Patent Publication Number: US-9891567-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to an electrophotographic image forming apparatus. 
     Description of the Related Art 
     An electrophotographic image forming apparatus uniformly charges a surface of an image-bearing member, such as a photosensitive drum, by charging means, then exposes it to form an electrostatic latent image, develops the electrostatic latent image with coloring toner, and forms a visible image (toner image). Then, the image forming apparatus transfers the formed toner image to a recording medium, such as a sheet of paper, directly or with an intermediate transfer member or other member disposed therebetween. Foreign matter remaining on the image-bearing member after the transfer, such as toner, is cleaned (removed) by a cleaning device, such as a cleaning blade. One known example of the cleaning device may be a configuration that uses a cleaning rotator, such as a fur brush, for cleaning foreign matter on the image-bearing member by rotating itself and rubbing the image-bearing member (Japanese Patent Laid-Open No. 2011-39427). 
     In a configuration in which a plurality of image-bearing members are provided with their respective fur brushes, when all the fur brushes are operated at the same timing, an electric power required for starting up the fur brushes is significantly large. 
     SUMMARY OF THE INVENTION 
     The present disclosure provides an image forming apparatus capable of reducing the intensive use of an electric power in starting up fur brushes. 
     The present disclosure provides an image forming apparatus including a first image forming portion, a second image forming portion, and a control portion. The first image forming portion includes a first image-bearing member, a toner image forming portion configured to form a first toner image on the first image-bearing member, and a first cleaning member being rotatable and configured to clean the first toner image on the first image-bearing member by rotation. The second image forming portion includes a second image-bearing member, a toner image forming portion configured to form a second toner image on the second image-bearing member, and a second cleaning member being rotatable and configured to clean the second toner image on the second image-bearing member by rotation. The control portion is configured to control a rotation start timing for each of the first and second cleaning members such that when an image forming operation starts after an image forming signal for forming an image on a recording medium is input, an output of a signal for starting the rotation of the first cleaning member and an output of a signal for starting the rotation of the second cleaning member are different. 
     According to other aspects of the present disclosure, one or more additional image forming apparatuses and one or more methods of using or controlling one or more image forming apparatuses are discussed herein. Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram that illustrates an image forming apparatus according to a first embodiment of the present disclosure. 
         FIG. 2  is a diagram that illustrates an image forming unit according to the first embodiment of the present disclosure. 
         FIG. 3  is a graph that illustrates a relationship between a surface potential of a photosensitive drum and a time for which a fur brush is driven according to the first embodiment of the present disclosure. 
         FIG. 4  is a block diagram that illustrates a configuration of a control portion in the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG. 5  is a flow chart that illustrates control processing performed by the control portion according to the first embodiment of the present disclosure. 
         FIG. 6  is a sequence chart that illustrates the control processing performed by the control portion according to the first embodiment of the present disclosure. 
         FIG. 7  is a flow chart that illustrates control processing performed by the control portion according to a second embodiment of the present disclosure. 
         FIG. 8  is a sequence chart that illustrates the control processing performed by the control portion according to the second embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     A first embodiment of the present disclosure is described below with reference to  FIGS. 1 to 6 . First, a general configuration of an image forming apparatus in the present embodiment is described with reference to  FIG. 1 . 
     [Outline of Image Forming Apparatus] 
       FIG. 1  is a diagram that illustrates an embodiment of the image forming apparatus in the present disclosure. As illustrated in  FIG. 1 , an image forming apparatus  100  is a full-color image forming apparatus using an electrophotographic technology. In the image forming apparatus  100 , image forming units  200 Y,  200 M,  200 C, and  200 K configured to form toner images of four colors are arranged. These four image forming units  200 Y,  200 M,  200 C, and  200 K have substantially the same configuration. In the following description, the configuration of the image forming unit  200 Y for yellow is described as a representative. For the other image forming units, the members having the same configurations and functions as those in the image forming unit  200 Y bear the same numerals and corresponding suffixes indicating their respective units. The toner images of four colors are made up of yellow (Y), magenta (M), cyan (C), and black (K) images. 
     The image forming unit  200 Y includes a photosensitive drum  1 Y as a rotationally drivable image-bearing member configured to bear a toner image, a charging device  2 Y, an exposing device  3 Y, a developing device  4 Y, a primary transfer roller  5 Y, and a cleaning device  30 Y. The surface of the photosensitive drum  1 Y is charged by the charging device  2 Y as charging means. The charged surface of the photosensitive drum  1 Y is exposed by the exposing device  3 Y as electrostatic latent image forming means based on image information, and an electrostatic latent image is formed thereon. The electrostatic latent image on the photosensitive drum  1 Y is developed as a toner image by the developing device  4 Y as developing means. The toner image on the photosensitive drum  1 Y is primarily transferred by the primary transfer roller  5 Y as primary transfer means at a primary transfer portion to an intermediate transfer belt  8  as an intermediate transfer member. Foreign matter remaining on the photosensitive drum  1 Y after the transfer, such as toner, is cleaned by the cleaning device  30 Y as cleaning means. The cleaning device  30 Y includes a fur brush  6 Y (cleaning rotator) configured to clean (remove) foreign matter on the photosensitive drum  1 Y and a cleaning blade  7 Y configured to remove foreign matter with attractive force reduced by the fur brush  6 Y. 
     The toner images of different colors formed by the image forming units  200 Y,  200 M,  200 C, and  200 K are transferred to the intermediate transfer belt  8  in an overlapping manner. The toner images transferred to the intermediate transfer belt  8  is made to arrive at a secondary transfer portion opposed a secondary transfer roller  10  by the intermediate transfer belt  8  rotationally driven in a direction indicated by the arrows in  FIG. 1 . The toner images on the intermediate transfer belt  8  are secondarily transferred to a recording medium  12  (a sheet of paper, a sheet material, such as a transparency, or the like) at the secondary transfer portion and are fixed on the recording medium  12  by a fixing device  11  as fixing means. Toner remaining on the intermediate transfer belt  8  after the secondary transfer (secondary transfer residual toner) is cleaned (removed) from the intermediate transfer belt  8  by an intermediate transfer belt cleaning device  9  as an intermediate transfer member cleaning portion. 
     Next, the elements in the above-described image forming unit  200 Y are described with reference to  FIG. 2 . In the following description, a configuration common to the image forming units is described without suffixes. A configuration specific to an image forming unit is described with a suffix. 
     [Toner] 
     In the present embodiment, the image forming apparatus  100  uses toner that is obtained by crushing and classifying a mixture in which a resin binder predominantly composed of polyester is kneaded with a pigment and that has an average particle diameter of approximately 6 μm. The toner is frictionally charged to negative polarity by rubbing with a magnetic carrier. The average charge amount of the toner attached to an electric potential at an exposure portion in the photosensitive drum  1  is approximately −30 μC/g. 
     [Photosensitive Drum] 
     The photosensitive drum  1  has a cylindrical shape with an axial length of 360 mm and an outside diameter of 84 mm and has a negatively chargeable organic photoconductor (OPC). Specifically, in the photosensitive drum  1 , a photosensitive layer including a photoconductive layer predominantly composed of an organic photoconductor is disposed on an electro-conductive base member. The OPC is typically a lamination in which a charge generation layer, charge transport layer, and surface protecting layer that are made of an organic material are laminated on a metal base member as an electro-conductive base member. In the present embodiment, a material described in Japanese Patent Laid-Open No. 2005-43806 is used in each layer. The photosensitive drum  1  is rotationally driven in a direction indicated by the arrow during image formation by a drum motor  50  (see  FIG. 4 ) as photoconductor driving means at a process speed (peripheral speed) of normally 300 mm/s. 
     [Charging Device] 
     The charging device  2  is a contact charging roller and is configured to charge the photosensitive drum  1  by employing an electric discharge phenomenon occurring in a minute gap between the charging device  2  and the photosensitive drum  1 . A cored bar in the charging device  2  is subjected to an applied charging bias voltage having preset conditions. For example, in the case where the applied DC bias is set at −500 V and AC bias is set at a peak-to-peak bias higher than or equal to twice a discharge start voltage under that environment, the charging device  2  performs charging processing such that an image forming portion in the photosensitive drum  1  is uniformly charged to approximately −500 V. A charging potential in the charging processing by the charging device  2  is negative (has the negative polarity) and charges the photosensitive drum  1  to the negative side. The DC bias applied during image formation is not limited to −500 V and is set at an electric potential suited for satisfactory image formation in accordance with the environment and circumstances, such as times for which the photosensitive drum  1  and charging device  2  are used, lifespans, and the like, as appropriate. The charging device  2  is not limited to the contact charging roller and may be another configuration, such as a noncontact charging roller or a device that uses corona charging. 
     [Exposing Device] 
     The exposing device  3  includes a semiconductor laser configured to perform image exposure on the photosensitive drum  1  with the surface uniformly charged by the charging device  2  based on image information. In the present embodiment, a potential of exposure with laser light is −200 V. The image forming unit  200  is provided with an electric potential measuring device (not illustrated) configured to measure the potential of the photosensitive drum  1  after the exposure and thus can check whether each of the charging potential and the exposure potential is actually a predetermined potential. In the present embodiment, the exposing device  3  is configured to perform image exposure by using the semiconductor laser. The exposing device  3  may be configured to perform image exposure by using another means, such as a light-emitting diode. 
     [Developing Device] 
     The developing device  4  includes a development container that stores two-component developer being a mixture of nonmagnetic toner and a magnetic carrier and a rotatable developing sleeve disposed at an opening portion in the development container. The developing sleeve has the functions of magnetically holding the developer in the development container by using a magnet fixed therein and of conveying the developer to a development portion being a gap portion between the developing sleeve and the photosensitive drum  1 . The axial length of the developing sleeve is 325 mm. The developing sleeve is connected to a high voltage power source configured to apply a development bias in which a direct-current voltage (−400 V) and an alternating-current voltage (Vp-p is 1600 V) are superimposed. By attaching toner to an electrostatic latent image by the development bias, developing processing is performed. The set value of the development bias is an example and may be set at a value adjusted in accordance with the charging potential or exposure potential for the photosensitive drum  1  as appropriate. 
     [Cleaning Device] 
     Next, the cleaning device  30  included in the image forming unit  200  is described in detail with reference to  FIG. 2 .  FIG. 2  is a cross-sectional view that illustrates the details of the cleaning device  30 . The cleaning device  30  includes the fur brush  6 , which has a brush shape, configured to scrape toner on the photosensitive drum  1  (image bearing member) and scrub the surface of the photosensitive drum  1 . The cleaning device  30  includes the cleaning blade  7  located downstream of the fur brush  6  in the rotational direction of the photosensitive drum  1  and configured to clean the surface of the photosensitive drum  1 . The cleaning device  30  houses the fur brush  6  and cleaning blade  7  inside a housing  31 . 
     The fur brush  6  has a rotating shaft with fibers implanted therein and, in the present embodiment, is produced by winding cloth with fibers implanted therein around a metal rotating shaft having a diameter of 12 mm. The fibers of the fur brush  6  in the present embodiment are bundles of 6-denier acrylic filaments and are implanted in a base material with a bristle implant density of 50 kF/inch 2  per filament. The fibers of the fur brush  6  have a length of 4.5 mm. The fur brush  6  is disposed in the image forming unit  200  such that the leading ends of the fibers enter the photosensitive drum  1  by approximately 0.4 mm. 
     The fur brush  6  is mounted such that the metal rotating shaft is grounded when it is mounted to the image forming apparatus. The photosensitive drum  1  and fur brush  6  are set such that both are rotated in the same direction at a contact nip portion where both are in contact with each other, as indicated by the arrows illustrated in  FIG. 2 . The rotation of the fur brush  6  is driven by a cleaning motor  60  as fur brush driving means and can be freely rotated by being controlled by a control portion  40 . In the present embodiment, the ratio of a peripheral speed of the fur brush  6  during steady rotation in which acceleration has been completed and the speed is constant to a peripheral speed of the photosensitive drum  1  during steady rotation is 110%. That is, the fur brush  6  and photosensitive drum  1  rotate at peripheral speeds at which the ratio of the peripheral speed of the fur brush  6  to that of the photosensitive drum  1  during steady rotation is 1.1. The peripheral speed at which the photosensitive drum  1  is in steady rotation is a speed at which images can be formed in the image forming apparatus  100 . 
     The fibers in the fur brush  6  are described in detail below. Each of the fibers in the fur brush  6  has a substantially circular cross-sectional shape and a surface shape having fine ridges and holes in places. The fine ridges provide the fur brush  6  with an increased area of a contact surface with foreign matter on the photosensitive drum  1 , and this leads to improved collecting performance. The cross-sectional shape of each of the fibers in the fur brush  6  is not limited to being circular and may be other shapes, such as oval, polygonal, or star shapes. It is useful that the cross-sectional shape of each of the fibers in the fur brush  6  may be selected in consideration of, in addition to the performance of colleting foreign matter, potential changes caused by contact with the surface layer of the photosensitive drum  1 , effects of rubbing on the surface layer of the photosensitive drum  1 , or other factors. 
     The fibers in the fur brush  6  in the cleaning device  30  are made of acrylic, which has characteristics of tending to become negative in the triboelectric series. The cleaning device  30  uses fibers that adjust resistance of the fibers by, for example, having a certain amount of carbon distributed in the fibers in the fur brush  6  and that possess electrical conductivity. The material that tends to be negatively charged is used in the fur brush  6  because an average charge amount in toner remaining on the photosensitive drum  1  in the image forming unit  200  can be of positive polarity (on the positive side). The state where the average charge amount in toner is on the positive side arises from the effects occurring when it passes through the primary transfer roller  5  and results from being able to switch from negative to positive in polarity caused by a significant decrease in the average charge amount in untransferred toner. 
     Thus, the use of the material that tends to become negative in the triboelectric series in the fibers in the fur brush  6  enables the cleaning device  30  to easily collet toner or the like that is made positive by the fur brush  6 . The use of the material that tends to become negative in the triboelectric series in the fibers in the fur brush  6  enables the cleaning device  30  to easily remove foreign matter from the surface of the photosensitive drum  1  and to assist the cleaning blade  7  in cleaning. The cleaning device  30  can perform discharging in which a negatively charged potential in the photosensitive drum  1  by causing the fur brush  6  to be in contact with the photosensitive drum  1  returns toward zero. The fibers in the fur brush  6  are not limited to acrylic and may be another material such as polyester, nylon, Teflon (registered trademark), or vinyl chloride. As the fibers in the fur brush  6 , a material that tends to become charged to polarity opposite to that of the charging potential of the toner and photosensitive drum  1  in the triboelectric series may be used in order to facilitate collecting negative toner or the like. 
     The cleaning blade  7  is made of urethane rubber and has an elastic force. The cleaning blade  7  in the present embodiment has an axial length of 340 mm and is in contact with the photosensitive drum  1  with a predetermined abutment pressure. 
     Next, the details of the removal of foreign matter on the photosensitive drum  1  by the cleaning device  30  are described. The cleaning device  30  weakens an attractive force to the photosensitive drum  1  by disturbing residues (foreign matter), such as toner (transfer residual toner), on the surface of the photosensitive drum  1  after a toner image is transferred by using the fur brush  6 . After weakening the attractive force of the foreign matter on the photosensitive drum  1  by using the fur brush  6 , the cleaning device  30  removes the foreign matter from the surface of the photosensitive drum  1  by using the cleaning blade  7 . The foreign matter removed from the surface of the photosensitive drum  1  is temporarily held on the fur brush  6  and then is transported to a scraper  32  being in contact with a circumferential surface of the fur brush  6  by rotation of the fur brush  6 . The foreign matter flies out of the fur brush  6  by a repulsive force of the fibers in the fur brush  6  elastically deformed by being in contact with the scraper  32  and falls on a conveying screw  33  or its vicinity. The foreign matter falling on the conveying screw  33  or its vicinity is conveyed in an axial direction of the photosensitive drum  1  by the conveying screw  33 , which extends along the rotational axial direction of the photosensitive drum  1 , passes through a collecting toner conveyance passage (not illustrated), and is collected by a toner collecting container (not illustrated). 
     The foreign matter removed by the fur brush  6  and cleaning blade  7  includes substances other than the transfer residual toner on the photosensitive drum  1 . The photosensitive drum  1  undesirably collects other substances, such as secondary transfer residual toner on the intermediate transfer belt  8  (on the intermediate transfer member), paper dust and filler containing paper fibers included in the recording medium  12 , calcium carbonate, or the like. Thus, the cleaning device  30  also cleans the secondary transfer residual toner and foreign matter such as components stemming from the recording medium  12 , together with the transfer residual toner on the photosensitive drum  1 . The secondary transfer residual toner and components stemming from the recording medium  12  on the intermediate transfer belt  8  should be cleaned by the intermediate transfer belt cleaning device  9 , but it is difficult to fully clean them. Foreign matter that has not been cleaned arrives at the photosensitive drum  1 . 
     When the components stemming from the recording medium  12  on the intermediate transfer belt  8  arrive at the cleaning blade  7  in the cleaning device  30 , they are caught in a contact portion between the cleaning blade  7  and the photosensitive drum  1 , and cleaning defects are likely to occur. Thus, it is useful to remove the components stemming from the recording medium  12  by using the fur brush  6  before they arrives at the cleaning blade  7 . When the intermediate transfer belt  8  is rotationally driven, the components stemming from the recording medium  12  may be conveyed to the photosensitive drum  1  from the intermediate transfer belt cleaning device  9 , and thus it is useful to drive the fur brush  6  in the cleaning device  30 . 
     [Frictional Charging by Fur Brush] 
     Next, frictional charging the photosensitive drum  1  by the fur brush  6  is described. As previously described, in the present embodiment, the fur brush  6  is made of an acrylic fiber, and the photosensitive drum  1  is made of an OPC. Accordingly, the fur brush  6  is on the negative side in the triboelectric series with respect to the photosensitive drum  1 . Thus, if the photosensitive drum  1  comes into contact with the fur brush  6  too many times, the photosensitive drum  1  is charged to polarity opposite to that of the charging potential. 
       FIG. 3  is a graph that illustrates a relationship between the time for which the fur brush  6  is driven (rubs) and the surface potential of the photosensitive drum  1  when the fur brush  6  is driven in a state where the photosensitive drum  1  is at rest.  FIG. 3  reveals that although the surface potential of the photosensitive drum  1  is substantially zero at a point in time before the fur brush  6  is driven, as the fur brush  6  is driven, the photosensitive drum  1  is charged to the positive side, which is opposite to the polarity of the charging potential. If the photosensitive drum  1  is charged at the positive side, it is difficult for the photosensitive drum  1  to be uniformly charged by the charging device  2 , and a potential difference locally arises on its surface. In this case, when image exposure light is emitted from the exposing device  3 , an electrostatic latent image with a different charging potential is formed locally, and when a development bias is applied by the developing device  4 , the amount of toner increases locally, and unevenness of image is likely to occur. Thus, it is useful that in a state where the photosensitive drum  1  is at rest, the fur brush  6  is not driven. In the following description, the state in which the photosensitive drum  1  is charged to polarity (positive) opposite to that of the charging potential (negative) is also described as being positively charged. The state in which the photosensitive drum  1  is charged to the same polarity as that of the charging potential (negative) is also described as being negatively charged. 
     When the fur brush  6  is new and near to its initial state, no contamination by transfer residual toner occurs, and the stiffness of the fibers is high and cleaning performance is high. Accordingly, the newer the fur brush  6  is, the more likely the photosensitive drum  1  is to be positively charged by the driving of the fur brush  6 . 
     In contrast, the more clogged the inside of the fur brush  6  is with toner, the less the effects on the charging potential of the photosensitive drum  1  by the driving of the fur brush  6  are. The effects on the charging potential of the photosensitive drum  1  by the driving of the fur brush  6  degraded by repeated use are also small. These phenomena are estimated to result from a change in the cleaning performance arising from a change in the driving torque of the fur brush  6  caused by a condition where toner is attached to the fur brush  6  or a change over time, such as an occurrence of a bend or twist in the fibers or shrinkage of an outer diameter of the fur brush  6 . 
     [Intermediate Transfer Belt] 
     The intermediate transfer belt  8  is an endless belt, is rotationally driven in the direction indicated by the arrows in  FIG. 1  by a driving roller  8 A, and has a three-tier structure in which a resin layer, an elastic layer, and a surface layer are positioned in this order from the back side. In the present embodiment, examples of a resin material that forms the resin layer in the intermediate transfer belt  8  may include polyimide and polycarbonate. The resin layer has a thickness of 70 μm to 100 μm. Examples of an elastic material that forms the elastic layer in the intermediate transfer belt  8  may include urethane rubber and chloroprene rubber. The elastic layer has a thickness of 200 μm to 250 μm. 
     It is useful that the surface layer in the intermediate transfer belt  8  is made of a material that can reduce the attractive force of toner to the surface of the intermediate transfer belt  8  and that facilitates transferring a toner image to the recording medium  12  at the secondary transfer roller  10 . In the present embodiment, the surface layer in the intermediate transfer belt  8  may be made of a resin material of any one of polyurethane, polyester, epoxy resin, and other resins or elastic materials of any two or more of elastic rubber, elastomer, butyl rubber, and other elastic materials. The intermediate transfer belt  8  may use one kind or two or more kinds of powder and grains, including fluorocarbon polymers, as a material for reducing a surface energy and increasing lubricity and distribute it in the material forming the surface layer. In the case where the intermediate transfer belt  8  uses powder and grains of fluorocarbon polymers or the like, the powder and grains may have nonuniform particle and grain sizes. 
     The surface layer in the intermediate transfer belt  8  in the present embodiment has a thickness of 5 μm to 10 μm. The surface layer in the intermediate transfer belt  8  includes an additive conductive material for adjusting a resistance value, such as carbon black, and its volume resistivity is 1e 8  Ωcm to 1e 14  Ωcm. [Intermediate Transfer Belt Cleaning Device] 
     The intermediate transfer belt cleaning device  9  is disposed at a location opposed to a tension roller  8 B for stretching the intermediate transfer belt  8  and includes two fur brushes whose material and shape are the same as those of the fur brush  6 , and the two fur brushes are disposed along the rotational drive direction of the intermediate transfer belt  8 . Of the two fur brushes included in the intermediate transfer belt cleaning device  9 , an upstream fur brush  9 A is arranged on the upstream side in the rotational drive direction of the intermediate transfer belt  8  and applies a negative bias to secondary transfer residual toner on the intermediate transfer belt  8 . Of the two fur brushes included in the intermediate transfer belt cleaning device  9 , a downstream fur brush  9 B is arranged on the downstream side in the rotational drive direction of the intermediate transfer belt  8  with respect to the upstream fur brush  9 A and applies a positive bias to the secondary transfer residual toner on the intermediate transfer belt  8 . The upstream fur brush  9 A and downstream fur brush  9 B are disposed such that the leading ends of the fibers enter the intermediate transfer belt  8  by approximately 0.8 mm. 
     In the present embodiment, a voltage that enables an electric current of −50 μA to flow is applied to the upstream fur brush  9 A to collect the secondary transfer residual toner on the intermediate transfer belt  8  and cause the secondary transfer residual toner to be negatively charged. A voltage that enables an electric current of +55 μA to flow is applied to the downstream fur brush  9 B to collect the secondary transfer residual toner made negative by the upstream fur brush  9 A. 
     The intermediate transfer belt cleaning device  9  cleans the secondary transfer residual toner and the components stemming from the recording medium  12  on the intermediate transfer belt  8  by using the upstream fur brush  9 A and downstream fur brush  9 B. The intermediate transfer belt cleaning device  9  collects components such as toner attached to the upstream fur brush  9 A and downstream fur brush  9 B by using a scraper (not illustrated), as in the case of the cleaning device  30 . The scraper may not be used, and the intermediate transfer belt cleaning device  9  may include a metallic roller being in contact with each of the upstream fur brush  9 A and downstream fur brush  9 B. In this configuration, the intermediate transfer belt cleaning device  9  collects foreign matter on the upstream fur brush  9 A and downstream fur brush  9 B by employing a potential difference between the metallic roller and each of the upstream fur brush  9 A and downstream fur brush  9 B. 
     The image forming apparatus  100  can collect most of the foreign matter on the intermediate transfer belt  8  by using the intermediate transfer belt cleaning device  9 . Unfortunately, however, the intermediate transfer belt cleaning device  9  may be unable to fully collect the foreign matter, and some may remain on the intermediate transfer belt  8 . In the intermediate transfer belt cleaning device  9 , the scraper may be unable to fully remove the foreign matter on the upstream fur brush  9 A and downstream fur brush  9 B. In such a case, the intermediate transfer belt cleaning device  9  may eject the foreign matter attached to the upstream fur brush  9 A and downstream fur brush  9 B to the intermediate transfer belt  8  when the upstream fur brush  9 A and downstream fur brush  9 B are driven. The foreign matter ejected to the intermediate transfer belt  8  may be conveyed to the image forming unit  200 . As previously described, if the foreign matter is conveyed to the image forming unit  200 , the cleaning performance by the cleaning blade  7  may degrade, and this may lead to the occurrence of cleaning defects. 
     [Control Portion] 
       FIG. 4  is a block diagram that illustrates a configuration of the control portion  40  for controlling the image forming apparatus  100 . As illustrated in  FIG. 4 , the control portion  40  includes a CPU  41 , which is a central processing unit for performing various control, a read-only memory (ROM)  42  configured to store programs executable by the CPU  41  and various data, and a random-access memory (RAM)  43  configured to temporarily retain results of computation by the CPU  41  and other data. 
     The CPU  41  controls a drum motor  50  configured to rotationally drive the photosensitive drum  1  and the cleaning motor  60  configured to rotationally drive the fur brush in each of the image forming units. The CPU  41  also controls a belt motor  70  as intermediate transfer belt driving means configured to rotationally drive the driving roller  8 A, which is configured to rotationally drive the intermediate transfer belt  8 . The CPU  41  also controls a belt cleaning motor  80  as intermediate transfer CLN driving means configured to rotationally drive the upstream fur brush  9 A and downstream fur brush  9 B included in the intermediate transfer belt cleaning device  9 . The CPU  41  emits driving OFF/ON signals to the motors in accordance with an executing program. 
     The CPU  41  controls an image formation high voltage applying portion  51  configured to control a charging voltage applied to the charging devices, a developing voltage applied to the exposing devices, and a transferring voltage applied to the primary transfer rollers. The CPU  41  also controls an intermediate transfer CLN high voltage applying portion  81  configured to apply a high-voltage potential to the intermediate transfer belt cleaning device  9 . The CPU  41  emits high-voltage OFF/ON signals to the high voltage applying portions. 
     [Fur Brush Control] 
     In the image forming apparatus  100  in the present embodiment, the image forming unit  200 Y is nearest the intermediate transfer belt cleaning device  9  in the rotational drive direction of the intermediate transfer belt  8 . The distance from the intermediate transfer belt cleaning device  9  to the fur brush  6 Y in the image forming unit  200 Y along the intermediate transfer belt  8  and photosensitive drum  1 Y is 90 mm. That is, the sum of the distance traveled by a cleaned portion  8   a , which is in contact with the intermediate transfer belt cleaning device  9  at the initiation of driving of the intermediate transfer belt  8 , to when it arrives at the photosensitive drum  1 Y and the distance traveled by a surface  1   a , which is opposed to the cleaned portion  8   a , of the photosensitive drum  1 Y to when it arrives at the fur brush  6 Y is 90 mm. Because the process speed of the image forming apparatus  100  in the present embodiment is 300 mm/sec, the foreign matter from the intermediate transfer belt cleaning device  9  arrives at the fur brush  6 Y in 300 ms. 
     Each of the drum motor  50  and belt motor  70  in the present embodiment is a stepping motor and can make the photosensitive drum  1  and intermediate transfer belt  8  reach a steady rotation where both are stably driven in 200 ms from the initiation of the driving. The image forming apparatus  100  can also make the photosensitive drum  1  and intermediate transfer belt  8  reach the steady rotation in less than 200 ms. It is useful that both reach the steady rotation in not less than 100 ms to avoid a large peripheral speed difference between the photosensitive drum  1  and intermediate transfer belt  8 . 
     The cleaning motor  60  in the present embodiment is a DC motor. Thus, the time it takes the fur brush  6  to reach a steady rotation since a driving ON signal is transmitted from the CPU  41  to the cleaning motor  60  varies depending on the state of the fur brush  6  or product variation of DC motors used in the cleaning motor  60 . The time required to reach the steady rotation varies depending on the state of the fur brush  6  because the state where the fur brush  6  is in contact with the photosensitive drum  1  varies and the driving torque of the fur brush  6  varies depending on the state of the fur brush  6 . 
     Therefore, the CPU  41  sets the timing when it drives the fur brush  6  on the assumption that the cleaning motor  60  is a DC motor in which the driving torque of the fur brush  6  is high and the startup characteristics are not good. It is found that the time it takes the cleaning motor  60  in the present embodiment to reach a steady rotation from the initiation of rotation is 100 ms at maximum. 
     Traditional image forming apparatuses typically start driving a fur brush and a photosensitive drum at the same time. However, if a driving shock occurs when an attempt to drive both at the same time is made, the fur brush may start rotating before the photosensitive drum starts being driven, the photosensitive drum may be much rubbed with the fur brush locally, an area charged to polarity opposite the charging potential (“memory”) may appear in the photosensitive drum. If the charging in the memory is slight, the image forming apparatus can recover the memory by idly rotating the photosensitive drum. If the photosensitive drum is idly rotated for a period of time required to recover the memory, the time for the idle rotation (down time) is taken before image formation starts, and this impairs usability. 
     To address this issue, in the present embodiment, the drum motor  50  and cleaning motor  60  are independent from each other, and both are independently controlled by the CPU  41 . The CPU  41  prevents the appearance of memory by controlling the drum motor  50  and cleaning motor  60  so as not to drive both at the same time. The details of the control of the drum motor  50  and cleaning motor  60  by the CPU  41  are described below. 
       FIG. 5  is a flow chart that illustrates control processing when the CPU  41  performs drive control for the photosensitive drum  1  and fur brush  6 .  FIG. 6  is a sequence chart that illustrates behaviors of the drum motor  50 , cleaning motor  60 , and belt motor  70  when the CPU  41  performs the control in accordance with the flow chart illustrated in  FIG. 5 . The sequence chart in  FIG. 6  schematically illustrates timings when signals for turning-on electrical driving are input into the motors. Thus, real operations of each of the motors may differ by the order of several tens of microseconds depending on the status of the torque of an object to be driven. It is necessary for the image forming apparatus  100  to determine the timing when the driving of each of the motors starts while checking the real operations for driving the motors. 
     As illustrated in  FIG. 5 , the CPU  41  first starts driving the drum motor  50  and belt motor  70  (S 101 ). In the processing at step S 101 , the CPU  41  outputs a driving ON signal to each of the drum motor  50  and belt motor  70  to start driving the drum motor  50  and belt motor  70  at the same time. In the image forming apparatus  100 , as illustrated in  FIG. 6 , the driving ON signal is output to each of the drum motor  50  and belt motor  70 , so that rotation of the photosensitive drum  1  and rotational drive of the intermediate transfer belt  8  start at the same time. The timing when the driving ON signal is output to each of the drum motor  50  and belt motor  70  and the rotation of the photosensitive drum  1  and rotational drive of the intermediate transfer belt  8  start at the same time is defined as first start timing A 1 . In the following description, the first start timing A 1  is used as the reference (0 ms) in the staring operation. 
     As previously described, the photosensitive drum  1  and intermediate transfer belt  8  are configured such that both reach a steady rotation in 200 ms since both start rotating. Thus, it is desirable that the fur brush  6 Y in the present embodiment start rotating after the time it takes the photosensitive drum  1  to start the steady rotation from the first start timing A 1  (=200 ms) or more elapses. 
     In the image forming apparatus  100 , as described above, foreign matter from the intermediate transfer belt cleaning device  9  arrives at the fur brush  6 Y in 300 ms. Thus, it is useful that the image forming apparatus  100  performs control such that the fur brush  6 Y can reach a steady rotation before 300 ms elapses from the initiation of rotational drive of the intermediate transfer belt  8 . As previously described, the fur brush  6  is configured such that it reaches the steady rotation in 100 ms from the initiation of rotation. 
     In consideration of the above, the CPU  41  determines the timing when it outputs the driving ON signal to the cleaning motor  60 . The CPU  41  proceeds to processing at step S 102  after the elapse of first time T 1  (=100 ms) from the processing at step S 101 . In the processing at step S 102 , the CPU  41  starts driving the belt cleaning motor  80  and starts applying a high voltage to the upstream fur brush  9 A and downstream fur brush  9 B by using the intermediate transfer CLN high voltage applying portion  81 . Then, the CPU  41  proceeds to processing at step S 103  after the elapse of second time T 2  (=100 ms) from the processing at step S 102 . In the processing at step S 103 , the CPU  41  starts rotation of the fur brush  6  by outputting a driving ON signal to the cleaning motor  60 . 
     In performing the processing at step S 103 , the CPU  41  outputs the driving ON signal to the cleaning motor  60  and starts rotation of all of the fur brushes  6  of the fur brushes  6 Y,  6 M,  6 C, and  6 K at the same time, as illustrated in  FIG. 6 . The timing when the driving ON signal is output to the cleaning motor  60  and all the fur brushes  6  start rotating is defined as second start timing A 2 . 
     In this way, after the elapse of the first time T 1  and second time T 2  from the processing at step S 101 , the CPU  41  performs the processing at step S 103 . Therefore, the CPU  41  can place a time interval equal to or longer than the time required to start a steady rotation of the photosensitive drum  1  (=200 ms) between the initiation of rotation of the photosensitive drum  1  and the initiation of rotation of the fur brush  6 . The CPU  41  starts rotation of the fur brush  6 Y at the point in time when 200 ms elapses from the initiation of rotational drive of the intermediate transfer belt  8 . Accordingly, the CPU  41  can make the fur brush  6 Y reach a steady rotation before the elapse of the time it takes foreign matter on the intermediate transfer belt  8  to arrive at the fur brush  6 Y from the initiation of rotational drive of the intermediate transfer belt  8  (=300 ms). The CPU  41  can rotate the fur brush  6  and photosensitive drum  1  at peripheral speeds at which the ratio of the peripheral speed of the fur brush  6  to that of the photosensitive drum  1  is always 1.1 or less by starting rotation of the fur brush  6  after the photosensitive drum  1  reaches the steady rotation. The first time T 1  and second time T 2  constitute a start waiting time in the present embodiment. 
     By executing the control at steps S 101  to S 103 , the CPU  41  prevents the photosensitive drum  1  and fur brush  6  from starting rotating at the same time. Thus, the image forming apparatus  100  can prevent the photosensitive drum  1  from being robbed with the fur brush  6  locally when each of the photosensitive drum  1  and fur brush  6  starts rotating and can reduce the appearance of memory on the photosensitive drum  1 . 
     After the processing at step S 103 , the CPU  41  ends the processing relating to the driving of the photosensitive drum  1  and fur brush  6 . After the elapse of third time T 3 , the CPU  41  performs various processing relating to image formation (S 104 ). It is useful that the third time T 3  is a time required to stabilize the driving of the photosensitive drum  1 , fur brush  6 , and intermediate transfer belt  8 . 
     Next, control processing performed by the CPU  41  when it stops the photosensitive drum  1  and fur brush  6  is described with reference to the sequence chart in  FIG. 6 . As previously described, the cleaning motor  60  in the present embodiment is a DC motor and stops after it rotates by only the amount corresponding to moment of inertia during driving after it is electrically turned off. The cleaning motor  60  is configured such that 300 ms is needed as first stop time S 1  taken to stop in the state where the driving torque of the fur brush  6  is low and the moment of inertia of the DC motor is the largest. 
     As described above, the drum motor  50  is a stepping motor, and it stops after it rotates by only the amount corresponding to moment of inertia of the stepping motor after it is electrically turned off. The drum motor  50  in the present embodiment is configured such that 200 ms is needed as second stop time S 2  taken to stop the photosensitive drum  1 . 
     In consideration of the above, the CPU  41  determines the timing when it outputs the driving OFF signal to each of the drum motor  50  and cleaning motor  60 . For example, to end an image forming operation and stop, the CPU  41  first outputs the driving OFF signal to the cleaning motor  60 . As illustrated in  FIG. 6 , the CPU  41  outputs the driving OFF signal to the cleaning motor  60  and stops rotation of all of the fur brushes  6 Y,  6 M,  6 C, and  6 K at the same time. The timing when the driving OFF signal is output to the cleaning motor  60  and a stopping operation for all the fur brushes  6  starts is defined as first stop timing A 3 . In the following description, the first stop timing A 3  is used as the reference (0 ms) in the stopping operation. 
     Next, because the first stop time S 1  (=300 ms) is needed to stop the fur brush  6  at maximum, the CPU  41  outputs the driving OFF signal to the drum motor  50  and belt motor  70  after the elapse of 300 ms or more from the first stop timing A 3 . In the present embodiment, the CPU  41  outputs the driving OFF signal to the drum motor  50  and belt motor  70  at the point in time when 400 ms elapses from the first stop timing A 3 . The timing when the driving OFF signal is output to the drum motor  50  and belt motor  70  and the stopping operation for the photosensitive drum  1  and intermediate transfer belt  8  starts is defined as second stop timing A 4 . 
     The CPU  41  can prevent a state where the fur brush  6  is rotating while the photosensitive drum  1  is at rest by starting the stopping operation for the photosensitive drum  1  at the second stop timing A 4 . Thus, the image forming apparatus  100  can prevent the photosensitive drum  1  from being rubbed with the fur brush  6  locally when the photosensitive drum  1  and fur brush  6  are at rest and can reduce the appearance of memory on the photosensitive drum  1 . In this way, the first stop time S 1  required to elapse from the first stop timing A 3  to the second stop timing A 4  constitutes a stop waiting time in the present embodiment. 
     As described above, the image forming apparatus  100  in the present embodiment starts rotation of the fur brush  6  after starting rotation of the photosensitive drum  1 . Thus, the image forming apparatus  100  can reduce the appearance of memory on the photosensitive drum  1  that would be caused by the fur brush  6  when the photosensitive drum  1  and fur brush  6  start rotating at the same time. That is, the image forming apparatus  100  can reduce the phenomenon in which the photosensitive drum  1  is charged by the fur brush  6  to polarity opposite the charging potential by the charging device  2 . 
     In the present embodiment, the CPU  41  is configured such that it outputs the driving ON signal to the cleaning motor  60  after the photosensitive drum  1  and intermediate transfer belt  8  reach a steady rotation. Other forms may also be used. If the fur brush  6  is driven for 100 ms or more in the state where the photosensitive drum  1  is at rest, the portion of the photosensitive drum  1  being in contact with the fur brush  6  is positively charged locally. If the peripheral speed difference between the peripheral speed of the fur brush  6  and that of photosensitive drum  1  is equal to or larger than a certain value, the photosensitive drum  1  is also positively charged. 
     The case where the photosensitive drum  1  is positively charged by being rubbed with the fur brush  6  rotating at a peripheral speed higher than that of the photosensitive drum  1  is described in detail below. An experiment on the relationship between the peripheral speed of the fur brush  6  with respect to that of the photosensitive drum  1  and the surface potential of the photosensitive drum  1  shows that the surface potential of the photosensitive drum  1  is positively charged to +15 V or more when the fur brush  6  rotates at a peripheral speed of 500% with respect to that of the photosensitive drum  1 . In the case where the fur brush  6  rotates at a peripheral speed of 450% with respect to that of the photosensitive drum  1 , the surface potential is positively charged to the order of +8 V. In contrast, in the case where the fur brush  6  rotates at a peripheral speed of 400% with respect to that of the photosensitive drum  1 , the surface potential of the photosensitive drum  1  is a potential of the order of −10 V to 0 V, and it is not substantially positively charged. 
     That is, during rotation of the fur brush  6 , including under acceleration and deceleration, when the peripheral speed of the fur brush  6  is always lower than 400% with respect to the peripheral speed of the photosensitive drum  1 , the image forming apparatus  100  can prevent the photosensitive drum  1  from being positively charged. Accordingly, the CPU  41  can start rotation of the fur brush  6  even before 200 ms elapses since rotation of the photosensitive drum  1  starts, at a timing at which the value of the ratio of the peripheral speed of the fur brush  6  to that of the photosensitive drum  1  is always less than 4.0. Specifically, the CPU  41  may be configured so as to perform the processing at step S 103  after the elapse of 150 ms since it performs the processing at step S 101  illustrated in  FIG. 5 . In this configuration, because the value of the ratio of the peripheral speed of the fur brush  6  to that of the photosensitive drum  1  is 2.5 at maximum, the fur brush  6  can be rotated within the range not larger than 4.0, which is the value of the peripheral-speed ratio where the surface potential of the photosensitive drum  1  is not positively charged. In this case, fourth time elapsed since the processing at step S 101  is performed (150 ms) constitutes the start waiting time. The value 4.0 of the ratio of the peripheral speed of the fur brush  6  to that of the photosensitive drum  1  constitutes a predetermined value. 
     With this configuration, the image forming apparatus  100  can properly rub the surface of the photosensitive drum  1  by using the fur brush  6 , can enhance the cleaning performance for the photosensitive drum  1 , and can prevent the cleaning blade  7  from being damaged by components stemming from the recording medium  12 . Because the image forming apparatus  100  can start rotation of the fur brush  6  before the photosensitive drum  1  reaches a steady rotation, the rise time required to start image formation can be reduced, and usability and productivity can be improved. 
     In rotation stopping operation, the image forming apparatus  100  may start a stopping operation for the photosensitive drum  1  before the fur brush  6  stops, at a timing at which the peripheral speed of the fur brush  6  is always lower than 400% with respect to that of the photosensitive drum  1 . With this configuration, the image forming apparatus  100  can reduce the fall time taken to complete image formation and can improve usability. In this case, fifth time elapsed from the initiation of the stopping operation for the fur brush  6  to the initiation of the stopping operation for the photosensitive drum  1  constitutes the stop waiting time. 
     Another control is described next with reference to  FIGS. 7 and 8 . In the above configuration, the fur brushes  6 Y,  6 M,  6 C, and  6 K start rotating at the same time and start their stopping operations at the same time. However, in the image forming apparatus  100 , components stemming from the recording medium  12  seldom reach the image forming units  200 C and  200 K, which are positioned on the downstream side in the rotational drive direction of the intermediate transfer belt  8 . 
     Specifically, of the total of foreign matter ejected from the intermediate transfer belt cleaning device  9  to the intermediate transfer belt  8 , approximately 60% attaches to the photosensitive drum  1 Y, and approximately 30% attaches to the photosensitive drum  1 M. That is, of the total of the foreign matter on the intermediate transfer belt  8 , approximately 90% attaches to the photosensitive drums  1 Y and  1 M, which are positioned on the upstream side in the rotation direction of the intermediate transfer belt  8 , and only approximately 10% arrives at the photosensitive drums  1 C and  1 K, which are positioned on the downstream side in the rotation direction. Thus, the foreign matter formed from the components stemming from the recording medium  12  on the intermediate transfer belt  8  affects the cleaning blades  7 C and  7 K little. Accordingly, in the present embodiment, the timing when the fur brush starts rotating is set for each of the image forming units. 
       FIG. 7  is a flow chart that illustrates control processing when the CPU  41  performs drive control for the photosensitive drum  1  and fur brush  6 .  FIG. 8  is a sequence chart that illustrates behaviors of the drum motor  50 , cleaning motor  60 , and belt motor  70  when the CPU  41  performs the control in accordance with the flow chart illustrated in  FIG. 7 . In the present embodiment, the photosensitive drum  1  and intermediate transfer belt  8  are configured such that both reach a steady rotation in 500 ms since both start rotating. In the present embodiment, the fur brush  6  is configured such that it reaches a steady rotation in 100 ms since it starts rotating. The image forming apparatus  100  in the present embodiment is configured such that its process speed is 225 mm/sec and foreign matter from the intermediate transfer belt cleaning device  9  arrives at the fur brush  6 Y in 400 ms. 
     As illustrated in  FIG. 7 , the CPU  41  first starts driving the drum motor  50  and belt motor  70  (S 201 ). In the processing at step S 201 , the CPU  41  outputs a driving ON signal to each of the drum motor  50  and belt motor  70  to start driving the drum motor  50  and belt motor  70  at the same time. In the image forming apparatus  100 , as illustrated in  FIG. 8 , the driving ON signal is output to each of the drum motor  50  and belt motor  70 , so that rotation of the photosensitive drum  1  and rotational drive of the intermediate transfer belt  8  start at the same time. The timing when the driving ON signal is output to each of the drum motor  50  and belt motor  70  and the rotation of the photosensitive drum  1  and rotational drive of the intermediate transfer belt  8  start at the same time is defined as first start timing B 1 . In the following description, the first start timing B 1  is used as the reference (0 ms) in the staring operation. 
     Next, the CPU  41  proceeds to the processing at step S 202  after the elapse of first time  110  (=100 ms) from the processing at step S 201 . In the processing at step S 202 , the CPU  41  starts driving the belt cleaning motor  80  and starts applying a high voltage to the upstream fur brush  9 A and downstream fur brush  9 B by using the intermediate transfer CLN high voltage applying portion  81 . Then, the CPU  41  proceeds to processing at step S 203  after the elapse of second time T 11  (=100 ms) from the processing at step S 202 . In the processing at step S 203 , the CPU  41  outputs a driving ON signal to the cleaning motor  60  and starts rotation of the fur brushes  6 Y and  6 M, which are near the intermediate transfer belt cleaning device  9  in the rotation direction of the intermediate transfer belt  8 . 
     In performing the processing at step S 203 , the CPU  41  outputs the driving ON signal to the cleaning motor  60  and starts rotation of the fur brushes  6 Y and  6 M at the same time, as illustrated in  FIG. 8 . The timing when the driving ON signal is output to the cleaning motor  60  and the fur brushes  6 Y and  6 M start rotating is defined as second start timing B 2 . In this way, after the elapse of the first time  110  and second time T 11  from the processing at step S 201 , the CPU  41  performs the processing at step S 203 . 
     Therefore, the CPU  41  can place a time interval equal to or longer than 200 ms between the initiation of rotation of the photosensitive drum  1  and the initiation of rotation of the fur brush  6 . In the case where the fur brushes  6 Y and  6 M start rotating at the second start timing B 2 , the value of the ratio of the peripheral speed of the fur brushes  6 Y and  6 M to that of the photosensitive drums  1 Y and  1 M is always at or below 2.5. That is, the CPU  41  can place a time interval equal to or longer than the time (=200 ms) required to achieve the state where the value of the ratio of the peripheral speed of the fur brushes  6 Y and  6 M to that of the photosensitive drums  1 Y and  1 M is always at or below a predetermined value before the initiation of the rotation of the fur brushes  6 Y and  6 M. Thus, even when the fur brushes  6 Y and  6 M rotate faster than the photosensitive drums  1 Y and  1 M, the image forming apparatus  100  can prevent the surface potential of the photosensitive drums  1 Y and  1 M from being positively charged by the fur brushes  6 Y and  6 M. 
     The CPU  41  starts rotation of the fur brush  6 Y at the point in time when 200 ms elapses from the initiation of rotational drive of the intermediate transfer belt  8 . Accordingly, the CPU  41  can make the fur brushes  6 Y and  6 M reach a steady rotation before the elapse of the time it takes components stemming from the recording medium  12  on the intermediate transfer belt  8  to arrive at the fur brush  6 Y from the initiation of rotational drive of the intermediate transfer belt  8  (=400 ms). Thus, the CPU  41  can increase the peripheral speed of the fur brush  6 Y up to a speed at which sufficient cleaning performance is achieved before foreign matter from the intermediate transfer belt cleaning device  9  arrives at the position of the fur brush  6 Y through the intermediate transfer belt  8  and photosensitive drum  1 . The first time T 10  and second time T 11  constitute a start waiting time in the present embodiment. 
     The CPU  41  proceeds to the processing at step S 204  after the elapse of third time T 12  (=100 ms). In the processing at step S 204 , the CPU  41  outputs a driving ON signal to the cleaning motor  60  and starts rotation of the fur brushes  6 C and  6 K, which are positioned on the downstream side in the rotation direction of the intermediate transfer belt  8 . 
     In performing the processing at step S 204 , the CPU  41  outputs the driving ON signal to the cleaning motor  60  and starts rotation of the fur brushes  6 C and  6 K at the same time, as illustrated in  FIG. 8 . The timing when the driving ON signal is output to the cleaning motor  60  and the fur brushes  6 C and  6 K start rotating is defined as third start timing B 3 . In this way, after the elapse of third time T 12  from the processing at step S 203 , the CPU  41  performs the processing at step S 204 . 
     By starting the rotation of the fur brushes  6 C and  6 K at the third start timing B 3 , the CPU  41  can make the peripheral speed of the photosensitive drums  1 C and  1 K at the third start timing B 3  higher than the peripheral speed of the photosensitive drums  1 Y and  1 M at the second start timing B 2 . That is, the CPU  41  can make the peripheral speed of the photosensitive drums  1 C and  1 K when the fur brushes  6 C and  6 K start rotating high and can more reliably prevent the surface potential of the photosensitive drums  1 C and  1 K from being positively charged. In the configuration according to the first embodiment, the surface potential of the photosensitive drums  1 C and  1 K is −9 V to +1 V. In the configuration according to the present embodiment, the surface potential of the photosensitive drums  1 C and  1 K can be −10 V to 0 V. 
     In the case where the fur brushes  6 C and  6 K start rotating at the third start timing B 3 , the value of the ratio of the peripheral speed of the fur brushes  6 C and  6 K to that of the photosensitive drums  1 C and  1 K is always at or below 2.5. Thus, even when the fur brushes  6 C and  6 K rotate faster than the photosensitive drums  1 C and  1 K, the image forming apparatus  100  can prevent the surface potential of the photosensitive drums  1 C and  1 K from being positively charged by the fur brushes  6 C and  6 K. 
     After the processing at step S 204 , the CPU  41  ends the processing relating to driving of the photosensitive drum  1  and fur brush  6 . After the elapse of fourth time T 13 , the CPU  41  executes control for stabilizing the driving of the photosensitive drum  1  and the driving of the intermediate transfer belt  8  (S 205 ). Then, the CPU  41  performs various processing relating to image formation. 
     Next, control processing performed by the CPU  41  when it stops the photosensitive drum  1  and fur brush  6  is described with reference to the sequence chart in  FIG. 8 . As in the first embodiment, the cleaning motor  60  in the present embodiment is a DC motor and stops after it rotates by only the amount corresponding to moment of inertia during driving after it is electrically turned off. The cleaning motor  60  in the present embodiment is configured such that 300 ms is needed as first stop time S 1  taken to stop in the state where the driving torque of the fur brush  6  is low and the moment of inertia of the DC motor is the largest. 
     As in the first embodiment, the drum motor  50  is a stepping motor, and it stops after it rotates by only the amount corresponding to moment of inertia of the stepping motor after it is electrically turned off. The drum motor  50  in the present embodiment is configured such that 200 ms is needed as second stop time S 2  taken to stop the photosensitive drum  1 . 
     In consideration of the above, the CPU  41  determines the timing when it outputs the driving OFF signal to each of the drum motor  50  and cleaning motor  60 . For example, to end an image forming operation and stop, the CPU  41  first outputs the driving OFF signal to the cleaning motor  60 . As illustrated in  FIG. 8 , the CPU  41  outputs the driving OFF signal to the cleaning motor  60  and stops rotation of all of the fur brushes  6 Y,  6 M,  6 C, and  6 K at the same time. The timing when the driving OFF signal is output to the cleaning motor  60  and a stopping operation for all the fur brushes  6  starts is defined as first stop timing B 4 . In the following description, the first stop timing B 4  is used as the reference (0 ms) in the stopping operation. 
     Next, because the first stop time S 1  (=300 ms) is needed to stop the fur brush  6  at maximum, the CPU  41  outputs the driving OFF signal to the drum motor  50  and belt motor  70  after the elapse of 300 ms or more from the first stop timing B 4 . In the present embodiment, the CPU  41  outputs the driving OFF signal to the drum motor  50  and belt motor  70  at the point in time when 400 ms elapses from the first stop timing B 4 . The timing when the driving OFF signal is output to the drum motor  50  and belt motor  70  and the stopping operation for the photosensitive drum  1  and intermediate transfer belt  8  starts is defined as second stop timing B 5 . 
     The CPU  41  can prevent a state where the fur brush  6  is rotating while the photosensitive drum  1  is at rest by starting the stopping operation for the photosensitive drum  1  at the second stop timing B 5 . Thus, the image forming apparatus  100  can prevent the photosensitive drum  1  from being rubbed with the fur brush  6  locally when the photosensitive drum  1  and fur brush  6  are at rest and can reduce the appearance of memory on the photosensitive drum  1 . 
     In this configuration, the image forming units  200 Y and  200 M, which are disposed on the upstream side in the rotational drive direction of the intermediate transfer belt  8  and in which the rotational drive of their fur brushes are started at the second start timing B 2 , constitute a first image forming unit. The image forming units  200 C and  200 K, which are disposed on the downstream side in the rotational drive direction with respect to the image forming units  200 Y and  200 M and in which the rotational drive of their fur brushes are started at the third start timing B 3 , constitute a second image forming unit. The number of units constituting the first image forming unit and the number of units constituting the second image forming units may not be the same. For example, the image forming unit  200 Y may constitute the first image forming unit, and the image forming units  200 M,  200 C, and  200 K may constitute the second image forming unit. As previously described, because approximately 30% of the components stemming from the recording medium  12  arrives at the image forming unit  200 M, it is useful that the first image forming unit and second image forming unit are configured as in the present embodiment. 
     In this way, the image forming apparatus  100  in the present embodiment first starts rotation of the fur brushes  6 Y and  6 M after starting rotation of the photosensitive drums  1 . Then, the image forming apparatus  100  starts rotation of the fur brushes  6 C and  6 K after starting rotation of the fur brushes  6 Y and  6 M. Accordingly, the image forming apparatus  100  can more reliably reduce the occurrence in which the photosensitive drums  1 C and  1 K are positively charged by the fur brushes  6 C and  6 K. 
     OTHER EMBODIMENTS 
     In the above-described embodiments, the image forming apparatus  100  starts rotation such that the fur brush  6 Y reaches a steady rotation after the rotational drive of the intermediate transfer belt  8  is started and before foreign matter on the intermediate transfer belt  8  arrives. Other forms may also be used. The fur brush  6  can sufficiently clean the photosensitive drum  1  when it rotates at a predetermined speed or more, even if the predetermined speed is below the peripheral speed in steady rotation. Thus, the image forming apparatus  100  may start rotation of the fur brush  6 Y at a timing that enables the fur brush  6 Y to be accelerated to a speed at which no cleaning defects occur after rotational drive of the intermediate transfer belt  8  is started and before foreign matter on the intermediate transfer belt  8  arrives. 
     In the above-described embodiments, the image forming apparatus  100  is configured such that the stopping operation for the photosensitive drum  1  is started after the stopping operation for the fur brush  6  is started. Other forms may also be used. The surface potential of the photosensitive drum  1  may be smaller than the potential change illustrated in  FIG. 3 , depending on the status of the fur brush  6 . Specifically, in the case where some toner is applied to the fur brush  6 , even when the photosensitive drum  1  is rubbed with the fur brush  6 , the occurrence in which it is positively charged can be reduced. Thus, by using a process for ejecting some toner from the developing device  4  to the fur brush  6 , the image forming apparatus  100  can avoid the photosensitive drum  1  from being positively charged even when the stopping operation for the photosensitive drum  1  is started before the stopping operation for the fur brush  6  is started. 
     The process for ejecting some toner from the developing device  4  to the fur brush  6  is described in detail below. The image forming apparatus  100  can eject some toner by repeating a series of operations of driving the developing sleeve in the developing device  4  for a short time and stopping it in the state where the charging potential and developing potential are 0 V. One example behavior of the developing device  4  may be repeating the operation of driving for 100 ms and stopping for 50 ms three times. 
     In the case where the toner is applied to the fur brush  6 , in the image forming apparatus  100 , even when the photosensitive drum  1  is at rest, the surface potential of the photosensitive drum  1  is not positively charged as long as the time for which the fur brush  6  is driven is at or below 300 ms. Thus, the image forming apparatus  100  can be configured such that the stopping operation for the photosensitive drum  1  is started before the stopping operation for the fur brush  6  is started. With this configuration, the image forming apparatus  100  can quickly stop the photosensitive drum  1  after image formation, and this leads to an extended life of the photosensitive drum  1 . 
     The image forming apparatus  100  in the above-described embodiments includes the fur brush  6  as a cleaning rotator. However, other forms may also be used. For example, a rotatable rolling member may also be used. 
     The image forming apparatus  100  in the above-described embodiments includes the drum motor  50  and cleaning motor  60  independently. Other forms may also be used. For example, the image forming apparatus  100  includes the drum motor  50  and cleaning motor  60  by using a single driving source. In this case, it is useful that the image forming apparatus  100  is configured such that the driving source is connected to a mechanism capable of switching power transmission, such as a clutch, to enable controlling the drum motor  50  and cleaning motor  60  independently. 
     The image forming apparatus  100  in the first and second embodiments is configured such that a toner image is transferred to the recording medium  12  by the secondary transfer roller  10 . Other forms may also be used. For example, the image forming apparatus  100  may be configured such that a toner image is transferred to the recording medium  12  by the primary transfer roller  5  in the image forming unit  200 . 
     The image forming apparatus according to the above-described embodiments is applicable to a copier, printer, facsimile machine, multifunction apparatus having the plurality of functions, and other similar apparatuses. 
     While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-144847 filed Jul. 22, 2015, which is hereby incorporated by reference herein in its entirety.