Patent Publication Number: US-7720429-B2

Title: Image forming apparatus including shutter arm unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Application No. 2005-129504, filed Dec. 26, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Aspects of the present invention relate to an image forming apparatus, and more particularly, to an image forming apparatus which prevents a multi-sheet feed along a transportation path. 
     2. Description of the Related Art 
     Generally, an image forming apparatus, such as, for example, a printer, a copier, a fax machine, or a multifunctional apparatus includes a printing unit which prints an image onto a recording medium and a transportation path which transports the recording medium inside the printing unit. This recording medium may be various different types of media, such as a sheet of paper, a sheet of photographic printing paper, OHP film, etc., and will be referred to as paper hereinafter, but is not limited to paper. Sheets of paper should be moved one by one through the printing unit during printing operations, from the time the sheets enter the printing unit until the time the sheets are discharged from the printing unit to a discharge tray. If two or more sheets of paper are moved through the printing unit simultaneously (a situation referred to as a multi-sheet feed), the extra sheets may cause a paper jam in the image forming apparatus, or may cause a printing defect in the images printed on the sheets of paper, such as by printing images intended to be printed on different sheets of paper onto the same sheet of paper. 
     An image forming apparatus of the related art may include a multi-sheet feed preventing unit which prevents several sheets of paper from being moved through the printer at the same time. Examples of multi-sheet feed preventing units are described below. 
     A knock up plate is disposed in a sheet-feeding cassette which stores the paper that is eventually supplied to the printing unit. The paper is stacked on the upper side of the knock up plate, and a knock up spring elastically biases the lower side of the knock up plate in an upwards direction. Sheets of paper stacked on the upper side of the knock up plate are picked up individually by a pick up roller, and then are supplied to the printing unit by a feed roller. 
     One example of a multi-sheet feed preventing unit includes finger members which are disposed at both sides of the knock up plate, such that the pick up roller picks up the sheets of paper individually (one by one). The finger members apply a transportation obstructing force on the sheets of paper by pressing both corners of the sheets of paper stacked on the knock up plate. The frictional force between the pick up roller and a top sheet of paper stacked on the knock up plate is greater than the frictional force between the top sheet of paper and a next sheet of paper stacked under the top sheet of paper. Furthermore, the frictional force between the pick up roller and a top sheet of paper stacked on the knock up plate is greater than the transportation obstructing force of the finger members. Thus, the frictional force generated by the pick up roller picks up the top sheet of paper, while the frictional force between the finger members and the sheets of paper prevents extra sheets of paper from being picked up, and the sheets are picked up individually. 
     A second example of a multi-sheet feed preventing unit includes a friction pad disposed on the upper side of the knock up plate which frictionally prevents two sheets of paper from being picked up simultaneously by the pick-up roller, when only two sheets of paper are stacked on the upper side of the knock up plate. In this second example, the friction pad is disposed at a position facing the pick up roller housed inside the frame of an image forming apparatus. The frictional force between the pick up roller and a top sheet of paper stacked on the knock up plate is greater than the frictional force between the top sheet of paper and the next sheet of paper stacked under the top sheet of paper. Furthermore, the frictional force between the pick up roller and a top sheet of paper stacked on the knock up plate is greater than the frictional force acting on the friction pad and the paper. Thus, the sheets of paper are picked up individually. 
     As described above, a sheet of paper picked up by the pick up roller is supplied to the printing unit by the feed roller. The sheet of paper is supplied to the printing unit along the transportation path, and an image is printed onto the sheet of paper. The sheet of paper with the printed image is transported along the transportation path and then discharged outside the image forming apparatus to a discharge tray. On the transportation path are rollers positioned at the entrance and exit of the printing unit and a guide member, positioned between the rollers, which guides the sheet of paper along the transportation path. The rollers contact each other at a roller portion where the rollers face each other, forming a nip which nips a sheet of paper between the two rollers, and the rollers include a guide member portion which connects the rollers. The multi-sheet feed preventing unit may be disposed at a position adjacent to the pick up roller, or may be disposed at any position along the transportation path. Although the various multi-sheet feed preventing units described above can prevent multiple sheets of paper from moving through the transportation path simultaneously, sometimes these various multi-sheet feed preventing units fail to prevent multi-sheet feeds. Thus, an improved multi-sheet feed preventing unit is necessary. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention provide an image forming apparatus which includes a multi-sheet feed preventing unit disposed at any position along a transportation path inside of the image forming apparatus. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     An aspect of the present invention provides an image forming apparatus including a printing unit which prints an image on a sheet of paper, wherein a transportation path of the sheet of paper is arranged between the entrance and exit of the printing unit, and a shutter arm unit which includes a shutter arm and a stopper arm. The shutter arm is biased on standby at a first position, and in standby one end of the shutter arm protrudes toward the transportation path. When a first sheet of paper moving along the transportation path contacts the front end of the sheet of paper, the shutter arm swings to a second position. A stopper arm disposed on the shutter arm prevents additional sheets from moving along the transportation path at the same time as the first sheet of paper by protruding toward the transportation path when the shutter arm swings to the second position and blocking the front ends of the additional sheets. 
     According to an aspect of the present invention, the stopper arm includes a free end which protrudes toward the transportation path at the second position, and a fixing end connected to the shutter arm by a hinge. 
     According to an aspect of the present invention, the shutter arm includes a protrusion cam which makes the free end protrude by pushing the stopper arm in one direction as the shutter arm swings to the second position. 
     According to an aspect of the present invention, the shutter arm includes an escape cam which retracts the free end from the transportation path by pushing the stopper arm in the other direction as the shutter arm returns to the first position. 
     According to an aspect of the present invention, the stopper arm rotates independent from the shutter arm before making contact with either the protrusion cam or the escape cam. 
     According to an aspect of the present invention, the shutter arm unit further includes a guide member which guides the sheet of paper along the transportation path, and a restriction groove formed in the guide member which opens toward the transportation path, wherein the stopper arm protrudes toward the transportation path through the restriction groove, and the restriction groove supports the stopper arm as the stopper arm blocks the front ends of sheets of paper moving simultaneously along the transportation path. 
     According to an aspect of the present invention, a pair of ends of the shutter arm, spaced apart from each other, are integrally formed together and face a direction perpendicular to the transportation direction of the paper, and may align the front end of the sheet of paper by contacting both sides of the paper as the paper moves along the transportation path. 
     According to an aspect of the present invention, the shutter arm is elastically biased to elastically return to the first position when the shutter arm no longer contacts the paper as the image forming apparatus discharges the paper. 
     According to an aspect of the present invention, the shutter arm unit further includes a sensing portion which detects the front end of the sheet of paper moving along the transportation path by sensing a swing position of the shutter arm. 
     According to an aspect of the present invention, the sensing portion includes a sensor arm integrally formed with the shutter arm which swings with the shutter arm, and an optical sensor, disposed in the swing path of the sensor arm, which senses the swing position of the sensor arm. 
     Another aspect of the present invention provides an image forming apparatus including a printing unit which prints an image onto a sheet of paper, wherein a transportation path of the sheet of paper is arranged between the entrance and exit of the printing unit, and a shutter arm unit. The shutter arm unit includes one roller out of a pair of rollers which transport the sheet of paper by forming a nip in the transportation path, a shutter arm assembled so as to freely rotate around a rotation shaft of the roller, wherein the shutter arm is elastically biased at a first or “standby” position when one end of the shutter arm protrudes toward the transportation path, is pushed by the front end of the sheet of paper as the sheet of paper passes through the nip, and swings to a second position after the sheet of paper passes through the nip, and a stopper arm formed on the shutter arm and positioned upstream of the nip, which blocks the front ends of extra sheets of paper moving simultaneously along the transportation path by protruding toward the transportation path when the shutter arm swings to the second position. 
     According to another aspect of the present invention, the stopper arm may include a free end which protrudes toward the transportation path at the second position, and a fixing end connected to the shutter arm by a hinge. 
     According to another aspect of the present invention, the shutter arm may include a protrusion cam which makes the free end protrude by pushing the stopper arm towards the transportation path as the shutter arm swings to the second position. 
     According to another aspect of the present invention, the shutter arm may include an escape cam which retracts the free end from the transportation path by pushing the stopper arm away from the transportation path as the shutter arm returns to the first position. 
     According to another aspect of the present invention, the stopper arm rotates freely around a hinge before making contact with either the protrusion cam or the escape cam. 
     According to another aspect of the present invention, the shutter arm unit further includes a guide member which guides the sheet of paper, and a restriction groove formed in the guide member which opens toward the transportation path, wherein the stopper arm protrudes toward the transportation path through the restriction groove, and the stopper arm blocks front ends of extra sheets of paper moving simultaneously with the sheet of paper along the transportation path by being supported in contact with the restriction groove. 
     According to another aspect of the present invention, the shutter arm unit further includes a pair of bosses, elastically biased toward the nip, which support both ends of the rotation shaft of the roller. 
     According to another aspect of the present invention, the shutter arm unit further includes an escape member which retracts the rotation shaft of the roller away from the nip. 
     According to another aspect of the present invention, the image forming apparatus further includes a body in which the printing unit is disposed, a door which opens and closes an area of the body near where the printing unit is disposed, a sheet-feeding cassette which is attachable to and detachable from the body and stores sheets of paper, and an escape member which contacts either of the sheet-feeding cassette or the door, and pops loose the rotation shaft of the roller when either the sheet-feeding cassette and/or the door is separated from the body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become more apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a side cross-sectional view which illustrates a main part of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is a perspective view which illustrates the installation of the shutter arm unit shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view which explains the function of a friction pad shown in  FIG. 1 ; 
         FIG. 4  is an assembled perspective view of the shutter arm unit shown in  FIG. 1 ; 
         FIG. 5  is an exploded perspective view of the shutter arm unit shown in  FIG. 1 ; 
         FIG. 6  is a perspective view which illustrates an assembly of the shutter arm and the roller shown in  FIG. 1 ; 
         FIGS. 7 through 9  are cross-sectional views which explain the operations of the shutter arm and the stopper arm shown in  FIG. 2 ; and 
         FIGS. 10 and 11  are cross-sectional views which illustrate the operation of the escape member shown in  FIGS. 4 and 5   
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
       FIG. 1  is a side cross-sectional view which illustrates a main part of an image forming apparatus  100  according to an embodiment of the present invention. The image forming apparatus  100  includes a printing unit (not shown), a body  101 , a sheet-feeding cassette  110 , a door  102 , a pick up roller  120 , and a shutter arm unit  500 . The printing unit and the door  102  are disposed in the body  101 . The door  102  is connected to a side of the body  101  by a hinge, and opens and closes the body  101  to allow access to the printing unit and the transportation path of paper P. The printing unit which prints an image on paper P includes a light-scanning unit  180 , a developing cartridge  140 , and a fixer  160 . 
     The light-scanning unit  180  scans light corresponding to image information onto a photosensitive drum  142  and forms an electrostatic latent image on an outer circumferential surface of the photosensitive drum  142 . Although not shown, the light-scanning unit  180  includes a light source which irradiates a laser beam and a beam deflector which deflects the laser beam irradiated from the light source. 
     The developing cartridge  140  is detachably mounted inside the body  101 . The developing cartridge  140  includes a charging roller  141 , a photosensitive drum  142 , a developing roller  143 , a supply roller  144 , an agitator  145 , and a toner storage  146 . According to another embodiment of the present invention, the photosensitive drum  142  and the charging roller  141  may be disposed outside the developing cartridge  140 . The toner storage  146  stores toner. The developing cartridge  140  is replaced when the toner stored in the toner storage  146  is exhausted. A user mounts the developing cartridge  140  in the body  101  by pushing a handle  147  of the developing cartridge  140  in a negative direction of the x-axis, as shown in  FIG. 1 . The developing cartridge  140  is separated from the body  101  by pulling the handle  147  in a positive direction of the x-axis. 
     The photosensitive drum  142  is positioned so that a part of the outer circumferential surface of the drum  142  is exposed out of the toner storage  146 , and the photosensitive drum  142  rotates in a predetermined direction. The outer circumferential surface of the photosensitive drum  142  is coated with a light-conductive material layer by a method such as vapor deposition. The charging roller  141  charges the photosensitive drum  142  to a predetermined electric potential. An electrostatic latent image, corresponding to an image which is to be printed, is formed on the outer circumferential surface of the photosensitive drum  142  by the light irradiated from the light-scanning unit  180 . 
     Solid powder toner is disposed on the surface of the developing roller  143 . The developing roller  143  transfers the toner onto the electrostatic latent image formed onto the photosensitive drum  142 , thereby developing the electrostatic latent image into a toner image. A developing bias voltage which supplies the toner to the photosensitive drum  142  is applied to the developing roller  143 . Then, the developing roller  143  and the outer circumferential surface of the photosensitive drum  142  contact each other to form a developing nip, or alternatively are spaced apart from each other to form a developing gap. The developing nip or the developing gap should be uniformly formed along an axial direction of the developing roller  143  and the photosensitive drum  142 . To develop the toner image, the toner transferred from the developing roller  143  moves through the developing nip or the developing gap to the photosensitive drum  142  by the developing voltage bias. 
     The toner supplied by the supply roller  144  is stuck onto the developing roller  143 . The agitator  145  constantly mixes the toner to prevent the toner in the toner storage  146  from hardening, and conveys the toner to the supply roller  144 . 
     A transfer roller  150  is positioned to face the outer circumferential surface of the photosensitive drum  142 . To transfer the toner image developed on the photosensitive drum  142  to the paper P, a transfer bias voltage which has an opposite polarity to the polarity of the toner image is applied to the transfer roller  150 . Then, the toner image is transferred onto the paper P by a combination of electrostatic force and mechanical contact pressure generated between the photosensitive drum  142  and the transfer roller  150 . 
     The fixer  160  includes a heating roller  161  and a pressing roller  162  facing each other. The fixer  160  fixes the toner image onto the paper P by applying heat from the heating roller  161  and pressure from the pressing roller  162  to the paper P. 
     After the fixer  160  fixes the toner image onto the paper P, the sheet-discharging roller  170  discharges the paper P with the fixed image outside the image forming apparatus  100 . The paper P discharged from the image forming apparatus  100  is stacked onto a discharge tray (not shown). 
     The transportation path of the paper P will be described below. The image forming apparatus  100  includes the sheet-feeding cassette  110  where the paper P is stacked. The pick up roller  120  picks up individual sheets of paper P stacked onto the sheet-feeding cassette  110  and pulls the individual sheet of paper P up along the transportation path. A pair of rollers  130  and  139 , facing each other and forming a nip, then transports the picked up paper P toward the developing cartridge  140 . When the paper P passes between the photosensitive drum  142  and the transfer roller  150 , the toner image is transferred onto the paper P and fixed by the heat and pressure of the fixer  160 . The paper P with the fixed image is finally discharged outside the image forming apparatus  100  by the sheet-discharging roller  170 . 
     The image forming apparatus  100  described above and shown in  FIG. 1  is a black and white electro-photographic image forming apparatus. However, the image forming apparatus  100  according to aspects of the present invention may instead be another type of image forming apparatus, such as a color electro-photographic image forming apparatus or an inkjet image forming apparatus. 
     A shutter arm unit  500  includes a shutter arm  520  and a stopper arm  510 . 
     The shutter arm  520  swings back and forth between first and second positions. In the first position, the shutter arm  520  is in a standby position, with one end  523  of the shutter arm  520  protruding toward the transportation path, as shown in  FIGS. 1 and 2 . As the sheet of paper P moves along the transportation path, a front end of the sheet of paper P pushes the end  523  of the shutter arm  520  and thus swings the shutter arm unit  500  to the second position due to the contact force of the front end of the sheet of paper P against the shutter arm  520 . 
     The stopper arm  510  is formed together with the shutter arm  520 . Since the stopper arm  510  is formed together with the shutter arm  520 , the stopper arm  510  and shutter arm  520  are positioned so that when the shutter arm  520  protrudes into the transportation path, the stopper arm retracts from the transportation path, and vice versa. Thus, the stopper arm  510  stays in a retracted position as long as the shutter arm  520  stays on standby at the first position. The stopper arm  510  protrudes toward the transportation path through a restriction groove  561  and blocks the front ends of extra sheets of paper moving simultaneously with the sheet of paper P along the transportation path when the shutter arm  520  swings to the second position. The swinging positions of the shutter arm  520  and the stopper arm  510  will be described later. The stopper arm  510  and shutter arm  520  do not have to be integrally formed together, and instead may be connected by any number of connecting devices, such as wires, rods, etc. 
     A friction pad  300  helps prevent extra sheets of paper moving simultaneously with the sheet of paper P along the transportation path from moving past the pick up roller  120  to the rollers  130  and  139 . The function of the friction pad  300  will be described in detail later. 
     Components inside of the image forming apparatus are located either “upstream” or “downstream” of each other along the transportation path. “Downstream” movement of the paper P is movement along the transportation path towards the discharge tray, and “upstream” movement is movement away from the discharge tray towards the sheet-feeding cassette  110 . For example, the shutter arm unit  500  is positioned downstream of the pick up roller  120 . However, the shutter arm unit  500  is not limited to the position shown in  FIG. 2 , and thus the shutter arm unit  500  may be positioned at a position A where the transfer roller  150  is disposed, a position B where the fixer  160  is disposed, a position between the positions A and B, i.e., between the transfer roller  150  and the fixer  160 , or a position downstream of B, i.e., between the fixer  160  and the sheet-discharging roller  170 . 
       FIG. 3  is a cross-sectional view which explains the function of the friction pad  300  shown in  FIG. 1 . The pick up roller  120  and the friction pad are shown in  FIG. 3 . Although the pick up roller  120 , the friction pad  300 , a first sheet of paper P 1  and a second sheet of paper P 2  are illustrated as being separated from one another in  FIG. 3 , the friction pad  300 , the first sheet of paper P 1  and the second sheet of paper P 2  can all actually contact each other when the pick up roller  120  picks up the first sheet of paper P 1 . The friction pad  300  is disposed downstream of the sheet-feeding cassette  110  in the body  101  of the image forming apparatus  100 . Since the friction pad  300  and the outer circumference of the pick up roller  120  are elastically pressed together, a normal force acts on the outer circumference of the pick up roller  120 , the first paper P 1 , the second paper P 2 , and the friction pad  300 . The normal force generates frictional forces Fp, Fs, and Ff. The frictional force Fp acts between the pick up roller  120  and the first sheet of paper P 1 . The frictional force Fs acts between the first sheet of paper P 1  and the second sheet of paper P 2 . The frictional force Ff acts between the second sheet of paper P 2  and the friction pad  300 . 
     Since the normal force generated by the elastic compression between the friction pad  300  and the pick up roller  120  is constant, the magnitudes of the frictional forces Fp, Fs, and Ff vary according to the specific coefficients of friction possessed by the frictional pad  300 , the first paper P 1  and the second paper P 2 , and the pick up roller  120 . By properly adjusting the coefficient of friction of the outer circumferential surface of the pick up roller  120  and the surface of the friction pad  300 , it is possible to make the magnitude of Fp sufficiently larger than the magnitude of Fs so that the pick up roller  120  frictionally picks up the first sheet of paper P 1  due to the large frictional force Fp, while the friction pad frictionally prevents the second sheet of paper P 2  from moving with the first sheet of paper P 1 . Thus, when the pick up roller  120  rotates, only the first sheet of paper P 1  is picked up by friction, and the second sheet of paper P 2  is not picked up by friction. 
     However, when an unexpected external force is applied to the image forming apparatus, such as a jolt or bump, or when the coefficients of friction for the pick up roller  120 , first sheet of paper P 1 , second sheet of paper P 2 , and/or friction pad  300  change, the largest frictional force and the smallest frictional force may change, and this may cause a transportation failure, i.e., a multi-sheet feed, in which the first sheet of paper P 1  and the second sheet of paper P 2  move simultaneously along the transportation path. The shutter arm unit  500  is disposed downstream of the pick up roller  120  and prevents multi-sheet feeds from occurring when the friction pad  300  cannot. 
       FIG. 4  is an assembled perspective view of the shutter arm unit  500  according to an embodiment of the present invention.  FIG. 5  is an exploded perspective view of the shutter arm unit  500  and  FIG. 6  is a perspective view which illustrates an assembly of the shutter arm  520  and roller  130 . 
     Referring to  FIGS. 4 through 6 , the shutter arm unit  500  includes the shutter arm  520  and the stopper arm  510 . 
     The shutter arm  520  includes a hinge hole  521 , a rotation shaft hole  522 , one end  523  which contacts the front end of the sheet of paper being moved along the transportation path, a hook  524 , a sensor arm  525 , a link  526 , a protrusion cam  527   a  (see  FIG. 7 ), and an escape cam  527   b  (see  FIG. 7 ). When the shutter arm  520  swings to the second position, a free end (not shown) of the stopper arm  510  which is connected to the shutter arm by a fixing end (not shown), which in turn is connected to the shutter arm  520  by a hinge  511 , protrudes toward the transportation path. The free end (not shown) of the stopper arm  510  slides through the restriction groove  561  formed into the guide member  560  and protrudes toward the transportation path to block the front ends of excess sheets from moving simultaneously with the sheet of paper P along the transportation path. The hinge  511  is disposed at the fixing end (not shown) of the stopper arm  510 , and rotates freely in the hinge hole  521  of the shutter arm  520 . 
     A rotation shaft  132  of the roller  130  is inserted into the rotation shaft hole  522 . However, a separate rotation shaft (not shown) may instead be used to bias the roller  130  into position. The shutter arm  520  rotates around the rotation shaft  132  of the roller  130  as the shutter arm  520  between the first and second positions. The shutter arm unit  500  is not limited to the position shown in the drawings, and may alternatively be disposed at any position on the transportation path of paper. When the shutter arm unit  500  is disposed downstream of the pick up roller  120  (see  FIG. 1 ), the shutter arm unit  500  includes the roller  130 , and the shutter arm  500  is freely rotatively inserted into the rotation shaft  132  of the roller  130 . 
     Although not shown, when the shutter arm unit  500  is disposed at the position represented by the box “A” (see  FIG. 1 ) of the transfer roller  150 , the shutter arm unit  500  includes the transfer roller  150  instead of the roller  130 , and the shutter arm  520  is freely rotatively inserted into the rotation shaft (not shown) of the transfer roller  150 . 
     Although not shown, when the shutter arm unit  500  is disposed at the position represented by the box “B” (see  FIG. 1 ) of the fixer  160 , the shutter arm unit  500  includes the heating roller  161  instead of the roller  130 , and the shutter arm  520  is freely rotatively inserted into the rotation shaft (not shown) of the heating roller  161 . 
     Although not shown, when the shutter arm unit  500  is disposed between the positions represented by the boxes “A” and “B” (see  FIG. 1 ), i.e., between the transfer roller  150  and the fixer  160 , the shutter arm unit  500  does not include the roller  130 , and the shutter arm  520  is freely rotatively inserted into a separately installed additional rotation shaft (not shown). 
     Although not shown, when the shutter arm unit  500  is disposed between the fixer  160  and the sheet-discharging roller  170  (see  FIG. 1 ), the shutter arm unit  500  does not include the roller  130 , and the shutter arm  520  is freely rotatively inserted into a separately installed additional rotation shaft (not shown). 
     When the sheet of paper P moves downstream along the transportation path, the front, or leading, end of the sheet of paper P contacts the end  523  of the shutter arm  520  because the shutter arm  520  is elastically biased into the first, or standby, position. 
     The shutter arm  520  is elastically biased in the standby position by a torsion spring  528  which elastically returns the shutter arm  520  to the first position after the sheet of paper P passes the shutter arm unit  500 . Both ends of the torsion spring  528  are supported by a hook  524  at the shutter arm  520  and a housing body  541 . 
     A sensing portion (not shown) senses a swing position of the shutter arm  520  and thus detects when the front end of the sheet of paper P is moving past the shutter arm  520 . The sensing portion may include a sensor arm  525  and an optical sensor  547 . Since the sensor arm  525  is integrally arranged with the shutter arm  520 , the sensor arm  525  has the same swing angle as that of the shutter arm  520 . The sensor arm  525  penetrates a sensor arm groove  545  formed in the housing  540 , and is inserted into a sensor groove  549  disposed at the optical sensor  547 . Although not shown, the optical sensor  547  includes a light-emitting portion and a light receiving portion, and detects the swing position of the sensor arm  525  by radiating light onto the sensor arm  525  and perceiving the difference in a quantity of the light reflected by the sensor arm  525 . 
     The shutter arm  520  has a pair of ends  523  spaced apart from each other and integrally connected to each other by a link  526 . The pair of ends  523  of the shutter arm  520  is elastically biased along a direction perpendicular to the transportation direction of paper. The pair of ends  523  of the shutter arm  520  functions to align the front end of the paper P by restraining the right and left sides of the front end of the sheet of paper P at the same level position with the same force. 
     As the shutter arm  520  swings to the second position, the protrusion cam  527   a  pushes the stopper arm  510  in a clockwise direction (see  FIG. 8 ) and forces the stopper arm  510  to protrude toward the transportation path. 
     As the shutter arm  520  elastically returns to the standby position, the escape cam  527   b  pushes the stopper arm  510  in a counter-clockwise direction (see  FIG. 9 ) and forces the stopper arm  510  to retract from the transportation path. 
     The stopper arm  510  swings freely around the hinge  511  of the shutter arm  520 . However, the protrusion cam  527   a  and the escape cam  527   b  limit the range in which the stopper arm  510  can swing to the space between the protrusion cam  527   a  and the escape cam  527   b.    
     When the protrusion cam  527   a  starts contacting the stopper arm  510  as a result of the sheet of paper P pushing against the shutter arm  520 , the stopper arm  510  protrudes toward the transportation path. Since the sheet of paper P pushing against the shutter arm  520  supplies the force which causes the stopper arm  510  to protrude towards the transportation path, the protruding force of the stopper arm  510  toward the transportation path is equivalent to the normal force of the sheet of paper pushing against the shutter arm  520 . When the sheet of paper P passes the shutter arm  520  as the sheet of paper P moves downstream, and the shutter arm  520  elastically returns to the standby position via the torsion spring  528 , the protrusion cam  527   a  does not create any resistance against the elastic movement of shutter arm  520  back to a standby position. The protrusion cam  527   a  does not cause resistance because the stopper arm  510  is positioned between the protrusion cam  527   a  and the escape cam  527   b  and swings freely as the shutter arm  520  elastically returns to the standby position. Thus, the return load of the shutter arm  520  is not increased by the protrusion cam  527   a.    
     On the other hand, when the sheet of paper P passes the shutter arm  520  as the sheet of paper P moves downstream, and the shutter arm  520  elastically returns to the standby position via the torsion spring  528 , the escape cam  527   b  contacts the stopper arm  510  and causes resistance against the shutter arm  520 . The escape force of the stopper arm  510  is equivalent to the elastic force of the torsion spring  528 . When the escape cam  527   b  starts contacting the stopper arm  510 , the return load of the shutter arm  520  increases, causing a resistance force against the elastic force of the torsion spring  528  and potentially preventing the shutter arm  520  from fully returning to the standby position. This return failure results in the shutter arm  520  returning to a position short of the standby position, which in turn results in the shutter arm  520  not being able to contact sheets of paper moving along the transportation path after the sheet of paper P. However, by spacing the escape cam  527   b  sufficiently apart from the protrusion cam  527   a , and positioning the escape cam  527   b  so that the escape cam does not, start contacting the stopper arm  510  until the shutter arm  520  is almost adjacent to the standby position, this return failure can be avoided. Thus, even though the escape cam  527   b  increases the return load of the shutter arm  520 , the return load on the shutter arm  520  caused by the escape cam  527   b  is not large enough to lead to the return failure of the shutter arm  520 . It is understood that the proximity of the protrusion cam  527   a  to the escape cam  527   b , as well as the position of the shutter arm  520  to the standby position, is adjustable. 
     The shutter arm unit  500  further includes a guide member  560  and a restriction groove  561 . The guide member  560  forms a part of the transportation path and guides the sheet of paper P along the transportation path. The restriction groove  561  is formed in the guide member  560  and opens toward the transportation path. The stopper arm  510  protrudes through the restriction groove  561 . An upper boundary of the restriction groove  561 , integrally formed into the shutter arm unit  500 , supports the stopper arm  510  protruding through the restriction groove  561  as the stopper arm  510  blocks the front ends of a plurality of excess sheets moving simultaneously with the sheet of paper P along the transportation path. 
     When the shutter arm unit  500  is positioned along the transportation path in a position which includes the roller  130  (see  FIG. 1 ), the shutter arm unit  500  may further include a boss  530  and an escape member  550 . Referring to  FIGS. 7 through 9 , the rollers  130  and  139  are positioned to contact or nearly contact each other at a nip N along the transportation path. The rollers  130  and  139  engage sheets of paper at the nip N. With respect to the relative position of the stopper arm  510  and the nip N, the stopper arm  510  is arranged upstream of the nip N. The stopper arm  510  blocks excess sheets of paper moving simultaneously with the sheet of paper P from entering the nip N. 
     Referring to  FIGS. 4 through 6 , a pair of bosses  530  is elastically biased toward the nip N. The bosses support both ends of the rotation shaft  132  of the roller  130 . The boss  530  includes a boss hole  532 , a boss rail  535 , and a boss spring  538 . The boss hole  532  rotatively supports the rotation shaft  132  of the roller  130 . The boss rail  535  is inserted into a boss rail guide  542  disposed in the housing  540 , and slides in a direction along the x-axis (see  FIG. 5 ). One end of the boss spring  538  is connected to one of the bosses  530 , and the other end of the boss spring  538  presses against the housing body  541 . The boss spring  538  elastically presses a boss  530  into the roller  130 , and the roller  130  elastically presses against the roller  139 , generating an adhesion force between the rollers  130  and  139  at the nip N which engages the sheet of paper P. 
     To prevent a paper jam or any interference between constitutional parts of the shutter arm unit  500  when attaching or detaching the shutter arm unit  500  from the image forming apparatus  100 , the pair of rollers  130  and  139  should be spaced apart from each other, and the shutter arm  520  and the stopper arm  510  need to retract away from the transportation path. To accomplish these purposes, the escape member  550  is provided. 
     The escape member  550  retracts the rotation shaft  132  of the roller  130  away from the nip N. The escape member  550  includes an escape member control portion  551 , escape member arms  552 , an escape member spring base  554 , and an escape member hinge  555 . The escape member  550  is assembled together with the guide member  560 . The escape member control portion  551  makes contact with a sheet-feeding cassette control portion  113  (see  FIG. 10 ) disposed on the sheet-feeding cassette  110  or a door control portion  103  (see  FIG. 11 ) disposed at the door  102 . The escape member arms  552  retract the rotation shaft  132  of the roller  130  from the transportation path by pushing the rotation shaft  132  in a negative direction along the x-axis. Both ends of the escape member spring  558  are supported by escape member spring bases  554  and  568  disposed on the escape member  550  and the guide member  560 , respectively, and provide the elastic escape force for the rotation shaft  132  of the roller  130 . The escape member hinge  555  is inserted into an escape member hinge groove  565  disposed on the guide member  560 , and the escape member  550  rotates around the escape member hinge  555 . 
     The guide member  560  includes the restriction groove  561 , the roller groove  562 , the escape member hinge groove  565 , an escape member boss  566 , a guide rib  567 , and the escape member spring base  568 . 
     The free end of the stopper arm  510  protrudes toward the transportation path through the restriction groove  561 . When the front ends of extra sheets of paper moving simultaneously with the sheet of paper P are caught in the free end of the stopper arm  510 , the extra sheets of paper push the stopper arm  510  in a downstream direction. As described above, a top boundary of the restriction groove  561  supports the stopper arm  510  from being pushed downstream by the extra sheets of paper. The roller  130  protrudes toward the transportation path through the roller groove  562 . The escape member  550  is rotatably mounted onto the escape member boss  566 . The guide rib  567  guides sheets of paper along the transportation path. 
       FIGS. 7 through 9  are cross-sectional views which explain, in sequence, the operations of the shutter arm  520  and the stopper arm  510 . 
       FIG. 7  illustrates a state when the first sheet of paper P 1  begins moving along the transportation path. In  FIG. 7 , the shutter arm  520  is positioned in the standby position. Thus, the stopper arm  510  is retracted from the transportation path. The pick up roller  120  frictionally engages the first sheet of paper P 1  and transports the first sheet of paper P 1  up the transportation path to the nip N. The first sheet of paper P 1  entering the nip N moves continuously along the transportation path and pushes the end  523  of the shutter arm  520  into the second position. 
       FIG. 8  illustrates a state when the first sheet of paper P 1  is passing through the nip N and a second sheet of paper P 2  is moving along together with the first sheet of paper P 1  in a multi-sheet feed. As shown in  FIG. 8 , the first sheet of paper P 1  pushes the end  523  of the shutter arm  520  into the second position by the force of the front end of the first sheet of paper P 1  moving along the transportation path. Although the torsion spring  528  generates an elastic return force in the direction of the x-axis which pushes the shutter arm  520  back towards the transportation path, the elastic return force is smaller than the normal force of the first sheet of paper P 1  generated by the surface of the first sheet of paper P 1  pushing against the shutter arm  520 . Thus, the shutter arm  520  does not return to the first position and stays in the second position. When the shutter arm  520  swings to the second position, the protrusion cam  527   a  pushes the stopper arm  510  so that the stopper arm  510  protrudes toward the transportation path. The stopper arm  510  blocks the second sheet of paper P 2  from moving simultaneously with the first sheet of paper P 1  along the transportation path. Since the stopper arm  510  is supported by the restriction groove  561 , and since the restriction groove  561  is integrally formed into the guide member  560 , the stopper arm  510  overcomes the transportation force of the second sheet of paper P 2  and prevents the second sheet of paper P 2  from further movement along the transportation path. 
       FIG. 9  illustrates a state just before the second paper P 2  passes through the nip N after the first sheet of paper P 1  has already passed through the nip N. The torsion spring  528  elastically biases the shutter arm  520  back to the standby position. The escape cam  527   b  retracts the stopper arm  510  from the transportation path by pushing the escape cam  527   b . After the first sheet of paper P 1  passes through the nip N, the stopper arm  510  retracts from the transportation path and the second sheet of paper P 2  resumes downstream movement along the transportation path. 
       FIG. 10  illustrates an operational state of the escape member  550  when a user attaches or detaches the sheet-feeding cassette  110 . The sheet-feeding cassette  110  illustrated by a solid line is in a state of being separated from the body  101  of the image forming apparatus  100 . The sheet-feeding cassette  110  illustrated by a dashed-dotted line is in a state of being installed into the body  101 . The escape member control portion  551 , which contacts the sheet-feeding cassette control portion  113  formed on the sheet-feeding cassette  110 , is released from the sheet-feeding cassette control portion  113  when the sheet-feeding cassette  110  is separated from the body  101 . When the escape member spring  558  pushes the escape member  550 , the escape member  550  swings around the escape member hinge  555  into an escape position. The escape member arms  552  push the rotation shaft  132  of the roller  130  in the negative direction of the x-axis. When the escape member arms  552  push the rotation shaft  132  of the roller of the roller  130  in the negative direction of the x-axis, the roller  130 , the shutter arm  520 , and the stopper arm  510  all move away from the transportation path of paper, making it easier to remove jammed paper and to attach and detach the shutter arm unit  500  to and from the body  101  of the image forming apparatus  100  without any interference from other constitutional parts. 
       FIG. 11  illustrates the operation of the escape member  550  when a user opens and closes the door  102 . The door  102  swings around a door hinge (not shown) and thereby opens and closes an area of the body  101  of the image forming apparatus  100 . In  FIG. 11 , the door  102  illustrated by a solid line represents the door  102  in an open state, and the door  102  illustrated by a dashed-dotted line represents the door  102  in a closed state. When the door  102  is in a closed state, the door control portion  103 , which is formed on the door  102 , keeps the escape member control portion  551  in a locked position. When a user opens the door  102 , the door control portion  103  swings open with the door  102 . As a result, the escape member control portion  551  in contact with the door control portion  103  elastically rotates in a counter-clockwise direction due to the elastic force of the escape member spring  558 . The escape member spring  558  pushes the escape member  550 , the escape member  550  swings around the escape member hinge  555 , and the escape member arms  552  move the rotation shaft  132  of the roller  130  in the negative direction of the x-axis. 
     According to aspects of the image forming apparatus of the present invention as described above, the stopper arm prevents a multi-sheet feed, the shutter arm and the stopper arm which are pushed by the front end of a sheet of paper have a simple structure, the increase in a return load of the shutter arm is prevented, the pair of ends of the shutter arm enables alignment of the front end of a sheet of paper moving along the transportation path, and the shutter arm unit may be disposed at any position on the transportation path. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.