Patent Publication Number: US-6704529-B2

Title: Image forming apparatus having improved driving arrangement

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from my application entitled DEVELOPING DEVICE OF ELECTROPHOTOGRAPHIC PRINTER filed with the Korean Industrial Property Office on the Feb. 21, 2001, and there duly assigned Ser. No. 2001-8776. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to image formation processes and apparatus generally, and, more particularly, to developing devices and processes for electrophotographic printers, laser printers, photocopiers. 
     2. Description of the Related Art 
     Generally, an electrophotographic printer includes a developing device that transfers images from a surface of a photosensitive drum to a printable medium such as paper fed from an internal or external feed device, and a fixing roller that heats the toner forming a latent image borne by the printable medium. 
     Conventional electrophotographic printers use a photosensitive drum, a developing roller, and a feeding roller. There must be nips, which are contact areas, between the photosensitive drum and the developing roller, and between the developing roller and the feeding roller. To maintain these nips, the electrophotographic printer has a power transmitting gear and the timing belt. Since the developing roller and the feeding roller are directly meshed with each other, it is difficult to transmit the power at a desirable velocity ratio while maintaining their nip within an acceptable range of length. 
     Direct contact between the feeding roller and the developing roller induces vibration and inconsistent velocity of the feeding roller while the feeding roller is transferring the developer to the developing roller. This, in turn, causes jitter, that is, irregular gaps between dots of a latent image on the exterior cylindrical surface of the photosensitive drum, so that the best printing quality cannot be assured. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved image forming apparatus and process. 
     It is another object to provide an image forming apparatus having an improved arrangement of gears. 
     It is still another object to provide a developing device and process in an electrophotographic printer that exhibits minimal jitter. 
     It is yet another object to provide a developing device and process in an electrophotographic printer that reduces occurrence of vibration while maintaining constant velocity of the feeding roller. 
     According to the present invention, an image forming apparatus may be constructed according to the principles of the present invention with a power transmitting gear meshed with a photosensitive drum gear formed on a photosensitive drum, and a transmission belt that transmits a driving force from the power transmitting gear to a developing roller gear formed on a developing roller. The power transmitting gear is meshed with an idler gear, and the idler gear is meshed with a feeding roller gear formed on a feeding roller that feeds a developer to the developing roller. With this configuration, the power transmitting gear transfers a driving force from the photosensitive drum to the feeding roller gear. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be 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 sectional view showing the gear arrangement in the conventional image forming apparatus; and 
     FIG. 2 is a sectional view showing [[the]] a first preferred gear arrangement in the image forming apparatus constructed according to the principles of the present invention. 
     FIG. 3 is a sectional view showing a second preferred gear arrangement in the image forming apparatus constructed according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the present invention will now be described with reference to the accompanying drawings. 
     As shown in FIG. 1, the conventional image forming apparatus  10  includes a photosensitive drum  11 , a developing roller  12 , and a feeding roller  13 . 
     By contact with the feeding roller  13 , developer (not shown) held in a casing  10   a  is subject to static electricity. Due to the static electricity, the developer is transferred from the feeding roller  13  onto the surface of the developing roller  12 . The developer attached onto the developing roller  12  is transmitted to electrostatic latent images formed on the surface of the photosensitive drum  11 . A nonmagnetic toner is usually used as the developer. 
     In the conventional image forming apparatus, the photosensitive drum  11 , the developing roller  12 , and the feeding roller  13  have a photosensitive drum gear  11   a , a developing roller gear  12   a , and a feeding roller gear  13   a  respectively. The photosensitive drum gear  11   a  is meshed with a power transmitting gear  14  which is connected to the toothed pulley on the developing roller gear  12   a  via a timing belt  15 . Driving force is transferred from a driving portion (not shown) to the photosensitive drum  11  and then to the developing roller  12  through the developing roller gear  12   a  and then to the feeding roller  13  through a feeding roller gear  13   a.    
     In order for the developer T to be conveyed to the photosensitive drum  21  under a relatively constant condition, nips must be maintained constantly between the photosensitive drum  11  and the developing roller  12 , and between the developing roller  12  and the feeding roller  13 . To maintain these nips, the conventional image forming apparatus  10  has a power transmitting gear  14  and a timing belt  15 . Since the developing roller  12  and the feeding roller  13  are directly meshed with each other, it is difficult to transmit the power at a desirable velocity ratio while maintaining their nip within an acceptable range of length. 
     Direct contact between the feeding roller  13  and the developing roller  12  induces vibration and inconsistent velocity of the feeding roller  13  while the feeding roller  13  is transferring the developer to the developing roller  12 . This problem causes jitter, that is, irregular gaps between dots of an image of the photosensitive drum  11 . 
     Referring now to FIG. 2, an image forming apparatus  10  is provided to an electrophotographic printer to transfer a developer T onto a printable medium  27 . The image forming apparatus  10  includes a photosensitive drum  21 , a developing roller  22 , a feeding roller  23 , a power transmitting gear  24 , and belt  25 . 
     The photosensitive drum  21  formed in a casing  10   a  is rotated by a driving force transferred from a driving portion (not shown). While the electrophotographic printer is being driven, the photosensitive drum  21  is exposed by an exposing device (not shown), and a predetermined electrostatic latent image is formed on the surface of the photosensitive drum  21 . The developing roller  22  transfers the developer T from the feeding roller  23  to the electrostatic latent image formed on the photosensitive drum  21 . When the photosensitive drum  21 , which is exposed through the image forming apparatus  10 , is contacted with a printable medium  27 , the developed image on the surface of the photosensitive drum  21  is transmitted to the printable medium  27 . 
     The feeding roller  23  is rotatably installed to supply the developer T held in the casing  10   a  to the developing roller  22 . The developer T is subject to static electricity due to contact with the feeding roller  23 , and this static electricity makes it possible to attach the developer T onto the outer circumference of the developing roller  22  by a certain thickness. 
     The developing roller  22  is rotatably installed between the feeding roller  23  and the photosensitive drum  21 , and transmits the developer T fed from the feeding roller  23  onto the electrostatic latent image formed on the surface of the photosensitive drum  21 . 
     The gears are arranged in such a manner that the feeding roller gear  23   a , the toothed pulley gear  22   a , and the photosensitive drum gear  21   a  are disposed on each rotary shaft for the feeding roller  23 , the developing roller  22 , and the photosensitive drum  21 . By the interaction between the respective gears, the feeding roller  23  and the developing roller  22  receive the power from the photosensitive drum  21 . 
     The power transmitting gear  24  transfers the driving force from the photosensitive drum  21  to the developing roller  22  and the feeding roller  23 . The power transmitting gear  24  is meshed with the photosensitive drum gear  21   a . The power transmitting gear  24  includes a toothed pulley gear  24   a  for power transmission. The toothed pulley  24   a  in the power transmitting gear  24  and the toothed pulley gear  22   a  in the developing roller  22  are connected with each other via the belt  25 . By connecting the power transmitting gear  24  and the developing roller  22  by the belt  25  through the toothed pulley  22   a  and toothed pulley gear  24   a , the photosensitive drum  21  and the developing roller  22  are maintained at a predetermined constant nip and velocity ratio. It is preferable that the nip between the photosensitive drum  21  and the developing roller  22  is approximately 0.1 mm long. Here, in order to indirectly transmit the power from the photosensitive drum  21  to the developing roller  22 , the belt  25  is used. By using the belt instead of using a plurality of transmitting gears, power loss by pitch error is significantly reduced because the pitch error is mainly caused by the use of a plurality of transmitting gears  24 . Furthermore, the use of belt reduces vibration at the developing roller  22  and the jitter caused by the operation at inconstant velocity. 
     In order to connect the power transmitting gear  24  with the feeding roller gear  23   a , more than one idler gear  26  can be meshed with all the gears. Since the developing roller  22  and the feeding roller  23  connected with each other indirectly, a constant nip is maintained between the developing roller  22  and the feeding roller  23 . Also, since the vibration or inconsistent velocity in the feeding roller  23  is generated by the direct contact between the developer T held in the casing  10   a  and the feeding roller  23 , the indirect connection between the developing roller  22  and the feeding roller  23  significantly reduces the transmission of the vibration or inconstant velocity from the feeding roller  23  to the developing roller  22 . 
     In transmitting the driving force to the feeding roller gear  23   a ,  2   n −1 (n=positive integer) of idler gear  26  is preferred for obtaining a uniform rotational direction of the feeding roller  23  as indicated by dot-lined arrows of FIG.  2  and FIG.  3 . FIG. 2 shows a preferred embodiment having one idler gear  26 . FIG. 3 shows another preferred embodiment having three idler nears  26   a .  26   b  and  26   c.    
     Further, in order to minimize the power loss due to increased number of idler gear  26  during the power transmission to the feeding roller  23 , it is more preferable that one idler gear  26  is used. 
     In the image forming apparatus  10  constructed as above, when the electrophotographic printer is driven, the photosensitive drum  21 , the developing roller  22 , and the feeding roller  23  are rotated in the direction indicated by the dot-lined arrow of FIG.  2 . The driving force generated from the photosensitive drum  21  is transmitted in the direction as indicated by a solid-lined arrow of FIG.  2 . More specifically, after the driving force generated from the photosensitive drum  21  is transmitted to the power transmitting gear  24 , the driving force is transmitted to the developing roller  22  through the belt  25 . The power transmitting gear  24  also transmits the driving force to the idler gear  26  meshed therewith, thereby transmitting the driving force to the feeding roller gear  23   a  that is meshed with the idler gear  26 . Accordingly, the developer T held within the casing  10   a  is conveyed to the developing roller  22  through the feeding roller  23 . The developer T transferred onto the developing roller  22  is then attached onto the electrostatic latent image formed on the photosensitive drum  21 , and then transferred onto the printable medium  27 . Where there are three idler gears as shown in FIG. 3, a first idler gear  26   a  is meshed with the power transmitting sear  24 , a second idler gear  26   b  is meshed with the first idler gear  26   a , and a third idler near  26   c  is meshed with the second idler gear  26   b  and the feeding roller gear  23   a.    
     As described above, since the driving force generated from the photosensitive drum  21  is indirectly transmitted to the developing roller  22  and the feeding roller  23 , the constant nip between the developing roller  22  and the feeding roller  23  is maintained. 
     Furthermore, since vibration and operation at inconstant velocity generated from the developing roller  22  and the feeding roller  23  are indirectly transmitted to the photosensitive drum  21 , the effect of the vibration and inconstant-velocity operation are minimized. 
     Thus, since the developer T is conveyed to the photosensitive drum  21  under a relatively constant condition, the printing quality improves. 
     Although the preferred embodiment of the present invention has been described, it will be understood by those skilled in the art that the present invention should not be limited to the described preferred embodiment, but various changes and modifications can be made within the spirit and scope of the present invention as defined by the appended claims.