be located at a central portion of the front wall in the right-left direction. A sheet supply cassette  21  of a sheet supplier  2  is mounted to a lower portion of the opening  11 . 
     As shown in  FIG. 2 , there are housed, in the housing  1   a , the sheet supplier  2 , a printer portion  3 , a controller  100 , and so on. The sheet supplier  2  is configured to supply sheets S stacked on a sheet supply tray  41  of the sheet supply cassette  21  to the printer portion  3  via a conveyance path  15 . The sheet supplier  2  will be later explained in detail. 
     The conveyance path  15  has a generally C-like shape in side view and is formed by a pair of guides that are opposed to each other with a suitable spacing interposed therebetween. The conveyance path  15  includes: a curved path  15   a  which is connected to a rear end portion of the sheet supply tray  41  so as to extend therefrom upward while curving toward the front side; and a straight path  15   b  which is connected to the curved path  15   a  and which extends substantially straight in the front-rear direction toward a sheet discharge tray  42 . 
     The printer portion  3  is an ink-jet printing device configured to print an image on the sheet S supplied by the sheet supplier  2 . The printer portion  3  includes a carriage  31 , a platen  32 , an ink-jet head  33 , and conveyance roller pairs  34 ,  35 . The carriage  31  is supported, above the straight path  15   b , by two guide rails  38 ,  39  extending in the right-left direction. The carriage  31  is configured to reciprocate in the right-left direction. When a carriage moving device (not shown) is driven under the control of the controller  100 , the carriage  31  moves along the guide rails  38 ,  39  in the right-left direction. 
     The platen  32  is disposed below the carriage  31 . The platen  32  supports, from below, the sheet S supplied from the sheet supplier  2 . The ink-jet head  33  is mounted on the carriage  31  and is configured to move in the right-left direction, together with the carriage  31 . The ink-jet head  33  has a plurality of nozzles  33   a  formed in its lower surface. Ink is ejected from the nozzles  33   a  toward the sheet S supported by the platen  32 . 
     The conveyance roller pairs  34 ,  35  are disposed such that the platen  32  is interposed therebetween in the front-rear direction. The two conveyance roller pairs  34 ,  35  are driven in synchronization with each other by a conveyance motor (not shown). When the two conveyance roller pairs  34 ,  35  are driven, the sheet S supported on the platen  32  is conveyed frontward along the straight path  15   b.    
     The controller  100  includes a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), an application specific integrated circuit (ASIC), which cooperate to control operations of the sheet supplier  2  and the printer portion  3 , for instance. 
     For instance, the controller  100  controls the sheet supplier  2  to execute a sheet supplying processing for supplying the sheet S to the printer portion  3 . Further, the controller  100  controls the printer portion  3  to execute a printing processing by alternatingly performing: an ejection operation in which ink is ejected from the nozzles  33   a  of the ink-jet head  33  toward the sheet S on the platen  32  during one movement of the carriage  31  in the scanning direction; and a conveyance operation in which the conveyance roller pairs  34 ,  35  convey the sheet S frontward by a predetermined distance, for printing an image on the sheet S. 
     The sheet supplier  2  will be next explained in detail. As shown in  FIG. 2 , the sheet supplier  2  includes the sheet supply cassette  21 , a sheet supply roller  22 , an arm  23 , a rotation roller  24 , and a supporter  25 . 
     The sheet supply cassette  21  includes: the sheet supply tray  41  capable of storing a plurality of sheets S; and the sheet discharge tray  42  which is disposed over the sheet supply tray  41  and to which is discharged the sheet S on which an image has been printed by the printer portion  3 . 
     The sheet supply tray  41  is shaped like a box opening upward. An upper surface of a bottom wall  41 B of the sheet supply tray  41  is a support surface  41 Ba on which a stack of a plurality of sheets S can be placed. The sheet supply tray  41  is capable of storing sheets of a plurality of types such as plain paper, glossy paper, transfer paper for iron printing and the like. 
     The arm  23  is supported by a housing  1   a  (as one example of “support portion”) such that the arm  23  is pivotable about a pivot shaft  23   x  provided at its basal portion. The pivot shaft  23   x  extends in the right-left direction and is disposed at a height level higher than the support surface  41 Ba of the sheet supply tray  41  in the up-down direction. 
     A roller shaft  22   x  extending in the right-left direction is provided at a distal portion of the arm  23 . The sheet supply roller  22  is rotatable about the roller shaft  22   x . A range over which the arm  23  is pivoted is set such that a lower limit of the range is defined by the support surface  41 Ba of the sheet supply tray  41  (the rotation roller  24 ), so as to permit the distal portion of the arm  23  to be always located more rearward than the basal portion thereof. Thus, the roller shaft  22   x  of the sheet supply roller  22  is always located more rearward than the pivot shaft  23   x . Further, the pivot shaft  23   x  is disposed at a height level higher than the roller shaft  22   x , namely, the pivot shaft  23   x  is located farther from the support surface  41 B than the roller shaft  22   x.    
     A position of the center of gravity of the arm  23  is located at a position of the arm  23  nearer to the distal portion than to the basal portion. With this configuration, there is generated, in the arm  23 , a rotational torque by its own weight in a direction in which the distal portion is located right under the pivot shaft  23   x . That is, the arm  23  is biased in a direction in which the distal portion of the arm  23  gets closer to the support surface  41 Ba (the rotation roller  24 ). Thus, the sheet supply roller  22  provided at the distal portion of the arm  23  contacts an uppermost one of the plurality of sheet S stacked on the support surface  41 Ba of the sheet supply tray  41  and presses the uppermost sheet S. In this respect, the arm  23  may be biased by a spring in the direction in which the distal portion of the arm  23  gets closer to the support surface  41 Ba. 
     A supply motor (not shown) is connected to the pivot shaft  23   x . In the arm  23 , a gear transmission mechanism (not shown) is provided for transmission of a drive force between the pivot shaft  23   x  and the roller shaft  22   x  of the sheet supply roller  22 . The gear transmission mechanism includes a plurality of gears and planetary gears. Under the control of the controller  100 , the supply motor is driven so as to rotate the pivot shaft  23   x , so that the sheet supply roller  22  rotates clockwise in  FIGS. 2 and 3 . Thus, the uppermost one of plurality of sheets S placed on the support surface  41 Ba is supplied in a direction toward the conveyance path  15 . (Hereinafter, the direction will be referred to as “sheet supply direction”.) 
     The sheet supply tray  41  includes a separation wall  41   w . The separation wall  41   w  is constituted by one of four walls of the sheet supply tray  41  that is located downstream of the sheet supply roller  22  in the sheet supply direction (on the left side in  FIG. 1 ). When a plurality of sheets S are supplied at one time in an overlapping state by rotation of the sheet supply roller  22 , the separation wall  41   w  comes into contact with one of the sheets S which is farthest from the sheet supply roller  22  and gives the farthest sheet S to a resistance to conveyance, so as to separate the uppermost sheet S contacting the sheet supply roller  22  from other sheets S that have been supplied with the uppermost sheet. To this end, the separation wall  41   w  includes a separation member (not shown) attached thereto. The separation member may be a plate member formed of a material having a large frictional resistance such as cork or rubber or may be a member having a plurality of protrusions formed of resin or metal. 
     A recess  41 Bb is formed in a bottom wall  41 B of the sheet supply tray  41  at a position at which the recess  41 Bb is opposed to the sheet supply roller  22  with the sheets S placed on the support surface  41 Ba interposed therebetween. The recess  41 Bb is open upward. A space in a rectangular parallelepiped shape is defined in the recess  41 Bb. 
     The rotation roller  24  is accommodated in the space defined in the recess  41 Bb. That is, the rotation roller  24  is opposed to the sheet supply roller  22  with the sheets S placed on the support surface  41 Ba interposed therebetween. With this configuration, the rotation roller  24  receives a pressing force from the sheet supply roller  22  by the arm  23  biased as described above. 
     The rotation roller  24  is formed of rubber or the like having high hardness (e.g., not lower than 90 degrees) at which a friction coefficient is unlikely to change even if an applied pressure changes. An outer circumferential surface of the rotation roller  24  is held in contact with a lowermost one of the sheets S placed on the support surface  41 Ba. The rotation roller  24  has a diameter smaller than a distance between a front wall and a rear wall of the recess  41 Bb in the front-rear direction. A rotation shaft  24   x  of the rotation roller  24  protrudes at its opposite end portions outward from the roller body in the right-left direction. An upper end of the rotation roller  24  is located at a height level higher than the support surface  41 Ba. 
     The supporter  25  supports the rotation roller  24  such that the rotation roller  24  is rotated by a movement, in the sheet supply direction, of the sheet S with which the rotation roller  24  is held in contact. As shown in  FIG. 3 , the supporter  25  includes two pairs of stoppers  51  (only one of which is illustrated in  FIG. 3 ) and a friction pad  52 . The two pairs of stoppers  51  are disposed so as to sandwich the roller body of the rotation roller  24  therebetween in the right-left direction. Each stopper  51  includes a pair of stopper walls  51   a ,  51   b . Each stopper wall  51   a ,  51   b  is a vertical wall extending upward from a bottom surface of the recess  41 Bb. The stopper walls  51   a ,  51   b  are disposed so as to sandwich the rotation shaft  24   x  of the rotation roller  24  therebetween in the front-rear direction. A distance by which the stopper walls  51   a ,  51   b  are spaced apart from each other in the front-rear direction is larger than a diameter of the rotation shaft  24   x  and is smaller than the diameter of the rotation roller  24 . The two pairs of stoppers  51  limit a movement of the rotation shaft  24   x  in the front-rear direction (i.e., a movement in the sheet supply direction and a movement in a direction opposite to the sheet supply direction) while allowing a movement of the rotation shaft  24   x  in the up-down direction. In other words, the two pairs of stoppers  51  limit a movement of the rotation roller  24  in the front-rear direction while allowing a movement of the rotation roller  24  in the up-down direction (i.e., a direction of pressing contact of the sheet supply roller  22  with respect to the rotation roller  24 ). Further, the two pairs of stoppers  51  position the rotation roller  24  such that the outer circumferential surface of the rotation roller  24  does not come into contact with the front wall and the rear wall of the recess  41 Bb. 
     In the present embodiment, in a state in which no sheets S are placed on the sheet supply tray  41 , the sheet supply roller  22  and the rotation roller  24  are held in contact with each other, and a position of the roller shaft  22   x  of the sheet supply roller  22  in the front-rear direction is the same as a position of the rotation shaft  24   x  of the rotation roller  24  in the front-rear direction. 
     The friction pad  52  is shaped like a plate. The friction pad  52  is disposed on the bottom surface of the recess  41 Bb so as to support the rotation roller  24  from below. That is, the friction pad  52  is held in contact with the outer circumferential surface of the rotation roller  24 . The friction pad  52  is formed of felt or the like having a small frictional resistance. The friction pad  52  has a friction coefficient smaller than that of a printing surface of each of the sheets S of every type placed on the sheet supply tray  41 . A load torque is given to the rotation roller  24  by the friction pad  52 . Thus, the rotation roller  24  does not rotate until a certain rotation force is given to the rotation roller  24 . As described above, the rotation roller  24  is allowed to move in the up-down direction. Consequently, when the sheet supply roller  22  applies the pressing force with respect to the rotation roller  24 , the pressing force is transmitted to the friction pad  52  via the rotation roller  24 . 
     With the configuration described above, when the sheet supply roller  22  is pressed with respect to the rotation roller  24  by the arm  23  in a state in which a plurality of sheets S are placed on the support surface  41 Ba, the same magnitude of a normal force P is generated, based on a relationship of action and reaction, on a line of action connecting the roller shaft  22   x  and the rotation shaft  24   x  between the sheet supply roller  22  and the sheet S, between the sheets S placed on the support surface  41 Ba, between the rotation roller  24  and the sheet S, and between the rotation roller  24  and the friction pad  52 . That is, even when the pressing force applied to the rotation roller  24  from the sheet supply roller  22  changes, the same magnitude of the normal force P is generated between the sheet supply roller  22  and the sheet S, between the sheets S placed on the support surface  41 Ba, between the rotation roller  24  and the sheet S, and between the rotation roller  24  and the friction pad  52 . 
     The sheet supplier  2  according to the present embodiment has a function of preventing the sheets S from being supplied at one time, namely, preventing multiple feeding of the sheets S, in the state in which the plurality of sheets S are placed on the support surface  41 Ba of the sheet supply tray  41 , by a rotation torque of the rotation roller  24 . Further, in a state in which only a single sheet S is placed on the support surface  41 Ba, a rotation force larger than the load torque described above is given to the rotation roller  24  when the sheet supply roller  22  rotates. Thus, the sheet supplier  2  has a function of supplying the sheet S by rotating the rotation roller  24  by a movement of the sheet S in the sheet supply direction. To achieve these functions, there is determined a value relationship among a static friction coefficient μ1 between the sheet supply roller  22  and the sheet S, a static friction coefficient μ2 between the sheets S placed on the support surface  41 Ba, a static friction coefficient μ3 between the rotation roller  24  and the sheet S, and a static friction coefficient μ4 between the rotation roller  24  and the friction pad  52 . 
     Before explaining the static friction coefficients in the sheet supplier  2  of the present embodiment, there will be explained a structure of a conventional sheet supplier  200  by referring to  FIG. 4 . 
     The conventional sheet supplier  200  has a friction pad  250  disposed on the bottom wall  41 B, instead of the rotation roller  24  and the supporter  25  of the sheet supplier  2  of the present embodiment. The friction pad  250  is a plate member formed of a material having a large frictional resistance (such as cork or rubber). The friction pad  250  is opposed to the sheet supply roller  22  with the sheets S placed on the support surface  41 Ba interposed therebetween. The friction pad  250  is held in contact with a lowermost one of the plurality of sheets S placed on the support surface  41 Ba. When the sheet supply roller  22  is pressed with respect to the friction pad  250  by the arm  23  in the state in which the plurality of the sheets S are placed on the support surface  41 Ba, the same magnitude of the normal force P is generated between the sheet supply roller  22  and the sheet S, between the sheets S placed on the support surface  41 Ba, and between the friction pad  250  and the sheet S. There will be next explained: a sheet supplying condition in a case where two sheets S, as one example of the plurality of sheets S, are placed on the support surface  41 Ba of the sheet supplier  200 ; and a sheet supplying condition in a case where a single sheet S is placed on the support surface  41 Ba. Further, a static friction coefficient between the friction pad  250  and the sheet S is defined as “static friction coefficient μ5”. 
     Initially, the sheet supplying condition in the case where two sheets S are placed on the support surface  41 Ba of the sheet supplier  200  will be explained. As shown in  FIG. 4A , an upper one of the two sheets S will be referred to as “sheet S 2 ”, and a lower one of the two sheets S will be referred to as “sheet S 1 ”. 
     For permitting the upper sheet S 2  to be moved (supplied) by rotation of the sheet supply roller  22 , a static frictional force (sheet supply force: μ1P) between the sheet supply roller  22  and the sheet S 2  needs to be larger than a static frictional force (μ2P) between the sheet S 1  and the sheet S 2 . On the other hand, for permitting the lower sheet S 1  to stay without being moved, a static frictional force (μ5P between the sheet S 1  and the friction pad  250  needs to be larger than the static frictional force (μ2P) between the sheet S 1  and the sheet S 2 . Thus, the sheet supplying condition in the case where the two sheets S 1 , S 2  are placed needs to include conditions represented by the following expressions (1) and (2):
 
μ1&gt;μ2  (1)
 
μ5&gt;μ2  (2)
 
     Next, the sheet supplying condition in the case where a single sheet S 1  is placed on the support surface  41 Ba will be explained. For permitting the sheet S 1  to be moved by rotation of the sheet supply roller  22 , a static frictional force (μ1P) between the sheet supply roller  22  and the sheet S 1  needs to be larger than the static frictional force (μ5P between the sheet S 1  and the friction pad  25 , as shown in  FIG. 4B . Thus, the sheet supplying condition in the case where the single sheet S 1  is placed needs to include a condition represented by the following expression (3):
 
μ1&gt;μ5  (3)
 
     As described above, the conveyance path  15  has a generally C-like shape in side view. Accordingly, the sheet S is placed on the sheet supply tray  41  such that a printing surface of the sheet S on which an image is to be printed by the printer portion  3  faces toward the support surface  41 Ba and a non-printing surface of the sheet S faces toward the sheet supply roller  22 . The printing surface of glossy paper used for photo printing, a transfer seal or the like has a friction coefficient (frictional resistance) larger than that of the non-printing surface thereof. Accordingly, in a case where the glossy paper is placed on the sheet supply tray  41 , the static friction coefficient μ5 between the friction pad  250  and the printing surface of the sheet S is larger than the static friction coefficient μ1 between the sheet supply roller  22  and the non-printing surface of the sheet S, so that there may arise a possibility that the condition represented by the above expression (3) is not satisfied. Consequently, in the case where a single sheet of the glossy paper is placed on the sheet supply tray  41 , the single sheet of the glossy paper cannot be supplied. 
     For enabling the single sheet of the glossy paper to be supplied even in the case where the single sheet of the glossy paper is placed on the sheet supply tray  41 , it may be considered that the friction pad  250  is formed of a material having a small frictional resistance so as to decrease the static friction coefficient μ 5 . In this case, however, the condition represented by the above expression (2) is not satisfied depending upon the type of the sheets S (e.g., plain paper) stored in the sheet supply tray  41 . As a result, in the case where the two sheets S are placed on the sheet supply tray  41 , there may be a risk that the two sheets S are supplied at one time when the sheet supply roller  22  rotates, namely, the multiple feeding of the sheets S may occur. Thus, the static friction coefficient μ5 changes depending upon the type of the sheets S placed on the sheet supply tray  41 . It is therefore very difficult to satisfy all of the conditions represented by the above expressions (1)-(3) for all types of the sheets S that can be placed on the sheet supply tray  41 . 
     When the last one of the plurality of sheets S placed on the sheet supply tray  41 , namely, the sheet S 1 , is supplied in the conventional sheet supplier  200 , the sheet S 1  is supplied while being in rubbing contact with the friction pad  250 , resulting in a damage to the printing surface of the sheet S 1  or causing a large rubbing noise. 
     In contrast, the sheet supplier  2  of the present embodiment includes the rotation roller  24  and the supporter  25 , in place of the friction pad  250 . In the sheet supplier  2 , the value relationship among the static friction coefficients μ1-μ4 is appropriately set. Thus, the sheet supplier  2  enables good conveyance even when only a single sheet S is placed on the sheet supply tray  41  while the sheet supplier  2  prevents or reduces an occurrence of the multiple feeding of the sheets S, irrespective of what type of the sheets S are placed on the sheet supply tray  41 . The sheet supplier  2  will be explained in detail. 
     Initially, there will be explained a sheet supplying condition in a case where the two sheets S 1 , S 2  are placed on the support surface  41 Ba of the sheet supplier  2  according to the present embodiment. As shown in  FIG. 5A , for permitting the upper sheet S 2  to be moved by rotation of the sheet supply roller  22 , the static frictional force (μ1P) between the sheet supply roller  22  and the sheet S 2  needs to be larger than the static frictional force (μ2P) between the sheet S 1  and the sheet S 2 . 
     On the other hand, for permitting the lower sheet S 1  to stay without being moved, a static frictional force (μ3P) between the sheet S 1  and the rotation roller  24  needs to be larger than the static frictional force (μ2P) between the sheet S 1  and the sheet S 2 . As described above, in the present embodiment, the multiple feeding of the sheets S is prevented by the rotation torque of the rotation roller  24 . That is, a static frictional force (μ4P) between the rotation roller  24  and the friction pad  52  is made larger than the static frictional force (μ2P) between the sheet S 1  and the sheet S 2 , so as to make the load torque larger than the rotation force that is given to the rotation roller  24 . Accordingly, the sheet supplying condition in the case where the two sheets S 1 , S 2  are placed includes conditions represented by the following expressions (4) and (5), in addition to the above expression (1):
 
μ3&gt;μ2  (4)
 
μ4&gt;μ2  (5)
 
     Next, there will be explained a sheet supplying condition in a case where a single sheet S 1  is placed on the support surface  41 Ba. In this instance, by giving, to the rotation roller  24 , the rotation force not smaller than the load torque, the rotation roller  24  is rotated by a movement of the sheet S in the sheet supply direction. Accordingly, both of: the static frictional force (μ1P) between the sheet supply roller  22  and the sheet S 1 ; and the static frictional force (μ3P) between the rotation roller  24  and the sheet S 1  need to be larger than the static frictional force (μ4P) between the rotation roller  24  and the friction pad  52 . Thus, the sheet supplying condition when the single sheet S 1  is placed need to include conditions represented by the following expressions (6) and (7):
 
μ1&gt;μ4  (6)
 
μ3&gt;μ4  (7)
 
     Summing up the expressions (1) and (4)-(7), it is needed to satisfy the condition represented by the expression (7) and a condition represented by the following expression (8):
 
μ1&gt;μ4&gt;μ2  (8)
 
     The static friction coefficient μ3 is the static friction coefficient between the rotation roller  24  and the sheet S, and the static friction coefficient μ4 is the static friction coefficient between the rotation roller  24  and the friction pad  52 . It is accordingly possible to satisfy the condition represented by the expression (7) by setting the friction coefficient of the friction pad  52  to be smaller than the friction coefficient of the printing surface of any type of the sheets S placed on the sheet supply tray  41 . 
     The static friction coefficient μ4 is the static friction coefficient between the rotation roller  24  and the friction pad  52  and does not depend on the type of the sheets S placed on the sheet supply tray  41 . It is thus possible to set the static friction coefficient μ4 to be larger than the static friction coefficient μ2 between the printing surface and the non-printing surface of any type of the sheets S placed on the sheet supply tray  41 . Further, by setting the friction coefficient of the sheet supply roller  22  to be larger than the friction coefficient of the non-printing surface of any type of the sheets S placed on the sheet supply tray  41 , it is possible to set the static friction coefficient μ1 to be larger than the static friction coefficient μ 2 . Consequently, the expression (8) can be satisfied. 
     Thus, the static friction coefficients μ1-μ4 can satisfy the conditions represented by the above expressions (7) and (8) irrespective of what type of the sheets S are placed on the sheet supply tray  41 . In the present embodiment, the respective friction coefficients of the sheet supply roller  22 , the rotation roller  24 , and the friction pad  52  are set so as to satisfy the conditions represented by the expressions (7) and (8). 
     For example, the static friction coefficient μ2 between the sheets S placed on the sheet supply tray  41  is 0.2 for plain paper and 0.8 for glossy paper. Thus, the static friction coefficient μ2 is set to fall within a range of 0.2-0.8. Accordingly, the static friction coefficient μ1 is set to fall within a range of 1.3-2.0, the static friction coefficient μ3 is set to fall within a range of 1.3-2.0, and the static friction coefficient μ4 is set to fall within a range of 0.8-1.3. 
     According to the present embodiment, the static friction coefficients μ1-μ4 are set to satisfy the conditions represented by the expressions (7) and (8). In the state in which the plurality of sheets S are placed on the sheet supply tray  41 , even when the sheet supply roller  22  is rotated while being held in contact with the uppermost sheet S, the lowermost sheet S is not supplied or moved owing to the rotation torque of the rotation roller  24 . In this instance, even in a case where three or more sheets S are placed on the sheet supply tray  41  and a plurality of sheets S other than the lowermost sheet S are supplied at one time, the sheets S are separated by the separation wall  41   w , so that the multiple feeding of the sheets S can be prevented. 
     In the state in which only a single sheet S is placed on the sheet supply tray  41 , the rotation roller  24  is rotated by the sheet supply force given to the sheet S by rotation of the sheet supply roller  22 , whereby the single sheet S can be supplied. When the single sheet S is supplied, the rotation roller  24  is rotated by the movement of the sheet S, making is possible to prevent the single sheet S from being damaged and to prevent a large rubbing noise from being generated. 
     In the embodiment explained above, the arm  23  is one example of “presser”, the rotation roller  24  is one example of “rotational member”, and the friction pad  52  is one example of “contact portion”. 
     Modified Embodiments 
     There will be next explained sheet suppliers according to various modified embodiments. In the following modified embodiments, the same reference signs as used in the illustrated embodiment are used to identify the corresponding components and functional portions, and a detailed explanation thereof is dispensed with. 
     A sheet supplier  102  shown in  FIG. 6A  will be explained. The sheet supplier  102  differs from the sheet supplier  2  in the structure of the supporter supporting the rotation roller  24 . A supporter  125  of the sheet supplier  102  includes an arm  151 , in place of the two pairs of the stoppers  51 . The arm  151  is accommodated in the recess  41 Bb. The arm  151  is supported by side walls of the recess  41 Bb so as to be pivotable about a pivot shaft  151   x  provided at a basal portion of the arm  151 . The pivot shaft  151   x  extends in the right-left direction. 
     The rotation shaft  24   x  of the rotation roller  24  is disposed at a distal portion of the arm  151 , and the rotation roller  24  is rotatably supported by the rotation shaft  24   x . In the sheet supplier  102  according to this modified embodiment, the arm  151  allows a movement of the rotation roller  24  in the up-down direction which is a direction of a pivotal movement of the arm  151  while limiting a movement of the rotation roller  24  in the front-rear direction. Consequently, when the sheet supply roller  22  applies the pressing force to the rotation roller  24 , the arm  151  is pivoted, and the pressing force can be transmitted to the friction pad  52  via the rotation roller  24 . 
     A sheet supplier  202  shown in  FIG. 6B  will be explained. The sheet supplier  202  differs from the sheet supplier  2  in the structure of the supporter supporting the rotation roller  24 . A supporter  225  of the sheet supplier  202  does not include the friction pad  52 . Instead, the supporter  225  includes a rotation shaft  224 X fixed to the housing  1   a . The rotation roller  24  is rotatably supported by the rotation shaft  224 X. An outer circumferential surface of the rotation shaft  224 X is processed to have a predetermined frictional resistance for giving the load torque to the rotation roller  24 . Specifically, the conditions represented by the expressions (7) and (8) are satisfied by using, as the static friction coefficient μ4, a static friction coefficient between the rotation roller  24  and the rotation shaft  224 X. With this configuration, the sheet supplier  202  offers advantages similar to those in the illustrated embodiment. In this modified embodiment, the rotation shaft  224 X is one example of “contact portion”. 
     A sheet supplier  302  shown in  FIG. 7A  will be explained. The sheet supply roller  22  is held in contact with an uppermost one of the plurality of sheets S placed on the support surface  41 Ba. When the number of the sheets S placed on the support surface  41 Ba changes, a position of the arm  23  in the up-down direction changes as a result of the pivotal movement of the arm  23  about the pivot shaft  23   x . Accordingly, when the arm  23  is pivoted, not only a position of the sheet supply roller  22  in the up-down direction but also a position of the sheet supply roller  22  in the front-rear direction changes. As a result, a contact position of the sheet supply roller  22  and the sheet S shifts in the front-rear direction. If the contact position thus shifts, there may arise a possibility that the sheet supply roller  22  cannot give the rotation roller  24  the pressing force or a possibility that the sheet S cannot be nipped between the sheet supply roller  22  and the rotation roller  24  in a case where only one rotation roller  24  is provided. 
     In the sheet supplier  302 , a plurality of the rotation rollers  24  are provided over a shift range which ranges, in the front-rear direction, from the contact position in a case where a maximum number of the sheets S are placed on the sheet supply tray  41  to the contact position in a case where a single sheet S is placed on the sheet supply tray  41 . A supporter  325  supporting the rotation rollers  24  includes: a friction pad  52  which is common to the rotation rollers  24  and which supports the rotation rollers  24  from below; and two pairs of the stoppers  51  (not illustrated in  FIG. 7A ) for each rotation roller  24 , for limiting a movement of the rotation rollers  24 . According to this modified embodiment, even when the number of the sheets S placed on the sheet supply tray  41  changes, the sheet supply roller  22  applies the pressing force with respect to at least any one of the rotation rollers  24 , whereby the sheet S can be nipped by the sheet supply roller  22  and the rotation roller  24  to which the pressing force is given by the sheet supply roller  22 . As a result, the sheet S can be appropriately conveyed even when only a single sheet S is placed on the sheet supply tray  41  while the sheet supplier  302  prevents an occurrence of the multiple feeding of the sheets S. 
     A sheet supplier  402  shown in  FIG. 7B  will be explained. The sheet supplier  402  differs from the sheet supplier  2  in the structures of the rotational member and the supporter. In the sheet supplier  402 , an endless belt  429  functions as the rotational member. A supporter  425  includes two pulleys  426 ,  427  and a friction pad  428 . The pulleys  426 ,  427  are disposed so as to be spaced apart from each other in the front-rear direction. The belt  429  is looped over the two pulleys  426 ,  427 . An outer surface of the belt  429  at an upper portion of the loop of the belt  429  is held in contact with the sheet S placed on the support surface  41 Ba. The friction pad  428  is held in contact with an inner surface of the belt  429  at the upper portion of the loop of the belt  429 , so as to support the belt  429  on an inner side of the loop of the belt  429 . The friction pad  428  is provided at least over the shift range of the contact position of the sheet supply roller  22  and the sheet S. 
     With the configuration described above, when the sheet supply roller  22  is pressed by the arm  23  with respect to the belt  429 , the same magnitude of the normal force P is generated between the sheet supply roller  22  and the sheet S, between the sheets S placed on the support surface  41 Ba, between the belt  429  and the sheet S, and between the belt  429  and the friction pad  428 . A static friction coefficient between the belt  429  and the sheet S is defined as the static friction coefficient μ3, and a static friction coefficient between the belt  429  and the friction pad  428  is defined as the static friction coefficient μ4, so as to satisfy the conditions represented by the expressions (7) and (8). Thus, the sheet supplier  402  according to this modified embodiment offers advantages similar to those in the illustrated embodiment. The friction pad  428  is provided over the shift range of the contact position of the sheet supply roller  22  and the sheet S. Accordingly, even when the number of the sheets S placed on the sheet supply tray  41  changes, the sheet supplier  402  achieves appropriate conveyance even in a situation in which only a single sheet S is placed on the sheet supply tray  41  while the sheet supplier  402  prevents an occurrence of the multiple feeding of the sheets S. 
     A sheet supplier  502  shown in  FIG. 8A  will be explained. In the sheet supplier  502  according to this modified embodiment, the rotation shaft  24   x  of the rotation roller  24  is located more frontward than the roller shaft  22   x  of the sheet supply roller  22 . That is, an angle θb formed by: (i) an upstream region  41 Ba 1  of the support surface  41 Ba located frontward of, namely, located upstream in the sheet supply direction of, a nip position (as one example of “nip portion”) at which the sheets S are nipped by the sheet supply roller  22  and the rotation roller  24 ; and (ii) a plane connecting a center axis of the roller shaft  22   x  and the nip position is an obtuse angle. With this configuration, the sheet S can be supplied by a smaller sheet supply force. This modified embodiment will be explained in detail. 
     A direction in which the sheet S is supplied by the sheet supply roller  22  and the rotation roller  24  coincides with a tangential direction of the sheet supply roller  22  and the rotation roller  24  at the nip position. As shown in  FIGS. 8B and 8C , a pressing force F when the sheet supply roller  22  presses the rotation roller  24  by the arm  23  is resolved into the normal force P and a biting force I parallel to the tangential direction. A direction of the biting force I is opposite to the supply direction of the sheet S. Accordingly, a larger sheet supply force is required with an increase in the biting force I. 
     The biting force I increases with an increase in an angle θt formed by: a straight line connecting the pivot shaft  23   x  and the nip position; and the tangential direction (the sheet supply direction). Accordingly, the angle θt is smaller in an arrangement of the sheet supplier  502  in which the rotation shaft  24   x  is located more frontward than the roller shaft  22   x , i.e., an arrangement in which the angle θb is an obtuse angle ( FIG. 8C ) than an arrangement in which the roller shaft  22   x  and the rotation shaft  24   x  are located at the same position in the front-rear direction, i.e., an arrangement in which the angle θb is 90° ( FIG. 8B ), so that the biting force I is smaller in the arrangement shown in  FIG. 8C  than the arrangement shown in  FIG. 8B . It is consequently possible to supply the sheet S by a smaller sheet supply force. 
     In the sheet supplier  502 , the sheet supply direction in which the sheet S is supplied by the sheet supply roller  22  and the rotation roller  24  includes a vertically downward component, as shown in  FIG. 8A . In view of this, a downstream region  41 Ba 2  of the support surface  41 Ba located downstream in the sheet supply direction of, namely, located rearward of, the nip position may have a portion which is recessed downward to a level lower than the upstream region  41 Ba 1  and which is located within an area distant from the nip position by a predetermined distance. In this instance, a conveyance load that the sheet S receives from the support surface  41 Ba is reduced, so that it is possible to convey the sheet S with a smaller sheet supply force. 
     While the embodiments of the present disclosure have been described above, it is to be understood that the disclosure is not limited to the details of the illustrated embodiments, but may be embodied with other various changes which may occur to those skilled in the art, without departing from the scope of the disclosure. The sheet supplying condition in the case where a single sheet S 1  is placed on the support surface  41 Ba may include a condition that the static frictional force (μ1P) between the sheet supply roller  22  and the sheet S 1  is larger than the static frictional force (μ3P) between the rotation roller  24  and the sheet S 1 , for preventing an occurrence of slippage between the sheet supply roller  22  and the sheet S. In other words, the condition represented by the following expression (9) may be included:
 
μ1&gt;μ3  (9)
 
     In the illustrated embodiment, the arm  23  functions as the presser configured to press the sheet supply roller  22  with respect to the rotation roller  24 . The present disclosure is not limited to this configuration. The presser may be configured otherwise as in a sheet supplier  602  shown in  FIG. 9 , for instance. In the sheet supplier  602 , the position of the sheet supply roller  22  is fixed. Further, the rotation roller  24  is disposed in a presser plate  640  which is provided near the bottom of the sheet supply tray  41  and which is biased upward. In this configuration, the rotation roller  24  is pressed toward the sheet supply roller  22 . That is, the sheet supplier  602  includes the presser plate  640  and a presser-plate moving mechanism (not shown). On the presser plate  640 , rear end portions (trailing end portions) of the sheets S placed on the sheet supply tray  41  are disposed. The presser plate  640  pivots about a rotation shaft provided at its front end portion, so that a rear end portion of the presser plate  640  is moved upward and downward. The presser-plate moving mechanism is configured to move the rear end portion of the presser plate  640  upward, under the control of the controller, by an amount corresponding to a decrease in the sheets S placed on the presser plate  640  in accordance with the decrease of the sheets S. In the sheet supplier  602 , the presser plate  640  includes the rotation roller  24  and the supporter  25 , and the rotation roller  24  can be pressed with respect to the sheet supply roller  22 . 
     In the illustrated embodiment, the conveyance path  15  has a C-like shape in side view. The present disclosure is not limited to this configuration. The conveyance path may be straight at a portion from the sheet supplier to the printer portion  3 . In this case, the printing surface of each of the sheets S placed on the sheet supply tray  41  faces toward the sheet supply roller  22 , and the non-printing surface thereof faces toward the support surface  41 Ba. 
     The printer portion  3  is not limited to the ink-jet printing device but may be a thermal or laser printing device. The present disclosure may be applicable to a facsimile, a copying machine, or a multi-function peripheral other than the printer. The sheet supplier need not necessarily have a recording portion. The sheet is not limited to paper but may be a cloth, for instance.