Patent Publication Number: US-10310441-B2

Title: Electrophotographic image forming apparatus, and electricity removing member used in image forming apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an electrophotographic image forming apparatus and an electricity removing member. 
     BACKGROUND ART 
     In electrophotographic image forming apparatuses, an electrostatic latent image is formed on a charged photoconductor, then it is developed by toner and a toner image is formed on the photoconductor, and after the toner image is transferred therefrom to a sheet, charges that have remained on the photoconductor are removed by an electricity removing device. As an example of the electricity removing device, there known a configuration for removing charges from the photoconductor by causing a grounded electricity removing member to contact the photoconductor (for example, see PTL 1). 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Patent Application Publication No. H01-154186 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     Meanwhile, in the configuration where the electricity removing member comes in contact with the photoconductor, electric characteristics of the electricity removing member, such as an inner resistance, may influence the electricity removing capability. However, not only the inner resistance of the electricity removing member but also a contact resistance of the electricity removing member may influence the electricity removing capability. 
     The present invention has been made in view of such conventional circumstances, and it is an object of the present invention to provide an image forming apparatus and an electricity removing member used in the image forming apparatus that can improve the electricity removing capability by taking the contact resistance into consideration. 
     Solution to the Problems 
     An image forming apparatus according to an aspect of the present invention includes a photoconductor and an electricity removing member electrically grounded and rotatably disposed to be in contact with a surface of the photoconductor. In the image forming apparatus, with regard to a resistance component of an inner impedance of the electricity removing member and a resistance component of a contact impedance of the electricity removing member that are calculated from a Cole-Cole plot obtained from measurement in a predetermined frequency range by an AC impedance method, the resistance component of the inner impedance is equal to or lower than a value that is obtained by multiplying a calculated resistance value by a first specific value, the calculated resistance value being calculated based on a predetermined formula as a DC resistance value of the electricity removing member that is required to reduce a pre-electricity-removal potential of the photoconductor to a predetermined post-electricity-removal potential during an electricity removal time that is obtained by dividing a contact width between the photoconductor and the electricity removing member by a linear speed of the photoconductor, the first specific value being calculated based on a ratio of a linear speed of the electricity removing member to the linear speed of the photoconductor, and the resistance component of the contact impedance is equal to or lower than a value that is obtained by multiplying the calculated resistance value by a second specific value that is calculated based on the ratio of the linear speed of the electricity removing member to a linear speed of the photoconductor. 
     Advantageous Effects of the Invention 
     According to the present invention, it is possible to provide an image forming apparatus and an electricity removing member used in the image forming apparatus that can improve the electricity removing capability by taking the contact resistance into consideration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration of an image forming apparatus according to a first embodiment of the present invention. 
         FIG. 2  is a diagram for explaining a main part of an image forming portion of the image forming apparatus according to the first embodiment of the present invention. 
         FIG. 3  is a diagram showing an equivalent circuit for explaining electric characteristics between a photoconductor and an electricity removing member of the image forming apparatus according to the first embodiment of the present invention. 
         FIG. 4  is a diagram showing a Cole-Cole plot of the electricity removing member of the image forming apparatus according to the first embodiment of the present invention. 
         FIG. 5  is a diagram showing an experiment device used to obtain the Cole-Cole plot of the electricity removing member of the image forming apparatus according to the first embodiment of the present invention. 
         FIG. 6  is a diagram showing the experiment device used to obtain the Cole-Cole plot of the electricity removing member of the image forming apparatus according to the first embodiment of the present invention. 
         FIG. 7  is a diagram showing invention examples and comparative examples. 
         FIG. 8  is a diagram showing relationship between a post-electricity-removal potential and a ratio of a linear speed of the electricity removing member to a linear speed of the photoconductor in the image forming apparatus according to the first embodiment of the present invention. 
         FIG. 9  is a diagram showing a configuration of a brush brittle of the electricity removing member of the image forming apparatus according to the first embodiment of the present invention. 
         FIG. 10  is a block diagram showing a system configuration of the image forming apparatus according to the first embodiment of the present invention. 
         FIG. 11  is a flowchart showing an example of a first speed change process executed in the image forming apparatus according to the first embodiment of the present invention. 
         FIG. 12  is a diagram showing relationship between a post-electricity-removal potential and the ratio of the linear speed of the electricity removing member to the linear speed of the photoconductor in the image forming apparatus according to the first embodiment of the present invention. 
         FIG. 13  is a diagram for explaining a main part of an image forming portion of an image forming apparatus according to a second embodiment of the present invention. 
         FIG. 14  is a block diagram showing a system configuration of the image forming apparatus according to the second embodiment of the present invention. 
         FIG. 15  is a flowchart showing an example of a contact pressure change process executed in the image forming apparatus according to the second embodiment of the present invention. 
         FIG. 16  is a block diagram showing a system configuration of an image forming apparatus according to a third embodiment of the present invention. 
         FIG. 17  is a flowchart showing an example of a second speed change process executed in the image forming apparatus according to the third embodiment of the present invention. 
         FIG. 18  is a diagram showing relationship between a cumulative printing rate and a contact resistance component of a contact impedance of the electricity removing member in the image forming apparatus according to the third embodiment of the present invention. 
         FIG. 19  is a diagram for explaining a main part of an image forming portion of an image forming apparatus according to a modification of the third embodiment of the present invention. 
         FIG. 20  is a block diagram showing a system configuration of an image forming apparatus according to a fourth embodiment of the present invention. 
         FIG. 21  is a flowchart showing an example of a third speed change process executed in the image forming apparatus according to the fourth embodiment of the present invention. 
         FIG. 22  is a diagram showing relationship between a cumulative number of prints and an outer diameter of the electricity removing member in the image forming apparatus according to the fourth embodiment of the present invention. 
         FIG. 23  is a block diagram showing a system configuration of an image forming apparatus according to a modification of the fourth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes embodiments of the present invention with reference to the accompanying drawings. It should be noted that the following embodiments are examples of specific embodiments of the present invention and should not limit the technical scope of the present invention. 
     First Embodiment 
     As shown in  FIG. 1 , an image forming apparatus  10  according to a first embodiment of the present invention is an electrophotographic monochrome printer and includes a control portion  1 , an image forming portion  2 , a sheet feed portion  3 , and a sheet discharge portion  4 . Other examples of the image forming apparatus according to the present invention include a facsimile, a copier, and a multifunction peripheral. In addition, the image forming apparatus according to the present invention is not limited to the image forming apparatus  10  supporting a monochrome printing as described in the first embodiment, but may be an electrophotographic color image forming apparatus of a tandem type or the like including a plurality of image forming portions for a plurality of colors. 
     The control portion  1  includes a CPU, a RAM, a ROM, and an EEPROM and controls the image forming apparatus  10  by causing the CPU to execute various processes in accordance with control programs stored in the ROM. 
     The image forming portion  2  is an electrophotographic image forming portion including a photoconductor drum  21 , a charging device  22 , a laser scanning device  23 , a developing device  24 , a transfer roller  25 , a cleaning member  26 , an electricity removing member  27 , and a fixing device  28 . It is noted that the photoconductor drum  21  is an example of the photoconductor, and the photoconductor is not limited to the photoconductor drum  21 , but may be a photoconductor belt, for example. 
     In the image forming apparatus  10 , the image forming portion  2 , under the control of the control portion  1 , executes an image forming process (print process) to form an image on a sheet such as a sheet of paper supplied from a sheet feed cassette  31  of the sheet feed portion  3 , and the sheet after the image forming process is discharged to the sheet discharge portion  4 . 
     Specifically, in the print process, the laser scanning device  23  scans a light beam on the surface of the photoconductor drum  21  charged by the charging device  22  so that an electrostatic latent image is formed based on image data. The electrostatic latent image formed on the surface of the photoconductor drum  21  is developed by the developing device  24 , and is transferred to the sheet by the transfer roller  25 . 
     Subsequently, the toner transferred to the sheet is fused and fixed to the sheet by the fixing device  28 . It is noted that the toner that has remained on the surface of the photoconductor drum  21  is cleaned by the cleaning member  26 . In addition, charges that have remained on the photoconductor drum  21  are removed by the electricity removing member  27  which is disposed on the downstream side of the cleaning member  26 . 
     The photoconductor drum  21  is, for example, an organic photoconductor (OPC) having a single-layer structure in which a photosensitive layer is formed around an aluminum tube, wherein the photosensitive layer contains a charge generating material and a charge transport material. The charge generating material is, for example, a perylene-based pigment, a phthalocyanine-based pigment or the like. The change transport material is, for example, a hydrazone-based compound, a fluorenone-based compound, an arylanime-based compound or the like. 
     Specifically, the photoconductor drum  21  is a positive-charged single layer photoconductor (PSLP) drum. It is noted that as other embodiments, the photoconductor drum  21  may be an organic photoconductor having a multi-layer structure, or may be a negative-charged organic photoconductor. 
     As shown in  FIG. 2 , the charging device  22  includes a charging roller  220  (an example of the charging member) that comes in contact with the photoconductor drum  21 . A power supply  221  applies a positive DC voltage to the charging roller  220 . This allows the charging roller  220  to apply a positive DC voltage to the photoconductor drum  21  so as to charge the photoconductor drum  21  to a predetermined charging potential. That is, the charging device  22  of the present embodiment is neither an AC-superposing-type charging device that superposes an AC voltage on a DC voltage, nor a contactless charging device, such as a scorotron charger, that charges the photoconductor drum  21  in a contactless manner. It is noted that as other embodiments, the charging device  22  may be an AC-superposing-type charging device or a contactless charging device. 
     The electricity removing member  27  is electrically grounded to the earth. In addition, the electricity removing member  27  is supported in such a way as to rotate while in contact with the surface of the photoconductor drum  21 . Specifically, the electricity removing member  27  is a brush-like roller member formed from a conductive metal material or resin material. As shown in  FIG. 2 , the electricity removing member  27  includes a basic body portion  270  and brush bristles  271 , wherein the basic body portion  270  is cylindrical, and one end of the brush bristles  271  is fixed to the basic body portion  270  and the other end is brought in contact with the surface of the photoconductor drum  21 . In addition, the electricity removing member  27  is not limited to a brush-like shape, but may be a cylindrical (roll-shaped) roller member formed from a conductive metal material or resin material. The resin material is, for example, rubber or sponge. 
     Meanwhile, in a configuration where the electricity removing member  27  comes in contact with the photoconductor drum  21 , as in the image forming apparatus  10 , electric characteristics of the electricity removing member  27 , such as the inner capacitance, may influence the potential stability and the memory image presence/absence on the surface of the photoconductor drum  21 . However, not only the inner capacitance of the electricity removing member  27 , but also the contact capacitence of the electricity removing member  27  may influence the potential stability and the memory image presence/absence. 
     In addition, in the image forming apparatus  10 , an electric characteristic, such as an inner resistance, of the electricity removing member  27  influences the electricity removing capability. However, not only the inner resistance of the electricity removing member  27  but also a contact resistance of the electricity removing member  27  may influence the electricity removing capability. Specifically, since the photoconductor drum  21  has a high surface resistance value, a horizontal flow of charges does not occur on the surface of the photoconductor drum  21 . As a result, even if the inner resistance of the electricity removing member  27  is low, if the contact resistance with the photoconductor drum  21  is high, charges cannot be removed effectively from the photoconductor drum  21 . 
     In a case where, as in the first embodiment, the contact-type charging device  22  that contacts the photoconductor drum  21  is used, generation of VOC (volatile organic compounds) is suppressed, compared to a contactless charging device such as the scorotron charger that charges the photoconductor drum  21  in a contactless manner. However, contact-type charging devices may be inferior to contactless charging devices in charging performances. In addition, the charging device  22  is of a type that applies a DC voltage. This may lower the charging performance. 
     On the other hand, as described below, the image forming apparatus  10  is configured such that the electric characteristic of the electricity removing member  27  satisfies a preset first specific condition. With this configuration, it is possible to improve the potential stability by taking the contact capacitance also into consideration and suppress an occurrence of the memory image. In addition, as described below, the image forming apparatus  10  is configured such that the electric characteristic of the electricity removing member  27  satisfies a preset second specific condition. With this configuration, it is possible to improve the electricity removing capability by taking into consideration also the contact resistance of the electricity removing member  27 . 
     As shown in  FIG. 3 , in an equivalent circuit  5  that indicates electric characteristics between the photoconductor drum  21  and the electricity removing member  27  of the image forming portion  2 , a resistor  51 , a capacitor  52 , and a resistor  53  are connected in parallel, wherein the resistor  51  corresponds to a DC resistance value R 1  of photoconductor drum  21 , the capacitor  52  corresponds to a capacitance C of the photoconductor drum  21 , and the resistor  53  corresponds to a DC resistance value R 2  of the electricity removing member  27 . 
     It is generally considered that in the equivalent circuit  5 , the lower the DC resistance value R 2  of the electricity removing member  27  is, the higher the electricity removing capability of the photoconductor drum  21  by the electricity removing member  27  is. However, it has been found that, in the actuality, not only the DC resistance value R 2  of the electricity removing member  27 , but also the contact resistance between the electricity removing member  27  and the photoconductor drum  21  influences the electricity removing capability. 
       FIG. 4  shows a Cole-Cole plot that was obtained by measuring an inner impedance Z 1  and a contact impedance Z 2  of the electricity removing member  27  by the AC impedance method in a predetermined frequency range of, for example, 0.05 Hz to 100 kHz. It payable to calculate, from this plot, an inner resistance component Ra and an inner capacitance component Ca of the inner impedance Z 1 , and a contact resistance component Rb and a contact capacitance component Cb of the contact impedance Z 2 . Here, in the Cole-Cole plot shown in  FIG. 4 , the inner impedance Z 1  and the contact impedance Z 2  are plotted as semicircles. However, they may each be plotted as a circular arc such as a semielliptical shape. 
     In the first embodiment, it is assumed that the resistance between the core metal of the photoconductor drum  21  and the photosensitive layer can be ignored. In addition, the DC resistance value R 1  of the photoconductor drum  21  is very high relative to the DC resistance value R 2  of the electricity removing member  27 . As a result, a combined resistance R 3  of the photoconductor drum  21  and the electricity removing member  27  can be considered the same as the DC resistance value R 2  of the electricity removing member  27 . 
     Suppose here that “t” denotes an electricity removal time during which the photoconductor drum  21  is in contact with the electricity removing member  27 , V 1  denotes a post-electricity-removal potential that is determined in advance as a target value of the surface potential of the photoconductor drum  21  after an elapse of the electricity removal time t, V 0  denotes a pre-electricity-removal potential of the photoconductor drum  21  at the start of the electricity removal by the electricity removing member  27 , and C denotes the capacitance of the photoconductor drum  21 . In this case, a theoretical value of DC resistance value R 2  of the electricity removing member  27  (hereinafter, the value is referred to as “calculated resistance value R 21 ”) by which the surface potential of the photoconductor drum  21  is changed by electricity removal from the pre-electricity-removal potential V 0  to the post-electricity-removal potential V 1  during the electricity removal time t, is calculated based on the following formula (1). It is noted that when S denotes a linear speed (surface speed) of the photoconductor drum  21 , and L denotes a contact width of the photoconductor drum  21  and the electricity removing member  27  in the rotation direction of the photoconductor drum  21 , the electricity removal time t is calculated as L/S.
 
[Math 1]
 
 V 1 =V 0 ×e   −t/(R2·C)   (1)
 
     However, as described above, the contact impedance of the electricity removing member  27  and the photoconductor drum  21  also influences the electricity removing capability of the electricity removing member  27 . As a result, in the image forming apparatus  10 , the electricity removing member  27  is configured in such a way as to satisfy the conditions of the following formulas (2) and (3) (the second specific condition)
 
[Math 2]
 
 Ra≤R 21×3×{1+(|1− Sr |×1.9)}  (2)
 
[Math 3]
 
 Rb≤R 21×1.2×{1+(|1− Sr |×1.9)}  (3)
 
     That is, in the image forming apparatus  10 , as shown in the formula (2), the inner resistance component Ra of the electricity removing member  27  is equal to or lower than a value obtained by multiplying the calculated resistance value R 21  of the electricity removing member  27  by a first specific value that is calculated on a ratio Sr which is a ratio of the linear speed of the electricity removing member  27  to the linear speed of the photoconductor drum  21 . In addition, in the image forming apparatus  10 , as shown in the formula (3), the contact resistance component Rb of the electricity removing member  27  is equal to or lower than a value obtained by multiplying the calculated resistance value R 21  of the electricity removing member  27  by a second specific value that is calculated based on the ratio Sr. 
     As described above, in the image forming apparatus  10 , the electric characteristics of the electricity removing member  27  are determined by taking into consideration not only the DC resistance value R 2  of the electricity removing member  27 , but also the inner resistance component Ra and the contact resistance component Rb. With this configuration, it is possible to improve the electricity removing capability of the electricity removing member  27 . On the other hand, the actual value of the DC resistance value R 2  of the electricity removing member  27  may be equal to or lower than the calculated resistance value R 21 , or higher than the calculated resistance value R 21 . 
     Specifically, the electricity removing capability of the electricity removing member  27  is improved since the inner resistance component Ra and the contact resistance component Rb of the electricity removing member  27  are respectively equal to or lower than values that are defined based on: the calculated resistance value R 21  which allows electricity to be removed to the post-electricity-removal potential V 1  during the electricity removal time t; and the ratio Sr of the linear speed of the electricity removing member  27  to the linear speed of the photoconductor drum  21 . It is noted that the first and second specific values are not limited to the above-mentioned values as far as similar effects are produced. 
     For example, in the image forming apparatus  10 , as shown in  FIG. 9 , each of the brush bristles  271  of the electricity removing member  27  includes a core portion  271 A and a surface layer portion  271 B. Here,  FIG. 9  is a cross section of one brush bristle  271 . The core portion  271 A is made of resin. The surface layer portion  271 B is made of carbon, and covers the surface of the core portion  271 A. For example, surface layer portion  271 B is formed together with the core portion  271 A when the brush bristle  271  is manufactured. In addition, the surface layer portion  271 B may be formed, after the core portion  271  is formed, by spraying carbon to the surface of the core portion  271 A. With this configuration, compared to a configuration where each of the brush bristles  271  is composed of only a resin layer that contains carbon, it is possible to reduce the inner resistance component Ra and the contact resistance component Rb of the electricity removing member  27 , while maintaining the strength of the brush bristles  271 . It is noted that the surface layer portion  271 B may contain a component other than carbon as far as the electricity removing member  27  satisfies the above-indicated formulas (2) and (3). In addition, the core portion  271 A may contain carbon. In addition, each of the brush bristles  271  may be composed of only a resin layer that contains carbon. 
     In addition, in the image forming apparatus  10 , the electricity removing member  27  rotates upon receiving a rotational driving force supplied from a first drive portion  272  (see  FIG. 10 ) such as a motor. For example, the electricity removing member  27  rotates at a faster linear speed than the photoconductor drum  21 . It is noted that the electricity removing member  27  may rotate at an equal speed to the photoconductor drum  21 , or at a slower speed than the photoconductor drum  21 . In addition, the electricity removing member  27  may rotate following the photoconductor drum  21  at a speed that is obtained be multiplying the linear speed of the photoconductor drum  21  by a predetermined ratio. 
     In addition, as described above, the contact impedance of the electricity removing member  27  with the photoconductor drum  21  also influences the potential stability and the image memory presence/absence on the surface of the photoconductor drum  21 . In the image forming apparatus  10 , the electricity removing member  27  is configured in such a way as to satisfy the conditions of the following formulas (4) and (5) (the first specific condition) as well.
 
Ca≤1.0E+05  (4)
 
0≤Cb/Ca≤0.4  (5)
 
     That is, in the image forming apparatus  10 , as shown in the formula (4), the inner capacitance component Ca of the electricity removing member  27  is equal to or lower than “1.0E+05” that is an example of the predetermined fourth specific value. In addition, in the image forming apparatus  10 , as shown in the formula (5), a capacitance ratio (Cb/Ca) that is obtained by dividing the contact capacitance component Cb of the electricity removing member  27  by the inner capacitance component Ca is equal to or lower than 0.4, wherein “0.4” is an example of the predetermined third specific value. 
     In this way, in the image forming apparatus  10 , with the configuration where the electric characteristics of the electricity removing member  27  are determined by taking into consideration the inner capacitance component Ca and the contact capacitance component Cb of the electricity removing member  27 , it is possible to improve the potential stability of the photoconductor drum  21  and suppress an occurrence of the image memory. Specifically, the inner capacitance component Ca is determined in such a way as to reduce the amount of charge that accumulates in the electricity removing member  27 , and the ratio of the contact capacitance component Cb to the inner capacitance component Ca is low, thus the charge is likely to leak from the electricity removing member  27 . This makes it possible to improve the potential stability and suppress an occurrence of the image memory. It is noted that the values of the third and fourth specific values are not limited to those described above as far as the similar effects are produced. 
     Examples 
     The following explains the measurement results of the image forming apparatus  10  with reference to  FIG. 5  to  FIG. 8 . 
       FIG. 5  and  FIG. 6  show an experiment device  90  that measures the inner resistance component Ra, the contact resistance component Rb, the inner capacitance component Ca and the contact capacitance component Cb of the electricity removing member  27 . The experiment device  90  includes two SUS rollers  91  and  92  aligned in the horizontal direction with 4 mm of distance therebetween, each of which is made of stainless steel and 18 mm in diameter. A film electrode  93  made of aluminum and having 150 mm of horizontal length is suspended between the SUS roller  91  and the SUS roller  92 . Each of the electricity removing members  27  of comparative examples 1 to 15 and invention examples 1 to 5 that are the experiment objects, is disposed to be in contact with the upper surface of the film electrode  93 . 
     In addition, the experiment device  90  includes a SUS roller  95  that has 30 mm of diameter and is disposed on the electricity removing member  27 . A weight  96  of 1 kg applies a downward load to the SUS roller  95 , and the load is applied to the electricity removing member  27  via the SUS roller  95 . The experiment is conducted in a state where the electricity removing member  27  and the SUS rollers  91 ,  92  and  95  are not rotating. The two SUS rollers  91  and  92  are connected to one electrode of an impedance measuring equipment  97  (LCR HiTESTER 3522 made by Hioki E. E. Corporation), and a base body  81  of the electricity removing member  27  is connected to the other electrode of the impedance measuring equipment  97 . In this state, the impedance measurement is performed by the impedance measuring equipment  97 . In this experiment, a sinusoidal AC voltage whose voltage value is 5.0 V is applied to ends of the two electrodes of the impedance measuring equipment  97 . The inner resistance component Ra, the contact resistance component Rb, the inner capacitance component Ca, and the contact capacitance component Cb of the electricity removing member  27  are measured while changing the frequency of the applied AC voltage in a range from 0.05 Hz to 100 kHz. The measurement was performed a plurality of times (2 to 16 times). The table of  FIG. 7  shows experiment results based on the average values of the measured values. 
       FIG. 7  also shows results of evaluation on the print process executed by the image forming apparatus  10  loaded with each electricity removing member  27  of the examples shown in  FIG. 7 , with regard to the electricity removing capability of the electricity removing member  27  on removing electricity from the photoconductor drum  21 , the potential stability, and the image memory presence/absence. 
     Here, with regard to the electricity removing capability, after the electricity removal of the photoconductor drum  21  had been performed by the electricity removing member  27 , an evaluation was made on whether the potential of the photoconductor drum  21  was reduced to a desired post-electricity-removal potential V 1 . In  FIG. 7 , “success” and “failure” are used to indicate the evaluation result of the electricity removing capability, wherein “success” indicates that the potential was reduced to the desired post-electricity-removal potential V 1 , and “failure” indicates that the potential was not reduced to the desired post-electricity-removal potential V 1   
     With regard to the potential stability, after a continuous printing of 60 minutes had been performed in the image forming apparatus  10 , the surface potential of the photoconductor drum  21  after charging by the charging device  22  was measured, and an evaluation was made on whether the surface potential was reduced by 10% or more from the initial surface potential that had been measured after charging by the charging device  22  before the start of the continuous printing. In  FIG. 7 , “success” and “failure” are used to indicate the evaluation result of the potential stability, wherein “success” indicates that the surface potential was not reduced by 10% or more from the initial surface potential, and “failure” indicates that the surface potential was reduced by 10% or more from the initial surface potential. The reason why the value “10%” was adopted is that when the surface potential is reduced by 10% or more from the initial surface potential, a problem such as a fog may occur. 
     With regard to the image memory presence/absence, after the image forming process had been performed by the image forming apparatus  10  to form a black patch of a predetermined shape on the front end of the print sheet and form a half image (gray image) on the other region of the print sheet, an evaluation was made visually on whether or not an image memory was generated. Specifically, when the shape of the black patch appeared in the half image region, it was determined that an image memory was generated. In  FIG. 7 , “success” and “failure” are used to indicate the evaluation result of the image memory presence/absence, wherein “success” indicates that an image memory was not generated, and “failure” indicates that an image memory was generated. 
     More specifically, a remodeled version of printer “FS-1320DN” made by KYOCERA Document Solutions Inc. was used as the image forming apparatus  10  in the experiment. In addition, in the image forming apparatus  10 , the pre-electricity-removal potential V 0  of the photoconductor drum  21  was 500 V, the surface speed (linear speed) of the photoconductor drum  21  was 0.15 m/s, and the contact width L was 0.005 m. In addition, the vacuum permittivity ε 0  was (8.9E−12) F/m, the relative permittivity εr of the photoconductor drum  21  was 3.5, and the film thickness d of the photoconductor drum  21  was 3.5E−05 m. In this case, the capacitance C of the photoconductor drum  21  was calculated as 8.85E−07 F from “ε0×εr/d”. 
     Furthermore, the post-electricity-removal potential V 1  was set to 100 V, wherein the post-electricity-removal potential V 1  is a desired potential after an electricity removal of the photoconductor drum  21  by the electricity removing member  27 . In this case, from the above-indicated formula (1), the calculated resistance value R 21  of the electricity removing member  27  is calculated as 2.34E+04 Ω. It is noted that the post-electricity-removal potential V 1  may be calculated by, for example, an expression “V 1 =V 0 ×0.2”, or, to provide a margin, may be calculated by, for example, an expression “V 1 =V 0 ×0.22+80”. 
     Here, in the comparative examples 1 to 13 and the invention examples 1 to 3, the surface speed (linear speed) of the electricity removing member  27  was set to 0.15 m/s that was the same as a linear speed S of the photoconductor drum  21 . As a result, in the comparative examples 1 to 13 and the invention examples 1 to 3, when the inner resistance component Ra of the electricity removing member  27  is equal to or lower than 7.02E+04 Ω that as three times the calculated resistance value R 21 , the formula (2) is satisfied. In addition, when the contact resistance component Rb of the electricity removing member  27  is equal to or lower than 2.81E +04 Ω that is 1.2 times the calculated resistance value R 21 , the formula (3) is satisfied. 
     On the other hand, in the comparative examples 14 to 15 and the invention examples 4 to 5, the linear speed of the electricity removing member  27  was set to a faster speed than the surface speed S of the photoconductor drum  21 . 
     Specifically, in the comparative example 14, the linear speed of the electricity removing member  27  was set to 0.24 m/s that was 1.6 times the linear speed S of the photoconductor drum  21 . As a result, in the comparative example 14, when the inner resistance component Ra of the electricity removing member  27  is equal to or lower than 1.502E+05 Ω that is 6.42 times the calculated resistance value R 21 , the formula (2) is satisfied. In addition, when the contact resistance component Rb of the electricity removing member  27  is equal to or lower than 6.01E +04 Ω that is 2.57 times the calculated resistance value R 21 , the formula (3) is satisfied. 
     In addition, in the comparative example 15, the linear speed of the electricity removing member  27  was set to 0.165 m/s that was 1.1 times the linear speed S of the photoconductor drum  21 . As a result, in the comparative example 15, when the inner resistance component Ra of the electricity removing member  27  is equal to or lower than 8.35E+04 Ω that is 3.57 times the calculated resistance value R 21 , the formula (2) is satisfied. In addition, when the contact resistance component Rb of the electricity removing member  27  is equal to or lower than 3.35E +04 Ω that is 1.43 times the calculated resistance value R 21 , the formula (3) is satisfied. 
     In addition, in the invention example 4, the linear speed of the electricity removing member  27  was set to 0.24 m/s that was 1.6 times the linear speed S of the photoconductor drum  21 . As a result, in the invention example 4, when the inner resistance component Ra of the electricity removing member  27  is equal to or lower than 1.502E+05 Ω that is 6.42 times the calculated resistance value R 21 , the formula (2) is satisfied. In addition, when the contact resistance component Rb of the electricity removing member  27  is equal to or lower than 6.01E+04 Ω that is 2.57 times the calculated resistance value R 21 , the formula (3) is satisfied. 
     In addition, in the invention example 5, the linear speed of the electricity removing member  27  was set to 0.255 m/s that was 1.7 times the linear speed S of the photoconductor drum  21 . As a result, in the invention example 5, when the inner resistance component Ra of the electricity removing member  27  is equal to or lower than 1.64E+05 Ω that is 6.99 times the calculated resistance value R 21 , the formula (2) is satisfied. In addition, when the contact resistance component Rb of the electricity removing member  27  is equal to or lower than 6.55E+04 Ω that is 2.80 times the calculated resistance value R 21 , the formula (3) is satisfied. 
     In the comparative example 1, the electricity removing member  27  whose brush bristles  271  were raw threads that were prepared by performing an opening and tearing process on a conductive acrylic fiber SA7 made by Toray Industries, Incorporated, was used. In the electricity removing member  27  of the comparative example 1, the raw thread resistance was 1.00E+07 Ω, the brush fineness was 30 μm, namely high (fiber was thick), and the brush density was 100 kF/inch 2 , namely low. It is noted that the comparative examples 1 to 9 were an entire distribution system where carbon of the fiber was distributed in the entire region of the raw thread. That is, in the electricity removing member  27  of the comparative examples 1 to 9, each of the brush bristles  271  is composed of only a resin layer that contains carbon. 
     In the comparative example 2, as in the comparative example 1, the electricity removing member  27  whose brush bristles  271  were raw threads that were prepared by performing an opening and tearing process on a conductive acrylic fiber SA7 made by Toray Industries, Incorporated, was used. In the electricity removing member  27  of the comparative example 2, the raw thread resistance was 1.00E+06 Ω, the brush fineness was 7 μm, namely low (fiber was thin), and the brush density was 500 kF/inch 2 , namely high. 
     In the comparative example 3, the electricity removing member  27  whose brush bristles  271  were raw threads of a conductive nylon UUN made by Unitika Limited was used. In the electricity removing member  27  of the comparative example 3, the raw thread resistance was 1.00E+06 Ω, the brush fineness was 7 μm, namely low (fiber was thin), and the brush density was 500 kF/inch 2 , namely high. It is noted that in the comparative examples 3 to 13 and the invention examples 1 to 3, the fiber cross sectional shape of the electricity removing member  27  was circular. 
     In the comparative examples 4 to 6, as in the comparative example 3, the electricity removing member  27  whose brush bristles  271  were raw threads of the conductive nylon UUN made by Unitika Limited was used. In the electricity removing member  27  of the comparative examples 4 to 6, the raw thread resistance was 1.00E+05 Ω, 1.04E+05 Ω, and 1.00E+05 Ω, respectively. In addition, in the electricity removing member  27  of the comparative examples 4 to 6, the brush fineness was 7 μm, 6 μm, and 6 μm, respectively. In addition, in the electricity removing member  27  of the comparative examples 4 to 6, the brush density was 500 kF/inch 2 , 550 kF/inch 2 , and 500 kF/inch 2 , respectively. 
     In the comparative examples 7 to 9, as in the comparative example 3, the electricity removing member  27  whose brush bristles  271  were raw threads of the conductive nylon UUN made by Unitika Limited was used. On the other hand, the electricity removing member  27  of the comparative examples 7 to 9 had more amount of carbon in the fiber than the comparative example 3 so that values of the inner resistance component Ra and the contact resistance component Rb were smaller. In the electricity removing member  27  of the comparative examples 7 to 9: the raw thread resistance was 1.00E+05 Ω, 1.00E+04 Ω, and 1.00E+05 Ω, respectively; the brush fineness was 6 μm, 7 μm, and 6 μm, respectively, namely low (fiber was thin); and the brush density was 550 kF/inch 2 , 500 kF/inch 2 , and 580 kF/inch 2 , respectively, namely high. 
     In the invention example 1, the electricity removing member  27  whose brush bristles  271  were raw threads of GBN fiber made by KB Seiren, Ltd was used. In the electricity removing member  27  of the invention example 1, the raw thread resistance was 1.00E+04 Ω, the brush fineness was 7 μm, namely low (fiber was thin), and the brush density was 500 kF/inch 2 , namely high. In addition, in the electricity removing member  27  of the invention examples 1 to 3 and the comparative examples 10 to 13, the carbon presence state in the fiber was not the entire distribution system, but was a two-layer structure where carbon was present in the outer portion of the fiber, and the contact resistance component had been reduced and the resistance ratio (Rb/Ra) had become low. That is, in the electricity removing member  27  of the invention examples 1 to 3 and the comparative examples 10 to 13, each of the brush bristles  271  includes the core portion  271 A and the surface layer portion  271 B. 
     In the comparative example 10, as in the invention example 1, the electricity removing member  27  whose brush bristles  271  were raw threads of GBN fiber made by KB Seiren, Ltd was used, but the comparative example 10 was higher in raw thread resistance than the invention example 1 by two digits. 
     In the comparative examples 11 to 13, the brush-like electricity removing member  27  whose brush bristles  271  were threads prepared by spraying carbon to polyester raw threads, was used. In the electricity removing member  27  of the comparative examples 11 to 13, carbon was sprayed to the polyester raw threads such that values of the inner resistance component Ra and the contact resistance component Rb were smaller. It is noted that in the comparative examples 11 to 13 and the invention example 3, the same amount of carbon was sprayed, and the comparative examples 11 to 13 differed from the invention example 3 in fineness and density of the polyester raw threads. 
     In the invention example 2, the brush-like electricity removing member  27  whose brush bristles  271  were polyester raw threads was used. In the electricity removing member  27  of the invention example 2, the raw thread resistance was 5.80E+03 Ω, the brush fineness was 7 μm, namely low (fiber was thin), and the brush density was 300 kF/inch 2 , namely high. In addition, in the invention example 2, as in the invention example 1, the electricity removing member  27  had the two-layer structure where carbon was present in the outer portion of the fiber, but carbon particles were directly sprayed to the outer portion of the fiber. With this configuration, the invention example 2 realized the same level of electric characteristic as the invention example 1, with a lower brush density than the invention example 1. 
     In the invention example 3, the brush-like electricity removing member  27  whose brush bristles  271  were polyester raw threads was used. In the electricity removing member  27  of the invention example 3, the raw thread resistance was 6.4E+03 Ω, the brush fineness was 7 μm, namely low (fiber was thin), and the brush density was 300 kF/inch 2 , namely high. In addition, in the invention example 3, as in the invention example 1, the electricity removing member  27  had the two-layer structure where carbon was present in the outer portion of the fiber, but carbon particles were directly sprayed to the outer portion of the fiber. It is noted that in the invention example 3, a smaller amount of carbon was sprayed than in the invention example 2. 
     In the comparative example 14, the same electricity removing member  27  as in the comparative example 10 was used. In addition, in the comparative example 15, the same electricity removing member  27  as in the comparative example 13 was used. In addition, in the invention example 4, the same electricity removing member  27  as in the comparative example 5 was used. In addition, in the invention example 5, the same electricity removing member  27  as in the comparative example 6 was used. 
     As shown in  FIG. 7 , in the comparative examples 1 to 6 and 10, since the inner resistance component Ra exceeds 7.02E+04 Ω that is three times the calculated resistance value R 21 , the formula (2) is not satisfied. On the other hand, in the comparative examples 7 to 9 and 11 to 13, since the inner resistance component Ra is equal to or lower than 7.02E+04 Ω that is three times the calculated resistance value R 21 , the formula (2) is satisfied. In addition, in the comparative example 14, since the inner resistance component Ra is equal to or lower than 1.502E+05 Ω that is 6.42 times the calculated resistance value R 21 , the formula (2) is satisfied. Furthermore, in the comparative example 15, since the inner resistance component Ra is equal to or lower than 8.35E+04 Ω that is 3.57 times the calculated resistance value R 21 , the formula (2) is satisfied. However, in the comparative examples 1 to 6, 10 and 13, since the contact resistance component Rb exceeds 2.81E+04 Ω that is 1.2 times the calculated resistance value R 21 , the formula (3) is not satisfied. The comparative examples 1 to 6, 10 and 13 were evaluated as “failure” with regard to the electricity removing capability. 
     On the other hand, in the invention examples 1 to 3, since the inner resistance component Ra of the electricity removing member  27  is equal to or lower than 7.02E+04 Ω that is three times the calculated resistance value R 21 , the formula (2) is satisfied, and since the contact resistance component Rb is equal to or lower than 2.81E+04 Ω that is 1.2 times the calculated resistance value R 21 , the formula (3) is satisfied. In addition, in the invention example 4, since the inner resistance component Ra of the electricity removing member  27  is equal to or lower than 1.502E+05 Ω that is 6.42 times the calculated resistance value R 21 , the formula (2) is satisfied, and since the contact resistance component Rb is equal to or lower than 6.01E+04 Ω that is 2.57 times the calculated resistance value R 21 , the formula (3) is satisfied. Furthermore, in the invention example 5, since the inner resistance component Ra of the electricity removing member  27  is equal to or lower than 1.64E+05 Ω that is 6.99 times the calculated resistance value R 21 , the formula (2) is satisfied, and since the contact resistance component Rb of the electricity removing member  27  is equal to or lower than 6.55E+04 Ω that is 2.80 times the calculated resistance value R 21 , the formula (3) is satisfied. The invention examples 1 to 5 were evaluated as “success” with regard to the electricity removing capability. 
     Here, with regard to the electricity removing capability, the comparative example 5 was evaluated as “failure”, while the invention example 4 that used the same electricity removing member  27  was evaluated as “success”, which shows improvement in the electricity removing capability. In addition, with regard to the electricity removing capability, the comparative example 6 was evaluated as “failure, while the invention example 5 that used the same electricity removing member  27  was evaluated as “success”, which shows improvement in the electricity removing capability. Similarly, with regard to the electricity removing capability, the comparative examples 10 and 13 were evaluated as “failure”, while the comparative examples 14 and 15 that used the same electricity removing member  27  were evaluated as “success”, which shows improvement in the electricity removing capability. These evaluation results show that it is possible to improve the electricity removing capability by setting the linear speed of the electricity removing member  27  to a speed that is faster than the linear speed S of the photoconductor drum  21 .  FIG. 8  shows relationships between the linear speed of the electricity removing member  27  and the post-electricity-removal V 1  in the image forming apparatuses  10  in which the electricity removing members  27  according to the comparative examples 5 to 6, 10, and 13 were mounted. 
     In this way, it was found that, in the image forming apparatus  10 , it is possible to obtain a desired electricity removing capability by taking into consideration not only the DC resistance value R 2  of the electricity removing member  27 , but also the inner impedance Z 1  and the contact impedance Z 2 . More specifically, a desired electricity removing capability was obtained when the above-indicated formulas (2) and (3) were satisfied. 
     As shown in  FIG. 7 , values of the capacitance ratio (Cb/Ca) were calculated from the Cole-Cole plot obtained from measurement performed by the experiment device  90  on the electricity removing member  27  of the comparative examples 1 to 15 and the invention examples 1 to 5, wherein the capacitance ratio (Cb/Ca) is a ratio of the contact capacitance component Cb to the inner capacitance component Ca. Here, in the comparative examples 1 to 4, 8 to 9, 12 to 13 and 15, the capacitance ration (Cb/Ca) is higher than 0.4, and the condition of the above-indicated formula (5) that the capacitance ratio (Cb/Ca) is equal to or higher than 0 and equal to or lower than 0.4, is not satisfied. On the other hand, in the comparative examples 5 to 7, 10 to 11 and 14, the capacitance ration (Cb/Ca) is equal to or lower than 0.4, and the condition of the above-indicated formula (5) that the capacitance ratio (Cb/Ca) is equal to or higher than 0 and equal to or lower than 0.4, is satisfied. However, in the comparative examples 1 to 3, 7 to 8, and 10 to 15, the inner capacitance component Ca of the electricity removing member  27  is higher than 1.0E+5.0, and the condition of the above-indicated formula (4) that the inner capacitance component Ca is equal to or lower than 1.0E+5.0, is not satisfied. With regard to the potential stability and the image memory presence/absence, an evaluation was made only on the samples whose electricity removing capability had been evaluated as “success”. Specifically, in the comparative examples 7 to 9, 11 to 12 and 14 to 15 whose electricity removing capability had been evaluated as “success”, the potential stability and the image memory presence/absence were evaluated as “failure”. 
     On the other hand, in the invention examples 1 to 5, the condition of the above-indicated formula (4) that the inner capacitance component Ca is equal to or lower than 1.0E+5.0, is satisfied, and the condition of the above-indicated formula (5) that the capacitance ratio (Cb/Ca) is equal to or higher than 0 and equal to or lower than 0.4, is satisfied. In addition, the invention examples 1 to 5 were evaluated as “success” with regard to the potential stability and the image memory presence/absence. 
     In this way, it was found that, in the image forming apparatus  10 , it is possible to improve the potential stability and suppress an occurrence of the image memory by taking into consideration not only the DC resistance of the electricity removing member  27 , but also the inner impedance Z 1  and the contact impedance Z 2 . More specifically, the potential stability was improved and an occurrence of the image memory was suppressed when the conditions of the above-indicated formulas (4) and (5) were satisfied. 
     Meanwhile, in the image forming apparatus  10 , an application voltage applied to the charging roller  220  that charges the photoconductor drum  21 , is changed. Here, in a case where the electricity removing capability of the electricity removing member  27  is set based on the maximum value of the application voltage in a configuration where the electricity removing member  27  contacts the photoconductor drum  21 , the wearing of the photoconductor drum  21  is accelerated and the life of the photoconductor drum  21  is shortened. On the other hand, in the image forming apparatus  10  according to the first embodiment of the present invention, as described below, it is possible to restrict the photoconductor drum  21  from wearing while securing necessary electricity removing capability. 
     Specifically, a first speed change program for causing the CPU to execute a first speed change process that is described below (see the flowchart of  FIG. 11 ) is stored in advance in the ROM of the control portion  1 . It is noted that the first speed change program may be recorded on a computer-readable recording medium such as a CD, a DVD, or a flash memory, and may be installed from the recording medium to a storage device such the EEPROM of the control portion  1  or the like. 
     As shown in  FIG. 10 , the control portion  1  includes a density detecting portion  11 , a voltage change portion  12 , and a first speed change portion  13 A. Specifically, the control portion  1  executes the first speed change program stored in the ROM by using the CPU, thereby functioning as the density detecting portion  11 , the voltage change portion  12 , and the first speed change portion  13 A. 
     The density detecting portion  11  executes a density detection process of detecting density of a patch image that is formed on the surface of the photoconductor drum  21  based on predetermined image data. 
     Specifically, in the image forming apparatus  10 , as shown in  FIG. 2 , a density sensor  29  is provided on a downstream side of the developing device  24  and on an upstream side of the transfer roller  25  in a rotation direction of the photoconductor drum  21 . For example, the density sensor  29  is an optical sensor including a light emitting portion and a light receiving portion. In the density sensor  29 , light emitted by the light emitting portion and reflected on the surface of the photoconductor drum  21  is received by the light receiving portion. The light receiving portion then outputs an electric signal that represents an amount of received light. 
     For example, when a predetermined first timing comes, the density detecting portion  11  forms the patch image on the surface of the photoconductor drum  21  by controlling the operation of each portion of the image forming portion  2 . The density detecting portion  11  then detects the density of the patch image by using the density sensor  29 . For example, the first timing is when the image forming apparatus  10  is powered on, when the image forming apparatus  10  returns to a normal state from a sleep state in which its partial functions are stopped, and when the print process is executed. 
     The voltage change portion  12  changes the application voltage that is applied from the power supply  221  to the charging roller  220 . 
     Specifically, the voltage change portion  12  changes the application voltage based on the density of the patch image detected by the density detecting portion  11 . It is noted that the voltage change portion  12  also changes a developing bias voltage that is applied to a developing roller provided in the developing device  24 , as well as the application voltage. 
     For example, in the image forming apparatus  10 , an initial setting value of the application voltage is set to 500 V. The voltage change portion  12  changes the application voltage from 500 V to 800 V when the density of the patch image detected by the density detecting portion  11  is thin in excess of a predetermined specific range. In addition, the voltage change portion  12  changes the application voltage from 500 V to 300 V when the density of the patch image is thick in excess of the specific range. 
     It is noted that the image forming apparatus  10  may include a temperature/humidity sensor that detects the temperature and humidity in the machine. In that case, the voltage change portion  12  may change the application voltage based on the temperature and humidity in the machine detected by the temperature/humidity sensor. 
     The first speed change portion  13 A increases a difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  as the application voltage applied to the charging roller  220  increases. Here, the first speed change portion  13 A is an example of the speed change portion of the present invention. 
     Specifically, the first speed change portion  13 A changes the linear speed of the electricity removing member  27  to a first specific speed (an example of the specific speed of the present invention) so that the ratio Sr satisfies the following formulas (6) and (7) and the difference from the linear speed of the photoconductor drum  21  becomes the minimum, wherein R 22  denotes a calculated resistance value calculated based on the above-mentioned formula (1) after the application voltage is changed by the voltage change portion  12 . It is noted that the pre-electricity-removal potential V 0  in the formula (1) is equal to the application voltage after a change by the voltage change portion  12  or is obtained by multiplying the application voltage after the change by a predetermined coefficient.
 
[Math 4]
 
 Ra≤R 22×3×{1+(|1− Sr|× 1.9)}  (6)
 
[Math 5]
 
 Rb≤R 22×1.2×{1+(|1− Sr|× 1.9)}  (7)
 
     For example, in the image forming apparatus  10 , as described above, the electricity removing member  27  rotates at a faster linear speed than the photoconductor drum  21 . As a result, the first speed change portion  13 A increases the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  by increasing the linear speed of the electricity removing member  27 . It is noted that in a case where the photoconductor drum  21  rotates at a faster linear speed than the electricity removing member  27 , the first speed change portion  13 A may increase the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  by decreasing the linear speed of the electricity removing member  27 . 
     For example, in the image forming apparatus  10 , the inner resistance component Ra, the contact resistance component Rb, and the calculated resistance values R 22  corresponding to application voltages that can be set in the image forming apparatus  10  are stored in the ROM of the control portion  1  in advance. In a case where the application voltage is changed by the voltage change portion  12 , the first speed change portion  13 A calculates a linear speed of the electricity removing member  27  that satisfies the above-described conditions, by using the inner resistance component Ra, the contact resistance component Rb, and the calculated resistance values R 22  stored in the ROM. The first speed change portion  13 A changes the linear speed of the electricity removing member  27  based on the calculation result. 
     It is noted that the first speed change portion  13 A may change the linear speed of the electricity removing member  27  so that the difference from the first specific speed becomes equal to or smaller than a preset allowed value. In addition, the first speed change portion  13 A may change the linear speed of the electricity removing member  27  so that the ratio Sr satisfies the above-described formulas (6) and (7). 
     In addition, in the image forming apparatus  10 , first table data may be stored in the ROM of the control portion  1  in advance, wherein the first table data indicates linear speeds of the electricity removing member  27  that correspond to the application voltages that can be set in the image forming apparatus  10 . In this case, when the application volume is changed by the voltage change portion  12 , the first speed change portion  13 A may change the linear speed of the electricity removing member  27  by using the first table data. For example, the first table data is generated based on experimental data that is obtained by an experiment conducted by using the image forming apparatus  10  to investigate the relationship between the post-electricity-removal potential V 1  and the ratios Sr corresponding to the pre-electricity-removal potentials V 0 . Here,  FIG. 12  shows an example of the experimental data obtained from the experiment. 
     In addition, the first speed change portion  13 A may increase the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  by changing the linear speed of the photoconductor drum  21 . 
     [First Speed Change Process] 
     In the following, with reference to  FIG. 11 , a description is given of an example of the procedure of the first speed change process executed by the control portion  1  in the image forming apparatus  10 . Here, steps S 11 , S 12 , . . . represent numbers assigned to the processing procedures (steps) executed by the control portion  1 . 
     &lt;Step S 11 &gt; 
     First, in step S 11 , the control portion  1  determines whether or not the first timing has come. 
     Here, upon determining that the first timing has come (Yes side at S 11 ), the control portion  1  moves the process to step S 12 . In addition, upon determining that the first timing has not come (No side at S 11 ), the control portion  1  waits at step S 11  for the first timing to come. 
     &lt;Step S 12 &gt; 
     In step S 12 , the control portion  1  executes the density detection process. Here, processes of steps S 11  and S 12  are executed by the density detecting portion  11  of the control portion  1 . 
     For example, the control portion  1  forms the patch image on the surface of the photoconductor drum  21  by controlling the operation of each portion of the image forming portion  2 . The control portion  1  then detects the density of the patch image by using the density sensor  29 . It is noted that in step S 12 , the control portion  1  may detect the temperature and humidity in the machine, the image forming apparatus  10 . 
     &lt;Step S 13 &gt; 
     In step S 13 , the control portion  1  changes the application voltage based on the density of the patch image detected in step S 12 . Here, process of step S 13  is executed by the voltage change portion  12  of the control portion  1 . 
     For example, when the density of the patch image detected in step S 12  is thin in excess of the specific range, the control portion  1  changes the application voltage to 800 V by rewriting data stored in a predetermined first storage area in the RAM that indicates the set value of the application voltage. In addition, when the density of the patch image is thick in excess of the specific range, the control portion  1  changes the application voltage to 300 V by rewriting the data in the first storage area. In addition, when the density of the patch image is within the specific range, the control portion  1  changes the application voltage to 500 V by rewriting the data in the first storage area. 
     &lt;Step S 14 &gt; 
     In step S 14 , the control portion  1  changes the linear speed of the electricity removing member  27  based on the application voltage after the change in step S 13 . Here, the process of step S 14  is executed by the first speed change portion  13 A of the control portion  1 . 
     Specifically, the control portion  1  changes the linear speed of the electricity removing member  27  to the first specific speed so that the ratio Sr satisfies the above-indicated formulas (6) and (7) and the difference from the linear speed of the photoconductor drum  21  becomes the minimum. For example, the control portion  1  changes the linear speed of the electricity removing member  27  by rewriting data stored in a predetermined second storage area in the RAM that indicates the set value of the linear speed of the electricity removing member  27 . 
     As described above, in the image forming apparatus  10  according to the first embodiment, the higher the application voltage applied to the charging roller  220  is, the larger the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  is. With this configuration, it is possible to restrict the photoconductor drum  21  from wearing while securing necessary electricity removing capability, compared to a configuration where the linear speed of the electricity removing member  27  is set based on the maximum value of the application voltage. 
     In addition, in the image forming apparatus  10  according to the first embodiment, the linear speed of the electricity removing member  27  is changed to the first specific speed so that the ratio Sr satisfies the above-indicated formulas (6) and (7) and the difference from the linear speed of the photoconductor drum  21  becomes the minimum. With this configuration, the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  is minimized within a range where the necessary electricity removing capability is secured. Accordingly, it is possible to restrict the wearing of the photoconductor drum  21  more effectively. 
     It is noted that as a modification of the first embodiment, the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  may be reduced as the surface potential of the photoconductor drum  21  is reduced due to deterioration over time or the like. For example, each time a predetermined period elapses, the first speed change portion  13 A may reduce the linear speed of the electricity removing member  27 . With this configuration, it is possible to restrict the wearing of the photoconductor drum  21  more effectively. 
     Second Embodiment 
     The following describes the image forming apparatus  10  according to a second embodiment of the present invention with reference to  FIG. 13  to  FIG. 15 . The second embodiment differs from the first embodiment in configuration of the electricity removing member  27  and the control portion  1  in the image forming apparatus  10 . Otherwise, the second embodiment has the same configuration as the first embodiment. 
     Specifically, in the image forming apparatus  10  according to the second embodiment, as shown in  FIG. 13 , the electricity removing member  27  is configured to move in a first direction D 1  and a second direction D 2 , wherein the first direction D 1  is a direction to approach the photoconductor drum  21 , and the second direction D 2  is opposite to the first direction D 1 . For example, in the image forming apparatus  10  according to the second embodiment, a bearing that supports a rotation shaft of the electricity removing member  27  is supported by a housing of the image forming apparatus  10  in such a way as to move in the first direction D 1  and the second direction D 2 . 
     In addition, as shown in  FIG. 14 , the control portion  1  includes a movement processing portion  14  in place of the first speed change portion  13 A. 
     Specifically, a contact pressure change program for causing the CPU to execute a contact pressure change process that is described below (see the flowchart of  FIG. 15 ) is stored in advance in the ROM of the control portion  1 . The control portion  1  executes the contact pressure change program stored in the ROM by using the CPU, thereby functioning as the density detecting portion  11 , the voltage change portion  12 , and the movement processing portion  14 . It is noted that the density detecting portion  11  and the voltage change portion  12  are the same as those described in the first embodiment, and description thereof is omitted. 
     The movement processing portion  14  makes the separation distance between the photoconductor drum  21  and the electricity removing member  27  shorter as the application voltage applied to the charging roller  220  is higher. That is, the higher the application voltage applied to the charging roller  220  is, the higher the contact pressure between the photoconductor drum  21  and the electricity removing member  27  is. This allows the contact resistance component Rb between the photoconductor drum  21  and the electricity removing member  27  to be decreased. 
     Specifically, when the voltage change portion  12  increases the application voltage, the movement processing portion  14  decreases the separation distance between the photoconductor drum  21  and the electricity removing member  27  by moving the electricity removing member  27  in the first direction D 1 . In addition, when the voltage change portion  12  decreases the application voltage, the movement processing portion  14  increases the separation distance between the photoconductor drum  21  and the electricity removing member  27  by moving the electricity removing member  27  in the second direction D 2 . 
     For example, as shown in  FIG. 14 , the image forming apparatus  10  is provided with a second drive portion  273 , such as a motor, configured to move the electricity removing member  27 . In addition, in the image forming apparatus  10 , second table data may be stored in the ROM of the control portion  1  in advance, wherein the second table data indicates positions of the electricity removing member  27  in its movable range that correspond to the application voltage that can be set in the image forming apparatus  10 . When the voltage change portion  12  changes the application voltage, the movement processing portion  14  moves the electricity removing member  27  by using the second table data. 
     [Contact Pressure Change Process] 
     In the following, with reference to  FIG. 15 , a description is given of an example of the procedure of the contact pressure change process executed by the control portion  1  in the image forming apparatus  10 . It is noted that among the steps included in the contact pressure change process, steps common to the first speed change process are indicated by the same reference signs, and description thereof is omitted. 
     &lt;Step S 15 &gt; 
     First, in step S 15 , the control portion  1  increases or decreases the separation distance between the photoconductor drum  21  and the electricity removing member  27  by moving the electricity removing member  27  in the first direction D 1  or the second direction D 2  depending on the application voltage after the change in step S 13 . Here, the process of step S 15  is executed by the movement processing portion  14  of the control portion  1 . 
     For example, when the application voltage is increased, the control portion  1  decreases the separation distance between the photoconductor drum  21  and the electricity removing member  27  by moving the electricity removing member  27  in the first direction D 1  based on the second table data. In addition, when the application voltage is decreased, the control portion  1  increases the separation distance between the photoconductor drum  21  and the electricity removing member  27  by moving the electricity removing member  27  in the second direction D 2  based on the second table data. 
     As described above, in the image forming apparatus  10  according to the second embodiment, the higher the application voltage applied to the charging roller  220  is, the smaller the separation distance between the photoconductor drum  21  and the electricity removing member  27  is. With this configuration, it is possible to restrict the photoconductor drum  21  from wearing while securing necessary electricity removing capability, compared to a configuration where the separation distance between the photoconductor drum  21  and the electricity removing member  27  is set based on the maximum value of the application voltage. 
     It is noted that the control portion  1  of the image forming apparatus  10  according to the second embodiment, may include the first speed change portion  13 A. Specifically, the image forming apparatus  10  according to the second embodiment may be configured such that the higher the application voltage applied to the charging roller  220  is, the smaller the separation distance between the photoconductor drum  21  and the electricity removing member  27  is, and the larger the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  is. 
     Meanwhile, in a configuration where the electricity removing member  27  comes in contact with the photoconductor drum  21 , external additives contained in the toner, such as silica, may adhere to the electricity removing member  27 . Here, when the amount of external additive adhered to the electricity removing member  27  increases, the contact resistance between the photoconductor drum  21  and the electricity removing member  27  increases, and the electricity removing capability is lowered. 
     Third Embodiment 
     The following describes the image forming apparatus  10  according to a third embodiment of the present invention with reference to  FIG. 16  to  FIG. 19 . The third embodiment differs from the first embodiment in configuration of the control portion  1  and the image forming portion  2  in the image forming apparatus  10 . Otherwise, the third embodiment has the same configuration as the first embodiment. 
     Specifically, in the image forming apparatus  10  according to the third embodiment, the image forming portion  2  does not include the density sensor  29 . 
     In addition, as shown in  FIG. 16 , the control portion  1  includes a first obtainment processing portion  15 A, a first variation amount obtaining portion  16 A, and a second speed change portion  13 B in place of the density detecting portion  11 , the voltage change portion  12 , and first speed change portion  13 A. 
     Specifically a second speed program for causing the CPU to execute a second speed change process that is described below (see the flowchart of  FIG. 17 ) is stored in advance in the ROM of the control portion  1 . The control portion  1  executes the second speed change program stored in the ROM by using the CPU, thereby functioning as the first obtainment processing portion  15 A, the first variation amount obtaining portion  16 A, and the second speed change portion  13 B. 
     The first obtainment processing portion  15 A obtains a cumulative value of consumption of toner (developer) based on a preset first obtainment condition. 
     For example, when a predetermined second timing comes, the first obtainment processing portion  15 A obtains the cumulative value of consumption of toner. For example, as is the case with the first timing, the second timing is when the image forming apparatus  10  is powered on, when the image forming apparatus  10  returns to the normal state from the sleep state in which its partial functions are stopped, and when the print process is executed. 
     For example, in the image forming apparatus  10 , a cumulative printing rate is stored in a predetermined third storage area in the EEPROM, wherein the cumulative printing rate is a cumulative value of a printing rate of each print that was output from the image forming apparatus  10 . For example, when the print process is executed, the control portion  1  calculate a printing rate of each print output in the print process, based on image data that is printed in the print process. In addition, when the size of a sheet on which an image is printed in the print process is different from a predetermined reference size, the control portion  1  converts the calculated printing rate into a printing rate for a sheet of the reference size. The control portion  1  then updates the cumulative printing rate stored in the third storage area based on a total of the calculated or converted printing rates. 
     Subsequently, the first obtainment processing portion  15 A obtains a cumulative value of consumption of toner based on the cumulative printing rate (an example of the first obtainment condition) stored in the third storage area. For example, the first obtainment processing portion  15 A obtains the cumulative value of consumption of toner by multiplying the cumulative printing rate read from the third storage area by a predetermined coefficient. 
     It is noted that the first obtainment processing portion  15 A may obtain the cumulative value of consumption of toner based on a cumulative number of prints (another example of the first obtainment condition) that is a cumulative value of the number of prints output from the image forming apparatus  10 . 
     The first variation amount obtaining portion  16 A obtains a variation amount ΔRb of the contact resistance component Rb of the contact impedance Z 2  of the electricity removing member  27  based on the cumulative value of consumption of toner obtained by the first obtainment processing portion  15 A. 
     For example, in the image forming apparatus  10 , third table data is stored in the ROM of the control portion  1  in advance, wherein the third table data indicates values of the variation amount ΔRb of the electricity removing member  27  that correspond to predetermined cumulative values of consumption of toner. The first variation amount obtaining portion  16 A obtains the variation amount ΔRb of the contact resistance component Rb of the electricity removing member  27  based on the cumulative value of consumption of toner obtained by the first obtainment processing portion  15 A and the third table data. For example, the third table data is generated based on experimental data that is obtained by an experiment conducted by using the image forming apparatus  10  to investigate the relationship between the cumulative value of consumption of toner in the image forming apparatus  10  and the contact resistance component Rb. Here,  FIG. 18  shows an example of the experimental data obtained by the experiment. It is noted that  FIG. 18  shows relationship between the contact resistance component Rb and a cumulative printing rate P used to calculate the cumulative value of consumption of toner. 
     It is noted that a formula (8) shown below may be stored in the ROM of the control portion  1  in advance, wherein the formula (8) indicates the variation amount ΔRb of the contact resistance component Rb and the cumulative printing rate P derived from the experimental data shown in  FIG. 18 . In this case, the first variation amount obtaining portion  16 A may obtain the variation amount ΔRb of the contact resistance component Rb based on the following formula (8) and the cumulative printing rate P read from the third storage area. In addition, the control portion  1  may not include the first obtainment processing portion  15 A. It is noted that in the following formula (8), F, G and H represent constants derived from the experimental data shown in  FIG. 18 .
 
[Math 6]
 
Δ Rb=F ÷(1 +G×e   −H×P )  (8)
 
     The second speed change portion  13 B increases the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  in correspondence with an increase of the cumulative value of consumption of toner obtained based on the first obtainment condition. 
     Specifically, the second speed change portion  13 B changes the linear speed of the electricity removing member  27  to a second specific speed so that the ratio Sr satisfies the above-indicated formula (2) and the following formula (9) and the difference from the linear speed of the photoconductor drum  21  becomes the minimum.
 
[Math 7]
 
 Rb+ΔRb≤R 21×1.2×{1+(|1− Sr|× 1.9)}  (9)
 
     For example, in the image forming apparatus  10 , as described above, the electricity removing member  27  rotates at a faster linear speed than the photoconductor drum  21 . As a result, the second speed change portion  13 B increases the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  by increasing the linear speed of the electricity removing member  27 . It is noted that in a case where the electricity removing member  27  rotates at a slower linear speed than the photoconductor drum  21 , the second speed change portion  13 B may increase the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  by decreasing the linear speed of the electricity removing member  27 . 
     For example, in the image forming apparatus  10 , the inner resistance component Ra, the contact resistance component Rb, and the calculated resistance value R 21  are stored in the ROM of the control portion  1  in advance. In a case where the first variation amount obtaining portion  16 A obtains the variation amount ΔRb of the contact resistance component Rb, the second speed change portion  13 B calculates the linear speed of the electricity removing member  27  that satisfies the above-described conditions, by using the inner resistance component Ra, the contact resistance component Rb, and the calculated resistance value R 21  stored in the ROM. The speed change portion  13 B changes the linear speed of the electricity removing member  27  based on the calculation results. 
     It is noted that the second speed change portion  13 B may change the linear speed of the electricity removing member  27  to a speed whose difference from the second specific speed is equal to or smaller than the allowed value. In addition, the second speed change portion  13 B may change the linear speed of the electricity removing member  27  to a speed so that the ratio Sr satisfies the above-indicated formulas (2) and (9). 
     In addition, in the image forming apparatus  10 , fourth table data may be stored in the ROM of the control portion  1  in advance, wherein the fourth table data indicates linear speeds of the electricity removing member  27  that correspond to predetermined cumulative values of consumption of toner. In this case, the second speed change portion  13 B may change the linear speed of the electricity removing member  27  by using the fourth table data and the cumulative value of consumption of toner obtained by the first obtainment processing portion  15 A. In addition, in this case, the control portion  1  may not include the first variation amount obtaining portion  16 A. For example, the fourth table data is generated based on: experimental data that is obtained by an experiment conducted by using the image forming apparatus  10  to investigate the relationship between the cumulative value of consumption of toner in the image forming apparatus  10  and the post-electricity-removal potential V 1 ; and experimental data that is obtained by an experiment conducted by using the image forming apparatus  10  to investigate the relationship between the ratio Sr and the post-electricity-removal potential V 1 . 
     In addition, the second speed change portion  13 B may increase the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  by changing the linear speed of the photoconductor drum  21 . In addition, the second speed change portion  13 B may increase the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  within a range of equal to or lower than a preset upper-limit value. 
     [Second Speed Change Process] 
     In the following, with reference to  FIG. 17 , a description is given of an example of the procedure of the second speed change process executed by the control portion  1  in the image forming apparatus  10 . 
     &lt;Step S 21 &gt; 
     First, in step S 21 , the control portion  1  determines whether or not the second timing has come. 
     Here, upon determining that the second timing has come (Yes side at S 21 ), the control portion  1  moves the process to step S 22 . In addition, upon determining that the second timing has not come (No side at S 21 ), the control portion  1  waits at step S 21  for the second timing to come. 
     &lt;Step S 22 &gt; 
     In step S 22 , the control portion  1  obtains the cumulative value of consumption of toner in the image forming apparatus  10 . Here, the processes of steps S 21  and S 22  are executed by the first obtainment processing portion  15 A of the control portion  1 . 
     Specifically, the control portion  1  obtains the cumulative value of consumption of toner by multiplying the coefficient by the cumulative printing rate read from the third storage area. 
     &lt;Step S 23 &gt; 
     In step S 23 , the control portion  1  obtains the variation amount ΔRb of the contact resistance component Rb of the electricity removing member  27  based on the cumulative value of consumption of toner obtained in step S 22 . Here, the process of step S 23  is executed by the first variation amount obtaining portion  16 A of the control portion  1 . It is noted that the process of step S 23  may be omitted. 
     Specifically, the control portion  1  obtains the variation amount ΔRb of the contact resistant component Rb of the electricity removing member  27  based on the cumulative value of consumption of toner obtained in step S 22  and the third table data. 
     &lt;Step S 24 &gt; 
     In step S 24 , the control portion  1  changes the linear speed of the electricity removing member  27  based on the variation amount ΔRb of the contact resistance component Rb of the electricity removing member  27  obtained in step S 23 . Here, the process of step S 24  is executed by the second speed change portion  13 B of the control portion  1 . 
     Specifically, the control portion  1  changes the linear speed of the electricity removing member  27  to the second specific speed so that the ratio Sr satisfies the above-indicated formulas (2) and (9) and the difference from the linear speed of the photoconductor drum  21  becomes the minimum. For example, the control portion  1  changes the linear speed of the electricity removing member  27  by rewriting data stored in the second storage area in the RAM that indicates the set value of the linear speed of the electricity removing member  27 . 
     As described above, in the image forming apparatus  10  according to the third embodiment, the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  increases in correspondence with an increase of the cumulative value of consumption of toner obtained based on the first obtainment condition. This makes it possible to restrict the electricity removing capability of the electricity removing member  27  from being lowered due to an increase of the amount of external additives adhered to the electricity removing member  27 . 
     In addition, in the image forming apparatus  10  according to the third embodiment, the linear speed of the electricity removing member  27  is changed to the second specific speed so that the ratio Sr satisfies the above-indicated formulas (2) and (9) and the difference from the linear speed of the photoconductor drum  21  becomes the minimum. With this configuration, the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  is minimized within a range where the necessary electricity removing capability is secured. Accordingly, it is possible to restrict the wearing of the photoconductor drum  21 . 
     It is noted that, as a modification of the third embodiment, in a configuration where a plurality of division areas are provided along a main scanning direction perpendicular to a conveyance direction of a sheet on which an image is formed, the cumulative printing rate is obtained for each of the plurality of division areas, and a division area that has the highest cumulative printing rate is referred to as a specific division area, the second speed change portion  13 B may increase the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  in correspondence with an increase of the cumulative value of consumption of toner which is obtained based on the cumulative printing rate of the specific division area. For example, a plurality of storage areas may be provided in the EEPROM of the control portion  1  so as to store the cumulative printing rates of the plurality of division areas respectively. In addition, the first obtainment processing portion  15 A may obtain the cumulative value of consumption of toner by multiplying the cumulative printing rate of the specific division area by the number of division areas and the coefficient. With this configuration, it is possible to set the linear speed of the electricity removing member  27  based on a portion of the electricity removing member  27  to which the largest amount of external additives adheres, among a plurality of portions of the electricity removing member  27  set along the main scanning direction. 
     In a addition, as another modification of the third embodiment, as shown in  FIG. 19 , the image forming apparatus  10  may include a cleaning member  274  configured to clean the surface of the electricity removing member  27 . For example, the cleaning member  274  is a blade-like member elongated in the axial direction of the rotation shaft of the photoconductor drum  21 , and is provided in contact with the brush bristles  271  of the electricity removing member  27 . For example, the cleaning member  274  is positioned so as to bite the electricity removing member  27  by 0.1 mm to 1.1 mm from the outer diameter of the electricity removing member  27 . With this configuration, it is possible to restrict the external additives from adhering to the electricity removing member  27 . It is noted that in a case where the cleaning member  274  is provided in the image forming apparatus  10 , the contents of the third table data, the formula (8), and the fourth table data may be corrected. 
     Meanwhile, in a configuration where the electricity removing member  27  comes in contact with the photoconductor drum  21 , tips of the brush bristles  271  that come in contact with the photoconductor drum  21  may be curved in such a way as to decrease the outer diameter of the electricity removing member  27  as the number of times the print process is executed increases. Here, when the outer diameter of the electricity removing member  27  is decreased, the contact area between the photoconductor drum  21  and the electricity removing member  27  is decreased, and the contact resistance between the photoconductor drum  21  and the electricity removing member  27  increases, resulting in reduction of the electricity removing capability of the electricity removing member  27 . 
     Fourth Embodiment 
     The following describes the image forming apparatus  10  according to a fourth embodiment of the present invention with reference to  FIG. 20  to  FIG. 23 . The fourth embodiment differs from the first embodiment in configuration of the control portion  1  and the image forming portion  2  in the image forming apparatus  10 . Otherwise, the fourth embodiment has the same configuration as the first embodiment. 
     Specifically, in the image forming apparatus  10  according to the fourth embodiment, the image forming portion  2  does not include the density sensor  29 . 
     In addition, as shown in  FIG. 20 , the control portion  1  includes a second embodiment processing portion  15 B, a second variation amount obtaining portion  16 B, and a third speed change portion  13 C in place of the density detecting portion  11 , the voltage change portion  12 , and the first speed change portion  13 A. 
     Specifically, a third speed change program for causing the CPU to execute a third speed change process that is described below (see the flowchart of  FIG. 21 ) is stored in advance in the ROM of the control portion  1 . The control portion  1  executes the third speed change program stored in the ROM by using the CPU, thereby functioning as the second obtainment processing portion  15 B, the second variation amount obtaining portion  16 B, and the third speed change portion  13 C. 
     The second obtainment processing portion  15 B obtains an outer diameter of the electricity removing member  27  based on a preset second obtainment condition. 
     For example, when a predetermined third timing comes, the second obtainment processing portion  15 B obtains the outer diameter of the electricity removing member  27 . For example, as is the case with the first timing, the third timing is when the image forming apparatus  10  is powered on, when the image forming apparatus  10  returns to the normal state from the sleep state in which its partial functions are stopped, and when the print process is executed. 
     For example, the second obtainment processing portion  15 B obtains the outer diameter of the electricity removing member  27  based on the cumulative number of prints (an example of the second obtainment condition) in the image forming apparatus  10 . 
     For example, in the image forming apparatus  10 , the cumulative number of prints in the image forming apparatus  10  is stored in a predetermined fourth storage area in the EEPROM. For example, each time the print process is executed, the control portion  1  updates the cumulative number of prints stored in the fourth storage area. 
     In addition, in the image forming apparatus  10 , fifth table data may be stored in the ROM of the control portion  1  in advance, wherein the fifth table data indicates outer diameters of the electricity removing member  27  that correspond to predetermined cumulative numbers of prints. The second obtainment processing portion  15 B obtains the outer diameter of the electricity removing member  27  based on the cumulative number of prints read from the fourth storage area and the fifth table data. For example, the fifth table data is generated based on experimental data that is obtained by an experiment conducted by using the image forming apparatus  10  to investigate the relationship between the cumulative number of prints and the outer diameter of the electricity removing member  27 . Here,  FIG. 22  shows an example of the experimental data obtained from the experiment. 
     It is noted that the second obtainment processing portion  15 B may obtain the outer diameter the electricity removing member  27  based on a cumulative number of rotations (another example of the second obtainment condition) of the electricity removing member  27 . In addition, the second obtainment processing portion  15 B may obtain the outer diameter of the electricity removing member  27  based on a current value (a further example of the second obtainment condition) of current flowing through the first drive portion  272  that drives the electricity removing member  27 . In addition, the second obtainment processing portion  15 B may obtain the outer diameter of the electricity removing member  27  based on any two or more of the cumulative number of prints, the cumulative number of rotations, and the current value of the current flowing through the first drive portion  272 . For example, the second obtainment processing portion  15 B may obtain, as the outer diameter of the electricity removing member  27 , an average value of an outer diameter of the electricity removing member  27  obtained based on the cumulative number of prints and an outer diameter of the electricity removing member  27  obtained based on the cumulative number of rotations. 
     The second variation amount obtaining portion  16 B obtains the variation amount ΔRb of the contact resistance component Rb of the contact impedance Z 2  of the electricity removing member  27  based on a decrease amount of the outer diameter of the electricity removing member  27  obtained by the second obtainment processing portion  15 B. 
     For example, in the image forming apparatus  10 , sixth table data may be stored in the ROM of the control portion  1  in advance, wherein the sixth table data indicates variation amounts ΔRb of the contact resistance component Rb of the electricity removing member  27  that correspond to predetermined decrease amounts of the outer diameter of the electricity removing member  27 . The second variation amount obtaining portion  16 B calculates the decrease amount of the outer diameter of the electricity removing member  27  based on: the outer diameter of the electricity removing member  27  obtained by the second obtainment processing portion  15 B; and an outer diameter of the electricity removing member  27  at the time of manufacture of the image forming apparatus  10  that is stored in the ROM in advance. Subsequently, the second variation amount obtaining portion  16 B obtains the variation amount ΔRb of the contact resistance component Rb of the electricity removing member  27  based on the calculated decrease amount of the outer diameter of the electricity removing member  27  and the sixth data table. For example, the sixth table data is generated based on experimental data that is obtained by an experiment conducted by using the image forming apparatus  10  to investigate the relationship between the decrease amount of the outer diameter of the electricity removing member  27  and the contact resistance component Rb. 
     The third speed change portion  13 C increases the difference between the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  in correspondence with a decrease in the outer diameter of the electricity removing member  27  obtained based on the second obtainment condition. 
     Specifically, the third speed change portion  13 C changes the linear speed of the electricity removing member  27  to a third specific speed that the ratio Sr satisfies the above indicated formulas (2) and (9) and the difference from the linear speed of the photoconductor drum  21  becomes the minimum. 
     For example, in the image forming apparatus  10 , as described above, the electricity removing member  27  rotates at a faster linear speed than the photoconductor drum  21 . As a result, the third speed change portion  13 C increases the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  by increasing the linear speed of the electricity removing member  27 . It is noted that in a case where the electricity removing member  27  rotates at a slower linear speed than the photoconductor drum  21 , the third speed change portion  13 C may increase the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  by decreasing the linear speed of the electricity removing member  27 . 
     For example, when the second variation amount obtaining portion  16 B obtains the variation amount ΔRb of the contact resistance component Rb, the third speed change portion  13 C calculates the linear speed of the electricity removing member  27  that satisfies the above-described conditions by using the inner resistance component Ra, the contact resistance component Rb, and the calculated resistance values R 21  stored in the ROM. The third speed change portion  13 C changes the linear speed of the electricity removing member  27  based on the calculation results. 
     It is noted that the third speed change portion  13 C may change the linear speed of the electricity removing member  27  so that the difference from the third specific speed becomes equal to or smaller than the allowed value. In addition, the third speed change portion  13 C may change the linear speed of the electricity removing member  27  so that the ratio Sr satisfies the above-described formulas (2) and (9). 
     In addition, in the image forming apparatus  10 , seventh table data may be stored in the ROM of the control portion  1  in advance, wherein the seventh table data indicates linear speeds of the electricity removing member  27  that correspond to predetermined decrease amounts of the outer diameter of the electricity removing member  27 . In this case, the third speed change portion  13 C may change the linear speed of the electricity removing member  27  by using the seventh table data and the decrease amount of the outer diameter of the electricity removing member  27  obtained by the second obtainment processing portion  15 B. In addition, in this case, the control portion  1  may not include the second variation amount obtaining portion  16 B. For example, the seventh table data is generated based on experimental data that is obtained by an experiment conducted by using the image forming apparatus  10  to investigate the relationship between the decrease amount of the outer diameter of the electricity removing member  27  and the post-electricity-removal potential V 1 , and experimental data that is obtained by an experiment conducted by using the image forming apparatus  10  to investigate the relationship between the ratio Sr and the post-electricity-removal potential V 1 . 
     In addition, the third speed change portion  13 C may increase the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  by changing the linear speed of the photoconductor drum  21 . In addition, the third speed change portion  13 C may increase the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  within a range of equal to or lower than a reset upper-limit value. 
     [Third Speed Change Process] 
     In the following, with reference to  FIG. 21 , a description is given of an example of the procedure of the third speed change process executed by the control portion  1  in the image forming apparatus  10 . 
     &lt;Step S 31 &gt; 
     First, in step S 31 , the control portion  1  determines whether or not the third timing has come. 
     Here, upon determining that the third timing has come (Yes side at S 31 ), the control portion  1  moves the process to step S 32 . In addition, upon determining that the third timing has not come (No side at S 31 ), the control portion  1  waits at step S 31  for the third timing to come. 
     &lt;Step S 32 &gt; 
     In step S 32 , the control portion  1  obtains the outer diameter of the electricity removing member  27 . Here, the processes of steps S 31  and S 32  are executed by the second obtainment processing portion  15 B of the control portion  1 . 
     Specifically, the control portion  1  obtains the outer diameter of the electricity removing member  27  based on the fifth table data and the cumulative number of prints read from the fourth storage area. 
     &lt;Step S 33 &gt; 
     In step S 33 , the control portion  1  obtains the variation amount ΔRb of the contact resistance component Rb of the electricity removing remember  27  based on the decrease amount of the outer diameter of the electricity removing member  27  obtained in step S 32 . Here, the process of step S 33  is executed by the second variation amount obtaining portion  16 B of the control portion  1 . It is noted that the process of step S 33  may be omitted. 
     Specifically, the control portion  1  calculates the decrease amount of the outer diameter of the electricity removing member  27  based on: the outer diameter of the electricity removing member  27  obtained in step S 32 : and an outer diameter of the electricity removing member  27  at the time of manufacture of the image forming apparatus  10  that is stored in the ROM in advance. Subsequently, the control portion  1  obtains the variation amount ΔRb of the contact resistance component Rb of the electricity removing member  27  based on the calculated decrease amount of the outer diameter of the electricity removing member  27  and the sixth data table. 
     &lt;Step S 34 &gt; 
     In step S 34 , the control portion  1  changes the linear speed of the electricity removing member  27  based on the variation amount ΔRb of the contact resistance component Rb of the electricity removing member  27  obtained in step S 33 . Here, the process of step S 34  is executed by the third speed change portion  13 C of the control portion  1 . 
     Specifically, the control portion  1  changes the linear speed of the electricity removing member  27  to the third specific speed so that the ratio Sr satisfies the above indicated formulas (2) and (9) and the difference from the linear speed of the photoconductor drum  21  becomes the minimum. For example, the control portion  1  changes the linear speed of the electricity removing member  27  by rewriting data stored in the second storage area in the RAM that indicates the set value of the linear speed of the electricity removing member  27 . 
     As described above, in the image forming apparatus  10  according to the fourth embodiment, the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  increases in correspondence with a decrease of the outer diameter of the electricity removing member  27  obtained based on the second obtainment condition. This makes it possible to restrict the electricity removing capability of the electricity removing member  27  from being lowered due to a decrease of the outer diameter of the electricity removing member  27 . 
     In addition, in the image forming apparatus  10  according to the fourth embodiment, the linear speed of the electricity removing member  27  is changed to the third specific speed so that the ratio Sr satisfies the above-indicated formulas (2) and (9) and the difference from the linear speed of the photoconductor drum  21  becomes the minimum. With this configuration, the difference between the linear speed of the photoconductor drum  21  and the linear speed of the electricity removing member  27  is minimized within a range where the necessary electricity removing capability is secured. Accordingly, it is possible to restrict the wearing of the photoconductor drum  21 . 
     It is noted that, as a modification of the fourth embodiment, as shown in  FIG. 23 , the image forming apparatus  10  may include a rotation control portion  17 . Specifically, each time the cumulative number of prints or the cumulative number of rotations increases by a predetermined reference value, the rotation control portion  17  rotates the electricity removing member  27  in a direction reverse to the rotation direction during execution of the print process, at a predetermined fourth timing that is different from a timing during the execution of the print process. For example, the rotation control portion  17  rotates the electricity removing member  27  in a direction revere to the rotation direction during execution of the print process, for a predetermined time period or a predetermined number of rotations. With this configuration, the curving of the tips of the brush bristles  271  is corrected at regular intervals, thereby making it possible to restrict the outer diameter of the electricity removing member  27  from decreasing. It is noted that in a case where the rotation control portion  17  is provided in the image forming apparatus  10 , the contents of the fifth table data may be corrected.