Abstract:
A toner cartridge has a toner chamber, agitator, damper, and position indicator. The agitator is disposed in the toner chamber and is free to rotate. The agitator is pushed to rotate and agitate the toner. The position indicator attached to the agitator and indicates a rotational position of the agitator. The damper is disposed in a path through which the agitator rotates, the damper defining an angular range through which the agitator rotates while the position indicator is detected by an external device. When the agitator rotates freely into the angular range, the damper damps the motion of the agitator. The damper is formed of a resilient film material and is bent into a base portion, and a raised portion base portion is fixed to an inner surface of the toner chamber and the raided portion extends into the path.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a toner cartridge and a mechanism for detecting remaining quantity of toner. 
     DESCRIPTION OF THE RELATED ART 
     FIG. 25 illustrates a conventional toner chamber and a toner agitator provided therein. 
     FIG. 26 illustrates a toner chamber of a conventional toner cartridge when the toner chamber is nearly empty of toner. 
     FIG. 27 illustrates the toner chamber of the conventional toner chamber when the toner chamber holds a sufficient amount of toner. 
     FIG. 28 illustrates the output signal of a toner sensor indicative of the remaining toner in the toner chamber. 
     Referring to FIG. 25, a toner agitator  92  is free to rotate in a toner chamber  91 . The toner chamber  91  has a gear, not shown, rotatably attached thereto. The gear  95  has a projection  94  that pushes the toner agitator  92  when the gear  95  rotates. As the gear  95  rotates in a direction shown by an arrow, the projection  94  pushes the toner agitator  92  so that the toner agitator  92  rotates together with the gear  95 . When the toner agitator  92  rotates past its highest position in the toner chamber  91 , the toner agitator  92  drops due to its own weight. If the toner chamber  91  holds a sufficient amount of toner therein as shown in FIG. 27, the toner agitator  92  leaves the projection  94  to land and rest on a pile of toner. If the toner chamber  91  holds little toner therein as shown in FIG. 26, when the toner agitator  92  rotates past its highest position, the toner agitator  92  drops and rotates to its lowest position due to its own weight. After the toner agitator  92  drops from its highest position, the gear  95  still continues to rotate so that the projection  94  pushes the toner agitator  92  again to rotate together with the toner agitator  92 . The toner agitator  92  stays at its lowest position for a longer time period when the toner chamber  91  holds little toner therein than when the toner chamber  91  holds a sufficient amount of toner therein. 
     A magnet-sensitive toner sensor, not shown, is disposed outside of the toner chamber  91 . The toner agitator  92  has a magnet attached thereto. As the toner agitator  92  rotates, the magnet passes the toner sensor. The toner sensor detects the magnetic flux of the magnet and outputs a sensor output as shown in FIG.  28 . 
     Referring to FIG. 26, when the toner agitator  92  is within an angular range α, the toner agitator  92  is substantially at its lowest position and the toner sensor outputs a signal indicative of “a toner-low.” 
     The conventional toner agitator  92  suffers from a problem that the toner agitator  92  tends to oscillate back and forth about its lowest position when the toner chamber  91  holds little toner therein. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to solve the drawbacks of the aforementioned conventional art. A toner cartridge includes a toner holding section, a toner agitator, a damper, and a position indicator. The toner holding section is a toner chamber that holds toner therein. The toner agitator is disposed in the toner holding section and is free to rotate. The toner agitator is driven in rotation to agitate the toner when it is pushed to rotate. The damper damps the motion of the toner agitator when the toner agitator rotates freely, the damper being disposed in the toner holding section. The position indicator is attached to the toner agitator and indicates the rotational position of the toner agitator. 
     The damper is disposed in a path through which the toner agitator rotates, the damper defining an angular range through which the toner agitator rotates while the position indicator is detected by an external device. 
     The damper is formed of a resilient film material and is bent into a base portion, and a raised portion. The base portion is fixed to an inner surface of the toner holding section and the raised portion extends into the path. 
     The raised portion exhibits a first resistance against a first force that causes the raised portion, to deform inwardly relative to the angular range and a second resistance against a second force that causes the raised portion to deform outwardly relative to the angular range. 
     The damper is one of two dampers. A first one of the two dampers is disposed at an upstream end of the angular range with respect to rotation of the toner agitator. A second one of the two dampers is disposed at a downstream end of the angular range with respect to rotation of the toner agitator. The raised portion of the second one of the two dampers lies in a plane at an angle with a plane in which the raised portion of the first one of the two dampers lies. 
     The toner agitator stays in the angular range for a shorter time when the toner cartridge holds a larger amount of toner therein than when the toner cartridge is nearly empty of toner. 
     The raised portion has a plurality of slits extending in a direction in which the raised portion extends. 
     The toner agitator is pushed by a drive member to rotate about a substantially horizontal axis. When the toner agitator is rotated past its vertical highest position, the toner agitator drops from the vertical highest position due to its own weight. When the toner agitator is in the angular range, the external device detects the magnetic field. 
     The position indicator is a magnet attached to the toner agitator. The magnet radiates a magnetic field that is detected by the external device. 
     A toner remaining detector for detecting a remaining amount of toner in a toner cartridge attached to an image-forming unit, the detector includes a toner agitator, a position indicator, and a damper. The toner agitator is disposed in the toner cartridge and is free to rotate about an axis. The toner agitator is driven in rotation to agitate the toner. The damper damps the motion of the toner agitator when the toner agitator rotates freely. The position indicator is attached to the toner agitator and transmits a signal indicative of a rotational position of the toner agitator. An external detector is provided on the image-forming unit and detects the signal when the toner agitator is rotated. 
     The damper is disposed in a path through which the toner agitator rotates, the damper damping the motion of the toner agitator when the toner agitator is within a range of rotational angle. 
     The damper is formed of a resilient film material bent into a base portion fixed to an inner surface of the toner cartridge and a raised portion extending into the path. 
     The damper is one of two dampers. A first one of the two dampers is disposed at an upstream end of the range of rotational angle with respect to rotation of the toner agitator. A second one of the two dampers is disposed at a downstream end of the range of rotational angle with respect to rotation of the toner agitator. 
     The raised portion of the first one of the two dampers has a plurality of slits formed in a direction in which the raised portion of the first one of the two dampers extends. 
     The second one of the two dampers is disposed such that the raised portion lies in a plane at an angle with the axis. 
     The toner agitator is rotatable freely about the axis. The second portion operates such that when the toner agitator rotates freely within the range of rotational angle, the second portion repels the toner agitator to stay within the range of rotational angle. The second portion operates such that when the toner agitator is rotated by the drive member, the second portion allows the toner agitator to move out of the range of rotational angle. 
     The toner agitator is pushed by a drive member to rotate about a substantially horizontal axis. When the toner agitator is rotated past its vertical highest position, the toner agitator drops from the vertical highest position due to its own weight and rotates about the substantially horizontal axis. When the toner agitator is substantially in the range of rotational angle, the signal is detected. 
     The position indicator is a magnet attached to the toner agitator and radiates a magnetic field. The detector is a magnetoresistive element and detects the magnetic field. 
     The toner agitator stays within the range of rotational angle for a shorter time when the toner cartridge holds a larger amount of toner therein than when the toner cartridge is nearly empty of toner. 
     An image-forming unit receives a toner cartridge attached thereto. The toner cartridge includes a toner agitator free to rotate and a position indicator that indicates a rotational position of the toner agitator. The toner agitator is driven in rotation when it is pushed to rotate. The image-forming unit includes a space that receives the toner cartridge therein and a detector that detects the position indicator when the position is at a certain rotational position. 
     The position indicator is a magnet that radiates a magnetic field and the detector is a magnetoresistive element that detects the magnetic field. 
     The toner agitator is pushed by a drive member to rotate about a substantially horizontal axis. When the toner agitator is rotated past its vertical highest position, the toner agitator drops from the vertical highest position due to its own weight. When the toner agitator is substantially in the range of rotational angle, the detector detects the magnetic field. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a perspective view illustrating a pertinent portion of an image-forming apparatus according a first embodiment; 
     FIG. 2 is a perspective view with a partial cross-sectional view, illustrating an image-forming unit of the image-forming apparatus and a toner cartridge according to the first embodiment; 
     FIG. 3 is a model representation of the image-forming unit of FIG. 2; 
     FIG. 4 is a perspective view with a partial cross-sectional view of the toner cartridge; 
     FIG. 5 is a cross-sectional view of the toner cartridge FIG. 4; 
     FIG. 6 illustrates the function of a damper according to the first embodiment; 
     FIG. 7 is a perspective view with a partial cross-sectional view of the toner cartridge; 
     FIG. 8 illustrates a mechanism that causes a toner agitator to rotate; 
     FIG. 9A illustrates the positional relation between a gear and the toner agitator when the toner cartridge holds a sufficient amount of toner; 
     FIG. 9B illustrates the positional relation between the gear and the toner agitator when the toner cartridge is almost empty of toner; 
     FIG. 10 is a cross-sectional view of a toner cartridge according to a second embodiment; 
     FIG. 11 is a perspective view with a partial cross-sectional view of the toner cartridge of FIG. 10; 
     FIGS. 12-14 illustrate the operation of dampers according to the second embodiment; 
     FIGS. 15,  16 , and  17  show modifications of the damper; 
     FIG. 18 is a perspective view with a partial cross-sectional view, illustrating a toner cartridge according to a third embodiment; 
     FIG. 19 illustrates dampers, as seen from above, according to the third embodiment; 
     FIG. 20 is a perspective view with a partial cross-sectional view of a toner cartridge according to a fourth embodiment; 
     FIG. 21 illustrates dampers, as seen from above, according to the fourth embodiment; 
     FIG. 22 is a perspective view of the damper; 
     FIGS. 23A and 23B are side views, illustrating a modification of the damper; 
     FIG. 24 is a cross-sectional side view, illustrating another modification of the damper; 
     FIG. 25 illustrates a conventional toner chamber and a toner agitator provided therein; 
     FIG. 26 illustrates a toner chamber of a conventional toner cartridge when the toner chamber is nearly empty of toner; 
     FIG. 27 illustrates the toner chamber of the conventional toner chamber when the toner chamber holds a sufficient amount of toner; and 
     FIG. 28 illustrates the output signal of a toner sensor indicative of the remaining toner in the toner chamber. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     {General Construction} 
     FIG. 1 is a perspective view illustrating a pertinent portion of an image-forming apparatus according a first embodiment. 
     FIG. 2 is a perspective view with a partial cross-sectional view, illustrating an image-forming unit of the image-forming apparatus and a toner cartridge according to the first embodiment. 
     FIG. 3 is a model representation of the image-forming unit of FIG.  6 . 
     Referring to FIGS. 1 and 2, an image-forming apparatus has an image-forming unit  70  and a toner cartridge  10 . The image-forming unit  70  is mounted on a side frame  80  assembled to the image-forming apparatus. 
     Referring to FIG. 3, the image-forming unit  70  includes a rotating photoconductive drum  72  and associated rotating structural elements. The photoconductive drum  72  rotates in a direction shown by arrow A. A charging roller  75  rotates in contact with the photoconductive drum  72  to charge the surface of the photoconductive drum  72  uniformly. An LED head  71  illuminates the charged surface of the photoconductive drum  72  in accordance with print data to form an electrostatic latent image on the photoconductive drum  72 . A developing roller  73  rotates in contact with the photoconductive drum  72  to develop the electrostatic latent image with toner into a toner image. A toner-supplying roller  74  rotates in contact with the developing roller  73  to supply the toner to the developing roller  73 . A toner cartridge  10  is disposed above the toner-supplying roller  74  to supply toner. A neutralizing and cleaning roller  76  rotates in contact with the photoconductive drum  72 . A transfer roller  77  rotates in contact with the photoconductive drum  72  with a print medium  78  sandwiched between the transfer roller  77  and the photoconductive drum  72 , so that the toner image is transferred electrostatically from the photoconductive drum  72  onto the print medium  78 . 
     {Tone Cartridge} 
     The toner cartridge  10  holds toner therein. The toner is discharged from a lower portion of the toner cartridge  10  and supplied to the toner-supplying roller  74 . The toner cartridge  10  includes a toner agitator  12  in the shape of a bar or rod, which is rotated by a gear  21  (FIG. 8) driven by a driving source, not shown. The toner agitator  12  agitates the toner to prevent the toner from clumping, so that the toner cartridge  10  discharges uniform toner particles. 
     The toner agitator  12  also serves as a part of a toner remaining indicator, which will be described later, that detects “a toner-low state” in which only a small amount of toner is left in the toner cartridge  10 . 
     The toner agitator  12  is in the shape of a crank shaft and has a shaft  12   c , a radial portion  12   b , and a bar  12   a  formed in one piece construction. The toner agitator  12  also has a short bar  13   a  that projects radially from the shaft  12   c  and has a magnet  13   b  attached to a tip thereof. The toner agitator  12  rotates about the shaft  12   c  in a direction shown by arrow A, so that the bar  12   a  describes a circular rotational path in a toner holder  17 . The radial portion  12   b  and the short bar  13   a  are angularly spaced apart by 90 degrees such that when the bar  12   a  is at its lowest position, the short bar  13   a  extends horizontally. A magnet-detecting sensor  15  is attached to the side frame  80  to oppose the magnet  13   b  at a position where the magnet takes up when the bar  12   a  is at its lowest position. 
     The magnet-detecting sensor  15  includes a magnetoresistive element such as Hall effect element that detects a magnetic field, and a connector that electrically connects the magnet-detecting sensor  15  and the control means, not shown, of the image-forming apparatus. 
     FIG. 4 is a perspective view with a partial cross-sectional view of the toner cartridge. 
     Referring to FIG. 4, the toner holder  17  is assembled into an outer case  16 . The toner holder  17  has a plurality of large openings in its upper portion through which the toner flows into the toner holder  17  from the outer case  16 . The toner holder  17  has a plurality of toner-discharging openings  18  formed in a bottom floor thereof. The toner is supplied through the toner-discharging openings  18  to the toner-supplying roller  74 . 
     A damper  14  is provided on the inner surface of the toner holder  17  and serves as a device that progressively diminishes oscillatory motion of the bar  12   a  back and forth about a location at which a magnet detecting sensor  15  is disposed. The damper  14  is formed of a resilient resin material such as polyester and has a base portion  14   b  and a raised portion  14   a . The base portion  14   a  is fixed to the inner surface of the toner holder  17  by, for example, an adhesive, the raised portion  14   a  extends into the path of the toner agitator and substantially traverses the circular rotational path of the bar  12   a . The damper  14  is disposed so that the raised portion  14   a  is on the right end of an angular range α of FIG. 5, i.e., the raised portion  14   a  is at a downstream end of the angular range (α) with respect to rotation of the toner agitator. 
     {Operation of Image Forming Unit} 
     When a printing operation is formed to form an image on the print medium, the photoconductive drum  72  (FIG. 2) is driven by a drive source, not shown, such as a motor to rotate clockwise. Then, the LED head  71  illuminates the surface of the photoconductive drum  72 , charged by the charging roller  75 , to form an electrostatic latent image. As the photoconductive drum  72  rotates, the electrostatic latent image is developed with toner by the developing roller  73  into a toner image. 
     The toner image is then transferred onto the print medium  78 , advanced on a carrier belt, not shown, into a transfer point defined between the photoconductive drum  72  and the transfer roller  77 . 
     Then, the print medium  78  is advanced to a fixing unit, not shown, where the toner image is fused into the print medium  78  into a permanent image. Then, the print medium  78  is discharged from the image-forming apparatus. 
     The cleaning roller  76  charges the surface of the photoconductive drum  72  on which the toner image was formed so that the surface is subjected to an electric field of a polarity opposite to that of the residual toner particles on the photoconductive drum  72 . Thus, the cleaning roller  76  removes the residual toner particles on the photoconductive drum  72  and neutralizes the surface of the photoconductive drum  72 . Thereafter, the surface of the photoconductive drum  72  is charged again uniformly by the charging roller  75 . 
     The photoconductive drum  72  further continues to rotate to repeat the aforementioned steps of electrophotography. 
     {Operation of toner remaining indicator} 
     FIG. 5 is a cross-sectional view of the toner cartridge. 
     FIG. 6 illustrates the function of the damper of the invention. 
     FIG. 7 is a perspective view with a partial cross-sectional view of the toner cartridge. 
     When the bar  12   a  of the toner agitator  12  rotates to its lowest position, it takes up a position as shown in FIG.  4 . When the bar  12   a  of the toner agitator  12  rotates to its highest position, it takes up a position as shown in FIG.  7 . When the bar  12   a  is within the angular range α as shown in FIG. 5, the magnet detecting sensor  15  detects the magnet  13   a.    
     FIG. 8 illustrates a mechanism that causes the toner agitator to rotate. 
     As shown in FIG. 8, when the gear  21  is driven in rotation by the drive source, not shown, a projection  20  on the gear  21  pushes the radial portion  12   b  in such a way that the toner agitator rotates in a direction shown by arrow. 
     FIG. 9A illustrates the positional relation between the gear and the toner agitator when the toner cartridge  10  holds a sufficient amount of toner. 
     If the toner cartridge  10  is almost full of toner, the projection  20  pushes the bar  12  to rotates together with the bar  12  until the bar  12   a  rotates past its highest position in the toner holder  17 . When the bar reaches its highest position, the bar  12   a  drops suddenly from its highest position due to its own weight but lands and rests on the pile of toner. In this case, the bar  12   a  is outside of the angular range α and therefore the magnet-detecting sensor  15  does not generate an output. As the gear  21  rotates, the projection  20  again abuts the radial portion  12   b  of the toner agitator  12  and pushes it to rotate together with the gear  20 , thereby agitating the toner. As the gear  21  further rotates, the bar  12   a  passes through the angular range α at the same speed as the gear, i.e., a fairly high speed. Therefore, the output of the magnet-detecting sensor  15  is of a short duration and does not indicate the “toner-low state.” 
     FIG. 9B illustrates the positional relation between the gear and the toner agitator when the toner cartridge  10  is almost empty of toner. 
     If the toner cartridge  10  is almost empty of toner, the projection  20  pushes the bar  12  to rotates together with the bar  12  until the bar  12   a  rotates past its highest position in the toner holder  17 . When the bar reaches its highest position, the bar  12   a  drops suddenly from its highest position due to its own weight to its substantially lowest position. 
     Thus, if the toner cartridge  10  is nearly empty of toner, the bar  12   a  that has dropped from its highest position does not receive very much resistance of toner and therefore travels through the angular range at a high speed. However, the bar  12   a  collides against the raised portion  14   a  of the damper  14 . 
     The raised portion  14   a  is disposed on the right side end of the angular range α of FIG.  6 . The damper  14  has a certain rigidity and resiliency such that the bar  12   a  cannot overcome the raised portion  14   a  but is repelled. As a result, the bar  12   a  stays within the angular range α until it is pushed by the projection  20  of the gear  21 . In other words, the bar  12   a  stays within the angular range α for a longer time period when the toner cartridge  10  is nearly empty of toner than when the toner cartridge  10  holds a large amount of toner therein. Thus, the output of the toner-detecting sensor  15  indicates the toner-low state. 
     Since the raised portion  14   a  of the damper  14  has rigidity such that the raised portion  14   a  repels the bar  12   a  but yieldingly flex to allow the bar  12   a  to overcome the raised portion  14   a  when the projection  20  of the gear  21  pushes the radial portion  12   b.    
     Second Embodiment 
     FIG. 10 is a cross-sectional view of a toner cartridge according to a second embodiment. 
     FIG. 11 is a perspective view with a partial cross-sectional view of the toner cartridge of FIG.  10 . 
     Elements similar to those of the first embodiment have been given the same reference numerals and the description thereof is omitted. 
     Another damper  24  is attached to an inner bottom surface of the toner holder  17 . Abase portion  24   b  is fixed to the inner bottom surface of the toner holder  17  by an adhesive. The damper  24  is formed of the same material as the damper  14  and is of the same structure as the damper  14 . The damper  24  is positioned such that a raised portion  24   a  is at the left end of the angular range α. In other words, the raised portion  24   a  and raised portion  14   a  are symmetric with respect to a vertical line passing through the shaft  12   c  such that the flat surface of the raised portion  14   a  opposes and is substantially parallel to the flat surface of the raised portion  24   a . The raised portion  24   a  is disposed at an upstream end of the angular range α with respect to rotation of the toner agitator and the raised portion  14   a  is disposed at a downstream end of the range α with respect to rotation of the toner agitator. 
     {Operation of the Dampers} 
     FIG. 12 illustrates the operation of the dampers  14  and  24 . 
     FIG. 13 illustrates the operation of the damper  24 . 
     FIG. 14 illustrates the operation of the damper  14 . 
     FIGS. 15,  16 , and  17  show the modification of the damper  24 . 
     When the toner cartridge  10  is almost empty of toner, the bar  12   a  that has dropped due to its own weight collides against the raised portion  24   a . The raised portion  24   a  yieldingly flexes so that the bar  12   a  overrides the raised portion  24   a  to move past the raised portion  24   a  toward the raised portion  14   a  of the damper  14 . 
     The bar  12   a  is then repelled by the raised portion  14   a  in a direction shown by arrow C to collide against the raised portion  24   a . The impact exerted on the raised portion  24   a  by the bar  12   a  is weak and therefore the raised portion  24   a  does not yieldingly flex but repels the bar  12   a . As a result, the bar  12   a  stays within the angular range α so that the output of the toner-detecting sensor  15  is of a long duration (FIG. 2) that indicates the toner-low state. 
     The dampers  14  and  24  are formed of a resin material such as polyester. The raised portion  24   a  exhibits less rigidity when the bar  12   a  collides against the raised portion  24   a  in such a direction as to open the damper (FIG. 13) than when the bar  12   a  collides against the raised portion  24   a  in such a direction as to fold the damper (FIG.  14 ). 
     Referring to FIG. 12, the dampers  14  and  24  are positioned relative to each other such that when the bar  12   a  collides against the raised portion  24   a , the distance X between the raised portion  24   a  and the raised portion  14   a  is much larger than the diameter of the bar  12   a . When the bar  12   a  overrides the raised portion  24   a  in the B direction, the raised portion  24   a  quickly regains its original position shortly after it flexes yieldingly. Therefore, when the bar  12   a  swings in the C direction of FIG. 12, the bar  12   a  does not override the raised portion  24   a.    
     In order to increase rigidity of the damper  47  in the C direction and decrease the rigidity in the B direction, a damper  47  may be formed such that the angle γ between the raised portion  47   a  and the base portion  47   b  is larger than 90 degrees as shown in FIG.  15 . 
     Alternatively, a damper  49  may be cut partly in a portion about which the damper  49  is bent into a raised portion  49   d  and a base portion  49   a  to make an angle of about 90 degrees. The damper  49  has smaller rigidity when the bar  12   a  collides against the damper  49  in the B direction and larger rigidity when the bar  12   a  collides against the damper  49  in the C direction. Thus, the damper  49  is difficult to be overridden by the bar  12   a  when the bar  12   a  collides against the damper  49  in the C direction. 
     As shown in FIG. 17, the damper  49  may be formed such that the raised portion  49   a  of the damper  49  is tapered and the raised portion  49   a  makes an angle θ larger than 90 degrees with the base portion  49   b . In this case, too, as shown in FIG. 17, the damper  49  has a small rigidity when the bar  12   a  collides against the damper  49  in the B direction and a large rigidity when the bar  12   a  collides against the damper  49  in the C direction. 
     The damper  14  may also be modified in the same way as shown FIGS. 15 to  17 . 
     Third Embodiment 
     FIG. 18 is a perspective view with a partial cross-sectional view, illustrating a toner cartridge according to a third embodiment. 
     FIG. 19 illustrates dampers as seen from above according to the third embodiment. 
     Elements similar to those of the first and second embodiments have been given the same reference numerals and the description thereof is omitted. 
     A damper  34  is positioned as shown in FIG. 19 such that the flat surface of the raised portion  34   a  lies in a plane at an angle β with a plane in which the raised portion  14   a  lies or a vertical plane passing through the shaft  12   c.    
     The dampers  14  and  34  are positioned relative to each other such that when the bar  12   a  collides against the raised portion  24   a , the distance X between the raised portion  34   a  and the raised portion  14   a  is much longer than the diameter of the bar  12   a . It is ensured that the distance X is within the angular range α of FIG.  10 . 
     If the toner cartridge  10  is almost empty of toner, when the bar  12   a  drops due to its own weight from its highest position, the bar  12   a  first swings in the B direction to collide against the raised portion  34   a  at a corner portion  34   b  and then progressively across entire raised portion  34   a . When the bar  12   a  collides against the corner portion  34   b , the rigidity is small and gradually increases as the bar  12   a  collides against the entire raised portion  34   a.    
     Therefore, even if the bar  12   a  has a small kinetic energy when it collides against the corner portion  34   b , the bar  12   a  is not repelled by the raised portion  34   b  but overrides the raised portion  34   b  into the angular range α, so that the toner-low state is reliably detected. 
     Fourth Embodiment 
     Elements similar to those of the first to third embodiments have been given the same reference numerals and the description thereof is omitted. 
     FIG. 20 is a perspective view with a partial cross-sectional view of a toner cartridge according to a fourth embodiment. 
     FIG. 21 illustrates dampers according to the fourth embodiment. 
     FIG. 22 is a perspective view of the damper. 
     A damper  44  includes a raised portion  44   a  and a base portion  44   b . The damper  44  is positioned as shown in FIG. 21 such that the flat surface of the raised portion  44   a  lies in a plane at an angle β with a plane in which the raised portion  14   a  lies or a vertical plane passing through the shaft  12   c . The raised portion  44   a  is separated by slits into a plurality of fingers f 1  to fn. The fingers have widths b1, b2, b3, . . . bn, which may be the same or different. 
     If the toner cartridge  10  is almost empty of toner, when the bar  12   a  swings in the B direction, the bar  12   a  collides against a corner portion  44   b  of the raised portion  44   a  and then swings further overriding the fingers progressively toward the damper  14 . The bar  12   a  receives progressively large repelling force b1, b1+b2, b1+b2+b3, b1+b2+b3+b4, . . . , b1+b2+ . . . +bn. 
     Thus, even if the bar  12   a  has a small kinetic energy when it collides against the corner portion  44   b , the bar  12   a  is not repelled by the corner portion  44   b  but overrides the corner portion  44   b  toward the raised portion  14 . The bar  12   a  then collides against the damper  14  and is repelled back toward the raised portion  44   a . The raised portion  44   a  repels the bar  12   a , so that the bar  12   a  can be within the angular range of FIG.  10 . In this manner, the toner-low state is reliably detected. 
     In the first to fourth embodiments, the dampers are formed of a resin material such as polyester and are attached to the inner surface of the toner holder  17  by an adhesive. The dampers may be of any type of retarding element, provided that when the bar  12   a  swings to collide against the corner portion  44   b , the rotation of the toner agitator  12  is retarded. 
     FIGS. 23A and 23B are side views, illustrating a modification of the damper. 
     FIG. 24 is a cross-sectional side view, illustrating another modification of the damper. 
     Referring to FIGS. 23A and 23B, a retarding member  51  is a flat spring formed of a flexible metal material element. The retarding member  51  is mounted on the inner surface of the toner holder  17 . As shown in FIGS. 23A and 23B, when the bar  12   a  collides against a folded portion of the retarding member  51 , the bar  12   a  causes the retarding member  51  to be deformed into a flatter shape. The deformation of the retarding member  51  absorbs the kinetic energy when the bar  12   a  collides against the retarding member  51 . 
     Referring to FIG. 24, projections  63  and  64  may be formed on the inner surface of the toner holder  17  so that the projectiosn  63  and  64  serve as a retarding member. The radial portion  12   b  of the toner agitator  12  has a film-like element that collides against the projections  63  and  64 , thereby being retarded in swinging motion. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art intended to be included within the scope of the following claims.