Patent Publication Number: US-11640133-B2

Title: Image forming apparatus capable of calculating deterioration quantity of photosensitive drum based on the number of rotations in contact and separation states

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priorities from Japanese Patent Application Nos. 2020-153549 filed Sep. 14, 2020 and 2020-153550 filed Sep. 14, 2020. The entire contents of the priority applications are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a drum cartridge including a photosensitive drum and a developing roller movable away from the photosensitive drum, and an image forming apparatus including the drum cartridge. 
     BACKGROUND 
     There has been known an image forming apparatus including a drum cartridge in which a developing roller is movable to separate from a photosensitive drum. In such an image forming apparatus, a period of time during which the developing roller makes contact with the photosensitive drum at the time of image forming operation is counted. When a total period of time during which the developing roller makes contact with the photosensitive drum reaches a threshold value, it is determined that the photosensitive drum reaches its end of service life. 
     SUMMARY 
     However, the conventional image forming apparatus may not perform accurate calculation of a deterioration quantity of the photosensitive drum, since a deterioration quantity of the photosensitive drum when the developing roller is in separation from the photosensitive drum is not taken into consideration in the above technique. 
     In view of the foregoing, it is an object of the present disclosure to provide a drum cartridge and an image forming apparatus in which accurate calculation of deterioration quantity of a photosensitive drum can be performed. 
     In order to attain the above and other objects, according to one aspect, the present disclosure provides an image forming apparatus including: a photosensitive drum; a developing roller; a separation mechanism; a main memory; and a controller. The photosensitive drum is rotatable about a first axis extending in an axial direction. The developing roller is rotatable about a second axis extending in the axial direction. The separation mechanism is configured to move at least one of the photosensitive drum and the developing roller to switch a state of the photosensitive drum and the developing roller between: a contact state in which an outer circumferential surface of the developing roller is in contact with an outer circumferential surface of the photosensitive drum; and a separation state in which the outer circumferential surface of the developing roller is in separation from the outer circumferential surface of the photosensitive drum. The main memory is configured to store therein a first rotation number and a second rotation number. The first rotation number is the number of rotations of the photosensitive drum in the contact state of the photosensitive drum and the developing roller. The second rotation number is the number of rotations of the photosensitive drum in the separation state of the photosensitive drum and the developing roller. The controller is configured to perform: calculating a deterioration quantity of the photosensitive drum based on the first rotation number and the second rotation number those stored in the main memory. 
     According to another aspect, the present disclosure also provides a drum cartridge including: a photosensitive drum; and a drum memory. The photosensitive drum is rotatable about a first axis extending in an axial direction. The photosensitive drum is switchable between: a contact state in which an outer circumferential surface of the photosensitive drum is in contact with an outer circumferential surface of a developing roller; and a separation state in which the outer circumferential surface of the photosensitive drum is in separation from the outer circumferential surface of the developing roller. The drum memory includes: a first storage area; and a second storage area. The first storage area is configured to store therein a first rotation number which is the number of rotations of the photosensitive drum in the contact state of the photosensitive drum. The second storage area is configured to store therein a second rotation number which is the number of rotations of the photosensitive drum in the separation state of the photosensitive drum. 
     According to still another aspect, the present disclosure also provides a drum cartridge including: a photosensitive drum; and a drum memory. The photosensitive drum is rotatable about a first axis extending in an axial direction. The photosensitive drum is switchable between: a contact state in which an outer circumferential surface of the photosensitive drum is in contact with an outer circumferential surface of a developing roller; and a separation state in which the outer circumferential surface of the photosensitive drum is in separation from the outer circumferential surface of the developing roller. The drum memory is configured to store therein a deterioration quantity of the photosensitive drum which is deteriorated due to rotation of the photosensitive drum. The deterioration quantity is determined based on a first rotation number and a second rotation number. The first rotation number is the number of rotations of the photosensitive drum in the contact state of the photosensitive drum. The second rotation number is the number of rotations of the photosensitive drum in the photosensitive drum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the embodiment(s) as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG.  1    is a diagram schematically illustrating a configuration of an image forming apparatus according to a first embodiment of the present disclosure; 
         FIG.  2    is an exploded perspective view of a drawer and a separation mechanism of the drawer in the image forming apparatus according to the first embodiment; 
         FIG.  3 A  is a perspective view of a developing cartridge in the image forming apparatus according to the first embodiment; 
         FIG.  3 B  is a side view of the developing cartridge in the image forming apparatus according to the embodiment; 
         FIG.  4 A  is a schematic top view illustrating the developing cartridge and components in the vicinity thereof in the image forming apparatus according to the first embodiment, and particularly illustrating a state where a developing roller is in its contact state; 
         FIG.  4 B  is a schematic top view illustrating the developing cartridge and the components in the vicinity thereof in the image forming apparatus according to the first embodiment, and particularly illustrating a state where the developing roller is in its separation state; 
         FIG.  5    is a view illustrating an inner portion of a side frame of the drawer to which the developing cartridge is attachable in the image forming apparatus according to the first embodiment; 
         FIG.  6    is a block diagram illustrating an electrical connection among a controller, a main memory, a drum memory, a temperature sensor, and motors in the image forming apparatus according to the first embodiment; 
         FIG.  7    is a table indicating information stored in a first storage area and a second storage area of the drum memory in the image forming apparatus according to the first embodiment; 
         FIG.  8    is a flowchart illustrating a process for storing information into the drum memory performed by the controller in the image forming apparatus according to the first embodiment; 
         FIG.  9    is a flowchart illustrating a lifetime determination process performed by the controller in the image forming apparatus according to the first embodiment; 
         FIG.  10    is a graph showing relationship between the total number of rotations of a photosensitive drum and a deterioration quantity of the photosensitive drum in the image forming apparatus according to the embodiment, in which a solid line indicates changes in the deterioration quantity of the photosensitive drum according to the first embodiment based on calculation, and a broken line indicates changes in a deterioration quantity of the photosensitive drum according to a conventional technique based on calculation; 
         FIG.  11    is a coefficient map according to a state of a photosensitive drum in an image forming apparatus according to a second embodiment; 
         FIG.  12    is a table indicating information stored in a first storage area and a second storage region in a drum memory in the image forming apparatus according to the second embodiment and a third embodiment of the present disclosure; 
         FIG.  13    is a flowchart illustrating a process for storing information in the drum memory performed by a controller in the image forming apparatus according to the second embodiment; 
         FIG.  14    is a flowchart illustrating a lifetime determination process performed by the controller in the image forming apparatus according to the second embodiment and the third embodiment; 
         FIG.  15    is a coefficient map according to a state of a photosensitive drum in the image forming apparatus according to the third embodiment; 
         FIG.  16    is a flowchart illustrating a process for storing information in a drum memory performed by the controller in the image forming apparatus according to the third embodiment; 
         FIG.  17    is a flowchart illustrating a process for storing information in a drum memory performed by a controller in an image forming apparatus according to a fourth embodiment of the present disclosure; and 
         FIG.  18    is a flowchart illustrating a lifetime determination process performed by the controller in the image forming apparatus according to the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     Hereinafter, an image forming apparatus  1  according to a first embodiment of the present disclosure will be described with reference to  FIGS.  1  through  10   . 
     As illustrated in  FIG.  1   , the image forming apparatus  1  is a color printer and includes a main casing  10 , a cover  11 , a sheet supply unit  20 , an image forming unit  30 , a controller  100 , a main memory  110 , a temperature sensor TS, a main motor M 1 , and a developing motor M 2 . 
     The main casing  10  has a first opening  10 A. The cover  11  is pivotally movable between a closed position (a position indicated by a solid line) in which the cover  11  closes the first opening  10 A and an open position (a position indicated by a two-dotted chain line) in which the cover  11  opens the first opening  10 A. 
     The sheet supply unit  20  is positioned at a lower internal portion of the main casing  10 . The sheet supply unit  20  includes a sheet tray  21  configured to accommodate therein a sheet(s) S, and a sheet supply mechanism  22  configured to supply a sheet S from the sheet tray  21  to the image forming unit  30 . The sheet tray  21  is detachable from the main casing  10  by pulling the sheet tray  21  out of the main casing  10 . 
     The sheet supply mechanism  22  includes a sheet pick-up roller  23 , a separation roller  24 , a separation pad  25 , and a pair of registration rollers  27 . The sheet S is a medium on which the image forming apparatus  1  can form an image. For example, plain paper, an envelope, a post card, thin paper, thick paper, calendered paper, a resin sheet, and a seal are available as the sheet S. 
     In the sheet supply unit  20 , the sheet(s) S accommodated in the sheet tray  21  is fed by the sheet pick-up roller  23 , and then separated one by one by the separation roller  24  and the separation pad  25 . Thereafter, a position of a leading edge of the sheet S is regulated by the registration rollers  27  whose rotation is stopped, and then, the sheet S is supplied to the image forming unit  30  by rotation of the registration rollers  27 . 
     The image forming unit  30  includes an exposure unit  40 , a drawer  90  as an example of a drum cartridge, a plurality of developing cartridges  60 , a conveying unit  70 , and a fixing unit  80 . 
     The exposure unit  40  includes a laser diode, a deflector, lenses, and mirrors those are not illustrated. The exposure unit  40  is configured to emit a plurality of laser beams which expose surfaces of respective photosensitive drums  50  to scan the surfaces. 
     The drawer  90  includes the plurality of photosensitive drums  50 , the plurality of developing cartridges  60 , and a drum memory  98 . That is, the drawer  90  is for use with the developing cartridges  60 . The developing cartridges  60  are attachable to and detachable from the drawer  90 . 
     Each of the photosensitive drums  50  is rotatable about a first axis  50 X extending in an axial direction of the photosensitive drum  50 . In the following description, the axial direction of the photosensitive drum  50  will be simply referred to as “axial direction”. A rotational driving force of the main motor M 1  is inputted into the photosensitive drums  50 . 
     The photosensitive drums  50  include a photosensitive drum  50 Y for a color of yellow, a photosensitive drum  50 M for a color of magenta, a photosensitive drum  50 C for a color of cyan, and a photosensitive drum  50 K for a color of black. Throughout the specification and the drawings, in a case where colors must be specified, members or components corresponding to the colors of yellow, magenta, cyan and black are designated by adding “Y”, “M”, “C” and “K”, respectively. 
     Each of the developing cartridges  60  includes a developing roller  61 . Each of the developing rollers  61  is configured to supply toner to a corresponding one of the photosensitive drums  50 . Specifically, the developing cartridges  60  include developing cartridges  60 Y,  60 M,  60 C and  60 K respectively including developing rollers  61 Y,  61 M,  61 C and  61 K. Also, these developing rollers  61 Y,  61 M,  61 C and  61 K correspond to the photosensitive drums  50 Y,  50 M,  50 C and  50 K, respectively, for the colors of yellow, magenta, cyan and black. 
     The temperature sensor TS is a sensor positioned inside the main casing  10  and at a position adjacent to the photosensitive drums  50 . The temperature sensor TS is configured to detect a temperature inside the main casing  10  of the image forming apparatus  1 . In the present embodiment, a temperature detected by the temperature sensor TS is regarded as a temperature of the photosensitive drums  50 . 
     The developing roller  61 Y, the developing roller  61 M, the developing roller  61 C, and the developing roller  61 K are arranged in this order from the upstream side toward the downstream side in a moving direction of the sheet S (hereinafter simply referred to as “sheet moving direction”). Each of the developing rollers  61  is rotatable about a second axis  61 X extending in the axial direction. 
     Each of the developing cartridges  60  is movable between a contact position (a position indicated by the solid line in  FIG.  1   ) where the developing roller  61  is in contact with the corresponding photosensitive drum  50  and a separated position (a position indicated by the two-dotted chain line in  FIG.  1   ) where the developing roller  61  is in separation from the corresponding photosensitive drum  50 . When the developing cartridge  60  is in the contact position, the developing roller  61  and the corresponding photosensitive drum  50  are in a contact state. When the developing cartridge  60  is in the separated position, the developing roller  61  and the corresponding photosensitive drum  50  are in a separation state. 
     As illustrated in  FIG.  2   , the photosensitive drums  50  are rotatably supported by the drawer  90 . Further, the developing cartridges  60  are supported by the drawer  90  so as to be attachable to and detachable from the drawer  90 . Moreover, the drawer  90  is attachable to and detachable from the main casing  10  through the first opening  10 A by opening the cover  11  (see  FIG.  1   ). As described above, the drawer  90  can be pulled out of the main casing  10  in the present embodiment. 
     The drawer  90  also includes a side frame  91 R, a side frame  91 L, a connection frame  92 , and a connection frame  93 . The side frame  91 R and the side frame  91 L are spaced apart from each other in the axial direction. The connection frame  92  connects one end portion of the side frame  91 R to one end portion of the side frame  91 L, and the connection frame  93  connects another end portion of the side frame  91 R to another end portion of the side frame  91 L. 
     The drawer  90  further includes chargers  52  and cleaning rollers  53  as illustrated in  FIG.  1   . Each of the chargers  52  is positioned to face the corresponding one of the photosensitive drums  50  and configured to charge the same. Each of the cleaning roller  53  is in contact with the corresponding one of the photosensitive drums  50  and configured to clean the photosensitive drum  50 . 
     Although not illustrated in the drawings in detail, the side frames  91 R and  91 L support one end portions and another end portions of the photosensitive drums  50 . Further, the side frame  91 L has a plurality of second openings  91 A. Each of the second openings  91 A is in a form of a notch recessed downward from an upper edge of the side frame  91 L and penetrates the side frame  91 L in the axial direction. With this configuration, each of the second opening  91 A allows the corresponding one of cam followers  170  (described later) to be positioned therein. 
     The image forming apparatus  1  further includes a plurality of separation mechanisms RK (see  FIG.  2   ). Each of the separation mechanism RK is configured to switch a state of the corresponding photosensitive drum  50  and the developing roller  61  between the contact position where an outer circumferential surface of the developing roller  61  is in contact with an outer circumferential surface of the corresponding photosensitive drum  50  and the separation state where the outer circumferential surface of the developing roller  61  is in separation from the outer circumferential surface of the corresponding photosensitive drum  50 . 
     Each of the separation mechanisms RK is configured to switch the state of the corresponding photosensitive drum  50  and the developing roller  61  between the contact state and the separation state by moving at least one of the corresponding photosensitive drum  50  and the developing roller  61 . 
     In the present embodiment, each of the separation mechanism RK is configured to move the corresponding developing roller  61  between the contact position where the developing roller  61  is in contact with the corresponding photosensitive drum  50  and the separated position where the developing roller  61  is in separation from the corresponding photosensitive drum  50 . These separation mechanisms RK are provided for the colors of Y, M, C and K. 
     Specifically, each of the separation mechanisms RK includes a support shaft  179 , a cam gear  150  ( 150 Y,  150 M,  150 C and  150 K), the cam follower  170 , a slide member  64 , and a spring  176  as illustrated in  FIGS.  2  to  4   . 
     The support shaft  179  is a shaft that extends in the axial direction. The support shaft  179  is provided at a side frame (not illustrated) of the main casing  10 . 
     The cam gear  150  is rotatable about a rotation axis  150 X extending in parallel to the second axis  61 X (see  FIG.  1   ) of the developing roller  61 . The cam gear  150  includes a gear portion  150 G, a disc portion  151 , and an end cam  152 . 
     The gear portion  150 G is positioned at an outer periphery of the disc portion  151 . The gear portion  150 G is configured to receive a driving force from the developing motor M 2 . The disc portion  151  has a substantially circular-plate shape, and a rotational driving force is inputted from the developing motor M 2  to the gear portion  150 G. Accordingly, the separation mechanism RK is operated by the driving force of the developing motor M 2 . 
     The cam follower  170  is slidingly movably supported by the support shaft  179 , and is slidingly movable in the axial direction due to contact with the end cam  152 . Specifically, as the cam gear  150  rotates, the cam follower  170  is guided by the end cam  152  to be slidingly movable between a first position (a position illustrated in  FIG.  4 B ) and a second position (a position illustrated in  FIG.  4 A ). The developing roller  61  and the photosensitive drum  50  are in the separation state when the cam follower  170  is at the first position, whereas the developing roller  61  and the photosensitive drum  50  are in the contact state when the cam follower  170  is at the second position. The cam follower  170  includes a slide shaft portion  171 , a contacting portion  172 , and a spring hook portion  174 . 
     The spring  176  illustrated in  FIG.  2    is a tension spring. The spring  176  has one end portion engaging with the spring hook portion  174 , and has another end portion engaging with the drawer  90  at a position lower than the spring hook portion  174 . Hence, the spring  176  urges the cam follower  170  in a direction from the first position toward the second position. As such, the spring  176  constantly urges the cam follower  170  toward the end cam  152 . 
     The slide shaft portion  171  engages with the support shaft  179 . The contacting portion  172  extends from the slide shaft portion  171 . The contacting portion  172  has an end face in the axial direction facing the end cam  152  and contactable with the end cam  152 . 
     As illustrated in  FIG.  3   , the slide member  64  is a member slidingly movable in the axial direction relative to a casing  63  of the developing cartridge  60 . That is, the slide member  64  is pressed by the cam follower  170  to be slidingly movable in the axial direction. 
     As illustrated in  FIGS.  4 A and  4 B , the slide member  64  includes a shaft  191 , a first abutment member  192  fixed to one end of the shaft  191 , and a second abutment member  193  fixed to another end of the shaft  191 . 
     The shaft  191  penetrates a hole formed in the casing  63  and extending in the axial direction, and is slidingly movably supported by the casing  63 . 
     The first abutment member  192  has a pressure receiving surface  192 A which is an end face thereof in the axial direction, and an inclined surface  192 B inclined relative to the axial direction. The pressure receiving surface  192 A is configured to be pressed by the cam follower  170 . 
     When the slide member  64  is pressed in the axial direction by the cam follower  170 , the inclined surface  192 B is configured to abut against a corresponding one of counterpart abutment portions  94  of the drawer  90  to urge the developing cartridge  60  ( 60 Y,  60 M,  60 C and  60 K) in a direction parallel to the sheet moving direction, thereby moving the developing cartridge  60 . The inclined surface  182 B is inclined in a direction from the photosensitive drum  50  toward the corresponding developing roller  61  as extending in a direction from the one end to the other end of the shaft  191  in the axial direction. 
     The second abutment member  193  has an inclined surface  193 B similar to the inclined surface  192 B of the first abutment member  192 . The inclined surface  193 B is configured to abut against a corresponding one of the counterpart abutment portions  94  of the drawer  90  when the slide member  64  is pressed in the axial direction by the cam follower  170 , to urge the developing cartridge  60  ( 60 Y,  60 M,  60 C and  60 K) in the sheet moving direction to move the same. 
     A spring  194  is positioned between the first abutment member  192  and the casing  63  to urge the slide member  64  toward the one side (i.e., toward the one end of the shaft  191 ) in the axial direction. The spring  194  is a compression spring disposed over the shaft  191  to allow the shaft  191  to be inserted therethrough. The spring  194  functions to urge the cam follower  170  toward the end cam  152  in the separation state of the developing roller  61  and the photosensitive drum  50 . 
     The counterpart abutment portions  94  are provided on upper portions of each of the side frames  91 R and  91 L of the drawer  90 . The counterpart abutment portions  94  are configured to abut the slide members  64 . Each of the counterpart abutment portions  94  is in a form of a roller rotatable about an axis extending in a third direction (an up-down direction) perpendicular to a first direction in parallel to the axial direction and a second direction in which the photosensitive drums  50  extend, for example. 
     The drawer  90  further includes pressure members  95  provided for each of the developing cartridges  60 . The pressure members  95  are positioned at positions in the vicinity of both end portions in the axial direction of the photosensitive drum  50 , respectively, for each of the developing cartridges  60 . Each of the pressure members  95  is urged by a spring  95 A (see  FIGS.  4 A and  4 B ). As a result of attachment of the developing cartridge  60  to the drawer  90 , the pressure members  95  presses protrusions  63 D (described later) of the developing cartridge  60 , respectively, due to a biasing force of the springs  95 A to allow the developing roller  61  to make contact with the corresponding photosensitive drum  50 . 
     As illustrated in  FIGS.  3 A and  3 B , each of the developing cartridges  60  ( 60 Y,  60 M,  60 C and  60 K) includes the casing  63  configured to accommodate toner therein, the slide member  64 , and a coupling  65 . 
     The casing  63  has one side surface in the axial direction on which a first protruding portion  63 A and a second protruding portion  63 B each protruding in the axial direction. The first protruding portion  63 A is coaxial with the second axis  61 X of the developing roller  61  and extending in the axial direction. The second protruding portion  63 B is positioned away from the first protruding portion  63 A by a prescribed distance. The second protruding portion  63 B is positioned above the first protruding portion  63 A in the present embodiment. 
     The first and second protruding portions  63 A and  63 B are rollers rotatable about axes extending in parallel to the axial direction. Although not illustrated in the drawings, the first and second protruding portions  63 A and  63 B are also provided at another side surface of the casing  63  at positions symmetrical with the first and second protruding portions  63 A and  63 B provided at the one side surface. 
     Further, the above-described protrusion  63 D configured to be pressed by the pressure member  95  is positioned at an upper front surface of the casing  63 . The protrusion  63 D protrudes in the axial direction outward from each side surface of the casing  63  in the axial direction. 
     The coupling  65  is configured to receive a rotational driving force of the developing motor M 2 . The developing roller  61  rotates in accordance with rotation of the coupling  65 . 
     As illustrated in  FIG.  5   , the side frame  91 L of the drawer  90  has an inner surface having first support surfaces  96 A and second support surfaces  96 B. The first support surface  96 A and the second support surface  96 B support the first protruding portion  63 A and the second protruding portion  63 B of the corresponding developing cartridge  60  from below when the developing cartridge  60  ( 60 Y,  60 M,  60 C and  60 K) is moved from the contact position to the separated position. Each of the first support surfaces  96 A and each of the second support surfaces  96 B extend in the sheet moving direction. 
     Each of the first support surfaces  96 A is positioned to support the corresponding first protruding portion  63 A. The first support surface  96 A is configured to guide the developing roller  61  and to fix a position thereof in the up-down direction when the developing cartridge  60  is attached to the drawer  90 . Each of the second support surfaces  96 B is positioned above the first support surface  96 A to support the corresponding second protruding portion  63 B. 
     Although not illustrated in the drawings, the first and second support surfaces  96 A and  96 B are also provided at an inner surface of the other side frame  91 R of the drawer  90  at positions symmetrical with the first and second support surfaces  96 A and  96 B of the left side frame  91 L. 
     Referring to  FIG.  5   , when the developing cartridge  60  is positioned at the contact position where the developing roller  61  is in contact with the corresponding photosensitive drum  50 , the first protruding portion  63 A is positioned closer to the downstream side in the sheet moving direction of the corresponding first support surface  96 A (see the first protruding portions  63 A of the developing cartridges  60 Y,  60 M and  60 C). On the other hand, when the developing cartridge  60  is at the separated position in which the developing roller  61  is separated away from the corresponding photosensitive drum  50 , the first protruding portion  63 A is positioned closer to the upstream side in the sheet moving direction of the corresponding first support surface  96 A (see the first protruding portion  63 A of the fourth developing cartridge  60 K). 
     In this way, the developing rollers  61 Y,  61 M,  61 C and  61 K of the developing cartridges  60 Y,  60 M,  60 C and  60 K move in a direction opposite the sheet moving direction (i.e., from the downstream side toward the upstream side in the sheet moving direction) when the separation mechanisms RK move the respective developing rollers  61 Y,  61 M,  61 C and  61 K from the contact positions to the separated positions. 
     Referring back to  FIG.  1   , the conveying unit  70  is positioned between the sheet tray  21  and the photosensitive drums  50 . The conveying unit  70  includes a drive roller  71 , a driven roller  72 , an endless belt as a conveyer belt  73 , and four transfer rollers  74 . The conveyer belt  73  is looped over the drive roller  71  and the driven roller  72  with taut, and has an outer peripheral surface facing each of the photosensitive drums  50 . Each of the transfer rollers  74  is positioned within a loop of the conveyer belt  73  to nip the conveyer belt  73  in cooperation with a corresponding one of the photosensitive drums  50 . 
     The conveying unit  70  conveys the sheet S as the conveyer belt  73  moves while the sheet S is mounted on an upper portion of the outer peripheral surface of the conveyer belt  73 , and at the same time, toner images formed on the respective photosensitive drums  50  are transferred onto the sheet S. 
     The fixing unit  80  is positioned at a downstream side of the photosensitive drums  50  and the conveying unit  70  in the moving direction of the sheet S. The fixing unit  80  includes a heat roller  81  and a pressure roller  82  positioned in facing relation to the heat roller  81 . A pair of conveyer rollers  15  are positioned above the fixing unit  80 , and a pair of discharge rollers  16  are positioned above the conveyer rollers  15 . 
     In the image forming unit  30  as configured above, the surface of each of the photosensitive drums  50  is uniformly charged by the corresponding charger  52 , and then, the surface is exposed to light with laser beam irradiated from the exposure unit  40 . Hence, an electrostatic latent image on a basis of image data is formed on the surface of each of the photosensitive drums  50 . 
     Further, toner accommodated in each of the casings  63  is carried on the surface of the corresponding developing roller  61 , and the toner is supplied from the developing roller  61  to the electrostatic latent image formed on the surface of the corresponding photosensitive drum  50  when the developing roller  61  makes contact with the photosensitive drum  50 . Hence, toner image corresponding to the electrostatic latent image is formed on the surface of each photosensitive drum  50 . 
     Then, the toner image formed on each of the photosensitive drums  50  is transferred onto the sheet S when the sheet S supplied on the conveyer belt  73  moves past a portion between the photosensitive drums  50  and the corresponding transfer rollers  74 . Then, the toner image transferred onto the sheet S is thermally fixed to the sheet S as the sheet S moves past a portion between the heat roller  81  and the pressure roller  82 . 
     The sheet S discharged from the fixing unit  80  is discharged by the conveyer rollers  15  and the discharge rollers  16  onto a discharge tray  13  positioned at an upper surface of the main casing  10 . 
     As illustrated in  FIG.  6   , the controller  100  includes a CPU  101 , a RAM  102 , a ROM  103 , an EEPROM  104 , and an input/output circuit. The controller  100  is configured to perform arithmetic processing based on information about the developing cartridge  60  attached to the main casing  10 , programs and data those stored in the RAM  102  and the ROM  103  to execute printing control. Incidentally, the RAM  102  and the EEPROM  104  are examples of the main memory  110 . Further, the RAM  102  is an example of a volatile memory, and the EEPROM  104  is an example of a nonvolatile memory. The CPU  101  is electrically connected to the RAM  102 , the ROM  103 , and the EEPROM  104 . 
     The controller  100  is electrically connected to the temperature sensor TS, the drum memory  98 , the main motor M 1 , and the developing motor M 2 . The main motor M 1  is configured to drive the photosensitive drums  50  through a gear train(s) (not illustrated). The developing motor M 2  is configured to drive the developing rollers  61  and the separation mechanisms RK through a gear train(s) and a clutch(s) those not illustrated. Note that transmission of electrical signal is indicated by a solid line, and transmission of a driving force is indicated by a broken line in  FIG.  6   . 
     The controller  100  is configured to acquire the temperature detected by the temperature sensor TS. Further, the controller  100  is configured to read data from the drum memory  98  and to write data into the drum memory  98 . 
     Also, the controller  100  is configured to count the number of rotations of the main motor M 1 , thereby calculating the number of rotations of the photosensitive drums  50  based on the counted number of rotations of the main motor M 1  and a gear ratio. The gear ratio is a ratio of the number of gear teeth of an output gear of the main motor M 1  to the number of gear teeth of an input gear of each photosensitive drum  50 , and stored in, for example, the EEPROM  104 . 
     Moreover, the controller  100  is configured to count the number of rotations of the developing motor M 2 . Accordingly, the controller  100  is configured to calculate the number of rotations the developing rollers  61  based on the counted number of rotations of the developing motor M 2  and a gear ratio. Note that the gear ratio is a ratio of the number of gear teeth of an output gear of the developing motor M 2  to the number of gear teeth of an input gear of each developing roller  61 , and stored in, for example, the EEPROM  104 . 
     The controller  100  is configured to count a first rotation number of the photosensitive drum  50 . The first rotation number is the number of rotations of the photosensitive drum  50  when the photosensitive drum  50  and the developing roller  61  is in the contact state. Further, the controller  100  is configured to count a second rotation number of the photosensitive drum  50 . The second rotation number is the number of rotations of the photosensitive drum  50  when the photosensitive drum  50  in the separation state. 
     Based on the first rotation number and the second rotation number, the controller  100  determines a deterioration quantity W of the photosensitive drum  50  indicative of a quantity by which the photosensitive drum  50  is deteriorated due to rotation of the photosensitive drum  50 . Further, the controller  100  is configured to calculate the lifetime of the photosensitive drum  50  based on the deterioration quantity W of the photosensitive drum  50 . In the following description, how the deterioration quantity W of the photosensitive drum  50 , the lifetime of the photosensitive drum  50 , and the remaining lifetime of the photosensitive drum  50  are calculated in the image forming apparatus  1  according to the first embodiment will be described in detail. 
     The controller  100  is configured to count the number of rotations of the main motor M 1  for a period of time from the main motor M 1  is turned ON until the main motor M 1  is turned OFF. The number of rotations of the main motor M 1  counted by the controller  100  is sequentially written into the RAM  102 . 
     The controller  100  is configured to calculate the number of rotations of the photosensitive drum  50  based on the counted number of rotations of the main motor M 1 . The number of rotations of the photosensitive drum  50  calculated by the controller  100  is also sequentially written into the RAM  102 . 
     The controller  100  is configured to determine whether the state between the photosensitive drum  50  and the developing roller  61  is the contact state or the separation state when the main motor M 1  is turned ON to separately count the first rotation number x m  and the second rotation number y n  within a prescribed period of time. The first rotation number x m  and the second rotation number y n  are sequentially written into the RAM  102 . 
     The controller  100  stores the counted first rotation number and the counted second rotation number into the drum memory  98 . As illustrated in  FIG.  7   , the drum memory  98  includes a first storage area  98 A and a second storage area  98 B. 
     The first storage area  98 A is configured to store therein the first rotation number. In the present embodiment, the first storage area  98 A stores therein a first total rotation number X as the first rotation number. The first total rotation number X is the accumulated number of rotations (x 1 +x 2 +x 3 + . . . +x m ) of the photosensitive drum  50  during the contact state since the photosensitive drum  50  is new. That is, the first total rotation number X is overwritten and stored in the first storage area  98 A each time the photosensitive drum  50  in the contact state makes rotation. 
     The second storage area  98 B is configured to store therein the second rotation number. In the present embodiment, the second storage area  98 B stores therein a second total rotation number Y as the second rotation number. The second total rotation number Y is the accumulated number of rotations (y 1 +y 2 +y 3 + . . . +y n ) of the photosensitive drum  50  during the separation state since the photosensitive drum  50  is a new one. That is, the second total rotation number Y is overwritten and stored in the second storage area  98 B each time the photosensitive drum  50  in the separation state rotates. 
     In order to calculate the deterioration quantity W of the photosensitive drum  50 , the controller  100  reads the first total rotation number X and the second total rotation number Y from the drum memory  98 . Then, the controller  100  calculates the deterioration quantity W of the photosensitive drum  50  based on the first total rotation number X and the second total rotation number Y those stored in the drum memory  98 . 
     Specifically, the controller  100  adds a number obtained by multiplying the first total rotation number X by a first coefficient a to a number obtained by multiplying the second total rotation number Y by a second coefficient b to calculate the deterioration quantity W of the photosensitive drum  50 . That is, W=aX+bY. The second coefficient b is smaller than the first coefficient a. Note that the first coefficient a and the second coefficient b are positive values obtained by experimental data prior to shipment of the image forming apparatus  1 . The first coefficient a and the second coefficient b are stored in advance in the drum memory  98  or the main memory  110  (for example, the EEPROM  104 ). 
     When the deterioration quantity W of the photosensitive drum  50  reaches a threshold value, the controller  100  determines that the photosensitive drum  50  reaches the end of service life. Note that the threshold value for the determination of the service life is stored in advance in the drum memory  98  or the main memory  110  (for example, the EEPROM  104 ). 
     The controller  100  calculates the remaining service life of the photosensitive drum  50  by subtracting the deterioration quantity W of the photosensitive drum  50  from a value indicative of an entire service life (life span) of the photosensitive drum  50 . The calculated remaining service life of the photosensitive drum  50  is displayed, for example, on a display (not illustrated) of the image forming apparatus  1 . 
     Next, one example of processes performed by the controller  100  will be described with reference to a flowchart illustrated in  FIG.  8   . The controller  100  repeatedly performs the processes in  FIG.  8    with respect to each of the four photosensitive drums  50  as long as the image forming apparatus  1  is powered ON. 
     As illustrated in  FIG.  8   , in  51  the controller  100  determines whether the main motor M 1  is turned ON. The controller  100  waits until the main motor M 1  is turned ON when the controller  100  determines that the main motor M 1  is not turned ON (S 1 : NO). 
     When the controller  100  determines that the main motor M 1  is turned ON (S 1 : YES), in S 2  the controller  100  determines whether the photosensitive drum  50  and the developing roller  61  are in the contact state. 
     When the controller  100  determines in S 2  that the photosensitive drum  50  and the developing roller  61  are in the contact state (S 2 : YES), in S 3  the controller  100  counts the first rotation number x m  of the photosensitive drum  50  for a prescribed period of time. The first rotation number x m  is sequentially written into the RAM  102 . Note that the prescribe period of time may be a certain period of time, or may be a period for performing print job once, or may be a period of time for rotating the photosensitive drum  50  by the prescribed number of rotations. 
     After performing the process of S 3 , in S 4  the controller  100  updates the first total rotation number X by adding the first rotation number x m  counted in the prescribed period of time to the first total rotation number X stored in the first storage area  98 A of the drum memory  98 . 
     Subsequently, in S 5  the controller  100  determines whether the state between the photosensitive drum  50  and the developing roller  61  (i.e., the contact state or the separation state) is changed from a state before the prescribed period of time elapses. 
     When the controller  100  determines in S 5  that the state between the photosensitive drum  50  and the developing roller  61  is changed (S 5 : YES), the controller  100  shifts to the process in S 2 . On the other hand, when the controller  100  determines in S 5  that the state between the photosensitive drum  50  and the developing roller  61  remains unchanged (S 5 : NO), in S 6  the controller  100  determines whether to turn OFF the main motor M 1 . 
     When the controller  100  determines in S 6  not to turn OFF the main motor M 1  (S 6 : NO), the controller  100  shifts to the process in S 3 . On the other hand, when the controller  100  determines in S 6  to turn OFF the main motor (S 6 : YES), the controller  100  ends the process in  FIG.  8   . 
     When the controller  100  does not determine in S 2  that the photosensitive drum  50  and the developing roller  61  are in the contact state (S 2 : NO), i.e., the photosensitive drum  50  and the developing roller  61  are in the separation state, in S 7  the controller  100  counts the second rotation number y n  of the photosensitive drum  50  for the prescribed period of time. 
     Subsequently, in S 8  the controller  100  adds the second rotation number y n  counted during the prescribed period of time in S 7  to the second total rotation number Y stored in the second storage area  98 B of the drum memory  98  to update the second total rotation number Y. 
     After performing the process of S 8 , in S 9  the controller  100  determines whether the state of the photosensitive drum  50  and the developing roller  61  is changed from a state before the prescribed period of time elapses. 
     When the controller  100  determines in S 9  that the state of the photosensitive drum  50  and the developing roller  61  is changed (i.e., the photosensitive drum  50  and the developing roller  61  is changed from the separation state to the contact state) (S 9 : YES), the controller  100  shifts to S 2 . On the other hand, when the controller  100  determines in S 9  that the state of the photosensitive drum  50  and the developing roller  61  is unchanged (S 9 : NO), in S 10  the controller  100  determines whether the main motor M 1  needs to be turned OFF. 
     When the controller  100  determines in S 10  that ON state of the main motor M 1  should be maintained (S 10 : NO), the controller  100  shifts to S 7 . On the other hand, when the controller  100  determines in S 10  that the main motor M 1  needs to be turned OFF (S 10 : YES), the controller  100  ends the process in  FIG.  8   . 
     Next, a lifetime determination process performed by the controller  100  in the image forming apparatus  1  according to the first embodiment will be described with reference to a flowchart illustrated in  FIG.  9   . The controller  100  repeatedly executes the lifetime determination process with respect to each of the four photosensitive drums  50  while the image forming apparatus  1  is powered ON. 
     As illustrated in  FIG.  9   , when performing the lifetime determination process, in S 11  the controller  100  reads the first total rotation number X and the second total rotation number Y from the drum memory  98  and stores the first total rotation number X and the second total rotation number Y into the RAM  102 . 
     After the process of S 11 , in S 12  the controller  100  calculates the deterioration quantity W of the photosensitive drum  50 . Specifically, the deterioration quantity W is the sum of: the number obtained by multiplying the first total rotation number X read from the RAM  102  by the first coefficient a; and the number obtained by multiplying the second total rotation number Y read from the RAM  102  by the second coefficient b (W=aX+bY). 
     After performing the process of S 12 , in S 13  the controller  100  determines whether the calculated deterioration quantity W is greater than or equal to the threshold value. 
     When the controller determines in S 13  that the calculated deterioration quantity W is greater than or equal to the threshold value (S 13 : YES), in S 14  the controller  100  determines that the photosensitive drum  50  reaches the end of service life, and then ends the lifetime determination process. On the other hand, when the controller  100  does not determine in S 13  that the calculated deterioration quantity W is greater than or equal to the threshold value, i.e., determines that the calculated deterioration quantity W is less than the threshold value (S 13 : NO), the controller  100  ends the lifetime determination process without determining that the photosensitive drum  50  reaches the end of service life. 
     According to the first embodiment described above, the first rotation number is stored in the first storage area  98 A and the second rotation number is stored in the second storage area  98 B of the drum memory  98 . Therefore, the number of rotations of the photosensitive drum  50  during the contact state and the number of rotations of the photosensitive drum  50  during the separation state can be separately stored in the drum memory  98 . 
     Hence, with respect to this drum cartridge (drawer  90 ), the deterioration quantity W of the photosensitive drum  50  can be accurately calculated by using the first rotation number and the second rotation number separately stored in the drum memory  98 . Accordingly, the controller  100  in the image forming apparatus  1  employing the above drum cartridge (drawer  90 ) can perform calculation of the deterioration quantity W of the photosensitive drum  50  based on the first total rotation number X and the second total rotation number Y. 
       FIG.  10    is a graph showing the relationship between the total numbers of rotations of the photosensitive drum  50  and the deterioration quantity W of the photosensitive drum  50  calculated by the controller  100 . The calculation obtained in the first embodiment is indicated by a solid line, and the calculation obtained in the conventional image forming apparatus is indicated by a broken line in  FIG.  10   . 
     According to the conventional image forming apparatus, no consideration was made with respect to the deterioration quantity in the separation state where the developing roller  61  is separated away from the photosensitive drum  50 , and the deterioration quantity in the contact state where the developing roller  61  is in contact with the photosensitive drum  50  was only counted. 
     In contrast, according to the first embodiment, the first rotation number x m  counted in the contact state and the second rotation number y n  counted in the separation state are distinguished when the deterioration quantity W is calculated. Specifically, the second rotation number y n  counted in the separation state is multiplied by the second coefficient b which is smaller than the first coefficient a. Hence, the deterioration quantity W of the photosensitive drum  50  can be calculated with high accuracy. 
     Second Embodiment 
     Next, an image forming apparatus according to a second embodiment of the present disclosure will be described with reference to  FIGS.  11  through  14   . 
     According to the first embodiment described above, the deterioration quantity W of the photosensitive drum  50  is calculated while taking the state of the photosensitive drum  50  and the developing roller  61  (the contact state or the separation state) into separate consideration. 
     The second embodiment is different from the first embodiment in that, the deterioration quantity W of the photosensitive drum  50  is calculated using a coefficient corresponding to the temperature of the photosensitive drum  50  while rotating in addition to consideration of the state (the contact state or the separation state) of the photosensitive drum  50  and the developing roller  61 . That is, according to the second embodiment, the deterioration quantity W is calculated using a first coefficient a m  and a second coefficient b n  which are changeable depending on the temperature of the photosensitive drum  50  while rotating. 
     The controller  100  is configured to determine the first coefficient a m  and the second coefficient b n  on the basis of the temperature of the photosensitive drum  50  that is acquired from the temperature sensor TS. The first coefficient a m  and the second coefficient b n  are variable values in accordance with the temperature of the photosensitive drum  50 . Depending on the materials of the photosensitive drum  50  and the cleaning roller  53 , the first coefficient a m  and the second coefficient b n  may become smaller or may become larger as the temperature of the photosensitive drum  50  acquired from the temperature sensor TS becomes higher. 
     The following description describes a case where the first coefficient a m  and the second coefficient b n  are determined to smaller values as the temperature of the photosensitive drum  50  acquired from the temperature sensor TS becomes higher. That is, the controller  100  determines the first coefficient a m  and the second coefficient b n  as being smaller values as the temperature of the photosensitive drum  50  acquired from the temperature sensor TS is higher. In other words, the first coefficient a m  and the second coefficient b n  are values determined as smaller values as the temperature of the rotating photosensitive drum  50  is higher in this case. 
     It is preferable that a map illustrated in  FIG.  11    is used to determine the first coefficient a m  and the second coefficient b n . The map indicates coefficients according to the temperature of the photosensitive drum  50  and the state of the photosensitive drum  50  (the contact state or the separation state). 
     As illustrated in  FIG.  11   , in the contact state of the photosensitive drum  50 , a H  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is a high temperature, a M  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is a medium temperature, and a L  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is a low temperature. 
     Similarly, in the separation state of the photosensitive drum  50 , b H  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is a high temperature, b M  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is a medium temperature, and b L  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is a low temperature. 
     As one example, the high temperature denotes a temperature higher than or equal to 30° C., the medium temperature denotes a temperature higher than or equal to 10° C. and lower than 30° C., and the low temperature denotes a temperature lower than 10° C. Further, the coefficients in the map of  FIG.  11    satisfy the following inequality relationship: a H &lt;a M &lt;a L , b H &lt;b M &lt;b L , a H &gt;b H , a M &gt;b M , and a L &gt;b L . 
     As illustrated in  FIG.  12   , the controller  100  is configured to store the first rotation number x m  during the prescribed period of time and the first coefficient a m  corresponding to the first rotation number x m  into the first storage area  98 A of the drum memory  98 . In other words, the first storage area  98 A is configured to store therein the first coefficient a m  corresponding to the first rotation number x m  in addition to the first rotation number x m . 
     Similarly, the controller  100  is configured to store the second rotation number y n  and the second coefficient b n  corresponding to the second rotation number y n  into the second storage area  98 B of the drum memory  98 . In other words, the second storage area  98 B is configured to store therein the second coefficient b n  corresponding to the second rotation number y n  in addition to the second rotation number y n . 
     The controller  100  reads and store all of the first rotation number x m , the first coefficient a m , the second rotation number y n , and the second coefficient b n  into the main memory  110  (for example, the RAM  102 ). Thereafter, the controller  100  calculates the deterioration quantity W of the photosensitive drum  50  by adding: an accumulation of a value obtained by multiplying the first rotation number x m  by the first coefficient a m  to an accumulation of a value obtained by multiplying the second rotation number y n  by the second coefficient b n (W=Σa m x m+ Σb n y n ). 
     In other words, the deterioration quantity W of the photosensitive drum  50  which is deteriorated due to the rotation of the photosensitive drum  50  is determined by addition of the accumulation of the value obtained by multiplying the first rotation number x m  by the first coefficient a m  to the accumulation of the values obtained by multiplying the second rotation number y n  by the second coefficient b n  smaller than first coefficient a m . 
     Next, one example of processes performed by the controller  100  in the image forming apparatus according to the second embodiment will be described with reference to a flowchart illustrated in  FIG.  13   . 
     As illustrated in  FIG.  13   , in S 21  the controller  100  determines whether the main motor M 1  is turned ON. The controller  100  waits until the main motor M 1  is turned ON when the controller  100  determines that the main motor M 1  is not turned ON (S 21 : NO). 
     When the controller  100  determines in S 21  that the main motor M 1  is turned ON (S 21 : YES), in S 22  the controller  100  determines the first coefficient a m  or the second coefficient b n  based on the state (the contact state or the separation state) of the photosensitive drum  50  and the acquired temperature of the photosensitive drum  50 . 
     After performing the process of S 22 , in S 23  the controller  100  counts the number of rotations of the photosensitive drum  50  for the prescribed period of time. The number of rotations of the photosensitive drum  50  is the first rotation number x m  or the second rotation number y n  depending on the state of the photosensitive drum  50  (the contact state or the separation state). 
     After performing the process of S 23 , in S 24  the controller  100  stores the first rotation number x m  and the determined first coefficient a m , or stores the second rotation number y n  and the determined second coefficient b n  into the drum memory  98 . 
     Thereafter, in S 25  the controller  100  determines whether at least one of the state of the photosensitive drum  50  (the contact state or the separation state) and acquired temperature of the photosensitive drum  50  changed from a state before the prescribed period of time elapses. Incidentally, “the acquired temperature of the photosensitive drum  50  changed” denotes that the acquired temperature changed from one of the high temperature, the medium temperature, and the low temperature (see  FIG.  11   ) to another of the high temperature, the medium temperature, and the low temperature. That is, in the process of S 25 , the controller  100  determines whether there is necessity of changing the coefficient. 
     When the controller  100  determines in S 25  that there are changes in at least one of the state of the photosensitive drum  50  and the acquired temperature of the photosensitive drum  50  (S 25 : YES), the controller  100  shifts to the process of S 22 . On the other hand, when the controller  100  determines in S 25  that there are no changes in the state of the photosensitive drum  50  and the acquired temperature of the photosensitive drum  50  (S 25 : NO), in S 26  the controller  100  determines whether to turn OFF the main motor M 1 . 
     When the controller  100  determines not to turn OFF the main motor M 1  (S 26 : NO), the controller  100  shifts to the process of S 23 . On the other hand, when the controller  100  determines in S 26  to turn OFF the main motor M 1  (S 26 : YES), the process is terminated. 
     Next, one example of a lifetime determination process performed by the controller  100  in the image forming apparatus according to the second embodiment will be described with reference to a flowchart illustrated in  FIG.  14   . 
     As illustrated in  FIG.  14   , for performing the lifetime determination process, in S 31  the controller  100  reads the first rotation number x m , the first coefficient a m , the second rotation number y n , and the second coefficient b n  from the drum memory  98 . 
     After the process of S 31 , in S 32  the controller  100  calculates the deterioration quantity W of the photosensitive drum  50 . Specifically, the deterioration quantity W of the photosensitive drum  50  is calculated by adding the accumulation of the value obtained by multiplying the first rotation number x m  by the first coefficient a m  to the accumulation of the value obtained by multiplying the second rotation number y n  by the second coefficient b n (W=Σa m x m+ Σb n y n ). 
     After performing the process of S 32 , in S 33  the controller  100  determines whether the calculated deterioration quantity W is greater than or equal to the threshold value. 
     When the controller  100  determines in S 33  that the calculated deterioration quantity W is greater than or equal to the threshold value (S 33 : YES), in S 34  the controller  100  determines that the photosensitive drum  50  reaches the end of service life, and ends the life determination process. On the other hand, when the controller  100  determines in S 33  that the calculated deterioration quantity W is less than the threshold value (S 33 : NO), the controller  100  ends the lifetime determination process without determining that the photosensitive drum  50  reaches the end of service life. 
     According to the second embodiment, the deterioration quantity W of the photosensitive drum  50  is calculated based not only on the state of the photosensitive drum  50  (the contact state or the separation state) but also on the first coefficient a m  corresponding to the first rotation number x m  and the second coefficient b n  corresponding to the second rotation number y n . Therefore, the deterioration quantity W of the photosensitive drum  50  while rotating can be calculated in accordance with the state of the photosensitive drum  50 . 
     Note that the deterioration quantity W of the photosensitive drum  50  due to its rotation varies depending on the temperature of the photosensitive drum  50 . Specifically, in a case where the photosensitive drum  50  is more likely to be scraped as the temperature of the photosensitive drum  50  is lower, the deterioration quantity W of the photosensitive drum  50  due to its rotation becomes smaller as the temperature of the photosensitive drum  50  becomes higher. 
     On the other hand, in a case where the photosensitive drum  50  is more likely to be scraped as the temperature of the photosensitive drum  50  is higher, the deterioration quantity W of the photosensitive drum  50  due to its rotation becomes larger as the temperature of the photosensitive drum  50  becomes higher. 
     Here, the relationship between the temperature of the photosensitive drum  50  and the likelihood of scrape of the photosensitive drum  50  varies depending on materials of the photosensitive drum  50  and the cleaning roller  53 . To this effect, according to the second embodiment, the deterioration quantity W is calculated on the basis of the state of the photosensitive drum  50  as well as the temperature of the photosensitive drum  50  during the rotation. Accordingly, the deterioration quantity W of the photosensitive drum  50  can be calculated accurately. 
     Third Embodiment 
     An image forming apparatus according to a third embodiment of the present disclosure will next be described with reference to  FIGS.  12  and  14  through  16   . 
     According to the second embodiment, the deterioration quantity W of the photosensitive drum  50  is calculated using the coefficient corresponding to the temperature of the photosensitive drum  50  in addition to consideration of the state of the photosensitive drum  50 . According to the third embodiment, the deterioration quantity W of the photosensitive drum  50  is calculated using a coefficient determined depending on a total rotation number Z of the photosensitive drum  50  in addition to the state of the photosensitive drum  50  and the temperature of the photosensitive drum  50 . 
     The total rotation number Z of the photosensitive drum  50  is a sum of the first total rotation number X and the second total rotation number Y (Z=X+Y). In a case where the total rotation number Z is zero (Z=0), the photosensitive drum  50  is a new product. The photosensitive drum  50  approaches the end of the service life as the total rotation number Z increases. 
     Specifically, the controller  100  determines the first coefficient a m  and the second coefficient b n  so that these first coefficient a m  and second coefficient b n  become larger values as the total rotation number Z of the photosensitive drum  50  from the new state increases. In other words, the first coefficient a m  and the second coefficient b n  are values determined to become larger values as the total rotation number Z of the photosensitive drum  50  from the new state increases. 
     It is preferable that a map illustrated in  FIG.  15    is used to determine the first coefficient a m  and the second coefficient b n , for example. Specifically, as illustrated in  FIG.  15   , in a state where the photosensitive drum  50  and the developing roller  61  are in the contact state and the total rotation number Z is low: a HS  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is the high temperature; a MS  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is the medium temperature; and a LS  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is the low temperature. 
     Further, in a state where the photosensitive drum  50  and the developing roller  61  are in the contact state and the total rotation number Z is medium: a HF  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is the high temperature, a MF  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is the medium temperature; and a LF  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is the low temperature. 
     Further, in a state where the photosensitive drum  50  and the developing roller  61  are in the contact state and the total rotation number Z is high: a HO  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is the high temperature; a MO  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is the medium temperature; and a LO  is determined as the first coefficient a m  when the acquired temperature of the photosensitive drum  50  is the low temperature. 
     Similarly, in a state where the photosensitive drum  50  and the developing roller  61  are in the separation state and the total rotation number Z is low: b HS  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is the high temperature, b MS  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is the medium temperature; and b LS  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is the low temperature. 
     Further, in a state where the photosensitive drum  50  and the developing roller  61  are in the separation state and the total rotation number Z is medium: b HF  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is the high temperature; b MF  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is the medium temperature; and b LF  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is the low temperature. 
     Further, in a state where the photosensitive drum  50  and the developing roller  61  are in the separation state and the total rotation number Z is high: b HO  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is the high temperature; b MO  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is the medium temperature; and b LO  is determined as the second coefficient b n  when the acquired temperature of the photosensitive drum  50  is the low temperature. 
     In one example, “the total rotation number Z is low” denotes that the total rotation number Z is in a range of from the photosensitive drum  50  is a new product (i.e., the number of rotations is 0 (zero)) to the rotation number of less than 10,000, “the total rotation number Z is middle” denotes that the total rotation number Z is in a range within greater than or equal to 10,000 and less than 20,000, and “the total rotation number Z is high” denotes that the total rotation number Z is not less than 20,000. Further, these coefficients in the map of  FIG.  15    satisfy the following relationship: a HS &lt;a MS &lt;a LS , a HF &lt;a MF &lt;a LF , a HO &lt;a MO &lt;a LO , a HS &lt;a HF &lt;a HO , a MS &lt;a MF &lt;a MO , a LS &lt;a LF &lt;a LO , b HS &lt;b MS &lt;b LS , b HF &lt;b MF &lt;b LF , b HO &lt;b MO &lt;b LO , b HS &lt;b HF &lt;b HO , b MS &lt;b MF &lt;b MO , and b LS &lt;b LF &lt;b LO . 
     Next, one example of processes performed by the controller  100  in the image forming apparatus according to the third embodiment will be described with reference to a flowchart illustrated in  FIG.  16   . 
     As illustrated in  FIG.  16   , in S 41  the controller  100  determines whether the main motor M 1  is turned ON. The controller  100  waits until the main motor M 1  is turned ON when the controller  100  determines that the main motor M 1  is not turned ON (S 41 : NO). 
     When the controller  100  determines in S 41  that the main motor M 1  is turned ON (S 41 : YES), in S 42  the controller  100  determines the first coefficient a m  or the second coefficient b n  in accordance with the state of the photosensitive drum  50  and the developing roller  61  (the contact state or the separation state), the acquired temperature of the photosensitive drum  50 , and the total rotation number Z of the photosensitive drum  50  referring to the map of  FIG.  15   . 
     After performing the process of S 42 , in S 43  the controller  100  counts the rotation number of the photosensitive drum  50  for the prescribed period of time. The rotation number is the first rotation number x m  or the second rotation number y n  depending on state of the photosensitive drum  50  and the developing roller  61  (the contact state or the separation state). 
     After the process of S 43 , in S 44  the controller  100  stores the first rotation number x m  and the determined first coefficient a m  or stores the second rotation number y n  and the determined second coefficient b n  into the drum memory  98 . 
     After performing the process of S 44 , in S 45  the controller  100  determines whether at least one of the state of the photosensitive drum  50  and the acquired temperature of the photosensitive drum  50  changes from a state before the prescribed period of time elapses. Incidentally, “the acquired temperature of the photosensitive drum  50  changed” denotes that the acquired temperature changed from one of the high temperature, the medium temperature, and the low temperature (see  FIG.  11   ) to another of the high temperature, the medium temperature, and the low temperature. That is, in the process of S 45 , the controller  100  determines whether there is necessity of changing the coefficient. 
     When the controller  100  determines in S 45  that at least one of the state of the photosensitive drum  50  and the acquired temperature of the photosensitive drum  50  changes (S 45 : YES), the controller  100  shifts to the process of S 42 . 
     On the other hand, when the controller  100  determines that there are no changes in the state of the photosensitive drum  50  and the acquired temperature of the photosensitive drum  50  (S 45 : NO), in S 46  the controller  100  determines whether the total rotation number Z exceeds a prescribed value. When the total rotation number Z becomes from the low to the medium, or when the total rotation number Z becomes from the medium to the high (see  FIG.  15   ), the controller  100  determines that the total rotation number Z exceeds the prescribed value. That is, in the process of S 46 , the controller  100  also determines whether there is necessity of changing the coefficient. 
     When the controller  100  determines in S 46  that the total rotation number Z exceeds the prescribed value (S 46 : YES), the controller  100  shifts to the process in S 42 . On the other hand, when the controller  100  determines in S 46  that the total rotation number Z does not exceed the prescribed value (S 46 : NO), in S 47  the controller  100  determines whether the main motor M 1  needs to be turned OFF. 
     When the controller  100  determines in S 47  not to turn OFF the main motor M 1  (S 47 : NO), the controller  100  shifts to the process of S 43 . On the other hand, when the controller  100  determines in S 47  to turn OFF the main motor M 1  (S 47 : YES), the controller  100  ends the process. 
     Note that a lifetime determination process performed by the controller  100  in the image forming apparatus according to the third embodiment is the same as that performed in the second embodiment (see  FIG.  14   ). 
     According to the third embodiment, the controller  100  calculates the deterioration quantity W of the photosensitive drum  50  based on the total rotation number Z of the photosensitive drum  50  as well as the state of the photosensitive drum  50  and the developing roller  61  (the contact state or the separation state) and the temperature of the photosensitive drum  50 , thereby realizing accurate calculation of the deterioration quantity of the photosensitive drum  50 . 
     Fourth Embodiment 
     Next, an image forming apparatus according to a fourth embodiment of the present disclosure will next be described with reference to  FIGS.  17  and  18   . 
     According to the first embodiment, the controller  100  stores the first total rotation number X and the second total rotation number Y of the photosensitive drum  50  into the drum memory  98 . The fourth embodiment is different from the first embodiment in that the controller  100  stores the deterioration quantity W of the photosensitive drum  50  into the drum memory  98 . 
     Specifically, the controller  100  is configured to store the deterioration quantity W of the photosensitive drum  50  into the drum memory  98 . When the photosensitive drum  50  is a new product, the deterioration quantity W is zero (W=0). 
     When the controller  100  causes the photosensitive drum  50  to rotate for a prescribed period of time, the controller  100  counts a rotation number z n  of the photosensitive drum  50  in the prescribed period of time. Further, the controller  100  determines a coefficient an in accordance with the state of the photosensitive drum  50  (the contact state or the separation state), the acquired temperature of the photosensitive drum  50 , and the total rotation number Z of the photosensitive drum  50  at a time of counting the rotation number z n . The coefficient α n  is the first coefficient a m  or the second coefficient b n  depending on the state of the photosensitive drum  50 . The map illustrated in  FIG.  15    is used for determining the coefficient α n  in a manner the same as the third embodiment. Note that coefficient other than that employed in the third embodiment (i.e., the coefficient employed in the first and second embodiments described above) may be used to determine the coefficient α n  in the present embodiment. 
     When the controller  100  controls the photosensitive drum  50  to rotate, the controller  100  stores an updated deterioration quantity W by adding a value obtained by multiplying the rotation number z n  by the coefficient α n  to the current deterioration quantity W. 
     Next, one example of processes performed by the controller  100  in the image forming apparatus according to the fourth embodiment will be described with reference to a flowchart illustrated in  FIG.  17   . 
     As illustrated in  FIG.  17   , in S 51  the controller  100  determines whether the main motor M 1  is turned ON. The controller  100  waits until the main motor M 1  is turned ON when the controller  100  determines in S 51  that the main motor M 1  is not turned ON (S 51 : NO). 
     When the controller  100  determines in S 51  that the main motor M 1  is turned ON (S 51 : YES), in S 52  the controller  100  determines the coefficient α n  in accordance with the state of the photosensitive drum  50  (the contact state or the separation state), the acquired temperature of the photosensitive drum  50 , and the total rotation number Z of the photosensitive drum  50 . 
     After performing the process of S 52 , in S 53  the controller  100  counts the rotation number z n  of the photosensitive drum  50  for the prescribed period of time. 
     After performing the process of S 53 , the controller  100  updates the deterioration quantity W by adding a value obtained by multiplying the rotation number z n  by the coefficient an to the deterioration quantity W of the photosensitive drum  50  already stored in the drum memory  98 , and stores the updated deterioration quantity W into the drum memory  98  (W←W+α n z n ). 
     After performing the process of S 4 , in S 55  the controller  100  determines whether the main motor M 1  should be turned OFF. 
     When the controller  100  does not determine in S 55  that the maim motor M 1  should be turned ON (S 55 : NO), the controller  100  shifts to the process of S 53 . On the other hand, when the controller  100  determines in S 55  that the main motor M 1  should be turned OFF (S 55 : YES), the process is ended. 
     Next, a lifetime determination process performed by the controller  100  in the image forming apparatus according to the fourth embodiment will be described with reference to a flowchart illustrated in  FIG.  18   . 
     As illustrated in  FIG.  18   , for performing the lifetime determination process, in S 61  the controller  100  reads the deterioration quantity W from the drum memory  98 . 
     After performing the process of S 61 , in S 62  the controller  100  determines whether the read deterioration quantity W is greater than or equal to the threshold value. 
     When the controller  100  determines in S 62  that the deterioration quantity W is equal to or greater than the threshold value (S 62 : YES), in S 63  the controller  100  determines that the photosensitive drum  50  reaches the end of service life, and the life determination process is ended. On the other hand, when the controller  100  determines in S 62  that the deterioration quantity W is less than the threshold value (S 62 : NO), the controller  100  determines that the photosensitive drum  50  does not reach the end of service life, and ends the life determination process. 
     According to the fourth embodiment, since the deterioration quantity W of the photosensitive drum  50  calculated based the number of rotations of the photosensitive drum  50  in the contact state (the first rotation number x m ) and the number of rotations of the photosensitive drum  50  in the separation state (the second rotation number y n ) are stored in the drum memory  98 , the deterioration quantity W of the photosensitive drum  50  can be calculated with high accuracy. 
     Further, not only the state of the photosensitive drum  50  and the temperature of the photosensitive drum  50  but also the total rotation number Z of the photosensitive drum  50  are considered for the calculation of the deterioration quantity W by the controller  100 . Therefore, the controller  100  can more accurately calculate the deterioration quantity W of the photosensitive drum  50 . 
     Modifications 
     While the description has been made with reference to the embodiments, it would be apparent to those skilled in the art that the present disclosure need not be limited to the above-described embodiments and various modifications can be made thereto. 
     For example, according to the second through fourth embodiments described above, the map showing the coefficients corresponding to the state of the photosensitive drum  50  is used to determine the first coefficient a m  and the second coefficient b n . However, these coefficients may be determined by a formula instead of the map. 
     Specifically, when the photosensitive drum  50  and the developing roller  61  are in the contact state, the coefficient an may be obtained by adding a value obtained by multiplying a first correction factor c 1  by the total rotation number Z to a first constant a 0 (α n =a 0 +c 1 Z). In the same way, when the photosensitive drum  50  is in the separation state, the coefficient α n  may be obtained by adding a value obtained by multiplying a second correction factor c 2  by the total rotation number Z to a second constant b 0 (α n =b 0 +c 2 Z). The first correction factor c 1  and the second correction factor c 2  are positive values. 
     The controller  100  may also calculate the deterioration quantity W of the photosensitive drum  50  by an accumulation of a value obtained by multiplying the rotation number z n  during the prescribed period of time by the coefficient α n (W=Σα n z n ). 
     Alternatively, the controller  100  may store the deterioration quantity W in the drum memory  98 , and may calculate an updated deterioration quantity W by adding a value obtained by multiplying the rotation number z n  by the coefficient an corresponding to the rotation number z n  to the already stored deterioration quantity W, and may store the updated deterioration quantity W into the drum memory  98  when rotating the photosensitive drum  50 . 
     Further, in the above-described embodiments, the map having three temperature sections including a section for the high temperature, a section for the medium temperature, and a section for the low temperature is used for determining the coefficient. However, a map having two temperature sections or not less than four temperature sections may be employed. 
     Further, in the above-described embodiments, the map having three sections for the total rotation number Z including a section for the low rotation number, a section for the medium rotation number, and a section for the high rotation number is used for determining the coefficient. However, a map having two sections or not less than four sections for the total rotation number Z may be used. 
     Further, in the above-described embodiments, the state of the photosensitive drum  50  and the developing roller  61  (the contact state or the separation state), the temperature of the photosensitive drum  50 , and the total rotation number Z of the photosensitive drum  50  are used for determining the coefficient. However, factors other than the above factors may be employed. 
     For example, in a case where the image forming apparatus  1  includes cleaning rollers each movable between a contact position where the cleaning roller is in contact with a corresponding photosensitive drum  50  and a separated position where the cleaning roller is in separation from the corresponding photosensitive drum  50 , the coefficient may be determined based on the state (the contact state or the separation state) of the cleaning roller. 
     Further, in a case where the image forming apparatus  1  includes charge rollers instead of the chargers  52  and each of the charge roller is movable between a contact position where the charge roller is in contact with a corresponding photosensitive drum  50  and a separated position where the charge roller is in separation from the corresponding photosensitive drum  50 , the coefficient may be determined based on the state (the contact state or the separation state) of the charge roller. 
     Further, in the above-described embodiment, each of the separation mechanisms RK performs the switching of the state of the photosensitive drum  50  and the developing roller  61  between contact state and the separation state by moving the developing roller  61 . However, the separation mechanism RK may perform the switching of the state of the photosensitive drum  50  and the developing roller  61  between contact state and the separation state by moving the photosensitive drum  50  or by moving both the developing roller  61  and the photosensitive drum  50 . 
     Further, according to the above third embodiment, the deterioration quantity W of the photosensitive drum  50  is calculated using the temperature of the photosensitive drum  50  and the coefficient corresponding to the total rotation number Z of the photosensitive drum  50 . However, the deterioration quantity W of the photosensitive drum  50  may be calculated without depending on the temperature of the photosensitive drum  50  but using the coefficient depending on the total rotation number Z of the photosensitive drum  50 . 
     Further, in the above-described embodiments, a motor for driving the photosensitive drums  50  and a motor for driving the developing rollers  61  are separately provided. However, one single motor may be provided to drive both the photosensitive drums  50  and the developing rollers  61 . 
     Further, in the above-described embodiments, the drum cartridge is the drawer  90  configured to be pulled out of the main casing  10 , and includes the four photosensitive drums  50  and the four developing cartridges  60  detachable from and attachable to the drum cartridge. However, other configurations may be available. 
     For example, the drum cartridge may not include the plurality of developing cartridges  60  and the plurality of photosensitive drums  50 , but may include one single developing cartridge and one single photosensitive drum. 
     Further, in the above-described embodiments, the drum cartridge is attachable to and detachable from the main casing in a horizontal direction. However, the drum cartridge may be attachable to and detachable from the main casing  10  from above, or in a diagonal direction. 
     Further, in the above-described embodiments, the drum cartridge allows the developing cartridge including the developing roller to be attachable thereto and detachable therefrom. However, a drum cartridge may be configured to allow a toner cartridge that does not include a developing roller to be attachable thereto and detachable therefrom. In the latter case, the drum cartridge may include the developing roller and the photosensitive drum, and the toner cartridge may not include the developing roller but include a toner accommodating portion for accommodating therein toner. 
     Further, in the above-described embodiments, the developing cartridge  60  is attachable to and detachable from the drum cartridge, and the drum cartridge to which the developing cartridge  60  is attached is attachable to and detachable from the main casing  10 . However, the developing cartridge  60  and the drum cartridge may be attachable to and detachable from the main casing  10  independent from each other. Further, a drum cartridge in which a developing cartridge is integrally formed with the drum cartridge so as not to be detachable from the drum cartridge may be attachable to and detachable from the main casing  10 . In the latter case, the drum cartridge may include a toner accommodating portion for accommodating therein toner, a developing roller, and a photosensitive drum. 
     Further, in the above-described embodiments, the image forming apparatus  1  is a color printer for forming a color image using toners of four colors. However, the image forming apparatus  1  may be a monochromatic printer, or a color printer that forms a color image using toners of three colors or more than five colors. 
     Further, the image forming apparatus  1  may be a multifunction peripheral or a copying machine. 
     Further, components and processes appearing in the embodiments and modifications described above may be suitably selected and combined as long as any conflicting combination is avoided.