Abstract:
An apparatus for forming an image is provided, in which a rotational member, such as a photoreceptor drum, is borne by using a rolling bearing assuring electric conduction between an inner ring and an outer ring by filling electroconductive grease. Rotation of a photoreceptor drum is borne by using a electroconductive rolling bearing having a resistance between the inner ring and the outer ring of about 15 kΩ or less in terms of a maximum value and about 8 kΩ or less in terms of an effective value under a dynamic condition, where a radial or thrust load acting between the inner ring and the outer ring is 10 N, and a relative difference in rotational velocity between the inner ring and the outer ring is 150 rpm, and the photoreceptor drum is rotated at about 75 rpm or more.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an apparatus for forming an image, such as an electrophotographic duplicator and a facsimile machine, and in particular, it relates to an improvement of an apparatus for forming an image, in which rotation of rotational members including an electrostatic latent image carrying member and other rotational members is borne by an electroconductive rolling bearing.  
           [0003]    2. Description of Related Art  
           [0004]    An apparatus for forming an image, such as an electrophotographic duplicator and a laser bean printer, contains various kinds of rotational members, such as a photoreceptor drum, a transferring roll, a magnet roll of a developing device, a heating roll of a fixing device and a transporting roll for a recording sheet or an original copy, and the rotational members are borne by a fixed member, such as a frame of the apparatus, through rolling bearings. In an electrophotographic process, a photoreceptor drum is uniformly charged, and then the charge is relieved only in the exposed part to form an electrostatic latent image. Therefore, the photoreceptor drum is necessarily grounded. In order to develop the electrostatic latent image formed on the photoreceptor drum, a developing bias is necessarily applied to the magnetic roll, and in order to transfer a toner image from the photoreceptor drum to the recording sheet, a transferring bias is necessarily applied to the transferring roll. Furthermore, the heating roll and the transporting roll for a recording sheet are sometimes charged due to the charge carried by the recording sheet, and when a discharge phenomenon due to the charge occurs, malfunction and unnecessary noise caused by radiated electric waves occur. In order to prevent the problems, the rolls are necessarily grounded. As a method for grounding the various kinds of rotational members or connecting them to a bias electric power source, such a method has been know that a spring is made in contact under sliding with one end or both ends of the rotational member, whereby connection and grounding of the various kinds of rotational members are attained.  
           [0005]    However, because the slide-contact between the spring and the rotational member brings about time-lapse deterioration, such as wear and alteration, fluctuation of the resistance between the spring and the rotational member is unavoidable, and therefore not only the initial performance cannot be maintained with the lapse of time, but also there is a possibility that grounding or connecting failure and abnormal noise occur.  
           [0006]    An electroconductive bearing has been known that assures electric conduction between an inner ring and an outer ring without provision of any particular slide-contacting member. When the rotation of the rotational member is borne by using the electroconductive bearing, the rotational member can be grounded or applied with a high bias voltage without providing a spring. The electroconductive bearing contains an outer ring fixed to a fixed member, such as a frame of an apparatus, an inner ring fixed to a rotational axis, and rolling elements, such as balls and rollers, rolling between the outer ring and the inner ring under receiving a load, in which the inner and outer rings and the rolling elements are formed with an electroconductive metal, such as steel. Grease filled between the inner ring and the outer ring is also imparted with electroconductivity, and even when an oil film is formed between the rolling elements and the inner ring or between the rolling elements and the outer ring, electric conduction can be assured.  
           [0007]    However, in the case where a rotational member, such as a photoreceptor drum, is grounded by using the conventional electroconductive bearing, another problem arises that unallowable radiation noise occurs at the electroconductive bearing, although the function of grounding the rotational member is exerted without any problem. An electronic office equipment, such as an electrophotographic duplicator and a facsimile machine, sometimes generate jamming (radiation noise) over wide frequency bands to cause disturbance in receivers, such as radios and televisions. In order to prevent the electric wave disturbance, a self-imposed standard has been determined by the manufacturer of the equipment in Japan. However, in the case where the conventional electroconductive bearing is used, the radiation noise generated by the bearing cannot be controlled within the range of the self-imposed standard. As a reason of the generation of radiation noise, it is considered that a slight gap is formed between the inner ring or the outer ring and the rolling elements during rotation of the rotational member, and electric leakage occurs at the gap.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention has been made in view of the problems associated with the conventional art and provides an apparatus for forming an image, in which a rotational member, such as a photoreceptor drum, is borne by using a rolling bearing assuring electric conduction between an inner ring and an outer ring by filling electroconductive grease, and generation of radiation noise from a rotational part using the bearing due to leakage can be effectively suppressed even when the rotational member is grounded without any other slide-contact member, such as an grounding spring.  
           [0009]    The invention relates to, as one aspect, an apparatus for forming an image containing an electrostatic latent image carrying member and an electroconductive rolling bearing that bears rotation of the electrostatic latent image carrying member; the electroconductive rolling bearing containing an outer ring and an inner ring, which are fixed to a pair of members relatively rotating, respectively, plural rolling elements rolling between the inner ring and the outer ring under receiving a load, and a lubricant filled between the outer ring and the inner ring that contains an electroconductive substance dispersed therein; a resistance between the inner ring and the outer ring being about 15 kΩ or less in terms of a maximum value and about 8 kΩ or less in terms of an effective value under a dynamic condition, where a radial or thrust load acting between the inner ring and the outer ring is 10 N, and a relative difference in rotational velocity between the inner ring and the outer ring is 150 rpm; and the electroconductive rolling bearing being used at a relative difference in rotational velocity between the inner ring and the outer ring of about 75 rpm or more.  
           [0010]    It is preferred that the resistance between the inner ring and the outer ring is 11 kΩ or less in terms of a maximum value and 3 kΩ or less in terms of an effective value under the dynamic condition.  
           [0011]    As confirmed by the inventors, when such a rolling bearing that is suppressed in resistance between the inner ring and the outer ring under the certain condition is used, the radiation noise generated from the bearing can be suppressed to such a level that causes no problem from the standpoint of the self-imposed standard of VCCI and of the standard of FCC.  
           [0012]    The reason why the dynamic condition for measuring the resistance is determined by the load and the relative difference in rotational velocity is as follows. In the case where a load applied on the rolling bearing is too large, an oil film is broken to make the three members, the inner ring, the rolling elements and the outer ring, each made of an electroconductive metal, in contact each other. In such a case, the electric conduction among the three members can be assured to such an extent that the radiation noise causes no problem. The resistance between the inner and outer rings of the rolling bearing having electroconductive grease filled therein tends to be increased when the relative difference in rotational velocity between the inner ring and the outer ring. In view of the circumstances, the dynamic condition for measuring the resistance is determined as a radial or thrust load acting between the inner ring and the outer ring of 10 N and a relative difference in rotational velocity between the inner ring and the outer ring of 150 rpm. Furthermore, because the radiation noise is liable to occur when the rotation number is increased, the electroconductive rolling bearing is preferably used at a rotation number of about 75 rpm or more, and more preferably 100 rpm or more. In the case where the rotation number is low, the conventional grounding method can be employed since noise is difficult to occur. When the electroconductive rolling bearings are used in two or more sites in the apparatus, the number of parts constituting the apparatus itself can be reduced, and miniaturization of the apparatus can be realized. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Preferred embodiments of the invention will be described in detail based on the following figures, wherein:  
         [0014]    [0014]FIG. 1 is a schematic diagram showing a constitution of a laser beam printer using an electroconductive rolling bearing according to the invention for bearing rotation of a photoreceptor drum;  
         [0015]    [0015]FIG. 2 is a schematic diagram showing an experimental apparatus for measuring a resistance of an electroconductive rolling bearing; and  
         [0016]    [0016]FIG. 3 is a circuit diagram of the experimental apparatus shown in FIG. 2.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    The invention will be described in more detail with reference to the attached drawings.  
         [0018]    [0018]FIG. 1 is a schematic diagram showing a tandem type color laser beam printer using the electroconductive rolling bearing according to the invention for bearing a photoreceptor drum. The laser beam printer has four image-forming engines  10 Y,  10 M,  10 C and  10 Bk for forming toner images of four colors, yellow, magenta, cyan and black, respectively, and an intermediate transfer belt (IBT)  20 , on which toner images are primarily transferred from the image-forming engines, and the toner images formed on the intermediate transfer belt  20  by multiple transfer is then secondarily transferred to a recording sheet P, so as to form a full color image.  
         [0019]    The intermediate transfer belt  20  is formed to have an endless form and is hanged on a pair of belt transporting rollers  21  and  22 , so as to receive the primary transfer of the toner images formed by the image-forming engines  10 Y,  10 M,  10 C and  10 Bk of the respective colors through rotating in the direction shown by the arrow. A secondary transferring roller  30  is arranged at a position opposite to one of the belt transporting roller  21  with respect to the intermediate transfer belt  20  intervening between them, and the recording sheet P is passed between the transferring roller  30  and the intermediate transfer belt  20 , which are in contact with each other under pressure, so as to receive the secondary transfer of the toner images from the intermediate transfer belt  20 . In other words, the belt transporting roller  21  functions as a backup roller of the transferring roller  30 . On the other hand, at a position opposite to the other belt transporting roller  22 , a belt cleaner  23  for the intermediate transfer belt  20  is arranged to remove the toner remaining and attached to the intermediate transfer belt  20  after the secondary transfer. The residual toner removed by the belt cleaner  23  is transported as a waste toner to the front side (the near side of the page plane in FIG. 1) with an auger  23   a  and is dropped in a recovery box described later.  
         [0020]    The four image-forming engines  10 Y,  10 M,  10 C and  10 Bk are arranged in parallel under the intermediate transfer belt  20 , and toner images formed corresponding to image information of the respective colors are primarily transferred to the intermediate transfer belt  20 . The four image-forming engines are arranged in the order of yellow  10 Y, magenta  10 M, cyan  10 C and black  10 Bk along the rotational direction of the intermediate transfer belt  20 , and the black image-forming engine  10 Bk, which is expected to be used most frequently, is arranged at the nearest position to the secondary transfer position. Under the image-forming engines  10 Y,  10 M,  10 C and  10 Bk, a raster scanning unit  40  is arranged to expose photoreceptor drums  11  installed in the respective image-forming engine corresponding to the image information. The raster scanning unit  40 , which is common to all the image-forming engines  10 Y,  10 M,  10 C and  10 Bk, contains four semiconductor lasers (not shown in the figure) emitting laser beams Bm modulated with the image information of the respective colors, and one polygonal mirror  41  rotating at high speed for scanning the four laser beams Bm along the axial direction of the photoreceptor drums  11 . The laser beams Bm scanned by the polygonal mirror  41  are reflected by mirrors (not shown in the figure) to proceed along the prescribed paths, and then expose the photoreceptor drums  11  of the image-forming engines  10 Y,  10 M,  10 C and  10 Bk through scanning windows  42  provided in an upper part of the raster scanning unit  40 .  
         [0021]    The image-forming engines  10 Y,  10 M,  10 C and  10 Bk each contains the photoreceptor drum  11 , a charging roller  12  for charging the photoreceptor drum  11  to a uniform background potential, a developing device  13  for developing an electrostatic latent image formed on the photoreceptor drum  11  by exposure with the laser beam Bm, so as to form a toner image, and a drum cleaner  14  for removing a residual toner and paper powder from the surface of the photoreceptor drum  11  after transferring the toner image to the intermediate transfer belt  20 , and toner images corresponding to the image information of the respective colors are formed on the photoreceptor drums  11 .  
         [0022]    Primary transferring rollers  15 Y,  15 M,  15 C and  15 Bk are arranged at positions opposite to the photoreceptor drums  11  of the image-forming engines  10 Y,  10 M,  10 C and  10 Bk with respect to the intermediate transfer belt  20  intervening between them. A prescribed bias voltage is applied to the transferring rollers  15 Y,  15 M,  15 C and  15 Bk to form electric fields between the photoreceptor drums  11  and the transferring rollers  15 Y,  15 M,  15 C and  15 Bk, and thus the toner images electrically charged on the photoreceptor drums  11  are transferred to the intermediate transfer belt  20  through the coulomb force.  
         [0023]    The recording sheet P is fed from a paper feeding cassette  2  contained in a lower part of the printer chassis  1  to the interior of the printer, specifically to the secondary transfer position, at which the intermediate transfer belt  20  and the secondary transferring roller  30  are in contact with each other. The paper feeding cassette  2  is set by sliding down into the lower part of the printer chassis  1  from the front side of the printer chassis, and a pickup roller  24  and a paper feeding roller  25  are arranged in parallel in an upper part of the paper feeding cassette  2  set in the printer chassis, so as to withdraw the recording sheet P contained in the cassette  2 . A retarding roller  26  is arranged at a position opposite to the paper feeding roller  25  to prevent duplicate feeding of the recording sheet P.  
         [0024]    A transporting path  27  of the recording sheet P inside the printer is arranged in a substantially vertical direction along the left side wall of the printer chassis  1 . The recording sheet P withdrawn from the paper feeding cassette  2  positioned in the bottom part of the printer chassis  1  is raised along the sheet transporting path  27 , and after receiving transfer of the toner image at the secondary transfer position, the recording sheet P is then sent to a fixing device  3  arranged immediately above the secondary transfer position. The recording sheet P having the toner image fixed thereto by the fixing device  13  is ejected through a ejecting roller  28  to a paper delivery tray  1   a  arranged on an upper part of the printer chassis  1  in a face down state. In FIG. 1, numeral  29  denotes a registration roller for controlling the entrance timing of the recording sheet P into the secondary transfer position.  
         [0025]    Upon forming a full color image by the color laser beam printer thus configured, the raster scanning unit  40  scans the photoreceptor drums  11  of the respective image-forming engines  10 Y,  10 M,  10 C and  10 Bk at a prescribed timing corresponding to the image information of the respective colors, whereby toner images corresponding to the image information are formed on the photoreceptor drums  11  of the image-forming engines  10 Y,  10 M,  10 C and  10 Bk. The toner images formed in the image-forming engines  10 Y,  10 M,  10 C and  10 Bk are transferred one by one to the rotating intermediate transfer belt  20 , and a multiplied toner image formed by accumulating the toner images of the respective colors is produced on the intermediate transfer belt  20 . The recording sheet P is dispatched at a prescribed timing from the paper feeding cassette  2  and is then passed between the secondary transferring roller  30  and the intermediate transfer belt  20  weighing the timing when the toner image primarily transferred to the intermediate transfer belt  20  reaches the secondary transfer position. Consequently, the multiplied toner image on the intermediate transfer belt  20  is secondarily transferred to the recording sheet P. The recording sheet P having been subjected to the secondary transfer is fixed for the toner image by the fixing device  3 , whereby the full color image is completed on the recording sheet P.  
         [0026]    In the laser beam printer, the electroconductive rolling bearings according to the invention are used for bearing the photoreceptors  11  of the image-forming engines, and for bearing the belt transporting rollers  21  and  22 , on which the intermediate transfer belt  20  is hanged, and ground the photoreceptor drums  11  and the belt transporting roller  22 , respectively. The electroconductive rolling bearings also connects the belt transporting roller  21  to a high bias voltage.  
         [0027]    The inventors investigate two kinds of electroconductive rolling bearings (Example and Comparative Example) as to whether or not radiation noise due to leakage is generated upon using the bearings under various using conditions. The two kinds of electroconductive rolling bearings each is a single line deep groove ball bearing having a bearing outer diameter of 42 mm, a width of 7 mm and a bearing inner diameter of 30 mm, but only the species of electroconductive grease filled between the inner ring and the outer ring are different from each other.  
         [0028]    The composition of the electroconductive grease filled in the rolling bearing of the Example is as follows:  
                                                       Base oil for lubricating oil   80%           Thickening agent   18%           (carbon black, average particle diameter: 60 nm)           Additive for lubricating oil (lithium soap)    2%                      
 
         [0029]    On the other hand, conventional electroconductive grease produced by Kyodo Yushi Co., Ltd. is filled in the rolling bearing of the Comparative Example.  
         [0030]    The inventors firstly carry out an experiment for measuring resistances of the rolling bearings of the Example and the Comparative Example. The experiment is carried out in such a manner as shown in FIG. 2 that the inner ring of the rolling bearing  101  as a sample is assembled on a rotational axis  100  capable of being rotated at a prescribed speed, whereas the outer ring of the rolling bearing  101  is assembled on a fixed housing  102 , and a resistance of 300 kΩ and a constant voltage power source of 30 V are connected in series to a line withdrawn from the outer ring. A line connected to the rotational axis  100  is grounded with a slip ring  103 . A voltage occurring between the outer ring of the rolling bearing  101  as a sample and the rotational axis  100  is input through a probe  104  of an attenuation ratio of 10/1 into a digital oscilloscope  105 , and the voltage is measured by using the digital oscilloscope  105 . The measurement is carried out while the rotation rate of the rotational axis  100  is changed to 70 rpm and 150 rpm, and the maximum voltage V max  and the effective voltage V eff  are measured for the respective rotation rates. The measurement is also carried out while a radial load of 10 N is applied to the rolling bearing  101 . The effective voltage V eff  is such a voltage that is defined by the following equation. 
         Effective voltage V eff ={square root}((momentary voltage) 2 /measurement time) 
         [0031]    The experimental apparatus shown in FIG. 2 has an equivalent circuit shown in FIG. 3, and thus the voltage V in FIG. 3 is measured by the oscilloscope. Therefore, the maximum value R max  and the effective value R eff  of the resistance of the rolling bearing are calculated by the following equations by using the maximum voltage V max  and the effective voltage V eff  thus obtained. 
         Maximum value of resistance  R   max =300 ( k Ω)× V   max /(30 −V   max ) 
         Effective value of resistance  R   eff =300 ( k Ω)× V   eff /(30 −V   eff ) 
         [0032]    The photoreceptor drums are borne by the rolling bearings of the Example and the Comparative Example on the apparatus frame of the laser beam printer. An alternating electric current of 1 mA and a direct electric current of −700 V are applied to the photoreceptor drums, whereas the apparatus frame is grounded, and it is measured as to whether or not leakage noise actually occurs from the rolling bearing by using a spectrum analyzer (produced by Hewlett-Packard Company). At this time, no slide-contacting member, such as an earthing spring, is provided for the photoreceptor drums. The measurement is carried out in an electric wave laboratory. The results are shown in Table 1 below along with the measured voltages and the calculated resistances.  
                                                                                               TABLE 1                                           Measured   Calculated               Rotation   voltage   resistance                number   V max     V eff     R max     R eff     State of occurrence of leakage           (rpm)   (V)   (mV)   (kΩ)   (kΩ)   noise                        Example   70   0.68   23   6.96   0.23   no leakage noise occurring           150   1.04   223   10.77   2.25   no leakage noise occurring       Comparative   70   14.50   1,290   280   13.48   radiation noise occurring at from       Example                       30 MHz to over 300 MHz           150   28.30   7,290   5,000   96.30   leakage noise occurring                  
 
         [0033]    As shown in Table 1, in the case where the rolling bearing of the Comparative Example is used, radiation noise due to leakage occurs at both rotation numbers of 70 rpm and 150 rpm. On the other hand, in the case where the rolling bearing of the Example is used, occurrence of radiation noise due to leakage is not observed. In the case where the apparatus using the rolling bearing of the Example is used as an apparatus for forming an image, when image formation is carried out by rotating the bearings of the photoreceptor drums at 123.5 rpm, occurrence of radiation noise due to leakage is not found, and full color images of good conditions can be obtained at high speed over a large number of sheets without formation of abnormal noise and malfunction.  
         [0034]    As described in the foregoing, according to the apparatus for forming an image of the invention, the resistance is suppressed to a prescribed value or less by filling electroconductive grease. In an apparatus for forming an image constituted by bearing rotational members, such as a photoreceptor drum, by using the rolling bearings, leakage noise occurring from the rolling bearings can be avoided to suppress the radiation noise to such a level that causes no problem from the standpoint of the self-imposed standard, even when the rotational members are grounded using no slide-contacting member, such as an earthing spring. Furthermore, the apparatus for forming an image can be further miniaturized by using the bearings for two or more of the rotational members.  
         [0035]    The entire disclosure of Japanese Patent Application No. 2001-083367 filed on Mar. 22, 2001 including specification, claims, drawings and abstract is incorporated herein by reference in its entirety.