Patent Publication Number: US-2016231668-A1

Title: Electric conductive roller, transfer device, and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2015-023295 filed Feb. 9, 2015. 
     BACKGROUND 
     Technical Field 
     The present invention relates to electric conductive rollers, transfer devices, and image forming apparatuses. 
     SUMMARY 
     An electric conductive roller according to an aspect of the invention includes a shaft, a cylindrical first elastic body having an electric conductivity and covering the shaft while being in contact with an outer periphery of the shaft, and a second elastic body having an annular shape, disposed at an end portion of the first elastic body, covering the shaft at a distance from the shaft, and having an electric conductivity, the second elastic body having a thickness that gradually increases from a portion between a free end of the second elastic body and a boundary between the second elastic body and the first elastic body to the free end while the second elastic body is retaining an outside diameter greater than or equal to an outside diameter of the first elastic body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic diagram (front view) of an image forming apparatus according to an exemplary embodiment; 
         FIG. 2  is a schematic diagram (partial cross-sectional view) in the vicinity of a second transfer portion in a transfer device according to an exemplary embodiment; 
         FIG. 3  is a schematic diagram (cross-sectional view) of a second roller constituting a transfer device according to an exemplary embodiment; 
         FIG. 4  is a schematic diagram (cross-sectional view) of a second roller constituting a transfer device according to a comparative mode; 
         FIG. 5  is a schematic diagram (partial cross-sectional view) in the vicinity of a second transfer portion in a transfer device according to a comparative mode; 
         FIG. 6  is a schematic diagram (cross-sectional view) of a modification example (first modification example) obtained by modifying the second roller according to the exemplary embodiment; 
         FIG. 7  is a schematic diagram (cross-sectional view) of a modification example (second modification example) obtained by modifying the second roller according to the exemplary embodiment; 
         FIG. 8  is a schematic diagram (cross-sectional view) of a modification example (third modification example) obtained by modifying the second roller according to the exemplary embodiment; 
         FIG. 9  is a schematic diagram (cross-sectional view) of a modification example (fourth modification example) obtained by modifying the second roller according to the exemplary embodiment; 
         FIG. 10  is a schematic diagram (cross-sectional view) of a modification example (fifth modification example) obtained by modifying the second roller according to the exemplary embodiment; 
         FIG. 11  is a schematic diagram (cross-sectional view) of a modification example (sixth modification example) obtained by modifying the second roller according to the exemplary embodiment; 
         FIG. 12  is a schematic diagram (cross-sectional view) of a modification example (seventh modification example) obtained by modifying the second roller according to the exemplary embodiment; and 
         FIG. 13  is a list of conditions and results of experiments conducted on examples and comparative modes. 
     
    
    
     DETAILED DESCRIPTION 
     General Description 
     The following describes a mode of embodying the invention (referred to as exemplary embodiment, below), followed by description of modification examples obtained by modifying the exemplary embodiment (first to seventh modification examples) and description of examples. 
     In the following description, directions denoted by arrows X and −X in the drawings indicate an apparatus width direction and directions denoted by arrows Y and −Y in the drawings indicate an apparatus height direction. Directions perpendicular to both the apparatus width direction and the apparatus height direction (directions denoted by arrows Z and −Z) indicate an apparatus depth direction. 
     EXEMPLARY EMBODIMENT 
     Referring now to the drawings, an exemplary embodiment is described below. Firstly, the entire configuration of an image forming apparatus  10  according to an exemplary embodiment is described, followed by description of the configuration of a characteristic portion (transfer device  30 ) according to the exemplary embodiment, description of the operation of the image forming apparatus  10  according to the exemplary embodiment, and description of effects of the exemplary embodiment. 
     Entire Configuration of Image Forming Apparatus 
     As illustrated in  FIG. 1 , the image forming apparatus  10  is an electrophotographic apparatus that includes a toner image forming unit  20 , a transfer device  30 , a transporting device  40 , a fixing device  50 , and a controller  60 . 
     Toner Image Forming Unit 
     The toner image forming unit  20  has a function of forming a toner image G held by a transfer belt TB constituting the transfer device  30  by performing steps of electric charging, exposure to light, and development. The transfer belt TB is described below. Here, the toner image forming unit  20  is an example of a forming unit. The toner image G according to the exemplary embodiment is formed with, for example, a negatively charged toner T. 
     The toner image forming unit  20  includes single-color units  21 Y,  21 M,  21 C, and  21 K that form toner images of different colors, that is, yellow (Y), magenta (M), cyan (C), and black (K). The single-color units  21 Y,  21 M,  21 C, and  21 K have the same configuration except for the colors of toner images G that they form. In the following description, the letters of the alphabet (Y, M, C, and K) in the symbols for the single-color units  21 Y,  21 M,  21 C, and  21 K are omitted when the single-color units  21 Y,  21 M,  21 C, and  21 K and their components do not need to be distinguished from one another. Each single-color unit  21  includes a photoconductor  22 , a charging device  24 , an exposure device  26 , and a developing device  28 . The photoconductor  22  is cylindrical. The photoconductor  22  is disposed so that its axis is aligned with the apparatus depth direction. In  FIG. 1 , reference symbols for the photoconductors  22 , the charging devices  24 , the exposure devices  26 , and the developing devices  28  of the single-color units  21  other than those of the single-color unit  21 K are omitted. 
     Transfer Device 
     The transfer device  30  has a function of rotating while holding toner images G of different colors that have been formed at the single-color units  21  and first-transferred to the transfer device  30  and a function of second-transferring, at a nip N 2 , the toner images F of the different colors onto a transported medium P, described below. The configuration of the transfer device  30  is described below. 
     Transporting Device 
     The transporting device  40  has a function of transporting a medium P so that the medium P passes through a nip N 2  and a nip N 3 , described below. 
     Fixing Device 
     The fixing device  50  has a function of heating and pressing, at the nip N 3 , toners T constituting toner images G that have been second-transferred to the medium P by the transfer device  30  to fix the toners T onto the medium P. The fixing device  50  includes a heating portion  50 A and a pressing portion  50 B. 
     Controller 
     The controller  60  has a function of controlling all the components of the image forming apparatus  10  other than itself. 
     The description given above is about the entire configuration of the image forming apparatus  10  according to the exemplary embodiment. 
     Configuration of Characteristic Portion 
     Referring now to the drawings, a characteristic portion (transfer device  30 ) according to the exemplary embodiment is described. 
     As illustrated in  FIG. 1 , the transfer device  30  includes a transfer belt TB, multiple first rollers  32 , a driving roller  34 , a second transfer portion  36 , and a power source PS. Here, the power source PS is an example of a voltage applying portion. 
     Transfer Belt, First Roller, and Driving Roller 
     The transfer belt TB is an endless belt. Each first roller  32  is disposed below the corresponding photoconductor  22  and forms a nip N 1  together with the photoconductor  22  by nipping the transfer belt TB therebetween. Each first roller  32  first-transfers a toner image G of the corresponding color formed on the corresponding photoconductor  22  to the transfer belt TB in response to an application of a voltage (first-transfer voltage) from a power source (not illustrated). The driving roller  34  is driven by a driving source (not illustrated) and rotates around its axis to rotate the transfer belt TB in the direction of arrow R. In this configuration, the transfer belt TB, while rotating in the direction of arrow R, receives toner images G of different colors first-transferred from the single-color units  21  and carries the toner images G of different colors on the outer perimeter to the nip N 1 . Here, the transfer belt TB is an example of a holding belt. The sheet resistance of the transfer belt TB according to the exemplary embodiment is, for example, 1.0×10 6  Ω/sq or higher and less than 1.0×10 10  Ω/sq. Here, “sq” denotes a unit volume of 1 m 3 . 
     Second Transfer Portion 
     The second transfer portion  36  has a function of second-transferring toner images G of different colors held by the transfer belt TB to a medium P transported by the transporting device  40 . As illustrated in  FIG. 2 , the second transfer portion  36  includes a second roller  70 , bearings  100 , a back-up roller  110  (hereinafter referred to as a BUR  110 ), and a movable portion (not illustrated). Here, the second roller  70  is an example of an electric conductive roller. The BUR  110  is an example of a contact portion.  FIG. 2  illustrates near-side portions of the second transfer portion  36  and the transfer belt TB in the apparatus depth direction. Far-side portions of the second transfer portion  36  and the transfer belt TB in the apparatus depth direction are symmetric with the near-side portions and have the same configuration. 
     Second Roller 
     As illustrated in  FIGS. 2 and 3 , the second roller  70  includes a shaft  80  and an elastic body  90 . Here, the shaft  80  is an example of a shaft. The shaft  80  according to the exemplary embodiment is made of metal. The second roller  70  illustrated in  FIG. 3  is in an unloaded state, that is, in the state of touching none of other components. In contrast, the second roller  70  illustrated in  FIG. 2  is in the state where the second transfer portion  36  is forming a nip N 2 . 
     The shaft  80  includes a cylindrical body  82 , having a diameter of D 3 , and protrusions  84 , protruding from both longitudinal ends of the body  82  and having a diameter of D 4  (&lt;D 3 ). As illustrated in  FIG. 3 , the shaft  80  is linearly symmetric with respect to its axis (dot-and-dash line C 1  in the drawings). The shaft  80  has stepped portions on both axial end portions. A grounded compression spring (not illustrated) in the compressed state touches one end surface of one of the protrusions  84 . 
     The elastic body  90  has electric conductivity. The elastic body  90  includes a first elastic body  92  and second elastic bodies  94 . In this description, having electric conductivity means, for example, that the volume resistivity is below 1.0×10 9  Ω·m. The elastic body  90  according to the exemplary embodiment has, for example, a volume resistivity of 1.0×10 4  Ω·m or higher and less than 1.0×10 8  Ω·m. An example of the elastic body  90  according to the exemplary embodiment is an electrically conductive foam (foam containing urethane foam and electrically conductive member). 
     As illustrated in  FIG. 3 , the first elastic body  92  is cylindrical (inner diameter of D 3  and outside diameter of D 1 ) in the unloaded state. The body  82  of the shaft  80  is fitted into the inner circumference of the first elastic body  92 . The first elastic body  92  is bonded to the body  82 . The first elastic body  92  thus touches the outer periphery of the body  82  and covers the body  82 . Here, the first elastic body  92  is a portion of the elastic body  90  that touches the outer periphery of the body  82  and covers the body  82 . The width of the first elastic body  92  (dimension in the direction of axis C 1 ) is equivalent to the width of the body  82  of the shaft  80 . 
     The second elastic bodies  94  are disposed on both longitudinal ends of the first elastic body  92 . As illustrated in  FIG. 3 , each second elastic body  94  is annular in the unloaded state. Here, being annular means being continuous around any axis (axis C 1  in the exemplary embodiment) so as to surround the shaft. The inner periphery of each second elastic body  94  has an inner diameter of D 3  from a boundary BP between itself and the first elastic body  92  to a free end FE (end opposite to the boundary BP). The outer periphery of each second elastic body  94  has an outside diameter of D 1  at the boundary BP and an outside diameter of D 2  (=1.03×D 1 ) at the free end FE. The outside diameter of each second elastic body  94  linearly (in relation to the linear function) increases from the boundary BP to the free end FE. In the above-described configuration, each second elastic body  94  covers the corresponding protrusion  84  of the shaft  80  at a distance from the protrusion  84 . Each second elastic body  94  gradually thickens from the boundary BP to the free end FE so that its outer diameter keeps being greater than or equal to D 1 , which is the outside diameter of the first elastic body  92 . The second elastic body  94  according to the exemplary embodiment is in the shape of a truncated cone having a hollow in the unloaded state. 
     As described above, the outside diameter of the first elastic body  92  and the outside diameter of each second elastic body  94  at the boundary BP are D 1 , and thus D 1  and D 2  satisfy the inequality D 2 /D 1 &gt;1. The outside diameter D 2  of each second elastic body  94  at the free end FE is 1.03×D 1 . Thus, D 1  and D 2  satisfy the inequality D 2 /D 1 ≦1.10. Accordingly, in the second roller  70  according to the exemplary embodiment, the outside diameter D 1  of the first elastic body  92 , the outside diameter D 1  of each second elastic body  94  at the boundary BP and the outside diameter D 2  of each second elastic body  94  at the free end FE satisfy the inequality 1&lt;D 2 /D 1 ≦1.10. 
     Bearings 
     The bearings  100  have a function of supporting the second roller  70 . As illustrated in  FIG. 2 , each bearing  100  is cylindrical. The bearings  100  support the second roller  70  in the state where the protrusions  84  at both end portions of the second roller  70  are fitted into the bearings  100 . The bearings  100  are attached to a frame (not illustrated) of the transfer device  30 . The bearings  100  according to the exemplary embodiment have insulating properties. In this description, having insulating properties means that, for example, the volume resistivity is greater than or equal to 1.0×10 12  Ω·m. The bearings  100  according to the exemplary embodiment have, for example, a volume resistivity of greater than or equal to 1.0×10 13  Ω·m. 
     The bearings  100  have a function of preventing electric-current leakages at the protrusions  84  of the shaft  80  and at the second elastic bodies  94  (particularly, at the protrusions  84  and at the free end FE of each second elastic body  94 ). Thus, as illustrated in  FIG. 2 , each bearing  100  is disposed so that part of the bearing  100  is located in a space (hollow portion) defined by the inner periphery of the corresponding second elastic body  94 . 
     BUR and Movable Portion 
     The BUR  110  has a function of causing the second roller  70  and the transfer belt TB to form a nip N 2  by coming into contact with the inner periphery of the transfer belt TB. Thus, as illustrated in  FIG. 1  and  FIG. 2 , the BUR  110  is disposed across from (above) the second roller  70  with the transfer belt TB interposed therebetween. 
     As illustrated in  FIG. 2 , the BUR  110  includes a shaft  112  and a resistor  114 . The shaft  112  is, for example, a column made of metal. The resistor  114  is cylindrical. The resistor  114  is fitted and bonded to the shaft  112 . Both end portions of the shaft  112  protrude beyond the resistor  114 . The dot-and-dash line C 2  in the drawings denotes the axis of the BUR  110 . 
     The resistor  114  is, for example, an electrically conductive solid rubber and is solider than the elastic body  90  of the second roller  70 . Thus, in the state where the nip N 2  is formed (in the state where the BUR  110  is in an operation position, described below), the resistor  114  presses the transfer belt TB and squashes the elastic body  90  together with the transfer belt TB. The resistor  114  according to the exemplary embodiment has, for example, a volume resistivity of greater than or equal to 1.0×10 3  Ω·m and less than 1.0×10 4  Ω·m. In other words, the volume resistivity of the resistor  114  is smaller than the volume resistivity of the elastic body  90 . 
     The movable portion (not illustrated) has a function of vertically moving the BUR  110 . Specifically, the movable portion includes a pair of bearings (not illustrated), a pair of tension springs (not illustrated), and a pair of cams (not illustrated). The bearings of the pair are fitted to both end portions of the shaft  112 . The tension springs of the pair pull the BUR  110  upward in the state where their ends are hooked on the corresponding bearings of the pair. The cams of the pair press the corresponding bearings of the pair in the state of being fixed to both end portions of the shaft (not illustrated). The pair of cams rotate while pressing the pair of bearings as a result of a driving source (not illustrated) rotating the shaft around its axis. In this configuration, the movable portion vertically moves the BUR  110 . 
     Here,  FIG. 1  and  FIG. 2  illustrate the state where the BUR  110  is disposed in the position (operation position) where an image forming operation is performed. On the other hand, while an image forming operation is not performed (for example, on standby before an image forming operation is performed), the BUR  110  is disposed in a stand-by position (not illustrated) above the operation position while remaining in contact with the inner periphery of the transfer belt TB. In the state where the BUR  110  is disposed in the stand-by position, the nip N 2  is unfastened. When the nip N 2  is unfastened, the elastic body  90  of the second roller  70  becomes separated from the transfer belt TB and enters the unloaded state illustrated in  FIG. 3 . 
     Power Source 
     The power source PS has a function of applying a voltage (second transfer voltage) to the BUR  110  and forming an electric field that causes toner images G held on the transfer belt TB to be transferred (second-transferred) to a medium that passes through the nip N 2 . 
     During a second transfer, the power source PS applies a voltage having a polarity (negative polarity) the same as the polarity of the toner T to portions of the shaft  112  of the BUR  110  protruding beyond the resistor  114  via leaf springs (not illustrated). As described above, the shaft  80  of the second roller  70  is in contact with the grounded compression spring. When the power source PS applies a negative voltage to the BUR  110  while the BUR  110  is in the operation position, the power source PS forms, at the nip N 2 , an electric field that causes the toner T held on the transfer belt TB to be transferred (second-transferred) to a medium P. The power source PS according to the exemplary embodiment applies to the shaft  112  of the BUR  110  a voltage within the range of approximately −3 to −2 kV in the standard environment (for example, an environment of a temperature of 23° C. and a humidity of 65%) and a voltage of approximately −9 kV in a low-temperature low-humidity environment (for example, an environment of a temperature of 10° C. and a humidity of 15%). 
     Supplement 
     The following provides supplement on description of the transfer device  30 . 
     Supplement 1 
     As illustrated in  FIG. 2 , portions of the transfer belt TB (both widthwise end portions of the transfer belt TB) and portions of the resistor  114  of the BUR  110  (both axial end portions of the BUR  110 ) protrude in the axial direction of the second roller  70  beyond the free ends FE of the second elastic bodies  94  of the second roller  70 . Portions of the transfer belt TB (both widthwise end portions of the transfer belt TB) protrude in the axial direction of the BUR  110  beyond both axial end portions of the resistor  114  of the BUR  110 . Thus, as illustrated in  FIG. 2 , a portion between the boundary BP and the free end FE of each second elastic body  94  of the second roller  70  forms a nip N 2  together with the resistor  114  of the BUR  110  with the transfer belt TB interposed therebetween. 
     Supplement 2 
     As described above (as illustrated in  FIG. 2 ), in the state where the nip N 2  is formed, the resistor  114  of the BUR  110  presses the transfer belt TB and squashes the elastic body  90  together with the transfer belt TB. From another point of view, the second roller  70  rotates around its axis in the state where the portion of the elastic body  90  (first elastic body  92  and the second elastic bodies  94 ) forming the nip N 2  is flattened. In this case, the portion of the first elastic body  92  forming the nip N 2  is compressed by being nipped by the transfer belt TB and the body  82  of the shaft  80 . In contrast, the portions of the second elastic bodies  94  forming the nip N 2  are deformed by being pressed by the transfer belt TB but their inner peripheries and their free ends FE do not touch other components (such as the shaft  80  or the bearings  100 ). As illustrated in  FIG. 2 , the portions of the second elastic bodies  94  forming the nip N 2  are deformed so as to come closer to the protrusions  84  (axis C 1 ) while being spaced apart from the protrusions  84  of the shaft  80 . As illustrated in  FIG. 2 , each second elastic body  94  forms the nip N 2  without having a gap between itself and the transfer belt TB throughout the area from the boundary BP and the free end FE. 
     Supplement 3 
     The width of a medium P used in the exemplary embodiment is smaller than the width, in the apparatus depth direction, of a portion of the nip N 2  formed by the transfer belt TB and the first elastic body  92 . Both widthwise end portions of the medium P thus pass through the nip N 2  without deviating from the nip N 2  formed by the transfer belt TB and the first elastic body  92 . 
     Supplement 4 
     As described above, the outer periphery of the second roller  70  (outer periphery of the elastic body  90 ) is disposed so as to come into contact with the transfer belt TB at the nip N 2 . In other words, the transfer belt TB and the second roller  70  (elastic body  90 ) have such a relation that the second roller  70  touches the transfer belt TB. In other words, the transfer belt TB is an object touched by the second roller  70  (elastic body  90 ). 
     The configuration of the image forming apparatus  10  has been described thus far. 
     Operation of Image Forming Apparatus 
     Referring now to the drawings, the operation of the image forming apparatus  10  according to the exemplary embodiment is described. 
     The controller  60  that has received image data from an outside apparatus (not illustrated) operates the toner image forming unit  20 . Then, each toner image forming unit  20  forms a toner image G on the corresponding photoconductor  22  as a result of the corresponding charging device  24  charging the photoconductor  22 , the corresponding exposure device  26  exposing the photoconductor  22  to light, and the corresponding developing device  28  developing the toner image G. 
     Subsequently, each first roller  32  receives an application of a first transfer voltage from the power source (not illustrated) and first-transfers the toner image G formed on the corresponding photoconductor  22  to the rotating transfer belt TB at the nip N 1 . The movable portion (not illustrated) moves the BUR  110 , in the stand-by position, to the operation position and the BUR  110  causes the second roller  70  and the transfer belt TB to form the nip N 2 . Thereafter, at the time when each toner image G that has been first-transferred to the rotating transfer belt TB and held on the transfer belt TB arrives at the nip N 2  together with the transfer belt TB, the transporting device  40  transports a medium P to the nip N 2 . The power source PS applies a second transfer voltage to the shaft  112  of the BUR  110  and forms an electric field that causes the toner images G held on the transfer belt TB to be transferred to the medium P that passes through the nip N 2 . As a result, the second transfer portion  36  second-transfers the toner images G on the transfer belt TB to the medium P that passes through the nip N 2 . Subsequently, the transporting device  40  transports the medium P to a nip N 3 . Then, the fixing device  50  heats the toner images G second-transferred to the medium P using the heating portion  50 A and presses the toner images G using the pressing portion  50 B to fix the toner images G to the medium P. The medium P to which the toner images G have been fixed is ejected by the transporting device  40  to the outside of the image forming apparatus  10 , whereby the operation of the image forming apparatus  10  is finished. 
     The operation of the image forming apparatus  10  has been described thus far. 
     Effects 
     Subsequently, effects of the exemplary embodiment are described. 
     Referring now to the drawings, effects (first and second effects) of the exemplary embodiment are firstly described in comparison with conceivable comparative modes described below. In the description of the comparative modes, components that are the same as those included in the exemplary embodiment are denoted by the same reference symbols. 
     Description of Comparative Modes 
     As illustrated in  FIG. 4 , second elastic bodies  94 A constituting a second roller  70 A according to a comparative mode are cylinders having an inner diameter of D 3  and an outside diameter of D 1 . Specifically, the inner diameter and the outside diameter of the second elastic bodies  94 A are the same as those of the first elastic body  92 . The second roller  70 A illustrated in  FIG. 4  is in the unloaded state. The second roller  70 A according to the comparative mode has the same configuration as the second roller  70  according to the exemplary embodiment except for the above-described points. A second transfer portion  36 A according to the comparative mode has the same configuration as the second transfer portion  36  according to the exemplary embodiment except that the second transfer portion  36 A includes the second roller  70 A according to the comparative mode in place of the second roller  70  according to the exemplary embodiment. A transfer device  30 A according to the comparative mode has the same configuration as the transfer device  30  according to the exemplary embodiment except that the transfer device  30 A includes the second transfer portion  36 A according to the comparative mode in place of the second transfer portion  36  according to the exemplary embodiment. An image forming apparatus  10 A according to a comparative mode has the same configuration as the image forming apparatus  10  according to the exemplary embodiment except that the image forming apparatus  10 A includes the transfer device  30 A according to the comparative mode in place of the transfer device  30  according to the exemplary embodiment. 
     When a second transfer is performed using the transfer device  30 A according to the comparative mode, an electric-current leakage may occur between the transfer belt TB and the shaft  80  of the second roller  70 A. When an electric-current leakage occurs, an electric current flows from the shaft  80  to the transfer belt TB, failing to form an electric field that causes the toner images G on the transfer belt TB to be second-transferred to a medium P. In addition, when an electric-current leakage occurs between the transfer belt TB and the shaft  80 , a second transfer error (a failure in transferring a toner image G to a medium P passing through the nip N 2  during the electric-current leakage) occurs at the second transfer portion  36 A (transfer device  30 A). In the image forming apparatus  10 A, an image forming failure attributable to the second transfer error occurs. Such electric-current leakages between the transfer belt TB and the shaft  80  particularly increasingly occur for example when a voltage of approximately −9 kV is applied to the BUR  110  in the low-temperature low-humidity environment. 
     When an electric-current leakage occurs between the transfer belt TB and the shaft  80 , an electric current flowing in response to the electric-current leakage presumably flows through a current passage formed between an end portion of the body  82  of the shaft  80  and the transfer belt TB. This current passage is presumed as the shortest route on the surface of each second elastic body  94 A (inner periphery and the free end FE) forming the nip N 2 . As described above, the reason why electric-current leakages between the transfer belt TB and the shaft  80  particularly increasingly occur in the low-temperature low-humidity environment is presumably because a larger quantity of moisture in the atmosphere is more likely to adhere to the surface of the second elastic bodies  94 A than in the case of the standard environment. The above-described presumption is believed to be reasonable from the evaluation results of the examples ( FIG. 13 ) described below. 
     First Effect 
     As illustrated in  FIG. 3 , each second elastic body  94  of the second roller  70  according to the exemplary embodiment gradually thickens from the boundary BP to the free end FE. In the exemplary embodiment, as illustrated in  FIG. 2 , the portion of each second elastic body  94  forming the nip N 2  is deformed so as to come closer to the corresponding protrusion  84  (axis C 1 ) of the shaft  80  while being spaced apart from the protrusion  84 . Thus, the electric path (shortest route on the surface, or on the inner periphery and the free end FE, of the second elastic body  94  forming the nip N 2 ) on each second elastic body  94  according to the exemplary embodiment is longer than the electric path on each second elastic body  94 A according to the comparative mode (see  FIG. 2  and  FIG. 5 ). The electric resistance of the electric path on each second elastic body  94  according to the exemplary embodiment is higher than the electric resistance of the electric path on each second elastic body  94 A according to the comparative mode. 
     Thus, the second roller  70  according to the exemplary embodiment, when constituting the second transfer portion  36 , is less likely to cause electric-current leakages (has a higher leakage voltage) at the second elastic bodies  94  than the second roller  70 A according to the comparative mode. Accordingly, the transfer device  30  according to the exemplary embodiment is more likely to minimize transfer errors attributable to the electric-current leakages than the transfer device  30 A according to the comparative mode. In addition, the image forming apparatus  10  according to the exemplary embodiment is more likely to minimize image forming failures attributable to the transfer errors than the image forming apparatus  10 A according to the comparative mode. Here, a leakage voltage is a voltage applied to one of the shaft  112  of the BUR  110  and the shaft  80  of the second roller  70 , while the other one of the shafts  112  and  80  is grounded, and a voltage at which an electric-current leakage occurs between the transfer belt TB and the shaft  80 . As described above, in the second transfer portion  36  according to the exemplary embodiment, the shaft  80  is grounded and a voltage is applied to the shaft  112 . In the second transfer portion  36  according to the exemplary embodiment, a second transfer voltage applied to the shaft  112  is naturally controlled so that it does not exceed the leakage voltage (so that it is smaller than an absolute value of the leakage voltage). 
     Second Effect 
     As illustrated in  FIG. 3 , unlike the second elastic bodies  94 A according to the comparative mode, each of the second elastic bodies  94  of the second roller  70  according to the exemplary embodiment gradually thickens from the boundary BP to the free end FE, so that the outside diameter of the second elastic body  94  increases from D 1  from the boundary BP to the free end FE. Thus, even when the second elastic bodies  94  according to the exemplary embodiment are deformed by being pressed by the transfer belt TB, each second elastic body  94  forms the nip N 2  without having a gap between itself and the transfer belt TB throughout the area from the boundary BP to the free end FE, as illustrated in  FIG. 2 . Unlike the second elastic bodies  94  according to the exemplary embodiment, the second elastic bodies  94 A according to the comparative mode do not protrude toward the transfer belt TB beyond the outer periphery of the first elastic body  92 . In addition, the second elastic bodies  94 A according to the comparative mode are thinner than the second elastic bodies  94  according to the exemplary embodiment. Thus, when a nip N 2  is formed, the second elastic bodies  94 A according to the comparative mode are deformed to a lesser extent and have a smaller reaction force against the transfer belt TB than the second elastic bodies  94  according to the exemplary embodiment. Thus, the second elastic bodies  94 A according to the comparative mode are more likely to form a gap between itself and the transfer belt TB over an area from the boundary BP to the free end FE. When a gap is formed between the outer periphery of each second elastic body  94  and the transfer belt TB, an electric-current leakage is more likely to occur in the gap between the outer periphery of the second elastic body  94  and the transfer belt TB. In contrast, the second elastic body  94  according to the exemplary embodiment is less likely to form a gap between itself and the transfer belt TB than the second elastic body  94 A according to the comparative mode. 
     Thus, the second roller  70  according to the exemplary embodiment, when constituting the second transfer portion  36 , is less likely to cause an electric-current leakage in a gap between the outer periphery of each second elastic body  94  and the transfer belt TB than the second roller  70 A according to the comparative mode. Accordingly, the transfer device  30  according to the exemplary embodiment is more likely to minimize transfer errors attributable to electric-current leakages than the transfer device  30 A according to the comparative mode. In addition, the image forming apparatus  10  according to the exemplary embodiment is more likely to minimize image forming failures attributable to transfer errors than the image forming apparatus  10 A according to the comparative mode. 
     Subsequently, third to fifth effects of the exemplary embodiment are described. 
     Third Effect 
     As described above, the second elastic bodies  94  of the second roller  70  according to the exemplary embodiment gradually thicken from the boundary BP to the free end FE and D 1  and D 2  satisfy the inequality D 2 /D 1 ≦1.10. 
     Here, it is presumed that a second roller (not illustrated) having D 1  and D 2  that satisfy the inequality 1.10&lt;D 2 /D 1  forms a nip N 2  together with the transfer belt TB. Second elastic bodies of this presumed second roller have an outside diameter that changes at a higher rate from the boundary BP to the free end FE than the second elastic bodies  94  according to the exemplary embodiment. Specifically, these second elastic bodies have a higher rate of change of the thickness than the second elastic bodies  94  according to the exemplary embodiment. Thus, these second elastic bodies are less likely to be deformed into a nip N 2  over an area from the boundary BP to the free end FE, whereby a gap is more likely to be formed between the outer periphery of each second elastic body and the transfer belt TB. In contrast, the second elastic bodies  94  according to the exemplary embodiment are less likely to form a gap between themselves and the transfer belt TB than the second elastic bodies of the second roller having D 1  and D 2  that satisfy the inequality 1.10&lt;D 2 /D 1 . 
     Thus, the second roller  70  according to the exemplary embodiment, when constituting the second transfer portion  36 , is less likely to cause electric-current leakages in a gap between the outer periphery of each second elastic body  94  and the transfer belt TB than the second roller having D 1  and D 2  that satisfy the inequality 1.10&lt;D 2 /D 1 . Accordingly, the transfer device  30  according to the exemplary embodiment is capable of reducing transfer errors attributable to the electric-current leakages further than the transfer device including the second roller having D 1  and D 2  that satisfy the inequality 1.10&lt;D 2 /D 1 . In addition, the image forming apparatus  10  according to the exemplary embodiment is more likely to minimize image forming failures attributable to the transfer errors than the image forming apparatus including the transfer device. 
     Fourth Effect 
     As described above, the second transfer portion  36  according to the exemplary embodiment includes a movable portion (not illustrated). Also as described above, while an image forming operation is not performed, the second roller  70  according to the exemplary embodiment is in the unloaded state, as illustrated in  FIG. 3 , with the nip N 2  being unfastened and the elastic body  90  of the second roller  70  being separated from the transfer belt TB. 
     Thus, in the second transfer portion  36  according to the exemplary embodiment, the second elastic bodies  94  are less likely to retain permanent deformation than a second transfer portion that does not include a movable portion (or a second transfer portion in which the second roller  70  forms a nip N 2  all the time together with the transfer belt TB). Thus, the second transfer portion  36  according to the exemplary embodiment is less likely to cause rotation failures (nonuniform peripheral speed during rotation) in association with the electric-current leakages at the second elastic bodies  94  and the remaining permanent deformation on the second elastic body  94  than a second transfer portion that does not include a movable portion. A second transfer portion that does not include a movable portion and a transfer device and an image forming apparatus that include the second transfer portion are included within the range of the technical scope of the invention. 
     Fifth Effect 
     As described above, the elastic body  90  according to the exemplary embodiment is made of an electrically conductive foam (foam containing urethane foam and an electrically conductive member). Thus, when being rubbed against the transfer belt TB, the second roller  70  according to the exemplary embodiment is less likely to be torn (has a greater tolerance) than, for example, a second roller having a similar shape as the second roller  70  including an elastic body  90  made of nitrile butadiene rubber (NBR or nitrile rubber). A second roller having a similar shape as the second roller  70  and including an elastic body  90  made of nitrile butadiene rubber (NBR or nitrile rubber), a second transfer portion including the second roller, and a transfer device and an image forming apparatus including the second transfer portion are included within the range of the technical scope of the invention. 
     MODIFICATION EXAMPLES 
     Referring now to  FIG. 6  to  FIG. 10 , modification examples in which the exemplary embodiment is modified are described. 
     First Modification Example 
     Configuration 
     As illustrated in  FIG. 6 , a second roller  70 B according to a first modification example has second elastic bodies  94 B having a shape different from the shape of the second elastic bodies  94  of the second roller  70  according to the exemplary embodiment. Specifically, the outside diameter of each second elastic body  94 B curvedly increases from the boundary BP to the free end FE. In a cross-sectional view taken throughout the axis C 1 , the outer peripheral edge of each second elastic body  94 B forms a curve extending from the boundary BP to the free end FE and recessed toward the axis C 1 .  FIG. 6  illustrates the second roller  70 B in the unloaded state. Except for the above-described points, the first modification example has the same configuration as the exemplary embodiment. Here, the second roller  70 B according to the first modification example is an example of an electric conductive roller. 
     Effects 
     The effects of the first modification example are similar to those of the exemplary embodiment. 
     Second Modification Example 
     Configuration 
     As illustrated in  FIG. 7 , a second roller  70 C according to a second modification example has second elastic bodies  94 C that have a shape different from the shape of the second elastic bodies  94  of the second roller  70  according to the exemplary embodiment. Specifically, the outside diameter of each second elastic body  94 C curvedly increases from the boundary BP to the free end FE. Here, in a cross-sectional view taken throughout the axis C 1 , the outer peripheral edge of each second elastic body  94 C forms a curve extending from the boundary BP to the free end FE and swelling in the radial directions.  FIG. 7  illustrates the second roller  70 C in the unloaded state. Except for the above-described points, the second modification example has the same configuration as the exemplary embodiment. Here, the second roller  70 C according to the second modification example is an example of an electric conductive roller. 
     Effects 
     The effects of the second modification example are the same as those of the exemplary embodiment. 
     Third Modification Example 
     Configuration 
     As illustrated in  FIG. 8 , a second roller  70 D according to a third modification example includes second elastic bodies  94 D having a shape different from the shape of the second elastic bodies  94  of the second roller  70  according to the exemplary embodiment. Specifically, the outside diameter of each second elastic body  94 D remains unchanged as D 1  from the boundary BP to the free end FE but the inner diameter of each second elastic body  94 D curvedly decreases. Here, in a cross-sectional view taken throughout the axis C 1 , the inner peripheral edge of each second elastic body  94 D forms a curve extending from the boundary BP to the free end FE and recessed in the radial directions.  FIG. 8  illustrates the second roller  70 D in the unloaded state. Except for the above-described points, the third modification example has the same configuration as the exemplary embodiment. Here, the second roller  70 D according to the third modification example is an example of an electric conductive roller. 
     Effects 
     The effects of the third modification example are the same as the first, third, and fifth effects of the exemplary embodiment. 
     Fourth Modification Example 
     Configuration 
     As illustrated in  FIG. 9 , a second roller  70 E according to the fourth modification example includes second elastic bodies  94 E having a shape different from the shape of the second elastic bodies  94  of the second roller  70  according to the exemplary embodiment. Specifically, the outside diameter of each second elastic body  94 E curvedly increases from the boundary BP to the free end FE whereas the inner diameter of each second elastic body  94 E curvedly decreases from the boundary BP to the free end FE. In a cross-sectional view taken throughout the axis C 1 , the inner peripheral edge of each second elastic body  94 E forms a curve extending from the boundary BP to the free end FE and recessed in the radial directions. In a cross-sectional view taken throughout the axis C 1 , the outer periphery of each second elastic body  94 E forms a curve extending from the boundary BP to the free end FE and recessed toward the axis C 1 .  FIG. 9  illustrates the second roller  70 E in the unloaded state. Except for the above-described points, the fourth modification example has the same configuration as the exemplary embodiment. Here, the second roller  70 E according to the fourth modification example is an example of an electric conductive roller. 
     Effects 
     The effects of the fourth modification example are the same as those of the exemplary embodiment. 
     Fifth Modification Example 
     Configuration 
     As illustrated in  FIG. 10 , a second roller  70 F according to a fifth modification example includes a shaft  80 F and a first elastic body  92 F of an elastic body  90 F having shapes different from the shapes of the shaft  80  and the first elastic body  92  of the elastic body  90  of the second roller  70  according to the exemplary embodiment. Specifically, the shaft  80 F is a column having a diameter of D 4  (the diameters of the body  82 F and the protrusion  84  are D 4 ). The elastic body  90 F includes a first elastic body  92 F and second elastic bodies  94 F and the inner diameter of the first elastic body  92 F is D 4 .  FIG. 10  illustrates the second roller  70 F in the unloaded state. Except for the above-described points, the fifth modification example has the same configuration as the exemplary embodiment. Here, the second roller  70 F according to the fifth modification example is an example of an electric conductive roller and the shaft  80 F according to the fifth modification example is an example of a shaft. 
     Effects 
     The effects of the fifth modification example are the same as those of the exemplary embodiment. 
     Sixth Modification Example 
     Configuration 
     As illustrated in  FIG. 11 , a second roller  70 G according to a sixth modification example includes second elastic bodies  94 G having a shape different from the shape of the second elastic bodies  94  of the second roller  70  according to the exemplary embodiment. Specifically, the outside diameter of each second elastic body  94 G remains unchanged as D 1  from the boundary BP to a fixed portion between the boundary BP and the free end FE (a portion located inward of the free end FE and outward of the boundary BP) and linearly increases from the fixed portion to the free end FE, that is, from the portion near the boundary BP to the free end FE.  FIG. 11  illustrates the second roller  70 G in the unloaded state. Except for the above-described points, the sixth modification example has the same configuration as the exemplary embodiment. Here, the second roller  70 G according to the sixth modification example is an example of an electric conductive roller. 
     Effects 
     The effects of the sixth modification example are the same as those of the exemplary embodiment. 
     Seventh Modification Example 
     Configuration 
     As illustrated in  FIG. 12 , a second roller  70 H according to a seventh modification example includes second elastic bodies  94 H having a shape different from the shape of the second elastic bodies  94  of the second roller  70  according to the exemplary embodiment. Specifically, the outside diameter of each second elastic body  94 H remains unchanged as D 1  from the boundary BP to the free end FE. The inner diameter of each second elastic body  94 H remains unchanged as D 3  from the boundary BP to a fixed portion between the boundary BP and the free end FE (a portion located inward of the free end FE and outward of the boundary BP) and linearly decreases from the fixed portion to the free end FE, that is, from the portion near the boundary BP to the free end FE.  FIG. 12  illustrates the second roller  70 H in the unloaded state. Except for the above-described points, the seventh modification example has the same configuration as the exemplary embodiment. Here, the second roller  70 H according to the seventh modification example is an example of an electric conductive roller. 
     Effects 
     The effects of the seventh modification example are the same as the first, third, and fifth effects of the exemplary embodiment. 
     The details of specific exemplary embodiments of the invention have been described thus far. The present invention, however, is not limited to the above-described exemplary embodiments. Other exemplary embodiments are conceivable within the range of the technical scope of the invention. 
     For example, the second roller  70  according to the exemplary embodiment has been described as a roller used for a second transfer. However, as long as the second roller  70  is usable for forming an electric field, the second roller  70  does not have to be an electric conductive roller used for a second transfer. The second roller  70  may be used as, for example, a charging roller that electrically charges the photoconductor  22 , a first transfer roller used for first-transferring a toner image G on the photoconductor  22  to the transfer belt TB, or a roller used for other purposes. The second rollers  70 B,  70 C,  70 D,  70 E,  70 F,  70 G, and  70 H according to the modification examples obtained by modifying the second roller according to the exemplary embodiment (first to seventh modification examples, or hereinafter referred to as second rollers according to modification examples) are also similarly usable for other purposes. 
     It has been described that the second roller  70  according to the exemplary embodiment is grounded and a second transfer voltage is applied to the BUR  110 . However, a second transfer voltage may be applied to the second roller  70  and the BUR  110  may be grounded, instead. 
     In the description of the exemplary embodiments, the second roller  70  according to the exemplary embodiment and the second rollers according to the modification examples have been separately described. However, a second roller obtained by combining the configurations of different second rollers is also included within the range of the technical scope of the invention. For example, the shape of the outer periphery of each second elastic body  94 E of the second roller  70 E according to the fourth modification example may be changed to the shape of the outer periphery of each second elastic body  94  of the second roller  70  according to the exemplary embodiment, each second elastic body  94 B of the second roller  70 B according to the first modification example, or each second elastic body  94 C of the second roller  70 C according to the second modification example. 
     In the description of the exemplary embodiments, the second elastic bodies  94 ,  94 B,  94 C,  94 D,  94 E,  94 F,  94 G, and  94 H have been described using the second roller  70  according to the exemplary embodiment and the second rollers according to the modification examples as examples. However, the shapes of the second elastic bodies  94 ,  94 B,  94 C,  94 D,  94 E,  94 F,  94 G, and  94 H are not limited to these shapes. Each second elastic body may have any shape as long as it covers the corresponding protrusion  84  of the shaft  80  while being spaced apart from the protrusion  84  and it gradually thickens from the boundary BP to the free end FE while the second elastic body is retaining an outside diameter greater than or equal to the outside diameter D 1  of the first elastic body  92 . 
     In the description of the second transfer portion  36  according to the exemplary embodiment, the BUR  110  is vertically moved by a movable portion (not illustrated). Instead, the BUR  110  may be disposed in the operation position without the movable portion being provided to the second transfer portion  36  and the second roller  70  may be made vertically movable by another movable portion (not illustrated). 
     EXAMPLES 
     Referring now to the drawings, examples and comparative examples are described. 
     General Description 
     Hereinbelow, an example and comparative examples (comparative examples 1 and 2) are described. Firstly, second rollers according to the example and the comparative examples were fabricated in the following manner. Then, leakage voltages that occur in the second rollers according to the example and the comparative examples were evaluated. Outstanding properties and evaluation results of the second rollers according to the example and the comparative examples are illustrated in the table in  FIG. 13 . 
     Evaluation Method of Leakage Voltage 
     Each of the second rollers according to the example and the comparative examples (comparative examples 1 and 2) was attached to DocuCentre-V C7775 (manufactured by Fuji Xerox Corporation). Under the low-temperature low-humidity environment (environment of a temperature of 10° C. and a humidity of 15%), a cyan halftone image was printed over the entirety of an image forming area of an A4-size medium P (plain paper copy paper) and the printed image was observed to evaluate the leakage voltage. 
     Second Rollers According to Example and Comparative Examples Second Roller According to Example 
     The second roller according to the example was shaped like the second roller  70  according to the exemplary embodiment (see  FIG. 3 ). Specifically, the diameter D 3  of the body  82  of the shaft  80  was determined as φ14 mm and the width of the body  82  was determined as 330 mm. The diameter D 4  of the protrusions  84  of the shaft  80  was determined as φ8 mm and the protrusion length was determined as 20 mm. The elastic body  90  was made of an electrically conductive urethane foam. An elastic body  90  having an outside diameter D 1  of φ20 mm and an inner diameter D 3  of φ13 mm in the unloaded state was prepared and the shaft  80  was inserted into the elastic body  90  with pressure. Thereafter, both end portions of the elastic body  90  were cut so that the elastic body  90  has a width of 340 mm. In the state where the shaft  80  was inserted into the elastic body  90  with pressure, the elastic body  90  was ground so that a portion of the elastic body  90  covering the body  82  has a thickness of 3 mm. The resultant portion was defined as the first elastic body  92 . In addition, the elastic body  90  was ground so that the free ends FE of the second elastic bodies  94  have a thickness of 3.5 mm. The resultant portions were defined as the second elastic bodies  94 . The width (dimension in the axial direction) of the second elastic bodies  94  formed on both end portions of the shaft  80  was 5 mm. 
     Second Roller According to Comparative Example 1 
     The second roller (not illustrated) according to the comparative example 1 has the same configuration as the second roller according to the example except that the second roller according to the comparative example does not include the second elastic bodies  94 . 
     Second Roller According to Comparative Example 2 
     A second roller according to the comparative example 2 has the same configuration as the second roller according to the example except that the outside diameter of the second elastic bodies  94  is equivalent to the outside diameter of the first elastic body  92  (that is, both thicknesses are 3 mm). The second roller according to the comparative example 2 is shaped like the second roller  70 A according to the comparative mode (see  FIG. 4 ). When the second roller according to the comparative example 2 was manufactured, the elastic body  90  was ground while a spacer having an outside diameter of φ14 mm and an inner diameter of φ8 mm was placed in a gap between each protrusion  84  of the shaft  80  and the corresponding second elastic body  94 . 
     Supplement on Table in FIG.  13   
     In the table illustrated in  FIG. 13 , the hollow portion means a gap between each protrusion  84  of the shaft  80  and the corresponding second elastic body  94 . The table illustrated in  FIG. 13  indicates that each of the second rollers according to the example and the comparative example 2 has a hollow portion, that is, a gap between each protrusion  84  and the corresponding second elastic body  94 . In addition, d 1  denotes the thickness of the first elastic body  92  and d 2  denotes the thickness of each second elastic body  94 . Here, d 2  in the comparative example 1 is described as zero. This means that the comparative example 1 does not include any second elastic body  94 . Thus, the second roller according to the comparative example 1 does not include a hollow portion (that is why the hollow portion column corresponding to the comparative example 1 is filled with “absent” in the table illustrated in  FIG. 13 ). 
     Consideration 
     As illustrated in the table in  FIG. 13 , the leakage voltage (V Leak ) in the second roller according to the example is −9.8 kV under the low-temperature low-humidity environment. Thus, the leakage voltage (V Leak ) (or the absolute value of the leakage voltage) in the second roller according to the example is higher than the leakage voltages (V Leak ) (or the absolute values of the leakage voltages) in the second rollers according to the comparative examples 1 and 2. Thus, in the second transfer portion  36  that includes the second roller according to the example (a form of the second roller  70  according to the exemplary embodiment), an electric-current leakage is less likely to occur between the shaft  80  and the transfer belt TB under the low-temperature low-humidity environment. When the comparative example 1 and the comparative example 2 are compared with each other, the leakage voltage in the second roller according to the comparative example 2 that includes the second elastic bodies  94  is higher than the leakage voltage in the second roller according to the comparative example 1 that does not include the second elastic bodies  94 . From these facts, the results on the table illustrated in  FIG. 13  are assumed to reflect the ground for the above-described presumption that an electric current flows through a current passage on each second elastic body  94  during an electric-current leakage, and reflect that the configuration of the second roller  70  according to the exemplary embodiment has the first to third effects. 
     The foregoing description of the exemplary 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 be defined by the following claims and their equivalents.