Patent Publication Number: US-5630199-A

Title: Reciprocation brush contact charger

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C §119 from an application entitled Brush Contact Charger earlier filed in the Korean Industrial Property Office on 8 Nov. 1994, which was duly assigned Ser. No. 94-29181 by that Office. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a charger for charging a photosensitive drum of an image forming apparatus, and more particularly, to a brush contact charger for charging the surface of a photosensitive drum at a nearly uniform electrical potential in an image forming apparatus adopting a charging system that employs a brush contact method. 
     In an image-forming apparatus using electrophotography to form an electrostatic latent image, the image forming apparatus uses high voltage to uniformly charge a photosensitive drum, exposes the photosensitive drum, and then terminates the charge. Then, the latent image is developed by an electrically charged toner and the developed image is fixed to a print medium by heating and pressing. 
     Meanwhile, most image forming apparatuses in current use adopt a corona discharging system employing a high voltage application as the method for charging the surface of a photosensitive drum. The corona discharging system, however, generates ozone (O 3 ), which causes environmental problems. Accordingly, a charging system employing a brush contact method has recently been under study. 
     Conventionally, a charger is electrically connected to a power supply and is installed so that a brush contacts a surface of a photosensitive drum which rotates together with a developing roller. Here, when voltage is applied from the power supply, the charger charges the surface of the photosensitive drum at a predetermined electrical potential via the brush. 
     As the charger is electrified however, the conductive fibers of the brush do not contact the surface of the photosensitive drum consistently, due to subtle length variations thereof. Further, it is difficult to maintain a regular spacing between the fibers. Thus, electrical potential varies across the surface of the photosensitive drum, and the photosensitive drum&#39;s surface is charged unevenly. This uneven charging degrades the quality of the final image. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a brush contact charger for uniformly charging a surface of a photosensitive drum of an image forming apparatus. 
     To accomplish the above object, there is provided a brash contact charger for charging a photosensitive drum of an image forming apparatus, the brush contact charger comprises a body having a brush for contacting and then charging the photosensitive drum, wherein the body is moved in a back and forth direction with respect to the longitudinal axis of the photosensitive drum, i.e., perpendicular to the rotating direction of the photosensitive drum, and a guide member for supporting the body while guiding the reciprocal movement of the body. 
     Since the above brush contact charger moves at a right angle with respect to the rotating direction, i.e., the proceeding direction, of the photosensitive drum, while charging the surface thereof, the surface of the photosensitive drum can be charged at a nearly uniform electrical potential. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention, and many of the attendant advantages thereof, will become readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 is a simplified schematic illustrating a charger adopting the brush contact method in an image forming apparatus; 
     FIG. 2 illustrates the electrical potential for each position along a photosensitive drum used in an image forming apparatus adopting the brush contact system of FIG. 1; 
     FIG. 3 schematically illustrates a brush contact charger constructed according to the principles of the present invention; 
     FIGS. 4A and 4B show operation of the brush contact charger constructed according to the principles of the present invention; 
     FIG. 5 illustrates the electrical potential for each position along a photosensitive drum, by using the brush contact charger according to the principles of the present invention; and 
     FIG. 6 is a partially extracted view showing another embodiment of the cam for use in the brush contact charger constructed according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a charger 11 is electrically connected to a power supply 15 and is installed so that a brush 11a contacts the surface of a photosensitive drum 12 which rotates together with a developing roller 13. Here, when voltage is applied from power supply 15, charger 11 charges the surface of photosensitive drum 12 at a predetermined electrical potential via brush 11a. 
     However, as charger 11 is electrified, the conductive fibers of brush 11a do not contact the surface of photosensitive drum 12 consistently, due to subtle length variations thereof. Further, it is difficult to maintain a regular spacing between the fibers. Thus, as shown in FIG. 2, a graph illustrates the varying electrical potential across the surface of photosensitive drum 12, indicating that the photosensitive drum&#39;s surface is charged unevenly. This uneven charging degrades the quality of the final image. 
     A brush contact charger 30 according to the principles of the present invention, and referring to FIG. 3, includes a charging brush 31a disposed adjacent to photosensitive drum 35, wherein charging brush 31a is attached to a body 31 electrically connected to a power supply (not shown). Body 31 is reciprocated in the longitudinal direction of photosensitive drum 35 in order to place a consistent charge on the surface of photosensitive drum 35 by moving charging brush 31a along the surface of photosensitive drum 35. Body 31 is slidably installed on two guide shafts 31g and 31g&#39;. A gear 34 installed on the shaft of photosensitive drum 35 is in meshed engagement with a smaller gear 33 having a cam 32 installed thereon. Cam 32 maintains contact with cam contact member 31c extending from one end portion of body 31. A compression spring 36 is mounted on a protrusion 40t extending from a frame 40 and a protrusion 31t extending from an opposite end portion of body 31. The biasing force exerted by spring 36 enables cam contact member 31c to maintain contact with cam 32. The rotation of gear 34 causes gear 33 and cam 32 to rotate thus driving body 31 along shafts 31g and 31g&#39; and against the biasing force of compression spring 36. Thus the combination of the rotation of cam 32 and the compression/decompression of spring 36 enables body 31 and brush 31a to move in a reciprocating manner along rotating photosensitive drum 35. 
     The operation of the thus-structured brush contact charger of the present invention will be explained with reference to FIGS. 3, 4A and 4B. 
     When power is supplied by a power supply (not shown) to charger 30, photosensitive drum 35 is charged by brush 31a. Here, the rotation of photosensitive drum 35 causes the rotation of gear 34 installed onto the shaft of photosensitive drum 35. Accordingly, gear 33, which is meshed with gear 34, rotates simultaneously with gear 34. Consequently, cam 32, which is fixed to gear 33, rotates so that body 31 of charger 30 moves reciprocally in accordance with the rotation of cam 32, as shown in FIGS. 4A and 4B. In other words, as shown in FIG. 4A, cam 32 rotates, and thus the peak of cam 32 contacts cam contact 31c of body 31 of charger 30 so as to push cam contact 31c. Then, body 31 moves in the direction of the arrow (FIG. 4A) along guide shafts 31g and 31g&#39;. At this time, body 31 compresses spring 36 to resiliently bias body 31. In the meantime, as shown in FIG. 4B, when cam 32 continues its rotation so that the recess thereof comes into play, body 31 of charger 30 moves in the direction of the arrow shown in FIG. 4B in response to the resilient (decorepressing) force of spring 36. 
     As such serial operations repeat, body 31 of the charger reciprocates at a right angle with respect to the rotating direction of photosensitive drum 35, and brush 31a charges the surface of photosensitive drum 35 while moving on the surface of photosensitive drum 35 according to a reciprocating movement. Therefore, as shown in FIG. 5, the surface of photosensitive drum 35 is charged at a nearly uniform electrical potential. 
     Cam 32 in FIGS. 3-4B has an slightly undulated contacting surface so that, during rotation of cam 32, brush 31a maintains a uniformly accelerated motion across the surface of photosensitive drum 35 for charging the surface with a nearly uniform electrical potential. Thus, brush 31c will make one uniform reciprocal motion across photosensitive drum 35 as cam 32 makes one complete cycle. 
     As shown in FIG. 6, however, a cam 32&#39;, which has more undulations in the contact surface than cam 32, can be employed to increase the frequency of the reciprocating movement of body 31. Here, when cam 32 is substituted with cam 32&#39; of FIG. 6, body 31 can be reciprocated at a much greater frequency, to improve the uniform charging effect of brush 31a, accordingly, and provide increased uniformity in the electrical potential distribution on the surface of photosensitive drum 35. 
     As described above, a brush contact charger of the present invention moves at a right angle with respect to the proceeding (rotating) direction of a photosensitive drum and charges the surface of a photosensitive drum. Accordingly, the surface of the photosensitive drum can be charged to an electrical potential with a nearly uniform distribution. Therefore, the photosensitive drum can obtain a more uniform distribution of toner resulting in a better image which can be obtained when the present invention is employed for an image forming apparatus. 
     While there have been illustrated and described what is to he considered to be the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out the present invention, but that the present invention includes all embodiments falling within the scope of the appended claims.