Patent Publication Number: US-8116648-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2008-052673, which was filed on Mar. 3, 2008, the contents of which are incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus having a transfer unit for transferring to a recording medium a toner image developed on a photoreceptor. 
     2. Description of the Related Art 
     An image forming apparatus employing an electrophotographic process generally includes a photoreceptor serving as an image bearing member which rotates, and a charging section, an exposure section, a developing section, a transfer section, a fixing section, a cleaning section, and an electricity removing section which are arranged around the photoreceptor. The charging section uniformly charges a surface of the photoreceptor. The exposure section irradiates the charged surface of the photoreceptor with light according to the image information and thereby forms an electrostatic latent image. The developing section stirs a toner therein to cause friction which electrifies the toner so that the toner is attached to the electrostatic latent image formed on the surface of the photoreceptor, and thereby forms a toner image. The transfer section provides a recording medium with charges of which polarity is opposite to that of the charges in the toner and thereby transfers the toner image to the recording medium. The fixing section fixes the transferred toner image to the recording medium with heat, pressure, or the like means. The cleaning section collects the toner which has failed to be transferred and thus remains on the surface of the photoreceptor. The electricity removing section removes the electricity of the photoreceptor, which remains after transferring of the toner image. The image forming apparatus employing the electrophotographic process constructed as above forms desired images on recording mediums. 
     A transfer unit, which serves as the transfer section for transferring the toner image from the photoreceptor to the recording medium, is composed of a transfer belt, a plurality of rollers on which the transfer belt is suspended in a tensioned manner and which turns the transfer belt around, and a transfer roller to which is applied an electric field having an opposite polarity to that of the charges of the toner image on the surface of the photoreceptor so that the toner image is transferred to recording paper on the transfer belt. The toner image on the photoreceptor is transferred to the recording paper conveyed to the running transfer belt. 
     In forming a color image in the electrophotographic process, toners of plural colors; for example, black (K), cyan (C), magenta (M), and yellow (Y), are used. In a color image forming apparatus, a structure having a photoreceptor as a center with other various sections such as those stated above, is provided for each color and a toner image is formed for every color. The toner images of all colors are transferred to a recording medium to be overlaid with each other and then fixed to the recording medium. A color image can be thus formed by subtractive color mixture. 
     To an intermediate transfer belt, toner images of respective colors are transferred in a manner that these images are sequentially overlaid with each other, thereby forming a multicolored toner image which will be then transferred to a recording medium. 
     In order to form images in such constitution as above, it is necessary to accurately adjust a rotating speed of each photoreceptor and a running speed of the transfer belt. An adjustment failure will lead to such transfer misregistration as position displacement of an image transferred to a recording medium, or deviation of image magnification, and in the case of forming color images, misregistration in overlaying toner images. 
     In an image forming apparatus disclosed in Japanese Unexamined Patent Publication JP-A 11-109827 (1999), one of a scanning speed of an exposure section, a rotating speed of an image bearing member, and a revolving speed of an intermediate transfer member, is controlled based on a count value, i.e., the number of sheets on which images have been printed, a temperature, and the like factor, in order to reduce fluctuation of the image magnification depending on durability of the intermediate transfer member as well as to reduce image change due to variation in electric resistance of the intermediate transfer member. 
     In an image forming apparatus disclosed in Japanese Unexamined Patent Publication JP-A 2004-252295, a revolving speed of a belt is directly measured at a transfer position and on the basis of the measurement result, the moving speed of the belt is controlled, in order to reduce position displacement of an image in spite of a change in thickness of, the elongation and vibration, etc. of the belt. 
     A toner may be attached to not only a photoreceptor but also a transfer belt, and this is why a cleaning section for transfer belt is provided to remove the toner attached to the transfer belt. A commonly-known cleaning section is composed of a cleaning blade which is made of an elastic material such as urethane rubber and disposed in contact with a surface of the transfer belt so as to scrape off the toner attached thereto. 
     For the transfer belt, a material having surface roughness Rz of around 6 μm is selected, and as the cleaning operation is performed with the cleaning blade in contact with the transfer belt, its microabrasion action undesirably causes the transfer belt to be gradually abraded away, resulting in the transfer belt having the surface roughness Rz of around 1 μm. 
     As the transfer belt has decreased surface roughness Rz, a frictional force increases at a contact portion between the cleaning blade and the transfer belt, and the cleaning blade may work as a brake, lowering the running speed of the transfer belt. 
     This causes mismatching between the rotating speed of the photoreceptor and the running speed of the transfer belt, which leads to some transfer misregistration. 
     In the image forming apparatus disclosed in JP-A 11-109827, the speed deterioration of the transfer belt due to the change in its surface roughness is not taken into consideration. In the image forming apparatus disclosed in JP-A 2004-252295 in which the speed of the belt is directly measured, it is possible to deal with the speed deterioration of the transfer belt due to the change in its surface roughness, but at the same time, it is necessary to provide a measurement section for directly measuring the speed of the belt. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an image forming apparatus with a simple structure in which a frictional force between a cleaning blade and transfer belt does not reduce a speed of a transfer belt. 
     The invention provides an image forming apparatus comprising a transfer unit for transferring to a recording medium a toner image developed on a photoreceptor, 
     the transfer unit having a transfer belt conveying the recording medium, and 
     the image forming apparatus comprising: 
     a cleaning section for cleaning a surface of the transfer belt by coming into contact therewith; and 
     a control section for controlling a running speed of the transfer belt based on a change in frictional force at a contact portion between the transfer belt and the cleaning section. 
     According to the invention, the control section controls the running speed of the transfer belt based on the change in frictional force at the contact portion between the transfer belt and the cleaning section. 
     Consequently, the increasing frictional force does not cause a decrease in the running speed of the transfer belt, with the result that the transfer belt can maintain its running speed at a constant level, thus allowing for formation of high-quality images without transfer misregistration. 
     Further, in the invention, it is preferable that the control section controls the running speed based on a correlation among the change in frictional force, a cumulative number of printed copies, and a distance that the transfer belt has run. 
     According to the invention, the control section controls the running speed based on the correlation among the change in frictional force, a cumulative number of printed copies, and the distance that the transfer belt has run. 
     The frictional force changes because the cleaning section scrubs the surface of the transfer belt of which surface roughness therefore changes. The change in friction force is therefore temporal change. Accordingly, the change in frictional force is correlated with the cumulative number of printed copies, the distance that the transfer belt has run, or the like factor relating to the temporal change, which is so-called a life count value. 
     Consequently, the use of the life count value enables the control on the running speed of the transfer belt in a simpler structure. 
     Further, in the invention, it is preferable that the control section controls the running speed for every predetermined increase in an cumulative number of printed copies or for every predetermined increase in a running distance of the transfer belt. 
     According to the invention, the control section controls the running speed for every predetermined increase in the cumulative number of printed copies or for every predetermined increase in the running distance of the transfer belt. 
     The frictional force changes so relatively slowly that these controls carried out at intervals of predetermined increase are enough to achieve accurate control as a whole. 
     Further, in the invention, it is preferable that the control section controls the transfer belt so that its running speed becomes equal to a circumferential speed of the photoreceptor. 
     According to the invention, the control section controls the transfer belt so that its running speed becomes equal to the circumferential speed of the photoreceptor. If the running speed of the transfer belt is slower than the circumferential speed of the photoreceptor, paper jam may occur. It is therefore preferable that the running speed of the transfer belt is set to be equal to, or at least around 1% higher than, the circumferential speed of the photoreceptor. 
     Further, in the invention, it is preferable that the transfer belt is composed of an endless belt member, and the running speed of the transfer belt is a revolving speed of the transfer belt. 
     According to the invention, the transfer belt is composed of an endless belt member, and the running speed of the transfer belt is the revolving speed of the transfer belt. 
     This makes it possible to easily control the running speed of the transfer belt by controlling only a rotating speed of a driving roller for turning the transfer belt. 
     Further, in the invention, it is preferable that the cleaning section has a plate-shaped cleaning blade made of urethane rubber and performs cleaning by bringing the cleaning blade into contact with the transfer belt. 
     According to the invention, the cleaning blade is used to clean the surface of the transfer belt. 
     The invention provides an image forming apparatus comprising a transfer unit for transferring to a recording medium a toner image developed on a photoreceptor, 
     the transfer unit having a transfer belt conveying the recording medium, and 
     the image forming apparatus comprising: 
     a cleaning section for cleaning a surface of the transfer belt by coming into contact therewith; and 
     a control section for controlling a running speed of the transfer belt based on a change in contact condition of the transfer belt and the cleaning section. 
     According to the invention, the control section controls the running speed of the transfer belt based on the change in contact condition of the transfer belt and the cleaning section. 
     This makes it possible to prevent the transfer belt from losing speed even when the transfer belt and the cleaning section come into contact with each other in a different way, so that the transfer belt can maintain its running speed at a constant level, which allows for formation of high-quality images without transfer misregistration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein: 
         FIG. 1  is a view showing a configuration of an image forming apparatus according to one embodiment of the invention; 
         FIG. 2  is an enlarged fragmentary view of a transfer unit in the image forming apparatus; 
         FIG. 3  is a block diagram showing an electric configuration of components relevant to a control on the transfer unit in the image forming apparatus; 
         FIG. 4  is a flowchart showing a speed control process of the transfer unit; and 
         FIG. 5  is a graph showing a relation between the number of printed copies and the revolving speed of a transfer belt. 
     
    
    
     DETAILED DESCRIPTION 
     Now referring to the drawings, preferred embodiments of the invention are described below. 
       FIG. 1  shows a view showing a configuration of an image forming apparatus  100  according to one embodiment of the invention. 
     The image forming apparatus  100  obtains image data read on document paper or image data received from external equipment, and forms monochrome images on recording paper according to the obtained image data. The image forming apparatus  100  roughly comprises an automatic document feeder (abbreviated as ADF)  101 , an image reading section  102 , a printing section  103 , a recording paper conveying section  104 , and a paper feeding section  105 . 
     In the automatic document feeder  101 , when document paper, at least one in number, is placed on a document placement tray  11 , the document paper is pulled and thereby introduced therefrom sheet by sheet into a document reading slot  102   a  of the image reading section  102 , through which the document paper is conveyed to be discharged onto a catch tray  12 . 
     Above the document reading slot  102   a , a contact image sensor (abbreviated as CIS)  13  is disposed. This CIS  13  repeatedly reads in a main scanning direction an image formed on the back side of the document paper passing through the document reading slot  102   a , and outputs image data of the image formed on the back side of the document paper. 
     In the image reading section  102 , the front side of the recording paper passing through the document reading slot  102   a  is irradiated with light of a lamp in the first scanning unit  15 , and the light reflected from the front side of the document paper is led by mirrors in the first and second scanning units  15  and  16  to an imaging lens  17  which is thus used to produce on a charge coupled device (abbreviated as CCD) image sensor  18  an image formed on the front side of the document paper. The CCD image sensor  18  repeatedly reads in the main scanning direction the image formed on the front side of the document paper and outputs the image data of the image formed on the front side of the document paper. It is thus possible to read images on both sides of the document paper. 
     Furthermore, in the case where the document paper is placed on a platen glass on top of the image reading section  102 , the first scanning unit  15  and the second scanning unit  16  are moved with their predetermined mutual speed relationship maintained so that the front side of the document paper placed on the platen glass is irradiated with light of the first scanning unit  15  and that the light reflected from the front side of the document paper is then led by the first and second scanning units  15  and  16  to the imaging lens  17  which is thus used to produce on the CCD image sensor  18  an image formed on the front side of the document paper. 
     The image data to be outputted from the CIS  13  or CCD image sensor  18  is subjected to various image processing in control circuits such as a microcomputer, before being outputted to the printing section  103 . 
     The printing section  103 , which forms images on recording paper according to image data, includes a photoreceptor  21 , a charger  22 , an optical writing unit  23 , a developing device  24 , a transfer unit  25 , a cleaning unit  26 , and a fixing unit  27 . 
     The photoreceptor drum  21  rotates in a single direction and has its surface cleaned by the cleaning unit  26  and then uniformly charged by the charger  22 . The charger  22  may be a non-contact charger-type charger or a contact roller-type or brush-type charger which comes into contact with the photoreceptor drum  21  to electrify it. 
     The optical writing unit  23  is a laser scanning unit (LSU) having two laser emitting portions  28   a  and  28   b  and two mirror groups  29   a  and  29   b . In the optical writing unit  23 , laser light according to image data inputted thereto is emitted from each of the laser emitting portions  28   a  and  28   b  and this laser light is then led to the photoreceptor drum  21  by way of each of the mirror groups  29   a  and  29   b  so that the uniformly charged surface of the photoreceptor drum  21  is exposed to the laser light, thereby forming an electrostatic latent image on the surface of the photoreceptor drum  21 . 
     To deal with the high-speed printing process, the optical writing unit  23  adopts a two beam system with the two laser emitting portions  28   a  and  28   b , to thereby lessen a burden associated with faster irradiation timing. 
     Note that for the optical writing unit  23 , the laser scanning unit may be replaced by an EL or LED writing head with an array of light-emitting elements. 
     The developing device  24  supplies the toner to the surface of the photoreceptor drum  21  to develop the electrostatic latent image, thereby forming a toner image on the surface of the photoreceptor drum  21 . The transfer unit  25  transfers the toner image on the surface of the photoreceptor drum  21  to recording paper conveyed by the paper conveying section  104 . The fixing unit  27  heats and pressurizes the recording paper so that the toner image thereon is fixed into place. After that, the recording paper is further conveyed by the paper conveying section  104  to a catch tray  47  where the recording paper is discharged. In addition, the cleaning unit  26  removes and collects a toner which remains on the surface of the photoreceptor drum  21  after the development and transfer operations. 
     A conductive elastic roller  34  is pressed against the surface of the photoreceptor drum  21  with the transfer belt  31  therebetween, with the result that the recording medium conveyed to the transfer belt  31  is pressed against the surface of the photoreceptor drum  21 . This conductive elastic roller  34  has an electric field of which polarity is opposite to that of charges in the toner image formed on the surface of the photoreceptor drum  21 . Owing to this electric field of opposite polarity, the toner image on the surface of the photoreceptor drum  21  is transferred to the recording paper on the transfer belt  31 . For examples, a positive (+) electric field is applied across the conductive elastic roller  34  when the toner image is negatively charged; or has minus (−) charges. 
     The fixing unit  27  has a heat roller  35  and a pressure roller  36 . Inside the heat roller  35 , a heat source is provided to heat a surface of the heat roller  35  to a predetermined temperature (the fixing temperature of around 160° C. to 200° C.). Further, a pressure member (not shown) is disposed on either side of the pressure roller  36  so that the pressure roller  36  comes into contact with the heat roller  35  at a predetermined pressure. The recording paper conveyed to a pressure-contact portion (referred to as a fixing nip portion) between the heat roller  35  and the pressure roller  36  is further conveyed by theses rollers  35  and  36  while an unfixed toner image on the recording paper is molten by heat and pressurized so that the toner image is fixed onto the recording paper. 
     The paper conveying section  104  includes plural pairs of conveying rollers  41  for conveying recording paper, a pair of registration rollers  42 , a conveying path  43 , a reversal conveying path  44 , a plurality of branch pawls  45 , and a pair of paper discharge roller  46 . 
     The recording paper is conveyed from the paper feeding section  105  to the conveying path  43  along which the recording paper is further conveyed until a leading end of the recording paper reaches the registration rollers  42 . At this time, the registration rollers  42  are temporarily halted, so that the recording paper curves with its leading end in contact with the registration rollers  42 . Using elastic force of this curved recording paper, the leading end of the recording paper and the registration rollers  42  are aligned in parallel with each other. After this, the registration rollers  42  start to rotate and thereby convey the recording paper to the transfer unit  25  in the printing section  103 , and the recording paper is then conveyed by the paper discharge roller  46  further to the catch tray  47 . 
     The registration rollers  42  each start and stop rotating by engaging and disengaging a clutch arranged between the registration roller  42  and its drive shaft or by switching on and off a motor which serves as a drive source of the registration roller  42 . 
     Moreover, in the case of recording an image on the back side of the recording paper as well, the plurality of branch pawls  45  turn to change routes of the conveying path  43  and the reversal conveying path  44  so that the recording paper is turned over in the reversal conveying path  44  and conveyed therein back to the registration rollers  42  in the conveying path  43 . An image is thus recorded also on the back side of the recording paper. 
     Along the conveying path  43  and the reversal conveying path  44 , sensors for detecting a position, etc. of the recording paper are provided in various positions. On the basis of the position of recording paper detected by these sensors, the conveying rollers and the registration rollers are driven under control for conveying and positioning the recording paper. 
     The paper feeding section  105  has a plurality of paper feeding trays  51 . The paper feeding trays  51  are each used for storing recording paper and disposed in a lower part of the image forming apparatus  100 . Each of the paper feeding trays  51  is provided with pickup rollers or the like component for pulling out the recording paper sheet by sheet, and the recording paper pulled out is then delivered to the conveying path  43  in the paper conveying section  104 . 
     Since the image forming apparatus  100  according to the present embodiment aims for high-speed printing process, the paper feeding trays  51  each have a capacity for 500 to 1,500 sheets of standard-size recording paper. 
     Moreover, on a lateral surface of the image forming apparatus  100 , a large capacity cassette (abbreviated as LCC)  52  is provided to store large piles of recording paper of different types, and a manual tray  53  is also provided to mainly feed nonstandard size recording paper. 
     The catch tray  47  is located on the other lateral surface of the image forming apparatus  100  than the lateral surface thereof on which the manual tray  53  is located. Instead of this catch tray  47 , an aftertreatment (e.g. stapling or punching) device for discharged paper, or a plurality of catch trays may be disposed as an option. 
       FIG. 2  is an enlarged fragmentary view of the transfer unit  25  in the image forming apparatus  100 . 
     The transfer unit  25  has the transfer belt  31 , a driving roller  32 , a driven roller  33 , and the conductive elastic roller  34 . The transfer belt is suspended and turned around in a tensioned manner on these rollers  32  to  34  and the other rollers. The transfer belt  31  is formed of an endless belt member with predetermined resistance (of 1×10 9  to 1×10 13  Ω/cm, for example), and carries the recording paper on its surface. 
     For the transfer belt  31 , a resin material so flexible but hardly stretchable that the transfer belt  31  can smoothly turn, is generally selected and formed into a film. The resin material as above includes heretofore known thermoplastic resin, thermoplastic elastomer, and thermosetting resin. 
     The transfer belt  31  is produced to have predetermined surface roughness in view of a releasing property of the recording paper to be conveyed by the transfer belt  31 . 
     The surface roughness is expressed by the roughness average (Ra) of center lines, the highest peak (Rmax), or the 10 point average roughness (Rz), of all sample parts selected at random in a surface. For the transfer belt  31 , a belt having 10 point average roughness (Rz) of around 1 μm to 7 μm is used, for example. 
     Below the driving roller, a cleaning unit for transfer belt (namely, transfer cleaning unit)  37  is disposed. The transfer cleaning unit  37  has a cleaning blade  38  and a removed-toner box  39  for accumulating the toner removed. The cleaning blade  38  is made of plate-shaped resin, such as urethane rubber, having moderate hardness and elasticity. 
     The cleaning blade  38  comes into contact with the transfer belt  31  to scrape off and thereby remove the toner attached to the surface of the transfer belt  31 . The difference in material hardness between the cleaning blade  38  and the transfer belt  31  causes the surface of the transfer belt  31  to be ground by the cleaning blade  38 . The abrasion on the surface of the transfer belt  31  indicates a decrease in its surface roughness. 
     The transfer belt  31  which is an elastic rubber belt with its surface layer coated with a fluorinated resin and has initial surface roughness Rz of 6 μm, operates with the cleaning blade  38  made of urethane rubber to print about 200,000 to 300,000 copies, that is to say, makes about 200,000 to 300,000 revolutions, and then it turns out that the surface roughness Rz of the transfer belt  31  has decreased to 1 μm. 
     As the transfer belt  31  has decreased surface roughness, a frictional force increases between the cleaning blade  38  and the transfer belt  31 , and the cleaning blade may work as a brake, lowering the running speed of the transfer belt  31 . 
     In the invention, how much speed the transfer belt  31  will lose is predicted, and the rotating speed of the driving roller  32  is adjusted so as not to cause a decrease in speed. To be specific, changes in speed decrease over time without adjustment have been measured in advance, and referring to this measurement result, the rotating speed of the driving roller  32  is increased so that the transfer belt  31  runs at constant speed. 
       FIG. 3  is a block diagram showing an electric configuration of components relevant to a control on the transfer unit in the image forming apparatus  100 . 
     A CPU  50  controls the whole image forming apparatus  100  including the transfer unit  25 . In controlling the transfer unit  25 , a read only memory (abbreviated as ROM)  61  stores a transfer unit control program for operating the driving roller  32 , etc., a photoreceptor control program for rotation control of the photoreceptor drum  21 , an image forming program for image forming operation, and the like program. A random access memory (abbreviated as RAM)  62  temporarily stores data required for operation according to the above programs. 
     A life management section  63  counts up the number of printed copies, the revolving speed of the transfer belt  31 , or the like element, and stores cumulative total values (life count values) of these elements, thereby managing usage of expendables such as a toner. A display  64  is mounted in an operation panel or the like part of the image forming apparatus  100  and shows the operation menu, a message indicative of exchanging a toner cartridge, a message indicative of occurrence of paper jam, etc. 
     A photoreceptor driving section  65  is composed of a motor for rotating the photoreceptor drum  21  of which rotating speed is controlled by the CPU  50 . A transfer belt driving section  66  is composed of a stepping motor for rotating the driving roller  32  in the transfer unit  25 , and the running speed of the transfer belt  31  is controlled by the CPU  50 . 
     The life count values and the running speed of the transfer belt  31 , that is, the rotating speed of the driving roller  32 , have been measured in advance as described above, and their correlation in form of a table, for example, may be stored in the life management section  63 . When the speed control is required, the CPU  50  will look up the table to control the rotating speed of the driving roller  32 . 
       FIG. 4  is a flowchart showing a speed control process of the transfer unit  25 . 
     In Step S 1 , the transfer unit  25  idles until a user inputs a print request, and if a print job request is transmitted to the transfer unit  25 , then the process goes to Step S 2 . In Step S 2 , the life count values stored in the life management section  63  are read out and checked. 
     In Step S 3 , the table of correlation between the life count values and the rotating speed of the driving roller  32  is looked up, and a rotating speed of the driving roller  32  corresponding to the life count values read out is determined at which the driving roller  32  is then set to rotate. 
     In Step S 4 , a printing process starts by rotating the driving roller  32  at the set speed so that the transfer belt  31  turns at a constant running speed. In Step S 5 , the requested printing process terminates and then in Step S 6 , the life count values are updated based on the number of printed copies and the revolving speed of the transfer belt  31 . 
     As above, by increasing the rotating speed of the driving roller  32  according to the life count values, it is possible to control the transfer belt  31  to keep itself from losing speed in spite of the contact between the transfer belt  31  and the cleaning blade  38 , thereby allowing for the transfer belt  31  to run at a constant speed. 
     As a result, no mismatching will be caused between the rotating speed of the photoreceptor drum  21  and the running speed of the transfer belt  31 , allowing for a high-quality-image forming process without transfer misregistration. 
     Although the life count values are collectively updated in Step S 6  after completion of the print job in the above description, the configuration is not limited thereto and the life count values may be sequentially updated after printing every single copy or after every single turning of the transfer belt  31 . 
     In the above flow, the rotating speed of the driving roller  32  is controlled based on the life count values at the moment when the print job is inputted. On the other hand, if the life count values are sequentially updated, then the speed control is carried out by checking the life count values for every update and looking up the table of correlation, thus allowing for more complete control. Especially in the case where a great number of copies are to be printed according to an inputted print job, it may possibly become necessary to increase the rotating speed of the driving roller  32  in the middle of one print job processing to keep the transfer belt  31  from losing speed, and it is therefore preferable to control the speed during one print job processing. 
     Alternatively, the speed control may be carried out for every predetermined increases in the life count values; for example, for every 100-sheet increase in the number of printed copies or for every 100-turn increase in the revolving speed of the transfer belt  31 . 
     Although the monochrome image forming apparatus is taken as an example to describe the invention in the above embodiment, note that application of the invention is not limited to such the monochrome image forming apparatus but may be a color image forming apparatus forming toner images of plural colors or any other image forming apparatus as long as it employs a transfer belt or intermediate transfer belt which is cleaned by a cleaning blade. 
     EXAMPLES 
     Examples of the invention will be described hereinbelow. 
     Using a commercially-available image forming apparatus, 300K copies (where K represents 1,000) were printed and at the time, the revolving speed (rpm) of the transfer belt  31  was measured. A set revolving speed, which was to be maintained at a constant level, of the transfer belt  31  was determined at 2,510 rpm. 
       FIG. 5  is a graph showing a relation between the number of printed copies and the revolving speed of the transfer belt. This graph shows the number of printed copies (sheets) on the horizontal axis with the revolving speed (rpm) of the transfer belt on the vertical axis. 
     At the outset, printing was done without speed control and a rate of decrease in speed was measured. The measurement result is plotted as a graph 1. It shows that the revolving speed of the transfer belt is lower as more copies are printed. 
     On the basis of the measurement result, difference from the set revolving speed of 2,510 rpm was calculated and defined as a correction value. 
     For example, when 300K copies had been printed, an actual measurement value of the revolving speed was 2,495 rpm, and a difference thereof from the set revolving speed of 2,510 rpm was therefore 15 rpm by evaluating the expression 2510-2495. Then, the correction value was determined as 0.60% by evaluating the expression 15/2495×100. The correction value may be used to control the rotating seed of the driving roller  32  so that the revolving speed of the transfer belt is higher by 0.60% with the life count value of 300K copies. 
     Referring to the actual measurement values, it is possible to create a table of correlation between the correction values and the number of printed copies serving as the life count value. Table 1 is one example of such tables of correlation. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 The number of printed 
               
               
                   
                 copies (sheets) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 0K 
                 100K 
                 150K 
                 200K 
                 250K 
                 300K 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Correction values 
                 0% 
                 0.40% 
                 0.50% 
                 0.60% 
                 0.60% 
                 0.60% 
               
               
                   
               
            
           
         
       
     
     The measurement result obtained in the case of printing under speed control based on the correction values as above is plotted as a graph 2. It shows that the transfer belt is able to maintain a constant revolving speed in spite of an increasing number of printed copies. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.