Patent Publication Number: US-8112026-B2

Title: Lubricant applying device, and image forming assembly and image forming apparatus which employ the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-166866 filed on Jun. 26, 2008. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a lubricant applying device, and an image forming assembly and image forming apparatus which employ the lubricant applying device. 
     2. Related Art 
     Image forming apparatuses having a mechanism for applying a solid lubricant to an image carrier have been proposed. 
     SUMMARY 
     According to an aspect of the invention, there is provided a lubricant applying device that faces a rotatable body, and that applies a solid lubricant to the rotatable body, the device including: a lubricating roll in which the solid lubricant for application is formed into a circular sectional shape around a rotatable support shaft, and that is contacted with the rotatable body; and a pressing mechanism that presses the lubricating roll toward the rotatable body, wherein the lubricating roll that is pressed against the rotatable body by the pressing mechanism applies the lubricant in a state where the lubricating roll is not rotated relative to the rotatable body, and, when a rotational torque due to the contact with the rotatable body exceeds a predetermined value, the lubricating roll is rotated in accordance with rotation of the rotatable body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1A  is a diagram schematically showing an exemplary embodiment of an image forming apparatus to which the invention is applied, and  FIG. 1B  is a diagram schematically showing a lubricant applying device in FIG.  1 A; 
         FIG. 2A  is a diagram showing an operation example of the lubricant applying device in the case where the rotational torque acting on a lubricating roll is lower than a restraining torque of a rotation control mechanism, and  FIG. 2B  is a diagram showing an operation example of the lubricant applying device in the case where the rotational torque acting on the lubricating roll is equal to or higher than the restraining torque of the rotation control mechanism; 
         FIG. 3  is a diagram showing the whole configuration of an image forming apparatus which is used in Exemplary embodiment 1; 
         FIG. 4  is a diagram showing the configuration of the periphery of an image forming portion which is used in Exemplary embodiment 1; 
         FIG. 5A  is a diagram showing in detail the lubricant applying device shown in  FIG. 4 , and  FIG. 5B  is a perspective view of main portions; 
         FIG. 6A  is a diagram showing an example of a torque limiter which is used in the exemplary embodiment, and  FIG. 6B  is a view looking in the direction of the arrow B in  FIG. 6A ; 
         FIG. 7  is a view showing an example of a characteristic model diagram of the torque limiter which is used in Exemplary embodiment 1; 
         FIG. 8  is a diagram showing an initial state of the lubricant applying device, and a temporal change state of the lubricant applying device; 
         FIG. 9  is a diagram showing a technique of setting the restraining torque of the torque limiter which is used in Exemplary embodiment 1; 
         FIG. 10A  is a diagram showing an operation example of the lubricant applying device which is used in Exemplary embodiment 1, in the case where the rotational torque acting on the lubricating roll is lower than the restraining torque of the torque limiter, and  FIG. 10B  is a diagram showing an operation example of the lubricant applying device which is used in Exemplary embodiment 1, in the case where the rotational torque acting on the lubricating roll is equal to or higher than the restraining torque of the torque limiter; 
         FIG. 11A  is a diagram showing an operation in a transferring portion caused by a lubricant layer formed by the lubricant applying device which is used in Exemplary embodiment 1, and  FIG. 11B  is a diagram showing an operation in the transferring portion in the case where the lubricant layer is not formed by the lubricant applying device; 
         FIG. 12  is a diagram showing a modification of the lubricant applying device which is used in Exemplary embodiment 1; 
         FIG. 13A  is a diagram schematically showing a lubricant applying device which is used in an image forming apparatus of Exemplary embodiment 2, and  FIG. 13B  is a diagram showing an example of a lock position changing mechanism; 
         FIG. 14  is a diagram showing an operation process of the lubricant applying device which is used in Exemplary embodiment 2; 
         FIG. 15  is a diagram schematically showing a lubricant applying device which is used in an image forming apparatus of Exemplary embodiment 3; 
         FIG. 16  is a diagram showing use conditions for setting the lubricating roll which is used in Example 1, to a resting state; and 
         FIG. 17  is a diagram showing rotational torques in a normal state and Abnormal states 1 to 3 which act on a lubricating roll under conditions used in Example 2. 
     
    
    
     DETAILED DESCRIPTION 
     Summary of Exemplary Embodiments 
     First, a summary of exemplary embodiments of an image forming apparatus, which is an example of the present invention, will be described with reference to  FIGS. 1A ,  1 B,  2 A, and  2 B. 
     An image carrier is exemplified in the following embodiments as an object of application of lubricant, but the object may not be limited thereto. Any rotatable body may be the object of application of lubricant. 
     Referring to  FIG. 1A , the image forming apparatus includes an image carrier  1  which can hold a toner image, and a lubricant applying device  2  which faces the image carrier  1 , and which applies a solid lubricant to the image carrier  1 . 
     In the technical means, any one of a photosensitive member, a dielectric member, and the like may be adequately selected as the image carrier  1  as far as it can hold a toner image, and the image carrier  1  may have any shape such as a drum-like shape or a belt-like shape. 
     In the case where the image forming system using the image carrier  1  employs the electrophotographic method or the electrostatic recording method, for example, an image forming apparatus is used which, as shown in  FIG. 1A , includes: a latent image writing device  3  which writes an electrostatic latent image onto the image carrier  1 ; a developing device  4  which makes the electrostatic latent image formed on the image carrier  1  to a visible image by a toner; a transferring device  5  which transfers the toner image on the image carrier  1  to a sheet  7  that is a recording member; and a cleaning device  6  which has a cleaning member  6   a  that is contacted with the image carrier  1 , and which cleans the image carrier by scraping off a foreign substance remaining on the image carrier  1  by the cleaning member  6   a.    
     The latent image writing device  3  is a device which performs an operation of writing an electrostatic latent image, and which is configured by, for example, a combination of a charging device and an exposing device, or alternatively may be a device which directly writes an electrostatic latent image by an ion stream. 
     The developing device  4  may be any device which can visualize an electrostatic latent image on the image carrier  1  by means of a toner. As a developer, a two-component developer containing a toner and a carrier, a single-component developer mainly consisting of a toner, or the like is used. 
       FIG. 1A  shows the transferring device  5  which transfers the toner image on the image carrier  1  to the sheet  7  that is a recording member. However, the invention is not restricted to this, and can be applied also to a mode where an intermediate transferring member which intermediately holds the toner image on the image carrier  1  before the image is transferred to the recording member. 
     Furthermore,  FIG. 1A  shows the mode where the cleaning device  6  uses the cleaning member  6   a  to clean the image carrier by scraping off a foreign substance remaining on the image carrier  1  by the cleaning member  6   a . However, the invention is not restricted to this, and can be applied also to a mode where a cleaning member having a brush-like shape or a roll-like shape is used separately or together with the cleaning member  6   a.    
     In the exemplary embodiment, the lubricant applying device  2  may be disposed separately from and independently of the image carrier  1 , or alternatively the image carrier  1  and the lubricant applying device  2  may be configured as an image forming assembly in which at least the image carrier and the apparatus are detachably attached to a casing of the image forming apparatus. 
     As shown in  FIG. 1B , the lubricant applying device  2  which is used in the exemplary embodiment includes: a lubricating roll  10  in which a solid lubricant for application is formed into a circular sectional shape around a rotatable support shaft  11 , and which is contacted with the image carrier  1 ; a pressing mechanism  12  which presses the lubricating roll  10  toward the image carrier  1 ; and a rotation control mechanism  13  which, when the rotational torque about the support shaft  11  of the lubricating roll  10  that is pressed against the image carrier  1  by the pressing mechanism  12  is lower than a predetermined rotational torque TS, restrains rotation of the lubricating roll  10 , and, when the rotational torque is equal to or higher than the predetermined rotational torque TS, causes the lubricating roll  10  to be rotated in accordance with rotation of the image carrier  1 . 
     The lubricating roll  10  is requested to have a mode where the solid lubricant is formed into a roll-like shape around the support shaft  11 . As the solid lubricant, any one of a wide variety of known materials such as ZnSt may be employed. 
     As the pressing mechanism  12 , any configuration including a mechanism which presses the lubricating roll  10  may be adequately selected. In the case where the own weight of the lubricating roll  10  is used, for example, a support shaft receiving portion is disposed in a member of supporting the lubricating roll  10  so that the own weight of the lubricating roll functions as a pressing force. 
     A rotational torque which is produced around the support shaft  11  of the lubricating roll  10  when a foreign substance such as a toner, a carrier, or paper dust is incorporated into the contact portion between the lubricating roll  10  and the image carrier  1  may be examined by experiments or the like. In the rotation control mechanism  13 , then, “predetermined rotational torque” may be predetermined. 
     According to the exemplary embodiment, as shown in  FIG. 2A , in the state where a rotational torque RT 1  which is lower than the predetermined rotational torque TS acts, the rotation control mechanism  13  restrains rotation of the lubricating roll  10 , and therefore the lubricating roll  10  is maintained in a resting state and the solid lubricant is applied to the surface of the image carrier  1 . By contrast, as shown in  FIG. 2A , in the state where a rotational torque RT 2  which is equal to or higher than the predetermined rotational torque TS acts, the lubricating roll  10  is caused to be rotated in accordance with rotation of the image carrier  1 . Therefore, the foreign substance  16  which is incorporated into the contact portion between the image carrier  1  and the lubricating roll  10  is moved together with rotation of the lubricating roll  10 , and discharged from the contact portion. 
     In the technical means, a typical mode of the pressing mechanism  12  is a mechanism which presses the lubricating roll  10  by the own weight or a pressing spring. 
     A preferred mode of the pressing mechanism  12  is a mechanism which suppresses a change of the pressing force irrespective of consumption of the solid lubricant of the lubricating roll  10 . Here, the term “suppresses a change of the pressing force” includes a mode where the pressing force is constant, and also that where the pressing force is slightly changed. It is requested only that the pressing force is maintained substantially constant. 
     A preferred structure of supporting the lubricating roll  10  has a mode where the support shaft  11  of the lubricating roll  10  is movable along the pressing direction of the pressing mechanism  12 . 
     A typical mode of the rotation control mechanism  13  is a torque limiter which is incorporated in the support shaft  11  of the lubricating roll  10 . The torque limiter which is used as the rotation control mechanism  13  is not restricted to a limiter in which the coil spring system is employed, and may be a limiter in which any one of various known systems is employed. 
     In the case where the contact portion between the image carrier  1  and the lubricating roll  10  is biased to one portion, the image forming apparatus may include a position changing mechanism  15  which makes the contact portion of the lubricating roll  10  with respect to the image carrier  1  variable. The position changing mechanism  15  may be manually operated, or may automatically perform the position changing operation at a predetermined timing. 
     In a preferred mode of the configuration where the position changing mechanism  15  is added, moreover, the configuration may include a position detector which can detect the rotational position of the lubricating roll  10 . According to this mode, the position detector can know the rotational position of the lubricating roll  10 . At this time, in the case where a rotational torque which is equal to or higher than the predetermined rotational torque acts on the lubricating roll  10 , for example, the lubricating roll  10  is rotated in accordance with rotation of the image carrier  1 . The rotational position of the lubricating roll  10  can be selectively changed only in the case where the lubricating roll  10  is in a non-rotation state for a long term. 
     The position where the lubricant applying device  2  is disposed may be adequately selected as far as the position is in the periphery of the image carrier  1 . From the viewpoint that the foreign substance  16  on the image carrier  1  is hardly incorporated into the contact portion between the image carrier and the lubricating roll  10 , it is preferable to dispose the lubricant applying device downstream from at least the cleaning member  6   a  of the cleaning device  6  in the moving direction of the image carrier  1 , and it is more preferable to dispose the lubricant applying device upstream from an electrostatic latent image writing position of the latent image writing device  3 , in the moving direction of the image carrier  1 . 
     A lubricant layer is formed on the surface of the image carrier  1  by the lubricant applying device  2 . In a layout where the lubricant layer passes through a transferring portion of the transferring device  5 , even when the sheet  7  that is a recording member is transported to the transferring portion of the transferring device  5 , the talc component of the sheet  7  is contacted with the lubricant layer of the image carrier  1 . Therefore, the talc component of the sheet  7  is little transferred toward the image carrier  1 . Consequently, the situation where the talc component of the sheet  7  is transferred toward the image carrier  1  to affect disturbance of a toner image hardly occurs. 
     Hereinafter, the invention will be described in more detail with reference to exemplary embodiments shown in the accompanying drawings. 
     Exemplary Embodiment 1 
     —Summary of Image Forming Apparatus— 
       FIG. 3  is a diagram schematically showing Embodiment 1 of an image forming apparatus to which the invention is applied. 
     Referring to the figure, in the image forming apparatus, an image forming engine  21  of the electrophotographic system or the like is mounted in an apparatus casing  20 , and a sheet feeding cassette  22  which can feed a sheet that is a recording member is disposed below the image forming engine  21  in the apparatus casing  20 . A sheet discharge tray  27  which can accommodate the sheet P that has undergone a recording operation is in a top portion of the apparatus casing  20 . A sheet conveying path  23  which guides the sheet P fed from the sheet feeding cassette  22  to the image forming engine  21  and the sheet discharge tray  27  is disposed in a substantially vertical direction on the side of a side face of the apparatus casing  20  (in the right side of  FIG. 3 ). 
     In the exemplary embodiment, the image forming engine  21  includes: a photosensitive member  31  which can hold a toner image, and which has a drum-like shape or the like; a charging device  32  which charges the photosensitive member  31 , and which is configured by a corona charger; an exposing device  33  which writes an electrostatic latent image onto the charged photosensitive member  31 , and which is configured by a laser scanning device or the like; a developing device  34  which makes the electrostatic latent image on the photosensitive member  31  to a visible image by a toner; a transferring device  35  which transfers the toner image on the photosensitive member  31  to the sheet P, and which uses a transferring roll or the like; a cleaning device  36  which removes foreign substances remaining on the photosensitive member  31 ; and a lubricant applying device  37  which is disposed between a charging portion of the charging device  32  and a portion where the electrostatic latent image is written by the exposing device  33 , and which applies a lubricant to the surface of the photosensitive member  31 . 
     A feeding roll  22   a  which feeds the sheet P one by one is disposed in the sheet feeding cassette  22 . Positioning rolls  24  which once position the sheet P, and which then convey the sheet are disposed in the sheet conveying path  23  and upstream from the photosensitive member  31 . A fixing device  25  is disposed in the sheet conveying path  23  and downstream from the photosensitive member  31 . Discharging rolls  26  are disposed immediately before the sheet discharge tray  27 . In  FIG. 3 , the reference numeral  28  denotes conveying rolls which are disposed in an appropriate number in the sheet conveying path  23 . 
     In the image forming engine  21  of the exemplary embodiment, particularly, the photosensitive member  31 , the charging device  32 , the cleaning device  36 , and the lubricant applying device  37  are integrated as an image forming assembly  38  (corresponding to a process cartridge) as shown in  FIG. 4 , and the image forming assembly, the developing device  34 , and the exposing device  33  are separately disposed. 
     In the charging device  32 , a discharge wire  322  is disposed in an insulative charging vessel  321 , and a charging current is supplied to the discharge wire  322 , thereby charging the surface of the photosensitive member  31 . 
     The developing device  34  is disposed below an exposure beam Bm emitted from the exposing device  33 , and has a developer container  41  which accommodates a two-component developer containing a toner and a carrier, and in which a portion on the side of the photosensitive member  31  is opened. A developing roll  42  is rotatably disposed in a portion facing the opening of the developer container  41 . A pair of stirring and conveying members  43 ,  44  are disposed in the developer container  41  along the axial direction of the developing roll  42 , and a partitioning plate  45  is disposed between the pair of stirring and conveying members  43 ,  44 . Through holes which are not shown are disposed in the vicinities of the both longitudinal ends of the partitioning plate  45 , respectively, to constitute a developer circulating path  46  through which the developer can be circularly conveyed. The reference numeral  47  denotes a layer restricting member which restricts a developer layer on the developing roll  42 . 
     The cleaning device  36  has a cleaning container  51  which is opened in the side of the photosensitive member  31 , and which houses foreign substances remaining on the photosensitive member  31 . A plate-like cleaning member  52  which elongates in a direction perpendicular to the rotational direction of the photosensitive member  31 , and which is placed in contact with the photosensitive member  31  is disposed in the opening edge of the cleaning container  51 . A waste conveying member along which foreign substances scraped off by the cleaning member  52  are guided to a waste container (not shown) that is on the side of one end of the cleaning container  51  is disposed in the cleaning container  51 . 
     —Lubricant Applying Device— 
     In the exemplary embodiment, as shown in  FIGS. 4 ,  5 A, and  5 B, the lubricant applying device  37  has a lubricating roll  60  in which the solid lubricant  62  is formed into a roll-like shape around a rotatable support shaft  61 . 
     As the solid lubricant  62 , ZnSt (zinc stearate) is used. In the lubricating roll  60 , the support shaft  61  is rotatably supported by end bearings  63 , and the end bearings  63  are held by guide holders  64  so as to be reciprocable in a radial direction of the photosensitive member  31 . 
     Each of the guide holders  64  has a substantially inverted U-shaped guide piece  65  which is opened while being opposed to the photosensitive member  31 . The end bearings  63  are embracingly held by the guide pieces  65 . Hook-like pawl portions  66  which can block the end bearings  63  are formed at the tip ends of the inverted U-shaped guide pieces  65 . In each of the guide holders  64 , a mounting piece  67  which extends from the guide piece  65  is integrally formed, and the mounting piece  67  is fixed through a fixing piece  68  such as a screw to, for example, a part of the cleaning container  51  of the cleaning device  36 . 
     The lubricating roll  60  is pressed toward the surface of the photosensitive member  31  by pressing mechanisms  70 . 
     In the exemplary embodiment, each of the pressing mechanisms  70  is realized by a combination of the own weight of the lubricating roll  60 , and a pressing spring  71  which is interposed between the substantially inverted U-shaped guide piece  65  of the guide holder  64  and the support shaft  61 . 
     In the exemplary embodiment, as shown particularly in  FIG. 5B , a torque limiter  80  serving as a rotation control mechanism is disposed in one end of the support shaft  61  of the lubricating roll  60 . 
     —Torque Limiter— 
     In the exemplary embodiment, as shown in, for example,  FIGS. 6A and 6B , the torque limiter  80  includes: an inner ring  81  which is coaxially fitted and attached to the support shaft  61  of the lubricating roll  60 ; an outer ring  82  which is disposed so as to cover the periphery of the inner ring  81 ; and a coil spring  83  which is pressingly inserted and wound between the inner ring  81  and the outer ring  82 . For example, the outer ring  82  is fixedly supported by a fixing member  85  through a rotation locking mechanism  84 , and the rotating operation of the inner ring  81  is suppressed by a pressing force which is applied to the inner ring  81  by the coil spring  83 . 
     In the rotation locking mechanism  84 , for example, a locking member  86  in which a restraining groove  87  having a substantially U-like shape is formed is attached to a part of the outer ring  82 , an engaging pin  88  is projected from the fixing member  85 , and the engaging pin  88  is engaged with the restraining groove  87 , whereby rotation of the outer ring  82  is locked. 
     In the exemplary embodiment, when a rotational torque which is equal to or higher than a predetermined level acts on the lubricating roll  60 , therefore, a slipping rotational force which exceeds the suppressing force that is applied on the inner ring  81  by the coil spring  83  is applied on the inner ring  81  by the torque limiter  80 , thereby causing the inner ring  81  to be relatively rotated with respect to the outer ring  82 . 
     The torque limiter  80  is not restricted to the above-described mode, and may employ any one of various known structures. 
     &lt;Torque Characteristics of Torque Limiter&gt; 
       FIG. 7  shows the torque characteristics of the torque limiter  80  in the exemplary embodiment. 
     In the figure, TS means a torque value (static friction torque value/static torque value) at an instant when the torque limiter  80  slips, TM means a torque value (dynamic friction torque value/dynamic torque value) during a period when the torque limiter  80  slips, and TR means a variation amount of a dynamic torque (torque ripple value/torque unevenness) during the period when the torque limiter  80  slips. 
     Namely, the torque limiter  80  operates so that, in the case where a rotational toque which is equal to or higher than the predetermined rotational torque is applied on the lubricating roll  60 , under conditions where the rotational toque exceeds the static torque value TS, the lubricating roll  60  is rotated in accordance with rotation of the photosensitive member  31 , and, under conditions where the rotational toque is lower that the static torque value TS, rotation of the lubricating roll  60  is restrained. When the rotational toque acting on the lubricating roll  60  becomes lower than the dynamic torque value TM after the lubricating roll  60  is rotated in accordance with rotation of the photosensitive member  31 , any portion of the surface of the lubricating roll  60  butts against the photosensitive member  31  to be restrained. 
     &lt;Temporal Change of Lubricating Roll&gt; 
     The lubricating roll  60  is pressed against the photosensitive member  31  by the predetermined pressing force, and hence the roll is gradually shaved and applied to the photosensitive member  31 . 
     The initial state of the lubricating roll  60  is shown in (a) of  FIG. 8 . 
     In the figure, the outer diameter of the lubricating roll  60  has an initial value Φ 0 , and hence the radius R 0  is a half of the value. The length SL of the pressing spring  71  from the positioning position S 0  maintains its initial value. Therefore, the resultant force NF of the initial load of the pressing spring  71  and the own weight of the lubricating roll  60  is applied to the contact portion N P  between the lubricating roll  60  and the photosensitive member  31 . The rotational torque RT 0  acting on the lubricating roll  60  is indicated by the following expression:
 
 RT   0   =μ×NF×R   0  
 
where μ is the coefficient of friction between the lubricating roll  60  and the photosensitive member  31 .
 
     By contrast, when the lubricating roll  60  is temporally changed, the state shown in (b) of  FIG. 8  is obtained. 
     When the lubricating roll  60  is shaved and applied to the photosensitive member  31 , namely, the radius R t  of the lubricating roll  60  is reduced in accordance with the temporal change of the lubricating roll  60 , and the resultant force NF t  of the temporally changing load of the pressing spring  71  and the own weight of the lubricating roll  60  is reduced by a degree corresponding to the elongation of the pressing spring  71 . When the radius R t  of the lubricating roll  60  is reduced by 1.4 mm, for example, the pressing spring  71  is elongated by 1.4 mm. In the case where the spring constant is 100 gf/mm, for example, the resultant force NF t  is reduced by 140 gf. Therefore, the rotational torque RT t  acting on the lubricating roll  60  is indicated by the following expression:
 
 RT   t   =μ×NF   t   ×R   t  
 
     In this case, NT t &lt;NT 0  and R t &lt;R 0 . 
     From the above, it will be understood that, when the rotational torque of the lubricating roll  60  is reduced by the temporal change, the lubricating roll  60  itself is hardly rotated. In view of this, therefore, the restraining torque (static torque value) TS of the torque limiter  80  must be set. 
     &lt;Method of Setting Restraining Torque of Torque Limiter&gt; 
     Next, a method of setting the restraining torque (static torque value) TS of the torque limiter  80  will be described with reference to  FIG. 9 . 
     In the figure, when the rotational torque in the initial state of the lubricating roll  60  is indicated by RT(MAX), the rotational torque at the end of life of the lubricating roll  60  (corresponding to the time when the solid lubricant of the lubricating roll  60  is consumed up and the support shaft  61  is exposed) is indicated by RT(MIN), the increment of the rotational torque caused by incorporation of the toner, paper dust, or the carrier that is a foreign substance into the contact portion between the lubricating roll  60  and the photosensitive member  31  is indicated by ΔRT, “RT(MAX)+ΔRT” is indicated by “RT A (MAX)”, and “RT(MIN)+ΔRT” is indicated by “RT A (MIN)”, the restraining torque (static torque value) TS of the torque limiter  80  is selected so as to enter the region m excluding boundaries with respect to RT A (MIN) and RT(MAX). 
     —Operation Process of Lubricant Applying Device— 
     It is assumed that the restraining torque TS of the torque limiter  80  is selected as described above. 
     First, as shown in  FIG. 10A , in the case where the rotational torque RT 1  acting on the lubricating roll  60  is lower than the restraining torque TS of the torque limiter  80 , the lubricating roll  60  is contacted with the photosensitive member  31  while maintaining the resting state, and hence the solid lubricant is shaved off from the lubricating roll  60 , and a lubricant layer M formed by the solid lubricant is formed on the surface of the photosensitive member  31 . 
     As shown in  FIG. 10B , in the case where a foreign substance W such as the toner, paper dust, or the carrier adheres to the photosensitive member  31  and the foreign substance W enters the contact portion between the lubricating roll  60  and the photosensitive member  31 , the coefficient of friction between the lubricating roll  60  and the photosensitive member  31  is increased by the existence of the foreign substance W. The rotational torque RT 2  acting on the lubricating roll  60  correspondingly becomes higher than the restraining torque TS caused by the torque limiter  80 . Then, the lubricating roll  60  which has been in the resting state is rotated in accordance with rotation of the photosensitive member  31 , and the foreign substance W which has been incorporated into the contact portion between the lubricating roll  60  and the photosensitive member  31  is passed through the contact portion to be discharged therefrom. 
     After the operation of discharging the foreign substance W, the foreign substance W does not exist in the contact portion between the lubricating roll  60  and the photosensitive member  31 , and hence the friction coefficient μ therebetween is reduced, the rotational torque RT 2  acting on the lubricating roll  60  correspondingly becomes lower than the dynamic torque value TM shown in  FIG. 7 , and the lubricating roll  60  is again restrained at any portion of the surface to enter the resting state. 
     —Function of Transferring Device in Transferring Portion— 
     It is assumed that, as shown in  FIG. 11A , the lubricant layer M is formed on the surface of the photosensitive member  31  by the lubricating roll  60 , and the lubricant layer M reaches the transferring portion of the transferring device  35 . 
     When, in this state, the sheet P that is a recording member enters the transferring portion of the transferring device  35 , the talc component PT of the sheet P is contacted with the surface of the photosensitive member  31 . 
     However, the surface of the photosensitive member  31  is covered by the lubricant layer M, and therefore the talc component PT of the sheet P does not adhere to the surface of the photosensitive member  31 , and returns to the sheet P. Consequently, the situation where the talc component PT of the sheet P adheres to the surface of the photosensitive member  31  hardly occurs. 
     In a comparative mode in which the lubricant layer M is not formed on the surface of the photosensitive member  31  as shown in  FIG. 11B , when the sheet P enters the transferring portion of the transferring device  35 , the talc component PT of the sheet P easily adheres to the surface of the photosensitive member  31 , and the talc component PT of the sheet P correspondingly remains on the surface of the photosensitive member  31 . In this case, since a toner image is hardly formed in a place where the talc component PT exists, there arises a fear that a toner image on the photosensitive member  31  is easily disturbed. 
     Modifications 
     In the exemplary embodiment, the combination of the own weight of the lubricating roll  60  and the pressing force of the pressing spring  71  is used in the pressing mechanisms  70 . However, the invention is not restricted to this. In the case where a layout in which the lubricating roll  60  is placed above the photosensitive member  31  is employed as shown in  FIG. 12 , for example, only the own weight of the lubricating roll  60  may be used. 
     In a layout in which the own weight of the lubricating roll  60  does not act, only the pressing force of the pressing spring  71  may be used. 
     Exemplary Embodiment 2 
       FIG. 13A  is a diagram showing Exemplary embodiment 2 of a lubricant applying device used in an image forming apparatus to which the invention is applied. 
     Referring to the figure, in a substantially same manner as Embodiment 1, the lubricant applying device  37  includes: the lubricating roll  60  which is contacted with the photosensitive member  31 ; the pressing mechanisms  70  which press the lubricating roll  60  toward the photosensitive member  31 ; and the torque limiter  80  serving as a rotation control mechanism which restrains rotation of the lubricating roll  60 . To the torque limiter  80 , a lock position changing mechanism  100  which changes the rotational position of the lubricating roll  60 , and an operation knob  110  which rotationally operates the torque limiter  80  are added. 
     As shown in  FIG. 13B , foe example, the lock position changing mechanism  100  is configured so that a circular through hole  102  through which the torque limiter  80  is passed is disposed in a fixing member  101 , lock holes  103  are disposed at predetermined angular intervals in the periphery of the through hole  102  of the fixing member  101 , a locking member  106  in which a restraining groove  107  having a substantially U-like shape is formed is attached to a part of the outer ring  82  of the torque limiter  80 , and the locking member  106  is adequately rotated, whereby a detachable lock pin  108  is engaged with the restraining groove  107  of the locking member  106 , and positionally inserted into one of the lock holes  103  of the fixing member  101 . 
     In the exemplary embodiment, the lubricating roll  60  is temporally changed (t 1 ) from the initial state (see (a) of  FIG. 14 ). When it is assumed that, as shown in (b) of  FIG. 14 , the lubricating roll  60  does not perform the rotating operation and remains to be placed at a predetermined position in the resting state for a long term, the contact portion of the lubricating roll  60  with respect to the photosensitive member  31  is shaved gradually and locally. 
     When this state is left as it is until the contact portion of the lubricating roll  60  with respect to the photosensitive member  31  is locally shaved to reach the support shaft  61 , the lubricating roll  60  finishes its life in the state where the roll is locally shaved. In the exemplary embodiment, as shown in (c) of  FIG. 14 , for example, the lock pin  108  of the look position changing mechanism  100  is pulled out, the operation knob  110  which is disposed coaxially with the outer ring  82  of the torque limiter  80  is rotated by a predetermined angle, the lock pin  108  of the lock position changing mechanism  100  is again engaged with the restraining groove  107  of the locking member  106 , and the lock pin  108  is positionally inserted into, for example, the lock hole  103  which is adjacent to the original lock hole, whereby the rotational position of the lubricating roll  60  is forcedly rotated by the predetermined angle to cause the new face  122  to be contacted with the photosensitive member  31 , in place of the shaved face  121 ( 1 ) of the lubricating roll  60 . 
     When, in this state, the lubricating roll  60  is temporally changed (t 2 ) as shown in (d) of  FIG. 14  and left at the predetermined position in the resting state for a long term while the roll does not perform the rotating operation, the contact portion of the lubricating roll  60  with respect to the photosensitive member  31  is gradually shaved. In this state, when the rotational position of the lubricating roll  60  is again changed by using the lock position changing mechanism  100 , the new face  122  is contacted with the photosensitive member  31 , in place of the shaved faces  121 ( 1 ),  121 ( 2 ) of the lubricating roll  60  as shown in (e) of  FIG. 14 . Thereafter, in the case where the lubricating roll  60  is locally shaved by the temporal change, the rotational position of the lubricating roll  60  is adequately rotated by using the lock position changing mechanism  100 . 
     In the exemplary embodiment, the operation knob  110  is disposed separately from the torque limiter  80 . It is a matter of course that, for example, the outer ring  82  of the torque limiter  80  may function also as the operation knob  110 . 
     Exemplary Embodiment 3 
       FIG. 15  is a diagram showing Exemplary embodiment 3 of a lubricant applying device used in an image forming apparatus to which the invention is applied. 
     Referring to the figure, the lubricant applying device  37  is approximately identical in basic configuration with Exemplary embodiment 2, but, unlike Exemplary embodiment 2, a position detector  130  is disposed in the periphery of the support shaft  61  of the lubricating roll  60  in order to detect whether the lubricating roll  60  is rotated or not. The components similar to those of Exemplary embodiment 2 are denoted by the same reference numerals as Exemplary embodiment 2, and their description is omitted. 
     In the exemplary embodiment, the position detector  130  has: a slitted disc  131  which is disposed on, for example, the support shaft  61  of the lubricating roll  60 ; and an optical sensor  132  in which a light emitting portion and a light receiving portion are opposed to each other across the slit of the slitted disc, and which detects whether the slit passes over the portion or not. 
     Detection information of the optical sensor  132  is supplied to a control device  140 . If the control device  140  determines that the lubricating roll  60  remains to be in the resting state for a predetermined long term, on the basis of, for example, the information from the optical sensor  132 , an alarm indicative of the determination is displayed on a display unit  141 . 
     Then, the user views the alarm display on the display unit  141 , and can change the rotational position of the lubricating roll  60  by using the lock position changing mechanism  100 . 
     In the exemplary embodiment, a manual operation is performed by using the lock position changing mechanism  100  to change the rotational position of the lubricating roll  60 . The invention is not restricted to this. Alternatively, if the control device  140  determines that the lubricating roll  60  remains to be in the resting state for the predetermined long term, for example, a drive transmitting mechanism which is not shown is selectively coupled to the lubricating roll  60 , and the lubricating roll  60  is caused to be automatically rotated through the drive transmitting mechanism, thereby changing the rotational position of the lubricating roll  60 . 
     EXAMPLES 
     Example 1 
     In the example, the lubricant applying device  37  of the image forming apparatus of Exemplary embodiment 1 is used, and use conditions (initial state) for setting the lubricating roll  60  to the resting state are obtained. 
     The use conditions in this case are listed in  FIG. 16 . 
     As the lubricating roll  60 , a roll in which a roll-like solid lubricant having an outer diameter of 14 mm is disposed around a support shaft that has a diameter of 6 mm in the initial state is used. As the pressing mechanisms  70  of the lubricating roll  60 , mechanisms configured by the combination of the own weight of the lubricating roll  60  and the pressing force of the pressing spring  71  are used. The friction coefficient μ between the lubricating roll  60  and the photosensitive member  31  is measured by, for example, Pendulum II type (50° C.) test method. 
     In the example, the restraining torque (static torque value) TS of the torque limiter  80  is set to 90 gf·cm, and the rotational torque (gf·cm) is measured under conditions which are applied to the lubricating roll  60 . The following results are obtained. 
     Normal state: 21 
     Abnormal state 1 (the toner is incorporated as a foreign substance): 82 
     Abnormal state 2 (paper dust is incorporated as a foreign substance): 103 
     Abnormal state 3 (the carrier is incorporated as a foreign substance): 185 
     In this case, it will be understood that, in Normal state and Abnormal state 1, the rotational torque RT is RT&lt;TS, and hence the lubricating roll  60  maintains the resting state with respect to the photosensitive member  31 , and, in Abnormal state 2 and Abnormal state 3, the rotational torque RT is RT≧TS, and hence the lubricating roll  60  is rotated in accordance with rotation of the photosensitive member  31 . 
     In Abnormal state 1, the toner is incorporated as a foreign substance into the contact portion between the lubricating roll  60  and the photosensitive member  31 . The coefficient of friction between the toner and the photosensitive member is lower as compared with paper dust or the carrier. Therefore, it is determined that, as an element which may damage the photosensitive member  31 , the toner does not exert a large influence. 
     Example 2 
     The example shows an example of a method in which the lubricant applying device of the image forming apparatus which is similar to that of Example 1 is used, the rotational torques in the initial state (the radius: 0.7 cm) of the lubricating roll  60  and the temporal change state (the radius: 0.4 cm, 0.3 cm) are measured, and the restraining torque (static torque value) TS of the torque limiter  80  is set. 
       FIG. 17  shows results. 
     In  FIG. 17 , as the pressing mechanisms  70 , mechanisms in which the own weight of the lubricating roll  60  and the pressing force (three kinds of 400 gf, 600 gf, and 800 gf) of the pressing spring  71  are combined with each other are used. The term “Total weight” means a total of the own weight of the lubricating roll  60  and the pressing force of the pressing spring  71 . 
     In the figure, the state where the lubricating roll  60  has the radius of 0.3 cm (corresponding to the radius of the support shaft) means that the lubricating roll is at the end of life. The rotational torque in the case where the pressing force of the pressing spring is, for example, 400 gf is checked, and the following results are obtained. 
     Abnormal state 2: 77.4 gf·cm (195.2 gf·cm in the initial state) 
     Abnormal state 3: 139.4 gf·cm (351.4 gf·cm in the initial state) 
     In the initial state of the lubricating roll  60 , the rotational torque in the normal state is 39.0 gf·cm. 
     In this case, the restraining torque (static torque value) TS of the torque limiter  80  may be adequately selected in a region between 39.0 gf·cm (the rotational torque in the normal state in the initial state) and 77.4 gf·cm (the rotational torque in Abnormal state 2 at the end of life) (excluding the boundary values). In the case where torque limiters respectively having three kinds of restraining torques of 40, 65, and 90 are prepared, for example, the torque limiters respectively having restraining torques of 40 and 65 may be selected from the three kinds of torque limiters. 
     The rotational torque in the case where the pressing force of the pressing spring is 600 gf is checked, and the following results are obtained. 
     Abnormal state 2: 107.4 gf·cm (265.2 gf·cm in the initial state) 
     Abnormal state 3: 193.4 gf·cm (477.4 gf·cm in the initial state) 
     In the initial state of the lubricating roll  60 , the rotational torque in the normal state is 53.0 gf·cm. 
     In this case, the restraining torque (static torque value) TS of the torque limiter  80  may be adequately selected in a region between 53.0 gf·cm (the rotational torque in the normal state in the initial state) and 107.4 gf·cm (the rotational torque in Abnormal state 2 at the end of life) (excluding the boundary values). For example, torque limiters respectively having restraining torques of 65 and 90 may be selected. 
     Namely, “Rotational torque in normal state” is expressed by μ×NF×R 
     where μ: the coefficient between the lubricating roll and the photosensitive member 
     NF: Total weight 
     R: the distance from the rotation center of the lubricating roll to the contact portion with respect to the photosensitive member. 
     The torques may be set so as to satisfy the relationships of “Rotational torque in normal state”&lt;“Restraining torque of torque limiter”&lt;“Rotational torques in Abnormal state 2 and Abnormal state 3”. 
     Here, the degree at which, when a foreign substance in Abnormal state 2 or 3 is incorporated, “μ (coefficient of friction between the lubricating roll  60  and the photosensitive member  31 )” is increased with respect to that in the normal state is important. In the example, “μ” in the normal state is 0.1, and “μ” in Abnormal state 2 and Abnormal state 3 are 0.5 and 0.9. 
     Based on the results of  FIG. 17 , the selection range of the restraining torque of the torque limiter  80  is approximately as follows: 
     Rotational torque in normal state: 40 to 70 (gf·cm) 
     Rotational torques in abnormal state (in this case, Abnormal state 2): 80 to 140 (gf·cm) 
     Restraining torque of torque limiter: 60 to 100 (gf·cm). 
     The foregoing description of the embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention defined by the following claims and their equivalents.